Hybrid Electric Vehicles
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Hybrid Electric Vehicles An alternative for the Swedish market?
Karl-Erik Egebäck Sören Bucksch
TITEL/TITLE ISBN 91-89511-08-5 Hybrid Electric Vehicle. An alternative for the ISSN 1104-2621 PUBLICERINGSDATUM/DATE PUBLISHED Swedish market? FÖRFATTARE/AUTHOR October, 2000 UTGIVARE/PUBLISHER Karl-Erik Egebäck, Autoemission K-E E Consultant, Sören Bucksch, KFB KFB – Swedish Transport and SERIE/SERIES Communications Research Board, Stockholm KFB-Report 2000:53 KFBs DNR 2000-388
SUMMARY See page 8
KFB Reports are sold through Fritzes’, S-106 47 Stockholm. Other KFB publications are ordered directly from KFB
Hybrid Electric Vehicles
An alternative for the Swedish market?
Karl-Erik Egebäck Sören Bucksch
This report was originally written in Swedish and it has now been updated and translated into English during July – September year 2000 by Karl-Erik Egebäck and Liz Egebäck Foxbrook
KFB Report 2000:53
CONTENTS TABLES.............................................................................................................................................................. 5 FIGURES ............................................................................................................................................................ 6 1
INTRODUCTION ....................................................................................................................................... 11
THE COMPLEXITY OF THE DEVELOPMENT OF FUELS AND VEHICLES.............................. 14
HYBRID SYSTEMS ................................................................................................................................... 17 4.1 4.2 4.3
Series hybrid systems ............................................................................................................................ 17 Parallel hybrid systems .......................................................................................................................... 18 Further developed hybrid systems ......................................................................................................... 19
FUELS .......................................................................................................................................................... 21 5.1 Fossil fuels............................................................................................................................................. 22 5.1.1 Standardization of gasoline, diesel oil and some other fuels......................................................... 22 5.1.2 Gasoline......................................................................................................................................... 23 5.1.3 Diesel oil........................................................................................................................................ 25 5.1.4 Liquefied Petroleum Gas (LPG) .................................................................................................... 26 5.1.5 Natural Gas ................................................................................................................................... 27 5.2 Flexible fuels ......................................................................................................................................... 28 5.2.1 Methanol........................................................................................................................................ 29 5.2.2 Dimethyl ether (DME) ................................................................................................................... 30 5.2.3 Synthetic gasoline and diesel oil.................................................................................................... 30 5.2.4 Hydrogen ....................................................................................................................................... 31 5.3 Not fossil fuels....................................................................................................................................... 32 5.3.1 Biogas............................................................................................................................................ 32 5.3.2 Ethanol and methanol.................................................................................................................... 33 5.4 Summary of automotive fuels................................................................................................................ 34
ENGINES – POWER UNIT ....................................................................................................................... 36 6.1 6.2 6.3 6.4
DEVELOPMENT OF BATTERIES.......................................................................................................... 58 7.1 7.2
Otto engines........................................................................................................................................... 36 Diesel engines........................................................................................................................................ 40 Alternative engines ................................................................................................................................ 45 Fuel cells................................................................................................................................................ 48 Present day batteries .............................................................................................................................. 58 Choice of batteries for hybrid vehicles .................................................................................................. 63
HYBRID VEHICLES ................................................................................................................................. 66 8.1
Potential for improved energy use in hybrid systems with different types of internal combustion engine 66 8..1.1 Theoretic background for the potential in improved energy use ................................................... 67 8.1.2 The interaction between the control unit, the energy transmitters and the mechanical power transmitters in a hybrid system...................................................................................................................... 69 8.1.3 Result of the PNGV program......................................................................................................... 72 8.1.4 The influence of hybrid systems on conventional engines ............................................................. 75 8.1.5 Series hybrid or parallel hybrid? .................................................................................................. 77 8.2 Examples of developed and demonstrated hybrid vehicles ................................................................... 78 8.2.1 Mercedes series hybrid .................................................................................................................. 78 8.2.2 DaimlerChrysler’s Necar fuel cell series ...................................................................................... 79 8.2.3 Toyota Prius, parallel hybrid ........................................................................................................ 80 8.2.4 Ford’s parallel hybrid vehicles ..................................................................................................... 82 8.2.5 Nissan’s parallel hybrid ............................................................................................................... 84 8.2.6 Some other light hybrid vehicles.................................................................................................... 86 8.3 Some examples of heavy-duty hybrid vehicles ..................................................................................... 87
Comparison between different fuels/drive trains................................................................................... 95
EFFICIENCY – FUEL ECONOMY ....................................................................................................... 100 9.1 The efficiency of hybrid systems......................................................................................................... 100 9.2 The use of energy and efficiency at different driving patterns. ........................................................... 102 9.2.1 Studies of hybrid systems for BMW ............................................................................................. 103 9.2.2 Energy use and efficiency of Mitsubishi hybrid trucks ................................................................ 104
10 10.1 10.2 10.3
FUEL AND DISTRIBUTION .............................................................................................................. 106 Conventional fuel ............................................................................................................................... 106 Alternative fuels ................................................................................................................................. 106 Electric energy.................................................................................................................................... 108
TEST METHODS ................................................................................................................................. 110
IMPACT ON THE EMISSIONS ......................................................................................................... 114
12.1 Theoretical background for a emission potential................................................................................. 114 12.2 Emissions related to the hybrid system................................................................................................ 116 12.2.1 The series hybrid from Mercedes-Benz ....................................................................................... 116 12.2.2 The parallel hybrid vehicle Prius from Toyota............................................................................ 117 12.3 The relationship between the driving pattern and the emissions ......................................................... 118 13 13.1 13.2 13.3
SUMMARY OF COSTS ....................................................................................................................... 120 Cost of the System............................................................................................................................... 121 Cost of the Batteries ............................................................................................................................ 121 Total costs............................................................................................................................................ 122
EFFECTS ON HEALTH ...................................................................................................................... 124
PROBLEMS - BALANCING ............................................................................................................... 130
SHORT TERM DEVELOPMENT ...................................................................................................... 132
DEVELOPMENT IN THE LONG TERM ......................................................................................... 135
REFERENCES .................................................................................................................................................. 137
TABLES Table 1. Environmentally classified parameters and components in gasoline in Sweden (MK1). ...........................23 Table 2. Environmentally classified parameters and components in diesel oil in Sweden ......................................25 Table 3. Composition of the Danish natural gas from the North See.......................................................................28 Table 4. Natural gas in the world – resources, production and ventilated/flared, year 1997..................................28 Table. 5. The composition of purified biogas according to an analysis...................................................................33 Table 6. Summary of physical and chemical characteristics of various engine fuels. .............................................34 Table 7. Emission Standards for light duty vehicles EU. Source: Auto/Oil II. ........................................................37 Table 8. EU-Standards for passenger cars and other light-duty vehicles................................................................40 Table 9. EU-standards for heavy-duty diesel fueled engines. Source: EU Directive 1999/96/EC...........................40 Table 10. EU-standards for heavy-duty diesel fuelled and gaseous fueled engines................................................41 Table 11. Comparison between different types of fuel cells. ....................................................................................51 Table 12. Anode and cathode reactions of SOFC ....................................................................................................52 Table 13. Anode and cathode reactions in a fuel cell with PEM. ............................................................................53 Table 14. The mass of the fuel cell and an estimation of how the mass can be reduced. .........................................55 Table 15. Development of lead acid batteries EU. Source: Cooper and Moseley, 1998. ........................................63 Table 16. Vehicle manufacturers and their choice of battery. .................................................................................64 Table 17. Comparison between a series hybrid and a conventional drive system. (Source: Mercedes (Abthoff et al., 1998). .........................................................................................................................................................79 Table 18. Fuel consumption (as gasoline) for Toyota Prius and some other vehicles with low fuel consumption. .82 Table 19. Some particulars for a number of hybrid buses and some trucks with hybrid systems. ...........................88 Table 20. Comparison between a series and a parallel hybrid vehicle respectively with a conventional vehicle.104
Table 21. The effect of charging strength on the working efficiency of charging and on the durability................109 Table 22. Japanese 10–15 mode emission standards and emissions for Toyota Prius. .........................................117 Table 23. Result from emission tests according to the US EPA FTP-75 test procedure. .......................................118 Table 24. Result from emission tests according to the US EPA HFET test procedure. .........................................118 Table 25. Estimation of the cost levels for various details in the hybrid system/vehicle........................................121 Table 26. Evaluation of various batteries. .............................................................................................................122 Table 27. Estimate of risks for yearly incidence of cancer associated with air pollution.. ....................................124 Table 28. Unit risk factors. Cancer mortality risks from life-long exposure to 1 µg/m3* .......................................128
FIGURES Figure 1. Series hybrid system. (Source: DOE, USA)..............................................................................................18 Figure 2. Parallel hybrid system. (Source: DOE, USA). .........................................................................................19 Figure 3. Specific energy used in a gasoline fuelled car and a series hybrid when driven on road (Iwai, 1998). ..20 Figure 4. Estimation concerning the use of various fuels in a fuel cell. Source: Sasaki, 1999................................35 Figure 5. The relationship between the displacement of the engine and its efficiency. ...........................................38 Figure 6. The specification (left) of the engine shown in the figure (right). ............................................................39 Figure 7. Diagram for control of engine, DIATA. Source: Breida 1998. ................................................................43 Figure 8. The HPCR fuel system with its components. Source: Breida 1998. .........................................................44 Figure 9. Typical torque curve and power curve respectively for a Stirling engine................................................46 Figure 10. Stirling engine. Source: USCAR, 1999. ................................................................................................47 Figure 11. GM’s gas turbine. Source: USCAR, 1999. .............................................................................................48 Figure 12. Basic principals of fuel cells. Source: US Department of Defence. ......................................................51 Figure 13. The three sections of a fuel cell: fuel processor, stack of fuel cell and DC/AC transformer. Source: US Department of Defence...............................................................................................................................52 Figure 14. Fuel cell with solid oxides (ceramic). Source. US Department of Defense............................................53 Figure 15. Components of a fuel cell system. ..........................................................................................................54 Figure 16. Organization for the development of batteries. Source: Sutula et al., 1998...........................................59 Figure 17. Specific effect respective to specific energy for various batteries. .........................................................65 Figure 18. Mussel diagram over fuel consumption for a 1.25 liter gasoline engine, modified by Ecotraffic (Sweden). Source: (Menne et al., 1996). ..........................................................................................................67 Figure 19. The Japanese 10-15 mode cycle............................................................................................................71 Figure 20. Different cases for calculation of the braking energy recovery (0-0.15g). ............................................71 Figure 21. Improvement of fuel consumption for a series hybrid when tested according to the ............................72 Figure 22. Potential for different automotive system. Source: NRC, 98..................................................................73 Figure 23. Mercedes hybrid car Necar 3, equipped with fuel cells. ........................................................................79 Figure 24. Mercedes hybrid car Necar 3, equipped with fuel cells. ........................................................................80 Figure 25. Schematic configuration of the hybrid system from Toyota. ..................................................................81 Figure 26. Schematic picture of Ford’s hybrid car(”LSR”). Source: Automotive Engineering International/February 1999). Reference: (Buchholz, 1999) ...........................................................................83 Figure 27. Ford’s hybrid system PTH with gasoline engine. Source: Buschhaus et al, 1998. ................................84 Figure 28. Scematic configuration of the hybrid system from Nissan. Kitada et al., 1998......................................85 Figure 29. Nova Transit Bus. Source: Whartman, 1998.........................................................................................90 Figure 30. Volvo hybrid delivery truck. ...................................................................................................................91 Figure 31. Power unit (APU) for hybrid vehicle.....................................................................................................93 Figure 32. Control systems, internal combustion engine and the emission control system. Source: Monrad och van der Weijer, 1998. .......................................................................................................................................94 Figure 33. Mitsubishi aerial working truck. Source: (Horii et al., 1998)................................................................95 Figure 34. Efficiency improvement by hybrid strategy. (Source Takaoka et al., 1998). .......................................101 Figure 35. Comparison of fuel consumption at different driving modes................................................................102 Figure 36. Mitsubishi service truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998. ..............................................................................................................................104 Figure 37. Mitsubishi working truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998. ..............................................................................................................................105 Figure 38. Energy efficiency of same important component of a HEV. Source: Horii et al., 1998. ......................105 Figure 39. Tests with parallel hybrid vehicle developed at the University of California Davis. Source: Duoba, M. and Larsen R., 1998 ..................................................................................................................................111 Figure 40. Tests with series hybrid vehicle developed at the West Virginia University. Source: Duoba ,M. and Larsen R., 1998 ..............................................................................................................................................112 Figure 41. Engine operational area and exhaust temperature. Source: Hirose et al., 1998. ................................115
Figure 42. Emissions for Mercedes series hybrid vehicle (prototype)...................................................................117 Figure 43. Cost of various driving systems. Source Mercedes Benz. .....................................................................120 Figure 44. Exemple of relevant aldehyds. Source: Egebäck and Westerholm, 1997.............................................126 Figure 45. Examples of relevant alkenes. Source: Egebäck and Westerholm, 1997. ............................................126 Figure 46. Examples of relevant alkyl nitrites. Source: Egebäck and Westerholm, 1997. ....................................127 Figure 47. Examples of relevant Monoaromatic compounds. ...............................................................................127 Figure 48. Examples of relevant PAC. Source: Egebäck and Westerholm, 1997..................................................128
1 SUMMARY According to the Swedish National Encyclopaedia the word hybrid comes from the Latin word (h)i’brida, hy’brida which means “cross” or “bastard” and its origin is the Greek word “bastard”. If one continues to the phrase hybrid vehicle the encyclopaedia describes this as “a vehicle which is fitted with more than one type of energy transformer and energy storage system for its propulsion, and where the drive or the regulating system of the vehicle determines which type shall be used. The energy converter can, for example, be a heat engine, a hydraulic engine, an electric engine or a fuel cell. Energy storage can be carried out by means of a chemical, kinetic, electric or hydrostatic energy storage system or by means of heat storage. In the future the development of hybrid electric vehicles will be of special interest since they provide an intermittent freedom of exhaust gases”. The object of this report, which has been produced within The Swedish Transport and Communications Research Board’s (KFB’s) Electric and Hybrid Vehicle Program, is to assemble information on and describe the situation for the development of hybrid vehicles and various alternatives within this field of development. In the report the description is concentrated mainly on the combination of combustion engine and electric battery, which is the most common combination in present day hybrid vehicles. In order to take a glimpse into the future even the combination of fuel cells and electric battery is described. Among the important factors for vehicle owners are the cost of the vehicle, the fuel and the use of the vehicle. For alternative vehicles the cost of the vehicle is usually higher than for an “ordinary” vehicle, quite simply because the alternatives are manufactured in smaller series. For the vehicle manufacturer and even for the buyer of the vehicle it can therefore be of great importance that as large a market as possible is obtained for the alternative vehicle, for example electric vehicle or hybrid vehicle. One way for the vehicle manufacturer to quickly create a large market can be to sell the cars at a lower than usual price, for a period of time. Examples of this are electric vehicles, which are manufactured by Ford and GM and the hybrid car Prius, which is manufactured by Toyota. One can ask the question as to whether the environmental advantages would be a sufficient reason for paying the higher price, if the vehicles were not subsidized or if the running costs had not been decreased by alterations in taxation. In present day hybrid systems one of the energy converters consists of electric batteries. A continued, comprehensive development of batteries is required in order to improve the technique and to reduce the costs. One important reason for introducing hybrid vehicles is to improve fuel economy, so that a considerable part of the report is devoted to this. The distribution of fuel and electricity is described briefly. The costs are also described briefly due to the fact that techniques of hybrid vehicles are so new that there is not yet sufficient information on which to base a clear picture of the costs. This can also be said of the testing methods for the hybrid vehicles in question, since no standardized testing methods have yet been determined. The effect on emissions is not completely clear, but present day development certainly seems to be leading in the right direction. It is obviously not especially difficult to find problems in a technique development which has not yet been tested on the open market other than on a small scale. The report suggests which are judged to require further technical development. The real barrier to a speedy introduction of the hybrid system is probably the cost. There are several questions, concerning vehicles, which have been in the limelight for the last 30 to 40 years, namely emissions, fuel consumption and safety. When we discuss running
hybrid vehicles and the possible advantages of them, it is chiefly the following points which give rise to a problem of greater or lesser magnitude. Vehicle emissions Energy conversion and fuel savings Battery development Costs The assessment which has been carried out and is presented in this report has provided a basis for the evaluation of some different lines of development for hybrid systems. In the short term (0-5 years) the picture is relatively clear since it takes time and money to develop and present new ideas. It seems clear that in the case of light-duty vehicles a continued effort will be put into developing the hybrid system, to some extent. In the case of heavy-duty vehicles there is a certain degree of uncertainty as far as the development of hybrid busses is concerned, due to the fact that it is generally private bus companies who carry out the development. Certainly this development generally takes place in conjunction with the manufacturers, but the question is how strongly the manufacturers are engaged in the development work. Under these circumstances strong support from the authorities is required in order for the development work to be carried out, since such activities are generally expensive. The light-duty hybrid electric vehicles that have hitherto been developed are mainly parallel hybrids. If the development of hybrid systems takes place it will most certainly concern lightduty vehicles, and these will be parallel hybrids equipped with an otto or a diesel engine, depending on which of these the manufacturers wish to back. The requirement for energy efficiency is easier to meet with a diesel engine in the hybrid system. Of all the hybrid systems which have been studied for this report, it is Ford’s parallel hybrid with a diesel engine which takes top place as far as energy is concerned. Generally speaking diesel engines have experienced a period of positive development, especially during the last decade. This is due, to a great extent, to the fact that diesel oil has been successively improved, and this is particularly true for Sweden. It now remains to be shown that the current development of purification techniques for the reduction of nitrogen oxides and particulate emissions will provide a system with such a good durability that it can be accepted for use during a long period. If such a development can be demonstrated and the development of diesel oil continues to give good results then the diesel engine can prove to be a suitable alternative for, amongst others, vehicles with hybrid systems (both light-duty and heavyduty). The efficiency of the otto engine will not reach the efficiency level of the diesel engine. However, continuous development of the otto engine is taking place in order to improve the fuel efficiency. The fuel consumption of an advanced otto engine with direct fuel injection is somewhere between that of the diesel engine and of the conventional otto engine. Where the emission of exhaust gases and noise are concerned one can reverse the argument. In the short term it can be difficult to achieve the same emission requirements for diesel driven vehicles as for petrol driven ones. Unfortunately the direct injected otto engines developed up to the present day have shown a higher level of the emissions of oxides of nitrogen and particles than their conventional counterparts. Vehicles with direct injected otto engines will also probably achieve the same emission performance as the vehicles fitted with conventional otto engines in the not too distant future. When determining the development of hybrid driven heavy-duty vehicles it should be remembered that there are clear differences between light-duty and heavy-duty vehicles. One difference is that the heavy-duty vehicles have, as a rule, more space available for additional and heavier equipment than have passenger cars. However there is no absolute solution, 9
especially for busses in city centers, which have a limited space unless some passenger space is sacrificed. The general opinion is, however, that heavy-duty vehicles can carry a relatively heavier packet of batteries than a passenger car. Providing that one can charge the batteries from the mains, this could mean that a system with series hybrids is advantageous. But it is not completely certain that this is a correct conclusion for all types of heavy-duty vehicles. If one considers the whole scale of heavy-duty vehicles, from light trucks to busses, there can be many cases where a parallel hybrid is a better alternative than a series hybrid. The recommendation is therefore that each type of vehicle and each possible use of the vehicle should be considered when choosing a hybrid system. In the long term it is not easy to foresee the development but in the case of hybrid electric vehicles some possible scenarios are already in sight, and these could be achieved during the coming 5 to 15 years: 1. Parallel hybrids will come to be the dominant system for light-duty vehicles and possibly even for certain other groups of vehicles. 2. Series hybrids will come to be used solely for heavy-duty vehicles. 3. The present day development of fuel cells will lead to more manufacturers paying greater attention to electric or hybrid vehicles with fuel cells. The development of fuel cells will most probably continue, at least at the present day level. Technical speaking it is likely that vehicles fitted with fuel cells will function satisfactorily until mass production is begun. A hinder for the development of a larger market is however the cost. Will the average vehicle owner be able to afford such a vehicle? A vehicle powered by a fuel cell can be considered to be an electric vehicle. If the fuel cell continues to be developed at the same pace as at present, so that a reasonable price level will be achieved on the market, the market for electric vehicles will also be favored. In the report the use of series hybrid vehicles is estimated to be limited to heavy-duty hybrid vehicles. Hybrids will not be likely to be relevant for heavy-duty vehicles, with the exception of those trucks which operate in city centers, i.e. trucks which are used for the distribution of goods to shops, as garbage vehicles and as certain types of working vehicle for service purposes. Continued development of the hybrid system for busses seems uncertain for various reasons. It is chiefly local bus companies and private contractors who develop hybrid busses, leading to uncertainty in the continuance of the development. If there is a technical breakthrough in the manufacture of batteries and simultaneously the manufacturers increase their efforts to develop hybrid vehicles, the situation can be changed so that there is a speedier introduction of hybrid systems for heavy-duty vehicles.
2 INTRODUCTION According to the Swedish National Encyclopaedia the word hybrid comes from the Latin word (h)i’brida, hy’brida which means “cross” or “bastard” and its origin is the Greek word “bastard”. If one continues to the phrase hybrid vehicle the encyclopaedia describes this as “a vehicle which is fitted with more than one type of energy transformer and energy storage system for its propulsion, and where the drive or the regulating system of the vehicle determines which type shall be used. The energy converter can, for example, be a heat engine, a hydraulic engine, an electric motor or a fuel cell. Energy storage can be carried out by means of a chemical, kinetic, electric or hydrostatic energy storage system or by means of heat storage. In the future the development of hybrid electric vehicles will be of special interest since they provide an intermittent freedom of exhaust gases”. The hybrid vehicles described in this report are hybrid electric vehicles (HEVs) – a commonly used definition of a hybrid electric propulsion system vehicle equipped with an internal combustion engine as one power source and electric traction motor as the other power source. Today there are in principal two types of hybrid system the “series hybrid” and “parallel hybrid”, see section 4 for a detailed description of these two systems. In this report the presentation about hybrid systems will primarily be concentrated to the combination internal combustion engine and electric motor which is today the most common combination for hybrid vehicles. However, when looking into the future, even the combination fuel cells and electric batteries will be described. A question that seems to be difficult to answer is what type of hybrid system will be the dominant one on the market for light-duty hybrid vehicles. Today it is likely that the development of hybrid systems for light-duty vehicles will be concentrated to the parallel system. However, this does not necessarily mean that series hybrid systems will be an uninteresting alternative in the long run. For heavy-duty vehicles it is likely that the series hybrid system will be the commonly used system. There are also other systems of types which can be seen to lie somewhere between series and parallel hybrids. Since the number of hybrid vehicles is limited and since some of them are only prototypes – except, possibly, in the case of Japan – the car owners are generally not familiar with this type of vehicles. It can also be said that long term testing of hybrid vehicles is so far rather limited. The fuel to be used in hybrid vehicles (and also in fuel cell vehicles) is a question of high importance since one strong motive for the use of the hybrid technology (and electric vehicles) is to reduce the emissions and to improve the fuel economy. In one section of this report dealing with the fuel cycle it is shown that the technology used for reforming the fuel in the vehicle will have a significant influence on the fuel economy. However, in this case, the practical possibility of storing fuel in the vehicle may be seen to be more important than energy efficiency. The classical example concerning the storage, distribution and use of gaseous fuels as fossil gases (natural gas and others) contra storage, distribution and use of liquid fuels is valid even in the case of fuel cells. Some of the key questions for the car owner are of course the cost of the vehicle when purchased, the fuel and the use of the vehicle. For alternative vehicles the of purchasing the vehicle is generally higher than for the commonly used commercial vehicles since the alternative vehicles are mostly produced in low quantities. For the car manufacturer but even for the purchaser of the vehicle it will be an advantageous to create a large market for an alternative such as the electric vehicle or the hybrid. One possibility for the manufacturer to create a larger market may be to sell the vehicles at a lower price during the introduction of the actual vehicle, resulting in a reduced profit. Examples which can be mentioned are the
electric vehicles manufactured and sold in the USA and the hybrid vehicle Prius manufactured by Toyota, Japan. It can also be asked whether the desire of the buyers of alternative vehicles to keep the pollution as low as possible would be strong enough for them to buy such vehicles if the costs of the purchasing and using these vehicles were to be subsidized. These and related questions are discussed in some papers which are referred to in this report. Since many of the articles and some reports of investigations in papers give somewhat similar information, a selection of the information has been carried out done (not systematic, however) in order to reduce the number of references. The fact that some of the information in the papers may old or not relevant for hybrid vehicles and therefore not valid for this report has been considered when selecting the literature references. The development of batteries is one of the subjects which are not easy to describe since there is a great number of report and other information to take notice of and that there may be a difference between the requirement for batteries for hybrid vehicles compared with requirements for batteries to be used in electric vehicles. In hybrid vehicles charging and discharging of the battery occurs in continuously repeated cycles during driving in traffic. This is true to some extent even for electric vehicles using regenerative braking (the energy released during decelerations – “braking”- is recycled to the battery) and this recharging must certainly be taken into account when developing batteries for hybrid vehicles and also for electric vehicles using a system for regenerative braking. One difference between a electric vehicle and a hybrid vehicle is that even the capacity for depth-of-discharge (DOD) may have to be larger for a battery used in an electric vehicle than for a battery used in a hybrid vehicle. A drawback for hybrid vehicles (and to a higher extent for electric vehicles) is that the cost of the battery is high. This is especially so for light batteries with high energy density; (in one case the cost of the battery pack was higher than purchase price of the vehicle). It seems therefore to be necessary to reduce the cost of the battery in order to use it in a vehicle produced in a large quantity. This should be made possible by an improvement in the production of the batteries. The USA constitutes a large part of the global market for vehicles and during the last decade the development of alternative vehicles has to a large extent been influenced by the program Partnership for a New Generation of Vehicles (PNGV) started 1994. The program is based on an agreement between the US Government (including 12 Departments) and Chrysler, Ford and General Motors. The program is directed towards passenger cars and the goal of the project is to improve the fuel economy of these vehicles to 80 MPG (approximately 3 liters /100 km). Further aims are as follows: The first goal is to: ”Significantly improve national competitiveness in manufacturing”. This means an improvement of the productivity of the US base for manufacturing by an significant upgrading of the US manufacturing technology, including adoption of agile and flexible manufacturing and reduction of costs and lead times, while reducing environmental impact and/or improving quality. The second goal is to: ”Implement commercially viable innovation from ongoing research on conventional vehicles”, which among other things means to pursue advances in vehicles leading to improvements in fuel efficiency and emissions while pursuing safety advances to maintain safety performance. The car industry will commit itself to applying those commercially viable technologies that are expected to significantly increase vehicle fuel efficiency and improve emissions. The third goal is to: ”Develop a vehicle to achieve up to 3 times the fuel efficiency of today’s comparable vehicle”. This is a “fuel efficiency improvement of up to three times the average of Concorde/Taurus/Lumina, with equivalent customer purchase price of today’s comparable sedans type of vehicles adjusted economics”. The requirement for fuel economy is 80 MPG
(approx. 3 l/100 km). Other requirements include emission standards as Tier II, which are; 0.125 for HC, 1.7 for CO and 0.2 for NOx, in g/mile at the odometer reading of 100 000 miles, which is equal to HC: 0.078, CO: 1.06 and NOx: 0.124 in g/km at the odometer reading of 160 000 km. According to PNGV at least 80 % of the vehicle has to be recyclable or to quote ”Achieve recyclability of at least 80 %”. This report is prepared for The Swedish Transport and Communications Research Board (KFB) and is based on the results and experiences which have been presented in the international literature. The impression is that there is a great optimism, especially about the development of fuel cells. Unfortunately this optimism may be misplaced since the barriers are described as fewer than those which will have to be solved in reality. This optimism may lead to the time frame for the remaining development of fuel cells being shortened unrealistically. Fuel cells are certainly not a new invention, but the application to our commonly used passenger cars requires that they be produced at a reasonable cost and that they can be proven to be efficient energy transformers without having negative effects on the environment or causing other problems. The aim of this report, which has been prepared within KFBs program for electric and hybrid vehicles, was to combine and describe the situation concerning the development of hybrid vehicles and the different alternative for this development. This report will constitute one of the reports on which the final report for the electric and hybrid vehicle will be based. The disposition of this report is as follows: firstly there will be a discussion of the complexity of the development and introduction of reformed or new automotive fuels and new technologies for propelling motor vehicles. After this there will be an introductory description of two different hybrid systems according to the terminology used for such systems. Since both gasoline and diesel oil and even alternative fuels may be used for the actual energy transformers, the internal combustion engine or fuel cells, a rather extensive description of the different fuels is given. In the case of the above mentioned two types of energy transformers, it is expected that a considerable development will occur in addition to that which has already occurred. Since this will also have an impact on the development of the vehicles, the expected changes will be discussed in two of the sections. In present day hybrid systems one of the energy transformers is the battery and in order to increase their life cycle, and thereby even the costs, further extensive research and development has to be carried out. An important aim of the introduction of hybrid vehicles is to improve the fuel economy and this will be discussed rather extensively in this report. The distribution of fuels and electricity will be briefly described, as will the costs, since the technology of hybrid vehicles is quite new and there is, so far, no clear information concerning costs. This also applies to testing methods for relevant hybrids, since no standardized methods have been determined. The effect of the emissions has not been thoroughly mapped out, but the trend seems to be decidedly positive. It must be mentioned that Peter Ahlvik, Ecotraffic, has supplied valuable contributions to the report and has also checked the report in its final stages, which is highly appreciated. The authors of the report, Karl-Erik Egebäck, Autoemission K-E E Consultant AB, tel. +46 (0)155 28 24 44 and Sören Bucksch, Sören Bucksch AB, tel. +46 (0)8 580 33 330 would like to thank Liz Egebäck Foxbrook for the help with the translation from Swedish to English and the languish check. A thanks is directed also to KFB, who has financed this work.
3 THE COMPLEXITY OF THE DEVELOPMENT OF FUELS AND VEHICLES During recent years there has been much activity in the development of both conventional vehicles, engines such as direct injected otto engines (GDI) and alternative vehicles such as electric vehicles, hybrid vehicles in which the electricity has been generated by fuel cells. Even concerning fuels much development has taken place. The conventional fuels, gasoline and diesel oil, are reformed in different ways. For example in the case of gasoline, in order to use more environmentally friendly components there has been a reduction in the content of benzene, an addition of an alcohol to the gasoline, a reduction in the vapor pressure etc. The improvements of diesel oil mean primary that the content of both sulfur and aromatics are reduced and that the cetane number is increased to the level of 48 to 50 or higher. Despite these changes of gasoline and diesel oil the fuels must of course still be of such quality that they can be used in existing vehicles on the market. The owners of the vehicles must as far as possible be kept indemnified and therefore it is important that no changes in the fuels are realized that will create a problem for the car owner. The gasoline must have a sufficiently high octane number in order to not cause knocking in the engine. After the introduction of lead-free gasoline, during the second part of 1980s, there has been a requirement that a gasoline with a lubricating additive should be available for the older cars in order to protect the valves in their engines. On the other hand it has been of great importance that the “lead-free” gasoline should not contain lead of such amount that there is deterioration of the emission control system. For the diesel oil it is important that its density is kept within specified limits so as not to have an undue influence on the engine power set by the engine manufacturer. It is also important that the diesel oil has such a lubricating quality that the wear of the engine and especially the fuel injection system does not increase. The introduction of a new technology not only requires certain changes in society, for example a new infrastructure if a new alternative fuel is to be introduced, but it will also have an impact on the user of the new technology. This is true not least in the area of automotive vehicles and the reason may be that the ownership and use of a motor vehicle is costly both for the private person and society, but also because of the fact that new technologies are linked to feelings of uncertainty when they are introduced. If we keep to area of automobiles the experiences are that even small problems in the introduction of a new technology can change a positive attitude within the car owners to a negative attitude. Of course the cost of the new technology is an important factor in this case. The above discussion indicates that the complexity of even small and positive changes will have an effect on the car owner and use of his or her vehicle. The improvement of the environmentally related quality of the fuel or the introduction of renewable fuels will probably be accepted as something positive for most of us including the car owner. In the case of the introduction of lead-free gasoline, which was a success in Sweden without causing any great problems, it was important that there was a good degree of co-operation between the authorities and the oil and car industry. Usually the car owner does not suffer if there are changes in the style or function of the car or if there are changes in the fuel composition unless there is an increase in the cost of purchasing the car and in running it. The latter may, however, be a problem for many car owners. Fortunately some changes can be advantageous even if they lead to higher costs for the car owner. One such advantage can be that a vehicle with high environmental qualities may be accepted for use in areas where the use of cars is restricted.
There may be advantages with some types of vehicles despite their being “odd”, provided they function well and especially if they are more fuel efficient than other vehicles. The reduction in the cost of using these cars may balance the higher cost for the purchase of the car. However, it is not easy to find such cases though progress in this direction can be expected to occur in the long term. If the interest in hybrid vehicles remains as keen as it has been, they may be interesting objects for purchasers of new cars. However, success for hybrid vehicles depends in the end on whether the car manufacturers make an effort to develop such reliable, well performing and not too costly hybrid concepts as will be attractive for the purchasers. During a transitional period it seems necessary for governments or governmental authorities in the progressive countries to financially or by other means support at least some part of the development and introduction of the hybrid vehicles. Such support was given for example in Sweden at the end of the 1980’s, on the introduction of the present-day efficient emission control system for light duty vehicles. One problem concerning hybrid vehicles is that the costs of developing these vehicles and the batteries to be used in them will be on a much higher level than for the above mentioned emission control system. The competition for customers and the increasing requirements concerning the emissions and fuel economy has led to the use of large resources among the car manufacturers and engine manufacturers in the search of new solutions for new types of engines and vehicles. Such trials may on the other hand lead to a shortness of resources, which require different priorities within the actual industry. For the car industry there may also be a dilemma that there are many alternatives to study in order to take the right decision and to maintain sound priorities. The shortness of resources and today’s focusing on fuel cells and the development of fuel cell vehicles may lead to a shortage in resources for the development of electric- and hybrid vehicles as compared to the case without this focusing on fuel cells. This focusing on fuel cells may also obstruct the development of alternative fuels for internal combustion engines since a successful development and introduction of alternative fuels is more closely related to a lower cost of these fuels, and especially biobased fuels, than it is to their having a potential for lowering the emissions of harmful substances. In the case of hybrid vehicles there is a possibility that the car manufacturers see these vehicles as transfer technology up to fuel cell vehicles, and that the only main change to be carried out is to use a stack of fuel cells instead of the internal combustion engine. Returning to the subject of hybrid vehicles, there is a question as to whether renewable fuels will be one of the possible alternatives, especially as improvements of gasoline and diesel oils have resulted in a higher potential for these fuels to meet the requirements concerning health and environment, except in the case of the so-called greenhouse gases carbon dioxide, methane, nitrous oxide etc. However there do not seem to be any technical obstacles to the use of renewable fuels for hybrid vehicles and, as has already been underlined, there is an obvious advantage in the reduction of greenhouse gases when using renewable fuels and even, in some respects, in the form of a reduction in the emission of NOx and particles. The latter is true also for other alternative fuels such as natural gas. Since the development of hybrid vehicles is taking place in the car manufacturers’ plants all over the world, the description of the hybrid vehicles is based on information found in the international literature and obtained through communication with different persons working in the field of development of hybrid systems. One problem is that the information from the car manufacturers is restricted and it is difficult to obtain information about the status of the development. In the meantime, from the start of a report like this many new inventions have been presented by the car manufacturers but not officially published and some new prototypes or other types of hybrid vehicles have been presented on the market. Unfortunately some of this information, which may be important, has not been included in this report. 15
So far most of the development of hybrid vehicles has occurred chiefly in Japan but even in the USA. So far there are only one or two prototypes of hybrid vehicles which have been presented by the European car manufacturers to the knowledge of the authors of this report. One of these is a Renault Kangoon and this vehicle will be available on the market year 2001. .
4 HYBRID SYSTEMS As has been described above the conception “hybrid”, related to motor vehicles, is a vehicle equipped with more than one energy transformer. By this definition it is not stated which type of energy transformers are used in the hybrid system. However, this does not mean that the number and types of energy transformers are unlimited when looking at practical, technical and economical possibilities. Fuel cells may, for example, be used in hybrid vehicles and such vehicles are already presented as prototypes, but are not today either so technically well developed or economically feasible as to be introduced on the market. The aim here is to describe the main alternatives of hybrid systems in greater detail, in order to provide information for those readers who are familiar with the hybrid technology for motor vehicles. It is usual today to have an internal combustion engine connected to an electric generator used as one of the (or the main) energy transformer system and a battery in combination with an electric motor as the other energy transformer system. The engine can most commonly be an otto engine or a diesel engine but other alternatives are possible such as sterling engines, gas turbines etc. The different types of engine are presented in Section 6. There are many different types of battery available such as special lead (acid) batteries, a valve regulated lead accumulator (VRLA), nickel metal hydride batteries (NiMH), natrium (sodium)-nickel chlorine batteries (named ZEBRA), zinc air batteries, lithium-ion and lithium-polymer batteries respective and some others. The use of capacitors and especially ultra-capacitors constitutes an important possibility of storing electric energy. The ongoing development of batteries is discussed in Section 7. The combination fuel cell and battery is a possible and attractive long-term alternative combination in hybrid vehicles. However, an even more attractive alternative would be a vehicle where the fuel cell is directly connected to the electric motor as the fuel cell can itself be regarded as a battery. The problem associated with such a system seems to be that electric energy is commonly not stored in a fuel cell system, such as the one mentioned, and therefore the fuel cells must rapidly convert the chemical energy in the fuel to electric energy, since the fuel cells has to produce a variably flow of electricity. The question is whether such a vehicle should be called a hybrid or an electric vehicle and one such vehicle is presented in Section 8.2.2. However, the energy efficiency of such a vehicle can exceed quite considerably that of a vehicle having both a fuel cell and a battery. The fuel cells are presented in Section 6. As already described there are two main types of hybrid systems, classified as series hybrids and parallel hybrids. There seems also to exist systems which could be classified as being of a type somewhere between series hybrids and parallel hybrids. However, in this report we are concentrating our description to the two main systems.
Series hybrid systems
By definition one can classify a series hybrid as a vehicle where an internal combustion engine (or some other type energy transformer) is placed in series with an electric motor (or more than one electric motor) for the traction of the vehicle. This implies in practice that the main function of the internal combustion engine is to generate electricity for the battery which in turn feeds the traction motor (or electric motor) either directly or by the battery, via a generator. In this manner there is no direct mechanical connection between the internal combustion engine and the driving wheels. Simply expressed one can also say that a series hybrid vehicle is basically powered by two sources. A common layout for a series hybrid system is shown in Figure 1. 17
The internal combustion engines used in a series hybrid system are usually rather small (compared with conventional traction system) and rarely deliver more than 50 % of the maximum power needed for propelling the vehicle*. Since there is no direct mechanical connection between the engine and the driving wheels, the engine in the most extreme case can be run more or less at constant load and speed. The condition - that the engine does not have to follow a certain dynamic driving cycle – means that the chances of obtaining low emission levels are good if for example an otto engine with a three-way catalyst system is used in the series hybrid system.
Figure 1. Series hybrid system. (Source: DOE, USA). In series hybrid vehicles an electric motor (or more than one motor) is used for the traction of the vehicle. Series hybrid systems are usually designed to be used in heavy-duty vehicles. In the case of buses where more than one electric motor is used these motors may be placed close to the driving wheels. One drawback of the series hybrid vehicle is that it has to be equipped with a rather powerful battery with a high energy density since all or at least 50% of the power may in extreme cases have to be delivered from the battery for the traction of the vehicle. The large batteries are heavy and add a considerable weight to the vehicle and are also considered to be costly.
4.2 Parallel hybrid systems The traction equipment of the parallel hybrid type of system is powered by two energy transformers, which work parallel with each other. In this case the internal engine is mechanically connected to the driving wheels via a gearbox and the electric motor supports the engine when more power is needed than can be delivered by the engine. Commonly the engine is larger and more powerful than in the series hybrid system while the electric motor is smaller and consequently less powerful. The internal combustion engine has to follow the type of dynamic driving conditions of the vehicle because of the mechanical connection to the driving wheels and this will somewhat reduce the potential for low emission levels. However, the impact of heavy transients can be a little less if a certain leveling by “peak shaving” is used in the hybrid system. One advantage with the parallel hybrid system is that the battery used is smaller and consequently lighter and less costly compared with the battery for series hybrids. A layout for a parallel hybrid system is shown in Figure 2. *
The most extreme variant of a series hybrid is an electric vehicle equipped with an auxiliary engine in order to increase the driving range of the vehicle.
Figure 2. Parallel hybrid system. (Source: DOE, USA).
Further developed hybrid systems
Today the development of hybrid vehicles seems to be towards the use of series hybrid systems, especially in heavy-duty vehicles and primarily in buses, while parallel hybrid systems are being developed for light duty vehicles. As has been pointed out earlier, the control of the internal combustion engine of a series hybrid vehicle involves a less varied load cycle than that is used for the engine of a parallel hybrid vehicle. Due to the less varied load of the engine a higher efficiency is achieved for an engine used in a series hybrid vehicle than for an engine in a parallel hybrid vehicle. This is discussed in more detail in section 6 and 8. Naturally it would be an advantageous in terms of fuel economy if the internal combustion engine in a parallel hybrid vehicle could be run in an area of the load cycle where the efficiency if the engine is highest. Areas with low loads should be avoided especially for otto engines since the efficiency of this type of engines drastically drops as the specific fuel consumption increases at low loads of the engine, which can be seen in Figure 3. In the figure different lines are shown for cases where the efficiency of the electric drive system (generator, charging/ discharging of the battery and the converter) is 77 % and 60 % resp. and where the maximum efficiency of the engine is as high as 35 %. In order to reduce the fuel consumption as far as possible the parallel hybrid system can, first of all, be designed according to the following characteristics: The internal combustion engine is switch off at the limit of low power needed for the traction of the vehicle. The battery is then used as the only energy source. The internal combustion engine switches off when the vehicle is stopped. The energy released during braking is fed back to the battery. The size of the internal combustion engine is adjusted to a lower requirement of power, as necessary. For acceleration and some other driving conditions, when the power of the internal combustion engine not is sufficient there will be support from the electric motor. Matching of the internal combustion engine with regard to the most fuel economic driving conditions by the use of a continuous variable transmission (CVT). Figure 3 shows the specific fuel consumption for a minivan and a series hybrid vehicle (SHEV*) system equipped with an internal combustion engine with an efficiency of 35 % and *
a generator. The specific fuel consumption was measured when driving on an even horizontal road. The SHEV exceeded the gasoline-fueled vehicle without hybrid system in fuel economy, i.e. in used energy under the following provisions; • when the efficiency of electric generation/charging/discharging of the battery is at least 60 %; • when the speed of the vehicle is less than 50 km/h; • when the efficiency of the electric generation/charging/discharging of the battery is at least 77%** and the speed of the vehicle is below 80 km/h. The performance of the SHEV was significantly better, concerning the use of energy, when the vehicle speed was less than 20 km/h. Iwai (Iwai, 1998) point out that a vehicle equipped with a parallel hybrid system (SPHV)*** has the best fuel economy i.e. energy efficiency at both low load and high load by driving the system as a SHEV at low load and as an internal combustion engine alternative at high load. According to Iwai it requires that the engine in a SHEV be propelled with high efficiency and an efficiency of at least 77 % and 60 % respectively for the electric drive system, for the SHEV to be able to surpass a gasoline fueled vehicle without hybrid system in the question of efficiency.
Figure 3. Specific energy used in a gasoline fuelled car and a series hybrid when driven on road (Iwai, 1998). The car manufacturers, for example Ford and Toyota, use the possibilities described by Iwai for the control of their hybrid systems. To be more specific, Ford uses them partly and Toyota uses them nearly full out. There are naturally further technical possibilities for improving the energy efficiency in the hybrid systems. Different hybrid vehicles are presented in Chapter 8. The above described study of the possibilities for further development and the two mentioned hybrid electric vehicles Toyota and Ford have shown that there is a potential for further improvements of hybrid systems.
Iwai has then calculated that the efficiency of the generation of electricity of 90 %, 95 % for the rectifier. Series-Parallel-Hybrid-Vehicle.
5 FUELS When developing the internal combustion engine for a hybrid vehicle the choice of fuel is an important question, because this is decisive for the type of engine to be used. The choice of fuel is also decisive for the efficiency of the engine as an energy transformer and its performance concerning the emissions of exhaust gases and noise. The question is then whether the hybrid vehicle should be optimized for high efficiency or low emission performance. This is decided, among other things, by the choice of fuel and the type of internal combustion engine to be used, but also by other components in the hybrid system and the interaction between these as directed by a control system. The authors of this report do not take any position concerning the choice of strategy regarding the best efficiency and the best emission performance by the hybrid system but have given some views about the role of fuel and engines in the coming sections. Hopefully the selection of the “right” fuel may contribute to achieving the goal of both an excellent efficiency and a good emission performance. The fuels which are actual for hybrid vehicles are the conventional fuels, gasoline and diesel oil, and those which are called alternative fuels. Of the latter the following may be used: Fossil fuels/fossil-based fuels (also called alternative fuels) such as natural gas (for example CNG or LNG), liquefied petroleum gas (LPG), dimethyl-ether (DME) synthetic gasoline and diesel oil and methanol based on natural gas. Bio-based fuels such as alcohols (ethanol and methanol), bio-gas and fatty oil esters (commonly called FAME). Hydrogen. It is technically possible to use different fuels, and therefore it is firstly a question as to whether a sufficient amount of fuel can be supplied, whether the fuel can be efficiently distributed and if the fuel is reasonable priced. However, the critical question is whether there is sufficient interest in investing in an alternative fuel. Which fuel will be elected to be used for fuel cells in the long run is, so far, an open question even if it from the point of energy should turn out that a gaseous fuel would be more efficient than methanol, which is a popular fuel for one of the popular fuel cells (PEM), see section 6.4. For hybrid vehicles gasoline or diesel oil commonly is used but for heavier vehicles, such as buses, gaseous fuels or an alcohol is used in some cases instead of diesel oil, see section 8. Fuels produced from either fossil-based material or bio-based materials are called flexible fuels in this report. In order to show one advantages of such a fuel, methanol can be taken as an example. In order to reduce the emission of the greenhouse gas CO2, it can be argued that methanol can be used in the long run, since it can be produced from natural gas to start with and later on from a bio-based material if an efficient and reliable method has been developed. However, there are various disadvantages of this, amongst others the risk that the introduction of bio-based methanol will be delayed many years if the cost of this fuel is regarded as being too high. In the following the discussion will be concentrated on gasoline, diesel oil, LPG, natural gas, bio-gas, ethanol, methanol, DME and two other synthetic fuels. Hydrogen will be mentioned as one alternative and then as a fuel for fuel cells but also as an energy carrier for other purposes. It can be of interest to mention that the government of Island has signed an agreement with the DaimlerChrysler-Ballard group, which may result in the production of hydrogen in Island, if the agreement is realized.
As has already been pointed out, there are many fuels within the fossil fuel group. However, in this context, it is of interest to primarily discuss gasoline and diesel oil, since these two fuels are commercially available in different countries all over the world. It seems also that they will be the primary fuels on the market for the transport sector, for a long time in the future. Natural gas and methanol based on natural gas are alternatives on an international market and they are therefore clearly objects of interest in this discussion. The development of automotive fuels in Sweden and even internationally has resulted in the present day situation where it is difficult to clearly define the over-all availability of the different gasoline blends and the different types of diesel oil. As a consequent, the specifications of these fuels changes from time to time since the increasingly severe emission standards not only affect the development of new vehicles but also of better fuels. In addition the specifications allow that the composition of fuels may show a variation not only from one country or group of countries to another group of countries – for example between the USA and Europe – but of course also between different classes of gasoline and diesel oil. In Sweden for example there is so far no requirement that all gasoline must contain a certain percentage of oxygenates, but on the other hand it not been explicitly declared that it is forbidden to blend gasoline with a certain amount of an alcohol or for instance MTBE or some other ether. Concerning MTBE the use of this oxygenate in gasoline is forbidden in California since there are evidence that the groundwater has been contaminated with MTBE in certain areas. Also in the federal USA a discussion has started as to whether MTBE should be fazed out from gasoline or whether the use of MTBE should be limited. These actions can result in the use of other ethers being questioned in the USA and California. It can be added that no similar actions have been taken in Europe on the EU level but a discussion has started which may also influence the use of MTBE in Sweden. However, to sum up the results of the development of gasoline and diesel oil, it is clear that both of these fuels have been improved during the last decade both in the USA and Europe and further improvements will be seen. 5.1.1
Standardization of gasoline, diesel oil and some other fuels
Neither gasoline nor diesel oil is a uniform mixture. Gasoline is produced by blending different hydrocarbons in order to meet the required specification primarily arrived at in cooperation between the car manufacturers and the oil industry. In Sweden this co-operation is organized by the Standards Institute (SIS-STG), which is also administratively responsible for the work, for the organization of meetings, for setting up the protocols and to printing and publishing the agreed standards. Authorities in Sweden such as the Environmental Protection Agency propose requirement for certain components in the fuel, especially if the fuel is going to be environmentally classified. In this case representatives for the agency or fuel experts usually participate in the work of preparation of standards. For a long time an extensive co-operation for international standardization of fuels for automobiles has been in existence. In Europe this work is organized within an organization called CEN (European Committee for Standardization). Experience has shown that there is an advantage in a co-operation within CEN, even if the requirements concerning some of the components or parameters in the fuel can differ from country to country. In areas with variable climatic conditions, such as the south of Europe and the Nordic countries, it is natural that the requirements for the automotive fuel differ. The connection between the composition of the fuel and health effects is briefly discussed in section 14.
In Sweden gasoline is classified environmentally. Some years ago there was no highest class MK1, i.e. a gasoline specified so as to have the best environmental quality, but today such gasoline exists on the Swedish market. The different classified parameters can be seen in the following summary, see Table 1. Gasoline has since long been regarded as a cleaner fuel than diesel oil. This judgment is certainly linked to the exhaust emissions from diesel-fueled vehicles. These have been seen as more dangerous from the health point of view than the exhaust emissions from gasoline fueled vehicles. The diesel vehicle exhaust has been shown to contain a greater mass of polycyclic aromatic hydrocarbons (PAH) than the gasoline vehicles. The greater mass of PAH is also linked to a higher mutagenic activity caused by the diesel vehicle exhaust when compared with gasoline vehicle exhaust. The diesel vehicle exhaust also contains a larger mass of particles which leads to a higher health risk when compared with gasoline vehicle exhaust. To this should be added that the smell of diesel vehicle exhaust is experienced as more unpleasant than the exhaust from gasoline fuelled vehicles. It is likely that many of these results and experiences can be referred to the difference in combustion process between the spark ignition engine (otto engine) and the compression ignition engine (diesel engine). On the other hand there are, or at least were, emissions which could be linked to components in the fuel. Not many years ago there were compounds with lead in gasoline in Sweden and additives are still used in gasoline in many other countries. It can also been underlined that gasoline contains benzene and other harmful hydrocarbons of which benzene is regarded as a carcinogen. Therefore it is wise to handle gasoline as a poison and also to be careful so as not to let gasoline touch the skin or to breathe gasoline vapor. Table 1. Environmentally classified parameters and components in gasoline in Sweden (MK1). Parameter Unit MK 1 ”temporary MK 1 from year 2000 blend of gasoline” (the date when EU spec. 1998 - 2000 2000 was implemented) Benzene
Max ppm (mass.)
Evaporated at150 °C1 Min. vol-% Additives
Not ash forming
Not ash forming
Source: STATOIL, Sweden.
The organic lead components belong to a group of additives which are regarded as the most poisonous components in gasoline. The existence of these organic lead pollutants in the environment was the main reason in Sweden for reducing lead additives in gasoline from 0.80 to 0.85 g/liter before 1970 to 0.70 from 1970, 0.40 from 1973, 0.15 g/liter 1980/81. “Green” (i.e. unleaded) regular gasoline was introduced year 1986 and 1995 the use of lead in gasoline was forbidden. Similar actions has been taken also in many other countries around the world. Later on the lead was faced out completely in the so called lead-free gasoline because lead was proven to be a poison for catalysts. Up to around the middle of the 1990’s there were one 23
or two blends of “green” gasoline in Sweden containing approximately 0.07 g/liter of lead additives. After that time another additive not containing lead (as used of lead was forbidden) has been used (see the second paragraph after this). Concerning the lead-free gasoline it can be of interest to know that even this gasoline may contain small amount of lead as long as leaded gasoline is distributed, due to the a risk for contamination during distribution of the fuel. The distribution lines for unleaded gasoline may not have been completely separated from those used for leaded gasoline. Therefore the concept “lead-free” is not relevant - a more adequate concept should be “unleaded gasoline”. Commonly the requirement for the non-leaded gasoline was that it should not contain more than 0.013 g lead per liter gasoline. Since no leaded gasoline exists in Sweden the blend of unleaded gasoline of today may be completely lead-free. There were two reasons for using lead-additives in gasoline of which one was that the octane number was increased and the second was that the valves in the engine were lubricated by lead. When the oil companies in Sweden decided to not use lead-additives in gasoline this additive was replaced with other additives – containing sodium or potassium - of which potassium was preferred since sodium gave high-temperature corrosion in the engine’s turbo aggregate. The opinion in the oil industry is that most of the cars in Sweden do not need any special lubrication for the valves and therefore they have decided to stop the blending of lubricating additive in gasoline. The replacement this time will be bottles containing a potassium additive, available at gas stations, which the owners of old cars can use when filling up gasoline. As already mentioned gasoline consists of a mixture of different hydrocarbons as can be seen in Table 1. One of the requirements for a good function of gasoline is that the motor octane number and the research octane number are high. For the Swedish unleaded (green) gasoline the research octane number has to be at least 95. For some years the oil companies in Sweden have provided the market with a premium gasoline with a research octane number of 98. Since the use of octane-increasing lead-components in the gasoline for catalyst cars introduced at the end of the 1980s was not allowed, oil companies had to use high octane components, such as isomerized hydrocarbons, at a higher rate than earlier. Aromatic hydrocarbons such as benzene and others also have a high octane number. The content of these hydrocarbons tended to increase and especially when 98 octane unleaded gasoline was introduced. This was also a consequence of a higher rate of the cracking of larger and heavier hydrocarbon molecules in order to have access to lighter hydrocarbon components for the production of gasoline. As can be seen in Table 1 the content of benzene is limited to 1% by volume according to EU specifications for gasoline which is an advantage. Since a high content of aromatics in gasoline is not desirable, when striving towards a blend of environmentally friendly automotive fuels, the content of aromatics and especially benzene (in gasoline) should be kept as low as possible. However, it is obvious that there is a correlation between certain hydrocarbons and the physical performance of the fuel such as between aromatics and the octane number of gasoline. Therefore other hydrocarbons, which do not expected to give negative health effects, should be used even if alternative production methods have to be used. In the case of aromatics the technology for production of other hydrocarbons such as alkylates is available but certainly more expensive and alkylates do not increase the octane number as aromatics do. According to information (Lindberg, 2000) the content of aromatics in the EU gasoline have to be reduced to 35% from year 2005. In Table 1 the difference can be seen between MK1 as “the transition quality” and gasoline according to the EU specification (the present day MK1 gasoline in Sweden) which has a maximum content of 42 % by volume (a rather high value). As noted above, benzene is a
component which favors the octane number but despite this has been decreased. Since the content of aromatics in gasoline will be limited to 35% within EU from year 2005 it may be difficult to produce since high octane the reformat used in the today’s 98 octane gasoline contains a high amount of aromatics. The environmental authorities in Sweden and others interested in the environment have seen it as important to keep the vapor pressure of gasoline as low as possible in order to limit the emission of evaporated fuel. However, the petroleum and car industry maintain that the vapor pressure must be kept at a high enough level enough to avoid cold start problems of especially old cars and this has resulted in the vapor pressure being considerably higher in the Swedish gasoline than in gasoline in, for example, USA. 5.1.3
There is also an environmental classification for diesel Sweden. Diesel oil is divided into three classes, MK1, MK2 and MK3 of which MK1 is defined as the most environmental friendly diesel oil. The environmental classification was implemented many years ago in Sweden and this has resulted in the stability of the system today. Preceding the classification, extensive investigations took place of more than ten different blends of diesel oils. The investigations clearly showed that there was a considerable potential in a new more environmentally directed specification of a diesel fuel. A decision could therefore be taken and the environmental classification was implemented without any delaying discussions. This led also to MK1 taking a large part of the market after only a few years. It should be mentioned that a part of the success of MK1 was that the introduction was supported by a tax incentive. The following requirements are valid for the different classes of diesel oil, see Table 2. Table 2. Environmentally classified parameters and components in diesel oil in Sweden (MK1, MK2 and MK3). Parameter/component MK 1 MK 2 MK 3 Density at 15 °C, kg/m3
Distillation, IBP, °C
Destination, 95 %, °C
PAC in aromatics, vol.-% Sulfur, mass-%
Source: The Swedish Environmental Protection Agency
Even within EU there has been a development of diesel fuels during recent years and new standards have been worked out and implemented even if these not as far reaching as the Swedish standards concerning the environmental improvement of diesel oil. The parameters which EU have given highest priority to, i.e. have seen to be most important to change, are the cetane number and the content of sulfur. According to the today’s standard the maximum content of sulfur in the fuel is 350 ppm (0.035 %) by weight, but a decision has been taken in order to limit the content of sulfur to 50 ppm (0.005 %) by weight from year 2005. Because of the negative effect of sulfur on catalysts (especially DeNOx catalysts), there is a demand from the car industry that the content of sulfur should be reduced to 30-35 ppm (0.0030-0.0035 %) by weight or as low as 5 ppm (0.0005 %) by weight. 25
Diesel oil has also been improved in many other countries. This started in California many years ago, but according to received information the environmental authorities in some eastern countries in the Far East and at least in Hong Kong have taken or will take actions which will lead to a reduction in sulfur content. A driving force for the reduction of sulfur is the need to reduce the emission of particles from diesel fueled engines. The European emission standards for diesel fueled heavy-duty engines are such that emissions have to be reduced to a level not regarded as possible some years ago. By this it seems necessary that new and very much improved diesel fuel will be introduced on the market. In this context it should be mentioned that in the early 1990’s a trial of synthetic diesel oil was carried out in order to prepare for the introduction of that fuel on the Swedish market. However, this attempt failed for at least two reasons. Firstly this synthetic diesel oil, which could in fact have reduced the particulate emissions and certain other emissions such as polycyclic aromatic hydrocarbons if used properly, required an adjustment of the fuel injection pump in order to increase the fuel flow and thereby achieving the same power output as when using commercial diesel fuel. On the other hand this should have resulted in a smoky engine when commercial fuel was used, which could happen quite often since the synthetic diesel oil was not distributed over the whole market in Sweden. The other reason for the failure of the introduction was that the synthetic fuel did not fulfil the requirements set for MK1 diesel fuel which meant that this fuel could not benefit from tax incentives from the Government. 5.1.4
Liquefied Petroleum Gas (LPG)
LPG as Liquefied Petroleum Gas is commonly called is one of the standardized automotive fuels and it must therefore meet the requirements concerning, among other things, the amount of the different of gaseous hydrocarbons – mostly propane and butane - specified in the standards. In Sweden the Standards Institute (SIS-STG) has adopted the standard for LPG called SSEN 589, which is also a European standard. According to the earlier standard for LPG, SS 115 54 20 the content of propane should be at least 70 %, the content of olefins not more than 10 % and the content of certain heavier hydrocarbons (C5 and heavier) should be a maximum of 15 % or a little more. In addition the old standard required a maximum and a minimum value of the vapor pressure. In the today’s standard (SSEN 589) the maximum value for vapor is 1550 kPa. The minimum value for vapor pressure is 250 kPA has been defined for four limit temperatures: Class A: -10 °C, Class B: -5 °C, Class C: 0 °C and Class D +10 °C and in Sweden Class A has been applied. There are two particular reasons for having certain requirement concerning the vapor pressure. First of all it is important to keep the vapor pressure as low as possible in order to restrict the evaporative emissions and secondly there must be a sufficiently high pressure in the gaseous fuels so that the fuel will flow into the engine as long as the fuel (liquefied LPG) is not pumped into the intake manifold of the engine. The composition of the gas must be such that the engine can be started and that the vehicle can be driven even at low temperatures. The main reason for the limiting of olefins in the gas is that olefins form deposits in the fuel system and the engine. During the 1970’s it was popular to use LPG in Sweden, especially in taxi cabs, as a consequence of there being a lower tax for LPG than for gasoline. However the rate of the tax was changed later on and this resulted in LPG was not being used to a sufficient extent to keep filling stations open and today LPG seems not to be a popular fuel in Sweden. There are many reasons for a very limited use of LPG in Sweden. There is no existing infrastructure for the distribution of LPG and it is not clear whether there are any economic advantages in the use of LPG. The vehicle has to be equipped with a special fuel system and it
is likely that many car owners would prefer to have one fuel system for LPG and one for gasoline if LPG was going to be used. In cases where the car is converted so as to add a LPG fuel system the gas tank may be placed in the luggage compartment. It is also uncertain whether the use of LPG will be favorable in terms of emissions especially in cases where “green” gasoline is available. In Sweden there have been incidences where a LPG fueled industrial truck has been used in halls for ice-hockey resulting in people, especially the players being poisoned from the emission of NO2. However, industrial trucks used for loading and unloading ships are often LPG-fuelled even if many of these trucks have now been replaced with electric powered industrial trucks. In order to sum up the discussion concerning the use of LPG it should be underlined that there is a good potential for achieving low emission levels if a proper technology is used for the adaptation of the vehicles to LPG. First of all the equipment used for LPG and also the engine must be dedicated for LPG and secondly the emission control system, including the catalyst, must be developed so as to be able to be used for LPG fueled vehicles. During recent years fuel systems have been developed with a function similar to fuel systems for today’s gasoline fueled vehicles. 5.1.5
Natural gas has a clear potential for being a more friendly automotive fuel than both gasoline and diesel oil, despite the fact that its use contributes to problems caused by greenhouse gases. These greenhouse gases are mostly carbon dioxide, emitted also by other fossil fuels and, in the case of natural gas, even methane. Up to now, however, the development has shown that there are certain technical difficulties in using gaseous fuels compared to the use of liquid fuels. This leads to the conclusion that there is a need for basic research and development of engines and the emission control system for the use of natural gas. However, the use of natural gas for engines in series hybrids can be estimated to be easier than its use in nonhybrid vehicles because it is no pronounced need for transient operation of the engine in a series hybrid. In Sweden it is a clear disadvantage in the use of vehicles fueled with natural gas, since there is no existing infrastructure for the distribution of natural gas in the main part of the country. The future for natural gas in Sweden is uncertain because no decision has been taken in order to build up a distribution system. It is true that natural gas is available for automobile use in the Southwest of Sweden but this distribution can only be maintained due to the fact that the main part of the gas is distributed to the industries in the area. It can also been said that no firm price of the natural gas used in vehicles has so far been decided. The price to be paid by driver of the car seems not to reflect the cost of the gas but is more or less linked to the price of the commercial fuels, especially gasoline. It is therefore not possible today to judge what the price of natural gas will be for the motorist, when more commonly used. Natural gas is estimated to be mainly a result of the decomposition of organic material but some events have indicated that natural gas also exists deep down under of the earth’s crust and that this gas is not a result of decomposition of organic material. If this is true, then there exists a huge quantity of natural gas in different parts of the world. However, even without the above mention natural gas under the earth’s crust, a great amount of natural gas can be found. Natural gas contains mostly methane but, depending on the well of the gas, it may contain up to 10 % or more of other saturated hydrocarbons such as butane, propane and ethane and even other combustible gases. Unfortunately natural gas also contains components which are to be regarded as pollutants, i.e. such a an amount of sulfur compounds that the gas has to be cleaned if used in for example fuel cell vehicles. Helium, which after hydrogen is the most 27
common element in the universe, is extracted from natural gas by lowering the temperature of the gas under high pressure. As already has been mentioned, the composition of natural gas varies from source to source. The Danish gas found in the North Sea has the following composition, especially of the following hydrocarbons and carbon dioxide and nitrogen see Table 3. Table 3. Composition of the Danish natural gas from the North See. COMPONENT CONTENT % Methane, CH4 91.6 Ethane, C2H6 4.7 Propane, C3H8 1.8 Higher hydrocarbons 1.1 Carbon dioxide, CO2 0.5 Nitrogen, N2 0.3 Source: The Swedish National Encyclopedia. Most of the huge quantity of natural gas can be found in the areas shown in Table 4. The greatest quantities can be found in Qatar (a peninsula in the Persian Gulf) and in Russia but also in the Far East, Africa and North America there are large amounts, as can be seen in the table. The production of natural gas was, according to BP 1 785.8 million tons in oil equivalence per year under the period 1988 – 1992, which is equal to approximately 50 % of the production of oil. Table 4. Natural gas in the world – resources, production and ventilated/flared, year 1997. Region Access* Production Ventilated/flared Billion m3 Billion m3 Billion m3 North America 15 (~1.6 %) 8 442 734 Central and South America 16 (~12.2 %) 6 272 83 Western Europe 4 (~1.3 %) 4 525 280 Eastern Europe and previous 53 903 No amount declared 701 USSR Middle East 26 (>9.3 %) 48 727 171 Africa 47 (~21.3 %) 10 059 102 Far East and Oceania Incomplete 12 355 243 declaration Total in the world 144 284 2 314 ---Source: US Department of Energy. *Amer. billion = 109 (1 000 000 000).
The proportion of natural gas in the total energy consumption varies considerably from country to country. In the Nordic Countries about the same amount of natural gas is consumed in Finland (gas from Russia) as in Denmark while Sweden consumes only a small part. Norway has a proportionally large production of the natural gas in Europe but it scarcely uses any of the gas itself at least up to now.
Flexibly produced automotive fuels are in this report characterized in that their production can be based either on a fossil stock, primarily natural gas, or renewable raw material. The fuels discussed in this report are methanol, DME, synthetic gasoline, synthetic diesel oil and 28
hydrogen. According to the definition used here even ethanol should be discussed here. However, the production of ethanol for use as automotive fuel is almost entirely based on renewable raw material and it will be discuss under the heading “Non fossil fuels”. The Fischer-Tropsch process is used for the production of synthetic gas from different raw materials or fossil stocks. These can be coal, natural gas but also bio-based material such as wood (wood products), rubbish, waste, refuse manufacturing losses or other materials built of molecules containing coal. These material are suitable for the production of other liquid fuels, other than methanol which today commonly is produced from coal and and/or natural gas. Gaseous fuels such as DME, which is a gas at normal pressure and temperature, and also other gases, can be produced, in addition to liquid fuels. The synthetic gas produced by the Fischer-Tropsch process can be used for the manufacture of different chemical products of which automotive fuels are one group. One advantage of these fuels is that they contain almost no impurities and have a well-defined composition since the production process essentially can be tailored with regards to which product is to be produced. 5.2.1
Methanol (CH3OH) is, as in the case of ethanol (C2H5OH), well defined, independent of whether it is produced from, for example, natural gas or is based on renewable raw material. Therefore there is no reason in this context to differentiate between the function of the fuels with respect to the base material used for their production. To be correct, methanol produced from renewable material should be discussed under the heading “Non-fossil fuels”. However, there is an important difference, depending on the material used for production, in that methanol produced from a fossil material contributes to a larger extent to the greenhouse gas CO2 than methanol produced from renewable materials. The Fischer-Tropsch process is used for the production of methanol and it is usually based on natural gas. Franz Fischer and Hans Tropsch developed the Fischer-Tropsch process in Germany 1925. The original use of the process was to produce gasoline from coke or lignite. According to received information, the last Fisher-Tropsch plant in Germany closed in 1992. In South Africa where there is a huge supply of coal the company Sasol has used the FischerTropsch process for many years and today both gasoline and diesel fuel are produced there. It was the long lasting embargo against South Africa which resulted in the starting up of the production of automotive fuels, and Sasol has been known worldwide for their use of the Fischer-Tropsch process. A byproduct of the production of gasoline and diesel oil according to the Fischer-Tropsch process is alcohols, for example ethanol. Some time earlier ethanol was mixed into gasoline and during a period when Brazil was short of ethanol this alcohol was exported to Brazil. According to received information (Ahlvik, 1999) Sasol has decided to return to the routine of blending ethanol in gasoline. It is well known that the FischerTropsch process is used also for the production of methanol. Today the Fisher-Tropsch process is used for production of automotive fuels, for example synthetic gasoline and diesel oil also from bio-based raw material. It should also been mentioned that two modified Fischer-Tropsch processes for the use of natural gas have been developed and studied (Borgwardt, 1998). These studies have shown that the production of methanol can be more efficient in the future. The use of methanol is briefly discussed under section 6.3.2. A comparison of different fuels is shown in Table 6.
Dimethyl ether (DME)
The process for production of DME was developed by Haldor Topsoe A/S (Denmark) and aimed at a production of DME as a propellant gas instead of CFC, which is not commonly used today as it is regarded as a strong greenhouse gas. By a co-incidence it was discovered that DME could be advantageously used as a fuel for compression ignition engines. DME has a high cetane number and the use of DME has been demonstrated to result in low levels of especially oxides of nitrogen and particles. Despite DME having such physical qualities that it can be compared with LPG (is a liquid at a moderate pressure) and despite its having created a manifested interest within the car industry, DME has not yet succeeded in being used as an automotive fuel. During the development of engines to be used for DME unexpected problem with fuel injection system occurred and this is certainly one reason for engines fueled with DME not having been introduced. However, according to the Volvo Bus Company, there is a good chance that engines adapted to the use of DME will be a success if governmental authorities support the development of such engines. 5.2.3
Synthetic gasoline and diesel oil
It is well known that synthetic gas can be used for the production of high-octane alkyl gasoline (not described here) for otto engines and a high cetane diesel fuel containing almost only paraffins, to be used in compression ignition engines. A clear advantage of this fuel is that it does not contain sulfur and therefore the emission control technology with catalysts can easily be used. Also this fuel does not lead to the formation of such as high rate of particles as commercial diesel oil. Because the above mentioned fuel commonly has a low density, present day diesel engines may have to be adapted to the fuel in order to maintain the power output from the engine. This problem does not need to be permanent if the diesel engines are developed so as to tolerate a variation of the fuel and thereby also the density of the fuel. The question is open as to whether there will be a place on the market for fuels produced from natural gas according to a method, which a Consultant within the oil industry has developed, based on the Fischer-Tropsch process (Syntroleum 1, 1999). In a press release from the 5th of January 1999 (Syntroleum 2, 1999) it is declared that Syntroleum’s fuel is produced by using the company’s process for converting natural gas to synthetic liquid hydrocarbons which do not contain sulfur and aromatics. These components commonly exist in petroleum-based fuels and both sulfur and aromatics cause emissions of unwanted pollutants. In addition sulfur is creates problems in the emission control system. According to Syntroleum their fuel also has the advantage that it can be used in present day engines without modifications and that it can be distributed by the existing distribution system. According to Newsletters from both US Department of Energy (DOE) and US Environmental Protection Agency (EPA) it can be seen that both of these authorities are interested in synthetic fuels, among others synthetic diesel fuels. It can also be said that a Swedish company Oroboros AB has plans for producing synthetic automotive fuels (bio-alkyl-fuels, a gasoline fuel, and a bio-paraffin, a diesel fuel) from wood. However, it seems not to be clear whether there will be a market for such fuels if the cost of the fuel is considerably higher than the cost of the commercial fuels available today. For hybrid electric vehicles of the future equipped with fuel cells it is estimated that the access to synthetic fuels may be important if these are hydrogen rich. As mentioned above there is an increasing interest in the USA in the possibility of producing synthetic fuels since the need for importing oil and petroleum fuels been growing (approximately 60 % today). From reports and notices in the literature the conclusion can be drawn that there are plans within DOE to 30
study the economic and environmental aspects of producing liquid synthetic fuels from natural gas and coal. As can be seen from Table 4 the access to natural gas is huge and, according to received information, the amount of natural gas in energy terms is larger than the access to crude oil. One disadvantage of the production of synthetic fuels from natural gas and coal is that it is likely that this will contribute to an increasing pollution by greenhouse gases and other pollutants. In the USA as in some other countries it is a large access to coal but also in Alaska there are large resources in natural gas. These resources are of course of vital importance today but the gas is not easy to transport as a gas from Alaska to the central of the USA. Therefore DOE has said that “Advances in Chemical conversion of natural gas suggest an add option for ANS (Alaska’s North Slope) to market. Using chemical GTL (Gas-to-Liquids) technology, the gas could be converted into a petroleum-like liquid that is more easily transported via oil pipeline and tanker to market”. According to received information the reserve of natural gas in the USA is about 4 680 billion cubic meter. Not yet discovered reserves of natural gas are supposed to be larger than the reserves of crude oil. 5.2.4
The use of hydrogen as an automotive fuel is not yet common but is likely to increase in the future. Primarily the intention is to use hydrogen for fuel cells but there is also an intention to use hydrogen in internal combustion engines. The use of hydrogen in internal combustion engines (otto engines) has a potential of drastically decreasing the emission of all pollutants except for oxides of nitrogen, as long as air is used as the oxygen supply for the combustion (air contains a high proportion of nitrogen). The high combustion temperature when using hydrogen will contribute to the formation of nitrogen oxide and therefore the oxide of nitrogen can be even larger compared to the emission when using gasoline. Provided that the engine is fed with hydrogen and oxygen the emissions will be close to zero but then it must be realized that there is a higher risk for explosion when hydrogen is mixed with oxygen (limits for explosion 4.7-94 % hydrogen in oxygen) compared with air (5-75 % hydrogen in air). The produced of hydrogen can either be petroleum based or based on renewable material. Therefore hydrogen can be classified as flexible fuel. Today it is most likely that it is most economic to produce hydrogen from natural gas in stationary establishments. Among those who are interested in the use of hydrogen as an automotive fuel there are expectations that hydrogen will be produced at current market prices. It is possibly that energy from the sun will at that time be more directly used for the production of hydrogen, and it will then be produced by electrolytic decomposition of water. If the production of hydrogen could be based on by electrolytic decomposition or some similar technology it is likely that there will not be any shortage of energy. For fuel cells there is a discussion going on as to whether hydrogen will be produced by reforming, for example, methanol from natural gas on board the vehicle or in stationary establishments. There are many problems to be solved before an efficient production, distribution and storage of hydrogen can be realized and on board the vehicle there are also problems to be studied in order to solve problems concerning the technology, economy and safety. It is certainly more efficient to produce hydrogen in stationary establishments than on board of the vehicle but if producing hydrogen in stationary establishments the problem with distribution has to be solved. According to a report from the Swedish Technical Attachés (KFB, 1999:30) trials with hydrogen are going on in some parts of the world – among others at BMW in Germany in internal combustion engines.
In Germany Shell has established a station for hydrogen in Hamburg and also started research on reformers for the production of hydrogen from gasoline in co-operation with, among others, DaimlerChrysler. At Munich airport Aral has assisted in the establishment of a station for hydrogen to be used in buses and cars. In the USA a station for hydrogen has been opened in Chicago and Illinois. These stations are used in a trial aimed at a study of tanking hydrogen in buses equipped with fuel cells. Ford has opened a station for hydrogen in Dearborn, Michigan. The station is linked to their research and development of fuel cells and internal combustion engines and it is equipped with two pumps for liquefied hydrogen and one pump for gaseous hydrogen. The aim is, among other things, to study in which form hydrogen should be filled up in the car (liquefied or gaseous). An addition plan, according to Ford, is to study which type of nozzle should be used and also the most suitable pressure in the fuel line during filling up. The cost of the establishment was $1.5 million inclusive of the construction, erecting, renting of equipment and supply of hydrogen under five years. In an article in (The Hydrogen & Fuel Cell Letter, 1998a) the plans for a co-operation between the Government of Island, DaimlerChrysler and Ballard Power Systems is described and the aim for the co-operation is to establish a hydrogen economy in Island within 15 to 20 years. One of the first steps is to carry out tests by using one of the hydrogen fueled buses developed by Daimler-Benz within one or two years. At the same time both sides of the partnership have to begin studies in order to investigate the impact on Island’s economy of the transformation of the country to a hydrogen-economy. According to the plans the aim of the concept is to convert Island’s very large fishing fleet – Island’s most important industry – for the use of fuel cells and to produce hydrogen locally. In addition the intention is that the country’s cars and buses gradually will be converted so as to be able to be fueled with hydrogen or methanol. The possibilities of producing hydrogen or methanol in Island for export to other countries are also to be investigated.
Not fossil fuels
Fuels based on bio-based material produced in Sweden today are ethanol, biogas and to a not certain degree some Rapeseed Methyl Ester (RME). The potential for future production of rapeseed oil in Sweden is strongly limited. The question whether methanol should be produced from wood instead of ethanol has been discussed and is still open even if there seems not be any great interest in starting a production of bio-methanol. On the other hand some new ideas have been presented in that as already has been mentioned the company, Oroboros AB has announced their intention to establish a production of bio-alkenes. There is also an investigation presented by KFB regarding production of bio-DME. Both of these two fuels can be produced by gasification of wood or some other biomass and the same technology can be used also for production of bio-methanol. Technology for production of various bio-fuels is available and therefore it is more or less a question of economy, access to natural resources and a market for the above bio-fuels whether a production should be established or not. Sweden is a proportionately small country and therefore there is no market for many of such fuels which are not established on an international market. 5.3.1
One advantage of the production of biogas is that the production is based on sludge from waste water purifying plants, slaughterhouse waste, waste from food industries and restaurants, waste from households and even farmyard manure. Almost all that is used for the production of biogas is such that society regards as waste, to be got rid of. It can, however, be used for the production of biogas to be used in industries and as an automotive fuel. Even 32
crops such as grass and clover can be used, but in this case the gas will be more expensive compared with the use of waste. Another advantage of biogas is that methane, the main part of the gas, which is a result of putrefaction, can be used instead of being emitted to the atmosphere, which would otherwise add to the greenhouse effect. Before using biogas as an automotive fuel it has to be cleaned from carbon dioxide and other unwanted components. Table 5 shows the result from an analysis of cleaned biogas. Table. 5. The composition of purified biogas according to an analysis. Component Oxygen Nitrogen Methane Carbon dioxide Hydrogen sulfide
Vol-% Vol-% Vol-% Vol-% ppm
55 9.0 ?
Vapor pressure (kPa), at 38°C Lower Calorific Value [MJ/kg] Boiling Point [°C] Vaporizing temp. (kJ/kg) at 20 °C Auto-ignition Temperature, (°C) Flash Point (°C) 3 Explosion Limits (% Gas in Air)
32 19.5 65 1110 450 12 5.5-26
? 27.1 78 904 420 ? 3.5-15
(mix of C4 to C14 hydrocarb.) ~0.72-0.75 95-98 14.8 Somewhat soluble 45-95 ~43-44 ~25-225 ~180 ~? 6 ~6.0-36.5
~43 ~180-300 ~250 ~250 ~70-75 ~0.6-6.5
50 -161 650 ? 5-15
27.6 -20 410 235 ? 3.4-18
kg/m3 (0 °C), 2) kg/l (15 °C) Observe that DME pass over in vapor phase at ca -20°), 3)Different values exist.
In Figure 4 an estimation of disadvantages and advantages are shown concerning fuels such as natural gas, methanol and gasoline for when they are intended to be used in fuel cells.
Figure 4. Estimation concerning the use of various fuels in a fuel cell. Source: Sasaki, 1999.
The octane number which is stated on the gasoline pump at the gas station. ”Air/Fuel ratio in the fuel mixture to the engine (kg air per kg fuel).
6 ENGINES – POWER UNIT The hybrid systems described in chapter 4 are mainly equipped with conventional internal combustion engines as one of their power units. Radical changes of these engines have not been discussed. In the case of hybrid electric vehicles alternative engines are discussed as a replacement of conventional engines. Fuel cells are also an alternative in the long run and the development of fuel cells will be discussed in Section 6.4. Since the alternative internal combustion engines have not yet been fully developed and they cannot be regarded as commercial, their potential for improved fuel economy is still somewhat unsure. However, certain indications concerning the magnitude of improvements can be seen for some of the hybrid systems. Since it will take a long time (approximately 10-20 years or more) before other alternatives than internal combustion engines (such as fuel cells) can compete economically as power units, all the alternatives should be compared with a further-improved internal combustion engine for use in a hybrid vehicle. Most types of internal combustion engines can also be fueled with alternative fuels. The choice of power unit influences the potential for alternative fuels and the influence is consequently not the same for alternative power units as for conventional power units. The power unit and the fuel to be used must therefore be considered as a whole. In existing hybrid systems an otto engine or a diesel engine is used as an energy transformer for liquid or gaseous fuels. Since these engines are commonly used in hybrids, an otto engine will be somewhat closer described in Section 6.1 and a diesel engine in Section 6.2. In Section 6.3 some alternative engines will be described and in Section 6.4 the most common types of fuel cells will be described. In addition to the rather short descriptions of otto engines and diesel engines two alternative engines, the Stirling engine and the gas turbine will be discussed. However, the estimation is that neither of these two engines will be used to any great extent in hybrid vehicles. Since fuel cells are being widely discussed with great interest a rather extensive description is given based on available literature. Despite there being some prototype vehicles equipped with fuel cells, it is estimated that hybrid vehicles with fuel cells will not be common in the streets during the coming 10 to 20 years, even if an introduction of such hybrid vehicles will be seen earlier.
The otto engine has gone through many transformations during time it has existed. Henry Ford, who started mass production of the engine, was of the opinion, at least during one period, that the engine should be fueled with ethanol. However, gasoline replaced ethanol and for the last 70 years gasoline has in reality been the dominant and almost only fuel for otto engines except for in some countries during the Second World War when the shortage of gasoline made it necessary to use other fuels. A new era for the development of otto engine began at the end of the 1950s when it became obvious, especially in California, that the use of internal combustion engines created both health effects and environmental problems due to their emissions. An intensive development of the otto engine has been carried out during the last 30 to 35 years in order to limit the pollution from the vehicles. The largest of the many steps was taken when the three-way catalyst system was introduced in the late 1970s. The three most important parts of the threeway catalyst system are the unit controlling, among others, the fuel flow, the oxygen sensor and the catalyst both of the last two being placed in the exhaust system. Before this system was introduced different equipment for emission control was used mainly in order to oxidize 36
CO and HC in the exhaust and among these there was even an oxidation catalyst. The threeway catalyst system is unique in that it also reduces oxides of nitrogen (in chemical terms) at the same time as CO and HC are oxidized. Today the otto engine is regarded as an engine with low emission levels at least when the engine is working well and is warmed up (by driving). However at low temperatures, at start and driving during the warming up phase of the engine and the catalyst, considerable emissions are released. In order to minimize these emissions, investigations and developments are in progress and some improvements have been achieved. One problem of the otto engine, compared with its competitor the diesel engine, is that the efficiency is low especially at low loads and idling. This is discussed in more detail in Section 8. That the low efficiency of the otto engine resulted in high fuel consumption was not seen as a serious problem, at least up to the first oil crisis year 1973. This was because the price paid for gasoline by the car owner was low and access to gasoline not considered to be restricted. Some years later a new oil crisis appeared and the price of gasoline increased somewhat more, in addition to the increase which followed the shortage of gasoline in 1973. The price of gasoline is now nearly 10 SKR (Swedish kronor which is between $1.10 and $1.15 at the present exchange rate) per liter. This has led to that fuel consumption of gasoline fueled cars is being regarded as one of the most important parameters for car owners, at least in Europe including the Nordic countries. However, it should be stressed that this does not mean that car manufacturers have previously not worked on the improvement of the fuel economy but it has now become more important than ever. The improvement of the fuel economy is in reality the driving force for the development of new types of power units for vehicles such as hybrid systems and fuel cells. This development is further described in the coming sections and for hybrid vehicles in Section 8. In the previous sections and even later on, the emissions from motor vehicles are described as one of the most important disadvantages for the people’s health and for the environment. This conclusion can also be drawn when studying the actions taken by the authorities and one example for light duty vehicles is shown in Table 7, where the present day (“Euro” 3) and coming Euro Standards are presented. Table 7. Emission Standards for light duty vehicles EU. Source: Auto/Oil II. GASOLINE Designation (Year and CO THC NOx HC+NOx month for implementation) Euro 2* (1996-10) 2.2 (3.2) (0.341) (0.252) 0.5 Euro 3 (2000-10) 2.3 0.20 0.15 Euro 4 (2005-10) 1.0 0.10 0.08 EEV(option 1) 0.6 0.04** 0.04 EEV(option 2) 0.6 0.04 * Test cycle with idle period of 40s before measurements ( ) Modified Dir 94/12. ** Up until Euro 4 (2005) this value is derogated to 0.05 g/km. *** Note: review of the measuring procedure for particulate emissions is necessary.
PM 0.01*** 0.01***
One example of an otto engine, which is adapted for use in a hybrid vehicle, is the engine that Toyota has developed to be used in their hybrid vehicle, Prius. There are several reports describing Prius and its engine and two of these are referred to here in this context (Takaoka et al., 1998; Hirose et al. 1998). During the process of investigating and evaluating the different alternative of engines which could be used in their hybrid vehicle, Toyota carried out extensive studies of the advantages and disadvantages of the different alternatives choices for their study. Finally the company decided that the best alternative was a gasoline-fueled engine with fuel injection into the 37
intake ports of the cylinders of the engine. Furthermore the engine should be optimized for a hybrid system having high expansion and low friction. The opinion of Toyota was, in fact, that the gasoline fueled engine has a lower energy efficiency than some other power units, that this could be overcome by the design of a new engine and the hybrid system. The engines which was regarded to be closest to the chosen engine alternative was a diesel fueled engine a direct injected gasoline fueled engine with stratified charge. Iwai (Iwai et al., 1998) has, in an educationally manner, described the matching between the internal combustion engine and the hybrid system. Schematic he shows how the matching between the drive train of the internal combustion engine and the hybrid system, see Figure 3 (SHEV means series hybrid) should be for two different efficiencies, 77 % and 60 %, of the battery, see also Figure 5. This comparison can also be used for a parallel hybrid system. Concerning the parallel hybrid, the opinion of Iwai is that it can be operated most efficiently as a series hybrid at both low and high loads. This is true for the second case (60% efficiency) if the efficiency is lower than the total efficiency, i.e. when the efficiency of the engine is multiplied by the efficiency of the battery system. In connection with this description, Iwai does discuss whether the battery should be loaded from the mains or by the internal combustion engine via a generator. This question will be discussed in a later section (Section 9). The responsible technicians at Toyota have certainly analyzed the questions and problems which have been discussed by Iwai. A presentation of the Toyota hybrid vehicle can be found in Section 8 among other hybrid vehicles. Here the presentation is limited to that which has been discussed about the Toyota engine for Prius and in this context only a few points will be mentioned which are regarded as being the most important. There are certainly other engines of interest to be informed about, since new inventions can now and then be found in the literature.
Figure 5. The relationship between the displacement of the engine and its efficiency. Source: Hirose et al., 1998.
In order to reach the goal of high thermal efficiency the technicians at Toyota based their decisions on the following three points: 1. The only restriction for the choice of the displacement of the engine was that it should meet the requirement of power and the restrictions for instability. This make it possible to use a high-expansion-ratio cycle with delayed intake valve closing, as well as to reduce friction loss by lowering the engine speed.
2. In order to achieve the highest reduction of the emissions the engine had to be run at λ=1 over its whole working area and a three-way catalyst system had to be used. 3. Active steps had to be taken during the development in order to reduce the weight and to increase the efficiency of the engine. The relationship between the displacement of an engine and its efficiency is shown in Figure 5. As can be seen the efficiency increases with the increase of the displacement but the fuel economy of the vehicle decreases from approximately 1500 cc and up. However, according to Hirose et al. the efficiency is higher for a small engines at low load than for larger engines. The decision taken by the technicians at Toyota was that the engine should meet the requirement set up for power output and for installation of the hybrid vehicle. This decision made it possible to keep to the intention of using a high-expansion-ratio cycle with delayed intake valve closing, as well as to reducing friction loss by lowering the engine speed. One problem was the vibrations during start and stop of the engine. These were efficiently reduced by the use of a special arrangement of the valve in the cylinders named VVT-i (“intelligent variable valve timing”). The engine and its specification are shown in Figure 6.
Figure 6. The specification (left) of the engine shown in the figure (right). Yaegashi et al., 1998.
Atkinson’s cycle The internal combustion engine in the Toyota hybrid vehicle is design for the use of the socalled Atkinson’s cycle, named after its inventor James Atkinson at the end of 19th century i.e. more than 100 years ago. In a conventional otto engine cycle the compressions ratio and the expansions ratio are nearly equal. In the original engine constructed by Atkinson a crank mechanism was used which gave a shorter crank motion for the compression than the crank motion for the expansion, i.e. lower compression ratio than the expansion ratio. This will theoretically result in a higher efficiency compared with a common otto engine having an equal expansion ratio. The expansion ratio for the engine in Toyota Prius is 13.5:1 while the effective compression ratio can be regarded as being approximately the same as for a conventional engine (i.e. about 10:1) Unfortunately it was shown that the crank mechanism used by Atkinson was too complicated to be practical. A variant of the theme is the so-called Miller system, invented by R. Miller in the 1940s. In this system a restriction of the compression ratio is caused by a control of the closing of the intake valves (earlier or later). This cycle is often named the “Miller cycle”, which is not completely correct since the cycle is related to the Atkinson cycle. However the Miller system implies a simplification of the apparatus compared with Atkinson’s original idea. Strictly speaking, it should be said the Toyota engine uses a cycle with a function according to the Atkinson cycle and that the Miller system is used in order to achieve a practical use of the cycle. The Miller system is often combined with the use of a super charger 39
for the intake air in order to reduce the losses of power and torque which are a result of the reduction of the efficient compression ratio. In the case of Toyota the displacement of the engine has been increased instead of using a turbo charger.
In Section 5.1.3 in which diesel fuel is discussed it is underlined that there is an interest in the US to use reformulated diesel fuels in order to reduce the emissions from diesel-fueled vehicles. A similar increasing interest can be found also in Europe. A great part of the increased interest is linked to an improved fuel economy where the diesel fueled compression ignition engine is superior to a gasoline fueled otto engine. The Achilles heel of the diesel engine is the emissions. In this respect the diesel engine is far from being able to compete with the otto engine. It is true that intensive investigations and research/development are going on which have considerably lowered the emission levels of the diesel engines but the question is whether this type of engines will ever be broadly accepted even with these improvements. According to emission standards agreed within EU, there now seems to be a final agreement within the EU-Commission also about the EURO 5 Standards including the EEV-Standards. As can be seen of Table 8, diesel engine are still being considerably improved, especially in order to meet the standards decided in California and Federal USA, meaning that the emission levels of diesel fueled vehicles will be quite low in the future. 1. EU Emission standards for passenger cars and light-duty trucks (g/km). Table 8. EU-Standards for passenger cars and other light-duty vehicles. Source: Auto oil, 2000.
DIESEL FUELLED Designation (Year and month for implementation) Euro 2 (1997-10) Euro 3 (2001-10) Euro 4 (2006-10) ( ) DI diesel engines.
CO g/km 1.0 0.64 0.50
HC+Nox g/km 0.7 (0.9) 0.56 0.3
NOx g/km 0.50 0.25
PM g/km 0.08 (0.1) 0.05 0.025
2. EU Emission standards for heavy-duty vehicles and buses. Diesel fuelled vehicles. Source: EU Directive 1999/96/EC.
Limit values – ESC and ELR tests The specific mass of the carbon monoxide, of the total hydrocarbons, of the oxides of nitrogen and of the particulates, as determined on the ESC test, and of the smoke opacity, as determined on the ELR test, shall not exceed the amounts shown in Table 9. Table 9. EU-standards for heavy-duty diesel fueled engines. Source: EU Directive 1999/96/EC. Mass of Mass of Mass of Mass of Smoke carbon monoxide hydrocarbons nitrogen oxides particulates (CO) (HC) (NOx) (PT) Row g/kWh g/kWh g/kWh g/kWh m-1 (a) A (2000) 2.1 0.66 5.0 0.8 0.10 0.13 B 1 (2005) 1.5 0.46 3.5 0.02 0.5 B 2 (2008) 1.5 0.46 2.0 0.02 0.5 C (EEV) 1.5 0.25 2.0 0.02 0.15 (a) For engines having a swept volume of less than 0.75 dm3 per cylinder and a rated power speed of more than 3000 min-1. "EEV" means Enhanced Environmentally Friendly Vehicle which is a type of vehicle propelled by an engine complying with the permissive emission limit values given in row C of the Tables 9 and 10.
2. Specifications Concerning the Emission of Gaseous and Particulate Pollutants and Smoke. Source: EU Directive 1999/96/EC. For type approval to row A of the Table 9 the emissions shall be determined on the ESC test (a new 13 mode steady state test procedure for diesel engines) and a ELR tests (a smoke test procedure for diesel engines) with conventional diesel engines including those fitted with electronic fuel injection equipment, exhaust gas recirculation (EGR), and/or oxidation catalysts. Diesel engines fitted with advanced exhaust aftertreatment systems including deNOx catalysts and/or particulate traps, shall additionally be tested on the ETC test (a transient test procedure for heavy-duty engines). For type approval testing to either row B1 or B2 or row C of the Table 9 the emissions shall be determined on the ESC, ELR and ETC tests. For gas engines, the gaseous emissions shall be determined on the ETC test Table 10. The ESC and ELR test procedures are described in Annex III, Appendix 1 and the ETC test procedure in Annex III, Appendices 2 and 3 of the EU Directive 1999/96/EC. 3. EU Emission standards for heavy-duty vehicles and buses. Diesel fuelled and gaseous fuelled vehicles. Source: EU Directive 1999/96/EC Limit values – ETC tests (b) For diesel engines that are additionally tested on the ETC test, and specifically for gas engines, the specific masses of the carbon monoxide, of the non-methane hydrocarbons, of the methane (where applicable), of the oxides of nitrogen and of the particulates (where applicable) shall not exceed the amounts shown in Table 10. Table 10. EU-standards for heavy-duty diesel fuelled and gaseous fueled engines. Source: EU Directive 1999/96/EC. Mass of Mass of non-methane carbon monoxide hydrocarbons (CO) (NMHC) g/kWh g/kWh
Mass of Mass of Mass of methane nitrogen oxides particulates (c) (d) Row (NOx) (CH4) (PT) g/kWh g/kWh g/kWh) (a) A (2000) 5.45 0.78 1.6 5.0 0.16 0.21 B1 (2005) 4.0 0.55 1.1 3.5 0.03 B2 (2008) 4.0 0.55 1.1 2.0 0.03 C (EEV) 3.0 0.40 0.65 2.0 0.02 (a) 3 For engines having a swept volume of less than 0.75 dm per cylinder and a rated power speed of more than 3000 min-1. (b) The conditions for verifying the acceptability of the ETC tests (see Annex III, Appendix 2, section 3.9) when measuring the emissions of gas fuelled engines against the limit values applicable in row A shall be re-examined and, where necessary, modified in accordance with the procedure laid down in Article 13 of Directive 70/156/EEC. (c) For NG engines only. (d) Not applicable for gas fuelled engines at stage A and stages B1 and B2.
The emission standards for heavy-duty vehicles are presented in two tables, Table 9 and Table 10, since there is one set of standards referring to the ESC test cycle and the ELR test cycle for diesel fueled engines (Table 9) and second set of standards referring to the ETC test cycle for diesel fueled and gaseous fueled engines. The decision taken by the EU-Commission is somewhat complicated and therefore it cannot be described here except concerning a few details. One example is that gaseous fueled engines and diesel engines fitted with advanced emission control systems including deNOx catalysts and/or particulate traps, shall additionally be tested on the ETC test (a transient test procedure for heavy-duty engines).
The requirements for meeting the above presented emission standards have also resulted in a discussion about the quality of the fuel to be used in the future. Among other requirements for the fuels there is a strong demand, expressed by the engine and car manufacturers, that the content of sulfur in the fuel must be reduced to a level far under the level which is common today. The real cause of the demand to reduce the content of sulfur in the fuel is the emission control technology to be used in order to meet the future emission standards. The systems of catalysts for reduction of especially oxides of nitrogen do not tolerate sulfur, which result in a fast deterioration of the system. Sulfur in the fuel will also cause problems in the control of particulate emissions. Of the literature presented in the US, etc it seems to be clear that the content of sulfur in the fuel has to be reduced. Going back in history it can be said that California was first in seeing the impact of sulfur and aromatics on the particulate emissions which lead to a requirement to reduce aromatics and sulfur in the diesel fuel. It should be underlined that reducing the content of aromatics will result in an increase of the cetane number which is also an advantage. Despite the improvements of the diesel fuel and the fact that the diesel engine is more energy efficient than the otto engine there seems to be opposition against diesel fuel vehicles in California, which has the most rigid emission standards in the world. Even in the Federal USA the interest for diesel fueled vehicles has been low among the authorities and others caring about the environment and people’s health. However, as in other countries in the world most of the heavy-duty vehicles are diesel fueled. In order to reduce the emissions from these types of vehicles the possibilities of improving diesel fuel is under investigation in many parts of the world and especially in the US and Europe, so as to be able to use more efficient emission control technology. Extensive research and investigations of the exhaust gas recirculation (EGR) technology in combination with other systems of filters have shown that it is possible to reduce the particle emissions to a level of 0.01 g/bhp-hr* (ca 0.014 g/kWh). Even the emissions of NOx + HC can be reduced to approximately 2.5 g/bhp-hr (ca 3.38 g/kWh) (Manufacturers, 1999a). Furthermore, investigations have also shown that the use of low sulfur containing diesel oil results in a larger reduction of emissions than diesel oils with higher sulfur levels. Diesel oil with 54 ppm sulfur was compared with diesel oil with 338 ppm sulfur. Intensive research is going on especially concerning the use of catalysts for the reduction of NOx-emissions. The different technologies of current interest are; the DeNOx technology, (”Lean-NOx”), ”NOx Adsorbers” i.e. (NOx ”Traps”) and the (SCR) technology. In addition to these the DeNOx and SCR (selective catalyst reduction) are well known at least in Sweden by the research carried out at the University of Lund (Sweden) whereas the technologies with ”NOx Adsorbers” and non-thermal plasma are less known. Combinations between these two technologies also exist. In short the meaning of the NOx Adsorber technology is to oxidize NO to NO2 and to store NO2 in order to use this component for the reduction of NOx. NOx can also be reduced by the non-thermal plasma technology but in this case the most interesting may be that it can be combined with DeNOX or the Adsorber variants. Because different nitrogen-oxygen components, for example N2O (a strong greenhouse gas) can be formed and that there is a risk for an increased emission of NO2 when using the technology with NOx Adsorbers and the technology with plasma, it could be of interest for among others the environmental protection authorities to follow the up the development and use of the here mentioned technologies. The ongoing research and investigations carried out have shown that there now exist an interest for using strong measures in order to reduce the emissions from diesel fueled vehicles. *
grams/brake horsepower hour
If the difficulties with the diesel engines related emissions could be solved an increased use of diesel engines even in light-duty vehicles could be of interest for both car owners and car manufacturers because of the fact that diesel engines are the most energy efficient vehicles of today. The fact that the compression ignition engine is more efficient than the spark ignition engine could be the reason why Ford choose a diesel engine for their hybrid vehicle instead of an otto engine (see next page and Section 8.2.4). Some time in the future the use of a technology with compression ignition of a premixed fuelair mixture may be developed. In principle this can be regarded as a diesel engine with low emission levels and high fuel efficiency. The system is called Homogenous Charge Compression Ignition (HCCI) presented in (Ahlvik and Brandberg, 1999). In order to get an idea about the possibilities of developing a diesel engine for a parallel hybrid studied a report (Jaura et al., 1998) has been studied which deals with the development of the diesel engine for Ford’s hybrid vehicle. There is a strong opinion that both the emissions and the energy use are extremely good for a hybrid vehicle. According to the report the responsible representative at Ford decided to develop a CIDI (Compression Ignition Direct Injected) engine denoted DIATA (Direct Injected Aluminium Through Bolt Assembly), which according to Ford is compact, has low weight, has a high power/weight ratio and is ”environmentally friendly”. The displacement of the engine is 1200 cc and it is equipped with four valves per cylinder (a 4 cylinder engine). It gives 55 kW at 4500 rpm and is also equipped with a turbo with variable geometry (VGT), an EGR system and has a fuel system with ”High Pressure Common Rail” (HPCR). In addition the engine is equipped with a series of electronic control units. Since the engine can be regarded as a primary concept for a future diesel engine some of the most important features of the engine will be described in somewhat closer detail. This description will be based on Figure 7.
Figure 7. Diagram for control of engine, DIATA. Source: Breida 1998. In Ford’s hybrid vehicle individual units or modules control the electric motor, ASM, and the internal combustion engine (DIATA). In order to operate the different devices in combination according to a control program which optimize the fuel economy. Emissions and engine performance, a control system (VSC) for the whole vehicle was developed. The accelerator and certain other functions of the engine are controlled by VSC. When the driver of the vehicle presses the accelerator, the VSC indicates the rate of charging of the battery, the speed of the vehicle and the engine temperature and decides on the strategy for the torque delivered by the engine and electric motor. VSC also indicates the stops of the vehicle and decides whether the engine has to be shut off. The exhaust manifold: In order to give space for the lean NOx catalyst, for reduction of the emissions, the design of the exhaust is such that the exhaust system fits in according to the WCR standards and so that the engine does not cause any turbulence and unnecessary backpressure. 43
Water inlets and outlets need sensors to govern the fans and these have been designed especially for the hybrid vehicle. The turbo charger (VGT) has an electric accelerator for fast response and low inertia. The accelerator has a linear control signal which controls the position of the vans by means of a turbo module in the control loop. ”High Pressure Common Rail” (HPCR) has been developed by Bosch (Figure 8). It is used in the engine for the hybrid vehicle. The injection pressure is independent of the engine speed and the power output. Using the pilot injection, which implies the injection of a small amount of fuel, it is possible to reduce the noise emissions and there is also an advantage when using EGR. HPCR includes a high-pressure radial jet fuel pump which is operated by the camshaft. The actual pump has three pumping elements placed radially. It has a high volumetric efficiency, requires only low torque for operation, is lubricated with the fuel and requires no synchronization between the pump and the engine shafts. The fuel is pumped using high pressure to the injectors and the volume has been optimized for fast response. The system is designed so that the oscillations caused by the pump and the injectors are damped. The highpressure injectors have an electromagnetic 2/2-valve actuator and the injection profile is adjusted by shaping the orifice. The time for opening and closing is 250 microseconds and the injectors fit into the injector bores of a conventional engine with four cylinders. The system for emission control is based on a lean NOx catalyst i.e. a catalyst which reduces NOx in the exhaust even when there is an excess of oxygen (for a three-way catalyst system there must not be any excess of oxygen in the exhaust). In the exhaust from a diesel engine there is always an excess of oxygen and a catalyst for such an engine must also be such that it reduces the emissions over a broad temperature range. In the case of the Ford CIDI engine an EGR system is used which reduces the NOx formation during the combustion to a certain extent. In a lean NOx catalyst a type of reductant such as urea, ammonia or hydrocarbons (can be diesel fuel in the case of a diesel engine) is used in order to support the catalytic reaction. The description of the Ford CIDI engine is presented as one example of the complexity of a diesel engine which has been designed for low emissions levels and high fuel efficiency. In this case, it is a diesel engine designed for a hybrid vehicle but many of the units, modules and other details can certainly be found even in other engines. It should be underlined that the engine presented here has many more important details than those presented in this section.
Figure 8. The HPCR fuel system with its components. Source: Breida 1998.
The engines described above are all engines with a reciprocating piston and where the combustion occurs within the cylinder of the engine, using conventional fuels. Neither have any thorough changes of these engines been discussed except the use of more electronic equipment in order to control the engine and auxiliary systems. For hybrid vehicles alternative engines (or energy transformers) are often discussed for used instead of the conventional internal combustion engines. A fuel cell package is also an alternative to be used in hybrid vehicles but it will be discussed separately in Section 6.4 since fuel cell engines are neither internal combustion engines or heat engines. Since the alternative engines, generally speaking, are not completely developed and are not regarded as commercial in this context, their potential for improved fuel economy and lowering emission levels is still unsure. Certain indications can, however, be seen for an estimation of the potential of some alternative system. There are for example such physical limitations in some of the alternative engines of energy transformers, which have to be taken into account when estimating whether their applicability is better than that of commercial engines. Seeing that it will take long time before alternatives to the present day commercial engines can economically compete, the alternative must be compared with further developed internal combustion engines. The present day so-called alternative fuels can without any great difficulties be used in almost any type of internal combustion engines provided the engine is adapted to the fuel. Since the potential of the fuel is affected by the engine used, the engine and the fuel must, as already has been underlined, be regarded as an entirety. A new emission control technology for NOx emissions when using an alcohol in a compression ignition engine can be mentioned as an example. In this case it has been shown that the function of the catalyst is most efficient within a limited temperature interval, when considering both regulated and non regulated (not limited by law) emissions. One possibility of avoiding or evening out a fluctuation of the exhaust temperature would be to use the engine in a hybrid system where the engine can be controlled so as to achieve the best conditions for the reduction of NOx. For the fuel economy it can generally be said that the fuel consumption, in terms of energy, for the same type and size of engine does not differ very much when either a commercial or alternative fuel is used. The types of engines discussed in this section are the Stirling engine and the gas turbine. Of these engines the Stirling engine can be of interest for use in series hybrids for both light-duty and heavy-duty vehicles while the gas turbine or turbo generator may be used in series hybrids for heavier vehicles (buses and trucks). During the literature studies no hybrid vehicle with a Stirling engine have been found except for a hybrid system presented in a later paragraph in this section (Rajashekara et al., 1998). In the case of the gas turbine only two reports have been found and in one of these a hybrid system with a gas turbine developed for a bus was reported (Malmquist et al., 1998). In the other report the use of a hybrid system with a turbo generator was presented (Brown et al., 1999). For reasons of costs and/or effectiveness or other reason it is unsure whether a Stirling engine or gas turbine (or turbo generator) in a hybrid system will be able to compete with the future otto engine or diesel engine in a hybrid system. Stirling engines are used today at least in one well-known application namely in submarines. In the 1970’s and somewhat later a Swedish company, United Stirling, introduced the Stirling engine as a power unit for passenger cars. Later on the engine was tested in different applications in the US and plans have been discussed concerning the use of the engine for electric generation and in this case by using solar energy as a direct energy source. The Stirling engine, which is a heat engine, differs basically from an internal combustion engine such as the otto engine and diesel engine in that it uses heated air or some other gas for 45
example helium as a working media. Since the working media is heated in a separate part of the engine any source producing heat can be used but in reality a liquid or a gaseous fuel is used. Trials have been carried out using solar energy as already mentioned. Nearly 200 years ago (1816) the Scottish priest and engineer Robert Stirling received a patent for his invention the heat engine named the Stirling engine. For reason of costs and also technical reasons the Stirling engine has not been accepted, so that it has not obtained a share of the market which its advantageous emission qualities in fact deserve. The authors of the report ”Control System for a Stirling Engine Driven Induction Generator” (Rajashekara et al., 1998) are of the opinion that the Stirling engine can be characterized by its high efficiency, low emission levels, that different fuels can be used and in passenger cars instead of otto engines. The authors of the report also mean that Stirling engine which is coupled to an electric generator can be used as a power unit in a hybrid vehicle as an Auxiliary Power Unit (APU). An APU is used in a series hybrid in order to load the batteries and to divide the power output from the engine and the batteries. The authors underline the fact that there is a difference in controlling the power from a Stirling engine compared to controlling the power from an internal combustion engine. In an Stirling engine the fuel flow does not have an impact on the mass of the working media (the gas) in the engine and therefore the flow of fuel cannot be used for controlling the power output. Another difference between a Stirling engine and an otto engine is that torque curve is rather flat in Stirling engine (see Figure 9) which can be seen as a special advantage. 120 100 80
60 40 20 0 10
70 x100 Speed
Figure 9. Typical torque curve and power curve respectively for a Stirling engine. Source: Rajashekara et al., 1998. . In order to control the power output from a Stirling engine different methods have been developed. These control methods usually control different parameters of the engine, such as temperature, piston stroke (swash plate* control), pressure, phase angle, speed, load or dead volume. Each scheme has its advantages and disadvantages; however, the temperature control and swash plate control methods are most commonly used. These affect the fuel flow and its effect on the temperature of the engine. Another method of power control is by changing the depth of the piston stroke at constant gas mass. This method is less complex more reliable and faster than the control method using the working gas mass.
The expression ”swash plate” is used for a rotating disc mounted with a certain angle and is attached with the piston (see Figure 10).
In the developed system the Stirling engine drives an inductive generator and the electric current from the generator is converted to a variable direct current by the use of a special transistor. The control variables for the generator are easy to manipulate and by this the control of the piston stroke, as a method for varying the power output from the generator, does not need to be used. The control method which has been developed uses a field oriented technique in order to control the generator system and vary the power output from the Stirling engine. By this the system for control of the power output is designed so as to control the generator instead of the Stirling engine. The Stirling engine – induction-generator-system was extensively tested in the laboratory and at present is running in a mini-van. For the operating power range of 8 – 40 kW and speed range of 3000 –12,000 rpm, the system efficiencies ranged from 85% to 92%. While the Stirling engine runs at approximately 40% efficiency, the predicted system efficiency was about 34% at most operating points. At low speeds, below 3000 rpm, the total APU efficiency was below 34% due to the nature of the operation of an induction machine. General Motors (GM) in the US has also been looking at the possibility of using a Stirling engine in a hybrid vehicle. Information from USCAR Media Center says that GM has been interested in using a Stirling engine in a series hybrid (USCAR, 1999). According to later information from other sources the conclusion can be drawn that this interest has not lead to any action concerning the use of Stirling engines. A model of the Stirling engine is shown in Figure 10.
Figure 10. Stirling engine*. Source: USCAR, 1999. Gas turbines are commonly used in airplanes but also in certain types of ships and for the generation of electricity. The gas turbine has also been of interest for passenger cars at, among others, Volvo. In its most simple execution the gas turbine system consists of turbine and one of the turbine-operated compressors and a combustion chamber. A simple small type of turbine is a part of a super charger primarily for diesel engines but during recent years also for passenger cars equipped with otto engines. In the report (Malmquist et al., 1998) a new generation of a gas turbine for a hybrid bus is described and this has been developed in co-operation between Volvo and ABB (Asea Brown Boweri). The development of the new hybrid bus is based on an earlier development of a hybrid bus with a gas turbine. The hybrid system in the bus is briefly presented in Section 8. *
A "swash plate* is shown in the figure.
In this section the Volvo hybrid system is briefly discussed, based on the report referred to above about the system for control of the power flow. Three different algorithms were determined for the generation of the power reference for the power module controller. The three algorithms were for line related control, energy balance control and average power control and they were all implemented for evaluation purposes. The sum of the three signal paths forms a reference signal which is then directed to the power module controller. The structure of the signal paths is chosen for the purpose of easy system evaluation and optimization. In another project for the evaluation of a hybrid bus, presented by NASA’s John H. Glenn Research Center a type of gas turbine fueled with natural gas is used but in this case it is named ”Turbogenerator”, see Figure 11. The reason for this may be that it was originally a jet engine from an airplane. The project is carried out in co-operation between the government, industry and scientists (Brown et al., 1999). The goal for the project is to improve the fuel consumption by 50 % (“double the fuel economy”) for buses in city traffic and to reduce the emissions by a tenth of the EPA (USA) emission standards. What is unique about the hybrid system for this bus is its system for storage of energy. For a buss with a maximum weight it may be advantageous to use ultracapacitors for energy storage since capacitors seem to be superior to batteries concerning accelerating for regenerative braking and low-temperature characteristics.
Figure 11. GM’s gas turbine. Source: USCAR, 1999. The car industry seems to have lost its interest in gas turbines for use in hybrid systems. However, according to an early information from USCAR Media Center, GM has expressed its interest in a gas turbine to be used in a hybrid vehicle. GM points out that a gas turbine is easy to fuel, “just burns anything that burns”, and that an increase in the energy efficiency of up to 50 % can be expected. The gas turbine is a strong candidate for use in order to meet the goal for the PNGV program. With regard to there being no great interest in gas turbines within the car industry this efficiency seems to be somewhat optimistic. The conclusion to be drawn here is that there must be a lack of clear information about the advantages and disadvantages of the gas turbine. A gas turbine is shown in Figure 11.
The technology of fuel cells is not new but there has been no cause to use it in the vehicle industry until recent years. The opinion is that it will not be ready for mass production before 10-20 years or more. There are both technical and economic problems to solve and both conventional and alternative engines, which have been described above will be used for a long 48
time in the future. If the introduction of fuel cells is successful it will mean that a new era of power unit for automobiles will arrive. This will happen when there is a transition from the 100 year old internal combustion engine to a quieter system with fewer moving parts and. One can also say that two important steps have been taken in that direction by the introduction of electric and hybrid vehicles. There is much interest in using the technology of fuel cells for developing new methods of propulsion for motor vehicles. One can see this from all the journals, articles and reports to be read. Take for example a recent article in Automotive Engineer where Allied Business Intelligence of New York State states that “a fuel cell will power four out of every 100 vehicles on US roads by the end of the decade, and proton exchange membrane (PEM) cells are set to take an 80 % share of the market”. (July/Aug 2000 edition). Information is also available on fuel cells on the Internet and in daily newspapers. A large part of the available information has been examined in order to provide a background to our evaluation of future hybrid systems. One problem with the information which is published in reports and other sources is that the development in the field of fuels cells appears to proceed faster that the development of other important systems in vehicles. There is a great risk that the information one receives one month is not quite “up to date” by the next month. Besides this, one reads optimistic statements about the development of fuel cells and the time for the introduction of systems ready to be put into production – statements which are difficult to evaluate. All this contributes to the uncertainty which is reflected in the descriptions of fuel cells and the fuel which can come to be used in them. All vehicle manufacturers are, however, not certain that resources should be used for the development and introduction of fuel cells. BMW can be taken as an example of these exceptions, since the company has stated that it will not use any resources for developing fuel cells but will use their resources for the development of hybrid vehicles, which is a more interesting alternative for vehicle manufacturers, at least in the short term (Maruo, K., 1998). Maruo has, on behalf of KFB, carried our a systematic evaluation of the technology for fuel cells and on-going development of fuel cells in various parts of the world. He states that one can describe three different scenarios – one optimistic, one realistic and one pessimistic (Maruo, K., 1998). For his description of an optimistic scenario Mauro takes DaimlerChrysler/Ballard as an example. The alliance seems to be using a great deal of resources and keeps on making presentations of versions of Necar. This puts a great deal of pressure on the other vehicle manufacturers. In his description of a realistic scenario Maruo says that vehicles with fuel cells will have reached the same state of development by 2004/2005 as electric vehicles were in 1997/1998. He also estimates that by 2004/2005 several hundred fuel cell vehicles will be produced per year at a cost of 40 000 dollars or more. For the pessimistic scenario Maruo points to a number of events which can occur and amongst other things that a decision that fuel cells cannot meet the expected demands in the case of emissions and good efficiency. Another difficulty is that the price if gasoline and diesel oil may remain at the present low level and the cost of using fuel cells will prove to be too high. Maruo also estimates that the greatest barrier to a successful introduction of fuel cells is the question of how the delivery of the fuel should be managed. The reason for this is that substantial investments, of tens and hundreds of billion dollars will be required in order to establish a production and distribution system of the new fuel, which could be methanol, natural gas or hydrogen. 49
Maruo also presents other alliances which have been built up in the vehicle industry for the development of vehicles using fuel cells. There is a general opinion that fuel cell techniques will eventual replace present day techniques using internal combustion engines (Stobart, R., 1999a). Fuel cells have the potential for being clean and quiet energy converters, which can be used in fields other than in vehicles. We have pointed to the American Energy Authority, (DOE), and the large amount of money they have invested in the development of fuel cells for stationary establishments for the production of electric energy (see further on in the text). The hope is that it will be possible to use the technique even as an energy converter in small establishments for example places where it is estimated to be too expensive to draw electric cables. In the case the fuel cells, hopefully at least within the coming 15-20 year period, will be more widespread in their use, there is a need for them to be more practical to use, and above other things, for them to cost less than at present. According to Stobert, (Stobert, R.K. 1999), the costs at the present time are in the region of 5 000 dollars/kW which means that a 50 kW stack of fuel cells would cost more than 250 000 dollars. According to the same source the goal of the Californian Air Resources Board (CARB) is that the costs can be cut to 60 dollars per kW, which even than would lead to a high cost in comparison with an internal combustion engine which is between 15-20 dollars per kW. Fuel cells must also be compact and not weigh too much in order to be attractive for various applications. Stobart has given, in one of his reports, (Stobart, 1999a) a short description of the technology for fuel cells under the headings: Fuel cell technology What more is required in order fore fuel cells to be a power system Where are the applications Future challenges and when can fuel cells come to be used more generally. Expressed simply, fuel cells convert a flow of fuel to electric energy. This can be compared to a battery where the production of electricity continues all the time. In the separate fuel cell the fuel takes part in the electrochemical process, where it is combined with oxygen, and does not only produce heat but primarily produces electric energy. In one type of fuel cell with PEM (Proton Exchange Membrane) every cell has been connected together to a membrane of a material which is both electrical isolating and conductive for protons (Stobart, R., 199a), (see also Figure 12). The figure shows that in fuel cells with PEM the membrane is pressed between two electrodes and the resulting membrane/proton aggregate is mounted between two bipolar plates. The fuel cell packet is assembled from a large number of separate fuel cells, and each contributes with ca 0.7-1 volt. In order to arrive at the required voltage a large number of cells are required probably several hundred, depending on the power one requires. In a brief document from the US Department of Defense (DoD) “Fuel Cell Descriptions” a description can be found of a system which can generate electric energy without consuming fuel and this is therefore the most attractive system from the point of view of the environment (DoD, 1999a). The attractive characteristics of the fuel cell are: high efficiency for energy conversion consists of modules very low level of chemical and acoustic pollution fuel flexibility 50
ability to work together fast load response All fuel cells work according to the same principle – applied fuel is subject to catalytic reaction (electrons move from the fuel elements) in the fuel cell in order to generate an electric current. The fuel cells consist of electrolytic material which is placed in a stack between two thin electrodes (porous anode and cathode) like a sandwich, (see Figure 12). The applied fuel passes over the anode (and oxygen over the cathode) where it catalytically splits into ions and electrons. The electrons pass through the external circuit in order to serve an electric load while the ions move through the electrolyte toward the oppositely charged electrode. At the electrode, ions combine to create by-products, primarily water and CO2. Depending on the input fuel and electrolyte, different chemical reactions will occur. (DoD, 1999a).
Figure 12. Basic principals of fuel cells. Source: US Department of Defence. According to the US Department of Defence there are basically four types of fuel cell, based on the electrolyte which is used, as follows: Fuel cell with phosphorus Fuel cell with carbonate (melted) Fuel cell with solid oxide (ceramic) Fuel cell with proton exchange membrane, PEM In the following table (Table 11) the four types of fuel cell are compared. Table 11. Comparison between different types of fuel cells. US Department of Defence. PAFC1 ELECTROLYTE OPERAT. TEMPERATURE FUELS REFORMING
Phosphoric acid Molten carbonate salt 375 ºF (190°C) 1200°F (650°C) (H2) Reformat H2/CO/ Reformat External External/Internal
1830°F (1000°C) 175°F(80°C) H2/CO2/CH4 Reformat H2 Reformat External/Internal External OXIDANT O2/Air CO2/O2/Air O2/Air O2/Air EFFICIENCY 40-50 % 50-60 % 45-55 % 40-50 % 1 2 3 Fuel cells with phosphoric acid. Fuel cells with carbonate (melted). Fuel cells with solid oxide. 4Fuelcells with proton exchange membrane fuel cell, PEM (Proton Exchange Membrane).
Figure 13. The three sections of a fuel cell: fuel processor, stack of fuel cell and DC/AC transformer. Source: US Department of Defence. Fuel cells are divided into three sections, (see Figure 13). 1. Fuel processors. 2. Section for production of electric energy (stack of fuel cell). 3, Transformer direct to alternating current. Hydrogen gas is produced from natural gas in the fuel processor. (Also the fuel is cleaned in the fuel processor, sulfur and CO being removed. These would otherwise decrease the working efficiency of the fuel cell. Fuel cells with PEM are especially sensitive to CO). The hydrogen-rich fuel is mixed with air and fed into the stack (the power section) where DC is generated, together with heat (which can be used for heating purposes). The DC is then transformed to AC in the transformer (power conditioner) and the “spikes” in the current are evened out. In the literature about fuel cells, the cells with PEM are especially mentioned as a strong alternative to fuel cell for vehicles. This is partly due to the fact that they are lighter in weight and faster to start up than other fuel cells (DoD, 1999b). This fuel cell comes from a development from a later period than SOFC, for example, which is another promising technology using ceramic electrodes, and which produces oxide ions (rather than protons) which meet the fuel at the cathode. The construction of SOFC is simple (see Figure 14) and has the advantage that it works with several different fuels. The disadvantage is that if it is to be used in vehicles the working temperature is up to 1000 degrees C and it takes a certain length of time to start up. The reaction in the fuel cell can be seen in Table 12. Table 12. Anode and cathode reactions of SOFC
Source: US Department of Defence
14. Fuel cell with solid oxides (ceramic). Source. US Department of Defense.
Fuel cells with PEM have, as stated above, been recently developed and are judged to be favorable due to their low weight, which means that they can more easily be moved around in, for example, a vehicle. According to Maruo (Maruo 1998) they are going to be developed by various vehicle manufacturers such as Chrysler, GM/Opel, Ford, Honda, Mazda, Mitsubushi, Nissan, Peugeot/Citroen, Renault, Toyota, VW/Volvo and Zevco. Maruo states, as already has been referred to, that BMW is not one of the manufacturers which are now developing fuel cells, because they think the working efficiency is too low and do not believe that fuel cells will be able to compete economically within the newt 20 years. According to information from another sources, BMW is using its resources for developing hybrid vehicles and is also working with a hydrogen fueled internal combustion engine. Fuel cells with PEM use hydrogen gas as fuel (DoD, 1999a). The stream of hydrogen meets the electrode (anode) where it is ionized. This results in the protons being led through the membrane. The electrons flow through the electrode and into the bipolar plate. These electrons continue through the cell and pas through the output circuit. Table 13. Anode and cathode reactions in a fuel cell with PEM.
Source; US Department of Defence
Tables 12 and 13 and Figures 11 and 13 illustrate which reactions take place in a fuel cell with PEM (Proton Exchange Membrane), and a Solid Oxide Fuel Cell; (see also Table 11). In document recently presented by Stobart, (Stobart, R.K., 1999b) Arthur D. Little, Cambridge, UK describes the fuel cell as the most promising alternative technology for vehicles. It has been named as the successor of the internal combustion engine, otto engines and diesel engines. Despite a great deal of investment and much interest among vehicle and engine manufacturers to get the fuel cell vehicles on the market, there remains a great many questions about their concept. Some of the questions concern the technology – can a fuel cell meet the demands for working efficiency and performance? Are vehicle owners prepared to pay the higher price for an environmentally superior alternative? A great many of the
advantages of this alternative are still thought, by the consumer, to be impossible to achieve, but there are other more valid, for example; “Clean” – the vehicle will probably be able to meet the SULEV* demands in California and several projects have demonstrated “non-measurable” emission of NOx. “Silent! – such as en electric vehicle: vehicles using fuel cells do not have high pressure or a significant source of noise, even if there are pumps and a need for cooling. “Available” – vehicles which use fuel cells, classed as EZEV (efficient zero emission vehicle) can make it possible for the owner to drive in a so-called environment zone in towns, which would otherwise not be allowed.
Figure 15. Components of a fuel cell system. In the above figure, Figure 15, can be seen the key components in a fuel cell system for a vehicle. Their function is briefly; A fuel processor which provides a stream of a hydrogen-rich gas mixture to the fuel cells. Generally the fuel processor supplies a mixture which has far too high a content of CO, which must be reduced to a low content (typically 10 – 30 ppm). A stack of fuel cells (in the case of Necar 4 the stack consists of 400 fuel cells) which are supplied with two streams of gas, fuel and oxygen (in the form of air). There are several types of fuel cell, but the demands seem to indicate that it will be PEM (fuel cell with proton exchange membrane) which will come to be used, or a type with solid polymers, which uses hydrogen gas as fuel and tolerates air as the source of oxygen. A compressor-expander unit, which supplies air under pressure to the fuel processor and to the use cells, which use the energy from the exhaust, gases. A water collection system which consists of condenser and water container. The weight of the fuel cell is a critical factor and the levels, which are involved, can be seen in Table 14.
SULEV has been suggested as a set of demands for the State of California for light-duty vehicles and they are 0.01 g/mi (0.0062 g/km) for organic gases excluding methane (NMOG), 1 g/mi (0.62 g/km) for CO and 0.02 g/mi (0.0124 g/km) for NOx.
Table 14. The mass of the fuel cell and an estimation of how the mass can be reduced. Power density of Specific power Power density of the fuel Specific power of fuel the power stack, density of the processor, kW/l processor, kW/kg kW/l power stack, kW/kg År 2000: 0.35 0.35 0.60 0.60 År 2004: 0.50 0.50 0.75 0.75 Source: The Hydrogen & Fuel Cell Letter, 1998b.
In a news letter it states that the DeNora S.p.A company in Milano has developed PEM fuel cells for a small truck, which supplies a private company called Coval H2 Partners in California, USA (The Hydrogen & Fuel Cell Letter, 1998b). The fuel cell stack contains 1000 separate fuel cells, which are operated by using hydrogen gas. It is stated that methanol will be used as fuel for the cells, and that this has a working efficiency of ca 21% at the present, but that there is a good potential for radically increasing this efficiency. The question is whether vehicles with fuel cells will come to replace battery driven electric vehicles and hybrid vehicles (Electric Vehicle Progress, June 1998). Certain vehicle manufacturers believe that they will have commercially available fuel cell vehicles in production by 2004, others see fuel cells in vehicles as a technology of the future. Since one can consider the fuel cell vehicle as being an electric vehicle, there is some thought that the development of fuel cells which is now taking place will be advantageous for the electric vehicle market. United Technologies Corporation (UTC) has stated that they plan to use PEM fuel cells for vehicles and busses (Electric Vehicle Progress, September 1998). UTC’s fuel cell technology has been used in NASA’s space project. The company’s newest fuel cell is 100 kW and is designed for busses. The fuel cell is a PAFC (phosphoric acid) cell, which will be driven using methanol as fuel. This is the first fuel cell for busses which uses a liquid fuel. Zevco is a company which aims to produce zero emission vehicles and they have now announced their plans for developing taxi vehicles for London. These vehicles will be equipped with fuel cells and have been christened “Millenium Taxis” (Electric Vehicle Progress, November 1998). These will be hybrid fuel cell/battery vehicles, and the fuel cell will be AFC (alkaline fuel cell) driven by hydrogen gas. The company thinks that this is favorable from the power/weight point of view when compared with PEM fuel cells. The fuel cell charges the battery of the taxi, which then drives an electric engine. A new organization has been started which consists of the foremost actors within the fuel cell industry. The active members are 3M, Daimler Benz, Ford MotorCorp, Energy Research Corp., Ballard Power Generation and the American Methanol Institute (Electric Vehicle Progress, Dec 1998). The members represent a cross-section of fuel cell technology and work with SOFC (solid oxide) fuel cells and a ceramic, MCFC (molten carbonate) fuel cell with molten carbonate PAFC (phosphoric acid) fuel cells and PEM (Proton Exchange Membrane) fuel cells. The Energy Department (DOE) has chosen 16 companies and educational institutions in nine states who will receive roughly 70 million dollars towards the cost of new research in advanced fuel cells and combustion engines with high working efficiency (Walsh, M., 1999). This reward supports the goal of a “Partnership for a New Generation of Vehicles” between industry and the government. The new project will also provide support for developing similar technology for fuel cells for use in related applications for the production of heat, for cooling and electricity, (see DOE, 199b). The project is being prepared and the aim is that it will be run for 2-3 years.
Research workers at Humboldt State University’s Schatz Research Center have developed a more effective fuel cell (probably a PEM fuel cell) which can be driven with high pressure input air, which reduces the demands for high effect for the compressor and thereby increases the working efficiency of the fuel cell. The process has been tested by an independent laboratory and found to better than two commercially developed fuel cells. Daimler/Chrysler, which has previously presented a new vehicle, Necar 3, driven with fuel cells has now presented its Necar 4 (Crosse, J., 1999). Necar 4 has PEM fuel cells and is fueled with liquid hydrogen gas. The system produces 55 kW and gives Necar 4 a top speed of 145 km/h and an operating distance of 450 km. Fuel consumption is stated to be 3.2 l/100 km in gasoline equivalent. The vehicle weighs 1 5880 kg against 1 170 kg for a corresponding gasoline driven vehicle. GM, which has already developed two electric vehicles – a passenger vehicle (EV1) and a truck (S10), states that they will be able to present a fuel cell driven vehicle in 2004 (Engine, 1/1999). GM carries out development work in co-operation with Exxon and at present they are evaluating how gasoline and natural gas can be used for the production of hydrogen gas. Renault, which is engaged in the FEVER project for the development of fuel cells, states that they use their Lagunda combi-vehicle for the development of fuel cells together with –de Nora, Ansaldo, VolvoTD, Ecole de Mines and Air Liquide (Engine, 1/1999). A transformer, which can reach a working efficiency of 92% for the greater part of its working range, transforms the voltage to 250V. The power will be transferred to a synchron engine through a fixed gear to a stack of batteries (Ni-MH). The specification states that Lagunda will have a working distance of 400 km. For the future there is great hope that the technology of fuel cells will be able to be combined with good environmental characteristics and an energy efficiency which is comparable with the that of the best diesel engines (Stobat, R and Linna, J R, 1/1999). A limited demonstration of fuel cells under constant use has been found to have a working efficiency of 30% and a potential for over 40%. It will not be easy to realize this potential for transient driving conditions and cold starts where one must compromise the effectivity. One also has to remember that research and development is under way on new concepts for combustion engines, in order to reach low levels of emission and high working efficiency. Some examples of these studies to be pointed out are the ultra-lean compression engines with homogenous fuel-air mixture and engines with throttle and spark plug ignition. According to an article by Crosse (Crosse, J., 1999) GM Opel has confirmed plans to commercial manufacture usable electric vehicles by 2004. GM is not the only manufacturer to use fuel cells as a source of electricity for there are plans at DaimlerChrisler, Honda, Toyota, Nissan and Ford to produce FCEV vehicles, but these have not spoken of their strategy and time scale as clearly as GM. GM’s efforts are connected with their GAPC (“Global Alternative Propulsion Center”) in Mainz-Kastel, Germany, which also has research resources at Warren, Michigan and Rochester, New York (Crosse, J., 1999). GM has a great deal of research underway on fuel cells. The chiefs of the research center are DR Byron McCormic and Dr Erhard Schubert. An important factor here is the co-operation with the oil company Exxon/Esso in order to have access to expert knowledge within the field of fuel technology and the infrastructure. A critical question of which fuel should be used. GM is, according to the information, neutral in this question, while DaimlerChrysler has already decided to use methanol (see however under “hybrid vehicles” in Chapter 2). DaimlerChrysler states that, at the present, do no exclude the use of gasoline for the production of hydrogen. In the question of the choice of fuel for fuel cells, there was lively discussion at a conference at Ypsilanti in the US during the 56
summer of 1999, without any conclusion being reached. The conclusion that one could draw from the discussion was that the vehicle industry still has an open mind in the question, and that that includes even DaimlerChrysler. Another open question is whether hydrogen should be produced on board the vehicle of whether it should be produced at stationary establishments.
7 DEVELOPMENT OF BATTERIES Electric propelled vehicles, industrial machines and especially industrial trucks have existed for a long time. With the increasing interest in electric fueled vehicles there has also been a great deal of attention paid to improving batteries and producing new types of batteries with higher energy density (Wh/kg) other than lead acid batteries. With the lead acid batteries the driving distance is limited to 70 to 80 km if an acceptable battery mass is used. The estimation today is that the driving distance will be considerably increased with the new types of batteries. The introduction of new batteries with higher energy density has, however, led to unexpected difficulties being encountered, which take time to solve. First of all it was necessary to adjust the development of the batteries for use in vehicles, so that the car owner does not have to spend too long recharging the batteries. This phase of development is not at all finished and therefore it is today not possible to determine, with any degree of certainty, whether the goals for the development will ever be reached. A capacity for the production of the new batteries must also be established. In order to reduce the cost of the batteries already in production and establish an acceptable level for the cost of new types of batteries the production efficiency must be increased. The present day cost of the more efficient batteries is regarded as being too high to be acceptable. A hindrance to cost reduction is that many of the material used in batteries (for example lithium) are expensive. The use of other, cheaper material may have to be restricted for environmental reasons, unless satisfactory routines for the use of these materials are established; cadmium for example is regarded as being one of the most dangerous poisons. A great deal of attention has to be paid to this fact when using cadmium. In the case of the production of batteries there may be some hindrance, such as a shortage of material or difficulties in acquiring certain materials. Later on in this section some more information will be presented concerning the development and production of batteries. The aim of this section was primarily to discover which types of batteries were most suitable for hybrid vehicles. One of the difficulties is to get access to the information about the latest and newly invented batteries and the state of the art especially about the development of batteries for hybrid vehicles. One special difficulty has been to find out what different batteries will cost in the future. It must be taken into account that the basis for a successful development, introduction and production of batteries is that the manufacturers can see an interested market. In this case no such market can be seen at present.
Present day batteries
With regard to the current state of the art the most prominent systems for hybrids are series hybrids and parallel hybrids. In these systems it is otto engines or diesel engines which are most commonly used in combination with a generator/electric motor or electric motors and a battery or batteries. Series hybrids usually have a larger package of batteries than parallel hybrids since they often serve larger vehicles and the internal combustion engine is not directly connected to the driving wheels. The power from the engine is used to load the batteries via the generator and during some part of the driving of the vehicle the batteries are the only energy source i.e. when the engine is shut off. This can be the case in heavily populated areas such as down town areas. The different types of hybrid systems have been presented in Section 4. From the description of the parallel hybrid system it can be seen that in this system there is an interaction between the two energy sources in that the battery supports the internal combustion engine via the 58
electric motor when more power is needed. The engine then “pays back” that energy when less power for propelling the vehicle is needed. In many cases such as in the case of the Toyota Prius, there is another difference between a parallel hybrid system and a series hybrid system concerning the charging of the battery or batteries. It is only the internal combustion engine which charges the battery in Prius. For the battery in a series hybrid it is common that this battery can also be charged from the mains. However, it is not certain that this difference exists in every case since the arrangement for the battery in a parallel hybrid can be such that it can be charged from the mains in addition to being charging from the engine via a generator. The question about the type of battery to be used in light-duty vehicles and heavy-duty vehicles, or whether the same type of battery can be used, seems not to have been satisfactorily answered. The mass of the battery in relation to its capacity for the storage of energy seems to be an important factor as does the cost of the battery. In Vol. 20 No. 22 of Electric Vehicle Progress (Nov. 1998) it is underlined that the cost of the batteries must be dramatically reduced for electric vehicles to be able to compete with vehicles powered with an internal combustion engine. It is also pointed out that the cost of the batteries can be such that the car manufacturers, at least in beginning of the period of introduction, lose money selling electric vehicles. Such a situation is however not uncommon when introducing a new product. The situation should not last too long, however, or the interest for the product may be lost. In the US the Department of Energy (DOE) is involved in a program called ”Advanced Automotive Technologies” (ATT) and the aim of this program is to bring together a broad spectrum of research activities to an integrated activity to develop advanced energy storage. The participators in PNGV (”Partnership of a New Generation of Vehicles”) are the leaders of this program and it is also served by, among others, Chrysler, Ford and GM through representatives for ”United States Council for Automotive Research” (USCAR). One task of the program is the development of batteries for electric vehicles and hybrid vehicles. Highpower batteries are developed for hybrid vehicles and high energy density batteries for electric vehicles. The structure of program can be seen in Figure 16 (Sutala et al., 1998).
Figure 16. Organization for the development of batteries. Source: Sutula et al., 1998. The aim of the U.S. Advanced Battery Consortium (USABC) is to develop high-energy batteries to meet the requirements of the emerging electric vehicle market in California. The 59
U.S. Department of Energy (DOE) supports an active program of long-range R&D to develop advanced energy storage and related systems technologies that will be necessary for the commercial viability of competitive hybrid electric vehicles. USABC is organized so as to support the PNGV program. By focusing on the hybrid electric vehicles the target of the PNGV program may be reached. The contributions within USABC and DOC are aimed at supporting the development of high-storage batteries such as nickel-metal hybrid and lithiumion batteries (Sutula et al., 1998). According to DOE one of the most important requirements for batteries in a hybrid system is a high-power to energy ratio. Batteries with power-to-energy ratios greater than 20kW/kWh, which use the USABC method of rating battery power and energy and with long cycle life are not available for high-power energy storage. The goal for energy storage is a 10-second power/energy ratio of 25 W/Wh already year 2000. The lifetime is not known but 10-year calendar life is required in order to achieve the goal for the overall system costs. R&D on ultra-capacitor and flywheel technologies for high-power storage devices has also been of interest. Within these activities for the development of batteries two interesting alternative were investigated, one of which was an ultra-capacitor and the other was a flywheel battery. Both of them have a rather high specific power (850 W/kg). However, VARTA is also developing batteries with a specific energy of 80Wh/kg. For these batteries the specific power is approximately 200 W/kg (Köhler and Niggermann, 1998). 62
• By a special adapted program for research and development carried out during the last years VARTA BATTERIE AG has been working with an upgrading of lithium-ion technology. By the use of lithium manganese spinel (MgAl2O4) and coal as active material, lithium-ion cells of 110 Wh and 250 Wh could be developed. The aim for future development is to increase the energy density of the battery cells and to reduce the relatively high weight and volume of the battery. By these means it is expected that the specific energy can be increased to 100 Wh/kg at the battery level (Brohm och Meissner, 1998). • The authors of the “Brite Euram Program” (Cooper and Mosely, 1998), funded by the European Commission points out that the ongoing discussion has been focused on the driving distance between chargings of the battery and by this the specific energy of the battery. The opinion of the authors is that range per charge is less important than the cost of the battery provided that the battery can be recharged within a short time. The fact that the energy density has been doubled since 1990 has shown that there has been a considerable development of lead acid batteries. The studies and trials which have been going on have also shown that it is essential that the recharging of a lead acid batteries firmly follows a correct procedure and also that a fast-charging of the battery can be beneficial for the lifetime of the battery. It has been shown that the battery can withstand up to 900 cycles of charging/discharging instead of the 250 cycles when using conventional charging/discharging. Table 15 shows the different steps of the development. Table 15. Development of lead acid batteries EU. Source: Cooper and Moseley, 1998.
Choice of batteries for hybrid vehicles
There are many reasons for the difficulties in choosing the “right” battery for a hybrid vehicle. First of all there are many alternative battery systems of which some are still under development. By further development even the cost can be reduced. In addition to other questions, it may be wise to ask whether efficient production has been developed for the type of battery of interest. A decisive factors which has an impact on the choice of the type of battery is its specific energy contra it specific power. With a hybrid vehicle - and especially a parallel hybrid where internal combustion engine or the alternative to that energy transformer has a relative low power – it is important that the battery via the electric motor can assist the engine driving when higher power is needed than the engine can deliver. In this case a battery with high 63
power is needed. For a pure electric vehicle a battery, high energy is needed in order to fulfill the requirement for along enough range between the chargings. If the use of energy in the vehicle has to be kept at a low level the mass of the battery must be kept as low as possible. Köhler and Niggermann discuss this matter (Köhler and Niggermann, 1998) and they have also underlined that the batteries have to be tailored with respect to the requirements. They point out, for example, that nickel-metal-hybrid batteries can be tailored with respect to the requirement of specific energy or specific power - high energy (HE), high power (HP) and ultra high power (UHP) respectively. UHP batteries have been under development and are today possibly in production. For batteries in a hybrid vehicle there is also a requirement that they must tolerate many (even small) chargings/dischargings – usually a higher number than for an electric vehicle even in the case where the electric vehicle is equipped with a system for regenerative charging. The development of batteries for especially hybrid vehicles must be directed towards a robust product which can withstand cycles of discharging/charging which typically can be as low as 3-5 % discharging/charging. The lifetime of batteries depends strongly on a positive development in this direction. Table 16 shows which batteries are used by some of the different car manufacturer and also shows the suppliers of the actual batteries. One of the goals for the development and use of hybrid vehicles is that use of energy is expected to be less than for a vehicle without hybrid system. It is not yet completely clear how far in this direction it is possible to go with a hybrid system since there are not many types of hybrid light-duty vehicles and hybrid heavy-duty vehicles in existence at the present time. Hybrid vehicles of today are either a few types on the market or they are presented as prototypes. Existing hybrid passenger cars are relatively small and are produced from lightweight material, which is a reason for the difficulties in comparing hybrid passenger cars with commercial passenger cars. A closer comparison between some different hybrid systems is presented in Section 8. The reason for pointing out this matter here is that the mass of the battery has a great impact on the mass of the vehicle. Whether or not lightweight material is use in the body of the vehicle it will still be heavier than the same vehicle without hybrid system. For hybrid vehicles it is therefore an advantage to choose a battery with respect to weight and specific power in order to reduce the weight of the car. However, this requires a careful analysis from one case to another. Table 16. Vehicle manufacturers and their choice of battery. Car Battery Battery manufacturer Honda Ni-MH Panasonic EV Energy Toyota Ni-MH Panasonic EV Energy Nissan Li-jon Sony GM Ni-MH GM-Ovonic Ford Ni-MH Panasonic EV Energy Chrysler Ni-MH SAFT Mercedes Benz Na-NiCl2 AEG BMW Na-NiCl2 AEG None Li-polymer LIBES Source: Sato, 1998. From the point of view of costs, it is most likely more advantageous in Sweden to charge the battery from the mains than to load the battery with electricity produced when using an internal combustion engine. In terms of energy the cost is much lower for electricity than for fossil fuel. On the other hand the cost may be lower when charging the battery with electricity produced by an engine than taken from the mains, if that electricity is produced in an electric generating plant fueled with a fossil fuel. However, in the latter case, the higher cost is not 64
paid by the user of electric in Sweden since the price of electricity is mainly the same and is independent who produces the electricity. (Only a very small fraction of electricity is produced from fossil fuel in Sweden). The fact that series hybrid vehicles are designed so as to have their batteries charged from the mains is of course, a matter for the car manufacturer. On the other hand it is not yet common practice that a parallel hybrid is designed in that way. The parallel hybrid is often equipped with a small battery since power from the battery is used only when supporting the internal combustion engine during accelerations or other driving conditions when an access of power is needed. W/kg
Specifika power fordifferent batteries
Specifika energy for different batteries
20 Pb (lead batt.)
Pb (lead batt.)
Figure 17. Specific effect respective to specific energy for various batteries. Source: Meeus och Gravenstein, 1998.
In Figure 17 the specific power and the specific energy for seven different batteries are shown graphically. In the case of specific power the nickel-metal-hydride battery is in a top position and in the case of specific energy, the lithium polymer battery is best. One problem in this context is that there is not yet any clear picture concerning the future cost of batteries with higher specific power and specific energy. According to one source of information the cost of a lead acid battery can be up to $100/kWh. The costs for both nickel-metal-hybrid batteries and lithium-ion batteries are higher than for lead acid batteries but the first mentioned batteries have a longer life cycle and they may therefore be less expensive to use in the long run.
8 HYBRID VEHICLES In Section 4 the two main alternatives of hybrid vehicles, series hybrids and parallel hybrids have been presented. Despite the fact that the first hybrid passenger car was presented on the market as late as year 1997, as a hybrid vehicle in production and also as some prototypes somewhat later (mostly passenger cars and buses), different alternative of hybrid vehicles can already be seen. The presentations have covered fuels to be used, internal combustion engines for hybrids, fuel cells, batteries, system and units to be used for the control of the hybrid vehicles and it can be said that a great interest has been shown in these types of vehicle. Electricity is produced for charging the battery by the APU (defined as the internal combustion engine and the generator). For a series hybrid there could be an advantage in designing the vehicle to be charged from the mains, an alternative which is not common for a parallel hybrid vehicles. Because in many cases that the battery pack represent a rather heavy mass, the parallel hybrid system is, in reality, the dominant system for light-duty vehicles. However, even for parallel vehicles the mass of the battery can be a problem and this is one of the driving forces for the development of improved or new types of batteries with higher specific power. For heavy-duty vehicles and especially buses operated in densely populated areas like city areas pure electric operation is an advantage for the environment and therefore these types of hybrids commonly are series hybrids with a rather heavy battery pack. For these types of vehicles there is a need for batteries with a high specific energy despite a heavy battery pack may not being the same burden for a heavy-duty vehicle as for a light-duty vehicle. In the following section the potential improved energy use (or improved fuel consumption) for hybrid vehicles is discussed and some of the engine technology and hybrid systems are also discussed. In addition a rather large number of prototypes of hybrid vehicles are presented, especially buses. A comparison between different fuel/hybrid system combinations is also presented.
Potential for improved energy use in hybrid systems with different types of internal combustion engine
Many different interactive possibilities exist for the improvement of fuel consumption or energy use* when considering every element in a vehicle or every factors having an impact on the use of motor vehicles. This fact has been clear to the car manufacturers for a long time and at the present time the improvement in the use of energy when driving the car is one of the main issues within the automotive industry even if it is not always seen in reality. However, the co-operation between the industry and the government expressed in the PNGV program (discussed in for example Section 8.1.3) has certainly had a great impact on the development of new ideas within the automotive industry and others involved. It has influenced not only the car industry in the USA but even in Japan and Europe and is one of the driving forces for improvement of vehicles in this respect even if the PNGV agreement with the US government only apply to Chrysler, Ford and GM. Unfortunately the use of less energy when driving the car has so far not had the required effect on the user of the car. Some of the factors having a great impact on the use of energy in the motor vehicles are the size of the body of the vehicle its weight and the size and power of the engine. It is a question *
In this context the expression ”energy use” is preferred since physically energy can only be used not consumed. When discussing the use of alternative (different fuels) it may be an advantage to use energy units.
as to whether the electric and hybrid vehicles which have so far been presented constitute a new trend regarding, among other things, the size of the passenger cars. Today there is a common opinion that the size of the car has to be reduced if the goal is to be reached, expressed in for example the PNGV program, of fuel economy of 80 miles per gallon. This is comparable to approximately 0.3 liter per 10 kilometer gasoline equivalent. The fuel consumption for some of the hybrid vehicles is presented in some of the following sections. 8..1.1
Theoretic background for the potential in improved energy use
Hybrid operation opens the possibility of improve the working efficiency of the of the internal combustion engine since the engine can be run in an advantageous areas from the point of view of fuel-efficiency. A three-dimensional diagram can illustrate this over the specific fuel consumption (in g/kWh) for a certain fuel engine combination. Such a diagram is called a mussel-diagram since the iso-lines (in this case for equal fuel consumption) together form a pattern which looks like a mussel-shell. Figure 18 is an example of a mussel-diagram. It is diagram for a 1.25 liters engine to be used in a passenger car from Ford (Menne et al., 1996). The diagram was primarily amended by Ecotraffic (Sweden) in order to show iso-lines for constant power.
Figure 18. Mussel diagram over fuel consumption for a 1.25 liter gasoline engine, modified by Ecotraffic (Sweden). Source: (Menne et al., 1996). When following the iso-line for constant power (the dashed lines going from the upper left to the bottom right) it can be clearly seen that the consumption of fuel varies for the different speeds. In the figure there is also a line (the dashed line going from zero torque and upwards to the right) showing the most favorable consumption of fuel. Certain considerations have been taken to the risk for vibrations and loss of comfort caused by engine speeds at high loads at such a low engine speed as 1200 r/min. For comfort reasons (vibrations and noise) it may not be tolerable to follow the dashed line to an engine speed as low 1200 r/min. and lower. However, in order to keep to as low fuel consumption as possible it would be favorable to 67
always follow the dashed line shown in the figure. Since normal driving in traffic rarely requires high power, the above-described driving conditions means that it should be necessary to always run the engine at speed in the range of 1200 to 2500 r/min. For practical reasons it is not possible to use such an extreme gear ratio of the gearbox (a gearbox with 6 to 7 gears) and a completely changed gear ratio for a car with such a small engine as 1.25 liter, is not reasonable. For such a gear ratio, as said above, the reserve of power should be close to zero and a very small slope of the road should force the driver to change the gear*. In practice the full potential of maximum energy efficiency of the internal combustion engine can be reached when using a common mechanical gearbox. A car driver would not accept such frequent gear changing that such an extreme gearbox would require**. In heavy-duty vehicles the situation is different (higher mass/power ratio) and in this cases up to 16 different gear ratios are used. An automatic gearbox makes use of somewhat lower gear ratios*** in the cardan transmission. However an automatic gearbox is less energy-efficient than a mechanical gear box and therefore the fuel consumption is usually higher for a given car with an automatic gearbox or is at best the same when compared with the same type of car with a mechanical gearbox. For an electric drive system for transmitting power from the electric motor to the wheels the same possibilities exist - or better ones - as for a car with an automatic gearbox, to run the internal combustion engine in a range with high efficiency. In addition it also has a better controllability (is faster). Therefore this type of system gives the basic possibilities to quickly move the operation of the engine to a more favorable area than that in which it has been running. A gear box with Continuously Variable Transmission (CVT,) or a series hybrid system will theoretically be able to follow the curve for best energy efficiency (or fuel consumption) shown in Figure 18. A parallel hybrid system combined with a CVT can also be operated similarly to the abovementioned combined system while the same possibility does not exist for a conventional mechanical gearbox (because the restricted number of gears). If the mechanical gearbox is equipped with a similar gear changing automatic system as in, for example, a Formula 1 car, the difference is less when compared with the CVT gearbox. The same comfort will not be obtained as with a CVT because the gear changing is not continuous. However, the automatic gear changing system for this type in passenger cars is now being introduced on a large scale. One example of this introduction is a new type of passenger car introduced by Volkswagen (VW Lupo 3L). Besides the fact that it is not possible to use an extreme final gear in combination with a mechanical gearbox there is another problem in that the internal combustion engine is overdimensioned for most of the driving conditions. As an example it can be said that power out from the engine must be 20 to 25 kW in order to achieve the highest efficiency according to Figure 18, which would mean that the driver would have to keep to a much higher speed than is allowed in Sweden. In a hybrid system on the other hand the size of the engine can be much smaller and, by these means, come closer to the most efficient area for operation of the *
In this context it should be reminded that at the end of the 1970’s and at the beginning of the 1980’s some of the car manufacturers had a very extreme gear ratio in their cars in order to reduce the fuel consumption but this also rendered in many complains from their customers. ** A frequent changing of the gear also results in certain energy losses. *** In this context ”lower gear ratio” is used to characterize a gear ratio which result in lower engine speed for a given speed of the vehicle. In terms of mathematics this is correct. In some popular literature there unfortunately is an misunderstanding of the concept when the wording “high gear ratio” is used as a synonym for a higher speed of the vehicle for a given engine speed.
engine. Even if the energy efficiency is somewhat lower than for a full-scale engine, it is compensated for by the more favorable operation of a smaller engine in a hybrid system. Both types of hybrid systems are usually designed so as to use the energy released during braking by the so called regenerative braking and this means that the system includes another possibility for improved energy use by the car, especially when driving in city areas. Despite there usually being less braking when driving on the road the system with regenerative braking make the hybrid system superior over a vehicle without a hybrid system. Unfortunately the losses during charging and discharging of the battery are relatively large (see a coming section), and this considerably reduces the potential of regenerative braking. The losses the rest of the electric system (excluding the battery) are also rather large since they occur both at charging (braking) and discharging (acceleration) affect the whole system. Despite that an electric controlled system for transmitting power from the electric motor to the wheels has about the same energy efficiency as an automatic gearbox it is clear that it has a considerable lower efficiency than the energy transmission by a conventional mechanical gearbox. Transmission of power via two gear wheels (grasping of teeth) in a gear reduction set result for example with efficiency of 98 % which can not be achieved for such an electric controlled system for transmission of energy. Even if the maximum efficiency for a well functioning electric motor of suitable size can have an efficiency as high as 90-95 %, it can not compete with a mechanical gearbox in this respect. In an electric drive system for transmission of energy also the losses in other components (generator inverter, battery and so on) must be accounted for. The weakest chain in this system is the batteries which have an efficiency of maximum 70 % for charging/discharging as a best and often much less (50 – 60 %) under the variety of driving conditions. For a situation where the battery is used a long time during the driving (as in a series hybrid) these losses will be considerable high. It is not surprising that the battery pack must be equipped with a cooler in order to avoid overheating during such driving conditions and to thereby reduce the deterioration of the batteries. It can be seen that there is a considerable potential to increase the energy efficiency provided that the efficiency of all components in the drive train are high. Even in this case the expression is valid that no chain is better than its weakest link, which leads to the strategy that improvements of the batteries is a good way to reduce losses of efficiency. 8.1.2
The interaction between the control unit, the energy transmitters and the mechanical power transmitters in a hybrid system
In the previous section the theoretical possibilities for a reduction of energy use in an automobile have been discussed but there are limitations for energy reductions in a hybrid system of chemical and physical reasons. In this section the interaction between the different unit in the system will be discussed and some example about factors influencing the energy efficiency will be given. The control unit in a hybrid system can be seen as its brain. With present day technology for electronic control there are many possibilities for the control of different functions and this technology can also be used for the control of many functions in a hybrid vehicle. The control unit can be “taught” to “feel” what the gear ratio should be, in for example a transmission like the CVT, so as to run a given electric motor or an internal combustion engine as efficiently as possible. The control unit can decide at what load the internal combustion engine shall be shut off and the vehicle be propelled only with the electric motor and also when to quickly start the engine so as to support the electric motor during such accelerations when more power is needed. In a parallel hybrid system the control unit can be programmed so as to propel the vehicle with only the internal combustion engine and to assist the engine with the electric
motor at higher load modes. For both of the systems the control unit has to facilitate the function of regenerative braking if that function is included in the hybrid system. In connection with the discussion about the control unit it should be said that there could be a problem to program the control unit for a hybrid vehicle so as to achieve reliability in operation of the vehicle and a comfortable ride. That means that no vibrations in the vehicle should occur and that there should be no sudden hesitations during driving caused by false information to the control unit resulting in that the control unit overestimating the physical/mechanic possibilities of the hybrid system. All mechanic systems or units for power transmission have their limitations but if they are operated within their area for good function they are mostly run reliably and are reliable during continuos operation. Since hybrid vehicles are a new type of vehicle with many new functions, the constructors have many new functions to design. In their work they must be especially careful that the interaction between the different functions is secured and that the different systems including the internal combustion engine and the electric motor are operated with high efficiency. In a report from Japan, certain factors influencing the energy efficiency in a hybrid system, are discussed (Iwai, 1998). A part of this was discussed in Section 4.3 and here the report will be discussed in somewhat more detail, especially some important factors in a hybrid system, which the author of the report has pointed out. The opinion of Iwai is that the thermal efficiency of a conventional engine is only about 15 % when the car is run according to the Japanese 10-15 mode cycle – a typical low load low speed cycle (Figure 19) and especially the part of the cycle which contain the 10 modes. He calls attention to the fact that the cars should run with an efficiency of 30-40 % provided that the following conditions are fulfilled; (1) the elimination of idling and operation of the internal combustion engine in areas where the thermal efficiency is low, so that the engine is operated only in areas with high thermal efficiency; (2) accumulation of the energy released during braking of the vehicle; (3) installation of a system for power transmission, which can be operated with high energy efficiency independent of area for operation; (4) the use of a bottoming cycle (Stirling cycle or steam cycle) which generates electric energy by driving a generator with the exhaust energy at low engine load. The studies that Iwai carried out, are based on experiments and vehicle tests. He has also carried out calculations based on certain assumptions and in one case he has calculated how much of the energy used during acceleration can be recovered during deceleration. What he shows, in Figure 20, is the percentage of the braking energy which can theoretically be recovered using an electric system with varying efficiency for the recovery, when the rate of deceleration equals the rate of acceleration. The figure is based on accelerations up to 40 km/h. The vehicle was free rolling during the decelerations i.e. the gear was not engaged. The different curves in Figure 20 are as follows; • the “EFF100%” curve shows the maximum theoretical recovery; • the “EFF 77%” curve shows the maximum theoretical recovery with a power generation efficiency of 90% and the inverter efficiency of 95%; • the “EFF 60%” curve shows the maximum theoretical recovery with a power generation efficiency of 90%, the inverter efficiency of 95% and a charging/ discharging efficiency of 70% for the battery.
Figure 19. The Japanese 10-15 mode cycle. The acceleration/deceleration that the calculations are based on are heavier (0.15 g) than the accelerations/decelerations of the ECE driving cycle (the low speed part of the complete ECE cycle of today) but they certainly occur in real traffic on the road. On the other hand a speed of 40 km/h must be seen as somewhat low and in the calculations carried out by Iwai this is the only case of acceleration/deceleration which is dealt with. Therefore the above calculations should be seen as an example presented in order to demonstrate some factors influencing the braking energy recovery and the impact of some parts in the hybrid system on the efficiency on the system. Iwai points out that when a lower acceleration/deceleration rate (0.08 g) is used as a base for the calculation of the braking energy recovery, the result will be only approximately 50 % recovery. Iwai indicates that the efficiency of charging/discharging of a battery for an electric car is 55% and that an efficiency of 70% is at the upper limit for a battery. A question not answered by the above calculations concerns the level of energy recovery in reality and the real efficiency of charging/discharging of batteries (see also Figure 20).
Figure 20. Different cases for calculation of the braking energy recovery (0-0.15g). Source: Iwai, 1998.
The tests, computer simulations and calculations carried out according to the above paragraphs have also resulted in a presentation of a figure showing the energy improvements for different driving conditions (see Figure 21). The figure represents improvements in km/l gasoline when the mass of the car as a hybrid has been normalized to the same car as a gasoline-fueled car. If the mass of the car is not normalized the comparisons would not be correct (according to Iwai) since the mass of the car is higher when the hybrid system is installed.
Figure 21. Improvement of fuel consumption for a series hybrid when tested according to the Japanese 10-15 mode cycle (1.8 ton ”minivan”). Source: Iwai, 1998. The designation SHEV” stands for series hybrid electric vehicle. Since the hybrid system has not been presented in detail it is not possible to determine how well the different alternatives depicted in Figure 21 are representative of an average series hybrid systems. However, the impression is that much work has been carried out for the above presented studies and calculations. Different hybrid systems can certainly show different efficiency depending on the design of the system, resulting in the energy transformations in one hybrid system differing from the energy transformations in another (see also Section 8.1.1). 8.1.3
Result of the PNGV program
Within the PNGV program the potential of hybrid systems has been evaluated with respect to reduced fuel consumption. The goal for the PNGV program is to improve the fuel economy for passenger cars from the present day level up to 80 miles per US gallon (3.785 liter. 80 miles/gallon=0.301 liter/10 km). This applies to a vehicle the size of a middle sized US 72
passenger car (type Ford Taurus) according to the PNGV requirement for fuel economy. The size of Ford Taurus is considerable larger than the size of European cars with the same fuel consumption, such as a car recently presented on the marketm the VW Lupo, 3 liter. Within PNGV there are also stringent requirements for emissions, safety, performance, costs etc. The program started 1994 and is a 10 year program. For the time schedule to be followed, prototypes of the specified cars, ready for production and meeting the stringent requirements, are to be presented in 2004. An evaluation of the PNGV has been presented in a report published in 1998 (NRC, 98) and thereby gives an estimation of the potential for reduced fuel consumption in different system combination. The results of a summary presented by Ecotraffic (Sweden) are shown in Figure 22. Each of the horizontal bars shown in the figure represents an interval of the uncertainty in the estimated fuel consumption (energy used). The vertical thicker line approximately represents the 80 miles/gallon goal for fuel economy – here in liter per 100 kilometer gasoline equivalents.
Figure 22. Potential for different automotive system. Source: NRC, 98. Of Figure 22 it can be seen that the ongoing development of the chassis and the body of the car with the goal to minimize the air drag and friction coefficient has the largest impact on the fuel consumption. The goal for the development of the chassis/body is far reaching. In the figure it can be seen that none of the conventional systems for propelling the vehicle (i.e. nonhybrid systems) are expected to have the potential to reach the goal for fuel consumption, not even the new advanced vehicles. A diesel hybrid (of parallel type) and the two variants of hybrid vehicles equipped with fuel cells are the only vehicles which are estimated to fulfil the PNGV requirement for fuel economy. It can be noticed that a stack of fuel cells including equipment for the production of hydrogen can be regarded as a hybrid system since both the fuel cells and the batteries are the energy sources for propelling the vehicle. The only hybrid alternative for such a vehicle is a series hybrid since the fuel cells generate electricity but not mechanical work. It can be seen as somewhat surprising that the potential for fuel cells is not estimated to be higher than shown in the figure, despite their high status as efficient energy transformers. Fuel 73
cell efficiencies of up to 70% are often reported which would be a remarkable improvement on the 30-40% for an internal combustion engine. The truth about the fuel cells is that a stack of fuel cells may have high efficiency in transforming the energy from hydrogen to electric energy but there are considerable energy losses in producing hydrogen in a fuel transformer and in other auxiliary equipment needed to run the fuel cells. It would be an advantage if the efficiency of the total system could be reported since it is that efficiency which is of interest when comparing systems for propelling the vehicle. In cases where reformers is used for the production of hydrogen on board a vehicle, fuels which are easier to handle than hydrogen (such as ethanol or methanol) can be used but even here the energy losses are large. There is an ongoing development of technology in order to use fuels such as natural gas, gasoline (Shell is engaged), even light diesel oil (jet fuel) and possibly even common diesel oil in fuel cells. However many vehicle manufacturers, regard the problems, including the infrastructure (for natural gas), as being larger for these fuels than for methanol, which is estimated to be the easiest fuel of all to be used for production of hydrogen on board of the vehicle. Regarding the energy losses for this production, it should be stated that high temperatures are needed for the production of hydrogen and there is also some emissions released. It cannot therefore be said that the vehicle is a zero emission vehicle. Concerning the emissions a study has been carried out on the request of KFB (Westerholm and Pettersson, 1999). When the PNGV program started in 1994 considerable resources were also used for the development of alternative engines such as the Stirling engine and gas turbines. The amount of resources spent on such engines seems to have been drastically cut, for two reasons. Firstly there are considerable technical problems with the engines (exotic materials such as ceramics have to be used) which leads to a risk for a delay in the time schedule. Secondly it is estimated that the use of these engines will result in less impact on fuel efficiency than that obtained by the use of fuel cells and diesel engines. The use of advanced otto engines (Parallel hybrid + Advanced otto” in the figure) has been one of the alternatives evaluated within the PNGV program. The expression “advanced” means in this context an engine designed with direct injection of gasoline – similar to the technology for the Mitsubishi GDI engine. At the beginning it was stated that even an advanced otto engine could not meet the requirements stated for fuel consumption within the PNGV program. It was therefore decided to only carry out observations of the development in the area and to spend only a small amount of resources for studies within the program. However, the positive progress and the resolution shown by Mitsubishi have resulted in more resources now being spent on this technology. Such engines may be of interest in a conventional vehicle and in the case of the required fuel efficiency not being met, the DI otto engine may fulfil some new somewhat leaner requirement for the fuel efficiency. A problem, which has not yet been solved, concerning the use of the DI otto technology is that the fuel must be nearly free from sulfur in order to meet the emission requirements, since the efficiency of the deNOx catalyst deteriorates very fast if the fuel contains sulfur. The most concrete form of prototype vehicles demonstrated today within the PNGV program are the prototypes of parallel hybrid vehicles presented by “the three big” (Chrysler, Ford and GM) and these vehicles have been demonstrated as test vehicles in traffic. The development of vehicles equipped with fuel cells is naturally somewhat less advanced. However, even in this case a few prototypes have been demonstrated, but not within the PNGV program as far as is known. For diesel engines the real problem is to meet the very stringent emission requirements. For example the standards for particulate emissions have been set at 0.01 grams/mile (0.0062 g/km) which must be regarded as a extremely low level for this type of engine. In this context the emission requirements stated by the PNGV program could be 74
compared with the EU particulate mission standards of 2000 and later, for passenger cars, which was 0.05 g/km. This is equal to 0.08 grams/mile respectively (see Section 6 for more information about the today and future EU European emission standards). The particulate standards in Europe for light-duty diesel vehicles are 0.025 g/km from year 2005 which is 0.04 grams/mile respectively. The comparison between the PNGV requirements and EU standards for year 2000 shows that the PNGV requirements are set at approximately a tenths of the current EU standards which means that the PNGV requirements are set at the level of a new three-way catalyst car. This level have been regarded as so low that no particulate standards have been set for gasoline catalyst cars in Europe. There is also a practical problem with the PNGV program in that the car manufacturer in the US have not developed and manufactured a small diesel engine since the middle of the 1980s. This means that they must again acquire the basic technology for developing a small diesel engine before they can start a production. Since the diesel engine is the only commercial engine having the potential for meeting the fuel efficiency requirements, that type of engine still may be a candidate for the PNGV program, together with the otto type of engine, provided it can meet the stringent emission requirements. If both diesel fuel and gasoline are considerable improved there may be a possibility for both types of engines if they are used in a hybrid system. For both fuels it is of great importance that the sulfur content is reduced to the level required for the use of efficient emission control and to reduce both NOx and the particulate emissions. The already initiated co-operation between the car manufacturers and the oil industry for improvement of the fuels must continue. In addition to the commercial fuels even alternative fuels including DME have been mentioned. 8.1.4
The influence of hybrid systems on conventional engines
In this section the potential for reducing the fuel consumption for the today and future different variants of reciprocating engines will shortly be discussed. Since the efficiency of the different engines varies as a function of engine speed and load (see Figure 18) the potential will not be the same for all systems. The four points below represent a ranking of the relative improvement of the engines, which will be discussed. It should be observed that it these are not absolute differences but only relative improvements. We do not take a firm position on the question of which hybrid system is the best for each type of engine (series or parallel) but the ranking is solely based on the prerequisite for the actual type of engine. 1. 2. 3. 4.
Conventional otto engine with three-way catalyst. Diesel engine (direct injected). Advanced direct injected otto engine (stoichiometric, EGR and TWC). Advanced direct injected otto engine (lean burn, EGR and deNOX).
The Achilles heel for the otto engine is that it is regulated by quantity, i.e. the engine is fed with the air-fuel mixture via a throttle*. Thereby the pumping losses are considerable at low loads. At full load these losses are smallest since the throttle then is fully open. A comparison between an otto engine and a DI** diesel engine will prove that the relative difference in *
The air/fuel ratio is stoichiometric or close to stoichiometric. Here the comparison is carried out between a direct injected diesel engine equipped with turbo since this technology has today approximately 50% of the market and is estimated to take over the diesel engine market. A comparison between the worst diesel alternative and an IDI engine without turbo would be considerably more advantageous for a gasoline engine.
energy use is approximately 17% less for the DI engine at full load (calculated as the ratio between the efficiencies at best of 41% for the diesel engine and 34% for the otto engine, based on Figure 18 for both cases). Because the pumping losses increase at low loads it is not surprising that the difference between the diesel engine and the otto engine increases by a factor 2 or more at low loads. In a certain driving cycle with typical mean values for the efficiencies of 14% and 18% respectively the relative difference will be approximately 20%. Based on the above discussion it can be clearly shown that the improvement in the efficiency by a transition from a conventional drive system to a hybrid system will be larger for an otto engine than for a diesel engine. There will still exist an absolute difference between an otto engine and a diesel engine which is less than 20% but larger than 17% - if the comparison is carried out for a hybrid system without regenerative braking. In the case of hybrid systems (with otto and diesel engines) where regenerative braking is used, which seems to be common and is accounted for, the absolute use of energy will be reduced by approximately the same amount for both types of engines. However the relative difference in energy efficiency will increase (mathematically) compared with a case without regenerative braking. For the advanced otto engines the goal for the development has been to decrease the fuel consumption at partial load by reducing or eliminating the pumping losses (avoid throttling i.e. the use of throttle) at part load. Thanks to the fact that direct injected otto engines are operated at stoichiometric air-fuel mixture (λ=1) at full load, exactly like conventional otto engine, the energy efficiency at full load will be the same for both types of engines or at best show a small improvement for the direct injected engine. Two main types of combustion systems for the direct injection engine seem to have been defined. One type, which has been commercialized by Mitsubishi and Toyota, uses a combination of excess of air (λ>1) and EGR. Through this method the pumping losses can be nearly eliminated at low loads. The other variant uses only EGR in order to achieve the same result. Despite the fact that the direct injection essentially increases the engine acceptance of EGR compared with a conventional otto engine, the pumping losses cannot be completely eliminated since the engine does not tolerate that high EGR rate that is required in order to completely eliminate the use of a throttle. In comparison with a diesel engine it is possible, within a small area of mussel diagram (see Figure 18) at low load, to nearly achieve the same efficiency as for a diesel engine when using the second variant of combustion system (see engine type 4 above). With the first variant of combustion system (see engine type 3 above) the same low fuel consumption cannot quite be achieved as for the second variant. Since the reason for using a hybrid system is to increase the average engine load (where the efficiency is higher) the engine will be operated relatively more in the area where the difference between a conventional otto engine and the direct injection engine is less than at low load or at medium load (Figure 18). Therefore the potential for a relative improvement of fuel consumption will consequently be less for the direct injection otto engine than for the conventional engine and probably also less than for a diesel engine. Another consideration to bear in mind is that a direct injection otto engine is likely to suit a parallel hybrid system better than a series hybrid system since the internal combustion engine is not operated at the same higher load in a parallel hybrid as in a series hybrid. If the comparisons for the four different types of engines are based on differences in fuel consumption the result for ranking of absolute fuel consumption will the following: 1. Diesel engine (direct injected). 2. Advanced direct injected otto engine (lean burn, EGR and deNOX). 3. Advanced direct injected otto engine (stoichiometric, EGR and TWC). 76
Conventional otto engine with three-way catalyst. 8.1.5
Series hybrid or parallel hybrid?
During the literature studies it has been noticed that the focus was more on series hybrids when the PNGV program started in 1994, but that later on it shifted over towards parallel hybrids. Higher costs and a lower potential for reduced fuel consumption for series hybrids than was first estimated may be the reason for this change of strategy. The hybrid systems which are now commercialized (Toyota, Honda, Audi and some other Japanese manufacturer) are all parallel hybrids despite the fact that the systems are somewhat different in their characteristics and construction. In some cases the classification “parallel hybrid” can be questioned since the systems may be seen as something between the two main hybrid systems, but which have chosen to be called parallel hybrids. Furthermore, the three types of hybrid systems presented within the PNGV program are all parallel hybrid types (more like parallel hybrids than the Toyota system). Signals from the European car manufacturers can be seen as an indication that the Toyota system will be the most popular in the short run. It should also be underlined that that hybrid systems present considerable more possibilities in the construction of the drive system than was earlier given by the commercial systems. In some cases the type of system cannot clearly be characterized. It is likely that many different systems will be used during the reasonably near future before the most efficient system (in the form of a compromise from different criteria) is determined. The factors which give a preference for parallel hybrid systems are the higher efficiency, the possibility of achieving good performance and a well-developed battery. For heavy-duty vehicles which are produced in limited numbers, there is also an advantage with the parallel hybrid system in that standard components can be used in many models, which supports the costs of the development. In order to sum up it can be said that there is a considerable potential, in using a hybrid system, for improved fuel economy (less energy use). Also the transition to the use of fuel cells is facilitated by the use of an electric drive system in hybrid vehicles. One idea to realize could be to use hydrogen in the vehicle for the fuel cells (to improve the fuel cell system) in order to eliminate the use of the battery as back up for the fuel cell (see Section 8.2.2). This is provided that the battery is not required for compensation of the power drop during start of the vehicle. The structural problem of handling hydrogen requires a development of the technology for the storage of hydrogen and improved methods for the production of hydrogen before a broader introduction will occur. For local fleets the situation may be somewhat more favorable but in this case hydrogen will be used only within for example a city area. Since fuel cells (like gas turbines) are used only in series hybrids, the parallel systems will be replaced when fuel cells take over the market for hybrid vehicles. BMW in co-operation with Dresden Technical University has evaluated and compared the series hybrid system with the parallel hybrid system (Friedmann et al., 1998) and found that there is an advantage with parallel systems in terms of “use of energy”. In the first place they point out that there are advantages with all hybrids in that they have a potential for “exhaust free” driving in environmental sensitive areas. They also have an additional power source and the hybrid can recover energy during braking. The authors also point out the advantage of and possibility to control the internal combustion engine in a way that is not possible with the engine in conventional vehicle. In Sections 9.1 and 9.2 more detailed presentations of the work, carried out by the authors of the above-referred report, can be found.
The sensational conclusion the authors of the report came to, concerning the fuel economy, is that the series hybrid with present day technology has a poorer energy use than a vehicle with a conventional engine despite the hybrid vehicle being equipped with regenerative braking “Braking Energy Recovery” (BERG). With the parallel hybrid (with BERG) the energy use can be improved, even in the case of electric driving, up to 40 km/h when driving according to the European driving cycle and up to 30 km/h when driving according to the European urban driving cycle (EUDC). Both of the systems (series and the parallel) are advantageous in terms of emissions compared with a conventional car, but even here the parallel hybrid is in front of the series hybrid. Iwai (Iwai, 1998) is also of the opinion that the parallel hybrid is superior to the series hybrid concerning the energy use, but this is valid under certain conditions. If the internal combustion engine can be operated in the area with highest efficiency there is a potential for the series hybrid to surpass the parallel hybrid in terms of energy use. However, the problem is that this operating condition is extremely difficult to maintain in reality.
Examples of developed and demonstrated hybrid vehicles
The hybrid passenger cars presented today are in most cases smaller than their counterparts without and the greater part of them are manufactured from lighter material than the material used in ordinary commercial vehicles. Despite the fact that the parallel hybrids are equipped with a smaller internal combustion engine and a lighter battery than the series hybrids, all the extra equipment including the electric motor to be used in a hybrid vehicle contribute to the fact that the parallel hybrid vehicle will be heavier than a vehicle of the same size without hybrid system. 8.2.1
Mercedes series hybrid
In a SAE report Mercedes presents results from measurements of emissions and fuel consumption (energy use) for a prototype car with a series hybrid system (Abthoff, J.O. et al., 1998). Data is presented only for the US EPA FTP-75 cycle, i.e. the same cycle as that used in Sweden from the middle of the 1970’s up to 1997, representing urban driving (see Table 17.). It should be added that Mercedes-Benz later on presented a series of vehicles equipped with fuel cells which according to the definition used here can be regarded as series hybrid vehicles (see below). As was earlier indicated, the reduction in the fuel consumption (or used energy) is small. For the hybrid vehicle it was 8.8 l/100 km when the system was optimized for low emission levels and 8.0 l/100 km when it was optimized for low energy use, which should be compared with 9.0 l/100 km for the commercial version of the same car. The reason for the small reduction of used energy is regarded as being related to the relatively high mass, 1 758 kg, of the hybrid vehicle which is an increase from 1 350 for the vehicle without hybrid system. Despite an increase in the men value of the efficiency of the hybrid system this improvement is eaten up by the higher mass of the vehicle. However, it should be taken into account that the presented car was a not finally developed prototype and is not representative of a series hybrid with a fully developed system. In the presented vehicle only minor changes have been introduced compared to the original vehicle and therefore it is quite clear that even changes of the body in the car would have improved the above-presented results. One advantage with this hybrid system from Mercedes, which is worth pointing out, is the battery pack which has an energy density of 1 000 W/kg and a total power of 55 kW. Despite these advantageous batteries the conclusion is that the increased mass of the vehicle is considerable.
Table 17. Comparison between a series hybrid and a conventional drive system. (Source: Mercedes (Abthoff et al., 1998). Fuel consumption in FTP-75 Characteristic
1.8 l class Series hybrid opt. Series hybrid, opt. production for emissions for fuel consumpt.
Acceleration 0-100 km/h (s)
Max. speed (km/h)
Emission levels (FTP-75)
Used energy (gasoline l/100 km)
Vehicle weight (kg) Notes:
The TLEV level exists in a commercial vehicle with a 2.3 liters engine but may also be possible to meet even with a 1.8 liters engine. 2 EZEV emissions, without the additional emissions required for distribution of the fuel, which has to be calculated for by the car manufacturer according to EZEV. (”TLEV” and ”EZEV” represent emissions requirements in California).
DaimlerChrysler’s Necar fuel cell series
The first car of the Necar series – an A-class car - was presented in 1994 and the third car, Necar 3, in 1997. Today a forth generation in the series, Necar 4, is now presented (see next paragraph), which may mean that first three in the series of hybrids are replaced by the new car. It should also be said that a new company has been formed by the joining of Mercedes Benz and Chrysler in 1998 into one company named DaimlerChrysler. (see Figure 23) and according to information from DaimlerChrysler that was the first car operated with fuel cells as the main power source. Necar 3 was equipped with a reformer used to produce hydrogen from methanol on board of vehicle.
Figure 23. Mercedes hybrid car Necar 3, equipped with fuel cells. Source: DaimlerChrysler home page.
The latest version vehicle of the Necar series is, as far as known, Necar 4 equipped with a fuel cell stack. Necar 4 is equipped as to be fueled with hydrogen and to be correct Necar 4 should not be regarded as a hybrid vehicle since it has no battery and is operated with a electric motor powered with electricity direct from the fuel cell. Consequently Necar 4 is an electric vehicle 79
by definition. The top speed of the car is 90 miles per hour (245 km/h) and the operation range is 450 km between refueling. As Necar 4 uses hydrogen as a fuel, which result in zero emissions according to DaimlerChrysler. Of figure 24 it can be seen that Necar 4 rather similar in shape to its forerunner Necar 3, Figure 23. According to information in the home page of DaimlerChrysler Necar 4 will be used for the transfer of “VIPs and pilots from the airport in Munich”.
Figure 24. Mercedes hybrid car Necar 4, equipped with fuel cells. Source: DaimlerChrysler home page
A representative for DaimlerChrysler said that they have decided to improve the cars, which may be interpreted as that the company is planning for new cars in the Necar series or some new car model equipped with fuel cells. DaimlerChrysler has declared that they are firmly determined to continue the development of fuel cell vehicles and points out that they are going to commercialize a fuel cell car in 2004. Furthermore they underline that at the present time the company has spent 1.4 billion dollars on this project which is at the same level as the sum of money spent on such high sellers as Chrysler 300 M, Chrysler Concorde, Chrysler LHS and Dodge Intrepid. At a seminar in November 1999 Dr Ferdinand Panik said that the question of an infrastructure for fuel cells must be seriously discussed as soon as possible “if a competitive advantage is to be secured in the U.S. and Germany”. Panik also said that the work carried out by his company DaimlerChrysler has proved that there is a technical feasibility in the use of fuel cells. More than 60 companies around the world are working in the field of fuel cells and are intending to launch vehicles with fuel cells within the next five years. Four of the largest Japanese automobile companies will have invested more than 546 million US dollars at the end of 1999 in the development of fuel cells. 8.2.3
Toyota Prius, parallel hybrid
Another interesting example to study is the Toyota hybrid Prius. According to early information Toyota started the introduction of Prius in December 1997 and has this year (year 2000) started introduction in Europe. A few cars used for demonstration have been seen both in Europe inclusive of Sweden and in the US since 1998. It has also been announced that the
production of Prius should be 1 000 cars per month to start with (it is assumed that the number of cars will be raised after the introduction of Prius on a broader market). Contrary to the example of Mercedes series hybrid demonstration, Toyota Prius is already a hybrid car ready for the market. The total mass of Prius is 1 240 kg which may be 100-150 kg more than a commercial car of the same type and size. The difference in mass is, however, much less than for the above-presented hybrid from Mercedes. According to data published by Toyota (Toyota, 1998; Takaoka et al., 1998) the fuel consumption of Prius is 3.6 l/100 km (67 mpg) gasoline when driving the vehicle according to the Japanese 10-15 mode cycle (see Figure 25). This level of fuel consumption must be regarded as a low for a low-speed driving cycle like the Japanese driving cycle. However, when a hybrid vehicle is compared with a conventional vehicle the difference in use of energy is generally largest at low load and speed, which favors the hybrid vehicles, since the internal combustion engine commonly is shut off at low loads and at idling and this is also the case with Prius. In addition the braking energy is mainly recovered in a hybrid vehicle. When driving on roads outside of cities etc. the difference between a hybrid vehicle and a commercial vehicle is less but Toyota has not presented any data for tests according to the European driving cycle. In the case of Toyota, where the battery is charged only by the engine, the energy delivered by the battery should not be accounted for when calculating the average energy used.
Figure 25. Schematic configuration of the hybrid system from Toyota. Source: Takaoka et al., 1998.
An investigation of the Toyota Prius has been carried out and published by US EPA (Hellman et al., 1998) and in this case energy use in terms of gasoline fuel economy was 49.8 miles/gallon (approximately 4.72 l/100 km) when the EPA test cycle was reported. The speed of the two parts of the test cycle is 45 and 55 miles/h (72 and 88 km/h) respectively. In the following table a recalculation from miles per gallon to liter per 100 kilometer for some different vehicles has been carried out (see Table 18). The differences between the results for fuel economy given above and the results for Toyota Prius presented in the table are related to varying data and a result of rounding off. As can be seen in Table 18 the used energy is at a considerable lower level for Toyota Prius than for Toyota Corolla. However, it should be noted that Toyota Prius is a rather small vehicle (a sub compact) compare with Toyota Corolla and it is of the same size class as Suzuki Metro but heavier than Suzuki which used energy at the lowest level of all of the vehicles in the table. The energy used for the diesel-fueled vehicles (Passat and Jetta) is also at a slightly lower level than for the Toyota Prius despite that they are both larger and heavier. The fuel economy* of 67.4 miles/gallon (28 km/l or 3.57 l/100 km) which is reported by *
In the USA the expression fuel economy commonly is used while in Europe the expression fuel consumption always is used. Here both these expressions are used in order to satisfy the presumptive readers.
Toyota is much lower than the figure for fuel consumption shown in Table 18. It is probable that this figure for fuel consumption been understood in Europe as the fuel consumption when measured according to the European test cycle and in the US as the fuel consumption measured according to the EPA FTP-75 test cycle. Even if the figure for gasoline fuel used per driving distance is remarkable good it is not as good as is stated in newspapers and magazines. An important aspect to take notice of when comparing different vehicles is still that the Toyota Prius hybrid system presents the same comfort as an automatic gearbox and therefore the comparisons should be carried out with conventional vehicles equipped with an automatic gearbox. Such a comparison would result in a more favorable position for Toyota Prius. Table 18. Fuel consumption (as gasoline) for Toyota Prius and some other vehicles with low fuel consumption. Gear box Inertia Fuel Fuel consumption weight, [l/100 km] [MJ/100 km] [kg] Suzuki Metro Sub-compact 964 Gasoline 4.32 ca 141 M5 VW New Beetle Sub-compact 1 418 Diesel oil 4.56 ca 163 M5 VW Passat Mid-size 1 531 Diesel oil 4.58 ca 163 M5 VW Jetta Compact 1 418 Diesel oil 4.62 ca 165 M5 Toyota Corolla Compact 1 247 Gasoline 6.44 ca 210 M5 Toyota Prius Sub-compact 1 361 Gasoline 4.84 156 AT (THS)a Notes: Energy content: gasoline=44 MJ7kg, diesel=43 MJ/kg. Density: gasoline=0.74 kg/l, diesel=0.83 kg/l. a Automatic gearbox does not exist in reality but the function in the hybrid system (THS, Toyota Hybrid System) can be compared with an automatic gearbox. Vehicle
Size of vehicle
Ford’s parallel hybrid vehicles
The new hybrid systems presented by the American car manufacturer Ford are the result of, among others things, computer simulations and work with models by which Ford identified two parallel hybrid systems. These are expected to meet the requirements for good driveability and good fuel economy at the same time as good emission performance is achieved. Ford has named one of the systems ”Post-Transmission Hybrid” (PTH) where the electric motor is direct connected to the driving wheels after the gearbox in the driveline. The ”PTH” hybrid vehicle is equipped with a 1.8 liter, 4-cylinder gasoline fueled engine and a 5speed transmission. . In the other hybrid system denoted ”Low Storage Requirement” (LSR) a small electric motor is used in combination with a nickel-metal-hybrid battery (which can be seen as a type of auxiliary unit) for storage of energy and also in order to support the internal combustion engine when more power is needed. In order to avoid a misunderstanding it should be pointed out that there are two alternative hybrid systems of which one, the LSR, is equipped with a CIDI diesel engine (see Section 6.2) denoted DIATA (”Direct Injected Aluminium Through Bolt Assembly”). The LSR hybrid system (see Figure 26) is mounted on a platform (a Contour/Mystique/Mondeo chassis), which is manufactured from low weight material. The CIDI diesel engine for the LSR has a displacement of 1.2 liters and a max. power of 55 kW at 4 500 rpm. The car is denoted P 2000 and it is equipped with comprehensive electronic control system. The total weight of the P 2000 is 2 000 lbs. (908 kg) which is 40% lower weight than Ford Taurus’ 1997 year’s model of 3318 lbs. (1506 kg). The comparison is valid for P 2000 without hybrid system, which in its present version will add approximately 990 lbs. (450 kg), but the goal for a well-adapted hybrid system is an additional weight of a little more than 600 lbs. (275 kg).
According to Ford (Buschhaus et al., 1998) energy use in terms of diesel oil of 3.5 l/100 km (68 miles/gallon fuel economy) has been achieved for the LSR vehicle with the CIDI diesel engine DIATA. Energy use of 3.4 l/100km (69 miles/gallon fuel economy) has been achieved for the LSR vehicle, when it is equipped with the same engine. It is noted in the report that both vehicles meet the Tier II emission standards. So far no information, in addition to that from Ford, has been available for this report and therefore all information used here is that from them. People at Ford dealing with research, investigations and development call attention to the fact that there are three predominant characteristics for hybrid vehicles. These are 1) that the internal combustion engine is shut off when the vehicle is stopped and that it is easy to start again; 2) the size of the engine is reduced and 3) that regenerative braking is included in the system. They also point out that there are other functions of importance, for example that the battery can be recharged faster than the existing 400 V belt driven inverter manages to do under operation. Furthermore the electric motor (”Starter/alternator”) can assist the internal combustion engine during accelerations (see also Figure 26).
Figure 26. Schematic picture of Ford’s hybrid car(”LSR”). Source: Automotive Engineering International/February 1999). Reference: (Buchholz, 1999)
Ford’s “LSR” hybrid vehicle has been equipped with a special ”Starter Alternator” which is not a new invention. It has not however been presented as an individual unit in for example Toyota’s and Nissan’s hybrid systems (see Nissan’s hybrid system below). In 1993 Ford received a contract from US Department of Energy (DOE), which stated that Ford had agreed to develop a hybrid system. It also stated that Ford should investigate and develop a synergistic combination of the technology for combustion and the technology for power drive. This was in order to increase the internal combustion engine technology, at the same time as the performance and the comfort of the vehicle had to meet the demands from the car owner. The plan for development of the above presented hybrid system, “PTH”, which meant that the vehicle should be equipped with an CIDI diesel engine, did not satisfy the requirements stated by DOE. Therefore in 1997 it was agreed between DOE and Ford that Ford should not use the DIATA engine in its PTH hybrid vehicle. Instead it was decided that a Mondeo based car, 83
which was already under development, should be used in the hybrid system to be delivered to DOE at the end of the time for the original contract. The hybrid system to be used in the car is schematically shown in Figure 27 and contains a PTH drive and an otto engine. As far as has been understood of the report (Buschhaus et al., 1998) the actual version of PTH in a Mondeo test bed (Figure 27) is equipped so that the battery can be charged from the mains. The construction of the “LSR” hybrid system in a P 2000 test bed (see Figure 26) is, however, such that alternative charging from the mains is not of any great use. Therefore this ability does not exist, even if this fact cannot clearly be seen in the figure or by the presentation of the system. The car cannot be run on only the battery since only a small battery is used.
Figure 27. Ford’s hybrid system PTH with gasoline engine. Source: Buschhaus et al, 1998. 8.2.5
Nissan’s parallel hybrid
Some of the fundamental parts of Nissan’s hybrid system are as follows. Internal combustion engine integrated with a CVT (”Continuously Variable Transmission”). Equipped with a lithium-ion battery. Fuel economy: + 100 % compared with conventional car. Acceleration: Equivalent to conventional car model. Planned introduction in Japan early year 2000. The basic construction of Nissan’s parallel hybrid vehicle is that the system (see Figure 28) is composed of a highly efficient otto type engine, a generator mounted on the front of the engine, a belt-drive continuously variable transmission (CVT), an electric motor and a lithium ion battery (Kitada et al., 1998). The system also includes a clutch with the basic function to transmit engine torque during the engine-powered mode and to isolate engine friction during the motor-powered mode (including regenerative braking). The use of a CVT has resulted in the electric motor and the internal combustion engine being able to be operated in areas with the highest efficiency (see Figure 18 mussel diagram). When driven the hybrid system is controlled according to the following six points: 84
(1) The internal combustion engine is shut off when the car is stopped; a creep torque is generated by the electric motor (traction motor) with the clutch activated. (2) Since the car is initially run by the traction motor a gear ratio is chosen (automatically) which allows the traction motor to operate with a high torque. When the speed of the car increases the generator is activated and it starts the internal combustion engine and activates the clutch. In cases with fast start the engine is started and the clutch is smoothly engaged, which result in the car being accelerated by both the motor and the engine in the same manner as during the cruising speed. (3) During cruising speed a gear ratio is chosen which allow the internal combustion engine to operate in its most efficient area. During this mode the traction motor/generator (one feature of the traction motor is that it can also operate as a generator) charges the batteries after the discharge during the startphase of the car. Nissan points out that the use of energy (expressed as fuel consumption) is reduced since the engine can be efficiently operated. Another reason is that operation at a smaller gear ratio is possible because of the large supplementary torque that can be expected from the traction motor. (4) During acceleration the traction motor assists the internal combustion engine and the batteries supply the energy needed. (5) During decelerations the function of the traction motor is to be a generator for charging the batteries. During this sequence the clutch is engaged in order to recover maximum energy to the batteries. (6) When moving the vehicle backward (reversing) the traction motor operates in reverse. Normally the batteries provide the traction motor with energy, but if the batteries are not fully charged the engine starts in order to charge the batteries.
Figure 28. Scematic configuration of the hybrid system from Nissan. Kitada et al., 1998. The reason for this detailed description of hybrid system from Nissan is that, in many details, it is different to the systems from for example Toyota and Ford. Probably there are no remarkable differences between the Nissan system and the other two systems but in the hybrid from Nissan the traction motor propels the car during the start phase and also when reversing the car which may decrease both the emissions and the fuel consumption. Whether the hybrid systems from Toyota and Ford have these two functions is not clear. In the hybrid system from Ford the diesel engine is not shut off during low loads, which is the case in the Nissan
system. The reason for not shutting off the engine in the Ford system may be that the fuel consumption is very low when a diesel engine is idling. With the control of the hybrid system used by Nissan it is clear that the fuel consumption (use of energy) is especially effected at low loads and idling. Unfortunately no data for either the fuel consumption or the emissions was available for this report. However in a report from Nissan (Kitada et al., 1998) it is said that the fuel economy is increased by 100%. This means that the fuel consumption is reduced by 50% when driving according to the Japanese 10-15 mode cycle while it is unchanged during accelerations from 0-100 km/h and at top speed, all compared with a conventional car. One problem concerning such information is that there is no information about whether the conventional car was a low or a high consumer. 8.2.6
Some other light hybrid vehicles
The focus on the use of hybrid vehicles as a possibility to reduce the use of energy (or fuel consumption) and emissions has attracted many car manufacturer, consulting companies working in the area of motor vehicles and institutions connected to universities. They have studied, analyzed and, in relevant cases, developed hybrid systems and vehicles with hybrid systems. At a conference ”EnV’99, Alternative Fuels and Advanced Technology Vehicles”, which dealt with alternative fuel technology and alternative vehicle technology, the presentations about hybrid systems were of special interest. In addition to the hybrid systems presented here it was shown that, among others, Honda and Southwest Research Institute (SwRI) have studied and developed hybrid systems. Representatives for both of these have presented plans/ideas for development and presumably also prototype hybrids. Honda announced that the company was planning an introduction of hybrid systems for the Japanese market. Honda: At the time for the preparation of this report no report was available which described in any detail the hybrid system from Honda. However, in the home page from Honda there was some brief information about a hybrid system named “Insight” which was under development. From this information it can be seen that the body of the car will be constructed from aluminum and that a 3-cylinder 1.0 liter internal combustion (gasoline fueled) lean burn engine (compression ratio 10.8:1 and 67 hp/5 700 rpm), will be used. The electric motor was an “ultra-thin” and carefully designed ”for outstanding performance and efficiency” motor – probably having that shape in order to fit the space at one end of the engine where it was going to be mounted. According to the plan the system would not be designed to permit charging the battery from the mains. The fuel economy was expected to be 61 miles/gallon (3.9 l/100 km) for city driving and 70 miles/gallon for highway driving. No fixed time for introduction was given and the intention of Honda may be, according to the given information, to be like Toyota and Nissan, in that the introduction be limited to the Japanese market. However the information says that: “The Honda Insight is the first gasoline- hybrid electric vehicle to be sold in the United States”. And further “Plus, it meets California’s stringent Ultra-Low Emission Vehicle (ULEV) standard, making it one of the world’s cleanest, most fuel-efficient gasoline-powered automobiles”. Southwest Research Institute (SwRI): The engineers at SwRI have developed ”A Parallel Hybrid Train” (SwRI, 1999). Since they not have mounted their system in a vehicle they have instead constructed a test bed and equipment for testing the hybrid system. This testing has been carried out in order to verify that the results meet what was expected during the planning of the system. The investigations carried out are, to a great extent, computer simulations but there have also been tests of different components of the system. The computer simulations of fuel consumption and emissions have been based on that the hybrid 86
system was mounted in a Ford Taurus – a rather large vehicle for Europe (1360 kg) and especially when compared with other hybrid vehicles. This has resulted in fuel consumption not being at the same level as for Toyota Prius – one of the hybrid vehicles which has met most interest to date. One features of special interest, despite not being new – is the ”Continuously Variable Transmission” (CVT) developed by Van Doorne in Holland. By the use of a CVT the hybrid system can be driven in four different modes (”Powertrain Power Flow”): Charging mode: The internal combustion engine (otto) is operated at full load at its most efficient area. The excess power (the power not required to propel the vehicle) is transferred by the planetary gear system to the electric motor in order to charge the battery. “Assist” mode: In this mode both the engine and the electric motor are used for traction. This combination delivers the maximum power to the drive wheels. It is the least fuel-efficient mode since the electric motor cannot deliver any excess power and the engine is not necessarily operated in its most efficient area. Electric mode: The electric motor supplies power to the drive wheels, exploiting its superiority to the otto engine. Regeneration mode: The deceleration (and braking) energy is recovered (i.e. delivered to the battery) via the electric motor. According to a representative from SwRI the function of the CVT is such that the internal combustion engine can be operated in its most efficient area and also that this parallel hybrid system can be controlled in a similar way as the engine for a series hybrid. Since data is only available from computer simulations in a laboratory there are some difficulties in estimating the performance of the system concerning energy efficiency and emission performance. According to the above summary different hybrid vehicles and hybrid systems for light duty vehicles are being developed and constructed and most of this work is being carried out in Japan and the US. Similar development work does not seem to be occurring in Europe, or at least to a much less extent. One exception presented above is Mercedes Benz or its successor DaimlerChrysler. According to the latest information found in their home page it seems like that DaimlerChrysler is most interested in fuel cells and their more recently presented passenger car is not a hybrid vehicle but is electric vehicle powered with a fuel cell. For the rest of Europe there is some activity in the area of hybrid light duty vehicles, at least at some of the car manufacturers’ plants, for example BWW (Friedmann et al., 1998), PSA (Peugeot/Citroen) (Beretta, 1998; Badin et al., 1998) and Audi (Hanauer, 1998). According to newly received information Renault will introduce a hybrid vehicle in year 2001.
Some examples of heavy-duty hybrid vehicles
Development of hybrid systems for heavy-duty vehicles has been in progress for many years but has been especially focused on buses. Most of the vehicles involved in this activity are still under development, which has recently included both heavy-duty trucks and light-duty trucks. The difference between buses and especially light-duty trucks is that some of the latter are parallel hybrids while all the buses so far known are series hybrids. The development of hybrid systems for trucks is still in an early phase and therefore it is not possible to estimate the future for the development of hybrid systems for this category of vehicles. Since the different hybrid systems have been described in detailed above only an overview of the different systems activities and vehicles will be presented in this section. There are many vehicles under development, or such vehicles that have been involved in the development of 87
hybrid systems. Most of these vehicles are different to each other. As far as is known only a few car manufacturers, but many bus operating companies, have been involved in the work with buses and there the hybrid systems are more or less tailored according to specifications provided by local bus authorities. In addition there are only a few cases where this activity has been found as presented in a proper report. The hybrid systems for buses are almost without exception series hybrids. Only one case of parallel hybrid system for a bus has been found and that bus was equipped with a lead acid battery, which seems to be a common battery used in buses presented in this section. Since no firm information was available in many cases, there is an uncertainty in the statement concerning the batteries. However, according to reports and other information at least one bus of these presented in Table 19 is equipped with a nickel-cadmium battery and a few buses are equipped with a high efficiency ultra-capacitor/battery. According to William West, Southern California Edison, new types of battery have been developed but they are estimated to be too expensive for the users. Now the situation is that many buses are equipped with a stack of batteries weighing as much as 4600 pounds (ca 2088 kg). The weight of another bus was 50 000 pounds (ca 22 700 kg) – a common weight of a bus without passengers is 26 400-28 600 pounds (ca 12 000-13 000 kg). However, it should be noted, that in these cases, as in many others, these are prototypes or the first generation of this kind of buses. In the case of the very heavy bus the company responsible for the conversion has promised that the weight of the bus could be close to 30 000 pounds (ca 13 620 kg) when further developed which seems to be a more appropriate weight. According to Table 19 different fuels, different APUs and some different units for energy storage (mostly lead acid batteries) are used (see Table 19). Table 19. Some particulars for a number of hybrid buses and some trucks with hybrid systems. Operator Deliverer of Fuel Int. combust. eng. Battery Comments hybrid system or other unit pack
Buses Bus company in Volvo/ABB Gothenburg, Swed. Not declared Mercedes-Benz Chicago Transit Authority Not declared
Gas turbine + battery 190 kW* Hydrogen Fuel cell 190 kW for drive etc. Hydrogen Fuel cell 275 hk ca 200 kW Diesel oil Diesel engine
Ballard for Fuel cells TNO Holland consult inst. ALSTOM/ Renault, France Not declared
CNG is proposed Diesel oil
Different bus companies Teansp. Energy Syst. Australia
SwRI in cooperate. with GM Not declared
2.2 lit. VW- engine (otto engine) 1. Otto engine 2.2 lit. 2. Otto engine 5 lit.
Diesel engine Otto engine /CNG Diesel engine
Lead acid battery Non
Volvo co-operate with ABB Tanks with 45 000 l hydrogen Non? Large progr. for fuel cells accord. to B High power CRT-filter battery DeNOx catalyst The mains Trolley bus
Not declared Not declared Lead acid battery Not declared Not declared
Passengers: 10 seats 30 standing Altra has tested one bus*** Problem with the engine control Emissions measurements*** +70 lit. pressure tank + flywheel etc.
Emiss. (City operation in Gothenb.): NOx=1,0, HC=0.1, CO=1,5 and PM=0.05, all in g/kWh. (Malmquist et al., 1998). Atesina, which operates electric mini buses in the center of Trento intents to introduce hybrid mini buses and has analyzed the costs etc. for operation with series hybrids. *** Emission measurements have been carried out and these have been discussed below. **
Alameda/Contra Costa Transit Augusta-Rich. County Transit Chattanooga Area RTA Cleveland RTA Mfl (Consort.) DUETS Indianapolis Internat. Airport MTA Los Angeles Different bus companies New York MTA Oahu Transit Authorities Orange County Transp. Autorit. Pittsburg Intern. Airport Vandenburg Air Force Base/APS Different bus companies
APS System USDOE particip. in the project Advanced Vehicle System NASA Lewis RTA A project with many participants General Motors
Rotary (Wankel-) engine, generator Hydrogen Standard otto engine (140 m3) CNG, Capstone turbine gasoline Natural Gas turbine gas Not Not declared declared CNG Probably otto engine
Advanced Tech. Transit Bus B la New Hampshire Techn. Inst. Orion Bus Industries El Dorado
CNG The sun
Diff. engines and alternative (fuel-cell) Sun cell
Diesel engine Parallel hybrid Not declared
New Flyer Industr. Ltd. El Dorado
Not declared Propane
Toyota Motor Corp Gasoline
Different bus companies Stockholm Bus
AMC and Korea National Univ. Lund tekn. Univ. Industry Electron. Malmö, Uppsala Lund tekn. Univ. Bus comp. Sweden Industry Electron.
Probably otto engine Probably gas turbine Probably otto engine
Saab otto engine
Cummins gas engine (otto principle)
Otto engine t
Various service companies
Service vehicle in Japan ASAB AB, AB Svelast, AB (Swed.) ASG AB, Västberga Deliver TGM in Gothenburg Sweden
Mitsubishi Motors Inc. Mercedes Benz
LPG, gasoline Diesel oil
Volvo Truck Comp.
Nickelcadmium Not declared Lead acid battery Ultra capacitor Not declared Not declared Ultra capacitor Not declared Not declared Lead batt. ”Plug in” Not declared Lead acid battery Not declared Not declared
Two electric motors Hydrogen is stored in metal hydride Larger turbines are under construction 900 000 dollars project 1.3 million dollars project Two buses exist - a third is ordered 51 million dollars 6 prototype busses Co-operation with many car manufact. A very heavy bus will be lighter Also an alternat. bus with NiCd-batt. One order has been written Problem with the bus Many are engaged in the project One smaller bus
Not declared Nickel-metal hydride Nickel-metal hydride
The project started in 1992 See KFB: Dnr 1997-0344 See KFB: Dnr 1997-0344
Lead acid battery Lead/gel battery Lead acid battery Nickelcadmium
Light truck, fuel cons. (see below) 6 trucks type Atego 1217 Truck type Vario 814 D 2 trucks type FL 615
Many of the above hybrid vehicle projects have an interesting profile and are therefore worth following up in order to generate more knowledge and experiences of hybrid operations with heavy-duty vehicles. For at least two reasons it is not possible to describe the project in this report. First of all there is a lack of required information and it seems not to be possible to collect this information. Secondly many of the project have been estimated to be in a phase when they still are either under development of the hybrid systems or under evaluation by operation in real traffic situations. Within a few years there may be interesting and valuably data and experiences available.
According to the collected information, those who operate the vehicles seem, in some cases, to have met rather intricate problems, which is no surprise since the hybrid technology for motor vehicles is rather new. The intention of using one or more hybrid vehicles in regular traffic is an important phase during the development. This will give the operator of the vehicle such information which is not possible to obtain during tests in a laboratory, even if the basic information about the system has been generated by such tests. Based on all information which has been collected for the preparation of this report the estimation is that the technical problems so far met are not such that they cannot be solved. Unfortunately there seem to be other problems (such as the energy efficiency) which may be a more serious barrier. These, together with costs, will be discussed in Section 13. Of the above-presented projects there are some which are of special interest to discuss in somewhat more detail, despite the fact that they may not be successful in reality or can already be used as example of failures. The projects to be discussed are as follows; hybrid busses developed by Volvo (Malmquist, et al., 1998); a truck developed by Volvo (KFB Rapport 2000:8); a bus from Mercedes-Benz (Merdedes-Benz home page, 1999); and a hybrid system from TNO Road-Vehicles Research Institute (Mourad och Weijer); two trucks from Mitsubishi (Hori et al., 1998); a bus from Allison Transmission (division within GM) in co-operation with Southwest Research Institute (SwRI) (Bass et al., 1999).
Figure 29. Nova Transit Bus. Source: Whartman, 1998. Volvo’s hybrid bus is, as are many other hybrid buses, equipped with a series hybrid (Malmquist et al., 1998). Unlike other buses this bus uses a gas turbine for generating the electricity for the battery. The use of a gas turbine can be seen as an example of the continuing interest at Volvo in the use of gas turbines since Volvo has already had some experience in the use of a gas turbine in a passenger car. Within the concern extensive experience exists since Volvo Aero Turbins is the manufacturer of jet turbines for aeroplanes. Consequently, the gas turbine for the bus was developed by Volvo Aero Turbins. The hybrid system for the bus was developed by ABB Hybrid Systems AB and the project was sponsored by KFB and NUTEK (Swedish National Board for Industrial and Technical Development). As one contribution from Volvo to KFB it was agreed that ethanol should be used for the gas turbine.
The aim of the project was to; (1) gain some experiences from a hybrid system by field tests (2) test new control algorithms (3) evaluate a different combination of the dynamic for gas turbine and battery The max power for the gas turbine is 110 kW and 130 kW for the asynchronous motor which propels the bus. The max energy content in the battery is 57 kWh and the maximum traction power of the system is 190 kW. According to reported information one of the purposes of the project was to operate the bus on a regular bus line in Gothenburg. According to non-official information, Volvo, for some reason was not going to continue after this bus was developed and tested. Hybrid electric truck from Volvo Volvo Truck Company has, with financial support from KFB, developed two hybrid electric trucks. After the development the trucks were rented to TGM in Gothenburg (Fast, 2000). The aim of the project was to construct the trucks for demonstration and to use them commercially. The trucks are of series hybrid type and are, in their construction, based on the truck model FL6-15 (see Figure 30). The original drive train of this truck is kept for the clutch and the gearbox. In the drive train a generator and an electric motor is installed for the traction of the vehicle via the ordinary rear axle. The vehicle is equipped with a diesel engine of type D6-200hk (EURO2) and a diesel filter of type CRT is installed in the exhaust system. The power of the generator is 110 kW and 130 kW of the electric motor (and a brief duration of 370 kW). A nickel-cadmium (200 Ah, 43 kWh and 216 V) is used for storage of energy produced by the engine via the generator. The capacity of the battery is sufficient for two trips per day in the electric mode. The loading capacity of the truck is 4 600 kg (approximately 5 700 lbs.) and a top speed of 90 km/h.
Figure 30. Volvo hybrid delivery truck. .
Mercedes-Benz (DaimlerChrysler) has developed a fuel cell bus named Nebus, which can be seen as an electric bus, if the information is correctly understood, since the electric motors are directly powered with electricity from the fuel cells. The fuel cell unit consists of ten fuel cell stacks of 25 kW each which result in power of totally 250 kW. Since the fuel cells 91
themselves need some power to be operated the total power output is 190 kW to the wheels and to the auxiliaries such as the on board electrical and air condition system (this means that the energy losses for the operation of the fuel cell is 60 kW and that efficiency of the fuel cells themselves is 76%). According to released information from Mercedes-Benz (Mercedes-Benz, 1999; Dircks, 1998) the fuel cells are fueled with hydrogen and oxygen and that the only emissions from the fuel cells is water vapor with a temperature of 55 °C. In the information available for the report there is no indication about the number of fuel cells in each stack, but it is estimated that the number of individual fuel cells in each stack 35 to 40 resulting in the total number of fuel cells being 350 to 400. The power electronics includes an AC-converter and a pulsewidth modulated inverter of which both are installed on the roof of the bus as control units for the wheel hub electric motors. As already indicated the fuel cells provide direct power to the two electric motors which have a capacity of 75 kW each, i.e. total 150 kW. The space for passengers can be increased since no internal combustion engine is needed and no axles, cardan shafts, alternators and fuel tank either. The fuel for the bus – 4 500-liter hydrogen is stored in seven 150 liters tanks on the roof of the bus at a pressure of 300 bars. In the above configuration the bus has an operating range of 250 km between refills. Since there is no internal combustion engine in the bus it is very quiet and according to Mercedes-Benz this is an important feature since the number of vehicles around the world is estimated to double by the year 2030, which requires fuelefficient vehicles which are quiet and low polluting. As a successor to Nebus another bus named Citaro was presented in April this year. The following information is quoted from the homepage of DaimlerChrysler: “The Citaro’s fuel cell unit delivers more than 250 kilowatts of power. It was developed and manufactured by the DaimlerChrysler subsidiary Xcellsis. The gas pressure bottles containing compressed hydrogen are mounted on the roof of the bus. The environmentally friendly bus can travel up to 300 kilometers at a top speed of 80 km/h and carry around 70 passengers. The electric motor, transmission, drive shaft and mechanical rear axle are all located at the rear of the bus. This ensures smooth low-floor design and easy access during maintenance. The bus also includes three doors for optimal passenger flow”. “EvoBus GmbH, a wholly-owned subsidiary of DaimlerChrysler, will supply the MercedesBenz Citaro low-floor urban buses with fuel cells at a price of 1.25 million euros each. The price includes comprehensive technical consulting and on-the-spot maintenance by EvoBus for a period of two years. While the infrastructure is being set up, DaimlerChrysler will provide the transport operators with guidance, knowledge and expertise”. “DaimlerChrysler is the first automaker worldwide to offer fuel cell vehicles on the market. The company plans to build some 20 to 30 urban buses with fuel cell drives during the next three years, and then offer them for sale to transport operating companies in Europe and abroad”. TNO-Road-Vehicles Research Institute has developed an APU (”Auxiliary Power Unit”) which in this case include a diesel engine and a generator (see Figure 31).
Figure 31. Power unit (APU) for hybrid vehicle. The reason for the presentation of this case is that the development of especially the internal combustion engine – a diesel engine – represents an example of how a diesel engine can be developed and adapted to be used in a hybrid system for heavy-duty vehicles. The clue in this case is a message saying that it, with regard to local or other conditions, must be decided from one case to another whether the actual engine is the best choice for the hybrid system to be used. If the local condition has created serious problem by exhaust emissions and noise such an APU must be used which will meet the special environmental requirements. In a case where the efficiency of the hybrid system is the most important parameter the most energyefficient APU can be used. The following is quoted from a report from TNO (Mourad och van de Weijer, 1998). ”The determination of the general specifications of the APU is based on a Program of Demands of the vehicle, e.g. performance requirements. Parallel to this the specific hybrid layout is taken into account as well as the preferred Energy Management Strategy (e.g. charge-sustaining). The resulting specifications include required power, efficiency, emission level, and acoustic behavior. The translation from Program of Demands into a Technical Specification is done using a simulation program. This simulation program also enables evaluation of the consequences of the design choices made. For instance, although the engine can be downsized when compared to a conventional vehicle (with all its efficiency and packaging advantages), the continuous maximum speed is somewhat reduced. The intermittent performance, however, may be equal or even better than that of a conventionally powered vehicle”. The hybrid technology offers a potential of clean vehicles but also energy efficient vehicles but with the present state of technology the requirement of good emission performance in terms of low noise and minimal exhaust emissions of a hybrid vehicle is somewhat contradictory to the requirement of an optimum energy economy. The different fuels such as gasoline, diesel, LPG, or CNG require different engine specifications and in this respect also a decision of what the demands for the vehicle are. In order keep to an alternative with high efficiency TNO chose the alternative diesel engine since this engine offers a higher efficiency than a otto engine despite the emissions of NOx and particles being worse from diesel engine than its counterpart (Mourad och van de Weijer, 1998). The diesel engine in TNO’s hybrid system is especially adapted (dedicated) for a hybrid system and equipped with both catalysts and a special particle filter (”CRT particulate trap”), which in connection to the hybrid system is schematically shown in Figure 32.
Figure 32. Control systems, internal combustion engine and the emission control system. Source: Monrad och van der Weijer, 1998. The hybrid system constructed by TNO, which as far as known not was realized in practice at the time of the presentation of their report, contains a control unit for the torque of the generator aiming at a reduction of rapid engine transients. The representatives for TNO point out that a generator-controlled torque is a special feature. The system also contains a battery not shown in the figure. Mitsubishi Motor Corporation (MMC) has developed one hybrid light-duty truck to be used as a delivery truck (presented 1995) and another truck equipped with special work equipment presented (see Figure 33) in a report (Horii et al., 1998). MMC’s goal for the development of the truck equipped with a hybrid system was, to quote; (1) “To provide dynamic performance equivalent to that of the basic diesel vehicle so that the work vehicle can keep up with the urban traffic flow, when traveling. (2) To assure a driving range of 200 km or more to allow for operation of the vehicle not only within the city but to also permit a round trip to a suburban area, (3) To introduce improvements for lighter weight and higher efficiency in order to reduce fuel consumption and emission, (4) To simplify operational controls for easier driving by taking advantage of motor drive, (5) To enable the vehicle to operate on battery power only in residential areas at night, (6) To provide the capability to drive the hydraulic pump from the electric power accumulated in the battery without having to use engine generated power so that emission can be eliminated and noise minimized to permit operation indoors, in tunnels, etc. as well as in residential areas”. After having studied and investigated different alternative and in the first place parallel hybrid systems contra series hybrid systems they decided to choose the latter type of system and this was for two main reasons: 1. Since the internal combustion engine is used only for the purpose of generating electric energy for the battery it can be operated in power load area where the efficiency is high. MCC believes that the total efficiency and the emission performance of a series hybrid is better than a parallel hybrid during city driving, which mostly consists of accelerations, decelerations and low speed modes. 2. Since the internal combustion engine is not directly connected to the driving wheels the series hybrid system has a simpler and freer drive train and will offers better possibilities for the mounting of hydraulic equipment etc. 94
The hybrid system developed and constructed by Mitsubishi does not differ very much, from what can be seen, from the “usual” system. Seeing that the hybrid vehicle is to be used as a working machine (see Figure 33) it has been fitted with a piece of equipment named ”Transmission with PTO”. This equipment serves the hydraulic pump which has a transmission ratio of 3.476.
Figure 33. Mitsubishi aerial working truck. Source: (Horii et al., 1998). The delivery truck presented by Mitsubishi in 1995 is fueled with an LPG fuelled otto engines and based on a truck called canter one of the mass production trucks. MMC has in detail studied the fuel consumption for a hybrid vehicle with the fuel consumption for a diesel fuel truck of the same type and the results of this study are presented in Section 9. MMC also points out that the emissions of NOx and black smoke have been considerable reduced from this vehicle when using the hybrid system. The intention of Mitsubishi to manufacture and sell their hybrid trucks can be seen from the fact that two hybrid trucks were presented in Beijing according to the following quoted information. “….this second Beijing PSE of the year focused on Electric Vehicles & Substitutional Fuel Vehicle Technology. Long engaged in developing alternative fuel vehicles, Mitsubishi Motors brought two hybrid electric vehicles (HEVs) to the event, the Mitsubishi Space Wagon HEV and the Mitsubishi Canter HEV. The Space Wagon HEV is equipped with an ultra-low emission CNG (Compressed Natural Gas) generator engine, high energy density lithium-ion batteries, and high-efficiency twin motors. In place of a diesel engine, the Canter HEV’s generator employs an LPG engine backed by organic electrolyte batteries”. In addition to the reports above, about the different hybrid heavy-duty vehicles presented in Table 19, the following reports (Jeanneret, et al., 1998), (Debal et al., 1999), (Dowell och Reddy, 1998), (Bullock och Hollis, 1998) have been used as a part of the information needed for this report – the authors and the title of all of the reports can be found in the reference list below.
Comparison between different fuels/drive trains
Several factors influence the choice of the combination fuel-power units for the hybrid system. There are especially two factors which are most important - the emissions and the energy efficiency of the system. The choice of the combination type of fuel contra type of 95
power unit for a hybrid system is also influenced by international agreements between different countries, or agreements within the different countries. This is especially true within areas or countries where the main production of automobiles occurs such as in the US, Japan and Europe. During recent years the question of fuel economy has grown very strong and in some cases it seems to be more important reducing fuel consumption than reducing the emissions of NOx, particles and other emission components in the exhaust. The many actors being more or less involved in the decision taking or in discussions about the importance of reducing the fuel consumption in vehicles contra reducing emissions and costs may contribute to an uncertainty about which way to go. When estimating the future development of hybrids the above factors have been taken notice of in addition to the technical possibilities for the two main hybrid systems to meet the two main requirements emissions performance and energy economy. If the development of the hybrid system is to be based only on the requirement of fuel (or energy) economy the fuel/power unit combination should certainly be based on the combination diesel oil/diesel engine today. It is true that the diesel engine is somewhat more expensive to produce and also heavier than the otto engine, but the difference in purchase price for the car manufacturer and the customer will soon be “paid back” by the higher efficiency of the diesel engine unless some unexpected requirement considerably increases the cost of using the combination diesel oil/diesel engine. This is especially valid for heavy-duty vehicles. It is true that there exist some alternative to the diesel engine (see Sections 7 and 8) but the problem is that “the state of the art” of these alternatives is so far not on the same level as the diesel engine and does not have the same market as the diesel engine. Therefore the only real candidate for meeting the fuel economy requirement (or use of energy) seems to be a diesel engine fueled with diesel oil. If the development of the hybrid system should be based only on the requirement of good emission performance the answer is not quite clear especially in an international perspective. From a study of the situation in Japan and in the US as a neutral observer, or from listening to the discussion in Europe including Sweden, it is quite clear that there are different opinions about the necessity of using a certain fuel engine combination for environmental reasons. In the USA the car manufacturer Ford presented in 1997 or 1998 a newly developed diesel engine for a hybrid vehicle meeting the Tier II requirements according to presented information – an emission level which, some years ago, not many experts in the field thought it was possible to reach. A study of the presented future emission standards also calls for rather remarkable emission reductions. Ford, who is one of the “big three” who has signed an agreement with the Government in the USA to do research and development within the PNGV program (see Section 8.1.3) has also signed an agreement with US Department of Energy (DOE) for the development of a hybrid vehicle. From a report (Buschhaus et al., 1998) presented by Ford (see Section 8.2.4) one conclusion could be that DOE was not satisfied with the presented plan to use a diesel engine in the new hybrid vehicle developed by Ford, since, at the time for the presentation of the above mentioned hybrid vehicle, it was agreed that Ford should develop a hybrid vehicle equipped with an otto engine. It is well known that the environmental authorities in the USA and especially in California have some doubt whether the combination diesel fuel/diesel engine of environmental reasons should be allowed to be used in at least light-duty vehicles. Similar reactions have also been heard in Sweden and up to some years ago only a small number of light-duty vehicles were diesel fueled vehicles. The opposition against the diesel oil/diesel engine is based on fact that a mass produced diesel engine has far from reached the same low emission levels as a mass produced otto engine. If
looking at the emission standards in Europe for example it can be seen that there is a clear difference between otto and diesel vehicles – vehicles with otto engines being at the lower level. The emissions from diesel engines are still expected to cause a higher health risk than the emissions from otto engines. In Section 8.1.4 a description of emission control systems for both diesel engines and otto engines can be found also for hybrid vehicles. However, the discussion in the previous paragraph about the diesel engine and the opposition within the environmental authorities may have had a decisive impact on the choice of the fuel/engine combination also for hybrid vehicles. The opinion expressed in many countries concerning the diesel engine have certainly resulted in that many companies have used the combination gasoline, ethanol or CNG/otto engine instead of diesel oil/diesel engine in their hybrid system. In cases when the alternative internal combustion engine is an otto engine there are at least two reasons for using gasoline as fuel. First of all an infrastructure has long existed for the distribution of gasoline and secondly the technology for the combination gasoline/otto engines can easily be copied when producing smaller engines for hybrid vehicles. The fuels which come closest for replacing gasoline are ethanol or methanol. In the question of the gaseous fuels, LPG and CNG, there may be different provisions in different countries but the situation in Sweden is such that natural gas, such as CNG, is certainly preferred to LPG since compressed biogas (which contains mostly methane like CNG) is already used in some cities in Sweden. Both biogas and CNG have are physically two of the “cleanest” fuels for otto engines and they have a good potential of contributing to lowest emissions of harmful pollutants when used in engines dedicated for the use of methane-containing gaseous fuels. However, experiences from the use of CNG and compressed biogas have shown that the technology for the use of gaseous fuels must be considerably improved in order to fully exploit the emission potential of these fuels. The conclusion from the above discussions is that within the nearest 5 years the primary combination will be the use of gasoline/otto engines for hybrid vehicles. The positive improvements of both gasoline and the otto engine will continue. There is still no clear indication that the direct injection otto engine will replace the conventional otto engine within 5 years as the gasoline fuelled alternative for hybrid systems. This is especially true if there will be a positive development of the control system for the hybrid vehicles in line with the presentation in for example Sections 8.1.1 and 8.1.4. Only Mitsubishi has announced that the combination direct injection gasoline engine and hybrid system with an electric motor will be used in order to “deliver super-efficient power generation and performance”. The development of diesel engine is estimated to have a remarkable impact on the emissions and therefore it will start to make a more positive contribution in the discussion about fuel/engine alternative but this positive movement towards a cleaner engine depends strongly on the cleaning up of the diesel oil. Since it is uncertain whether the diesel engine will have that advantage, attention in a shorter perspective should be called to the fact that there are alternatives available which may take some part of the market from gasoline and diesel oil. The alternative, in the first place for light-duty hybrid vehicles, is expected to be the ethanol/otto engine and for a heavy-duty hybrid vehicle the CNG/otto engine or the Biogas/otto engine could be a strong candidate. In Section 5.2 the huge resources of natural gas have been discussed. For a country like Sweden the cost for an infrastructure for natural gas over the whole country would be extremely high and in addition it is uncertain whether there is a common interest among the owners of vehicles in using natural gas as an automotive fuel. However, looking at the situation in Sweden there may exist an interest in using natural gas in certain cities or areas. One possibility which has been discussed is to convert natural gas to methanol and to produce ethanol from wood as a biofuel. 97
By the development of intelligent control units and better batteries for the hybrid systems, the internal combustion engine will be operated without rapid transients, which will result in more freedom for the car manufacturer to improve the interaction between the two power systems in the hybrid vehicle. The average energy efficiency of the system will increase, especially during driving in city traffic. In addition to the possibilities discussed in Section 8.2 and Section 12 the model for development presented by TNO may give many positive results. These improvements of the energy efficiency etceteras may also result in that the car owner more easily will tolerate the higher cost of using alternative fuels. In a Swedish perspective, building up an infrastructure for natural gas and biogas to be used in vehicles would result in both natural gas and biogas being more attractive as automotive fuels. The technology for the use of these gaseous fuels in engines would certainly also be affected. Whether there will be a development in that direction is not certain, as the organizing of a such infrastructure must be based on a political decision. If a decision is taken in Sweden in this direction the use of CNG in hybrid vehicles is estimated to be limited to heavy-duty vehicles. Since there are also some negative factors connected with hybrid systems, especially in that the vehicle contains more parts and units than a conventional vehicle, the extra weight and the cost of the vehicle is an extra burden for the car owner. The hybrid system may also result in an increasing need for maintenance. There is an urgent need for batteries with higher power and energy densities and the question is when such batteries are to be seen. These disadvantages and especially the cost of the hybrid vehicle certainly have a negative effect on the market, and for a further positive development of the hybrid systems there is a need for large market. The fact that fuel cell technology may have a more positive development and be an attractive alternative sooner than expected may be an obstacle to the development of hybrid technology and especially the drive trains with an internal combustion engine. In a longer perspective (10 to 20 years) many different ways for the development can be seen and therefore it is not possible to point out a certain direction. The PNGV program and the strong requirement to use “cleaner” vehicles using less energy will have a strong impact on the development of fuel, engines and the body of the vehicle. The development of hybrid vehicle which has been seen has positively influenced the required improvements of the vehicle. There is good possibility, through the use of hybrid technology, to use the internal combustion engine and especially the otto engine more efficiently. Since the hybrid system gives extra weight to the vehicle a lighter body vehicle has been developed and new production technology has been exploited in order to compensate for this extra weight. In addition many units for the control of the different systems of the vehicle have been developed and all of this in a positive way. In order to sum up the above discussion and the expected development in the time frame of 5 to 10 years it should be underlined that; -
a well pronounced interest in the hybrid technology has been shown among car manufacturers and these who are responsible for the development of hybrid electric vehicles;
it has been shown that many actors have been involved in the development of hybrid systems for city buses. Unfortunately only a few car manufacturers have been involved in that work. Therefore it is unsure whether the development of hybrid systems for city buses will continue;
many well functioning hybrid systems for light-duty vehicles have been developed by the car manufacturers especially in Japan and in the USA. This well organized development seems to continue at least in the short time frame; 98
the PNGV agreement between the US Government and the three car manufacturers Chrysler, Ford and GM and also other agreements have positively affected the decisions taken among car manufacturers not only in the USA and Japan but also in other countries in order to develop fuel efficient vehicles and among these hybrid vehicles.
In order to sum up the above discussion and the expected development in the time frame of 10 to 20 years it should be underlined that; -
it is not possible make a firm estimation about the development of hybrid vehicles after 2010 to 2015 since there is a considerable uncertainty in some factors affecting a such development;
two scenarios can be seen concerning the development of energy efficient motor vehicles, namely: (a) a positive continuation of the development of hybrid electric vehicles parallel to research and development of fuel cells; (b) a brake through for the development of fuel cells resulting in (1) the use in fuel cells in hybrid electric vehicles with a battery used for storage of electricity and (2) the use of fuel cells as the only source for the delivery of electric power to the traction motor in a vehicle without battery for the storage of electricity use for the traction motor.
Today it seems most likely that the development of highly fuel efficient vehicle will follow scenario (a) since it is estimated that the fuel cell technology will be to expensive to be used in mass produced vehicles within the time frame of 20 years.
9 EFFICIENCY – FUEL ECONOMY Iwai, who was presented in Section 8.1.2 (Iwai et al., 1998), has been studying the efficiency of different parts in a hybrid system and also discussing the importance of controlling the interaction between the internal combustion engine and the electric drive train. According to Iwai the efficiency, and by this the fuel economy for a vehicle with internal combustion engine, be improved by; 1) eliminating of engine idling and running the engine at low loads where the thermal efficiency is low and by operating the engine only in points of the engine operational area with high efficiency; 2) accumulating the braking energy at deceleration; 3) installing a power system that can operate with a high degree of efficiency, regardless of the driving cycle; 4) using a bottoming cycle that creates electric energy by driving the power generator with exhaust energy, making it possible to achieve higher efficiency of the system.
The efficiency of hybrid systems
BMW has, as was mentioned in Section 8.1.5, engaged Dresden Technical University to study and evaluate the efficiency of hybrid systems by comparisons with an conventional vehicle (Friedmann et al., 1998). The comparisons were based on tests using a BMW and on results which had been generated by EUCAR on a series hybrid and a parallel hybrid and in addition to some other studies. The researchers who carried out the comparisons at the university, have analyzed how the different hybrid systems will be controlled in order to achieve the highest efficiency and they have then also considered how the internal combustion engine has to be controlled for the purpose of achieving high efficiency. In line with what was said in Section 8.1.1 the scientists at the university point out that the efficiency in a drive train of hybrid vehicle, which incorporates a generator and an electric motor is considerably less than that of a mechanical drive train. This is in cases where the engine is direct connected to the driving wheels via a mechanical drive train. The above statement is especially valid for series hybrids where the mechanical energy has to be transformed to electric energy and then again to mechanical energy for the traction of the vehicle. In the case of a parallel hybrid, the branch of the drive train the electric motor etceteras can be directly compared with drive train of a series hybrid. There is, however, an important difference between the two systems in that one branch of the drive train of a parallel hybrid is directly coupled to the driving wheels. Consequently the conclusion of the evaluation is that a control strategy must be used where the internal combustion engine is as often as possible operated in its most efficient zones of the engines working area, in order to achieve the maximum efficiency of the hybrid system. In other words the control of the hybrid system must be such that the average efficiency of, in the first place, the internal combustion engine is increased by the engine “taking back” the losses in the part of the drive train equipped with an electric motor which is fed by the engine and the battery. In the best cases the losses are eliminated. An interesting scenario discussed by the authors in their report (Friedmann et al., 1998) and which also have been discussed by others, is that computer simulations and the use of different control units will be important tools during the phases in the further development of hybrid vehicles. In the present phase of the development and when some hybrid systems have been demonstrated, it is important to evaluate the result achieved so far. However, and important question is to what extent the gained experiences will be used as a base for the 100
optimization of the future hybrid system. This seems to be most important in the case of hybrid systems for heavy-duty vehicles. This is because the development systems for especially buses, in many cases, seem to be suffering from the same type of analyses which have been the basis for the development of light duty hybrid vehicles. Testing prototypes under the phase of development is important in order to learn about the function of the different units in the system but many costly experiments can be replaced by a design based on computer simulation. This seems to be the message from those who have been analyzing the efficiency and optimization of hybrid systems. In this report the importance of using an optimized control algorithm for the hybrid system in order to achieve a high average efficiency has been repeatedly underlined. It has also been shown that a direct connection from the internal combustion engine via a mechanical drive train causes less losses of efficiency than an electric drive train. On the other hand the average efficiency of the engine depends on how the engine is operated. The impact of this and other possibilities to be used in the hybrid systems can be seen in Figure 34. Matching the different units in the system is important for a good performance of the vehicle, low emission levels and good fuel economy. In the case of the possibilities of improving the hybrid system the potential for improvements is probable higher for the electric system and especially the battery than for the internal combustion engine.
Figure 34. Efficiency improvement by hybrid strategy. (Source Takaoka et al., 1998). A certain modification of the hybrid system seems to be necessary. This has been pointed out by some of those who have studied the hybrid system in detail, such as Iwai and the scientists at Dresden Technical University. A question to be answered within the next few years is whether diesel engines will be acceptable for use in some hybrid applications, for example in series hybrids for heavy-duty vehicles, in order to compensate the lower efficiency of the electric drive train compared to the mechanical drive train. If the expected considerable improvements of the diesel engine are realized the acceptance of the diesel engine may increase. A possible alternative would be to use an alternative fuel such as ethanol in the new generation of diesel engines in order to improve the exhaust emission compared with the 101
exhaust from diesel engines. In addition this alternative would improve the energy equivalent fuel economy. An engine in a well-designed series hybrid can be operated with less rapid transients than the engine in a conventional vehicle. Therefore there is a better possibility for adapting the diesel engine in a hybrid vehicle for the use of an alcohol than to adapt a diesel engine in a conventional vehicle for the use of an alcohol. The Toyota hybrid vehicle Prius was one of the first hybrid vehicles to be presented and certainly the first hybrid vehicle to be ready for the market. It can therefore be expected that some important improvements of the hybrid system would improve the fuel economy. Despite this early introduction on the market it should be recognized that Toyota has shown that a good matching between the different units in the hybrid system and the use of a new component in the drive train the, the planetary gear, resulted in a fuel-efficient system. From Figure 34 it can be seen that the efficiency curve is much higher for the hybrid vehicle when compared to a conventional vehicle and that low speeds and idling of the engine are cut off. Rapid accelerations of the engine are avoided by the assistance of the electric motor. According to Figure 34 the average efficiency of Toyota Prius has been increased by 80% from the average efficiency of a conventional vehicle and that there is an additional increase of 20% from the regenerative braking. However no clear indication has been given about the driving condition. It is estimated that the comparison was carried out for the Japanese 10-15 mode cycle. In Section 8.2.3 it has been shown that the difference between the hybrid vehicle and the conventional vehicle decreases with the increase of the average speed. However when keeping to the Japanese 10-15 mode cycle the information from Toyota says that the fuel economy for Prius compared with a Toyota Carina is 28 km/l contra 14 km/l. The displacement of the engine is the same for both vehicles but the Carina was an automatic. In Figure 35 the comparison of the two vehicles is shown for some different speeds.
Figure 35. Comparison of fuel consumption at different driving modes. Source Takaoka et al., 1998
The use of energy and efficiency at different driving patterns.
In section 9.1 the need for computer simulations were discussed and also their practical use. To use only a test matrix and tests in a laboratory as a basis for taking decisions as to whether a series hybrid or a parallel hybrid is the best alternative is both time consuming and costly and therefore hardly any car manufacturer will do so. Today the planning of for example a new model is based on an investigation of many factors and not only on the design of the vehicle but also on a study of the market, cost factors and so on. In the case of systems such as hybrid vehicles the situation may be somewhat different. Here some irrelevant factors may give a need to present new ideas and expectations to achieve such result of the development 102
which are unrealistic in reality. On the other hand many new ideas have been realized by trial and errors and in the case of hybrid vehicles there still is a lack of hardware with as good performance as it is possible to achieve. One example is the batteries, which with today’s knowledge could be much better. Careful planning of the different factors, not only the hardware but also the use of the vehicle, can result in more sound and realistic expectations. This will go a long way to achieving a certain result. As a basis for the decision to be taken as to whether a series hybrid or parallel hybrid is the best system for the fulfillment of the expected goal, computer simulations can be carried out. These should use factors such as the driving pattern for the area, where the vehicle will be operated and the basic parameters for different hybrid systems. One such computer simulation in connection with laboratory measurements is presented and discussed in the following paragraph. 9.2.1
Studies of hybrid systems for BMW
In order to get an idea of how an evaluation of hybrid systems can be carried out the model used by Dresden Technical University during their investigations and studies carried out for BMW presented is Section 8.1.5 and Section 9.1 respectively. The studies were especially concentrated to the evaluation of the energy efficiency of series hybrids and parallel hybrids and were, for the computer simulations, based on calculations, in one phase, of the difference between a series hybrid vehicle and a conventional vehicle. A similar calculation and comparison was carried out for the evaluation of a parallel hybrid system. Then a series hybrid vehicle was compared with a parallel hybrid vehicle. The authors of the report (Friedman et al, 1998) point out that there is a loss of efficiency in the electric drive train. They say that, in the case of a parallel hybrid, the electric drive (which transmits force parallel to the mechanical drive train) works like the drive train in a series hybrid. The result of the evaluation is presented in Table 20. From the table it can be seen that the regenerative braking has a considerable influence on the fuel economy of both the series hybrid and the parallel hybrid. However, the authors claim that regenerative braking (which is engaged during decelerations) shortens the “exhaust emission free driving” achieved by free rolling of the vehicle during decelerations. According to the authors of the report (Friedman et al, 1998) regenerative braking does therefore not favor the reduction of exhaust emissions (here it should be added that this problem could be handled by the hybrid control unit). From the table it can also be seen that for the series hybrid, according to the calculations there is a decrease in fuel consumption (or energy used) for only one combination, namely “0” for electric operation range, 0.92 for Degree of coupling and “Yes” for BERG (braking energy generation). For all other combinations there are increases in fuel consumption from +8% up to 50% when compared with a conventional vehicle. In the case of the parallel hybrid the picture is somewhat different concerning the savings and losses in fuel consumption where savings are from 0% up to 15% and the losses from 0% up to 10% according to the calculations. It should be noted that these results are based on computer simulations using data or experiences from their own experiments or data from the literature. The example is interesting in that some of the important parameters of the hybrid vehicles are defined and used in a mathematical model. In this case it has been shown that a parallel hybrid vehicle is more efficient than a series hybrid vehicle for a driving pattern such as the driving cycle used in this example. This is in line with what that others have pointed out, as for example Iwai (Iwai, 1998) who underlines that it is more difficult achieve high efficiency for a series hybrid vehicle than for a parallel hybrid vehicle. However, it should be noted that the figures 103
presented by the authors of the report (Friedman et al, 1998) will certainly be different to those from other driving cycles and other efficiency estimations. Table 20. Comparison between a series and a parallel hybrid vehicle with a conventional vehicle. Driving cycle Series
Electric driving [km]
Series City City ”0” City City ”0” On road. On road. ”0” On road On road ”0” City City 15 City City 15 On road On road 15 On road On road 15 City City 30 City City 30 On road On road 30 On road On road 30
Parallel ”0” ”0” ”0” ”0” 20 20 20 20 40 40 40 40
Regenerative braking Series Yes No Yes No Yes No Yes No Yes No Yes No
Degree of engine driving Paral- Series Paral lel lel Yes 0.92 0.83 No 0.94 0.85 Yes 0.98 0.95 No 0.98 0.95 Yes 0.38 0.78 No 0.42 0.79 Yes 0.45 0.90 No 0.52 0.90 Yes 0.36 0.76 No 0.37 0.76 Yes 0.42 0.90 No 0.44 0.82
Fc* for conventional car l/100 km 10.2 10.2 6.6 6.6 10.2 10.2 6.6 6.6 10.2 10.2 6.6 6.6
Fc for hybrid vehicle
Changes in fuel consumption
10.0 11.0 7.1 7.3 11.5 13.0 8.7 8.8 12.6 12.6 9.7 9.9
8.66 10.3 6.3 6.6 9.2 11.0 6.5 6.9 9.3 11.1 6.7 7.3
-2 +8 +8 +11 +13 +27 +32 +34 +24 +44 +47 +50
-15 +1 -6 ±0 -10 +8 -2 +5 -9 +9 +2 +10
*Fc: Fuel consumption (Use of energy). Minus sign: Reduced Fc. Plus sign: Increased Fc. 9.2.2
Energy use and efficiency of Mitsubishi hybrid trucks
In Section 8.3 two hybrid trucks – one delivery truck and one aerial working truck developed by Mitsubishi were presented. In the report (Horii et al., 1998) two figures showing the fuel consumption and energy efficiency were also presented. The values presented by Mitsubishi have been slightly rearranged and are shown here in Figure 36 and Figure 37.
Figure 36. Mitsubishi service truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998. Three different driving conditions have been compared – idling, 40 km/h and 80 km/h and in the figures even the efficiency, η, has been filled in for the two trucks and the different conditions. The values for the energy used was calculated in the way quoted from the report (Horii, 1998). The energy consumption of the HEV was calculated by the use of simulated values. “The actually measured efficiency values of the various parts of the delivery truck 104
HEV were partially modified to match the work truck HEV, and the efficiency values of all the components were multiplied together to work out vehicle efficiency. In addition, theoretical running energy was divided by vehicle efficiency to calculate energy consumption”. The energy used was presented in kWh/km and “Theor” and “Act” in the figures below means “Theoretical” and “Actual” respectively.
Figure 37. Mitsubishi working truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998.
Figure 38. Energy efficiency of same important component of a HEV. Source: Horii et al., 1998. Figure 38 shows the power/energy flow and the efficiency of the individual components used by Mitsubishi as a basis for the calculation of vehicle efficiency. Since the details of regenerated power and generated power that are used (that portion of the energy which is used without passing though the batteries and that which is used for recharging the batteries) vary according to the operation mode, the regeneration efficiency and battery efficiency were changed according to the mode of operation.
10 FUEL AND DISTRIBUTION It is only in the case of electricity that distribution differs from that for conventional vehicles, as long as conventional and common alternative engines are used. New alternative fuels can come into existence if other types of internal combustion engine and sources of power (e.g. fuel cells) come to be used. This will most likely not occur for a long time (10-20 years or more). One question which needs to be thought about is how will electric energy for hybrid vehicles be distributed, so that this will be as effective as possible and is going to meet the demands which must be placed on it.
The conventional fuels are gasoline and diesel oil. The distribution of these is generally secure for whole Sweden as in other countries countries by the stations, which have been given the not completely adequate name of “gasoline stations”. There are at the present time (year 2000) three different qualities of gasoline in Sweden – 95 octane unleaded (usually called lead free) gasoline, 98 octane unleaded gasoline and a blend between 95 octane and 98 octane in order to achieve a 96 octane quality. The latter will be phased out very soon, unless this has already taken place. When unleaded gasoline was introduced a discussion took place as to how it would be distributed, since there were already three different blends of gasoline. It was found to be very expensive to fit all the gasoline stations over the whole country with new tanks and pumps for the distribution of unleaded gasoline. This problem was solved by that the middle variety, 96 octane, no longer being distributed through its own pumps but by means of mixture-pumps. Since unleaded gasoline was introduced other important changes have been made (i.e. the introduction of 98 octane unleaded gasoline), and the question is what the future distribution of gasoline will be like. New demands are being made for gasoline from both the car manufacturers and the authorities. The truth of the matter is that there is a risk it will be no space for further alternatives unless an expensive extension of the distribution net takes place. Three qualities of diesel oil have been defined, MK1, MK2 and a third blend of diesel oil designated MK3 (see Table 2). Since MK1 took the greatst part of the market within a few years of being introduced and completely dominates the market at the present time, it is not easy to sort out how the other part of the distribution looks. According to received information (Lindberg, 2000) MK2 is no longer distributed but a small amount of MK3. During recent years a “European Diesel Oil” has been specified after a great deal of discussions and deliberations but so far not any blend of “European diesel oil” is distributed on the Swedish market. The future will show how much of the market diesel oil will have in the future. There can also be discussions about which sort of diesel oil, with the addition of RME, water emulsion etc., will be available on the market and if this fuel should be treated as diesel fuel or as an alternative fuel.
For hybrid vehicles there are a range of fuels available due to there are various alternative engines – Stirling and gas turbines – and even fuel cell that are being considered for use in hybrid vehicles. In reality it is quite uncertain that the two named engine alternatives will at all be used to any great extent. Even in the case of fuel cells not much indicates that these will be used as alternatives to otto or diesel engines other than to a very restricted extent, within 106
the coming 10-year period. In fleet trials and in single test vehicle prototype fuel cells can be used for various tests. Since one of the object of the hybrid vehicle is to reduce pollution it is estimated that they will be adapted for the use for environmentally suitable fuels. This especially since newer types of catalyst require ”cleaner” fuel than the gasoline available today and in an international view this applies even the diesel oil. Some form of both gasoline and diesel oil can be relevant as alternatives also for fuel cells. Concerning the distribution of fuels that are considered to belong to the group of alternative fuels, the following as alternatives can be of interest. Natural gas/biogas: Both these fuels can be relevant to be used in hybrid vehicles even in Sweden. however, in the short term, their use will be limited to fleets of heavy-duty hybrid vehicles used locally in built-up areas. One condition for a widespread use of natural gas in Sweden is that distribution pipes are drawn to strategic places in the whole country. The possibility of distributing biogas and natural gas in the same pipes should be discussed. Natural gas can be an attractive alternative fuel even for fuel-cell vehicles in built-up areas, chiefly for the production of hydrogen in special establishments. Today tanking of natural gas/biogas takes place at tanking stations according to a network which has been developed in Linköping, Stockholm, Trollhättan, Uppsala (for biogas) and for certain towns in the county of Skåne and on the west coast (for natural gas from Denmark). Methanol/ethanol: No certain conclusion has been drawn in Sweden about whether it is more advantageous to distribute both of these fuels, or whether only one should be distributed and in this case, which of them. If and when the distribution network is built up, with tanks and tanking stations, it is important that they are constructed so that either methanol or ethanol can be distributed without there being any problems with the material used for the stations. The greatest demands concerning the material, from the corrosion point of view, apply especially when methanol is to be used. It is also important that routines are established and instructions given for the prevention of accidents, for the protection of the personnel and others who handle the distribution or otherwise come into contact with alcohol fuels. LPG/DME: Even if LPG is best used in otto engines and DME in diesel engines, they can be considered similar from the point of view of the techniques of distribution. No negative characteristics are known about these fuels except those which also apply to gasoline and diesel oil. LPG consists to a large extent of propane (a gaseous fuel which is uses for heating and cooking in caravans). One should, however, be aware that both LPG and DME are heavier than air and can therefore collect in depressions or near the floor in the case of leakage in closed areas and that they can be easily ignited by sparks from, for example, an electric switch. It is for this reason that smelling substance, mercaptan, is added to LPG (this should also be the case for DME). To the knowledge of the authors DME has not been used commercially in internal combustion engines. Both LPG and DME can be stored in tanks, in liquid form under low pressure. Both fuels must be tanked in a closed system but they do not require as high a pressure as natural gas or biogas. LPG has not had any great success on the market in Sweden but it is used in large quantities in Holland, Italy and certain other countries in Europe. It is not expect that LPG will be used to any great extent, as a vehicle fuel, in Sweden. In the case of DME it is not easy to speculate. Hydrogen: At the present time hydrogen is not used commercially as a vehicle fuel. This situation can change if hydrogen comes to be used in fuel cells. However, there are some engine manufacturers who are developing internal combustion engines for hydrogen (BMW for example). Fuel cells are run on hydrogen but the question is whether the hydrogen is to be produced inside or outside the vehicle. Mercedes-Benz has considerable experience in this field and has one time developed a hydrogen driven vehicle using an internal combustion engine. Today prototypes of both passenger cars and buses equipped with fuel cells have been 107
developed to be tanked with hydrogen (see Section 8). The future will show whether M-B (now DaimlerChrysler) will continue to use hydrogen as a direct fuel or whether there will be a development so that busses will be tanked with hydrogen but that a liquid fuel, such as methanol, will be used for passenger vehicles (see previous discussion on this). It is expected that there will be a development in this direction since both patience and a certain amount of care are required when tanking hydrogen, and it is not all vehicle owners who have these qualities. In the case of distribution and tanking of hydrogen there are techniques for minimizing the risks which are otherwise associated with hydrogen. Air mixtures with 5-75% hydrogen are explosive. Hydrogen can be stored and transported as both gas and liquid. Because of hydrogen’s ability to combine with other substances, a technique has been developed where hydrogen is allowed to bind to metal hydrides for storage and transport. The disadvantage of this is that the transported fuel will be heavy and there is thus an ongoing development of lighter metal hydrides which is expected to reduce the problem. The question is whether this is the solution since there is no certain indication that this technique will be used in motor vehicles. Today DaimlerChrysler store hydrogen in pressure tanks for their busses with fuel cells. Research is under way, however, and other more efficient storage methods may be found which can alter the picture radically. Despite hydrogen being used in test vehicles with fuel cells it is still uncertain whether the gas will be able to compete with the liquid fuels. The question can come to be settled within the coming years by the “California Fuel Cell Partnership” – a co-operation has been established between the fuel cell manufacturers Ballard, and American, German and Japanese vehicle manufacturers and also a number of oil companies. Its object is to demonstrate the use of fuel cell vehicles in real traffic. The project seems still to be in the development stage but DaimlerChrysler and Ford have promised to each of them contribute with 5 vehicles equipped with hydrogen tanks. Each of them will also if possible contribute with 10 vehicles equipped with methanol tanks. However, the intention within the mentioned partnership is that the trials will take place from 2000 to 2003.
In Sweden electric energy is widely available and it has the advantage of not being produced, at present, by the use of fossil fuels. For the private person the cost of electricity is approximately 40% or more lower than the cost of gasoline, calculated in terms of energy. This is mainly due to the lower tax on electricity. This comparison applies if vehicle owners charge their batteries in their own garages or from their flats. The comparison is, however, not completely adequate in the case of a hybrid vehicle equipped for charging the battery from the mains. This can give an indication of the economic advantage, in the case of a private person, of using electricity as compared with using gasoline. The difference will not be as great when comparing electricity with diesel oil. It has previously been mentioned that the energy transmission efficiency is better when generating electricity using an internal combustion engine than when using a fossil fuel driven generator for electricity in a stationary establishment. In this context it should be noted that the use of electric energy generated in Finland, Norway and Sweden does not generally contribute to the emission of pollutants, except in exceptional cases, since most of the electricity in Sweden or at least 50% is produced in hydraulic (water) power stations and the rest in nuclear power plants. Questions on the use of electricity in KFB’s “Electric and hybrid program” are mainly focused on the purely electric vehicle. Information concerning the distribution of electricity for hybrid vehicles can generally speaking be obtained from investigations concerning the 108
distribution of electricity to electric vehicles. We shall therefore only briefly mention that there is not a great deal decided about how batteries should be charged. Since the cost of the batteries is an important part of the running costs, for a series hybrid especially, it is important to charge and maintain the batteries in a way which does not cause the batteries to deteriorate. A great deal of research is being carried out on this subject and a control function has even been developed for the battery, so that charging and discharging take place in the best possible way. In Japan such a control function has been developed for a nickel-metal hybrid (Noboru et al., 1998; Tojima et al., 1998). This control function protects the battery against being overheated, which would not only reduce the working efficiency if the battery but would also have a negative effect on its lifetime. In a report (Noboru et al., 1998) it has been pointed out that the mechanism behind the gradual deterioration of the charging capacity through undercharging (which is here interpreted that one does not carry out the charging process using a sufficiently large capacity) has not been established. It is stated that there is a difference between different types of battery, so that certain batteries can be charged in a shorter period of time than others do. In an investigation of the charging time of a valve regulated lead battery (Cooper and Mosely, 1998) it was found that fast charging dramatically increased the lifetime of the battery, which can be seen in Table 21. Table 21. The effect of charging strength on the working efficiency of charging and on the durability Parameter Charge Regime Discharge Regime Capacity Check after every 50 Cycles Charge Efficiency Cycles Lifetime Discharge Status
Slow Charge 5 hour rate At 2 hour rate to 11.6V 80% Discharge to 10.5V and fully charged for 3 cycles 87 % 250 10 000 Ah Failed
Fast Charge 12 minute rate At 2 hour rate to 11.6V 80% Discharge to 10.5V and fully charged for 3 cycles 97 % 900+ 30 000 Ah Still Healthy
11 TEST METHODS A whole new situation has arisen concerning the methods for evaluation of the effects of the fuel consumption (or the use of energy) and of exhaust gas emissions when using hybrid systems, compared with such evaluations when using present day, conventional vehicles. Firstly the energy used during the test of a hybrid vehicle cannot always be related solely to the use of the specific fuel and not either the emissions released from the vehicle to be correct, since the status of the energy storage battery must be exactly known both before and after the test. Instead the energy used during the test has to be calculated for both the internal combustion engine and for the electric motor (motors). Nor can one expect that the two basic systems, series and parallel, will be able to be tested according to the same method, without the measurements becoming unnecessarily comprehensive. Such differences between the two basic systems can be found that it can be more rational to test them according to somewhat different methods. The Society of Automotive Engineers (SAE) has produced a preliminary standard for the measurement of energy consumption and emissions of hybrid vehicles. The standard is called “Draft SAE J1711”. It is not officially available so we have not been able to study the preliminary standard. In a SAE report, SAE-paper 981080 (Duoba, M. And Larsen, R., 1998) it is stated that the preliminary standard SAE J1711 has been changed a number of times since it first appeared in 1992. It is thought that a key problem with measurement methods for hybrid vehicles is that there are too few hybrid vehicles on the market for the concept of the test method to be able to be verified. Technicians at the Argonne National Laboratory (ANL) also have some comments on the preliminary SAE standard. They think that the SAE J1711 measurement method is too long for their purposes, and that the suggested standard means that one will be dependent on the vehicle manufacturers to specify how the vehicle reacts for the various tests. They also think that the initial installation of the system in the vehicle must be adjusted in order for the driving cycles and measurement to be able to be carried out. After their first draft of the measurement method the committee who was involved with it decided that hybrid vehicles must be classified in various categories and be tested according to different methods suited to the function of the relevant hybrid. According to them this is not in line with the wishes of either the authorities or the vehicles manufacturers with many vehicle models. For technical reasons these want a comprehensive measurement method which is suited to all types of hybrid vehicle. Technicians at Argonne think, on the other hand, that the function of the hybrid is too complex for it to be possible to characterize all types of hybrid vehicle using only one measurement method. The following steps for characterization of and measurements on hybrid vehicles have been specified in a report (Duoba, M. and Larsen R., 1998): CHARACTERIZATION OF VARIOUS FUNCTIONS OF HYBRID VEHICLES – is a complete characterization of each discrete way that the hybrid vehicle works. It includes both the various function steps that are connected in automatically and those which the driver of the vehicle can connect in by switching etc. “ZEV MODE” – if the vehicle is to driven by electricity for a whole driving cycle, this will be regarded as a zero emission stretch or as having “ZEV” possibilities. “HEV MODES”– HEV functions is defined in the report (Duoba, M. and Larsen, R., 1998) as a single energy-supply strategy – a strategy during which the electric motor and the internal combustion engine work together to supply the vehicle with sufficient power for driving it. 110
“ON BOARD CHARGING” – a HEV function, which means that the vehicle’s batteries are charged by a special generator while driving. ”CHARGE-DEPLETING* HEV MODES” – results collected from this stage of the test cannot be corrected so that the electric energy, which is used is eliminated – electric energy will always be depleted for this test. For this reason the characterization of this step must include the results of the emissions from the internal combustion engine and also the fuel consumption. The relationship between these and the amount of electric energy used from the battery during the test shall be calculated. “CHARGE-SUSTAINING HEV MODES” – is either the steps in the test where there is insufficient capacity in the battery for the internal combustion engine to be able to be switched off, or when the internal combustion engine is sometimes switched on and sometime not. Because the engine an electric drivetrain are working together during HEV mode this usually means that the battery is continuously storing and releasing energy throughout the operation. This leads to unpredictable behavior that requires more test time to characterize a particular mode accurately “SOC-CORRECTIONS” – this is an important aid which has been developed to solve the problem of cycle to cycle variations for transient HEV driving. Measurement of the charging level of the batteries, “state of the charge” (“SOC”), is a method of combining the results from the various cycles in order to balance out the charging and discharging of the battery during a test. This process produced data concerning the fuel economy (energy consumption) and emissions, which corresponds to the zero net change in SOC. “LINEAR REGRESSION SOC METHOD” - occurs in the first draft of the preliminary SAE standard which then comprised a method with linear regression for correcting the battery’s charging level. This method is most useful for vehicles which do not have zero net charge with a significant stretch using electricity. The functions or steps in the measurement method which are described above are those which we think should be included when measuring energy consumption and emissions from hybrid vehicles. Variations in the design of the various steps can obviously occur and the idea with suiting the method to the type of hybrid system and function of the hybrid system should certainly be considered when finalizing design of a standard method for measurements on hybrid vehicles.
Figure 39. Tests with parallel hybrid vehicle developed at the University of California Davis. Source: Duoba, M. and Larsen R., 1998 A parallel hybrid system developed at the University of California Davis and a series hybrid developed at West Virginia University were tested in one of the US EPA laboratories and the complexity in the tests is reflected in Figures 39 and Figure 40. The abbreviations in the figures are as follows: 111
ZEV: A step in the driving cycle where the vehicle is driven as an electric vehicle, i.e. only using energy from the battery. UDDS: Urban Dynamometer Driving Schedule. HWF(E)DS: Highway Fuel Economy Dynamometer Schedule HWY prep*: Warming up cycle before HWFEDS (The same driving cycle as for determination of fuel consumption, HWFEDS, is driven once in order to warm up the vehicle immediately before the HWFEDS test is carried out). A-h; Ampere hour kWh: Kilowatt-hour AC: Alternating Current MPG: Miles per gallon (1 mile = 1.609 km, 1 gallon = 3.785 l). Cold-start: The starting temperature of the test is usually ca 22 degrees C. Hot-start: The vehicle is usually run so that the internal combustion engine reaches driving temperature. SOC Corrected: Corrected State of Charge (level for charging the battery, see above). Comments to Figure 39: The energy consumption for tests with cold-start and warm-start can be seen in the right part of the figure. For each pair of squares in the figure the first square gives the energy consumption in l/100 km (and MPG, i.e. number of miles per 3.785 l fuel, exclusive of the use of electricity). The other square gives the amount of electricity used in Ah and kWh and this energy consumption has to be added to the consumption of fuel. There is a corresponding situation for reporting the results at warm start and even for tests according to HWFEDS. HWY stands for warming up the vehicle before the actual HWFEDS test is carried out.
Figure 40. Tests with series hybrid vehicle developed at the West Virginia University. Source: Duoba ,M. and Larsen R., 1998
Comments to Figure 40: The results are given here in a somewhat more complicated form. This is because one test (the first square in the left part of the figure) is carried out with the vehicle run only in the electric vehicle position. This gives the possibility of comparing the energy consumption for two test cases - running as electric vehicle and running with the internal combustion engine via the electric system. The other square on the left side of the figure gives the energy consumption in the internal combustion engine of 11.46 l/100 km (25.52 MPG). Since the battery is charged with 1.8 Ah by running the internal combustion engine the real energy consumption during the test must be corrected to 9.14 l/100 km (25.52 112
MPG). Compare the values in square 3 and square 4 in the right part of the figure. There is a corresponding situation for test with warm start. However the calculations for the two HWFEDS tests which were carried out resulted in a minus value for the charging level of the battery in the first test and a positive value for the charging level of the battery in the second test.
12 IMPACT ON THE EMISSIONS The studies carried out have shown that there is a realistic potential for a reduction of the emissions by replacing conventional vehicles with hybrid vehicles. However, there are many “ifs” which have to be fulfilled in order to surpass the light duty conventional mass produced gasoline fueled vehicle of the future. Therefore when making comparisons with a such vehicle there is a higher potential for improved fuel economy than for improved emissions especially when comparing data generated according to standard test procedures. When looking at the real traffic situations and the use of the vehicle during different meteorological conditions the situation for the hybrid system seems much more positive. The main reason for this is that there are more alternatives and a higher freedom for the parameters and conditions which have a significant impact on the emissions. These conditions and/or parameters are, for example, the cold start situations (which are a problem in the Nordic countries) during which the hybrid technology offers some realistic opportunities to minimize the impact on the emissions. The hybrid vehicle can be designed so as to start the trip in the electric mode. This may require that the battery is charged from the mains and that the compartment of the vehicle is warmed up by an electric fan. There are also many other possibilities in a hybrid system to take care of the emissions at periods with low temperatures. More of the technology to be used will be discussed in Section 12.1 With a well designed and efficient electric drive train and an optimal matching between the electric drive train and the internal combustion engine the engine can be operated so as to avoid rapid accelerations which is a risk, especially in a parallel hybrid system. For both types of hybrid systems and especially for series hybrids there is an urgent requirement to have access to highly efficient batteries, in order to improve the weak link in the electric drive train. In Section 12.2 the emission performance is presented for some of the systems discussed earlier in the report. Unfortunately there are only two systems for which emission data are available for this report and it is also a drawback that no emission data for a second generation of hybrid vehicles has been available.
12.1 Theoretical background for a emission potential Since the hybrid systems have not been commercially available, except in Japan until this year, a closer evaluation of their potential for low emission levels has not been possible. In Section 8.2 some result found in the literature have been presented and discussed and these results indicate a rather positive emission potential for hybrid light-duty vehicles. For heavyduty vehicles there is a lack of reliable data for a realistic estimation of the potential. When dealing with this matter it must be kept in mind that the hybrid systems for which emission data have been available are the first generation of these types of vehicles. When trying to find the right level for an estimation of the emission potential a question is raised as to whether there are any fundamental factors that have a negative or a positive impact on the emissions from the hybrids. Both with series hybrids and parallel hybrids there is a possibility to operate the internal combustion engine in a different regime than the regime it would be operated in if the driving cycle should be completely followed by the engine. Since the emission control system (TWC) for otto engines rather is sensitive to transients (regardless of fuel) it can be estimated that close-to-constant driving favors the emission reduction. It has been shown by experiments that rapid accelerations cause higher emission levels. New systems for reduction of NOx from diesel engines (fueled with diesel oil or an alternative for diesel oil) the so-called deNOx catalysts, may also benefit from reduced transients. This is also valid for other emission 114
control systems for example EGR (exhaust gas recirculation), where the governing (for both otto engines and diesel engines) is considerable simplified at steady-state operation of the engine. The greatest impact of reduced transients would be achieved for an engine with a completely steady state operation. One such example is the so called “Range Extender” which is basically an electric car equipped with a very small internal combustion engine, used in order to maintain the electric capacity of the battery. Unfortunately there are indications which show that the engine in a present day hybrid system is operated with the same number of transients as an engine in a conventional vehicle. Whether this is true or not is a matter for closer studies of the hybrid systems on the market. The number of transients may be the same but not the shape since the engine in for example the Toyota Prius according to Toyota (Hirose et al., 1998) is operated within a close area of the conventional engine working area (see Figure 41).
Figure 41. Engine operational area and exhaust temperature. Source: Hirose et al., 1998. Since the size of the engine in a hybrid system is reduced the average load of the engine is increased with the result that the average temperature would increase. This will result in a risk for higher strain of the catalyst unless it is designed for a higher degree of conversion. However, the authors of the report (Hirose et al., 1998) claim that the exhaust temperature distribution of their THS engine for Prius is lower than for a conventional engine. The greatest advantage with the reduced size of an engine controlled with a three-way catalyst system (TWC) is most likely that the heating of the system is faster which result in lower level of emissions during the warming up phase. For a diesel engine, however, a higher exhaust gas temperature is a great advantage since the temperature in the exhaust of a diesel engine is usually too low to achieve a reaction in the catalyst during lengthy periods. One example to point out is that during driving according to the low speed part of the European driving cycle (4 km of totally 11 km) the conversion in the catalyst for light-duty vehicles is very poor. This results in the average conversion ratio for the whole driving cycle being lower than 50%. With a hybrid system there is a great possibility to increase the conversion ratio in the catalyst. One disadvantage with a higher average load of an internal combustion engine is that these commonly result in an increased NOx emission from the engine (before the catalyst) which occurs in both otto and diesel engines. For otto engines the increase is caused by a higher rate of NOx formation* (increased temperature) and by the fact that EGR is not usually used at higher loads (in the case where the engine is equipped with this type of system). In the case of diesel engines the increased ratio of NOx formation is caused by the lower EGR rate (if an *
In the case above the increase of NOx emission before the catalyst can be in the order of a factor 4. Typical values are from 5 g/kWh to 20 g/kWh.
EGR system is used). For both engines there is a need to rate EGR at higher loads in order to decrease the ratio of NOx formation. This applies when these types of engines are to be used in hybrid systems or in vehicles equipped with continuously variable transmission (CVT) where the systems, in both cases, are optimized for a minimum of fuel consumption. The problem with the increased NOx emissions has also been observed by for example Ossis et al. (Ossis et al., 1996). Electric preheated catalysts have been developed in order to reduce the emissions from otto engines during cold start conditions. Even if the commercialization of these types of catalysts is not yet been carried out on a broad bases (BMWs V12 engine is, however, one example) it is quite clear that hybrid electric systems are advantageous in this respect. The situation is that even if the requirement of electricity has been drastically reduced (one tenth of what was originally needed in 6 to 7 years), more electric energy is needed than a commonly used battery can deliver in order to heat the catalyst. This is especially true for cold starts at low temperatures. Unfortunately there is an indication that the control strategies used today shut off the electric heating at considerable low temperatures in order to be able to meet the need for electricity to the start the engine. In the case of hybrid vehicles with their larger size of the battery there is no problem for the battery to deliver the electricity needed to heat the catalyst even at low temperatures (see also Section 8.2.5 about Nissan hybrid system). To summarize this section is can be underlined that a considerable potential exists for the reduction of emissions by the use of the hybrid technology. Moreover, for the hybrid system where the batteries can be charged with electricity from the mains the emissions can be reduced by the fuel consumption being reduced. This is of course valid under the assumption that the electricity is generated with low emission levels.
12.2 Emissions related to the hybrid system In this section the emission-related abilities of two of the above presented hybrid systems will be analyzed and discussed in somewhat more detail. It should be kept in mind that Mercedes – Benz by their new organization DaimlerChrysler has most likely replaced the use of an internal combustion engine with fuel cells. However there is certainly some interest in studying the result achieved with the M-B series hybrid vehicle equipped with an internal combustion engine. 12.2.1 The series hybrid from Mercedes-Benz It is well known fact that the potential to achieve low emission levels is highest for series hybrids. In the SAE report (Abthoff, J.O. et al., 1998) from Mercedes-Benz the result from emission tests on their series hybrid have been presented. In Figure 42 the result from these emission tests are shown and also the emission standards for ULEV and EZEV. From the figure it can be clearly seen that significant emission reductions have been achieved with the series hybrid vehicle. Compared with the European emission standards of today the California ULEV standards are considerable more stringent. The standards which are valid for year 2000 are somewhat less stringent than the ULEV standards and close to the California standards for LEV. The ULEV standards are, in comparison with the first generation of the first catalyst (TWC) cars in Sweden (model 1987-1989), considerable more stringent in that the levels of the standards are 5-10 times lower. In comparison with the ULEV standards the proposals for EZEV standards even more stringent in that the emission levels proposed are only 10% of the ULEV standards. From Figure 42 it can be seen that the emission levels of the tested M-B series hybrid vehicle are lower than the EZEV standards.
Figure 42. Emissions for Mercedes series hybrid vehicle (prototype). Source: Abthoff et. Al.
12.2.2 The parallel hybrid vehicle Prius from Toyota Emission data for the parallel hybrid vehicle Prius have been presented by Toyota (tests according to the Japanese 10-15 mode cycle) and by US EPA (UA EPA FTP-75 tests). Unfortunately no results from emission test according to the European test cycle have been available for this report. The results presented by Toyota can be seen in Table 22. Table 22. Japanese 10–15 mode emission standards and emissions for Toyota Prius. Emissions (g/km) Emission demand/vehicle
Japan 10 –15
If the above presented emission data was also valid for tests according to the European test procedure (and with the European specification of the vehicle), the emission performance would be much better than that of the vehicles which meet the current European emission standards and rather closer to the ULEV standards. Unfortunately there are too few hybrid vehicles available for carrying out the investigations which are required in order to establish a reliable emission level for hybrid vehicles based on the present day technology. In order to improve this short emission evaluation, data generated by US EPA (Hellman et al., 1998) has been summarized in a table in Section 12.3 where the relationship between the driving pattern of the vehicle and the emissions is discussed.
12.3 The relationship between the driving pattern and the emissions In the different sections above it has been underlined that the fuel consumption “savings” in a hybrid vehicle compared to a conventional vehicle are largest at low speeds and loads of the vehicle. A question was raised as to whether this is also true for the emissions. In order to get an indication, despite the lack of specific data needed, the emission data generated at US EPA was used for this purpose, in addition to that it was of interest to present emission values from tests according to the FTP-75 driving cycle. Results from the tests carried out by US EPA are summarized in Table 23 and Table 24. It should be observed that the values presented here are converted to SI units. Table 23. Result from emission tests according to the US EPA FTP-75 test procedure. Test date 980403 980417 980423 980424
FID HC g/km 0.037 0.031 0.037 0.037
NMHCE g/km 0.037 0.025 0.031 0.031
CO g/km 0.249 0.249 0.249 0.249
NOx g/km 0.031 0.031 0.044 0.031
CO2 g/km 112.5 110.0 113.1 108.1
Fc l/100 km 4.95 4.84 4.98 4.77
Net charg. * Amp-h. -0.285 -0.252 -0.132 -0.038
*Negative values are related to the net charges of the battery during the driving cycle. Source: Hellman et al., 1998. Table 24. Result from emission tests according to the US EPA HFET test procedure. Test date* FID HC g/km 980403 0.025 980403 0.025 980417 No analysis 980417 0.006 980423 0.006 980423 0.006 980424 0.012 980424 0.012
NMHCE g/km 0.025 0.025 No analysis 0.006 0.006 0.006 0.012 0.012
CO g/km 0.186 0.186 No analysis 0.124 0.124 0.124 0.124 0.062
NOx g/km 0.031 0.037 No analysis 0.025 0.037 0.031 0.025 0.031
CO2 g/km 110.0 101.9 No analysis 103.2 111.97 103.8 110.0 102.6
Fc l/100 km 4.835 4.472 No analysis 4.534 4.905 4.572 4.838 4.520
Net charg.**. Amp-h. -0.488 -0.08 -0.444 -0.052 -0.37 -0.014 -0.338 -0.097
*The first value in each column of HFET (Highway Fuel Economy Test – a test for measurement of fuel consumption) refers to the first test of FTP-75, the second value to the second FTP-75 test and so on (compare the test dates in the tables). In this case measurement of emissions and fuel consumption have also been carried out for the preparatory driving according to the HFET cycle, which is not a common procedure. ** Negative values are related to the net charges of the battery during the driving cycle. Source: Hellman et al., 1998.
The result of the tests carried out by EPA show that the emissions of HC at FTP-75 are, on average, more than 40% larger than the values presented by Toyota (Table 23) but as an average approximately 50% less than the Toyota values when driving according to the HFET cycle. It should be kept in mind that the data presented by Toyota were generated according to the Japanese 10-15 mode cycle (see Section 8.1.2). The emission of CO does not vary for the FTP-75 tests and is at the same level as the CO emission values reported by Toyota. For the HFET cycle the average values are nearly 50% less than the values reported by Toyota (compare Table 23 with table 24). On average, the emission of NOx is 0.034 g/km for the FTP-75 tests and 0.031 g/km for the HFET tests i.e. 37% and 24% respectively higher levels than the Toyota NOx values.
Comparing the results from FTP-75 (which represent a lower average speed than the HFET cycle) with the results from the HFET cycle there is an indication of a negative correlation between the average speed and the emissions i.e. the emission levels are higher at a lower average speed than at a higher speed. According to the tests carried out by US EPA the fuel consumption is considerable higher than data reported by Toyota (see also Section 8.2.3) Since there is a good potential for obtaining low emission levels and high fuel efficiency, the hybrid vehicle alternative can be seen as a more attractive alternative today than an electric vehicle. However, the development of the future may give the answer as to which of the alternative hybrid or electric vehicle is more attractive. If there will be a successful development and reasonable cost of fuel cell vehicles which work like a electric vehicle, the answer may be definitely “electric vehicle”. In line with the ranking for best fuel economy (see Section 8.1.4) the ranking can be carried out for the emission performance for conventional vehicles. To keep it simple, the ranking is carried out only for the emissions of NOx and particles. The result of such a ranking will be as follows: Conventional otto engine with TWC emission control. Advanced otto engine with direct injection (stoichiometric, EGR and TWC) Advanced otto engine with direct injection (lean-burn, EGR and deNOX) Diesel engine (direct injection) Comments: It is possible to carry out a similar ranking even for other emission components than for NOx and particles but then the picture will be more complicated depending on the technology used for emission control etceteras and it has been excluded for reasons of space.
13 SUMMARY OF COSTS A more thoroughly costs estimates was not really included in the enquiry for the preparation this report since there is no on-going production of hybrid electric vehicles and that the extra costs involved for the hybrid vehicles therefore are difficult to estimate. One cannot, however, ignore the fact that this type of vehicle involves the cost of a double driving system. Since there must also be batteries in the system the cost are further increased in the case of the batteries having to be very large. A cautious estimation is that, if a conventional vehicle, with the size of the Toyota Prius, now costs 150,000 Swedish crowns a cost up to 100,000 Sw.cr. would have to be added for a hybrid vehicle, even if there was a relatively large-scale production (10 000 units per year). Chrysler has estimated that their prototype (PNGV) would cost 15,000 dollars more (approximately 140,000 Sw.cr. in year 2000 exchange rate). This is however a considerable improvement on the previous prognosis from the same manufacturer of +60,000 dollars. To this it can be added that the difference in the actual cost for a Ford Focus flexible fuel vehicle and a Toyota Prius (both of them are to be sold in Sweden) is approximately 80 000 Swedish crowns (approximately $8 400 in the today’s exchange rate). A calculation of cost given by Mercedes for their previously mentioned hybrid system (see section 8.2.1) is shown in Figure 43.
Figure 43. Cost of various driving systems. Source Mercedes Benz. Comparison of production costs for various types of driving system (basis for calculations: 10,000 units) Conventional driving system 100%, Convention driving system with EZEV emission level 124% , Series hybrid (EZEV) 204%, Electric vehicle (ZEV) 226%, Source Mercedes Benz
As can be seen in Figure 43 the cost of a driving system for both a hybrid vehicle and a purely electric vehicle exceed the cost of a conventional driving system. In the future and when larger series are involved, the extra cost could probably be reduced to ca 25,000 Sw.cr., but even this extra cost can be too high to be able to motivate a large-scale use of the technique. It can be seen from the figure that a conventional driving system developed to meet the EZEV limits would be considerably less expensive. Since the parallel hybrid system has the potential
of being considerably cheaper than the series hybrid system, this would be a strong reason for it being more successful in the short and relatively long-term. The opinion of the authors concerning the hybrid system for passenger vehicles is that the extra cost of series hybrids will be far too high, within the foreseeable future (to 2010) for it to be able to compete with a conventional driving system. Even if a parallel hybrid system can be somewhat cheaper, it seems that the extra cost even for that system is also a considerable hinder. A number of technical break-throughs must thus take place for the hybrid system to be able to compete with a conventional system for passenger vehicles. The great potential for reduced fuel consumption will however probably lead to a great deal of interest for these driving systems in the future.
13.1 Cost of the System The above paragraph deals with a vehicle manufacturer’s calculated extra cost for a hybrid vehicle. A comparison of costs between a conventional driving system, a conventional driving system with very low emissions levels, a series hybrid with very low emission levels and an electric vehicle were discussed and shown in Figure 43. The discussion and the percentages shown in the figure clearly show that there will be a difference in costs between the various driving systems. In the table below, Table 25, an attempt has been made to give a rough estimation of the costs of certain vital parts of the hybrid system. These are stated as “high”, “medium” and “comp conv” (comparable with conventional vehicle). Table 25. Estimation of the cost levels for various details in the hybrid system/vehicle. Comments Equipment or detail in the High Medium * Compare convent.** hybrid system/vehicle Body + chassis Int. combustion engine Electric motor/generator Battery Control electronics Cont. variable transmission Production method
X X X X X X X
Use of light material Smaller but special Alternator unit Depends on type of battery Large function requirements Vital part of the system Several mom. at production.
*One has to calculate a larger or smaller extra cost, due to there being differences compared with a conventional vehicle **There can be certain differences from the cost of a conventional internal combustion engine the chassis/body respectively. Similar material will commonly be used in the conventional vehicles
These estimations are based on small series of vehicles.
13.2 Cost of the Batteries There is a great degree of uncertainty concerning what the batteries will cost, and this makes it very difficult to make a prognosis. As in most cases the costs depend on the amount of units which the manufacturer can sell. In the case of batteries there seems to be three things which affect the basic costs; the cost for a development and production unit; development costs; costs of materials. The cost for the development and production building and for the development of batteries can be greatly influenced by the number of items sold. However, the costs of materials will 121
not be affected in a positive direction other than in special cases. On the contrary, the cost of materials will increase if the number of items sold is large and there is a limited availability of material (see Table 26). Table 26. Evaluation of various batteries. Parameter Unit Lead NickelNickel-me- Zebra Acid cadmium tal hydride Specific energy Specific power Life cycles
No. of cycles at 80 % ”OEM” price Euro/kWh Factor of Wh x kW x “Merit” No. of Cycles/ “OEM” price
In a report (Cornu, 1998) “Factor of Merit” characteristics have been developed for a number of batteries and these factors have served as the basis for the determination of interest in these batteries. The interest among vehicle manufacturers in lead batteries was found to be lowest and for lithium-ion batteries it was found to be highest. (See Table 26).
13.3 Total costs Certain total costs can easily be calculated, such as the fuel costs and the cost of electricity if the batteries are to be charged from the mains. There are however three costs which are especially uncertain, namely; Depreciation cost for the vehicle; Costs for exchanging batteries; Service and maintenance costs. Since these three costs are so uncertain, at this stage of the development of hybrid vehicles, there is of no value for the reader of this report in the authors making any speculations about the costs. Hopefully certain experiences will be generated as soon as Toyota’s hybrid vehicle comes on the market in Sweden, among other places. In the case of series hybrids, which usually have a built-in possibility of being charged from the mains, this charging is expected to lead to lower running costs than if the batteries were to be charged from an internal combustion engine while being driven. This must however be weighed against the fact that, in this case, a greater battery capacity is required than in the case of a parallel hybrid, which is usually not equipped for charging from the net. Running costs can be affected in a way, which is related to the hybrid system/vehicle compared with running cost for a conventional vehicle, and in any case for the present day hybrid vehicles such as Toyota Prius, Ford’s hybrid vehicles, Nissan’s hybrid vehicles and certain others which have been developed recently. The investigation of the hybrid system, which has been presented in this report shows that there is a potential for a reduction in fuel consumption (or use of energy) when driving hybrid vehicle compared to driving conventional vehicles and especially for parallel hybrids in light-duty vehicles. Several of the hybrid vehicles have lighter weight material and other improvements have been made, which are positive for running costs. In the future we do not expect that the use of light weight material in vehicles and the fact that hybrid vehicles are smaller in size than the conventional 122
“average vehicle” will be unique. The driving system of a hybrid vehicle represents a greater mass than the driving system of a conventional vehicle. The question is whether and by how much the mass of the driving system of a hybrid vehicle can be reduced compared to a conventional vehicle.
14 EFFECTS ON HEALTH Health risk estimations have long been used as an aid in investigating the need for limiting emissions from traffic. Some major studies of health risks have been carried out in Sweden, as well as some of lesser magnitude. To name just two of the major studies, one investigation was conducted by a parliamentary committee, “The Swedish Government Committee on Automotive Air Pollution” (Motor Vehicle and Cleaner Air, 1983), and comprised a medical assessment of various exhaust gas pollutants under the leadership of Professor Lars Friberg. The other large-scale study, “The Swedish Ambient Air Project” (Environmental Health Perspectives, 1994), led by Professor Jan-Åke Gustafsson, which in reality took 12 years to carry out, dealt with cancer risks associated with exposure to air pollutants in built-up areas. The Swedish Environmental Protection Agency initiated and financed the latter research project in the different fields of automotive emissions, characterization of pollutants in the exhaust, biological testing and health risk assessments. When studying the final reports from the project, it should be kept in mind that the material on which the final report is based was developed during the 1980s and 1990s as well as during the late 1970s. Many important steps have been taken since then, including positive changes in the fuel matrix and in vehicle emission performance, in order to improve the air quality in populated areas. Consequently, it is of no benefit in this presentation concerning health effects to go too deeply into the results from these studies. However, it should also be mentioned that the substances found in the exhaust and also in the air (in 1983) are more or less the same at the present time but at a lower level. In addition, it can be noted that since 1983, one pollutant in the exhaust gas from vehicles has been nearly completely eliminated in Sweden, namely emissions related to lead in gasoline. With respect to the above discussion concerning the substances, it can be of value to quote a passage from the Vehicle Exhaust Gas Committee’s report where it was pointed out that, “The various substances causing air pollution can produce different types of effects on human health. They may have effects of either an acute or chronic nature on lungs and respiratory passages”. “Moreover, different substances can enter the bloodstream, where they can have an adverse effect on virtually any bodily organ (toxaemia)” It can also be of some value to refer to a table presented in the final report from the Swedish Ambient Air Project, which provides a comparison of estimated cases of cancer per million inhabitants in the USA and in Sweden (see Table 25). For scientists in Sweden doing research in the field of heath risks related to exhaust from automobiles, it has been of advantage to refer to experiences and data generated in the USA, since many important health effect studies have been carried out in both the federal USA and in California. Evaluations of health risks caused by various substances in the environment are based firstly on results of experiments in which human, animal or biological test systems were exposed to the substances of concern, but also on epidemiological studies by which the health of different sectors of the country’s population is studied. While not going more deeply into these studies, it can nevertheless be said that both have considerable shortcomings and it is therefore difficult to draw accurate conclusions from them. One problem in experimenting on animals and humans is that only a limited number of individuals can be examined. Experiments on humans can be performed only for short periods and at moderate levels of exposure. This makes it difficult to clearly establish the effect thresholds for various substances on the basis of experimentation.
Since animals experiments are time consuming and costly to perform, methods for short-term bioassay tests have been developed and extensively used in order to compare the total mutagenic activity of samples taken in the motor vehicle exhaust and in ambient air. In Sweden, for example, two types of short-term biological tests have been used of which one is the well known Ames test in Salmonella typhimurium, which has been shown to be a useful tool. The other test method used involves the dioxin receptor affinity tests, a less known biological test which, however, can be seen as a complement to the Ames salmonella test. Studies have shown that extracts of particulate matter and samples taken in the semi-volatile phase in motor vehicle exhaust and in ambient air contain components with high affinity for the receptor in rat liver which specifically binds 2,3,7,8-tetra chloro dibenzo-p-dioxin (TCDD). Both of these biological test methods are described in a report (Westerholm and Egebäck, 1991), as well as elsewhere. Concerning epidemiological studies, there are two main approaches. In one, a section of the population suffering from a specific effect is compared with a reference group that is unaffected, and an attempt is also made to try to determine the various factors, such as exposure to air pollution, that explain the difference. The second approach is to compare population groups exposed, for example, to different levels of air pollution in order to find out whether the state of health of the two groups differs and, if so, to determine whether differences in exposure can provide an explanation. Table 27. Estimate of risks for yearly incidence of cancer associated with air pollution.. Source: Möller et al., 1994). Cancer cases/year/million inhabitants a Pollution in the air USA Swedenb Particulate phase POM, PAH [as B(a)P] 1.0 11.6 POM .[and different equivalents], all cancer cases -34.9 Gas phase 5.8 0.9-1.0 1-3 Butadiene 0.6 0.4-0.6 Benzene, leukemia 1.2 0.4-0.6 All cancer cases 2.9 0.1-0.2 Formaldehyde