Hybrid Vehicles

Hybrid Electric Vehicles: The Next Generation Automobile Revolution Prof. Ravindra P. Joshi and Prof. Anil P. Deshmukh* Abstract – Electric vehicles have been around since the inception of the automobiles. But the internal combustion engines turned out to be the most suitable for automobiles due to availability of cheap fuel at that time, high energy density of gasoline, robust design, longer driving range, no battery charging requirements, rapid acceleration requirements of an automobile. Over a period of time, due to rising fuel costs, shortage of fuel and also due to the pressing need to reduce the air pollution, electric hybrid vehicles appear to be a clean and green alternative to IC engines. In this paper, various battery technologies such as Lead-acid batteries, Nickel Metal Hydride (Ni-MH) batteries, Zinc-air fuel cells are evaluated in terms of technical and commercial viability for an electric vehicle application. Performance of electric vehicle is simulated under various speeds, gradients etc. and it can be concluded that Zinc-air fuel cells appear to be the best possible source of energy for hybrid electric vehicles as of today.

Over the past two decades, power electronics has been developed that enables ultra-efficient electric motor propulsion with regenerative braking. Permanent magnet motors and controllers are available from various developers that can provide ideal EV propulsion.

Index Terms—Battery powered vehicles, torque, energy density, fuel cells, IC engine.

In spite of the above technical advantages and environment friendliness of the electric vehicles, they are not yet successfully commercialized since it requires confluence of technology, market, economic and political factors that could transform electric vehicles into an attractive choice for consumers. Factors which affected successful commercialization of electric vehicles were, weight and volume of batteries, high initial cost as well as high battery maintenance costs, safety and security considerations of batteries, less driving range and lack of proper infrastructure to support battery charging.

Major advantages of electric vehicles are they are energy efficient, environment friendly, offer better performance, reduce dependency on fossil fuel, balanced weight distribution can be achieved, less maintenance required, operating cost is substantially less, they produce less noise and vibrations. But there are a few disadvantages as well, such as limited driving range per charge due to limited energy storage capacity, longer recharge time, high initial battery cost, high battery maintenance/ replacement cost, bulky and heavy weight as compared to gasoline vehicles.

1. INTRODUCTION Electric or battery powered vehicles have been in the research focus for many years. A lot of research has been carried out for numerous years in order to achieve technical goals such as optimizing efficiency of the vehicle propulsion system as well as battery system, achieving longer distances per charge, rapid acceleration, high torque at low speeds etc. Technology advancements can be summarized in four major areas such as advancements in power electronic drive systems, battery technology, electronic control systems, materials and body structures of electric vehicles. Advancements in power electronics and inventions of new power electronic devices such as thyristors, IGBT’s, GTO’s etc. for heavy duty power applications has led to the possibility of controlling large amounts of power with the help of electronic circuits. This coupled with advancements in VLSI electronics, microelectronics have made controlling of the electric vehicle much more efficient and reliable.

Hybrid electric vehicles are being developed which can combine the benefits of IC engines and electric motors and can be configured to obtain different objectives, such as better fuel economy, less pollution, reducing wastage of energy, recapturing energy during braking, reducing size and capacity of IC engine, reduce idling time of engine etc. Hybrid vehicles carry a relatively smaller IC engine, rechargeable energy storage system (an onboard electric generator along with batteries) and electric propulsion system. In hybrid electric vehicles, operating range of an IC engine can be used in more optimum manner for electricity generation and charging batteries, since IC engine can operate at its maximum efficiency point at high speeds rather than delivering power to the mechanical transmission system over a considerable speed range.

* Authors are faculty members of the International

Institute of Information Technology (I2IT), Hinjawadi, Pune – 411057. 0-7803-9794-0/06/$20.00 ©2006 IEEE

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When cruising or on flat road, where light thrust is needed, full hybrids can use the IC engine to generate electricity by spinning an electrical generator to either recharge the battery or directly feed power to an electric motor that drives the vehicle. This is different than a pure battery operated electric vehicle. Hybrid vehicles still require fuel source, but it can be gassoline or diesel or ethanol or hydrogen. There are two types of hybrid vehicles: series hybrid and parallel hybrid. Their functioning and block diagrams are mentioned below.

Engine Clutch

Transmission Clutch Batteries

Fig.2 : Parallel Hybrid vehicle

Series Hybrid Vehicle: In this type, the IC engine drives an electric generator, which can charge the batteries or power an electric motor that is connected to the transmission system as per the driving conditions. Electric motor alone powers the transmission under light loads and when starting from rest and battery provides additional drive power. During deceleration/braking, electric motor acts as a generator and the electricity generated is used to charge the batteries. This can save the mechanical energy during deceleration, which is otherwise wasted in brake drums. Fig. 1 shows the block diagram of series hybrid vehicle.

Engine

Generator

Batteries

Motor

Motor

Some of the advanced technologies typically used by hybrids include, 1) Regenerative Braking- The electric motor applies resistance to the drive train causing the wheels to slow down. In return, the energy from the wheels turns the motor, which functions as a generator, converting energy normally wasted during coasting and braking into electricity, which is stored in a battery until needed by the electric motor. 2) Electric motor Drive/assist- The electric motor provides additional power to assist the engine in accelerating, passing or hill climbing. This allows a smaller, more efficient engine to be used. In some vehicles, the motor alone provides power for low-speed driving conditions where IC engines are least efficient.

Transmission

3) Automatic start/ shut off: It automatically shuts off the engine when the vehicle comes to a stop and restarts it when the accelerator is pressed. This prevents wasted energy from idling.

Fig. 1: Series hybrid vehicle

II. DESIGN CONSIDERATIONS The nature of electric vehicles is governed mainly by the characteristics of the batteries used. The weight and volume of the batteries is usually a deciding factor. For a car, usually the battery weight could range from 300 to 600 Kg and they require a 200 to 300 Liters of adjoining volume. Recapturing kinetic energy with regenerative braking during deceleration can neutralize negative effect of battery weight for urban driving. Hence electric vehicle will be always preferred for urban driving with a limited drive range and for slow to medium speeds. For long distance driving, electric vehicle will not be able to match the performance of conventional IC engine in terms of distance

Parallel Hybrid Vehicle: In this type, the IC engine drives an electric generator and the transmission system at the same time. It is capable of working simultaneously with the motor or independently. Fig. 2 shows block diagram of a parallel hybrid vehicle.

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traveled in a single charge and quick refueling time.

zinc oxide and water is released back into the system.

1) Selection of battery:

Zinc-air batteries have properties of fuel cells as well as batteries. The zinc is the fuel and the rate of the reaction can be controlled by controlling the air flow, and used zinc/electrolyte paste can be removed from the cell and replaced with fresh paste [3].

Several choices are available such as Lead-acid (PbA) batteries, Nickel Metal Hydride (Ni-MH) batteries, Zinc-air batteries etc. The choice of battery depends on several factors such as energy density, energy/weight, energy/volume, energy/cost, power/weight ratios, battery efficiency, initial and maintenance cost, availability of batteries in a particular country etc. [1] Table 1 shows typical values of battery efficiency, energy/weight, energy/volume ratios for Lead-acid batteries from manufacturer’s specification sheets. Lead acid Batteries Energy/weight 30-40 Wh/ Kg Energy/volume 60-75 Wh/Liter Power/weight 180 W/ Kg Efficiency 70 to 90 % Energy/$ 5 to 8 Wh/$

Fig. 3 Zinc-air battery These batteries also have very high energy densities and are relatively inexpensive compared to Ni-MH batteries. Table 3 shows typical values of battery efficiency, energy/weight, energy/volume ratios for Zinc-air batteries from manufacturer’s specification sheets.

Table 1: Lead-acid battery specifications

Zinc-air batteries Energy/weight 200 Wh/ Kg Energy/volume 220 Wh/Liter Power/weight 200 W/ Kg Efficiency 50 to 60 % $/ KW $ 205/- per kW

Nickel Metal Hydride (Ni-MH) battery is a type of rechargeable battery where hydrogen absorbing alloy is used for the anode. They have high energy/weight and energy/volume ratios compared to Lead-acid batteries, hence they are very compact but also very expensive. Table 2 shows typical values of battery efficiency, energy/weight, energy/volume ratios for Ni-MH batteries from manufacturer’s specification sheets.

Table 3: Zinc-air battery specifications Fuel cells are designed for continuous replenishment of the reactants consumed, i.e. it produces electricity from external supply of fuel and oxygen as opposed to limited internal capacity of a battery [4]. They make use of reactants such as hydrogen on the anode side and oxygen on the cathode side (e.g. hydrogen cell). Usually reactants flow in and reaction products flow out. This can be maintained over very long periods of time as long as the flows are maintained. One disadvantage is that they have low overall efficiency (only about 50 to 55% as compared to 80% efficiency of Leadacid batteries). A lot of research is going on in the fuel cell technology. Table 4 below shows technical targets of fuel cell technology to be achieved by year 2015. This fuel cell technology could turn out to be a low cost, light weight and highly efficient alternative to the conventional batteries to be used for automobiles in future [7].

Ni-MH Batteries Energy/weight 30-80 Wh/ Kg Energy/volume 140-300 Wh/Liter Power/weight 250-1000 W/ Kg Efficiency 60 to 70 % Energy/$ 1.25 to 2 Wh/$ Table 2: Ni-MH battery specifications Zinc-air batteries (or sometimes referred as Zinc-air fuel cells) are non-rechargeable electrochemical batteries powered by oxidation of Zinc with oxygen from the air. The cell comprises of central static replaceable anode comprising of electrochemically generated Zinc particles in a potassium hydroxide solution. Water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zincate at which point electrons are released that travel to the cathode. The zincate decays into

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Technical targets (Hydrogen Fuel cell system) System Level

Hydrogen storage

2010

2015

Efficiency

%

60

60

Cost

$/ kW

45

30

Specific energy density

kWh/Kg

2

3

Energy density

kWh/L

1.5

2.7

Cost

$/ kWH

4

2

Refueling rate

KgH2/min

1.5

2

For the purpose of simulations, we have assumed a four wheeled, 19 seat capacity vehicle, with an unladen weight of 2000 Kg. It is considered because it can be a potential candidate for carrying urban traffic in a metropolitan city. III.

SIMULATION OF HYBRID ELECTRIC VEHICLE

Simulation is done using an empirical, but well known formula, Power = a*W*S + b*F*S3 + G*W*S Where, a = Coefficient of rolling resistance (can be considered between 0.008 to 0.01 for all practical purposes with rubber tyres)

Table 4: Technical targets for Fuel cell research 2) Selection of drive system:

W = Weight of the vehicle in Newton

Drive system includes selection of drive motor, motor controller unit, gear box assembly, transmission system etc. Location of these components also play a vital role since weight balance needs to be achieved in order to maintain stability of the vehicle.

S = Speed of the vehicle in m/sec. b = Drag factor (Kg/m3) considered as 0.6 for a square shaped front portion of a vehicle. F = Front area in sq. meters (this value is considered as 7 sq. m for a 19 seater vehicle).

3) Design of chassis: This includes selection of materials for chassis, deciding on the size and shape of chassis, ensuring stability and weight balance, while maintaining the aerodynamic shape and structure, minimizing drag coefficient etc.

G = Gradient in percentage (values considered are 0%, 3% and 6%) Appropriate values of coefficients are used and values of power required from the vehicle, battery weight and battery volumes are obtained for Leadacid batteries, Ni-MH batteries and Zinc-air batteries. Battery efficiency values are considered to be 80% for Lead-acid batteries, 65% for Ni-MH batteries, and 55% for Zinc-air batteries after referring to the specification sheets of these batteries. Electric vehicle speed is considered from 20 Kmph to 80 Kmph at gradients of 0%, 3% and 6% which are most common in practice. Table 5 shows the results. It is assumed that the vehicle weight is 2000 Kg and distance covered per charge is 100 Km for all the speeds. Hence battery energy capacity can be calculated and the number of batteries, weight and volume of batteries required can be estimated.

4) Simulations under different operating conditions: Before making selection of batteries and other electrical and mechanical components as mentioned above, it is extremely important to study the behavior of the electric vehicle under different operating conditions such as cruising at different speeds on a plain surface, effect of rolling resistance and drag coefficient, or climbing a gradient. The calculations below tremendous amount of variation in power requirement from the vehicle at these varied operating conditions. Especially on gradient, vehicle requires tremendous amount of extra power which must be catered to while designing a vehicle [5]. Especially on gradient, vehicle requires tremendous amount of extra power which must be catered to while designing a vehicle.

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By applying the above formula at various vehicle speeds and with various gradients of 0%, 3% and 6%, respectively, all the results are calculated and are shown in Tables 5,6 and 7 respectively in Appendix.

Sample calculation for a Zinc-air battery is shown below, Zinc-air battery cell:

1Vdc, 280 AH

Zinc-air battery:

12 cells connected in series (12Vdc)

Comparison of battery weight and battery volume required for the same distance covered (in our case it is 100 Km), shows that the trade-off exists between the battery weight and volume. For example, Ni-MH batteries are heavier but they consume less volume; while Zinc-air batteries are lighter but require more volume. The electric vehicle designer will have to make a choice while selecting batteries regarding the weight, volume and cost of the batteries.

One string of Zinc-air battery: 12 Zinc-air batteries in series (144 Vdc) Total energy of battery bank:

3*144*280 = 120.96 KW-hours (3 strings)

Assuming 55% efficiency of the battery system, energy converted into useful work is, Net energy = 120.96 * 0.55 = 66.53 KW-hours

IV. CONCLUSION

If an electric vehicle consumes 15 KW of power, the net energy will last for 4.4 hours.

In this paper, various battery technologies such as Lead-acid batteries, Nickel Metal Hydride (NiMH) batteries, Zinc-air batteries and fuel cells are evaluated in terms of technical and commercial viability for an electric vehicle application. Performance of electric vehicle is simulated under various speeds, gradients etc. It can be concluded that Zinc-air batteries appear to be the best possible source of energy for hybrid electric vehicles as of today. But with promising inventions in fuel cell technology, it could turn out to be a low cost, light weight and highly efficient alternative to the conventional batteries to be used for automobiles and future cars.

If the vehicle speed is considered as 40 Kmph, it will travel distance of 177 Km. # In this way we can estimate the distance covered by a vehicle per one charge. Depending on the speed of the vehicle, distance covered will vary for a given battery energy capacity. Hence if the desired distance per charge is decided, the battery energy capacity as well as number of batteries required can be found out for a given Ah capacity of the battery [2]. Following calculation is used for finding out the power required by an electric vehicle at a given speed and gradient condition,

V. REFERENCES [1]

Thomas B. Gage, “Lead-acid batteries: Key to Electric Vehicle Commercialization”, Long Beach Battery Conference, 2000.

Power required = a*W*S + b*F*S3 + G*W*S

[2]

D.V. Battul, “Study of Electric vehicle performance” Conf. Rec. IEEE/IAS, 1999.

Where,

[3]

Zinc-air battery specifications, Electric Fuel Corporation, Delaware, USA.

[4]

John Cooper, “Powering future Vehicles with the Refuelable Zinc-air battery”, Science and Technology review, October 1995.

[5]

J. Russell Lemon, “Simulation of an Electric Vehicle”, July 1993.

[6]

E. J. Carlson, P. Kopf, J. Sinha, S. Shriramulu and Y. Yang, “Cost Analysis of PEM fuel cell systems for transportation”, National Renewable Energy Laboratory, Sept. 2005.

[7]

K. Kamlesh Kumar Sethy, K. Sandeep Prakash, Prof. Anil P. Deshmukh, “Study and review guidelines for use of CFD softwares for Fuel cell model tools”, School of Engineering, International Institute of Information Technology, Pune.

For a gradient of 3% and speed of 60 Kmph:

a = coefficient of rolling resistance = 0.01 W = weight of the vehicle = 2000 Kg * 9.8 = 19600 Newton S = speed of vehicle = 60 kmph = 16.67 m/sec b = drag factor = 0.6 for a square edged shape F = Front area of vehicle = 8 * 9 = 72 sq. feet i.e. approximately, 7 sq. m. Power required = (0.01*19600 * 16.67) + (0.6 * 7 * 16.673) + (0.03* 19600 * 16.67) = 32.524 KW #

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VI. BIOGRAPHIES Anil P. Deshmukh is a fellow of IIPE, Pune Chapter. He has completed his Bachelor’s degree from Indore University & pursued his M.Tech from Indian Institute of Technology, Kharagpur, India. He has over 31 +years of experience in Industries with 20+ years with Auto Majors like TATAS, M& M ,Greaves Auto Ltd & was teaching for sizable period to UG& PG’S at Engineering College in the areas of Mechanical , Production & High Tech Subjects like Robotics, Cad/ Cam, Quality Engg. He has performed from Jr Executive to VicePresident positions in Industries during 31+ years. .He was CoPatentor on Tools Profile & presented 12 National Papers & One International at Singapore for STLE , (1997). He is working as senior Professor - Automotive Engineering at International Institute of Information Technology, Hinjawadi, Pune for a year & running M.SAE -Program with LTU , USA

R. P. Joshi is an IEEE member since 1990. He has completed his Bachelor’s degree from Pune University and then pursued his M.S. degree from the University of Tennessee, Knoxville, USA. He has total of 19 years of work experience, out of which 9 years from various industries and 10 years of academic experience in the areas of power electronics, analog and digital electronics, Networking and Telecommunications. He has worked in Singapore for more than 8 years in power electronics as well as computer networking areas. He is working as an Associate Professor in the International Institute of Information Technology, Hinjawadi, Pune for the past 3 years.

VII. APPENDIX Km/ Hr

Power required (kW)

kWh rating

Weight (Kg)

Volume (Ltr.)

From vehicle

From Lead Acid Batteries

From NiMH Batteries

From Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

20

1.80

2.26

2.78

3.29

11.3

13.9

16.45

282

173

82

150

46.3

74

40

7.91

9.90

12.18

14.4

24.75

30.45

36

618

380

180

330

101.5

163

60

22.72

28.4

35.0

41.3

47.43

58.45

68.97

1185

730

344

473

194

313

80

50.43

63.04

77.6

91.7

78.8

97

114.63

1970

1212

573

1050

323

521

Table 5: Km/ Hr

Power required (kW)

Results with 0% gradient

kWh rating

Weight (Kg)

Volume (Ltr.)

From vehicle

From Lead Acid Batteries

From NiMH Batteries

From Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

20

5.07

6.337

7.80

9.22

31.68

39.0

46.09

792

487.5

230.5

422.2

130

209.5

40

14.45

18.06

22.23

26.26

45.14

55.57

65.65

1128

694.6

328.3

601.8

185.2

298.4

60

32.53

40.66

50.03

59.14

67.9

83.55

98.75

1697

1044

493

905.3

278.5

448.8

80

63.39

79.23

97.52

115.3

99.0

121.9

114.1

2475

1523

720

1320

406

655

Table 6: Km/ Hr

Power required (kW)

Results with 3% gradient

kWh rating

Weight (Kg)

Volume (Ltr.)

From vehicle

From Lead Acid Batteries

From NiMH Batteries

From Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

Lead Acid Batteries

Ni-MH Batteries

Zinc Air Batteries

20

8.32

10.4

12.8

15.14

41.62

64.0

75.67

1040

800

378

555

213.3

343.9

40

20.97

26.21

32.27

38.13

65.54

80.66

95.33

1638.5

1008

476.6

873.8

268.8

433.3

60

42.33

52.9

65.12

76.96

88.34

108.75

128.5

2208.5

1359

642.5

1177.8

362.5

584.1

80

76.54

95.67

117.75

139.16

159.77

147.18

173.9

3990

1839

869.8

2130.2

490.6

790

Table 7:

Results with 6% gradient

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