Variable Compression Ratio Engines

The Project Report on

VARIABLE COMPRESSION RATIO ENGINES Submitted in partial fulfilment of requirements for the term work of the subject

(IC ENGINES) [TE Mechanical – SEM: V]

By Sr. No.

1. 2. 3.

Name Viraj Dabhade Shubham Dere Sameer Dode

Roll No. 33 36 38

Under the guidance of (Prof. Swapnil Kadam)

(Department of Mechanical Engineering)

Don Bosco Institute of Technology Mumbai University (2014-2015)

Table of Contents

Sr. No

Title

Page no

Abstract

2

1.

Introduction

3

2.

Analysis of VCR engine 2.1 Need of VCR engine

4

2.2 Methods of Obtaining VCR

5

2.2 Control Strategy for VCR

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2.4 Benefits of VCR

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3.

Limitations

10

4.

Conclusion

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5.

Future scope

6.

5.1 Porsche’s Patented Design

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5.2 Future Activities

13

References

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Abstract Conventional gasoline engines operate at a fixed compression ratio, which is set low enough to prevent premature ignition of the fuel, or “knock,” at high power levels under fast acceleration, high speeds, or heavy loads. However, gasoline engines usually operate at relatively low power levels under slow acceleration, lower speed, or light loads. If the compression ratio were increased at low-power operation, gasoline engines could achieve higher fuel efficiency. A variable compression ratio (VCR) engine is able to operate at different compression ratios, depending on the particular vehicle performance needs. The VCR engine is optimized for the full range of driving conditions, such as acceleration, speed, and load. At low power levels, the VCR engine operates at high compression to capture fuel efficiency benefits, while at high power levels, it operates at low compression levels to prevent knock. To further improve fuel economy, the VCR engine is small, with about onethird the displacement volume of a conventional gasoline engine. A supercharger boosts engine peak power when needed for occasional hard acceleration or hill climbing. Increasingly stringent emissions and fuel economy standards have long remained a source of challenges for research in automobile engine technology development towards the more thermally efficient and less polluting engine. Variable compression ratio (VCR) technology has long been recognized as a method for improving the fuel economy of SI engines. Spark ignition (SI) engines have lower part-load efficiency when compared with the diesel engines .To improve this efficiency operate SI engine with high compression ratio at part load and with low compression ratio at high load. So we proposed a design of modified connecting rod to increase or decrease the compression ratio at two stages instead of continuously varying the compression ratio .By this method we can able to get a high share of the potential fuel savings in comparison to other variable system. The connecting rod is modified by designing two hydraulic cylinders in its main body with pistons connected with the eccentric, at small end of connecting rod to achieve desired compression ratio.

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Chapter 1

Introduction

Worldwide pressure to reduce automotive fuel consumption and CO2 emissions is leading to the introduction of various new technologies for the automotive engine. The concept of variable compression ratio (VCR) promises improved engine performance, efficiency, and reduced emissions. VCR is identified as the key enabling technology of downsized engines. The search for a feasible VCR engine has been driven by the compromise between WOT (Wide Open Throttle) and part-throttle which exists on any fixed CR engine. Variable Compression Ratio (VCR) is a system which is used to adjust the compression ratios of the internal combustion engine. In simple terms, it changes the combustion chamber size of the cylinder according to various different operating conditions such as speed, load, acceleration and torque. Car emissions and fuel economy are two challenges for the automotive sector in which VCR engine is a very decent technology approaching low fuel consumption and pollutant emission reduction. The car manufacturers have to look forward for more thermally efficient and less polluting engine. In present world the automotive sector spends over millions of dollars for approaching low emission as well as low fuel consumption cars. Various different ways are been discovered such as hybrid cars, fuel cell cars, solar cars and many more as future development. VCR engine would practically prove to be boon for automotive sector (Evolution Perspectives). The Fixed Compression Ratio (FCR) engine has a fixed compression ratio without any kind of change in the size of the combustion chamber in cylinder. The FCR engines have high emissions due to the fact that when it comes to high speed or load, FCR use more fuel which produces more emissions but VCR engine provides increase in the fuel efficiency under varying loads and speed. Most of the cars used recently are Spark plug (SI) or diesel Ignition engines. The different ways the SI engine can increase its efficiency is by higher compression ratio, reducing throttle losses, low friction, variable timing valve and down-sizing. The concept of VCR engine significantly contributes its benefits to thermo-dynamic efficiency. The concept of the VCR is that it continuously operates at different compression ratio as per the need of the performance. The change in the combustion chamber volume continuously takes place with the varying compression ratio. Therefore the thermodynamic advantages are been encountered through the engine map. At low level of power, VCR engine drives at higher compression ratio which can capture high fuel efficiency and at the high power levels the VCR engine runs at low compression ratio to prevent knocking. Unlikely as FCR engine, the VCR engine keeps the engine temperature more or less at the optimum with high combustion efficiency. It tends to keep high power at the same engine dimensions.

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Chapter 2

VCR Engine Analysis

2.1 Need of VCR Engine Variable Compression Ratio (VCR) is a system which is used to adjust the compression ratios of the internal combustion engine. In simple terms, it changes the combustion chamber size of the cylinder according to various different operating conditions such as speed, load, acceleration and torque. Car emissions and fuel economy are two challenges for the automotive sector in which VCR engine is a very decent technology approaching low fuel consumption and pollutant emission reduction. One of the key features affecting thermal efficiency is the compression ratio which is always compromise in fixed compression ratio spark ignition engines if the compression ratio is higher than the design limit the fuel will pre ignite causing knocking, which could damage the engine. Generally, the operating conditions of SI engines vary widely, such as stop and go city traffic, highway motoring at constant speed, or high-speed freeway driving. Unfortunately, most of the time SI engines in city driving conditions operate at relatively low power levels under slow accelerations, low speeds, or light loads, which lead to low thermal efficiency and hence higher fuel consumption. As the engine load decreases, the temperature in the end gas drops, so that high compression ratio could be employed without the risk of knocking in naturally aspirated or boosted engines. Raising the compression ratio from 8 to 14 produces an efficiency gain from 50 to 65 per cent , whereas going from 16 to 20 produces a gain from 67 to 70 per cent fig shows the effect of compression ratio with respect to thermal efficiency.

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      

Need of high specific power output accompanied by good reliability and longer engine life. Use of high pressure turbo charging results induces high thermal load. Turbocharger doesn’t have good adiabatic efficiency. High peak pressure problem occurs at full load. Can be minimized by reducing CR. But CR should be sufficiently high for good starting and part load operation. VCR concept is beneficial in low load, for better multi fuel capacity

2.2 WAYS TO MODIFY CR 1. 2. 3. 4. 5. 6.

Moving the cylinder head. Variation of combustion chamber volume. Variation of piston deck height. Modification of connecting rod geometry. Moving the crankpin within the crankshaft (effectively varying the stroke). Moving the crankshaft axis.

A: articulated cylinder head B: hydraulic pistons C: eccentrics on bearings

E: additional piston in cylinder head

D: multilink rod-crank mechanisms

F: gear-based mechanisms

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1.Moving the cylinder head The moving head concept (Saab Automobile AB) combines a cylinder head with cylinder liners into a monohead construction, which pivots with respect to the remainder of the engine. The lower half of the block includes the crankcase and engine mounts, and carries the crankshaft, gear box, oil cooler, and auxiliaries. The upper half includes the cylinders, their liners, camshafts, and an integrally cast cylinder head. This part is referred to as the monohead. Saab has enabled a tilting motion to adjust the effective height of the piston crown at TDC. The linkage serves to tilt the monohead relative to the crankcase in order to vary the TDC position of the piston. By means of actuator and linkage mechanism the compression ratio can be varied from 8 to 14. A screw type supercharger provides a 2:1 boost pressure when wide open throttle conditions occur. This system gives wide fuel flexibility, with reduced CO2 emissions proportional to fuel consumption. Saab recognized that the fuel efficiency of the VCR engine would be low without high-pressure supercharging.

Variation of Combustion Chamber Volume Using A Secondary Piston Or Valve

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2.To vary combustion chamber volume by using a secondary piston or valve. The piston could be maintained at an intermediate position, corresponding to the optimum compression ratio for a particular condition. The volume of the combustion chamber is increased to reduce the compression ratio by moving a small secondary piston which communicates with the chamber, however, this would require a finite length bore in which the piston could travel, which raises questions of sealing, packaging, and durability.

Ford VCR Engine Varying combustion chamber geometry compromises the area available for intake and exhaust valves, while moving the cylinder head and barrel is feasible in a research engine but harder to accomplish in a production vehicle. The cylinder head cooling needs to be improved by an efficient cooling system and the auxiliary piston needs proper lubrication for efficient functioning of the VCR engine.

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2.3 CONTROL STRATEGY FOR VCR The following figure presents a 2-D representation of the Pro-drive strategy for boost and CR response to variations in load (driver demand).

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From the above figure, it can be seen that points 1, 2, 4, 5 lie on the plane of low compression. Point 2 lies on the plane of high compression. The engine is started at low CR and zero boosts (point 1). When the driver accelerates, load and boost increase to point 2.When the driver throttles back into a light load cruise (point 2), load and boost reduce and CR increases. When the throttle is re-opened from this condition, CR reduces as boost and load increase, reaching point 4 and, ultimately, point 5 (WOT).For simplicity, the figure assumes only 2 available CR values (high and low). The same logic can be applied to intermediate values of CR by considering the transition between intermediate planes of CR.

2.4 BENEFITS OF VCR    

 

          

Hence the important benefits of the VCR engine can be summarized as follows: — Optimum combustion efficiency in the whole load and speed range. Low fuel consumption and low exhaust emissions. The VCR provides better control over pollutant generation and after-treatment than a conventional fixed compression ratio (FCR) engine also extends the life expectancy of a three way catalytic converter. As the geometrical volumetric ratio is under control on VCR engines, the engine always operates below the knock limit, whatever the load. The VCR engine provides excellent fuel flexibility, since the compression ratio can be varied and adjusted to suit the properties of the fuel, and therefore the engine will always run at the compression ratio best suited to the fuel being used for bi-fuel (compressed natural gas (CNG)/gasoline) power-trains; the realization of VCR is of specific interest. High fuel flexibility, with optimal combustion efficiency. Very smooth idle and full load accelerations are achieved. It provides better indicated thermal efficiency than that of FCR engines. It allows for a significant idle speed reduction because of reduced misfiring and cyclic irregularities, resulting in low vibration levels. Reduction in low-frequency noise because of constant peak pressures. Smoother combustion because the rate of heat release is the same (short) both at low and high compression ratios. Cold starting emissions can be reduced greatly by early catalyst warm-up in the catalytic converter. Improvement in the low end torque of a petrol engine without the risk of detonation. Potential technology for future high-boosting super lean burn engines. Low CO2 emissions by down-sizing for the same power output. Good idling performance at low ambient temperatures. Constant frictional losses owing to almost constant peak pressures.

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Chapter 3

Limitations

Variable compression ratio engines have not yet reached the market, despite patents and experiments dating back over decades. Indeed, several prototypes of VCR engines and vehicles have been tested. In many cases, the deviation from conventional production engine structure or layout represents a significant commercial barrier to widespread adoption of the technology. Some of the commercial barriers are listed below:1. The available methods require major changes to the base engine architecture or layout and represent significant commercial barriers to widespread adoption of the technology. 2. Introduction of additional elements within the crowded combustion chamber environment threatens to compromise ideal geometry and layout of the valves and ports. 3. Engine-out emissions performance is likely to be undermined by additional crevice volumes which obstruct complete burning, thereby increasing hydrocarbon emissions. 4. There is a significant increase in reciprocating mass in the case of a variable height piston. 5. Some approaches lead to an increase in vibrations owing to intermediate members in the connecting rod. 6. In some cases, reworking of the entire engine structure is necessary. 7. Variable compression ratio designs consist of multilink rod-crank mechanisms, which may also present a near-to-sinusoidal motion unfavorable to cylinder filling at low speeds and fine-scale turbulence.

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Chapter 4

Conclusion

The VCR engine has great potential for improving part-load thermal efficiency, more efficient operation, ability to down size the engine, multi fuel flexibility and reducing the harmful emissions when compared to other competing technologies. The main obstacles to adoption of VCR are incompatibility with major components in current production and difficulties of combining VCR and non-VCR manufacturing within existing plant. The Potential of these technologies needs to be evaluated by a trade -off between cost and consumption benefit. It is potentially one of the profitable sources to investigate for the automotive industry. VCR systems with certain modification in connecting rod design i.e. hydraulic pistons in connecting rod body & eccentric in connecting rod small end is the most economical, simplest, & most beneficial method to achieve two stage variable compression ratio compared to conventional continuous variable compression ratio system .from this paper we conclude that by this method in VCR system fuel consumption can be reduced to certain level. This VCR system has great potential for improving part-load thermal efficiency and reducing greenhouse when they are used in combination with down-sizing gas emissions. Variable compression ratio promises more efficient operation, the ability to down-size the engine, multi-fuel flexibility, and the potential to revise emission characteristics. Following results are found:1. Under full load conditions, the performance and efficiency of an engine with a compression ratio that is adapted to load demands is capable of reducing knock susceptibility. 2. In addition, the risk of pre-ignition, mega-knocking effects and engine jerking, as the result of retarded combustion phases, can be reduced. 2. The VCR also provides further potential to control the exhaust gas temperature, contributing to protecting component temperatures. Combustion, a reduced CR results in a lower peak firing pressure.

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Chapter 5 FUTURE SCOPE 5.1 PORSCHE’S PATENTED DESIGN Porsche has received a patent on a new form of the variable-compression-ratio engine. This would be especially useful with turbocharged engines that today run at lower compression ratios, to avoid over-stressing the engine when the turbocharger is forcing more air into the combustion chambers. The patent was sought by Porsche’s huge consulting arm along with Porsche client Hilite International, suggesting the engine technology might be offered to other automakers if and when it’s made commercially viable. Why variable compression ratio matters? Today’s gasoline-engine cars compress the outside air to about a tenth its original volume, a 10:1 ratio, inside each cylinder. Compress it more and you get detonation – also called knock or ping – before the spark plug ignites the air-fuel mixture at or near top dead center, when the piston is at the top of its travel. Technology has raised compression ratios to 11:1 or 12:1 and as high as 12:1 in Mazda SkyActiv cars. Premium-grade gasoline allows for higher compression ratios than regular. Knock sensors can adjust ignition timing to avoid detonation. Still, design engineers have to back off on the base compression ratio when there’s a turbocharger involved, which affects efficiency at low rpm, which reduces mpg in the vehicle and its desirability to the buyer. It also makes the car feel like a pig when your first tromp on the throttle (turbo lag). Enter the variable-compression-ratio engine and Porsche’s new technology. How Porsche does it: Adjustable length connecting rods Porsche and Hilite conceived a way to adjust the apparent length of the connecting rods, the metal arms that connect to the crankshaft and drive the pistons up and down. A solenoid allows small oil-pressure-driven rods and an eccentric adjuster to raise or lower the bearing supporting the piston. The patent diagram appears to show a high and low position currently, not a variable height. The car starts off with the piston in the high position. When the turbo begins injecting pressurized air, the piston drops to the low position. That reduces the compression ratio momentarily, allowing for more turbocharger boost and more power. The Porsche-Hilite design appears to be comparatively simple, at least compared to other variable-compressionratio efforts that date back a century.

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Development still needed From patent to engine in production could be several years. Even a relatively simple design needs to be tested for durability and quirks that might show up outside the lab. Still, engines of the last 25 years have become increasingly more complex without any falloff in basic reliability. Hilite International makes components used in variable valve timing controls (VVT) that run reliably despite their complexity. Since Porsche is a consulting group as well as an automaker, and in some years in the past made more money off consulting, this is likely to be a technology with the possibility of being adopted throughout the industry on small engines — just as Mitsubishi’s balancer shafts are now common on almost every fourcylinder engine. These designs also show that the gasoline internal combustion engine will continue to be the dominant powerplant in passenger cars. 5.2 FUTURE DEVELOPMENTS Future developments in VCR engine could be as follows: 1. Develop an optimized cylinder head for the VCR engine that is capable of realizing the full potential of this technology. 2. Conduct an optimization assessment and determine the best engine configuration and develop optimized components for a second-generation engine. 3. Develop a second-generation engine by using assessment results and optimized components.

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Chapter 6 References 1. 2. 3. 4. 5.

6. 7. 8.

Heywood JB, Internal Combustion Engine Fundamentals, Tata McGraw Hill. M. L. Mathur & R. P. Sharma, A Course in Internal Combustion Engines, Dhanpat Rai Domkundwar, I.C.Engines, Dhanpat Rai & Sons Tadeusz J. Rychter, Andrzej Teodorczyk (Institute of Heat Engineering, Warsaw University of Technology), variable compression ratio engine - VR/LE concept A research article on Optimisation of Connecting Rod Design to Achieve VCR (www.ijera.com) http://www.ott.doe.gov http://www.nptel.com Mahesh P. Joshi, Aparna V. Kulkarni, Variable Compression Ratio (VCR) Engine- A Review Of Future Power Plant For Automobile (www.ijmerd.com)

9. Yamaha Environment-Friendly "SD (Super Diesel) Engine" 10. Green Car Congress: Lotus, QUB and Jaguar to Develop Variable Compression Ratio, 2-Stroke OMNIVORE Research Engine [1] 11. Lotus Engineering Omnivore Variable Compression Ratio Engine to Debut in Geneva [2] 12. Motortrend.com, February 25, 2009 [2]. 13. US patent 5025757, Gregory J. Larsen, "Reciprocating piston engine with a varying compression ratio", issued 1991-06-25

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