Lecture-32

Prepared under QIP-CD Cell Project

Internal Combustion Engines

Ujjwal K Saha, Ph.D.

Department of Mechanical Engineering

Indian Institute of Technology Guwahati 1

Introduction • • • • • •

Need of High specific power output accompanied by good reliability and longer engine life. Use of high pressure turbo charging results induces high thermal loads. Turbocharger doesn’t have good adiabatic efficiency. High peak pressure problem occurs at full load Can be minimized by reducing CR But also CR should be sufficiently high for good starting and part load operation. 2

VCR engine • High compression ratio is used for good stability and low load operation • Low compression ratio used at full load to boost the turbocharger intake pressure • Load increases – engine exhaust increases – boost available more • At full load turbocharger boost capacity is high so reduction in CR is necessary for more efficiency and to reduce thermal stresses. • Used mainly with turbocharged diesel engines -- VCR concept is beneficial at low load -- better multifuel capacity -- also spark engine can produce knock due to sudden change from high CR to low CR.

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Theoretical Analysis • For part load and high load CR is low in VCR than FCR • Expansion is slower at low compression ratios. • Gas temperature is lower than the for constant compression ratio engines for full compression stroke and up to 500 after tdc . After this the temperature drop is slower due to slower expansion • Exhaust valves in VCR run hotter. • Boost pressure and mean cycle temperature increases with load. • Both bsfc and isfc increases with load. • Pre-turbine gas temperature is higher – but limited by metallurgical considerations.

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Methods of obtaining VCR

• •

Variable compression ratio can be obtained by altering: The clearance volume. Both the clearance volume and the swept volume.

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Various VCR Concepts • A very new and efficient method is slidable piston head and cylinder. • Variation of combustion chamber volume. • Variation of piston deck height. • Modification of connecting rod geometry. • Moving the crankpin within the crankshaft. • Moving the crankshaft axis. • Traverse type mechanism.

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Moving Head (SVC) By combining head and liners into a semimonobloc construction which pivots with respect to the remainder of the engine, SAAB have enabled a tilting motion to adjust the effective height of the piston crown at TDC.

high compression ratio 14:1

low compression ratio 8:1

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Variation of Combustion Chamber Volume Typically the volume of combustion chamber is increased to reduce the CR by moving a secondary piston : • Ford type VCR Head: Ford patent for compression adjustment using a secondary piston or valve. 8

• Volvo/Alvar type VCR Head: Alvar engine concept in which each secondary piston moves continuously at half crankshaft speed and could, potentially, share drive with a camshaft. Phase variation between the secondary pistons and the crankshaft assembly enables the required variation in CR.

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Variable Height Piston Variation in compression height of the piston offers potentially the most attractive route to a production VCR engine since it requires relatively minor changes to the base engine architecture.

Ford VCR Piston

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Daimler – Benz VCR Piston

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A popular approach has been to replace the conventional con rod with a 2 piece design in which an upper member connects with the piston while a lower member connects with the crankshaft. By constraining the freedom of the point at which the two members join, the effective height of the con rod can be controlled and, hence, the compression volume. All the compound con rod designs result in modified piston motion when compared to a conventional engine, since the piston is connected to a rod whose other end is no longer moving in a circular orbit.

Connecting Rod Geometry

Nissan VCR Engine 12

•Peugeot VCR Engine

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• Mayflower e3 VCR Engine

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Movement of Crankshaft or Crankpin Several systems have been proposed which either carry the crankshaft main bearings in an eccentric assembly or move the crankpins eccentrically to effect a Stroke change at TDC.

• Gomesys VCR engine in which moveable crankpins form an eccentric sleeve around the conventional crankpins and are driven by a large gear.

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• Rapan VCR engine in which the crankshaft main bearings are carried in an eccentric housing which can be rotated by an actuator, via a mechanism, to vary the crankshaft position with respect to the cylinder head.

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Traverse diesel engine T-01

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Control strategy Basic Relationship: Points 1, 2, 4, 5 lie on the plane of low compression. Point 3 lies on the plane of high compression. ‰ The engine is started at low CR and zero boost (point 1). ‰ When the driver accelerates, load and boost increase to point 2. When the driver throttles back into a light load cruise (point 3), load and boost reduce and CR increases. ‰ When the throttle is reopened from this condition, CR reduces as boost and load increase, reaching point 4 and, ultimately, point 5 (WOT). ‰

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• Tip-in/Tip-out strategy:

Suppression of unwanted throttle input 19

Accomplishments • VCR shows the high efficiency at lower engine power levels. • Favorable burn rate and coefficient of variance, which allow the application of lean burn technology. • Favorable and consistent emission level.

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Accomplishments AVDS-1100

• VCR engine is very compact and has a higher power to weight ratio. • VCR principle causes low thermal and structural loads. • bsfc of the VCR engine is as good as that of conventional engine. • VCR engine has a very less low frequency noise.

Gross b.p., (kW)

AVCR-1100

186.5

1100

10

26

Displacement, (sq. cm)

18300

18300

Compression ratio

22 : 1

10:1, 22:1

1385 3.5

1385

317

686

Bmep. ( bar)

Weight (kg) Weight, kg/Gross b. p. b.p./sq. m Maximum torque, N-m at rpm Min, sfc, kg/gross kW/hr

13.5

1490/2000 3860/2000

0.232

0.232 21

Accomplishments • Due to use of high compression ratio at low loads the VCR engine has a good starting and idling performance. • Due to higher compression ratio at starting and part load operation the VCR engine has good multifuel capability.

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References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Crouse WH, and Anglin DL, DL (1985), Automotive Engines, Tata McGraw Hill. Eastop TD, and McConkey A, (1993), Applied Thermodynamics for Engg. Technologists, Addison Wisley. Fergusan CR, and Kirkpatrick AT, (2001), Internal Combustion Engines, John Wiley & Sons. Gill PW, Smith JH, and Ziurys EJ, (1959), Fundamentals of I. C. Engines, Oxford and IBH Pub Ltd. Heisler H, (1999), Vehicle and Engine Technology, Arnold Publishers. Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill. Heywood JB, and Sher E, (1999), The Two-Stroke Cycle Engine, Taylor & Francis. Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines, Dhanpat Rai & Sons, New Delhi. Pulkrabek WW, (1997), Engineering Fundamentals of the I. C. Engine, Prentice Hall. Rogers GFC, and Mayhew YR, YR (1992), Engineering Thermodynamics, Addison Wisley. Stone R, (1992), Internal Combustion Engines, The Macmillan Press Limited, London. Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice, Vol. 1 & 2, The MIT Press, Cambridge, Massachusetts. 23

Web Resources 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

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