Off-Road chap7_8 - Chapters 7 SI vs CI Performance...

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1 Chapters 7 SI vs CI Performance Comparison Performance Comparison of CI and SI Engines ± The CI engine cycle can be carried out in either 2 or 4 strokes of the piston, with the 4-cycle CI engine being more common. ± The air and fuel are not united in a CI engine until fuel is injected into the combustion chamber. The fuel injected into a CI engine (typically starting at about 20 o before TDC) has very little time to mix with air. Therefore, the mixture in the combustion chamber is heterogeneous (very rich within fuel spray plumes and very lean outside the plumes). Performance Comparison of CI and SI Engines con’t ± The air-fuel mixture of a SI engine has much more time to mix and is nearly homogeneous by the time of ignition in the combustion chamber. The fuel-air equivalence ratio of the homogeneous mixture in a SI engine must remain close enough to unity to be combustible. ± The compression ratio of a CI engine must be high enough to cause auto-ignition of the air-fuel mixture. The compression ratio of a SI engine must be low enough to prevent auto-ignition.
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2 Performance Comparison of CI and SI Engines con’t ± The high compression ratio increases the stress on a CI engine, so it must be constructed more robustly than a SI engine. ± A spark ignites the mixture in a SI engine, and a flame front sweeps smoothly across the combustion chamber ± Initial combustion in a CI engine is rough and uncontrolled because the mixture may ignite spontaneously at more then one place in the combustion chamber. Performance Comparison of CI and SI Engines con’t ± The power output of a CI engine is limited by it’s air- handling ability, which tends to limit the power output of the engine. ± A CI engine can produce about the same power output as a SI engine of equal displacement running at the same speed. Goering et. al., 2003, Of -road Vehicles, ASAE Performance Comparison of CI and SI Engines con’t ± CI engines are more efficient than SI engines at both full load and part load. The excess air supplied to the CI engine and its higher compression ratio help to increase the indicated thermal efficiency. ± The indicated thermal efficiency of a diesel engine improves at part load because fuel injection is ended sooner. The SI engine has no corresponding improvement in indicated thermal efficiency at part load.
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3 Performance Comparison of CI and SI Engines con’t ± The brake thermal efficiencies for the SI (gasoline) and CI (diesel) engines at full load are, respectively, 24.9% and 32.0%.
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This document was uploaded on 11/21/2011.

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Off-Road chap7_8 - Chapters 7 SI vs CI Performance...

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