Similar questions

Compare abnormal combustion in SI and CI engines.

Marks: 8 M, 4 M

Year: Dec 2014, May 2015

1. In S.I. engines detonation is caused by the auto-ignition of end gas towards the end of combustion whereas knocking in CI engines is due to auto-ignition of the first charge at the start of combustion. Therefore, to avoid detonation in SI engines auto ignition of end gas has to be prevented, and in CI engines earlier possible auto ignition should occur.
2. The air fuel mixture that self ignites in SI engines is homogeneous. Due to this rate of pressure rise and hence intensity of detonation is substantial compared to CI engines, where there is no homogeneous air-fuel mixture present at the time of self-ignition.
3. Compression ratio has to be limited in SI engines beyond which detonation would occur. But for CI engines higher the compression ratio, lesser is the ignition delay and lesser probability of detonation (or diesel knock) occurring.
4. Larger size of cylinder promotes detonation in SI engines whereas detonation in CI engines is reduced with the same. For this reason engines with cylinder diameters larger than 250 mm are almost always of CI type.
5. Fuels found good to prevent detonation in SI engines have longer ignition delay time which proves to be poor for preventing detonation in CI engines. This means fuels having higher octane rating have poor cetane rating and vice-versa.