Mechanical Engineering - Automobile Engineering - Discussion

Diesel engines are designed to operate at higher compression ratios, usually between 15 to 20, while petrol engines have lower compression ratios usually between 8 to 10. Since the higher the compression ratio of an engine the higher the thermal efficiency.

The correct air fuel ratio for the S.I enegine is aprox 15:1 (specified 14.6 : 1).

Since Petrol is very volatile, the 'Compression Ratio' for Petrol engines is typically lower. Thus, it varies from 10:1 to 14:1.

The petrol engine compresses the air & fuel with 10:1 to 14:1 ratio. The petrol engine mixes petrol with air & compresses this mixture in the combustion chamber.