The air-fuel ratios used in a compression ignition engine lie in the range of 18 to 25.

Compression-Ignition Engine

• In C.I. engine combustion occurs by the high temperature produced by the compression of the air, ie., it is an auto-ignition. For this, a minimum compression ratio of 12 is required.
• The efficiency of the cycle increases with higher values of compression ratio but the maximum pressure reached in the cylinder also increases. This requires heavier construction.
• The upper limit of compression ratio in a C.I. engine is due to mechanical factors and is a compromise between high efficiency and low weight and cost.
• The normal compression ratios are in the range of 14 to 17 but maybe upto 23.
• The air-fuel ratios used in the C.I. engine lie between 18 and 25 as against about 14 in the S.I. engine, and hence C.I. engines are bigger and heavier for the same power than S.I. engines.