Compression Ignition Engines (or C.I. Engine):

It works on the basis of dieses cycle. In diesel engines, energy addition occurs at constant pressure but energy rejection at constant volume. Here spark plug can be replaced by fuel injector. The ratio of compression is from 12 to 25. Engine has low speed as compared to Spark Ignition Engines. It is having maintenance cost but low running cost. These are called as compression ignition engines as the ignition is accomplished by the heat of compression.

The upper limit of compression ratio in S.I. Engine is made fix by anti knock quality of fuel. While in Compression Ignition Engines upper limit of compression ratio is restricted by thermal and mechanical stresses of cylinder material. That's why compression ratio of S.I. engine is having more compression ratio as compared to S.I. Engine.