Differentiate between SI and CI engines.

Spark Ignition Engines (S.I. Engine) 

It works on otto cycle. In Otto cycle, the energy supply and rejection occur at constant volume process and the compression and expansion occur isentropically. The engines working on Otto cycle use petrol as the fuel and incorporate a carburetor for the preparation of mixture of air fuel vapor in correct proportions for rapid combustion and a spark plug for the ignition of the mixture at the end of compression. The compression ratio is kept 5 to 10.5. Engine has generally high speed as compared to C.I. engine. Low maintenance cost but high running cost. These engines are also called spark ignition engines or simply S.I. Engine.

 
Compression Ignition Engines (C.I. Engine) 

It works on diesel cycle. In diesel engines, the energy addition occurs at constant pressure but energy rejection at constant volume. Here spark plug is replaced by fuel injector. The compression ratio is from 12 to 25. Engine has generally low speed as compared to S.I. engine. High maintenance cost but low running cost. These are known as compression ignition engines, (C.I) as the ignition is accomplished by heat of compression.

The upper limit of compression ratio in S.I. Engine is fixed by anti knock quality of fuel. While in C.I. Engine upper limit of compression ratio is limited by thermal and mechanical stresses of cylinder material. That’s way the compression ratio of S.I. engine has more compression ratio as compared to S.I. Engine.