In spark ignition engine or SI engines the compression ratio varies from 6 to 10. Here the compression ratio upper limit is set by the anti-knock properties of the fuel in use. i.e. the octane rating for the fuel.Whereas in diesel engines or compression ignition engine the compression ratio varies from 16 to 20. Here the upper limit of compression ratio is determined by the weight of the engine.
Speed: Spark ignition engines or petrol engines are high-speed engines.this is because of their low weight and homogeneous combustion.On the other hand, compression ignition engine is low-speed engines owing to their heavy weight and heterogeneous combustion process.
Thermal efficiency: Since spark ignition engines have lower compression ratios the maximum value of thermal efficiency is lower than a compression ignition engine.
Weight: Spark ignition engines are lighter than compression ignition engine. The reason for this is simple. In compression ignition engine very high operating pressures are present. Hence they have to have thicker and stronger construction when compared to spark ignition engines.
Time of knocking: In spark ignition system, knocking takes place at the end of the combustion process. Whereas in case of compression ignition system knocking takes place in starting phase of combustion. Now, what is knocking anyways? Knocking happens when there is an autoignition of fuel in the chamber. This happens when there are carbon deposits in the engine. What happens is that carbon absorbs heat and does not release it.This results in fuel getting ignited before the desired time. Knocking can be harmful to the engines. Hence so much emphasis is laid on regular maintenance of the engine.