In S.I engine, the knocking occurs at the end of combustion whereas in C.I engine knocking occurs at the very beginning of combustion.

The detonation in the S.I engine is of a homogeneous charge causing very high rate of pressure rise and very high maximum pressure. In the C.I engine, the fuel and air are heterogeneous and hence the rate of pressure rise is normally lower than that in the detonating part of the charge in the S.I engine.

In the C.I engine, the fuel is injected into the cylinder only at the end of the compression stroke, there is no question of pre-ignition as in S.I engine.

In the C.I engine, serve audible knock is always present during knocking as compared S.I engine.