Explanation- The knock phenomenon of C.I. engine depends upon delay period. If delay period is small then less amount of fuel is admitted into cylinder. When small amount of fuel is burns then there is smooth pressure rise, so there is no knocking. If the delay period is very long, then more amount of fuel is accumulated in the combustion chamber. When it actually burns, sudden pressure rise will cause the cylinder wall to vibrate, thus it produces noise and this is said to be knocking. Knocking occurs near the beginning of combustion. i.e. at the end of first stage of combustion.
Knocking in C I engine is of imperfectly mixed charged and hence the rate of pressure rise is normally lower than that in the detonation in S I Engine. Fuel is injected into the cylinder only at the end of the compression stroke and there is no question of preignition or premature ignition. Knocking is not easy to distinguish from normal combustion. Diesel knock is reduced with increase in size of cylinder. In C.I. engines, higher compression ratio causes lesser ignition delay and hence lesser possibility of diesel knocks.