Detonation/ knocking

Detonation/ knocking :  the rapid auto-ignition of a portion of a fuel causes a pressure wave of high intensity to be set up in the cylinder of an I.C. engine. This pressure wave of high intensity propagates rapidly through the gas gives a violet blow to the walls of the cylinder, to the combustion chamber and to the piston. A loud pulsating noise known as knocking is produced which gives violent vibration to the engine. The term detonation is used to indicate presedce of gas vibration and term knock is used to include all phenomenon that arise from auto-ignition. The knock in S.I. engines is characterized by sudden auto-ignition of the charge farthest away from the spark plug while in case of C.I. engines it is characterized by the sudden auto-ignition of the mixture at the very beginning of the combustion process.