i) Detonation: - Detonation is auto-ignition of last part of homogeneous charge occurring near the end of combustion, before the flame front reaches it. In auto- ignition, the burning is almost instantaneous which results in extremely rapid release of energy causing pressure of the end gas to rise almost 3 to 4 times , from about 50 bar to 150 -200 bar. This large pressure differential gives rise to a severe pressure wave which strikes the cylinder wall and sets it vibrating, giving rise to a characteristic high pitched metallic ringing sound as if stroke struck by light hammer.
ii) Pre-ignition:- Pre-ignition is the ignition of the homogeneous mixture in the cylinder, before the timed ignition spark occurs, caused by the local overheating of the combustible mixture. Pre ignition is initiated by some overheated projecting part such as the sparking plug electrodes, exhaust valve head, metal corners in the combustion chamber, carbon deposits etc.