08-17-2017, 12:40 AM
Knock is the result of the rapid combustion of the end-gas ahead of the flame front which undergoes chemical reactions prior to normal combustion. This cause high frequency pressure oscillations inside the cylinder that produce the sharp metallic noise called knock.
Two theories have been postulated to explain the origin of knock :
1. The autoignition theory which states that when the air/fuel mixture in the end- gas region is compressed to sufficiently high pressure and temperature the fuel oxidation process - starting with the preflame chemistry and ending with rapid energy release - can occur spontaneously in parts or all of the end-gas region.
2.The detonation theory which holds that,under knocking conditions, the advancing flame front accelerates to sonic velocities and consumes the end-gas at a rate much faster than would occur with normal flame speeds.
The detonation theory led many to call knock "detonation" but the more general term "knock" is preferred since the engine phenomenon includes more than the end-gas energy release and since there is much less evidence to support the detonation theory than the auto-ignition theory.