Knocking, Detonation

In normal combustion the flame front travels in a direction across the combustion chamber from 1 to 4. As it advances it compresses the unburned charge (Air + fuel mixture) thus raining its temperature. The temperature also increases due to the reaction taking place in the unburned charge. If the temperature of the unburned charge is less than the auto ignition temperature than the flame front will cross 2 2’ and move through the farthest point (4) of the chamber in a normal manner but when the unburned charge across 2 2’ reaches its auto ignition temperature before reaching the flame front the unburned charge will ignite.

 
In auto ignition the burning is almost instantaneous which increases the pressure of end gas to almost 3 to 4 times. This large pressure differential give rise to pressure wave which sets the cylinder wall vibrating as if stroke struck by light hammer. This phenomenon is known as knocking or detonation.
Reasons for detonation:
·          Increased compression ratio
·          Raising the inlet temperature of charge
·          Supercharging the engine
·          Advancing in spark timing
·          Location of spark plug
·          Low octane rating of fuel
Control of detonation:
·          Increasing the engine rpm
·          Retarding spark
·          Reducing inlet pressure in inlet manifold
·          Using too lean or too rich mixture
·          Increasing the turbulence
·          Using lower compression ratio
·          Using two or more spark plugs
·          Using high octane rating fuel