KNOCKING OR DETONATION IN S.I ENGINES

 If  the temperature of air - fuel mixture is raised high enough, the mixture will self-ignite without the need of a sparkplug. This phenomenon is called as self-ignition or Auto Ignition. The temperature above which self-ignition occurs called the self - ignition temperature.

If the temperature of the unburnt mixture exceeds the self - ignition temperature during the ignition delay period, auto - ignition occurs at various locations in the cylinder. This will generate pressure pulses. These pressure pulses can cause damage to the engine and to the engine and quite often are in the audible frequency range. This phenomenon is often called knocking or detonation. 

EFFECTS OF KNOCKING : 

*  The impact on the engine components and structures may cause          failure and creates undesirable noise which is always                          objectionable.

*  The lack of control of combustion process leads to pre ignition          and   local over heating. Therefore,  piston  may be damaged              by  overheating.

*  The pressure differences in the combustion chamber cause the gas      to vibrate and scrub the chamber   walls causing increased loss         of  heat to the coolant. 

*  Detonation results in increased carbon deposits on the wall of  the      cylinder.

*  Due to increase in the rate of heat transfer, the power output as          well as efficiency of  the engine will  decrease.