Abnormal Combustion (Surface ignition) Knocking or detonation is a combustion knock which is due to end charge combustion by self-ignition before reaching the flame front. It is also known as spark knock.

What is the cause of abnormal Combustion or Detonition in IC Engine

Abnormal combustion occurs by surface ignition. In surface ignition, ignition will not occur by spark plug but due to any hot spot in combustion chamber.

• During combustion some of the part receives heat from combustion and becomes very hot and it acts as a spark plug. This hot part may be exhaust valve head, any carbon particle deposited on the piston or cylinder head or spark plug electrode.

-Carbon deposits also occupy some space inside the cylinder. So increases the compression ratio which causes for high temperature. Also carbon deposits are poor heat conductor which acts as an insulator leads to decreases the heat transfer and finally causes high in cylinder temperature.−

-The surface ignition occurs before (pre-ignition) or after (post-ignition) normal ignition.Pre ignition is very dangerous as it creates the negative work which may damage the engine parts like piston, piston rod, and crank shaft. Pre-ignition and post-ignition mayor may not causes knocking.

-Different type of combustion phenomenon available by this surface ignition are:1. Run-on surface ignition2. Run-away surface ignition 3. Wild ping4. Rumble1. Run-on surface ignition:

-S. I. engine can be stop by switch-off the ignition system means power supply to sparkplug is cut-off and hence spark does not occur by spark plug.

-Theoretically engine should stop but actually it runs due to any hot surface (which may act as a spark plug) inside the engine cylinder. This phenomenon is known as “Run-onsurface ignition”.2. Run-away surface ignition:

-Defective spark plug or exhaust valve receive the heat from combustion cycle and this heated spot causes pre-ignition. This type of surface ignition is very dangerous which may seizure or melt the piston and cylinder. The engine may catch fire, when fire enter in suction intake manifold.3. Wild ping:− Some hot carbon deposits moves free inside the combustion chamber which provide source for combustion.−

-This combustion occurs erratic and unpredictable way produces very sharp knocking which is known as wild ping.4. Rumble:− Due to hot spot inside the combustion chamber, combustion starts at a number of points (like diesel engine). It may be before (pre-ignition) or after (post-ignition) normal spark.

• As combustion starts at number of points, heavy explosion of mixture take place which produces large erratic noise. High pressure waves produces resulting in engine vibration& noise which is known as engine rumble.

Engine detonation, however, can be caused by excess heat and pressure, leading to engine knock. Computers in modern vehicles can detect engine knock and compensate by adjusting engine timing. While this saves your engine from destroying itself, performance and fuel economy can suffer.