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Pre - ignition process in S.I Engine

Pre - Ignition is defined as the phenomenon of ignition of the charge before the ignition spark occurs. This type of  ignition is caused when some parts of the combustion space e.g. spark plug , exhaust valve, carbon particles in the combustion chamber are over heated under certain operating conditions. When pre-ignition occurs , it is the equivalent of an advanced spark and may cause detonation. Conversely, when detonation is severe and long continued, it may heat up the spark plug points or carbon particles. 



Pre - Ignition may cause high rates of pressure rise due to multiple ignition points and advanced timing together with irregular detonation. Pre - Ignition leads to increase in pressure before the piston the reaches TDC and hence the piston movement is opposed and the power output is reduced.

In Heavy - duty engine, excessive heating due to the pre ignition may cause piston and cylinder damage. In multi cylinder engines pre - ignition may develop in one or more cylinders, then the remaining cylinder will take full load and hence probably result in piston seizure followed by the breaking up of the piston with the catastrophic results to the whole engine.  

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