Internal Combustion Engines
Pre-ignition is caused by the spontaneous combustion of the mixture before the end of the compression stroke, and is due to

Any one of the listed here
Red hot carbon deposits on cylinder walls
Overheated spark plug points
Cylinder walls being too hot

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Internal Combustion Engines
In a four stroke cycle diesel engine, the exhaust valve

Opens at 30° before bottom dead centre and closes at 10° after top dead centre
Opens at 30° after bottom dead centre and closes at 10° before top dead centre
Opens at bottom dead centre and closes at top dead centre
May open and close anywhere

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