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
Cylinder walls being too hot
Overheated spark plug points
Red hot carbon deposits on cylinder walls

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Internal Combustion Engines
The exhaust valve in a four stroke cycle petrol engine

May open and close anywhere
Opens at 50° after bottom dead center and closes at 15° before top dead center
Opens at bottom dead center and closes at top dead center
Opens at 50° before bottom dead center and closes at 15° after top dead center

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