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

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

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Internal Combustion Engines
The fuel valve in a four stroke cycle diesel engine

May open and close anywhere
Opens at 15° before top dead center and closes at 20° after top dead center
Opens at top dead center and closes at bottom dead center
Opens at 15° after top dead center and closes at 20° before bottom dead center

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