Internal Combustion Engines
The knock in diesel engine occurs due to

Instantaneous auto ignition of last part of charge
Reduction of delay period
Delayed burning of the first part of the charge
Instantaneous and rapid burning of the first part of the charge

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

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

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