Internal Combustion Engines
The knock in diesel engine occurs due to

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

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Internal Combustion Engines
In a four stroke cycle petrol engine, the inlet valve

Opens at 20° after top dead center and closes at 20° before bottom dead center
Opens at 20° before top dead center and closes at 40° after bottom dead center
May open or close anywhere
Opens at top dead center and closes at bottom dead center

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