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
May open and close anywhere
Opens at bottom dead centre and closes at top dead centre

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Internal Combustion Engines
Scavenging air in diesel engine means

Forced air for cooling cylinder
Air used for forcing burnt gases out of engine's cylinder during the exhaust period
Air used for combustion sent under pressure
Burnt air containing products of combustion

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