Internal Combustion Engines
Scavenging air in diesel engine means

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

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Internal Combustion Engines
In a typical medium speed 4-stroke cycle diesel engines

May open and close anywhere
Exhaust valve opens at bottom dead center and closes at top dead center
Exhaust valve opens at 35° before bottom dead center and closes at 20° after top dead center
Exhaust valve opens just after bottom dead center and closes just before top dead center

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