Internal Combustion Engines
The reason for supercharging in any engine is to

Increase power
Effect fuel economy
Reduce weight and bulk for a given output
Increase efficiency

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Internal Combustion Engines
Scavenging air in diesel engine means

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

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