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

Starts at top dead center and ends at bottom dead center
May start and end anywhere
Starts at 30° after top dead center and ends at 50° after bottom dead center
Starts at 30° before top dead center and ends at 50° before bottom dead center

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Internal Combustion Engines
Compression ratio of I.C. Engines is

Volume displaced by piston per stroke and clearance volume in cylinder
The ratio of volumes of air in cylinder before compression stroke and after compression stroke
Ratio of pressure after compression and before compression
Swept volume/cylinder volume

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