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

Starts at 30° after top dead center and ends at 50° after bottom dead center
May start and end anywhere
Starts at top dead center and ends at 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
Detonation is harmful due to

High intensity of knock causes crankshaft vibration and the engine runs rough
Increase in the rate of heat transfer, there is a reduction in the power output and efficiency of the engine
Excessive turbulence which removes most of the insulating gas boundary layer from the cylinder walls
None of these

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