Internal Combustion Engines
Pre-ignition is caused by the spontaneous combustion of the mixture before the end of the compression stroke, and is due to

Any one of the listed here
Red hot carbon deposits on cylinder walls
Overheated spark plug points
Cylinder walls being too hot

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Internal Combustion Engines
As compared to air standard cycle, in actual working, the effect of variation in specific heats is to

Reduce maximum pressure and maximum temperature
Increase maximum pressure and decrease maximum temperature
Decrease maximum pressure and increase maximum temperature
Increase maximum pressure and maximum temperature

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