Automobile Engineering in ME
The compression ratio in petrol engine is kept less than in Diesel engine because

It is just customary to have less compression ratio in petrol engines
It makes petrol engines lighter
Less compression ratio gives better performance
Higher or equivalent compression ratio in petrol engines is not possible due to pre-ignition

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Automobile Engineering in ME
It is necessary to maintain the valve clearances as they

Allow for lengthening of the valves owing to the heat of combustion
Make the crankshaft turn smoothly
Reduce the resistance to sliding that occurs between the cam and the tappet
Increase the speed at which the valves move up and down

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