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

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

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Automobile Engineering in ME
The function of a radiator fan in the cooling system is that

It draws heat out of the engine compartment
It blows air through the radiator when necessary
It cools the engine by blowing air onto the cylinder block
It is turned by wind force as the vehicle moves forward, and its rotation drives the water pump

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