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

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

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Automobile Engineering in ME
The function of positive crankcase ventilation (PCV) system is that it

Returns blow by gases from the crankcase to the intake system
Mixes fuel with air
Promotes combustion by creating a swirling movement in the air-fuel mixture
Feeds blow by gases to the exhaust manifold

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