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
In a ventilated disc brake

Disc contains many small holes for optimum cooling performance
A duct directs air towards the caliper for cooling while the vehicle is moving
Disc contains radial vanes between its rubbing surfaces for optimum cooling performance
Caliper is covered with cooling fins

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