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

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
It is just customary to have less compression ratio in petrol engines

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Automobile Engineering in ME
The main task of a battery in automobiles is to

Act as a reservoir or stabilizer of electricity
Supply electricity to the vehicle's electrical system at all times while the engine is running
Supply a large amount of power to turn the starter motor when the engine is being started
Supply electricity to the alternator

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