Automobile Engineering in ME
The basic purpose of a four wheel drive (4WD) system is that it

Ensures that effective braking can be performed, even on slippery surfaces
Ensures effective transmission of engine torque to all four wheels, even on slippery road surfaces
Delivers improved cornering on dry road surfaces
Eliminates the need of snow tyres, tyre chains, etc.

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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
It makes petrol engines lighter
Less compression ratio gives better performance
It is just customary to have less compression ratio in petrol engines

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