Automobile Engineering in ME
Two advantages of using helical gears rather than spur gears in a transmission system are

Strength and cost
Strength and less end thrust
Noise level and strength
Noise level and economy

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

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

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