Automobile Engineering in ME
The main function of the tread pattern on tyre is that

The crests between the tread grooves absorb road noise
The tread pattern protects the tyre's inner carcass from small stones and pieces of glass
The tread grooves pass air between the tyre and road surface, thereby preventing tyre from overheating
In wet conditions, the tread grooves expel water that is drawn between the tyre and road surface

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

It makes petrol engines lighter
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
Less compression ratio gives better performance

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Automobile Engineering in ME
The ignition in a spark ignition engine takes place when the piston is

Approaching the T.D.C. position on its compression stroke
Approaching the T.D.C position on its exhaust stroke
Leaving the T.D.C. position on its compression stroke
Exactly at the T.D.C. position on its compression stroke

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