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

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

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Automobile Engineering in ME
The function of a second ring is that it is a

Compression ring and aids the top ring in sealing and cooling
Cushion ring which prevents piston slap and knock
Back up oil control ring which will work in place of the bottom ring when it is broken
Back up compression ring which will work in place of the top ring when it is broken

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