Automobile Engineering in ME
If the spark plug deposit indicates black coating of soot, it indicates that the engine has been generally operating on

Too lean mixture
Too rich mixture
Stoichiometric mixture
Most economical mixture

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

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