Automobile Engineering in ME
The main characteristics of a maintenance free (MF) battery is that

A maintenance free battery has a relatively short shelf life when compared with standard batteries
Recharging of a maintenance free battery is neither required nor possible
Since it is sealed, the water in maintenance free battery is not lost through evaporation thus accordingly it is not necessary to top up the cells with water
A maintenance free battery requires little maintenance during normal use and it is sufficient to add water instead of an electrolyte containing sulphuric acid

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

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