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

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

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Automobile Engineering in ME
The auto ignition in a spark ignition engine means

Ignition of the charge before the passage of flame front
Automatic ignition of the charge at the end of compression
Ignition induced to supplement the process of normal combustion
Ignition induced by the passage of a spark

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