Automobile Engineering in ME
The two kinds of piston rings are

Compression and oil control rings
Pressure and sealing rings
Oil scrapper and oil control rings
Compression and sliding seal rings

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

Back up compression ring which will work in place of the top ring when it is broken
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

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