sealing the combustion chamber
supporting heat transfer from the piston to the cylinder wall
regulating motor oil consumption
Piston rings are subject to wear as they rub up and down the cylinder bore. To minimise this, they are made of wear
resistant materials (cast irons and steels) and coated or treated to enhance their wear resistance.
Typically top and oil control rings will be coated with chrome, or nitrided or have a PVD (physical vapour deposit)
ceramic coating. The lower oil control ring is designed to leave a film of lubricating oil a few micrometres thick on
the bore as the piston descends. When fitting new rings to a used engine to improve compression and oil consumption
without reboring the cylinder, special "ridge dodger" rings are sometimes used for the top compression ring. These have a
small step of iron removed from the top section to avoid making contact with any wear ridge at the top of the cylinder,
which could break a conventional ring. Generally speaking, these are not to be recommended, as they are probably not
required and may give inferior oil consumption.
A piston ring is an open-ended ring which fits into a groove on the outer diameter of a piston in an internal combustion engine.
The three main functions of piston rings in internal combustion engines are: