Piston ring

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Spring-loaded piston rings.

A piston ring is an open-ended ring that fits into a groove on the outer diameter of a piston in an internal combustion engine.

1 Explanation
2 Wear
3 Fitting new piston rings
4 External links

[edit] Explanation

The three main functions of piston rings in internal combustion engines are:

Sealing the combustion chamber.
Supporting heat transfer from the piston to the cylinder wall.
Regulating motor oil consumption.

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder. Most automotive pistons have three rings: The top two whilst also controlling oil are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings). Typical compression ring designs will have an essentially rectangular cross section or a keystone cross section. The periphery will then have either a barrel profile (top compression rings) or a taper or taper napier form (second compression rings. There are some taper faced top rings and on some old engines simple plain faced rings were used. Oil control rings typically are of 3 types. Single piece cast iron, helicoil spring backed cast iron or steel, multipiece steel. The spring backed oil rings and the cast iron oil rings have essentially the same range of peripheral forms which consist of 2 scraping lands of various detailed form. The multipiece oil control rings usually consist of 2 rails or segments (these are thin steel rings) with a spacer expander spring which keeps the two rails apart and provides the radial load.

[edit] Wear

Piston rings are subject to wear as they rub up and down the cylinder bore. To minimise this, they are made wear resistant materials (cast irons and steels) and coated or treated to enhance the wear resistance. Typically top and oil control rings will be coated with Chromium, or Nitrided possibly plasma sprayed or have a PVD (physical vapour deposit) ceramic coating. For enhanced scuff resistance and further improved wear most modern diesel engines will have top rings coated with a modified chromium coating known as CKS, a patented coating from Goetze. 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.

[edit] Fitting new piston rings

When fitting new piston rings, the end gap is a crucial measurement. In order that a ring may be fitted to the piston, it is not continuous but is broken at one point on its circumference. The ring gap may be checked by putting the ring into the bore/liner and measuring with a feeler gauge and should be within recommended limits. Too small a gap may be completely taken up under hot running conditions, leading to seizure of the piston. Too large a gap will give unacceptable levels of blow-by gasses or oil consumption. If being measured in a used bore it may indicate excessive bore wear. If 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 these are not to be recommended as they are probably not required and may give inferior oil consumption.