Compression rings are metal seals that fit between pistons and cylinder walls in internal combustion engines. Each compression ring fits into a groove around the outer diameter of a piston. Therefore the ring, rather than the piston itself, makes direct contact with the wall of the cylinder bore. The main purpose of a compression ring is to prevent fuel, air and combustion gases from escaping into the crankcase. The compression ring can also facilitate heat transfer between the pistons and the cylinder walls. The closely-associated oil control rings help to coat the cylinder walls with a thin layer of oil.
Pistons in many engines have three piston rings, though the number can differ from application to application. The upper ring or rings are usually referred to as the compression rings. The lower rings are often known as oil control rings. Each ring has a small break in its continuity, which allows it to be stretched over the piston and placed into the piston groove. The break also allows the ring to compress when the piston is installed into a cylinder. Together, these rings serve to separate each piston from the wall of the cylinder into which it fits.
Without compression rings, combustion gases could escape into the crankcase. Oil from the crankcase could also freely enter the combustion chambers. Similar effects may also be observed when a compression ring fails. An effect sometimes known as blow-by may result when a compression ring begins to weaken. When this occurs, combustion gases may escape into the crankcase. Oil can then be blown into the PCV system, air intake and elsewhere. This loss of combustion gases into the crankcase may also result in low or uneven compression. Compression such as this can cause an engine to run poorly. Another symptom that may be observed is known as burning or using oil. When a failing compression ring allows oil into the combustion chamber, it may burn and result in thick, blue exhaust. This can also cause an engine to run poorly, as the spark plugs may become oil-fouled.
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The ability of compression rings to effectively pass heat from the pistons to the cylinder walls may also be important. Massive heat can be generated within internal combustion engines. In many engines cooling is handled by circulating water around the exterior of the thin cylinder walls. Since compression rings contact both piston and cylinder wall, they can provide a path for the heat to dissipate. In this way, the compression rings can help prevent the pistons from being damaged by excessive heat.