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 coat the cylinder walls with a thin film of oil.
At the top of the cylinder-shaped pistons are piston rings that seal the combustion chamber and its contents from the rest of the engine.
The pistons in many engines have three piston rings, although the number can vary from application to application. The upper ring or rings are commonly referred to as compression rings. The bottom rings are often referred to as oil control rings.
Each ring has a small break in its continuity, allowing it to be stretched over the piston and placed in the piston groove. The break also allows the ring to compress when the piston is installed in a cylinder. Together these rings serve to separate each piston from the cylinder wall in 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 can also be observed when a compression ring fails.
An effect sometimes known as leakage can occur when a compression ring begins to weaken. When this happens, combustion gases can escape into the crankcase. Then oil can be blown into the PCV system, air intake and other places. This loss of combustion gases in the crankcase can also result in low or uneven compression. Compression like this can cause an engine to malfunction.
Another symptom that can be observed is known as oil burning or use. When a faulty compression ring allows oil to enter the combustion chamber, it can ignite and produce a thick, blue exhaust. This can also cause the engine to malfunction, as the spark plugs can get dirty with oil.
The ability of the compression rings to efficiently pass heat from the pistons to the cylinder walls can also be important. Massive heat can be generated within internal combustion engines. In many engines, cooling is managed by circulating water around the outside of the thin cylinder walls.
Since compression rings come into contact with the piston and cylinder wall, they can provide a path for heat to dissipate. In this way, compression rings can help prevent pistons from being damaged by excessive heat.