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As its name implies, a piston ring is a metal ring affixed to an engine's piston. In four-stroke automobile engines, the piston ring system is responsible for sealing the combustion chamber and its contents from the rest of the engine, as well as regulating oil circulation and consumption. In simple two-stroke engines, like those found on motorcycles and scooters, piston rings do not circulate oil because lubrication is provided by oil mixed directly in with the fuel. In both cases, piston rings play a vital role in the smooth operation of an engine.
In most four-stroke engines, a series of three rings is used per piston. Rather than entirely encircling the piston, each piston ring is instead open-ended, so that it can compress when installed in the cylinder, to ensure a tight seal. They do not all perform the same function, however. In order from top to bottom on a piston head are the compression ring, the wiper ring, and the oil control ring. Each one is typically made out of cast iron or steel, but they are shaped differently and have unique design features.
The topmost ring, the compression ring, functions mainly as the seal — keeping the fuel-air mixture in the combustion chamber from escaping to other parts of the engine. Viewed end on, the compression ring can look like a rectangle or a keystone. In the middle, the wiper ring is designed with a tapered edge angled downwards to lubricate the cylinder wall, and direct excess oil down to the crankcase. Finally, the oil control ring, which is often not a single ring but rather two thin rails, incorporates drilled holes that allow oil to sluice through.
Of all the parts of an engine subjected to wear and tear, the piston rings are among the most constantly stressed. With an engine's pistons acting as the movable end of the combustion chamber, the pressure of each ignition must be contained by the rings. Despite wear-resistant coatings and treatments, a piston ring nevertheless is prone to wearing down through normal use.
As a piston ring wears down, undesirable behavior can affect engine performance. Previously segregated oil and fuel can intermix, causing pressure to build up in the crankcase as gasoline dilutes the oil. In addition, oil that gets into the combustion chamber can ignite, causing the car's exhaust system to emit blue smoke. Finally, worn piston rings can negatively impact an engine's compression, resulting in wasted fuel, excessive oil consumption, and decreased horsepower. Relatively inexpensive by itself, the labor required to replace a piston ring can make it an expensive and time-consuming proposition, though a necessary one for optimal performance.