What’s a piston ring?

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Piston rings are metal rings attached to an engine’s piston that seal the combustion chamber and regulate oil circulation. In four-stroke engines, there are three rings per piston, each with a unique design and function. Worn piston rings negatively affect engine performance, causing fuel waste, oil consumption, and decreased power.

As its name implies, a piston ring is a metal ring attached to the piston of an engine. 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, such as those found in motorcycles and scooters, the piston rings do not circulate oil because lubrication is provided by oil mixed directly with the fuel. In both cases, the piston rings play a vital role in the proper functioning of an engine.

In most four-stroke engines, a series of three rings per piston is used. Instead of completely encircling the piston, each piston ring is instead open so that it can be compressed when installed in the cylinder, to ensure a tight seal. However, not all perform the same function. In order from top to bottom in the head of a piston are the compression ring, wiper ring, and oil control ring. Each is typically made of cast iron or steel, but they are shaped differently and have unique design features.

The top ring, the compression ring, functions primarily as the seal, preventing the fuel-air mixture in the combustion chamber from escaping to other parts of the engine. Viewed from the front, the compression ring may look like a rectangle or a corner stone. In the middle, the wiper ring is designed with a downward-angled tapered edge to lubricate the cylinder wall and direct excess oil into 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 pass through.

Of all the parts of an engine subjected to wear, the piston rings are among the most stressed. With an engine’s pistons acting as the moving 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 is prone to wear with normal use.

As a piston ring wears, undesirable behavior can affect engine performance. Previously segregated oil and fuel can mix, causing pressure to build up in the crankcase as the gasoline dilutes the oil. Also, oil entering the combustion chamber can ignite and cause the car’s exhaust system to emit blue smoke. Finally, worn piston rings can negatively affect engine compression, resulting in wasted fuel, excessive oil consumption, and decreased power. Relatively inexpensive on its own, the labor required to replace a piston ring can make it a time-consuming and expensive proposition, though necessary for optimum performance.

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