Britannica Sources used in this answer

How do piston rings prevent gases and oil from leaking in an engine?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

Piston rings are springy, circular metal components that fit into grooves machined into the outer surface of a piston. They function by ensuring a snug fit between the piston and the cylinder wall, creating a necessary seal for engine operation.123

These rings serve two primary purposes:

Gas Sealing: The upper grooves of the piston contain plain compression rings. These rings prevent high-pressure combustion gases from blowing past the piston during the combustion cycle.3
Oil Control: The lower rings are designed to distribute and limit the amount of lubricating oil on the cylinder wall. By regulating this lubrication, they prevent excess oil from entering the combustion chamber.342

By maintaining this seal, piston rings help preserve the engine's compression ratio, which is essential for efficient performance.2