A piston lies inside a hollowed-out area called a cylinder. The crown of a piston faces an enclosed compression chamber where gasoline ignites and forces the piston downward. A series of grooves lie below the crown. These grooves contain the piston rings, which help an engine run smoothly.
Sealing Compression Chambers
The elastic piston rings fit snugly against the walls of the cylinder. When gasoline ignites and the resultant gas fills the compression chamber, the piston rings prevent gas from escaping into areas where it will not contribute to the downward thrust of the piston.
The lower piston rings are called oil-control rings or scraper rings. The area between the skirt and the cylinder becomes oily as the piston moves upward. Then the oil-control rings scrape most of the oil off the cylinder walls when gases force the cylinder downward, according to "The Fundamentals of Motor Vehicle Technology." This prevents oil from getting into the compression chamber, where it will burn and cause the engine to emit smoke.
The burning gasoline makes the crown of the piston very hot. The piston rings cool the piston head by conducting the heat away from it to the cylinder wall. The cylinder wall passes the heat on, until it dissipates in the outside air.
Stabilising the Piston
The elastic piston rings expand till they reach the walls of the cylinder. This stabilises the piston and keeps it from knocking against the sides of the cylinder.