Piston rings keep a tight seal within the cylinder to stop the heat and pressure in there from escaping.
They also stop oil passing up into the combustion chamber.
New rings and cylinders have minor irregularities and when these wear off, the rings will make a better fit.
To help this along, the rings can be given a porous coating. It’s softer and wears more quickly than the ring material which is usually cast iron.
To prevent wear, the face of the piston ring can be coated with a harder material like chromium that operates well against cast iron without scuffing.
They are split so they can be fitted into grooves in the piston, and to expand against the cylinder walls.
When they’re removed, their diameter’s larger than the piston’s. So when they’re installed they’re compressed and the gap is almost closed. Tension in the rings keeps them against the walls.
There are 2 main types of piston rings - compression rings and oil rings.
Compression rings must seal against compression loss during the compression stroke, or the air-fuel mixture won’t be fully compressed.
They must also seal properly during the power stroke, or combustion gases are forced past the piston into the crankcase. This is called blowby.
A plain compression ring has a rectangular section. It is held against the walls by combustion pressure behind the rings.
A tapered ring seals against pressure too but its slightly tapered face helps scrape oil off the walls as well.
Faced rings are designed to better resist heat and wear.
A ring that is chamfered or grooved exerts increased pressure against the walls. It is also called a torsional ring.
Its shape creates internal forces in the ring so that when it is installed, it twists slightly upwards.
During intake, the ring scrapes surplus oil off the walls.
During compression, they tend to slide over the oil and not carry it into the combustion chamber. In the power stroke, combustion pressure forces down on the top of the ring and also against its back. This straightens it so that it has full-face contact with the cylinder walls for effective sealing.
Oil-control rings prevent excessive oil working up into the combustion chambers.
It can be a one-piece ring that depends on its own tension to hold it against the cylinder walls. Slots in the ring and holes in the piston behind the ring let oil return to the sump.
Many oil-rings are segmental types with 3 or 4 segments. It has 2 side rails and an expander, which also acts as a spacer for the rails. They depend on the expander to hold them against the walls. The expander is made of thin steel with a series of crimps to give it an outward spring force.