What's a Piston Ring in an engine?

A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.

The three main functions of piston rings in reciprocating engines are :

1. Sealing the combustion chamber so that there is no transfer of gases from the combustion chamber to the crank.
2. Supporting heat transfer from the piston to the cylinder wall.
3. Regulating engine oil consumption.

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Piston rings are a major source of hint to identify if the engine is two stroke or four stroke.Three piston rings suggest that it is a four stroke engine while two piston rings suggest that it is a two stroke engine.

There are2 types of piston ring ,oil ring and compression ring.

from top compression ring and below there oil ring

Piston rings are made to maintain the cylinder and combustion pressure of the automobile. They prevent any oil from seeping into the combustion chamber as well as sealing in the air and fuel to be able to compress them. There are typically three piston rings to every piston, and each one of these rings does different things to help the engine operate.

The first ring is known as the top ring; also know as the compression ring. This ring helps the piston work, because during the combustion process it doesn’t allow it to lose any pressure. This ring maintains any pressure build up as the piston arrives at the top of the stroke. This is possible because the top ring acts as a barrier.

The second ring is also known as the secondary compression ring. This ring is a back up compression ring with a slightly tighter tolerance to help the top compression ring in sealing the combustion chamber.

The last ring is called the oil ring. These work with the piston in the engine to lubricate the cylinder, walls, pistons, rings, and wrist pins. This ring keeps the oil out of the combustion process. The oil rings also help the thermal control because it cools the piston by directing oil around it. The piston rings are located on the piston. The piston is located in the cylinder, and multiple cylinders are usually arranged side by side in a bank. This bank is known as the engine block.

It is ring set installed around the piston to prevent the oil from entering the fire champers.

I agree with Mr. Vinod answer.