There are two types of piston rings
1. Compression Rings
2. Oil Control Rings
The top portion of the piston is called as a piston crown.
In an engine, the temperature of the piston will be more at the crown of the piston. The piston crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason, the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures.
The function of a first compression ring (top ring) is that it maintains a seal and prevents escape of burned gases and loss of pressure in the combustion chamber.
The function of a second ring is that it is a compression ring and aids the top ring in sealing and cooling.
The function of oil control rings is that it prevents engine oil from going into the combustion chamber and returns excess lubricating oil off the cylinder wall through the return holes in the piston to the oil pan.