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Purpose is to improve the sealing performance of the piston and to keep engine oil separate from the combustion chamber.
The down side is the piston rings increase friction (and so losses).
There are three types of piston rings:
The compression ring improves the sealing of the piston to the cylinder so increasing compression. The compression ring also helps control oil and conduct heat away from the piston to the cylinder and then the cooling system.
The scraper ring is designed to remove oil from the cylinder wall and so return it to the crankcase. This could be included within the classification "Oil Control Ring".
The oil control ring stops oil passing into the combustion chamber and so getting burnt.
In steam engines the piston rings are compression rings and are there to increase compression.