Piston rings purpose

Piston rings seal the combustion chamber. They are set with precision so as to apply the correct pressure on the cylinder wall or liner, which ensures there is a consistent film of oil across the cylinder's working surfaces. This provides sufficient lubrication and protects against wear.
Piston rings from Perkins come as three rings. They are the top compression ring, then the intermediate compression ring and finally the oil control ring. These parts are relatively small in size but play a large role in the main cylinder block of your engine.

Their function is to seal off gases generated in the internal combustion process, help with transferring heat to the cylinder wall and then to both lubricate and scrape down oil from it. Getting the quantities of oil right is vital. Too much oil will cause it to burn off during combustion potentially making your engine produce blue smoke, too little will cause the engine ultimately to seize.

The primary role of the top compression ring is to seal off the majority of the combustion gases to ensure you get the maximum power output from your engine. Any failure or weakening of the piston ring in this area means your engine is working less efficiently than it should.
The bottom ring is responsible for most of the oil control, helping to make sure the right amount of oil is used to lubricate the working surfaces of the cylinder, while the intermediate ring helps with both functions, playing a finishing role in the combustion sealing as well as the downward oil scraping.