A piston ring is an open ring embedded in the piston ring groove, usually made of alloy cast iron. Although small in size, it shoulders three major missions:

　　1. Sealing: This is the primary task. The outer circular surface of the piston ring tightly adheres to the cylinder wall, preventing high-temperature and high-pressure gas from leaking into the crankcase through the gap between the piston and the cylinder, ensuring combustion chamber pressure. Once the seal fails, the engine power will significantly decrease.

　　2. Heat transfer: About 70-80% of the heat absorbed by the top of the piston needs to be transferred to the cylinder wall through the piston ring, and then carried away by the cooling system. If heat transfer is not smooth, the piston may burn or cause detonation due to overheating.

　　3. Oil control: The piston ring (especially the oil ring) is responsible for scraping off excess oil on the cylinder wall to prevent it from entering the combustion chamber and participating in combustion (i.e. "burning oil"), while leaving a thin oil film on the cylinder wall to ensure lubrication.

　　A typical piston ring set includes two compression rings and one oil ring. During installation, the openings of each ring must be staggered, usually separated by 120 degrees or 180 degrees, to prevent the formation of "leakage channels".

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