Piston rings are divided into two types: one is compression ring and the other is oil ring. The piston ring is a metal elastic ring with outward expansion, the compression ring is used to seal the combustible gas in the fuel chamber with meters, and the oil ring is used to scrape off excess oil on the cylinder. Rotating the piston ring back and forth depends on the pressure ratio difference between gas and liquid. 　　

The role of piston ring: 　　

1. Sealing and heat transfer To ensure the seal between the piston and the cylinder wall, prevent the combustible mixture and high temperature gas in the cylinder from leaking into the crankcase, and transfer the heat received by the top of the piston to the cylinder wall to avoid overheating of the piston. The main function of the oil ring is to scrape off the excess oil splashing on the cylinder wall and to coat a uniform oil film on the cylinder wall. The piston ring is affected by the high temperature and high pressure gas in the cylinder when it works, and it slides at high speed in the cylinder under the condition of poor lubrication. Due to the shape error of the cylinder wall, the piston ring slides up and down while also producing radial movement in the ring groove. This not only increases the wear of the ring and the ring groove, but also makes the piston ring subject to alternating bending stress and easily break. 　　

2. Piston ring material and surface treatment According to the function and working conditions of the piston ring, the material of the piston ring should have good wear resistance, thermal conductivity, heat resistance, impact toughness, elasticity and sufficient mechanical strength. Currently widely used piston ring materials include high-quality gray cast iron, ductile cast iron, alloy cast iron and steel belts. The outer surface of a piston ring is usually chrome-plated or sprayed with molybdenum. The porous chromium layer has high hardness and can store a small amount of oil, which can lubricate and reduce wear. Molybdenum has a high melting point and is also porous, so spraying molybdenum can also improve the wear resistance of piston rings.