Piston ring function includes sealing, regulating oil (oil control), heat conduction (heat transfer), guidance (support) four roles. Sealing: refers to sealing gas, do not let the combustion chamber gas leakage to the crankcase, the gas leakage is controlled at a minimum, improve thermal efficiency. Air leakage will not only reduce the power of the engine, but also make the oil deterioration, which is the main task of the gas ring; Adjust the oil (oil control) : the excess lubricating oil on the cylinder wall is scraped off, and the cylinder wall is covered with a thin oil film to ensure the normal lubrication of the cylinder and piston and ring, which is the main task of the oil ring. In modern high-speed engines, special attention is paid to the role of piston ring control oil film; Heat conduction: the heat of the piston is transmitted to the cylinder liner through the piston ring, that is, the cooling effect. According to reliable data, 70 ~ 80% of the heat received by the piston top of the uncooled piston is dispersed through the piston ring to the cylinder wall, and 30 ~ 40% of the cooling piston is dispersed through the piston ring to the cylinder wall; Support: The piston ring keeps the piston in the cylinder, prevents direct contact between the piston and the cylinder wall, ensures smooth movement of the piston, reduces friction resistance, and prevents the piston from knocking the cylinder. Generally, the piston of the gasoline engine uses two gas rings and one oil ring, while the diesel engine usually uses two oil rings and one gas ring.