1.Forces
The forces acting on the piston rings include gas pressure, the ring's own elastic force, the inertia of the ring's reciprocating motion, the friction between the ring and the cylinder and the ring groove, etc., as shown in the figure. Due to the action of these forces, the ring will produce basic movements such as axial movement, radial movement, and rotational movement. In addition, due to its movement characteristics, the piston ring is accompanied by irregular movement, and it is inevitable that there will be suspension and axial vibration caused by axial irregular movement, radial irregular movement and vibration, twisting movement, etc. These irregular movements often hinder the piston ring from playing its role. When designing the piston ring, it is necessary to give full play to the favorable movement and control the unfavorable side.
2.Thermal conductivity
The high heat generated by combustion is transferred to the cylinder wall through the piston ring, so it can play a role in cooling the piston. The heat dissipated from the piston ring to the cylinder wall can generally reach 30~40% of the heat borne by the top of the piston.
3.Air tightness
The first function of the piston ring is to maintain the seal between the piston and the cylinder wall and control the air leakage to a minimum. This function is mainly undertaken by the gas ring, that is, under any operating conditions of the engine, the leakage of compressed air and gas must be controlled to a minimum to improve thermal efficiency; prevent the cylinder and piston or cylinder and ring from being stuck due to leakage; prevent failures caused by the deterioration of lubricating oil, etc.
4.Oil control
The second function of the piston ring is to properly scrape off the lubricating oil attached to the cylinder wall and maintain normal oil consumption. When too much lubricating oil is supplied, it will be sucked into the combustion chamber, increasing the oil consumption, and the carbon deposits produced by combustion will have a very bad effect on the engine performance.
5.Support
Because the piston is slightly smaller than the inner diameter of the cylinder, if there is no piston ring, the piston will be unstable in the cylinder and it will not be able to move freely. At the same time, the ring must also prevent the piston from directly contacting the cylinder and play a supporting role. Therefore, the piston ring moves up and down in the cylinder, and its sliding surface is entirely supported by the ring.
