The entire piston can be divided into three parts: the piston top, the piston head and the piston skirt.
The main function of the piston is to withstand the combustion pressure in the cylinder and transmit this force to the crankshaft through the piston pin and connecting rod. In addition, the piston also forms a combustion chamber together with the cylinder head and the cylinder wall.
The top of the piston is a component of the combustion chamber, so it is often made into different shapes. The top of the piston of the gasoline engine is mostly flat or concave, so that the combustion chamber is compact, the heat dissipation area is small, and the manufacturing process is simple. The crown piston is commonly used in two-stroke gasoline engines. The piston crown of a diesel engine is often made into various pits.
The portion from the top of the piston to the lowermost piston ring groove is called the piston head. Its role is to withstand gas pressure and prevent air leakage. Heat is transferred to the cylinder wall through the piston ring. The piston head is cut with a number of ring grooves for seating the piston ring. The upper 2 and 3 slots are used to place the air ring, and the lower 1 and 2 slots are used to install the oil ring. A small number of small holes are drilled in the bottom of the oil ring groove, so that excess lubricating oil scraped from the cylinder wall can flow back to the oil sump through the small hole.
All parts below the piston ring groove are called piston skirts. Its role is to guide the piston to reciprocate in the cylinder and to withstand side pressure. When the engine is working, the piston will be bent and deformed due to the pressure of the gas in the cylinder. After the piston is heated, the amount of expansion is greater than that of other parts due to the large amount of metal at the piston pin. In addition, the piston will also undergo extrusion deformation under the action of side pressure. The combined result of the above deformation causes the piston skirt section to become an ellipse with the long axis in the direction of the piston pin. In addition, since the distribution of temperature and mass of the piston in the axial direction is not uniform, the thermal expansion of each section is large and small.