The entire piston can be mainly divided into three parts: piston top, piston head and 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 the combustion chamber together with the cylinder head and the cylinder wall.
The piston top is an integral part of the combustion chamber, so it is often made into different shapes. The gasoline engine piston top mostly adopts a flat top or a concave top, so as to make the combustion chamber compact, the heat dissipation area is small, and the manufacturing process is simple. Raised top pistons are commonly used in two-stroke gasoline engines. Piston tops of diesel engines are often made of various pits.
The piston head is the part above the piston pin seat. The piston head is equipped with a piston ring to prevent high temperature and high pressure gas from entering the crankcase and preventing oil from entering the combustion chamber; most of the heat absorbed by the piston top also passes through the piston head. The part is transmitted to the cylinder, which is then transmitted away through the cooling medium.
The piston head is machined with several ring grooves for installing the piston rings. The number of piston rings depends on the requirements of the seal, which is related to the speed of the engine and the cylinder pressure. The number of rings of high-speed engines is less than that of low-speed engines, and the number of rings of gasoline engines is less than that of diesel engines. Generally, gasoline engines use 2 air rings and 1 oil ring; diesel engines use 3 air rings and 1 oil ring; low-speed diesel engines use 3 to 4 air rings. In order to reduce friction loss, the height of the ring belt should be reduced as much as possible, and the number of rings should be reduced under the condition of ensuring sealing.
All parts below the piston ring groove are called piston skirts. Its function is to guide the piston to reciprocate in the cylinder and to withstand side pressure. When the engine is working, due to the action of the gas pressure in the cylinder, the piston will be bent and deformed. After the piston is heated, the amount of expansion of the piston pin is larger than that of other places due to the large amount of metal at the piston pin. In addition, the piston will produce extrusion deformation under the action of side pressure. The combined result of the above deformation makes the section of the piston skirt become an ellipse whose major axis is in the direction of the piston pin. In addition, due to the uneven distribution of temperature and mass along the axis of the piston, the thermal expansion of each section is large at the top and small at the bottom.
