The whole piston structure can be divided into three parts: piston top, piston head and piston skirt.
The main function of the piston is to bear the combustion pressure in the cylinder and transmit this force to the crankshaft through the piston pin and connecting rod. In addition, the piston, together with the cylinder head and cylinder wall, forms a combustion chamber.
Piston top is an integral part of combustion chamber, so it is often made into different shapes. Flat or concave top is usually used for piston top of gasoline engine in order to make combustion chamber compact, small heat dissipation area and simple manufacturing process. The convex piston is often used in two-stroke gasoline engines. The piston top of diesel engine is often made into various pits.
The piston head is above the piston pin seat. The piston head is equipped with piston rings to prevent high-temperature and high-pressure gas from entering the crankcase and oil from entering the combustion chamber. Most of the heat absorbed by the top of the piston is transmitted to the cylinder through the piston head, and then to the cylinder through the cooling medium.

The piston head is machined with several ring grooves for installing piston rings. The number of piston rings depends on the sealing requirements. It is related to the engine speed and cylinder pressure. High-speed engine has fewer rings than low-speed engine, and gasoline engine has fewer rings than diesel engine. Generally, gasoline engines use two air rings and one oil ring; diesel engines use three air rings and one oil ring; low-speed diesel engines use three to four air rings. In order to reduce friction loss, the ring height should be reduced as far as possible, and the number of rings should be reduced under the condition of ensuring sealing.
In Piston Structure, All parts below the piston ring groove are called piston skirts. Its function is to guide the piston to and fro in the cylinder and bear side pressure. When the engine is working, the piston will bend because of the gas pressure in the cylinder. When the piston is heated, the expansion of the piston pin is larger than that of other parts because of the large amount of metal in the piston pin. In addition, the piston will also produce extrusion deformation under lateral pressure. As a result of the above deformation, the piston skirt section becomes an ellipse of the long axis in the piston pin direction. In addition, due to the uneven distribution of temperature and mass along the axis of the piston, the thermal expansion of each section is larger and smaller.

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