The entire piston can be 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 a combustion chamber together with the cylinder head and cylinder wall.
The piston top is a component of the combustion chamber, so it is often made into different shapes. The top of the gasoline engine piston is mostly flat or concave, so that the combustion chamber structure is compact, the heat dissipation area is small, and the manufacturing process is simple. Convex top pistons are often used in two-stroke gasoline engines. The piston top of a diesel engine is often made into various pits.
02:2 Do you know the installation skills of motorcycle piston rings? You will know how simple it is after reading it
The piston head is the part above the piston pin seat. The piston head is installed with a piston ring to prevent high-temperature, high-pressure gas from entering the crankcase and prevent engine oil from entering the combustion chamber; most of the heat absorbed by the piston top must also be transferred to the cylinder through the piston head, and then transferred through the cooling medium.
The piston head is processed with several ring grooves for installing piston rings. The number of piston rings depends on the sealing requirements, which is related to the engine speed and cylinder pressure. The number of rings in high-speed engines is less than that in low-speed engines, and the number of rings in gasoline engines is less than that in 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 part 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 withstand lateral pressure. When the engine is working, due to the effect of the gas pressure in the cylinder, the piston will bend and deform. After the piston is heated, due to the large amount of metal at the piston pin, its expansion is greater than other parts. In addition, the piston will also be squeezed and deformed under the action of lateral pressure. The combined result of the above deformation makes the cross section of the piston skirt become an ellipse with the long axis perpendicular to the direction of the piston pin. In addition, since the temperature and mass distribution of the piston along the axial direction are uneven, the thermal expansion of each section is larger at the top and smaller at the bottom.
