The whole piston can be divided into three parts: piston crown, piston head and piston skirt.
The main role 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 with the cylinder head and cylinder wall.
The piston crown is a component of the combustion chamber, so it is often made into different shapes. Most of the gasoline engine piston crowns use flat or concave roofs to make the combustion chamber compact, radiating area small, and simple manufacturing process. Convex pistons are commonly used in two-stroke gasoline engines. The piston crown of diesel engines is often made into 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 top of the piston also passes through the piston head It is transferred to the cylinder, and then passed through the cooling medium.
The piston head is machined with several 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. High-speed engines have fewer rings than low-speed engines, and gasoline engines have fewer rings than diesel engines. General gasoline engines use 2 gas rings and 1 oil ring; diesel engines have 3 gas rings and 1 oil ring; low-speed diesel engines use 3 to 4 gas 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 the seal.
All parts below the piston ring groove are called piston skirts. Its role is to guide the piston to reciprocate in the cylinder and 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, because the piston pin has more metal, its expansion is greater than elsewhere. In addition, the piston will also squeeze and deform under the action of side pressure. The combined result of the above-mentioned deformation makes the cross section of the piston skirt become an ellipse with the long axis in the direction of the piston pin. In addition, due to the uneven temperature and mass distribution of the piston along the axis, the thermal expansion of each section is large and small.