What is a car piston
The piston of a car is the core moving part in an internal combustion engine that converts thermal energy into mechanical energy . It transmits the combustion pressure to the crankshaft through reciprocating motion and participates in the formation of the combustion chamber.
Youdaoplaceholder0 Core definitions and functions
A car piston is a cylindrical component installed inside the engine cylinder. Its core functions include:
Youdaoplaceholder0 Energy conversion : The gas pressure generated by fuel combustion is transmitted to the crankshaft through the piston pin and connecting rod, converting it into rotational power.
Youdaoplaceholder0 Combustion chamber composition : Together with the cylinder head and cylinder wall, it forms a closed combustion space.
Youdaoplaceholder0 Motion coordination : The four-stroke cycle of intake, compression, power generation and exhaust is completed through the coordination of reciprocating motion.
Youdaoplaceholder0 Structure and material properties
The piston adopts a split design:
Youdaoplaceholder0 top : Directly bearing the combustion pressure, gasoline engines mostly adopt a flat top structure, while diesel engines are often designed with pits to meet the combustion requirements.
Youdaoplaceholder0 head : A key part for installing piston rings, featuring 3 to 4 ring grooves (gas rings and oil rings) to achieve sealing and lubrication.
Youdaoplaceholder0 Skirt : Guiding function, surface coating reduces friction, modern design tends to be "short and plump" to enhance rigidity and heat dissipation.
The main material is aluminum alloy, which combines lightweight (reducing inertial force) and high-temperature resistance. Some high-performance models adopt nickel plating or ceramic coating
Youdaoplaceholder0 Work environment and technical challenges
Pistons operate under extreme conditions:
Youdaoplaceholder0 High temperature and high pressure : The instantaneous temperature of the combustion chamber exceeds 2500K, and the top continuously bears a pressure of 3-9MPa.
Youdaoplaceholder0 high-speed motion : The gasoline engine can reach a rotational speed of 6000rpm, corresponding to a linear speed exceeding 20m/s.
Youdaoplaceholder0 Complex forces : subjected to periodic impact loads, lateral pressure and thermal stress deformation.
Modern pistons optimize their structure through finite element analysis and adopt technological innovations such as cooling oil channels and heat insulation coatings to address these challenges .
The entire piston can mainly be divided into three parts: the piston top, the piston head and the piston skirt.
The main function of the piston is to bear the combustion pressure in the cylinder and transfer this force to the crankshaft through the piston pin and connecting rod. In addition, the piston, together with the cylinder head and the cylinder wall, forms the combustion chamber.
The piston top is a component of the combustion chamber and is often made in different shapes. In gasoline engines, the piston top mostly adopts a flat top or a concave top to make the combustion chamber structure compact, the heat dissipation area small, and the manufacturing process 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.
The piston head is the part above the piston pin seat. Piston rings are installed on the piston head to prevent high-temperature and high-pressure gas from entering the crankcase and to stop engine oil from entering the combustion chamber. Most of the heat absorbed by the top of the piston is also transferred to the cylinder through the piston head and then carried away by 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 and is related to the engine speed and cylinder pressure. The number of rings in a high-speed engine is fewer than that in a low-speed engine, and the number of rings in a gasoline engine is fewer than that in a diesel engine. A general gasoline engine uses two gas rings and one oil ring. The diesel engine has three gas rings and one oil ring. Low-speed diesel engines use 3 to 4 gas rings. To reduce frictional losses, the height of the annular part should be lowered as much as possible. Under the condition of ensuring sealing, efforts should be made to minimize the number of rings.
All parts below the piston ring groove are called piston skirts. Its function is to guide the piston to make reciprocating motion in the cylinder and bear the lateral pressure. When the engine is running, due to the effect of the gas pressure inside the cylinder, the piston will bend and deform. After the piston is heated, because there is more metal at the piston pin, its expansion is greater than that at other parts. In addition, the piston will also undergo compressive deformation under the action of lateral pressure. The combined result of the above deformations causes the cross-section of the piston skirt to become an ellipse with the long axis perpendicular to 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 cross-section is larger at the top and smaller at the bottom.
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