What is the function of an automobile piston?

[OZoe]

Piston function: The piston's role is to withstand gas pressure and transmit it through the piston pin to the connecting rod, which drives the crankshaft to rotate. The top of the piston also forms part of the combustion chamber. Working principle of the piston: Operating conditions: Pistons operate under conditions of high temperature, high pressure, high speed, and poor lubrication. They are in direct contact with high-temperature gases, with instantaneous temperatures reaching above 2500K. Consequently, they are subjected to severe heating, yet have poor heat dissipation conditions, leading to very high operating temperatures—up to 600-700K at the top—with highly uneven temperature distribution. The piston top endures significant gas pressure, especially during the power stroke, where gasoline engines can reach 3-5MPa and diesel engines 6-9MPa. This results in piston impact and the bearing of lateral forces. Pistons move back and forth in the cylinder at high speeds (8-12m/s), with constantly changing velocities, generating substantial inertial forces and imposing significant additional loads on the piston. Under these harsh conditions, pistons can deform, experience accelerated wear, and develop additional loads and thermal stresses, all while being subjected to chemical corrosion from combustion gases. Piston structure: Generally, pistons are cylindrical in shape. Depending on the engine's operating conditions and requirements, their construction varies. Typically, pistons are divided into three parts: the head, the skirt, and the piston pin boss. Head: The head refers to the top of the piston and the ring groove section. The piston top is entirely determined by the combustion chamber's requirements. A flat or nearly flat top design helps reduce the piston's contact area with high-temperature gases, ensuring uniform stress distribution. Piston skirt: The piston skirt is the lower part of the piston, whose role is to maintain as vertical a posture as possible during reciprocating motion, serving as the guiding part of the piston. Piston pin boss: The piston pin boss is the supporting part where the piston connects to the connecting rod via the piston pin, located above the piston skirt. In high-speed engines, the piston pin boss may not align with the piston's central axis plane but can be slightly offset to one side—specifically, toward the side that contacts the cylinder wall during the power stroke. This offset reduces piston slap against the cylinder wall when the piston changes direction at top dead center, thereby decreasing engine noise.