Piston

A piston is a cylindrical mechanical component that moves linearly within a cylinder. In reciprocating engines, its motion is driven by the pressure of expanding combustion gases and converted into rotational motion by a connecting rod attached to a crankshaft. Pistons also serve in hydraulic and pneumatic cylinders, where fluid pressure moves the piston to produce linear force.

A piston typically has a crown at the top, a skirt along the sides, and grooves near the top for piston rings—a set of compression rings and an oil-control ring. The piston is connected to the connecting rod by a wrist pin (gudgeon pin). Materials have evolved from cast iron to aluminum alloys with steel rings, and some designs incorporate cooling features such as oil jets or cooling channels in the crown.

During the compression and power strokes, gas pressure acts on the piston crown, creating a force that drives the piston within the cylinder. The rings seal the combustion chamber, control oil migration, and smooth the cylinder wall. The linear motion is transformed into rotational motion by the crankshaft through the connecting rod. Clearances and lubrication are critical to limit wear and temperature.

Piston design varies with engine type. Crown shape influences compression ratio and combustion efficiency. Piston wear, ring sticking, or piston slap can occur if lubrication or clearances are inadequate. In hydraulic and pneumatic cylinders, seals and bore finish determine efficiency and life.