Kolben

Kolben is the German word for piston, a cylindrical component that moves back and forth inside a cylinder of an internal-combustion engine, compressor, or other reciprocating machine. In engines, the Kolben separates the combustion chamber from the crankcase and transmits the force from expanding gases to the connecting rod, which turns the crankshaft.

A piston is fitted with rings that seal the combustion chamber and control the oil film. Compression rings seal the high-pressure gas and prevent gas leakage, while oil rings scrape and regulate lubrication. The piston is connected to the connecting rod by a piston pin (gudgeon pin) that allows rotation about the connecting rod axis.

Pistons are designed with a crown, which can be flat, concave, or domed to influence compression and flame front propagation; a skirt provides stability and reduces side thrust. They are typically made from aluminum alloy for lightness, with higher-strength steel or cast iron used in heavy-duty or high-temperature applications. Cooling of the piston is achieved via oil passages or spray jets in many designs.

Two-stroke and four-stroke engines use different piston shapes and ring arrangements; diesel pistons typically have thicker crowns and special cooling features to withstand higher compression and temperatures, while gasoline engine pistons emphasize weight and thermal properties.

Beyond engines, pistons are used in steam engines, air or gas compressors, and hydraulic cylinders to convert fluid pressure into mechanical motion and back again.

The term Kolben is widely used in German-language technical literature; the English equivalent is piston.