Piston

A piston is a component found in reciprocating engines, reciprocating pumps, gas compressors, and hydraulic and pneumatic cylinders, among other similar mechanisms. It is the moving part held inside a cylinder and sealed gas tight by piston rings. In an engine, the piston transfers force from expanding gas in the cylinder to the crankshaft through a piston rod or connecting rod; in a pump the process runs in reverse, with force transferred from the crankshaft to the piston to compress or eject the fluid in the cylinder. In some engine designs the piston also acts as a valve, covering and uncovering ports in the cylinder wall as it moves.