What is Piston?
{{For|the American composer|Walter Piston}} {{For|the use of the term in optics|piston (optics)}} [[Image:Four stroke engine diagram.jpg|thumbnail|right|Components of a typical, [[four stroke cycle]], DOHC piston engine. (E) Exhaust [[camshaft]], (I) Intake camshaft, (S) [[Spark plug]], (V) [[poppet valve|Valve]]s, (P) Piston, (R) [[Connecting rod]], (C) [[Crankshaft]], (W) Water jacket for coolant flow.]] In general, a '''piston''' is a lubricated sliding [[shaft]] that fits tightly inside the opening of a [[Cylinder (engine)|cylinder]]. Its purpose is to change the [[volume]] enclosed by the cylinder, to exert a [[Force (physics)|force]] on a [[fluid]] inside the cylinder, to cover and uncover ports, or some combination of these. A rubber seal is sometimes used to keep the lubricate within the shaft. Due to the constant motion of the machine this seal wears quickly and should be replaced with every servicing. If the seal should break during usage there can be disastrous long lasting consequences for the machine. ==Creating force== There are two ways that a piston engine can transform combustion into motive power. These are two-stroke and four-stroke. {{Main|Two-stroke engine}} {{Main|Four-stroke engine}} A single cylinder two-stroke engine produces power every crankshaft revolution, while a single cylinder four-stroke engine produces power once every two revolutions. Older designs of small two-stroke engines produced more pollution than four stroke engines. However, modern two-stroke designs, like the [[Vespa ET2 Injection]] use [[fuel-injection]] and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel injection and produce low emissions. One of the biggest internal combustion engines in the world, the [[Wärtsilä-Sulzer RTA96-C]] is a two-stroke; it is bigger than most two-story houses, has pistons nearly 1 [[metre]] in [[diameter]] and is one of the most efficient mobile engines in existence. In theory, a four stroke engine has to be larg