A cylinder in the central working part of a reciprocating engine, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminium or iron before precision features are machined into it. The cylinders may then be lined with sleeves of some harder metal, or given a wear-resistant coating such as Nikasil. A cylinder's displacement, or swept volume, is its cross-sectional area times the distance the piston travels within the cylinder (the stroke). The engine displacement is the swept volume of one cylinder times the number of cylinders in the engine. A piston is seated outside each cylinder by several metal piston rings which fit around its outside surface in machined grooves: typically two for compressional sealing and one to seal the oil. They are made of spring steel and sit flush with the hard walls of the sleeve, riding on a thin layer of lubricating oil, which is essential to prevent the engine from seizing up. This contact, and the resulting wear, explains the need for the hard lining on the inner surface of the cylinder.
Heat engines
Heat engines, including Stirling engines, are sealed machines using pistons within cylinders to transfer energy from a heat source to a colder reservoir, often using steam or another gas as the working substance.
Internal combustion engines operate on the inherent volume change accompanying oxidation of gasoline (petrol), diesel fuel (or some other hydrocarbon) or ethanol: an expansion which is greatly enhanced by the heat produced. They are not classical heat engines, since they expel the working substance, which is also the combustion product, into the surroundings.
The reciprocating motion of the pistons is translated into crankshaft rotation via connecting rods. As a piston moves back and forth, a connecting rod changes its angle; its distal end has a rotating link to the crankshaft. In addition to cylinder-piston engines, there are also rotary turbines.