A piston is seated inside 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 (In steam engines only compressional sealing rings are used of which there can be from two to five on the piston; a fine vapour of oil is usually maintained suspended in the steam working in the cylinder. The rings make near contact with the hard walls of the liner, riding on a thin layer of lubricating oil which is essential to keep 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. The breaking in or running in of an engine is a process whereby tiny irregularities in the metals are encouraged to form congruent grooves. An engine job or rebore is a process in which the cylinders are machined out to a slightly larger diameter, and new sleeves and piston rings installed.
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. (See Carnot cycle.) The first illustration depicts a longitudinal section of a cylinder in a steam engine. The sliding part at the bottom is the piston, and the upper sliding part is a distribution valve (in this case of the D slide valve type) that directs steam alternately into either end of the cylinder. Refrigerators and air conditioners are heat engines driven in reverse cycle as pumps.
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. The Wankel engine is a rotary adaptation of the cylinder-piston concept which has been used by Mazda in automobiles. Rotary engines are relatively quiet because they lack the clatter of reciprocating motion.
Air-cooled engines generally use individual cases for the cylinders to facilitate cooling. Inline motorcycle engines are an exception, having two-, three-, four-, or even six-cylinder air-cooled units in a common block. Water-cooled engines with only a few cylinders may also use individual cylinder cases, though this makes the cooling system more complex. The Ducati motorcycle company, which for years used air-cooled motors with individual cylinder cases, retained the basic design of their V-twin engine while adapting it to water-cooling.
In some engines, especially French designs, the cylinders have "wet liners". They are formed separately from the main casting so that liquid coolant is free to flow around their outsides. Wet-lined cylinders have better cooling and a more even temperature distribution, but this design makes the engine as a whole somewhat less rigid.
A typical four-cylinder automobile engine has a single row of water-cooled cylinders. V engines (V6 or V8) use two angled cylinder banks. The "V" is designed to minimize vibration through destructive interference of harmonic overtones. (The "straight-eight" engine is a thing of the past.) Many other engine configurations exist.
During use, the cylinder is subject to wear from the rubbing action of the piston rings and piston skirt. This is minimized by the thin oil film which coats the cylinder walls, but eventually the cylinder becomes worn and slightly oval in shape, usually necessitating a rebore to an oversize diameter and the fitting of new, oversize pistons. The cylinder does not wear above the highest point reached by the top compression ring of the piston, which can result in a detectable ridge. If an engine is only operated at low rpm for its early life (e.g. in a gently driven automobile) then abruptly used in the higher rpm range (e.g. by a new owner), the slight stretching of the connecting rods at high speed can enable the top compression ring to contact the wear ridge, breaking the ring. For this reason it is important that all engines, once initially run-in, are occasionally "exercised" through their full speed range to develop a tapered wear profile rather than a sharp ridge.