Cylinder (engine)

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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 aluminum or iron before precision features are machined into it. (Ceramics have also been tried, so far unsuccessfully.) 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 (the square of half the bore times pi ) 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 make near contact with the hard walls of the sleeve, 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 of an engine is a process whereby tiny irregularities in the metals form congruent grooves. An engine job is a process in which the cylinders are machined out to a slightly larger diameter, and new sleeves and piston rings are installed.

[edit] Heat engines

Main article: Heat engine

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 cross-section of a cylinder in a steam engine. The sliding part at the bottom is the piston, and the upper sliding part is a poppet valve that directs steam alternately into either end of the cylinder. Refrigerators and air conditioners are heat engines run in reverse, and the noise of the pistons moving inside cylinders is often a nuisance.

[edit] Internal combustion engines

Main article: Internal combustion engine

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.