Editing Reciprocating engine (section)

==Common features in all types==
[[File:Engine movingparts.jpg|thumb|Ray-traced image of a piston engine]]
There may be one or more pistons. Each piston is inside a [[cylinder (engine)|cylinder]], into which a gas is introduced, either already under pressure (e.g. [[steam engine]]), or heated inside the cylinder either by [[ignition system|ignition]] of a fuel air mixture ([[internal combustion engine]]) or by contact with a hot heat exchanger in the cylinder ([[Stirling engine]]). The hot gases expand, pushing the piston to the bottom of the cylinder. This position is also known as the bottom [[Dead centre (engineering)|dead center]] (BDC), or where the piston forms the largest volume in the cylinder. The piston is returned to the cylinder top (top dead center) (TDC) by a [[flywheel]], the power from other pistons connected to the same shaft or (in a [[Single- and Double-acting cylinder#Double-acting|double acting cylinder]]) by the same process acting on the other side of the piston. This is where the piston forms the smallest volume in the cylinder. In most types the expanded or "[[Exhaust gas|exhausted]]" gases are removed from the cylinder by this [[Stroke (engines)|stroke]]. The exception is the [[Stirling engine]], which repeatedly heats and cools the same sealed quantity of gas. The stroke is simply the distance between the TDC and the BDC, or the greatest distance that the piston can travel in one direction.

In some designs the piston may be powered in both directions in the cylinder, in which case it is said to be [[Single- and Double-acting cylinder#Double-acting|double-acting]].

[[Image:Steam engine nomenclature.png|thumb|left|300px|'''Steam piston engine'''<br />A labeled schematic diagram of a typical single-cylinder, simple expansion, double-acting high pressure steam engine. Power takeoff from the engine is by way of a belt.
{{ordered list
 | Piston
 | Piston rod
 | Crosshead bearing
 | Connecting rod
 | Crank
 | Eccentric valve motion
 | Flywheel
 | Sliding valve
 | Centrifugal governor
}}]]

In most types, the linear movement of the piston is converted to a rotating movement via a [[connecting rod]] and a [[crankshaft]] or by a [[swashplate]] or other suitable mechanism. A [[flywheel]] is often used to ensure smooth rotation or to store energy to carry the engine through an un-powered part of the cycle. The more cylinders a reciprocating engine has, generally, the more vibration-free (smoothly) it can operate. The power of a reciprocating engine is proportional to the volume of the combined pistons' displacement.

A seal must be made between the sliding [[piston]] and the walls of the [[Cylinder (engine)|cylinder]] so that the high pressure gas above the piston does not leak past it and reduce the efficiency of the engine. This seal is usually provided by one or more [[piston ring]]s. These are rings made of a hard metal, and are sprung into a circular groove in the piston head. The rings fit closely in the groove and press lightly against the cylinder wall to form a seal, and more heavily when higher combustion pressure moves around to their inner surfaces.

It is common to classify such engines by the number and alignment of cylinders and total volume of [[Engine displacement|displacement]] of gas by the pistons moving in the cylinders usually measured in cubic centimetres (cm<sup>3</sup> or cc) or [[litre]]s (l) or (L) (US: liter). For example, for internal combustion engines, single and two-cylinder designs are common in smaller vehicles such as [[motorcycles]], while [[automobile]]s typically have between four and eight, and [[locomotive]]s and [[ships]] may have a dozen cylinders or more. Cylinder capacities may range from 10&nbsp;cm<sup>3</sup> or less in model engines up to thousands of liters in ships' engines.<ref>Hanlon, Mike. [http://www.gizmag.com/go/3263/ Most powerful diesel engine in the world] ''GizMag''. Accessed: 14 April 2017.</ref>

The [[compression ratio]] affects the performance in most types of reciprocating engine. It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

The [[Stroke ratio|bore/stroke ratio]] is the ratio of the diameter of the piston, or "[[Bore (engine)|bore]]", to the length of travel within the cylinder, or  "stroke". If this is around 1 the engine is said to be "square". If it is greater than 1, i.e. the bore is larger than the stroke, it is "oversquare". If it is less than 1, i.e. the stroke is larger than the bore, it is "undersquare".

Cylinders may be aligned [[Straight engine|in line]], in a [[V engine|V configuration]], [[Flat engine|horizontally opposite]] each other, or [[Radial engine|radially]] around the crankshaft. [[Opposed-piston engine]]s put two pistons working at opposite ends of the same cylinder and this has been extended into triangular arrangements such as the [[Napier Deltic]]. Some designs have set the cylinders in motion around the shaft, such as the [[rotary engine]].

[[Image:BetaStirlingTG4web.svg|thumb|250px|'''Stirling piston engine''' [[Rhombic drive|Rhombic Drive]] – Beta Stirling Engine Design, showing the second displacer piston (green) within the cylinder, which shunts the working gas between the hot and cold ends, but produces no power itself.
{{ordered list
 | {{colorbox|#e47ab0}} Hot cylinder wall
 | {{colorbox|#8aa7a6}} Cold cylinder wall
}}{{ordered list
 | start=5
 | {{colorbox|#7ff280}} Displacer piston
 | {{colorbox|#046fe2}} Power piston
 | {{colorbox|#74e0e0}} Flywheels
}}]]

In some steam engines, the cylinders may be of varying size with the smallest bore cylinder working the highest pressure steam. This is then fed through one or more, increasingly larger bore cylinders successively, to extract power from the steam at increasingly lower pressures. These engines are called [[compound engine]]s.

Aside from looking at the power that the engine can produce, the mean effective pressure (MEP), can also be used in comparing the power output and performance of reciprocating engines of the same size. The mean effective pressure is the fictitious pressure which would produce the same amount of net work that was produced during the power stroke cycle. This is shown by:
: <math>W_{net}= \text{MEP}\cdot A_pS = \text{MEP} \cdot V_d</math>

where <math>A_p</math> is the total piston area of the engine, <math>S</math> is the stroke length of the pistons, and <math>V_d</math> is the total displacement volume of the engine. Therefore:
: <math>\text{MEP} = \frac{W_{net}}{V_d}</math>

Whichever engine with the larger value of MEP produces more net work per cycle and performs more efficiently.<ref name="Cengal" />