A reciprocating engine, also often known as a piston engine, is typically a
heat engine that uses one or more
reciprocating pistons to convert high temperature and high
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
into a
rotating motion. This article describes the common features of all types. The main types are: the
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
, used extensively in
motor vehicles; the
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
, the mainstay of the
Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
; and the
Stirling engine for niche applications. Internal combustion engines are further classified in two ways: either a
spark-ignition (SI) engine, where the spark plug initiates the combustion; or a
compression-ignition (CI) engine, where the air within the cylinder is compressed,
thus heating it, so that the heated air ignites fuel that is injected
then
Then may refer to:
* Then language, spoken in Guizhou province of China
* "Then", a song on the 1970 album ''Time and a Word'' by English rock band Yes
* ''Then'' (Canadian series), a 1999 compilation album released in Canada
* ''Then'' (Misako ...
or
earlier.
[''Thermodynamics: An Engineering Approach'' by Yunus A. Cengal and Michael A. Boles]
Common features in all types

There may be one or more pistons. Each piston is inside a
cylinder, into which a gas is introduced, either already under pressure (e.g.
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
), or heated inside the cylinder either by
ignition of a fuel air mixture (
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
) 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 Center (BDC), or where the piston forms the largest volume in the cylinder. The piston is returned to the cylinder top (
Top Dead Centre) (TDC) by a
flywheel, the power from other pistons connected to the same shaft or (in a
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 "
exhausted" gases are removed from the cylinder by this
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
double-acting.
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 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 rings. 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
displacement of gas by the pistons moving in the cylinders usually measured in cubic centimetres (cm
3 or cc) or
litres (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
A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods.
The year 1886 is regarded ...
s typically have between four and eight, and
locomotives, and
ships may have a dozen cylinders or more. Cylinder capacities may range from 10 cm
3 or less in model engines up to thousands of liters in ships' engines.
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
bore/stroke ratio is the ratio of the diameter of the piston, or "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
in line, in a
V configuration,
horizontally opposite each other, or
radially around the crankshaft.
Opposed-piston engines 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.
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 engines.
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:
:
where
is the total piston area of the engine,
is the stroke length of the pistons, and
is the total displacement volume of the engine. Therefore:
:
Whichever engine with the larger value of MEP produces more net work per cycle and performs more efficiently.
Operations
In steam engines and internal combustion engines, valves are required to allow the entry and exit of gases at the correct times in the piston's cycle. These are worked by cams, eccentrics or cranks driven by the shaft of the engine. Early designs used the
D slide valve but this has been largely superseded by
Piston valve or
Poppet valve
A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...
designs. In steam engines the point in the piston cycle at which the steam inlet valve closes is called the
cutoff and this can often be controlled to adjust the
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...
supplied by the engine and improve efficiency. In some steam engines, the action of the valves can be replaced by an
oscillating cylinder.
Internal combustion engines operate through a sequence of strokes that admit and remove gases to and from the cylinder. These operations are repeated cyclically and an engine is said to be
2-stroke,
4-stroke or
6-stroke depending on the number of strokes it takes to complete a cycle.
The most common type is 4-stroke, which has following cycles.
# Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
# Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
# Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a
spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
# Exhaust: Also known as outlet. During the ''exhaust'' stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.
History
An early known example of rotary to reciprocating motion is the
crank
Crank may refer to:
Mechanisms
* Crank (mechanism), in mechanical engineering, a bent portion of an axle or shaft, or an arm keyed at right angles to the end of a shaft, by which motion is imparted to or received from it
* Crankset, the compone ...
mechanism. The earliest hand-operated cranks appeared in
China during the
Han Dynasty
The Han dynasty (, ; ) was an Dynasties in Chinese history, imperial dynasty of China (202 BC – 9 AD, 25–220 AD), established by Emperor Gaozu of Han, Liu Bang (Emperor Gao) and ruled by the House of Liu. The dynasty was preceded by th ...
(202 BC–220 AD).
[Needham, Joseph. (1986). Science and Civilization in China: Volume 4, Part 2, Mechanical Engineering. Taipei: Caves Books, Ltd. Pages 118–119.] The Chinese used the crank-and-connecting rod for operating querns as far back as the Western Han dynasty (202 BC - 9 AD). Eventually crank-and-connecting rods were used in the inter-conversion of rotary and reciprocating motion for other applications such as flour-sifting, silk-reeling machines, treadle spinning wheels, and furnace bellows driven either by horses or waterwheels.
Several
saw mills in
Roman Asia and
Byzantine Syria during the 3rd–6th centuries AD had a crank and
connecting rod mechanism which converted the rotary motion of a
water wheel
A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or buck ...
into the linear movement of saw blades.
In 1206, Arab engineer
Al-Jazari invented a
crankshaft.
The reciprocating engine developed in Europe during the 18th century, first as the
atmospheric engine
The atmospheric engine was invented by Thomas Newcomen in 1712, and is often referred to as the Newcomen fire engine (see below) or simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creati ...
then later as the
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
. These were followed by the
Stirling engine and
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
in the 19th century. Today the most common form of reciprocating engine is the internal combustion engine running on the combustion of
petrol
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic c ...
,
diesel,
Liquefied petroleum gas (LPG) or
compressed natural gas
Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in ...
(CNG) and used to power
motor vehicles and
engine power plant An engine power plant is a power station in which power comes from the combination of a reciprocating engine and an alternator.
Thanks to very short start-up time, Engine power plants can provide full output within few minutes (high flexibility) a ...
s.
One notable reciprocating engine from the World War II Era was the 28-cylinder,
Pratt & Whitney R-4360 Wasp Major radial engine. It powered the last generation of large piston-engined planes before jet engines and turboprops took over from 1944 onward. It had a total engine capacity of , and a high power-to-weight ratio.
The largest reciprocating engine in production at present, but not the largest ever built, is the
Wärtsilä-Sulzer RTA96-C turbocharged two-stroke diesel engine of 2006 built by
Wärtsilä. It is used to power the largest modern container ships such as the
Emma Mærsk. It is five stories high (), long, and weighs over in its largest 14 cylinders version producing more than 84.42 MW (114,800 bhp). Each cylinder has a capacity of , making a total capacity of for the largest versions.
Engine capacity
For piston engines, an engine's capacity is the
engine displacement, in other words the volume swept by all the pistons of an engine in a single movement. It is generally measured in
litres (l) or cubic inches (c.i.d., cu in, ''or'' in
3) for larger engines, and cubic centimetres (abbreviated cc) for smaller engines. All else being equal, engines with greater capacities are more powerful and consumption of fuel increases accordingly (although this is not true of every Reciprocating engine), although power and fuel consumption are affected by many factors outside of engine displacement.
Power
Reciprocating engines can be characterized by their
specific power, which is typically given in kilowatts per litre of
engine displacement (in the U.S. also
horsepower per cubic inch). The result offers an approximation of the peak power output of an engine. This is not to be confused with
fuel efficiency, since high efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 75 kW/L (1.65 hp/in
3).
Other modern non-internal combustion types
Reciprocating engines that are powered by compressed air, steam or other hot gases are still used in some applications such as to drive many modern torpedoes or as pollution-free motive power. Most steam-driven applications use
steam turbines, which are more efficient than piston engines.
The French-designed FlowAIR vehicles use compressed air stored in a cylinder to drive a reciprocating engine in a local-pollution-free urban vehicle.
Torpedoes may use a working gas produced by
high test peroxide or
Otto fuel II, which pressurize without combustion. The
Mark 46 torpedo, for example, can travel underwater at fuelled by Otto fuel without
oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ...
.
Reciprocating quantum heat engine
Quantum heat engines are devices that generate power from heat that flows from a hot to a cold reservoir.
The mechanism of operation of the engine can be described by the laws of
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...
.
Quantum refrigerators are devices that consume power with the purpose to pump heat from a cold to a hot reservoir.
In a reciprocating quantum heat engine, the working medium is a quantum system such as spin systems or a harmonic oscillator.
The
Carnot cycle and
Otto cycle are the ones most studied.
The quantum versions obey the laws of
thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
. In addition, these models can justify the assumptions of
endoreversible thermodynamics.
A theoretical study has shown that it is possible and practical to build a reciprocating engine that is composed of a single oscillating atom. This is an area for future research and could have applications in
nanotechnology.
Can a car engine be built out of a single particle?
Physorg, November 30, 2012 by Lisa Zyga. Accessed 01-12-12
Miscellaneous engines
There are a large number of unusual varieties of piston engines that have various claimed advantages, many of which see little if any current use:
* Free-piston engine
* Opposed-piston engine
* Swing-piston engine
*IRIS engine
The IRIS Engine is a design for a new type of internal combustion engine. Its inventors say that engines constructed using this design can be smaller, lighter and significantly more efficient than traditional engines of comparable horsepower and d ...
* Bourke engine
* Thermo-magnetic motor
See also
* Heat engine for a view of the thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
involved in these engines.
*For a contrasting approach using no pistons, see the pistonless rotary engine.
*For an historical perspective see Timeline of heat engine technology.
*Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...
** Steam locomotive
A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...
* Stirling engine
*Internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
** Otto cycle
** Diesel cycle
** Engine configuration for a discussion of the layout of the major components of a reciprocating piston internal combustion engine.
**Diesel engine
The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-ca ...
** Gasoline engine
Notes
External links
Combustion video
– in-cylinder combustion in an optically accessible, two-stroke engine
at Infoplease.
at the US Centennial of Flight Commission.
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Engine technology