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{{short description|Machine that converts one or more forms of energy into mechanical energy (of motion)}}
{{hatnote group|
{{Other uses}}
{{redirect|Motor}}
}}

[[File:4StrokeEngine Ortho 3D Small.gif|thumb|right|225px|An animation showing the four stages of the [[Four-stroke engine|four-stroke]] gasoline-fueled internal combustion cycle with [[spark plug|electrical ignition source]]: {{ordered list |Induction ''(Fuel enters)'' |Compression |Ignition ''(Fuel is burnt)''|Emission ''(Exhaust out)''}}]]
[[File:Jet engine.svg|thumb|right|450px|[[Jet engines]] use the [[heat of combustion]] to generate a high-velocity exhaust as a form of [[reaction engine]]. [[Mechanical energy]] to power the aircraft's electrical and [[hydraulic]] systems can be taken from the turbine shaft, but [[thrust]] is produced by expelled exhaust gas.]]

An '''engine''' or '''motor''' is a [[machine]] designed to convert one or more forms of [[energy]] into [[motion (physics)|mechanical energy]].<ref>{{cite web |url=http://dictionary.reference.com/browse/motor |title=Motor |quote=a person or thing that imparts motion, esp. a contrivance, as a steam engine, that receives and modifies energy from some source in order to use it in driving machinery. |publisher=Dictionary.reference.com |access-date=2011-05-09 |archive-date=2008-04-07 |archive-url=https://wayback.archive-it.org/all/20080407154945/http://dictionary.reference.com/browse/Motor |url-status=live }}</ref><ref>[http://dictionary.reference.com/browse/motor Dictionary.com: (World heritage)] {{Webarchive|url=https://wayback.archive-it.org/all/20080407154945/http://dictionary.reference.com/browse/Motor |date=2008-04-07 }} "3. any device that converts another form of energy into mechanical energy so as to produce motion"</ref>

Available energy sources include [[potential energy]] (e.g. energy of the Earth's [[gravitational field]] as exploited in [[hydroelectric power generation]]), heat energy (e.g. [[Geothermal energy|geothermal]]), [[chemical energy]], [[electric potential]] and nuclear energy (from [[nuclear fission]] or [[nuclear fusion]]). Many of these processes generate heat as an intermediate energy form; thus [[heat engine]]s have special importance. Some natural processes, such as atmospheric [[convection cell]]s convert environmental heat into motion (e.g. in the form of rising air currents). Mechanical energy is of particular importance in [[transportation]], but also plays a role in many industrial processes such as cutting, grinding, crushing, and mixing.

Mechanical heat engines convert [[heat]] into work via various thermodynamic processes. The [[internal combustion engine]] is perhaps the most common example of a mechanical heat engine in which heat from the [[combustion]] of a [[fuel]] causes rapid pressurisation of the gaseous combustion products in the combustion chamber, causing them to expand and drive a [[piston]], which turns a [[crankshaft]]. Unlike internal combustion engines, a [[reaction engine]] (such as a [[jet engine]]) produces [[thrust]] by expelling [[reaction mass]], in accordance with [[Newton's third law of motion]].

Apart from heat engines, [[electric motor]]s convert electrical energy into [[machine (mechanical)|mechanical]] motion, [[pneumatic motor]]s use [[compressed air]], and [[clockwork motor]]s in [[wind-up toy]]s use [[elastic energy]]. In biological systems, [[molecular motor]]s, like [[myosin]]s in [[muscle]]s, use [[chemical energy]] to create forces and ultimately motion (a chemical engine, but not a heat engine).

Chemical heat engines which employ air (ambient atmospheric gas) as a part of the fuel reaction are regarded as airbreathing engines. Chemical heat engines designed to operate outside of Earth's atmosphere (e.g. [[rocket]]s, deeply submerged [[submarine]]s) need to carry an additional fuel component called the [[oxidizer]] (although there exist [[Superoxidant|super-oxidizers]] suitable for use in rockets, such as [[fluorine]], a more powerful oxidant than oxygen itself); or the application needs to obtain heat by non-chemical means, such as by means of [[nuclear reaction]]s.

==Emission/Byproducts==
All chemically fueled heat engines emit exhaust gases. The cleanest engines emit water only. Strict [[zero-emissions]] generally means zero emissions other than water and water vapour. Only heat engines which combust pure hydrogen (fuel) and pure oxygen (oxidizer) achieve zero-emission by a strict definition (in practice, one type of rocket engine). If hydrogen is burnt in combination with air (all airbreathing engines), a side reaction occurs between atmospheric oxygen and atmospheric [[nitrogen]] resulting in small emissions of {{NOx|link=yes}}. If a [[hydrocarbon]] (such as [[Alcohol fuel|alcohol]] or gasoline) is burnt as fuel, {{CO2|link=yes}}, a [[greenhouse gas]], is emitted. Hydrogen and oxygen from air can be reacted into water by a [[fuel cell]] without side production of {{NOx}}, but this is an [[electrochemical cell|electrochemical]] engine not a heat engine.

== Terminology ==
The word ''engine'' derives from  [[Old French]] {{wikt-lang|fro|engin|engin}}, from the [[Latin]] {{lang|la|ingenium}}–the root of the word {{wikt-lang|la|ingenious|ingenious}}. Pre-industrial weapons of war, such as [[catapult]]s, [[trebuchet]]s and [[battering ram]]s, were called ''[[siege engine]]s'', and knowledge of how to construct them was often treated as a military secret. The word ''gin'', as in  ''[[cotton gin]]'', is short for ''engine''.  Most mechanical devices invented during the [[Industrial Revolution]] were described as engines—the steam engine being a notable example. However, the original steam engines, such as those by [[Thomas Savery]], were not mechanical engines but pumps. In this manner, a [[fire engine]] in its original form was merely a water pump, with the engine being transported to the fire by horses.<ref>{{Cite web|title=World Wide Words: Engine and Motor|url=http://www.worldwidewords.org/articles/engine.htm|website=World Wide Words|language=en-gb|access-date=2020-04-30|archive-date=2019-04-25|archive-url=https://web.archive.org/web/20190425010153/http://www.worldwidewords.org/articles/engine.htm|url-status=live}}</ref>

In modern usage, the term ''engine'' typically describes devices, like steam engines and internal combustion engines, that burn or otherwise consume fuel to perform [[mechanical work]] by exerting a [[torque]] or linear [[force]] (usually in the form of [[thrust]]). Devices converting heat energy into motion are commonly referred to simply as ''engines''.<ref>{{cite web |title=Engine |work=Collins English Dictionary |access-date=2012-09-03 |url=http://www.collinsdictionary.com/dictionary/english/Engine |archive-date=2012-08-29 |archive-url=https://web.archive.org/web/20120829024422/http://www.collinsdictionary.com/dictionary/english/engine |url-status=live }}</ref> Examples of engines which exert a torque include the familiar automobile gasoline and diesel engines, as well as [[turboshaft]]s. Examples of engines which produce thrust include [[turbofan]]s and [[rocket]]s.

When the internal combustion engine was invented, the term ''motor'' was initially used to distinguish it from the steam engine—which was in wide use at the time, powering locomotives and other vehicles such as [[steamroller|steam rollers]]. The term ''[[wikt:motor|motor]]'' derives from the Latin verb {{Wikt-lang|la|moto#Verb|moto}} which means 'to set in motion', or 'maintain motion'. Thus a motor is a device that imparts motion.

''Motor'' and ''engine'' are interchangeable in standard English.<ref>Dictionary definitions:
* {{OED|motor}}
* {{OED|engine}}
* {{Cite Merriam-Webster|motor}}
* {{Cite Merriam-Webster|engine}}
* {{Cite Dictionary.com|motor}}
* {{Cite Dictionary.com|engine}}</ref> In some engineering jargons, the two words have different meanings, in which ''[[wikt:engine|engine]]'' is a device that [[combustion|burns]] or otherwise consumes fuel, changing its chemical composition, and a motor is a device driven by [[Electric motor|electricity]], [[pneumatic motor|air]], or [[hydraulic motor|hydraulic]] pressure, which does not change the chemical composition of its energy source.<ref>"Engine", ''McGraw-Hill Concise Encyclopedia of Science and Technology'', Third Edition, Sybil P. Parker, ed. McGraw-Hill, Inc., 1994, p. 714.</ref><ref>{{Cite web |url=http://www.worldwidewords.org/articles/engine.htm |title=World Wide Words: Engine and Motor |last=Quinion |first=Michael |website=Worldwide Words |access-date=2018-02-03 |archive-date=2019-04-25 |archive-url=https://web.archive.org/web/20190425010153/http://www.worldwidewords.org/articles/engine.htm |url-status=live }}</ref> However, [[High-power rocketry|rocketry]] uses the term [[Model rocket motor classification|rocket motor]], even though they consume fuel.

A heat engine may also serve as a ''[[wikt:prime mover|prime mover]]''—a component that transforms the flow or changes in pressure of a [[Fluid mechanics|fluid]] into [[mechanical energy]].<ref>"Prime mover", ''McGraw-Hill Concise Encyclopedia of Science and Technology'', Third Edition, Sybil P. Parker, ed. McGraw-Hill, Inc., 1994, p. 1498.</ref> An [[automobile]] powered by an internal combustion engine may make use of various motors and pumps, but ultimately all such devices derive their power from the engine. Another way of looking at it is that a motor receives power from an external source, and then converts it into mechanical energy, while an engine creates power from pressure (derived directly from the explosive force of combustion or other [[chemical]] reaction, or secondarily from the action of some such force on other substances such as air, water, or steam).<ref>{{cite book|editor-last=Goldstein|editor-first=Norm|title=The Associated Press Stylebook and Briefing on Media Law |year=2007|publisher=Basic Books|location=New York |isbn=978-0-465-00489-8|page=84|edition=42nd}}</ref>{{better source needed|reason= You are citing an Associated Press style guide? Really? Surely there is a better source.|date=February 2024}}

== History ==
=== Antiquity ===
[[Simple machine]]s, such as the [[Mace (bludgeon)|club]] and [[oar]] (examples of the [[lever]]), are [[Prehistory|prehistoric]]. More complex engines using [[human power]], [[Working animals|animal power]], [[Water wheel|water power]], [[Windmill|wind power]] and even steam power date back to antiquity. Human power was focused by the use of simple engines, such as the [[Capstan (nautical)|capstan]], [[windlass]] or [[Treadwheel|treadmill]], and with [[rope]]s, [[pulley]]s, and [[block and tackle]] arrangements; this power was transmitted usually with the forces [[mechanical advantage|multiplied]] and the speed [[gear reduction|reduced]]. These were used in [[Crane (machine)|cranes]] and aboard [[ship]]s in [[Ancient Greece]], as well as in [[Mining|mines]], [[Pump|water pumps]] and [[siege engines]] in [[Ancient Rome]]. The writers of those times, including [[Vitruvius]], [[Frontinus]] and [[Pliny the Elder]], treat these engines as commonplace, so their invention may be more ancient. By the 1st century AD, [[cattle]] and [[horse]]s were used in [[Mill (grinding)|mills]], driving machines similar to those powered by humans in earlier times.

According to [[Strabo]], a water-powered mill was built in Kaberia of the [[Parthian Empire|kingdom of Mithridates]] during the 1st century BC. Use of [[water wheel]]s in mills spread throughout the [[Roman Empire]] over the next few centuries. Some were quite complex, with [[aqueduct (bridge)|aqueducts]], [[dam]]s, and [[sluice]]s to maintain and channel the water, along with systems of [[gears]], or toothed-wheels made of wood and metal to regulate the speed of rotation. More sophisticated small devices, such as the [[Antikythera Mechanism]] used complex trains of gears and dials to act as calendars or predict astronomical events. In a poem by [[Ausonius]] in the 4th century AD, he mentions a stone-cutting saw powered by water. [[Hero of Alexandria]] is credited with many such [[wind]] and [[steam]] powered machines in the 1st century AD, including the [[Aeolipile]] and the [[vending machine]], often these machines were associated with worship, such as animated altars and automated temple doors.

=== Medieval ===
Medieval Muslim engineers employed [[gear]]s in mills and water-raising machines, and used [[dam]]s as a source of water power to provide additional power to watermills and water-raising machines.<ref name="Hassan">{{cite book|first1=Ahmad Y. |last1=Hassan |author-link1=Ahmad Y. Hassan |chapter-url=http://www.history-science-technology.com/Articles/articles%2071.htm |title=Transfer of Islamic Technology to the West, Part II |chapter=Transmission of Islamic Engineering |archive-url=https://web.archive.org/web/20080218171021/http://www.history-science-technology.com/Articles/articles%2071.htm |archive-date=2008-02-18 }}</ref> In the [[Islamic Golden Age|medieval Islamic world]], such advances made it possible to [[Mechanization|mechanize]] many industrial tasks previously carried out by [[manual labour]].

In 1206, [[al-Jazari]] employed a [[Crank (mechanism)|crank]]-[[conrod]] system for two of his water-raising machines. A rudimentary [[steam turbine]] device was described by [[Taqi al-Din Muhammad ibn Ma'ruf|Taqi al-Din]]<ref name="Hassan1">[[Ahmad Y Hassan|Hassan, Ahmad Y.]] (1976). ''Taqi al-Din and Arabic Mechanical Engineering'', pp. 34–35. Institute for the History of Arabic Science, [[University of Aleppo]].</ref> in 1551 and by [[Giovanni Branca]]<ref name="Giovanni">{{cite web|url=http://www.history.rochester.edu/steam/thurston/1878/Chapter1.html |title=University of Rochester, NY, ''The growth of the steam engine'' online history resource, chapter one |publisher=History.rochester.edu |access-date=2010-02-03 |archive-url=https://web.archive.org/web/20120204034636/http://www.history.rochester.edu/steam/thurston/1878/Chapter1.html |archive-date=2012-02-04 }}</ref> in 1629.<ref>{{cite book |url=https://books.google.com/books?id=Cv9LH4ckuEwC&pg=PA432 |title=Power plant engineering |first=P.K. |last=Nag |year=2002 |publisher=[[Tata McGraw-Hill]] |page=432 |isbn=0-07-043599-5}}</ref>

In the 13th century, the solid [[rocket motor]] was invented in China. Driven by gunpowder, this simplest form of internal combustion engine was unable to deliver sustained power, but was useful for propelling weaponry at high speeds towards enemies in battle and for [[fireworks]]. After invention, this innovation spread throughout Europe.

=== Industrial Revolution ===
[[Image:Boulton and Watt centrifugal governor-MJ.jpg|thumb|upright|Boulton & Watt engine of 1788]]
The [[Watt steam engine]] was the first type of steam engine to make use of steam at a pressure just above [[atmospheric pressure|atmospheric]] to drive the piston helped by a partial vacuum. Improving on the design of the 1712 [[Newcomen steam engine]], the Watt steam engine, developed sporadically from 1763 to 1775, was a great step in the development of the steam engine. Offering a dramatic increase in [[fuel efficiency]], [[James Watt]]'s design became synonymous with steam engines, due in no small part to his business partner, [[Matthew Boulton]]. It enabled rapid development of efficient semi-automated factories on a previously unimaginable scale in places where waterpower was not available. Later development led to [[steam locomotive]]s and great expansion of [[Rail transport|railway transportation]].

As for internal combustion [[piston engine]]s, these were tested in France in 1807 by [[de Rivaz]] and independently, by the [[Nicéphore Niépce|Niépce brothers]]. They were theoretically advanced by [[Nicolas Léonard Sadi Carnot|Carnot]] in 1824.{{citation needed|date=May 2011}} In 1853–57 [[Eugenio Barsanti]] and [[Felice Matteucci]] invented and patented an engine using the free-piston principle that was possibly the first 4-cycle engine.<ref>{{cite web|title=La documentazione essenziale per l'attribuzione della scoperta|url=http://www.barsantiematteucci.it/inglese/documentiStorici.html|quote=A later request was presented to the Patent Office of the Reign of Piedmont, under No. 700 of Volume VII of that Office. The text of this patent request is not available, only a photo of the table containing a drawing of the engine. This may have been either a new patent or an extension of a patent granted three days earlier, on 30 December 1857, at Turin.|access-date=24 February 2014|archive-date=25 February 2017|archive-url=https://web.archive.org/web/20170225042248/http://www.barsantiematteucci.it/inglese/documentiStorici.html}}</ref>

The invention of an [[internal combustion engine]] which was later commercially successful was made during 1860 by [[Etienne Lenoir]].<ref>Victor Albert Walter Hillier, Peter Coombes – [https://books.google.com/books?id=DoYaRsNFlEYC&dq=cc+engine&pg=PA34 ''Hillier's Fundamentals of Motor Vehicle Technology'', Book 1] Nelson Thornes, 2004  {{ISBN|0-7487-8082-3}} [Retrieved 2016-06-16]</ref>

In 1877, the [[Otto cycle]] was capable of giving a far higher [[power-to-weight ratio]] than steam engines and worked much better for many transportation applications such as cars and aircraft.

[[File:Mercedes V6 DTM Rennmotor 1996.jpg|thumb|upright=1.2|A V6 [[internal combustion engine]] from a [[Mercedes-Benz]]]]

=== Automobiles ===
The first commercially successful automobile, created by [[Karl Benz]], added to the interest in light and powerful engines. The lightweight gasoline internal combustion engine, operating on a four-stroke Otto cycle, has been the most successful for light automobiles, while the thermally more-efficient [[Diesel engine]] is used for trucks and buses. However, in recent years, [[turbocharged]] Diesel engines have become increasingly popular in automobiles, especially outside of the United States, even for quite small cars.

==== Horizontally-opposed pistons ====
In 1896, Karl Benz was granted a patent for his design of the first engine with horizontally opposed pistons. His design created an engine in which the corresponding pistons move in horizontal cylinders and reach top dead center simultaneously, thus automatically balancing each other with respect to their individual momentum. Engines of this design are often referred to as “flat” or “boxer” engines due to their shape and low profile. They were used in the [[Volkswagen Beetle]], the [[Citroën 2CV]], some Porsche and Subaru cars, many [[BMW]] and [[Honda]] [[motorcycle]]s.  Opposed four- and six-cylinder engines continue to be used as [[aircraft engine|a power source]] in small, [[propeller (aeronautics)|propeller-driven]] [[aircraft]].

==== Advancement ====
The continued use of internal combustion engines in automobiles is partly due to the improvement of engine control systems, such as on-board computers providing engine management processes, and electronically controlled fuel injection. Forced air induction by turbocharging and supercharging have increased the power output of smaller displacement engines that are lighter in weight and more fuel-efficient at normal cruise power. Similar changes have been applied to smaller Diesel engines, giving them almost the same performance characteristics as gasoline engines. This is especially evident with the popularity of smaller diesel engine-propelled cars in Europe.  Diesel engines produce lower [[hydrocarbon]] and {{CO2}} emissions, but greater [[Atmospheric particulate matter|particulate]] and {{NOx|link=yes}} pollution, than gasoline engines.<ref name=Harrison2001>{{Citation |title= Pollution: Causes, Effects and Control |first= Roy M. |last= Harrison |author-link=Roy M. Harrison|edition=4th |publisher= [[Royal Society of Chemistry]] |year= 2001 |isbn= 978-0-85404-621-8 }}</ref> Diesel engines are also 40% more fuel efficient than comparable gasoline engines.<ref name=Harrison2001/>

==== Increasing power ====
In the first half of the 20th century, a trend of increasing engine power occurred, particularly in the U.S. models.{{Clarify|reason=As opposed to what models?|date=June 2012}} Design changes incorporated all known methods of increasing engine capacity, including increasing the pressure in the cylinders to improve efficiency, increasing the size of the engine, and increasing the rate at which the engine produces work. The higher forces and pressures created by these changes created engine vibration and size problems that led to stiffer, more compact engines with V and opposed cylinder layouts replacing longer straight-line arrangements.

==== Combustion efficiency ====
Optimal combustion efficiency in passenger vehicles is reached with a coolant temperature of around {{convert|230|F|C|order=flip|0}}.<ref>{{Cite web|last=McKnight|first=Bill|date=August 2017|title=The Electrically Assisted Thermostat|url=https://www.motor.com/magazine-summary/electrically-assisted-thermostat|url-status=live|access-date=2021-03-13|website=Motor.com|language=en-US|archive-date=2021-05-03|archive-url=https://web.archive.org/web/20210503012608/https://www.motor.com/magazine-summary/electrically-assisted-thermostat}}</ref>

==== Engine configuration ====
Earlier automobile engine development produced a much larger range of engines than is in common use today. Engines have ranged from 1- to 16-cylinder designs with corresponding differences in overall size, weight, [[engine displacement]], and cylinder [[Bore (engine)|bores]]. Four cylinders and power ratings from 19 to 120&nbsp;hp (14 to 90&nbsp;kW) were followed in a majority of the models. Several three-cylinder, two-stroke-cycle models were built while most engines had straight or in-line cylinders. There were several V-type models and horizontally opposed two- and four-cylinder makes too. Overhead [[camshaft]]s were frequently employed. The smaller engines were commonly air-cooled and located at the rear of the vehicle; compression ratios were relatively low. The 1970s and 1980s saw an increased interest in improved [[Fuel economy in automobiles|fuel economy]], which caused a return to smaller V-6 and four-cylinder layouts, with as many as five valves per cylinder to improve efficiency. The [[Bugatti Veyron]] 16.4 operates with a [[W16 engine]], meaning that two [[V8 engine|V8]] cylinder layouts are positioned next to each other to create the W&nbsp;shape sharing the same crankshaft.

The largest internal combustion engine ever built is the [[Wärtsilä-Sulzer RTA96-C]], a 14-cylinder, 2-stroke turbocharged diesel engine that was designed to power the ''[[Emma Mærsk]]'', the largest container ship in the world when launched in 2006. This engine has a mass of 2,300 tonnes, and when running at 102&nbsp;rpm (1.7&nbsp;Hz) produces over 80 MW, and can use up to 250 tonnes of fuel per day.

== Types ==
An engine can be put into a category according to two criteria: the form of energy it accepts in order to create motion, and the type of motion it outputs.

=== Heat engine ===
{{Main|Heat engine}}

==== Combustion engine ====
Combustion engines are [[heat engine]]s driven by the heat of a [[combustion]] process.

==== Internal combustion engine ====
[[File:A three-horsepower internal combustion engine that ran on coal gas LCCN2006691790.jpg|thumb|254x254px|A three-horsepower internal combustion engine that ran on coal gas]]
{{Main|Internal combustion engine}}The ''internal combustion engine'' is an engine in which the [[combustion]] of a fuel (generally, [[fossil fuel]]) occurs with an oxidizer (usually air) in a [[combustion chamber]]. In an internal combustion engine the expansion of the high [[temperature]] and high [[pressure]] gases, which are produced by the combustion, directly applies [[force]] to components of the engine, such as the [[piston]]s or [[turbine blade]]s or a [[propulsive nozzle|nozzle]], and by moving it over a distance, generates mechanical [[work (physics)|work]].<ref name=r1 >{{cite encyclopedia|first=Charles Lafayette II |last=Proctor |url=https://www.britannica.com/EBchecked/topic/290504/internal-combustion-engine |title=Internal Combustion engines |encyclopedia=Encyclopædia Britannica Online |access-date=2011-05-09}}</ref><ref name=r2 >{{cite web |url=http://www.answers.com/topic/internal-combustion-engine?cat=technology |title=Internal combustion engine |publisher=Answers.com |access-date=2011-05-09 |archive-date=2011-06-28 |archive-url=https://web.archive.org/web/20110628184840/http://www.answers.com/topic/internal-combustion-engine?cat=technology |url-status=live }}</ref><ref name=r3>{{cite web|url=http://inventors.about.com/gi/dynamic/offsite.htm?site=http://www.bartleby.com/65/in/intern-co.html |archive-url=https://archive.today/20120721063446/http://inventors.about.com/gi/dynamic/offsite.htm?site=http://www.bartleby.com/65/in/intern-co.html |archive-date=2012-07-21 |title=Columbia encyclopedia: Internal combustion engine |publisher=Inventors.about.com |access-date=2011-05-09 }}</ref><ref name=r4 >{{cite web |url=http://www.infoplease.com/ce6/sci/A0825332.html |title=Internal-combustion engine |publisher=Infoplease.com |year=2007 |access-date=2011-05-09 |archive-date=2011-05-15 |archive-url=https://web.archive.org/web/20110515152409/http://www.infoplease.com/ce6/sci/A0825332.html |url-status=live }}</ref>

==== External combustion engine ====
{{Main|External combustion engine}}
An ''external combustion engine'' (EC engine) is a [[heat engine]] where an internal working [[fluid]] is heated by combustion of an external source, through the engine wall or a [[heat exchanger]]. The [[fluid]] then, by expanding and acting on the [[Mechanism (engineering)|mechanism]] of the engine produces motion and usable [[Mechanical work|work]].<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/external%20combustion |title=External combustion |publisher=Merriam-Webster Online Dictionary |date=2010-08-13 |access-date=2011-05-09 |archive-date=2018-06-27 |archive-url=https://web.archive.org/web/20180627202610/https://www.merriam-webster.com/dictionary/external%20combustion |url-status=live }}</ref> The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).

"[[Combustion]]" refers to [[burning]] fuel with an [[oxidizer]], to supply the heat. Engines of similar (or even identical) configuration and operation may use a supply of heat from other sources such as nuclear, solar, geothermal or exothermic reactions not involving combustion; but are not then strictly classed as external combustion engines, but as external thermal engines.

The working fluid can be a gas as in a [[Stirling engine]], or [[steam]] as in a steam engine or an organic liquid such as n-pentane in an [[Organic Rankine cycle]].  The fluid can be of any composition; gas is by far the most common, although even single-phase [[liquid]] is sometimes used.  In the case of the steam engine, the fluid changes [[Phase (matter)|phases]] between liquid and gas.

==== Air-breathing combustion engines ====

''Air-breathing combustion engines'' are combustion engines that use the [[oxygen]] in atmospheric air to [[oxidise]] ('burn') the fuel, rather than carrying an [[oxidiser]], as in a [[rocket]]. Theoretically, this should result in a better [[specific impulse]] than for rocket engines.

A continuous stream of air flows through the air-breathing engine. This air is compressed, mixed with fuel, ignited and expelled as the [[exhaust gas]]. In [[reaction engine]]s, the majority of the combustion energy (heat) exits the engine as exhaust gas, which provides thrust directly.

;Examples
Typical air-breathing engines include:
* [[Reciprocating engine]]
* [[Steam engine]]
* [[Gas turbine]]
* [[Airbreathing jet engine]]
* [[Turboprop|Turbo-propeller engine]]
* [[Pulse detonation engine]]
* [[Pulse jet]]
* [[Ramjet]]
* [[Scramjet]]
* [[Liquid air cycle engine]]/[[Reaction Engines SABRE]].

==== Environmental effects ====
The operation of engines typically has a negative impact upon [[air quality]] and ambient [[noise pollution|sound levels]]. There has been a growing emphasis on the pollution producing features of automotive power systems. This has created new interest in alternate power sources and internal-combustion engine refinements. Though a few limited-production battery-powered electric vehicles have appeared, they have not proved competitive owing to costs and operating characteristics.{{Citation needed|date=November 2012}} In the 21st century the diesel engine has been increasing in popularity with automobile owners. However, the gasoline engine and the Diesel engine,  with their new emission-control devices to improve emission performance, have not yet been significantly challenged.{{Citation needed|date=November 2012}} A number of manufacturers have introduced hybrid engines, mainly involving a small gasoline engine coupled with an electric motor and with a large battery bank, these are starting to become a popular option because of their environment awareness.

==== Air quality ====
[[Exhaust gas]] from a spark ignition engine consists of the following: [[nitrogen]] 70 to 75% (by volume), [[water vapor]] 10 to 12%, [[carbon dioxide]] 10 to 13.5%, [[hydrogen]] 0.5 to 2%, [[oxygen]] 0.2 to 2%, [[carbon monoxide]]: 0.1 to 6%, unburnt [[hydrocarbons]] and partial [[oxidation]] products (e.g. [[aldehyde]]s) 0.5 to 1%, [[nitrogen monoxide]] 0.01 to 0.4%, [[nitrous oxide]] <100 ppm, [[sulfur dioxide]] 15 to 60 ppm, traces of other compounds such as fuel additives and lubricants, also halogen and metallic compounds, and other particles.<ref>Paul Degobert, Society of Automotive Engineers (1995), ''Automobiles and Pollution''</ref> Carbon monoxide is highly toxic, and can cause [[carbon monoxide poisoning]], so it is important to avoid any build-up of the gas in a confined space. [[Catalytic converter]]s can reduce toxic emissions, but not eliminate them. Also, resulting greenhouse gas emissions, chiefly [[carbon dioxide]], from the widespread use of engines in the modern industrialized world is contributing to the global [[greenhouse effect]] – a primary concern regarding [[global warming]].

==== Non-combusting heat engines ====
{{Main|heat engine}}
Some engines convert heat from noncombustive processes into mechanical work, for example a nuclear power plant uses the heat from the nuclear reaction to produce steam and drive a steam engine, or a gas turbine in a rocket engine may be driven by decomposing [[hydrogen peroxide]]. Apart from the different energy source, the engine is often engineered much the same as an internal or external combustion engine.

Another group of noncombustive engines includes [[thermoacoustic heat engine]]s (sometimes called "TA engines") which are thermoacoustic devices that use high-amplitude sound waves to pump heat from one place to another, or conversely use a heat difference to induce high-amplitude sound waves. In general, thermoacoustic engines can be divided into standing wave and travelling wave devices.<ref>{{cite book |url=https://www.scribd.com/doc/147785416/Experimental-Investigations-on-a-Standing-Wave-Thermoacoustic-Engine#fullscreen |first=Mahmoud |last=Emam |title=Experimental Investigations on a Standing-Wave Thermoacoustic Engine, M.Sc. Thesis |publisher=Cairo University |location=Egypt |year=2013 |access-date=2013-09-26 |archive-date=2013-09-28 |archive-url=https://web.archive.org/web/20130928064211/http://www.scribd.com/doc/147785416/Experimental-Investigations-on-a-Standing-Wave-Thermoacoustic-Engine#fullscreen |url-status=live }}</ref>

[[Stirling engines]] can be another form of non-combustive heat engine. They use the Stirling thermodynamic cycle to convert heat into work. An example is the alpha type Stirling engine, whereby gas flows, via a [[recuperator]], between a hot cylinder and a cold cylinder, which are attached to reciprocating pistons 90° out of phase. The gas receives heat at the hot cylinder and expands, driving the piston that turns the [[crankshaft]]. After expanding and flowing through the recuperator, the gas rejects heat at the cold cylinder and the ensuing pressure drop leads to its compression by the other (displacement) piston, which forces it back to the hot cylinder.<ref>{{cite journal |first=Khaled M. |last=Bataineh |title=Numerical thermodynamic model of alpha-type Stirling engine |journal=Case Studies in Thermal Engineering |volume=12 |year=2018 |pages=104–116 |issn=2214-157X |doi=10.1016/j.csite.2018.03.010|doi-access=free }}</ref>

=== Non-thermal chemically powered motor ===
Non-thermal motors usually are powered by a chemical reaction, but are not heat engines. Examples include:
* [[Molecular motor]] – motors found in living things
* [[Synthetic molecular motor]].

=== Electric motor ===
{{Main|Electric motor|Electric vehicle}}
An ''electric motor'' uses [[electrical energy]] to produce [[mechanical energy]], usually through the interaction of [[magnetic fields]] and [[electrical conductor|current-carrying conductors]]. The reverse process, producing electrical energy from mechanical energy, is accomplished by a [[Electrical generator|generator]] or [[dynamo]]. [[Traction motor]]s used on vehicles often perform both tasks. Electric motors can be run as generators and vice versa, although this is not always practical.
Electric motors are ubiquitous, being found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, [[power tools]], and [[hard drive|disk drives]].  They may be powered by direct current (for example a [[battery (electric)|battery]] powered portable device or motor vehicle), or by [[alternating current]] from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the thousands of [[watt (unit)|kilowatts]]. Electric motors may be classified by the source of electric power, by their internal construction, and by their application.
[[File:Electric motor.gif|thumb|Electric motor]]
The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as 1821. Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electrical distribution networks.

To reduce the electric [[energy consumption]] from motors and their associated [[carbon footprint]]s, various regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher efficiency electric motors. A well-designed motor can convert over 90% of its input energy into useful power for decades.<ref>{{cite web |title=Motors |publisher=American Council for an Energy-Efficient Economy |url=http://www.aceee.org/topics/motors |archive-url=https://web.archive.org/web/20121023182642/http://aceee.org/topics/motors |archive-date=2012-10-23 }}</ref> When the efficiency of a motor is raised by even a few percentage points, the savings, in [[kilowatt hour]]s (and therefore in cost), are enormous. The electrical [[Efficient energy use|energy efficiency]] of a typical industrial [[induction motor]] can be improved by: 1) reducing the electrical losses in the [[stator]] windings (e.g., by increasing the cross-sectional area of the [[Electrical conductor|conductor]], improving the [[Inductor|winding]] technique, and using materials with higher [[Electrical conductivity|electrical conductivities]], such as [[copper]]), 2) reducing the electrical losses in the [[Rotor (electric)|rotor]] coil or casting (e.g., by using materials with higher electrical conductivities, such as copper), 3) reducing magnetic losses by using better quality magnetic [[steel]], 4) improving the [[aerodynamic]]s of motors to reduce mechanical windage losses, 5) improving [[Bearing (mechanical)|bearings]] to reduce [[friction loss]]es, and 6) minimizing manufacturing [[Engineering tolerance|tolerances]]. ''For further discussion on this subject, see [[Premium efficiency]]).''

By convention, ''electric engine'' refers to a railroad [[electric locomotive]], rather than an electric motor.

=== Physically powered motor ===
Some motors are powered by potential or kinetic energy, for example some [[funicular]]s, [[gravity plane]] and [[ropeway conveyor]]s have used the energy from moving water or rocks, and some clocks have a weight that falls under gravity. Other forms of potential energy include compressed gases (such as [[pneumatic motor]]s), springs ([[clockwork motor]]s) and [[Elastic band#Model use|elastic bands]].

Historic [[military]] [[siege engines]] included large [[catapult]]s, [[trebuchet]]s, and (to some extent) [[battering ram]]s were powered by potential energy.

==== Pneumatic motor ====
{{main|Pneumatic motor}}
A ''pneumatic motor'' is a machine that converts potential energy in the form of [[compressed air]] into [[mechanical work]]. Pneumatic motors generally convert the compressed air to mechanical work through either linear or rotary motion.  Linear motion can come from either a diaphragm or a piston actuator, while rotary motion is supplied by either a vane type air motor or piston air motor. Pneumatic motors have found widespread success in the hand-held tool industry and continual attempts are being made to expand their use to the transportation industry. However, pneumatic motors must overcome efficiency deficiencies before being seen as a viable option in the transportation industry.

==== Hydraulic motor ====
{{main|Hydraulic motor }}
A ''hydraulic motor'' derives its power from a [[pressure|pressurized]] [[liquid]]. This type of engine is used to move heavy loads and drive machinery.<ref>{{cite web
 |url=http://reference.howstuffworks.com/hydraulic-engine-encyclopedia.htm
 |title=Howstuffworks "Engineering"
 |publisher=Reference.howstuffworks.com
 |date=2006-01-29
 |access-date=2011-05-09
 |archive-url=https://web.archive.org/web/20090821175006/http://reference.howstuffworks.com/hydraulic-engine-encyclopedia.htm
 |archive-date=2009-08-21
}}</ref>

====Hybrid====
Some motor units can have multiple sources of energy. For example, a [[plug-in hybrid electric vehicle]]'s electric motor could source electricity from either a battery or from [[fossil fuels]] inputs via an internal combustion engine and a generator.

== Performance ==
The following are used in the assessment of the performance of an engine.

=== Speed ===
Speed refers to crankshaft rotation in piston engines and the speed of compressor/turbine rotors and electric motor rotors. It is typically measured in [[revolutions per minute]] (rpm).

=== Thrust ===
[[Thrust]] is the force exerted on an airplane as a consequence of its propeller or jet engine accelerating the air passing through it. It is also the force exerted on a ship as a consequence of its propeller accelerating the water passing through it.

=== Torque ===
[[Torque]] is a turning [[Moment (physics)|moment]] on a shaft and is calculated by multiplying the force causing the moment by its distance from the shaft.

=== Power ===
[[Power (physics)|Power]] is the measure of how fast work is done.

=== Efficiency ===
{{Main|Engine efficiency}}
Efficiency is a proportion of useful energy output compared to total input.

=== Sound levels ===
Vehicle noise is predominantly from the engine at low vehicle speeds and from tires and the air flowing past the vehicle at higher speeds.<ref>{{cite journal|first=C. Michael |last=Hogan |title=Analysis of Highway Noise |journal=Journal of Water, Air, and Soil Pollution |volume=2 |issue=3 |pages=387–92 |date=September 1973 |issn=0049-6979 |doi=10.1007/BF00159677 |bibcode=1973WASP....2..387H |s2cid=109914430 }}</ref> Electric motors are quieter than internal combustion engines. Thrust-producing engines, such as turbofans, turbojets and rockets emit the greatest amount of noise due to the way their thrust-producing, high-velocity exhaust streams interact with the surrounding stationary air.
Noise reduction technology includes intake and exhaust system [[muffler]]s (silencers) on gasoline and diesel engines and noise attenuation liners in turbofan inlets.

== Engines by use ==
Particularly notable kinds of engines include:
{{div col|colwidth=22em}}
* [[Aircraft engine]]
* [[Automobile engine]]
* [[Model engine]]
* [[Motorcycle engine]]
* [[Marine propulsion]] engines such as [[Outboard motor]]
* [[Non-road engine]] is the term used to define engines that are not used by vehicles on roadways.
* [[Railway locomotive]] engine
* [[Spacecraft propulsion]] engines such as [[Rocket engine]]
* [[Traction engine]]
{{div col end}}

== See also ==
{{div col|colwidth=22em}}
* [[Aircraft engine]]
* [[Automobile engine replacement]]
* [[Electric motor]]
* [[Engine cooling]]
* [[Engine swap]]
* [[Gasoline engine]]
* [[HCCI|HCCI engine]]
* [[Hesselman engine]]
* [[Hot bulb engine]]
* [[IRIS engine]]
* [[Micromotor]]
** [[Flagella]] – biological motor used by some microorganisms
** [[Nanomotor]]
** [[Molecular motor]]
** [[Synthetic molecular motor]]
** [[Adiabatic quantum motor]]
* [[Multifuel]]
* [[Reaction engine]]
* [[Solid-state engine]]
* [[Timeline of heat engine technology]]
* [[Timeline of motor and engine technology]]
{{div col end}}

== References ==
=== Citations ===
{{Reflist}}

=== Sources ===
{{Refbegin}}
* J.G. Landels, ''Engineering in the Ancient World'', {{ISBN|0-520-04127-5}}
{{Refend}}

== External links ==
{{Commons category|Engines}}
{{Wiktionary|engine}}
{{Wiktionary|motor}}
* {{US patent|194047}}
* {{usurped|1=[https://web.archive.org/web/20071106074405/http://www.animatedpiston.com/ Detailed Engine Animations]}}
* [http://www.gbm.dk/gbm/Motor-e.htm Working 4-Stroke Engine – Animation] {{Webarchive|url=https://web.archive.org/web/20190425010224/http://www.gbm.dk/gbm/Motor-e.htm |date=2019-04-25 }}
* [http://www.animatedengines.com Animated illustrations of various engines]
* [http://www.popularmechanics.com/cars/news/industry/5-alternative-engine-architectures?click=main_sr 5 Ways to Redesign the Internal Combustion Engine]
* [https://www.turfmagazine.com/maintenance/rebirth-of-the-gasoline-engine/ Article on Small SI Engines.]
* [https://www.turfmagazine.com/maintenance/rise-of-the-compact-diesel-engine-2/ Article on Compact Diesel Engines.]
* [https://web.archive.org/web/20211016054925/https://www.autocurious.com/2020/04/types-of-engines.html?m=1 Types Of Engines]
* [https://www.gutenberg.org/ebooks/42369 Motors] (1915) by [https://www.gutenberg.org/ebooks/author/635 James Slough Zerbe].

{{Authority control}}
{{Heat engines|state=uncollapsed}}
{{Thermodynamic cycles}}
{{Machines}}

[[Category:Engine technology| ]]
[[Category:Engines| ]]