Editing Engine (section)

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