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{{Английская Википедия/Панель перехода}}
{{Short description|Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber}}
{{Redirect|ICEV|the form of water ice |Ice V|the high speed train|Intercity Experimental{{!}}ICE V}}
{{Redirect|Internal combustion||Internal Combustion (disambiguation){{!}}Internal Combustion}}
{{Use American English|date=April 2021}}
{{Use dmy dates|date=November 2021}}

[[File:Four stroke engine diagram.jpg|thumb|Diagram of a cylinder as found in an overhead cam 4-stroke gasoline engine: {{ubl
    |'''C'''&nbsp;–&nbsp;[[crankshaft]]
    |'''E'''&nbsp;–&nbsp;exhaust [[camshaft]]
    |'''I'''&nbsp;–&nbsp;inlet [[camshaft]]
    |'''P'''&nbsp;–&nbsp;[[piston]]
    |'''R'''&nbsp;–&nbsp;[[connecting rod]]
    |'''S'''&nbsp;–&nbsp;[[spark plug]]
    |'''V'''&nbsp;–&nbsp;[[poppet valve|valves]]. red: exhaust, blue: intake.
    |'''W'''&nbsp;–&nbsp;[[water jacket|cooling water jacket]]
    |''gray structure''&nbsp;–&nbsp;[[engine block]]
}}]]

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 combustion engine, the expansion of the high-[[temperature]] and high-[[pressure]] gases produced by combustion applies direct [[force]] to some component of the engine. The force is typically applied to [[piston]]s ([[Reciprocating engine|piston engine]]), [[turbine blade]]s ([[gas turbine]]), a [[Wankel engine|rotor (Wankel engine)]], or a [[propulsive nozzle|nozzle]] ([[jet engine]]). This force moves the component over a distance, transforming [[chemical energy]] into [[kinetic energy]] which is used to propel, move or power whatever the engine is attached to.

The first commercially successful internal combustion engine was created by [[Étienne Lenoir]] around 1860,<ref name="EB1">{{cite encyclopedia|encyclopedia=Encyclopædia Britannica |url=https://www.britannica.com/EBchecked/topic/1350805/history-of-technology/10451/Internal-combustion-engine |title=History of Technology: Internal Combustion engines |publisher=Britannica.com |access-date=2012-03-20}}</ref> and the first modern internal combustion engine, known as the [[Otto engine]], was created in 1876 by [[Nicolaus Otto]]. The term ''[[wikt:internal combustion engine|internal combustion engine]]'' usually refers to an engine in which combustion is [[Intermittency|intermittent]], such as the more familiar [[two-stroke]] and [[four-stroke]] piston engines, along with variants, such as the [[Six-stroke engine|six-stroke]] piston engine and the [[Wankel engine|Wankel rotary engine]]. A second class of internal combustion engines use continuous combustion: [[gas turbine]]s, [[jet engine]]s and most [[rocket engine]]s, each of which are internal combustion engines on the same principle as previously described.<ref name="EB1" /><ref name=pulkrabek>{{cite book|title=Engineering Fundamentals of the Internal Combustion Engine|url=https://archive.org/details/engineeringfunda00pulk|url-access=limited|last=Pulkrabek|first=Willard W.|year=1997|page=[https://archive.org/details/engineeringfunda00pulk/page/n15 2]|isbn=978-0-13-570854-5|publisher=Prentice Hall}}</ref> ([[Firearm]]s are also a form of internal combustion engine,<ref name=pulkrabek /> though of a type so specialized that they are commonly treated as a separate category, along with weaponry such as mortars and anti-aircraft cannons.) In contrast, in [[external combustion engine]]s, such as [[steam engine|steam]] or [[Stirling engine]]s, energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids for ''external'' combustion engines include air, hot water, [[pressurized water reactor|pressurized water]] or even [[boiler]]-heated [[Sodium-cooled fast reactor|liquid sodium]].

While there are many stationary applications, most ICEs are used in mobile applications and are the primary power supply for [[vehicle]]s such as [[cars]], [[aircraft]] and [[boats]]. ICEs are typically powered by [[hydrocarbon]]-based fuels like [[natural gas]], [[gasoline]], [[diesel fuel]], or [[ethanol]]. [[Renewable fuels]] like [[biodiesel]] are used in compression ignition (CI) engines and [[Ethanol fuel|bioethanol]] or [[ETBE]] (ethyl tert-butyl ether) produced from bioethanol in spark ignition (SI) engines. As early as 1900 the inventor of the diesel engine, [[Rudolf Diesel]], was using peanut oil to run his engines.<ref>{{cite web|url=https://www.sciencedirect.com/topics/engineering/rudolf-diesel |title=Rudolf Diesel – an overview |publisher=Sciencedirect.com – topics |date=2016-01-01 |accessdate=2022-02-17}}</ref> Renewable fuels are commonly blended with fossil fuels.  [[Hydrogen]], which is rarely used, can be obtained from either fossil fuels or renewable energy.

== History ==
{{Main|History of the internal combustion engine}}
Various [[scientist]]s and [[engineer]]s contributed to the development of internal combustion engines. In 1791, [[John Barber (engineer)|John Barber]] developed the [[gas turbine]]. In 1794 Thomas Mead patented a [[gas engine]]. Also in 1794, Robert Street patented an internal combustion engine, which was also the first to use [[liquid fuel]], and built an engine around that time. In 1798, [[John Stevens (inventor, born 1749)|John Stevens]] built the first American internal combustion engine. In 1807, [[France|French]] engineers [[Nicéphore Niépce]] (who went on to invent [[photography]]) and [[Claude Niépce]] ran a prototype internal combustion engine, using controlled dust explosions, the [[Pyréolophore]], which was granted a patent by [[Napoleon|Napoleon Bonaparte]]. This engine powered a boat on the [[Saône]] river in France.<ref name="Nicéphore Niépce's House Museum 2021">{{cite web | title=The Pyréolophore: a new engine principle | website=Nicéphore Niépce's House Museum | date=2021-02-17 | url=https://photo-museum.org/the-pyreolophore-a-new-engine-principle/ | access-date=2021-04-03}}</ref><ref name="Paléo-Energétique 2019">{{cite web | title=The Pyreolophore engine | website=Paléo-Energétique | date=2019-09-09 | url=https://paleo-energetique.org/en/paleoinventions/the-pyreolophore-engine/ | access-date=2021-04-03}}</ref> In the same year, [[Switzerland|Swiss]] engineer [[François Isaac de Rivaz]] invented a hydrogen-based internal combustion engine and powered the engine by electric spark. In 1808, De Rivaz fitted his invention to a primitive working vehicle – "the world's first internal combustion powered automobile".<ref>{{cite book |last1=Eckermann |first1=Erik |title=The World History of the Automobile |date=2001 |publisher=Society of Automotive Engineers |location=Germany |isbn=978-0-7680-0800-5 |page=371 |url=https://books.google.com/books?id=yLZeQwqNmdgC&q=%22Fran%C3%A7ois+Isaac+de+Rivaz%22&pg=PA18 |access-date=21 September 2020}}</ref> In 1823, [[Samuel Brown (engineer)|Samuel Brown]] patented the first internal combustion engine to be applied industrially.

In 1854 in the UK, the Italian inventors [[Eugenio Barsanti]] and [[Felice Matteucci]] obtained the certification: "Obtaining Motive Power by the Explosion of Gases". In 1857 the Great Seal Patent Office conceded them patent No.1655 for the invention of an "Improved Apparatus for Obtaining Motive Power from Gases".<ref>{{cite book |url=https://books.google.com/books?id=m8TsygLyfSMC&q=patent+barsanti+matteucci+1857&pg=PA78 |title=Biographical Dictionary of the History of Technology|isbn=978-1-134-65020-0|last1=Day|first1=Lance|last2=McNeil|first2=Ian|year= 2002|publisher=Routledge }}</ref><ref>{{cite book |url=https://books.google.com/books?id=SWYfxp-VNgwC&q=patent+barsanti+matteucci+1857&pg=PA585|title=The Steam-Engine and Other Heat-Engines|isbn=978-1-107-61563-2|last1=Alfred Ewing|first1=J.|year=2013|publisher=Cambridge University Press }}</ref><ref>{{cite book |url=https://books.google.com/books?id=drZDDwAAQBAJ&q=patent+barsanti+matteucci+1857&pg=PA204|title=Physics of Energy|isbn=978-1-107-01665-1|last1=Jaffe|first1=Robert L.|last2=Taylor|first2=Washington|year=2018|publisher=Cambridge University Press }}</ref><ref>{{cite patent |country=GB |number=185401072 |title=Obtaining motive power by the explosion of gases |invent1=Barsanti, Eugenio |invent2=Matteucci, Felice  |pridate=1854-05-13}}</ref> Barsanti and Matteucci obtained other patents for the same invention in France, Belgium and Piedmont between 1857 and 1859.<ref>{{cite web |url=http://www.brovedanigroup.com/wp-content/uploads/2015/03/Calendario-2015.pdf |title=The invention of the internal combustion engine. A spark of italian creativity |access-date=20 June 2019 |archive-date=20 June 2019 |archive-url=https://web.archive.org/web/20190620052429/http://www.brovedanigroup.com/wp-content/uploads/2015/03/Calendario-2015.pdf |url-status=dead }}</ref><ref>{{cite web|url=http://www.barsantiematteucci.it/en/i-brevetti/#1543246398059-22e86b19-662e.|title=The patents|access-date=20 June 2019|archive-date=14 June 2020|archive-url=https://web.archive.org/web/20200614065848/http://www.barsantiematteucci.it/en/i-brevetti/#1543246398059-22e86b19-662e.|url-status=dead}}</ref> In 1860, [[Belgium|Belgian]] engineer [[Etienne Lenoir|Jean Joseph Etienne Lenoir]] produced a gas-fired internal combustion engine.<ref name="Encyclopedia Britannica Étienne Lenoir">{{cite web | title=Étienne Lenoir | website=Encyclopedia Britannica | url=https://www.britannica.com/biography/Etienne-Lenoir | access-date=2021-04-03}}</ref> In 1864, [[Nicolaus Otto]] patented the first atmospheric gas engine. In 1872, American [[George Brayton]] invented the first commercial liquid-fueled internal combustion engine. In 1876, [[Nicolaus Otto]] began working with [[Gottlieb Daimler]] and [[Wilhelm Maybach]], patented the compressed charge, four-cycle engine. In 1879, [[Karl Benz]] patented a reliable [[two-stroke]] gasoline engine. Later, in 1886, Benz began the first commercial production of motor vehicles with an internal combustion engine, in which a three-wheeled, four-cycle engine and chassis formed a single unit.<ref name="The Library of Congress">{{cite web | title=Who invented the automobile? | website=The Library of Congress | url=https://www.loc.gov/item/who-invented-the-automobile/ | access-date=2021-04-03 | archive-url=https://web.archive.org/web/20210201211947/https://www.loc.gov/everyday-mysteries/item/who-invented-the-automobile/ | archive-date=2021-02-01}}</ref> In 1892, [[Rudolf Diesel]] developed the first compressed charge, compression ignition engine. In 1926, [[Robert Goddard (scientist)|Robert Goddard]] launched the first liquid-fueled rocket. In 1939, the [[Heinkel He 178]] became the world's first [[jet aircraft]].

== Etymology ==
At one time, the word ''engine'' (via [[Old French]], from [[Latin]] ''ingenium'', "ability") meant any piece of [[machinery]]—a sense that persists in expressions such as ''[[siege engine]]''. A "motor" (from Latin ''motor'', "mover") is any machine that produces mechanical [[Power (physics)|power]]. Traditionally, [[electric motor]]s are not referred to as "engines"; however, combustion engines are often referred to as "motors". (An ''[[electric locomotive|electric engine]]'' refers to a [[locomotive]] operated by electricity.)

In boating, an internal combustion engine that is installed in the hull is referred to as an engine, but the engines that sit on the transom are referred to as motors.<ref>{{cite web |url=http://www.worldwidewords.org/articles/engine.htm |title=World Wide Words: Engine and Motor |website=World Wide Words |date=1998-12-27 |access-date=2016-08-31}}</ref>

== Applications ==
[[File:1970 AMX BBG-2part3.JPG|thumb|[[Reciprocating engine]] of a car]]
[[File:Montreal power backup.jpg|thumb|[[Diesel generator]] for backup power]]
Reciprocating piston engines are by far the most common power source for land and water [[vehicle]]s, including [[automobile]]s, [[motorcycle]]s, [[ship]]s and to a lesser extent, [[locomotive]]s (some are electrical but most use diesel engines<ref>{{cite book|title=Technology Today and Tomorrow|last=James|first=Fales|page=344}}</ref><ref>{{cite book|title=Extreme Machines on Land|last=Armentrout|first=Patricia|page=8}}</ref>). Rotary engines of the Wankel design are used in some automobiles, aircraft and motorcycles.  These are collectively known as internal-combustion-engine vehicles (ICEV).<ref name="DeLuchi1991">{{cite book|author=M. A. DeLuchi|title=Emissions of Greenhouse Gases from the Use of Transportation Fuels and Electricity: Main text|url=https://books.google.com/books?id=ZcTmKEe7ofcC&pg=PA100|year=1991|publisher=Center for Transportation Research, Argonne National Laboratory|pages=100–}}</ref>

Where high power-to-weight ratios are required, internal combustion engines appear in the form of [[combustion turbine]]s, or sometimes Wankel engines. [[Powered aircraft]] typically use an ICE which may be a reciprocating engine. Airplanes can instead use [[jet engine]]s and [[helicopter]]s can instead employ [[turboshaft]]s; both of which are types of turbines. In addition to providing propulsion, [[airliner]]s may employ a separate ICE as an [[auxiliary power unit]]. Wankel engines are fitted to many [[unmanned aerial vehicle]]s.

ICEs drive large electric generators that power electrical grids. They are found in the form of [[combustion turbine]]s with a typical electrical output in the range of some 100&nbsp;MW. [[Combined cycle power plant]]s use the high temperature exhaust to boil and superheat water steam to run a [[steam turbine]]. Thus, the efficiency is higher because more energy is extracted from the fuel than what could be extracted by the combustion engine alone.
Combined cycle power plants achieve efficiencies in the range of 50–60%. In a smaller scale, [[stationary engine]]s like [[gas engine]]s or [[diesel generator]]s are used for backup or for providing electrical power to areas not connected to an [[electric grid]].

[[Small engine]]s (usually 2‐stroke gasoline/petrol engines) are a common power source for [[lawnmower]]s, [[string trimmer]]s, [[chain saw]]s, [[leafblower]]s, [[pressure washer]]s, [[snowmobile]]s, [[jet ski]]s, [[outboard motor]]s, [[moped]]s, and [[motorcycle]]s.

== Classification ==
There are several possible ways to classify internal combustion engines.

=== Reciprocating ===
By number of strokes:
* [[Two-stroke engine]]
** Clerk cycle<ref name="first-hand.info_2_stroke_diesel">{{cite web |url=http://www.first-hand.info/TwoStrokeCycleDieselEngine.html |title=Two Stroke Cycle Diesel Engine |website=First Hand Info |access-date=2016-09-01 |archive-url=https://web.archive.org/web/20160823101051/http://www.first-hand.info/TwoStrokeCycleDieselEngine.html |archive-date=2016-08-23 |url-status=dead }}</ref>
** Day cycle
* [[Four-stroke engine]] ([[Otto cycle]])
* [[Six-stroke engine]]

By type of ignition:
* [[Compression-ignition engine]]
* [[Spark-ignition engine]] (commonly found as [[gasoline engine]]s)

By mechanical/thermodynamic cycle (these cycles are infrequently used but are commonly found in [[hybrid vehicles]], along with other vehicles manufactured for [[fuel efficiency]]<ref>{{cite web|last=Haseli|first=Yousef|title=Atkinson Cycle|url=https://www.sciencedirect.com/topics/engineering/atkinson-cycle|access-date=8 May 2022|website=Science Direct}}</ref>):
* [[Atkinson cycle]]
* [[Miller cycle]]

=== Rotary ===
{{Hatnote|For rotating-crankcase [[radial engine|radial]]-cylindered engines, see [[Rotary engine]].}}
* [[Wankel engine]]
*[[Pistonless rotary engine]]

=== Continuous combustion ===
* [[Gas turbine]] engine
** [[Turbojet]], through a propelling nozzle
** [[Turbofan]], through a duct-fan
** [[Turboprop]], through an unducted propeller, usually with variable pitch
** [[Turboshaft]], a gas turbine optimized for producing mechanical torque instead of thrust
* [[Ramjet]],<ref>{{cite web|last=Hall|first=Nancy|title=Editor|url=https://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.html|access-date=26 June 2020|website=NASA}}</ref> similar to a turbojet but uses vehicle speed to compress (ram) the air instead of a compressor.
* [[Scramjet]], a variant of the ramjet that uses supersonic combustion.
* [[Rocket engine]]

== Reciprocating engines ==
{{see also|Diesel engine|Gasoline engine|Reciprocating engine}}

=== Structure ===
[[File:Clevelandblock.jpg|thumb|Bare cylinder block of a V8 engine]]
[[File:Piston and connecting rod.jpg|thumb|left|upright=0.6|Piston, piston ring, gudgeon pin and connecting rod]]

The base of a reciprocating internal combustion engine is the [[engine block]], which is typically made of [[cast iron]] (due to its good wear resistance and low cost)<ref name="Heywood 2018 p. 11">{{harvnb|Heywood|2018|p=11}}</ref> or [[aluminum]]. In the latter case, the cylinder liners are made of cast iron or steel,<ref name="Denton 2011 p. 109">{{harvnb|Denton|2011|p=109}}</ref> or a coating such as [[nikasil]] or [[alusil]]. The engine block contains the [[cylinder (engine)|cylinders]]. In engines with more than one cylinder they are usually arranged either in 1 row ([[straight engine]]) or 2 rows ([[boxer engine]] or [[V engine]]); 3 or 4 rows are occasionally used ([[W engine]]) in contemporary engines, and other [[engine configuration]]s are possible and have been used. [[Single cylinder engine]]s (or ''thumpers'') are common for motorcycles and other small engines found in light machinery. On the outer side of the cylinder, passages that contain cooling fluid are cast into the engine block whereas, in some heavy duty engines, the passages are the types of removable cylinder sleeves which can be replaceable.<ref name="Heywood 2018 p. 11" /> Water-cooled engines contain passages in the engine block where cooling fluid circulates (the [[water jacket]]). Some small engines are air-cooled, and instead of having a water jacket the cylinder block has fins protruding away from it to cool the engine by directly transferring heat to the air. The cylinder walls are usually finished by [[honing (metalworking)#Cross-hatch finish|honing to obtain a cross hatch]], which is able to retain more oil. A too rough surface would quickly harm the engine by excessive wear on the piston.

The [[piston]]s are short cylindrical parts which seal one end of the cylinder from the high pressure of the compressed air and combustion products and slide continuously within it while the engine is in operation. In smaller engines, the pistons are made of aluminum; while in larger applications, they are typically made of cast iron.<ref name="Heywood 2018 p. 11" /> In performance applications, pistons can also be titanium or forged steel for greater strength. The top surface of the piston is called its ''crown'' and is typically flat or concave. Some two-stroke engines use pistons with a [[deflector piston|deflector head]]. Pistons are open at the bottom and hollow except for an integral reinforcement structure (the piston web). When an engine is working, the gas pressure in the combustion chamber exerts a force on the piston crown which is transferred through its web to a [[gudgeon pin]]. Each piston has [[piston ring|rings]] fitted around its circumference that mostly prevent the gases from leaking into the crankcase or the oil into the combustion chamber.<ref name="Yamagata 2005 p. 6">{{harvnb|Yamagata|2005|p=6}}</ref> A [[crankcase ventilation system|ventilation system]] drives the small amount of gas that escapes past the pistons during normal operation (the blow-by gases) out of the crankcase so that it does not accumulate contaminating the oil and creating corrosion.<ref name="Heywood 2018 p. 11" /> In two-stroke gasoline engines the crankcase is part of the air–fuel path and due to the continuous flow of it, two-stroke engines do not need a separate crankcase ventilation system.

[[File:Diesel engine valve train.JPG|thumb|Valve train above a diesel engine cylinder head. This engine uses rocker arms but no pushrods.]]

The [[cylinder head]] is attached to the engine block by numerous [[bolt (fastener)|bolts]] or [[stud (fastener)|studs]]. It has several functions. The cylinder head seals the cylinders on the side opposite to the pistons; it contains short ducts (the ''ports'') for intake and exhaust and the associated intake [[poppet valves|valves]] that open to let the cylinder be filled with fresh air and exhaust valves that open to allow the combustion gases to escape. However, 2-stroke crankcase scavenged engines connect the gas ports directly to the cylinder wall without poppet valves; the piston controls their opening and occlusion instead. The cylinder head also holds the [[spark plug]] in the case of spark ignition engines and the [[injector]] for engines that use direct injection. All CI (compression ignition) engines use fuel injection, usually direct injection but some engines instead use [[indirect injection]]. SI (spark ignition) engines can use a [[carburetor]] or fuel injection as port injection or [[gasoline direct injection|direct injection]]. Most SI engines have a single spark plug per cylinder but [[dual ignition|some have 2]]. A [[head gasket]] prevents the gas from leaking between the cylinder head and the engine block. The opening and closing of the valves is controlled by one or several [[camshaft]]s and springs—or in some engines—a [[desmodromic valve|desmodromic mechanism]] that uses no springs. The camshaft may press directly the stem of the valve or may act upon a [[rocker arm]], again, either directly or through a [[pushrod]].

[[File:18XER engine block.jpg|thumb|Engine block seen from below. The cylinders, oil spray nozzle and half of the main bearings are clearly visible.]]

The crankcase is sealed at the bottom with a [[sump]] that collects the falling oil during normal operation to be cycled again. The cavity created between the cylinder block and the sump houses a [[crankshaft]] that converts the reciprocating motion of the pistons to rotational motion. The crankshaft is held in place relative to the engine block by [[main bearing]]s, which allow it to rotate. Bulkheads in the crankcase form a half of every main bearing; the other half is a detachable cap. In some cases a single ''main bearing deck'' is used rather than several smaller caps. A [[connecting rod]] is connected to offset sections of the crankshaft (the [[crankpin]]s) in one end and to the piston in the other end through the gudgeon pin and thus transfers the force and translates the reciprocating motion of the pistons to the circular motion of the crankshaft. The end of the connecting rod attached to the gudgeon pin is called its small end, and the other end, where it is connected to the crankshaft, the big end. The big end has a detachable half to allow assembly around the crankshaft. It is kept together to the connecting rod by removable bolts.

The cylinder head has an [[intake manifold]] and an [[exhaust manifold]] attached to the corresponding ports. The intake manifold connects to the [[air filter]] directly, or to a carburetor when one is present, which is then connected to the [[air filter]]. It distributes the air incoming from these devices to the individual cylinders. The exhaust manifold is the first component in the [[exhaust system]]. It collects the exhaust gases from the cylinders and drives it to the following component in the path. The [[exhaust system]] of an ICE may also include a [[catalytic converter]] and [[muffler]]. The final section in the path of the exhaust gases is the [[tailpipe]].

=== Four-stroke engines ===
{{Main|Four-stroke engine}}
[[File:4StrokeEngine Ortho 3D Small.gif|thumb|Diagram showing the operation of a 4-stroke SI engine. Labels:<br />'''1''' ‐ Induction<br />'''2''' ‐ Compression<br />'''3''' ‐ Power<br />'''4''' ‐ Exhaust]]

The ''top dead center'' (TDC) of a piston is the position where it is nearest to the valves; ''bottom dead center'' (BDC) is the opposite position where it is furthest from them. A ''stroke'' is the movement of a piston from TDC to BDC or vice versa, together with the associated process. While an engine is in operation, the crankshaft rotates continuously at a nearly constant [[angular speed|speed]]. In a 4-stroke ICE, each piston experiences 2 strokes per crankshaft revolution in the following order. Starting the description at TDC, these are:{{sfn|Stone|1992|pp=1–2}}{{sfn|Nunney|2007|p=5}}

# '''Intake''', '''induction''' or '''suction''': The intake valves are open as a result of the cam lobe pressing down on the valve stem. The piston moves downward increasing the volume of the combustion chamber and allowing air to enter in the case of a CI engine or an air-fuel mix in the case of SI engines that do not use [[gasoline direct injection|direct injection]]. The air or air-fuel mixture is called the ''charge'' in any case.
# '''Compression''': In this stroke, both valves are closed and the piston moves upward reducing the combustion chamber volume which reaches its minimum when the piston is at TDC. The piston performs [[work (physics)|work]] on the charge as it is being compressed; as a result, its pressure, temperature and density increase; an approximation to this behavior is provided by the [[ideal gas law]]. Just before the piston reaches TDC, ignition begins. In the case of a SI engine, the spark plug receives a high voltage pulse that generates the spark which gives it its name and ignites the charge. In the case of a CI engine, the fuel injector quickly injects fuel into the combustion chamber as a spray; the fuel ignites due to the high temperature.
# '''Power''' or '''working stroke''': The pressure of the combustion gases pushes the piston downward, generating more [[kinetic energy]] than is required to compress the charge. Complementary to the compression stroke, the combustion gases expand and as a result their temperature, pressure and density decreases. When the piston is near to BDC the exhaust valve opens. In the ''blowdown'', the combustion gases expand [[irreversible process|irreversibly]] due to the leftover pressure—in excess of [[back pressure]], the gauge pressure on the exhaust port.
# '''Exhaust''': The exhaust valve remains open while the piston moves upward expelling the combustion gases. For naturally aspirated engines a small part of the combustion gases may remain in the cylinder during normal operation because the piston does not close the combustion chamber completely; these gases dissolve in the next charge. At the end of this stroke, the exhaust valve closes, the intake valve opens, and the sequence repeats in the next cycle. The intake valve may open before the exhaust valve closes to allow better scavenging.

=== Two-stroke engines ===
{{Main|Two-stroke engine}}
The defining characteristic of this kind of engine is that each piston completes a cycle every crankshaft revolution. The 4 processes of intake, compression, power and exhaust take place in only 2 strokes so that it is not possible to dedicate a stroke exclusively for each of them. Starting at TDC the cycle consists of:
# '''Power''': While the piston is descending the combustion gases perform work on it, as in a 4-stroke engine. The same [[thermodynamics]] for the expansion apply.
# '''Scavenging''': Around 75° of crankshaft rotation before BDC the exhaust valve or port opens, and blowdown occurs. Shortly thereafter the intake valve or transfer port opens.  The incoming charge displaces the remaining combustion gases to the exhaust system and a part of the charge may enter the exhaust system as well. The piston reaches BDC and reverses direction. After the piston has traveled a short distance upwards into the cylinder the exhaust valve or port closes; shortly the intake valve or transfer port closes as well.
# '''Compression''': With both intake and exhaust closed the piston continues moving upwards compressing the charge and performing work on it. As in the case of a 4-stroke engine, ignition starts just before the piston reaches TDC and the same consideration on the thermodynamics of the compression on the charge apply.

While a 4-stroke engine uses the piston as a [[positive displacement pump]] to accomplish scavenging taking 2 of the 4 strokes, a 2-stroke engine uses the last part of the power stroke and the first part of the compression stroke for combined intake and exhaust. The work required to displace the charge and exhaust gases comes from either the crankcase or a separate blower. For scavenging, expulsion of burned gas and entry of fresh mix, two main approaches are described: Loop scavenging, and Uniflow scavenging. SAE news published in the 2010s that 'Loop Scavenging' is better under any circumstance than Uniflow Scavenging.<ref name="first-hand.info_2_stroke_diesel"/>

==== Crankcase scavenged ====
[[File:Two-Stroke Engine.gif|thumb|Diagram of a crankcase scavenged valveless 2-stroke engine in operation]]

Some SI engines are crankcase scavenged and do not use poppet valves. Instead, the crankcase and the part of the cylinder below the piston is used as a pump. The intake port is connected to the crankcase through a [[reed valve]] or a rotary disk valve driven by the engine. For each cylinder, a transfer port connects in one end to the crankcase and in the other end to the cylinder wall. The exhaust port is connected directly to the cylinder wall. The transfer and exhaust port are opened and closed by the piston. The reed valve opens when the crankcase pressure is slightly below intake pressure, to let it be filled with a new charge; this happens when the piston is moving upwards. When the piston is moving downwards the pressure in the crankcase increases and the reed valve closes promptly, then the charge in the crankcase is compressed. When the piston is moving downwards, it also uncovers the exhaust port and the transfer port and the higher pressure of the charge in the crankcase makes it enter the cylinder through the transfer port, blowing the exhaust gases. Lubrication is accomplished by adding ''[[two-stroke oil]]'' to the fuel in small ratios. ''Petroil'' refers to the mix of gasoline with the aforesaid oil. This kind of 2-stroke engine has a lower efficiency than comparable 4-strokes engines and releases more polluting [[exhaust gases]] for the following conditions:
* They use a ''[[total-loss oiling system]]'': all the lubricating oil is eventually burned along with the fuel.
* There are conflicting requirements for scavenging: On one side, enough fresh charge needs to be introduced in each cycle to displace almost all the combustion gases but introducing too much of it means that a part of it gets in the exhaust.
* They must use the transfer port(s) as a carefully designed and placed nozzle so that a gas current is created in a way that it sweeps the whole cylinder before reaching the exhaust port so as to expel the combustion gases, but minimize the amount of charge exhausted. 4-stroke engines have the benefit of forcibly expelling almost all of the combustion gases because during exhaust the combustion chamber is reduced to its minimum volume. In crankcase scavenged 2-stroke engines, exhaust and intake are performed mostly simultaneously and with the combustion chamber at its maximum volume.

The main advantage of 2-stroke engines of this type is mechanical simplicity and a higher [[power-to-weight ratio]] than their 4-stroke counterparts. Despite having twice as many power strokes per cycle, less than twice the power of a comparable 4-stroke engine is attainable in practice.

In the US, 2-stroke engines were banned for road vehicles due to the pollution. Off-road only motorcycles are still often 2-stroke but are rarely road legal. However, many thousands of 2-stroke lawn maintenance engines are in use.{{citation need|date=July 2016}}

==== Blower scavenged ====
[[File:Diesel engine uniflow.svg|thumb|upright|Diagram of uniflow scavenging]]

Using a separate blower avoids many of the shortcomings of crankcase scavenging, at the expense of increased complexity which means a higher cost and an increase in maintenance requirement. An engine of this type uses ports or valves for intake and valves for exhaust, except [[opposed piston engine]]s, which may also use ports for exhaust. The blower is usually of the [[Roots blower|Roots-type]] but other types have been used too. This design is commonplace in CI engines, and has been occasionally used in SI engines.

CI engines that use a blower typically use ''[[uniflow scavenging]]''. In this design the cylinder wall contains several intake ports placed uniformly spaced along the circumference just above the position that the piston crown reaches when at BDC. An exhaust valve or several like that of 4-stroke engines is used. The final part of the intake manifold is an air sleeve that feeds the intake ports. The intake ports are placed at a horizontal angle to the cylinder wall (I.e: they are in plane of the piston crown) to give a swirl to the incoming charge to improve combustion. The largest reciprocating IC are low speed CI engines of this type; they are used for marine propulsion (see [[marine diesel engine]]) or [[electric power generation]] and achieve the highest thermal efficiencies among internal combustion engines of any kind. Some diesel-electric [[locomotive engine]]s operate on the 2-stroke cycle. The most powerful of them have a brake power of around 4.5&nbsp;[[watt|MW]] or 6,000&nbsp;[[horsepower|HP]]. The [[EMD SD90MAC]] class of locomotives are an example of such. The comparable class [[GE AC6000CW]], whose prime mover has almost the same brake power, uses a 4-stroke engine.

An example of this type of engine is the [[Wärtsilä-Sulzer RTA96-C]] turbocharged 2-stroke diesel, used in large container ships. It is the most efficient and powerful reciprocating internal combustion engine in the world with a [[thermal efficiency]] over 50%.<ref>{{cite web |url=http://www.ansys.com/assets/testimonials/siemens.pdf |title=CFX aids design of world's most efficient steam turbine |access-date=2010-08-28 |url-status=dead |archive-url=https://web.archive.org/web/20101116145711/http://ansys.com/assets/testimonials/siemens.pdf |archive-date=2010-11-16 }}</ref><ref>{{cite web |url=http://pepei.pennnet.com/display_article/152601/6/ARTCL/none/none/1/New-Benchmarks-for-Steam-Turbine-Efficiency/ |title=New Benchmarks for Steam Turbine Efficiency – Power Engineering |publisher=Pepei.pennnet.com |date=2010-08-24 |access-date=2010-08-28 |archive-date=18 September 2009 |archive-url=https://web.archive.org/web/20090918071420/http://pepei.pennnet.com/display_article/152601/6/ARTCL/none/none/1/New-Benchmarks-for-Steam-Turbine-Efficiency |url-status=dead }}</ref><ref>{{cite journal|url=https://www.mhi.co.jp/technology/review/pdf/e451/e451021.pdf |first1=Tatsuo |last1=Takaishi |first2=Akira |last2=Numata |first3=Ryouji |last3=Nakano |first4=Katsuhiko |last4=Sakaguchi |title=Approach to High Efficiency Diesel and Gas Engines |journal=Mitsubishi Heavy Industries Technical Review |volume=45 |issue=1 |date=March 2008 |access-date=2011-02-04}}</ref> For comparison, the most efficient small four-stroke engines are around 43% thermally-efficient (SAE 900648);{{citation needed|date=October 2014}} size is an advantage for efficiency due to the increase in the ratio of volume to surface area.

See the [[#External links|external links]] for an in-cylinder combustion video in a 2-stroke, optically accessible motorcycle engine.

====Historical design====
[[Dugald Clerk]] developed the first two-cycle engine in 1879. It used a separate cylinder which functioned as a pump in order to transfer the fuel mixture to the cylinder.<ref name="first-hand.info_2_stroke_diesel" />

In 1899 [[Joseph Day (inventor)|John Day]] simplified Clerk's design into the type of 2 cycle engine that is very widely used today.<ref name="first-hand.info_2_spark_ignition">{{cite web |url=http://www.first-hand.info/TwoStrokeSparkIgnitionEngine.html |title=Two Stroke Spark Ignition (S.I) Engine |website=First Hand Info |access-date=2016-09-01 |archive-url=https://web.archive.org/web/20160809121327/http://www.first-hand.info/TwoStrokeSparkIgnitionEngine.html |archive-date=2016-08-09 |url-status=dead }}</ref>
Day cycle engines are crankcase scavenged and port timed. The crankcase and the part of the cylinder below the exhaust port is used as a pump. The operation of the Day cycle engine begins when the crankshaft is turned so that the piston moves from BDC upward (toward the head) creating a vacuum in the crankcase/cylinder area. The carburetor then feeds the fuel mixture into the crankcase through a [[reed valve]] or a rotary disk valve (driven by the engine). There are cast in ducts from the crankcase to the port in the cylinder to provide for intake and another from the exhaust port to the exhaust pipe.  The height of the port in relationship to the length of the cylinder is called the "port timing".

On the first upstroke of the engine there would be no fuel inducted into the cylinder as the crankcase was empty. On the downstroke, the piston now compresses the fuel mix, which has lubricated the piston in the cylinder and the bearings due to the fuel mix having oil added to it. As the piston moves downward it first uncovers the exhaust, but on the first stroke there is no burnt fuel to exhaust. As the piston moves downward further, it uncovers the intake port which has a duct that runs to the crankcase. Since the fuel mix in the crankcase is under pressure, the mix moves through the duct and into the cylinder.

Because there is no obstruction in the cylinder of the fuel to move directly out of the exhaust port prior to the piston rising far enough to close the port, early engines used a high domed piston to slow down the flow of fuel. Later the fuel was "resonated" back into the cylinder using an expansion chamber design. When the piston rose close to TDC, a spark ignited the fuel. As the piston is driven downward with power, it first uncovers the exhaust port where the burned fuel is expelled under high pressure and then the intake port where the process has been completed and will keep repeating.

Later engines used a type of porting devised by the Deutz company to improve performance. It was called the [[Schnuerle porting|Schnurle Reverse Flow]] system. DKW licensed this design for all their motorcycles. Their [[DKW RT 125]] was one of the first motor vehicles to achieve over 100&nbsp;mpg as a result.<ref>{{cite web |url=http://www.audi.com/corporate/en/company/history/models/dkw-rt-1252h-1954.html |title=DKW RT 125/2H, 1954 > Models > History > AUDI AG |publisher=Audi |access-date=2016-09-01}}</ref>

=== Ignition ===
Internal combustion engines require ignition of the mixture, either by [[Spark-ignition engine|spark ignition (SI)]] or [[diesel engine|compression ignition (CI)]]. Before the invention of reliable electrical methods, hot tube and flame methods were used. Experimental engines with [[laser ignition]] have been built.<ref>{{cite news |url=http://www.physorg.com/news/2011-04-laser-revolution-internal-combustion.html |title=Laser sparks revolution in internal combustion engines |work=Physorg.com |date=2011-04-20 |access-date=2013-12-26}}</ref>

==== Spark ignition process ====
[[File:Bosch magneto (Army Service Corps Training, Mechanical Transport, 1911).jpg|thumb|Bosch magneto]]{{Main|Spark-ignition engine}}
[[File:Point Ignition.JPG|thumb|Points and coil ignition]]

The spark-ignition engine was a refinement of the early engines which used Hot Tube ignition. When Bosch developed the [[ignition magneto|magneto]] it became the primary system for producing electricity to energize a spark plug.<ref>{{cite web |url=http://theoldmotor.com/?p=135204 |title=The Early History of the Bosch Magneto Company in America |website=The Old Motor |date=2014-12-19 |access-date=2016-09-01}}</ref> Many small engines still use magneto ignition. Small engines are started by hand cranking using a [[recoil start]]er or hand crank. Prior to [[Charles F. Kettering]] of Delco's development of the automotive starter all gasoline engined automobiles used a hand crank.<ref>{{cite web |url=http://www.autolife.umd.umich.edu/Gender/Walsh/1911Auto_Comfort.htm |title=Hand Cranking the Engine |website=Automobile in American Life and Society |publisher=University of Michigan-Dearborn |access-date=2016-09-01}}</ref>

Larger engines typically power their [[Starter (engine)|starting motors]] and [[ignition system]]s using the electrical energy stored in a [[lead–acid battery]].  The battery's charged state is maintained by an [[Alternator#Automotive alternators|automotive alternator]] or (previously) a generator which uses engine power to create electrical energy storage.

The battery supplies electrical power for starting when the engine has a [[Starter (engine)|starting motor]] system, and supplies electrical power when the engine is off. The battery also supplies electrical power during rare run conditions where the alternator cannot maintain more than 13.8 volts (for a common 12V automotive electrical system). As alternator voltage falls below 13.8 volts, the lead-acid storage battery increasingly picks up electrical load. During virtually all running conditions, including normal idle conditions, the alternator supplies primary electrical power.

Some systems disable alternator field (rotor) power during wide-open throttle conditions. Disabling the field reduces alternator pulley mechanical loading to nearly zero, maximizing crankshaft power. In this case, the battery supplies all primary electrical power.

Gasoline engines take in a mixture of air and gasoline and compress it by the movement of the piston from bottom dead center to top dead center when the fuel is at maximum compression. The reduction in the size of the swept area of the cylinder and taking into account the volume of the combustion chamber is described by a ratio. Early engines had [[compression ratio]]s of 6 to 1. As compression ratios were increased, the efficiency of the engine increased as well.

With early induction and ignition systems the compression ratios had to be kept low. With advances in fuel technology and combustion management, high-performance engines can run reliably at 12:1 ratio. With low octane fuel, a problem would occur as the compression ratio increased as the fuel was igniting due to the rise in temperature that resulted. [[Charles Kettering]] developed a [[Tetraethyllead|lead additive]] which allowed higher compression ratios, which was progressively [[Gasoline#Additives|abandoned for automotive use]] from the 1970s onward, partly due to [[lead poisoning]] concerns.

The fuel mixture is ignited at different progressions of the piston in the cylinder. At low rpm, the spark is timed to occur close to the piston achieving top dead center. In order to produce more power, as rpm rises the spark is advanced sooner during piston movement. The spark occurs while the fuel is still being compressed progressively more as rpm rises.<ref>{{cite web |url=http://www.innovatemotorsports.com/resources/myths.php |title=Spark Timing Myths Debunked – Spark Timing Myths Explained: Application Notes |publisher=Innovate Motorsports |access-date=2006-09-01}}</ref>

The necessary high voltage, typically 10,000 volts, is supplied by an [[induction coil]] or transformer. The induction coil is a fly-back system, using interruption of electrical primary system current through some type of synchronized interrupter. The interrupter can be either contact points or a power transistor. The problem with this type of ignition is that as RPM increases the availability of electrical energy decreases. This is especially a problem, since the amount of energy needed to ignite a more dense fuel mixture is higher. The result was often a high RPM misfire.

[[Capacitor discharge ignition]] was developed. It produces a rising voltage that is sent to the spark plug. CD system voltages can reach 60,000 volts.<ref>{{cite web |url=http://www.jetav8r.com/Vision/Ignition/CDI.html |title=Electronic Ignition Overview |website=Jetav8r |access-date=2016-09-02}}</ref> CD ignitions use step-up [[transformer]]s. The step-up transformer uses energy stored in a capacitance to generate [[electric spark]]. With either system, a mechanical or electrical control system provides a carefully timed high-voltage to the proper cylinder. This spark, via the spark plug, ignites the air-fuel mixture in the engine's cylinders.

While gasoline internal combustion engines are much easier to start in cold weather than diesel engines, they can still have cold weather starting problems under extreme conditions. For years, the solution was to park the car in heated areas. In some parts of the world, the oil was actually drained and heated overnight and returned to the engine for cold starts. In the early 1950s, the gasoline Gasifier unit was developed, where, on cold weather starts, raw gasoline was diverted to the unit where part of the fuel was burned causing the other part to become a hot vapor sent directly to the intake valve manifold.  This unit was quite popular until electric [[engine block heater]]s became standard on gasoline engines sold in cold climates.<ref>{{cite magazine |url=https://books.google.com/books?id=zdwDAAAAMBAJ&pg=PA149 |title=Gasifier Aids Motor Starting Under Arctic Conditions |magazine=[[Popular Mechanics]] |date=January 1953 |page=149 |publisher=[[Hearst Magazines]] }}</ref>

==== Compression ignition process ====
{{Main|Diesel engine}}
For ignition, diesel, [[Partially premixed combustion|PPC]] and [[Homogeneous charge compression ignition|HCCI]] engines rely solely on the high temperature and pressure created by the engine in its compression process. The compression level that occurs is usually twice or more than a gasoline engine. Diesel engines take in air only, and shortly before peak compression, spray a small quantity of diesel fuel into the cylinder via a fuel injector that allows the fuel to instantly ignite. HCCI type engines take in both air and fuel, but continue to rely on an unaided auto-combustion process, due to higher pressures and temperature. This is also why diesel and HCCI engines are more susceptible to cold-starting issues, although they run just as well in cold weather once started. Light duty diesel engines with [[indirect injection]] in automobiles and light trucks employ [[Glow plug (diesel engine)|glowplugs]] (or other pre-heating: see [[Cummins B Series engine|Cummins ISB#6BT]]) that pre-heat the [[combustion chamber]] just before starting to reduce no-start conditions in cold weather. Most diesels also have a battery and charging system; nevertheless, this system is secondary and is added by manufacturers as a luxury for the ease of starting, turning fuel on and off (which can also be done via a switch or mechanical apparatus), and for running auxiliary electrical components and accessories. Most new engines rely on electrical and electronic [[engine control unit]]s (ECU) that also adjust the combustion process to increase efficiency and reduce emissions.

=== Lubrication ===
[[File:Overhead cam engine with forced oil lubrication (Autocar Handbook, 13th ed, 1935).jpg|thumb|Diagram of an engine using pressurized lubrication]]
{{Commons category|Internal combustion piston engine lubrication systems}}

Surfaces in contact and relative motion to other surfaces require [[lubrication]] to reduce wear, noise and increase efficiency by reducing the power wasting in overcoming [[friction]], or to make the mechanism work at all. Also, the lubricant used can reduce excess heat and provide additional cooling to components. At the very least, an engine requires lubrication in the following parts:
* Between pistons and cylinders
* Small bearings
* Big end bearings
* Main bearings
* Valve gear (The following elements may not be present):
** Tappets
** Rocker arms
** Pushrods
** Timing chain or gears. Toothed belts do not require lubrication.

In 2-stroke crankcase scavenged engines, the interior of the crankcase, and therefore the crankshaft, connecting rod and bottom of the pistons are sprayed by the [[two-stroke oil]] in the air-fuel-oil mixture which is then burned along with the fuel. The valve train may be contained in a compartment flooded with lubricant so that no [[Oil pump (internal combustion engine)|oil pump]] is required.

In a ''splash lubrication system'' no oil pump is used. Instead the crankshaft dips into the oil in the sump and due to its high speed, it splashes the crankshaft, connecting rods and bottom of the pistons. The connecting rod big end caps may have an attached scoop to enhance this effect. The valve train may also be sealed in a flooded compartment, or open to the crankshaft in a way that it receives splashed oil and allows it to drain back to the sump. Splash lubrication is common for small 4-stroke engines.

In a ''forced'' (also called ''pressurized'') ''lubrication system'', lubrication is accomplished in a closed-loop which carries [[motor oil]] to the surfaces serviced by the system and then returns the oil to a reservoir. The auxiliary equipment of an engine is typically not serviced by this loop; for instance, an [[alternator (automotive)|alternator]] may use [[ball bearings]] sealed with their own lubricant. The reservoir for the oil is usually the sump, and when this is the case, it is called a ''[[wet sump]]'' system. When there is a different oil reservoir the crankcase still catches it, but it is continuously drained by a dedicated pump; this is called a ''[[dry sump]]'' system.

On its bottom, the sump contains an oil intake covered by a mesh filter which is connected to an oil pump then to an [[oil filter]] outside the crankcase. From there it is diverted to the crankshaft main bearings and valve train. The crankcase contains at least one ''oil gallery'' (a conduit inside a crankcase wall) to which oil is introduced from the oil filter. The main bearings contain a groove through all or half its circumference; the oil enters these grooves from channels connected to the oil gallery. The crankshaft has drillings that take oil from these grooves and deliver it to the big end bearings. All big end bearings are lubricated this way. A single main bearing may provide oil for 0, 1 or 2 big end bearings. A similar system may be used to lubricate the piston, its gudgeon pin and the small end of its connecting rod; in this system, the connecting rod big end has a groove around the crankshaft and a drilling connected to the groove which distributes oil from there to the bottom of the piston and from then to the cylinder.

Other systems are also used to lubricate the cylinder and piston. The connecting rod may have a nozzle to throw an oil jet to the cylinder and bottom of the piston. That nozzle is in movement relative to the cylinder it lubricates, but always pointed towards it or the corresponding piston.

Typically forced lubrication systems have a lubricant flow higher than what is required to lubricate satisfactorily, in order to assist with cooling. Specifically, the lubricant system helps to move heat from the hot engine parts to the cooling liquid (in water-cooled engines) or fins (in air-cooled engines) which then transfer it to the environment. The lubricant must be designed to be chemically stable and maintain suitable viscosities within the temperature range it encounters in the engine.

=== Cylinder configuration ===
Common cylinder configurations include the [[straight engine|straight or inline configuration]], the more compact [[V engine|V configuration]], and the wider but smoother [[flat engine|flat or boxer configuration]]. [[Aircraft engine]]s can also adopt a [[radial engine|radial configuration]], which allows more effective cooling. More unusual configurations such as the [[H engine|H]], [[U engine|U]], [[X engine|X]], and [[W engine|W]] have also been used.

[[File:Schemes.png|thumb|Some popular cylinder configurations:<br />
a – straight<br />
b – V<br />
c – opposed<br />
d – W]]

Multiple cylinder engines have their valve train and crankshaft configured so that pistons are at different parts of their cycle. It is desirable to have the pistons' cycles uniformly spaced (this is called ''even firing'') especially in forced induction engines; this reduces torque pulsations{{sfn|Nunney|2007|p=15}} and makes [[Straight engine|inline engines]] with more than 3 cylinders statically [[engine balance|balanced]] in its primary forces. However, some [[engine configuration]]s require odd firing to achieve better balance than what is possible with even firing. For instance, a 4-stroke [[Straight-twin engine|I2 engine]] has better balance when the angle between the crankpins is 180° because the pistons move in opposite directions and inertial forces partially cancel, but this gives an odd firing pattern where one cylinder fires 180° of crankshaft rotation after the other, then no cylinder fires for 540°. With an even firing pattern, the pistons would move in unison and the associated forces would add.

Multiple crankshaft configurations do not necessarily need a [[cylinder head]] at all because they can instead have a piston at each end of the cylinder called an [[opposed piston]] design. Because fuel inlets and outlets are positioned at opposed ends of the cylinder, one can achieve uniflow scavenging, which, as in the four-stroke engine is efficient over a wide range of engine speeds. Thermal efficiency is improved because of a lack of cylinder heads. This design was used in the [[Junkers Jumo 205]] diesel aircraft engine, using two crankshafts at either end of a single bank of cylinders, and most remarkably in the [[Napier Deltic]] diesel engines. These used three crankshafts to serve three banks of [[opposed-piston engine|double-ended cylinders]] arranged in an equilateral triangle with the crankshafts at the corners. It was also used in single-bank [[locomotive engine]]s, and is still used in [[marine propulsion]] engines and marine auxiliary generators.

=== Diesel cycle ===
{{Main|Diesel cycle}}
[[File:DieselCycle PV.svg|thumb|[[P-V diagram]] for the ideal diesel cycle. The cycle follows the numbers 1–4 in clockwise direction.]]

Most truck and automotive diesel engines use a cycle reminiscent of a four-stroke cycle, but with temperature increase by compression causing ignition, rather than needing a separate ignition system. This variation is called the diesel cycle. In the diesel cycle, diesel fuel is injected directly into the cylinder so that combustion occurs at constant pressure, as the piston moves.

=== Otto cycle ===
The [[Otto cycle]] is the most common cycle for most cars' internal combustion engines that use gasoline as a fuel. It consists of the same major steps as described for the four-stroke engine: Intake, compression, ignition, expansion and exhaust.

=== Five-stroke engine ===
In 1879, [[Nicolaus Otto]] manufactured and sold a double expansion engine (the double and triple expansion principles had ample usage in steam engines), with two small cylinders at both sides of a low-pressure larger cylinder, where a second expansion of exhaust stroke gas took place; the owner returned it, alleging poor performance. In 1906, the concept was incorporated in a car built by EHV ([[Eisenhuth Horseless Vehicle Company]]);<ref>{{cite book |title=The Romance of Engines |first=Takashi |last=Suzuki |pages=87–94 |publisher=SAE |year=1997}}</ref> and in the 21st century [[Ilmor]] designed and successfully tested a 5-stroke double expansion internal combustion engine, with high power output and low SFC (Specific Fuel Consumption).<ref>{{cite web |url=http://www.ilmor.co.uk/capabilities/5-stroke-engine |title=5-Stroke Concept Engine Design and Development |publisher=Ilmor Engineering |access-date=2015-12-18}}</ref>

=== Six-stroke engine ===
The [[six-stroke engine]] was invented in 1883. Four kinds of six-stroke engines use a regular piston in a regular cylinder (Griffin six-stroke, Bajulaz six-stroke, Velozeta six-stroke and Crower six-stroke), firing every three crankshaft revolutions. These systems capture the waste heat of the [[four-stroke]] Otto cycle with an injection of air or water.

The [[Beare Head]] and "piston charger" engines operate as [[opposed-piston engine]]s, two pistons in a single cylinder, firing every two revolutions rather than every four like a four-stroke engine.

=== Other cycles ===
The very first internal combustion engines did not compress the mixture. The first part of the piston downstroke drew in a fuel-air mixture, then the inlet valve closed and, in the remainder of the down-stroke, the fuel-air mixture fired. The exhaust valve opened for the piston upstroke. These attempts at imitating the principle of a [[steam engine]] were very inefficient.
There are a number of variations of these cycles, most notably the [[Atkinson cycle|Atkinson]] and [[Miller cycle]]s.

[[Split-cycle engine]]s separate the four strokes of intake, compression, combustion and exhaust into two separate but paired cylinders. The first cylinder is used for intake and compression. The compressed air is then transferred through a crossover passage from the compression cylinder into the second cylinder, where combustion and exhaust occur. A split-cycle engine is really an [[air compressor]] on one side with a combustion chamber on the other.

Previous split-cycle engines have had two major problems—poor breathing (volumetric efficiency) and low thermal efficiency. However, new designs are being introduced that seek to address these problems.
The [[Scuderi Engine]] addresses the breathing problem by reducing the clearance between the piston and the cylinder head through various turbocharging techniques. The Scuderi design requires the use of outwardly opening valves that enable the piston to move very close to the cylinder head without the interference of the valves. Scuderi addresses the low thermal efficiency via firing after top dead center (ATDC).

Firing ATDC can be accomplished by using high-pressure air in the transfer passage to create sonic flow and high turbulence in the power cylinder.

== Combustion turbines ==

=== Jet engine ===
{{Main|Jet engine}}
[[File:Turbofan operation.svg|thumb|Turbofan jet engine]]

Jet engines use a number of rows of fan blades to compress air which then enters a [[combustor]] where it is mixed with fuel (typically JP fuel) and then ignited. The burning of the fuel raises the temperature of the air which is then exhausted out of the engine creating thrust. A modern [[turbofan]] engine can operate at as high as 48% efficiency.<ref>{{cite web|title=Aviation and the Global Atmosphere|url=http://www.ipcc.ch/ipccreports/sres/aviation/index.php?idp=97|website=Intergovernment Panel on Climate Change|access-date=2016-07-14}}</ref>

There are six sections to a turbofan engine:
* Fan
* Compressor
* Combustor
* Turbine
* Mixer
* Nozzle<ref>{{cite web |url=https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/engines.html |title=Engines |publisher=NASA Glenn Research Center |location=US |date=2014-06-12 |access-date=2016-08-31}}</ref>

=== Gas turbines ===
{{Main|Gas turbine}}
[[File:Gas Turbine Power Plant.JPG|thumb|Turbine power plant]]

A gas turbine compresses air and uses it to turn a [[turbine]]. It is essentially a jet engine which directs its output to a shaft.  There are three stages to a turbine: 1) air is drawn through a compressor where the temperature rises due to compression, 2) fuel is added in the [[combuster]], and 3) hot air is exhausted through turbine blades which rotate a shaft connected to the compressor.

A gas turbine is a rotary machine similar in principle to a [[steam turbine]] and it consists of three main components: a compressor, a combustion chamber, and a turbine. The temperature of the air, after being compressed in the compressor, is increased by burning fuel in it. The heated air and the products of combustion expand in a turbine, producing work output. About {{frac|2|3}} of the work drives the compressor: the rest (about {{frac|1|3}}) is available as useful work output.<ref name="GePowerGen">{{cite web|title=How a Gas Turbine Works|url=https://powergen.gepower.com/resources/knowledge-base/what-is-a-gas-turbine.html|website=General Electric Power Generation|publisher=[[General Electric]]|access-date=2016-07-14}}</ref>

Gas turbines are among the most efficient internal combustion engines. The General Electric 7HA and 9HA turbine [[combined cycle]] electrical plants are rated at over 61% efficiency.<ref name="GasTurbineWorld">{{cite web|title=Air-cooled 7HA and 9HA designs rated at over '''61%''' CC efficiency|url=http://www.gasturbineworld.com/ge-7ha.html|website=Gasturbineworld|access-date=2016-07-14|archive-url=https://web.archive.org/web/20160720084706/http://www.gasturbineworld.com/ge-7ha.html|archive-date=2016-07-20|url-status=dead}}</ref>

=== Brayton cycle ===
{{Main|Brayton cycle}}
[[File:Brayton cycle.svg|thumb|300px|Brayton cycle]]

A gas turbine is a rotary machine somewhat similar in principle to a steam turbine. It consists of three main components: compressor, combustion chamber, and turbine. The air is compressed by the compressor where a temperature rise occurs. The temperature of the compressed air is further increased by combustion of injected fuel in the combustion chamber which expands the air. This energy rotates the turbine which powers the compressor via a mechanical coupling. The hot gases are then exhausted to provide thrust.

Gas turbine cycle engines employ a continuous combustion system where compression, combustion, and expansion occur simultaneously at different places in the engine—giving continuous power. Notably, the combustion takes place at constant pressure, rather than with the Otto cycle, constant volume.

== Wankel engines ==
[[File:Wankel Cycle anim en.gif|thumb|upright|The Wankel rotary cycle. The shaft turns three times for each rotation of the rotor around the lobe and once for each [[orbital revolution]] around the eccentric shaft.]]
{{Main|Wankel engine}}

The Wankel engine (rotary engine) does not have piston strokes. It operates with the same separation of phases as the four-stroke engine with the phases taking place in separate locations in the engine. In [[Thermodynamics|thermodynamic]] terms it follows the [[Otto engine]] cycle, so may be thought of as a "four-phase" engine. While it is true that three power strokes typically occur per rotor revolution, due to the 3:1 revolution ratio of the rotor to the eccentric shaft, only one power stroke per shaft revolution actually occurs. The drive (eccentric) shaft rotates once during every power stroke instead of twice (crankshaft), as in the Otto cycle, giving it a greater power-to-weight ratio than piston engines. This type of engine was most notably used in the [[Mazda RX-8]], the earlier [[Mazda RX-7|RX-7]], and other vehicle models. The engine is also used in unmanned aerial vehicles, where the small size and weight and the high power-to-weight ratio are advantageous.

== Forced induction ==
{{main|Forced induction}}
Forced induction is the process of delivering compressed air to the intake of an internal combustion engine. A forced induction engine uses a [[gas compressor]] to increase the pressure, temperature and [[density of air|density of the air]]. An engine without forced induction is considered a [[naturally aspirated engine]].

Forced induction is used in the automotive and aviation industry to increase engine power and efficiency. It particularly helps aviation engines, as they need to operate at high altitude.

Forced induction is achieved by a [[supercharger]], where the compressor is directly powered from the engine shaft or, in the [[turbocharger]], from a turbine powered by the engine exhaust.

== Fuels and oxidizers ==
All '''internal combustion engines''' depend on [[combustion]] of a [[chemical fuel]], typically with oxygen from the air (though it is possible to inject [[nitrous oxide]] to do more of the same thing and gain a power boost). The combustion process typically results in the production of a great quantity of thermal energy, as well as the production of steam and carbon dioxide and other chemicals at very high temperature; the [[Adiabatic flame temperature|temperature reached]] is determined by the chemical make up of the fuel and oxidizers (see [[stoichiometry]]), as well as by the compression and other factors.

=== Fuels ===
The most common modern fuels are made up of [[hydrocarbon]]s and are derived mostly from [[fossil fuel]]s ([[petroleum]]). Fossil fuels include [[diesel fuel]], [[gasoline]] and [[liquefied petroleum gas|petroleum gas]], and the rarer use of [[propane]]. Except for the fuel delivery components, most internal combustion engines that are designed for gasoline use can run on [[natural gas]] or liquefied petroleum gases without major modifications. Large diesels can run with air mixed with gases  and a pilot diesel fuel ignition injection. Liquid and gaseous [[biofuel]]s, such as [[ethanol fuel|ethanol]] and [[biodiesel]] (a form of diesel fuel that is produced from crops that yield [[triglyceride]]s such as [[soybean]] oil), can also be used. Engines with appropriate modifications can also run on [[hydrogen]] gas, [[wood gas]], or [[wood gas|charcoal gas]], as well as from so-called [[wood gas|producer gas]] made from other convenient biomass. Experiments have also been conducted using powdered solid fuels, such as the [[magnesium injection cycle]].

Presently, fuels used include:
* [[Petroleum]]:
** Petroleum spirit ([[North American English|North American]] term: [[gasoline]], [[British English|British]] term: petrol)
** [[Diesel fuel]].
** [[Autogas]] ([[liquified petroleum gas]]).
** [[Propane]].
** [[Compressed natural gas]].
** [[Jet fuel]] ([[aviation fuel]])
** [[Residual fuel]]
* [[Coal]]:
** Gasoline can be made from carbon (coal) using  the [[Fischer–Tropsch process]]
** Diesel fuel can be made from carbon using  the [[Fischer–Tropsch process]]
* Biofuels and vegetable oils:
** [[Peanut oil]] and other [[vegetable oil fuel|vegetable oils]].
** [[Woodgas]], from an onboard [[wood gasifier]] using solid wood as a fuel
** Biofuels:
*** [[Biobutanol]] (replaces gasoline).
*** [[Biodiesel]] (replaces petrodiesel).
*** [[Dimethyl Ether]] (replaces petrodiesel).
*** [[Bioethanol]] and [[Biomethanol]] ([[wood alcohol]]) and other [[biofuel]]s  (see [[Flexible-fuel vehicle]]).
*** [[Biogas]]
* [[Hydrogen]] (mainly [[spacecraft]] [[rocket engine]]s)

Even fluidized metal powders and explosives have seen some use. Engines that use gases for fuel are called gas engines and those that use liquid hydrocarbons are called oil engines; however, gasoline engines are also often colloquially referred to as "gas engines" ("[[petrol engine]]s" outside North America).

The main limitations on fuels are that it must be easily transportable through the [[fuel injection|fuel system]] to the [[combustion chamber]], and that the fuel releases sufficient [[energy]] in the form of [[heat]] upon [[combustion]] to make practical use of the engine.

[[Diesel engine]]s are generally heavier, noisier, and more powerful at lower speeds than [[gasoline engine]]s. They are also more fuel-efficient in most circumstances and are used in heavy road vehicles, some automobiles (increasingly so for their increased [[fuel efficiency]] over gasoline engines), ships, [[railway]] [[locomotive]]s, and light [[aircraft]]. Gasoline engines are used in most other road vehicles including most cars, [[motorcycle]]s, and [[moped]]s. In [[Europe]], sophisticated diesel-engined cars have taken over about 45% of the market since the 1990s. There are also engines that run on [[hydrogen car|hydrogen]], [[methanol]], [[ethanol]], [[liquefied petroleum gas]] (LPG), [[biodiesel]], [[wood|paraffin]] and [[tractor vaporizing oil]] (TVO).

==== Hydrogen ====
{{Main|Hydrogen internal combustion engine vehicle}}

Hydrogen could eventually [[Hydrogen economy|replace conventional fossil fuels]] in traditional internal combustion engines. Alternatively [[fuel cell]] technology may come to deliver its promise and the use of the internal combustion engines could even be phased out.

Although there are multiple ways of producing free hydrogen, those methods require converting combustible molecules into hydrogen or consuming electric energy. Unless that electricity is produced from a renewable source—and is not required for other purposes—hydrogen does not solve any [[energy crisis]]. In many situations the disadvantage of hydrogen, relative to carbon fuels, is [[Hydrogen economy#Storage|its storage]]. [[Liquid hydrogen]] has extremely low density (14 times lower than water) and requires extensive insulation—whilst gaseous hydrogen requires heavy tankage. Even when liquefied, hydrogen has a higher [[specific energy]] but the volumetric energetic storage is still roughly five times lower than gasoline. However, the energy density of hydrogen is considerably higher than that of electric batteries, making it a serious contender as an energy carrier to replace fossil fuels. The 'Hydrogen on Demand' process (see [[direct borohydride fuel cell]]) creates hydrogen as needed, but has other issues, such as the high price of the [[sodium borohydride]] that is the raw material.

=== Oxidizers ===
[[File:Moore-single-cylinder-gasoline-engine.jpg|thumb|upright|One-cylinder gasoline engine, {{c.|1910}}]]

Since air is plentiful at the surface of the earth, the oxidizer is typically atmospheric oxygen, which has the advantage of not being stored within the vehicle. This increases the power-to-weight and power-to-volume ratios. Other materials are used for special purposes, often to increase power output or to allow operation under water or in space.
* Compressed air has been commonly used in [[torpedo]]es.<ref>{{cite book |url=https://maritime.org/doc/whitehead3/pg06.htm |title=The Whitehead Torpedo, notes on handling etc. |publisher=Bureau of Ordnance |location=US |year=1890 |access-date=2017-05-15 |via=San Francisco Maritime National Park Association |quote=After assembling, the air-flask shall be charged to 450 lbs. pressure}}</ref>
* Compressed [[oxygen]], as well as some compressed air, was used in the Japanese [[Type 93 torpedo]]. Some [[Air-independent propulsion|submarines]] carry pure oxygen. Rockets very often use [[liquid oxygen]].<ref>
{{cite web|url=http://liftoff.msfc.nasa.gov/news/2003/news-goddard.asp |title=Re-Creating History |publisher=NASA |url-status=dead |archive-url=https://web.archive.org/web/20071201210444/http://liftoff.msfc.nasa.gov/news/2003/news-goddard.asp |archive-date=2007-12-01 }}</ref>
* [[Nitromethane]] is added to some racing and [[Model engine|model]] fuels to increase power and control combustion.
* [[Nitrous oxide]] has been used—with extra gasoline—in tactical aircraft, and in specially equipped cars to allow short bursts of added power from engines that otherwise run on gasoline and air. It is also used in the Burt Rutan rocket spacecraft.
* [[Hydrogen peroxide]] power was under development for German World War II submarines. It may have been used in some non-nuclear submarines, and was used on some rocket engines (notably the [[Black Arrow]] and the [[Messerschmitt Me 163]] rocket fighter).
* Other chemicals such as chlorine or fluorine have been used experimentally, but have not been found practical.

== Cooling ==
{{main|Internal combustion engine cooling}}
Cooling is required to remove excessive heat—high temperature can cause engine failure, usually from wear (due to high-temperature-induced failure of lubrication), cracking or warping. Two most common forms of engine cooling are [[air-cooled engine|air-cooled]] and [[Water-cooled#Automotive usage|water-cooled]]. Most modern automotive engines are both water and air-cooled, as the water/liquid-coolant is carried to air-cooled fins and/or fans, whereas larger engines may be singularly water-cooled as they are stationary and have a constant supply of water through water-mains or fresh-water, while most power tool engines and other small engines are air-cooled. Some engines (air or water-cooled) also have an [[oil cooling|oil cooler]]. In some engines, especially for [[Turbine blade#Cooling|turbine engine blade cooling]] and [[Regenerative cooling (rocket)|liquid rocket engine cooling]], fuel is used as a coolant, as it is simultaneously preheated before injecting it into a combustion chamber.

== Starting ==
{{main|Starter motor}}
[[File:InleHandCrank.webm|thumb|right|Hand-cranking a boat diesel motor in [[Inle Lake]] ([[Myanmar]])]]

[[File:Automobile starter.JPG|thumb|Electric starter as used in automobiles]]

Internal combustion engines must have their cycles started. In reciprocating engines this is accomplished by turning the crankshaft (Wankel Rotor Shaft) which induces the cycles of intake, compression, combustion, and exhaust.  The first engines were started with a turn of their [[flywheel]]s, while the first vehicle (the Daimler Reitwagen) was started with a hand crank.  All ICE engined automobiles were started with hand cranks until [[Charles Kettering]] developed the electric starter for automobiles.<ref>{{cite press release |url=http://media.gm.com/media/us/en/gm/home.detail.html/content/Pages/news/us/en/2012/Feb/0215_cad_starter.html |title=Cadillac's Electric Self Starter Turns 100 |publisher=General Motors |location=US |access-date=2016-09-02}}</ref> This method is now the most widely used, even among non-automobiles.

As diesel engines have become larger and their mechanisms heavier, [[Air-start system|air starters]] have come into use.<ref>{{cite web |url=http://www.ingersollrandproducts.com/am-en/products/air-starters |title=Ingersoll Rand Engine Starting – Turbine, Vane and Gas Air Starters |publisher=Ingersoll Rand |access-date=2016-09-05 |archive-url=https://web.archive.org/web/20160913233007/http://www.ingersollrandproducts.com/am-en/products/air-starters |archive-date=2016-09-13 |url-status=dead }}</ref> This is due to the lack of torque in electric starters. Air starters work by pumping compressed air into the cylinders of an engine to start it turning.

Two-wheeled vehicles may have their engines started in one of four ways:
* By pedaling, as on a bicycle
* By pushing the vehicle and then engaging the clutch, known as "run-and-bump starting"
* By kicking downward on a single pedal, known as "kick starting"
* By an electric starter, as in cars

There are also starters where a spring is compressed by a crank motion and then used to start an engine.

Some small engines use a pull-rope mechanism called "recoil starting", as the rope rewinds itself after it has been pulled out to start the engine. This method is commonly used in  pushed lawn mowers and other settings where only a small amount of torque is needed to turn an engine over.

Turbine engines are frequently started by an electric motor or by compressed air.

== Measures of engine performance ==
Engine types vary greatly in a number of different ways:
* [[Fuel efficiency|energy efficiency]]
* fuel/propellant consumption ([[brake specific fuel consumption]] for shaft engines, [[thrust specific fuel consumption]] for jet engines)
* [[power-to-weight ratio]]
* [[thrust to weight ratio]]
* [[Machine torque|torque curves]] (for shaft engines), [[thrust lapse]] (jet engines)
* [[compression ratio]] for piston engines, [[overall pressure ratio]] for jet engines and gas turbines

=== Energy efficiency ===
Once ignited and burnt, the [[combustion]] products—hot gases—have more available [[thermal energy]] than the original compressed fuel-air mixture (which had higher [[chemical energy]]). This available energy is manifested as a higher [[temperature]] and [[pressure]] that can be converted into [[kinetic energy]] by the engine. In a reciprocating engine, the high-pressure gases inside the cylinders drive the engine's pistons.

Once the available energy has been removed, the remaining hot gases are [[exhaust gas|vented]] (often by opening a [[poppet valve|valve]] or exposing the exhaust outlet) and this allows the piston to return to its previous position (top dead center, or TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any thermal energy that is not translated into work is normally considered a waste product and is removed from the engine either by an air or liquid cooling system.

Internal combustion engines are considered [[heat engine]]s (since the release of chemical energy in combustion has the same effect as [[heat]] transfer into the engine) and as such their theoretical efficiency can be approximated by idealized [[thermodynamic cycle]]s. The thermal efficiency of a theoretical cycle cannot exceed that of the [[Carnot cycle]], whose efficiency is determined by the difference between the lower and upper [[operating temperature]]s of the engine. The upper operating temperature of an engine is limited by two main factors; the thermal operating limits of the materials, and the auto-ignition resistance of the fuel. All [[metal]]s and [[alloy]]s have a thermal operating limit, and there is significant research into [[ceramic]] materials that can be made with greater thermal stability and desirable structural properties. Higher thermal stability allows for a greater temperature difference between the lower (ambient) and upper operating temperatures, hence greater thermodynamic efficiency. Also, as the cylinder temperature rises, the fuel becomes more prone to auto-ignition. This is caused when the cylinder temperature nears the flash point of the charge. At this point, ignition can spontaneously occur before the spark plug fires, causing excessive cylinder pressures. Auto-ignition can be mitigated by using fuels with high auto-ignition resistance ([[octane rating]]), however it still puts an upper bound on the allowable peak cylinder temperature.

The [[thermodynamic limit]]s assume that the engine is operating under ideal conditions: a frictionless world, ideal gases, perfect insulators, and operation for infinite time. Real world applications introduce complexities that reduce efficiency. For example, a real engine runs best at a specific load, termed its [[power band]]. The engine in a car cruising on a highway is usually operating significantly below its ideal load, because it is designed for the higher loads required for rapid acceleration.{{citation needed|date=June 2012}} In addition, factors such as [[wind resistance]] reduce overall system efficiency.  Vehicle [[fuel economy in automobiles|fuel economy]] is measured in [[miles per gallon]] or in liters per 100 kilometers. The volume of hydrocarbon assumes a standard energy content.

Even when aided with turbochargers and stock efficiency aids, most engines retain an ''average'' efficiency of about 18–20%.<ref>{{cite web|url=http://courses.washington.edu/me341/oct22v2.htm |title=Improving IC Engine Efficiency |publisher=Courses.washington.edu |access-date=2010-08-28}}</ref> However, the latest technologies in [[Formula One engines]] have seen a boost in thermal efficiency past 50%.<ref>{{cite web|url=https://www.motorauthority.com/news/1112999_mercedes-amg-f1-engine-achieves-50-percent-thermal-efficiency |title=Mercedes AMG F1 engine achieves 50 percent thermal efficiency |first=Sean |last=Szymkowski |work=Motor Authority |location=US |date=2017-10-01 |access-date=2020-08-23}}</ref>
There are many inventions aimed at increasing the efficiency of IC engines. In general, practical engines are always compromised by trade-offs between different properties such as efficiency, weight, power, heat, response, exhaust emissions, or noise. Sometimes economy also plays a role in not only the cost of manufacturing the engine itself, but also manufacturing and distributing the fuel. Increasing the engine's efficiency brings better fuel economy but only if the fuel cost per energy content is the same.

=== Measures of fuel efficiency and propellant efficiency ===
For stationary and shaft engines including propeller engines, fuel consumption is measured by calculating the [[brake specific fuel consumption]], which measures the mass flow rate of fuel consumption divided by the power produced.

For internal combustion engines in the form of jet engines, the power output varies drastically with airspeed and a less variable measure is used: [[thrust specific fuel consumption]] (TSFC), which is the mass of propellant needed to generate [[Impulse (physics)|impulses]] that is measured in either pound force-hour or the grams of propellant needed to generate an impulse that measures one kilonewton-second.

For rockets, TSFC can be used, but typically other equivalent measures are traditionally used, such as [[specific impulse]] and [[effective exhaust velocity]].

== Air and noise pollution ==
{{Pollution sidebar|Air}}

=== Air pollution ===
{{unreferenced section|date=November 2022}}
Internal combustion engines such as reciprocating internal combustion engines produce [[air pollution]] emissions, due to incomplete combustion of [[carbon]]aceous fuel. The main derivatives of the process are [[carbon dioxide]] {{chem|CO|2}}, water and some [[soot]]—also called [[atmospheric particulate matter|particulate matter]] (PM). The effects of inhaling particulate matter have been studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. There are, however, some additional products of the combustion process that include [[nitrogen oxide]]s and [[sulfur]] and some uncombusted hydrocarbons, depending on the operating conditions and the fuel-air ratio.

Carbon dioxide emissions from internal combustion engines (particularly ones using [[fossil fuels]] such as gasoline and diesel) contribute to human-induced [[climate change]].  Increasing the engine's [[fuel efficiency]] can reduce, but not eliminate, the amount of {{chem|CO|2}} emissions as carbon-based fuel combustion produces {{chem|CO|2}}.  Since removing {{chem|CO|2}} from engine exhaust is impractical, there is increasing interest in alternatives.  Sustainable fuels such as [[biofuel]]s, [[synfuel]]s, and [[electric motor]]s powered by batteries are examples.

Not all of the fuel is completely consumed by the combustion process. A small amount of fuel is present after combustion, and some of it reacts to form oxygenates, such as [[formaldehyde]] or [[acetaldehyde]], or hydrocarbons not originally present in the input fuel mixture. Incomplete combustion usually results from insufficient [[oxygen]] to achieve the perfect [[stoichiometric]] ratio. The flame is "quenched" by the relatively cool cylinder walls, leaving behind unreacted fuel that is expelled with the exhaust. When running at lower speeds, quenching is commonly observed in diesel (compression ignition) engines that run on natural gas. Quenching reduces efficiency and increases knocking, sometimes causing the engine to stall. Incomplete combustion also leads to the production of [[carbon monoxide]] (CO). Further chemicals released are [[benzene]] and [[1,3-butadiene]] that are also [[hazardous air pollutant]]s.

Increasing the amount of air in the engine reduces emissions of incomplete combustion products, but also promotes reaction between oxygen and [[nitrogen]] in the air to produce [[nitrogen oxide]]s ({{NOx}}). {{NOx}} is hazardous to both plant and animal health, and leads to the production of [[ozone]] ({{chem|O|3}}). Ozone is not emitted directly; rather, it is a secondary air pollutant, produced in the atmosphere by the reaction of {{NOx}} and [[volatile organic compound]]s in the presence of sunlight. Ground-level ozone is harmful to human health and the environment. Though the same chemical substance, ground-level ozone should not be confused with [[stratospheric ozone]], or the [[ozone layer]], which protects the earth from harmful ultraviolet rays.

Carbon fuels containing sulfur produce [[sulfur monoxide]]s (SO) and [[sulfur dioxide]] ({{chem|SO|2}}) contributing to [[acid rain]].

In the United States, nitrogen oxides, [[Particulates|PM]], carbon monoxide, sulfur dioxide, and ozone, are regulated as [[criteria air pollutant]]s under the [[Clean Air Act (United States)|Clean Air Act]] to levels where human health and welfare are protected. Other pollutants, such as benzene and 1,3-butadiene, are regulated as [[hazardous air pollutant]]s whose emissions must be lowered as much as possible depending on technological and practical considerations.

{{NOx}}, carbon monoxide and other pollutants are frequently controlled via [[exhaust gas recirculation]] which returns some of the exhaust back into the engine intake.  [[Catalytic converter]]s are used to convert exhaust chemicals to {{chem|CO|2}} (a [[greenhouse gas]]), {{chem|H|2|O}} (water vapour, also a greenhouse gas) and {{chem|N|2}} (nitrogen).

{{see also | Air suction valve}}

==== Non-road engines ====
{{Main|Non-road engine#Emission standards}}
The emission standards used by many countries have special requirements for [[non-road engine]]s which are used by equipment and vehicles that are not operated on the public roadways. The standards are separated from the road vehicles.<ref>{{cite web|title=2013 Global Sourcing Guide|url=http://www.gsgnet.net/gsgpdfs/EmissionsStandards.pdf|publisher=Diesel & Gas Turbine Publications|access-date=2013-12-26|archive-url=https://web.archive.org/web/20130925005145/http://gsgnet.net/gsgpdfs/EmissionsStandards.pdf|archive-date=2013-09-25|url-status=dead}}</ref>

=== Noise pollution ===
Significant contributions to [[noise pollution]] are made by internal combustion engines. Automobile and truck traffic operating on highways and street systems produce noise, as do aircraft flights due to jet noise, particularly supersonic-capable aircraft. Rocket engines create the most intense noise.

=== Idling ===
{{main|Idle reduction}}
Internal combustion engines continue to consume fuel and emit pollutants while idling. Idling is reduced by [[Start-stop system|stop-start systems]].

== Carbon dioxide formation ==
A good way to estimate the mass of carbon dioxide that is released when one litre of diesel fuel (or gasoline) is combusted can be found as follows:<ref>{{cite book |last=Hilgers |first=Michael |title=The Diesel Engine, in series: commercial vehicle technology |publisher=Springer |year=2020 |isbn=978-3-662-60856-2 |location=Berlin/Heidelberg/New York}}</ref>

As a good approximation the chemical formula of diesel is {{chem|C|n|H|2n}}. In reality diesel is a mixture of different molecules. As carbon has a molar mass of 12&nbsp;g/mol and hydrogen (atomic) has a molar mass of about 1&nbsp;g/mol, the fraction by weight of carbon in diesel is roughly {{frac|12|14}}.

The reaction of diesel combustion is given by:

2{{chem|C|n|H|2n}} + 3n{{chem|O|2}} {{eqm}} 2n{{chem|C|O|2}} + 2n{{chem|H|2|O}}

Carbon dioxide has a molar mass of 44 g/mol as it consists of 2 atoms of oxygen (16&nbsp;g/mol) and 1 atom of carbon (12 g/mol). So 12&nbsp;g of carbon yields 44&nbsp;g of carbon dioxide.

Diesel has a density of 0.838&nbsp;kg per litre.

Putting everything together the mass of carbon dioxide that is produced by burning 1 litre of diesel can be calculated as:

<math>0.838 kg/L \cdot {\frac{12}{14}}\cdot {\frac{44}{12}}= 2.63 kg/L</math>

The figure obtained with this estimation is close to the values found in the literature.

For gasoline, with a density of 0.75&nbsp;kg/L and a ratio of carbon to hydrogen atoms of about 6 to 14, the estimated value of carbon dioxide emission from burning 1 litre of gasoline is:

<math> 0.75 kg/L \cdot {\frac{6 \cdot 12}{6\cdot 12 + 14}\cdot 1} \cdot {\frac{44}{12}}= 2.3 kg/L </math>

== Parasitic loss ==
The term ''parasitic loss'' is often applied to devices that take energy from the engine in order to enhance the engine's ability to create more energy or convert energy to motion. In the internal combustion engine, almost every mechanical component, including the [[drivetrain]], causes parasitic loss and could thus be characterized as a parasitic load.

===Examples===
[[Bearing (mechanical)|Bearings]], oil pumps, [[piston ring]]s, valve springs, [[flywheel]]s, [[transmission (mechanics)|transmissions]], [[driveshaft]]s, and [[differential (mechanics)|differential]]s all act as parasitic loads that rob the system of power. These parasitic loads can be divided into two categories: those inherent to the working of the engine and those drivetrain losses incurred in the systems that transfer power from the engine to the road (such as the transmission, driveshaft, differentials and axles).

For example, the former category (engine parasitic loads) includes the oil pump used to lubricate the engine, which is a necessary parasite that consumes power from the engine (its host). Another example of an engine parasitic load is a [[supercharger]], which derives its power from the engine and creates more power for the engine. The power that the supercharger consumes is parasitic loss and is usually expressed in [[kilowatt]] or [[horsepower]]. While the power that the supercharger consumes in comparison to what it generates is small, it is still measurable or calculable. One of the desirable features of a [[turbocharger]] over a supercharger is the lower parasitic loss of the former.<ref>{{Cite web|url=https://www.caranddriver.com/features/a25412500/turbocharger-vs-supercharger-definition|title=Turbochargers vs. Superchargers: Which Is Better?|last=Witzenburg|first=Gary|date=2018-12-12|website=Car and Driver|language=en-US|access-date=2020-04-21}}</ref>

Drivetrain parasitic losses include both steady state and dynamic loads. Steady state loads occur at constant speeds and may originate in discrete components such as the [[torque converter]], the transmission [[Oil pump (internal combustion engine)|oil pump]], and/or [[clutch]] drag, and in seal/bearing drag, churning of lubricant and gear [[windage]]/[[friction]] found throughout the system. Dynamic loads occur under acceleration and are caused by [[inertia]] of rotating components and/or increased friction.<ref name=":0">{{Cite web|url=http://www.superstreetonline.com/how-to/engine/modp-1005-drivetrain-power-loss|title=Drivetrain Power Loss – The 15% 'Rule'|last=Pratte|first=David|date=2020-03-09|website=SuperStreetOnline|language=en|access-date=2020-04-21}}</ref>

=== Measurement ===
While rules of thumb such as a 15% power loss from drivetrain parasitic loads have been commonly repeated, the actual loss of energy due to parasitic loads varies between systems. It can be influenced by powertrain design, lubricant type and temperature and many other factors.<ref name=":0" /><ref>{{Cite book|last1=Ko|first1=Yoshiyuki|last2=Hosoi|first2=Kenzo|title=SAE Technical Paper Series|date=1984-02-01|chapter=Measurements of Power Losses in Automobile Drive Train|volume=1|chapter-url=https://www.sae.org/content/840054/|pages=840054|doi=10.4271/840054}}</ref> In automobiles, drivetrain loss can be quantified by measuring the difference between power measured by an [[Engine test stand|engine dynamometer]] and a [[chassis dynamometer]]. However, this method is primarily useful for measuring steady state loads and may not accurately reflect losses due to dynamic loads.<ref name=":0" /> More advanced methods can be used in a laboratory setting, such as measuring in-cylinder pressure measurements, flow rate and temperature at certain points, and testing of individual parts or sub-assemblies to determine friction and pumping losses.<ref name=":2">{{Cite journal|last1=Thiruvengadam|first1=Arvind|last2=Pradhan|first2=Saroj|last3=Thiruvengadam|first3=Pragalath|last4=Besch|first4=Marc|last5=Carder|first5=Daniel|date=October 2014|title=Heavy-Duty Vehicle Diesel Engine Efficiency Evaluation and Energy Audit|url=https://theicct.org/sites/default/files/publications/HDV_engine-efficiency-eval_WVU-rpt_oct2014.pdf|journal=Center for Alternative Fuels, Engines & Emissions|via=West Virginia University}}</ref>

For example, in a dynamometer test by [[Hot Rod (magazine)|''Hot Rod'' magazine]], a [[Ford Mustang]] equipped with a modified 357ci [[Ford small block engine|small-block Ford V8]] engine and an automatic transmission had a measured drivetrain power loss averaging 33%. In the same test, a [[Buick]] equipped with a modified 455ci V8 engine and a 4-speed manual transmission was measured to have an average drivetrain power loss of 21%.<ref name=":1">{{Cite web|url=https://www.hotrod.com/articles/ccrp-0311-drivetrain-power-loss/|title=Drivetrain Power Loss|last=Smith|first=Jeff|date=2003-11-01|work=[[Car Craft Magazine]]|language=en|access-date=2020-04-21}}</ref>

Laboratory testing of a heavy-duty diesel engine determined that 1.3% of the fuel energy input was lost to parasitic loads of engine accessories such as water and oil pumps.<ref name=":2" />

===Reduction===
Automotive engineers and tuners commonly make design choices that reduce parasitic loads in order to improve efficiency and power output.  These may involve the choice of major engine components or systems, such as the use of [[dry sump]] lubrication system over a [[wet sump]] system. Alternately, this can be effected through substitution of minor components available as aftermarket modifications, such as exchanging a directly engine-driven fan for one equipped with a [[fan clutch]] or an electric fan.<ref name=":1" /> Another modification to reduce parasitic loss, usually seen in track-only cars, is the replacement of an engine-driven water pump for an electrical water pump.<ref>{{Cite web|url=http://www.mustangandfords.com/how-to/engine/mmfp-0611-reduce-parasitic-drag|title=Reduce Parasitic Drag – Pump Up The Power – Dr. Dyno|last=Holdener|first=Richard|date=1 November 2006|website=Mustang 360}}</ref> The reduction in parasitic loss from these changes may be due to reduced friction or many other variables that cause the design to be more efficient.{{Citation needed|date=August 2009}}

== See also ==
{{Portal|Energy}}
{{cmn|colwidth=22em|
* [[Adiabatic flame temperature]]
*[[Aluminum internal combustion engine]]
* [[Air-fuel ratio]]
* [[Bore (engine)|Bore]]
* [[Component parts of internal combustion engines]]
* [[Crude oil engine]] – a [[two-stroke engine]]
* [[Deglazing (engine mechanics)]]
* [[Diesel engine]]
* [[Dieselisation]]
* [[Direct fuel injection|Direct injection]]
* [[Dynamometer]]
* [[Electric vehicle]]
* [[Engine test stand]] – information about how to check an internal combustion engine
* [[External Combustion Engine]]
* [[Fossil fuel]]s
* [[Gasoline direct injection]]
* [[Gas turbine]]
* [[Heat pump]]
* [[Homogeneous Charge Compression Ignition]]
* [[Hybrid vehicle]]
* [[Indirect injection]]
* [[Jet engine]]
* [[Magnesium injection cycle]]
* [[Model engine]]
* [[Multi-valve]]
* [[Petrofuel]]
* [[Piston engine]]
* [[Pistonless rotary engine]]
* [[Reciprocating engine]]
* [[Stroke (engine)|Stroke]]
* [[Turbocharger]]
* [[Variable displacement]] / [[Variable compression ratio]]
}}

== References ==
{{reflist}}

==Bibliography==
* {{cite book |last=Anyebe |first=E.A |year=2009 |title=Combustion Engine and Operations, Automobile Technology Handbook |volume=2 |ref=none}}
* {{cite book | last=Denton | first=T. | title=Automobile Mechanical and Electrical Systems | publisher=Taylor & Francis | series=Automobile Mechanical and Electrical Systems: Automotive Technology : Vehicle Maintenance and Repair | year=2011 | isbn=978-1-136-27038-3 | url=https://books.google.com/books?id=mXAQBAAAQBAJ | page= }}
* {{cite book | last=Heywood | first=J. | title=Internal Combustion Engine Fundamentals 2E | publisher=McGraw-Hill Education | year=2018 | isbn=978-1-260-11611-3 | url=https://books.google.com/books?id=OmJUDwAAQBAJ | page= }}
* {{cite book |title=Light and Heavy Vehicle Technology |first=Malcolm J. |last=Nunney |year=2007 |edition=4th |isbn=978-0-7506-8037-0 |publisher=Elsevier Butterworth-Heinemann }}
* {{cite book |title=The High-Speed Internal Combustion Engine |first=Harry |last=Ricardo |year=1931 |url=http://es.scribd.com/doc/40610101/The-High-Speed-Internal-Combustion-Engine-by-Sir-Harry-Ricardo |ref=none}}
* {{cite book |first=R.K. |last=Singal |title=Internal Combustion Engines |publisher=Kataria Books |location=New Delhi, India |isbn=978-93-5014-214-1 |ref=none}}
* {{cite book|title=Introduction to Internal Combustion Engines |first=Richard |last=Stone |year=1992 |edition=2nd |isbn=978-0-333-55083-0 |publisher=Macmillan }}
* {{cite book | last=Yamagata | first=H. | title=The Science and Technology of Materials in Automotive Engines | publisher=Elsevier Science | series=Woodhead Publishing in materials The science and technology of materials in automotive engines | year=2005 | isbn=978-1-84569-085-4 | url=https://books.google.com/books?id=7rekAgAAQBAJ | page= }}
* Patents:
** {{ cite patent
  |country=ES
  |number=433850
  |title=Perfeccionamientos en Motores de Explosion, con Cinco Tiem-Pos y Doble Expansion
  |pubdate=1976-11-01
  |invent1=Ubierna Laciana
  |ref=none
}}
** {{ cite patent
  |country=ES
  |number=230551
  |title=Un Nuevo Motor de Explosion
  |pubdate=1957-03-01
  |invent1=Ortuno Garcia Jose
  |ref=none
}}
** {{ cite patent
  |country=ES
  |number=249247
  |title=Motor de Carreras Distintas
  |pubdate=1959-09-01
  |invent1=Ortuno Garcia Jose
  |ref=none
}}

== Further reading ==
* {{cite book |last1=Singer |first1=Charles Joseph |last2=Raper |first2=Richard|title=A History of Technology: The Internal Combustion Engine |editor-last1=Charles |editor-first1=Singer |publisher=Clarendon Press |year=1978 |pages=157–176|display-editors=etal |isbn=978-0-19-858155-0}}
* {{cite book |last=Setright |first=LJK |title=Some unusual engines |year=1975 |publisher=The Institution of Mechanical Engineers |location=London |isbn=978-0-85298-208-2}}
* {{cite book |last=Suzuki |first=Takashi |title=The Romance of Engines |year=1997 |publisher=Society of Automotive Engineers |location=US |isbn=978-1-56091-911-7}}
* {{cite book |first=Horst O. |last=Hardenberg |title=The Middle Ages of the Internal Combustion Engine |year=1999 |publisher=Society of Automotive Engineers |location=US}}
* {{cite book |last=Gunston |first=Bill |title=Development of Piston Aero Engines |year=1999 |publisher=PSL |isbn=978-1-85260-619-0}}

== External links ==
{{Commons category|Internal combustion engines}}
{{Wikiversity|Internal combustion engines}}
{{Wiktionary|combustion engine}}
* [https://www.youtube.com/watch?v=ObmAOJA__1k Combustion video] – in-cylinder combustion in an optically accessible, 2-stroke engine
* [http://www.animatedengines.com Animated Engines] – explains a variety of types
* [http://www.autoeducation.com/rm_preview/engine_intro.htm Intro to Car Engines] – Cut-away images and a good overview of the internal combustion engine
* [https://web.archive.org/web/20110720090822/http://me.engin.umich.edu/autolab/Projects/index.html Walter E. Lay Auto Lab] – Research at The University of Michigan
* [https://www.youtube.com/watch?v=W2eILCrW53M&NR YouTube] – Animation of the components and built-up of a 4-cylinder engine
* [https://www.youtube.com/watch?v=2QB7XPMeLnA YouTube] – Animation of the internal moving parts of a 4-cylinder engine
* [https://web.archive.org/web/20090627061021/http://www.popularmechanics.com/automotive/new_cars/4261289.html?series=19 Next generation engine technologies] retrieved May 9, 2009
* [http://www.howstuffworks.com/engine1.htm How Car Engines Work]
* [http://www.douglas-self.com/MUSEUM/POWER/unusualICeng/unusualICeng.htm Unusual Internal-Combustion Engines]
* Aircraft Engine Historical Society (AEHS) – [http://www.enginehistory.org AEHS Home]

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{{DEFAULTSORT:Internal Combustion Engine}}
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