Editing Atkinson cycle

{{Short description|Thermodynamic cycle}}
{{thermodynamics|cTopic=Processes and Cycles}}
The '''Atkinson-cycle engine''' is a type of [[internal combustion engine]] invented by [[James Atkinson (inventor)|James Atkinson]] in 1882. The Atkinson cycle is designed to provide [[Energy conversion efficiency|efficiency]] at the expense of [[power density]]. 

A variation of this approach is used in some modern automobile engines. While originally seen exclusively in [[hybrid electric]] applications such as the earlier-generation [[Toyota Prius]], later hybrids and some non-hybrid vehicles now feature engines with [[variable valve timing]], which can run in the Atkinson cycle as a part-time operating regimen, giving good economy while running in Atkinson cycle, and conventional power density when running as a conventional, [[Otto cycle]] engine.

==Design==
Atkinson produced three different designs that had a short compression stroke and a longer expansion stroke. The first Atkinson-cycle engine, the ''differential engine'', used opposed pistons. The second and best-known design was the ''cycle engine'', which used an over-center arm to create four piston strokes in one crankshaft revolution. This unusual reciprocating engine had the intake, compression, power, and exhaust [[Stroke (engine)|strokes]] of the [[four-stroke cycle]] in a single turn of the [[crankshaft]], and was designed to avoid infringing certain patents covering [[Otto-cycle]] engines.<ref name=pat367496/> Atkinson's third and final engine, the ''utilite engine'', operated much like a two-stroke engine. 

Apart from the features implemented to avoid Otto patents, the truly unique Atkinson's design is that the engines have an expansion stroke that is longer than the compression stroke, and by this method the engine achieves greater [[thermal efficiency]] than a traditional piston engine. Atkinson's engines were produced by the British Gas Engine Company and also licensed to other overseas manufacturers. 

Many modern engines now use a crankshaft center-line offset from the cylinder center-line (making them [[Desaxe]] engines) and/or unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke. Miller applied this technique to the four-stroke engine, so it is sometimes referred as the Atkinson/[[Miller cycle]], US patent 2817322 dated Dec 24, 1957.<ref>{{US patent|2817322}}</ref> In 1888, Charon filed a French patent and displayed an engine at the Paris Exhibition in 1889. The Charon gas engine (four-stroke) used a similar cycle to Miller, but without a supercharger. It is referred to as the "Charon cycle".<ref>{{cite book |url={{Google books|8d03AAAAMAAJ|page=152|keywords=charon |plainurl=yes}} |title=A text-book on gas, oil and air engines: or, Internal combustion motors without boiler |page=152 |first=Brian |last=Donkin |publisher=C. Griffin and company, limited |year=1896}}</ref>

[[Hugo Güldner]] argued in his 1914 book that Körting was the first firm to build a gas engine with a short compression stroke and a longer expansion phase in 1891, based on a design first proposed by [[Otto Köhler (engineer)|Otto Köhler]] in 1887. This engine also had an engine-load dependent valve train which increased the intake and compression stroke with increasing engine load. On the other hand, the compression was decreasing at low and medium loads, which ultimately reduced the efficiency.<ref>{{cite book |last=Güldner |first=Hugo |title=Das Entwerfen und Berechnen der Verbrennungskraftmaschinen und Kraftgas-Anlagen |trans-title=The design and calculation of internal combustion engines and power gas systems |publisher=Springer-Verlag |publication-place=Berlin, Heidelberg |year=1914 |isbn=978-3-662-24387-9 |doi=10.1007/978-3-662-26508-6 |page=64}}</ref>

[[Roy Fedden]], at [[Bristol]], tested an arrangement in the [[Bristol Jupiter]] IV engine in 1928, with variable retard timing allowing part of the charge to be blown back into the intake manifold, in order to have sustainable reduced operation pressures during takeoff.{{cn|date=June 2021}}

Modern engine designers are realizing the potential fuel-efficiency improvements the Atkinson-type cycle can provide.<ref>{{cite web |url=http://www.autos.ca/auto-tech/auto-tech-atkinson-cycle-engines-and-hybrids/ |title=Auto Tech: Atkinson Cycle engines and Hybrids |work=Autos.ca |date=2010-07-14 |access-date=2013-02-23}}</ref>

==Atkinson "Differential Engine"==
The first implementation of the Atkinson cycle was in 1882; unlike later versions, it was arranged as an [[opposed-piston engine]], the Atkinson differential engine.<ref>{{cite book | last = Gingery | first = Vincent | title = Building the Atkinson Differential Engine | year = 2000 | publisher = David J. Gingery Publishing, LLC | isbn = 1878087231 }}</ref><ref name=pat336505 /> In this, a single crankshaft was connected to two opposed pistons through a toggle-jointed linkage that had a nonlinearity; for half a revolution, one piston remained almost stationary while the other approached it and returned, and then for the next half revolution, the second-mentioned piston was almost stationary while the first approached and returned. 

Thus, in each revolution, one piston provided a compression stroke and a power stroke, and then the other piston provided an exhaust stroke and a charging stroke. As the power piston remained withdrawn during exhaust and charging, it was practical to provide exhaust and charging using valves behind a port that was covered during the compression stroke and the power stroke, and so the valves did not need to resist high pressure and could be of the simpler sort used in many steam engines, or even [[reed valve]]s.

{{gallery
|File:US336505-Atkinson Opposed Piston Engine.png|Patent drawing of the Atkinson "Differential Engine", 1882
|File:Atkinson Opposed Piston Engine.gif|Animation of the Atkinson differential engine, 1882
}}

==Atkinson "Cycle Engine"==
The next engine designed by Atkinson in 1887 was named the "Cycle Engine" This engine used poppet valves, a cam, and an over-center arm to produce four piston strokes for every revolution of the crankshaft. The intake and compression strokes were significantly shorter than the expansion and exhaust strokes. 

The "Cycle" engines were produced and sold for several years by the British Engine Company. Atkinson also licensed production to other manufacturers. Sizes ranged from a few up to 100 horsepower.

{{gallery
|File:Atkinson-Gas-Engine.png|Atkinson gas engine as shown in US Patent 367496, 1887
|File:Atkinson Engine with Intake.gif|Animation of the Atkinson "Cycle Engine", 1887
}}

==Atkinson "Utilite Engine"==
[[File:Atkinson utilite engine.jpg|thumb|Atkinson Utilite Engine]]
[[File:Atkinson's Utilite' engine 1892.png|thumb|Atkinson's Utilite engine 1892]]

Atkinson's third design was named the "Utilite Engine".<ref>{{cite book |title=The gas, petrol, and oil engine, Volume 2 |first=Dugald |last=Clerk |page=210 |url=https://books.google.com/books?id=aB9YAAAAYAAJ&q=The+gas%2C+petrol%2C+and+oil+engine%2C+Volume+2+By+Dugald+Clerk |year=1913 |publisher=J. Wiley}}</ref> Atkinson's "Cycle" engine was efficient; however, its linkage was difficult to balance for high speed operation. Atkinson realized an improvement was needed to make his cycle more applicable as a higher-speed engine. 

With this new design, Atkinson was able to eliminate the linkages and make a more conventional, well balanced engine capable of operating at speeds up to 600 rpm and capable of producing power every revolution, yet he preserved all of the efficiency of his "Cycle Engine" having a proportionally short compression stroke and a longer expansion stroke. The Utilite operates much like a standard two-stroke except that the exhaust port is located at about the middle of the stroke. 

During the expansion/power stroke, a cam-operated valve (which remains closed until the piston nears the end of the stroke) prevents pressure from escaping as the piston moves past the exhaust port. The exhaust valve is opened near the bottom of the stroke; it remains open as the piston heads back toward compression, letting fresh air charge the cylinder and exhaust escape until the port is covered by the piston. 

After the exhaust port is covered the piston begins to compress the remaining air in the cylinder. A small piston fuel pump injects liquid during compression. The ignition source was likely a hot tube as in Atkinson's other engines. This design resulted in a two-stroke engine with a short compression and longer expansion stroke. 

The Utilite Engine tested as even more efficient than Atkinson's previous "differential" and "cycle" designs. Very few were produced, and none are known to survive. The British patent is from 1892, #2492. No US patent for the Utilite Engine is known.

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==Ideal thermodynamic cycle==
[[File:T cycle AtkinsonMiller.png|thumb|right|Figure 1: Atkinson gas cycle]]

The ideal Atkinson cycle consists of:
* 1–2 [[Isentropic process|Isentropic]], or [[Reversible process (thermodynamics)|reversible]], [[Adiabatic process|adiabatic]] compression
* 2–3 [[Isochoric process|Isochoric]] heating (Qp)
* 3–4 [[Isobaric process|Isobaric]] heating (Qp')
* 4–5 Isentropic expansion
* 5–6 Isochoric cooling (Qo)
* 6–1 Isobaric cooling (Qo')

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==Modern Atkinson-cycle engines==
[[File:Atkinson-cycle engine.jpg|thumb|A small engine with Atkinson-style linkages between the piston and flywheel. Modern Atkinson-cycle engines do away with this complex energy path.]]

In the late 20th century, the term "Atkinson cycle" began to be used to describe a modified [[Otto-cycle]] engine—in which the intake valve is held open longer than normal, allowing a reverse flow of intake air into the intake manifold. This "simulated" Atkinson cycle is most notably used in the [[Toyota NZ engine#1NZ-FXE|Toyota 1NZ-FXE]] engine from the early [[Prius]] and the [[Toyota Dynamic Force engine]]s. 

The effective compression ratio is reduced—for the time the air is escaping the cylinder freely rather than being compressed—but the ''expansion'' ratio is unchanged (i.e., the compression ratio is smaller than the expansion ratio). The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy—meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density. Due to a smaller portion of the compression stroke being devoted to compressing the intake air, an Atkinson-cycle engine does not take in as much air as would a similarly designed and sized Otto-cycle engine. Four-stroke engines of this type that use the same type of intake valve motion but also utilize [[forced induction]] to make up for the loss of power density are known as [[Miller cycle|Miller-cycle]] engines.

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==Rotary Atkinson-cycle engine==
[[File:WikiDartEngine.gif|thumb|right|Rotary Atkinson-cycle engine]]

The Atkinson cycle can be used in a [[pistonless rotary engine|rotary engine]]. In this configuration, an increase in both power and efficiency can be achieved when compared to the Otto cycle. This type of engine retains the one power phase per revolution, together with the different compression and expansion volumes of the original Atkinson cycle.

Exhaust gases are expelled from the engine by compressed-air scavenging. This modification of the Atkinson cycle allows the use of alternative fuels such as diesel and hydrogen. 

Disadvantages of this design include the requirement that rotor tips seal very tightly on the outer housing wall and the mechanical losses suffered through friction between rapidly oscillating parts of irregular shape. See [[#External_links|external links]] below for more information.

The [[Sachs Motorcycles|Sachs]] KC-27 Wankel engine in the [[Hercules W-2000]] motorcycle used the Atkinson cycle. A depression capsule opens a secondary path for the incoming charge.{{cn|date=June 2021}}

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==Vehicles using Atkinson-cycle engines==
[[File:2018 Hyundai Ioniq SE HEV S-A 1.6 Front.jpg|alt=|thumb|[[Hyundai Ioniq]] hybrid]]
[[File:2010 Ford Fusion Hybrid.jpg|thumb|2010 [[Ford Fusion Hybrid]] (North America)]]

While a modified Otto-cycle piston engine using the Atkinson cycle provides good [[fuel efficiency]], it is at the expense of a lower power-per-displacement as compared to a traditional four-stroke engine.<ref>Heywood, John B. ''Internal Combustion Engine Fundamentals'', p. 184-186.</ref> If demand for more power is intermittent, the power of the engine can be supplemented by an [[electric motor]] during times when more power is needed. This forms the basis of an Atkinson cycle-based [[hybrid electric]] drivetrain. These electric motors can be used independently of, or in combination with, the Atkinson-cycle engine, to provide the most efficient means of producing the desired power. This drive-train first entered production in late 1997 in the [[Toyota Prius (XW10)|first-generation Toyota Prius]].

{{As of|2018|July}}, many production [[hybrid vehicle drivetrain]]s use Atkinson-cycle concepts—for example, in:

*[[Chevrolet Volt]]
*[[Chrysler Pacifica (minivan)|Chrysler Pacifica]] (front-wheel drive) plug-in hybrid model minivan
*Fiat 500 Hybrid and Fiat Panda Hybrid 
*[[Ford C-Max#Hybrid and plug-in hybrid|Ford C-Max]] (front-wheel drive / US market) hybrid and plug-in hybrid models
*[[Ford Escape Hybrid|Ford Escape]]/[[Mercury Mariner]]/[[Mazda Tribute]] electric (front- and four-wheel drive) with a compression ratio of 12.4:1
*[[Ford Fusion Hybrid]]/[[Mercury Milan Hybrid]]/[[Lincoln MKZ Hybrid]] electric (front-wheel drive) with a compression ratio of 12.3:1
*[[Ford Maverick (2022)|Ford Maverick]]<ref>{{cite news |url=https://jalopnik.com/ford-brings-back-small-cheap-trucks-with-the-20-000-20-1847052190 |title=2022 Ford Maverick Is A $20,000 Hybrid |first=Jason |last=Torchinsky |work=Jalopnik |location=US |date=2021-06-08 |access-date=2021-06-09}}</ref>
*[[Honda Accord Plug-in Hybrid]]<ref>{{cite news |url=http://www.worldcarfans.com/113012152931/honda-accord-plug-in-hybrid-earns-the-title-for-being-the |title=Honda Accord Plug-in Hybrid earns the title for being the most fuel-efficient sedan in America |work=worldcarfans.com |first=Michael |last=Gauthier |date=2013-01-21 |access-date=2013-01-22}}</ref>
*[[Honda Accord Hybrid]] (front-wheel drive)
*[[Honda City]] (hybrid)
*[[Honda Clarity#Plug-in hybrid|Honda Clarity Plug-In Hybrid]]<ref>{{Cite web|url=https://www.honda.ca/clarity/2018/performance-capability-features|title=2018 Honda Clarity Plug-in Hybrid|website=www.honda.ca|language=en|access-date=2018-01-25|archive-url=https://web.archive.org/web/20180126021408/https://www.honda.ca/clarity/2018/performance-capability-features|archive-date=2018-01-26|url-status=dead}}</ref>
*[[Honda CR-V]] (hybrid 2020–present)
*[[Honda Insight]] (front-wheel drive)<ref>{{Cite web|url=https://www.honda.ca/insight/specs|title=2018 Honda Insight Hybrid|website=www.honda.ca|language=en|access-date=2018-07-14}}</ref>
*[[Honda Fit]] (front-wheel drive) some of the 3rd generation engines switch between Atkinson and Otto cycles.
*[[Hyundai Sonata Hybrid]] (front-wheel drive)
*[[Hyundai Elantra]] Atkinson-cycle models
*[[Hyundai Grandeur]] hybrid (front-wheel drive)
*[[Hyundai Ioniq]] hybrid, plug-in hybrid (front-wheel drive)
*[[Hyundai Palisade]] 3.8 L Lambda II V6 GDi
*[[Infiniti M35h]] hybrid (rear-wheel drive)
*[[Kia Forte]] 147 hp 2.0 petrol only (front-wheel drive)
*[[Kia Niro]] hybrid, plug-in hybrid (front-wheel drive)
*[[Kia Optima Hybrid]] [[Kia K5]] hybrid 500h (front-wheel drive) with a compression ratio of 13:1
*[[Kia Cadenza]] Hybrid [[Kia K7]] hybrid 700h (front-wheel drive)
*[[Kia Telluride]] 3.8 L Lambda II V6 GDi
*[[Kia Seltos]] 2.0L (front-wheel drive)
*[[Lexus CT 200h]] (front-wheel drive)
*[[Lexus ES 300h]] (front-wheel drive)
*[[Lexus GS 450h]] hybrid electric (rear-wheel drive) with a compression ratio of 13:1
*[[Lexus RC F]] (rear-wheel drive)
*[[Lexus GS F]] (rear-wheel drive)
*[[Lexus HS 250h]] (front-wheel drive)
*[[Lexus IS 200t]] (2016)<ref>{{cite web |url=http://www.lexus.com/models/IS/performance |title=2016 Lexus IS – Performance |publisher=Lexus |location=US |access-date=2016-08-09}}</ref>
*[[Lexus NX]] hybrid electric (four-wheel drive)
*[[Lexus RX 450h]] hybrid electric (four-wheel drive)
*[[Lexus UX]] hybrid electric (four-wheel drive)
*[[Lexus LC]] (rear-wheel drive)
*[[Mazda Mazda6]]  (2013 for the 2014 model year)
*[[Mercedes-Benz M-Class#ML450 Hybrid|Mercedes ML450 Hybrid]] (four-wheel drive) electric
*[[Mercedes-Benz S-Class#S400 BlueHYBRID|Mercedes S400 Blue Hybrid]] (rear-wheel drive) electric
*[[Mitsubishi Outlander PHEV]] (2018 for the 2019 model year, plug-in hybrid four-wheel drive)<ref>{{cite web |url=http://www.mitsubishi-motors.com/en/newsrelease/2018/detail1103.html |title=2019 Outlander PHEV |publisher=Mitsubishi |location=US |access-date=2018-02-23}}</ref>
*[[Subaru Impreza#XV / Crosstrek 2|Subaru Crosstrek Hybrid]] (2018 for the 2019 model year, all-wheel drive)
*[[Toyota Camry Hybrid]] electric (front-wheel drive) with a compression ratio of 12.5:1
*[[Toyota Avalon Hybrid]] (front wheel drive) 
*[[Toyota Highlander Hybrid#Hybrid|Toyota Highlander Hybrid]] (2011 and newer)<ref>{{cite web|last=Edmunds |first=Dan |url=http://www.edmunds.com/toyota/highlander-hybrid/2011/road-test.html |title=2011 Toyota Highlander Hybrid Road Test |publisher=Edmunds.com |date=2010-09-24 |access-date=2012-07-04}}</ref>
*[[Toyota Prius]] [[hybrid electric]] (front-wheel drive) with a (purely geometric) compression ratio of 13.0:1
*[[Toyota Vitz#Hybrid|Toyota Yaris Hybrid]] (front-wheel drive) with a compression ratio of 13.4:1
*[[Toyota Auris#Auris Hybrid|Toyota Auris Hybrid]] (front-wheel drive)
*[[Toyota Tacoma]] V6 (beginning in 2015 for the 2016 model year)
*[[Toyota RAV4 Hybrid]] (beginning in 2015 for the 2016 model year)
*[[Toyota Sienna]] (2016 for the 2017 model year, hybrid beginning for 2021 model year)
*[[Toyota Venza]] (hybrid beginning for 2021 model year)
*[[Toyota C-HR|Toyota C-HR Hybrid]] (2016–present)
*[[Toyota Yaris Cross|Toyota Yaris Cross Hybrid]] (2021–present)
*[[Toyota Innova (AG10)|Toyota Innova Hybrid]] (2022–present)

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==Patents==
The 1887 patent (US 367496) describes the mechanical linkages necessary to obtain all four strokes of the four-stroke cycle for a gas engine within one revolution of the crankshaft.<ref name=pat367496>{{cite patent|country=US|number=367496|inventor=J. Atkinson|title=Gas Engine|gdate=1887-08-02|url=https://www.google.com/patents/US367496}}</ref> There is also a reference to an 1886 Atkinson patent (US 336505), which describes an [[opposed-piston engine|opposed-piston]] gas engine.<ref name=pat336505 >{{cite patent|country=US|number=336505 |inventor=J. Atkinson |title=Gas Engine |gdate=1886-02-16}}</ref> The British patent for the "Utilite'" is from 1892 (#2492).

==See also==
* [[History of the internal combustion engine]]
* [[Variable valve timing]]
* [[Variable valve lift]]

==References==
{{reflist}}

==External links==
* [https://web.archive.org/web/20070815030209/http://www.ornl.gov/~webworks/cpr/v823/rpt/106982.pdf Comparison of Prime Movers Suitable for USMC Expeditionary Power Sources], [[Oak Ridge National Laboratory]]
* [https://libralato.co.uk Libralato Engines] – developing a rotary Atkinson cycle engine
* [https://white-smoke.wikifoundry.com/page/Rotary+Engine Rotary Atkinson cycle engine] {{Webarchive|url=https://web.archive.org/web/20200619181506/http://white-smoke.wikifoundry.com/page/Rotary+Engine |date=2020-06-19 }} – gives details of this engine as well as comparisons with conventional and Wankel engines
* [https://www.greencarreports.com/news/1014183_the-priuss-not-so-secret-gas-mileage-secrets The Prius's Not So Secret Gas-Mileage Secrets] – how the Prius uses the Atkinson cycle to get better results than an Otto cycle engine
* [https://www.findagrave.com/memorial/198780164/james-atkinson James Atkinson at Find A Grave] – personal details

{{Thermodynamic cycles|state=uncollapsed}}

[[Category:Thermodynamic cycles]]
[[Category:Hybrid vehicles]]