Editing Flat-twin engine

{{Short description|Piston engine with two cylinders in opposing directions}}
[[File:Douglas motorcycle engine, Abergavenny steam rally 2012.jpg|thumb|[[Douglas (motorcycles)|Douglas]] 80 Plus motorcycle engine (circa 1950)]]
{{redirects here|Flat Twin|Soviet radar nicknamed "flat twin"|5N65 radar}}
A '''flat-twin engine''' is a two-cylinder [[internal combustion engine]] with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the '''boxer-twin engine''', where both pistons move inwards and outwards at the same time.

The flat-twin design was patented by [[Karl Benz]] in 1896 and the first production flat-twin engine was used in the ''Lanchester 8 hp Phaeton'' car released in 1900. The flat-twin engine was used in several other cars since, however a more common usage is in motorcycles; early models oriented the cylinders in line with the frame, however later models switched to the cylinders being perpendicular to the frame to provide even cooling across both cylinders.

Flat-twin engines were also used in several aircraft up until the 1930s and in various stationary applications from the 1930s to the 1960s.

The Australian lawnmower manufacturer Victa also produced a flat-twin engine push mower from August 1975 to 1980 dubbed the ‘Twin 500’, and later the ‘Supreme’. These engines were manufactured in Canada. They are very sought after as only small numbers were produced, most likely due to ignition- and fuel-related problems in early models. In the Supreme (the later model) all these problems were fixed with a rear-domed piston, crankcase mixers and refined ignition system{{cn|date=February 2024}}.

== Typical design ==
[[File:Boxerengineanimation.gif|thumb|[[Flat_engine#Boxer_engine|Boxer]] crankshaft configuration]]

Most flat-twin engines use a [[Flat_engine#Boxer_engine|boxer]] configuration for the [[crankshaft]] and are therefore called "boxer-twin" engines. In a boxer-twin engine, the 180° crankshaft moves the pistons in phase with each other, therefore the forces generated by one piston are cancelled out by the other, resulting in excellent primary balance. The evenly spaced [[firing order]] also assists in reducing vibration. The equal and opposite forces in a boxer-twin engine do however generate a [[Couple (mechanics)|rocking couple]], due to the offset distance between the pistons along the crankshaft.<ref name="Wilson"/>{{refpage|page=27}}

[[File:Allumage 2cv.svg|thumb|[[Wasted spark]] ignition system]]
A commonly used ignition system is [[wasted spark]],<ref>{{cite web |title=BMW Motorcycle Engine Animation |url=http://www.animatedpiston.com/BMW.htm |website=www.animatedpiston.com |url-status=usurped |archive-url=https://web.archive.org/web/20170627021204/http://www.animatedpiston.com/BMW.htm |archive-date=27 June 2017}}</ref> which is a simple ignition system using a double-ended coil firing both spark plugs on each revolution (i.e. during both the compression and exhaust strokes). This system is distributorless and requires only a single contact breaker and coil for the engine.<ref name="2CVStuff">{{cite web |url      =http://www.2cvstuff.com/Files/ignition.pdf |title      =2CV Stuff: A Series Ignition System - Specifications |website      =2CV Stuff |publisher      =Oui2 |location      =Grantham, Lincolnshire, UK |archive-url      =https://web.archive.org/web/20120316122922/http://www.2cvstuff.com/Files/ignition.pdf |archive-date      =2012-03-16 |url-status      =dead |access-date      =2013-05-18 }}</ref>

=== Crankcase pressure ===
The boxer-twin configuration can cause pressuring of the [[crankcase]] during each inward piston stroke and de-pressurisation during each outward piston stroke, since both pistons are moving inwards or outwards at the same time. This crankcase pumping effect (also found on single-cylinder engines and 360° parallel-twin engines) is usually addressed by means of a crankcase breather.<ref name="Cameron">{{cite journal|last= Cameron|first= Kevin|author-link= Kevin Cameron (journalist)|title= TDC: Pumped|date= January 1992|page=14|journal= [[Cycle World]]|volume= 31|issue= 1|editor-last= Edwards|editor-first= David|publisher= Hachette Magazines|location= Newport Beach, CA US|issn= 0011-4286|url= {{Google books|eToxUA1LZbsC|TDC: Pumped|page=14|plainurl=yes}}|access-date= 17 April 2015}}</ref>

The [[Citroën 2CV]] boxer-twin engine took advantage of this pumping effect to maintain a partial vacuum inside the crankcase, in order to reduce oil leaks when an oil seal malfunctions. This was achieved by using a [[Check valve|one-way valve]] (a leather or rubber flap over a hole in the crankcase), to let air escape the crankcase but not enter it.<ref>{{cite web |url=https://discourse-cdn-sjc1.com/cartalk/uploads/default/optimized/2X/0/0e996467337b68f98a6b64844e16ba92610daec2_2_1035x567.gif |title= Citroen 2CV engine operation animation |website=www.discourse.com |url-status=dead |archive-url= https://web.archive.org/web/20190119174443/https://discourse-cdn-sjc1.com/cartalk/uploads/default/optimized/2X/0/0e996467337b68f98a6b64844e16ba92610daec2_2_1035x567.gif |archive-date=19 January 2019}}</ref>

==Applications==
===Automobiles===
[[File:Citroen A-series 2CV 602cc Engine (18770871302).jpg|thumb|[[Citroën 2CV]] engine (viewed from rear)]]

The beginnings of the flat-twin engine were in 1896, when Karl Benz obtained a patent for the design. A year later, his company [[Benz & Cie]] unveiled the first flat-twin engine, a boxer design called the "contra engine".<ref name="English">{{cite news|last= English|first= Bob|title= The engine that Benz built still survives|url= https://www.theglobeandmail.com/globe-drive/car-life/classic-cars/the-engine-that-benz-built-still-survives/article4317376/|access-date= 2013-12-19|newspaper= The Globe and Mail|location= Toronto, Canada|date= 2010-04-29|url-status= dead|archive-url= https://web.archive.org/web/20131220085552/http://www.theglobeandmail.com/globe-drive/car-life/classic-cars/the-engine-that-benz-built-still-survives/article4317376/|archive-date= 2013-12-20}}</ref>

In 1900, The [[Lanchester Motor Company|Lanchester Engine Company]] began production of the ''[[Lanchester 8&nbsp;hp Phaeton]]'', which used a flat-twin engine.<ref name="Rogliatti">{{cite book| last       = Rogliatti| first      = Gianni| title         = Period Cars| year      = 1973| publisher = Hamlyn| location  = Feltham, Middlesex, UK| isbn      = 0-600-33401-5| page     = 140}}</ref><ref name="Smith">{{cite web |url=http://www.caranddriver.com/features/the-10-most-unusual-engines-of-all-time-feature |title=The 10 Most Unusual Engines of All Time |last=Smith |first=Sam |date=October 2010 |website=[[Car and Driver]] |publisher=Hearst |at= Lanchester Twin-Crank Twin|archive-url=https://web.archive.org/web/20120703192824/http://www.caranddriver.com/features/the-10-most-unusual-engines-of-all-time-feature |archive-date=2012-07-03 |url-status=live |access-date=2013-05-17 |quote=One crank lived above the other, and each piston had three connecting rods—two light outer ones and a heavier one in the centre. The light rods went to one crank, the heavy rods to the other, and the two shafts counterrotated. }}</ref> This engine had an unusual design of two counter-rotating crankshafts, with each piston attached to its crankshaft by a thick connecting rod.<ref name="Rogliatti"/> Each piston was also connected to the other crankshaft by two thinner connecting rods, causing the two pistons to move on the same axis.<ref name="Rogliatti"/><ref name="Smith"/> It also had the torque reaction of one crankshaft cancel the torque reaction of the other, cancelling torque reaction in the engine.<ref name="Rogliatti"/> Lanchester used this engine design until 1904.<ref name="Smith"/>

Other early uses of flat-twin engines were 1903-04 [[Ford Model A (1903–1904)|Ford Model A]], the 1904-1905 [[Ford Model C]], the 1905-1906 [[Ford Model F]].<ref name="Kimes">{{cite book|last=Kimes |first=Beverly |title=Standard Catalog of American Cars 1805-1942 |year=1996 |publisher=Krause Publications |isbn=0-87341-428-4 |page=572 }}</ref><ref name="Brooke">{{cite book| last = Brooke| first = Lindsay| title = Ford Model T: The Car that Put the World on Wheels| url = {{Google books|dvZjf-4qbeAC|Ford Model T: The Car that Put the World on Wheels|page=38|plainurl=yes}}| access-date            = 2013-05-18| year = 2008| publisher = Motorbooks| location = Minneapolis, MN USA| isbn = 978-0-76032-728-9|chapter = Chapter 1 Before the Model T| chapter-url            = {{Google books|dvZjf-4qbeAC|Ford Model T: The Car that Put the World on Wheels|page=29|plainurl=yes}}}}</ref>{{refpage|pages=38–44}} and several [[Jowett Cars]] models from 1910 to 1937.<ref>[{{Google books|hVZWnZbUJOYC|The Complete Catalogue of British Cars 1895 - 1975|page=189|plainurl=yes}} Specification tables]</ref><ref name="Culshaw&Horrobin">{{cite book |last1= Culshaw|first1= David|last2= Horrobin|first2= Peter|year= 2013|orig-year= 1974|title= The Complete Catalogue of British Cars 1895 - 1975|edition= e-book|url= {{Google books|hVZWnZbUJOYC|The Complete Catalogue of British Cars 1895 - 1975|plainurl=yes}}|location= Poundbury, Dorchester, UK|publisher= Veloce Publishing |isbn= 978-1-845845-83-4}}</ref>{{refpage|pages=188–189}}

The [[Citroën 2CV]], produced from 1948 to 1990, was one of the first front-wheel drive cars to use a flat engine. The 2CV was powered by an air-cooled boxer-twin engine. Also in 1948, the [[Panhard Dyna X]] was released with front-wheel drive and an air-cooled boxer-twin engine. Other cars following World War II using boxer-twin engines were the 1945-1954 [[Jowett Bradford]] van,<ref name="Culshaw&Horrobin"/>{{refpage|page=373}}, the 1961-1976 [[DAF Daffodil]], the 1961-1978 [[Toyota Publica]], the 1965-1969 [[Toyota Sports 800]] sportscar and several front-wheel drive models from [[Citroën]] and [[Panhard]]. Several rear-engined cars were also produced with boxer-twin engines originally designed for motorcycles, such as the 1957-1975 [[Puch 500]], the 1957-1959 [[BMW 600]] and the 1959-1965 [[BMW 700]]. The Brazilian manufacturer [[Gurgel|Gurgel Motores]] used an in-house developed water-cooled boxer-twin engine (Enertron engine) and the Volkswagen air-cooled boxer-four in several models from 1988 to 1994.

The [[Toyota U engine]] was an air-cooled flat-twin engine produced from 1961 to 1976. Introduced in the [[Toyota Publica]] subcompact car, the U engine was also used in the [[Toyota MiniAce]] small commercial vehicle and the [[Toyota Sports 800]] sports car.

===Motorcycles===
==== Transverse mounting ====
[[File:Flat twin Douglas 1912 N 3.jpg|thumb|1912 [[Douglas (motorcycles)|Douglas N3]] engine]]

The benefits of using a flat-twin engine mounted with the crankshaft running perpendicular to the frame (therefore the cylinders being in line with the frame) are a low centre of gravity<ref name="Willoughby">{{cite book|last=Willoughby|first=Vic|title=Classic Motorcycles|edition=Third impression|orig-year=1975|year=1977|publisher=The Hamlyn Publishing Group|isbn=0-600-31870-2|page=23|chapter=Douglas}}</ref> and that a belt-drive or chain-drive system can be used to transmit drive to the rear wheel.<ref name="Willoughby"/><ref name="Norbye">{{cite book|last=Norbye|first=Jan P.|title=BMW - Bavaria's Driving Machines|year=1984|publisher=Beekman House|location=New York, NY, USA|isbn=0-517-42464-9|page=[https://archive.org/details/bmwbavariasdrivi00norb/page/15 15]|chapter=The Origins of BMW: From Flying Machines to Driving Machines|chapter-url-access=registration|chapter-url=https://archive.org/details/bmwbavariasdrivi00norb/page/15}}</ref> However, the downsides are uneven heat distribution (the front cylinder is more heavily cooled than the rear cylinder)<ref name="Willoughby"/><ref name="Norbye"/> and a longer wheelbase is often required due to the length of the engine.<ref name="Willoughby"/>

The first flat-twin [[motorcycle engine]] was built in 1905 by the Light Motors Company in the United Kingdom. Originally named the Fée (renamed "Fairy" soon after its introduction),<ref name=TMCfuture>{{cite journal |date= October 5, 1916|title= The engine of the future|page= 283|journal= [[The Motor Cycle (magazine)|The Motor Cycle]]|publisher= Iliffe & Sons Ltd.}}</ref> it was designed as a "bicycle engine system" which transmitted power to a pulley on the rear wheel via a chain.<ref name="Partridge">{{Citation |last= Partridge|first= Michael|year= 1976|title= Motorcycle Pioneers: The Men, the Machines, the Events 1860-1930|publisher= David & Charles (Publishers)|isbn= 0-7153-7-209-2|page= 42|chapter= 1905 2 1⁄2hp Joseph Barter motorcycle}}</ref> Manufacture of the Fairy was taken over by the [[Douglas (motorcycles)|Douglas Engineering Company]], one of Light Motors' suppliers, when the Light Motors Company folded in 1907.<ref name="Wilson">{{cite book|last=Wilson|first=Hugo|title=The Encyclopedia of the Motorcycle|year=1995|publisher=Dorling Kindersley|location=London, UK|isbn=0-7513-0206-6 |chapter=The A-Z of Motorcycles}}</ref>{{refpage|pages=218-219}} Later in 1907, Douglas changed the drivetrain from the chain and pulley design to a belt-drive system driven directly from the engine.{{Sfn|Partridge|1976|p=42}} Later developments of the Douglas motorcycle were made with the cylinders in line with the frame until the Second World War.<ref name="Wilson"/>{{refpage|page=51}}

Other early flat-twin motorcycles used a similar layout, with their cylinders aligned along the frame and therefore with the crankshaft running transverse to the frame.{{cn|date=February 2024}}

In 1914 the main supplier of rear-hub gearboxes, [[Sturmey-Archer]], introduced a 3-speed countershaft gearbox with integral kick-starter,<ref name="MC1914">{{cite journal |date= 27 August 1914|title= A Sturmey-Archer Countershaft Gear|page= 274|journal= [[The Motor Cycle (magazine)|The Motor Cycle]]|publisher= Iliffe & Sons Ltd.}}</ref> which posed a design problem for motorcycles with transversely-mounted flat-twin engines. This gearbox could be relatively easily located behind a single-cylinder or V-twin engine, however this arrangement would result in an excessively long wheelbase for flat-twin engines. Solutions to this problem included using a countershaft below the engine (as used by the Douglas Fairy),<ref>[http://www.motorcycles20thcentury.com/motors/drawing/Douglas-1911.jpg Drawing of 1911 Douglas]</ref> or a gearbox located above the engine,<ref name="Willoughby"/><ref>[http://www.motorcycles20thcentury.com/motors/normal/Douglas-K32-1932.jpg Drawing of 1932 Douglas K32]</ref> although in some cases the cylinders were short enough to use the gearbox in the traditional location behind the engine.<ref>[http://www.motorcycles20thcentury.com/motors/normal/Douglas-1926.jpg Drawing of 1926 Douglas]</ref>

In 1916, most flat-twin motorcycles still used a transversely-mounted engine. The European models at this time included the [[Bradbury Motor Cycles|Bradbury]] {{cvt|3.5|hp|kW}}, the [[Brough Motorcycles|Brough HB]], the [[Douglas (motorcycles)|Douglas]] {{cvt|2.75|hp|kW}} and {{cvt|4|hp|kW}} models, the [[Humber motorcycles|Humber]] 3.5 hp and {{cvt|6|hp|kW}} models, the [[Matchless]] 6&nbsp;hp, the [[Montgomery Motorcycles|Montgomery]] 6&nbsp;hp, [[Williamson Flat Twin]] {{cvt|8|hp|kW}}, and the Bayerische Flugzeugwerke Helios (the predecessor to BMW's first motorcycle). Models produced in the United States included the [[Indian Model O]] and the [[Harley-Davidson Model W]].<ref name="MC1916">{{cite journal |date= November 9, 1916|title= Flat Twins|pages= 400–403|journal= [[The Motor Cycle (magazine)|The Motor Cycle]]|publisher= Iliffe & Sons Ltd.}}</ref><ref name="Norbye"/><ref name="Wilson"/>{{refpage|page=26}}<ref name="Indian and Harley">
*{{cite journal|editor-last= Wood|editor-first= Bill|title= Classics: 1917 Indian Model O|page= 71|journal= American Motorcyclist|date= October 2001b|volume= 55|issue= 10|issn= 0277-9358|publisher= [[American Motorcyclist Association]]|location    = Pickerington, OH US|url= {{Google books|DfsDAAAAMBAJ|Classics: 1917 Indian Model O|page=71|plainurl=yes}}|access-date= 19 April 2015|quote= Like the early Douglases, the Model O had its engine placed in the frame with the cylinders facing fore and aft, rather than sticking out to each side...}}
*{{cite book|last= Mitchel|first= Doug|title= Harley-Davidson Chronicle|year= 1997|publisher= Publications International|location= Lincolnwood, Illinois, USA|isbn= 0-7853-2514-X|pages= [https://archive.org/details/harleydavidsonch0000mitc_u3q3/page/44 44–45]|chapter= The Early Years (1903–1928)|chapter-url= https://archive.org/details/harleydavidsonch0000mitc_u3q3/page/44}}
*{{cite journal|editor-last= Wood|editor-first= Bill|title= Classics: 1922 Harley-Davidson Sport Twin|page= 127|journal= American Motorcyclist|date= March 2001a|volume= 55|issue= 3|issn= 0277-9358|publisher= [[American Motorcyclist Association]]|location    = Pickerington, OH US|url= {{Google books|HPsDAAAAMBAJ|Classics: 1922 Harley-Davidson Sport Twin|page=127|plainurl=yes}}|access-date= 19 April 2015|quote = In building the Sport Twin, Harley took Douglas' lead in orienting the cylinders in line with the frame.}}</ref>

==== Longitudinal mounting ====
{{multiple image
| align             = right
| direction         = vertical
| image1            = Xa-engine.jpg
| caption1          = 1942 [[Harley-Davidson XA]] engine
| image2            = Opposed_cylinders_on_a_1967_BMW_motorcycle.jpg
| caption2          = 1967 [[BMW R60/2|BMW R 60/2]] engine
}}

The main benefit of mounting a flat-twin engine with the crankshaft in line with the frame (therefore the cylinders sitting sideways in the frame) is that an air-cooled engine receives the same amount of cooling for each cylinder.<ref name="Wilson"/>{{refpage|pages=26-27}}<ref name="Willoughby"/> The [[Harley-Davidson XA]], which used a flat-twin engine with the cylinders across the frame, maintained an oil temperature 100&nbsp;°F (56&nbsp;°C) cooler than a [[Harley-Davidson WLA]] with a V-twin with the cylinders in line with the frame.<ref name="Wood">{{cite journal|editor-last= Wood|editor-first= Bill|title = Classics: 1942 Harley-Davidson XA|page=127|journal= American Motorcyclist|date= March 1999|volume= 53|issue= 3|issn= 0277-9358|url = {{Google books|A_cDAAAAMBAJ|Classics: 1942 Harley-Davidson XA|page=127|plainurl=yes}}|access-date  =2015-04-19|publisher = [[American Motorcyclist Association]]|location    = Pickerington, OH US|quote = Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA's oil temperature 100 degrees cooler than a standard Harley 45.}}</ref> A side benefit is that the cylinders provide protection to the rider in the event of a collision or fall, and keeps their feet warm in cold weather.<ref name="Willoughby"/><ref name="Wilson"/>{{refpage|page=27}} The downsides are that the engine cannot be mounted as close to the ground (otherwise the cylinders can scrape the ground during cornering)<ref name="Cocco">{{cite book|last1= Cocco|first1= Gaetano|title= Motorcycle Design and Technology|url= https://books.google.com/books?id=Ydk0bgq2_3YC|access-date= 2013-09-09|edition= English|year= 2004|publisher= Motorbooks International|location= St. Paul, MN USA|isbn= 0-7603-1990-1|page= 118|chapter= Chapter 11: The Engine|chapter-url= https://books.google.com/books?id=Ydk0bgq2_3YC&pg=PA113|quote= However, it does create some problems for longitudinal development of the bike because the boxer cylinders have to be positioned high up from the ground in order to protect them from scraping the ground when leant over in turns.}}{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> and that it exposes the cylinders and valve covers to the danger of collision damage.<ref name="Willoughby"/><ref name="Wilson"/>{{refpage|page=27}}

A transverse mounting also means that the torque reaction will twist the motorcycle to one side (such as on sharp acceleration/deceleration or when opening the throttle in neutral) instead of shifting the weight balance between the front and rear wheels. However, many modern motorcycles reduce this  effect by rotating flywheels or alternators in the opposite direction to that of the crankshaft.<ref name="motorcyclecruiser.com">{{cite web| url = http://www.motorcyclecruiser.com/roadtests/sport_cruisers_comparison/index.html| title = Sport Cruisers Comparison - Seven Sport-Cruiser Motorcycles| first1 = Art| last1 = Friedman| first2 = Andrew| last2 = Trevitt| first3 = Andrew| last3 = Cherney| first4 = Jamie| last4 = Elvidge| first5 = Evans| last5 = Brasfield| date = April 2000| work = Motorcycle Cruiser| publisher = Source Interlink Media| at = "Take a Spin" section, paragraph 4| access-date = 2010-09-10| quote = Though the Valkyrie also has a longitudinal crankshaft, this torque reaction has been eliminated by making some of the components, such as the alternator, spin the opposite direction of the engine.| archive-url = https://web.archive.org/web/20100502180324/http://www.motorcyclecruiser.com/roadtests/sport_cruisers_comparison/index.html| archive-date = 2010-05-02| url-status = dead}}</ref><ref name="Battisson">{{cite web|url=http://www.breganzane.com/vm/history/v6/page3.html |title=Developing the V6 - Taming The Beast |first=Stephen |last=Battisson |year=1997 |work= The Laverda V6 |publisher=Stephen Battisson |page=3 |access-date=2010-09-10 |quote=By arranging the rest of the engine internals to rotate in the opposite direction to the crankshaft their forces are cancelled out without having to resort to the weight, complexity and friction associated with two crankshafts.  |url-status=dead |archive-url=https://web.archive.org/web/20110708092245/http://www.breganzane.com/vm/history/v6/page3.html |archive-date=July 8, 2011 }}</ref>

One of the first motorcycles with a longitudinally-mounted flat-twin engine was the 1916 [[ABC motorcycles|ABC]], which was built in the United Kingdom.<ref name="MC1916" /> To accommodate chain drive, the ABC used a [[bevel gear|bevel drive]] at the gearbox to change the direction of the drive through ninety degrees.<ref name="Wilson"/>{{refpage|page=10}} BMW's first motorcycle, the 1923 [[BMW R32|BMW R 32]] was another early example of a longitudinally-mounted flat-twin engine, although it this case the power was transmitted to the rear wheel via a shaft drive.<ref name="Wilson"/>{{refpage|page=26}}

Over time, longitudinal mounting became more common for flat-twin engines. [[BMW Motorrad|BMW]] has a long history of flat-twin engine motorcycles,<ref name="Wilson"/>{{refpage|pages=26-32}}<ref name=BMWMotorcycles>{{Cite web|url=http://www.bmwmotorcycles.com/bikes/index.jsp?bikeSection= |title=BMW Motorrad USA - Bikes |access-date=2008-12-22 |work=bmwmotorcycles.com/ |archive-url=https://web.archive.org/web/20080822022309/http://www.bmwmotorcycles.com/bikes/index.jsp?bikeSection= |archive-date=2008-08-22 |url-status=dead }}</ref> as do [[IMZ-Ural|Ural]] (Russia) and [[Dnepr (motorcycle)|Dnepr]] (Ukraine).
{{clear right}}

===Aviation===
[[File:Bristol Cherub II.jpg|thumb|upright|[[Bristol Cherub]] II installed in aircraft]]

In 1902, the [[Pearse monoplane]] (which would later become one of the first aircraft to achieve flight) was powered by a flat-twin engine built on a farm by a hobbyist inventor.<ref>{{cite web |title=Richard Pearse |url=https://nzhistory.govt.nz/culture/richard-pearse |website=www.nzhistory.govt.nz |access-date=10 August 2019}}</ref><ref name="SciMedia">{{cite web|url= http://sciencelearn.org.nz/Contexts/Flight/Sci-Media/Images/Richard-Pearse-s-monoplane|title= Richard Pearse's monoplane|publisher= University of Waikato|date= 2012-04-26|ref={{harvid|SciMedia|2012}} }}</ref><ref>{{cite web |title=Richard Pearse Designed and Built Early Flying Machine |url=https://worldhistory.us/american-history/richard-pearse-designed-and-built-early-flying-machine.php |website=www.worldhistory.us |access-date=10 August 2019 |date=1 July 2017}}</ref> This engine used the unusual design of a single shared [[crank pin]] and double acting pistons.<ref>[http://www.billzilla.org/ffeng.gif Drawing of replica engine]</ref><ref>{{cite web |title=Richard Pearse : New Zealand Pioneer Aviator (1877 - 1953) |url=http://www.ctie.monash.edu.au/hargrave/pearse1.html |website=www.monash.edu.au |access-date=10 August 2019}}</ref> In 1908, the French company Dutheil-Chalmers began production of flat-twin aircraft engines, which used two counter-rotating crankshafts.<ref>{{cite web |title=Dutheil-Chalmers Éole Opposed-Piston Aircraft Engine |url=https://oldmachinepress.com/2017/07/20/dutheil-chalmers-eole/ |website=www.oldmachinepress.com |access-date=10 August 2019 |language=en |date=20 July 2017}}</ref><ref>{{cite web |title=Principal Aero Engines of 1910: Page 141 - Horizontal Engines |url=http://www.oldengine.org/members/diesel/Duxford/1910.htm |website=www.oldengine.org |access-date=10 August 2019 |archive-date=3 August 2019 |archive-url=https://web.archive.org/web/20190803093713/http://www.oldengine.org/members/diesel/Duxford/1910.htm |url-status=dead }}</ref> The Dutheil-Chlamers engine was used by the 1907 [[Santos-Dumont Demoiselle]] ''No. 20'' experimental airplane, with later versions of this airplane being produced with flat-twin engines from [[Darracq]] and [[Clément-Bayard]].

Most piston-engined aircraft used more than two cylinders, however other flat-twin aircraft engines from the 1920s and 1930s include the American [[Aeronca E-107]] and [[Aeronca E-113]], the British [[Bristol Cherub]], and the Czechoslovakian [[Praga B2]]. The [[HKS 700E]] is an oil-cooled flat twin for ultralight aircraft that is currently{{when?|date=February 2024}} in production.<ref>{{cite web |title=HKS Aviation Engines |url=http://www.hksengines.com/ |website=www.hksengines.com |access-date=10 August 2019}}</ref>

In larger aircraft, flat-twin engines have been used in [[auxiliary power unit]]s (APUs). A notable example was made by [[ABC Motors]] in the 1920s and 1930s.<ref name="Chaplin">{{cite journal| last1 = Chaplin| first1 = R. H.| last2 = Nixon| first2 = F.| editor-last =Poulsen| editor-first =C. M. | date = 1939-04-06| title = Ancillary Power Services| journal = Flight| volume = 35| issue = 1580| pages = 357–359| location = London| access-date = 2010-12-29| url = http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201031.html| quote = Both lecturers discussed the claims of the auxiliary engine for supplying service power. This is a well-known British example, the A.B.C. flat twin. }}</ref> During World War II, the [[Norbert Riedel|Riedel]] firm in Germany designed and manufactured a two-stroke flat-twin engine as [[Aircraft engine starting#Gas turbine engines|jet engine starter motors]] for the Junkers Jumo 004, BMW 003 and Heinkel HeS 011 jet engines<ref>{{cite web |url=http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |title=Design Analysis of BMW 003 Turbojet - "Starting the Engine" |last1=Schulte |first1=Rudolph C. |date=1946 |website=legendsintheirowntime.com |publisher=United States Army Air Force - Turbojet and Gus Turbine Developments, HQ, AAF |access-date=September 3, 2016 |quote=Starting procedure is as follows: Starting engine is primed by closing electric primer switch, then ignition of turbojet and ignition and electric starting motor of [[Norbert Riedel|Riedel engine]] are turned on (this engine can also be started manually by pulling a cable). After the Riedel unit has reached a speed of about 300 rpm, it automatically engages the compressor shaft of the turbojet. At about 800 rpm of the starting engine, the starting fuel pump is turned on, and at 1,200 rpm the main (J-2) fuel is turned on. The starter engine is kept engaged until the turbojet attains 2,000 rpm, at which time the starter engine and starting fuel are turned off, the turbojet rapidly accelerating to rated speed of 9,500 rpm on the J-2 fuel |archive-date=September 29, 2018 |archive-url=https://web.archive.org/web/20180929074301/http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |url-status=dead }}</ref><ref name="Gunston">{{cite book |last1=Gunston |first1=Bill |author-link1=Bill Gunston|title=The Development of Jet and Turbine Aero Engines |edition=Second |year=1997 |orig-year=1995 |publisher=Patrick Stephens |location=Cambridge, England  |isbn=1-85260-586-3 |page= 141 }}
</ref>

=== Other uses ===
The [[Maytag]] 'Model 72' flat-twin engines— produced from 1937 until some time between 1952 and 1960— were used in various applications including clothes washing machines.<ref name="Shelton">{{cite journal| last1 = Shelton| first1 = Charles L.| date = March–April 1999| title = Maytag Twins or 'Look-a-Likes'?| journal = Gas Engine Magazine| location = Topeka, Kansas, United States| publisher = Ogden Publications| format = aspx| access-date = 2010-12-28| url = http://gasengine.farmcollector.com/Equipment/Maytag-Twins-or-Look-a-Likes.aspx| quote = The twin, or 72 as it was commonly referred to, was used primarily as a source of power for the Maytag washing machines. Even as late as the early '30s, some brands of washers were hand operated; thus a ready power source such as the twin had a great deal of influence on Americans' work habits.| archive-date = 2011-10-06| archive-url = https://web.archive.org/web/20111006154039/http://gasengine.farmcollector.com/Equipment/Maytag-Twins-or-Look-a-Likes.aspx| url-status = dead}}</ref><ref name="Kinney">{{Cite web| url = http://www.herculesengines.com/Maytag/Default.htm| title = Maytag Engine-Driven Wringer Washer| access-date = 2009-01-08| last = Kinney| first = Keith| date = 2007-02-27| work = Old Iron and Other Americana: The collections of the Kinney family| archive-url = https://web.archive.org/web/20100117011736/http://www.herculesengines.com/Maytag/Default.htm| archive-date = 2010-01-17| url-status = dead}}</ref><ref>*{{cite web| url = http://www.oldengine.org/docs/Maytag_SM.pdf| title = Maytag Service Instructions| pages = 11–16| access-date = 2010-12-27| archive-date = 2011-01-25| archive-url = https://web.archive.org/web/20110125120854/http://www.oldengine.org/docs/Maytag_SM.pdf| url-status = dead}}</ref><ref name="Hunn">{{cite book| last = Hunn| first = Peter| title = The Small-Engine Handbook | url = https://books.google.com/books?id=ZKz21Fy2_r8C&q=Maytag+two+cylinder+engine| access-date = 2012-07-05| series = Motorbooks Workshop| date = Jun 13, 2005 | publisher = MotorBooks International| isbn = 978-0-76032-049-5| page = 42| chapter = Short Profiles of Manufacturers| chapter-url = https://books.google.com/books?id=ZKz21Fy2_r8C&q=Maytag+two+cylinder+engine&pg=PA24| quote = Often equipped with a foot pedal kick-starter, Maytag motors were available in both single-cylinder and opposed-twin formats.}}</ref><ref name="MaytagClub37">{{Cite web| url = http://www.maytagclub.com/page-37.htm| title = Maytag Multi-Motor Engines| access-date = 2009-01-08| publisher = Maytag Collector's club| archive-url = https://web.archive.org/web/20090527131149/http://www.maytagclub.com/page-37.htm| archive-date = 2009-05-27| url-status = dead}}</ref>

Electrical generators using flat-twin engines were built by Norman Engineering Company from 1932 to 1968 and by Douglas during World War II.<ref name="Brown">{{cite journal| last1 = Brown| first1 = Roland|date=November–December 2007| title = 1955 Douglas Dragonfly| journal = Motorcycle Classics| publisher = Ogden Publications| access-date = 2010-12-28| url = http://www.motorcycleclassics.com/motorcycle-reviews/1955-douglas-dragonfly.aspx}}</ref> ''Enfield Industrial Engines'' (part of [[Royal Enfield]]) produced flat-twin two-stroke petrol engines during World War II which were used for generators and other military uses. After 1945, Enfield produced flat-twin diesel engines, with applications including farm and marine use. [[Coventry Victor]] introduced a diesel version of its existing 688&nbsp;cc petrol flat-twin in 1932, and went on to produce flat-twin diesel and petrol engines for a variety of industrial and marine uses into the 1950s.<ref name="Baldwin">{{Cite book|last= Baldwin|first= Nick|title= The World guide to automobile manufacturers|year= 1987|publisher= Facts on File Publications|isbn= 0816018448|url= {{Google books|iJa1AAAAIAAJ|The World guide to automobile manufacturers|page=111|plainurl=yes}}|access-date= 4 March 2015}}</ref>{{refpage|page=111}}

Two-stroke flat-twins were often used as [[outboard motor]]s for boats,{{when|date=August 2019}} as they were smoother than single-cylinder engines. In the 1940s, they were largely replaced by straight-twin two-stroke engines, which were easier to start and no longer had excessive amounts of vibration.<ref name="Holcolmb">{{cite journal | last1 =Holcolmb | first1 =Hank | editor-last =Juettner | editor-first =Walter R. |date=October 1964 | title =Inside Today's Outboards | journal =MotorBoating | volume =114 | issue =4 | pages =34–35 | location =New York, NY USA | publisher =Hearst | issn =1531-2623 | access-date =2013-05-18 | url ={{Google books|QYSfHsX__vgC|MotorBoating October 1964|page=34|plainurl=yes}} }}
</ref>

== References ==
{{reflist|30em}}

{{Commons category|Flat-twin engines}}

{{Piston engine configurations}}

[[Category:Motorcycle engines]]
[[Category:Automobile engines]]
[[Category:Flat engines|02]]
[[Category:Piston engine configurations]]