Editing Straight-twin engine

{{short description|Inline piston engine with two cylinders}}
[[File:Rovena 250 1964 - Hispano Villiers Engine.JPG|thumb|right|1964 [[Sanglas#History|Sanglas Rovena]] motorcycle engine (built by Hispano Villiers)]]
[[File:1997SuzukiGS500E-engine.jpg|thumb|1997 [[Suzuki GS500]] motorcycle engine]]

A '''straight-twin engine''', also known as an '''inline-twin''', '''vertical-twin''', '''inline-2''', or '''parallel-twin''', is a two-cylinder [[Reciprocating engine|piston engine]] whose cylinders are arranged in a line along a common crankshaft.

Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, [[personal watercraft|jet ski]]s, all-terrain vehicles, tractors and ultralight aircraft.

Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees.

== Terminology ==
The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin".<ref name="Duckworth2012" /> Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation; however, they are often also used interchangeably.<ref name="Duckworth2012" /><ref name="Tuttle2005" />

In the United Kingdom, the term "parallel-twin" is traditionally used for engines with a crankshaft angle of 360 degrees, since the two pistons are in the same direction (i.e. parallel to each other).{{citation needed|date=October 2020}} "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees,{{citation needed|date=October 2020}} which causes the pistons to travel in opposite directions. The terms "straight-twin" and "inline-twin" were used more generically for any crankshaft angle.

For motorcycles, "inline-twin" has sometimes referred to either a [[longitudinal engine]] orientation (i.e. with the crankshaft in line with the chassis)<ref name="Wilson1995" /><ref name="Henshaw2008" /> or a U-engine ([[U engine#Tandem twin engine|tandem twin]]) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).<ref name="Walker2000" />

== Design ==
Compared with [[V-twin engine]]s and [[flat-twin engine]]s, straight-twins are more compact, a simpler design and cheaper to produce.<ref name="ICE_TnP" /> Straight-twin engines can be prone to vibration, either because of the irregular [[Firing order#Firing interval|firing interval]] present in 180° crank engines or the large uncountered reciprocating mass in 360° crank engines.  Inline-twins also suffer further from torsional torque reactions and vibration.<ref name="heavy" />

=== Crankshaft angle ===
[[File:Straight-twin engine with different crank shaft angles.gif|thumb|Animation with different crankshaft angles]]

The most common [[crankshaft]] configurations for straight-twin engines are 360 degrees, 180 degrees, 270 degrees,<ref name="ABM1911" /><ref name="MotorcycleWorks" /><ref name="feel" /> and less common are 90 degrees.

; 360 degrees
In an engine with a 360 degree crankshaft, both pistons move up and down at the same time. However, the firing interval is offset between cylinders, with one of the cylinders firing during the first crankshaft rotation and then the other cylinder in the following rotation. This set up results an even 360 degree firing interval unlike other crank configurations in inline twin engines. The 360 degree engines can use a single ignition system for both cylinders, using a [[wasted spark]] system.

The imperfect [[primary balance]] is as per a single-cylinder engine of equivalent reciprocating mass. Early engines attempted to reduce vibration through counterweights on the crankshaft; however, later methods also included [[balance shaft]]s and a separate weighted connecting rod. Compared with a single-cylinder engine, the more frequent firing interval (360 degrees compared with 720 degrees) results in smoother running characteristics, despite the similar dynamic imbalance.

From the 1930s, most British four-stroke straight-twin motorcycle engines used a 360 degree crankshaft,<ref name="JAE1971" /> since this avoided the uneven intake pulsing of other configurations, thus preventing the need for twin carburettors. In the 1960s, even though Japanese motorcycles mostly switched to 180 degree crankshafts for engines sized from 250 to 500&nbsp;cc, various smaller and larger engines continued to use a 360 degree crankshaft. Vibration was less of an issue for smaller engines, such as the 1965 [[Honda C92, CB92, C95 Benly|Honda CB92]] and 1979 [[Honda CM series|Honda CM185]]. Larger engines, such as the 1969 [[Yamaha XS 650]] and 1972 [[Yamaha TX750]], often used balance shafts to reduce the vibration.<ref name="balls" /> The later 1978–1984 [[Honda CB250N/CB400N]] engines also used a 360 degree crankshaft, as does the 1989 [[Yamaha XTZ 750|Yamaha XTZ750 Super Ténéré]]. The 2008 [[BMW F series parallel-twin]] motorcycles also use 360 degree crankshafts, with a third "vestigial" connecting rod (acting as a counterbalance) and a rev limit of 9,000&nbsp;rpm to reduce vibrations.<ref name="HolmstromBMW" /> In 2009 [[Fiat]] launched [[Multiair]] inline twin car engines that use 360 degree crankshaft which relied on balance shafts to reduce the vibrations.

; 180 degrees
In an engine with a 180 degree crankshaft, one piston rises as the other falls. In a four-stroke engine, the firing interval is uneven, with the second cylinder firing 180 degrees after the first, followed by a gap of 540 degrees until the first cylinder fires again. The uneven firing interval causes vibrations and results in a 'lumpy' power delivery. A 180° engine also requires a separate ignition system for each cylinder.

Perfect [[primary balance]] is possible with a 180 degree straight-twin engine; however, the design creates a [[Couple (mechanics)|rocking couple]] which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a [[secondary balance|secondary imbalance]] (similar to an inline-four engine); however, the lower reciprocating mass means that this often does not require treatment.

A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, because displacement of the crankcase is relatively unchanged as the pistons move.

In the 1960s, Japanese motorcycle manufacturers favoured the use of 180 degree crankshafts, since the increased smoothness allowed higher [[revolutions per minute|rpm]] and thus higher power outputs. For example, the 1966 [[Honda CB450]] 180 degree crankshaft engine has a similar power output to contemporary British 360 degree crankshaft engines, despite having a smaller displacement of 450&nbsp;cc compared with 650&nbsp;cc.<ref name="WFMCutts" /><ref name="Turner_Threat" /><ref name="ClassicMac" /> Both the 1973 [[Yamaha TX500]] and the 1977 [[Suzuki GS series|Suzuki GS400]] had a 180 degree crankshaft and a balance shaft. Since 1993, most Honda straight-twin motorcycle engines use 180 degree crankshafts.

[[Two-stroke engine]]s typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The [[fundamental frequency]] of vibration is twice that of an equivalent single-cylinder engine; however, the amplitude is halved.<ref name="MotorBoatingOct1964" /> Two-stroke engines that do not use a 180 degree crankshaft include the 1972 [[Yankee (motorcycle)|Yankee]].

; 270 degrees
In an engine with a 270 degree crankshaft, one piston follows three quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 270 degrees after the first, followed by a gap of 450 degrees until the first cylinder fires again. This is the same pattern as a 90 degree [[V-twin engine]], and both configurations have a similar 'pulsing' exhaust sound as a result. The pistons in a 270 degree straight-twin engine are never both stationary at the same time (as per a 90 degree V-twin engine), thereby reducing the net momentum exchange between the crank and pistons during a full rotation.

An imperfect primary balance is created in a 270 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this. The secondary balance of a 270 degree engine is perfect; however, the configuration does result in an unbalanced rocking couple.

The first production 270 degree straight-twin motorcycle engines were fitted to the 1996 [[Yamaha TRX850]] and [[Yamaha TDM]].<ref name="MotorcycleWorks" /><ref name="CW2007" /><ref name="feel" /> Later examples include the 2009 [[Triumph Thunderbird (2009)|Triumph Thunderbird]], 2010 [[Norton Commando#961 Commando|Norton Commando 961]], 2012 [[Honda NC700 series]], 2014 [[Yamaha MT-07]], 2016 [[Triumph Thruxton|Triumph Thruxton 1200]], 2018 [[Royal Enfield Interceptor 650|Royal Enfield Interceptor 650 & Continental GT]] and 2019 [[BMW F900R]].<ref>{{Cite web |title=The new BMW F900R and F900XR. |url=https://www.press.bmwgroup.com/canada/article/detail/T0302311EN/the-new-bmw-f900r-and-f900xr?language=en |access-date=2025-05-26 |website=www.press.bmwgroup.com |language=en}}</ref> This architecture is proving popular among manufacturers, which are upgrading models that were previously equipped with other engine types, such as the 2016 [[Honda Africa Twin]] (formerly a [[V-twin]]), 2023 [[Honda CB600F|Honda Hornet]] (formerly an [[Straight-four engine|inline-4]]) or 2023 [[Suzuki V-Strom 800|V-Strom 800]] (the older design being equipped with a [[V-twin]]).

; 90 degrees
In an engine with a 90 degree crankshaft, one piston follows quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 90 degrees after the first, followed by a gap of 630 degrees until the first cylinder fires again.

Probably [[CFMoto]] on year [[2030s]] use this crankshaft

=== Main bearings ===
Each cylinder in a straight-twin engine has a separate [[crankpin]], unlike V-twin engines which can use a common crankpin for both [[connecting rod]]s. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two [[main bearing]]s.<ref name="BargerRide" /> Beginning in the late 1950s, most Honda straight-twin engines had four main bearings.<ref name="EveryHonda" /><ref name="FrankHonda" /> Subsequent straight-twin engines had four or occasionally three main bearings.<ref name="balls" />

== Usage in motorcycles ==
===History===
[[File:Triumph Speed Twin.jpg|thumb|1938–1966 [[Triumph Speed Twin]] ]]
[[File:Quail Motorcycle Gathering 2015 (17135471883).jpg|thumb|1973–1975 [[Yamaha RD350]] two-stroke motorcycle]]

The world's first production motorcycle, the 1894 [[Hildebrand & Wolfmüller]] used a straight-twin engine. The cylinders lay flat and forward-facing, with the pistons connected directly to the rear wheel using a locomotive-style connecting rod. In 1903, the [[Werner Motors|Werner Motocyclette]] became the second production motorcycle model, using a straight-twin engine with vertical cylinders. The Werner engine uses cast-iron cylinders with integral heads, side valves and has a displacement of 500&nbsp;cc.<ref>http://www.moto-histo.com/france/fr1/fr1.htm|Motos Francaises, Werner Brothers Moto Bicyclettes</ref>

The 1938 [[Triumph Speed Twin]] was a successful straight-twin motorcycle which also led to straight-twin engines becoming more widely used by other brands. The engine was designed by [[Edward Turner (motorcycle designer)|Edward Turner]] and [[Val Page]], and was initially used in the 1933 [[Triumph 6/1]] sidecar hauler (which won the [[International Six Days Enduro|International Six Days Trial]] silver medal and the 1933 [[Maudes Trophy]]).<ref>{{cite web |url= http://www.motorcycleclassics.com/classic-british-motorcycles/triumph-6-1-zm0z13mjzbea.aspx?PageId=2#ArticleContent|title= Triumph's First Twin: Triumph 6/1|last1= Davis|first1= Simon|date= May–June 2013|website= Motorcycle Classics|publisher= Ogden Publications|location= Topeka, Kansas, USA|access-date= 2014-04-26|page=2}}</ref> During the development of the engine, it was found that a 360 degree crank angle was better suited to the use of a single carburettor than a 180 degree crank angle.<ref>{{cite web |url= http://www.motorcycleclassics.com/classic-british-motorcycles/triumph-6-1-zm0z13mjzbea.aspx#axzz2zzmFO38W|title= Triumph's First Twin: Triumph 6/1|last1= Davis|first1= Simon|date= May–June 2013|website= Motorcycle Classics|publisher= Ogden Publications|location= Topeka, Kansas, USA|access-date= 2014-04-26|page=1}}</ref>

Following the trend created by the Triumph Speed Twin, the most common design used by British motorcycle manufacturers until the mid-1970s was a four-stroke straight-twin engine with a 360 degree crankshaft.<ref name="Setright, Parallel twin" /><ref name="TriumphPassion" /> The manufacturers producing these motorcycles included [[Birmingham Small Arms Company|BSA]], [[Norton Motorcycle Company|Norton]], [[Triumph Engineering|Triumph]], [[Ariel Motorcycles|Ariel]], [[Matchless]] and [[AJS]]. Straight-twin engines were also produced by Italian and German manufacturers,<ref name="Wilson1995p53" /><ref name="BaconDominator" /><ref name="ParallelUniverse" /> along with the American manufacturer [[Indian Motocycle Manufacturing Company|Indian]].<ref name="Wilson1995pp108_9" />

In 1949, the [[AJS Porcupine|AJS E-90 Porcupine]] won the 500&nbsp;[[1949 Grand Prix motorcycle racing season|1949 Grand Prix World Championship]], becoming the first and only straight-twin motorcycle to win the championship. This engine is one of few four-stroke straight-twins to use cylinders oriented horizontally rather than vertically.<ref name="parallel-twin" />

Since the 2000s, BMW and several Japanese manufacturers have continued to produce straight-twin engines, mostly for middleweight models.<ref name="ClarkeMMT" /> Several large [[Scooter (motorcycle)|scooters]] have also used straight-twin engines, such as the 2001 [[Yamaha TMAX]]<ref name="MCN_TMax" /> and the 2001 [[Honda Silver Wing (scooter)|Honda Silver Wing]].<ref name="MotorcycleSilverWingTest" /> Straight-twin engines are also used in motocross sidecar racing.<ref name="WestlakeGiants" />

=== Transverse-engined models ===
[[File:NortonCommandoMKIIA.jpg|thumb|1973 [[Norton Commando|Norton Commando Interstate]] ]]

Many large British motorcycles from 1945 to the 1960s used a straight-twin [[transverse engine]] (i.e. oriented with the crankshaft perpendicular to the frame), such as the [[Triumph Bonneville]] and [[Norton Commando]]. This layout is well suited to air-cooling, since both cylinders receive equal airflow and the exhaust can exit in the well-cooled location at the front of each cylinder.<ref>{{Citation |last1=Coombs |first1=Matthew |last2=Haynes |first2=John |last3=Shoemark |first3=Pete |title=Motorcycle Basics | edition=2nd |publisher=Haynes | year=2002| page=1•31| isbn= 978-1-85960-515-8 | url=https://books.google.com/books?id=D0DhHAAACAAJ}}</ref>

The transverse-engine straight-twin design has been largely replaced by [[V-twin engine]]s; however, the straight-twin design has the advantage of easier packaging of ancillaries (such as the air-filter, carburettor and ignition components), which also improves access to ancillaries for maintenance/repairs.<ref>"Fast Bike" magazine August 1995 page 20</ref> A straight-twin engine using a 270 degree crankshaft can have a similar sound and feel to a V-twin engine with an uneven firing order.
{{clear right}}

=== Longitudinal-engined models ===
[[File:Sunbeam S8 1951.jpg|thumb|1951 [[Sunbeam S7 and S8|Sunbeam S8]] ]]

[[Longitudinal engine]] straight-twin motorcycles are less common; however, examples include the 1930–1938 [[Dresch|Dresch Monobloc]]<ref>{{cite web |title=Dresch Monobloc 1930 500cc 2 cyl sv |url=https://www.classic-motorcycle.com/Content/Bike/dresch-monobloc-1930-500cc-2-cyl-sv/a0002 |website=www.classic-motorcycle.com |access-date=13 October 2020}}</ref> and the 1949–1956 [[Sunbeam S7 and S8]].<ref>487 cc Sunbeam. Walneck's Classic Cycle Trader, Dominion Enterprises, Dec 1996 reprinted from Classic Bike Magazine, 5 January 1950.</ref><ref name="DealerNewsNov1996" />

This engine orientation allows for a motorcycle as narrow as a single-cylinder engine,<ref name="MDATHandbook" /> which reduces the aerodynamic drag, especially for the purpose of motorcycle racing. However, the main disadvantage for air-cooled engines is that the rear cylinder runs hotter than the front cylinder.<ref name="heavy" /><ref name="UltiHistFast" />
{{clear right}}

== Usage in automobiles ==
[[File:Fiat 500 engine.jpg|thumb|[[Longitudinal engine|Longitudinal]] straight-twin engine at the rear of a [[Fiat 500]]]]
Although two-cylinder engines are quite uncommon in cars, the straight-twin layout has been used for several automobile engines over time.

The first known straight-twin engine was a variant of the [[Gottlieb Daimler#Daimler Motors and the Phönix engine (1890 to 1900)|Daimler Motors' ''Phoenix'']] engine introduced in 1895; these engines were used in [[Panhard]] cars that year.<ref>{{cite book |author= Brazendale|year= 1983|title= Classic Cars: 50 Years of the World's Finest Automotive Design|url= {{Google books|g6y9umQmtfUC|Classic Cars: 50 Years of the World's Finest Automotive Design|page= 57|plainurl=yes}}|publisher= Bookthrift|pages= 49, 57|isbn= 0-6710-5103-2|access-date= 26 April 2014}}</ref> Another early straight-twin engined car was the 1898 [[Decauville automobile|Decauville Voiturelle]], which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat.<ref name="AntiqueSedgewick" />

Straight-twin engines have been used in various small cars, such as the 1957 [[Fiat 500]], 1958 [[Subaru 360]], 1958 [[NSU Prinz]],<ref name="LightCarCaunter" /><!--ref name="MotorV165" /--> 1962 [[Mitsubishi Minica]], 1967 [[Honda N360]], 1970 [[Honda Z600]], 1972 [[Fiat 126]],<ref name="HandbookNorbye" /><ref name="PopSciFeb75" /> 1988 [[VAZ-1111|VAZ Oka]],<ref>{{cite journal|title=Noutăți. VAZ 111 |journal=Autoturism |issue=1/1989}} p. 8.</ref> 1988 [[Dacia Lăstun]],<ref>{{cite journal |last=Teodor|first=Pompiliu |title=Dacia 500 Lăstun |journal=Autoturism |issue=2/1989}} p. 5.</ref> 1980 [[Daihatsu Cuore]],<ref>{{Cite book | title = Quattroruote: Tutte le Auto del Mondo 1985 | editor = Mastrostefano, Raffaele | publisher = Editoriale Domus S.p.A | ref = TAM85 | year = 1985 | pages = 233–234 | language = it | location = Milano | isbn = 88-7212-012-8}}</ref> and the 2008 [[Tata Nano]].<ref name="WorldsCheapestAutocar" />

As of January 2024, petrol straight-twin engines used in production cars currently just include the [[Fiat TwinAir engine]] (used in various models from Fiat, Lancia and Alfa Romeo). 

The [[Piaggio Porter]] made use of a diesel straight-twin engine until 2020.<ref>{{cite web |url= http://www.piaggioveicolicommerciali.it/veicolicommerciali/IT/it/home/modelli/quattroruote/porter-max.html|title= Piaggio Porter Maxxi |website= piaggioveicolicommerciali.it |language= it|access-date=21 January 2014}}</ref>

== Usage in marine vessels ==
[[File:Easthope Marine Engine.jpg|thumb|Easthope marine engine, circa 1960]]
Straight-twin engines have been often used as [[inboard motor]]s, [[outboard motor]]s and [[jet pump]] motors.<ref name="JetVee" />

In the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay Prawners[[Nobby (boat)#Lancashire nobby|Lancashire Nobbys]] would sometimes retrofit an inboard engine, such as the Lister or the ''[[Kelvin Diesels|Kelvin]] E2'' 3.0 litre petrol-paraffin engine.

From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the [[propeller shaft]]. The ''Suzuki 15'' outbound motor was introduced in 1989.<ref name="PopMechFeb1989" />
{{clear right}}

== Other uses ==
[[File:Rotax 503 display.JPG|thumb|[[Rotax 503]] aircraft engine]]
Other uses include [[tractor]]s (such as various John Deere models until 1960),<ref name="JohnDeereArt" /> [[snowmobile]]s,<ref name="PS_May1984" /><ref name="SnowmobileServiceManual" /> [[personal watercraft]]s,<ref name="PopMechMay2003FighterJets" /><ref name="FieldAndStreamOct1972" /> and [[all-terrain vehicle]]s.<ref name="AM_TG_Feb2003" /><ref name="BigBore" /><ref name="Best2UpATV" /> Design variations include two-stroke, four-stroke, petrol, diesel,<ref name="OGPEMay2006" /> [[Engine cooling#Air-cooling|air-cooling]], [[Engine cooling#Liquid cooling|water-cooling]], [[naturally aspirated engine|natural aspiration]] and [[turbocharger|turbocharging]].<ref name="AutoEngOct2008" />

[[Ultralight aviation|Ultralight aircraft]], single seat gyro-copters and small homebuilt aircraft have also used straight-twin engines,<ref name="UltralightCliche" /> often using engines originally designed for snowmobiles such as the [[Hirth 2704]] and Cuyuna 430-D.<ref name="PS_May1984" /><ref name="MachineDesignSnowAir" /><ref name="BergerBurr" /> Purpose-built engines for ultralight aircraft include the [[Rotax 503]] and [[Rotax 582]].<ref name="RotaxOperManual" /> Straight-twin engines are sometimes also used in large scale [[radio-controlled aircraft]].<ref name="InTwinSpecSheet" /><ref name="TaplinDesignManufacturingProfiles" />
{{Clear right}}

==See also==
{{commons category|Straight-two engines}}
* [[Flat-twin engine]]
* [[List of motorcycles by type of engine]]
* [[V-twin engine]]

==References==
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<ref name="ClarkeMMT">{{cite book |last1=Clarke |first1=Massimo |title=Modern Motorcycle Technology: How Every Part of Your Motorcycle Works |url= https://books.google.com/books?id=u7U524TamcUC |access-date=2012-09-08 |edition=English |series=Motorbooks Workshop |date=17 April 2010 |orig-year=2008 |publisher=MotorBooks International |isbn=978-0-76033-819-3}}</ref>
<ref name="MCN_TMax">{{cite web |url=http://www.motorcyclenews.com/MCN/bikereviews/searchresults/Bike-Reviews/Yamaha/Yamaha-YP500-T-Max-2001-current/|title=Yamaha T-Max (2001-2011) |newspaper=[[Motor Cycle News]] |publisher=Bauer Media |archive-url=https://web.archive.org/web/20100930065905/http://www.motorcyclenews.com/MCN/bikereviews/searchresults/Bike-Reviews/Yamaha/Yamaha-YP500-T-Max-2001-current/ |archive-date=2010-09-30 |url-status=live |access-date=2012-09-08|quote=Engine specification 4 stroke, parallel twin, auto}}</ref>
<ref name="MotorcycleSilverWingTest">{{cite web | url = http://www.motorcycle.com/manufacturer/honda/2008-honda-silver-wing-abs-review-80995.html| title = 2008 Honda Silver Wing ABS Review | date = Jun 12, 2008| website= Motorcycle.com | publisher = Verticalscope |archive-url = https://web.archive.org/web/20100727070525/http://www.motorcycle.com/manufacturer/honda/2008-honda-silver-wing-abs-review-80995.html | archive-date = 2010-07-27 | url-status = live | access-date = 2012-07-11 | quote = The engine is no longer an oversized, vibration-prone single but a smooth and reasonably powerful in-line twin.}}</ref>
<ref name="WestlakeGiants">{{cite book |last1=Westlake |first1=Andy |title=Off-Road Giants!: Heroes of 1960s Motorcycle Sport |url=https://books.google.com/books?id=JSZ7nf7iqHIC |access-date=2013-02-01 |date=15 November 2008 |publisher=Veloce Publishing |location=Dorchester, UK |isbn=978-1-84584-190-4 |pages=117–122 |chapter=Chapter 17: Wasp - A Sting in the Tail|chapter-url=https://books.google.com/books?id=JSZ7nf7iqHIC&pg=PA117}}</ref>
<ref name="ABM1911">American bicyclist and motorcyclist, Volume 6. Cycling Press, 1911</ref>
<ref name="MotorcycleWorks">{{cite book| last = Henshaw| first = Peter | title = How Your Motorcycle Works: Your Guide to the Components & Systems of Modern Motorcycles| date = Sep 15, 2012| publisher = Veloce Publishing| isbn = 978-1-84584-494-3| page = 19| chapter = Two–Engine| quote = Unmachined forging of a 270-degree crankshaft for parallel-twin engine. (Honda)}}</ref>
<ref name="feel">{{cite journal | last1 = Ried | first1 = Rick |date=December 2001 | title = British Heritage, American Style | journal = American Motorcyclist | volume = 55 | issue = 12 | pages = 18–20 | location = Pickerington, OH USA | publisher = [[American Motorcyclist Association]] | issn = 0277-9358 | quote = ...the America utilizes a 270-degree crank. That gives the America a lumpier cadence at idle that's a fair replica of a V-twin beat, although updated balancer shafts keep the vibrations under control.}}</ref>
<ref name="JAE1971">JAE: the journal of automotive engineering, Volume 2. Institution of Mechanical Engineers (Great Britain). Automobile Division. Automobile Division, Institution of Mechanical Engineers., 1971</ref>
<ref name="WFMCutts">{{cite book |last1=Cutts|first1=John|last2=Scott|first2=Michael|title=World's Fastest Motorcycles|date=1 August 1991|publisher=Book Sales|isbn=1-5552-1708-7}}</ref>
<ref name="Turner_Threat">{{cite book| last = Clew| first = Jeff| title = Edward Turner: The Man Behind the Motorcycles| edition = illustrated, revised| date = 1 February 2007| publisher = Veloce Publishing | location = Dorchester, UK| isbn = 978-1-84584-065-5}}</ref>
<ref name="ClassicMac">{{cite book |last1=McDiarmid|first1=Mac|title=Classic superbikes from around the world|date=1 January 1995|publisher=Parragon|isbn=0-7525-1017-7|pages=52–53 |chapter=Honda CB450 'Black Bomber'}}</ref>
<ref name="HolmstromBMW">{{cite book |last1=Holmstrom|first1=Darwin|others=Brian J. Nelson, photographer|title=BMW Motorcycles|url=https://books.google.com/books?id=yQnrSn8nNVUC|access-date=2012-09-10|date=4 December 2009 |publisher=MotorBooks International |isbn=978-0-76033-748-6}}</ref>
<ref name="CW2007">[[Cycle World]], Volume 46. CBS Publications, 2007</ref>
<ref name="MotorBoatingOct1964">{{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 =https://books.google.com/books?id=QYSfHsX__vgC}}</ref>
<ref name="DealerNewsNov1996">{{cite journal |last1= Watanabe|first1= Laurie|last2= Mitchel|first2= Doug|date=November 1996|title= That magnificent man on his riding machines|journal= Dealernews|volume= 32|issue= 12|type= Trade Publication |quote= If you press him, though, Jim is likely to expound on his 1947 S7 Sunbeam – "twin-cylinder, rubber-mounted engine. They were an inline two-cylinder, not crosswise in the frame, like conventional twins are."}}</ref>
<ref name="MDATHandbook">{{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|pages= 117–118|chapter= Chapter 11: The Engine|chapter-url= https://books.google.com/books?id=Ydk0bgq2_3YC&pg=PA113|quote= In some construction layouts the transverse width is the same as a single-cylinder engine, which allows very narrow frames and bodywork with small frontal areas.|ref= MDATHandbook}}</ref>
<ref name="UltiHistFast">{{cite book |last1=Brown |first1=Roland|title=Ultimate History of Fast Bikes|date=1 September 2003|publisher=Parragon |isbn=978-1-40541-592-7}}</ref>
<ref name="AntiqueSedgewick">[https://books.google.com/books?id=zCAWvcmlfmIC&q=%22inline-twin%22 Antique cars] Michael Sedgwick, 1980 - 93 pages "The engine, an inline twin, lived in a box like a meat-safe at the rear."</ref>
<ref name="HandbookNorbye">{{cite book| last = Norbye| first = Jan P.| title = The complete handbook of automotive power trains| url = https://books.google.com/books?id=YSpKEyATeM8C&q=editions:-VzpBOB8_d8C| access-date = 2013-05-18| year = 1981| publisher = Tab Books| isbn = 0-8306-2069-9}}</ref>
<ref name="PopSciFeb75">{{cite journal | last1 =Norbye | first1 =Jan P. | editor-last =Luckett | editor-first =Hubert P. |date=February 1975 | title =The new logic in small-car engineering | journal =[[Popular Science]] | volume =206 | issue =2 | pages =56–59 | location =New York, NY USA | publisher =Times Mirror Magazines | issn =0161-7370 | access-date =2013-05-18 | url = https://books.google.com/books?id=WwEAAAAAMBAJ }}</ref>
<ref name="LightCarCaunter">{{cite book| last = Caunter| first = Cyril Francis| others = Science Museum (Great Britain)| title = The light car: a technical history of cars with engines of less than 1600 c.c. capacity| url = https://books.google.com/books?id=JG1TAAAAMAAJ&q=NSU+parallel+twin | edition = Second illustrated| year = 1958| publisher = H.M.S.O.| isbn = 9780112900030}}</ref>
<ref name="WorldsCheapestAutocar">{{cite journal | url = http://www.autocar.co.uk/car-news/new-cars/worlds-cheapest-car-launched-tata-nano | title = World's cheapest car launched: Tata Nano | journal = [[Autocar (magazine)|Autocar]] | publisher = Haymarket Consumer Media | access-date = 2012-07-12 | quote = The twin-cylinder, 623cc petrol engine powering it drives the rear wheels, and sits directly above them to the right of the rear of the car, with a four-speed manual gearbox immediately to its left.}}</ref>
<ref name="PopMechFeb1989">[https://books.google.com/books?id=KOQDAAAAMBAJ&dq=%22inline-twin%22&pg=PA109 Popular Mechanics] – Feb 1989 – Page 109. Vol. 166, No. 2 – 136 pages – Magazine "Also new from Suzuki are the inline twin 15 and the inline 3-cylinder 25. Both are equipped with loop-charging, oil injection and external-mount tilt and trim. The 25 also has pre-atomized oil injection and an overrev limiter."</ref>
<ref name="JetVee">{{cite web|url=http://www.jetvee.com/images/JetVee_BoatEngine4.pdf|title=2 Stroke International Marine Engines & Jet Pumps|website=jetvee.com|access-date=4 April 2018}}</ref>
<ref name="UltralightCliche">{{cite book |last1=Cliche |first1=Andre |title=Ultralight Aircraft Shopper's Guide |edition=8th |year=2001 |publisher=Cybair Limited Publishing |isbn=0-9680628-1-4 |pages=G–3 and G–4}}</ref>
<ref name="PS_May1984">{{cite journal |last=Markowski |first=Michael A.|date=May 1984|title=Build your own ultralight? –tips from an expert |journal=[[Popular Science]] |volume=224 |issue=5 |pages=103–107, 158 |location=New York, NY USA |publisher=Times Mirror Magazines|editor1-first=C. P.|editor1-last=Gilmore|editor2-first=Everett H. |editor2-last=Ortner |issn=0161-7370 |access-date=2012-09-13 |url=https://books.google.com/books?id=lgAAAAAAMBAJ}}</ref>
<ref name="MachineDesignSnowAir">[https://books.google.com/books?id=KnMrAQAAMAAJ&q=%22inline-twin%22 Machine Design]: Volume 54, Issues 1–6; Volume 54, Issues 1–6 1982 "A modified snowmobile power plant, the 30-hp inline twin has been detuned by lowering the compression ratio and modifying the carburetion"</ref>
<ref name="BergerBurr">Berger-Burr's ultralight and microlight aircraft of the world, Volume 16. P.185. Alain-Yves Berger, Norman Burr. Haynes Publishing Group, 1 Jan 1983</ref>

<ref name="RotaxOperManual">{{cite web |url=http://www.rotax-aircraft-engines.com/portaldata/5/dokus/d04495.pdf |title= Operators Manual for Engine Types 447, 503, 582|year=2010 |publisher=BRP-Powertrain |type=operators manual |access-date=2012-10-05}}</ref>
<ref name="InTwinSpecSheet">{{cite web|url=http://www.aircraftinternational.com/catalog/motors/3w150ir2/3w150ir2.htm|title=3W-150iR2 Inline Twin|website=www.aircraftinternational.com|access-date=4 April 2018}}</ref>
<ref name="TaplinDesignManufacturingProfiles">{{cite web |url=http://modelenginenews.org/drj/taplin_mk3.html |title=Model Engine Designer and Manufacturing Profiles: Taplin Twin |last=Janson |first=David R. |access-date=2012-10-05}}</ref>
<ref name="AM_TG_Feb2003">{{cite journal | last1 = Parsons | first1 = Grant | editor-last = Wood | editor-first = Bill |date=February 2003 | title = Top Gear: Polaris 600 Sportsman Works Hard and Plays Hard | journal = American Motorcyclist | volume = 57 | issue = 2 | pages = 16 | location = Pickerington, OH USA | publisher = [[American Motorcyclist Association]] | issn = 0277-9358 | quote = Underneath the bodywork, you'll find a 597cc overhead-valve parallel twin with two valves per cylinder, fed by a 34mm carb.}}</ref>
<ref name="PopMechMay2003FighterJets">{{cite journal | last1 = Taylor | first1 = Rich | editor-last = Oldham | editor-first = Joe |date=May 2003 | title = Fighter Jets | journal = Popular Mechanics | volume = 180 | issue = 5 | pages = 114–117 | publisher = Hearst Magazines | issn = 0032-4558 | quote = Powered by a turbocharged 4-stroke Parallel Twin that has half the displacement of the Sea-Doo's supercharged GTX, the lightweight Polaris MSX 150 is almost as fast and $2200 less expensive.}}</ref>
<ref name="FieldAndStreamOct1972">{{cite journal | editor-last = Samson | editor-first = Jack |date=October 1972 | title = Quieter & Safer: the '73 machines | journal = Field & Stream | volume = 77 | issue = 6 | pages = S10 – S14 | location = New York, NY USA | publisher = CBS Publications| issn = 8755-8599 | quote = The important power plants will now be 399 to 440cc parallel twins.}}</ref>
<ref name="JohnDeereArt">{{cite book| last = Klancher| first = Lee| title = The Art of the John Deere Tractor: Featuring Tractors from the Walter and Bruce Keller Collection| date = 13 May 2011| publisher = Voyageur Press| isbn = 9780760339497}}</ref>
<ref name="SnowmobileServiceManual">{{Google books|Rpy2LpP6wTgC|Snowmobile Service Manual|page=297}} Intertec Publishing - 1986 - 488 pages "Engines consist of two types; the axial fan, inline twin and the simultaneous firing opposed twin.</ref>
<ref name="BigBore">{{cite web |url= http://atvmag.com/article.asp?nid=2141&rez=1280&reload=true|title= Which Big Bore Engine Layout Is Best? |date= 3 August 2011 |magazine= Dirt Trax Magazine |access-date=6 August 2012|archive-date=17 December 2013 |url-status=live |archive-url=https://web.archive.org/web/20131217221233/http://atvmag.com/article.asp?nid=2141&rez=1280&reload=true}}</ref>
<ref name="Best2UpATV">{{cite web |url=http://www.atvmag.com/article.asp?nid=2318&rez=1280&reload=true |title= BEST IN CLASS: BEST 2-UP ATV|date= 9 March 2012 |magazine= Dirt Trax Magazine |access-date=6 August 2012 |archive-date=17 December 2013 |archive-url=https://web.archive.org/web/20131217221238/http://www.atvmag.com/article.asp?nid=2318&rez=1280&reload=true |url-status=live}}</ref>
<ref name="OGPEMay2006">"Emission-compliant diesel engines cover 83 to 1,350 bhp in 10 models." Oil, Gas, & Petrochem Equipment May 2006: 18. General OneFile. Web. 29 June 2012. "Cat petroleum diesels line includes eight inline two, three, and four-cylinder engines</ref>
<ref name="AutoEngOct2008">{{cite journal |date=October 2008|title= PSA fires up two-cylinder turbo |journal= Automotive Engineer|volume= 33 |issue= 9 |page= 5 |quote= Taking an 1,124cc naturally-aspirated inline-four as a baseline, it selected the inline-twin configuration as offering the lowest friction and thermal losses.}}</ref>
}}

{{Piston engine configurations}}

[[Category:Straight engines|2]]
[[Category:Straight-twin engines| ]]