Straight-twin engine: Difference between revisions

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			{{short description\|Inline piston engine with two cylinders}}
	{{Refimprove\|date=September 2007}}
			] motorcycle engine (built by Hispano Villiers)]]
	A '''parallel-twin engine''' (aka '''straight twin''', '''straight-two cylinder engine''', '''inline twin''', or '''vertical twin''') is a two-cylinder ] that has its cylinders arranged side by side.
			] motorcycle engine]]
	There are three crankshaft configurations for this engine: 360°, 180°, and the new 270°:
	* In a 360° engine, both pistons rise and fall together. The dynamic balance is identical to that of a single-cylinder engine, but with twice the number of ignition pulses.
	* In a 180° engine, one piston rises as the other falls. This gives good primary balance, albeit with a rocking couple; but results in irregular ignition pulses.
	* In a 270° engine, one piston follows a quarter of a turn behind the other. This is a compromise between the first two types, yielding results similar to a ].

			A '''straight-twin engine''', also known as an '''inline-twin''', '''vertical-twin''', '''inline-2''', or '''parallel-twin''', is a two-cylinder ] whose cylinders are arranged in a line along a common crankshaft.
	Unlike ]s, straight-twin engines do not use a common crank pin for both ]s; each cylinder has its own crank pin. Most British straight-twin motorcycle engines from the 1930s through the 1980s, such as Triumph, BSA, Norton and Royal Enfield, had two main bearings. An exception was the AJS/Machless, which had a third, center main bearing.

			Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, ]s, all-terrain vehicles, tractors and ultralight aircraft.
	Honda straight-twin engines, which began appearing in the late 1950s, had four main bearings. Subsequent straight-twin engines have four, or occasionally three, main bearings.

			Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees, but rarely use 450 degrees and 90 degrees
	==Automobile use==
	]]]

			== Terminology ==
	In the past, straight-twin engines have been used in very small cars (e.g. ], ], and ]s like the ] and ]) and in farm equipment, notably by ] whose large two-cylinder engines were used in their line of farm ]s up until 1960.
			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.
	From 1967 to 1972, Honda produced the ] and its successors N400 and N600 with straight-twin engines in 360 cc, 400 cc, and 600 cc sizes. The ] was produced from 1970 to 1972. From 1958 to 1971, Subaru produced the ] with a rear-mounted, rear-drive 358 cc air-cooled engine.

			For motorcycles, "inline-twin" has sometimes referred to either a ] orientation (i.e. with the crankshaft in line with the chassis)<ref name="Wilson1995" /><ref name="Henshaw2008" /> or a U-engine (]) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).<ref name="Walker2000" />
	Current production cars that use an inline twin engine include the ], which has a 623 cc engine; and the ] TwinAir, which has a ] 875 cc engine.

			== Design ==
	] briefly considered creating an inline two-cylinder engine for ] use in the 1950s. ] worked with ] and ] on ] but abandoned the development after some tests of the engine in the F1-racer of that time. They hoped that the good low-end torque would make good for the lesser maximum power on twisting street-courses, but the fast escalating maximum power of the multi-cylinder-engines and the growing number of high-speed-courses killed the project.<ref>"Schnelle Motoren" (Author = Helmut Hütten)</ref>
			Compared with ]s and ]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 ] 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 ===
	==Motorcycle use==
			]
	], a sohc parallel-twin]]
	] Interstate, an OHV 360° parallel-twin]]
	]
	], a dohc 270° parallel-twin]]

			The most common ] configurations for straight-twin engines are 360 degrees, 180 degrees and 270 degrees,<ref name="ABM1911" /><ref name="MotorcycleWorks" /><ref name="feel" /> and even can 90 degrees and 450 degrees but uncommon.
	The motorcycle world generally refers to these engines as "parallel twins" or "vertical twins", and the term "straight-two" is obsolete. "Parallel twin" refers to an engine mounted transversely across the frame; and the term "inline twin" refers to exclusively to an engine mounted inline with the frame, such as the ]. In four-stroke designs, the parallel twin is usually vertical or near vertical. An exception is the racing-only ], which featured nearly horizontal cylinders, and Suzuki two-stroke, small bore, street-bikes.

			; 360 degrees
	]'s 1937 ] started a trend, and up to the mid-1970s ] parallel twins were the most common type of British motorcycles, being produced by Triumph, BSA, Norton, Ariel, Matchless and AJS. Italian, German and USA manufacturers have also made parallel twins, but BMW have made the ] their trademark, while Moto-Guzzi, Ducati and Harley-Davidson have specialised in V-twins. Japanese manufacturers still make parallel twins, particularly for middleweight bikes. USA-made parallel twins include the 1949 440 cc Indian Scout and the 1950 500 cc Indian Warrior.
			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 ] system.

			The imperfect ] 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 ]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.
	Although the rise in popularity of the large V-twin motorcycle has seen the parallel twin fall out of favour, the latter retains these advantages over the former:<ref>''Fast Bike'' magazine August 1995 page 21</ref>
	* A parallel twin is cheaper to make, having only one cylinder block and one cylinder head.
	* Both cylinders can have the exhaust pipe exiting at the front, in the cool air stream.
	* Siting of ancillaries (air-filter, carburetters, ignition etc.) is simpler.
	* This simpler layout can potentially make maintenance access easier.
	* The parallel twin is both lighter and shorter, allowing a lighter frame and shorter wheelbase.
	* The motorcycle's centre of gravity can be sited optimally, i.e. lower and further forward.
	* Provided a 270° crank is used, a parallel twin can simulate the "feel" of a V-twin.

			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 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 ] and 1979 ]. Larger engines, such as the 1969 ] and 1972 ], often used balance shafts to reduce the vibration.<ref name="balls" /> The later 1978–1984 ] engines also used a 360 degree crankshaft, as does the 1989 ]. The 2008 ] motorcycles also use 360 degree crankshafts, with a third "vestigial" connecting rod (acting as a counterbalance) and a rev limit of 9,000 rpm to reduce vibrations.<ref name="HolmstromBMW" /> In 2009 ] launched ] inline twin car engines that use 360 degree crankshaft which relied on balance shafts to reduce the vibrations.
	===Crank angle (360° and 180°)===
	All classic British four-stroke parallel twin motorcycles used a ] of 360°, which allowed the use of a single carburetor. (180° and 270° twins need twin carburetors). An early Kawasaki was a clone of the 360° British ]; but in the 1960s, Japanese manufacturers favoured the 180° whose smoothness allowed higher rpm and thus more power. For example, the 1966 Honda 450 cc dohc 180° “Black Bomber" could outperform contemporary British 650 cc 360° twins.

			; 180 degrees
	Many small motorcycles of less than 250 cc use a 360° crankshaft as the vibration issue was less significant; examples includes Honda's CB92, CB160, CA72, CA77s, and CM185. Larger twins over 500 cc, such as the Yamaha's XS650 and TX750, have used 360° crankshafts, but such parallel twins tend to feature balance shafts. The Honda CB-series in the 250 to 500 cc range used 180° crankshafts. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 featured a 180° crankshaft and a balance shaft, while the 1974 Kawasaki KZ400 used a 360° crankshaft and a balance shaft.
			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 ] is possible with a 180 degree straight-twin engine; however, the design creates a ] which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a ] (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, as displacement in the crankcase stays roughly constant. However, a 180° engine requires a separate ignition system, points or otherwise, for each cylinder. The 360° twins can have a single ignition system for both cylinders, with a ] on each cylinder's exhaust stroke. The ] parallel twin motorcycle is a 360° design. Inherent vibration in the BMW F800 means its engine is limited to 9,000 rpm. BMW reduced the vibration using a third "vestigial" connecting rod to act as a counterbalance.

			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 ]s, the crank angle is generally 180° with a working cycle every 180°. Such an engine will produce fewer vibrations. An exception is the ], which featured a 360° crankshaft. The Yankee's configuration, which featured separate combustion chambers for the two cylinders, should not be confused with that of a ].

			In the 1960s, Japanese motorcycle manufacturers favoured the use of 180 degree crankshafts, since the increased smoothness allowed higher ] and thus higher power outputs. For example, the 1966 ] 180 degree crankshaft engine has a similar power output to contemporary British 360 degree crankshaft engines, despite having a smaller displacement of 450 cc compared with 650 cc.<ref name="WFMCutts" /><ref name="Turner_Threat" /><ref name="ClassicMac" /> Both the 1973 ] and the 1977 ] had a 180 degree crankshaft and a balance shaft. Since 1993, most Honda straight-twin motorcycle engines use 180 degree crankshafts.
	===Crank angle (270°)===

	A modern development is the 270° crank, which imitates the sound and feel of a 90° V-twin. A 270° engine requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise that allows a more regular firing pattern than a 180° crank, and less vibration than a 360° crank. Also, just like a 90° V-twin (but unlike the 180° and 360°), the 270° crank never has both pistons stationary, thus aiding crankshaft momentum.
			]s typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The ] 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 ].

			; 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 ], 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 ] and ].<ref name="MotorcycleWorks" /><ref name="CW2007" /><ref name="feel" /> Later examples include the 2009 ], 2010 ], 2012 ], 2014 ], 2016 ] and 2018 ]. This architecture is proving popular among manufacturers, which are upgrading models that were previously equipped with other engine types, such as the 2016 ] (formerly a ]), 2023 ] (formerly an ]) or 2023 ] (the older design being equipped with a ]).

			; 90 degrees
			In an engine with a 90 degree crankshaft, one piston follows one 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. thereby reducing the net momentum exchange between the crank and pistons during a full rotation.

			An imperfect primary balance is created in a 90 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this, probably similar like a 270 degree straight-twin. The secondary balance of a 90 degree engine is probably perfect similar like a 270 degree; however, the configuration does result in an unbalanced rocking couple.

			Motorcycle which use 90 degree straight-twin are doesn't exist for now, probably will exist on year ] or future.

			; 450 degrees
			In an engine with a 450 degree crankshaft, one piston follows full and one quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 450 degrees after the first, followed by a gap of 270 degrees until the first cylinder fires again. Probably this is the same pattern as a ] (especially ] and ]), and both configurations have a similar 'pulsing' exhaust sound as a result. thereby reducing the net momentum exchange between the crank and pistons during a full rotation. Probably this is opposite pattern of 270 degree crankshaft.

			An imperfect primary balance is created in a 450 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this, probably similar like a 270 degree straight-twin. The secondary balance of a 450 degree engine is probably perfect similar like a 270 degree; however, the configuration does result in an unbalanced rocking couple.

			Same like a 90 degree straight-twin, motorcycle which use 450 degree straight-twin are doesn't exist for now, probably will exist on year ] or future.

			=== Main bearings ===
			Each cylinder in a straight-twin engine has a separate ], unlike V-twin engines which can use a common crank pin for both ]s. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two ]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===
			] ]]
			] two-stroke motorcycle]]

			The world's first production motorcycle, the 1894 ] 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 ] 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 cc.<ref>http://www.moto-histo.com/france/fr1/fr1.htm\|Motos Francaises, Werner Brothers Moto Bicyclettes</ref>

			The 1938 ] 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 ] and ], and was initially used in the 1933 ] sidecar hauler (which won the ] silver medal and the 1933 ]).<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 ], ], ], ], ] and ]. 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 ].<ref name="Wilson1995pp108_9" />

			In 1949, the ] won the 500 ], 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 ] have also used straight-twin engines, such as the 2001 ]<ref name="MCN_TMax" /> and the 2001 ].<ref name="MotorcycleSilverWingTest" /> Straight-twin engines are also used in motocross sidecar racing.<ref name="WestlakeGiants" />

			=== Transverse-engined models ===
			] ]]

			Many large British motorcycles from 1945 to the 1960s used a straight-twin ] (i.e. oriented with the crankshaft perpendicular to the frame), such as the ] and ]. 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 ]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 ===
			] ]]

			] straight-twin motorcycles are less common; however, examples include the 1930–1938 ]<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 ].<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 ==
			] straight-twin engine at the rear of a ]]]
			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 ] engine introduced in 1895; these engines were used in ] 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 ], 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 ], 1958 ], 1958 ],<ref name="LightCarCaunter" /><!--ref name="MotorV165" /--> 1962 ], 1967 ], 1970 ], 1972 ],<ref name="HandbookNorbye" /><ref name="PopSciFeb75" /> 1988 ],<ref>{{cite journal\|title=Noutăți. VAZ 111 \|journal=Autoturism \|issue=1/1989}} p. 8.</ref> 1988 ],<ref>{{cite journal \|last=Teodor\|first=Pompiliu \|title=Dacia 500 Lăstun \|journal=Autoturism \|issue=2/1989}} p. 5.</ref> 1980 ],<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 ].<ref name="WorldsCheapestAutocar" />

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

			The ] 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 ==
			]
			Straight-twin engines have been often used as ]s, ]s and ] motors.<ref name="JetVee" />

			In the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay Prawners] would sometimes retrofit an inboard engine, such as the Lister or the ''] 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 ]. The ''Suzuki 15'' outbound motor was introduced in 1989.<ref name="PopMechFeb1989" />
			{{clear right}}

			== Other uses ==
			] aircraft engine]]
			Other uses include ]s (such as various John Deere models until 1960),<ref name="JohnDeereArt" /> ]s,<ref name="PS_May1984" /><ref name="SnowmobileServiceManual" /> ]s,<ref name="PopMechMay2003FighterJets" /><ref name="FieldAndStreamOct1972" /> and ]s.<ref name="AM_TG_Feb2003" /><ref name="BigBore" /><ref name="Best2UpATV" /> Design variations include two-stroke, four-stroke, petrol, diesel,<ref name="OGPEMay2006" /> ], ], ] and ].<ref name="AutoEngOct2008" />

			], 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 ] and Cuyuna 430-D.<ref name="PS_May1984" /><ref name="MachineDesignSnowAir" /><ref name="BergerBurr" /> Purpose-built engines for ultralight aircraft include the ] and ].<ref name="RotaxOperManual" /> Straight-twin engines are sometimes also used in large scale ].<ref name="InTwinSpecSheet" /><ref name="TaplinDesignManufacturingProfiles" />
			{{Clear right}}

			==See also==
			{{commons category\|Straight-two engines}}
			* ]
			* ]
			* ]

	==References==		==References==
			{{reflist\|1=30em\|refs=
	{{Reflist}}
			<ref name="ICE_TnP">{{cite book \|title= The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design.\|last= Taylor \|first= Charles Fayette \|date= 19 Mar 1985 \|publisher= MIT Press \|isbn= 0-262-70027-1 \|edition= 2 \|volume= 2}}</ref>
			<ref name="heavy">{{cite book \|last1=Nunney \|first1=M. J. J.\|title=Light and Heavy Vehicle Technology\|edition=4, illustrated, revised\|date=31 May 2012\|publisher=Taylor & Francis \|isbn= 978-0-75068-037-0}}</ref>
			<ref name="BargerRide">{{cite book\| first1 = Sonny\| last1 = Barger\| author1-link = Sonny Barger\| first2 = Darwin\| last2 = Holmstrom\| title = Let's Ride: Sonny Barger's Guide to Motorcycling\| date = Jun 8, 2010\| publisher = HarperCollins\| isbn = 978-0-06196-426-8}}</ref>
			<ref name="EveryHonda">{{cite book\| last = Mitchel\| first = Doug\| title = Honda Motorcycles: Everything You Need to Know About Every Honda Motorcycle Ever Built\| date = Oct 18, 2005\| publisher = Krause Publications\| isbn = 978-0-87349-966-8\| pages = 12–15\| chapter = 1959}}</ref>
			<ref name="FrankHonda">{{cite book\| last = Frank\| first = Aaron\| editor-last = Holmstrom\| editor-first = Darwin\| title = Honda Motorcycles\| date = Jul 12, 2003\| publisher = MotorBooks International\| isbn = 978-0-76031-077-9\| page = 34\| chapter = Chapter Three: Honda Who?}}</ref>
			<ref name="balls">{{cite book \|last1=Vandenheuvel\|first1=Cornelis\|title=Pictorial History of Japanese Motorcycles\|date=18 July 1997\|publisher=Bay View Books\|location=Devon U.K.\|isbn=1-8709-7997-4}}</ref>
			<ref name="Duckworth2012">{{Citation \| editor-first=Mick \| editor-last=Duckworth \| first1=Phil \| last1=Hunt \| url=https://books.google.com/books?id=cQ6-B-tbcqMC&pg=PA126 \| last2=McKay \| first2=Malcolm \| first3=Hugo \| first4=James \| last3=Wilson \| last4=Robinson \| title=Motorcycle: The Definitive Visual History \| publisher=], ] \| year=2012 \| pages=126, 210 \| isbn=978-1-46540-088-8}}</ref>
			<ref name="Wilson1995">{{cite book \| chapter=Glossary \| url=https://archive.org/details/encyclopediaofmo00wils_1 \| title=The Encyclopedia of the Motorcycle \| publisher=Dorling Kindersley \| last=Wilson \| first=Hugo \| url-access=registration \| year=1995 \| location=London \| pages= \| isbn=0-7513-0206-6 \| quote='''in-line''' Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine... '''parallel twin''' A two-cylinder engine layout in which both cylinders are side by side and mounted across the frame. }}</ref>
			<ref name="Henshaw2008">{{cite book \| access-date=July 13, 2012 \| chapter=Super Bantam \| chapter-url=https://books.google.com/books?id=R83apsln2NIC&pg=PA42 \| title=The BSA Bantam Bible: All Models 1948 to 1971 \| publisher=Veloce Publishing \| date=Jun 15, 2008 \| last=Henshaw \| first=Peter \| page=42 \| isbn=978-1-84584-159-1 \| quote=...and the Sunbeam S7, that civilised but low-powered shaft-drive in-line twin that BSA hoped would fill a niche as a gentleman's machine...}}</ref>
			<ref name="Walker2000">{{Citation \| author-link=Mick Walker (motorcycling) \| last=Walker \| first=Mick \| title=Mick Walker's European Racing Motorcycles \| publisher=Redline Books \| year=2000 \| isbn=978-0-95313-113-6 \| ref=Walker2000}}</ref>
			<ref name="Setright, Parallel twin">{{cite book \| title=Motorcycles \| publisher=Weidenfeld & Nicolson \| last=Setright \| first=L.J.K. \| author-link=L. J. K. Setright \| year=1976 \| location=London \| page=48 \| isbn=0-297-77234-1 }}</ref>
			<ref name="Tuttle2005">{{Citation \| last=Tuttle \| first=Mark Jr. \| magazine=Rider \| title=BMW F800S \| date=December 2005 \| page=15}}</ref>
			<ref name="parallel-twin">{{cite book \| first1=Dain \| last1=Gingerelli \| url=https://books.google.com/books?id=Kabox9JS1OEC \| title=365 Motorcycles You Must Ride \| publisher=MBI Publishing \| date=10 January 2011 \| access-date=2012-09-08 \| last2=Everitt \| first2=Charles \| last3=Michels \| first3=James Manning \| page=8 \| isbn=978-0-76033-474-4}}</ref>
			<ref name="TriumphPassion">{{cite book \| first1=Lindsay \| last1=Brooke \| url=https://books.google.com/books?id=I8sIulyI7xoC \| title=Triumph: A Century of Passion and Power \| publisher=MotorBooks International \| date=5 January 2003 \| access-date=2012-09-08 \| chapter=Chapter Two: Speed Twin and Tiger 100 \| pages=36–37 \| isbn=978-0-76030-456-3 \| quote=Stylish, light, responsive, smooth, and refined, the Speed Twin changed the direction of motorcycle design and dominated it for thirty years.}}</ref>
			<ref name="Wilson1995p53">{{harvnb\|Wilson\|1995\|pages=53, 120, 122}}</ref>
			<ref name="BaconDominator">{{cite book \| title=Norton Dominator Twins \| last=Bacon \| first=Roy \| year=1990 \| page=7 \| publisher=PRC \| isbn=1-85648-306-1 }}</ref>
			<ref name="ParallelUniverse">{{cite journal \| title=Parallel Universe \| publisher=American Motorcyclist Association \| date=December 2004 \| access-date=2012-09-08 \| editor1-first=Wood \| editor1-last=Bill \| url=https://books.google.com/books?id=xvUDAAAAMBAJ&q=American+Motorcyclist+Dec+2004 \| last=Oliver \| first=Lance \| journal=] \| volume=58 \| issue=12 \| location=Pickerington, Ohio \| page=13 \| issn=0277-9358}}</ref>
			<ref name="Wilson1995pp108_9">{{harvnb\|Wilson\|1995\|pages=108–109}}</ref>
			<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=] \|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 = ] \| 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">], 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"> 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 =] \| 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 = ] \| 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"> – 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=] \|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">: 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>
	==External links==
			<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>
	* on the ] of a straight twin engine.
			<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 = ] \| 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>
	{{Piston engine configurations}}
			<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}}

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Latest revision as of 06:13, 8 December 2024

Inline piston engine with two cylinders

A straight-twin engine, also known as an inline-twin, vertical-twin, inline-2, or parallel-twin, is a two-cylinder 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, jet skis, 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, but rarely use 450 degrees and 90 degrees

Terminology

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

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). "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees, 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) or a U-engine (tandem twin) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely).

Design

Compared with V-twin engines and flat-twin engines, straight-twins are more compact, a simpler design and cheaper to produce. Straight-twin engines can be prone to vibration, either because of the irregular 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.

Crankshaft angle

The most common crankshaft configurations for straight-twin engines are 360 degrees, 180 degrees and 270 degrees, and even can 90 degrees and 450 degrees but uncommon.

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 shafts 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, 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 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 CB92 and 1979 Honda CM185. Larger engines, such as the 1969 Yamaha XS 650 and 1972 Yamaha TX750, often used balance shafts to reduce the vibration. The later 1978–1984 Honda CB250N/CB400N engines also used a 360 degree crankshaft, as does the 1989 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 rpm to reduce vibrations. 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 rocking couple which requires use of a balance shaft to reduce the vibration. A 180 degree straight-twin engine has a 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 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 cc compared with 650 cc. Both the 1973 Yamaha TX500 and the 1977 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 engines 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. Two-stroke engines that do not use a 180 degree crankshaft include the 1972 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. Later examples include the 2009 Triumph Thunderbird, 2010 Norton Commando 961, 2012 Honda NC700 series, 2014 Yamaha MT-07, 2016 Triumph Thruxton 1200 and 2018 Royal Enfield Interceptor 650 & Continental GT. 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 Hornet (formerly an inline-4) or 2023 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 one 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. thereby reducing the net momentum exchange between the crank and pistons during a full rotation.

An imperfect primary balance is created in a 90 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this, probably similar like a 270 degree straight-twin. The secondary balance of a 90 degree engine is probably perfect similar like a 270 degree; however, the configuration does result in an unbalanced rocking couple.

Motorcycle which use 90 degree straight-twin are doesn't exist for now, probably will exist on year 2030s or future.

450 degrees

In an engine with a 450 degree crankshaft, one piston follows full and one quarters of a rotation behind the other. This results in an uneven firing interval where the second cylinder fires 450 degrees after the first, followed by a gap of 270 degrees until the first cylinder fires again. Probably this is the same pattern as a Ducati L-twin engine (especially Ducati Panigale 1199 and Ducati Panigale 1299), and both configurations have a similar 'pulsing' exhaust sound as a result. thereby reducing the net momentum exchange between the crank and pistons during a full rotation. Probably this is opposite pattern of 270 degree crankshaft.

An imperfect primary balance is created in a 450 degree straight-twin engine, due to a combination of free force and rocking couple; a balance shaft is often used to compensate for this, probably similar like a 270 degree straight-twin. The secondary balance of a 450 degree engine is probably perfect similar like a 270 degree; however, the configuration does result in an unbalanced rocking couple.

Same like a 90 degree straight-twin, motorcycle which use 450 degree straight-twin are doesn't exist for now, probably will exist on year 2030s or future.

Main bearings

Each cylinder in a straight-twin engine has a separate crank pin, unlike V-twin engines which can use a common crank pin for both connecting rods. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two main bearings. Beginning in the late 1950s, most Honda straight-twin engines had four main bearings. Subsequent straight-twin engines had four or occasionally three main bearings.

Usage in motorcycles

History

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 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 cc.

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 and Val Page, and was initially used in the 1933 Triumph 6/1 sidecar hauler (which won the International Six Days Trial silver medal and the 1933 Maudes Trophy). 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.

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. The manufacturers producing these motorcycles included BSA, Norton, Triumph, Ariel, Matchless and AJS. Straight-twin engines were also produced by Italian and German manufacturers, along with the American manufacturer Indian.

In 1949, the AJS E-90 Porcupine won the 500 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.

Since the 2000s, BMW and several Japanese manufacturers have continued to produce straight-twin engines, mostly for middleweight models. Several large scooters have also used straight-twin engines, such as the 2001 Yamaha TMAX and the 2001 Honda Silver Wing. Straight-twin engines are also used in motocross sidecar racing.

Transverse-engined models

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.

The transverse-engine straight-twin design has been largely replaced by V-twin engines; 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. 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.

Longitudinal-engined models

Longitudinal engine straight-twin motorcycles are less common; however, examples include the 1930–1938 Dresch Monobloc and the 1949–1956 Sunbeam S7 and S8.

This engine orientation allows for a motorcycle as narrow as a single-cylinder engine, 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.

Usage in automobiles

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 Daimler Motors' Phoenix engine introduced in 1895; these engines were used in Panhard cars that year. Another early straight-twin engined car was the 1898 Decauville Voiturelle, which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat.

Straight-twin engines have been used in various small cars, such as the 1957 Fiat 500, 1958 Subaru 360, 1958 NSU Prinz, 1962 Mitsubishi Minica, 1967 Honda N360, 1970 Honda Z600, 1972 Fiat 126, 1988 VAZ Oka, 1988 Dacia Lăstun, 1980 Daihatsu Cuore, and the 2008 Tata Nano.

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.

Usage in marine vessels

Straight-twin engines have been often used as inboard motors, outboard motors and jet pump motors.

In the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay PrawnersLancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the 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.

Other uses

Other uses include tractors (such as various John Deere models until 1960), snowmobiles, personal watercrafts, and all-terrain vehicles. Design variations include two-stroke, four-stroke, petrol, diesel, air-cooling, water-cooling, natural aspiration and turbocharging.

Ultralight aircraft, single seat gyro-copters and small homebuilt aircraft have also used straight-twin engines, often using engines originally designed for snowmobiles such as the Hirth 2704 and Cuyuna 430-D. Purpose-built engines for ultralight aircraft include the Rotax 503 and Rotax 582. Straight-twin engines are sometimes also used in large scale radio-controlled aircraft.

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Engine configurations for piston engines

Type

Stroke cycles

Cylinder layouts

Inline / straight	I1 I2 I3 I4 I5 I6 I7 I8 I9 I12 I14
Flat / boxer	F2 F4 F6 F8 F10 F12 F16
V / Vee	V2 V3 V4 V5 VR5 V6 VR6 V8 V10 V12 V14 V16 V18 V20 V24
W	W3 W6 W8 W12 W16 W18 W24 W30
Other	Deltic H Radial Split-single U X

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