A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder
piston engine
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common featu ...
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 Ski
Jet Ski is the brand name of a personal watercraft (PWC) manufactured by Kawasaki, a Japanese company. The term is often used generically to refer to any type of personal watercraft used mainly for recreation, and it is also used as a verb to ...
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". 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
In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back.
Use
This type of motor is usually used for rear-wheel drive cars, except ...
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 engine
A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
Although widely associated with motorcycles (installed either transversely or longi ...
s and
flat-twin engine
A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same ti ...
s, 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
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
configurations for straight-twin engines are 360 degrees, 180 degrees and 270 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. The 360 degree engines can use a single ignition system for both cylinders, using a
wasted spark
A wasted spark system is a type of ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a ...
system.
The imperfect
primary balance
Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary bala ...
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
Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in opposite direction to each other, which generates a net vertical force.
Th ...
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, 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
The Yamaha XS650 is a mid-size motorcycle made by Yamaha Motor Company. The standard model was introduced in October 1969,and produced through 1979. The "Special" cruiser model was introduced in 1978 and produced through 1985. The XS650 began with ...
and 1972
Yamaha TX750
The TX750 is a two-cylinder standard motorcycle built by Yamaha. The bike was released in 1972. Significant reliability problems affected the engines in early bikes. Yamaha made several changes to solve the problems but the bike was withdrawn fro ...
, often used balance shafts to reduce the vibration. The later 1978-1984
Honda CB250N/CB400N
The Honda CB250N and CB400N Super Dream are motorcycles manufactured by the Honda Motor Company from 1978 to 1986. The successor to the short lived Dream model, it had a series of revisions including a six-speed transmission and what Honda termed ...
engines also used a 360 degree crankshaft. The 2008
BMW F series parallel-twin
The BMW F series is a family of parallel-twin engine dual-sport motorcycles manufactured in Berlin, Germany by BMW Motorrad. Launched in 2008, the range comprises the F650GS, F700GS'', ''F800GS, and F800GSA. In 2012, the F700GS replaced the ...
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.
; 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
Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary bala ...
is possible with a 180 degree straight-twin engine, however the design creates a
rocking couple Rocking may refer to:
*Rocking chair
*Uprock, the street dance known as "Rocking"
Music
Albums
*Rockin' (The Guess Who album)
*Rockin' (Frankie Laine album) 1957
Songs
*"Hajej, nynjej" Czech children's carol, recorded as "Rocking" by Julie Andre ...
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
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
and thus higher power outputs. For example, the 1966
Honda CB450
The Honda CB450 is a standard motorcycle made by Honda from 1965 to 1974 with a 180° DOHC straight-twin engine. Producing 45 bhp (some 100 bhp/ litre), it was Honda's first "big" motorcycle, though it did not succeed in its goal of competing di ...
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
The Yamaha TX500 is a two-cylinder standard motorcycle built by Yamaha and sold in 1973 and 1974. Early models closely resembled the Triumph Bonneville in style. In 1975 the bike was renamed XS500 and then continued to be updated until 1978 when s ...
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 engine
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of t ...
s typically use a 180 degree crankshaft, since this results in two evenly-spaced power strokes per revolution. The
fundamental frequency
The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. In ...
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
The term ''Yankee'' and its contracted form ''Yank'' have several interrelated meanings, all referring to people from the United States. Its various senses depend on the context, and may refer to New Englanders, residents of the Northern United St ...
.
; 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
A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
Although widely associated with motorcycles (installed either transversely or longi ...
, 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
The Yamaha TRX850 is a sports motorcycle with a 10-valve DOHC 849 cc 270° parallel-twin engine. First released in Japan in 1995, a version for the European market was available from 1996 to 2000.
Design and development
The TRX has a half ...
and
Yamaha TDM
The Yamaha TDM is a sport touring motorcycle built by Yamaha Motor Company between 1991 and 2011.
Design
The TDM was not intended as an off-road machine but as a comfortable yet manoeuvrable all-rounder. A TDM is too bulky and heavy for serio ...
. Later examples include the 2009
Triumph Thunderbird
The Triumph Thunderbird is a British motorcycle that was introduced by Triumph in 1949 and produced in many forms until 1966. The name was used three more times for new and distinct Triumph models.
Original Triumph: 6T Thunderbird
To capture ...
Honda NC700 series
The Honda NC700 series is a family of motorcycles produced by Honda since 2012. NC700 series was a 'new concept', being unlike conventional motorcycles, a bike designed for commuters, new or veteran riders. The series also includes the motorcycle/ ...
, 2014
Yamaha MT-07 Yamaha may refer to:
* Yamaha Corporation, a Japanese company with a wide range of products and services, established in 1887. The company is the largest shareholder of Yamaha Motor Company (below).
** Yamaha Music Foundation, an organization estab ...
Each cylinder in a straight-twin engine has a separate
crank pin
A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to t ...
, unlike V-twin engines which can use a common crank pin for both
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
s. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two
main bearing
Main may refer to:
Geography
* Main River (disambiguation)
**Most commonly the Main (river) in Germany
* Main, Iran, a village in Fars Province
*"Spanish Main", the Caribbean coasts of mainland Spanish territories in the 16th and 17th centuries
...
s. 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
The Hildebrand & Wolfmüller was the world's first production motorcycle. Heinrich and Wilhelm Hildebrand were steam-engine engineers before they teamed up with Alois Wolfmüller to produce their internal combustion ''Motorrad'' in Munich in 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 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
The Speed Twin 5T is a standard motorcycle that was made by Triumph at their Coventry, and later Meriden factories. Edward Turner, Triumph’s Chief Designer and Managing Director, launched the Triumph Speed Twin at the 1937 National Motorcyc ...
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
Maudes Trophy
The Maudes Trophy is a motorcycle award established in 1923 by George Pettyt, owner of Maudes Motor Mart based in Great Portland Street, London who promoted an impartially-observed endurance test for motorcycles and provided a challenge award to t ...
). 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 Norton may refer to:
Places
Norton, meaning 'north settlement' in Old English, is a common place name. Places named Norton include: Canada
*Rural Municipality of Norton No. 69, Saskatchewan
*Norton Parish, New Brunswick
**Norton, New Brunswick, a ...
,
Triumph
The Roman triumph (Latin triumphus) was a celebration for a victorious military commander in ancient Rome. For later imitations, in life or in art, see Trionfo. Numerous later uses of the term, up to the present, are derived directly or indirectl ...
,
Ariel
Ariel may refer to:
Film and television
*Ariel Award, a Mexican Academy of Film award
* ''Ariel'' (film), a 1988 Finnish film by Aki Kaurismäki
* ''ARIEL Visual'' and ''ARIEL Deluxe'', 1989 and 1991 anime video series based on the novel series ...
,
Matchless
Matchless is one of the oldest marques of British motorcycles, manufactured in Plumstead, London, between 1899 and 1966. A wide range of models were produced under the Matchless name, ranging from small two-strokes to 750 cc Four-stroke cy ...
and
AJS
A. J. Stevens & Co. Ltd was a British automobile and motorcycle manufacturer in operation from 1909 to 1931. The company was founded by Joe Stevens in Wolverhampton, England. After the firm was sold, the name continued to be used by Matchless, A ...
. Straight-twin engines were also produced by Italian and German manufacturers, along with the American manufacturer
Indian
Indian or Indians may refer to:
Peoples South Asia
* Indian people, people of Indian nationality, or people who have an Indian ancestor
** Non-resident Indian, a citizen of India who has temporarily emigrated to another country
* South Asia ...
.
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
Scooter may refer to:
Vehicles
Ground
Human or gravity powered
* Eccentric-hub scooter, propelled by a standing rider making a bouncing motion
* Kick scooter, propelled by a standing rider pushing off the ground
* Knee scooter, a mobility device ...
have also used straight-twin engines, such as the 2001
Yamaha TMAX
The Yamaha TMAX (or T-Max) series of maxi-scooters has been manufactured by Yamaha Motor Company for the European market since its debut at July 2000 press events in Naples, Italy and Iwata, Japan, combining motorcycle performance with the co ...
and the 2001 Honda Silver Wing. Straight-twin engines are also used in motorcross sidecar racing.
Transverse-engined models
Many large British motorcycles from 1945 to the 1960s used a straight-twin
transverse engine
A transverse engine is an engine mounted in a vehicle so that the engine's crankshaft axis is perpendicular to the direction of travel. Many modern front-wheel drive vehicles use this engine mounting configuration. Most rear-wheel drive vehicles ...
(i.e. oriented with the crankshaft perpendicular to the frame), such as the
Triumph Bonneville
The Triumph Bonneville is a standard motorcycle featuring a parallel-twin four-stroke engine and manufactured in three generations over three separate production runs.
The first two generations, by the defunct Triumph Engineering in Meriden, Wes ...
and
Norton Commando
The Norton Commando is a British Norton-Villiers motorcycle with an OHV pre-unit parallel-twin engine, produced by the Norton Motorcycle company from 1967 until 1977. Initially having a nominal ''750 cc'' displacement, actually , in 197 ...
. 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 engine
A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
Although widely associated with motorcycles (installed either transversely or longi ...
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. 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
In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back.
Use
This type of motor is usually used for rear-wheel drive cars, except ...
straight-twin motorcycles are less common, however examples include the 1930-1938 Dresch Monobloc and the 1949-1956
Sunbeam S7 and S8
The Sunbeam S7 and S8 are British motorcycles designed by Erling Poppe with styling loosely based on the BMW R75 designs that were acquired as war reparations by BSA (full rights to the Sunbeam brand had been acquired from AMC in 1943).
...
.
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
Panhard was a French motor vehicle manufacturer that began as one of the first makers of automobiles. It was a manufacturer of light tactical and military vehicles. Its final incarnation, now owned by Renault Trucks Defense, was formed ...
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
The Fiat 500 ( it, Cinquecento, ) is a rear-engined, four-seat, small city car that was manufactured and marketed by Fiat Automobiles from 1957 until 1975 over a single generation in two-door saloon and two-door station wagon bodystyles.
Launch ...
, 1958
Subaru 360
The Subaru 360 is a rear-engined, two-door city car manufactured and marketed from 1958 to 1971 by Subaru. As the company's first automobile, production reached 392,000 over its 12-year model run.
Noted for its small overall size, 1,000 lb ...
, 1958
NSU Prinz The NSU Prinz (Prince) is an automobile which was produced in West Germany by the NSU Motorenwerke AG from 1958 to 1973.
NSU Prinz I, II & III
The first post-war NSU car, the Prinz I, was launched at the Frankfurt Motor Show in September 1957 ac ...
, 1962
Mitsubishi Minica
The is a kei car produced by Mitsubishi Motors mainly for the Japanese domestic market from 1962 to 2011. It was first built by Shin Mitsubishi Heavy-Industries, one of Mitsubishi Heavy Industries' three regional automotive companies until they we ...
, 1967
Honda N360
The Honda N360 is a small front-engine, front-wheel drive, two-door, four-passenger car manufactured and marketed by Honda from March 1967 through 1970 in Japan's highly regulated kei class — as both a two-door sedan and three-door wagon.
...
, 1970
Honda Z600
The Honda Z (marketed also as the Z600) is a two-door hatchback kei car/city car manufactured and marketed by the Honda Motor Company, from 1970 until 1974. Exports mostly ended after 1972, when the domestic market models received redesigned pil ...
, 1972
Fiat 126
The Fiat 126 (Type 126) is a four-passenger, rear-engine, city car manufactured and marketed by Fiat over a twenty-eight year production run from 1972 until 2000, over a single generation. Introduced by Fiat in October 1972 at the Turin Auto Show ...
, 1988
VAZ Oka
The Lada Оkа (VAZ-1111, SeAZ-1111, КаmАZ-1111, Astro 11301) (russian: Ока (ВАЗ-1111, СеАЗ-1111, КамАЗ-1111)) is a city car designed in the Soviet Union in the later part of the seventies by AvtoVAZ. It entered production in 198 ...
, 1988
Dacia Lăstun
The Dacia 500 Lăstun () was a small city car manufactured by Tehnometal, Timișoara, under the auto marque Dacia. The name derives from the Romanian for house martin, a small bird related to swallows.
History
The Lăstun was a low-cost Romanian ...
, and 1980
Daihatsu Cuore ''Cuore'', , the Italian-language word for "heart", may refer to:
* CUORE Experiment, a particle physics facility in the Laboratori Nazionali del Gran Sasso in Italy
* ''Cuore'' (magazine), a Spanish women's magazine established in 2006
* ''Cuore ...
.
Petrol straight-twin engines currently used in production cars include the
Tata Nano
The Tata Nano is a compact city car that was manufactured and marketed by Indian automaker Tata Motors over a single generation, primarily in India, as an inexpensive rear-engined hatchback intended to appeal to current riders of motorcycles a ...
and the
Fiat TwinAir engine
Fiat's TwinAir engine is a Straight-twin engine designed by Fiat Powertrain Technologies as part of its Small Gasoline Engine (SGE) family — employing Fiat’s MultiAir hydraulically actuated variable valve timing and lift technology.
O ...
(used in various models from Fiat, Lancia and Alfa Romeo). Diesel straight-twin engines currently in production include the
Piaggio Porter
The Piaggio Porter is a cab over microvan and pick-up produced and sold by the Italian company Piaggio since 1992 under the ''Piaggio Commercial Vehicle'' brand.
First generation (1992)
Development
The collaboration project between Piaggio and ...
engine.
Usage in marine vessels
Straight-twin engines have been often used as
inboard motor
An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside the hull of the craft, an ''inboard motor'' is an engine enclosed within the hull of the boat, usually connected to a pr ...
s,
outboard motor
An outboard motor is a propulsion system for boats, consisting of a self-contained unit that includes engine, gearbox and propeller or jet drive, designed to be affixed to the outside of the transom. They are the most common motorised method ...
s and
jet pump
An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...
motors.
In the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay Prawners Lancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the ''
Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
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
A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connect ...
. The ''Suzuki 15'' outbound motor was introduced in 1989.
Other uses
Other uses include
tractor
A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most common ...
s (such as various John Deere models until 1960),
snowmobile
A snowmobile, also known as a Ski-Doo, snowmachine, sled, motor sled, motor sledge, skimobile, or snow scooter, is a motorized vehicle designed for winter travel and recreation on snow. It is designed to be operated on snow and ice and does not ...
s,
personal watercraft
A personal watercraft (PWC), also called water scooter or jet ski, is a recreational watercraft that a rider sits or stands on, not within, as in a boat. PWCs have two style categories, first and most popular being a runabout or "sit down" whe ...
s, and
all-terrain vehicle
An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike, or simply a quad, as defined by the American National Standards Institute (ANSI); is a vehicle that travels on low-pressure tires, with a seat that is stra ...
s. Design variations include two-stroke, four-stroke, petrol, diesel,air-cooling,
water-cooling
Cooling tower and water discharge of a nuclear power plant
Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water is inexpensive and non ...
,
natural aspiration
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
and
turbocharging
In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...
.
Ultralight aircraft
Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and conventional three-axis control aircraft with ailer ...
, 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
The Hirth 2704 and 2706 are a family of in-line twin cylinder, two stroke, carburetted aircraft engines, with optional fuel injection, designed for use on ultralight aircraft and especially two seat ultralight trainers, single seat gyrocop ...
and Cuyuna 430-D. Purpose-built engines for ultralight aircraft include the
Rotax 503
The Rotax 503 is a , inline 2-cylinder, two-stroke aircraft engine, built by BRP-Rotax GmbH & Co. KG of Austria for use in ultralight aircraft.Raisner, William: ''LEAF catlog'', pages 6-105. Leading Edge Airfoils, 1995.
As of 2011 the Ro ...
and
Rotax 582
The Rotax 582 is a two-stroke, two-cylinder, rotary intake valve, oil-in-fuel or oil injection pump, liquid-cooled, gear reduction-drive aircraft engine manufactured by BRP-Rotax GmbH & Co. KG. It is for use in non-certified aircraft operating ...
. Straight-twin engines are sometimes also used in large scale
radio-controlled aircraft
A radio-controlled aircraft (often called RC aircraft or RC plane) is a small flying machine that is controlled remotely by an operator on the ground using a hand-held radio transmitter. The transmitter continuously communicates with a receiver ( ...
.
See also
*
Flat-twin engine
A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same ti ...
*
List of motorcycles by type of engine
List of motorcycles by type of engine is a list of motorcycles by the type of motorcycle engine used by the vehicle, such as by the number of cylinders or configuration.
A transverse engine is an engine mounted in a vehicle so that the engine's c ...
*
V-twin engine
A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
Although widely associated with motorcycles (installed either transversely or longi ...