![Douglas motorcycle engine, Abergavenny steam rally 2012](https://upload.wikimedia.org/wikipedia/commons/7/7b/Douglas_motorcycle_engine%2C_Abergavenny_steam_rally_2012.jpg)
A flat-twin engine is a two-cylinder
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same time.
The flat-twin design was patented by
Karl Benz
Carl Friedrich Benz (; 25 November 1844 – 4 April 1929), sometimes also Karl Friedrich Benz, was a German engine designer and automotive engineer. His Benz Patent Motorcar from 1885 is considered the first practical modern automobile and fir ...
in 1896 and the first production flat-twin engine was used in the ''Lanchester 8 hp Phaeton'' car released in 1900. The flat-twin engine was used in several other cars since, however a more common usage is in motorcycles; early models oriented the cylinders in line with the frame, however later models switched to the cylinders being perpendicular to the frame to provide even cooling across both cylinders.
Flat-twin engines were also used in several aircraft up until the 1930s and in various stationary applications from the 1930s to the 1960s.
The Australian lawnmower manufacturer Victa also produced a flat-twin engine push mower from August 1975 to 1980 dubbed as the ‘Twin 500’ and ‘Supreme’ these motors were manufactured in Canada, they are very sought after as production only saw few numbers most likely due to ignition and fuel related problems in early models, the supreme (being the later model) had fixed all these problems with a rear domed piston, crankcase mixers and refined ignition system
Typical design
![Boxerengineanimation](https://upload.wikimedia.org/wikipedia/commons/8/8a/Boxerengineanimation.gif)
Most flat-twin engines use a
boxer Boxer most commonly refers to:
* Boxer (boxing), a competitor in the sport of boxing
*Boxer (dog), a breed of dog
Boxer or boxers may also refer to:
Animal kingdom
* Boxer crab
* Boxer shrimp, a small group of decapod crustaceans
* Boxer snipe ee ...
configuration for the
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 ...
and are therefore called "boxer-twin" engines. In a boxer-twin engine, the 180° crankshaft moves the pistons in phase with each other, therefore the forces generated by one piston are cancelled out by the other, resulting in excellent primary balance. The evenly spaced
firing order
The firing order of an internal combustion engine is the sequence of ignition for the cylinders.
In a spark ignition (e.g. gasoline/petrol) engine, the firing order corresponds to the order in which the spark plugs are operated. In a diesel engi ...
also assists in reducing vibration. The equal and opposite forces in a boxer-twin engine do however generate a
rocking couple, due to the offset distance between the pistons along the crankshaft.
![Allumage 2cv](https://upload.wikimedia.org/wikipedia/commons/0/0b/Allumage_2cv.svg)
A commonly used ignition system is
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 ...
, which is a simple ignition system using a double-ended coil firing both spark plugs on each revolution (i.e. during both the compression and exhaust strokes). This system is distributorless and requires only a single contact breaker and coil for the engine.
Crankcase pressure
The boxer-twin configuration can cause pressuring of the
crankcase
In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block.
Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/a ...
during each inward piston stroke and de-pressurisation during each outward piston stroke, since both pistons are moving inwards or outwards at the same time. This crankcase pumping effect (also found on singles-cylinder engines and 360° parallel-twin engines) is usually addressed by means of a crankcase breather.
The
Citroën 2CV
The Citroën 2CV (french: link=no, deux chevaux(-vapeur), , lit. "two steam horse(power)s", meaning "two ''taxable'' horsepower") is an air-cooled front-engine, front-wheel-drive, economy family car, introduced at the 1948 Paris Mondial de l ...
boxer-twin engine took advantage of this pumping effect to maintain a partial vacuum inside the crankcase, in order to reduce oil leaks when an oil seal malfunctions. This was achieved by using a
one-way valve
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid ( liquid or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have ...
(a leather or rubber flap over a hole in the crankcase), to let air escape the crankcase but not enter it.
Applications
Automobiles
![Citroen A-series 2CV 602cc Engine (18770871302)](https://upload.wikimedia.org/wikipedia/commons/7/73/Citroen_A-series_2CV_602cc_Engine_%2818770871302%29.jpg)
The beginnings of the flat-twin engine were in 1896, when Karl Benz obtained a patent for the design. A year later, his company
Benz & Cie unveiled the first flat-twin engine, a boxer design called the "contra engine".
In 1900, The
Lanchester Engine Company began production of the ''
Lanchester 8 hp Phaeton
The Lanchester 8 hp Phaeton is a brass era automobile designed by Frederick W. Lanchester, developed and built by the Lanchester Motor Company
The Lanchester Motor Company Limited was a car manufacturer located until early 1931 at Armourer M ...
'', which used a flat-twin engine.
This engine had an unusual design of two counter-rotating crankshafts, with each piston attached to its crankshaft by a thick connecting rod.
Each piston was also connected to the other crankshaft by two thinner connecting rods, causing the two pistons to move on the same axis.
It also had the torque reaction of one crankshaft cancel the torque reaction of the other, cancelling torque reaction in the engine.
Lanchester used this engine design until 1904.
Other early uses of flat-twin engines were 1903-04
Ford Model A, the 1904-1905
Ford Model C
The Ford Model C is an automobile that was produced by Ford Motor Company
Ford Motor Company (commonly known as Ford) is an American multinational automobile manufacturer headquartered in Dearborn, Michigan, United States. It was founded ...
, the 1905-1906
Ford Model F
The Ford Model F is an automobile produced by Ford. It was a development of the Model A and Model C, but was larger, more modern, and more luxurious. Production started in 1905 and ended in 1906 after about 1,000 were made. It was built at the ...
.
and several
Jowett Cars
Jowett was a manufacturer of light cars and light commercial vehicles in Bradford, West Riding of Yorkshire, England from 1906 to 1954.
Early history
Jowett was founded in 1901 by brothers Benjamin (1877–1963) and William (1880–1965) Jowe ...
models from 1910 to 1937.
The
Citroën 2CV
The Citroën 2CV (french: link=no, deux chevaux(-vapeur), , lit. "two steam horse(power)s", meaning "two ''taxable'' horsepower") is an air-cooled front-engine, front-wheel-drive, economy family car, introduced at the 1948 Paris Mondial de l ...
, produced from 1948 to 1990, was one of the first front-wheel drive cars to use a flat engine. The 2CV was powered by an air-cooled boxer-twin engine. Also in 1948, the
Panhard Dyna X
The Panhard Dyna X was a lightweight berline designed by the engineer Jean Albert Grégoire and first exhibited as the AFG ''(Aluminium Français Grégoire)'' Dyna at the Paris Motor Show in 1946.
Conception and development
Mindful of the preca ...
was released with front-wheel drive and an air-cooled boxer-twin engine. Other cars following World War II using boxer-twin engines were the 1945-1954
Jowett Bradford van,
, the 1961-1976
DAF Daffodil
The DAF Daffodil is an economy small family car that was manufactured by DAF from 1961 until 1967. Together with the DAF 750, launched at the same time, it replaced the DAF 600. DAFs 750 was essentially the same car but with even fewer luxurious ...
, the 1961-1978
Toyota Publica
The is a small car manufactured by the Japanese automaker Toyota from 1961 until 1978. Conceived as a family car to fulfill the requirements of the Japanese Government's "national car concept", it was the smallest Toyota car during that period an ...
, the 1965-1969
Toyota Sports 800
The is Toyota's first production sports car. The prototype for the Sports 800, called the Publica Sports, debuted at the 1962 Tokyo Auto Show, featuring a space age sliding canopy and utilizing the powertrain of the Publica 700, a Japanese mark ...
sportscar and several front-wheel drive models from
Citroën
Citroën () is a French automobile brand. The "Automobiles Citroën" manufacturing company was founded in March 1919 by André Citroën. Citroën is owned by Stellantis since 2021 and previously was part of the PSA Group after Peugeot acquired ...
and
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 ...
. Several rear-engined cars were also produced with boxer-twin engines originally designed for motorcycles, such as the 1957-1975
Puch 500
The Puch 500 is a city car produced by the Austrian manufacturer Puch, a subsidiary of Steyr-Daimler-Puch in Graz. It was built under licence from Fiat and was based on the Fiat 500.
The beginning
In 1954 it was decided at Steyr-Puch to resume c ...
, the 1957-1959
BMW 600
The BMW 600 is a four-seater microcar produced by the German automaker BMW from mid-1957 until November 1959. Partially based on the BMW Isetta two-seater, it was BMW's first postwar four-seater economy car. It was not a sales success, but it b ...
and the 1959-1965
BMW 700
The BMW 700 is a small rear-engined car which was produced by BMW in various models from August 1959 to November 1965. It was the first BMW automobile with a monocoque structure. The 700 was a sales success at a time when BMW was close to fina ...
. The Brazilian manufacturer
Gurgel Motores used a in house developed water-cooled boxer-twin engine (Enertron engine) and the Volkswagen air-cooled boxer-four in several models from 1988 to 1994.
The
Toyota U engine
The Toyota U engine is a series of flat engines produced by Toyota. The original version of this engine was produced in the 1960s and 1970s in flat-twin configuration. The engine series was extended in 2012 for the engine derived from the Subaru f ...
was an air-cooled flat-twin engine produced from 1961 to 1976. Introduced in the
Toyota Publica
The is a small car manufactured by the Japanese automaker Toyota from 1961 until 1978. Conceived as a family car to fulfill the requirements of the Japanese Government's "national car concept", it was the smallest Toyota car during that period an ...
subcompact car, the U engine was also used in the
Toyota MiniAce
The Toyota MiniAce was a small utility vehicle built by Toyota from November 1967 until November 1975. It shared many parts with the Toyota Publica, especially the Publica P20 Pickup. In Japan, it was sold through the ''Toyota Corolla Store'' and ' ...
small commercial vehicle and the
Toyota Sports 800
The is Toyota's first production sports car. The prototype for the Sports 800, called the Publica Sports, debuted at the 1962 Tokyo Auto Show, featuring a space age sliding canopy and utilizing the powertrain of the Publica 700, a Japanese mark ...
sports car.
Motorcycles
Transverse mounting
![Flat twin Douglas 1912 N 3](https://upload.wikimedia.org/wikipedia/commons/d/df/Flat_twin_Douglas_1912_N_3.jpg)
The benefits of using a flat-twin engine mounted with the crankshaft running perpendicular to the frame (therefore the cylinders being in line with the frame) are a low centre of gravity
and that a belt-drive or chain-drive system can be used to transmit drive to the rear wheel.
However, the downsides are uneven heat distribution (the front cylinder is more heavily cooled than the rear cylinder)
and a longer wheelbase is often required due to the length of the engine.
The first flat-twin
motorcycle engine
A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.
Although some moped ...
was the built in 1905 by the Light Motors Company in the United Kingdom. Originally named the Fée (renamed "Fairy" soon after its introduction),
it was designed as a "bicycle engine system" which transmitted power to a pulley on the rear wheel via a chain.
Manufacture of the Fairy was taken over by the
Douglas Engineering Company, one of Light Motors' suppliers, when the Light Motors Company folded in 1907.
Later in 1907, Douglas changed the drivetrain from the chain and pulley design to a belt-drive system driven directly from the engine. Later developments of the Douglas motorcycle were made with the cylinders in line with the frame until the Second World War.
Other early flat-twin motorcycles used a similar layout, with their cylinders aligned along the frame and therefore with the crankshaft running transverse to the frame.
In 1914 the main supplier of rear-hub gearboxes,
Sturmey-Archer
Sturmey-Archer was a manufacturing company originally from Nottingham, England. It primarily produced bicycle hub gears, brakes and a great many other sundry bicycle components, most prominently during their heyday as a subsidiary of the Ralei ...
, introduced a 3-speed countershaft gearbox with integral kick-starter,
which posed a design problem for motorcycles with transversely-mounted flat-twin engines. This gearbox could be relatively easily located behind a single-cylinder or V-twin engine, however this arrangement would result in an excessively long wheelbase for flat-twin engines. Solutions to this problem included using a countershaft below the engine (as used by the Douglas Fairy), or a gearbox located above the engine,
although in some cases the cylinders were short enough to use the gearbox in the traditional location behind the engine.
In 1916, most flat-twin motorcycles still used a transversely-mounted engine. The European models at this time included the
Bradbury , the
Brough HB, the
Douglas
Douglas may refer to:
People
* Douglas (given name)
* Douglas (surname)
Animals
*Douglas (parrot), macaw that starred as the parrot ''Rosalinda'' in Pippi Longstocking
*Douglas the camel, a camel in the Confederate Army in the American Civil W ...
and models, the
Humber
The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal rivers Ouse and Trent. From there to the North Sea, it forms part of the boundary between th ...
3.5 hp and models, the
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 ...
6 hp, the
Montgomery 6 hp,
Williamson Flat Twin , and the Bayerische Flugzeugwerke Helios (the predecessor to BMW's first motorcycle). Models produced in the United States included the
Indian Model O
The Indian Model O was a lightweight motorcycle made by the Hendee Manufacturing Company from 1917 to 1919. Indian expected to sell the Model O to young or thrifty riders. The United States' entry into World War I caused Indian to shift manufactu ...
and the
Harley-Davidson Model W
The Harley-Davidson Model W, also known as the ''Sport Twin'', is a motorcycle made by Harley-Davidson from 1919 to 1923. Unusually for a Harley-Davidson motorcycle, the Model W had a flat-twin engine and a trailing link fork. The Model W set spe ...
.
[
*
*
*]
Longitudinal mounting
The main benefit of mounting a flat-twin engine with the crankshaft in line with the frame (therefore the cylinders sitting sideways in the frame) is that an air-cooled engine receives the same amount of cooling for each cylinder.
The
Harley-Davidson XA
The Harley-Davidson XA (Experimental Army) was a flat-twin, shaft drive motorcycle made by Harley-Davidson for the US Army during World War II.
Development and production
During WWII, the U.S. Army asked Harley-Davidson to design a specification ...
, which used a flat-twin engine with the cylinders across the frame, maintained an oil temperature 100 °F (56 °C) cooler than a
Harley-Davidson WLA
The Harley-Davidson WLA is a Harley-Davidson motorcycle that was produced to US Army specifications in the years during and around World War II. It was based on an existing civilian model, the WL, and is of the 45 solo type, so called due to its ...
with a V-twin with the cylinders in line with the frame.
A side benefit is that the cylinders provide protection to the rider in the event of a collision or fall, and keeps their feet warm in cold weather.
The downsides are that the engine cannot be mounted as close to the ground (otherwise the cylinders can scrape the ground during cornering)
and that it exposes the cylinders and valve covers to the danger of collision damage.
Longitudinal mounting also means that the torque reaction will twist the motorcycle to one side (such as on sharp acceleration/deceleration or when opening the throttle in neutral) instead of shifting the weight balance between the front and rear wheels. However, many modern motorcycles reduce this effect by rotating flywheels or alternators in the opposite direction to that of the crankshaft.
One of the first motorcycles with a longitudinally-mounted flat-twin engine was the 1916
ABC
ABC are the first three letters of the Latin script known as the alphabet.
ABC or abc may also refer to:
Arts, entertainment, and media Broadcasting
* American Broadcasting Company, a commercial U.S. TV broadcaster
** Disney–ABC Television ...
, which was built in the United Kingdom.
To accommodate chain drive, the ABC used a
bevel drive at the gearbox to change the direction of the drive through ninety degrees.
BMW's first motorcycle, the 1923
BMW R 32 was another early example of a longitudinally-mounted flat-twin engine, although it this case the power was transmitted to the rear wheel via a shaft drive.
Over time, longitudinal mounting became more common for flat-twin engines.
BMW has a long history of flat-twin engine motorcycles,
as do
Ural
Ural may refer to:
*Ural (region), in Russia and Kazakhstan
*Ural Mountains, in Russia and Kazakhstan
*Ural (river), in Russia and Kazakhstan
* Ual (tool), a mortar tool used by the Bodo people of India
*Ural Federal District, in Russia
*Ural econ ...
(Russia) and
Dnepr Dnepr may refer to:
*Dnieper, a river flowing through Russia, Belarus and Ukraine to the Black Sea
* Dnepr (motorcycle), a Ukraininan motocycle brand
* Dnepr (rocket), a 1999 space launch vehicle
*Dnepr radar Dnepr may refer to:
*Dnieper, a river ...
(Ukraine).
Aviation
![Bristol Cherub II](https://upload.wikimedia.org/wikipedia/commons/e/ee/Bristol_Cherub_II.jpg)
In 1902, the
Pearse monoplane (which would later become one of the first aircraft to achieve flight) was powered by a flat-twin engine built on a farm by a hobbyist inventor.
This engine used the unusual design of a single shared
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 ...
and double acting pistons. In 1908, the French company Dutheil-Chalmers began production of flat-twin aircraft engines, which used two counter-rotating crankshafts. The Dutheil-Chlamers engine was used by the 1907
Santos-Dumont Demoiselle
The Santos-Dumont ''Demoiselle'' was a series of aircraft built in France by world aviation pioneer Alberto Santos-Dumont. They were light-weight monoplanes with a wire-braced wing mounted above an open-framework fuselage built from bamboo. The ...
''No. 20'' experimental airplane, with later versions of this airplane being produced with flat-twin engines from
Darracq
A Darracq and Company Limited owned a French manufacturer of motor vehicles and aero engines in Suresnes, near Paris. The French enterprise, known at first as A. Darracq et Cie, was founded in 1896 by Alexandre Darracq after he sold his Gladi ...
and
Clément-Bayard
Clément-Bayard, Bayard-Clément, was a French manufacturer of automobiles, aeroplanes and airships founded in 1903 by entrepreneur Gustave Adolphe Clément. Clément obtained consent from the Conseil d'Etat to change his name to that of his b ...
.
Most piston-engined aircraft used more than two cylinders, however other flat-twin aircraft engines from the 1920s and 1930s include the American
Aeronca E-107
The Aeronca E-107 was one of the first low-cost reliable engines of the post-World War I era.
Design and development
The E-107A was a production aviation flathead engine designed to replace a Morehouse engine on the first prototype of the Aer ...
and
Aeronca E-113
The E-113 was a small flat-twin piston engine developed by Aeronca for use in some of their light aircraft. It was an overhead valve development of the flathead configuration E-107.
Design and development
Originally fitted with a single ignit ...
, the British
Bristol Cherub
The Bristol Cherub is a British two-cylinder, air-cooled, aircraft engine designed and built by the Bristol Aeroplane Company. Introduced in 1923 it was a popular engine for ultralight and small aircraft in the 1930s.
Variants
;Cherub I
:Init ...
, and the Czechoslovakian
Praga B2
The Praga B2 was a low powered aero engine suitable for very light aircraft. It dates from the mid-1930s.
Design
The Praga B2 was a dual ignition, air-cooled horizontal twin producing 40 horsepower (30 kW) aimed at lightweight aircraft. ...
. The
HKS 700E
The HKS 700E is a twin-cylinder, horizontally opposed, four stroke, carburetted aircraft engine, designed for use on ultralight aircraft, powered parachutes and ultralight trikes. The engine is manufactured by HKS, a Japanese company noted f ...
is an oil-cooled flat twin for ultralight aircraft that is currently in production.
In larger aircraft, flat-twin engines have been used in
auxiliary power unit
An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115&n ...
s (APUs). A notable example was made by
ABC Motors
ABC Motors Limited ("All British (Engine) Company") of Hersham, Surrey, England was a manufacturer of cars, aircraft, motor scooters, and engines for road and air. Established by Ronald Charteris in Hersham, Surrey in 1912, its chief designer ...
in the 1920s and 1930s.
During World War II, the
Reidel firm in Germany designed and manufactured a two-stroke flat-twin engine as
jet engine starter motors for the Junkers Jumo 004, BMW 003 and Heinkel HeS 011 jet engines
[
]
Other uses
The
Maytag
The Maytag Corporation is an American home and commercial appliance company owned by Whirlpool Corporation since April 2006.
Company history
The Maytag Washing Machine Company was founded in 1893 by businessman Frederick Maytag. In 1925, ...
'Model 72' flat-twin engines— produced from 1937 until some time between 1952 and 1960— were used in various applications including clothes washing machines.
[*]
Electrical generators using flat-twin engines were built by Norman Engineering Company from 1932 to 1968 and by Douglas during World War II.
''Enfield Industrial Engines'' (part of
Royal Enfield) produced flat-twin two-stroke petrol engines during World War II which were used for generators and other military uses. After 1945, Enfield produced flat-twin diesel engines, with applications including farm and marine use.
Coventry Victor
Coventry Victor was a British motorcycle and car manufacturer. Originally Morton & Weaver, a proprietary engine manufacturer in Hillfields, Coventry, founded in 1904, the company changed its name to Coventry Victor Motors in 1911. The company cl ...
introduced a diesel version of its existing 688 cc petrol flat-twin in 1932, and went on to produce flat-twin diesel and petrol engines for a variety of industrial and marine uses into the 1950s.
Two-stroke flat-twins were often used as
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 for boats, as they were smoother than single-cylinder engines. In the 1940s, they were largely replaced by straight-twin two-stroke engines, which were easier to start and no longer had excessive amounts of vibration.
[
]
References
{{Piston engine configurations
Motorcycle engines
Automobile engines
02
Piston engine configurations