Flat-twin
Encyclopedia
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
with the cylinders arranged on opposite sides of the crankshaft. It is part of the class of flat engine
Flat engine
A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
s, sub-type "boxer", and shares most characteristics of those engines.
Motorcycle use
BMW MotorradBMW Motorrad
BMW Motorrad is the motorcycle brand of the German company BMW, part of its Corporate and Brand Development division. The current General Director of the unit is Hendrik von Kuenheim....
manufactures a number of flat-twin engine motorcycles,
as do Ural
IMZ-Ural
IMZ-Ural is a Russian maker of heavy sidecar motorcycles. Recently the company has developed a solo motorcycle with no sidecar.In 1940, the Soviet Union acquired the design and production techniques for BMW R 71 motorcycles and sidecars. The first M-72 model was finished in 1941...
and Dnepr
Dnepr (motorcycle)
Dnepr is the brand name of heavy motorcycles produced in Kiev, Ukraine. It has been in use since 1967.Motorcycles have been produced in Kiev since 1946 at the Kiev Motorcycle Plant . Initial production was of a 98 cc two-stroke model that was confiscated from the German firm Wanderer as...
.
The geometry gives good primary balance
Engine balance
Engine balance is the design, construction and tuning of an engine to run smoothly. Improving engine balance reduces vibration and other stresses and can improve the overall performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery...
, but there is an unbalanced moment on the crankshaft caused by the pistons being offset from each other.
Cylinders along frame
Douglas (motorcycles)
Douglas was a British motorcycle manufacturer from 1907–1957 based in Kingswood, Bristol, owned by the Douglas family, and especially known for its horizontally opposed twin cylinder engined bikes and as manufacturers of speedway machines...
in the United Kingdom, Helios of Germany, and Harley-Davidson
Harley-Davidson
Harley-Davidson , often abbreviated H-D or Harley, is an American motorcycle manufacturer. Founded in Milwaukee, Wisconsin, during the first decade of the 20th century, it was one of two major American motorcycle manufacturers to survive the Great Depression...
of the United States, had their cylinders aligned along the frame, and therefore with the crankshaft running transverse to the frame.
This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel. Another advantage of this layout is that it has a low centre of gravity. However, in this layout, the front cylinder is more heavily cooled than the rear cylinder, and the wheelbase tends to be excessive due to the length of the engine. The wheelbase can be reduced by placing the transmission above the rear cylinder, as done on some Douglas motorcycles.
Cylinders across frame
ABC motorcycles
ABC motorcycles was a British motorcycle manufacturer established in 1914 by Ronald Charteris in London. Several British motorcycle firms started up with the name "ABC", including Sopwith. The All British Engine Company Ltd...
introduced a motorcycle with a flat-twin engine with the cylinders across the frame, and therefore with the crankshaft running longitudinally when referenced to the frame. To accommodate chain drive, the ABC used a bevel drive
Bevel gear
Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped.Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well...
at the gearbox to change the direction of the drive through ninety degrees. The 1923 BMW R32
BMW R32
The BMW R32 was the first motorcycle produced by BMW under the BMW name. An aircraft engine manufacturer during World War I, BMW was forced to diversify after the Treaty of Versailles banned the German air force and German aircraft manufacture...
used a similar engine position with a drive shaft using bevel gears to power the rear axle.
This position allowed both cylinders to protrude into the airflow, providing excellent air cooling for each cylinder. The Harley-Davidson XA
Harley-Davidson XA
The Harley-Davidson XA was a flat-twin shaft drive motorcycle made by Harley-Davidson for the US Army during World War II....
, 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 with a V-twin with the cylinders in line with the frame.
Many motorcyclists appreciate the way the cylinders in this layout provide protection to the rider in the event of a collision or fall, and keeps their feet warm in cold weather.
A disadvantage of this layout is that it exposes the cylinders and valve covers to the danger of collision damage. Longitudinal crankshaft mounting is also associated with a torque reaction that tends to twist the motorcycle to one side on sharp acceleration or when opening the throttle in neutral and in the opposite direction on sharp deceleration. Many modern motorcycle manufacturers correct for this effect by rotating flywheels or alternators in the opposite direction to that of the crankshaft.
Automotive use
Flat-twin engines were used in several economy carEconomy car
An economy car is an automobile that is designed for low cost operation. Typical economy cars are small, light weight, and inexpensive to buy. Economy car designers are forced by stringent design constraints to be inventive...
s, including the Citroën 2CV
Citroën 2CV
The Citroën 2CV |tax horsepower]]”) was an economy car produced by the French automaker Citroën between 1948 and 1990. It was technologically advanced and innovative, but with uncompromisingly utilitarian unconventional looks, and deceptively simple Bauhaus inspired bodywork, that belied the sheer...
, the Panhard Dyna X
Panhard Dyna X
The Panhard Dyna X was a lightweight compact saloon car designed by the visionary engineer Jean Albert Grégoire and first exhibited as the AFG Dyna at the Paris Motor Show in 1946....
and Dyna Z
Panhard Dyna Z
The Panhard Dyna Z was a lightweight motor car made by Panhard of France. It was first presented to the press at a Paris restaurant named "Les Ambassadeurs" on 17 June 1953 and went into production the following year...
, Steyr-Puch 500
Puch 500
Puch 500 was a small car produced by Steyr-Daimler-Puch in the city of Graz, Austria. It was manufactured under license from Fiat and was based on the Fiat 500.- The beginning :In 1954 it was decided at Steyr-Puch to resume car production after the war...
, DAF Daffodil
DAF Daffodil
The DAF 750 is a small family car that was manufactured by DAF from 1961 until 1963. It replaced the DAF 600. At the same time as launching the 750, DAF launched the DAF Daffodil which was essentially the same car but with more luxurious fittings and a lot more chrome trim on the outside...
, BMW 600
BMW 600
The BMW 600 is a four-seat microcar produced by BMW from mid-1957 until November 1959. Partially based on the BMW Isetta two-seater, it was BMW's first postwar four seat economy car...
, several Jowett
Jowett
Jowett was a manufacturer of light cars and light commercial vehicles in Bradford, West Yorkshire, England from 1906 to 1954.-Early history:Jowett was founded in 1901 by brothers Benjamin and William Jowett with Arthur V Lamb. They started in the cycle business and went on to make V-twin engines...
cars between World Wars I and II, and the Toyota Publica
Toyota Publica
The Toyota Publica was a small car manufactured by the Japanese automaker Toyota from 1961 to 1978. Conceived as a family car to fulfil the requirements of the "national car concept", it was the smallest Toyota car during that period and was superseded in that role by the Toyota Starlet, which...
and Toyota Sport 800. Flat-twin engines were also used in several early cars, including the Ford
Ford Motor Company
Ford Motor Company is an American multinational automaker based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford and Lincoln brands, Ford also owns a small stake in Mazda in Japan and Aston Martin in the UK...
1903-04 Model A, Model C
Ford Model C
The Ford Model C is a version of the first Ford Model A with more modern look. It has a slightly more powerful engine and 15 cm longer wheelbase. It was the entry-level car in the Ford model lineup, slotting below the upscale Model B. Production ended in 1905 with 800 cars made...
, and Model F
Ford Model F
The Ford Model F is an automobile produced by the Ford Motor Company. It was a development of the Model A and Model C, but was larger, more modern, and more luxurious. It was a four-seater phaeton with running boards and a side-entrance tonneau standard. Production started in 1905 and ended in 1906...
.
Other uses
MaytagMaytag
Maytag Corporation is an American home and commercial appliance company, headquartered in Newton, Iowa, that is a division of the Whirlpool Corporation.-Company history:...
used its Model 72 flat-twin engines to power washing machines, although they were used as proprietary engines for other purposes as well. Maytag began manufacturing the Model 72 engine in 1937 and, after a break in production from May 1942 to June 1945 due to World War II, continued manufacturing them until the 1950s. Production ended some time between 1952 and 1960.
During World War II, motorcycle manufacturer Douglas
Douglas (motorcycles)
Douglas was a British motorcycle manufacturer from 1907–1957 based in Kingswood, Bristol, owned by the Douglas family, and especially known for its horizontally opposed twin cylinder engined bikes and as manufacturers of speedway machines...
built generators powered by their flat-twin engines, while ABC Motors
ABC Motors
ABC Motors Limited 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 was the young and talented Granville Bradshaw...
used one in an Auxiliary power unit
Auxiliary power unit
An auxiliary power unit is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.-Function:...
(APU).
Ignition systems
Flat-twin engines are well suited to the wasted sparkWasted spark
The wasted spark system is an ignition system used with some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs even though one is on its compression stroke and one is on its exhaust stroke. The extra spark on the exhaust stroke has no effect and...
ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
, a distributor-less ignition system using a double-ended coil firing both spark plugs on each revolution, that is, on both the compression stroke and the exhaust stroke. This system requires only a single contact breaker and single coil to run two cylinders.