Flat-twin

A flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft.

Motorcycle use

The best knownFact|date=September 2008 example is in the 'traditional' BMW motorcycle, which has used engines of this layout since 1923. This geometry gives (uniquely, amongst twin cylinder engines) virtually perfect horizontal and vertical balance.Fact|date=September 2008 There remains a vibrating torque reaction which is quite noticeable in the "Boxer" layout at tick-over, though it cannot be described as disturbing.Fact|date=September 2008

In the very early days of motorcycles, this layout was sometimes used along the line of the motorcycle (Douglas and Helios) with obvious advantages for an air-cooled engine.Fact|date=September 2008

Referring to the World-War-II-vintage Harley-Davidson XA, which has an opposed twin engine, the American Motorcyclist Association's "Motorcycle Hall of Fame Museum" stated:

Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature 100 degrees cooler than a standard Harley 45. [ [http://www.motorcyclemuseum.org/classics/bike.asp?id=71 "1942 Harley-Davidson XA"] ]

The latter was a V-twin with its cylinders mounted in line with the frame, such that the air flow to the cylinders, especially the rear cylinder, is restricted.

In motorcycles, the flat-twin engine allows a low center of gravity - though some of this advantage cannot be realised, since the engine needs to be high in the motorcycle to provide banking clearance in turns. Fact|date=September 2008 Another benefit (much appreciated by some users) is that the cylinders provide considerable protection to both rider and motorcycle (fairing) in the event of a collision or fall.Fact|date=September 2008

Automotive use

In automobiles, the flat-twin engine was once popular since it adapts easily to forced air-cooling (the flat-fours of the VW Beetle also notable in this respect).Fact|date=September 2008 Two disadvantages, not considered very serious in Europe before and after the war, were significant mechanical noise and poor adaptation to heating the inside of the car.Fact|date=September 2008 Advocates of these air-cooled enginesWho|date=September 2008 remind us of the large number of car break-downs directly caused by failures of the water-cooling, and also the big penalty in weight.

Notable examples of small cars with flat-twin engines include the BMW 600 of 1957, which used the BMW R67 motorcycle engine, and the BMW 700 of 1959, which used a shrouded and bored-out version of the same engine. [ [http://rribreau.club.fr/techniqueCOUPE.htm "BMW 700"] ]

Also notable is the French Citroën 2CV, produced from the 1930s to the 1990s. Considered most under-powered by modern standards, the last versions of this engine were reasonable (giving 32 hp from 602cc) considering their small engine displacement and overall light weight.Fact|date=September 2008

Panhard built flat-twin engines which were used in competition on DB and CD cars. These engines were known to have a very high power-to-weight ratio despite their rather simple conception.Fact|date=September 2008

Toyota used one such engine in the Toyota Publica and a derivative of it, the Toyota Sport 800.Fact|date=September 2008

Several Jowett cars from 1910 to the 1950s had a flat-twin or a flat-four engine.Fact|date=September 2008

Maytag used a small flat twin as a washing machine engine.Fact|date=September 2008

References

See also

Motorcycles
* ABC Motorcycle
* BMW motorcycles
* BMW R51/3
* BMW R60/2
* BMW R75/5
* BMW R1200RT
* Douglas motorcycles
* Harley-Davidson XA

Automobiles
* BMW 600
* BMW 700
* Citroen 2CV
* Panhard Dyna X
* Panhard Dyna Z

External links

* [http://www.animatedpiston.com/BMW.htm Detailed BMW Motorcycle Flat Twin Engine Animations]