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Flat-twin
From Wikipedia, the free encyclopedia
A flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft. The best known example is the 'traditional' (since 1923) BMW motorcycle engine, where it is known as the Boxer. This geometry gives (uniquely, amongst twin cylinder engines) virtually perfect horizontal and vertical balance. There remains a vibrating torque reaction which is quite noticeable in the "Boxer" layout at tick-over, though it cannot be described as disturbing. In the very early days of motorcycles, this same layout was sometimes used along the line of the motorcycle (Douglas and BMW) with obvious disadvantages for an air-cooled engine.
According to the American Motorcyclist Association's Motorcycle Hall of Fame Museum, regarding the World-War-II-vintage Harley-Davidson XA, which has an opposed twin engine — "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."[1] The latter was a transversely mounted V-twin, in which the airstream cannot reach the cylinders, especially the rear cylinder, as efficiently as on a longitudinally mounted flat twin.
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. 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.
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). 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. Particularly 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. Advocates of these air-cooled engines 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.
Panhard built some 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.
Toyota used one such engine in the Toyota Publica and a derivative of it, the Toyota Sport 800.
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| Straight | Single · 2 · 3 · 4 · 5 · 6 · 8 · 9 · 10 · 12 · 14 |
| Flat | 2 · 4 · 6 · 8 · 10 · 12 · 16 |
| V | 2 · 4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 24 |
| W | 8 · 12 · 16 · 18 |
| Other inline | H · U · Square four · VR · Opposed · X |
| Other | Hemi · Radial · Rotary · Pistonless (Wankel) |