Flat-twin
From Wikipedia, the free encyclopedia
A flat-twin is a two cylinder internal combustion engine with the cylinders arranged in a flat configuration, often called a Boxer engine. This geometry is thought to be the best for minimizing vibrations in a two cylinder engine.
On motorcycles, flat twins can be positioned either with the crankshaft positioned longitudinally, as in the BMW twins, or tranversely, as in the Douglas. The cylinders of a longitudinally mounted flat twin motorcycle engine protrude sideways into the air stream.
Engine cooling is an advantage of the longitudinally mounted flat twin. 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.
The flat-twin engine allows a low center of gravity but this advantage is, in part, balanced by the fact that the engine should be placed rather high in the motorcycle to prevent the cylinders from touching the ground in turns, a side benefit is that the cylinders will prevent the motorcycle from crushing the rider in the event of an accident. In practice a longitudinal V-twin such as on Moto-Guzzi will give a lower center of gravity, but with more vibrations (from odd firing) and less efficient cooling it has different advantages.
In automobiles, the flat-twin was popular on small French cars after World War II. 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. The Citroën 2CV was also powered by a flat-twin. Although the 2CV engine was built with low cost and reliability in mind, the last versions of this engine were relatively powerful (giving 32 hp for 602cc) considering their small engine displacement.
[edit] See Also
| Piston engine configurations | |
|---|---|
| Straight | Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14 |
| V | 2, 4, 5, 6, 8, 10, 12, 16, 20, 24 |
| Flat | 2, 4, 6, 8, 10, 12, 16 |
| W | 8, 9, 12, 16, 18 |
| Other inline | H, VR, Opposed, U (Square), X |
| Other | Radial, Rotary, Pistonless (Wankel) |
