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 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. 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 layout was sometimes used along the line of the motorcycle (Douglas and Helios) with obvious advantages for an air-cooled engine.
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.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. 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. 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.
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.
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.
Several Jowett cars from 1910 to the 1950s had a flat-twin or a flat-four engine.
Maytag used a small flat twin as a washing machine engine.