A flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft. It is a flat engine with two cylinders. Used in motorcycles for more than ninety years, flat-twins have also been used in automobiles, light aircraft, stationary powerplants, and household appliances.
Early flat-twin motorcycles had their engines mounted with the cylinders in line with the frame. This caused uneven cooling of the cylinders and required the motorcycle to have a long wheelbase. Later flat-twin motorcycles had their engines mounted with their cylinders across the frame for better air cooling and a shorter wheelbase. Disadvantages of this layout include torque reaction in the motorcycle in turns and the potential to damage cylinder heads.
There are two possible crank configurations for the flat twin. The more common is the boxer twin, with a 180 degree crankshaft with two crankpins, such that the pistons move in and out simultaneously. In some older applications, and in some flat-twin lawnmower engines, there is only one crank throw with a shared pin. This type of engine is not a boxer, but a 180 degree V-engine.
Flat-twin engines were used in several of Henry Ford's early cars, including the Ford 1903-04 Model A, Model C, and Model F.
Flat-twin engines were later used in several economy cars, including the Citroën 2CV, the Panhard Dyna X and Dyna Z, Steyr-Puch 500, DAF Daffodil, BMW 600, BMW 700, several Jowett cars between World Wars I and II, and the Toyota Publica and Toyota Sport 800.
Maytag 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 built generators powered by their flat-twin engines.
Boxer-twin engines are well suited to the wasted spark ignition system, 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.