Flat-twin engine

A flat-twin engine is an internal combustion engine with two cylinders placed on opposite sides of the crankshaft. The commonest form is the boxer twin, in which both pistons move inwards and outwards together, so that the forces from one piston largely cancel those from the other and the engine has excellent primary balance, although the offset between the cylinders leaves a rocking couple. The layout was patented by Karl Benz in 1896, and the first production flat twin powered the Lanchester 8 hp Phaeton of 1900. It later served in small cars such as the Citroen 2CV, in motorcycles from Douglas, ABC and BMW, in early aircraft and auxiliary power units, and in stationary and marine engines for generators, washing machines and outboard motors.