Flat-twin engine

A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same time.

The flat-twin design was patented by Karl Benz in 1896 and the first production flat-twin engine was used in the ''Lanchester 8 hp Phaeton'' car released in 1900. The flat-twin engine was used in several other cars since, however a more common usage is in motorcycles; early models oriented the cylinders in line with the frame, however later models switched to the cylinders being perpendicular to the frame to provide even cooling across both cylinders.

Flat-twin engines were also used in several aircraft up until the 1930s and in various stationary applications from the 1930s to the 1960s.

The Australian lawnmower manufacturer Victa also produced a flat-twin engine push mower from August 1975 to 1980 dubbed as the ‘Twin 500’ and ‘Supreme’ these motors were manufactured in Canada, they are very sought after as production only saw few numbers most likely due to ignition and fuel related problems in early models, the supreme (being the later model) had fixed all these problems with a rear domed piston, crankcase mixers and refined ignition system

Typical design

Most flat-twin engines use a boxer configuration for the crankshaft and are therefore called "boxer-twin" engines. In a boxer-twin engine, the 180° crankshaft moves the pistons in phase with each other, therefore the forces generated by one piston are cancelled out by the other, resulting in excellent primary balance. The evenly spaced firing order also assists in reducing vibration. The equal and opposite forces in a boxer-twin engine do however generate a