Flat-twin engine

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. Most of the flat-twin engines built are air cooled.

Flat-twins are used on motorcycles, especially BMWs. The main advantage of this geometry in motorcycles is the superior air cooling of the cylinders. The engine itself will also allow 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.