A flat engine or boxer engine or horizontally opposed engine is a type of engine where the pistons lie horizontally opposed, with pairs of cylinders on the left and the right, as opposed to most modern engines where all pistons are inline, or the alternating pistons are positioned in a "V"-angle.
Experts distinguish between V-at-180° engines and real boxer engines (horizontally opposed engines). The horizontally opposed engine has one crank pin per piston while in the V-at-180° engine two pistons share the same crank pin.
The benefit of using an horizontally opposed engine engine versus a V engine is that it provides perfect balance because each piston's movement is exactly counterbalanced by the corresponding piston movement of the opposite side. These engines can run perfectly smooth and free of vibrations with a four-stroke cycle, regardless of number of cylinders, and do not require the counterweights on the crankshaft to balance the weight of the reciprocating parts which are required in other engine configurations.
http://upload.wikimedia.org/wikipedia/en/d/de/Boxer_engine_diagram.jpg
Automobiles and motorcycles powered by a flat engine generally have a lower center of gravity, giving better stability and control. However, these engines are also wider than normal engines and are more expensive to build than traditional engines and, thus, are not widely used by automobile manufacturers.
The flat configuration fits very well with air cooling and aircraft engines. Air-cooled designs such as in the VW Beetle used a flat-4, as did the Porsche 356 and 912. The Chevrolet Corvair used an air-cooled flat-6, a rarity in American designs. Both the older and newer models of the 911 use a flat-6, at first air cooled but later models water-cooled. These automobiles placed the engine in the rear rather than the front, where its width would not interfere with the steering of the front wheels; the use of air-cooling obviated the need for connecting the engine to radiators, as well as reducing the weight even more.
Subaru, on the other hand, uses water-cooled front mounted engines, of flat-4 and flat-6 design, although referring to them as an "H engine". Ferrari used a 180° version of the Colombo V12 on the Testarossa and Berlinetta Boxer models. Air-cooled flat-twin engines were used by Citroën in their model 2CV and its derivatives, while the Citroën GS used a flat-4, and a flat-6 was proposed for the Citroën DS but rejected due to impractability. BMW uses a flat-twin in many of their motorbikes
