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. Theses engines can run perfectly smooth and free of vibrations with a four-stroke cycle, regardless of number of cylinders.
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 Porsche356 and 912. Both the older and newer models of the 911 use a flat-6, while Subaru use the flat-4 and flat-6 design extensively, although referring to it as an "H engine". Ferrari used a flat_12 on the Testarossa. 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.
The modern SUBARUengines manufactured by Fuji Heavy Industries in Japan, a company involved in aerospace, is a very logical choice for an aircraft builder looking for a modern engine for his or her modern airplane.
In an in-line engine the crankshaft is affixed to the crankcase by five main bearing caps bolted to the crankcase in a distance of only eleven inches (this puts a main bearing every two inches).
The heads (there are two in a boxerengine) are made from lightweight cast-aluminum alloy for quick even warm-up with the crankcase and to aid in combustion-chamber cooling efficiency.
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