Boxer engine
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 the alternating pistons are positioned in a "V"-angle. Some experts distinguish between V-at-180° engines and real boxer engines. The real boxer has one crank pin per piston while in the V-at-180° engine two pistons share the same crank pin. Real boxer engines can run perfectly smooth and free of vibrations with a four-stroke cycle, regardless of number of cylinders.
The benefit of using a boxer 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. Boxer-engined automobiles also generally have a lower center of gravity, giving better stability and control. However, boxer 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. 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.
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