High Quality Content by WIKIPEDIA articles! A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896 by engineer Karl Benz. It should not to be confused with the opposed-piston engine, in which each cylinder has a piston at both ends and no cylinder head.