
A flat-4 or horizontally-opposed-4 is a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase. The pistons are usually mounted on the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching his gloves together before a fight, which has led to it being referred to as a boxer engine.The low centre of gravity of the engine is an advantage.

SUBARU engine used in single man Airplane!

