Square-four engine

A four cylinder engine in which each cylinder axis forms one corner of a square. This represents a way of doubling parallel twin cylinders to make a four cylinder unit, with two separate parallel crankshafts connected by gears or chain. This arrangement allows the engine width to be kept to that of a twin cylinder unit with only a small increase in length, thus providing the advantages of an in-line four without the problem of width. Applied in four-stroke and two-stroke motorcycles; in two-strokes mainly to permit rotary valve induction