Animation of a U engine
A U engine is a piston engine made up of two separate straight engines (complete with separate crankshafts) placed side-by-side and coupled to a shared output shaft. When viewed from the front, the engine block resembles the letter "U".
Although much less common than the similar V engine design, several U engines were produced from 1915 to 1987 for use in airplanes, racing cars, racing and road motorcycles, locomotives, and tanks.