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 1915-1987 for use in airplanes, racing cars, racing and road motorcycles, locomotives, and tanks. Design The main benefit of a U engine layout is the ability to share common parts with straight engine upon which is it based. Additionally, if the two crankshafts rotate in opposite directions, the gyroscopic effect of the rotating components in each cylinder bank cancel each other out. However, a V engine is typically more compact and lighter than a U engine (in part due to the lack of a second crankshaft), therefore V engines are far more common than U engines. The H engine layout uses a similar concept to U engines, whereby two flat engines are stacked vertically. History Petrol engines The first U engine known to have been built was the 1915-1916 Bugatti U-16 aircraft engine, which had 16 cylinders and a displacement of . Approximately 40 engines were built at the Duesenberg factory in the United States during World War I. A small number of engines based on the Bugatti U engine were also produced after the war by Breguet Aviation in France. The Fiat 806 was a 1927 Grand Prix racing car that was powered by a twelve-cylinder U engine. This engine, designated the 'Type 406', used a supercharger and had a single centrally-mounted intake camshaft which operated the intake valves located on the inside of each cylinder bank. Two separate camshafts operated the exhaust valves (one per bank). On test the unit delivered 187 bhp at 8,500 rpm at maximum boost. The Fiat 806 car competed in only one race, the Milan Grand Prix on 4 September 1927 (not to be confused with the 500 km 1927 Italian Grand Prix held on the same day). The race was won by the Fiat 806, however Fiat then retired from Grand Prix racing and the Type 806 did not race again. The 1931-1959 Ariel Square Four motorcycle used a four-cylinder engine (also called a 'square four' engine). The engine was compact and had as narrow a frontal area as a 500 cc, parallel twin, however the rear