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.
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.
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 24.3 L (1,483 cu in). 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 1.5 L (92 cu in) 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 50 km (31 mi) 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 pair of cylinders on this air-cooled engine were prone to overheating.
The 1985-1987 Suzuki RG500 motorcycle, and several related racing motorcycles, used a water-cooled four-cylinder U engine. Although some racing success was achieved, the road bikes did not sell well and the design was phased out in favour of a conventional inline-four engine.
In the 1930s Sulzer Brothers Ltd. began production of an 'LD series' twelve-cylinder U engine for use in rail locomotives. The LD series was replaced by the LDA series, for a combined production period of over 50 years. The Sulzer 12LDA twin-bank engine formed the mainstay of British locomotives built in the 1960s, with over 700 used in the Peak and Class 47 locomotives. The Sulzer LDA engine used a smaller gear for the central output shaft than the two gears attached to the crankshaft. This resulted in the output shaft rotating at approximately 1000 rpm while the crankshafts rotated at approximately 750 rpm. The purpose of this gearing was to allow the use of a smaller, and lighter, electrical generator when the engine was used in a diesel-electric locomotive.
The General Motors 6046 is a twin-engine setup that was used by Sherman tanks during World War II. The 6046 was built using two straight-six engines that were separately clutched to a single output shaft, which was itself clutched to the transmission unit. A total of 10,968 6046D-powered M4A2 Shermans were produced. After World War II, several Soviet Union tanks powered by 16-cylinder and 18-cylinder engines that were reverse-engineered from the General Motors 6046 engine. These Soviet engines were designated Russkiy Dizel (Diesel Energo) DPN23/2H30 and the DRPN23/2H30.
A tandem twin engine, occasionally used in motorcycles and go-karts, is a two-cylinder engine which uses a similar design to U engines. The motor has two crankshafts, one for each cylinder which are joined and kept in co-ordination by load carrying, crank-phasing gears connecting the two cylinders.
The tandem twin layout is used only with two-stroke engines since these must have a discrete crankcase chamber per cylinder. The prime advantage of a tandem-twin two-stroke is that the engine can be very narrow while allowing chain final drive without a power-wasting 90° turn.
Between 1975 and 1982, Kawasaki used the design to win four 250 cc and four 350 cc world championships before they retired from Grand Prix racing. The engine design was also used for a road legal production motorcycle inspired by the racer. The Kawasaki KR models were instrumental in establishing the company as a manufacturer of high-performance motorcycles.
Rotax developed a similar tandem twin design, the model 256, which it sold to independent constructors. The CCM Armstrong 250 cc, Waddon, EMC, Hejira, Decorite, and Cotton racers used this engine. CCM Armstrong developed a 350 cc version of the engine. Aprilia's 1985 GP racing bikes also used the Rotax model 256.
An unusual variation on the U engine is the use of a single crankshaft which is linked to the pistons in both cylinder banks by rocking beams. This system was used in an eight-cylinder petrol engine produced by the All-British Car Company between 1906 and 1908.
Both have at their heart a twin-cylinder two-stroke engine that is, in effect, two 125cc Singles mounted one behind the other in a common crankcase, their cranks connected by large gears.
Sato, like Kaaden, realised that the convention twin-cylinder two-stroke, with its cylinders across the frame, coupled to the outward facing carburettors of a disc-valve twin had a major drawback, because of excessive width.
These hopes were due to the introduction of the Model 256 inline twin...