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A U engine is a piston engine made up of two separate straight engines (complete with separate crankshafts) joined by gears or chains. It is similar to the H engine which couples two flat engines. The design is also sometimes described as a "twin bank" or "double bank" engine, although these terms are sometimes used also to describe V engines.
This configuration is uncommon, as it is heavier than a V design. The main interest in this design is its ability to share common parts with straight engines. However, V engines with offset banks can also share straight engine parts (except for the crankshaft), and this is therefore a far more common design today when both engine forms are produced from the same basic design.
A prototype British heavy tank proposed in April 1916 was intended to be powered by a doubled version of the 105 hp Daimler 'Silent Knight' engine, with two banks of cylinders side by side sharing a common crankcase. However this was not a true 'U' engine, as the cylinder banks were to have been independent, each separately driving its own crankshaft and four-speed gearbox. Neither the tank nor the engine were ever made.
The first U engine known to have been built was the 16-cylinder 24.3 litre displacement Bugatti U-16 aero engine designed and patented by Ettore Bugatti in 1915–1916. Bugatti licensed the design to Duesenberg in America, who produced about 40 of a modified version as the King-Bugatti, and Breguet of France, who built a few in the years after the end of World War 1. Bugatti later used the same engine layout in the Bugatti Type 45 of 1928, but only two were produced.
Starting in 1925, FIAT began preparing an engine and car for the new 1926 1.5 litre racing formula, adopted in both Europe and the USA. For the engine, designated the 'type 406', FIAT chose to mount two 750 cc six-cylinder banks side by side on a common aluminium crankcase, with their crankshafts geared to a common output shaft. A single Roots-type supercharger driven from the nose of the right-hand crankshaft delivered mixture to a pair of intake manifolds located between the cylinder blocks at 13 lbs/sq. in boost. The left-hand crankshaft drove the single water pump. One centrally mounted camshaft operated the induction valves, which were on the inside of each cylinder bank; two camshafts mounted outboard drove the exhaust valves. Twin Bosch magnetos were driven from the tail of the central camshaft. The engine weighed 381 lbs, not excessive for the time. On test the unit delivered 187 bhp at 8,500 rpm at maximum boost.
The type 406 powered the Type 806 car to wins in the heats and the final of the 1926 Milan Grand Prix. However, opposition in the FIAT management to further racing, and the expiry of the 1.5 litre formula in 1927 meant the car and its engine were scrapped beyond recovery.
Matra developed a high-end Bagheera prototype powered by a 2.6 litre U8 engine made of two Simca 1000 Rallye 2 straight-4s connected by chains around 1974. However because of the petroleum crisis this car was never put in production.
Several types of U-form diesel engine have been historically produced, by companies such as Lister Blackstone and Sulzer Brothers Ltd. A twin bank diesel engine for marine use is described in US Patent 4167857. However, no further documentation has been found for any ship or marine application of such an engine.
Sulzer Brothers developed a diesel engine for rail traction of this type, the LD series, in the 1930s, that was in production for more than fifty years. Several cylinder sizes were produced, including the 19 (bore 190 mm), 22 (bore 220 mm), 25 (bore 250 mm), 28 (bore 280 mm) and 31 (bore 310 mm). The engines of the LD and later, the LDA series, were commonly found in 6 and 8 cylinders inline and 12 cylinders U form. The U form engines were installed in railway locomotives operating in several countries, including Britain, Bulgaria, China, France, Poland and Romania. For further information on the British Railways locomotives (classes 44, 45, 46, 47) see List of British Rail modern traction locomotive classes.
Sulzer Brothers later discontinued their rail traction engine business.
The M4A2 version of the Sherman tank was powered by the General Motors 6046D twin diesel engine, a 12-cylinder twin bank version of the General Motors series 71 six cylinder supercharged two-stroke diesel. Each six cylinder engine unit displaced 6,965cc, and was separately clutched to a single output shaft, which was itself clutched to the transmission unit. The whole engine weighed 2,323 kg (5,110 lbs) dry weight, and produced up to 410 horsepower at 2,900 rpm with both units running. A total of 10,968 6046D-powered M4A2 Shermans were produced.
A series of 16 and 18-cylinder U engines were developed in Russia during the Soviet era designated Russkiy Dizel (Diesel Energo) DPN23/2H30 DRPN23/2H30  These are two-stroke, opposed-piston turbocharged diesel engines of high power density (5600 kw, 16-cylinder, 6300-6500 kw, 18-cylinder) with the four crankshafts turning at 896-900 rpm and the output shaft turning at 1000 rpm. Each cylinder has a 230mm bore, with the two pistons in each cylinder each having a stroke of 300 mm in either direction. These engines were used as propulsion units in various Russian and Eastern Bloc ships, and also as stationary power generators.
Direction of rotation 
If the crankshaft on one bank of cylinders is made to rotate in the opposite direction of the crankshaft on the other bank, then the gyroscopic effects of the rotating components would be canceled. However, counter-rotating crankshafts would make the joining of the power outputs very difficult.
Gear ratio 
In the Sulzer LDA engine, a gear wheel on each crankshaft meshed with a slightly smaller gear wheel on the central output shaft. The crankshafts ran at around 750 rpm but the output shaft ran at about 1,000 rpm. This allowed the use of a smaller, and lighter, electrical generator when the engine was used in a diesel-electric locomotive.
Square four engine 
A square four is a U engine with two cylinders on each side. This configuration was used on the Ariel Square Four motorcycle from 1931 to 1959. This design was revived as a two-stroke version on some racing Suzukis, and their subsequent road-going version the Suzuki RG500. Although some racing success was achieved, the road bikes did not sell well and the design was phased out in favour of inline four-stroke designs, as engineering and marketing resources were being applied to more common four-stroke designs at the time.
An experimental square four outboard motor was built for evaluation, but the design was not used due to the complexity of the drivetrain.
Tandem twin engine 
In tandem twins, the motor has two crankshafts, one for each cylinder which are joined and kept in co-ordinated by load carrying, crank-phasing gears connecting the two cylinders. The design has been used in motorcycling, and go-kart racing.
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,
Single crankshaft 
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.
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- L’Ebé Bugatti, 1966, 'The Bugatti Story', Editions de la Table Ronde & L'Action Automobile, first British edition 1967, Len Ortzen (translator) and Souvenir Press Ltd, London, pps 70-72, p162
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