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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.

An engine of this type was the 16-cylinder engine found on the Bugatti Type 45, and only two were produced. However, Bugatti licensed the design to Duesenberg in America, who produced about 40, and Breguet of France, who both intended the engine for aircraft use.

Matra developed a high-end Bagheera prototype powered by a 2.6 L U8 engine made of two Simca 1000 Rallye 2 Straight-4s connected by chains around 1974. However with 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 [1] and Sulzer Brothers Ltd. A twin bank diesel engine for marine use is described in US Patent 4167857 [2]. 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. Sulzer Brothers later discontinued the rail traction engine business. [3]

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.

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 didn't sell in great numbers, and the design was phased out in favour of in-line, four-stroke designs, as at the time two stroke engines were quickly being superseded by more economical, reliable, and emissions-friendly four-strokes.

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