U engine

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.

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