A U-engine is a reciprocating piston engine made up of two in- line engines placed side by side . It has two crankshafts that rotate in opposite directions and are coupled via a gear drive. The H-engine is similar to the U-engine , in which two 180 ° V-engines are connected to each other. Other names for the U-engine are, for example, "double row engine " .
U-engines are very rare because they are heavier than a corresponding V-engine . On the other hand, from 4 cylinders upwards, they have the advantage of a perfect balance of the free inertia forces and moments of 1st and 2nd order.
Some diesel engines for locomotives were built as underground engines by Sulzer from Winterthur in Switzerland . In the 1930s they developed a series of diesel U12 engines (the "LD series" and the later "LDA series" ) that were produced for more than 50 years. Different cylinder sizes were built - for example "19" with a 190 mm bore, "22" with a 220 mm bore, "25" with a 250 mm bore, "28" with a 280 mm bore and "31" with a 310 mm bore. Sulzer supplied these engines to various countries such as Great Britain, Bulgaria, China, France, Poland and Romania.
Another example is the Bugatti U16 aero engine, developed by Bugatti in 1916 but not accepted by the French Air Force . It was licensed to Duesenberg in the USA at the end of 1917 and further developed there by the engineer Charles Brady King into the King-Bugatti U-16 . Of the planned 2000 units, 40 units were built near Duesenberg by the beginning of 1919. However, the U-16 was no longer used in any aircraft. The U-16 Type 34 based on it and constructed in 1925 was never built. However, double-compressor-charged 16-cylinder U-engines were used in the Bugatti "Type 47" and "Type 45" sports and racing cars . They were water-cooled four-stroke engines that were designed and produced in 1929/1930.