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 A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...

design, several U engines were produced from 1915-1987 for use in airplanes, racing cars,

racing In sport, racing is a competition of speed, in which competitors try to complete a given task in the shortest amount of time. Typically this involves traversing some distance, but it can be any other task involving speed to reach a specific goa ...

and

road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...

motorcycles, locomotives, and tanks.

Design

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 A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rota ...

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 A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

), therefore V engines are far more common than U engines. The

H engine An H engine is a piston engine comprising two separate flat engines (complete with separate crankshafts), most often geared to a common output shaft. The name "H engine" is due to the engine blocks resembling a letter "H" when viewed from the fr ...

layout uses a similar concept to U engines, whereby two

flat engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, ...

s are stacked vertically.

History

Petrol engines

The first U engine known to have been built was the 1915-1916

Bugatti U-16 The Bugatti U-16 was a 16-cylinder water-cooled double-8 vertical in-line "U engine", designed by Ettore Bugatti in 1915-1916 and built in France in small numbers. The US Bolling Commission bought a license to build the engine in the US, and sm ...

aircraft engine, which had 16 cylinders and a displacement of .L’Ebé Bugatti, 1966, 'The Bugatti Story', Editions de la Table Ronde & L'Action Automobile, first British edition 1967,

Len Ortzen Len Ortzen was an English writer and translator from French. Life Ortzen grew up in the East End of London, and his first novel, ''Down Donkey Row'' (1938), was appreciatively reviewed by Hugh Massingham as "a picture, at once faithful and amus ...

(translator) and Souvenir Press Ltd, London, pps 70-72, p162 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 Breguet or Bréguet may refer to: * Breguet (watch), watch manufacturer **Abraham-Louis Breguet (1747–1823), Swiss watchmaker ** Louis-François-Clement Breguet (1804–1883), French physicist, watchmaker, electrical and telegraph work * Brégue ...

in France. The ''Fiat 806'' was a 1927 Grand Prix racing car that was powered by a 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 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 The Square Four is a motorcycle produced by Ariel between 1931 and 1959, designed by Edward Turner, who devised the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufactur ...

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.

Diesel

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 A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conv ...

. The General Motors 6046 is a twin-engine setup that was used by

Sherman tank } The M4 Sherman, officially Medium Tank, M4, was the most widely used medium tank by the United States and Western Allies in World War II. The M4 Sherman proved to be reliable, relatively cheap to produce, and available in great numbers. It w ...

s 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.

Variations

Tandem twin engine

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 engine A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of ...

s 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 Rotax is the brand name for a range of internal combustion engines developed and manufactured by the Austrian company BRP-Rotax GmbH & Co KG (until 2016 BRP-Powertrain GmbH & Co. KG), in turn owned by the Canadian Bombardier Recreational Produc ...

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.Italian Racing Motorcycles
Mick Walker. Redline Books, 1 Jan 2000

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 The All-British was an automobile manufacturer based at Bridgeton, Glasgow, Scotland, from 1906 to 1908. The company was founded by George Johnston, formerly of Arrol-Johnston, primarily for the manufacture of a 54 horsepower eight-cylinder ca ...

Car Company between 1906 and 1908.

References

{{Piston engine configurations Piston engine configurations Engines by cylinder layout