H engine

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Animation of an H engine H-engine.gif
Animation of an 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 front. The most successful "H" engine in this form was the Napier Dagger and its derivatives. The name was also applied to engines of the same basic layout, but rotated through 90 degrees—most famously the Napier Sabre series. A variation on the "H" theme were the Fairey Prince (H-16) & Fairey P.24 Monarch, where the two engines retained separate drives, driving Contra-rotating propellers through separate concentric shafts. Although successful, they only existed in prototype form.


The H engine is a relatively rare layout, with its main use being in aircraft engines during the 1930s and 1940s. The 1966 Lotus 43 Formula One car used a 16-cylinder H engine, and an 8-cylinder H engine was used for powerboat racing in the 1970s.


The benefits of an H engine are the ability to share common parts with the flat engine upon which it is based, and the good engine balance which results in less vibration (which is difficult to achieve in many other types of four-cylinder engines). [1]

However, H engines are relatively heavy and have a high centre of gravity. The latter is not only due to the second crankshaft being located near the top of the engine, but also the engine must be high enough off the ground to allow clearance underneath for the exhaust pipes[ citation needed ].

The U engine layout uses a similar concept, placing two straight engines side-by-side.


Aircraft engines

Napier Sabre H-24 engine (starboard side) CASM - Napier Sabre - 030906.jpg
Napier Sabre H-24 engine (starboard side)

Formula One racing engines

The British Racing Motors (BRM) H-16 Formula One engine won the 1966 US Grand Prix in a Lotus 43 driven by Jim Clark. [2] It was also used by the unsuccessful 1966 BRM P83 car driven by Graham Hill and Jackie Stewart. As a racing-car engine it was hampered by a high center of gravity, and it was heavy and complex, with gear-driven twin overhead cams for each of four cylinder heads, two gear-coupled crankshafts, and mechanical fuel injection. [3] [4]

Motorcycle engines

Brough Superior H-4 motorcycle engine Brough Superior Golden Dream (close up).JPG
Brough Superior H-4 motorcycle engine

The Brough Superior Golden Dream motorcycle, first shown in 1938. [5] A 1,000 cc H-4 design and a few units were produced in early 1939. Any development planned was interrupted by World War II and subsequent years of austerity.

Wooler built a motorcycle prototype with a similar configuration to the Brough Superior Golden Dream and exhibited it at the British International Motor Show at Earls Court Exhibition Centre in 1948 and again in 1951. This was replaced by a flat-four engined prototype at the 1953 show.

Powerboat racing engines

German firm Konig, who specialised in racing outboard motors, [6] built a few 1000cc H-8s in the 1970s, which were basically two of their VC500 flat fours mounted one above the other, with the direction of rotation reversed on one of them. Each half of the engine was a water cooled 2-stroke with rotating disc valve driven by a toothed belt via two 45/90 degree pulleys, plus two siamesed expansion chamber exhausts, fed by two single choke carbs. Both cylinders at each end of each engine fired at the same time, hence the siamesed exhausts for each pair.

Other engines named "H"

Subaru has marketed its flat-four and flat-six engines as "H4" and "H6" respectively. The letter "H" in this case refers to "horizontally-opposed", an alternative term for flat engines; these engines can also be said to look like a "H" or conjoined "H"s, albeit from the top and in schematic form.

The Saab H engine is a straight-four engine produced from 1981 to 2009. The letter "H" represents "high compression".

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Radial engine Reciprocating engine with cylinders arranged radially from a single crankshaft

The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is called a "star engine" in some languages. The radial configuration was commonly used for aircraft engines before gas turbine engines became predominant.

Sleeve valve

The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve valve engines saw use in a number of pre-World War II luxury cars and in the United States in the Willys-Knight car and light truck. They subsequently fell from use due to advances in poppet-valve technology, including sodium cooling, and the Knight system double sleeve engine's tendency to burn a lot of lubricating oil or to seize due to lack of it. The Scottish Argyll company used its own, much simpler and more efficient, single sleeve system (Burt-McCollum) in its cars, a system which, after extensive development, saw substantial use in British aircraft engines of the 1940s, such as the Napier Sabre, Bristol Hercules, Centaurus, and the promising but never mass-produced Rolls-Royce Crecy, only to be supplanted by the jet engines.

Napier Lion

The Napier Lion was a 12-cylinder, petrol-fueled 'broad arrow' W12 configuration aircraft engine built by D. Napier & Son from 1917 until the 1930s. A number of advanced features made it the most powerful engine of its day and kept it in production long after other contemporary designs had been superseded. It is particularly well known for its use in a number of racing designs, for aircraft, boats and cars.

D. Napier & Son

D. Napier & Son Limited was a British engineering company best known for its luxury motor cars in the Edwardian era and for its aero engines throughout the early to mid-20th century.

Straight-four engine Inline piston engine with four cylinders

A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

Coventry Climax

Coventry Climax was a British forklift truck, fire pump, racing, and other specialty engine manufacturer.

British Racing Motors Formula One team

British Racing Motors (BRM) was a British Formula One motor racing team. Founded in 1945 and based in the market town of Bourne in Lincolnshire, it participated from 1951 to 1977, competing in 197 grands prix and winning seventeen. BRM won the constructors' title in 1962 when its driver Graham Hill became world champion. In 1963, 1964, 1965 and 1971, BRM came second in the constructors' competition.

Overhead camshaft engine

An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.


In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.

The British Racing Motors V16 was a supercharged 1.5 litre V-16 cylinder racing engine built by British Racing Motors (BRM) for competing in Formula One motor racing in the immediate aftermath of World War II. Designed in 1947 and raced until 1954–55, it produced 600 bhp (450 kW) at 12,000 rpm, although test figures from Rolls-Royce suggested that the engine would be able to be run at up to 14,000rpm.

Rolls-Royce Eagle (1944)

The Rolls-Royce Eagle Mk XXII was a British 24-cylinder, sleeve valve, H-block aero engine of 46 litre displacement. It was designed and built in the early-1940s by Rolls-Royce Limited and first ran in 1944. It was liquid-cooled, of flat H configuration with two crankshafts and was capable of 3,200 horsepower at 18 psi boost.

Motorcycle engine Engine that powers a motorcycle

A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.

Douglas (motorcycles) British motorcycle manufacturer

Douglas was a British motorcycle manufacturer from 1907–1957 based in Kingswood, Bristol, owned by the Douglas family, and especially known for its horizontally opposed twin cylinder engined bikes and as manufacturers of speedway machines. The company also built a range of cars between 1913 and 1922.

Lotus-Ford Twin Cam Motor vehicle engine

The Lotus-Ford Twin Cam is an inline-four petrol engine developed by Lotus for the 1962 Lotus Elan. A few early examples displaced 1.5 litres, but the majority were 1.55-litre (1557ml) engines. It used a Ford 116E iron cylinder block and a new aluminium cylinder head with dual overhead camshafts. The Twin Cam was used in a variety of vehicles until Lotus stopped production in 1973. It was succeeded by the Lotus 907 engine.

Napier Rapier 1920s British piston aircraft engine

The Napier Rapier was a British 16-cylinder H pattern air-cooled aero engine designed by Frank Halford and built by Napier & Son shortly before World War II.

Anzani 20 1910s French aircraft piston engine

The 1913 20-cylinder Anzani air-cooled radial engine was the first four row radial and one of the most powerful engines of its period, though few were used.

Fairey Prince (H-16)

The Fairey P.16 Prince was a British experimental 1,500 hp 16-cylinder H-type aircraft engine designed and built by Fairey in the late 1930s. The engine did not go into production.

The BRM P115 was a Formula 1 racing car built by British Racing Motors in 1967.


The BRM P83 was a Formula One racing car designed by Tony Rudd and built by British Racing Motors for the new engine regulations of 1966. It used a highly unorthodox H16 engine which caused problems throughout the car's racing life, and despite the best efforts of Graham Hill and Jackie Stewart took BRM from championship contenders to also-rans, leading it to be regarded alongside the BRM Type 15 as another embarrassing failure for the British marque caused by overcomplicated engineering.

German carmaker Porsche built several series of flat-eight engines of differing displacements over the course of many years. They were mainly used in Porsche's racing cars.


  1. Willoughby, Vic (1989). Classic motor cycles. Ivy Leaf. ISBN   0-86363-005-7.
  2. "BRM engines H16". Members.madasafish.com. Retrieved 2010-09-12.
  3. Taylor, Roger. "Lotus 43 & B.R.M. 83". Model Cars (July 1967): 327. Archived from the original on 26 March 2016.
  4. Taylor, Roger. "Lotus 43 & B.R.M. 83". Model Cars (July 1967): 328. Archived from the original on 4 March 2016.
  5. Wilson, Hugo (1995). "Brough Superior Dream" . The Encyclopedia of the Motorcycle. London: Dorling Kindersley. pp.  34–35. ISBN   0-7513-0206-6.
  6. "Quincy vs Konig". www.quincylooperracing.us. Retrieved 16 November 2019.