An H engine is a

piston engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...

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 The Napier Sabre is a British H engine, H-24-cylinder, coolant, liquid-cooled, sleeve valve, piston aircraft engine, aero engine, designed by Frank Halford, Major Frank Halford and built by D. Napier & Son during World War II. The engine evolv ...

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 BRM 16-cylinder H engine, and an 8-cylinder H engine was used for powerboat racing in the 1970s.

Design

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 Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and Balance shaft#Overview, balance shafts, to prevent unpleasant and potentially dam ...

which results in less vibration (which is difficult to achieve in many other types of four-cylinder engines). 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. The U engine layout uses a similar concept, placing two straight engines side-by-side.

History

Aircraft engines

* Lycoming, US ** Lycoming H-2470 H-24 " hyper engine" (1930s) *

Fairey Aviation The Fairey Aviation Company Limited was a British aircraft manufacturer of the first half of the 20th century based in Hayes, Hillingdon, Hayes in Middlesex and Heaton Chapel and RAF Ringway in Cheshire that designed important military aircraft ...

, UK ** Fairey Prince (H-16) (1939) – ** Fairey Monarch (1939) – H-24 * Klöckner-Humboldt-Deutz DZ 720 – H-32, 102.9 litres diesel * D. Napier & Son, UK. ** Napier Rapier (1929) – H-16 air-cooled vertical, 8.83 litres ** Napier Dagger (1934) – H-24, air-cooled vertical, 16.85 litres , a development of the Rapier **

(1938) – H-24, water-cooled horizontal sleeve valves, 36.7 litres . *

Pratt & Whitney Pratt & Whitney is an American aerospace manufacturer with global service operations. It is a subsidiary of RTX Corporation (formerly Raytheon Technologies). Pratt & Whitney's aircraft engines are widely used in both civil aviation (especially ...

, US ** XH-2240 - H-24, liquid-cooled, ** XH-2600 - H-24, liquid-cooled, ** XH-3130 - H-24, liquid-cooled, ** XH-3730 - H-24, liquid-cooled, * Rolls-Royce Eagle (1944) – H-24, 46.2 litres, .

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. 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. File: BRM H16 engine.jpg , BRM H-16 engine (64-valve version)

Motorcycle engines

The Brough Superior Golden Dream motorcycle, first shown in 1938. 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 Earls Court Exhibition Centre was a major international exhibition and events venue in London, England. At its peak it is said to have generated a £2 billion turnover for the economy. It replaced exhibition and entertainment grounds, original ...

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, 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".

References

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