An H engine is a

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common feat ...

comprising two separate

flat engines Flat or flats may refer to: Architecture * Flat (housing), an apartment in the United Kingdom, Ireland, Australia and other Commonwealth countries Arts and entertainment * Flat (music), a symbol () which denotes a lower pitch * Flat (soldier), ...

(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-24-cylinder, liquid-cooled, sleeve valve, piston aero engine, designed by Major Frank Halford and built by D. Napier & Son during World War II. The engine evolved to become one of the most powerful inline p ...

series. A variation on the "H" theme were the

Fairey Prince (H-16) The Fairey P.16 Prince was a British experimental 1,500 hp (1,118 kW) 16-cylinder H-type aircraft engine designed and built by Fairey in the late 1930s. The engine did not go into production. Design and development The Prince P.16 was a r ...

& 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 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 engine The straight or inline engine is an internal combustion engine with all cylinders aligned in one row and having no offset. Usually found in four, six and eight cylinder configurations, they have been used in automobiles, locomotives and aircraf ...

s side-by-side.

History

Aircraft engines

* Lycoming, US ** Lycoming H-2470 H-24 "

hyper engine The hyper engine was a 1930s study project by the United States Army Air Corps (USAAC) to develop a high-performance aircraft engine that would be equal to or better than the aircraft and engines then under development in Europe. The project goa ...

" (1930s) 2,300 hp * Fairey Aviation, UK **

(1939) – 1,500 hp **

Fairey Monarch The Fairey P.24 Monarch was a British experimental 2,000 hp (1,490 kW) class H-24 aircraft engine designed and built by Fairey in the late 1930s. The engine did not go into production. Design and development The P.24 Monarch was a 24-cylin ...

(1939) – H-24 2,240 hp * Klöckner-Humboldt-Deutz DZ 720 – H-32, 102.9 litres diesel * D. Napier & Son, UK. **

Napier Rapier 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. Design and development The Rapier was the first of Napier's H cylinder engines. Th ...

(1929) – H-16 air-cooled vertical, 8.83 litres 340 hp ** Napier Dagger (1934) – H-24, air-cooled vertical, 16.85 litres 890 hp, a development of the Rapier **

(1938) – H-24, water-cooled horizontal sleeve valves, 36.7 litres 3,500 hp. *

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

, 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, 3,200 hp.

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 In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...

, 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) File: BRM H16 in P83.jpg , BRM H-16 engine in a 1966 BRM P83

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 just west of central London. At its peak it is said to have generated a £2 billion turnover for the economy. It replaced exhibition and entertainment grounds, ...

in 1948 and again in 1951. This was replaced by a

flat-four engine A flat-four engine, also known as a horizontally opposed-four engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, ...

d 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 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; 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