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[[Image:BRM H16 engine.jpg|thumb|The [[British Racing Motors|BRM]] H16 [[Formula One]] engine in its final, 64-[[Poppet valve#Internal combustion engine|valve]] incarnation.]]
An '''H engine''' (or H-block) is an [[engine configuration]] in which the [[cylinder (engine)|cylinder]]s are aligned so that if viewed from the front, they appear to be in a vertical or horizontal letter [[H]].

An H engine can be viewed as two [[flat engine]]s, one atop or beside the other. The "two engines" each have their own [[crankshaft]], which are then geared together at one end for power-take-off. This leads to a worse [[power-to-weight ratio]] than simpler configurations with only one shaft. The only obvious advantage of the H configuration is to allow the building of reasonably short engines with more than 12 cylinders, their compact size being useful as [[aircraft engine]]s where their small size allows for better [[aerodynamics]] - see [[Lycoming engine|Lycoming]]. The [[U engine]] is a similar concept, except that it uses two [[straight engine]]s.

==Examples==
The H configuration is very uncommon. Known examples are:

===Road vehicles===
[[Image:BRM H16 in P83.jpg|thumb|right|250px|A BRM H16 engine, mounted in the back of a [[BRM P83]] [[Formula One]] car.]]
*The [[British Racing Motors]] (BRM) H-16 Formula One engine; this was a major failure. The engine was powerful, but the design was hampered not only by its low [[torque]] and a high [[center of gravity]] but also was heavy, unreliable and very complex, with gear-driven twin overhead cams for each of four cylinder heads, two gear-coupled crankshafts, and mechanical fuel injection. [[Jackie Stewart]] is believed to have said "This piece of metal is better used as a ship's anchor than as a power plant". Despite its poor reputation [[Jim Clark]] managed to win the 1966 US Grand Prix with a [[Lotus 43]] powered by this engine. [http://members.madasafish.com/~d_hodgkinson/brm-e-H16.htm]
*The [[Brough Superior]] 1000cc Golden Dream motorcycle, first shown in 1938. This was an H-4 design but only a few units were produced in early 1939.

===Aircraft engines===
*The [[Lycoming Engines|Lycoming]] [[H 2470]] [[hyper engine]],  that did not go into production.
*[[Napier & Son]]
** [[Napier Rapier|Rapier]] H-16, 8.83 litres 340 hp
** [[Napier Dagger|Dagger]] H-24, 16.85 litres 890 hp
**[[Napier Sabre|Sabre]] H-24, 36.7 litres 3,500 hp
*[[Rolls-Royce Eagle (1944)|Rolls-Royce Eagle]] H-24, 46.2 litres, 3,200 hp
*[[Fairey Aviation]] [[Fairey H-16]] "Prince", 1,500 hp
Unlike the BRM, the Lycoming and the Eagle, the Sabre was eventually developed to be a successful engine.


==Other uses of H term==

[[Subaru]] produces water-cooled [[flat-4]] and [[flat-6]] "Horizontal" engines that are marketed as H-4 and H-6, despite the fact that their configuration has nothing to do with a real H engine.

{{Piston engine configurations}}

[[Category:Boxer engines]]