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For the Saab "H" engine (a straight-4) see Saab H engine. For the EMD engine, see EMD SD90MAC.

An H engine (or H-block) is an engine configuration in which the cylinders are aligned so that if viewed from the front appear to be in a horizontal letter H.

An H engine can be viewed as two flat engines, one atop 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 engines where their small size allows for better aerodynamics - see Lycoming. The U engine is a similar concept, except that it uses two straight engines.

Contents 1 Examples 1.1 Road vehicles 1.2 Aircraft engines 2 Other uses of H term

The H configuration is very uncommon. Known examples are:

• The British Racing Motors H-16 Formula One engine

this was a major failure. This engine was powerful but heavy and unreliable, had low torque and a high center of gravity. 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.

• 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.

• The Lycoming H 2470 hyper engine, that did not go into production.
• Napier & Son
  • Rapier H-16
  • Dagger H-24
  • Sabre H-24, 36.7 litres 3,500 hp
  • Rolls-Royce Eagle 22 H-24 46.2 litres, 3,200 hp

Unlike the BRM and the Lycoming and the Eagle, the Sabre eventually matured into a superb design.

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
Straight	Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V	2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat	2, 4, 6, 8, 10, 12, 16
W	8, 9, 12, 16, 18
Other inline	H, VR, Opposed, U (Square), X
Other	Radial, Rotary, Pistonless (Wankel)