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H engine

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.

The H configuration is therefore very uncommon. Known examples are:

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