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, they appear to be in a vertical or horizontal letter H.

An H engine can be viewed as two flat engines, 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. The H configuration allows the building of multi-cylinder engines that are shorter than the alternatives, sometimes delivering advantages on aircraft. For race-car applications there is the disadvantage of a higher centre of gravity, not only because one crankshaft is located atop the other, but also because the engine must be high enough off the ground to allow clearance underneath for a row of exhaust pipes. The power-to-weight ratio is not as good as simpler configurations employing one crankshaft. There is excellent mechanical balance, especially desirable and otherwise difficult to achieve in a four-cylinder engine.

Two straight engines can be similarly joined to provide a U engine.

Subaru produces water-cooled flat-four and flat-six "Horizontally-opposed" engines that are marketed as H4 and H6 which are not to be confused with H-block engines. The naming scheme refers to engine description, similar to inline engines being named I4 or I6, rather than their appearance front-on.

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