For the Saab "H" engine (a straight-4) see Saab 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 - see Lycoming.

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

• The British Racing Motors H-16 Formula One engine, which 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".
• The Lycoming H 2470 hyper engine, that did not go into production.
• The Napier Rapier , Dagger and Sabre airplane engines. Unlike the BRM and the Lycoming, the Sabre eventually matured into a superb design.

Subaru produce 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.