The H engine is a type of internal combustion engine oriented so that its cylinders appear to form the letter "H." The design of this engine makes it thin and suitable for long, compact spaces where a large number of cylinders can be implemented to produce lots of pulling power. It is often used in locomotives and propeller-driven aircraft. The term "H engine" can also refer to certain automobile engines with an "H" designation, although these bear no design relation to the H-shaped engine.
Engines often take their classification from the orientation of their cylinders. A V-8 engine, for example, has two rows of four cylinders shaped in a "V" formation. In the H engine, each leg of cylinders has an independent crankshaft that is geared into the other engine half's crankshaft, forming the "H" shape. Also called H-block, the true "H" shape is generally considered unsuitable for automobile use due to its shape and the fact it has a low torque ratio created by having two crankshafts, making for slow initial acceleration.
A tractor-maker, Farmall®, as well as several automakers, including Honda®, Subaru® and Saab®, have created engines they call an H engine. All of these manufacturers use the "H" to designate inline four-cylinder, and sometimes six-cylinder, engines. Inline means that all of the cylinders sit in a straight line, as opposed to a true H where the cylinders sit in two rows. In this case, engines are designated not by shape, but by alphabetical lettering. For Honda®, the H followed the F and was replaced by the K. Saab® first issued the H as a replacement of the B engine in 1981.
The most well known of the automobile H engines were those produced by Honda®, produced between 1993 and 2001. The first of this type engine, styled as the H22™, was used in the Prelude® and some Accord® models. The Honda® "H" series was considered a high performance engine and was also used in Honda® racing cars. Unmodified, the base H22™ created 190 to 220 horsepower and 156 to 163 ft-lb of torque with a top rpm of 7,500.
Historically, the H-shaped engine's most notable usage was in the 1930s and 1940s in propeller-driven aircraft, as it was considered a compact, aerodynamic engine. The most famous use of the H engine in aircraft was in World War II British fighters. The Napier Sabre™, a 24-cylinder, H engine producing 3,500 rpm, was used in Typhoon and Tempest fighter planes. The 24-cylinder Rolls-Royce® Eagle 22™, producing 3,200 rpm, was used in the Wyvern fighter/torpedo bomber.