An X engine is a piston engine design that places two V-type engines opposite each other while sharing a single crankshaft. When viewed from the front, the two V-type engines become an X engine. Designed and intended to save space in military aircraft, most X engine prototype designs were abandoned in favor of V-type or radial type engine designs due to mechanical unreliability. The height of X engine curiosity came during the World War II era and focused primarily on converting existing V-12 engines into V-24 aircraft engines for use in both fighters and heavy bombers. The Honda racing division is rumored to have been working on a V-32 racing engine in the 1960s that was slated to race in the Formula 1 series, however, the engine was never produced for racing purposes.
The most common method of producing more power from a piston engine is to increase the engine's displacement. In a V-type engine configuration, adding cylinders accomplishes this, however, it also increases the engine's size. By changing from a V-type engine design to an X engine design, the designers effectively doubled the displacement of the piston-driven engine without adding any length to the engine package. Weight was a negative factor with the X engine prototypes, and development of the turbofan aircraft engine effectively put an end to further development of any X-type engine packages.
Rolls-Royce developed an X-24 engine package which was based loosely on the V-12 Peregrine engine used in British World War II fighter aircraft. The V-12 was a troublesome engine, and using it to create the X-24 only proved to double the problems. The intention of the X engine was to increase the horsepower of the Peregrine V-12 from 750 to 1,500 by doubling the engine's displacement. The creation of the Rolls-Royce Merlin V-12, which produced 1,100 horsepower, all but ended the further development of the X-24 by Rolls-Royce.
One of the side effects of war is often the increase in technology required to build bigger and better weapons. The X engine was part of this technology, with nearly all waring countries in World War II attempting to develop an X-type engine. The invention of the gas turbine engine all but ended the need for further refinement of an X-type piston engine. Turbocharging as well as supercharging replaced the need to increase the displacement of a piston engine to create higher horsepower numbers.