An X engine is a piston engine comprising twinned V-block engines horizontally-opposed to each other. Thus, the cylinders are arranged in four banks, driving a common crankshaft. Viewed head-on, this would appear as an X. X engines were often coupled engines derived from existing powerplants.
This configuration is extremely uncommon, primarily due its weight and complexity as compared to a radial engine. However, it was more compact (per number of cylinders) than a V-engine.
Most examples of X engines are from the World War II era, and were designed for large military aircraft. The majority of these are X-24's based on existing V-12's. The following are examples of this engine type:
- Ford, as an X-8 prototype during the 1920s that led the way to the company's eventual Flathead V-8.<ref>Hemmings Auto Blogs » Blog Archive » SIA Flashback - Experimental Ford Engines</ref><ref>Eli Apolzon and daughter Ruth Family Page * * * Austin, Texas - Henry Ford's X-8 engine and how it will be used to raise dollars for Autistic and other Special Needs children</ref>
- Daimler Benz DB 604, developed for the Luftwaffe’s Bomber B program. Development suspended.
- Isotta-Fraschini Zeta R.C. 24/60, developed for the Caproni F6 fighter, but never fully completed before Italy’s surrender in 1943.
- Rolls-Royce Vulture, based on two Peregrines and the powerplant of the ill-fated Avro Manchester bomber and the Hawker Tornado fighter.
- Rolls-Royce Exe, an air-cooled sleeve valve prototype engine.
- Napier Cub, a water cooled X-16 engine of the 1920s, which powered the prototype Blackburn Cubaroo torpedo bomber.
- Honda is said to have experimented with an X-32 engine configuration in the 1960s for their Formula One racing efforts, but abandoned the design as being too complex and unreliable.