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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. It was more compact (per number of cylinders) than a V-engine, however. Shorter crankshafts relative to a inline or V design also appealed to early 20th century engineers like Henry Ford, given the less developed metallurgical technology of the time.
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-24s based on existing V-12s. 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.
Daimler-BenzDB 604, developed for the Luftwaffe’s Bomber B program. Development suspended.
Isotta-FraschiniZeta R.C. 24/60, developed for the Caproni F6fighter, 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 Manchesterbomber and the Hawker Tornado fighter.
Rolls-Royce Exe, an air-cooledsleeve valve prototype engine.
Napier Cub, a water-cooled X-16 engine of the 1920s, which powered the prototype Blackburn Cubarootorpedo 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.