X engine

A simplified diagram of an X24 engine

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. Since 24-cylinder models were the predominant configuration, it is most likely (but not known) that the angles between banks would have been 60-120-60-120, and not 90-90-90-90, since V-12 engines most commonly use a 60-degree bank to improve engine vibration characteristics. X-engines were often coupled engines derived from existing powerplants.

Isotta-Fraschini Zeta R.C. 24/60

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. In practice, the X-engine inherited the drawbacks of both inline and radial designs rather than their advantages. Overheating was a perpetual problem for X engines, and a major reason for the failure of the type.

Most examples of X-engines are from the World War II era, and were designed for large military aircraft. The following are examples of this engine type:

• Isotta-Fraschini Zeta R.C. 24/60, an X-24 developed for the Caproni F6 fighter, but never fully completed before Italy’s surrender in 1943.
• Daimler Benz DB 604, an X-24 developed for the Luftwaffe’s Bomber B program. Development suspended.
• Rolls-Royce Vulture, an X-24 based on two Peregrines and the powerplant of the ill-fated Avro Manchester bomber and the Hawker Tornado fighter.
• Napier Cub, a water cooled X-16 engine of the 1920's, which powered the prototype Blackburn Cubaroo torpedo bomber.

Piston engine configurations
Straight	Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V	2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat	2, 4, 6, 8, 10, 12, 16, H
W	8, 9, 12, 16, 18
Other inline	H, VR, Opposed, U (Square), X
Other	Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)