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.
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)|
|Stroke cycles |
One • Two • Four • Six •
|Engine types |
Gas turbine • Piston • Jet • Rocket engine • Steam engine • Stirling engine • Tschudi• Twingle
Rotary • Wankel • Free-piston • Britalus • Coomber • Swing-piston • Orbital • Quasiturbine
Cylinder head porting • D slide • Four-stroke • Manifold • Multi • Piston • Poppet • Sleeve
|Piston layouts |
Single cylinder • Straight • Opposed • Flat • V • W • H • Deltic • Radial • Rocket engine nozzle • Rotary • Stelzer • Controlled Combustion • Bourke
|Motion mechanisms |
Cam • Connecting rod • Coomber rotary • Crank • Crank substitute • Crankshaft • Linkages (Evans • Peaucellier-Lipkin • Sector straight-line • Watt) • Double acting/differential cylinder