X 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. 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 an 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:

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Wikipedia