X engine

An X engine is a piston engine with four banks of cylinders around a common crankshaft, such that the cylinders form an "X" shape when viewed front-on.

The advantage of an X engine is that it is shorter than a V engine of the same number of cylinders, however the drawbacks are greater weight and complexity as compared to a radial engine. Therefore, the configuration has been rarely used.

Several of the X engine designs were based on combining two V engines.