An axial engine (sometimes known as a barrel engine or Z-crank engine) is a type of reciprocating engine with pistons arranged around an output shaft with their axes parallel to the shaft. Barrel refers to the cylindrical shape of the cylinder group (result of the pistons being spaced evenly around the central crankshaft and aligned parallel to the crankshaft axis) whilst the Z-crank alludes to the shape of the crankshaft.

As a cam engine, an axial engine can use either a swashplate or a wobble plate to translate the piston motion to rotation. A wobble plate is similar to a swashplate, in that the pistons press down on the plate in sequence, imparting a lateral moment that is translated into rotary motion. This motion can be simulated by placing a compact disc on a ball bearing at its centre and pressing down at progressive places around its circumference. The difference is that while a wobble plate nutates, a swash-plate rotates. An alternative design, the Rand cam engine, replaces the plate with one or more sinusoidal cam surfaces. Vanes mounted parallel to a shaft mounted inside a cylindrical 'barrel' that are free to slide up and down ride the sinuous cam, the segments formed by rotor, stator walls and vanes constituting combustion chambers. In effect these spaces serving the same purpose as the cylinders of an axial engine, and the sinuous cam surface acts as the face of the pistons. In other respect this form follows the normal cycles of internal combustion but with burning gas directly imparting a force on the cam surface, translated into a rotational force by timing one or more detonations. This design eliminates the multiple reciprocal pistons, ball joints and swash plate of a conventional 'barrel' engine but crucially depends on effective sealing provided by sliding and rotating surfaces.

The key advantage of the axial design is that the cylinders are arranged in parallel around the output/crank shaft in contrast to radial and inline engines, both types having cylinders at right angles to the shaft. As a result, it is a very compact, cylindrical engine, allowing variation in compression ratio of the engine while running. In a swashplate engine the piston rods stay parallel with the shaft, and piston side-forces that cause excessive wear can be eliminated almost completely. The small-end bearing of a traditional connecting rod, one of the most problematic bearings in a traditional engine, is eliminated.

While axial engines are challenging to make practicable at typical engine operating speeds some cam engines have been tested that offer extremely compact size (approximating to a six-inch (150mm) cube) yet producing approximately forty horsepower at c 7000 rpm, useful for light aerial applications. The attraction of lightweight and mechanically simple (far fewer major moving parts, in the form of a rotor plus twelve axial vanes forming twenty-four combustion chambers) engines, even with a finite working life, have obvious application for small unmanned aircraft.

In 1911 the Macomber Rotary Engine Company of Los Angeles marketed one of the first axial internal-combustion engines, manufactured by the Avis Engine Company of Allston, Massachusetts. A four-stroke, air-cooled unit, it had seven cylinders and a variable compression ratio, altered by changing the wobble-plate angle and hence the length of piston stroke. It was called a "rotary engine", because the entire engine rotated apart from the end casings.

Ignition was supplied by a Bosch magneto directly driven from the cam gears. The high voltage current was then taken to a fixed electrode on the front bearing case, from which the sparks would jump to the spark plugs in the cylinder heads as they passed within 1⁄16 inch (1.6 mm) from it. According to Macomber's literature, it was "guaranteed not to overheat".

The engine was claimed to be able to run at 150 to 1,500 rpm. At the normal speed of 1,000 rpm, it reportedly developed 50 hp. It weighed 230 pounds (100 kg) and it was 28 inches (710 mm) long by 19 inches (480 mm) in diameter.

Pioneer aviator Charles Francis Walsh flew an aircraft powered by a Macomber engine in May 1911, the "Walsh Silver Dart".