internal-combustion engine in which the combustion chambers and cylinders rotate with the driven shaft around a fixed control shaft to which pistons (piston and cylinder) are affixed; the gas pressures of combustion are used to rotate the shaft. Some of these engines have pistons that slide in toroidal (doughnut-shaped) cylinders; others have single- and multiple-lobed rotors. Early rotary engines were used in World War I aircraft. They were air-cooled, with cylinders arranged circularly around a crankshaft fastened rigidly to the fuselage. The propeller was attached directly to the circular frame on which the rotating cylinders were mounted. Various inefficiencies in these engines led to their abandonment after the war.

After World War II the development of a new kind of rotary engine reawakened interest. The Wankel is the most fully developed and widely used of the rotary engines. In the Wankel engine the rotor, in the form of an equilateral triangle, rotates with an orbital motion in a specially shaped casing and forms rotating crescent-shaped combustion chambers between its sides and the curved wall of the casing. The three apexes of the rotor are provided with spring-loaded sealing plates that maintain a continuous sliding contact with the concave inner surface of the casing, and the combustion chambers increase and decrease successively in size as the rotor turns. The fuel charge from a carburetor enters the chamber through an intake port, is compressed as the size of the chamber is reduced by the rotation of the rotor, and at the appropriate time is ignited by a spark plug.

The Wankel engine was first tested for use in automobiles in 1956. It has since come to be used for such industrial applications as driving air compressors, where small, light-weight, high-speed engines with mechanical simplicity are needed. See also gasoline engine.