rotary engine, internal-combustion engine whose cycle is similar to that of a piston engine, but which produces rotary motion directly without any conversion from reciprocating motion. A major problem associated with engines of this type is preventing the leakage of combustion gases. The only type of rotary engine currently considered to be of practical value is the Wankel engine (see internal-combustion engine internal-combustion engine, one in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power.
..... Click the link for more information. ). Although the gas turbine produces rotary motion directly, it is not generally considered a rotary engine because it functions differently.

rotary engine

Internal-combustion engine in which the combustion chambers and cylinders rotate with the driven shaft around a fixed control shaft to which pistons are attached. The gas pressures of combustion are used to rotate the shaft. In the Wankel engine, the most fully developed and widely used rotary engine, a triangular rotor 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.

rotary engine [′rōd·ə·rē ′en·jən]

(mechanical engineering)

A positive displacement engine (such as a steam or internal combustion type) in which the thermodynamic cycle is carried out in a mechanism that is entirely rotary and without the more customary structural elements of a reciprocating piston, connecting rods, and crankshaft.

Rotary engine

Internal combustion engine that duplicates in some fashion the intermittent cycle of the piston engine, consisting of the intake-compression-power-exhaust cycle, wherein the form of the power output is directly rotational.

Four general categories of rotary engines can be considered&colon; (1) cat-and-mouse (or scissor) engines, which are analogs of the reciprocating piston engine, except that the pistons travel in a circular path; (2) eccentric-rotor engines, wherein motion is imparted to a shaft by a principal rotating part, or rotor, that is eccentric to the shaft; (3) multiple-rotor engines, which are based on simple rotary motion of two or more rotors; and (4) revolving-block engines, which combine reciprocating piston and rotary motion. See Automobile, Combustion chamber, Diesel engine, Gas turbine, Internal combustion engine, Otto cycle