Abstract: The present invention provides a rotary internal combustion engine comprising a stator, and a rotor journalled for rotation in said stator, said stator considered in a plane normal to its axis of rotation having a cross-section which is approximately an equilateral triangle with curvilinear sides, each side of said rotor having its external peripheral surface defining with the internal periphery of said stator a respective combustion chamber, the external peripheral surface of each side of said rotor including considered in the direction of rotation of said rotor an upstream surface portion having a relatively great radial component so as to receive maximum rotary thrust from the expanding combustion gases, an intermediate surface portion which is curvilinear and which extends from said upstream surface portion so as with said upstream surface portion to define relative to the internal periphery of the stator a part of the combustion chamber and receive less thrust than said upstream portion, and a downstream