Abstract: A rotary engine including a pair of concentric rotors one being rotatable within the other, one of said rotors being driven by the output shaft of the engine and the other rotor connected to drive said output shaft. The outer rotor having radially inwardly extending abutments and the inner rotor having radially outwardly extending abutments, the abutments of the two rotors defining therebetween a plurality of expansible gas chambers. The inner rotor having means to hold same stationary so as to provide a reaction member for said outer rotor and intermittent drive means driven by said output shaft and adapted to intermittently drive said inner rotor to create relative rotation between said rotors whereby expansion and contraction of said chambers will take place.