PURPOSE: To effectively convert thermal energy into mechanical torque by providing two concentric rotors with pressure receiving plates which slide on and seal an annular cylinder surface, and conducting a series of cycle consisting of compression, expansion, exhaust, and suction by varying the center-to-center distance in the circumferential direction among the pressure receiving plates.

CONSTITUTION: When an output shaft 23 rotates, the motion of the shaft 23 is transmitted to an intermediate shaft sprocket 21 via a chain 26, and to an intermediate shaft sprocket 22 via a chain 27. In addition, the rotation of the intermediate shaft sprocket 22 is transmitted to a rotor 14 via an intermediate shaft 19, an intermediate shaft gear 20, and a rotor shaft gear 16. In addition, the rotation of the intermediate shaft sprocket 21 is transmitted to a rotor 12 via an intermediate shaft 17, an intermediate shaft gear 18, and a rotor shaft gear 13. Accompanied with this, a distance between both pressure receiving plates 12a and 14a is varied, and, when the distance becomes minimum, the confined, fresh air is compressed and at the same time both ignition and combustion are carried out by means of a fuel injection valve 30 and an ignition plug 31. Thus, the power is generated, and the output shaft 23 is started and accelerated. At the same time, in the position where the distance between both pressure receiving plates 12a and 14a becomes maximum, the rotor pressure receiving plate 14a is made pass an exhaust port 28 so as to start scavenging.