This paper presents a numerical model of the fluid dynamic processes and mechanical losses of a Wankel-type expansion device for microgeneration purposes. The prototype was built at the University of Pisa using the rotating parts of a gasoline-fueled Wankel engine. Considering the geometrical features of the prototype, the model simulates the fluid-dynamic losses across the valves, the thermal exchange with the outer ambient and the mechanical losses of the whole mechanism. The numerical results were then validated by the comparison with the pressure cycle obtained at the dynamic test bench using compressed air.