A prime mover for transforming energy from the pressure form to the mechanical form. The prime mover comprises a pair of toothed rotors which engage one another so as to be self-timing. Each rotor has a first portion of relatively large diameter and a second portion of relatively small diameter, each of the portions extending approximately 180.degree.. The rotors are enclosed in respective intersecting cylindrical chambers in a housing, with the tips of the teeth of the small diameter portion of each rotor being displaced radially inwardly from the peripheral surface of the corresponding chamber, and with the tips of the teeth of the large diameter portion of each rotor being directly adjacent to the inner surface of the corresponding chamber. First and second sealing members are mounted in the housing, and these engage the tips of successively passing teeth of the large diameter portion of each rotor, as the rotors turn. A drive fluid is introduced into an inlet in the housing located at the common tangent of the two rotors, and the drive fluid exits through a diametrically opposed aligned outlet.