A pair of tangential rotors (14, 16) are provided on separate shafts dependently rotatable in a housing, one having a vane (60) and the other a notch (70) for allowing passage of the vane, to form a fluid-tight segmented annular region through which the vane moves. A valve admits (106) a mass of high pressure compressible fluid to the region through a triangular port f98) for expansion, or from the region after compression, the mass of fluid being confined in a portion of the region between the vane and the surface of the notched rotor and changing in pressure because of the change in arcuate length and thus volume of the confined region portion. Multiple pairs of rotors may be included on the one rotor and two notches on the other. Two vaned rotors may cooperate with one notched rotor, the vaned rotors being on separate shafts.