A volumetric expander (20) configured to transfer a working fluid and generate useful work includes a housing. The housing includes an inlet port (24) configured to admit relatively high-pressure working fluid and an outlet port (26) configured to discharge to a relatively low-pressure working fluid. The expander also includes first and second twisted meshed rotors (30,32) rotatably disposed in the housing and configured to exp/and the relatively high-pressure working fluid into the relatively low-pressure working fluid. Each rotor has a plurality of lobes, and when one lobe of the first rotor is leading with respect to the inlet port, one lobe of the second rotor is trailing with respect to the inlet port. The expander additionally includes an output shaft (38) rotated by the relatively high-pressure working fluid as the fluid undergoes expansion. A system for generating work using the expander in a Rankine cycle is also disclosed.