Abstract: The transcritical carbon dioxide rolling piston expressor (expander-compressor) is developed to replace the throttling valve in the CO2 transcritical cycle for the improvement of the performance efficiency of the CO2 transcritical cycle and decreasing larger throttling losses. The design consideration and the feature of the rolling piston type expressor are presented. The torque of the shaft of this type of expressor are calculated and analyzed. The researches show that the value and direction of forces on the shaft produced by the fluid in two cylinders are different due to the opposite flow process in the expander from that in the compressor and the proper phase of two–rolling piston can allow the expressor to take over the low-vibration characteristics. So the optimum phase of two cranks should be taken into account in the design. In addition, the analytical results also show that the shaft between two cranks is the part that is easy to break.

Key words: Carbon dioxide, Expressor, Transcritical-cycle, Curve-face gear, Design guideline, Tooth profile, Transmission model