Design and Simulation of an SCO2 Opposing Piston Polygon Expansion Engine

The expansion engine described herein is designed to operate on a waste heat driven Rankine cycle utilizing super critical carbon dioxide (SCO2) as the working fluid. The end use for this system is as a means of recapturing waste heat generated in the power production process. In this capacity, the system will utilize leftover waste heat to generate more usable power, thereby increasing the efficiency of the power plant at which it is installed. The polygon engine opposed piston expander design was chosen due to its higher possible power output when compared to more typical layouts. The SCO2 power cycle was chosen due to the high efficiency of supercritical power cycles. The system presented herein was found to have a power to weight ratio of 1.979 HP/lbm with a maximum output power of 190 HP at about 1000 RPM and roughly 970 lbf-ft of torque. The optimum valve timing was found to consist of an intake opening angle of 2.5°, an intake closure angle of 110°, an exhaust opening angle of 170°, and an exhaust closure angle of 360°. The existing cycle states with inlet at 200 °C/20 MPa/258.82 kg/m3 and outlet at 163 °C/12.7 MPa/176.78 kg/m3 were found to be ideal. Furthermore, the existing piston design consisting of a 3-inch piston with a 2-inch stroke was also found to be acceptable.