Thermodynamic analysis of the isentropic and polytropic expansion profiles of typical working fluids was carried out in order to design a highly efficient displacement-type expander for a low-temperature Rankine cycle. First, expansion profiles were analyzed for three typical working fluids: HFC245fa, ammonia, and supercritical CO 2 . The hot-side temperature ranged from 60 ° to 120 °C, and the cold-side temperature was 10 °C. In the analysis, isentropic and polytropic expansion processes were assumed to behave thermodynamically. In the analysis results, we noted similarities among the expansion profiles for different hot-side temperatures. This similarity allowed us to introduce the unique concept of a variable mechanism for expansion profile fitting in displacement-type expanders. This variable expansion mechanism can be achieved by simply adjusting the position of the inlet and/or outlet port of the expander.