Devices and methods for moving a working fluid through a controlled thermodynamic cycle in a positive displacement fluid-handling device (20, 20′, 20″) with minimal energy input include continuously varying the relative compression and expansion ratios of the working fluid in respective compressor and expander sections without diminishing volumetric efficiency. In one embodiment, a rotating valve plate arrangement (40, 42, 44, 46) is provided with moveable apertures or windows (48, 50, 56, 58) for conducting the passage of the working fluid in a manner which enables on-the-fly management of the thermodynamic efficiency of the device (20) under varying conditions in order to maximize the amount of mechanical work needed to move the target quantity of heat absorbed and released by the working fluid. When operated in refrigeration modes, the work required to move the heat is minimized. In power modes, the work extracted for the given input heat is maximized.