Abstract: This invention uses a body force to trap the liquid component of a fluid in local potential minimums in a continuous cavity in an expander. Shaping of the cavity traps the vapor components of the fluid between these "liquid pistons". In the external combustion embodiment, the cavities have a continuously increasing cross section. Therefore, the surface pressure of the fluid generates an unbalanced force on the containing expander. The cavities are shaped such that components of the unbalanced forces combine to generate a torque, which rotates the expanders. In the preferred embodiment, some of this rotational force is fed back by gearing to revolve the expanders around a rotor axis. This revolving generates a centrifugal body force on the fluid in the expander cavities. In the internal combustion embodiment, the expander stages are preceded by decreasing cross section stages which compress the fuel air mixture. The mixture is ignited and expands in the following stages.