Abstract: An engine for rotating an output shaft includes a housing which defines an inner cylindrical surface coaxial with the output shaft. A roller member is disposed within the housing and includes an inner cylindrical bearing surface and an outer cylindrical surface spaced from the inner surface of the housing. Longitudinal axis of the rolling member is radially offset from the axis of the housing and the shaft. A mechanism is provided for preventing relative rotation and movement between the housing and the rolling member, and a plurality of chambers are defined between the housing and the rolling member. Devices are provided for selectively introducing expansive gas or fluid into the chamber in rotationally sequential fashion, and devices are further provided for exhausting expanding gas or fluid from the chambers in rotationally sequential fashion to urge the roller member away from the housing and expand the chambers at the site of the expansive gas or fluid introduction.