Abstract: The invention relates to energy conversion devices for converting between fluid flow energy and rotational mechanical energy, utilizing "Coriolis" forces which are produced when there is a relative motion velocity non-parallel to an instant rotation axis direction. In one embodiment, two piston members move around in an annular chamber of a housing member journalled on a support member for rotation relative thereto about a first axis extending in a diametral plane of the annular chamber, the support member being journalled for rotation about a second axis intersecting and transverse to the first axis, the second axis being a stationary axis forming the principal axis of rotation. With pressurized fluid applied to move the pistons around, the device operates as an engine. In a second embodiment, means are provided for effecting combustion of an air-fuel mixture within the annular chamber, to provide an internal combustion engine.