Energy conversion devices employing gases and/or liquids as medium, constructed with a hollow, preferably toroidal cylinder having two ports and two, or more, identical ferromagnetic pistons moveably sealing the hollow cylinder. Movements of the pistons are electromagnetically selectable from the outside of the cylinder to either impart varying unidirectional forces on the medium and/or to influence a magnetic circuit placed outside the cylinder for the generation of motive or electromotive forces. In a pump, a compressor and an externally heated engine, the selective movement of the pistons always maintains a piston between the two ports. In an internal-combustion engine, addition is made of more pistons and one piston is selectively stopped and approached by the adjacent piston, compressing a gaseous mixture to effect combustion; electronic detection of the combustion allows the rapid electromagnetic release of the stopped piston and stops the adjacent piston which was compressing prior to the combustion.