a method and apparatus for converting radiation power , especially solar radiation , into mechanical power . a piston is provided that is movable in a cylinder accompanied by the formation of a variable - sized working chamber . during a compression stroke , the piston is used to press a working medium , accompanied by a reduction in the size of the working chamber , into a radiation absorber / heat exchanger that is disposed directly in the working chamber , receives radiation energy , and essentially completely accommodates the working medium when the working chamber is at a minimum . during a working stroke , the working medium , which is compressed by the reduction in size of the working chamber and which is heated up by absorbing thermal energy from the radiation absorber / heat exchanger , is used to move the piston , upon expansion of the working medium , into a position that enlarges the working chamber . at least a portion of the working medium is exchanged following the working stroke and prior to a subsequent compression stroke .