A method and a device for recovering heat from an engine and for converting the heat into mechanical energy using an expansion machine. A heat accumulator fluid is guided in a primary circuit by means of a primary pump and is firstly heated by the waste heat of the combustion engine by means of a heat exchanger, is transferred into a heat accumulator and recirculated to the first heat exchanger, and, secondly, the heat accumulator fluid is guided in a secondary circuit by said heat accumulator fluid being extracted in the vapor state from the heat accumulator and being supplied to the expansion machine, and being condensed by means of a condenser downstream thereof and being recirculated into the heat accumulator by means of a secondary pump. The primary circuit of the heat accumulator fluid is connected to the secondary circuit exclusively via the heat accumulator.