The invention relates to a device (1) for converting waste heat of an internal combustion machine (2) into mechanical energy. The device comprises a piston machine (3) that converts the waste heat of the internal combustion machine (2) during an OCR process into mechanical energy which can be transmitted onto a shaft (7) driven by the internal combustion machine (2). Furthermore, a variable gear (6) is provided via which the piston machine (3) transmits the mechanical energy onto the shaft (7) of the internal combustion machine (2). The variable gear (6) translates an initial rotational speed of the piston machine (3) into a rotational speed of the shaft (7) driven by the internal combustion machine (2). In this way, the ORC process can be carried out in an optimal manner.