A variable displacement hydrostatic power unit (7) is connected with an internal combustion engine (2). The power unit (7) operates as a pump to deliver hydraulic fluid to at least one consumer (V) and operates as a motor as a hydraulic starter for the internal combustion engine (2). Hydraulic fluid from a hydraulic accumulator (25) is supplied to the power unit (7) operated as a motor. The power unit (7) includes a spring device (55) which, when the internal combustion engine (2) is shut off, actuates a displacement volume control device (50) into a position with the maximum displacement volume. When the power unit (7) is actuated with hydraulic fluid from the hydraulic accumulator (25), the internal combustion engine (7) is immediately started by the power unit (7) without the need for an immediately preceding adjustment of the displacement volume control device (50) of the power unit (7).