A variable displacement hydrostatic power unit (7) is in a drive connection with an internal combustion engine (2). When operated as a pump, the power unit (7) delivers hydraulic fluid to at least one consumer (V). When operated as a motor, the power unit (7) is a hydraulic starter for the internal combustion engine (2) and is supplied with hydraulic fluid from a hydraulic accumulator (25). The displacement volume of the power unit (7) is set by a displacement volume control device (50) actuated by a positioning device (52). The displacement volume control device (50) is displaced into a position with maximum displacement volume by a corresponding actuation of the positioning device (52) chronologically prior to the shutoff of the internal combustion engine (2). A securing device (60) holds the displacement volume control device (50) in the maximum displacement volume position when the internal combustion engine (2) is shut off.