A hydrostatic power unit (2) comprises a variable displacement machine with a continuously variable displacement volume and is operated as a pump and motor and is in a drive connection with an internal combustion engine (3). When operated as a pump, the power unit sucks hydraulic fluid out of a tank (9) and delivers into a delivery side (P), and, when operated as a motor, functions as a hydraulic starter to start the internal combustion engine (3). When operated as a motor, the power unit is supplied with hydraulic fluid from a hydraulic accumulator (30). The displacement volume of the power unit (2) is set by a displacement volume control device (60) actuated by a positioning piston device (61) supplied with hydraulic fluid from a charging pressure circuit (23). The power unit (2) includes a supplemental positioning piston device (80) in an operative connection with the displacement volume control device (60) and which is actuated directly by the pressure present in the hydraulic accumulator (30). By means of a connection of the supplemental positioning piston device (80) with the hydraulic accumulator, the power unit (2) can be adjusted to increase the displacement volume to start the internal combustion engine (3).

A fuel pump system for an aerial refueling system including: a variable displacement motor operable to be driven by a hydraulic fluid pressure; a fuel pump operable to be driven by the variable displacement motor; and a drive system controller (DSC) connected to the variable displacement motor, wherein the DSC is operable to direct an operation of the fuel pump in modes including: a flow control mode operable to maintain an output fuel flow rate from the fuel pump to a predetermined maximum inlet pressure at a reception coupling for a receiver aircraft; a fuel pressure control mode operable to regulate the output fuel flow rate to not exceed the predetermined maximum inlet pressure; and a priority mode operable to reduce the output fuel flow rate in response to a decrease in the hydraulic pressure. Also, a method of refueling a receiver aircraft.