brake energy is recovered by the machine when the piston is driven through an external load this results in an increased efficiency of the control apparatus for the cylinder unit avoiding any problems arising from heat application_number US 74121685 A description fig1 shows a cylinder unit 10 which cylinder space associated with the smaller piston face is connected to a pressure line 12 and which cylinder space 13 associated with the larger piston face is connected via a line 14 and a hydraulic machine 15 to the pressure line 12 which is still connected to a hydraulic accumulator 16 the hydraulic machine 15 is coupled to a prime mover 17 for example an electrical machine which may operate in the motor and generator mode a predetermined system pressure p0 is maintained constant in the pressure line 12 by a pressure source which is not shown in fig1 the hydraulic machine 15 is an axial piston type machine for example having a variable absorption or respectively displacement volume the stroke of the machine may be adjusted either manually or by a servo motor not shown the machine 15 operates as a pump when driven by the machine 17 assuming that the pressure in the line 14 is higher than in the pressure line 12 the piston 18 of the cylinder 10 will.