piston back side rooms 13 13 39 and to the expansion piston back side rooms 14 14 39 diaphragms 18 18 39 divide the buffers 19 19 39 and the crank room 15 the movements of the compressor pistons 7 7 39 and the expansion pistons 2 2 39 change the volume of the piston back side rooms 13 13 39 14 14 39 and the crank room 15 this volume changes make a pressure difference at the diaphragms 16 16 39 17 17 39 the fluid at the piston back side rooms 13 13 39 14 14 39 flow into the buffers 19 19 39 the pressure differences between the buffers 19 19 39 and the crank room 15 are absorbed by the diaphragms 18 18 39 this keeps the diaphragms 16 16 39 17 17 39 of the rods 11 11 39 12 12 39 away from the pressure so the diaphragms 16 16 39 17 17 39 are in safe without any damages as shown in figure in dotted line a line 20 with an oil mist filter or with an absorber 21 may connect the piston back side rooms 13 13 39 14 14 39 and the buffers 19 19 39 with the crank room 15 if the pressure of the piston back side room drops and the pressure occurs between the crank room the pressure is released to the crank room 15 through the line 20 this prevents the diaphragms from the excess pressure by keeping the pressure constant although the invention has been described in its preferred form with a certain degree of particularity obviously many changes and variations are possible therein it is therefore to be understood that the present invention may.