a closed working fluid system for a regenerative stirling engine is disclosed . the system employs double - acting pistons arranged with each low temperature space connected to one hot space of an adjacent piston . the low temperature spaces are all connected to a reservoir system employing two separate chambers , one at a high pressure and another at a relatively low pressure . control means select the reservoir for communication with the working system depending on the torque demand of the engine ; the control means also permits fluid flow to pass from any one low temperature space to the selected reservoir when the pressure condition in the low temperature space exceeds the associated reservoir pressure . independent communication is provided between each pair of adjacent low temperature spaces ; the communication is controlled by a valve operating in phase with the phase changes of the double - acting pistons so that only one pair of low temperature spaces are in communication at any one time . the latter communication operates to displace the independent pumping mechanisms employed by the prior art . the apparatus herein allows the integrated compression spaces to increase the pressure of the working fluid system in series .