Expansion machines can improve the efficiency of refrigeration cycles especially when applying carbon dioxide as the refrigerant. To achieve this, the authors favor the application of modified reciprocating machines. One of the main challenges when realizing such machines is the control of the fluid flows in and out of the machine's cylinders. In contrast to reciprocating compressors where self-acting valves are used, piston-position-dependent inlets and outlets have to be applied for reciprocating expansion machines. A concept is presently being investigated, which is based on the modification of an axial piston machine of the wobble plate or swash plate type, with a rotating disc as control unit. One of the remarkable features of the concept is the simple design without electronic devices. The system may be designed as compressor-expander unit to obtain a compact and economic competitive solution. In this paper, the concept will be discussed and first evaluations of the design will be shown.
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