The concept of a free-piston expansion/compression unit with a variable Built-in Volume Ratio (BVR) is proposed. This device has no crankshaft mechanism which provides a possibility to optimize the expansion process free of mechanical limitations. An additional degree of freedom is used, namely the rotation to control the in- and the outlet ports timing. Further, the operation in the expander mode will be described.In most of the existing linear expanders/compressors, bouncing chambers or devices are used to reverse the piston movement at extreme positions. This approach is characterized by relatively high energy losses due to irreversibility of such a process. As an alternative, a fully controlled movement of the piston is proposed. This paper is focused on the control algorithm based on rules, which have been obtained and based on the insight in the system. Including the rotation timing, resulting in an optimal expansion process with an outlet pressure matching with the required one.
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