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A 2D axissymmetric CFD model of a prototype free-piston engine has been developed using a commercial code. The dynamics of a passive inlet valve mounted on the moving piston was the main interest in the investigation. Preliminary results of cylinder and compressor pressure show good agreement with experimental pressure data for both fired and motoring conditions. Insight into the probable lift characteristics of the passive inlet valve was obtained. The CFD model achieves the simulation of complex multi-body movement through structured mesh deformation. Momentum source terms are utilized to modify flow in certain domains in the model. The paper reports the difficulty of specifying appropriate loss terms for a 2D model of 3D flow. It concludes by assessing the suitability of simplified 2D CFD to model gas dynamics. In cases where the flow departs significantly from a 2D form, either 3D CFD or 1D gas dynamics is probably a better choice.