The CFD-ACE commercial code has been utilized for a 2-D model of a Free Piston Stirling Engine (FPSE). Several code validations were conducted including laminar flow in oscillatory pipe and parallel plate flows. The CFD results showed good agreement with available experimental data as well as analytical solutions. The 2-D model consisted of an Expansion Space (ES), Heater (HR), Regenerator (RG), Cooler (CR) and Compression Space (CS). The HR and CR were modeled as concentric fins, while the RG utilizes the CFD-ACE porous media model. CFD data were obtained for the PV power from the ES and CS as well as heat in and out of the heat exchangers. The model for the FPSE was conducted for two grids, coarse (64,823 cells) and fine (133,078 cells) and includes the CS, CR, RG, HR and ES. Both the power piston and displacer were also modeled. Results were obtained for energy in and out from each component including enthalpy flux at both sides of the regenerator. The CFD-ACE porous media model was utilized (which is known not to accurately represent the unsteady heat transfer process in the regenerator due to the assumption of gas-solid temperature equilibrium). The results obtained for 110 cycles (coarse grids) and 100 cycles (fine grids) were compared with Sage results. The codes for both cases seem to head towards the right direction of energy balance. The similarities and differences among the different cases were examined and discussed in the paper. Two approaches have been proposed to accelerate the convergence process: 1) Replace the solid walls (as much as possible, e.g. in the heater and cooler) by a uniform temperature surface. This might accelerate the convergence of the CFD process but will alter the B.C., which would result in different heat transfer rates, 2) Model computationally each component separately and merge them one at a time using user subroutine in CFD-ACE. Both approaches have been attempted and the results are encouraging.