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A novel concept which couples ID and 2D CFD models in a simulation of unsteady lC engine flows was investigated, and such a coupled model was developed. Two unified solution procedures which are capable of predicting mixed compressible and incompressible flow fields found in an engine were developed and comparatively studied. One is the pressure correction algorithm, the other is the block implicit algorithm. They provided platforms for the implementation of coupled models. Second order spatial and Euler backward time differencing schemes were adopted. The comprehensive comparative studies were performed on a variety of benchmark flows ranging from steady to unsteady, incompressible to compressible. The data documented have shown that the prediction qualities of the two algorithms were comparable in all calculations. The block implicit procedure required more storage memory generally but it converged faster in all cases except the incompressible flow calculations. General strategies to couple the ID CFD model with the 2D CFD model in one calculation were proposed. They were successfully incorporated in both of the unified solution procedures. The predictions from these coupled models for a series of unsteady benchmark flows were competitive in quality with those from single 2D CFD models, however, the computing costs involved were comparatively much lower. In these calculations, the coupled models integrated in the block implicit procedure produced faster convergence than those in the pressure correction procedure, but required more computing resource. In addition, the implicit coupling stragety was more efficient compared to the explicit counterpart. A ID and 2D coupled model integrated in the pressure correction procedure was applied to simulate a realistic cylinder-valve-pipe flow. The overal prediction quality is satisfactory compared with experimental measurements. Some discrepancies which occurred were largely attributed to numerical representations of valve mechanism and the lack of turbulence models. For this engine application, the coupled model has shown advantages in computing cost or straightforwardness over a conventional uniform 2D model or boundary condition model.