A new method for the analysis of the gas flow in an internal combustion engine has been developed. It is based on the interactive coupling between commercially available three (STAR-CD) and one dimensional (PROMO) fluid dynamics codes. With this method the detailed transient flow distribution for any engine component of interest can be calculated taking into account the overall gas dynamic interaction with other engine components.The underlying physics and numerics are outlined. A description of the coupling procedure ensuring proper communication between the two computer codes is given. Also addressed is the averaging procedure adopted at the 3D boundaries, including the influence of the 1D/3D interface placement.A first application of this new method is presented, in which the gas flow in a turbo-charged DI-diesel-engine is simulated. The intake manifold and ports are treated as a single 3D-model in this study, while the rest of the engine is represented by a system of 1D-components. Comparisons to experimental data obtained from pressure measurements are presented. Future steps towards a more precise gas dynamics representation of the overall engine are discussed.The work presented is part of an ongoing PhD-project (Mr. R. Sinclair) at VW.