Direct Coupled 1D/3D-CFD-Computation (GT-Power/Star-CD) of the Flow in the Switch-Over Intake System of an 8-Cylinder SI Engine with External Exhaust Gas Recirculation 2002-01-0901

The setting of boundary conditions on the boundaries of a 3D-CFD grid under certain conditions is a source of significant errors. The latter might occur by numerical reflection of pressure waves on the boundary or by incorrect setting of the chemical composition of the gas mixture in recirculation zones (e.g. in the intake manifold of internal combustion engines when the burnt gas from the cylinder enters the intake manifold and passes the boundary of the CDF-grid. When the flow direction is changed the setting of pure new charge on the boundary leads to errors). This type of problems should receive attention in operation points with low engine speed and load.

The direct coupling of a 3D-CFD program (Star-CD) with a 1D-CFD program (GT-Power) is done by integration of the 3D-grid of the engine component as a „CFD-component” of the 1D computational model of a complete engine. The exchange of boundary data takes place on time step level, which replace the usual setting of fixed values/arrays of boundary data on the 3D-grid. This integrated modeling on one hand allows the combination of the advantages of 1D-CFD program (fast computation, high flexibility, adaptability of the model for the complete range of engine operation points) with those of the 3D-CFD program (spatial resolution of flow field, computation of mixture processes and other inhomogeneous effects, e.g. chemical reactions, extended predict-ability). On the other hand, by computing the travelling waves and tracing the mixture composition in both programs, the mentioned sources of errors are almost eliminated. But there are some additional requests on the geometry of such an internal boundary as an interface between the two programs. The paper will discuss the problem of integrated CFD modeling of Star-CD and GT-Power applied to the switch-over intake system of an 8-cylinder SI engine with exhaust gas recirculation. As a supplement, some computations on other cases (e.g. intake manifold of an 4-cylinder engine) are presented.