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The paper describes the experimental and simulation work recently carried out to investigate the effects of secondary air injection on the emission conversion in the exhaust after-treatment system of a S.I. automotive engine. The modeling of the 1D unsteady reacting flows in the complete exhaust system of a spark ignition engine, designed to satisfy the Euro IV limits, has been performed including the secondary air injection system, to predict the possible shortening of catalyst light-off time and the speed-up of the after-treatment system warm-up. The transport of chemical species with reactions in gas phase (post-oxidation of unburned HC in the exhaust manifold) and in solid phase (conversion of pollutants in the catalyst) with and without secondary air has been simulated by the 1D thermo-fluid dynamic model GASDYN, developed by the authors.

A new approach for the fluid-dynamic simulation of the Diesel Particulate Filters (DPF) has been developed. A mathematical model has been formulated as a system of nonlinear partial differential equations describing the conservation of mass, momentum and energy for unsteady, compressible and reacting flows, in order to predict the hydrodynamic characteristics of the DPF and to study the soot deposition mechanism. In particular, the mass conservation equations have been solved for each chemical component considered, and the advection of information concerning the chemical composition of the gas has been figured out for each computational mesh. A sub-model for the prediction of the soot cake formation has been developed and predictions of soot deposition profiles have been calculated for different loading conditions. The results of the simulations, namely the calculated pressure drop, have been compared with the experimental data.