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. The code has been extended to simulate the injection of air in the exhaust manifold and predict the consequent post-oxidation of pollutants in the ducts. The main chemical reactions arising in gas phase, in the upper part of the exhaust manifold, have been included, considering the oxidation of C3H6, C3H8 and CO and the steam-reforming of C3H6 and C3H8. The heat released in the gas due to the exothermal reactions has been taken into account, to evaluate the exhaust gas temperature along the ducts with injection of air.
A Fiat-Alfa Romeo four-stroke, four-cylinder 2.0L automotive S.I. engine complying with the Euro IV regulations has been modeled, in order to predict the chemical specie concentration along the exhaust system. A large set of experimental data concerning this engine (cylinder pressure, pressure pulses, wall and gas temperatures, gas chemical composition along the system) with different secondary air mass flows has enabled a comprehensive comparison between predictions and measurements, in order to validate the model in different operating conditions.