Secondary Air Injection in the Exhaust After-Treatment System of S.I. Engines: 1D Fluid Dynamic Modeling and Experimental Investigation 2003-01-0366

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.