An Integrated Simulation Model for the Prediction of S.I. Engine Cylinder Emissions and Exhaust After-Treatment System Performance

The calculation of the main pollutant emissions discharged into the atmosphere by means of numerical codes requires the development of integrated models, including either an accurate thermodynamic in-cylinder analysis and the simulation of reacting unsteady flows in the duct system. This paper describes the main features of the numerical model GASDYN developed by the authors, which in the last years has been enhanced in order to achieve this kind of objectives. A multi-zone approach has been adopted to predict the combustion process in s.i. engines, whereas the so called super-extended Zeldovich mechanism has been introduced to perform a more detailed description of all the chemical reactions involved in the NO_x production process. The simulation of the reacting flows in the exhaust manifold has been completed by the introduction of further enhancements to predict the chemical behavior of gases inside the catalytic converters. The oxidation of CO, C₃H₆, C₃H₈, H₂ and reduction of NO, the steam-reforming reactions of C₃H₆, C₃H₈, the water-gas shift reaction of CO have been considered. Moreover, an oxygen-storage sub-model has been introduced, to account for the behavior of cerium oxides. A detailed thermal model of the converter takes into account the heat released by the exothermic reactions as a source term in the heat transfer equations. A wide comparison between the simulation and the experimental measurements taken on a Fiat-AlfaRomeo four-cylinder, 2.0L automotive s.i. engine, is presented to validate the proposed model.