The Prediction of 1D Unsteady Flows in the Exhaust System of a S.I. Engine Including Chemical Reactions in the Gas and Solid Phase

The paper describes the research work concerning the simulation of 1D unsteady reacting flows in s.i. engine pipe-systems, including pre-catalysts and main catalysts. The numerical model GASDYN has been developed to enable the concurrent prediction of the wave motion in the intake and exhaust ducts, the chemical composition of the gas discharged by the cylinder of a s.i. engine, the chemical and thermal behavior of catalytic converters. The effect of considering 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) on the predicted wave motion is reported. A detailed fluid dynamic, chemical and thermal modeling of the catalytic converters and the whole exhaust system allows the optimization of geometrical dimensions, characteristics and position of pre-catalysts and main catalysts, in order to achieve the optimal response in terms of conversion efficiency, warm-up, light-off time and engine gas exchange process. The 1D fluid dynamic model developed has been applied to the simulation of the unsteady flows with transport of reacting chemical species in the exhaust system of a Fiat-AlfaRomeo four-cylinder, 2.0L automotive s.i. engine. Predicted results concerning pressure losses, pressure pulses, wall temperatures and exhaust gas compositions in different locations of the exhaust system are compared with experimental data, pointing out a satisfactory agreement.