Development of a Quasi-Steady Approach Based Simulation Tool for System Level Exhaust Aftertreatment Modeling 2008-01-0866

This article describes a system level 1D simulation tool that has been constructed on the Quasi-steady (QS) method. By assuming that spatial changes are much greater than the temporal ones, rigorous 1D governing equations can be considerably simplified thus becoming less computationally demanding to solve and therefore suitable for control oriented modeling purposes.

With the proposed tool exhaust pipe wall temperature profiles, including multiple-wall-layer configurations, are solved through a finite difference scheme. Momentum equation is included for predicting pressure losses due to frictions and geometric irregularity. Exhaust fluid properties (transport and thermodynamic) are evaluated according to NASA or JANAF polynomial thermal data basis. The proposed tool allows the consideration of an arbitrary number of chemical species and reactions in the entire system. A novel semi-automatic approach was developed to handle catalytic reaction kinetics intuitively. Global reaction mechanisms and their rate formulations can be input or modified via a GUI system and, without any compiling requirements to generate a new executive command, are ready to use.

Over the course of development, several aspects of the QS approach have been investigated and are discussed in this article, including numerical stability, system accuracy and computational efficiency. The current implementation permits a typical aftertreatment (AT) model to run about 10 to 100 times faster than real time on a regular personal computer, depending mainly on the physical nature (i.e. stiffness of governing equations) and on the required accuracy level.

To demonstrate its applicability for AT modeling, three case studies have been conducted: (a) development of an intrinsic SCR mechanistic model from micro-reactor measurements with V₂O₅-WO₃/TiO₂ catalysts; (b) kinetic calibration and emissions prediction for a DOC; and (c) simulation of a system level AT model consisting of a fuel reformer, a LNT, a DPF and a SCR for NOx control. Kinetic analyses, simulation results and comparisons to experiments are presented and discussed.