System Level Modeling of Lean Exhaust Aftertreatment Devices

Syed  Wahiduzzaman, Gamma Technologies Inc.

In order to meet the increasingly stringent emissions standard it is imperative that a two pronged approach is pursued for reduction of tailpipe emissions. In this regard emissions, and often the exhaust compositions, are needed to be controlled both at its source and then subsequently cleaned up at the exhaust system. In addition, an aftertreatment system often consists of array of catalysts and its performance depends on the transient characteristics of the exhaust gas composition and thermodynamic state. To complicate the matter furthermore, relevant technologies are still evolving at a rapid pace. Consequently, an aftertreatment modeling approach should not only be system based but also offer a high level of configurability. Thus a system approach that includes a model of an engine/vehicle may provide an efficient means to analyze system performance and examine relative effects of competing phenomena and technologies. Additionally, it could also be an excellent tool for interpretation of experimental data. Another aspect of aftertreatment modeling is that appropriate kinetics is a highly sensitive function of formulation and physical description of the catalysts. Often a set of global kinetics that is able to predict a particular catalyst performance may not be valid for others having similar functionality and usage. These mechanisms are often closely guarded by the manufacturers and may not be readily available to engine systems modelers. There are also great variations in formats and conventions in expressing catalytic reaction mechanisms and thus, in general, the task of inputting and interpreting these mechanisms can be a daunting task.

In order to address these issues, a highly configurable, intuitive kinetics modeling environment (Kinetics template Library) has been developed for the GT-SUITE engine/vehicle modeling tool. The library is capable of accepting a variety of catalytic reaction mechanisms of different formats such as global kinetics with arbitrary Langmuir-Hinshelwood (L-H) inhibition functions and surface reactions with coverage terms describing the dynamic behaviors between asbsorption/desorption. This tool can be used to model various aftertreatment devices either in a standalone mode or as a system of interconnected and coupled components including engine and vehicle. The tool has various advanced features. For example, an arbitrary set of chemical mechanism can be incorporated into the catalyst models which are then parsed and integrated into the flow and thermal conservation equations. The resulting ODE/DAE system is solved with integrators capable of handling stiff kinetics. Additionally, a built-in DOE/optimizer facility allows use of experimental data for extraction/identification of kinetic parameters. In this work chemistry library application examples to model aftertreatment systems such as DOC, SCR, LNT, TWC and associated control systems will be modeled and analyzed.

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