Abstract
Mathematical models to simulate the operation of various catalytic converter platforms are successfully used as tools to support catalyst design and optimization as well as for control purposes. Modelling the chemical phenomena is possibly the most challenging task of such models, due to the complicated reaction mechanisms that occur on the catalytic surface under the transient conditions of real exhaust gas. Typically, there is a trade-off between the level of detail of chemical relations and the ease of model parameterization. Models employing reaction schemes with global reactions have a good chemical background, and require reasonable parameterization effort to be applied for the purposes of design and optimization of exhaust after-treatment systems. Further simplifications of the reaction kinetics usually disregard the chemical background and are therefore mainly applicable for catalyst control purposes.
The present work proposes an approach to reducing the parameterization effort associated with global reaction models, without completely disregarding the chemical background. The underlying assumption is that dynamic storage phenomena govern the pollutant conversion under real-world operating conditions. Therefore, global reaction schemes can be simplified by focusing only on dynamic storage and release reactions. This results in a compact reaction scheme with a relatively small number of tuneable parameters. The proposed reaction scheme is implemented in a 1D solution of the energy and species balance equations in catalytic monoliths, based on well-established approaches from the literature. The application range and limitations of this approach are examined for two different catalyst platforms: three-way catalysts and diesel oxidation catalysts.
For three-way catalysts (TWC), the proposed reaction scheme involves five oxygen storage and release reactions and one steady-state reaction. The reaction scheme accounts both for gasoline and for natural gas engine operation. It is validated against a large database of real-world driving cycles, carried out with different vehicles. The model can capture the main trends in pollutant conversion both in hot-mode and in cold-start operation. The model is validated and used for the prediction of the emissions of a fleet of vehicles, an application where the low parameterization effort of the model is of critical importance.
For diesel oxidation catalysts (DOC), the physical importance of oxygen storage reactions is limited, as no oxygen storage components are typically used in the washcoat. However, the proposed simplified approach is still usable with the following modifications: a very small value is assigned to the oxygen storage capacity and an additional reaction for NO oxidation to NO2 is included. Again, the model is validated against real-world driving cycles, carried out with light-duty commercial vehicles equipped with DOC, and captures pollutant conversion in hot-mode operation well.
Keywords: three-way catalyst, diesel oxidation catalyst, oxygen storage, transient, validation