A Modelling Approach to the Design Optimization of Catalytic Converters of IC Engines
In this paper, a modelling approach to the design optimization of catalytic converters is presented. The first step of the optimization is the model-assisted sizing of catalysts. For a given inlet exhaust condition, a semi-empirical, experimentally calibrated, 0-D steady state catalyst model is employed to sort through a data base of catalysts under given restraints, yielding few successful candidates. Following this screening process, a 1-D transient plug-flow catalyst model is used to analyze the species concentrations and the temperature variation across the catalyst.
The second step deals with the flow optimization of the catalyst converter under the given geometric restraints. A commercially available CFD package is employed to simulate isothermal flow and to evaluate flow uniformity characteristics in the catalytic converter. The substrate is modelled as porous media, where viscous and initial resistances are specified via empirical formula. With the help of the CFD tool, the flow in the converter can be optimized using appropriate boundary layer control methods. In a specific example, the effects of perforated plate on the flow separation in a wide-angle diffuser are demonstrated. This paper also addresses the issue of flow resistance of perforated plates.
The design of oxidation catalytic converters, 3-way catalytic converters and/or SCR systems involves the determination of the loading of precious metals, and washcoat formulations. Material choice, and geometric configuration of the components of exhaust systems, (including manifolds, diffusers, setting chambers, converter housings, substrates, and contractions), and the positioning of the converter in the exhaust lines are other important design aspects. Since the 1970s significant knowledge regarding the manufacturing and utilization of catalytic converters for internal combustion engines has been acquired. However, the design of catalytic converters is still more of an empirical practice than a science. The balance of the situation may need to be shifted toward the latter as the emission standards become more stringent. Recent publications indicate that the complexity of the exhaust gas lines is increased as a result of the use of closed coupled oxidation catalysts, storage catalysts and/or particulate filters , and the merging of the exhaust aftertreatment with powertrain management leads to the integrated emission reduction approaches . The greater complexity associated with the advanced emission control strategies leads to a desire to incorporate modelling approaches in the design process to improve the performance of products, shorten prototype development periods and reduce overall costs.
1-D steady state plug-flow catalyst models have been widely used to predict the performance of catalytic converters . At DCL International Inc., a 0-D and a 1-D catalyst model are employed for the catalyst sizing and performance prediction. Although these models are robust and reliable for some ranges of applications, they are not capable of dealing with complex geometries and the associated processes, i.e. multi-dimensional flow and heat transfer. Flow maldistribution not only lowers the local Damköhler number in the higher velocity zones, but also produces larger aging effects  in these zones, which subsequently result in reduced overall conversion efficiency of the species.