A Three-Parameter Transient 1D Catalyst Model 2011-01-1306

Interactions between in-cylinder combustion and emission aftertreatment need to be understood for optimizing the overall powertrain system. Numerical investigations can aid this process. For this purpose, simple and numerically fast, but still accurate models are needed for in-cylinder combustion and exhaust aftertreatment. The chemical processes must be represented in sufficient detail to predict engine power, fuel consumption, and tailpipe emission levels of NOx, soot, CO and unburned hydrocarbons. This paper reports on a new transient one-dimensional catalyst model. This model makes use of a detailed kinetic mechanism to describe the catalytic reactions.

A single-channel or a set of representative channels are used in the presented approach. Each channel is discretized into a number of cells. Each cell is treated as a perfectly stirred reactor (PSR) with a thin film layer for washcoat treatment. Heat and mass transport coefficients are calculated from Nusselt and Sherwood laws. Either detailed or global surface chemistry is applied in the thin film layer. Three global parameters are used to align the detailed chemistry model with a given catalyst topology and composition; one parameter for heat transfer, one for mass transfer and one for overall reaction efficiency. This allows considering detailed surface chemistry, molecular diffusion and heat conductivity while maintaining affordable CPU time. Detailed, usually unknown, specifications of the catalyst material are insignificant for the presented approach.

The models' applicability is demonstrated for a single-channel of a NOx-storage catalyst (NSC). The detailed surface chemistry by Koop and Deutschmann is utilized. Good agreement between experimental data and model results is achieved. The investigation of surface site fractions shows, that CO and C₃H₆ from exhaust gases inhibit NO oxidation by the same process; in both cases surface bound CO blocks the sites for NO oxidation. The inhibition effect is mainly determined by the total concentration of carbon atoms contained in CO and HC in the exhaust stream. Oxidation by surface bound oxygen was further found to be the major pathway for HC conversion. The lasting inhibition effect of unburned hydrocarbons on NO oxidation was studied by a transient calculation. In this test a sudden cutoff of unburned hydrocarbons in the exhaust stream was assumed. The response time for NO oxidation was found to be 5.5 seconds. The high response time proofs the necessity of using a transient model of sufficient detail to simulate catalytic oxidation during transient engine processes or under cyclic variations.