Simulation of a NOx Sensor for Model-Based Control of Exhaust Aftertreatment Systems

Optimal control of the exhaust aftertreatment components is an important aspect for minimizing pollutants. Nitrogen oxides are therefore monitored using amperometric gas sensors. However, they show marked cross-sensitivities to ammonia, which can be disadvantageous, e.g., if they are applied in NH3 selective catalytic reduction systems. In this study, a model that simulates the operation of a NOx sensor is developed. The three oxygen pumping cells and the upstream diffusion barriers were implemented in a 1D geometry, with the relevant electrode reactions taking place over its length. The diffusion barriers were calculated quasi-stationary. The dynamics of incoming gas components as a function of the exhaust mass flow were integrated with a first order linear time invariant system. The problem was calculated using the convection-diffusion equation. The reaction kinetics were determined based on stationary measurements in laboratory atmosphere, whereby initially only the reaction with respect to NO and NO2 was considered. Then, the cross-sensitivities to oxygen, water and ammonia were examined. The developed data set could then be used for transient boundary conditions. The simulated and the measured sensor signals in the case of engine test bench conditions with real driving cycles agree very well.