An electrochemical model of an amperometric NOx sensor

To help design future amperometric NOx sensors, a physics-based sensor model that includes diffusion and electrochemical submodels is developed. It is shown that NO is partly reduced in the O-2 sensing chamber which affects NO concentration in the O-2 sensing and in the NOx sensing chamber. Therefore, the electrochemical model is developed to simulate partial reduction of NOx on the O-2 sensing electrode and reduction of NOx on the NOx sensing electrode. A transport model that simulates diffusion of the gas species through the sensor diffusion barriers and sensor chambers is coupled to the electrochemical submodels. A fully controlled sensor test-rig that provides controlled gas mixtures is employed to carry out experiments to estimate model parameters. Then, the sensor is installed on the exhaust system of a medium duty Diesel engine and then on a port injection spark ignition engine. Experiments at different engine operating conditions with different NOx concentrations from 0 to 2820 ppm have been performed to validate the model accuracy at different operating conditions. Through the validation process, the NOx sensing cell voltage is changed experimentally at different NOx concentrations to evaluate the model accuracy at different cell voltages. The model results closely match the experiments with the maximum 12% error for the NOx sensing pumping current.