Production engine emission sensor modeling for in-use measurement and on-board diagnostics

Abstract

Production engine emission sensors have become essential for on-board measurement in the exhaust gas and for engine feedback control. To help design future amperometric sensors, first the diffusion mechanism of a zirconia-based amperometric NOx sensor was examined by studying the effect of sensor temperature on sensor output. The multi component molecular diffusion mechanism was experimentally found to be the dominant diffusion mechanism that affects the diffusive flow through the sensor diffusion barriers. A sensor model was developed based on this dominant diffusion mechanism to predict NOx concentration which was validated with the experiments at different Diesel engine operating conditions with different species concentrations. Then, a physics-based sensor model that includes diffusion and electrochemical submodels is developed. It is shown that NO is partly reduced in the O2 sensing chamber which affects NO concentration in the O2 sensing chamber and in the NOx sensing chamber. Therefore, the electrochemical model is developed to simulate partial reduction of NOx on the O2 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. 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. The model results closely match the experiments with a maximum 12% error for the NOx sensing pumping current. Cross-sensitivity of electrochemical sensors to the other exhaust gas contaminations, especially NH3, is still a challenge for the automotive industry. A dynamic NOx sensor model is developed to remove ammonia cross sensitivity from production NOx sensors mounted downstream of Diesel-engine selective catalytic reduction