Research On Mechanism Modeling And Dynamic And Static Process Analysis For NO_x Sensor Performance Management

NO_x sensor is of great significance to automotive exhaust gas aftertreatment systems.The sixth-generation NO_x sensor needs to measure the concentration of oxygen and nitrogen oxides（NO_x）in the exhaust gas.The working process of the NO_x sensor is complicated.There are three measuring chambers and three pump voltages.During the working process,the pump voltage needs to be controlled to ensure that all O₂ in the chambers can be pumped out,while ensuring that all NO₂ is decomposed into NO.And NO cannot be decomposed in advance,and the most important thing is to ensure that the sensor can respond quickly to changes in exhaust gas,so higher requirements are placed on the control system.Now,due to insufficient research on the influencing factors of the dynamic and static responses of the NO_x sensor,when designing the control algorithm,the method of trial and error is generally adopted to constantly change the control parameters and compare the control effects.Therefore,it is difficult to find the best control parameters.As a result,the response speed of the NO_x sensor is slow,and the change in the air-fuel ratio and the NO_x concentration in the exhaust gas cannot be quickly fed back to the automobile control center.Therefore,in order to be able to deeply understand the physical processes involved in the operation of the sensor,it is necessary to construct a mechanism model.This thesis analyzes the dynamic and static response characteristics of the sensor by studying the static model of the NO_x sensor and the dynamic model of the electrochemical reaction at the three-phase boundary.First,according to the influence of the pump voltage on the pump current in the steady state,a steady-state response model was constructed,including the gas diffusion process model through the diffusion barrier layer and the electrochemical model of the oxygen reduction process.By comparing the experimental data and simulation data of static response,the validity of the mechanism model is verified,and the prediction error can basically be maintained within 30%.Combined with the static model and experimental results,the static response characteristics are analyzed.Then,by analyzing the dynamic process of the electrochemical reaction occurring at the three-phase boundary,the corresponding dynamic process mechanism model was constructed.The method of electrochemical impedance spectroscopy is used to verify the validity of the mechanism model constructed.And according to the transfer function of the response,the description of the electronic components in the equivalent circuit is obtained,so that the factors that affect the reaction process can be explained from the physical mechanism.Finally,according to the mechanism model describing the dynamic process,the dynamic response characteristics of the sensor under different working conditions are analyzed,including the effects of different oxygen partial pressure and temperature on the charge transfer resistance,polarization resistance and oxygen coverage on the electrode surface.Determine the rate determining steps of the electrochemical reaction in the Electrode/Electrolyte system under different conditions.According to the obtained dynamic and static mechanism model,the response characteristics of the sensor can be analyzed,which provides guidance for the subsequent control algorithm design,facilitates finding the best control parameters,and improves the response speed.