Modelling of a Solid-Electrolyte Oxygen Sensor

The limiting values for NOx and HC concentrations in the exhaust gas of SI engines will be further lowered by legislation in many countries during the next years. This necessitates an improvement of the pollution control systems, which is achieved by including the dynamics of the three way catalyst into the control system. Before a control system can be designed, the dynamic behaviour of the exhaust after treatment system including the sensors has to be properly analyzed. As a first step a dynamic model of a solid-electrolyte oxygen sensor has been derived. It was the goal to obtain a better understanding of the cross sensitivities towards both reducing and oxidizing exhaust gas components such as H₂, CO, O₂ and NO. The model consists of three parts. Firstly, the porous protection layer, where only diffusion is assumed to occur, secondly the porous catalytic electrodes where the redox reactions take place and thirdly the solid electrolyte, where the electric potential is generated. The catalytic electrodes are mainly modelled with Langmuir-Hinshelwood kinetics, partly extended by structures obtained from earlier investigations of catalytic converters. The static characteristics including their sensitivities towards the exhaust gas components can be well reproduced within the range of interest, i.e. as expected to occur in the exhaust gas system of an SI engine. It can be shown that the air-to-fuel ratio rather than the actual oxygen fraction in the exhaust gas is responsible for the output voltage. The behaviour of the sensor output is very sensitive to reducing species, especially H₂.