Lambda detection with thin-film metal oxides using synthetic exhaust gas mixtures

Abstract

New exhaust gas sensors based on the gas dependence of the electric conductivity of metal oxide thin films have been developed to realize a cylinder-selective lambda measurement in internal combustion engines. To understand these results, the lambda characteristics of the sensors were measured in synthetic gas mixtures with concentrations similar to the exhaust of an internal combustion engine. The typical lambda characteristics of the sensors show a strong change in resistance near the stoichiometric point, and a slight dependence on the lambda value in the rich (lambda <1) and lean (lambda > 1) regions. The lambda dependence is similar to that of the computed oxygen partial pressure of the used gas mixtures in equilibrium. This allows us to assume that on the sensor surface the thermodynamic equilibrium is established between the gases and the metal oxide determined by the concentration of oxygen vacancies. From a comparison of the computed with the measured lambda characteristic of the sensor materials, it can be assumed that at temperatures of about 1000 °C the resistance of thin-film metal oxides is determined by the concentration of oxygen vacancies as a function of the oxygen partial pressure. A dispersion of catalytically active platinum can enhance the reaction rate to establish the equilibrium even at temperatures lower than 1000 °C.