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Results of laboratory studies of the static characteristics of several different commercially available heated exhaust gas oxygen sensors are described. In these studies, the emf of the sensors was measured as a function of temperature and of the composition of calibrated gas mixtures. Several different binary gas mixtures (H2/N2, CO/N2, C3H6/N2, C3H8/N2, and CH4/N2) were used together with a variable amount of O2. In addition to laboratory studies, the same sensors were also studied in the exhaust gas of an engine. Whereas at high temperatures thermodynamic equilibrium appears to prevail, clear departures from thermodynamic equilibrium are observed at some lower temperatures (the value of which depends on the specific sensor and the specific gas mixture used). This behavior is manifested by shifts of the emf step away from stoichiometry, broadening of the step, abnormally high emf values in excess oxygen mixtures, and abnormally low emf values in reducing gas mixtures.

A steady state model of the ZrO2 exhaust gas oxygen sensor response to a simple (O2,CO,H2,N2) gas mixture has shown that while the detailed shape of the curve for sensor emf output versus inverse redox ratio for the gas mixture depends on many parameters, the step from a relatively high emf to a lower emf that occurs at a critical gas composition can be located from conservation constraints on the individual atomic species. In this paper, these conservation constraints are generalized; a Rule of Mixtures is developed that relates the inverse redox ratio of the gas at the ZrO2 sensor switch point (Rs′) to a weighted average of the corresponding switch points for individual oxygen /gas-component mixtures (Rsjo′): where j denotes a specific reductant species, zj is the stoichiometric factor of the j species for complete oxidation, pj∞ is the partial pressure of gas species j in the mixture, and Δ is a well defined property of the O2 and NO oxidizing gases and the sensor electrodes.