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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.

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.

An oxygen sensor based on electrochemical pumping is described which is capable of measuring with high sensitivity A/F over an extended range from very lean to very rich A/F values. The sensor output is approximately proportional to A/F, has a weak dependence on temperature and is essentially independent of the gas pressure.

An engine-dynamometer study was performed to quantify the air-fuel ratio (A/F) offset between the window of a three-way catalyst (TWC) and the closed-loop control point of an exhaust gas oxygen (EGO) sensor. In this study, the effects of rpm, torque, EGR, and A/F modulation were explored along with the age of the TWC and EGO sensor. In general, it was determined that the closed-loop EGO sensor control point shifts lean as a function of increasing feedgas NOx concentration, thus causing the engine A/F to move away from the high NOx conversion efficiency regime of the TWC.

The transient response of ZrO2 and TiO2 EGO sensors has been investigated under actual engine operating conditions. The results of this study show that the response of an EGO sensor is dependent upon the characteristics of the engine and feedback control system with which it is used. Specifically, sensor response time is a function of the magnitude and frequency of the A/F changes and the initial and final values of A/F to which the sensor is exposed. ZrO2 and TiO2 sensors show similar transient behavior and have practically equivalent response times.

This paper discusses the properties of EGO sensors based on thick TiO2 films. These devices are fabricated by depositing a TiO2 film on an insulating or conducting substrate. In addition to being inherently inexpensive, planar techniques provide flexibility in the fabrication of multicomponent structures incorporating temperature compensation and heating elements. Furthermore, TiO2 film sensors are found to possess faster transient response than available ZrO2 and ceramic TiO2 sensors. Results of engine studies of the properties of these new TiO2sensors will be presented and discussed.

The change in the resistance of titanium dioxide with oxygen partial pressure is utilized to obtain an air-to-fuel ratio sensor. TiO2 material properties, sensor components and performance characteristics are discussed. Some results of engine dynamometer and vehicle tests of sensor performance and durability are presented.

An experimental exhaust sensor using CoO ceramic is being developed for measuring and controlling air-to-fuel ratio (A/F) of internal combustion engines operating under lean-burn conditions. The operation of the sensor is based on the fact that at elevated temperatures the electrical resistance of CoO depends on the oxygen partial pressure (PO2). At controlled temperatures, the combination of good PO2 sensititivty, high reproducibility of the electrical properties and small temperature coefficient of resistance of CoO materials make this sensor suitable for the lean A/F range (15-20) where exhaust PO2 depends only weakly on A/F ratio. At low PO2 (A/F<13.5) and high temperatures, CoO reduces to Co; therefore, prolonged exposure of the sensor to these conditions must be avoided. The material properties and the sensor characteristics are discussed and results of tests on car-mountable CoO sensors are presented.