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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 brief physical description of ZrO2 and TiO2 exhaust gas oxygen sensors is presented, followed by a comparison of the two sensors operating under actual open-loop and closed-loop conditions on an engine/dynamometer setup. The sensitivity of the ZrO2 sensor to engine load and cylinder-to-cylinder maldistribution is explored, and the compatibility of the ZrO2 switch point and a three-way catalyst window is considered.

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.