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Laboratory studies were performed to examine the effect of NOx on the static switch point of current (1996 MY) production-type heated exhaust gas oxygen (HEGO) sensors. These sensors are used on vehicles for feedback control of the A/F. Several binary gas mixtures and simulated exhaust gas mixtures were used to measure the static response to NO and NO2 in the laboratory. The results of the laboratory studies indicate that most of these HEGO sensors completely equilibrate NO, but only partially equilibrate NO2. To validate this result, engine studies were performed to investigate whether the A/F control point shifts when the amount of EGR is changed. Results from both the laboratory and engine studies are presented.

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.