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Conventional thimble type lambda sensors combined with a pulse discharge technique were used for NOx detection in a simulated combustion exhaust gas with varying oxygen and moisture levels. Open circuit discharge characteristics after defined polarization pulses with alternating polarity show strong dependencies on NOx (112 mV / decade) in the lowest ppm range (0.5 – 50 ppm). Increase in NOx concentration accelerates sensor discharge, whereby the discharge curves following negative pulses are more affected by the NOx content compared to positive pulses. The sensitivities to NO and NO2 are equal and measurement of total NOx is possible. The discharge curves are affected by the oxygen and the water content. An increase in O2 concentration results in a shift of the discharge curves to negative voltages. Additionally a decreased sensor response to NOx is obtained for both polarization polarities. The water concentration has the opposite effect. All discharge curves are shifted to positive voltages with increasing H2O content. As a result, the sensor response increases strongly. The voltage shifts due to O2 and H2O are expected from Nernstian behavior of a lambda probe, whereby moisture influence is stronger compared to O2. The feasibility of the method as an exhaust gas total NOx measurement system is confirmed.

P1.6.4 NOx-Detection by Pulsed Polarization of Lambda Probes