Sabine Fischer

NO Detection by Pulsed Polarization of Lambda Probes-Influence of the Reference Atmosphere

The pulsed polarization measurement technique using conventional thimble type lambda probes is suitable for low ppm NOx detection in exhaust gas applications. To evaluate the underlying sensor mechanism, the unknown influence of the reference atmosphere on the NO sensing behavior is investigated in this study. Besides answering questions with respect to the underlying principle, this investigation can resolve the main question of whether a simplified sensor element without reference may be also suitable for NO sensing using the pulsed polarization measurement technique. With an adequate sensor setup, the reference atmosphere of the thimble type lambda probe is changed completely after a certain diffusion time. Thus, the sensor response regarding NO is compared with and without different gas atmospheres on both electrodes. It is shown that there is still a very good NO sensitivity even without reference air, although the NO response is reduced due to non-existing overlying mixed potential type voltage, which is otherwise caused by different atmospheres on both electrodes. Considering these results, we see an opportunity to simplify the standard NOx sensor design by omitting the reference electrode.

NOx-detection by pulsed polarization of lambda probes

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.

NOx - Detektion an YSZ-Sensoren mittels Spannungspulsen

Zusammenfassung Diese dynamische Messmethode mit wechselseitiger Polarisation der Elektroden und Auswertung der Entladekurven bietet den grosen Vorteil, dass an einfachen Sensorelementen Stickoxide in kleinem Konzentrationsbereich detektiert werden konnen. Bei dieser Spannungs-Puls-Methode ist die Form und Lage der Selbstentladekurve charakteristisch fur die Gaskonzentration. Da bei allen Anwendungen der Wassergehalt eine grose Rolle spielt, wurde in dieser Studie dieser Einfluss genauer betrachtet.