Keiichi Saji

Influence of H2O, CO2 and various combustible gases on the characteristics of a limiting current-type oxygen sensor

Current-voltage characteristics of limiting current-type oxygen sensors were investigated. The sensor showed a two-stage current plateau in current-voltage characteristics in H2O−O2−N2 and CO2−O2−N2 mixtures. The sensor current in the first stage corresponded to O2 concentration and was practically independent of H2O and CO2 concentration in the gas mixtures. The sensor current in the second stage increased linearly with the H2O or CO2 concentration, for a sensor with high electrode activity. The behavior of the sensor suggests that the deoxidization of H2O or CO2 occurs at the sensor cathode. For nonequilibrium gas mixtures containing combustible gas and O2, the sensor current in the first stage decreased linearly with combustible gas concentration. The decrease of the sensor current differed from that corresponding to the O2 concentration consumed by the reaction of these gases in the ambient gas, depending on the kind of combustible gas. The reduction of the sensor current is explained by a model assuming that the reaction of these gases occurs at the cathode, and the diffusion of the combustible gas in the porous coating is a rate-limiting step.

Thin film air-fuel ratio sensor

Abstract A thin film air-fuel ratio (A/F) sensor for wide range combustion control has been developed. This sensor consists of a limiting-current-type detective part for wide-range A/F, a resistive-type detective part for stoichiometric A/F and a heating part. The limiting-current-type detective part is oprated by changing the polarity of the applied sensor voltage, and detects A/F from rich to lean range without an additional reference chamber for pumping or introducing oxygen. The operating mechanism in the rich range of the part was analyzed by measuring its current-voltage characteristics in various gas mixtures. The analysis revealed that the limiting-current characteristics corresponding to combustible gas concentration rely on the oxygen dissociated from H 2 O.