Capacitive air-fuel ratio sensor

A new mechanism for determining the stoichiometric air-fuel ratio (A/F) of the concentrations of oxidizing and reducing species in a gas phase is presented. The mechanism, which appears suitable for use in the automotive application, is the reversible change in the work function of Pt (by as much as 0.7 V). This change occurs at elevated temperatures when the Pt is alternately exposed to dilute mixtures of oxidizing (oxygen) and reducing (hydrocarbons and related molecules) species that are rich or lean of their stoichiometric ratio. The change can be electrically monitored by the Kelvin vibrating capacitor method or other capacitive techniques. The sensitivity to A/F is verified by comparing the Kelvin signal with that of a ZrO2 electrochemical A/F sensor whose working Pt electrode is simultaneously the active surface in the Kelvin measurement. With the observation that O2 pumping to or from the active Pt surface with the ZrO2 cell can also significantly affect the work function, the possibility exists for developing an extended range A/F sensor using this mechanism.