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Technical Paper

A New Mechanism for Measuring Exhaust A/F

1993-11-01
932957
Exhaust gas air-fuel ratio (A/F) sensors are common devices in powertrain feedback control systems aimed at minimizing emissions. Both resistive (using TiO2) and electrochemical (using ZrO2) mechanisms are used in the high temperature ceramic devices now being employed. In this work a new mechanism for making the measurement is presented based on the change in the workfunction of a Pt film in interaction with the exhaust gas. In particular it is found that the workfunction of Pt increases reversibly by approximately 0.7 V at that point (the stoichiometric ratio) where the exhaust changes from rich to lean conditions. This increase arises from the adsorption of O2 on the Pt surface. On returning to rich conditions, catalytic reaction of the adsorbed oxygen with reducing species returns the workfunction to its original value. Two methods, one capacitive and one thermionic, for electrically sensing this workfunction change and thus providing for a practical device are discussed.
Technical Paper

Extended Range Air-to-Fuel Ratio Sensor

1984-09-01
841250
An oxygen sensor based on electrochemical pumping is described which is capable of measuring with high sensitivity A/F over an extended range from very lean to very rich A/F values. The sensor output is approximately proportional to A/F, has a weak dependence on temperature and is essentially independent of the gas pressure.
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