Ford Motor Company Limited

Allen L Schamp

Robert E Hetrick

A sensor for determining the stoichiometric air to fuel ratio (A/F) at the input to the cylinders of an internal combustion engine which is based on measuring the change in the workfunction of a material which occurs when the oxidizing and reducing species in the adjacent gas phase are at or near their stoichiometric ratio. In one method the sensor includes a material which is capable of thermionically emitting alkali metal or other appropriate ions into an exhaust gas atmosphere where they are subsequently collected by a nearby collector electrode and the magnitude of the emission current is measured. The interaction of the emitting surface with the gas phase reversibly changes the workfunction of the emitting surface from large to smaller values as that gas phase is varied through the stoichiometric ratio with respect to the amounts of the oxidizing and reducing species in the gas. Such a change in workfunction of the surface is accompanied by a change in the rate of the thermionic emission at the stoichiometric ratio thereby sensing that ratio in the gas phase which is proportional to the A/F at the input to the cylinders.

Workfunction based A/F sensor

A sensor determines a wide range of air-to-fuel ratio values (A/F) about the stoichiometric A/F in a gaseous mixture which has oxidizing and reducing species as might be found in an automotive exhaust. The sensor includes a surface that serves simultaneously as the working electrode of a solid-state oxygen concentration cell and as a surface whose workfunction changes from a higher to a lower value as the oxidizing and reducing species in the adjacent gas phase, and in interaction with the surface, pass from an excess oxidizing to the excess reducing condition about the stoichiometric ratio. In one embodiment, the surface exposed to the gas in question is the emitter electrode of a thermionic diode which emits positively charged alkali ions into the exhaust ambient where they are collected by a nearby collector electrode. Changes in the ratio of the oxidizing and reducing species about the stoichiometric ratio produce corresponding changes in the thermionic emission. Oxygen pumping to or from the working electrode modifies the thermionic emission in a systematic way to provide a pumping method of operation in which the magnitude of the pump current required to maintain the thermionic emission current at some preset value despite changes in the A/F ratio of the ambient leads to a relationship between pump current and A/F that can be used to sense the A/F.

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Deaborn, MI, United States of America

Allen L Schamp