1995-02-01

Estimation of the Switch Point of an Exhaust Gas Oxygen Sensor in General Exhaust Environments 950531

A steady state model of the ZrO2 exhaust gas oxygen sensor response to a simple (O2,CO,H2,N2) gas mixture has shown that while the detailed shape of the curve for sensor emf output versus inverse redox ratio for the gas mixture depends on many parameters, the step from a relatively high emf to a lower emf that occurs at a critical gas composition can be located from conservation constraints on the individual atomic species. In this paper, these conservation constraints are generalized; a Rule of Mixtures is developed that relates the inverse redox ratio of the gas at the ZrO2 sensor switch point (Rs′) to a weighted average of the corresponding switch points for individual oxygen /gas-component mixtures (Rsjo′):
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where j denotes a specific reductant species, zj is the stoichiometric factor of the j species for complete oxidation, pj is the partial pressure of gas species j in the mixture, and Δ is a well defined property of the O2 and NO oxidizing gases and the sensor electrodes.
Within this paper, data is accumulated from the literature to extend this Rule of Mixtures to gas compositions appropriate to automotive exhaust (including N2, O2, NO, CO, H2, C3H6, C3H8, CH4 and/or CHy where y is the average hydrogen-to-carbon ratio for a mixture of hydrocarbons). The Rule is validated by laboratory experiments on simple gas systems and then applied to predict sensor response in several different contexts. It is shown that the Rule of Mixtures can be useful in investigating sensor response to different fuels and in assessing sensor behavior when exposed to different exhaust compositions derived from the same fuel but different engines, engine operating conditions or sensor locations.

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