Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1996-04-16
1998-03-03
Bell, Bruce F.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204424, 204425, G01N 2726
Patent
active
057230307
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This application is a 371 of PCT/DE95/01387 filed Dec. 10, 1995.
The invention relates to an electrochemical measuring sensor for determining the oxygen content of gases, in particular, for determining the oxygen content in exhaust gases of internal combustion engines, having a tube-shaped sensor element formed of a solid electrolyte at whose outer side a measuring electrode is arranged which is exposed to the measuring gas and at whose inner side a reference electrode is arranged to a method for making the measuring sensor.
PRIOR ART
Electrochemical measuring sensors of the generic type are known. These are embodied, for example, in a so-called finger design, wherein a solid electrolyte body forms a sensor element which, as a closed tube, is tightly fixed in a metallic housing. Here, an outer measuring electrode of the sensor element is directly exposed to the gas to be measured, whereas an inner electrode serving as reference electrode is exposed to a reference gas, for example, the atmospheric oxygen. The electrodes are connected to an evaluation circuit via conductor tracks which, on the inside and on the outside, are guided on the closed tube. If the measuring electrode is acted upon by a gas to be measured, for example, by the exhaust gas of a motor vehicle, a different partial oxygen pressure appears at the measuring electrode and at the reference electrode so that a voltage signal can be tapped between the electrodes. This voltage signal serves to determine the oxygen content in the exhaust gas so that conclusions can be drawn regarding the operation of the internal combustion engine. Particularly the so-called lambda value can be determined which represents a measure for the composition of the air-fuel mixture with which the internal combustion engine is operated. Depending on whether it is the air or the fuel that is present in stoichiometric excess, the lambda value is larger or smaller than 1.
The known electrochemical measuring sensors have in common that they are directly exposed to the exhaust gas path so that the reference electrode needs to be sufficiently sealed vis-a-vis the exhaust gas path. For this purpose it is known to arrange a plurality of special seals which allow, on the one hand, a sealing of the reference electrode against the exhaust gas and, on the other hand, a supply of the reference gas, namely of the atmospheric oxygen, to the reference electrode. The seal arrangements have a complicated and complex design. Unburnt fuel reaches the exhaust gas path practically washing around the measuring sensor particularly during specific operating situations of the motor vehicle, for example, during startup, which means that even for gastight seals it cannot be excluded that the fuel enters the reference region.
SUMMARY AND ADVANTAGES OF THE INVENTION
In contrast to the known measuring sensors, the measuring sensor according to the invention for determining the oxygen content of gases, in particular, for determining the oxygen content in exhaust gases of internal combustion engines, has a tube-shaped sensor element formed of a solid electrolyte at whose outer side a measuring electrode is arranged which is exposed to the measuring gas and at whose inner side a reference electrode is arranged, and wherein the reference electrode is coated with a ceramic layer having a high gas diffusion resistance such that a pumping reference volume forms at the reference electrode. The measuring sensor according to the invention offers the advantage that a sealing of the reference electrode is possible in a simple manner. Since the reference electrode is allocated a pumping reference volume covered with a layer having a high gas diffusion resistance, it is possible in a simple manner to limit the sealing of the reference electrode to the layer that covers the pumping reference volume. The pumping reference volume forms an internal oxygen supply source for the reference electrode. By applying a pumping voltage to the reference electrode and a measuring electrode that
REFERENCES:
patent: 4174258 (1979-11-01), Bode
Bell Bruce F.
Robert & Bosch GmbH
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