Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
1999-11-16
2001-03-27
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
C073S023320, C073S031050, C422S090000
Reexamination Certificate
active
06205843
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensing element used in a gas sensor installable in an exhaust gas passage of an internal combustion engine of an automotive vehicle for detecting a specific gas component, such as a NOx gas concentration, contained in the exhaust gas, or an air-fuel ratio sensor incorporated in an air-fuel ratio (A/F) control system of the internal combustion engine, or a sensor for detecting an oxygen gas concentration.
Automotive vehicles exhaust harmful emission gases, such as NOx, HC, CO, that cause serious air pollution. Nowadays, the global warming phenomenon caused by CO2 is a big social problem to be solved.
To prevent air pollution, law regulations have become severe and strict to substantial reduce of harmful emissions exhausted from automotive vehicles and also in detection of deteriorated catalytic converters which may not function to purify the exhaust gases.
To stop the global warming phenomenon, effective countermeasures will include reduction of CO
2
, restriction in fuel consumption, and favorable treatment in tax.
In the market of automotive vehicles, reduction of harmful emissions and improvement of fuel economy are main goals to be attained to respond to the requirements from the modern society.
To satisfy such requirements, lean burn engines including direct-injection type engines (injecting fuel directly in a combustion chamber) have been developed as prospective engines having the capability of improving the fuel economy of the gasoline engines.
The lean burn techniques are characterized in that the air-fuel ratio (A/F ratio) is set to be a higher level compared with a stoichiometric (or theoretical) value, i.e., 14.7, of the ordinary engines. In general, shifting of the air-fuel ratio to a higher level increases the NOx amount contained in the exhaust gas. The conventional three-way catalytic converters were chiefly developed to purify the stoichiometric exhaust gases. Thus, it is necessary to develop a new exhaust gas purification system effectively applicable to advanced lean burn techniques.
Meanwhile, there are advanced diesel engines that are electronically controlled. Reduction of NOx gas is also important for the diesel engines. In this respect, development of so-called DeNOx catalytic converter systems will be indispensable for the advanced diesel engines.
To develop the NOx catalyst based purification systems, it is important to accurately control the purification rate of the NOx catalyst or monitor the deterioration of the NOx catalyst. To realize this, it is desirable to directly detect the NOx gas concentration of the exhaust gas.
Unexamined Japanese patent publication No. 64-39545 (No. 1-39545) discloses a gas sensing element that is capable of directly detecting the NOx gas concentration of the exhaust gas. This conventional gas sensing element comprises two sets of cells, each consisting of an oxygen pump cell and a sensor cell. Each oxygen pump cell includes electrodes provided on opposite sides of a solid electrolytic member disposed between a sample gas chamber and an exhaust gas passage. Each sensor cell includes electrodes provided on opposite sides of a solid electrolytic member disposed between the sample gas chamber and a reference gas chamber. The exhaust gas is introduced via an introducing port into sample gas chamber. The NOx gas concentration is detectable by measuring an electric current value between the sensor cells.
FIGS. 9A and 9B
are views showing another conventional gas sensing element (for example, disclosed in the Unexamined Japanese patent publication No. 8-271476). A gas sensing element
9
comprises two solid electrolytic members
901
and
902
between which a spacer is interposed. A sample gas chamber is formed in this spacer. The sample gas chamber consists of a first chamber
903
and a second chamber
904
.
The sample gas is introduced via an introducing passage
905
into the first chamber
903
. An oxygen sensor cell
91
detects the oxygen gas concentration in the first chamber
903
. A drive voltage of a first oxygen pump cell
92
is feedback controlled so as to equalize a detected oxygen gas concentration with a predetermined value.
The oxygen sensor cell
91
includes two electrodes
911
and
912
provided on a surface of the solid electrolytic member
902
. One electrode
911
is exposed to air in an air introducing passage
907
, and the other electrode
912
is exposed to the gas in the first chamber
903
. The first oxygen pump cell
92
comprises two electrodes
921
and
922
provided on opposite surfaces of the solid electrolytic member
901
. One electrode
921
is exposed to the sample gas, and the other electrode
922
is exposed to the gas in the first chamber
903
.
The second chamber
904
communicates with the first chamber
903
via a passage
906
. A second oxygen pump cell
93
is provided in the second chamber
904
to discharge the oxygen gas from the second chamber
904
. The second oxygen pump cell
93
comprises two electrodes
911
and
932
provided on the surface of the solid electrolytic member
902
. The electrode
932
, having NOx deoxidizing properties, is exposed to the gas in the second chamber
904
.
In the second chamber
904
, NOx contained in the exhaust gas is deoxidized and decomposed so as to newly generate oxygen gas. The pump current flowing through the second oxygen pump cell
93
increases or decreases in response to the generated oxygen gas.
The oxygen gas, contained in the sample gas diffusing from the first chamber
903
to the second chamber
904
, has a constant concentration. From this fact, it is believed that the increase or decrease of the pump current is dependent on the deoxidization of NOx. In other words, the NOx gas concentration is detectable by measuring the pump current.
However, the introducing passage of the above-described gas sensing element is a pinhole which is usually formed by machining. The pinhole is formed by opening a through hole extending vertically across a sheet serving as a solid electrolytic member.
The gas amount diffusing through the pinhole is dependent on the ambient temperature T. Usually, when the pinhole is formed by machining, the gas diffusion amount is proportional to T
1.75
.
For this reason, the output current of the sensor element has temperature dependency. When the exhaust gas temperature changes in a wide range, a significant measuring error will be caused.
This is a common problem raised in gas sensor elements which are used for detecting the concentration of specific gas components, such as O
2
, HC and CO, involved in the sample gas.
SUMMARY OF THE INVENTION
In view of the foregoing problems encountered in the prior art, the present invention has an object to provide a gas sensing element which is capable of eliminating measuring errors in severe circumstances where the temperature of the sample gas changes in a wide range.
Another object of the present invention is to provide a method for measuring a specific gas concentration as well as an oxygen gas concentration of a sample gas with a sensing element having a simplified structure.
In order to accomplish this and other related objects, the present invention provides a gas sensing element comprising a sample gas chamber into which a sample gas is introduced, a reference gas chamber into which a reference gas is introduced, a sensor cell located in both of the sample gas chamber and the reference gas chamber for detecting the concentration of a specific gas contained in the sample gas, an oxygen pump cell located in the sample gas chamber for pumping oxygen gas from or to the sample gas chamber, and an introducing passage including at least one pinhole provided on an outer surface of the oxygen pump cell for introducing the sample gas into the sample gas chamber. The outer surface of the oxygen pump cell faces an outside of the gas sensing element. And, a porous diffusion resistive layer is provided on the outer surface of the oxygen pump cell so as to cover a portion corresponding to the introducing pa
Hada Satoshi
Imamura Shinichiro
Mizutani Keigo
Sugiyama Tomio
Tanaka Akio
Denso Corporation
Pillsbury & Winthrop LLP
Wiggins David J.
Williams Hezron
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