Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-03-30
2003-07-08
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S408000, C204S426000, C204S427000
Reexamination Certificate
active
06589409
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multilayered gas sensing element capable of detecting a specific gas (such as NOx) concentration and preferably employable in an exhaust system for an internal combustion engine of an automotive vehicle.
Harmful gases emitted from automotive internal combustion engines cause serious air pollution the modem society now faces. Various laws and regulations require automotive manufacturers to satisfy severe standards for promoting purification of emission gases. Under such circumferences, it is known that the emission gas purification can be effectively performed by directly detecting the NOx concentration to feedback control the engine combustion as well as to monitor the catalyst condition based on the detected NOx value.
FIG. 17
shows a conventional multilayered gas sensing element
9
which is installable in the exhaust system of an automotive internal combustion engine and is capable of detecting the NOx concentration in the exhaust gas.
The multilayered gas sensing element
9
comprises solid electrolytic sheets
963
and
965
and insulating sheets
964
and
966
stacked to form a first chamber
911
and a second chamber
912
into which an objective gas to be measured is introduced.
A first diffusion resistive passage
910
connects the first chamber
911
to an outside of the gas sensing element
9
. A second diffusion resistive passage
920
connects the first chamber
911
to the second chamber
912
. Furthermore, the multilayered gas sensing element
9
comprises a reference gas chamber
914
into which a reference gas is introduced.
A first electrochemical cell
902
, located or provided in the first chamber
911
, pumps in and out oxygen in accordance with an applied voltage. A second electrochemical cell
903
, located or provided in the second chamber
912
, is responsive to application of a predetermined voltage for generating a sensor current representing a NOx concentration in the objective gas.
The second electrochemical cell
903
has an electrode on which NOx is reducible. Therefore, the NOx concentration can be measured based on an ion current caused by the reduction of NOx.
Accordingly, when excessive oxygen resides in the second chamber
912
and the oxygen concentration is fluctuating, the second electrochemical cell
903
produces an sensor output corresponding to a sum of NOx concentration and fluctuating oxygen concentration in the second chamber
912
. Thus, the NOx concentration detected by the second electrochemical cell
903
is inaccurate.
To solve this problem, the first electrochemical cell
902
is provided on the surface of the first chamber
911
. The first electrochemical cell
902
discharges oxygen from the first chamber
911
to the outside of the gas multilayered sensing element
9
so as to maintain the oxygen concentration in the first and second chambers
911
and
912
to a constant level.
However, according to the above-described conventional multilayered gas sensing element, when the air-fuel ratio of the exhaust gas to be measured is shifted to the rich side, it is necessary to introduce oxygen into the first chamber
911
to oxidize the reducible gas, such as propane, contained in the measured exhaust gas. However, one of the electrodes of the first electrochemical cell
902
is not brought into contact with the reference gas in the reference gas chamber
914
. This makes it impossible to detect the air-fuel ratio being switched to the rich side. Hence, the first electrochemical cell
902
cannot function properly in the rich side of air-fuel ratio.
Furthermore, in this case, the inside of second chamber
912
is filled by a rich environment. An inverse electromotive force is applied to the second electrochemical cell
903
. This makes it impossible to detect the NOx concentration.
Similar problem will arise in general multilayered sensing elements having a sensing mechanism for detecting a specific gas concentration based on an ion current caused in response to oxygen ion caused when the specific gas is decomposed on the electrode of the second electrochemical cell.
SUMMARY OF THE INVENTION
To solve the above-described problems, an object of the present invention is to provide a multilayered gas sensing element capable of accurately detecting a specific gas concentration of an exhaust gas emitted from an automotive engine even when combustion condition changes so widely that the air-fuel ratio changes in a wide range from a lean side to a rich side.
In order to accomplish the above and other related objects, the present invention provides a multilayered gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced, a first diffusion resistive passage connecting the first chamber to an outside of the gas sensing element, a second diffusion resistive passage connecting the first chamber to the second chamber, a first electrochemical cell having one end located in the first chamber and the other end provided in a reference gas chamber for pumping in and out oxygen in accordance with an applied voltage, and a second electrochemical cell having one end located in the second chamber and being responsive to application of a predetermined voltage for generating a sensor current representing a specific gas concentration in the objective gas.
The multilayered gas sensing element of the present invention is characterized in that the first electrochemical cell has one end located in the first chamber and the other end provided in the reference gas chamber and the second electrochemical cell has one end located in the second chamber.
The multilayered gas sensing element of the present invention functions in the following manner.
The first electrochemical cell faces both the first chamber and the reference gas chamber.
When the air-fuel ratio is shifted to the rich side, such switching of air-fuel ratio can be detected as the one end of the first electrochemical cell is located in the reference gas chamber. Accordingly, the first electrochemical cell can pump oxygen into the first chamber.
Furthermore, pumping operation of the first electrochemical cell makes it possible to maintain the oxygen concentration in the second chamber to a constant level. Accordingly, no inverse electromotive force is applied to the second electrochemical cell. This makes it possible to measure a specific gas concentration of the measured gas in a wide range from the lean side to the rich side.
As apparent from the foregoing, the present invention provides a multilayered gas sensing element capable of accurately detecting a specific gas concentration in a measured gas exhausted from an internal combustion engine even if combustion condition is controlled with the air-fuel ratio varying in a wide range from the lean side to the rich side.
Furthermore, according to the multilayered gas sensing element of the present invention, the oxygen pumping in and out operation causes an oxygen ion current flowing in an electric circuit of the first electrochemical cell.
The oxygen ion current has a current value representing the air-fuel ratio. Therefore, the first electrochemical cell can be used as an air-fuel ratio detecting cell.
In other words, the multilayered gas sensing element of the present invention can measure both the specific gas concentration and the air-fuel ratio simultaneously.
Application of the multilayered gas sensing element of the present invention is not limited to measurement of NOx concentration. Therefore, the multilayered gas sensing element of the present invention is applicable to other types of gas sensors, such as a CO sensor and a HC sensor.
Furthermore, according to the present invention, it is preferable that the second electrochemical cell has the other end located in the reference gas chamber.
This arrangement is effective to stabilize an electric potential of the positive electrode side of the second electrochemical cell. Thus, the specific gas concentration is accurately detectable.
Furthermore, it is preferable that the re
Miyashita Akira
Mizutani Keigo
Saito Toshitaka
Tanaka Akio
Denso Corporation
Nixon & Vanderhye PC
Tung T.
LandOfFree
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