Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-03-19
2003-04-29
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S427000
Reexamination Certificate
active
06554983
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensing element capable of detecting emission gas such as NOx 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 air pollution as a serious problem the modern society now faces. Various laws and regulations require automotive manufacturers to satisfy severe standards for promoting emission gas purification. 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.
FIGS. 9 and 10
show conventional gas sensing elements.
A pump cell
3
faces a first chamber
11
. To perform pumping of oxygen between the first chamber
11
and the outside of the sensing element, a voltage is applied to the pump cell
3
. A monitor cell
95
detects a concentration of oxygen in the first chamber
11
. The pump cell
3
is feedback controlled based on a detected value of monitor cell
95
to maintain a constant oxygen concentration.
A sensor cell
2
faces a second chamber
12
. The sensor cell
2
measures oxygen ions produced from NOx in the second chamber
12
and produces a sensor signal (i.e., oxygen ion current) representing a NOx concentration based on the measured oxygen ions. As the oxygen concentration in the second chamber
12
is constant, an amount of oxygen ions moving across the sensor cell
2
is proportional to the NOx concentration. In other words, the oxygen ion current of the sensor cell
2
is proportional to the NOx concentration.
Thus, the NOx concentration can be accurately measured irrespective of change of the oxygen concentration in the measured exhaust gas.
In this case, the sensor cell
2
is made of a material capably of decomposing NOx into oxygen ions and nitrogen ions to measure the NOx concentration. However, when the sensor cell
2
is made of other material, the sensor cell
2
will be able to measure another gas concentration.
However, the conventional gas sensing elements have the following problems.
The monitor cell, provided in the first chamber, cannot accurately monitor the oxygen concentration in the vicinity of the sensor cell provided in the second chamber. A significant difference will appear between the oxygen concentration of the first chamber and that of the second chamber.
The second chamber communicates with the first chamber via a narrow passage (i.e., diffusion resistive passage). Presence of such a narrow passage possibly delays transmission of oxygen concentration change to the second chamber compared with transmission to the first chamber. Accordingly, when the monitor cell is provided in the second chamber, the control of the first chamber is delayed. The response of control is worsened.
SUMMARY OF THE INVENTION
To solve the above-described problems, an object of the present invention is to provide a gas sensing element having excellent response and capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
In order to accomplish the above and other related objects, the present invention provides a first gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced. A first diffusion resistive passage connects the first chamber to an outside of the gas sensing element. A second diffusion resistive passage connects the first chamber to the second chamber. A pump cell, provided on a surface defining the first chamber, performs pumping of oxygen in accordance with an applied voltage. A first monitor cell, provided on a surface defining the first chamber, generates an electromotive force representing an oxygen concentration in the first chamber. A second monitor cell, provided on a surface defining the second chamber, generates an electromotive force representing an oxygen concentration in the second chamber. A sensor cell, provided on a surface defining the second chamber, is responsive to application of a predetermined voltage for generating a sensor current representing a specific gas concentration in the objective gas. And, the voltage applied to the pump cell is controlled based on the electromotive forces obtained from the first and second monitor cells.
According to the first gas sensing element, the first and second monitor cells face the first and second chambers respectively. The voltage applied to the pump cell is controlled based on the electromotive forces obtained from the first and second monitor cells.
The first gas sensing element of the present invention operates in the following manner.
The first monitor cell interposes between the first chamber and the reference gas chamber. The second monitor cell interposes between the second chamber and the reference gas chamber. Each of the first and second monitor cells generates an electromotive force in response to a measured oxygen concentration.
When the oxygen concentration in the measured gas is stable, there is no substantial difference between the oxygen concentration in the first chamber and the oxygen concentration in the second chamber. Thus, the electromotive force of the first monitor cell is substantially identical with that of the second monitor cell.
In this case, the voltage applied to the pump cell is controlled based on the electromotive force of the second monitor cell because the second monitor cell can accurately monitor the oxygen concentration in the vicinity of the sensor cell due to their positional relationship.
When the oxygen concentration in the measured gas is varying widely, the change of oxygen concentration is transmitted first to the first chamber and then transmitted with a larger delay to the second chamber. In other words, the electromotive force of the first monitor cell is apparently different from that of the second monitor cell.
When the oxygen concentration in the measured gas is increasing gradually, the electromotive force of the first monitor cell becomes smaller than that of the second monitor cell. On the other hand, when the oxygen concentration in the measured gas is decreasing gradually, the electromotive force of the first monitor cell becomes larger than that of the second monitor cell. This is due to time delay required when the measured gas passes through the diffusion resistive passage connecting the first chamber to the second chamber.
In such a transient state, to suppress adverse influence caused by deterioration in response, the voltage applied to the pump cell is controlled based on the electromotive force of the first monitor cell because the first monitor cell can promptly monitor the change of oxygen concentration caused in the measured gas.
As described above, the present invention provides the first gas sensing element which has excellent response and is capable of accurately detecting a specific gas concentration in a measured gas irrespective of unpredictable or unstable distribution of oxygen gas concentration.
Each of the sensor cell, the pump cell, and the monitor cell consists of a pair of electrodes with each electrode being made of a material individually selected considering the position where the cell is provided.
For example, the sensor cell has an electrode facing the second chamber. This electrode is required to have a function of generating oxygen ions from the specific gas to be detected.
The pump cell and the monitor cell have electrodes facing the first and second chambers. Preferably, these electrodes are inactive against the specific gas to be detected.
With this arrangement, it becomes possible to cause the decomposition of the specific gas in a limited region on the sensor cell, thereby enabling accurate measurement of specific gas concentration.
According to the first gas sensing element of the present invention, it is preferable that the first monito
Imamura Hiroo
Makino Daisuke
Mizutani Keigo
Saito Toshitaka
Denso Corporation
Nixon & Vanderhye P.C.
Tung T.
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