Device for measuring concentration of nitrogen oxide in combustion gas

• US 5,034,112 A
• Filed: 05/16/1989
• Issued: 07/23/1991
• Est. Priority Date: 05/19/1988
• Status: Expired due to Term

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Nissan Motor Co., Ltd.
• Inventors: Ito, Takao, Murase, Isao, Moriyama, Akinobu, Shimozono, Akira
• Primary Examiner(s): Niebling, John F.
• Assistant Examiner(s): Leader, William T.
• Application Number:
  US07/352,580
• Time in Patent Office: 798 Days
• Field of Search: 204/1 N, 204/410, 204/412, 204/425, 204/406, 204/153.14, 422/98
• US Class Current: 204/406
• CPC Class Codes: G01N 27/4065 Circuit arrangements specially adapted therefor
• CPC Class Codes: G01N 27/4074 for detection of gases other than oxygen
• CPC Class Codes: G01N 27/417 using cells , i.e. more than one cell and probes with solid electrolytes
• CPC Class Codes: G01N 33/0037 NOx
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

For use on automobiles, the invention provides a compact device for measuring the concentration of a nitrogen oxide in a combustion gas such as an exhaust gas of an internal combustion gas. The device has an ion pumping cell using an oxygen ion conductive solid electrolyte to adjust the partial pressure of oxygen in a gas diffusion chamber and a sensor such as an oxygen concentration cell using a solid electrolyte to detect the partial pressure of oxygen in the chamber. The pumping cell or the concentration cell is provided with a catalyst which decomposes the nitrogen oxide in the diffusion chamber only when the partial pressure of coexisting oxygen is sufficiently low. In the device the current supplied to the pumping cell is measured when the partial pressure of oxygen in the diffusion chamber is low enough for the activity of the catalyst and also when the oxygen partial pressure is made too high, and the concentration of the nitrogen oxide in the gas is computed from the two values of the current and the known relationship between the concentration of oxygen in the gas and the current necessary for keeping each of the high and low partial pressures of oxygen. It is convenient to use a pair of combinations of the pumping cell and the concentration cell to keep the high partial pressure of oxygen in one combination and the low partial pressure of oxygen in the other.

Claims

1. A device for measuring the concentration of a nitrogen oxide in a combustion gas, comprising:

an electrochemical cell which comprises a member of an oxygen ion conductive solid electrolyte and first and second electrodes attached to the solid electrolyte member;

a diffusion rate determining means for diffusing a fraction of the combustion gas, with a predetermined resistance to the gas diffusion, into a space in which the first electrode of the electrochemical cell is exposed;

a catalyst which is provided in or in the vicinity of the first electrode of said cell and which decomposes the nitrogen oxide only when the partial pressure of oxygen coexisting with the nitrogen oxide is lower than a predetermined level;

means for supplying a variable DC current to said electrochemical cell to flow in the solid electrolyte between the first and second electrodes to thereby cause migration of oxygen ions in a predetermined direction through the solid electrolyte between the first and second electrodes for adjusting the partial pressure of oxygen in the vicinity of the first electrode of said cell;

means for detecting the partial pressure of oxygen in the vicinity of the first electrode of said cell;

a discriminating means for deciding whether the detected partial pressure of oxygen is lower than said predetermined level or not;

a first current measuring means for measuring the current supplied to said electrochemical cell when it is decided that the detected partial pressure of oxygen is not lower than said predetermined level;

a second current measuring means for measuring the current supplied to said electrochemical cell when it is decided that the detected partial pressure of oxygen is lower than said predetermined level; and

means for computing the concentration of the nitrogen oxide in the combustion gas by using the two current values measured by the first and second current measuring means, respectively, the relationship between the concentration of oxygen in said gas and the current measured by the first current measuring means, and the relationship between said concentration of oxygen and the current measured by said second current measuring means, according to the following equation;

space="preserve" listing-type="equation">X.sub.NO =[(I.sub.P(B) -α

.sub.(B))-(γ

.sub.O(B) /γ

.sub.O(A))×

(I.sub.P(A) -α

.sub.(A))]/η

.sub.NO ·

γ

.sub.O(B)

where X_NO is the concentration of the nitrogen oxide, I_P(A) is the current measured by the first current measuring means, I_P(B) is the current measured by the second current measuring means, α

.sub.(A) is a current measured by the first current measuring means when the device is operated in a reference gas containing neither oxygen nor the nitrogen oxide, α

.sub.(B) is a current measured by the second current measuring means when the device is operated in said reference gas, γ

_O(A) is a predetermined coefficient indicating the ratio of the current measured by the first current measuring means to the concentration of oxygen in the combustion gas, γ

_O(B) is a predetermined coefficient indicating the ratio of the current measured by the second current measuring means to the concentration of oxygen in the combustion gas, and η

_NO is a constant.

2. A device according to claim 1, wherein said means for detecting the partial pressure of oxygen comprises an oxygen concentration cell comprising a plate of an oxygen ion conductive solid electrolyte, a measurement electrode on one side of the solid electrolyte plate and a reference electrode on the opposite side of the solid electrolyte plate, the concentration cell being arranged such that said measurement electrode is exposed in said space and located in the vicinity of said first electrode of said electrochemical cell while said reference electrode is exposed to a reference gas.

3. A device according to claim 2, wherein said discriminating means comprises means for detecting the output voltage of said concentration cell and comparing the detected output voltage with at least one reference voltage.

4. A device according to claim 2, wherein said electrochemical cell and said concentration cell are united into a single element such that said space is defined between the solid electrolyte member of said electrochemical cell and the solid electrolyte plate of said concentration cell.

5. A device according to claim 1, wherein said catalyst is selected from the group consisting of platinum and rhodium.

6. A device for measuring the concentration of a nitrogen oxide in a combustion gas, comprising:

first and second electrochemical cells each of which comprises a member of an oxygen ion conductive solid electrolyte and first and second electrodes attached to the solid electrolyte member;

a first diffusion rate determining means for diffusing a fraction of the combustion gas, with a predetermined resistance to the gas diffusion, into a first space in which the first electrode of the first electrochemical cell is exposed;

a second diffusion rate determining means for diffusing a fraction of the combustion gas, with a predetermined resistance to the gas diffusion, into a second space in which the first electrode of the second electrochemical cell is exposed;

a catalyst which is provided in or in the vicinity of the first electrode of each of the first and second electrochemical cells and which decomposes the nitrogen oxide only when the partial pressure of oxygen coexisting with the nitrogen oxide is lower than a predetermined level;

first and second current supplying means for supplying a variable DC current to the first and second electrochemical cells, respectively, to flow in the solid electrolyte between the first and second electrodes to thereby cause migration of oxygen ions through the solid electrolyte between the first and second electrodes;

a first control means for detecting the partial pressure of oxygen in the vicinity of the first electrode of the first electrochemical cell and controlling the current supplied to the first cell such that the detected partial pressure of oxygen becomes constant and higher than said predetermined level;

a first current measuring means for measuring the current supplied to the first electrochemical cell while the partial pressure of oxygen detected by said first control means is higher than said predetermined level;

a second control means for detecting the partial pressure of oxygen in the vicinity of the first electrode of the second electrochemical cell and controlling the current supplied to the second cell such that the detected partial pressure of oxygen becomes constant and lower than said predetermined level;

a second current measuring means for measuring the current supplied to the second electrochemical cell while the partial pressure of oxygen detected by said second control means is lower than said predetermined level; and

means for computing the concentration of the nitrogen oxide in the combustion gas by using the two current values measured by the first and second current measuring means, respectively, the relationship between the concentration of oxygen in said gas and the current measured by the first current measuring means, and the relationship between said concentration of oxygen and the current measured by the second current measuring means, according to the following equation;

space="preserve" listing-type="equation">X.sub.NO =[(I.sub.P(B) -α

.sub.(B))-(γ

.sub.O(B) /γ

.sub.O(A))×

(I.sub.P(A) -α

.sub.(A))]/η

.sub.NO ·

γ

.sub.O(B) (I)

where X_NO is the concentration of the nitrogen oxide, I_P(A) is the current measured by the first current measuring means, I_P(B) is the current measured by the second current measuring means, α

.sub.(A) is a current measured by the first current measuring means when the device is operated in a reference gas containing neither oxygen nor the nitrogen oxide, α

.sub.(B) is a current measured by the second current measuring means when the device is operated in said reference gas, γ

_O(A) is a predetermined coefficient indicating the ratio of the current measured by the first current measuring means to the concentration of oxygen in the combustion gas, γ

_O(B) is a predetermined coefficient indicating the ratio of the current measured by the second current measuring means to the concentration of oxygen in the combustion gas, and η

_NO is a constant.

7. A device according to claim 6, wherein said first and second control means respectively comprise first and second oxygen concentration cells each of which comprises a plate of an oxygen ion conductive solid electrolyte, a measurement electrode on one side of the solid electrolyte plate and a reference electrode on the opposite side of the solid electrolyte plate, the first concentration cell being arranged such that its measurement electrode is exposed in said first space and located in the vicinity of the first electrode of said first electrochemical cell, the second concentration cell being arranged such that its measurement electrode is exposed in said second space and located in the vicinity of the first electrode of said second electrochemical cell.

8. A device according to claim 7, wherein said first electrochemical cell and said first concentration cell are united into a single element such that said first space is defined between the solid electrolyte member of said first electrochemical cell and the solid electrolyte plate of said first concentration cell, said second electrochemical cell and said second concentration cell being united into a single element such that said second space is defined between the solid electrolyte member of said second electrochemical cell and the solid electrolyte plate of said second concentration cell.

9. A device according to claim 8, wherein the united combination of said first electrochemical cell with said first concentration cell and the united combination of said second electrochemical cell with said second concentration cell are integrated on a substrate into a single part.

10. A device acccording to claim 7, wherein each of the first and second control means comprises means for comparing the output voltage of the assigned one of said first and second concentration cells with a reference voltage for controlling the current supplied to the assigned electrochemical cell so as to keep said output voltage equal to the reference voltage.

11. A device according to claim 6, wherein said catalyst is selected from the group consisting of platinum and rhodium.

12. A device for measuring the concentration of a nitrogen oxide in a combustion gas, comprising:

an electrochemical cell which comprises a member of an oxygen ion conductive solid electrolyte and first and second electrodes attached to the solid electrolyte member;

a diffusion rate determining means for diffusing a fraction of the combustion gas, with a predetermined resistance to the gas diffusion, into a space in which the first electrode of the electrochemical cell is exposed;

a catalyst which is provided in or in the vicinity of the first electrode of said cell and decomposes the nitrogen oxide only when the partial pressure of oxygen coexisting with the nitrogen oxide is lower than a predetermined level;

means for supplying a variable DC current to said electrochemical cell to flow in the solid electrolyte between the first and second electrodes to thereby cause migration of oxygen ions in a predetermined direction through the solid electrolyte between the first and second electrodes for adjusting the partial pressure of oxygen in the vicinity of the first electrode of said cell;

a first reference means for producing a first electrical signal which represents a relatively high first reference partial pressure;

a second reference means for producing a second electrical signal which represents a relatively low second reference partial pressure;

a control means for detecting the partial pressure of oxygen in the vicinity of the first electrode of said electrochemical cell and controlling the current supplied to the electrochemical cell such that the detected partial pressure of oxygen becomes constant and higher than said predetermined level while said first electrical signal is supplied to the control means and becomes constant and lower than said predetermined level while said second electrical signal is supplied to the control means;

a switching means for alternately supplying said first electrical signal and said second electrical signal to said control means at predetermined time intervals;

a current measuring means for measuring the current supplied to said electrochemical cell while the partial pressure of oxygen detected by said control means is higher than said predetermined level and also while the partial pressure detected by said control means is lower than said predetermined level; and

means for computing the concentration of the nitrogen oxide in the combustion gas by using the current value measured by said current measuring means while the detected partial pressure of oxygen is higher than said predetermined level, the current value measured by said current measuring means while the detected partial pressure of oxygen is lower than said predetermined level, the relationship between the concentration of oxygen in the gas and the current measured by said current measuring means while the detected partial pressure of oxygen is higher than said predetermined level, and the relationship between said concentration of oxygen and the current measured by said current measuring means while the detected partial pressure of oxygen is lower than said predetermined level, according to the following equation;

space="preserve" listing-type="equation">X.sub.NO =[(I.sub.P(B) -α

)-(γ

.sub.O(B) /γ

.sub.O(A))×

(I.sub.P(A) -α

)]/η

.sub.NO ·

γ

.sub.O(B) (II)

where X_NO is the concentration of the nitrogen oxide, I_P(A) is the current measured by the current measuring means while the detected partial pressure of oxygen is higher than said predetermined level, I_P(B) is the current measured by the current measuring means while the detected partial pressure of oxygen is lower than said predetermined level, α

is a current measured by the current measuring means when the device is operated in a reference gas containing neither oxygen nor the nitrogen oxide, γ

_O(A) is a predetermined coefficient indicating the ratio of the current measured by the current measuring means while the detected partial pressure of oxygen is higher than said predetermined limit to the concentration of oxygen in the combustion gas, γ

_O(B) is a predetermined coefficient indicating the ratio of the current measured by the current measuring means while the detected partial pressure of oxygen is lower than said predetermined limit to the concentration of oxygen in the combustion gas, and η

_NO is a constant.

13. A device according to claim 12, wherein said control means comprises an oxygen concentraion cell comprising a plate of an oxygen ion conductive solid electrolyte, a measurement electrode on one side of the solid electrolyte plate and a reference electrode on the opposite side of the solid electrolyte plate, the concentration cell being arranged such that said measurement electrode is exposed in said space and located in the vicinity of said first electrode of said electrochemical cell while said reference electrode is exposed to a reference gas.

14. A device according to claim 13, wherein said electrochemical cell and said concentration cell are united into a single element such that said space is defined between the solid electrolyte member of said electrochemical cell and the solid electrolyte plate of said concentration cell.

15. A device according to claim 13, wherein each of said first and second electrical signals is a constant voltage signal, said control means comprising means for comparing the output voltage of said concentration cell with either of the first and second voltage signals for controlling the current supplied to the electrochemical cell so as to keep said output voltage equivalent to the compared voltage signal.