Method and apparatus for measuring oxygen concentration and nitrogen oxide concentration

• US 6,214,207 B1
• Filed: 11/06/1997
• Issued: 04/10/2001
• Est. Priority Date: 11/08/1996
• Status: Expired due to Term

• Current Assignee: NGK Spark PLUG Company Limited (Niterra Co., Ltd.)
• Original Assignee: NGK Spark PLUG Company Limited (Niterra Co., Ltd.)
• Inventors: Inagaki, Hiroshi, Miyata, Shigeru, Kondo, Noriaki
• Primary Examiner(s): Warden, Sr., Robert J.
• Assistant Examiner(s): Olsen, Kaj K.
• Application Number:
  US08/965,512
• Time in Patent Office: 1,251 Days
• Field of Search: 204/426, 204/425, 204/408, 205/781, 205/783.5, 205/784, 205/784.5
• US Class Current: 205/781
• 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

Abstract

Using a sole Nox sensor Nox concentration and oxygen concentration are measured accurately. A measurement device for measuring NOx concentration and oxygen concentration comprises an NOx sensor 2 having a first measurement chamber 20 communicating via a diffusion rate regulating layer 4d with the gas under measurement and a second measurement chamber 26 communicating with the first measurement chamber 20 via diffusion rate regulating layers 6d, 22d. A first pump cell 4 and an oxygen concentration measurement Vs cell 6 are formed on the first measurement chamber 20, while a second pump cell 8 is formed on the second measurement chamber 26. Inside of the first measurement chamber 20 is controlled to a constant low oxygen concentration by controlling the first pump current IP1 so that output of Vs cell 6 will equal a reference voltage VC0. By applying a constant voltage across the second pump cell 8 NOx in the second measurement chamber 26 is decomposed to pump out oxygen. Thus NOx concentration and oxygen concentration are measured from the second pump current IP2 and from the first pump current IP1, respectively. During measurement, sensor temperature is detected from the internal resistance of the Vs cell for controlling heater current supplied to heaters 12, 14 to maintain a constant sensor temperature.

Claims

1. A method for measuring an oxygen concentration and a nitrogen oxide concentration in a gas under measurement comprising:

(a) providing a NOx sensor comprising a first measurement chamber, a second measurement chamber, a first oxygen pumping cell, a second oxygen pumping cell and a heater,

said first measurement chamber comprising said first oxygen pumping cell and an oxygen concentration measuring cell and communicating with the gas under measurement via a first diffusion rate regulating portion, said first oxygen pumping cell comprising an oxygen ion conductive solid electrolyte layer sandwiched between a pair of porous electrodes, said second oxygen pumping cell comprising another oxygen ion conductive solid electrolyte layer sandwiched between another pair of porous electrodes, said second measurement chamber communicating with the first measurement chamber via a second diffusion rate regulating portion, and said heater being adapted for heating the cells, said solid electrolyte layers being disposed in lamination;

wherein an oxygen concentration measurement cell for measuring the oxygen concentration of the gas under measurement supplied to the second measurement chamber is disposed between said first and second oxygen pumping cells in a laminating direction of said solid electrolyte layers;

(b) causing a current flow in said first oxygen pumping cell so that a deviation from a target value of an output voltage of the oxygen concentration measurement cell will be zero, thereby controlling an oxygen concentration in the first measurement chamber to a constant value;

(c) applying a constant voltage across said second oxygen pumping cell in a direction of pumping oxygen out of the first measurement chamber;

(d) measuring a value of a current flowing in the second oxygen pumping cell which is indicative of the nitrogen oxide concentration in the gas under measurement;

(e) simultaneously measuring a value of the current flowing in the first oxygen pumping cell which is indicative of the oxygen concentration in the gas under measurement and

(f) measuring a temperature of the oxygen concentration measuring cell and correcting the current flowing in the second pumping cell based on said measured temperature.

2. The method as defined in claim 1,

wherein said heater is arranged on both sides of a laminate of said solid electrolyte layers at positions superposing said first and second oxygen pumping cells and said oxygen concentration measurement cell; and

wherein an amount of the current supplied to the heater is controlled by directly detecting internal resistance of said oxygen concentration measurement cell so that a temperature of the oxygen concentration measurement cell in the NOx sensor will be a target temperature.

3. The method as defined in claim 2, wherein measured results of the oxygen concentration and the nitrogen oxide concentration are corrected for a deviation from the target temperature of the oxygen concentration measurement cell.

4. The method of claim 1, wherein the oxygen concentration measuring cell alternatively functions as a temperature sensor of the oxygen concentration measuring cell and as an oxygen concentration measuring sensor for the gas under measurement supplied to the second measurement chamber.

6. The device of claim 4, further comprising:

heater current supply controlling means for controlling a current supplied to said heater so that the temperature of said oxygen concentration measurement cell as detected by said temperature detection means will be a target temperature.

7. The device of claim 6, further comprising:

correction means for temperature compensation of measured results of the oxygen concentration and the nitrogen oxide concentration by correcting said measured results in response to a deviation from said target temperature of the oxygen concentration measurement cell as detected by said temperature detection means.

8. The device of claim 7, wherein said temperature correction means comprises a computer which calculates an amount of temperature correction of the oxygen concentration and the nitrogen oxide concentration based on a correction map as a function of the deviation from the target temperature.

9. The device of claim 7, wherein the oxygen concentration measurement cell has a porous electrode facing the first measurement chamber.

10. The device of claim 6, wherein said temperature detection means detects the temperature of the oxygen concentration measurement cell by detecting an internal resistance of the oxygen concentration measurement cell and wherein said heater current supply controlling means controls a current supplied to said heater so that the internal resistance of the oxygen concentration measurement cell will be of a value corresponding to the target temperature.

11. The device of claim 10, further comprising a porous electrode on a side of the oxygen concentration measurement cell, said porous electrode being opposite to the first measurement chamber and closed, and part of oxygen in a closed space can be leaked out via a leakage resistance;

and

wherein said pump current control means causes a small amount of current to flow in said oxygen concentration measurement cell in a direction of pumping oxygen out of said first measurement space into said closed space to control an amount of current flowing in said first oxygen pumping cell so that as said closed space is caused to function as an internal oxygen reference source, an electromotive force generated across said oxygen concentration measurement cell will be of a constant value;

said temperature detection means causing periodic interruption of a connection between the pump current control means and the oxygen concentration measurement cell and during said interruption an amount of current larger than said small current for detecting the internal resistance of said oxygen concentration measurement cell is caused to flow in said oxygen concentration measurement cell in a direction opposite to that of said small current, the internal resistance of said oxygen concentration measurement cell being detected from a voltage generated across the electrodes of the oxygen concentration measurement cell during said interruption.

12. The device of claim 11, wherein said temperature detection means causes the current for detecting the internal resistance of said oxygen concentration measurement cell to flow across said oxygen concentration measurement cell in one direction and then in an opposite direction.

13. The device of claim 10, wherein the oxygen concentration measuring cell alternatively functions as a temperature sensor of the oxygen concentration measuring cell and as said oxygen concentration measuring cell.

14. The device of claim 13, wherein the oxygen concentration measuring cell is connected to a switching circuit that changes over between two modes, wherein a first mode is for supplying a constant voltage for the measuring the oxygen concentration and a second mode is for measuring said internal resistance of the oxygen concentration measurement cell.

15. The device of claim 14, wherein in said first mode, the oxygen concentration measuring cell is connected to an input of a comparator (AMP) for comparing the output voltage of the oxygen concentration cell with a target value (VC0), the comparator outputting an output signal indicative of said deviation from the target value of the output voltage of the oxygen concentration measurement cell, thereby causing said current to flow in the first oxygen pumping cell.

16. The device of claim 15, wherein said current is measured and evaluated by an electronic control unit (ECU) to determine the oxygen concentration of the gas under measurement, while said electronic control unit (ECU) determines the nitrogen oxide concentration based on the value of the current flowing in the second oxygen pumping cell.

17. The device of claim 16, wherein said electric control unit (ECU) controls said switching circuit for alternately changing over between said first and second modes.

5. A device for measuring an oxygen concentration and a nitrogen oxide concentration in a gas under measurement, comprising:

(a) a NOx sensor comprising a first measurement chamber, a second measurement chamber, a first oxygen pumping cell, a second oxygen pumping cell and a heater, said first measurement chamber comprising said first oxygen pumping cell and an oxygen concentration measuring cell and communicating with the gas under measurement via a first diffusion rate regulating portion, said first oxygen pumping cell comprising an oxygen ion conductive solid electrolyte layer sandwiched between a pair of porous electrodes, said second oxygen pumping cell comprising another oxygen ion conductive solid electrolyte layer sandwiched between another pair of porous electrodes, said second measurement chamber communicating with the first measurement chamber via a second diffusion rate regulating portion, and said heater being adapted for heating the cells;

said solid electrolyte layers being disposed in lamination;

wherein an oxygen concentration measurement cell for measuring the oxygen concentration of the gas under measurement supplied to the second measurement chamber is disposed between said first and second oxygen pumping cells in a laminating direction of said solid electrolyte layers;

(b) pump current control means for causing a current to flow in said first oxygen pumping cell so that a deviation from a target value of an output voltage of the oxygen concentration measurement cell will be zero, thereby controlling an oxygen concentration in the first measurement chamber to a constant value;

(c) constant voltage application means for applying a constant voltage across said second oxygen pumping cell in a direction of pumping oxygen out of the second measurement chamber;

(d) nitrogen oxide concentration measurement means for measuring the nitrogen oxide concentration in the gas under measurement based on a value of a current flowing in the second oxygen pumping cell;

(e) oxygen concentration measurement means for simultaneously measuring the oxygen concentration in the gas under measurement based on a value of the current flowing in the first oxygen pumping cell

(f) temperature detection means for directly detecting a temperature of the oxygen concentration measurement cell, and

(g) correction means for correction of the current flowing in the second pumping cell based on a temperature as measured by the oxygen concentration measuring cell

wherein said heater is arranged on both sides in a laminating direction of said solid electrolyte layers at positions superposing said first and second oxygen pumping cells and said oxygen concentration measurement cell.

18. The device of claim 5, wherein the oxygen concentration measurement cell comprises a solid electrolyte layer,

said first measurement chamber and the second measurement chamber are made up by laminating solid electrolyte layers interposed with a small gap.

19. The device of claim 18, wherein said second diffusion rate regulating layer overlaps at least a portion of the first diffusion rate regulating layer and said oxygen concentration measurement cell is provided in a vicinity of said second diffusion rate regulating layer.

20. The device of claim 18, wherein said heater comprises two heater substrates in the form of thin plates with heater wiring embedded therein.

21. The device of claim 20, wherein the first diffusing rate regulating layer is arranged in the solid electrolyte layer of the first oxygen pumping cell so that said first diffusion rate regulating layer is disposed between said heater substrates.

22. The device of claim 5, further comprising porous electrodes of the oxygen concentration measurement cell, wherein one of said porous electrodes is disposed in the first measurement chamber.

23. The device of claim 5, wherein one of said porous electrodes of the first oxygen pumping cell is disposed in the first measurement chamber.

24. The device of claim 5, wherein said oxygen concentration measurement means comprises an electronic computing unit having a computer which calculates the oxygen concentration of the gas under measurement based on the value of the current flowing in the first oxygen pumping cell.