Methods and apparatus for measuring NOx gas concentration, for detecting exhaust gas concentration and for calibrating and controlling gas sensor

• US 20020130053A1
• Filed: 02/06/2002
• Published: 09/19/2002
• Est. Priority Date: 03/21/1997
• Status: Active Grant

• Current Assignee: NGK Spark PLUG Company Limited
• Original Assignee: NGK Spark PLUG Company Limited
• Inventors: Ando, Masashi, Sato, Yoshikuni, Ishida, Noboru, Sugaya, Satoshi, Nadanami, Norihiko, Oshima, Takafumi, Otsuka, Takaki, Kuzuya, Yasuhisa, Okumura, Tatsuo
• Granted Patent: US 6,743,352 B2
• Time in Patent Office: Days
• US Class Current: 205/781
• CPC Class Codes: F01N 11/00 Monitoring or diagnostic devices for exhaust-gas treatment apparatus , e.g. for catalytic activity safety, indicating or supervising devices for internal combustion engines F02B77/08; testing of machines G01M13/00
• CPC Class Codes: F01N 2550/02 Catalytic activity of catalytic converters
• CPC Class Codes: F01N 2550/03 of sorbing activity of adsorbents or absorbents
• CPC Class Codes: F01N 2560/026 for measuring or detecting NOx
• CPC Class Codes: F01N 2570/14 Nitrogen oxides
• CPC Class Codes: F01N 2610/03 the substance being hydrocarbons, e.g. engine fuel
• CPC Class Codes: F01N 3/0842 Nitrogen oxides
• CPC Class Codes: F01N 3/0871 Regulation of absorbents or adsorbents, e.g. purging by electrically controlling the supply of combustible mixture or its constituents only F02D41/0235
• CPC Class Codes: F01N 3/2066 Selective catalytic reduction [SCR]
• CPC Class Codes: F02D 41/146 the characteristics being an NOx content or concentration
• CPC Class Codes: F02D 41/2474 Characteristics of sensors
• CPC Class Codes: F02D 41/28 Interface circuits
• CPC Class Codes: G01N 27/4175 Calibrating or checking the analyser
• CPC Class Codes: G01N 27/419 Measuring voltages or currents with a combination of oxygen pumping cells and oxygen concentration cells
• CPC Class Codes: G01N 33/0037 for NOx
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• CPC Class Codes: Y02T 10/12 Improving ICE efficiencies
• CPC Class Codes: Y02T 10/40 Engine management systems

Abstract

A method for detecting the concentration of a specific gas (NOx) in an engine exhaust gas using a gas sensor and a process of correcting the gas sensor. According to the correcting process, the amount of moisture in the exhaust gas is estimated, then a a gas concentration detection signal output by the gas sensor on the basis of an estimated amount of moisture is corrected.

Finally, the effect of the amount of moisture in the exhaust gas is removed or reduced from the detected value of gas concentration.

Claims

1. A method for measuring the nitrogen oxide concentration using a nitrogen oxide concentration detector having:

a first measurement chamber into which is introduced a measurement gas via a first diffusion resistance;

an oxygen concentration detection electrode for measuring the oxygen concentration in the detection gas in first measurement chamber;

a first oxygen ion pump cell for pumping oxygen contained in the measurement gas from said first measurement chamber to outside and/or from outside to inside of said first measurement chamber based on the potential of said oxygen concentration detection electrode;

a second measurement chamber into which the gap is introduced from said first measurement chamber via a second diffusion resistance; and

a second oxygen ion pump cell having a pair of electrodes across which a voltage is applied to decompose the nitrogen oxide in said second measurement chamber to pump out dissociated oxygen to cause a current corresponding to the nitrogen oxide concentration, referred to as second oxygen pump current, to flow in the second oxygen ion pump cell;

wherein said first oxygen ion pump cell sufficient pumps out oxygen from the first measurement chamber to an extent not to allow all nitrogen oxide to decompose oxygen in said measurement gas;

wherein the nitrogen oxide concentration obtained based on said second oxygen pump current is corrected responsive to the oxygen concentration in the detection gas.

2. The method as defined in claim 1

wherein variation of the nitrogen oxide concentration is a function of variation of said second oxygen pump current and wherein a coefficient of the variation of the second oxygen pump current referred to as gain, in said function is varied responsive to the oxygen concentration in the detection gas to find the nitrogen oxide concentration.

3. The method as defined in claim 2

wherein said gain is a function of the oxygen concentration in the detection gas;

wherein a detection gas having a known oxygen concentration and a known nitrogen oxide concentration is injected into said detector to measure the second oxygen pump current;

wherein a variation of the second oxygen pump current versus to the variation of the nitrogen oxide concentration at a pre-set oxygen concentration, referred to as gain at a pre-set oxygen concentration is found, using values of said nitrogen oxide concentration and value of the second oxygen pump current; and

wherein a coefficient of said oxygen concentration in a function of the gain and the oxygen concentration in the detection gas is determined using the value of the gain in said pre-set oxygen concentration.

4. The method as defined in claim 2

wherein said gain is represented as a function of the oxygen concentration in the detection gas, the detection gas having a known oxygen concentration and a known nitrogen oxide concentration is injected to measure the second oxygen pump current;

wherein a variation of the second oxygen pump current versus the variation of the nitrogen oxide concentration at a pre-set oxygen concentration, referred to as gain at a pre-set oxygen concentration is found, using the values of said nitrogen oxide concentration and the value of the second oxygen pump current; and

wherein the gain for said pre-set oxygen concentration is selected and used responsive to the oxygen concentration in the detection as said gain.

5. The method as defined in claim 3

wherein said gain is represented in terms of a logarithm of the oxygen partial pressure in the detection gas.

6. The method as defined in any one of claims 2 to 5;

wherein the oxygen concentration in the detection gas is varied to measure the second oxygen pump current values for substantially zero concentration and a pre-set concentration of the nitrogen oxide, the gain at said pre-set oxygen concentration and said second oxygen concentration, referred to as offset for the pre-set oxygen concentration, are measured, and

wherein the nitrogen oxide concentration is found from said gain and the offset corresponding to the oxygen concentration in the detection gas and from the second oxygen pump current.

7. The method as defined in any one of claims 1 to 5 wherein the nitrogen oxide concentration obtained based on said second oxygen pump current is corrected based on an output of said oxygen concentration detecting electrode which varies in accordance with the oxygen concentration in the detection gas.

27. The method for correcting the gas sensor as defined in claim 1 wherein said gas sensor is a NOx sensor.

8. An apparatus for measuring the nitrogen oxide concentration comprising:

(a) a nitrogen oxide concentration detector having

(a1) a first measurement chamber into which a detection gas is introduced via a first diffusion resistance;

(a2) an oxygen concentration detection electrode measuring the oxygen concentration in the detection gas in said first measurement chamber;

(a3) a first oxygen ion pump cell sufficiently pumping oxygen contained in the detection gas from said first measurement chamber to outside and/or from outside to inside of said first measurement chamber based on the potential of said oxygen concentration detection electrode to an extent not to allow all nitrogen oxide to decompose oxygen in said measurement gas;

(a4) a second measurement chamber into which the gas is introduced from said first measurement chamber via a second diffusion resistance; and

(a5) a second oxygen ion pump cell having a pair of electrodes across which a voltage is applied to decompose the oxygen nitride in said second measurement chamber to pump out dissociated oxygen to cause the current corresponding to the nitrogen oxide concentration referred to as second oxygen pump current, to flow in the second oxygen ion pump cell;

(b) gain selection means for selecting a coefficient, referred t as gain, of variation of said second oxygen pump current in a function responsive to an output of said oxygen concentration detection electrode, provided that said variation in said nitrogen oxide concentration is said function of variation of said second oxygen pump current; and

(c) operation means for calculating the nitrogen oxide concentration based on said second oxygen pump current and the gain selected by said gain selection means.

9. A method for controlling a nitrogen oxide concentration detector having:

a first measurement chamber into which a measurement gas is introduced via a first diffusion resistance;

a first oxygen ion pump cell sufficiently pumping oxygen in the measurement gas from said first measurement chamber to outside and/or from outside to inside of said measurement chamber to an extent not to allow all the nitrogen oxide to decompose in the gas under measurement;

a second measurement chamber into which a measurement gas is introduced from the first measurement chamber via a second diffusion resistance; and

a second oxygen ion pump cell having a pair of electrodes provided on the inside and outside of the second measurement chamber, an electrical voltage being applied across said electrodes to decompose the nitrogen oxide in said second measurement chamber to dissociate oxygen, said second oxygen ion pump cell pumping out dissociated oxygen to cause a flow of current corresponding to the nitrogen oxide concentration, referred to as second oxygen ion pump current;

said method comprising;

correcting a detection output of the nitrogen oxide concentration of said nitrogen oxide concentration detector derived from said second oxygen ion pump current responsive to changes caused in the oxygen partial pressure in the measurement gas.

10. The method, as defined in claim 9, for controlling a nitrogen oxide concentration detector having an oxygen partial pressure detection cell including an oxygen partial pressure detection electrode for detecting the oxygen partial pressure in said first measurement chamber, said method comprising:

correcting a detection output of the nitrogen oxide concentration based on an output of said oxygen partial pressure detection cell.

11. The method as defined in claim 10 wherein the voltage applied across said first oxygen pump cell is controlled based on an output of said oxygen partial pressure detection cell and wherein a detection output of the nitrogen oxide concentration is corrected based on a variation of the current flowing in said first oxygen ion pump cell, referred to as first oxygen ion pump current.

15. The control apparatus as defined in claim 14 comprising:

a constant oxygen partial pressure controller which controls said first oxygen ion pump cell to provide a constant output of said oxygen partial pressure detection cell; and

a storage unit having pre-stored therein the relation between variation of output of said oxygen partial pressure detection cell and offset of said second oxygen ion pump current;

wherein pre-set data is read from said storage unit responsive to a variation of an output of said oxygen partial pressure detection cell, and an offset value of the second oxygen ion pump current is varied based on said data to correct a detection output of said nitrogen oxide concentration.

20. The method as defined in claim 19,

wherein said NOx sensor has first and second air gaps, first and second diffusion resistance units and first and second oxygen ion pump cells;

wherein said exhaust gases are diffused via said first diffusion resistance into said first gap, said first oxygen ion pump cell pumping out oxygen from said first gap so that oxygen in the gas diffused via said first diffusion resistance into said first gap will be of a specified oxygen concentration; and

wherein the gas having the specified oxygen concentration is diffused from said first gap via said second diffusion resistance into said second gap;

NOx is decomposed in said second gap;

said second oxygen ion pump cell pumping out dissociated oxygen ions; and

the NOx concentration is detected from a current flowing in said second oxygen ion pump cell.

21. The method as defined in claim 19 or 20 wherein said NOx sensor is mounted downstream of a NOx occlusion catalyst and wherein said zero point is calibrated based on a detection output of said NOx sensor when an air-to-fuel ratio is temporarily set to a rich side for cleaning NOx occluded in said NOx occlusion catalyst.

12. A method for controlling a nitrogen oxide concentration detector having:

a first measurement chamber into which a measurement gas is introduced via a first diffusion resistance;

a first oxygen ion pump cell sufficiently pumping oxygen in the measurement gas from said first measurement chamber to outside and/or from outside to inside of said measurement chamber to an extent not to allow all the nitrogen oxide to decompose oxygen in the gas under measurement;

a second measurement chamber into which a measurement gas is introduced from the first measurement chamber via a second diffusion resistance; and

a second oxygen ion pump cell having a pair of electrodes provided on the inside and outside of the second measurement chamber, an electrical voltage being applied across said electrodes to decompose the nitrogen oxide in said second measurement chamber to dissociate oxygen, said second oxygen ion pump cell pumping out dissociated oxygen to cause a flow of current corresponding to the nitrogen oxide concentration, referred to as second oxygen ion pump current;

said method comprising;

controlling the voltage applied across the second oxygen ion pump cell depending on variations in the oxygen partial pressure in the measurement gas.

13. The method as defined in claim B4 wherein, if the oxygen partial pressure is low or high, the voltage applied across the second oxygen ion pump cell is lowered or raised, respectively.

14. An apparatus for measuring nitrogen oxide concentration comprising:

(a) a nitrogen oxide concentration detector having;

(a1) a first measurement chamber into which a measurement gas is introduced via a first diffusion resistance;

(a2) a first oxygen ion pump cell sufficiently pumping oxygen in the measurement gas from said first measurement chamber to outside and/or from outside to inside of said measurement chamber to an extent not to allow all the nitrogen oxide to decompose in the gas under measurement;

(a3) a second measurement chamber into which a measurement gas is introduced from the first measurement chamber via a second diffusion resistance; and

(a4) a second oxygen ion pump cell having a pair of electrodes provided on the inside and outside of the second measurement chamber an electrical voltage being applied across said electrodes to decompose the nitrogen oxide in said second measurement chamber to provide dissociated oxygen, said second oxygen ion pump cell pumping out the dissociated oxygen to cause a flow of current corresponding to the nitrogen oxide concentration, referred to as second oxygen ion pump current

said apparatus further comprising;

(b) an oxygen partial pressure detection cell detecting the oxygen partial pressure in said first measurement chamber or in said second measurement chamber; and

(c) a nitrogen oxide concentration processor receiving an output of said oxygen partial pressure detection cell and an output of said second oxygen ion pump cell as an input, said nitrogen oxide concentration processor correcting a detection output of the nitrogen oxide concentration of the nitrogen oxide concentration detector derived from the second oxygen ion pump current when the output of said oxygen partial pressure detection cell is changed.

16. A method for detecting the concentration of exhaust gas using a gas sensor which detects the concentration of a specific component in a gas discharged from an internal combustion engine, the method comprising:

calibrating a zero point of a detection output of the gas sensor specifying the zero concentration of said specific component, based on a detection output of the gas sensor in atmosphere, and

detecting the concentration of said specific component based on the calibrated detection output.

19. The method as defined in any one of claims 16 to 18 wherein said gas sensor is a NOx sensor.

17. A method for detecting the concentration of exhaust gas using a gas sensor which detects the concentration of a specific component in a gas discharged from an internal combustion engine, comprising:

calibrating the zero point of a detection output of the gas sensor specifying the zero concentration of the specific component, based on a detection output of the gas sensor obtained on cutting fuel supply to said internal combustion engine for setting the concentration of said specific component in the gas introduced into said gas sensor substantially to zero or to substantially the same level as the atmosphere; and

detecting the concentration of said specific component based on the calibrated detection output.

18. A method for detecting the concentration of exhaust gases using a gas sensor which detects the concentration of a specific component in a gas discharged from an internal combustion engine, the method comprising:

calibrating the zero point of a detection output of the gas sensor specifying the zero concentration of the specific component, based on a detection output of the gas sensor obtained on setting a rich air-to-fuel ratio for said internal combustion engine to reduce said specific component and to set the concentration of said specific component in the gas introduced into said gas sensor substantially to zero or to substantially the same level as the atmosphere; and

detecting the concentration of said specific component based on the calibrated detection output.

22. A method for detecting the concentration of exhaust gases using a gas sensor which detects the concentration of a specific component in a gas discharged from an internal combustion engine, comprising:

driving the internal combustion engine under conditions which enable the concentration of said specific component to be estimated or which render said concentration known;

calibrating a detection output of said gas sensor based on a detection output of said gas sensor under a driving condition for said engine; and

detecting the concentration of said specific component based on the calibrated detection output.

23. An apparatus for detecting the concentration of exhaust gases comprising:

a gas sensor detecting the concentration of a specific component in a gas discharged from an internal combustion engine;

driving condition setting means for setting driving conditions for the engine which enable the concentration of said specific component to be estimated or which render said concentration known; and

calibration means for calibrating a detection output of said gas sensor based on a detection output of said gas sensor under said driving conditions as set by said driving condition setting means.

24. An apparatus for detecting the concentration of exhaust gases comprising:

a NOx occlusion catalyst disposed in an exhaust pipe of an internal combustion engine;

a NOx sensor mounted downstream of said NOx occlusion catalyst in the exhaust pipe to detect he NOx concentration in the exhaust gases;

driving condition setting means for transiently providing a rich air-to-fuel ratio for cleaning NOx occluded in said NOx occlusion catalyst; and

means for detecting a state of deterioration of said NOx occlusion catalyst based on changes in a detection output of said NOx sensor before and after setting the rich air-to-fuel ratio.

25. An apparatus for detecting the concentration of exhaust gases comprising:

a selectively NOx reducing catalyst arranged in an exhaust pipe of an internal combustion engine;

a NOx sensor mounted downstream of said catalyst in the exhaust pipe to detect the NOx concentration in the exhaust gases;

means for adding HC to exhaust gases of said internal combustion engine; and

means for detecting the state of deterioration of said catalyst based on changes in a detection output of said NOx sensor before and after HC addition.

26. In a method for detecting the concentration of a specific gas in an engine exhaust gas, a method for correcting a gas sensor comprising:

estimating the amount of moisture in the exhaust gas from driving conditions of the engine;

correcting a gas concentration detection signal output by the gas sensor responsive to an estimated amount of moisture; and

removing or reducing the effect of the amount of moisture in said exhaust gas from the detected value of the gas concentration.

28. The method for correcting the gas sensor as defined in claim D1 or D2 wherein said gas sensor is a sensor exploiting the oxygen ion pumping action of a solid electrolyte.

29. The method for correcting the gas sensor as defined in any one of claims D1 to D3 further comprising:

estimating an air to fuel ratio supplied to said engine to be corresponding to the amount of moisture in said exhaust gas;

pre-formulating a map containing correction data for said gas concentration detection signal corresponding to said estimated ratio based on said estimated ratio, said gas concentration detection signal and true gas concentration value;

reading out, from said map, specific correction data corresponding to said estimated ratio as determined by an engine driving condition; and

correcting said gas concentration detection signal by the read-out correction data.

30. In a method for detecting the NOx concentration using a NOx sensor having at least two solid electrolyte oxygen ion pump cells, in which a voltage is applied across a first oxygen ion pump cell for pumping out oxygen in the measurement gas, a current corresponding to the oxygen concentration in the measurement gas is caused to flow in said first oxygen ion pump cell, a second oxygen ion pump cell decomposes NOx in the gas not pumped by the first oxygen ion pump cell to pump out dissociated oxygen, with a current corresponding to the NOx gas concentration flowing in the second oxygen ion pump cell, a method for correcting a gas sensor;

wherein the voltage is applied across the first oxygen ion pump cell for sufficiently pumping out oxygen in the measurement gas to an extent not to allow all NOx to decompose in the measurement gas;

wherein a NOx gas concentration detection signal corresponding to the electric current value obtained from said second oxygen ion pump cell is corrected, responsive to a signal corresponding to an electric current value obtained from the first oxygen ion pump cell; and

wherein the effect of the amount of moisture in said measurement gas is removed or reduced from the NOx gas concentration as found based on said NOx gas concentration detection signal.

31. The method for correcting the gas sensor as defined in claim D5,

wherein the electric current value in said first oxygen ion pump cell is estimated to correspond to the amount of the moisture in said measurement gas,

wherein a map containing correction data of said NOx gas concentration detection signal corresponding to the electric current value of the first oxygen ion pump cell is pre-formulated based on the electric current value of said first oxygen ion pump cell,

wherein electric current value obtained from said second oxygen ion pump cell and a true NOx gas concentration, pre-set correction data corresponding to the electric current value of the first oxygen ion pump cell is read out from said map, and

wherein the NOx gas concentration detection signal is corrected by the read-out correction data.

32. The method as defined in claim D5 wherein the voltage is applied across the first oxygen ion pump cell for sufficiently pumping out oxygen in the measurement gas to an extent substantially not to allow NOX in the measurement gas to decompose.

33. Apparatus for detecting the concentration of a specific gas in an engine exhaust gas comprising:

a gas sensor;

means for estimating the amount of moisture in the exhaust gas from driving conditions for an engine;

means for correcting a gas concentration detection signal output from the gas sensor responsible to an estimated amount of moisture; and

means for removing or reducing the effect of the amount of moisture in said exhaust gas from the detected value of the gas concentration.

34. The apparatus as defined in claim D8 wherein said gas sensor is a NOx sensor.

35. D10. The apparatus as defined in claim D8 further comprising:

means for estimating an air to fuel ratio supplied to said engine to be corresponding to the mount of moisture in said exhaust gas;

memory means for storing a map containing correction data for said gas concentration detection signal corresponding to said estimated ratio based on said estimated ratio, said gas concentration detection signal and true gas concentration value;

reading means for reading out, from said map, specific correction data corresponding to said estimated ratio as determined by an engine driving condition; and

correcting means for correcting said gas concentration detection signal by the read-out correction data.

36. Apparatus for detecting the NOx concentration comprising a NOx sensor having at least two solid electrolyte oxygen ion pump cells, in which a voltage is applied across a first oxygen ion pump cell for pumping out oxygen in the measurement gas, a current corresponding to the oxygen concentration in the measurement gas is caused to flow in said first oxygen ion pump cell, a second oxygen ion pump cell decomposes NOX in the gas not pumped by the first oxygen ion pump cell to pump out dissociated oxygen, with a current corresponding to the NOx gas concentration flowing in the second oxygen ion pump cell;

wherein the voltage is applied across the first oxygen ion pump cell for sufficiently pumping out oxygen in the measurement gas to an extent not to allow all NOx to decompose oxygen in the measurement gas;

wherein a NOx gas concentration detection signal corresponding to the electric current value obtained from said second oxygen ion pump cell is corrected, responsive to a signal corresponding to an electric current value obtained from the first oxygen ion pump cell; and

wherein the effect of the amount of moisture in said measurement gas is removed or reduced from the NOx gas concentration as found based on said NOx gas concentration detection signal.

37. A method for determining NOx concentration is a measurement gas, under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions, comprising:

(1) introducing the measurement gas into the flow channel;

(2) forming in said flow channel a residual gas having a NOx concentration different from that before entrance into said flow channel by extracting an oxygen gas from the measurement gas of step (1) through the ceramic body to outside of the flow channel;

(3) dissociating NOx of said NOx-concentrated residual gas into nitrogen and oxygen by applying a voltage across electrodes formed on the ceramic body;

(4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by the electrochemical action of the oxygen dissociated from NOx in step (3);

(5) determining a basic NOx concentration of the measurement gas, based on the electric current measured in step (4), and

(6) correcting said basic NOx concentration determined by step (5) responsive to the oxygen concentration in the measurement gas.

38. A method for determining NOx concentration in a measurement gas, under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions, comprising:

(1) introducing the measurement gas into the flow channel;

(2) forming in said flow channel a residual gas having a NOx concentration different from that before entrance into said flow channel by extracting an oxygen gas from the measurement gas of step (1) through the ceramic body to outside of the flow channel;

(3) dissociating NOx of said NOx-concentrated residual gas into nitrogen and oxygen by applying a voltage across electrodes formed on the ceramic body;

(4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by the electrochemical action of the oxygen dissociated from NOx in step (3);

(b 5) determining a basic NOx concentration of the measurement gas, based on the electric current measured in step (4), and

(6) correcting said basic NOx concentration of step (5) responsive to said change if the oxygen partial pressure in the measurement gas is changed.

39. A method for determining NOx concentration in a measurement gas, under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions, comprising:

(1) introducing the measurement gas into the flow channel;

(2) forming in said flow channel a residual gas having a NOx concentration different from that before entrance into said flow channel by extracting an oxygen gas from the measurement gas of the step (1) through the ceramic body to outside of the flow channel;

(3) dissociating NOx of said NOx-concentrated residual gas into nitrogen and oxygen by applying a voltage across electrodes formed on the ceramic body;

(4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by the electrochemical action of the oxygen dissociated from NOx in step (3);

(5) determining a basic NOx concentration of the measurement gas, based on the electric current measured in step (4); and

(6) measuring an output in step (4) introducing atmospheric air, and calibrating zero point of said basic NOx concentration.

40. A method for determining the NOx concentration in a measurement gas, under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions, comprising:

(1) introducing the measurement gas into the flow channel;

(2) forming in said flow channel a residual gas having a NOx concentration different from that before entrance into said flow channel by extracting an oxygen gas from the measurement gas of step (1) through the ceramic body to outside of the flow channel;

(3) dissociating NOx of said NOx-concentrated residual gas into nitrogen and oxygen by applying a voltage across electrodes formed on the ceramic body;

(4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by the electrochemical action of the oxygen dissociated from NOx in step (3);

(5) determining a basic NOx concentration of the measurement gas, based on the electric current measured in step (4); and

correcting the basic NOx concentration wherein the amount of moisture in said measurement gas is estimated from a driving condition of an engine, and said NOx concentration of step (5) is corrected depending on an estimated amount of moisture.

41. A method for measuring a nitrogen oxide concentration using a nitrogen oxide concentration detector having:

a first measurement chamber formed in said detector, into which first measurement chamber a measurement gas is introduced via a first diffusion resistance formed at an entrance of the first measurement chamber;

an oxygen concentration detection electrode for measuring an oxygen concentration in the measurement gas in said first measurement chamber;

a first oxygen ion pump cell for pumping out oxygen from said first measurement chamber to form a residual gas in the first measurement chamber in accordance with the potential of said oxygen concentration detection electrode;

a second measurement chamber formed in said detector into which second measurement chamber the residual gas is introduced via a second diffusion resistance formed between the first and second chambers; and

a second oxygen ion pump cell having two output electrodes across which electrodes a voltage is applied to decompose the oxygen nitride in the residual gas and to pump out oxygen dissociated from the oxygen nitride to cause a current that corresponds to the nitrogen oxide concentration, said current being referred to as a second oxygen pump current flowing in the second oxygen ion pump cell;

wherein said first oxygen ion pump cell sufficiently pumps out oxygen from the first measurement chamber to an extent not to allow all nitrogen oxide in the residual gas to decompose; and

wherein the nitrogen oxide concentration obtained responsive to said second oxygen pump current is corrected in accordance to the oxygen concentration in the measurement gas, said oxygen concentration being detected by a current flowing through the first oxygen ion pump cell.