Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
1999-01-15
2002-02-05
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C073S023310, C204S425000
Reexamination Certificate
active
06344134
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for measuring the concentration of NOx in a gas to be measured (hereinafter referred to as a measurement gas), such as exhaust gas emitted from combustion equipment or an internal combustion engine, as well as to an NOx concentration sensor.
Also, the present invention relates to an NOx concentration sensor for measuring the concentration of NOx in exhaust gas emitted from an internal combustion engine used in transportation equipment such as an automobile, ship, or an airplane, or industrial equipment such as a boiler, as well as to an apparatus and a method for measuring NOx concentration.
BACKGROUND OF THE INVENTION
Conventionally, a method has been proposed for measuring NOx concentration using an NOx concentration sensor including two measurement spaces and two oxygen ion pumping cells (hereinafter referred to as pumping cells). The conventional method includes the steps of: pumping out oxygen from one measurement space by means of a first pumping cell to an extent so as not to cause dissociation of NO; causing NO to completely dissociate in the other measurement space by means of a second pumping cell; and determining NOx concentration based on a pumping current induced by oxygen ions generated by dissociation of NO.
For example, Japanese Patent Application Laid-Open (Kokai) No. 2-1543 proposes a method for measuring the concentration of NOx, typically nitrogen monoxide (NO), contained in exhaust gas, using an NOx concentration sensor composed of two measurement spaces, each of which is equipped with a pumping cell. The method includes the steps of: pumping out only oxygen from one measurement space to an extent so as not to cause dissociation of NO; measuring a current, Ip
1
, flowing through a pumping cell (hereinafter referred to as a pumping current) associated with the measurement space; causing O
2
and NO to dissociate in the second measurement space; pumping out, from the second measurement space, oxygen generated by dissociation of O
2
and NO; measuring a pumping current, Ip
2
, flowing through the second pumping cell; and determining NOx concentration based on the difference between the pumping currents (Ip
2
−Ip
1
).
Japanese Patent Application Laid-Open (Kokai) No. 2 122255 proposes a method for measuring the concentration of a certain gas component contained in a mixed gas which contains an oxygen compound other than the gas component to be measured. The method includes the steps of: causing dissociation of the oxygen compound having a dissociation voltage lower than that of the gas component to be measured, by applying a certain fixed voltage (which does not cause dissociation of the gas component to be measured) to a first pumping cell; pumping out oxygen generated by dissociation of the oxygen compound; causing dissociation of the gas component to be measured by applying a certain voltage to a second pumping cell; detecting the amount of oxygen generated by dissociation of the gas component to be measured in terms of a second pumping current, Ip
2
; and determining the concentration of the gas component to be measured based on the second pumping current Ip
2
.
Japanese Patent Application Laid-Open (Kokai) No. 8 271476 proposes an improvement in the above-described method for measuring NOx concentration. The method includes the steps of: eliminating excess oxygen from a first space by controlling the amount of oxygen contained in the first space so as not to substantially affect the measurement of NOx concentration and so as not to cause substantial dissociation of NO; causing dissociation of NO in a second space; and determining NOx concentration based on a pumping current, Ip
2
, induced by oxygen generated by dissociation of NO.
The above-described methods have been developed in order to measure the concentration of NOx, for example, in an automobile exhaust gas. However, recently, there has arisen a need for measuring the concentration of NOx in a catalytically purified exhaust gas, in order to verify the purification performance of an exhaust gas purification catalyst. In this case, the accuracy required for measuring NOx concentration is not greater than ± several tens of ppm with respect to several hundreds of ppm in NOx concentration. The above-described methods fail to sufficiently satisfy this accuracy requirement and thus there is a need for further improvement.
The method for measuring NOx concentration using the aforementioned NOx concentration sensor significantly depends on oxygen concentration and temperature, and therefore it is difficult to accurately measure the NOx concentration.
In the NOx sensor proposed in the above-mentioned Japanese Patent Application Laid-Open (Kokai) No. 2-1543, the first pumping current Ip
1
is relatively strong. Thus, the differential pumping current (Ip
2
−Ip
1
) is considerably influenced by variation of the first pumping current Ip
1
. Therefore, due to variation among sensors and temperature, measurement accuracy is impaired.
In the above method described in Japanese Patent Application Laid-Open (Kokai) No. 8-271476, oxygen in the first space is held at a partial pressure of 10
−4
to 10
−6
atm so as not to cause dissociation of NO within the first space. Thus, a considerable amount of oxygen is introduced into the second space. The relatively large amount of oxygen present within the second space hinders sufficient dissociation of NO within the second space. Also, the absolute quantity of variation in the oxygen concentration of gas introduced into the second space increases. As a result, an electric signal issued by the second pumping cell includes a measurement error, thus causing a failure to attain the required measurement accuracy.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the above problems of the prior art. It is therefore an object of the present invention to provide a method capable of accurately measuring NOx concentration, as well as an NOx concentration sensor capable of accurately measuring NOx concentration.
Nitrogen monoxide (NO) is dominant among NOx contained, for example, in an automobile exhaust gas. In other words, the content of NOx species other than NO, that is, the content of N
2
O and NO
2
, is very low. Therefore, the following description relates to the measurement of NO concentration. The term “NOx concentration” substantially means “NO concentration”.
The above object of the present invention has been achieved by providing a two-serial-space NOx concentration sensor comprising a measurement gas space (ambient atmosphere to be measured) in series communication with a first internal space a second internal space. A measurement gas is introduced from the measurement gas space into the first space and then into the second space.
In the NOx concentration sensor, by activating a first pumping cell, a portion of oxygen (O
2
) and nitrogen monoxide (NO) contained in the first space dissociates to generate oxygen ions (O
2−
). The thus-generated oxygen ions are transferred (pumped out) into the measurement gas space. By contrast, oxygen can also be pumped into the first space from the measurement gas space. By activating a second pumping cell, residual O
2
and NO contained in gas diffusing within the second space dissociate to generate oxygen ions, which are also removed by pumping. The first pumping cell and the second pumping cell may each comprise a solid electrolyte substrate and a pair of porous electrodes disposed on opposite sides or a single side of the substrate. By applying a predetermined voltage between the porous electrodes, oxygen can be transferred from one of the porous electrodes to the other.
To achieve the above object, a first mode of a first aspect of the present invention provides a method for measuring NOx concentration using a two-serial-space NOx concentration sensor. The sensor includes a first pumping cell and a second pumping cell each comprising a solid electrolyte. In the sensor, a measurement gas space, a first space,
Ando Masashi
Hayakawa Nobuhiro
Ishida Noboru
Matsuura Toshitaka
Nadanami Norihiko
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
Sughrue & Mion, PLLC
Warden, Sr. Robert J.
LandOfFree
Method for measuring NOx concentration and NOx concentration... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for measuring NOx concentration and NOx concentration..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for measuring NOx concentration and NOx concentration... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2945522