Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
1998-03-23
2002-04-23
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C205S784500, C204S425000, C204S426000
Reexamination Certificate
active
06375828
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a gas sensor and, more particularly, to a NOx gas sensor and a method and apparatus therefor, used for detecting the NOx gas concentration in exhaust gases of an internal combustion engine intended for industrial use and for driving an automobile, vessel or aircraft, or in combustion gases of a boiler or the like.
This invention also relates to a method and apparatus for measuring the composition of exhaust gases of a combustion device or an internal combustion engine, in particular the NOx gas concentration.
This invention also relates to a method and apparatus for controlling a NOx gas sensor used for measuring the NOx gas concentration in an ambient atmosphere in which the oxygen concentration in a measurement gas is changed acutely.
This invention also relates to a method and apparatus for detecting the exhaust gas concentration for measuring the concentration of harmful gas components contained in the exhaust gas from the internal combustion engine and, more particularly, to a method and apparatus for calibrating the zero point of an output of the NOx gas sensor.
RELATED ART
Recently, in dealing with an intensification of exhaust gas regulations, research has been constructed into controlling the operation of an engine or the catalyst based on a direct measurement of the NOx in e.g., engine exhaust gases. The resulting NOx gas sensor, which uses an oxygen ion conductor, such as ZrO
3
causes the measurement gas (detection gas) to be introduced into a first measurement chamber and a first oxygen ion pumping cell in the first measurement chamber pumps oxygen out, to an extent such as not to decompose NOx at all. Then, the gas lowered in oxygen concentration is introduced into a second measurement chamber where oxygen is further pumped out by a second oxygen ion pumping cell to cause NOx from the NOx-gas contained in the remaining gas to be decomposed. This decomposition is sensed as an electric current. In such NOx gas sensor, the NOx gas concentration is detected based on the current flowing across a pair of electrodes provided in the second oxygen ion pumping cell.
In general, a pair of electrodes are arranged on both sides of the oxygen ion conductor in such sensor. One of the electrodes is exposed to the atmosphere in the first measurement chamber, while the other electrode is exposed to a reference atmosphere containing oxygen. The electrical voltage applied across the first oxygen ion pumping cell is varied, based on an output of an oxygen partial pressure detection cell adapted for measuring the oxygen concentration in the first measurement chamber, in order to control the oxygen concentration in the first measurement chamber to a constant value.
SUMMARY OF THE DISCLOSURE
However, various problems have been encountered in the course of investigations toward the present invention.
First, the output gain of the NOx gas sensor varies depending on the oxygen concentration upon measuring the NOx gas concentration, which results in an error in the NOx gas concentration measurement.
The present inventors have further formed that, if the oxygen concentration in the measurement gas is varied significantly, delay is caused in voltage control for the first oxygen ion pumping cell, thereby making it difficult to measure the NOx gas concentration correctly. In particular, in lean burn engines using gasoline or a diesel fuel, which recently are used in increasing numbers, the oxygen concentration in the exhaust gas is varied significantly under the driving conditions. Thus, if the PID control is performed based on the potential of an oxygen partial pressure detection cell, it is difficult to correctly measure the NOx concentration in the exhaust gas if only the method of varying and controlling the pump capability of the first oxygen ion pumping cell is used.
In the above-described current limiting type sensor, one problem is that the detection current flowing in the oxygen ion pumping cell is as small as several &mgr;A. Moreover, since decomposition of harmful gas components (such as NOx) is controlled or suppressed by the catalytic action on the electrode surface, another problem is that, on prolonged use, catalytic activity is changed causing a shift in the zero point (i.e., a detection output of the gas sensor specifying that the concentration of the pre-set component is substantially zero).
On the other hand, in an oxide semiconductor type gas sensor, which is based on the principle that changes in electrical resistance of an oxide semiconductor is proportional to the amount of a pre-set gas component adsorbed on the oxide semiconductor, suffers from poor reproducibility. Further, if moisture in the measurement gas is electrolyzed, causing it to be decomposed or dissociated, then in the second oxygen ion pumping cell, a current will flow due to oxygen yielded on electrolysis not only of NOx but also of moisture, resulting in an incorrect measurement of the NOx concentration. Moreover, in a lean burn engine using gasoline or a diesel fuel, the moisture in the exhaust gas is varied significantly under driving conditions. Consequently, it is difficult to make a correct measurement of the NOx concentration in the exhaust gases.
Thus, none of these types of gas sensor is reliably used as a sensor for a prolonged time and under significantly varying environmental conditions, as in the case of a sensor mounted on an internal combustion engine, in particular the sensor mounted on an exhaust gas system of a vehicle.
It is therefore an object of the present invention to provide a method and apparatus for accurately measuring NOx gas concentration in engine exhaust.
It is also an object of the present invention to provide a method and apparatus for controlling the NOx gas sensor which enables the NOx gas concentration to be measured accurately even if the oxygen partial pressure in the measurement gas is changed.
It is also an object of the present invention to provide a method and apparatus for measuring an exhaust gas concentration for a prolonged time and, in particular, a method and apparatus which enables calibrating a zero point of the detection output of an NOx gas sensor.
It is a further object of the present invention to provide a method and apparatus for calibrating a gas sensor in which the effect of the amount of moisture in the measurement gas on detection of the gas concentration is reduced or eliminated, in order to enable a correct measurement of gas concentration values.
Still further objects of the present invention will become apparent in the entire disclosure.
According to the present invention one or more of the foregoing objects are achieved by several different features of the invention, based on one or more aspects of each feature as disclosed and claimed herein.
According to a first feature of the present invention, there is provided a measurement method and apparatus for measuring the NOx gas concentration in which the NOx gas concentration obtained as a NOx gas sensor output is corrected in response to the oxygen concentration in the detection gas (measurement gas). This is based on a correspondence between the gas sensor output and a current caused to flow by pumping out oxygen dissociated on decomposition of NOx.
An NOx gas sensor according to this first feature includes a first measurement chamber into which a detection gas is introduced via a first diffusion resistance, an oxygen concentration detection electrode for measuring the oxygen concentration in the detection gas in the first measurement chamber, and a first oxygen ion pump cell for pumping oxygen contained in the detection gas out from the first measurement chamber to the outside and/or inside of the first measurement chamber based on the potential of an oxygen concentration detection electrode. The pumping is generally to an extent such that NOx is not entirely decomposed, or in some case to an extent such that nitrogen oxide does not decompose into oxygen in the measurement gas. There also is a second measurement chamber into which the
Ando Masashi
Ishida Noboru
Kuzuya Yasuhisa
Nadanami Norihiko
Okumura Tatsuo
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
Sughrue & Mion, PLLC
Warden, Sr. Robert J.
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