Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
1997-12-02
2001-04-10
Warden, Jill (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C436S118000, C436S151000, C436S152000, C073S023310, C073S031050
Reexamination Certificate
active
06214208
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a method for measuring the concentrations of the NOx gas and an apparatus or sensor for carrying out the measurement method. More particularly, it relates to a method for measuring the NOx gas concentration used in exhaust gases containing a large quantity of H
2
O, and a NOx gas concentration sensor.
BACKGROUND OF THE INVENTION
Recently, researches into directly measuring the concentration of the NOx gas for controlling the infernal combustion engine or catalysts are proceeding in meeting with intensification of regulations on vehicle exhaust gases.
In particular, researches are going on extensively for a NOx gas concentration sensor of the type in which the oxygen is pumped out by a first oxygen ion pump cell using a solid electrolyte (oxygen conductor) such as zirconia, so as not to cause decomposition of the NOx gas (2NO→N
2
+O
2
) by the first oxygen ion pump cell and in which oxygen is further pumped out from the NOx-gas-containing residual gas by the second oxygen ion pump cell for decomposing NOx and measuring the current generated by dissociated oxygen ions for detecting the NOx gas concentration because measurement of the NOx gas concentration is thought to be easily made without being affected by interfering gases contained in the exhaust gases, such as HC or CO.
SUMMARY OF THE DISCLOSURE
In the course of investigation toward the present invention following problem has been encountered.
According to the finding reached by the present inventors, the above method has a drawback that, if H
2
O is mixed into the NOx gas concentration sensor, the NOx concentration cannot be measured accurately.
It is therefore an object of the present invention to provide a method for measuring the NOx gas concentration for enabling accurate measurement of the NOx gas concentration in a H
2
O containing atmosphere and a NOx gas concentration sensor for carrying out the method.
Other problems and objects of the present invention will become apparent in the entire disclosure.
Among various aspects of the present invention there are the following elements. The elements included in aspect (1—1) are as follows: there is a first step of introducing the gas under measurement containing NOx into the flow channel. The oxygen gas is drawn or evacuated to outside of the flow channel via a ceramic body having an electrically controllable conducting rate of oxygen. A NOx concentrated residual gas is formed in the flow channel. A voltage is applied to the ceramic body to an extent that does not substantially dissociate H
2
O in the residual gas. Particularly, a voltage lower than the dissociation voltage between hydrogen and oxygen of at least H
2
O (H—O dissociation voltage) in the residual gas is impressed across the electrodes formed on the outside body. NOx in the NOx concentrated residual gas is dissociated into nitrogen and oxygen. The current caused by NOx dissociation flowing in the ceramic body is measured across the electrodes. The current is generated by the electro-chemical action of oxygen dissociated from the NOx gas during the dissociation step. The NOx concentration is determined based on this current.
According to further aspect based on aspect (3-1), the flow of the gas into the flow channel is restricted (aspect 1-2). The flow of the residual gas to the electrode for the NOx dissociation is restricted (aspect 1-3).
The flow chart comprises a first flow channel and a second flow channel communicating with the first flow channel, the formation of the residual gas for concentrating NOx without dissociating H
2
O is performed in the first flow channel, and NOx is dissociated in the second flow channel (aspect 1-4).
According to aspect (2-1), there is provided a NOx gas concentration sensor for measuring the NOx concentration on a gas under measurement. The sensor comprises a ceramic body having an electrically controllable conducting rate of oxygen ion; a flow channel provided facing said ceramic body so that that NOx containing gas under measurement will flow therein, with the NOx gas in the gas under measurement being varied in the concentration during the process of movement of the gas under measurement therethrough; and means (generally pumping cell) for extracting the oxygen gas from the gas under measurement via said ceramic body to outside of the flow channel for forming a residual gas, in said flow channel, having a NOx concentration different from before entering the flow channel; and a plurality of electrodes formed on said ceramic body so that NOx in the residual gas is dissociated into nitrogen and oxygen by applying a voltage to an extent that H
2
O in the residual gas is substantially not dissociated. The sensor further comprises generally circuit means, i.e., means for measuring a current flowing in said ceramic body across said electrodes, said current being generated by the electrochemical operation of oxygen dissociated from the NOx gas; and means for determining the NOx concentration in the gas under measurement based on said measured current.
According to aspect (2—2), in the flow channel there is disposed an electrode for extracting oxygen gas out of the flow channel along the flowing direction of the measurement gas, and
wherein the electrode has a length relative to the entire length of the flow channel being:
electrode/entire length=¼ to ¾.
According to aspect (2-3), the flow channel comprises a first flow channel in which residual gas is formed, and a second flow channel in which NOx is dissociated, the second flow channel having an inlet communicating to the first flow channel, and the electrode disposed in the flow channel for extracting the oxygen gas out of the flow channel extends from the vicinity of a gas flow restrictor disposed in the first flow channel until at longest a position closer to said gas flow restrictor than the inlet of the second flow channel.
In the NOx gas concentration sensor, the sensor output (IP
2
) usually has temperature dependency and oxygen concentration dependency, thus obstructing accurate measurement of the NOx gas concentration. The NOx gas concentration sensor according to the aspects 2—2 or 2-3 exhibits low oxygen concentration dependency to enable accurate measurement of the NOx gas concentration. If the sensor according to the aspects 2—2 or 2-3 is mounted on the exhaust system of an internal combustion engine driven in the vicinity of a lean range or a theoretical value point (stoichiometrical point), the NOx gas concentration detection output is stabilized in case of lean-rich switching, that is in case of acute change in the oxygen concentration in the exhaust gases.
According to further aspect (2-4), the NOx gas concentration sensor is formulated as follow: The ceramic body is formed of plural ceramic layers laminated together; the flow channel includes a first flow channel for forming residual gas and a second flow channel for dissociating NOx, the first and second flow channels extending along the surface of said laminated ceramic layers; and the first and second flow channels communicate with each other by a flow channel extending in the laminating direction of the ceramic layers.
According to aspect (2-5), in the NOx gas concentration sensor
1
, an oxygen concentration detecting electrode for detecting the oxygen concentration in the flow channel is formed on the ceramic body facing the flow channel, wherein an oxygen concentration for generating a reference potential versus the oxygen concentration detecting electrode is formed on the ceramic body outside of the flow channel; the oxygen concentration reference electrode is an auto-generating reference electrode across which a voltage is impressed for transporting oxygen towards the reference electrode for providing a constant oxygen concentration around this reference electrode.
According to aspect (2-6), the electrode provided outside the channel for drawing the oxygen gas to outside of the flow channel is sealed or covered. This electrode communicates with atmosphere via
Ando Masashi
Ishida Noboru
Nadanami Norihiko
Okumura Tatsuo
Ootuka Takaki
Morrison & Foerster / LLP
NGK Spark Plug Co. Ltd.
Warden Jill
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