Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
2000-10-30
2002-04-16
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C204S425000, C204S426000, C205S784500
Reexamination Certificate
active
06372120
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention relates to a method for determining the NO
x
concentration in a gas, in particular in the exhaust gas of an internal combustion engine. The concentration is determined with a measuring sensor having a solid electrolyte. The measuring sensor has a first measuring cell, into which the gas passes via a diffusion barrier, in which the oxygen concentration is measured via a first Nernst voltage between a first electrode and a reference electrode, exposed to the ambient air, and is controlled to a first oxygen concentration by means of a first oxygen-ion pumping current between the first electrode and an external electrode. The measured value of the first Nernst voltage is used for controlling the first oxygen-ion pumping current. The sensor further has a second measuring cell, which is connected to the first measuring cell via a diffusion barrier and in which the oxygen concentration is measured via a second Nernst voltage between a second electrode and the reference electrode and is controlled to a second oxygen concentration by means of a second oxygen-ion pumping current (Ip
1
) between the second electrode and the external electrode. The NO
x
concentration is measured at the same time by a measuring electrode in the second measuring cell.
For measuring the NO
x
concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to use a thick-film measuring sensor. Such a measuring sensor is described in the publication by N. Kato et al., “Performance of Thick Film NO
x
Sensor on Diesel and Gasoline Engines,” Society of Automotive Engineers, publication 970858, 1997, or in N. Kato et al., “Thick Film NO
x
Sensor for the Measurement of Low No
x
Concentration”, Society of Automotive Engineers, publication 98D170, 1998. That measuring sensor has two measuring cells and consists of a zirconium oxide that conducts oxygen ions. The system implements the following measuring concept: in a first measuring cell, which is fed the gas to be measured via a diffusion barrier, a first oxygen concentration is set by means of a first oxygen-ion pumping current, with no decomposition of NO
x
taking place. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is further reduced by means of a second oxygen-ion pumping current and NO
x
decomposes at a measuring electrode. The oxygen generated in this way is sensed as a measure of the NO
x
concentration. The entire measuring sensor is in this case brought to an elevated temperature, for example 430° C., by means of an electric heater. The measuring error of the measuring sensor described in the publication corresponds to an NO
x
concentration of 22 ppm.
The measuring error is of a systematic nature for the MO following reasons: for setting the first oxygen concentration in the first measuring cell, the oxygen-ion pumping current Ip
0
flows between a first electrode and an external electrode through the thick-film material, which in this case is a solid electrolyte of ZrO
2
. Between the solid electrolyte ZrO
2
and the first electrode there is a transitional resistance R, through which the oxygen-ion pumping current Ip
0
flows. The resultant voltage drop is measured at the same time as the determination of a first Nernst voltage V
0
in the first measuring cell, which is used for controlling the first oxygen-ion pumping current Ip
0
. The measured voltage V
0
thus comprises the actual Nernst voltage V
N0
and the described additive voltage drop across the transitional resistance R
0
. This makes it more difficult for controlling to the first oxygen concentration to be carried out in the first measuring cell and leads to a systematic error, since too low a first oxygen concentration may under certain circumstances cause NO
x
that was actually intended to be decomposed and measured only in the second measuring cell to be decomposed already at the first electrode. If the first oxygen concentration is too high, too much oxygen diffuses into the second measuring cell, in which the residual oxygen content becomes too high as a result. This likewise leads to a falsification of the NO
x
measurement.
In the prior art, this problem has been solved by using the fact that, with an ideally set first oxygen concentration at the first measuring cell, the setpoint value for the second oxygen-ion pumping current Ip
1
, with which the second oxygen content in the second measuring cell is set, is known. This setpoint value for the second oxygen-ion pumping current Ip
1
can then be used for correcting the measured value V
0
of the first Nernst voltage to the extent that the second oxygen-ion pumping current Ip
1
achieves this setpoint value. The control required for this cannot, however, be designed to be very fast, for reasons of stability, so that when there are rapid changes in NO
x
concentration in the gas to be measured compensation is not completely possible. Then the first oxygen concentration in the first measuring cell is not controlled to the desired value, which, as described, leads to measuring errors in the sensing of the NO
x
concentration.
It has become known from U.S. Pat. No. 6,036,842 (see Japanese application JP 170160/96 and European published application EP 0 816 836 A2) to incorporate a voltage derived from the first pumping current into the control of the first pumping current for correcting a transitional resistance.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method for determining a NO
x
concentration in a gas which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows a more exact sensing and measurement of the NO
x
concentration in a gas using the above-described measuring sensor.
With the above and other objects in view there is provided, in accordance with the invention, a method of determining a NO
x
concentration in a gas, in particular in an exhaust gas of an internal combustion engine. The method comprises:
providing a measuring sensor with a solid electrolyte, a first measuring cell with a first electrode, a second measuring cell with a second electrode, a diffusion barrier separating the first measuring cell from the second measuring cell, and a reference electrode exposed to ambient air;
allowing a gas to pass via a diffusion barrier into the first measuring cell, measuring an oxygen concentration in the first measuring cell via a first Nernst voltage between the first electrode and the reference electrode, and regulating the oxygen concentration to a first oxygen concentration with a first oxygen-ion pumping current between the first electrode and an external electrode, and thereby using a measured value of the first Nernst voltage for controlling the first oxygen-ion pumping current;
measuring an oxygen concentration in the second measuring cell via a second Nernst voltage between the second electrode and the reference electrode, and regulating the oxygen concentration in the second measuring cell to a second oxygen concentration with a second oxygen-ion pumping current between the second electrode and the external electrode;
also measuring the NO
x
concentration with a measuring electrode in the second measuring cell; and
correcting an error occurring in a setting of the first oxygen-ion pumping current by correcting the measured value of the first Nernst voltage with a product of a correction value and the first oxygen-ion pumping current, wherein the correction value approximates a transitional resistance between the first electrode and the solid electrolyte being supplied at least partially by a controller, wherein the controller uses as a reference variable a prescribed value (Ip
1
set
) of the second oxygen-ion pumping current and as a controlled variable a current value of the second oxygen-ion pumping current.
In other words, the measured value V
0
of the first Nernst voltage is corrected by a controller, which uses as the reference variable
Rössler Jürgen
Zhang Hong
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Tung T.
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