Patent Application
20090242426
The invention relates to a gas sensor for detecting the quantity of a measurement gas contained in a gas mixture, comprising:
A gas sensor of this type is known from KATO, N; KURACHI, H; HAMADA, Y: Thick Film ZrO2 NOx Sensor for the Measurement of Low NOx Concentration. In: SAE TECHNICAL PAPER SERIES 980170. The known gas sensor comprises a substrate based on zirconium oxide, a prechamber and a measuring chamber being formed in said substrate. An external air duct with a reference electrode arrangement therein is furthermore situated in the substrate. The exhaust gas emitted by an internal combustion engine passes through a first diffusion barrier into the prechamber. A control device detects the Nernst voltage between the reference electrode in the external air duct and a measuring electrode assigned to the prechamber and controls a pump cell for oxygen in such a way that the measured Nernst voltage is set to a predetermined value. The known gas sensor involves regulating the Nernst voltage in the prechamber to 300 mV. This corresponds to an oxygen concentration of 1 ppm. At this oxygen concentration, the free oxygen is almost completely removed in the prechamber. The oxygen bonded in the nitrogen oxides is not liberated, however.
The prechamber is followed by a measuring chamber, which is separated from the prechamber by a further diffusion barrier. A further control device assigned to the measuring chamber detects the Nernst voltage between the reference electrode in the external air duct and a measuring electrode assigned to the measuring chamber and regulates a pump cell assigned to the measuring chamber in such a way that the Nernst voltage is set to a predetermined value. The known gas sensor involves regulating the Nernst voltage in the measuring chamber to a value of 400 mV. This corresponds to an oxygen concentration of 0.01 ppm.
Furthermore, a detection device is provided in the measuring chamber, said detection device being formed by the reference electrode in the external air duct and a detection electrode. The detection electrode is produced on the basis of rhodium. The detection electrode therefore acts as a reducing catalyst that brings about a decomposition of the nitrogen oxides. The oxygen formed by the decomposition of nitrogen oxides is pumped away by the detection device, the pump current required for this being a measure of the concentration of nitrogen oxides in the measuring cell.
The known gas sensor is suitable in particular for the detection of nitrogen oxides in the context of exhaust gas after treatment. By way of example, the nitrogen oxide emission from diesel engines is intended to be reduced further by means of suitable exhaust gas after treatment systems. The so-called selective catalytic reduction (=SCR) technology is provided, inter alia, for this purpose, which involves injecting a urea solution into a catalyst. The ammonia produced after the decomposition of urea adsorbs on the catalyst. The nitrogen oxides from the exhaust gas react with the ammonia adsorbed on the catalyst to form nitrogen and water.
In order, then, to be able to precisely regulate the injection of the urea solution, an ammonia sensor which is as precise as possible and which if possible has no cross-sensitivity to further exhaust gas components is required.
Proceeding from this prior art, therefore, the invention is based on the object of providing a selective gas sensor which is as precise as possible for a measurement gas whose concentration cannot be detected by conventional gas sensors which are sensitive to a detection gas.
This object is achieved by means of a gas sensor comprising the features of the independent claim. Advantageous configurations and developments are specified in claims dependent on said independent claim.
The gas sensor for detecting the quantity of a measurement gas contained in a gas mixture has, like conventional gas sensors for detecting a detection gas, a prechamber and a measuring chamber including the pump and detection devices required for detecting the detection gas. In addition, the gas sensor for detecting the measurement gas is provided with an inlet chamber, which is arranged upstream of the prechamber in the flow direction and is separated from the prechamber by a further diffusion barrier. A pump device is formed in the inlet chamber, by means of which pump device the partial pressure of the free gas component of the detection gas can be set to a predetermined value below the value of the partial pressure prevailing in the prechamber. Furthermore, in the prechamber of the gas sensor, the partial pressure of the gas component is set to a value at which the measurement gas can be converted into the detection gas by reaction with the free gas component. In the gas sensor, the free gas component is largely removed in the inlet chamber. As a result, the detection gas decomposes in the measuring chamber. The quantity of the detection gas produced in the prechamber by reaction between the free gas component and the measurement gas therefore essentially corresponds to the quantity of the measurement gas that entered into the inlet chamber. The detection gas contained in the prechamber can then be detected in a conventional manner, the quantity of detected detection gas being a measure of the quantity of the measurement gas to be detected.
By switching off the pump device in the inlet chamber, the gas sensor for the measurement gas can be converted into a gas sensor for the detection gas.
The gas component is preferably an oxidizing gas, in particular oxygen. Accordingly, the detection gas involves gaseous oxides, in particular nitrogen oxides, for which sensor arrangements are known which, by means of an inlet chamber being disposed upstream, can be reconfigured as a precise and selective gas sensor for ammonia.
The detection device preferably comprises a catalyst for the reduction of the detection gas. The detection device furthermore comprises a voltage source, which keeps the voltage between a detection electrode provided with the catalyst and a pump electrode with a predetermined value, and also a current measuring unit, which measures the current required for this purpose. Detection devices of this type are available in particular for the detection of gaseous oxides.
The inlet chamber, the prechamber and the measuring chamber preferably each comprise a measuring unit which measures the voltage between a reference electrode exposed to ambient air and a respective electrode assigned to the inlet chamber, the prechamber and the measuring chamber. The measuring units assigned to the inlet chamber, the prechamber and the measuring chamber each act on current sources which control the current through pump cells. By varying the current flowing through the pump cells, the current sources keep the voltages detected by the voltage measuring units at predetermined values. The pump cells comprise a respective electrode assigned to the inlet chamber, the prechamber and the measuring chamber and also a common pump electrode exposed to exhaust gases, a solid electrolyte, in particular based on zirconium oxide, being situated between said electrodes.
In one preferred embodiment of the gas sensor, the current source assigned to the inlet chamber adjusts the voltage measured by the voltage measuring unit to a value of between 600 and 800 mV. The current source assigned to the prechamber furthermore keeps the voltage detected by the associated voltage measuring unit to a value of between 100 and 200 mV. Finally, the current source assigned to the measuring chamber adjusts the voltage detected by the associated measuring unit preferably to a value of between 350 and 450 mV.
Further properties and advantages of the invention will become apparent from the description below, in which exemplary embodiments of the invention are explained in detail with reference to the accompanying drawing, in which:
In the inlet chamber 6, the nitrogen oxides from the exhaust gas are reduced to nitrogen at a first electrode 13. The oxygen thereby obtained is removed, together with the oxygen already present in the exhaust gas, from the inlet chamber 6 by means of a pump cell 14. The pump cell 14 comprises a further pump electrode 15 besides the first electrode 13. The pump electrode 15 is exposed to the exhaust gas.
The pump cell 14 is operated with a current I1 from a current source 16, which sets the current I, such that a voltage U1 measured by a voltmeter 17 remains at a predetermined value. The voltage U1 is set to a value of between 600 and 800 mV, preferably to a value in the region of 700 mV. A Nernst voltage dimensioned in this way sets an air ratio corresponding to λ<1 in the inlet chamber 6. Accordingly, an outflow 18 of oxygen generally takes place in the pump cell 14 if λ>1 holds true in the exhaust gas. Occasionally, however, an inflow 19 of oxygen can also be brought about by the pump cell 14 if λ<<1 in the exhaust gas.
In the exemplary embodiment illustrated in the drawing, a single electrode, namely the first electrode 13, which is preferably produced on the basis of a noble metal such as platinum, for example, is used for the reduction of the nitrogen oxides and for setting the oxygen content in the inlet chamber.
The gas mixture contained in the inlet chamber 6 flows through the diffusion barrier 11 into the prechamber 7, where a second electrode 20 together with the pump electrode 15 forms a second pump cell 21. A current source 22 supplies the pump cell 21 with a current I2, which is set by the current source 22 such that a lean air ratio corresponding to λ>1 is set in the prechamber 7. For this purpose, the current is regulated by the current source 22 in such a way that a voltage U2—measured by a voltmeter 23—between the second electrode 20 and the reference electrode 5 is set to a predetermined value within the range of between 100 and 200 mV. The Nernst voltage U2 between the second electrode 20 and the reference electrode 5 is preferably set to a value in the region of 150 mV, however.
A lean air ratio, corresponding to λ>1, thus prevails in the prechamber 7. As a result, the reducing exhaust gas components such as carbon monoxide and hydrocarbons are oxidized to form carbon dioxide and water and ammonia is oxidized to form nitrogen oxides. Accordingly, an inflow 24 of oxygen generally takes place through the pump cell 21.
In the measuring chamber 8, the oxygen diffusing through the diffusion barrier 12 passes to a third electrode 25, which together with the pump electrode 15 forms a third pump cell 26. A current source 27 is connected to the pump cell 26 and regulates the current through the pump cell 26 in such a way that a voltage U3—measured by a voltmeter 28—between the third electrode 25 and the reference electrode 5 remains at a predetermined value within the range of 350 and 450 mV. The Nernst voltage U3 in the measuring chamber 8 is preferably kept at a value in the region of 400 mV. As a result, the air ratio with regard to oxygen is set approximately to stoichiometry.
On account of a suitable doping of the third electrode 25 produced on the basis of a noble metal such as platinum, no reduction of the nitrogen oxides takes place at the electrode 25. Rather, the nitrogen oxides are reduced to nitrogen and oxygen at a measuring electrode 29. The oxygen produced at the measuring electrode 29 is removed from the measuring chamber 8 with the aid of a current source 30. The pump voltage U4 required for this is applied between the measuring electrode 29 and the pump electrode 15. The pump voltage U4 is likewise set to a value within the range of between 350 and 450 mV, preferably to a value of approximately 400 mV.
The measuring electrode 29 together with the pump electrode 15 thus forms a measuring cell 31. The current I4 through the measuring cell 31 is detected by an ammeter 32. The magnitude of the current I4 is a direct measure of the ammonia concentration in the exhaust gas.
In general, therefore, from the measuring chamber 8 an outflow 33 takes place through the pump cell 26 and a further outflow 34 takes place through the measuring cell 31. Occasionally, however, an inflow 35 of oxygen can also occur through the pump cell 26.
The measuring electrode 29 is preferably produced on the basis of a noble metal such as platinum.
In the prechamber 7, ammonia is at least partly converted into nitrogen oxides and these nitrogen oxides are detected in the measuring chamber 8. The ammonia measurement is therefore taken back to a nitrogen oxide measurement. For the measurement of the concentration of nitrogen oxides, tried and tested devices and methods are available which permit nitrogen oxides to be measured with high selectivity and great accuracy even when used in series.
It should be pointed out that no serious cross-sensitivity to nitrogen oxides is to be expected since gaseous nitrogen is produced both in the inlet chamber 6 and in the prechamber 7 and the measuring chamber 8, such that it is not necessarily the case that the nitrogen originating from the reduction of the nitrogen oxides in the inlet chamber 6 flows into the prechamber 7 and is oxidized there to form nitrogen oxides. If nitrogen does flow from the inlet chamber 6 into the prechamber 7 and is oxidized there to form nitrogen oxide, this becomes apparent in an offset of the current I4 which can be calibrated out.
The gas sensor 1 described here can optionally be used as an ammonia sensor or as a nitrogen oxide sensor. The gas sensor 1 operates as a nitrogen oxide sensor in particular when the regulation of the oxygen concentration in the inlet chamber 6 is deactivated by the current source 16 being disconnected from the pump cell 14.
The principle of a gas sensor described here can fundamentally be applied to further hydrogen-containing gases which can be oxidized to form gaseous oxides. By way of example, a gas sensor of this type can fundamentally also be used for measuring the concentration of hydrogen sulfides or hydrocarbons.
Furthermore, the principle can also be applied to gases which do not contain hydrogen and which can be oxidized by addition of a further free gas component, where the oxidized gas can be dissociated by reduction in the measuring chamber.
Furthermore, it should be pointed out that further measurement principles known to the person skilled in the art can also be applied instead of the amperometric measurement principle used in the measuring chamber 8.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 029 556.8 | Jun 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/063119 | 6/13/2006 | WO | 00 | 12/21/2007 |
