Patent Grant
8852950
The invention relates to a method for determining a NOx concentration in a measurement gas, wherein a sensor signal of a gas sensor that is sensitive for NOx and is brought into contact with the measurement gas is captured and a measurement value for the NOx concentration is determined from the sensor signal, and to a device for carrying out this method.
Such methods are already known, wherein chemiluminescence detectors (CLD) are preferably used as the gas sensors. For example, such methods are used for testing legal limit values, for example, for determining the specific nitrogen oxide emission as an exhaust gas characteristic of a combustion engine, but these methods are also used in other applications.
Legal limit values for nitrogen oxide content in a measurement gas, for example, the exhaust gas of a combustion engine, have often been established for reasons of climate protection or for other political reasons, wherein sanctions can be imposed for non-compliance. Such regulations often refer to a specifically predetermined measurement method, in order to rule out legal uncertainties.
The invention is based on the objective of creating an improved method for determining a NOx concentration in a measurement gas that is suitable for inspecting the compliance of legal or other regulations.
For meeting this objective, in a method of the type noted above it is provided that a measurement value is determined for the concentration of a second component in the measurement gas and that a numerical value for the NOx concentration in the measurement gas is determined from the measurement value for the concentration of the second component in the measurement gas. The method according to the invention offers the advantage that by determining the concentration of a second component in the measurement gas, a legally specified gas sensor, and thus a legally specified measurement method or a gas sensor type established as a reference or an established reference measurement method can be simulated or systematic measurement errors of a specified measurement method can be corrected for other measurements, wherein the usability of the method being applied for conclusions on the compliance of a specified limit value or for the comparison with other numerical values is not negatively affected or is enabled. The method according to the invention could also be used advantageously for engine settings for which a technician should use a comparable measurement device that allows a comparison with measurement values recorded in other ways or with other measurement methods.
According to one construction of the invention, it could be provided that the second component is oxygen. Tests have shown that for the frequently used chemiluminescence detectors, a systematic measurement error that can be caused, for example, by the presence of O2 occurs when determining the concentration of NOx. It has been shown that the CLD method that has proven effective for low NOx concentrations exhibits systematic deviations for high O2 and NOx concentrations. The method according to the invention thus determines a measurement value for the NOx concentration and a numerical value for the NOx concentration, wherein the numerical value can be used as a corrected measurement value or as a difference or factor for a corrected measurement value. Alternatively or additionally, as the second component an oxygen compound, for example, CO2, could be used. Here, for example, the O2 percentage can be determined indirectly via the CO2 percentage and the λ value of the measurement gas or the direct influence of CO2 on the NOx concentration determined with a specified measurement method is taken into consideration.
According to one construction of the invention, it could be provided that the measurement value and the numerical value for the NOx concentration in the measurement gas are displayed. Thus, the user of the method can decide for himself which of the two displayed values he wants to use for his actual application, wherein the numerical value can be a corrected measurement value or a difference value that is to be added to the measurement value or subtracted from this measurement value or a factor with which the measurement value is to be multiplied.
For monitoring regulations for combustion engines, it can be provided that the measurement gas is taken from the exhaust gases of a combustion engine, for example, of a diesel engine. Here it is advantageous that the method can also be used when the exhaust gas has a high O2 percentage.
The determination of the concentration of the second component in the measurement gas can be carried out by a separate analysis method or by some other input of the relevant values. For an automated execution of the method according to the invention, however, it is advantageous when a second gas sensor is provided for the determination of the concentration of the other component in the measurement gas.
For determining the measurement value for the NOx concentration from the sensor signal of the gas sensor it can be provided that the measurement value for the NOx concentration in the measurement gas is determined with reference to a stored first characteristic line from the sensor signal of the first gas sensor.
Alternatively or additionally it could be provided that the numerical value for the NOx concentration in the measurement gas is determined with reference to a stored second characteristic line from the sensor signal of the first and/or second gas sensor. Thus, the numerical value for the NOx concentration can be determined as a corrected measurement value as a function of the first and second sensor signal and/or as a difference value for the measurement value determined with the first gas sensor for the NOx concentration from the sensor signal of the second gas sensor.
An especially simple determination of the measurement values or numerical values is produced when the first characteristic line and/or the second characteristic line is/are formed by interpolation or extrapolation of calibration or support points each of a characteristic line curve. In this way it is advantageously achieved that for setting up the method according to the invention, the recording of only a small number of calibration points is required. The characteristic lines are advantageously defined and/or determined by calibration and/or comparison measurements.
For achieving an improved measurement accuracy, it can be provided that the first characteristic line and/or the second characteristic line is/are stored as data series, that is, with a plurality of measurement points. This is especially advantageous when, for an interpolation or extrapolation based on the specific properties of the gas-sensor type being used, undesired deviations and/or systematic errors are to be expected.
According to one advantageous embodiment of the invention, it can be provided that the first characteristic line is the characteristic line of an electrochemical sensor, an infrared sensor, a UV sensor, or a semiconductor sensor, and that the second characteristic line maps the deviations of a chemiluminescence detector from the first characteristic line for different values of concentration of the other component in the measurement gas. Tests have shown that infrared sensors, UV sensors, or electrochemical sensors that advantageously have constructions sensitive to NOx exhibit a measurement response deviating from frequently specified chemiluminescence detectors for a high O2 percentage in the measurement gas, in which an undesired systematic error source is eliminated or at least reduced. Instead of the electrochemical sensors, semiconductor sensors could also be used.
According to another advantageous embodiment, it could be provided that the first characteristic line is the characteristic line of a chemiluminescence detector and that the second characteristic line maps the deviations of the characteristic line of an electrochemical sensor, an infrared sensor, ITV sensor, or semiconductor sensor from the first characteristic line for different values of the concentration of the other component in the measurement gas. In this way it is advantageously achieved that even for the use of a chemiluminescence detector that is frequently specified legally or for other reasons, corrected measurement values for the NOx concentration that take into consideration the described systematic errors can be provided with the method according to the invention.
The infrared sensor is advantageously constructed as a non-dispersive IR sensor. The CLD sensor is advantageously constructed sensitive to NOx.
One especially advantageous construction is produced when an electrochemical sensor is used for the first characteristic line or for the second characteristic line.
For a simplified monitoring of the legal regulations it can be provided that the exceeding and/or falling below of limit values for the measurement value and/or numerical value for the NOx concentration in the measurement gas are signaled separately. The signaling can be realized optically, acoustically, or by printout in a log or in some other way. Thus, the user can advantageously limit himself to information on the NOx concentration corrected by systematic errors, while the method according to the invention ensures that the exceeding and/or falling below of limit values that are specified legally or for some other reasons is signaled independent of this limitation.
For example, for a construction of the method according to the invention it can be provided that the difference between measurement value and numerical value for the NOx concentration in the measurement gas increases or decreases with increasing concentration of the second component in the measurement gas.
A further improved method is given in that at least one additional measurement and/or characteristic value for the concentration of an additional component in the measurement gas is calculated and/or input and that a correction value for the NOx concentration in the measurement gas is determined from the additional measurement and/or characteristic value. For example, it could be provided that through the concentration of the additional component in the measurement gas, the moisture content in the measurement gas is determined, especially the additional component is water or water vapor. Alternatively or additionally, it could be provided that the additional component is CO2. Through the consideration of additional components, systematic errors due to the presence of additional components are taken into account even better or the influence of the moisture content on the measurement value is taken into account. The determination of the at least one component is carried out, for example, by a correspondingly constructed gas sensor.
For the further improvement of the method it can be provided that the determination of the numerical value for the NOx concentration in the measurement gas from the measurement value for the concentration of the second component and/or the additional component is carried out as a function of the moisture content of the measurement gas. The moisture content of the measurement gas can be carried out here, for example, by the input of a numerical value and/or by the input of a dew point in the measurement gas and/or by a measurement on the measurement gas, for example, with a ZrO2 probe. Tests have shown that the influence of the moisture content in the measurement gas is considerable when increased requirements are placed on the measurement accuracy, because oxygen can dissolve in the water contained in the measurement gas, wherein a correction of cross-sensitivities to other substances is advantageous more than ever. For a simple realization, it can be provided that the moisture content of the measurement gas is taken into account only in steps, for example, in two, three, or more than three steps.
For the use of a gas dehydration unit, for example, a gas cooler connected before the gas sensor, the consideration of the moisture content in the measurement gas is required when the NOx concentration for moist measurement gas is to be calculated. Conversely, the moisture content could be used to calculate a NOx concentration for dried measurement gas for a measurement without gas dehydration.
If the measurement gas involves the exhaust gas of a combustion engine, the moisture content could also be calculated from the parameters of the chemical substances fed to the combustion engine, especially the fuel and/or the air, and from the combustion products. Alternatively, in this way, the moisture content could also be measured directly.
For taking into account the moisture content of the measurement gas, here it could be provided that, for the fed chemical substances, elementary components, in particular, H and/or C, can be determined by means of an elementary analysis and/or the moisture content of the measurement gas is calculated, in particular, from the determined elementary components. Advantageously, in the elementary analysis, concentrations of the elementary components are determined from which the moisture content can be calculated. Alternatively or additionally, the moisture content of the air fed to the combustion engine can be determined.
For meeting the objective and especially for carrying out the method according to the invention, for a device for determining a NOx concentration in a measurement gas that has a gas sensor that is sensitive for NOx and can be brought into contact with the measurement gas and an evaluation unit for determining a measurement value for the NOx concentration in the measurement gas from the sensor signal of the gas sensor it is provided that an additional gas sensor is provided that is constructed for determining the concentration of a second component in the measurement gas and that the evaluation unit has means for determining a numerical value for the NOx concentration in the measurement gas from the measurement value determined with the first gas sensor for the NO concentration in the measurement gas and in the sensor signal of the second gas sensor. Advantageously, means for carrying out the described method according to the invention are constructed in the device.
In one advantageous construction it can be provided that the device has a portable construction, for example, in that the components of the device are arranged in a common housing.
According to one construction of the invention it can be provided that a storage means is provided in or on the device, wherein a first characteristic line for the evaluation of the sensor signal of the first gas sensor and a second characteristic line for the evaluation of the second sensor signal are stored in this storage means, wherein a difference value by which the numerical value deviates from the measurement value for the NOx concentration in the measurement gas or a factor with which the measurement value for the NOx concentration is to be multiplied for determining a corrected measurement value, that is, the numerical value can be inferred from the second characteristic line. Thus, the correction of systematic measurement errors due to the influences by the second component or the simulation of a measurement method that is legally specified or already established can be carried out in an especially easy way and the device according to the invention can be easily adapted to changing legal regulations or the selection of a different reference method. The stored characteristic lines are advantageously obtained through reference measurements and stored as data series and/or as functional dependencies in a memory.
In one refinement, different characteristic lines are stored for different reference measurement methods, so that for a plurality of measurement methods the appropriate resulting measurement value can be assigned for the NO concentration. For simplification, a characteristic line or its parameters averaged across different measurement methods could also be stored.
For achieving an improved consideration of the influences caused by additional components of the measurement gas it can be provided that means for the input and/or determination of the concentration of an additional component in the measurement gas are constructed and that, for different numerical values of this concentration of the additional component, different characteristic lines are stored from each of which the difference value or factor by which the numerical value deviates from the measurement value for the NOx concentration in the measurement gas can be inferred. This difference value or factor can be given for the invention, in general, in that the measurement value achieved with the measurement method and the numerical value corrected by systematic measurement errors are stored, wherein the difference is produced by the formation of a difference or that a factor is stored with which the measurement value is to be multiplied, in order to achieve the corrected measurement value, wherein the factor is produced by the formation of a quotient, or that the difference value or the factor is stored directly. For many application requirements, it is already sufficient if the input of the concentration of an additional component in the measurement gas is allowed in discrete steps. For example, the additional component could be the water content in the measurement gas and thus the moisture content of the measurement gas.
In one construction of the invention it can be provided that a gas cooler 25 and/or a condensate trap 26 in the gas flow of the measurement gas is arranged before the gas sensor 22 or the gas sensors 22, 23. Here it is advantageous that a defined moisture content of the measurement gas is set by the upstream gas cooler 25 or the upstream condensate trap 26, wherein systematic measurement errors are reduced.
For the consideration of the influence of the moisture content in the measurement gas it can be provided that means for the identification 27 of the presence of a gas cooler 25 or a condensate trap 26 in the gas flow of the measurement gas and/or means for the identification and/or input of the operating temperature of the gas cooler or the condensate trap are provided. Through these means, a conclusion on the moisture content that can be used for the correction of systematic measurement errors is possible in the measurement gas at least in coarse steps.
For determining the moisture content in the measurement gas, a moisture sensor, for example, a ZrO2 probe could be brought into contact with the measurement gas before the gas cooler or, in general, a gas dehydration device or instead of the gas dehydration device. With the measurement values for the moisture, the corrected measurement values for the NOx concentration in dry measurement gas can be converted to moist measurement gas and vice versa.
If the device according to the invention is used for determining the NOx percentage in the exhaust gas of a combustion engine, then sensors and/or input means could be provided for determining the moisture content, wherein parameters of the fuel being used, for example, the H or C percentage, and of the fed air, for example, the O2 percentage, could be calculated or input with these sensors or input means. In this case, means that allow a calculation of the moisture content in the exhaust gas from these parameters are constructed on the device.
The invention will now be described with reference to embodiments, but is not limited to these embodiments. Additional embodiments are given through the combination of features of the claims with each other and/or with features of the embodiments.
Shown are:
The arrangement here has a probe 4 with which a portion of the measurement gas 3 can be extracted and fed via an unheated hose 5 to the device 2. The feeding of the measurement gas 3 via the hose 5 is carried out as described in DE 196 31 002 C2, in particular, on Page 3, Line 29 to Page 4, Line 10 and in the patent claims of this publication.
In another embodiment, the hose 5 can be heated.
For extracting the measurement gas sample in
The device 2 is equipped according to the requirements for determining the concentration of O2, CO, NO, NO2, and SO2 with the help of electrochemical sensors and also has available a CO2—IR sensor including absolute pressure measurement.
The device 2 has available an evaluation unit 8 with means for determining a numerical value for the NOx concentration in the measurement gas from the measurement value determined with the first gas sensor 22 for the NOx concentration in the measurement gas 3 and the sensor signal of the second gas sensor 23 for the O2 concentration in the measurement gas 3.
The evaluation unit 8 is connected via a data line 9 to the base body 10 of the device 2 that encloses, forming a housing, the sensors for determining the concentrations of the components in the portion of the measurement gas 3 extracted via the probe 4.
On the evaluation unit 8 that can be separated from the base body 10 there are input means 11 for the input of control commands and/or numerical values, display means 12 for the display of control steps, input requirements, or measurement values and/or calculated numerical values, and output means 13 for the output of measurement logs and measurement values.
In the evaluation unit 8 there is a storage means 24 or memory in which a first characteristic line for the evaluation of the sensor signal of the NOx gas sensor 22 and a second characteristic line for the evaluation of the second sensor signal of the O2 sensor 23 are stored.
Another measurement device 14 is connected via another data line 15 to the evaluation unit 8 for determining ambient moisture and ambient temperature.
For the further processing of the calculated measurement values and/or the measurement log, a PC 16 is connected via a connection line 17 to the evaluation unit 8. The components of the arrangement 1 can be stored in an accessories cabinet 18 for transport.
In the device 2 shown in
In another embodiment it can be provided that the evaluation unit 8 in
With the described arrangement, a method for determining a NOx concentration in the measurement gas 3 can be carried out that will be described in detail below.
In the evaluation unit 8, a sensor signal of a gas sensor is captured, wherein this sensor is arranged in the base body 10 and is sensitive to NOx and is brought into contact with the measurement gas 3 via the probe 4 and the hose 5.
From this detected sensor signal, a measurement value for the NOx concentration is then determined.
By means of an additional sensor in the base body 10 of the device 2, a measurement value for the concentration of O2 in the measurement gas 3 is determined.
From the two determined measurement values for the concentration of NOx and O2, a numerical value for the NOx concentration in the measurement gas 3 is now determined that would be produced for the use of a CLD sensor instead of the electrochemical sensor.
The measurement value measured with the electrochemical sensor for the NOx concentration in the measurement gas 3 and the determined numerical value that would be produced for the use of a CLD sensor are displayed on the display means 12 of the evaluation unit 8.
For determining the measurement values and numerical values, characteristic lines are stored in the evaluation unit 8.
The device 2 also has available, in the base body 10, a gas cooler that is not visible in the illustration and is connected before the gas sensors in the measurement gas flow. Thus, the gas sensors in the base body 10 measure the dry measurement gas flow. With the knowledge of the moisture content of the measurement gas 3 before entry into the gas cooler, the NOx concentration for moist measurement gas can be calculated from this knowledge.
In another embodiment, the gas cooler has a separate construction.
Here, the abscissa represents the values measured with the different methods for the NOx concentration at different oxygen concentrations in the measurement gas. Deviations with increasing oxygen content can be seen clearly.
It can be clearly seen that the measurement values achieved with the non-dispersive infrared sensor 19 agree with the measurement values achieved by means of the electrochemical sensor 21 in the scope of statistical deviations, while deviations to the measurement values 20 of the CLD sensor are produced that increase in magnitude with increasing oxygen content.
From this information, a characteristic line can be derived with which the NOx concentration determined from a CLD sensor can be corrected with knowledge of the O2 concentration or with which the measurement with a CLD sensor can be simulated on the basis of a measurement of the NOx concentration with an IR or electrochemical sensor.
The mentioned information is stored in the form of characteristic lines in the evaluation unit 8 and is used for determining the numerical value for the NOx concentration that is displayed on the display means 12 and is optionally output with the output means 13.
In the evaluation unit 8, limit values specified legally or as a reference are stored for NOx concentrations and the exceeding and/or falling below of these limit values for each of the measurement value and the numerical value for the NOx concentration in the measurement gas is signaled separately on the display means 12 or logged in a measurement log. In the evaluation unit 8, for additional components, especially CO2 and water, in the measurement gas 3 analogous to
In the evaluation unit 8, the sensor signals of gas sensors are finally processed in the base body 10 such that a calculation of the moisture content of the measurement gas 3 is possible before entry into the gas cooler.
In the method for determining a NOx concentration in a measurement gas, it is provided that a measurement value for the NOx concentration is determined from the sensor signal of a gas sensor and that a measurement value for the concentration of a second component in the measurement gas is determined, wherein, a corrected value for the NOx concentration in the measurement gas is determined from the measurement values and wherein the measurement value and the corrected measurement value for the NOx concentration are displayed and/or output.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/007190 | 9/3/2008 | WO | 00 | 3/3/2011 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/025745 | 3/11/2010 | WO | A |
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