RU2623003C2 - Detection method of sulphur poisoning within exhaustion cleaning system - Google Patents

Abstract

FIELD: engines and pumps.

SUBSTANCE: exhaustion additional cleaning system for the internal combustion engine comprises at a minimum one diesel oxydation catalysts (DOC) and/or at a minimum one diesel particulate filter (DPF), at a minimum one selective reduction catalyst (SCR-catalyst), an electronating agent feeding device, first NO_x-detector element (12) that is upstream of the noted DOC and/or DPF located, second NO_x-detector element (14) that is downstream of the noted SCR-catalyst located, and at a minimum one temperature-sensitive element (16), designed with a possibility of the temperature measurement of the exhaust flow and with a possibility of at a minimum first signal conditioning on this temperature basis (T1). The method involves stages, at which: one measures the containing of compounds NOX2 nitrogen oxides that are downstream of the noted SCR-catalyst when the electronating agent is not delivered; one measures the temperature T when measuring NOX2; one compares NOX2 or value, that is tied to NOX2, with the predefined detection criterion that relates to the measured temperature. One provides the directive signal on the comparison basis. The directive signal is meant to point out that the noted at a minimum one DOC is sulphur poisoned, and that the noted ta a minimum one diesel particulate filter (DPF) is sulphur poisoned.

EFFECT: improving acquisition accuracy.

14 cl, 3 dwg

Description

Technical field

The present invention relates to an exhaust aftertreatment system and to a method for such a system according to the restrictive parts of the independent claims.

In particular, the method and system are adapted to detect sulfur poisoning of a Dioxide Oxidation Converter (DOC) in an exhaust aftertreatment system.

An internal combustion engine burns a mixture of air and fuel in order to generate a driving torque. The combustion process generates exhaust gases that are delivered from the engine to the atmosphere. The exhaust gases contain nitrogen oxides (NO _x ), carbon dioxide (CO ₂ ), carbon monoxide (CO) and particles. NO _x is a composite term to mean exhaust gases, which primarily consist of nitric oxide (NO) and nitrogen dioxide (NO ₂ ). The exhaust aftertreatment system cleans the exhaust emissions in order to reduce them before they are released into the atmosphere. In an exemplary exhaust aftertreatment system, a metering system injects a reducing agent into the exhaust gases upstream of a selective catalytic reduction catalyst (SCR catalyst). The mixture of exhaust gases and a reducing agent reacts in an SCR catalyst and thus reduces the amount of NO _x emitted to the atmosphere.

One example of a reducing agent is liquid urea, commercially available in the form of AdBlue®. This liquid is a non-toxic solution of urea in water, which is used to chemically reduce emissions of nitrogen oxides, in particular for freight vehicles with diesel engines.

The reducing agent reacts with NO _x in the SCR catalyst to influence the reduction of NO _x . More specifically, the reducing agent breaks down and forms ammonia (NH ₃ ), which then reacts with NO _x to form water and nitrogen gas (N ₂ ).

In order to achieve the described reduction in NO _x , NH ₃ must be stored in an SCR catalyst. In order for the catalytic converter to work efficiently, this storage must be at an appropriate level. In more detail, lower NO _x conversion efficiency depends on storage level.

Maintaining high conversion efficiency under various operating conditions depends on maintaining the storage of NH ₃ . The level of NH ₃ , however, should gradually decrease as the temperature of the SCR catalyst rises to avoid NH ₃ emissions (i.e., excess NH ₃ emitted from the catalyst), which can reduce the conversion efficiency of the catalyst.

In short, to meet more stringent environmental requirements, vehicle manufacturers are increasingly using SCR catalyst systems to remove nitrogen oxides (NO _x ) from diesel engine exhaust. This is accomplished by injecting an ammonia solution into the SCR catalyst to help convert NO _x particles to nitrogen gas and water. It is necessary to provide an exhaust treatment strategy so that a sufficient amount of NO _{x is} converted, while at the same time trying not to inject too much ammonia, both for environmental reasons and for reasons of economy.

At least one Dioxide Oxidizer (DOC) is also used in exhaust aftertreatment systems, as well as one or more diesel particulate filter (DPF) filters, which are often catalytic coated. The purpose of the coating, among other things, is to form a sufficient amount of NO ₂ to achieve passive oxidation of the carbon black that is captured by DPF. This is done, inter alia, by the reaction C + NO ₂ → CO + NO.

The formation of NO ₂ in the DOC will depend, inter alia, on the mass flow of exhaust gases and the temperature in the DOC. In addition to depending on flow and temperature, the DOC and / or catalytic coating in DPF retains sulfur (S), which may be present in the exhaust gas, at lower temperatures, and emits sulfur at temperatures typically above 400 ° C. If operating conditions force the absorption of a lot of sulfur, the DOC will be poisoned, i.e. NO ₂ formation will be inhibited. The content of NO ₂ after DPF will also depend on the state of DPF with respect to sulfur poisoning. Sulfur, therefore, is the main reason that the formation of NO ₂ decreases in DOC and on the catalytic coating of DPF. Actual sulfur absorption and sulfur emission temperatures depend on the particular catalyst mixture and specific operating conditions.

Low sulfur diesel fuel (below 10 mg / l), which is currently generally available in Europe and the USA, can be used for a sufficient number of hours or days of engine operation without exceeding the exhaust temperature of 400 ° C before than there will be a noticeable decrease in the formation of NO ₂ in DOC and / or DPF coated with a catalytic material. Heavy vehicles moving in this way are unusual, but can occur. However, sulfur poisoning of DOC and / or DPF coated can occur faster if the driver uses a fuel that has a higher sulfur content, for example when driving in countries where there is no low sulfur fuel, or if the vehicle accidentally refuel with high sulfur content.

Therefore, it is important to detect such poisoning and eliminate sulfur from the DOC. Sulfur is removed from the DOC and / or DPF coated by heating the catalysts above 400 ° C for a sufficient time, for example more than five minutes, which can be accomplished by injecting fuel into the exhaust gases or by activating the burner.

The temperature implied by desulfurization does not affect the SCR catalyst, which will be at that time at a temperature at which it works very efficiently, and there is minimal effect on the ratio between NO ₂ and NO.

Sensors used to measure the content of nitrogen oxides from exhaust gases are often very expensive components. NO _x sensors are often made of ceramic metal oxides, usually yttrium stabilized zirconium (YSZ). YSZ is pressed to form a solid ceramic that conducts oxygen ions at high temperatures, from about 400 ° C. To receive a measurement signal, a pair of noble metal electrodes is placed on the surface, making it possible to measure changes in the voltage or current of the electrical signal as a function of NO _x concentration.

High requirements must be met for the sensor in order to achieve the sensitivity and reliability required for measurements in exhaust gas streams. The cost of NO _x sensors is therefore high.

The NO _x sensor generates an output signal that represents a combination of the contents of NO and NO ₂ .

Upstream of the DOC (see FIG. 1), the exhaust gases contain about 90% (± 5%) NO and the rest NO ₂ . This ratio can be estimated from theoretical models.

It is known that the NO _x sensor has different sensitivities to NO and NO ₂ ; its sensitivity to NO is greater than to NO ₂ . The output signal S _NOX from the NO _x sensor can then be expressed as S _NOX = A × [NO] + B _x × [NO ₂ ], where A> B.

WO 2010/068147 describes how the diagnosis of sulfur poisoning is performed in a system with one NO _x sensor before the ASC (the last part of some SCR catalysts with special coatings) and the other after the ASC.

Diagnostics then uses NO _x levels to detect and regulate the SCR catalyst. WO 2010/068147 refers to a method for post-treatment of exhaust gases in cases where the system contains DOC and DPF. The method describes the possibility of detecting sulfur poisoning in DOC and DPF by measuring their ability to form NO ₂ . This can, among other things, be measured by NO _x sensors and then compared with the calculated values.

US 2008/216466, US 2003/032188 and US 2005/109022 are examples of patent specifications that refer to various methods for removing unwanted substances (e.g. sulfur) from a catalyst. For example, the degree of sulfur poisoning is calculated by measuring the ability of the catalyst to eliminate NO _x -gases. This is achieved by measuring the contents of NO _x gases before and after the catalytic converter.

The sulfur poisoning of DOC and DPF with catalytic coatings is a known problem that can, among other things, be caused by the use of fuels with very high sulfur content. Currently, there are no reliable methods for obtaining indications that sulfur poisoning has occurred, and the aim of the present invention is to indicate such a method.

At present, it may happen that a warning lamp is fired erroneously to indicate too high emission levels, which is indirectly caused by sulfur poisoning. Another objective of the invention is to provide an early indication that sulfur poisoning may have occurred, and thereby make possible the initiation of countermeasures.

SUMMARY OF THE INVENTION

The above problems are solved using the invention defined by the independent claims.

Preferred embodiments are defined by the dependent claims.

The NO _x sensor is adapted to produce an output signal that represents a combination of NO and NO ₂ contents, and the present invention exploits the fact that the NO _x sensors used have different sensitivities to NO and NO ₂ as the basis for the diagnosis of sulfur poisoning . The ability of DOC and DPF to convert NO to NO ₂ is a good indicator of sulfur poisoning.

When the proportion of NO ₂ relative to the proportion of NO increases due to oxidation, which is usually performed in DOC / DPF, when it is not poisoned, this means that the output signal strength from the NO _x sensor is downstream of the SCR catalyst (NOX2) (see Fig. 1) will decrease at temperatures above 150 ° C. However, the decrease is not linear over the temperature range and flow range, but reaches a maximum before a decrease in activity, but never drops below 150 ° C.

If the DOC / DPF is poisoned, the decrease will therefore not be so large. If it is completely poisoned, there will be practically no change in the signal. This means, for example, that the calculated virtual NO _x signal after the DOC / DPF will be erroneously corrected and may, therefore, cause the indication of a generally too high signal power, i.e. too high NO / NO ₂ content.

By comparing the actual NO _x signals measured downstream of the SCR catalyst with the expected signals and associating them with any previous measured signals and fuel refueling, it is possible according to the invention to detect a deterioration in the conversion of NO to NO _x and subsequent sulfur poisoning.

An advantage of the invention is that existing sensors can be used to detect a behavior model that would otherwise require a separate sensor or a workshop visit. Thus, it is possible at an earlier stage to receive an indication of sulfur poisoning and initiate earlier countermeasures.

Brief Description of the Drawings

FIG. 1 is a schematic illustration of an exhaust aftertreatment system according to the present invention;

FIG. 2 is a graph of the content of nitric oxide in exhaust gases in the first and second NO _x sensors; and

FIG. 3 is a schematic flowchart illustrating the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described with reference to the accompanying drawings.

FIG. 1 is a schematic illustration of an exhaust aftertreatment system according to the present invention.

It depicts an exhaust aftertreatment system 2 for an internal combustion engine 4, which generates an exhaust gas stream 6. System 2 comprises at least one oxidative oxidizer (DOC), designed, inter alia, to convert NO to NO ₂ , and at least one selective catalytic reduction catalyst (SCR catalyst). A diesel particulate filter (DPF) is preferably also provided downstream of the DOC. This filter may be uncoated or provided with a catalytic coating. Soot and ashes are collected here, and some conversion of NO to NO _{2 is} also performed.

The system further comprises a reducing agent device 8 adapted to supply a reducing agent 10, for example urea or ammonia, to the exhaust gas stream 6 upstream of said SCR catalyst, where NO and NO ₂ (NO _x ) are then converted to N ₂ . The amount of reducing agent supplied is controlled by a control means (not shown) depending, inter alia, on the observed content of nitrogen oxides and the temperature of the exhaust gases.

The first NO _x sensor 12 is provided upstream of the aforementioned DOC and is adapted to measure the content of nitrogen oxide mixtures (NO _x ) in the exhaust gas stream and form a first output NO _x signal (NOX1) based thereon. The second NO _x sensor 14 is provided downstream of the SCR catalyst and is adapted to measure the contents of the mixtures (NO _x ) of nitrogen oxides in the exhaust stream and to form a second output NO _x signal (NOX2) and at least one temperature sensor 16 is provided to monitor the temperature of the exhaust gas stream and generate at least a first temperature signal (T1) based thereon. The diagram shows four temperature sensors 16 for measuring the temperature of the exhaust gas stream at various points in the exhaust aftertreatment system. They are placed before and after the DOC and before and after the SCR catalyst and form the corresponding temperature signals T1, T2, T3 and T4.

The exhaust aftertreatment system 2 according to the invention comprises a computing unit 18 to which the first and second output NO _x signals (NOX1, NOX2) and said first temperature signal (T1) or one or more temperature signals can be transmitted. The first and second NO _x sensors 12, 14 are adapted to measure the content of NOX1 mixtures of nitrogen oxides upstream of said DOC and practically simultaneously measure the content of NOX2 mixtures of nitrogen oxides downstream of the SCR catalyst when the reducing agent 10 is not supplied to the stream 6 exhaust gases from the reducing agent device 8. The temperature sensor or sensors 16 are also adapted to measure the temperature T when measurements NOX1 and NOX2 are performed.

Computing unit 18 then compares NOX2, or the value associated with NOX2, with a detection criterion that relates to the measured temperature, and generates an indication signal 20 based on the comparison.

In one embodiment, the detection criterion is a predetermined threshold value NOXtr associated with the measured temperature, and an indication signal 20 is generated if NOX2 is greater than NOXtr. In this embodiment, NOXtr may, for example, be selected to be in a predetermined relationship with NOX1.

FIG. 2 is a graph showing the content of nitric oxide in exhaust gases in the first and second NO _x sensors. It should be emphasized that the graph is primarily intended to illustrate aspects that are important to illustrate the invention, and therefore is simplified.

As can be seen in the drawing, the content of nitric oxide NOX1 measured by the first NO _x sensor 12 is constant regardless of temperature. Measurement by the second NO _x sensor 14, when no reducing agent is supplied, will show the amplitude of the output signal decreasing if the DOC (and, where applicable, DPF) works as expected, i.e. if sulfur poisoning has not occurred. The proportion of NO ₂ in NO _x will therefore increase with respect to the proportion of NO and the different sensitivities of the sensor to NO and NO ₂ will cause a decrease in the amplitude of NOX2. This is illustrated in the drawing by decreasing NOX2 with increasing temperatures.

By comparing NOX1 and NOX2 at the same temperature (350 ° C in the drawing) with the threshold value NOXtr, which is applicable at this temperature (in this case 350 ° C), it is possible to get an indication of whether NOX2 represents the expected decrease. In the drawing, the NOX2 signal of the sensor at 350 ° C is less than NOXtr, which indicates that no sulfur poisoning has occurred.

The graph also shows the NOX2 'curve (dash-dot line) representing the output signals received from the second NO _x sensor 14 in a different situation. Here, the output of the NO _x sensor is higher than NOXtr at 500 ° C, which may indicate sulfur poisoning by DOC (and / or DPF).

In another embodiment, the computing unit 18 is adapted to compare by determining ΔNO _X = | NOX1-NOX2 |, comparing ΔNO _X with a predetermined threshold value NOXtr ', which serves as a detection criterion, and generating an indication signal 20 if ΔNOX below NOXtr '. In this embodiment, the difference between NOX1 and NOX2 is thus compared with the threshold value NOXtr '.

The threshold values are preferably selected such that some deviation of NOX2 is required for indication to be provided. This may mean in the embodiment of FIG. 2 that NOXtr, for example, should be 10% higher than the corresponding "normal" values of NOX2. And in another embodiment, when the difference between NOX1 and NOX2 is compared with NOXtr ', NOXtr', for example, is selected 10% smaller than the "normal" difference.

In both embodiments, a table or a simple database is preferably provided, for example, in the computing unit 18, containing agreed values for the temperatures of the exhaust gas stream and said predetermined threshold values NOXtr or NOXtr '. The definition of NOXtr implies starting with NOX1 and allowing the threshold to be a predefined fraction of NOX1 that is related to temperature. Of course, it is also possible to calculate these threshold values directly on the basis of the relationship between temperature and NO _x content.

In one embodiment, the indicating signal 20 is adapted to indicate that DOC and / or DPF is sulfur poisoned. It can take the form of an alarm for the driver that the vehicle should call into the workshop. It may also mean that a countermeasure is immediately triggered to remove sulfur from the DOC and / or DPF, for example, by increasing the temperature of the exhaust gas in a controlled manner.

Since it is known that the output from the NO _x sensor is differently sensitive to NO and NO ₂ , the output from the second NO _x sensor is adjusted appropriately to obtain a “true” NO _x value, which can then be set relative to output signal from the first NO _x sensor. In practice, this means applying a correction value to NOX2, which thereby implies a higher value. If the DOC and / or DPF is poisoned, NOX2 will therefore increase and the application of a correction value will then indicate a NO _x content that is too high and therefore erroneous.

In a further embodiment, the exhaust aftertreatment system is adapted to calibrate the NOX2 output signal from the second NO _x sensor 14 with respect to the NOX1 output signal from the first NO _x sensor 12. This is done by passing exhaust gases through the system at a low temperature, preferably below 150 ° C when oxidation practically does not occur in the DOC, and the reducing agent 10 is not applied to the exhaust stream from the device 8 with the reducing agent.

This is done by measuring NOX1 and NOX2, determining NOX2k = NOX1-NOX2 and determining the calibrated value for NOX2 in the form NOX2 '= NOX2 + NOX2k. Then, by means of the computing unit 18, a comparison is made with the calibrated value NOX2 ′ instead of NOX2.

The present invention also relates to a method for an exhaust aftertreatment system for an internal combustion engine that generates an exhaust gas stream. The method is illustrated by a schematic flowchart of FIG. 3.

The components of the exhaust aftertreatment system and their functions are described in detail above, and reference is made here to this description.

The method according to the invention includes the steps of:

A - measurements of the content of NOX1 mixtures of nitrogen oxides upstream of the DOC and measurements at almost the same time as the content of NOX2 mixtures of nitrogen oxides downstream of the SCR catalyst when the reducing agent is not applied to the exhaust gas stream from the device with the reducing agent,

B - temperature measurements T, when measurements are made NOX1 and NOX2,

C - comparisons of NOX2 or a value associated with NOX2 with a detection criterion related to the measured temperature,

D - formation of the indicating signal based on comparison.

In one embodiment, the detection criterion in step C is a predetermined threshold value NOXtr associated with the measured temperature, and said indicating signal is generated in step D if NOX2 is greater than NOXtr.

In another embodiment, step A further comprises determining ΔNO _X = | NOX1-NOX2 |, said detection criterion in step C is a predetermined threshold value NOXtr 'that is compared with ΔNO _X , and said indicating signal is generated in step D if ΔNO _X less NOXtr '.

Predefined threshold values, for example, are stored in a table or database that contains agreed values for exhaust gas flow temperatures and said predefined threshold values (NOXtr and NOXtr '). As mentioned above, they can also be calculated.

In one embodiment, the indicating signal is adapted to indicate that DOC and / or DPF are sulfur poisoned. It can take the form of an alarm for the driver that the vehicle should call into the workshop. It can also trigger a countermeasure immediately triggered in order to remove sulfur from the DOC and / or DPF, for example, by increasing the temperature of the exhaust gas in a controlled manner.

In an additional embodiment of the present invention, the method comprises the step of calibrating NOX2 at a low temperature, preferably below 150 ° C, when oxidation practically does not occur in the DOC and the reducing agent is not applied to the exhaust gas stream from the reducing agent device. This is achieved by measuring NOX1 and NOX2, determining the calibrated value NOX2k = NOX1-NOX2 and determining the calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, the process steps described above are carried out with the calibrated value NOX2' instead of NOX2.

The present invention is not limited to the preferred embodiments described above. Various alternatives, modifications, and equivalents may be used. The above described embodiments are therefore not considered to limit the protective scope of the invention as defined by the appended claims.

Claims (18)

1. A method for an exhaust aftertreatment system for an internal combustion engine that generates an exhaust gas stream, the system comprising at least one oxidative diesel exhaust gas (DOC) converter and / or at least one diesel particulate particulate filter (DPF) at least one selective reduction catalyst (SCR catalyst), a reducing agent device configured to supply a reducing agent to an exhaust gas stream upstream of said SCR of the catalyst, the first ΝΟ _x sensor (12) located upstream of the said DOC and / or DPF and configured to measure the content of nitrogen oxides mixtures (NO _x ) in the exhaust gas stream and generate, on its basis, the first output ΝΟ _x signal (NOX1), a second ΝΟ _x sensor (14) located downstream of the SCR catalyst and configured to measure the content of (NO _x ) mixtures of nitrogen oxides in the exhaust stream and form a second output ΝΟ _x signal on its basis (NOX2) and at least one temperature sensor to (16) adapted to measure the temperature of the exhaust stream and forming on its base at least the first temperature signal (Τ1), characterized in that it comprises the steps of: A - measure the content of NOX2 mixtures of nitrogen oxides downstream of the SCR catalyst when the reducing agent is not supplied to the exhaust stream from the device with the reducing agent, B - measure the temperature T when the NOX2 measurement is performed, C - compare NOX2, or the value associated with NOX2, with a predefined detection criterion related to the measured temperature, and D - a indicating signal is generated based on a comparison, the indicating signal is intended to indicate that said at least one DOC is sulfur-poisoned and / or that said at least one particulate filter (DPF) is sulfur-poisoned. 2. The method according to p. 1, characterized in that step A further includes the step of measuring the content of the mixtures of NOX1 nitrogen oxides upstream of said DOC and / or DPF at substantially the same time as the content of the mixtures of NOX2 oxides nitrogen downstream of said SCR catalyst. 3. The method according to claim 1, characterized in that the detection criterion in step C is a predetermined threshold value NOXtr associated with the measured temperature, and said indicating signal is generated in step D if NOX2 is greater than NOXtr. 4. The method according to p. 2, characterized in that stage A further includes a stage at which ΔΝO _x = | ΝOΧ1-NOX2 | is determined, and stage C includes a stage at which ΔΝO _{x is} compared with a predetermined threshold value NOXtr ', which serves as said detection criterion, and said indicating signal is generated in step D if ΔΝO _{x is} less than NOXtr '. 5. The method according to p. 3 or 4, characterized in that they provide a table that contains agreed values for the temperature of the exhaust stream and the aforementioned predetermined threshold values (NOXtr, NOXtr '). 6. The method according to any one of paragraphs. 2-4, characterized in that NOX2 is calibrated at a low temperature, preferably below 150 ° C, when oxidation essentially does not occur in the DOC and the reducing agent is not delivered to the exhaust stream from the reducing agent device by means of the steps where NOX1 is measured and NOX2, determine the calibrated value NOX2k = NOX1-NOX2 and determine the calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, the process steps according to any of the above items being carried out with the calibrated value NOX2' instead of NOX2. 7. An exhaust aftertreatment system (2) for an internal combustion engine (4), which generates an exhaust gas stream (6), the system (2) comprising at least one diesel exhaust gas oxidizer (DOC) and / or at least one diesel particulate filter (DPF), at least one selective reduction catalyst (SCR catalyst), a reducing agent device (8) configured to supply a reducing agent (10) to the exhaust gas stream (6) above flow from said SCR catalyst, the first ΝΟ _x sensor (12) located upstream of the said DOC and / or DPF and configured to measure the content of nitrogen oxide mixtures (NO _x ) in the exhaust stream and form the first NO _x output on its basis -signal (NOX1), the second ΝΟ _x- sensor (14), located downstream of the SCR catalyst and configured to measure the content of mixtures (NO _x ) of nitrogen oxides in the exhaust stream and the formation of its second output ΝO _x signal (NOX2), and at least one temperature d tchik (16) adapted to measure the flow of exhaust gas temperature and formation on its base at least the first temperature signal (Τ1), characterized in that it comprises a computing unit (18), which are transmitted said first and second output ΝΟ _x -signals (NOX1, NOX2) and said first temperature signal (T1), the first and second ΝΟ _x sensors (12, 14) being configured to measure the content of NOX1 mixtures of nitrogen oxides upstream of said DOC and / or DPF and measure essentially at the same time content mixtures of NOX2 nitrogen oxides downstream of the SCR catalyst when the reducing agent (10) is not supplied to the exhaust gas stream (6) from the device (8) with the reducing agent, while the temperature sensor (16) is configured to measure temperature T when measurements of NOX1 and NOX2 are carried out, and the computing unit (18) is configured to compare NOX2 or a value associated with NOX2 with a detection criterion related to the measured temperature and generating, based on the comparison, an indication signal (20) intended To indicate that said at least one DOC poisoned by sulfur and / or that said at least one particulate filter (DPF) is poisoned by sulfur. 8. The exhaust aftertreatment system according to claim 7, characterized in that said detection criterion is a predetermined threshold value NOXtr, and a indicating signal (20) is generated if NOX2 is greater than NOXtr. 9. An exhaust aftertreatment system according to claim 7, characterized in that the computing unit (18) is configured to contain a determination of ΔΝO _x = | NOX1-NOX2 |, to compare ΔΝO _x with a predetermined threshold value NOXtr ', which serves as the mentioned criterion detecting and generating a signal (20) if ΔΝO _{x is} less than NOXtr '. 10. The exhaust aftertreatment system according to claim 8 or 9, characterized in that there is a table that contains the agreed values for the exhaust gas flow temperatures and the aforementioned predetermined threshold values NOXtr or NOXtr '. 11. An exhaust aftertreatment system according to claim 7, characterized in that it is configured to calibrate NOX2 at a low temperature, preferably below 150 ° C, when oxidation essentially does not occur in the DOC and the reducing agent (10) is not supplied to the exhaust gas stream from the device (8) with a reducing agent, by measuring NOX1 and NOX2 and determining the calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, and the comparison is carried out by means of a computing unit (18) with the calibrated value NOX2' instead of NOX2. 12. An exhaust aftertreatment system according to claim 8, characterized in that it is configured to calibrate NOX2 at a low temperature, preferably below 150 ° C, when oxidation essentially does not occur in the DOC and the reducing agent (10) is not supplied to the exhaust gas stream from the device (8) with a reducing agent, by measuring NOX1 and NOX2 and determining the calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, and the comparison is carried out by means of a computing unit (18) with the calibrated value NOX2' instead of NOX2. 13. The exhaust aftertreatment system according to claim 9, characterized in that it is configured to calibrate NOX2 at a low temperature, preferably below 150 ° C, when oxidation essentially does not occur in the DOC and the reducing agent (10) is not supplied to the exhaust gas stream from the device (8) with a reducing agent, by measuring NOX1 and NOX2 and determining the calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, and the comparison is carried out by means of a computing unit (18) with the calibrated value NOX2' instead of NOX2. 14. The exhaust aftertreatment system according to any one of paragraphs. 7-9 and 11-13, characterized in that the diesel particulate filter (DPF) is located downstream of said DOC.

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