EP2885513A1

EP2885513A1 - Method for detecting sulphur poisoning in an exhaust treatment system

Info

Publication number: EP2885513A1
Application number: EP13784466.8A
Authority: EP
Inventors: Ola Stenlåås
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2012-05-03
Filing date: 2013-04-24
Publication date: 2015-06-24
Also published as: EP2885513A4; SE1250441A1; RU2623003C2; BR112014026956A2; CN104271909A; SE538378C2; WO2013165302A1; RU2014148590A

Abstract

The invention relates to an exhaust post-treatment system (2) for a combustion engine (4) which produces an exhaust flow (6), which system (2) comprises at least one diesel oxidation catalyst (DOC) and/or at least one diesel particle filter (DPF), and at least one selective reduction catalyst (SCR catalyst). The system further comprises a reducing agent device (8) adapted to supplying a reducing agent (10) to the exhaust flow (6) upstream of said SCR catalyst, a first NO_x sensor (12) situated upstream of said DOC and/or DPF and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a first NOx output signal (NOX1), a second NO_x sensor (14) situated downstream of said SCR catalyst and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a second NO_x output signal (NOX2), and at least one temperature sensor (16) adapted to monitoring the temperature of the exhaust flow and to producing on the basis thereof at least a first temperature signal (T1). The exhaust post- treatment system (2) comprises a calculation unit (18) to which said first and second NOx output signals (NOXl, NOX2) and said first temperature signal (T1) are arranged to be conveyed, said first and second NOx sensors (12, 14) are adapted to measuring the content of nitrogen oxide compounds NOX1 upstream of said DOC and/or DPF and to measuring at substantially the same time the content of nitrogen oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent (10) is being supplied to the exhaust flow (6) from the reducing agent device (8), and said temperature sensor (16) is adapted to measuring the temperature T when the measurements of NOXl and NOX2 are taking place, which calculation unit (18) is adapted to comparing NOX2 with a detection criterion, which preferably comprises comparing with a predetermined threshold value NOXtr related to the measured temperature, and to generating on the basis of the comparison an indicating signal (20).

Description

Title

Method for detecting sulphur poisoning in an exhaust treatment system Field of the invention

The present invention relates to an exhaust post-treatment system, and a method pertaining to such a system, according to the preambles of the independent claims.

In particular, the method and the system are adapted to detecting sulphur poisoning of a diesel oxidation catalyst (DOC) in the exhaust post-treatment system. A combustion engine burns a mixture of air and fuel in order to generate a propulsive torque. The combustion process generates exhaust gases which are delivered from the engine to the atmosphere. The exhaust gases comprise nitrogen oxides (ΝΟχ), carbon dioxide (C0₂), carbon monoxide (CO) and particles. NO_x is a composite term to denote exhaust gases which consist primarily of nitrogen oxide (NO) and nitrogen dioxide (N0₂). An exhaust post-treatment system treats exhaust discharges in order to decrease them before they are released to the atmosphere. In an exemplifying exhaust post-treatment system, a dosing system injects a reducing agent into the exhaust gases upstream of a selective catalytic reduction catalyst (SCR catalyst). The mixture of exhaust gases and reducing agent reacts in the SCR catalyst and thereby reduces the amounts of ΝΟχ released 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 urea solution in water which is used to chemically reduce discharges of nitrogen oxides, particularly for diesel-engined heavy vehicles.

The reducing agent reacts with ΝΟχ in the SCR catalyst to effect the NO_x reduction. More specifically, the reducing agent is broken down and forms ammonia (NH₃) which then reacts with ΝΟχ to form water and nitrogen gas (N₂). To achieve the NO_x reduction described, NH₃ has to be stored in the SCR catalyst. For the catalyst to work effectively, this storage has to be at an appropriate level. In more detail, the NO_x reduction, the conversion efficiency, depends on the storage level. Maintaining high conversion efficiency in different operating situations depends on maintaining the store of NH₃. The NH₃ level does however have to be decreased progressively as the temperature of the SCR catalyst rises, to avoid NH₃ discharges (i.e. surplus NH being released from the catalyst) which might lower the conversion efficiency of the catalyst.

In brief, to meet stricter environmental requirements, vehicle manufacturers are increasingly using SCR catalyst systems to remove nitrogen oxides (ΝΟχ) from diesel exhaust gases. This is done by injecting ammonia solution into an SCR catalyst to help to convert ΝΟχ particles to nitrogen gas and water. The exhaust cleaning strategy needs to cater for sufficient ΝΟχ to be converted while at the same time trying not to inject too much ammonia, for both environmental and operational economy reasons.

At least one diesel oxidation catalyst (DOC) is also used in exhaust post-treatment systems, as too are one or more diesel particle filters (DPF) which are often provided with a catalytic coating. The object of the coating is inter alia to generate a sufficient amount of N0₂ to achieve passive oxidation of soot which is captured by DPF. This takes place inter alia by the reaction C + N0₂→ CO + NO. The formation of N0₂ in DOC will depend inter alia on the mass flow of the exhaust gases and the temperature in DOC. As well as the dependency on flow and temperature, DOC and/or the catalytic coating in DPF store sulphur (S), which may be present in the exhaust gases, at lower temperatures and release sulphur at temperatures typically over 400°C. If the operating conditions cause it to absorb much sulphur, DOC will be poisoned, i.e. the formation of N0₂ will be inhibited. The N0₂ content after DPF will also depend on DPF's state with regard to sulphur poisoning. Sulphur is thus the main reason for the formation of N0₂ to decrease in DOC and at the catalytic coating of DPF. The actual temperatures of sulphur absorption and sulphur release depend on the specific catalyst mix and the specific operating conditions.

Diesel fuel with low sulphur content (below 10 ppm), which is now generally available in Europe and the USA, may be used for quite a number of hours or days of engine operation without exceeding exhaust temperatures of 400°C before there is an appreciable decrease in the formation of N0₂ in DOC and/or in the DPF coated with catalytic material.

Running heavy vehicles in such a way is unusual but may happen. However, sulphur poisoning of DOC and/or the coated DPF may occur sooner if the driver uses fuel which has a higher sulphur content, e.g. when driving in countries where there is no low-sulphur fuel or if the vehicle is inadvertently provided with fuel with a high sulphur content.

It is therefore important to detect such poisoning and remove the sulphur from DOC. Sulphur is removed from DOC and/or the coated DPF by heating the catalysts to over 400°C for a substantial time, e.g. more than five minutes, which may be achieved by injecting fuel into the exhaust gases or by activating a burner.

The temperature involved in the desulphuration does not affect the SCR catalyst, which will during it be at a temperature at which it operates very effectively and there is minimal effect upon the ratio between N0₂ and NO.

The sensors used for measuring the nitrogen oxides content of exhaust gases are often very expensive components. NO_x sensors are often made of ceramic metal oxides, usually yttrium-stabilised zirconium (YSZ). YSZ is pressed to form a solid ceramic which conducts oxygen ions at high temperatures, from about 400°C. To obtain a measurement signal, a pair of noble metal electrodes are placed on the surface, making it possible for the variations in voltage or current of an electrical signal to be measured as a function of the ΝΟχ concentration.

High requirements have to be met for the sensor to achieve the sensitivity and robustness required for measurements in exhaust flows. The cost of ΝΟχ sensors is therefore high.

The NO_x sensor produces an output signal which represents the aggregate of the NO and N0₂ contents. Upstream of DOC (see Figure 1) the exhaust gases contain about 90% (+/- 5%) of NO and the remainder N0₂. This ratio may be estimated from theoretical models. It is known that the NO_x sensor has different sensitivities to NO and N0₂, its sensitivity to NO being greater than to N0₂. The output signal SN_OX from the NO_x sensor may be then expressed as SN_OX = A*[NO] + B* [ N0₂], where A > B. WO-2010/068147 describes how a sulphur poisoning diagnosis is made in a system with one NO_x sensor before ASC (the last part of certain SCR catalysts with special coatings) and another after.

The diagnosis then uses the 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 comprises DOC and DPF. The method describes the possibility of detecting sulphur poisoning in DOC and DPF by measuring their ability to form N0₂. This may inter alia be measured by NO_x sensors and then be compared with estimated values.

US-2008/216466, US-2003/032188 and US-2005/109022 are examples of patent specifications referring to different methods for removing undesirable substances (e.g. sulphur) from a catalyst. For example, the degree of sulphur poisoning is calculated by measuring the catalyst's ability to remove NO_x gases. This is achieved by measuring the content of NO_x gases before and after the catalyst. Sulphur poisoning of DOC and of DPF with catalytic coatings is a known problem which may inter alia be caused by using fuel with too much sulphur content. There are at present no reliable methods for obtaining indications that there is sulphur poisoning, and the object of the present invention is to indicate such a method.

It may at present happen that a warning lamp lights up erroneously to indicate too high discharge levels, which is indirectly caused by sulphur poisoning. Another object of the invention is to provide an early indication that there may be sulphur poisoning and thereby make it possible to initiate countermeasures.

Summary of the invention

The above objects are achieved with the invention defined by the independent claims.

Preferred embodiments are defined by the dependent claims. An NO_x sensor is adapted to producing an output signal which represents the aggregate of the NO and N0₂ contents, and the present invention utilises the fact that the NO_x sensors used have different sensitivities to NO and N0₂ as a basis for arriving at a diagnosis of sulphur poisoning. The ability of DOC and DPF to convert NO to N0₂ is a good indicator of sulphur poisoning.

As the proportion of N0₂ relative to that of NO is increased by the oxidation which normally takes place in DOC/DPF when not poisoned, this means that the strength of the output signal from the NO_x sensor downstream of the SCR catalyst (NOX2) (see Figure 1) will decrease at temperatures over 150°C. However, the decrease is not linear across the temperature range and flow range but reaches a maximum before falling back, but never goes below 150°C.

If DOC/DPF is poisoned, the decrease will therefore not be so great. If it is completely poisoned, there will be substantially no change in the signal. This means for example that a calculated virtual NO_x signal after DOC/DPF will be erroneously corrected and may therefore be caused to indicate an altogether too high signal strength, i.e. too high an NO N0₂ content.

By comparing actual NO_x signals measured downstream of the SCR catalyst with expected signals and relating them to any previous measured signals and refuellings it is possible according to the invention to detect impaired NO to NO_x conversion and consequent sulphur poisoning.

An advantage of the invention is that existing sensors may be used to detect a pattern which would otherwise require a separate sensor or a workshop visit. It is thus possible at an earlier stage to have an indication of sulphur poisoning and to initiate earlier countermeasures.

Brief description of drawings

Figure 1 is a schematic illustration of an exhaust post-treatment system according to the present invention. Figure 2 is a graph of nitrogen oxide contents in the exhaust gases at the first and second NO_x sensors.

Figure 3 is a schematic flowchart illustrating the present invention. Detailed description of preferred embodiments of the invention

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

Figure 1 is a schematic illustration of an exhaust post-treatment system according to the present invention.

It depicts an exhaust post-treatment system 2 for a combustion engine 4 which produces an exhaust flow 6. The system 2 comprises at least one diesel oxidation catalyst (DOC) intended inter alia to convert NO to N0₂, and at least one selective catalytic reduction catalyst (SCR catalyst). A diesel particle filter (DPF) is preferably also provided downstream of DOC. This filter may be uncoated or be provided with a catalytic coating. Here soot and ash are gathered and a certain conversion of NO to N0₂ also takes place here. The system further comprises a reducing agent device 8 adapted to supplying a reducing agent 10, e.g. urea or ammonia, to the exhaust flow 6 upstream of said SCR catalyst where NO and N0₂ (NO_x) are then converted to N₂. The amount of reducing agent supplied is controlled by a control means (not depicted) depending inter alia on monitored contents of nitrogen oxides and the temperature of the exhaust gases.

A first NO_x sensor 12 is provided upstream of said DOC and is adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a first NO_x output signal (NOX1). A second NO_x sensor 14 is provided downstream of said SCR catalyst and is adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a second NO_x output signal (NOX2), and at least one temperature sensor 16 is provided to monitor the temperature of the exhaust flow and to produce on the basis thereof at least a first temperature signal (Tl). The diagram shows four temperature sensors 16 for measuring the temperature of the exhaust flow at different points in the exhaust post- treatment system. They are situated before and after DOC and before and after the SCR catalyst and produce respective temperature signals Tl , T2, T3 and T4.

The exhaust post-treatment system 2 comprises according to the invention a calculation unit 18 to which the first and second NO_x output signals (NOXl , NOX2) and said first temperature signal (Tl), or one or more of the temperature signals, are arranged to be conveyed. The first and second NO_x sensors 12, 14 are adapted to measuring the content of nitrogen oxide compounds NOXl upstream of said DOC and to substantially simultaneously measuring the content of nitrogen oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent 10 is being supplied to the exhaust flow 6 from the reducing agent device 8. The temperature sensor or sensors 16 are also adapted to measuring the temperature T when the measurements of NOXl and NOX2 are taking place.

The calculation unit 18 then compares NOX2, or a value related to NOX2, with a detection criterion which is related to the measured temperature, and generates on the basis of the comparison an indicating signal 20. In one embodiment the detection criterion is a predetermined threshold value NOXtr related to the measured temperature, and the indicating signal 20 is generated if NOX2 is greater than NOXtr. In this embodiment, NOXtr may for example be chosen such as to be in a predetermined relation with NOXl . Figure 2 is a graph showing the nitrogen oxide contents of the exhaust gases at the first and second NO_x sensors. It should be emphasised that the graph is primarily intended to illustrate the aspects which are important for illustrating the invention and is therefore simplified. As may be seen in the diagram, the content of nitrogen oxide NOXl measured by the first NO_x sensor 12 is constant irrespective of temperature. Measurement by the second NO_x sensor 14 when no reducing agent is being supplied will show the amplitude of the output signal decreasing if DOC (and, where applicable, DPF) works as intended, i.e. if no sulphur poisoning has taken place. The proportion of N0₂ in NO_x will therefore increase relative to the NO proportion, and the different sensitivities of the sensor to NO and N0₂ will cause the amplitude of NOX2 to decrease. This is illustrated in the diagram by NOX2 decreasing with increasing temperatures.

By comparing NOX1 and NOX2 at the same temperature (350°C in the diagram) with a threshold value NOXtr which is applicable at that temperature (in this case 350°C) it is possible to obtain an indication of whether NOX2 presents an expected decrease. In the diagram the sensor signal NOX2 at 350°C is less than NOXtr, which indicates that there is no sulphur poisoning.

The diagram also shows a curve NOX2' (chain-dotted line) representing output signals taken from the second NO_x sensor 14 in a different situation. Here the NO_x sensor's output signal is higher than NOXtr at 500°C, which may indicate sulphur poisoning of DOC (and/or DPF).

In another embodiment, the calculation unit 18 is adapted to conducting the comparison by determining ΔΝΟχ = | NOX1-NOX2 | , comparing ΔΝΟχ with the predetermined threshold value NOXtr' which serves as the detection criterion, and generating the indicating signal 20 if ΔΝΟΧ is lower than NOXtr'. In this embodiment the difference between NOX1 and NOX2 is thus compared with a threshold value NOXtr'.

The threshold values are preferably chosen such that a certain deviation of NOX2 is required for an indication to be given. This may mean in the embodiment depicted in Figure 2 that NOXtr has for example to be 10% higher than corresponding "normal" values of NOX2. And in the other embodiment, when the difference between NOX1 and NOX2 is compared with NOXtr', NOXtr' is for example chosen at 10% less than the "normal" difference. In both embodiments a table or a simple database is preferably provided, e.g. in the calculation unit 18, containing coordinated values for temperatures of the exhaust flow and said predetermined threshold values NOXtr or NOXtr'. Determining NOXtr involves starting from NOX1 and allowing the threshold value to be a predetermined portion of NOX1 which is related to temperature. It is of course also possible to calculate these threshold values directly on the basis of the relationship between temperature and the NO_x content.

In one embodiment the indicating signal 20 is adapted to indicating that DOC and/or DPF are sulphur-poisoned. It may take the form of an alarm signal to the driver that the vehicle should call at a workshop. It may also mean that a countermeasure is immediately initiated to remove sulphur from DOC and/or DPF, e.g. by raising in a controlled way the temperature of the exhaust gases.

As it is known that the output signal from an NO_x sensor is sensitive differently to NO and N0₂, the output signal from the second NO_x sensor is subjected to a suitable adjustment value to arrive at a "true" NO_x value which may then be set against the output signal from the first NO_x sensor. In practice this means applying an adjustment value to NOX2, which thereby assumes a higher value. If DOC and/or DPF are poisoned, NOX2 will therefore increase, and applying an adjustment value will then indicate an NO_x content which is too high and therefore erroneous. In a further embodiment, the exhaust post-treatment system is adapted to calibrating the output signal NOX2 from the second NO_x sensor 14 with respect to the output signal NOX1 from the first NO_x sensor 12. This is done by having exhaust gases pass through the system at a low temperature, preferably below 150°C, when substantially no oxidation is taking place in DOC and no reducing agent 10 is being supplied to the exhaust flow from the reducing agent device 8.

This is done by measuring NOX1 and NOX2, determining NOX2k = NOX1 -NOX2 and determining a calibrated value for NOX2 in the form NOX2'=NOX2+NOX2k. The comparison by the calculation unit 18 is then conducted with the calibrated value NOX2' instead of NOX2. The present invention relates also to a method for an exhaust post-treatment system for a combustion engine which produces an exhaust flow. The method is illustrated by the schematic flowchart in Figure 3.

The constituent parts of the exhaust post-treatment system and their functions are described in detail above and that description is here cited.

The method according to the invention comprises the steps of

A - measuring the content of nitrogen oxide compounds NOX1 upstream of DOC and measuring at substantially the same time the content of nitrogen oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent is being supplied to the exhaust flow from the reducing agent device,

B - measuring the temperature T when the measurements of NOX1 and NOX2 are taking place,

C - comparing NOX2, or a value related to NOX2, with a detection criterion related to the measured temperature,

D - generating an indicating signal on the basis of the comparison.

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

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

The predetermined threshold values are for example stored in a table or a database which contains coordinated values for temperatures of the exhaust flow and said predetermined threshold values (NOXtr and NOXtr'). As mentioned above, they may also be calculated.

In one embodiment the indicating signal is adapted to indicating that DOC and/or DPF are sulphur-poisoned. It may take the form of an alarm signal to the driver that the vehicle should call at a workshop. It may also entail a countermeasure being immediately initiated to remove sulphur from DOC and/or DPF, e.g. by raising in a controlled way the temperature of the exhaust gases. In a further embodiment of the present invention, the method comprises the step of calibrating NOX2 at a low temperature, preferably below 150°C, when substantially no oxidation is taking place in DOC and no reducing agent is being supplied to the exhaust flow from the reducing agent device. This is achieved by measuring NOX1 and NOX2, determining a calibration value NOX2k = NOX1-NOX2 and determining a calibrated value for NOX2 as NOX2 '=NOX2+NOX2k, method steps described above being conducted with the calibrated value NOX2' instead of NOX2.

The present invention is not restricted to the preferred embodiments described above. Sundry alternatives, modifications and equivalents may be used. The above embodiments are therefore not to be regarded as limiting the invention's protective scope which is defined by the attached claims.

Claims

1. A method for an exhaust post-treatment system for a combustion engine which produces an exhaust flow, which system comprises at least one diesel oxidation catalyst (DOC) and/or at least one diesel particle filter (DPF), at least one selective reduction catalyst (SCR catalyst), a reducing agent device adapted to supplying a reducing agent to the exhaust flow upstream of said SCR catalyst, a first NO_x sensor (12) situated upstream of said DOC and/or DPF and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a first NO_x output signal (NOXl ), a second NO_x sensor (14) situated downstream of said SCR catalyst and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a second NO_x output signal (NOX2), and at least one temperature sensor ( 16) adapted to measuring the temperature of the exhaust flow and to producing on the basis thereof at least a first temperature signal (Tl), c h a r a c t e r i s e d in that the method comprises the steps of

A - measuring the content of nitrogen oxide compounds NOX2 downstream of said SCR catalyst when no reducing agent is being supplied to the exhaust flow from the reducing agent device,

B - measuring the temperature T when the measurement of NOX2 is taking place, C - comparing NOX2, or a value related to NOX2, with a predetermined detection criterion related to the measured temperature,

D - generating an indicating signal on the basis of the comparison.

2. The method according to claim 1, in which step A further comprises measuring the content of nitrogen oxide compounds NOXl upstream of said DOC and/or DPF at substantially the same time as the content of nitrogen oxide compounds NOX2 downstream of said SCR catalyst is being measured.

3. The method according to claim 1 or 2, in which said detection criterion at step C is a predetermined threshold value NOXtr related to the measured temperature, and said indicating signal is generated at step D if NOX2 is greater than NOXtr.

4. The method according to claim 2 or 3, in which step A further comprises determining ΔΝΟχ = | NOX1-NOX2 | , step C comprises comparing ΔΝΟχ with a predetermined threshold value NOXtr' which serves as said detection criterion and said indicating signal is generated at step D if ΔΝΟχ is less than NOXtr'.

5. The method according to either of claims 3 and 4, in which a table is provided which contains coordinated values for temperatures of the exhaust flow and said predetermined threshold values (NOXtr, NOXtr').

6. The method according to any one of the foregoing claims, in which said indicating signal is adapted to indicating that said at least one DOC is sulphur-poisoned and/or that said one at least one particle filter (DPF) is sulphur-poisoned.

7. The method according to any one of claims 2-6, comprising the steps of calibrating NOX2 at a low temperature, preferably below 150°C, when substantially no oxidation is taking place in DOC and no reducing agent is being supplied to the exhaust flow from the reducing agent device, by measuring NOX1 and NOX2, determining a calibration value NOX2k = NOX1-NOX2 and determining a calibrated value for NOX2 as NOX2^,=NOX2+NOX2k, method steps according to any one of the foregoing claims being conducted with the calibrated value NOX2' instead of NOX2.

8. An exhaust post-treatment system (2) for a combustion engine (4) which produces an exhaust flow (6), which system (2) comprises at least one diesel oxidation catalyst (DOC) and/or at least one diesel particle filter (DPF), at least one selective reduction catalyst (SCR catalyst), a reducing agent device (8) adapted to supplying a reducing agent (10) to the exhaust flow (6) upstream of said SCR catalyst, a first NO_x sensor (12) situated upstream of said DOC and/or DPF and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a first NO_x output signal (NOX1 ), a second NO_x sensor (14) situated downstream of said SCR catalyst and adapted to measuring the content of nitrogen oxide compounds (NO_x) in the exhaust flow and to producing on the basis thereof a second NO_x output signal (NOX2), and at least one temperature sensor (16) adapted to measuring the temperature of the exhaust flow and to producing on the basis thereof at least a first temperature signal (Tl),

c h a r a c t e r i s e d in that the exhaust post-treatment system (2) comprises a calculation unit (18) to which said first and second NO_x output signals (NOXl , NOX2) and said first temperature signal (Tl) are arranged to be conveyed, said first and second NO_x sensors (12, 14) are adapted to measuring the content of nitrogen oxide compounds NOXl upstream of said DOC and/or DPF and to measuring at substantially the same time the content of nitrogen oxide compounds NOX2 downstream of said SCR catalyst when no reducing agent (10) is being supplied to the exhaust flow (6) from the reducing agent device (8), and that said temperature sensor (16) is adapted to measuring the temperature T when the measurements of NOXl and NOX2 are taking place, which calculation unit (18) is adapted to comparing NOX2, or a value related to NOX2, with a detection criterion related to the measured temperature, and to generating on the basis of the comparison an indicating signal (20).

9. The exhaust post-treatment system according to claim 8, in which said detection criterion is a predetermined threshold value NOXtr, and said indicating signal (20) is generated if NOX2 is greater than NOXtr.

10. The exhaust post-treatment system according to claim 8, in which the calculation unit (18) is adapted to comprising determining ΔΝΟχ = | NOX1 -NOX2 | , to comparing ΔΝΟχ with a predetermined threshold value NOXtr' which serves as said detection criterion and to generating the indicating signal (20) if ΔΝΟχ is less than NOXtr'.

1 1. The exhaust post-treatment system according to either of claims 9 and 10, in which a table is provided which contains coordinated values for temperatures of the exhaust flow and said predetermined threshold values NOXtr or NOXtr'.

12. The exhaust post-treatment system according to any one of claims 8-1 1, in which said indicating signal (20) is adapted to indicating that said at least one DOC is sulphur-poisoned and/or that said one at least one particle filter (DPF) is sulphur-poisoned.

13. The exhaust post- treatment system according to any one of claims 8-12, which system is adapted to calibrating NOX2 at a low temperature, preferably below 150°C, when substantially no oxidation is taking place in DOC and no reducing agent (10) is being supplied to the exhaust flow from the reducing agent device (8), by measuring NOX1 and NOX2, and to determining a calibration value for NOX2 as

NOX2'=NOX2+NOX2k, the comparison being conducted by the calculation unit (18) with the calibrated value NOX2' instead of NOX2.

14. The exhaust post-treatment system according to any one of claims 8-13, in which said diesel particle filter (DPF) is situated downstream of said DOC.