METHOD AND SYSTEM FOR DIAGNOSING EXHAUST SENSORS

Abstract

Provided is a method for diagnosing exhaust sensors, where at least one substance resulting from combustion is reduced by an additive. A first sensor intended to measure an occurrence of said substance upstream said supply of additive, and a second sensor intended to measure an occurrence of said substance downstream said supply of additive. The method comprises: determining whether the locations of said first and second sensors are reversed by: determining if a second measurement value of said second sensor exceeds a corresponding first measurement value of said first sensor at least to a first extent, and when this condition occurs, determining that the locations of said first and second sensors sensor are reversed, said measurement values are determined when a supply of additive is set to obtain at least a first reduction of said at least one substance to be reduced.

Claims

1. A method for diagnosing exhaust sensors subjected to exhaust gases resulting from combustion, wherein an aftertreatment system is arranged for reduction of at least one substance resulting from said combustion by supplying an additive comprising a reagent to an exhaust gas stream resulting from said combustion, wherein a first exhaust sensor is subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location upstream of said supply of additive, and a second exhaust sensor being subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location downstream of said supply of additive, the method comprising: determining whether the locations of said first sensor and second sensor are reversed, the determination comprising: determining if a second measurement value of said second sensor exceeds a corresponding first measurement value of said first sensor at least to a first extent; and when said second measurement value of said second sensor exceeds said corresponding first measurement value of said first sensor at least to said first extent, determining that the locations of said first and second sensor are reversed, wherein said measurement values are determined when a supply of additive is set to obtain at least a first reduction of said at least one substance to be reduced.

2. A method according to claim 1, further comprising: when it is determined that said first and second sensors are reversed, using sensor signals from said first sensor as though they are sensor signals from said second sensor, and sensor signals from said second sensor as though they are sensor signals from said first sensor.

3. A method according to claim 1, further comprising: utilizing signals from said first and second sensors in a control system controlling a plurality of functions; and when it is determined that said first and second sensors are reversed, identifying sensor signals from said first sensor as though they are sensor signals from said second sensor, and sensor signals from said second sensor as though they are sensor signals from said first sensor, when used in said control system.

4. A method according to claim 1, further comprising: utilizing signals from said first and second sensors in a control system controlling a plurality of functions; and when it is determined that said first and second sensors are reversed, reverse identities for said first and second sensor in said control system.

5. A method according to claim 1, further comprising: determining said first measurement value as an accumulation of an occurrence of said at least one substance to be reduced measured by said first sensor; and determining said second measurement value as an accumulation of an occurrence of said at least one substance to be reduced measured by said second sensor, said accumulation having a length in time.

6. A method according to claim 5, wherein said length in time is determined by any one or more from the group: a period of time, a period during which an accumulated work produced by an internal combustion engine amounts to a first amount of work during accumulation, or an accumulation of reagent and/or additive being supplied to the exhaust gas stream amounting at least to a first amount of reagent/additive.

7. A method according to claim 1, further comprising: setting said first reduction of said at least one substance to a level that is expected to result in a difference between said first and second measurement values exceeding differences that may be caused by tolerances in sensor accuracy of said first and second sensors.

8. A method according to claim 1, further comprising: setting said first reduction of said at least one substance to a level that results in a reduction of said at least one substance equivalent to a reduction, in percent, that at least equals or exceeds: (((1,A)*(1,B))1)*100, where A is a tolerance in sensor accuracy of said first sensor expressed in percent, and B is a tolerance in sensor accuracy of said second sensor expressed in percent.

9. A method according to claim 1, further comprising: determining that the locations of said first and second sensors are reversed when the second measurement value of said second sensor exceeds the first measurement value of said first sensor at least by a factor equivalent to (1,A)*(1,B), where A is a tolerance in sensor accuracy of said first sensor expressed in percent, and B is a tolerance in sensor accuracy of said second sensor expressed in percent.

10. A method according to claim 1, further comprising, when reversing said signals form said first and second sensors, setting one or more trouble codes indicating faulty sensor locations of said first and second sensors.

11. A method according to claim 1, wherein said additive being an additive at least partly comprising urea.

12. A method according to claim 1, wherein said first and second sensors are sensors configured for measuring an occurrence of nitrogen oxides in said exhaust gas stream.

13. (canceled)

14. (canceled)

15. A system for diagnosing exhaust sensors subjected to exhaust gases resulting from combustion, wherein an aftertreatment system is arranged for reduction of at least one substance resulting from said combustion by supplying an additive comprising a reagent to an exhaust gas stream resulting from said combustion, wherein a first exhaust sensor is subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location upstream said supply of additive, and a second exhaust sensor being subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location downstream said supply of additive, the system comprising: means adapted to determine whether the locations of said first sensor and second sensor are reversed, the determination comprising: determining if a second measurement value of said second sensor exceeds a corresponding first measurement value of said first sensor at least to a first extent; and when said second measurement value of said second sensor exceeds said corresponding first measurement value of said first sensor at least to said first extent, determining that the locations of said first and second sensor are reversed, wherein said measurement values are determined when a supply of additive is set to obtain at least a first reduction of said at least one substance to be reduced.

16. A system according to claim 15, further comprising a catalytic converter is arranged downstream of said supply of additive and upstream an intended location of said second sensor.

17. A vehicle comprising a system for diagnosing exhaust sensors subjected to exhaust gases resulting from combustion, wherein an aftertreatment system is arranged for reduction of at least one substance resulting from said combustion by supplying an additive comprising a reagent to an exhaust gas stream resulting from said combustion, wherein a first exhaust sensor is subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location upstream said supply of additive, and a second exhaust sensor being subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location downstream said supply of additive, the system comprising: means adapted to determine whether the locations of said first sensor and second sensor are reversed, the determination comprising: determining if a second measurement value of said second sensor exceeds a corresponding first measurement value of said first sensor at least to a first extent; and when said second measurement value of said second sensor exceeds said corresponding first measurement value of said first sensor at least to said first extent, determining that the locations of said first and second sensor are reversed, wherein said measurement values are determined when a supply of additive is set to obtain at least a first reduction of said at least one substance to be reduced.

18. A vehicle according to claim 17, wherein said system further comprises a catalytic converter arranged downstream of said supply of additive and upstream an intended location of said second sensor.

19. A computer program product comprising computer program code stored on a non-transitory computer-readable medium, said computer program product for diagnosing exhaust sensors subjected to exhaust gases resulting from combustion, wherein an aftertreatment system is arranged for reduction of at least one substance resulting from said combustion by supplying an additive comprising a reagent to an exhaust gas stream resulting from said combustion, wherein a first exhaust sensor is subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location upstream of said supply of additive, and a second exhaust sensor being subjected to the exhaust gas stream and intended to measure an occurrence of said at least one substance at a location downstream of said supply of additive, said computer program product comprising computer instructions to cause said at least on control unit to perform the following operations: determining whether the locations of said first sensor and second sensor are reversed, the determination comprising: determining if a second measurement value of said second sensor exceeds a corresponding first measurement value of said first sensor at least to a first extent; and when said second measurement value of said second sensor exceeds said corresponding first measurement value of said first sensor at least to said first extent, determining that the locations of said first and second sensor are reversed, wherein said measurement values are determined when a supply of additive is set to obtain at least a first reduction of said at least one substance to be reduced.

20. A computer program product according to claim 19, further comprising: when it is determined that said first and second sensors are reversed, using sensor signals from said first sensor as though they are sensor signals from said second sensor, and sensor signals from said second sensor as though they are sensor signals from said first sensor.

21. A computer program product according to claim 19, further comprising: utilizing signals from said first and second sensors in a control system controlling a plurality of functions; and when it is determined that said first and second sensors are reversed, identifying sensor signals from said first sensor as though they are sensor signals from said second sensor, and sensor signals from said second sensor as though they are sensor signals from said first sensor, when used in said control system.

22. A computer program product according to claim 19, further comprising: utilizing signals from said first and second sensors in a control system controlling a plurality of functions; and when it is determined that said first and second sensors are reversed, reverse identities for said first and second sensor in said control system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1A illustrates a powertrain of an exemplary vehicle in which embodiments of the invention advantageously can be utilized;

[0044] FIG. 1B illustrates an example of a control unit in a vehicle control system;

[0045] FIG. 2 illustrates an example of an aftertreatment system where a supply of additive is utilized and with which embodiments of the invention advantageously can be utilized.

[0046] FIG. 3 illustrates an exemplary method for reversing sensor signals.

[0047] FIG. 4 illustrates variations in supply of additive over time.

[0048] FIG. 5 illustrates an exemplary method according to embodiments of the invention for determining whether sensors are reversed.

DETAILED DESCRIPTION OF THE INVENTION

[0049] In the following detailed description, the present invention will be exemplified for a vehicle. The invention is, however, applicable also for other kinds of transportation means, such as air and water crafts. The invention is also applicable to fixed installations.

[0050] Furthermore, the present invention is exemplified below in connection with supply of a urea based additive for reduction of nitrogen oxides. The present invention is, however, applicable for any kind of system where an additive is supplied, where the additive can be arranged for reduction of any substance/compound in the exhaust gas stream, and hence not necessarily nitrogen oxides.

[0051] Further, in the present description and the appended claims the expression substance is defined to include chemical compounds, compositions and mixtures.

[0052] FIG. 1A schematically depicts a powertrain of an exemplary vehicle 100. The powertrain comprises a power source, in the present example an internal combustion engine 101, which, in a conventional manner, is connected via an output shaft of the internal combustion engine 101, normally via a flywheel 102, to a gearbox 103 via a clutch 106. An output shaft 107 from the gearbox 103 propels drive wheels 113, 114 via a final gear 108, such as a common differential, and half shafts 104, 105 connected to said final gear 108.

[0053] The internal combustion engine 101 is controlled by the vehicle control system via a control unit 115. The clutch 106 and gearbox 103 are also controlled by the vehicle control system by means of a control unit 116.

[0054] FIG. 1A, consequently, discloses a powertrain of a specific kind, but the invention is applicable in any kind of powertrain and also e.g. in hybrid vehicles. The disclosed vehicle further comprises an aftertreatment system 130 for aftertreatment (purifying) of exhaust gases that results from combustion in the internal combustion engine 101. The functions of the aftertreatment system 130 are controlled by means of a control unit 204.

[0055] The aftertreatment system 130 can be of various kinds and designs, and according to the disclosed embodiment an additive is supplied to the exhaust gas stream. An exemplary aftertreatment system 130 in which the present invention can be utilized is shown more in detail in FIG. 2, and in the disclosed exemplary embodiment, the aftertreatment system 130 comprises a selective catalytic reduction (SCR) catalytic converter 201. The aftertreatment system can also comprise further non-disclosed components, such as, for example, further catalytic converters and/or particle filters which can be arranged upstream or downstream the SCR catalytic converter 201.

[0056] The supply of additive can, according to the above, for example, be used in the reduction of the concentration of nitrogen oxides NO.sub.x in the exhausts from the internal combustion engine through the use of an SCR catalytic converter.

[0057] This additive can, as according to the disclosed embodiment, inter alia be an additive comprising urea as reagent and e.g. consist of AdBlue which constitutes a frequently used additive and which consists of a mixture of approximately 32.5% urea dissolved in water. Urea forms ammonium when heated, and the ammonium then reacts with nitrogen oxides NO.sub.x in the exhaust gas stream. The present invention is applicable when using AdBlue, as well as when using any other urea based additive. As was mentioned above, the invention is also applicable when using any kind of additive comprising other reagents, and where any suitable substance in the exhaust gas stream is reduced/treated using the additive.

[0058] Apart from said catalytic converter 201, FIG. 2 further discloses an additive dosing system, in the disclosed example a urea dosing system (UDS), which comprises a urea, or dosing, tank 202, which is connected to an injection nozzle 205 through the use of which additive is injected into the exhaust gas stream 119. The dosing of urea is controlled by a control unit 204 controlling exhaust gas aftertreatment, which generates control signals for controlling the supply of additive so that a desired amount is injected into the exhaust gas stream 119 from the tank 202 using the injection nozzle 205.

[0059] Dosing systems for the supply of additive are in general well described in the prior art, and the precise manner in which the supply of additive is dosed/performed is therefore not described in detail herein. The dosing may be subject to adaptation, or correction. In the following the term correction is used, and correction can be performed e.g. at regular intervals and aims to ensure that the injected amount of additive corresponds to the demand. In general, the dosing varies, in principle, continuously as the operating conditions changes and the generation, in this example, of nitrogen oxides therewith.

[0060] The dosing is in general performed on the basis of the presence of the substance to be reduced upstream the supply of additive. The presence of NO.sub.x upstream the supply of additive and downstream the catalytic converter 201 and hence downstream the supply of additive, respectively, can, for example, be determined through the use of NO.sub.x sensors 207, 208.

[0061] Sensor signals from sensor 208 can be used to determine whether a desired conversion, i.e. reduction, is taking place and thereby whether the supply of additive can be assumed to be performed in a desired manner.

[0062] The amount of additive actually needed may in reality differ from the predetermined amount. For example, the quality/concentration of the additive may differ from the quality/concentration of the additive for which dosing amounts were determined. Also, e.g. wear and/or aging and/or faulty components, such as the catalytic converter, may affect the actual amount being injected. In order to account for factors of this kind adaptation, or correction, can be performed, where e.g. a correction factor is applied to account for such variations.

[0063] Such correction may usually be performed until a maximum limit has been reached. When the maximum limit has been reached it is considered that no further correction can be performed by further increasing the injected quantity.

[0064] However, as has been explained above, proper functionality with regard to supply of additive and correction requires that said sensor signals actually represent measurements from the position in the exhaust gas stream where the sensors presumably are located.

[0065] The invention relates to a method for diagnosing exhaust sensors subjected to exhaust gases resulting from combustion. In particular, the invention provides a method where it is determined whether two sensors being subjected to an exhaust gas stream have been reversed, i.e. interchanged. Further, according to embodiments of the invention, if this is the case, sensor signals from the sensors may be reversed such that sensor signals from the sensors instead are interpreted, defined, as if the sensor locations where reversed. In this way, e.g. a control system such as a vehicle control system can use the sensor signals as had the sensors been located at their intended positions to ensure proper functionality until the sensors can be properly relocated.

[0066] An exemplary method 300 regarding reversing the sensor signals is shown in FIG. 3, which method can be implemented at least partly e.g. in the control unit 204 for controlling aftertreatment of exhaust gases. Similarly, the method according to embodiments of the invention disclosed in FIG. 5 can be implemented at least partly e.g. in the control unit 204. As indicated above, the functions of a vehicle are, in general, controlled by a number of control units, and control systems in vehicles of the disclosed kind generally comprise a communication bus system consisting of one or more communication buses for connecting a number of electronic control units (ECUs), or controllers, to various components on board the vehicle. Such a control system may comprise a large number of control units, and the control of a specific function may be divided between two or more of them.

[0067] For the sake of simplicity, FIGS. 1A, 2 depicts only control units 115-116, 204, but vehicles 100 of the illustrated kind are often provided with significantly more control units, as one skilled in the art will appreciate. Control units 115-116, 204 are arranged to communicate with one another and various components via said communication bus system and other wiring, partly indicated by interconnecting lines in FIG. 1A.

[0068] The present invention can be implemented in any suitable control unit in the vehicle 100, and hence not necessarily in the control unit 204. The sensor diagnostics according to the present invention will usually depend on signals being received from other control units and/or vehicle components, and it is generally the case that control units of the disclosed type are normally adapted to receive sensor signals from various parts of the vehicle 100. The control unit 204 will, for example, receive signals from NO.sub.x sensors 207, 208 and possibly data relating e.g. to the work being produced by the combustion engine. Control units of the illustrated type are also usually adapted to deliver control signals to various parts and components of the vehicle, e.g. to the engine control unit or other suitable control unit when tests indicate that performance of the vehicle should be restricted.

[0069] Control of this kind is often accomplished by programmed instructions. The programmed instructions typically consist of a computer program which, when executed in a computer or control unit, causes the computer/control unit to exercise the desired control, such as method steps according to the present invention. The computer program usually constitutes a part of a computer program product, wherein said computer program product comprises a suitable storage medium 121 (see FIG. 1B) with the computer program 126 stored on said storage medium 121. The computer program can be stored in a non-volatile manner on said storage medium. The digital storage medium 121 can, for example, consist of any of the group comprising: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit etc, and be arranged in or in connection with the control unit, whereupon the computer program is executed by the control unit. The behaviour of the vehicle in a specific situation can thus be adapted by modifying the instructions of the computer program.

[0070] An exemplary control unit (the control unit 204) is shown schematically in FIG. 1B, wherein the control unit can comprise a processing unit 120, which can consist of, for example, any suitable type of processor or microcomputer, such as a circuit for digital signal processing (Digital Signal Processor, DSP) or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC). The processing unit 120 is connected to a memory unit 121, which provides the processing unit 120, with e.g. the stored program code 126 and/or the stored data that the processing unit 120 requires to be able to perform calculations. The processing unit 120 is also arranged so as to store partial or final results of calculations in the memory unit 121.

[0071] Furthermore, the control unit 204 is equipped with devices 122, 123, 124, 125 for receiving and transmitting input and output signals, respectively. These input and output signals can comprise waveforms, pulses or other attributes that the devices 122, 125 for receiving input signals can detect as information for processing by the processing unit 120. The devices 123, 124 for transmitting output signals are arranged so as to convert calculation results from the processing unit 120 into output signals for transfer to other parts of the vehicle control system and/or the component (s) for which the signals are intended. Each and every one of the connections to the devices for receiving and transmitting respective input and output signals can consist of one or more of a cable; a data bus, such as a CAN bus (Controller Area Network bus), a MOST bus (Media Oriented Systems Transport) or any other bus configuration, or of a wireless connection.

[0072] Returning to the exemplary method 300 illustrated in FIG. 3, the method starts in step 301, where it is determined whether sensor location is to be diagnosed. The method remains in step 301 for as long as this is not the case. The method continues to step 302 when it is determined that the sensor location is to be diagnosed. The transition from step 301 to step 302 can, for example, be initiated according to various criteria. For example, the diagnostics can be arranged to be performed each time there is an indication of an error in the supply of additive, when certain types of errors in the supply of additive is indicated and/or when there is an indication that reduction is insufficient.

[0073] As has been mentioned above and will be explained further in the following, if the locations of the sensors 207, 208 accidentally have been reversed and hence are located on each other's intended positions, this will give rise to behaviours that, in turn, may cause setting of various trouble codes regarding the supply of additive.

[0074] As was also mentioned above, the supply of additive is oftentimes controlled on the basis of the presence of the substance to be reduced upstream the supply of additive. This means that if the sensors accidentally have been reversed, the supply will unintentionally be based on sensor signals from the sensor being located downstream the supply of additive.

[0075] Consequently, when additive is supplied, the reduction will reduce the occurrence of NO.sub.x downstream the supply. However, since this will be taken as the occurrence in the exhaust gases upstream the supply of additive, due to the misplaced sensor, the reduced occurrence will be taken as a reduced production of NO.sub.x in the combustion engine, with the result that supply of additive is reduced instead of being increased further as would in reality be required to obtain the desired reduction. This is exemplified for illustrative purposes in FIG. 4, which shows a scenario of the reduction when the sensors upstream and downstream the supply of additive has been reversed.

[0076] FIG. 4 discloses changes in time for the supply of additive, indicated by dashed line 401. Further, solid line 402 represents measurements from the sensor that in reality is located downstream the supply of additive, and dashed line 403, which for purposes of illustration is a constant line in this example, represents signals from the sensor that the system believes is the sensor located downstream the supply of additive but which in reality represents emissions upstream the supply of additive and hence emissions from the internal combustion engine 101. Dashed line 404 represents the target reduced NO.sub.x level, i.e. target level to be measured by sensor 208, and e.g. representing a level required to fulfil legislation.

[0077] At system startup, t=t0, there is no injection of additive (urea) and the NOx emission levels are, assuming that no reagent is stored in the catalytic converter 201, the same both upstream and downstream the point of supply of additive. Consequently, the system determines that there is a need for supply of additive to reduce the emission to approved levels and substantially at t=t0 supply of additive is commenced. At time t=t1 reduction of NO.sub.x using the additive has commenced and the levels indicated by NO.sub.x sensor 208 in FIG. 2 which according to the above the system believes is sensor 207 has decreased to a level E.sub.1. As was mentioned above, the supply of additive is in general controlled as a function of the occurrence of NO.sub.x leaving the engine, engine-out NO.sub.x. That is, the occurrence of the substance at the (assumed) position of sensor 207 in FIG. 2. This means that when sensor 208 is taken as the sensor indicating engine out NO.sub.x the system comes to the conclusion that engine-out NO.sub.x has been reduced, and therefore the need for additive should also be reduced.

[0078] Because of this, at t=t.sub.1, the increase in supply of additive is stopped and at t=t.sub.2 a reduction in the supply of additive is commenced even though the actual emission level still is far higher than the desired level indicated by E.sub.target in FIG. 4. As a result, the resulting NO.sub.x levels measured by sensor 208 will begin to increase. This in turn, makes the system believe that the engine out NO.sub.x levels increases, so that at time t.sub.3 the amount of additive being supplied is not further decreased but instead, at t.sub.4, the amount of supplied additive will again be increased in response to the increased engine out levels.

[0079] Consequently, when the NO.sub.x sensors have been reversed the system will exhibit an oscillating, cyclic, behaviour of the kind shown in FIG. 4 and it may not be apparent for the control system to determine the reason for the system behaving as it does. The determination of the supply of additive may also comprise an influence of the measured NO.sub.x downstream the supply, which according to the example is very high, but the possible correction using this factor is in general limited e.g. to 10-100% of the already determined amount. The resulting injection of additive will therefore be far too small to obtain the desired reduction even when a maximum correction factor is applied.

[0080] Since sensor signals from sensors 207, 208 may be used also by other functionality in the control system, e.g. to determine whether the vehicle fulfils set criteria regarding emission levels, the reversed sensors may cause various undesired effects, such as vehicle performance being restricted (reduced) by the control system due to the vehicle not fulfilling legislative requirements. According to embodiments of the invention problems of this kind are solved or at least mitigated by a system where it is determined by means of signals from sensors 207, 208 whether the sensors have been reversed. This is performed in step 302 of FIG. 3.

[0081] If it is determined in step 302 that the sensors in fact are reversed, the method continues to step 303 where the sensor signals are reversed in the control system so that any functionality utilizing sensor signals from sensors 207, 208 automatically will use sensor signals from a sensor at a correct location in the system irrespective of whether sensors have been reversed during assembly. Simultaneously, any suitable diagnostic trouble codes may be set so that the system can be corrected e.g. by physically relocating the sensors and resetting the control system in this regard when the vehicle is taken in for service. The method is then ended in step 304. If it is determined in step 302 that the sensors have not been reversed the method may be ended or return to step 301. In this case other fault detection mechanisms may disclose other kinds of problems.

[0082] A method according to embodiments of the invention for determining whether the sensors have been reversed will be disclosed in the following with reference to FIG. 5.

[0083] The method 500 of FIG. 5 for determining whether the sensors have been reversed starts in step 501, where the method remains until it is to be determined whether the sensors are reversed, which can be initiated, e.g., by the method of FIG. 3, or by any other trigger. For example, the method according to FIG. 5 can be arranged to be carried out when any or certain types of faults in the aftertreatment are indicated.

[0084] In step 502 a set of measures are set to initial values, e.g. zero. These measures include:

[0085] m.sub.outlet a measure representing an accumulation of NO.sub.x at an assumed location upstream the supply of additive, i.e. an accumulation of the NO.sub.x leaving the combustion engine,

[0086] m.sub.tail: a measure representing an accumulation of NO.sub.x at the assumed position of sensor 208, i.e. downstream the supply of additive and essentially representing the amount of NO.sub.x discharged to the ambient air of the vehicle, and

[0087] UREA.sub.acc: a measure representing the amount of additive, in this example urea, that has been supplied to the exhaust gas stream during accumulation of m.sub.outlet, m.sub.tail.

[0088] The additive can be arranged to be supplied according to the above, i.e. in dependence of the (presumed) occurrence of NO.sub.x upstream the supply. According to one embodiment, the supply can be arranged to be based on sensor signals from the sensor that is interpreted as being the sensor downstream the supply of additive, since, in case the sensors in fact are reversed, a higher amount of additive will be supplied when dosing is based on signals from this sensor.

[0089] In step 503 dosing of additive is set to a dosing that is presumed to reduce NO.sub.x at least to a predetermined extent. For example, the dosing of additive can be set to an amount that is expected to reduce the occurrence of NO.sub.x e.g. at least by 35% or to any other suitable degree. This step can be omitted e.g. in case intended reduction is always set to some level, or set in any other way. The reduction is set to a level that accounts for measurement differences that might prevail due to tolerances in sensor accuracy.

[0090] According to embodiments of the invention, the reduction can be set to a level that results in a reduction of the occurrence of NO.sub.x that may be determined in any suitable way but that is equivalent to a reduction, expressed in percent, that at least equals or exceeds:

(((1,A)*(1,B))1)*100, where

[0091] A is the tolerance in sensor accuracy of one of the sensors expressed in percent, and B is the tolerance in sensor accuracy of the other sensor expressed in percent.

[0092] For example, NO.sub.x sensors may have a tolerance in accuracy of e.g. 15%, and if the sensors are at different extremes in terms of accuracy, this factor alone may render a difference in accumulated NO.sub.x in the order of 30%. In particular, using the above relation with A=15 (15% accuracy) results in a possible difference of 1.15*1.15=1.32=32% from two sensors subjected to the same exhaust gas stream. According to embodiments of the invention, at least this reduction is set.

[0093] In step 504 accumulation is started, and in step 505 it is determined whether the work E.sub.engine produced by the combustion engine during accumulation has reached a work E.sub.R. This work can be set to some suitable level, e.g. some suitable number of kWh, e.g. in the order of 10-100 kWh, to ensure that representative measurements are obtained. If too small amounts of additive are supplied the tolerances of the sensors may provide unreliable results. Alternatively, or in addition, it can be determined if a minimum amount of additive m.sub.urea since start of accumulation amounts to or exceeds an amount m.sub.urea,min representing an amount which is considered large enough to ensure that the reduction resulting from the supply of additive will have a desired impact on the measures m.sub.outlet and m.sub.tail above. Also, accumulation can be arranged to be performed until any of m.sub.outlet and m.sub.tail reaches a set limit provided the reduction is set to a suitable level according to the above. According to embodiments of the invention, the accumulation may be arranged to, alternatively or in addition, be performed for at least a first number of minutes, e.g. 15-60 minutes.

[0094] When it is determined in step 505 that the accumulated work E.sub.engine produced by the combustion engine has reached the limit E.sub.R, or any other applied criteria has been fulfilled, the resulting accumulated measures m.sub.outlet and m.sub.tail are evaluated in step 506. According to the present example, it is also determined in step 505 whether the supplied amount of reagent m.sub.urea at least amount to m.sub.R described below in order to ensure that a sufficient amount of reagent has been supplied to allow sensor locations to be distinguished. The evaluation of sensor location in step 506 may, for example, be performed according to the following:

[0095] According to the disclosed example, the sensors may be deemed to be reversed (switched) if the accumulated masses over the first engine work, E.sub.R, differs so that:

[00001]$m_{\mathrm{tail}}>C_R{m}_{\mathrm{outlet}}.Math..Math.\mathrm{for}.Math..Math.a.Math..Math.\mathrm{given}.Math..Math.m_{\mathrm{urea}}>m_R$$\mathrm{where}.Math.\text{:}$$m_{\mathrm{outlet}}=\underset{E_R}{.Math.}.Math..Math.\left(d.Math..Math.m_{\mathrm{outlet}}{\mathrm{po}}_{\mathrm{engine}}\right),$

[0096] dm.sub.outlet represents measured Engine Out NOx (according to what the system assumes is the emissions leaving the internal combustion engine) for a particular time period t.sub.i of the total time t during which the work E.sub.R is produced. Any suitable time periods t.sub.i can be used in the summation until the accumulated work from the combustion engine reaches E.sub.R. In the present example, dm.sub.outlet is expressed in g/kWh, and engine power, po.sub.engine, in kW.

[0097] Consequently, summation is performed during a time t it takes for the internal combustion engine to deliver the work E.sub.R.

[00002]$m_{\mathrm{tail}}=\underset{E_R}{.Math.}.Math..Math.\left(d.Math..Math.m_{\mathrm{tail}}{\mathrm{po}}_{\mathrm{engine}}\right),$

where dm.sub.tail represents measured NO.sub.x tailpipe/downstream SCR (according to the assumed sensor location) for a particular time period t.sub.i of the total time t during which the work ER is produced, t.sub.i being same as or different from t.sub.i above, although accumulation is till performed for the same production of work from the internal combustion engine.

[0098] Similarly, the injected amount can be determined as:

[00003]$m_{\mathrm{urea}}=\underset{E_R}{.Math.}.Math..Math.\left(d.Math..Math.m_{\mathrm{urea}}{t}_i\right),\mathrm{where}.Math..Math.d.Math..Math.m_{\mathrm{urea}}.Math..Math.\mathrm{represents}.Math..Math.\mathrm{urea}.Math..Math.\mathrm{dosage},e.g..Math.\mathrm{expressed}.Math..Math.\mathrm{in}.Math..Math.g/h.$

dm.sub.urea can be already present in the control system, or be determined directly from the control of e.g. injection nozzles. It is to be noted that it is the amount of urea, i.e. reagent, being supplied that is accumulated, and hence the supply of reagent may be higher. For example, the concentration of urea in adBlue is about 32.5%.

[0099] The factor C.sub.R can be determined as:

C.sub.R(NOx sensor accuracy).sup.2>1.

[0100] For example, limit values according to the following can be used E.sub.R=30 kWh, m.sub.R=0.35 m.sub.tail, and C.sub.R=1.32 (sensor accuracy of 15%).

[0101] That is, m.sub.R/m.sub.tail can be set to the equal the minimum NO.sub.x conversion efficiency being used, in this example 35%. Further, it takes one ammonium molecule to reduce one NO.sub.x molecule, and the well known chemistry regarding the reduction of NO.sub.x by NH.sub.3 e.g. in regard of weight of additive being required for reduction of e.g. a gram of NO.sub.x, is utilized in the above equations.

[0102] In sum, the measured NO.sub.x is integrated for both sensors, and then, subject to the requirements of the measurements, the quotient m.sub.tail/m.sub.outlet cannot be greater than C.sub.R unless sensors are switched. If it is determined in step 507 that the quotient m.sub.tail/m.sub.outlet is greater than C.sub.R, the sensors are deemed to be switched in step 508. Otherwise the sensors are deemed to be correctly positioned in step 509. The method is then ended in step 510.

[0103] Consequently, according to the disclosed example, sensor location can be evaluated in a manner that accounts for e.g. differences due to tolerances by ensuring that the impact on measurements of the supply of additive will exceed the possible differences caused by tolerances so that an unambiguous result regarding actual sensor location can be obtained.

[0104] Finally, the present invention has been exemplified for a vehicle. The invention is, however, applicable in any kind of craft, such as, e.g., aircrafts, watercrafts and spacecrafts. The invention is also applicable for use in combustion plants. Also, the aftertreatment system may comprise further components such as one or more particle filters, one or more oxidation catalytic converters as is known per se. It is also contemplated that the aftertreatment system may comprise more than one SCR catalytic converter.