EP3430249A1

EP3430249A1 - Method for determining a corrected nitrogen oxide value and ammonia value in an internal combustion engine

Info

Publication number: EP3430249A1
Application number: EP17703718.1A
Authority: EP
Inventors: Hong Zhang
Original assignee: Continental Automotive GmbH
Current assignee: Continental Automotive GmbH
Priority date: 2016-03-16
Filing date: 2017-02-02
Publication date: 2019-01-23
Also published as: CN108779696A; WO2017157569A1; CN108779696B; DE102016204323A1; DE102016204323B4; US20190112955A1; US10738677B2

Abstract

The invention relates to a method for determining a corrected nitrogen oxide value or corrected ammonia value in an internal combustion engine which has an SCR catalytic converter (20), a nitrogen oxide sensor (22), a urea injection device (26) and an ammonia sensor (24). The method comprises determining that the internal combustion engine is in an overrun cut-off phase, interrupting the urea injection, determining a nitrogen oxide reference value from a nitrogen oxide reference signal generated by the nitrogen oxide sensor (22) or determining an ammonia reference value from an ammonia reference signal generated by the ammonia sensor (24) and detecting a corrected nitrogen oxide value from a nitrogen oxide signal generated by the nitrogen oxide sensor (22) during normal operation of the internal combustion engine, taking into account the nitrogen oxide reference value or determining a corrected ammonia value from an ammonia signal generated by the ammonia sensor (24) during normal operation of the internal combustion engine, taking into account the ammonia reference value.

Description

description

Method for determining a corrected nitrogen oxide value and ammonia value in an internal combustion engine

The present invention relates to a method for determining a corrected nitrogen oxide value and ammonia value in an internal combustion engine, more particularly to a method of determining a corrected nitrogen oxide value and ammonia value in an internal combustion engine having an SCR catalyst in which a nitrogen oxide sensor and an ammonia sensor are disposed downstream of the SCR catalyst ,

In internal combustion engines, especially diesel internal combustion engines, it is known to use so-called SCR (selective catalytic reduction) catalysts, which are used for the reduction of nitrogen oxides in the exhaust gases of the internal combustion engine. The chemical reaction on the SCR catalyst is selective, which means that the nitrogen oxides (NO, O ₂ ) are preferably reduced, while undesirable secondary reactions, such as the oxidation of sulfur dioxide to sulfur trioxide, are largely suppressed.

For the chemical reaction, a urea upstream of the SCR catalyst is injected into the exhaust gas, which then at least partially decomposes into ammonia, which can rea ^¬ greieren with the exhaust gas to water and nitrogen within the SCR catalyst ^¬ . For controlling the quantity of urea to be injected, for example, nitrogen oxide sensors and ammonia sensors are used to determine the respective proportions in the outlet tract of the

To measure internal combustion engine and then control the correct amount of urea to be injected. It is an object of the invention to provide a method with which the nitrogen oxide values and ammonia values can be determined on a vehicle-specific basis and as accurately as possible. This object is achieved according to the method of independent claims 1 and 2 and the outlet tract according to claim 9. Before ^¬ some embodiments are specified in the dependent claims. The present invention is based on the idea during a fuel cut-off phase of the internal combustion engine, the

To calibrate nitrogen oxide sensors and the ammonia sensor, that is, that with the method according to the invention a possible aging effect of the sensors can be reduced. In particular, the calibration is performed during a Betriebszu ^¬ stands of the engine, namely during the overrun cut-off phase, while the outlet zone in no or almost no nitrogen oxides are present. Consequently, the so-called zero offset of the respective nitrogen oxide sensors, but also of the ammonia sensor, can be calibrated and adjusted. Further, the present invention, the idea is based on the characteristics of a downstream of the SCR catalyst disposed Stick ^¬ oxidsensors and the characteristic of a downstream of the

SCR catalyst arranged urea sensor by the combination of both signals to adapt.

According to the invention, a method for determining a corrected nitrogen oxide value in an internal combustion engine is provided, which comprises an SCR catalytic converter, downstream of the

SCR catalyst arranged nitrogen oxide sensor and arranged upstream of the SCR catalyst urea injection arrangement. The method according to the invention comprises determining that the internal combustion engine is in a fuel cut-off phase, interrupting the fuel injection injection of urea by means of the urea injection device during the fuel cut-off phase, determining a nitrogen oxide reference value from a nitrogen oxide signal generated by the nitrogen oxide sensor during the fuel cut-off phase and determining a corrected nitrogen oxide value from one of

Nitrogen oxide sensor during normal operation of the internal combustion ^¬ machine generated nitrogen oxide signal taking into account the nitrogen oxide reference value. During the fuel cut phase, for example downhill, fuel injection into the cylinders of the internal combustion engine is interrupted, thereby substantially passing air through the engine. In addition, urea injection (for example AdBlue dosing) is also interrupted, as a result of which nitrogen oxide emissions and urea emissions fall. As a result of this reduction in nitrogen oxide emissions and urea emissions, the respective signals from both the nitrogen oxide sensor upstream of the SCR catalyst and the signals from the nitrogen oxide sensor downstream of the SCR catalyst are also reduced. In similarity ^¬ Licher way, the signal of the sensor reduces ammonia downstream of the SCR catalyst.

The present disclosure makes further use of the fact that the downstream of the SCR catalyst disposed nitrogen oxide sensor is cross-sensitive to ammonia, that is, the signal (hereinafter referred to as "nitrogen oxide signal") of the embroidery ^¬ oxidsensors the sum of NOx and Ammoni ^¬ ak Indicates concentrations.

According to the invention is another method for determining a corrected value ammonia in an internal combustion engine of ^¬ fenbart that an SCR catalyst, an upstream of the SCR catalyst disposed urea injection device, and a arranged downstream of the SCR catalyst Ammoni ^¬ aksensor. The inventive method according to this aspect of the present invention comprises determining that the internal combustion engine is in a fuel cut-off phase, interrupting the injection of urea by means of the urea injection device during Schubabschal ^¬ tion phase, determining an ammonia reference value from one of the ammonia sensor during the fuel cut-off phase generated ammonia signal and determining a corrected ammonia value from a generated by the ammonia sensor during normal operation of the ^¬ combustion ^¬ ammonia signal, taking into account the ammonia reference value.

According to a further aspect of the present invention, the two methods according to the invention are combined, that is to say that the method for determining a corrected nitrogen oxide value and the method for determining a corrected ammonia value run simultaneously. The respective processes are preferably executed only when the nitrogen oxide reference value is smaller than a certain nitrogen oxide before ^¬ threshold or if the ammonia reference value is smaller than a predetermined Ammoniakschwel ^¬ lenwert. It is even more preferable if the determination of a corrected nitrogen oxide value or the determination of a corrected ammonia value takes place only when an absolute change gradient of the reference signals of the nitrogen oxide sensor is less than a predetermined nitrogen oxide change threshold or if an absolute change gradient of the reference signals of the ammonia sensor is smaller than is a predetermined ammonia change threshold.

In the context of the present disclosure, an absolute change gradient describes the change in time within a predetermined time interval briefly in succession detected signals.

According to a preferred embodiment, the method further comprises determining an ammonia difference between an ammonia value determined during the normal operation of the internal combustion engine from the ammonia signal generated by the ammonia sensor and a further ammonia value determined during normal operation of the internal combustion engine from the nitrogen oxide signal generated by the nitrogen oxide sensor. The ammonia difference may be an indication of drift of the ammonia sensor. Then, the following steps during a subsequent fuel cut phase are executed when that is, during normal operation of the internal ^¬ combustion engine ammonia difference is higher than a predetermined ammonia difference threshold value: activating the urea injector for injecting a predetermined amount of urea during a predetermined period, determining at least a first ammonia value of from Ammonia sensor generated ammonia signals, determining at least a second ammonia value from signals generated by the nitrogen oxide sensor, determining that the change gradient of the first ammonia signals is less than a first change ^¬ threshold, determining that the change gradient of the second ammonia values is less than a second change threshold and Adaptation of a slope of the characteristic curve of the ammonia sensor by means of the second ammonia values, if the gradients of change of the first and second ammonia w each are smaller than the associated change threshold. In such a preferred embodiment of the nitrogen oxide sensor is cross-sensitive to ammonia, that is, the signal of the nitrogen oxide sensor indicative of the sum of nitrogen oxide and Ammoni ^¬ ak concentrations. In particular, such a design from ^¬ from a functioning SCR catalyst and at high Ammonia slip (for example, in ammonia over 40 ppm) is preferred, in which the signal of the downstream of the

SCR catalyst arranged nitrogen oxide sensor substantially indicates an ammonia value, since the nitrogen oxides are converted in the SCR catalyst and the concentration downstream of the

SCR catalyst is significantly reduced. Thus, by measuring ammonia by the nitrogen oxide sensor which can measure more accurate than the ammonia sensor in Wesent ^¬ union, the slope of the characteristic of the ammonia sensor to be adapted.

According to a preferred embodiment of the method according to the invention, the nitrogen oxide reference value is determined as follows:

^N0 X _R ef, new = ^N0 X _R ef, aU ~ ^" NO _x with

Nitrogen oxide reference value for the correction, nitrogen oxide reference value of a previous correction,

Weighting factor between 0 and 1 depending on the operating time of the internal combustion engine between two corrections, and

currently determined nitrogen oxide value (determined during the fuel cut-off phase of the internal combustion engine from a nitrogen oxide signal generated by the nitrogen oxide sensor).

Similarly, the ammonia reference value may preferably be determined as follows: = NH ₃ Ref _f , al, t - K ₂ (T _t Z _{l L} - _t Ζ ₂ Δ) ^■ NH ₃ 5 With : Ammonia reference value for the correction, ^NH 3 ammonia reference value of a previous correction, Weighting factor between 0 and 1 depending on the

Operating time of the internal combustion engine between two corrections, and

currently determined ammonia value (during Schubab ^¬ switching phase of the internal combustion engine determined from an ammonia signal generated by the ammonia sensor).

The weighting factors Ki (T _t i _t 2) and K 2 (T _t i _t 2) are dependent on the engine operating state and the engine operating time between two adaptations and are preferably in a range between 0 and 1.

In a further embodiment of the method according to the invention, the internal combustion engine comprises a further nitrogen oxide sensor, which is arranged upstream of the SCR catalytic converter. In this case, the method further comprises determining another

A nitrogen oxide reference value from a nitrogen oxide signal generated by the further nitrogen oxide sensor during the fuel cut-off phase and determining a corrected further nitrogen oxide value from a nitrogen oxide signal generated by the further nitrogen oxide sensor during normal operation of the internal combustion engine, taking into ^account the nitrogen oxide ^{reference value} .

In a further advantageous embodiment, the corrected nitrogen oxide value, which is determined from a nitrogen oxide signal generated by the nitrogen oxide sensor, is corrected with the ammonia value, which is calculated from an ammonia signal generated by the ammonia sensor as follows: NO _{x ff} = NO _x - NH ₃ with

NO _Xnetto corrected and adjusted ammonia nitrogen oxide value, NO _x nitrogen oxide determined from a value produced by the nitrogen oxide sensor nitric oxide signal, and NH ₃ determined from a signal generated from the ammonia sensor signal ammonia ammonia value.

According to a further aspect of the present disclosure, an exhaust duct for an internal combustion engine is disclosed, to produce the one SCR catalyst, a downstream of the SCR catalyst at ^¬ parent nitrogen oxide sensor which is adapted to a nitrogen oxide value indicative of nitric oxide signal, upstream of the SCR Catalyst disposed urea injection device, which is designed to inject a predetermined amount of urea, one downstream of the

The SCR catalyst arranged ammonia sensor, which is formed to the ammonia value downstream of the

SCR catalyst indicating ammonia signal, and having a control unit which is adapted to receive the nitrogen oxide signal and the ammonia signal and to carry out a method according to the present disclosure.

In a preferred embodiment, the outlet tract further comprises a further nitrogen oxide sensor, which is arranged upstream of the SCR catalytic converter, and which is designed to accommodate the nitrogen oxide sensor

To generate nitrogen oxide value indicating further nitrogen oxide signal.

Other objects and features of the invention would be apparent to those skilled in the art upon consideration of the accompanying drawings, in which: 1 shows a part of an exemplary disclosed exhaust ^¬ tract of an internal combustion engine,

FIG. 2 shows a flow chart according to an example method for determining a corrected nitrogen oxide value or a corrected ammonia value, and FIG

Fig. 3 is a Flussidagramm for adjusting the slope of a

Characteristic of an ammonia sensor according to the vorlie ^¬ ing disclosure shows.

1 shows schematically a part of an outlet tract 10 of an internal combustion engine (not shown in detail). The exhaust tract 10 has an SCR catalytic converter 20, which is designed to carry out a chemical reaction so that the nitrogen oxides in the exhaust gas can be reduced. Upstream of the SCR catalyst 20 is a particulate filter, such as a diesel particulate filter. Downstream of the

SCR catalyst 20, a nitrogen oxide sensor 22 and a Am ^¬ moniaksensor 24 are arranged, which are adapted to generate ent ^¬ speaking signals. In particular, the stick ^¬ oxide sensor 22 is adapted to generate a nitrogen oxide value to a ^¬ pointing nitric oxide signal. Similarly, the ammonia sensor 24 is configured to generate an ammonia signal indicative of ammonia value.

In a preferred embodiment, the nitrogen oxide sensor 22 and the ammonia sensor 24 are integrated in a sensor.

Upstream of the SCR catalyst 20 is disposed a urea injection device 26 adapted to inject a predetermined amount of urea at predetermined times. The urea solution is designed to be from Exhaust gas to be decomposed so that at least partially ammonia is formed, which can react chemically in the SCR catalyst 20 and thus reduce the nitrogen oxides in the exhaust gas. According to the exemplary embodiment of the outlet tract 10 illustrated in FIG. 1, a further nitrogen oxide sensor 32 is additionally provided upstream of the particle filter 20, which is designed to generate a further nitrogen oxide signal indicating a nitrogen oxide value.

A control unit 40, which may be part of the control of the internal combustion engine, for example, is with the

Nitrous oxide sensor 22, the ammonia sensor 24, the urea injection device 26 and the other nitrogen oxide sensor 32 connected and adapted to receive signals from these devices or to send them to control the same.

For example, the controller 40 is configured to execute a method according to FIG. 2. The method according to FIG. 2 begins at step 200 and determines at step 210 whether the internal combustion engine is in a fuel cut-off ^phase . If it is determined at step 200 that the focal ^¬ combustion engine is in a normal operation with thrust, the procedure moves to step 270 where it is terminated.

However, if at step 210 determines that the focal ^¬ combustion engine is in a fuel cut-off phase, downhill at ^¬ play, at a drive, the method moves to step 220, at the next interruption of the Kraftstoffe- the urea feed inspritzung additionally by means of the urea injection device 26 during the fuel cut-off phase is interrupted. Then, a reference value nitric oxide from a nitric oxide produced by the sensor 22 during the overrun cut-off phase reference signal nitric oxide and / or a Ammoniakrefe be at step 230 ^¬ Limit value of one from the ammonia sensor 24 during the

Fuel cut phase generated ammonia ^reference signal he ^¬ mediates.

At step 240, it is checked whether the nitrogen oxide value and the ammonia value downstream of the SCR catalyst 20 are smaller than a nitrogen oxide threshold value and smaller than a predetermined ammonia threshold, respectively. If only one of the two values is above the respective threshold value, the process goes to step 270, where it is terminated. At this point, it should be noted that because of the linear sensor characteristics at step 240, instead of comparing the values with a predetermined threshold, the signals produced by the respective sensors may be directly compared to a corresponding predetermined threshold signal. However, the two values are less than their associated predetermined threshold, then the method proceeds to step 250, at which it is queried whether an absolute change ^¬ gradient of the respective values is smaller than a threshold value are associated. Specifically, it is queried at 250 whether an absolute value of gradient of change of the reference signals of the nitrogen oxide sensor 22 is smaller than a predetermined nitrogen oxide is dänderungsschwellenwert or whether an absolute change ^¬ gradient of the reference signals of the ammonia sensor 24 is less than a predetermined variation threshold value is ammonia. Again, the query at step 250 can be done directly with the signals generated by the sensors 22, 24.

If only one of the two absolute change gradients is above its corresponding predetermined change Threshold, the process proceeds to step 270, where the process is terminated.

However, determined at step 250 that both change ^¬ gradient is smaller than their respective change threshold value, the method continues with the step 260 at which both of the nitrogen oxide reference value according to the formula (1) and the Ammo ^¬ niakreferenzschwellenwert according to the following formula (2 ) be adjusted.

^N0 X «ef, new = ^N0 X« ef, aU ~ NO _x (1)

NH, NH 3 Ref.neu _{Ref lt} -K ₂ T _ _tl _t2) -NH ₃ (2) comprising:

NO _x ref.new nitrogen oxide reference value for correction,

NO ^x Ref, old nitrogen oxide reference value of a previous correction,

Ki (T _tl _ _t2 ) Weighting factor between 0 and 1 depending on the

Operating time of the internal combustion engine between two corrections,

NO _> currently determined nitrogen oxide value (during the

Fuel cut-off phase of the internal combustion engine is determined from a nitrogen oxide signal generated by the nitrogen oxide sensor),

NH ^ Ref.ne ammonia reference value for correction,

NH ₃ ^ _D Re _F fa, ammonia reference value of a previous correction, K ₂ (Ttl-t2) weighting factor between 0 and 1 depending on the

Operating time of the internal combustion engine between two corrections, and NH ₃ currently determined ammonia value (during Schubab ^¬ switching phase of the internal combustion engine from an ammonia sensor (24) generated ammonia signal determined).

Referring to FIG. 3, another exemplary method according to the present disclosure is shown as a flowchart beginning at 300. At step 310, a query is made as to whether the internal combustion engine is in normal operation. It is meant with the normal operation that the

Internal combustion engine operates under combustion of fuel and accordingly provides thrust.

If it is determined at step 310 that no normal operation of the internal combustion engine, such as a fuel cut ^¬ phase is present, the process moves to step 390 and ends there.

Further, it is determined at step 310 whether the ammonia slip of the SCR catalyst is above a predetermined threshold, for example, above 40 ppm. Since the nitric oxide sensor is cross-sensitive to ammonia, the nitric oxide sensor measures the sum of nitrogen oxides and ammonia. Accordingly, it can be assumed with a sufficiently high ammonia slip that the signal of the ammonia sensor 24 is substantially equal to the signal of the nitrogen oxide sensor 32, provided that the SCR catalyst is operating properly and can decorate the nitrogen oxide significantly redu ^¬. If it is determined at step 310 that the engine is in normal operation and that the ammonia slip is greater than a predetermined threshold, at step 320, both the nitrogen oxide sensor 22 and the ammonia sensor 24 generate corresponding signals. In the subsequent step 330, an ammonia difference between the ammonia value determined from the nitrogen oxide signal and the ammonia value determined from the ammonia ^signal is formed. At step 330 it is further queried whether this ammonia difference exceeds a predetermined ammonia difference threshold. If it is determined at step 330 that the ammonia difference threshold value is not exceeded, ge ^¬ reached the process to step 390 and is ended. However, if at step 330 determines that the ammonia difference exceeds the ammonia difference threshold value, the process proceeds to step reaches 340. In particular, determining whether the signal generated by the ammonia sensor 24 sig ^¬ cantly is oxidsignal of the generated from the nitrogen oxide sensor 22 of nitrogen, at step 330, which during indicates the prevailing operating state of the internal combustion engine substantially ammonia, deviates. This deviation can be checked, for example, with the ammonia difference. At step 340, it is determined whether the engine has changed from the normal operation to the fuel cut operation. If it is determined at step 340 that the internal combustion engine continues ^¬ in normal operation, the process moves to step 390 and is ended.

However, if at step 340 determines that the internal combustion engine from normal operation is open ^¬ changes to a fuel cut operation, the process moves to step 350 at which the urea injection device 26 is controlled such that it despite the overrun cut-off phase for a predetermined period (for example a few seconds ) injects a predetermined amount of urea, so that at a position downstream of the SCR catalyst 20, an ammonia concentration from above a threshold, for example greater than about 40 ppm.

However, it is assumed that downstream of the two sensors 22, 24 an ammonia blocking catalyst and / or another SCR catalyst for the oxidation of ammonia or for the storage of SCR exist, so that the ammonia emissions do not exceed the legal limit. Subsequently, the nitrogen oxide sensor 22 and the ammonia sensor 24 produce corresponding signals during the Schubab ^¬ shifting phase at step 360th It should be noted that due to the fuel cut-off phase, the signals of the nitrogen oxide sensor 22 substantially indicate an ammonia value, since there are essentially no nitrogen oxides.

At the next step 370, it is determined that the first ammonia values determined from the signals of the nitrogen oxide sensor 22 and the second ammonia values determined from the ammonia signals of the ammonia sensor 24 are at least partially stable, ie that the gradients of the first and second ammonia values are each less than a predetermined first or second ammonia value second change threshold. If at step 370 be true ^¬ that the first and second ammonia levels are at least partly unstable, the process moves to step 390 and is ended.

However, if at step 370 determines that the first and second ammonia levels are at least partially stable, the process moves to step 380 at which the signals of the embroidery ^¬ oxidsensors 22, indicating the levels of ammonia, used for adapting the slope of the characteristic of the ammonia sensor 24 becomes. In particular, the fact is taken into account that the nitrogen oxide sensor 22 can measure more accurately than the ammonia sensor 24. At step 390, the activated at step 350 the urine ^¬ fuel injection is then terminated, and the method for adapting the characteristic of the ammonia sensor 24 by means of the signals of the nitrogen oxide sensor 22 is completed.

Claims

claims

A method for determining a corrected nitrogen oxide value in an internal combustion engine, the

SCR catalyst (20), a nitrogen oxide sensor (22) arranged downstream of the SCR catalytic converter (20) and a urea injection device (26) arranged upstream of the SCR catalytic converter (20), with:

Determining that the internal combustion engine is in a fuel cut-off phase,

Interrupting the injection of urea by means of the urea injection device (26) during the fuel cut-off phase,

Determining a nitrogen oxide reference value from a nitrogen oxide ^{reference signal generated} by the nitrogen oxide sensor (22) during the fuel cut-off ^phase , and

Determining a corrected nitrogen oxide value from a nitrogen oxide signal generated by the nitrogen oxide sensor (22) during normal operation of the internal combustion engine, taking into account the nitrogen oxide reference value.

2. A method for determining a corrected ammonia value in an internal combustion engine comprising an SCR catalyst (20), a urea injection device (26) arranged upstream of the SCR catalytic converter (20) and a downstream of the

SCR catalyst (20) arranged ammonia sensor (24), comprising:

Determining that the internal combustion engine is in a fuel cut-off phase,

Interrupting the injection of urea by means of the urea injection device (26) during the fuel cut-off phase, Determining an ammonia ^{reference value} from an ammonia ^{reference signal generated} by the ammonia sensor (24) during the fuel cut-off ^phase , and

Determine a corrected value from a ammonia from the ammonia sensor (24) during normal operation of the internal combustion engine ammonia signal generated taking into account the ammonia ^¬ reference value.

3. The method according to any one of the preceding claims, wherein determining a nitrogen oxide reference value value or the He ^¬ transmit an ammonia reference value takes place only when the nitrogen oxide value is less than a predetermined Stickoxidschwel ^¬ lenwert is or if the ammonia value is smaller than a certain prior ^¬ ammonia threshold ,

4. The method according to any one of the preceding claims, wherein determining a nitrogen oxide reference value and determining an ammonia reference value takes place only when a starting ^¬ soluter change gradient of the reference signals of the nitrogen oxidsensors (22) is smaller than a predetermined Stickoxidän- is alteration threshold or when an absolute variation gradient of the reference signals of the ammonia sensor (24) is less than a predetermined ammonia change ^threshold value.

5. The method according to any one of the preceding claims, further comprising:

Determining an ammonia difference between a generated during the normal operation of the internal combustion engine from the ammonia sensor (24) Ammonia signal detected ammonia value and a detected from the signal generated by the nitrogen oxide sensor nitric oxide signal during normal operation of the internal ^¬ combustion engine further ammonia value,

Performing the following steps during a subsequent fuel cut-off phase when the during the Normal operation of the internal combustion engine determined ammonia ^¬ difference is greater than a predetermined ammonia difference ^threshold is:

Activating the urea injector to inject a predetermined amount of urea for a predetermined period of time,

Determining at least a first ammonia value from ammonia signals generated by the ammonia sensor (24) and determining that the change gradient of the at least one first ammonia value is less than a predetermined first change threshold,

Determining at least one second ammonia value from signals generated by the nitrogen oxide sensor (22) and determining that the change gradient of the at least one second ammonia value is less than a predetermined second change threshold, and

The at least one first or second ammonium niakwerts is smaller than the predetermined first and second change threshold adaption of a slope of the characteristic of the ammonia sensor means of the second ammonia values when the Ände ^¬ rungsgradient.

6. The method according to any one of the preceding claims, wherein the nitrogen oxide reference value is determined as follows:

^N0 X _R ef, new = ^N0 X _R ef, aU ~ NO _x with:

Nitrogen oxide reference value for the correction, nitrogen oxide reference value of a previous correction, Ki (TrL- _t 2 weighting factor between 0 and 1 depending on the operating time of the internal combustion engine between two corrections, and

NO _x currently determined nitrogen oxide value (during the fuel cut-off phase of the internal combustion ^¬ machine from a nitrogen oxide sensor (22) generated nitrogen oxide signal determined); or wherein the new ammonia reference value is determined as follows:

NH ₃ , = NH _{3 f} , - K ₂ ( _tl _ _t2 ) ^■ NH ₃ with

NHs ", ammonia reference value for correction,

N _3Refalt ammonia _{reference value of} a previous one

Correction,

K ₂ (T _ _tl _t2) weighting factor between 0 and 1 depending on the operation time of the internal combustion engine between two corrections, and

NH ₃ currently determined ammonia value (during

Fuel cut-off phase of Brennkraftma ^¬ machine determined from an ammonia sensor (24) generated ammonia signal).

7. The method of claim 1, wherein the internal combustion engine comprises a further nitrogen oxide sensor disposed upstream of the SCR catalyst, the method further comprising:

Determining a further nitrogen oxide reference value from a nitrogen oxide signal generated by the further nitrogen oxide sensor (32) during the fuel cut-off phase, and Determining a corrected further nitrogen oxide value from a nitrogen oxide signal generated by the further nitrogen oxide sensor (32) during normal operation of the internal combustion engine taking into account the nitrogen oxide reference value.

8. The method according to any one of the preceding claims, wherein the corrected nitrogen oxide value is determined nitric oxide signal generated from a by the nitrogen oxide sensor (22) is corrected with the Ammo ^¬ niakwert, the ammonia signal generated from a from the ammonia sensor (24), as follows:

NO _{x ff} = NO _x - NH ₃ with

NO _Xnetto corrected and ammonia- _adjusted

Nitrogen oxide value

NO _x from one of the nitrogen oxide sensor (22) generated

Nitrogen oxide value determined by the nitrogen oxide signal, and

NH ₃ from one generated by the ammonia sensor (24)

Ammonia signal determined ammonia value.

9. exhaust tract (10) for an internal combustion engine, comprising:

an SCR catalyst (20),

^¬ be associated with a downstream of the SCR catalyst (20), nitrogen oxide sensor (22) which is adapted to generate a nitrogen oxide value downstream of the SCR catalyst (20) displ ^¬ constricting nitric oxide signal,

injecting an upstream of the SCR catalyst (20) at ^¬ parent urea injector (26) which is adapted to a predetermined amount of urea, a downstream of the SCR catalyst (20) arranged ^¬ ammonia sensor (24) which is adapted to a value of the ammonia downstream of the SCR catalyst (20) indicative signal to generate ammonia, and

- A control unit (40) which is adapted to receive the nitrogen oxide signal and the ammonia signal and to carry out a method according to any one of the preceding claims.

10. The exhaust tract (10) of claim 9, further comprising a further nitrogen oxide sensor (12) disposed upstream of the SCR catalyst (20) and adapted to generate a further nitrogen oxide signal indicative of the amount of nitrogen oxide upstream of the SCR catalyst (12).