Method for estimating the NOx and NH3 concentration fraction downstream of at least one SCR catalyst arranged in an exhaust stream of an internal combustion engine

Abstract

Method for estimating the NOx and NH3 concentration fraction downstream of at least one SCR catalyst (22) arranged in an exhaust stream (11) of an internal combustion engine (10).

Description

State of the art

[0001] DE 10 2014 201 304 A1 relates to a method for operating a catalyst system intended for reducing nitrogen oxides in exhaust gases of an internal combustion engine, which catalyst system comprises at least one SCR catalyst (150) which is operated with a reactant which can be stored in the SCR catalyst (150) so that a specific reactant fill level is present in the SCR catalyst (150), wherein the metered addition of reactant is carried out on the basis of models (604, 606) with the aid of an NH3 fill level controller (602), modeling errors are compensated with the aid of an NH3 fill level observer (605). Disclosure of the invention

[0002] In a first aspect, the invention relates to a method for estimating a NOx and NH3 concentration fraction downstream of at least one SCR catalyst arranged in an exhaust system of an internal combustion engine, wherein a first NOx concentration sensor value is determined upstream of the SCR catalyst and downstream of the internal combustion engine by means of a first NOx sensor, wherein a second NOx concentration sensor value is determined downstream of the SCR catalyst (22) by means of a second NOx sensor, wherein the first and second NOx sensors have a cross-sensitivity to ammonia, wherein, depending on the first and second NOx concentration sensor values, a division into a modeled NOx concentration component and a modeled NH3 concentration component is carried out by means of a model, in particular at the location of the second NOx sensor, wherein an emission phase is detected depending on the second NOx concentration sensor value if the second NOx concentration sensor value exceeds a first predeterminable threshold value, where the model determines and stores modeled NH3 and NOx concentration fractions at discrete times, wherein a ratio between the determined modeled NH3 concentration fraction and the second NOx concentration sensor value is determined, wherein a first counter is incremented when the ratio exceeds a predeterminable NH3 threshold value and is decremented when the ratio falls below a predeterminable NOx threshold value, wherein an assignment of the NOx and NH3 concentration components for the second NOx concentration sensor signal determined in the emission phase is carried out as a function of the first counter.

[0003] The method has the particular advantage that a statistical assignment of the NOx and NH3 concentration components can be carried out if a sufficiently reliable database is available. The method improves the reliable assignment of the NOx and NH3 cross-sensitivities of the NOx sensor arranged downstream of the SCR catalyst. This enables an assignment of the NOx and NH3 concentration components over the entire driving cycle, as required by emissions legislation such as the EU7 OBM. The method begins with a statistical evaluation of the NOx and NH3 concentration components at the start of a detected emission phase, whereby an assignment of the NOx and NH3 concentration components is only carried out once a sufficiently reliable result for the assignment is available.

[0004] In a special embodiment, a second counter is incremented as long as the model returns valid measurements, wherein if the second NOx concentration sensor value falls below the first predeterminable threshold value, an end of the emission phase is detected and the second counter is stopped.

[0005] In a further embodiment, a valid emission phase exists if the second counter exceeds a predefined validity threshold. Furthermore, a valid emission phase does not exist if the second counter falls below the predefined validity threshold, so that a substitute value is used for the assignment. The method offers the particular advantage that, depending on the second counter, it can be determined when sufficient valid measurements are available for the allocation of the NOx and NH3 concentration components or not. In case of an insufficiently valid emission phase, a substitute value can be used to assign the NOx and NH3 concentration fractions.

[0006] In an advantageous embodiment, a proportional allocation of the NOx and NH3 concentration share is carried out if the first counter lies between an upper predeterminable NH3 maximum threshold value and a lower predeterminable NOx minimum threshold value.

[0007] In a special embodiment, if the first counter is greater than or equal to an upper predeterminable NH3 maximum threshold value, only NH3 concentration components are assigned for the emission phase. If the second counter reaches or exceeds the upper NH3 maximum threshold, there is a sufficiently strong indication that the emissions detected in the emission phase are NH3 emissions.

[0008] In an advantageous embodiment, if the first counter is less than or equal to a lower predeterminable NOx minimum threshold value, only NOx concentration components are assigned for the emission phase.

[0009] If the second counter reaches or falls below the lower NOx minimum threshold, there is a sufficiently strong indication that the emissions detected in the emission phase are NOx emissions.

[0010] In a further embodiment, a third counter is incremented with the detection of the emission phase, wherein the third counter is stopped with the end of the emission phase, wherein if the third counter falls below a predeterminable time threshold value, an assignment is made exclusively of NOx concentration components.

[0011] In further aspects, the invention relates to a device, in particular a control unit and a computer program, which are configured, in particular programmed, to execute one of the methods. In yet another aspect, the invention relates to a machine-readable storage medium on which the computer program is stored. Short description of the drawings

[0012] The invention is explained in more detail below using an exemplary embodiment illustrated in the figures. They show: Fig. 1 shows a schematic representation of an SCR system with an SCR catalyst of an internal combustion engine, Fig. 2 a) shows an exemplary measurement of the second NOx sensor plotted over time t, b) shows an example of the determination of the NH3 concentration fractions r determined by the model M NH3,M shown over time t, c) shows an exemplary determination of the first counter with the upper presettable NH3 maximum threshold value and the lower presettable NOx minimum threshold value and, d) shows an exemplary determination of the second counter with the predeterminable validity threshold S Val . Fig. 3 a schematic flow diagram of an exemplary embodiment of the method according to the invention for estimating the NOx and NH3 concentration downstream of at least one SCR catalyst of an internal combustion engine. Embodiments of the invention

[0013] An internal combustion engine 10 has an SCR system 20 in its exhaust system 11, which Fig. 1. This has a reducing agent dosing unit 21, with which a urea-water solution (AdBlue) can be injected into the exhaust system 11. Ammonia is released from this at the high temperatures of the exhaust gas. Downstream of the reducing agent dosing unit 21 is arranged a first SCR catalyst 22. Further SCR catalysts can be arranged downstream of the first SCR catalyst 22. A first NOx sensor 31 and a temperature sensor 12 are arranged upstream of the reducing agent dosing unit 21 and downstream of an internal combustion engine 10 in the exhaust system 11. The first NOx sensor 31 measures a first NOx concentration sensor value NOx1, preferably as a NOx concentration or as a NOx mass flow. A second NOx sensor 32 is arranged downstream of the first SCR catalyst 22 and measures a second NOx concentration sensor value NOx2, preferably as a NOx concentration or as a NOx mass flow.In particular, this also applies to the NH3 concentration or an NH3 mass flow.

[0014] In an optional embodiment, an NH3 sensor 33 can also be installed downstream of the SCR catalyst 22. The NH3 sensor can measure an NH3 concentration fraction r NH3 determine.

[0015] All NOx sensors 31, 32 transmit their signals to an electronic control unit 100. Since the NOx sensors 31, 32 are cross-sensitive to ammonia in addition to nitrogen oxides, their signals are combined signals of nitrogen oxides and ammonia. However, the first NOx sensor 31 is located upstream of the reducing agent dosing unit 21, so that it reliably measures only the amount of nitrogen oxide in the exhaust gas. The reducing agent dosing unit 21 also reports the amount of ammonia metered into the exhaust system 11 to the control unit 100.

[0016] Furthermore, model M is stored on the control unit 100, which, depending on the first NOx concentration sensor value NOx1 and the second NOx concentration sensor value NOx2, calculates a NOx concentration fraction r NOx,M and an NH3 concentration fraction r NH3,M Optionally, the exhaust gas temperature of the temperature sensor 12 can be used as an input variable.

[0017] However, the described model M does not provide continuous NOx and NH3 concentration components and is based on the first and second NOx sensor signals upstream and downstream of the SCR catalyst 22.

[0018] In the Fig. Figure 2 shows an example measurement for the presented method. The time axis t as the abscissa applies to all diagrams shown (a) - (d). The upper diagram a) shows an example measurement for the second NOx sensor 32 in parts per million (ppm) over time t. Furthermore, the first predeterminable threshold value S NOx2 shown, if the second NOx concentration sensor value NOx2 exceeds this threshold value, an emission phase is detected and the emission phase is ended when the second NOx concentration sensor value NOx2 exceeds the first predefinable threshold value S NOx2 The detection of the end of the emission phase can preferably also be filtered so that the second NOx concentration sensor value NOx2 exceeds the first predeterminable threshold value S NOx2 must be below for a minimum period.

[0019] In diagram b) the modelled NH3 concentration fractions r determined by model M are shown. NH3,M in parts of a million (ppm) over time t.

[0020] In diagram c) the first counter Z1 (with the number n) with the upper preset NH3 maximum threshold value NH3 max and the lower preset NOx minimum threshold value NOx min shown.

[0021] It can be seen that the first counter Z1 only shows a change if the model M can determine valid measured values.

[0022] In particular, the first counter Z1 counts depending on the determined NH3 concentration fractions r NH3,M the model M.

[0023] In this case, the first counter Z1 initially remains unchanged until the model M detects a visible increase in the NH3 concentration fractions r NH3,M The first counter Z1 is continuously incremented and takes the value of the maximum threshold NH3 max a.

[0024] In an alternative embodiment, the first counter Z1 can also have a value greater than the maximum threshold value NH3 maxand values ​​smaller than the minimum threshold NOx min assume.

[0025] In diagram d) the second counter Z2 (with the number n) and predeterminable validity threshold S Val It can be seen that the second counter Z2 is only incremented when valid measurements of model M for the NH3 concentration fractions r NH3,M The second counter Z2 can be reset to 0 when the end of emission phases is detected.

[0026] The measurement shows that the second counter Z2 exceeds the preset validity threshold S Val In an advantageous embodiment, by exceeding the validity threshold S Val by the second counter Z2 a validation of the allocation of the NOx and NH3 concentration fractions r NOx ; r NH3 for the second NOx concentration sensor signal NOx2 determined in the emission phase.

[0027] In the Fig. 3 shows the exemplary sequence of the method for estimating a NOx and an NH3 concentration downstream of at least one SCR catalyst 22 of an internal combustion engine 10 arranged in an exhaust system 11 using a flow chart.

[0028] In a first step 200, the first and second NOx concentration sensor values ​​NOx1;NOx2 are continuously determined by the control unit 100. If the second NOx concentration sensor value NOx2 exceeds a first predeterminable threshold value S NOx2 an emission phase is detected. The first preset threshold value S NOx2 can preferably be 5 ppm.

[0029] In a special embodiment, exceeding the first predeterminable threshold value S NOx2 by the second NOx concentration sensor value NOx2 also filtered, in particular by a PT1 filter, over a predeterminable time.

[0030] The method then continues in a step 210.

[0031] In a step 210, upon detection of the emission phase, the model M stored on the control unit 100 is started. Depending on the first and second NOx concentration sensor values ​​NOx1;NOx2, the model M determines a modeled NH3 concentration fraction r at discrete times. NH3,M and a modeled NOx concentration fraction r NOx,M .

[0032] The modeled NH3 and NOx concentration fraction corresponds to an NH3 and NOx concentration signal downstream of the SCR catalyst 22, in particular at the location of the second NOx sensor 32.

[0033] Optionally, the temperature signal from temperature sensor 12 can also be used as an input variable for the M model.

[0034] In the further embodiment, the method is based on the determined modeled NH3 concentration fraction rNH3,M described.

[0035] In an alternative embodiment, the NH3 concentration fraction, in particular the modeled NH3 concentration fraction r NH3,M , can be determined by an NH3 sensor 33 installed downstream of the SCR catalyst 22.

[0036] The method then continues in a step 220.

[0037] In a step 220, a ratio V between the determined modeled NH3 concentration fraction r NH3,M and the second NOx concentration sensor value NOx2. This is done for each discrete measured value of the Model M.

[0038] If the ratio V exceeds a predefined NH3 threshold value S NH3 If a first counter Z1 is incremented, the ratio V falls below a predeterminable NOx threshold value S NOxthe first counter Z1 is decremented. Furthermore, the model M returns a status bit for each measured value indicating whether the measurement was valid.

[0039] As long as the status bit returns a valid measurement, a second counter Z2 is incremented.

[0040] The method then continues in a step 230.

[0041] In a step 230, if the second NOx concentration sensor value NOx2 exceeds the first predeterminable threshold value S NOx2 again falls below the end of the emission phase.

[0042] At this point in time, the second counter Z2 is checked against a predefined validity threshold S Val compared.

[0043] If the second counter Z2 is greater than or equal to the preset validity threshold S Val an allocation of the NOx and NH3 concentration fractions r NOx ; r NH3depending on the first counter Z1 for the second NOx concentration sensor signal NOx2 determined in the emission phase.

[0044] If the second counter Z2 falls below the preset validity threshold S Val the allocation is suspended because the emission phase is preferably too short.

[0045] Alternatively, if the predefined validity threshold S Val by the second counter an allocation of NOx and NH3 concentration fractions r NOx ; r NH3 depending on a predefined substitute value. For emission phases that are too short, it is preferably assumed that only NOx concentrations r NOx is.

[0046] In a special embodiment, if the first counter Z1 is greater than or equal to an upper predeterminable NH3 maximum threshold value NH3 maxis an assignment exclusively of NH3 concentration fractions r NH3 for the second NOx concentration sensor signal NOx2 determined in the emission phase.

[0047] This assignment can be maintained even though the second counter Z2 exceeds the predeterminable validity threshold S Val falls below.

[0048] In a further embodiment, if the first counter Z1 is less than or equal to a lower predeterminable NOx minimum threshold value NOx min is an allocation exclusively of NOx concentration fractions r NOx for the second NOx concentration sensor signal NOx2 determined in the emission phase.

[0049] This assignment can be maintained even though the second counter Z2 exceeds the predeterminable validity threshold S Val falls below.

[0050] In a special embodiment, a third counter Z3 is incremented upon detection of the emission phase and terminated when the emission phase is terminated.

[0051] If the third counter Z3 reaches a preset time threshold value (S time,min ), an allocation exclusively of NOx concentration fractions r NOx for the second NOx concentration sensor signal NOx2 determined in the emission phase.

[0052] The process can then be terminated or started again in step 200. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2014 201 304 A1

[0001]

Claims

[1] Method for estimating a NOx and NH3 concentration fraction downstream of at least one SCR catalyst (22) of an internal combustion engine (10) arranged in an exhaust system (11), wherein a first NOx concentration sensor value (NOx1) upstream of the SCR catalyst (22) and downstream of the internal combustion engine (10) is determined by means of a first NOx sensor (31), wherein a second NOx concentration sensor value (NOx2) is determined downstream of the SCR catalyst (22) by means of a second NOx sensor (32), wherein the first and second NOx sensors (31, 32) have a cross-sensitivity to ammonia (NH3), wherein, depending on the first and second NOx concentration sensor values ​​(NOx1;NOx2), a model (M) is used to divide the NOx concentration into a modeled NOx concentration fraction (r Nox,M ) and a modeled NH3 concentration fraction (r NH3,M), in particular at the location of the second NOx sensor (32), wherein, depending on the second NOx concentration sensor value (NOx2), an emission phase is detected when the second NOx concentration sensor value (NOx2) exceeds a first predeterminable threshold value (S Nox2 ) exceeds where the model (M) represents the NH3 and NOx concentration fractions (r NH3,M ; r NOx,M ) is determined and stored, whereby a ratio (V) between the determined modeled NH3 concentration fraction (r NH3,M ) and the second NOx concentration sensor value (NOx2), wherein a first counter (Z1) is incremented when the ratio (V) exceeds a predeterminable NH3 threshold value (S NH3 ) and is decremented when the ratio (V) exceeds a predeterminable NOx threshold value (S NOx ), wherein, depending on the first counter (Z1), an allocation of the NOx and NH3 concentration fractions (r NOx ; r NH3 ) for the second NOx concentration sensor signal (NOx2) determined in the emission phase. [2] Method according to claim 1, characterized by that a second counter (Z2) is incremented as long as the model (M) returns valid measurements, whereby if the second NOx concentration sensor value (NOx2) exceeds the first predeterminable threshold value (S NOx2 ), an end of the emission phase is detected and the second counter (Z2) is stopped. [3] Method according to claim 2, characterized by that a valid emission phase exists if the second counter (Z2) has a predeterminable validity threshold (S Val ) and there is no valid emission phase, if the second counter (Z2) exceeds the predeterminable validity threshold (S Val) and a replacement value is used for the assignment. [4] Method according to claim 1, characterized by that a proportional allocation of the NOx and NH3 concentration share (r NOx , r NH3 ) is carried out when the first counter (Z1) is between an upper predeterminable NH3 maximum threshold value (NH3 max ) and a lower, predeterminable NOx minimum threshold value (NOx min ) lies. [5] Method according to claim 1, characterized by that if the first counter (Z1) is greater than or equal to an upper preset NH3 maximum threshold value (NH3 max ), only NH3 concentration components for the emission phase can be assigned. [6] Method according to claim 1, characterized by that if the first counter (Z1) is less than or equal to a lower predeterminable NOx minimum threshold value (NOx min ), only NOx concentration components for the emission phase are assigned. [7] Method according to claim 1, characterized by that a third counter (Z3) is incremented upon detection of the emission phase, wherein the third counter (Z3) is stopped upon the end of the emission phase, wherein when the third counter (Z3) reaches a predeterminable time threshold value (S time,min ) falls below an allocation exclusively of NOx concentration fractions (r NOX ) is carried out. [8] Method according to claim 1, characterized by that the NOx and NH3 concentration fractions are converted into NOx mass flows and NH3 mass flows. [9] Computer program which is designed to carry out a method according to one of claims 1 to 8. [10] An electronic storage medium comprising a computer program according to claim 9. [11] Device, in particular control device (100), which is designed to carry out a method according to one of claims 1 to 8.

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