Method for finding ammonia mass flow

Abstract

A method for determining an ammonia mass flow or an ammonia concentration between two SCR catalytic converters (21, 22) arranged successively in an exhaust gas line (10), wherein a first metering unit (31) is arranged upstream of the first SCR catalytic converter (21) and a second metering unit (32) is arranged between the two SCR catalytic converters (21, 22). The determination is performed based on a signal (SensDs1) of a NOx sensor (41) arranged between the two SCR catalytic converters (21, 22), a signal (SensDs2) of a NOx sensor (42) arranged downstream of the second SCR catalytic converter (22), a nitrogen oxide emission (NOxUs) upstream of the first SCR catalytic converter (21), an ammonia metered amount by means of the first metering unit (31) and an ammonia metered amount by means of the second metering unit (32).

Description

Technical Field

[0001] The invention relates to a method for determining an ammonia mass flow between two SCR catalytic converters arranged one behind the other in an exhaust gas line. The invention also relates to a computer program that implements each step of the method and to a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control unit that is configured to implement the method. Background Art

[0002] A very promising method for reducing nitrogen oxides in oxygen-rich exhaust gases is selective catalytic reduction (SCR) with the aid of ammonia or agents that release ammonia. The efficiency of the SCR catalyst depends on its temperature, on the exhaust gas space velocity and, crucially, on the filling level of ammonia adsorbed on its upper surface. In addition to the directly metered ammonia, the adsorbed ammonia is also used to reduce the nitrogen oxides, whereby the efficiency of the SCR catalyst is increased compared to an empty catalyst. The storage properties depend on the respective operating temperature of the catalyst. The lower the temperature, the greater the storage capacity.

[0003] If an SCR catalytic converter has completely filled its storage, ammonia slip can occur in the event of a load jump in the internal combustion engine, whose exhaust gases are reduced by means of the SCR catalytic converter, even if ammonia or an agent that releases ammonia is no longer metered into the exhaust gas line. However, if the highest possible nitrogen oxide conversion is to be achieved, it is unavoidable to operate the SCR system with a high ammonia filling level. If the temperature of a completely filled SCR catalytic converter increases due to a load jump in the internal combustion engine, its ammonia storage capacity decreases, which leads to ammonia slip.

[0004] This effect is particularly prominent in that the SCR catalytic converter is constructed near the internal combustion engine so that the SCR catalytic converter quickly reaches its operating temperature after a cold start of the internal combustion engine. A second SCR catalytic converter downstream of the first SCR catalytic converter can therefore be arranged in the exhaust gas line in order to adsorb and then convert ammonia in the ammonia slip of the first catalytic converter.

[0005] The on-board diagnostic (OBD) standard requires that both SCR catalytic converters must be monitored simultaneously. For this purpose, there is usually a nitrogen oxide sensor downstream of each of the two SCR catalytic converters. In some SCR catalytic converter systems, only one metering unit is constructed upstream of the first SCR catalytic converter in order to meter the reducing agent solution that releases ammonia into the exhaust gas line. Therefore, the ammonia filling of the second SCR catalytic converter is achieved only by ammonia leakage from the first SCR catalytic converter. DE 10 2016 201 602 A1 proposes a method for calculating the ammonia mass flow between two SCR catalytic converters. However, there are also SCR catalytic converter systems that have a metering unit upstream of each SCR catalytic converter. Summary of the invention

[0006] The method can determine the ammonia mass flow or the ammonia concentration between two SCR catalytic converters arranged successively in an exhaust gas line. In this case, a first metering unit is arranged upstream of a first SCR catalytic converter and a second metering unit is arranged between the two SCR catalytic converters. The determination is performed from the signal of a NOx sensor arranged between the two SCR catalytic converters, the signal of a NOx sensor arranged downstream of the second SCR catalytic converter, the nitrogen oxide emissions upstream of the first SCR catalytic converter, the ammonia metered amount by means of the first metering unit and the ammonia metered amount by means of the second metering unit. In this case, the nitrogen oxide emissions and the ammonia metered amount can be provided in particular as mass flows or concentrations.

[0007] The ammonia mass flow or the ammonia concentration ascertained in this way can be used for a model correction of the first SCR catalytic converter and / or as a model input for the second SCR catalytic converter.

[0008] In particular, when determining the amount of ammonia ammonia atomized, a calculation is performed in which the amount of ammonia atomized by means of the first metering unit is divided by the sum of the two ammonia atomized amounts.

[0009] If the amount of ammonia metered in by means of the first metering unit is zero, the signal of the NOx sensor arranged downstream of the second SCR catalytic converter is preferably ignored during the ascertainment. This can simplify the ascertainment.

[0010] When the amount of ammonia metered in by means of the second metering unit is zero, the nitrogen oxide emissions upstream of the first SCR catalytic converter are preferably neglected during the ascertainment. This can likewise simplify the ascertainment.

[0011] If an SCR catalytic converter system is used in which an exhaust gas recirculation line branches off from the exhaust gas line at a branching point between the first SCR catalytic converter and the second metering unit, preferably, the exhaust gas mass flow upstream of the branching point and the exhaust gas mass flow downstream of the branching point are also taken into account when determining. This allows errors in the determination to be avoided by changing the exhaust gas mass flow between the two SCR catalytic converters when an exhaust gas recirculation valve of the exhaust gas recirculation line is at least partially open.

[0012] The computer program is configured to execute each step of the method, in particular when the computer program is run on a computer or an electronic control unit. The computer program can implement the method on a conventional electronic control unit without having to modify its structure. For this purpose, the computer program is stored on a machine-readable storage medium. By running the computer program on a conventional electronic control unit, an electronic control unit is obtained, which is configured to determine the ammonia mass flow or ammonia concentration between two SCR catalytic converters arranged successively in an exhaust gas line by means of the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0014] Figure 1 An SCR catalytic converter system having two SCR catalytic converters is schematically shown, of which an ammonia mass flow or an ammonia concentration between the two SCR catalytic converters can be ascertained using a method according to an exemplary embodiment of the present invention.

[0015] Figure 2 A further SCR catalytic converter system having two SCR catalytic converters is schematically shown, of which the ammonia mass flow or the ammonia concentration between the two SCR catalytic converters can be ascertained using a method according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0016] The internal combustion engine has an SCR catalytic converter system in its exhaust gas line 10, which is Figure 1. Its nitrogen oxide emission NOxUs is introduced into this SCR catalyst system. The SCR catalyst system has a first SCR catalyst 21 and a second SCR catalyst 22. The first metering unit 31 is arranged upstream of the first SCR catalyst 21, and the urea aqueous solution can be injected into the exhaust pipe 10 by means of the first metering unit. When the temperature of the exhaust gas is high, ammonia NH3Dos1 is released from the urea aqueous solution. The second metering unit 32 is arranged upstream of the second SCR catalyst 22 so as to release additional ammonia NH3Dos2 from the other urea aqueous solution. The first NOx sensor 41 is arranged between the SCR catalysts 21 and 22 upstream of the second metering unit 32. The second NOx sensor 42 is arranged downstream of the second SCR catalyst 22. Since the first NOx sensor 41 is cross-sensitive to ammonia, its signal SensDs1 is for nitrogen oxides NOxDs1. Sens and ammonia NH3Ds1 Sens The second NOx sensor 42 is also cross-sensitive to ammonia. However, it can be assumed that no ammonia is present downstream of the second SCR catalyst 22, so that the second SCR catalyst only measures nitrogen oxides NOxDs2 Sens The model can be used to estimate the nitrogen oxides NOxDs1 in the exhaust gas between the SCR catalysts 21 and 22. Mdl and ammonia NH3Ds1 Mdl share.

[0017] With the help of the specific metering agent requirement facDosEff, the ratio of the mass of metered ammonia to the mass of converted nitrogen oxides can be determined at any time in the SCR catalytic converter system and compared with the chemical ratio. It is thus possible to determine the nitrogen oxide reducing capacity of the metered medium. If ammonia leaks out of the first SCR catalytic converter 21, the converted nitrogen oxides appear to be reduced, because the sensor signal SensDs1 downstream of the first SCR catalytic converter 21 contains an ammonia portion in addition to the nitrogen oxide portion. At the operating point where no ammonia slip occurs, the specific metering agent requirement of the first SCR catalytic converter 21 corresponds to the specific metering agent requirement of the two SCR catalytic converters. The specific metering agent requirement only for the first SCR catalytic converter 21 can be calculated according to formula 1:

[0018] .

[0019] The first NOx sensor 41 measures not only nitrogen oxides, but also ammonia due to its cross-sensitivity. Therefore, the nitrogen oxide concentration displayed by the first NOx sensor 11 can be partly attributed to ammonia. Written in units of concentration (prefix r), according to formula 2, it applies:

[0020] .

[0021] It is assumed here that a cross sensitivity of 1:1 to ammonia is displayed as nitrogen oxides in ppm. If the cross sensitivity is not 1:1, then theoretically a factor is also inserted before rNH3, which maps this cross sensitivity.

[0022] The amount of ammonia metered in which the nitrogen oxide conversion is effectively contributed can be converted into the amount of nitrogen oxide according to formula 3, which theoretically converts this amount of ammonia:

[0023] .

[0024] in this case, is the stoichiometric factor, is the amount of nitrogen oxides converted. The latter can be calculated from the amount of nitrogen oxides flowing into the SCR catalyst 21 according to Formula 4: and the amount of nitrogen oxides flowing out therefrom :

[0025] .

[0026] Formula 5 is obtained from Formula 3 and 4, where is the nitrogen oxide mass at the first nitrogen oxide sensor 41 , which can be known with the aid of a model:

[0027] .

[0028] It is also possible to define Equation 5 by mass flow instead of mass in Equation 6:

[0029] .

[0030] The mass flow can be converted into concentration units in equations 7 and 8, where dmEG is the exhaust gas mass flow, or are the stoichiometric ratios of ammonia or nitrogen oxides to the exhaust gas mass flow, respectively:

[0031]

[0032] .

[0033] because equal and The quotient of , thus formulas 9 and 10 are obtained:

[0034] .

[0035] If a specific metering agent requirement is made for the entire system, then equation 11 for the mass or equation 12 for the mass flow is obtained. In this case, equation 12 takes into account that it can be assumed that no ammonia is measured downstream of the second SCR catalytic converter 22 in the second nitrogen oxide sensor 42, which actually only measures nitrogen oxides:

[0036] .

[0037] If the mass flow is converted again into concentration units here, then formula 13 results:

[0038] .

[0039] If only the balance of the first SCR catalytic converter 21 is used to calculate a specific dosing agent requirement for the entire system, this can be defined as a mass flow according to Formula 14 or as a concentration according to Formula 15:

[0040]

[0041] .

[0042] Formula 16 is obtained from Formula 13 and Formula 15:

[0043] .

[0044] Equation 17 is obtained from Equation 10 and 16:

[0045] .

[0046] Formula 18 is obtained from Formula 2 (for the first SCR catalytic converter 21) and Formula 17, by means of which the ammonia concentration between the two SCR catalytic converters 21, 22 can be calculated from the signals SensDs1, SensDs2 of the two NOx sensors 41, 42, the nitrogen oxide emission NOxUs upstream of the first SCR catalytic converter 21 and the two ammonia metered quantities NH3Dos1, NH3Dos2 :

[0047] .

[0048] When rNH3Dos1=0, since ammonia metering is performed only by the second metering unit 32, when calculating the ammonia concentration When the signal SensDs2 of the NOx sensor 42 arranged downstream of the second SCR catalyst 23 is ignored, the formula 18 is simplified to the formula 19:

[0049] .

[0050] When rNH3Dos1=1, since ammonia metering is performed only by means of the first metering unit 31, when calculating the ammonia concentration When the ammonia concentration upstream of the first SCR catalyst 21 is ignored , because Formula 18 is simplified to Formula 20. In this case, it is considered that the NOx sensor 42 arranged downstream of the second SCR catalyst 23 only displays nitrogen oxides, so that the difference in the nitrogen oxide concentration measured by the two nitrogen oxide sensors 41, 42 indicates the ammonia concentration between the two SCR catalysts 21, 22 .

[0051]

[0052] Figure 2 Show according to Figure 1 A modification of the SCR catalytic converter system. The exhaust gas recirculation line 50 for low-pressure exhaust gas recirculation branches at the branch point 11 between the first SCR catalytic converter 21 and the second metering unit 32. According to formula 21, the variable γ represents the ratio between the exhaust gas mass flow dmEG1 upstream of the branch point and the exhaust gas mass flow dmEG2 downstream of the branch point 11:

[0053] .

[0054] The NOx mass flow which is fed back into the air system of the internal combustion engine via the exhaust gas recirculation line 50 can be calculated according to formula 22:

[0055] .

[0056] Similarly, equation 23 is obtained for the NH 3 mass flow into the exhaust gas recirculation line 50:

[0057] .

[0058] In order to calculate the specific dosing agent requirement, the losses through the exhaust gas recirculation line 50 are now calculated according to formula 24. This applies to ammonia in the numerator and nitrogen oxides in the denominator:

[0059] .

[0060] Formula 25 is obtained from Formula 15 and 24:

[0061] .

[0062] The variable ε introduced there makes it possible to transform Formula 25 into Formula 26:

[0063] .

[0064] Formula 2 (for the first SCR catalytic converter 21) and 17 give Formula 27, by means of which the ammonia concentration between the two SCR catalytic converters 21, 22 can be calculated from the signals SensDs1, SensDs2 of the two NOx sensors 41, 42, the nitrogen oxide emission NOxUs upstream of the first SCR catalytic converter 21 and the two ammonia metered quantities NH3Dos1, NH3Dos2 :

[0065] .

[0066] If the low-pressure exhaust gas recirculation valve (not shown) of the exhaust gas recirculation line 50 is closed and therefore γ=1, equation 18 results again from equation 27.

[0067] For simplification, when using low-pressure exhaust gas recirculation, instead of equation 27, equation 17 is used. However, for this purpose, the ammonia metering amounts NH3Dos1, NH3Dos2 must also be corrected by the different exhaust gas mass flows—if the ammonia metering amounts are available as mass flows. In this case, equation 28 results:

[0068] .

[0069] Equation 28 and the conversion from concentration to mass flow yields equations 29 and 30:

[0070]

[0071] .

Claims

1. For determining the ammonia mass flow (dmNH3Ds1) between two SCR catalytic converters (21, 22) arranged successively in an exhaust gas line (10) Sens ) or ammonia concentration (rNH3Ds1 Sens ), wherein The first metering unit (31) is arranged upstream of the first SCR catalyst (21), and the second metering unit (32) is arranged between the two SCR catalysts (21, 22), characterized in that the signal (SensDs1) of the NOx sensor (41) arranged between the two SCR catalysts (21, 22), the signal (SensDs2) of the NOx sensor (42) arranged downstream of the second SCR catalyst (22), the nitrogen oxide emission (NOxUs) upstream of the first SCR catalyst (21), the ammonia metering amount (NH3Dos1) by means of the first metering unit (31) and the ammonia metering amount (NH3Dos2) by means of the second metering unit (32) are calculated by means of the following formula: rSensDs1=rNOxDs1Sens+rNH3Ds1Sens in, rSensDs1 represents the concentration and signal of the nitrogen oxide concentration and the ammonia concentration arranged between the two SCR catalysts (21, 22); rNOxDs1 Sens Indicates the nitrogen oxide concentration arranged between the two SCR catalysts (21, 22); rNH3Ds1 Sens represents the ammonia concentration arranged between the two SCR catalysts (21, 22); rNH3Dos1 represents the concentration of the ammonia metered addition amount of the first metering unit (31); rNH3Dos2 represents the concentration of the ammonia metered addition amount of the second metering unit (32); rNOxUs represents the concentration of nitrogen oxide emissions upstream of the first SCR catalyst (21); rNOxDs2 Sens represents the concentration of nitrogen oxides downstream of the second SCR catalyst (22).

2. The method according to claim 1, characterized in that When the amount of ammonia metered in (NH3Dos1) by means of the first metering unit (31) is zero, the signal (SensDs2) of the NOx sensor (42) arranged downstream of the second SCR catalytic converter (22) is ignored during the ascertainment.

3. The method according to claim 1, characterized in that When the amount of ammonia metered in (NH3Dos2) by means of the second metering unit (32) is zero, the nitrogen oxide emissions (NOxUs) upstream of the first SCR catalytic converter (21) are neglected during the ascertainment.

4. The method according to claim 1, characterized in that: An exhaust gas recirculation line (50) branches off from the exhaust gas line (10) at a branching point (11) between the first SCR catalytic converter (21) and the second metering unit (32), wherein an exhaust gas mass flow (dmEG1) upstream of the branching point (11) and an exhaust gas mass flow (dmEG2) downstream of the branching point (11) are also taken into account when determining the exhaust gas mass flow.

5. The method according to claim 4, characterized in that The ratio of the two exhaust gas mass flows (dmEG1, dmEG2) is formed. 6 . A computer program product configured to execute each step of the method according to claim 1 .

7. A machine-readable storage medium having a computer program stored thereon, which is configured to execute each step of the method according to any one of claims 1 to 5.

8. An electronic control unit, which is configured to determine the ammonia mass flow (dmNH3Ds1) between two SCR catalytic converters (21, 22) arranged successively in an exhaust gas line (10) by means of a method according to any one of claims 1 to 5 Sens ) or ammonia concentration (rNH3Ds1 Sens ).

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