CN111810279A

CN111810279A - Method for determining an ammonia mass flow

Info

Publication number: CN111810279A
Application number: CN202010274429.5A
Authority: CN
Inventors: F.施魏策; A.弗里奇; S.奇瓦纳基斯
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-04-10
Filing date: 2020-04-09
Publication date: 2020-10-23
Also published as: DE102019205107A1

Abstract

Method for determining an ammonia mass flow or an ammonia concentration between two SCR catalysts (21, 22) arranged one behind the other in an exhaust gas line (10), a first metering unit (31) being arranged upstream of the first SCR catalyst (21) and a second metering unit (32) being arranged between the two SCR catalysts (21, 22). The determination is carried out from a signal (SensDs 1) of a NOx sensor (41) arranged between the two SCR catalysts (21, 22), a signal (SensDs 2) of a NOx sensor (42) arranged downstream of the second SCR catalyst (22), an emission of nitrogen oxides (NOxUs) upstream of the first SCR catalyst (21), an ammonia dosing amount by means of the first metering unit (31) and an ammonia dosing amount by means of the second metering unit (32).

Description

Method for determining an ammonia mass flow

Technical Field

The invention relates to a method for determining the ammonia mass flow between two SCR catalysts arranged one behind the other in an exhaust gas line. In addition, the invention relates to a computer program which carries out each step of the method and to a machine-readable storage medium which stores the computer program. Finally, the invention relates to an electronic control unit which is provided for carrying out the method.

Background

One promising method for reducing nitrogen oxides in oxygen-rich exhaust gases is by Selective Catalytic Reduction (SCR) of ammonia or ammonia-releasing agents. The efficiency of the SCR catalyst depends on its temperature, on the space velocity of the exhaust gas and decisively on the filling conditions of the ammonia adsorbed on its upper surface. In addition to the directly metered ammonia, the adsorbed ammonia also serves to reduce nitrogen oxides, as a result of which the efficiency of the SCR catalyst is increased compared to an empty catalyst. The storage characteristics are dependent on the respective operating temperature of the catalytic converter. The lower the temperature, the greater the storage capacity.

If the SCR catalyst has completely filled its accumulator, ammonia slip can occur in the event of a load jump in the internal combustion engine whose exhaust gas is reduced by means of the SCR catalyst, even if ammonia or an agent which releases ammonia is no longer metered into the exhaust gas line. However, if the highest possible nitrogen oxide conversion should be achieved, it is unavoidable to operate the SCR system with high ammonia filling conditions. If the temperature of the fully filled SCR catalyst rises due to a load jump of the internal combustion engine, its ammonia storage capacity decreases, which leads to ammonia slip.

This effect is distinguished in particular in that the SCR catalytic converter is configured in the vicinity of the internal combustion engine, so that the SCR catalytic converter reaches its operating temperature quickly after a cold start of the internal combustion engine. A second SCR catalyst downstream of the first SCR catalyst can therefore be arranged in the exhaust gas line in order to adsorb and then convert ammonia in the ammonia slip of the first catalyst.

On-board diagnostics (OBD) standards require that two SCR catalysts must be monitored simultaneously. For this purpose, in each case one nox sensor is usually present downstream of the two SCR catalysts. In some SCR catalyst systems, only one metering unit is provided upstream of the first SCR catalyst in order to meter the ammonia-releasing reducing agent solution into the exhaust gas line. Thus, ammonia filling of the second SCR catalyst is achieved only by ammonia slip of the first SCR catalyst. DE 102016201602 a1 proposes a method for calculating the ammonia mass flow between two SCR catalysts. However, SCR catalyst systems also exist, which each have a metering unit upstream of each SCR catalyst.

Disclosure of Invention

The method enables the ammonia mass flow or the ammonia concentration between two SCR catalysts arranged one behind the other in the exhaust gas line to be determined. In this case, the first metering unit is arranged upstream of the first SCR catalytic converter, and the second metering unit is arranged between the two SCR catalytic converters. The determination is carried out from a signal of a NOx sensor arranged between the two SCR catalysts, a signal of a NOx sensor arranged downstream of the second SCR catalyst, a nitrogen oxide emission upstream of the first SCR catalyst, an ammonia dosing amount by means of the first metering unit and an ammonia dosing amount by means of the second metering unit. In this case, the nitrogen oxide emissions and the metered ammonia addition can be provided in particular as mass flows or concentrations.

The ammonia mass flow or ammonia concentration determined in this way can be used for model correction of the first SCR catalyst and/or as a model input for the second SCR catalyst.

In particular, a calculation is carried out during the determination, in which the ammonia metered amount by means of the first metering unit is divided by the sum of the two ammonia metered amounts.

When the ammonia dosing amount by means of the first dosing unit is zero, the signal of a NOx sensor arranged downstream of the second SCR catalyst is preferably ignored in the determination. This can simplify the finding.

When the ammonia dosing amount by means of the second dosing unit is zero, the nitrogen oxide emissions upstream of the first SCR catalyst are preferably ignored in the determination. This can also simplify the determination.

If an SCR catalyst system is used in which the exhaust gas recirculation line branches off from the exhaust gas line at a branching point between the first SCR catalyst and the second metering unit, it is preferred that the determination takes into account also the exhaust gas mass flow upstream of the branching point and the exhaust gas mass flow downstream of the branching point. Thus, errors in the determination can be avoided by changing the exhaust gas mass flow between the two SCR catalysts when the exhaust gas recirculation valve of the exhaust gas recirculation line is at least partially open.

The computer program is provided for carrying out each step of the method, in particular when the computer program runs on a calculator or an electronic controller. The computer program enables the method to be implemented on a conventional electronic controller without structural changes thereto. For this purpose, the computer program is stored on a machine-readable storage medium. By running this computer program on a conventional electronic control unit, an electronic control unit is obtained which is provided for determining the ammonia mass flow or the ammonia concentration between two SCR catalytic converters arranged one behind the other in the exhaust gas line by means of the method.

Drawings

Embodiments of the invention are illustrated in the drawings and are set forth in detail in the following description.

Fig. 1 shows schematically an SCR catalyst system with two SCR catalysts, between which an ammonia mass flow or an ammonia concentration of which can be determined by means of a method according to an embodiment of the invention.

Fig. 2 shows schematically a further SCR catalyst system with two SCR catalysts, between which an ammonia mass flow or an ammonia concentration of which can be determined by means of a method according to an embodiment of the invention.

Detailed Description

Internal combustion engine inIn its exhaust line 10, there is an SCR catalyst system, which is shown in fig. 1. The nox emission NOxUs of the SCR catalyst system is fed to this SCR catalyst system. The SCR catalyst system has a first SCR catalyst 21 and a second SCR catalyst 22. A first metering unit 31 is arranged upstream of the first SCR catalyst 21, with which an aqueous urea solution can be injected into the exhaust line 10, from which ammonia NH3Dos1 is released when the temperature of the exhaust gas is high. The second metering unit 32 is arranged upstream of the second SCR catalyst 22 in order to release further ammonia NH3Dos2 from the further aqueous urea solution. A first NOx sensor 41 is arranged upstream of the second metering unit 32 between the

SCR catalysts

21, 22. A second NOx sensor 42 is arranged downstream of the second SCR catalyst 22. Since the first NOx sensor 41 is sensitive to ammonia crossover, its signal SensDs1 is for the nitrogen oxide NOxDS1_SensAnd ammonia NH3Ds1_SensA sum signal of (a). 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, which therefore measures only the nitrogen oxides NOxDs2_Sens. By means of the model, it is possible to estimate NOxDS1 in the exhaust gas between the

SCR catalysts

21, 22_MdlAnd ammonia NH3Ds1_MdlThe fraction of (c).

With the aid of a specific dosing requirement facDosEff, the ratio of the metered ammonia mass to the mass of the nitrogen oxides to be converted can be determined at any time in the SCR catalyst system and compared with the chemical ratio. This makes it possible to determine the nitrogen oxide reducing ability of the medium to be metered. If ammonia escapes from the first SCR catalyst 21, the nitrogen oxides converted appear to be less, since the sensor signal SensDs1 contains a proportion of ammonia in addition to a proportion of nitrogen oxides downstream of the first SCR catalyst 21. In operating points in which no ammonia slip occurs, the specific dosing requirement of the first SCR catalyst 21 corresponds to the specific dosing requirements of both SCR catalysts. The specific dosing requirement can be calculated according to equation 1 only with respect to the first SCR catalyst 21:

。

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 partially attributed to ammonia. Written in units of concentration (prefix r), applicable according to equation 2 is:

。

it is assumed here that the cross-sensitivity to ammonia 1:1 is shown as nitrogen oxides in ppm. If the cross-sensitivity is not 1:1, then theoretically a coefficient is also put before rNH3 that maps this cross-sensitivity.

The metered ammonia amount, which effectively contributes to the nitrogen oxide conversion, can be converted into a nitrogen oxide amount according to equation 3, equation 3 theoretically converting this ammonia amount:

。

in this case, it is preferable that the air conditioner,

is a stoichiometric factor that is a function of the specific chemical,

is the amount of nitrogen oxides being converted. The latter can be determined from the amount of nitrogen oxides flowing into the SCR catalyst 21 according to equation 4

And the amount of nitrogen oxides flowing out therefrom

：

。

From equations 3 and 4Equation 5 is derived, where,

is the nox mass at the first nox sensor 41, which can be determined by means of a model:

。

equation 5 can also be defined in equation 6 by mass flow rather than mass:

。

the mass flow can be switched in equations 7 and 8 in units of concentration, where dmEG is the exhaust gas mass flow,

or

The stoichiometric ratio of ammonia or nitrogen oxides to the exhaust gas mass flow, respectively:

。

due to the fact that

Is equal to

And

thus, equations 9 and 10 result:

。

if a specific dosing requirement is set forth for the entire system, equation 11 for mass or equation 12 for mass flow is obtained. In this case, it can be assumed in equation 12 that ammonia is not measured downstream of the second SCR catalyst 22 in the second nox sensor 42, and that the second nox sensor 42 actually measures only nitrogen oxides:

。

if the mass flow is again converted here to units of concentration, equation 13 results:

。

if only the balance of the first SCR catalyst 21 is used to calculate a specific dosing requirement for the entire system, this can be defined as mass flow according to equation 14 or as concentration according to equation 15:

。

equation 16 is derived from equation 13 and equation 15:

。

equation 17 is derived from equations 10 and 16:

。

from equation 2 (for the first SCR catalyst)21) And equation 17 yields equation 18, by means of which equation 18 the ammonia concentration between the two

SCR catalysts

21, 22 can be calculated from the signals SensDs1, SensDs2 of the two

NOx sensors

41, 42, the NOx emissions NOxUs upstream of the first SCR catalyst 21 and the two ammonia metered additions NH3Dos1, NH3Dos2

：

。

When rNH3Dos1=0, since ammonia metering is performed only by means of the second metering unit 32, the ammonia concentration is being calculated

While, signal SensDs2 of NOx sensor 42 disposed downstream of second SCR catalyst 23 is ignored, because equation 18 is simplified to equation 19:

。

when rNH3Dos1=1, since ammonia metering is performed only by means of the first metering unit 31, the ammonia concentration is calculated

While ignoring the ammonia concentration upstream of the first SCR catalyst 21

Since equation 18 is simplified to equation 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

。

Fig. 2 shows a modification of the SCR catalyst system according to fig. 1. An exhaust gas recirculation line 50 for low-pressure exhaust gas recirculation branches off at the branching point 11 between the first SCR catalyst 21 and the second metering unit 32. According to equation 21, the variable γ represents the ratio between the exhaust gas mass flow dmEG1 upstream of the branching point and the exhaust gas mass flow dmEG2 downstream of the branching point 11:

。

the NOx mass flow which is again fed into the air system of the internal combustion engine via the exhaust gas recirculation line 50 can be calculated according to equation 22:

。

similarly, equation 23 is derived for mass flow of NH3 into EGR line 50:

。

to calculate a particular dosing requirement, the loss through the exhaust gas recirculation line 50 is now calculated according to equation 24. This applies to ammonia in the molecule and nitrogen oxides in the denominator:

。

from equations 15 and 24, equation 25 is derived:

。

the variable implementation introduced there transforms equation 25 into equation 26:

。

from the equations 2 (for the first SCR catalyst 21) and 17, the equation 27 is derived, by means of which equation 27 the ammonia concentration between the two

SCR catalysts

21, 22 can be calculated from the signals SensDs1, SensDs2 of the two

NOx sensors

：

。

When the low-pressure egr valve, not shown, of the egr line 50 is closed and therefore γ =1, equation 18 is derived again from equation 27.

For simplicity, equation 27 is not used when using low pressure exhaust gas recirculation, but equation 17 is used. However, for this purpose, the ammonia metering NH3Dos1, NH3Dos2 must also be corrected by a different exhaust gas mass flow if the ammonia metering is to be used as a mass flow. In this case, equation 28 is obtained:

。

equations 29 and 30 are derived from equation 28 and the conversion from concentration to mass flow:

。

Claims

1. for determining the ammonia mass flow (dmNH 3Ds 1) between two SCR catalysts (21, 22) arranged one behind the other in an exhaust gas line (10)_Sens) Or ammonia concentration (rNH 3Ds 1)_Sens) Wherein a first metering unit (31) is arranged upstream of the first SCR-catalyst (21) and a second metering unit (32) is arranged between the two SCR-catalysts (21, 22), characterized in that the determination is carried out from a signal (SensDs 1) of a NOx sensor (41) arranged between the two SCR-catalysts (21, 22), a signal (SensDs 2) of a NOx sensor (42) arranged downstream of the second SCR-catalyst (22), a nitrogen oxide emission (NOxUs) upstream of the first SCR-catalyst (21), an ammonia dosing amount (NH 3Dos 1) by means of the first metering unit (31) and an ammonia dosing amount (NH 3Dos 2) by means of the second metering unit (32).

2. Method according to claim 1, characterized in that the ammonia dosing amount (NH 3Dos 1) by means of the first dosing unit (31) is divided by the sum of two ammonia dosing amounts (NH 3Dos1, NH3Dos 2).

3. Method according to claim 1, characterized in that when the ammonia dosing amount (NH 3Dos 1) by means of the first dosing unit (31) is zero, the signal (SensDs 2) of a NOx sensor (42) arranged downstream of the second SCR catalyst (23) is ignored in the evaluation.

4. Method according to any one of claims 1 to 3, characterized in that when the ammonia dosing amount (NH 3Dos 2) by means of the second dosing unit (32) is zero, the nitrogen oxide emissions (NOxUs) upstream of the first SCR catalyst (21) are ignored in the determination.

5. Method according to any one of claims 1 to 4, characterized in that an exhaust gas recirculation line (50) branches off from the exhaust gas line (10) at a branch point (11) between the first SCR catalyst (21) and the second metering unit (32), wherein the evaluation is carried out taking into account also the exhaust gas mass flow (dmEG 1) upstream of the branch point (11) and the exhaust gas mass flow (dmEG 2) downstream of the branch point (11).

6. The method according to claim 5, characterized in that the ratio of the two exhaust gas mass flows (dmEG 1, dmEG 2) is formed.

7. Computer program arranged to perform each step of the method according to any one of claims 1 to 6.

8. A machine-readable storage medium having stored thereon a computer program according to claim 7.

9. Electronic controller, which is provided for ascertaining a mass flow of ammonia (dmNH 3Ds 1) between two SCR catalysts (21, 22) arranged one behind the other in an exhaust gas duct (10) by means of a method according to one of claims 1 to 6_Sens) Or ammonia concentration (rNH 3Ds 1)_Sens）。