Determining the amount of nitrogen oxides in an exhaust system with exhaust gas recirculation

Abstract

Methods for determining the amount of nitrogen oxides (NOx) _out in the exhaust gas exiting directly from an internal combustion engine (2), wherein the exhaust gas is passed through an exhaust gas tract (4), an exhaust gas aftertreatment system with at least one nitrogen oxide storage catalyst (9a) and at least one nitrogen oxide reduction catalyst (10a) and optionally at least partially through an exhaust gas recirculation line (12) branching off from the exhaust gas tract downstream of the exhaust gas aftertreatment system, wherein the nitrogen oxide content is determined on the basis of a quantity of nitrogen oxides determined downstream of the nitrogen oxide reduction catalyst (10a) in the exhaust gas, characterized in that the nitrogen oxide content NOx _out with the equation NOx _out = NOx _all − f ( ( NOx _all − NOx 1 ) , EGR rate ) is determined, whereby NOx _out the amount of nitrogen oxide in the exhaust gas directly exiting the internal combustion engine is, NOx _allthe amount of nitrogen oxide in the exhaust gas exiting directly from the internal combustion engine, provided that no nitrogen oxide has been converted in the exhaust gas aftertreatment system and all nitrogen oxide is contained in the exhaust gas recirculated through the exhaust gas recirculation line (12), (NOx _all - NOx1), EGR rate the difference between the nitrogen oxide recirculated in the exhaust gas recirculation line (12) and the total nitrogen oxide exiting the internal combustion engine (2), and NO x1 the nitrogen oxide concentration at the branch of the exhaust gas recirculation line (12).

Description

[0001] The invention relates to a method for determining a quantity of nitrogen oxide in an arrangement with an internal combustion engine and exhaust gas recirculation in the exhaust gas exiting the internal combustion engine on the basis of a quantity of nitrogen oxide and ammonia determined downstream of a nitrogen oxide reduction device.

[0002] For internal combustion engines, catalytic aftertreatment of exhaust gases has become standard practice to comply with legally mandated emission limits. Modern internal combustion engines often operate with lean fuel-air mixtures with an excess of oxygen to increase efficiency. Nitrogen oxides (NOx) produced cannot be oxidized under lean conditions, as their catalytic reduction is only possible under rich conditions. Therefore, under lean conditions, the NOx in the exhaust gas are temporarily stored in a lean NOx trap (LNT). Once the LNT's storage capacity is exhausted, a cycle with a rich exhaust gas mixture, or operating under substoichiometric conditions (λ < 1), is performed to regenerate the LNT. This regeneration process is also known as a rich purge.In this cycle, the temporarily stored nitrogen oxides are oxidized to nitrogen in a nitrogen oxide reduction catalyst, and the catalyst is freed up again for the storage of nitrogen oxides.

[0003] In publication EP 3 006 686 A1, a method for determining the amount of nitrogen oxide in exhaust gas exiting directly from an internal combustion engine is described, in which the exhaust gas is passed through an exhaust tract, a NOx storage catalyst and at least partially through exhaust gas recirculation, wherein the nitrogen oxide content is determined on the basis of an amount of ammonia determined downstream of the nitrogen oxide reduction catalyst in the exhaust gas.

[0004] In publication DE 10 2014 018 037 A1, the nitrogen oxide content is also determined as a function of the amount of nitrogen oxides downstream of the NOx storage catalyst. The raw emission of nitrogen oxides from an internal combustion engine is also determined as a function of how much nitrogen oxide is recirculated to the cylinders via a return line.

[0005] Further prior art relating to the background of the present invention is provided in publications DE 10 2014 210 448 A1 and DE 10 2014 018 032 A1.

[0006] To control the regeneration of the LNT (Low-temperature nitrogen) system and reduce the temporarily stored nitrogen oxides, it is important to determine the nitrogen oxide concentration in the exhaust gas tract immediately downstream of the internal combustion engine. Nitrogen oxide concentrations can be determined using nitrogen oxide sensors, the installation of which is quite expensive. However, a nitrogen oxide sensor immediately downstream of the internal combustion engine could be eliminated if the amount of nitrogen oxide in the exhaust gas exiting the engine directly could be determined based on a model.

[0007] The task is therefore to determine the amount of nitrogen oxide in the exhaust gas exiting directly from an internal combustion engine.

[0008] This problem is solved by a method having the features of claim 1. Further advantageous embodiments and configurations of the invention will become apparent from the dependent and sub-claims, the figures, and the exemplary embodiments.

[0009] A first aspect of the invention relates to a method for determining a quantity of nitrogen oxide (NOx). _out in the exhaust gas exiting directly from an internal combustion engine, in which the exhaust gas is routed through an exhaust tract, an exhaust aftertreatment system with at least one nitrogen oxide storage catalyst and at least one nitrogen oxide reduction catalyst and optionally at least partially through an exhaust gas recirculation line branching off from the exhaust tract downstream of the exhaust aftertreatment system, wherein the nitrogen oxide content is determined on the basis of a quantity of nitrogen oxides determined downstream of the nitrogen oxide reduction catalyst in the exhaust gas, wherein the nitrogen oxide content NOx _out with the equation NOx_out=NOx_all−f((NOx_all−NOx1), EGRrate) is determined, whereby NOx _out the amount of nitrogen oxide in the exhaust gas directly exiting the internal combustion engine is, NOx _allthe amount of nitrogen oxide in the exhaust gas exiting directly from the internal combustion engine, provided that no nitrogen oxide has been converted in the exhaust aftertreatment system and all nitrogen oxide is contained in the exhaust gas recirculated through the exhaust gas recirculation line, (NOx _all - NOx1), EGR rate the difference between the nitrogen oxide recirculated in the exhaust gas recirculation line and the total nitrogen oxide emitted from the internal combustion engine, and NO x1 The nitrogen oxide concentration at the branch of the exhaust gas recirculation line is measured.

[0010] This method advantageously allows the use of exhaust gas recirculation to determine the amount of exhaust gas exiting the internal combustion engine. This enables reliable control of the catalysts responsible for removing nitrogen oxides from the exhaust gas, namely the nitrogen oxide storage catalyst and / or the nitrogen oxide reduction catalyst. Furthermore, the regeneration frequency of the nitrogen oxide storage catalyst can be optimized by running the internal combustion engine on a richer fuel mixture, thus saving fuel. The method also advantageously allows for the optimization of the dosing of the aqueous urea solution.

[0011] The values ​​for NO x1 NOx_all and NOx_all are each determined or set by controlling the load and speed points of the internal combustion engine.

[0012] It has been shown that there is a correlation between the amount of nitrogen oxides (NOx) contained in the exhaust gas from the internal combustion engine and the portion of ammonia slip from the NOx reduction catalyst that is recirculated in the exhaust gas recirculation (EGR) line. The NOx reduction catalyst can store a certain amount of ammonia, which is added to the exhaust gas upstream of the catalyst in the form of an aqueous urea solution for NOx reduction. Once the storage capacity is exhausted, i.e., when more ammonia is present than is needed for NOx reduction in the exhaust gas, ammonia can escape from the catalyst. It has been shown that in exhaust gas recirculation systems, an increase in the amount of NOx in the exhaust gas exiting directly from the internal combustion engine coincides with the onset of ammonia slip.Furthermore, it has been shown that in exhaust gas recirculation systems, the amount of nitrogen oxides in the exhaust gas decreases when the amount of nitrogen oxides in the recirculated exhaust gas is reduced. From this, a determination of the amount of nitrogen oxides in the exhaust gas exiting directly from the internal combustion engine can be derived.

[0013] It is therefore preferred if the amount of ammonia in the exhaust gas is additionally determined in the process. The determination of the amount of ammonia can be carried out using suitable sensors, calculated from the measured nitrogen oxide concentration, or in another manner known to those skilled in the art. It is therefore particularly preferred if, in the process according to the invention, a correction factor fac(NH3, EGR rate) for the amount of ammonia in the recirculated exhaust gas is additionally added to equation (I), so that the nitrogen oxide content NOx out with the equation NOx_out=NOx_all−f((NOx_all−NOx1), EGR rate)+fac(NH3, EGR rate) is determined. The determination of the nitrogen oxide content NOx _out It also advantageously allows for optimal control of the dosage of urea in the exhaust tract upstream of the nitrogen oxide reduction catalyst.

[0014] Furthermore, it is preferred if the exhaust gas recirculation is a low-pressure exhaust gas recirculation system.

[0015] A second aspect of the invention relates to an arrangement for carrying out the method according to the invention, comprising an internal combustion engine, an exhaust system, a first exhaust aftertreatment device with a nitrogen oxide storage catalyst, a second exhaust aftertreatment device with a nitrogen oxide reduction catalyst, an exhaust gas recirculation system, a control device and a regulating device. wherein the control device is designed to record values ​​of quantities of nitrogen oxides downstream of the nitrogen oxide reduction catalyst and to calculate a quantity of nitrogen oxide NOx from this _out to determine, and wherein the control unit is designed to receive values ​​from the control unit and adjust them according to the determined amount of nitrogen oxides (NOx). _out to regulate the amount of ammonia or urea fed into the exhaust system.

[0016] The advantages of the arrangement correspond to those of the method according to the invention.

[0017] Preferably, in the arrangement according to the invention, the control device is further configured to record values ​​of amounts of ammonia downstream of the nitrogen oxide reduction catalyst (10a) and to use them in determining the amount of nitrogen oxide NOx. _out to incorporate it.

[0018] Preferably, in the arrangement according to the invention, the control device is further configured to control the regeneration of the nitrogen oxide storage catalyst arranged in the first exhaust aftertreatment device.

[0019] A third aspect of the invention relates to a motor vehicle with an arrangement according to the invention.

[0020] The invention is explained in more detail using the figures. They show: Fig. 1 a schematic diagram of an embodiment of an arrangement according to the invention. Fig. 2 a flowchart of an embodiment of a method according to the invention.

[0021] An arrangement 1 according to the invention has, in the embodiment shown in the illustration, Fig. 1. An internal combustion engine 2. The internal combustion engine 2 can be a compression-ignition or spark-ignition engine. The internal combustion engine 2 has three cylinders 3, but can also have a different number, e.g., two, four, five, six, or eight cylinders. The internal combustion engine 2 is connected to an exhaust tract 4. A turbine 5 of a turbocharger is arranged in the exhaust tract 4. The turbine 5 is connected via a shaft 6 to a compressor 7, which is arranged in the intake tract 8. A first catalytic device 9 is arranged downstream of the turbine 5 in the exhaust tract 4. This device particularly includes a nitrogen oxide storage catalyst 9a, but may also include an oxidation catalyst, a three-way catalyst, and / or a particulate filter. A second catalytic device 10 is arranged downstream of the first catalytic device 9. This device particularly includes a nitrogen oxide reduction catalyst 10a.Upstream of the second catalytic unit 10, there is a feed line from a reducing agent container 11, through which an aqueous urea solution can be introduced into the exhaust gas tract 4.

[0022] Downstream of the second catalytic unit 10, an exhaust gas recirculation line 12 (EGR line) of a low-pressure exhaust gas recirculation system branches off from the exhaust tract 4. The EGR line 12 opens into the intake tract 8 upstream of the compressor 7. The EGR line 12 has an exhaust gas recirculation valve 13, through which the EGR line 12 is connected to the exhaust tract 4.

[0023] In the area of ​​the branch of the EGR line 12, a nitrogen oxide sensor 14 and an ammonia sensor 15 are arranged. Both sensors are connected to a control unit 16 to record the amounts of ammonia and nitrogen oxides in the area of ​​the branch of the EGR line 12 and to calculate a nitrogen oxide quantity (NOx) from this. _outto determine. The control unit 16 is connected to a control unit 17, which can receive values ​​from the control unit 16 and adjust them according to the determined amount of nitrogen oxides (NOx). _out can initiate the introduction of an adequate amount of aqueous urea solution from the reducing agent container 11 into the exhaust gas tract 4.

[0024] In one embodiment of the method according to the description of Fig. 2. A quantity of nitrogen oxide (NOx) will be determined. _out determined in the exhaust gas exiting directly from the internal combustion engine 2. The method can be carried out according to Fig. The process is described in two steps. In step S1, the control unit 16 uses sensors 14 and 15 to record values ​​for the amounts of ammonia and nitrogen oxides in the area of ​​the branch of the EGR line 12. In a second step S2, the control unit 16 calculates these values ​​based on the formula... NOx_out=NOx_all−f((NOx_all−NOx1), EGRrate) the amount of nitrogen oxides NOx _out Preferably, the ammonia that escaped from the nitrogen oxide reduction catalyst 10a and is contained in the recirculated exhaust gas is also included in the NOx calculation. _out one. This results in the formula NOx_out=NOx_all−f((NOx_all−NOx1), EGR rate)+fac(NH3, EGR rate)

[0025] The obtained value of NOx _out In a third step, S3, the control unit 16 transmits the signal to the control unit 17. The control unit 17 then sends a control command to the reducing agent tank 11, which in turn triggers a response corresponding to the NOx level. _out The appropriate amount of aqueous urea solution is introduced into the exhaust gas tract 4. The urea is hydrolyzed into ammonia and carbon dioxide, and the ammonia is stored in the nitrogen oxide reduction catalyst 10a in an amount necessary to reduce the current amount of nitrogen oxides in the exhaust gas. Reference symbol list 1. Arrangement 2 Internal combustion engine 3 cylinders 4 Exhaust system 5 Turbine 6 wave 7 Compressor 8 Intake tract 9 first catalytic device 9a Nitrogen oxide storage catalyst 10 second catalytic unit 10a Nitrogen oxide reduction catalyst 11 reducing agent containers 12 Exhaust gas recirculation line 13 Exhaust gas recirculation valve 14 Nitrogen oxide sensor 15 Ammonia sensor 16 Control device 17 Control unit

Claims

[1] Method for determining a quantity of nitrogen oxide NOx _out in the exhaust gas emerging directly from an internal combustion engine (2), in which the exhaust gas is passed through an exhaust tract (4), an exhaust gas aftertreatment system with at least one nitrogen oxide storage catalyst (9a) and at least one nitrogen oxide reduction catalyst (10a) and optionally at least partially through an exhaust gas recirculation line (12) branching off from the exhaust tract downstream of the exhaust gas aftertreatment system, wherein the nitrogen oxide content is determined on the basis of a quantity of nitrogen oxides determined in the exhaust gas downstream of the nitrogen oxide reduction catalyst (10a), characterized by that the nitrogen oxide content NOx _out with the equation NOx_out=NOx_all−f((NOx_all−NOx1), EGRrate) is determined, whereby NOx _out the amount of nitrogen oxide in the exhaust gas directly emitted from the internal combustion engine, NOx _allthe quantity of nitrogen oxide in the exhaust gas directly exiting the internal combustion engine is under the condition that no nitrogen oxide has been converted in the exhaust gas aftertreatment system and all nitrogen oxide is contained in the exhaust gas recirculated through the exhaust gas recirculation line (12), (NOx _all - NOx1), EGR rate the difference between the nitrogen oxide recirculated in the exhaust gas recirculation line (12) and the total nitrogen oxide exiting the internal combustion engine (2), and NO x1 the nitrogen oxide concentration at the branch of the exhaust gas recirculation line (12). [2] Method according to claim 1, wherein the amount of ammonia in the exhaust gas is additionally determined. [3] Method according to claim 2, wherein a correction factor fac(NH3, EGR rate) for the amount of ammonia in the recirculated exhaust gas is added to equation (I) so that the nitrogen oxide content NOx _out with the equation NOx_out=NOx_all−f((NOx_all−NOx1), EGR rate)+fac(NH3, EGR rate) is determined. [4] Method according to claim 1 or 2, wherein the exhaust gas recirculation (12) is a low-pressure exhaust gas recirculation. [5] Arrangement (1) for carrying out a method according to one of the preceding claims, comprising an internal combustion engine (2), an exhaust tract (4), a first exhaust gas aftertreatment device (9) with a nitrogen oxide storage catalyst (9a) and a second exhaust gas aftertreatment device (10) with a nitrogen oxide reduction catalyst (10a), an exhaust gas recirculation (12), a control device (16) and a control device (17), wherein the control device (16) is designed to record values ​​of quantities of nitrogen oxides downstream of the nitrogen oxide reduction catalyst (10a) and to calculate therefrom a nitrogen oxide quantity NOx _outand wherein the control device (17) is designed to receive values ​​from the control device (16) and, in accordance with the determined amount of nitrogen oxide NOx _out to regulate the amount of ammonia or urea fed into the exhaust tract (4). [6] Arrangement (1) according to claim 5, wherein the control device (16) is further designed to record values ​​of quantities of ammonia downstream of the nitrogen oxide reduction catalyst (10a) and to include them in the determination of the nitrogen oxide quantity NO x_out to incorporate. [7] Arrangement (1) according to claim 5 or 6, wherein the control device (16) is further designed to control the regeneration of the nitrogen oxide storage catalyst (9a) arranged in the first exhaust gas aftertreatment device (9). [8] Motor vehicle with an arrangement according to one of claims 5-7.

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