DE102017200851B4 - Cylinder deactivation to reduce ammonia slip - Google Patents

Abstract

Method for controlling exhaust gas aftertreatment in a system comprising an internal combustion engine (2) with at least two cylinders (3), the internal combustion engine (2) being designed to operate all cylinders (3) in the active state in a first operating state and in a second operating state to switch off at least one cylinder (3), - an exhaust tract (4) with at least one exhaust gas aftertreatment device (9), wherein the exhaust gas aftertreatment device (9) comprises at least one catalytic converter for selective catalytic reduction (9a), - an exhaust gas recirculation line (10) of a low-pressure Exhaust gas recirculation system, which branches off from the exhaust tract (4) downstream of the exhaust gas aftertreatment device (9) and fluidly connects the exhaust tract (4) to the intake tract (7) of the internal combustion engine (2), comprising the steps of: - operating the internal combustion engine (2), - determining a Ammonia slip from the selective catalytic reduction catalyst (9a),- comparing the ammonia iak slip with a predetermined threshold value of the ammonia content in the exhaust tract (4) downstream of the first selective reduction catalyst (9a),- deactivating at least one cylinder (3) when the threshold value is exceeded, with at least one cylinder (3) remaining activated,- or Continuing to operate the internal combustion engine (2) if the threshold value is not exceeded.

Description

The invention relates to a method for reducing ammonia slip from a catalytic converter for selective catalytic reduction within an exhaust gas recirculation circuit.

In the case of internal combustion engines, catalytic after-treatment of the exhaust gases has become established in order to comply with the legally prescribed emission values. To increase efficiency, modern internal combustion engines often work with lean fuel-air mixtures with an excess of oxygen. Nitrogen oxides that occur cannot be reduced in lean operation because their catalytic reduction is only possible under substoichiometric operating conditions. The nitrogen oxides in the exhaust gas are therefore temporarily stored in a NOx storage catalytic converter (lean NOx trap, LNT) during lean operation. If the absorption capacity of the LNT is exhausted, a cycle with a rich exhaust gas mixture or in sub-stoichiometric operation (λ < 1) is carried out to regenerate the LNT. Such a regeneration is also referred to as a rich purge. In this cycle, the intermediately stored nitrogen oxides in the LNT and, if present, in a nitrogen oxide reduction catalytic converter are reduced, with the nitrogen oxide storage capacity of the LNT being regenerated for the storage of nitrogen oxides.

A nitrogen oxide reduction catalytic converter, also referred to as a catalytic converter for selective catalytic reduction or SCR catalytic converter, can store a certain amount of ammonia, which is added to the exhaust gas in the form of an aqueous urea solution or in gaseous form upstream of the SCR catalytic converter to reduce nitrogen oxides.

Ammonia in a form as described above can, for example, be provided from an external source or also be formed in the area of the exhaust gas tract from nitrogen and hydrogen in an exhaust gas aftertreatment device. In the pamphlet DE 10 2010 008 052 A1 it is disclosed that hydrogen and nitrogen oxides for producing ammonia are provided by means of selected cylinders. In some cases, cylinders are switched off to increase the load on the cylinders that remain activated and thus increase the temperature in the relevant cylinders in order to enable the production of certain exhaust products such as hydrogen. The ammonia is then formed from the substances mentioned, hydrogen and nitrogen oxides, in a three-way catalytic converter which is arranged in the exhaust system.

If the memory function is exhausted, i. H. if there is more ammonia than is required to reduce the nitrogen oxides in the exhaust gas, ammonia can escape from the catalytic converter. This unwanted escape of ammonia is called ammonia slip. There is a higher likelihood of ammonia slip under high flow and/or high temperature conditions. If the SCR catalytic converter is arranged upstream of a branch of an exhaust gas recirculation line, the ammonia can be recirculated into the internal combustion engine. The loss of an amount of ammonia through slip must be replaced by an amount that is significantly higher than a factor of one of the recirculated amount, since part or a large part of the ammonia recirculated with the exhaust gas is converted into additional nitrogen oxides in the internal combustion engine . As the SCR catalytic converter ages, the amount of ammonia that can be stored decreases, so that the ammonia slip increases even more.

The object is to limit the amount of ammonia in the recirculated exhaust gas.

This object is solved by a method having the features of claim 1. Further advantageous configurations of the invention result from the secondary and subclaims, the figures and the exemplary embodiments.

• - eine Brennkraftmaschine mit mindestens zwei Zylindern, wobei die Brennkraftmaschine ausgebildet ist, in einem ersten Betriebszustand alle Zylinder im aktiven Zustand zu betreiben und in einem zweiten Betriebszustand mindestens einen Zylinder abzuschalten,
• - einen Abgastrakt mit mindestens einer Abgasnachbehandlungseinrichtung, wobei die Abgasnachbehandlungseinrichtung mindestens einen Katalysator zur selektiven katalytischen Reduktion umfasst,
• - eine Abgasrückführungsleitung eines Niederdruck-Abgasrückführungssystems, die stromabwärts der Abgasnachbehandlungseinrichtung vom Abgastrakt abzweigt und den Abgastrakt fluid mit dem Ansaugtrakt der Brennkraftmaschine verbindet,

umfassen die Schritte:

• - Betreiben der Brennkraftmaschine,
• - Ermitteln eines Ammoniakschlupfes aus dem ersten Katalysator zur selektiven katalytischen Reduktion,
• - Vergleichen des Ammoniakschlupfes mit einem vorbestimmten Schwellenwert des Ammoniakgehalts im Abgastrakt stromabwärts des Katalysators zur selektiven Reduktion,
• - Deaktivieren mindestens eines Zylinders, wenn der Schwellenwert überschritten wird, wobei mindestens ein Zylinder aktiviert bleibt,
• - oder Weiterbetreiben der Brennkraftmaschine, wenn der Schwellenwert nicht überschritten wird.

A first aspect of the invention relates to a method for controlling exhaust gas aftertreatment in a system

• - an internal combustion engine with at least two cylinders, wherein the internal combustion engine is designed to operate all cylinders in the active state in a first operating state and to switch off at least one cylinder in a second operating state,
• - an exhaust tract with at least one exhaust gas aftertreatment device, wherein the exhaust gas aftertreatment device comprises at least one catalyst for selective catalytic reduction,
• - an exhaust gas recirculation line of a low-pressure exhaust gas recirculation system, which branches off from the exhaust tract downstream of the exhaust gas aftertreatment device and fluidly connects the exhaust tract to the intake tract of the internal combustion engine,

include the steps:

• - operation of the internal combustion engine,
• - determining an ammonia slip from the first catalytic converter for selective catalytic reduction,
• - Comparing the ammonia slip with a predetermined threshold of Ammonia content in the exhaust tract downstream of the catalytic converter for selective reduction,
• - deactivating at least one cylinder when the threshold is exceeded, leaving at least one cylinder activated,
• - Or continue to operate the internal combustion engine if the threshold value is not exceeded.

The method advantageously reduces the consumption of ammonia to be introduced, particularly in the form of a commercially available urea solution such as AdBlueO, which saves costs. Furthermore, the method is also economical in terms of space, since the size of the urea solution or ammonia tank can be kept small in comparison with conventional ones. In addition, carbon dioxide emissions are minimized.

Preferably, in the method, the ammonia slip is determined using a kinetic model for predicting ammonia slip from the first selective catalytic reduction catalyst. In particular, the kinetic model is created as a function of the temperature, the volume flow and/or the amount of nitrogen oxide stored.

In the method, the ammonia slip is preferably determined by means of at least one ammonia sensor. The ammonia sensor can be used as an alternative to the kinetic model. The ammonia sensor can also be used in addition to the kinetic model, in particular a value predicted by the model can be confirmed.

Furthermore, the ammonia slip can also be determined using at least one nitrogen oxide sensor. Nitrogen oxide sensors have a high affinity for ammonia; the person skilled in the art knows how to distinguish between values caused by nitrogen oxides and ammonia. The nitrogen oxide sensor can be used as an alternative or in addition to the kinetic model.

Multiple cylinders are preferably deactivated in the method. It is assumed that the internal combustion engine has more than one cylinder, e.g. B. two, three, four or more. Deactivation of multiple cylinders is advantageous because, depending on the specific conditions, it may be necessary when there are operating conditions where there is a risk of ammonia slip. In particular, several cylinders can be gradually deactivated if the conditions make this necessary. Depending on the number of cylinders and the operating conditions, e.g. B. in a four-cylinder engine one, two or three cylinders can be deactivated to achieve conditions that counteract ammonia slip.

In a preferred further embodiment of the method, an ammonia slip is additionally determined by means of a kinetic model under the condition that at least one cylinder has been deactivated. The same kinetic model as above is used. The values are adjusted to the selected conditions. In the process, an optimal operating mode is determined, which is present at a minimum ammonia slip, that is to say ideally at an ammonia slip of zero. The criteria for the ammonia slip can be combined with other criteria, e.g. B. for the reduction of nitrogen oxides, the particle emissions of the internal combustion engine, etc.

In a preferred further embodiment of the method, a model-predictive control is additionally used to determine an ammonia slip. A time-discrete dynamic model of the process to be controlled is used to calculate the future behavior of the process depending on the corresponding input signals. Specifically, a selective catalytic reduction kinetic model is used here to predict future ammonia slip based on driver demand. Advantageously, the prediction window can be short in time if the prediction is only based on one value, e.g. B. based on the exhaust gas volume flow.

A second aspect of the invention relates to an arrangement for carrying out a method according to one of the preceding claims, comprising a multi-cylinder internal combustion engine designed for cylinder deactivation with an exhaust gas tract with at least one exhaust gas aftertreatment device, the exhaust gas aftertreatment device having at least one catalyst for selective catalytic reduction, and an exhaust gas recirculation line of a low pressure -Exhaust gas recirculation system, which branches off from the exhaust tract downstream of the second exhaust gas aftertreatment device and fluidly connects the exhaust tract to the intake tract of the internal combustion engine. The advantages of the arrangement according to the invention correspond to the advantages of the method according to the invention.

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

• 1 eine schematische Darstellung einer Ausführungsform der erfindungsgemäßen Anordnung.
• 2 ein Fließdiagramm einer Ausführungsform des erfindungsgemäßen Verfahrens.

The invention is explained in more detail with reference to the figures. Show it:

• 1 a schematic representation of an embodiment of the arrangement according to the invention.
• 2 a flow diagram of an embodiment of the method according to the invention.

An inventive arrangement 1 has in the embodiment shown in FIG 1 an internal combustion engine 2 on. The internal combustion engine 2 can be a self-igniting or spark-ignited internal combustion engine. The arrangement 1 is provided in particular in a motor vehicle, but can alternatively also be provided in other vehicles. The internal combustion engine 2 has three cylinders 3, but can also have a different number, e.g. B. two, four, five, six or eight cylinders. The internal combustion engine 2 is connected to an exhaust system 4 . A turbine 5 of a turbocharger is arranged in the exhaust tract 4 . The turbine 5 is connected via a shaft to a compressor 6 which is arranged in the intake duct 7 . A first exhaust gas aftertreatment device 8 is arranged in the exhaust tract 4 downstream of the turbine 5 . The first exhaust gas aftertreatment device 8 has an oxidation catalyst, e.g. B. a diesel oxidation catalyst, and / or a passive nitrogen oxide adsorber (PNA) and / or a nitrogen oxide storage catalyst (LNT). A second exhaust gas aftertreatment device 9 is arranged downstream of the first exhaust gas aftertreatment device 8 and has in particular a first catalytic converter for selective catalytic reduction (SCR catalytic converter) 9a. The first SCR catalytic converter 9a is combined with a particle filter 9b. The particle filter 9b is z. B. coated with the SCR catalyst 9a. Alternatively, the particle filter 9b and the SCR catalytic converter 9a can also be arranged individually within the second exhaust gas aftertreatment device 9 . It could also be arranged only the SCR catalytic converter 9a in the second exhaust gas treatment device 9, and the particle filter 9b at a different point in the exhaust tract.

An exhaust gas recirculation line 10 (EGR line) of a low-pressure exhaust gas recirculation system branches off from the exhaust tract 4 downstream of the second exhaust gas aftertreatment device 9 . The EGR line 10 opens into the intake tract 7 upstream of the compressor 6 . The EGR line 10 has an exhaust gas recirculation valve 11 . Furthermore, the EGR line 10 has an exhaust gas recirculation cooler device 12 . The cooling device 12 can have a bypass. A combination valve, which is designed for mixing fresh air and recirculated exhaust gas (not shown), can be arranged in the area where the exhaust gas recirculation line 10 opens into the intake tract 7 . A throttle device can also be arranged in this area (not shown).

A third exhaust gas aftertreatment device 13 is shown downstream of the branch of the EGR line 10, which is optional and has, for example, a second SCR catalytic converter. The second SCR catalytic converter is provided for further nitrogen oxide reduction. Furthermore, he can also store ammonia, the z. B. escaped upstream from the first SCR catalytic converter 9a and was not returned to the intake tract 7 via the EGR line 10 . A feed line from a reducing agent tank, via which an aqueous urea solution or gaseous ammonia can be introduced into the exhaust tract 4 (not shown), can be arranged upstream of the third exhaust gas aftertreatment device 13 . A feed line for a reducing agent can also be arranged upstream of the second exhaust gas aftertreatment device 9 (not shown).

An ammonia sensor 14 and a nitrogen oxide sensor 15 are arranged in the exhaust tract 4 downstream of the second exhaust gas aftertreatment device 9 . Corresponding sensors 14, 15 can also be arranged at other points in the exhaust system 4, e.g. B. downstream of the third exhaust gas treatment device 13 or in the EGR line 10. Other sensors, z. B. temperature sensors and lambda probes can be arranged at various points in the exhaust tract 4. The sensors 14, 15 are connected to a control device (not shown), which can receive measured values from the sensors and, within the meaning of the invention, can determine an ammonia slip from the first SCR catalytic converter 9a. On the basis of the measured values, the concentration of the escaped ammonia can be determined in addition to the detection of an ammonia slip itself.

Furthermore, the arrangement 1 has an intercooler 16 which is arranged in the intake duct 7 . Furthermore, the arrangement 1 has a high-pressure exhaust gas recirculation line 17 which branches off from the exhaust gas tract 4 upstream of the turbine 5 and opens into the intake tract 8 downstream of the intercooler 16 .

In a method for controlling an exhaust gas aftertreatment with an arrangement 1 described above, the internal combustion engine 2 is operated in a first step S1. All cylinders are in the activated state. In a second step S2, it is determined whether there is ammonia slip from the first SCR catalytic converter 9a. In principle, an ammonia slip can be determined on the basis of sensor values by means of the control device. The control device is designed to determine the ammonia concentration both from the values measured by means of the ammonia sensor 14 and from the values measured by means of the nitrogen oxide sensor 15 .

In particular, ammonia slip can be predicted based on a kinetic model. The temperature of the first SCR catalytic converter 9a and/or the exhaust gas, the exhaust gas volume flow and the quantity of ammonia in the first SCR catalytic converter 9a (as a measure of the utilization of the ammonia storage capacity ) as functions in the kinetic model.

In one embodiment of the invention, an ammonia slip can also be determined in step S2 using a kinetic model under the condition that at least one cylinder has been deactivated. In other words, depending on the number of deactivated cylinders, a corresponding ammonia slip is calculated on the basis of a model. The same kinetic model is used as above, some values, in particular the values taken downstream of the turbine 5, being of course different. In the process, an optimal operating mode is determined, which is present at a minimum ammonia slip, that is to say ideally at an ammonia slip of zero. The criteria for the ammonia slip can be combined with other criteria, e.g. B. for the reduction of nitrogen oxides, the particle emissions of the internal combustion engine, etc.

In a further embodiment of the invention, an ammonia slip can additionally be created in step S2 using a model-predictive control for determining an ammonia slip. A selective catalytic reduction kinetic model is used to predict future ammonia slip based on driver demand. For example, in the case of a full throttle request, when the driver of a corresponding motor vehicle presses the accelerator pedal, an increase in the exhaust gas volume flow is provided for the model before the volume flow has actually increased. In this way, ammonia slip can be predicted up to several seconds before it actually occurs. Accordingly, if ammonia slip is predicted to be greater than a threshold, one or more cylinders may be preemptively deactivated. On the basis of the volume flow, it is therefore possible to work with a very short forecast window.

Since the temperature and the ammonia loading of the SCR catalyst change more slowly than the volume flow, a longer forecast window can be used on the basis of these parameters. Temperature values and volume flow values of the exhaust gas recorded under fully activated cylinders of internal combustion engine 2 and under gradually deactivated cylinders can flow into the model in order to predict ammonia slip. In this way, a request from the driver can also be included in the calculation of exhaust gas temperatures. A calculation of the ammonia loading of the first SCR catalytic converter 9a can be determined on the basis of technical values of the catalytic converter 9a and the amount of ammonia introduced, with the manner of calculating being known to those skilled in the art.

If ammonia slip is determined, the determined value is compared in a third step S3 with a first threshold value of the ammonia content in the exhaust gas, which was predetermined. The first threshold corresponds z. B. a certain permissible value that should not be exceeded. If the first threshold value of the ammonia content in the exhaust gas is exceeded, a cylinder 3 of the internal combustion engine 2 is deactivated in a fourth step S4a. Depending on the determined concentration of ammonia, two cylinders 3 can be deactivated if z. B. the concentration has reached a value which is significantly higher than the first threshold value. This can e.g. B. a second threshold can be predetermined, when reached the two cylinders are deactivated. Different threshold values can therefore be specified, which correspond to the deactivation of a specific number of cylinders. For internal combustion engines with more than three cylinders, the method can be continued accordingly, so that z. B. with six cylinders depending on the level of ammonia slip up to five cylinders can be gradually deactivated.

However, if the first threshold is not exceeded, no cylinder is deactivated. The method is then returned from step S3 in an alternative fourth step S4b to step S2 until an ammonia content in the exhaust gas downstream of the second exhaust gas aftertreatment device 9 is determined.

reference list

1

arrangement

2

internal combustion engine

3

cylinder

4

exhaust tract

5

turbine

6

compressor

7

intake tract

8th

first exhaust aftertreatment device

9

second exhaust aftertreatment device

9a

first catalyst for selective catalytic reduction

9b

particle filter

10

Low pressure exhaust gas recirculation line

11

exhaust gas recirculation valve

12

exhaust gas cooler device

13

third exhaust aftertreatment device

14

ammonia sensor

15

nitrogen oxide sensor

16

Intercooler device

17

High pressure exhaust gas recirculation line

Claims (11)

A method for controlling exhaust gas aftertreatment in a system comprising - an internal combustion engine (2) with at least two cylinders (3), wherein the internal combustion engine (2) is designed to operate all cylinders (3) in the active state in a first operating state and to switch off at least one cylinder (3) in a second operating state, - an exhaust tract (4) with at least one exhaust gas aftertreatment device (9), wherein the exhaust gas aftertreatment device (9) comprises at least one catalyst for selective catalytic reduction (9a), - an exhaust gas recirculation line (10) of a low-pressure exhaust gas recirculation system, which branches off from the exhaust tract (4) downstream of the exhaust gas aftertreatment device (9) and fluidly connects the exhaust tract (4) to the intake tract (7) of the internal combustion engine (2), comprising the steps: - Operating the internal combustion engine (2), - Determining an ammonia slip from the catalyst for selective catalytic reduction (9a), - Comparing the ammonia slip with a predetermined threshold value of the ammonia content in the exhaust tract (4) downstream of the first catalytic converter for selective reduction (9a), - deactivating at least one cylinder (3) if the threshold is exceeded, leaving at least one cylinder (3) activated, - Or continue to operate the internal combustion engine (2) if the threshold value is not exceeded. procedure after claim 1 wherein the ammonia slip is determined using a kinetic model for predicting ammonia slip from the selective catalytic reduction catalyst (9a). procedure after claim 2 , whereby the kinetic model is created as a function of the temperature, the volume flow and/or the amount of ammonia input into the model. Method according to one of the preceding claims, wherein the ammonia slip is determined by means of at least one ammonia sensor (14). Method according to one of the preceding claims, in which the ammonia slip is determined by means of at least one nitrogen oxide sensor (15). Method according to one of the preceding claims, in which several cylinders (3) are deactivated. procedure after claim 6 , whereby the cylinders (3) are gradually deactivated. Method according to one of the preceding claims, wherein the ammonia slip is additionally determined by means of a further kinetic model under the condition that at least one cylinder (3) has been deactivated. Method according to one of the preceding claims, wherein a model predictive control is additionally used. Arrangement (1) for carrying out a method according to one of the preceding claims, comprising a multi-cylinder internal combustion engine (2) designed for cylinder deactivation and having an exhaust gas tract (4) with at least one exhaust gas aftertreatment device (9), the exhaust gas aftertreatment device (9) having at least one catalytic converter for selective catalytic Reduction (9a) and an exhaust gas recirculation line of a low-pressure exhaust gas recirculation system (10), which branches off from the exhaust tract (4) downstream of the exhaust gas aftertreatment device (9) and fluidly connects the exhaust tract (4) to the intake tract (7) of the internal combustion engine (7). Motor vehicle with an arrangement according to claim 10 .

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