DE102005050709B4 - Method for operating an exhaust gas aftertreatment system - Google Patents

Abstract

Method for operating an exhaust gas aftertreatment system of an internal combustion engine, the exhaust gas aftertreatment system having: an SCR catalytic converter (10) for the selective catalytic reduction of nitrogen oxides, a reducing agent addition device (12) upstream of the SCR catalytic converter (10), one downstream of the SCR catalytic converter (10) arranged exhaust gas sensor (16) which is sensitive to nitrogen oxides and ammonia, and a control device (14) for regulating the reducing agent addition device (12) based on an ammonia fill level model of the SCR catalytic converter (10) in such a way that the ammonia fill level of the SCR catalytic converter (10) is set to a predeterminable level The target filling level is regulated, characterized by the method steps: - detecting an output signal (N1) of the exhaust gas sensor (16) downstream of the SCR catalytic converter (10) during overrun operation of the internal combustion engine; and- if the detected output signal (N1) exceeds a predetermined threshold value (T1), in an adaptation step, adapting the current modeled ammonia fill level of the SCR catalytic converter (10) to a predetermined limit fill level.

Description

The invention relates to a method for operating an exhaust gas aftertreatment system of an internal combustion engine according to the preamble of claim 1 and an exhaust gas aftertreatment system of an internal combustion engine according to the preamble of claim 10.

To reduce harmful emissions from motor vehicles, catalytic converters are used in the exhaust system. These also include a so-called SCR catalytic converter for the selective catalytic reduction of nitrogen oxides using a reducing agent added to the exhaust gas stream. In the case of urea-based SCR catalysts, an aqueous urea solution (HWL) is added to the exhaust gas stream upstream of the SCR catalyst. If the exhaust gas temperatures are sufficient, the urea ultimately breaks down into ammonia (NH ₃ ). The ammonia is adsorbed in the SCR catalytic converter and reduces the nitrogen oxides (NO _x ) in the exhaust gas stream on the catalyst surface, provided the catalytically active components have been chemically activated by sufficient temperatures.

The ammonia dosage should be in a stoichiometric ratio to the nitrogen oxide emission at the catalyst inlet in order to compensate for storage depletions due to nitrogen oxide reduction. Especially at low conversion rates, which occur, for example, due to high space velocities and/or temperatures, it can happen that the adsorption storage is already full and the ammonia leaves the SCR catalyst without contributing to the nitrogen oxide reduction. This phenomenon leads to an emission of ammonia at the catalyst outlet, the so-called “ammonia slip”.

The DE 101 26 456 A1 shows a device and a method for removing nitrogen oxides from the exhaust gas of lean-burn internal combustion engines using a reducing agent containing ammonia, in which a nitrogen oxide reduction catalyst divided into at least two separate parts is used. A sensor is provided on the output side of each catalyst part, which detects the ammonia slip of the respective catalyst part. The reducing agent is added in a controlled manner based on the ammonia slip recorded in this way. In this way, a differentiated assessment of the entire catalyst volume is possible, and the nitrogen oxide conversion can be improved compared to an integral measurement of a catalyst volume of the same size. However, the costs for the sensors are disadvantageous. In addition, the ammonia slip represents a quantity that only indirectly characterizes the catalyst state. Furthermore, slip-controlled addition of reducing agent proves to be difficult if ammonia slip is to be completely avoided.

If the exhaust aftertreatment system with SCR catalytic converter and reducing agent addition device is controlled by a control unit based on a fill level model for the SCR catalytic converter, the fill level model must be based on a trade-off between a maximum nitrogen oxide reduction and a minimum ammonia slip.

From the DE 103 47 130 A1 an exhaust gas aftertreatment system with an SCR catalytic converter and an HWL addition device is known, in which the amount of ammonia stored in the SCR catalytic converter (fill level) is estimated in the form of a complex fill level model. Furthermore, the nitrogen oxide concentrations are measured upstream and downstream of the SCR catalytic converter and an actual fill level of the SCR catalytic converter is determined from the difference between these measured values. The fill level model can then be adapted to the actual fill level of the SCR catalytic converter determined in this way.

Further describes the DE 102 28 660 A1 the applicant provides an exhaust gas aftertreatment system with an SCR catalytic converter, in which the ammonia fill level model of the SCR catalytic converter is corrected as a function of a rate of change in the temperature of the exhaust gas, a rate of change in the temperature of the SCR catalytic converter and/or a rate of change in the exhaust gas mass flow, in order to simultaneously minimize reducing agent slip with a high nitrogen oxide reduction.

The DE 42 17 552 C1 discloses an exhaust gas aftertreatment system for internal combustion engines with a catalyst for the selective catalytic reduction of nitrogen oxides from exhaust gases with superstoichiometric addition of ammonia or ammonia-releasing substances. A first sensor that detects the ammonia concentration contained in the exhaust gas interrupts the addition of the amount of ammonia when a predetermined upper threshold value is reached. A second sensor, which detects the ammonia adsorbed in the catalyst, restarts the addition of ammonia when a predetermined lower threshold value is reached.

From the EP 775 013 B1 a method is known in which a reducing agent is introduced into the exhaust gas in the flow direction of the exhaust gas in front of an SCR catalytic converter, the reducing agent only being used during the starting phase of an internal combustion engine and when operating with a falling and possibly almost constant exhaust gas temperature, taking into account the temperature-dependent Storage capacity of the SCR catalyst for the reducing agent is metered in super-stoichiometrically in relation to the nitrogen oxide concentration and the reducing agent is otherwise metered in sub-stoichiometrically.

The object of the invention is to provide a method for operating an exhaust gas aftertreatment system with an SCR catalytic converter, which minimizes ammonia slip by appropriately adapting the fill level model to actual states of the SCR catalytic converter. In a continuation, this minimization should also be possible in the long term.

This object is achieved by a method for operating an exhaust gas aftertreatment system of an internal combustion engine, wherein the exhaust gas aftertreatment system has an SCR catalytic converter for the selective catalytic reduction of nitrogen oxides, a reducing agent addition device upstream of the SCR catalytic converter, and an exhaust gas sensor which is arranged downstream of the SCR catalytic converter and is sensitive to nitrogen oxides and ammonia and a control device for regulating the reducing agent addition device based on an ammonia fill level model of the SCR catalytic converter such that the ammonia fill level of the SCR catalytic converter is adjusted to a predeterminable target fill level. The method is characterized by detecting an output signal of the exhaust gas sensor downstream of the SCR catalytic converter during overrun operation of the internal combustion engine; and, if the detected output signal exceeds a predetermined threshold value, in an adaptation step, adapting the current modeled ammonia fill level of the SCR catalytic converter to a predetermined limit fill level.

The nitrogen oxide concentration measured, for example, using a NO _x sensor is known to have a cross-sensitivity to ammonia, so that an ammonia slip can be detected in this way with the exhaust gas sensor. The method evaluates the nitrogen oxide concentration downstream of the SCR catalytic converter during overrun operation of the internal combustion engine, ie during a phase in which no fuel injection takes place. If the measured nitrogen concentration during overrun shows values greater than zero, this is clearly ammonia slip. This means that the SCR catalytic converter has reached or exceeded its ammonia storage capacity. The current modeled ammonia level is therefore adjusted to a predetermined limit level. This prevents the addition of the reducing agent in order to ultimately reduce the ammonia slip. Due to the evaluation of the actual ammonia concentrations during overrun operation of the internal combustion engine, ammonia slip can be clearly identified in comparison to conventional systems, whereby the operation of the exhaust gas aftertreatment system can be optimized more reliably. The target fill level of the SCR catalytic converter is the highest possible ammonia fill level (generally smaller than the predetermined limit fill level or the storage capacity of the catalytic converter), at which ammonia slip can be safely avoided in any case (ie for different operating points of the internal combustion engine).

In the invention, the current modeled ammonia level of the SCR catalytic converter is adapted to the predetermined limit level (so-called short-term adaptation) using the method described above.

In a further embodiment of the invention, the number of times the threshold value is exceeded is additionally recorded by the detected nitrogen oxide concentration, so that if in a predetermined route or time interval a number of overrun operating states with an adjustment of the current modeled ammonia fill level exceeds a predetermined threshold value, a correction factor for reduction the amount of reducing agent added and/or the target filling level is determined (so-called long-term adaptation). Here, the specific correction factor is preferably stored in a non-volatile memory of the control device so that it is also taken into account the next time the internal combustion engine is started.

The above correction factor can be determined, for example, depending on a time until the threshold value is exceeded by the number of exceedances and / or on the size of the threshold value with regard to the detected nitrogen oxide concentration.

In a further embodiment of the invention, the current modeled ammonia fill level of the SCR catalytic converter is only adapted to the limit fill level if the detected nitrogen oxide concentration exceeds the predetermined threshold value for a predetermined minimum period of time. This measure means that random, non-significant fluctuations in the measurement results of the nitrogen oxide concentration cannot lead to an incorrect correction of the level model.

In a yet further embodiment of the invention, a nitrogen oxide concentration upstream of the SCR catalytic converter is also detected. In this case, the current modeled ammonia level of the SCR catalyst can be adjusted to the limit level if the size ratio of Aus output signal of the exhaust gas sensor and nitrogen oxide concentrations upstream of the SCR catalytic converter exceeds a predetermined threshold value. This measure further increases the accuracy of the method, since the nitrogen oxide emissions of the internal combustion engine are not always exactly zero, even in overrun mode.

In this embodiment, it is further possible to adapt the current modeled ammonia level of the SCR catalyst to the limit level of the SCR catalyst only if the size ratio exceeds the predetermined threshold for a predetermined minimum period of time. This allows random, non-significant fluctuations in the measurement results of the nitrogen oxide concentration to be eliminated.

• 1 eine schematische Darstellung des Aufbaus eines Abgasnachbehandlungssystems, bei welchem die vorliegende Erfindung angewendet werden kann;
• 2 ein Flussdiagramm des Verfahrensablaufs zum Betrieb des Abgasnachbehandlungssystems von 1 gemäß einem ersten Ausführungsbeispiel der Erfindung;
• 3 ein Flussdiagramm des Verfahrensablaufs zum Betrieb des Abgasnachbehandlungssystems von 1 gemäß einem zweiten Ausführungsbeispiel der Erfindung; und
• 4 ein Flussdiagramm des Verfahrensablaufs zum Betrieb des Abgasnachbehandlungssystems von 1 gemäß einem dritten Ausführungsbeispiel der Erfindung.

The above and other features and advantages of the invention will be better understood from the following description of preferred, non-limiting embodiments of the invention with reference to the accompanying drawings. Show in it:

• 1 a schematic representation of the structure of an exhaust aftertreatment system to which the present invention can be applied;
• 2 a flowchart of the process sequence for operating the exhaust gas aftertreatment system 1 according to a first embodiment of the invention;
• 3 a flowchart of the process sequence for operating the exhaust gas aftertreatment system 1 according to a second embodiment of the invention; and
• 4 a flowchart of the process sequence for operating the exhaust gas aftertreatment system 1 according to a third embodiment of the invention.

Referring to 1 The components of an exhaust gas aftertreatment system of an internal combustion engine that are relevant to the present invention will now be explained.

The exhaust gas aftertreatment system in particular has an SCR catalytic converter 10 arranged in the exhaust system of an internal combustion engine (not shown) for the selective catalytic reduction of nitrogen oxides (NO _x ) in the exhaust gas stream. Upstream of this SCR catalyst 10, a urea metering valve 12 is provided in a known manner as a reducing agent addition device for adding an aqueous urea solution (HWL) into the exhaust gas stream. At sufficiently high temperatures of the exhaust gas stream, the injected HWL is finally broken down into ammonia (NH ₃ ), which is adsorbed in the SCR catalytic converter 10. The reduction of nitrogen oxides then takes place with the adsorbed ammonia on the catalyst surface.

A first exhaust gas sensor 16 is provided downstream of the SCR catalytic converter 10, and a second exhaust gas sensor 18 in the form of an NO _x sensor is provided upstream of the urea injector 12 (and thus also upstream of the SCR catalytic converter 10). The first exhaust gas sensor 16 is designed, for example, as an amperometric gas sensor with sensitivity to both nitrogen oxides and ammonia. A suitable exhaust gas sensor of this type is, for example, in DE 103 20 257 A1 described by the applicant.

Furthermore, two temperature sensors 20, 22 are provided downstream and upstream of the SCR catalytic converter 10. The catalyst temperature can be determined based on the temperatures measured by the two temperature sensors 20, 22. Alternatively, other means for detecting or determining the catalyst temperature can of course also be used.

The measured values of the exhaust gas sensors 16, 18 and the temperature sensors 20, 22 are fed to a control device 14 of the exhaust gas aftertreatment system as input values. This control device 14 controls, among other things, the urea metering valve 12 of the exhaust gas aftertreatment system based on these input values, further input values relating to the operating state of the internal combustion engine and a fill level model for the SCR catalytic converter 10. The fill level model forms a compromise between the maximum possible NO _x conversion in the SCR catalytic converter 10 and the minimum possible ammonia slip. However, since the level model itself is not the subject of the present invention, for the sake of brevity, reference is only made to relevant literature.

Although in 1 Not shown, the exhaust gas aftertreatment system can of course contain additional components, such as additional catalytic converters, particle filters, sound attenuation elements, additional sensors and the like.

With reference to 2 until 4 The following describes the process sequences of various exemplary embodiments for the optimized operation of the exhaust gas aftertreatment system described above 1 explained.

In the process flow of 2 A case will first be considered in which the second exhaust gas sensor 18 is omitted in the exhaust gas aftertreatment system.

In 2 After starting an ammonia slip detection function in the control device 14, overrun operation of the internal combustion engine is initially set in step S10 if an operating situation intended for this exists. In overrun mode, no fuel is injected, so that no combustion occurs in the combustion chambers of the internal combustion engine and it can therefore be assumed that no nitrogen oxides are generated.

Then, in step S12, the nitrogen oxide concentration N1 is detected downstream of the SCR catalytic converter 10 using the first exhaust gas sensor 16. Next, in step S14, it is judged whether or not the detected output signal N1 of the exhaust gas sensor 16 exceeds a predetermined threshold value T1. Since ideally no nitrogen oxides are emitted by the internal combustion engine, the threshold value T1 is preferably chosen to be relatively small, but taking into account a measurement tolerance of the exhaust gas sensor 16. The threshold value T1 preferably corresponds to a defined tolerance limit for ammonia slip of the SCR catalytic converter 10.

If the detected output signal N1 is below the threshold value T1 (NO in step S14), the process returns to step S12 or, alternatively, can also be ended directly. If the decision in step S14 is YES, i.e. if the detected nitrogen oxide concentration N1 exceeds the threshold value T1, a timer for time recording is started in step S16.

The output signal N1 of the exhaust gas sensor 16 is then detected again in step S18 and compared with the threshold value T1 in step S20. The two steps S18 and S20 are repeated until it is judged in step S20 that the detected nitrogen oxide concentration N1 has fallen below the threshold value T1 again. Then, in step S22, it is checked whether the time t measured by the timer, during which the detected nitrogen oxide concentration N1 was above the threshold value T1, exceeds a predetermined threshold value T2.

If the time t of the timer is smaller than the threshold value T2 (NO in step S22), it is judged that the detected increased output signal N1 of the exhaust gas sensor 16 is due to non-significant fluctuations in the nitrogen oxide concentration. In this case, the process flow returns to step S12 or alternatively can also be ended.

However, if there is a sufficiently long, i.e. significant increase in the output signal N1 above the threshold value T1 (YES in step S22), then a significant measurement effect caused by ammonia in the exhaust gas is recognized. This is interpreted to mean that the ammonia storage capacity of the SCR catalytic converter 10 has been reached or exceeded and therefore there is ammonia slip. Therefore, in step S24, the operation of the exhaust gas aftertreatment system is adapted by adapting the current modeled ammonia level to a predetermined limit level. This prevents or reduces the addition of the HWL through the urea metering valve 12 and thus prevents or minimizes the ammonia slip. This is the case of the so-called short-term adaptation of the level model.

As described above, the current modeled ammonia level is generally adapted to a predetermined limit level. For example, this predetermined limit level corresponds to the measured ammonia storage capacity of the SCR catalytic converter. However, it is also possible to set the limit level depending on the size of the output signal N1, i.e. depending on the size of the ammonia slip. In any case, the limit level should be set depending on the conditions on the SCR catalytic converter in the respective overrun mode, with the temperature having the greatest influence here. The limit filling level is preferably determined in such a way that under the prevailing operating conditions it would lead to an ammonia slip of zero, i.e. that ammonia slip would be avoided. For this purpose, suitable maps must be determined and saved in advance.

If the ammonia slip detection described above occurs several times within a certain period of time, a correction factor K is formed, which affects the target level of the level model and/or the amount of reducing agent HWL added, as in 3 shown.

In 3 are the same process steps as in the process of 2 provided with the same reference numbers, and a further description of the same is omitted.

As in 3 shown, after the start of overrun operation of the internal combustion engine in step S10 and before the first detection of the output signal N1 of the first exhaust gas sensor 16 downstream of the SCR catalytic converter 10 in step S12, a counter Z is first initialized (step S26). After the operation of the exhaust gas aftertreatment system in the case of ammonia slip has been adjusted to the predetermined limit level as described above (step S24), the counter Z, which counts the number of times the output signal N1 is exceeded, ie the detected ammonia slip, is incremented by 1 in step S28.

In step S30 it is then assessed whether the counter Z exceeds a predetermined threshold value T3 progresses or not. If an ammonia slip has not yet been detected too frequently within a certain time or distance window (NO in step S30), the process flow is ended. However, if it is a repeated detection of an ammonia slip (YES in step S30), then, as already mentioned, a correction factor K is determined and stored in a non-volatile memory of the control device 14 (step S32), so that it is also the same at the next start the internal combustion engine is available again. This is referred to as long-term adaptation of the level model for the SCR catalytic converter 10.

The correction factor K can take a value between 0 and 1. The value of the correction factor K is determined, for example, based on the time until the threshold value T3 is reached, on the size of the threshold value T3, on the size of the threshold value T1 and the like.

Based on 4 A process flow for an exhaust gas aftertreatment system with the two NO _x sensors 16 and 18 will now be presented 1 described.

The process flow of 4 corresponds to the process flow of 3 with the determination of the correction factor K, and the same reference numbers are used for the same process steps, without explaining them again in more detail. Alternatively, the process flow of 4 but also analogous to the process flow of 2 can only be carried out for a short-term adaptation of the level model.

After initializing the counter Z in step S26, a nitrogen oxide concentration N2 is detected upstream of the HWL injection using the second NO _x sensor 18 upstream of the urea metering valve 12 (step S34). Then, in step S36, it is judged whether or not this nitrogen oxide concentration N2 is smaller than a threshold value T5.

If the measured nitrogen oxide concentration N2 exceeds the threshold value T5, i.e. if there are significant nitrogen oxide emissions from the internal combustion engine, the process flow returns to step S10 or is ended because the prerequisites for the detection of ammonia slip according to the invention are not present, for example because the internal combustion engine is not in the overrun mode.

If the detected nitrogen oxide concentration N2 is below the threshold value T5 (YES in step S36), the process flow can be continued as above.

As a precaution, the nitrogen oxide concentration N2 is also compared again with the threshold value T5 in the loop of steps S18 and S20 (step S38) in order to cancel the ammonia slip detection if necessary (NO in step S38). In this case, step S18 is also replaced by a step S40, in which, in addition to the output signal N1 of the first exhaust gas sensor 16 downstream of the SCR catalytic converter 10, the nitrogen oxide concentration N2 upstream of the HWL metering valve 12 is also detected.

If the second NO _x sensor 18 is present and used, there is also a further variation of the process sequences 2 or. 3 possible.

As in 4 shown is in 4 the step S14 of 2 or. 3 replaced by a step S42, in which a size ratio, for example in the form of a difference value ΔN (=N1-N2) between the nitrogen oxide concentration N1 downstream of the SCR catalytic converter 10 and the nitrogen oxide concentration N2 upstream of the urea metering valve 12 is compared with a threshold value T4. Step S20 of the process sequences is also carried out analogously 2 or. 3 replaced by a corresponding step S44. The ammonia slip detection works reliably even if the nitrogen oxide emissions of the internal combustion engine are not ideally zero when it is coasting.

In an even further modification, it can also be considered to use both the nitrogen oxide concentration N1 and the difference value ΔN as assessment criteria for the ammonia slip detection.

Claims (12)

Method for operating an exhaust gas aftertreatment system of an internal combustion engine, the exhaust gas aftertreatment system having: an SCR catalytic converter (10) for the selective catalytic reduction of nitrogen oxides, a reducing agent addition device (12) upstream of the SCR catalytic converter (10), a downstream of the SCR catalytic converter (10) arranged exhaust gas sensor (16) which is sensitive to nitrogen oxides and ammonia, and a control device (14) for regulating the reducing agent addition device (12) based on an ammonia level model of the SCR catalytic converter (10) in such a way that the ammonia fill level of the SCR catalytic converter (10) is at one predeterminable target filling level is regulated, characterized by the method steps: - Detecting an output signal (N1) of the exhaust gas sensor (16) downstream of the SCR catalytic converter (10) during overrun operation of the internal combustion force machine; and - if the detected output signal (N1) exceeds a predetermined threshold value (T1), in an adaptation step, adapting the current modeled ammonia fill level of the SCR catalytic converter (10) to a predetermined limit fill level. Procedure according to Claim 1 , characterized in that if in a predetermined route or time interval a number (Z) of overrun operating states with an adjustment of the current modeled ammonia fill level exceeds a predetermined threshold value (T3), a correction factor (K) for reducing the amount of reducing agent added and / or the target fill level is determined. Procedure according to Claim 2 , characterized in that the specific correction factor (K) is stored in a non-volatile memory of the control device (14). Procedure according to Claim 2 or 3 , characterized in that the correction factor (K) is determined depending on a time until the threshold value (T3) is exceeded by the number (Z) of exceedances and / or on the size of the threshold value (T1) with respect to the detected nitrogen concentration (N1). becomes. Method according to one of the preceding claims, characterized in that the current modeled ammonia filling level of the SCR catalytic converter (10) is only adjusted to the limit filling level if the detected output signal (N1) exceeds the predetermined threshold value (T1) for a predetermined minimum period of time (T2). exceeds. Method according to one of the preceding claims, characterized in that a nitrogen oxide concentration (N2) is detected upstream of the SCR catalytic converter (10); and that if the detected nitrogen oxide concentrations (N2) upstream of the SCR catalytic converter (10) exceeds a predetermined threshold value (T5), the output signal (N1) of the exhaust gas sensor (16) depends on the detected nitrogen oxide concentration (N2) upstream of the SCR catalytic converter (10) is corrected. Procedure according to Claim 6 , characterized in that the nitrogen oxide concentration (N2) is detected upstream of the reducing agent addition device (12). Procedure according to Claim 6 or 7 , characterized in that the current modeled ammonia level of the SCR catalytic converter (10) is only adjusted to the limit level if the size ratio (ΔN) of the output signal (N1) of the exhaust gas sensor (16) and nitrogen oxide concentration (N2) upstream of the SCR catalytic converter (10) exceeds a predetermined threshold value (T4). Procedure according to Claim 6 or 7 , characterized in that the current modeled ammonia level of the SCR catalytic converter (10) is only adjusted to the limit level if the size ratio (ΔN) of the output signal (N1) of the exhaust gas sensor (16) and nitrogen oxide concentration (N2) upstream of the SCR catalytic converter (10) exceeds a predetermined threshold value (T4) for a predetermined minimum period of time (T2). Exhaust gas aftertreatment system of an internal combustion engine, with - an SCR catalytic converter (10) for the selective catalytic reduction of nitrogen oxides, - a reducing agent addition device (12) upstream of the SCR catalytic converter (10), - a control device (14) for regulating the reducing agent addition device (12) based on an ammonia level model of the SCR catalytic converter (10), and - an exhaust gas sensor (16) which is arranged downstream of the SCR catalytic converter (10) and is sensitive to nitrogen oxides and ammonia, characterized in that the control device (14) during overrun operation of the internal combustion engine, if a Output signal (N1) of the exhaust gas sensor (16) exceeds a predetermined threshold value (T1), the current modeled ammonia level of the SCR catalytic converter (10) is adjusted to a predetermined limit level. Exhaust gas aftertreatment system Claim 10 , characterized in that the exhaust gas sensor (16) is designed as an amperometric gas sensor. Exhaust gas aftertreatment system Claim 10 or 11 , characterized in that an NO _x sensor (18) is provided upstream of the SCR catalytic converter (10).

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