Exhaust aftertreatment device and method for detecting reducing agent slip

Abstract

Method for detecting reducing agent slip in an exhaust aftertreatment device (10) for an internal combustion engine (11), wherein - the exhaust aftertreatment device (10) comprises an SCR catalyst (13) with an instantaneous amount and capacity of reducing agent, an exhaust passage (12) and a low-pressure exhaust gas recirculation (LP-EGR) system branching off downstream from the exhaust passage (12) to the SCR catalyst (13), wherein - exhaust gas (27) exiting from the SCR catalyst (13) is guided through the low-pressure exhaust gas recirculation (19), and wherein - a momentary NOx content in the exhaust gas in the lead-up to the SCR catalyst (13) is detected, characterized in that - in the run-up to the SCR catalyst (13) within a very short and limited time period and with a limited amount of reducing agent, a positive impulse injection (I+) of a reducing agent is carried out, the amount added (Qi) of which is significantly larger than a nominal amount added (Qn) of the reducing agent and the amount added (Qi) of which is smaller than a buffer amount, so that under nominal conditions the instantaneous amount of reducing agent stored does not exceed the instantaneous amount of reducing agent stored, - and wherein in a time interval before and after the positive impulse injection (31, I+) the dosage is essentially equal to the nominal dosage (Qn), wherein - a reducing agent slip is detected when the instantaneous upstream NOx level increases after the positive impulse injection (I+) (NOx+).

Description

[0001] The present disclosure relates to an exhaust aftertreatment technology comprising a method and a device for detecting reducing agent slip of an SCR catalyst within an exhaust aftertreatment device for an internal combustion engine, according to the preamble of the respective independent claim.

[0002] To comply with increasingly stringent emissions regulations, emissions tests are carried out under real driving conditions to check whether the actual exhaust emissions are kept within permitted limits.

[0003] Currently known exhaust aftertreatment methods and devices are not optimally designed with regard to capturing reducing agent slip from an SCR catalyst. Reducing agent slip corresponds to the phenomenon of (excess) reducing agent leaving the SCR catalyst.

[0004] The object of the present invention is to provide an improved exhaust aftertreatment technology, in particular a method for detecting reducing agent slip, an exhaust aftertreatment device and a control unit designed to execute the method, and a software product comprising instructions for executing the method. This object is achieved by the characterizing features of the respective independent claim.

[0005] Reducing agent slip can have various negative effects on both the environment and the physical components of an exhaust aftertreatment system. In particular, reducing agent slip can cause corrosion on physical components of the exhaust aftertreatment system, especially on low-pressure exhaust gas recirculation valve components when exhaust gas is recirculated into the combustion engine.

[0006] NOx emissions tend to increase at very high combustion temperatures in the cylinder. To reduce the peak cylinder temperature and consequently lower the NOx emission rate exiting the engine, exhaust gas recirculation, particularly low-pressure exhaust gas recirculation (LPEG), is a preferred NOx reduction technique. A portion of the exhaust gas after the SCR catalyst is recirculated back into the combustion engine via an LPEG path. Additionally, the LPEG gas can be cooled to increase the volume of the exhaust gas before it is fed into the engine. However, the presence of reducing agent in the recirculated exhaust gas can cause corrosion in the LPEG valve components and lead to the formation of even more NOx in the engine due to an oxidation process. Therefore, the occurrence of reducing agent slip is undesirable while low-pressure exhaust gas recirculation is active.In particular, uncontrolled permanent reducing agent slip can cause damage, whereas controlled and temporarily and / or quantitatively limited reducing agent slip may be permissible.

[0007] In the present disclosure, the terms "in the lead-up" and "in the trailing" are used in reference to a normal flow direction of exhaust gas or intake air under nominal conditions. Intake air flows essentially in one direction from an air inlet (atmosphere) to an internal combustion engine. The normal flow direction of exhaust gas points from an engine outlet to an exhaust manifold.

[0008] Intake air can be pure air or any mixture of air with recirculated exhaust gas. Any gas exiting the engine is called exhaust gas. The exhaust gas can contain various combustion products and additional substances, especially added reducing agents.

[0009] The detection method according to the present disclosure is designed to detect reducing agent slip in an exhaust aftertreatment device. That is, to detect the phenomenon or extent of the escape of unreacted reducing agent from an SCR catalyst, which then flows into the atmosphere.

[0010] In an exhaust aftertreatment system, exhaust gas containing nitrogen oxides (NOx) emitted by an internal combustion engine is directed to a selective catalytic reduction (SCR) catalyst located in an exhaust gas passage. To reduce NOx, the SCR catalyst performs a selective reduction process known as the SCR process. For the SCR process to occur, the SCR catalyst is supplied with a reducing agent, in particular by means of a reducing agent injection device. The reducing agent added to the exhaust gas is transported to the SCR catalyst and stored on its active surface. The stored reducing agent reacts selectively with the NOx present in the exhaust gas, especially as the exhaust gas passes through the SCR catalyst and once the catalyst has reached a specific threshold temperature.

[0011] As a result, NOx (and reacted reducing agent) contained in the exhaust gas are converted into neutral components, primarily water and molecular nitrogen. Precise control of the reducing agent addition is necessary to ensure a balanced reduction process in which all or most of the NOx is converted into these neutral components. Insufficient reducing agent addition can lead to a lower NOx reduction rate, while excessive reducing agent addition can cause unreacted reducing agent fractions to pass through the SCR catalyst.

[0012] Reducing agent slip can be caused by various physical and non-physical effects in the exhaust aftertreatment system, and particularly in the SCR catalyst. Physical effects can include contamination of the active surface where reducing agent is stored, aging, and deterioration of the chemical quality of the active surface. Non-physical effects can include (undetected or misinterpreted) fluctuations in the exhaust gas's space velocity and temperature, (undetected) fluctuations in the amount or capacity of the reducing agent stored in the SCR catalyst, (undetected) fluctuations in the SCR catalyst's temperature, (undetected) fluctuations in the reducing agent's mixing rate with the exhaust gas, and so on.The exhaust gas's space velocity and temperature influence the reducing agent storage capacity and consequently affect NOx conversion efficiency. An increase in space velocity shortens the reducing agent storage time, potentially causing an early onset of reducing agent slip. As explained below, most or all of the aforementioned parameters and effects can be modeled or estimated for nominal exhaust aftertreatment operation in such a way that reducing agent slip can be essentially avoided.

[0013] However, if a physical component, detection result, or estimation / modeling deviates from the nominal condition, unexpected reducing agent slip can still occur and be easily and quickly detected with the available technology. In particular, if there is only a small deviation from the nominal conditions, the occurrence of reducing agent slip can be induced, but only within a very short and limited time period and with a limited amount of reducing agent escaping from the SCR catalyst. This allows countermeasures or corrections to be implemented based on the detection of the (induced) reducing agent slip. Specifically, after the detection of (induced) reducing agent slip, the exhaust aftertreatment system can be returned to its nominal behavior, which will greatly increase the likelihood of preventing persistent reducing agent slip.

[0014] The exhaust aftertreatment device according to the present disclosure comprises an exhaust gas passage for guiding exhaust gas and at least one SCR catalyst arranged on or in the exhaust gas passage. It further comprises at least one reducing agent injection device arranged upstream of the first SCR catalyst, at least one detection device for capturing nitrogen oxides (NOx) upstream of the SCR system, and at least one branch to a low-pressure exhaust gas recirculation (LP-EGR) system arranged downstream of the SCR catalyst.

[0015] Furthermore, the exhaust aftertreatment device can include a second SCR catalyst, and optionally a second reducing agent addition device in the upstream section to the second SCR catalyst.

[0016] The (first) SCR catalyst performs a SCR process that chemically converts nitrogen oxides (NOx) present in the exhaust gas into neutral components (i.e., molecular nitrogen and water vapor) using a reducing agent. A second SCR catalyst can be positioned downstream of the first SCR catalyst in the exhaust gas path. The second SCR catalyst can be positioned to reduce nitrogen oxides leaving the first SCR catalyst. The first and second SCR catalysts can operate separately or overlapping. They can be used individually or in combination to reduce the maximum amount of nitrogen oxides in the exhaust gas.

[0017] The second SCR catalyst can be used particularly when the first SCR catalyst overheats and its temperature rises above a certain threshold. In such a case, chemical reactions for NOx reduction and / or storage of the reducing agent may no longer be possible to the desired extent within the first SCR catalyst. Consequently, the NOx conversion capacity of the first SCR catalyst may be reduced, and eventually some of the reducing agent stored within the SCR catalyst may be released.

[0018] The detection method according to the present disclosure comprises the following steps. A positive-pulse injection of a reducing agent is performed upstream of the SCR catalyst, the amount of which is significantly larger than the nominal amount of reducing agent. Exhaust gas exiting the SCR catalyst is routed through the low-pressure exhaust gas recirculation (19). An instantaneous NOx content in the exhaust gas is detected upstream of the SCR catalyst (13). Reducing agent slip is detected if the instantaneous upstream NOx content increases after the positive-pulse injection.

[0019] The detection method utilizes a recently discovered phenomenon. In an SCR process, reducing agent injections are carried out upstream of an SCR catalyst using different dosing control strategies, i.e., a conservative dosing scheme or an aggressive dosing scheme. In the case of a conservative dosing scheme, the storage of the reducing agent within the SCR catalyst is not maximized, but rather controlled according to a predetermined or continuously monitored target quantity. The target quantity provided by a conservative dosing scheme is significantly below the instantaneous maximum storage capacity of the SCR catalyst. The instantaneous maximum storage capacity of the SCR catalyst is its quantitative ability to store reducing agent molecules, i.e., the maximum amount of reducing agent molecules (mass / volume) that can be stored.For example, conservative dosing sets the target amount for the reducing agent addition in such a way that the amount of reducing agent actually stored in the SCR catalyst will be in the range of 30% to 80% of the current maximum storage capacity.

[0020] Although the technology disclosed here can be implemented with any type of dosing scheme, it is generally preferable to use the conservative dosing scheme. The main advantage of conservative dosing is that it avoids undesirable overfilling of the SCR catalyst, thus keeping the instantaneous storage of reducing agent in the SCR catalyst below its estimated storage capacity. Therefore, any reducing agent slippage through the SCR catalyst can be avoided. However, the conservative dosing scheme is not effective under certain conditions, particularly if the SCR catalyst is aged or contaminated. In such cases, the instantaneous storage quantity can approach or even equal the actual storage capacity.

[0021] The reducing agent can be added via any dosing device and according to any dosing rate calculation algorithm. The addition of the reducing agent (for the first SCR catalyst) is generally carried out upstream of the (first) SCR catalyst. The added reducing agent can either be the reactant, i.e., a chemical substance that (directly) participates in a chemical reaction in which nitrogen oxides are reduced. Alternatively or additionally, the added reducing agent can be a precursor that is converted into the reactant after addition. The reactant can preferably be ammonia. A precursor can be urea, which dissociates into ammonia within the exhaust gas. Furthermore, the reducing agent, the reactant, and the precursor can alternatively be any other suitable chemical substances.

[0022] The reducing agent added to the exhaust gas is transported to and into the (first) SCR catalyst. There, it is stored, primarily by adhering to a catalyst surface. The stored reducing agent, especially a stored reactant (ammonia), can chemically react with nitrogen oxides contained in the exhaust gas entering the SCR catalyst. Following the chemical reaction, so-called neutral products such as atomic nitrogen (N₂), oxygen (O₂), and water (H₂O) escape from the SCR catalyst. The chemical reaction converts some of the stored reducing agent into other products. The addition of further reducing agent can compensate for this conversion, thus keeping the current amount of stored reducing agent (reactant / urea) more or less stable.

[0023] The storage capacity of an SCR catalyst can be limited and can depend on various parameters that may change during operation or over its lifetime. Consequently, the instantaneous storage capacity can vary and must be estimated.

[0024] In a conservative dosing scheme, the reduction agent addition is intended to be kept within limits such that the currently stored amount of the reduction agent remains above a minimum storage threshold and below the (estimated) instantaneous storage capacity. It is further recommended that the instantaneous amount of reduction agent stored be monitored using a simulation model or another suitable estimation method. The dosing should be carried out in such a way that the (estimated) currently stored amount of the reduction agent is kept below the instantaneous storage capacity by a certain threshold (slip-avoidance threshold). A technology for implementing such a conservative dosing scheme is described in the unpublished patent application DE 10 2017 110 234 A1.

[0025] The addition of reducing agent can be controlled solely to compensate for the amount of reacted reducing agent. Alternatively or additionally, it can also be controlled to increase or decrease the instantaneous storage quantity in accordance with a detected increase or decrease in the estimated instantaneous storage capacity of the (first) SCR catalyst. In other words, a target storage quantity can be defined that can be adjusted during operation. The control of the reducing agent addition can be implemented such that an estimated instantaneous storage quantity is approximated to the target storage quantity.

[0026] Despite the aforementioned measures to enable a conservative dosing scheme, the currently stored amount of reducing agent can become too large if too much reducing agent is injected, exceeding the current storage capacity. In such a case, stored reducing agent may be released back from the SCR catalyst and / or recently added reducing agent may pass through the SCR catalyst without being stored there. Consequently, reducing agent slippage could occur.

[0027] Although simulation models and evaluation techniques, as described in DE 10 2017 110 234 A1, have reached a high level of quality and generally allow for very efficient reduction of nitrogen oxides, the phenomenon of reducing agent slippage can still occur, even with conservative dosing. One reason for this can be that a reducing agent injection device ages or deteriorates, causing it to add an excess of the reducing agent to the exhaust gas. Another reason can be that the SCR catalyst becomes contaminated, ages, or is damaged, resulting in an actual instantaneous storage capacity for the reducing agent that is lower than an estimated storage capacity. In addition, various other factors could cause reducing agent to pass through from the (first) SCR catalyst.

[0028] Positive pulse injections (I+), which contain higher amounts of reducing agent than the nominal amount, are performed upstream of the SCR catalyst. When a positive pulse injection is performed, some of the reducing agent exceeding the storage capacity of the SCR catalyst can escape unreacted. Before the pulse injections are performed, the low-pressure EGR path is opened to allow exhaust gas recirculation. The unreacted reducing agent leaving the first SCR catalyst is recirculated back into the engine via the low-pressure EGR path.

[0029] The technology described herein makes it possible to detect reducing agent slip, i.e., unreacted reducing agent escaping from a (first) SCR catalyst, without requiring an additional sensor (directly) downstream of the SCR catalyst. Consequently, the legal requirements for efficient exhaust aftertreatment can be met with a simplified and relatively inexpensive system.

[0030] Due to the exhaust gas recirculation, the reducing agent contained in the recirculated exhaust gas is oxidized, causing the formation of more NOx in the engine cylinder. Consequently, a significant increase, or in other words, a spike in NOx emissions from the engine is observed at the exhaust NOx sensor. Therefore, if NOx slip occurs, the resulting increase in NOx quantity can be detected, indicating the presence of reducing agent slip.

[0031] The low-pressure EGR (LP-EGR) passage can branch off from the exhaust gas passage. This branch can be located downstream of the first SCR catalyst. When the LP-EGR is active, at least a portion of the exhaust gas leaving the first SCR catalyst can be recirculated back to the intake passage and consequently into the combustion engine. The LP-EGR can include a control valve. When open, this control valve allows exhaust gas to pass through. Additionally, the LP-EGR passage can cool the exhaust gas to enable a larger volume of recirculated gas.

[0032] From DE 10 2016 219 548 A1 and DE 10 2014 018 032 A1, exhaust aftertreatment systems with SCR catalyst and exhaust gas recirculation are known, in which reducing agent slip can be detected via a NOx sensor upstream of the SCR catalyst. From DE 10 2005 062 120 A1, an exhaust aftertreatment system with catalyst and without exhaust gas recirculation is known, in which the reducing agent rate is increased until ammonia breakthrough is detected by means of an ammonia-sensitive sensor in the flow direction downstream of the catalyst.

[0033] The present disclosure proposes to implement a positive-pulse injection of the reducing agent. The positive-pulse injection is performed upstream of the (first) SCR catalyst, in particular by operating the first reducing agent addition device. The positive-pulse injection has an addition quantity that is significantly larger than a nominal addition quantity of the reducing agent during nominal operation, in particular larger than a nominal addition quantity set based on a conservative dosing scheme. In other words, during the positive-pulse injection, a much larger quantity of the reducing agent is injected than would be required to maintain the (estimated) instantaneous amount of reducing agent stored in the (first) SCR catalyst at a stable level or within the range of the target storage quantity.The amount added during positive-pulse injection can, however, be smaller than a buffer amount, corresponding to the aforementioned slip-avoidance threshold. Consequently, when exhaust aftertreatment is performed under nominal conditions, positive-pulse injection can increase the instantaneous amount of reducing agent stored within the (first) SCR catalyst. However, the instantaneous amount of reducing agent stored would not exceed the (actual) instantaneous storage capacity. As a result, no reducing agent slip should occur.

[0034] However, if the exhaust aftertreatment is not operating under nominal conditions, for example, if the SCR catalyst is dirty or aged, the actual instantaneous storage capacity may be lower than the estimated value. And / or the (actual) injection quantity may be greater than the estimated value. Starting from positive-pulse injection, the (actual) instantaneous amount of reducing agent stored may therefore exceed the (actual) instantaneous reducing agent storage capacity, and consequently, reducing agent slip may occur.

[0035] Then at least some of the reducing agent that escapes from the (first) SCR catalyst can be transported along with the exhaust gas through the low-pressure EGR and enter the combustion engine. In other words, in the event of reducing agent slippage, reducing agent will be present in the recirculated gas, i.e., in the intake manifold. This recirculated reducing agent will then enter the combustion engine.

[0036] There, the recirculated reducing agent can be burned, which can increase the NOx content in the exhaust gas leaving the combustion engine. Alternatively, the recirculated reducing agent can pass through the combustion engine unreacted.

[0037] The upstream NOx detection device, located in the exhaust gas path (in the downstream section to the engine and in the upstream section to the SCR catalyst), will come into contact with the recirculated reducing agent or with the respective increased NOx concentration. Currently known NOx sensors exhibit cross-sensitivity to reducing agents. Therefore, both the increased NOx concentration due to the combustion of the reducing agent and the sensor's contact with unreacted recirculated reducing agent will cause an increase in the sensor signal. Whether the recirculated reducing agent is combusted or unreacted can be determined based on the type of reducing agent (for example, urea / ammonia is combusted) and based on the known operating conditions of the combustion engine (temperature, pressure, etc., within the combustion chamber).

[0038] Therefore, if the instantaneous NOx content in the exhaust gas is measured in the run up to the SCR catalyst (while at least part of the exhaust gas is recirculated), reducing agent slip can be detected if the instantaneous upstream nitrogen oxide content or the respective sensor signal increases after the positive pulse injection.

[0039] As a result, a particularly simple and cost-effective technology is provided that can detect the occurrence of reducing agent slip. If reducing agent slip is present, additional measures can be taken to improve exhaust aftertreatment. For example, the extent of the reducing agent slip can be determined and / or adjustments can be made to the dosing scheme or to the estimation and control procedures used for reducing agent addition.

[0040] The upstream NOx detection device is preferably arranged in the exhaust gas passage in a section downstream of the engine, upstream of the (first) SCR catalyst, and upstream of a (first) reducing agent injection device. The upstream NOx detection device is preferably a physical sensor and may hereinafter be referred to as the first NOx sensor. It is designed to detect the NOx content in the exhaust gas. The first NOx sensor can detect the NOx content flowing upstream of the first SCR catalyst and, in particular, upstream of a (first) reducing agent injection device. It can detect the NOx content before it enters the first SCR catalyst.

[0041] The method for detecting reducing agent slip according to the present disclosure can be used in combination with an exhaust aftertreatment device comprising only one SCR catalyst. However, it can also be advantageously applied in combination with an exhaust aftertreatment device comprising at least one second SCR catalyst arranged downstream of the first SCR catalyst. The second (or further) SCR catalyst can preferably be arranged in the exhaust gas passage and downstream of the low-pressure exhaust gas recirculation (LPEG) branch. A separate and additional reducing agent injection can be implemented upstream of the second SCR catalyst. In particular, a second reducing agent injection device can be arranged between the LPEG branch and the second SCR catalyst.

[0042] If reducing agent slip is detected (based on the increase in the upstream nitrogen oxide concentration), the reducing agent feed to the second SCR catalyst can be adjusted to compensate for the excess reducing agent that escaped from the first SCR catalyst. This excess reducing agent (excluding the recirculated portion) can presumably have migrated to the second SCR catalyst, where it can be stored and used for further nitrogen oxide reduction. In any case, no further addition of reducing agent by the second reducing agent feed device is required to this extent.

[0043] A further NOx detection device, called a trailing NOx detection device, can be arranged in the exhaust gas passage. It can also be a physical NOx sensor and consequently called a second NOx sensor. According to current regulations, the second NOx sensor can be positioned to detect the NOx content in exhaust gas leaving the exhaust aftertreatment system, particularly at or within the exhaust manifold. It can be located specifically in the trailing section to the second SCR catalyst. A detection result or sensor reading from the second NOx detection device can be used to verify whether NOx conversion in the second SCR catalyst or in the entire exhaust aftertreatment system is consistent with the various estimates and simulation models.

[0044] The exhaust aftertreatment method can be in the form of a software product that includes instructions for executing a procedure. The software product can be stored on a physical data carrier and / or executed within a data processing device, in particular within a control unit of an exhaust aftertreatment device. By executing the software product on a computer, in particular on a control unit according to the present disclosure, the computer or control unit is operated to execute the detection procedure.

[0045] Further advantageous variations and details are included in the dependent claims and are described below with reference to the drawings.

[0046] The invention is illustrated in the drawings in an exemplary and schematic manner. They show: Fig.1: a representation of an exhaust aftertreatment device according to the present disclosure; Fig. 2: an illustration showing a dual SCR exhaust aftertreatment device; Fig. 3-6: various schematic representations explaining the recording procedure of the present disclosure; Fig. 7: a schematic representation showing opportunities for enabling conditions;

[0047] Fig.Figure 1 shows a schematic view of an exhaust aftertreatment device (10). This includes an exhaust passage (12) connected to an internal combustion engine (11). The exhaust gas emitted by the internal combustion engine (11) is routed along or through the exhaust passage (12) to the atmosphere. The exhaust passage (12) can be configured as a single-line structure extending to an exhaust outlet (24), or it can additionally include one or more bypasses (not shown) or branch passages such as a low-pressure exhaust gas recirculation (LP-EGR) (19), from where a portion of the exhaust gas can be recirculated to the intake passage (21) of the internal combustion engine (11).

[0048] A number of additional components, directly or indirectly involved in exhaust aftertreatment, can be arranged on, in, or within the exhaust gas passage. Furthermore, one or more turbines (15) can be arranged on or in the exhaust gas passage (12), which is / are driven by the exhaust gas flow.

[0049] An engine intake of the internal combustion engine (11) can further be connected to an intake passage (21). A turbine can be functionally coupled to at least one compressor (14) located in the intake passage (21). The at least one compressor (14) can be operated by the energy provided by the turbine (15) to compress intake air and thus increase the air mass supplied to a combustion chamber (22) of the internal combustion engine (11).

[0050] The internal combustion engine (11) can be of any type and configuration. It is preferably a diesel engine. It can include any means for adding fuel to the intake air. Preferably, at least one fuel injector (16) is arranged on a combustion chamber (22) of the engine. The fuel injector (16) is designed to perform direct injections of fuel into the combustion chamber so that the fuel is immediately combusted with the intake air (26) contained in the combustion chamber. A gas resulting from the combustion process is called exhaust gas and is forced out of the combustion chamber through an engine outlet to enter the exhaust passage (12).

[0051] At least one SCR catalyst (13) is arranged in the exhaust gas passage (12). The example in Fig. 1 comprises exactly one SCR catalyst, which is arranged between the combustion engine (11) and a branch (39) to a low-pressure exhaust gas recirculation. Fig. Figure 2 shows another example comprising the first SCR catalyst (13) and at least one additional SCR catalyst, which is called the second SCR catalyst (28). The second (or further) SCR catalyst is preferably arranged in the exhaust gas passage (12) downstream of the branch (39).

[0052] A section of the exhaust gas passage located upstream of the first SCR catalyst can be called the upstream passage, while the other section of the exhaust gas passage (12) can be called the downstream passage. The terms upstream passage and downstream passage thus refer to the direction of exhaust gas flow (29) with respect to the first SCR catalyst (13).

[0053] A diesel particulate filter (DPF) that is not in Fig. 1 but in Fig.The SCR catalyst shown in Figure 2 can be additionally integrated with at least one of the SCR catalysts. It can be designed to remove soot from the exhaust gas flowing through the exhaust aftertreatment system. Preferably, the diesel particulate filter (DPF) is provided or integrated at least on the first SCR catalyst. An SCR catalyst with an integrated DPF can be called an SDPF.

[0054] The second SCR catalyst (which is only in Fig.The second SCR catalyst (28) can be located downstream of the first SCR catalyst (13), i.e., in the downstream section, to assist the aftertreatment of exhaust gas that has already passed through the first SCR catalyst (13). For example, if the first reducing agent catalyst (13) is too hot to sufficiently convert the NOx in the exhaust gas directly downstream of the combustion engine (11), i.e., in a section of the exhaust gas passage (12) where the temperature is generally high, the second SCR catalyst can assist or take over the NOx reduction process to compensate for the efficiency loss of the first SCR catalyst (13). The distance between the first SCR catalyst (13) and the second SCR catalyst (28) can be much greater than shown in Figure 2. Fig.2 appears. It can be one or more meters in length. The section of the exhaust gas passage (12) extending from the first SCR catalyst (13) to the second SCR catalyst (28) can be in direct or indirect contact with a cooling medium, in particular the outside air. As a result, a heat loss phenomenon can occur during the passage of exhaust gas from the first SCR catalyst (13) to the second SCR catalyst (28), which reduces the local temperature of the exhaust gas. Consequently, the second SCR catalyst (28) is generally supplied with exhaust gas that has a reduced temperature compared to the temperature of the exhaust gas entering the first SCR catalyst (13).

[0055] A NOx content, i.e., an instantaneous quantity of nitrogen oxides (NOx) (measured, for example, per unit volume) in the exhaust gas, can be measured by means of at least one NOx sensor. A NOx detection device, in particular a first NOx sensor, is preferably arranged upstream of the first SCR catalyst (13), especially where NOx emissions from the combustion engine are detected for the first time. As in Fig. As shown in Figure 1, it is possible for a further NOx sensor (17'') to be arranged in the exhaust gas passage (12) directly in the downstream section to the (first) SCR catalyst in order to detect an instantaneous NOx content there, i.e., in the exhaust gas escaping from the first SCR catalyst (13) (without or before entering a second SCR catalyst). However, according to the present disclosure, it is not necessary to provide such a further NOx sensor, particularly not in a single-SCR system as shown in Figure 1. Fig. 1 shown.

[0056] The exhaust aftertreatment device (10) can comprise at least one reducing agent addition device (18). This device can be configured in any desired manner. Preferably, at least one reducing agent addition device, referred to as the first reducing agent addition device (18), is arranged upstream of the first SCR catalyst (13). The (first) reducing agent addition device (18) can be arranged directly upstream of the SCR catalyst (13), i.e., without any intermediate components. Alternatively, at least one intermediate component can be arranged between the (first) reducing agent addition device (18) and the (first) SCR catalyst (13). Such an intermediate component can be an exhaust gas mixer (not shown), which can improve the homogeneity of the mixture of exhaust gas and reducing agent.

[0057] Alternatively or additionally, an intermediate component can be a diesel particulate filter, as described above.

[0058] The exhaust aftertreatment device may further comprise a second reducing agent addition device (18') arranged (directly or indirectly) upstream of the second SCR catalyst (28). However, this second reducing agent addition device (18') is preferably arranged downstream of the branch (39) to the low-pressure exhaust gas recirculation (LP-EGR, 19), so that a reducing agent added to the exhaust gas by the second reducing agent addition device (18') does not – in nominal operation – enter the LP EGR or contact the LP EGR valve (23).

[0059] The exhaust aftertreatment device (10) preferably comprises a control unit (20), in particular an electronic control unit. The control unit (20) is preferably connected to sensors and / or actuators of the exhaust aftertreatment device (10). In particular, it can receive a sensor signal or a sensor value from the first NOx sensor (17) and / or from a further NOx sensor (17'). It can also be connected in a controlling manner to the valve of the LP EGR and at least the first reducing agent injection device (18). Further connections can be provided to at least one of the other sensors and / or actuators described in this disclosure, in particular to the second NOx sensor (17') and / or a second or further reducing agent injection device (18') and / or a further NOx sensor (17").

[0060] The control unit (20) can also be connected to or integrated with an internal combustion engine control unit, in particular an injection control unit, to control injections (not shown). Furthermore, the control unit (20) can collect or receive a number of other parameters relating to the internal combustion engine, the exhaust gas, the engine control timing, and / or the various temperature values. Based on these parameters, an enable condition can be established that is representative of a stable combustion and / or exhaust aftertreatment condition. During such a stable condition, any changes in the measured exhaust gas parameters, in particular an increase in the upstream nitrogen oxide content measured by the first NOx sensor (17), can be clearly attributed to a system manipulation intentionally applied to the exhaust aftertreatment.Such system manipulation can in particular involve the administration of a positive impulse injection (I+), which is explained below.

[0061] The method according to the present disclosure can be carried out, in part or in full, by the control unit (20). The control unit (20) can therefore generate command values ​​accordingly to actuate the at least one reducing agent addition device (18, 18') and finally the low-pressure EGR valve (23).

[0062] The Fig. Sections 3-6 explain various aspects of the data collection process. Fig. 3-5 explain the execution of positive pulse injection in combination with monitoring the NOx quantity at NOx sensors.

[0063] Fig.Figure 3 shows the curve of an instantaneous amount of reducing agent added (1st Uinj) by the first reducing agent addition device (18). In the time interval before and after the positive pulse injection (31, I+), the amount added is essentially equal to the nominal injection amount (Qn). However, during the positive pulse injection (31, I+), the respective higher injection amount (Qi) is added.

[0064] Fig.Section 4 explains – to illustrate the technical process – an instantaneous NOx content (NOX) and an instantaneous reducing agent content (NH3) of the exhaust gas in the downstream phase to the first SCR catalyst (13), which can be determined under laboratory conditions. With the exhaust aftertreatment technology according to this disclosure, it is not necessary to measure these parameters, although it is possible to measure at least the downstream NOx content with the other NOx sensor (17'') already mentioned. Due to the increased amount of reducing agent added to the exhaust gas with the positive pulse injection (31, I+), a temporarily increased NOx reduction performance may occur. Consequently, the downstream NOx content may be reduced during or after the positive pulse injection (31, I+). Fig.4. It is assumed that reducing agent slip occurs. Thus, a post-run reducing agent content in the exhaust gas greater than 0 is caused for a limited time period during or after the positive impulse injection (31, I+).

[0065] As already explained, the ND-EGR is active during the execution of the detection procedure and during positive-pulse injection (31, I+). Consequently, at least some of the excess reducing agent passing through the first SCR catalyst (13) will be recirculated and thus enter the combustion engine (11). There, it will in most cases be burned or oxidized, causing a further increase in the NOx content in the exhaust gas escaping the combustion engine. This (temporary) increase (NOx+) of the instantaneous NOx content (33), detected by the first NOx sensor (17), i.e., upstream of the first SCR catalyst (13), is in Fig. 5 shown.

[0066] The characteristic curve in Fig.5 can further be considered for the quantitative estimation of the reducing agent / ammonia slip according to the additional NOx quantity (NOx+) detected at the first NOx sensor (17) following the pulse injection (31, I+), and in particular after the exhaust gas escaping from the first SCR catalyst (13) is recirculated through the low-pressure EGR (19) path into the combustion engine (11). Based on the detected additional NOx quantity (NOx) and a known low-pressure EGR mass flow, an excess quantity of reducing agent that escaped from the first SCR catalyst (13) during or after the positive pulse injection (31, I+) can be calculated. Consequently, a differential reducing agent quantity can be calculated, i.e., the difference between the instantaneous reducing agent storage quantity and the instantaneous reducing agent storage capacity before the positive pulse injection (31, I+) is performed.Based on this difference in the amount of reducing agent, the calculation scheme for the nominal amount of reducing agent can be adjusted and / or an estimation or simulation model can be calibrated to estimate the SCR storage behavior; that is, the estimated instantaneous reducing agent storage capacity and / or the estimated instantaneous amount of reducing agent can be adjusted. After this calibration, the reducing agent addition control / dosing scheme control is executed with increased precision, thereby preventing further undesirable occurrences of (uncontrolled) reducing agent slippage.

[0067] Fig.Section 6 explains the setting of an enable / disable flag, which is instructed by the control unit (20). An enable / disable flag can be set in any way. It can represent one or more parameter monitoring processes used to determine a stable condition. Various examples of setting an enable / disable flag are described in the unpublished patent application DE 10 2017 110 234 A1.

[0068] Fig. Section 7 explains an example of finding opportunities to set an enable condition. In this example, the enable condition is set as an essential criterion for performing or avoiding a pulse injection. The characteristic curves in the Fig. 2-6 and the example in Fig. 7 will be explained further below. Fig. 7 shows the order from top to bottom • a representation illustrating the vehicle speed (Vsp) of a vehicle incorporating the exhaust aftertreatment technology according to the present disclosure; • a diagram that explains the vehicle's engine speed (ESP); • a representation explaining the instantaneous NOx level (US SCR NOx) as determined by the first NOx sensor (17); • a result of an equilibrium state detection (SSD).

[0069] The positive impulse injection (I+) and the detection procedure can be performed during the time interval indicated by the dashed-lined rectangle in Fig.7 is covered. During this time period, several criteria can be met which – alone or in combination – can cause the enable condition(s) to be met. Specifically, an equilibrium state of vehicle speed (35), an equilibrium state of engine speed (36), an equilibrium state of the upstream nitrogen oxide content (37), and consequently a time interval for equilibrium state detection with a specific minimum duration occur. Under these conditions, the detection procedure can be carried out, and the detection of an increase in the upstream nitrogen oxide content during or after the positive pulse injection can be attributed to the addition of the excess reducing agent quantity during the pulse (I+).

[0070] Alternatively or additionally, apart from monitoring the enable condition, the acquisition procedure illustrated above can be executed at any time during engine operation or at any arbitrary time, provided that various operating conditions (e.g., cold start, city driving, and highway driving conditions) are met. Furthermore, it can be executed at fixed time intervals or based on events, e.g., after another learning cycle for updating or calibrating the estimation / simulation model has been completed. Preferably, a target time window for executing the acquisition procedure can be defined based on operating conditions, fixed time intervals, or external events. Within the target time window, the actual execution of a positive impulse injection can be triggered based on the enable condition(s).

[0071] To execute the positive pulse injection (I+), one or more enable conditions are predefined, in particular an equilibrium state of one or more engine-related parameters, such as engine speed, vehicle speed, and engine output NOx emissions, and / or parameters relating to the aftertreatment system, such as the amount of reducing agent added, the instantaneous storage capacity, the temperature of the SCR catalyst (13), etc., to ensure that any irregular system responses are not caused by a change in the operating point, but rather by the changed amount of reducing agent added. The enable conditions are monitored to identify opportunities for equilibrium states of the predefined parameters before the positive pulse injection is performed.The pulse injections of the reducing agent dosing are carried out upstream of the first SCR catalyst (13), in the event that an enable condition, in particular an equilibrium state of the NOx quantity at the first engine exhaust NOx sensor (17), is met, and additionally when the ND-EGR control valve (23) is in an open position to allow at least part of the exhaust gas (27) to flow through the ND-EGR (19) path into the engine (11).

[0072] When a spike in NOx emissions from the engine (11) is detected, it is assumed that reducing agent is present in the recirculated gas, or in other words, that reducing agent slip from the first SCR catalyst (13) is detected. The reducing agent dosing upstream of the first SCR catalyst (13) is adjusted as soon as reducing agent slip is identified, particularly if a significant increase in NOx levels is detected at the first NOx sensor (17) following the pulse injection. This increase in NOx levels is relatively much higher than that detected before the pulse injection, and especially when an equilibrium state of engine exhaust NOx levels is observed.

[0073] In a dual-SCR configuration, as in Fig.As shown in Figure 2, the exhaust gas containing reducing agent slip emitted by the first SCR catalyst (13) is routed to the second SCR catalyst (28). The second SCR catalyst (28), located downstream of the first SCR catalyst and downstream of the EGR branch (39), can utilize the excess reducing agent from the reducing agent slip for NOx reduction. Based on the known reducing agent slip, the control unit (20) can instruct the second reducing agent injection device (18') upstream of the second SCR catalyst (28) to adjust the reducing agent dosage accordingly. In other words, a compensation control can be implemented by which the amount of reducing agent to be added by the second reducing agent addition device (18') is reduced after the detection of the reducing agent slip from the first SCR catalyst (13).As already described, the extent of the compensatory reduction can depend on the quantitative estimate of the reducing agent slip.

[0074] The following section explains specific details of the present disclosure, which can be used in any combination with the basic explanations above.

[0075] A reducing agent dosing device (18, 18') can supply the first or second SCR catalyst (13, 28) with individual, time-distributed injections of reducing agent, i.e., a pattern of successive injections with intervening pauses. Alternatively, a continuous dosing of reducing agent can be provided, preferably with an adjustable dosing rate (amount of dosing per unit of time). Regardless of the type of injection(s) within a defined time period, the numerous individual injections or the continuous dosing can contribute a dosing amount corresponding to the explanations in Fig. 3 corresponds. The added reducing agent can be uniformly mixed with the exhaust gas due to turbulence, especially caused by a mixer, within the exhaust gas passage.

[0076] A diesel particulate filter (DPF) can be positioned at any suitable location in the exhaust system. It is preferably designed to remove soot from the exhaust gas. The DPF can be installed individually, in combination with an SCR catalyst, or both.

[0077] The exhaust gas temperature can contribute to the generation of reducing agent slip, particularly when the temperature rises above a certain range. NOx conversion is highly efficient within a specific temperature window. At temperatures above a certain storage active temperature, the storage capacity of the SCR catalyst for storing reducing agent decreases because desorption (the opposite reaction to adsorption) of the stored reducing agent occurs. As a result, the reaction kinematics can slow down. In the detection method according to the present disclosure, an enabling condition can be that the exhaust gas temperature (upstream of the first SCR catalyst (13)) is below, and in particular by a tolerance value below, the storage active temperature.

[0078] Temperature can also affect the mixing of the reducing agent and the exhaust gas. At temperatures below a certain permissible threshold, the mixing rate may be insufficient, and consequently, the reducing agent may not adhere to the SCR catalyst, which can also contribute to reducing agent slip. However, such reducing agent slip due to insufficient temperature could not be attributed to pulse injection. In the detection method according to the present disclosure, a disable condition can be set / no enable condition can be set if the instantaneous exhaust gas temperature is below the permissible threshold.

[0079] Adequate mixing of the reducing agent with the exhaust gas can contribute to NOx reduction performance. Even if the reducing agent dosage matches the actual demand, a uniform distribution and complete mixing of the reducing agent with the gas stream is advantageous, or may even be necessary, to achieve the nominal NOx conversion efficiency. In the detection method according to the present disclosure, sufficient uniformity of temporally separated reducing agent injections can be a parameter for setting the enable condition(s). In other words, the positive-pulse injection (I+) can be avoided if the reducing agent injections within a specified time window have not been performed with sufficient uniformity, i.e., with excessively long intervals or with unacceptably different individual injection quantities.

[0080] The kinematics of the reduction reactions within the SCR catalyst cannot be accurately predicted for the entire lifetime of the catalyst. Aging conditions can cause a decrease in NOx conversion efficiency. Therefore, a nominal or ideal reaction performance of the reducing agent with NOx cannot be predicted, especially when the operating state of the SCR changes due to the factors mentioned above. However, by using learning functions and by performing the acquisition procedure as disclosed herein, good control precision can be achieved, even when aging conditions or other undesirable phenomena occur.

[0081] Pulse injections of urea / ammonia can be used for very short durations to monitor the operating conditions of an SCR catalyst. These pulse injections can be performed to identify the actual reducing agent dosing requirement. The reducing agent dosing can then be further adjusted once the dosing requirement has been determined based on the current operating status of the SCR catalyst. Sufficiently large quantities of reducing agent are selected for the pulse injections to create an excess of reducing agent relative to the storage capacity of the SCR catalyst. The excess amount of reducing agent, which is greater than the instantaneous storage capacity of the SCR catalyst (or in other words, when more is present than can be consumed in SCR reactions), is consequently forced to pass through the SCR catalyst.Based on the pulse injections and the subsequently determined reducing agent slip, a target value for the reducing agent dosage can be determined.

[0082] The technology disclosed here can also be used with a non-conservative dosing scheme. In the case of an aggressive dosing scheme, for example, the target amount of reducing agent added upstream of the (first) SCR catalyst can be set greater than 80% or 90% of the maximum storage capacity (estimated instantaneous storage capacity). The values ​​for the target amounts of reducing agent mentioned in this application are given as an example and may vary depending on the size and efficiency of the SCR catalyst. With an aggressive dosing scheme, excess reducing agent amounts that exceed what can be adsorbed within the SCR catalyst can cause premature overfilling of the SCR catalyst and consequently reducer slippage.However, corrosion-resistant components can be used within the exhaust passage, the ND-EGR and even the intake passage and the engine, so that the aforementioned negative consequences of reducing agent slip will not occur. REFERENCE MARK LIST 10 Exhaust gas after Exhaust aftertreatment treatment device device 11 Combustion engine internal combustion engine 12 Exhaust gas passage Exhaust passage 13 First SCR catalyst First SCR catalyst 14 Compressor compressor 15 turbine turbine 16 Fuel injector Fuel injector 17 First nitrogen oxide First nitrogen oxide (NOx) (NOx) sensor sensor 17' Second nitrogen oxide Second nitrogen oxide (NOx) (NOx) sensor sensor 17" Further nitrogen oxide Further nitrogen oxide (NOx) sensor (NOx) sensor 18 First reductant First reducing agents additional device Add-on device 18' Second reductant Second reducing agent additional device Add-on device 19 Low pressure exhaust gas recirculation (LP-EGR) Low-pressure exhaust gas recirculation (LP-EGR) 20 Control unit control unit 21 Intake passage Intake passage 22 Combustion chamber combustion chamber 23 Exhaust gas recirculation control valve Exhaust gas recirculation control valve 24 Tail pipe Exhaust (tailpipe) 25 Exhaust gas cooler Exhaust gas cooler 26 Fresh air Fresh air 27 Exhaust gas exhaust 28 Second SCR catalyst Second SCR catalyst 29 Exhaust gas flow direction Exhaust gas flow direction 30 Diesel particulate filter (DPF) Diesel particulate filter 31 Positive impulse injection Positive impulse injection 32 Reductant / reactant (ammonia) downstream to SCR catalyst Reducing agent / reaction agent (ammonia) in the tailings to the SCR catalyst 33 NOx content downstream to (first) SCR catalyst NOx content in the trailing area to the (first) SCR catalyst 34 Upstream NOx content / NOx content upstream to (first) SCR catalyst NOx content in the lead-up to the (first) catalyst 35 Steady-state of vehicle speed steady state of vehicle speed 36 Steady state engine speed steady state of engine speed 37 Steady state of upstream NOx emissions from engine steady state of the NOx emissions from the engine 38 Time interval of steady state Time interval of the steady state 39 Branching point Junction Uinj Reductant / preproduct (Urea) injection Reducing agent / precursor (urea) injection DS SCR NOx content downstream Nitrogen oxide content in NOx to SCR catalyst After-run to the SCR catalyst US SCR NOx content upstream Nitrogen oxide content in NOx to SCR catalyst Preliminary stage to the SCR catalyst CD Controller detection Control unit detection F Flag Flag Vsp Vehicle speed Vehicle speed ESP Engine speed Engine speed SSD Steady-state detection Recording of stationary state I+ Positive impulse injection Positive impulse injection NOx+ US NOx content increase Increase in the upstream nitrogen oxide content Up Upstream NOx content before positive impulse injection NOx level before positive impulse injection Up' Upstream NOx content after positive impulse injection NOx level before positive pulse injection Qi Addition quantity / Addition quantity during positive impulse Addition amount / Addition amount during positive impulse Qn Nominal addition quantity Nominal addition amount

Claims

[1] Method for detecting reducing agent slip in an exhaust gas aftertreatment device (10) for an internal combustion engine (11), wherein - the exhaust gas aftertreatment device (10) comprises an SCR catalyst (13) with a current reducing agent storage quantity and a current reducing agent storage capacity, an exhaust gas passage (12) and a low-pressure exhaust gas recirculation (LP-EGR) branched off from the exhaust gas passage (12) downstream to the SCR catalyst (13), wherein - exhaust gas (27) emerging from the SCR catalyst (13) is guided through the low-pressure exhaust gas recirculation (19), and wherein - a current NOx content in the exhaust gas in the flow to the SCR catalyst (13) is recorded, characterized by , that - in the feed line to the SCR catalyst (13) within a very short and limited period of time and with a limited amount of reducing agent, a positive pulse injection (I+) of a reducing agent is carried out, the added quantity (Qi) of which is significantly greater than a nominal added quantity (Qn) of the reducing agent and the added quantity (Qi) of which is smaller than a buffer quantity, so that under nominal conditions the instantaneous reducing agent storage quantity does not exceed the instantaneous reducing agent storage capacity, - and wherein in a time interval before and after the positive pulse injection (31, I+) the addition quantity is substantially equal to the nominal addition quantity (Qn), wherein - a reducing agent slip is detected when the instantaneous pre-run NOx content increases (NOx+) after the positive pulse injection (I+). [2] Method according to claim 1, wherein an instantaneous pre-run NOx content (Up,Up') is monitored before and after the positive pulse injection (I+). [3] Method according to one of the preceding claims, wherein predefined enable conditions are monitored, and wherein the positive pulse injection (I+) - is avoided if no enable condition is met, or - is executed if at least one enable condition is met. [4] Method according to one of the preceding claims, wherein the exhaust gas recirculation is active at least after and preferably already before the positive pulse injection (I+). [5] Method according to one of the preceding claims, wherein a momentary reducing agent slip in the downstream flow to the SCR catalyst (13) is determined based on a change in the momentary pre-run NOx content (Up, Up') starting from a first state before or during the positive pulse injection (I+) to a second state after the positive pulse injection (I+). [6] Method according to one of the preceding claims, wherein the reducing agent slip is determined when a peak value in the pre-run NOx content (Up') is detected at the first NOx sensor (17) after the positive pulse injection (I+). [7] Method according to one of the preceding claims, wherein the reducing agent addition in the feed to the SCR catalyst (13) is adjusted based on the increase in the feed NOx content (Up') after the positive pulse injection (I+) and / or based on the extent of the detected reducing agent slip. [8] Method according to one of the preceding claims, wherein the exhaust gas aftertreatment device comprises a second SCR catalyst (28) arranged in the exhaust gas passage downstream of the branching off of the low-pressure EGR, and wherein a momentary amount of reducing agent which has passed through the first SCR catalyst (13) is fed to the second SCR catalyst (28). [9] Method according to one of the preceding claims, wherein a NOx content in the exhaust gas leaving the second SCR catalyst (28) is monitored at a second NOx sensor (17') arranged downstream of the second SCR catalyst (28). [10] Method according to one of the preceding claims, wherein a further reducing agent addition is carried out in the downstream section to the first SCR catalyst (13) and in the upstream section to the second SCR catalyst (28), and wherein the dosage of the further reducing agent addition is controlled based on the detection of the reducing agent slip from the first SCR catalyst (13). [11] A software product that is storable or stored on a physical data carrier, wherein the software product is for execution on a computer, in particular on a control unit (20) for an exhaust gas aftertreatment device (10), in order to have the computer or the control unit (20) execute a method, and wherein the software product comprises instructions for executing a method according to any one of claims 1-10. [12] Electronic control unit for controlling an exhaust gas aftertreatment device (10), wherein the electronic control unit is designed to carry out a method according to one of claims 1-10. [13] Exhaust gas aftertreatment device comprising: - An exhaust passage (12) for conducting exhaust gas; - A first SCR catalyst (13) arranged in the exhaust passage (12); - A low-pressure exhaust gas recirculation (19) with a branch (39) from the exhaust gas passage (12), wherein the branch (39) is arranged in the downstream section to the first SCR catalyst (13); - A (first) reducing agent addition device (18) which is arranged on or in the exhaust gas passage (12) in the feed line to the first SCR catalyst (13); - A first nitrogen oxide (NOx) sensor (17) for detecting a current feed-through nitrogen oxide content (Up, Up'), wherein the first NOx sensor (17) is arranged in the feed-through to the first SCR catalyst (13) in the exhaust passage (12); characterized by that the exhaust gas aftertreatment system comprises a control unit according to the previous claim. [14] Exhaust gas aftertreatment device according to claim 13, wherein a second SCR catalyst (28) is arranged in the exhaust passage (12) downstream of the first SCR catalyst (13), in particular downstream of the branch (39). [15] Exhaust gas aftertreatment device according to claim 13 or 14, wherein a second reducing agent addition device (18') is arranged in the upstream line to the second SCR catalyst (28), and in particular in the downstream line to the branch (39). [16] Exhaust gas aftertreatment device according to one of claims 13 to 15, wherein the exhaust gas aftertreatment device comprises a second NOx sensor (17') arranged downstream of the second SCR catalyst (28).

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