Method for operating an exhaust system, in particular of a motor vehicle

Abstract

The invention relates to a method for operating an internal combustion engine through a flow-through exhaust system (10), wherein the exhaust system comprises a first SCR catalyst (14), a second SCR catalyst (16) arranged downstream of the first SCR catalyst (14), a particulate filter (18) arranged upstream of the first SCR catalyst (16), and at least one metering point (D) arranged upstream of the particulate filter (18), at which a reducing agent for denitrification of the exhaust gas can be introduced, in which then, - if the soot loading of the particulate filter (18) exceeds a predefinable first threshold but is below a second predefinable threshold and the exhaust gas temperature is within a predefinable temperature range, in a first step the quantity of reducing agent introduced into the exhaust gas at the metering point (D) is increased beyond the current target fill level of the first SCR catalyst (14), and in a second step the introduction of the reducing agent at the metering point (D) is stopped and passive regeneration of the particulate filter (18) is carried out according to the principle of the continuously regenerating particle trap.

Description

[0001] The invention relates to a method for operating an exhaust system through which exhaust gas from an internal combustion engine flows.

[0002] From EP 2 907 984 B1, a method for operating an exhaust system of an internal combustion engine, by means of which, for example, a motor vehicle can be powered, is known. The exhaust system comprises a first SCR catalyst, which is designed as an SCR coating of a particulate filter, and a second SCR catalyst, which is arranged downstream of the first SCR catalyst in the direction of exhaust gas flow through the exhaust system. The respective SCR catalyst is designed to effect or support selective catalytic reduction (SCR), whereby nitrogen oxides (NOx) contained in the exhaust gas are reduced during the SCR process. xThe nitrogen oxides are converted into nitrogen and water, particularly with the help of a reducing agent introduced into the exhaust gas. This process removes the nitrogen oxides from the exhaust gas. Nitrogen oxide removal means, in particular, that the nitrogen oxides contained in the exhaust gas are at least partially removed. To effect or support the SCR process, the respective SCR catalyst has, for example, a coating that is catalytically effective with regard to the SCR process.

[0003] The exhaust system from EP 2 907 984 B1 has at least one metering point arranged upstream of the particulate filter, at which the said reducing agent can be introduced into the exhaust gas for the purpose of denitrification, or is introduced.

[0004] It is generally known that the particulate filter can passively regenerate at temperatures above 300 degrees Celsius. EP 2 907 984 B1 mentions the possibility of limiting the denitrification efficiency of the first SCR catalyst by providing a second dosing point for the reducing agent mentioned in EP 2 907 984 B1 downstream of the first SCR catalyst, in order to ensure sufficient passive regeneration of the particulate filter.

[0005] German patent DE 10 2011 017 482 A1 discloses a method for operating an exhaust system through which exhaust gas from an internal combustion engine flows, wherein the exhaust system comprises an SCR-coated particulate filter, an SCR catalyst arranged downstream of the SCR-coated particulate filter, and a metering point arranged upstream of the SCR-coated particulate filter at which a reducing agent for denitrification of the exhaust gas can be introduced. If the soot load of the particulate filter exceeds a threshold value, in a first step the quantity of the reducing agent introduced into the exhaust gas at the metering point is increased beyond the current target fill level of the SCR-coated particulate filter and the SCR catalyst, and in a second step following the first step, at least the increased introduction of the reducing agent at the metering point is stopped and active regeneration of the particulate filter is carried out.Furthermore, if the exhaust gas temperature is within a predefinable temperature range above 300 degrees Celsius, passive regeneration of the particulate filter can be carried out according to the principle of the continuously regenerating particle trap.

[0006] German patent DE 10 2015 012 736 A1 discloses a method for operating an exhaust system through which exhaust gas from an internal combustion engine flows, wherein the exhaust system comprises an SCR-coated particulate filter, an SCR catalyst arranged downstream of the SCR-coated particulate filter, and a first metering point arranged upstream of the SCR-coated particulate filter, at which a reducing agent for denitrification of the exhaust gas can be introduced. A second metering point is provided downstream of the SCR-coated particulate filter and upstream of the SCR catalyst. Depending on the temperature of the SCR-coated particulate filter, a control unit regulates the introduction of the reducing agent via the first metering point or via the second metering point. The object of the present invention is to further develop a method of the type mentioned above in such a way that particularly low-emission operation with low fuel and reducing agent consumption is achievable.

[0007] This problem is solved by a method with the features of claim 1. Advantageous embodiments with expedient further developments of the invention are specified in the remaining claims.

[0008] The inventive method according to claim 1 provides that then, - if the soot load of the particulate filter exceeds a predefinable first threshold but is below a second predefinable threshold, and the exhaust gas temperature is in a predefinable temperature range above 300 degrees Celsius, in a first step the quantity of reducing agent introduced into the exhaust gas at the metering point is increased beyond the current target fill level of the first SCR catalyst, and in a second step following the first step the introduction of the reducing agent at the metering point is stopped and passive regeneration of the particulate filter is carried out according to the principle of the continuously regenerating particle trap, - if the soot load of the particulate filter exceeds a predefinable second threshold, in a first step an increase is made in the quantity of the reducing agent introduced into the exhaust gas at the dosing point beyond the current target fill level of the first SCR catalyst, and in a second step following the first step at least the increased introduction of the reducing agent at the dosing point is stopped and an active regeneration of the particulate filter is carried out.

[0009] The temperature of the exhaust gas is also referred to as the exhaust gas temperature. Soot loading characterizes or describes the amount of soot particles captured in the particulate filter, which are also simply called soot or particles. As the exhaust system operates, the particulate filter becomes increasingly clogged with soot particles from the exhaust gas, since these particles are filtered out by the particulate filter. The soot particles filtered from the exhaust gas by the particulate filter are deposited inside the filter, so that the soot loading of the particulate filter increases with continued operation of the exhaust system.

[0010] In the first step of the process, that is, when the first step is carried out, the amount of reducing agent injected into the exhaust gas at the dosing point is increased, thereby raising the reducing agent loading of the first SCR catalyst to a predefined target value. The loading of the first SCR catalyst with the reducing agent is also referred to as the reducing agent loading and describes or characterizes the amount of reducing agent, particularly unused, contained or absorbed by the first SCR catalyst. The reducing agent loading is also referred to as the fill level or reducing agent level of the first SCR catalyst.

[0011] The respective SCR catalyst is designed to effect or support selective catalytic reduction (SCR). For this purpose, the SCR catalyst has, for example, at least one coating that is catalytically effective with regard to SCR. During SCR, nitrogen oxides (NOx) contained in the exhaust gas are reduced. xThe nitrogen oxides are converted into nitrogen and water with the aid of the reducing agent, in particular with the help of ammonia from the reducing agent, thereby removing the nitrogen oxides from the exhaust gas. The removal of the nitrogen oxides from the exhaust gas means, in particular, that the nitrogen oxides contained in the exhaust gas are at least partially converted into nitrogen and water and thus removed from the exhaust gas. The unused reducing agent means, in particular, that the unused reducing agent has not yet participated in the SCR process and is therefore still available to participate in the SCR process and to convert the nitrogen oxides contained in the exhaust gas into nitrogen and water within the framework of the SCR process.

[0012] The method according to the invention further comprises a second step, which is carried out after the first step. In the second step of the method, the introduction of the reducing agent at the metering point is stopped. In other words, the introduction of the reducing agent at the metering point, also referred to as metering or dosing, is stopped when the reducing agent load, i.e., the fill level of the first SCR catalyst, has reached a predetermined maximum fill level or a predetermined percentage increase from the current target fill level, and regeneration of the particulate filter is carried out. The respective regeneration of the particulate filter according to the invention reduces its soot load.At least partially during the regeneration of the particulate filter, in particular during a predominant part of the regeneration period or during the entire duration of the regeneration, the introduction of reducing agent at the dosing point is omitted.

[0013] The first step represents a preliminary procedure for the regeneration of the particulate filter, since the first step is carried out, at least partially, and in particular predominantly or completely, before the actual regeneration of the particulate filter and is thus carried out in preparation for the regeneration.

[0014] Increasing the reducing agent loading to the target value means, in particular, that the reducing agent loading of the first SCR catalyst is increased compared to a point in time prior to the execution of the second step. In other words, the reducing agent loading of the first SCR catalyst has a certain initial value at the aforementioned point in time prior to the execution of the first step. Through the first step, or by performing the first step, the reducing agent loading of the first SCR catalyst is increased from this initial value to the specified maximum loading level. This initial value is, for example, the current target loading level, which is determined, in particular, calculated, using a computational model.The current target fill level and / or the increase or the specified maximum fill level value is determined in particular depending on the exhaust gas temperature and the aging of the first SCR catalyst.

[0015] It has proven particularly advantageous to increase the reducing agent loading of the first SCR catalyst by 50 to 100 percent compared to the specified target level, i.e., compared to the initial value, so that, for example, the target value is at least 150 percent, and especially at least 200 percent, of the initial value, or the target value lies within a range of 150 percent to 200 percent of the initial value. This ensures a sufficient level reserve, enabling low-reducing agent consumption and low-emission operation.

[0016] Within the scope of the invention, an SCR catalyst, i.e., the first SCR catalyst and the second SCR catalyst, is understood to be an exhaust gas aftertreatment element that is catalytically effective with respect to SCR and by means of which SCR can be effected or, in particular, catalytically supported. By increasing the reduction agent loading of the first SCR catalyst, a fill level build-up occurs in the first SCR catalyst, thereby creating a supply of reducing agent.If the soot load in the particulate filter exceeds the first threshold but remains below the second predefinable threshold, and the exhaust gas temperature is above 300 degrees Celsius within the predefinable temperature range, the reduced amount of reductant stored in the first SCR catalyst is shifted to the second SCR catalyst at elevated temperatures. There, while passive regeneration of the particulate filter takes place, the exhaust gas can be denitrified. By eliminating the need to introduce the reducing agent at the metering point according to the invention, sufficient NO₂ is advantageously available in the particulate filter for soot combustion, and the regeneration of the particulate filter can be carried out particularly effectively. This allows pollutant emissions, reduced reductant consumption, and fuel consumption of the internal combustion engine to be kept especially low.In other words, it is possible to shift reducing agent from the first SCR catalyst to the second SCR catalyst by building up the fill level, while no reducing agent is introduced at the dosing point. Particularly when the exhaust system has only one dosing point, namely the one mentioned above, and thus this is the only point where reducing agent can be introduced into the exhaust gas, this fill level build-up can lead to a significant, cyclical overdosing of the reducing agent. This significant overdosing results in the aforementioned fill level build-up in the first SCR catalyst and subsequently in a fill level build-up in the second SCR catalyst, as reducing agent can then pass from the first SCR catalyst to the second.

[0017] The inventive method for the active regeneration of the particulate filter also allows for an overall saving of reducing agent due to the comparatively good SCR efficiency of the second SCR catalyst. The inventive introduction of a reduction agent reserve onto the second SCR catalyst prior to active regeneration of the particulate filter aids in the reduction of nitrogen oxides, since the lower temperature level in the second SCR catalyst, located further from the engine, provides better conditions for good SCR efficiency compared to the first SCR catalyst located closer to the engine. The inventive method eliminates the need for an overdose of reducing agent for SCR in the first SCR catalyst, which would otherwise be necessary to compensate for increased ammonia oxidation occurring at high temperatures.

[0018] To achieve particularly low-emission operation, it is provided that exhaust gas recirculation, in particular low-pressure exhaust gas recirculation (LP-EGR), is terminated when reducing agent slip of the first SCR catalyst is detected. Reducing agent slip of the first SCR catalyst means that the first SCR catalyst allows an excessive amount of unused reducing agent to pass through, so that downstream of the first SCR catalyst, and especially upstream of the second SCR catalyst, an excessively large amount of unused reducing agent is contained in the exhaust gas. In the process according to the invention, this regularly occurs when, after an increased introduction of reducing agent at the metering point, the reducing agent is pushed onto the second SCR catalyst.The idea of ​​the invention is, in particular, that a strategy for carrying out exhaust gas recirculation is adapted to any reducing agent slip that may occur in the first SCR catalyst, in order to avoid excessive recirculation of unused reducing agent and thus of unused ammonia. This allows raw nitrogen oxide emissions, N₂O emissions, and corrosion in an exhaust gas recirculation return line to be kept within low limits.

[0019] In one embodiment of the method according to the invention, the temperature range extends from 330 degrees Celsius up to and including 470 degrees Celsius, and in particular from 350 degrees Celsius up to and including 450 degrees Celsius. Within this temperature range, the regeneration of the particulate filter can be carried out with a comparatively high conversion rate as passive regeneration according to the principle of the continuously regenerating particle trap (CRT). Passive regeneration according to the principle of the continuously regenerating particle trap is a passive regeneration of the particulate filter, since it can be carried out without special measures, in particular without actively increasing the exhaust gas temperature.During passive regeneration, the soot particles captured in the particulate filter are converted, primarily on a continuous basis, to carbon dioxide (CO2) and nitrogen monoxide (NO) using nitrogen dioxide (NO2) contained in the exhaust gas. This conversion occurs through the oxidation of the soot particles with nitrogen dioxide. The reduction of soot loading is also referred to as soot removal. The nitrogen monoxide produced during soot removal can be removed from the exhaust gas by at least one of the SCR catalysts and, in the process, converted into nitrogen and water, particularly with the help of a reducing agent. The temperature range in which passive regeneration is carried out is not actively set by increasing the exhaust gas temperature; rather, passive regeneration occurs when the exhaust gas temperature is within this range, without any active increase in the exhaust gas temperature.

[0020] This allows fuel consumption and emissions to be kept particularly low.

[0021] In a further embodiment of the invention, the first threshold value is advantageously three grams of soot loading per liter of volume of the particulate filter. This means that the particulate filter has a volume, in particular a filter volume, for filtering the soot particles, and the threshold value of the soot loading is three grams per liter of volume of the particulate filter.

[0022] In one embodiment of the inventive method, the introduction of reducing agent is restarted after passive regeneration has been carried out when the soot loading falls below a lower limit value of approximately 2 grams per liter compared to the first threshold value.

[0023] In one embodiment of the inventive method, the reintroduction of reducing agent occurs after the completion of the increased introduction of the reducing agent for passive regeneration, if passive regeneration is aborted, particularly if the exhaust gas temperature leaves the aforementioned temperature range, also referred to as the temperature window. Advantageously, the operation of an exhaust system according to this embodiment of the invention allows the emission of the internal combustion engine to be kept particularly low, since the introduction of reducing agent remains switched off only under optimal operating conditions for passive regeneration reactions of the soot in the particulate filter, i.e., only when the passive regeneration reaction in the particulate filter exhibits a high conversion rate.As soon as the operating conditions of the exhaust system are outside the temperature range, it is advantageous to introduce a reducing agent that is tailored to the currently prevailing operating conditions.

[0024] In one embodiment of the inventive method, the re-introduction of reducing agent occurs after the completion of the increased introduction of the reducing agent for passive regeneration, if passive regeneration is aborted, particularly if a quantity of reducing agent previously introduced into the second SCR catalyst has been used up. Advantageously, the operation of an exhaust system according to this embodiment of the invention allows the emission of nitrogen oxides from the internal combustion engine to be kept particularly low.

[0025] In one embodiment of the method according to the invention, the second threshold value is at least approximately 6 grams per liter of volume of the particle filter.

[0026] Active regeneration of the particulate filter means, in particular, that the exhaust gas temperature is actively increased, especially through measures within the engine, to a value of more than approximately 500 degrees Celsius. This allows soot deposited in the particulate filter to be burned off, primarily through an oxidation reaction with the residual oxygen in the exhaust gas. This increase in exhaust gas temperature allows, for example, reducing agents to be transferred from the first SCR catalyst to the second SCR catalyst, which is significantly cooler, and used there to reduce nitrogen oxide emissions from the exhaust gas.It was found that, particularly at the beginning of active regeneration, the second SCR catalyst can be approximately 100 to 200 degrees Celsius colder than the particulate filter and / or the first SCR catalyst. This is because the second SCR catalyst is located downstream of both the particulate filter and the first SCR catalyst, and therefore further away from the combustion engine than the particulate filter and the first SCR catalyst. Consequently, the heating of the second SCR catalyst is delayed compared to the particulate filter and the first SCR catalyst. This is also possible at high temperatures when using the second dosing point or when the fill level is built up directly in the second SCR catalyst, for example, by directly driving over or bypassing the first SCR catalyst.

[0027] Overall, it is extremely advantageous that the inventive method for passive regeneration of the particulate filter allows the time intervals between the active regenerations of the particulate filter necessary to comply with emission regulations, which result in a significant increase in fuel consumption (CO2), to be extended.

[0028] If, for example, the exhaust system has a second dosing point located downstream of the first SCR catalyst and upstream of the second SCR catalyst, it is conceivable to discontinue, and in particular refrain from, the introduction of reducing agent at the first dosing point, while reducing agent is introduced into the exhaust gas at a second dosing point, thus ensuring continuous dosing of reducing agent at the second dosing point. In other words, it is conceivable that alternating operation of the SCR catalysts is provided. Such alternating operation is particularly relevant when both the first and second dosing points are in use.For example, depending on the temperature and exhaust gas mass flow, either the first front SCR catalyst or the second rear SCR catalyst is used to remove nitrogen oxides from the exhaust gas. Therefore, if the exhaust gas is processed by the first SCR catalyst or the second SCR catalyst, nitrogen oxide removal by the second SCR catalyst or the first SCR catalyst, respectively, does not occur. For example, at high temperatures, nitrogen oxide removal is performed primarily, and in particular completely, by the second SCR catalyst, or a reducing agent is introduced into the exhaust gas at the second dosing point. At low temperatures, however, nitrogen oxide removal is performed primarily, and in particular completely, by the first SCR catalyst, or a reducing agent is introduced into the exhaust gas at the first dosing point.This allows a particularly advantageous temperature window to be used for the denitrification of the exhaust gas, whereby the denitrification of the exhaust gas is also known as NO. x The term "turnover" is used. The alternating operation or the distribution of the reducing agent dosage across the dosing points can depend on various factors such as aging, temperatures, exhaust gas mass flow, load range, operating mode, etc.

[0029] However, providing exactly one metering point, especially in the form of the first metering point, is advantageous insofar as the number of parts and thus the costs, weight and installation space requirements of the exhaust system can be kept particularly low.

[0030] Further advantages, features, and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing. The features and combinations of features mentioned above in the description, as well as those mentioned below in the figure description and / or shown in the figures alone, can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the invention.

[0031] The drawing shows in the single figure a schematic representation of an exhaust system which is operated by means of a method according to the invention.

[0032] The single figure shows a schematic representation of an exhaust system 10 for an internal combustion engine, in particular of a motor vehicle such as a car, and preferably a passenger car. The motor vehicle is powered by the internal combustion engine. The internal combustion engine has an intake manifold through which air flows, and the air flowing through the intake manifold is guided into at least one combustion chamber of the internal combustion engine, which is designed, in particular, as a cylinder. The combustion chamber is supplied with air and fuel for operating the internal combustion engine, so that a fuel-air mixture is formed in the combustion chamber. The fuel-air mixture is burned, resulting in exhaust gas from the internal combustion engine. In particular, the internal combustion engine is designed, for example, as a diesel engine or as another type of internal combustion engine.The exhaust system 10 is used to discharge the exhaust gas from the combustion chamber. The exhaust gas from the internal combustion engine flows through the exhaust system 10, with an arrow 12 in the figure illustrating the exhaust gas flowing through the exhaust system 10.

[0033] The exhaust system 10 comprises a first SCR catalyst 14 and a second SCR catalyst 16, which are designed to effect or support selective catalytic reduction (SCR). This means that the respective SCR catalyst 14 or 16 has at least one coating that is catalytically effective with regard to SCR. During SCR, nitrogen oxides (NOx) contained in the exhaust gas are reduced. xThe nitrogen oxides are converted into nitrogen and water, particularly with the aid of a reducing agent, and especially with ammonia from the reducing agent. This process removes the nitrogen oxides from the exhaust gas. Therefore, removing the nitrogen oxides from the exhaust gas means that at least some of them are removed by converting them into water and nitrogen.

[0034] The figure shows that the second SCR catalyst 16 is arranged downstream of the first SCR catalyst 14 in the direction of exhaust gas flow through the exhaust system 10. The second SCR catalyst 16 is, for example, a so-called underbody catalyst or underbody SCR, since it is located, for example, in or below the underbody of the vehicle. The underbody is formed, for example, by the vehicle's superstructure, which may be a self-supporting body. The first SCR catalyst 14 is located, for example, together with the internal combustion engine in an engine compartment of the superstructure. Thus, the second SCR catalyst 16 is significantly further away from the internal combustion engine than the first SCR catalyst 14.

[0035] The exhaust system 10 also includes a particulate filter 18, which is designed, for example, as a diesel particulate filter (DPF). The particulate filter 18 filters soot particles contained in the exhaust gas. The soot particles filtered from the exhaust gas by the particulate filter 18 are deposited inside the particulate filter 18, so that the soot loading of the particulate filter 18 increases with the ongoing operation of the exhaust system 10. The soot loading thus characterizes the quantity of soot particles that have been or are being filtered from the exhaust gas by the particulate filter 18. The particulate filter 18 is designed as an SCR-coated particulate filter. Therefore, the particulate filter 18 is designed as a so-called SCR-coated particulate filter. The SCR-coated catalyst has at least one coating that is catalytically active with regard to the SCR process.Thus, the particle filter 18 is also designed to support or effect the SCR, especially catalytically.

[0036] The exhaust system 10 also has a metering point D at which the aforementioned reducing agent can be introduced into the exhaust gas, in particular by injection. For this purpose, at least one metering element 20 is arranged at the metering point D, by means of which the reducing agent can be introduced into the exhaust gas or is introduced. The introduction of the reducing agent into the exhaust gas is also referred to as metering or injection.

[0037] The reducing agent is, in particular, a liquid reducing agent, which may be in the form of an aqueous urea solution (HWL). Especially after the reducing agent is introduced into the exhaust gas, ammonia (NH3) can result from the reduced agent, as the reducing agent decomposes into ammonia. During the SCR process, the nitrogen oxides contained in the exhaust gas can react with the ammonia and thereby be converted into water and nitrogen.

[0038] Preferably, the exhaust system 10 has exactly one metering point in the form of metering point D, at which the reducing agent can be introduced into the exhaust gas. In other words, the exhaust system 10 has, for example, metering point D as the only metering point at which reducing agent can be introduced into the exhaust gas. Optionally, the exhaust system 10 may have a second metering point D2, which is located downstream of the first metering point D and downstream of the first SCR catalyst 14 and upstream of the second SCR catalyst 16. For example, if the exhaust system 10 has the second metering point D2, reducing agent can be introduced into the exhaust gas at the second metering point D2. The exhaust system 10 also has a storage catalyst 22, which is located upstream of the particulate filter 18 and, in particular, upstream of metering point D. The storage catalyst 22 is also referred to as a NO storage catalyst. x-Storage catalyst, as the storage catalyst can store 22 nitrogen oxides from the exhaust gas.

[0039] In the figure, arrow 24 illustrates any reducing agent slip that may occur in the first SCR catalyst 14, where the reducing agent slip is also simply referred to as slip. Reducing agent slip means that the first SCR catalyst 14 allows an excessively large amount of unused reducing agent to pass through, so that an excessively large amount of unused reducing agent is present in the exhaust gas at a location downstream of the first SCR catalyst 14 and upstream of the second SCR catalyst 16. Unused reducing agent specifically means that the unused reducing agent has not (yet) participated in the SCR process and is therefore still available to carry out the SCR and thus to reduce the nitrogen oxide emissions from the exhaust gas.

[0040] The exhaust system 10 also has a branch point A for exhaust gas recirculation. The exhaust gas recirculation is, for example, a low-pressure exhaust gas recirculation (LP-EGR) system and includes, for example, a recirculation line (not shown in the figure). At branch point A, at least a portion of the exhaust gas flowing through the exhaust system 10 can be diverted from the exhaust system 10 and returned to the intake manifold via the recirculation line. The diverted exhaust gas flowing through the recirculation line is returned to the intake manifold and, in particular, introduced into it. The exhaust gas introduced into the intake manifold is carried along by the air flowing through the intake manifold and transported into the combustion chamber. The exhaust gas recirculation system includes at least one exhaust gas recirculation valve 26, for example, a flap, by means of which the amount of exhaust gas flowing through the recirculation line can be adjusted.In other words, the amount of exhaust gas to be recirculated can be adjusted using the exhaust gas recirculation valve 26.

[0041] The figure clearly shows that the branch point A is located downstream of the first SCR catalyst 14 and upstream of the second SCR catalyst 16, and is specifically positioned closer to the first SCR catalyst 14 than to the second SCR catalyst 16. Furthermore, the exhaust system 10 includes an ammonia slip catalyst 28, also known as an ASC. The ASC catalytically converts unused reducing agent, particularly unused ammonia, which may still be present in the exhaust gas downstream of the SCR catalysts 14 and 16, into nitrogen and water, thus preventing excessive reducing agent or ammonia emissions.

[0042] The following describes a method for operating the exhaust system 10, whereby particularly fuel-efficient, low-reducing agent-consumption, and low-emission operation can be achieved. In this method, a first step is carried out when the soot load of the particulate filter 18 exceeds a predefinable first threshold of 3 grams per liter of the particulate filter 18 and the exhaust gas temperature, also referred to as the exhaust gas temperature, is within a predefinable temperature range between 350 and 450 degrees Celsius, particularly at a point located upstream of the particulate filter 18.In the first step of the process, that is, when the first step is carried out, the quantity of reducing agent introduced into the exhaust gas at metering point D, in particular by means of the metering element 20, is increased, thereby increasing the reducing agent loading of the first SCR catalyst 14 to a predefinable maximum level. The reducing agent loading of the first SCR catalyst 14 describes the level of, in particular, unused reducing agent in the first SCR catalyst 14. In other words, the reducing agent loading describes or characterizes the quantity of reducing agent, in particular unused, that has been absorbed by the first SCR catalyst 14.At a point in time prior to the execution of the first step, the reducing agent loading of the first SCR catalyst 14, for example, has a first value. Through the first step, or by carrying out the first step, the reducing agent loading is increased from this first value to the specified maximum fill level value, by increasing the amount of reducing agent introduced into the exhaust gas at the dosing point D. The increase in the amount of reducing agent introduced into the exhaust gas at the dosing point D means, in particular, that before the execution of the first step, or at the aforementioned point in time, the amount of reducing agent introduced into the exhaust gas at the dosing point D has a second value.By increasing the amount of reducing agent introduced into the exhaust gas at dosing point D, the quantity is increased from the second value to a correspondingly larger third value, so that during the execution of the first step a larger quantity of reducing agent is introduced into the exhaust gas at dosing point D than before the execution or before the start of the execution of the first step.

[0043] The process further comprises a second step, which is carried out after the first step and specifically when the reducing agent loading of the first SCR catalyst has reached its maximum level. In the second step, the introduction of the reducing agent at metering point D is stopped, and a passive regeneration of the particulate filter 18 is performed or effected, thereby at least reducing the soot loading of the particulate filter 18. During the entire period, no reducing agent is introduced at metering point D, so that the particulate filter regenerates at least partially. This allows the passive regeneration to be carried out particularly effectively and thus with low fuel consumption, while maintaining sufficient NOx reduction in the exhaust gas.This is achieved by increasing the reducing agent loading, or the fill level, of the SCR catalyst 14 before the passive regeneration of the particulate filter 18. This creates a reserve level in the first SCR catalyst 14 during the first step of the process, ensuring that, despite the fact that no reducing agent is introduced into the exhaust gas during passive regeneration, the second SCR catalyst 16 is supplied with a sufficient quantity of reducing agent to remove nitrogen oxides from the exhaust gas. Alternatively or additionally, creating this reserve level allows for a sufficient quantity of unused reducing agent to be stored in the SCR catalyst 14 to achieve adequate nitrogen oxide removal from the exhaust gas.

[0044] It has proven particularly advantageous if the maximum fill level is approximately 150 percent to 200 percent of the first value, where the first value is the current target fill level of the SCR catalyst 14. Thus, a percentage increase in the fill level is provided. Furthermore, the maximum fill level of the SCR catalyst 16, including the SCR coating of the particulate filter 18 with reducing agent, can also be specified. The SCR catalyst 14 and the particulate filter 18 of the present invention form an exhaust aftertreatment module, in particular in the form of a front combination unit, by means of which the exhaust gas can be aftertreated particularly advantageously.

[0045] Passive regeneration is aborted when the exhaust gas temperature leaves the specified temperature range and terminates when the soot load of the particulate filter 18 falls below a predefinable limit of 2 grams per liter volume of the particulate filter 18.

[0046] If the soot load of the particulate filter 18 exceeds a predefinable second threshold of 6 grams per liter of particulate filter volume 18, such that active regeneration of the particulate filter 18 is required, prior to actively raising the exhaust gas temperature to approximately 500 degrees Celsius (particularly by means of in-engine measures), an increase in the quantity of reducing agent introduced at metering point D is carried out according to the first step of the method according to the invention. In the second step of the method, at least the increased introduction of the reducing agent at metering point D is stopped, and active regeneration of the particulate filter 18 is carried out or effected.

[0047] Additionally, exhaust gas recirculation is always prevented when reducing agent slip from the first SCR catalyst 14 is detected. This prevents an excessive amount of unused reducing agent from being recirculated and introduced into the intake manifold. This allows for particularly low-emission operation. Reference symbol list 10 Exhaust system 12 Arrow 14 first SCR catalyst 16 second SCR catalyst 18 particulate filters 20 dosing element 22 Storage catalyst 24 Arrow 26 Valve element 28 Ammonia slip catalyst A junction D Dosing point D2 second dosing point

Claims

[1] Method for operating an exhaust system (10) through which exhaust gas from an internal combustion engine flows, wherein the exhaust system (10) comprises a first SCR catalyst (14), a second SCR catalyst (16) arranged downstream of the first SCR catalyst (14), a particulate filter (18) arranged upstream of the first SCR catalyst (14) or in a common block with the first SCR catalyst (14), and at least one metering point (D) arranged upstream of the particulate filter (18) at which a reducing agent for denitrification of the exhaust gas can be introduced into it, in which then, - if the soot load of the particulate filter (18) exceeds a predefinable first threshold but is below a second predefinable threshold and the exhaust gas temperature is in a predefinable temperature range above 300 degrees Celsius, in a first step the quantity of reducing agent introduced into the exhaust gas at the metering point (D) is increased beyond the current target fill level of the first SCR catalyst (14), and in a second step following the first step the introduction of the reducing agent at the metering point (D) is stopped and passive regeneration of the particulate filter (18) is carried out according to the principle of a continuously regenerating particle trap, - if the soot load of the particulate filter (18) exceeds the second threshold value, in a first step the quantity of reducing agent introduced into the exhaust gas at the metering point (D) is increased beyond the current target fill level of the first SCR catalyst (14), and in a second step following the first step at least the increased introduction of the reducing agent at the metering point (D) is stopped and an active regeneration of the particulate filter (18) is carried out, and - a low-pressure exhaust gas recirculation (LP-EGR) is terminated when a reducing agent slip of the first SCR catalyst (14) is detected. [2] Method according to claim 1, characterized by that the temperature range extends from 330 degrees Celsius inclusive to 470 degrees Celsius inclusive. [3] Method according to claim 1 or 2, characterized by, that the first threshold is at least approximately three grams per liter of volume of the particulate filter (18). [4] Method according to any one of the preceding claims, characterized by , that the introduction of reducing agent is restarted after passive regeneration if the soot load falls below a lower limit of approximately 2 grams per liter compared to the first threshold. [5] Method according to any one of the preceding claims, characterized by , that the introduction of reducing agent is restarted after passive regeneration if the exhaust gas temperature leaves the predefinable temperature range. [6] Method according to any one of the preceding claims, characterized by, that the introduction of reducing agent is restarted after passive regeneration has been carried out when a quantity of reducing agent previously introduced into the second SCR catalyst (16) has been used up. [7] Method according to claim 1, characterized by , that the second threshold is at least approximately 6 grams per liter of volume of the particulate filter (18).

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