Functional diagnosis of a secondary catalyst of an exhaust aftertreatment system of an internal combustion engine

Abstract

The invention relates to a method, a control device (7), and a computer program (11) for functional diagnostics of a secondary catalyst (SK) of an internal combustion engine (1) comprising a first exhaust catalyst (3) and a downstream second secondary catalyst (SK). The method is carried out by means of a control device (7) that receives a first and a second sensor signal (Sig1, Sig2) that are representative of at least one pollutant fraction in the exhaust mass flow (11a).A diagnostic routine is performed to check the first exhaust catalyst (3) by repeatedly switching between a lean operating phase and a rich operating phase depending on the first sensor signal (Sig1). At least one pollutant fraction in the exhaust mass flow (11a) downstream of the secondary catalyst (SK) and a pollutant mass integral (SMI_SK) for the relevant pollutant are determined based on the second sensor signal (Sig2) over the duration of the diagnostic routine. If the pollutant mass integral value (SMI_SK) exceeds an assigned pollutant integral limit (S_IGw), the secondary catalyst (SK) is diagnosed as "no longer sufficiently functional" or "not present." This allows for an advantageous functional check of the secondary catalyst (SK).

Description

The invention relates to a method, a control device, and a computer program for performing a functional diagnosis of a secondary catalyst of an exhaust aftertreatment system (AgNbgs device) of an internal combustion engine (ICU), which has at least a first exhaust catalyst located close to the ICU and a second exhaust catalyst located downstream of the first exhaust catalyst and further away from the engine in the exhaust mass flow, the latter being referred to here and subsequently as the secondary catalyst. Since such a catalyst is generally located on the underbody of a motor vehicle (MV) powered by an internal combustion engine, the terms underfloor catalyst (UB-cat) or underfloor catalyst (UFC) are also commonly used. The two catalysts are generally designed as oxygen-storage three-way catalysts. The increasingly strict Europe-wide legislation regarding pollutant emissions from internal combustion engines, especially combustion engines in motor vehicles, requires not only the use and increasingly comprehensive design of AgNbgs devices, but also the constant monitoring of the proper functioning of all device components that influence pollutant emissions. To ensure compliance with the required emission limits, AgNbgs devices are therefore equipped with combinations of multiple exhaust gas catalysts and particulate filters. These are complemented by exhaust gas sensors, such as lambda probes to determine the oxygen content in the exhaust gas or NOx sensors, from whose signals a lambda value can be calculated in addition to the NOx and NH3 concentrations. The values ​​supplied by the various sensors are used, on the one hand, for the control and regulation of the internal combustion engine, especially with regard to pollutant emissions, and for monitoring pollutant emissions, and on the other hand, also for functional monitoring of the individual components of the AgNbgs device. In order to comply with the emission limits based on the latest legislation, it is essential to arrange, in addition to a first exhaust gas catalyst located close to the internal combustion engine at the upstream end of the AgNbgs device, for example a three-way catalyst, at least a second exhaust gas catalyst or secondary catalyst at the downstream end of the AgNbgs device, for example also designed as a three-way catalyst, in order to eliminate pollutant components that could not be eliminated up to that point due to malfunction or insufficient function of the upstream catalysts in relation to the exhaust gas mass flow.For the final monitoring of the AgNbgs device and in particular the emission of pollutants into the environment, a NOx sensor is arranged downstream of the secondary catalyst, the signal of which can indicate a breakthrough of pollutants, in particular NOx or NH3, on the basis of which an error message can in turn be generated, output or stored. Due to the error message, a check and, if necessary, a repair of the AgNbgs device is indicated, and the operation of the BKM may be limited to emergency operation until then. However, such an error message currently provides no information about which individual component of the AgNbgs device is defective or impaired due to age. This can significantly increase the effort required for repairs, and it is desirable for a vehicle- or control-system-internal diagnostic system to provide information about which of the exhaust catalysts is defective or impaired. In particular, a so-called full measurement, in which intrusive lambda stimulation (rich / lean cycles) of the secondary catalyst is performed until NOx breakthrough occurs, cannot be carried out for diagnosing the secondary catalyst, as such NOx breakthroughs are included in the emissions balance and inevitably increase emissions to an impermissible level.Furthermore, because the first catalyst located close to the engine must always be conditioned along with the fuel, and because the secondary catalyst is located at the downstream end of the AgNbgs system, continuous, uninterrupted stimulation must be ensured. During this time, no higher-priority actions, such as component protection, overrun fuel cut-off, or load changes may occur. Since such functions take precedence over diagnostics, it becomes increasingly unlikely that an operating cycle can be completed without interrupting the diagnostic process. This results in an unacceptably low "InUse Monitoring Performance Ratio" (minimum frequency of diagnostics across operating cycles). Furthermore, it is desirable that such a diagnosis does not require additional sensors or other costly measures and can be carried out solely with the existing means of the exhaust aftertreatment device. The present invention is therefore based on the objective of creating a means by which it is possible to diagnose the functional state or the defectiveness, in particular of a secondary catalyst of an AgNbgs device of the type described above, in a simple manner and without additional hardware components, whereby a sufficient frequency of these diagnoses can be ensured in the ongoing, intended operation of the BKM. This problem is solved by the subject matter of the independent claims. The inventive method takes advantage of the fact that, during the diagnostic routine for checking the function of the first exhaust catalyst, a breakthrough of further pollutants also occurs during each lean-burn phase until oxygen breakthrough at the first exhaust catalyst. These pollutants then reach the secondary catalyst and, if functioning correctly, are neutralized by it to a minimum. If the pollutant breakthrough thus generated at the first exhaust catalyst is nevertheless detectable or measurable by means of the exhaust gas sensor arranged downstream of the secondary catalyst in the exhaust stream, it can be assumed that the secondary catalyst is no longer sufficiently functional or is even missing. The advantages of the invention lie in the fact that, in the event of failure of the exhaust aftertreatment device, not only can the failure of the exhaust aftertreatment device itself be detected via "On Board Diagnostics" (OBD), but also, advantageously, the condition of the secondary catalyst located at the downstream end of the exhaust aftertreatment device can be diagnosed, thus making it easier to identify the component of the exhaust aftertreatment device that is responsible for the failure. Advantageous embodiments, further developments and details of the present invention will become apparent from the dependent claims, the description and the drawing. The inventive method for functional diagnosis of a secondary catalyst of an exhaust aftertreatment device of an internal combustion engine relates to an exhaust aftertreatment device comprising a first oxygen-storage-capable exhaust catalyst and a second oxygen-storage-capable catalyst, referred to as a secondary catalyst, located downstream in the exhaust stream. The procedure is carried out by means of at least one electronic control device associated with the exhaust aftertreatment device and the internal combustion engine, by performing the steps described below. According to a first embodiment of the method, for example during the intended operation of the BKM and in particular for carrying out the diagnostic procedure, a first sensor signal is continuously received and evaluated, which is representative of a first oxygen fraction in the exhaust mass flow immediately after the first exhaust catalyst, and a second sensor signal is received and evaluated, which is representative of at least one pollutant fraction in the exhaust mass flow immediately after the secondary catalyst. A diagnostic routine is then initiated to check the function of the first exhaust catalyst. This routine is characterized by multiple transitions between a lean operating phase (λ>1) and a rich operating phase (λ<1) of the internal combustion engine, depending on the first sensor signal indicating the oxygen content of the exhaust gas leaving the first catalyst. For example, if the first sensor signal during a lean operating phase indicates that the oxygen content in the exhaust gas rises above a certain threshold, the system switches to a rich operating phase, and vice versa. During the aforementioned diagnostic routine, the proportion of at least one pollutant in the exhaust gas mass flow downstream of the secondary catalyst is determined in parallel, based on the second sensor signal, which is supplied, for example, by a NOx sensor. At least one pollutant mass integral is then calculated for the pollutant in question over the duration of the diagnostic routine. In other words, the mass of the pollutant in question emitted into the environment over the duration of the diagnostic routine is summed. The value of the pollutant mass integral determined in this way is then compared with a predefined, assigned pollutant integral limit value. If the value of the determined pollutant mass integral exceeds the assigned pollutant integral limit value, the secondary catalyst is diagnosed as "no longer sufficiently functional" or "not present". Using the method described above, a second exhaust catalyst, referred to here as a secondary catalyst, arranged downstream of a first exhaust catalyst in the exhaust aftertreatment device, can now be advantageously tested for proper function independently of the function of the other components of the exhaust aftertreatment device and, in case of failure, repaired or replaced in a targeted manner. The terms "first exhaust catalyst" and "second exhaust catalyst" or "secondary catalyst" refer to two exhaust catalysts arranged sequentially in the exhaust mass flow, for example, three-way catalysts, between which further components of the exhaust aftertreatment system, such as a particulate filter, may be arranged, but no further oxygen storage component may be located. These terms are not intended to limit the total number of exhaust catalysts arranged in the exhaust aftertreatment system to two.In a common configuration, however, the first exhaust catalyst is a three-way catalyst (TWC) located close to the engine, i.e., in the immediate vicinity of the internal combustion engine, and the second exhaust catalyst, i.e., the secondary catalyst, is a so-called underfloor catalyst (UFC) located far from the engine, on the underbody of a motor vehicle powered by the internal combustion engine, also designed as a three-way catalyst (TWC). A second embodiment of the inventive method for functional diagnosis of a secondary catalyst is characterized in that the secondary catalyst of the exhaust aftertreatment device is diagnosed as "fully functional" if the pollutant mass integral value does not reach the predetermined pollutant integral limit value. This also allows for simple confirmation that a sufficiently functional secondary catalyst is present or installed. A third embodiment of the method for the functional diagnosis of a secondary catalyst is characterized in that the at least one pollutant is either nitrogen oxide (NOx) or ammonia (NH3). However, it can be either nitrogen oxide (NOx) or ammonia (NH3), the proportions of which in the exhaust gas are detected simultaneously, possibly by a sum sensor signal. This advantageously ensures that emissions, especially those pollutants considered particularly harmful to the environment, can be reduced to the limits prescribed by law or to a minimum. A further development of the third embodiment described above is characterized by the fact that, based on the second sensor signal, both the nitrogen oxide content and the ammonia content in the exhaust gas mass flow downstream of the secondary catalyst are determined, and based on this, a nitrogen mass integral and an ammonia mass integral are calculated over the duration of the diagnostic routine. This advantageously allows monitoring of the secondary catalyst's function with regard to pollutants considered particularly harmful to the environment. In a further development of the aforementioned enhancement, the secondary catalyst of the exhaust aftertreatment system is diagnosed as "no longer sufficiently functional" or "not present" if at least one of the nitrogen mass integral and ammonia mass integral values ​​exceeds a predefined, associated pollutant integral limit. In this way, a malfunction of the secondary catalyst is reliably detected even if only one of the aforementioned pollutants is present. Alternatively, in a sixth embodiment, a further development of the third embodiment, the method for the functional diagnosis of a secondary catalyst can be characterized in that, based on the second sensor signal, a sum value for the nitrogen oxide content and the ammonia content in the exhaust gas mass flow downstream of the secondary catalyst is determined, and based on this, a pollutant mass integral is calculated over the duration of the diagnostic routine, which includes the sum of the aforementioned pollutant content. Since the second sensor signal can already be a sum signal of the pollutants nitrogen oxide and ammonia, this embodiment advantageously avoids the potentially error-prone separation of the different pollutants for separate analysis.When considering the sum signal, a higher signal amplitude can also be expected (in total), which increases the robustness of the method against system-related deviations. Another embodiment of the inventive method for functional diagnostics of a secondary catalyst is characterized in that the duration of the diagnostic routine is determined by the duration of the lean-burn and rich-burn phases of the internal combustion engine during at least two transitions between the lean-burn and rich-burn phases, including a delay period. However, depending on requirements, three or more transitions between the lean-burn and rich-burn phases can also be provided. The durations of the respective lean-burn and rich-burn phases depend on the course of the first sensor signal, and the delay period is given by the time lag with which the response of the second sensor signal to a respective transition to a lean-burn or rich-burn phase of the internal combustion engine is completed.This time delay results, among other things, primarily from the travel time required for the exhaust gas sample to travel from the exhaust outlet of the combustion engine to the exhaust gas sensor located downstream of the secondary catalyst. According to this design, the duration of the diagnostic routine is variably adjusted to the respective operating conditions at the first exhaust catalyst, which change over its operating or service life, thus advantageously avoiding miscalculations of the respective pollutant mass integrals. Finally, a further embodiment of the method according to the invention is characterized in that, if the secondary catalyst is diagnosed as "no longer sufficiently functional" or "not present," at least one of the following measures is initiated by the electronic control device: - A warning indicator is activated to alert the user of the combustion engine, for example, the driver of a vehicle powered by the combustion engine, to the malfunction of the secondary catalyst; - A corresponding error message is entered into an error memory to enable a quick diagnosis by a repair shop; - A restart of the internal combustion engine (1) is prevented after operator deactivation to prevent continued operation with impermissibly increased emissions. Operator deactivation here means actively switching off or...To understand switching off the BKM by an operator; - A restriction of the operation of the internal combustion engine to a low-emission operating range in order to enable safe and easy transport, for example of a vehicle powered by the BKM, to a parking position or a workshop. These measures can be implemented individually or in any combination and ensure that non-compliant operation of the respective combustion engine cannot go undetected and cannot be ignored by a user / operator of the combustion engine in the long term. This effectively ensures that the environmental impact of pollutants in the exhaust gas remains as low as possible. The electronic control device according to the invention for functional diagnostics of a secondary catalyst of an exhaust aftertreatment device of an internal combustion engine comprises at least the following components: - An electronic storage device in which at least one computer program product with program instructions for carrying out the method is provided and - an input / output interface which is set up for receiving and outputting electrical signals, as well as - an electronic computing unit (processor) for executing program instructions, with access to the at least one electronic storage device and the input / output interface. Thus, the electronic control device is configured such that the program instructions, when executed by means of the electronic processing unit, cause the electronic control device to execute the method according to the invention, according to one of the embodiments described above. The aforementioned advantages of the method according to the invention can be realized with the electronic control device. The computer program product according to the invention for functional diagnostics of a secondary catalyst of an exhaust aftertreatment device of an internal combustion engine comprises program instructions for execution by the electronic computing unit of the electronic control device described above, wherein the program instructions, when executed by the electronic computing unit, cause the electronic control device to carry out the method according to the invention in accordance with one of the embodiments and configurations described above. In other words, the invention relates to a method, a control device, and a computer program for the functional diagnosis of a secondary catalyst of an exhaust aftertreatment system of an internal combustion engine, which has two oxygen-storage-capable exhaust catalysts arranged one after the other in the exhaust mass flow. The method is carried out by means of an electronic control device, wherein the control device receives a first sensor signal and a second sensor signal, which are representative of an oxygen content and at least a pollutant content in the exhaust mass flow, respectively.As part of the procedure, a diagnostic routine is performed to check the function of the first exhaust catalyst. This routine is characterized by multiple cycles between a lean and a rich operating phase of the internal combustion engine, depending on the first sensor signal. At least one pollutant fraction in the exhaust mass flow downstream of the secondary catalyst is determined using the second sensor signal. Based on this, at least one pollutant mass integral is calculated for the pollutant in question over the duration of the diagnostic routine. If the value of this pollutant mass integral exceeds a predefined, assigned pollutant integral limit, the secondary catalyst of the exhaust aftertreatment system is diagnosed as "no longer sufficiently functional" or "not present."This allows for a functional check of the secondary catalyst in a conveniently simple manner and without additional hardware requirements, thus ensuring low-emission operation of the internal combustion engine. The features and combinations of features of the embodiments of the invention mentioned above in the description or below in the description of the figures are to be applied individually, in part, or in their entirety, including in combination or complementary form, in further developments of the invention, without departing from the scope of the invention, insofar as they are not alternatively applicable or mutually exclusive. Features and details described in connection with the method naturally also apply in connection with the electronic control device and the computer program product, and vice versa, so that mutual reference can always be made to the individual aspects of the invention with regard to the disclosure of this invention. The figures below illustrate particularly advantageous embodiments, details or further developments of the invention, although the subject matter of the invention is not intended to be limited to these examples. Figure 1 shows a simplified schematic representation of an internal combustion engine with an exhaust aftertreatment device. Figure 2 shows a simplified flowchart of the method according to the invention. Figure 3 shows a simplified signal diagram of the process flow in an embodiment of the method with an intact secondary catalyst. Figure 4 shows a further simplified signal diagram of the process flow in an embodiment of the method with a non-functional secondary catalyst. Figure 5 shows a further simplified signal diagram of the process flow in an embodiment of the method in which a pollutant mass integral is calculated, which includes the sum of the aforementioned pollutant fractions in the exhaust gas mass flow. The figures in this application are to be considered schematic only, and the relative sizes of the individual figures and the elements depicted within them are not to scale. Rather, individual elements may be exaggerated for clarity and / or to improve comprehensibility. Elements with identical functions, names, or similar effects are identified across all figures by the same reference symbols. In some figures, individual reference symbols may be omitted to improve clarity. Fig. 1 shows a combustion engine 1, which can be designed, for example, as a reciprocating internal combustion engine with four cylinders (indicated). The combustion engine 1 is supplied with air-fuel mixture 10a from the intake side via an air-fuel mixture supply unit 10. The air-fuel mixture supply unit 10 represents, for example, a fuel injection system in conjunction with a throttle valve, which is connected to the electronic control device 7 via signal lines 8 and is controlled in such a way that an air-fuel mixture 10a is supplied to the combustion chambers of the combustion engine 1 in a predetermined quantity and composition.Thus, the electronic control device 7, according to the program instructions contained therein, can influence both the power supplied and the composition of the exhaust gas mass flow 11a, in particular the so-called lambda value, which provides information about the oxygen content in the exhaust gas, and the NOx concentration in the exhaust gas. Connected to the outlet side of the engine block 1 (ECU 1) is an exhaust gas recirculation (EGR) device 2 comprising a first exhaust gas catalyst 3, a second exhaust gas catalyst 4, and a particulate filter 5, which are connected to the ECU 1 via an exhaust pipe 11 and interconnected. The exhaust gas mass flow 11a is directed through the exhaust pipe 11 to the aforementioned components of the EGR device 2. The exhaust gas mass flow 11a is represented by arrow symbols indicating the direction of flow. The first exhaust gas catalyst 3 is located close to the engine, directly at the exhaust gas outlet of the ECU 1, and is, for example, designed as a three-way catalyst (TWC).Following the first exhaust catalyst 3, with respect to the exhaust mass flow 11a downstream, is first a particulate filter 5, which is designed, for example, as a gasoline particulate filter (GPF), and then the second exhaust catalyst 4, referred to here as secondary catalyst SK, which can also be designed as a three-way catalyst (TWC) and can, for example, be arranged as a so-called underbody catalyst (UbKat) on the underbody of a motor vehicle. In the exhaust pipe 11, a first exhaust gas sensor 6a is arranged upstream of the first exhaust gas catalyst 3, and a second exhaust gas sensor 6b, which provides the first sensor signal Sig1, is arranged downstream of the first exhaust gas catalyst 3. A third exhaust gas sensor 6c, which provides the second sensor signal Sig2, is arranged downstream of the secondary catalyst SK, in the exhaust gas mass flow 11a. Additionally, a temperature sensor 9 for measuring the temperature of the exhaust gas mass flow 11a is arranged in the exhaust pipe 11 between the first exhaust gas catalyst 3 and the particulate filter 5. The aforementioned sensors 6a-c and 9 are connected via electrical signal lines 8 to the input / output interface 7c of an electronic control device 7 according to the invention for transmitting the sensor signals to the control device 7. The air-fuel mixture supply unit 10 of the internal combustion engine 1 is also connected via electrical signal lines 8 to the input / output interface 7c of the electronic control device 7 according to the invention. The electronic control device 7 comprises an electronic computing unit 7a, also referred to as a processor, an electronic storage device 7b, and an input / output interface 7c. In this embodiment, the electronic storage device 7b includes a program memory 7b1 in which a computer program product 20 according to the invention, containing program instructions, is available. Furthermore, the electronic storage device 7b includes an operating data memory 7b2 in which predefined operating data 21, as well as operating data acquired during operation, are stored, for example, arranged in characteristic maps. The electronic computing unit 7a is configured to carry out the method according to the invention in the embodiments described above and, if applicable, in addition thereto. For this purpose, the electronic computing unit 7a has access to, or is in a data connection (symbolized by connecting arrows) with, the input / output interface 7c, the program memory 7b1, and the operating data memory 7b2. When the program instructions are executed by the electronic computing unit 7a, using the relevant operating data 21 and various threshold values ​​for the oxygen content, the pollutant content, or the pollutant integral limit values ​​(S_IGw) in the exhaust gas mass flow 11a, the electronic control device 7 is caused to execute the method according to the invention for the functional diagnosis of a secondary catalyst SK of an exhaust aftertreatment device 2 of a BKM 1.The electronic control device 7, including the corresponding computer program product 20 and the exhaust gas sensor 6c arranged at the exhaust gas outlet of the AgNbgs device 2, which can be designed, for example, as a NOx sensor, as an NH3 sensor, as a CmHn sensor or as a combined sensor for several pollutants including oxygen, forms a so-called pollutant emission mass detection device 30, which is set up to carry out the method according to the invention. In accordance with the program instructions, sensor data, for example the signals from the exhaust gas sensors 6a-c which represent, for example, the oxygen content or the various pollutant content in the exhaust gas mass flow 11a, operating data 12 which are representative of the size of the exhaust gas mass flow or from which the size of the exhaust gas mass flow can be determined, and further signals, for example from the temperature sensor 9, are received via the input / output interface 7c and the programmed calculation operations for lambda control and in particular for the functional diagnosis of a secondary catalyst SK are carried out according to the method according to the invention.Based on this, control commands are then issued, for example to control the air-fuel mixture 10a to the air-fuel mixture supply unit 10 of the internal combustion engine 1, and, in the case of a secondary catalyst diagnosed as no longer functioning sufficiently, control commands to activate a warning indicator 25 and / or to enter a corresponding error message in an error memory, which may, for example, be part of the operating data memory 7b2, and / or to prevent a restart of the BKM 1 after an operator deactivation and / or to restrict the operation of the BKM 1 to a low-emission operating range. Fig. 2 shows a simplified flowchart illustrating the essential process steps S1 - S6 and E3 of an embodiment of the method according to the invention. In the first process step S1, the procedure for functional diagnosis of a secondary catalyst SK of an exhaust aftertreatment device 2 of an internal combustion engine 1 is started. The two key process steps S2a and S2b then occur simultaneously. During process step S2a, a first sensor signal, Sig1, representing the initial oxygen content in the exhaust gas mass flow 11a immediately downstream of the first exhaust catalyst 3, is continuously received and evaluated. This signal is symbolically represented by the arrow labeled Sig1. Simultaneously, a second sensor signal, Sig2, representing at least one pollutant content in the exhaust gas mass flow 11a immediately downstream of the secondary catalyst SK, is received and evaluated. This signal is symbolically represented by the arrow labeled Sig2. The process step S2a represents the execution of a diagnostic routine, DG_K1, to check the function of the first exhaust catalyst 3, which is characterized by multiple changes between a lean operating phase and a rich operating phase of the internal combustion engine 1 depending on the first sensor signal Sig1. The process step S2b represents the determination of at least one pollutant fraction in the exhaust gas mass flow 11a downstream of the secondary catalyst SK based on the second sensor signal Sig2 and the determination of at least one pollutant mass integral, SMI_SK, for the pollutant in question, over a period of time of the diagnostic routine, D_Diag. In decision step E3, it is determined whether the value of at least one pollutant mass integral, SMI_SK, exceeds a predefined, assigned pollutant integral limit value, S_IGw, which is represented by SMI_SK > S_IGw. If the value of at least one pollutant mass integral SMI_SK does not exceed the specified, assigned pollutant integral limit S_IGw, the secondary catalyst SK of the exhaust aftertreatment device 2 is diagnosed as "fully functional", SK=ok, in process step S4 and the process is aborted or a return to the beginning of the process sequence occurs and the process can be restarted automatically, for example at a certain time interval. If the value of at least one pollutant mass integral SMI_SK exceeds the specified, assigned pollutant integral limit S_IGw, the secondary catalyst SK of the exhaust aftertreatment device 2 is diagnosed as "no longer sufficiently functional" or "not present", SK=nok, in process step S5. In the subsequent process step S6, at least one of the following measures can then be initiated by the electronic control device 7: - Activation of a warning indicator 25, represented by a lamp symbol; - Entry of a corresponding error message into an error memory, for example in the operating data memory 7b2; - Prevention of a restart of the internal combustion engine (1) after an operator deactivation, not shown; or - Restriction of the operation of the internal combustion engine to a low-emission operating range, not shown. Naturally, several of these measures can also be initiated in parallel or in combination. Figures 3 and 4 each show a simplified signal diagram of the sequence of one execution of the procedure, but for different diagnostic results. Above the time axis t, in the upper part of the respective diagrams in Figures 3 and 4, the course of the λ-value of the exhaust gas mass flow 11a leaving the BKM 1, also referred to as λ_BMK, is shown in the individual operating phases of the BKM 1. Below this, the signal course Sig1 of the first exhaust gas sensor 6b, which is located immediately downstream of the first exhaust gas catalyst 3, is recorded, whereby the signal course Sig1 is representative of the oxygen content in the exhaust gas mass flow 11a, immediately downstream of the first catalyst 3. The behavior of the λ value, λ_BMK, shows that at the start of the diagnostic routine DG_K1 for the first catalyst 3, a lean-burn phase with λ > 1 is initiated from a neutral operating state of BKM 1 at λ = 1. As soon as the oxygen saturation of the first catalyst 3 increases during this lean-burn phase, the value of the first sensor signal Sig1 drops, indicating an increasing oxygen content in the exhaust gas mass flow 11a, immediately downstream of the first catalyst 3, and thus an oxygen breakthrough at the first catalyst 3. As soon as the value of the first sensor signal Sig1 falls below a lower signal limit Sig1_UG, the BKM 1 switches to a rich operating phase with λ < 1, which is maintained until the first sensor signal Sig1 has risen again to the point where it exceeds a higher signal limit Sig1_OG, indicating that there is no longer any oxygen breakthrough at the first catalyst 3.At this point, BKM 1 is switched back to a lean operating phase, as at the beginning of the diagnostic routine DG_K1, and the previous cycle is repeated until the next rich operating phase is completed. BKM 1 can then resume normal operation at λ=1. Further transitions between lean and rich operating phases can also be performed according to the same pattern. The timing of these transitions can, for example, provide information about the oxygen storage capacity and thus the aging state of the first catalyst, as is known in the past. The signal curve recorded in the same area as the first sensor signal Sig1 for a partial signal for the oxygen content Sig2_O2, which represents the oxygen content in the exhaust gas mass flow 11a immediately downstream of the secondary catalyst SK, is based on the second sensor signal Sig2 of the exhaust gas sensor 6c located downstream of the secondary catalyst SK. In a suitable configuration, the exhaust gas sensor 6c, located downstream of the secondary catalyst SK, can, for example, be designed as a NOx sensor, providing a total signal for the pollutant components contained in the exhaust gas mass flow 11a. This signal can be decomposed into the individual components for oxygen and the various pollutants using a computational model. Of course, it is also possible to arrange several exhaust gas sensors downstream of the secondary catalyst SK, each directly providing a separate signal value for one of the pollutants contained in the exhaust gas mass flow 11a. However, this appears to be undesirable for cost reasons. In the area below the sensor signal Sig1 and the signal curve of the partial signal for the oxygen fraction Sig2_O2 in the diagrams of Figs. 3 and 4, a signal curve for a partial signal for the nitrogen fraction Sig2_NOx and a signal curve for a partial signal for the ammonia fraction Sig2_NH3 are plotted against the time axis t. The signal curve for the partial signal Sig2_NOx represents the NOx fraction determined from the second sensor signal Sig2, and the signal curve for the partial signal Sig2_NH3 represents the NH3 fraction determined from the second sensor signal Sig2 in the exhaust gas mass flow 11a, immediately downstream of the secondary catalyst SK. Directly below the signal curve for the partial signal Sig2_NOx, an integral curve is plotted against the time axis t, representing the pollutant mass integral for nitrogen SMI_SK_NOx, determined based on the signal curve Sig2_NOx.Furthermore, directly below the signal curve for the sub-signal Sig2_NOx, another integral curve is plotted over the time axis t, which represents the pollutant mass integral for ammonia SMI_SK_NH3 determined on the basis of the signal curve for the sub-signal Sig2_NH3. In this embodiment, the duration of the diagnostic routine D_Diag extends from the start of the first lean-burn phase until a delay period D_Vzg extends beyond the end of the second rich-burn phase. The delay period D_Vzg is defined as the time lag with which the response of the second sensor signal Sig2 to a respective change to a lean-burn or rich-burn phase of the BKM 1 is expected or completed. In Fig. 3, the signal curve for the partial signal Sig2_O2 initially shows that, until the end of each lean-burn phase, no relevant oxygen breakthrough is measurable downstream of the secondary catalyst SK; the oxygen breakthrough after the first catalyst 3 is therefore absorbed by the secondary catalyst SK. Sufficient oxygen storage capacity of the secondary catalyst SK thus appears to be present, and a secondary catalyst therefore seems to be present in principle. Furthermore, the signal curve for the partial signal Sig2_NOx shows a slight increase in the NOx content in the exhaust gas mass flow 11a downstream of the secondary catalyst SK, specifically during the transition from the lean-burn to the subsequent rich-burn phase. The NOx masses emitted during this process are summed over the diagnostic duration D_Diag and integrated into the pollutant mass integral for nitrogen, SMI_SK_NOx. As can be seen from the graph, a predefined pollutant integral limit for nitrogen, S_IGw_NOx, is not reached within the duration of the diagnostic routine D_Diag. Therefore, the secondary catalyst SK is diagnosed as functioning adequately, or OK, at least with regard to NOx emissions. Furthermore, the signal curve for the partial signal Sig2_NH3 shows a slight increase in the NH3 content in the exhaust gas mass flow 11a downstream of the secondary catalyst SK, specifically during the transition from the rich operating phase to the subsequent lean operating phase. The NH3 masses emitted during this process are summed over the diagnostic duration D_Diag and integrated into the pollutant mass integral for ammonia, SMI_SK_NH3. As can be seen from the diagram, a predefined pollutant integral limit for ammonia, S_IGw_NH3, is not reached within the duration of the diagnostic routine D_Diag. Therefore, the secondary catalyst SK is diagnosed as functioning adequately, or at least with regard to NH3 emissions. Fig. 4 shows the case of a defective, malfunctioning, or missing secondary catalyst SK. In Fig. 4, the signal curve for the partial signal Sig2_O2 initially shows that, towards the end of each lean-burn phase, oxygen breakthrough is measurable downstream of the secondary catalyst SK. The oxygen breakthrough after the first catalyst 3 thus propagates to the exhaust gas sensor 6c downstream of the secondary catalyst SK. Sufficient oxygen storage capacity of the secondary catalyst SK therefore initially appears to be lacking; a secondary catalyst thus appears to be either defective or completely absent. Furthermore, the signal curve for the partial signal Sig2_NOx shows an increased rise in the NOx fraction in the exhaust gas mass flow 11a downstream of the secondary catalyst SK, specifically during the transition from the lean-burn to the subsequent rich-burn phase. The NOx masses emitted during this process are summed over the diagnostic duration D_Diag and integrated into the pollutant mass integral for nitrogen, SMI_SK_NOx. As can be seen from the diagram, a predefined pollutant integral limit for nitrogen, S_IGw_NOx, is significantly exceeded within the duration of the diagnostic routine D_Diag. Therefore, the secondary catalyst SK is diagnosed as no longer sufficiently functional or even absent (nok), at least with regard to NOx emissions. Furthermore, the signal curve for the partial signal Sig2_NH3 also shows an increased rise in the NH3 fraction in the exhaust gas mass flow 11a downstream of the secondary catalyst SK, specifically during the transition from the rich operating phase to the subsequent lean operating phase. The NH3 masses emitted in this process are summed over the diagnostic duration D_Diag and integrated into the pollutant mass integral for ammonia, SMI_SK_NH3. As can be seen from the diagram, a predefined pollutant integral limit for ammonia, S_IGw_NH3, is also exceeded within the duration of the diagnostic routine D_Diag. Therefore, the secondary catalyst SK is diagnosed as no longer sufficiently functional or even absent, at least with regard to NH3 emissions. Fig. 5 shows a signal diagram for an embodiment of the method, characterized in that, based on the second sensor signal Sig2, a signal sum value Sig2_Sum for the nitrogen oxide content and the ammonia content in the exhaust gas mass flow 11a downstream of the secondary catalyst SK is determined, and based on this, a pollutant sum mass integral SMI_SK_Sum is formed over the duration D_Diag of the diagnostic routine, which includes the sum of the aforementioned pollutant content. Here, too, the situation involves a defective, malfunctioning, or missing secondary catalyst SK. Figure 5 also shows, from the signal curve for the partial signal Sig2_O2, that an increased oxygen fraction is measurable downstream of the secondary catalyst SK towards the end of each lean-burn operating phase. The transition between the lean and rich operating phases is accompanied, albeit with a slight time lag, by increased signal values ​​for the nitrogen and ammonia fractions in the exhaust gas mass flow 11a, which are summarized in the signal sum value Sig2_Sum. Regardless of the type of pollutant, the pollutant masses emitted are summed over the diagnostic duration D_Diag and integrated into the pollutant mass integral, SMI_SK_Sum.As can be seen from the diagram, a predefined pollutant sum integral limit, S_IGw_Sum, is exceeded here as well within the duration of the diagnostic routine D_Diag. Therefore, the secondary catalyst SK is also diagnosed here as no longer sufficiently functional or not present, nok. Reference symbol list 1 Internal combustion engine 2 Exhaust aftertreatment device 3 First exhaust catalyst 4 / SK Second exhaust catalyst / Secondary catalyst 5 Particulate filter 6a - c Exhaust gas sensors 7 Electronic control device 7a Electronic processing unit 7b Electronic storage device 7b1 Program memory 7b2 Operating data memory 7c Input / output interface 8 Signal lines 9 Temperature sensor 10 Air-fuel mixture supply unit 10a Air-fuel mixture 11 Exhaust pipe 11a Exhaust mass flow 20 Computer program product 21 Operating data 25 Warning indicator 30 Pollutant emission mass detection device GPF Gasoline particulate filter Δp Differential pressure sensor TWC Three-way catalyst UbKat Underfloor catalyst S1 - S6 Process steps E3 Decision step λ Exhaust gas λ value λ_BMK Exhaust gas λ value at the internal combustion engine outlet Sig1 First sensor signal Sig1_OG Upper limit first Sensor signal Sig1_UG lower limit first sensor signal Sig2 second sensor signal Sig2_O2 sub-signal for the oxygen fractionfrom Sig2 Sig2_NOx partial signal for the nitrogen component of Sig2 Sig2_NH3 partial signal for the ammonia component of Sig2 Sig2_Sum signal sum value of Sig2 DG_K1 diagnostic routine D_Diag duration of the diagnostic routine D_Vzg delay duration SMI_SK pollutant mass integral SMI_SK_NOx nitrogen mass integral SMI_SK_NH3 ammonia mass integral SMI_SK_Sum pollutant sum mass integral S_IGw pollutant integral limit S_IGw_NOx pollutant integral limit for nitrogen S_IGw_NH3 pollutant integral limit for ammonia S_IGw_Sum pollutant sum integral limit ok sufficiently functional nok no longer sufficiently functional / not present t time axis

Claims

Method for functional diagnosis of a secondary catalyst (SK) of an exhaust aftertreatment device (2) of an internal combustion engine (1), which has a first oxygen-storage-capable exhaust catalyst (3) and the second secondary catalyst (SK) designed as an oxygen-storage-capable exhaust catalyst downstream in the exhaust mass flow (11a), by carrying out the following steps by means of at least one electronic control device (7) assigned to the exhaust aftertreatment device (2) and / or the internal combustion engine (1): - Continuously receiving and evaluating a first sensor signal (Sig1) that is representative of a first oxygen fraction in the exhaust mass flow (11a) immediately after the first exhaust catalyst (3) and receiving and evaluating a second sensor signal (Sig2) that is representative of at least one pollutant fraction in the exhaust mass flow (11a) immediately after the secondary catalyst (SK);- Performing a diagnostic routine (DG_K1) to check the function of the first exhaust catalyst (3), which is characterized by multiple changes between a lean operating phase and a rich operating phase of the internal combustion engine (1) depending on the first sensor signal (Sig1); - Determining at least one pollutant fraction in the exhaust mass flow (11a) downstream of the secondary catalyst (SK) based on the second sensor signal (Sig2) and determining at least one pollutant mass integral (SMI_SK) for the pollutant in question over a period of time of the diagnostic routine (D_Diag); - Diagnosing the secondary catalyst (SK) as "no longer sufficiently functional" or "not present" (SK=nok) if the value of the at least one pollutant mass integral (SMI_SK) exceeds a predetermined, assigned pollutant integral limit value (S_IGw). Method for functional diagnosis of a secondary catalyst according to claim 1, characterized by diagnosing the secondary catalyst (SK) of the exhaust aftertreatment device (2) as "fully functional" (SK=ok) when the value of the pollutant mass integral (SMI_SK) does not reach the specified pollutant integral limit value (S_IGw). Method for functional diagnosis of a secondary catalyst (SC) according to claim 1, characterized in that the at least one pollutant is nitrogen oxide, NOx, and / or ammonia, NH3. Method for functional diagnosis of a secondary catalyst (SK) according to claim 3, characterized in that, based on the second sensor signal (Sig2), both a nitrogen oxide content and an ammonia content in the exhaust gas mass flow (11a) downstream of the secondary catalyst (SK) are determined and, based on this, a nitrogen mass integral (SMI_SK_NOx) and an ammonia mass integral (SMI_SK_NH3) are formed over the duration (D_Diag) of the diagnostic routine. Method for functional diagnosis of a secondary catalyst (SK) according to claim 4, characterized in that the secondary catalyst (SK) of the exhaust aftertreatment device (2) is diagnosed as "no longer sufficiently functional" or "not present" if at least one of the values ​​of the nitrogen mass integral (SMI_SK_NOx) and the ammonia mass integral (SMI_SK_NH3) exceeds a respective predetermined, associated pollutant integral limit value (S_IGw_NOx, S_IGw_NH3). Method for functional diagnosis of a secondary catalyst (SK) according to claim 3, characterized in that, based on the second sensor signal (Sig2), a signal sum value (Sig2_Sum) for the nitrogen oxide content and the ammonia content in the exhaust gas mass flow (11a) downstream of the secondary catalyst (SK) is determined and, based on this, a pollutant sum mass integral (SMI_SK_Sum) is formed over the duration (D_Diag) of the diagnostic routine, which includes the sum of the aforementioned pollutant content. Method for functional diagnosis of a secondary catalyst (SK) according to one of the preceding claims, characterized in that the duration of the diagnostic routine (D_Diag) is determined by the duration of the lean operating phases and the rich operating phases of the internal combustion engine (1) during at least two changes between lean operating phase and rich operating phase, including a delay period (D_Vzg), wherein the duration of the respective lean operating phases and the rich operating phases depend on the course of the first sensor signal (Sig1), and wherein the delay period (D_Vzg) is given by the time delay with which a reaction of the second sensor signal (Sig2) to a respective change to a lean operating phase or to a rich operating phase of the internal combustion engine (1) is completed. Method for functional diagnosis of a secondary catalyst (SK) according to one of the preceding claims, characterized in that, in the event of a diagnosis of the secondary catalyst (SK) as "no longer sufficiently functional" or "not present" (nok), at least one of the following measures is initiated by the electronic control device (7): - Activation of a warning indicator (25); - Entry of a corresponding error message into an error memory; - Prevention of a restart of the internal combustion engine (1) after operator deactivation; - Restriction of the operation of the internal combustion engine (1) to a low-emission operating range. Electronic control device (7) for functional diagnostics of a secondary catalyst (SK) of an exhaust aftertreatment device (2) of an internal combustion engine (1), comprising at least one electronic storage device (7b) in which at least one computer program product (11) with program instructions for carrying out the method is provided, and an input / output interface (7c) configured for receiving and outputting electrical signals, as well as an electronic computing unit (processor) (7a) for executing program instructions, with access to the at least one electronic storage device (7b) and the input / output interface (7c); wherein the program instructions, when executed by means of the electronic computing unit (7a), cause the electronic control device (7) to execute the method according to one of the preceding claims. Computer program product (11) for functional diagnosis of a secondary catalyst (SK) of an exhaust aftertreatment device (2) of an internal combustion engine (1), comprising program instructions for execution by the electronic computing unit (7a) of the electronic control device (7) according to claim 8, wherein the program instructions, when executed by the electronic computing unit (7a), cause the electronic control device (7) to carry out the method according to any one of claims 1 to 8.

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