SE530435C2 - Method and apparatus for monitoring the function of an exhaust after-treatment system - Google Patents

Description

530 435 temperature values, such as the difference between them, it is determined whether or not the SCR catalyst and its injector function satisfactorily and as expected. Similar methods are also previously known from DE 4122 787 A1, US 5,860 277 A and EP 0 756 071 A2. A problem associated with this type of monitoring procedure that relies on a calculation model is that the calculation model under normal conditions can give fairly correct calculation values but under certain operating conditions can give calculation values that deviate significantly from the corresponding ones. fair values. Thus, erroneous error indications can be generated based on test samples recorded under the latter operating conditions.

The function of an SCR catalyst can also be monitored by means of a NOX sensor which is arranged to measure the NOX content in the exhaust gases downstream of the catalyst. This NOK sensor can be supplemented by another NOX sensor which is arranged to measure the NOX content in the exhaust gases upstream of the catalyst in order to provide a comparative value. However, a NOX sensor is an expensive component and an alternative to cope without a NOX sensor upstream of the catalyst is to use a calculation model to calculate the amount of NOX produced by the vehicle engine.

Furthermore, another calculation model can be used to calculate the expected conversion of NOX in the catalyst in order to obtain calculated values of the NOX content in the exhaust gases downstream of the catalyst. These calculation values can then be compared with measured values from the NOX sensor downstream of the catalyst to detect any disturbances in the catalyst and the associated equipment. The use of calculation models for calculating the magnitude of the NOx content in the exhaust gases can, however, give calculation values that deviate significantly from the corresponding fair values, which in turn can result in the generation of incorrect error indications.

Due to increasingly strict regulations regarding the above-mentioned type of vehicle-borne monitoring, there is a great need for a reliable method and a reliable device for monitoring the function of an exhaust after-treatment system.

DESCRIPTION OF THE INVENTION The object of the present invention is to provide a new and reliable way of monitoring an exhaust aftertreatment system of a motor vehicle.

This object is achieved by means of a method having the features defined in claim 1 and a monitoring device having the features defined in claim 12.

According to the solution according to the invention, the function of the exhaust after-treatment system is monitored based on a measurement signal from a sensor regarding the size of a parameter related to exhaust gases flowing out of an exhaust after-treatment device, for example in the form of a catalyst or filter included in the exhaust after-treatment system. . A frequency analysis of the measurement signal provided by the sensor over a certain period of time is performed and information regarding the operation of the exhaust aftertreatment system is generated based on this frequency analysis.

The above-mentioned time period is hereinafter referred to as the “evaluation period”.

The solution according to the invention is based on the insight that a properly functioning exhaust gas aftertreatment device in the form of a NOx-reducing catalyst has an inertia which causes a low-pass filtering effect on the NOX content in exhaust gases passing through the catalyst, and that a properly mode-operated exhaust after-treatment device in the form of a catalyst or a particle filter has a temperature inertia which causes a low-pass filtering effect on the temperature of exhaust gases passing through the catalyst / filter. This is because the catalyst and the filter, when functioning properly, attenuate fluctuations in the NOX content and the temperature occurring upstream of the catalyst / filter, and the faster fluctuations are usually attenuated to such an extent that they have a reduced effect on the value of the NOX content / temperature downstream of the catalyst / filter. The measurement signal from a NOX sensor or temperature sensor arranged downstream of a catalyst or a particulate filter thus comprises more high-frequency parts when the catalyst / filter is exposed to a disturbance which reduces its efficiency compared with the case when the catalyst / filter works properly way. There are several methods well known to a person skilled in the art which can be used for evaluating the low-pass filtration capacity of the exhaust after-treatment device in question, and thus its function, based on frequency analysis of the measurement signal from a sensor arranged downstream of said device.

By basing the monitoring of the exhaust aftertreatment system on a frequency analysis of the measurement signal from a sensor, such as a temperature sensor or NOx sensor, arranged downstream of the exhaust aftertreatment device, it becomes possible to generate information regarding the function of the exhaust aftertreatment system. a complex and sometimes incorrect calculation model, and without being dependent on measured values from a corresponding sensor arranged upstream of the exhaust after-treatment facility. Furthermore, the monitoring becomes insensitive to deviations between the actual parameter values and the corresponding measured values provided by the sensor, which deviations can be caused, for example, by offset errors in the sensor due to sensor drift.

An evaluation value that reflects the nature of the high frequency part of the measurement signal provided by the sensor during the evaluation period is suitably determined by said frequency analysis, after which the evaluation value is compared with a comparison value or a given limit value to generate information regarding the operation of the processing system. According to a first alternative, the evaluation value is selected to represent the signal energy of a high-pass filtered measurement signal obtained by filtering the measurement signal provided by the sensor during the evaluation period by means of a high-pass filter. In this way, the evaluation value can be determined in a simple and effective way. Alternatively, the evaluation value can be determined by evaluating a frequency spectrum determined by the said frequency analysis. According to another alternative, the evaluation value is selected as a value representing the calculated cut-off frequency of a low-pass filter model, which represents the signal filtering characteristics of the exhaust after-treatment system with respect to said parameter and which is determined by evaluating the measurement signal from the sensor.

The comparison value is advantageously a stored value, which is determined in advance and associated with specific operating conditions of the motor vehicle. In this case, a monitoring operation is performed only when the prevailing operating conditions of the motor vehicle correspond to operating conditions for which a comparative value has been determined and stored. The comparison value can alternatively be determined in connection with the performance of a monitoring operation based on measured or calculated values of the size of said parameters of exhaust gases flowing into the exhaust aftertreatment device or based on calculated values of the size of said parameter of exhaust gases flowing out of exhaust gases. the treatment facility.

Further advantageous features of the monitoring method and the monitoring device according to the invention are stated in the dependent claims and the description below.

The invention also relates to a computer program having the features defined in claim 16, a computer program product having the features defined in claim 18 and an electronic control unit having the features defined in claim 19. BRIEF DESCRIPTION OF THE EMBODIMENTS The invention will be described in more detail below by means of exemplary embodiments with reference to the accompanying drawings, in which: Fig. 1 is a schematic sketch of an internal combustion engine with an associated exhaust after-treatment system, illustrating an embodiment a monitoring device according to the present invention, Fig. 2 is a diagram showing an exemplary measurement signal from a sensor as a function of time, Fig. 3 is a diagram showing the frequency spectrum of the measurement signal according to Fig. 2, Fig. 4 is an exemplary Bode diagram. representing the frequency response of a catalyst included in an exhaust aftertreatment device, Fig. 5 is a schematic schematic diagram of an electronic control unit for implementing a process according to the invention, and Fig. 6 is a flow chart illustrating a process according to an embodiment of the invention.

EMBODIMENTS OF THE INVENTION A monitoring method according to the invention and a monitoring device according to the invention will hereinafter be described as implemented / implemented for monitoring the function of an exhaust after-treatment system which comprises an exhaust after-treatment device in the form of a catalyst. However, the invention is in no way limited to this application, but the invention can be used for monitoring an exhaust after-treatment system having any type of exhaust after-treatment device which has a low-pass filtering effect on a measurable exhaust parameter. The invention can be used, for example, for monitoring the operation of an exhaust after-treatment system which comprises an exhaust after-treatment device in the form of a filter, such as a particle filter or an open-structure filter.

An internal combustion engine 1 with an associated exhaust after-treatment system 2 is shown schematically in Fig. 1. The exhaust gases leaving the internal combustion engine 1 are led through an exhaust line 3 and discharged to the environment via an exhaust outlet 4. A catalyst 5 is arranged in the exhaust line 3. The exhaust gases from the internal combustion engine 1 pass through the catalyst 5 before being released to the environment via the exhaust outlet 4.

In the example illustrated in Fig. 1, the catalyst 5 is an SCR catalyst. In this case, reducing agent is injected by means of an injector 6 into the exhaust gases in the exhaust line 3 upstream of the catalyst 5. The injector 6 comprises one or more injectors 7 arranged in the exhaust line 3 in the form of injectors or the like, and a storage container 8 connected thereto reducing agents. The injection device 6 also comprises a control means 9, for example in the form of a control valve, which is arranged to regulate the supply of reducing agent to said one or more injection means 7, and a control means 10 connected to the control means 9. The control means 9 is controlled by said control means 10, which determines the amount of reducing agent to be injected into the exhaust gases based on the prevailing operating conditions of the internal combustion engine 1 and the catalyst 5. The injection device 6 may also comprise further components, such as a dosing device, etc. The solvent may be urea (CO (NH2) 2), ammonia (NH3) or hydrocarbon (fuel).

A sensor 11 is arranged in the exhaust line 3 downstream of the catalyst 5. In this example, the sensor 11 is a NOx sensor which is arranged to generate a measuring signal representing the NOx content 530 435 in the exhaust gases flowing out of the catalyst 5, i.e. the NOX content. in the exhaust gases at the outlet of the catalyst. The measurement signal may be a continuous signal representing the continuous changes of the measured parameter, i.e. forming a continuous stream of measured values regarding the magnitude of the parameter, but usually recorded as a discrete time signal forming a series of consecutive and discrete measured values regarding the size of the parameter. The monitoring device 20 comprises processing means 21 arranged to receive the measurement signal from the sensor 11 regarding the measured magnitude of the NOX content in the exhaust gases downstream of the catalyst 5. The processing means 21 is arranged to perform a frequency analysis of the measurement signal provided by the sensor 11 during a time period Tev, herein referred to as evaluation period, and to generate information regarding the function of the exhaust aftertreatment system 2 based on said frequency analysis.

If the catalyst functions properly, under normal operating conditions of the internal combustion engine 1 and the catalyst 5, it has a generally reducing effect on the NOX content of the exhaust gases passing through the catalyst and also dampens peak values and rapidly varying fluctuations of the NOK content occurring in the exhaust gases upstream of the catalyst 5. in such a way that these peak values and fluctuations have been eliminated or at least significantly reduced when the exhaust gases leave the catalyst. In this respect, the catalyst resembles a low-pass filter and has a low-pass filtering effect on the magnitude of the NOX content of the exhaust gases passing through the catalyst 5. Thus, the measurement signal from the sensor 11 resembles a low-pass filtered signal. The low-pass filtered effect varies with variations in the NOX-reducing effect of the catalyst. By evaluating the low-pass filtering ability of the catalyst 5, for example by evaluating the character of the high-frequency part of the measurement signal provided by the sensor 11, it is thus possible to detect whether the catalyst 5 and the associated exhaust after-treatment system 2 are working. in a satisfactory and expected manner eiler not. Based on the above-mentioned frequency analysis of the measurement signal provided by the sensor 11, the processing means 21 is suitably arranged to determine an evaluation value Vev which reflects the character of the high-frequency part of the measurement signal provided by the sensor during the evaluation period Tev. The processing means 21 is then arranged to compare this evaluation value Vev with a comparison value Vco or a given limit value Vjh to generate information regarding the function of the catalyst 5 and the associated exhaust after-treatment system 2. There are several different methods well known to a person skilled in the art. for evaluating the measurement signal from the sensor 11 to determine an evaluation value of suitable shape. Some of these possible methods are described below.

According to a first method, the measurement signal provided by the sensor 11 is filtered during the evaluation period Tev by means of a high-pass filter 22, and the evaluation value Vev is selected to represent the signal energy of the high-pass filtered measurement signal. This signal energy can be calculated in any suitable and conventional way by means of an algorithm known to a person skilled in the art. The determined evaluation value Vev can then be compared with a signal energy-representing comparison value Vw or a given limit value Vth. In this case, the comparative value Vw may, for example, be chosen to represent the signal energy of a previously high-pass filtered measurement signal from the sensor 11 recorded during a period of time when the catalyst and the associated exhaust after-treatment system were known to function properly or the signal energy of a corresponding high-pass filtered signal determined using appropriate calculation models. The processing means 21 is suitably arranged to generate an error indication if the difference or ratio between the evaluation value Vev and the comparison value Vw exceeds a limit level in order thereby to indicate a malfunction of the system. According to a second method, a frequency spectrum of the measurement signal provided by the sensor 11 is determined during the evaluation period by frequency analysis of said measurement signal. This frequency spectrum can be determined in any suitable and conventional manner by means of an algorithm known to a person skilled in the art, such as the algorithm called Fast Fourier Transform (FFT). As an illustrative example, Fig. 3 shows a frequency spectrum 23 determined by FFT based on the measurement signal 24 shown in Fig. 2. The frequency spectrum shows the magnitude of each frequency component of the measurement signal. The frequency spectrum of the measurement signal from the sensor 11 is the product of the frequency spectrum of a signal representing the magnitude of the parameter in question upstream of the catalyst 5 and the frequency response of the catalyst. The determined frequency spectrum 23 can be evaluated in any suitable way to determine the evaluation value Vev.

The evaluation value can for example be determined from a frequency spectrum by means of the formula: Crank = 2highY (f) / ZtotY (f) where: - Crank is the evaluation value, - Y (f) is the y-axis value for the frequency fi said frequency spectrum - ZhÖQYG ) is the sum of Y (f) over a selected high frequency part of said frequency spectrum, and - ZMYU) is the sum of Y (f) over the low frequency part and the selected high frequency part of said frequency spectrum. i If this evaluation value Crank exceeds a given limit value Vm, this indicates a malfunction of the exhaust after-treatment system. The processing means 21 is suitably arranged to generate an error indication if the evaluation value Vev exceeds the limit value Vth in order thereby to indicate a malfunction of the exhaust after-treatment system. 5311] 435 11 According to a third method, a low-pass filter model is established which represents the signal filtering characteristics of the exhaust after-treatment system with respect to said parameter by evaluating the measurement signal from the sensor 11 during the evaluation period Tev. This low-pass filter model can be determined in any suitable and conventional manner by means of an algorithm known to a person skilled in the art, such as, for example, by autoregressive modeling or autoregressive moving average modeling. In this case, the evaluation value Crank is selected as a value representing the calculated cut-off frequency of the low-pass filter model. As an illustrative example, Fig. 4 shows a Bode diagram 25 associated with an established low-pass filter model in the form of a first-order autoregressive model.

The breaking frequency fbryt is indicated in the Bode diagram according to Fig. 4. The determined evaluation value Vev can then be compared with a comparison value V60 which represents the breaking frequency of a comparative low-pass filter model or a given limit value Vth. In this case, for example, the comparative value Vw may be chosen to represent the cut-off frequency of a comparative low-pass filter model previously determined based on a measurement signal from the sensor 11 recorded during a period when the catalyst and associated exhaust aftertreatment system were known to function properly or based on a corresponding signal determined by means of appropriate calculation models.

The length of the Tev evaluation period and the interval between each monitoring operation can, of course, vary widely from case to case depending on the nature of the system to be monitored.

The evaluation period can, for example, be in the order of -30 seconds or longer.

An appropriate comparative value Vw, or limit value VU, can be determined empirically with the help of practical tests and / or theoretically based on appropriate calculations. The comparison value Vw or the limit value Vth is suitably a stored value, which is determined in advance and associated with specific operating conditions of the motor vehicle. The comparison value Vw can alternatively be determined in connection with the execution of a monitoring operation based on measured or calculated values of the magnitude of said parameters of exhaust gases flowing into the exhaust aftertreatment device 5 or based on calculated values of the magnitude of said parameter of exhaust gases flowing out of the exhaust aftertreatment device 5. The monitoring device 20 suitably comprises some type of indicating device for displaying an error message and / or emitting an acoustic or luminous warning signal when a fault of the monitored system has been detected. An abnormal evaluation value Crank indicates that the catalyst 5 and / or any other component of the exhaust after-treatment system 2, such as for example the injector 6 or the sensor 11, is not working satisfactorily and may, for example, have one or more of the following causes: - the catalyst 5 or a part of it has been removed, - the catalyst 5 is degenerate or damaged, - the sensor 11 is degenerate or damaged, - the injector 6 does not inject the expected amount of the expected reducing agent, - an error in a used calculation model, - one or more input signals to a used calculation model is incorrect.

The possible causes of errors should thus be checked when an error indication has been generated to find and correct the error in question.

The processing means 21 is advantageously a computer unit, for example in the form of an electronic control unit of a motor vehicle.

It will be apparent to one skilled in the art that the embodiments described above in connection with the monitoring of an exhaust aftertreatment system provided with an exhaust aftertreatment device in the form of an SCR catalyst could be easily modified to monitor the operation of an exhaust aftertreatment system. provided with any other type of exhaust aftertreatment device having an inertia with respect to the measured parameter in question, such as another type of catalyst or a filter. The parameter could be, for example, the temperature of the exhaust gases flowing out of a catalyst or a filter, in which case the above-mentioned sensor is a temperature sensor arranged downstream of the catalyst / filter.

A flow chart illustrating a method according to an embodiment of the invention is shown in Fig. 6. In a first step S1 a measurement signal from a sensor 11 is recorded for a certain period of time regarding the magnitude of a parameter related to exhaust gases flowing out of an exhaust aftertreatment device. In a second step S2, a frequency analysis of the registered measurement signal is performed. In a subsequent step S3, information is generated regarding the operation of the exhaust after-treatment system based on said frequency analysis, after which the monitoring cycle ends in step S4.

Computer program code for implementing a method according to the invention is suitably included in a computer program, which can be read into the internal memory of a computer, such as the internal memory of an electronic control unit of a motor vehicle which comprises an exhaust after-treatment system to be monitored. Such a computer program is suitably provided via a computer program product which comprises a data storage medium which can be read by an electronic control unit, which data storage medium has the computer program stored thereon. Said data storage medium is, for example, an optical data storage medium in the form of a CD-ROM, a DVD, etc., a magnetic data storage medium in the form of a hard disk, a floppy disk, a cassette tape, etc., or a memory of the ROM type. PROM, EPROM or EEPROM or a flash memory.

A computer program according to an embodiment of the invention comprises computer program code for bringing a computer: - receiving a measurement signal from a sensor regarding the size of a parameter related to exhaust gases flowing out of an exhaust after-treatment device included in an exhaust after-treatment system of a motor vehicle; - performing a frequency analysis of the measurement signal provided by said sensor for a certain period of time; and - generating information regarding the operation of the exhaust aftertreatment system based on said frequency analysis.

Fig. 5 very schematically illustrates an electronic control unit 30 comprising an execution means 31, such as a central processing unit (CPU), for executing computer software. The execution means 31 communicates with a memory 33, for example of the RAM type, via a data bus 32. The control unit 30 also comprises data storage medium 34, for example in the form of a memory of the type ROM, PROM, EPROM or EEPROM or a flash memory. The execution means 31 communicates with the data storage medium 34 via the data bus 32. A computer program comprising computer program code for implementing a method according to the invention is stored on the data storage medium 34.

The invention is of course not in any way limited to the embodiments described above, but a number of possibilities for modifications thereof should be obvious to a person skilled in the art, without departing from the basic idea of the invention as defined in the appended claims.

Claims (1)

A method for monitoring the operation of an exhaust aftertreatment system (2) of a motor vehicle based on a measurement signal from a sensor (11) regarding the magnitude of a parameter related to exhaust gases flowing out of an exhaust aftertreatment device (5), for example in the form of a catalyst or a filter, which is included in the exhaust aftertreatment system (2), a frequency analysis of the measurement signal provided by the sensor (11) for a certain period of time (Tev ) is performed, said time period here being called evaluation period, and information regarding the function of the exhaust after-treatment system (2) is generated based on said frequency analysis, characterized in that an evaluation value (Vev) which reflects the character of the high frequency part of the measurement signal which provided by the sensor (11) during the evaluation period (Tev) is determined by said frequency analysis, and that the evaluation value (Vev) is compared with a comparison value rde (Vw) or a given limit value (Vth) to generate information regarding the function of the exhaust after-treatment system (2). Method according to claim 1, characterized in that the measurement signal provided by the sensor (11) during the evaluation period (Tev) is filtered by means of a high-pass filter, said evaluation value (Vev) representing the signal energy of the high-pass filtered measurement signal. Method according to claim 1, characterized in that a frequency spectrum (23) is determined by said frequency analysis, and in that the frequency spectrum (23) is evaluated in order to determine said evaluation value (Vev). Method according to claim 3, characterized in that the evaluation value is determined by means of the formula: VeV = EhÖgY (f) / Etf, tY (f) where: - Crank is the evaluation value, - Y (f) is the y-axis value of the frequency fi of said frequency spectrum - Zhögl / (f) is the sum of Y (f) over a selected high frequency part of said frequency spectrum, and - ZWYG) is the sum of Y (f) over the low frequency part and the selected high frequency part the part of said frequency spectrum. Method according to claim 1, characterized in that: - a low-pass filter model representing the signal filtering characteristics of the exhaust after-treatment system with respect to said parameter is determined by evaluating the measurement signal from the sensor (11); and - that the evaluation value (Crank) is selected as a value representing the calculated cut-off frequency of the low-pass filter model. Method according to Claim 5, characterized in that the low-pass filter model is determined by means of autoregressive modeling or autoregressive moving average value modeling. Method according to one of Claims 1 to 6, characterized in that the comparison value (Vw) or the limit value (Vw) is a stored value, which is determined in advance and associated with specific operating conditions of the motor vehicle. Method according to any one of claims 1-6, characterized in that the comparison value (Vw) is determined in connection with performing a monitoring operation, based on measured or calculated values of the magnitude of said parameter of exhaust gases flowing into the exhaust aftertreatment device or based on calculated values of the magnitude of said parameter of exhaust gases flowing out of the exhaust after-treatment device. A method according to any one of claims 1-8, wherein said exhaust after-treatment device (5) is a catalyst or a filter, characterized in that the evaluation value (Vev) is determined based on a measurement signal. from a temperature sensor regarding the temperature of exhaust gases flowing out of the exhaust after-treatment device (5). A method according to any one of claims 1-8, wherein said exhaust gas aftertreatment device (2) is a catalyst, for example an SCR catalyst, characterized in that the evaluation value (Vev) is determined based on a measurement signal from a NOX sensor (11) regarding NOX the content of exhaust gases flowing out of the catalyst (5). Monitoring device for monitoring the function of an exhaust after-treatment system of a motor vehicle based on a measurement signal from a sensor (11) regarding the magnitude of a parameter related to exhaust gases flowing out of an exhaust after-treatment device, for example in the form of a catalyst or filter the monitoring system, the monitoring device (20) comprising processing means (21) arranged to receive a measurement signal from the sensor (11) regarding the measured magnitude of said parameter, the processing means (21) being arranged to perform a frequency analysis of the measurement signal provided by the sensor for a certain period of time (Tev), herein referred to as evaluation period, and to generate information regarding the function of the exhaust after-treatment system based on said frequency analysis, characterized in that the processing means (21) is arranged to determine an evaluation value (Vevlar) the nature of the high frequency one part of the measurement signal provided by the sensor (11) during the evaluation period (Tev), and to compare the evaluation value (Vev) with a comparison value (Vw) or a given limit value (Vm) to generate information regarding the function of the exhaust after-treatment system. Monitoring device according to claim 11, characterized in that the parameter is the temperature of exhaust gases flowing out of an exhaust after-treatment device in the form of a catalyst or a filter included in the exhaust after-treatment system. wherein the sensor is a temperature sensor arranged to measure said temperature and the processing means (21) is arranged to determine the evaluation value (Vev) based on a measurement signal from the temperature sensor regarding said temperature. Monitoring device according to claim 11, characterized in that the parameter is the NOX content of exhaust gases flowing out of an exhaust after-treatment device in the form of a catalyst, for example an SCR catalyst, which is part of the exhaust after-treatment system, the sensor (11) being a NOX sensors arranged to measure said NOX content and the processing means (21) are arranged to determine the evaluation value (Vev) based on a measurement signal from the sensor (11) regarding said NOX content. Computer program that can be read to the internal memory of a computer in a motor vehicle comprising an exhaust after-treatment system and a sensor for measuring the size of a parameter related to exhaust gases flowing out of an exhaust after-treatment device, for example in the form of a catalyst or a filters included in the exhaust aftertreatment system, the computer program comprising computer program code for causing the computer to: - receive a measurement signal from said sensor regarding the magnitude of said parameter; - performing a frequency analysis of the measurement signal provided by said sensor during a certain time period (Tev), said time period here being called evaluation period, by determining an evaluation value (Vev) which reflects the character of the high frequency part of the measurement. 16. 17. 530 435 19 the signal provided by said sensor during the evaluation period (Tev); and - comparing the evaluation value (Vev) with a comparison value (Vw) or a given limit value (Vth) to generate information regarding the function of the exhaust after-treatment system. A computer program product comprising a data storage medium readable by an electronic control unit (30), wherein a computer program according to claim 15 is stored on said data storage medium. Electronic control unit (30) comprising an execution means (31), a memory (33) connected to the execution means and a data storage medium (34) connected to the execution means, a computer program according to claim 15 being stored on said data storage medium (34).