Method and arrangement for monitoring the functioning of an exhaust gas aftertreatment system

TECHNICAL FIELD

The present invention relates to a method and a monitoring arrangement for monitoring the functioning of an exhaust gas aftertreatment system of a motor vehicle based on a measuring signal from a sensor as to the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance, for instance in the form of a catalyst or a filter, included in the exhaust gas aftertreatment system. Furthermore, the invention relates to a computer program comprising computer program code for implementing a method according to the invention, a computer program product comprising a data storage medium readable by an electronic control unit and having said computer program stored thereon, and an electronic control unit.

BACKGROUND ART

In order to fulfil existing and future regulations regarding On- Board Diagnostics of an exhaust gas aftertreatment system of a heavy motor vehicle, it is for instance necessary to control that a catalyst, e.g. in the form of an SCR catalyst (SCR = Selective Catalytic Reduction), included in the system is functioning properly.

A method for monitoring an SCR catalyst is previously known from WO 2004/109072 A1 . According to this known method, a temperature value representing the temperature of exhaust gases flowing out of the SCR catalyst is calculated by means of a calculation model and compared with a temperature value measured in the exhaust line downstream of the SCR catalyst. Based on the correspondence between these temperature values, such as for instance the difference between them, it is established whether or not the SCR catalyst and its injection device are functioning in a satisfying and expected manner.

Similar methods are also previously known from DE 4 122 787 A1 , US 5 860 277 A and EP 0 756 071 A2. A problem associated with this type of monitoring methods relying on a calculation model is that the calculation model normally might give rather correct calculation values but at some operating conditions might give calculation values deviating substantially from the corresponding real values. Thus, incorrect fault indications might be generated based on test samples recorded during the last mentioned operating conditions.

The functioning of an SCR catalyst may also be monitored by means of a NO _x sensor arranged to measure the NO _x content in the exhaust gases downstream of the catalyst. This NO _x sensor may be supplemented by another NO _x sensor arranged to measure the NO _x content in the exhaust gases upstream of the catalyst in order to provide a comparison value. However, a NO _x sensor is an expensive component and one option for dispensing with a NO _x sensor upstream of the catalyst is to use a calculation model for calculating the amount of NO _x produced by the vehicle engine. Furthermore, another calculation model may be used for calculating the expected conversion of NO _x in the catalyst so as to obtain calculated values of the NO _x content in the exhaust gases downstream of the catalyst. These calculated values may then be compared with the measuring values from the NO _x sensor downstream of the catalyst in order to detect possible disorders of the catalyst and the associated equipment. The use of calculation models for calculating the magnitude of the NO _x content in the exhaust gases may however give calculation values deviating substantially from the corresponding real values, which in its turn may result in the generation of incorrect fault indications.

Due to more and more strict regulations regarding the above- indicated type of On-Board Diagnostics, there is a great need of a reliable method and arrangement for monitoring the functioning of an exhaust gas aftertreatment system.

DISCLOSURE OF THE INVENTION

The object of the present invention is to propose a new and reliable manner of monitoring an exhaust gas 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 arrangement having the features defined in claim 12.

According to the inventive solution, the functioning of the exhaust gas aftertreatment system is monitored based on a measuring signal from a sensor as to the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance, for instance in the form of a catalyst or a filter, included in the exhaust gas aftertreatment system. A frequency analysis of the measuring signal supplied by the sensor during a certain period of time is performed so as to establish an evaluation value that reflects the character of a frequency part, preferably the high frequency part, of the measuring signal supplied by the sensor during this period of time. The evaluation value is compared with a comparison value or a given threshold value in order to generate information regarding the functioning of the exhaust gas aftertreatment system

The above-indicated period of time is in the following denominated "evaluation period".

The inventive solution is based on the realization that a properly functioning exhaust gas aftertreatment appliance in the form of a

NO _x reducing catalyst has an inertness that will cause a low-pass filtering effect on the NOx content in exhaust gases passing the catalyst, and that a properly functioning exhaust gas aftertreatment appliance in the form of a catalyst or particle filter has a temperature inertness that will cause a low-pass filtering effect on the temperature of exhaust gases passing the catalyst/filter. This is due to the fact that the catalyst and filter,

when functioning properly, will dampen fluctuations of the NO _x content and the temperature, respectively, occurring upstream of the catalyst/filter, and the more rapid fluctuations will normally be dampen to such an extent that they will have a reduced effect on the value of the NO _x content/temperature downstream of the catalyst/filter. Thus, the measuring signal from a NO _x sensor or temperature sensor arranged downstream of a catalyst or particle filter will comprise more high-frequency parts when the catalyst/filter is subjected to a disorder that reduces the efficiency thereof as compared to the case when the catalyst/filter is functioning properly. There are several different methods well known to a person skilled in the art that may be used for evaluating the low-pass filtering capacity of the exhaust gas aftertreatment appliance in question, and thus the functioning thereof, based on frequency analysis of the measuring signal from a sensor arranged downstream of said appliance.

By basing the monitoring of the exhaust gas aftertreatment system on a frequency analysis of the measuring signal from a sensor, such as a temperature sensor or NO _x sensor, arranged downstream of the exhaust gas aftertreatment appliance, it will be possible to generate information regarding the functioning of the exhaust gas aftertreatment system without being dependent on comparative calculation values from a complex and sometimes inaccurate calculation model, and without being dependent on measuring values from a corresponding sensor arranged upstream of the exhaust gas aftertreatment appliance. Furthermore, the monitoring will be insensitive to deviations between the real parameter values and the corresponding measuring values supplied by the sensor, which deviations for instance may be caused by offset errors of the sensor due to sensor drifting.

According to a first alternative, the evaluation value is chosen so as to represent the signal energy of a high-pass filtered measuring signal obtained by filtering the measuring signal

supplied by the sensor during the evaluation period by means of a high-pass filter. Hereby, the evaluation value may be established in a simple and efficient manner. Alternatively, the evaluation value may be established by evaluation of a frequency spectrum established by said frequency analysis. According to another alternative, the evaluation value is chosen as a value representing the estimated cutoff frequency of a low-pass filter model, which represents the signal filtering characteristics of the exhaust gas aftertreatment system with respect to said parameter and which is established by evaluation of the measuring signal from the sensor.

The comparison value is with advantage a stored value, which is established in advance and associated with specific operating conditions of the motor vehicle. In this case, a monitoring operation will only be carried out when the prevailing operating conditions of the motor vehicle correspond to operating conditions for which a comparison value has been established and stored. The comparison value may alternatively be established in connection with the execution of a monitoring operation, based on measured or calculated values of the magnitude of said parameter of exhaust gases flowing into the exhaust gas aftertreatment appliance or based on calculated values of the magnitude of said parameter of exhaust gases flowing out of the exhaust gas aftertreatment appliance.

Further advantageous features of the monitoring method and the monitoring arrangement according to the invention are indicated in the dependent claims and the following description.

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 17 and an electronic control unit having the features defined in claim 18.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be more closely described by means of embodiment examples, with reference to the appended drawings, where:

Fig 1 is a schematic diagram of a combustion engine with an associated exhaust gas aftertreatment system, illustrating an embodiment of a monitoring arrangement according to the present invention,

Fig 2 is a diagram showing an exemplifying measuring signal from a sensor as a function of time,

Fig 3 is a diagram showing the frequency spectrum of the measuring signal of Fig 2,

Fig 4 is an exemplifying Bode plot representing the frequency response of a catalyst included in an exhaust gas aftertreatment appliance,

Fig 5 is a schematic outline diagram of an electronic control unit for implementing a method according to the invention, and

Fig 6 is a flow diagram illustrating a method according to an embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION

An inventive monitoring method and monitoring arrangement will in the following be described as implemented to monitor the functioning of an exhaust gas aftertreatment system comprising an exhaust gas aftertreatment appliance in the form of a catalyst.

However, the invention is in no way limited to this utilization. On the contrary, the invention may be utilized for monitoring an exhaust gas aftertreatment system having any type of exhaust gas aftertreatment appliance that has a low-pass filtering effect on a measurable exhaust gas parameter. The invention may for

instance be used for monitoring the functioning of an exhaust gas aftertreatment system comprising an exhaust gas aftertreatment appliance in the form of a filter, such as a particle filter or an open structure filter.

A combustion engine 1 with an associated exhaust gas aftertreatment system 2 is schematically shown in Fig 1 . The exhaust gases leaving the combustion engine 1 are conveyed through an exhaust line 3 and are discharged into the surroundings via an exhaust outlet 4. A catalyst 5 is arranged in the exhaust line 3. The exhaust gases from the combustion engine 1 will pass the catalyst 5 before being discharged into the surroundings 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 injection device 6 into the exhaust gases in the exhaust line 3 upstream of the catalyst 5. The injection device 6 comprises one or several injection members 7 in the form of injection nozzles or the like arranged in the exhaust line 3, and a reducing agent storage container 8 connected thereto. The injection device 6 also comprises a regulating member 9, e.g. in the form of a control valve, arranged to regulate the supply of reducing agent to said one or several injection members 7, and a control means 10 connected to the regulating member 9. The regulating member 9 is controlled by said control means 10, which determines, on the basis of the prevailing operating conditions of the combustion engine 1 and the catalyst 5, the amount of reducing agent to be injected into the exhaust gases. The injection device 6 may also comprise further components, such as a dosing appliance etc. The reducing agent may be urea (CO(NH ₂ ) ₂ ), ammonia (NH ₃ ) or hydrocarbon (fuel).

A sensor 1 1 is arranged in the exhaust line 3 downstream of the catalyst 5. In this example, the sensor 1 1 is a NO _x sensor arranged to generate a measuring signal representing the NO _x content in the exhaust gases flowing out of the catalyst 5, i.e. the

NO _x content in the exhaust gases at the outlet of the catalyst. The measuring signal may be a continuous signal representing the continuous changes of the measured parameter, i.e. forming a continuous stream of measuring values as to the magnitude of the parameter, but is normally recorded as a discrete-time signal forming a series of consecutive and discrete measuring values as to the magnitude of the parameter.

The monitoring arrangement 20 comprises processing means 21 arranged to receive the measuring signal from the sensor 1 1 as to the measured magnitude of the NO _x content in the exhaust gases downstream of the catalyst 5. The processing means 21 is adapted to perform a frequency analysis of the measuring signal supplied by the sensor 1 1 during a certain period of time T _ev , here denominated evaluation period, and to generate information regarding the functioning of the exhaust gas aftertreatment system 2 based on said frequency analysis.

If the catalyst is functioning properly, it will under normal operating conditions of the combustion engine 1 and the catalyst 5 have a general reducing effect on the NO _x content of the exhaust gases passing through the catalyst and will also dampen peak values and rapidly varying fluctuations of the NO _x content occurring in the exhaust gases upstream of the catalyst 5 in such a manner that these peak values and fluctuations have been eliminated or at least substantially reduced when the exhaust gases leave the catalyst. In this respect, the catalyst resembles a low-pass filter and will have a low-pass filtering effect on the magnitude of the NO _x content of the exhaust gases passing through the catalyst 5. Thus, the measuring signal from the sensor 1 1 resembles a low-pass filtered signal. The low-pass filtering effect varies with variations in the NO _x reducing efficiency of the catalyst. Thus, by evaluating the low-pass filtering capacity of the catalyst 5, for instance by evaluating the character of the high frequency part of the measuring signal supplied by the sensor 1 1 , it will be possible to detect whether or

not the catalyst 5 and the associated exhaust gas aftertreatment system 2 is functioning in a satisfying and expected manner.

Based on the above-indicated frequency analysis of the measuring signal supplied by the sensor 1 1 , the processing means 21 is adapted to establish an evaluation value V _ev that reflects the character of a frequency part, preferably the high frequency part, of the measuring signal supplied by the sensor during the evaluation period T _ev . The processing means 21 is then adapted to compare this evaluation value V _ev with a comparison value V _co or a given threshold value V _th in order to generate information regarding the functioning of the catalyst 5 and the associated exhaust gas aftertreatment system 2. There are several different methods well known to a person skilled in the art that may be used for evaluating the measuring signal from the sensor 1 1 in order to establish an evaluation value of a suitable form. Some of these conceivable methods will be described below.

According to a first method, the measuring signal supplied by the sensor 1 1 during the evaluation period T _ev is filtered by means of a high-pass filter 22 and the evaluation value V _ev is chosen so as to represent the signal energy of the high-pass filtered measuring signal. This signal energy may be calculated in any suitable and conventional manner with the aid of an algorithm known to a person skilled in the art. The established evaluation value V _ev may then be compared with a signal energy representing comparison value V _co or a given threshold value V _th . In this case, the comparison value V _co may for instance be chosen so as to represent the signal energy of a previously high-filtered measuring signal from the sensor 1 1 recorded during a period of time when the catalyst and the associated exhaust gas aftertreatment system were known to function properly or the signal energy of a correspondingly high-pass filtered signal established by means of suitable calculation models. The processing means 21 is suitably adapted generate a fault indication if the difference or ratio between the evaluation value

V _ev and the comparison value V _co exceeds a threshold level so as to thereby indicate a functional disorder of the system.

According to a second method, a frequency spectrum of the measuring signal supplied by the sensor 1 1 during the evaluation period is established by frequency analysis of said measuring signal. This frequency spectrum may be established in any suitable and conventional manner with the aid of an algorithm known to a person skilled in the art, such as for instance the algorithm denominated Fast Fourier Transform (FFT). As an illustrative example, Fig 3 shows a frequency spectrum 23 established by FFT based on the measuring signal 24 shown in Fig 2. The frequency spectrum shows the magnitude of each frequency component of the measuring signal. The frequency spectrum of the measuring signal from the sensor 1 1 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 established frequency spectrum 23 may be evaluated in any suitable manner in order to establish the evaluation value V _ev -

The evaluation value may for instance be established from a frequency spectrum by the formula:

V _ev =σ _high Y(f)/σ _tot Y(f)

where:

- V _ev is the evaluation value,

- Y(f) is the y-axis value for the frequency f in said frequency spectrum,

- σ _high Y(f) is the sum of Y(f) across a selected high frequency part of said frequency spectrum, and

- ∑totY(f) is the sum of Y(f) across the low frequency part and the selected high frequency part of said frequency spectrum. If this evaluation value V _ev exceeds a given threshold value V _th , this will indicate a functional disorder of the exhaust gas

aftertreatment system. The processing means 21 is suitably adapted generate a fault indication if the evaluation value V _ev exceeds the threshold value V _th so as to thereby indicate a functional disorder of the exhaust gas aftertreatment system.

According to a third method, a low-pass filter model representing the signal filtering characteristics of the exhaust gas aftertreatment system with respect to said parameter is established by evaluation of the measuring signal from the sensor 1 1 during the evaluation period T _ev - This low-pass filter model may be established in any suitable and conventional manner with the aid of an algorithm known to a person skilled in the art, such as for instance by Autoregressive Modeling or Autoregressive Moving Average Modeling. In this case, the evaluation value V _ev is chosen as a value representing the estimated cutoff frequency of the low-pass filter model. As an illustrative example, a Bode plot 25 associated with an established low-pass filter model in the form of an Autoregressive Model of the first order is shown in Fig 4. The cutoff frequency f _cut is indicated in the Bode plot of Fig 4. The established evaluation value V _ev may then be compared with a comparison value V _co representing the cutoff frequency of a comparative low-pass filter model or a given threshold value V _th . In this case, the comparison value V _co may for instance be chosen so as to represent the cutoff frequency of a comparative low-pass filter model previously established based on a measuring signal from the sensor 1 1 recorded during a period of time when the catalyst and the associated exhaust gas aftertreatment system were known to function properly or based on a corresponding signal established by means of suitable calculation models.

The length of the evaluation period T _ev and the interval between each monitoring operation may of course vary widely from case to case depending on the nature of the system to be monitored. The evaluation period may for instance be in the order of 10-30 seconds or longer.

A suitable comparison value V _co or threshold value V _th may be established empirically by practical tests and/or theoretically on the basis of suitable calculations. The comparison value V _co or threshold value V _th is with advantage a stored value, which is established in advance and associated with specific operating conditions of the motor vehicle. The comparison value V _co may alternatively be established in connection with the execution of a monitoring operation, based on measured or calculated values of the magnitude of said parameter of exhaust gases flowing into the exhaust gas aftertreatment appliance 5 or based on calculated values of the magnitude of said parameter of exhaust gases flowing out of the exhaust gas aftertreatment appliance 5.

The monitoring arrangement 20 suitably comprises some sort 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 V _ev indicates that the catalyst 5 and/or any other component of the exhaust gas aftertreatment system 2, such as for instance the injection device 6 or the sensor 1 1 , is not functioning satisfyingly, and may for instance be due to one or more of the following causes:

- the catalyst 5 or a part thereof has been removed, - the catalyst 5 is degenerated or damaged,

- the sensor 1 1 is degenerated or damaged,

- the injection device 6 does not inject the expected amount of the expected reducing agent,

- an error in a utilized calculation model, - one or more of the input signals to a utilized calculation model is incorrect.

Thus, the possible fault causes should be checked when a fault indication has been generated in order to find and correct the fault in question.

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

It should be evident to a person skilled in the art that the embodiment examples described above in connection with the monitoring of an exhaust gas aftertreatment system provided with an exhaust gas aftertreatment appliance in the form of an SCR catalyst easily could be modified in order to monitor the functioning of an exhaust gas aftertreatment system provided with any other type of exhaust gas aftertreatment appliance having an inertness with respect to the measured parameter in question, such as another type of catalyst or a filter. The parameter could for instance be the temperature of the exhaust gases flowing out of a catalyst or a filter, in which case the above-indicated sensor is a temperature sensor arranged downstream of the catalyst/filter. The parameter could also be the ammonia content of the exhaust gases flowing out of an SCR catalyst operated with a reducing agent in the form of urea or ammonia, in which case the above-indicated sensor is an ammonia sensor arranged downstream of the catalyst.

A flow diagram illustrating a method according to an embodiment of the invention is shown in Fig 6. In a first step S1 , a measuring signal from a sensor 1 1 as to the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance is recorded during a certain period of time T _ev - In a second step S2, a frequency analysis of the recorded measuring signal is performed so as to establish an evaluation value V _ev that reflects the character of a frequency part, preferably the high frequency part, of the measuring signal supplied by the sensor 1 1 during said period of time T _ev - In a subsequent step S3, the evaluation value V _ev is compared with a comparison value V _co or a given threshold value V _th in order to generate information regarding the functioning of the exhaust gas aftertreatment system, whereupon the monitoring cycle is terminated in step S4.

Computer program code for implementing a method according to the invention is suitably included in a computer program, which is loadable into the internal memory of a computer, such as the internal memory of an electronic control unit of a motor vehicle comprising an exhaust gas aftertreatment system to be monitored. Such a computer program is suitably provided via a computer program product comprising a data storage medium readable by an electronic control unit, which data storage medium has the computer program stored thereon. Said data storage medium is for instance an optical data storage medium in the form of a CD-ROM disc, a DVD disc etc, a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc, or a memory of the type ROM, PROM, EPROM or EEPROM or a Flash memory.

A computer program according to an embodiment of the invention comprises computer program code for causing a computer:

- to receive a measuring signal from a sensor as to the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance included in an exhaust gas aftertreatment system of a motor vehicle;

- to perform a frequency analysis of the measuring signal supplied by said sensor during a certain period of time T _ev so as to establish an evaluation value V _ev that reflects the character of a frequency part, preferably the high frequency part, of the measuring signal supplied by the sensor during this period of time T _ev ; and

- to compare the evaluation value V _ev with a comparison value V _co or a given threshold value V _th in order to generate information regarding the functioning of the exhaust gas aftertreatment system.

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 instance of the

type RAM, via a data bus 32. The control unit 30 also comprises data storage medium 34, for instance 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 restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.