WO2014200413A1

WO2014200413A1 - Method for detection of a blocked flow

Info

Publication number: WO2014200413A1
Application number: PCT/SE2014/050663
Authority: WO
Inventors: Johan WÄNGDAHL; Kurt KÄLLKVIST
Original assignee: Scania Cv Ab
Priority date: 2013-06-10
Filing date: 2014-06-02
Publication date: 2014-12-18
Also published as: SE1350705A1; DE112014002374T5; SE537396C2; DE112014002374B4

Abstract

Method to detect a flow blockage (205) in a fluid system (201), which flow blockage is arranged between a first fluid conduit (203) and a second fluid conduit (204) connected to the first fluid conduit via the flow blockage (205). The method comprises to control a flow source (202) connected to the first fluid conduit (203) so that at least one in-flow (208) into the first fluid conduit (203) is created, and to control a dosage device (206) connected to the second fluid conduit (204) so that at least one out-flow (209, 219) from the second fluid conduit is created. The method comprises to create one of said flows (209) as a time dependent flow of known size and with a time constant τ, and the remaining said at least one flow (208, 219) as a substantially constant flow over the time constant τ. so that pressure fluctuations arise in at least one of said fluid conduits (203, 204), to measure the amplitude of the pressure fluctuations and based on the measured amplitude to detect the flow blockage (205).

Description

Method for detection of a blocked flow

FI ELD OF THE I NVENTION The present invention pertains to a method for detection of a blocked flow. The invention is particularly, but not exclusively, focused on the performance of such a method to detect the degree to which a filter in an SCR-system for exhaust purification is clogged . The invention also pertains to a computer program product comprising computer program code to implement a method according to the invention, and an electronic control device.

BACKGROUN D TECHNOLOGY

In order to meet current requirements regarding exhaust purification , today's motor vehicles are usually equipped with a catalytic converter in the exhaust pipe, in order to achieve a catalytic conversion of environmentally harmful exhaust components into less environmentally harmful substances. One method used to achieve an effective catalytic conversion is based on injecting a reduction agent into the exhausts upstream of the catalytic converter. A reduction substance included in, or created by, the reduction agent is introduced into the catalytic converter by the exhausts, where it is absorbed in active sites in the catalytic converter, giving rise to an accumulation of the reduction agent in the catalytic converter. The accumulated reduction agent may either desorb, i .e. loosen from the active sites, or react with an exhaust substance for conversion of such exhaust substance into a harmless su bstance. Such a reduction catalytic converter may for example be of SCR-type (SCR = Selective Catalytic Reduction). This type of catalytic converter is referred to hereafter as an SCR-catalytic converter. An SCR- catalytic converter reduces NO_x selectively in the exhausts, but not the oxygen in the exhausts. I n an SCR-catalytic converter, a urea- or ammonium-based reduction agent is usually injected , e.g . AdBlue, in the exhausts upstream of the catalytic converter. When urea is injected into the exhausts, ammonia is formed , and it is this ammonia which constitutes the reduction agent contributing to the catalytic conversion in the SCR-catalytic converter.

SE 1 1 50862 and WO201 1 142708 describe SCR-systems for injection of reduction agent in an exhaust system upstream of an SCR-catalytic converter. In this type of SCR system a container holding a reduction agent is included . The SCR system also has a pump, arranged to pump said reduction agent from the container via a suction hose and to add it via a pressurised hose to a dosage device, arranged at an exhaust system in the vehicle, such as at an exhaust pipe of the exhaust system . The dosage device is arranged to inject a required amount of reduction agent into the exhaust conduit upstream of the SCR catalytic converter, according to operational procedures stored in a control device of the vehicle. In order to control the pressure at small or no dosage amounts more easily, the system may also have a return flow hose, arranged to go from a pressure side of the system back to the container. According to this configuration, it is possible to cool the dosage device with the reduction agent, which when cooled flows from the container via the pump and the dosage device back to the container. In this manner, an active cooling of the dosage device is provided . The return flow from the dosage device to the container is substantially constant. The SCR-system also comprises a filter to filter the reduction agent before the dosage with the dosage device. This filter is arranged to protect the dosage device from being clogged by particles, such as e.g . earth particles, dirt, etc. The filter may be a paper filter, but other types of filter may obviously be used .

One problem with such filters is that these may easily be clogged , which leads to a flow blockage, so that the reduction agent may not be dosed as intended . Such a flow blockage must be detected and fixed , since the emissions of NO_x-gases otherwise increases. I n WO201 1 142708 this problem is resolved by continuously measuring the accumulated amount of liquid which is supplied via a dosage device, and based on these measurements to determine whether the filter needs to be replaced . This method is based on assumptions about a certain amount of supplied reduction agent leading to a certain degree of clogging of the filter. In order to avoid filter replacements unnecessarily, and impaired exhaust purification, it is desirable to achieve a way of detecting whether the filter is clogged .

One way of detecting a flow blockage, for example in the form of a clogged filter, is to measure the fall in pressure over the clogged or partly clogged filter. With knowledge about the flow through the filter, it is possible to determine the size of the blockage. However, the pressures in the fluid conduits adjacent to the filter on both sides of the filter must be known, and additionally the flow through the blockage. Therefore, hardware in the form of pressure sensors are required in connection with both the fluid conduits. Alternatively, at least the fluid conduit on one side must consist of a closed volume with a known elasticity. One example of the latter is a pressurised container of known size containing a pressurised medium, which container is emptied through a blockage, for example in the form of a throttle. With knowledge about the medium, the size of the blockage may then be estimated . Unfortunately this technology may not be used in a fluid system where there are simultaneous unknown in- and outflows, since the net flow in or out of the container determines the pressure fall .

SUMMARY OF THE I NVENTION

One objective of the present invention is to present a method to be able to detect, at an early stage and with simple means, a flow blockage in a fluid system without impacting the fluid system's normal function .

According to one aspect of the invention , this objective is achieved with a method to detect a flow blockage in a fluid system, which flow blockage is arranged between a first fluid conduit and a second fluid conduit connected to the fist fluid conduit via the flow blockage, wherein the method comprises to control a flow source connected to the first fluid conduit so that at least one in-flow into the first fluid conduit is created , and to control a dosage device connected to the second fluid conduit so that at least one out-flow from the second fluid conduit is created . The method also comprises to create one of said flows as a time dependent flow of known size and with a time constant T , and the remaining said at least one flow as a constant or substantially constant flow over the time constant τ, so that pressure fluctuations arise in at least one of said fluid conduits, to measure the amplitude of the pressure fluctuations in one of said fluid conduits wherein pressure fluctuations arise, and based on the measured amplitude to detect the flow blockage.

Since the amplitude of the pressure fluctuations depend on the size of the flow blockage, it is possible to detect whether there is a flow blockage by measuring this. Absence of a flow blockage entails a smaller amplitude of the pressure fluctuations. The amplitude of the pressure fluctuations grows the larger the flow blockage becomes, since both fluid conduits are more and more delimited from each other through the blockage. With the method according to the invention , a flow blockage may be discovered at an early stage, before it becomes critical and risks to give rise to serious consequences. This also is effected by only measuring the pressure in one of the fluid conduits connected to the blockage. It is thus sufficient with one pressure sensor, which reduces the need for hardware compared to the prior art described above, where a pressure fall is measured over the blockage. When the method is carried out in e.g . an SCR-system such as described above, it also does not impact the system's normal function, since the method may be carried out independently of constant or su bstantially constant flows to and from both fluid conduits respectively, and since a time dependent flow source is already in place. The flow blockage may thus be detected at the same time as fluid flows as usual through the system. According to one embodiment, the flow blockage is detected based on the measured amplitude, the size of the time dependent flow, and the fluid system's volume V and elasticity E. By knowing these characteristics a more reliable detection of the flow blockage is obtained .

According to one embodiment, the measured amplitude is used as a test variable t-ι , which is tested against an alarm criterion, and , given that the alarm criterion is met, an error code is generated . In this manner, a control device, arranged to implement the method automatically, may generate an alarm when the flow blockage needs to be corrected . By directly using the measured amplitude, no calculations of the flow blockage's actual size need to be carried out by the control device.

According to another embodiment, the flow blockage's size is calculated based on the measured amplitude, the size of the time dependent flow, the first fluid conduit's volume V and elasticity and the second fluid conduit's volume V₂ and elasticity E₂. With knowledge about said characteristics the size of a blockage may be calculated , which is useful if the blockage only becomes critical when it exceeds a certain size. It is then possible to take measures at the correct time and to avoid replacing e.g . a filter, since the filter is only mildly clogged and the fluid system still functions satisfactorily.

According to one embodiment the calculated size is used as a test variable t-i , which is tested against an alarm criterion, and , given that the alarm criterion is met, an error code is generated . In this manner, a control device, arranged to implement the method automatically, may generate an alarm when the flow blockage needs to be corrected .

According to one embodiment, the alarm criterion is met if the test variable t exceeds a predetermined threshold value. This is an easy way of determining whether the flow blockage needs to be corrected . Regardless of whether the calculated size of the flow blockage or the measured amplitude of the pressure fluctuations are used as a test variable t-i , the threshold value may advantageously be set so that it is optimised for the given fluid system, i .e. with respect to e.g . the fluid system's input volumes, elasticity, the fluid's features, the ambient parameters such as pressure and temperature, the time dependent flow's size, etc.

According to another embodiment, the method is carried out for a system where said fluid conduits differ in volume. Preferably the volume of one of said fluid conduits is at least ten times larger than the volume of another of said fluid conduits. The pressure fluctuations may in such a system become more pronounced in the smaller of the both fluid conduits, which facilitates the detection of the flow blockage.

According to another embodiment, the time dependent flow is created in that of said fluid conduits which has the least volume. This usually provides the most pronounced pressure fluctuations.

According to another embodiment, the amplitude of the pressure fluctuations is measured in that of said fluid conduits which has the least volume. According to another embodiment, the amplitude of the pressure fluctuations is measured in that of said fluid conduits wherein the time dependent flow is created . In these manners, it is easiest to detect the pressure fluctuations since they usually become most pronounced in both these cases. Naturally, that of said fluid conduits having the least volume may coincide with that of said fluid conduits where the time dependent flow is created .

According to another embodiment, at least one said in-flow is created into the first fluid conduit by controlling a flow source in the form of a pump, so that fluid is pumped from a container into said first fluid conduit. By using a pump said in-flow may be created either as a time dependent or as a constant or substantially constant flow.

According to one variant of this embodiment, fluid is returned from the second fluid conduit to said container in a said out-flow in the form of a return flow, and fluid is led out of the fluid system in a said out-flow in the form of a consumption flow. In this way, a through-flow of fluid is created in the system , which is advantageous if any part in the fluid system is dependent on through-flow to be cooled down. In , for example, a system for injection of reduction agent upstream of an SCR-catalytic converter, a part of the reduction agent is often used for dosage to the exhaust system via the dosage device's out-flow, and the part of the reduction agent which is not supplied is returned in a return flow to the container. This part of the reduction agent is used advantageously as a coolant to avoid overheating of the dosage device. The reduction agent in the return flow is filtered , and the SCR-system therefore becomes self-cleaning to some extent. The returned filtered reduction agent thereby does not contribute to further clogging the filter.

According to one embodiment, the dosage device is controlled to create said time dependent flow. This is advantageous in applications where one wants fluid to leave the fluid system in the form of discrete doses instead of through a continuous outflow, such as is often the case in a system for injection of reduction agent upstream of an SCR-catalytic converter.

According to another embodiment, the time dependent flow is created in the form of a time periodical flow. Preferably, the time periodical flow is created in the form of a flow similar to a square wave. Such a flow is easily achieved by, for example, opening and closing a valve, wherein a flow similar to a square wave with a certain time constant is created . Preferably, the time periodical flow is created with a periodical time of 0.1 -1 0 s, more preferably 0.2-5 s, and even more preferably 0.25-1 s. According to one embodiment, the method is carried out for a fluid system comprising a flow blockage in the form of a filter which is clogged to some degree, wherein the degree of clogging of the filter is detected . According to another embodiment, the method is carried out for a system for injection of a reduction agent, such as a urea or ammonia based reduction agent, upstream of an SCR-catalytic converter in an exhaust conduit from a combustion engine. The method is well suited to be carried out for such a system, since the system usually comprises a time dependent flow source, such as a dosage device which doses reduction agent in discrete doses to the exhaust conduit, and the clogging of a filter in such a system may need to be measured independently of the substantially constant flows, which also exist in the system. The method is also advantageous since no extra pressure transmitter is required , but the flow blockage may be detected with hardware existing in the system.

According to another aspect of the invention, the objective is achieved with a computer program which may be downloaded to an internal memory of a computer, comprising software to control the steps according to the method suggested above, when said program is executed on a computer.

According to another aspect of the invention, the objective is achieved with a computer program product comprising a data storage medium which is readable by a computer, the computer program code of a computer program according to the above being stored on the data storage medium .

According to another aspect of the invention, the objective is achieved through an electronic control device comprising an execution means, a memory connected to the execution means and a data storage medium connected to the execution means, the computer program code in a computer program according to the above being stored on said data storage medium.

According to another aspect of the invention, the objective is achieved through a motor vehicle comprising an electronic control device according to the above. Other advantageous features of the invention and advantages the latter are set out in the description below.

BRI EF DESCRI PTION OF TH E DRAWI NGS

The invention is described below with the help of exam embodiments, with reference to the enclosed drawings, where

Fig . 1 shows a schematic diagram of a system in which the method according to the invention may be carried out,

Fig . 2 shows a schematic diagram of a system for injection of reduction agent upstream of an SCR-catalyst, and Fig . 3 is a schematic diagram of a control device for implementation of a method according to the invention.

DETAI LED DESCRI PTION OF EMBODI M ENTS ACCORD I NG TO THE I NVENTION

A flow blockage herein means a flow blockage of some degree, thus also a partial blockage of a flow. The partial blockage may sit in a filter or be localised in some other place in the fluid system, e.g . at a throttle or similar. Fluid system herein means a liquid or a gas system . Preferably, but not necessarily, a system herein is designed for through-flow of a liquid . Preferably, but not necessarily, a fluid system herein is in a vehicle. The fluid system may for example be a system for injection of reduction agent upstream of an SCR-catalytic converter in an exhaust conduit from a combustion engine, a fuel system designed to supply a combustion engine with fuel , a hydraulic brake system in a vehicle, a hydraulic system, a pneumatic system etc. Fluid conduit herein means a passage to hold and transport a fluid , such as a reduction agent in liquid form . The pipe may be a pipe of any dimension. The pipe may consist of any suitable material , such as plastic, rubber or metal . A dosage device herein means a device comprising a valve device of some type, which may be controlled to create a constant or time dependent flow of a known size.

A constant, or substantially constant, flow means a flow that has a time constant which is considerably larger than the time dependent flow's time constant, so that the constant ,or substantially constant, flow's variations over time occur much slower than the time dependent flow's variations over time. A first fluid system 1 01 where the method according to the invention may be carried out is displayed schematically in Fig . 1 . The fluid system comprises a flow source in the form of a pressurised fluid container 102 with a volume V_B, a first fluid conduit 103 with a first volume V-i , a second fluid conduit 1 04 with a second volume V₂ and a flow blockage 105 arranged between the first and the second fluid conduit. A dosage device 106 is arranged in connection with the second fluid conduit, as well as a pressure sensor 1 07. In order to calculate the size of the flow blockage 105, an in-flow 108 from the pressurised container 102 is created in the first fluid conduit 1 03. With the help of the dosage device 1 06, a time dependent out-flow 109 of a known size and with a time constant T (i .e. the time it takes for the flow to increase/decrease to around 63% of its maximum level) is created from the second fluid conduit 1 04. Pressure fluctuations then arise in the fluid system 101 . With the help of the pressure sensor 107, the pressure in the second fluid conduit 104 is measured , and the amplitude of the pressure fluctuations is determined . Since the pressure fluctuations' amplitude is a function of the size of the flow blockage, a blockage may be detected for example by observing how the amplitude changes over time. The larger the flow blockage, the larger the amplitude of the pressure fluctuations becomes. With knowledge about the fluid system's ingoing volumes V_B, V and V₂ and the container's 1 02 elasticity E_B, and the fluid conduits' respective elasticity E-i and E₂,as well as the size of the outflow, the size of the flow blockage may be estimated . In case throttles of the same size as or larger than the flow blockage 105 occur between the pressurised container 1 02 and the first fluid conduit 1 03 in the fluid system 1 01 , the container's 102 volume and elasticity may be disregarded .

In a fluid system similar to the above described system, instead of a pressurised container a pump connected to a fluid container may be used as a flow source. The pump may be controlled to create a time dependent inflow, and the dosage device may then be arranged to create a constant outflow, or an outflow which is substantially constant over the inflow's time constant τ. The pressure sensor may be connected to either of the both fluid conduits, but suitably the fluid system is arranged so that the pressure sensor is connected to that of both fluid conduits which has the smaller volume, since the pressure fluctuations become most pronounced in this fluid conduit. Preferably, but not necessarily, this fluid conduit coincides with the fluid conduit where the time dependent flow is created , which may lead to even more pronounced pressure fluctuations.

A fluid system 201 , for injection of reduction agent upstream of an SCR-catalytic converter (not displayed) in an exhaust conduit from a combustion engine is schematically illustrated in Fig . 2. The system comprises a flow source in the form of a pump 202, arranged to pump reduction agent via a fluid conduit 21 1 from a container 21 2, and into a fluid conduit 203. In direct connection to the fluid conduit 203, a main filter 205 is arranged , and downstream of this there is a fluid conduit 204. In connection with this, there is a dosage device 206, which may be electrically controlled with a control device (not displayed), and which is arranged to inject reduction agent into said not displayed exhaust conduit via a dosage valve 210. The dosage device 206 comprises an electronic control card (not displayed), arranged to handle communications with the control device. The dosage device 206 may also comprise plastic and/or rubber components, which may melt or be impacted negatively in some other way at too high temperatures. A pressure sensor 207 is arranged to measure the pressure in the second fluid conduit 204. From the dosage device 206 a return conduit 213 is arranged , which via a throttle 214 leads back to the container 212. Apart from the main filter 205 there are a number of filters 21 5, 216, 21 7 arranged in the fluid system . The filter 21 7 is in this case considerably larger than the main filter 205.

When used , a su bstantially constant in-flow 208 of reduction agent into the fluid conduit 203 is created with the help of the pump 202. The reduction agent is filtered at the main filter 205 and passes into the fluid conduit 204. Downstream of the fluid conduit 204 the reduction agent is supplied in doses to the exhaust conduit with the help of the dosage device 206, which is controlled to create an out-flow 209 of a known size from the second fluid conduit 204, in the form of a flow similar to a square wave, by opening and closing the dosage valve 210. Via the dosage device 206 non-administered reduction agent is also led back to the container 21 2 in a substantially constant return flow 219, via the throttle 214. Thanks to the return flow 219, the dosage device 206 may continuously be cooled down with the help of the reduction agent. When the dosage valve 21 0 is closed , a pressure is built up toward the throttle 214 in the fluid conduit 204 with the help of the pump 202 , and when the dosage valve 21 0 is open the pressure eases, since the reduction agent then flows out through both the dosage valve and via the throttle 214. The pressure fluctuations are measured by the pressure sensor 207. With the help of the control device, the pressure fluctuations' amplitude is calculated , and based on this the degree of clogging of the main filter 205 may be determined . Based on the measured amplitude, the size of the time dependent flow, the fluid conduit's 203 volume V and elasticity and the fluid conduit's 204 volume V₂and elasticity E₂, the degree of clogging of the main filter 205 is calculated . In this embodiment of the method according to the invention, the fluid system 201 is arranged with a first fluid conduit 203, whose volume V₁ is around ten times larger than the volume V₂ of the second fluid conduit 204. The time dependent flow 209 is created in the form of a square wave in the second fluid conduit 204, at which the pressure is also measured . The pressure fluctuations therefore become easy to detect with the help of the pressure sensor 207. The degree of clogging of the main filter 205 may be determined independently of the su bstantially constant in- and out-flows 208, 219 of reduction agent in the fluid conduits 203, 204. The fact that the flows are su bstantially constant entails in this case, that the time constant of the time dependent, in this case square wave shaped , flow 209 is significantly smaller than any potential time constants of the other flows 208, 219. This may for example entail that the time constant τ for the time dependent flow is in the range of tenths of seconds, while the other flows vary over a considerably longer time, such as with a time constant in the interval 5-30 s. The calculated size of the flow blockage may suitably be used as a test variable t-i , which with the help of the control device is tested against an alarm criterion . Given that the alarm criterion is met, an error code is generated . The alarm criterion may for example be set so that it is met if the test variable t₁ exceeds a predetermined threshold value.

Instead of calculating the size of the flow blockage, it may be suitable to use the measured amplitude of the pressure fluctuations as a test variable t-| . In this case, the threshold value may be set so that the fluid system's characteristics are taken into consideration, such as the fluid conduits' volumes and elasticity, so that an actual value for the flow blockage's size does not need to be calculated . Suitable threshold values may e.g . be determined empirically or with the help of simulation. For certain applications the fluid system's characteristics may however vary with for example temperature and pressure. In, for example, a fuel system the pressure may vary between around 500-2000 bar and the elasticity of the system becomes highly dependent on the pressure. In such cases it may be advantageous to either calculate the size of the flow blockage and to use this as a test variable t or to use a threshold value which is e.g . pressure dependent.

A computer program code for the implementation of a method according to the invention is suitably included in a computer program, loadable into the internal memory of a computer, such as the internal memory of an electronic control device of a motor vehicle. Such a computer program is suitably provided via a computer program product comprising a data storage medium readable by an electronic control device, the data storage medium having the computer program stored thereon. Said data storage medium is e.g . 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 drive, a floppy disc, a cassette, etc. , or a Flash memory or a ROM , PROM , EPROM or EEPROM type memory.

Fig . 3 illustrates schematically an electronic control device 40 comprising execution means 41 , such as a central processor unit (CPU ), for the execution of a computer software. The execution means 41 communicates with a memory 42, e.g . a RAM memory, via a data bus 43. The control device 40 also comprises a data storage medium 44, e.g . in the form of a Flash memory or a ROM , PROM , EPROM or EEPROM type memory. The execution means 41 communicates with the data storage means 44 via the data bus 43. A computer program comprising computer program code for the implementation of a method according to the invention is stored on the data storage medium 44. The invention is not limited in any way to the embodiments described above, but numerous possible modifications thereof should be obvious to a person skilled in the area, without such person departing from the spirit of the invention as defined by the appended claims.

Claims

CLAI MS

1 . Method to detect a flow blockage (105, 205) in a fluid system (101 , 201 ), which flow blockage is arranged between a first fluid conduit (103, 203) and a second fluid conduit (104, 204) connected to the fist fluid conduit via the flow blockage (105, 205), wherein the method comprises to control a flow source (102 , 202) connected to the first fluid conduit (1 03, 203) so that at least one in-flow (1 08, 208) into the first fluid conduit (1 03, 203) is created , and to control a dosage device (106, 206) connected to the second fluid conduit (204, 204) so that at least one out-flow ( 109, 209, 21 9) from the second fluid conduit is created ,

characterised in that :

the method comprises to create one of said flows (1 09, 209) as a time dependent flow of known size and with a time constant τ, and the remaining said at least one flow (108, 208, 21 9) as a constant or substantially constant flow over the time constant τ, so that pressure fluctuations arise in at least one of said fluid conduits (103, 104, 203, 204), to measure the amplitude of the pressure fluctuations in one of said fluid conduits (104, 204) wherein pressure fluctuations arise, and based on the measured amplitude to detect the flow blockage ( 105, 205).

2. Method according to claim 1 , characterised in that the flow blockage (105, 205) is detected based on the measured amplitude, the size of the time dependent flow (109, 209) and the fluid system's volume V and elasticity E.

3. Method according to claim 1 or 2 , characterised in that the measured amplitude is used as a test variable t-i , which is tested against an alarm criterion, and that, given that the alarm criterion is met, an error code is generated .

4. Method according to claim 1 , characterised in that the flow- blockage's (1 05, 205) size is calculated based on the measured amplitude, the size of the time dependent flow ( 109, 209), the first fluid conduit's volume V and elasticity E-i , and the second fluid conduit's volume V₂ and elasticity E₂.

5. Method according to claim 4, characterised in that the calculated size is used as a test variable t-i , which is tested against an alarm criterion, and that, given that the alarm criterion is met, an error code is generated .

6. Method according to claim 3 or 5, characterised in that the alarm criterion is met if the test variable t exceeds a predetermined threshold value.

7. Method according to any one of the previous claims, characterised in that it is carried out for a system, wherein said fluid conduits (1 03, 1 04, 203, 204) differ in volume.

8. Method according to claim 7, characterised in that it is carried out for a system, wherein the volume of one of said fluid conduits (203) is at least ten times larger than the volume of another of said fluid conduits (204).

9. Method according to claim 7 or 8, characterised in that the time dependent flow is created in that of said fluid conduits (104, 204) with the smallest volume.

10. Method according to any one of claims 7 to 9, characterised in that the amplitude of the pressure fluctuations is measured in that of said fluid conduits (1 04, 204) which has the smallest volume.

1 1 . Method according to any one of the previous claims, characterised in that the amplitude of the pressure fluctuations is measured in that of said fluid conduits (104, 204), wherein the time dependent flow (109, 209) is created .

12. Method according to any one of the previous claims, characterised in that at least one said in-flow (208) into the first fluid conduit (203) is created by controlling a flow source in the form of a pump (202), so that fluid is pumped from a container (212) into said first fluid conduit (203).

13. Method according to claim 1 2, characterised in that fluid is returned from the second fluid conduit (204) to said container (212) in a said out-flow in the form of a return flow (219), and fluid is led out of the fluid system (201 ) in a said out-flow in the form of a consumption flow (209).

14. Method according to any one of the previous claims, characterised in that the dosage device (1 06, 206) is controlled to create said time dependent flow (109, 209).

15. Method according to any one of the previous claims, characterised in that the time dependent flow (109, 209) is created in the form of a time periodical flow.

16. Method according to any one of the previous claims, characterised in that it is carried out for a fluid system comprising a flow blockage (105, 205) in the form of a filter which is clogged to some degree, wherein the degree of clogging of the filter (105, 205) is detected .

1 7. Method according to any one of the previous claims, characterised in that the method is carried out for a system (201 ) for injection of reduction agent upstream of an SCR-catalytic converter in an exhaust conduit from a combustion engine.

1 8. Computer program comprising computer program code to bring a computer to implement a method according to any one of claims 1 -1 7 when the computer program code is executed in the computer.

1 9. A computer program product comprising a data storage medium which is readable by a computer, wherein the computer program code of a computer program according to claim 18 is stored on the data storage medium.

20. Electronic control device (40) comprising an execution means (41 ), a memory (42) connected to the execution means and a data storage medium (44) connected to the execution means, the computer program code in a computer program according to claim 18 being stored on said data storage medium (44).

21 . Motor vehicle comprising an electronic control device (40) according to claim 20.