SE541093C2

SE541093C2 - Dosing system for reduction agent

Info

Publication number: SE541093C2
Application number: SE1750957A
Authority: SE
Inventors: Kurre Källkvist; Max Lindfors; Per Bremberg; Peter Engelke
Original assignee: Scania Cv Ab
Priority date: 2017-07-24
Filing date: 2017-07-24
Publication date: 2019-04-02
Also published as: SE1750957A1; DE102018005638A1

Abstract

The present invention relates to a dosing system (20) for adding a reducing agent to an exhaust gas flow from an internal combustion engine (2) and comprises a storage tank (21) for the reducing agent, a pressurizing supply pump (23) arranged downstream of the tank (21) for supplying the reducing agent via at least a first filter device (26) to a first dosing unit (22) and to a second dosing unit (24), each dosing unit (22; 24) comprising a respective dispensing device (220, 240) for the reducing agent, the first and second dosing units (22; 24) being arranged in series and the first filter device (26) being arranged upstream of the first dosing unit (22). A bypass pipe (28) comprising a flow obstruction device (280) is arranged to connect a supply pipe (31) from the supply pump (23) upstream of the first filter device (26) with an intermediate pipe (32) between the first dosing unit (22) and the second dosing unit (24) and at least a portion of the reducing agent flow is arranged to by-pass the first filter device (26) and the first dosing unit (22) via the by-pass pipe (28) in case the first filter device (26) is at least partly clogged. The dosing system is preferably included in the exhaust system (10) of a vehicle (1).

Description

Dosing system for reduction agent TECHNICAL FIELD The present invention relates to a dosing system for a reducing agent, an exhaust gas system and a vehicle comprising the exhaust gas system as defined in the appended claims.

BACKGROUND ART Due to strict environmental regulations internal combustion engines, such as diesel engines, are provided with an exhaust gas purification system which comprises one or several devices to reduce both particles and emissions of environmentally harmful gases which result from the combustion of fuel during the operation of the internal combustion engine. Environmentally harmful discharges include e.g. carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (???) and particulate matter (PM). To regulate emissions or discharges from vehicles there are various standards and legal requirements which govern the permissible levels for exhaust discharges.

Emission reduction technologies suitable for diesel vehicles include exhaust gas recirculation (EGR), particulate filters, diesel oxidation catalysts (DOC), and selective catalytic reduction (SCR), which is used to reduce gaseous nitrogen oxide (NOx) -emissions from internal combustion engines. The SCR-system comprises a dosing system or arrangement for dosing a reducing agent and a catalyst with an SCR-substrate. The reducing agent may be e.g. a mixture of 32.5% urea and water, often marketed with a trade name AdBlue<®>. Injecting the agent and mixing it with the exhaust gases results in a chemical reaction whereby nitrogen gas and water are formed over the SCR-catalyst.

The dosing system for reducing agent normally comprises a tank for the reducing agent connected to a pump device and filter devices to protect a dosing unit comprising a dispensing or injection device which injects the reducing agent into the exhaust gas duct. During the operation of the internal combustion engine the exhaust gases reach usually a temperature which is high enough to vaporise the urea solution so that ammonia is formed. However, in case the filter device clogs, there is a risk for insufficient supply of reducing agent and thus operational disturbances in the exhaust gas system. In a worst case the whole dosing system fails. Thus, there is a need for redundancy in such dosing systems so that sufficient reducing agent supply can be ensured during all times.

Clogging problems in connection with valves have been realized for example in EP2955352 and said document describes a control system for dosing which takes into account clogging of a valve. However, how to ensure supply of reducing agent in a dosing system in which a filter device is clogged and thus prevent passage to the injecting device have not been discussed in the document. Thus, there is still a need to improve the existing dosing systems to ensure operational robustness.

SUMMARY OF THE INVENTION In view of the problems in connection with the prior art dosing systems and exhaust gas systems it is an objective of the present invention to provide a dosing system and an exhaust gas system with improved operational robustness.

The inventors of the present disclosure have identified a problem in connection with dosing systems comprising at least two serially connected dosing units, namely that when a first filter device upstream of the first dosing unit in the series is clogged, the supply of the reducing agent is also prevented to the dosing unit or units downstream of the first dosing unit. Therefore, a further objective of the present invention is to ensure supply of the reducing agent in dosing systems comprising at least two dosing units in series.

It is a further objective of the present invention to ensure that emission levels under the allowed pre-determined levels are reached during the operation of the internal combustion engine. It is also an objective to provide an exhaust gas system which enables operation of a vehicle with reduced fuel consumption.

The objectives above are attained by a dosing system as defined in the appended claims. The dosing system is suitable for adding a reducing agent to an exhaust gas flow from an internal combustion engine and the dosing system comprises a storage tank for the reducing agent, a pressurizing supply pump arranged downstream of the tank for supplying the reducing agent via at least a first filter device to a first dosing unit and to a second dosing unit, each dosing unit comprising a dispensing device for the reducing agent. The first and second dosing units are arranged in series and the first filter device is arranged upstream of the first dosing unit. A bypass pipe comprising a flow obstruction device is arranged to connect a supply pipe from the supply pump upstream of the first filter device with an intermediate pipe between the first dosing unit and the second dosing unit. At least a portion of the reducing agent flow is arranged to by-pass the first filter device and the first dosing unit via the by-pass pipe in case the first filter device is at least partly clogged. In case the first dosing unit additionally fails, the reducing agent flow can be arranged to by-pass the first filter device and the first dosing unit via the bypass pipe. In this way the operation of the dosing system can be assured even if the first filter device were completely clogged and if the first unit failed. Also, by arranging the dosing units in series, relatively simple construction with reduced amount of pipe connections compared to dosing units arranged in parallel can be obtained.

The flow obstruction device can be a throttle valve connected to the by-pass pipe. Thus, passive or active control of the flow obstruction device can be provided.

Suitably, more than 20% and up to 100% of the reducing agent flow is arranged to flow through the by-pass pipe depending on the clogging grade of the filter device. Thus, if the filter device is only partly clogged, a portion of the reducing agent can be arranged to flow through the first filter device to the dosing unit while the remaining portion can by-pass the first filter device and the first dosing unit. Thus, operational disturbances can be avoided. The flow obstruction device can be dimensioned so that 0-20% of the reducing agent flow is arranged to flow through the by-pass pipe during normal operation of the dosing system. In this way the by-pass pipe provides for robustness during the operation of the system.

An outlet of the by-pass pipe is suitably connected to the intermediate reducing agent pipe and a check valve is arranged in the intermediate reducing agent pipe between the outlet of the bypass pipe and the first dosing unit to prevent the flow of the reducing agent back to the first dosing unit. Thus, unfiltered reducing agent cannot flow to the dosing unit and the unit is therefore effectively protected from particulate matter.

According to a variant of the invention, the first and second dosing units are arranged to be cooled by means of the reducing agent. Thus, a separate cooling system is not necessary and in this way simple construction for the system can be provided.

The second dosing unit can be connected to a return pipe through which part of the reducing agent is arranged to flow back to the tank. In this way closed circuit fora portion of the reducing agent flow can be provided.

According to an aspect, a second filter device is suitably arranged downstream of the first dosing unit and upstream of the second dosing unit, and wherein a main filter device is arranged downstream of the supply pump and upstream of the first filter device. By arranging several filter devices in the dosing system, the components of the dosing system are protected against impurities.

The present invention also relates to an exhaust gas system comprising a first selective catalytic reduction (SCR) catalyst and a second selective catalytic reduction (SCR) catalyst, the first and second selective catalytic reduction (SCR) catalysts being fluidly connected to a respective first and second dosing unit arranged in series for adding a reducing agent. The exhaust gas system further comprises a diesel particulate filter and optionally a diesel oxidation catalyst and an ammonia slip catalyst. The first dosing unit is arranged upstream of the first SCR-catalyst and the second dosing unit is arranged downstream of the diesel particulate filter and upstream of the second SCR-catalyst. The exhaust gas system is robust and simple in construction and provides redundancy in case of clogging of the first filter device or operational disturbances in the first dosing unit, since there are two dosing units and SCR-catalysts arranged in series. In the exhaust gas system, a first filter device is arranged upstream of the first dosing unit and a bypass pipe comprising a flow obstruction device is arranged upstream of the first filter device and wherein at least a portion of the reducing agent flow is arranged to by-pass the first filter device and the first dosing unit via the by-pass pipe in case the first filter device is at least partly clogged. Also, if the dosing unit has failed the portion of the reducing agent flow can be arranged to by-pass the first filter device and the first dosing unit via the by-pass pipe. In this way the operation of the exhaust system can be ensured even if the first filter device were clogged completely and if the first dosing unit has failed.

Suitably, the exhaust gas system comprises a turbocharger turbine upstream of the first selective catalytic reduction (SCR) catalyst. In this way energy efficiency of the exhaust gas system can be improved.

Preferably, the exhaust gas system comprises a diesel particulate filter downstream of the first selective catalytic reduction (SCR) catalyst and upstream of the second selective catalytic reduction (SCR) catalysts. In this way the SCR catalysts are arranged in a manner where the temperature variations in the exhaust gas system can be utilized during the operation of the exhaust gas system.

Suitably, the exhaust gas system is comprised in a silencer of a vehicle. Thus, a space saving construction can be provided. The exhaust gas system preferably comprises or is connected to the dosing system described above.

The present invention also relates to a vehicle comprising the exhaust gas system as described above.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the respective drawings, and in which: Fig. 1 schematically illustrates a vehicle comprising an exhaust system according to an embodiment of the invention.

Fig. 2 schematically illustrates a coupling scheme for dosing system according to an embodiment of the invention.

Fig. 3 schematically illustrates a coupling scheme for an exhaust gas system according to an embodiment of the invention.

DETAILED DESCRIPTION The present invention is based upon the realisation by the inventors that redundancy to a dosing system comprising at least two dosing units with respective filter device, which dosing units are arranged in series can be provided by arranging a by-pass pipe comprising a flow obstruction device to connect a supply pipe from the supply pump upstream of the filter device with an intermediate pipe between the first dosing unit and the second dosing unit. Thus, the first filter device and the first dosing unit can be by-passed while the further components of the dosing system are provided with reducing agent in case the first filter device is clogged and if the first dosing unit is failed. To protect the first dosing unit, a check valve can be arranged in the intermediate reducing agent pipe between an outlet of the by-pass pipe and the first dosing unit to prevent the flow of the reducing agent back to the first dosing unit. According to a variant, the by-pass pipe, the flow obstruction device and the check valve can be integrated in the first dosing unit. In this way compact construction can be obtained. Also, in this way the first dosing unit can be cooled by means of the reducing agent even in the case the first filter device is clogged.

To ensure that main portion of the reducing agent is supplied to the first dosing unit during normal operation, the by-pass pipe is provided with a flow obstruction device. When the filter device is clean and thus not clogged the pressure provided by the pump downstream of a storage tank for the reducing agent is sufficient to press the reducing agent through the first filter device and all additional filter devices to the dosing unit. However, when the first filter device is clogged or partially clogged, the back-pressure caused by the clogging in the filter will increase, thereby making it more difficult to pass the filter with retained pressurizing force from the supply pump. The reducing agent flow will therefore choose a way with least resistance, and thus at least a portion of the reducing agent will flow through the by-pass pipe. In this way the reducing agent flow is arranged to by-pass the first filter device and the first dosing unit via the by-pass pipe. The more the filter is clogged, the greater is the portion of the flow through the by-pass pipe. The flow obstruction device is suitably dimensioned so that 0-20% of the reducing agent flow passes through the by-pass pipe during the normal operation of the dosing system. When the clogging grade of the filter increases the portion of the reducing agent flow through the by-pass pipe also increases. This is due to the increased back-pressure caused by the increased clogging grade of the filter device. Thus, in this way the volume of the reducing agent passing through the by-pass pipe is arranged to be dependent of the clogging grade, and can be controlled passively by means of a self-adjusting flow obstruction device or actively by means of electrical control devices and e.g. an electrical control valve as a flow obstruction device. Suitably, when the filter device starts to clog, more than 20% and up to 100% of the reducing agent flow is arranged to flow through the by-pass pipe depending on the clogging grade of the filter device. The flow obstruction device may be e.g. a throttle valve. The throttle valve may be for example a spring loaded valve connected to the bypass pipe.

The reducing agent dosing system of the present disclosure is suitably located in or connected to an exhaust gas system for an internal combustion engine. The internal combustion engine may be any internal combustion engine, but is preferably a four-stroke combustion engine, even more preferably a compression ignition four-stroke combustion engine. The engine may be used in any application commonly known for internal combustion engines. It may for example be merchandized as a free-standing engine, for use for example in power generation or industrial settings. However, application in a vehicle is preferred. By vehicle it is meant any machine utilising an internal combustion engine to provide motive force, either directly, or indirectly as in the case of series hybrid vehicles. This includes, but is not limited to, motor vehicles such as cars, trucks and buses; railed vehicles such as trains and trams; watercraft such as ships and boats; and aircraft. The reducing agent is preferably diesel exhaust fluid comprising a solution of urea in water, in accordance with standard AUS 32 of ISO 22241. However, the reducing agent may also be another liquid additive added to the exhaust stream, and may comprise hydrocarbon fuel to "burn off" a diesel particulate filter arranged downstream in the exhaust system. The reducing agent may alternatively be termed "reductant" or "reduction agent".

The dosing unit comprises or consists of a dispensing device which may be a liquid-only device, otherwise known as an airless injector. This means that the dispensing device does not utilize compressed air in order to facilitate injection of the reductant into the exhaust conduit. Since a compressor requires energy to run, this represents an energy saving compared to air-assisted systems. Moreover, some applications such as marine applications do not necessarily have a ready source of compressed air to hand, and thus the use of extra, costly components can be avoided. However, the dispensing device of the reductant dosing system may alternatively be an air-assisted device, i.e. a device that utilises compressed air to facilitate injection of the reductant. Pressurised reductant may be supplied to the dispensing device via a supply pipe. The dispensing device suitably comprises a controllable valve means for dosing the required amount of reductant to the exhaust system. A dosing pipe may be connected to the dispensing device along which the reductant is transported to an outlet of the dosing pipe. The dosing pipe may traverse a wall of the exhaust gas duct in the exhaust gas system at any suitable location, preferably downstream of a turbocharge turbine.

To prevent overheating of the dosing units and especially the dispensing devices, e.g. injection valves thereof, they are suitably arranged to be cooled by means of the reducing agent. In case the first filter device is clogged it is essential that the supply of reducing agent is maintained to the dosing units downstream of the first dosing unit and therefore the first dosing unit is bypassed. The first filter device has mainly a purpose of protecting the first dispensing device, e.g. injection valve in the first dosing unit from particulate matter enter the valve. However, according to a variant, the supply of the reducing agent could be arranged such that cooling of the first dosing unit is enabled, i.e. a small portion of the reducing agent flow is allowed to bypass the filter device and be fed through a cooling circuit of the first dosing unit. However, the non-filtered reductant should be prevented from entering a supply line for the dispensing device. Also, a main portion of the flow is suitably arranged to by-pass the first filter device and the first dosing unit. In this way the first dosing unit could be cooled while the dispensing device is protected against particulate matter. Alternatively, as mentioned above, the by-pass pipe, the flow obstruction device and the check valve can be integrated in the first dosing unit whereby at least a portion of the first dosing unit can be cooled by means of the reducing agent.

In the dosing system, the second dosing unit is suitably connected to a return pipe through which part of the reducing agent is arranged to flow back to the tank. In this way part of the reductant which is mainly used for cooling is arranged in a closed circuit.

The dosing system may comprise several filters or filter devices, e.g. a second filter device can be arranged downstream of the first dosing unit and upstream of the second dosing unit. Further a main filter device can be arranged downstream of the supply pump and upstream of the first filter device. Also, a storage tank filter may be arranged to separate larger particulate matter from the reductant. Also a supply pump filter may be arranged upstream of the supply pump to protect the pump from particulate matter. Each of the filters improves the purity of the reducing agent to be injected into the exhaust gas system.

The present invention also relates to a robust exhaust gas system which comprises a first selective catalytic reduction (SCR) catalyst and a second selective catalytic reduction (SCR) catalyst which are fluidly connected to a respective first and second dosing unit arranged in series for adding a reducing agent. Further, the exhaust gas system comprises a diesel particulate filter and optionally a diesel oxidation catalyst and ammonia slip catalyst. The first dosing unit is arranged upstream of the first SCR-catalyst and the second dosing unit is arranged downstream of the diesel oxidation catalyst and/or the diesel particulate filter and upstream of the second SCR-catalyst. In this way it can be assured that reducing agent can be provided to the exhaust flow at least before reaching the second SCR catalysts. The advantage of arranging the dosing units in series is that they can be operated alternately or simultaneously in series. Since the dosing units can be operated alternately, it is possible to choose which dosing unit and the SCR catalyst is to be used depending on the temperature of the exhaust gases: the position is chosen depending on whether the exhaust gas temperature is highest downstream or upstream of the diesel particulate filter. To optimize vaporization of the reductant it is advantageous to choose the dosing unit and catalyst located at the highest temperature area. In this way it is possible to reduce the need for operating the engine in a heat generating way, e.g. by delaying the combustion or at high engine rotation speeds, and thus it is possible to reduce fuel consumption.

The exhaust gas system preferably comprises a dosing system as described above, wherein it is possible to allow at least a portion of the reducing agent flow to by-pass the first filter device and the first dosing unit if the first filter device is at least partly clogged.

The exhaust gas system also suitably comprises a turbocharger turbine upstream of the first selective catalytic reduction (SCR) catalyst. The exhaust gas system is equipped with a turbocharger for recovering energy from the exhaust gases. An exhaust duct is connected downstream of the turbocharger for conveying exhaust gas from the turbocharger through the exhaust system towards downstream components, such as for example particulate filters, the selective reduction catalysts, ammonia slip catalyst, etc. The exhaust duct is suitably a pipe having a cylindrical duct wall with a longitudinally extending central axis. By cylindrical it is meant a cylinder having any cross-sectional shape known in the art, but most commonly circular-cylindrical. Any exhaust duct known in the art may be used. The exhaust gas system or at least some of the components thereof can be comprised in a silencer of a vehicle.

Figure 1 illustrates schematically a side view of a vehicle 1 according to an embodiment of the invention. The vehicle 1 includes an internal combustion engine 2, a first exhaust duct 4 leading to a turbocharger turbine 6, and a second exhaust duct 8 leading from the turbocharger turbine 6 to an exhaust gas system 10 comprising SCR purification system 30 comprising an SCR catalyst. The exhaust gas system may be at least partly housed in a silencer 11 of the vehicle 1. The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car. The vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 2.

Figure 2 illustrates schematically a coupling scheme for a dosing system according to a variant of the present invention. The dosing system 20 comprises a storage tank 21 for the reducing agent and a pressurizing supply pump 23 arranged downstream of the tank 21 for supplying the reducing agent to a first dosing unit 22 via a supply pipe 31. The reducing agent is supplied to the first dosing unit 22 via at least a first filter device 26 located upstream of the first dosing unit 22. In the embodiment shown in Fig. 2, the dosing system comprises a supply pump filter device 37 upstream of the supply pump 23 and a main filter device 25 downstream of the supply pump 23. A tank filter 35 is arranged in the storage tank 21 in connection with an inlet to the supply pipe 31. The supply pump 23 pressurizes the reducing agent flow with sufficient pressure to ensure supply through all components of the dosing system during normal operation. The dosing system 20 further comprises a second dosing unit 24. A second filter device 27 is arranged upstream of the second dosing unit 24 and downstream of the first dosing unit 22. The first dosing unit 22 comprises a first dispensing device 220 and the second dosing unit comprises a second dispensing device 240 for the reducing agent. The first and second dosing units 22, 24 are arranged in series and connected to each other by means of an intermediate pipe 22.

According to the invention a by-pass pipe 28 comprising a flow obstruction device 280 is arranged to connect the supply pipe 31 from the supply pump 23, downstream of the tank 21, and upstream of the first filter device 26 with the intermediate pipe 32 extending between the first dosing unit 22 and the second dosing unit 24. At least a portion of the reducing agent flow is arranged to by-pass the first dosing unit 22 and the first filter device 26 via the by-pass pipe 28 in case the first filter device 26 is at least partly clogged. The flow obstruction device 280 is suitably a throttle valve. As shown in figure 2, an outlet 281 of the by-pass pipe 28 is connected to the intermediate reducing agent pipe 32 and a check valve 29 is arranged in the intermediate reducing agent pipe 32 between the outlet 281 and the first dosing unit 22 to prevent the flow of the reducing agent back to the first dosing unit 22. According to an embodiment, the by-pass pipe 28, the flow obstruction device 280 and the check valve 29 are integrated components of the first dosing unit 22.

Furthermore, the second dosing unit 24 is connected to a return pipe 33 through which part of the reducing agent is arranged to flow back to the tank 21. In this way the reducing agent used to cool the dosing units is returned to the tank.

Figure 3 illustrates schematically a coupling scheme for an exhaust gas system according to a variant of the invention. The exhaust gases are generated by combustion of fuel in an internal combustion engine 2. The exhaust gases are collected in an exhaust gas collector connected to a first exhaust duct 4 leading the exhaust gases to a turbocharger turbine 6. A second exhaust gas duct 8 is arranged to lead the exhaust gases from the turbocharger turbine 6 to a silencer 11. The exhaust gas system 10 comprises a first selective catalytic reduction (SCR) catalyst 32 and a second selective catalytic reduction (SCR) catalyst 34. The first and second SCR catalysts are fluidly connected to a respective first dosing unit 22 and second dosing unit 24 arranged in series for adding the reducing agent. The exhaust gas system 10 further comprises a diesel particulate filter (DPF) 40. The exhaust gas system may optionally comprise a diesel oxidation catalyst and an ammonia slip catalyst which are not shown in the illustrated example. The first dosing unit 22 is arranged upstream of the first SCR catalyst 32 and the second dosing unit 24 is arranged downstream of the diesel particulate filter 40 and upstream of the second SCR catalyst 34. All components could be arranged in the silencer 11 of a vehicle, but in the illustrated example the first dosing unit is arranged upstream of the silencer 11. As generally explained above, the dosing units 22 and 24 can be operated alternately or in series as described above. The purified exhaust gases are then led out of the silencer 11 via an exhaust gas pipe 36.

Further variations of the invention are possible within the scope defined in the appended claims.

Claims

1. Dosing system (20) for adding a reducing agent to an exhaust gas flow from an internal combustion engine (2), the dosing system (20) comprising a storage tank (21) for the reducing agent, a pressurizing supply pump (23) arranged downstream of the tank (21) for supplying the reducing agent via at least a first filter device (26) to a first dosing unit (22) and to a second dosing unit (24), each dosing unit (22; 24) comprising a respective dispensing device (220, 240) for the reducing agent, the first and second dosing units (22; 24) being arranged in series and the first filter device (26) being arranged upstream of the first dosing unit (22), characterized in that a by-pass pipe (28) comprising a flow obstruction device (280) is arranged to connect a supply pipe (31) from the supply pump (23) upstream of the first filter device (26) with an intermediate pipe (32) between the first dosing unit (22) and the second dosing unit (24) and wherein at least a portion of the reducing agent flow is arranged to by-pass the first filter device (26) and the first dosing unit (22) via the by-pass pipe (28) in case the first filter device (26) is at least partly clogged.

2. Dosing system according to claim 1, wherein the flow obstruction device (280) is a throttle valve connected to the by-pass pipe (28).

3. Dosing system according to claim 1 or 2, wherein more than 20% and up to 100% of the reducing agent flow is arranged to flow through the by-pass pipe (28) depending on the clogging grade of the filter device (26).

4. Dosing system according to any one of the preceding claims, wherein an outlet (281) of the by-pass pipe (28) is connected to the intermediate reducing agent pipe (32) and a check valve (29) is arranged in the intermediate reducing agent pipe (32) between the outlet (281) of the by-pass pipe (28) and the first dosing unit (22) to prevent the flow of the reducing agent back to the first dosing unit (22).

5. Dosing system according to any one of the preceding claims, wherein the first and second dosing units (22; 24) are arranged to be cooled by means of the reducing agent.

6. Dosing system according to any one of the preceding claims, wherein the second dosing unit (24) is connected to a return pipe (33) through which part of the reducing agent is arranged to flow back to the tank (21).

7. Dosing system according to any one of the preceding claims, wherein a second filter device (27) is arranged downstream of the first dosing unit (22) and upstream of the second dosing unit (24), and wherein a main filter device (25) is arranged downstream of the supply pump (21) and upstream of the first filter device (26).

8. An exhaust gas system (10) comprising a first selective catalytic reduction (SCR) catalyst (32), a second selective catalytic reduction (SCR) catalyst (34), the first and second selective catalytic reduction (SCR) catalysts (32; 34) being fluidly connected to a respective first and second dosing unit (22; 24) arranged in series for adding a reducing agent, a diesel particulate filter (40) and optionally a diesel oxidation catalyst and an ammonia slip catalyst, wherein the first dosing unit (22) is arranged upstream of the first SCR-catalyst (32) and the second dosing unit (24) is arranged downstream of the diesel particulate filter (40) and upstream of the second SCR-catalyst (34), characterized in that a first filter device (26) is arranged upstream of the first dosing unit (22) and a by-pass pipe (28) comprising a flow obstruction device (280) is arranged upstream of the first filter device (26) and wherein at least a portion of the reducing agent flow is arranged to by-pass the first filter device (26) and the first dosing unit (22) via the by-pass pipe (28) in case the first filter device (26) is at least partly clogged.

9. The exhaust gas system of claim 8, wherein the exhaust gas system (10) comprises a turbocharger turbine (6) upstream of the first selective catalytic reduction (SCR) catalyst (32).

10. The exhaust gas system of any one of claims 8 or 9, wherein the exhaust gas system (10) comprises a diesel particulate filter (40) downstream of the first selective catalytic reduction (SCR) catalyst (32) and upstream of the second selective catalytic reduction (SCR) catalysts (34).

11. The exhaust gas system of any one of claims 8 to 10, wherein the exhaust gas system (10) is comprised in a silencer (11) of a vehicle (1).

12. Vehicle (1) comprising the exhaust gas system (10) according to any one of claims 9-11.