GB2452249A

GB2452249A - An exhaust system

Info

Publication number: GB2452249A
Application number: GB0716078A
Authority: GB
Inventors: Klaus Rusch; Clive Telford; Nicholas Sever-Topping
Original assignee: Emcon Technologies Germany Augsburg GmbH; Emcon Technologies UK Ltd
Current assignee: Faurecia Emissions Control Technologies Germany GmbH; Faurecia Emissions Control Technologies UK Ltd
Priority date: 2007-08-17
Filing date: 2007-08-17
Publication date: 2009-03-04
Also published as: GB0716078D0; WO2009024815A3; WO2009024815A2

Abstract

An exhaust system 20 having a flow path including an emission control device 1 in a first portion of the flow path, a liquid injector 3 positioned to inject a liquid into a second portion 4 of the flow path after the first portion and a catalytic emission control device 2 in a third portion of the flow path after the second portion, the second portion defining a collecting chamber 40, acting to collect exhaust gas once it has expanded upon exiting the emission control device, said collecting chamber feeding the exhaust gas into a contraction area 46 of the second portion, the contraction area including a pipe having a part which projects into the collecting chamber. Preferably, the emission control device is a diesel particulate filter and the catalyst an selective catalytic reduction catalyst; wherein the second portion serves to thoroughly mix the injected liquid, preferably urea, with the exhaust gases. A static mixer 4 may be provided and the second portion of the flow path may be divided into a first stream A2 and a second stream A3 which bypasses the part of the pipe.

Description

An Exhaust System The present invention relates to an exhaust system in particular for use with a diesel internal combustion engine.

Diesel internal combustion engines produce various pollutants including NO and soot.

Whilst the engines themselves are controlled in a manner so as to minimise such pollutants, nevertheless it is necessary to treat the exhaust gas so as to remove such pollutants and meet legal requirements.

One known option is to provide an exhaust system where an exhaust gas is fed through a diesel particulate filter (DPF) and then through a selective catalytic reduction catalyst (SCR catalyst). The DPF operates to remove soot whereas the SCR catalyst operates to remove NOR.

In order for the SCR catalyst to operate, a liquid reagent, in this case a reducing agent, such as urea is injected into the exhaust gas upstream of the SCR catalyst but downstream of the DPF.

Furthermore, since the above mentioned known system has been applied to commercial vehicles which traditionally have a supply of compressed air for use with the vehicle brakes, it has been convenient to inject the reducing agent using an air assisted injector with air at typically 5 bar pressure. This gives a relatively small particle size to the reducing agent droplets. However, certain vehicles (such as smaller commercial vehicles that use hydraulic brakes and/or saloon cars etc) may not have available a supply of compressed air. Thus, there are known injector nozzles (used in exhaust systems that do not have a DPF) that do not utilise compressed air. However, such nozzles produce a relatively large droplet size of reducing agent and these larger droplets take longer to be filly disbursed within the exhaust gas stream prior to entering the SCR catalyst.

Future legal requirements will require a high conversion rate of NOR. One option would be to inject excess urea. However, if this is done, any urea that has not reacted with NO will be vented to atmosphere with the treated exhaust gas. This will result in an unpleasant ammonia smell. Accordingly, a system must be provided that injects just sufficient urea (or other reducing agent) to react with the NO in the exhaust gas. The problem with such an arrangement is that the urea and exhaust gas must be thoroughly mixed before they reach the SCR catalyst. However, any static mixers and the like provided to assist mixing must not produce significant back pressure.

Furthermore where space is at a premium, it is necessary to position the DPF and the SCR catalyst close together which in turn limit the amount of volume for that part of the exhaust system that connects the exit from the DPF to the inlet to the SCR catalyst.

Thus, an object of the present invention is to provide a system that efficiently mixes a liquid with an exhaust gas.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:-Figure 1 shows a schematic representation of a first embodiment according to the present invention, and Figures 2 to 14 show further schematic views of further embodiments according to the present invention.

With reference to figure 1 there is shown an exhaust system 20 and an internal combustion diesel engine 22. Exhaust gases exiting from the diesel engine 22 enter the exhaust system 20 via exhaust inlet 8 and then pass through an emission control device in the form of a diesel particulate filter (DPF) 1. Upon exiting the DPF the exhaust gases pass through an injecting/mixing region 24 before entering a catalytic emission control device in the form of a selective catalytic reduction (SCR) catalyst 2. Exhaust gases then exit the SCR catalyst and exit the exhaust system via exhaust outlet 9 to atmosphere.

Arrows Al to A5 show the general flow of exhaust gas.

In this case the DPF is made from an extruded ceramic material. The DPF has a series of longitudinal tubes, half of which are blanked off at the inlet end 30 and the other half of which are blanked off at the outlet end 31. The exhaust gas therefore must pass through a tube wall when flowing through the exhaust system, the tube walls being porous and acting to filter any soot within the exhaust gas. In this case the cross section of the DPF is circular.

The injecting/mixing region 24 has several components as follows.

A collecting chamber 40 is defined between the outlet end 31 of the DPF and the outer wall 41 and inner wall 42. A contraction area 44 is defined by outer tube 45. An expansion area 47 is defined between inner wall 42, outer wall 41 and inlet end 34 of the SCR catalyst.

1 5 In this case the SCR catalyst is also a ceramic material made by extrusion. The SCR catalyst includes several tubes which, in this case, are open at both ends, i.e. open at inlet end 34 and open at outlet end 35. The exhaust system includes an inlet expansion area 50 and outlet expansion area 51.

As the exhaust gas passes through the exhaust system it expands and contracts as follows:-The cross section of the flow path in the inlet expansion area 50 is larger than the cross section of the exhaust inlet pipe 8, so that as the exhaust gas passes from the exhaust inlet pipe 8 to the inlet expansion area 50 it expands.

The open area of the inlet end 30 of the DPF 1 is typically 20% to 40%. This is because, as mentioned above, half of the tubes are blocked and the tubes have a defined wall thickness. Thus, when the exhaust gas is passed from the inlet expansion area 50 into the DPF I it inevitably contracts.

Upon exiting the DPF into the collecting chamber 40, the exhaust gases will expand.

The collecting chamber collects the gases together and passes them through outer tube 45 and inner tube 46. It will be appreciated that the cross section area of the outer tube 45 is smaller than the flow of cross section area in the collecting chamber 40 and hence as the gases pass through the contraction area 44 they will inevitably be contracted.

As the exhaust gas exits the lower end 45B of outer tube 45 and enter the expansion area 47, the cross section area of flow increases thereby expanding the exhaust gases.

The open area of inlet end 34 of the SCR catalyst is typically between 50% and 65% and hence the exhaust gases will contract as they enter the SCR catalyst.

As the exhaust gas exits the SCR catalyst into the outer expansion area 51, the exhaust gases will expand and will then contract as it enters the exhaust outlet 9.

The above mentioned expansion and contraction of the exhaust gases tends to attenuate low frequency noise generated by the engine 22.

Looking at the injecting/mixing region 24 in more detail, inner tube 46 includes a bell mouth portion 55, a curved portion 56 and a straight portion 57. The bell mouth portion 55 acts as a convergent section and, in this case is frustoconical. By providing the upstream end of inner tube 46 as a convergent section pressure losses within the exhaust gas stream can be reduced.

The curved portion acts to turn that part A2 of the gas flow through 90 degrees. A static mixer 4 is provided at the junction between the curved portion 56 and straight portion 57.

A reducing agent injector 3 is provided mounted on outer wall 42. It is orientated so as to spray reducing agent 3A into inner tube 46. A funnel 60 ensures that all the reducing agent enters the inner tube 46. In this case the injector is an airless injector, i.e. an injector that only injects the reducing agent and does not inject air. In other words the injector is a single phase injector since it only injects a liquid and does not inject a gas.

In further embodiments the reducing agent injector could be an air assisted injector i.e. an injector that injects both reducing agent and compressed air, the compressed air acting as the means for propelling the reducing agent. Such an injector is a twin phase injector since it injects both a liquid and a gas.

The reducing agent spray 3A impinges directly on the static mixer 4, this tends to break up the droplets of reducing agent within the reducing agents spray into smaller droplets, thereby improving subsequent mixing.

As mentioned above, the flow of exhaust gas A2 enters the inner tube 46. However, the flow of exhaust gas A3 bypasses the inner tube and passes down the annulus 61 formed between the outer tube and the inner tube. A secondary static mixer 12 is provided at the end 57B of the straight portion 57 of the inner tube 46. The outer tube 45 projects below this secondary static mixer and it is in this lower portion of the outer tube 45 that the flows 2A and 3A of exhaust gas recombine to form flow 4A.

The exhaust gas flows into the expansion area 47 from outlet 7.

It will be appreciated that the straight portion 57, the static mixer 4 and the curved portion 56 together form a primary mixing chamber 5 which mixes the reducing agent with portion A2 of the exhaust gas flow. A secondary mixing chamber 6 is provided where the exhaust gas streams 2A and 3A recombine. Furthermore, the expansion area 47 also acts to mix the recombined streams 3A and 2A. Only once the gas stream has entered the discreet tubes of the SCR catalyst can no general mixing occur.

It will be appreciated that the inner tube 46 projects into the collecting chamber 40, as does the upper part of the outer tube 45. By doing this, it is possible to start the mixing of the exhaust gas stream with the reducing agent spray whilst the exhaust gas stream is still in the collecting chamber 40. Note that the injector 3 injects the reducing agent into the collecting chamber 40 in this case into a part of the inner tube 46 which is positioned within the collecting chamber. Note that the reduction agent injector 3 is positioned so as to inject the reducing agent downstream of the open end 46A of the inner tube 46.

An example of a static mixer can be seen in patent application EP047 11494 (Publication Number EP 1716917) In further embodiments that static mixer could take alternate forms.

It is clear that portion A3 of the exhaust gas flow bypasses the inner tube 46, and in particular bypasses the static mixer 4.

Note that the end 57B of straight portion 57 projects into the expansion area 47, as does the lower end of the outer tube.

It will be appreciated that the exhaust gas flow through the DPF is from left to right when viewing figure 1 and the exhaust gas flow through the SCR. catalyst is from right to left when viewing figure 1, in other words the DPF and the SCR catalyst are positioned parallel to each other so that the flow path is reversed. This provides for a compact exhaust system.

As previously mentioned, it is important to provide efficient mixing of the exhaust gas with the reducing agent prior to it entering the SCR catalyst. The diesel engine 22 will have a maximum exhaust mass flow rate and by matching the volume of the injecting/mixing region 24 with this maximum exhaust mass flow rate can provide for efficient mixing. By way of example, a 12 litre diesel engine typically might have a maximum exhaust mass flow rate of 2000kg per hour. Such an engine will require the injecting/mixing region to have a volume of 12 litres, i.e. 6 litres per 1000kg per hour of maximum exhaust mass flow rate. However, it is clear that the system can still be provided which is efficient and has a different volume when compared with the engine maximum exhaust mass flow rate. Thus, the injecting/mixing region 24 might have a mixing volume of between 4 and 20 litres per 1000kg per hour of maximum exhaust mass flow rate of engine 22, preferably it has a mixing volume of between 4 and 8 litres per 1000kg per hour of maximum exhaust mass flow rate of engine 22, more preferably it has a mixing volume of between 5 and 7 litres per 1000kg per hour of maximum exhaust mass flow rate of engine 22.

It is clear from the above that the exhaust system has three major parts, namely an emission control device in the form of a diesel particulate filter in a first portion of the flow path, an injecting/mixing region 24 in a second portion of the flow path and a catalytic emission control device in the form of an SCR catalyst in a third portion of the flow path.

Figure 2 shows a second embodiment of an exhaust system 72. Components the equivalent of those in exhaust system 20 are labelled identically. In this case the bell mouth portion 55 is larger. Note that it is spaced from the outlet end 31 of the DPF 1.

This allows the exhaust gas stream A3 to bypass the tube 46. In this case the contraction area 44 is defined between the end of inner wall 42 and the opposing outer wall 41. In this case there is no outer tube equivalent to outer tube 45. The tube 46 has a lower portion 70 which has perforations 66 to allow the exit of exhaust stream A2. The lower end of tube 46 is blanked off by a plate 68. Mixing of exhaust gas streams A3 and A4 occurs in the expansion area 47.

Figure 3 shows a third embodiment 73 from an exhaust system with features the equivalent I 5 of those in exhaust system 20 being labelled identically. In this case DPF 1 is in the form of two distinct diesel particulate filters IA and lB. Each DPF is similar in constructions to DPF I of figure 1. In this case DPF IA and I B both have circular cross sections. In this case the end of bell mouth portion 55 is close to the outlet end 31A of DPF IA. The bell mouth 55 therefore acts as a collecting chamber 40A for the gas passing through DPF IA.

A collecting chamber 40B is provided for the exhaust gas passing through DPF lB. The exhaust gas stream A3 passes through tube 62. Exhaust gas streams A2 and A3 combine at the secondary mixing chamber 6 which includes perforations 66 to allow the gas to pass into the expansion area 47.

Figure 4 shows a fourth embodiment of an exhaust system 74 in which components that fulfil the same function as those of exhaust system 20 are labelled identically. In this case the bell mouth portion 55 is connected directly to the straight portion 57, there being no curved portion the equivalent of 56 of figure 1. Whilst the injector 3 is still orientated so as to inject the reducing agent into tube 46, in this case injection takes place upstream of the end 46A of tube 46, i.e. upstream of the open end of tube 46. In this case tube 46 includes perforations 63 to allow the ingress of exhaust gas stream A3 into tube 46. The lower end of tube 46 includes perforations 66 to allow the combined exhaust gas stream A4 to enter the expansion area 47, and blanking plate 68.

Figure 5 shows a fifth embodiment of an exhaust system 75 which features performing the same function as those of exhaust system 72 of figure 2 labelled identically. In this case the open end of the bell mouth 55 sits close to the output ends of DPFs 1A and lB.

However, the bell mouth further includes perforations 64 to allow exhaust gas stream A3 to bypass the static mixer 4 and portions of tube 46 that lie downstream of the injector 3.

In this case the SCR catalyst 2 consists of two distinct SCR catalysts 2A and 2B. These are manufactured by a similar process to SCR catalyst 2. In this case SCR catalysts 2A and 2B are circular in cross section.

Figure 6 shows a sixth embodiment of an exhaust system 76 in which components that fulfil the same function as those of exhaust system 20 are labelled identically. In this case the DPF I is identical to that shown in figure 3 and the SCR catalyst 2 is identical to that shown in figure 5. In this case outer tube 45 includes perforations 63 in that part 1 5 positioned within collecting chamber 40 and also perforations 66 in that part positioned within expansion area 47. Outer tube 45 includes blanking plate 69.

Figure 7 shows a seventh embodiment of an exhaust system 77 in which components that fulfil the same function as those of exhaust system 20 are labelled identically. In this case DPF I A and I B is identical to DPF shown in figure 3. SCR catalyst 2A and 2B is identical to the SCR catalyst shown in figure 5. Exhaust system 77 does not have the equivalent of outer tube 45 of exhaust system 20. In this case a portion of the tube 46 positioned within the collecting chamber 40 but downstream of the injector 3 and static mixer 4 includes perforations 63 to allow ingress of exhaust gas stream A3.

Figure 8 shows an eighth embodiment of an exhaust system 78 in which components that fulfil the same function as those of exhaust system 72 are labelled identically. DPF IA and I B are identical to those shown in figure 3. SCR catalysts 2A and 2B are identical to those shown in figure 5. Consideration of figure 2 shows inner wall 42 projecting into the injecting/mixing region 24. This inner wall is not present in the exhaust system 78, nevertheless the arrangement is such as to provide a collecting chamber 40, a contraction area 44 and an expansion area 47, i.e. as the exhaust gases leave the DPF they expand into collecting chamber 40, they then contract as they pass contraction area 44 and then expand as they enter expansion area 47.

Figure 9 shows a ninth embodiment of an exhaust system 79 in which components that fulfil the same function as those of exhaust system 74 are labelled identically. DPF IA and I B are identical to those shown in figure 3. SCR catalysts 2A and 2B are identical to those shown in figure 5.

Figure 0 shows a tenth embodiment of an exhaust system 80 in which components that fulfil the same function as those of exhaust system 75 are labelled identically. As shown in figure 5 the inner wall 42 projects into the injecting/mixing region 24, however, in figure this is not the case. Nevertheless, the exhaust system 80 includes a collecting chamber (compare figures 8 and 10).

Figure 11 shows an eleventh embodiment in which components that fulfil the same function as those of exhaust system 74 are labelled identically. DPF I A and I B are identical to those shown in figure 3. SCR catalysts 2A and 2B are identical to those shown in figure 5. The straight portion 57 of tube 46 is identical with that shown in figure 4. The bell mouth portion 55, curved portion 56 and static mixer 4 are identical to those shown in figure 6.

Figure 12 shows the spatial positioning of the DPFs and SCR catalysts of figure 11. As previously mentioned the DPF IA and lB each have a circular cross section and define axes P and Q respectively. The SCR catalysts 2A and 2B also have a circular cross section (as previously mentioned) and define axes R and S respectively. The axes P, Q, R and S are all parallel to each other. In particular the axes are positioned on the corners of rectangle X. Such an arrangement provides for a compact exhaust system which nevertheless has the capacity to be used on relatively large diesel engines. In particular, such an exhaust system can be positioned near the chassis rails of a tractor unit of articulated vehicle, typically in front of the fuel tank. The space envelope limitations of such an area are, on an underside ground clearance, on a top side vehicle cab clearance, on an outer side vehicle width restrictions, and on an inner side vehicle chassis restrictions.

As such the space envelope tends to be rectangular and by using two distinct DPF and two distinct SCR catalysts and arranging them spatially as shown in figure 12 provides for a compact system that will fit the available space envelope. Note also that, due to production limitations it is difficult to provide a single DPF or a single SCR catalyst that is large enough to be used with relatively large vehicle engine without unduly restricting the back pressure. By using two separate DPFs and/or two separate SCR catalysts the flow capabilities of the DPF and/or SCR catalysts can be matched with larger engine capacities.

In further embodiments it is possible to use three or more distinct emission control devices in the first portion of flow path, for example three or more distinct diesel particulate filters.

In further embodiments it is possible to have three or more catalytic emission control devices in the third portion of the flow path such as three or more distinct selective catalytic reduction catalysts.

For the avoidance of doubt, the term rectangle as used in the present application includes a square.

Figure 13 shows a thirteenth embodiment of an exhaust system 83 in which components that fulfil the same function as those of exhaust system 74 are labelled identically. Tube 46 includes perforations 63 to allow ingress of exhaust gas stream A3 into tube 46. End 64 of tube 46 is blanked and thereby ensuring that exhaust gas stream A4 passes generally radially out of tube 46 via perforations 66.

Figure 14 shows a fourteenth embodiment of an exhaust system 84 in which components that fulfil the same function as those of exhaust system 20 are labelled identically. In this case the DPF 1, injecting/mixing region 24 and SCR 2 are all in line with the exhaust gas flowing generally from left to right when viewing the figure. Such an arrangement can be used where the space envelope for the exhaust system is longer and thinner than the space envelope required for the embodiments shown in figures 1 to 13.

The invention has been described in terms of exhaust gases passing through an emission control device in the form of a diesel particulate filter, an injecting/mixing region where a reducing agent is injected, and then through a selective catalytic reduction catalyst.

However, in further embodiments the injecting/mixing region could be positioned between different types of emission control device and can mix the exhaust gas with different types of liquid.

Thus, in further embodiments the emission control device I shown in the figures can be in the form of a selective catalytic reduction catalyst. The injector 3 can inject hydrocarbons, and the catalytic emission control device 2 could be an active diesel particulate filter. An active diesel particulate filter is a diesel particulate filter which has a catalyst applied thereto (as opposed to a separate oxidation catalyst and diesel particulate filter which will be described below). Under these circumstances the hydrocarbon will be periodically injected in order to burn off soot which has collected within the active DPF.

In further embodiments the emission control device I shown in the figures can be a selective catalytic reduction catalyst, the injector 3 can inject hydrocarbons and the catalytic emission control device 2 can be an oxidation catalyst, such as a diesel oxidation catalyst. Under these circumstances the exhaust gas exiting the catalytic emission control device passes to a diesel particulate filter (not shown). The hydrocarbon will be periodically injected in order to burn off soot which has collected within the active DPF.

In further embodiments, the emission control device I shown in the figures can be a diesel particulate filter (including an active diesel particulate filter), the injector 3 could inject hydrocarbons and the catalytic emission control device 2 could be a lean NO trap. The lean NO trap provides a means of removing NON,. Hydrocarbons are periodically injected into the exhaust stream by injector 3 as a means of chemically reducing the NO absorbed by the lean NO trap.

Claims

Claims I. An exhaust system having a flow path including an emission control device in a first portion of the flow path, a liquid injector positioned to inject a liquid into a second portion of the flow path after the first portion and a catalytic emission control device in a third portion of the flow path after the second portion, the second portion defining a collecting chamber, acting to collect exhaust gas once it has expanded upon exiting the emission control I 0 device, said collecting chamber feeding the exhaust gas into a contraction area of the second portion, the contraction area including a pipe having a part which projects into the collecting chamber.
2. An exhaust system as defined in claim I in which an upstream end of said part of said pipe has a convergent section.
3. An exhaust system as defined in claim 2 in which said convergent section is frustoconical.
4. An exhaust system as defined in any preceding claim in which the liquid injector is positioned so as to inject a liquid into the collecting chamber.
5. An exhaust system as defined in claim 4 in which the liquid injector is positioned so as to inject a liquid into said part of said pipe.
6. An exhaust system as defined in any preceding claim in which the liquid agent injector is positioned spaced from the end of the pipe so as to inject a liquid into said end of said part of said pipe.
7. An exhaust system as defined in any one of claims 1 to 5 in which the liquid injector is positioned so as to inject a liquid downstream of said end of said part of said pipe.
8. An exhaust system as defined in any preceding claim in which the liquid injector is positioned so as to inject a liquid onto a static mixer.
9. An exhaust system as defined in any preceding claim in which the second portion of the flow path has a first stream which passes through an end of said part of said pipe and a second stream which bypasses said end of said part of said pipe.
10. An exhaust system as defined in claim 9 in which said pipe includes perforations through which said second stream can pass.
11. An exhaust system as defined in claim 10 in which said perforations are in said part which projects into said collecting chamber.
12. An exhaust system as defined in any preceding claim in which the second portion 1 5 includes an expansion area downstream of said contraction area.
13. An exhaust system as defined in claim 12 in which the pipe has a further part which projects into the expansion area.
14. An exhaust system as defined in claim 13 in which the further part of the pipe includes perforations through which at least a part of the exhaust gas stream passes.
15. An exhaust system as defined in claim 14 in which all of the exhaust gas stream passes through the perforations of the further part of the pipe.
16. An exhaust system as defined in any preceding claim in which the first portion is positioned adjacent to the third portion so that the flow path is reversed.
17. An exhaust system as defined in any preceding claim in which the emission control device is defined by a first and second distinct emission control devices positioned parallel to each other, preferably the first and/or second emission control device has a circular cross section.
I 8. An exhaust system as defined in claim 1 7 when dependent upon claim 9 in which a majority of the first stream is provided by exhaust gas exiting the first emission control device and/or a majority of the second stream is provided by gas exiting the second emission control device.
19, An exhaust system as defined in any preceding claim in which the catalytic emission control device is defined by first and second distinct catalytic emission control devices provided parallel to each other, preferably the first and/or second catalytic emission control device has a circular cross section.
20. An exhaust system as defined in claim 19 when dependent upon claim 17 in which the first and second emission control device and the first and second catalytic emission control device each define an axis and the first and second emission control device and the first and second catalytic emission control device are positioned such that an axis is provided substantially on each corner of a rectangle.
21. An exhaust system as defined in any one of claims I to 15 in which thefirst, second and third portions are positioned linearly relative to each other.
22. An exhaust system as defined in any preceding claim in which the liquid injector is an airless injector or an air assisted injector.
23. An exhaust system as defined in any preceding claim in which said emission control device in the first portion of the flow path is a diesel particulate filter or a selective catalytic reduction catalyst.
24. An exhaust system as defined in any preceding claim in which the injector injects a reducing agent such as aqueous urea solution or any other pre cursor for ammonia.
25. An exhaust system as defined in any one of claims 1 to 23 in which the injector injects hydrocarbons such as diesel.
26. An exhaust system as defined in any preceding claim in which the catalytic emission control device in the third portion of the flow path is an active diesel particulate filter, or an oxidation catalyst, such as a diesel oxidation catalyst, or a lean NO trap.
27. A diesel engine having a maximum exhaust mass flow rate and being connected to an exhaust system as defined in any preceding claim, the second portion of the flow path having a mixing volume of at least 4 litres per thousand kilograms per hour of maximum exhaust mass flow rate, preferably a mixing volume of between 4 and 8 litres per thousand kilograms per hour of maximum exhaust mass flow rate.