SE1651253A1

SE1651253A1 - Axially distributed mixing device

Info

Publication number: SE1651253A1
Application number: SE1651253A
Authority: SE
Inventors: Petersson Kim; Yitbarek Zemichael
Original assignee: Scania Cv Ab
Priority date: 2016-09-22
Filing date: 2016-09-22
Publication date: 2018-03-23
Also published as: SE542488C2; DE102017008643A1

Abstract

The present invention relates to a mixing device (10) for an exhaust system of an internal combustion engine (2). The mixing device comprises a plurality of primary blades (12, 612, 712) arranged around an mixer axis (C). Each primary blade (12, 612, 712) extends from a first end (14) in proximity to the mixer axis (C) to a second end (16) at a circumference of the mixing device. The primary blades are arranged such that each primary blade (12, 612, 712) is offset by an offset angle (θ) in a direction around the mixer axis (C) in relation to an immediately preceding and/or immediately following primary blade (12, 612, 712), and such that each primary blade (12, 612, 712) is translationally offset by an offset distance (d) in a direction along the mixer axis (C) in relation to an immediately preceding and/or immediately following primary blade (12, 612, 712), wherein the offset angle (θ) and offset distance (d) are both greater than zero.The invention further relates to an exhaust system comprising such a mixing device (10), and a vehicle (1) comprising an exhaust system comprising such a mixing device (10).

Description

Axially distributed mixing device TECHNICAL FIELD The present invention relates to a mixing device for an exhaust system of an internalcombustion engine. The invention further relates to an exhaust system comprising such a mixing device, and a vehicle comprising an exhaust system comprising such a mixing device.

BACKGROUND ART Emissions standards for motor vehicles are becoming increasingly stringent. Such standardstypically specify maximum emission levels for a number of tailpipe pollutants including carbonmonoxide (CO), hydrocarbons (HC), nitrogen oxides (NOX) and particulate matter (PM). lnorder to meet the requirements of the present and presumably future standards, vehicles arerequired to be equipped with emissions reduction technologies. Such emissions reductiontechnologies suitable for diesel vehicles include exhaust gas recirculation (EGR), particulatefilters, diesel oxidation catalysts (DOC), and selective catalytic reduction (SCR). Eachtechnology has its own distinct advantages and disadvantages, and may increase the incidenceof one pollutant while reducing the incidence of another. For example, EGR may reduce NOXemissions, but reduce fuel efficiency and increase particulate matter. Therefore, a number of technologies are commonly applied together in order to meet emissions standards.

Selective catalytic reduction (SCR) is an effective technology to reduce tailpipe nitrogen oxides(NOX) emissions. lt involves adding a reductant, such as ammonia, to the vehicle exhauststream. The reductant, with the aid of a catalyst, reduces NOX in the exhaust stream tonitrogen gas (NZ) and water. ln practical implementations in motor vehicles, an aqueous ureasolution is used as a reductant and this urea solution is thermally decomposed to ammonia and carbon dioxide in the hot exhaust stream.

Since SCR is implemented downstream ofthe engine as an exhaust aftertreatment, it does notaffect the combustion performance ofthe engine in the same manner as for example EGR does. Therefore, it is desirable to be able to remove substantially all NOX from the exhaust 2 stream using only SCR, without the need for EGR. However, this is not without difficulties. lnorder to produce the quantities of ammonia required to reduce substantially all NOX, largequantities of urea solution must be injected into the exhaust stream. This urea must beeffectively mixed with the exhaust stream in order to be evenly distributed over a number ofSCR catalyst substrates arranged in the exhaust conduit. However, the space available formixing is extremely limited and the reductant is commonly injected into the exhaust streamshortly upstream ofthe SCR catalyst substrates. ln order to improve mixing a mixing device,often resembling a turbine blade, is arranged in the exhaust pipe. However, even when using a mixing device, it is difficult to achieve sufficiently uniform mixing.

Non-uniform mixing can lead to deposition of urea and urea by-products on surfacesdownstream ofthe injection site. I\/|oreover, if the reductant is not uniformly distributedamong the SCR catalysts, the catalysts will need to be dimensioned to suit the catalystreceiving the maximum flux of reductant, meaning that some catalysts will be over-dimensioned, which is environmentally and economically wasteful. Furthermore, excess ureawill have to be injected to fully supply all catalysts with reductant. This excess urea will requirethe use of a larger ammonia slip catalyst in order to prevent tailpipe ammonia emissions.Therefore, there is a desire for mixing devices that provide a more uniform mixing of reductant with the exhaust stream.

A further application of mixing devices is mixing an injected fuel jet with an exhaust stream upstream of a diesel oxidation catalyst.

A mixing and/or evaporating device for an exhaust system is described in US 2008/0308083.The device comprises a tubular body with one axial end having a plurality of radially inwardlyprojecting blades, the said blades being arranged adjacent to one another and having an angleof incidence in relation to an axial direction. The device may be produced at low cost by manufacturing from a single sheet metal body by shaping.There remains a need for improved mixing devices for use in exhaust systems.SUMMARY OF THE INVENTION The inventors of the present invention have identified a number of shortcomings with regard to the prior art. Prior art mixing devices provide insufficient mixing, tending instead to merely 3rotate the flow entering the mixing device instead of mixing it. Prior art devices also tend toobstruct the flow of the exhaust stream, causing a significant pressure drop over the mixingdevice, higher pressures upstream of the mixing device, and ultimately decreased engine efficiency and increased fuel consumption. lt is an object of the present invention to provide a mixing device that provides improvedmixing of an exhaust stream with a substance injected into the exhaust stream, and/or causes a lower pressure drop over the mixing device when arranged in an exhaust stream.

These objects are achieved by a mixing device for an exhaust system of an internal combustion engine in accordance with the appended claims.

The mixing device comprises a plurality of primary blades arranged around a mixer axis, eachprimary blade extending from a first end in proximity to the mixer axis to a second end at acircumference ofthe mixing device. The primary blades are arranged such that each primaryblade is offset by an offset angle in a direction around the mixer axis in relation to animmediately preceding and/or immediately following primary blade. Each primary blade istranslationally offset by an offset distance in a direction along the mixer axis in relation to animmediately preceding and/or immediately following primary blade. The offset angle andoffset distance are both greater than zero. lt is important to note that the mixer axis is atheoretical construct, and the mixing device may or may not comprise a physical component, such as a central shaft, which corresponds to the mixer axis.

Such a mixing device has blades spatially arranged in a manner similar to the treads of a spiralstaircase. Corresponding points on the outer ends ofthe blades, such as for example the outer mid-point, trace a helical line.

Such an arrangement provides improved mixing of a reactant added to an exhaust stream ascompared to prior art mixers. Without wishing to be bound by theory, the reason for this ispossibly that immediately after injection, the concentration of reactant is highly localised inthe exhaust stream. ln prior art mixers, this highly concentrated area is carried by the exhauststeam to the mixing device. However, prior art mixers tend to merely rotate the incomingstream without sufficient mixing, meaning that the stream exiting the mixer may be rotated, but still has a highly concentrated patch of reactant. ln contrast, the axially distributed mixer 4as defined above breaks the impinging exhaust stream into a series of streams where rotationof each stream is initiated at different points along the length ofthe mixer. This leads to bettermixing of the streams and a more even distribution of the reactant in the exhaust stream upon leaving the mixer.

I\/|oreover, the axially distributed mixer causes a lesser pressure drop across the mixer ascompared to prior art mixers. This may be because in prior art mixers, the non-axiallydistributed blades block a significant proportion ofthe cross-sectional area of the exhaust pipewhere the blades are situated. ln contrast, in the axially distributed mixer, the cross-sectionalarea ofthe exhaust pipe that is obstructed at any single cross-section is limited, thus allowing the exhaust stream to flow easier thorough the mixer.

The mixer axis may be a straight line forming a central longitudinal axis ofthe mixing device,or it may be a curved line extending primarily in a longitudinal direction through the mixingdevice. For example, the mixer axis may oscillate with a sinusoidal component in one or two dimensions, and may therefore resemble a sinusoidal curve or helix.

The mixing device may further comprise a central shaft, wherein the longitudinal axis ofthecentral shaft is coincident with the mixer axis, and wherein the first end of each of theplurality of primary blades is fixed to the central shaft. Such a shaft assists in maintaining thespatial arrangement of the blades and supports the inner ends of the blades, minimising vibration and lessening noise.

The mixing device may further comprise a tubular support extending longitudinally in thedirection along the central axis, wherein the tubular support encompasses the plurality ofprimary blades, and wherein the second end of each ofthe plurality of primary blades is fixedto the tubular support. Such a tubular support assists in maintaining the spatial arrangementof the blades and supports the outer ends of the blades, minimising vibration and lessening noise.

Each primary blade forms a pitch angle relative to the mixer axis. The pitch angle (a) may bethe same for all primary blades. By appropriate choice of pitch angle, improved mixing at thedesired exhaust flow rates may be obtained, and pressure drop across the mixing device reduced. 5The mixing device may have at least three primary blades, such as at least six primary blades.The number of blades may be chosen as appropriate in order to obtain a desired balance between mixing and pressure drop.

Each primary blade may be straight, or each primary blade may be curved. The shape ofthe blades may be optimised in order to improve mixing of the exhaust stream.

The mixing device may further comprise a plurality of secondary blades. These secondaryblades may further divide and combine the exhaust streams passing through the mixing device, leading to improved mixing.

Each secondary blade may be rotationally offset by an offset angle in a direction around themixer axis in relation to an immediately preceding and/or immediately following secondaryblade, wherein the offset angle is greater than or equal to zero. The offset angle ofthesecondary blades may be the same or different to the offset angle of the primary blades. Theoffset angle of the secondary blades may be the same as the offset angle of the primary blades, or it may be opposite to the offset angle of the primary blades.

Each secondary blade may be translationally offset by an offset distance in a direction alongthe mixer axis in relation to an immediately preceding and/or immediately followingsecondary blade, wherein the offset distance is greater than or equal to zero. Thus, if thesecondary blades are offset both rotationally and translationally, they may constitute a second helical arrangement, forming a double helical arrangement with the primary blades.

The offset distance of the secondary blades may be the same as the offset distance ofthe primary blades.

Each secondary blade forms a pitch angle relative to the mixer axis. This pitch angle may bethe same for all secondary blades. The pitch angle ofthe secondary blades may be the same asthe pitch angle of the primary blades, or the pitch angle ofthe secondary blades may be opposite to the pitch angle of the primary blades.

According to a further aspect of the present invention, the above objects are achieved by anexhaust system for an internal combustion engine, comprising a mixing device as disclosed above. 6According to yet another aspect of the present invention, the above objects are achieved by avehicle comprising an exhaust system, the exhaust system comprising a mixing device as disclosed above.

Further objects, advantages and novel features of the present invention will become apparent to one ski||ed 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 accompanyingdrawings, in which the same reference notations denote similar items in the various diagrams, and in which: Fig 1 schematically illustrates a vehicle comprising a mixing device according to thepresent invention.

Fig 2a schematically illustrates a perspective view of a mixing device according to anembodiment of the invention.

Fig 2b schematically illustrates a frontal view of a mixing device according to anembodiment of the invention.

Fig 2c schematically illustrates a side view of a mixing device according to anembodiment of the invention.

Fig 3a schematically illustrates a side view of swirl patterns arising from the flow of afluid around a prior art mixing device.

Fig 3b schematically illustrates a top view of swirl patterns arising from the flow of afluid around a prior art mixing device.

Fig 4a schematically illustrates a side view of swirl patterns arising from the flow of a fluid around a mixing device according to an embodiment ofthe present invention.

Fig 4b Fig 5 Fig 6a Fig eb Fig 6c Fig 7a Fig 7b Fig 7c Fig 8a Fig sb Fig 9 7schematically illustrates a top view of swirl patterns arising from the flow of afluid around a mixing device according to an embodiment ofthe present invention. shows a graph of ammonia mole flux against time for both a prior art mixingdevice and a mixing device according to an embodiment ofthe present invention. schematically illustrates a perspective view of a mixing device according to another embodiment of the invention. schematically illustrates a frontal view of a mixing device according to another embodiment of the invention. schematically illustrates a side view of a mixing device according to another embodiment of the invention. schematically illustrates a perspective view of a mixing device according to a further embodiment of the invention. schematically illustrates a frontal view of a mixing device according to a further embodiment of the invention. schematically illustrates a side view of a mixing device according to a further embodiment of the invention. schematically illustrates a side view of a mixing device according to yet a further embodiment of the invention. schematically illustrates a side view of a mixing device according to yet a further embodiment of the invention. schematically illustrates a side view of a mixing device according to yet another embodiment of the invention.

DETAILED DESCRIPTION The present invention is based on the discovery that whenever the blades of a mixing deviceare distributed in an axial direction, instead of being distributed only around thecircumference of an axis as in prior art mixers, then the properties of the mixing device areimproved. More specifically, improved mixing is obtained between the added reactant and theexhaust stream, and the mixing device causes a lower pressure drop as compared to prior art devices.

The mixing device of the present invention comprises a number of blades arranged to bespatially distributed in the exhaust pipe of an engine exhaust system whenever the exhaustsystem is equipped with the mixing device. The spatial distribution of the blades break the laminar flow of the exhaust stream and provide good mixing.

Prior art mixing devices having blades typically resemble a spoked wheel with the bladesextending radially from a central axis of the device outwardly towards the outer circumferenceof the device. ln prior art devices, the blades are therefore distributed only circumferentially as per the spokes of a wheel. ln the presently disclosed mixing device, the blades are distributed not only circumferentially,but also axially along the axis of the device. Thus, the blades of the present device resemble aspiral stair arrangement, with each blade being progressively offset both rotationally aroundthe mixer axis and also translationally along the mixer axis from the preceding blade. lfmapping out corresponding points of the blades three-dimensionally, such as the cornerformed between the leading edge and the outer edge of each blade, these points trace ahelical line. lf the mixer axis is a longitudinal central axis of the mixing device, then this central axis may also be the helical axis.

The mixer axis may be a straight line extending in a longitudinal direction through the centreof the mixing device, in a manner similar to prior art mixing devices. However, the mixer axismay also take another form, such as a curved line or helix. lt should be emphasised that themixer axis referred to here is a theoretical construct, and the mixing device may have a voidrunning centrally through the device coincident with the mixer axis. ln such a case, the blades of the mixing device may be held in position by fixing to a tubular support that encompasses 9the blades. For example, the blades may be fixed by their outer (circumferential) edge to thetubular support. The tubular support may comprise an intact length of pipe, such as a lengthof exhaust pipe. However, it may also comprise a mesh, strips, wire, or similar support structure for positioning within a length of exhaust pipe.

The mixing device may have a central shaft essentially coincident with the mixer axis of thedevice. The edges of the blades in closest proximity to the shaft may be fixed to the shaft inorder to hold the blades in position. The shaft in turn may be fixed to an inner surface of an exhaust pipe when configuring the mixing device in an exhaust system.

The mixing device may comprise both a central shaft and a tubular support as describedabove. The mixing device may lack both a central shaft and tubular support, the bladespossibly being fixed to each other instead. The blades, support, shaft and/or any furthercomponents of the mixing device may be manufactured separately and assembled.

Alternatively, the entire mixing device may be cut, cast or 3D-printed as a single component.

The blades of the mixing device may resemble the blades of any conventional mixing deviceknown in the art. The blades may for example be entirely planar, or they may be provided withbends or curves. They may for example be formed from a square, rectangular trapezoid orotherwise shaped piece of material. All of the blades may have the same shape, or the shapeand/or dimensions of the blades may vary from blade to blade. The number of blades in themixing device may be chosen as appropriate, but at least three blades arranged in a helicalconformation is preferred. Due to the lower pressure drop obtained by having translationaloffset, a greater number of mixing blades, such as 10 blades, or even 20 blades, may be used without necessarily negatively impacting the pressure drop as compared to prior art devices.

Each blade is offset relative to adjacent blades around the mixer axis of the mixing device, sothat when seen along the direction of the axis, the blades may resemble the spokes of awheel. The blades are preferably evenly spaced around the circumference so as to provide atleast one complete helical rotation of blades. For example, for a mixing device having eightblades, a rotational offset of 45° between blades may provide a single complete 360° rotation.However, the blades may be arranged to provide more than a single 360° rotation. Forexample, for a mixing device again having eight blades, if a 90° rotational offset is used then two complete rotations of the helix are obtained. Exactly how the offset angle is measured is unimportant, as long as corresponding points are used on all blades. For example, the offsetangle may be determined as the angle formed between the leading edge of a blade and theleading edge of the subsequent blade when viewed along the principal direction of the mixeraxis between the two blades in question. The offset angle is always greater than zero, and mayfor example be greater than 10°, such as greater than 20°. The offset angle is always less than 360°, and may for example be less than 350°, such as less than 340°.

Depending on the sign or magnitude of the rotational offset, a right-handed helix or lefthanded helical arrangement may be obtained. For example, if an offset of +45° is usedbetween blades, a right-handed helix is obtained. However, using an offset of 315° (or -45° if expressed in another manner), then a left-handed helix is obtained.

The blades are also offset relative to adjacent blades translationally along the mixer axis of themixing device. The combination of offsetting the blades both around the mixer axis and alongthe mixer axis means that the blades are spatially arranged in a manner similar to the treads ofa spiral staircase. The size of the translational offset between each blade may be chosen asappropriate. The size of the translational offset may affect both the mixing obtained and thepressure drop at a specific exhaust flow rate. The offset distance may be determined forexample by measuring the vector along the mixer axis between where the leading edge of afirst blade intersects the axis and where the leading edge of the subsequent blade intersectsthe axis. The offset distance is always greater than zero, and may for example be greater than5% of the total axial length of the mixing device, such as greater than 10% of the total axiallength of the mixing device. The offset length is always less than 90% of the total axial lengthof the mixing device, for example less than 80% of the total length of the mixing device, such as less than 70% of the total length of the mixing device.

A suitable pitch angle may be chosen for the blades. Pitch angle may be varied by rotatingeach blade around its longitudinal axis, which may be an axis extending from the mixer axis ofthe mixing device radially outwards towards the circumference of the mixing device. Since theincident direction of the exhaust stream on the mixing device may be affected by devicesupstream of the mixing device in the exhaust system, such as a turbine or pre-mixer, the pitchangle is easiest to determine relative to the fixed mixer axis of the mixing device. lf the blade and mixer axis are straight, the pitch angle may be determined as the angle formed by the 11intersection of the plane of the blade with the mixer axis of the mixing device. lf the blade ormixer axis are curved, the pitch may be defined in some other suitable manner, for example by defining a chord line for the blade.

The blades may all have the same pitch angle, as is typically the case for prior art mixingdevices. However, due to the use of a translational offset there is an increased distancebetween blades, thus potentially allowing the blades to have differing pitches. For example,the pitch of the blades may progressively increase or decrease from the leading blade to thetrailing blade in the direction of exhaust flow, wherein the leading blade is the blade that the exhaust flow first impinges upon, and the trailing blade is the blade that the exhaust flow last impinges upon.

All blades of the mixing device disclosed herein may be arranged in a helical fashion aspreviously described. However, the mixing device may alternatively have a primary set ofblades arranged in a helical fashion, and further sets of blades, such as secondary or tertiaryblades, may be arranged in another fashion. For example, the mixing device may have twosets of helically-arranged blades, a primary set and a secondary set. The sets may have thesame handedness, i.e. the mixing device may resemble a double-helix arrangement.Alternatively, the sets may differ in handedness, with one set being left-handed and the other being right-handed.

The secondary and further sets of blades may not be arranged in a helical fashion at all. Forexample, the secondary blades may be arranged with only a rotational offset and notranslational offset, i.e. resemble a conventional prior art blade arrangement. Such anarrangement may be positioned at the leading side of the mixing device (i.e. upstream end), atthe trailing side of the mixing device (i.e. the downstream end), or somewhere in the middleof the helical arrangement of primary blades. The secondary blades may be arranged with onlya translational offset and no rotational offset. The secondary and tertiary blades do notnecessarily have to be the same shape or size as the primary blades. The mixing device mayhave no secondary or tertiary blades at all, or may comprise from one to ten secondary andtertiary blades. The secondary and/or tertiary blades do not need to be offset relative to theprimary blades: i.e. a secondary blade may occupy the same translational position along the central axis as a primary blade. 12The mixing device may be produced from any material known in the art for mixing devices.The material may preferably to|erate the elevated operating temperatures of the exhaustconduit and be substantially inert to exhaust gases at the relevant temperatures. The mixingdevice is typically produced from steel, but it may be produced using other materials such asmetals (e.g. aluminium), ceramics, and/or thermoplastic or thermosetting resins or composites that can to|erate the elevated temperatures of the exhaust gases (e.g. epoxy).The present invention will now be further illustrated with reference to the appended figures.

Figure 1 shows schematically a side view of a vehicle 1 according to an embodiment of theinvention. The vehicle 1 includes a combustion engine 2 and an exhaust conduit 4 leading fromthe engine 2 to atmosphere via a turbocharger 6. Arranged in the exhaust conduit 4 is areductant dosing device 6 and an SCR catalyst 8. A mixing device 10 according to the presentdisclosure is arranged in the exhaust conduit 4 between the reductant dosing device 6 and SCRcatalyst 8. The vehicle illustrated is a heavy goods vehicle, but the invention is applicable toother vehicles vehicle, including but not limited to, busses, commercial vehicles and passenger Ca FS.

Figures 2a-2c schematically illustrate a mixing device 10 according to an embodiment of thepresent invention. Figure 2a depicts the device in a perspective view; Figure 2b depicts thedevice as seen from the front, along the central axis; and Figure 2c depicts the device as seenfrom the side. The mixing device 10 comprises eight blades 12 arranged around a central axisC such that the first ends 14 of the blades are in proximity to the central axis C and the secondends 16 are located at the circumference of the mixing device 10. The blades 12 aredistributed around the central axis C with an offset angle 0 between each blade ofapproximately 45°. lt can be seen that this constitutes a single full rotation. The blades 12 arealso distributed along the central axis C with an offset distance d between each blade. The firstends 14 of the blades 12 are affixed to a central shaft 18. lt can be seen in Figure 2c that thatblade labelled 12b is depicted more or less side-on, allowing the blade chord line 15 to bedrawn between the leading edge of the blade and the trailing edge of the blade. The bladepitch angle oL may be determined as the angle formed between the blade chord line 15 and the central axis C. All ofthe blades 12 have the same pitch angle oL. 13Figures 3a, 3b, 4a and 4b schematically illustrate the results of computational fluid dynamicscalculations that compare exhaust flow around the mixing device depicted in Figure 2 with aprior art mixing device having no offset along the direction of the central axis. lt can be seenfrom Figures 3a (side view) and 3b (from above) that the prior art mixer causes fluids enteringthe mixer to swirl with a uniform intensity and direction, meaning that poor mixing isobtained. ln contrast, the mixer shown in Figure 2 creates several swirl flows having differentintensities, as can be seen from Figures 4a (side view) and 4b (from above). These several swirl flows are non-parallel and collide with each other, providing improved mixing.

The results ofthe CFD calculations were confirmed experimentally in an exhaust system wherea mixing device was arranged upstream of three parallel SCR catalysts. A typical operationaldosage of urea solution was added to a typical operational exhaust flow (approx. 1700 kg/h).Figure 5 shows a graph of NH3 mole flux against time for the axially distributed mixercompared to the conventional prior art non-axially distributed mixer. Lines 501, 502 and 503show the mole flux at the respective SCR catalyst substrates when using the prior art catalyst.lt can be seen from arrow 504 that there is a significant variation of the mole flux between thethree substrates. This difference is calculated to be approximately 23 %. Looking at the systemwhere an axially distributed mixer in accordance the present disclosure has been used (lines505, 506 and 507), the variation in mole flux is significantly lessened (arrow 508), and iscalculated to be approximately 6 %. Thus, the use of an axially distributed mixer according tothe present disclosure provides a much more even distribution of reductant to all of theparallel SCR catalysts. This means that less urea needs to be dosed to the exhaust system, thecatalysts do not need to be over-dimensioned to the same extent, and the need for an ammonia slip catalyst is lessened.

Looking at the pressure drop caused by the mixers under the experimental conditionsdescribed above, it was observed that the prior art mixer caused a pressure drop of 150 mbar.The axially distributed mixer according to the present disclosure caused a lesser pressure dropof 134 mbar, an 11 % reduction. Without wishing to be bound by theory, it is thought that thegreater pressure drop caused by prior art mixing devices is due to the fact that at the sectionof the exhaust pipe where the mixing device is positioned, in prior art devices the exhaust pipeis significantly obstructed due to the compact arrangement of the blades. ln contrast, the mixing devices of the present disclosure have a greater special distribution and therefore do 14not block the exhaust pipe as much at any given cross-section of the exhaust pipe. The lowerpressure drop caused by the axially distributed mixers leads to an improved fuel economy for the engine.

Figures 6a-6c schematically illustrates a mixing device 10 according to another embodiment ofthe invention. Figure 6a depicts the device in a perspective view; Figure 6b depicts the deviceas seen from the front, along the central axis; and Figure 6c depicts the device as seen fromthe side. ln this embodiment, a series of eight primary blades 612 have been supplementedwith a series of eight secondary blades 613. The primary blades 612 are arranged in a helicalconformation with an angular offset 0 of approximately 45° between blades. The secondaryblades 613 are also arranged in a helical conformation with an angular offset 02 of 45°between blades. The translational offset d along the central axis C between each blade of theset of primary blades 612 is the same as the translational offset d2 for the secondary blades613. The leading blade 613a of the secondary set of blades 613 is more-or-less diametricallyopposed to the leading blade 612a of the primary set of blades 612, and is somewhat offsetalong the central axis C with respect to 612a. The two sets of blades 612 and 613 form an essentially parallel double-helical arrangement.

Figures 7a-7c schematically illustrates a mixing device 10 according to a further embodimentof the invention. Figure 7a depicts the device in a perspective view; Figure 7b depicts thedevice as seen from the front, along the central axis; and Figure 7c depicts the device as seenfrom the side. ln this embodiment, a primary set of four blades 712 has been supplementedwith a secondary set of four blades 713. The primary blades 712 are arranged in a helicalconformation with an angular offset 0 of approximately 90° between blades. The secondaryblades 713 are also arranged in a helical conformation with an angular offset 02 ofapproximately 90° between blades. The translational offset d of the primary blades 712 is thesame as the translational offset d2 of the secondary blades 713. The leading secondary blade713a is separated along the central axis C by approximately a distance equal to the offsetdistance d. That is to say that the leading secondary blade 713a placed at approximately thesame distance along the central axis C as the second primary blade 712b. The resultingarrangement resembles two parallel helices with a common helical axis, wherein one of the helices is somewhat offset along the direction of the helical axis compared to the other helix.

Figures 8a and 8b schematically illustrate mixing devices according to yet furtherembodiments ofthe invention. Here, the mixing devices are seen from the side. The mixer axisC is curved in both Figures. ln Figure 8a the period of the curve of the mixer axis C isapproximately equal to the length of the mixing device 10. ln Figure 8b the period of the curveof the mixer axis C is approximately equal to double the length of the mixing device. Theblades 12 vary in size depending on the proximity of the mixer axis C to the exhaust conduit wall 4 at the relevant offset distance along the mixer axis C.

Figure 9 schematically illustrates a mixing device according to yet another embodiment of theinvention. The blades 12 are not fixed to a central shaft, but are instead fixed to a tubular support 20 that circumferentially encompasses the blades.

Claims

16CLAll\/IS

1. A mixing device (10) for an exhaust system of an internal combustion engine (2), the mixing device comprising a plurality of primary blades (12, 612, 712) arranged around amixer axis (C), each primary blade (12, 612, 712) extending from a first end (14) inproximity to the mixer axis (C) to a second end (16) at a circumference of the mixingdevice, wherein the primary blades are arranged such that each primary blade (12,612, 712) is offset by an offset angle (6) in a direction around the mixer axis (C) inrelation to an immediately preceding and/or immediately following primary blade (12,612, 712), characterised in that each primary blade (12, 612, 712) is translationallyoffset by an offset distance (d) in a direction along the mixer axis (C) in relation to animmediately preceding and/or immediately following primary blade (12, 612, 712),wherein the offset angle (6) and offset distance (d) are both greater than zero. A mixing device according to claim 1, wherein the mixer axis (C) is a straight lineforming a central longitudinal axis of the mixing device, or wherein the mixer axis (C) isa curved line extending primarily in a longitudinal direction through the mixing device.A mixing device according to any one of claims 1-2, wherein the mixing device (10)further comprises a central shaft (18), wherein the longitudinal axis of the central shaft(18) is coincident with the mixer axis (C), and wherein the first end (14) of each of theplurality of primary blades (12, 612, 712) is fixed to the central shaft (18). A mixing device according to any one of the preceding claims, wherein the mixingdevice further comprises a tubular support extending longitudinally in the directionalong the mixer axis (C), wherein the tubular support encompasses the plurality ofprimary blades (12, 612, 712), and wherein the second end (16) of each of the pluralityof primary blades (12, 612, 712) is fixed to the tubular support. A mixing device according to any one of the preceding claims, wherein each primaryblade (12, 612, 712) forms a pitch angle (a) relative to the mixer axis (C), and whereinthe pitch angle (oL) is the same for all primary blades (12, 612, 712). A mixing device according to any one of the preceding claims, wherein the mixingdevice has at least three primary blades (12, 612, 712). A mixing device according to any one of the preceding claims, wherein each primary blade (12, 612, 712) is straight, or wherein each primary blade is curved. 8. 10. 11. 1

2. 1

3. 1

4. 1

5. 1

6. 17 A mixing device according to any one of the preceding claims, further comprising aplurality of secondary blades (613, 713). A mixing device according to claim 8, wherein each secondary blade (613, 713) isrotationally offset by an offset angle (G2) in a direction around the mixer axis (C) inrelation to an immediately preceding and/or immediately following secondary blade(613, 713), wherein the offset angle (G2) is greater than or equal to zero. A mixing device according to claim 9, wherein the offset angle (G2) of the secondaryblades (613, 713) is the same or opposite to the offset angle (G) of the primary blades(12, 612, 712). A mixing device according to any one of claims 8-10, wherein each secondary blade(613, 713) is translationally offset by an offset distance (d2) in a direction along themixer axis (C) in relation to an immediately preceding and/or immediately followingsecondary blade (613, 713), wherein the offset distance (d2) is greater than or equal tozero. A mixing device according to claim 11, wherein the offset distance (d2) of thesecondary blades (613, 713) is the same as the offset distance (d1) of the primaryblades (12, 612, 712). A mixing device according to any one of the preceding claims, wherein each secondaryblade (613, 713) forms a pitch angle (d2) relative to the mixer axis (C), and wherein thepitch angle (d2) is the same for all secondary blades (613, 713). A mixing device according to claim 13, wherein the pitch angle (d2) of the secondaryblades (613, 713) is the same as the pitch angle (a) of the primary blades (12, 612,712), or wherein the pitch angle (d2) of the secondary blades (613, 713) is opposite tothe pitch angle (oL) of the primary blades (12, 612, 712). An exhaust system for an internal combustion engine (2), comprising a mixing device(10) according to any one of claims 1-14. A vehicle (1) comprising an exhaust system, the exhaust system comprising a mixing device (10) according to any one of claims 1-14.