Abstract:
A fluid dosing device for introducing a fluid into a gas flow having a given flow direction, the dosing device comprising: input means for receiving the fluid from a source of fluid; nozzle means for injecting the fluid into the gas flow, the nozzle means comprising valve means arranged to move between a first position in which the valve means seals the nozzle means and a second open position in which injection of fluid into the gas flow can take place wherein the fluid dosing device is arranged to inject fluid into the gas flow in a direction having at least a component which is counter to the flow direction of the gas flow.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a device for dosing a gas flow with a fluid. In particular, the invention relates to a device for achieving optimum dispersion and mixing of a reagent in a flow of exhaust gases in order to reduce harmful exhaust gas emissions. 
       BACKGROUND ART 
       [0002]    The emission of nitrogen oxide (NO x ) compounds in engine exhausts has long been the focus for health professionals and regulatory agencies worldwide. In many locations, regulations require stringent reductions of NO x  levels in new equipments. NO x  emissions may be found in a variety of systems such as internal combustion engines, gas turbine exhaust, lean burn engines, industrial boilers, process heaters or other process streams. 
         [0003]    In order to reduce NO x  emissions, it is known to use a Selective Catalyic Reduction (SCR) device to treat an exhaust flow and to significantly reduce nitrous oxide (NO x ) emissions. In an SCR system a reducing agent, for example urea solution, is dosed into the exhaust gas flow upstream of an SCR catalyst. This reducing agent is then usually reacted in the presence of a catalyst downstream of the injection point in an SCR device. Within the SCR device NO x  compounds are then reduced to nitrogen. WO2004111401 discloses such a device. 
         [0004]    The general operation of an SCR device is shown in  FIGS. 1 and 2 . 
         [0005]    In  FIG. 1 , a diesel engine  1  produces an exhaust flow comprising various exhaust gases  3 . The exhaust gases are conveyed through an exhaust system, indicated generally at  5 , comprising an oxidation catalyst device  7 , a selective reduction catalyst device  9  and a slip catalyst  11 . 
         [0006]    The oxidation catalyst device  7  is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. This layer contains a small, well dispersed amount of precious metals such as platinum or palladium. As the exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles (unburned fuel and oil) are oxidized, thereby reducing harmful emissions. 
         [0007]    The SCR device  9  performs Selective Catalytic Reduction (SCR) of nitrogen oxide (NOx) using ammonia (derived from a source of urea) as a chemical reductant. SCR is a proven technical and economic solution for the heavy-duty market to meet Euro IV and Euro V emission requirements for diesel-powered commercial vehicles in Europe. 
         [0008]    The slip catalyst  11  is located downstream of the SCR device  9  to clean up any unreacted ammonia. 
         [0009]    Urea for the SCR device  9  is stored in a tank  13  which is in fluid communication with the exhaust system  5 . A pump  15  is provided to pump urea from the tank  13  to the exhaust system  5 . The supply of urea is controlled by a control unit  17 , for example the engine control unit, which receives engine speed and other engine parameters from the engine  1 . An injection device  19  (also referred to herein as a fluid dosing device) is used to inject the urea into the exhaust flow. 
         [0010]    The operation of the SCR device is shown in more detail in  FIG. 2 . Exhaust gas  3 , comprising for example NO and NO 2 , mixes with urea solution  21  from the urea tank  13 . When the urea solution  21  and the hot exhaust gas  3  mix hydrolysis and thermal decomposition processes produce ammonia  23 , NH 3 . 
         [0011]    The ammonia  23  and exhaust gases  3  then enter the SCR device  9  which reduces the nitrogen oxides to nitrogen. Downstream of the SCR device  9  the exhaust flow comprises a mixture of nitrogen and water, as indicated at  25 . 
         [0012]    For maximum efficiency in the reduction reaction it is important that the reducing agent is dispersed evenly throughout the exhaust flow over a wide range of exhaust and reducing agent flow speeds and conditions. As shown in  FIG. 1 , there is often another catalyst upstream of the urea injection point, which tends to smooth out the exhaust flow, causing low turbulences in the exhaust stream. In such a low turbulence environment there is a tendency for there to be incomplete mixing between the exhaust gases and the reducing agent. 
         [0013]    DE10060808 describes a turbulence generating feature which comprises a rotating blade that acts to deviate the exhaust gas from its flow direction and create inhomogeneities in the exhaust gas stream. However, such a device tends to create a permanent back-pressure within the exhaust system which leads to a loss of engine efficiency. 
         [0014]    DE19856366 shows a further arrangement in which the spray of reducing agent is perpendicular to the exhaust flow. In such arrangements, if the spray is optimised to fill the exhaust pipe at relatively low exhaust velocities then when the exhaust velocity is increased to higher levels the spray is deflected resulting in incomplete mixing. 
         [0015]    It is therefore an object of the present invention to provide a fluid dosing device for introducing a fluid to a gas flow that substantially overcomes or mitigates the above mentioned problems. 
       DISCLOSURE OF THE INVENTION 
       [0016]    According to a first aspect of the present invention there is provided a fluid dosing device for introducing a fluid into a gas flow having a given flow direction, the dosing device comprising: input means for receiving the fluid from a source of fluid; nozzle means for injecting the fluid into the gas flow, the nozzle means comprising valve means arranged to move between a first position in which the valve means seals the nozzle means and a second open position in which injection of fluid into the gas flow can take place wherein the fluid dosing device is arranged to inject fluid into the gas flow in a direction having at least a component which is counter to the flow direction of the gas flow. 
         [0017]    The first aspect of the invention provides for a fluid dosing device that injects a fluid into a gas flow in such a manner that at least a component of the fluid injection direction is counter to the gas flow. Injecting the fluid counter to the gas flow allows far more efficient mixing of the fluid and the gases in the gas flow than in prior art arrangements. 
         [0018]    Since the nozzle means is, at least partially, facing into the gas flow the possibility of contaminants or material deposits entering the fluid dosing device via the nozzle means is increased compared to prior art systems. In order to counteract this, the nozzle means is provided with a valve member which is arranged to move between a first (closed) position in which it seals the nozzle means from the gas flow and a second (open) position which allows injection into the gas flow to take place. It is noted that the opening and closing action of the valve means in use tends to clean the valve means and nozzle means of any material deposits that may have formed. 
         [0019]    Conveniently, the nozzle means comprises a sealing surface that interacts with a complementary shaped valve surface on the valve means to seal the nozzle means in the first position to prevent fluid from exiting the fluid dosing device and to prevent gas from the gas flow entering the nozzle means. 
         [0020]    Conveniently, the valve means is biased towards the first position by a suitable biasing means, such as a spring. Fluid pressure within the fluid dosing device may be arranged to provide a force opposing the biasing means such that when the fluid pressure exceeds a threshold valve the valve means is able to move to the second (open) position and fluid is therefore injected into the gas flow. 
         [0021]    The valve means and nozzle means can be shaped such that the injected fluid assumes the form of a hollow conical spray. Such a spray pattern is likely to mix better than the individual sprays described in the prior art. 
         [0022]    A further means of preventing material deposits from forming on the nozzle of the fluid dosing device is to form a pocket a clean air around the nozzle means. The fluid dosing device may therefore conveniently be provided with an air supply means which injects (clean) air into the gas flow in the region of the nozzle means. 
         [0023]    The air supply means constitutes an alternative solution to the problem of material deposits on the nozzle means (the provision of a valve member being the alternative solution). 
         [0024]    Therefore, according to a second aspect of the present invention there is provided a fluid dosing device for introducing a fluid into a gas flow having a given flow direction, the dosing device comprising: input means for receiving the fluid from a source of fluid; nozzle means for injecting the fluid into the gas flow, and; air supply means arranged to inject air into the gas flow in the region of the nozzle means wherein the fluid dosing device is arranged to inject fluid into the gas flow in a direction having at least a component which is counter to the flow direction of the gas flow. 
         [0025]    Conveniently, the fluid dosing device is received in a complementary shaped mounting. The mounting arrangement may be designed such that there is an air gap between the dosing device and the mounting. The air gap may conveniently be connected to a source of air and be open to the gas flow in the region of the nozzle means. The air gap and source of air together form an air supply means that, in use, supplies air from the air source through the air gap to the nozzle region of the dosing device in order to form an air pocket around the nozzle. 
         [0026]    Conveniently, the fluid dosing device may be arranged to inject fluid into the gas flow in a direction which is substantially counter to the gas flow direction. 
         [0027]    Conveniently, the gas flow may be conveyed by an exhaust pipe. The fluid dosing device may be linear in shape and received in a complementary shaped mounting formed in a wall of the exhaust pipe. 
         [0028]    For such a linear device, the nozzle means may be closer to the wall of the exhaust pipe than the centre of the pipe. 
         [0029]    The fluid dosing device may alternatively be substantially C shaped and arranged such that the nozzle of the device is substantially in the centre of the exhaust pipe. 
         [0030]    The gas flow may comprise NOx-containing exhaust gases. 
         [0031]    According to a third aspect of the present invention there is provided an exhaust pipe for conveying a gas flow in a given flow direction, the pipe comprising a mounting arrangement defining a bore for receiving a complementary shaped fluid dosing device according to the first or second aspects of the invention, the mounting arrangement being arranged such that the fluid dosing device injects fluid into the exhaust pipe in a direction having at least a component which is counter to the flow direction. 
         [0032]    Conveniently, the exhaust pipe further comprises an air inlet formed in the wall of the exhaust pipe upstream of the fluid dosing device and connected to a source of air. The air inlet and source of air together form an air supply means arranged such that, in use, air may be injected from the source of air into the exhaust pipe such that a pocket of air is formed in the region of the nozzle means of the fluid dosing device. 
         [0033]    Alternatively, instead of an air inlet located separate from the fluid dosing device, the air supply means may comprise a clearance (air gap) between the inner surface of the mounting arrangement and the outer surface of the fluid dosing device. The clearance may be open to the exhaust pipe in the region of the nozzle means and additionally may be connected to a source of air. A pocket of clean air may therefore be formed in the region of the nozzle means by injecting air from the air source through the clearance into the exhaust flow. 
         [0034]    According to a fourth aspect of the present invention there is provided a catalytic reduction system comprising a fluid dosing device according to the first or second aspects of the present invention; an exhaust pipe according to the third aspect of the present invention and at least one selective catalytic reduction converter, said converter being located in the exhaust pipe downstream of the fluid dosing device. 
         [0035]    The catalytic reduction system may further comprise a reducing agent reservoir for holding a reducing agent (i.e. the “fluid” injected by the fluid dosing device) and a reducing agent pump for delivering reducing agent from the reservoir to the fluid dosing device. 
         [0036]    According to a fifth aspect of the present invention there is provided a method of introducing a fluid into a gas flow having a given flow direction, the method comprising: receiving fluid from a source of fluid; injecting the fluid into the gas flow wherein the fluid is injected into the gas flow in a direction having at least a component which is counter to the flow direction of the gas flow. 
         [0037]    According to a sixth aspect of the present invention there is provided a carrier medium for carrying a computer readable code for controlling a processor, computer or other controller to carry out the method of the fourth aspect of the invention. 
         [0038]    The invention extends to a vehicle comprising a fluid dosing device according to the first aspect of the present invention and also to an engine management unit for use with a vehicle, the unit comprising a fluid dosing device according to the first or second aspect of the present invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: 
           [0040]      FIG. 1  is a schematic representation of a known exhaust system comprising a selective catalytic reduction (SCR) device; 
           [0041]      FIG. 2  is an enlarged view of the SCR device shown in  FIG. 1 ; 
           [0042]      FIG. 3  shows a known fluid dosing device for introducing a reducing agent into an exhaust flow; 
           [0043]      FIG. 4  shows the fluid dosing device of  FIG. 3  at an increased exhaust flow velocity; 
           [0044]      FIG. 5  shows a further fluid dosing device for introducing a reducing agent into an exhaust flow; 
           [0045]      FIG. 6  shows the fluid dosing device of  FIG. 5  at an increased exhaust flow velocity; 
           [0046]      FIG. 7  shows a fluid dosing device in accordance with an embodiment of the present invention; 
           [0047]      FIG. 8  shows a valve member of the nozzle region of the fluid dosing device of  FIG. 7  in a first position; 
           [0048]      FIG. 9  shows the valve member of the nozzle region of the fluid dosing device of  FIG. 7  in a second position; 
           [0049]      FIG. 10  shows the fluid dosing device of  FIG. 7  at an increased exhaust flow velocity; 
           [0050]      FIG. 11  shows a further fluid dosing device in accordance with an embodiment of the present invention; 
           [0051]      FIG. 12  shows the fluid dosing device of  FIG. 11  at an increased exhaust flow velocity; 
           [0052]      FIG. 13  shows an exhaust pipe including a further fluid dosing device in accordance with an embodiment of the present invention and an additional air supply passage; 
           [0053]      FIG. 14  shows a further fluid dosing device incorporating an air supply means in accordance with an embodiment of the present invention; and 
           [0054]      FIG. 15  shows a C-shaped fluid dosing device incorporating an air supply means in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]      FIG. 3  shows a known spray arrangement for introducing a reducing agent into an exhaust flow. In  FIG. 3 , a section through an exhaust pipe  30  is shown. A fluid dosing device, which shall be referred to with reference to the drawings as an injection device (generally indicated as  3   1 ), for injecting the reducing agent is provided. The injection device  31  comprises a nozzle body  32  which is shown passing through the lower wall of the pipe  30  and is of a generally C shaped construction. A spray  34  of reducing agent is shown entering the interior of the exhaust pipe  30  from a nozzle  36  of the injection device  3   1 . The nozzle  36  is generally in the centre of the pipe  30 . 
         [0056]    The direction of the exhaust flow is indicated by the arrow  38 . It is noted that the injection device  31  introduces the reducing agent in the same direction as the exhaust gas flow  38 . 
         [0057]    In  FIG. 3  the velocity of the exhaust flow is relatively low and it can be seen that the spray  34  is optimised to achieve full mixing of the spray with the exhaust gases, as indicated by the fact that the spray  34  extends across the diameter  40  of the pipe  32 . 
         [0058]      FIG. 4  shows the same spray arrangement as  FIG. 3  and like numerals have been used to denote like features. In  FIG. 4 , the direction of the exhaust flow is the same as in  FIG. 3 . However, the velocity of the exhaust flow is greater than in  FIG. 3 , as indicated by the larger arrow  42 . This results in a problem since the diameter  44  of the spray  34  at the point that it evaporates is now less than the diameter  40  of the pipe  30 . There is therefore incomplete mixing of the reducing agent within the increased velocity exhaust flow in the conventional spray arrangement of  FIGS. 3 and 4 . Further it is noted that in its gaseous form, the reducing agent now requires a long distance of exhaust pipe to fully mix with the exhaust gases. In restricted space environments, for example the underside of a car, it is often not possible to provide enough pipe length between catalyst devices to achieve full mixing. 
         [0059]      FIG. 5  shows a further known arrangement for introducing a reducing agent into an exhaust flow. In  FIG. 5 , a generally C shaped injection device  31  once again passes through the wall of the exhaust pipe  30  such that the nozzle  36  is generally in the centre of the pipe  30 . The direction of the exhaust gas flow rate is once again indicated by arrow  38 . The velocity of the exhaust gas flow is relatively low (as indicated by the size of the arrow  38 ). 
         [0060]    In  FIG. 5  however the nozzle is arranged to introduce the reducing agent in two or more separate sprays  46 ,  48 . The reducing agent in this case is introduced perpendicular to the direction of the exhaust gas flow. Such an arrangement is shown in DE10060808. 
         [0061]    It can be seen in  FIG. 5  that the injection device  31  is optimised to introduce the reducing agent at this low flow velocity since the sprays  46 ,  48  extend to the walls of the pipe  30 . 
         [0062]      FIG. 6  shows the same arrangement as  FIG. 5 . In  FIG. 6  however the velocity of the exhaust gas flow has increased as indicated by the larger arrow  42 . The reducing agent sprays  46 ,  48  are deflected in this higher velocity exhaust gas flow such that there is no longer complete mixing of the spray with the exhaust gases before the spray evaporates. 
         [0063]      FIG. 7  shows an arrangement for introducing a reducing agent into an exhaust flow in accordance with the first aspect of the present invention.  FIG. 7  shows a section through an exhaust pipe  50 . Upper  52  and lower  54  pipe walls are visible. 
         [0064]    Exhaust gases flow through the pipe  50  in a generally right to left direction as indicated by the arrow  56 . 
         [0065]    An injection device  57  in accordance with an embodiment of the present invention comprises a generally tubular nozzle body  58  and is shown entering the pipe  50  through the lower wall  54  of the pipe  50  in the orientation shown. A sealing arrangement  60  clamps the injection device  57  in place and ensures that no exhaust gases can escape at the point where the injection device  57  enters the pipe  50 . 
         [0066]    The injection device  57  comprises a nozzle  62  which injects reducing agent  64  into the centre of the pipe  50  in a direction that opposes the direction  56  of exhaust gas flow within the pipe  50 . The injection device  57  is shaped so as to form substantially a C-shape along its length. End  66  of the injection device  57  is in fluid communication with a source (not shown) of reducing agent, e.g. the urea tank  13  of  FIG. 1 . A bore (not shown in  FIG. 7 ) runs through the nozzle body  58  such that a fluid communication path exists between the fluid source/end  66  and the nozzle  62 . 
         [0067]    The spray  64  introduced into the pipe  50  by the injection arrangement ( 58 ,  62 ) is optimised to substantially reach the full cross section of the exhaust pipe  50 . 
         [0068]    The nozzle  62  of the injection device shown in  FIG. 7  comprises a valve arrangement. For ease of reference and the clarity of the drawings the valve arrangement is not shown in  FIG. 7  but is depicted in  FIGS. 8 and 9 . 
         [0069]    Turning to  FIG. 8 , a cross section through the injection device  57  is shown in the region of the nozzle  62 . The nozzle body  58 /nozzle  62  comprises a nozzle bore  130  within which a nozzle valve element (indicated generally as  132 ) is located. 
         [0070]    The nozzle valve element  132  comprises a piston  134  located towards the end of the nozzle bore  130  and a shaft  136  located on the other side of the piston  134  than the tip of the nozzle  62 . The piston  134  is generally a close clearance fit in the nozzle bore  130  such that the nozzle valve element  132  may slide within the nozzle bore. An annular grove  138  is provided in a downstream portion of the piston  134  and the nozzle bore  130 . An end of the piston  134  adjacent the tip of the nozzle  62  is flared outwards to define a valve surface  140  of the piston having a diameter greater than the nozzle bore  130 . The nozzle bore  130  adjacent the tip of the nozzle is flared to define a sealing surface  142  complementary to the shape of the valve surface  140  of the piston. A nozzle chamber  144  is defined by the annular grove  138  and the bore  130  of the nozzle. 
         [0071]    The end of the piston  134  distant from the tip of the nozzle is attached to the shaft  136  of the valve nozzle element  132 . A bush  146  is provided within the nozzle bore  130 , and the shaft  136  is a clearance fit within the bush  146 . The shaft extends into the bore of the injection device. A retaining collar  148  is carried on and fixed to the shaft  136  close to the end of the shaft distant from the piston  134 . A nozzle spring  150 , comprising a compression spring, is disposed between the retaining collar  148  and the bush  148  so as to bias the valve surface  140  of the piston against the sealing surface  142  of the nozzle. 
         [0072]    Helical grooves  152  are provided on an upstream portion of the piston to define helical passages between the piston  134  and the nozzle bore  130 . The source of reducing agent is therefore in communication with the tip of the nozzle by way of the helical passages  152  and a clearance  154  between the shaft and the bush. 
         [0073]    Referring to  FIGS. 8 and 9 , as reducing agent is pumped from the source of reducing agent through the bore  130  of the injection device  57 , an increase in pressure of the reducing agent is experienced in the nozzle chamber  144  of the nozzle  62 . The pressure of the reducing agent within the nozzle chamber  144  acts upon the valve surface  140  of the piston so as to impart a force on the valve nozzle element  132  opposed to the biasing force of the nozzle spring  150 . When the pressure of reagent exceeds a threshold value, the force acting on the valve nozzle element  132  due to the reducing agent pressure is sufficient to overcome the biasing force of the nozzle spring  150  and to create a clearance  156  between the valve surface  140  of the piston and the sealing surface  142  of the nozzle ( FIG. 9 ). 
         [0074]    When the valve nozzle element is in an open position (the position described above in relation to  FIG. 9 ), reducing agent is expelled from the nozzle  62  by way of the clearance  156 . The size of the clearance and the shape of the sealing surface  142  and valve surface  140  are adapted so that, upon expulsion of the reducing agent, the reducing agent forms a hollow conical spray to aid dispersion and mixing of the reducing agent within the exhaust gas stream. 
         [0075]    The nozzle valve element  132  described above has the advantage that no reducing agent is exposed to the exhaust gases when the nozzle is not injecting. This reduces the chance of the reducing agent decomposing and blocking the nozzle  62  (or any exhaust contaminants blocking it). It is noted that the movement of the nozzle as it opens tends to be self-cleaning. The hollow conical spray produced is also likely to mix better than the individual sprays in the prior art. 
         [0076]    It is noted that the nozzle valve element  132  described in relation to  FIGS. 8 and 9  is located in the nozzle  62  of each of the arrangements shown in  FIGS. 7 &amp; 10-   12 . It may also additionally be located in the arrangements depicted  FIGS. 13 to 15  relating to the second aspect of the present invention. 
         [0077]      FIG. 10  shows the same injection arrangement ( 58 ,  62 ) of  FIG. 7 . In this Figure however the exhaust gas flow has a greater velocity than in  FIG. 7 . This greater velocity flow is indicated by arrow  68 . It is noted that the direction of the exhaust flow remains from right to left in the Figure. 
         [0078]    In the higher velocity flow of  FIG. 10  the spray  64  of reducing agent that is introduced from the nozzle  62  of the injection device  57  is deflected backwards (relative to the direction in which it is injected) such that the direction of the spray  64  begins to approximate the direction of the exhaust flow (the changed direction of the spray  64  being indicated by the arrows  70 ). 
         [0079]    It is noted however that, as the distance the spray  64  travels before reaching the walls  52 ,  54  of the pipe  50  is not substantially altered, the spray  64  still substantially fills the full pipe cross section. 
         [0080]    There is a high differential velocity between the spray  64  and the exhaust gas flow  68  at the centre of the pipe in the arrangement of  FIG. 10 . Close to the pipe walls however there is a low differential velocity between the spray  64  and the exhaust gas flow  68 . This varying velocity differential across the pipe  50  results in the formation of vortices  72  downstream from the nozzle  62  of the injection device  57 . These vortices  72  act to provide further mixing of the reducing agent  64  with the exhaust gases. It is noted that since these vortices are only generated while the injection device is spraying reducing agent much less energy is required to create them than is the case if the turbulence is generated by deflecting the exhaust gases directly (by for example using an arrangement similar to that shown in DE10060808). 
         [0081]    To achieve the most even mixing it is advantageous to inject reducing agent into the centre of the exhaust pipe as shown in Figures.  7  and  10 . However, this may be undesirable for reasons of heat management and it is noted that heating of the nozzle of the injection device by the exhaust gases can result in the urea solution exceeding the temperature (around 70° C.) at which crystals precipitate out of it. 
         [0082]    To reduce heating of the nozzle and injection device it is known to inject reducing agent from close to the wall of the pipe. An example of this arrangement is shown in DE19856366. 
         [0083]    An embodiment of the present invention in which reducing agent is introduced from close to one of the walls of the exhaust pipe is shown in  FIGS. 11 and 12 . 
         [0084]    In  FIG. 11 and 12 , reducing agent is introduced into the pipe  50  from a linearly shaped injection device  74 . The nozzle  76  of this device  74  is shown to be closer to the lower wall  54  of the pipe  50  than the centre of the pipe. The axis of the injection device  74  is orientated such that the spray  78  of reducing agent from the nozzle  76  has an injection direction  80  with a substantial component that opposes the direction  56  of the exhaust gas flow. The angle between the axis of the injection device  74  and the axis of the pipe  50  may generally be in the range  15 - 45  degrees to ensure good mixing of the reducing agent with the exhaust gases. 
         [0085]    The injection device  74  is mounted and received in a complementary shaped port  75  formed by the lower wall  54  of the pipe  50 . 
         [0086]      FIG. 11  shows a pipe  50  in which the exhaust gases have a relatively low velocity. It can be seen from this Figure that the injection arrangement ( 74 ,  76 ) is optimised to produce a spray  78  that substantially fills the entire cross section of the pipe. 
         [0087]      FIG. 12  shows the same arrangement as in  FIG. 11  except that the velocity of the exhaust gas flow is relatively higher (as indicated by the larger arrow  68 ). 
         [0088]    The initial injection direction  80  is the same as in  FIG. 11  but the increased exhaust gas flow velocity causes the spray  78  to curl backwards until the spray direction changes (as indicated by arrow  82 ) such that it has a component that is travelling substantially in the same direction as the exhaust gas flow. 
         [0089]    It is noted however that the fact that the initial injection occurs substantially against the flow of exhaust gas means that the spray  78  still substantially fills the cross section of the pipe  50 . The relatively high velocity differential between the spray  78  and the exhaust gases at the bottom of the pipe  50  (as shown in the orientation illustrated in  FIGS. 11 and 12 ) and the relatively low differential velocity between the spray and exhaust gases at the top of the pipe (as shown in the orientation shown in  FIGS. 11 and 12 ) generates a vortex  84  which further aids mixing of the spray and the exhaust gas. 
         [0090]    As noted above, the exhaust flow from an engine may contain contaminants. Furthermore, it is possible for material to deposit out of the reducing agent once it has been injected into the exhaust pipe. Such contaminants and materials may deposit onto the nozzle of an injection device thereby having an adverse affect on the ability of the injection device to form a spray of reducing agent. Such a problem is particularly relevant in the case of the present invention where reducing agent is injected counter to the exhaust flow direction. 
         [0091]      FIGS. 13 to 15  show various injection device arrangements designed to reduce the chance of material depositing onto the nozzle of the injection device in accordance with the second aspect of the present invention. It is noted that the arrangements shown in  FIGS. 13-15  may also include the valve arrangement described above in relation to  FIGS. 8 and 9 . 
         [0092]      FIG. 13  shows a first arrangement which is generally similar to that depicted in  FIG. 12  (it is noted that like numerals have been used to denote like features between  FIGS. 12 and 13 ). In  FIG. 13  however the lower pipe wall  54  defines a further injection means  86  located upstream from the injection device  74 . 
         [0093]    Injection means  86  (air inlet  86 ) is arranged to introduce a flow of air  88  into the exhaust pipe  50  in substantially the direction of the exhaust flow  68 . This air flow  88  forms a pocket  90  of clean air around the nozzle  76  of the injection device  74  thereby reducing the risk of material deposits forming on the nozzle  76  opening. 
         [0094]      FIG. 14  shows an alternative design for the injection device  74  shown in  FIG. 13 .  FIG. 14  therefore shows an injection arrangement  92  in which an injection device  94  is mounted and received in a substantially complementary shaped port  96  formed by the lower wall  54  of the pipe  50 . The injection device comprises a nozzle  98  within the pipe  50  and serves to convey reducing agent from a source of reducing agent (not shown) to the pipe  50  (the direction of reducing agent being generally indicated by the arrow  100 ). 
         [0095]    An annular clearance  102  surrounds the injection device  94  as defined by the outer surface of the injection device  94  and the inner surface of the socket  96 . 
         [0096]    The port  96  further comprises an air inlet  104  having an air supply bore  106  that connects the annular clearance  102  to a source of air (indicated by the air flow arrow  108 ). 
         [0097]    The air inlet  104 , clearance  102  and source of air together form an air supply means that, in use allows air to flow through the air supply passage  106  and the annular passage  102  to the nozzle opening  98 . This flow of air once again forms a pocket  110  of relatively clean air around the injection device  94  and the nozzle  98  thereby reducing the risk that deposits will form on the nozzle  98  and impair injection performance. 
         [0098]      FIG. 15  shows a variation of the injection device arrangement of  FIGS. 5 and 6  (it is noted that like numerals have been used to denote like features between  FIGS. 5 ,  6  and  15 ). 
         [0099]    In  FIG. 15 , the injection device  58  is surrounded by a sleeve  112 . The sleeve is clamped in position by the sealing arrangement  60 . The sleeve  112  is designed such that a gap  114  is defined between the inner surface of the sleeve  112  and the outer surface of the injection device  58 . 
         [0100]    An air inlet  116 , comprising an air supply bore  118 , is defined by the sleeve member  112  which serves to connect the gap  114  to a source of air (not shown in  FIG. 15 ). 
         [0101]    The source of air and gap  114  together form an air supply means and, in use, a flow of air  120  is passed through the air supply bore  118  and gap  114  to the region of the nozzle  62  of the injection device  58 . In this manner a pocket  122  of relatively clean air is formed around the nozzle region of the injection device  58 . 
         [0102]    It is noted that as a relatively low pressure of air is needed to form the air pockets of  FIGS. 13-15 , it may conveniently be bled from anywhere in the inlet passages of the engine downstream of the turbocharger&#39;s compressor wheel. 
         [0103]    It is also noted in the embodiments of  FIGS. 13-15  that the air serves to reduce the temperature of the nozzle ( 76 ,  98 ,  62 ) which further minimises the tendency for crystals to from in the reducing agent solution within the nozzle. 
         [0104]    The embodiments of the present invention described above are illustrated injecting into straight, parallel sided sections of the exhaust. However, it will be appreciated by the skilled person that the same principles can be applied for injection into a bend of the exhaust pipe and/or into a pipe of varying cross section. If required, the pipe cross-section can be deliberately varied near the point of injection of the reducing agent in order to optimise the exhaust velocity to achieve the best spray mixing. For instance, the spray could be introduced into the throat region of a venturi in order to inject into a high velocity exhaust flow without generating excessive back pressure. 
         [0105]    It will be understood that the embodiments described above are given by way of example only and are not intended to limit the invention, the scope of which is defined in the appended claims. It will also be understood that the embodiments described may be used individually or in combination with the other embodiments described herein.