Abstract:
An armature for a solenoid actuator is disclosed. The armature comprises a first face comprising a recess suitable for receiving a biasing spring in use of the armature; a second face opposite the first face; and fluid communication passages for providing a fluid flow path through the armature between the recess and the second face in use of the armature. The first face is uninterrupted by the fluid communication passages. The invention reduces the risk of cavitation damage during operation of the actuator. In one application, the actuator is used in a fluid pump for a selective catalytic reduction system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an armature suitable for use in a solenoid actuator. In particular, but not exclusively, the invention relates to an armature for use in a pump forming part of a selective catalytic reduction system. 
     BACKGROUND TO THE INVENTION 
     One strategy for reducing nitrogen oxide exhaust gas emissions in an internal combustion engine involves the introduction of a reagent comprising a reducing agent, typically a liquid ammonia source such as an aqueous urea solution, into the exhaust gas stream. This method is known as selective catalytic reduction or SCR. The reducing agent is injected into the exhaust gas upstream of an exhaust gas catalyst, known as an SCR catalyst. Nitrogen oxides in the exhaust gas undergo a catalysed reduction reaction with the ammonia source on the SCR catalyst, forming gaseous nitrogen and water. 
     Typically, in a selective catalytic reduction (SCR) system, injection of the reagent into the exhaust gas stream is achieved by pumping the reagent from a supply tank to an injection nozzle disposed within the exhaust gas stream using a suitable pump, such as described in the present applicant&#39;s co-pending European Patent Application Publication No. EP-A-1878920. 
       FIG. 1  shows, schematically and in simplified form, a known pump  20  suitable for pumping reagent in an SCR system. The pump  20  comprises a solenoid actuator  22  disposed within a generally cylindrical housing  24 . The actuator  22  comprises a tubular pole member  26 , formed integrally with the housing  24 , and a wire winding or coil  28  disposed around the pole member  26 . One end of the pole member  26  forms an annular pole face  30  of the actuator  22 . 
     An armature  32  is provided in an armature chamber  34  adjacent to the pole face  30 . The armature  32  is connected to a pumping plunger  36 . The plunger  36  is slidably received in a central bore  38  of a sleeve  40  disposed centrally within the pole member  26 . An end face  42  of the sleeve  40  is spaced from the pole face  30 , so as to form a spring chamber  44  adjacent to the pole face  30 . A biasing spring  46  is partially received in the spring chamber  44  to bias the armature  32  away from the pole face  30 . One end of the biasing spring  46  abuts the end face  42  of the sleeve  40 , and the other end of the biasing spring  46  abuts a central region  48  of the armature  30 . 
     The pump  20  further comprises, at an upstream end, an inlet  50 , which receives fluid from a source such as a tank (not shown) and, at a downstream end, an outlet  52  that is in communication with a delivery nozzle (not shown). In use of the pump  20  in an SCR system for an internal combustion engine, the fluid is a reagent and the delivery nozzle is disposed within an exhaust pipe of the engine. 
     A supply passage  54  is provided within the pump  20  to convey fluid from the inlet  50  to the outlet  52 . In this example, the supply passage  54  comprises an annular space  56  between the pole member  26  and the coil  28 , and further comprises radial passages  58  that extend through the pole member  26  and the sleeve  40  to communicate with the bore  38  of the sleeve  40 . When the pumping plunger  36  is in the position shown in  FIG. 1 , with the armature  32  biased away from the pole face  30 , the radial passages  58  communicate with a delivery chamber  60  formed at a downstream end of the bore  38  of the sleeve  40 . 
     Fluid flow from the delivery chamber  60  to the outlet  52  is controlled by an outlet valve  62 , which is arranged to open when the pressure of fluid in the delivery chamber  60  exceeds a threshold level. 
     In operation, in a pumping stroke of the pump, the coil  28  is energized to generate a toroidal magnetic field around the coil  28 . As a result, the armature  32  moves towards the pole face  30 , against the force of the biasing spring  46 , such that the downstream end of the plunger  36  interrupts fluid flow between the radial passages  58  and the delivery chamber  60 . The downstream end of the plunger  36  reduces the volume of the delivery chamber  60 , so that the pressure of fluid in the delivery chamber  60  increases. Once the threshold pressure is reached, the outlet valve  62  opens to cause delivery of fluid from the outlet  52  of the pump  20 . 
     The coil  28  is then de-energized, whereupon the magnetic forces acting on the armature  32  diminish. The force of the biasing spring  46  causes the armature  32  to move away from the pole face  30 , so as to increase the volume of the pumping chamber  60  and re-open fluid communication between the radial passages  58  and the delivery chamber  60 . Fluid can then re-fill the delivery chamber  60 , ready for the next pumping stroke. 
     In an SCR system, it is desirable to provide rapid, frequent injections of fluid into the exhaust pipe. For example, to ensure sufficient atomization of the fluid as it leaves the delivery nozzle, the velocity of the pumping plunger  36  must be relatively high, typically of the order of 2 meters per second. As will be appreciated from  FIG. 1 , the armature  32  must move through fluid within the armature chamber  34 . Since the diameter of the armature  32  is relatively large, a significant quantity of fluid is displaced when the armature  32  moves. The displacement of this fluid tends to slow the movement of the armature  32  and therefore the pumping plunger  36 . 
     To allow the armature  32 , and hence the plunger  36 , to move fast enough in the armature chamber  34 , it is known to provide vent holes  64  in the armature  32 . The vent holes  64  extend axially through the armature  32  from the face of the armature  32  nearest the pole face  30  to the opposite face, furthest from the pole face  30 . During movement of the armature  32 , fluid can flow through the vent holes  64  as well as around the periphery of the armature  32 , thereby reducing the fluid drag on the armature  32 . 
     It has been found that, when the coil  28  is de-energized and the armature  32  moves away from the pole face  30 , the pressure in the spring chamber  44  is caused to drop rapidly. This can lead to cavitation damage to the actuator  20 , caused by the collapse of cavities in the fluid in the spring chamber  44  that form as a result of the pressure drop. 
     Accordingly, it would be desirable to provide an armature for an actuator that overcomes or mitigates this problem. 
     SUMMARY OF THE INVENTION 
     Against this background, the present invention resides in an armature for a solenoid actuator including a first face having a recess suitable for receiving a biasing spring in use of the armature, and a second face opposite the first face. The armature further includes means for fluid communication through the armature between the recess and the second face in use of the armature. The first face is uninterrupted by the fluid communication means. 
     By providing a fluid flow path between the recess and the second face, the pressure difference across the armature upon movement of the armature can be minimized. Consequently, the risk of cavitation damage is lower, particularly in the region of the recess. The armature is therefore suitable for use in an actuator that operates at high speeds and/or frequencies, such as in a pump in a selective catalytic reduction system. 
     Furthermore, because the first face of the armature is uninterrupted by the fluid communication means, the presence of the fluid communication means does not significantly affect the magnetic behavior of the armature when used in an actuator. In particular, the ability of the armature to carry a magnetic field in the material of the armature adjacent to the front face is not significantly reduced by the presence of the fluid communication means. To this end, the fluid communication means may be spaced from the first face of the armature. 
     The fluid communication means may communicate with a peripheral wall of the recess. The armature may define a central axis normal to the first face, and the fluid flow path may have a first component in a direction parallel to the axis and a second component extending radially with respect to the axis. In one embodiment, a vector described by the fluid flow path can be resolved into a first vector component parallel to the axis and a second vector component extending radially with respect to the axis. Preferably, the fluid flow path is inclined with respect to the axis. The armature may be generally cylindrical or disc-shaped, and the axis may be a cylinder axis of the armature. 
     In one embodiment, the fluid communication means includes one or more passages extending from the second face to the recess. 
     In another embodiment, the fluid communication means includes one or more channels in the second face of the armature that open into the recess. The depth of the or each channel may increase moving towards the recess. In this way, the channel may define a fluid flow path that is inclined with respect to the axis of the armature. 
     The or each channel may be arranged such that, during manufacture of the armature, the or each channel is formable by relative movement of a tool and the armature in a direction normal to the first face. In this case, the armature is thereby designed such that the shape of the armature allows for straightforward fabrication, for example by molding or pressing. In one example, the armature is shaped such that the entirety of the or each channel is open to the second face in the direction normal to the first face. 
     The fluid communication means may further include an indentation in an end face of the recess adjacent to the opening of the or each channel into the recess. The indentation may be a dimple. Advantageously, the indentation serves to increase the cross-sectional area of the fluid flow path. 
     The armature may further include venting means for providing a fluid flow path between the first face and the second face. In one embodiment, the fluid communication means intersects the venting means. 
     The armature may further include an aperture for receiving a plunger in use of the armature. The aperture preferably extends between the second face and the recess. In use, the aperture is closed by the plunger. A piston, valve element or other control member could be used in place of the plunger. 
     The invention also extends to a solenoid actuator including an armature according to the invention as described above. The actuator may further include a pole member having a pole face, wherein the first face of the armature is opposed to the pole face. The actuator may further include a biasing spring. A first end of the biasing spring may be received within the recess of the armature. The biasing spring may bias the armature away from the pole face. 
     The invention also extends to a fluid pump for a selective catalytic reduction system. The fluid pump includes an armature according the invention as described above, and/or a solenoid actuator according to the invention as described above. 
     The invention also resides in a fluid pump for a selective catalytic reduction system, including a solenoid actuator having an armature, a pole member having a pole face, and a biasing spring for biasing the armature away from the pole face. The armature includes a first face including a recess for receiving the biasing spring; a second face opposite the first face; and fluid communication means for providing a fluid flow path through the armature between the recess and the second face in use of the pump, the first face being uninterrupted by the fluid communication means. The pump further includes a pumping plunger associated with the armature. The solenoid actuator is operable to cause reciprocable movement of the pumping plunger. 
     The invention further resides in an armature for a solenoid actuator, the armature including a first face including a recess suitable for receiving a biasing spring in use of the armature, the recess having a peripheral wall; a second face opposite the first face; and at least one passage extending from the second face to the peripheral wall for providing a fluid flow path through the armature between the recess and the second face in use of the armature. 
     Preferred and/or optional features of each embodiment of the invention may be present in the other embodiments of the invention also, alone or in appropriate combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference has already been made to  FIG. 1  of the accompanying drawings, which is a cross-sectional view of a known fluid pump. 
       Embodiments of the present invention will now be described, by way of example only, with reference to the remaining accompanying drawings, in which like reference numerals are used for like parts, and in which: 
         FIG. 2  is a cross-sectional view of a fluid pump having an armature according to a first embodiment of the invention; 
         FIG. 3  is an enlarged cross-sectional view of the armature of the fluid pump shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a fluid pump having an armature according to a second embodiment of the invention; 
         FIG. 5  illustrates the armature of the fluid pump in  FIG. 4  in greater detail, showing (a) a perspective view of one face of the armature; (b) a perspective view of another, opposite face of the armature; and (c) a cross-sectional view on line A-A of the armature; and 
         FIG. 6  illustrates an armature according to a third embodiment of the invention, showing (a) a perspective view of one face of the armature; (b) a perspective view of another, opposite face of the armature; and (c) a cross-sectional view on line A-A of the armature. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a fluid pump  100  suitable for pumping reagent in an SCR dosing system of an internal combustion engine. Many of the components of the pump  100  are similar to those described above with reference to the known pump  20  of  FIG. 1 , and like reference numerals are used for like parts. Consequently, only the differences between the invention shown in  FIG. 2  and the known pump  20  of  FIG. 1  will be described in detail. 
     The pump  100  comprises an actuator  122  having an armature  132  according to a first embodiment of the invention. Referring additionally to  FIG. 3 , the armature  132  comprises a generally disc-shaped body  168  defining a central axis (labeled P in  FIG. 3 ) at the diametric centre of the disc. The armature  132  is made from a suitable soft magnetic material, such as a ferritic iron alloy. The armature includes a first face  170  that opposes the pole face  30  of the actuator, and a second face  172  opposite the first face  170 . 
     A generally cylindrical recess  174  is provided in the first face  170  of the armature  132 . The recess  174  is disposed coaxially with the body  168  of the armature  132 . An aperture  176  extends from the recess  174  to the second face  172 . 
     Vent holes  164  extend through the body  168  between the first and second faces  170 ,  172  in a direction parallel to the armature axis P. Only one such axial vent hole  164  is visible in  FIGS. 2 and 3 , but preferably several axial vent holes  164  are provided, spaced equi-angularly around the armature  132 . 
     Furthermore, several vent passages  178  extend through the body  168  from the second face  172  to the recess  174 . The vent passages  178  comprise drillings disposed at an angle, or inclined, relative to the axis P of the armature  132 . These inclined vent passages  178  open into a peripheral wall  180  of the recess  174 . One inclined vent passage  178  is visible in cross-section in  FIGS. 2 and 3 , whilst only the openings into the wall  180  of three other inclined vent passages  178  are visible. Seven passages  178  in total are provided in this example. 
     Except for the recess  174 , the first face  170  of the armature  132  is generally planar. The second face  172  comprises an annular groove or depression  182  arranged around a central land  184 , through which the aperture  176  emerges. The axial vent holes  164  and the inclined vent passages  178  communicate with or intersect the groove  182 . 
     In use, as shown in  FIG. 2 , the armature  132  is located in the armature chamber  34  of the pump  100 . The pumping plunger  136  comprises a plunger shaft  186  and, at an upstream end thereof, a cylindrical plunger head  188  and an end plate  190 . Preferably, the end plate  190  is integral with the plunger head  188 . The plunger head  188  is received within the aperture  176  of the armature  132 . The end plate  190  has a diameter larger than the plunger head  188 , so that the end plate  190  abuts the land  184  on the second face  172  of the armature. 
     The plunger head  188  is a tight fit in the aperture  176 , and may be a threaded or interference fit. The end plate  190  may be welded or otherwise connected to the armature  136 . The plunger head  188  and the end plate  190  together block the flow of fluid through the aperture  176  in use of the pump  100 . 
     As in the known pump of  FIG. 1 , the tubular pole member  26  of the pump receives a sleeve  140 . The sleeve  140  comprises a central bore  138  within which the shaft  186  of the plunger  136  is slidable. An upstream end face  142  of the sleeve  140  is set back slightly from the pole face  30 , in a downstream direction. 
     The upstream end of the spring  46  is received in the recess  174  and abuts the upstream end face  175  thereof. Thus, in this embodiment of the invention, the armature  132  comprises a spring chamber in the form of the recess  174 . The downstream end of the spring  46  abuts the end face  142  of the sleeve  140 . 
     Operation of the pump  100  is as described for the pump of  FIG. 1 . However, the inclined passages  178  provide fluid communication means that allow fluid to flow between the second face  172  of the armature and the recess  174  as the armature  132  reciprocates within the armature chamber  34 . Advantageously, therefore, when the armature  132  approaches the pole face  30 , fluid can still flow between the spring chamber defined by the recess  174  and the armature chamber  34 . As a result, the pressure drop on the downstream side of the armature  132 , particularly in the recess  174  and adjacent to the end face  142  of the sleeve  140 , is minimized, and cavitation damage is unlikely to arise. 
     If the passages  178  were absent, the fluid volume in the recess  174  could become isolated from the armature chamber  34  if the armature  132  were to abut the pole face  30 . This would result in a significantly higher pressure drop arising on the downstream side of the armature  132 , leading to an increased risk of cavitation damage. 
     On energization of the coil  28 , the magnetic field passes from the housing  24  into the peripheral edge  192  of the armature, then through the body of the armature  168  to its first face  170 , before passing into the pole face  30  of the pole member  26 . 
     It is to be noted that the inclined passages  178  do not intersect the first face  170  of the armature  132 . Instead, the inclined passages  178  open into the recess  174 , leaving the first face  170  uninterrupted by the passages  178 . Similarly, the peripheral edge  192  of the armature is uninterrupted by the passages  178 . Consequently, the path of the magnetic field within the armature  132  on energization of the coil  28  is largely unaffected by the presence of the inclined passages  178 , and so the inclined passages  178  do not appreciably reduce the force imparted to the armature  132 , even when, as is preferable, the inclined passages  178  have a relatively large diameter to provide a large flow area. 
     It will also be appreciated that the provision of the passages  178  advantageously reduces the mass of the armature  132 . By reducing the mass of the armature  132 , the inertia of the armature  132  is reduced so that the plunger  136  can move at higher speed. However, the bending stiffness of the armature  132  is not significantly reduced by the presence of the passages  178 . 
     Furthermore, because the inclined passages  178  open into the wall  180  of the recess, the end face  175  of the recess  174  is uninterrupted by the openings of the passages  178  so as to provide a planar surface against which the spring  46  can be stably located. Likewise, the passages  178  do not encroach on the aperture  176 , so that the fit of the plunger head  188  in the aperture  176  is not affected by the presence of the passages  178 . 
     Each inclined passage  178  extends in a direction having only radial and axial components, with respect to armature axis P. As a result, the flow of fluid through the inclined passages  178  upon movement of the armature  132  does not give rise to rotational forces on the armature  132 , as would be the case if the passages  178  extended in a direction having a non-radial component. 
     The armature  132  of  FIGS. 2 and 3  could be manufactured by machining from a solid bar or rod of suitable material. The axial vent holes  164  and inclined vent passages  178  could be formed by drilling. 
       FIG. 4  shows a pump  200  having an armature  232  according to a second embodiment of the invention. The pump  200  of  FIG. 4  differs from the pump  100  of  FIG. 2  only in the design of the armature  232 , and like reference numerals are used for like parts. Only the differences between the first and second embodiments will be described. 
     As shown additionally in  FIG. 5 , in this second embodiment the armature  232  comprises a body  268 , a first face  270  opposed to the pole face  30  of the actuator  54  in use, and a second face  272  opposite the first face  270 . 
     A recess  274  is provided in the first face  270  to receive the upstream end of the spring  46 . In this embodiment of the invention, a chamfered region  277  of the recess  274  connects the end face  275  and the peripheral wall  280  of the recess. The spring  46  abuts the generally planar end face  275  of the recess  274 . 
     As in the first embodiment of the invention, the second face  272  of the armature  232  comprises an annular groove  282  disposed around a central land  284 . An aperture  276  extends from the recess  274  to the second face  272 . In use, the plunger  136  is received in the aperture  276  so as to prevent fluid flow through the aperture  276 . 
     The armature  232  comprises five axial vent holes  264 , arranged equi-angularly around the armature  232  and extending through the armature  232  in a direction parallel to the armature axis P. Each of the vent holes  264  communicates with the groove  282 , and allows fluid communication between the first and second faces  270 ,  272  of the armature  232 . 
     The armature  232  further comprises five radially-extending grooves or channels  210  in the second face  272 . The channels  210  are generally U-shaped in cross section, and the depth of each channel  210  increases moving towards the centre of the armature  232  so that a base  214  of each channel  210  extends at an inclined angle with respect to the axis P of the armature  232 . Each channel  210  intersects or opens into the peripheral wall  280  of the recess  274 , downstream of the central land  284 , so that the channels  210  define fluid communication means that allow fluid to flow between the second face  272  and the recess  274  in use of the armature. 
     As seen most clearly in  FIGS. 5(   b ) and ( c ), where each channel  210  meets the wall  280  of the recess  274 , the chamfered region  277  is absent so as to allow fluid flow between the recess  274  and the channels  210 . Furthermore, in order to increase the flow area through the channels  210 , the intersection between each channel  210  and the recess  274  is enlarged by the provision of an indentation or dimple  212  in the end face  275  of the recess  274 . The intersection between each channel  210  and the recess  274  is therefore generally circular. 
     The channels  210  in this second embodiment of the invention serve the same purpose as the inclined passages  178  in the first embodiment of the invention, and share the same advantages. 
     Additionally, it is to be noted that the shape of the channels  210  in the second embodiment is such that the entirety of each channel  210  is open to the second face  272  of the armature  232  in the axial direction. In other words, every part of each channel  210  is in view when looking at the second face  272  of the armature  232  along the axis P. Similarly, the entirety of each dimple  212  is open to the first face  270  of the armature  232 . Consequently, during manufacture of the armature  232 , the channels  210  and the dimples  212  are respectively formable by relative movement of a tool and the armature  232  in a direction parallel to the armature axis P. 
     The armature  232  can therefore be manufactured readily by metal injection molding, without the need for retractable pins to form inclined channels, or by a pressing and sintering process, in which only axial movement of the punches and dies is possible. 
       FIG. 6  illustrates an armature  332  according to a third embodiment of the invention. The armature  332  is similar to the armature of the second embodiment illustrated in  FIG. 5 . Only the differences between the third and second embodiments will be described. 
     In this third embodiment of the invention, three channels  310  are provided in the second face  272  of the armature  332 , to provide fluid communication means between the second face  272  and the recess  374  in the first face  370  of the armature  332 . Additionally, six axial vent holes  364  are provided to allow fluid communication between the first and second faces  370 ,  372 . 
     The channels  310  intersect three of the axial vent holes  364 . The channels  310  can therefore extend deeper into the body  368  of the armature  332 , so that the area of intersection between each channel  310  and the peripheral wall  380  of the recess  374  is larger than in the armature shown in  FIGS. 4 and 5 . The base  314  of each channel, which leads from the periphery of an axial vent hole  364  to the wall  380  of the recess, extends at an inclined angle with respect to the axis of the armature  332 . 
     The chamfered region  377  between the end face  375  and the peripheral wall  380  of the recess  374  is absent in the region of the intersection between each channel  310  and the recess  374 , so as to increase the flow area. However, because the channels  310  extend further towards the first face  370  of the armature  332 , it is not necessary to provide dimples in the end face  375  of the recess  374  in this embodiment. 
     It will be appreciated that any suitable means for fluid communication between the recess and the second face of the armature may be provided, so long as the fluid communication means does not interrupt, intersect or extend along or into the first face of the armature. Examples of such means include drillings, bores, passages, channels, grooves, notches, conduits, indentations, depressions and so on. The form of the fluid communication means may be selected based on the preferred manufacturing method for the armature. 
     Any suitable number of fluid communication means could be provided in the armature. For example, between three and seven passages, channels or other such means may be provided. Similarly, any suitable number of axial vent holes may be provided. Providing more passages advantageously increases the total cross-sectional area available for fluid communication through the armature. Preferably, the axial vent holes and the fluid communication means are uniformly distributed around the armature, but this need not be the case. 
     The fluid communication means may intersect one or more of the axial vent holes, as in the third embodiment of the invention, or alternatively the fluid communication means may be separate from the axial vent holes. Conceivably, the axial vent holes could be omitted, since adequate fluid flow through the armature may be available via the fluid communication means. 
     Several further modifications and variations to the embodiments of the invention described above are also possible, without departing from the scope of the invention as defined in the appended claims.