Abstract:
A metering system for metering a liquid has an electric motor ( 32 ) for setting a desired feed rate by modifying the rotation speed of the electric motor. It furthermore has an eccentric drive ( 52, 56 ), drivable by said electric motor ( 32 ), for a pump ( 53 ) that has two delivery directions. It also has a pump ring ( 62 ) made of an elastomeric material and a stationary ring ( 70 ) which is arranged relative to the pump ring ( 62 ) and to the eccentric drive ( 52, 56 ) in such a way that a pump chamber ( 120 ), extending in a circumferential direction, is formed between the stationary ring ( 70 ) and pump ring ( 62 ), which chamber changes shape upon rotation of the electric motor ( 32 ) in order to deliver a liquid to be metered through the pump chamber ( 120 ). A stationary seal ( 142 ) is provided in the pump chamber ( 120 ), between two fluid ports.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a metering system for metering a liquid. 
       BACKGROUND 
       [0002]    Toxic exhaust gases and nitrogen oxides (NOx) occur in the context of the combustion process in diesel engines. To eliminate or break down these nitrogen oxides, it is known to inject a urea solution, by means of a metering pump, into the previously purified exhaust gas stream. The ammonia that is thereby released converts up to 80% of the nitrogen oxides into harmless nitrogen and water in a downstream SCR catalytic converter. 
         [0003]    Because a urea solution is a chemically aggressive and very low-viscosity medium that has a tendency to crystallize, special pumps, in which the urea solution does not come into contact with the drive equipment of the metering pump, are used to deliver it. The delivery space is separated from the equipment space by, for example, a membrane or another flexible part. 
         [0004]    The pump runs constantly during vehicle operation, establishing a pressure of, for example, 5 bar. Urea is present in the lines and systems. If the ambient temperature drops below the freezing point after the vehicle is shut off, the system would completely freeze up. Since not all components can withstand freezing, the urea solution must be pumped back into a reservoir container after the vehicle is shut off. In known systems, this occurs by means of a 4/2-way valve that reverses the delivery direction. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the invention to make a novel metering system available. 
         [0006]    According to the invention, this object is achieved by using a reversible variable-speed electric motor to drive the eccentric pump rotor, the rotor including an elastomeric ring, a portion of which forms a seal against the opposite wall of the pump chamber. It is thereby possible to make available a metering system that has a very compact construction and that, in the one rotation direction of the electric motor, draws the liquid to be metered out of the reservoir container and transports it to the consumption point, and, in the other rotation direction, draws that liquid out of the lines of the system and transports it back to the reservoir container. 
         [0007]    The problems that have arisen in practice when a 4/2-way valve is used are thereby avoided, i.e. after the internal combustion engine is shut off, the rotation direction of the electric motor is reversed for a predetermined time period. Because said motor has no contact with the urea solution, reversal of the flow direction using the motor is robust, since such motors have a very long service life. The result is to prevent the urea solution from freezing in cold weather, since with such a motor it is very easy to pump the pump, lines, injection valves, etc. largely to an empty state when no urea solution is being injected, i.e. for example after the engine is shut off. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0008]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiment, in no way to be understood as a limitation of the invention, that is described below and depicted in the drawings. 
           [0009]      FIG. 1  is a three-dimensional depiction of an embodiment of a metering system  30  that serves in this example to meter urea, the delivery direction being determined by the rotation direction of a multi-phase collectorless external-rotor motor  32  and the delivery rate per second being determined by the rotation speed of said electric motor  32 , enabling very precise and economical adjustment of the desired metered amount; 
           [0010]      FIG. 2  is a plan view from above of the metering system of  FIG. 1 , viewed in the direction of arrow II of  FIG. 1 ; 
           [0011]      FIG. 3  is a longitudinal section through metering system  30 , viewed along line III-III of  FIG. 2 ; 
           [0012]      FIG. 4  is a plan view that shows the metering system of  FIG. 3  from the right, viewed along line IV-IV of  FIG. 2 ; 
           [0013]      FIG. 5  is a plan view looking along line V-V of  FIG. 2 ; 
           [0014]      FIG. 6  is an enlarged section viewed along line VI-VI of  FIG. 5 ; this section applies to the rotor position of  FIG. 5  and looks different at other rotor positions; 
           [0015]      FIG. 7  is an enlarged section viewed along line VII-VII of  FIG. 5 ; as with the section of  FIG. 6 , this section applies to the rotor position depicted in  FIG. 5 ; 
           [0016]      FIG. 8  is an enlarged section viewed along line VIII-VIII of  FIG. 5 ; as with the sections according to  FIGS. 6 and 7 , this section applies to the rotor position of  FIG. 5 ; 
           [0017]      FIG. 9  is an enlarged section viewed along line IX-IX of  FIG. 5 ; as with the sections according to  FIGS. 6 ,  7 , and  8 , this section applies to the rotor position of  FIG. 5 ; and 
           [0018]      FIGS. 10A to 10J  are depictions to explain the mode of operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a three-dimensional depiction of a preferred embodiment of a metering system  30  as used, for example, to inject a urea solution as required into the exhaust gas stream of a diesel engine. 
         [0020]    To drive it, the metering system has a multi-phase collectorless external-rotor motor  32  whose rotation speed behavior can be controlled by means of a PWM control signal, as is known e.g. from EP 1 413 045 B1 and corresponding U.S. Pat. No. 7,068,191, KUNER &amp; SCHONDELMAIER. This makes it possible to control the rotation speed and rotation direction of the motor, in accordance with the rotation speed and power demand of the vehicle on which metering system  30  is located. The elements for this are defined by the manufacturer of the engine controller, depending on the requirements of the particular vehicle, and can differ greatly, depending on the type of vehicle (passenger car, truck, aircraft, helicopter, ship, etc.). An advantage of the present invention is that metering system  30  is suitable for very different applications. 
         [0021]    Motor  32  has an electronic drive system, e.g. a three-phase inverter. This electronic system is in turn controlled by an arrangement that serves to decode the pulse duty factor pwm of a PWM signal that is delivered via a lead, and thereby to control the motor in terms of its rotation direction and rotation speed. If the pulse duty factor is referred to as “pwm,” the following correspondences then result (as a non-binding example): 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 pwm 
                 Operating state 
               
               
                   
               
             
             
               
                 0% to 5%  
                 not permitted 
               
               
                 95% to 100% 
                 not permitted 
               
               
                 5% to 85% 
                 Metering mode. Rotation direction = pumping;  
               
               
                   
                 n = 500 to 3500 rpm 
               
               
                 85% to 95%  
                 Back-suction mode. Rotation direction = 
               
               
                   
                 suction; n = 3500 rpm 
               
               
                   
               
             
          
         
       
     
         [0022]    An example of a corresponding decoding circuit is described in detail in EP 1 413 045 B1 and U.S. Pat. No. 7, 068,191, to whose content reference is made, in order to avoid excessive length. All known circuits can of course be used to modify the rotation speed of an electric motor. 
         [0023]      FIG. 1  shows an example of a simple mechanical construction of a metering system  30  that is of course suitable for a wide variety of applications, e.g. including in the pharmaceutical industry and for the manufacture of foods, or e.g. in breweries, to name only a few examples. 
         [0024]    System  30  here has a base  40  on which is arranged, on the right, a first support  42  which carries a bearing element  44  that is depicted here as a ball bearing. 
         [0025]    Arranged at a distance from support  42  is a second support  46  that, according to  FIG. 3 , carries a bearing element  48  that is likewise depicted as a ball bearing. 
         [0026]    As  FIG. 3  shows, bearing elements  44 ,  48  are arranged so that they align with one another. Journaled in them is a shaft  50  on which is mounted, between bearing elements  44 ,  48 , an eccentric bushing  52  that also serves as a spacer between bearing elements  44 ,  48 . Bushing  52  serves to drive a pump  53  that is therefore arranged between bearing supports  42  and  46 . 
         [0027]    Mounted on eccentric bushing  52  is inner ring  54  of an eccentric bearing  56  whose outer ring  58  is mounted on the inner side of a ring  60  that serves as a support for a pump ring  62 . 
         [0028]    Pump ring  62  is manufactured from a suitable synthetic rubber (elastomer) and is mounted by plastic injection molding in an annular groove  64  of ring  60  so that it follows the motions of ring  60 . The latter can be manufactured e.g. from steel, nickel, or bronze. 
         [0029]    In experiments, a synthetic rubber referred to by the abbreviation PEDM (polyester-ethylene-diene monomer) has proved advantageous as an elastomer. 
         [0030]    As shown, for example, in  FIGS. 8 and 9 , pump ring  62  is surrounded on its outer side by a stationary ring  70  that, according to  FIG. 4 , is connected by means of bolts  84  to base  40  and has a T-shaped cross section, namely an edge portion  76  parallel to rotation axis  74  of the metering system, and a holding portion  78  that extends perpendicular to rotation axis  74  and whose radially inner edge is labeled  80 . 
         [0031]    As  FIGS. 4 and 5  show, stationary ring  70  is widened in its lower region and is connected to base part  40  by means of two bolts  84 . Stationary ring  70  is thus located, in the installed state, between supports  42 ,  46 , i.e. bearings  44 ,  48  are arranged closely against one another and can therefore serve as bearings for the entire metering system  30 . 
         [0032]    A support tube  90  through which shaft  50  extends (see  FIG. 3 ) is provided on support  46 . Shaft  50  is therefore journaled only by bearings  44  and  48 . Mounted at its left end (in  FIG. 3 ) is the cup-shaped magnetic yoke  92  of rotor  94  of motor  32 . A ring magnet  96 , which is separated by an air gap  98  from internal stator  100  of motor  32 , is located on the inner side of yoke  92 . Internal stator  100  is mounted on the outer side of support tube  90 . 
         [0033]    Motor  32  also has a circuit board  102  on which electronic components of motor  32  are located. Circuit board  102  is connected via a cable  104  to a plug connector  106 . Motor  32  is supplied via cable  104  with energy, usually with DC voltage from a battery, and a control lead through which the rotation speed and rotation direction of motor  32  are controlled is also located in cable  104 . 
         [0034]    A great advantage of a collectorless motor, in particular in a vehicle, is the high efficiency that can be achieved with such an arrangement. 
         [0035]    Motor  32  drives eccentric bushing  52  via shaft  50 , and said bushing imparts an eccentric motion to eccentric bearing  54 , so that said eccentric motion is likewise imparted to ring  60 . 
         [0036]    A pump chamber  120  is located between the radial outer side of pump ring  62  and the radial inner side  80  of holding portion  78  (see  FIGS. 5 and 7 ). 
         [0037]    Because pump ring  62  is in continuous rolling contact with its outer side  80  on the inner side of holding part  78 , pump chamber  120  is constantly changing shape and thereby transports the metered fluid, that is present in pump chamber  120 , from an inlet to an outlet. 
         [0038]    To prevent this liquid from simply circulating in pump chamber  120 , two connectors  122 ,  124 , that are connected to the portions there of pump chamber  120 , are provided at a suitable site (see  FIG. 5 ). 
         [0039]    When shaft  50  is rotating clockwise, as shown by arrow  128  of  FIG. 5 , the left part of pump chamber  120  thus becomes smaller, so that liquid is pushed out through connector  122  (see arrow  130  of  FIG. 5 ), and the right part of pump chamber  120  becomes larger, so that liquid is drawn in through connector  124  (see arrow  132  of  FIG. 5 ). 
         [0040]    When shaft  50  is rotating oppositely to the direction of arrow  128 , i.e. counterclockwise, the processes occur in the reverse direction, i.e. in this case, liquid is pushed out of connector  124  and liquid is drawn in through connector  122 . The same pump  53  can thus be used to meter liquid and also to pump liquid out. 
         [0041]      FIGS. 1 ,  3 , and  4  to  6  show that a wedge  140  is provided in an opening of pump ring  62 , said wedge having two functions: 
         [0042]    a) It spreads pump ring  62  in a radial direction so that it constantly abuts sealingly with its spread outer portion  142  against inner side  80  of stationary ring  70 , thus preventing pumped fluid from flowing directly back to the suction side. 
         [0043]    b) It prevents pump ring  62  from rotating relative to stationary ring  70 , so that pump chamber  120  (between stationary ring  70  and pump ring  62 ) is sealed and no fluid can escape from it. 
         [0044]    As shown, for example, by  FIG. 8 , pump ring  62  has lateral extensions or flanges  142 ,  144  that extend along flanks  146 ,  148  of holding part  78  and are pressed by pressure plates  151 ,  152  against said flanks, so that pump chamber  120  is held (immobilized) and sealed against holding part  78  (see  FIG. 8 ). At the transition from edge  80  to flanks  146 ,  148 , holding portion  78  has a respective bead-like enlargement  145 ,  145 ′ that further improves sealing there. 
         [0045]    Pressure plates  146 ,  148  are pressed toward one another by bolts  150 , one of which is depicted in  FIG. 6 . Pump chamber  120 , which in an embodiment has a maximum height of less than a millimeter, is thus in communication with the outside world only through connectors  122 ,  124 , and is otherwise hermetically sealed. 
         [0046]      FIGS. 10A to 10J  serve to explain the mode of operation. The reference characters are the same as in  FIGS. 1 to 9 , except that ring  60 , on which pump ring  62  is mounted, is not depicted separately. 
         [0047]    For illustration, a position pointer  170  is shown in each Figure, indicating the position of the maximum of eccentric bushing  52  in the context of a clockwise rotation, as follows:
     FIG. 10A  12 o&#39;clock     FIG. 10B  1:30     FIG. 10C  3:00     FIG. 10D  4:30     FIG. 10E  6:00   FIG.  1 OF 7:30     FIG. 10G  9:00     FIG. 10H  10:30     FIG. 10J  12:00     FIGS. 10A and 10J  are consequently identical.   
 
         [0058]    Eccentric bearing  56  thus causes pump ring  62  to be compressed, continuously in a circumferential direction and successively at the locations (for example) 12:00 ( FIG. 10A ), 1:30 ( FIG. 10B ), 3:00 ( FIG. 10C ), etc., sufficiently strongly that pump chamber  120  no longer allows passage there, and the fluid in pump chamber  120  is consequently transported forward (in a clockwise direction) and is pumped outward through connector  122 . At the same time, new fluid is drawn in through connector  124 . 
         [0059]    In the context of a counterclockwise rotation, connector  122  becomes the suction connector and connector  124  becomes the discharge connector; this is not depicted, since it corresponds simply to a mirror image of  FIGS. 10A to 10J . 
         [0060]    Metering system  30  described above is very maintainable, since pump  53  can easily be replaced. Many variants and modifications are, of course, possible in the context of the present invention.