Patent Publication Number: US-8123459-B2

Title: Impeller, fuel pump having the impeller, and fuel supply unit having the fuel pump

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and incorporates herein by reference Japanese Patent Applications No. 2007-227717 filed on Sep. 3, 2007 and No. 2008-168445 filed on Jun. 27, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to an impeller for a fuel pump. The present invention further relates to a fuel pump having the impeller. The present invention further relates to a fuel supply unit having the fuel pump. 
     BACKGROUND OF THE INVENTION 
     A turbine-type fuel pump known in the past is mounted in a fuel pump of a vehicle so as to feed fuel under pressure into a vehicle engine. 
     Such a type of fuel pump is mounted within a sub-tank provided on a bottom of a fuel tank. In the present structure, even when a vehicle turns or goes up a slope, and a liquid level of fuel in a fuel tank tilts, or even when the liquid level of fuel in the fuel tank is reduced by the fuel consumption, fuel is securely drawn or discharged. The sub-tank is a fuel container that is filled with fuel from a fuel tank, so that the fuel container can store fuel at a liquid level independent of a liquid level in the fuel tank. 
     As a structure for filling the sub-tank with fuel, for example, U.S. Pat. No. 5,596,970 discloses pump chambers of a fuel pump. The pump chambers of a fuel pump are coaxially formed in two rows. In the present structure, an outer pump chamber provided at an outer side is used for feeding fuel under pressure into a vehicle engine, and an inner pump chamber provided at an inner side is used for filling the sub-tank with fuel. Furthermore, JP-A-2007-132196 discloses enhancement of pump efficiency of a fuel pump by specifying a backward tilt angle or a forward tilt angle of a rear surface located at a rear side in a rotation direction of a vane groove of an impeller. The backward tilt angle of the rear surface is defined between a line, which connects a radially inner end of the rear surface with a radially outer end of the rear surface, and a line extending in a radial direction from the radially inner end. The forward tilt angle of the rear surface is defined between a line, which connects a center in a rotation axis direction of the rear surface with one of ends in the rotation axis direction of the rear surface, and a line extending in a rotational tangent direction from the center in the rotation axis direction of the rear surface. 
     As in U.S. Pat. No. 5,596,970, when pump chambers are coaxially formed in two rows, and an inner pump chamber is used for filling the sub-tank with fuel, circumferential speed of an impeller decreases in the inner pump chamber compared with in the outer pump chamber. Therefore, suction negative-pressure is reduced in the inner pump chamber compared with in the outer pump chamber. 
     Therefore, for example, when residual quantity of fuel in a fuel tank decreases, so that a liquid level of fuel in the fuel tank is reduced compared with a pump mounting position, and finally fuel runs out of the inner pump chamber, suction negative-pressure in the inner pump chamber becomes extremely low. Consequently, fuel cannot be drawn up from the fuel tank into the inner pump chamber. Even when fuel can be drawn up into the inner pump chamber at low suction negative-pressure, unless gas (air) is exhausted from the inner pump chamber to produce a pump effect, the fuel cannot be pumped up into the sub-tank. 
     In order to solve the present problem, a vane groove configuration disclosed in JP-A-2007-132196 may be applied as a vane groove configuration of the impeller for the inner pump chamber in U.S. Pat. No. 5,596,970 so as to enhance pump efficiency. However, in the present combination, fuel to be pumped up into the sub-tank is rather excessively boosted in pressure. Such excessive boost in pressure leads to increase in drive torque of a fuel pump, causing increase in current consumption. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing and other problems, it is an object of the present invention to produce a fuel pump impeller configured to steadily pump fuel with low torque. It is another object of the present invention to produce a fuel pump having the impeller and configured to steadily pump fuel with low torque. It is another object of the present invention to produce a fuel supply unit having the fuel pump and configured to steadily pump fuel with low torque. 
     According to one aspect of the present invention, an impeller for a fuel pump having an outer pump chamber and an inner pump chamber being substantially coaxial with each other, the impeller comprises a plurality of partition walls provided at least in a region corresponding to the inner pump chamber and arranged in the rotative direction, each of the plurality of partition walls partitioning inner vane grooves, which are adjacent to each other. A rear surface is located at a rear side in a rotative direction of each of the inner vane grooves. At least a radially inner side of the rear surface inclines rearward in the rotative direction from a radially inner side to a radially outer side. A first line connects a radially inner end of the rear surface with a radially outer end of the rear surface. A second line extends in a radial direction from the radially inner end of the rear surface. The first line and the second line therebetween define a backward tilt angle α 2 . The backward tilt angle α 2  satisfies a relationship of 30°≦α 2 ≦80°. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a cross sectional view showing a fuel supply unit of a first embodiment; 
         FIG. 2  is an enlarged cross sectional view showing the periphery of a pump portion of a fuel pump of the fuel supply unit of the first embodiment; 
         FIG. 3A  shows a general front view of an impeller in the first embodiment, and  FIG. 3B  shows an enlarged view of  FIG. 3A ; 
         FIG. 4  is an oblique cross sectional view showing the pump portion of the fuel pump of the first embodiment; 
         FIG. 5  is an enlarged view of an outer vane groove in the impeller of the first embodiment; 
         FIG. 6  is an enlarged view of an inner vane groove of the impeller in the first embodiment; 
         FIG. 7  is a graph showing a relationship between a backward tilt angle α 2  and suction negative-pressure; 
         FIG. 8A  shows a general front view of an impeller in a second embodiment, and  FIG. 8B  shows an enlarged view of  FIG. 8A ; 
         FIG. 9  is a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B ; 
         FIG. 10  is a graph showing a relationship between an inclination angle β and a pumping flow rate; 
         FIG. 11  is a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B  in a third embodiment; 
         FIG. 12  is a graph showing a relationship between a forward tilt angle γ and pump efficiency; 
         FIG. 13  is an enlarged view of an inner vane groove of an impeller in a fourth embodiment; 
         FIG. 14  is an enlarged view of an inner vane groove of an impeller in a fifth embodiment; 
         FIG. 15  is an enlarged view of an inner vane groove of an impeller in a sixth embodiment; 
         FIG. 16  is an enlarged view of an inner vane groove of an impeller in a seventh embodiment; 
         FIG. 17  is a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B  in an eighth embodiment; 
         FIG. 18  is a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B  in a ninth embodiment; 
         FIG. 19  is a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B  in a tenth embodiment; 
         FIG. 20  is an enlarged view of an impeller of an eleventh embodiment; 
         FIG. 21  is a view seen in an arrow XXI direction in  FIG. 20 ; 
         FIG. 22  is a cross sectional view taken along the line XXII-XXII of  FIG. 20 ; 
         FIG. 23  is a cross sectional view taken along the line XXIV-XXIV of  FIG. 20 ; 
         FIG. 24  is a cross sectional view taken along the line XXIII-XXIII of  FIG. 20 ; and 
         FIG. 25  is a graph showing a relationship between an inclination angle β 2  and a pumping flow rate. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     A fuel supply unit  1  for a vehicle of the present embodiment is described according to  FIGS. 1 to 7 . 
     As shown in  FIG. 1 , the fuel supply unit  1  is accommodated in a fuel tank  10  to supply fuel from the fuel tank  10  into a fuel consumption unit outside the fuel tank  10 . In the present embodiment, the fuel consumption unit is, for example, a vehicle engine. The fuel supply unit  1  has a sub-tank  20 , which is provided on a bottom of the fuel tank  10 , and a fuel pump  30 , which is accommodated in the sub-tank  20 . 
     The fuel tank  10  is for storing fuel. In the present embodiment, the fuel is, for example, gasoline. The subtank  20  is a fuel container that is provided on the bottom of the fuel tank  10  so that the sub-tank  20  can store fuel at a liquid level, independent of a liquid level of fuel in the fuel tank  10 . 
     Specifically, the sub-tank  20  is formed of resin in a bottomed, cylindrical or box-like shape. In the present embodiment, the sub-tank  20  is in a cylindrical shape. A through hole  22  is provided in a bottom (sub-tank bottom)  21  of the sub-tank  20 , and the inside of the fuel tank  10  communicates with the inside of the sub-tank  20  via the through hole  22 . 
     A gap space  23  is formed between the sub-tank bottom  21  and the bottom of the fuel tank  10 . The gap space  23  is formed in a size that enables accommodation of a suction filter  90 , which filtrates fuel flowing into the fuel pump  30  to remove a foreign substance, and the gap space communicates with the inside of the fuel tank  10 . 
     The through hole  22  is inserted with an inner suction tube  58  that communicates with an inner pump chamber  50   b  of the fuel pump  30  described later. The inner suction tube  58  extends into the gap space  23  and is connected to the suction filter  90 . 
     A check valve  58   a  is provided within the inner suction tube  58 , which allows fuel to flow substantially only from a gap space  23  to an inner pump chamber  50   b . The check valve  58   a  restricts backflow of fuel from the sub-tank  20  into the fuel tank  10  via the inner pump chamber  50   b  and the inner suction tube  58 . 
     A suction filter  91  is also provided on an upper surface of the sub-tank bottom  21  in the sub-tank  20  for filtrating fuel flowing into the fuel pump  30  to remove a foreign substance. The suction filter  91  is connected to an outer suction tube  59  that communicates with an outer pump chamber  50   a  of the fuel pump  30  described later. 
     The fuel pump  30  is configured to have a motor portion  40 , a pump portion  50 , a resin cover end  70 , and the like. The motor portion  40  is supplied with electric power for rotation. The pump portion  50  is supplied with rotational drive force from the motor portion  40  for drawing and discharging fuel. The resin cover end  70  forms a discharge passage for guiding fuel discharged from the pump portion  50  from the inside of the fuel pump  30  to the outside of the fuel tank  10 . 
     First, the motor portion  40  is a known DC electromotive motor with brushes. Specifically, the motor portion is in a configuration where an armature  43  is rotatably provided at the radially inner side of permanent magnets  42 , which are provided annually along an inner circumferential surface of a cylindrical housing  41 . Further, a coil (not shown) of the armature  43  is applied with an electric current whereby the armature  43  itself rotates. A brushless motor may be used for the motor portion  40 . 
     The coil of the armature  43  is supplied with electric power from an external power supply via a terminal of a connector portion  72  provided on the cover end  70 , brushes provided in the cover end  70 , and a commutator provided in the armature  43  (any of them is not shown). The cover end  70  is fixed to one end side of the housing  41  by being caulked or the like. More specifically, the cover end  70  is fixed to an upper end side of the housing  41  in a mounting condition as shown in  FIG. 1 . 
     A rotational shaft  44  of the armature  43  is supported by a bearing provided in the center of both the cover end  70  and the pump portion  50 . Furthermore, an end of the rotational shaft  44  at the side of the pump portion  50  of the rotational shaft  44  is connected to an impeller  51  of the pump portion  50 . 
     In the present structure, when the motor portion  40  is applied with an electric current to rotate the armature  43 , the impeller  51  rotates together with the armature  43 , so that the pump portion  50  conducts a pump operation. Fuel, which has flowed from the pump portion  50  into a fuel chamber  45  in the housing  41  by the pump operation of the pump portion  50 , flows out to the outside of the fuel tank  10  through a discharge passage formed in a cylindrical discharge port  71  of the cover end  70 . 
     The pump portion  50  is configured to have the impeller  51 , a pump chamber casing  52 , and a pump chamber cover  53 . More specifically, the impeller  51  is rotatably accommodated about the rotational shaft  44  within a casing formed by the pump chamber casing  52  and the pump chamber cover  53 . 
     The impeller  51  is described in detail according to  FIGS. 3 to 6 .  FIG. 3A  shows a general front view of the impeller  51  seen in a rotation axis direction.  FIG. 3B  shows an enlarged view of the periphery of the impeller  51  of  FIG. 3A .  FIG. 4  shows an oblique cross sectional view in a condition that the impeller  51  is accommodated in the casing. 
     The impeller  51  is a disk-shaped member formed of resin. As shown in  FIGS. 3A ,  3 B, the impeller  51  has multiple outer vane grooves  54  and inner vane grooves  55  formed thereon for transmitting momentum to fuel. The outer vane grooves  54  and the inner vane grooves  55  are coaxially provided in two rows in a rotative direction. 
     More specifically, a ring  51   a  is provided at an outermost circumference of the impeller  51 . The outer vane grooves  54  are provided at a radially inner side of the ring  51   a . The inner vane grooves  55  are provided at a radially inner side of the outer vane grooves  54 . 
     First, the outer vane grooves  54  are described. As shown in  FIGS. 3A ,  3 B, and  4 , the outer vane grooves  54  adjacent to each other in a rotative direction are partitioned by a V-shape partition wall  54   a . As shown in  FIG. 4 , the V-shape partition wall  54   a  inclines forward in the rotative direction from approximately the center in a rotation axis direction (thickness direction) of the impeller  51  to an end face  51   b  at both sides in the rotation axis direction of the impeller  51 . That is, the partition wall  54   a  is formed substantially in the V shape such that both the sides of the end face  51   b  inclines forward in the rotative direction in a cylindrical section around a rotation axis. 
     In each of the outer vane grooves  54 , a partition wall protrudes from a radially inner side of the outer vane groove  54  to a radially outer side thereof. The partition wall  54   b  partitions a part of the groove  54  at the radially inner side in the rotation axis direction. Therefore, in a radially outer side of the partition wall  54   b  of the outer vane groove  54 , both spaces defined by the end faces  51   b  of the impeller  51  communicate with each other. 
     Furthermore, as shown in the enlarged view of the outer vane groove  54  of  FIG. 5 , in a rear surface  54   c  located at a rear side in the rotative direction of the outer vane groove  54 , at least a radially inner side inclines rearward in the rotative direction from the radially inner side to the radially outer side. That is, in a surface located at a front side in the rotative direction of the partition wall  54   a , at least the radially inner side inclines rearward in the rotative direction from the radially inner side to the radially outer side. 
     A backward tilt angle α 1  is defined between a line  101  and a line  102 . The line  101  connects a radially inner end  54   d  of the rear surface  54   c  to a radially outer end  54   e  thereof in a plane perpendicular to the rotation axis. The line  102  extends in a radial direction of the impeller  51  from the radially inner end  54   d . The backward tilt angle α 1  is approximately in a range of 15°≦α 1 ≦30°. 
     Next, the inner vane grooves  55  are described. A configuration of the inner vane grooves  55  is basically the same as that of the outer vane grooves  54 . Specifically, the inner vane grooves  55  adjacent to each other in the rotative direction are partitioned by a V-shape partition wall  55   a  that inclines forward in the rotative direction. A part of each inner vane groove  55  at the radially inner side is partitioned by a partition wall  55   b.    
     Furthermore, as shown in the enlarged view of the inner vane groove  55  of  FIG. 6 , in a rear surface  55   c  located at a rear side in the rotative direction of the inner vane groove  55 , at least a radially inner side inclines rearward in the rotative direction from the radially inner side to a radially outer side. That is, in a surface located at a front side in the rotative direction of the partition wall  55   a , at least the radially inner side inclines rearward in the rotative direction from the radially inner side to the radially outer side. 
     A backward tilt angle α 2  is defined between a line (first line)  103  and a line (second line)  104 . The line  103  connects a radially inner end  55   d  of the rear surface  55   c  with a radially outer end  55   e  thereof in a plane perpendicular to the rotation axis. The line  104  extends in a radial direction of the impeller  51  from a radially inner end  55   d . The backward tilt angle α 2  is approximately in a range of 30°≦α 2 ≦80°. 
     Referring to  FIG. 3 , a D-shape hole  51   c  is formed at a radially inner side of each inner vane groove  55  of the impeller  51 . The D-shape hole  51   c  penetrates through both end faces  51   b  of the impeller  51 . The D-shape hole  51   c  is fitted with a substantially D-shaped portion of the rotational shaft  44  of the motor portion  40 . 
     As shown in  FIG. 2 , a pump chamber casing  52  and a pump chamber cover  53  are formed of metal typified by aluminum (for example, aluminum dye cast), or a resin material having excellent fuel resistance and high strength. First, the pump chamber casing  52  is formed substantially in a cylindrical shape for accommodating the impeller  51 . A concave portion  52   a  is formed within the pump chamber casing  52 . 
     The concave portion  52   a  has a depth in the rotation axis direction, and the depth is deeper by about 5 μm to 50 μm than a thickness of the impeller  51 . In the present structure, a dimension in the rotation axis direction of the casing formed by the pump chamber casing  52  and the pump chamber cover  53  and a dimension in the rotation axis direction of the impeller  51  are set to therebetween define a predetermined gap. 
     Furthermore, an outer pump channel  52   b  and an inner pump channel  52   c  are arcuately formed substantially in a surface of the concave portion  52   a  over a predetermined angle range, the surface facing the impeller  51 . The channels allow passage of fuel in accordance with a rotation of the impeller  51 . 
     The outer pump channel  52   b  and the inner pump channel  52   c  are formed at positions respectively corresponding to arrays of the outer vane grooves  54  and the inner vane grooves  55  of the impeller  51 . A discharge port  52   d  for a fuel chamber is provided at a trailing end in the rotative direction of the outer pump channel  52   b  of the pump chamber casing  52 . The discharge port  52   d  communicates with the fuel chamber  45  in the housing  41 . 
     On the other hand, the pump chamber cover  53  is formed approximately in a disk shape, and fixed by being caulked or the like together with the pump chamber casing  52 . The pump chamber cover  53  is provided at a lower end side in the mounting condition shown in  FIG. 1  and located at the side opposite to a side where the cover end  70  of the housing  41  is mounted. The pump chamber cover  53  is positioned at a predetermined location with respect to the pump chamber casing  52 . 
     In a surface facing the impeller  51  of the pump chamber cover  53 , as shown in  FIG. 2 , an outer pump channel  53   b  and an inner pump channel  53   c  are also arcuately formed over a predetermined angle range. In the present structure, the channels allow passage of fuel in accordance with rotation of the impeller  51 . The outer pump channel  53   b  and the inner pump channel  53   c  are also formed respectively at positions corresponding to arrays of the outer vane grooves  54  and the inner vane grooves  55  of the impeller  51 . 
     In the pump chamber cover  53 , the outer suction tube  59  and the inner suction tube  58  are integrally formed. In addition, a leading end of the outer pump channel  53   b  in the rotative direction of the impeller  51  communicates with a suction passage in the outer suction tube  59 , and a leading end in the rotative direction of the inner pump channel  53   c  communicates with a suction passage in the inner suction tube  58 . Furthermore, a discharge port for sub-tank  53   d  communicating with the sub-tank  20  is provided at a trailing end in the rotative direction of the inner pump channel  53   c.    
     In the present structure, an outer pump chamber  50   a  is formed by the outer pump channel  52   b  of the pump chamber casing  52 , outer vane grooves  54  of the impeller  51 , and outer pump channel  53   b  of the pump chamber cover  53 . Moreover, an inner pump chamber  50   b  is formed by the inner pump channel  52   c  of the pump chamber casing  52 , inner vane grooves  55  of the impeller  51 , and inner pump channel  53   c  of the pump chamber cover  53 . 
     Furthermore, in the present embodiment, similarly to the described U.S. Pat. No. 5,596,970, the inner pump chamber  50   b  is used for filling the sub-tank  20  with fuel supplied from the fuel tank  10 , and the outer pump chamber  50   a  is used for feeding fuel under pressure from the sub-tank  20  into the fuel consumption unit. 
     Next, description is made on an operation of the fuel supply unit of the present embodiment having the above configuration. When a not-shown vehicle start switch is turned on, so that electric power is supplied from the battery to the fuel pump  30  via the connector  72 , the armature  43  of the motor portion  40  rotates. Then, the impeller  51  rotates together with the rotational shaft  44  of the armature  43 . 
     When the impeller  51  rotates, and thus the inner pump chamber  50   b  conducts a pump operation, fuel in the fuel tank  10  sequentially flows through the gap space  23 , the suction filter  90 , the inner suction tube  58 , the inner pump chamber  50   b , and the discharge port  53   d  for the sub-tank  20 , and finally fills the sub-tank  20 . 
     Furthermore, when the outer pump chamber  50   a  conducts a pump operation, fuel in the sub-tank  20  sequentially flows through the suction filter the outer suction tube  59 , the outer pump chamber  50   a , and the discharge port  52   d  for the fuel chamber  45 , and finally is discharged into the fuel chamber  45 . The fuel discharged into the fuel chamber  45  passes through the periphery of the armature  43  while cooling the armature  43 , and is led out to the outside of the fuel tank  10  from the cylindrical discharge port  71 . 
     Here, a principle of the operation of the fuel pump  30  in the present embodiment is described. Since the principle of the operation of the outer pump chamber  50   a  is essentially the same as that of the inner pump chamber  50   b , only the principle of the operation of the outer pump chamber  50   a  is described according to  FIG. 4 . 
     Fuel drawn from the outer suction tube  59  into the outer pump chamber  50   a  flows through the outer pump channels  52   b  and  53   b  from a side of the outer suction tube  59  to a side of the discharge port  52   d  for the fuel chamber  45  in accordance with rotation of the impeller  51 . In such flow of fuel, fuel flows while being guided by the partition wall  54   b  to cause a swirl flow  300  where fuel rotates symmetrically between both sides in the rotation axis direction of the impeller  51 . 
     By producing the swirl flow  300 , fuel repeats flowing from the outer pump channels  52   b  and  53   b  into each outer vane groove  54  and flowing from each outer vane groove  54  into the outer pump channels  52   b  and  53   b . Whereby, momentum in the rotative direction is transmitted from the outer vane groove  54  to the fuel, so that the fuel is increased in pressure. 
     In the present embodiment, since the backward tilt angle α 1  of the outer vane groove  54  is set to be in the range about 15°≦α 1 ≦30° as described before, high pump efficiency can be produced by the outer pump chamber  50   a  as previously disclosed in U.S. Pat. No. 5,596,970. On the other hand, since the backward tilt angle α 2  of the inner vane groove  55  is set to be in the range of 30°≦α 2 ≦80°, suction negative-pressure required for pumping up fuel into the inner pump chamber  50   b  can be stably generated. 
     The present operation is described in a more detailed manner according to  FIG. 7 .  FIG. 7  is a graph showing a relationship between the backward tilt angle α 2  of the inner vane groove  55  and the suction negative-pressure. More specifically, the graph shows a result of measurement of suction negative-pressure in the case where the impeller  51  idled at 5000 rpm when the fuel liquid level  400  shown in  FIG. 1  is lower than a pump mounting position  401 , and gas (air) fills the inner pump chamber  50   b . The pump mounting position  401  corresponds to a lowermost surface position of the impeller  51 . 
     As indicated by  FIG. 7 , the backward tilt angle α 2  is set to be 30°≦α 2 , thereby stable suction negative-pressure required for pumping up fuel into the inner pump chamber  50   b  can be generated. On the other hand, when the angle α 2  is set to be α 2 ≦30°, the sub-tank  20  cannot be filled with fuel since suction negative-pressure is small, and fuel cannot be sufficiently drawn up. 
     When the angle α 2  is set to be 80°&lt;α 2 , the rear surface  55   c  of the inner vane groove  55  cannot be effectively formed since the rear surface  55   c  of each inner vane groove  55  inclines rearward in the rotative direction (radially inner side) compared with a tangent of an inscribed circle  402  formed by ends at inner diameter sides of the inner vane grooves shown in  FIG. 3B . Therefore, the backward tilt angle α 2  of the inner vane groove  55  is set to be 30°≦α 2 ≦80°, thereby even when fuel does not exist in the inner pump chamber  50   b , fuel can be pumped up from the fuel tank  10  into the sub-tank  20 . 
     Furthermore, the inner pump chamber  50   b  is provided at a radially inner side compared with the outer pump chamber  50   a . Therefore, in the outer pump chamber  50   a , circumferential speed of the impeller  51  is used to efficiently increase pressure of fuel so that fuel can be fed under pressure from the sub-tank  20  to the outside of the fuel tank  10 . In addition, in the inner pump chamber  50   b , unnecessary boost of fuel pressure can be restricted. 
     As a result, increase in drive torque is suppressed in the inner pump chamber  50   b , and consequently fuel can be pumped up from the fuel tank  10  into the sub-tank  20  at low torque. 
     Second Embodiment 
     In the first embodiment, a basic configuration of the inner vane grooves  55  is substantially the same as that of the outer vane groove  54 , and the outer and inner vane grooves  54 ,  55  respectively have the backward tilt angles α 1  and α 2  being different from each other. On the contrary, in the present embodiment, as shown in  FIG. 8A ,  8 B, description is made on an example where inner vane grooves  55   x  having a different configuration from that of the outer vane grooves  54  in the first embodiment are used. 
       FIG. 8A ,  8 B shows views respectively corresponding to  FIGS. 3A ,  3 B in the first embodiment, wherein  FIG. 8A  shows a general front view seen in the rotation axis direction of the impeller  51  in the present embodiment, and  FIG. 8B  shows an enlarged view of the periphery of the impeller  51  of  FIG. 8A . In  FIG. 8A ,  8 B, portions, which are substantially similar to or equal to those in the first embodiment, are denoted with the identical signs respectively. This is substantially the same in other embodiments described below. 
     As shown in  FIG. 8A ,  8 B, the partition wall  55   b  is not provided in each of the inner vane grooves  55   x  in the present embodiment. Therefore, the swirl flow  300  described in  FIG. 4  is hardly generated in an inner pump chamber  50   b  in the present embodiment compared with the structure in the first embodiment. Furthermore, a rear surface  55   cx  of the inner vane groove  55   x  inclines rearward in the rotative direction from one end side to the other end in the rotation axis direction. 
     More specifically, as shown in  FIG. 9 , the rear surface  55   cx  inclines rearward in the rotative direction from an end at a side of a pump chamber cover  53  to an end at a side of a pump chamber casing  52  in a cylindrical surface around a rotation axis.  FIG. 9  is a cylindrical sectional view around the rotation axis taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B . 
     On the cylindrical surface around the rotation axis, an inclination angle β is defined between a line (third line)  105  and a line (fourth line)  106 . The line  105  connects the end  55   fx  of the rear surface  55   cx  at the side of the pump chamber cover  53  with the end  55   gx  of the rear surface  55   cx  at the side of the pump chamber casing  52 . The line  106  extends from the end  55   fx  at the side of the pump chamber cover  53  in a direction of a tangent at a rear side in the rotative direction. The inclination angle β is in a range of 65°≦β&lt;90° in the whole area in a radial direction of the rear surface  55   cx.    
     In the present embodiment, the inclination angle β is set to be approximately the same in the whole area in the radial direction of the rear surface  55   cx . Alternatively, one inclination angle β on the cylindrical surface at the radially inner circumferential side may be different from another inclination angle β on the cylindrical surface at the radially outer circumferential side. For example, the inclination angle β may be gradually reduced from the inner circumferential side to the outer circumferential side. 
     Other configurations are substantially the same as those in the first embodiment. Therefore, when the fuel supply unit  1  of the present embodiment is started, the outer pump chamber  50   a  operates substantially in the same way as in the first embodiment. 
     Furthermore, in the present embodiment, the inclination angle β of the inner vane groove  55   x  is set to be 65°≦β&lt;90°. In the present structure, when the fuel surface  400  is lower than the pump mounting position  401 , and gas (air) fills the inner pump chamber  50   b , air can be exhausted from the inner pump chamber  50   b , so that the inner pump chamber  50   b  can produce a certain pump effect. 
     The present operation is described according to  FIG. 10 .  FIG. 10  is a graph showing a relationship between the inclination angle β of the inner vane groove  55   x  and a pumping flow rate of the inner pump chamber  50   b . A test condition is substantially the same as in the case of  FIG. 7 . As indicated from  FIG. 10 , the inclination angle β is set to be 65°≦β&lt;90°, thereby the pumping flow rate can be sufficiently secured. Thus, fuel can be sufficiently pumped up from the fuel tank  10  into the sub-tank  20 . On the other hand, when the angle β is set to be β&lt;65°, the flow rate of pumping into the inner pump chamber  50   b  is drastically reduced. 
     When the inclination angle β=90° is given, the rear surface  55   cx  is parallel to the rotation axis direction. In this case, the rear surface  55   cx  of the inner vane groove  55   x  does not incline rearward in the rotative direction from one end side to the other end in the rotation axis direction. Even in this case, as shown in  FIG. 10 , fuel can be pumped up from the fuel tank  10  into the sub-tank  20 . 
     According to the present embodiment, even when fuel does not exist in the inner pump chamber  50   b , fuel can be securely pumped up from the fuel tank  10  into the sub-tank  20  at low torque. 
     Third Embodiment 
     In the present embodiment, description is made on an example where a shape of a V-shape partition wall  55   a  of the inner vane groove  55  is specified, thereby high pump efficiency ηb can be produced by the inner pump chamber  50   b  compared with the first embodiment. 
     Specifically, as shown in  FIG. 11 , a forward tilt angle γ is defined between a line (ninth line)  107  and a line (tenth line)  108 . The line  107  connects a center  55   h  in the rotation axis direction of a rear surface  55   c  on a cylindrical surface around a rotation axis with one of ends  55   i  in the rotation axis direction of the rear surface  55   c . The line  108  extends in a direction of a tangent at the front side in the rotative direction from the center  55   h  in the rotation axis direction of a rear surface  55   c . The forward tilt angle γ is in a range of 70°≦γ&lt;90°.  FIG. 11  shows a cross sectional view corresponding to a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX of  FIG. 8B  in the present embodiment. 
     Other configurations are substantially the same as in the first embodiment. Therefore, when the fuel supply unit  1  of the present embodiment is started, the outer pump chamber  50   a  operates similarly in the same way as in the first embodiment. 
     Furthermore, in the present embodiment, since the forward tilt angle γ of the inner vane groove  55  is set to be 70°≦γ&lt;90°, even when the fuel surface  400  is lower than the pump mounting position  401 , and gas (air) fills the inner pump chamber  50   b , pump efficiency of the inner pump chamber  50   b  can be stably maintained high. 
     The present operation is described according to  FIG. 12 .  FIG. 12  is a graph showing a relationship between the forward tilt angle γ of the inner vane groove  55  and the pump efficiency ηb of the inner pump chamber  50   b . A test condition is substantially the same as in the case of  FIG. 7 . As indicated from  FIG. 12 , the forward tilt angle γ is set to be in a range of 70°≦γ&lt;90°, thereby the pump efficiency can be stably maintained high. 
     The present effect is produced because the forward tilt angle γ is set to be 70°≦γ&lt;90°, thereby fuel can be transported without generating an excessive swirl flow in the inner pump channels  52   c  and  53   c  of the inner pump chamber  50   b  in which fuel need not be excessively increased in pressure. On the other hand, when the angle γ is set to be γ&lt;70°, an excessive swirl flow is induced, leading to drastic reduction in pump efficiency. 
     The pump efficiency ηb of the inner pump chamber  50   b  is given by the following expression F1.
 
η b =( P*Q )/( Tb*R )  (F1)
 
     P denotes discharge pressure of the inner pump chamber  50   b , Q denotes the pumping flow rate of the inner pump chamber  50   b , Tb denotes drive torque of the inner pump chamber  50   b , and R denotes the number of rotations of the motor portion  40 . When the forward tilt angle γ is 90°, while the partition wall  55   a  of the inner vane groove  55  is not in a V shape, high pump efficiency can be produced as shown in  FIG. 12 . 
     As described above, according to the present embodiment, even when fuel does not exist in the inner pump chamber  50   b , fuel can be pumped up from the fuel tank  10  into the sub-tank  20  at low torque while the pump efficiency ηb is stably maintained high. 
     Fourth to Seventh Embodiments 
     Fourth to seventh embodiments are modifications of the first to third embodiments respectively. That is, the backward tilt angle α 2  between the line  103 , which connects the radially inner end  55   d  of the rear surface  55   c  with the radially outer end  55   e  of the rear surface  55   c , and the line  104 , which extends in the radial direction of the impeller  51  from a radially inner end  55   d , is set in the range of 30°≦α 2 ≦80°, similarly to the embodiments. In the present embodiment, a configuration of a surface to be actually formed into the rear surface  55   c  is modified. 
     Specifically, in the fourth embodiment, as shown in  FIG. 13 , the inner vane groove  55  is shaped to be R-chamfered at a corner of a peripheral configuration. 
     In the fifth embodiment, as shown in  FIG. 14 , a peripheral configuration of the inner vane groove  55  is formed linearly at a radially inner side, and formed arcuately at a radially outer side. 
     In the sixth embodiment, as shown in  FIG. 15 , a peripheral configuration of the inner vane groove  55  is formed arcuately at a radially inner side, and formed linearly at a radially outer side. 
     Furthermore, in the seventh embodiment, as shown in  FIG. 16 , a peripheral configuration of the inner vane groove  55  is formed linearly. 
       FIGS. 13 to 16  are enlarged views showing the inner vane groove  55  in the fourth to seventh embodiments respectively, and the inner vane groove  55  in each embodiment corresponds to the inner vane groove  55  in  FIG. 6 . In each of the fifth to seventh embodiments, as shown in  FIGS. 14 to 16 , the radially inner end  55   d  corresponds to an intersection between a circular arc, which is formed by inner diameter side ends of the inner vane grooves  55 , and an extension of a linear portion of the rear surface  55   c . Further, the radially outer end  55   e  corresponds to an intersection between a circular arc formed by outer diameter side ends of the inner vane grooves  55  and the extension of a linear portion of the rear surface  55   c.    
     As shown in  FIGS. 13 to 16 , even when the peripheral configuration of the inner vane groove  55  is modified, the backward tilt angle α 2  is set to be the range of 30°≦α 2 ≦80°, thereby the same advantages as in the first to third embodiments can be obtained. 
     Eighth to Tenth Embodiments 
     Each of eighth to tenth embodiments are modifications of the second embodiment. That is, on the cylindrical surface around the rotation axis, an inclination angle β is defined between a line  105  and the line  106 . The line  105  connects the end  55   fx  of the rear surface  55   cx  at the side of the pump chamber cover  53  with the end  55   gx  of the rear surface  55   cx  at the side of the pump chamber casing  52 . The line  106  extends in the direction of the tangent at the rear side in the rotative direction from the end  55   fx  at the side of the pump chamber cover  53 . The inclination angle β is in a range of 65°≦β≦90°. In the present embodiment, a configuration of a surface to be actually formed into the rear surface  55   cx  is modified. 
     Specifically, in the eighth embodiment, as shown in  FIG. 17 , an outer circumferential end of the rear surface  55   cx  is formed by multiple straight lines. In the ninth embodiment, as shown in  FIG. 18 , the pump chamber cover  53  side of the rear surface  55   cx  is formed by a curved line. Furthermore, in the tenth embodiment, as shown in  FIG. 19 , substantially only the rear surface  55   cx  is inclined. Each of  FIGS. 17 to 19  shows a cross sectional view corresponding to a cross sectional view taken along the line IX, XI, XVII, XVIII, XIX-IX, XI, XVII, XVIII, XIX in  FIG. 8B  in each of the present embodiments. 
     As shown in  FIGS. 17 to 19 , even when the configuration of the outer circumferential end of the rear surface  55   cx  is modified, the inclination angle β is set to be 65°≦β&lt;90°, thereby the same advantage as in the second embodiment can be obtained. 
     Eleventh Embodiment 
     The present embodiment includes modifications of the second embodiment. In the second embodiment, description was made on the example where the inclination angle β of the rear surface  55   cx  of the inner vane groove  55   x  was approximately the same in the whole area in the radial direction. On the contrary, in the present embodiment, as shown in  FIGS. 22 to 24 , description is made on an example where an inclination angle β 1  at a radially inner circumferential side of the rear surface  55   cx  is made different from an inclination angle β 2  at a radially outer circumferential side of the rear surface  55   cx.    
       FIG. 20  is an enlarged view of the periphery of the impeller  51  in the present embodiment, which corresponds to  FIG. 8B .  FIG. 21  shows a view seen in an arrow XXI direction of  FIG. 20 , that is, a view of the rear surface  55   cx  seen in the rotative direction.  FIGS. 22 ,  23 , and  24  respectively show a cylindrical cross sectional view taken along the line XXII-XXII of  FIG. 20 , a cylindrical cross sectional view taken along the line XXIII-XXIII of  FIG. 20 , and a cylindrical cross sectional view taken along the line XXIV-XXIV of  FIG. 20 , the cylindrical cross sectional views being around the rotation axis. 
     In the present embodiment, the rear surface  55   cx  is formed by multiple surfaces intersecting with each other. Specifically, the rear surface  55   cx  is formed by two surfaces of an inner area surface  551  and an outer area surface  552 . As shown in  FIG. 21 , the inner area surface  551  intersects with the outer area surface  552  at a bending portion  55   j  extending obliquely with respect to a radial direction. 
     Furthermore, the inner area surface  551  is formed by a plane parallel to the rotation axis direction. Therefore, as shown in  FIG. 22 , an inclination angle β 1 , which is defined between a line (fifth line)  105   a  and a line (sixth line)  106   a , is given to be β 1 =90°. Here, the line  105   a  connects an end  551   f , which is at one end side in the axial direction, with an end  551   g , which is at the other end side in the axial direction, at a radially innermost circumferential side of the rear surface  55   cx . The line  106   a  extends in a direction of a tangent at a rear side in the rotative direction from the end  551   f  at the one end side in the axial direction. 
     On the other hand, the outer area surface  552  is formed by a plane inclining to a rear side in the rotative direction from the bending portion  55   j . Furthermore, as shown in  FIG. 23 , an inclination angle β 2 , which is defined between a line (seventh line)  105   b  and a line (eighth line)  106   b , is given to be 55°≦β 2 &lt;90°. The line  105   b  connects an end  552   f , which is at one end side in the axial direction, with an end  552   g , which is at the other end side in the axial direction, at a radially innermost circumferential side of the rear surface  55   cx . The line  106   b  extends in a direction of a tangent at a rear side in the rotative direction from the end  552   f  at the one end side in the axial direction. 
     In the present structure, the inner area surface  551  and the outer area surface  552  obliquely intersect with each other at the bending portion  55   j , as shown in  FIG. 24 . An inclination angle β 3 , which is defined between a line  105   c  and a line  106   c , is also given to be 55°≦β 3 &lt;90°. The line  105   c  connects an end  553   f , which is at one end side in the axial direction, with an end  553   g , which is at the other end side in the axial direction, at a radially outer side from an approximately central portion in a radial direction of the rear surface  55   cx . The line  106   c  extends in a direction of a tangent at a rear side in the rotative direction from the end  553   f  at the one end side in the axial direction. 
     Other configurations are substantially the same as in the second embodiment. As in the present embodiment, the inclination angle β 1  and the inclination angle β 2  of the rear surface  55   cx  are respectively modified. Even in the present structure, the inclination angle β 2  is set to be 55°≦β 2 &lt;90°, thereby the similar advantage to in the second embodiment can be obtained. 
     The present operation is described according to  FIG. 25 .  FIG. 25  is a graph showing a relationship between the inclination angle β 2  of the inner vane groove  55   x  and a pumping flow rate of the inner pump chamber  50   b . A test condition is substantially the same as in the case of  FIG. 7 . As indicated from  FIG. 25 , the inclination angle β 2  is set to be 55°≦β 2 &lt;90°, thereby fuel can be sufficiently pumped up from the fuel tank  10  into the sub-tank  20 . On the other hand, when the angle β 2  is set to be β 2 &lt;55°, the pumping flow rate into the inner pump chamber  50   b  is drastically reduced. 
     Therefore, according to the present embodiment, even when fuel does not exist in the inner pump chamber  50   b , fuel can be securely pumped up from the fuel tank  10  into the sub-tank  20  at low torque. Furthermore, the inclination angle β 2  can be set throughout a wide range compared with the inclination angle β in the second embodiment and the eighth to tenth embodiments, and consequently the degree of design freedom can be enhanced. 
     In the above description, each of the inner area surface  551  and the outer area surface  552  is formed by a plane in the present embodiment. Alternatively, at least one of the inner area surface  551  and the outer area surface  552  may be formed by a curved surface. Furthermore, substantially only the outer area surface  552  may be formed by a curved surface so that the inner area surface  551  and the outer area surface  552  smoothly intersect with each other. 
     Other Embodiments 
     In the embodiments, the partition wall  55   b  is provided in the inner vane groove  55  in the first and third embodiments. Alternatively, the partition wall  55   b  may not be provided as in the second embodiment. 
     In the first to third embodiments, the outer pump chamber  50   a  is used for feeding fuel under pressure from the sub-tank  20  to the outside of the fuel tank  10 , and the inner pump chamber  50   b  is used for filling the sub-tank  20  with fuel from the fuel tank  10 . Alternatively, when the outer pump chamber  50   a  is used for filling the sub-tank with fuel, and the inner pump chamber  50   b  is used for feeding fuel under pressure, it suffices that a shape is reversed between the outer vane groove  54  and the inner vane groove  55 . 
     The above structures of the embodiments can be combined as appropriate. 
     It should be appreciated that while the processes of the embodiments of the present invention have been described herein as including a specific sequence of steps, further alternative embodiments including various other sequences of these steps and/or additional steps not disclosed herein are intended to be within the steps of the present invention. 
     Various modifications and alternations may be diversely made to the above embodiments without departing from the spirit of the present invention.