Patent Publication Number: US-10767645-B2

Title: Fuel pump

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of International Application No. PCT/JP2016/002088 filed Apr. 19, 2016, which designated the U.S. and claims priority to Japanese Patent Application No. 2015-142167 filed on Jul. 16, 2015, the entire contents of each of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a fuel pump that draws fuel into a gear housing chamber and discharges the fuel. 
     BACKGROUND ART 
     Patent Literature 1 discloses a pump that draws fuel into a gear housing chamber and discharges the fuel. The pump includes: an outer gear having inner teeth; an inner gear having outer teeth and meshing with the outer gear in eccentric state; and a pump housing that defines a cylindrical gear housing chamber housing the outer gear and the inner gear to be rotatable from both sides in the axial direction. The outer gear and the inner gear rotate, while expanding and contracting a volume of a pump chamber formed plurally between the outer gear and the inner gear, to sequentially draw fluid into and discharge from each of the pump chambers. 
     The pump housing has a spiral-shaped groove formed from a radially-inside corner part opposing a radially-outside corner part of the outer gear toward a central part. 
     PRIOR ART LITERATURES 
     Patent Literature 
     Patent Literature 1: JP 2009-144689 A 
     SUMMARY OF INVENTION 
     However, a complicated processing is required for forming the spiral-shaped groove. Moreover, it is difficult to fully absorb a positional deviation of the outer gear which may be produced, for example, when fuel is discharged out of a pump chamber, and pulsation cannot fully be controlled. As a result, a fuel pump having a high pump efficiency cannot be offered. 
     The purpose of the present disclosure is to provide a fuel pump having high pump efficiency. 
     According to an aspect of the present disclosure, a fuel pump includes: an outer gear having a plurality of inner teeth; an inner gear having a plurality of outer teeth and eccentrically meshing with the outer gear; and a pump housing that defines a cylindrical gear housing chamber housing the outer gear and the inner gear to be rotatable, from both sides in an axial direction. The outer gear and the inner gear rotate, while expanding and contracting a volume of a plurality of pump chambers formed between the outer gear and the inner gear, to sequentially draw fuel into and discharge from each of the pump chambers. An inner circumference part of the pump housing has a radially-inside corner part opposing a radially-outside corner part of an outer circumference part of the outer gear. The pump housing has an annular groove formed in an annular shape all around the radially-inside corner part. 
     Accordingly, the pump housing defines the cylindrical gear housing chamber. The gear housing chamber houses both the gears to be rotatable by sandwiching the outer gear and the inner gear from both sides in the axial direction. When the outer gear and the inner gear rotate, fuel is sequentially drawn into the pump chamber between the gears and is discharged. A positional deviation such as inclination of the outer gear may occur, for example, at a time of the discharging. 
     In the present disclosure, the pump housing has the annular groove formed in the annular shape around all the circumferences of the radially-inside corner part opposing the radially-outside corner part of the outer gear. If a position deviation of the outer gear occurs in a state where fuel has flowed into the annular groove through a clearance between the gears and the pump housing, damper effect can be applied to the outer circumference part of the outer gear to resolve the positional deviation by the fuel in the annular groove. A pulsation caused by rotation of the outer gear and the inner gear can be eased by the annular groove, and the sliding resistance can be restricted because the outer gear and the inner gear rotate stably. Accordingly, a fuel pump with high pump efficiency can be offered. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial sectional view illustrating a fuel pump according to a first embodiment. 
         FIG. 2  is a cross-sectional view taken along a line II-II of  FIG. 1 . 
         FIG. 3  is a cross-sectional view taken along a line of  FIG. 1 . 
         FIG. 4  is a cross-sectional view taken along a line IV-IV of  FIG. 1 . 
         FIG. 5  is a cross-sectional view illustrating a pump casing of the first embodiment, which is taken along a line V-V of  FIG. 3 . 
         FIG. 6  is an enlarged view illustrating a part of  FIG. 5  with an outer gear. 
         FIG. 7  is a front view illustrating a joint component of the first embodiment. 
         FIG. 8  is a view of a second embodiment corresponding to  FIG. 6 . 
         FIG. 9  is a graph illustrating a comparison in flow rate in experiments between the fuel pump of the second embodiment and a fuel pump of a comparative example not having an annular groove. 
         FIG. 10  is a graph illustrating a comparison in current value in experiments between the fuel pump of the second embodiment and a fuel pump of a comparative example not having an annular groove. 
         FIG. 11  is a view of a first modification corresponding to  FIG. 6 . 
         FIG. 12  is a view of an example of a second modification corresponding to  FIG. 6 . 
         FIG. 13  is a view of another example of the second modification corresponding to  FIG. 6 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described hereafter referring to drawings. In the embodiments, a part that corresponds to a matter described in a preceding embodiment may be assigned with the same reference numeral, and redundant explanation for the part may be omitted. When only a part of a configuration is described in an embodiment, another preceding embodiment may be applied to the other parts of the configuration. The parts may be combined even if it is not explicitly described that the parts can be combined. The embodiments may be partially combined even if it is not explicitly described that the embodiments can be combined, provided there is no harm in the combination. 
     First Embodiment 
     A fuel pump  100  according to a first embodiment is a trochoid pump of positive displacement, as shown in  FIG. 1 . The fuel pump  100  is a diesel pump mounted in a vehicle, and is used for pumping light oil having viscosity higher than gasoline, for combustion in an internal-combustion engine. The fuel pump  100  includes an electric motor  80  and a pump main part  10  housed inside a cylindrical pump body  2 , and a side cover  5  is projected outward away from the pump main part  10  while the electric motor  80  is interposed between the side cover  5  and the pump main part  10  in the axial direction Da. In the fuel pump  100 , a rotation shaft  80   a  of the electric motor  80  is driven to rotate through an electric connector  5   a  of the side cover  5 . An outer gear  30  and an inner gear  20  rotate using the driving force of the rotation shaft  80   a  in the pump main part  10 . Light oil corresponding to fuel is drawn into a gear housing chamber  56  housing both the gears  20  and  30 , pressurized, and discharged out of the gear housing chamber  56  to flow through a fuel passage  6  and a discharge port  5   b  of the side cover  5 . 
     In this embodiment, an inner rotor type brushless motor is adopted as the electric motor  80 , in which a four-pole magnet and a six-slot coil are arranged. For example, when the ignition of a vehicle is turned on, or when the accelerator of a vehicle is pressed, a positioning control is performed by the electric motor  80  by rotating the rotation shaft  80   a  to a drive rotation side or a drive rotation reverse side. Then, a drive control is performed to rotate the rotation shaft  80   a  to the drive rotation side from the position positioned in the positioning control. 
     The drive rotation side represents a side corresponding to a forward direction of a rotational direction Rig to be mentioned later (see  FIG. 4 ). The drive rotation reverse side represents a side corresponding to a reverse direction of the rotational direction Rig (see  FIG. 4 ). 
     Hereafter, the pump main part  10  is explained in detail, also using  FIGS. 2-7 . The pump main part  10  includes a pump housing  11 , an inner gear  20 , a joint component  60 , and an outer gear  30 . 
     The pump housing  11  has a pump cover  12  and a pump casing  16  arranged in the axial direction Da to define a cylindrical gear housing chamber  56  housing both the gears  20  and  30  to be rotatable, from both sides in the axial direction Da. 
     The pump cover  12  shown in  FIGS. 1-2, and 4  is one component of the pump housing  11 . The pump cover  12  is formed in a disk shape having wear resistance by performing surface treatments, such as plating, to a base material made of metal which has rigidity, such as steel material. The pump cover  12  is projected outward from the end of the pump body  2  away from the electric motor  80  in the axial direction Da. 
     The pump cover  12  defines a cylindrical intake port  12   a  and an intake passage  13  having an arc groove shape, to draw fuel from the outside. The intake port  12   a  passes through the pump cover  12  in the axial direction Da, at a specific opening part Ss eccentrically arranged relative to an inner central line Cig of the inner gear  20 . The intake passage  13  is defined in the pump cover  12 , and faces the gear housing chamber  56 . As shown in  FIG. 2 , an inner periphery edge  13   a  of the intake passage  13  is extended in the rotational direction Rig of the inner gear  20  with a length less than the semicircle. An outer periphery edge  13   b  of the intake passage  13  is extended in the rotational direction Rog of the outer gear  30  (see  FIG. 4 ) with a length less than the semicircle. 
     The width of the intake passage  13  is increased as extending from a start end  13   c  to a finish end  13   d  in the rotational direction Rig, Rog. Moreover, the intake passage  13  communicates with the intake port  12   a , since the intake port  12   a  is defined at the opening part Ss of the slot bottom  13   e . As shown in  FIG. 2 , the width of the intake passage  13  is set smaller than the width of the intake port  12   a  throughout the opening part Ss where the intake port  12   a  is open. 
     The pump casing  16  shown in  FIGS. 1, and 3-6  is one component of the pump housing  11 . The pump casing  16  is formed in a based cylindrical shape having wear resistance by performing surface treatments, such as plating, to a base material made of metal which has rigidity, such as steel material. An opening  16   a  of the pump casing  16  is covered with the pump cover  12 , so as to be closed all the circumferences. An inner circumference part  22  of the pump casing  16  is formed in a cylindrical bore shape arranged eccentrically relative to the inner central line Cig. 
     The pump casing  16  defines a discharge passage  17  having an arc hole shape to discharge fuel from the gear housing chamber  56 . The discharge passage  17  passes through a concave bottom part  16   c  of the pump casing  16  in the axial direction Da. As shown in  FIG. 3 , an inner periphery edge  17   a  of the discharge passage  17  is extended in the rotational direction Rig of the inner gear  20  with a length less than the semicircle. An outer periphery edge  17   b  of the discharge passage  17  is extended in the rotational direction Rog of the outer gear  30  with a length less than the semicircle. The width of the discharge passage  17  is decreased as extending from a start end  17   c  to a finish end  17   d  in the rotational direction Rig, Rog. 
     The pump casing  16  has a reinforcing rib  16   d  at the discharge passage  17 . The reinforcing rib  16   d  is formed integrally with the pump casing  16 , and reinforces the pump casing  16  by extending over the discharge passage  17  in a direction intersecting the rotational direction Rig of the inner gear  20 . 
     As shown in  FIG. 3 , the concave bottom part  16   c  of the pump casing  16  has an intake groove  18  having an arc shape and opposing the intake passage  13  across a pump chamber  40  defined between the gears  20  and  30  (to be explained in detail) to correspond with the form of the intake passage  13  projected in the axial direction Da. Thereby, the discharge passage  17  and the intake groove  18  are formed symmetric with respect to a line symmetry in the outline at a side of the pump casing  16  adjacent to the gear housing chamber  56 . 
     A sliding surface part  16   e  of the concave bottom part  16   c  has a plane shape, and slides with the inner gear  20  which rotates at the inner circumference side, and slides with the outer gear  30  which rotates at the outer circumference side. 
     As shown in  FIG. 2 , the pump cover  12  has a discharge groove  14  having an arc shape at a position opposing the discharge passage  17  across the pump chamber  40  to correspond with the form of the discharge passage  17  projected in the axial direction Da. Thereby, the intake passage  13  and the discharge groove  14  are formed symmetric with respect to a line symmetry in the outline through the joint housing chamber  58  at a side of the pump cover  12  adjacent to the gear housing chamber  56 . 
     The joint housing chamber  58  is recessed in the axial direction Da from the sliding surface part  12   b  of the pump cover  12  at a position opposing the inner gear  20  on the inner central line Cig. In this way, the joint housing chamber  58  communicates with the gear housing chamber  56 , at one side of the gear housing chamber  56  in the axial direction Da, thereby housing rotatably the main body  62  of the joint component  60  to be mentioned later. 
     The sliding surface part  12   b  of the pump cover  12  has a plane shape adjacent to the gear housing chamber  56 , and slides with the inner gear  20  which rotates at the inner circumference side, and slides with the outer gear  30  which rotates at the outer circumference side. 
     As shown in  FIG. 1 , a radial bearing  50  is fixed by fitting with the concave bottom part  16   c  of the pump casing  16  on the inner central line Cig, and supports the rotation shaft  80   a  of the electric motor  80  in the radial direction, while the rotation shaft  80   a  passes through the concave bottom part  16   c . Further, a thrust bearing  52  is fixed by fitting with the pump cover  12  on the inner central line Cig, and supports the rotation shaft  80   a  in the axial direction Da. 
     Moreover, as shown in  FIGS. 2 and 5 , the pump casing  16  has a radially-inside corner part  70  at a location where the inner circumference part  22  and the sliding surface part  16   e  of the concave bottom part  16   c  are connected to each other in an annular shape. The pump casing  16  has an annular groove  72  at the radially-inside corner part  70 . That is, the annular groove  72  is formed at a side opposite from the joint housing chamber  58  through the gear housing chamber  56  in the axial direction Da. 
     Specifically, the annular groove  72  is formed in the annular shape all around the circumference. The annular groove  72  of this embodiment is recessed from the outermost circumference of the concave bottom part  16   c  in the axial direction Da away from the gear housing chamber  56 . As shown in  FIG. 6 , which is an enlarged view, a bottom  73  of the annular groove  72  is formed in an arc shape in the cross-section vertically along the radial direction of the pump casing  16 . The arc shape in this embodiment is an ellipse shape. 
     The annular groove  72  is formed to have a width dimension Wg and a depth dimension Dg which are set approximately uniform all around the circumference. As shown in  FIG. 5 , a width dimension Wg 1  of a portion open to the gear housing chamber  56  is larger than twice of the depth dimension Dg, and smaller than or equal to three times of the depth dimension Dg. 
     Each of the inner gear  20  and the outer gear  30  is a trochoid gear in which teeth are made to have trochoid curves. 
     Specifically, the inner gear  20  shown in  FIGS. 1 and 4  is arranged eccentrically in the gear housing chamber  56  by setting the inner central line Cig to be in common with the rotation shaft  80   a . Moreover, the thickness dimension of the inner gear  20  is formed slightly smaller than the corresponding dimension of the cylindrical gear housing chamber  56 . In this way, the inner circumference part  22  of the inner gear  20  is supported by the radial bearing  50  in the radial direction, and the both sides in the axial direction Da are respectively supported by the sliding surface part  16   e  of the pump casing  16  and the sliding surface part  12   b  of the pump cover  12 . 
     Moreover, the inner gear  20  has the insertion hole  26  recessed in the axial direction Da at a position opposing the joint housing chamber  58 . The insertion hole  26  is defined at plural positions in the circumference direction at equal intervals, and each of the insertion holes  26  passes through the inner gear to a position adjacent to the concave bottom part  16   c.    
     The joint component  60  shown in  FIGS. 1, 2, 4, and 7  is formed, for example, of synthetic resins, such as polyphenylene sulfide (PPS) resin, and rotates both the gears  20  and  30  by connecting the rotation shaft  80   a  to the inner gear  20 . The joint component  60  has the main body  62  and the insertion part  64 . The main body  62  is fitted with the rotation shaft  80   a  through the fitting hole  62   a  in the joint housing chamber  58 . The insertion part  64  is formed at plural locations corresponding to the insertion holes  26 . Specifically, the number of the insertion holes  26  or the insertion parts  64  of this embodiment is five which is a prime number by avoiding the number of poles and the number of slots of the electric motor  80  to reduce the influence of torque ripple of the electric motor  80 . Each of the insertion parts  64  is extended in the axial direction Da from a position on the outer circumference side of the fitting hole  62   a  of the main body  62 . 
     The insertion part  64  is inserted in the corresponding insertion hole  26  through a clearance. When the rotation shaft  80   a  rotates to the drive rotation side, the insertion part  64  pushes on the insertion hole  26 , thereby transmitting the driving force of the rotation shaft  80   a  to the inner gear  20  through the joint component  60 . That is, the inner gear  20  is rotatable in the rotational direction Rig about the inner central line Cig. 
     The outer circumference part  24  of the inner gear  20  has the outer teeth  24   a  arranged in the rotational direction Rig at equal intervals. The outer teeth  24   a  are able to oppose each of the passages  13 ,  17  and each of the grooves  14 ,  18  in the axial direction Da, in response to rotation of the inner gear  20 , so as to be restricted from adhering onto the sliding surface part  12   b ,  16   e.    
     As shown in  FIGS. 1 and 4 , the outer gear  30  is eccentric to the inner central line Cig of the inner gear  20 , and is arranged coaxially in the gear housing chamber  56 . Thereby, the inner gear  20  is eccentric to the outer gear  30  in an eccentric direction De as one radial direction of the outer gear  30 . 
     The outer diameter and the thickness dimension of the outer gear  30  are slightly smaller than the corresponding dimensions of the cylindrical gear housing chamber  56 . In this way, the outer circumference part  34  of the outer gear  30  is supported by the inner circumference part  16   b  of the pump casing  16 , and the both side in the axial direction Da are respectively supported by the sliding surface parts  12   b  and  16   e . Moreover, the outer circumference part  34  of the outer gear  30  has the radially-outside corner part  36  opposing the radially-inside corner part  70  of the pump housing  11 . The radially-outside corner part  36  of the outer gear  30  has a chamfering part  36   a  shaped in a taper shape all around the circumference. Thus, the outer gear  30  is rotatable in the fixed rotational direction Rog about the outer central line Cog which is eccentric from the inner central line Cig, with the inner gear  20 . 
     The inner circumference part  32  of the outer gear  30  has the inner teeth  32   a  arranged in the rotational direction Rog at equal intervals. The number of the inner teeth  32   a  of the outer gear  30  is set to be larger than the number of the outer teeth  24   a  of the inner gear  20  by one. In this embodiment, the number of the inner teeth  32   a  is ten, and the number of the outer teeth  24   a  is nine. Each of the inner teeth  32   a  is able to oppose each of the passages  13 ,  17 , and each of the grooves  14 ,  18  in the axial direction Da, in response to rotation of the outer gear  30 , so as to be restricted from adhering onto the sliding surface part  12   b ,  16   e.    
     The inner gear  20  meshes with the outer gear  30  due to the relative eccentricity in the eccentric direction De. Thereby, plural pump chambers  40  are formed to continue with each other, between the gears  20  and  30  in the gear housing chambers  56 . When the outer gear  30  and the inner gear  20  rotate, the volume of the pump chambers  40  expands and contracts. 
     The volume of the pump chamber  40  communicated with the intake passage  13  and the intake groove  18  by opposing is expanded in response to rotation of both the gears  20  and  30 . As the result, fuel is drawn from the intake port  12   a  through the intake passage  13  into the pump chamber  40  inside the gear housing chamber  56 . At this time, since the width of the intake passage  13  is increased as extending from the start end  13   c  to the finish end  13   d  (see  FIG. 2 ), the amount of fuel drawn through the intake passage  13  corresponds to the increase in the volume of the pump chamber  40 . 
     The volume of the pump chamber  40  communicated with the discharge passage  17  and the discharge groove  14  by opposing decreased in response to rotation of both the gears  20  and  30 . As the result, simultaneously with the intake function, fuel is discharged out of the gear housing chamber  56  through the discharge passage  17  from the pump chamber  40 . At this time, since the width of the discharge passage  17  is decreased as extending from the start end  17   c  to the termination part  17   d  (see  FIG. 3 ), the amount of fuel discharged out through the discharge passage  17  corresponds to the decrease in the volume of the pump chamber  40 . 
     Thus, the fuel sequentially drawn through the intake passage  13  into the pump chamber  40  and discharged out through the discharge passage  17  is discharged out from the discharge port  5   b  through the fuel passage  6 . Due to the above-mentioned pumping action, a pressure of fuel adjacent to the discharge passage  17  becomes higher than a pressure of fuel adjacent to the intake passage  13 . 
     On the other hand, a part of the fuel drawn into the gear housing chamber  56  leaks from each of the pump chambers  40  due to a dimension relationship between the outer gear  30  and the inner gear  20 , and the gear housing chamber  56 . The leak fuel forms an oil film between the gear  20 ,  30  and the sliding surface part  12   b ,  16   e , and flows into the joint housing chamber  58  and the annular groove  72 . 
     The annular groove  72  exists to make an area on a radially outer side of the intake passage  13  and an area on a radially outer side of the discharge passage  17  to communicate with each other. Further, due to the setting of the width dimension Wg 1  of the annular groove  72 , a distance between the pump chamber  40  and the annular groove  72  becomes the optimal, for securing the sealing of the pump chamber  40 , to adjust the inflow amount of the fuel to the annular groove  72 . As a result, comparatively uniform fuel pressure can be maintained in the annular groove  72  where fuel flowed in, all around the circumference. 
     Now, one pump chamber  40  formed between the gears  20  and  30  inside the gear housing chamber  56  is moved from the intake passage  13  toward the discharge passage  17  in response to rotation of both the gears  20  and  30 . When both the gears  20  and  30  reach a predetermined phase, the pump chamber  40  communicates with the discharge passage  17 . At the moment of the communication, reaction caused by fuel discharged to the discharge passage  17  acts on the outer gear  30  and the inner gear  20 . The reaction may be produced at the same number as the number of the outer tooth  24   a  per one rotation of the inner gear  20  (nine times in this embodiment). 
     The action and effect in the first embodiment is explained below. 
     According to the first embodiment, the pump housing  11  defines the cylindrical gear housing chamber  56 . The gear housing chamber  56  houses both the gears  20  and  30  to be rotatable from both sides in the axial direction Da. When the outer gear  30  and the inner gear  20  rotate, fuel is drawn sequentially into the pump chamber  40  between the gears  20  and  30  and is discharged. A positional misalignment such as inclination of the outer gear  30  may occur at a time of the discharging. 
     In the fuel pump  100 , the pump casing  16  of the pump housing  11  has the annular groove  72 , at the radially-inside corner part  70  opposing the radially-outside corner part  36  of the outer gear  30 , formed in the annular shape all around the circumference. When a positional misalignment of the outer gear  30  occurs in the state where fuel flowed into the annular groove  72  through the clearance between the gears  20 ,  30  and the pump housing  11 , the fuel which flowed into the annular groove  72  causes the damper effect to the outer circumference of the outer gear  30  to correct the positional misalignment. The annular groove  72  can ease pulsation generated in response to rotation of the outer gear  30  and the inner gear  20 , and the sliding resistance can be reduced because the outer gear  30  and the inner gear  20  rotate stably. By the above, the fuel pump  100  can be offered with high pump efficiency. 
     According to the first embodiment, the annular groove  72  is recessed toward the axial direction Da. When the position of the outer gear  30  is displaced, the fuel which flowed in the annular groove  72  can apply an action pressure to the outer gear  30  in the axial direction Da. Thereby, the damper effect can be efficiently exerted on the outer circumference of the outer gear  30 . 
     According to the first embodiment, the joint housing chamber  58  housing the joint component  60  communicates with the gear housing chamber  56 , at one side of the gear housing chamber  56  in the axial direction Da, and the annular groove  72  is formed at a side opposite from the joint housing chamber  58 . The fuel which flowed into the joint housing chamber  58 , and the fuel which flowed into the annular groove  72  exert the damper effect on the outer gear  30  and the inner gear  20  from both sides, such that the balance between the gears  20  and  30  can be maintained in the axial direction Da. Therefore, the sliding resistance can be reduced at a time of rotating both the gears  20  and  30 . By the above, the pump efficiency increases. 
     According to the first embodiment, the insertion part  64  extended in the axial direction Da from the main body  62  of the joint component  60  is inserted in the insertion hole  26  of the inner gear  20  recessed in the axial direction Da, through a clearance. When the rotation shaft  80   a  is axially misaligned, for example, by vibration of a vehicle, the axial misalignment can be absorbed by the clearance adjacent to the insertion hole  26 . Therefore, since the sliding resistance can be reduced at a time of rotating the outer gear  30  and the inner gear  20 , the pump efficiency increases. 
     According to the first embodiment, the bottom  73  of the annular groove  72  has an arc shape in the cross-section. Since a flow of the fuel at the bottom  73  becomes smooth by the annular groove  72  having the cross-section shaped in the arc, the action pressure can be efficiently transmitted to the outer circumference of the outer gear  30 . 
     Second Embodiment 
     As shown in  FIGS. 8-10 , a second embodiment is a modification of the first embodiment. The second embodiment is described focusing on a different point from the first embodiment. 
     The annular groove  272  in the fuel pump  200  of the second embodiment is formed in the annular shape all around the circumference, similarly to the first embodiment. As shown in  FIG. 8 , the annular groove  272  is recessed from the outermost circumference of the concave bottom part  16   c  in the axial direction Da away from the gear housing chamber  56 . 
     The annular groove  272  is formed so that each of the width dimension Wg and the depth dimension Dg is approximately uniform all around the circumference. However, the width of the annular groove  272  in one radial direction is made smaller as extending to the bottom  273 . Specifically, the annular groove  272  of the second embodiment is shaped in a triangle tapering as extending to the bottom  273  in the cross-section vertically along the radial direction of the pump casing  16 . An external wall  275  of the annular groove  272  is formed to extend in the axial direction Da, and an internal wall  274  of the annular groove  272  inclines to the outer circumference side as extending to the bottom  273 . The bottom  273  of the annular groove  272  has an arc shape in the cross-section, similarly to the first embodiment. 
     Results of comparison experiments are explained below using  FIGS. 9 and 10 , between the fuel pump  200  of the present embodiment and a fuel pump of a comparative example in which the annular groove  272  is not formed in the fuel pump  200 . The comparison experiments were conducted on the conditions at which the fuel is JIS No. 2 light oil and the fuel temperature is 25° C. In  FIGS. 9 and 10 , Hi mode represents a case where the supply voltage to the electric motor  80  is 12V, for example, used in the state of a full throttle. Lo mode represents a case where the supply voltage to the electric motor  80  is 6V, for example, used in the state of an idling. The fuel pressure in  FIGS. 9 and 10  represents a fuel pressure adjusted in a pressure regulator of an internal-combustion engine. In  FIGS. 9 and 10 , a solid line represents data of the fuel pump  200  of the present embodiment, and a dashed line represents data of the comparative example. 
     In  FIG. 9 , the flow rate of the present embodiment is higher than the flow rate of the comparative example, at each fuel pressure, in each mode. In  FIG. 10 , the current value of the present embodiment is less than the current value of the comparative example at each fuel pressure in the Hi mode. In the Lo mode, when the fuel pressure is 600 kPa, there is no significant difference in the current value between of the present embodiment and the comparative example, but the current value of this embodiment becomes lower than the current value of the comparative example as the fuel pressure is lowered. 
     According to the second embodiment, since the pump casing  16  of the pump housing  11  has the annular groove  272  formed in the annular shape all around the circumference, at the radially-inside corner part  70 , it becomes possible to achieve the action and effect similar to the first embodiment. 
     According to the second embodiment, the annular groove  272  has the triangle shape which tapers off as extending to the bottom  273 , in the cross-section. Therefore, since the volume of the annular groove  272  can be reduced relative to a pressure receiving area at the position where the annular groove  272  opposes the outer gear  30 , the action pressure can be efficiently transmitted to the outer circumference of the outer gear  30 , while controlling the leak amount of the fuel to the annular groove  272 . 
     Other Embodiment 
     The present disclosure is not limited to the embodiments, and can be applied to various embodiment and combination within a range not deviated from the scope of the present disclosure. 
     Specifically, as a first modification, as shown in  FIG. 11 , the annular groove  72  may be formed in a semicircle shape in the cross-section, which is an example where the bottom  73  of the annular groove  72  has an arc shape in the cross-section. In this example, the width dimension Wg 1  is just twice of the depth dimension Dg. 
     As a second modification, the annular groove  72  may be recessed in a direction other than the axial direction Da. The annular groove  72  of  FIG. 12  is recessed in the slant direction. In this case, when the position of the outer gear  30  is displaced, it becomes possible to apply the action pressure to the outer gear  30  along the slant direction. The annular groove  72  of  FIG. 13  is recessed in the radial direction. In this case, when the position of the outer gear  30  is displaced, it becomes possible to apply the action pressure to the outer gear  30  along the radial direction. 
     As a third modification, the bottom  73  of the annular groove  72  may be formed in a rectangle shape. 
     As a fourth modification, the pump housing  11  may have the annular groove  72  at the respective sides of the gear housing chamber  56  in the axial direction Da. In this case, it is not necessary to form the joint housing chamber  58 . 
     As a fifth modification, the fuel pump  100  may draw and discharge gasoline other than light oil, or liquid fuel similarly to this, as fuel.