Patent Publication Number: US-8523513-B2

Title: Fuel pump

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2006-251619 filed on Sep. 15, 2006, the contents of which are hereby incorporated by reference into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel pump that draws in a fuel, increases the pressure thereof, and discharges the pressurized fuel. 
     2. Description of the Related Art 
     A known fuel pump generally comprises a casing and an impeller rotatably disposed within the casing. A first group of concavities is formed in a lower surface of the impeller. A second group of concavities is formed in an upper surface. The concavities are repeated in a circumferential direction. A first groove is formed in a first inner surface of the casing in an area that faces the first group of concavities of the impeller. A second groove is formed in a second inner surface of the casing in an area that faces the second group of concavities of the impeller. The first and second grooves extend in a circumference direction from an upstream end to a downstream end, respectively. A pump channel is formed inside the casing by the groups of concavities of the impeller and the grooves of the casing. An intake hole and a discharge hole are formed in the casing. The intake hole links the upstream end of the pump channel and the exterior of the casing. The discharge hole links the downstream end of the pump channel and the exterior of the casing. When the impeller rotates, the fuel is drawn from the intake hole into the pump channel. The fuel drawn into the pump channel flows from the upstream end to the downstream end of the pump channel, while the pressure thereof is increased. The pressurized fuel is discharged to the outside of the casing via the discharge hole. 
     In this known fuel pump, a clearance is provided between the casing and the impeller. Where the clearance is large, the fuel in the pump channel easily leaks from the pump channel to the clearance and high pump efficiency cannot be obtained. Therefore, in order to obtain high pump efficiency, it is necessary to decrease the clearance and reduce fuel leakage from the pump channel. However, if the clearance is too small, the casing and the impeller come into surface contact and sliding resistance increases, thereby decreasing pump efficiency. Thus, because of this trade-off relationship between the reduction in sliding resistance and reduction in fuel leakage, a technology is required that can realize the two at the same time. 
     Japanese Laid-open Patent Application Publication No. 6-213195 discloses a fuel pump in order to solve this problem. In this fuel pump, a plurality of concave portions is formed in the entire inner surface of the casing. The concave portions are formed as dots or grooves, and the concave portions are separated from each other. By forming a plurality of concave portions in the inner surface of the casing, the surface tension of the fuel increases and the adhesion force of the fuel inside the clearance between the casing and the impeller increases. As a result, sealing capacity can be improved without unnecessarily reducing the clearance between the casing and the impeller. Thus, with this fuel pump, both the reduction of fuel leakage and the reduction of sliding resistance can be realized. 
     BRIEF SUMMARY OF THE INVENTION 
     In the above-described fuel pump, because a plurality of concave portions is formed in the inner surface of the casing, it is necessary to perform complex cutting of the casing. Moreover, because a high degree of precision is required for this cutting operation, manufacturing cost increases. Moreover, where the concave portions are formed by mechanical processing such as cutting, there is a variation in the shape of concave portions. As a result, there is a variation in pump efficiency. For these reasons, with the above-described technology, it is difficult to realize both the reduction in fuel leakage and the reduction in sliding resistance at a low manufacturing cost and with good stability. 
     It is one object of the present teachings to provide a fuel pump that can be manufactured at a low cost and in which both the reduction of fuel leakage and the reduction in sliding resistance can be realized with good stability. 
     In one aspect of the present teachings, a fuel pump is provided with a casing and a substantially disc-shaped impeller rotatably disposed within the casing. A first group of concavities is formed in a lower surface of the impeller. The concavities forming the first group are arranged in concentric circles with respect to the rotational axis of the impeller. A second group of concavities is formed in an upper surface of the impeller. The concavities forming the second group are arranged in concentric circles with respect to the rotational axis of the impeller. A first groove is formed in a first inner surface of the casing. The first groove extends from an upstream end to a downstream end in an area that faces the first group of concavities. A second groove is formed in a second inner surface of the casing. The second groove extends from an upstream end to a downstream end in an area that faces the second group of concavities. An intake hole is formed in the casing. The intake hole passes from the exterior of the casing to the upstream end of the first groove. A discharge hole is formed in the casing. The discharge hole passes from the exterior of the casing to the downstream end of the second groove. At least one seal portion is disposed on at least one of the first inner surface, the second inner surface, the lower surface and the upper surface. The seal portion comprises one layer or a plurality of layers of thin film. 
     In this fuel pump, the seal portion that seals leakage of the fuel comprises one layer or a plurality of thin films. The formation of a thin film is easier and processing accuracy is higher than in the case of shaping by mechanical processing such as cutting. Thus, even if a seal portion has a complex shape, the seal portion can be manufactured at a low cost by using a thin film. Furthermore, a seal portion using a thin film has high accuracy. As a result, both the reduction in fuel leakage and the reduction in sliding resistance can be realized at a low manufacturing cost and with good stability. 
     Other objects, features and advantages of the present teachings will be readily understood after reading the following detailed description together with the accompanying drawings and claims. The additional features and aspects disclosed herein may be utilized singularly or, in combination with the above-described aspect and features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of the fuel pump of the first embodiment. 
         FIG. 2  is a cross sectional view along the II-II line in  FIG. 1 . 
         FIG. 3  is an enlarged view of the main portion of the fuel pump of the first embodiment. 
         FIG. 4  is a drawing, which corresponds to the cross sectional view along the II-II line of  FIG. 1 , of the second embodiment. 
         FIG. 5  is an enlarged view of the main portion of the fuel pump of the second embodiment. 
         FIG. 6  is a drawing, which corresponds to the cross sectional view along the II-II line of  FIG. 1 , of the third embodiment. 
         FIG. 7  is an enlarged view of the main portion of the fuel pump of the third embodiment. 
         FIG. 8  is a vertical sectional view of the fuel pump of the fourth embodiment. 
         FIG. 9  is a cross sectional view along the IX-IX line in  FIG. 8 . 
         FIG. 10  is an enlarged view of the main portion of the fuel pump of the fourth embodiment. 
         FIG. 11  is a drawing, which corresponds to the cross sectional view along the IX-IX line of  FIG. 8 , of the fifth embodiment. 
         FIG. 12  is an enlarged view of the main portion of the fuel pump of the fifth embodiment. 
         FIG. 13  is a drawing, which corresponds to the cross sectional view along the IX-IX line of  FIG. 8 , of the sixth embodiment. 
         FIG. 14  is an enlarged view of the main portion of the fuel pump of the sixth embodiment. 
         FIG. 15  is a drawing, which corresponds to the cross sectional view along the IX-IX line of  FIG. 8 , of the seventh embodiment. 
         FIG. 16  is an enlarged view of the main portion of the fuel pump of the seventh embodiment. 
         FIG. 17  is a drawing, which corresponds to the cross sectional view along the IX-IX line of  FIG. 8 , of the eighth embodiment. 
         FIG. 18  is a drawing, which corresponds to the cross sectional view along the IX-IX line of  FIG. 8 , of the ninth embodiment. 
         FIG. 19  is a plan view of the impeller of the tenth embodiment. 
         FIG. 20  is a plan view of the impeller of the eleventh embodiment. 
         FIG. 21  is a vertical sectional view illustrating examples of the casing or impeller in the area where the seal layer is formed. 
         FIG. 22  is a vertical sectional view illustrating examples of the casing or impeller in the area where the seal layer is formed. 
         FIG. 23  is a vertical sectional view illustrating examples of the casing or impeller in the area where the seal layer is formed. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present teaching will be described below. 
     (Feature 1) 
     A seal layer is formed from one thin film. 
     (Feature 2) 
     A seal layer is formed from two or more laminated thin films, and the cross section of the seal layer in the laminated portion has a step-like shape. 
     Embodiment 1 
     A first embodiment of the present teachings will be explained below. A fuel pump of the present embodiment is a fuel pump for an automobile. The fuel pump is disposed within a fuel tank and serves to supply fuel to the automobile engine. As shown in  FIG. 1 , a fuel pump  10  comprises a motor unit  12  and a pump unit  14  accommodated in a housing  16 . The motor unit  12  has a rotor  18 . The rotor  18  has a shaft  20 , a laminated iron core  22  that is fixed to the shaft  20 , a coil (not shown in the figure) that is wound about the laminated iron core  22 , and a commutator  24  connected to the ends of the coil. The shaft  20  is supported by bearings  26 ,  28  so that it can rotate with respect to the housing  16 . A permanent magnet  30  is fixed inside the housing  16  so as to surround the rotor  18 . Terminals (not shown in the figure) are provided at a top cover  32  attached to the upper portion of the housing  16 . The terminals are electrically connected to the rotor  18 . When electric current is supplied to the coil via a brush  34  and the commutator  24 , the rotor  18  and the shaft  20  rotate. 
     The pump unit  14  is accommodated in the lower portion of the housing  16 . The pump unit  14  comprises a substantially disk-shaped impeller  36 . A group of concavities  36   a  is provided in the upper surface of the impeller  36 . The concavities  36   a  are arranged side by side at the outer peripheral edge of the impeller  36 . A group of concavities  36   b  are provided in the lower surface of the impeller  36 . The concavities  36   b  is arranged side by side at the outer peripheral edge of the impeller  36 . A through hole is provided in the center of the impeller  36 . The shaft  20  fits into the through hole of the impeller  36 . Therefore, when the shaft  20  rotates, the impeller  36  also rotates. 
     The casing  37  that accommodates the impeller  36  is composed of a pump cover  38  and a pump body  40 . In the pump cover  38 , a groove  38   a , having a width almost twice as large as the width of the group of concavities  36   a  in the radial direction, is formed in the area that faces the outer peripheral edge of the impeller  36 . As shown in  FIG. 2 , the groove  38   a  is formed to have an almost C-like shape extending from an upstream end to a downstream end along the rotation direction of the impeller  36 . A discharge hole  50  passing from the downstream end of the groove  38   a  to the upper surface of the pump cover  38  is formed in the pump cover  38 . The discharge hole  50  links the interior and exterior (inner space of the motor unit  12 ) of the casing  37 . 
     As shown in  FIG. 1 , a groove  40   a , having a width almost twice as large as the width of the group of concavities  36   b  in the radial direction, is formed in the pump body  40  in the area that faces the outer peripheral edge of the impeller  36 . Similarly to the groove  38   a , the groove  40   a  is formed to have an almost C-like shape extending from an upstream end to a downstream end along the rotation direction of the impeller  36 . An intake hole  42  passing from the lower surface of the pump body  40  to the upstream end of the groove  40   a  is formed in the pump body  40 . The intake hole  42  links the interior of the casing  37  with the exterior (exterior of the fuel pump). Furthermore, a vapor jet  41  is formed in the pump body  40 . The vapor jet  41  discharges vapor generated inside the groove  40   a  to the outside of the pump. The groups of concavities  36   a  and  36   b , groove  38   a , and groove  40   a  form a pump channel  44  so as to cover the outer peripheral edge of the impeller  36 . 
     When the impeller  36  rotates inside the casing  37 , fuel is drawn into the pump channel  44  via the intake hole  42 . While the fuel flows in the pump channel  44 , the fuel pressure is increased. The pressurized fuel is discharged out from the discharge hole  50  toward the motor unit  12 . The discharged fuel passes through the motor unit  12  and is discharged out from a discharge port  48  formed in the top cover  32 . 
     As shown in  FIG. 2  and  FIG. 3 , a seal layer  52  is formed on a surface (referred to hereinafter as “inner surface”) of the pump cover  38  that faces the upper surface of the impeller  36 . The seal layer  52  is convex toward the impeller  36 . The seal layer  52  is, for example, a thin film with a thickness of 1 to 200 μm made from a synthetic resin such as a phenolic resin, an epoxy resin, or a polyamidoimide resin. The seal layer  52  can be formed using various well-known methods (e.g., screen printing, ink jet printing, sheet bonding) for forming thin films. For example, a thin film is disposed by screen printing on the pump cover  38  and then the thin film is fixed to the pump cover  38  by curing (i.e., drying, heat treatment, photocuring, etc.). As a result, a thin film (i.e., a seal layer) can be formed on the inner surface of the casing  37 . The seal layer  52  has a ring shape. The outer diameter of the seal layer  52  is slightly less than the inner diameter of the groove  38   a , and the inner diameter of the seal layer  52  is almost intermediate between the outer diameter of the shaft  20  and the inner diameter of the groove  38   a . The seal layer  52  is disposed to be concentric with the shaft  20  on the inner side of the groove  38   a . A very small clearance C 1  (see  FIG. 3 ) is formed between the pump cover  38  and the impeller  36  in an area where the seal layer  52  is not formed. The thickness of the seal layer  52  is less than the clearance C 1 . A very small clearance C 2  that is less than the clearance C 1  is formed between the seal layer  52  and the impeller  36 . 
     As shown in  FIG. 3 , a seal layer  54  is also formed on a surface of the pump body  40  that faces the lower surface of the impeller  36 . The seal layer  54  is also a thin film with a thickness of 1 to 200 μm made from a synthetic resin such as a phenolic resin, an epoxy resin, or a polyamidoimide resin. The seal layer  54  is formed by the same method as the seal layer  52 . The seal layer  54  has a ring shape similar to the seal layer  52  and the shape thereof is almost identical to that of the seal layer  52 . The seal layer  54  is disposed to be concentric with the shaft  20  on the inner side of the groove  40   a . A very small clearance C 3  is formed between the pump body  40  and the impeller  36  in an area where the seal layer  54  is not formed. The thickness of the seal layer  54  is less than the clearance C 3 . A very small clearance C 4  that is less than the clearance C 3  is formed between the seal layer  54  and the impeller  36 . 
     In the fuel pump of the present embodiment, seal layers  52 ,  54  are formed on the inner surfaces of the casing  37 . The seal layers  52 ,  54  are formed directly inside the grooves  38   a ,  40   a . As a result, the leakage of the fuel from the pump channel  44  into the clearances between the casing  37  and impeller  36  can be reduced. Furthermore, by forming the seal layers  52 ,  54 , it is possible to ensure large clearances C 1 , C 3  in the areas where the seal layers  52 ,  54  have not been formed. Therefore, sliding resistance can be also reduced, while reducing the leakage. The seal layers  52 ,  54  are formed by well-known thin film forming technology. Such method makes it possible to attain processing accuracy higher than that attained with shaping methods based on mechanical processing such as cutting. Therefore, the occurrence of variation in product performance can be inhibited. In addition, because the processing operations in thin film formation are simpler than those of cutting, the manufacturing cost can be reduced. 
     Embodiment 2 
     A second embodiment of the present teachings will be explained below. The fuel pump of the second embodiment is a partial modification of the fuel pump of the first embodiment. Accordingly, only the difference between the fuel pump of this embodiment and that of the first embodiment will be explained to avoid redundant explanation. Furthermore, components that are common for the fuel pump of the second embodiment and the fuel pump of the first embodiment will be denoted by similar reference symbols. The same is true for the below-described third to eleventh embodiments. 
     As shown in  FIG. 4  and  FIG. 5 , a seal layer  62  is formed on a surface of a pump cover  68  that faces an upper surface of an impeller  36 . Similarly to the first embodiment, the seal layer  62  is a thin film made from a synthetic resin. The seal layer  62  can be formed by the same method as used in the first embodiment. The seal layer  62  is formed by laminating a lower layer  62   a  and an upper layer  62   b . A width of the lower layer  62   a  in the radial direction is larger than a width of the upper layer  62   b  in the radial direction. The seal layer  62  has a step-shaped cross section. A very small clearance is also formed between the seal layer  62  and the impeller  36 . 
     As shown in  FIG. 5 , a seal layer  64  is also formed on a surface of an pump body  70  that faces a lower surface of the impeller  36 . The seal layer  64  is a thin film with a thickness of 1 to 200 μm made from a synthetic resin. The seal layer  64  is formed by the same method as the seal layer  62 . The seal layer  64  is also formed by laminating a lower layer  64   a  and an upper layer  64   b  and has a step-shaped cross section. A very small clearance is also formed between the seal layer  64  and the impeller  36 . 
     In the fuel pump of the second embodiment, the seal layers  62 ,  64  are formed on the surfaces of casing  67 . As a result, the fuel leakage from the pump channel  74  can be reduced. The cross section of the seal layers  62  and  64  has a step-like shape. The upper layers  62   b ,  64   b  on the side of the impeller  36  serve as seal portions. The width of the upper layers  62   b ,  64   b  is less than the width of the lower layers  62   a ,  64   a  on the side of the casing  67 . Therefore, a large clearance can be ensured in the area outside the upper layers  62   b ,  64   b . Therefore, sliding resistance can be effectively reduced, while reducing the fuel leakage. The seal layers  62 ,  64  are also formed by well-known thin film forming technology. Therefore, the seal layers  62 ,  64  can be shaped with good accuracy and at a low manufacturing cost despite a complex step-like shape. 
     Embodiment 3 
     A third embodiment of the present teachings will be explained below. As shown in  FIG. 6  and  FIG. 7 , a seal layer  82  is formed on a surface of a pump cover  88  that faces an upper surface of an impeller  36 . The seal layer  82  is a thin film made from a synthetic resin. The seal layer  82  is formed by the same method as in the above-described embodiments. The seal layer  82  is formed to have an almost ring shape and is formed between a discharge hole  50  and an upstream end of the groove  88   a  (i.e., an intake hole  42 ). Thus, the seal layer  82  is formed also in an area  82   c  located between the discharge hole  50  and the intake hole  42 . The outer end of the seal layer  82  extends close to a circle to which a central line in the radial direction of a groove  88   a  can be extended in the circumferential direction. As shown in  FIG. 7 , the diameter of the circle serving as a central line in the radial direction of the groove  88   a  almost matches the outer diameter of the impeller  36 . Further, the seal layer  82  comprises a lower layer  82   a  and an upper layer  82   b  and has a step-shaped cross section. A very small clearance is formed between the seal layer  82  and the upper surface of the impeller  36 . 
     As shown in  FIG. 7 , a seal layer  84  is formed on a surface of an pump body  90  that faces a lower surface of the impeller  36 . The seal layer  84  is a thin film made from a synthetic resin. The seal layer  84  is formed to have almost the same shape as the seal layer  82 . Thus, similarly to the seal layer  82 , the seal layer  84  is also formed to have an almost ring shape and is formed between a downstream end of the groove  90   a  (i.e., the discharge hole  50 ) and the intake hole  42 . The seal layer  84  also comprises a lower layer  84   a  and an upper layer  84   b . A very small clearance is formed between the seal layer  84  and the lower surface of the impeller  36 . 
     Since the pressure of the fuel at the discharge hole  50  is highest and the pressure of the fuel at the intake hole  42  is lowest, the fuel leakages between the discharge hole  50  and the intake hole  42  is larger than the fuel leakage in other areas. In the fuel pump of the third embodiment, the seal layers  82 ,  84  are formed in the surfaces of the casing  87 . Further, the seal layers  82 ,  84  reach the areas between the discharge hole  50  and the intake hole  42 . As a result, sealing ability between the discharge hole  50  and the intake hole  42  is improved and the fuel leakage can be reduced even more effectively. Furthermore, the width of the upper layers  82   b ,  84   b  is less than the width of the lower layers  82   a ,  84   a . Therefore, a large clearance can be ensured in the area outside the upper layers  82   b ,  84   b . Therefore, sliding resistance can be effectively reduced. 
     Embodiment 4 
     A fourth embodiment of the present teachings will be explained below. As shown in  FIG. 8 , a pump unit  14  comprises an impeller  106 . A group of concavities  106   a  is provided in the vicinity of the outer peripheral edge of the upper surface of the impeller  106 . A group of concavities  106   b  is provided in the vicinity of the outer peripheral edge of the lower surface of the impeller  106 . Each of the groups of concavities  106   a ,  106   b  is formed in an area at a predetermined distance from the outer periphery of the impeller  106 . Furthermore, each of the concavities  106   a  communicates with corresponding concavity  106   b  at the bottom portions thereof. A through hole that is engaged with a shaft  20  is provided in the center of the impeller  106 . 
     The casing  107  is composed of a pump cover  108  and an pump body  110 . In the pump cover  108 , a groove  108   a  is formed in an area that faces the group of concavities  106   a . As shown in  FIG. 9 , the groove  108   a  extends from an upstream end to a downstream end along the rotation direction of the impeller  106 . A discharge hole  50  is formed in the pump cover  108 . The discharge hole  50  passes from the downstream end of the groove  108   a  to the upper surface of the pump cover  108 . As shown in  FIG. 8 , a pump channel  114  is formed by the group of concavities  106   a  and the groove  108   a . In the pump body  110 , a groove  110   a  is formed in an area that faces the group of concavities  106   b . Similarly to the groove  108   a , the groove  110   a  extends from an upstream end to a downstream end along the rotation direction of the impeller  106 . An intake hole  42  is formed in the pump body  110 . The intake hole  42  passes from the lower surface of the pump body  110  to the upstream end of the groove  110   a . Another pump channel  116  is formed by the group of concavities  106   b  and the groove  110   a.    
     When the impeller  106  rotates, fuel is drawn into the pump channels  114  and  116 . While the drawn fuel flows in the pump channels  114 ,  116 , the fuel pressure is increased. The pressurized fuel is discharged out from the discharge hole  50  toward the motor unit  12 . 
     As shown in  FIGS. 9 ,  10 , a seal layer  102  is formed in a surface of the pump cover  108  that faces the upper surface of the impeller  106 . Similarly to the above mentioned embodiments, the seal layer  102  is a thin film made from a synthetic resin. The seal layer  102  is formed by the method similar to that of the above-described embodiments. The seal layer  102  has an almost ring shape and is formed between the discharge hole  50  and the upstream end of the groove  108   a . The outer end of the seal layer  102  formed between the discharge hole  50  and the upstream end of the groove  108   a  extends beyond the outside of a circle to which the outer peripheral edge of the groove  108   a  can be extended. Thus, the outer end of the seal layer  102  extends to about the outer periphery of the impeller  106  (see  FIG. 10 ). The seal layer  102  comprises a lower layer  102   a  and an upper layer  102   b . A very small clearance is formed between the seal layer  102  and the impeller  106 . 
     As shown in  FIG. 10 , a seal layer  104  is also formed on a surface of the pump body  110  that faces the lower surface of the impeller  106 . The seal layer  104  is also a thin film made from a synthetic resin. The seal layer  104  is formed in the same manner as the seal layer  102 . The seal layer  104  is formed to have an almost ring shape and is formed between the downstream end of the groove  110   a  and the intake hole  42 . As shown in  FIG. 10 , an outer end of the seal layer  104  formed between the downstream end of the groove  110   a  and the intake hole  42  almost matches the outer diameter of the impeller  106 . The seal layer  104  also comprises a lower layer  104   a  and an upper layer  104   b . A very small clearance is formed between the seal layer  104  and the lower surface of the impeller  106 . 
     In the fuel pump of the fourth embodiment, the seal layers  102 ,  104  are formed between the upstream ends and downstream ends of the grooves  108   a ,  110   a . As a result, sealing ability in the discharge hole  50  and intake hole  42  can be improved and the fuel leakage can be effectively reduced. 
     Embodiment 5 
     A fifth embodiment of the present teachings will be explained below. As shown in  FIG. 11  and  FIG. 12 , a seal layer  122  is formed in a surface of a pump cover  128  that faces an upper surface of an impeller  106 . Similarly to the above mentioned embodiments, the seal layer  122  is a thin film made from a synthetic resin. In this embodiment, the seal layer  122  is formed in an area excluding the vicinity of a groove  128   a  and a through hole (i.e., shaft  20 ). Thus, the seal layer  122  is formed not only in the area inside the groove  128   a , but also outside the groove. A very small clearance is formed between the seal layer  122  and the upper surface of the impeller  106 . 
     As shown in  FIG. 12 , a seal layer  124  is formed on a surface of an pump body  130  that faces the lower surface of the impeller  106 . The seal layer  124  is also a thin film made from a synthetic resin. Similarly to the seal layer  122 , the seal layer  124  is formed in an area (inside and outside the groove  130   a ) excluding the vicinity of the groove  130   a  and the through hole (the shaft  20 ). A very small clearance is formed between the seal layer  124  and the impeller  106 . 
     In the fuel pump of the fifth embodiment, the seal layers  122 ,  124  are formed around the respective grooves  128   a  and  130   a . As a result, the fuel leakage from the pump channels  134 ,  136  can be reduced. In particular, because the seal layers  122 ,  124  are also formed on the outside of the grooves  128   a ,  130   a , the fluid can be prevented from leaking to the clearance between the outer peripheral edges of the impeller  106  and the pump cover  128 . 
     Embodiment 6 
     A sixth embodiment of the present teachings will be explained below. As shown in  FIGS. 13 and 14 , a seal layer  142  is formed on a surface of a pump cover  148  that faces an upper surface of an impeller  106 . The seal layer  142  is formed of three thin films  142   a ,  142   b ,  142   c  made from a synthetic resin. The thin film  142   a  has a ring shape and is disposed concentrically with a shaft  20  inside a groove  148   a . The thin film  142   b  has a ring shape and is disposed concentrically with the shaft  20  outside the groove  148   a . The thin film  142   c  is formed to have an almost trapezoidal shape. An end portion on the inner side of the thin film  142   c  is in the form of a circular arc that follows the inner peripheral edge of the thin film  142   a , and the end portion on the outer side of the thin film  142   c  is in the form of a circular arc that follows the outer peripheral edge of the thin film  142   b . The thin film  142   c  is disposed between a discharge port  50  and the upstream end of the groove  148   a . In the seal layer  142 , the thin film  142   c  is formed by lamination on the thin film  142   a  and the thin film  142   b.    
     As shown in  FIG. 14 , a seal layer  144  is formed on a surface of an pump body  150  that faces the lower surface of the impeller  106 . The seal layer  144  is also formed of three thin films  144   a ,  144   b ,  144   c . The shape and arrangement of thin films  144   a ,  144   b ,  144   c  are similar to the shape and arrangement of thin films  142   a ,  142   b ,  142   c , respectively. 
     In the fuel pump of the sixth embodiment, the seal layers  142 ,  144  are formed around the respective grooves  148   a ,  150   a  and in the area between the upstream end and downstream end of grooves  148   a ,  150   a . As a result, the fuel leakage from pump channels  154 ,  156  can be reduced. Furthermore, the seal layers  142 ,  144  are formed of three thin films ( 142   a ,  142   b ,  142   c ), ( 144   a ,  144   b ,  144   c ), respectively. These thin films ( 142   a ,  142   b ,  142   c ), ( 144   a ,  144   b ,  144   c ) have a simple shape and may be disposed in respective adequate locations. Therefore, the seal layer of a complex shape can be easily formed with good accuracy and at a low cost. If necessary, the locations in which the thin films are disposed and the number of laminated thin films can be changed. Therefore, the degree of freedom in designing the clearance between the casing  147  and the impeller  106  can be increased. 
     Embodiment 7 
     A seventh embodiment of the present teachings will be explained below. As shown in  FIGS. 15 ,  16 , a seal layer  162  is formed on a surface of a pump cover  168  that faces an upper surface of an impeller  106 . The seal layer  162  is formed of two thin films  162   a ,  162   b  made from a synthetic resin. The thin film  162   a  is disposed in a region except the vicinity of a groove  168   a  and a through hole (a shaft  20 ). The thin film  162   b  has an almost ring shape and is disposed in an area inside the groove  168   a  and also in an area  162   c  between the upstream end and downstream end (i.e., discharge hole  50 ) of the groove  168   a . The outer end of the thin film  162   b  disposed in the area  162   c  extends to the outside of the circle to which the outer peripheral edge of the groove  168   a  can be extended and approximately matches the outer diameter of the impeller  106 . In the thin film  162   b  disposed in the area  162   c , there are three notches  162   d  extending in the circumferential direction from the end portion of the thin film on the side of the discharge hole  50 . The length of the notches  162   d  is about one third of the distance between the discharge hole  50  and the upstream end of the groove  168   a . The thin film  162   b  is laminated on the thin film  162   a.    
     As shown in  FIG. 16 , a seal layer  164  is formed on a surface of an pump body  170  that faces a lower surface of the impeller  106 . The seal layer  164  is formed of two thin films  164   a ,  164   b . The shape and arrangement of thin films  164   a ,  164   b  are almost identical to the shape and arrangement of thin films  162   a ,  162   b , respectively. Thus, the thin film  164   a  is disposed in a region except the vicinity of a groove  170   a  and a shaft  20 . The thin film  164   b  is formed to have an almost ring shape and is disposed in an area inside the groove  170   a  and also in an area between the upstream end (intake hole) and the downstream end of the groove  170   a . The outer end of the thin film  164   b  disposed between the upstream end and downstream end of the groove  170   a  approximately matches the outer diameter of the impeller  106 . In the thin film  164   b  disposed between the upstream end and downstream end of the groove  170   a , there are three notches (not shown in the figure) extending in the circumferential direction from the end portion of the thin film on the side of the intake hole. The thin film  164   b  is laminated on the thin film  164   a.    
     In the fuel pump of the seventh embodiment, the seal layers  162 ,  164  are formed around the respective grooves  168   a ,  170   a  and in the area between the upstream end and downstream end of grooves  168   a ,  170   a . As a result, the fuel leakage from pump channels  174 ,  176  can be reduced. Further, the seal layers  162 ,  164  are formed of two thin films ( 162   a ,  162   b ), ( 164   a ,  164   b ), respectively. In the thin films  162   b ,  164   b  that are laminated as top film, notches are formed in the vicinity of the discharge hole  50  or intake hole  42 . When viewed as a longitudinal cross-section along the radial direction of the impeller, cross-sectional area of the clearances between the thin films  162   b ,  164   b  and the casing  167  changes gradually in the vicinity of the discharge hole  50  or intake hole  42 . As a result, periodic pressure fluctuations caused by rotation of the impeller  106  can be relaxed and noise generation can be reduced. 
     Embodiment 8 
     An eighth embodiment of the present teachings will be explained below. As shown in  FIG. 17 , a seal layer  182  is formed on a surface of a pump cover  188  that faces an upper surface of an impeller  106  (see  FIG. 8 ). The seal layer  182  is formed of a thin film made from a synthetic resin. The seal layer  182  has an almost ring shape and is disposed on the inner side from a groove  188   a . A group of notches  182   a  is formed in the seal layer  182 . The notches  182   a  are substantially same to each other in shape and size and are disposed equidistantly along the outer periphery of the seal layer  182 . Each notch  182   a  extends as a curve (spirally) from the outer peripheral side of the seal layer  182  toward the center (closed end). An end portion of the notch  182   a  on the center side (i.e., a closed end of the notch  182   a ) is shifted in the rotation direction (i.e., the direction of arrow A) of the impeller  106  with respect to the end portion on the outer periphery side (i.e., an open end of the notch  182   a ). In other words, the end portion of the notch  182   a  on the center side is located at a position forward of a position of the end portion of the notch  182   a  on the outer periphery side in the rotation direction of the impeller  106 . A very small clearance is formed between the seal layer  182  and the impeller  106 . A seal layer (not shown in the figure) similar to the seal layer  182  is formed on a surface of the pump body that faces a lower surface of the impeller  106 . 
     In the fuel pump of the eighth embodiment, a group of spiral notches  182   a  is formed in the seal layer  182  of the pump cover  188 , and a similar group of notches is formed in the seal layer of the pump body. As a result, respective groups of spiral grooves (these grooves will be hereinafter referred to as spiral grooves) are formed in the surface of the pump cover  188  that faces the impeller  106  and in the surface of the pump body that faces the impeller  106 . The end portion on the center side of the spiral groove is shifted with respect to the end portion on the outer periphery side in the rotation direction of the impeller  106 . As a result, when the impeller  106  rotates, the fuel in the clearance between the impeller  106  and the casing is drawn into the spiral grooves and flows from the end portion on the outer periphery side of the spiral groove to the end portion on the center side of the spiral groove. When the fuel drawn into the spiral grooves flows from the outer periphery side toward the center, the pressure of the fuel in the grooves acts upon the upper and lower surfaces of the impeller  106 , and the impeller  106  is held between the pump cover  188  and the pump body. Thus, even if a clearance between the casing and the impeller  106  is decreased, increasing the sliding resistance is prevented. Therefore, both the reduction in the fuel leakage and the reduction in sliding resistance can be realized. Further, because the seal layer is formed by a well-known thin-film forming technology, a high processing accuracy can be obtained at a low cost. Therefore, a group of notches  182   a  can be formed with good accuracy at a low cost. 
     Embodiment 9 
     A ninth embodiment of the present teachings will be explained below. As shown in  FIG. 18 , a seal layer  202  is formed in a surface of a pump cover  208  that faces an upper surface of an impeller  106  (see  FIG. 8 ). The seal layer  202  is formed of a thin film made from a synthetic. The seal layer  202  is formed in an area except the vicinity of a groove  208   a  and a shaft  20 . A group of notches  202   a  is formed in the seal layer  202 . The notches  202   a  have substantially identical shape and size and are disposed between the outer diameter of the shaft  20  and the inner diameter of the groove  208   a . Each notch  202   a  extends as a curve (spirally) from the center side of the seal layer  202  toward the outer periphery side. An end portion of the notch  202   a  on the outer periphery side is shifted in the rotation direction (arrow B) of the impeller  106  with respect to the end portion on the center side. In other words, the end portion of the notch  202   a  on the outer periphery side (i.e., closed end of the notch  202   a ) is located at a position forward of a position of the end portion of the notch  202   a  on the center side (i.e., open end of the notch  202   a ) in the rotation direction of the impeller  106 . A very small clearance is formed between the seal layer  202  and the impeller  106 . A seal layer (not shown in the figure) similar to the seal layer  202  is formed in a surface of the pump body that faces a lower surface of the impeller  106 . 
     In the fuel pump of the ninth embodiment, group of spiral notches are formed in the seal layers of the pump cover  208  and pump body. As a result, when the impeller  106  rotates, the fuel in the clearance between the impeller  106  and the casing is drawn into the spiral grooves and flows from the end portion on the center side of the spiral groove to the end portion on the outer periphery side of the spiral groove. When the fuel inside the spiral grooves flows from the center side toward the outer periphery side, the pressure of the fuel in the spiral grooves acts upon the upper and lower surfaces of the impeller  106 , and the impeller  106  is held between the pump cover  208  and the pump body. Because contact between the impeller  106  and the casing is prevented by the pressure of fuel flowing in the spiral grooves, the clearance between the casing and the impeller  106  can be reduced. Therefore, both the reduction in leak flow rate and the reduction in sliding resistance can be realized. 
     Embodiment 10 
     A tenth embodiment of the present teachings will be explained below. 
     As shown in  FIG. 19 , a seal layer  222  is formed on an upper surface of an impeller  236  (see  FIG. 8 ). The seal layer  222  is formed of two thin films  222   a ,  222   b  made from a synthetic resin. The thin film  222   a  has a ring shape, the outer diameter thereof is somewhat less than the inner diameter of the group of concavities  236   a , and the inner diameter of the thin film  222   a  is about in the middle between the outer diameter of a shaft  20  and the inner diameter of the group of concavities  236   a . The thin film  222   a  is disposed concentrically with the shaft  20 . The thin film  222   b  also has a ring shape, the outer diameter thereof is less than the outer diameter of the thin film  222   a , and the inner diameter is larger than the inner diameter of the thin film  222   a . The thin film  222   b  is laminated on the thin film  222   a . A seal layer (not shown in the figure) similar to the seal layer  222  is also formed on a lower surface of the impeller  236  (see  FIG. 8 ). 
     In the fuel pump of the tenth embodiment, the seal layer  222  is formed on the upper surface of the impeller  236 , and a seal layer similar to the seal layer  222  is also formed in the lower surface of the impeller  236 . Therefore, the fuel leakage from the pump channel can be reduced. Furthermore, the seal layers are formed by a well-known thin film formation technology. As a result, they can be formed with high accuracy at a low cost. Because the seal layers can be formed with high accuracy, the clearance between the impeller and the casing can be decreased and pump efficiency can be increased. 
     Embodiment 11 
     An eleventh embodiment of the present teachings will be explained below. As shown in  FIG. 20 , a seal layer  242  is formed in a surface of an upper surface of an impeller  256  (see  FIG. 8 ). The seal layer  242  is formed of a thin film made from a synthetic resin. The seal layer  242  has a ring shape and is disposed on the inner side of a group of concavities  256   a . A group of notches  242   a  is formed in the seal layer  242 . The notches  242   a  have substantially same shape. Each notch  242   a  extends as a curve (spirally) from the center side of the seal layer  242  toward the outer periphery side. An end portion of the notch  242   a  on the center side is shifted in the rotation direction (arrow C) of the impeller  256  with respect to the end portion on the outer periphery side. In other words, the end portion of the notch  242   a  on the center side (i.e., open end of the notch  242   a ) is located at a position forward of a position of the end portion on the outer periphery side (i.e., closed end of the notch  242   a ) in the rotation direction of the impeller  256 . A seal layer (not shown in the figure) similar to the seal layer  242  is formed in a lower surface of the impeller  256  (see  FIG. 8 ). It is noted that the end portion of the notch  242   a  on the outer periphery side may be located at a position forward of a position of the end portion on the center side in the rotation direction of the impeller  256 . 
     In the fuel pump of the eleventh embodiment, since the seal layers are formed in the upper and lower surfaces of the impeller  256 , the fuel leakage from the pump channel can be reduced. Furthermore, because groups of spiral notches are formed in these seal layers, when the impeller  256  rotates, the fuel in the clearance between the impeller  106  and the casing  107  is drawn into the spiral grooves and flows from the end portion on the center side of the spiral groove to the end portion on the outer periphery side of the spiral groove, and the pressure of the fuel in the spiral grooves acts upon the upper and lower surfaces of the impeller  256 . As a result, the impeller  256  is held between the pump cover  108  and the pump body  110 . Since contact between the casing  107  and the impeller  106  is prevented by the pressure of fuel flowing in the spiral grooves, the clearance between the casing  107  and the impeller  106  can be decreased. Therefore, both the reduction in fuel leakage and the reduction in sliding resistance can be realized. 
     In the fuel pumps of the above-described embodiments 1 to 11, the seal layers use a synthetic resin as a material for a thin film constituting the seal layer. However, the present teachings are not limited such configuration, For example, a material obtained by adding an additive to a synthetic resin may be also used as a material for the thin film. When graphite, PTFE (polytetrafluoroethylene (trade name: Teflon)), and molybdenum disulfide that increase sliding ability are used as the additive, sliding resistance can be reduced and wear of the seal layer can be inhibited. Furthermore, where an inorganic filler such as talc and silica is used as an additive to adjust viscosity, an appropriate viscosity can be obtained and operability can be improved. 
     Furthermore, as shown in  FIG. 21 , before a thin film is formed on the surface of the casing or impeller, peaks and valleys may be formed on the surface of a base material (i.e., casing or impeller)  270 . As a result, bonding strength between the thin film  260  of a seal layer and the base material  270  is increased by an anchor effect. Note that peaks and valleys on the surface of the base material  270  can be formed by etching. 
     Alternatively, a porous material may be used as a base material  272 , as shown in  FIG. 22 . As a result, because peaks and valleys are present on the surface of the base material  272 , bonding of the thin film  260  of a seal layer and the base material  272  can be increased without processing the surface of the base material  272 . 
     Furthermore, as shown in  FIG. 23 , an intermediate layer  280  composed of a material having affinity for both a base material  274  and the seal layer  260  may be provided between the seal layer  260  and the base material  274 . For example, where the base material  274  is aluminum or a synthetic resin and the seal layer  260  is from a stainless steel foil, an epoxy resin and/or an acrylic resin can be used as a material of the intermediate layer  280 . In this case, the epoxy resin and/or acrylic resin acts as an adhesive that can bond the seal layer  260  and the base material  274 . 
     As described hereinabove, with the fuel pumps of embodiments 1 to 11, a seal layer is formed on the surface of a casing or an impeller by a thin film formation technology that enables accurate processing in an easy manner and at a low cost. As a result, two problems that have in a trade-off relationship, namely, the reduction in fuel leakage and the reduction in sliding resistance, can be resolved with good stability. Therefore, pump efficiency can be significantly improved. 
     Several preferred embodiments of the present teachings have been described above, but these embodiments are merely illustrating examples and do not limit the scope of the claims. Various alternatives and modifications to the above specific examples are included in the technology described in the scope of the patent claims. 
     Furthermore, the technological elements disclosed in the present specification and appended drawings have technical utility individually or in various combinations thereof and are not limited to the combinations described in the claims at the time of filing. Moreover, the art disclosed in the present specification and appended drawings achieve a plurality of objects simultaneously, and have technical utility by achieving one of these objects.