Motor-driven fuel pump
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor-driven fuel pump for pumping fuel from a fuel tank through a pressure regulator and an injector into an internal combustion engine. 
     2. Description of the Prior Art 
     An engine fuel injection system has been known which includes a motor-driven fuel pump adapted to be disposed in a fuel tank so as to pump liquid fuel from the tank through a pressure regulator and a filter to an injector which is operative to inject the fuel into an engine. A typical prior art motor-driven fuel pump of the class specified has been desinged to provide a discharge pressure of more than 200 Kpa. The discharge requirement for the prior art fuel pump was solely to meet the lower limit of the discharge pressure. In designing the prior art fuel pump, the designer was not required to pay any particular attention to the upper limit of the pump discharge. 
     Recent automotive fuel injection systems manufactured by automobile manufacturers, however, need fuel pumps to be operated to provide discharge pressure of medium levels around 100 Kpa. Because of this greatly lowered fuel pump discharge pressure, the pressure regulators of associated fuel injection systems are required to provide greatly increased pressure-regulating abilities so as to more strictly control the upper and lower limits of the pressure of fuel to be injected. However, pressure-regulators which satisfy this requirement are very difficult to manufacture and tend to be very expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a motor-driven fuel pump which does not need a highly increased pressure-regulating ability of an asociated presure regulator but is operate itself to provide a controlled discharge pressure which falls within a predetermined desirable pressure range, i.e., a discharge pressure having an upper limit within the predetermined pressure range. 
     The motor-driven fuel pump according to the present invention is operated to pump liquid fuel from a fuel tank through a fuel line including a pressure-regulator and an injector into an internal combustion engine. The fuel pump includes an electric motor to be energized by an electric power source, such as batteries mounted on an associated automobile, a pump housing defining therein a pump chamber provided with fuel inlet and outlet, an impeller rotatively disposed in the pump chamber and drivingly connected to the motor to suck the fuel from the tank through the fuel inlet into the pump chamber and discharge the fuel through the outlet, means defining a fuel discharge passage extending in fluid-flow communication with the pump chamber outlet, a check valve disposed in the fuel discharge passage to prevent backward flow of the fuel toward the pump chamber outlet, and overflow means operative to allow a part of the fuel in the fuel discharge passage upstream of the check valve to overflow at a controlled rate. 
     The overflow means may preferably comprise a plug hole and a plug member therein. The plug member and the plug hole may preferably be so shaped and dimensioned as to define therebetween an overflow passage having a fluid-flow cross-sectional area which is determined by the position of the plug member relative to the plug hole. Accordingly, when the component parts of the motor-driven fuel pamp have been assembled into a unit, the pump may be experimentarily operated by the manufacturer to check the discharge characteristic of the pump. If the pump discharge thus checked is higher or lower than a predetermined desirable range, the position of the plug member relative to the plug hole can be adjusted by the pump manufacturer to adjust the rate of overflow of the fuel and thus the pump discharge. Preferably, the plug member is rotatable at that time relative to the plug hole to enable the plug member to be rotated by the manufacturer by means of a tool, such as a screw driver, to adjust the rotational position of the plug member relative to the plug hole. 
     The fuel pump may preferably be provided with an adjustment-locking member which is attached, after the adjustment of the overlfow, to the pump housing and to the plug member to threby prevent any further rotation of the plug member relative to the plug hole. 
     As the overflow means provided in the fuel pump according to the present invention can be adjusted by the manufacturer to assure that the pump provides a discharge characteristic having not only the lower dishcarge limit but also the upper discharge limit both falling within a pressure range which has been predetermined to avoid the necessity for a highly precise pressure-regulating ability of an expensive pressure regulator to be used with the fuel pump in a fuel injection system for an internal combustion engine. 
     The above and other objects, features and advantages of the present invention will be made more apparent by the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a fuel injection system including an embodiment of the fuel pump according to the present invention; 
     FIG. 2 is an enlarged axial sectional view of the fuel pump shown in FIG. 1; 
     FIG. 3 is a bottom view of the fuel pump shown in FIG. 2; 
     FIG. 4 is a fragmentary sectional view of the pump taken along line IV--IV in FIG. 2; 
     FIG. 5 is a plan view of a lower pump housing member as viewed from line V--V in FIG. 2; 
     FIG. 6 is an enlarged fragmentary plan view of the lower pump housing member showing a discharge end portion of a fuel discharge passage and a fuel overflow mechanism disposed therein; 
     FIG. 7 is a sectional view of the overflow mechanism taken along line VII--VII in FIG. 6; 
     FIG. 7A is a view similar to FIG. 7 but illustrates a plug hole with a plug removed therefrom; 
     FIG. 8 is a side elevational view of the plug; 
     FIG. 9 is a bottom view of the plug; 
     FIG. 10 is a side elevational view of a plug adjustment-locking member; 
     FIG. 11 is a bottom view of the adjustment-locking member; 
     FIG. 12 is a view similar to FIG. 7 but shows the adjustment-locking member inserted into the bottom end of the plug hole and attached to the plug; and 
     FIG. 13 is a bottom view of the overflow mechanism and the locking member shown in FIG. 12. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring first to FIG. 1 of the drawings, a fuel injection system for an internal combustion engine E includes an embodiment of a motor-driven fuel pump 2 according to the present invention. The pump 2 is disposed in liquid fuel contained in a fuel tank 1 and includes a filter 12 disposed at an inlet section of the pump, a discharge pipe 3 and an overflow mechanism 4 to be described in detail later. The discharge pipe 3 is connected by a coupling member 5 to one end of a fuel conduit 6 the other end of which is connected to a fuel pressure regulator 8. A fuel filter 7 is disposed in the fuel line 6 upstream of the pressure regulator 8. A branch conduit 10 is connected at one end to the fuel line 6 between the filter 7 and the pressure regulator 8 and has the other end connected to a fuel injector 9 associated with an intake manifold of the engine E. A return pipe 11 extends between the pressure regulator 8 and the fuel tank 1. The fuel pump 2 is electrically connected to an electrical power source 14. 
     Referring to FIGS. 2 and 3, the fuel pump 2 includes a regenerative type impeller 20 provided with two circumferential rows of vane grooves 20a formed in the end faces of the impeller along the circumferential edges of the end faces. The vane grooves 20a in one of the impeller end faces are circumferentially staggered relative to the vane grooves 20a in the other impeller end face. 
     The impeller 20 is rotatably disposed in a pump chamber 21 defined in a pump housing 22 which comprises an outer pump housing member 22a with an integral suction port 23, an inner pump housing member 22b disposed in abutment contact with the outer pump housing member 22a to cooperate therewith to define the pump chamber 21 therebetween, a motor brush housing 22c retaining a brush unit 26 in sliding contact with a commutator 25 of an electric motor 24, and a cylindrical metallic yoke housing 22d surrounding the peripheries of the outer and inner pump housing members 22a and 22b and a part of the periphery of the brush housing 22c to secure them into a unitary structure. For this purpose, the yoke housing 22d has a plurality of integral tabs 22d 1  depending from the bottom end of the yoke housing and bent over the outer surface of the outer pump housing member 22a. 
     The motor 24 is a DC motor including permanent field magnets 24a disposed around a rotor 61 and held between the brush housing 22c and the inner pump housing member 22b. Electric terminals 60 are mounted on the brush housing 22c and electrically connected to the brush unit 26 and thus to the commutator 25 so that the rotor 61 can be rotated. The rotor 61 is supported by a rotor shaft 64 having an upper end rotatably supported by an upper bearing 62 fixed to the brush housing 22c, the lower end portion of the rotor shaft 64 being rotatably supported by a lower bearing 63 fixed to the inner pump housing member 22b. The lower end portion of the rotor shaft 64 has an extension 64awhich is drivingly connected to the impeller 20 and extends therethrough into rotatable engagement with a thrust bearing 65 fixed to the outer pump housing member 22a. 
     Referring to FIGS. 2 to 6, a circumferential fluid passage 21b is defined between the grooved outer peripheral region of the bottom end face of the impeller 20 and the axially opposed region of the outer pump housing member 22a, as best shown in FIG. 5. A similar circumferential fluid passage (not shown) is defined between the upper end face of the impeller 20 and the inner pump housing member 22b. When the impeller 20 is rotated by the motor 24, the fuel is sucked from the fuel tank 1 through the suction port 23 moved through the fluid passages 21b in counterclockwise direction as viewed in FIG. 5 while the fuel pressure is progressively increased until the pressurized fuel flows through an outlet 21a of the pump chamber 21, as indicated by an arrow F in FIG. 5. The outer pump housing member 22a is formed therein with an aperture 21b&#39; communicating the fuel passage 21b with the interior of the tank 1 to facilitate removal of air bubbles contained in the fuel being pressurized. 
     The outlet 21a is divergent toward its downstream end 21c as shown in FIG. 5. The above-mentioned discharge pipe 3 is integral with the inner pump housing member 22b and has a lower end open to the diverged downstream end of the outlet 21a of the fuel passage 21b. A tubular tip 3a is sealingly fixed to the upper end of the discharge pipe 3. A check valve 30 is disposed within the discharge pipe 3 and comprises a generally conical valve member 30c and a compression coil spring 30a extending between the inner end of the tubular tip 3a and the valve member 30c to downwardly urge the same into sealing engagement with an annular valve seat 30b formed in the inner pump housing member 22b (see FIG. 2). The valve 30 is adapted to be normally opened by the fuel pressure in the outlet during operation of the fuel pump 2. 
     The above-mentioned overflow mechanism 4 is provided in the outer pump housing member 22a adjacent to the diverged downstream end portion 21c of the fuel outlet 21a to permit excess fuel to be returned therefrom back into the fuel tank 1. For this purpose, the overflow mechanism 4 includes a plug hole 40 formed in the outer pump housing member 22a and communicated with the diverged downstream end portion 21c of the fuel outlet 21a and a plug member 41 inserted into the plug hole 40, as best seen in FIG. 4. The plug member 41 has an enlarged top with which the bottom face of the inner pump housing member 22b is in abutment engagement to retain the plug member 41 against removal from the plug hole 40, as best shown in FIG. 4. 
     The overflow mechanism 4 will be described in more detail hereunder with reference to FIGS. 4 to 9. The plug hole 40 includes a larger upper end section 40a communicated with the diverged end portion 21c of the fuel outlet 21a, a larger lower end section 40b open in the underside of the outer pump housing member 22a and a substantially cylindrical intermediate section 40c interconnecting the upper and lower end sections 40a and 40b, as best seen in FIG. 7A. In the inner cylindrical peripheral surface of the intermediate section 40c, there is formed a recess or groove 40a&#39; which extends circumferentially of the plug hole 40, has its top open to the larger upper end section 40a of the hole 40 and has a bottom 46 which extends obliquely relative to the axis of the hole 40, as best seen in FIG. 7A. 
     The plug member 41 includes a generally C-shaped flange 41a extending around a part of the periphery of the top portion of a cylindrical main section 41b and having circumferentially opposed circumferential ends which define an opening or notch 41a&#39; therebetween, as best seen in FIGS. 6 and 9. An elongated axial groove 43 is formed in the outer peripheral surface of the cylindrical section 41b and is disposed in axial alignment with but axially spaced from the notch 41a&#39; in the flange 41a. The top face of the plug member 41 is planar so as to be abutted by the inner pump housing member 22b as shown in FIG. 4, but the bottom face of the plug member 41 is formed therein with a cross-shaped, tool-engagement recess 42 (see FIG. 9) which is substantially axially co-extensive with the elongated groove 43 (see FIG. 9) and communicated therewith (see FIG. 9). 
     The plug hole 40 and the plug member 41 are arranged such that the flange 41a is loosely received in the larger upper section 40a while the cylindrical section 41b is snugly received in the cylindrical intermediate section 40c of the plug hole 40 with the outer peripheral surface of the cylindrical section 41b being in sealing engagement with but slidable relative to the cylindrical section 40c of the plug hole 40, as will be seen in FIG. 7. In addition, the upper part of the elongated axial groove 43 in the plug member 41 can be brought into overlapping relationship to the axially inclined circumferential groove 40a&#39; formed in the inner peripheral surface of the plug hole 40, as will be seen in FIG. 7. It will be appreciated that the area of the axial groove 43 which is overlapped with the inclined groove 40a&#39; is variable depending upon the rotational position of the plug member 41 relative to the plug hole 40. The notch 41a&#39; in the flange 41a is operative to communicate the diverged end 21c of the fuel outlet 21a  of the pump chamber 21 with the axially inclined circumferential groove 40a&#39;. Accordingly, the rate of overflow of the fuel from the fuel outlet 21a through the overflow mechanism 4 back into the fuel tank 1 can be adjusted by rotating the plug member 41 relative to the plug hole 40 by means of a tool such as a screw driver (not shown) which is engageable with the tool-engagement groove 42 in the plug member 41. If the plug member 41 is rotated to a positoin in which the axial groove 43 in the plug member is not overlapped with the axially inclined circumferential groove 40a&#39; in the outer pump housing member 22a, the fuel cannot overflow through the overflow mechanism 4. 
     The steps of assembling the component parts of the fuel pump 2 except for the plug member 41 are well known in the art and it will be sufficient to make it clear that the plug member 41 is fitted into the plug hole 40 before the outer and inner pump housing members 22a and 22b are assembled together. After all of the component parts of the fuel pump 2 described above have been assembled into a semi-finished pump unit, the unit is subjected to a test with respect to the discharge characteristic thereof. More specifically, the pump unit is experimentarily operated by the manufacturer to adjust the pump discharge. 
     The rate of discharge through the discharge pipe 3 is measured while the plug member 41 of the overflow mechanism 4 is rotated by a tool such as a screw driver to adjust the rate of overflow of pumped fuel through the overflow mechanism. If the fuel pressure at the discharge end of the discharge pipe 3 is higher than a predetermined desirable level, then the plug member 41 can be rotated to increase the overflow through the overflow mechanism 4 to lower the upper limit of fuel discharge pressure of the pump 2, and vice versa. As such, not only the lower limit of the pump discharge pressure but also the upper limit threof can be adjusted to fall within a predetermined discharge pressure range to thereby avoid any undue pressure rise in the fuel injection system due to insufficient pressue regulating ability of the pressure regulator in the fuel injection system. 
     Referring to FIGS. 10 to 13, the fuel pump 2 is provided with means for locking the adjustment of the overflow mechanism 2 thus obtained. The locking means comprises an adjustment locking member 45 driven into the lower hole section 40b in the outer pump housing member 22a and into the tool-engagement recess 42 in the plug member 41, as shown in FIGS. 12 and 13, to prevent any further accidential or intentional rotation of the plug member 41 relative to the plug hole 40 to thereby lock the adjustment of the overflow characteristic of the overflow mechanism 4. The adjustment-locking member 45 is formed from a plastic material and has a pair of legs 45a engaged with the tool-engagement recess 42 in the plug member 41 and a base portion having radial arms 45b firmly engaged with the inner peripheral surface of the lower section 40b of the plug hole 40. 
     It will be noted that the adjustment-locking member 45 is driven into the plug hole section 40b and into the tool-engagement reces in the plug member 41 after all of the other component parts of the fuel pump 2 have been assembled and the rate of the overflow of the fuel through the overflow mechanism 4 has been adjusted.