1.  Field of the Invention
This invention relates to a fuel supply apparatus, and more particularly to a fuel supply apparatus for supplying a fuel under a high pressure to a fuel injection type internal combustion engine, for example, an automobile engine. 
2.  Description of the Related Art
FIGS.  8 to 14 are drawings describing a related art general fuel supply system for a fuel injection type internal combustion engine.  FIG.  8 is a schematic illustration of this fuel supply system, FIG.  9 a sectional view of a principal portion of a fuel supply apparatus included in this fuel supply system, FIG.  10 a sectional view taken along the line Xxe2x80x94X in FIG.  9, FIG.  11 a partial enlarged sectional view of what is shown in FIG.  9, taken along a plane Y-Z and illustrating the contacting condition of a driving cam and a tappet of the fuel supply apparatus, FIG.  12 a sectional view taken along a plane Y-X with respect to the plane Y-Z in FIG.  11, FIG.  13 a graph showing the condition of the deformation of a pressure receiving surface of the tappet which receives a force of the driving cam, and FIG.  14 a graph showing the condition of the distribution of Hertz stress on the pressure receiving surface. 
Referring to FIGS.  8 to 9, the fuel supply system includes a fuel tank 1, a fuel supply apparatus 6 and fuel injection valves 10 as main elements, the fuel supply apparatus 6 having a filter 11, a low-pressure damper 12, a suction valve 13, an electromagnetic valve 17, a pump 16, and a discharge valve 14. 
Fuel 2 in the fuel tank 1 is sent out by the low-pressure pump 3, pressure regulated by a low-pressure regulator 5 via the filter 4, and supplied to the fuel supply apparatus 6.  Only such a quantity of the fuel 2 thus supplied to the apparatus that is necessary for fuel injection is pressure-increased by the fuel supply apparatus 6, and supplied to a common rail 9 of an internal combustion engine (not shown), the fuel being then injected as a high-pressure atomized fuel from the fuel injection valves 10 into cylinders (not shown) of the internal combustion engine.  The quantity of fuel needed during this time is determined by a control unit (not shown) and controlled by the electromagnetic valve 17, and an excess fuel is discharged from the electromagnetic valve 17 to the portion of a fuel passage which is between the low-pressure damper 12 and suction valve 13.  A reference numeral 7 in FIG.  8 denotes a filter, and 8 a high-pressure relief valve, which is opened when the pressure in the interior of the common rail becomes abnormally high, to prevent the common rail 9 and fuel injection valve 10 from being broken. 
Referring to FIG.  9 showing a principal portion of the fuel supply apparatus 6, the pump 16 includes a cylinder 25 incorporated in a cylinder casing 30 and provided with a pressure chamber 24 therein which has a fuel suction port 22 and a fuel discharge port 23; a piston 26 moving slidingly in the axial direction thereof in the cylinder 25 to vary the volume of the pressure chamber 24; a columnar tappet 28 joined to the piston 26; and a bolt 29 fitted slidably around the tappet 28 and having a threaded portion engaged with the cylinder casing 30.  Referring to FIGS.  10 to 12, a driving cam 41 mounted on a cam shaft 40 of the engine contacts a pressure receiving surface 28a at a lower end in the drawing of the tappet 28, and a rotational force of the driving cam 41 occurring due to the rotation of the cam shaft 40 is transmitted to the tappet 28 and piston 26 via the pressure receiving surface 28a as a driving force.  Owing to the driving force thus transmitted to the piston 26, the piston 26 is moved vertically to vary the volume of the pressure chamber 24. 
A surface 261, which contacts the tappet 28, of the piston 26 bulges slightly toward the tappet 28 as shown in FIGS.  11 and 12.  The reason why the surface 261 is thus bulged resides in that, when the tappet 28 is moved slidingly in the axial direction owing to the rotation of the driving cam 41, inclination of the tappet 28 occurs due to a clearance set between the tappet 28 and bolt 29, which inclination reduces a lateral force transmitted from an upper surface 28b of the tappet 28 to the piston 26. 
Referring to FIG.  11, all of arrows a, b, c represent positions from which a force from the driving cam 41 is applied to the pressure receiving surface 28a.  Out of these arrows, the arrow b represents a position from which the force is applied to the portion of the pressure receiving surface 28a which is close to the center thereof, while both of the arrows a, c represent positions from which the force is applied to the portions of the pressure receiving surface 28a which are on somewhat inner side of the outer circumference thereof.  As shown in FIG.  11, the driving cam 42 is generally formed wider than the tappet 28.  In an initial stage of an operation of the driving cam 41, the condition of the application of the force by the driving cam 41 to the receiving surface 28a is uniform over the whole of the same surface 28a.  Accordingly, the levels of the force applied to the force applying positions represented by the arrows a, b, c are also uniform. 
However, as described above, the portion of the upper surface 28b of the tappet 28 which is around the force applying position represented by the arrow b contacts the bulging portion of the surface 261 of the piston 26, while the portions of this surface 28b which are around the force applying positions of the arrows a, c have a narrow clearance between the upper surface 28b and the surface 261.  Due to the existence of this clearance, the pressure receiving surface 28a is deformed as shown by a solid line in FIG.  13, and the distribution of Hertz stress during this time becomes as shown by a solid line in FIG.  14.  FIGS.  13 and 14 show data obtained when a fuel discharge pressure is as high as 15 MPa. 
What are shown in FIGS.  13 and 14 will now be described.  The lateral axis of each of FIGS.  13 and 14 represents a position of the tappet 28 in the direction of Z-axis, and the longitudinal axis of each of FIGS.  13 and 14 a displacement distance (xcexcm) based on the deformation of the pressure receiving surface 28a and measured from an initial position thereof, and Hertz stress (MPa).  Each of the solid curves in FIGS.  13 and 14 shows the distribution of Hertz stress recorded when the fuel discharge pressure is 15 MPa.  The a, b, c in each of these drawings represent displacement distances (FIG.  13) and Hertz stress (FIG.  14) in the force applying positions of the arrows a, b, c. As is clear from FIG.  13, the displacement distance becomes maximal around the positions of arrows a, c, and decreases at an outer circumference.  As a result, the Hertz stress becomes maximal at inflexion points of the displacement distance around the arrows a, c as is clear from FIG.  13. 
When the fuel discharge pressure is thus high, the abrasion of the driving cam 41 and tappet 28 increases due to the high Hertz stress occurring locally in positions around those of the arrows a, c, i.e. the positions near the outer circumference of the pressure receiving surface 28a.  In order to deal with this problem, the related techniques employed a method of reducing Hertz stress by increasing the outer diameter of the tappet 28 and the width and outer diameter of the driving cam 41, but this method caused the dimensions and weight of the fuel supply apparatus 6 to increase. 
The present invention has been made in view of the above-mentioned circumstances, and provides a fuel supply apparatus capable of reducing the abrasion of a driving cam and a tappet without increasing the dimensions and weight of the apparatus. 
The fuel supply apparatus for supplying a fuel to an engine according to the present invention includes a cylinder, a piston and a tappet.  The cylinder is provided with a fuel pressurization chamber having a fuel suction port and a fuel discharge port.  The piston is moving slidingly in the axial direction thereof in the cylinder and thereby increasing and decreasing the volume of the fuel pressurization chamber.  The tappet has a pressure receiving surface for contacting a driving cam of the engine and receiving a driving force of the driving cam, and which transmits the driving force to the piston.  The tappet has a groove on its outer surface.  The groove is positioned in the region which corresponds to the vicinity of an outer circumference of the pressure receiving surface for preventing the local concentration of stress thereon. 
Accordingly, the groove gives an easily deformable portion of a low rigidity.  The easily deformable portion is positioned in the region which corresponds to the vicinity of the outer circumference of the pressure receiving surface.  The easily deformable portion works to relax the Hertz stress, and, owing to this action of the easily deformable portion, an effect of reducing the abrasion of the driving cam and tappet is obtained. 
Preferably, in the fuel supply apparatus, the tappet includes a larger-diameter portion and a smaller-diameter portion.  The larger diameter portion is engaged with a tappet stopper provided on an opened end portion of a cylinder casing.  The smaller-diameter portion is capable of passing through the tappet stopper and has the pressure receiving surface.  The groove is formed on the outer surface of the smaller-diameter portion. 
Still preferably, in the fuel supply apparatus, the tappet has a board-like portion between the pressure-receiving surface and the groove.  Accordingly, the board-like portion functions as a pressure receiving portion which effectively receives the driving force from the driving cam. 
Still preferably, the fuel supply apparatus in which the outer diameter of the larger-diameter portion is 10 mm to 15 mm with the thickness of the board-like portion is 0.5 mm to 1.5 mm. 
Accordingly, the board-like portion functions as a pressure receiving portion which effectively receives the driving force from the driving cam, without being broken even when the board-like portion receives the driving force from the driving cam. 
Still preferably, the fuel supply apparatus in which the depth of the groove measured from the outer surface of the lager-diameter portion is 0.5 mm to 2 mm. 
Still preferably, the fuel supply apparatus in which the groove has a V-shaped, semicircular or U-shaped cross section. 
Accordingly, the easily deformable portion given by the groove maintains a low rigidity and, moreover, does not have the problem of the occurrence of the breakage thereof even when the easily deformable portion receives the driving force from the driving cam.