Patent Publication Number: US-6213094-B1

Title: High-pressure fuel pump

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-pressure fuel pump installed in a high-pressure fuel supply assembly used in a cylinder-injected engine, for example. 
     2. Description of the Related Art 
     FIG. 5 is a block diagram of a conventional high-pressure fuel supply assembly  100 , and FIG. 6 is a cross section thereof. This high-pressure fuel supply assembly  100  includes: a low-pressure damper  2  for absorbing surges in low-pressure fuel, the low-pressure damper  2  being connected to a low-pressure fuel intake passage  1  through which flows low-pressure fuel from a low-pressure fuel pump (not shown); a high-pressure fuel pump  3  for pressurizing low-pressure fuel from the low-pressure damper  2 ; a high-pressure damper  5  for absorbing surges in the high-pressure fuel flowing through a high-pressure fuel discharge passage  4  connected to the high-pressure fuel pump  3 ; and a check valve for improving the starting of an engine by maintaining fuel in delivery pipes  8  at high pressure even when the engine is stopped, the check valve being disposed between the high-pressure damper  5  and a fuel supply port  7  and opening when the fuel pressure on the delivery pipe  8  side is lower than the fuel pressure on the high-pressure damper  5  side. Moreover, in the drawings,  17  is a passage connecting to a high-pressure regulator (not shown) from between the fuel supply port  7  and the delivery pipe  8 . 
     The above low-pressure damper  2  is mounted in a first recess  10   a  in a casing  10 . The low-pressure damper  2  includes: a cylindrical holder  14 ; a base  13  having a ball  11  disposed in a bore  12 ; and a metal bellows  15  disposed inside the holder  14 . 
     The above high-pressure fuel pump  3  includes: a valve assembly  20  for opening and closing the low-pressure fuel intake passage  1  and the high-pressure fuel discharge passage  4 ; and a high-pressure fuel supply body  21  for pressurizing low-pressure fuel and discharging it into the high-pressure fuel discharge passage  4 . 
     FIG. 7 is a cross section of the valve assembly  20 , the valve assembly  20  being composed of a first plate  22 , a second plate  23 , and a thin, flat valve main body  19  positioned between the first and second plates  22  and  23 . A first fuel inlet  24  connected to the low-pressure fuel intake passage  1  and a first fuel outlet  25  connected to the high-pressure fuel discharge passage  4  are formed in the first plate  22 , the inside dimensions of the first fuel outlet  25  being larger than the inside dimensions of the first fuel inlet  24 . A second fuel inlet  26  having inside dimensions larger than those of the first fuel inlet  24  and a second fuel outlet  27  having inside dimensions smaller than those of the first fuel outlet  25  are formed in the second plate  23 . The valve main body  19  is provided with an intake-side tongue  28  interposed between the first fuel inlet  24  and the second fuel inlet  26 , and a discharge-side tongue  29  interposed between the first fuel outlet  25  and the second fuel outlet  27 . 
     The high-pressure fuel supply body  21  includes: a casing  10  housing the valve assembly  20  inside a second recess  10   b;  a cylindrical sleeve  30  housed in surface contact with the second plate  23  of the valve assembly  20 ; a piston  33  slidably inserted inside the sleeve  30  forming a fuel pressurization chamber  32  in cooperation with the sleeve  30 ; and a first spring  36  disposed between a recessed bottom surface  34  of the piston  33  and a holder  35 , the spring  36  applying force to the piston  33  in a direction which expands the volume of the fuel pressurization chamber  32 . 
     The high-pressure fuel supply body  21  also includes: a housing  37  fitted over the sleeve  30 ; a ring-shaped securing member  38  securing the valve assembly  20 , the sleeve  30 , and the housing  37  inside the second recess  10   b  of the casing  10  by fitting over the housing  37  and engaging the second recess  10   b  of the casing  10  by a male thread portion formed on an outer circumferential surface of the securing member  38 ; a metal bellows  40  disposed between the housing  37  and a receiving portion  39 ; a second spring  41  compressed and disposed around the outside of the bellows  40  between the housing  37  and a holder  42 ; and a bracket  43  disposed to surround the second spring  41 , the bracket  43  being secured to the casing  10  by a bolt (not shown). 
     The high-pressure fuel supply body  21  also includes: a tappet  44  slidably disposed in a slide bore  43   a  in an end portion of the bracket  43 ; a pin  45  rotatably suspended in the tappet  44 ; a bush  46  rotatably disposed on the pin  45 ; and a cam roller  47  rotatably disposed on the bush  46 , the cam roller  47  contacting a cam (not shown) secured to a cam shaft (not shown), following the shape thereof, and reciprocating the piston  33 . 
     The above high-pressure damper  5  is screwed into a third recess  10   c  in the casing  10 . The high-pressure damper  5  includes: a first case  50 ; a second case  51  disposed opposite the first case  50 , the second case  51  forming a space in cooperation with the first case  50 ; and a thin, flat disk-shaped stainless steel diaphragm  54  dividing the space into a back-pressure chamber  52  charged with high-pressure gas and a buffer chamber  53 . The diaphragm  54  moves so that the pressure of the fuel flowing into the buffer chamber  53  from the high-pressure fuel discharge passage  4  is equalized with the pressure of the high-pressure gas in the back-pressure chamber  52 , thereby changing the volume inside the buffer chamber and absorbing surges in the fuel in the high-pressure fuel discharge passage  4 . 
     In a high-pressure fuel supply assembly  100  having the above construction, the piston  33  is reciprocated by the rotation of the cam secured to the cam shaft of an engine (not shown) by means of the cam roller  47 , the bush  46 , the pin  45 , and the tappet  44 . 
     When the piston  33  is descending (during the fuel intake stroke), the volume of the inside of the fuel pressurization chamber  32  increases and the pressure inside the fuel pressurization chamber  32  decreases. When the pressure inside the fuel pressurization chamber  32  falls below the pressure at the first fuel inlet  24 , the intake-side tongue  28  of the valve main body  19  bends towards the second fuel inlet  26 , allowing fuel in the low-pressure fuel supply passage  1  to flow through the first fuel inlet  24  into the fuel pressurization chamber  32 . 
     When the piston  33  is ascending (during the fuel discharge stroke), the pressure inside the fuel pressurization chamber  32  increases, and when the pressure inside the fuel pressurization chamber  32  rises above the pressure at the first fuel outlet  25 , the discharge-side tongue  29  of the valve main body  19  bends towards the first fuel outlet  25 , allowing fuel in the fuel pressurization chamber  32  to flow through the first fuel outlet  25  and the fuel discharge passage  4  into the high-pressure damper  5 , where fuel pressure surges are absorbed. High-pressure fuel is then supplied to the delivery pipes  8  via the check valve  6  and the fuel supply port  7 , and thereafter supplied to the fuel injection valves  9 , which inject fuel into each of the cylinders (not shown) of the engine. 
     In the high-pressure fuel pump  3  of the high-pressure fuel supply assembly  100  of the above construction, the housing  37 , the sleeve  30 , and the valve assembly  20  are held inside the second recess  10   b  by the securing member  38 . Because the securing member  38  presses on an outer circumferential portion of the housing  37 , the valve assembly  20  is subjected to a large load from the casing  10  at an outer circumferential portion of the valve assembly  20 , and to an extremely small load at a central portion of the valve assembly  20 . FIG. 6 shows the distribution of the load at that time, and it can be seen that the load increases radially outwards. 
     At the central portion of the valve assembly  20 , the pressure bearing on the valve assembly  20  is extremely low, and during the fuel intake stroke, when the load acting on a peripheral portion  27   a  of the second fuel outlet  27  on the second plate  23  through the discharge-side-tongue  29  at the mouth of the first fuel outlet  25  corresponds to the cross-sectional area of the mouth multiplied by the discharge pressure, there is a risk that the second plate  23  will be deformed by the load towards the piston  33  in the vicinity of the central portion where the pressure bearing on the peripheral portion  27   a  is extremely low. 
     Similarly, during the fuel discharge stroke, when the load acting on a peripheral portion  24   a  of the first fuel inlet  24  on the first plate  22  through the intake-side tongue  28  at the mouth of the second fuel inlet  26  due to the high pressure in the fuel pressurization chamber  32  corresponds to the cross-sectional area of the mouth multiplied by the pressure inside the fuel pressurization chamber, there is a risk that the first plate  22  will be deformed by the load towards the high-pressure damper  5  in the vicinity of the central portion where the pressure bearing on the peripheral portion  24   a  is extremely low. 
     When the second plate  23  or the first plate  22  bend in this manner, even though there should not normally be any gap between the second plate  23  and the discharge-side tongue  29  during the fuel intake stroke, a gap forms between the second plate  23  and the discharge-side tongue  29  in the vicinity of the central portion where the bearing pressure is extremely low. Similarly, even though there should not normally be any gap between the first plate  22  and the intake-side tongue  28  during the fuel discharge stroke, a gap forms between the first plate  22  and the intake-side tongue  28  in the vicinity of the central portion where the bearing pressure is extremely low. Consequently, when the discharge pressure is high, one problem has been that fuel leaks out from between the second plate  23  and the discharge-side tongue  29  during the fuel intake stroke, and out from between the first plate  22  and the intake-side tongue  28  during the fuel discharge stroke, dramatically reducing volumetric efficiency {(the actual amount of fuel discharged into the high-pressure fuel discharge passage  4  from the fuel pressurization chamber  32  during one stroke of the piston  33 )/(the cross-sectional area of the piston  33 × the stroke distance)}. Another problem has been that due to the formation of the above gaps, fretting occurs in places other than the intake-side tongue  28  and the discharge-side tongue  29  of the valve main body  19 , such as between elements of the casing  10 , the valve assembly  20 , and the sleeve  30 , giving rise to fuel leaks from gaps there and reducing the discharge flow. 
     SUMMARY OF THE INVENTION 
     The present invention aims to solve the above problems and an object of the present invention is to provide a high-pressure fuel pump with improved volumetric efficiency in which valve fretting is prevented. 
     To this end, according to the present invention, there is provided a high-pressure fuel pump comprising: a valve assembly disposed between a low-pressure fuel intake passage and a high-pressure fuel discharge passage, the valve assembly opening and closing the low-pressure fuel intake passage and the high-pressure fuel discharge passage; and a high-pressure fuel supply body for pressurizing low-pressure fuel flowing from the low-pressure fuel intake passage and discharging pressurized fuel into the high-pressure fuel discharge passage, the valve assembly including: a first plate having a first fuel inlet connected to the low-pressure fuel intake passage, and a first fuel outlet connected to the high-pressure fuel discharge passage; a second plate having a second fuel inlet having inside dimensions larger than inside dimensions of the first fuel inlet, and a second fuel outlet having inside dimensions smaller than inside dimensions of the first fuel outlet; and a thin, flat valve main body positioned between the first plate and the second plate, the valve main body having an intake-side tongue interposed between the first fuel inlet and the second fuel inlet opening only when fuel flows from the low-pressure fuel intake passage into the high-pressure fuel supply body, and a discharge-side tongue interposed between the first fuel outlet and the second fuel outlet opening only when fuel flows from the high-pressure fuel supply body into the high-pressure fuel discharge passage, the high-pressure fuel supply body including: a casing housing the valve assembly in a recess; a sleeve housed in the recess in surface contact with the valve assembly; a piston slidably inserted into the sleeve forming a fuel pressurization chamber in cooperation with the sleeve, the piston pressurizing fuel flowing into the fuel pressurization chamber from the low-pressure fuel intake passage; and a securing member securing the valve assembly and the sleeve inside the recess by pressing on an outer circumferential portion of the sleeve, a back-pressure chamber connected to the high-pressure fuel discharge passage being formed in the casing so as to face a central portion of the first plate. 
     According to another aspect of the present invention, there is provided a high-pressure fuel pump comprising: a valve assembly disposed between a low-pressure fuel intake passage and a high-pressure fuel discharge passage, the valve assembly opening and closing the low-pressure fuel intake passage and the high-pressure fuel discharge passage; and a high-pressure fuel supply body for pressurizing low-pressure fuel flowing from the low-pressure fuel intake passage and discharging pressurized fuel into the high-pressure fuel discharge passage, the valve assembly including: a first plate having a first fuel inlet connected to the low-pressure fuel intake passage, and a first fuel outlet connected to the high-pressure fuel discharge passage; a second plate having a second fuel inlet having inside dimensions larger than inside dimensions of the first fuel inlet, and a second fuel outlet having inside dimensions smaller than inside dimensions of the first fuel outlet; and a thin, flat valve main body positioned between the first plate and the second plate, the valve main body having an intake-side tongue interposed between the first fuel inlet and the second fuel inlet opening only when fuel flows from the low-pressure fuel intake passage into the high-pressure fuel supply body, and a discharge-side tongue interposed between the first fuel outlet and the second fuel outlet opening only when fuel flows from the high-pressure fuel supply body into the high-pressure fuel discharge passage, the high-pressure fuel supply body including: a casing housing the valve assembly in a recess; a sleeve housed in the recess in the casing in surface contact with the valve assembly; a piston slidably inserted into the sleeve forming a fuel pressurization chamber in cooperation with the sleeve; and a securing member securing the valve assembly and the sleeve inside the recess by pressing on an outer circumferential portion of the sleeve, a pressing member being provided for integrating the casing and the valve assembly and for pressing the valve assembly towards the casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is cross section of a high-pressure fuel supply assembly incorporating a high-pressure fuel pump according to Embodiment 1 of the present invention; 
     FIG. 2 is a graph showing the relationship between fuel discharge pressure and volumetric efficiency in a high-pressure fuel pump; 
     FIG. 3 is a cross section of a high-pressure fuel supply assembly incorporating a high-pressure fuel pump according to Embodiment 2 of the present invention; 
     FIG. 4 is a cross section of a high-pressure fuel supply assembly incorporating a high-pressure fuel pump according to Embodiment 3 of the present invention; 
     FIG. 5 is a block diagram showing the construction of a conventional high-pressure fuel supply assembly; 
     FIG. 6 is a cross section of a conventional high-pressure fuel supply assembly; and 
     FIG. 7 is a cross section of the valve assembly of the high-pressure fuel pump in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A high-pressure fuel supply assembly  200  according to the present invention will be explained below. Parts the same as or corresponding to those in FIGS. 5 to  7  above will be given the same numbering. 
     Embodiment 1 
     FIG. 1 is a cross section of a high-pressure fuel supply assembly  200 . This high-pressure fuel supply assembly  200  includes: a low-pressure damper  2  for absorbing surges in low-pressure fuel, the low-pressure damper  2  being connected to a low-pressure fuel intake passage  1  through which flows low-pressure fuel from a low-pressure fuel pump (not shown); a high-pressure fuel pump  60  for pressurizing low-pressure fuel from the low-pressure damper  2 ; a high-pressure damper  5  for absorbing surges in the high-pressure fuel flowing through a high-pressure fuel discharge passage  4  connected to the high-pressure fuel pump  3 ; and a check valve for improving the starting of an engine by maintaining fuel in delivery pipes  8  at high pressure even when the engine is stopped, the check valve being disposed between the high-pressure damper  5  and a fuel supply port  7  and opening when the fuel pressure on the delivery pipe  8  side is lower than the fuel pressure on the high-pressure damper  5  side. 
     The above low-pressure damper  2  is mounted in a first recess  62   a  in a casing  62 . The low-pressure damper  2  includes: a cylindrical holder  14 ; a base  13  having a ball  11  disposed in a bore  12 ; and a metal bellows  15  disposed inside the holder  14 . 
     The above high-pressure fuel pump  60  includes: a valve assembly  20  for opening and closing the low-pressure fuel intake passage  1  and the high-pressure fuel discharge passage  4 ; and a high-pressure fuel supply body  61  for pressurizing low-pressure fuel and discharging it into the high-pressure fuel discharge passage  4 . 
     As shown in FIG. 7 above, the valve assembly  20  is composed of a first plate  22 , a second plate  23 , and a thin, flat valve main body  19  positioned between the first and second plates  22  and  23 . A first fuel inlet  24  connected to the low-pressure fuel intake passage  1  and a first fuel outlet  25  connected to the high-pressure fuel discharge passage  4  are formed in the first plate  22 , the inside dimensions of the first fuel outlet  25  being larger than the inside dimensions of the first fuel inlet  24 . A second fuel inlet  26  having inside dimensions larger than those of the first fuel inlet  24  and a second fuel outlet  27  having inside dimensions smaller than those of the first fuel outlet  25  are formed in the second plate  23 . The valve main body  19  is provided with an intake-side tongue  28  interposed between the first fuel inlet  24  and the second fuel inlet  26 , and a discharge-side tongue  29  interposed between the first fuel outlet  25  and the second fuel outlet  27 . 
     The high-pressure fuel supply body  61  includes: a casing  62  housing the valve assembly  20  in a second recess  62   b;  a cylindrical sleeve  30  housed in the second recess  62   b  in surface contact with the second plate  23 ; a piston  33  slidably inserted into the sleeve  30  forming a fuel pressurization chamber  32  in cooperation with the sleeve  30 ; and a first spring  36  disposed between a recessed bottom surface  34  of the piston  33  and a holder  35 , the spring  36  applying force to the piston  33  in a direction which expands the volume of the fuel pressurization chamber  32 . 
     The high-pressure fuel supply body  61  also includes: a housing  37  fitted over the sleeve  30 ; a ring-shaped securing member  38  securing the valve assembly  20 , the sleeve  30 , and the housing  37  inside the second recess  62   b  of the casing  62  by fitting over the housing  37  and engaging the second recess  62   b  of the casing  62  by a male thread portion formed on an outer circumferential surface of the securing member  38 ; a metal bellows  40  disposed between the housing  37  and a receiving portion  39 ; a second spring  41  compressed and disposed around the outside of the bellows  40  between the housing  37  and a holder  42 ; and a bracket  43  disposed so as to surround the second spring  41 , the bracket  43  being secured to the casing  62  by a bolt (not shown). 
     The high-pressure fuel supply body  61  also includes: a tappet  44  slidably disposed in a slide bore  43   a  in an end portion of the bracket  43 ; a pin  45  rotatably suspended in the tappet  44 ; a bush  46  rotatably disposed on the pin  45 ; and a cam roller  47  rotatably disposed on the bush  46 , the cam roller  47  contacting a cam (not shown) secured to a cam shaft (not shown), following the shape thereof, and reciprocating the piston  33 . 
     The above high-pressure damper  5  is screwed into a third recess  62   c  in the casing  62 . The high-pressure damper  5  includes: a first case  50 ; a second case  51  disposed opposite the first case  50 , the second case  51  forming a space in cooperation with the first case  50 ; and a thin, flat disk-shaped stainless steel diaphragm  54  dividing the space into a back-pressure chamber  52  charged with high-pressure gas and a buffer chamber  53 . The diaphragm  54  moves so that the pressure of the fuel flowing into the buffer chamber  53  from the high-pressure fuel discharge passage  4  is equalized with the pressure of the high-pressure gas in the back-pressure chamber  52 , thereby changing the volume inside the buffer chamber and absorbing surges in the fuel in the high-pressure fuel discharge passage  4 . 
     A connecting passage  63  having one end connected to the high-pressure discharge passage  4  is formed in the casing  62  on the axis of the piston. A back-pressure chamber  64  facing a central portion of the first plate  22  is also formed in the casing  62 . This back-pressure chamber  64  is connected to the connecting passage  63 . An O-ring  65  for forming an airtight seal between the first plate  22  and an outer circumferential portion of the back-pressure chamber  64  is disposed on the outer circumferential portion of the back-pressure chamber  64 . 
     In a high-pressure fuel supply assembly  200  having the above construction, the piston  33  is reciprocated by the rotation of the cam secured to the cam shaft of an engine (not shown) by means of the cam roller  47 , the bush  46 , the pin  45 , and the tappet  44 . 
     When the piston  33  is descending (during the fuel intake stroke), the volume of the inside of the fuel pressurization chamber  32  increases and the pressure inside the fuel pressurization chamber  32  decreases. When the pressure inside the fuel pressurization chamber  32  falls below the pressure at the first fuel inlet  24 , the intake-side tongue  28  of the valve main body  19  bends towards the second fuel inlet  26 , allowing fuel in the low-pressure fuel supply passage  1  to flow through the first fuel inlet  24  into the fuel pressurization chamber  32 . 
     When the piston  33  is ascending (during the fuel discharge stroke), the pressure inside the fuel pressurization chamber  32  increases, and when the pressure inside the fuel pressurization chamber  32  rises above the pressure at the first fuel outlet  25 , the discharge-side tongue  29  of the valve main body  19  bends towards the first fuel outlet  25 , allowing fuel in the fuel pressurization chamber  32  to flow through the first fuel outlet  25  and the fuel discharge passage  4  into the high-pressure damper  5 , where fuel pressure surges are absorbed. High-pressure fuel is then supplied to the delivery pipes  8  via the check valve  6  and the fuel supply port  7 , and thereafter supplied to the fuel injection valves  9 , which inject fuel into each of the cylinders (not shown) of the engine. 
     Whereas in a conventional assembly the pressure bearing on the central portion was extremely low, in a high-pressure fuel pump  60  of a high-pressure fuel supply assembly  200  of the above construction, as shown in FIG. 1, the pressure bearing on the central portion is increased by subjecting the central portion to the load of the discharged high-pressure fuel through the back-pressure chamber  64 , and the pressure bearing on the outer circumferential portion is also maintained at the level of a conventional assembly, ensuring bearing pressure over the entire surface, so that the formation of undesirable gaps between the second plate  23  and the discharge-side tongue  29  is suppressed during the fuel intake stroke, and similarly, the formation of undesirable gaps between the first plate  22  and the intake-side tongue  28  is suppressed during the fuel discharge stroke. Consequently, the volumetric efficiency will not drop suddenly even if the fuel discharge pressure is raised. 
     Moreover, the magnitude of the load at the central portion of the valve assembly  20  can be controlled by changing the fuel discharge pressure and the radial dimensions of the back-pressure chamber  64 . 
     FIG. 2 is a graph showing the relationship between the discharge pressure of the fuel from the fuel pressurization chamber  32  and volumetric efficiency and is based on data obtained in experiments conducted by the present inventors comparing a comparative example with Embodiment 1 of the present invention under conditions where an engine was running at 3000 rpm. From these results, it can be seen that drops in volumetric efficiency when the fuel discharge pressure was high were significantly reduced in Embodiment 1 of the present invention compared to the comparative example. 
     Embodiment 2 
     FIG. 3 is a cross section of a high-pressure fuel supply assembly  300  according to Embodiment 2 of the present invention, in which an equalizing member  72  is disposed in a back-pressure chamber  70  formed in a casing  71 . An O-ring  73  for forming a tight seal between an outer wall of the equalizing member  72  and an inner wall of the back-pressure chamber  70  is disposed between the outer wall and the inner wall. 
     The rest of the construction is the same as for Embodiment 1 and explanation thereof will be omitted. 
     In this embodiment, high-pressure fuel flowing into the back-pressure chamber  70  from the high-pressure fuel discharge passage  4  is stopped by the equalizing member  72 , and is further prevented from flowing to the first plate  22  side by the O-ring  73 . 
     Furthermore, because the load resulting from the discharged fuel acts on the first plate  22  through the equalizing member, a uniform load is applied to the first plate  22 , suppressing the formation of gaps in the valve assembly  20  proportionately. 
     Embodiment 3 
     FIG. 4 is a cross section of a high-pressure fuel supply assembly  400  according to Embodiment 3 of the present invention, in which a casing  80  and the valve assembly  20  are integrated by a pressing member  81 . 
     The pressing member  81  has a projection  82  to which one end of the spring  36  is attached, and a thread portion  83  engaging the casing  80  and the first plate  22 . The pressing member  81  presses the central portion of the valve assembly  20  towards the high-pressure damper  5  by means of the projection  82 . 
     In Embodiment 3, an outer circumferential portion of the valve assembly  20  is subjected to a load pressing towards the high-pressure damper  5  by means of the securing member  38 , and the central portion thereof is subjected to a load pressing towards the high-pressure damper  5  by means of the pressing member  81  so that the valve assembly  20  is firmly held all over by the casing  80 , the sleeve  30 , and the projection  82 . Consequently, the formation of gaps between the second plate  23  and the discharge-side tongue  29  during the fuel intake stroke and the formation of gaps between the first plate  22  and the intake-side tongue  28  during the fuel discharge stroke are suppressed, and thus the volumetric efficiency will not drop significantly even if the fuel discharge pressure is high. 
     As explained above, a high-pressure fuel pump according one aspect of the present invention comprises a back-pressure chamber connected to the high-pressure fuel discharge passage being formed in the casing so as to face a central portion of the first plate. Therefore, load is also applied to the central portion so that the formation of undesirable gaps between the second plate and the discharge-side tongue is suppressed during the fuel intake stroke in the vicinity of the central portion where the bearing pressure is conventionally extremely low, and similarly, the formation of undesirable gaps between the first plate and the intake-side tongue are suppressed during the fuel discharge stroke in the vicinity of the central portion where the bearing pressure is conventionally extremely low. Consequently, the volumetric efficiency will not drop significantly even if the fuel discharge pressure is raised. The amplitude of any drops in volumetric efficiency can also be minimized. Furthermore, the occurrence of fretting in the valve assembly due to the formation of gaps is prevented. 
     According to one form of the high-pressure fuel pump, the back-pressure chamber may be disposed on the axis of the piston. Therefore, biases in the pressing load distribution acting on the valve assembly can be prevented, and the formation of gaps can be further suppressed. 
     According to another form of the high-pressure fuel pump, an O-ring for forming an airtight seal between the first plate and an outer circumferential portion of the back-pressure chamber may be disposed between the first plate and the outer circumferential portion. Therefore, high-pressure fuel is prevented from flowing from the back-pressure chamber to the valve assembly side so that the volumetric efficiency will not drop significantly even if the fuel discharge pressure is raised. 
     According to still another form of the high-pressure fuel pump, an equalizing member for uniformly pressing the first plate may be disposed in surface contact with the first plate within the back-pressure chamber. Therefore, high-pressure fuel flowing into the back-pressure chamber from the high-pressure fuel discharge passage is stopped by the equalizing member and is prevented from flowing to the first plate side so that the volumetric efficiency will not drop significantly even if the fuel discharge pressure is raised. Furthermore, a uniform load is applied to the first plate, suppressing the formation of gaps in the valve assembly proportionately. 
     According to one form of the high-pressure fuel pump, an O-ring for forming an airtight seal between an outer wall of the equalizing member and an inner wall of the back-pressure chamber may be disposed between the outer wall and the inner wall. Therefore, high-pressure fuel is prevented from flowing from the back-pressure chamber to the valve assembly side so that the volumetric efficiency will not drop significantly even if the fuel discharge pressure is raised. 
     According to another aspect of the present invention, the high-pressure fuel pump comprises a pressing member being provided for integrating the casing and the valve assembly and for pressing the valve assembly towards the casing. Therefore, load is also applied to the central portion so that the formation of undesirable gaps between the second plate and the discharge-side tongue is suppressed during the fuel intake stroke, and similarly, the formation of undesirable gaps between the first plate and the intake-side tongue are suppressed during the fuel discharge stroke, and consequently the volumetric efficiency will not drop significantly even if the fuel discharge pressure is raised. Furthermore, the occurrence of fretting in the valve assembly due to the formation of gaps is prevented. 
     According to one form of the high-pressure fuel pump, the pressing member may comprise: a projection for attaching one end of a spring which elastically presses the piston, the projection pressing the second plate; and a thread portion engaging the casing. Therefore, the holder conventionally holding the spring can be modified and used as a pressing member, enabling improvements to the volumetric efficiency without increasing the number of parts.