Abstract:
A pulsation damper mounted in a fuel chamber ( 23 ) of a high-pressure fuel pump ( 20 ) is provided with a diaphragm ( 11 ) having a flat section ( 11   a ) displaced when fuel pressure is applied thereto, a pump cover ( 10 ) for supporting the diaphragm ( 11 ), and a gas chamber ( 12 ) formed by the diaphragm ( 11 ) and the pump cover ( 10 ). Pressure pulsation occurring in the fuel chamber ( 23 ) is suppressed by displacement of the flat section ( 11   a ). The diaphragm ( 11 ) is formed in a closed-bottomed tubular shape with the flat section ( 11   a ) located at the bottom and has a projection ( 11   b ) provided to the periphery of the flat section ( 11   a ) and projecting to the side opposite to the pump cover ( 10 ). A tubular peripheral section extending from the outer periphery of the projection ( 11   b ) so as to be vertical to the flat section ( 11   a ) is fitted over the pump cover ( 10 ). The externally fitting portion of the tubular peripheral section is a joint section ( 11   c ) joined to the pump cover ( 10 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a National Stage of International Application No. PCT/JP2009/055202 filed Mar. 17, 2009, the contents of all of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a pulsation damper, particularly to a pulsation damper that is integrally provided to a high-pressure fuel pump for feeding high pressure fuel to the delivery pipe of an in-cylinder injection internal combustion engine that uses gasoline as fuel, and reduces pulsations generated by the operation of the pump. 
     BACKGROUND ART 
     As is known, an in-cylinder injection internal combustion engine using gasoline as fuel includes a high-pressure fuel pump that receives fuel pumped up from a fuel tank by a fuel pump, pressurizes the fuel to a pressure higher than the discharge pressure of the fuel pump, and sends the pressurized fuel to a delivery pipe (high-pressure piping) connected to an injector serving as a fuel injection device. Typically, in an internal combustion engine having such a high-pressure fuel pump, the pressure of fuel that has been pumped up from the fuel tank by the fuel pump is maintained at a “feed pressure”, which is not more than 400 kPa when the fuel is supplied to a fuel chamber formed in the high fuel pressure fuel pump. Fuel that has been supplied to the fuel chamber is then sent from the fuel chamber to a pressurizing chamber in a cylinder via an electromagnetic valve. When the amount of fuel in the pressurizing chamber is adjusted to a predetermined amount by an upward motion of a plunger vertically reciprocating in the cylinder, the electromagnetic valve is closed. When the electromagnetic valve is closed, the fuel is pressurized as the plunger is moved upward, and sent under pressure to the delivery pipe via a check valve. The pressure of fuel sent under pressure from the pressurizing chamber is variable between 4 to 13 MPa in accordance, for example, closing timing of the electromagnetic valve. Then, the fuel of which the pressure has been accumulated in the delivery pipe, is directly injected into the cylinders of the engine by valve opening of the injector. At this time, the amount of fuel that flows into the fuel chamber of the high-pressure fuel pump from the fuel pump per unit time is not necessarily equal to the amount of fuel that flows out from the fuel chamber to the pressurizing chamber in the cylinder per unit time. The difference in the fuel amount causes pulsations in the fuel pressure in the fuel chamber. Also, in such a high-pressure fuel pump, fuel that is being pressurized after being sent from the fuel chamber to the pressurizing chamber of the cylinder is returned to the fuel chamber, so that the amount of fuel sent from the pump to the delivery pipe is adjusted. Therefore, the pressure difference between the fuel in a section including the fuel chamber and the fuel that is being pressurized also generates pulsations of the fuel pressure in the fuel chamber. Such pressure pulsation of fuel, in other words, variation in pressure, varies the amount of fuel sent from the fuel chamber to the pressurizing chamber in the cylinder. This contributes to degradation of the adjustment accuracy of the amount of fuel sent from the high-pressure fuel pump to the delivery pipe. 
     Accordingly, high-pressure fuel pumps disclosed in Patent Documents 1 and 2 each have a pulsation damper that absorbs pressure pulsation of fuel in a fuel chamber, so as to reduce pressure pulsation described above. 
     The pulsation damper disclosed in Patent Document 1 has a cross-sectional structure shown in  FIG. 9 . That is, the pulsation damper has two sets of two diaphragms  71   a ,  71   b  provided in a fuel chamber  75  defined in a housing  70 . The diaphragms  71   a ,  71   b  have outer peripheral joint sections  73   a ,  73   b , which are welded to each other and supported by a support member  74 . Each set of the diaphragms  71   a ,  71   b  has a gas chamber  72   a ,  72   b  between two diagrams. The gas chambers  72   a ,  72   b  are filled with inert gas of a predetermined pressure, for example, argon gas or nitrogen gas. The volume of the gas chambers  72   a ,  72   b  changes in accordance with the fuel pressure in the fuel chamber  75 , so that pressure pulsation as described above is absorbed. The fuel chamber  75  receives fuel from a fuel tank (not shown) via a fuel passage  76  connected to the fuel chamber  75 . 
     The pulsation damper disclosed in Patent Document 2 has a cross-sectional structure shown in  FIG. 10  and includes a plate member  83  and a diaphragm  81 . The plate member  83  forms a fuel chamber  85  with a housing  84 . The plate member  83  and the diaphragm  81  are welded to each other at a joint section  81   a  at the periphery. An annular member  86  is provided along the joint section  81   a . The plate member  83  is covered with a pump cover  80 . A gas chamber  82  defined by the plate member  83  and the diaphragm  81  is filled with inert gas of a predetermined pressure, like the pulsation damper disclosed in Patent Document 1. In accordance with the fuel pressure in the fuel chamber  85 , the diaphragm  81  is displaced into the fuel chamber  85  or toward the plate member  83 , thereby absorbing pressure pulsation of fuel. 
     With either of the pulsation damper of Patent Document 1 or 2, when pressure pulsation of fuel occurs in the fuel chamber, the diaphragm is deformed in accordance with the pressure pulsation in a direction to increase or reduce the volume of the gas chamber. This absorbs the pressure pulsation, thereby reducing changes in the fuel pressure. 
     In either of these pulsation dampers, when the volume of the gas chamber changes due to deformation of the diaphragm, a force resulting from the pressure of gas filling the gas chamber acts on members forming the outer periphery of the gas chamber including the joint sections, that is, acts on the diaphragms and the plate member. The force acts from within the gas chamber toward the outside of the gas chamber. Thus, when the force acts on the joint sections, it acts to separate joined members, specifically, the joined diaphragms or the joined diaphragm and plate member. Such a force acts on the joint section each time the diaphragms are deformed due to pressure pulsation. Although the force does not completely separate the joined members from each other, the force causes delamination from the innermost parts of the joint sections. In other words, joint loosening occurs. Therefore, these pulsation dampers need to have members for preventing joint loosening such as the support member  74  (Patent Document 1) or the annular member  86  (Patent Document 2), which apply force for pressing joined members against each other. 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2008-19728 
     Patent Document 2: Japanese Laid-Open Patent Publication No. 2008-2361 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a pulsation damper that, despite a simple structure, is capable of maintaining high reliability at a joint section of a diaphragm that is integrated with a high-pressure fuel pump and operates together with a gas chamber to inhibit pressure pulsations of fuel. 
     To achieve the foregoing objective and in accordance with the present invention, a pulsation damper for a fuel chamber of a high-pressure fuel pump is provided. The pulsation damper includes a diaphragm and a support member. The diaphragm has a displacement section that is displaced by pressure acting there against. The diaphragm reduces pressure pulsation in the fuel chamber by means of displacement of the displacement section. The support member supports the diaphragm, and, together with the diaphragm, forms a gas chamber. The diaphragm is shaped like a lidded cylinder and has a bottom formed by the displacement section and a cylindrical circumferential section extending perpendicularly from the displacement section. The cylindrical circumferential section has a fitting section that is joined to the support member while being fitted to the support member. 
     In the above configuration, the cylindrical circumferential section extends from the displacement section of the diagram at a right angle. While being fitted to the support member for the diaphragm, the fitting portion of the cylindrical circumferential section is joined to the support member. Accordingly, the joint section and the displacement section are perpendicular to each other. That is, if the pressure caused by changes in volume of the gas chamber due to displacement of the displacement section acts on the joint section between the cylindrical circumferential section and the support member, the pressure does not act in a direction for separating the fitting portion from the support section. Therefore, the reliability at the joint section between the diaphragm and the support member is maintained at a high level. 
     According to one aspect of the present invention, the displacement section includes an annular projection and a flat section surrounded by the projection. The annular projection is continuous to the cylindrical circumferential section and has an arcuately bulging cross-sectional shape in the direction opposite to the support member. The cylindrical circumferential section is perpendicular to the flat section. 
     The stress generated in the diaphragm by pressure applied to the displacement section thereof concentrates on a part that is continuous to the cylindrical circumferential section, which extends in a direction perpendicular to the displacement section, that is, on the periphery of the displacement section. In this regard, the projection that has an arcuately bulging cross-sectional shape in the direction opposite to the support member is formed on the periphery of the displacement section, on which stress is concentrated. Also, the remainder of the displacement section is formed to be flat to increase the area for receiving stress concentrated on the periphery. This relaxes the stress acting on the diaphragm. This allows the reliability at the joint section to be maintained at a high level, and therefore further improves the pressure tolerance as a pulsation damper. 
     According to one aspect of the present invention, the support member is a pump cover for the high-pressure fuel pump. 
     According to this configuration, the pump cover of the high-pressure fuel pump, to which the pulsation damper is attached, is used as the support member for the diaphragm of the pulsation damper. Thus, compared to a configuration with an additional support member for supporting the diaphragm, the number of components of the high-pressure fuel pump is reduced, and the size of the high-pressure fuel pump is minimized. 
     In accordance with one aspect of the present invention, the pump cover partially has a low rigidity section with low rigidity. 
     According to this configuration, the low rigidity section of the pump cover correspondingly increases the amount of displacement of the pump cover in response to the pressure applied to the displacement section of the diaphragm. That is, in addition to the diaphragm having the displacement section, the cover serving as the support member can absorb pressure changes in fuel, in other words, pressure pulsation. This increases the range of pressure pulsation that can be absorbed by the entire pulsation damper, and therefore improves pulsation reducing performance. 
     The low rigidity section is, for example, formed by attaching the pump cover to the upper end cylindrical section of a housing of the high-pressure fuel pump, and reducing the thickness of the part that is attached to the upper end cylindrical section so that it has a lowered rigidity. Alternatively, the thickness is reduced in a part of the pump cover to which the cylindrical circumferential section of the diaphragm is joined to form the low rigidity section. Further, the thickness is reduced in a part of the pump cover that faces the displacement section of the diaphragm to form the low rigidity section. These possible structures are all effective. 
     According to these configurations, it is possible to expand the range of pressure that can be absorbed by the pulsation damper simply by reducing the thickness in a part of the material of the pump cover to form a low rigidity section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view with a block diagram, showing a high-pressure fuel pump and surrounding configuration, in which a pulsation damper according to one embodiment of the present invention is used; 
         FIG. 2  is a cross-sectional view showing the cross-sectional structure of the pulsation damper according to the same embodiment; 
         FIG. 3  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to a modification of the same embodiment; 
         FIG. 4  is a graph showing a relationship between a pressure difference calculated by subtracting the pressure of gas sealed in a gas chamber from a fuel pressure, and corresponding changes in volume of the gas chamber; 
         FIG. 5  is a graph showing a relationship between the pressure difference and the stress per unit amount of change in volume; 
         FIG. 6  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to another embodiment; 
         FIG. 7  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to another embodiment; 
         FIG. 8  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to another embodiment; 
         FIG. 9  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to prior art; and 
         FIG. 10  is a cross-sectional view showing the cross-sectional structure of a pulsation damper according to another prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A pulsation damper according to one embodiment of the present invention will now be described with reference to  FIGS. 1 and 2 . 
       FIG. 1  schematically shows a high-pressure fuel pump  20  having a pulsation damper according to the present embodiment and a surrounding structure, or a fuel supply system. The high-pressure fuel pump  20  is attached, for example, to a cylinder head cover of an in-cylinder injection internal combustion engine that uses gasoline as fuel. 
     As shown in  FIG. 1 , the high-pressure fuel pump  20  has a housing  21 , in which a fuel inlet  22   a  and a fuel chamber  23  are provided. Fuel that has been pumped by a fuel pump (feed pump)  41  flows into the fuel inlet  22   a . The fuel is then temporarily retained in the fuel chamber  23 . Also, fuel retained in the fuel chamber  23  is sent to a pressurizing chamber  22   c  in a cylinder via a fuel communication passage  22   b  and an electromagnetic valve  24 . The fuel is then pressurized by a plunger  25  in the pressurizing chamber  22   c , and the pressurized fuel is sent under pressure to a delivery pipe  50  via a check valve  26  and a fuel outlet  22   d.    
     In this high-pressure fuel pump  20 , the fuel chamber  23  has an opening upper end, and the opening is covered with a pulsation damper. The pulsation damper includes a pump cover  10  and a diaphragm  11  joined to the pump cover  10 . The diaphragm  11  has a flat section  11   a , a projection  11   b , and a joint section  11   c . The projection  11   b  is formed to surround the flat section  11   a  and has an arcuate cross-sectional shape bulging toward the fuel chamber  23 . The joint section  11   c  is joined to the pump cover  10 . The electromagnetic valve  24  is located in the fuel communication passage  22   b , which connects the fuel chamber  23  and the pressurizing chamber  22   c  to each other. The electromagnetic valve  24  is a normally closed open type. That is, the electromagnetic valve  24  is closed only when the coil is energized, and closes the fuel communication passage  22   b . Energization of the electromagnetic valve  24  is controlled by an electronic control unit  60 , which controls the operational state of the in-cylinder injection internal combustion engine. Further, a plunger  25  is provided in the cylinder. An end of the plunger  25  opposite to the pressurizing chamber  22   c  is coupled to a lifter  27 , while the plunger  25  is urged toward the bottom dead center by a spring  28 . The bottom of the lifter  27  is pressed against a pump cam  30 , which is provided on and rotates integrally with a camshaft. Each time the cam nose of the pump cam  30  lifts the lifter  27 , the plunger  25  is moved upward to pressurize fuel in the pressurizing chamber  22   c.    
     In the fuel supply system including the high-pressure fuel pump  20  as described above, fuel stored in the fuel tank  40  is supplied to the fuel inlet  22   a  of the high-pressure fuel pump  20  at a discharge pressure, for example, of 400 kPa by the fuel pump (feed pump)  41 . The fuel that has been supplied to the high-pressure fuel pump  20  is temporarily retained in the fuel chamber  23 , and is then delivered to the pressurizing chamber  22   c  via the fuel communication passage  22   b  on condition that the plunger  25  is moving downward in the cylinder and that the electromagnetic valve  24  is in the open state (non-energized state). Thereafter, as the plunger  25  is moved upward, the fuel that has been sent to the pressurizing chamber  22   c  starts being pressurized. While the electromagnetic valve  24  is open, the fuel is not provided to the fuel outlet  22   d , but is returned to the fuel chamber  23  via the fuel communication passage  22   b . Then, when the electromagnetic valve  24  is closed based on energization by the electronic control unit  60 , the pressure of fuel in the pressurizing chamber  22   c  is increased, for example, to 4 to 13 MPa. The pressurized fuel is provided under pressure from the fuel outlet  22   d  to the delivery pipe  50  via the check valve  26 . In the high-pressure fuel pump  20  as described above, it is possible to control the amount and pressure of fuel delivered under pressure to the delivery pipe  50  by controlling the valve closing timing of the electromagnetic valve  24  when the plunger  25  is moved upward. In this manner, fuel stored under pressure in the delivery pipe  50  is injected into the cylinders of the engine when the injector  51  is opened. 
     In the above described fuel supply system, the amount of fuel supplied per unit time to the high-pressure fuel pump  20 , particularly to the fuel chamber  23  by the fuel pump  41  is not necessary equal to the amount of fuel supplied to the pressurizing chamber  22   c  from the fuel chamber  23  via the electromagnetic valve  24 . Therefore, due to the difference between the amount of fuel supplied to and the amount of fuel discharged from the fuel chamber  23 , variation of fuel pressure, or pressure pulsation occurs. In addition, the fuel that is being pressurized as the plunger  25  is moved upward in the pressurizing chamber flows back to the fuel chamber  23  before the electromagnetic valve  24  is closed. This is also a cause of pressure pulsation. Such pressure pulsation is absorbed by the pulsation damper provided to cover the opening of the fuel chamber  23 . 
     Next, the configuration of the pulsation damper, which absorbs pressure pulsation of fuel in the high-pressure fuel pump  20  and the mechanism of absorption of pressure pulsation will be described with reference to  FIG. 2 . 
       FIG. 2  shows the cross-sectional structure of the pulsation damper according to the present embodiment. As shown in  FIG. 2 , the pulsation damper includes the pump cover  10 , which covers the opening of the high-pressure fuel pump  20  ( FIG. 1 ), and the diaphragm  11 , which is supported by the pump cover  10 . The diaphragm  11  contacts fuel retained in the fuel chamber  23  ( FIG. 1 ) and is therefore acted upon by the pressure of the retained fuel. In the present embodiment, the diaphragm  11  is formed like a lidded cylinder with the flat section  11   a  and the annular projection  11   b  surrounding the flat section  11   a . The flat section  11   a  occupies most of the surface area of the diaphragm  11 . The pressure of the fuel applied to the flat section  11   a  in a concentrated manner. The projection  11   b  bulges into the fuel chamber  23  and has an arcuate cross-sectional shape. That is, a cylindrical circumferential section is provided on the outer periphery of the projection  11   b . The cylindrical circumferential section is perpendicular to the flat section  11   a  forming the bottom and extends in a direction opposite to the bulging direction of the projection  11   b . The diaphragm  11  is formed of stainless steel material such as SUS631 (precipitate hardened steel), for example, through pressing to have the described shape. The pump cover  10  also includes a flat section  10   a  and an annular projection  10   b  surrounding the flat section  10   a . When the pulsation damper is assembled, the flat section  10   a  of the pump cover  10  is parallel to the flat section  11   a  of the diaphragm  11 , and the projection  10   b  bulges toward the diaphragm  11 . Also, a circumferential section is provided on the outer periphery of the projection  10   b . The circumferential section extends in a direction opposite to the bulging direction of the projection  10   b . A hook section  10   c  is provided at the upper end of the circumferential section. The hook section  10   c  is hooked to the upper end of the opening of the housing  21  ( FIG. 1 ). The pump cover  10  is formed of stainless steel material such as SUS430 (ferritic stainless steel), for example, through pressing to have the described shape. 
     When assembling the pump cover  10  and the diaphragm  11  together, the distal end of the circumferential section of the diaphragm  11  that is perpendicular to the flat section  11   a  and extends in the direction opposite to the bulging direction of the projection  11   b  is press-fitted about the circumferential section of the pump cover  10  that is perpendicular to the flat section  10   a  and extends in the direction opposite to the bulging direction of the projection  10   b . The press-fitted section is fixed to the circumferential section of the pump cover  10 , which serves as a support member, by welding. In  FIGS. 1 and 2 , a part of the diaphragm  11  that is fixed by welding is referred to as the joint section (fitting section)  11   c . When these members are fitted to each other, the gas chamber  12 , which is defined by the pump cover  10  and the diaphragm  11 , is filled with inert gas such as argon gas or nitrogen gas, at predetermined pressure, such as 400 kPa. The gas is sealed in the gas chamber  12 . When the pump cover  10  and the diaphragm  11  are welded to each other, laser welding can be employed in which laser energy of carbon dioxide gas laser or YAG laser is used. Alternatively, resistance welding can be employed in which two members to be welded are pressed against each other and provided with electric current, so that resistance heat melts the members to be welded. 
     In the pulsation damper, which is configured as described above to be integrally assembled with the high-pressure fuel pump  20  ( FIG. 1 ), the flat section  11   a  of the diaphragm  11 , which is exposed to the fuel in the fuel chamber  23  ( FIG. 1 ), receives pressure pulsation of fuel, which is generated when the above described high-pressure fuel pump  20  ( FIG. 1 ) operates. Since the applied fuel pressure, particularly the pressure of fuel that is being pressurized in the pressurizing chamber  22   c  ( FIG. 1 ) is normally higher than the pressure of the inert gas sealed in the gas chamber  12 , the flat section  11   a  of the diaphragm  11  is deformed toward the pump cover  10 . That is, the deformation reduces the volume of the gas chamber  12 . This absorbs the pressure of fuel. Further, in the pulsation damper according to the present embodiment, when welding the diaphragm  11  to the pump cover  10 , a part of the joint section  11   c  where these members are overlapped is perpendicular to the flat section  11   a , which receives the pressure of fuel. Thus, when pressure pulsation of fuel occurs, the joint section  11   c  only receives shearing load. Also, due to the decrease in the volume of the gas chamber  12 , the pressure of the sealed gas acting on the joint section  11   c  acts in a direction substantially parallel to the joint section  11   c . Since such pressure never acts to separate overlapped parts of the pump cover  10  and the diaphragm  11  in the joint section  11   c , the above described joint loosening is not likely to occur. 
     The present inventors found out that when the same pressure was applied to both the prior art pulsation damper configured as shown in  FIG. 9  and the pulsation damper of the present embodiment, joint loosening, or delamination of the overlapped parts reached 300 μm at maximum in the prior art pulsation damper, and joint loosening was significantly smaller at 0.05 μm in the pulsation damper of the present embodiment. 
     In the case of the prior art pulsation damper shown in  FIG. 10 , when fuel pressure is applied to the flat section of the diaphragm  81 , the stress generated due to deformation of the diaphragm  81  concentrates on the bent section. In contrast, in the pulsation damper according to the present embodiment, the projection  11   b  is provided about the flat section  11   a  of the diaphragm  11 . The stress generated due to deformation of the diaphragm  11  is relaxed by the projection  11   b . That is, compared to the prior art pulsation damper, the area in which stress is concentrated can be enlarged, so that the maximum value of the stress is lowered. Therefore, when designing pulsation dampers assuming that the maximum value of stress that acts on the section is the same, the separation damper of the present embodiment can have a diaphragm of a larger diameter or a less thickness than that in the prior art pulsation damper. The amount of displacement of a diaphragm is proportional to the 4th power of its radius and inversely proportional to the 3rd power of the thickness. Accordingly, the pulsation damper of the present embodiment can have a larger displacement amount than the prior art pulsation damper. In other words, without increasing the number of the diaphragm  11 , the displacement amount of the volume can be increased. 
     The pulsation damper of the present embodiment may be modified as shown in  FIG. 3 . In this modification, a number of, for example, three, projections  11   b  are provided about the flat section  11   a . However, the inventors have found out that the smaller the number of the projections  11   b , the more remarkable the stress relaxing effect became. That is, as shown in  FIG. 2 , the structure in which only one projection  11   b  is provided in the periphery of the diaphragm  11  achieves the most remarkable stress relaxing effect. Hereafter, the results of experiments performed by the inventors will be described with reference to  FIGS. 4 and 5 . The experiments were related to the relationship between the number of projections  11   b  provided about the flat section  11   a  of the diaphragm  11  and the stress relaxing effect. 
       FIG. 4  is a graph showing the relationship between a pressure difference, or the pressure obtained by subtracting the pressure of the inert gas sealed in the gas chamber  12  from the fuel pressure, and the amount of change in volume of the gas chamber  12 , that is, the amount of displacement of the flat section  11   a  of the diaphragm  11 . The black dots in the graph represent sampled values obtained from the structure shown in  FIG. 2 , and the black squares represent sampled values obtained from the structure shown in  FIG. 3 . 
     As obvious from  FIG. 4 , the amount of change in volume per unit pressure acting on the diaphragm  11  has a greater value when only one projection  11   b  is provided in the periphery of the diaphragm  11  either in a case where the pressure difference has a positive value, that is, when the fuel pressure is greater than the pressure of the inert gas sealed in the gas chamber  12 , and the diaphragm  11  is deformed toward the pump chamber  23 , or in a case where the pressure difference has a negative value, that is, when the diaphragm  11  is deformed toward the fuel chamber  23 . 
     On the other hand,  FIG. 5  is a graph showing the relationship between the pressure difference and the value obtained by dividing, by the amount of change in volume, the maximum value of stress generated when the diaphragm  11  is deformed. In this graph, as in  FIG. 4 , the black dots represent values obtained from the structure shown in  FIG. 2 , and the black squares represent values obtained from the structure shown in  FIG. 3 . 
     As obvious from  FIG. 5 , in a case where the pressure difference has a positive value, the stress per unit amount of change in volume is substantially the same between the structure shown in  FIG. 2  and the structure shown in  FIG. 3 , when the pressure difference is 300 kPa. In contrast, in a case where the pressure difference is 400 kPa, the structure shown in  FIG. 3  has smaller stress per unit amount of change in volume than the structure shown in  FIG. 2 . However, the difference is substantially equal to zero. When the pressure difference has a positive value, and between 100 and 200 kPa, the structure shown in  FIG. 2  has a smaller stress per unit amount of change in volume. On the other hand, in a case where the pressure difference has a negative value, the smaller the absolute value of the pressure difference, the greater the difference by which the stress per amount of change in volume of the structure shown in  FIG. 2  is smaller than that of  FIG. 3  becomes. Further, in the range of the pressure difference between −100 to −400 kPa, the stress per unit amount of change in volume of the structure shown in  FIG. 2  is 1.5 times smaller than the structure shown in  FIG. 3 . 
     With reference to the results shown in  FIGS. 4 and 5 , regardless whether the pressure difference has a positive or negative value or the magnitude of the pressure difference, the structure shown in  FIG. 2  achieves a greater amount of change in volume than the structure shown in  FIG. 3 . Also, the structure shown in  FIG. 2  generally has smaller stress per unit amount of change in volume than that of  FIG. 3 . Even if the stress per unit amount of change is greater in  FIG. 2 , the different is substantially zero. That is, by providing only one projection  11   b  about the diaphragm  11 , the stress relaxing effect and the effect of amount of change in volume are remarkable compared to a case where a multiple, for example, three projections  11   b  are formed. 
     As described above, the pulsation damper according to the present embodiment has the following advantages. 
     (1) The cylindrical circumferential section, which perpendicularly extends from the flat section  11   a  of the diaphragm  11  via the projection  11   b , is fitted about the pump cover  10 . In this state, the fitting section of the cylindrical circumferential section is welded to the pump cover  10 . That is, the diaphragm  11  and the pump cover  10  are assembled such that the joint section  11   c  and the flat section  11   a  are perpendicular to each other. Thus, even if the pressure caused by changes in volume of the gas chamber  12  due to displacement of the flat section  11   a  acts on the welded section between the cylindrical circumferential section and the pump cover  10 , the pressure does not act in a direction for separating the joint section  11   c  from the pump cover  10 . Therefore, the reliability of the joint between the pump cover  10  and the joint section  11   c  is maintained at a high level. 
     (2) The projection  11   b , which has an arcuate cross-sectional shape bulging in a direction opposite to the pump cover  10 , is formed in a part surrounding the flat section  11   a , on which stress is concentrated when the diaphragm  11  is displaced, that is, in a periphery continuous to the cylindrical circumferential section of the diaphragm  11 . This relaxes the stress concentrated on the periphery, and thus maintains the reliability of the joint section  11   c  at a high level. That is, this further improves the pressure tolerance of the entire pulsation damper. 
     (3) As in the modification of the present embodiment shown in  FIG. 3 , a plurality of projections  11   b  may be provided in the periphery of the diaphragm  11 . When only one projection  11   b  is provided in the periphery of the diaphragm  11 , a remarkable stress relaxing effect is achieved, and the reliability at the joint section  11   c  can be maintained at a high level. 
     (4) As a support member for the diaphragm  11 , the pump cover  10  of the high-pressure fuel pump  20  is employed. The number of components of the high-pressure fuel pump  20  can be reduced, and the size of the high-pressure fuel pump  20  is maintained to be minimized. 
     The above described embodiment and its modification may be modified as shown below. 
     As shown in  FIG. 2  or  FIG. 3 , which show a modification, the pump cover  10  forming the pulsation damper substantially has a constant thickness. However, the rigidity of the pump cover  10  may be reduced by any of the following configurations. 
     a. As shown in  FIG. 6 , which corresponds to  FIG. 2 , the hook section  10   c  may have a thin section  10   d , which is thinner than the remainder of the pump cover  10 . 
     b. As shown in  FIG. 7 , which corresponds to  FIG. 2 , a thin section  10   e  may be provided in a circumferential section that is perpendicular to the flat section  10   a  and projects in a direction opposite to the bulging direction of the projection  10   b , that is, in a part to which the diaphragm  11  is welded. 
     c. As shown in  FIG. 8 , which corresponds to  FIG. 2 , a thin section  10   f  may be formed in the flat section  10   a  of the pump cover  10 . 
     These configurations provide the following advantage in addition to the above advantages (1) to (4). 
     (5) The amount of displacement of the pulsation damper in accordance with pressure applied to the flat section  11   a  of the diaphragm  11  can be increased by the amount of flexing of low rigidity sections, or the thin sections  10   d ,  10   e ,  10   f.  That is, in addition to displacement of the diaphragm  11 , the pump cover  10  serving as a support member can absorb pressure pulsation generated in fuel, so that the pressure pulsation reduction effect is maintained at a high level. 
     Instead of reducing the rigidity of the pump cover  10  by providing the thin sections  10   d ,  10   e ,  10   f , the parts that correspond to the thin sections may be formed of a material different from the material of the remaining parts, or of a material having a lower rigidity than the remaining parts, so that the rigidity of the pump cover  10  is reduced. However, different types of stainless steel materials, which are preferable as the materials for the pump cover  10 , do not vary significantly in rigidity. Also, forming the pump cover  10  of different materials requires complicated processes. Thus, reduction of the rigidity of the pump cover  10  is practically most easily and effectively achieved by providing the thin section  10   d ,  10   e , or  10   f.    
     In the illustrated embodiment, the diaphragm  11  is fitted about the pump cover  10 . However, the diaphragm  11  may be fitted inside the pump cover  10 . 
     When assembling the pump cover  10  and the diaphragm  11  together, the distal end of the periphery of the diaphragm  11  is press-fitted about the periphery of the pump cover  10 , and then the press-fitted section is welded to fix the diaphragm  11  to the pump cover  10 . However, the diaphragm  11  may be joined to the pump cover  10  by a method other than welding. For example, the diaphragm  11  may be joined to the pump cover  10  by fixing the press-fitted section by adhesive or brazing. 
     The pump cover  10  of the high-pressure fuel pump  20  also functions as a support member supporting the diaphragm  11 . However, the diaphragm  11  may be supported by an additional member provided separately from the pump cover  10 . 
     In the pulsation damper according to the modification shown in  FIG. 3 , the diaphragm  11  has three projections  11   b  of the same widths. However, the widths of the projections may be different. Nevertheless, the pulsation damper shown in  FIG. 2  is most favorable for relaxing the stress as described above. 
     The diaphragm  11  has at least one projection  11   b  in the periphery surrounding the flat section  11   a . However, a diaphragm having no projection  11   b  may be used. That is, a diaphragm may be used in which a flat section  11   a  includes a displacement section having an appropriate curvature and continuous to the cylindrical circumferential section.