Patent Publication Number: US-7216631-B2

Title: Flow damper for common rail fuel injection apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2004-317277 filed on Oct. 29, 2004, the content of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a flow damper (safety valve) to be fastened to a common rail body of a common rail fuel injection apparatus. 
   BACKGROUND OF THE INVENTION 
   Conventional flow damper is described referring to  FIG. 16 . 
   A flow damper J 1  in  FIG. 16  is provided with: an approximately cylinder-shaped valve body J 2  in which a fuel passage is formed; a piston J 4  that is slidable in an axial direction along a piston slide hole J 3  formed in the valve body J 2 ; a spring J 5  that urges the piston J 4  to an upstream side of a fuel flow; and a stopper J 6  that restricts a travel of the piston J 4  to the upstream side. 
   In the piston J 4  is formed an aperture path J 7  that communicates an upstream side and a downstream side of the fuel passage. When any abnormal condition such as excessive fuel outflow occurs in the injector, a downstream flow amount increases to increase a pressure difference before and after the aperture path J 7 , and the piston J 4  moves to the downstream side (injector side) to seat a valve portion J 8  of the piston J 4  on a valve seat J 9  of the valve body J 2 . In this manner, the flow damper J 1  stops the outflow of the high-pressure fuel when any malfunction occurs accidentally (refer to U.S. Pat. No. 6,357,415-B and its counterpart JP-3521811-B, for example). 
   The conventional flow damper J 1  has the following issues. 
   (1) The valve body J 2  is one to be fastened to a common rail body J 10 . The common rail body J 10  accumulates high-pressure fuel, so that intimate contact surfaces of the valve body J 2  and the common rail body J 10  must be highly oil tight seal surfaces, and the valve body J 2  is fastened to the common rail body J 10  at a large axial force. 
   The valve body J 2  is fastened to the common rail body J 10  at a high strength, so that even a slight deviation in accuracy or shape of a seat surface can distort the valve body J 2  in a rotational side at the large axial force. 
   The valve body J 2  supports the piston J 4  therein in a slidable state, therefore, if the valve body J 2  is distorted by the above-described cause to deform the piston slide hole J 3  radially inward, a slide clearance between the valve body J 2  and the piston J 4  decreases to spoil a slide motion of the piston J 4 . 
   In addition, the intimate contact surfaces of the valve body J 2  and the common rail body J 10  (or the stopper J 6 ) require high work accuracy such as a high flatness, which is a cause of a cost increase. 
   (2) A female screw (a hole for inserting the valve body J 2  thereinto) J 11  of the common rail body J 10  may have strain such as deformation by any kind of cause. Correspondingly, as shown in  FIG. 16 , a male screw J 12  at a side of the valve body J 2  is provided on an outer circumference of a direct slide range J 2  in which the valve body J 2  and the piston J 4  are in direct slide contact with each other. 
   Thus, when the valve body J 2  is fastened to the common rail body J 10  at the large axial force, the strain that occurs in the female screw J 11  of the common rail body J 10  is transmitted via a screw-fastening portion to the valve body J 2 . As a result, the valve body J 2  is distorted and the piston slide hole J 3  is distorted, too. 
   In this manner, the distortion of the piston slide hole J 3  spoils the slide motion of the piston J 4 . 
   SUMMARY OF THE INVENTION 
   The present invention is achieved in view of the above-described issues, and has an object to provide a flow damper in which a piston slide motion is not spoiled even if a valve body is fastened to a common rail body at a large axial force. 
   The flow damper has: a valve body to be fastened to a port of a common rail body of a common rail fuel injection apparatus, the valve body having an approximately cylinder-shaped piston hole in one end portion thereof so that the piston hole is coaxially aligned to the valve body to open to the port and to provide a cylindrical wall between an outer circumference of the valve body and an inner circumference of the piston hole; a piston that slides in the piston hole to start or block a fuel flow through the valve body; and a piston operation securing means that secures a slide motion of the piston against a force for the common rail to press the valve body to occur a distortion in the cylindrical wall when the valve body is fastened to the port of the common rail body. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will be appreciated, as well as methods of operation and the function of the related parts, from a study of the following detailed description, the appended claims, and the drawings, all of which form a part of this application. In the drawings: 
       FIG. 1  is a cross-sectional view showing a flow damper according to a first embodiment of the present invention; 
       FIG. 2  is a system construction diagram showing a common rail fuel injection apparatus according to the first embodiment. 
       FIG. 3  is a cross-sectional view showing a flow damper according to a second embodiment of the present invention; 
       FIG. 4  is a cross-sectional view showing a flow damper according to a third embodiment of the present invention; 
       FIG. 5  is a cross-sectional view showing a flow damper according to a fourth embodiment of the present invention; 
       FIG. 6  is a cross-sectional view showing a flow damper according to a fifth embodiment of the present invention; 
       FIG. 7  is a cross-sectional view showing a flow damper according to a sixth embodiment of the present invention; 
       FIG. 8  is a cross-sectional view showing a flow damper according to a seventh embodiment of the present invention; 
       FIG. 9  is a cross-sectional view showing a flow damper according to an eighth embodiment of the present invention; 
       FIG. 10  is a cross-sectional view showing a flow damper according to a ninth embodiment of the present invention; 
       FIG. 11A  is a cross-sectional view showing a flow damper according to a tenth embodiment of the present invention; 
       FIG. 11B  is an enlarged cross-sectional view showing a leading end of a valve body of the flow damper according to the tenth embodiment; 
       FIG. 11C  is an enlarged cross-sectional view showing a deformed state of the leading end of a valve body of the flow damper according to the tenth embodiment; 
       FIG. 12  is a cross-sectional view showing a flow damper according to an eleventh embodiment of the present invention; 
       FIG. 13  is a cross-sectional view showing a flow damper according to a twelfth embodiment of the present invention; 
       FIG. 14  is a cross-sectional view showing a flow damper according to a thirteenth embodiment of the present invention; 
       FIG. 15  is a cross-sectional view showing a flow damper according to a fourteenth embodiment of the present invention; and 
       FIG. 16  is a cross-sectional view showing a conventional flow damper. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   In a first embodiment, a system construction of a common rail fuel injection apparatus is described referring to  FIG. 2 , then a flow damper is described referring to  FIG. 1 . 
   The common rail fuel injection apparatus shown in  FIG. 2  is a system for performing fuel injections in respective cylinders of an engine (for example, a diesel engine: not shown). The common rail fuel injection apparatus is composed of: a common rail  1 ; an injector  2 ; a supply pump  3 ; an ECU (engine control unit)  4 ; an EDU (driving unit); and so on. 
   The common rail  1  is an accumulation container to accumulate high-pressure fuel to be supplied to the injector  1  therein. The common rail  1  is connected via a high-pressure pump pipe  6  to an outflow port of the supply pump  3  to pressure-feed the high-pressure pump so as to accumulate a common rail pressure corresponding to a fuel injection pressure therein. Further, the common rail  1  is connected to a plurality of injector pipes  7  to supply the high-pressure fuel to the respective cylinders. 
   A flow damper  31  is provided at a connection portion of the common rail  1  and the injector pipe  7 . A detailed description of the flow damper  31  is given later. 
   In a relief pipe  9  to return the fuel from the common rail  1  to a fuel tank  8  is installed a pressure limiter  10 . The pressure limiter  10  is a pressure safety valve that opens when a fuel injection pressure in the common rail  1  exceeds a limit set value to limit the fuel injection pressure in the common rail  1  within the limit set value. 
   Further, on the common rail  1  is installed a pressure reduction valve  11 . The pressure reduction valve  11  opens in accordance with a valve open signal applied by the ECU  4  to reduce the common rail pressure via the relief pipe  9  rapidly. In this manner, by installing the pressure reduction valve  11  on the common rail  1 , the ECU  4  can perform a rapid reduction control of the common rail pressure to a pressure in accordance with a vehicle driving state. Some common rail  11  is not provided with the pressure reduction valve  11 . 
   The injector  2  is installed in every cylinder of the engine to supply and inject the fuel in the cylinder. The injectors  2  are connected to downstream ends of a plurality of injector pipes  7  that branch out from the common rail  1 . In the injector  2  are installed a fuel injection nozzle that supplies and injects the high-pressure fuel accumulated in the common rail  1 , an electromagnetic valve that performs a lift control of a needle installed in the fuel injection nozzle, and so on. 
   Leakage fuel from the injector  2  returns via the relief pipe  9  to the fuel tank  8 , too. 
   The supply pump  3  is a high-pressure fuel pump to pressure-feed the high-pressure fuel to the common rail  1 . On the supply pump  3  is installed a feed pump that sucks the fuel in the fuel tank  8  via a filter  12  to the supply pump  3 . The supply pump  3  compresses the fuel sucked by the feed pump to high-pressure, then pressure-feeds the fuel to the common rail  1 . The feed pump and the supply pump  3  are driven by an identical camshaft  13 . The camshaft  13  is rotatably driven by the engine. 
   On the supply pump  3 , a SCV  14  (suction control valve) is installed on a fuel passage to lead the fuel into a pressurization chamber to pressurize the fuel to the high-pressure, to adjust an opening degree of the fuel passage. The SCV  14  adjusts a fuel suction amount sucked into the pressurization chamber and changes a fuel discharge amount to be pressure-feed to the common rail  1 , by being driven by a pump driving signal from the ECU  4 . That is, the ECU  14  adjusts the common rail pressure to a pressure in accordance with a vehicle driving state by controlling the SCV  14 . 
   The ECU  4  is provided with: a CPU that performs a control process and a calculation process; a storage device (a memory device such as a ROM, a standby RAM, an EEPROM and RAM) to store respective programs and data; and a microcomputer with conventional construction including functions such as an input circuit, an output circuit, and a power supply circuit. Then, the ECU  4  performs respective calculation processes in accordance with sensor signals (engine parameters: signals in accordance with driver&#39;s driving state, engine driving state, and so on) that are read in the ECU  4 . 
   To the ECU  4  are connected, as detection means to detect the driving state and the like, sensors and the like such as an acceleration sensor to detect an opening degree of an accelerator, a rotational frequency sensor to detect a number of rotation of the engine, a coolant temperature sensor to detect a temperature of a coolant of the engine, in addition to a common rail pressure sensor  15   
   A specific example of a calculation in the ECU  4  is shown. The ECU  4  performs controls of an injector control system, which performs a drive control of the injector  2 , and of a common rail pressure control system, which performs a drive control of the SCV  14 . 
   In each fuel injection, the injector control system calculates an injection pattern, a target injection amount and an injection start timing in accordance with the programs stored in the ROM and the sensor signals (engine parameters) read in the RAM, then calculates an injector valve-open signal. 
   The common rail pressure control system calculates a target common rail pressure in accordance with the programs stored in the ROM and the sensor signals (engine parameters) read in the RAM, then calculates a SCV driving signal to equalize an actual common rail pressure, which is calculated by the common rail pressure sensor  15 , to the target common rail pressure. 
   The EDU  5  is provided with: an injector drive circuit that applies a valve-open driving current to the electromagnetic valve of the injector  2  in accordance with a injector valve-open signal applied by the ECU  4 ; and a pump drive circuit that applies a drive current value to the SCV  14  in accordance with a SCV drive signal (duty signal) applied by the ECU  4 . The EDU  5  may be installed in a casing together with the ECU  4 . 
   The common rail  1  is a common rail body  20  having a pipe-shape to accumulate ultra high-pressure fuel therein and provided with a pipe connection means  21  to connect the high-pressure pump pipe  6 , the relief pipe  9 , the injector pipes  7  thereto. In addition to the pipe connection means  21 , the common rail body  20  is provide with a functional component connection portions  22  to install the pressure limiter  10 , the pressure reduction valve  11 , the common rail pressure sensor  15 , and so on. 
   As shown in  FIG. 2 , the common rail body  20  may be one formed by forging and on which respective holes and flat surface portions (after-mentioned intra common rail passage, inside and outside communication holes  23 , a first flat surface  24 , and so on) are worked after the forging. As an alternative of the one shown in  FIG. 2 , the common rail body  20  may be constructed of a low-cost pipe material and on which a number of the pipe connection means  21  in an axial direction of the pipe material, to decrease a manufacturing cost. 
   The common rail body  20  is made of hard metal such as steel. The common rail body  20  is provided therein with an intra common rail passage (high-pressure accumulation chamber, not shown) along a longitudinal direction of the common rail body  20 . 
   Further, on a side of the common rail body  20  are formed a plurality of the inside and outside communication holes  23  to communicate its outer circumference and the intra common rail passage (refer to  FIG. 1 ). The inside and outside communication holes  23  are to be communicated with the high-pressure pump pipe  6 , the relief pipe  9 , the injector pie  7 , and so on. The inside and outside communication holes  23  are bored at adequate intervals in the axial direction of the common rail body  20 . An outer side of each inside and outside communication hole  23  opens approximately at a center of the first flat surface  24  formed on the side surface of the common rail body  20 . 
   The outer opening (outside opening portion) of the inside and outside communication hole  23  is provided with a chamfered portion that extends radially outward, to increase an opening area of the outer opening of the inside and outside communication hole  23 . 
   Further, on an inner face of the hole around the first flat surface  24  is formed a first female screw  26  to fasten the pipe connection means  21  (a valve body  32  in an after-mentioned flow damper  31 ) thereto (refer to  FIG. 1 ). An example in which the first female screw  26  is integrally provided with the common rail body  20 , however, the first female screw  26  may be a female screw part such as a nut that is fixed on (combined with) the common rail body  20  by welding and the like. 
   A part of the pipe connection means  21  to connect the common rail body  20  and the injector pipes  7  is provided with a flow damper  31  shown in  FIG. 1 . 
   The flow damper  31  is provided with: a valve body  32  that is to be fastened to the common rail body  20 ; a piston  33  that slides in the valve body  32 ; a spring  34  that urges the piston  33  to an upstream side of fuel flow; and a stopper  35  that restricts a travel of the piston  33  to the upstream side. 
   In the piston  33  is formed an aperture path  36  that communicates an upstream side and a downstream side of the fuel passage. When any abnormal condition such as excessive fuel outflow occurs in the injector  2 , a downstream flow amount increases to increase a pressure difference before and after the aperture path  36 , and the piston  33  moves to the downstream side (injector  2  side) to seat a valve portion  37  of the piston  33  on a valve seat  38  of the valve body  32 . In this manner, the flow damper  31  stops the outflow of the high-pressure fuel when any malfunction occurs accidentally. 
   Respective parts of the flow damper  31  are described in detail in the following. In the following description, one side of the flow damper  31  to be connected to the common rail body  20  is referred to as “lower side”, and the other side, to which the injector pipe  7  is connected, is referred to as “upper side”. 
   The valve body  32  is made of hard metal such as steel, and has an approximately cylinder-shape in which the fuel passage is formed. 
   At the lower side on an outer circumference of the valve body  32  is formed a first male screw  41  to be screwed into the first female screw  26  of the common rail body  20 . At the upper side on an outer circumference of the valve body  32  is formed a second male screw  42  to fix the injector pipe  7  thereon. 
   On a leading end surface of the first male screw  41  is formed a surface that surrounds the opening of the piston slide hole  43 . An upper and lower surfaces of the stopper  35  are provided in parallel with each other. The lower surface of the stopper  35  aligns with the first surface  24  of the common rail body  20 , and the upper surface of the stopper  35  aligns with the leading end surface of the first male screw  41 . Thus, by screw-fastening the first male screw  41  of the valve body  32  tightly to the first female screw  26  of the common rail body  20 , the first surface  24 , the stopper  35  and the leading end surface of the first male screw  41  are pushed to each other to form a body seal surface (oil tight surfaces: intimate contact surfaces). 
   On a leading end surface of the second male screw  42  is formed a pressure reception seat surface  45  having a conically tapered shape into which a conical portion  44 , which is formed on a leading end of the injector pipe  7 , is inserted. On the bottom portion of the pressure reception seat surface  45  opens an upper fuel passage  46 . 
   To the second male screw  42  is screw-fastened a second female screw  48  that is formed on an inner circumference of a pipe fastening screw member  47 . 
   The pipe fastening screw member  47  is screwed into the second male screw  42  in a state of being engaged with a step  44   a  on the rear of the conical portion  44  of the injector pipe  7 . By screw-fastening the pipe fastening screw member  47  tightly to the second male screw  42 , the conical portion  44  of the injector pipe  7  is strongly pushed to the pressure reception seat surface  45  to form a pipe seal surface (oil tight surfaces: intimate contact surfaces). 
   Correspondingly, at a center of the valve body  32  is formed the piston slide hole  43  from a lower end to an approximately central portion to support the piston  33  slidably to provide a cylindrical wall  32   a  between an outer circumference of the valve body  32  and an inner circumference of the piston hole  43 . Further, at the center at an upper portion of the valve body  32  is formed the upper fuel passage  46  that is communicated with an upper end of the piston slide hole  43 . The upper fuel passage  46  and the piston slide hole  43  constitute the fuel passage. 
   At a boundary between the upper fuel passage  46  and the piston slide hole  43  is formed a valve seat  38  having an approximately conical shape to extend downward. The piston slide hole  43  and the upper fuel passage  46  are coaxially disposed, to locate the valve portion  37  of the piston  33  and the valve seat  38  of the valve body  32  coaxially. 
   The piston  33  is made of a material such as steel, aluminum and resins that is resistant to high-pressure fuel. The piston  33  is supported in the piston slide hole  43  of the valve body slidably in the axial direction. The piston  33  is provided with a lower large diameter portion  51  that directly slides on the piston slide hole  43 , and an upper protruding portion  52  of which a diameter is small to form a step between the large diameter portion  51  and itself. At an upper end of the protruding portion  52  is provided with the valve portion  37  that can block the upper fuel passage  46  by seating on the valve seat  38  of the valve body  32 . On the step between the large diameter portion  51  and the protruding portion  52  seats a lower end of the spring  34 , so that the spring  34  urges the piston  33  downward. 
   In the piston  33  is formed the aperture path  36  that communicates a lower portion (a center hole  35   a  of the stopper  35 ) with an upper portion (an inner space of the piston slide hole  43  above the piston  33 ). The aperture path  36  comprises: a lower center hole  53  that is formed at the center in the lower side of the large diameter portion  51 ; an upper communication groove  54  that is formed on the side surface of the large diameter portion  51 ; and an aperture (orifice)  55  that communicates the lower center hole  53  with the upper center hole  54 . 
   When the fuel flow amount that flows downstream in a normal operation time and the like, the urging force of the spring  34  seats the lower end of the piston  33  on the stopper  35 , so that the fuel flow that has passed through the center hole  35   a  of the stopper  35  is supplied to the injector only via the aperture path  36 . 
   When the fuel flow amount that flows downstream increases, the pressure difference before and after the aperture path  36  increases, and the piston  33  moves upward to lift the piston  33  off the stopper  35 . Then, the fuel that has passed through the center hole  35   a  of the stopper  35  is supplied to the injector  2  via both the aperture path  36  and a slide clearance between the large diameter portion  51  of the piston  33  and the piston slide hole  43 . 
   When the fuel flow amount that flows downstream increases, the pressure difference before and after the aperture path  36  further increases by a malfunction occurrence of an excessive fuel discharge into the injector  2  and the like, the pressure difference before and after the aperture path  36  further increases. Then, the piston  33  moves further upward to seat the valve portion  37  at the upper end of the protruding portion  53  on the valve seat  38  of the valve body  32  to block the upper fuel passage  46 . 
   In this manner, the flow damper  31  stops the high-pressure fuel discharge when the fuel flow amount flowing downstream increases over a set amount by any accidental malfunction occurrence. 
   The stopper is made of hard metal such as steel and copper, which has a fine seal performance, and has a disc shape having the center hole  35   a  at the center of which to pass the fuel therethrough. As described above, the center hole  35   a  is the fuel passage to communicate the inside and outside communication hole  23  of the common rail body  20  and the lower center hole  53  of the piston  33 . The stopper  35  is a seal member (gasket) that forms the above-described body seal surfaces being interposed between the first flat surface  24  of the common rail body  20  and the leading end surface of the first male screw  41 . The stopper  35  also has a stopper function to restrict the piston  33  to move downward in the piston slide hole  43 . 
   The spring  34  is a compression coil spring to urge the piston  33  downward. A compression load of the spring  34  determines an operation value (a set value for the flow damper  31  to interrupt the high-pressure fuel discharge) of the flow damper  31 . 
   The valve body  32  is tightly fastened to the common rail body  20  to prevent the high-pressure fuel from leaking securely. However, in the case that the valve body  32  is tightly fastened to the common rail body  20 , if a slight deviation in accuracy or a shape of the seat surface is there, the large axial force and rotational slide can deform the valve body  32 . Specifically, the cylindrical wall  32   a  in the lower end portion of the valve body  32 , which forms the body seal surface, is deformed. 
   As described above, the lower portion of the valve body  32  slidably supports the large diameter portion  51  of the piston  33  therein. The slide clearance between the large diameter portion  51  of the piston  33  and the piston slide hole  43  is small (around 10 μm to 20 μm, for example) to increase accuracy in a coaxial alignment. Thus, if the cylindrical wall  32   a  in the lower end portion of the valve body  32  deforms radially inward, the slide clearance decreases to spoil the slide motion of the piston  33 . 
   Thus, the first embodiments provides a clearance α between the valve body  32  and the piston  33  to absorb a distortion (deformation) that occurs when the valve body  32  is fastened to the common rail body  20 . 
   Specifically, in the first embodiment, a total outer circumference of the lower side of the large diameter portion  51  of the piston  33  is provided with a clearance (cut portion)  56   a  as shown in  FIG. 1  to provide the clearance α to absorb the deformation of the cylindrical wall  32   a  of the valve body  32  in proximity to the lower end thereof. A size of the clearance equals clearances α in the after-mentioned embodiments. Alternatively, the size of the clearance capable of absorbing the deformation occurring in the valve body  32  is acceptable. The size varies in accordance with a kind of the material forming the valve body  32 , a fastening torque, and so on. For example, the size If the clearance in the present embodiment is set as: approximately 5 mm to 10 mm in the axial direction from the lower end of the large diameter portion  51 ; and approximately 0.1 mm to 1.0 mm of width in the radial direction. 
   By providing the flow damper  31  as in the first embodiment, even if a slight deviation in accuracy or a shape of the seat surface occurs a radially inward deformation of the cylindrical wall  32   a  of the valve body  32  in proximity to a lower end thereof when the valve body  32  is fastened to the common rail body  20  at a large axial force, the clearance α between the valve body  32  and the piston  33  absorbs the deformation. Thus, the deformation of the valve body  32  does not affect the slide motion of the piston  33 . That is, to fasten the valve body  32  to the common rail body  20  at the large axial force does not spoil the slide motion of the piston  33 . 
   Further, the clearance α between the valve body  32  and the piston  33  absorbs the deformation of the valve body  32  occurred by the fastening at the large axial force. Thus, as in the first embodiment, it is possible to limit working accuracies of the body seal surfaces (intimate contact surfaces) of the valve body  32  and the stopper  35  for cost decrease. 
   Second Embodiment 
   A second embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 3 . In the following embodiments, the same reference numerals as in the above-described first embodiments denote components having the same function as in the first embodiment. 
   In the second embodiment, as in the first embodiment, a clearance (cut portion)  25   b  is provided over an inner circumference to provide a clearance α to absorb the deformation of the cylindrical wall  32   a  the valve body  32  in the proximity of the lower end thereof. 
   Third Embodiment 
   A third embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 4 . 
   In the third embodiment, an outer diameter size of the large diameter portion  51  of the piston  33  is smaller than an inner diameter size of the piston sliding hole  43  to provide a clearance α between the large diameter portion  51  and the piston  33  to absorb the deformation of the cylindrical wall  32   a  of the valve body  32  in the proximity of the lower end thereof. 
   When the outer diameter size of the large diameter portion  51  of the piston  33  is smaller than the inner diameter size of the piston sliding hole  43  as in the third embodiment, an axial center of the large diameter portion  51  of the piston  33  does not always align with that the piston sliding hole  43 . Then, an axial center of the protruding portion  52  of the piston  33  does not align with that of the upper fuel passage  46  of the valve body  32 . That is, a coaxial alignment of the valve seat  38  and the valve body  37  is spoiled. 
   In the third embodiment, an upper surface of the stopper  35  is provided with a sliding guide  57  for the piston  33 . Thus, the axial center of the large diameter portion  51  of the piston  33  aligns with that of the piston sliding hole  43 , to secure the coaxial alignment of the valve seat  38  and the valve portion  37 . The sliding guide  57  is a support member that slidably supports an inner surface of the lower center hole  53  of the piston  33  in the axial direction, and a fuel passage is provided at the center of the sliding guide  57 . 
   Fourth Embodiment 
   A fourth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 5 . 
   In the fourth embodiment, a collar  58  (corresponding to an additional member), which slidably supports the piston  33 , is disposed between the valve body  32  and the piston  33 . Thus, a clearance α is provided between the valve body  32  and the collar  58  to absorb a deformation occurring in the valve body  32  when the valve body  32  is fastened to the common rail body  20 . 
   Specifically, the collar  58  is a cylindrical body that slidably supports the large diameter portion  51  of the piston  33 , and made of hard metal such as steel, etc. In the valve body  32  is formed a collar insertion hole  59  in which the collar  58  is inserted. An inner circumference of the collar insertion hole  59  and an outer circumference of the collar  58  provide the clearance α therebetween to absorb the deformation occurring in the valve body  32  when the valve body  32  is fastened to the common rail body  20 . 
   By providing the flow damper  31  as in the first embodiment, even if a slight deviation in accuracy or a shape of the seat surface deforms the valve body  32  when the valve body  32  is fastened to the common rail body  20  at a large axial force, the clearance α between the valve body  32  and the collar  58  absorbs the deformation. Thus, the deformation of the valve body  32  does not affect the piston sliding hole  43  provided on an inner circumference of the collar  58 . That is, to fasten the valve body  32  to the common rail body  20  at the large axial force does not spoil the slide motion of the piston  33 . 
   Further, the clearance α between the valve body  32  and the collar  58  absorbs the deformation of the valve body  32  occurred by the fastening at the large axial force. Thus, as in the first embodiment, it is possible to limit working accuracies of the body seal surfaces (intimate contact surfaces) of the valve body  32  and the stopper  35  for cost decrease. 
   Fifth Embodiment 
   A fifth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 6 . 
   In the fifth embodiment, an elastic body  60  is disposed between the collar  58  and the stopper  35  to get rid of a lash of the collar  58 . In  FIG. 6  is shown a conical spring as an example of the elastic body  60 , however, other kinds of the elastic body such as a wave washer and ring rubber may be used. 
   Sixth Embodiment 
   A sixth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 7 . 
   In the sixth embodiment, the collar  58  and the stopper  35  are integrally provided, so as to decrease the number of parts, to get rid of a lash of the collar  58 , and to improve a coaxial alignment of a piston  33  and a valve body  32  (that is, a coaxial alignment of a valve seat  38  and a valve portion  37 . 
   Seventh Embodiment 
   A seventh embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 8 . 
   A collar  58  in the seventh embodiment is provided with not only a piston sliding hole  43  but also a valve seat  38  on which a valve portion  37  at the leading end of the protruding portion  52  of the piston  33  seats. By providing the collar  58  in this manner, it is possible to improve a coaxial alignment of a valve seat  38  and a valve portion  37 . 
   Eighth Embodiment 
   An eighth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 9 . 
   In the eighth embodiment, a restriction member is press-fitted into a piston sliding hole  43  of the valve body  32  to prevent a deformation, which occurs in the valve body  32  when the valve body  32  is fastened to the common rail body  20 , from extending radially inward in the piston sliding hole  43 . 
   Specifically, in the eighth embodiment is shown an example in which a stopper  35  is press-fitted as the restriction member in the piston sliding hole  43 . Alternatively, another restriction member other than the stopper  35  may be press-fitted to an inner circumference of the piston sliding hole  43 . 
   In the present embodiment, a lower end face the cylindrical wall  32   a  of the valve body  32  is in an intimate contact with a first flat surface  24  of the common rail body  20  to form body seal surfaces (oil tight surfaces: intimate contact surfaces). 
   By the configuration as in the eighth embodiment, even when the valve body  32  is fastened to the common rail body  20  at a large axial force, the stopper  35  (restriction member), which is press-fitted to the inner circumference of the piston sliding hole  43 , prevents the piston sliding hole  43  from deforming radially inward. That is, even when the valve body  32  is fastened to the common rail body  20  at the large axial force, it is possible to prevent the sliding clearance between the valve body  32  and the piston  33  from decreasing so as not to spoil a slide motion of the piston  33 . 
   Further, the stopper  35  (restriction member) prevents the piston sliding hole  43  from being deformed radially inward by the fastening at the large axial force, so that it is possible to limit working accuracies of the intimate contact surfaces of the valve body  32  and the common rail body  20  for cost decrease. 
   Ninth Embodiment 
   A ninth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 10 . 
   In the ninth embodiment, the above-described stopper  35  in the third embodiment is provided on its upper surface with a press-fitting portion  61  (restriction member) that is press-fitted into an inner circumference of the piston sliding hole  43 . 
   Tenth Embodiment 
   An eighth embodiment is described referring to a cross-sectional view of a flow damper  31  and a enlarged view of a principal portion of a leading end of a valve body shown in  FIGS. 11A to 11C . 
   In the tenth embodiment are provided: (1) a distortion diverting out means  62  that diverts a distortion, which occurs radially outward in the valve body  32  when the valve body  32  is fastened to the common rail body  20 ; and (2) a clearance α between the valve body  32  and the common rail body  20  to absorb the radially outward distortion by the distortion diverting out means  62 . 
   Specifically, when the valve body  32  is fastened to the common rail body  20 , a slight deviation in accuracy or a shape of the seat surface occurs the deformation in the cylindrical wall  32   a  of the valve body  32  in proximity to a lower end thereof. 
   In the tenth embodiment, as shown in  FIG. 11B , a lower end surface of the cylindrical wall  32   a  of the valve body  32 , which is subjected to a rotational slide under a large axial force in a fastening time of the valve body  32 , is provided with tapering surfaces (inner circumferential tapering width  62   a &gt;Outer circumferential tapering width  62   b ) to deviate the deformation radially outward. Thus, a lower end of the cylindrical wall  32   a  is disposed radially outside of a midpoint in the thickness of the cylindrical wall  32 . 
   Alternatively, the distortion diverting out means  62  may be provided with one tapering surface at the lower end surface of the cylindrical wall  32   a  so that the lower end of the tapering surface is disposed on a radially outer periphery of the lower end surface of the cylindrical wall  32 . Further, the distortion diverting out means  62  may be provided with a rounding at the lower end surface of the cylindrical wall  32   a  so that the lower end of the rounding is disposed radially outside of a midpoint in the thickness of the cylindrical wall  32 . 
   By providing the distortion diverting out means  62  by the tapering surfaces, when the valve body  32  is tightly screw-fastened to the common rail body  20 , the cylindrical wall  32   a  of the valve body  32  in the proximity of the lower end deforms radially outward as shown in  FIG. 11C . 
   Correspondingly, the clearance α is provided between the valve body  32  and the common rail body  20  (a hole for inserting the valve body  32 ) to absorb the deformation of the cylindrical wall  32   a  the valve body  32  in the proximity of the lower end that occurs radially outward by the distortion diverting out means  62 . 
   Specifically, in the tenth embodiment, as shown in  FIG. 11A , the clearance (cut portion)  56   c  is provided to extend over the outer circumference of the lower side of the valve body  32 , so that the clearance α is provided to absorb the radially outward deformation of the cylindrical wall  32   a  of the valve body  32  in the proximity of the lower end thereof. 
   By the configuration as in the tenth embodiment, the deformation in the fastening time of the valve body  32  to the common rail body at the large axial force, occurs radially outward. Then, the deformation is absorbed by the clearance α between the valve body  32  and the common rail body  20 . As a result, it inhibits a problem for the piston sliding hole  43  to deform radially inward. That is, even when the valve body  32  is fastened to the common rail body  20  at a large axial force, it is possible to prevent the sliding clearance between the valve body  32  and the piston  33  from decreasing, not to spoil a slide motion of the piston  33 . 
   Further, the distortion diverting out means  62  and the clearance α between the valve body  32  and the common rail body  20  prevent the sliding hole in the valve body  32  from being deformed radially inward by the fastening at the large axial force, so that it is possible to limit working accuracies of the intimate contact surfaces of the valve body  32  and the stopper  35  for cost decrease. 
   Eleventh Embodiment 
   An eleventh embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 12 . 
   In the eleventh embodiment: (1) an axial force applying portion β, which applies an axial force toward a common rail body  20  to a valve body  32  when the valve body  32  is fastened to the common rail body  20 , and a direct sliding range γ, in which a piston  33  directly slides on the valve body  32 , are provided at a distance from each other in an axial direction; and (2) a clearance α is provided between the direct slide range γ in the valve body  32  and the common rail body  20  (a hole for inserting the valve body  32 ) to prevent the common rail body  20  from pressing the valve body  32  (a clearance α to absorb a distortion occurring in the hole for inserting the valve body  32 ). 
   Specifically, as shown in  FIG. 12 , (1) a first male screw  41  (axial force applying portion β) is formed on an outer circumference of the valve body  32  in the proximity of a midpoint in the axial direction, and a portion of the valve body  32  below the first male screw  41  (direct sliding range γ) is provided to be inserted in the hole of the common rail body  20 , so that and the axial force applying portion b and the direct sliding range γ are provided at a distance from each other in the axial direction, and (2) a clearance (cut portion)  56   d  is provided over an entire outer circumference of the valve body  32  below the first male screw portion  41 , so that the a clearance α is provided to prevent the common rail body  20  from pressing the valve body  32 . The size of the clearance that can absorb the distortion occurring in the hole for inserting the valve body  32  is acceptable, and the size is determined as appropriate in accordance with manufacturing deviations. 
   A shape of the hole for inserting the valve body  32  can have a distortion such as a deformation by any cause such as heat applied before an installation of the valve body  32  or an external load. 
   Therefore, by a configuration as in the eleventh embodiment, even when the valve body  32  is fastened to the common rail body  20  at a large axial force, the distortion occurring in the hole for inserting the valve body  32  is absorbed by the clearance α between the valve body  32  and the common rail body  20 . Thus, it is possible to inhibit a problem for the distortion occurring in the hole for inserting the valve body  32  to be transmitted to the valve body  32 . Accordingly, it is possible to avoid a problem of a distortion of the piston sliding hole  43  so as not to spoil a slide motion of the piston  33 . 
   Twelfth Embodiment 
   A twelfth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 13 . 
   In the twelfth embodiment, a clearance (cut portion)  56   e  is provided to extend over an inner circumference of the hole of the common rail body  20  for inserting a lower portion of the first female screw  26 , so that the clearance α is provided between a portion of a valve body  32  below the first male screw  41  (the direct sliding range γ in the valve body  32 ) and the common rail body  20  to prevent toe common rail body  20  from pressing the valve body  32 . 
   Thirteenth embodiment 
   A thirteenth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 14 . 
   In the above-described eleventh and twelfth embodiments are shown examples in which the clearance α is extended by providing at least one of the valve body  32  and the common rail body  20  with the clearance (cut portion)  56   d ,  56   e.    
   Correspondingly, in the thirteenth embodiment, instead of providing the valve body  32  or the common rail body  20  with the clearance (cut portion)  56   d ,  56   e , a diameter of the hole for inserting the portion of the valve body  32  below the first male screw  41  (the direct sliding range γ of the valve body  32 ) is extended, and an outer diameter of the portion of the valve body  32  below the first male screw  41  (the direct sliding range γ of the valve body  32 ) is narrowed, so that it is intended to increase an insertion clearance for the valve body  32 , and the insertion clearance is used as the clearance α to prevent the common rail body  20  from pressing the valve body  32 . 
   Fourteenth embodiment 
   A fourteenth embodiment is described referring to a cross-sectional view of a flow damper  31  shown in  FIG. 15 . 
   In the fourteenth embodiment, (1) a male screw  63  is formed on an outer circumference of the cylindrical portion of the common rail body  20 , in which the hole for inserting the valve body  32  is formed, and (2) a female screw  66  of a nut  65 , which is associated with a flange  64  provided on the outer circumference of the valve body in the proximity of a midpoint in the axial direction, is tightly screw-fastened to the above-described male screw  63 , so that the lower end of the cylindrical wall  32   a  of the valve body  32  is strongly pressed on the first flat surface  24  pf the common rail body  20 . That is, the association portion between the flange  64  and the nut  65  serves as the axial force applying portion β. By this construction, the axial force applying portion β and the direct sliding range γ are provided at a distance from each other in the axial direction. 
   In the fourteenth embodiment, as in the above-described thirteenth embodiment, the diameter of the hole for inserting the valve body  32  is extended, and the outer diameter of the portion of the valve body  32  below the flange  64  is slightly narrowed, so that it is intended to increase the insertion clearance for the valve body  32 , and the insertion clearance is used as the clearance α to prevent the common rail body  20  from pressing the valve body  32 . 
   This description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.