Abstract:
To increase a valve-opening pressure at which a pressure relief valve is opened in order to deal with a higher pressure of the fuel, a large pressure relief valve is installed in a high-pressure fuel supply pump. This upsizes the high-pressure fuel supply pump. A pressure relief valve is installed in a discharge joint. This can provide a high-pressure fuel supply pump that is not large too much and sufficiently performs a relief function by efficiently using the excessive space in the pump even when the fuel pressure is increased.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the configuration of a high-pressure fuel supply pump for an internal-combustion engine of a vehicle. 
       BACKGROUND ART 
       [0002]    High-pressure fuel supply pumps that increase the pressure of the fuel are widely used for direct-injection internal-combustion engines in which the fuel is directly injected to the inside of the combustion chamber among internal-combustion engines, for example, of vehicles. 
         [0003]    The high-pressure fuel supply pump is sometimes provided with a pressure relief valve mechanism that opens when an excessive high pressure is generated in a high-pressure pipe in the downstream part of the discharge valve so as to communicate the downstream high-pressure fuel path of the discharge valve with the upstream low-pressure fuel path of the discharge valve and protect the high-pressure pipes including a common rail. 
         [0004]    JP 2009-257197 A describes a high-pressure fuel supply pump in which a pressure relief valve mechanism is integrally and vertically or horizontally provided to the pump body (see PTL 1). 
         [0005]    JP 2013-167259 A is another Patent Literature. 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1: JP 2009-257197 A 
         [0007]    PTL 2: JP 2013-167259 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    Recently, in order to deal with environmental regulations, there is the increasing demand for increasing the pressure of the fuel in a direct-injection internal-combustion engine in which the fuel is directly injected to the inside of the combustion chamber among internal-combustion engines, for example, of vehicles. In order to deal with a higher pressure of the fuel, it is necessary to increase the valve-opening pressure to open the pressure relief valve. In order to increase the valve-opening pressure, it is necessary to strengthen the relief biasing string. As a result, the size of the pressure relief valve is adversely increased. Thus, in conventional techniques, the size of the high-pressure fuel supply pump is increased so that such an upsized pressure relief valve is installed in the high-pressure fuel supply pump. For example, in PTL 2, the pressure relief valve mechanism is not provided to the protruding joint, and the discharge valve is integrated with the pressure relief valve mechanism. This makes it difficult to strengthen the relief biasing string. 
         [0009]    Additionally, such an upsized high-pressure fuel supply pump makes it difficult to leave space for installing the high-pressure fuel supply pump depending on engines, or makes the layout of the high-pressure pipes complicated and increases the cost. 
         [0010]    An objective of the present invention is to provide a high-pressure fuel supply pump in which the pressure relief valve can be installed in the pump body with a simple structure and the pump body can be reduced in size even when the high-pressure fuel supply pump deals with a high fuel pressure. 
       Solution to Problem 
       [0011]    Installing the pressure relief valve in the discharge joint can achieve the objective of the present invention. 
       Advantageous Effects of Invention 
       [0012]    According to the present invention having the configuration described above, a high-pressure fuel supply pump that is not large too much and sufficiently performs a relief function by efficiently using the excessive space in the pump even when the high-pressure fuel supply pump deals with a higher fuel pressure. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a vertical cross-sectional view of the whole of a high-pressure fuel supply pump according to a first embodiment of the present invention. 
           [0014]      FIG. 2  is a horizontal cross-sectional view of the whole of the high-pressure fuel supply pump according to the first embodiment of the present invention. 
           [0015]      FIG. 3  is a vertical cross-sectional view of the whole of the high-pressure fuel supply pump according to the first embodiment of the present invention. 
           [0016]      FIG. 4  illustrates an exemplary fuel supply system using the high-pressure fuel supply pump according to the first embodiment of the present invention. 
           [0017]      FIG. 5  illustrates the pressure waveforms in each part and a common rail of the high-pressure fuel supply pump according to the first embodiment of the present invention. 
           [0018]      FIG. 6  illustrates an exemplary fuel supply system using the high-pressure fuel supply pump according to a second embodiment of the present invention. 
           [0019]      FIG. 7  is a vertical cross-sectional view of the whole of the high-pressure fuel supply pump according to the second embodiment of the present invention. 
           [0020]      FIG. 8  is a vertical cross-sectional view of the whole of a high-pressure fuel supply pump according to a third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Hereinafter, an embodiment according to the present invention will be described. 
       First Embodiment 
       [0022]    The configuration and operation of a system will be described with reference to the view of the whole configuration of the system illustrated in  FIG. 4 . 
         [0023]    A part surrounded by a dashed line is the body of a high-pressure fuel supply pump (hereinafter, referred to as a high-pressure pump). The mechanism and parts in the dashed line are integrally embedded in a high-pressure pump body  1 . The fuel in a fuel tank  20  is pumped up by a feed pump  21 , and fed via an intake pipe  28  to an intake joint  10   a  of the pump body  1 . 
         [0024]    After passing through the intake joint  10   a,  the fuel passes through a pressure pulsation reducing mechanism  9 , and an intake path  10   b,  and reaches an intake port  30   a  of an electromagnetic inlet valve  30  included in a flow rate control mechanism. The pulsation preventing mechanism  9  will be described below. 
         [0025]    The electromagnetic inlet valve  30  includes an electromagnetic coil  308 . When the electromagnetic coil  308  does not conduct electricity, the difference between the biasing force of an anchor spring  303  and the biasing force of a valve spring  304  biases an inlet valve body  301  in a valve-opening direction in which the inlet valve body  301  is opened, and this opens the intake opening  30   d.  Note that the biasing force of the anchor spring  303  and the biasing force of the valve spring  304  are set so that 
         [0000]    the biasing force of the anchor spring  303 &gt;the biasing force of the valve spring  304 
 
holds.
 
         [0026]    When the electromagnetic coil  308  conducts electricity, a state in which an anchor  305  is moved to the left side of  FIG. 4  and the anchor spring  303  is compressed is maintained. An inlet valve body  301  with which the tip of an electromagnetic plunger  305  coaxially has contact seals the intake opening  30   d  connected to a pressurizing chamber  11  of the high-pressure pump using the biasing force of the valve spring  304 . 
         [0027]    The operation of the high-pressure pump will be described hereinafter. 
         [0028]    When the rotation of a cam described below displaces a plunger  2  downward in  FIG. 1  and the plunger  2  is in an intake process, the volume of the pressurizing chamber  11  is increased and the fuel pressure in the pressurizing chamber  11  is decreased. In the intake process, when the fuel pressure in the pressurizing chamber  11  is reduced to a pressure lower than the pressure in the intake path  10   b  (the intake port  30   a ), the fuel passes through the opened intake opening  30   d  and flows into the pressurizing chamber  11 . When the plunger  2  completes the intake process and moves to a compression process, the plunger  2  moves to the compression process (a state in which the plunger  2  moves upward in  FIG. 1 ). At that time, a state in which the electromagnetic coil  308  does not conduct electricity is maintained, and thus magnetic biasing force does not act. Thus, the inlet valve body  301  is still opened by the biasing force of the anchor spring  303 . The volume of the pressurizing chamber  11  decreases with the compressing motion of the plunger  2 . In such a state, the fuel sucked in the pressurizing chamber  11  is returned through the opened inlet valve body  301  to the intake path  10   b  (the intake port  30   a ). Thus, the pressure in the pressurizing chamber is not increased. This process is referred to as a return process. 
         [0029]    When a control signal from an engine control unit  27  (hereinafter, referred to as ECU) is applied to the electromagnetic inlet valve  30  in the return process, a current flows through the electromagnetic coil  308  of the electromagnetic inlet valve  30 . The magnetic biasing force moves the electromagnetic plunger  305  to the left side of  FIG. 4  and a state in which the anchor spring  303  is compressed is maintained. As a result, the biasing force of the anchor spring  303  does not act on the inlet valve body  301 . The fluid force due to the biasing force of the valve spring  304  and the flow of the fuel into the intake path  10   b  (the intake port  30   a ) acts. This closes the inlet valve  301  and thus closes the intake opening  30   d.  When the intake opening  30   d  is closed, the fuel pressure in the pressurizing chamber  11  starts increasing with the upward motion of the plunger  2 . When the fuel pressure is larger than or equal to the pressure in the discharge joint  12 , the fuel remaining in the pressurizing chamber  11  is discharged at high pressure through the discharge valve mechanism  8 , and fed to the common rail  23 . This process is referred to as a discharge process. 
         [0030]    In other words, the compression process of the plunger  2  (a process in which the plunger  2  rises from a lower starting point to an upper starting point) includes the return process and the discharge process. Controlling the timing at which the electromagnetic coil  308  of the electromagnetic inlet valve  30  conducts electricity can control the amount of the high-pressure fuel to be discharged. When the timing at which the electromagnetic coil  308  conducts electricity is hastened, the proportion of the return process is low and the proportion of the discharge process is high to the compression process. In other words, the amount of fuel to be returned to the intake path  10   b  (the intake port  30   a ) is decreased and the amount of fuel to be discharged at high pressure is increased. On the other hand, when the timing at which the electromagnetic coil  308  conducts electricity is delayed, the proportion of the return process is high and the proportion of the discharge process is low to the compression process. In other words, the amount of fuel to be returned to the intake path  10   b  is increased and the amount of fuel to be discharged at high pressure is decreased. The timing at which the electromagnetic coil  308  conducts electricity is controlled by the instructions from the ECU. 
         [0031]    The configuration described above controls the timing at which the electromagnetic coil  308  conducts electricity. This can control the amount of fuel to be discharged at high pressure in accordance with the amount of fuel that the internal-combustion engine requires. 
         [0032]    The outlet of the pressurizing chamber  11  is provided with a discharge valve mechanism  8 . The discharge valve mechanism  8  includes a discharge valve seat  8   a,  a discharge valve  8   b,  and a discharge valve spring  8   c.  When there is no fuel differential pressure between the pressurizing chamber  11  and the discharge joint  12 , the discharge valve  8   b  is pressed and fixed to the discharge valve seat  8   a  and closed by the biasing force of the discharge valve spring  8   c.  When the fuel pressure in the pressurizing chamber  11  exceeds the fuel pressure in the discharge joint  12 , the discharge valve  8   b  is opened against the discharge valve spring  8   c  and the fuel in the pressurizing chamber  11  is discharged at high pressure through the discharge joint  12  to the common rail  23 . 
         [0033]    As described above, the fuel guided to the intake joint  10   a  is pressurized at high pressure by the reciprocation of the plunger  2  in the pressurizing chamber  11  of the pump body  1  as much as necessary, and fed from the discharge joint  12  to the common rail  23  by the pressure. 
         [0034]    Injectors  24  for direct injection (namely, a direct-injection injectors) and a pressure sensor  26  are attached to the common rail  23 . The number of the attached direct-injection injectors  24  corresponds to the number of cylinder engines of the internal-combustion engine. The direct-injection injectors  24  open and close in accordance with the control signal from the engine control unit (ECU)  27  so as to inject the fuel in the cylinder. 
         [0035]    The pump body  1  is further provided with a discharge flow path  110  communicating the downstream part of the discharge valve  8   b  with the pressurizing chamber  11  and bypassing the discharge valve, separately from the discharge flow path. The discharge flow path  110  is provided with a pressure relief valve  104  that limits the flow of the fuel only to a direction from the discharge flow path to the pressurizing chamber  11 . The pressure relief valve  104  is pressed to the pressure relief valve seat  105  by the relief spring  102  that generates pressing force. When the difference between the pressure in the pressurizing chamber and the pressure in a relief path is larger than or equal to a predetermined pressure, the pressure relief valve  104  moves away from the pressure relief valve seat  105  and opens. 
         [0036]    For example, when a failure of the direct-injection injector  24  causes an excessive high pressure in the common rail  23  and the differential pressure between the discharge flow path  110  and the pressurizing chamber  11  is larger than or equal to the valve-opening pressure at which the pressure relief valve  104  is opened, the pressure relief valve  104  opens and the discharge flow path at the excessive high pressure is returned from the discharge flow path  110  to the pressurizing chamber  11 . This protects a high-pressure pipe such as the common rail  23 . 
         [0037]    Hereinafter, the configuration and operation of the high-pressure fuel pump will be described in more detail with reference to  FIGS. 1 to 4 . A general high-pressure pump is air-tightly sealed and fixed to the flat surface of a cylinder head  41  of the internal-combustion engine with a flange  1   e  provided to the pump body  1 . An O-ring  61  is fitted to the pump body  1  so that the airtightness between the cylinder head and the pump body is retained. 
         [0038]    A cylinder  6  is attached to the pump body  1 . The cylinder  6  is formed in a cylinder with a bottom on an end so that the cylinder  6  guides the back-and-forth movement of the plunger  2  and the pressurizing chamber  11  is formed in the cylinder  6 . The pressurizing chamber  11  is provided with a plurality of communication holes  11   a  so that the pressurizing chamber  11  communicates with the electromagnetic inlet valve  30  configured to feed the fuel and the discharge valve mechanism  8  configured to discharge the fuel from the pressurizing chamber  11  to the discharge path. 
         [0039]    The outer diameter of the cylinder  6  includes a large-diameter part and a small-diameter part. The small-diameter part is pressed and inserted in the pump body  1 . The surface of a width difference  6   a  between the large-diameter part and the small-diameter part is pressed and fixed to the pump body  1 . This prevents the fuel pressurized in the pressurizing chamber  11  from leaking to the low-pressure side. 
         [0040]    The lower end of the plunger  2  is provided with a tappet  3  that converts the rotation movement of a cam  5  attached to a camshaft of the internal-combustion engine into up-and-down movement, and transmits the up-and-down movement to the plunger  2 . The plunger  2  is pressed and fixed to the tappet  3  through a retainer  15  with a spring  4 . This can move (reciprocate) the plunger  2  up and down with the rotation movement of the cam  5 . 
         [0041]    A plunger seal  13  held on the lower end of the inner periphery of the seal holder  7  has slidably contact with the outer periphery of the plunger  2  on the lower end of the cylinder  6  in the drawing. This seals the blow-by gap between the plunger  2  and the cylinder  6  and prevents the fuel from leaking to the outside of the pump. Meanwhile, this prevents the lubricant (including engine oil) that smoothly moves a sliding part of the internal-combustion engine from leaking through the blow-by gap into the pump body  1 . 
         [0042]    The fuel sucked by the feed pump  21  is fed through the intake joint  10   a  coupled with the intake pipe  28  to the pump body  1 . 
         [0043]    A damper cover  14  is coupled with the pump body  1  and forms a low-pressure fuel chamber  10 . The fuel passing through the inlet joint  10   a  flows into the low-pressure fuel chamber  10 . In order to remove an obstacle such as a metal powder in the fuel, a fuel filter  102  is attached to the upstream part of the low-pressure fuel chamber  10 , for example, while being pressed and inserted in the pump body  1 . 
         [0044]    A pressure pulsation reducing mechanism  9  is installed in the low-pressure fuel chamber  10  so that the pressure pulsation reducing mechanism  9  reduces the spread of the pressure pulsation generated in the high-pressure pump to a fuel pipe  28 . When the fuel sucked in the pressurizing chamber  11  is returned through the opened inlet valve body  301  to the intake path  10   b  (the intake port  30   a ) under a state in which the flow rate of the fuel is controlled, the fuel returned to the intake path  10   b  (the intake port  30   a ) generates the pressure pulsation in the low-pressure fuel chamber  10 . However, the pressure pulsation is absorbed and reduced by the expansion and contraction of a metal damper  9   a  forming the pressure pulsation reducing mechanism  9  provided to the low-pressure fuel chamber  10 . The metal damper  9   a  is formed of two corrugated metal disks of which outer peripheries are bonded together. Inert gas such as argon is injected in the metal damper  9   a.  Mounting hardware  9   b  is configured to fix the metal damper  9   a  on the inner periphery of the pump body  1 . 
         [0045]    The electromagnetic inlet valve  30  is a variable control mechanism that includes the electromagnetic coil  308 . The electromagnetic inlet valve  30  is connected to the ECU through the terminal  307  and repeats conduction and non-conduction of electricity so as to open and close the inlet valve and control the flow rate of the fuel. 
         [0046]    When the electromagnetic coil  308  does not conduct electricity, the biasing force of the anchor spring  303  is transmitted to the inlet valve body  301  through the anchor  305  and the anchor rod  302  integrally formed with the anchor  305 . The biasing force of the valve spring  304  installed in the inlet valve body is set so that 
         [0000]    the biasing force of the anchor spring  303 &gt;the biasing force of the valve spring  304 
 
holds. As a result, the inlet valve body  301  is biased in a valve-opening direction in which the inlet valve body  301  is opened. The intake opening  30   d  is opened. Meanwhile, the anchor rod  302  has contact with the inlet valve body  301  at a part  302   b  (in a state illustrated  FIG. 1 ).
 
         [0047]    The setting for the magnetic biasing force generated by the electricity conduction through the coil  308  is configured to enable the anchor  305  to overcome the biasing force of the anchor spring  303  and be sucked into a stator  306 . When the coil  308  conducts electricity, the anchor  303  moves toward the stator  306  (the left side of the drawing) and a stopper  302   a  formed on an end of the anchor rod  302  has contact with an anchor rod bearing  309  and is seized. At that time, the clearance is set so that 
         [0000]    the travel distance of the anchor  301 &gt;the travel distance of the inlet valve body  301 
 
holds. The contact part  302   b  opens between the anchor rod  302  and the inlet valve body  301 . As a result, the inlet valve body  301  is biased by the valve spring  304  and the intake opening  30   d  is closed.
 
         [0048]    The electromagnetic inlet valve  30  is fixed to the pump body  1  while an inlet valve seat  310  is hermetically inserted in a tubular boss  1   b  so that the inlet valve body  301  can seal the intake opening  30   d  to the pressurizing chamber. When the electromagnetic inlet valve  30  is attached to the pump body  1 , the intake port  30   a  is connected to the intake path  10   b.    
         [0049]    The discharge valve mechanism  8  is provided with a plurality of discharge paths radially drilled around the sliding axis of the discharge valve body  8   b.  The discharge valve mechanism  8  includes a discharge valve seat member  8   a  and a discharge valve member  8   b.  The discharge valve seat member  8   a  is provided with a bearing that can sustain the sliding reciprocation of the discharge valve body  8   b  at the center of the discharge valve seat member  8   a.  The discharge valve member  8   b  has the central axis so as to slide with respect to the bearing of the discharge valve seat member  8   a,  and has a circular contact surface on the outer periphery. The circular contact surface can retain the airtightness by having contact with the discharge valve seat member  8   a.  Furthermore, a discharge valve spring  33  is inserted and held in the discharge valve mechanism  8 . The discharge valve spring  33  is a coil spring that biases the discharge valve member  8   b  in a valve-closing direction in which the discharge valve member  8   b  is closed. The discharge valve seat member, for example, is pressed, inserted and held in the pump body  1 . The discharge valve member  8   b  and the discharge valve spring  33  are further inserted in the pump body  1 . A sealing plug  17  seals the pump body  1 . This forms the discharge valve mechanism  8 . The discharge valve mechanism  8  is formed as described above. The formation causes the discharge valve mechanism  8  to function as a check valve that controls the direction in which the fuel flows. 
         [0050]    The operation of the pressure relief valve mechanism will be described in detail. As illustrated, a pressure relief valve mechanism  100  includes a pressure relief valve housing  101 , a relief spring  102 , a relief holder  103 , a pressure relief valve  104 , and a pressure relief valve seat  105 . After the pressure relief valve seat  105  is pressed, inserted and fixed to the pressure relief valve housing  101 , the pressure relief valve  104 , the relief holder  103 , and the relief spring  102  are sequentially inserted. The set load of the relief spring  102  is determined depending on the position at which the pressure relief valve seat is fixed. The valve-opening pressure at which the pressure relief valve  104  is opened is determined depending on the set load of the relief spring  102 . The pressure relief valve mechanism  100  unitized as described above is fixed to the pump body  1  by the press-insertion of the pressure relief valve seat  105  to the inner peripheral wall of a cylindrical pass-through slot  1 C provided to the pump body  1 . Subsequently, the discharge joint  12  is fixed so that the discharge joint  12  blocks the cylindrical pass-through slot  1 C of the pump body  1  so as to prevent the fuel from leaking from the high-pressure pump to the outside and to enable the pressure relief valve mechanism  100  to be connected to a common rail. Meanwhile, the pressure relief valve mechanism  100  is partially stored in the discharge joint  12 . 
         [0051]    The discharge valve mechanism  8  and the pressure relief valve mechanism  100  are installed in the pump body so that the central axes of the discharge valve mechanism  8  and the pressure relief valve mechanism  100  are radially arranged around the pressurizing chamber  11 . This can make the process easy while the pump body  1  is produced. 
         [0052]    The overshoot generated in the pressurizing chamber will be described with reference to  FIG. 5 . When the motion of the plunger  2  starts decreasing the volume of the pressurizing chamber  11 , the pressure in the pressurizing chamber increases with the decrease in volume. When the pressure in the pressurizing chamber finally exceeds the pressure in the discharge flow path  110 , the discharge valve mechanism  8  is opened and the fuel is discharged from the pressurizing chamber  11  to the discharge flow path  110 . From the moment the discharge valve mechanism  8  is opened to the time immediately after the opening, the pressure in the pressurizing chamber overshoots and becomes very high. The very high pressure propagates in the discharge flow path and the pressure in the discharge flow path simultaneously overshoots. If the outlet of the pressure relief valve mechanism  100  is connected to the intake flow pass  10   b  at the overshoot, the overshoot of the pressure in the discharge flow path causes the pressure difference between the inlet and outlet of the pressure relief valve  104  to exceed the valve-opining pressure at which the pressure relief valve mechanism  100  is opened. This causes an error in the pressure relief valve. In light of the foregoing, the outlet of the pressure relief valve mechanism  100  of the embodiment is connected to the pressurizing chamber  11 , and thus the pressure in the pressurizing chamber acts on the outlet of the pressure relief valve mechanism  100  and the pressure in the discharge flow path  110  acts on the inlet of the pressure relief valve mechanism  11 . The pressure overshoot occurs simultaneously in the pressurizing chamber and the discharge flow path. Thus, the pressures difference between the inlet and outlet of the pressure relief valve does not exceed the valve-opining pressure at which the pressure relief valve is opened. In other words, an error in the pressure relief valve does not occur. 
         [0053]    When the motion of the plunger  2  starts increasing the volume of the pressurizing chamber  11 , the pressure in the pressurizing chamber decreases with the increase in volume. When the pressure in the pressurizing chamber falls below the pressure in the intake path  10   b  (the intake port  30   a ), the fuel flows from the intake path  10   b  (the intake port  30   a ) into the pressurizing chamber  11 . When the motion of the plunger  2  starts decreasing the volume of the pressurizing chamber  11  again, the fuel is pressurized at high pressure and discharged due to the mechanism described above. 
         [0054]    Next, an example in which failure of the direct-injection injector  24  generates an excessive high pressure in the common rail  23  will be described in detail. 
         [0055]    In the event of failure of the direct-injection injector, in other words, when the injection function of the direct-injection injector stops and the direct-injection injector does not feed the fuel fed in the common rail  23  into the combustion chamber of the internal-combustion engine, the fuel accumulates between the discharge valve mechanism  8  and the common rail  23 . This causes an excessive high pressure of the fuel. When the fuel pressure moderately increases to the excessive high pressure, the pressure sensor  26  provided to the common rail  23  detects the abnormal pressure. Then, the electromagnetic inlet valve  30  that is a flow rate control mechanism provided in the intake path the intake path  10   b  (the intake port  30   a ) is controlled by feedback control. The feedback control operates as a safety function to decrease the amount of the fuel to be discharged. However, the feedback control with the pressure sensor is not effective in dealing with an instantaneous excessive high pressure. When the electromagnetic inlet valve  30  is out of order and keeps the maximum flow rate in an operation state in which the fuel is not required so much, the pressure at which the fuel is discharged excessively increases. In such a case, the excessive high pressure is not dissolved because of the failure of the flow rate control mechanism even when the pressure sensor  26  of the common rail  23  detects the excessive high pressure. 
         [0056]    When the excessive high pressure described above occurs, the pressure relief valve mechanism  100  of the embodiment functions as a safety valve. 
         [0057]    When the motion of the plunger  2  starts increasing the volume of the pressurizing chamber  11 , the pressure in the pressurizing chamber decreases with the increase in volume. When the pressure in the inlet of the pressure relief valve mechanism  100 , namely, in the discharge flow path is higher than or equal to the pressure in the outlet of the pressure relief valve, namely, in the pressurizing chamber  11  by the valve-opening pressure at which the pressure relief valve mechanism  100  is opened, the pressure relief valve mechanism  100  is opened and returns the fuel at an excessive high pressure in the common rail to the pressurizing chamber. This return prevents the fuel pressure from being higher than or equal to a predetermined pressure even when an excessive high pressure occurs. This prevention protects the high-pressure pipe system including the common rail  23 . 
         [0058]    In the present embodiment, the mechanism described above prevents the pressure difference between the inlet and outlet of the pressure relief valve mechanism  100  from being higher than or equal to the valve-opening pressure at which the pressure relief valve mechanism  100  is opened, and thus, the pressure relief valve mechanism  100  is not opened in the discharge process. 
         [0059]    In the intake process and the return process, the fuel pressure in the pressurizing chamber  11  decreases to a low pressure identical to the pressure in the intake pipe  28 . On the other hand, the pressure in the relief chamber  112  increases to a pressure identical to the pressure in the common rail  23 . When the differential pressure between the relief chamber  112  and the pressurizing chamber is higher than or equal to the valve-opening pressure at which the pressure relief valve  104  is opened, the pressure relief valve  104  is opened and the fuel at an excessive high pressure is returned from the relief chamber  112  to the pressurizing chamber  11 . This protects the high-pressure pipe system including the common rail  23 . 
       Second Embodiment 
       [0060]    Next, a second embodiment will be described with reference to  FIGS. 6 and 7 . 
         [0061]    In the second embodiment, a pressure relief valve mechanism  100  provided to a pump body  1  communicates the downstream part of a discharge valve  8   b  with an intake path  10   b . A pressure relief valve  104  is pressed to a pressure relief valve seat  105  by a relief spring  102  generating pressing force. When the pressure difference between the intake path and a relief path is higher than or equal to a predetermined pressure, the pressure relief valve  104  moves away from the pressure relief valve seat  105  and opens. 
         [0062]    When, for example, failure of a direct-injection injector  24  generates an excessive high pressure, for example, in a common rail  23  and the differential pressure between the discharge flow path  110  and the intake path  10   b  is higher than or equal to the valve-opening pressure at which the pressure relief valve  104  is opened, the pressure relief valve  104  is opened and the discharge flow path at the excessive high pressure is returned from the discharge flow path  110  to the pressurizing chamber  11 . This protects the high-pressure pipe system including the common rail  23 . 
       Third Embodiment 
       [0063]    Next, a third embodiment will be described with reference to  FIGS. 8 and 9 . 
         [0064]    In the third embodiment, a pressure relief valve mechanism  100  includes a pressure relief valve stopper  101 , a pressure relief valve  102 , a pressure relief valve seat  103 , a relief spring stopper  104 , and a relief spring  105  as illustrated. The pressure relief valve seat  103  includes a bearing that enables the pressure relief valve  102  to slide. The pressure relief valve  102  integrally including a sliding shaft is inserted in the pressure relief valve seat  103 . After that the position of the relief spring stopper  104  is determined so that the relief spring  105  has a desired load, and the relief spring stopper  104  is fixed to the pressure relief valve  102 , for example, by press and insertion. The valve-opening pressure at which the pressure relief valve  102  is opened is determined depending on the pressing force of the relief spring  105 . The pressure relief valve stopper  101  is inserted between the pump body  1  and the pressure relief valve seat  103  so as to function as a stopper that controls how much the pressure relief valve  102  is opened. The pressure relief valve mechanism  100  unitized as described above is fixed to the pump body  1  by the press and insertion of the pressure relief valve seat  103  to the inner peripheral wall of a cylindrical pass-through slot  1 C provided to the pump body  1 . In other words, the pressure relief valve is an inward-opening valve. The relief spring  105  is provided on a side of the pressure relief valve  102  facing the discharge joint  12  as described above. This prevents the increase in volume of the pressurizing chamber  11  even when the outlet of the pressure relief valve  104  of the pressure relief valve mechanism  100  is opened toward the pressurizing chamber  11 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  pump body 
           2  plunger 
           6  cylinder 
           8  discharge valve mechanism 
           9  pressure pulsation reducing mechanism 
           11  pressurizing chamber 
           30  electromagnetic inlet valve 
           100  pressure relief valve mechanism 
           101  pressure relief valve housing 
           102  relief spring 
           103  relief holder 
           104  pressure relief valve 
           105  pressure relief valve seat