Abstract:
A fuel filtration device includes a first filter normally used to filter foreign substances in fuel, a second filter used to filter foreign substances in fuel only in a state where a degree of clogging of the first filter is larger than a predetermined level, and a regulating device for regulating a flow of fuel through the second filter. The regulating device disables the flow of fuel through the second filter to limit the filtering of foreign substances through the second filter when the degree of clogging of the first filter is equal to or smaller than the predetermined level. The regulating device enables the flow of fuel through the second filter to permit the filtering of foreign substances through the second filter when the degree of clogging of the first filter is larger than the predetermined level.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and incorporates herein by reference Japanese Patent Application No. 2009-23342 filed on Feb. 4, 2009. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a fuel filtration device having a filter for filtering off foreign substances in fuel, and to a fuel supply system including the device. 
         [0004]    2. Description of Related Art 
         [0005]    It is common for a vehicle to include a filter for filtering off foreign substances in fuel which is used for combustion of an internal combustion engine in the vehicle. As described in, for example, JP-UM-A-53-090126, when a differential pressure between before and after the filter exceeds a predetermined pressure, or when a travel distance of the vehicle exceeds a predetermined distance, for example, a driver is alarmed with a warning lamp or the like so as to prompt the driver to replace the filter. 
         [0006]    However, when clogging of the filter becomes advanced, output of the engine sometimes decreases sharply due to shortage in supply of fuel to the engine. In this case, even though the alarm that prompts filter replacement is given, operation of the engine in an output reduced state is necessitated until the filter has been replaced, and in the worst case, the engine is stopped. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention addresses at least one of the above disadvantages. According to the present invention, there is provided a fuel filtration device including a first filter, a second filter, and a regulating means. The first filter is normally used to filter foreign substances contained in fuel. The second filter is used to filter the foreign substances contained in the fuel only in a state where a degree of clogging of the first filter is larger than a predetermined level. The regulating means is for regulating a flow of the fuel through the second filter. The regulating means disables the flow of the fuel through the second filter to limit the filtering of the foreign substances through the second filter when the degree of clogging of the first filter is equal to or smaller than the predetermined level. The regulating means enables the flow of the fuel through the second filter to permit the filtering of the foreign substances through the second filter when the degree of clogging of the first filter is larger than the predetermined level. 
         [0008]    According to the present invention, there is also provided a fuel supply system for an internal combustion engine, including a fuel tank, a low pressure fuel pump, a high pressure fuel pump, and the fuel filtration device. The fuel tank receives fuel. The low pressure fuel pump pumps the fuel out of the fuel tank. The high pressure fuel pump further pressurizes and pumps the fuel, which is supplied from the low pressure fuel pump. The fuel filtration device is placed in one of a fuel conduit that connects between the fuel tank and the low pressure fuel pump, and a fuel conduit that connects between the low pressure fuel pump and the high pressure fuel pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
           [0010]      FIG. 1  is a diagram illustrating a fuel supply system in which a fuel filtration device in accordance with a first embodiment of the invention is disposed; 
           [0011]      FIG. 2A  is a diagram illustrating an operating state of the fuel filtration device in accordance with the first embodiment at normal time; 
           [0012]      FIG. 2B  is a diagram illustrating an operating state of the fuel filtration device in accordance with the first embodiment when clogging is caused; 
           [0013]      FIG. 3  is a diagram illustrating structure of the fuel filtration device in accordance with the first embodiment; 
           [0014]      FIG. 4  is a diagram illustrating a fuel supply system in which a fuel filtration device in accordance with a second embodiment of the invention is disposed; 
           [0015]      FIG. 5A  is a diagram illustrating an operating state of the fuel filtration device in accordance with the second embodiment at normal time; and 
           [0016]      FIG. 5B  is a diagram illustrating an operating state of the fuel filtration device in accordance with the second embodiment when clogging is caused. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Embodiments of the invention will be described below with reference to the accompanying drawings. The same numerals are used in the drawings to indicate the same or equivalent parts in the following embodiments, and the preceding description of the component having the same numeral is referred to when explaining the parts with the same numerals. 
       First Embodiment 
       [0018]    A fuel filtration device according to a first embodiment of the invention is provided for instance, for a common rail type fuel supply system for a diesel engine (internal combustion engine) for an automobile. 
         [0019]    With reference to  FIG. 1 , the common rail type fuel supply system of the first embodiment will be roughly described below. Generally, this system is configured such that an electronic control unit (ECU)  10  receives sensor outputs from various sensors, and controls devices that constitute the fuel supply system based on the sensor outputs. The ECU  10  feedback-controls fuel pressure in a common rail  50  (current fuel pressure measured by a rail pressure sensor  51 ) to be a target value (target fuel pressure) by adjusting an amount of current supplied to a suction regulating valve (to be described in greater detail hereinafter) so as to control a fuel discharged amount from a pumping device  40  to be a desired value. Based on the above fuel pressure, the ECU  10  controls fuel injection quantity toward a predetermined cylinder of a target engine and eventually an output of the engine (rotational speed or torque of an output shaft), to be a desired value. 
         [0020]    The fuel supply system includes a fuel tank  20 , a fuel filtration device  30 , the pumping device  40 , the common rail  50 , and injectors  60  (fuel injection valves), which are arranged in this order in a flow direction of the fuel (i.e., from the upstream side to the downstream side). In this system, fuel in the fuel tank  20  is pressure-fed through the pumping device  40 , so that fuel is supplied to a target device in the system. 
         [0021]    Next, the structure of the pumping device  40  will be explained below. 
         [0022]    The pumping device  40  includes a feed pump  41  (fuel pump) and a high pressure pump  42 , which will be described below, and is configured such that fuel pumped up from the fuel tank  20  by the feed pump  41  is pressurized through the high pressure pump  42  to be discharged. Meanwhile, the amount of fuel fed to the high pressure pump  42  is regulated by a suction regulating valve (not shown) disposed on a fuel-suction side of the high pressure pump  42 . 
         [0023]    The feed pump  41  functions as a “low pressure supply pump”. The feed pump  41  suctions the fuel in the fuel tank  20  through a inlet  43  to be fed into the high pressure pump  42 , and a trochoid pump may be used for the feed pump  41 . The suction regulating valve is controlled by the ECU  10  to regulate an amount of fuel suctioned from the feed pump  41  into the high pressure pump  42 . More specifically, the amount of the fuel fed by the feed pump  41  is regulated to be a required discharge amount through this suction regulating valve, and then the fuel flows into the high pressure pump  42 . Accordingly, a discharge amount of fuel that is pressure-fed from the pumping device  40  to the common rail  50  is controlled. 
         [0024]    The high pressure pump  42  is a plunger pump for pressurizing fuel, whose amount is regulated through the suction regulating valve, and for discharging the fuel into the outside. The feed pump  41  and the high pressure pump  42  are driven by rotation of a drive shaft  44 , and the drive shaft  44  is driven in accordance with rotation of a crankshaft (output shaft of the engine). Therefore, the feed pump  41  and the high pressure pump  42  are driven by power from the engine output. As described above, when the pumping device  40  is activated, the fuel in the fuel tank  20  is pumped up through the operation of the feed pump  41 , and then pressurized and supplied (pressure-fed) into the common rail  50  through the operation of the high pressure pump  42  after the regulation of its flow rate through the suction regulating valve. 
         [0025]    The common rail  50  stores the fuel, which has been pressure-fed from the above-described pumping device  40 , in a high pressure state, and supplies the fuel to the injector  60  of each cylinder through a high pressure pipe  52  provided for each of cylinders # 1  to # 4 . The injector  60  is provided for each combustion chamber in the cylinders # 1  to # 4 , and injects high pressure fuel which is pressure-accumulated and held in the above common rail  50 . An excess amount of fuel supplied to the injector  60  is returned into the fuel tank  20  through a low pressure pipe  53 . 
         [0026]    Constitution of the fuel filtration device  30 , which is a main feature of the present embodiment, will be described below with reference to  FIGS. 2A to 3 .  FIGS. 2A and 2B  illustrate a route through which fuel flows in the fuel filtration device  30 , and  FIG. 3  is a sectional view illustrating structure of the fuel filtration device  30 . 
         [0027]    As shown in  FIGS. 1 to 2B , the fuel filtration device  30  is disposed on a upstream side of the feed pump  41  in the fuel flow direction, and fuel flows through the inside of the fuel filtration device  30  by suction negative pressure of the feed pump  41 . The fuel filtration device  30  includes two filters (a first filter  31  and a second filter  32 ) as filters for filtering off foreign substances in fuel. A first flow route  30   a  in which the first filter  31  is disposed and a second flow route  30   b  in which the second filter  32  is disposed, are arranged in a communication passage formed inside the fuel filtration device  30 . Both these flow routes  30   a ,  30   b  communicate with each other in parallel arrangement. 
         [0028]    A valve  33  (regulating means) that opens and closes the second flow route  30   b  is disposed in a portion of the second flow route  30   b  on a downstream side of the second filter  32 . The valve  33  is a mechanical valve including a valve plug  331  for opening and closing the second flow route  30   b  and a spring  332  for urging the valve plug  331  in a valve closing direction. The valve plug  331  opens the second flow route  30   b  when a differential pressure between before and after the valve  33  exceeds a set pressure. 
         [0029]    Urging force of the spring  332  is set such that the valve plug  331  of the valve  33  closes the second flow route  30   b  at normal time when the first filter  31  is not clogged (see  FIG. 2A ). In other words, the above-described set pressure for the valve  33  is higher than the differential pressure between before and after the valve  33  at normal time. 
         [0030]    When the clogging of the first filter  31  becomes advanced, a differential pressure between before and after the first filter  31  becomes large. Accordingly, suction negative pressure of the feed pump  41  becomes high, so that pressure on a downstream side of the first filter  31  in the fuel flow direction decreases. When the pressure on a downstream side of the first filter  31  is reduced in the above manner, pressure on a downstream side of the valve  33  in the fuel flow direction decreases simultaneously. Therefore, the differential pressure between before and after the valve  33  becomes large, and as illustrated with  FIG. 2B , the valve plug  331  of the valve  33  is operated to open the second flow route  30   b  against the urging force of the spring  332 . In other words, the urging force of the spring  332  is set such that the valve plug  331  opens the second flow route  30   b  at the time that the first filter  31  is so clogged that the first filter  31  needs to be replaced (hereinafter referred to simply as “clogging generated time”). Thus, the above-described set pressure for the valve  33  is set to coincide with the differential pressure between before and after the valve  33  at the clogging generated time of the first filter  31 , and is thereby set based on the above-described negative pressure at the clogging generated time. 
         [0031]    Accordingly, at normal time, fuel flows into the first filter  31 , while a flow into the second filter  32  is blocked by the valve  33 . Hence, fuel flows along the first flow route  30   a  as illustrated with a dashed line in  FIG. 2A , and the second filter  32  is bypassed. On the other hand, at the clogging generated time, fuel flows into the second filter  32  as indicated by a dashed line in  FIG. 2B  in accordance with the opening operation of the valve  33 . 
         [0032]    A specific structure of the fuel filtration device  30  will be explained below. 
         [0033]    As shown in  FIG. 3 , the fuel filtration device  30  includes the first and second filters  31 ,  32 , a metal filter head  34 , and a filter holding member. A fuel inlet passage  341  (fuel inlet) and a fuel outlet passage  342  (fuel outlet) are formed in the filter head  34 . The filter holding member, which is described in greater detail hereinafter, holds the first and second filters  31 ,  32 , and is attacked to the filter head  34 . In the following description, a filter holding member-side (lower side of  FIG. 3 ) of the filter head  34  is referred to as a “lower side”, and an opposite side of the filter head  34  from the filter holding member (upper side of  FIG. 3 ) is referred to as an “upper side.” 
         [0034]    The filter head  34  has a cylindrical shape, and the fuel inlet passage  341  and the fuel outlet passage  342  open on an outer circumferential surface of the filter head  34 . An entrance pipe  54  (see  FIGS. 1 to 2B ) that communicates with the fuel tank  20  is connected to the fuel inlet passage  341 , and an outlet pipe  55  (see  FIGS. 1 to 2B ) that communicates with the pumping device  40  is connected to the fuel outlet passage  342 . 
         [0035]    An annular inlet groove  343  that communicates with the fuel inlet passage  341 , and an annular outlet groove  344  that communicates with the fuel outlet passage  342  are formed on a lower end surface of the filter head  34 . The fuel outlet passage  342  branches into a first flow passage  345  and a second flow passage  346 , and the outlet groove  344  communicates with the fuel outlet passage  342  via the first flow passage  345 . The first flow passage  345  and the outlet groove  344  serve as a part of the first flow route  30   a  which is described above using  FIGS. 2A and 2B , and the second flow passage  346  serves as a part of the second flow route  30   b  which is illustrated with  FIGS. 2A and 2B . 
         [0036]    A valve accommodating chamber  347  that accommodates the valve plug  331  of the valve  33  and the spring  332  is formed in the second flow passage  346 . The second flow passage  346  is closed as a result of engagement of the valve plug  331  with a seat surface  348  of the valve accommodating chamber  347 , and the second flow passage  346  is opened as a result of disengagement of the valve plug  331  from the seat surface  348 . 
         [0037]    The valve accommodating chamber  347  further includes a switch  333  (sensing means) for outputting a valve opening signal when the valve  33  opens the second flow passage  346 . A contact-type switch which is pressed and moved by the valve plug  331  disengaging from the seat surface  348  is used for the switch  333 . An ON signal (valve opening signal) of the switch  333  is transmitted to the ECU  10  through a communication line  334 . Upon receiving the valve opening signal, the ECU  10  provides a notification to a driver of a vehicle to prompt replacement of the first filter  31 . The above alarm is given for instance, by indicating a warning on a display of an instrument panel on which various instruments such as a vehicle speed meter are arranged. 
         [0038]    The filter holding member includes a case  35 , a lid member  36 , an outer pipe  361 , an inner pipe  362 , a partition plate  363 , an upper stay  371 , an intermediate stay  372 , and a lower stay  373 , which are described in greater detail hereinafter. All the members  35 ,  36 ,  361  to  363 , and  371  to  373  that constitute the filter holding member are made of metal, and the members are joined to each other by welding, for instance. 
         [0039]    The case  35  is formed in a cylindrical shape having a bottom part whose upper portion opens, and an opening of the case  35  is closed by the lid member  36 . An accommodating space  35   a  that accommodates the first and second filters  31 ,  32  is defined inside the case  35  and the lid member  36 . 
         [0040]    The lid member  36  has a disc shape whose peripheral edge is welded to the case  35 , and an inlet port  36   a , a first outlet hole  36   b , and a second outlet hole  36   c , which will be described below, are formed on the lid member  36 . The inlet port  36   a  is a through hole that communicates between the inlet groove  343  and the accommodating space  35   a , and formed annularly along the inlet groove  343 . The first outlet hole  36   b  is a through hole that communicates between the outlet groove  344  and the accommodating space  35   a , and formed annularly along the outlet groove  344 . The second outlet hole  36   c  is a through hole that communicates between the second flow passage  346  and the accommodating space  35   a . The first outlet hole  36   b  and the outlet groove  344  are located respectively annularly inward of the inlet port  36   a  and the inlet groove  343 , and the second outlet hole  36   c  and the second flow passage  346  are located respectively annularly inward of the first outlet hole  36   b  and the outlet groove  344 . 
         [0041]    The outer pipe  361  and the inner pipe  362  are formed in a cylindrical shape extending in upper and lower directions, and the inner pipe  362  is arranged inside the outer pipe  361 . An upper end portion of the inner pipe  362  is welded onto a lower surface of the lid member  36  such that a cylinder interior portion  362   a  communicates with the second outlet hole  36   c . An upper end portion of the outer pipe  361  is welded onto a lower surface of the lid member  36  such that a cylinder interior portion  361   a  communicates with the first outlet hole  36   b . Accordingly, the cylinder interior portion  362   a  of the inner pipe  362  functions as a part of the second flow route  30   b  which is illustrated with  FIGS. 2A and 2B , and the cylinder interior portion  361   a  of the outer pipe  361  functions as a part of the first flow route  30   a  which is illustrated with  FIGS. 2A and 2B . 
         [0042]    The partition plate  363  is formed in a disc shape that divides the cylinder interior portion of the outer pipe  361  in the upper and lower directions. A part (portion indicated by the numeral  361   a ) of the cylinder interior portion of the outer pipe  361  above the partition plate  363  communicates with the first outlet hole  36   b  to constitute the first flow route  30   a  as describe above, and a part (portion indicated by the numeral  361   b ) of the cylinder interior portion of the outer pipe  361  below the partition plate  363  communicates with the cylinder interior portion  362   a  of the inner pipe  362  to serve as a part of the second flow route  30   b.    
         [0043]    The upper stay  371  and the intermediate stay  372  are formed in a disc shape extending radially outward from an outer circumferential surface of the outer pipe  361 , and the lower stay  373  has a disc shape that is welded to an open end of the outer pipe  361  so as to close the cylinder interior portion  361   b  of the outer pipe  361 . The first filter  31  is fitted and held between the upper stay  371  and the intermediate stay  372 , and the second filter  32  is fitted and held between the intermediate stay  372  and the lower stay  373 . 
         [0044]    First communicating holes  361   c  that constitute the first flow route  30   a  are formed on a portion of the outer pipe  361  opposed to the first filter  31 , and second communicating holes  361   d  that constitute the second flow route  30   b  are formed on a portion of the outer pipe  361  opposed to the second filter  32 . In addition, an outer peripheral surface of the first filter  31  is referred to as an inlet  361   e , and an outer peripheral surface of the second filter  32  is referred to as an inlet  361   f.    
         [0045]    By virtue of the above-described constitution, fuel, which has flowed into the fuel filtration device  30  from the entrance pipe  54 , flows into the accommodating space  35   a  through the fuel inlet passage  341 , the inlet groove  343 , and the inlet port  36   a . Then, after a flow of the fuel through the first flow route  30   a  or the second flow route  30   b , which is described in greater detail hereinafter, the fuel flows out of the device  30  to the outlet pipe  55  through the fuel outlet passage  342 . 
         [0046]    The first flow route  30   a  which leads fuel into the first filter  31  includes the first communicating hole  361   c , the cylinder interior portion  361   a  of the outer pipe  361 , the first outlet hole  36   b , the outlet groove  344 , and the first flow passage  345  in this order from the upstream side in the fuel flow direction. The second flow route  30   b  through which fuel flows into the second filter  32  includes the second communicating holes  361   d , the cylinder interior portion  361   b  of the outer pipe  361 , the opening  362   b  and the cylinder interior portion  362   a  of the inner pipe  362 , the second outlet hole  36   c , and the valve accommodating chamber  347  in this order from the upstream side in the fuel flow direction. 
         [0047]    The first filter  31  and the second filter  32  are filter elements which are composed, for instance, by pasting two or more sheets of filter material together, and the first and second filters  31 ,  32  are the elements formed separately from each other. A filtration area of the second filter  32  is made smaller than a filtration area of the first filter  31 . The filter element has a function of capturing foreign substances included in fuel (light oil), a function of removing moisture in fuel, and a function of removing a wax component in fuel. As time of use passes, the captured foreign substances and wax component, for example, deposit in the filter element so as to clog these first and second filters  31 ,  32 . The filter element that is clogged above a certain level has reached the end of the lifetime, so that it needs replacement. 
         [0048]    When the first filter  31  is replaced, the filter holding member is detached from the filter head  34 . By screwing and joining the upper end portion  351  of the case  35  to the filter head  34 , the filter holding member is detachably attached to the filter head  34 . In a screwed state (state illustrated with  FIG. 3 ), a lower surface of the filter head  34  and an upper surface of the lid member  36  are in a closely-attached state therebetween. In particular, clearance between the inlet groove  343  and the inlet port  36   a , clearance between the outlet groove  344  and the first outlet hole  36   b , and clearance between the second flow passage  346  and the second outlet hole  36   c , are sealed with a sealing member (e.g., O ring) to prevent leakage of fuel. 
         [0049]    When the clogged first filter  31  is replaced with a new first filter  31 , the upper end portion  351  of the case  35  is detached from the filter head  34 , and a new filter holding member holding new first and second filters  31 ,  32  is attached to the filter head  34 . Therefore, when the first filter  31  is replaced by a new filter, the first filter  31  is changed together with the filter holding member and the second filter  32 . 
         [0050]    Operation of the fuel filtration device  30  illustrated in  FIG. 3  will be described below. 
         [0051]    At normal time when the first filter  31  is not clogged, the second flow passage  346  leading to the second filter  32  is closed because the valve  33  closes the second flow route  30   b  (state illustrated in  FIGS. 3 and 2A ). Accordingly, fuel, which has flowed into the accommodating space  35   a  from the fuel inlet passage  341  through the inlet groove  343  and the inlet port  36   a , does not flow in the second flow route  30   b , but flows along the first flow route  30   a . Therefore, the fuel flows through the first filter  31 , and a flow of fuel through the second filter  32  is closed. 
         [0052]    Then, when the clogging of the first filter  31  becomes advanced, the suction negative pressure of the feed pump  41  becomes high, so that the pressure on the downstream side of the first filter  31  (pressure in the fuel outlet passage  342 ) decreases. As a result, the pressure on the downstream side of the valve  33  (pressure in the valve accommodating chamber  347 ) is reduced, and thereby the valve  33  opens the second flow route  30   b  (state illustrated in  FIG. 2B ). For this reason, fuel, which has flowed in through the fuel inlet passage  341 , does not flow toward the first flow route  30   a  having a high pressure loss due to the clogging of the first filter  31 . Instead, the fuel flows through the second filter  32  which is not clogged and thus has a lower pressure loss than the first filter  31 . 
         [0053]    According to the present embodiment explained in full detail above, the following advantageous effects are produced.
       (1) The fuel filtration device  30  includes the second filter  32  in addition to the first filter  31  that is used at normal time. At normal time, fuel flows through the first filter  31 , whereas the flow into the second filter  32  is blocked by the valve  33 . When the first filter  31  is clogged, fuel flows through the second filter  32  in accordance with the opening operation of the valve  33 . Therefore, the first filter  31  fulfills a filtering function, and the second filter  32  is not made to capture foreign substances in fuel at normal time, and the second filter  32 , which is not used at normal time, is made to fulfill a filtering function when the first filter  31  is clogged. Accordingly, if the first filter  31  is clogged, the engine is operated with the second filter  32  in use until the first filter  31  has been replaced. Thus, shortage in supply of fuel to the engine is relieved, so that a state of reduction in output of the engine is avoided.   (2) The present embodiment is aimed at the fuel filtration device  30  disposed on the upstream side of the feed pump  41 , and fuel flows through the inside of the fuel filtration device  30  due to the suction negative pressure of the feed pump  41 . In such a suction negative pressure-type fuel filtration device  30 , the passage downstream of the first filter  31  is connected with the downstream side of the valve  33 . The valve  33  is set to open the second flow route  30   b , when the pressure on the downstream side of the first filter  31  decreases in accordance with the generation of clogging of the first filter  31  so that the pressure on the downstream side of the valve  33  falls below a predetermined pressure. Accordingly, the fuel filtration device  30  switches between the first and second flow routes  30   a ,  30   b  at normal time or at the clogging generated time by employing a mechanical valve without using a magnet-type valve for the valve  33 . As a result, electronic control means such as an electronic circuit needed when the switching is electronically controlled using the magnet-type valve becomes unnecessary, and cost reduction in the fuel filtration device  30  is achieved using a cheap mechanical valve.   (3) Although the second filter  32  is not for normal use, when the second filter  32  is used repeatedly because of the clogging of the first filter  31 , the second filter  32  is also clogged, so that the filter  32  requires replacement, too. Accordingly, if the second filter  32  is used continuously without replacement, a state of reduction in output of the engine until the first filter  31  has been replaced, cannot be avoided. In the present embodiment, in view of this, both the first and second filters  31 ,  32  are held by a single filter holding member, and the filter holding member is attached to and removed from the filter head  34  with both the filters  31 ,  32  being held. Therefore, when the first filter  31  is replaced by a new filter, the first filter  31  is changed together with the filter holding member and the second filter  32 . Or, after the filter holding member is removed from the filter head  34 , both the filters  31 ,  32  that are held by the filter holding member are replaced. Hence, forgetting to replace the second filter  32  is limited compared to a case where both the filters  31 ,  32  are separately detached from the filter head  34  to be replaced. Furthermore, according to the present embodiment, replacement of the second filter  32  is carried out simultaneously with replacement of the first filter  31 . As a result, workability in replacement of both the filters  31 ,  32  improves.   (4) In the present embodiment, when the valve  33  opens the second flow route  30   b  in accordance with the generation of clogging of the first filter  31 , the valve opening signal is outputted to the ECU  10  from the switch  333 . Consequently, the ECU  10  recognizes time for replacement of the first filter  31  based on whether this valve opening signal has been sent. More specifically, the time for replacement (clogging generated time) of the first filter  31  is recognized using the valve  33  for switching between the first and second flow routes  30   a ,  30   b . In consequence, a dedicated detecting means for detecting a clogging state of the first filter  31  (e.g., differential pressure sensor for detecting the differential pressure between before and after the first filter  31 ) is made unnecessary.   (5) In the present embodiment, the filtration area of the second filter  32  is smaller than the filtration area of the first filter  31 . More specifically, sizes of both the filters  31 ,  32 , which have a cylindrical shape, in their radial direction are made the same, and a size of the second filter  32  in a direction of its cylindrical shaft (upper and lower directions) is made smaller than the first filter  31 . Because the second filter  32  is less frequently used compared to the first filter  31 , by making smaller the second filter  32  than the first filter  31  in the above-described manner, the filtering function of the first filter  31  at normal time and the filtering function of the second filter  32  at the clogging generated time are fully carried out, and an installation space for the fuel filtration device  30  in the upper and lower directions is made small.       
 
       Second Embodiment 
       [0059]    In the above first embodiment illustrated in  FIG. 1 , the pumping device  40  is configured such that the feed pump  41  and the high pressure pump  42  are accommodated in the same case, and the fuel filtration device  30  is disposed on the upstream side of the feed pump  41  in a fuel flow direction. Therefore, the fuel filtration device  30  of the first embodiment is a suction negative pressure-type filtration device in which fuel flows through the first and second flow routes  30   a ,  30   b  due to the suction negative pressure of the feed pump  41 . 
         [0060]    On the other hand, a pumping device  40  according to the present embodiment illustrated with  FIG. 4  is configured such that a feed pump  41  and a high pressure pump  42  are separately arranged, and a fuel filtration device  30  is disposed on a downstream side of the feed pump  41  in the fuel flow direction. Therefore, the fuel filtration device  30  of the present embodiment is a discharge pressure (positive pressure)-type filtration device in which fuel flows through the first and second flow routes  30   a ,  30   b  by discharge pressure from the feed pump  41 . In addition, structure of the fuel filtration device  30  of the second embodiment is the same as the above first embodiment illustrated in  FIG. 3 , and only a set pressure for a valve  33 , which is described in greater detail hereinafter, is different from the first embodiment. 
         [0061]    More specifically, similar to the first embodiment, the fuel filtration device  30  of the present embodiment includes two filters (i.e., a first filter  31  and a second filter  32 ) as illustrated with  FIGS. 5A and 5B . A first flow route  30   a  in which the first filter  31  is disposed and a second flow route  30   b  in which the second filter  32  is disposed, are arranged along a flow route formed inside the fuel filtration device  30 , and both these flow routes  30   a ,  30   b  communicate with each other in parallel arrangement. The valve  33  that opens and closes the second flow route  30   b  is disposed in a portion of the second flow route  30   b  on a downstream side of the second filter  32  in the fuel flow direction, and this valve  33  opens the second flow route  30   b  when a differential pressure between before and after the valve  33  exceeds a set pressure. 
         [0062]    Urging force of a spring  332  is set such that a valve plug  331  of the valve  33  closes the second flow route  30   b  at normal time when the first filter  31  is not clogged (see  FIG. 5A ). In other words, the above-described set pressure for the valve  33  is higher than the differential pressure between before and after the valve  33  at normal time. 
         [0063]    As the clogging of the first filter  31  becomes advanced, pressure on an upstream side of the first filter  31  in the fuel flow direction, i.e., pressure (positive pressure) on a discharge side of the feed pump  41  becomes higher. When the positive pressure of the feed pump  41  becomes high in the above-described manner, pressure on upstream and downstream sides of the second filter  32  increases, so that pressure on an upstream side of the valve  33  also increases simultaneously. Accordingly, the differential pressure between before and after the valve  33  becomes large, and as shown in  FIG. 5B , the valve plug  331  of the valve  33  is operated to open the second flow route  30   b  against the urging force of the spring  332 . In other words, the urging force of the spring  332  is set such that the valve plug  331  opens the second flow route  30   b  at the clogging generated time of the first filter  31 . Thus, the above-described set pressure for the valve  33  is set to coincide with the differential pressure between before and after the valve  33  at the clogging generated time of the first filter  31 , and is thereby set based on the above-described positive pressure at the clogging generated time. 
         [0064]    According to the present embodiment explained in full detail above, the above-described advantageous effects (1) and (3) to (5) are produced, and the following advantageous effects are produced similar to the above effect (2).
       (6) The present embodiment is aimed at the fuel filtration device  30  disposed on the downstream side of the feed pump  41 , and fuel flows through the inside of the fuel filtration device  30  due to the discharge pressure of the feed pump  41 . In such a discharge positive pressure-type fuel filtration device  30 , the passage downstream of the second filter  32  is connected with the upstream side of the valve  33 . The valve  33  is set to open the second flow route  30   b , when the pressure on the downstream side of the second filter  32  increases in accordance with the generation of clogging of the first filter  31  so that the pressure on the upstream side of the valve  33  exceeds a predetermined pressure. Accordingly, the fuel filtration device  30  switches between the first and second flow routes  30   a ,  30   b  at normal time or at the clogging generated time by employing a mechanical valve without using a magnet-type valve for the valve  33 . As a result, electronic control means such as an electronic circuit needed when the switching is electronically controlled using the magnet-type valve becomes unnecessary, and cost reduction in the fuel filtration device  30  is achieved using a cheap mechanical valve.       
 
         [0066]    Moreover, according to the above-described fuel filtration device  30  of the first and second embodiments, only by changing the set pressure for the valve  33  between in the case of the fuel filtration device  30  disposed on the upstream side of the feed pump  41  (first embodiment) and in the case of the fuel filtration device  30  disposed on the downstream side of the feed pump  41  (second embodiment), the invention is applied to the fuel filtration device  30  in both the cases. More specifically, in the case of the suction negative pressure-type filtration device (first embodiment), the valve  33  may be selected based on the pressure (negative pressure) on the downstream side of the valve  33  at the clogging generated time, and in the case of the positive pressure-type filtration device (second embodiment), the valve  33  may be selected based on the pressure (positive pressure) on the upstream side of the valve  33  at the clogging generated time. As a result, the constitution of the fuel filtration device  30  except the valve  33  is made common to both the above types. 
         [0067]    Modifications of the above embodiments will be described below. The invention is not limited to the descriptions in the above-described embodiments, and may be embodied through the modifications as follows. 
         [0068]    In the above embodiments, the first and second filters  31 ,  32  are elements that are formed separately from each other. Alternatively, a single element may be used for the first and second filters  31 ,  32 . More specifically, by eliminating the intermediate stay  372  illustrated in  FIG. 3  and by dividing the first flow route  30   a  from the second flow route  30   b  with the partition plate  363  on a downstream side of a single element, a part of the single element above the partition plate  363  may function as the first filter  31 , and a part of the single element below the partition plate  363  may function as the second filter  32 . Accordingly, compared with when different elements are employed respectively for the first and second filters  31 ,  32 , the structure of the fuel filtration device  30  is simplified, e.g., the intermediate stay  372  becomes unnecessary. However, in the case where the first and second filters  31 ,  32  is divided off by the intermediate stay  372  as in the first and second embodiments, a degree of sail of the second filter  32  because of foreign substance in fuel at normal time is reduced. 
         [0069]    In the above embodiments, when the first filter  31  is replaced, the first filter  31  is replaced together with the second filter  32 . Alternatively, the first and second filters  31 ,  32  may be made respectively attachable to and detachable from the filter holding member. After the filter holding member is detached from the filter head  34 , the first and second filters  31 ,  32  may be respectively replaced separately. Accordingly, if the clogging of the second filter  32  has not become advanced at the time of replacement of the first filter  31 , the second filter  32  continues to be used without replacement. 
         [0070]    In the above embodiments, the first and second filters  31 ,  32  are held by a single filter holding member. Alternatively, the first and second filters  31 ,  32  may be held respectively by different filter holding members. 
         [0071]    In the above embodiments, the switch  333  is provided for the valve  33 , and the generation of clogging of the first filter  31  is detected based on whether the valve opening signal has been sent from the switch  333 . However, the invention is not limited to such a detection method. For instance, a differential pressure sensor for detecting the differential pressure between before and after the valve  33  may be provided. Based on a detection signal from the differential pressure sensor, the generation of clogging may be detected. Or, the generation of clogging may be detected based on a travel distance of the vehicle. 
         [0072]    Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.