Abstract:
A pressure-holding valve for a fuel injection system with a high-pressure region and a low-pressure region, the valve having with a valve housing having a first connection that can be connected to the low-pressure region and a second connection that can be connected to the return of a fuel injection valve device. The housing contains a reciprocating valve cup prestressed in opposition to the force of a first spring device and a through opening which can be closed by a closing element prestressed in opposition to the force of a second spring device to maintain a minimum pressure in the return. The valve housing, between the first connection and the valve cup includes with a pressure relief device that can be actuated from outside the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 2004/000930 filed on May 3, 2004. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a pressure-holding valve for a fuel injection system with a high-pressure region and a low-pressure region, which valve has a valve housing equipped with a first connection that can be connected to the low-pressure region and a second connection that can be connected to the return of a fuel injection valve device, which valve housing contains a reciprocating valve cup that is prestressed in opposition to the prestressing force of a first spring device and has a through opening, which can be closed by a closing element that is prestressed in opposition to the prestressing force of a second spring device in order to maintain a minimum pressure in the return. The present invention also relates to a tool for reducing the pressure in an above-described pressure-holding valve. The present invention also relates to a set including an above-described pressure-holding valve and an above-described tool. The present invention also relates to a fuel injection system that has a low-pressure region and a high-pressure region from which fuel is supplied to a fuel injection valve device that is connected to the low-pressure region via a return. 
   2. Description of the Prior Art 
   In conventional fuel injection systems, piezoelectric actuators can be used to assure a defined injection of fuel. Such a piezoelectric actuator can be coupled to a control valve element by means of a hydraulic coupling device. In order to assure a filling of the coupling device, a minimum pressure can be maintained in the return of the fuel injection valve device in the fuel injection system, even after the system itself or an associated internal combustion engine is switched off. This minimum pressure can be maintained by means of a pressure-holding valve in the return of the fuel injection valve device. The minimum pressure to be maintained by the pressure-holding valve can lead to an uncontrolled spraying of fuel during installation and/or maintenance work on the fuel injection system. 
   The object of the present invention is to provide a pressure-holding valve, a tool, a set, and/or a fuel injection system, which make it easily possible to selectively reduce the minimum pressure to be maintained by the pressure-holding valve. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   In a pressure-holding valve for a fuel injection system with a high-pressure region and a low-pressure region, which valve has a valve housing equipped with a first connection that can be connected to the low-pressure region and a second connection that can be connected to the return of a fuel injection valve device, which valve housing contains a reciprocating valve cup that is prestressed in opposition to the prestressing force of a first spring device and has a through opening, which can be closed by a closing element that is prestressed in opposition to the prestressing force of a second spring device in order to maintain a minimum pressure in the return, the object is attained in that a pressure relief device that can be actuated from the outside is provided in the valve housing, between the first connection and the valve cup. The pressure relief device can be used to selectively reduce the pressure in the return with the aid of a tool. This reliably prevents an undesired spraying of the fuel. The spring devices are preferably helical compression springs that are supported at opposite ends of the valve housing. 
   A preferred exemplary embodiment of the pressure-holding valve is characterized in that the pressure relief device has a pressure pin that protrudes from the first connection toward the valve cup. The pressure pin serves to bridge over the distance between the first connection and the valve cup. It is, however, not necessary for the pressure pin to be in contact with the valve cup. 
   Another preferred exemplary embodiment of the pressure-holding valve is characterized in that the pressure pin protrudes from a positioning disk that is clamped between the second spring device and the valve housing. The positioning disk serves to position the pressure pin in the valve housing. 
   Another preferred exemplary embodiment of the pressure-holding valve is characterized in that between the positioning disk and the valve housing, there is a fixing disk that serves to fix a filter element between the positioning disk and the fixing disk. The contact pressure required to fix the filter element between the two disks is assured by the second spring device, which presses the fixing disk, the filter element, and the positioning disk against the valve housing. The resulting clamping prevents an undesired slippage of the filter element. 
   Another preferred exemplary embodiment of the pressure-holding valve is characterized in that through openings are provided in the fixing disk and the positioning disk. The through openings permit fuel to flow through from the first connection to the valve cup and vice versa. 
   Another preferred exemplary embodiment of the pressure-holding valve is characterized in that the through openings in the fixing disk and the positioning disk are designed and arranged so as to assure a flow of fuel through the fixing disk and the positioning disk regardless of a relative rotation of the two disks in relation to each other. A suitable size, number, and position of the through openings in the disks simplify assembly of the pressure-holding valve. The two disks can be offset from each other by any angle without hindering the passage of fuel. 
   Another preferred exemplary embodiment of the pressure-holding valve is characterized in that on the side oriented away from the positioning disk, the fixing disk has an annular bead at its radial outside. The annular bead functions as a spacer element and assures that the fixing disk does not rest directly against the opening of the first connection. This assures that the through flow of fuel from the first connection to the valve cup and vice versa is not hindered by the fixing disk itself. 
   With a tool for reducing the pressure in an above-described pressure-holding valve, the above-indicated object is attained in that the tool has a cup-shaped base body with a bottom from which an essentially circular, cylindrical circumferential wall extends, whose inner diameter is slightly greater than the outer circumference of the pressure-holding valve in the region of the first connection. When the fuel is bled from the valve housing, the tool is slid onto the first connection so that the circumferential surface of the cup partially encompasses the valve housing in order to catch the escaping fuel. 
   A preferred exemplary embodiment of the tool is characterized in that on the inside of the tool, an arbor extends from the bottom in the direction of the tool axis, whose outer diameter is slightly smaller than the inner diameter of the first connection and whose length is greater than the length of the first connection. The arbor, which is preferably oriented in the direction of the longitudinal axis of the valve housing, can be inserted through the first connection, which is preferably embodied in the form of a connection fitting, into the interior of the valve housing until it comes into contact with the fixing disk. If the arbor is inserted further into the interior of the valve housing, then this causes the fixing disk, the positioning disk, and the pressure pin to lift away from the valve housing counter to the action of the prestressing force exerted by the second spring device. If the arbor is inserted even further into the interior of the valve housing, then the pressure pin comes into contact with the valve cup or the closing element. With further insertion of the arbor into the valve housing, the valve cup is lifted away from its seat, as a result of which, the pressure in the return connected to the second connection is relieved in the direction of the first connection. The inside of the tool catches the pressurized fuel escaping from the first connection. 
   The above-indicated object is also attained by a set that includes an above-described pressure-holding valve and an above-described tool. If the pressure in the return line is to be reduced, then the connection line attached to the first connection is removed and the arbor is inserted into the first connection until the valve cup lifts away from its seat. The fuel tool catches the fuel that consequently escapes from the first connection. 
   In a fuel injection system, which includes a low-pressure region and a high-pressure region that supplies a fuel injection valve, which valve is connected via a return to the low-pressure region, the above-mentioned object is attained in that an above-described pressure-holding valve is connected to the return of the fuel injection valve device and to the low-pressure region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages, features, and details of the present invention ensue from the following description in which various exemplary embodiments are described in detail in conjunction with the drawings, which: 
       FIG. 1  is a schematic depiction of a fuel injection system equipped with a pressure-holding valve; 
       FIG. 2  is a perspective view of a section through a pressure-holding valve equipped with a fixing disk and a positioning disk; 
       FIG. 3  is a top view of the fixing disk from the pressure-holding valve shown in  FIG. 2 ; 
       FIG. 4  shows a section along the line IV-IV in  FIG. 3 ; 
       FIG. 5  is a top view of the positioning disk from the pressure-holding valve shown in  FIG. 2 ; 
       FIG. 6  is a view of a section along the line VI-VI in  FIG. 5 , and 
       FIG. 7  shows a longitudinal section through a tool for relieving the pressure in the pressure-holding valve shown in  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  schematically depicts a common rail fuel injection system. From a low-pressure receptacle  1 , which can also be referred to as the fuel tank, a fuel-supply pump  2  supplies fuel via a connecting line  3  to a high-pressure pump  4 . The connecting line  3  contains an overflow valve  6 . The low-pressure receptacle  1 , the fuel-supply pump  2  and the connecting line  3  are acted on with low pressure and are thus associated with the low-pressure region. 
   The high-pressure pump  4  has a pressure control valve  8  attached to it, which is connected via a line  9  to the low-pressure receptacle  1 . The high-pressure pump  4  also has a high-pressure line  10  leading from it, which supplies the highly pressurized fuel to a high-pressure reservoir  12 , which is also referred to as the common rail. The high-pressure reservoir  12 , with the interposition of flow limiters  13 , is connected to high-pressure lines  14 , which supply the highly pressurized fuel from the high-pressure reservoir  12  to injection valves  15 , which are also referred to as injectors and of which only one is shown in  FIG. 1  for the sake of clarity. The high-pressure line  10 , the high-pressure reservoir  12 , the high-pressure line  14 , and the injection valve  15  contain highly pressurized fuel and are thus associated with the high-pressure region of the fuel injection system. 
   From the fuel injection valve  15 , a return line, which has two segments  16  and  17 , leads to the low-pressure receptacle  1 . A pressure-holding valve  18  is connected between the two segments  16  and  17  of the return line. The pressure-holding valve  18  serves to maintain a minimum pressure of approximately 10 bar in the segment  16  of the return line, which permits a coupling chamber between a piezoelectric actuator and a control valve element in the fuel injection valve  15  to be filled regardless of the operating state of the fuel injection system. The design and function of the pressure-holding valve  18  will be explained below. An electronic control unit  19  controls the operation of the fuel injection system. 
     FIG. 2  shows the pressure-holding valve  18  from  FIG. 1  in a perspective sectional view. The pressure-holding valve  18  shown in  FIG. 2  has a cup-shaped valve housing  20  with a bottom  21  and a cover  22 . A first connection fitting  23  extends outward from the cover  22 . In addition, a second connection fitting  24  extends outward from the bottom  21 . The segment  17  of the return line (shown in  FIG. 1 ) is attached to the first connection fitting  23 . The segment  16  of the return line (shown in  FIG. 1 ) is attached to the second connection fitting  24 . 
   The valve housing  20  to  22 , which has an essentially circular cylindrical shape, contains a valve cup  25  that can reciprocate between two stops  26  and  27 . The stop  26  is embodied on the cup-shaped valve housing part  20  and the stop  27  is embodied on the valve housing cover  22 . The valve cup  25  has an essentially circular, disk-shaped base body that rests against an O-ring  28  on its radial outside, which is partially accommodated in a corresponding groove between a tubular shoulder of the housing cover  22  and the valve housing part  20 . The disk-shaped base body of the valve cup  25  is held in sealed contact with the O-ring  28  with the aid of a first spring device  29 . The first spring device  29  is a prestressed helical compression spring. 
   The essentially circular, cylindrical base body of the valve cup  25  has a central opening from which a tubular extension  30  leads, which has an opening  31  at its end. The tubular extension  30  contains a valve ball  32  that a second spring device  33  holds in contact with the opening  31  at the end of the tubular extension  30 . The second spring device  33  is a prestressed helical compression spring. 
   The valve ball  32  prestressed by the second spring device  33  unblocks the opening  31  if the pressure in the segment  16  of the return line (see  FIG. 1 ) exceeds a predetermined value. This assures the reduction of any undesirable excess pressure possibly present in the segment  16  of the return line. 
   One end of a pressure pin  35  is situated close to the valve cup  25  and its other end is integrally connected to a positioning disk  36 . The positioning disk  36  cooperates with a filter disk  37  in order to fix a filter sheet  38  in place. The second spring device  33  holds the positioning disk  36 , the fixing disk  37 , and the interposed fixing sheet  38  against the inside of the valve housing cover  22 . On the side of the fixing disk  37  oriented away from the positioning disk  36 , an annular bead  39  is provided toward the radial outside, which rests against the inside of the valve housing cover  22 . This produces a space between the fixing disk  37  and the valve housing cover  22 , which assures that fuel can travel to and from the connection fitting  23 . 
     FIGS. 3 and 4  show that the fixing disk  37  has four through openings  41  to  44  let into it, which assure that fuel can pass through. 
     FIGS. 5 and 6  show that six through openings  51  to  56  are let into the positioning disk  36 , which likewise assure that fuel can pass through. The through openings  41  to  44  and  51  to  56  in the two disks  37  and  36  are designed and arranged so that regardless of the rotation angle of the two disks in relation to each other, there are always at least two through openings in the two disks at least partially overlapping or congruent to each other. This assures that fuel can always pass through regardless of the installation position of the disks. 
     FIG. 7  shows a longitudinal section through a tool  70 , which has an essentially circular, cylindrical base body  72  that is closed at one end by a bottom  73 . The tool  70  therefore has a cup-shaped form, with a longitudinal tool axis  74 . On the inside of the cup-shaped tool, an arbor  75  extends from the bottom  73  in the direction of the longitudinal tool axis  74 . The length of the arbor  75  is less than the overall length of the essentially circular, cylindrical base body  72  in the direction of the longitudinal tool axis  74 . The arbor  75  therefore does not protrude out from the tool  70 . 
   When the pressure-holding valve  18  is in the installed position, the segment  16  of the return line (in  FIG. 1 ) is connected to the connection fitting  24  and the segment  17  of the return line (in  FIG. 1 ) is connected to the connection fitting  23 . For example, the pressure in the segment  16  of the return line is approximately 10 bar. If it is necessary to reduce this pressure for maintenance and/or installation work, then the segment  17  of the return line, which is acted on by low-pressure or is unpressurized, is removed from the connection fitting  23 . Due to the action of the first spring device  29  and the pressure prevailing in the segment  16  of the return line, the valve cup  25  rests against the O-ring  28 . In addition, due to the action exerted on it by the second spring device  33 , the valve ball  32  rests against the opening  31  of the valve cup  25 . The pressure in the segment  16  of the return line and on the inside of the valve housing to which it is connected, between the bottom  21  and the valve cup  25 , is maintained even if the first connection fitting  23  is open to the outside. 
   In order to relieve the pressure in the segment  16  of the return line and the inside of the pressure-holding valve between the bottom  21  and the valve cup  25 , the arbor  75  of the tool  70  is inserted into the first connection fitting  23  until the free end of the arbor  75  comes into contact with the fixing disk  37 . Then the arbor  75  is inserted further into the valve housing  20 , which moves the fixing disk  37 , the positioning disk  36 , and the pressure pin  35  toward the valve cup  25  until the pressure pin  35  comes into contact with the valve cup  25  or the valve ball  32 . If the pressure pin  35  is moved further in the same direction, i.e. toward the second connection fitting  24 , then the valve cup  25  lifts away from the O-ring  28  and the fuel contained in the valve housing  20  between the valve cup  25  and the bottom  21  can bypass the valve cup  25  and escape through the first connection fitting  23  into the cup-shaped tool  70 . In this state, the circumference edge of the tool  70  oriented away from the bottom  73  is situated in the region of the valve housing cover  22  with enough overlap to reliably prevent an undesired escape of pressurized fuel from the cup-shaped tool  70 . 
   The present invention makes it possible to selectively decrease the pressure in the segment  16  of the return line within seconds, without an uncontrolled spraying of fuel. The function of the pressure-holding valve is maintained. This prevents injuries to service personnel due to uncontrolled spraying of hot fuel when the return rail is opened. It also prevents escaping fuel from coming into contact with hot engine parts or hot exhaust system parts.