Patent Publication Number: US-6981653-B2

Title: Fuel injection device for an internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 02/04455 filed on Dec. 5, 2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is directed to an improved fuel injection system for an internal combustion engine and more particularly to such a system having a high pressure pump connected to a fuel injection valve for each cylinder of the engine. 
   The invention is based on a fuel injection system for an internal combustion engine as generically defined by the preamble to claim  1 . 
   2. Description of the Prior Art 
   A fuel injection system known from EP 0 987 431 A2 has a high-pressure fuel pump that is connected to a fuel injection valve for each cylinder of the internal combustion engine. The high-pressure fuel pump has a pump piston that defines a pump working chamber and is driven into a stroke motion by the engine. The fuel injection valve has a pressure chamber connected to the pump working chamber and an injection valve element that controls at least one injection opening; the pressure prevailing in the pressure chamber can move the injection valve element in the opening direction counter to a closing force in order to open the at least one injection opening. A first electrically actuated control valve controls a connection of the pump working chamber to a relief chamber. A second electrically actuated control valve controls a connection of a control pressure chamber to a relief chamber. A control piston defines the control pressure chamber; the pressure prevailing in the control pressure chamber causes the control piston to act on the injection valve element in a closing direction and this control piston can move in concert with the injection valve element. The control pressure chamber has a connection to the pump working chamber. For a fuel injection, the first control valve is closed and the second control valve is opened so that high pressure cannot build up in the control pressure chamber and the fuel injection valve can open. When the second control valve is open, though, fuel flows out of the pump working chamber via the control pressure chamber so that the fuel quantity available for the injection is reduced along with the fuel quantity supplied by the pump piston and the pressure available for the injection is reduced as well. It follows from this that the efficiency of the fuel injection system is not optimal. 
   A fuel injection system of this kind is known from EP 0 987 431 A2. This fuel injection system has a high-pressure fuel pump that is connected to a fuel injection valve for each cylinder of the internal combustion engine. The high-pressure fuel pump has a pump piston that defines a pump working chamber and is driven into a stroke motion by the engine. The fuel injection valve has a pressure chamber connected to the pump working chamber and an injection valve element that controls at least one injection opening; the pressure prevailing in the pressure chamber can move the injection valve element in the opening direction counter to a closing force in order to open the at least one injection opening. A first electrically actuated control valve is provided, which controls a connection of the pump working chamber to a relief chamber. A second electrically actuated control valve is also provided, which controls a connection of a control pressure chamber to a relief chamber. A control piston defines the control pressure chamber; the pressure prevailing in the control pressure chamber causes the control piston to act on the injection valve element in a closing direction and this control piston can move in concert with the injection valve element. The control pressure chamber has a connection to the pump working chamber. For a fuel injection, the first control valve is closed and the second control valve is opened so that high pressure cannot build up in the control pressure chamber and the fuel injection valve can open. When the second control valve is open, though, fuel flows out of the pump working chamber via the control pressure chamber so that the fuel quantity available for the injection is reduced along with the fuel quantity supplied by the pump piston and the pressure available for the injection is reduced as well. It follows from this that the efficiency of the fuel injection system is not optimal. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   The fuel injection system according to the invention has the advantage over the prior art that when the injection valve element is in its open position, a smaller area of the control piston is acted on by the pressure prevailing in the control pressure chamber and consequently a weaker force acts on the injection valve element in the closing direction than when the injection valve element is in its closed position so that the second control valve can be closed during the fuel injection and no loss in fuel quantity or fuel pressure occurs during the injection, which therefore improves the efficiency of the fuel injection system. 
   Advantageous embodiments and modifications of the fuel injection system according to the invention are disclosed. One embodiment permits the pressure-exposed end surface of the control piston to be reduced in a simple way. 
   The fuel injection system according to the invention, with the characterizing features of claim  1 , has the advantage over the prior art that when the injection valve element is in its open position, a smaller area of the control piston is acted on by the pressure prevailing in the control pressure chamber and consequently a weaker force acts on the injection valve element in the closing direction than when the injection valve element is in its closed position so that the second control valve can be closed during the fuel injection and no loss in fuel quantity or fuel pressure occurs during the injection, which therefore improves the efficiency of the fuel injection system. 
   Advantageous embodiments and modifications of the fuel injection system according to the invention are disclosed in the dependent claims. The embodiment according to claim  2  permits the pressure-exposed end surface of the control piston to be reduced in a simple way. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described herein below, with reference to the drawings, in which: 
       FIG. 1  schematically depicts a fuel injection system for an internal combustion engine according to the invention, 
       FIG. 2  shows an enlarged detail, labeled II in  FIG. 1 , of the fuel injection system when an injection valve element is in a closed position, 
       FIG. 3  shows the detail II when the injection valve element is in an open position, 
       FIG. 4  shows the detail II of the fuel injection system according to a modified embodiment when the injection valve element is in a closed position, 
       FIG. 5  shows the detail II according to the modified embodiment of the fuel injection system when the injection valve element is in an open position, and 
       FIG. 6  shows a graph of the pressure at injection openings of a fuel injection valve of the fuel injection system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 to 5  show a fuel injection system for an internal combustion engine of a motor vehicle, preferably an autoignition engine in which the fuel injection system is preferably embodied as a so-called unit fuel injector and, for each cylinder of the engine, has a high-pressure fuel pump  10  and a fuel injection valve  12  connected to it, which comprise a common component. Alternatively, the fuel injection system can also be embodied as a so-called unit pump system, in which the high-pressure fuel pump and the fuel injection valve of each cylinder are disposed separate from each other and are connected to each other via a line. The high-pressure fuel pump  10  has a pump body  14  with a cylinder bore  16  in which a pump piston  18  is guided in a sealed fashion, which piston is set into a stroke motion counter the force of a return spring  19 , at least indirectly by means of a cam  20  of a camshaft of the engine. In the cylinder bore  16 , the pump piston  18  defines a pump working chamber  22  in which fuel is compressed at high pressure during the delivery stroke of the pump piston  18 . The pump working chamber  22  is supplied with fuel from a fuel tank  24  of the motor vehicle. 
   The fuel injection valve  12  has a valve body  26  that is connected to the pump body  14  and can be composed of a number of parts; an injection valve element  28  can be guided in a bore  30  in this valve body  26 . In its end region oriented toward the combustion chamber of the cylinder of the engine, the valve body  26  has at least one, preferably several injection openings  32 . In its end region oriented toward the combustion chamber, the injection valve element  28  has a sealing surface  34  that is conical, for example, which cooperates with a valve seat  36  embodied in the end region of the valve body  26  oriented toward the combustion chamber; the injection openings  32  branch off from this valve seat  26  or branch off downstream of it. In the valve body  26 , between the injection valve element  28  and the bore  30 , toward the valve seat  36 , there is an annular space  38 , which in its end region oriented away from the valve seat  36 , by means of a radial enlargement of the bore  30 , transitions into a pressure chamber  40  that encompasses the injection valve element  28 . At the level of the pressure chamber  40 , the fuel injection valve  28  has a pressure shoulder  42  formed by a cross sectional reduction. The end of the injection valve element  28  oriented away from the combustion chamber is engaged by a prestressed closing spring  44 , which presses the injection valve element  28  toward the valve seat  36 . The closing spring  44  is disposed in a spring chamber  46  of the valve body  26 , which adjoins the bore  30 . 
   At its end oriented away from the bore  30 , the spring chamber  46  is adjoined by an additional bore  48  in the valve body  26 , in which a control piston  50  is guided in a sealed fashion, which is connected to the injection valve element  28 . The control piston  50  functions as a moving wall to define a control pressure chamber  52  in the bore  48 . The control piston  50  is connected to the injection valve element  28  by means of a piston rod  51  with a diameter smaller than that of the control piston. The control piston  50  can be of one piece with the injection valve element  28 , but for assembly reasons is preferably embodied as a separate part that is attached to the injection valve element  28 . 
   A conduit  60  leads from the pump working chamber  22 , through the pump body  14  and the valve body  26  to the pressure chamber  40  of the fuel injection valve  12 . A conduit  62  leads from the pump working chamber  22  or from the conduit  60 , to the control pressure chamber  52 . The control pressure chamber  52  is also fed by a conduit  64 , which produces a connection to a relief chamber, which function can be served at least indirectly by the fuel tank  24  or another region in which a low pressure prevails. A connection  66  leads from the pump working chamber  22  or the conduit  60  to a relief chamber  24  and is controlled by means of a first electrically actuated control valve  68 . The control valve  68  can, as shown in  FIG. 1 , be embodied as a 2/2-port directional control valve. The connection  64  of the control pressure chamber  52  to the relief chamber  24  is controlled by a second electrically actuated control valve  70 , which can be embodied as a 2/2-port directional control valve. A throttle restriction  63  can be provided in the connection  62  of the control pressure chamber  52  to the pump working chamber  22  and a throttle restriction  71  can be provided in the connection of the control pressure chamber  52  to the relief chamber  24 . The control valves  68 ,  70  can have an electromagnetic actuator or a piezoelectric actuator and are triggered by an electronic control unit  72 . 
   When the fuel injection valve  12  is closed, then the annular sealing surface  34  of the injection valve element  28  rests against the valve seat  36 . In this case, only the surface area of the pressure shoulder  42  of the injection valve element  28  is acted on in the opening direction  29  by the pressure prevailing in the pressure chamber  40  and no other forces act on the injection valve element  28  in the opening direction  29 . When the fuel injection valve  12  opens, then the sealing surface  34  of the injection valve element  28  lifts up from the valve seat  36  thus opening an annular flow cross section  37  between the sealing surface  34  and the valve seat  36 . When the sealing surface  34  of the injection valve element  28  is lifted up from the valve seat  36 , then a pressure likewise acts on the end surface of the injection valve element  28 , i.e. the annular sealing surface  34  and the remaining surface area of the injection valve element  28  surrounded by this sealing surface  34 , thus exerting a force on the injection valve element  28  in the opening direction  29 . When the sealing surface  34  of the injection valve element  28  is spaced only a small distance apart from the valve seat  36  and is consequently only opened by a partial stroke, then only a small flow cross section  37  is opened, which throttles the fuel flowing through, resulting in a pressure drop. Then the only force acting on the end surface of the injection valve element  28  in the opening direction is the pressure that is reduced by the throttling action in comparison to the pressure prevailing in the pressure chamber  40 . When the injection valve element  28  executes its maximal opening stroke, then its sealing surface  34  is spaced a greater distance apart from the valve seat  36  so that a correspondingly greater flow cross section  37  is opened. With the greater flow cross section, a less intense throttling action occurs so that a correspondingly higher pressure acts on the end surface of the injection valve element  28  in the opening direction  29 . When the fuel injection valve  12  is open, the injection valve element  28  is consequently also acted on with the force exerted in the opening direction  29  by the pressure acting on the end surface of the injection valve element  28  in addition to the force exerted on the pressure shoulder  42  by the pressure prevailing in the pressure chamber  40 . 
   The end surface of the control piston  50  that is acted on by the pressure prevailing in the control pressure chamber  52  is sized so that when the second control valve  70  is closed, when the control pressure chamber  52  is closed off from the relief chamber  24 , and during the delivery stroke of the pump piston  18 , high pressure builds up in the pump working chamber  22  and therefore also in the control pressure chamber  52  and, in addition to the force of the closing spring  44 , a force acting on the injection valve element  28  in the closing direction is produced, which is greater than the force acting on the injection valve element  28  in the opening direction  29  when the injection valve element  28  is in its closed position or, as explained above, is opened with only a partial stroke and its sealing surface  34  is lifted up from the valve seat  36 . In this case, the fuel injection valve  12  is closed or remains closed. 
     FIGS. 2 and 3  show an enlarged detail II of the fuel injection system;  FIG. 2  shows the control piston  50  when the injection valve element  28  is in its closed position and  FIG. 3  shows the control piston when the injection valve element  28  is in the position in which it is opened by its maximal stroke. The conduit  62  that serves as a connection to the pump working chamber  22  and the conduit  64  that serves as a connection to the relief chamber  24  each feed into the control pressure chamber  52 , viewed in the direction of its longitudinal axis  49 , close to the edge of the control pressure chamber  52 , for example on opposite sides from each other. In the boundary  53  of the control pressure chamber  52  at the opposite end from the control piston  50 , viewed in the direction of its longitudinal axis  49 , at least approximately coaxial to the control piston  50 , a recess  54  is provided, which has a connection  55  to a relief chamber  24  leading from it and contains a throttle constriction  56 . The cross section of the recess  54  is preferably circular and is provided with a seat  57  on the edge at which it transitions into the boundary  53 . From the end surface of the control piston  50  that defines the control pressure chamber  52 , a projection  58  protrudes coaxial to the recess  54 , tapering toward the boundary  53  in the direction of the longitudinal axis  49  of the control piston  50 , and is embodied, for example, in an at least approximately conical form. The projection  58  is provided with a sealing surface  59  that cooperates with the seat  57 . Alternatively, however, it is also possible for the boundary  53  of the control pressure chamber  52  to be provided with a projection that protrudes into the control pressure chamber  52  and whose end has a seat embodied on it that cooperates with a sealing surface provided at the end of the control piston  50 . When the injection valve element  28  according to  FIG. 2  is in its closed position, then the control piston  50  is in its corresponding stroke position, with its sealing surface  59  spaced apart from the seat  57 . When the second control valve  70  is closed, then high pressure prevails in the control pressure chamber  52  as well as in the pump working chamber  22 , which acts on the entire surface area of the control piston  50  and correspondingly exerts a large force on the injection valve element  28  in the closing direction. When the injection valve element  28  according to  FIG. 3  is in the position in which it is opened by its maximal stroke, then the projection  58  of the control piston  50  protrudes partially into the recess  54  and rests with its sealing surface  59  against the seat  57 . The recess  54 , as part of the control pressure chamber  52 , and the part of the end surface of the control piston  50  disposed inside the seat  57  are then closed off from the rest of the control pressure chamber  52  and are pressure relieved in the direction of the relief chamber  24  via the connection  55  to the throttle restriction  56 . The pressure prevailing in the rest of the control pressure chamber  52  then only acts on an annular part of the end surface of the control piston  50  encompassing the sealing surface  59  and the seat  57  so that a correspondingly weaker force is exerted on the injection valve element  28  in the closing direction. If it is necessary to open the fuel injection valve  12  starting from a closed position of the injection valve element  28 , then the second control valve  70  must be opened in order to relieve the pressure in the control pressure chamber  52  since the pressure acts on the entire end surface of the control piston  50 . If it is necessary to close the fuel injection valve  12  starting from a position of the injection valve element  28  in which it is opened by its maximal stroke, then if the second control valve  70  is closed, the first control valve  68  must be opened so that the pressure prevailing in the pressure chamber  40  and a pressure acting on the injection valve element  28  in the opening direction  29  falls below the sum of the force of the closing spring  44  and the force in the closing direction exerted by the pressure prevailing in the control pressure chamber  52 . 
     FIGS. 4 and 5  show the detail II of the fuel injection system according to a modified embodiment. The control pressure chamber  152  here has a diametrically reduced extension  154 , which connects to both the conduit  62  that serves as a connection to the pump working chamber  22  and the conduit  64  that serves as a connection to the relief chamber  24 . The extension  154  is situated at least approximately coaxial to the control piston  150 . At the transition from the control pressure chamber  152  to the extension  154 , the diametrical reduction forms an annular boundary  153  of the control pressure chamber  152 . The boundary  153  extends laterally, for example at least approximately perpendicular to the longitudinal axis  49  of the control piston  150 , is embodied as flat, and constitutes a flat seat  157 . The end of the control piston  150  oriented toward the boundary  153  has an annular projection  158  protruding from it, with an annular sealing surface  159  at the end. When the injection valve element  28  is in its closed position and the control piston  150  is in the corresponding position according to  FIG. 4 , then the control piston  150  is disposed with its sealing surface  159  spaced apart from the seat  157  at the boundary  153  and the pressure prevailing in the control pressure chamber  152  acts on the entire end surface of the control piston  150 . When the injection valve element  28  is in its open position and the control piston  150  is in the corresponding position according to  FIG. 5 , then the sealing surface  159  of the control piston  150  rests against the seat  157 . The part of the control pressure chamber  152  disposed outside the sealing surface  159  is then closed off from the extension  154  so that the part  152  of the control pressure chamber is no longer connected to the pump working chamber  22 . The part  152  of the control pressure chamber is thus pressure relieved and is connected to a relief chamber  24 , for example by means of a gap  155  between the control piston  150  and the bore  48  that constitutes a throttle restriction or by means of a separate connection that contains a throttle restriction. The pressure prevailing in the extension  154  of the control pressure chamber then acts only on the part of the end surface of the control piston  150  inside the annular sealing surface  159 . 
   The embodiment of the control piston  50  having the projection  58  with the conical sealing surface  59  according to  FIGS. 2 and 3  can also be used in the embodiment according to  FIGS. 4 and 5  instead of the annular projection  158  provided there. Likewise, the embodiment of the control piston  150  with the annular projection  158  and the annular sealing surface  159  according to  FIGS. 4 and 5  can also be used in the embodiment according to  FIGS. 2 and 3  instead of the projection  58  provided there. The essential difference between the embodiment according to  FIGS. 2 and 3  and the embodiment according to  FIGS. 4 and 5  is that in the embodiment according to  FIGS. 2 and 3 , when the injection valve element  28  is in its open position, the pressure prevailing in the control pressure chamber  52  acts on an annular part of the end surface of the control piston  50  surrounding the seat  57 , while in the embodiment according to  FIGS. 4 and 5 , when the injection valve element  28  is in its open position, the pressure prevailing in the extension  154  of the control pressure chamber  52  acts on a part of the end surface of the control piston  150  disposed inside the seat  157 . 
   The function of the fuel injection system will be explained below.  FIG. 6  shows the graph of the pressure p at the injection openings  32  of the fuel injection valve  12  over time t during an injection cycle. During the intake stroke of the pump piston  18 , it is supplied with fuel from the fuel tank  24 . During the delivery stroke of the pump piston  18 , the fuel injection begins with a preinjection, in which the control unit  72  closes the first control valve  68  so that the pump working chamber  22  is closed off from the relief chamber  24 . The control unit  72  also opens the second control valve  70  so that the control pressure chamber  52  or  152  is connected to the relief chamber  24 . In this instance, high pressure cannot build up in the control pressure chamber  52  or  152  since it is pressure relieved in the direction of the relief chamber  24 . However, a small quantity of fuel can flow out of the pump working chamber  22  to the relief chamber  24  via the throttle restrictions  63  and  71  so that the entire high pressure that would build up if the second control valve  70  were closed cannot build up in the pump working chamber  22 . If the pressure in the pump working chamber  22  and therefore in the pressure chamber  40  of the fuel injection valve  12  is great enough for the compressive force that it exerts on the injection valve element  28  via the pressure shoulder  42  to exceed the sum of the force of the closing spring  44  and the compressive force exerted on the control piston  50  or  150  by the residual pressure prevailing in the control pressure chamber  52  or  152 , then the injection valve element  28  moves in the opening direction  29  and unblocks the at least one injection opening  32 . The injection valve element  28  then only opens with a partial stroke so that the sealing surface  59  or  159  of the control piston  50  or  150  does not come into contact with the seat  57  or  157 . Only a relatively low pressure acts on the end surface of the injection valve element  28  in the opening direction  29  and the pressure prevailing in the control pressure chamber  52  or  152  acts on the entire end surface of the control piston  50  or  150 . In order to terminate the preinjection, the control unit closes the second control valve  70  so that the control pressure chamber  52  or  152  is closed off from the relief chamber  24 . The first control valve  68  remains in its closed position. As a result, the same high pressure as in the pump working chamber  22  builds up in the control pressure chamber  52  or  152  so that a powerful compressive force acts on the control piston  50  or  150  in the closing direction. The fact that as a result of the partial stroke of the injection valve element  28 , only a slight force is exerted on the injection valve element  28  in the opening direction  29 , which is less than the sum of the force of the closing spring  44  and the force of the pressure acting on the control piston  50  or  150 , causes the fuel injection valve  12  to close. The preinjection corresponds to an injection phase labeled I in  FIG. 6 . 
   For a subsequent main injection that corresponds to the injection phase labeled II in  FIG. 6 , the control unit  72  opens the second control valve  70 . The fuel injection valve  12  then opens due to the reduced compressive force on the control piston  50  or  150  and the injection valve element  28  moves for its maximal opening stroke until the sealing surface  59  or  159  of the control piston  50  or  150  comes into contact with the seat  57  or  157 . The seat  57  or  157  consequently also constitutes a stop for limiting the stroke of the control piston  50  or  150  and therefore the opening stroke motion of the injection valve element  28 . If the injection valve element  28  is opened by its maximal opening stroke, then the control unit  72  can close the second control valve  70  so that the control pressure chamber  52  or  152  is closed off from the relief chamber  24 . Then the same high pressure as in the pump working chamber  22  does in fact build up in the control pressure chamber  52  or  152 , but due to the small end surface of the control piston  50  or  150  that is actually subjected to pressure, the force in the closing direction, which is the sum of the force of the pressure acting on the control piston  50  or  150  and the force of the closing spring  44 , is less than the force in the opening direction  29  generated by the force on the pressure shoulder  42  and the end surface of the injection valve element that is open by its maximal stroke, thus causing the fuel injection valve  12  to remain open. As long as the second control valve  70  remains open, the fuel injection occurs at a reduced pressure since a small fuel quantity flows out of the pump working chamber  22  and into the relief chamber  24  via the open control valve  70 . When the second control valve  70  is closed, then no more fuel can flow out of the pump working chamber  22  and the fuel injection occurs at a higher pressure, as indicated in  FIG. 6 . The time at which the control unit  72  closes the second control valve  70  preferably varies as a function of operating parameters of the internal combustion engine, in particular as a function of the engine speed. It is possible for the control unit  72  to close the second control valve  70  at an earlier time when engine speeds are low and for the control unit  72  to close the second control valve  70  at a later time as engine speeds increase. This allows limits to be placed on the maximal pressure of the fuel injection at high engine speeds. 
   In order to terminate the main injection, the control unit  72  brings the first control valve  68  into its open switched position so that the pump working chamber  22  communicates with the relief chamber  24  and only a slight pressure-induced force acts on the injection valve element  28  in the opening direction  29 ; the fuel injection valve  12  closes due to the force of the closing spring  44  and the force exerted by the residual pressure in the control pressure chamber  52  or  152 , which once again acts on the entire end surface of the control piston  50  or  150  after the sealing surface  59  or  159  of the control piston  50  or  150  lifts up from the seat  57  or  157 . The second control valve  70  can be in either its open position or its closed position upon termination of the main injection. 
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.