Abstract:
The invention relates to a fuel injection valve comprising a housing ( 1 ) in which pressure is applied to a nozzle needle ( 8 ), in a control chamber ( 28 ), at least indirectly with a closing force in the direction of a valve seat ( 10 ). The pressure in the control chamber ( 28 ) can be adjusted using a control valve ( 40 ) as said control chamber ( 28 ) is able to be connected to a low pressure chamber ( 46 ) via an outlet restrictor ( 31 ) and be filled with fuel at high pressure via an inlet restrictor ( 30 ). A longitudinally-displaceable control piston ( 29 ) is arranged in the control chamber ( 28 ) and divides said chamber ( 28 ) into a first control sub-chamber and a second control sub-chamber ( 228 ), the first control sub-chamber ( 128 ) being able to be connected to the low pressure chamber ( 46 ) by means of said outlet restrictor ( 31 ). A sealing surface ( 38 ) is formed on the control piston ( 29 ) and interacts with a sealing seat ( 39 ) in the control chamber ( 28 ) such that the inlet restrictor ( 30 ) is hydraulically disconnected from the second control sub-chamber ( 228 ) when the sealing surface ( 38 ) comes to rest against the sealing seat ( 39 ). Said first control sub-chamber ( 128 ) and second control sub-chamber ( 228 ) are constantly hydraulically interconnected by means of a restrictor connection ( 34 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a fuel injection valve for internal combustion engines, such as is preferably used for the injection of fuel into a combustion chamber of an auto-ignition internal combustion engine. 
     Injection systems for the injection of fuel into combustion chambers at high pressure are known from the prior art. Here, fuel is compressed by a high-pressure pump and is temporarily stored in a high-pressure accumulator, a so-called rail. Said high-pressure accumulator provides a feed to one or more fuel injection valves which inject the required fuel into the respective combustion chamber. It is an aim here for exactly the required fuel quantity to be introduced into the combustion chamber as finely atomized and spatially uniform a manner as possible. DE 100 24 702 A1, for example, presents an injection system of said type together with injection valve. 
     To control the injection, the known fuel injection valves have a nozzle needle which is arranged in longitudinally displaceable fashion in a housing and which, to open and close at least one injection opening, interacts with a nozzle seat. The movement of the nozzle needle is in this case controlled by the pressure in a control chamber, said pressure acting on that face surface of the nozzle needle which faces away from the valve seat. By means of a control valve, the pressure in the control chamber is lowered or raised, which correspondingly changes the closing force on the nozzle needle, such that said nozzle needle, driven by the hydraulic force of the fuel which is at injection pressure and which surrounds the nozzle needle, moves in a longitudinal direction. 
     The pressure in the control chamber is achieved through the inflow and outflow of pressurized fuel. In the known fuel injection valves, the control chamber is permanently connected to pressurized fuel via an inflow throttle which connects the control chamber to a high-pressure line within the fuel injection valve. To lower the pressure in the control chamber, the control chamber can be connected via an outflow throttle to a low-pressure chamber, wherein the outflow throttle can be opened or closed by means of a control valve. When the outflow throttle is open, pressurized fuel thus flows out of the control chamber into the low-pressure chamber, wherein the pressure in the control chamber, and thus the closing force on the nozzle needle, decrease. This so-called discharge quantity is basically unavoidable owing to the construction principle. 
     When the control valve is open, fuel flows constantly into the control chamber via the inflow throttle, said fuel expanding in the control chamber and flowing onward into the low-pressure chamber. Said fuel must be compressed by the high-pressure pump in addition to the fuel intended for injection, which reduces the efficiency of the injection system. 
     To increase the efficiency of the injection system, DE 101 31 617 A1 has disclosed a fuel injection valve in which the fuel pressure in the control chamber is controlled by way of a 3/2 directional valve. Depending on the position of said control valve, fuel either flows from a high-pressure line via an inflow and outflow throttle into the control chamber, or is discharged into a low-pressure chamber. The control by way of a 3/2 directional valve is however cumbersome and expensive. Furthermore, the build-up and dissipation of pressure in the control chamber is relatively slow in the case of this embodiment. 
     SUMMARY OF THE INVENTION 
     The fuel injection valve according to the invention is, by contrast, capable of considerably reducing the fuel discharge quantity using simple means without the dynamics of the control being adversely affected. For this purpose, a control piston is arranged in longitudinally displaceable fashion in the control chamber, which control piston divides the control chamber into a first control chamber part and a second control chamber part, wherein the first control chamber part can be connected to a low-pressure chamber via an outflow throttle and the first control chamber part is permanently hydraulically connected to the second control chamber part via a connecting throttle. On the control piston there is formed a sealing surface which interacts with a sealing seat such that, when the sealing surface is in contact with the sealing seat, the second control chamber part is hydraulically separated from the inflow throttle. Here, the control piston is not moved by further actuators or other control devices; it is moved exclusively by the hydraulic forces acting on it. The construction is correspondingly easy and inexpensive to realize. 
     As a result of the closure of the inflow throttle, it is the case during a major part of the opening phase of the fuel injection valve that no fuel flows into the control chamber and is expanded onward from there, without further benefit to the fuel injection system, into the low-pressure chamber. This increases the efficiency of the fuel injection system and thus reduces the fuel consumption of a motor vehicle equipped with a fuel injection system of said type. 
     In a first advantageous embodiment of the invention, the connecting throttle is formed in a valve piece in which the control piston is guided. This permits a wide range of variability with regard to the arrangement of the control piston and valve piece, and reduces the production costs in relation to a configuration with guidance directly in the housing of the fuel injection valve. Furthermore, the connecting throttle can advantageously be formed in the valve piece. Alternatively, the connecting throttle may also be formed in the control piston itself. 
     In a further advantageous embodiment of the invention, there is formed in the outer surface of the control piston an annular groove into which the inflow throttle issues. The annular groove is separated from the second control chamber part when the sealing surface is in contact with the sealing seat. Since the annular groove has the same effective hydraulic surface area in both directions of longitudinal movement of the control piston, the fuel in the annular groove does not give rise to a hydraulic force acting on the control piston in the longitudinal direction, which would have to be compensated. 
     In a further advantageous embodiment, the control piston is subjected, by a spring element, to an opening force away from the sealing seat. Here, the force of the spring element serves to move the control piston into a defined initial position in order to ensure a distinct function. The spring element may in this case advantageously be arranged in the second control chamber part, such that the first control chamber part can be kept very small, which permits rapid switching of the fuel injection valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and advantageous refinements of the invention will emerge from the description and from the drawing. 
       Multiple exemplary embodiments of the invention are illustrated in the drawing, in which: 
         FIG. 1  shows a fuel injection valve according to the invention in longitudinal section, 
         FIG. 2  shows an enlarged detail of  FIG. 1  in the region of the control chamber, and 
         FIGS. 3, 4, 5 and 6  show further exemplary embodiments of the invention in the same form of illustration as  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a fuel injection valve according to the invention in longitudinal section. The fuel injection valve has a housing  1  which has a holding body  3  and a nozzle body  4  which are braced against one another by means of a clamping nut  5 . In the holding body  3  and in the nozzle body  4  there is formed a pressure chamber  7  which can be filled with fuel at high pressure via a high-pressure port  32 . In the pressure chamber  7  there is arranged, in longitudinally displaceable fashion, a nozzle needle  8  which, on its end which faces toward an internal combustion engine in an installed position, has a valve sealing surface  11  by means of which the nozzle needle  8  interacts with a nozzle seat  10  and thereby controls the connection of multiple injection openings  12 , which are formed in the nozzle body  4 , to the pressure chamber  7 . In this case, the nozzle needle  8  is guided, in the region of the nozzle body  4 , in a guide section  16 , wherein the fuel flow through the pressure chamber  7  in the direction of the injection openings  12  past the guide section  16  is ensured by means of one or more ground portions on the guide section  16 . Also arranged in the nozzle body  4  is a closing spring  9  which surrounds the nozzle needle  8  and which is supported, under compressive preload, with one end against the holding body  3  and with the other end against a shoulder  14  formed on the nozzle needle  8 , said closing spring thereby exerting a closing force on the nozzle needle  8  in the direction of the valve seat  10 . 
     At the end remote from the nozzle seat, there is arranged in the pressure chamber  7  a valve piece  20  which is supported against a throttle plate  24 , which throttle plate forms that end of the pressure chamber  7  which faces away from the valve seat, and which throttle plate may also be formed in one piece with the valve piece  20 . In this case, the throttle plate  24  is braced by means of a clamping screw  25  against a shoulder in the holding body  3 . In the valve piece  20  there is formed a stepped bore  21 ,  22  which comprises a guide bore  22  of reduced diameter and a bore section  21  of widened diameter. The nozzle needle  8 , by way of its end which faces away from the nozzle seat and on which the face side  17  is formed, projects into the widened bore section  21  and is guided radially therein. The guide bore  22 , the throttle plate  24  and the face side  17 , facing away from the valve seat, of the nozzle needle  8  delimits a control chamber  28 . In this case, the control chamber  28  is connected to the pressure chamber  7  by an inflow throttle  30  formed in the valve piece  20  and to a low-pressure chamber  46  via an outflow throttle  31 , said low-pressure chamber being connected to a return line (not illustrated in the drawing) such that a low pressure prevails in the low-pressure chamber  46  at all times. 
     For the opening and closing of the outflow throttle  31 , a control valve  40  is arranged in the housing  1  on that side of the throttle plate  24  which faces away from the control chamber  28 . The control valve  40  comprises a magnet armature  42 , wherein a sealing ball  43  is arranged on that end of said magnet armature which faces toward the throttle plate  24 , by means of which sealing ball the magnet armature  42  lies on a seat formed in the throttle plate  24  and thereby closes the outflow throttle  31 . The magnet armature  42  is subjected to a closing force in the direction of the throttle plate  24  by a spring  45  and can, by means of an electromagnet  44 , be pulled counter to the force of the spring  45  into an open position, such that the sealing ball  43  opens up the outflow throttle  31  and fuel can flow out of the control chamber  28  into the low-pressure chamber  46 . 
       FIG. 2  shows the region of the control chamber  28  of  FIG. 1  once again on an enlarged scale. To restrict the flow of fuel into the control chamber  28  through the inflow throttle  30 , a control piston  29  is arranged in the control chamber  28 . The control piston  29  is longitudinally movable in the control chamber  28  and is guided by way of a cylindrical section  129  in the guide bore  22 . That end of the control piston  29  which faces toward the nozzle needle  8  is of widened form and, on an outer surface, forms a sealing surface  38  which interacts with a sealing seat  39  formed on the valve piece  20 . The movement of the control piston  29  is in this case limited in the direction of the nozzle needle  8  by a stop  35 . The control piston  29  divides the control chamber  28  into a first control chamber part  128  and a second control chamber part  228 , wherein the first control chamber part  128  is formed between the first face surface  36  of the control piston  29  and the throttle plate  24 , and the second control chamber part  228  is formed between the second face surface  37  of the control piston  29  and the face side  17  of the nozzle needle  8 . To connect the two control chamber parts  128 ,  228 , there is provided in the valve piece  20  a connecting throttle  34  via which pressure equalization between the two control chamber parts  128 ,  228  is possible. 
     In the first control chamber part  128  there is arranged a closing spring  33  which subjects the control piston  29  to an opening force in the direction of the nozzle needle  8  and presses said control piston against the stop  35 . Between the cylindrical section  129  of the control piston  29  and the sealing surface  38 , there is formed on the outer side of the control piston  29  an annular groove  49  into which the inflow throttle  30  issues. When the control piston  29  is situated in its open position, that is to say is in contact with the stop  35 , the second control chamber part  228  is hydraulically connected to the inflow throttle  30  via the annular groove  49 , as illustrated in  FIG. 2 . 
     The described fuel injection valve functions as follows. At the start of the injection, the control valve  40  is deenergized, such that the magnet armature  42 , driven by the closing spring  45 , closes the outflow throttle  31 . Owing to the connection between the first control chamber part  128  and the second control chamber part  228  via the connecting throttle  34 , the same high pressure prevails throughout the control chamber  28 , because there is a connection via the inflow throttle  30  to the pressure chamber  7 , in which fuel is present at high pressure. If the electromagnet of the control valve  40  is energized, the magnet armature  42  is lifted from the throttle plate  24  and opens up the outflow throttle  31 , via which fuel then flows out of the first control chamber part  128  into the low-pressure chamber  46 . The pressure in the first control chamber part  128  thereupon falls very rapidly, which reduces the hydraulic forces on the first face side  36  of the control piston  29 , whereas the pressure in the second control chamber part  228  remains considerably higher owing to the throttling action of the connecting throttle  34  and the fuel flowing in from the inflow throttle  30 . The resulting high hydraulic force on the second face side  37  of the control piston  29  pushes the control piston away from the stop  35  in the direction of the throttle plate  24  until the sealing surface  38  of said control piston comes into contact with the sealing seat  39  and separates the annular groove  49  from the second control chamber part  228 . The inflow throttle  30  is now sealed off and the high fuel pressure prevails only within the annular groove  49 , while the pressure in the second control chamber part  228  now falls further, also reducing the closing force on the face side  17  of the nozzle needle  8 , until said nozzle needle—driven by the hydraulic forces in the pressure chamber  7 —is lifted from the nozzle seat  10  and fuel flows out of the pressure chamber  7  to the injection openings  12  and emerges from the fuel injection valve through these. 
     To end the injection, the energization of the electromagnet  40  is ended, such that the magnet armature  42  travels back into its closed position and closes the outflow throttle  31  again. Driven by the spring  33 , the control piston is lifted from the sealing seat  39 , as a result of which the annular groove  49  is connected to the second control chamber part  228  again. The pressure in the two control chamber parts  128 ,  228  then rapidly increases, and the nozzle needle  8  is pushed back into its closed position against the nozzle seat  10 , thus ending the injection. The control piston  29  moves in the direction of the nozzle needle  8  until it bears against the stop  35  again. 
     Since the inflow throttle  30  remains closed by the control piston  29  practically during the entire injection, only a small amount of fuel passes into the low-pressure chamber  46  as a result of the opening and closing of the outflow throttle  31 . This reduces the requirement for compressed fuel that would otherwise flow into the control chamber  28  through the inflow throttle  31  during the entire injection. Furthermore, the thermal loading of the control valve  40  is reduced in this way because the fuel that is compressed to high pressure releases a large amount of heat energy as it expands, which heat energy must be dissipated. 
       FIG. 3  shows a further exemplary embodiment, wherein identical parts are denoted by the same reference signs. This exemplary embodiment differs from the exemplary embodiment of  FIG. 2  merely in that the connecting throttle  34  is formed not in the valve piece  20  but as a longitudinal bore in the valve piston  29 , which is generally easier and less expensive to produce than an angled or oblique bore in the valve piece  20 . 
       FIG. 4  shows a further exemplary embodiment in the same form of illustration as  FIG. 2  and  FIG. 3 . The control piston  29  is in this case equipped with a further shoulder on the nozzle-side end, wherein the closing spring  33  is arranged between said further shoulder and the valve piece  20 , and said closing spring is correspondingly omitted from the first control chamber part  128 . This arrangement of the closing spring  33  makes it possible for the first control chamber part  128  to be made very small, whereby the pressure falls very rapidly when the control valve is open and the nozzle needle  8  opens correspondingly rapidly after activation of the control valve  40 . 
     A further exemplary embodiment is illustrated in  FIG. 5 . In this case, the control piston  29  does not have an annular groove  40  but is of cylindrical form as far as the sealing surface  38 . The closing spring  33  bears, by way of its end facing toward the nozzle needle  8 , against the sealing surface  38 , whereas the other end of the closing spring bears against a shoulder  41  of the valve piece  20 . The sealing surface  38  and the shoulder  41  of the valve piece  20  thus form an inflow chamber  47  into which the inflow throttle  30  issues and which accommodates the closing spring  33 . In this arrangement, the control piston  29  is of relatively simple form, and the volume of the inflow chamber is further reduced by the closing spring  33 , which is advantageous for rapid switching of the control piston  29 . 
       FIG. 6  shows a further exemplary embodiment of the invention, which differs from the exemplary embodiment of  FIG. 3  primarily by the omission of the closing spring. The opening of the fuel injection valve is identical to the process discussed above. To nevertheless realize a force on the control piston  8  in the direction of the nozzle needle  8  when said control piston is in contact with the sealing seat  39 , the sealing seat  39  can be relocated radially outward slightly. In this way, the resultant hydraulic force acting on the control piston  29  in the longitudinal direction owing to the pressure in the annular groove  49  is no longer zero, and instead, there is a resultant force in the direction of the nozzle needle  8 . Now, when the control valve  40  is closed, said force is sufficient to push the control piston  29  away from the sealing seat  39  and produce the connection between the inflow throttle  30  and the second control chamber part  228 .