Patent Publication Number: US-2009230220-A1

Title: Injector  for  a  fuel  injection  system

Description:
PRIOR ART 
     The invention is based on an injector for an injection system of an internal combustion engine, having a nozzle needle and a control valve, as known for instance from German Patent Disclosure DE 10 2004 058 184.3. 
     DISCLOSURE OF THE INVENTION 
     To improve emissions from internal combustion engines still further, it is necessary to increase the injection pressure. To keep the amount of leakage low in that situation, injectors are used whose nozzle needle has no pressure step, so that the entire nozzle needle is surrounded by fuel that is at rail pressure. As a result, only slight closing forces are available, which leads to problems in metering small injection quantities, since the performance graphs of such injectors are very steep. 
     The object of the invention is to furnish an injector which makes a sufficiently high closing force available even though the nozzle needle is embodied without a pressure step. 
     In an injector for an injection system of an internal combustion engine, having a nozzle needle and having a control valve, the nozzle needle being guided in an injector housing and cooperating with a nozzle needle seat of the injector housing, and a high-pressure chamber being present between the control valve and the nozzle needle seat, this object is attained according to the invention in that a closing piston is provided on the nozzle needle or on a control piston; that the closing piston subdivides the high-pressure chamber into a first region and a second region; and that the two regions of the high-pressure chamber communicate hydraulically by means of a closing throttle restriction. 
     By means of the closing piston according to the invention and the closing throttle restriction, the closing force engaging the nozzle needle and the control piston during the closing motion is increased because the effective pressure area is increased. 
     Because of the increased pressure area, the pressure difference required for furnishing a sufficiently high closing force can be reduced. Consequently, given a suitable design of the injector of the invention or of the closing piston and/or the closing throttle restriction, it is also possible to increase the needle closing speed and to reduce the steepness of the performance graph. 
     In an advantageous feature of the invention, the closing piston is guided in the radial direction in the injector housing; and that between the closing piston and the nozzle needle, there is play in the radial direction. As a result, a complicated double guidance of the nozzle needle and control piston with the resultant production costs can be avoided. Moreover, it is assured that seizing of the nozzle needle from an axial offset between the guidance of the nozzle needle and the guidance of the closing piston in the injector housing will not occur. 
     It has proved especially advantageous if the closing piston has a slit, and this slit is simultaneously embodied as a closing throttle restriction. 
     In this embodiment, the closing piston can be guided, similarly to a piston ring of an internal combustion engine, with a certain prestressing in the injector housing, so that even with increasing wear of the injector and closing piston, play-free guidance of the closing piston in the injector housing is always assured. Simultaneously, if the slit is suitably dimensioned, it acts as a closing throttle restriction. As a result, the closing throttle restriction can be furnished without additional production effort and expense. 
     Alternatively, it is understood also to be possible to provide a throttle bore in the closing piston, or to adjust the desired throttling action by way of the play between the closing piston and the injector housing. 
     To make it possible to attain the advantages of the invention, the outside diameter of the closing piston is greater than the diameter of the nozzle needle in the region of the nozzle needle guide. 
     In a further advantageous feature of the injector, the control valve, in a first switching position, interrupts a hydraulic communication between a control chamber and a fuel return, while in a second switching position, it establishes a hydraulic communication between the control chamber and a fuel return. 
     The control piston defines a control chamber and is coupled with the nozzle needle. 
     It has furthermore proved advantageous if the control piston has a through bore, and on an end toward the control chamber it has a sealing edge that surrounds the through bore, and a flat seat cooperating with the sealing edge is embodied on the control chamber body. 
     It is structurally easily possible for the valve body and the control chamber body to be embodied in one piece. It is equally possible to embody the control piston and the nozzle needle in one piece. 
     Further advantages and advantageous embodiments can be learned from the following drawings, their description and the claims. All the characteristics described in the drawings, in their description, and in the claims can be essential to the invention, both individually and in arbitrary combination with one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Shown are: 
         FIG. 1 , the schematic illustration of a common rail injection system; 
         FIG. 2 , a first exemplary embodiment of an injector according to the invention; 
         FIG. 3 , a second exemplary embodiment of an injector according to the invention; and 
         FIG. 4 , a plan view on one exemplary embodiment of a closing piston. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     In  FIG. 1 , a common rail injection system is shown schematically. From a fuel tank  1 , fuel is pumped with the aid of a pump unit  2  into a high-pressure fuel reservoir  3  and subjected to high pressure. The fuel subjected to high pressure is then allocated as a function of demand to the individual cylinders of the internal combustion engine to be supplied. The injection of the fuel subjected to high pressure is effected through injectors  4 ,  5 ,  6  and  7 . In  FIG. 1 , only the injector  7  is shown, for the sake of simplicity. 
     The injector  7  communicates with the common rail  3  via a high-pressure connection  11 . The injector  7  moreover communicates hydraulically with the tank  1  via a fuel return  13 , which is virtually pressureless. The injector  7  will be described in further detail below in conjunction with  FIGS. 2 and 3 . 
     The injector  7  includes a housing  15 , in which a nozzle needle  17  is guided. A control valve  19  according to the invention and an electromagnetic actuator  21  are disposed on the end, remote from the combustion chamber, of the injector  7 . 
     Below the control valve  19 , a control chamber  23  is embodied in a control chamber body  25 . The control chamber  23  is defined by a control piston  27 . 
     In the present exemplary embodiment, the control piston  27  is joined to the nozzle needle  17  by a weld seam  29 . It is understood that the possibility also exists of embodying the control piston  27  and the nozzle needle  17  in one piece. 
     To compensate for errors of alignment between the control chamber  23  and the nozzle needle  17 , the cross section of the control piston  27  is reduced in some regions. This reduction in the cross section is effected in the exemplary embodiment shown in  FIG. 2  by two grooves  31  and  33 , and the grooves  31  and  33  are offset from one another by an angle of 90°. The grooves  31  and  33  reduce the bending stiffness of the control piston  27  and function similarly to a cardan joint. 
     In the exemplary embodiment shown in  FIG. 2 , a first pair of grooves  31  and  33  is provided below the control chamber body  25 , while a second pair of grooves  31  and  33  is provided above the nozzle needle  17 . By the cooperation of the two “cardan joints” formed by the grooves  31  and  33 , errors of alignment between the control chamber  23  and the nozzle needle  17  can be compensated for especially well. 
     In the exemplary embodiment shown in  FIG. 2 , the control chamber body  25  and a valve body  35  of the control valve  19  are embodied as separate components. It is alternatively possible to embody the valve body  35  and the control chamber body  25  in one piece. 
     Both the control chamber body  25  and part of the valve body  35  are located in a high-pressure chamber  37  in the injector  7 . The high-pressure connection  11  discharges into the high-pressure chamber  37  and is directly in communication hydraulically with the common rail injection system  3 . This means nothing other than that the rail pressure prevails in the high-pressure chamber  37 . The high-pressure chamber  37  can also take on the function of a pressure reservoir. 
     The control chamber body  25  is pressed against the valve body  35  via a closing spring  39 . The closing spring  39  furthermore serves to press the nozzle needle  17  against a nozzle needle seat  69 , when the injector is pressureless. For that purpose, there is a sleeve  41 , which rests on a closing piston  43 . An annular chamber  45  is embodied in the valve body  35 . This annular chamber  45  is in communication hydraulically with the high-pressure chamber  37 , via a bore  47  that can also be embodied as a throttle restriction. 
     The control chamber  23  and the high-pressure chamber  37  communicate hydraulically with one another via an inflow throttle restriction  49 . The control chamber  23  and the annular chamber  45  communicate hydraulically with one another via an outflow throttle restriction  51 . 
     A control piston  53  has a through bore  55 , which is hydraulically in communication with the fuel return  13  (see  FIG. 2 ). 
     One face end of the control chamber body  25  is embodied as a flat seat. This flat seat cooperates with a sealing edge  59 , disposed on the end of the control piston  53  toward the valve body  25 . 
     The closing piston  43  is braced in the closing direction of the nozzle needle  17  against a shoulder  61  of the nozzle needle  17 . Simultaneously, the shoulder  61  forms a sealing seat that cooperates with the closing piston  43 . The sealing seat, in the exemplary embodiment of  FIG. 1 , is embodied as a flat seat. A flat seat is especially advantageous in the present case, since the closing piston is guided in the radial direction not by the sealing seat but solely in the injector housing  15 . 
     Below this shoulder  61 , the nozzle needle  17  is guided axially displaceably in a nozzle needle guide  63 . So that fuel from the upper part of the high-pressure chamber  37  can reach the injection ports  65  of the injector  7 , a plurality of flattened faces  67  are provided on the nozzle needle  17 , in the region of the nozzle needle guide  63   
     The injection ports  65  are disposed in a nozzle needle seat  69  such that they are closed when the nozzle needle  17  rests on the nozzle needle seat  69 . 
     In the exemplary embodiment shown in  FIG. 2 , there is a closing throttle restriction  71  in the form of a longitudinal bore in the closing piston  43 . 
     The closing piston  43  is guided in the injector housing  15 . The diameter of the injector housing, in the region in which the closing piston  43  is guided, is given as D 1  in  FIG. 2 . The diameter of the nozzle needle guide  63  is marked D 2  in  FIG. 1 . Comparing the diameters D 1  and D 2  clearly shows that the diameter D 1  of the closing piston  43  is greater than the diameter D 2  of the nozzle needle guide  63  (D 1 &gt;D 2 ). 
     The injector  7  according to the invention functions as follows: 
     In the first switching position, shown in  FIG. 2 , the actuator  21  is not being supplied with current, so that a compression spring  73 , which acts on an armature plate  75  and thus also on the control piston  53 , presses the control piston  53  against the control chamber body  25 . As a result, the hydraulic communication between the control chamber  23  and the fuel return  13  is closed, Consequently, rail pressure prevails in the control chamber  23 , and the nozzle needle  17  is closed. 
     As soon as an electromagnet  77  of the electric actuator is supplied with current, the armature plate  75  is drawn upward in  FIG. 2 , so that the sealing edge  59  of the valve body  43  lifts from the control chamber body  25 , and thus a hydraulic communication is established between the control chamber  23  and the fuel return  13 , via the outflow throttle restriction  55  and the through bore  57  in the valve body  43 . 
     Since the outflow throttle restriction  55  has a lesser flow resistance than the inflow throttle restriction  53 , the pressure in the control chamber drops in this second switching position of the control valve  8 . As a consequence, the nozzle needle  17  lifts from its nozzle needle seat  69 , and fuel is injected into the combustion chamber of an internal combustion engine. Since the closing piston  43  moves upward in  FIG. 2  along with the nozzle needle  17  as soon as the nozzle needle  17  opens, fuel flows from a first region  79  of the high-pressure chamber  37  through the closing throttle restriction  71  into a second region  81  of the high-pressure chamber  37 . Because of the flow resistance of the closing throttle restriction  71 , the opening motion of the nozzle needle is slowed somewhat. By a suitable adaptation of the inflow throttle restriction  49 , outflow throttle restriction  51 , and if present the bore  47 , however, this effect can be compensated for entirely or in part. 
     As long as the nozzle needle  17  is open, fuel flows out of the first region  79  of the high-pressure chamber  37  via the closing throttle restriction  71  and the second region  81  of the high-pressure chamber  37  into the combustion chamber of the engine through the injection ports  65 . As a consequence, a certain pressure reduction takes place in the closing throttle restriction  71 . The pressure forces exerted on the closing piston  43  by the fuel located in the first region  79  of the high-pressure chamber  37  are therefore greater than the pressure forces exerted on the closing piston  43  by the fuel located in the second region  81  of the high-pressure chamber  37 . The resultant force is oriented downward in  FIG. 2 , that is, in the closing direction of the nozzle needle  17 . This force is also operative during the closure of the nozzle needle  17 . Since the effective area of the closing piston  43  is relatively large, a slight pressure drop at the closing throttle restriction  71  suffices to furnish a sufficiently strong closing force. This slight pressure drop has no effect, or only a very slight effect, on the atomization of the fuel in the injection ports  65 . 
     When the injection is to be terminated, the electromagnet  77  is switched to be currentless. As a consequence, the control piston  43  moves downward in  FIG. 2 , until the sealing edge  59  rests on the face end of the control chamber body  25 . As a result, the hydraulic communication between the control chamber  23  and the fuel return  13  is undone again, and fuel at high pressure can flow into the control chamber  23  via the inflow throttle restriction  53 . 
     If the bore  47  is present, then fuel can flow into the control chamber  23  via the outflow throttle restriction  55  as well. As a consequence, the nozzle needle  17  is again pressed against its nozzle needle seat. 
     Because the closing piston  43  has a large cross-sectional area, whose outside diameter is equivalent to the diameter D 1 , a slight pressure drop at the closing throttle restriction  71  is sufficient to engender a strong closing force. Hence the pressure loss at the closing piston  43  can be reduced, and at the same time the closing speed can be increased. 
     The closing piston  43  is guided axially in the injector housing. Between the closing piston  43  and the nozzle needle  17 , there is play in the radial direction, so that seizing of the nozzle needle cannot occur for instance if the nozzle needle guide  63  and the region of the injector housing  15  in which the closing piston  43  is disposed have a slight error of alignment. 
     As an alternative to the exemplary embodiment shown in  FIG. 2 , in which the nozzle needle  17  is coupled to the control valve  19  via the control piston  27 , it would also be possible to couple the control piston  27  and the nozzle needle  17  hydraulically with one another. Alternatively, it is also possible to embody the nozzle needle  17  and the control piston  27  in one piece. 
       FIG. 3  shows an exemplary embodiment of a closing piston  43  according to the invention. In this exemplary embodiment, the closing piston is embodied as a slit ring. In the non-installed state, the outside diameter of the closing piston  43  can be somewhat greater than the diameter D 1  of the injector housing  15 , so that the closing piston  43  is inserted into the injector housing with a certain prestressing. The slit  83  is dimensioned such that it takes on the task of the closing throttle restriction. As a result of the prestressing of the closing piston  43  in the bore of the injector housing  15 , the closing piston  43  always rests without play in the injector housing  15 , even if over the course of time, wear should occur at the closing piston  43  or in the injector housing.