Abstract:
A common rail injector is proposed which is very compact in structure and nevertheless brings high closing forces to bear at the end of the injection. This is attained, among other provisions, in that the closing piston has a larger diameter than the nozzle needle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE00/02825 filed on Aug. 18, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an injector for a common rail injection system for internal combustion engines, having a valve control chamber, defined by the end face of a nozzle needle, in which the fuel inlet takes place via an inlet throttle and the fuel outlet takes place via an outflow C throttle, and there is a closing piston in the valve control chamber. 
   2. Description of the Prior Art 
   To reduce the structural length of conventional injectors, various efforts have been made, with the goal of X constructing injectors in which the nozzle needle discharges N directly into the valve control chamber, and no valve piston is necessary. From European Patent 0 426 205, an injector is known in which the nozzle needle discharges directly into the valve control chamber. Located in the valve control chamber are a control element and a closing piston. A disadvantage of this design is that the closing piston and the control element with an inlet throttle and outflow throttle are disposed in line with one another, so that despite the omission of the valve piston, the structural length of the injector is still comparatively great. Furthermore, the closing forces at the end of injection are relatively slight. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to furnish an injector that is especially compact in structure and simple in design, and in which the closing forces at the end of injection are high. 
   According to the invention, this object is attained by an injector for a common rail injection system for internal combustion engines, having a valve control chamber, defined by the end face of a nozzle needle, in which the fuel inlet takes place via an inlet throttle and the fuel outlet takes place via an outflow throttle, and there is a closing piston, which has a greater diameter than the nozzle needle, in the valve control chamber. 
   This injector has the advantage that its structural length is especially short, since there is only one closing piston in the valve control chamber. Furthermore, in the injector of the invention the closing force at the end of injection is especially high, because the diameter of the closing piston is greater than the diameter of the nozzle needle. Finally, by reducing the number of components of the injector, a simple design of the injector has been achieved. 
   A variant of the injector of the invention provides that the closing piston is disposed between the inlet throttle and outflow throttle on one side and the nozzle needle on the other, so that the closing piston also takes on control tasks. 
   In another embodiment, it is provided that the closing piston has a first bore, extending between its end faces, so that the positive displacement work which the nozzle needle must perform upon opening of the injection nozzle counter to the pressure in the valve control chamber is slight. 
   In an advantageous feature of the invention, the closing piston has a throttle bore extending between its end faces, so that after the end of injection, the closing piston can be returned to its outset position at a defined speed. 
   In a supplement to the invention, a stroke stop is provided in the valve control chamber and limits the displaceability of the closing piston in the direction of the inlet throttle and the outflow throttle, so that the fuel can flow unhindered into and out of this portion of the valve control chamber. 
   In a further version, a closing spring is present, which is braced against the closing piston and the nozzle needle, so that after the end of injection the closing piston is moved into its outset position by the spring force. 
   In an advantageous feature, it is provided that the closing spring is disposed in the valve control chamber, so that a simple design is assured, and the spring force acts directly on the closing piston. 
   In a supplement to the invention, it is provided that the closing spring is braced against the end face of the nozzle needle, so that the nozzle needle is simple in design. 
   Another variant provides that the nozzle needle has a pin protruding in the direction of its longitudinal axis and past its end face, so that the portion of the valve control chamber defined by the closing piston and the end face of the nozzle needle does not fail to attain a minimum volume predetermined by the length of the pin. Because of the elasticity of the fuel, this minimum volume brings about a certain elasticity or “softness” of the injector in the valve control chamber and the walls of the valve control chamber. 
   In another variant of the invention, the first bore of the closing piston is closable by the pin, so that with the the injection nozzle open, the pressure in the valve control chamber between the closing piston and the nozzle needle drops no more than necessary, and the leakage losses between the nozzle needle and the valve control chamber are reduced. 
   In a supplement to the invention, it is provided that the first bore of the closing piston has a sealing seat on the face end toward the nozzle needle, and the pin has a corresponding sealing cone, so that especially good sealing between the pin and the closing piston is achieved. 
   A variant provides that the inlet throttle and/or the outflow throttle is disposed in a housing of the injector, so that the dimensions of the injector are reduced still further. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and advantageous features of the invention can be learned from the ensuing description, taken with the drawings, in which: 
       FIG. 1  is a cross section through an injector according to the invention; and 
       FIG. 2  is an enlarged detail X of FIG.  1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , an injector according to the invention is shown. Via a high-pressure connection stub  1 , fuel  3  is carried via an inlet conduit  5  to an injection nozzle  7  and via an inlet throttle  9  into a valve control chamber  11 . The valve control chamber  11  communicates with a fuel return  17  via an outflow throttle  13 , which can be opened by a magnet valve  15 . The fuel  3  is shown in  FIG. 1  as a black area. 
   The valve control chamber  11  is defined by a nozzle needle  21 . The nozzle needle  21  prevents the fuel  3 , which is under pressure, from flowing into the combustion chamber, not shown, between injections. This is achieved by the provision that the nozzle needle  21  is pressed into a nozzle needle seat  22  and seals off the inlet conduit  5  from the combustion chamber, not shown. 
   The nozzle needle  21  has a cross-sectional change  23  from a larger diameter  25  to a smaller diameter  27 . The nozzle needle  21  is guided with its larger diameter  25  in a housing  29 . The cross-sectional change  23  defines a pressure chamber  31  of the injection nozzle  7 . 
   In  FIG. 2 , an enlarged detail X of  FIG. 1  of the injector of the invention is shown. In this view it can be seen that the valve control chamber  11  is defined by an end face  33  of the nozzle needle  21 . A closing piston  34  is located in the valve control chamber  11  and has a first, larger bore  35  and a second, smaller throttle bore  36 . The stroke of the closing piston  34  in the direction of the magnet valve  15  is limited by a stroke stop  37 . A pin  38  with a conical tip that fits into a complimentary sealing seat  39  of the closing piston  34  protrudes from the end face  33  of the nozzle needle  21 .  FIG. 2  shows a state of the injector in which the closing piston  34  rests on the stroke stop  37 , and the nozzle needle is seated on its nozzle needle seat  22 , not shown in FIG.  2 . In this position, there is a gap between the pin  38  and the sealing seat  39  of the closing piston  34 , so the fuel  3 , not shown in  FIG. 2 , can flow through the first bore  35  of the closing piston  34  into the part of the valve control chamber  11  located between the closing piston  34  and the nozzle needle  21 . 
   When the outflow throttle  13  is closed, the hydraulic force acting on the end face  33  of the nozzle needle  21  is greater than the hydraulic force acting the cross-sectional change  23 , because the end face  33  of the nozzle needle  21  is larger than the annular face of the area of the cross-sectional change  23 . If the high-pressure pump, not shown, of the fuel injection system is not driven because the engine is at a stop, then a closing spring  40 , acting on the end face  33  of the nozzle needle  21 , presses the nozzle needle  21  against the nozzle needle seat  22  shown in FIG.  1  and thus closes the injection nozzle  7  or injector. 
   When the outflow throttle  13  is opened, which happens when a ball  41  of the magnet valve  15 , not described in detail, is lifted from a ball seat  42 , the pressure in the valve control chamber  11  drops. As a consequence, the hydraulic force acting on the end face  33  drops as well. As soon as this hydraulic force is less than the hydraulic force acting on the cross-sectional change area  23 , the nozzle needle  21  moves in the direction of the closing piston  34 , until the pin  38  rests on the sealing seat  39 . As a result, the injection nozzle  7  shown in  FIG. 1  is opened, and the fuel  3  is injected into the combustion chamber. The opening travel of the nozzle needle  21  is represented in  FIG. 2  by the nozzle needle stroke “h”. 
   The inlet throttle  9  prevents a complete pressure equalization between the inlet conduit  5  and the valve control chamber  11 . The opening speed of the nozzle needle  21  is determined by the difference in flow between the inlet throttle  9  and the outflow throttle  13 . 
   This indirect triggering of the nozzle needle  21  via a hydraulic force booster system is necessary, because the forces required for rapid opening of the nozzle needle  21  cannot be generated directly with the magnet valve  15 . The so-called “control quantity” required in addition to the fuel quantity injected into the combustion chamber reaches the fuel return  17  via the inlet throttle  9 , the valve control chamber  11 , and the outflow throttle  13 . In addition to the control quantity, leakage also occurs at the nozzle needle guide. The control and leakage quantities can amount to up to 50 mm 3  per stroke. They are returned to the fuel tank, not shown, via the magnet valve  15 . 
   To terminate the injection, the outflow throttle  13  is closed by the ball  41  of the magnet valve  15 , in a known manner not explained in further detail. As a result of the closure of the outflow throttle  13 , virtually the same rail pressure builds up again via the inlet throttle  9  in a portion  43  of the valve control chamber  11  that is defined by the closing piston  34  and the outflow throttle  13 . This pressure exerts a hydraulic force on the nozzle needle  21  via the end face  45  of the closing piston  34  and via the pin  38  resting on the sealing seat  39 . As soon as this hydraulic force exceeds the hydraulic force acting on the cross-sectional change area  23 , the nozzle needle  21  closes. Because the end face  45  of the closing piston is markedly larger in comparison to the annular face area of the cross-sectional change  23 , the closing motion takes place very fast and with great force. Simultaneously with the closing motion, a small portion of the fuel, flowing into the portion  43  of the valve control chamber  11 , flows through the throttle bore  36  into the valve control chamber  11  defined by the closing piston  34  and by the end face  33  of the nozzle needle  21 . The closing motion takes place so fast that before a pressure equalization is reached, the nozzle needle  21  rests on the nozzle needle seat  22  again, and the injection is terminated. The closing speed of the nozzle needle  21  is determined essentially by the flow through the inlet throttle  9 . 
   In order for the closing piston  34  to move to the outset position against the stroke stop  37  after the end of injection, the portion of the valve control chamber  11  defined by the closing piston  34  and the end face  33  of the nozzle needle  21  is filled with fuel through the throttle bore  36 , while the closing spring  40  presses the closing piston  34  upward. It is also conceivable to omit the throttle bore  36  and to dimension the play of the closing piston  34  in the housing  29  in such a way that the fuel flows through the annular gap between the closing piston  34  and the housing  29 . The second end face  47  of the closing piston  34  can also, as shown in  FIG. 2 , have a shoulder, which serves for instance to guide the closing spring  40 . 
   The foregoing relates to a preferred exemplary embodiment 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.