Abstract:
A fuel injection system includes a pressure chamber having a displacable piston which can be subjected to pressure via a low-pressure-side pressure booster chamber for compressing the fuel in a high-pressure-side pressure booster chamber to be delivered to an injector. The stroke of the piston is controllable essentially by the pressure in a differential chamber of the pressure chamber and is used to vary the fuel pressure delivered to the injector. Provision is made for enlarging the cross section of the outlet cross section out of the differential chamber of the pressure chamber are provided. The fuel pressure during the injection can be varied. A pressure increase can be simply achieved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 02/01792 filed on May 17, 2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an improved fuel injection system employing a pressure booster for use in an internal combustion engine. 
   2. Description of the Prior Art 
   For better comprehension of the specification and claims, several terms will now be defined: The fuel injection system of the invention can be embodied as either stroke-controlled or pressure-controlled. Within the context of the invention, the term stroke-controlled fuel injection system is understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable nozzle needle, on the basis of the hydraulic cooperation of the fuel pressure in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the nozzle needle. Alternatively, the deflection of the nozzle needle can be effected by a final control element (actuator). In a pressure-controlled fuel injection system of the invention, the nozzle needle is moved counter to the action of a closing force (spring) by the fuel pressure prevailing in the nozzle chamber of an injector, so that the injection opening is uncovered for an injection of the fuel from the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called injection pressure, while the term system pressure means the pressure at which fuel is available or kept on hand inside the fuel injection system. The term fuel metering means furnishing a defined fuel quantity for injection. The term leak fuel, or leakage is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a guide leakage) and that is not used for injection and is pumped back to the fuel tank. The pressure level of this leak fuel can have a standing pressure, and after that the fuel is depressurized to the pressure level of the fuel tank. 
   Many engine manufacturers want to have a shallow pressure increase edge at the onset of the injection. Often, a boot phase is also wanted, for reducing emissions. In fuel injection systems with pressure chambers of the kind known for instance from German Patent Disclosure DE-A1 199 10 970, the pressure chamber can be used for shaping the course of injection. Thus the desired course of injection can be realized without additional parts such as deflection pistons. To vary the pressure course, the motion of the piston of the pressure chamber can be utilized. Varying the inlet cross section to the high-pressure-side pressure booster chamber as a function of stroke is known from U.S. Pat. No. 5,568,317. hereby incorporated by reference. This US patent proposes a multi-stage control of the inlet cross section. 
   SUMMARY OF THE INVENTION 
   The present invention enables varying the fuel pressure during the injection and attaining a pressure increase with simple means. If for example two outlet cross sections (a larger one and a smaller one) out of the differential chamber of the pressure chamber are uncovered in succession as a function of the piston stroke of the pressure chamber, then a so-called boot injection can be performed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Three exemplary embodiments of the fuel injection system of the invention are explained herein below, with reference to the drawings, in which: 
       FIG. 1  is a stroke-controlled fuel injection system according to the invention with a pressure chamber with a two-stage outlet cross section; 
       FIG. 2  shows a first means for continuously variable changing the outlet cross section; and 
       FIG. 3  shows a second means for continuously variable changing the outlet cross section. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the first exemplary embodiment, shown in  FIG. 1 , of a stroke-controlled fuel injection system  1 , a quantity-regulated fuel pump pumps fuel from a supply tank via a supply line into a central pressure reservoir (common rail), from which a plurality of pressure lines  2 , corresponding in number to the number of individual cylinders, lead away to the individual injectors  3  (injection devices) protruding into the combustion chamber of the internal combustion engine to be supplied. In  FIG. 1 , only one of the injectors  3  is shown. With the aid of the fuel pump, a first system pressure is generated and stored in the common rail. This first system pressure is used for preinjection and is needed both for postinjection (HC enrichment for the sake of exhaust gas posttreatment or soot reduction) and to define an injection course with a plateau (boot injection). For injecting fuel at a second, higher system pressure, each injector  3  is assigned a respective local pressure booster  4  having a check valve  5  and a displaceable piston  6 . Such fuel injection systems are known for instance from DE-A1 199 10 970. 
   For controlling the pressure booster  4 , the pressure in the differential chamber  7 , which is formed by a transition from a larger to a smaller piston cross section, is used. For refilling and deactivating the pressure chamber  9 , the differential chamber  7  is subjected to a supply pressure (rail pressure). Then, the same pressure conditions (rail pressure) prevail at all pressure faces of a piston  6 . The piston  6  is pressure-equalized. By means of an additional spring  8 , the piston  6  is pressed into its outset position. For activating the pressure booster  4 , the differential chamber  7  is pressure-relieved, and the pressure booster  4  generates a pressure boost in accordance with the ratio of surface areas. With this type of control, it can be attained that a low-pressure-side pressure booster chamber  10  need not be pressure-relieved in order to restore the pressure booster  4  and refill a high-pressure-side pressure chamber  9 . The depressurization losses in a small hydraulic boost can thus be reduced sharply. 
   For controlling the pressure chamber, instead of a complicated 3/2-way valve, a throttle  11  and a simple 2/2-way valve  12  are used. A throttle connects the differential chamber  7  with fuel, which is at supply pressure from a common rail, and a throttle  11  connects the differential chamber  7  to the valve  12 . The 2/2-way valve  12  thus connects the differential chamber  7  to a leak fuel line  13 . The throttle  11  should be designed to be as small as possible, yet still large enough that the piston  6  returns to its outset position between injection cycles. A guide leakage of the piston  6  can be used as the throttle. When the 2/2-way valve  12  is closed, no leakage occurs in the guides of the piston  6 , since the leakage from the differential chamber  7  is subjected to pressure. The throttle can also be integrated with the piston. 
   If the 2/2-way valves  12  and  14  are closed, the injector  3  is under the pressure of the common rail. The pressure booster  4  is in its outset position. An injection at rail pressure can now be effected by means of the valve  14 . If an injection at higher pressure is desired, then the 2/2-way valve  12  is triggered (opened), and a pressure boost is thus attained. 
   The injection is effected via a fuel metering, with the aid of a nozzle needle  15  that is axially displaceable in a guide bore and has a conical valve sealing face on one end, with which it cooperates with a valve seat face on the housing of the injector  3 . On the valve seat face of the injector housing, injection openings are provided. Inside a nozzle chamber  16 , a pressure face pointing in the opening direction of the nozzle needle  15  is exposed to the pressure prevailing there, which is delivered to the nozzle chamber  16  via a pressure line. Coaxially to a valve spring  17 , a thrust piece  18  also engages the nozzle needle  15  and with its face end remote from the valve sealing face it defines the control chamber  19 . The control chamber  19  has an inlet with a first throttle from the fuel pressure connection and an outlet with a second throttle, which is controlled by the 2/2-way valve  14 , to a pressure relief line  20 . 
   Fuel at the first or second system pressure constantly fills the nozzle chamber  16  and the control chamber  19 . Upon actuation (opening) of the 2/2-way valve  14 , the pressure in the control chamber  19  can be reduced, so that as a consequence the pressure force in the nozzle chamber  16  acting on the nozzle needle  15  in the opening direction exceeds the pressure force acting on the nozzle needle  15  in the closing direction. The valve sealing face lifts from the valve seat face, and fuel is injected. The process of pressure relief of the control chamber  19  and thus the stroke control of the nozzle needle  15  can be varied by way of how the throttles are dimensioned. 
   The end of the injection is initiated by re-actuating (closing) the 2/2-way valve  14 , which disconnects the control chamber  19  from the leak fuel line  20 , so that a pressure builds up again in the control chamber  19  that can move the thrust piece  18  in the closing direction. 
   To modify the pressure increase from the pressure booster  4 , the outlet cross section from the differential chamber  7  can be embodied to have multiple stages. In the outset position of the piston  6 , only the outlet path  21  is opened. As a result, upon opening of the valve  12 , a slow pressure drop inside the differential chamber  7  occurs, and this creates a damped motion of the piston  6 , and thus a slow pressure increase in the pressure chamber  9  to a medium pressure level is effected. After a stroke h, a second, larger outlet path  22 , which communicates with a circumferential groove in the wall of the differential chamber  7 , becomes operative because the circumferential groove is uncovered. The result is a faster pressure drop inside the differential chamber  7  and a corresponding faster, undamped motion of the piston  6 , with a resultant maximum pressure level increase in the pressure in chamber  9 . After the closure of the valve  12 , the piston  6  is moved back into its outset position. The pressure booster  4  is deactivated. 
     FIG. 1  shows structure which generates a stepwise increase in the cross section of the outlet from differential chamber  7 . Instead of the stepwise increase in the cross section of the outlet from the differential chamber  7 , a continuous increase in cross section can also be embodied (FIGS.  2  and  3 ), essentially by orienting the groove vertically along the differential chamber  7 , instead of its being circumferential as shown in FIG.  1 . By this means, a continuously varying rate of pressure increase without interfering pressure fluctuations can be achieved. In  FIG. 2 , piston  24  corresponds to piston  6  in FIG.  1 . By means of motion of the piston in its direction of motion  23 , and depending on the position of the piston  24 , only a part of face  25  of a slotlike opening  26 , which is oriented vertically along differential chamber  7  of the pressure booster  4 , is uncovered by a control edge of piston  24 , while the remaining face  27  of the opening  26  is covered. The opening  26  in the wall face of the differential chamber  7  forms the communication of the differential chamber  7  (see  FIG. 1 ) with the leak fuel line  22  (see  FIG. 1 ) and is closable by the piston  24 . As the piston stroke increases in length, a larger outlet cross section of slotlike opening  26  is uncovered. 
   In  FIG. 3 , piston  30  corresponds to piston  6  of FIG.  1 . Here a slotlike, generally vertical opening  28  in the wall face of the differential chamber  7  of a pressure booster has a cross-sectional area that is variable in the direction of motion  29  of the piston  30 . The piston  30  itself has a recess  31 , which communicates with generally vertical slotlike opening  28  in the side of the differential chamber  7  (see FIG.  1 ), and slot  28  communicates with the leak fuel line  22 , see FIG.  1 . The recess  31  forms a kind of control window that slides along the slot  28 . The outlet cross section can be varied arbitrarily by way of the course of the piston stroke. Alternatively, the slotlike opening  28  can also be embodied in the piston  30 , and the control edge again cooperates with a recess  31  that can be embodied in the wall face of the differential chamber  7 . 
   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.