Abstract:
A fuel injection system includes a pressure booster having a displaceable piston which can be subjected to pressure via a pressure booster chamber on the low-pressure side for compressing the fuel in a pressure booster chamber on the high-pressure side to be delivered to an injector. The stroke of the piston is controllable by the pressure in a differential chamber of the pressure booster and is used to vary the fuel pressure delivered to the injector. Means for continuously variable definition of the inlet cross section to the pressure booster chamber of the pressure booster on the low-pressure side or of the outlet cross section from the differential chamber of the pressure booster are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 02/01801 filed on May 18, 2002. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an improved fuel injection system, including a pressure booster, for use in an internal combustion engine. 
     2. Description of the Prior Art 
     For better comprehension of the description and claims, some terms will now be defined: The fuel injection system of the invention can be embodied as either stroke-controlled or pressure-controlled. Within the scope of the invention, a stroke-controlled fuel injection system is to be understood to mean that the opening and closing of the injection opening with the aid of a displaceable nozzle needle is effected on the basis of the hydraulic cooperation of the fuel pressures 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 means of a final control element (actuator). In a pressure-controlled fuel injection system according to the invention, the nozzle needle is moved by the fuel pressure, prevailing in the nozzle chamber of an injector, counter to the action of a closing force (spring), so that the injection opening is opened 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 the infection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or kept on hand inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term leakage, or leak fuel, is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a guide leakage) but is not used for injection and so is returned to the fuel tank. The pressure level of this leak fuel can be a static or fixed pressure, after which the fuel is depressurized to the pressure level of the fuel tank. 
     Many engine manufacturers require a shallow leading edge of the pressure at the onset of the injection. Often, a boot phase is also desired in order to lower emissions. In fuel injection systems with a pressure booster, of the kind known for instance from German Patent Disclosure DE-A1-19910970, the pressure booster can be used to shape the course of injection. Thus the desired injection course can be achieved without such additional parts as deflection pistons. To vary the pressure course, the motion of the piston of the pressure booster can be used. Varying the inlet cross section to the pressure booster chamber on the low-pressure side as a function of pressure is known from U.S. Pat. No. 5,868,317. That US patent proposes controlling the inlet cross section in multiple stages. 
     SUMMARY OF THE INVENTION 
     The present invention enables varying the fuel pressure during the injection and achieving a flat pressure increase without interfering pressure fluctuations. As the piston stroke increases, a greater cross section is opened and thus a greater injection quantity is made possible, so that continuously variable shaping of the injection course is possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Two exemplary embodiments of the fuel injection system of the invention are described herein below, with reference to the drawings, in which: 
         FIG. 1  is a first continuously variable change in cross section of the inlet to or outlet from a chamber of a pressure booster in a fuel injection system according to the invention; 
         FIG. 2  is a second continuously variable change in cross section of the inlet to or outlet from a chamber of a pressure booster in a fuel injection system; and 
         FIG. 3  is a stroke-controlled fuel injection system with a pressure booster of the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the known stroke-controlled fuel injection system  1  shown in  FIG. 3 , a quantity-regulated fuel pump  2 ′ pumps fuel from a tank via a supply line into a central common rail  2 ″, from which a plurality of pressure lines  2  corresponding to the number of individual cylinders lead to the individual injectors  3  (injection devices) protruding one into each combustion chamber of the engine to be supplied. In  FIG. 1 , only one of the injectors  3  is shown. With the aid of the fuel pump  2 ′, a first system pressure is generated and stored in the common rail  2 ″. This first system pressure is used for preinjection and, as needed, for postinjection (HC enrichment for the sake of exhaust gas posttreatment or soot reduction) as well as to produce 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  with a check valve  5  and a displaceable piston  6 . Such fuel injection systems are known for instance from DE-A1-19910970. 
     For controlling the pressure booster  4 , the pressure in the differential chamber  7 , embodied by a transition from a larger to a smaller piston cross section, is used. For refilling and deactivating the pressure booster, the differential chamber  7  is subjected to a supply pressure (rail pressure). Then the same pressure conditions (rail pressure) prevail at all the pressure faces of a piston  6 . The piston  6  is in pressure equilibrium. 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 depending on the ratio of surface areas. With this type of control, restoring the pressure booster  4  and refilling a pressure chamber  9  do not require that a pressure booster chamber  10  on the low-pressure side be pressure-relieved. Then, given a small hydraulic step-up, depressurization losses can be sharply reduced. 
     For controlling the pressure booster  4 , instead of a complicated 3/2-way valve, a throttle  11  and a simple 2/2-way valve  12  can be used. The throttle  11  connects the differential chamber  7  with fuel, at supply pressure, from the common rail  2 ″ to the valve  12 . The 2/2-way valve  12  connects the differential chamber  7  to a leakage 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 also be used as a throttle. With the 2/2-way valve  12  closed, no leakage in the guides of the piston  6  occurs, because 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, then the injector  3  is at the pressure of the common rail  2 ″. The pressure booster  4  is in its outset position. Now, by means of the valve  14 , an injection at rail pressure can occur. 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 fuel metering with the aid of a nozzle needle  15 , which is axially displaceable in a guide bore, and which has a conical valve sealing face on one end and with this face it cooperates with a valve seat face on the injector housing of the injector  3 . At 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 , which with its face end remote from the valve sealing face defines a control chamber  19 , also engages the nozzle needle  15 . The control chamber  19  has an inlet with a first throttle, from the fuel pressure connection direction, and an outlet to the leakage line  13  that is controlled by the 2/2-way valve  14 . 
     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 operation of pressure relief of the control chamber  19  and thus the control of the stroke of the nozzle needle  15  can be varied by way of the dimensioning of the throttles. 
     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 leakage line  13  again, so that in the control chamber  19  a pressure again builds up that can move the thrust piece  18  in the closing direction. 
     In  FIGS. 1 and 2 , the inlet to the pressure booster chamber  10  on the low-pressure side and/or the outlet from the differential chamber  7  (see  FIG. 1 ) are provided with a continuous cross-sectional enlargement. A shallow pressure increase without interfering pressure fluctuations can be achieved. In  FIG. 1 , by the direction of motion  23  of a piston  24  (longitudinal direction of the opening and of the piston), depending on the position of the piston  24 , only a partial area  25  of a slotlike opening  26  is uncovered as far as a control edge  24 ′, while a partial area  27  of the opening  26  is covered. The opening  26  in the wall face of a pressure booster chamber (differential chamber or low-pressure chamber) establishes the communication of the differential chamber  7  (see  FIG. 1 ) with the leakage line, or the communication of the pressure booster chamber  10  on the low-pressure side with the pressure line  2  (see FIG.  1 ), and is closable by the piston. As the piston stroke increases, a larger inlet or outlet cross section is uncovered. In  FIG. 2 , a slotlike opening  28  in the wall face of a pressure booster chamber 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 establishes the continuous communication of the differential chamber  7  (see  FIG. 1 ) with the leakage line, or the communication between the low-pressure chamber  10  and the pressure line  2 . The recess  31  forms a kind of control window that slides along the slot  28 . 
     Alternatively, the slotlike opening  28  can be embodied in the piston, and the control edge  24 ′ or a recess  31  can be embodied in the wall face. 
     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.