Abstract:
A fuel injection system for internal combustion engines has at least one stroke-controlled injector. A pressure booster that has a movable piston is connected between the at least one injector and a high-pressure working medium source. The movable piston divides a primary chamber, which can be connected to the high-pressure working medium source, from a pressure chamber that communicates with the at least one injector and is filled with fuel. The pressure booster generates a first fuel system pressure in the injector, which is used for the injection. The fuel injection system has means for furnishing a further, second fuel system pressure, and these means can be used for injection without activating the pressure booster.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 02/00860, filed on Mar. 12, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an improved fuel injection system for internal combustion engines. 
     2. Description of the Prior Art 
     For introducing fuel into direct-injection Diesel engines, both stroke- and pressure-controlled fuel injection systems are known. For better comprehension of the description and claims, several terms will first be explained: The fuel injection according to the invention can be done by either stroke or pressure control. Within the scope of the invention, a stroke-controlled fuel injection is understood to mean that the opening and closing of the injection opening is accomplished with the aid of a displaceable valve member because of the hydraulic cooperation of the pressures in a nozzle chamber and in a control chamber. A pressure reduction within the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be accomplished by a final control element (actuator). In a pressure-controlled fuel injection according to the invention, the valve member 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 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 injection pressure, while a system pressure is understood to mean the pressure at which fuel is available or is kept on hand within the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term or leak fuel, is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a reference leakage) and is not used for injection and is pumped back to the fuel tank. This leak fuel can have a standing pressure, after which the fuel is depressurized to the pressure level of the fuel tank. 
     It is also known to use a pressure booster, in order to have not only the rail pressure but a further, different injection pressure available. The use of a separate working medium (such as hydraulic oil) for actuating the pressure booster has the disadvantage that it is no longer possible to use the rail pressure as the injection pressure. 
     SUMMARY OF THE INVENTION 
     For embodying a flexible fuel injection system which uses a separate working medium (hydraulic oil) for actuating the pressure booster, a fuel injection system is proposed according to the invention. 
     To enhance the flexibility of a fuel injection system, besides the fuel pressure of the hydraulic oil-actuated pressure booster (first system pressure), a further, second (low) fuel system pressure is generated, which can be used for injection. The second system pressure is stored as needed in a pressure reservoir and is applied constantly to the injector. A flexible shaping of the injection course and multiple injection can be attained. For generating pressure, a separate high-pressure pump can be used. However, it is also possible for the fuel pressure to be generated with a central pressure booster. Advantageously, the second system pressure can also be furnished by means of storage of a portion of the fuel compressed by the pressure booster. 
     If the fuel pressure is selected to be higher than the oil pressure in the pressure reservoir, then a hydraulic restoring force acts on a piston of the local pressure booster. The requisite restoring spring can thus be reduced in size or even omitted. This has a major advantage in terms of installation space, which is important especially for integrating the pressure booster with the injector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Seven exemplary embodiments of the fuel injection system of the invention are described in detail herein below, with reference to the drawings, in which: 
     FIG. 1, the use of hydraulic oil for actuating a local pressure booster and for triggering an injector; 
     FIG. 2, the use of hydraulic oil for actuating the local pressure booster and of fuel for triggering the injector; 
     FIG. 3, a different triggering of the pressure booster, using hydraulic oil for actuating the local pressure booster and for triggering the injector; 
     FIG. 4, the use of hydraulic oil for actuating the local pressure booster and triggering the injector that is connected to a central pressure reservoir; 
     FIG. 5, the use of hydraulic oil for actuating the local pressure booster and of fuel for triggering the injector that is connected to a central pressure reservoir; 
     FIG. 6, the use of a central pressure booster; and 
     FIG. 7, a further triggering of the local pressure booster. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the first exemplary embodiment of a fuel injection system  1 , shown in FIG. 1, a supply container  2  for a working medium (such as hydraulic oil) and a supply container  3  for fuel are used. A high-pressure pump  4  pumps the working medium, that is, hydraulic oil, into a central pressure reservoir  5 , in which the hydraulic oil is compressed to a controllable system pressure of approximately 50 bar to 250 bar and is stored. Thus the pressure reservoir  5  furnishes a high-pressure working medium source. 
     A low-pressure fuel pump  6  pumps fuel  3  via a supply line  7  into a pressure chamber  8  of a pressure booster  9 . Each injector  10  is assigned one local pressure booster  9 . In FIG. 1, only one pressure booster  9  and one injector  10  are shown. With the aid of a 3/2-way valve  11 , the triggering of the pressure booster  9  can be done, in that a supply line  12  to a primary chamber  13  of the pressure booster  9  can be connected either to an oil return  14  or to the pressure reservoir  5 . The pressure chamber  8  communicates via a check valve  15  with a nozzle chamber  16  of the injector  10 , so that a pressure buildup in the nozzle chamber  16  can take place. A control chamber  17  of the injector  10  is connected to the pressure reservoir  5  and, with the aid of a 2/2-way valve  18  and a pressure relief throttle  19 , can be made to communicate with an oil return  20 , so that the pressure in the control chamber  17  can be varied. 
     The injection is effected via a fuel metering, with the aid of a nozzle needle  21 , which is axially displaceable in a guide bore and cooperates with a valve seat face on the housing of the injector  10 . On the valve seat face of the injector housing, injection openings are provided. Inside the nozzle chamber  16 , a pressure face pointing in the opening direction of the nozzle needle  21  is exposed to the pressure prevailing there, which is delivered to the nozzle chamber  16  via the supply line  22 . Also engaging the nozzle needle, coaxially with a valve spring  23 , is a thrust member  24 , which defines the control chamber  17 . From the fuel pressure connection, the control chamber  17  has an inlet with a first throttle  25 , and it has an outlet via the oil return  20  and the 2/2-way valve  18 . 
     The nozzle chamber  16  continues, via an annular gap between the nozzle needle  21  and the guide bore, as far as the valve seat face of the injector housing. Via the pressure in the control chamber  17 , the thrust member  24  is subjected to pressure in the closing direction. 
     The control of the injector  10  is effected hydraulically by the cooperation of the pressures in the nozzle chamber  16  and in the control chamber  17  (given suitable design of the pressure faces). When the valve  20  is opened, the pressure in the control chamber  17  drops, and the nozzle needle  21  uncovers the injection openings. The injection begins. When the valve  20  is closed, a rail pressure builds up again in the control chamber  17 , and the nozzle needle  21  closes the injection openings. 
     For injection of fuel at a system pressure that is elevated compared to the pressure reservoir  5 , each injector  10  is assigned its own local pressure booster  9 . The pressure booster  9  includes the 3/2-way valve  11  for triggering, as well as a check valve and a piston  26 . The movable piston  26  divides the primary chamber  13 , which is connectable to the pressure reservoir  5 , from a fuel-filled pressure chamber  8  that communicates with the at least one injector  10 . The piston  26  can be acted upon by pressure on one end. A differential chamber  27  is pressure-relieved by means of a leak fuel line, so that the piston  26  can be displaced in order to reduce the volume of the pressure chamber  8 . The piston  26  is moved in the compression direction, so that the fuel located in the pressure chamber B is compressed and delivered to the control chamber  17  and to the nozzle chamber  16 . A check valve prevents the return flow of compressed fuel to the fuel tank. By means of a suitable ratio of surface area in the primary chamber  13  and the pressure chamber  8 , an elevated pressure can be generated. If the primary chamber  13  is connected to the leak fuel line  14  with the aid of the valve  11 , the restoration of the piston and the refilling of the pressure chamber  8  are effected. To improve the restoration performance, one or more springs may be provided. By means of the pressure boost, a first fuel system pressure is thus generated. 
     By means of the check valve  15 , the nozzle chamber  16  and a local pressure reservoir  28  remain under pressure when the pressure booster is pressure-relieved by the valve  11 . Thus a constant fuel pressure is applied to the injector  10 . An injection at arbitrary times is possible, even if the pressure booster  9  is not triggered and thus is not compressing any fuel in the compression chamber  8 . A second, low fuel system pressure is generated, which can be used for the injection. The pressure in the pressure reservoir  28  can be set to a desired level by means of an overpressure valve  29 . To that end, the pressure in the pressure reservoir  28  can drop, via the valve  29 , down to its opening pressure. Thus a low pressure level of approximately 300 to 500 bar can preferably be set. In that case, a preinjection, boot phase of a main injection, and a graduated postinjection can be defined for regenerating exhaust gas posttreatment systems, for instance. The size of the pressure reservoir  28  must be designed to suit the desired injection course. Preferably, the local pressure reservoir is used only for a small preinjection and a short boot phase. Then it can be very small and may even be formed by the existing lines and spaces. 
     For triggering the injector, in the embodiment of FIG. 2 (fuel injection system  30 ), compressed fuel from the nozzle region is used, instead of the hydraulic oil from the pressure reservoir  5 . The pressure reservoir  28  is designed accordingly. 
     FIG. 3 shows a different triggering of the pressure booster  9 , with a 2/2-way valve  31  in a fuel injection system  32 . The piston  26 , in the deactivated state upon restoration, is not completely hydraulically pressure-equalized. An increased spring force compensates for this. 
     To solve this problem differently, an elevated fuel pilot pressure can be used. In FIG. 4, in a fuel injection system  34 , a second (low) fuel system pressure is provided, which furnishes a basic fuel pressure in the system. The second fuel system pressure is generated by a fuel high-pressure pump  39 . As needed, this second fuel system pressure can be stored in a central pressure reservoir  33 . 
     The second fuel system pressure is connected to the pressure chamber  8  and to the nozzle chamber  16 . The nozzle chamber  16  is therefore always subjected to fuel pressure. This fuel pressure can be used at any time for an injection and can thus be used for instance for a preinjection or a boot phase. 
     For the pressure reservoir  33 , a pressure control may be provided. If the second system pressure is selected as higher than the oil pressure of the working medium, then the piston experiences a hydraulic restoring force, and if there are installation space problems, a restoring spring can be dispensed with. 
     A fuel injection system  35  in FIG. 5 is equivalent to that of FIG.  4 . Instead of the hydraulic oil, fuel is used here for triggering the injector  10 . 
     For generating the second fuel system pressure (basic fuel pressure), instead of a high-pressure pump a central pressure booster  36  can also be used (fuel injection system  37  in FIG.  6 ). For pressure control and/or vibration damping, once again a pressure reservoir  33  can be used. 
     FIG. 7 shows a further circuitry option; the 3/2-way valve  11  is provided for controlling the pressure booster  9  in a fuel injection system  38  with a central pressure reservoir  33 . In this circuitry option, the piston experiences a hydraulic restoring force, and if there are installation space problems, a restoring spring can be dispensed with. 
     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.