Abstract:
A fuel injection system has one or more unit fuel injectors or pump-line-nozzle units, corresponding in number to the cylinders, for compressing the fuel. The fuel injection system includes means for generating two different injection pressures during the injection and at least one valve for controlling the injection with a cross sectional control. The fuel injection with the aid of the unit fuel injector or a pump-line-nozzle unit can be achieved over a wide rpm range with great precision.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 00/02735 filed on Aug. 12, 2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel injection system and more particularly to an improved fuel injection system which produces two different injection pressures. 
     2. Description of the Prior Art 
     For the sake of better understanding of the description and claims, several terms will now be explained. The fuel injection system according to the invention can be embodied as either stroke-controlled or pressure-controlled. Within the scope of the invention, the term stroke-controlled fuel injection system will be understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable valve member as a result 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 valve member. Alternatively, the deflection of the valve member can be effected by a final control element (or actuator). In a pressure-controlled fuel injection system 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 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 is called the injection pressure, while the term system pressure is understood to be the pressure at which fuel is available or is stored inside the fuel injection system. Fuel metering means delivering fuel to the nozzle chamber by means of a metering valve. In combined fuel metering, a common valve is used to meter various injection pressures. In the pump-nozzle unit (PDE), also called a unit fuel injector, the injection pump and the injector form a unit. One such unit per cylinder is built into the cylinder head and driven either directly via a tappet or indirectly via tilting levers by the engine camshaft. The pump-line-nozzle system (PLD) operates by the same method. In this case, a high-pressure line leads to the nozzle chamber or nozzle holder. 
     A unit fuel injector is known for instance from German Patent Disclosure DE 195 175 78 A1. In this fuel injection system, the system pressure is generated via a piston that can be acted upon by pressure and whose motion is controlled by a cam drive. A variable fuel injection of different quantities for the sake of preinjection, main injection and postinjection is only limitedly feasible by means of this kind of fuel injection system. 
     SUMMARY OF THE INVENTION 
     To achieve fuel injection with the aid of a unit fuel injector or a pump-line-nozzle unit over a wide rpm range with great precision, a fuel injection system according to the invention is proposed. Refinements make it possible to remove pollutant exchange and more-flexible preinjection and optionally a postinjection by means of a unit fuel injector or a pump- line-nozzle system. If a valve with a cross sectional control, for instance by means of a piezoelectric actuator, is used for the fuel metering, then improved metering of the injected fuel quantity can be achieved. This creates a good minimum-quantity capability in the preinjection. The development of the injection course in the main injection can be varied in a targeted way. Each unit fuel injector or pump-line-nozzle unit can contain a pressure storage chamber, which can be decoupled from the unit and filled with fuel during the pumping stroke of the compression device. By means of the pressure storage chamber, control of the injection pressure can be done relatively independently of the engine rpm. The time between the triggering of the pressure buildup and the injection can be selected freely within wide ranges. The time of the onset of the pressure buildup determines the pressure level attained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing advantages and features of the invention will be apparent from the detailed description contained herein below, taken with the drawings in which: 
     FIG. 1, is a schematic view, partially in section of a stroke-controlled fuel injection system and; 
     FIG. 2, is a view similar to FIG. 1 showing a second embodiment of a stroke-controlled fuel injection system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the first exemplary embodiment, shown in FIG. 1, of a stroke-controlled fuel injection system  1 , a prefeed pump  2  pumps fuel  3  from a tank  4  via a feed line  5  to a plurality of unit fuel injectors  6  (injection devices), corresponding in number to the number of individual cylinders of an internal combustion engine to be supplied and protruding into the combustion chamber of the engine. In the drawing, only one of the unit fuel injectors  6  is shown. 
     Each unit fuel injector  6  is composed of a fuel compressing device  7  and means for injection. Per engine cylinder, one unit fuel injector  6  is built into a cylinder head. The compression device  7  is driven either directly via a tappet or indirectly via tilting levers by an engine camshaft. Electronic regulating devices make it possible to exert targeted influence on the quantity of injected fuel (injection course) in a known manner. 
     The fuel compressing device  7  can compress fuel in a compression chamber  8 . Check valves  9  and  10  and a 2/2-way valve  11  prevent the return flow of fuel in the direction of the feed pump  2  to the low-pressure region. The fuel compressing device  7  can be part of a unit fuel injector (PDE) known per se or of a pump-line-nozzle unit (PLD). The fuel compressing device  7  serves to generate an injection pressure. The pressure buildup is achieved with the aid of the 2/2-way valve  11 . 
     During the pumping stroke of the fuel compressing device  7 , the pressure storage chamber  12  can be filled with fuel and decoupled from the pressure generation region via the check valves  9  and  10 . 
     The injection is effected via fuel metering with the aid of a pistonlike valve member  13 , which is axially displaceable in a guide bore and has a conical valve sealing face  14  on one end, with which face it cooperates with a valve seat face on the injector housing of the injector unit  6 . Injection openings are provided on the valve seat face of the injector housing. A nozzle chamber  15  and a control chamber  16  are formed. Inside the nozzle chamber  15 , a pressure face pointing in the opening direction of the valve member  13  is exposed to the pressure prevailing there, which is delivered to the nozzle chamber  15  via a pressure line  17 . The valve member  13  is furthermore engaged coaxially to a compression spring  18  by a tappet  19 , which with its face end  20  remote from the valve sealing face  14  defines the control chamber  16 . From the direction of the fuel pressure connection, the control chamber  16  has an inlet with a throttle  21  and an outlet to a pressure relief line  22 , which is controlled by a valve unit  24 . 
     The nozzle chamber  15  continues across an annular gap between the valve member  13  and the guide bore, up to the valve seat face of the injector housing. The tappet  19  is urged by pressure in the closing direction, via the pressure in the control chamber  16 . By throttling of the valve cross section inside the valve unit  24 , an injection pressure that is variable during injection and thus a shaping of the course of injection can be achieved by means of a cross sectional control, in which the pressure in the control chamber  16  is varied and thus throttling of the injection pressure is achieved at the valve sealing face  14  via the valve member  13 . To achieve a continuous cross sectional control, both piezoelectric actuators and fast-acting magnet actuators are conceivable. By providing multi-stage valves, instead of a continuous shaping of the injection pressure, a plurality of different injection pressure levels can be generated during injection by means of various throttle positions. Analogously, throttling at the valve cross section of the valve  11  would also be conceivable for forming the course of injection, as shown in the second embodiment illustrated in FIG.  2 . 
     The valve unit  24  is actuated by an electromagnet or piezoelectric actuator to open or close or switch over. The actuator is triggered by a control unit, which is capable of monitoring and processing various operating parameters (engine rpm, and so forth) of the engine to be supplied. 
     Fuel at a system pressure constantly fills the nozzle chamber  15  and the control chamber  16 . Upon actuation of the valve unit  24 , the pressure in the control chamber  16  can be lowered, so that as a consequence, the pressure in the nozzle chamber  15  exerted in the opening direction on the valve member  13  predominates over the pressure acting in the closing direction on the valve member  13 . The valve sealing face  14  lifts from the valve seat face, and fuel is injected. The pressure relief process for the control chamber  16  and thus the control of the stroke of the valve member  13  can be varied by way of the dimensioning of the first throttle  21  and second throttle in valve unit  24  as well as additional throttling in the valve seat. 
     The end of injection is initiated by re-actuating (closing) the valve unit  24 ; this decouples the control chamber  16  from a leakage line  25  again, so that in the control chamber  16 , a pressure builds up again that can move the valve member  13  in the closing direction. 
     The pressure drop during the main injection is compensated for by the fact that the fuel compressing device  7  further fills the pressure storage chamber  12 . The size of the pressure storage chamber  12  is preferably selected such that the preinjection and postinjection can be performed by means of pumping of fuel that is done from the pressure storage chamber  12 . The compression chamber  8  of the fuel compressing device  7  can be re-filled independently of the region of the fuel injection. The pressure buildup in the region of the fuel metering is determined by actuation of the 2/2-way valve  11 . For limiting the maximum pressure within the fuel injection system, a pressure limiting valve (not shown in the exemplary embodiment) can be used in the region of the pressure storage chamber. 
     The first exemplary embodiment of a fuel injection system  1  and the second exemplary embodiment of a fuel injection system  31  in FIG. 2 have in common the fact that an advantageous unit fuel injector  6  or  36  is combined with a local pressure storage chamber and a cross sectional control of the fuel-metering valve unit. 
     The first exemplary embodiment of a fuel injection system  1  and the second exemplary embodiment of a fuel injection system  1  in FIG. 2 have in common the fact that an advantageous unit fuel injector  6  is combined with a local pressure storage chamber and a cross sectional control of the fuel-metering valve unit. 
     The local pressure storage chamber  12  is utilized to store the pressure, to make a flexible instant of injection possible for a preinjection or postinjection outside the cam stroke of the unit fuel injector  6 . The pressure storage chamber  12  enables the control of the injection pressure independently of the rpm of the internal combustion engine. This is done by regulating the time between the triggering of the pressure buildup and the triggering of the injection. The time for filling the pressure storage chamber  12  determines the pressure level attained. Separate valve units are used for the buildup of the injection pressure and for the control of the injection. 
     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.