Abstract:
An injector, actuated by a piezoelectric actuator with a hydraulic booster, for a fuel injection system for internal combustion engines is disclosed, in which the pressure booster is always hydraulically prestressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 00/03582 filed on Oct. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an injector for a fuel injection system for internal combustion engines, having a pressure booster that has a first piston guided in a first guide bore and a second piston guided in a second guide bore, in which the first piston and the second piston define a pressure chamber that is under at least a supply pressure, and having a leaking oil line, the first piston being actuated by an actuator and the first piston at least indirectly actuating the final control element of a control valve, and to a fuel injection system for internal combustion engines using the injector. 
     2. Description of the Prior Art 
     To make it possible to assure a defined operating state of the injector and consequently good operating performance, and above all favorable engine starting performance, the second piston of the hydraulic pressure booster has until now been pressed against the final control element of the control valve by a spring, such as a cup spring. 
     This arrangement assures that the second piston of the hydraulic pressure booster will always assume a defined position, but air inclusions or the like can have the effect that a defined operating state of the injector is still lacking, and in particular that the engine starting performance is inadequate. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The primary object of the invention is to furnish an injector for a fuel injection system for internal combustion engines that has a pressure booster with improved operating performance. 
     This object is attained according to the invention by an injector for a fuel injection system for internal combustion engines, having a pressure booster that has a first piston guided in a first guide bore and a second piston guided in a second guide bore, wherein the first piston and the second piston define a pressure chamber that is under at least a supply pressure, and having a leaking oil line, wherein the first piston is actuated by an actuator and the first piston at least indirectly actuates the final control element of a control valve, in which the second guide bore communicates hydraulically with the leaking oil return and the pressure p 1  in the leaking oil return is lower than the supply pressure. 
     As a result of the prevailing pressure difference between the pressure chamber and the leaking oil return, a hydraulic force that presses the second piston against the final control element of the control valve always acts on the second piston. 
     As a result, not only is a defined position of the second piston of the hydraulic pressure booster assured, but in addition, there is a constant exchange of the fuel, or possible air inclusions, in the pressure booster. 
     Because of the low pressure level in the injector directly downstream of the control valve, the operating performance of the outlet throttle disposed between the control chamber and the control valve also improves. 
     In variants of the invention, the first guide bore, the first piston and the housing are sealed off from their surroundings, in particular by a bellows, so that no leakage can occur, and the supply pressure in the pressure chamber remains high. 
     Further in the invention, it is provided that supply pressure in the pressure chamber is furnished by a supply line discharging into the chamber formed by the housing, bellows and first piston, so that all the lines ( 31 ,  25 ,  27 ) and chambers ( 34 ,  17 ,  29 ) always have a flow of fuel through them. 
     In a further feature of the invention, a pressure holding valve is disposed in the supply line, so that pressure p 2  in the supply line is limited, in particular to 50 bar. 
     Another embodiment has a recess which is in communication with the leaking oil return at the second guide bore, so that the control and leakage quantities can be returned without problems. 
     In a further variant of the invention, the second piston actuates the final control element of the control valve via a tappet, so that the portions of the second guide bore that lead between the second piston and the tappet need not be precisely aligned, and different tolerances are possible. 
     Further in the invention, the outlet side of the control valve discharges into the leaking oil return, so that the control quantity is carried away. 
     In another feature of the invention, the actuator is a piezoelectric actuator, thus making major control forces available. 
     Another embodiment provides for mounting the housing, the control valve, and the actuator in a retaining body, thus on the one hand facilitating manufacture and mounting and on the other making a play-free, prestressed connection of the components possible. 
     The object stated above is also attained by a fuel injection system for internal combustion engines, having a high-pressure fuel pump, having a high pressure fuel reservoir, and having at least one injector according to the invention, so that the advantages of the invention come into play in this fuel injection system as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further objects and advantages of the invention can be learned from the ensuing description, taken in conjunction with the single figure of the drawing which is a sectional view schematically showing an injector embodying the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawing, one exemplary embodiment of an injector of the invention, with a hydraulic pressure booster, is shown. In a two-part housing  1  and  3 , a first guide bore  5  and a second guide bore  7  are disposed coaxially to one another. A first piston  9  is guided in the first guide bore  5 . The second guide bore  7  guides a second piston  11  and a tappet  13 . Via the tappet  13 , the second piston  11  acts on the spherical final control element  15  of the control valve, not shown, of the injector. Because of the two-piece design of the second piston  11  and tappet  13 , the two portions of the second guide bore  7  need not be aligned precisely with one another; furthermore, the play between the second piston  11  and the second guide bore  7  and between the tappet  13  and the second guide bore  7  can be selected to be different. 
     The first piston  9  and the second piston  11  define a pressure chamber  17  of a hydraulic pressure booster  19 . A piezoelectric actuator  21  acts on the first piston  9 . When the piezoelectric actuator  21  is actuated, it presses the first piston  9  in the direction of the arrow into the pressure chamber  17 . As soon as the consequently rising pressure in the pressure chamber  17  suffices to exert a hydraulic force on the end face of the second piston  11  protruding into the pressure chamber  17  that is greater than the hydraulic force exerted by the fuel under control pressure in the control chamber  23  on the final control element  15 , the final control element  15  opens and establishes a hydraulic communication between the control chamber  23  and a leaking oil return  25 . This communication is effected via a portion of the second guide bore  7 , a supply line  27  and a recess  29  of the second guide bore  7 . As a result, the pressure in the control chamber  23  drops, and tie injection nozzle, not shown, opens. 
     As soon as the actuation of the piezoelectric actuator  21  ends, the piezoelectric actuator moves, together with the first piston  9 , back into its outset position counter to the direction of the arrow. As a result, the pressure in the pressure chamber  17  drops. A supply line  31 , in which a supply pressure p 2  prevails, supplies the pressure chamber  17  with fuel indirectly, via a bore in the housing  1  and a bellows  33 . 
     The housing  1  and the first piston  9  are sealed off from their surroundings by the bellows  33 . The supply line  31  discharges into the chamber  34  defined by the housing  1 , first piston  9  and bellows  33  and fills this chamber with the fuel, which is at the pressure p 2 . Through the annular gap between the first guide bore  5  and the first piston  9 , fuel reaches the pressure chamber  17 , so that this pressure chamber is always filled with fuel, and also, at least approximately, a pressure in the amount p 2  always prevails in the pressure chamber  17 . 
     In the recess  29  as in the leaking oil return  25 , a pressure p 1  prevails, which is lower than the supply pressure p 2 . Consequently, a hydraulic force oriented from the piezoelectric actuator  21  to the final control element  15  acts on the second piston  11 . As a consequence of this exertion of force, the second piston  11  and the final control tappet  13  are pressed against the final control elements  15 , so that no play can arise between the second piston  11 , tappet  13  and final control element  15 . Instead, the pressure booster  19  is prestressed. 
     Since only the pressure pi prevails in the recess  29 , fuel flows out of the pressure chamber  17  into the recess  29  through the annular gap between the second guide bore  7  and the second piston  11 . Besides the aforementioned effect that the pressure difference between the pressure chamber  17  and the recess  29  connects the piston to the final control element  15  without play, the described flow of the fuel from the supply line  31  as far as the leaking oil return  25  has the effect that the entire pressure booster always experiences a flow of fresh, relatively cool fuel through it, thus lessening the risk of seizing; furthermore, any air or vapor bubbles that may be present are always conveyed out of the pressure booster  19  into the leaking oil return  25 . 
     Another advantage of the embodiment according to the invention is that in contrast to the prior art, the bellows is always subjected to pressure only statically. As a result, an indication of the supply pressure p 2  without having to modify the bellows  33  is possible, and thus also an increase in the prestressing of the piezoelectric actuator. The supply pressure p 2  can for instance amount to 50 bar. 
     The entire injector, comprising the piezoelectric actuator  21 , pressure booster  19  and the control valve, not shown, and the actual injection nozzle itself, are held together by a retaining body  35 . The fuel flowing out of the supply line  31  through the retaining body  35  is prevented from flowing out into the surroundings by two O-rings, which are disposed between the retaining body  35  and the housing  1 . 
     In the supply line  31 , there is a pressure holding valve  37 , which assures that an allowable maximum pressure in the supply line  31  and in the chambers  17  of the pressure booster that communicate hydraulically with the supply line  31  is never exceeded. As soon as this maximum pressure is reached, the pressure holding valve  37  opens. The outlet side of the pressure holding valve can be in communication with the leaking oil return  25 , for instance. 
     The foregoing relates to 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.