Abstract:
A fuel injector, especially an injector valve for fuel injection equipment in internal combustion engines, includes a piezoelectric or magnetostrictive actuator and a valve closing body, operable by an actuator with the aid of a valve needle, which cooperates with a valve seat surface to form a sealing seat, and a valve housing. The actuator is prestressed by a compression spring and, together with this, is surrounded by an actuator housing which is supported by fluid at both its ends.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel injector. 
     BACKGROUND INFORMATION 
     Ordinarily, changes in length of a piezoelectric actuator in a fuel injector are compensated for by the influence of temperature using hydraulic devices or by choosing suitable material combinations. 
     European Published Patent Application No. 0 869 278 describes a fuel injector in which the longitudinal change of the actuator is compensated for by an appropriate material combination. The fuel injector as in this document has an actuator, positioned in an actuator chamber, which is connected with form locking to a pressure shoulder via which the actuator acts upon the valve needle in opposition to the force of a pressure spring. The actuator is supported at one end on a pressure plate, and at the other end on a control element. During operation of the actuator, the valve needle is activated in the direction of spray-off. 
     In the document named, compensation for the longitudinal change of the actuator, caused by temperature, is achieved by a plurality of compensation discs positioned between the pressure plate and the end face of the actuator. These have a temperature expansion coefficient corresponding with opposite sign to that of the actuator element. During a shortening of the actuator caused by rising temperature, the compensation discs expand, and thereby compensate for the thermal longitudinal change of the actuator. 
     This design has a disadvantage above all in connection with cost of manufacture, having relatively high costs conditional especially on the choice of materials (e.g. INVAR). The compensation for longitudinal changes by hydraulic devices is known, for instance, from European Patent 0 477 400. With designs of this kind, the fundamental disadvantage is that large volumes of liquid have to be displaced, and, because of that, there is a greater tendency to cavitation. 
     SUMMARY OF THE INVENTION 
     The fuel injector according to the present invention on the other hand, has the advantage of simple construction of the component parts, from a standpoint of production engineering. This guarantees a fail-safe and precise method of operation of the fuel injector. Of particular advantage are the liquid support on both sides and the low damping volume for avoiding cavitational damage. 
     Especially of advantage are the encapsulation and prestressing of the actuator, since the quasi-static thermal linear deformation of the actuator does not have to be compensated for by costly material combinations, but is compensated for by a change in initial stress of the compression spring. Thereby, the overall length of the actuator housing is not influenced by thermal changes in length. For that reason, only a change in position of the actuator housing relatively to the valve housing still has to be compensated. 
     Sealing the actuator housing from the valve housing has the advantage that the actuator cannot be attacked by the chemically aggressive fuel. 
     The use of fuel as hydraulic medium is of advantage, since leakage losses can be compensated permanently by fuel supply. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows an axial section through an exemplary embodiment of a fuel injector according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In an axial section, FIG. 1 illustrates an exemplary embodiment of fuel injector  1  according to the present invention. This is about a so-called top feed injection valve having central fuel supply via a fuel inlet  28  which opens toward the inside. 
     In an actuator housing  2 , an actuator  3  of ring-shaped design, having a central hollow recess  29  and being made of disc-shaped piezoelectric or magnetostrictive elements  4  and a compression spring  5 , are located. The actuator  3  is operated by an external voltage source via a plug connection  12 . To make it simple, only one single contact  13  is shown in FIG.  1 . The actuator housing is closed at its ends by a first outer flange  6  and a second outer flange  7 , which are sealed from a valve housing  10  surrounding the actuator  3  by a first sealing element  8  and a second sealing element  9 . 
     The first outer flange  6  includes a first disc  31  and a first sleeve  32 . The first disc  31  lies at a first end face  24  of actuator  3 . The second outer flange  7  includes a second disc  33  and a second sleeve  34 . The second disc  33  abuts a first end  26  of compression spring  5 . A second end face  25  of actuator  3  and a second end  27  of compression spring  5  are supported on a middle flange  11 . Actuator  3  is held under prestress by compression spring  5  via middle flange  11 . 
     Middle flange  11  is preferably connected with force-locking to an operating body  15  by a weld  14 . The operating body  15  is located in the central recess  29  of actuator  3 , and is in contact, via extension an  35 , with a valve needle  17 , at which a valve closing body  30  is formed. During lifting off of the valve closing body  30  from a valve seat surface  18  of a valve seat body, fuel is sprayed off through a spray-off opening  19 . Operating body  15  is supported on the inlet side on a return spring  20  and grips from behind with its spray-off side extension  35  a flange  36  of valve needle  17 . Between flange  36  of valve needle  17  and operating body  15  a spring  16  is clamped. During the closing movement, operating body  15  can swing through with respect to valve needle  17 , so that only the inert mass of valve needle  17  strikes against valve seat surface  18 . This avoids bounce pulses. The fuel flows through an inner hollow recess  37  of the operating body  15 , transverse borings  38  upstream from flange  36  and at least one passage  39  to the sealing seat. 
     Between first sleeve  32  of first outer flange  6  and valve housing  10  there is a first damping chamber  21 . Between the second sleeve  34  of second outer flange  7  and valve housing  10  there is a second ring-shaped damping chamber  22 . Damping chambers  21  and  22  are in contact with fuel inlet  28  via guide slot  23  partially throttled, and are thereby filled with fuel as damping medium. They buffer actuator housing  2  against valve housing  10 . When needed, damping medium is supplied or given off via guide slot  23 . Actuating housing  2  is thus axially freely, slidingly movable in valve housing  10 , under oppositely changing volumes in first damper chamber  21  and second damper chamber  22 . 
     When an electrical operating voltage is connected to actuator  3  of fuel injector  1  according to the present invention shown in FIG. 1, the disc-shaped elements  4  of actuator  3  expand, whereby middle flange  11  is moved counter to the flowing direction of the fuel. Compression spring  5  is further pressed together, counter to the already present prestressing. Valve closing body  30  lifts off valve seat surface  18  and fuel is sprayed off through spray-off opening  19 . 
     Because of the great operating frequency of actuator  3  during the operation of fuel injector  1  according to the present invention in an internal combustion engine, the damping medium between the outer flanges  6  and  7  of actuator housing  2  and valve housing  10  in damping chambers  21  and  22  behaves as an incompressible fluid, since the expansion of actuator  3  during its operation occurs too rapidly for the damping medium to escape through guide slot  23 . 
     A fuel injector  1  experiences great temperature fluctuations during operation. On the one hand, the entire fuel injector  1  heats up through contact with the combustion chamber of an internal combustion engine, and on the other hand, local temperature effects appear, for instance, from the power loss during deformation of piezoelectric actuator  3  or from electrical charge movement. This results in a thermal length reduction of disc-shaped elements  4 , since piezoelectric ceramics have negative temperature expansion coefficients, that is, they contract while heating up and expand while cooling. 
     Such a shortening of actuator  3  by heating is compensated inside actuator housing  2  by the expansion of prestressed compression spring  5 . The shortening of actuator  2  leads to a lengthening of compression spring  5 . Since middle flange  11  is stopped at operating body  15  by weld  14 , the change of length of actuator  3  results in a positional change of actuator housing  2 . This positional change of actuator housing  2  is opposed by the fluid storage of actuator housing  2  within valve housing  10 , since, during quasi-static positional changes of actuator housing  2  relatively to valve housing  10  through temperature influences, the movement of actuator housing  2  takes place so slowly, that damper medium can escape through guide slot  23  or can continue flowing. 
     The present invention is not limited to the illustrated exemplary embodiment, but can also be carried out in a multitude of other methods of construction of fuel injectors.