Abstract:
A fuel injector, in particular a fuel injector for fuel-injection systems of internal combustion engines, including a piezoelectric or magnetostrictive actuator, which, via a valve needle, actuates a valve-closure member arranged on the valve needle, the valve-closure member cooperating with a valve-seat surface to form a sealing seat, the fuel injector having an hydraulic compensation chamber. A pressure piston cooperates with the compensation chamber, which is filled with hydraulic fluid via an hydraulic fluid inlet. The actuator is arranged between the pressure piston and the valve needle and displaceable in the axis of the valve needle and the pressure piston.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a fuel injector.  
         BACKGROUND INFORMATION  
         [0002]    European Patent Application No. 0 477 400 A1 discusses a system for an adaptive mechanical tolerance compensation, which is effective in the lift direction and intended for a path transformer of a piezoelectric actuator for a fuel injector. In this case, the actuator acts on a master (transmitting) piston, which is connected to an hydraulic chamber, and a slave (receiving) piston, which moves a mass to be driven and positioned, is moved via the pressure increase in the hydraulic chamber. This mass to be driven is a valve needle of a fuel injector, for example. The hydraulic chamber is filled with an hydraulic fluid. When the actuator is deflected and the hydraulic fluid in the hydraulic chamber is compressed, a small portion of the hydraulic fluid leaks at a defined leakage rate. This hydraulic fluid is replenished in the rest phase of the actuator.  
           [0003]    German Patent Application No. 195 00 706 A1 discusses an hydraulic path transformer for a piezoelectric actuator of a fuel injector, which is positioned between the actuator and a valve needle of the fuel injector. A master piston and a slave piston are arranged on a common axis of symmetry, and an hydraulic chamber lies between the two pistons. A spring, which presses the master cylinder and the slave piston apart, is located in the hydraulic chamber, the master piston being prestressed in the direction of the actuator and the slave piston in a working direction against a valve needle. When the actuator transmits a lifting movement to the master cylinder, this lifting movement is transmitted to the slave piston by the pressure of an hydraulic fluid in the hydraulic chamber, since the hydraulic fluid in the hydraulic chamber is not compressible and only a small portion of the hydraulic fluid is able to escape during the short duration of a lift through ring gaps between the master piston and a guide bore, and a slave piston and a guide bore.  
           [0004]    In the rest phase, when the actuator does not exert any compressive force on the master cylinder, the master piston and the slave piston are pushed apart by the spring, and, due to the produced vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the ring gaps and refills it. In this way, the path transformer automatically adapts to linear deformations and pressure-related expansions of a fuel injector.  
           [0005]    Disadvantageous in this related art is that the path transformer gets very hot from the waste heat of an internal combustion engine. The path transformer is located in a region of the fuel injector that lies deep in an installation bore once the fuel injector is installed and thus in close proximity to the combustion chamber. In the rest phases of the actuator, the fuel may evaporate and thus cause a failure of the fuel injector, since the evaporated fuel is compressible and the valve needle is not opened for that reason.  
           [0006]    This danger exists in particular after a hot internal combustion engine has been shut off. The fuel-injection system then loses its pressure, and the fuel evaporates particularly easily. This may have the result that in a renewed effort to start the internal combustion engine the lifting movement of the actuator is no longer transmitted to a valve needle and the fuel injector no longer functions.  
         SUMMARY OF THE INVENTION  
         [0007]    In contrast, the fuel injector according to the present invention has the compensation chamber near a fuel-distributor line and at a distance from the side of the fuel injector contacting a combustion chamber of an internal combustion engine. The fuel injector according to an exemplary embodiment of the present invention thus has a lower temperature in the region of the compensation chamber than the related art. Furthermore, a larger unit volume is available for the compensation-chamber design.  
           [0008]    In an exemplary embodiment of the present invention a chamber spring is located on the compensation-chamber side of the pressure piston and exerts a prestressing force on the pressure piston, which pushes the pressure piston out of the compensation chamber or out of a guide bore of the pressure piston connected to the compensation chamber. The chamber spring may be a membrane spring, a disk spring or a helical spring.  
           [0009]    During the rest phase when no voltage is applied to the magnetostrictive or piezoelectrical actuator, the actuator does not exert any pressure on the pressure piston. Instead, because of the chamber spring, the pressure piston is pressed against the actuator, which is supported so as to be movable and displaceable and is advanced in the direction of the valve needle until is comes to rest against it. Due to the attendant volume increase of the compensation chamber, a vacuum pressure is produced and hydraulic fluid flows into the compensation chamber via the hydraulic fluid inlet until the vacuum pressure is compensated. In this manner, the loss of hydraulic fluid during the working phase of the actuator and the superpressure this produces is compensated. Linear deformations of the housing and the transmission path from the valve needle via the actuator up to the actuator support are thus compensated for, since the actuator is braced on the pressure piston, which always advances to the maximum extent in the direction of the valve needle.  
           [0010]    In an exemplary embodiment of the present invention, the compensation chamber is also able to be supplied with an hydraulic fluid that is under a higher pressure than the pressure of the fuel on the actuator side of the pressure piston.  
           [0011]    This exerts pressure on the pressure piston during the rest phases of the actuator, without a chamber spring being required, the force moving the pressure piston, and thereby the float-mounted actuator, toward the valve needle up to the stop. This also compensates for the leakage losses during the working phase of the actuator and for the linear deformations of the housing and the linear deformations of the actuator and the valve needle caused by the heating and fuel pressure during the rest phases of the actuator.  
           [0012]    The hydraulic fluid inlet may have an intake throttle, which allows only a small portion of the compensation chamber volume of hydraulic fluid to flow back during activation of the actuator.  
           [0013]    During the brief activation phase of the actuator, only little hydraulic fluid can drain and flow back, whereas during the long rest phase of the actuator sufficient hydraulic fluid is able to flow in to ensure a play compensation and to replenish the compensation chamber at all times.  
           [0014]    The hydraulic fluid inlet may have a check valve, thereby allowing a particularly rapid replenishing during the rest phases. If the check valve is configured as a rapidly responding check valve, return-flow losses during the working phase of the actuator may effectively be prevented.  
           [0015]    In an exemplary embodiment of the present invention, the hydraulic fluid inlet is a controllable intake valve, which is closed in the non-controlled state.  
           [0016]    Since such an intake valve may release a large cross section, the compensation chamber may be filled very rapidly by a control pulse during the rest phase.  
           [0017]    The compensation chamber may have an hydraulic fluid outlet with a discharge throttle. As in the case of the hydraulic fluid inlet, the loss during the control phase of the actuator and the attendant pressure increase is only slight; however, a continuous flushing of the compensation chamber and an advantageous cooling of the compensation chamber may occur during the rest phase of the actuator.  
           [0018]    Alternatively, the compensation chamber has an hydraulic fluid outlet with a controllable discharge valve, which is closed in the non-controlled state in a preferred specific embodiment. In this way, an especially large cross section and increased flushing may be achieved during the rest phase.  
           [0019]    As an alternative, the hydraulic fluid outlet of the compensation chamber may have a pressure limiting valve. By increasing the pressure above the limiting pressure of the pressure limiting valve a flushing may be achieved during the rest phase of the actuator. If the pressure limiting valve is designed in such a way that the response lag of the pressure limiting valve is greater than the time duration of a working phase of the actuator, hydraulic fluid losses during the working phase may be kept to a minimum.  
           [0020]    In an exemplary embodiment of the fuel injector according to the present invention, the hydraulic fluid outlet in the compensation chamber is located at the highest point in the installation position of the fuel injector. In this way, any gas bubbles that may be present are removed during flushing. In particular during the start of an internal combustion engine, which was switched off earlier in a hot operating state, a functioning of the fuel injector may be ensured. Gas bubbles that may be produced by evaporating fuel and may prevent a pressure generation in the compensation chamber because of their compressibility, are removed in a reliable and rapid manner.  
           [0021]    The compensation chamber may be filled with fuel, or may alternatively also be connected to an oil circuit of the internal combustion engine. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 shows a schematic section through a first exemplary embodiment of a fuel injector according to an exemplary embodiment of the present invention.  
         [0023]    [0023]FIG. 2 shows a schematic section through a second exemplary embodiment of a fuel injector according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    [0024]FIG. 1 schematically shows a fuel injector  1  in a section and a block diagram. It is a fuel injector  1  having an outwardly opening valve needle  2 , which is connected to a valve-closure member  3 . A valve-seat support  5 , integrally formed or constructed with a valve body  4 , has a valve-seat surface  6 , which forms a sealing seat  7  together with valve-closure member  3 . Valve needle  2  has a spring stop  8  on which valve needle  9  is braced. At its second end, valve spring  9  rests against a guide sleeve  10  for valve needle  2 . Via spring stop  8 , valve spring  9  exerts an initial stress on valve needle  2 , which presses valve-closure member  3  against sealing seat  6 .  
         [0025]    An actuator  11  is connected to an actuator tappet  13  guided in a partition shield  12 . Actuator  11  may be supplied with a current via connecting lines  14 . At its end facing away from sealing seat  6 , actuator  11  is connected to a pressure piston  15 , which seals a compensation chamber  17  from valve body  4  by an elastic seal  16 . The interconnected and cooperating unit made up of actuator tappet  13 , actuator  11  and pressure piston  15  is supported in a moveable and float-mounted manner in the longitudinal axis of fuel injector  1  by partition disk  12  via actuator tappet  13 , and by elastic seal  16  via pressure piston  15 . Compensation chamber  17  is continually supplied with fuel as hydraulic fluid by way of a fuel inlet  19  and an inflow throttle  20 . A negligible quantity of fuel is also drained continuously via a discharge throttle  21  und a fuel discharge  22 .  
         [0026]    Also via fuel inlet  19  and inflow bores  23   a ,  23   b  and  23   c , fuel is flowing to sealing seat  6 .  
         [0027]    If actuator  11  is energized via connecting lines  14 , it expands in length and attempts to press pressure piston  15  into compensation chamber  17 . Since the fuel contained in compensation chamber  17  is only slightly compressible as a fluid and inflow throttle  20  and discharge throttle  21  have small diameters, such as 20 μm, only small quantities of fuel may escape, and high pressure is rapidly generated in compensation chamber  17  against which pressure piston  15  is braced. In this way, valve spring  9  at the other end of actuator  11  is acted on with an opening force, via actuator tappet  13 , and valve needle  2  with valve-closure member  3  is actuated, so that valve-closure member  3  lifts off from sealing seat  6 . Once the current has been switched off, valve spring  9  moves valve needle  2  back into its original position. At the same time, chamber spring  18  exerts a compressive force on pressure piston  15 , which retains actuator  11  with actuator tappet  13  at spring stop  8  of valve needle  2 . The spring forces adjust actuator  11  between the hydraulic cushion and the valve needle in a play-free manner. In the process, fuel continues to flow via inflow throttle  20  into compensation chamber  17  until it is completely filled with fuel again. If the heating causes linear deformations of valve body  4  or actuator  11 , actuator  11  with actuator tappet  13  and pressure piston  15  will thus always be displaced in the longitudinal direction of fuel injector  1  until it comes to rest against spring stop  8  of valve needle  2 .  
         [0028]    Since fuel continually flows through compensation chamber  17 , even during the rest phase of actuator  11  in which actuator  11  is not energized via connecting lines  14 , this compensation chamber  17  is cooled. Furthermore, in an exemplary embodiment of the present invention no parts of a coupler are dynamically displaced in fuel injector  1 , since compensation chamber  17  is only subjected to a static support force via pressure piston  15 . The response characteristic of fuel injector  1  is thus improved. If fuel discharge  22  is arranged in such a way that an outlet  24  lies at the highest point in the installation position of fuel injector  1  of an internal combustion engine (not shown here), any possibly produced gas bubbles are effectively removed from compensation chamber  17 . In particular, once a hot internal combustion engine has been turned off, this prevents that evaporated fuel in compensation chamber  17  forms a gas bubble during restarting, since such gas bubbles are removed via inflow throttle  20  and pushed into fuel discharge  2  when the fuel supply commences  2 . It cannot happen that pressure piston  15  is unable to generate pressure in compensation chamber  17  due to compressed gas bubbles, and valve needle  2  thus fails to open.  
         [0029]    Alternatively, it is possible to use a check valve instead of inflow throttle  20 , which releases a large flow cross section when vacuum pressure exists in compensation chamber  17 . Also as an alternative, a pressure limiting valve may be used instead of discharge throttle  21 , which, due to its inertia, does not respond during the brief activation phase of actuator  11 , but opens when a certain adjustable superpressure exists in compensation chamber  17  and releases a large discharge cross section.  
         [0030]    [0030]FIG. 2 shows an exemplary embodiment of a fuel injector  1  according to the present invention. Components that are identical to FIG. 1 have been provided with the same reference numerals. Valve-closure member  3  is in operative connection with valve needle  2 , forming a sealing seat  6  together with valve sealing-seat surface  6  on valve-sealing section  5  formed on valve body  4 . Via valve spring  9  and valve-spring stop  8 , valve needle  2 , which is guided in guide sleeve  10 , is pulled into sealing seat  6  by way of its valve-closure member  3 . Actuator  11  is arranged between actuator tappet  13 , guided in partition disk  12 , and pressure piston  15  held by elastic seal  16  and is interconnected to them and may be energized via connecting lines  14 . Fuel is supplied to sealing seat  6  via fuel inlet  19  and supply bores  23   a ,  23   b  and  23   c . Chamber spring  18  is arranged in compensation chamber  17 .  
         [0031]    Via an oil inlet  25 , which has a switching valve  26  and is connected to the oil circuit of the internal combustion engine (not shown here), oil is supplied to compensation chamber  17  as hydraulic fluid. This oil can flow off via an additional switching valve  27  and an oil outlet  28 .  
         [0032]    Switching valves  26 ,  27  may release large flow cross sections. After actuator  11  is de-energized, switching valve  26  of oil inlet  25  allows a rapid refilling of the compensation chamber by a large inflow cross section. It is also possible, at the same time and controllable in the extent, to release oil outlet  28  by a switching valve  27 , attaining a flushing and cooling of compensation chamber  17 . In the same manner, it is possible to prevent the formation of bubbles, both after a start and during operation. This danger is additionally reduced by the use of the medium oil as the hydraulic fluid.