Abstract:
A fuel injector, in particular a fuel injector for fuel-injection systems of internal combustion engines, including a piezoelectric or magnetostrictive actuator, actuates a valve-closure member formed on a valve needle via a hydraulic coupler, the valve-closure member cooperating with a valve-seat surface to form a valve-sealing seat. The coupler includes a master piston and a slave piston which are connected to a pressure chamber, and at least one coupler-spring element which in each instance produces a prestressing force on the master piston, counter to a working direction, and on the slave piston, in a working direction. The pressure chamber of the coupler is connected to a fuel inflow in the flow-through direction to the pressure chamber via an inflow bore and a check valve.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a fuel injector.  
         BACKGROUND INFORMATION  
         [0002]    European Patent Application No. 0 477 400 discusses a system for an adaptive, mechanical tolerance compensation, acting in the lift direction, for a path transformer of a piezoelectric actuator for a fuel injector. In this case, the actuator acts on a master (transmitter) piston connected to an hydraulic chamber, and a slave (receiving) piston, moving a mass to be driven and positioned, is moved via the pressure increase in the hydraulic chamber. This mass to be driven is, for example, a valve needle of a fuel injector. The hydraulic chamber is filled with an hydraulic fluid. When the actuator is deflected and the hydraulic fluid in the hydraulic chamber compressed, a small portion of the hydraulic fluid leaks at a defined leakage rate. In the rest phase of the actuator, this hydraulic fluid is replenished.  
           [0003]    German Published Patent Application No. 195 00 706 discusses a hydraulic path transformer for a piezoelectric actuator in which a master piston and a slave piston are lying on a common axis of symmetry and the hydraulic chamber is located between the two pistons. A spring, which presses apart the master cylinder and the slave piston, is located in the hydraulic chamber, the master piston being prestressed in the direction of the actuator and the slave piston being prestressed in a working direction of 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 a hydraulic fluid in the hydraulic chamber since the hydraulic fluid in the hydraulic chamber is not compressible and during the short duration of a lift only a very small portion of the hydraulic fluid is able to escape 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 pressure on the master piston, the spring pushes apart the master piston and the slave piston, and, due to the produced vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the ring gaps and refills it. In this manner, the path transformer automatically adapts to longitudinal deformations and pressure-related extensions of a fuel injector.  
           [0005]    In other systems, hydraulic fluid may evaporate during a relief period in which no high pressure prevails in the hydraulic chamber. However, gas is compressible and generates an appropriately high pressure only after a substantial reduction in volume. The master cylinder may then be pressed into its guide bore without a force being transmitted to the slave piston.  
           [0006]    This danger exists, in particular, in a fuel injector used for injecting gasoline as fuel, in those instances where the gasoline is also used as the hydraulic fluid. This danger is increased even further in the case of a directly injecting fuel injector for gasoline once a hot internal combustion engine has been switched off. A fuel-injection system then loses its pressure, and the gasoline evaporates particularly easily. In a renewed effort to start the internal combustion engine, this may lead to the lifting movement of the actuator no longer being transmitted to a valve needle and the fuel injector no longer functioning.  
           [0007]    A cavitation of the fuel may occur if the spring exerts a high clamping force upon the master cylinder and the slave cylinder and the movement of the actuator into its original position occurs very rapidly. The vacuum pressure being generated in the hydraulic chamber may then lead to cavitation and to damage of components resulting therefrom.  
         SUMMARY OF THE INVENTION  
         [0008]    The fuel injector according to the present invention may provide that, given vacuum pressure in the pressure chamber, the check valve opens and releases a connection to the fuel inflow. The coupler-spring element exerts a force upon the master piston and the slave piston in an attempt to increase the volume of the pressure chamber when the coupler does not assume the maximally possible length as transmission element between the actuator and the valve needle. Due to the relatively large cross section of the inflow bore, it is then possible for fuel to continue flowing into the pressure chamber until the check valve closes at pressure parity in the pressure chamber and the fuel inflow, and the coupler assumes the maximally possible length as transmission element between the actuator and the valve needle.  
           [0009]    The rapid refilling of the hydraulic chamber may be advantageous in those cases when, after considerable loading and, thus, high temperature of the fuel injector, gas has formed in the pressure chamber following a standstill of an internal combustion engine. Since no, or only low, pressure prevails in the fuel inflow in the shut-off state of the internal combustion engine, it may happen that the fuel, due to the gas of the possibly evaporating fuel, is pressed into the fuel inflow through the ring gap between the master piston and the slave piston and the respective guide bores. When the internal combustion engine is started, the actuator exerts a lifting force on the coupler. However, since gas is compressible, this lifting movement is no longer transmitted to the valve needle. In contrast, in the fuel injector according to the present invention, the check valve is opened as soon as the fuel pressure in the fuel inflow rises, and fuel under overpressure flows into the pressure chamber. This fuel compresses the gas and at the same time cools the pressure chamber, thereby causing the evaporated fuel to condense.  
           [0010]    Furthermore, the fuel injector according to the present invention may provide that expansions of the fuel injector caused by temperature changes and changes in the fuel pressure, are automatically compensated in the transmission path between the actuator and valve needle. The lift of the valve needle is always unchanged.  
           [0011]    The master piston and the slave piston may lie on a common axis and in a common guide bore, the pressure chamber being arranged between them.  
           [0012]    This example embodiment of the fuel injector according to the present invention is simple to produce since only one precise bore is required for the master piston and the slave piston.  
           [0013]    The check valve may be a ball-check valve and a valve seat of the ball-check valve is formed on the slave piston, the inflow bore penetrating the slave piston.  
           [0014]    In an example embodiment, the ball-check valve is stressed by a ball-valve spring which is arranged in a spring bore of the master piston. Relative to the guide bore, the spring bore has a diameter such that the wall thickness of the master piston that remains relative to the diameter of the guide bore is low.  
           [0015]    A considerable part of the installation volume of the check valve may be located inside the master piston, so that the coupler as a whole may have a shorter configuration in its longitudinal extension. Furthermore, due to the fuel pressure, the master piston may be expanded in the region of the spring bore, since the remaining wall thickness is only low, and the ring gap leading to leakage losses is reduced.  
           [0016]    The ball-valve spring may simultaneously be the coupler-spring element.  
           [0017]    An additional component may be saved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 shows a schematic section through an example embodiment of a fuel injector configured according to the present invention.  
         [0019]    [0019]FIG. 2 shows a schematic section, in region II of FIG. 1, through the fuel injector configured according to the present invention.  
         [0020]    [0020]FIG. 3 shows an hydraulic circuit diagram of the coupler of the fuel injector shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 shows a schematic section through an example embodiment of a fuel injector  1  configured according to the present invention. An actuator  4  is located in a valve body  2  in an actuator chamber  3 , actuator  4  abutting against an actuator-support element  5 . Two connecting bores  6  are used to supply electrical connecting lines for actuator  4 . Actuator  4  is controlled via the connecting lines (not shown). Actuator  4  transmits its lifting movement to an actuator head  7 , which is integrally formed with a tappet  8 . An actuator spring  9 , which abuts against a first spring system  10  of actuator head  7  and a second spring system  11  of an intermediate piece  12 , exerts a prestressing force on actuator head  7 , so that actuator head  7  rests against actuator  4 . A sealing ring  13  seals intermediate piece  12  from valve body  2 . Tappet  8  penetrates intermediate piece  12  and transmits a lifting movement of actuator  4  and actuator head  7  to a master piston  14 . A corrugated tube  15  is sealingly connected to the intermediate piece at one side. The other side of corrugated tube  15  is likewise sealingly connected to master piston  14 . Actuator chamber  3  is sealingly sealed from an upper fuel chamber  16   a  by sealing ring  13 , intermediate piece  12 , corrugated tube  15  and master piston  14 .  
         [0022]    Master piston  14  is inserted in a guide bore  17  of a coupler support  18 . Inserted in the same guide bore  17  is a slave piston  19  which is penetrated in its longitudinal axis by an inflow bore  20 . Inflow bore  20  is sealed by a ball  21  of a ball check valve, which is prestressed by a ball spring  22 . Coupler support  18 , master piston  14 , slave piston  19  and ball spring  22  as well as ball  21  form hydraulic coupler  23  whose structure is described in FIG. 2 below.  
         [0023]    Slave piston  19  transmits its lifting movement to a valve needle  24  via a valve-needle head  28 . Valve needle  24  includes a valve-closure member  25 , which is integrally formed with valve needle  24  and cooperates with a valve-seat surface  26  formed on a valve-seat support  29  to form a valve-sealing seat  27 . Fuel injector  1  includes a valve needle  24  that opens toward the outside and lifts off from valve-sealing seat  27  toward a combustion chamber, releasing an annular spray-discharge orifice once fuel injector  1  opens. A valve spring  30  abuts against a first spring system  31  of valve-seat support  29  and, via a second spring system  32  formed at valve-needle head  28 , exerts an initial stress on valve spring  30  in a closing direction, which presses valve-closure member  25  against valve-sealing seat  27 .  
         [0024]    Via a fuel-inflow bore  33  in valve body  2 , the fuel may flow from a fuel inflow (not shown) to upper fuel chamber  16   a . The fuel flows to lower fuel chamber  16   b  and further to valve-sealing seat  27  via openings  34  in valve body  2  and fuel bores  35  in coupler support  18 .  
         [0025]    [0025]FIG. 2 shows a schematic section through fuel injector  1  configured according to the present invention, in region II of FIG. 1. Components already discussed in connection with FIG. 1 have been provided with the same reference numerals. The cut-out section shows hydraulic coupler  23  with master piston  14  and slave piston  19 . Master piston  14  and slave piston  19  are inserted in a shared guide bore  17  of coupler support  18 . Coupler support  18  in turn is inserted in a bore  36  of valve body  2  and sealed by a ring  37  made of an elastomeric material. Via connecting bores  38  in coupler support  18 , fuel-inflow bore  33  in valve body  2  is connected to upper fuel chamber  16   a . Fuel flows to lower fuel chamber  16   b  via the openings in valve body  2  and fuel bores  35  in coupler support  18 .  
         [0026]    Tappet  8  which is integrally formed with actuator head  7  in FIG. 1, penetrates intermediate piece  12  and abuts against master piston  14  by manner of a molded part  39 . A corrugated tube  15  is sealingly connected to the intermediate piece on one side. The other side of corrugated tube  15  is likewise sealingly connected to master piston  14 . These connections consist, for instance, of a slight pressure fit or soldering, welding or bonding of sleeve-shaped sections  40  of corrugated tube  15  to master piston  14  and/or intermediate piece  12 . Sealing ring  13 , intermediate piece  12 , corrugated tube  15  and master piston  14  sealingly seal actuator chamber  3  from upper fuel chamber  16   a.    
         [0027]    Master piston  14  includes a spring bore  41  whose diameter is smaller than the diameter of guide bore  17  to only such an extent that the wall thickness of master piston  14  that remains in the region of spring bore  41  is relatively small. Inside spring bore  41  and in guide bore  17 , between master piston  14  and slave piston  19 , is a pressure chamber  42 .  
         [0028]    Slave piston  19  is penetrated in its longitudinal axis by inflow bore  20 . Inflow bore  20  is sealed by ball  21  which is prestressed by ball spring  22  and forms a ball-sealing seat  44  together with outlet  43  of inflow bore  20 . Ball-check valve  49  is made up of ball-sealing seat  44 , ball  21  and ball spring  22 . Inflow bore  20  is connected to lower fuel chamber  16   b  via a transverse bore  45  in slave piston  19 . Ball spring  22 , via a spring-pressure piece  46  which includes a spring-guide section  47 , abuts against master piston  14 . By manner of its other end, ball spring  22  is braced on ball  21  via a ball-pressure piece  48 . Thus, ball spring  22  presses ball  21  into ball-sealing seat  44  and simultaneously provides master piston  14  with an initial stress in the direction of actuator  4  and slave piston  19  with an initial stress in the direction of valve needle  24 .  
         [0029]    [0029]FIG. 3 shows an hydraulic circuit diagram of the coupler of fuel injector  1  of FIG. 1. Master piston  14  and slave piston  19  are represented in a schematized form as pistons acting on pressure chamber  42  arranged between them. In order to make it easier to find the components that correspond to the circuit symbols, the circuit symbols are denoted by the reference numerals corresponding to the components in FIG. 1 and FIG. 2. Via inflow bore  20 , fuel is able to flow as hydraulic fluid from fuel-inflow bore  33  via ball-check valve  49 , made up of ball-sealing seat  44 , ball  21  and ball spring  22 , in the flow-through direction of ball-check valve  49  into pressure chamber  42 . The ring gap existing between master piston  14  and guide bore  17  of coupler support  18  in FIG. 2 acts as a master-piston throttle  50  by manner of which pressure chamber  42  is connected to upper fuel chamber  16   a . The ring gap existing between slave piston  19  and guide bore  17  of coupler support  18  in FIG. 2 likewise acts as a slave-piston throttle  51  by manner of which pressure chamber  42  is connected to lower fuel chamber  16   b.    
         [0030]    In response to a voltage being applied to actuator  4 , actuator  4  exerts a lifting force on actuator head  7  and tappet  8  in FIG. 1. This lifting force is transmitted to master piston  14  which is moved in guide bore  17  toward slave piston  19 . This causes the pressure in pressure chamber  42  to rise rapidly since the fuel contained in pressure chamber  42  is incompressible as fluid. Slave piston  19  is pushed out of guide bore  17  onto valve needle  24  and lifts valve needle  24  out of valve-sealing seat  27 . Since the duration of the lift is relatively short, during the lift only a relatively small quantity of fuel is able to flow into upper fuel chamber  16   a  or lower fuel chamber  16   b  via the ring gap between master piston  14  and guide bore  17  and between slave piston  19  and guide bore  17 . This corresponds to the flow rate of the fuel from pressure chamber  42  via master-piston throttle  50  into upper fuel chamber  16   a  and the flow rate of the fuel via slave-piston throttle  51  into lower fuel chamber  16   b  in the hydraulic circuit diagram of FIG. 3, as a function of the overpressure prevailing in pressure chamber  42 . Ball-check valve  49  is acted upon in its blocking direction by the overpressure in pressure chamber  42  relative to lower and upper fuel chambers  16   a ,  16   b  and fuel-inflow bore  33 , and closes.  
         [0031]    When the voltage drops at actuator  4 , actuator spring  9  presses actuator head  7  into its rest position onto actuator  4 , and valve needle  24  is pressed into valve-sealing seat  27 . A coupler-spring element, which simultaneously is ball spring  22  in the present example embodiment, exerts a force upon master piston  14  and slave piston  19  in an attempt to increase the volume of pressure chamber  42  when hydraulic coupler  23  fails to assume the maximally possible length as transmission element between actuator  4  and valve needle  24 .  
         [0032]    Due to ball-check valve  49  and inflow bore  20  of slave piston  19 , it is now possible for fuel to continue flowing into pressure chamber  42  until ball-check valve  49  closes at pressure parity in pressure chamber  42  and the fuel inflow, and coupler  23  assumes the maximally possible length as transmission element between actuator  4  and valve needle  24 . The rapid refilling of pressure chamber  42  may be advantageous in those instances when, following a standstill of an internal combustion engine after considerable loading and, thus, high temperature of the fuel injector, gas has formed in pressure chamber  42 . As soon as the fuel pressure in fuel-inflow bore  33  rises, ball-check valve  49  is opened and fuel under overpressure flows into pressure chamber  42 . This fuel compresses the gas and simultaneously cools pressure chamber  42 , thereby condensing the evaporated fuel.  
         [0033]    A cavitation of the fuel may be avoided when the volume of pressure chamber  42  increases rapidly, since a negative pressure in pressure chamber  42  is quickly compensated by the fuel that continues to flow via ball-check valve  49 . Therefore, fuel injector  1  according to the present invention may allow the use of an hydraulic coupler  23  that may allow temperature and expansion compensation at simultaneously very rapid opening and closing movements of valve needle  24 .  
         [0034]    Due to the low wall thickness of master piston  14  in the region of spring bore  41 , a widening of the ring gap of master piston  14  relative to guide bore  17  in response to overpressure in pressure chamber  42  is reduced and the corresponding flow rate of fuel through master-piston throttle  50  of the circuit diagram of FIG. 3 minimized.