Abstract:
A fuel injector, in particular a fuel injector for fuel-injection systems of internal combustion engines, has a piezoelectric or magnetostrictive actuator which actuates, via a hydraulic coupler a valve-closure member formed on a valve needle, the valve-closure member cooperating with a valve-seat surface to form a sealing seat. The coupler is made up of a pressure cylinder, a pressure-cylinder support joined to the pressure cylinder, and a pressure piston guided in this pressure cylinder, which form a pressure chamber; and of a coupler spring element between the pressure piston and the pressure cylinder which generates a prestressing force that forces the pressure piston out of the pressure cylinder.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a fuel injector of the type having a piezoelectric or magnetostrictive actuator. 
   BACKGROUND INFORMATION 
   EP 0 477 400 discloses a system for an adaptive, mechanical tolerance compensation, effective in the lift direction, for a path transformer of a piezoelectric actuator for a fuel injector. The actuator lift is transmitted via a hydraulic chamber in this case. The hydraulic chamber has a defined leakage with a defined leakage rate. The lift of the actuator is initiated into the hydraulic chamber via a transmitter master piston and transmitted to an element to be operated via a receiver slave piston. This element, for example, is a valve needle of a fuel injector. 
   EP 0 477 400 discloses a path transformer for a piezoelectric actuator in which the actuator transmits a lifting force to a transmitter cylinder which is sealed by a cylinder support. Guided in this transmitter cylinder is a receiver piston which likewise seals the transmitter cylinder and thereby forms the hydraulic chamber. A spring which pushes the transmitter cylinder and the receiver piston apart is positioned in the hydraulic chamber. The receiver piston mechanically transmits a lifting movement to a valve needle, for instance. When the actuator transmits a lifting movement to the transmitter cylinder, this lifting movement is transmitted to the receiver 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 very small portion of the hydraulic fluid is able to escape through the annular gap during the short duration of a lift. In the rest phase, when the actuator does not exert a pressure force on the transmitter cylinder, the spring presses the receiver piston out of the cylinder and, due to the generated vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the annular gap and refills it. In this way, the path transformer automatically adapts to longitudinal deformations and pressure-related extensions of a fuel injector. 
   This known art is disadvantageous in that the hydraulic chamber can only be filled slowly. Long injection times occur especially in a cold start at low pressure, so that more hydraulic fluid escapes via the annular gap and must subsequently be refilled in a shorter period of time at low pressure. If this is not done, the fuel injector loses lift in each injection until it is entirely unable to function. 
   It is also disadvantageous that the hydraulic fluid can evaporate if insufficient pressure prevails in the hydraulic chamber. However, gas is compressible and generates an appropriately high pressure only after a considerable reduction in volume. 
   This poses a particular danger when shutting off a hot internal combustion engine which uses a fuel injector for gasoline and in which the gasoline is simultaneously used as the hydraulic fluid. A fuel injection system then loses its pressure, and the gasoline evaporates particularly easily. In a new effort to start the internal combustion engine, this may result in the lifting movement of the actuator not being transmitted to the needle since the following flow of cool fuel does not reach the hydraulic chamber soon enough. 
   SUMMARY OF THE INVENTION 
   The fuel injector according to an embodiment of the present invention has a coupler valve-seat member that lifts off from the coupler valve seat once the coupler fails to assume the potential length as the transmission element between the actuator and the valve needle, in this way releasing a potential inflow for the fuel to the pressure chamber via the inflow bore. Since the cross-sectional area taken up by the coupler valve-sealing seat is smaller than the cross-sectional area of the pressure piston, both the coupler spring element and also the increased pressure in the coupler chamber during the activation exert a closing effect on the coupler valve-sealing seat. Due to the relatively large cross section of the inflow bore, fuel may now quickly flow into the pressure chamber until the coupler spring element, at pressure parity in the pressure chamber and the fuel inflow, has forced the pressure piston out from the pressure cylinder to such an extent that the coupler valve-closure member sets down on the coupler valve-seat surface. In this way, the coupler valve-sealing seat interrupts the inflow of fuel from the fuel inflow into the pressure chamber. This is particularly advantageous in those cases where, following a standstill of an internal combustion engine after considerable loading and, thus, high temperature of the fuel injector, gas has formed in the pressure chamber. Since no, or only low, pressure prevails in the fuel inflow in the shut-off state of the internal combustion engine, the fuel, due to the gas of the evaporating fuel, is forced into the fuel inflow through the annular gap between the pressure piston and the pressure cylinder. 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 not transmitted further to the valve needle. In contrast, in the fuel injector configured according to the present invention it is advantageous that, as soon as the fuel pressure rises in the fuel inflow, the coupler valve-closure member is lifted off from the coupler valve-seat surface, the coupler valve-sealing seat is released and fuel under overpressure flows into the pressure chamber. This fuel compresses the gas and cools the pressure chamber at the same time, thereby causing the evaporated fuel to condense. 
   If, for instance, during a cold start, the fuel injector is activated for an extended period of time so that the coupler volume has been reduced by leakage via the annular gap, the coupler valve-sealing seat is released when the actuator is reset. In this way, the coupler chamber is quickly refilled until it has again obtained its original position, and the coupler valve-sealing seat closes. 
   Furthermore, in the fuel injector according to an embodiment of the present invention, expansions of the fuel injector, due to 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 able to remain the same. 
   The coupler valve-closure member may be embodied as a spherical surface and the corresponding coupler valve-seat surface at the valve needle as a conical surface. 
   In additional embodiments, the inflow bore is formed in the pressure-cylinder support, and the coupler valve-closure member is formed in one piece with the pressure-cylinder support and the pressure cylinder. 
   A small design is able to be achieved in accordance with the present invention. In addition, by the gradient of the conical surface and the form design of the hemispherical surface, it is possible to constructionally define how large the effective surface is that is sealed from the fuel inflow by the cross-sectional area of the coupler valve-sealing seat. For the functioning of the fuel injector according to the present invention, this effective area must be smaller than the effective surface of the pressure piston. 
   In an additional embodiment, the coupler valve-seat surface is formed at the valve needle and the pressure piston is joined to a guide piston guided in a bore in a partition shield that shields the fuel inflow from an actuator chamber. Moreover, a corrugated tube is provided at the guide piston to seal this actuator chamber. This embodiment combines components and saves unit volume of the fuel injector. 
   In an another embodiment, the lift of the valve needle may be restricted by a stop of an actuator head or, alternatively, by a stop of the valve needle or, as an alternative, by a stop of the pressure piston or the pressure cylinder. 
   When the lift restricted by the stop is always less than the minimum lift of the actuator in all operating states, an always identical and defined lift of the valve needle is able to be achieved, regardless of the expansion and elongation of a valve member of the fuel injector. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  depicts a schematic section through an exemplary embodiment of a fuel injector configured according to the present invention. 
   

   DETAILED DESCRIPTION 
   Fuel injector  1 , schematically shown in  FIG. 1 , has a valve needle  2  which is joined to a valve-closure member  3  and cooperates via this valve-closure member  3  with a valve-seat surface  5  formed in a valve member  4  to form a valve-sealing seat. Fuel injector  1  is on outwardly opening fuel injector provided with a valve needle  2  that opens toward the outside. Valve needle  2  is guided in a valve-needle guide  10  by a guide section  7  which includes a spring setup  8  for a valve-closure spring  9 . Valve-closure spring  9  is braced against a second spring system  11  at valve member  4  and provides valve needle  2  with an initial stress which presses valve-closure member  3  against valve-seat surface  5 . A sealing ring  13  positioned in a groove  12  provides a sealing of the ring gap (not shown here) between valve member  4  and a bore (likewise not shown) in a cylinder head of an internal combustion engine. 
   To actuate valve needle  2 , a piezoelectric or magnetostrictive actuator  14  is positioned in a valve-member upper section  17 , which is able to be provided with a voltage via a bore  15  in valve-member upper section  17  and an electrical supply line  16 . Actuator  14  has a larger overall length so as to obtain a perceptible lift when a voltage is applied to actuator  14 . The largest part of the overall length of actuator  14  is not represented in FIG.  1 . Adjoining actuator  14  is an actuator head  18  provided with a spring contact surface  19  at which an actuator tension spring  20  rests, which in turn is braced against a partition shield  21 . Actuator spring  20  provides an initial stress to actuator  14 , so that, in response to voltage being applied to electrical supply line  16 , the lift of actuator  14  is transmitted to actuator head  18 . Formed on actuator head  18  is a pressure tappet  22 , which is integrally formed with actuator head  18  and transmits the lift of actuator  14 . Actuator head  18  is guided in valve-member upper section  17  by an actuator-head sleeve  23  and, following a maximum valve travel h, this actuator-head sleeve  23  strikes against partition shield  21 , thereby limiting maximum valve travel h of actuator  14 . 
   Actuator-head tappet  22  transmits the lifting movement of actuator  14  to a pressure-piston support  24  into which a blind-hole bore  25  has been centrally introduced. Pressure-piston support  24  is guided by a guide bore  27  which penetrates support plate  21 . Support plate  21  is sealed from valve-member upper section  17  by a sealing ring  26 . A corrugated tube  28  concentrically encloses pressure-piston support  24  and is affixed to pressure-piston support  24  by a welded seam  29 . On the other side, corrugated tube  28  is attached to support plate  21  by a welded seam  30 . In response to a lifting of actuator  14  and an attendant movement of actuator head  18  having actuator-head tappet  22  formed thereon, pressure-cylinder support  24  is moved in the longitudinal direction, corrugated tube  28  following this movement and expanding correspondingly. At the same time, corrugated tube  28  which, by welded seams  30  and  29 , has sealed ends with respect to pressure-cylinder support  24  and support plate  21 , seals an actuator chamber  31  from a fuel chamber  32 . 
   Formed in one piece with pressure-piston support  24  is a pressure piston  33  functioning as the transmitter piston, which is guided inside a pressure cylinder  34  functioning as the receiver cylinder. Pressure cylinder  34  is integrally formed with a pressure-cylinder support  35 . Centrally guided through pressure-cylinder support  35  is an inflow bore  36 . Inside pressure cylinder  34 , which is sealed by pressure piston  33 , is a pressure chamber  37 . Pressure piston  33 , pressure cylinder  34  and pressure-cylinder support  35  form hydraulic coupler  35   a . Concentrically around pressure piston  33  and pressure cylinder  34 , hydraulic coupler  35   a  is provided with a coupler helical spring  38  between a spring stop  39  at pressure-cylinder support  35  and an additional spring stop  40  at pressure-piston support  24 . Inflow bore  36  is separated from fuel chamber  32  by a coupler valve-closure member, which is embodied as a hemispherical surface on pressure-cylinder support  35 , and by a coupler valve-seat surface  42 , which is embodied as a conical surface on guide section  7  of valve needle  2 , forming a coupler valve-sealing seat. A discoid surface having diameter d results from the coupler valve-sealing seat, this surface not being acted upon by the pressure of the fuel held in fuel chamber  32 . The fuel flows into fuel chamber  32  via a fuel-inflow bore  44 . 
   In response to voltage being applied to actuator  14  via the electrical supply, actuator  14  expands in the longitudinal direction of fuel injector  1  and presses actuator head  18  with actuator tappet  22  formed thereon in the direction of valve seat  6 . The lift is restricted to a lift h by the stop of actuator-head sleeve  23  at partition shield  21 . The movement is transmitted to pressure-piston support  24  and pressure piston  33 . The fuel contained in pressure chamber  37 , being a fluid, is unable to be compressed and, thus, transmits the movement to pressure-cylinder support  35 . Due to the spring force of coupler helical spring  38  and the force of actuator  14 , coupler valve-closure member  41  is pressed onto coupler valve-seat surface  42 . This causes coupler valve-sealing seat  43  to close sealingly, and no fuel is able to escape from pressure chamber  37 . Valve needle  2  opens to the outside, lifting off from valve-sealing seat  6 . During the lift, only a gap-loss fuel quantity may escape from pressure chamber  37  through the annular gap between pressure piston  33  and pressure cylinder  34 . At the conclusion of the lift, the actuator is pressed back by actuator spring  23 , and valve-needle spring  9  presses valve needle  2  into its valve-sealing seat  6 . Corrugated tube  28 , which has-been provided with an initial stress, keeps pressure-piston support  24  sealingly against actuator-head tappet  22 . Since a small quantity of fuel from pressure chamber  37  has reached fuel chamber  32  via the annular gap and since the fuel in fuel chamber  32  is under superpressure, coupler valve-sealing seat surface  43  opens now because the diameter of the cross-sectional surface sealed by coupler valve-sealing seat surface  43  from the fuel pressure in fuel chamber  32  is smaller than the diameter of pressure piston  33 , and the spring force of coupler helical spring  38  is overcome. Pressurized fuel is now able to flow from fuel chamber  32  past coupler valve-sealing seat  43  through inflow bore  36  into pressure chamber  37 . As soon as the pressure is equalized in pressure chamber  37  and in fuel chamber  32 , coupler helical spring  38  pulls pressure piston  33  out of pressure cylinder  34  until coupler valve-closure member  41  comes to rest on coupler valve-seat surface  42  and coupler valve-sealing seat  43  is closed again. 
   Fuel injector  1  configured according to the present invention and having the described transmission path of the lifting force from actuator  14  to valve needle  2 , in this way advantageously adjusts to the expansions of valve member  4  and of valve-member upper section  17  in response to pressure fluctuations in the fuel pressure. Temperature-related expansions are also compensated for. 
   Furthermore, a malfunction of fuel injector  1 , for instance during a renewed start, may be prevented in an advantageous manner after an internal combustion engine has been turned off while still warm from operating. Fuel chamber  32  slowly loses fuel pressure once an internal combustion engine has been turned off while still warm from operation. This may lead to the evaporation of fuel in pressure chamber  37 . Without fuel injector  1  configured according to the present invention, the evaporated fuel in pressure chamber  37  would be compressed as gas during a renewed start, without generating the required pressure to open valve needle  2 . During a start of the internal combustion engine, an external pump (not shown here) first pressurizes the fuel in combustion chamber  32 . Subsequently, as described before, in a fuel injector  1  configured according to the present invention, coupler valve-sealing seat  43  is opened and fuel flows into pressure chamber  37  via inflow bore  36 . This causes cooling, and the evaporated fuel condenses.