Patent Publication Number: US-6213414-B1

Title: Fuel injector

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel injector having an actuator for generating a lifting movement and a lift translator (lift transfer element) filled with a hydraulic medium and deformable in the lifting direction, for transferring the lifting movement of the actuator to a valve needle. 
     BACKGROUND INFORMATION 
     German Patent No. 195 00 706 describes a fuel injector. In this context, provision is made for a lift translator or travel transformer which converts a relatively small actuator travel, for example, of a piezoelectric actuator, into a longer lift of the valve needle. In this context, the expansion of the actuator is introduced into an amplifier chamber via a working piston of the lift translator, and transmitted to the valve needle via a lift piston, the valve executing a lift which is increased in the ratio of the piston areas bordering the amplifier chamber at the end faces. To compensate for temperature influences, wear, and manufacturing tolerances on the actuator travel, the amplifier chamber is provided with a defined leak, which is implemented by a ring gap between the piston and a valve-housing wall, and has a sufficiently high resistance to flow so that the movements of the actuator are transmitted to the valve needle in an essentially undamped manner. Used as hydraulic medium of the lift transmission is the fuel itself, diesel fuel in the case of German Patent No. 195 00 706. However, this creates the problem that the defined leak must be very small because of the low viscosity of the fuel. If the intention is for the known fuel injector to be used for Otto spark ignition engines, the further problem arises that the gasoline boils at comparatively low temperatures and can then no longer work as hydraulic medium because of the formation of bubbles. 
     SUMMARY OF THE INVENTION 
     The fuel injector according to the present invention has a compensating chamber for the hydraulic medium, the compensating chamber being connected to an interior space of the lift translator via a throttle opening and enclosed by a casing, and the fuel injector having the advantage that quasistatic deformations of the actuator because of temperature influences can be compensated for by an exchange of the hydraulic medium between the interior space of the lift translator and the compensating chamber. By choosing a sufficiently small throttle opening, however, the hydraulic medium contained in the lift translator acts as an incompressible liquid during the relatively fast actuator actuation so that the fast movements of the actuator are transmitted to the valve needle in an essentially undamped manner. 
     The fuel injector according to the present invention has the advantage that the hydraulic medium can be selected independently of the fuel so that the hydraulic medium can have a high boiling point and a relatively high viscosity. In this manner, the hydraulic medium can be prevented from evaporating. A higher viscosity permits a larger opening diameter of the throttle opening, which, because of this, can be manufactured in a simpler manner from a standpoint of production engineering. 
     The preferably elastic casing of the compensating chamber is preferably composed of a thin-walled metal body which is welded to the lift translator in a liquid-tight manner. The compensating chamber can radially surround a small-diameter section of the lift translator, thereby enabling a compact design. 
     The outer wall of the lift translator is preferably designed as a bellows. This has the advantage for the outside wall to be deformable only in the lifting direction and not in the radial direction. Because of this, the lifting movements of the actuator are transmitted to the valve needle without damping. 
     Suitable as hydraulic medium is a liquid which boils at high temperature so that the hydraulic medium is prevented from evaporating. The diameter of the throttle opening is preferably selected as a function of the viscosity of the hydraulic medium in such a manner that the lifting movement of the actuator is transmitted in an essentially undamped manner; however, quasistatic deformations are compensated for by exchange of the hydraulic medium with the elastic compensating chamber. 
     The actuator is preferably a stacked piezoelectric actuator permitting lifting movements which are defined in time and space. 
     The fuel injector according to the present invention is particularly suitable for an injection system for an Otto spark ignition engine, since gasoline is not suited as hydraulic medium of the lift transmission because of the low boiling point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a longitudinal section through an exemplary embodiment of a fuel injector according to the present invention in a simplified representation. 
     FIG. 2 shows an enlarged representation of a lift translator of the fuel injector of FIG.  1 . 
     FIG. 3 shows an enlarged representation of section III in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an exemplary embodiment of a fuel injector  1  according to the present invention in a longitudinal section. A housing  11  accommodates a, for example, stacked piezoelectric actuator  2  in a radial bore, the piezoelectric actuator being caused to generate defined lifting movements (schematically represented by an arrow  13 ) via an electric control device (not shown). The electric control signal for controlling actuator  2  is supplied via a plug-in connection  14 . The hydraulic connection is carried out, for example, via a cross bore  25 . A hydraulic lift translator  3  transmits the lifting movement from actuator  2  to an axially movable valve needle  4 . A valve-closure member  15  joined to valve needle  4 , together with a valve-seat face  16  formed on a valve seat  5 , forms a sealing seat. A restoring spring  12  keeps the sealing seat closed. 
     FIG. 2 shows lift translator  3  of fuel injector  1  in an enlarged representation. In this context, actuator  2 , via a small lift h 1 , acts upon a large area A 1  of lift translator  3 , which is designed as a bellows  7  filled with a hydraulic medium  6 , and, because of the hydraulic filling, generates a correspondingly increased lift h 2 =h 1 ·A 1 /A 2  at the smaller area A 2  at the end facing away from actuator  2 , and transmits lift h 2  to valve needle  4 . In this context, bellows  7  must be designed sufficiently rigidly so that a motion is possible only in the lifting direction but not in the radial direction. Thus, the forces can be transmitted from piezoelectric actuator  2  to valve needle  4  dynamically and essentially without damping. In this context, bellows  7  has a plurality of alternating regions  19  and  20  which are narrowed and widened, respectively, in the radial direction. In section  21  adjoining actuator  2 , in the exemplary embodiment over three widened regions  20  and two narrowed regions  19 , bellows  7  has a larger diameter. Contiguously joining up thereto is a small-diameter section  17  which has a reinforcement  22  at the extremity. 
     Small-diameter section  17  of lift translator  3  is radially surrounded by an elastic casing  10  made of thin-walled metallic material and welded to lift translator  3  in a liquid-tight manner. In this context, casing  10  can be welded to a shoulder  24  which forms the transition between section  21  having a larger diameter and small-diameter section  17 . Casing  10  encloses a closed compensating chamber  9 , which is connected to an interior space  18  of lift translator  3  via a throttle opening  8  so that hydraulic medium  6  can flow back and forth between interior space  18  of lift translator  3  and compensating chamber  9  through throttle opening  8 , as follows particularly from FIG.  3 . 
     Dividing wall  23  between compensating chamber  9  and interior space  18  is formed by bellows  7 . Thus, compensating chamber  9  forms an elastic reservoir for hydraulic medium  6  for compensating for quasistatic influences on the travel, particularly by temperature-related linear expansion, but also because of manufacturing tolerances or symptoms of wear. However, throttle opening  8  is selected, as a function of the viscosity of used hydraulic medium  6 , to be small enough so that hydraulic medium  6  contained in lift translator  3  represents an incompressible liquid during a short time interval. Thus, actuator movements having a duration in the millisecond range are transmitted without a relevant pressure compensation with compensating chamber  9  taking place. However, thermal expansions of actuator  2  (&gt;1 second) can be compensated for by liquid displacements. In this context, the connected system composed of actuator  2 , lift translator  3 , and valve needle  4 , is preloaded by restoring spring  12  (FIG.  1 ). 
     Fuel injector  1  according to the present invention allows a medium different from the fuel to be used as hydraulic medium  6  in lift translator  3 . In this manner, a hydraulic medium  6  can be selected which is optimally suitable for the purpose of application with regard to viscosity and evaporation properties. Suited as hydraulic medium is, in particular, a relatively viscous liquid having a high boiling point such as a hydraulic oil. 
     Fuel injector  1  according to the present invention allows slow deformations in the lifting direction, which are caused, for example, by temperature influences, to be compensated for, while the comparatively fast lifting movements of actuator  2  are transmitted to valve needle  4  dynamically and essentially without damping in an amplified manner.