Injector for fuel injector systems

An injector for a fuel injection system is provided with an injector housing in which a piezoelectric stack is located and with a valve housing connected with the injector housing in which a valve closing device with a jet needle is displaceably located. The valve closing device can be actuated by the piezoelectric stack. The valve closing device can be reset by a return device. A hydraulic following amplifier is located between the piezoelectric stack and the jet needle of the valve closing device. The amplifier has a displacement piston actuated by the piezoelectric stack. A control piston located downstream from the displacement piston and increases the displacement travel. A working piston that actuates the jet needle and increases the actuating force.

BACKGROUND AND SUMMARY OF THE INVENTION
 This application claims the priority of German application 198 17 320.2,
 filed in Germany on Apr. 18, 1998, the disclosure of which is expressly
 incorporated by reference herein.
 The invention relates to an injector for fuel injection systems of the type
 comprising
 an injector housing in which a piezoelectric stack is located,
 a valve housing connected with the injector housing in which a valve
 closing device, which can be operated by the piezoelectric stack, and
 provided with a jet needle, is displaceably mounted,
 a return device being provided by means of which the valve closing device
 can be returned,
 a displacement piston actuated by the piezoelectric stack being located
 between the piezoelectric stack and the jet needle of the valve closing
 device, and
 a control piston located downstream from the displacement piston that
 increases the adjustment travel.
 An injector of the above noted general type is known from German Patent
 Document DE 195 19 191 C2. A hydraulic distance transformation unit is
 located between a piezoelectric stack and the jet needle of the injector.
 This unit has a displacement piston and a control piston located
 downstream from the displacement piston. However, the fact that the
 actuating force for the jet needle decreases during the travel
 transformation is disadvantageous.
 A fuel injector for internal combustion engines is known from German Patent
 Document DE 195 00 706 A1, said valve having a hydraulic travel amplifier
 for converting a travel of the piezoelectric actuator. In this valve,
 passages that supply a fluid and carry fluid away are separate from one
 another, with the fluid being guided into an annular space by a passage
 located in the valve housing. However, the disadvantage of this injector
 is that, although the travel is amplified, the actuating force is reduced
 at the same time by the law of the lever. It is also disadvantageous that
 the passage of the fuel injector is subjected to a bending stress while
 fuel is being supplied to the annular chamber.
 Reference is made regarding additional prior art to European Patent
 Document EP 0 218 895 B1, from which a metering valve for metering fluids
 or gases with a piezoelectric actuator is known. The pressure with which
 the valve is actuated acts on the piezoelectric actuator directly. At the
 pressures of approximately 1000 bars that develop in fuel injection
 systems, exact function of the valve is no longer guaranteed because of
 losses in the actuating travel of the jet needle. It is also
 disadvantageous that, after the jet needle lifts out of the valve seat,
 the fuel sprays uncontrollably into the combustion chamber through the
 resulting gap.
 A goal of the present invention is to provide an injector of the type
 referred to above with which fuel injection can be performed with high
 accuracy and precision and without loss of fuel by transformation of the
 travel.
 According to the invention, this goal is achieved by providing an
 arrangement wherein a working piston is provided for hydraulic following
 amplification that actuates the jet needle and increases the actuating
 force.
 By using a hydraulic follower amplifier in the form of a working piston it
 is possible to decouple the system in terms of force. The travel of the
 piezoelectric stack is transmitted to a displacement piston. A control
 piston connected downstream from the displacement piston which increases
 the adjustment travel produced by the piezoelectric stack moves at a
 specified transformation ratio toward the jet needle. The jet needle is
 then actuated by a working piston that increases the actuating force.
 The travel amplification according to the invention is decoupled from the
 force because the application of force to open the jet needle comes only
 from the system pressure, for example a rail pressure. Since there is no
 loss of power in the transformation, the actuation of the piezoelectric
 stack also does not have a negative influence on the opening of the jet
 needle.
 In a highly advantageous improvement of certain preferred embodiments of
 the invention, provision is made such that a pressure compensating chamber
 is located for a hydraulic length compensation of the piezoelectric stack
 between the displacement piston and the control piston, said chamber being
 connected on one side with an overflow line of the control piston and on
 the other side with an overflow line of the displacement piston.
 The pressure compensating chamber according to the invention together with
 its hydraulic compensating volume serves to compensate temperature and
 elongation effects of the piezoelectric stack.
 In another likewise highly advantageous feature of certain preferred
 embodiments of the invention, provision can also be made for a pressure
 pad to be located between the jet needle and the working piston for
 hydraulic length compensation for the jet needle, with a length
 compensating chamber with a compensating spring being located between the
 pressure pad and the working piston.
 As a result of this design according to the invention, hydraulic length
 compensation is achieved for the jet needle, due to thermal and hydraulic
 changes in length.
 The injector according to the invention is suitable for jet needles that
 open outward as well as those that open inward using the same operating
 principle.
 Other objects, advantages and novel features of the present invention will
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS
 The injector 1 shown in FIG. 1 has an injector housing 2, a piezoelectric
 guide 3 in which a piezoelectric stack 4 is located and a valve housing
 connected with injector housing 2 by means of a union nut 5. A valve
 closing device 7 is displaceably mounted in valve housing 6.
 Valve closing device 7 has tappet 8 as a jet needle with a valve stem 9
 into which tappet 8 fits.
 At the end of the valve stem 9 facing the combustion chamber, a sealing
 member is provided in the form of a shoulder 10. Valve housing 6, shoulder
 10, and a separating device connected with valve stem 9, which is designed
 as a pressure compensating cylinder 11, form an annular gap 12 that is
 filled with fuel during operation. When valve 1 is open, a precisely
 metered quantity of fuel is sprayed from annular gap 12 into a combustion
 chamber, not shown in the drawing. For this purpose, a flow restricter 13
 is used that is pressed by a spring device 14 against a cross-sectional
 area of shoulder 10 of valve stem 9. Spring device 14 abuts a cylindrical
 stop 15.
 An annular chamber 16 is formed between piezoelectric guide 3 and injector
 housing 2, in which chamber a line 17 that supplies fuel to valve 1
 terminates. From here the fuel flows through bores 18 into annular gap 12.
 Piezoelectric stack 4 is located completely in the low-pressure area of
 passages that carry fuel away and therefore is not adversely affected by
 the fuel supplied at very high pressure. The reverse flow of fuel in this
 pressure area takes place in an annular chamber 19 where it escapes from
 the end of piezoelectric stack 4 that faces away from the combustion
 chamber.
 If a control voltage is applied to piezoelectric stack 4, it produces in
 known fashion an elongation of piezoelectric stack 4, causing valve
 closing device 7 to open, since a corresponding gap results between
 shoulder 10 of valve stem 9 and a valve seat 6 and/or the flow restricter
 13. To end the injection process, the control voltage is switched off,
 whereupon piezoelectric stack 4 again shrinks to its original length. Jet
 needle 8 is returned by a jet needle spring 51 that abuts an annular bead
 55 of jet needle 8.
 FIG. 2 shows the transmission of force from piezoelectric stack 4 to jet
 needle 8 to open it. Piezoelectric stack 4 is surrounded by a protective
 tube 20 provided with a seal 20A on the end. The sealing cap 20A of
 protective tube 20 is located axially between piezoelectric stack 4 and a
 displacement piston 21, and thus actuates the piston when piezoelectric
 stack 4 lengthens. A control piston 22 is located axially in front of
 displacement piston 21 relative to the combustion chamber. Control piston
 22 has a smaller effective pressure area than displacement piston 21. The
 hydraulic transformation ratios result from the different geometries
 and/or diameter ratios of the displacement piston 21 and control piston
 22. A piezoelectric stack pretensioning is produced by a plurality of cup
 springs 23 arranged one behind the other, said springs being located in a
 pressure compensating chamber 24. Pressure compensating chamber 24 is
 filled with test oil or with fuel. Filling and/or pressure compensation
 are performed by deliberate leaks between control piston 22, displacement
 piston 21, and the surrounding cylindrical housing 25. A feed 26
 terminates in cylindrical housing 25, said feed being connected with the
 annular supply chamber 16. In this fashion, cylindrical housing 25 is
 mounted axially and nonrotatably. As a result of the specified
 transformation ratio between displacement piston 21 and control piston 22,
 control piston 22 is moved more than displacement piston 21.
 An annular chamber 29 is supplied with system pressure (rail pressure) from
 annular chamber 16 from supply line 26 by an annular groove 27 and a
 diagonal bore 28 located in control piston 22. Annular chamber 29 is
 formed between control piston 22 and a sliding sleeve 30.
 If piezoelectric stack 4 receives a control voltage, the protective tube
 20, displacement piston 21, and control piston 22 are displaced in the
 direction of arrow B. A leading control edge 31 opens between control
 piston 22 and sliding sleeve 30, producing a high-pressure connection
 through annular chamber 29 with a bore 32 in sliding sleeve 30 and
 therefore to a working cylinder and/or working pressure chamber 33
 connected therewith, which is located radially between sliding sleeve 30
 with trailing control edge 36 and cylindrical housing 25 and axially
 between one end of cylindrical housing 25 and a working piston 34. As a
 result of working pressure chamber 33 being charged with high pressure,
 working piston 34 is displaced in the same direction as control piston 22
 in the direction of arrow B. As a result of the pretensioning spring 35,
 sliding sleeve 30 follows working piston 34 and seals off pressure chamber
 33 with trailing control edge 36. Sliding sleeve 30 follows the working
 piston 34 until it again strikes the leading control edge 31 between
 control piston 22 and sliding sleeve 30 and/or blocks this control edge.
 As a result, the working pressure chamber 33 is hydraulically tight and a
 working piston remains in this position. As may be seen, displacement
 piston 21 specifies the path for the following amplifier consisting of
 displacement piston 21, control piston 22, sliding sleeve 30, and working
 piston 34, which is then switched to jet needle 8.
 Because of the differences in diameter of the effective piston areas
 between displacement piston 21 and control piston 22, control piston 22
 travels a greater distance.
 If the control voltage is removed from piezoelectric stack 4, displacement
 piston 21 will be pushed back by the cup springs 23. The increase in
 volume in pressure compensating chamber 24 enables return spring 52,
 pretensioned between jet needle 8 and an axial depression in the end of
 control piston 22, to push control piston 22 backward together with
 sliding sleeve 30 against the direction of arrow B. As a result, an
 annular gap 38 is produced between trailing control edge 36 and working
 piston 34 that makes it possible for oil to flow out from working cylinder
 33 in the direction of pressure pad 42 and further into annular chamber
 19. The escaped amount allows working piston 34 to return to its starting
 position.
 A hydraulic length compensating chamber 39 for jet needle 8, produced by
 thermal and hydraulic changes in length, is thus formed by cylindrical
 housing 25, working piston 34, compensating spring 40, compensating bore
 41, and pressure pad 42. Changes in length and therefore changes in volume
 are compensated by bore 41. In this manner, even if jet needle 8 is
 compressed, working piston 34 always abuts the return control edge.
 Protective tube 20 has the purpose of ensuring that the piezoelectric stack
 4 does not come in contact with fuel.
 A hydraulic length compensation of piezoelectric stack 4 is achieved by the
 deliberate leakage 73 of control piston 22 and a capillary 74 machined in
 the outside diameter of displacement piston 21 through which leakage
 reaches the return line and/or annular chamber 19.
 For practical purposes, there are two systems, one on the piezoelectric
 stack side and the other on the jet needle side, with the parts always
 being under pretension and therefore always ensuring a contact, regardless
 of lengthwise expansion effects or temperature differences. It is also
 important in this respect that the overflow feed into pressure
 compensating chamber 24 roughly corresponds to the amount that escapes
 from it through the overflow line in displacement piston 21 (capillary).
 This also means that the pressure in pressure compensating chamber 24 must
 be lower than the spring force of return spring 52. Cup springs 23 ensure
 that the displacement piston 21 always abuts the piezoelectric stack 4 and
 the piezoelectric stack 4 is simultaneously pretensioned.
 The mechanical performance of piezoelectric stack 4 is used exclusively for
 valve positioning. In other words, this means that the increase in force
 has nothing directly to do with piezoelectric stack 4. Therefore, it is
 not the piezoelectric force that is used to actuate jet needle 8, but only
 the pressure developed in the pressure chamber of working cylinder 33, and
 this pressure is proportional to the actuating force.
 The embodiment described above relates to a jet needle 8 that opens
 outward, while the direction of travel of piezoelectric stack 4
 corresponds to the direction of travel of the opening of the jet. It is
 advantageous to keep the loss of oil through lengthwise groove 19 to 3 to
 5 bars counterpressure (cavity formation, cavitation).
 FIGS. 3 and 4 show an injector in which jet needle 8' opens inward to
 inject fuel. This means that the actuating direction of piezoelectric
 stack 4' is opposite to the direction of actuation of jet needle 8'. In
 this embodiment, we have used the same reference numbers with a
 corresponding superscript for those parts that have the same functions as
 in the embodiment according to FIGS. 1 and 2. That is, the injector 1',
 injector housing 2', guide 3', valve housing 6', valve closing device 7'
 and protective tube 20' correspond in function to FIGS. 1 and 2.
 In contrast to the embodiment according to FIG. 1, an annular line 16 is
 not provided for supplying rail pressure, but a stub 43. An overflow line
 44 is provided to return fuel. The piezoelectric pretensioning can be set
 in pressure compensating chamber 24' by cup springs or coil springs 23'.
 In this injector system, the direction of travel must be reversed when
 piezoelectric stack 4' is actuated. In this case, the space in which a
 spring 56 is located is only a vent space. The pressure compensating
 chamber 24' on the other hand is compressed with a control voltage on
 piezoelectric stack 4.
 In addition, a difference in diameter is operational in pressure
 compensating chamber 24'. The difference in the diameters of the effective
 piston areas of compensating piston 21' and control piston 22' in order to
 achieve the desired transformation ratios and hence a greater travel for
 control piston 22', result from a smaller effective end area 46 that acts
 in the direction of piezoelectric stack 4', by comparison with an
 effective end area of 21', which is directed toward jet needle 8'. If the
 pressure compensating chamber 24' is made smaller by a control voltage on
 piezoelectric stack 4', a pressure buildup occurs in this chamber that
 actuates control piston 22' opposite to the direction of action of
 piezoelectric stack 4' in the direction of arrow C. With control piston
 22' in this displacement direction, it carries sliding sleeve 30' in
 direction C as well. As a result of this displacement, pressure release
 occurs in a working cylinder 33' which corresponds to the working cylinder
 in the embodiment shown in FIGS. 1 and 2. The pressure relief occurs in
 working cylinder 33' into overflow line 44 through bores 48 in working
 piston 34'. Since the direction is reversed in this embodiment, it means
 that the leading control edge 31' closes jet needle 8' and trailing
 control edge 36' between sliding sleeve 30' and working piston 34' opens
 jet needle 8' and hence creates a connection between supply line 43 and
 injection holes 49 for injecting fuel.
 To close injection holes 49 following elimination of the control voltage
 from piezoelectric stack 4', a pressure buildup again occurs via leading
 control edge 31' in working cylinder 33', since sliding sleeve 30'
 encounters working cylinder 34' by return control edge 36', interrupting
 the connection to overflow line 44. This means that when jet needle 8' is
 in its closed position, the full system pressure is available in the
 pressure chamber of working cylinder 33', since the pressure chamber of
 working cylinder 33' is supplied with the full system pressure through
 diagonal bores 53 in sliding sleeve 30' by means of leading control edge
 31' in conjunction with supply line 26' and an annular chamber 50 between
 sliding sleeve 30' and control piston 22'. If working piston 34' shifts
 slightly, leading control edge 31' opens immediately and forms the
 connection to the high-pressure side at this edge. It is only when control
 piston 22' is displaced in direction C as a result of a control voltage
 being applied to piezoelectric stack 4' that the pressure in working
 cylinder 33' drops accordingly and jet needle 8' can open to inject fuel.
 The fuel supply for the pressure compensating chamber 24' comes through a
 connecting passage 54 in control piston 22' to the feed 26.
 Just as in the case of the coil spring 35 in the embodiment shown in FIGS.
 1 and 2, sliding sleeve 30 is pressed by a cup spring 35' against working
 piston 34'. The control piston 22' is returned by a cup spring 52' that
 abuts working piston 34'.
 It is also advantageous in this regard to keep the flow of overflow oil
 through lengthwise groove 19 to 3 to 5 bars counterpressure.
 The foregoing disclosure has been set forth merely to illustrate the
 invention and is not intended to be limiting. Since modifications of the
 disclosed embodiments incorporating the spirit and substance of the
 invention may occur to persons skilled in the art, the invention should be
 construed to include everything within the scope of the appended claims
 and equivalents thereof.