The present invention relates to a metering device for dosing pressurized fluids, particularly an injection valve for a fuel injection system in an internal combustion engine. The metering device is of the type which comprises a housing having an end part provided with an outlet passage terminating with a metering opening, an axially moveable valve needle passing through the outlet passage and controlling opening and closing of the metering opening, and a piezoelectric actuator in axial alignment with the valve needle and cooperating with the valve needle to control its axial movement. The invention further relates to a method for setting a flow rate of such a metering device.
The European Patent application EP 1 046 809 A2 discloses an injection valve of this type. For these injection valves it is essential that the flow rate delivered by the injector can be set to a defined value at the end of the assembly process in the factory. Presently, the calibration of such an injector is carried out in a state where the injector is not completely assembled and welded.
The injector to be calibrated is introduced in an appropriate blocking tool. The regulation of the flow rate through the injector is carried out by inserting one or more calibrated spacer rings in the valve spring blockage and repeatedly measuring the flow rate until a desired flow rate is reached. After the calibration process is completed, the injector proceeds to the final welding of the inlet fitting on the housing. Such an iterative operational sequence is time consuming and expensive and may not be adapted to the mass production in a factory.
The current process is also extremely complex, because several process variables such as the length and the elongation of the piezoelectric stack, the dimensional tolerances of the parts involved, and the chamber height of the thermal compensator have to be taken into account. Further, as the spacer rings have to be changed repeatedly, it is not possible to calibrate the injector during a flow phase of the fluid.
A major drawback of the current solution arises from the inability to adjust the flow of the injector after the completion of the whole assembly process. The current type of flow adjustment is feasible only before the final welding of the housing and the valve body of the injector. Therefore, if it is found in the post calibration run that an injector does not meet the flow requirements, no further corrections are possible, and the injector has to be discarded.
DE 37 35 288 A1 discloses an electromagnetic fuel injection valve with a beam which is fixed to a valve upper part by laser welding. The injection now further comprises a needle, which is preloaded into a closing position by a spring. In addition to that a coil is provided being part of an electromagnetic circuit exerting a force on the needle if a current is applied to the coil. For adjusting a lift of the needle punches are pushed through recesses of the valve upper body to plastically deform the beam and in that way to change its axial length.
WO 01/90570 A1 discloses a fuel injector with a check valve member, which is hydraulically operated by respectively changing a pressure of a fuel. In order to perform a preinjection or a microinjection a solid state motor is provided to lower a check stop, so that when fuel pressure in the nozzle chamber is raised the check valve member is limited to lift a much smaller distance.
US 6,385,848 B1 discloses a method of setting a distance between a first body and a second body in a fuel injector. The method includes providing an intermediate body having a first end, a second end and a longitudinal axis, the first end being fixedly connected to the first body and a second end being fixedly connected to the second body. The intermediate body is compressed towards the longitudinal axis. The compression axially elongates the intermediate body such that the first body is separated from the second body.
DE 199 32 762 A1 discloses an injection valve with a needle, that rests in a closing position on a seat, the seat being connected to a valve seat carrier. The valve seat carrier is plastically deformed in order to change the lift of the needle.
In view of the foregoing, it is an object of the present invention to improve the flow adjustment process for a metering device of the above mentioned type.
This object is achieved by a metering device with the features of appended claim 1, and by the method for setting a flow rate of a metering device of independent claim 8.
Advantageous embodiments of the invention are disclosed in the dependent claims.
According to the invention, in a metering device of the type mentioned above, a plastically deformed adjustment element is arranged axially aligned between a bottom end piece of the piezoelectric actuator and a head of the valve needle, wherein a plastic deformation of the adjustment element regulates the axial spacing between the piezoelectric actuator and the valve needle, thereby setting a flow rate for the metering device.
The invention is thus based on the idea to provide a deformable adjustment element in the metering device allowing for a flow rate calibration even after the metering device is completely assembled.
In a preferred embodiment of the invention, the adjustment element comprises a metal element shaped such that a radial compression of the element causes an axial elongation thereof. The flow rate can then be set by driving two or more shaped punches radially inward through the housing of the metering device, thereby radially compressing and axially elongating the adjustment element. As the relative movement of the upper and lower face of the metal element is based on a controlled deformation of the metal element, the desired lift of the needle and thus the desired flow rate can be achieved.
Advantageously, the adjustment element has a first contact area for mounting the adjustment element to the bottom end piece of the piezoelectric actuator, a plastically deformable crimping area, the radial compression of which causes an axial elongation of the adjustment element, and a second contact area for contacting the head of the valve needle.
In a further preferred embodiment of the invention, the first contact area of the adjustment element and the bottom end piece of the piezoelectric actuator comprise corresponding engagement means to mount the adjustment element to the piezoelectric actuator.
The second contact area of the adjustment element may advantageously comprise a hardened hemispherical head pin to provide good contact with the needle head.
In an especially preferred embodiment, the adjustment element is formed as a hollow frustroconical and biconical body with a central cylindrical area.
Preferably, two or more through holes are provided in the housing of the metering device, which are radially aligned with a crimping area of the adjustment element. The through holes provide access to the adjustment element and allowing plastic deformation thereof after the complete assembly of the metering device.
According to the invention, in a method for setting a flow rate of any of the metering devices described above, the flow rate through a completely assembled metering device is repeatedly measured and the adjustment element is gradually radially compressed until a predetermined flow rate through the metering device is achieved.
In order to radially compress the adjustment element, preferably one or more punches are inserted in the through holes of the housing and driven inwardly, thereby inducing an axial adjustment of the spacing between the piezoelectric actuator and the valve needle.
In a preferred embodiment, the punch or the punches are driven inwardly by imposed strokes. Alternatively, the punch or the punches may be driven inwardly by an imposed load.
The advantages gained by the technical features of the invention include
The invention, both its construction an its method of operation together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawings, wherein
Figure 1 illustrates an injection valve 10 for direct-injection gasoline engines. The injection valve 10 has a housing 12, whose lower part has an outlet passage 14 terminating with a metering opening 16.
Among other elements that are not pertinent to the present invention, the housing 12 contains a piezoelectric actuator 18. An excitation voltage is applied to the piezoelectric actuator 18 to open the injection valve 10 and inject gasoline into the engine cylinder. In response to the excitation voltage, the piezoelectric actuator 18 increases in length in axial direction by a predetermined amount. The length extension is transmitted to a valve needle 20 disposed in the outlet passage 14. The needle 20 depresses a biasing spring and lifts from its seat to start the injection of pressurized gasoline in the engine cylinder. When the excitation voltage is terminated, the length of the piezoelectric actuator 18 decreases to its normal value and the valve needle 20 is pushed back in its closing position.
In the embodiment of Fig. 1, a plastically deformable adjustment element 22 is arranged axially aligned between a bottom action cap 24 of the piezoelectric actuator 18 and a head 26 of the valve needle 20. As best seen in the close-up view of Fig. 2, the adjustment element 22 is formed as a hollow frustroconical and biconical metal body 30 with a central cylindrical area 32. As evident from Fig. 2, the metal body 30 has a shape for which a radial compression of the body 30 causes an axial elongation of the adjustment element 22.
Fig. 3 shows an exploded view of the adjustment element 22 together with the piezoelectric actuator 18. The bottom end piece 24 of the piezoelectric actuator 18 comprises an engagement element 38, which allows the metal body 30 to be easily mounted to the piezoelectric actuator 18 by geometrical interference of the contact area 39 (Fig. 2) of the adjustment element 22 with the corresponding engagement element 38.
The adjustment element 22 further comprises a hardened hemispherical head pin 34 to optimize the contact with the needle head 26. As the piezoelectric actuator 18, the hemispherical head pin 34 comprises an engagement element 36 allowing the metal body 30 to be easily mounted to the hemispherical head pin 34 by geometrical interference.
Now again with reference to Fig. 1, two through holes 28 and 29 are provided in the housing 12 of the injection valve 10 to provide access to the adjustment element 22 and allowing plastic deformation thereof after the complete assembly of the injection valve 10. The two through holes 28 and 29 are radially aligned with the central crimping area 32 of the adjustment element and are spaced by 180° along the circumference of the housing 12. While only two through holes are shown in the embodiment of Fig. 1, it will be appreciated that three or more appropriately spaced through holes may be provided in the housing 12 as well.
The flow rate of the injector valve 10 can then be adjusted after it is completely assembled and welded. In this state the through holes 28 and 29 are initially closed by plastic plugs for packaging purposes.
At first, the pre-adjusted lift and/or flow rate is measured. If the desired flow rate is not attained, the plastic plugs are removed and one or more punches are inserted in the through holes 28 and 29 and driven inwardly by applying strokes or a constant load. Thereby, the metal body 30 of the adjustment element 22 is gradually deformed.
The resulting plastic deformation of the metal body caused by the action of the punches induces an axial elongation of the of adjustment element 22 and therefore increases the spacing between the piezoelectric actuator 18 and the needle 20. As the relative movement of the upper and lower face of the metal body 30 is based on a controlled deformation, the desired lift and the desired flow rate can be achieved.