Patent Application
20070023542
In addition to unit injector fuel injection systems, modern autoignition internal combustion engines also currently use high-pressure accumulator (common rail) injection systems. In high-pressure accumulator injection systems, the individual fuel injectors respectively associated with the cylinders of the internal combustion engine are supplied with fuel from a high-pressure accumulator (common rail). The fuel injectors can be actuated either by means of solenoid valves or by means of piezoelectric actuators. If the fuel injectors are actuated by means of piezoelectric actuators, then it is also possible to produce an injection valve member that can be actuated directly by means of the piezoelectric element.
DE 697 20 145 C2 has disclosed an injection valve. The injection valve has a valve needle that a spring contained inside a spring chamber presses against a seat surface. The spring is clamped between a movable stop and a spring support connected to the valve needle. A constricted flow path is provided, through which a limited quantity of fuel can flow from the spring chamber at a limited speed. The injection valve also has a valve that includes a moving stop surface; this valve can be actuated during operation of the injection valve in such a way that a second, higher quantity of fuel can flow from the spring chamber at a second, higher speed. The valve is comprised of a seat surface that is situated around an opening that communicates with the spring chamber; the movable stop can come into contact with the seat surface so that it is possible to control the fuel flow through the opening. The movable stop can be designed to move in reaction to the fuel pressure inside a pump chamber.
In fuel injectors with directly controllable injection valve members, in order for an actuator to be able to open the injection valve member, it is necessary for the actuator to overcome a powerful opening force. The powerful opening force required, which is to be exerted by means of the actuator, is due to the fact that the injection valve member, which can be embodied in the form of a nozzle needle, is pressed into its seat through exertion of system pressure (pressure level in the high-pressure accumulator). The forces required to lift the injection valve member away from its seat are usually several hundred N, for example 400 N. In order to assure a sufficient fuel flow through the injection openings into the combustion chamber of an autoignition internal combustion engine when the injection valve member is fully open, it is also necessary for the injection valve member to execute a maximum stroke of several 100 μm, e.g. in the range between 200 μm and 300 μm. The above-mentioned values, i.e. the force of several hundred N required to open the injection valve member and the maximum achievable stroke of the injection valve member from its fully closed position into its fully open position, are essentially the determining parameters for the size of a piezoelectric actuator to be integrated into a fuel injector. Although the length/diameter ratio of the piezoelectric actuator can in fact be varied by means of an integrated hydraulic transmission, the size of the actuator, also referred to as actuator volume, is essentially proportional to the opening force to be exerted and to the maximum stroke to be executed by the injection valve member, which can be embodied in the form of a needle.
By means of a variable transmission mechanism, the embodiment according to the present invention makes it possible for the forces that are required to move the needle-shaped injection valve member to be adapted to the forces of an actuator that is integrated into the fuel injector for direct triggering of the injection valve member. It is thus possible to make optimal use of the actuator volume, i.e. its size, and to keep the actuator, which is to be integrated into the fuel injector, very small. In addition, with the embodiment according to thefv present invention, in a fuel injector that is actuated by a piezoelectric actuator and triggers the injection valve member directly, small injection quantities can be achieved in a stable fashion since the variable transmission mechanism functions like an intermediate stroke stop for the injection valve member that can be embodied in the form of a needle. In general, intermediate stroke positions of an injection valve member, i.e. intermediate strokes, that must be produced in ballistic operating positions of the injection valve member without the latter being supported against a stop, are difficult to maintain and extremely difficult to reproduce. The variable transmission mechanism according to the present invention makes it possible to stabilize this critical operating state of an injection valve member embodied in the form of a needle, i.e. makes it reproducible.
The achievement of an intermediate stroke position situated between the closed position of the injection valve member and the open position of the injection valve member also takes into account the fact that if a preinjection is to be executed or if small quantities are to be injected, a definite stroke of the injection valve member must be produced, even when there are fluctuations, i.e. variations, in the triggering voltage of the piezoelectric actuator.
The embodiment according to the present invention will be explained in detail below in conjunction with the drawings.
It is clear from the depiction according to Fig. that a fuel injector 1 has an injector body 2 and a nozzle body 3. The nozzle body 3 and the injector body 2 are connected to each other in a sealed fashion by means of a clamping sleeve 4, for example at a screw connection 5.
The injector body 2 of the fuel injector 1 is provided with a high-pressure fitting 6 in which fuel at system pressure, i.e. fuel at the pressure prevailing in the high-pressure accumulator (common rail), flows into a cavity 7 of the injector body 2. The system pressure is labeled pCR. From the cavity 7, which contains a piezoelectric actuator 8, the fuel at system pressure flows through a high-pressure inlet 22 to a high-pressure chamber 21. From there, the fuel at system pressure pCR flows via open areas 19 embodied on the circumference of a needle-shaped injection valve member 9, to an annular gap 20 whose end oriented toward the combustion chamber can be provided with injection openings not shown in
Between the injector body 2 and the nozzle body 3, is a disk 12 that functions as a guide for a piston 10 of the variable transmission mechanism. The piston 10 is encompassed by the disk 12 and prestressed by means of a piston spring 11. One end of the piston spring 11 rests against the upper, flat side of the disk 12 and the other end rests against the underside of the piezoelectric actuator 8. The diameter of the piston 10 is labeled dA.
The end of the piston 10 protrudes into a coupling chamber 13. The coupling chamber 13 contains a spring element 15 that can be embodied, for example, in the form of a coil spring. Underneath the coupling chamber 13, the nozzle body 3 contains a preliminary stroke sleeve labeled with the reference numeral 17. A spring element 23 presses the preliminary stroke sleeve against a collar 14 in the upper region of the injection valve member 9 that can be embodied in the form of a needle. In the coupling chamber 13, the spring element 15 is centered on a centering pin 16 above the collar 14 on the injection valve member 9. The spring element 15, which can be embodied for example in the form of a coil spring, acts on the injection valve member 9, which can be embodied in the form of a needle, in the closing direction, i.e. moves it into its seat, which is labeled with the diameter dS.
The diameter of the engine valve member 9, which can be embodied in the form of a needle, is labeled dN and the outer diameter of the preliminary stroke sleeve 17 is labeled dV.
The spring element 15 presses the injection valve member 9 into its seat at the combustion chamber end of the nozzle body 3. The spring element 23 in the high-pressure chamber 21 continuously presses the preliminary stroke sleeve 17 against the collar 14 on the injection valve member 9, which can be embodied in the form of a needle, which establishes a definite starting position for the preliminary stroke sleeve 17, represented by the definite distance hV. In the following, it is assumed that the piezoelectric actuator 8 of the fuel injector 1 is being supplied with voltage, i.e. its piezoelectric crystals have lengthened in the vertical direction.
If the voltage U in the piezoelectric actuator 8 is reduced, then the piston 10 moves out from the coupling chamber 13 due to the action of the piston spring 11 on the piston 10. This reduces the pressure p in the coupling chamber. The more the voltage U in the piezoelectric actuator 8 is reduced, the greater the reduction in the pressure p in the coupling chamber. When a critical opening pressure pÖ has been reached, the needle-shaped injection valve member 9 opens. The opening pressure pÖ,1 is defined by the system pressure pCR, the seat diameter dS of the injection valve member 1, which can be embodied in the form of a needle, and the diameter dV of the preliminary stroke sleeve 17 according to the following equation:
pÖ,1=PCR(dV2−dS2)/dV2.
A relatively large definite diameter dV of the preliminary stroke sleeve 17 results in rather a high opening pressure pÖ of the injection valve member 9, which can be embodied in the form of a needle. Because of this fact, the voltage U in the piezoelectric actuator 8 need only be reduced slightly until the needle-shaped injection valve member 9 opens (also see
The needle-shaped injection valve member 9 then moves together with the preliminary stroke sleeve 17 and thus at a slower speed than the piston 10. The resulting speed transmission ratio is determined by the transmission ratio i1=dA2/dV2.
On the other hand, the opening force resulting from the pressure infiltration of the seat (dS) of the injection valve member 9, which can be embodied in the form of a needle, is reduced at the same ratio and acts on the piezoelectric actuator 8.
Only when the upper end of the preliminary stroke sleeve 17 has reached the stop 18 at the flat surface at the bottom of the disk 12 does the transmission ratio change from i1 to i2=dA2/dN2.
Further opening of the injection valve member 9, which can be embodied in the form of a needle, requires further reduction of the pressure in the coupling chamber 13, i.e. further reduction of the voltage U in the piezoelectric actuator 8.
Once a second critical opening pressure pÖ,2 has been reached, the nozzle needle-shaped injection valve member 9 opens and follows the movement of the piezoelectric actuator 8 with the currently effective transmission ratio i2. The second critical opening pressure pÖ,2 depends essentially on the level of the pressure below the (partially open) nozzle seat dS and therefore cannot be precisely indicated.
If it is necessary to inject only a second, smaller injection quantity to the combustion chamber of the autoignition internal combustion engine, then the needle-shaped injection valve member 9 advantageously remains against the stop 18 of the preliminary stroke sleeve 17 on the flat surface at the bottom of the disk 12 until the voltage U in the piezoelectric actuator 8 is increased again, which results in a closing of the needle-shaped injection valve member 9.
By contrast with the embodiment according to
The collar 14 is situated in the upper region of the needle-shaped injection valve member 9. A first shim ring 32 is accommodated between the upper end of the preliminary stroke sleeve 17 and the bottom of the collar 14 and a second shim ring 33 is situated at the bottom end of the preliminary stroke sleeve 17. The second shim ring 33 is provided with one or more openings 34 so that fuel flowing into the high-pressure chamber 21 via the high-pressure inlet 22 at system pressure pCR can pass through the second shim ring 33. From the high-pressure chamber 21, the fuel flows along the annular gap 20 in the direction toward the injection openings situated at the combustion chamber end of the fuel injector 1. The injection openings can be embodied in the form of a single row of holes or in the form of several rows of holes situated concentric to one another.
The sequence of
At time t0, the actuator voltage U in the piezoelectric actuator would be Umax, i.e. the piezoelectric crystals of the piezoelectric actuator are supplied with the maximum current and therefore are elongated to a maximum degree. At time t0, the actuator stroke HA is h1 and the coupling chamber pressure p at time t0 is PCR (rail pressure). At time t0, the injection valve member 9, which can be embodied in the form of a needle, is fully open.
At time t1, the maximum voltage Umax falls to a critical value Ukrit. As a result, the elongation of the piezoelectric crystals of the piezoelectric actuator 8 diminishes by a small amount. The piston 10 travels out from the preliminary stroke sleeve 17 so that at time t1, a pressure p in the coupling chamber 13 falls by Δp1. As a result, the injection valve member 9, which can be embodied in the form of a needle, begins its opening movement. At time t2, the pressure p in the coupling chamber 13 is pH, which corresponds to a holding pressure, see pH=pÖ,2.
With a further decrease in the actuator voltage U in the piezoelectric actuator 8, the piston 10 travels farther out of the coupling chamber 13 so that a second pressure decrease Δp2 in the coupling chamber 13 occurs until time t3. At time t3, the injection valve member 9 has passed the definite distance hV, i.e. has executed a preliminary stroke and is now slightly open. Time t3 marks the end of the decrease region A in which the injection valve member 9, which can be embodied in the form of a needle, moves together with the preliminary stroke sleeve 17 and more slowly than the piston 10. The speed transmission in region A is determined by the transmission ratio i1=dA2/dV2.
If the definite distance hV has been passed, i.e. if the upper end of the preliminary stroke sleeve 17 is resting against the stop 18 at the bottom end of the disk 12, then the transmission ratio changes to i2=dA2/dN2. As the actuator voltage U decreases further, the transmission ratio i1 changes to i2 so that when a second critical opening pressure pÖ,2 has been reached, the needle-shaped injection valve member 9 opens in the increase region. At time t4, the actuator voltage Umax has fallen to its minimum value Umin, i.e. the piezoelectric crystals of the piezoelectric actuator 8 are then no longer being supplied with current so that the elongation of the actuator equals 0. According to
At time t5, current is once again supplied to the actuator so that its stack of piezoelectric crystals once again begins to lengthen. As a result, the pressure in the coupling chamber decreases again in the time interval between t5 and t6, and the injection valve member 9, which can be embodied in the form of a needle, is moved from its maximum opening stroke hmax in the direction toward its closed position until the definite distance hV has been reached at time t6. Between times t6 and t7, the top of the preliminary stroke sleeve 17 and the stop 18 on the flat surface at the bottom of the disk 12 once again assume the definite distance hV, which corresponds to a pressure increase by Δp2 in the coupling chamber 13 between the piston 10 and the top end of the injection valve member 9.
Between times t7 and t8, a pressure increase by Δp1 occurs in the coupling chamber 13 because the actuator voltage U in the piezoelectric actuator 8 increases once again to the maximum voltage Umax.
In
The opening force curve 40 for a piezoelectric actuator that actuates a fuel injector without a stepped transmission demonstrates that the opening pressure pÖ,3 of the fuel injector lies significantly below the opening pressure pÖ,1 of a fuel injector that operates with a piezoelectric actuator equipped with a stepped transmission. According to the opening force curve 40, a piezoelectric actuator operating without a stepped transmission requires a switching energy depicted by the shaded region represented by the triangle a-b-c in
The second opening pressure pÖ,2 of the injection valve member 9, which can be embodied in the form of a needle, is thus significantly lower in a fuel injector equipped with a piezoelectric actuator and a stepped transmission. This also means that a lower actuating force is required in order to move the injection valve member, thus permitting a piezoelectric actuator 8 of this kind to have a small volume.
According to the opening force curve 41 for a fuel injector equipped with a piezoelectric actuator 8 and a stepped transmission, the pressure p in the coupling chamber decreases when a definite distance hV is reached, then increases again sharply in a pressure jump, and then decreases again degressively toward the system pressure pCR. When the maximum opening stroke of the injection valve member 9, which can be embodied in the form of a needle, has been reached, the pressure p in the coupling chamber 13 is identical to the system pressure pCR.
A fuel injector whose injection valve member 9 is triggered directly with a piezoelectric actuator 8—see reference numeral 42 and the dash-shaded region in
The embodiment according to the present invention achieves an optimum utilization of the properties of the piezoelectric actuator 8 and adapts them to the stroke/force characteristic curve of an injection valve member 9 by means of a variable transmission. Consequently, it is also possible to achieve stable, extremely low injection quantities by means of an intermediate stroke stop that is defined by the edge 18 (see definite distance hV between the edge 18 and the upper end of the preliminary stroke sleeve 17).
| Number | Date | Country | Kind |
|---|---|---|---|
| 102004028522.5 | Jun 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/51168 | 3/15/2005 | WO | 11/18/2005 |
