EP1342913B1 - Fuel injector - Google Patents

Abstract

The injector (2) comprises a body (4) with an orifice (10) at the end to deliver fuel, a moving needle (12), a piezoelectric actuator (26) and a return spring (18). Movement amplification between the actuator and the needle is provided by a hydraulic chamber (32) with one end closed by the moving surface (28) of the actuator and the other end closed by a surface (24') attached to the needle.

Description

The present invention relates to a fuel injector.

It aims, more specifically, the injection of a fuel delivered at a high pressure, using a piezoelectric type actuator.

We know, by US Patent 6,079,641 , a fuel injector comprising a body, in which is formed a first passage for the supply of a fuel at a high pressure, which comes for example from a pump. This passage opens into an outlet port, made at the end of the body, while another passage allows the return of fuel at a lower pressure, which is returned to the reservoir.

There is also provided a needle, which is mounted movably in the body between two positions, respectively corresponding to the opening and closing of the aforementioned outlet orifice. A piezoelectric actuator makes it possible to bring this needle into its open position, while a spring ensures the return of this needle in its closed position.

Since the size of the piezoelectric actuator is necessarily limited, its displacement is reduced to a few tens of micrometers. It is thus conceived that, to generate a sufficient stroke of the needle, it is necessary to provide amplification between the respective movements of the actuator and the needle.

According to the teaching of this American document, such amplification is performed by means of a hydraulic valve, which controls the filling or emptying of a hydraulic control chamber, in which slides the needle.

This injector of known type, however, has certain disadvantages.

Indeed, it is of a relatively complex structure, since it uses a large number of constituent elements.

Moreover, in use, four different forces act on the needle of the injector. Therefore, a precise displacement of this needle is relatively difficult to implement.

US-A-6,119,952 describes all the features of the preamble of appended claim 1. However, this document does not provide a particular filling mode of the hydraulic amplification chamber, which is used.

The invention proposes to remedy the various disadvantages of the prior art mentioned above.

For this purpose, it relates to a fuel injector according to claim 1 attached.

• la figure 1 est une vue schématique de face, illustrant un injecteur de carburant conforme à l'invention, dans la position de fermeture de son pointeau ; et
• la figure 2 est une vue schématique de côté, illustrant l'injecteur de carburant de la figure 1, dans la position d'ouverture de son pointeau.

The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example, and with reference to the appended drawings, in which:

• the figure 1 is a schematic front view, illustrating a fuel injector according to the invention, in the closed position of its needle; and
• the figure 2 is a schematic side view, illustrating the fuel injector of the figure 1 , in the open position of its needle.

The fuel injector 2, shown in the figures, comprises a body 4 in which is formed, in known manner, a first passage 6.

The latter opens, on the one hand, into a lateral opening 8 placed in communication with a source of high pressure fuel, such as a pump. This passage 6 also opens, at an axial end of the injector, into an outlet orifice 10, which makes it possible to inject the fuel into a combustion chamber, not shown.

A needle 12 is mounted movably with respect to the body 4, between respective closing positions of the orifice 10 ( figure 1 ) and opening this hole ( figure 2 ). More specifically, the needle has a thinned end, or nose 14, which closes the orifice 10 in the aforementioned closing position.

This needle 12 is also provided with a flange 16 against which bears a first end of a spring 18. The other end of this spring 18 is in abutment against a bearing surface 20 belonging to an annular element 22 , fixed with respect to the body 4.

A cylindrical movable member 24, which is integral with the needle at its end opposite the nose 14, is free to slide relative to the inner wall of this fixed element 22.

The injector 2 also comprises a piezoelectric actuator 26, of a type known per se, which is provided opposite the nose 14. This actuator 26 is secured to a second movable member 28, which can slide relative to a second element fixed 30, also of annular shape.

It should be noted that the surface 28 'of the movable member 28, integral with the actuator, has a section substantially greater than that of the surface facing 24 'belonging to the movable member 24, integral with the needle 12. The ratio between these sections is at least 4, preferably at least 5.

These surfaces 24 'and 28' delimit, with the facing surfaces 22 'and 30' of the fixed elements 22 and 30, a hydraulic chamber 32. The latter is laterally bordered by two walls, in which are formed two calibrated nozzles 34, 36 .

The first 34 of these nozzles is in communication with the high pressure passage 6, while the second 36 of these nozzles is in communication with another passage 38, formed in the body. In known manner, this second passage 38, which allows the transport of fuel at lower pressure, opens into an orifice 40, placed in communication with the tank of the vehicle, which is not shown.

It should be noted that the passage section of the high pressure nozzle 34 is substantially greater than that of the low pressure nozzle 36. The ratio between these sections is at least 10, preferably at least 100.

It should be noted that the low-pressure nozzle can also be made by the choice of clearance and the guide length between the movable member 28 and the fixed element 30. Similarly, the high pressure nozzle can also be produced by the choice of the clearance and the guide length between the movable member 24 and the fixed element 22.

The operation of the injector described above, during its start-up phase, as well as its static and dynamic uses, will be explained in what follows.

During start-up, if the increase in pressure of the supply rail is carried out suddenly, the pressure exerted on the nose 14 of the needle is greater than that of the hydraulic chamber 32, which is experiencing a rise in pressure slower, given the presence of the nozzle 34. Without special precautions, this phenomenon is likely to cause a small displacement of the needle, to the left in the figures, which leads it to leave its closed position.

Under these conditions, it is advantageous to exert a compression action of the hydraulic chamber 32, by controlling the piezoelectric actuator 26. In other words, this actuator, as well as the movable member 30, are moved to the right, looking at the figure 1 , which keeps the nose 14 in its closed position of the orifice 10.

In this regard, the compression force exerted by the actuator 26 is accompanied by that of the return spring 18, which pushes the nose 14 to the right. However, it must be sufficiently weak, to maintain depression of the hydraulic chamber 32, to continue filling the latter.

However, it should be noted that, in general, the pressurization of the rail is slow enough to ensure a gradual filling of the chamber 32. The occurrence of the problem described above is therefore relatively limited.

In static operation, in the absence of pressure in the injector, the needle is held in its position of the figure 1 , under the action of the return spring 18. The characteristics of the latter are such that it does not allow the opening of the needle below a predetermined pressure in the combustion chamber.

When the injector is fed with high pressure, the hydraulic chamber 32 progressively reaches this high pressure, by passing the fluid through the calibrated nozzle 34. In this way, the needle 12 is subjected to this high pressure, at the same time at its nose 14 and the surface 24 'of the movable member 24.

Advantageously, the dimensions of the different pressure surfaces of the pressure on the needle, as well as those of the calibrated nozzles, are calculated so that the needle 12 remains in its closed position, regardless of the feed pressure of the nozzle. the injector.

On the other hand, it should be noted that, when the hydraulic chamber 32 rises in pressure, the piezoelectric crystal 26 is pushed to the left by the movable member 28. In this way, this crystal is subjected to a setting effort permanent, proportional to the value of the high pressure prevailing in the injector.

When it is desired to move the needle towards its open position, illustrated in FIG. figure 2 , an electric voltage is applied to the piezoelectric actuator 26. This therefore induces a displacement of this actuator 26 towards the left, according to the arrow f , accompanied by a corresponding movement of the movable member 28.

A depression is then created inside the chamber 32, which itself causes a displacement to the left, according to the arrow F, of the movable member 24 and the needle 12. Since the section of the surface 24 'is substantially less than that of the surface 28', the displacement D of the needle 12 is significantly higher than that of the piezoelectric member.

The hydraulic chamber 32 thus provides a function of amplification of the movement. In addition, the piezoelectric crystal 26 has a direct action on the needle 12, in that it directly controls the stroke thereof, by means of the hydraulic amplification allowed by the chamber 32.

To close the injector, it is sufficient to cancel (or decrease) the voltage applied to the actuator 26: it then moves to the right, looking at the figures, which creates an overpressure in the chamber 32, this overpressure causing the displacement to the right of the needle 12.

Note however that the depression created in the chamber 32, by the displacement of the actuator 26, is not permanent in nature, because of the presence of calibrated leaks 34, 36. Moreover, it should be noted that these The latter make it possible to purge any gaseous pockets that may form in the chamber 32.

Therefore, when the needle 12 is in the open position ( figure 2 ), it slowly returns to its closed position, with a time constant which is mainly a function of the flow rate coming from the high-pressure opening 8. Once the needle has returned to its closed position ( figure 1 ), the pressure of the hydraulic chamber 32 rises, with a time constant given by the flow from the high pressure opening 8.

The hydraulic leaks 34, 36 are such that the filling time constant of the chamber 32 is much higher than the time required to bring the needle 12 into its open position. Consequently, the amplification of movement, authorized by this hydraulic chamber 32, is accompanied by a parasitic high-pass type effect.

Thus, the nature of the movement of the needle 12 depends on how the voltage is applied to the actuator 26.

When this tension is applied quickly, the movement of the needle is complete while, when this tension is applied slowly, the movement of the needle may be only partial or nonexistent. Indeed, in the latter case, the depression created in the chamber 32 tends to be compensated by the arrival of liquid, via the nozzle 34.

The lifting of the needle, namely the distance separating its nose 14 from the orifice 10, tends to decrease over time. This phenomenon can be compensated by a specific control of the piezoelectric crystal. However, if this lift is performed so as to saturate the permeability of the nose of the injector, such a correction is not necessary.

Moreover, when returning to the closed position of the needle, the voltage to be applied to the piezoelectric crystal is lower than the voltage of the control of the opening. This phenomenon must be taken into account by the electronic control system of the piezoelectric crystal. This control system is, moreover, capable of managing the return to the closed position of the needle so as to reduce its speed of impact on its seat.

In addition, the lifting of the needle can be controlled by means of a corresponding control of the voltage applied to the piezoelectric crystal 26. This therefore allows variable lifting values, insofar as the parasitic effects, such as resonance, leakage and hysteresis, are suitably compensated by the electronic control.

It should be recalled that, by nature, the piezoelectric crystal 26 is much more fragile in tension than in compression. In this injector 2, this crystal is thus preloaded hydraulically by the supply pressure, and then works only in compression in the vicinity of this prestressing. Since the hydraulic pressure which compresses this crystal is small compared to the compressive strength of the latter, the actuator 26 operates permanently in its safety zone.

It should be noted that, when an injector is put into service, the hydraulic pressure inside the chamber 32 induces an electric charge displacement of the crystal 26, which then behaves like a sensor. Such information may advantageously be used by the electronic control system to evaluate rapid changes in hydraulic pressure.

It will be noted here that in operation, the hydraulic pressure of the source is caused to change. The piezoelectric crystal 26 can also in these moments serve as a sensor in its inactive phases, that is to say when the pressure of the chamber 32 is equal to the pressure from the source.

The invention makes it possible to achieve the objectives mentioned above.

Indeed, since the stroke of the needle is controlled directly by the piezoelectric actuator, a precise displacement of the needle can be obtained, more easily than in the prior art.

Furthermore, the injector of the invention is of simple design, since it involves a small number of constituent elements.

In addition, the use of the piezoelectric crystal makes it possible to generate waves, which can be transmitted to the nose of the needle, either hydraulically or via the same material of the injector.

Thus, the high frequency vibrations, transmitted by the fluid, are capable of ensuring atomization thereof, producing capillary waves at the interfaces between the fluid and the material.

In addition, the vibrations transmitted by the material, from the body to the nose of the needle, induce a scrubbing effect, which prevents any clogging of the nozzles by the carbon particles of the combustion.

Claims (5)

1. Fuel injector (2) comprising a body (4) equipped with an opening (8) in communication with a source of pressurized fuel, with an injection orifice (10) and with an orifice (40) in communication with a tank of fuel at low pressure, a needle (12) able to move with respect to the body, between positions in which this orifice is closed (Figure 1) and open (Figure 2), a piezo electric actuator (26) capable of bringing the needle (12) into its open position, return means, particularly elastic return means (18), able to return the needle (12) to its closed position, and means of amplifying the movement of the said actuator (26) allowing the needle to be moved over a travel greater than that of this actuator, these amplifying means comprising a hydraulic amplification chamber (32) which is at least partially closed by a first surface (24') belonging to a first moving member (24) secured to the needle (12), and by a second surface (28') belonging to a second moving member (28) secured to the piezo electric actuator, the first and second surfaces (24', 28') being positioned facing one another and the first surface (24') secured to the needle (12) having a cross section appreciably smaller than that of the second surface (28') secured to the actuator (26), the hydraulic movement amplifying chamber (32) being placed in communication, via a first calibrated nozzle (34), with a passage (6) for conveying fuel from the opening (8) to the injection orifice (10), characterized in that the hydraulic chamber (32) is placed in communication via a second nozzle (36) with a passage (38) able to transport the fuel to the low pressure fuel tank, and in that the first nozzle (34) has a passage cross section appreciably greater than that of the second nozzle (36).
2. Injector according to Claim 1, characterized in that the first and second moving members (24, 28) are cylindrical and in particular are coaxial.
3. Injector according to either one of the preceding claims, characterized in that the hydraulic amplifying chamber (32) is also delimited by first and second additional surfaces (22', 30') belonging to first and second elements (22, 30) that are fixed relative to the body (4), the first and second moving members (24, 28) being able to slide respectively with respect to interior walls of said fixed elements.
4. Injector according to Claim 3, characterized in that the return means comprise a spring (18) which rests against a bearing surface (20) of the first fixed element (22), opposite the first additional surface (22').
5. Injector according to one of the preceding claims, characterized in that the nozzles (34, 36) are formed in walls of the chamber (32) which run substantially perpendicular to the surfaces (24', 28') of the moving members (24, 28).

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