Piezoelectric actuator with a sheathing composed of a composite material

A piezoelectric actuator for flowing-around media, with a piezo element arranged between two end caps. The piezo element is surrounded by a ring-shaped sheathing connected to the end caps. The sheathing consists, at least in portions, of a composite material with at least two layers, at least one layer consisting of a metallic material and at least one layer consisting of a polymer.

This application is a national stage of International Application No. PCT/EP/2007/003084, filed Apr. 5, 2007, which claims priority under 35 U.S.C. §119 to German Patent Application Nos. 10 2006 020 300.5, filed Apr. 28, 2006 and 10 2006 025 820.7, filed Jun. 2, 2006, the entire disclosures of which are herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a piezoelectric actuator for high pressure media which flow around it.

In order to reduce the emissions of internal combustion engines, injection systems are increasingly used for the fuel supply, in which fuel is conveyed with the aid of a high-pressure pump into an accumulator and is injected from there into the combustion space with the aid of an injector. The activation of the injector takes place by means of an electrically activated actuator, preferably a piezoelectric actuator. On account of the very short switching times of such piezoelectric actuators, the injection operations can be controlled and metered with high accuracy; in particular, when piezoelectric actuators are employed, a plurality of nozzle needle strokes (injection operations) per engine revolution are possible.

A piezoelectric actuator contains a piezo element consisting of quartz ceramic or PZT ceramic (lead/zirconate/titanate ceramic), the active main faces of which are connected to an actuator cover and the actuator bottom. By an electrical voltage being applied to the actuator cover or actuator bottom, the length of the piezo element can be varied. This length change is transmitted, during operation, to a valve in the fuel injector.

The problem is that the piezoelectric actuator is often in direct contact with the media or fuel, since it is arranged in the pressure space of the injector and fuel under a high hydrostatic pressure washes around it there. In order to prevent oil or fuel from penetrating into the interior of the piezoelectric actuator, therefore, the latter has to be provided with a protective or sealing-off arrangement. This may, for example, be a sleeve-shaped housing jacket which is fastened on the end face to the actuator bottom and to the actuator cover respectively. In the German patent document DE 102 30 032 A1, to protect the piezoelectric actuator against media flowing around it, it is proposed to provide the actuator with a sheathing composed of an electrically insulating, flaccid and/or elastic material which surrounds the piezo element. Furthermore, it is known from (German patent application 10 2006 012 845.1) to configure the sheathing as a shrunk-on hose which is firmly pressed on the end faces against the outer circumference of the actuator bottom and of the actuator cover with the aid of peripheral ring elements.

By means of a sheathing composed of an electrically insulating material, in particular of a shrunk-on film, the piezo element can, indeed, be protected against the fuel which washes around the actuator (and which is under high pressure). However, a completely diffusion-tight sheathing often cannot be implemented by means of such polymer films.

One object of the invention, therefore, is to provide a piezoelectric actuator of the type described above, in which the piezo element of the actuator is effectively protected against flowing-around media.

This and other objects and advantages are achieved by the piezoelectric actuator according to the invention, in which the sheathing consists, at least in portions, of a composite material with at least two layers, at least one layer consisting of a metallic material and at least one layer consisting of a polymer. Such a composite material is electrically conductive; the sheathing can therefore be wired up electrically (in particular, by grounding the sheathing) in such a way that it is impossible for ions to penetrate the sheathing. The sheathing thus constitutes a diffusion-tight protective layer and protects the piezo element against penetrating liquids, in particular against fuel, water and ions.

On the other hand, the polymer layer of the composite material can be configured in such a way that it ensures a chemical passivity of the sheathing.

The sheathing has to satisfy high requirements: it must protect the piezo element effectively against penetrating media (fuel, water, ions, etc.) and, particularly also in the joining region between the sheathing film and end caps, ensure permanent diffusion tightness. Furthermore, even under cyclic load, it must be insensitive to the hydrostatic pressure acting in the pressure space, that is to say must possess a sufficient expansion capacity and high elasticity. Such properties must be ensured within the overall temperature range of between −40° C. and 120° C.

In order to fulfill these high requirements, the use of a composite material for the sheathing offers the possibility of suitably selecting or adapting the number, sequence and thickness of the individual layers. Furthermore, the metal layer and/or the polymer layer of the sheathing may be provided with reinforcing fabrics and/or fibers. Moreover, the composite material of the sheathing may have, in addition to the at least one polymer layer and one metal layer, further layers (consisting, in particular, of paper and/or cardboard and/or fabric and/or fibers) which in this case each perform a particular function. Layers or layer systems into which ion getters are implanted may also be used. In an advantageous combination of individual layers, it is possible to cover the entire requirement spectrum of the sheathing.

The metallic layer or metallic layers of the composite material may, in particular, be roll-bonded or electrodeposited or generated by means of physical vapor deposition (PVD) or chemical vapor deposition (CVD).

A major advantage of using composite materials is the possibility of applying or processing coatings which could not be applied or could not be used as individual layers standing alone so as to ensure process reliability. One example of this is metal films consisting of pure aluminum or aluminum alloys, which can be produced in a film thickness of 6 μm, but, with this low layer thickness, cannot fulfill the requirements placed on an actuator sheathing or cannot be applied reliably; if, however, such a thin aluminum film is drawn onto a tear-resistant, expandable polymer film, the composite material which in this case occurs is suitable for actuator sheathing.

Furthermore, by an advantageous combination and sequence of the various layers in the composite material, it is possible to achieve properties or functions which are possible only due to this combination. Thus, for example, an aluminum-coated polymer film has an increased expansion and shear capacity, as compared with a pure aluminum film.

In an advantageous refinement of the invention, the sheathing comprises a (sheathing) film composed of a composite material, which film is connected, diffusion-tight, to the end caps of the actuator. In this context, “film” is to be understood as meaning a sheet-like, flexible material portion expandable or elastic within certain limits and having a small wall thickness. The sheathing film seals off the inner space of the sleeve with respect to the outside space and therefore protects the piezo element from the flowing-around media, such as fuel or fuel constituents and water. On account of its elastic properties, it allows pressure compensation between the actuator inner space, enclosed by the sheathing film, and the surroundings.

In a further advantageous refinement of the invention, the sheathing comprises a sleeve having locally delimited perforations which are spanned by an elastic and expandable film connected sealingly to the sleeve. The film seals off the inner space of the sleeve with respect to the outside space and therefore protects the piezo element against the flowing-around media, such as fuel or fuel constituents and water. At the same time, because of its elastic expandable properties, the film allows pressure compensation between the two spaces even in the case of high ambient pressures (up to 2000 bar). The sleeve and/or the film in this case consist/consists of a composite material.

As is known from German patent document 10 2006 012 845.1, the sheathing may be fastened to the end caps of the actuator by means of ring-shaped clamping elements. Alternatively, the sheathing may be adhesively bonded to the end caps or be connected to the end caps by means of a soldered or welded joint (for example, WIG welding, laser or electron beam welding).

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1is a diagrammatic sectional illustration of a piezoelectric actuator1for actuating the injection valve of a fuel injector in an internal combustion engine. The basic set-up of such a fuel injector is explained, for example, in (German patent application 10 2006 012 845.1), the disclosure content of which is hereby incorporated into the present patent application. During operation, the actuator1is arranged in a pressure space6of the fuel injector and fuel which is under high pressure washes around it there.

The actuator1comprises two end caps3, an actuator bottom3aand an actuator cover3b, between which a piezo element2is arranged. The piezo element2consists, for example, of a plurality of plies of piezo layers consisting of a piezoelectric ceramic and receives control signals from a control apparatus by means of electrical lines7. The piezo element2is insulated electrically with respect to the end caps3by means of support plates4and is sealed off against the fuel in the pressure space6of the injector by means of a fuel-tight or fuel-repellent sheathing5, with the result that electrical short circuits in the electrical components of the actuator1are avoided. In the exemplary embodiment illustrated here, a cavity9formed between the piezo element2and the inner wall8of the sheathing5is filled with an electrically insulating fluid10, for example a silicone oil, so that pressure compensation between the inner space9and the outside space6can be made possible.

The sheathing5comprises a sheathing film11composed of a composite material which comprises at least one metallic layer12(consisting, for example, of aluminum or of an aluminum alloy) and at least one polymer layer13(consisting, in particular, of a polymer) (see the illustration of a detail inFIG. 2). The sheathing film is electrically conductive, expandable and flexible.

In order to seal off, diffusion-tight, the inner space of the actuator1with respect to the pressure space6, the sheathing film11is soldered peripherally to the end caps3of the actuator1in the exemplary embodiment illustrated here. In the present exemplary embodiment, the end caps3have ceramic basic bodies14provided with ring-shaped metal elements15, to which the sheathing film11is soldered; the material (or a coating) of the metal element15is coordinated with the materials of the sheathing film11in such a way that a simple, diffusion-tight and permanent soldering of the sheathing film11to the metal element15can be ensured. Alternatively, the sheathing film may be welded, adhesively bonded or connected mechanically (for example, via a clamping connection having ring elements) to the end caps3.

FIG. 3shows a further exemplary embodiment of the actuator1′ according to the invention. Here, the sheathing5′ comprises a cylindrical sleeve17consisting of a metal sheet, the wall of which is provided with perforations18. The sleeve17surrounds the piezo element2and the support plates4of the actuator1′ completely and is welded, soldered, adhesively bonded or mechanically connected to the end caps3′.

The sleeve17is looped around on the outside by a flexible and elastic film19composed of a composite material11′, said film lying flat on the outer wall of the sleeve17; it spans the perforations18in the form of a membrane and, when the actuator1is in operation, serves the pressure compensation between the fluid10enclosed in the cavity9and the (high) pressure, acting on the actuator1′ from outside (that is to say, from the pressure space6of the injector), of the fuel to be injected. So that such pressure compensation can be mastered even under cyclic load and under high pressures in continuous operation, the composite material of the film11′ must possess a sufficiently high expansion capacity or elasticity.