WO2006128842A1

WO2006128842A1 - Piezoelectric actuator unit with improved heat dissipation and fuel injector

Info

Publication number: WO2006128842A1
Application number: PCT/EP2006/062666
Authority: WO
Inventors: Günter LEWENTZ; Carsten Schuh; Thorsten Steinkopff; Claus Zumstrull
Original assignee: Siemens Vdo Automotive Ag
Priority date: 2005-06-01
Filing date: 2006-05-29
Publication date: 2006-12-07
Also published as: EP1886361A1; DE102005025137A1

Abstract

The invention relates to a piezoelectric actuator unit (13) and to a fuel injector (11) that is configured with a piezoelectric actuator unit (13). The problem of generic structures of this kind is that the heat dissipated in the actuator unit (13), especially during multiple injections, is not dissipated into the environment in an optimum manner, thereby leading to an overheating which may impair the function of the actuator unit (13) and of the fuel injector (11). The aim of the invention is therefore to improve heat dissipation. For this purpose, a cavity (16) is defined between an actuator module (8) and a plastic sleeve (2) configured as a casting mold and is filled with a casting compound (6). Said casting compound (6) has a plurality of pores and/or interstices (9) and is an open-pore compound to a degree well over the percolation limit. In this manner, not only thermal conductivity is improved but also the actuator module (8) is electrically passivated, especially if the pores or interstices (9) are filled with a liquid.

Description

description

Piezoelectric actuator unit with improved heat dissipation and fuel injector

The invention is based on a piezoelectric actuator unit, which is formed with at least one actuator module, according to the preamble of claim 1.

Furthermore, the invention relates to a fuel injector according to the independent claim 12, which is formed with the piezoelectric actuator unit according to the invention.

It is already known that in the production of a piezoelectric actuator unit, the actuator module i.a. is arranged in a trained as a mold plastic sleeve. Subsequently, a cavity formed between the actuator module and an inner wall of the plastic sleeve is filled with a potting compound. The potting compound has the task of acting on the one hand as a passivation for the actuator module, in particular to prevent electrical voltage flashovers on the outer actuator surface. On the other hand, the potting compound is needed to fix the actuator module in position and to protect it from damage.

It is also known that the actuator module is electrically connected before casting with two Kontaktierungspins. The Kontaktierungspins are also fixed in the plastic sleeve and potted together with the actuator module. The whole unit is then covered with a bourdon tube. Furthermore, the actuator module is formed with a top plate and with a bottom plate, which complete the upper and the lower end of the tube spring and are non-positively connected to the actuator module. The entire unit is arranged in an actuator housing, which is attached to the top plate. It forms the piezoelectric actuator unit. Furthermore, it is already known to use fuel injectors for a fuel injection system, in particular for a Coramon Rail injection system, in which a piezoelectric actuator unit is installed. For injecting gasoline or diesel fuels into a combustion chamber of an internal combustion engine, such fuel injectors are frequently also used for multiple injection in order to carry out one or more pre-injections, main injections and post-injections at intervals of a few μ seconds. The problem is that due to the necessary drive energy within the actuator module, a correspondingly high power loss occurs, which must be dissipated as heat loss to the outside. Especially at high drive frequencies, the power loss occurring in the interior of the actuator module can lead to a considerable self-heating of the actuator module. This self-heating is very harmful to the actuator unit and can lead to malfunction or even destruction of the actuator unit in extreme conditions.

Another problem is that for future injection systems stricter exhaust conditions for the internal combustion engine to be met. To comply with the stricter exhaust gas conditions, however, a higher specific driving energy is necessary, through which an even higher power loss or heat loss arises. Known fuel injectors that are formed with today's piezoelectric actuator units are therefore not usable in future injection systems, since they would not withstand the higher thermal loads.

The invention is based on the object to improve the thermal capacity in a piezoelectric actuator unit or in a fuel injector. This object is achieved with the features of the juxtaposed claims 1 and 12. In the case of the piezoelectric actuator unit according to the invention or in the case of the fuel injector with the characterizing features of the independent claims 1 and 12, there is the advantage that the heat dissipation via the open-pore casting compound is considerably improved. It is considered to be particularly advantageous that nevertheless the electrical passivation of the surface of the actuator module is maintained. This is of importance particularly in the case of irregular or rough surfaces of the actuator module, since electrical voltage flashovers can preferably occur on rough surfaces. Furthermore, it appears to be advantageous that, despite the pores and the cavities, the potting compound has sufficient strength to ensure the fixation, in particular of the actuator module.

Advantageous refinements and improvements of the piezoelectric actuator unit or of the fuel injector specified in the independent claims 1 and 12 are given by the measures listed in the dependent claims. A particularly favorable solution is considered that the pores and / or interstices are in mesh with each other. This results in an irregular line network, via which the heat loss can be removed.

A further improvement of the heat dissipation can be achieved if the pores and / or interspaces are filled with a highly thermally conductive fluid whose thermal conductivity is greater than that of the potting compound.

It also appears advantageous that the fluid has a high dielectric strength. Thereby, the fluid supports the insulating effect of the potting compound, so that in particular electrical flashovers on the actuator surface can be reliably prevented. The fluid is preferably formed with a low viscosity. As a result, it can circulate in the network of pores and / or interstices and thereby better dissipate the heat loss to the external environment.

As the fluid, a particularly low-viscosity transformer oil can be used, which both has good electrical insulation properties and furthermore forms a good heat conductor.

To produce the pores and / or interstices, it is provided to use a polymer granulate as potting compound whose melting temperature is higher than the operating temperature of the actuator module. This prevents that the granules can change and in particular melt, when the actuator module reaches a maximum operating temperature. On the other hand, the melting temperature should be smaller than the Curie temperature of the piezoceramic disks from which the actuator module is formed. The predetermined upper temperature limit ensures that the piezoceramic disks can not be damaged in terms of their function at this temperature load.

In the case of the polymer granules, by briefly heating within the abovementioned temperature range, the individual grains can melt on their edges and cakes to form a solid unit, without the entire granules changing into a liquid phase. The melted granulate thus forms a solid, open-pore structure in which, on the one hand, the fluid can derive the heat loss of the actuator module through intensive circulation. On the other hand, the caked polymer granules form a stable form, by which the actuator module is securely fixed.

An alternative possibility for the formation of pores and / or gaps is to introduce, for example, micro- or nanotubes into the potting compound. An open-pored structure can also be produced if hollow spheres are introduced into the potting compound. The casting compound is heated together with the hollow spheres until the hollow spheres burst open, so that a coherent structure results, through which the fluid can circulate.

Another method for producing an open-pore structure is to introduce into the potting compound materials that release a gas during a temperature treatment. During the heat treatment, it releases the released, the corresponding bubbles and cavities forms and which are formed due to their irregular shape predominantly contiguous. These hollow structures can then also be used for a circulating fluid. Carbonates, in particular sodium carbonate, which release gas during the heat treatment can be used as materials.

An embodiment of the invention is illustrated in the drawing and will be explained in more detail in the following description.

1 shows an inventive actuator module with a plastic sleeve in three-dimensional representation,

FIG. 2 shows a plan view of the plastic sleeve,

Figure 3 shows a sectional view through the plastic sleeve with the actuator module and

FIG. 4 shows a fuel injector in which the actuator module according to the invention is installed.

FIG. 1 shows a three-dimensional representation of an actuator unit 13 with an actuator module 8 and with a plastic sleeve 2, which is preferably cylindrical in shape as a casting mold. The top of the plastic sleeve 2 is open, so here the actuator module 8, which is preferably designed as a piezoelectric stack actuator with a plurality of piezoceramic layers, can be introduced. Furthermore, two contacting pins 5, which are connected to the actuator module 8 via electrical leads (not shown in FIG. 1), are inserted into the plastic sleeve 2. The plastic sleeve 2 is sealed in connection with a bottom plate of the actuator module 8 downwards, so that the plastic sleeve 2 is closed towards the bottom. However, the bottom plate of the actuator module 8 is led out at the bottom so that the change in length of the actuator module 8 occurring during activation can be tapped off here. As a result, a cup-shaped cavity 16 is formed between the inner wall of the plastic sleeve 2 and the actuator module 8, which is later filled with a casting compound. Furthermore, 16 holders are mounted in the cavity, which fix the inserted actuator module 8 and the Kontaktierungspins 2 in position until it is shed.

The actuator module 8 is arranged in the center of the plastic sleeve 2 and is inserted so that its top plate and bottom plate can not be covered by the potting compound. On the side of the plastic sleeve 2, the two contacting pins 5 are arranged at two opposite points. Their upper ends protrude out of the plastic sleeve 2. They are needed for the subsequent electrical contacting of the molded actuator unit 13 and are also not covered by the potting compound.

Figure 2 shows the previously described actuator unit 13 with the plastic sleeve 2 in plan view before the casting. The actuator module 8 is arranged in the center of the plastic sleeve 2. The two Kontaktierungspins 5 are for example introduced into a bore 4 of a bottom surface 7 and are secured against lateral slipping. The contacting pins 5 are preferably formed so long that their contact surfaces arranged above protrude from the plastic sleeve 2 can. In this state, the device is ready for subsequent encapsulation with the potting compound.

FIG. 3 shows in a three-dimensional representation a longitudinal section and a cross section through the previously described plastic sleeve 2 with the actuator module 8. The cavity 16 between the inner wall of the plastic sleeve 2 and the actuator module 8 was filled with a potting compound 6. It is essential to the invention that the potting compound 6 has a structure with pores and / or interstices 9 that is open-pored to a high degree above the percolation limit, the pores and / or interstices 9 being connected to one another like a net. The percolation limit is reached when, with a homogeneous distribution of the granules in the potting compound 6, the individual grains abut each other and touch each other. The potting compound 6 thus forms an actuator enclosure for the actuator module 8, which on the one hand protects the actuator module 8 against electrical flashovers and, on the other hand, ensures a firm mechanical hold of both the actuator module and the contact pins 5. The pores preferably have diameters in the range of 10 μm to several millimeters. In addition, the pores form a network of channels that form motion paths for the flow of a liquid or gas.

Furthermore, it is provided that the pores and / or interstices 9 are filled with a good heat-conducting fluid. In particular, the fluid should have a low viscosity and a high dielectric strength. A suitable fluid is, for example, a corresponding transformer oil. The fluid preferably has a thermal conductivity coefficient that is greater than 1 W / (m * k). In addition, the fluid has a dielectric strength, which is preferably greater than 10 kV / mm. In addition, the fluid has a kinematic viscosity, which is preferably less than 30 mm ² / s (cSt). The high degree of open porosity ensures a significantly improved

Heat removal via the fluid, because of its low viscosity within the network of pores, ie the channels and Gaps 9 particularly good can dissipate the heat loss of the actuator module 8 by flow.

In this case, the heat-conducting fluid flows through the network of pores and transports heat from the actuator module 8 to the outside.

After encapsulation, the actuator unit 13 can be surrounded, for example, with a preloaded tube spring and an actuator housing. After further steps such as polarization, embossing, etc., the actuator unit 13 can be installed as a drive for a fuel injector.

For the production of the open-pore structure with pores and / or gaps 9, various methods can be used. It is intended to use for the open-pored structure a polymer granulate whose melting temperature is above the operating temperature of the actuator module 8 and below the Curie temperature of the piezoceramic disks used in the actuator module 8. The introduced into the cavity 16 granules is briefly melted to effect a permanent connection on the points of contact of the granules. This results in percolating networks for both the shell granules and the interstices that form a channel network for flow of the heat-conducting fluid. Instead of the polymer granules, any other type of material in the form of granules can be used, which can be processed via a temperature process into a sewer network.

As filling materials for the potting compound 6, for example, alternatively micro or nanotubes can be used.

Alternatively, hollow spheres can be introduced into the potting compound, which rupture during melting and thereby form a percolating structure. Another alternative possibility for forming the open-pore structure is that 6 materials are entered into the potting compound, which release gases at a corresponding temperature treatment and thereby form bubble structures. For example, carbonates, especially sodium carbonate can be used.

4 shows a fuel injector 11 is shown as a sectional view, as it can be used for example in a common rail injection system for a diesel or gasoline engine (internal combustion engine) use. In the upper part of the fuel injector 11, the fuel is supplied via a fuel inlet 12. In the middle part of the fuel injector 11, the inventive piezoelectric Akto- purity 13 is arranged. At its upper end there are the connection surfaces for the contact pins 5. At the lower end of the actuator unit 13, a valve unit 14 is arranged. The arrangement of the actuator unit 13 is designed such that it is in operative connection with the valve unit 14. By applying an electrical voltage to the contact pins 5, the actuator unit 13 is extended by a small amount. This extension causes the valve unit 14 to lift its nozzle needle and to open the injection holes 15 located in the lower part of the fuel injector 11. Thereby, the fuel under high pressure can be injected into a cylinder of the internal combustion engine. These parts are known per se and have not been shown in detail in Figure 4 for reasons of clarity.

Claims

claims

1. Piezoelectric actuator unit (13) with at least one actuator module (8), with a plastic sleeve (2) and with a potting compound (6), wherein the actuator module (8) in the plastic sleeve (2) is arranged, wherein between the actuator module (8) and an inner wall of the plastic sleeve (2) a cavity (16) is formed, and wherein the cavity (16) with the potting compound (6) is filled, characterized in that to improve the heat conduction, the potting compound (6) with a Variety of pores and / or spaces (9) is formed.

2. Piezoelectric actuator unit according to claim 1, characterized in that the pores and / or interstices (9) are at least partially in mesh with each other.

3. Piezoelectric actuator unit according to one of the preceding claims, characterized in that the pores and / or interstices (9) of the potting compound (6) are filled with a fluid whose heat conduction coefficient is greater than that of the potting compound (6).

4. Piezoelectric actuator unit according to claim 3, characterized in that the fluid has a high dielectric strength, which is greater than 10 kV / mm.

5. Piezoelectric actuator unit according to one of claims 3 or 4, characterized in that the fluid has a low viscosity, which is smaller than 30 mm ² / s.

6. Piezoelectric actuator unit according to one of claims 3 to 5, characterized in that the fluid has a transformer oil.

7. Piezoelectric actuator unit according to one of the preceding claims, characterized in that the casting compound (6) comprises a polymer granules whose melting temperature above the operating temperature of the actuator module (8) and below the Curie temperature of the actuator module (8) arranged piezoceramic discs , thereby forming a network of channels.

8. Piezoelectric actuator unit according to claim 7, characterized in that the polymer granules are glued after introduction into the cavity (16) by brief melting with adjacent grains.

9. Piezoelectric actuator unit according to one of claims 1 to 6, characterized in that in the Vergussmaße

(6) Micro or Nanotubes are arranged.

10. Piezoelectric actuator unit according to one of claims 1 to 6, characterized in that in the potting compound (5) hollow balls are arranged, which burst during the melting and form a network of channels.

11. Piezoelectric actuator unit according to one of claims 1 to 6, characterized in that in the potting compound (6) materials are arranged, which release a gas during a temperature treatment.

12. Fuel injector for a common rail injection system, formed with at least one nozzle unit (14) and with a piezoelectric actuator unit (13) according to one of the preceding claims.