DE102005011366A1 - Piezoelectric actuator for internal combustion engine fuel injector has casing made of material with defined level of heat conductivity - Google Patents

Abstract

The piezoelectric actuator (2) has a plastics housing (5) with a cast infill (6) and with two contact pins (7) in electrical connection with the actuator module. There is a cylindrical spring (8) surrounding the plastics housing and a cap plate (3) and base plate (4) form locked with the actuator module. At least one of the components is made of a material with a heat transfer coefficient of greater than point three W/(k.m). An independent claim is also included for a fuel injector of a common rail injection system, using the piezoelectric actuator.

Description

The Invention is based on a piezoelectric actuator unit with a Actuator module according to the preamble of claim 1, which in a with filled with a potting compound Plastic sleeve is arranged. The actuator module is electrically powered with two Kontaktierungspins connected and is covered by a Bourdon tube. Furthermore is the actuator module is connected to a top plate and to a bottom plate, which complete the upper and the lower end of the Bourdon tube and with the actuator module non-positively are connected. The entire unit is arranged in an actuator housing, which is attached to the top plate. Furthermore, the invention relates a fuel injector according to the independent claim 10, the with the piezoelectric according to the invention Actuator is formed.

It is already known for a fuel injection system, in particular for a common rail injection system To use fuel injectors, in which a piezoelectric Actuator unit is installed. For the injection of gasoline or diesel fuels into one Combustion chamber of an internal combustion engine become such fuel injectors often also for a multiple injection used at intervals of several milliseconds to perform one or more pre, main and post injections. Especially at high driving frequencies it may be due to the inside of the Actuator module occurring power loss and as a result of limited electrical efficiency to a considerable self-heating of the Actuator module come. This self-heating is very harmful for the actuator unit and can cause malfunction or even destruction of the extreme conditions lead piezoelectric actuator unit.

One Another problem is that for future injection systems stricter exhaust conditions for the Internal combustion engine to meet are. For adherence to the tightened Exhaust conditions, however, becomes a higher specific driving energy required, through which an even higher power dissipation respectively heat loss arises. Known fuel injectors with today's piezoelectric Actuator units are formed, therefore, in future Injection systems can not be used as they are more exposed to thermal loads would not resist.

Of the Invention is based on the object, in a piezoelectric Actuator or in a fuel injector the thermal To improve resilience. This task comes with the features the sibling claims 2 to 10 solved.

at the piezoelectric according to the invention Actuator with the characterizing features of claim 1 respectively in the fuel injector according to claim 10 results in the advantage that within the piezoelectric Actuator resulting heat loss better and more reliable after be discharged outside can, as this is possible with known actuator units. As a particularly advantageous It is considered that to improve the heat dissipation at least one Component of the piezoelectric actuator unit made of a material which is not just a required mechanical or electrical Property taken into account is, but also an optimized, i. largest possible Wärmeleitkoeffizient trained is. Often have materials with similar mechanical properties size Differences in the heat transfer coefficient on. For example, a mild steel can be 3 to 4 times have better thermal conductivity as an alloyed steel. By using and optimizing these properties it succeeds in a very advantageous way very much bes ser, the resulting heat loss outward dissipate and thereby an invalid Temperature rise in the interior of the piezoelectric actuator unit or prevents the fuel injector. This is especially true therefore very important because the fuel injector because of its installation be called anyway Engine block against overheating particularly at risk is.

By those in the dependent claims listed activities are advantageous developments and improvements in the sibling claims 2 to 10 specified piezoelectric actuator unit or given the fuel injector. The top plate looks special advantageous that preferably used as a material steel is, whose Wärmeleitkoeffizient λ at least the value 40 W / (K · m) having. The steel can be formed with the required hardness be to the length changes the actuator can be transferred lossless on the injector. There The head plate is further connected to an actuator housing, it forms in conjunction with the injector a particularly effective heat sink out, so that a lot of heat loss are dissipated to the outside environment can.

One another possibility to improve heat dissipation is seen in that the bottom plate of a material, preferably is made of a steel whose Wärmeleitkoeffizient λ at least the value 40 W / (K · m) reached. As a rule, the bottom plate in direct mechanical Contact with, for example, a servo valve or a nozzle needle stands, can also over the bottom plate and the adjacent components a considerable heat dissipated become. In addition, the bottom plate of the usually cooling Fuel flows around will, so too over The fuel correspondingly much heat energy can be derived can.

Of the Contacting pin is made of an electrically conductive material, for example, made of a metal alloy. There metal usually a good conductor of heat it seems cheap, also the contact pin for heat dissipation consulted. The Kontaktierungspin forms comparatively only a small heat sink. But this can also be used to control the energy balance of the to improve piezoelectric actuator unit.

A another possibility to improve heat dissipation The piezoelectric actuator unit also consists of the Bourdon tube made of a material whose thermal conductivity λ at least the value 15 W / (K · m) reached. The Bourdon tube surrounds the actuator module and thus forms to the environment a relatively large Surface. About these size surface can then be a correspondingly large heat be dissipated.

The actuator module encased by the Bourdon tube is for mechanical and electrical protection arranged in a cylindrical actuator housing. The actuator housing is on its outer surface of the relatively cool Fuel flows around. It thus forms a large one Heat sink. In training of the actuator housing a material is used whose thermal conductivity coefficient λ at least the value 13 W / (K · m) achieved as much as possible huge Part of the resulting heat loss lead away to be able to.

Of the Cavity between the actuator module and a plastic sleeve is usually filled with a potting compound. Suitable as potting compound a silicone, with a coefficient of thermal conductivity λ of at least 0.3 W / (K · m) is trained. The silicone not only achieves that the actuator module against electrical voltage flashovers on its outer wall protected is. Rather, over a highly thermally conductive Vergusssilikon the heat forwarded and in particular transferred to the Bourdon tube become.

to Improvement of heat conduction the Vergusssilikons can be added to a corresponding filler. The filler For example, as granules with different grain size on a maximum heat transfer coefficient be matched. By doing so leaves the heat dissipation over the Further improve casting silicone.

to Recording of the actuator module is usually a mounting sleeve used to the actuator module with the electrical connection lines to the Contact pins before casting in a desired Position to fix. The receiving cage is made of a plastic manufactured, for example, a polyamide plastic (PA plastic). Alternatively, the plastic also with a good heat conducting Filler mixed be.

One embodiment The invention is illustrated in the drawing and will become apparent in the following description explained in more detail.

1 shows a sectional view through a piezoelectric actuator unit according to the invention and

2 shows a fuel injector with a piezoelectric actuator unit according to the invention according to the 1 in a schematic representation.

1 shows a sectional view through a piezoelectric actuator unit according to the invention 1 , which is particularly useful as a drive unit for actuating an injection nozzle in a fuel injector. The piezoelectric actuator unit 1 has an actuator module 2 on, which is composed of several hundred piezoelectric ceramic layers in the form of a Stapelaktors. Between each two ceramic layers, an inner electrode is arranged, which alternately with two on the outer wall of the actuator module 2 vertically extending outer electrodes are electrically connected. Furthermore, two contacting pins are parallel to the two outer electrodes 7 arranged, via appropriate lines with the two external electric who are contacted. The actuator module is used for mounting 2 with the wired two contact pins 7 in a plastic sleeve (mounting sleeve) 5 used and fixed before the components used 2 . 7 be potted with a potting compound, such as with a Vergusssilikon or elastomer. The plastic sleeve 5 is made of a plastic, for example made of a PA plastic (polyamide).

At an upper end of the actuator module 2 is a headstock 3 arranged with the actuator module 2 positively connected. The headstock 3 thus limits the actuator module 2 up. In the headstock 3 are two isolated feedthroughs for the two contacting pins 7 designed so that the protruding ends of the two Kontaktierungspins 7 later for the electrical supply of the actuator module 2 can be connected to a control voltage of a corresponding control unit.

The lower end of the actuator module 2 is from a floor plate 4 limited, which also with the actuator module 2 positively connected. Between the headstock 3 and the bottom plate 4 is a bourdon tube 8th sleeve-shaped and surrounds the molded actuator module 2 , The Bourdon tube 8th is formed with a biasing force acting on the actuator module 2 acts as a restoring force and thus a provision of the non-actuated actuator module 2 supported in his basic position. The whole assembly is powered by an actuator housing 9 encased sleeve-shaped. The upper end of the actuator housing 9 is with the top plate 3 firmly and sealingly connected. The lower end of the actuator housing 9 is however opposite the bottom plate 4 arranged axially movable, but also sealed fuel-tight. When controlling the actuator module 2 thus arises between the bottom plate 4 and the underside of the actuator housing 9 a small, axial stroke movement, which can be used to control a servo valve, a nozzle needle or the like.

dQ/dt

ist der Wärmestrom,

λ

ist der Wärmeleitkoeffizient in W/(K·m),

δ

ist die Dicke der Wand eines Bauteils,

F

ist die Fläche, durch die die Wärme strömt,

t_w1

ist die Temperatur der wärmeren Wandoberfläche und

t_w2

ist die Temperatur der kälteren Wandoberfläche.

When controlling the actuator module 2 arises in the piezoelectric actuator unit 1 due to the limited efficiency, a corresponding loss of heat, which leads to a self-heating of the surrounding components ( 3 to 9 ), since each component ( 3 to 9 ) can dissipate due to its geometric dimensions (area F) and its Wärmeleitkoeffizienten λ only a certain heat energy to its environment. The heat flow dQ / dt occurring per unit time of the individual components ( 3 to 9 ) can be calculated according to the formula: dQ / dt = λ / δ · F · (t w1 - t W2 )

/ Dt dQ

is the heat flow,

λ

is the heat conduction coefficient in W / (K · m),

δ

is the thickness of the wall of a component,

F

is the area through which the heat flows

t _w1

is the temperature of the warmer wall surface and

t _w2

is the temperature of the colder wall surface.

As can be seen from the formula, the heat flow of a component increases proportionally with increasing coefficient of thermal conductivity λ. This means that with a larger coefficient of thermal conduction λ, more heat energy can be transmitted or dissipated to the environment, in particular to an adjacent, cooler component. The invention therefore proposes for at least one or more components 3 to 9 the piezoelectric actuator unit 1 to select a material that not only meets the mechanical or electrical requirements, but also has an optimal, ie as large as possible Wärmeleitkoeffizienten λ.

As big the Differences in the thermal conductivity coefficient λ of similar Can be materials, is shown in the table below. In the table are as an example some materials made of steel and steel alloys listed, the partially very different Wärmeleitkoeffizienten λ have. The Wärmeleitkoeffizienten λ can be thereby differ by more than four times the value. So has the Thermal conductivity coefficient λ for the alloy Invar Ni36 only has a value of about 13 W / (K · m), while the value for unalloyed Steel St35 at 57 W / (K · m) lies.

Table:

at Plastics can the Wärmeleitkoeffizienten λ also be very different, especially if they are with appropriate fillers are mixed and / or different particle sizes or molecular structures exhibit.

In order to use these different coefficients of thermal conductivity λ, it is provided according to the invention, the top plate 3 preferably made of a steel, the coefficient of thermal conductivity λ reaches at least 40 W / (K · m). Accordingly, the bottom plate 4 preferably be made of a steel whose Wärmeleitkoeffizient λ also reaches at least the value 40 W / (K · m).

Another important component with regard to heat dissipation is the potting compound 6 According to the invention it is proposed to use a Vergusssilikon whose Wärmeleitkoeffizient λ reaches at least 0.3 W / (K · m). The Wärmeleitkoeffizienten λ can be further improved if a corresponding good thermal conductivity filler, granules or the like is attached. Also, a corre sponding grain size can be selected. It should also be noted that requirements are met, for example, with respect to the insulating properties against electrical flashovers, mechanical extensibility, moisture absorption, surface adhesion, temperature stability, etc.

A further improvement of the heat dissipation can also be achieved by a suitable choice of material for the Bourdon tube 8th be achieved. For the Bourdon tube 8th For example, a material, for example a special alloy, whose heat conduction coefficient λ reaches at least 15 W / (K · m) can be selected. The Bourdon tube 8th forms a relatively large surface over which the heat energy can be well forwarded.

As an improved heat conductor and the actuator housing 9 be educated. Its material is, for example, an Invar alloy whose thermal conductivity coefficient λ reaches at least the value 13 W / (K · m). Since the actuator housing is thermally coupled to the housing of the fuel injector and the injector housing is already heavily loaded due to the high ambient temperatures of the engine, it is provided to optimize its Wärmeleitkoeffizienten λ particularly well.

To further optimize the heat dissipation is provided, the plastic sleeve 5 from a material with good thermal conductivity λ form. Its λ value should at least reach the value 0.35 W / (K · m).

As well as the Kontaktierungspins 7 can contribute to the heat dissipation, it is provided that the Wärmeleitkoeffizient λ reaches at least the value 75 W / (K · m).

Optimal heat dissipation is achieved if possible all affected components 3 to 9 are optimized with respect to their Wärmeleitkoeffizienten λ. This reduces the component temperature, in particular that of the actuator module 2 so deep that the piezoelectric actuator unit 1 is suitable for higher quality requirements and can be operated with a higher driving energy. For example, then the number of injection pulses per cycle to five or more injection pulses can be increased without the piezoelectric actuator unit or the fuel injector are thermally overloaded. The solution according to the invention advantageously results in new free spaces for improved injection strategies.

In 2 is a fuel injector 11 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 via a fuel inlet 12 fed. In the middle part of the fuel injector 11 is the piezoelectric actuator unit 1 and below this is a valve unit 14 arranged. The arrangement of the actuator unit 1 is designed such that it communicates with the valve unit 14 is in active connection.

At the upper end of the piezoelectric actuator unit 1 are the connections for the contactie rungspins 7 , By applying an electrical voltage to the Kontaktierungspins 7 extends the piezoelectric actuator unit 1 a small piece. This extension causes the valve unit 14 raises its nozzle needle and the lower part of the nozzle unit 14 located spray holes 15 opens. Thereby, the fuel can be injected at high pressure into a cylinder of the internal combustion engine.

Claims (10)

Piezoelectric actuator unit with an actuator module ( 2 ), which in one with a potting compound ( 6 ) filled plastic sleeve ( 5 ) is arranged, with two Kontaktierungspins ( 7 ) connected to the actuator module ( 2 ) are electrically connected, with a Bourdon tube ( 8th ), the plastic sleeve ( 5 ), with a top plate ( 3 ) and with a bottom plate ( 4 ) at the top and bottom of the bourdon tube ( 8th ) and with the actuator module ( 2 ) are positively connected and with an actuator housing ( 9 ), which the bourdon tube ( 8th ) and covered with the top plate ( 3 ), characterized in that at least one for the piezoelectric actuator unit ( 1 ) used component ( 3 to 9 ) is made of a material formed with a heat conduction coefficient λ greater than 0.3 W / (K · m). Piezoelectric actuator unit according to claim 1, characterized in that the top plate ( 3 ) is made of a material whose heat conduction coefficient λ reaches at least 40 W / (K · m). Piezoelectric actuator unit according to claim 1 or 2, characterized in that the bottom plate ( 4 ) is made of a material whose heat conduction coefficient λ reaches at least 40 W / (K · m). Piezoelectric actuator unit according to one of the preceding claims, characterized in that a Kontaktierungspin ( 7 ) is made of an electrically conductive material whose Wärmeleitkoeffizient λ reaches at least 75 W / (K · m). Piezoelectric actuator unit according to one of the preceding claims, characterized in that the bourdon tube ( 8th ) is made of a material whose thermal conductivity λ reaches at least 15 W / (K · m). Piezoelectric actuator unit according to one of the preceding claims, characterized in that the actuator housing ( 9 ) is made of a material whose thermal conductivity λ reaches at least 13 W / (K · m). Piezoelectric actuator unit according to one of the preceding claims, characterized in that the potting compound ( 6 ) has a silicone whose heat conduction coefficient λ reaches at least 0.3 W / (K · m). Piezoelectric actuator unit according to claim 7, characterized in that the potting compound ( 6 ) is filled with a good heat-conducting filler and that the coefficient of thermal conductivity λ reaches at least 0.35 W / (K · m). Piezoelectric actuator unit according to one of the preceding claims, characterized in that the plastic sleeve ( 5 ) is made of a plastic, preferably made of a PA plastic whose thermal conductivity λ reaches at least 0.35 W / (K · m). Fuel injector for a common rail injection system, formed with a piezoelectric actuator unit ( 1 ) according to any one of the preceding claims.