Abstract:
A fuel injection valve for a fuel injection system of an internal combustion engines includes a valve closing body cooperating with a valve seat body to form a sealing seat, and a piezoelectric actuator for actuating the valve closing body. The piezoelectric actuator includes piezo layers and one or more temperature compensation layers. The temperature compensation layers have a temperature expansion coefficient having an operational sign opposite the sign of the temperature expansion coefficient of the piezo layers.

Description:
FIELD OF THE INVENTION 
     The present invention concerns fuel injection valves. 
     BACKGROUND INFORMATION 
     German published Patent Application No. 195 38 791 concerns a fuel injection valve for fuel injection systems for internal combustion engines in which the valve closing body is actuated by a piezoelectric actuator. The piezoelectric actuator has a plurality of piezo layers made of a piezoelectric material. Electrodes are arranged between the piezo layers so that an electrical voltage can be applied to the piezo layers, causing the piezoelectric actuator, used for actuating the valve closing body, to expand. 
     A problem with using piezoelectric actuators is believed to be thermal expansion. Piezoelectric materials, unlike materials such as steel or plastic, have a negative temperature expansion coefficient. Therefore, the piezoelectric actuator contracts with increasing temperature, while the surrounding housing expands. The different temperature expansion coefficients of the piezoelectric actuator and the housing result in a temperature-dependent valve lift if not compensated using appropriate measures. 
     For temperature compensation, German Published Patent Application No. 195 38 791 apparently proposes that the valve housing be designed as two parts made of two different materials. For example, it is proposed that one housing part be made of steel and the other housing part be made of Invar. By an appropriate selection of the length of the first housing part made of steel and of the second housing part made of Invar, the overall thermal expansion of the housing should be matched to the thermal expansion of the piezoelectric actuator and thus the piezoelectric actuator and the housing surrounding the piezoelectric actuator expand and contract in the same manner. 
     It is believed that a disadvantage of this measure is the cost of manufacturing the valve housing and the relatively high cost of the material of the second housing part, which is preferably made of Invar. Furthermore, it must be taken into consideration that the valve housing and the actuator may be of different temperatures. Thus the piezoelectric actuator may heat up due to its heat losses, in particular, when the fuel injection valve is frequently actuated, and its temperature is only slowly transferred to the valve housing. On the other hand, the temperature of the valve housing is influenced by the heat transferred from the internal combustion engine on which the fuel injection valve is mounted. This type of temperature compensation is therefore not believed to be satisfactory. 
     German Patent No. 195 19 192 purportedly concerns a hydraulic lift transformer arranged between the piezoelectric actuator and the valve needle that actuates the valve closing body. Temperature compensation results from the fact that the lift transformer only responds to the relatively quick movement resulting in the intended opening of the fuel injection valve, whereas a relativity slow, temperature-dependent expansion or contraction of the piezoelectric actuator may cause the hydraulic fluid to leak out via guide gaps. It is believed that a disadvantage of this design is, however, the relatively high cost of the hydraulic lift transformer. 
     Other temperature compensation methods include forced tempering of the piezoelectric actuator using a liquid or gaseous medium, which is held at a constant temperature in a closed circuit, or a series arrangement of piezoelectric actuators with a temperature-compensating piece, which is arranged between the piezoelectric actuator and a valve needle actuating the valve closing body, for example. While the first method may be relatively costly, the use of a compensating piece arranged in series is believed to have the disadvantage that the piezoelectric actuator and the compensating piece, as mentioned before, are not necessarily subjected to the same temperature or the same temperature variation; therefore, temperature compensation is relatively inaccurate. 
     SUMMARY OF THE INVENTION 
     The fuel injection valve according to an exemplary embodiment of the present invention is believed to have the advantage that the piezoelectric actuator of the fuel injection valve has considerably improved temperature compensation. According to an exemplary embodiment of the present invention, one or more temperature compensation layers having a temperature expansion coefficient with opposite signs with respect to the temperature expansion coefficient of the piezo layers are provided directly in the piezoelectric actuator. Through an appropriate selection of the number and thickness of the temperature compensation layers, accurate temperature compensation can be achieved. 
     By embedding the temperature compensation layers in the piezo layers of the piezoelectric actuator, it is at least better ensured that the temperature compensation layers are subjected to the same temperature and the same temperature variation as the piezo layers of the actuator. In particular, a large contact surface exists between the piezo layers and temperature compensation layers, so that the temperature of the temperature compensation layers and that of the piezo layers are quickly equalized. This is important because the heat loss of the piezoelectric actuator can be subjected to considerable fluctuation when the actuation frequency of the fuel injection valve varies as a result of a variation in the internal combustion engine speed. Due to the large contact surface between the temperature compensating layers and the piezo layers and the proximity of the temperature compensating layers to the piezo layers, temperature compensation by the temperature compensation layers can quickly follow these fluctuations. In addition, a change in the temperature of the actuator due to fluctuating amounts of heat transferred from the internal combustion engine can be quickly compensated by using the method according to the present invention. Expensive forced tempering of the piezoelectric actuator is not necessary. 
     In a particularly advantageous manner, the temperature compensation layers can be used simultaneously as electrodes for activating the piezo layers if the temperature compensation layers are made of a metallic material. 
     Thus, the piezoelectric actuator is temperature compensated to a high degree. However, the valve housing surrounding the actuator, which is usually made of metal or plastic, can still be subjected to thermal expansion resulting in a temperature-dependent position shift of the valve seat body with respect to the valve closing body connected to the actuator. In order to avoid this, an equalizing sleeve made of a ceramic material is advantageously provided, which either surrounds the piezoelectric actuator or is itself surrounded by the piezoelectric actuator. The piezoelectric actuator is either in contact with the valve housing via the equalizing sleeve or actuates the valve closing body via the equalizing sleeve and, optionally, a valve needle. If the equalizing sleeve has the same axial length as the piezoelectric actuator, the temperature of the valve housing has no effect on the axial position of the valve closing body with respect to the axial position of the valve seat body, i.e., temperature compensation of the valve housing is achieved. 
     According to an exemplary embodiment, a first end of the piezoelectric actuator is connected to the valve closing body via a valve needle and a first end of the equalizing sleeve is in contact with the valve housing. A connecting element, which may be plate-shaped for example, is held in contact with the second end of the equalizing sleeve and the second end of the piezoelectric actuator by a spring. According to another exemplary embodiment design, the first end of the piezoelectric actuator is in contact with the valve housing, and the first end of the equalizing sleeve is connected to the valve closing body via a valve needle. A plate-shaped connecting element is held in contact with the second end of the equalizing sleeve and the second end of the piezoelectric actuator by a spring also in this case. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section through a fuel injection valve according to an exemplary embodiment of the present invention. 
     FIG. 2 shows detail II in FIG. 1 in a detailed sectional view. 
     FIG. 3 shows detail II in FIG. 1 in a detailed sectional view according to a variant of the exemplary embodiment of FIG.  2 . 
     FIG. 4 shows a section through a fuel injection valve according to another exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A section of an injection valve according to an exemplary embodiment of the present invention is shown in FIG.  1 . Fuel injection valve  1  is used to inject fuel in particular into an externally ignited compressed-mixture internal combustion engine. 
     Fuel injection valve  1  has a valve closing body  2  designed in one piece with valve needle  3  and forms a sealing seat together with a valve seat body  4 . The embodiment of fuel injection valve  1  shown in FIG. 1 is a fuel injection valve  1  opening outward. A valve seat surface  5  is therefore arranged on the outside of valve seat body  4 . 
     Valve seat body  4  is inserted in an axial longitudinal bore  6  of a valve housing  7  and sealingly connected to valve housing  7  through welding, for example. Fuel enters via a fuel inlet opening  8  in valve housing  7  and goes to the sealing seat formed by valve closing body  2  and valve seat body  4  via a spring support space  9 . A restoring spring  10  arranged between valve seat body  4  and a flange  11  of valve needle  3  is arranged in spring support space  9  formed by axial longitudinal bore  6  of valve housing  7 . Restoring spring  10  transmits a restoring force to valve needle  3  in the closing direction of fuel injection valve  1 . 
     Valve needle  3  and valve closing body  2  are actuated via a piezoelectric actuator  12  whose first end  13  is in flush contact with end face  14  of flange  11  of valve needle  3 . When piezoelectric actuator  12  is electrically excited, it expands in its axial longitudinal direction and displaces valve needle  3  and valve closing body  2  designed in one piece with valve needle  3  downward in FIG. 1, so that fuel injection valve  1  opens. After the electrical excitation voltage is turned off, piezoelectric actuator  12  contracts again, so that valve closing body  2  is moved back into its closing position by restoring spring  10 . 
     A feature according to an exemplary embodiment of the present invention is the layered design of piezoelectric actuator  12 . An exemplary embodiment of the layered structure of piezoelectric actuator  12  is shown enlarged in FIG.  2 . 
     Piezoelectric actuator  12  has a plurality of stacked piezo layers  21  made of a piezoelectric material. Electrodes are applied on piezo layers  21  such as, for example, by sputtering or vapor deposition, so that an electric voltage can be applied to piezo layers  21  resulting in an electrical field being formed in piezo layers  21  in the direction of longitudinal axis  22  of fuel injection valve  1 , causing piezoelectric actuator  12  to expand. 
     The expansion and contraction of piezo layers  21  depend not only on the electrical field intensity applied, but also, to a considerable degree, on the temperature. Unlike usual materials, piezoelectric substances have a negative thermal expansion coefficient (α&lt;0), which means that piezoelectric materials contract with increasing temperature. In order to prevent an unintended valve motion caused by temperature fluctuations, this temperature-dependent expansion of piezo layers  21  must be compensated. Therefore, according to an exemplary embodiment of the present invention, at least one, or a plurality of temperature compensation layers  20  are arranged between piezo layers  21 . Temperature compensation layers  20  have a temperature expansion coefficient whose operational sign is opposite to that of the temperature expansion coefficient of piezo layers  21 , which means that temperature compensation layers  20  are made of a material with a positive temperature expansion coefficient (α&gt;0), while piezo layers  21 , have a negative temperature expansion coefficient (α&lt;0). By selecting the appropriate number and thickness of temperature compensation layers  20 , the sum of contractions or expansions of all temperature compensation layers  20  corresponds in absolute value to the sum of expansions or contractions of all piezo layers  21 , but with the opposite sign. Effective temperature compensation is achieved in this manner. 
     In the exemplary embodiment illustrated in FIG. 2, a piezo layer  21  and a temperature compensation layer  20  are alternatingly sandwiched in a piezoelectric actuator  12 . FIG. 3 shows, as an enlargement of detail II in FIG. 1, a piezoelectric actuator  12  having an alternatively layered structure, in which a temperature compensation layer  20  is arranged between a plurality of piezo layers  21 . 
     A material having a high positive temperature expansion coefficient such as aluminum, copper, or a suitable plastic is well-suited for temperature compensation layers  20 ; materials having a good thermal conductivity and a low heat capacity, so that the temperature of temperature compensation layer  20  is quickly equalized to the temperature of piezo layers  21 , are also advantageous. 
     If temperature compensation layers  20  are made of a metallic material, temperature compensation layers  20  may advantageously also be used as electrodes for piezo layers  21 . 
     Since temperature compensation layers  20  are arranged in close proximity to piezo layers  21 , rapid equalization of the temperature of temperature compensation layers  20  to the temperature of piezo layers  21  is ensured, so that temperature compensation is not subject to any considerable delay. 
     Through the measures described according to the exemplary embodiment of the present invention, effective temperature compensation of piezoelectric actuators  12  is achieved, so that the resulting temperature expansion coefficient of piezoelectric actuator  12  is at least approximately equal to zero. However, if piezoelectric actuator  12  is in direct contact with a solid component of valve housing  7 , unintended relative displacement of valve seat body  4  with respect to valve closing body  2  may occur, which may result in unintended valve opening due to the temperature expansion or contraction of the areas of valve housing  7  surrounding actuator  12 . Therefore it is proposed according to the present invention that the thermal expansion of valve housing  7  be also compensated. For this purpose, an equalizing sleeve  23  surrounding piezoelectric actuator  12  is provided. One end  24  of equalizing sleeve  23  is in contact with first step  25  of valve housing  7 . First end  13  of piezoelectric actuator  12  acts via valve needle  3 , as described above, upon valve closing body  2 . Second end  26  of equalizing sleeve  23 , opposite first end  24 , and second end  27  of piezoelectric actuator  12 , opposite first end  13  are connected via a connecting element  28  which has a plate-shaped design in the exemplary embodiment. Connecting element  28  is movable in the axial direction in valve housing  7  and is held in contact both with second end  26  of equalizing sleeve  23  and second end  27  of piezoelectric actuator  12  by spring  29  designed in the present embodiment as a flat spring. Valve housing  7  is terminated by an end plate  30 , which is in contact with spring  29  and which may be connected to main body  31  of valve housing  7 , for example, by welding. 
     Equalizing sleeve  23  has the same axial length as piezoelectric actuator  12  and is made of a material having an extremely low temperature expansion coefficient, preferably a ceramic material or a glass material. Since piezoelectric actuator  12 , as described above, is temperature compensated, both equalizing sleeve  23  and piezoelectric actuator  12  are subject to virtually no temperature-dependent longitudinal expansion. Connecting element  28  is therefore always in the same axial position regardless of the operating temperature of fuel injection valve  1  with respect to step  25  of valve housing  7 , and regardless of a possible temperature-dependent longitudinal expansion to which the areas of valve housing  7  surrounding equalizing sleeve  23  and piezoelectric actuator  12  are subjected. Therefore, temperature-dependent expansion of these areas of valve housing  7  causes no axial displacement of valve seat body  7  with respect to valve closing body  2 . If valve needle  3  and a section of valve housing  7  between step  25  and valve seat body  4  are made of the same material, a change in temperature in this area also causes no relative change in the position of valve closing body  2  with respect to valve seat body  4 , so that fuel injection valve  1  as a whole is effectively temperature compensated. 
     FIG. 4 shows another embodiment of fuel injection valve  1  according another exemplary embodiment of the present invention. In the exemplary embodiment shown in FIG. 4, temperature compensation is implemented in a fuel injection valve  1  opening inward. In order to facilitate identification, elements described previously are provided with the same reference symbols, so that a redundant description is unnecessary. 
     In the exemplary embodiment illustrated in FIG. 4, piezoelectric actuator  12  has a sleeve shape. It has, however, the same layered structure as shown in FIGS. 2 and 3, i.e., temperature compensation layers  20  are arranged between piezo layers  21 , so that piezoelectric actuator  12  is temperature compensated. The effective temperature expansion coefficient of actuator  12  is therefore essentially equal to zero. In the exemplary embodiment illustrated in FIG. 4, an equalizer sleeve  23  made of a ceramic material and surrounded by piezoelectric actuator  12  may be provided. Fuel inlet opening  8  is formed at a fuel inlet nozzle  40  at the end of fuel injection valve  1  opposite valve seat body  4 . Fuel is supplied to the sealing seat via an axial bore  41  in fuel inlet nozzle  40 , a cutout  42  in plate-shaped connecting element  28 , an axial longitudinal cutout  43  in equalizing sleeve  23 , through bores  44  in flange  11  of valve needle  3 , and spring support space  9 . The pulling spring  45  is provided in spring support space  9 . 
     Furthermore, a connector plug  46  used for electrical contacting of piezoelectric actuator  12  is illustrated in FIG.  4 . Connector plug  46  may be designed as an injection molded plastic part, for example. 
     When piezoelectric actuator  12  is electrically actuated, its first end  13  is in contact with step  25  of valve housing  7  and displaces plate-shaped connecting element  28  in FIG. 4 upward against spring  29 . Flange  11  of valve needle  3  is held in contact with first end  24  of equalizing sleeve  23  by pulling spring  45 . At the same time, second end  26  of equalizing sleeve  23  is permanently held in contact with plate-shaped connecting element  28 . Therefore, the expansion of piezoelectric actuator  12  causes valve closing body  2  to lift and fuel injection valve  1  to thereby open. It is essential that the elastic force of spring  29  be greater than the elastic force of pulling spring  45 . When the electrical excitation voltage is turned off, piezoelectric actuator  12  contracts again so that spring  29  again brings valve closing body  2  in contact with valve seat body  4  via plate-shaped connecting element  28 , equalizing sleeve  23 , and valve needle  3  and thus closes fuel injection valve  1 . 
     Since equalizing sleeve  23  has the same axial length as piezoelectric actuator  12  and both piezoelectric actuator  12  and equalizing sleeve  23  have an extremely low temperature expansion coefficient, valve lift is almost temperature-independent. In particular, the area of valve housing  7  surrounding piezoelectric actuator  12  and equalizing sleeve  23  have no influence on the valve lift, since its thermal expansion is compensated by spring  29 . 
     A conducting compound can be applied between actuator  12  and equalizing sleeve  23  either in the exemplary embodiment of FIG. 1 or in the exemplary embodiment of FIG. 4 for improved heat capacity between equalizing sleeve  23  and actuator  12 . 
     Instead of pulling spring  45 , the flush contact between flange  11  of valve needle  3  with first end  24  of equalizing sleeve  23  and the flush contact with second end  26  of equalizing sleeve  23  with plate-shaped connecting element  28  can also be implemented by gluing or pressing, for example. 
     Since fuel flows through the center of fuel injection valve  1  according to the exemplary embodiment illustrated in FIG. 4, rotation-symmetric components can be used, which allows inexpensive manufacturing. Fuel injection valve  1 , with fuel flowing through its center, requires no lateral fuel inlet opening  8 . Therefore installation on an internal combustion engine using normal hydraulic connecting methods is simplified. Due to the fact that no parts subject to wear are used, a long-lasting fuel injection valve  1  results according to the exemplary embodiments of the present invention.