Abstract:
In an actuator unit for an injection system of an internal combustion engine, the difference in the longitudinal extension when a change of temperature of the piezoelectric actuator element occurs affecting the actuator unit is equalized compared to the actuator housing due to different thermal expansion coefficient values in that a fastening element is arranged on the actuator housing. The fastening element is made of a different material than the actuator housing and thus affects a force acting counter to the change of length of the actuator housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2008/059204 filed Jul. 15, 2008, which designates the United States of America, and claims priority to German Application No. 10 2007 033 033.4 filed Jul. 16, 2007, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates to an actuator unit for an injection system of an internal combustion engine. 
     BACKGROUND 
     Actuator units with piezoelectric actuator elements are used, for example, for injecting fuel into combustion chambers of internal combustion engines. The piezoelectric actuator elements preferably consist of ceramic and are disposed in an actuator housing consisting, for example, of invar. As the different materials of the actuator housing and piezoelectric actuator element have different values for the respective coefficient of thermal expansion, different length changes of the piezoelectric actuator element and actuator housing may occur in the event of a change in the temperature acting on the actuator unit. This may shift the position of the piezoelectric actuator element with respect to a final control element so that the operative connection between the piezoelectric actuator element and the final control element is affected accordingly. 
     Patent specification DE 199 09 106 C2 discloses an actuator unit in which length changes resulting from different coefficients of thermal expansion for the different materials of actuator housing and piezoelectric actuator element can be equalized by the actuator housing being composed of an inner, middle and outer sleeve, at least two sleeves having a different coefficient of thermal expansion. The materials of the sleeves are selected such that, as a result of the different coefficients of thermal expansion of the individual sleeves, the different length changes in the actuator housing and piezoelectric actuator element are equalized. 
     SUMMARY 
     According to various embodiments, an alternative solution for an actuator unit can be provided by means of which different linear expansions of the actuator housing and piezoelectric actuator element can be equalized as simply as possible. 
     According to an embodiment, an actuator unit comprises an actuator housing enclosing a piezoelectric actuator element, wherein the actuator housing and the piezoelectric actuator element are made from different materials with correspondingly different coefficients of thermal expansion, and a fastening element disposed on the actuator housing whose material is different from the material of the actuator housing and from the material of the piezoelectric actuator element in respect of the coefficient of thermal expansion, wherein the actuator housing and fastening element materials are selected such that one of the materials has a higher coefficient of thermal expansion than the coefficient of thermal expansion of the piezoelectric actuator element and the other material has a lower coefficient of thermal expansion than the coefficient of thermal expansion of the piezoelectric actuator element, and the fastening element engages in a thread provided on the actuator housing. 
     According to a further embodiment, the fastening element can be clamped axially on the actuator housing. According to a further embodiment, the fastening element can be embodied as a clamping nut. According to a further embodiment, the length of the part of the clamping nut engaging in the thread may depend on the coefficient of thermal expansion of the piezoelectric actuator element. According to a further embodiment, the fastening element can be made of steel, the actuator housing of invar and the piezoelectric actuator element of ceramic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail with reference to the accompanying drawings in which: 
         FIG. 1 : shows a sectional view of an actuator unit. 
     
    
    
     DETAILED DESCRIPTION 
     The advantages achieved with the various embodiments are in particular that, on the basis of a known coefficient of thermal expansion of the piezoelectric actuator element, the differential linear expansion of the actuator housing and piezoelectric actuator element in the axial direction is equalized by disposing on the actuator housing a fastening element producing a force-compensating effect. This is provided by selecting the coefficient of thermal expansion of the actuator housing and fastening element such that one of the materials has a coefficient of thermal expansion that is higher than the coefficient of thermal expansion of the piezoelectric actuator element, and the other material has a coefficient of thermal expansion that is lower than the coefficient of thermal expansion of the piezoelectric actuator element. 
     According to a further embodiment, it may be provided that the fastening means is embodied as a clamping nut and is fastened to a thread provided on the actuator housing. Disposing the clamping nut on the actuator housing is advantageous in that the different length changes of the actuator housing and piezoelectric actuator element take place without deformation of the actuator unit, thereby enabling the actuator unit to be re-machined inexpensively and quickly. 
     According to a further embodiment, the equalization of the different linear expansions of the actuator housing and piezoelectric actuator element can be finely adjusted via the length of the part of the clamping nut engaging in the thread. The length of the part of the clamping nut engaging in the thread of the actuator housing depends on the coefficient of thermal expansion of the piezoelectric element. The advantage is therefore that different length changes of the same material, which may occur e.g. in the case of the same material being supplied by different manufacturers, are prevented solely by changing the length of the part of the clamping nut engaging in the thread of the actuator housing. 
       FIG. 1  shows a sectional view of an actuator unit. The actuator unit  1  has a piezoelectric actuator element  13  which is disposed inside an actuator housing  14 . The piezoelectric actuator element  13  is connected at one end to a head element  11  and at the opposite end to a base element  12 . A fastening element  10  is also disposed on the actuator housing  14 . Said fastening element  10  is, for example, a clamping nut. In an advantageous embodiment of the actuator unit, the piezoelectric actuator element  13  consists of ceramic, the actuator housing  14  of nickel steel, and the fastening element  10  of steel. 
     Because of the different coefficients of thermal expansion for the materials of the piezoelectric actuator element  13  and actuator housing  14 , in the event of a change in temperature a differential length change of the actuator housing  14  and piezoelectric actuator element  13  of the actuator unit occurs in the axial direction. The position of the piezoelectric actuator element  13  may now change in relation to a final control element not shown in  FIG. 1 . This may cause an operative connection between the piezoelectric actuator element  13  and the final control element not shown in  FIG. 1  to be influenced such that, in the case of injection valves, for example, control of the amount of fuel to be injected will be impaired. 
     The differential length change between the piezoelectric actuator element  13  and actuator housing  14  occurring in the actuator unit  1  in the event of a temperature change is equalized by the fastening element  10 . Equalization of the differential linear expansion by the fastening element  10  takes place insofar as a force component is exerted on the actuator housing  14  by the fastening element  10  in the axial direction, counteracting the movement of the actuator housing  14 . As the fastening element  10  is embodied as a clamping nut, for example, the force exerted on the actuator housing  14  is greater the longer the part of the clamping nut engaging in the thread of the actuator housing  14 . 
     The fastening element  10 , the actuator housing  14  and the piezoelectric actuator element  13  are preferably made of different materials having a different coefficient of thermal expansion. To enable the different length changes between the actuator housing  1  and the fastening element  10 , in relation to the length change of the piezoelectric actuator element  13 , to be equalized, the materials of the fastening element  10  and actuator housing  1  are selected such that one of the two materials has a higher coefficient of thermal expansion than the coefficient of thermal expansion of the piezoelectric actuator element  13 , and the other material has a lower coefficient of thermal expansion than the coefficient of thermal expansion of the piezoelectric actuator element  13 . 
     The case shall now be considered that the coefficient of thermal expansion of the fastening element  10  is lower than the coefficient of thermal expansion of the piezoelectric actuator element  13 , and at the same time the coefficient of thermal expansion of the actuator housing  14  is higher than the coefficient of thermal expansion of the piezoelectric actuator element  13 . Therefore, because of the higher coefficient of thermal expansion of the actuator housing  14 , an excessively large linear expansion of the actuator housing  14  compared to the piezoelectric actuator element  13  occurs. This excessively large linear expansion compared to the piezoelectric actuator element  13  is equalized by the fastening element  10  in that now, because of the lower coefficient of thermal expansion of the fastening element  10 , a force opposing the axial direction of movement of the actuator housing  14  is exerted on the actuator housing  14 . 
     In some embodiments, the fastening element  10  is embodied as a clamping nut and is fastened to a thread  16  provided on the actuator housing. The equalization of the different linear expansions of the actuator housing  14  and piezoelectric actuator  13  can be finely adjusted by adjusting the length of the part of the clamping nut engaging in the thread  16 . The length of the part of the clamping nut engaging in the thread  16  of the actuator housing depends on the coefficient of thermal expansion of the piezoelectric element.