Abstract:
A valve for controlling fluids in which the valve is actuatable by a piezoelectric actuator that acts directly on a valve member of the control valve. To compensate for changes in length of different magnitudes resulting from temperature factors, a Peltier element is incorporated into the heat flow from the piezoelectric actuator to a heat-dissipating part of the housing, and the Peltier element is triggered by an electric control unit in such a way that temperature-dictated changes in length of the piezoelectric actuator are at least partly compensated for compared with those of the housing receiving the piezoelectric actuator.

Description:
PRIOR ART 
     The invention is based on a valve for controlling fluids as generically defined hereinafter. In one such valve, known from European Patent Disclosure EP A 0 371 469, the actuation of the valve member is accomplished by a piezoelectric actuator, by the provision of a hydraulic chamber between the piezoelectric actuator and the valve member, by way of which chamber tolerances can be to compensated for. Such valves have the disadvantage that care must be taken to assure that the hydraulic chamber is always adequately filled with adjusting fluid. Furnishing a hydraulic chamber also means major expense for sealing the chamber off. If conversely the valve for controlling fluids is to be actuated directly by a piezoelectric actuator, then problems arise from the fact that the work of a piezoelectric actuator, because of a supply of current to it, produces considerable heat. The heat leads to changes in a length of the piezoelectric actuator itself and to thermal expansions of the housing that surrounds the piezoelectric actuator and heats the piezoelectric acuator. Over the course of operation of such a valve, it can thus happen that because of the different thermal expansions, the valve can no longer reach its closing position or a defined position. 
     ADVANTAGES OF THE INVENTION 
     The valve according to the invention for controlling fluids has an advantage over the prior art that a simple valve can be furnished which is actuated directly by the piezoelectric actuator; by the compensating elements provided, temperature-dictated changes in length of the piezoelectric actuator are substantially compensated for compared to those of the housing receiving the piezoelectric actuator. Advantageously, this is achieved by incorporating a Peltier element, which is triggered by an electric control unit in such a way that temperature-dictated changes in length of the piezoelectric actuator are at least partly compensated for compared with those changes in the housing that receives the piezoelectric actuator, into the heat flow from the piezoelectric actuator to a heat-dissipating part of the housing. Depending upon how electrical current is supplied, such a Peltier element can effect either cooling or heating in a known manner at the intended installation location and can thus counteract undesired changes of length, especially those in the actuating direction of the piezoelectric actuator, because either heat is supplied or heat is dissipated, depending on the state of operation or the construction specifications. 
     Advantageously, compensation is accomplished by using compensating elements whose material has a different coefficient of thermal expansion from that of the surrounding materials. A material with a high coefficient of thermal expansion has the advantage that by means of heating and/or cooling by the Peltier element, relatively large compensating changes in length in the actuating direction of the piezoelectric actuator are attained, which can absorb the large changes in length resulting in the region of heat development of the piezoelectric stack that is in operation, and thus a compensation for these changes in length is achieved compared to the lesser changes in length in the surrounding housing because of the lesser heat flow density there. 
     The heating or cooling of the compensating elements can be done by means of an axially or radially adjoining Peltier element. In the latter version, the advantages are a smaller axial installation space required and a larger contacting surface area between the compensating elements and Peltier element and the heat-absorbing housing that furnishes a heat sink. The heat outflow from the piezoelectric element to the heat sink can advantageously be improved by interposing the cup-shaped housing as set forth hereinafter, so as to achieve the least possible temperature dependency of the actuating stroke of the piezoelectric actuator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One exemplary embodiment of the invention is shown in the drawings and described in further detail below. 
     FIG. 1 shows a schematic illustration of a fuel injection valve, in which the valve according to the invention for controlling fluids can be employed; 
     FIG. 2 is a simplified illustration of a first version of the valve of the invention; 
     FIG. 3 shows a second version of the invention using compensating elements; 
     FIG. 4 shows a third exemplary embodiment of the invention, with a Peltier element resting circumferentially on the compensating elements of FIG. 3; and 
     FIG. 5 shows a fourth exemplary embodiment of the invention, with a piezoelectric actuator supported in a cup-shaped housing part. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified illustration of a fuel injection valve  1 , which has an injection valve housing  2  with a stepped bore  3 , in which an injection valve member  5  is guided. This valve member has a conical sealing face  6  on one end, which cooperates with a conical valve seat  7  on the end of the stepped bore  3 . Downstream of the valve seat, there are fuel injection openings  8 , which are disconnected from a pressure chamber  9  when the sealing face  6  is placed on the valve seat  7 . The pressure chamber extends via an annular chamber  10 , formed around the smaller-diameter part  13  of the injection valve member  5  adjoining the sealing face  6  upstream, as far as the valve seat  7 . Via a pressure line  12 , the pressure chamber  9  communicates with a high-pressure fuel source in the form of a high-pressure fuel reservoir  14 , which is supplied with fuel, brought to injection pressure, from a tank  11 , for example by a high-pressure pump  4  that pumps at a variable pumping rate. The high-pressure fuel reservoir supplies a plurality of the injection valves shown. In the region of the pressure chamber  9 , the smaller-diameter part  13  of the injection valve member changes over, at a pressure shoulder  16  pointing toward the valve seat  7 , into a larger-diameter part  18  of the injection valve member. This part is guided tightly in the stepped bore  3  and continues, on the side remote from the pressure shoulder  16 , in an intermediate part  19  until the intermediate part reaches a piston-like end  20  of the injection valve member. In the region of the intermediate part  19 , this part has a spring plate  22 , and a compression spring  21  that urges the fuel injection valve in the closing direction the compression spring is fastened between the spring plate and the housing  2  of the fuel injection valve. 
     The piston-like end  20 , with a face end  24  whose surface area is greater than that of the pressure shoulder  16 , defines a control chamber  25  in the housing  2  of the fuel injection valve; via a first throttle  26 , this control chamber communicates constantly with the high-pressure fuel reservoir  14 , and via a second throttle  27 , disposed in an outflow conduit  28 , this chamber communicates with a relief chamber  29 . The flow through the outflow conduit  28  is controlled by a control valve  31 , embodied as a 2/2-way valve, in such a way that the outflow conduit is either opened or closed. 
     The triggering of the control valve  31  serves to control the injection quantity and the instant of injection of fuel into the combustion chambers of an associated internal combustion engine, in particular a diesel engine. When the control valve is closed, because of the constant communication of the control chamber  25  with the high-pressure fuel reservoir, the pressure prevailing there is at a high level. Because the surface area of the face end  24  is greater than the surface area of the pressure shoulder  16 , and the pressure acting on both of these faces is of equal magnitude at this moment, there is a resultant force, reinforced by the compression spring  21 , that keeps the fuel injection valve member  5  in the closed position. If the control valve  31  is opened in order to trip an injection, then the control chamber  25  can be relieved to the relief chamber  29 , so that being decoupled from the high-pressure fuel reservoir by the first throttle  26 , a pressure at a lower level is established in the control chamber  25 . In this case, the compressive forces acting in the opening direction on the pressure shoulder  16  predominate, and the fuel injection valve is opened for injection, whereby the instant of injection and the injection onset are defined. By re-closure of the control valve  31 , the original high-fuel pressure is very quickly reestablished in the control chamber  25 , since the fuel can continue to flow in via the first throttle  26 . As a result, the fuel injection valve member  5  returns to its outset or closing position to terminate the high-pressure injection. 
     The triggering of the fuel injection valve is effected via a control unit  36 , which as a function of operating parameter triggers the control valves  31  of the individual fuel injection valves, and which with a pressure sensor  37  also detects the pressure in the high-pressure fuel reservoir and controls the variably pumping high-pressure fuel pump  4  as a function of the deviation from a desired set-point value. Parallel to this pump, a pressure limiting valve  38  can be provided, which is also triggerable as a pressure control valve as a function of operating parameters, depending on the design of the high-pressure fuel quantity delivery. The high-pressure fuel pump can also pump the same quantity constantly, and via the pressure limiting valve, which in this case should explicitly be considered to be a pressure control valve, the pressure in the high-pressure fuel reservoir  14  can be regulated. 
     The valve according to the invention for controlling fluids can be used as a control valve  31 . In FIG. 2, part of the fuel injection valve of FIG. 1 is shown, along with the injection valve housing  2 , in which the control valve  31  is also integrated, and along with the control chamber  25 , which is enclosed in the housing by the face end  24  of the piston-like end  20 . The inflow to the control chamber  25  is effected via the first throttle  26 , and the outflow is effected via the outflow conduit  28 , in which the second throttle  27  is seated. 
     The control valve has a valve member  40 , with a shaft  41  and a valve head  42  that protrudes into a valve chamber  43 . On the end of the shaft  41  remote from the valve head, a spring plate  44  is provided, on which a compression spring  45  rests, which on the other end is braced against the housing and seeks to keep the valve member in the closing position. This is accomplished by the contact of a sealing face  47 , provided on the valve head, with a conical seat  46 , which is located at the transition between the valve chamber  43  and a guide bore  48  of the adjoining shaft  41 . Adjacent to the sealing face  47 , the shaft has an annular recess  49 , which makes it possible, when the valve head  42  is lifted from the valve seat  46 , for the valve chamber  43  to communicate with a portion of the outflow conduit  128  that branches off from the guide bore  48 . This outflow conduit discharges into a spring chamber  51 , which receives the compression spring and the end, protruding from the guide bore  48 , of the shaft  41  along with the spring plate  46 , and from which a line  228  leads to the relief chamber  29 . By means of the compression spring  45 , the valve member  40  is normally kept in the closing position, so that the valve chamber  43  and the control chamber  25  are closed on the outflow side, and the high pressure of the high-pressure fuel reservoir can build up in the control chamber  25  in order to close the fuel injection valve member. 
     An actuation of the valve member  40  in the opening direction is accomplished by means of the aforementioned piezoelectric actuator  53 . The piezoelectric actuator comprises a piezoelectric element in the form of a piezoelectric stack  56 , which is enclosed in axial terms by a bottom disk  57  and a top disk  58 ; as the actuating part, the bottom disk  57 , with a piston like end  59 , can be made to contact the shaft  41 . Since piezoelectric components can be stressed permanently and reliably only for pressure, the piezoelectric stack is prestressed by spring element  60 . The supply of electricity to the piezoelectric stack is not shown in the drawing and is accomplished in the usual way. The actuator, thus formed of the piezoelectric stack, bottom disk  57 , top disk  58 , and spring elements  60 , is tightly enclosed in an actuator chamber  54  by a resilient diaphragm element  61 . The diaphragm element  61  closes the actuator chamber  54  off from the spring chamber  51  and also keeps the actuator, with its top disk  58 , in contact with a Peltier element  62 . The Peltier element rests with flat faces on one side on the top disk  58  and on the other on the housing wall  63  pointing in the axial direction of the valve member  40 , and it has electric lead lines  65  and  66  for current flows controlled by the control unit. 
     As a result, if heat develops in the piezoelectric stack, there is a good outflow of heat between the top disk  58  and the Peltier element  62 , and between the Peltier element and the housing wall of the injection valve housing  2 . When voltage is supplied to the piezoelectric stack, the piezoelectric actuator lengthens, displacing the valve member  40  in the opening direction, which returns to its closing position upon retraction of the excitation of the piezoelectric stack by the force of the compression spring  45  and the attendant reduction in length of the piezoelectric stack. These operating events of the piezoelectric stack create heat, which lead to expansion of material, taking into account the applicable coefficients of thermal expansion for the various materials. Despite good heat dissipation, the temperature in the region of the piezoelectric actuator, which as a heat source has a high-heat flow stress, will become greater during operation of the fuel injection valve than the temperature of the housing surrounding the actuator chamber  54 , the housing having a lesser heat flow stress. With increasing time in operation, the piezoelectric actuator would accordingly increase in length compared with the specified length of the housing into which it is installed, and could affect the position of the valve member  40 . To allow the valve member  40  to regularly return to its closing position, a prestroke hv is provided here first, which the piston-like end  59  must cover in order to attain contact with the valve member. This prestroke can partly absorb the aforementioned temperature-dictated differences in length, so that the mode of operation of the control valve is unaffected thereby. In addition, however, the Peltier element also offers a further capability of compensating for differences in length, because when controlled actively it has influence on the temperature of the piezoelectric actuator. In a known manner, depending on the current flow direction through a Peltier element, a temperature drop (cooling) or a temperature increase can be accomplished. These elements are therefore also known as electric semiconductor heat pumps. With a regulated supply and/or dissipation of heat, the temperature of the piezoelectric actuator can be regulated within limits. Depending on the structural specification of the length of the piezoelectric actuator in relation of the valve to be actuated, the spacing of the piezoelectric actuator from this valve can be kept constant by heating or cooling. 
     To reinforce the action of the Peltier element, a compensating element  68  can be provided in accordance with FIG. 3 between the piezoelectric actuator and the Peltier element in the direction of a change of length of the piezoelectric actuator  53 , or in its actuating direction; the compensating element, with its flat face ends  69 ,  70 , rests flush on the piezoelectric actuator  53  and the Peltier element  62 , respectively, so that good heat flow is assured. The compensating element has a particularly high coefficient of thermal expansion. By variably heating or cooling via the Peltier element  62 , it is possible with this high coefficient of thermal expansion to compensate for a major change in length of the piezoelectric actuator  53 . For example, when the engine or the valve is not yet at operating temperature, if the Peltier element is heated, then the piezoelectric actuator at the beginning of operation assumes an intended working position relative to the shaft  41  of the valve member  40 . As the heating increases, the compensating elements  68  can then be cooled, which leads to a loss in terms of a change of length, which compensates for the increase in terms of change in length of the piezoelectric actuator. 
     In an alternative version shown in FIG. 4, however, the compensating elements  68  can also be brought into direct contact with the adjacent housing wall  63 . In this case, the Peltier element  62 ′ is disposed circumferentially on the compensating elements  68 ′, thus forming a large heat-contact area. On the other hand, the annular Peltier element is then in good thermal contact with the annularly surrounding wall  71  of the housing. Because of the large heat-introducing cross section, this wall makes it possible for the heat, which must be passed from the piezoelectric actuator  53  to the environment, to be dissipated quickly. This wall communicates with corresponding heat sinks, which can either be coolant fluid or cooling faces. 
     In a third version, which is shown in FIG. 5, the location of the piezoelectric actuator  53  in a cup-shaped housing part  73  can be fixed geometrically. This housing part offers a large heat-dissipating cross section, and its outer jacket or sidewall surface communicates circumferentially, via a Peltier element  62 ″, with the cooled housing or with special cooling bodies  75 . Once again, exact positioning of the piezoelectric actuator is accomplished. By the heat flow via the cup-shaped housing part, the piezoelectric actuator is already partially cooled, and with the support of the Peltier element  62 ′, it undergoes a compensation in length. Furthermore, the cup-shaped housing part can itself be operative as a compensating element, analogous the version of FIG.  4 . The version of FIG. 5 offers a still further improved heat flow compared with that of FIG. 4, because larger dissipating surface areas and larger areas of heat contact with the Peltier element  62 ″ are available. 
     In the construction presented here of a valve for controlling fluids, a piezoelectric actuator can be used in a simple way; a piezoelectric actuator has the advantage of exactly controllable opening strokes and also opening instants with respect to the opening travel. To that end, it offers the major advantage of allowing the realization of high switching speeds, which are capable of controlling even small preinjection quantities by means of brief and/or slight relief of the control chamber. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.