Abstract:
An injector includes an actuator arranged in an actuator space, a piston guide having a bore hole, and a piston arranged in the bore hole. The piston has a first end face facing the actuator and delimiting a first space in and/or on the bore hole, and a second end face lying opposite the first space and delimiting an adjoining second space in and/or on the bore hole. The piston is arranged between the first and second spaces, and a gap extends around the circumference of the piston between the piston and the bore hole. The piston includes a first material and the piston guide includes a second material, the first and second materials having different thermal expansion properties such that when the piston guide and/or piston are heated, the gap width of the gap decreases to limit fuel leakage between the first space and second spaces.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2014/060535 filed May 22, 2014, which designates the United States of America, and claims priority to DE Application No. 10 2013 210 843.5 filed Jun. 11, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to an injector having an actuator chamber in which an actuator is arranged, a control plate in which a control piston bore is provided, a control piston which is arranged in the control piston bore of the control plate, wherein the control piston has a first face side facing toward the actuator, wherein a section of the control piston bore delimited by the first face side forms a control chamber, wherein a section of the control piston bore situated opposite the control chamber forms a spring chamber, wherein the control piston is arranged between the control chamber and the spring chamber, wherein, at the circumference of the control piston, a gap with a gap width is provided between the control piston and the control piston bore. 
       BACKGROUND 
       [0003]    For the injection of fuel into internal combustion engines, use is made inter alia of direct fuel injection. For this purpose, use is made of piezo injectors, the nozzle needle of which is driven by way of a piezo actuator. Here, virtually clearance-free coupling between the piezo actuator and nozzle needle is necessary, though this is difficult to maintain owing to thermal changes in length in the piezo injector. To eliminate this problem, the nozzle needle is coupled hydraulically to the piezo actuator. For this purpose, the piezo injector has an actuator chamber in which the piezo actuator is arranged. A control piston is arranged in a control piston bore. The control piston has a first face side facing toward the piezo actuator. A section of the control piston bore delimited by the first face side forms a first control chamber. A section of the control piston bore situated opposite the first control chamber forms a spring chamber. The control piston is arranged between the first control chamber and the spring chamber. A nozzle needle has a second face side. The nozzle needle guides a nozzle needle sleeve, wherein the nozzle needle sleeve and the second face side delimit a second control chamber. Furthermore, a connecting bore is provided between the first control chamber and the second control chamber. A leakage pin, which is arranged between the piezo actuator and the first face side and in a leakage pin bore, effects coupling of the piezo actuator and of the control piston. If the piezo actuator is actuated, the leakage pin presses against the control piston and displaces the latter in the direction of the nozzle needle. 
         [0004]    Owing to the pressure difference between the control chamber and the spring chamber, a fluid flow takes place laterally through a gap between the control piston and the control piston plate. Here, the fluid flow is dependent on the gap width and the temperature of the fuel. Owing to the great temperature differences during the operation of the injector with cold and hot fuels and with a cold and hot internal combustion engine, the fluid flow changes in a manner dependent on the temperature of the injector and of the fuel. This can lead to changed operating characteristics. 
       SUMMARY 
       [0005]    One embodiment provides an injector having an actuator chamber in which an actuator is arranged, a piston guide in which a bore is provided, a piston which is arranged in the bore of the piston guide, wherein the piston has a first face side facing toward the actuator, wherein the piston, by way of the first face side, delimits a first chamber which is arranged in and/or at the bore, wherein the piston, by way of a second face side situated opposite the first chamber, delimits a second chamber which is adjacent in and/or at the bore, wherein the piston is arranged between the first chamber and the second chamber, wherein, at the circumference of the piston, a gap with a gap width is provided between the piston and the bore, wherein the piston has a first material and the piston guide has a second material, wherein the first material, when warmed up, exhibits first thermal expansion, and the second material, when warmed up, exhibits second thermal expansion which differs from the first thermal expansion, and wherein the first material is selected relative to the second material such that, when the piston guide and/or the piston are/is warmed up, the gap width of the gap decreases in order to limit fuel leakage between the first chamber and the second chamber. 
         [0006]    In a further embodiment, the first material and the second material are selected such that, when the piston guide and/or the piston are/is warmed up, leakage of the fuel through the gap is substantially constant over the course of the warming-up of the piston guide and/or of the piston. 
         [0007]    In a further embodiment, the first material has a first coefficient of thermal expansion and the second material has a second coefficient of thermal expansion, wherein the first material and the second material are selected such that the first coefficient of thermal expansion is lower than the second coefficient of thermal expansion. 
         [0008]    In a further embodiment, the first material has a first coefficient of thermal expansion and the second material has a second coefficient of thermal expansion, wherein the two materials are selected such that the two coefficients of thermal expansion have a difference of 3 to 12·10 −6  K −1 , in particular 5 to 10·10 −6  K −1 . 
         [0009]    In a further embodiment, the first material has a first coefficient of thermal expansion of 5 to 25·10 −6  K −1  and the second material has a second coefficient of thermal expansion of 10 to 30·10 −6  K −1 . 
         [0010]    In a further embodiment, one of the two materials is hard metal, in particular in a composition having at least 70 percent, preferably at least 90, tungsten carbide and 1 to 30 percent, preferably 1 to 10 percent, cobalt or nickel-chromium or nickel-chromium-cobalt, and the other of the two materials is steel, in particular an unalloyed steel or low-alloy steel. 
         [0011]    In a further embodiment, one of the two materials has titanium, in particular at least 50 percent, preferably 80 percent titanium, and the other of the two materials has a steel which comprises at least one of the following metals: chromium, nickel, manganese, copper. 
         [0012]    In a further embodiment, one of the two materials has steel with a coefficient of thermal expansion of 12 to 16·10 6  K 1  and the other of the two materials has a manganese steel, in particular MnNi10Cu18 or MnNi16Cu10. 
         [0013]    In a further embodiment, the piston guide is a control plate or a leakage pin bore or a nozzle needle sleeve, and in that the piston, correspondingly to the piston guide, is a control piston or a leakage pin or a nozzle needle. 
         [0014]    In a further embodiment, the actuator chamber is a piezo actuator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Example embodiments of the invention are described in detail below with reference to the drawings, in which: 
           [0016]      FIG. 1  shows a sectional view of an upper part of an injector; 
           [0017]      FIG. 2  shows a sectional view of a lower part of the injector; 
           [0018]      FIG. 3  shows an enlarged view of the sectional view of the injector shown in  FIG. 2 ; and 
           [0019]      FIG. 4  shows a diagram of a kinetic viscosity of a fuel, plotted versus a temperature of the fuel. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Some embodiments of the invention provide an injector including an actuator chamber in which an actuator is arranged, a piston guide in which a bore is provided, and a piston which is arranged in the bore of the piston guide. The piston has a first face side facing toward the actuator. The piston, by way of the first face side, delimits a first chamber which is arranged in and/or at the bore. The piston, by way of a second face side situated opposite the first chamber, delimits a second chamber which is adjacent in and/or at the bore, wherein the piston is arranged between the first chamber and the second chamber. At the circumference of the piston, a gap with a gap width is provided between the piston and the bore. The piston has a first material and the piston guide has a second material, wherein the first material, when warmed up, exhibits first thermal expansion, and the second material, when warmed up, exhibits second thermal expansion which differs from the first thermal expansion. The first material is selected relative to the second material such that, when the piston guide and/or the piston are/is warmed up, the gap width of the gap decreases in order to limit fuel leakage between the first chamber and the second chamber. 
         [0021]    This configuration has the advantage that, when the fuel and/or the injector are/is warmed up, the fuel leakage flow between the first chamber and the second chamber is reduced owing to the decreasing gap width. In this way, the injector exhibits improved operating characteristics, and can be actuated in a more precise and targeted manner. At the same time, the gap width can be adapted in targeted fashion to the lubrication characteristics of the fuel in an improved manner. 
         [0022]    In a further embodiment, the first material and the second material are selected such that, when the piston guide and/or the piston are/is warmed up, leakage of the fuel through the gap is substantially constant over the course of the warming-up of the piston guide and/or of the piston. In this way, particularly stable operating characteristics and particularly good actuation characteristics of the injector are provided. 
         [0023]    In a further embodiment, the first material has a first coefficient of thermal expansion and the second material has a second coefficient of thermal expansion, wherein the first material and the second material are selected such that the first coefficient of thermal expansion is lower than the second coefficient of thermal expansion. In this way, distortion of the piston guide within a housing of the injector is prevented, and at the same time, the gap width is reduced in the event of warming up. 
         [0024]    It may be advantageous if the first material has a first coefficient of thermal expansion and the second material has a second coefficient of thermal expansion, wherein the two materials are selected such that the two coefficients of thermal expansion have a difference of 3 to 12·10 −6  K −1 , in particular 5 to 10·10 −6  K −1 . 
         [0025]    It may be advantageous if the first material has a first coefficient of thermal expansion of 5 to 25·10 −6  K −1  and the second material has a second coefficient of thermal expansion of 10 to 30·10 6  K 1 . 
         [0026]    It may be advantageous if one of the two materials is hard metal, in particular in a composition having at least 70 percent, preferably at least 90, tungsten carbide and 1 to 30 percent, preferably 1 to 10 percent, cobalt or nickel-chromium or nickel-chromium-cobalt, and the other of the two materials is steel, in particular an unalloyed steel or low-alloy steel. 
         [0027]    It may be advantageous if one of the two materials has titanium, in particular at least 50 percent, preferably 80 percent titanium, and the other of the two materials has a steel which comprises at least one of the following metals: chromium, nickel, manganese, copper. 
         [0028]    It may be advantageous if one of the two materials has steel with a coefficient of thermal expansion of 12 to 16·10 −6  K −1  and the other of the two materials has a manganese steel, in particular MnNi10Cu18 or MnNi16Cu10. 
         [0029]    In a further embodiment, the piston guide is a control plate or a leakage pin bore or a nozzle needle sleeve, and the piston, correspondingly to the piston guide, is a control piston or a leakage pin or a nozzle needle. 
         [0030]    It may be advantageous if the actuator is a piezo actuator. 
         [0031]      FIG. 1  shows a sectional view of an upper part  10  of an injector  15 .  FIG. 2  shows a sectional view of a lower part  20  of the injector  15  shown in  FIG. 1 .  FIG. 3  shows an enlarged sectional view of the injector  15  shown in  FIG. 2 .  FIG. 4  shows a diagram of a kinetic viscosity cSt of a fuel, plotted versus a temperature T of the fuel. Below,  FIGS. 1 to 4  will, for better understanding, be discussed jointly. The injector  15  is, in the embodiment, in the form of a piezo injector. The injector  15  may serve for the injection of fuel into an internal combustion engine, in particular for the injection of diesel fuel into a common-rail internal combustion engine. 
         [0032]    The injector  15  has an injector housing  25 . In the injector housing  25  there is provided a high-pressure bore  30 . Furthermore, a high-pressure port  35  is provided at the top side on the injector housing  25 , through which high-pressure port a pressurized fuel can be supplied into the high-pressure bore  30 . The high-pressure bore  30  runs substantially in a longitudinal direction through the injector housing  25  as far as a high-pressure region  40  in the lower part  20  of the injector  15 . Furthermore, the injector housing  25  has an actuator chamber  45  in the upper part  10  of the injector  15 . An actuator  50  is arranged in the actuator chamber  45 . In the embodiment, the actuator  50  in the form of a piezo actuator. Here, it is particularly advantageous for the piezo actuator to be in the form of a fully active piezo stack. Other actuators, in particular electrical actuators  50 , are self-evidently also conceivable. The actuator  50  is of substantially cylindrical form and can be charged with an electrical voltage by way of an electrical terminal  55 . If the electrical voltage is changed, a length of the actuator  50  in the longitudinal direction of the injector  15  can be varied. 
         [0033]    In the lower part  20  (cf.  FIGS. 2 and 3 ), the injector  15  has a control plate  60  in which a control piston bore  65  is arranged. Furthermore, a control piston  70  is arranged in the control piston bore  65 . The control piston  70  has a first face side  75  pointing in the direction of the actuator  50 . A section of the control piston bore  65  delimited by the first face side  75  forms a first control chamber  80 . At that longitudinal end of the control piston  70  which is situated opposite the first control chamber  80 , the control piston, by way of its second face side  86 , forms a spring chamber  85  in the control piston bore  65 . The control piston  70  is arranged in axially displaceable fashion between the first control chamber  80  and the spring chamber  85 . 
         [0034]    In the spring chamber  85  there is arranged a control piston spring  90  which, in the embodiment, is for example in the form of a helical compression spring. Here, a first longitudinal end of the control piston spring  90  is supported at the top side on the control piston  70  and at the bottom side on a face side of the control piston bore  65 . Here, the control piston spring  90  acts on the control piston  70  with a force which acts in the longitudinal direction or in the direction of the first control chamber  80 . The spring chamber  85  is connected by way of a high-pressure connection  95  to the high-pressure region  40 . During the operation of the injector  15 , the high-pressure region  40  is constantly flooded with fuel via the high-pressure bore  30 . Furthermore, during the operation of the injector  15 , the pressure which prevails in the high-pressure region  40  is present at all times in the spring chamber  85 . Depending on the operating state, the fuel present in the high-pressure region  40  assumes different temperatures. The different temperatures T result in a different kinetic viscosity cSt of the fuel (cf.  FIG. 4 ). 
         [0035]    Between the actuator  50  and the control piston bore  65  there is provided a leakage pin  100 . The leakage pin  100  is in this case dimensioned such that an increase in length of the actuator  50  is transmitted via the leakage pin  100  to the control piston  70 . Here, the leakage pin  100  is arranged in a leakage pin bore  105  of a leakage pin plate  106 , and forms a piston. The leakage pin bore  105  serves in this case as a (piston) guide of the leakage pin  100 . 
         [0036]    In the lower part  20  of the injector  15 , the high-pressure bore  95  opens into the high-pressure region  40 . Furthermore, a nozzle needle  110  is arranged in the high-pressure region  40 . The nozzle needle  110  guides a nozzle needle sleeve  115 , but itself forms a piston in the nozzle needle sleeve  115 . A longitudinal end, pointing in the direction of the control piston plate  60 , of the nozzle needle  110  has a face side  120 . The face side  120  forms, together with the nozzle needle sleeve  115  and the control plate  60 , a second control chamber  125 . The second control chamber  125  is fluidically connected via a connecting bore  130  to the first control chamber  80 . 
         [0037]    The nozzle needle  110  has a circumferentially encircling collar  135 . Between the collar  135  and the nozzle needle sleeve  115  there is arranged a nozzle needle spring  140 . Here, the nozzle needle spring  140  is supported by way of a first longitudinal end on the nozzle needle sleeve  115  and by way of a second longitudinal end on the collar  135 . Here, the nozzle needle spring  140  acts on the nozzle needle with a force directed away from the second control chamber  125  or a force directed away from the upper part  10 . 
         [0038]    In the closed state of the injector  15 , the nozzle needle  110  bears against a lower tip  145  of the lower part  20  of the injector  15 . Here, the actuator  50  is discharged, and is thus at its shortest length. In this state, no fuel is injected by way of the injector  15  into a combustion chamber of the internal combustion engine. This state is illustrated in  FIGS. 1 to 3 . 
         [0039]    If the actuator  50  is charged with electrical energy via the electrical terminal  55 , the length of the actuator  50  increases. Here, via the leakage pin  100 , a force of the actuator  50  is transmitted to the control piston  70 . As a result of the force, the control piston  70  is displaced in the control piston bore  65  in the direction of the nozzle needle  110 . As a result, the volume of the first control chamber  80  increases, whereby the pressure in the first control chamber  80 , and also in the second control chamber  125  which is coupled by the connecting bore  130 , is reduced. As a result, owing to the reduced pressure in the second control chamber  125 , a reduced force acts on the second face side  120  of the nozzle needle  110 . In a lower region of the nozzle needle  110 , the nozzle needle  110  continues to be acted on in the direction of the second control chamber  125  by the pressure of the high-pressure region  40 . Owing to the pressure drop in the second control chamber  125  and the fact that the pressure at the lower end of the nozzle needle  110  remains constant, the nozzle needle  110  is lifted, and the injector  15  is opened, such that fuel is injected from the high-pressure region  40  into an internal combustion engine. 
         [0040]    If the actuator  50  is subsequently deactivated and thus decreases in length, the high pressure prevailing in the spring chamber  85  and the force exerted on the control piston  70  by the control piston spring  90  effect a movement of the control piston  70  in the direction of the first control chamber  80 . As a result, the pressure in the first control chamber  80 , and also in the second control chamber  125  owing to the connecting bore  130  that exists between the first control chamber  80  and the second control chamber  125 , is increased. Owing to the elevated pressure in the second control chamber  125 , the nozzle needle  110  is forced in the direction of the tip  145  of the lower part  20  of the injector  15 , such that the injector  15  is closed and the fuel injection into the combustion chamber is ended. 
         [0041]    To prevent seizing of the control piston  70  in the control piston bore  65  of the control plate  60 , the control piston  70  has, at the circumference, a gap  150  arranged between the control piston  70  and the control piston bore  65 . The gap  150  itself has a gap width b. If, as already discussed above, the control piston  70  is forced in the direction of the nozzle needle by the leakage pin  100 , fuel flows out of the spring chamber  85  via the gap  150  into the first control chamber  80 . This leads to pressure equalization between the spring chamber  85  and the first control chamber  80 . 
         [0042]    The volume flow of the fuel flowing through the gap  150  is dependent on the viscosity of the fuel. Here, as shown in  FIG. 4 , the fuel has a kinetic viscosity cSt which decreases sharply with increasing temperature. Normally, the fuel, in particular the diesel fuel, may have a temperature of  −3 0° C. to 100° C. This has the effect that, in the case of a uniform gap width b of the gap  150 , the leakage losses through the gap  150  increase with increasing temperature T. 
         [0043]    The spring force exerted on the control piston  70  by the control piston spring  90  ensures that, in the closed state of the injector  15 , the control piston  70  bears against the leakage pin  100 . In this way, the actuator  50 , the leakage pin  100  and the control piston  70  are coupled to one another without clearance. 
         [0044]    The leakage pin  100  together with the leakage pin bore  105  realizes a first pairing clearance  155 . The first pairing clearance  155  is in this case selected such that a second gap (not illustrated) is provided at the circumference between the leakage pin  100  and the leakage pin bore  105 , and a first leakage  160  from the first control chamber  80  in the direction of the actuator chamber  45  can take place between the leakage pin bore  105  and the leakage pin  100 . From the actuator chamber  45 , the first leakage  160  can escape from the injector  15  via a leakage port  165 . 
         [0045]    Through the gap  150  between the control piston  70  and the control piston bore  65 , if the pressure in the first control chamber  80  is lower than the pressure in the spring chamber  85 , a second leakage  70  takes place from the spring chamber  85  into the control chamber  80  along the control piston  70  through the gap  150 . 
         [0046]    Furthermore, the control piston  70  may have a throttle bore  75  which fluidically connects the spring chamber  85  to the first control chamber  80 . In this case, a third leakage  180  takes place through from the spring chamber  85  into the first control chamber  80  through the throttle bore  175 . 
         [0047]    The nozzle needle  110  is guided in the nozzle needle sleeve  115  by way of a second pairing clearance  185 . The second pairing clearance  185  is in this case selected such that a second gap (not illustrated) is provided, on the circumference, between the nozzle needle  110  and the nozzle needle sleeve  115 . By way of the second pairing clearance  185 , it is possible, if the pressure in the second control chamber  125  is lower than the pressure in the high-pressure region  40 , for a fourth leakage  190  from the high-pressure region  40  into the second control chamber  125  to take place. 
         [0048]    In the closed state of the injector  15 , the first leakage  160  along the leakage pin  100  gives rise to an outflow of fuel out of the first control chamber  80 . To prevent a pressure drop in the first control chamber  80 , which can lead to an inadvertent opening of the nozzle needle  110 , the fuel flowing out as a result of the first leakage  160  must be compensated by way of the second leakage  170 , the third leakage  180  and/or the fourth leakage  190 . If the throttle bore  117  is not provided, the third leakage  180  is omitted, such that the sum of the second leakage  170  and the fourth leakage  190  is at least as great as the first leakage  160 . If the throttle bore  175  is provided, the sum of the second leakage  170 , the third leakage  180  and the fourth leakage  190  is at least as great as the first leakage  160 . 
         [0049]    In the open state of the nozzle needle  110  and thus of the injector  15 , the second leakage  170 , the third leakage  180  and/or the fourth leakage  190  give rise to an inflow of fuel into the first control chamber  80  and into the second control chamber  125 . The inflow of fuel causes a pressure increase in the first control chamber  80  and in the second control chamber  125 . Here, to prevent inadvertent premature closure of the nozzle needle  110  and thus of the injector  15 , it must be ensured that the increase in pressure in the first control chamber  80  and in the second control chamber  125  is kept small. 
         [0050]    Furthermore, the second leakage  170  and the fourth leakage  190  must be selected such that an inadvertent opening of the nozzle needle  110  in the event of very steep pressure rises in the high-pressure region  40  is prevented. As already discussed above and shown in  FIG. 4 , the fuel has a viscosity which changes with temperature. In configuring the gap width b of the gap  150  of the control piston  70 , but also of the pairing clearances  155 ,  185 , it must be ensured that the control piston  70  does not become jammed in the control piston bore  65 , giving rise to seizing of the control piston  70  in the control piston bore  65 , in the presence of high temperatures of the fuel. At the same time, it must be ensured that the second leakage  170  through the gap  150  is adequately great. 
         [0051]    This likewise applies analogously to the leakage pin  100  in the leakage pin bore  105  and the first leakage  160 , and to the fourth leakage between the nozzle needle sleeve  115  and the nozzle needle  110 . 
         [0052]    To influence the leakages  160 ,  170 ,  180 ,  190  in targeted, temperature-dependent fashion, it is provided in the embodiment that, by way of example, the control piston  70  has a first material, and the control piston plate  60  has a second material. 
         [0053]    The first material, when warmed up, exhibits first thermal expansion. The second material, when warmed up, exhibits second thermal expansion. The second thermal expansion differs from the first thermal expansion. Here, the first material and the second material are selected such that, when the control plate  60  and the control piston  70  are warmed up, the gap width b of the gap  150  decreases in order to limit the second leakage  170  between the spring chamber  85  and the first control chamber  80  with increasing temperature T of the fuel. 
         [0054]    It is pointed out that the leakage pin  100  and the leakage pin plate  106 , in which the leakage pin bore  105  is arranged, likewise have a material combination of said type. The same also applies to the nozzle needle sleeve  115  and the nozzle needle  110 , wherein the nozzle needle  115  has the second material and the nozzle needle  110  has the first material. In this way, it is also possible for the first leakage  160  at the leakage pin  100 , and/or the fourth leakage  190  between the nozzle needle sleeve  115  and the nozzle needle  110 , to be reduced by way of an expansion of the material of the nozzle needle  110  and/or of the leakage pin  100  when the nozzle needle  110  and/or the leakage pin  100  are warmed up, as the pairing clearances  155 ,  185  between the leakage pin  100  and the leakage pin bore  105  and between the nozzle needle  110  and the nozzle needle sleeve  115  become smaller with progressive warming-up, and the gaps present in each case between leakage pin  100  and leakage pin bore  105  and between nozzle needle  110  and nozzle needle sleeve  115  become narrower. 
         [0055]    Here, the materials may be selected such that, when the control plate  60  and/or the control piston  70  are/is warmed up, the second leakage  170  through the gap  150  is substantially constant over the course of the warming-up of the control plate  60  and/or of the control piston  70 . Also, in the embodiment, the materials of the nozzle needle  110 , nozzle needle sleeve  115 , leakage pin  100  and leakage pin plate  106  are selected analogously to the control piston  70  and the control plate  60 . In this way, particularly good control characteristics of the injector  15  can be attained; in particular, an undesired opening or closing of the nozzle needle  110  can be avoided. 
         [0056]    The first material has a first coefficient of thermal expansion, and the second material has a second coefficient of thermal expansion. Here, the materials of the control piston  70  and/or of the control plate  60  are selected such that the first coefficient of thermal expansion is lower than the second coefficient of thermal expansion. In particular if the first material has hard metal, in particular in a composition with at least 70, preferably 90 percent tungsten carbide and 1 to 30 percent, preferably 1 to 10, cobalt, and if the second material is steel, in particular an unalloyed or low-alloy steel, it is possible in this way for the operating characteristics of the injector  15  to be kept uniform over the course of the warming-up of the injector  15 . 
         [0057]    As an alternative to the above-stated fraction of the cobalt of the first material, this may be replaced with a nickel-chromium fraction or a nickel-chromium-cobalt fraction, such that the first material has at least 70 percent, preferably 90 percent tungsten carbide and 1 to 30 percent, preferably 1 to 10 percent nickel-chromium or nickel-chromium-cobalt. 
         [0058]    By virtue of the fact that the control plate  60  is composed of steel, it exhibits similar warming-up characteristics to the injector housing  25 . At the same time, the gap  150  is reduced in terms of its gap width b by the control piston  70  composed of hard metal when the latter is warmed up. 
         [0059]    Even though the invention has been illustrated and described in more detail on the basis of the preferred exemplary embodiment, the invention is not restricted to the examples disclosed, and other variations may be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention. 
         [0060]    It is emphasized that, as an alternative to the configuration of the injector  15  described above, it is also the case that only one or two of the three combinations of piston guide (control plate  60 , leakage pin plate  106 , nozzle needle  110 ) and piston (control piston  70 , leakage pin  100 , nozzle needle sleeve  115 ) has the above-stated material combination. What is particularly simple to implement is the configuration in which only the control piston  70  has the first material and the control piston plate  60  has the second material in order to limit the second leakage  170  between the spring chamber  85  and the first control chamber  80 . 
         [0061]    As an alternative to the materials stated immediately above, it is also conceivable for materials other than those stated immediately above to be used for the control piston  70  and the control plate  60  and/or for the leakage pin plate  106  and the leakage pin  100  and/or for the nozzle needle sleeve  115  and for the nozzle needle  110 . Here, it is particularly advantageous if the materials are selected such that the two materials have, in terms of coefficient of thermal expansion, a difference of 3 to 10·10 −6  K −1 , in particular 5 to 10·10 −6  K −1 . 
         [0062]    Here, it is particularly advantageous if the first coefficient of thermal expansion of the first material is 5 to 25·10 −6  K −1  and the second coefficient of thermal expansion of the second material is 10 to 30·10 −6  K −1 . 
         [0063]    As an alternative to the above-stated material combination of hard metal and steel, a material combination of titanium is conceivable, in particular if the first material has 50 percent, preferably 80 percent titanium and the other material is steel. Here, it is advantageous for the steel to comprise at least one of the following metals: chromium, nickel, manganese, copper. 
         [0064]    It is alternatively also conceivable for a material combination for the first material and the second material to be selected, in the case of which at least one of the two materials has steel with a coefficient of thermal expansion of 12 to 10·10 −6  K −1  and the other of the two materials has a manganese steel. Here, it is particularly advantageous if the manganese steel is MnNi10Cu18 or MnNi16Cu10. 
         [0065]    It is pointed out that the stated material combinations of first material and second material are suitable both for the control plate  60  and the control piston  70  but also for the leakage pin  100  and the leakage pin plate  106  and/or for the nozzle needle sleeve  115  and the nozzle needle  110 . 
         [0066]    It is also conceivable for the above-stated material combinations to be combined with one another.