Abstract:
A piezo injector includes (a) an actuator chamber in which a piezo actuator is arranged, (b) a control piston bore in which a control piston having a first end side facing the piezo actuator is arranged, wherein a portion of the control piston bore delimited by the first end side forms a first control chamber and an opposing portion of the control piston bore forms a spring chamber, and wherein the control piston is arranged between the first control chamber and the spring chamber, (c) a nozzle needle having a second end side, wherein the nozzle needle guides a nozzle needle sleeve, wherein the nozzle needle sleeve and the second end side delimit a second control chamber, (d) a connecting bore between the first control chamber and second control chamber, and (e) a leakage pin arranged between the piezo actuator and the first end side in a leakage pin bore.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2012/063753 filed Jul. 13, 2012, which designates the United States of America, and claims priority to DE Application No. 10 2011 079 468.9 filed Jul. 20, 2011, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a piezo injector, e.g., for an internal combustion engine. 
       BACKGROUND 
       [0003]    Internal combustion engines with direct fuel injection are known. For direct fuel injection, piezo injectors are used, a nozzle needle of which is driven by means of a piezo actuator. In this case, it is necessary to have between the piezo actuator and nozzle needle virtually play-free coupling which, however, is difficult to maintain because of thermal length changes in the piezo injector. Too small an idle stroke between the piezo actuator and nozzle needle will result in incomplete closing of the nozzle needle. Too large an idle stroke between the piezo actuator and nozzle needle leads to an increase in the activation energy necessary for activating the piezo injector. In the prior art, attempts were made to compensate thermal length changes by a suitable choice of material and by the geometry. However, this results in high manufacturing costs and greatly restricts structural freedom in the configuration of the piezo injector. 
       SUMMARY 
       [0004]    One embodiment provides a piezo injector, with an actuator space in which a piezo actuator is arranged, a control piston bore in which a control piston is arranged, the control piston having a first end face confronting the piezo actuator, a portion, delimited by the first end face, of the control piston bore forming a first control space, a portion, opposite the first control space, of the control piston bore forming a spring space, the control piston being arranged between the first control space and the spring space, a nozzle needle with a second end face, the nozzle needle guiding a nozzle needle sleeve, the nozzle needle sleeve and the second end face delimiting a second control space, a connecting bore between the first control space and the second control space, and a leakage pin which is arranged between the piezo actuator and the first end face in a leakage pin bore. 
         [0005]    In a further embodiment, a first leakage out of the first control space being made possible, a second leakage out of the spring space into the first control space being made possible, a third leakage out of a high-pressure region into the second control space being made possible, a sum of the second leakage and of the third leakage being at least as large as the first leakage, and the sum of the second leakage and of the third leakage being so small that, with the nozzle needle open, a pressure rise in the second control space brought about by the second leakage and the third leakage does not lead to closure of the nozzle needle. 
         [0006]    In a further embodiment, the piezo injector has a high-pressure bore, the high-pressure bore being connected to the high-pressure region, and the high-pressure region being connected to the spring space. 
         [0007]    In a further embodiment, the spring space has arranged in it a control piston spring which acts upon the control piston with a force acting in the direction of the first control space. 
         [0008]    In a further embodiment, the piezo injector having a nozzle spring which acts upon the nozzle needle with a force directed away from the second control space. 
         [0009]    In a further embodiment, a first pairing play is present between the leakage pin and the leakage pin bore, the first pairing play making the first leakage possible, and the first pairing play amounting to less than 2 μm. 
         [0010]    In a further embodiment, a third pairing play is present between the nozzle needle and the nozzle needle sleeve, the third pairing play making the third leakage possible, and the third pairing play amounting to between 5 μm and 8 μm. 
         [0011]    In a further embodiment, a second pairing play is present between the control piston and the control piston bore, the second pairing play making the second leakage possible, and the second pairing play amounting to between 5 μm and 8 μm. 
         [0012]    In a further embodiment, the control piston having a throttle bore running between the first control space and the spring space, and the throttle bore making the second leakage possible. 
         [0013]    In a further embodiment, the throttle bore is closed by the leakage pin when the leakage pin bears against the control piston. 
         [0014]    In a further embodiment, a throttle is arranged in the connecting bore between the first control space and the second control space. 
         [0015]    In a further embodiment, the piezo actuator is a fully active piezo stack. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Example embodiments of the invention are described below in more detail with reference to the accompanying figures in which: 
           [0017]      FIG. 1  shows a sectional view of an upper part of a piezo injector; and 
           [0018]      FIG. 2  shows a sectional view of a lower part of the piezo injector. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments of the present invention provide a piezo injector in which length changes of the piezo injector are compensated automatically. 
         [0020]    According to some embodiments, a piezo injector comprises an actuator space in which a piezo actuator is arranged, a control piston bore in which a control piston is arranged which has a first end face confronting the piezo actuator, a portion, delimited by the first end face, of the control piston bore forming a first control space, a portion, opposite the first control space, of the control piston bore forming a spring space, and the control piston being arranged between the first control space and the spring space, a nozzle needle with a second end face, the nozzle needle guiding a nozzle needle sleeve, the nozzle needle sleeve and the second end face delimiting a second control space, a connecting bore between the first control space and the second control space, and a leakage pin which is arranged between the piezo actuator and the first end face in a leakage pin bore. Advantageously, in this piezo injector, there is hydraulic coupling between the piezo actuator and the nozzle needle. This hydraulic coupling advantageously has the effect of play compensation and stroke step-up. Advantageously, as a result, length changes in the piezo injector which are caused by temperature effects, wear at contact points in the drive and changes in the state of polarization of the piezo actuator can be compensated. This advantageously enables the injector to be manufactured from any material, without thermal expansion properties of the material having to be taken into account. Advantageously, therefore, a material especially resistant to high pressure can be used. Advantageously, complicated processes for setting an idle stroke when the piezo injector is being mounted are dispensed with, thus reducing the manufacturing costs of the piezo injector. By an idle stroke being dispensed with, energy required for activating the piezo injector is also reduced. A further advantage of the piezo injector is improved injection quantity stability during dynamic engine operation. It is likewise advantageous that pressure losses in the piezo injector are reduced, as compared with the prior art. 
         [0021]    It is expedient that a first leakage out of the first control space is made possible, a second leakage out of the spring space into the first control space is made possible and a third leakage out of a high-pressure region into the second control space is made possible. 
         [0022]    In this case, a sum of the second leakage and of the third leakage is at least as large as the first leakage. Moreover, the sum of the second leakage and of the third leakage is so small that, with the nozzle needle open, a pressure rise in the second control space brought about by the second leakage and the third leakage does not lead to closure of the nozzle needle. Advantageously, the second leakage and the third leakage prevent the first leakage from causing the nozzle needle to open inadvertently. The second and the third leakage also advantageously prevent unwanted opening of the nozzle needle in the event of very steep pressure rises in the high-pressure region. 
         [0023]    Preferably, the piezo injector has a high-pressure bore which is connected to the high-pressure region. In this case, the high-pressure region is connected to the spring space. Advantageously, the high pressure of the high-pressure bore then constantly prevails in the spring space. 
         [0024]    It is expedient that the spring space has arranged in it a control piston spring which acts on the control piston with a force acting in the direction of the first control space. Advantageously, the control piston spring causes the control piston to return into its initial position after an injection operation has been terminated. 
         [0025]    It is likewise expedient that the piezo injector has a nozzle spring which acts upon the nozzle needle with a force directed away from the second control space. Advantageously, the nozzle spring then assists closure of the nozzle needle in order to terminate an injection operation. 
         [0026]    In one embodiment of the piezo injector, there is between the leakage pin and the leakage pin bore a first pairing play which makes the first leakage possible. In this case, the first pairing play amounts to less than 2 μm. Advantageously, experiments and model computations have shown that such a first pairing play leads to a sufficiently small first leakage. 
         [0027]    In one embodiment of the piezo injector, there is between the nozzle needle and the nozzle needle sleeve a third pairing play which makes the third leakage possible. In this case, the third pairing play amounts to between 5 μm and 8 μm. Advantageously, it has been shown in model computations and experiments that a third pairing play of this order of magnitude leads to a suitable third leakage. 
         [0028]    In one embodiment of the piezo injector, there is between the control piston and the control piston bore a second pairing play which makes the second leakage possible. In this case, the second pairing play amounts to between 5 μm and 8 μm. Advantageously, model computations and experiments have shown that a second pairing play having such dimensioning leads to a second leakage of suitable size. 
         [0029]    In another embodiment of a piezo injector, the control piston has a throttle bore which runs between the first control space and the spring space and which makes the second leakage possible. Advantageously, such a throttle bore also makes a second leakage of suitable size possible. 
         [0030]    In some embodiments, the throttle bore is closed by the leakage pin when the leakage pin bears against the control piston. Advantageously, the second leakage is then interrupted when the nozzle needle is in the open state, with the result that the risk of undesirable closure of the nozzle needle brought about by the second leakage is reduced. 
         [0031]    In one embodiment of the piezo injector, a throttle is arranged in the connecting bore between the first control space and the second control space. 
         [0032]    In some embodiments, the piezo actuator is a fully active piezo stack. Advantageously, the piezo actuator can be separated hermetically from the fuel and therefore does not have to have special fuel resistance. 
         [0033]    A sectional view of a piezo injector  100  is illustrated in  FIGS. 1 and 2 .  FIG. 1  shows an upper part  101  of the piezo injector  100 .  FIG. 2  shows a lower part  102  of the piezo injector  100 . The piezo injector  100  can serve for injecting fuel in an internal combustion engine. The piezo injector  100  can serve, for example, for injecting diesel fuel in a common rail internal combustion engine. 
         [0034]    The piezo injector  100  has an injector housing  110 . The injector housing  110  may be composed of essentially any material, since the thermal expansion properties of the injector housing  110  are unimportant. In particular, the injector housing  110  does not have to be composed of Invar. 
         [0035]    In the injector housing  110 , a high-pressure bore  120  is arranged, to which fuel which is under high pressure can be delivered via a high-pressure connection  121 . The high-pressure bore  120  runs in the longitudinal direction through the injector housing  110  as far as a high-pressure region  178 , also dealt with below, in the lower part  102  of the piezo injector  100 . The upper part  101  of the piezo injector  100  has, furthermore, a leakage connection  111 . 
         [0036]    Further, the injector housing  110  has in the upper part  101  of the piezo injector  100  an actuator space  131  in which a piezo actuator  130  is arranged. The piezo actuator  130  is preferably a fully active piezo stack. The piezo actuator  130  has approximately a cylindrical shape and can be acted upon via an electrical connection  132  with electrical voltage in order to change the length of the piezo actuator  130  in the longitudinal direction. 
         [0037]    The piezo injector  100  has in the lower part  102  a control piston bore  151  in which a control piston  150  is arranged. The control piston  150  has a first end face  152  pointing in the direction of the piezo actuator  130 . A portion, delimited by the first end face  152 , of the control piston bore  151  forms a first control space  153 . The control piston bore  151  forms at its longitudinal end opposite the first control space  153  a spring space  154 . The control piston  150  is thus arranged between the first control space  153  and the spring space  154 . 
         [0038]    Located in the spring space  154  is a control piston spring  155  which may be designed, for example, as a helical compression spring. A first longitudinal end of the control piston spring  155  is supported on the control piston  150 . A second longitudinal end of the control piston spring  155  is supported on an end face of the control piston bore  151 . The control piston spring  155  acts upon the control piston  150  with a force acting in the direction of the first control space  153 . 
         [0039]    The spring space  154  is connected to the high-pressure region  178  via a high-pressure connection  157 . Thus, when the piezo injector  100  is operation, fuel having the pressure prevailing in the high-pressure bore  120  and in the high-pressure region  178  is constantly present in the spring space  154 . 
         [0040]    A leakage pin  140  is arranged in a leakage pin bore  141  between the piezo actuator  130  and the control piston bore  151 . The length of the leakage pin  140  is in this case dimensioned such that an increase in the length of the piezo actuator  130  is transmitted to the control piston  150  via the leakage pin  140 . 
         [0041]    Further, the high-pressure region  178 , into which the high-pressure bore  120  issues, is arranged in the lower part  102  of the piezo actuator  100 . A nozzle needle  170  which guides a nozzle needle sleeve  171  is arranged in the high-pressure region  178 . One longitudinal end of the nozzle needle  170 , said longitudinal end pointing in the direction of the upper part  101  of the piezo injector  100 , has a second end face  172 . Above the second end face  172  is formed a second control space  173  which is delimited by the second end face  172  and by the nozzle needle sleeve  171 . The second control space  173  is connected to the first control space  153  via a connecting bore  160 . 
         [0042]    The nozzle needle  170  has a peripheral collar  174  connected fixedly to the nozzle needle  170 . Between the collar  174  and the nozzle needle sleeve  171  is arranged a nozzle spring  175  which may be designed, for example, as a helical compression spring. A first longitudinal end of the nozzle spring  175  is supported on the nozzle needle sleeve  171 . A second longitudinal end of the nozzle spring  175  is supported on the collar  174 . The nozzle spring  175  acts upon the nozzle needle  170  with a force directed away from the second control space  173 . 
         [0043]    When the piezo injector  100  is in the closed state, the nozzle needle  170  bears against a lower tip of the lower part  102  of the piezo injector  100 . The piezo actuator  130  is discharged and has its minimum length. The piezo injector  100  does not carry out any fuel injection. 
         [0044]    When the piezo actuator  130  is charged via the electrical connection  132  and the length of the piezo actuator  130  is thereby increased, the piezo actuator  130  exerts via the leakage pin  140  upon the control piston  150  a force by which the control piston  150  is moved in the control piston bore  151  in the direction of the spring space  154 . The volume of the first control space  153  is thereby increased, with the result that the pressure in the first control space  153  and in the second control space  173  decreases. The reduced pressure in the second control space  173  therefore exerts a then reduced force upon the second end face  172  of the nozzle needle  170 . 
         [0045]    The high pressure, still acting upon the lower end of the nozzle needle  170 , of the high-pressure region  178  consequently causes an upward movement of the nozzle needle  170  in the direction of the second control space  173 . The piezo injector  100  is thereby opened in order to inject fuel. 
         [0046]    The ratio of the diameter of the control piston  150  and consequently of the diameter of the first control space  153  to the diameter of the nozzle needle  170  on its second end face  172  and consequently to the diameter of the second control space  173  defines a step-up ratio between a length change of the piezo actuator  130  and a stroke of the nozzle needle  170 . If the diameter of the control piston  150  amounts, for example, to 5 mm and the diameter of the nozzle needle  170  on its second end face  172  amounts, for example, to 3.5 mm, a step-up ratio of about 2 is obtained. 
         [0047]    After the opening of the nozzle needle  170 , the stroke of the nozzle needle  170  can be controlled via a variation in the length of the piezo actuator  130 . The length of the piezo actuator  130  can be varied, in turn, via a variation in the energy delivered to the piezo actuator  130  via the electrical connection  132 . 
         [0048]    When the piezo actuator  130  subsequently is discharged, and therefore shortened, the high pressure prevailing in the spring space  154  and the force exerted upon the control piston  150  by the control piston spring  155  cause a movement of the control piston  150  in the direction of the first control space  153 . The pressure in the first control space  153  thereby rises and, because of the connecting bore  160  present between the first control space  153  and second control space  173 , the pressure in the second control space  173  also rises. This results in a movement of the nozzle needle  170  back to the lower end of the lower part  102  of the piezo injector  100 , as a result of which the piezo injector  100  is closed and fuel injection is terminated. 
         [0049]    The spring force exerted upon the control piston  150  by the control piston spring  155  ensures that, when the piezo injector  100  is in the closed state, the control piston  150  constantly bears against the leakage pin  140  and the drive formed by the piezo actuator  130 , leakage pin  140  and control piston  150  is always play-free. The result of this is that varying thermal boundary conditions, length changes of the piezo actuator  130  and wear phenomena in the contact regions have no appreciable influence upon the injection quantities dispensed by the piezo injector  100 . 
         [0050]    The leakage pin  140  is fitted into the leakage pin bore  141  with a first pairing play  142 . On account of the first pairing play  142 , a first leakage  143  out of the first control space  143  takes place along the leakage pin  140  into a region of the piezo injector  100  which is arranged above the leakage pin  140  and from where the first leakage  143  can escape via the leakage connection  111 . On account of the high pressure prevailing in the first control space  153 , the first pairing play  142  selected must be small in order to obtain a small first leakage  143 . The first pairing play  142  preferably amounts to less than 2 μm, especially preferably to approximately 1 μm. 
         [0051]    The control piston  150  is fitted into the control piston bore  151  with a second pairing play  158 . When the pressure in the first control space  153  is lower than the pressure in the spring space  154 , a second leakage  159  from the spring space  154  along the control piston  150  into the first control space  153  occurs because of the second pairing play  158 . The control piston  150  may also have a throttle bore  156  which runs from the spring space  154  through the control piston  150  to the first control space  153 . In this case, a fourth leakage  180  from the spring space  154  into the first control space  153  is possible through the throttle bore  156 . If the throttle bore  156  is absent, the second pairing play  158  preferably amounts to between 3 μm and 10 μm, especially preferably to between 5 μm and 8 μm, in order to make a sufficient second leakage  159  possible. If the throttle bore  156  is present and therefore the fourth leakage  180  is made possible, the second pairing play  158  selected can be very small and amount, for example, to 1 μm. 
         [0052]    The nozzle needle  170  is fitted into the nozzle needle sleeve  171  with a third pairing play  176 . When the pressure in the second control space  173  is lower than the pressure in the high-pressure region  178 , a third leakage  177  out of the high-pressure region  178  into the second control space  173  can occur along the nozzle spring  175  as a result of the third pairing play  176 . The third pairing play  176  preferably amounts to between 3 μm and 10 μm, especially preferably to between 5 μm and 8 μm. If the throttle bore  156  is present, the third leakage  177  may be dispensed with and the third pairing play  176  may likewise be made very small, for example of a size of about 1 μm. 
         [0053]    When the piezo injector  100  is in the closed state, the first leakage  143  along the leakage pin  140  causes fuel to flow out of the first control space  153 . So that this outflow of fuel from the first control space  153  does not lead to a pressure drop in the first control space  153 , the result of which would be an inadvertent opening of the nozzle needle  170 , the fuel loss caused by the first leakage  143  must be compensated by the second leakage  159 , the third leakage  177  and/or the fourth leakage  180 . If the throttle bore  156  is absent and therefore the fourth leakage  180  does not take place, the sum of the second leakage  159  and of the third leakage  177  must be at least as large as the first leakage  143 . If the throttle bore  156  is present, the sum of the second leakage  159 , of the third leakage  177  and of the fourth leakage  180  must be at least as large as the first leakage  143 . 
         [0054]    When the nozzle spring  175  and therefore the piezo injector  100  are in the open state, an inflow of fuel into the first control space  153  and the second control space  173  occurs due to the second leakage  159 , the third leakage  177  and/or the fourth leakage  180 . The inflow of fuel causes a pressure rise in the first control space  153  and in the second control space  173 . However, the pressure increase must be so small that there is no inadvertent premature closure of the nozzle needle  170  and therefore of the piezo injector  100 . 
         [0055]    Especially preferably, the nozzle bore  156  and the leakage pin  130  are designed such that the leakage pin  140  closes the throttle bore  156  when the nozzle needle  170  is opened. As a result, with the nozzle needle  170  open, the fourth leakage  180  is prevented, so that premature undesirable closure of the nozzle needle  170  is ruled out. 
         [0056]    A throttle may be arranged in the connecting bore  160  between the first control space  153  and the second control space  173 . 
         [0057]    The second leakage  159  and the third leakage  177  are also necessary in order to prevent unwanted opening of the nozzle needle  170  in the event of very steep pressure rises in the high-pressure region  178 .