Abstract:
The disclosure relates to a fuel injection valve that includes a valve needle, a closing spring applying a spring force to the valve needle, which spring force loads the valve needle in the direction of a closing position, and an actuator assembly. A coil of the actuator assembly produces a magnetic force on a magnet armature of the actuator assembly such that, as the magnet armature travels toward a pole piece of the actuator assembly,. the magnet armature covers an idle stroke to a stop element of the valve needle and then carries the valve needle toward the pole piece. A spring constant of the closing spring and the magnetic force are matched such that the magnitude of the resultant force of the spring force and the magnetic force decreases with increasing distance of the valve needle from the closing positionand remains the same with increasing the distance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of PCT Application PCT/EP2015/071202 filed Sep. 16, 2015, which claims priority to German Application DE 10 2014 220 877.7, filed Oct. 15, 2014. The disclosures of the above applications are incorporated herein by reference 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates to a fuel injection valve. 
       BACKGROUND 
       [0003]    Fuel injection valves are known, in which the armature covers a predefined idle stroke before it engages on the valve needle, in order to move the latter out of the closed position. For example, WO 2012/041984 A1 discloses a fuel injection valve of this type. 
         [0004]    In fuel injection valves of this type, an oscillating profile of the needle position around a predefined open position can occur when the predefined open position of the valve needle is reached. The oscillating profile influences the fuel quantity which is output by the fuel injection valve. The exact profile of the oscillations can be predicted only with difficulty. In addition, the oscillations can lead to the fuel quantity no longer being dependent linearly or even no longer monotonously on the valve opening time. The non-linear, in particular oscillating profile of the quantity characteristic curve is also called an “S-shape”. 
       SUMMARY 
       [0005]    It is therefore an object of the present disclosure to specify a fuel injection valve, by way of which particularly precise fuel metering and/or a particularly simple actuating capability may be achieved. 
         [0006]    One aspect of the disclosure provides a fuel injection valve. The fuel injection valve has a valve assembly. The valve assembly has a displaceably mounted valve needle and a valve seat. The valve needle and the valve seat interact mechanically with one another, in order to prevent fuel flow through an injection opening of the fuel injection valve in a closed position of the valve needle and to release the fuel flow in other positions of the valve needle. 
         [0007]    Implementations of the disclosure may include one or more of the following optional features. In some implementations, the fuel injection valve has a prestressed closing spring that exerts a spring force on the valve needle. The spring force loads the valve needle in the direction of the closed position. Moreover, the fuel injection valve has an electromagnetic actuator assembly which has a coil, a pole piece and a magnet armature. 
         [0008]    In some implementations, the magnet armature is mounted such that it can be displaced with respect to the valve needle. The valve needle has a stop element, from which the magnet armature is spaced apart in a rest state of the fuel injection valve. In some examples, the armature is spring-loaded by an armature restoring spring in the direction away from the stop element. 
         [0009]    The coil is configured to generate a magnetic force on the magnet armature when it is energized for releasing the fuel flow through the injection opening, in order to move the magnet armature toward the pole piece. On its path toward the pole piece, the magnet armature at first covers an idle stroke toward the stop element and subsequently enters into a positively locking connection with the latter, with the result that it drives the valve needle in the direction of the pole piece. Here, it transmits, in particular, the magnetic force at least partially to the valve needle, to accelerate the latter and/or to move it counter to the flow resistance as a result of the fuel in the valve assembly. 
         [0010]    A spring constant of the closing spring and the magnetic force of the actuator assembly are adapted to one another such that the magnitude of the summed force of the spring force and the magnetic force remains constant with an increasing spacing of the valve needle from the closed position or, preferably, decreases with an increasing spacing of the valve needle from the closed position. In other words, the difference between the magnetic force and the spring force is independent of the spacing of the valve needle from the closed position, or the difference between the magnetic force and the spring force preferably drops with an increasing spacing of the valve needle from the closed position. 
         [0011]    In some implementations, the fuel injection valve has a stopper that limits the movement of the magnet armature and/or the valve needle toward the pole piece. The stopper is formed, for example, by the pole piece or is fastened in a stationary manner with respect to the pole piece, for examples, the stopper is fastened to the pole piece. The spring constant of the closing spring and the magnetic force of the actuator assembly may be adapted to one another in such a way that the summed force is directed away from the closed position when the magnet armature and/or the valve needle come/comes into contact with the stopper. In some examples, when the magnet armature and/or the valve needle come/comes into contact with the stopper, the summed force has a magnitude that is at most half as great as the magnitude of the summed force on the valve needle when leaving the closed position, for example, in the case of an identical current through the coil. 
         [0012]    In some examples, the magnitude of the magnetic force remains constant with an increasing spacing of the valve needle from the closed position. To this end, the coil may be loaded, for example, with an operating current that drops with an increasing spacing of the valve needle from the closed position. In this case, the magnitude of the summed force drops with an increasing spacing of the valve needle from the closed position. In some examples, the magnitude of the magnetic force increases with an increasing spacing of the valve needle from the closed position. In this case, the coil may be loaded at least temporarily with a constant operating current during the opening movement of the valve needle. 
         [0013]    In some implementations, the magnitude of the summed force of the spring force and the magnetic force decreases with an increasing spacing of the valve needle from the closed position in the case of a constant current through the coil, or at least remains constant. In some examples, the constant current through the coil is the maximum operating current, for which the actuator assembly is designed. The maximum operating current has, for example, a value between 10 A and 18 A, the limit values being included. In some implementations, the constant current through the coil is the holding current which can be applied to the coil, in order to hold the valve in an open position, in which the magnet armature or the valve needle bears against the stopper and the stop element of the valve needle is in contact with the magnet armature. The holding current has, for example, a value between 3 A and 5 A, the limit values being included. The adaptation of the magnetic force takes place, for example, in the refinements, for example, by way of the selection of the inductance and rated current of the coil. 
         [0014]    In conventional fuel injection valves, the closing spring has only a relatively low spring constant, with the result that the spring force is scarcely dependent on the position of the valve needle, and the rise in the magnetic force on account of the approach of the magnet armature to the pole piece can pre-dominate. The acceleration of the valve needle then rises with the spacing from the closed position. In contrast, the acceleration of the valve needle in the present fuel injection valve advantageously drops during the course of the opening movement of the valve needle. For example, the velocity of the needle (and therefore its kinetic energy) is particularly low when the magnet armature or the valve needle comes into contact with the stopper. In this way, the oscillations of the needle about the open position, in which the magnet armature or the valve needle bears against the stopper and the stop element of the valve needle is in contact with the magnet armature, advantageously have a particularly low amplitude or are even completely suppressed. 
         [0015]    The actuator assembly may expediently be designed to hold the fuel injection valve in the open position. In some implementations, the closing spring is designed in such a way that the spring force is increased to at least 1.5 times, for example, at least doubles with respect to the prestress during the course from the closed position to the open position. In contrast, the spring force in conventional fuel injection valves is usually substantially constant over the entire needle stroke. In some examples, the prestress has a value between 5 N and 25 N, preferably between 10 N and 20 N. 
         [0016]    In some implementations, the spring constant of the closing spring and the magnetic force of the actuator assembly are adapted to one another in such a way that the valve needle brakes by way of the flow resistance of the fuel in the valve assembly if the spacing of the valve needle from the closed position exceeds a predefined value which is, in particular, smaller than the needle stroke. For example, in the case of a predefined constant current through the coil, at first the valve needle is accelerated along part of the needle stroke when leaving the closed position and is subsequently braked over a further part of the needle stroke if the spacing exceeds the predefined value. In this way, the risk of oscillations of the valve needle about the open position is particularly low. Here, the needle stroke is the distance which the valve needle covers from the closed position as far as the open position. 
         [0017]    In some examples, the spring constant of the closing spring and the magnetic force of the actuator assembly are adapted to one another in such a way that the magnitude of the summed force for example, in the case of a constant current through the coil. In some implementations, the fuel injection valve is designed for operation at a fuel pressure of 700 bar or less, for example, a fuel pressure of 500 bar or less. The electromagnetic actuator assembly may be designed to be so weak that it is unsuitable for moving the needle away from the closed position counter to a higher pressure of the fuel in the valve assembly. The spring constant of the closing spring may be 100 N/mm or more, for examples, the spring constant may be 300 N/mm or more. 
         [0018]    In some implementations, the valve is designed by means of the idle stroke and the magnet armature in such a way that the force of the magnet armature on the valve needle during the entering of the positively locking connection between the magnet armature and the stop element has a value of 1.3 or more of the sum of the prestress of the closing spring and the hydraulic force which is exerted on the valve needle by the fuel in the closed position. The value may be 1.5 or more and, for example, the value may be between 2 times and 4 times the sum of the prestress of the closing spring and the hydraulic force which is exerted on the valve needle by the fuel in the closed position. In some examples, the current through the coil is at a maximum when the magnet armature comes into contact with the stop element. For example, it is then at a value which corresponds to the maximum operating current. 
         [0019]    In this way, the valve needle is lifted up reliably from the closed position, and the valve needle passes rapidly through, in particular, at least the first third of the needle stroke. For example, the valve needle passes rapidly through at least the first 50 μm of the needle stroke. 
         [0020]    The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  shows a known fuel injection valve, 
           [0022]      FIG. 2  shows a diagrammatic longitudinal section of a detail of a fuel injection valve according to the disclosure, 
           [0023]      FIG. 3  shows a diagram of the fuel quantity which is output by the known fuel injection valve in a manner which is dependent on the opening duration, 
           [0024]      FIG. 4  shows a diagram of the needle position and the position of the magnet armature in a manner which is dependent on the opening duration in the known fuel injection valve, 
           [0025]      FIG. 5  shows a diagram of the needle position and the position of the magnet armature in a manner which is dependent on the opening duration in the fuel injection valve according to the disclosure, and 
           [0026]      FIG. 6  shows a diagram of the magnetic force, the spring force and the hydraulic force in a manner which is dependent on the opening duration in the fuel injection valve according to the disclosure. 
       
    
    
       [0027]    Identical, equivalent or identically acting elements are provided with the same designations in the figures. In some figures, individual designations can be omitted in order to improve the clarity. The figures and the proportions of the elements which are shown in the figures among one another are not to be considered to be true to scale, unless units are expressly specified. Rather, individual elements can be shown in an exaggeratedly large manner for improved representability and/or for improved comprehension. 
       DETAILED DESCRIPTION 
       [0028]      FIG. 1  shows a fuel injection valve  100  for an internal combustion engine according to the prior art in a longitudinal sectional illustration through a longitudinal axis  140  of the fuel injection valve  100 . 
         [0029]    The fuel injection valve  100  has a valve assembly  105 . The valve assembly  105  has a hollow valve body  160 , a valve needle  135 , and a valve seat  150 . 
         [0030]    The valve needle  135  is received in the valve body  160  and is mounted such that it can be displaced with respect to the valve body  160 . The valve needle  135  interacts mechanically with the valve seat  150 , in order to close an injection opening  115  of the fuel injection valve  100 . To this end, at its end which faces the injection opening  115 , the valve needle  135  has a closing element  145  which has the basic shape of a ball in the present case. 
         [0031]    In addition, the valve needle  135  has a stop element  165  which is arranged at an end of the valve needle  135  which lies opposite the closing element  145 . A closing spring  155  of the fuel injection valve  100  acts on the stop element  165 . The closing spring  155  is prestressed, in order to exert a spring force Ff on the valve needle  135 , which spring force Ff preloads the valve needle  135  in the direction of a closed position  230 . In the closed position  230 , the closing element  145  is in sealing contact with the valve seat  150 . 
         [0032]    In addition, the fuel injection valve  100  has an electromagnetic actuator assembly  120  which has a coil  125 , a pole piece  127  and a magnet armature  130 . The pole piece  127  is stationary with regard to the valve body  160 . For example, it is fastened to the valve body  160  or is configured in one piece with the latter. 
         [0033]    The magnet armature  130  is arranged in the valve body  160  and is mounted such that it can be displaced with respect to the valve body  160 . In addition, it is mounted such that it can be displaced with respect to the valve needle  135 , to be precise between the stop element  165  and a disk  180  which are both connected fixedly to a shank of the valve needle  135 . 
         [0034]    The coil  125  is configured to generate a magnetic force Fm on the magnet armature  130  when it is energized for releasing the fuel flow through the injection opening  115 . The magnetic force Fm moves the magnet armature  130  toward the pole piece  127 , the magnet armature  130  first of all covering an idle stroke  220  toward the stop element  165 , from which it is spaced apart in a currentless rest state of the fuel injection valve  100 , before it enters into a positively locking connection with the stop element  165 , with the result that it drives the valve needle  135  on its further path in the direction of the pole piece  127 , with the result that the valve needle  135  is moved away from the closed position  230  and thus releases the fuel flow through the injection opening  115 . 
         [0035]    A stopper  170  which is formed in the present case by an end of the pole piece  127  which faces the magnet armature  130  limits the axial movability of the magnet armature  130  toward the pole piece  127 . In an open position  215 ,  235  of the valve assembly  105 , the magnet armature  130  bears against the stopper  170 , and the stop element  165  bears against the magnet armature  130 . 
         [0036]    In the fuel injection valve  100  shown in  FIG. 1 , the idle stroke  220  is ensured by means of a permanent magnet  175  which exerts an armature restoring force Fr which is directed away from the pole piece  127  on the magnet armature  130 . The armature restoring force Fr which is exerted by the permanent magnet  175  decreases greatly with the spacing of the magnet armature  130  from the permanent magnet  175 , with the result that it does not damp, or damps merely slightly, in particular, the contact of the magnet armature  130  on the pole piece  127  at the end of the opening movement. This leads to overshooting of the valve needle  135  and to oscillations of the valve needle  135  about the open position  235  of the valve needle  135 , in which open position  235  the stop element  165  is in positively locking contact with the magnet armature  130 , whereas the magnet armature  130  is in positively locking contact with the pole piece  127 . 
         [0037]    These oscillations are shown by way of example in  FIG. 4  which shows the spacing nl of the valve needle  135  from the valve seat  150  (dashed line) and the position of the magnet armature  130  (solid line) in a manner which is dependent on the opening duration Ti of the fuel injection opening  115 . Here, the opening duration Ti is, in particular, the duration of an electric current signal which is applied to the coil  125 . 
         [0038]    The oscillatory profile of the needle position can be seen in the region which is marked by means of a circle. It leads to the non-linearity of the injection quantity mf which is marked in  FIG. 3  by means of the dashed ellipse in a manner which is dependent on the opening duration Ti in the fuel injection valve  100  of  FIG. 1 . 
         [0039]    In contrast,  FIG. 2  shows a detail of a fuel injection valve  100  according to the disclosure which corresponds fundamentally to the known fuel injector valve  100  which is described in conjunction with  FIG. 1 .  FIG. 2  shows a detail of the valve assembly  105  of the fuel injection valve  100  in a longitudinal section. Here, only the right-hand half (in the plane of the drawing) of the valve assembly  105  is shown for reasons of simplification. 
         [0040]    In contrast to the fuel injection valve  100  of  FIG. 1 , the idle stroke  220  of the magnet armature  130  is ensured by means of an armature restoring spring  240 . The permanent magnet  175  described in  FIG. 1  can advantageously be dispensed with. Opposite ends of the armature restoring spring  240  can be supported, for example, against the stopper  170  and against the magnet armature  130 , as shown in  FIG. 2 . Cutouts  245 ,  250  may be provided in the magnet armature  130  and in the stopper  170  in order to receive the respective ends. 
         [0041]    If the stopper is different than the pole piece  127 , one end of the armature restoring spring  240  can also be supported against the pole piece  127  itself. In the present case, the armature restoring spring  240  follows the magnet armature  130  and the stopper  170  in the radial direction. As an alternative, the ends of the armature restoring spring  240  can also be arranged in central passage openings of the magnet armature  130  and/or the stopper  170  or the pole piece  127 . In some examples, one end of the armature restoring spring is not supported against the stopper  170 , but rather against the valve needle  135 , for example, against the stop element  165 . Details with respect to the arrangement and design of the armature restoring spring  240 , stop element  165  (also called driver), magnet armature  130  and stopper  170  or pole piece  127  are disclosed in German patent application no. 10 2013 219 974.0, the disclosure of which is to this extent included herewith in the present disclosure by way of back-reference. 
         [0042]    In the non-energized rest state of the fuel injection valve  100 , the magnet armature  130  is situated in a first position  205  with respect to the spring loading by way of the armature restoring spring  240 , in which first position  205  it is spaced apart from the stop element  165  of the valve needle  135 . A magnetic force Fm is generated on the magnet armature  130  by means of loading of the coil  125  with an operating current, in order to axially displace the magnet armature  130  with respect to the valve needle  135  by the idle stroke  220  from the first position  205  counter to the spring force of the armature restoring spring  240 , until it comes into contact with the stop element  165  in a second position  210 . 
         [0043]    From there, it is displaced by means of the magnetic force Fm by the actuating path  225  to a third position  215 , in which it comes into contact with the stopper  170 . On the actuating path  225 , the magnet armature  130  drives the valve needle by means of the stop element  165  from the closed position  230  to the open position  235  counter to the spring force Ff of the closing spring  155 . In the open position of the valve assembly  105 , the magnet armature  130  is in positively locking contact with the stopper  170  in the third position  215 , and the valve needle  135  is in the open position  235 , its stop element  165  being in positively locking contact with the magnet armature  130 . 
         [0044]    The fuel injector valve  100  is designed, for example, for operation at a nominal pressure of 350 bar. In some examples, fuel injector valve  100  is designed to open up to a maximum fuel pressure of 420 bar. in some examples, the spring constant of the closing spring  155  has a value of 100 N/mm or more, such as, but not limited to 300 N/mm. In contrast, in the known fuel injection valve of  FIG. 1 , the spring constant of the closing spring 155 is only 14 N/mm. In some examples, the closing spring is prestressed with a force between 10 N and 30 N, the limit values being included. In some examples, the prestress is 20 N. 
         [0045]    The actuator assembly is designed, for example, for a maximum operating current with a value between 10 A and 20 A, the limit values being included. in some implementations, actuator assembly is designed for a maximum operating current of 12 A. In some examples, the inductance of the coil  125  has a value between 0.1 mH and 10 mH, the limit values being included, for example a value of 2 mH. Here, the inductance is measured, for example, in the closed state of the fuel injector valve  100  (that is to say, magnet armature  130  in the first position  205  and valve needle  135  in the closed position  230 ). The inductance in the closed state may have a value between 2 mH and 4 mH. If the magnet armature  130  is in the second position  210  and the coil is loaded with the maximum operating current of 12 A, the inductance drops, for example, to a value between 0.16 mH and 0.8 mH. 
         [0046]    The idle stroke  220  has, for example, a value between 40 μm and 120 μm, the limit values being included; in the present case, it is 40 μm. The actuating path  225  has, for example, a length of 40 μm or more, for example, between 40 μm and 100 μm, the limit values being included. In the present case, the actuating path  225  has a length of approximately 90 μm. 
         [0047]    The spring constant of the closing spring  155  and the magnetic force of the actuator assembly  120  are adapted to one another in such a way that, in the case of a constant magnetic force Fm, the magnitude of the summed force of the spring force Ff and the magnetic force Fm decreases monotonously with an increasing spacing of the valve needle  135  from the closed position  230 . In the open position  215 ,  235 , the magnitude of the summed force has a value of 50% or less of the magnitude of the summed force when the valve needle  135  leaves the closed position  230  (magnet armature  130  in the second position  210 ). Here, the summed force in the open position  215 ,  235  is directed away from the closed position  230 , with the result that the magnet armature  130  and the valve needle  135  are held in the open position  215 ,  235  by means of the magnetic force Fm. 
         [0048]    At the same time, the summed force is so low that, after running through approximately 50 μm of the actuating path  225 , the valve needle is accelerated only weakly toward the pole piece  127  and is braked by way of the flow resistance of the fuel in the valve body  160  for at least 15 μm, for example, for at least 30 μm ahead of the contact of the magnet armature  130  on the stopper  170 . The velocity of the magnet armature  130  during contact with the stopper  170  is advantageously lower, for example lower by at least 30%, more specifically, lower by at least 60%, than in a position which corresponds to running through the first 50 μm of the actuating path. 
         [0049]      FIG. 6  shows the profile of the force F in a manner which is dependent on the opening duration Ti, that is to say on the duration of the electric current signal which is applied to the coil  125 , for the magnetic force Fm, the spring force Ff and the force Fh which is exerted on the valve needle  135  by the fuel. Here, only the (positive) magnitudes of the spring force Ff and magnetic force Fm are plotted for improved clarity, although the two forces act in opposite directions. Against the same time axis,  FIG. 5  shows the spacing nl of the valve needle  135  from the closed position  230  in a manner which is dependent on the opening duration Ti (dashed line) and the position of the magnet armature  130  (solid line). 
         [0050]    After the coil current is switched on, the magnetic force Fm rises at first until, at a first time T 1 , first of all the movement of the magnet armature  130  away from the first position  205  begins and the magnet armature  130  subsequently comes into contact with the stop element  165  at a second time T 2 . At the second time T 2 , the movement of the valve needle  135  away from the closed position  230  begins. The magnetic force Fm is kept at least approximately constant from the time T 2 , by the operating current through the coil being lowered continuously from a maximum value to a holding current. In contrast, starting from the prestress, the spring force Ff rises from time T 2  until it reaches its maximum value when the valve passes into the open position  215 ,  235  at a following, third time T 3 . In some examples, the maximum value of the spring force Ff corresponds approximately to 2.5 times the prestress. 
         [0051]    The summed force of spring force Ff and magnetic force Fm has a value of approximately 40 N when the valve needle leaves the closed position  230  at the time T 2  and only a value of approximately 15 N when the magnet armature  130  or the valve needle  135  comes into contact with the stopper  170  at the time T 3 . This corresponds to a reduction of the summed force to less than 40%. 
         [0052]    In some examples, the magnitude of the magnetic force Fm is greater in both cases than the magnitude of the spring force Ff, with the result that the summed force is directed in each case in the opening direction. When the magnet armature  130  or the valve needle  135  comes into contact with the stopper  170  at time T 3 , the magnitude of the summed force is smaller, however, than the magnitude of the hydraulic force Fh, with which the fuel counteracts the movement of the valve needle  135 , with the result that the valve needle is braked by way of the flow resistance of the fuel before reaching the open position  235 . 
         [0053]    The overshooting of the needle  135  beyond the open position  235  when the magnet armature  130  comes into contact with the stopper  170  is thus particularly low, and oscillations of the needle position are avoided. This is shown in  FIG. 5  using the dashed line which represents the spacing nl of the valve needle  135  from the closed position  230  in a manner which is dependent on the opening duration Ti for the present fuel injection valve  100 . The position of the magnet armature  130  also advantageously does not have an oscillating profile (see the solid line of  FIG. 5 ). The injection quantity mf can thus be set in a particularly precise manner. The fuel injection valve  100  may be actuated particularly simply in order to achieve a predefined injection quantity Mf. 
         [0054]    The force Fm of the magnet armature  130  of approximately 65 N on the valve needle  135  during the entering of the positively locking connection between the magnet armature  130  and the stop element  165  at time T 2  corresponds approximately to 1.3 times the sum of the prestress of the closing spring  155  of 20 N in the present case and the hydraulic force of approximately 30 N which is exerted on the valve needle  135  by the fuel in the closed position  230 . A sufficient force impact is thus ensured by means of the idle stroke and the magnet armature, in order to move the valve needle  135  away from the closed position  230 . 
         [0055]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.