Patent Publication Number: US-2006000924-A1

Title: Pump-nozzle unit and method for regulating the opening pressure of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation of copending International Application No. PCT/DE03/04094 filed Dec. 11, 2003 which designates the United States, and claims priority to German Application No. 2002 10260346.4 filed Dec. 20, 2002.  
     TECHNICAL FIELD  
      The invention relates to a method for setting the nozzle opening pressure of a pump-nozzle unit for feeding fuel into a combustion chamber of an internal combustion engine, the pump-nozzle unit having: a fuel injection nozzle, which has a nozzle needle which can move in reciprocating fashion between a closed position and an open position, a spring, which exerts a closure force, the level of which depends on a prestressing force exerted on the spring, on the nozzle needle, and a first pressure space, to which a first pressure can be applied, with an opening force being exerted on the nozzle needle by the first pressure. Furthermore, the invention relates to a pump-nozzle unit for feeding fuel into a combustion chamber of an internal combustion engine, in particular a pump-nozzle unit whereof the nozzle opening pressure has been set using the method according to the invention, having a fuel injection nozzle, which has a nozzle needle which can move in reciprocating fashion between a closed position and an open position, and a spring, which exerts a closure force, the level of which depends on a selected prestressing force exerted on the spring, on the nozzle needle.  
     BACKGROUND  
      Pump-nozzle units of this type are used in particular in combination with pressure-controlled injection systems. One significant feature of a pressure-controlled injection system consists in the fact that the fuel injection nozzle opens as soon as an opening force, which is influenced at least by prevailing pressures, is exerted on the nozzle needle. Pressure-controlled injection systems of this type are used for fuel metering, fuel preparation, the shape of the injection profile and to seal off the supply of fuel with respect to the combustion chamber of the internal combustion engine. Pressure-controlled injection systems allow the time profile of the quantitative flow during the injection to be controlled in an advantageous way. This allows the power, fuel consumption and pollutant emissions of the engine to be influenced in a positive way.  
      In the pump-nozzle units of the generic type, the fuel pump and the fuel injection nozzle are formed as an integrated component. For each combustion chamber of the internal combustion engine, there is at least one pump-nozzle unit, which is generally installed in the cylinder head. The fuel pump typically comprises a fuel pump piston which can move in reciprocating fashion in a fuel pump cylinder and is driven either directly by means of a tappet or indirectly by means of rocker levers of a camshaft of the internal combustion engine. That portion of the fuel pump cylinder which usually forms a second pressure space can be connected via a control valve to a fuel low-pressure region, when the control valve is open, fuel is sucked into the second pressure space from the fuel low-pressure region, and if the control valve stays open, this fuel is then forced back from the second pressure space into the fuel low-pressure region. As soon as the control valve is closed, the fuel pump piston compresses the fuel which is present in the second pressure space, and thereby builds up the pressure. The second pressure space is in communication with a first pressure space, with a first pressure, prevailing in the first pressure space, exerting an opening force on the nozzle needle, for example on a portion of the nozzle needle which has a shoulder.  
      The first pressure, which prevails in the first pressure space and at which the nozzle needle opens and an injection takes place, is referred to as the nozzle opening pressure. To satisfy the functional demands imposed on a pump-nozzle unit, it is necessary to set the nozzle-opening pressure; the higher the nozzle-opening pressures, the more accurate the setting tolerance needs to be. To set the nozzle opening pressure, it is known to preassemble at least the component which accommodates the spring and the spring as well as the fuel injection nozzle on a base disk, and then to measure the actual nozzle-opening pressure on a pressure test bench. The required thickness of a washer which is fitted in the pump-nozzle unit in order to prestress the spring and therefore to change the nozzle-opening pressure, can be calculated from the measured actual nozzle-opening pressure, a predetermined desired nozzle-opening pressure and the spring constant of the spring. In the prior art, the pump-nozzle unit has to be at least partially dismantled in order for the washer to be fitted, and then reassembled after the adjustment washer has been fitted. In many cases, it is then necessary for the nozzle-opening pressure, which has been altered by the washer being fitted, to be tested again on the pressure test bench. If the desired nozzle-opening pressure is not achieved with the fitted washer, it is necessary for the pump-nozzle unit to be dismantled again and for a washer of a different thickness to be fitted.  
      The method explained above is very complex, on account of the dismantling and assembly steps required and the measurement of the actual nozzle-opening pressure, which is separate from the dismantling and assembly steps in terms of time, and therefore entails high costs.  
     SUMMARY  
      The invention is based on the object of developing the methods of the generic type and the pump-nozzle units of the generic type in such a manner that an exact nozzle-opening pressure is ensured in an inexpensive way.  
      This object can be achieved by a method for setting the nozzle opening pressure of a pump-nozzle unit for feeding fuel into a combustion chamber of an internal combustion engine, the pump-nozzle unit having a fuel injection nozzle, which has a nozzle needle which can move in reciprocating fashion between a closed position and an open position, a spring, which exerts a closure force, the level of which depends on a prestressing force exerted on the spring, on the nozzle needle, and a first pressure space, to which a first pressure can be applied, with an opening force being exerted on the nozzle needle by the first pressure, in which the following steps are carried out simultaneously:—applying a first pressure to the first pressure space,—varying the prestressing force exerted on the spring until a selected prestressing force, at which the nozzle needle moves into the open or closed position at the desired level of the first pressure, is reached, wherein the end portion of the spring is locked in the selected position by a prestressing element used in the pump-nozzle unit which forces the end portion of the spring into the selected position, the prestressing element is locked in a selected position in order to force the end portion of the spring into the selected position, and the prestressing element is locked in its selected position by frictional locking; and wherein the prestressing element is designed as a conical stopper, and the prestressing element is arranged in a conical portion of the pump-nozzle unit.  
      An end portion of the spring can be locked in a selected position which the end portion of the spring adopts when the selected prestressing force is exerted on the spring. The dimensions of the prestressing element can be suitable for forcing the end portion of the spring into the selected position. The prestressing element can be locked in its selected position by positive locking. The prestressing element can be deformed in order to achieve the frictional and/or positive lock. The prestressing element can be designed in the form of a sleeve or a slotted sleeve. The prestressing element can be designed in the form of a cup with a hole provided in its base. The prestressing force can be varied by changing the position of the prestressing element. The prestressing force can be varied by changing the position of a mandrel.  
      The object can also be achieved by a pump-nozzle unit for feeding fuel into a combustion chamber of an internal combustion engine, comprising a fuel injection nozzle, which has a nozzle needle which can move in reciprocating fashion between a closed position and an open position, and a spring, which exerts a closure force, the level of which depends on a selected prestressing force exerted on the spring, on the nozzle needle, wherein the level of the selected prestressing force is dependent on a selected position of a prestressing element which is locked in the pump-nozzle unit, wherein the prestressing element forces an end portion of the spring into a selected position and wherein the prestressing element is locked in its selected position by frictional locking, and wherein the prestressing element is designed as a conical stopper, and the prestressing element is arranged in a conical portion of the pump-nozzle unit.  
      The prestressing element can be deformed in order to achieve the frictional and/or a positive lock. The prestressing element can be designed in the form of a sleeve or a slotted sleeve. The prestressing element can be designed in the form of a cup with a hole in its base.  
      The method according to the invention builds on the generic prior art by virtue of including the following steps, which are carried out simultaneously: applying a first pressure to the first pressure space, and varying the prestressing force exerted on the spring until a selected prestressing force, at which the nozzle needle moves into the open or closed position at the desired level of the first pressure, is reached. In this context, in addition to the prestressing force exerted on the spring, it is if appropriate also possible to vary the first pressure, should this prove advantageous. Furthermore, it should be borne in mind that the first pressure for setting the nozzle-opening pressure is preferably not generated by means of the fuel pump assigned to the pump-nozzle unit at least in the ready-to-operate state, but rather is generated externally. The movement of the nozzle needle into the open or closed position can, for example, be detected directly and/or by means of the profile of the first pressure. The method according to the invention makes it possible to set the nozzle-opening pressure very accurately without (repeated) dismantling and assembly steps, and results in a stable pump function. Furthermore, the method according to the invention can be carried out completely automatically at least in preferred embodiments.  
      In preferred embodiments of the method according to the invention, it is also provided that an end portion of the spring is locked in a selected position which the end portion of the spring adopts when the selected prestressing force is exerted on the spring. The locking of the end portion of the spring is preferably likewise fully automatic, either while the selected prestressing force is being determined or thereafter.  
      In the context outlined above, a preferred refinement of the method according to the invention provides that the locking of the end portion of the spring in the selected position is effected by a prestressing element which is inserted into the pump-nozzle unit and forces the end portion of the spring into the selected position. To force the end portion of the spring into the selected position, the prestressing element can act either directly or indirectly, for example via a further element, on the end portion of the spring.  
      In certain embodiments of the method according to the invention, it is possible to provide that the dimensions of the prestressing element are suitable for forcing the end portion of the spring into the selected position. A pump-nozzle unit whereof the nozzle-opening pressure has been set in accordance with this embodiment, in the fully assembled state, under certain circumstances cannot be distinguished from a pump-nozzle unit whereof the nozzle-opening pressure has been set using the known method explained in the introduction. Nevertheless, even this embodiment of the method according to the invention can be carried out very much more easily than the known methods, since the appropriate dimensions of the prestressing element, unlike in the prior art, do not have to be calculated by means of the spring constant of the spring and so on, but rather can be determined directly, for example by length measurement or hydraulic opening pressure measurement, so that repeated dismantling and assembly can be avoided under all circumstances.  
      However, in preferred embodiments of the method according to the invention, it is provided that the prestressing element is locked in a selected position in order to force the end portion of the spring into the selected position. Since in this case the prestressing element is locked in a selected position, which ensures the selected prestressing force, with this embodiment it is not necessary to provide prestressing elements with defined length classes.  
      In the context explained above, it is particularly preferable for the prestressing element to be locked in its selected position by frictional and/or positive locking. A frictional lock is particularly preferred, for example a frictional lock with a coefficient of friction of 0.1÷0.2.  
      If appropriate, in the context explained above, it is also possible to provide that the prestressing element be deformed in order to achieve the frictional and/or positive lock. Deformation of this type can be achieved, for example, by pressing the prestressing element into a conical portion of the spring chamber.  
      Furthermore, it may be advantageous for the prestressing element to be designed in the form of a sleeve or a slotted sleeve.  
      In addition or as an alternative, it is possible to use embodiments in which it is provided that the prestressing element is designed in the form of a cup with a hole in its base.  
      Furthermore, it is possible to provide that the prestressing element be of conical design at least in portions.  
      As has already been indicated above, in certain embodiments of the method according to the invention, it is also possible to provide that the prestressing element be arranged in a conical portion of the pump-nozzle unit.  
      According to a first preferred embodiment of the method according to the invention, it is provided that the prestressing force is varied by changing the position of the prestressing element. For this purpose, the prestressing element can be pressed continuously or in steps into the spring chamber, for example with the aid of a draw-in ram, until the desired nozzle-opening pressure results. The force which is required to press in the prestressing element is in this case preferably significantly higher than the selected prestressing force. This can be ensured, for example, by providing a suitable coefficient of sliding friction or suitable add on friction value.  
      In an alternative embodiment of the method according to the invention, it is provided that the prestressing force is varied by changing the position of a mandrel. In this embodiment it is possible, for example, to provide that the draw-in ram which is provided for pressing in the prestressing element has a bore through which the mandrel extends, it being possible for an end portion of the mandrel to act directly or indirectly on the end portion of the spring. With this solution, it is possible to increase the prestressing force in order to determine the selected prestressing force and then also to reduce it again. The mandrel may act on the end portion of the spring, for example via a perforated disk. The prestressing element is preferably only moved into its final selected position by the draw-in ram when the selected prestressing force has been determined.  
      The pump-nozzle unit according to the invention builds on the generic prior art by virtue of the fact that the level of the selected prestressing force is dependent on a selected position of a prestressing element in which the prestressing element is locked in the pump-nozzle unit. The pump-nozzle units according to the invention differ from the known pump-nozzle units by virtue of the fact that the level of the selected prestressing force is defined not by a prestressing element with defined dimensions, for example a washer with a defined thickness, but rather by means of the location or position of the prestressing element in the pump-nozzle unit. Pump-nozzle units of this type can be produced at lower cost than the known pump-nozzle units, since the setting of the nozzle-opening pressure can be carried out at low cost by means of the position of the prestressing element, for example by means of the method according to the invention.  
      In the pump-nozzle unit according to the invention, it is preferably also provided that the prestressing element forces an end portion of the spring into a selected position.  
      In preferred embodiments of the pump-nozzle unit according to the invention, it is also provided that the prestressing element be locked in its selected position by frictional and/or positive locking.  
      In this context, it is also possible to provide that the prestressing element be deformed in order to achieve the frictional and/or positive lock.  
      Similarly to the method according to the invention, the pump-nozzle unit according to the invention also encompasses embodiments in which it is provided that the prestressing element be designed in the form of a sleeve or a slotted sleeve.  
      Alternatively, it is possible to provide for the prestressing element to be designed in the form of a cup with a hole in its base.  
      With the pump-nozzle unit according to the invention, it is likewise possible to provide for the prestressing element to be of conical design at least in portions.  
      In addition or as an alternative, it is possible to provide that the prestressing element be arranged in a conical portion of the pump-nozzle unit.  
      The discovery that the nozzle-opening pressure can be set very accurately and at low cost if the prestressing force of the spring when pressure is applied to the first pressure space is varied until the desired injection properties are achieved, is crucial to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be explained by way of example with reference to the appended drawings and on the basis of preferred embodiments.  
       FIG. 1  diagrammatically depicts an embodiment of the pump-nozzle unit according to the invention.  
       FIG. 2  shows a diagrammatic illustration presenting the setting of the nozzle-opening pressure in accordance with a first embodiment of the method according to the invention.  
       FIG. 3   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for the first embodiment of the method according to the invention.  
       FIG. 3   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the first embodiment of the method according to the invention.  
       FIG. 3   c  shows a graph illustrating a possible profile of the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 3   a  and the pressure profile illustrated in  FIG. 3   b.    
       FIG. 3   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 3   a  and the pressure profile illustrated in  FIG. 3   b.    
       FIG. 4  shows a diagrammatic illustration presenting the setting of the nozzle-opening pressure in accordance with a second embodiment of the method according to the invention.  
       FIG. 5   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for the second embodiment of the method according to the invention.  
       FIG. 5   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the second embodiment of the method according to the invention.  
       FIG. 5   c  shows a graph illustrating a possible profile for the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 5   a  and the pressure profile illustrated in  FIG. 5   b.    
       FIG. 5   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 5   a  and the pressure profile illustrated in  FIG. 5   b.    
       FIG. 6   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for a third embodiment of the method according to the invention.  
       FIG. 6   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the third embodiment of the method according to the invention.  
       FIG. 6   c  shows a graph which illustrates a possible profile for the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 6   a  and the pressure profile illustrated in  FIG. 6   b.    
       FIG. 6   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 6   a  and the pressure profile illustrated in  FIG. 6   b.    
       FIG. 7  diagrammatically depicts a prestressing element in the form of a sleeve or a slotted sleeve.  
       FIG. 8  diagrammatically depicts a prestressing element in the form of a deep-drawn or extruded and perforated cup.  
       FIG. 9  diagrammatically depicts a prestressing element in the form of a conical stopper.  
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a diagrammatically depicted embodiment of the pump-nozzle unit according to the invention. The pump-nozzle unit  10  illustrated, for feeding fuel  12  into a combustion chamber  14  of an internal combustion engine, has a fuel pump  32 - 40 . A fuel pump piston  36  can move in reciprocating fashion in a fuel pump cylinder  34 . The fuel pump piston  36  is driven directly or indirectly via a camshaft (not shown) of the internal combustion engine. The compression space of the fuel pump cylinder  34  forms a second pressure space  32 . The second pressure space  32  is connected to a piezoelectrically operated control valve  40 , which is known per se, via a fuel line  38 . The control valve  40  is used to either close the fuel line  38  or connect it to a fuel low-pressure region  42 , from which fuel  12  can be sucked up. In its open at-rest position, in the event of an upwardly directed movement of the fuel pump piston  36 , as seen in  FIG. 1 , fuel  12  is sucked out of the fuel low-pressure region  42  into the second pressure space  32 . Provided that the control valve  40  is still in its open at-rest position in the event of a downwardly directed movement of the fuel pump piston  36 , as seen in  FIG. 1 , fuel  12  which has previously been sucked into the second pressure space  32  can be forced back into the fuel low-pressure region  42 . If the control valve  40  is actuated, it closes the fuel line  38 . As a result, the fuel  12  which has been sucked into the second pressure space  32  is compressed in the event of a downwardly directed movement of the fuel pump piston  36 , thereby generating the second pressure p 32  in the second pressure space  32 . The pump-nozzle unit  10  illustrated also comprises a fuel injection nozzle, which is denoted overall by  16  and has a nozzle needle  18  which can move in reciprocating fashion between a closed position and an open position. The upper end portion of the nozzle needle  18 , as seen in  FIG. 1 , has a disk  48  and a guide pin  56 , which in the embodiment illustrated in  FIG. 1  is guided in a spring chamber  30 . A spring  20  is arranged in the spring chamber  30  and exerts a downwardly directed closure force on the disk  48  and the guide pin  56  and therefore the nozzle needle  18 . The upper end portion  24  of the spring  20  is supported against a cup-shaped prestressing element  26 , which is locked in a selected position Y S  in the spring chamber  30 . The selected position Y S  of the closure element  26 , which is referenced to the upper edge of the closure element  26  in  FIG. 1 , forces the end portion  24  of the spring  20  into a selected position X S . The selected position Y S  of the closure element  26  or the selected position X S  of the end portion  24  of the spring  20  is used to set a selected prestressing force F S  which is exerted on the spring  20 . The level of this selected prestressing force F S  influences the nozzle-opening pressure, as will be explained in more detail below. A first pressure space  22  surrounds a portion of the nozzle needle  18  which has a shoulder  46 . The first pressure space  22  is in communication with the second pressure space  32  via a connecting line  44 . Therefore, fuel which is at a first pressure P 22  in the first pressure space  22  exerts an opening force on the nozzle needle  18 . This opening force counteracts the closure force exerted by the spring  20  on the disk  48  and the guide pin  56 . It can be seen that the selected position Y S  of the prestressing element  26  or the selected position X S  of the end portion  24  of the spring  20  defines the level of the first pressure P 22S  required in the first pressure space  22 , which leads to the nozzle needle  18  opening and therefore to an injection operation. The closure element  26  is locked in the spring chamber  30  by frictional locking; by way of example, it is possible to provide for a coefficient of friction of 0.1÷0.2. Although this is not illustrated in  FIG. 1 , the closure element  26  and/or the spring chamber  30  may be conical in form at least in portions, in order to facilitate locking of the closure element  26  in the spring chamber  30 . Furthermore, although this is likewise not illustrated, it is also possible for a pressure to be applied to the spring chamber  30  and/or another region of the pump-nozzle unit, in order to influence the opening behavior of the nozzle needle  18 . By way of example, a pressure prevailing in the spring chamber  30 , in addition to the closure force generated by the spring  20 , would exert a further closure force on the disk  48  and the guide pin  56 . The method according to the invention can advantageously be used to determine the selected position Y S  of the prestressing element  26  and to lock the prestressing element  26  in this position, as will be explained in more detail below.  
       FIG. 2  shows a diagrammatic illustration of the setting of the nozzle-opening pressure in accordance with a first embodiment of the method according to the invention. In this embodiment, a spring  20 , the lower end portion (not shown) of which exerts a closure force on a nozzle needle, is arranged in a spring chamber  30 , only part of which is illustrated. A prestressing element  26 , which is designed in the form of a perforated cup, forces the upper end portion  24  of the spring  20  into a selected position X S  and exerts a selected prestressing force F S  on the spring  20 . The arrangement is selected in such a manner that the prestressing element  26  can be pressed into the spring chamber  30  by a draw-in ram  50 , the force F E  which is required to do this being significantly higher than the selected prestressing force F S . In other words, the prestressing element  26  is frictionally locked in the spring chamber  30  with respect to the selected prestressing force F S , but can be pressed further into the spring chamber  30  by a significantly higher force F E  exerted via the draw-in ram  50 . Based on the illustration presented in  FIG. 2 , the prestressing element  26  is pushed or pressed down into the spring chamber  30  until it adopts a selected position Y S  in which the end portion  24  of the spring  20  is fixed in a selected position X S , in which the selected prestressing force F S  is exerted on the spring  20 . The setting of the selected prestressing force F S  is explained in more detail below with reference to  FIGS. 3   a  to  3   d.    
       FIG. 3   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for the first embodiment of the method according to the invention,  FIG. 3   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the first embodiment of the method according to the invention,  FIG. 3   c  shows a graph illustrating a possible profile for the nozzle-space pressure or the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 3   a  and the pressure profile illustrated in  FIG. 3   b,  and  FIG. 3   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 3   a  and the pressure profile illustrated in  FIG. 3   b.  It can be seen from  FIG. 3   b  that the system is subject to a constant second pressure p 32  of, in the case illustrated,  700  bar during the setting operation. The second pressure p 32  for setting the nozzle-opening pressure is generated not by the fuel pump  32  to  42  (cf  FIG. 1 ), but rather externally. It can be seen from the illustration represented in  FIG. 3   a  that the prestressing force exerted on the spring  20  is increased in steps. Based on the illustration presented in  FIG. 2 , the prestressing force F is increased by moving the draw-in ram  50  downward in steps, so that the spring  20  is prestressed further in steps by means of the prestressing element  26 .  FIG. 3   c  shows the profile of the first pressure p 22  within the first pressure space  22  (cf.  FIG. 1 ).  FIG. 3   c  also shows the opening and closing characteristics of the nozzle needle, the first pressure p 22  at which the nozzle needle opens and also the first pressure p 22  at which the nozzle needle closes again increasing as the prestressing force rises. The period of time between the nozzle needle  18  opening and closing in each case defines the duration of an injection, as can be seen from  FIG. 3   d.  The prestressing force F is increased in steps until the nozzle needle  20  opens at a selected first (opening) pressure p 22S . The prestressing force F which is exerted on the spring  20  at this time corresponds to the selected prestressing force F S . As soon as this selected prestressing force F S  has been reached, the draw-in ram  50  (cf  FIG. 2 ) can be removed, since the prestressing element  26  is in its selected position Y S , in which it locks the end portion  24  of the spring  20  in the selected position X S  thereof. As an alternative to increasing the prestressing force F in steps, as illustrated in  FIG. 3   a,  it is also possible for the prestressing force F to be increased continuously.  
       FIG. 4  shows a diagrammatic illustration of the setting of the nozzle-opening pressure in accordance with a second embodiment of the method according to the invention. In this embodiment too, a spring  20  is arranged in a spring chamber  30 , only part of which is illustrated; the lower end portion (not shown) of this spring  20  exerts a closure force on a nozzle needle  18 . A perforated disk  54  is arranged between a prestressing element  26  and the upper end portion  24  of the spring  20 . The prestressing element  26  is designed in the form of a perforated cup, with a mandrel  28  extending through the recess in the prestressing element  26  and thereby being able to exert a prestressing force F on the perforated disk  54  and therefore the spring  20 . The mandrel  28  furthermore extends through a hole  52  provided in a draw-in ram  50 , in such a manner that the mandrel  28  can be moved up and down in a direction Z independently of the draw-in ram  50 . In accordance with the illustration presented in  FIG. 4 , the prestressing element  26  is already in its selected position Y S , in which the upper end portion  24  of the spring  20  is forced into its selected position X S  via the perforated disk  54 . During the determination of the selected prestressing force F S  or of the selected position X S , the prestressing element  26  and the draw-in ram  50  are arranged further upward than in the illustration presented in  FIG. 4 , so that the mandrel  28  can be moved up and down independently of the draw-in ram  50  and the prestressing element  26 , until the mandrel  28  is in a selected position Z S , in which the selected prestressing force F S  is exerted on the spring  20 . As soon as the mandrel  28  has adopted its selected position Z S , in which the end portion  24  of the spring  20  is in its selected position X S , the draw-in ram  50 , and with it the prestressing element  26 , are moved downward by a force F E , which must preferably be significantly higher than the selected prestressing force F S , being exerted on the draw-in ram  50  in order to move the prestressing element  26 . Then, the mandrel  28  and the draw-in ram  50  can be removed, since the prestressing element  26  has been frictionally locked in the spring chamber  30  with respect to the prestressing force F S .  
      As an alternative to locking the prestressing element  26  in the spring chamber  30 , it is also possible, for example, to record the distance which the mandrel  28  has moved into the spring chamber  30  by the time the selected prestressing force F S  is reached, and then to fit a prestressing element of a defined length, which ensures the selected position X S  of the end portion  24  of the spring  20 . However, the following explanation of the determining and setting of the selected prestressing force F S  relates to an embodiment of the invention in which a prestressing element  26  which is locked in a selected position Y S  in the spring chamber  30  is used, in accordance with the illustration presented in  FIG. 4 .  
       FIG. 5   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for the second embodiment of the method according to the invention,  FIG. 5   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the second embodiment of the method according to the invention,  FIG. 5   c  shows a graph illustrating a possible profile for the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 5   a  and the pressure profile illustrated in  FIG. 5   b,  and  FIG. 5   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 5   a  and the pressure profile illustrated in  FIG. 5   b.  In this embodiment too, the second pressure p 32  in the second pressure space  32  is preferably provided by an external pressure source, in order to determine the selected prestressing force F S  and/or set the nozzle-opening pressure to p 22S . As can be seen from a comparison of  FIGS. 5   b  and  5   c,  first of all an initial setting sequence is carried out, during which the nozzle needle  18  opens and closes a number of times (cf.  FIG. 5   c ). For this purpose, a second pressure p 32  of 500 bar is applied to the system (cf.  FIG. 5   b ). At the same time, a relatively low prestressing force of 500 N is exerted on the spring  20  via the mandrel  28  in accordance with  FIG. 4 . Then, both the second pressure p 32  and the prestressing force F are increased; to increase the prestressing force F, the mandrel  28  is moved further downward with respect to the illustration presented in  FIG. 4 . At this time, the draw-in ram  50  and the prestressing element  26  from  FIG. 4  are still in a higher position than that illustrated, so that the spring  20  can be compressed to a greater or lesser extent by an upward and downward movement of the mandrel  28  and can thereby be prestressed. After the prestressing force F has been increased to more than 700 N, this force is gradually reduced again (cf.  FIG. 5   a ) by the mandrel  28  from  FIG. 4  being moved back upward. As soon as the nozzle needle  18  opens at a desired opening pressure p 22S  in the first pressure space, the selected prestressing force F S  has been reached and the upward movement of the mandrel  28  is stopped. Then, the draw-in ram  50 , and with it the prestressing element  26 , are moved downward, with respect to  FIG. 4 , until the prestressing element  26  is locked in its selected position Y S , by frictional locking with the spring chamber  30  based on the selected prestressing force F S . After the prestressing element  26  has been locked in its selected position Y S , in order to force the end portion  24  of the spring  20  into its selected position X S , both the draw-in ram  50  and the mandrel  28  are removed.  
       FIG. 6   a  shows a graph illustrating a possible profile of the prestressing force exerted on the spring as a function of time for a third embodiment of the method according to the invention,  FIG. 6   b  shows a graph illustrating a possible profile for the pump pressure as a function of time for the third embodiment of the method according to the invention,  FIG. 6   c  shows a graph illustrating a possible profile for the first pressure and the opening and closing characteristics of the nozzle needle as a function of time, for the force profile illustrated in  FIG. 6   a  and the pressure profile illustrated in  FIG. 6   b,  and  FIG. 6   d  shows the injection characteristics of the pump-nozzle unit as a function of time, for the force profile illustrated in  FIG. 6   a  and the pressure profile illustrated in  FIG. 6   b.  In the embodiment of the method according to the invention which is illustrated in  FIGS. 6   a  to  6   d,  the prestressing force F is altered continuously, in the present case increased, in accordance with the profile presented in  FIG. 6   a.  Furthermore, in the case illustrated in  FIG. 6   b,  the pressure p 32  which is preferably also generated externally in this case is 750 bar. By way of example under these conditions, the nozzle needle  18  starts to oscillate, i.e. the nozzle needle  18  opens and closes at short intervals (cf.  FIG. 6   c ). This allows the desired opening pressure p 22S  to be determined exclusively on the basis of the profile of the first pressure p 22 . For this purpose, the prestressing force F can, for example, be increased continuously (cf.  FIG. 6   a ) or in small steps until the desired opening pressure p 22S  of, in the case illustrated, 700 bar is reached for the first time (cf  FIG. 6   c ). This procedure can if desired be combined with the first embodiment illustrated in  FIG. 2  and with the second embodiment illustrated in  FIG. 4 .  
      In all the embodiments in which the prestressing element  26  is locked by frictional locking, it is preferable for it to be the case, for the force F E  required to move the prestressing element  26 , that F E ≧10*F S , where F S  is the selected prestressing force to be set.  
       FIG. 7  diagrammatically depicts a prestressing element in the form of a sleeve or a slotted sleeve,  FIG. 8  diagrammatically depicts a prestressing element in the form of a deep-drawn or extruded and perforated cup, and  FIG. 9  diagrammatically depicts a prestressing element in the form of a conical stopper. Although this is not necessary in every case, all the embodiments of the prestressing element  26  illustrated have an aperture. An aperture of this type may be required, for example, if the spring chamber  30  is also filled with pressurized fuel from above, in order for a further closure force to be exerted on the nozzle needle  18 .  
      The features of the invention which have been disclosed in the present description, in the drawings and in the claims may be pertinent to realization of the invention both on their own and in any desired combination.