Abstract:
A method for producing a throttle point, particularly in a channel of a valve piece of a fuel injector is disclosed. The following are performed: first, the outlet channel is created by machining. Then, the throttle point is stamped by a stamping die inserted into a guide hole. The pre-stamped throttle point is electrical-discharge machined in such a way that said throttle point has an electrical-discharge machined inlet area and comprises a stamped outlet area, which lies before a continuous cross-section expansion of the outlet channel as viewed in the flow direction.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2009/064068, filed Oct. 26, 2009, which claims the benefit of priority to Serial No. DE 10 2008 044 096.5, filed Nov. 27, 2008 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The disclosure proceeds from known ball seat or needle valves for adjusting a throughflow of fluid media. Such valves with a spherical or needle-shaped closing element are used in many fields of technology in which a throughflow of fluid media, such as for example gases or liquids, must be adjusted, for example in the field of hydraulic regulating systems. 
     An important application example of such ball seat or needle valves can be found in the field of automotive engineering, in particular in the field of injection technology. Such valves are used in numerous injection devices for hydraulic pressure regulation and/or control of the injection behavior of such systems. In particular in the field of high-pressure accumulator injection systems (common rail system), these valves are used to control the stroke of an injection valve member which opens or closes injection openings. Examples of such devices are specified in DE 101 52 173 A1 and DE 196 50 865 A1. Here, a control chamber which directly or indirectly influences the stroke of the injection valve member is connected to the valve directly or via a further bore via an inlet. Aside from the closing element, the valve has an actuator which presses the closing element into a valve seat or raises the closing element from said valve seat in order to separate the control chamber from a relief chamber, or connect said control chamber to the relief chamber. 
     As presented for example in DE 101 52 173 A1, valves for adjusting throughflow known from the prior art generally have a prethrottle on that side of the inlet which faces toward the control chamber. Said prethrottle is adjoined in the direction of the valve by one or more widenings of the inlet, which may take different forms. The flow through the prethrottle is generally configured so as to be lower than the volume flow which passes between the valve seat and the closing element when the ball seat valve or needle valve is fully open. As a result of the prethrottle, the throughflow is substantially independent of the closing element stroke tolerance to be set. 
     In practice, however, conventional valves known from the prior art which have a spherical or needle-shaped closing element have the disadvantage of intense erosion. If there is a large pressure gradient between the entry and exit of a throttle bore, such as is often the case in throttle bores in high-pressure injection valves which operate with pressures of up to 3500 bar, the medium accelerates in the throttle bore so intensely that the pressure falls to approximately 0 bar, and cavitation bubbles form. To prevent said cavitation bubbles from imploding on the surface of a component, or even on the valve seat, and thereby damaging the surface, it has proven to be advantageous for a large amount of the cavitation bubbles to be converted in a controlled manner into liquid phase by means of a second prethrottle, and for damage to functionally critical components to thereby be prevented, as described in DE 10 2007 004 553 A1. 
     The flow speed of the medium increases intensely in the inlet region of a throttle bore. At the same time, the pressure in the medium falls. If the pressure falls to the vapor pressure of the medium, cavitation bubbles form. Since the pressure can fall no further below the vapor pressure of the medium, the mass flow through the throttle bore remains constant after the vapor pressure is reached. The vapor-filled cavitation bubbles take up a larger volume than the liquid medium. This results in a pressure recovery in the onward flow through the throttle bore. The pressure recovery is further intensified by the contact of the flow against the throttle wall. In throttles with relatively high pressure recovery, a higher back pressure can be applied downstream of the throttle before flow no longer passes in a cavitating manner through the throttle. The greater the pressure recovery, the more independent the throttle is of the conditions downstream of the throttle. 
     To make throttle bores as insensitive to pressure fluctuations and geometric influences as possible, a cavitating throughflow is desirable. Ideally, the cavitation transition point should lie as close as possible to the start of the throttle, because the attainable pressure recovery is greater and therefore the tendency for a cavitating flow through the throttle increases. In contrast, if the cavitation transition point is situated at the end of the throttle bore in the region of the throttle outlet, the pressure recovery can take place only in a diffuser bore. Since the latter however has a relatively large cross section, the attainable pressure recovery is lower. If the throttle duct widens slowly in the flow direction, then the cavitation transition point lies at the start of the throttle, because the narrowest cross section of the throttle duct lies in the vicinity of the throttle inlet. 
     Throttle bores which are presented in the fuel injectors according to the prior art may be produced for example by means of cutting production processes such as drilling. It is however also possible for throttle bores to be produced by erosion. In the case of erosion, as a result of the more intense burn-off of the electrode at the tip, a throttle bore is generated which tapers slowly in the erosion direction. The tapering of the throttle bore in the erosion direction takes place on the micrometer scale. An eroded throttle can accordingly have the cavitation transition point in the region of the inlet only if flow passes through said throttle counter to the erosion direction. In contrast, if erosion counter to the throughflow direction is not possible or uneconomical or if there are other reasons against this, a throttle duct is formed which tapers slowly in the throughflow direction. As a result, a cavitation transition point is generated a short distance upstream of the throttle outlet, which leads to the abovementioned reduced maximum attainable pressure recovery. 
     To produce a throttle bore which, contrary to the “natural” erosion process, widens slowly in the erosion direction, the approach at present is to set the component to be eroded in a tumbling motion such that the origin of the tumbling motion is situated in the vicinity of the throttle inlet. This hitherto practiced method is however associated with the disadvantage that small throttle throughflow tolerances can be realized only with difficulty. 
     SUMMARY 
     The present disclosure describes a method for producing a throttle point, by means of which method it is possible to produce a throttle bore which has a cavitation transition point from non-cavitating to cavitating throughflow as close as possible to the throttle inlet, and which can be produced by means of erosion even in the flow direction. 
     In the solution proposed according to the disclosure, a throttle bore which has the advantages of a cavitation transition point from non-cavitating to cavitating throughflow and which widens in the throughflow direction is produced by impressing a diffuser bore in the soft state. Hardening of the component obtained in this way is subsequently carried out. In the case of a throttle bore which widens in the flow direction, the entry region of the throttle determines the throughflow. The diameter is therefore produced very precisely in said region. The length of the inlet region is preferably between one and two times the throttle diameter. The inlet region of the throttle bore may be cylindrical or may widen on the micrometer scale in the flow direction. The outlet region of the throttle bore has no influence on the throughflow, but determines the level of the possible pressure recovery. The outlet region must have a greater diameter than the inlet region of the throttle bore and widen on the micrometer scale in the flow direction. While the inlet region of the throttle bore is preferably produced by means of an erosion process after the hardening of the component, the outlet region of the throttle is generated by means of an impressing process in the soft state. 
     After a first cutting machining process, an already existing diffuser bore is reduced in diameter by impressing to such an extent that the smallest cross section is slightly smaller than the throttle diameter to be produced later by an erosion process. Here, a throttle form is generated which is similar to a de Laval nozzle. Subsequently, that tapering region of the throttle bore which corresponds to the inlet region of a de Laval nozzle, and which is detrimental to a cavitating flow through the throttle bore, is replaced by the formation of a pre-hole, preferably by means of drilling. Here, as sharp-edged a transition as possible is formed between the pre-hole and the throttle bore. After the hardening of the component, a precisely defined throttle diameter is then produced by means of an erosion process. Here, the electrode is advantageously adapted to the funnel shape generated during the impressing process. 
     The method proposed according to the disclosure for producing a throttle bore is characterized in particular in that the impressing punch is self-centered in a guide bore which delimits the control chamber and in which the injection valve member is mounted. In the method proposed according to the disclosure, standard erosion processes can be used to produce the throttle bore contour. The impressing process can be integrated into the already provided process step of cutting production. 
     A cavitation transition point in the throttle inlet region of a throttle bore can be attained by means of the sharp-edged transition between the pre-hole and the throttle bore and also a minimum throttle bore length, which is dependent on the throttle diameter at the throttle point. Furthermore, a cavitation transition point in the throttle inlet region is promoted by means of a throttle cross section which slowly increases in size in the flow direction through the throttle bore. 
     The method proposed according to the disclosure for producing the throttle bore with a cavitation transition point in the throttle inlet region can be applied to throttle bores in fuel injectors of all fuel injection systems for auto-ignition internal combustion engines in which erosion of the throttle bore in the valve piece counter to the flow direction is not possible, or a conical contour of the throttle should be more accentuated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be described in more detail below on the basis of the drawing, in which: 
         FIG. 1  shows an illustration of a component in which an outlet duct runs, at the preliminary turning stage, 
         FIG. 2  shows the component illustrated at the preliminary turning stage in  FIG. 1  with an impressing punch impression, 
         FIG. 3  shows the removal of the impressing punch impression by cutting, and the production of a pre-hole with a sharp-edged transition to the throttle bore, 
         FIG. 4  shows the eroded hard stage with final, highly precisely set throttle diameter, 
         FIG. 5  shows an embodiment of an outlet duct which extends through two components, a throttle plate and a guide body, 
         FIG. 6  shows the illustration of a component which is designed in particular as a valve piece and in which the throttle point produced according to the disclosure is formed. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a component, in particular a valve piece of a fuel injector, in which a duct for discharging a pressurized medium is formed. 
     The component  10  illustrated in  FIG. 1  is in particular the valve piece of a fuel injector in which a control chamber is situated. The control chamber is charged by a pressurized medium, in particular fuel at system pressure. The system pressure of the fuel is generated for example in a high-pressure accumulator body (common rail) of a high-pressure accumulator injection system by means of a high-pressure pump which acts on the high-pressure accumulator body (common rail). An inlet throttle  22  will be produced subsequently at a counterbore  21  (cf. illustration of  FIG. 3 ), through which inlet throttle the medium at system pressure, such as for example fuel, flows into the control chamber (cf.  FIG. 3 ). 
     A release of pressure from the control chamber formed in the valve piece  10  takes place through a duct, in particular an outflow duct  14 . The duct which runs in the valve piece  10  opens out at a valve seat  20  which is formed in a funnel  12  on a planar side of the valve piece  10 . A spherical or needle-shaped closing element (not illustrated here) is held in the valve seat  20 , which closing element closes off the outlet duct  14  in the closed state and, for a release of pressure from the control chamber (cf.  FIGS. 3 and 6 ), opens said outlet duct  14  under the action of a solenoid valve or a piezoelectric actuator or the like. 
     In  FIG. 1 , the component  10 , which is in particular a valve piece, is provided with a guide bore  18  which adjoins the control chamber, and said component  10  is illustrated at the preliminary turning stage before the impressing process. 
     In the illustration of  FIG. 2 , the component, which is preferably designed as a valve piece, according to  FIG. 1  is illustrated at a further machining stage. 
       FIG. 2  shows that the component  10 , which is designed as a valve piece, has the guide bore  18  which extends to the control chamber  16  symmetrically with respect to the axis of symmetry  28 . The guide bore  18  serves to center and guide an impressing punch. The impressing punch has an impressing head on the end side. The impressing punch centered in the guide bore  18  of the valve piece  10  can be inserted, in conjunction with the cutting turning machining process during which the outlet duct  14  is produced in the valve piece  10 , into the guide bore  18 , and said impressing punch deforms the end-side delimiting wall of the guide bore  18  through which the outlet duct  14  extends, as illustrated in  FIG. 2 . The impressing of the throttle point  32  is carried out preferably in the soft state of the valve piece  10 , that is to say at a time at which the surface of said valve piece has not yet been hardened. As a result of the introduction of the impressing punch into the guide bore  18 , upsetting  40  of the material of the valve piece  10  occurs in the region of the outlet duct  14 , and a throttle point  32  is formed by upset material  40 . In the upset material  40 , a residual cross section remains which is slightly smaller than the throttle diameter  54  produced in a highly precise manner at a later time. On account of the contour impressed uniformly in the material of the component  10 , that is to say formed in the material of the valve piece  10 , a throttle point  32  is produced which is adjoined, as viewed in the axial direction, by a continuous cross-sectional widening  38  of the outlet duct  14 . The outlet duct  14  opens out—as illustrated in FIG.  2 —at the valve seat  20  in the funnel  12  of the valve piece  10 . A seat throttle  50  in the outlet duct  14  has the effect that some of the cavitation bubbles in particular in the edge region condense, thereby preventing the cavitation bubbles from destroying the valve seat formed in the valve piece  10 . The seat throttle  50  is situated in the outlet duct  14  directly upstream of the opening-out point in the valve seat  20 . As illustrated in  FIG. 2 , the insertion of the impressing punch into the guide bore  18  permits self-centering of the impressing punch during the impressing process. 
       FIG. 3  shows subsequent cutting machining of the valve piece to form the control chamber. 
     As illustrated in  FIG. 2 , during the impressing process, a throttle point contour is formed which is similar to that of a de Laval nozzle. Said contour which is similar to that of a de Laval nozzle has both a rounded inlet region and also a rounded outlet region. The inlet region with its rounded edges is unfavorable for the formation of a cavitating flow of the medium through the throttle point. A sharp-edged throttle inlet is in fact required. For this reason, the impressing punch impression is completely removed, which is preferably achieved by means of a cutting machining process. The pre-hole  44  is situated at the point where the inlet region, which was detrimental to a cavitating throughflow, of the impressed contour was situated before the cutting machining process, and said pre-hole forms a sharp-edged transition to the throttle point  32 . 
     The illustration of  FIG. 3  shows that the impressed contour is produced on account of the upset material  40 . The upset material  40 , with its side adjoining the outlet duct  14  in the valve piece  10 , delimits the throttle point  32  in the outlet duct  14 . Since the impressing punch is centered in the guide bore  18  of the valve piece  10 , as in the illustration of  FIG. 2 , the deformation of the material of the valve piece  10  to form the throttle point  32  is introduced symmetrically into the valve piece  10 , that is to say the upsetting  40  of the material of the valve piece  10  runs symmetrically with respect to the axis of symmetry of the valve piece  10 . At the subsequent cutting machining stage, the control chamber  16  is turned out of the valve piece  10  in the region in which the impressing punch impression was situated in the machining step according to  FIG. 2 . During the cutting machining process according to  FIG. 3 , firstly the control chamber  16  is formed adjacent to the inlet throttle  22  and secondly a pre-hole  44  which constitutes a sharp-edged throttle inlet is produced. Said pre-hole  44  is situated at the entry side of the throttle point  32 . The cavitation transition point is situated in the entry region of the throttle point  32  and is generated during the erosion process. The throttle point  32  is formed as a result of the upsetting of the material  40 . 
     The sharp-edged pre-hole  44  according to the illustration in  FIG. 3  is situated between the control chamber  16  and an inlet  31  into the throttle point  32 . 
     The illustration of  FIG. 4  shows the eroded hard stage of the valve piece. 
     Erosion is subsequently carried out, as a result of which the defined diameter  54  which determines the throughflow is produced in a highly precise manner at the throttle point  32 . During the erosion of the valve piece  10 , a throttle point  32  with a very precise diameter  54  is finally formed which opens out into a slowly widening outlet region (cf. position  38 ) generated during the impressing process. The smallest residual cross section generated during the impressing process should advantageously be set so as to be slightly smaller than the throttle diameter generated during the erosion process. This provides as smooth a transition as possible between the eroded contour and the impressed contour. During the erosion of the valve piece  10 , a throttle point  32  is formed which widens in the erosion direction and which merges into the continuous cross-sectional widening  38 , which has an extremely favorable influence on the throughflow characteristics of the throttle point  32  produced according to the disclosure in the outlet duct  14 . The erosion electrode used for the erosion process, that is to say for producing the final throttle diameter  54 , is advantageously adapted to the contour of the throttle point  32  previously formed during the impressing process. In this way, it is possible to realize extremely short cycle times within the erosion process. 
     Before the erosion process, which preferably takes place in an erosion direction which is the same as the flow direction through the throttle point  32  in the outlet duct  14  to the valve seat  20 , hardening of the valve piece  10  is carried out. The desired precisely well-defined throttle diameter  54  of the throttle point  32 , that is to say the fine adjustment of the throttle cross section and the throttle action attained as a result, is set by means of the erosion process and subsequent hydroerosive rounding. 
     By means of the above-illustrated machining process for producing a throttle point  32  in the outlet duct  14  of a valve piece  10 , it is possible to obtain a cavitation transition point at the throttle inlet region, which generates a transition from non-cavitating throughflow to cavitating throughflow with a relatively high back pressure. In this way, the throttle point  32  is insensitive with regard to pressure and geometrical influences. 
     As shown in  FIGS. 2 ,  3  and  4 , the continuously widening cross-sectional widening  38  which is advantageous for the cavitating flow through the throttle point  32  can be obtained by virtue of the erosion electrode used for the finish machining of the throttle point  32  being adapted to the impressed contour already provided by the impressing process, that is to say a de Laval nozzle contour or a funnel shape downstream of the throttle point  32 . In this way, it is advantageously obtained that the cavitation transition point lies in the inlet region of the throttle point  32 . The length of the subsequently eroded throttle bore is of particular significance. The inlet region of the throttle bore may be cylindrical or may widen on the micrometer scale in the flow direction. The outlet region of the throttle bore has no influence on the throughflow, but determines the level of the possible pressure recovery. The outlet region must have a greater diameter than the inlet region of the throttle bore, and widen on the micrometer scale in the flow direction. 
     The illustrated manufacturing process for producing a throttle point  32  with a cavitation transition point in the throttle inlet region may be implemented for throttle points  32  which are used in outlet ducts  14  for control chambers  16  for actuating fuel injectors, and in which erosion of the throttle point  32  counter to the flow direction is not possible. 
     In the case of injection nozzles at the combustion-chamber-side end of fuel injectors, the conicity of the injection openings which open into the combustion chamber can be described by the K factor, that is to say the conicity factor. With increasing K factor, the cross section of the injection opening tapers to an increasing extent in the flow direction. In the case of injection openings, the conicity thereof is utilized to delay the cavitation time and thereby increase the jet penetration depth during the injection of fuel into the combustion chamber of the internal combustion engine. The above-described impressing process can also be taken into consideration for producing conically running injection openings more cost-effectively. 
       FIG. 5  shows an outlet duct, running through two mutually separate components, for the release of pressure from a control chamber. 
       FIG. 5  shows that, along a contact surface  66 , a throttle point  58  is connected to a guide body  60 . The injection valve member  62  which is preferably of needle-shaped design is guided in the guide bore  18  in the guide body  60 . The planar side  64  of said injection valve member  62  delimits the control chamber  16 , which in turn is charged with fuel at system pressure via the inlet throttle  22 . The outlet duct  14  which extends from the control chamber  16  in the guide body  60  to the valve seat  20  in the throttle plate  58  comprises the pre-hole  44  and the throttle point  32 , which adjoins said pre-hole  44  as viewed in the throughflow direction. The throttle point  32  merges—as viewed in the throughflow direction—into a diffuser  52 , which in turn opens out in a seat throttle  50  provided below the valve seat  20 . The valve seat  20  according to the embodiment in  FIG. 5  is closed off by a spherical or conical closing element  46  which is held in a closing element receptacle  58 . The closing element receptacle  58  may be formed on the underside of an armature pin which can be actuated by means of a solenoid valve or a piezoelectric actuator. 
     In the embodiment illustrated in  FIG. 5 , the outlet duct  14  runs through two separate components, that is to say the throttle plate  58  and the guide bore  60 . The outlet duct  14  is produced in the throttle plate  58  by means of cutting process steps, and in the guide body  60  likewise by cutting machining. A disadvantage here is that two components are required, and therefore production is more expensive overall, and an additional sealing surface is formed. The impressing process is not used here, such that the valve piece illustrated in  FIG. 5  comprises two components, that is to say the throttle plate  58  and the guide body  60 . 
     In contrast,  FIG. 6  shows a throttle point produced in the outlet duct  14  by means of the method proposed according to the disclosure. 
     As shown in  FIG. 6 , in contrast to the illustration of  FIG. 5 , the valve piece  10  is formed in one piece, that is to say the throttle plate  58  and the guide body  60  for guiding the preferably needle-shaped injection valve member  62  form one component. 
     The needle-shaped injection valve  62  is guided in the guide bore  18  of the valve piece  10  and, by means of its planar side  64 , delimits the control chamber  16 . The control chamber  16  is in turn charged with fuel at system pressure through the inlet throttle  22 . The release of pressure from the control chamber  16  takes place in the outlet duct  14 , with the pre-hole  44  serving as an admission chamber of the throttle duct  14  and the throttle point  32  contained therein. The sharp-edged transition  44  is formed by the edge between the pre-hole  44  and the throttle. The sharp-edged transition  44  (pre-hole) is situated upstream of the throttle point  32  in the flow direction through the outlet duct  14 . As shown in the illustrations of  FIGS. 1-4 , said throttle point  32  is produced by means of cutting process steps and by means of impressing. The diameter constriction  34 ,  54  of the outlet duct  14 , which forms the actual throttle point  32 , is realized by means of upset material  40  which, during the impressing process, delimits the throughflow cross section of the outlet duct  14  at the throttle point  32 . According to the machining steps illustrated in  FIGS. 1-4 , the throttle point  32  comprises firstly the eroded inlet region and secondly the impressed outlet region which, in the throughflow direction of the medium to be conducted to the valve seat  20 , lies upstream of the continuous cross-sectional widening  38 . 
     The continuous cross-sectional widening  38  of the outlet duct  14  merges into a seat throttle  50 , also referred to as a prethrottle, below the valve seat  20  which, in the illustration of  FIG. 6 , is closed off by the closing element  46 , which is of spherical design here. The closing element  46  is guided in a closing element receptacle  48  which in turn is actuated by means of a solenoid valve or by means of a piezoelectric actuator, to name two examples. During the opening of the valve seat  20 , the closing element  46  is raised out of the valve seat  20 , such that via the seat throttle  50  arranged below the valve seat  20 , a control quantity can be discharged out of the control chamber  16  via the outlet duct  14  with throttle point  32  integrated therein.