Abstract:
A nozzle needle for an injector for injecting fuel includes at least one guiding section with a recess and a guiding surface configured to rest against an inner surface of a nozzle body of the injector, the recess at least partially delimiting the guiding surface and being configured to allow a flow of fuel along the guiding section, and the guiding surface being guided about the longitudinal axis of the nozzle needle in a helical manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2012/076195 filed Dec. 19, 2012, which designates the United States of America, and claims priority to DE Application No. 10 2011 090 148.5 filed Dec. 30, 2011, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention concerns a nozzle needle for an injector for injecting fuel, and an injector with such a nozzle needle, having at least one guide portion and a recess with a guide surface, wherein the guide surface is configured to rest on an inner face of a nozzle body of the injector, and the recess at least partially delimits the guide surface and is configured to allow a fuel flow along the guide portion. 
     BACKGROUND 
     Nozzle needles are known for injectors for injecting fuel into cylinder combustion chambers of an internal combustion engine, which have a guide portion. The guide portion here has guide surfaces which are separated from each other by recesses. The guide surfaces serve to lie on an inner face of the nozzle body of the injector and guide the nozzle needle in its stroke movement. The recesses are provided to allow fuel to flow by, along the nozzle needle in the nozzle body, parallel to the longitudinal axis of the nozzle needle. The recesses are normally oriented parallel to a longitudinal axis of the nozzle needle and are usually formed in the nozzle needle by ground grooves or by removal of the circumferential surface into a three- or four-sided profile by material removal in a hard-machining process. 
     SUMMARY 
     One embodiment provides a nozzle needle for an injector for injecting fuel, having at least one guide portion and a recess with a guide surface, wherein the guide surface is configured to rest on an inner face of a nozzle body of the injector, and the recess at least partly delimits the guide surface and is configured to allow a fuel flow along the guide portion, wherein the guide surface is guided in a helical manner about the longitudinal axis of the nozzle needle. 
     In a further embodiment, the at least one guide surface overlaps with itself or with a further guide surface on a projection in a projection plane which is arranged at right angles to the longitudinal axis of the nozzle needle. 
     In a further embodiment, the at least one guide surface forms a closed circuit in the projection plane. 
     In a further embodiment, the guide surface extends substantially over an angular segment of 60° to 90°, in particular from 65° to 75° in the projection plane. 
     In a further embodiment, the guide portion has at least one guide tooth which is guided in a helical manner about the longitudinal axis, wherein the guide surface is arranged radially on the outside at the guide tooth. 
     In a further embodiment, the guide tooth has an involute toothing. 
     In a further embodiment, the ratio of the area of the recess to the total area of the nozzle needle in the profile section is 0.15 to 0.30, in particular 0.18 to 0.25. 
     In a further embodiment, the guide surface has a width in relation to an angular segment established by the guide surface, in the peripheral direction of the nozzle needle, of 1.5° to 5°, in particular 2° to 3°, in particular 2.2° to 2.3°. 
     In a further embodiment, the guide portion can be produced in the nozzle needle by means of cold forming, in particular by means of rolling. 
     Another embodiment provides an injector for injecting fuel into the cylinder combustion chambers of an internal combustion engine, with a nozzle body with an inner face and a nozzle needle as disclosed above, wherein the guide surface of the nozzle needle lies at least partly on the inner face of the nozzle body and guides the nozzle needle in its movement. 
     Another embodiment provides a method for production of a nozzle needle for an injector for injecting fuel into the cylinder combustion chambers of an internal combustion engine, wherein a base body of the nozzle needle is soft-machined, in particular turned, wherein at least one recess and at least one guide surface are produced in the base body of the nozzle needle by rolling, wherein the base body is annealed, and wherein the guide surface of the guide portion is hard-machined, in particular hard-turned or ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments are discussed below with reference to the drawings, in which: 
         FIG. 1  shows a side view of a nozzle needle, 
         FIG. 2  shows a section view of an injector with the nozzle needle shown in  FIG. 1 , 
         FIG. 3  shows a partial extract from the injector shown in  FIG. 2  with a guide portion of the nozzle needle, and 
         FIG. 4  shows a cross section through the injector and the nozzle needle in the region of the guide portion. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention allow an improved stabilization of the nozzle needle in the nozzle body of the injector, and simpler production of the nozzle needle. 
     In some embodiments the nozzle needle has a guide portion to guide the nozzle needle in a stroke movement parallel to a longitudinal axis in a nozzle body of the injector, wherein the guide portion comprises at least one guide surface and a recess. 
     The guide surface lies on an inner face of the nozzle body of the injector. The guide surface is guided in a helical manner about the longitudinal axis of the nozzle needle, so as to obtain an improved support of the nozzle needle inside the nozzle body of the injector. The recess is configured to allow a fuel flow along the guide portion of the nozzle needle. Furthermore, deaxialization is minimized due to the helical design of the guide surface and the mutual offset in relation to possible further guide portions. 
     In a further embodiment, the at least one guide surface overlaps with itself or a further guide surface on a projection in a projection plane which is arranged at right angles to the longitudinal axis of the nozzle needle, so that the nozzle needle can be stabilized particularly well. 
     It may be particularly advantageous for stabilizing the nozzle needle if the guide surface and the further guide surface form a closed circuit in the projection plane. 
     In a further embodiment, the guide surface extends substantially over an angular segment from 60° to 90°, in particular from 65° to 75° in the projection plane, so as to ensure a reliable support of the nozzle needle even over a small number of guide surfaces. 
     In a further embodiment, the guide portion has at least one guide tooth which is guided in a helical manner about the longitudinal axis of the nozzle needle, wherein the guide surface is arranged radially on the outside at the guide tooth. 
     Due to the formation of the guide surface at the guide tooth, fuel can be transported reliably along the nozzle needle on its path over the guide portion along the guide tooth. 
     In a further embodiment, the guide tooth has an involute toothing. In this manner, the guide tooth can easily be produced in the nozzle needle by a cold forming process. 
     In a further embodiment, the ratio of the area of the recess of the guide portion to a total area of the nozzle needle in the profile section or in the profile plane is 0.15 to 0.30, in particular between 0.18 and 0.25, so that a pressure reduction on delivery of fuel along the nozzle needle, along the guide portion through the recess, is kept small. 
     In a further embodiment, the guide portion is produced in the nozzle needle by means of cold forming, in particular by means of the rolling production process. In this way, the nozzle needle can economically be produced even in large series. 
     Other embodiments provide a method for production of a nozzle needle. For this a base body of the nozzle needle is soft-machined, in particular turned, in order to give the nozzle needle a predefined base form. To form the guide portion or produce at least one recess and at least one guide surface, a tool with negative form is rolled on the base body of the nozzle needle and thus the shape of the guide portion, or its guide surface and the recess, is produced in the nozzle needle. The base body of the nozzle needle is annealed and then at least the guide surfaces are hard-machined, in particular hard-turned or ground. In this way the nozzle needle can be produced economically since the guide surface or the recess of the guide portion can be produced by means of an economic soft machining of the base body of the nozzle needle. This reduces the cycle time in production of the nozzle needle. 
       FIG. 1  shows a side view of a nozzle needle  1 , and  FIG. 2  shows a longitudinal section through an injector  2  with a nozzle needle  1  shown in  FIG. 1 . 
     The nozzle needle  1  has a spring portion  5  which in mounted state is surrounded by a spring  40  in the injector  2 . Adjacent to this is a pressing portion  6 , on which a ring  50  is pressed in mounted state of the injector  2 . The ring  50  serves to transfer twist forces from the spring  40  into the nozzle needle  1 . Downwards from the pressing portion  6 , the nozzle needle  1  tapers over the tapering segment  7  towards a constriction  8 . A first guide portion  11  is arranged adjacent to this. On the underside, the first guide portion  11  is delimited by a further constriction  8 , to which a shaft  12  is attached. This is followed by a second guide portion  13  which is delimited by further constrictions  8  on both sides. A further shaft  14  is arranged from the bottom constriction  8 . After this, the nozzle needle  1  tapers over several conical portions towards a tip  16  of the nozzle needle  1 . A seat region  15  is provided between the further shaft  14  and the tip  16 , and lies on the nozzle body  3  of the injector  2  when the injector  2  is in the closed position. 
     The injector is configured for injecting fuel into the cylinder combustion chambers of an internal combustion engine. Both the injection time and the fuel quantity to be injected are determined by a position of the nozzle needle  1  in the injector  2 . To control the injection process, the nozzle needle  1  is raised and lowered along its longitudinal axis  10  in a nozzle body  3  of the injector. 
     The first guide portion  11  and the second guide portion  13  serve to guide the nozzle needle in the nozzle body  3  of the injector in its stroke movement and to prevent an axial deflection of the nozzle needle  1  on its stroke movement. Furthermore in an intermediate space  31  between the nozzle body and the peripheral surface of the nozzle needle  1 , fuel is delivered to the tip  16  of the nozzle needle  1  and, when the nozzle needle is in the open position, injected into the cylinder combustion chamber under high pressure via an opening  32  at the lower end of the injector  2 . 
       FIG. 3  shows an extract from the injector  2  which is indicated in dotted lines in  FIG. 2 , and  FIG. 4  shows a cross section through the injector  2  along a section plane B-B shown in  FIG. 2 . The guide portion  11  comprises several guide teeth  20  which are separated from each other by individual recesses  21 . The guide teeth  20  have the contour of an involute toothing, so that the teeth flanks  32  of the guide teeth  20  are convex and the tooth base  23  is formed concave. A guide surface  14 ,  15 ,  16  is arranged lying radially on the outside at each of the guide teeth  20 . A first guide surface  14  or a second guide surface  15  or a third guide surface  16  each have a predefined width B defined in the peripheral direction of the nozzle needle  1 . Preferably the width of the guide surface  14 ,  15 ,  16  in the peripheral direction of the nozzle needle  1 , in relation to an angular segment established by the guide surface  14 ,  15 ,  16 , is 1.5° to 5° wide. Advantageously width B in particular is between 2° and 3°, in a particularly advantageous embodiment between 2.2° and 2.3°. 
     The guide teeth  20  or the guide surfaces  14 ,  15 ,  16  and the recesses  21  are arranged in a helical manner in relation to the longitudinal axis on the peripheral surface of the nozzle needle  1 . The recesses  21  are provided in the first guide portion  11  to provide a fuel flow along the first guide portion  11 , from the top in the  FIGS. 2 and 3  in the direction of the tip  16  of the nozzle needle  1 . 
     In mounted state of the nozzle needle  1 , the guide surfaces  14 ,  15 ,  16  lie on an inner face, in this embodiment on a corresponding inner circumferential face  30  of the nozzle body  3 . Each guide surface  14 ,  15 ,  16  encloses a corresponding guide segment  140 ,  150 ,  160 . The contact of the guide surfaces  14 ,  15 ,  16  stabilizes the nozzle needle  1  in its stroke movement, i.e. in the movement in the direction of longitudinal axis  10 , which overlaps with a longitudinal axis of the injector  2  (not shown). The helical configuration of the guide surfaces  14 ,  15 ,  16 , despite a small width B of the guide surfaces  14 ,  15 ,  16 , increases the width of the guide segment  140 ,  150 ,  160  corresponding to the respective guide surface  14 ,  15 ,  16 , so that the nozzle needle  1  makes contact in a broad angular or circumferential segment of the nozzle needle  1  and is therefore guided reliably in its stroke movement. 
       FIG. 3  furthermore shows with dotted lines a projection plane A-A which is arranged at right angles to the longitudinal axis  10 , or the plane vector of which runs parallel to longitudinal axis  10 . If the guide surfaces  14 ,  15 ,  16  are projected into projection plane A-A (shown below the extract from the injector  2  in  FIG. 3 ), each guide segment  14 ,  15 ,  16  depicts a corresponding circle segment. It is particularly advantageous for stabilizing the nozzle needle  1  if the corresponding circle segments overlap in the projection plane A-A. 
     In an overlap region  145 ,  146  between a first guide segment  140  and a second guide segment  150 , or between the first guide segment  140  and a third guide segment  160 , the first guide surface  14  and the second and third guide surfaces  15  and  16  lie axially at the same height at the inner circumferential face  30  of the nozzle body  3  of the injector  2 . Thus for example for a first overlap region  145  between the first guide segment  140  and the second guide segment  150 , the first guide surface  14  in an upper region of the first guide portion  11 , and the second guide surface  15  in a lower region of the guide surface  15 , lie at axially the same height at the inner circumferential face  30  of the nozzle body  3  of the injector  2 . This further stabilizes the nozzle needle and prevents the deaxialization of the nozzle needle  1 . 
     To guarantee an overlap, the guide teeth  20  or the radially outermost guide surfaces  14 ,  15 ,  16  have a pitch of 17° to 25°, in particular from 20° to 22°. Because of the number of six guide teeth  20  selected in this embodiment in the first guide segment  11 , the guide surfaces  14 ,  15 ,  16  overlap in the projection plane A-A through the projected guide segments  140 ,  150 ,  160  such that in the projection plane A-A, they form a closed circuit. In this way, a particularly good stabilization of the nozzle needle  1  by the first guide portion  11  is possible. 
     In a particularly advantageous configuration of this embodiment, the guide surfaces  14 ,  15 ,  16  or the corresponding guide segments  140 ,  150 ,  160  each enclose an angular segment of 60° to 90°, in particular from 65° to 75°. It is also conceivable that the guide surfaces completely wind at least once around the nozzle needle and thus enclose an angular segment of at least 360°. This would have the consequence that, for an angular segment of more than 360°, the guide surface would overlap with itself in the projection plane A-A. 
     The second guide portion  13  shown in  FIGS. 1 and 2  is formed similarly to the first guide portion  11 . The only difference of the second guide portion  13  from the first guide portion  11  is that the axial extension of the second guide portion  13  in the direction of the longitudinal axis  10  is shorter than that of the first guide portion  11 . With the same pitch of the guide surfaces of the second guide portion  13 , this has the consequence that these, or the guide segments they enclose, do not overlap in the projection plane B-B. 
     In order to guarantee an overlap of the guide segments corresponding to the guide surfaces in the second guide portion  13 , it would be conceivable to increase the pitch of the guide surfaces or guide teeth or increase the axial extension of the second guide portion  13 . 
     In this embodiment, the pitch of the two guide portions  11 ,  13  is the same. Alternatively it is also conceivable to select opposing pitches, in order to reduce an eddying of the fuel in the intermediate space  31  between the nozzle body  3  and the nozzle needle  1  due to the first guide portion  11 , or adapt to a desired atomization of the fuel on the injection process. 
     Also, in this embodiment two guide portions  11 ,  13  are provided which are physically separated from each other by the shaft  12 . In this way the deaxialization of the nozzle needle  1  can be reduced. Alternatively it is also conceivable to merge the two guide portions  11 ,  13  into one guide portion with no physical separation, in order to provide a reliable guidance of the nozzle needle  1 . 
     To allow an adequate fuel flow at the guide portions  11 ,  13 , the ratio of all combined areas A A  of recesses  21  to a total area A D  of the nozzle needle in the profile section B-B (see  FIG. 4 ) is 0.15 to 0.30, but in particular 0.18 to 0.25. In this way it can be guaranteed that the fuel flow to the tip  16  of the injector  2  is not adversely affected by the guide portion  11 ,  13 . Furthermore, the fuel serves as lubrication at the guide surfaces  14 ,  15 ,  16  up to the inner circumferential face  30  of the nozzle body  3  of the injector  2 , in order to prevent increased wear between the nozzle needle  1  and the nozzle body  3 . 
     To produce the nozzle needle  1  in a base body (not shown) or a blank, the nozzle needle  1  is given a basic shape by soft machining, in particular by turning. After turning, the first guide portion  11  or the second guide portion  13  can be produced by cold forming, in particular by rolling. Here it is particularly advantageous if only a single tool is used to roll the first guide portion  11  and second guide portion  13 , so that both guide portions  11 ,  13 , or their recesses  21  and the guide surfaces  14 ,  15 ,  16 , can be produced in the nozzle needle  1  in one production step. In order to allow the material displaced on rolling to escape, the constrictions  8  are provided at the top and bottom of the first guide portion  11  and second guide portion  13 . During rolling, the material of the base body is displaced into this constriction  8  without the shaft  12  thickening and thus reducing a predefined distance between the nozzle needle  1  and the inner circumferential face  30  of the nozzle body  3  due to bead formation. Here the selection of an involute toothing for the guide portion  11 ,  13  is of particular advantage, since here the tool substantially rolls on the nozzle needle  1  and a slip process between the tool and the nozzle needle  1  is avoided. This results in a high machining accuracy and in low wear on the tool. 
     Alternatively it is conceivable to roll a corresponding guide portion  11 ,  13  each with a separate tool, in order thus to be able to accommodate any necessary geometry differences, in particular a different pitch, during production. 
     Following rolling, the base body of the nozzle needle  1  is annealed i.e. subjected to heat treatment in order to give the surface of the nozzle needle  1  the properties necessary for the operating conditions, in particular a predefined hardness. Then the first guide portion  11  and second guide portion  13 , the seat region  15  and the pressing portion  6 , are machined by means of hard machining, in particular by means of a hard turning process or a grinding process, to guarantee a high dimensional accuracy and surface quality of said portions  6 ,  11 ,  13 ,  15 . 
     Although the invention has been illustrated and described in detail in the form of the preferred exemplary embodiment, the invention is not restricted by the examples disclosed and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention.