Patent Publication Number: US-9835124-B2

Title: Fuel injector

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a fuel injector for internal combustion engines, of the kind which can be used for injecting fuel under high pressure into the combustion chamber of an internal combustion engine. 
     A fuel injection nozzle or fuel injector for internal combustion engines is known from German Laid-Open Application DE 29 20 100 A1. In the known fuel injector, a nozzle needle is arranged in a longitudinally movable manner in an injector body and interacts by means of a sealing edge formed on the nozzle needle with a nozzle needle seat formed on the injector body and opens and closes a plurality of first injection openings by means of its longitudinal movement. Adjoining this at the combustion-chamber end, the nozzle needle has a pin region, which projects into a blind hole formed in the injector body and thereby closes a plurality of second injection openings. Up to a partial stroke of the nozzle needle, fuel flows into the combustion chamber of the internal combustion engine only through the first injection openings, while the pin region seals off the second injection openings. After the partial stroke, the pin region emerges from the blind hole and thus exposes the second injection openings. A step-shaped injection characteristic which includes good suitability for very small quantities can thereby be achieved. However, the sealing function of the pin region when projecting into the blind hole requires high accuracy of manufacture and high wear resistance. 
     SUMMARY OF THE INVENTION 
     In contrast, the fuel injector according to the invention exhibits less wear with a similar injection characteristic and, at the same time, requires less accuracy of manufacture. 
     To achieve this, the fuel injector has a pressure chamber, which is formed in an injector body and in which a nozzle needle is arranged in a longitudinally movable manner, which nozzle needle has, at the combustion-chamber end thereof, a cone region, which is tapered in a combustion chamber direction, and a pin region of constant diameter d 23 . The injector body furthermore has a substantially conical nozzle needle seat, from which a first injection opening extends, and a blind hole, which adjoins the nozzle needle seat at the combustion-chamber end and has a cylindrical segment having the diameter d 31  and a hole base, from which a second injection opening extends. The cone region of the nozzle needle interacts with the nozzle needle seat and thereby opens and closes the first injection opening and the second injection opening with respect to the pressure chamber. At least during a partial stroke of the nozzle needle, the first injection opening and the second injection opening are connected to one another via a throttle gap, which is formed in the blind hole between the pin region and the wall of the blind hole, and the throttle gap remains constant at least over the partial stroke. Owing to the throttle gap, there is no contact or only slight contact between the pin region and the nozzle needle and hence also little or no wear in these regions. Moreover, there can be larger tolerances in manufacture than if the pin region had to perform a sealing function. 
     In an advantageous embodiment of the fuel injector according to the invention, the difference between the diameter d 31  of the blind hole and the diameter d 23  of the pin region is greater than 6 μm and less than 30 μm. Thus, the throttle gap is larger by 3 μm on average, and the selected tolerance chain between the pin region and the wall of the blind hole can be relatively large, at up to 3 μm, as long as the nozzle needle is not subject to transverse forces. At the same time, the gap width must be less than 15 μm to achieve sufficient throttling by the throttle gap. 
     In another advantageous embodiment, one or more second injection openings are present, and the flow cross section through the throttle gap is smaller than the total flow cross section through the second injection opening or through all the second injection openings. The flow cross section through the throttle gap preferably amounts to 15% . . . 70% of the total flow cross section through the second injection opening or through all the second injection openings over the partial stroke. The fuel supply to the second injection openings is thereby throttled for as long as the throttle gap is present, and this means that the fuel injector is well suited to very small quantities. 
     It is advantageous if the pin region emerges from the blind hole and the flow cross section into the blind hole is enlarged relative to the throttle gap in the case of strokes which are greater than the partial stroke. As a result, more fuel is supplied to the second injection openings, this being necessary to achieve higher engine power outputs. 
     In another advantageous embodiment, the nozzle needle has an end region which adjoins the pin region at the combustion-chamber end. This enables the transition from partial engine load to full engine load to be made smoother and hence more economical since the curve of the injection rate against time or stroke is shallower in this transition. 
     In an advantageous embodiment, the end region is embodied as a cone. As a result, the fuel quantity supplied to the second injection openings increases linearly after the partial stroke, leading to an advantageous injection characteristic, depending on the application. 
     In another advantageous embodiment, the end region is embodied so as to be substantially cylindrical and has at least one lateral recess. As a result, the nozzle needle projects into the blind hole with a portion widened relative to the pin region, even after the partial stroke, and therefore the axial misalignments between the injector body and the nozzle needle are smaller and hence there is also a lower risk of wear during the closing of the nozzle needle. The shape of the lateral recesses can be configured according to the application, ensuring that the fuel supplied to the second injection openings increases quickly or less quickly after the partial stroke. 
     It is advantageous if the at least one recess is embodied as a ground flat. The desired reduction in the throttling function after the partial stroke can thereby be achieved in a simple manner through a manufacturing technique. 
     In another advantageous embodiment, the at least one recess is embodied so as to be substantially semicircular in cross section. The potential area of contact between the end region and the wall of the blind hole is thereby enlarged, leading to better guidance of the nozzle needle in the blind hole and hence also to a lower risk of wear. 
     It is advantageous if the flow cross section into the blind hole is larger than the total flow cross section through all the second injection openings in the case of a maximum stroke of the nozzle needle which is greater than the partial stroke. As a result, the injection characteristic in the case of the maximum stroke is determined substantially by the geometry of the first and second injection openings; there is virtually no longer any throttling function between the injector body and the nozzle needle. The accuracy of manufacture of the two injection openings is therefore decisive for the maximum stroke, while the tolerances of the throttle gap are of subordinate importance in this respect. 
     It is advantageous if a plurality of first injection openings and/or a plurality of second injection openings is/are present. Uniform injection of the fuel into the combustion chamber can thereby be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a detail of the fuel injector according to the invention in longitudinal section, wherein only the essential regions are shown. 
         FIG. 2  shows another illustrative embodiment of the fuel injector according to the invention in longitudinal section, wherein likewise only the essential regions are shown. 
         FIG. 3  shows a cross section through another illustrative embodiment of the fuel injector according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the end of a fuel injector  100 , which projects into the combustion chamber  110  of an internal combustion engine in the installed position. The fuel injector  100  has an injector body  10  with a pressure chamber  30 , which is connected via a high-pressure passage (not shown) to a fuel source under high pressure (not shown), e.g. a common rail. 
     A nozzle needle  20  is arranged in a longitudinally movable manner in the pressure chamber  30 . In the detail shown, the nozzle needle  20  has a central part  21  and a cone region  22  arranged on the combustion-chamber end thereof, a pin region  23  and an end region  24 . The cone region  22  and the end region  24  are embodied so as to taper in the direction of the combustion chamber  110 , and the pin region  23  is embodied so as to be cylindrical with the diameter d 23 . 
     In the detail shown, the injector body  10  has a cylindrical body stem  11  and, adjoining the latter at the combustion-chamber end, a conical nozzle needle seat  17  and a blind hole  31 , which represents part of the pressure chamber  30 . At least one first injection opening  1  of diameter d 1  leads into the combustion chamber  110  from the blind hole  31 , and at least one second injection opening  2  of diameter d 2  leads into the combustion chamber  110  from the nozzle needle seat  17 . The blind hole  31  has a cylindrical section of diameter d 31  and, adjoining the latter at the combustion-chamber end, a hole base, which is of rounded design in the illustrative embodiment shown. There can be both one or more first injection openings  1  and one or more second injection openings  2 . 
     In the closed operating state shown, the nozzle needle  20  interacts with the nozzle needle seat  17  at a sealing edge  22   a  formed at the transition from the central part  21  to the cone region  22  and thus closes the hydraulic connection from the pressure chamber  30  to the first injection opening  1  and the second injection opening  2 ; the blind hole  31  is thereby separated hydraulically from the remainder of the pressure chamber  30 . The cylindrical pin region  23  of diameter d 23  projects into the cylindrical section of the blind hole  31  of diameter d 31  and thus forms a throttle gap  32  of width t/2 with the wall of the blind hole  31 , where t=d 31 −d 23 . The first injection opening  1  and the second injection opening  2  are continuously connected hydraulically via the throttle gap  32 . 
     To inject fuel through the two injection openings  1 ,  2 , the nozzle needle  20  is moved in the opening direction  29  by a control operation (not shown), e.g. the lowering of a pressure in a control chamber at the end of the nozzle needle  20  remote from the combustion chamber, with the result that the cone region  22  and the sealing edge  22   a  rise from the nozzle needle seat  17  and the hydraulic connection from the pressure chamber  30  to the two injection openings  1 ,  2  and the blind hole  31  is freed. 
     Up to a partial stroke s of the nozzle needle  20 , the pin region  23  projects into the blind hole  31 , and therefore the throttle gap  32  exists in the blind hole  31  between the pin region  23  and the wall of the blind hole  31 . During this partial stroke s, the throttling effect due to the throttle gap  32  is greater than the throttling effect due to the second injection opening  2  or the total throttling effect due to all the second injection openings  2 ; the flow cross section through the throttling gap  32  is thus smaller than the total flow cross section through all the second injection openings  2 . To achieve this, the width t/2 of the throttle gap  32  and the clearance t for the pin region  23  within the blind hole  31  should be designed as follows: 
     The flow cross section through throttle gap A DS  is: 
               A   DS     =           π   4     ·     d   31   2       -       π   4     ⁢       (       d   31     -   t     )     2         =       π   4     ·     (       2   ·     d   31     ·   t     -     t   2       )               
where d 31 &gt;&gt;t:
 
     
       
         
           
             
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     Flow cross section through all x second injection openings A 2.EÖ : 
     
       
         
           
             
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     Up to the partial stroke (s), the following should apply: A DS &lt;A 2.EÖ   
     
       
         
           
             
               
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     Up to the partial stroke (s), the following should preferably apply: 
     
       
         
           
             
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     Up to the partial stroke s, the injection characteristic is thus determined substantially by the geometry of the first injection opening  1  and of the throttle gap  32 . 
     After the partial stroke s, the pin region  23  emerges from the blind hole  31 , but initially the end region  24  remains in the blind hole  31 . Owing to the conical shape of the end region  24 , the flow cross section between the blind hole  31  and the nozzle needle  20  widens as the stroke increases. At a maximum stroke v, the pin region  23  and the end region  24  have emerged from the blind hole  31  to such an extent that the flow cross section between the injector body  10  and the nozzle needle  20  is larger than the total flow cross section through all the second injection openings  2 . At the maximum stroke v, the injection characteristic is thus determined substantially by the geometry of the first and second injection openings  1 ,  2 . 
     The illustrative embodiment in  FIG. 2  differs from that in  FIG. 1  in the embodiment of the end region  24 . All the other features are embodied in the same way as in the illustrative embodiment in  FIG. 1  and are therefore not described again. 
       FIG. 2  shows the end region  24 , embodied so as to be substantially cylindrical, which has the same diameter d 23  as the pin region  23 . Recesses  27  are formed laterally on the end region  24 , with the result that the flow cross section between the injector body  10  and the nozzle needle  20  is enlarged after the partial stroke s. For this purpose, three recesses  27 —in the form of ground flats in the illustrative embodiment shown—are usually distributed over the circumference, ensuring approximately uniform inflow to the second injection openings  2  while simultaneously providing good guidance of the end region  24  in the blind hole  31 . At the maximum stroke v of the nozzle needle  20 , however, the end region  24  can have emerged from the blind hole  31 . 
       FIG. 3  shows the section A-A from  FIG. 2 . The section lies in the plane of the transition from the pin region  23  to the end region  24 . The throttle gap  32  of width t/2 is formed in the blind hole  31  of the injector body  10  between the injector body  10  and the nozzle needle  20 , forming, together with the lateral recesses  27  arranged on the end region  24 , the flow cross section in the blind hole  31 . In the embodiment shown, there are three recesses  27 , and the recesses  27  are of semicircular configuration in cross section.