Abstract:
Proposed is a fuel injector for injecting fuel into a combustion chamber of an internal combustion engine. The fuel injector has a housing in which are arranged an injection valve element with a nozzle needle, a pressure boosting device, and a first control valve, and a second control valve. The first control valve controls a control space of the nozzle needle and the second control valve controls a differential pressure space of the pressure boosting device. A first return flow connection is provided for discharging the control quantity of the control space of the nozzle needle, and a second return flow connection is provided for discharging the control quantity of the pressure boosting device. The two return flow connections are arranged on different housing parts of the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2008/050022 filed on Jan. 3, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is based on a fuel injector. 
     2. Description of the Prior Art 
     A fuel injector with an integrated pressure booster is known for instance from German Patent Disclosure DE 103 35 340 A1. The pressure booster has a pressure booster piston, guided in a housing of the fuel injector, that acts on a compression chamber, a differential pressure chamber, and a high-pressure chamber. With a first control valve, a rear control chamber of a nozzle needle is triggered, and the control volume is diverted into a low-pressure/return flow system. A second control valve connects the differential pressure chamber of the pressure booster with the low-pressure/return flow system as well. As a result of the change of pressure in the differential pressure chamber, the pressure booster piston presses into the compression chamber and compresses the fuel there, which as a result experiences a pressure increase that is transmitted to a pressure shoulder of the nozzle needle, so that the high pressure acting on the pressure shoulder lifts a nozzle needle from the nozzle needle seat and injects the fuel, at the fuel pressure elevated above the system pressure, into the combustion chamber of an internal combustion engine. 
     In German Patent Application DE 100 206 038 840.2, it is shown to provide one return flow connection each, for communication with the low-pressure/return flow system, for both the first control valve that triggers the nozzle needle and the second control valve that triggers the pressure booster. 
     Since the return flow systems for the first control valve of the nozzle needle and for the second control valve of the pressure booster are exposed to different pressure levels, and furthermore the return flow system of the pressure booster is subjected to severe pressure surges, a technologically appropriate decoupling of the two return flow circuits inside the fuel injector is necessary. In addition, a suitable disposition of the two return flow connections on the injector housing is necessary. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The fuel injector of the invention has the advantage that functional influence of the control valves on one another is prevented. This assures a stable quantitative performance graph of the fuel injector, which is necessary if an optimal course of injection as a function of the power demand made of the engine is to be assured. By the disposition of the two return flow connections in different housing parts of the housing, simple separation and embodiment of the two return flow connections is possible. 
     It is especially advantageous to embody the first return flow connection for the control quantity or for the control volume of the nozzle needle by means of at least one bore, which is extended to the outside through a first housing part, and the housing part is the nozzle lock nut, and to embody the second return flow connection for the control quantity or for the control volume of the pressure-boosting device by means of at least one further bore, which is extended to the outside through a second housing part. The second bore discharges into an annular indentation embodied on the outer wall of the housing part, and the indentation is defined by a first housing portion and a second housing portion, and there is one sealing ring each in the first housing portion and in the second housing portion. 
     The first control valve has a first low-pressure chamber, which communicates via a hydraulic connecting line with a first chamber that is located between the nozzle lock nut and a further housing part. The second control valve has a second low-pressure chamber, which communicates hydraulically with at least one outflow bore that leads into a second chamber, into which the further bore of the second return flow connection discharges. A leak fuel chamber, which communicates hydraulically with the first low-pressure chamber of the first control valve via hydraulic connecting lines extending through the housing, is associated with the second control valve. Advantageously, the leak fuel chamber communicates hydraulically with the second low-pressure chamber of the second control valve by means of a bypass conduit, with which a throttle restriction is integrated. 
     In the installed state in the internal combustion engine, each of the two return flow connections communicates with a respective return flow conduit integrated with a cylinder head of the engine, and each return flow conduit is connected to a respective low-pressure/return flow system of the engine. To that end, a stepped bore is expediently embodied in the cylinder head, and the housing of the fuel injector protrudes at least partway into this bore, and inside the stepped bore, there are two hydraulically separate portions, each with annular chambers, and the first return flow connection discharges into the one annular chamber and the second return flow portion discharges into the other annular chamber, and the annular chambers each communicate hydraulically with a respective return flow conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One exemplary embodiment of the invention is described in further detail below in conjunction with the drawings, in which: 
         FIG. 1  is a sectional view through a fuel injector of the invention; and 
         FIG. 2  is a sectional view through the fuel injector of the invention in the installed state in a cylinder head of an internal combustion engine. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fuel injector shown in  FIG. 1  for instance has a housing  10  with a first housing part  11 , a second housing part  12 , a third housing part  13 , a fourth housing part  14 , and a connection part  15 , as well as a nozzle body  16 . The nozzle body  16 , the housing part  14 , and the housing part  13  are hydraulically tightly fastened by means of a nozzle lock nut  17 . The nozzle body  16  contains an injection valve member  20 , with a nozzle needle  21  that is axially displaceably guided in the nozzle body  16 . The nozzle needle  21  cooperates with a nozzle needle seat, not shown in detail and embodied on the nozzle body  16 , which with the nozzle needle  21  forms a sealing seat that in turn separates a nozzle needle pressure chamber  22  from injection openings  23 , in the closed state of the nozzle needle  21 . A control chamber sleeve  24  is guided on the nozzle needle  21  and a presses against a sealing face by means of a compression spring  25  and thereby surrounds a control chamber  26 . With a control face  27  acting in the closing direction, the nozzle needle  21  is exposed to the control chamber  26 . A high-pressure bore  18  is disposed in the housing part  14  and leads into the nozzle needle pressure chamber  22 . The housing  10  has a high-pressure connection  19 , for instance on the housing part  12 , and has a high-pressure supply line  29  by which the fuel injector is connected to a common rail of a diesel injection system. 
     Also disposed in the housing  10  of the fuel injector, for boosting the system pressure of the common rail, is a pressure-boosting device  30  with a pressure booster piston  31  embodied as a stepped piston. The pressure booster piston  31  is exposed to a work chamber  32 , a differential pressure chamber  33 , and a compression chamber  34 . The work chamber  32  and the compression chamber  34  communicate, via a connecting conduit  35 , with a check valve  36 . The high-pressure line  29  leads into the work chamber  32 , so that the system pressure of the common rail is constantly present in the work chamber. An upper piston portion  37  of the pressure booster piston  31  also protrudes into the work chamber  32  and is surrounded by a restoring spring  38 , which returns the pressure booster piston  31  to the outset position shown in  FIG. 1 . The upper piston portion  37  serves as a stop upon the return of the pressure booster piston  31  to the outset position. From the compression chamber  34 , a further high-pressure bore  39  branches off, which communicates hydraulically with the high-pressure bore  18 , so that the pressure of the compression chamber  34  is transmitted to the nozzle needle pressure chamber  22 . 
     The fuel injector furthermore includes a first control valve  40 , embodied as a servo valve, and a second control valve  50 , also embodied as a servo valve. The first control valve  40  is a 2/2-way valve and includes a first electromagnetic control element  41 , having a magnet armature  42  that is connected to a first valve piston  43 . The valve piston  43  acts on a sealing seat  44 , which separates a control bore  45 , communicating with the control chamber  26 , from a low-pressure chamber  46 . A first low-pressure connection  461  extending out of the low-pressure chamber  46  of the first control valve  40  leads into a first annular chamber  48 , which surrounds the housing part  14  and into which a bore  49  extending through the nozzle lock nut  17  leads. The bore  49  forms a first return flow connection  71 , which is hydraulically in communication with a first low-pressure/return flow system. 
     Via the first return flow connection  71 , the control quantity from the control chamber  26  of the nozzle needle  21 , which quantity is switched by the first control valve  40 , is carried away into the first low-pressure/return flow system, and the control quantity flows out into the low-pressure/return flow system at a substantially constant pressure level of only about 1 bar. 
     The second control valve  50  includes a second electromagnetic control element  52 , having a second magnet armature  53  that is connected to a second valve piston  54 . The valve piston  54  has a first sealing seat  55 , embodied for instance as a slide seat, and a second sealing seat  56 , embodied for instance as a flat seat, so that the second control valve  50  operates as a 3/2-way valve. The first sealing seat  55  separates a high-pressure line  57 , communicating with the work chamber  32 , from a valve chamber  58 . The valve chamber  58  communicates via a hydraulic connection  59  with the differential pressure chamber  33  and is hydraulically separated from a further low-pressure chamber  61  by means of the second sealing seat  56 . Two outflow bores  62 , for instance, lead from the low-pressure chamber  61  to a branching chamber  63 , which is in communication via further hydraulic connections  64  with a further annular chamber  65 . From the annular chamber  65 , bores  66 , for instance two in number, extending through the housing part  13  branch off, which form a second return flow connection  72 . The second return flow connection  72  is in communication with a second low-pressure/return flow system. Above the bore  66  is an upper annular housing portion  75 , and below the bore  66  is a lower annular housing portion  76 , each with a respective sealing ring  77 . Thus between the housing portions  75 ,  76 , an indentation  78  extending around the housing  10  is formed, into which the two bores  66  discharge. The function of the housing portions  75 ,  76  and the sealing rings  77  will be addressed in conjunction with  FIG. 2 . By way of the second return flow connection  72 , the control quantity, switched by the second control valve  50 , of the pressure-boosting device  30  is carried away into the second low-pressure/return flow system. The control quantity from the pressure-boosting device  30  is greater than the control quantity from the control chamber  26  and has substantially greater pressure surges. 
     The low-pressure chamber  46  of the first control valve  40  communicates hydraulically with a leak fuel chamber  51  of the second control valve  50  via the first low-pressure connection  461  embodied in the housing part  14 , a second low-pressure connection  462  extending through the housing part  13 , a third low-pressure connection  463  extending through the housing part  12 , and a fourth low-pressure connection  464  embodied in the housing part  11 . The leak fuel chamber  51  stretches in the second control element  52  as far as the second valve piston  54 . Between the leak fuel chamber  51  and the further low-pressure chamber  61 , there is a bypass conduit  67  with a throttle restriction  68 , so that the two hydraulic chambers communicate in hydraulically throttled fashion. 
     Because of the presence of two separate return flow connections  71 ,  72  for the return flow of the control volume from the pressure-boosting device  30  and of the control volume from the control chamber  26  of the nozzle needle  21 , it is assured that the control volume from the pressure-boosting device  30 , which is burdened by major pressure surges, does not affect the first control valve  40  for triggering the nozzle needle  21 . 
       FIG. 2  shows the fuel injector in the installed state in a cylinder head  80  of an internal combustion engine. The cylinder head  80  has a stepped bore  81 , with a first portion  82 , a second portion  83 , and a third portion  84 , as well as an annular contact face  85 . A sealing ring  86  rests on the annular contact face  85 , and the fuel injector with the nozzle lock nut  17  rests on the sealing ring. In the first portion  82 , the stepped bore  81  has a first annular chamber  87 , into which the second return flow connection  72  discharges. The first annular chamber  87  is hydraulically sealed off at the top and bottom by means of the respective sealing rings  77 . In the second portion  83 , a second annular chamber  88  is embodied, which is hydraulically sealed off at the top by the sealing ring  77  of the second housing portion  76  and at the bottom by the sealing ring  86 . The first return flow connection  71  discharges into the second annular chamber  88 . From the second annular chamber  88 , a first return flow conduit  91  leads outward, which hydraulically connects the first return flow connections, not shown, of farther fuel injectors to one another. The first return flow conduit  91  is connected to the first low-pressure/return flow system. A second return flow conduit  92  leads into the first annular chamber  87 , so that by means of the second return flow conduit  92 , further, second return flow connections, not shown, of further fuel injectors are hydraulically coupled. The second return flow conduit  92  communicates hydraulically with the second low-pressure/return flow system. 
     The fuel injector is secured to the cylinder head  80  by means of a locking claw  95 , which engages a flangelike annular face  96  on the housing  10  of the fuel injector, by means of a locking screw  97 , in such a way that by means of the locking claw  95  and the sealing ring  86 , a gas-tight contact of the fuel injector with the combustion chamber of the engine is created at the contact face  85 . 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.