Abstract:
The invention relates to an injector for a fuel injection system of an internal combustion engine, particularly in a motor vehicle, with an injector body which has a pressure booster section and a needle section. At least one injection hole is provided in the needle section. A nozzle needle which has an adjustable stroke is disposed in the needle section for controlling an injection of fuel through the at least one injection hole. A pressure booster is provided for increasing a fuel injection pressure relative to a system pressure. For this purpose, the pressure booster has a double-diameter or stepped piston, a control rod, and a pressure booster bottom which together form the boundary of a coupling chamber. A coupling path extends into the control rod and connects the coupling chamber, over a valve device which is located outside or within the injector, to a high-pressure supply of fuel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/055522 filed on May 6, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an injector for a fuel injection system of an internal combustion engine, in particular in a motor vehicle. 
     2. Description of the Prior Art 
     In order to be able to further reduce pollutant emissions of internal combustion engines, further development has been primarily focused on increasing the injection pressure. In this connection, a large fuel volume in the injector body is advantageously sought in order to be able to keep pressure pulsations in multiple injections to a minimum. A reduction in hydraulic pulsations also has a favorable effect with regard to wear on the nozzle seat. The increase in the injection pressure in known injectors is usually achieved through execution of a pressure boosting, which is used to act on the fuel with a pressure that is greater than the pressure of the system, i.e. is acted on with a multiple of the atmospheric pressure, and at this high pressure, is metered into the combustion chamber. A supply of fuel to the pressure booster in this case is usually carried out via a plurality of interconnected bores, but these weaken the injector body, thus negatively affecting its service life, and are also susceptible to leaks. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The injector according to the invention has the advantage over the prior art that no bores for a hydraulic connection of a pressure boosting arrangement have to be provided in the injector body, thus making it possible to prolong the service life of the injector according to the invention. As in a conventional design, the injector according to the invention has a pressure boosting section, also referred to as the actuator section, and a needle section, the latter of which accommodates a nozzle needle that is able to execute a stroke motion in order to control an injection of fuel through at least one injection orifice. The pressure booster used to increase the fuel injection pressure has a stepped piston, a control rod, and a pressure booster bottom that cooperate with one another to delimit a coupler chamber. In lieu of bores in the injector body, in the injector according to the invention, a coupling path extending in the control rod is provided, which connects the coupler chamber to a high-pressure fuel supply via a valve device situated outside the injector. The essential advantage therefore lies in the simple central connection of the fuel supply to the pressure booster. This makes it possible to implement a relatively high injection pressure with a simultaneously moderate system pressure. In particular, the injector according to the invention also has a significantly improved multiple injection capacity because of its large high-pressure injector volume and reduced pressure pulsations thanks to its lack of control lines. Furthermore, it is also possible to achieve a rapid switching or actuation of the nozzle needle. The fact that it is possible to eliminate complex bores inside the injector body, which negatively affect the service life of the injector and are leakage-prone, significantly prolongs the service life of the injector according the invention. 
     The end of the control rod oriented toward the nozzle needle suitably reaches into a cavity provided in the pressure booster bottom; this cavity is hydraulically connected via a connecting path to a needle control chamber that is in turn delimited by the nozzle needle, a nozzle needle sleeve encompassing the needle, and the pressure booster bottom. The connecting path passing axially through the pressure booster piston, which can be embodied in the form of a bore for example, is situated centrally in comparison to conventional bores situated in the injector body and is therefore significantly easier to manufacture and seal. In particular, this design makes it possible to eliminate a hydraulic line routing in an injector body wall or outside the injector, leading to the needle control chamber, which constitutes a structurally simple and well-engineered embodiment. 
     In an advantageous embodiment of the design according to the invention, the stepped piston is encompassed by a filling sleeve that is able to execute a stroke motion on it or by a stationary annular wall; the stepped piston, the pressure booster bottom, and the filling sleeve or stationary annular wall cooperate with one another to delimit a pressure booster chamber, commonly also referred to as an intensifier chamber. In the embodiment with a filling sleeve, which is supported so that it is able to execute a stroke motion on the stepped piston, it is also possible for a prestressing spring to be provided, one end of which rests against a stop on the injector body and the other end of which rests against the filling sleeve, prestressing the latter against the pressure booster bottom. The stepped piston, the pressure booster bottom, and the filling sleeve or annular wall, together with the prestressing spring provided in the case of the filling sleeve, form a boosting device for boosting the pressure prevailing in the coupler chamber to a significantly higher pressure required for the injection process in the pressure booster chamber. A boosting action is produced by the significant size differences between the coupler chamber and the pressure booster chamber. This makes it possible to achieve a high injection pressure with a simultaneously moderate system pressure, thus permitting reduction of the pollutant emissions of the internal combustion engine equipped with the injector according to the invention. 
     In another advantageous embodiment of the design according to the invention, the injector body is provided with a high-pressure chamber in which the control rod, the stepped piston, and the filling sleeve or annular wall are situated. The high-pressure chamber in this case is significantly larger in comparison to the coupler chamber, the pressure booster chamber, and the needle control chamber and has the greatest volume. A large-volumed high-pressure chamber has a positive effect on pressure pulsations in multiple injections, which can be kept to a minimum. 
     The high-pressure fuel supply is suitably connected directly to the high-pressure chamber via a hydraulic line and indirectly to the coupling path in the control rod via the valve device. In this case, both the direct supply to the high-pressure chamber and the indirect supply to the coupling path in the control rod via the valve device extend at least partially parallel to each other in an injector end plate so that a connection of the injector according to the invention to the high-pressure fuel supply is possible via only one side, namely the injector end plate. It is therefore unnecessary to provide an additional, structurally complex line routing, for example to the pressure booster chamber or needle control chamber. 
     Other important defining characteristics and advantages of the injector according to the invention ensue from the dependent claims, the drawings, and the associated description of the figures given in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the injector according to the invention are shown in the drawings and will be explained in detail in the subsequent description. 
         FIG. 1  is a very simplified schematic longitudinal section through an injector according to the invention, 
         FIG. 2  is a schematic depiction like the one in  FIG. 1 , but of a different embodiment, 
         FIG. 3  is also a schematic depiction like the one in  FIG. 1 , but of a different embodiment, and 
         FIG. 4  is a very simplified schematic longitudinal section through another embodiment of the injector according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to  FIGS. 1 through 4 , the injector  1  according to the invention includes an injector body  2 , which is usually composed of two sections, namely a needle section  3  situated at the bottom and a pressure booster section  4  situated above the former. The two sections  3  and  4  can be attached to each other by a suitable connecting technique, for example a welded connection or a screw connection. In the exemplary embodiments shown, a clamping nut  5  is provided, which encompasses the needle section  3  and clamps it against the pressure booster section  4 . The clamping nut  5  is preferably screwed onto the pressure booster section  4 . 
     The injector  1  is supplied by a high-pressure fuel supply  6 , which is connected directly to a high-pressure chamber  8  situated in the injector  1  via a hydraulic line  7  and is connected indirectly to a coupling path  11  situated in a control rod  10  via a hydraulic line  7 ′ equipped with a valve device  9 . 
     The needle section  3  is provided with at least one injection orifice  12  and a nozzle needle  13  supported so that it is able to execute a stroke motion in order to control an injection of fuel through the at least one injection orifice  12 . At an end oriented away from the at least one injection orifice  12 , the nozzle needle  13  has a nozzle needle sleeve  14  encompassing it, which is prestressed against a pressure booster bottom  16  by a closing compression spring  15 , one end of which rests against the nozzle needle sleeve  14  and the other end of which rests against the nozzle needle  13  or against a stop situated there. At the same time, the closing compression spring  15  prestresses the nozzle needle  13  into its closed position. The nozzle needle  13  is situated so that it is able to execute a stroke motion in a nozzle chamber  28 , which is hydraulically connected to a pressure booster chamber  27  via at least one through opening  29  provided in the pressure booster bottom  16 . The pressure booster section  4  contains a pressure booster  17  for increasing a fuel injection pressure in relation to a system pressure. The pressure booster  17  has a stepped piston  18 , the control rod  10 , and the pressure booster bottom  16 , which cooperate with one another to delimit a coupler chamber  19 . According to the invention, the coupling path  11  extends inside the control rod  10  and connects the coupler chamber  19  to the high-pressure fuel supply  6  via the valve device  9  situated outside the injector  1 . The valve device  9  here can for example be embodied in the form of a solenoid valve or a piezoelectric actuator or also in the form of a 2/2-way or 3/2-way solenoid valve or piezoelectric valve that has a 3/2-way functionality in combination with a servo valve. 
     With its end oriented toward the nozzle needle  13 , the control rod  10  reaches into a cavity  20  provided in the pressure booster bottom  16 , which cavity is hydraulically connected to a needle control chamber  22  via a connecting path  21 . The needle control chamber  22  here is delimited by the nozzle needle  13 , the nozzle needle sleeve  14  encompassing the latter, and the pressure booster bottom  16 . At the same time, the cavity  20  is connected to the coupler chamber  19  via the coupling path  11 ; the coupling path  11  has radial openings  23  in the region of the coupler chamber  19 . As is shown in  FIGS. 2 through 4 , it is possible for the cavity  20  to contain a control rod spring  24 , which prestresses the control rod  10  in the direction oriented out from the cavity  20 , i.e. upward. In the pressure booster bottom  16 , a connecting line  30  is also provided, which is embodied for example in the form of a bore and hydraulically connects the pressure booster chamber  27  to the needle control chamber  22 . The connecting line  30  and/or the connecting path  21  can optionally be provided with a throttle device  31 ; for example, the throttle device  31  in the connecting path  21  can be embodied in the form of an outlet throttle and the throttle device  31  in the connecting line  30  can be embodied in the form of an inlet throttle. 
     According to  FIGS. 1 through 3 , the stepped piston  18  of the pressure booster  17  is encompassed by a filling sleeve  25  that is supported so that it is able to execute a stroke motion on the stepped piston  18 . According to  FIG. 4 , the stepped piston  18  is encompassed by a stationary annular wall  26 . The annular wall  26  in this case can be embodied as separate from or of one piece with the pressure booster bottom  16 . The stepped piston  18 , the pressure booster bottom  16 , and a filling sleeve  25  or stationary annular wall  26  cooperate with one another to delimit a pressure booster chamber  27 . 
     According to  FIGS. 1 ,  2 , and  4 , a stepped piston spring  32  is provided, one end of which rests against a stop  33  on the injector body or collar  38  ( FIG. 4 ) and the other end of which rests against the stepped piston  18 . According to  FIGS. 1 and 2 , the stepped piston spring  32  presses the stepped piston  18  upward, thus clamping it in a nonoperating state against a stop  33 ′, which is embodied as an annular external step on the control rod  10 . At the same time, this presses the control rod  10  against an end plate  34 , thus sealing the coupling path  11  in relation to the high-pressure chamber  8 . 
     The stop  33  on the injector body is provided with at least one axial through opening  35 , which hydraulically connects the high-pressure chamber  8  to its section  8 ′ ( FIG. 1 ) situated below the stop  33 . 
     According to  FIGS. 1 and 2 , a prestressing spring  36 , which prestresses the filling sleeve  25  against the pressure booster bottom  16 , rests against a side of the stop  33  oriented away from the stepped piston spring  32 . In the embodiment according to  FIG. 3 , the prestressing spring  36  is embodied in the form of a clamping spring, one end of which rests against the stepped piston  18  and the other end of which rests against the filling sleeve  25 , prestressing the latter against the pressure booster bottom  16 . 
     In the embodiment of the injector  1  according to  FIG. 2 , the stepped piston  18  is embodied in the form of a so-called “free-flying piston,” which has no stroke stop on the control rod  10 . As in  FIGS. 3 and 4 , the control rod spring  24  here clamps and seals the control rod  10  against the end plate  34 . The advantage here is that rapid pressure changes are compensated for directly by means of stroke changes of the stepped piston  18 , thus making it possible to assure that the injector  1  does not open unintentionally, particularly in the event of a rapid decrease in system pressure. 
     The depiction in  FIG. 3  shows a variant in which the filling and resetting of the pressure booster  17  is assured not by an opening of the filling sleeve  25 , but by a modified nozzle needle sleeve  14 . In this case, a sealing edge of the nozzle needle sleeve  14  is situated radially toward the outside in comparison to the embodiments of the injector  1  according to  FIGS. 1 and 2 . A sealing diameter of the nozzle needle sleeve  14  therefore lies on a larger diameter, which achieves an opening when a pressure in the needle control chamber  22  is greater than in the nozzle chamber  28 . 
     In the variant according to  FIG. 4 , a stepped piston resetting by means of the stepped piston spring  32  has also been redesigned to make it possible to achieve an advantage in terms of space. For this reason, the stepped piston spring  32  rests against the injector body  2  via an annular collar  38  and presses against the stepped piston  18  via a washer  39  in order to reset the stepped piston after the end of the injection process. 
     The function of the injector  1  according to the invention can be described as follows: 
     First, all of the volumes of the injector  1  are at the system pressure level. If the pressure in the coupling path  11  is reduced through actuation of the valve device  9 , then the pressure in the needle control chamber  22  and the pressure in the coupler chamber  19  decrease. On one hand, this causes an increase in the forces acting in the opening direction on the nozzle needle  13 , causing it to open. On the other hand, a pressure increase occurs in the pressure booster chamber  27  as a result of a pressure decrease in the coupler chamber  19 . Consequently, the pressure in the nozzle chamber  28  also increases and the injector  1  injects fuel into a combustion chamber at an injection pressure that is higher than the system pressure. 
     In order to close the injector  1 , the valve device  9  is actuated, in particular closed, causing the pressures in the needle control chamber  22  and in the coupler chamber  19  to rise to system pressure again. If the pressures have returned to the system pressure level, then the stepped piston spring  32  produces a slight negative pressure in the pressure booster chamber  27 , causing the filling sleeve  25  to open and the resetting of the stepped piston  18  in combination with a volume compensation, causes a resetting of the injector  1  into its initial position. 
     One particular advantage of the injector  1  according to the invention is the central location of the coupling path  11  inside the control rod  10 , which permits the elimination of high-pressure bores in the injector body  2 . This makes it possible to achieve a high injection pressure with a simultaneously moderate system pressure by means of only a single valve device  9 . At the same time, it is possible to achieve a rapid switching of the nozzle needle  13  and a significantly improved multiple injection capacity due to a large volume of the high-pressure chamber  8  and reduced pressure pulsations through the elimination of control lines. 
     The foregoing relates to the preferred exemplary embodiments 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.