Abstract:
A fuel injection system for an internal combustion engine with direct injection includes a fuel injection device which can inject the fuel directly into a combustion chamber of the engine has a valve element bordering on a work chamber, and the position of the valve element depends on the pressure in the work chamber. A pressure booster piston borders on a control chamber on one side and on a high-pressure chamber on the other. A fuel supply can subject the control chamber to various pressures. The pressure booster piston is integrated with the fuel injection device and that the high-pressure chamber is integrated with the work chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 02/03318 filed on Sep. 6, 2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a fuel injection system for an internal combustion engine with direct injection, having a fuel injection device which can inject the fuel directly into a combustion chamber of the engine and a valve element which borders on a work chamber, and the position of the valve element depends on the pressure in the work chamber; having a pressure booster piston, which on one side borders on a control chamber and on the other side borders on a high-pressure chamber; and having a fuel supply, which can subject the control chamber to various pressures. 
   2. Description of the Prior Art 
   One fuel injection system, known from German Patent Disclosure DE 199 45 785 A1, includes a fuel pump which has a high-pressure and a low-pressure outlet. The highpressure outlet communicates with a control chamber of a pressure booster device. A high-pressure chamber of the pressure booster device communicates via a check valve with the low-pressure outlet of the fuel pump. A high-pressure line leads from the high-pressure chamber to a work chamber of a fuel injection device, by way of which the pressure is transmitted from the high-pressure chamber into this work chamber. Depending on the pressure in the high-pressure chamber or in the work chamber, a valve element of the fuel injection device is moved from a closed position to an opened position, or vice versa. The advantage of a pressure booster device or a hydraulic booster device is primarily that a relatively simple fuel pump can be used, yet the fuel can still be injected at very high pressure into a combustion chamber of the engine. This is important for the sake of favorable emissions performance of the engine. 
   From German Patent Disclosure DE 197 38 804 A1, a fuel injection system with a hydraulic booster device is likewise known. In it as well, the hydraulic booster device communicates with the fuel injection device via a high-pressure line. 
   The object of the present invention is to further develop a fuel injection system of the type defined at the outset such that when it is used, the emissions performance of the engine is even better, and as little energy as possible is needed to operate the fuel injection system. The temperature of the fuel injection device in operation should also be as low as possible. 
   This object is attained, in a fuel injection system of the type defined at the outset, in that the pressure booster piston is integrated with the fuel injection device, and the high-pressure chamber is integrated with the work chamber. 
   SUMMARY OF THE INVENTION 
   By integrating the pressure booster piston with the fuel injection device and the high-pressure chamber with the work chamber, a separate high-pressure line that leads from the pressure booster device to the fuel injection device is no longer necessary. As a result, the total volume to be compressed by the pressure booster device is reduced. 
   This accelerates the motion of the pressure booster piston in both directions, and as a consequence speeds up the buildup (and reduction) of the pressure in the high-pressure chamber. This in turn prevents fuel, for instance at the onset or end of an injection, from reaching the combustion chamber of the engine with only little pressure or little impetus. The high impetus with which the fuel in the fuel injection system of the invention is injected leads to an improvement in the emissions performance of the engine. 
   The energy to be brought to bear to operate the fuel injection system of the invention is also relatively slight, since because of the reduced volume that has to be compressed and expanded, only slight energy losses occur upon this compression and expansion. This also lessens the unwanted heating of the fuel injection device during its operation, since less entropy occurs in the compression and decompression work of the fuel. 
   The pumping volume that has to be furnished for compressing the fuel in the high-pressure chamber is also less, so that a fuel supply with less capacity can be provided. The loads on all the components used in the fuel injection system are also reduced, since the high pressure is now applied to essentially only in the combined high-pressure and work chamber. Less-expensive components can therefore be used for producing the fuel injection system of the invention. 
   In a first refinement, the pressure booster piston is disposed coaxially to the valve element. A fuel injection device of this kind is relatively compact, above all in the radial direction. 
   In a refinement of this, the valve element is guided in the pressure booster piston. This additionally has the advantage that the axial dimensions of the fuel injection device can also be kept comparatively slight. Moreover, because of the pressure booster piston, guidance for the valve element is created, so that the valve element cooperates very precisely with a valve seat assigned to it. 
   It is also advantageous if a longitudinal bore, by which the high-pressure chamber is supplied with fuel, is present in the valve element. This leads to a further reduction in the radial size of the fuel injection device that is used in the fuel injection system of the invention. 
   In an especially preferred feature of the fuel injection system of the invention, a check valve which opens toward the high-pressure chamber is present between the longitudinal bore in the valve element and the high-pressure chamber. Such a check valve is very simple in structure and assures that the highpressure chamber is reliably disconnected from the fuel supply during a compression. This is highly advantageous for the function of the pressure booster. The disposition near the high-pressure chamber reduces the loads on the longitudinal bore in operation, so that an inexpensive material can be selected for the valve element. 
   The longitudinal bore in the valve element can communicate with a low-pressure fuel supply. Thus whenever a high pressure does not prevail in the high-pressure chamber, the high-pressure chamber can be supplied with fuel that is available at a uniform, low pressure. Filling of the high-pressure chamber with fuel thus takes place uniformly and securely, and the loads on the longitudinal bore and thus on the valve element are kept slight. 
   As an alternative to this, it is possible that the longitudinal bore in the valve element communicates with a high-pressure fuel supply, which can also subject the control chamber to various pressures. In this case, a separate low-pressure fuel supply can be dispensed with. This reduces the costs of the fuel injection system of the invention. Moreover, incorporating this system into an internal combustion engine is facilitated, since it is no longer necessary to manipulate a separate low-pressure line. 
   For concrete realization of this, it is proposed that the control chamber coaxially surrounds the valve element, and that the longitudinal bore in the valve element communicates with the control chamber via a radially extending opening. This is space-saving and can be produced economically. 
   In a further variant, the pressure booster piston is braced via a spring on a nozzle body of the fuel injection device. The effect of this is that the pressure booster piston is reliably urged into its outset position. 
   A hollow chamber, which is present between the pressure booster piston and the nozzle body and is variable in volume upon a motion of the pressure booster piston, preferably communicates with a leak fluid outlet via a check valve, and the check valve opens toward the leak fluid outlet. The spring that acts on the pressure booster piston can for instance be accommodated in such a hollow chamber. 
   By means of the leak fluid outlet with the check valve, upon the first stroke of the pressure booster piston, any fuel present in the hollow chamber is pumped toward the leak fluid outlet. In all the further strokes of the pressure booster piston, then only the remaining fuel vapor in the hollow chamber has to be compressed, and any leakage that may occur between injections is pumped to a leakage removal line. As a result, pressure fluctuations in the low-pressure loop are avoided, and the energy expenditure needed to operate the fuel injection system of the invention is reduced still further. 
   It is also possible for the control chamber to be capable of being made to communicate via a check valve with a low-pressure fuel supply, where the check valve opens toward the control chamber. If in iperation the minimal pressure in the control chamber is below the pressure furnished by the low-pressure fuel supply, then when the minimal pressure in the control chamber is reached, the check valve is opened, and a slight quantity of fuel flows from the low-pressure fuel supply into the control chamber. 
   This is based on the following thought: The fluid located in the control chamber is constantly being compressed and decompressed again by the high-pressure fuel supply. This creates entropy or heat, which causes heating of the entire fuel injection device. This can impair the functioning of the fuel injection device. Because in the refinement proposed here “fresh” and thus cool fuel constantly reaches the control chamber from the low-pressure fuel supply, the temperature of the fuel located in the control chamber is lowered, and the overall heating of the fuel injection device in operation of the fuel injection system of the invention is reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the present invention are described in detail herein below, in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a basic sketch of a first exemplary embodiment of a fuel injection system for an internal combustion engine with a fuel injection device; 
       FIG. 2 , a fragmentary section through the fuel injection device of  FIG. 1 , with a pressure booster piston in a first position; 
       FIG. 3 , a view similar to  FIG. 2  of a region of the fuel injection device of  FIG. 1  with the pressure booster piston in a second position; 
       FIG. 4 , is a basic sketch similar to  FIG. 1  of a second exemplary embodiment of a fuel injection system for an internal combustion engine with a fuel injection device; 
       FIG. 5 , a fragmentary section through the fuel injection device of  FIG. 4 , with a pressure booster piston in a first position; and 
       FIG. 6 , a view similar to  FIG. 5  of a region of the fuel injection device of  FIG. 4  with the pressure booster piston in a second position. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a fuel injection system according to the invention is identified overall by reference numeral  10  and includes a fuel tank  12 , from which a fuel pump  14  pumps the fuel to a fuel injection device  16 . The fuel injection device is an injector that injects the fuel directly into a combustion chamber  18  of an internal combustion engine (not further shown). 
   The fuel pump  14  includes a control pressure outlet  20  and a low-pressure outlet  22 . A control pressure line  24  is connected to the control pressure outlet  20 . A control valve  26  is disposed in the control pressure line. From the control valve  26 , a diversion line  28  leads back to the fuel tank  12 . The control pressure line  24  leads to a control pressure connection  30  on the fuel injection device  16 . The control valve  26  should be switched such that in one switching position, the control pressure outlet  20  of the fuel pump  14  communicates with the control pressure connection  30  of the fuel injection device  16 , while conversely in another switching position of the control valve  26 , the control pressure connection  30  communicates with the fuel tank  12  via the diversion line  28 . 
   From the low-pressure outlet  22  of the fuel pump  14 , a low-pressure line  32  leads to a low-pressure connection  34  on the fuel injection device  16 . A leak fluid line  38  leads from a leak fluid outlet  36  of the fuel injection device  16  back to the fuel tank  12 . 
   The precise structure of the fuel injection device  16  can be seen from  FIGS. 2 and 3  where it is seen that the fuel injection device  16  includes a nozzle body  40 , which comprises a lower part  42 , in terms of  FIGS. 2 and 3 , and an upper part  44  which is braced by a nozzle lock nut  46  against a connection part  48 . The upper part  44  of the nozzle body  40  is sleevelike. The lower part  42  of the nozzle body  40  has a stepped blind bore  50 . On the lower end, in terms of  FIG. 2 , of the lower part  42  of the nozzle body  40 , outlet openings  52  lead to the outside from the blind bore  50 . All the parts described thus far are furthermore rotationally symmetrical parts of circular-cylindrical cross section. 
   In the blind bore  50  of the lower part  42  of the nozzle body  40  and in the sleevelike upper part  44  of the nozzle body  40 , a pressure booster piston  54  is received axially displaceably with sliding play. This part likewise comprises an upper part  56  and a lower part  58 , in terms of  FIG. 2 . An annular collar  60  is formed onto the upper end of the lower part  58  of the pressure booster piston  54 . A compression spring  62  is braced on the annular collar, and its other end rests on the lower part  42  of the nozzle body  40 . The compression spring  62  urges the lower part  58  of the pressure booster piston  54 , with the annular collar  60 , against a shoulder  64  in the upper part  44  of the nozzle body  40 . The compression spring  62  is received in an annular chamber  65 . A lower axial end face  66 , that is, lower in terms of  FIG. 2 , on the lower part  58  of the pressure booster piston  54  is smaller overall than an upper axial end face  68  on the upper part  56  of the pressure booster piston  54 . 
   The pressure booster piston  54  is penetrated by a recess. A portion of a valve needle  70  is guided in the recess, and this valve needle cooperates with a valve seat (not identified by reference numeral) on the lower end of the blind bore  50 , in the region of the outlet openings  52 . The valve needle  70  and the pressure booster piston  54  are thus disposed coaxially to one another. The valve needle  70  extends through the pressure booster piston  54 , upward in terms of  FIG. 2 , into a blind bore  74  in the connection part  48  of the fuel injection device  16 . Between the upper end, in terms of  FIG. 2 , of the valve needle  70  and the end of the blind  74 , a compression spring  72  is fastened that urges the valve needle  70  against the valve seat in the region of the outlet openings  52 , or in other words in the closing direction. 
   The axial length of the lower part  58  of the pressure booster piston  54  is dimensioned such that the pressure booster piston  54 , in the upper outset position shown in  FIG. 2 , ends at the bottom before a cross-sectional narrowing (not identified by reference numeral) of the stepped blind bore  50  in the nozzle body  40 . An annular high-pressure chamber  76  is formed between the valve needle  70 , the lower end face  66  of the pressure booster piston  54 , and the wall of the stepped blind bore  50  in the nozzle body  40 . 
   The valve needle  70  extends through the high-pressure chamber  76 . In the region of the high-pressure chamber  76 , there is a cross-sectional enlargement on the valve needle  70  that forms a pressure face  78 , whose force resultant opposes the pressure force exerted by the compression spring  72 , or in other words points in the opening direction of the valve needle  70 . The space surrounding the pressure face is called the work chamber  79 . It coincides with the high-pressure chamber  76 . From the high-pressure chamber  76 , an annular chamber (not identified by reference numeral), which is formed between the valve needle  70  and the lower region of the blind bore  50  in the nozzle body  40 , extends as far as the valve seat, that is, the outlet openings  52 . 
   In the valve needle  70 , a radial bore  82  extends into the high-pressure chamber  76  from a spring chamber  80  that is located in the valve needle  70  in the region of the high-pressure chamber  76 . A spring-loaded check valve  84  that opens toward the spring chamber  80  is disposed in the spring chamber  80 . From the check valve  84 , a low-pressure conduit  86  that is coaxial with the longitudinal axis of the valve needle  70  extends in the valve needle  70  as far as the upper end, in terms of  FIG. 2 , of the valve needle  70 , where it discharges into the blind bore  74  in the connection part  48 . The blind bore  74  communicates, via a radial bore  88  in the wall of the connection part  48 , with an annular conduit  90  in a connecting part  92 . The connecting part, via the low-pressure connection  34 , establishes a communication with the low-pressure line  32 . 
   From the control pressure connection  30 , which in terms of  FIG. 2  is located at the upper end of the connection part  48 , an overall eccentric control conduit  94  leads to a control chamber  96 . This control chamber  96  is formed as an annular chamber between the upper axial end face  68  of the pressure booster piston  54 , the outer jacket face of the valve needle  70 , and the connecting part  48  of the nozzle body  40  and is thus disposed coaxially with the valve needle  70 . Via a springloaded check valve  98  that opens toward the control chamber  96 , the control chamber communicates with a scavenging conduit  100  that discharges into the radial bore  88  in the connection part  48 . 
   In  FIG. 3 , the lower part of the fuel injection device  16  is shown. The view in  FIG. 3  is rotated by 90° about the longitudinal axis of the fuel injection device  16 , compared to  FIG. 2 . Moreover, in  FIG. 3  the pressure booster piston  54  is in its lower end position, while conversely in  FIG. 2  it is in its upper outset position. 
   As can be seen from  FIG. 3 , from the boundary region between the annular collar  60  on the lower part  58  of the pressure booster piston  54  and the shoulder  64  of the upper part  44  of the nozzle body  40 , a longitudinal groove  102  leads between the nozzle lock nut  46  and the upper part  44  of the nozzle body  40 . It leads to a spring-loaded check valve  104 . The check valve blocks in the direction toward the longitudinal groove  102 . From the check valve  104 , a conduit not shown in the drawing leads to the leak fluid outlet  36 . 
   The fuel injection system  10  shown in  FIGS. 1–3  functions as follows: 
   Before an injection of fuel into the combustion chamber  18  by the fuel injection device  16 , the high-pressure chamber  76  is filled with fuel. To that end, fuel is pumped from the low-pressure outlet  22  of the fuel pump  14  to the low-pressure connection  34  of the fuel injection device  16 . From there, the fuel reaches the high-pressure chamber  76 , via the low-pressure conduit  86  in the valve needle  70 , the check valve  84 , the spring chamber  80 , and the conduit  82 . Once the pressure in the high-pressure chamber  76  is approximately equivalent to the pressure at the low-pressure outlet  22  of the fuel pump  14 , the check valve  84  closes. 
   The control valve  26  is at first switched such that the control pressure connection  30  of the fuel injection device  16  communicates with the fuel tank  12 . The control chamber  96  is accordingly extensively pressureless, and the pressure booster piston  54  is in the upper outset position shown in  FIG. 2 . For performing an injection, the control valve  26  is switched such that the control pressure connection  30  communicates with the control pressure outlet  20  of the fuel pump  14 . The corresponding pressure now prevails, via the control conduit  94 , in the control chamber  96  as well. The pressure at the control pressure outlet  20  of the fuel pump  14  is considerably higher than the pressure at the low-pressure outlet  22 . 
   For this reason, and because of the ratios in the surface areas of the axial end faces  66  and  68  of the pressure booster piston  54 , the result at the pressure booster piston  54  is a force oriented toward the high-pressure chamber  76 , so that the pressure booster piston  54  moves in the direction of the high-pressure chamber  76 . As a result, the fuel present in the high-pressure chamber  76  is compressed, and a very high pressure in the high-pressure chamber  76  is generated. In the lower end position of the pressure booster piston  54 , shown in  FIG. 3 , the pressure in the high-pressure chamber  76  can be as high as approximately 1800 bar. 
   Because of the high pressure in the high-pressure chamber  76 , that is, in the work chamber  79 , the result at the pressure face  78  of the valve needle  70  is a force oriented in the opening direction of the valve needle  70 , counter to the direction of action by the compression spring  72 . Because of this force, the valve needle  70  is lifted from the valve seat, and as a result the outlet openings  52  are made to communicate with the high-pressure chamber  76 . Thus the fuel reaches the combustion chamber  18  from the outlet openings  52  at very high pressure. 
   If the injection is to be terminated, the control valve  26  is switched against in such a way that the control pressure connection  30  of the fuel injection device  16  communicates with the fuel tank  12 . This causes a sudden relief of the control chamber  96 . By means of the compression spring  62 , the pressure booster piston  54  is pressed upward again in terms of  FIGS. 2 and 3 . As a result, the pressure in the high-pressure chamber  76 , that is, the work chamber  79 , also drops, so that the valve needle  70  closes. Once the pressure in the high-pressure chamber  76  has dropped enough, the check valve  84  opens. Replenishing fuel can then flow into the high-pressure chamber  76  through the low-pressure conduit  86 . 
   As a result of the sudden pressure drop in the control chamber  96 , a relief pressure wave is generated. This causes the check valve  98  to open briefly, and cold fuel from the scavenging conduit  100  reaches the control chamber  96 . This has the advantage that the temperature increase of the fuel enclosed in the control chamber  96 , caused by the repeated compression and decompression, is compensated for by the delivery of cool fuel, and thus the temperature increase of the entire fuel injection device  16  in operation can be kept within certain limits. 
   As a result of certain leaks between the parts that move relative to one another, fuel also reaches the space  65  between the lower part  42  of the nozzle body  40  and the lower part  58  of the pressure booster piston  54 , in which the compression spring  62  is disposed. When upon an injection the pressure booster piston  54  moves downward in terms of  FIGS. 2 and 3 , the volume of this space also decreases. Fuel present in it is therefore carried away to the leak fluid outlet  36  via the longitudinal groove  102  and the check valve  104 . 
   In the ensuing injections or reciprocating motions of the pressure booster piston  54 , essentially no further fuel is pumped out of the space to the leak fluid outlet  36 . Instead, fuel vapor forms in this space, and this vapor is compressed during the reciprocating motions of the pressure booster piston  54  from vapor pressure to approximately ambient pressure. As a result, pressure fluctuations in the low-pressure loop are avoided. 
   In  FIGS. 4–6 , a second exemplary embodiment of a fuel injection system  10  is shown. Parts, elements and regions that have equivalent functions to parts, elements and regions of the exemplary embodiment shown in  FIGS. 1–3  are identified by the same reference numerals. They are not described here again in detail. 
   One essential difference between the fuel injection system  10  shown in  FIG. 4  and the above system is that the fuel pump  14  now has only a control pressure outlet  20  but no low-pressure outlet. Correspondingly, the fuel injection device  16  has only a control pressure connection  30  and a leak fluid outlet  36 . Consequently, in  FIG. 5 , there is no low-pressure outlet. 
   In the fuel injection device  16  shown in  FIGS. 5 and 6 , there is no check valve between the longitudinal groove  102  and the leak fluid outlet  36 . Instead, the longitudinal groove  102  extends via a leakage conduit  106  directly to the leak fluid outlet  36 . The leak fluid outlet is furthermore in communication, via a radial bore  108  in the wall of the connection part  48 , with the interior of the blind bore  74  in the connection part  48 . 
   Supplying the high-pressure chamber  76  with fuel is effected in the fuel injection device  16  shown in  FIGS. 5 and 6  via the control chamber  96 . To that end, there is a radial inlet bore  110  in the wall of the valve needle  70 , at the level of the control chamber  96 . In addition, the conduit  86  in the valve needle  70  extends from the check valve  84  only to the level of the control chamber  96 . The advantage of this exemplary embodiment is that a low-pressure system (low-pressure outlet at the fuel pump, low-pressure line, low-pressure connection at the fuel injection device, etc.) can be dispensed with. 
   The high-pressure chamber  76 , as already noted above, is the chamber in which an enclosed fluid is compressed by the pressure booster piston  54 , and a very high pressure is thus generated. The work chamber  79  is the chamber in which, by a pressure change at the pressure face  78  of the valve needle  70 , a force is generated that leads to a motion of the valve needle  70 . In both fuel injection devices  16  described above, the high-pressure chamber  76  of the pressure booster piston  54  is integrated with the work chamber  79  of the valve needle  70 . The two chambers accordingly coincide. Thus upon an injection through the fuel injection device  16 , only a comparatively small total volume is compressed, which reduces unwanted effects of elasticity of the fuel enclosed in the high-pressure chamber  76 . 
   The foregoing relates to 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.