Abstract:
A fuel injection system for internal combustion engines, in particular Diesel engines, having a fuel reservoir which is supplied from a high-pressure pump and supplies fuel to a number of injectors corresponding to the number of cylinders of the engine, and has a pressure limiting valve, which is connected sealingly to the fuel reservoir, the pressure limiting valve having a valve housing, a high-pressure region, a low-pressure region, a valve seat oriented toward the inlet, an axially displaceable valve body, and a valve spring urging the valve body in the direction of the valve seat. The pressure limiting valve is integrated—at least partially—with the fuel reservoir, in such a way that at least the valve seat and the valve body are spatially associated with the fuel reservoir, and the seal sealing off the pressure limiting valve from the fuel reservoir is associated with the low-pressure region of the pressure limiting valve.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a fuel injection system of the type employing a common rail and a pressure limiting valve for lowering system pressure  
           [0003]    2. Description of the Prior Art  
           [0004]    Fuel reservoir injection systems (so-called common rail systems) are known in principle. A distinction is made between common rail systems with a quantity-regulated high-pressure pump (known as CP3 systems), in which a pressure reduction in the fuel pressure reservoir (rail) is possible only via the injection and control quantity and leakage, and common rail systems in which a pressure limiting valve on the high-pressure side is used to lower the system pressure. The subject of the present invention is the latter type of system, that is, one with a pressure limiting valve for lowering the system pressure.  
           [0005]    With respect to the prior art for such pressure limiting valves, reference may be made for instance to European Patent Disclosure EP 0 267 162 B1 and German Patent Disclosure DE 198 22 671 A1. The known pressure limiting valves are intended exclusively for mounting not only outside the fuel injection pump (the high-pressure pump) but also outside the fuel reservoir (rail) per se. Typically, the pressure limiting valve is secured to the end of the fuel reservoir remote from the high-pressure pump. Because of the high pressure prevailing in the fuel reservoir, until now a valve housing with a threaded stub for screwing the pressure limiting valve to the fuel reservoir was practically indispensable. Because of the high pressure, problems arise in particular in sealing off the pressure limiting valve from the fuel reservoir. Other inadequacies of the known system result from the requirement that on account of the high pressure, the valve housing must be very massive (high costs!) and because of the comparatively large installation space required.  
           [0006]    OBJECT AND SUMMARY OF THE INVENTION  
           [0007]    The object of the invention is to make suitable provisions for eliminating or at least significantly reducing the problems of sealing, components and installation space, at reasonable engineering effort and expense.  
           [0008]    By means of the integration, according to the invention, of the valve seat and the valve body and thus of the essential functional elements of the pressure limiting valve with the fuel reservoir, the region where the pressure limiting valve is sealed off is shifted from the (former) high-pressure region to the low-pressure region, which makes the sealing task considerably easier to perform. At the same time, this provision of the invention advantageously provides a corresponding reduction in the installation space required and also affords the possibility of designing the pressure limiting valve housing, and its fastening to the fuel reservoir, in a less complicated way than before. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings.  
         [0010]    [0010]FIG. 1, in the form of a block circuit diagram, shows one embodiment of a fuel injection system of the type in question;  
         [0011]    [0011]FIG. 2, in vertical longitudinal section, shows one embodiment of a pressure limiting valve; and  
         [0012]    [0012]FIG. 3 shows another embodiment of a pressure limiting valve, in a view corresponding to FIG. 2.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    In FIG. 1, reference numeral  10  overall designates a fuel tank, with an electric fuel pump  11 , prefilter  12 , and filter  13 . A high-pressure pump is identified by reference numeral  14 , and a fuel reservoir (or so-called common rail) is identified overall by reference numeral  15 . From the fuel tank  10 , the fuel is pumped by the fuel pump  11  via a low-pressure line  16  to the high-pressure pump  14 , which brings the fuel to high pressure and delivers it to the fuel reservoir  15  via a high-pressure line  17 . The fuel reservoir  15  supplies four injectors  22 - 25  of a four-cylinder internal combustion engine (not shown)—each via a respective high-pressure line  18 ,  19 ,  20  and  21 —with the fuel they require. Fuel that is (perhaps) not required by the injectors  22 - 25  reaches a return collection line  30  via a respective return line  26 ,  27 ,  28  and  29 , and from the line  30  it is returned to the tank  10 . A certain (slight) leakage quantity also returns to the fuel tank  10  from the high-pressure pump  14 , via a return line  31  and the return collection line  30 .  
         [0014]    For ascertaining the quantity of fuel required by the injectors  22 - 25  in a given operating state of the engine, six sensors in all are provided, identified by reference numerals  32 - 37 . The sensor  32  is associated with the crankshaft and accordingly monitors the applicable engine rpm. The sensor  33 , associated with the camshaft, serves to monitor the phase of the engine at a given time. The sensor  34  is associated with the accelerator pedal and accordingly takes the acceleration desired by the driver at a given time into account. The sensor  35  takes the engine charge pressure into account, while sensor  36  detects the air temperature and sensor  37  detects the coolant temperature. Corresponding signals from the sensors  32 - 37  travel over signal lines  38 - 43  to reach a control unit  44 , which processes the signals. A further sensor  45  ascertains the applicable pressure in the fuel reservoir  15  and—via a signal line  46 —sends signals accordingly to the control unit  44 . Taking the incoming signals into account, the control unit  44  triggers both the high-pressure pump  14  and the fuel pump  11  via respective control lines  47  and  48 .  
         [0015]    On the end of the fuel reservoir  15  remote from the high-pressure inlet  17 , there is a pressure limiting valve  49 —shown only schematically in FIG. 1—which has the task of preventing the pressure in the fuel reservoir  15  from exceeding a fixed maximum value. The fuel quantity that is—optionally—diverted from the fuel reservoir  15  for this purpose in the opening position of the pressure limiting valve  49  returns to the fuel tank  10 , via a return line  50  that discharges into the return collection line  30 .  
         [0016]    One possible embodiment of a pressure limiting valve can be seen in FIG. 2, where it is identified overall by reference numeral  49   a . It comprises a valve housing  51 , a valve spring  52 , a platelike spring support  53 , a ball-shaped valve body  54 , a conical valve seat  55 , an inlet bore  56  that tapers in stages in the direction toward the valve seat  55  and discharges into the valve seat, and a return connector  57  formed integrally onto the back end of the valve housing  51 .  
         [0017]    One special feature is that the pressure limiting valve  49   a  is integrated partially, specifically in terms of its essential parts, that is, the valve seat  55  and the inlet bore  56 , with the fuel reservoir (shown only in part in FIG. 2) identified overall by reference numeral  15   a . The fuel reservoir—in the usual way—has a cylindrical inner chamber  58 , from which fuel conduits  59 ,  60  extend at a right angle, discharging into line connectors  61  and  62 , respectively. To the line connectors  61 ,  62  (and to corresponding further line connectors not shown in FIG. 2), pressure lines (not shown) can be connected, which—as can be seen from FIG. 1, for example—lead to injectors (marked  22 - 25  in FIG. 1).  
         [0018]    A (further) special feature is that the fuel reservoir  15   a —on its end face  63  toward the valve and graduated relative to this end face—has a connector neck  64 , which is disposed concentrically to the cylindrical inner chamber  58  of the fuel reservoir  15   a , the inner chamber  58  extending into the connector neck  64 . The cylindrical inner chamber  58 , on its end, merges in stepped fashion with the inlet bore  56  of the pressure limiting valve  49   a . The connector neck  64  serves only to sealingly secure the valve housing  51 , which to that end, with a portion (a) of its length, fits over and surrounds the connector neck  64 . The (remaining) portion of the valve housing  51  that does not fit over the connector neck  64  remains, for receiving the valve spring  52  and the platelike spring support  53 .  
         [0019]    The valve spring  52  is embodied as a compression spring; on the front end it is operatively connected to the spring support  53 , and on the back end it is braced on a bottom  65  of the valve housing  51 . Via the spring support  53 , the valve spring  52  exerts a force, determined by its spring constant and its prestressing, on the ball-shaped valve body  54  in the closing direction (arrow  66 ) of the pressure limiting valve  49 . The closing force exerted by the valve spring  52  acts on the valve body  54  counter to an opening pressure (arrow  67 ) determined by the pressure inside the fuel reservoir  15  (the so-called rail pressure). If the opening force effected by the rail pressure exceeds the closing force of the valve spring  52 , then the valve body  54  moves in the direction of the arrow  67 , and the pressure limiting valve  49   a  opens. A certain (comparatively slight) fuel quantity then flows out of the inner chamber  58  of the fuel reservoir  15   a  into the valve housing  51 , and from there it flows back into the fuel tank ( 10 , FIG. 1) via the return connector  57  and the return line ( 50  in FIG. 1). This process lasts until such time as the rail pressure in the inner chamber  58  of the fuel reservoir  15   a  has dropped back below its maximum value again.  
         [0020]    To assure the pressure-limiting process described above, a good fixation and sealing of the valve housing  51  on the fuel reservoir  15   a  is necessary. The fixation, in the exemplary embodiment shown in FIG. 2, is effected at  68  in a simple way, for instance by shearing, crimping, welding, and so forth. By a suitable choice or variation of the slip-on measurement a of the pressure limiting valve housing  51  relative to the connector neck  64 , and the thus-determined prestressing of the valve spring  52 , it is possible to set or—if needed—adjust the opening pressure of the pressure limiting valve  49   a  without major effort.  
         [0021]    The sealing off of the pressure limiting valve housing  51  from the fuel reservoir  15   a  is effected at  69 ; the special feature here is that this is a low-pressure seal. This substantial advantage (compared to the high-pressure sealing of pressure limiting valves in the prior art) is made possible by the (partial) integration, shown here and described above, of the pressure limiting valve  49   a  with the fuel reservoir  15   a . With respect to the sealing provisions for the low-pressure sealing in detail, simple O-ring seals can for instance be used—as shown in FIG. 2 (see reference numeral  69 ).  
         [0022]    The fundamental principle of a (partial) integration of the pressure limiting valve with the fuel reservoir and the advantages thus obtained, especially in terms of shifting the sealing out of the high-pressure region to the low-pressure region of the valve, are also achieved in the embodiment of FIG. 3. There are differences, however, structurally. In FIG. 3, the pressure limiting valve is identified overall by reference numeral  49   b  and the fuel reservoir is identified overall by reference numeral  15   b . Otherwise, elements of the fuel reservoir  15   b  that correspond to those of the embodiment of FIG. 2 are identified by the same reference numerals as in FIG. 2. For instance, the fuel reservoir  15   b —similarly to the embodiment of FIG. 2—has a connector neck  64 , onto which the housing  51   b  of the pressure limiting valve  49   b  is slipped, secured (at  68   b , for instance by shear crimping), and sealed off (at  69 , for instance by an O-ring seal). A structural difference from FIG. 2 is that in the variant of FIG. 3, the valve seat  55  and valve body  54  are shifted farther into the interior of the fuel reservoir  15   b . To enable actuation of the valve body  54  in the valve closing direction  66  by means of the valve spring  52  via a spring support  53   b , the spring support  53   b  is solidly connected to a rodlike actuating element  70 , which ends in an extension  71  of narrowed diameter that cooperates with the valve body  54 . The actuating element  70  is supported, in a manner capable of reciprocation, in a guide bore  72  of the fuel reservoir  15   b  and of its connector neck  64 .  
         [0023]    Since in the variant of FIG. 3, unlike the embodiment of FIG. 2, the valve seat  55  and valve body  54  are not disposed in the connector neck  64  but instead are shifted into the interior of the fuel reservoir  15   b , and since—because of the actuating element  70  disposed sealingly in the guide bore  72 —there is no hydraulic communication with inner chamber of the pressure limiting valve housing  51   b , a return connector  73  is provided not on the pressure limiting valve housing  51   b  but instead on the fuel reservoir  15   b . In this case—again inside the fuel reservoir  15   b —a return line  74  is provided on the low-pressure side of the valve seat  55 , laterally adjoining the bore  72  that contains the actuating element  70 ; this return bore  74  discharges into a return connector  73  on the outer circumference of the fuel reservoir  15   b.    
         [0024]    Another special feature of the variant of FIG. 3 is that the pressure limiting valve housing  51   b  has a plastically deformable bottom  65   b , on which—as in the embodiment of FIG. 2 as well—the valve spring  52  is braced. By plastic deformation of the housing bottom  65   b  in the axial direction  66  or  67 , the prestressing of the valve spring  52  and thus the opening pressure of the pressure limiting valve  49  can thus be set or adjusted. (A corresponding plastic deformability for the sake of settability or adjustability of the spring prestressing and thus of the valve opening pressure can naturally be provided in the case of the valve housing bottom  65  of FIG. 2 as well. However, the return connector  57  should not be impaired thereby.).  
         [0025]    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.