Abstract:
The present invention relates to a fuel injection device for combustion engines, preferably Diesel engines, with at least one pilot valve by means of which a control piston is displaceable by a pressure medium in order to convey fuel through at least one line/channel toward a combustion chamber of the internal combustion engine, whereby at least one accumulator is provided in the flow path of the fuel or the pressure medium, and an accumulator space is connected to the connecting line/channel for the fuel or the pressure medium.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a fuel injection device for combustion engines, preferably Diesel engines, with at least one pilot valve by means of which a control piston is displaceable by a pressure medium in order to convey fuel through at least one line/channel toward a combustion chamber of the internal combustion engine. 
     For a reliable and clean mixture formation, internal combustion engines, Diesel engines in particular, require an injection process consisting of several individual injection actions. The injection processes are divided into pre-injection and main injection of the fuel quantity. FIG. 3 illustrates the injection process. First, triggered by a switching pulse of the engine control, a pilot or pre-injection occurs. After a time period t, the pilot or pre-injection is terminated by a cut-off pulse and after a time period t, the main injection of the fuel is started by a switching pulse. The main injection has a longer duration than the pre-injection stage. Also, significantly more fuel is injected during the main injection than during the pre-injection stage. Particularly for producing the pre-injection quantity, a control unit is employed which, however, requires a high-cost electronic control system and which shows energetic losses. Frequently, a damper is employed for producing the pre-injection fuel quantity. This damper, however, cannot be fully utilized in each step of the operation. The reason for this is that the response times of the control hydraulic are too long in the event of small injection quantities due to the design of the control elements and a small pre-injection quantity can, therefore, be produced only with the help of a significant control-technical structural design and expenditure. 
     Therefore, it is an object of the present invention to provide a fuel injection device of the aforementioned kind such that a small pre-injection quantity can be produced with the fuel injection device having a simple structural design and only requiring a small control-technical expenditure and design. 
     SUMMARY OF THE INVENTION 
     This object is solved by the inventive fuel injection device by providing at least one accumulator in the flow path of the fuel or the pressure medium and connecting the accumulator space of the at least one accumulator with the connecting line for the fuel or the pressure medium. 
     With the inventive fuel injection device, the quantity of the fuel to be conveyed is, in addition to a minimum activation time of the control valve, minimized by the accumulator. A portion of the fuel to be conveyed to the combustion chamber or a portion of the pressure medium enters the accumulator. The accumulator volume for fuel or pressure medium is instantly available during a pre-injection action. When fuel is conveyed to the accumulator, only a minimum quantity of fuel needs to be conveyed to the combustion chamber via the connecting line. It is sufficient if the accumulator volume is smaller than the minimum quantity of fuel to be conveyed during the pre-injection stage. Therefore, only such a quantity of fuel needs to be conveyed to the combustion chamber during the pre-injection stage by the control piston which quantity equals the difference between the fuel amount that can be removed from the accumulator and the required pre-injection quantity. The accumulator can also be arranged such that it receives a portion of the pressure medium which acts upon the control piston. Also in this manner, the injection quantity of fuel can be limited. The accumulator volume can be selected to be equal to, larger or smaller than the minimum fuel volume to be conveyed during the pre-injection stage. If the accumulator volume is larger, any quantity of fuel can be conveyed. The accumulator does not require a high-cost electronic control system and no costly designed control elements. Therefore, even the smallest injection quantities can be reliably produced with the inventive injection device with a simple structural design and at lowest control-technical design and expenditure. 
     The accumulator can be provided within the inventive injection device, however, it can also be provided externally of the injection device. The accumulator volume can be designed to be fixed, however, it can also be designed to be variable. The accumulator makes a volume variation possible which results, depending on the respective adjustment of the accumulator, in a reduced quantity of fuel exiting when a hydraulic intake volume of fuel is predetermined time-wise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the present invention will appear more clearly from the following specification in conjunction with the accompanying schematic drawings in which: 
     FIG. 1 shows a longitudinal section of an inventive injection-quantity limiting device; 
     FIG. 1A is a detailed view of the accumulator within the insertion members; 
     FIG. 2 shows a diagram of the time sequence of an injection process in which the inventive injection quantity limiting device is used; 
     FIG. 3 shows a flow quantity/time diagram of an injection process. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with the aid of several specific embodiments utilizing FIGS. 1 through 3. 
     The injection quantity limiting device is a part of an injection device with a pressure regulator with the aid of which an injection valve  4  of internal combustion engines, Diesel engines in particular, is activated. The pressure regulator has a control piston  1  which is displaceable within a channel/bore  2  of a housing member  3 . When the injection valve  4  is closed (FIG.  1 ), the control piston  1  abuts a stop  6  under the force of a spring  5 . The stop  6  can be adjustable, e.g., embodied as a screw member or as an insertable socket member. It is also possible to provide as a stop  6  a retaining ring which is inserted in the wall of the bore  2 . FIG. 1 shows the control piston  1  in a starting position, displaced by the force of the spring  5 . In this starting position, a needle-shaped valve body  23  of the injection valve  4  closes off nozzle openings  24 . Fuel is fed via the nozzle openings  24  to the combustion chamber of the internal combustion engine. 
     The control piston  1  has a piston surface  7  which is acted upon by system pressure p 1 . At its opposite end, the control piston  1  is provided with a recess  8  the bottom surface  9  of which is abutted by a pressure transferring or intensifying piston  10 . It has a smaller diameter than the control piston  1  and projects into a second bore  11  of the housing means  3 . The second bore  11  has a smaller diameter than the bore  2 . The pressure p 1  is intensified by the pressure intensifying piston  10  creating the larger pressure p 2  which acts upon the injection valve  4 . 
     The hydraulic medium acting upon the piston surface  7  is fed by a channel/pressure line  12  to which a pilot valve  26  is connected which can also be embodied as a piezo valve. The pilot valve  26  is connected to a pressure supply which is controlled by a control valve  13  by means of which the initial pressure is adjusted. The pilot valve  26  and the control valve  13  can be controlled directly or indirectly by means of the engine control. The pilot valve  26  is connected to a control unit  25  which controls and monitors the operation of the pilot valve  26  and the control valve  13  and which is connected to the engine control. In the starting position of the control piston  1  illustrated in FIG. 1, the bore  2  is relieved to the tank T via the pressure line  12  and the pilot valve  26 . 
     When the internal combustion engine is operated, the pilot valve  26 , controlled by the control unit  25 , is switched such that the hydraulic medium is pressurized. The hydraulic medium reaches the piston surface  7  via the pressure line  12 . Thus, the system pressure p 1  acts upon the piston surface  7 . The recess  8  opposite the piston surface  7  is relieved of pressure and is connected to the atmosphere by a bore opening  16  penetrating the housing member  3 . The air within the recess  8  and within the space containing the spring is displaced through the bore opening  16 . The control piston  1  is displaced against the force of the spring  5  by the system pressure p 1 . Thereby, the pressure intensifying piston  10  is also displaced whereby the fuel within the second bore  11  is pressed into a channel  14  by a fixedly connected distribution plate  17 . The channel  14  is provided within an insertion member  22  which is received by a threaded socket member  19 . The threaded socket member  19  is screwed onto the housing member  3  and receives the injection valve  4  which projects out of the threaded socket member  19 . The distribution plate  17  is clamped by means of the threaded socket member  19  between the insertion member  22  and the housing member  3 . The channel  14  extends from the distribution plate  17  through the insertion member  22  and the injection valve  4  to an injection chamber  15  which is penetrated by the valve body  23 . An axial bore  31  is provided, adjoining the injection chamber  15  and leading to the nozzle openings  24  and it has a larger diameter than the portion of the valve body  23  which projects into the axial bore  31 . The valve body  23  projects into a central receiving cavity  32  of the insertion member  22 . The central receiving cavity  32  is closed off at the opposite side by the distribution plate  17 . One end of a compression spring  21  is supported on the distribution plate  17  and its other end rests on a shoulder member  33 . The shoulder member  33  is provided at the end portion of the valve body  23  that is positioned within the central receiving cavity  32  and has a central projection  18  for centering the compression spring  21 . The valve body  23  projects with an enlarged portion  34  into the injection chamber  15 . Within the injection chamber  15  the enlarged portion  34  merges into a thinner end portion  35 . 
     Pressure is exerted upon the enlarged portion  34  of the valve body  23  by the fuel entering the injection chamber  15 , and the valve body  23  is thereby pushed back against the force of the compression spring  21 . The nozzle openings  24  are thus released so that the fuel can enter the combustion chamber. 
     Subsequent to this injection process, the pilot valve  26  is switched by the control unit  25  so that the pressure line  12  is released to the tank T via the pilot valve  26 . The control piston  1  is, therefore, via the pressure intensifying piston  10 , pushed back to the stop  6  by the force of the spring  5 . Furthermore, the valve body  23  is pushed back by the compression spring  21  to the closing position illustrated in FIG.  1 . Subsequently, a new injection cycle is started in the manner described. 
     Via a back pressure valve  20  provided within the distribution plate  17 , fuel is taken in from a fuel container (not illustrated) during the return stroke of the pistons  1 ,  10  through an opening  36  within the threaded socket member  19  and within the insertion member  22 . The fuel reaches the second bore  11  via the distribution plate  17  so that it can be conveyed to the nozzle openings  24  during the next stroke of the pressure intensifying piston  10  in the manner described. The opening  36  also opens into the central receiving cavity  32 . During the return stroke of the pressure intensifying piston  10  the back pressure valve  20  is opened up by the low pressure that is created whereby fuel is taken in. 
     The channel  14  is connected to an accumulator  38  within the insertion member  22  via a lateral bore  37  within the insertion member  22 . In the illustrated embodiment, the accumulator  38  is formed by an accumulator piston  27  and an accumulator compression spring  28  which is supported on an adjusting screw  30 . It is screwed into a threaded bore  39  within the insertion member  22 . The force ofthe accumulator compression spring  28  can be continuously adjusted by the adjusting screw  30 . The adjusting screw  30  has a central screw bore  29  penetrating the adjusting screw  30 . The space  40  receiving the accumulator compression spring  28  is connected to the atmosphere via the central screw bore  29 . The accumulator piston  27  is positioned to be sealed off within a piston space  41  into which the lateral bore  37  opens up. 
     FIG. 1 shows the starting position of the accumulator piston  27  which abuts the bottom surface  42  of the piston space  41  under the force of the accumulator compression spring  28 . 
     The accumulator  38  has the effect that the quantity of fuel to be conveyed is minimized in addition to a minimum activation time of the pilot valve  26 . When the pilot valve  26  is switched by the control unit  25  in the manner described, from its starting position illustrated in FIG. 1, the control piston  1  is displaced in the manner described whereby the higher pressure p 2  acting upon the fuel to be conveyed is created by the pressure intensifying piston  10 . Since the activation time of the pilot valve  26  cannot be reduced any further, an excessive amount of fuel is conveyed into the accumulator  38  via the lateral bore  37 . The pressure p 2  is larger than the pressure exerted upon the accumulator piston  27  by the accumulator compression spring  28  so that the accumulator piston  27  is pushed backward by the excessive fuel amount, against the force of the accumulator compression spring  28 . Thereby, the excessive fuel amount can be received by the piston space  41 . The air within the space  40  is displaced to the atmosphere via the central screw bore  29 . The conveying duration for the accumulator volume is kept available for the entire conveying duration of the fuel. Therefore, any accumulator volume can be varied by a longer control signal, and, thus, a longer opening duration of the nozzle openings  24 . 
     As can be seen from FIG. 2, the pilot valve  26  is controlled at the time t 0 . Thereby, the pressure intensifying piston  10  is displaced by the control piston  1  in the manner described. The pressure intensifying piston  10  presses the fuel within the second bore  11  via the distribution plate  17  into the channel  14 . A portion of this fuel quantity reaches the accumulator  38  via the lateral bore  37 . This accumulator volume is designated the reference numeral  43  in FIG.  2 . The minimum fuel injection volume  44  is reached at the time t min . Subsequent to the time t 1 , the pilot valve  26  is again activated whereby the injection process is terminated in the manner described. Accordingly, the injection quantity Q decreases to zero after a certain time delay. By employing the accumulator  38 , the injection quantity is limited in a structurally simple manner. If the accumulator volume is selected to be larger than the minimum volume  44  conveyed, any volume can be conveyed, starting at 0 mm 3 . When the force of the accumulator compression spring  28  is selected to be larger than the opening force of the valve body  23 , a reduced fuel volume can be injected into the combustion chamber until the accumulator  38  is entirely filled. 
     When the pilot valve  26  is positioned in the closing position according to FIG. 1, the fuel quantity within the piston space  41  is reconveyed into the channel  14  by the accumulator piston  27  via the lateral bore  37  because the accumulator piston  27  is displaced by the accumulator compression spring  28  into its starting position illustrated in FIG.  1 . However, as soon as the injection process is again started by switching the pilot valve  26 , a portion of the fuel acted upon by the pressure p 2  within the channel  14  is conveyed into the piston space  41  via the lateral bore  37  whereby the accumulator piston  27  is pushed back accordingly against the force of the accumulator compression spring  28 . 
     FIG. 2 illustrates that the volume of the accumulator  38  can vary by the amount  45 . Depending on the magnitude of the pressure p 2  within the channel  14 , a larger or a smaller amount of fuel is conveyed into the accumulator  38 . The force of the accumulator compression spring  28  can be optimally adjusted by the adjusting screw  30  to the respective requirement. Corresponding to the spring force, the accumulator piston  27  is pushed back within the piston space  41  to a variable extent depending on the pressure p 2  within th channel  14 . The fuel quantity which can be received by the accumulator  38  can thereby be varied. By employing the accumulator  38 , the fuel volume to be conveyed can be minimized when the pilot valve  26  has a minimum activation time. A structurally complicated control unit is not required for achieving this result. By employing the accumulator  38 , a very small quantity of fuel can, thus, be injected into the combustion chamber. Thereby, particularly the pre-injection followed by a main injection of fuel can be carried out inexpensively. The accumulator  38  itself can have any suitable structural design. It does not have to comprise the accumulator piston  27  and the accumulator compression spring  28 . For example, it is possible without difficulty to form the accumulator by a membrane and a bubble accumulator. 
     The accumulator  38  can also be connected to the bore  2  or the pressure line  12 . In that case, the accumulator  38  does not receive fuel, but a hydraulic medium. Also in that case, a limitation of the injection quantity of fuel can be achieved. Receiving a portion of the hydraulic medium for acting upon the control piston  1  by the accumulator has the same effect as receiving a portion of the fuel. 
     Finally, it is possible to vary the pre-compression force of the accumulator compression spring  28  by the control unit  25 . For example, a spindle can be provided at which the adjusting screw is positioned and which is turned as a function of signals of the control unit  25 . The adjusting screw  30  is shifted as a function of the turning direction of the spindle and thus, the pre-compression force of the accumulator compression spring  28  is altered. 
     The specification incorporates by reference the disclosure of German priority document 198 50 016.5 of Mar. 3, 1999. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.