Abstract:
A method for refilling an emptied hydraulic coupler of a fuel injector comprises the steps of: connecting a pressure generator to a return line of the fuel injector; operating the pressure generator in order to generate a return pressure in the return line; operating the fuel injector. The method can also include the step of monitoring the pressure level in the return line.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method and an apparatus for refilling and for checking the leak tightness of a hydraulic coupler, as used in fuel injectors and in particular piezoelectrically driven fuel injectors. 
         [0002]    BACKGROUND INFORMATION 
         [0003]    The purpose of the injectors in a fuel injection system is to inject the fuel volume requested by the control unit into the combustion chamber of the engine at a predefined point in time. Common rail diesel injection systems having so-called piezo injectors, which have a piezoelectrically driven actuator module, have been in production use for several years. One of the special design features of piezo injectors is the hydraulic coupler. The hydraulic coupler equalizes temperature-related changes in the length of the actuator module. One prerequisite for proper functioning of the injector is that the coupler space be completely filled with fuel during operation. An incompressible liquid volume in the coupler space then allows force to be transferred from the actuator to the switching valve of the injector. 
         [0004]    A further prerequisite for proper functioning of the injector is that a counter-pressure be present in the injector return line. The counter-pressure is generated as a rule by a constant pressure valve (CPV) or a throttling element located in the injector return line. 
         [0005]    If the coupler space is not completely filled with fuel, and if instead an air cushion has accumulated in the coupler space, this results in negative effects on injector function. Force transfer from the actuator to the switching valve is impaired as a result of the compressibility of the air/fuel mixture in the coupler space. The injection volume decreases as a function of the size of the enclosed air volume; in the worst case the injection volume becomes zero. The result of this is that the engine is no longer capable of running. 
         [0006]    The air inclusion can result, for example, from repair work on the fuel system, running the tank dry, defects in the constant pressure valve, fuel degassing effects, or incorrect installation or storage of the injector. 
         [0007]    Lack of counter-pressure in the return line, as well as failure to inject because of an empty coupler, result in engine missing and failure to start. Neither defect can be detected by the control unit; this makes fault localization very difficult. 
         [0008]    Also known are numerous end-of-the-line faults in which vehicles can no longer be started because of empty couplers or defective constant pressure valves. The subsequent troubleshooting in such cases is always complex. 
         [0009]    In order to restore injector function and allow the engine to run again, the only possibility that remains in many cases is to replace emptied injectors with new parts. As delivered, the injectors are filled at the factory with a testing oil in order to ensure injector function. 
         [0010]    British Published Patent Appln. No. 2 277 386 describes a diagnostic system for detecting malfunctioning fuel injectors for an internal combustion engine, having an injector controller for individual application of control to the injectors, which are connected to a fuel distributor. Pressure sensors are mounted on the fuel distributor in order to detect pressure waves that result from the operation of the individual injectors. 
         [0011]    A signal processing apparatus is provided in order to process the pressure signals of the pressure sensors. The output signal of the signal processing apparatus corresponds to the fuel flow rate through the fuel injectors. An output signal of this kind can be used to inform an operator of the engine of a malfunction of the fuel injectors. The output signal can also be delivered to a fuel injector control apparatus in order to adapt the actuation duration of the injectors so that a desired flow rate for each activation is achieved. 
       SUMMARY 
       [0012]    An object of the invention is to make available a standard shop method and a standard shop device for refilling completely or partly emptied couplers of fuel injectors, and/or for checking the leak tightness of the injector and of the return system. 
         [0013]    A method according to the present invention for refilling an emptied hydraulic coupler of a fuel injector encompasses the steps of connecting a pressure generator to a return line of the fuel injector; operating the pressure generator in order to generate a return pressure in the return line; and lastly operating the fuel injector at the elevated return pressure in the return line. 
         [0014]    The invention also encompasses a pressure generator that is embodied for connection to the return line of a fuel injector. 
         [0015]    With the method according to the present invention utilizing a pressure generator according to the present invention it is possible, in particular in injection systems having piezo fuel injectors, to build up in the injector the counter-pressure necessary for operation when the injector has previously been emptied. The result is that the functionality of the injector is restored and the engine can once again run. Emptied injectors therefore do not need to be replaced with new parts. 
         [0016]    Defects in the return system of the injectors, for example impermissible pressure losses at the constant pressure valve, can also be detected. 
         [0017]    The invention makes it possible to isolate the fault quickly and, as necessary, to replace specific components and refill the coupler. 
         [0018]    The invention is easy to utilize, and the time required is short. The enormous costs resulting from the replacement of injectors in the event of a fault can be eliminated by the invention. 
         [0019]    In an embodiment of the method, the pressure generator is operated in such a way that the return pressure is equal to 5 to 25 bar. In particular, a predefined desired return pressure in the range between 5 and 25 bar is generated by suitable operation of the pressure generator. In this fashion, the injector to be filled is always operated at its optimum return pressure, which is predefined by its design. 
         [0020]    In an embodiment of the method, the return pressure is held constant during operation of the fuel injector. Optimum operation of the fuel injector is thereby ensured. 
         [0021]    In an alternative embodiment, an alternating-pressure load is applied in the return line. 
         [0022]    In an embodiment, the pressure generator generates the elevated pressure in the return line by compressing a gas, in particular air. Pressure generation by compressing air is particularly easy and economical to implement, since air can be taken from the environment at no cost. In addition, the compressibility of the gas equalizes undesired pressure fluctuations to a certain extent. 
         [0023]    In a further embodiment, a pressure threshold load is applied on the injector return. The result is that venting of the injector is optimized or, depending on the injector design, made possible in the first place. 
         [0024]    In an alternative embodiment, the return pressure is generated by compressing a liquid, e.g. (diesel) fuel or a test oil. Because a liquid is practically incompressible as compared with a gas, an elevated pressure can be generated particularly efficiently because it is not necessary to compress the liquid itself in order to generate an elevated pressure in the return line. 
         [0025]    In an embodiment, a constant pressure valve is provided in the return line, and the method according to the present invention includes checking the function of the constant pressure valve. 
         [0026]    Thanks to a constant pressure valve in the return line, the pressure in the return line can always be set to the optimum value for operation of the fuel injector, so that the fuel injector can be operated particularly effectively. 
         [0027]    A method according to the present invention also makes it possible to check a constant pressure valve of this kind in particularly simple and efficient fashion, in order to allow malfunctions of the fuel injectors or of the engine that are attributable to a defective constant pressure valve to be detected and, as applicable, eliminated. 
         [0028]    In an embodiment of a pressure generator according to the present invention, the pressure generator has a pressure measuring device (manometer) that is embodied to monitor the pressure generated by the pressure generator or the pressure in the return line. A pressure measuring device of this kind allows the desired pressure in the return line to be established exactly, and in particular allows excessive pressure in the return line, which can cause damage to components of the injection system, to be avoided. 
         [0029]    The pressure generator can additionally have an overpressure valve that automatically opens when a predefined pressure is exceeded, in order to avoid a hazardous overpressure in the return line. The pressure at which an overpressure valve of this kind opens can be, for example, 30 bar, so that a return pressure in the range from 5 to 25 bar can be established in the return line but a considerable and hazardous exceedance of the maximum desired return pressure of 25 bar is reliably and automatically avoided. 
         [0030]    The pressure generator can have an electrically operated compressor that is embodied to generate the overpressure in the return line. An electrically operated compressor allows the elevated pressure in the return line to be generated in particularly simple and convenient fashion; in particular, a compressor of this kind can have control applied to it via a suitable control circuit in such a way that a predefined pressure is generated in the return line. 
         [0031]    Alternatively or additionally, a manual pump can be provided, for example, for pressure generation. A pressure generator having a manual pump is particularly cost-effective and low-maintenance, and moreover allows an elevated pressure to be generated in the return line even without additional electrical power, for example for use in a breakdown situation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic view of a fuel injection system having a pressure generator according to the present invention. 
           [0033]      FIG. 2  shows the minimum necessary counter-pressure in the return line as a function of fuel injection pressure and engine speed. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  schematically depicts an exemplifying embodiment of a fuel injection system having a pressure generator  36  according to the present invention. 
         [0035]    The injection system has a tank  2  for storing fuel  3  that is to be injected. 
         [0036]    During operation, fuel  3  is withdrawn from tank  2  through a pre-filter  4  that is disposed inside tank  2 , and through a fuel withdrawal line  6 . 
         [0037]    Fuel  3  withdrawn from tank  2  is conveyed through a fuel filter  14  to a fuel pump  16  that is preferably embodied as a high-pressure fuel pump  16 . 
         [0038]    Provided between fuel filter  14  and the inlet of fuel pump  16  is a temperature sensor  15  that measures the temperature of fuel  3  that is flowing into fuel pump  16 , and supplies the measured temperature value via a signal lead  15   a  to a control unit  44 . 
         [0039]    Fuel pump  16  causes the desired elevated injection pressure to be applied to the fuel, and conveys the fuel at elevated pressure via a pressure line to a fuel distributor (“common rail”)  22 . 
         [0040]    Excess fuel is conveyed via a return line  18  back to the inlet of fuel pump  16 . 
         [0041]    Fuel distributor  22  is of substantially tubular configuration, a pressure sensor  24  and a pressure control valve  25  being respectively mounted at the two end faces of tube  22 . Pressure sensor  24  and pressure control valve  25  are connected via electrical control leads  24   a,    25   a  to control unit  44 . Pressure sensor  24  measures the pressure of fuel  3  in fuel distributor  22  and outputs a corresponding electrical signal to control unit  44 . Control unit  44  compares the pressure ascertained by pressure sensor  24  with a predefined desired fuel pressure in fuel distributor  22 , and regulates fuel pump  16 , and/or pressure control valve  25  disposed at an opposite end of fuel distributor  22 , in such a way that the desired fuel pressure is established within fuel distributor  22 . Excess fuel is removed from fuel distributor  22  through a discharge line  23  and conveyed back to the inlet of fuel pump  16 . 
         [0042]    Multiple connectors  26  (four, in the example shown in  FIG. 1 ) each for connecting one fuel injector  30  are embodied on the circumference of the tubular fuel distributor  22 . 
         [0043]    In the exemplifying embodiment shown in  FIG. 1 , the fuel injector  30  shown is one that is hydraulically connected to fuel distributor  22  via a throttling element  28  and a fuel line  29 . 
         [0044]    Fuel injector  30  has control applied to it via an electrical control lead  31  that is connected to control unit  44 . 
         [0045]    A return line  32  is connected via an injector connector  45  to fuel injector  30  and leads from fuel injector  30  to the outlet side of fuel filter  14 , in order to bring excess fuel  3  out of fuel injector  30  back to the inlet side of high-pressure pump  16  so that it can be introduced again into fuel injector  30 . 
         [0046]    A constant pressure valve  35 , which is embodied to establish and maintain a defined pressure in return line  32  during operation, is provided in return line  32 . 
         [0047]    Provided before constant pressure valve  35  in the flow direction is a shutoff apparatus  34  that is embodied, for example, as a Matra shutoff. Shutoff apparatus  34  makes it possible, for diagnostic purposes, to shut off the discharge of fuel from return line  32  or keep it at a specific pressure, for example in order to detect leaks in return line  32  or in fuel injectors  30 , or defects in constant pressure valve  35 . 
         [0048]    Pressure generator  36  according to the present invention is connected to return line  32 . Connection can occur with the aid of an adapter  33  between a injector connector  45  and return line  32 . Adapter  33  can be embodied with an automatic or manually actuated valve in order to enable connection and removal of pressure generator  36  with no pressure loss in return line  32 . 
         [0049]    A pressure generator  36  according to the present invention is connected to return line  32 . Pressure generator  36  has a pressure cylinder  37  having a pressure piston  39  movable therein, such that an elevated pressure can be generated in pressure cylinder  37  by moving pressure piston  39 . Pressure cylinder  37  is filled with a gas, e.g. air, or with a liquid, e.g. (diesel) fuel or a suitable test oil. 
         [0050]    Pressure piston  39  can be driven manually (manual operation) or by a preferably electric motor (not shown in  FIG. 1 ) in order to elevate the pressure in pressure cylinder  37 . 
         [0051]    Also possible are other forms (not shown) of a pressure container  37  in which an elevated pressure is generated by a suitable pump apparatus  39 . 
         [0052]    A pressure measuring apparatus  38  (manometer), at which the pressure existing in pressure piston  37  can be read, is provided on pressure piston  37 . 
         [0053]    Pressure measuring apparatus  38  can be embodied as an electrical pressure gauge  38 . 
         [0054]    Provided between pressure piston  37  and adapter  33  is a relief valve  40  which is embodied in such a way that it opens, and allows gas or liquid to flow out of pressure piston  37  through a relief line  41 , when a predefined pressure in pressure piston  37  is exceeded. Relief valve  40  serves as a safety valve in order to prevent damage to return line  32 , to fuel injector  30 , or to other components of the injection system as a result of excessive pressure in return line  32 . 
         [0055]    A filter  42  is provided on return line  32  between relief valve  40  and connection point  33  of pressure generator  36  in order to prevent dirt particles from traveling out of pressure piston  37  into the return line, where they can cause clogging and possibly damage to constant pressure valve  35  and/or to fuel pump  16 . 
         [0056]    In order to refill a partly or completely emptied injector  30 , pressure generator  36  is connected to return line  32  with the aid of adapter  33 , as shown in  FIG. 1 . Actuation of pressure piston  39  causes a desired elevated pressure to be generated in pressure piston  37  and therefore also in return line  32 . The system makes it possible to build up the pressure in return line  32  up to the maximum pressure of constant pressure valve  35 . 
         [0057]    The pressure buildup in return line  32  and in pressure piston  37  can be read from and monitored on pressure gauge  38 . 
         [0058]    Once the desired pressure has been built up in return line  32 , the engine (not shown in the Figure) is started, and fuel is injected through injector  30  into the combustion chamber of the engine. 
         [0059]    When a compressor is used for pressure generation, the pressure level can be held constant during the starting operation, if applicable, by supplying further gas or liquid into pressure cylinder  37 . With hand pumps having a manually operated pressure piston  39 , it is possible and necessary to continue pumping as applicable. 
         [0060]    The pressure thereby built up in return line  32  causes generation of a pressure cushion in the coupler of injector  30 , which results in a “stiffening” of the coupler unit. This ensures that the actuator stroke is transferred to the switching valve inside injector  30 , and injector  30  is again functional. 
         [0061]    As operation continues, the air that has collected in the coupler of injector  30  is discharged through return line  32 , so that after a certain amount of operating time injector  30  is operational even without the additional pressure generator  36 . 
         [0062]    Pressure measuring device  38  of pressure generator  36  can also be used simultaneously to check the leak tightness and “switching pressure” of constant pressure valve  35 . 
         [0063]    In the case of a constant pressure valve  35  affected by leakage, or in systems that have a throttling element instead of a constant pressure valve  35 , the pressure buildup can be achieved by shutting off return line  32  with the aid of a shutoff apparatus  34  or an adapted constant pressure valve. Shutoff apparatus  34  can be disposed in the flow direction before or after the original constant pressure valve  35 . 
         [0064]    A partly or completely emptied injector  30  can be refilled in simple and economical fashion by using a pressure generator  36  in a method according to the present invention; in particular, the complexity and cost involved in installing and removing or replacing an injector  30  are eliminated. At the same time, the return system, including injector  30 , can be checked for leak tightness. 
         [0065]      FIG. 2  is a diagram showing the pressure in return line  32  necessary for correct functioning of the engine, as a function of the rotation speed U (X axis) of the engine and the pressure p (Y axis) in fuel distributor  22 . 
         [0066]    The diagram shows that the pressure necessary in return line  32  rises both with the engine speed U and with the pressure p in fuel distributor  22 , so that, in particular, high engine speeds U and high pressures p in fuel distributor  22  necessitate high pressures of up to 10 bar in return line  32 . 
         [0067]    At low engine speeds U and/or a lower pressure p in fuel distributor  22 , on the other hand, a lower pressure of e.g. 3 bar in return line  32  is sufficient.