Patent Publication Number: US-2016230691-A1

Title: Method for operating a fuel injector

Description:
CROSS REFERENCE 
     The present application claims the benefit under 35 U.S.C. §119 of German Patent Application No. 102015202389.3 filed on Feb. 11, 2015, which is expressly incorporated herein by reference in its entirety. 
     BACKGROUND INFORMATION 
     The present invention is directed to a method for operating a fuel injector. The subject matter of the present invention is also a computer program, a machine-readable memory medium, a control unit, and a program code. 
     A method for operating a fuel injector is described in German Patent Application No. DE 10 2009 002 483 A1. In this method, a valve needle is driven with the aid of an electromagnetic actuator. A variable characterizing the acceleration of a magnet armature of the electromagnetic actuator is formed as a function of at least one electrical operating parameter. Based on this variable characterizing the acceleration, the operating state of the fuel injector is inferred. 
     In the fuel injector described there, the magnet armature is not fixedly connected to the valve needle, but instead is in an overhung position between two stops. 
     The axial play between the magnet armature and the two stops is referred to as the armature free travel. A compression spring ensures that the magnet armature is always in contact with the combustion-chamber-side stop in the idle state and thus the complete armature free travel is available as an acceleration distance when the injector is activated. 
     It is advantageous in a system of this type that, due to the impulse of the armature generated during opening at the same magnetic force, the valve needle may also be safely opened even at higher fuel pressures. Due to the decoupling of the masses between the valve needle and the armature, the impact force in the seat is divided into two impulses. 
     It is disadvantageous in this system that the armature rebounds after striking the lower stop during closing of the injector. It may thereby occur that the complete armature free travel is run through again and the armature still has so much energy upon renewed impact on the upper stop that the valve needle is lifted out of the seat once again for a short time. This results in undesirable post-injections, increased pollutant emissions and increased consumption by the vehicle. Even if the armature does not run through the complete armature free travel during the rebound, it still requires some time until it is settled again. 
     If the armature is activated again before the final settling, a less robust function of the fuel injector results. This is disadvantageous, in particular in the case of multiple injections with short pauses between the individual injections. The case may thereby occur that the impact impulses are correspondingly increased or decreased. 
     SUMMARY 
     An example method according to the present may have the advantage over the related art that two injections may be triggered quickly one after the other. In the process, very short pause times may be implemented. Additional sensors are not necessary for the method according to the present invention, since variables from other functionalities may be used. 
     These advantages are thus achieved in that the triggering of a second injection begins at a first point in time. At this first point in time, a valve needle reaches its closed position during a first injection. This means that as soon as the valve needle reaches its closed position during the first injection, the triggering of the second injection begins. If the interval between the end of the triggering and the point in time of the closing of the valve needle is referred to as the closing time, and the interval between the end of the triggering for the first injection and the beginning of the second injection is referred to as the pause time, then the closing time and the pause time are practically of the same length. 
     It may be advantageous if two injections may follow each other at a very short interval. The first injection hereby positively influences the second injection. Thus, since the second injection coincides with the closing point in time of the first injection, a defined state of the system virtually arises and the second triggering provides a reproducible injection. 
     It may be advantageous if the triggering of the second injection begins after the first point in time and before the second point in time at which a magnet armature reaches its end position. In this specific embodiment, the demands for accuracy are lower. The advantages are, however, achieved to the greatest possible extent. 
     The most advantageous triggering results if the triggering of the second injection begins directly after the first point in time at which the valve needle closes. 
     It may be further advantageous if the first point in time is ascertained based on an operating parameter of the fuel injector. 
     It may be advantageous if the current which flows through the fuel injector, and/or the voltage which is applied at the fuel injector is evaluated as the operating parameter. These variables are either easy to ascertain or are already available in the control unit for other tasks. 
     If the first point in time is read out from an engine characteristic map based on parameters of the internal combustion engine, the ascertainment of the point in time may be dispensed with. In addition, the method is also usable in operating states in which the first point in time is not ascertainable or is only ascertainable with difficulty. 
     It may be particularly advantageous hereby if the first point in time is ascertained in the presence of certain parameters of the internal combustion engines based on operating parameters of the fuel injector and is entered into the engine characteristic map. Thus, the engine characteristic map may be adapted during ongoing operation to aging effects or other changes. 
     In an additional aspect, the present invention relates to program code together with processing instructions for compiling a computer program executable on a control unit, in particular, source code with compiler and/or linking instructions, the program code resulting in the computer program for carrying out all steps of a described method, if it is converted into an executable computer program according to the processing instructions, i.e., in particular, compiled and/or linked. This program code may be provided in particular by source codes which, for example, are downloadable from a server on the internet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention are represented in the figures and are explained in greater detail below. 
         FIG. 1  shows a schematic representation of an internal combustion engine including multiple fuel injectors operated according to an example embodiment of the present invention, 
         FIGS. 2 a  through 2 c    show a schematic representation of one fuel injector from  FIG. 1  in three different operating states. 
         FIG. 3  shows different signals plotted over the time. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     An internal combustion engine bears as a whole the reference numeral  10  in  FIG. 1 . It includes a tank  12 , from which a conveying system  14  conveys fuel into a common rail  16 . Multiple electromagnetically actuated fuel injectors  18   a  through  18   d  are connected to the common rail which inject the fuel directly into their assigned combustion chambers  20   a  through  20   d.  The operation of internal combustion engine  10  is controlled or regulated by a control and regulating unit  22 , which also activates fuel injectors  18   a  through  18   d,  among other things. 
       FIGS. 2 a  through 2 c    schematically show fuel injector  18   a  according to  FIG. 1  in a total of three different operating states. Additional fuel injectors  18   b,    18   c,    18   d  shown in  FIG. 1  have a corresponding structure and functionality. 
     Fuel injector  18   a  includes an electromagnetic actuator which has a solenoid coil  26  and a magnet armature  30  interacting with solenoid coil  26 . Magnet armature  30  is connected to a valve needle  28  of fuel injector  18   a  in such a way that, with respect to a vertical moving direction of valve needle  28  in  FIG. 2 a   , it is movable with a non-negligible mechanical play relative to valve needle  28 . 
     Thus, a two-part mass system  28 ,  30  arises which effectuates the drive of valve needle  28  by electromagnetic actuator  26 ,  30 . This two-part configuration improves the installability of fuel injector  18   a  and an undesirable rebound of valve needle  28  during impact on its valve seat  38  is reduced. 
     In the present configuration illustrated in  FIG. 2 a   , the axial play of magnet armature  30  on valve needle  28  is limited by two stops  32  and  34 . However, at least lower stop  34  in  FIG. 2 a    might also be implemented by an area of the housing of fuel injector  18   a.    
     Valve needle  28  is acted upon by a valve spring  36 , as is shown in  FIG. 2 a   , by an appropriate spring force against valve seat  38  in the area of housing  40 . In  FIG. 2 a   , fuel injector  18   a  is shown in its open state. In this open state, magnet armature  30  is moved upward due to energization of solenoid coil  26  in  FIG. 2 a    so that, upon engaging in stop  32 , valve needle  28  is moved out of its valve seat  38  against the spring force. Thus, fuel  42  may be injected by fuel injector  18   a  into combustion chamber  20   a  ( FIG. 1 ). 
     As soon as the energization of solenoid coil  26  is ended by control unit  22  ( FIG. 1 ), valve needle  28  moves, due to the effect of the spring force exerted by valve spring  36 , toward its valve seat  38  and entrains magnet armature  30 . A power transmission from valve needle  28  to magnet armature  30  takes place here in turn by upper stop  32 . 
     As soon as valve needle  28  completes its closing movement with the impact on valve seat  38 , magnet armature  30 , as shown in  FIG. 2 b   , may move further downward, due to the axial play in  FIG. 2 b   , until it contacts second stop  34 , as is illustrated in  FIG. 2   c.    
     In  FIG. 3 , different variables are plotted over the time; two injections are depicted here. In the first line, current I flowing through solenoid coil  26  is shown. In the second line, lift AH of magnet armature  30  is plotted, and in the third line, lift NH of valve needle  28  is plotted over the time. This representation of the progressions is selected only by way of example. 
     The energization of solenoid coil  26  begins at point in time t 0 . After a short delay, magnet armature  30  begins to move and entrains valve needle  28 . Both reach their maximum lift after a short time. 
     At point in time t 1 , the energization is discontinued and the magnet armature begins to fall back. Valve needle  28  is simultaneously moved into its closed position by the spring. The valve needle reaches its closed position at point in time t 2 . Due to its inertia, the magnet armature has not yet reached its stop at point in time t 2 . 
     It is provided according to the example embodiment of the present invention, that the energization of the next injection begins at point in time t 2 , at which valve needle  28  reaches its closed position. This results in that the armature moves again in its other direction and the valve needle changes over again into its open position. 
     It is provided in one particularly simple specific embodiment, that point in time t 2 , at which the valve needle reaches its closed state, is measured for each injection. The progression of current I flowing through the solenoid coil or the voltage applied at the solenoid coil is preferably evaluated to ascertain point in time t 2 . Numerous conventional methods are available for this purpose. A corresponding method is described in the related art. 
     In one additional specific embodiment, it may also be provided that point in time t 1  or the period between the end of the energization at point in time t 1  and point in time t 2  is stored in a memory in the control unit. This value is preferably stored in an engine characteristic map as a function of the operating state of the internal combustion engine and/or of the driven vehicle. This is advantageous since it is not possible to ascertain point in time t 2  in all operating states with sufficient accuracy, or the ascertainment requires computing time. 
     It may be advantageous in this specific embodiment if the period is stored and the triggering of the second injection begins by this period after the end of the triggering of the first injection. 
     It may also be provided in one embodiment that point in time t 2  or the period is ascertained in specific operating states of the internal combustion engine and is stored in a memory or an engine characteristic map for later use. 
     The triggering of the second injection should take place so soon after the end of the triggering of the first injection that the valve needle has reached its closed position; however, the magnet armature is still in motion. It is also possible here that the triggering of the second injection already begins when the valve needle has not yet completely reached its closed position.