Patent Publication Number: US-8996280-B2

Title: Method for operating a fuel injector of an internal combustion engine, and control device for an internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control device and a method for operating a fuel injector of an internal combustion engine. 
     2. Description of Related Art 
     Internal combustion engines operating according to the Otto or Diesel method and injecting fuel directly into the combustion chamber of the internal combustion engine are especially advantageous with regard to efficiency, emission behavior and power output. In order to utilize the advantages of this so-called direct injection to the fullest extent possible, highest demands are made on the metering accuracy of the injectors, especially when small injection quantities are involved, in particular in the case of jet-directed combustion methods. 
     The metering of minute fuel quantities is required especially during multiple injections, in particular for the startup, warm-up and heating of the catalytic converter of the internal combustion engine. Furthermore, the requirements on the metering precision are increased even further by the increasing injection pressures. From published German patent application document DE 10 2004 015745 A1, a method for operating an injector and for determining the time of flight of the valve needle of the injector is known, to which reference is made hereby. 
     The injectors known from the related art have a characteristic curve of the time of flight of the valve element of the injector as a function of the actuation period, which basically is able to be subdivided into three ranges. As a rule, there is a direct correlation between the time of flight and the injected fuel quantity: the longer the time of flight, the greater the injected fuel quantity under unchanged marginal conditions. 
     In a first range, the so-called partial travel range, the injector is actuated only very briefly, and a characteristics curve segment results that rises monotonously but not always linearly. In a second range, the so-called transition range, the time of flight drops again with an increasing actuation period of the fuel injector, so that a first point of inflection, or a local maximum, is attained between partial travel range and the transition range. 
     This transition range ends at a second point of inflection, or a local minimum. A third characteristics curve segment begins at an actuation period that is greater than actuation period T 2  associated with the second point of inflection, in which third segment the characteristic curve of the time of flight rises monotonously again and has an extremely linear characteristic. 
     Since the position of the transition range and the times of flights of the valve element associated with the first and second points of inflection are individual for each injector and also vary across the service life of the injector, it is currently not possible to represent the partial travel range and the transition range of the characteristic curve for actuating the injector, in particular for metering minute injection quantities, with the required accuracy. This is the reason why currently only the so-called full travel range is triggered with regard to the characteristic curve, which makes it impossible to meter minute fuel quantities. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is based on the objective of expanding the application range of the injectors especially in the direction of small and minute injection quantities, and of increasing the metering precision. 
     According to the present invention, this objective is achieved in that the transition range of the characteristic curve is determined individually for each injector and suppressed or skipped during operation of the internal combustion engine. As a result of the method according to the present invention, a monotonously rising characteristic curve is formed between the actuation period and the time of flight, or the valve element of the injector or the injection quantity. The operating or application range within which fuel injection quantities are able to be metered is able to be expanded considerably in this way. In particular, shorter actuating periods and consequently smaller injection quantities are realizable as a result. Another advantage is that the metering precision is improved. 
     In a further advantageous development of the present invention, transition range ÜB is delimited by a first point of inflection WP 1  and a second point of inflection WP 2 , or a local maximum and a local minimum of a characteristic curve of the time of flight of a valve element of the fuel injector as a function of the actuating period. 
     Both the points of inflection and the local extreme values are able to be determined by a multitude of methods known from the related art, using the nodes of the characteristics curve, thereby making it possible to determine the transition range for each injector individually. Moreover, it is also possible to determine the points of inflection and/or the extreme values regularly during operation of the internal combustion engine and across the entire service life of the injectors, and corrections may be made, if necessary, so that drift of the operational performance of the injectors is able to be detected and taken into account for the actuating period. This makes it possible to realize constantly high metering precision across the entire service life of the internal combustion engine and the injector, and thus also to comply with the legally mandated emission limit values across the entire service life of the internal combustion engine. 
     The method according to the present invention is based on methods for determining the time of flight of the valve element of an injector, which are known from published German patent application document DE 10 2004 015745 A1, for instance. The time of flight of the valve element is ultimately determined in that the current and/or the voltage characteristic at the terminals of the injector is recorded with high time resolution and then analyzed. Thus, this too, requires no additional hardware, and the method is able to be repeated regularly while the internal combustion engine is running, so that the determination of the characteristics curves is able to take place at regular intervals across the entire service life of the internal combustion engine, and the resulting points of inflection or local maximums/minimums are able to be determined. 
     A relatively simple method for determining the first point of inflection and/or the second point of inflection of the characteristics curve of the fuel injector provides that the times of flights associated with different actuating periods of the injector be determined, and the actuating periods and the associated times of flights be used for generating a characteristics curve. In a further step, this characteristics curve is subdivided into ranges which feature monotonous changes in the times of flight at varied actuating periods, especially a partial travel range TH, a transition range ÜB, and a full travel range VH. According to the present invention, these ranges are delimited from each other by a point of inflection or a local extreme value. Thus, according to the present invention, the transition range is able to be ascertained by methods known per se in order to determine points of inflection and/or local extreme values. As a result, the method according to the present invention makes it easy to determine the first and the second point of inflection or a local maximum and a local minimum at all times while the internal combustion engine is in operation, without requiring additional hardware, thereby determining the transition range of the characteristics curve on the basis of these values and implementing the method according to the present invention. 
     A specific time of flight FDWP 1  is able to be assigned to the first point of inflection or the local maximum. Correspondingly, it is possible to assign a time of flight FDWP 2  to the second point of inflection or the local minimum. In this context, time of flight FDWP 1  at the first point of inflection is greater than time of flight FDWP 2  at the second point of inflection. For only then will there be a transition range in the characteristics curve, within which the characteristics curve does not rise monotonously. In order to then arrive at a monotonous characteristics curve of the injector, in the present invention a switch takes place from using the characteristics curve in partial travel range, to using the characteristics curve in full travel range, if the desired time of flight resulting from the required injection quantity is greater than the time of flight FDWP 2  at the second point of inflection and smaller than the time of flight at the first point of inflection. This ensures that a switch to the characteristics curve in the full travel range takes place when it is already possible to trigger the injector in the full travel range in such a way that the desired time of flight is achieved. 
     To avoid instabilities of the method, the switch from using the characteristic curve in the partial travel range, to the characteristics curve in the full travel range always takes place for as long as the desired time of flight is less than the time of flight at the first point of inflection, minus a first minimum distance ΔFD, 1 . This ensures that the method never uses a node of the characteristics curve that is located in the direct vicinity of, or directly at, the first point of inflection, which could lead to instabilities of the method. First minimum distance ΔFD, 1  is advantageously selected such that it absorbs the drift of the characteristics curve to be expected between two cycle-based detections of the characteristics curve during normal operation, which means that a stable control of the injector is possible at all times. 
     In analogous manner, it is also provided that a switch from the characteristics curve in the full travel range to the characteristics curve in the partial travel range takes place no later than at the instant when the desired time of flight is less than the time of flight at the second point of inflection, plus a second minimum distance ΔFD, 2 . This, too, ensures that the characteristics curve in the direct proximity of the second point of inflection will not be used and that the method according to the present invention runs in a stable manner. 
     To allow the detection of drift of the characteristics curve that occurs during operation, the first point of inflection and/or the second point of inflection, or the local maximum and the local minimum, are/is determined anew at regular intervals. For instance, it is possible to count a certain operation period of the internal combustion engine, and to detect the characteristics curve of the injector including the points of deflection and the local extreme values after a predefined operating period has elapsed, and to update the times of flight associated with the points of inflection and to store them in a memory. 
     Furthermore, in order to fully utilize the advantages of the method according to the present invention, each injector of an internal combustion engine is operated according to the method of the present invention and the points of inflection or the local extreme values are determined individually for each injector. This makes it possible to operate each cylinder of the internal combustion engine in optimal manner across the entire service life, so that the total emissions of the internal combustion engine are also at a constantly low level. 
     Since the switch from the characteristics curve of the partial travel range to the characteristics curve of the full travel range, and vice versa, is set up to occur at different limits, that is to say, the distance to the first point of inflection or the second point of inflection, a hysteresis results in the switch between the ranges of the characteristics curve, so that the method dwells longer in a particular range of the characteristics curve and the number of changes from one range of the characteristics curve to another range of the characteristics curve is able to be reduced. Furthermore, so-called toggling in the direct vicinity of the first point of inflection and the second point of inflection is avoided. This toggling, too, is undesired since it reduces the stability of the control of the injector. 
     Of special importance is the realization of the method according to the present invention in the form of a computer program which is able to run on a computer or a processing unit of a control device, and which is suitable for executing the method. The computer program may be stored on an electronic storage medium, for example, the storage medium in turn being part of the control device, for instance. 
     Further advantages, features and details result from the following description, in which different exemplary embodiments of the present invention are shown with reference to the drawing. In this context, the features mentioned in the claims and the description may be essential to the present invention either individually in isolation or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a - 1   c  show schematic illustrations of an injector suitable for implementing the method according to the present invention. 
         FIG. 2  shows an exemplary, schematic illustration of the characteristics curve of an injector. 
         FIG. 3  shows the characteristics curve according to  FIG. 2 , with a suppressed transition range. 
         FIG. 4  shows an explanation of the method according to the present invention, including a hysteresis 
         FIG. 5  shows a flow chart of one specific embodiment of the method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1   a  through  1   c  show a specific development of a fuel injector  10  provided for the injection of fuel, into an internal combustion engine in different operating states of an injection cycle. 
       FIG. 1   a  shows injector  10  in its neutral state, in which it is not actuated by control device  22  assigned to it. A solenoid valve spring  111  presses a valve ball  105  into a seat of outlet restrictor  112  provided for this purpose, so that a fuel pressure corresponding to the rail pressure is able to be generated in valve control chamber  106 , as it also prevails in the region of high-pressure port  113 . 
     The rail pressure is also applied in chamber volume  109  which surrounds valve needle  116  of injector  10 . Valve needle  116  is kept closed against an opening force acting on pressure shoulder  108  of valve needle  116  by the forces applied to the end face of control plunger  115  by the rail pressure, and the force of nozzle spring  107 . 
       FIG. 1   b  shows fuel injector  10  in its open state, which it assumes when actuated in the following manner by control device  22 , starting from the neutral state shown in  FIG. 2   a : Electromagnetic actuator  102 ,  104 , which in this case is made up of solenoid coil  102  denoted in  FIG. 2   a  and solenoid armature  104  cooperating with solenoid coil  102 , is acted upon by control device  22  by an actuation current I forming an actuation signal, in order to open solenoid valve  104 ,  105 ,  112  operating as control valve in the case at hand. The magnetic force of electromagnetic actuator  102 ,  104  exceeds the spring force of valve spring  111  ( FIG. 1   a ), which causes solenoid armature  104  to lift valve ball  105  off its valve seat and thereby opens outlet restrictor  112 . 
     As soon as outlet restrictor  112  opens, fuel is able to drain into a fuel reservoir (not shown) from valve control chamber  106  in the cavity situated above in  FIG. 1   b , see the arrows, and via a fuel return line  101 . Inlet restrictor  114  prevents complete pressure equalization between the rail pressure applied in the region of high-pressure port  113  and the pressure in valve control chamber  106 , so that the pressure in valve control chamber  106  drops. As a result, the pressure in valve control chamber  106  becomes lower than the pressure in chamber volume  109 , which continues to correspond to the rail pressure. The reduced pressure in valve control chamber  106  causes a correspondingly reduced force on control plunger  115  and thus leads to opening of injector  10 , i.e., to valve needle  116  being lifted off its valve needle seat in the region of spray-discharge orifices  110 . This operating state is illustrated in  FIG. 1   b.    
     Then, i.e., once the valve needle has lifted off from the valve needle seat, valve needle  116  executes an essentially ballistic trajectory, primarily under the influence of the hydraulic forces in chamber volume  119  and in valve control chamber  106 . Given an actuation period of sufficient length, during which actuation current I is applied to solenoid coil  102 , however, valve needle  116  may also reach a needle travel stop (not shown) in its opening movement, which defines the maximum needle travel. In this case, injector  10  is said to be operated in its full travel range. 
     As soon as electromagnetic actuator  102 ,  104  ( FIG. 1   a ) is no longer actuated by control device  22  at an end of the actuation period, valve spring  111  exerts downward pressure on solenoid armature  104 , as shown in  FIG. 1   c , so that valve ball  105  subsequently seals outlet restrictor  112 . This causes the renewed generation of rail pressure in control chamber  106 . This pressure in control chamber  106 , which is now increased, exerts greater force on control plunger  115 , which, in conjunction with the force of nozzle spring  107 , exceeds the force acting on valve needle  116  in the region of chamber volume  109 , and which therefore returns valve needle  116  to its closing position again. 
     The fuel injection has ended as soon as valve needle  116  reaches its valve needle seat in the region of spray-discharge orifices  110  and seals them, see  FIG. 1   c .  FIG. 2  shows the characteristics curve of an injector  10  by way of example, actuation period T A  being plotted on the X-axis, and time of flight FD being plotted on the Y-axis. 
     Characteristics curve  25  is subdividable into three ranges. The first range begins in the direct vicinity of the origin and ends at instant T 1 . This first range is denoted as partial travel range TH, due to the fact that valve needle  13  does not open completely in this range and does not strike the travel stop. In partial travel range TH, characteristics curve  25 . 1  is relatively steep and frequently non-linear. However, to simplify matters, the first range of characteristics curve  25 . 1  is shown as a straight line in  FIG. 2 . A characteristic of first range TH is that characteristics curve  25 . 1  rises in monotonous manner. At an actuation period t A =T 1 , characteristics curve  25  has a first point of inflection WP 1 , or a first local maximum. With actuation periods t A &gt;T 1 , time of flight FD drops again, to the point at which a second turn of inflection WP 2 , or a second local maximum, is attained at an actuation period t A =T 2 . 
     If actuation period t A &gt;T 2  is then selected, characteristics curve  25 . 3  rises again monotonously and usually has a very linear characteristic. This means that the actuation of the injector using actuation periods t A &gt;T 2  is easy to control in terms of control technology, and that an excellent linear correlation exists between the actuation period and time of flight FD or the injected fuel quantity resulting therefrom. 
     Until now, the operating range of the injector has been restricted to full travel range VH at actuation periods t A &gt;T 2 , since especially in transition range ÜB, the metering accuracy drops and, in particular, the deviation between different specimens of injectors having the same design increases considerably. This, too, reduces the metering accuracy. 
     In order to circumvent this problem, the present invention provides for a suppression of transition range  25 . 2  of the characteristics curve, and for composing a monotonously rising characteristics curve from ranges  25 . 1  and  25 . 3  of characteristics curve  25 . Such a combined, monotonously rising characteristics curve is shown in  FIG. 3 . In order to achieve a monotonously rising characteristics curve, a switch between the two components  25 . 1  and  25 . 3  of characteristics curve  25  must take place at a specific time of flight, i.e., at the so-called switchover time of flight FD U  (see  FIG. 2 ). This means that with an actuation period FD&lt;FD U , first range  25 . 1  of the characteristics curve will be utilized, and at actuation periods or times of flight FD&gt;FD U , range  25 . 3  of the characteristics curve will be analyzed. In this way trigger durations t A &lt;TU 1  may be used to control fuel injector  10  in the case of small injection quantities. For greater injection quantities, the actuation period is t A &gt;TU 2 . The range between TU 1  und TU 2  is never actuated, except for determining the points of inflection, so that transition range ÜB is suppressed. This makes it possible to increase the metering accuracy and thus the operating behavior of the internal combustion engine. 
     An essential feature of the transition range is that a first point of inflection WP 1  and/or a local maximum is present between first range  25 . 1  and second range  25 . 2  of characteristics curve  25 . According to the present invention, this first point of inflection WP 1  or the local maximum may be used for separating partial travel range TH from transition range ÜB. Analogously, it is possible to use second point of inflection WP 2 , which is situated between second range  25 . 2  and third range  25 . 3  of characteristics curve  25 , to separate these ranges from each other. 
     In the simplified illustration according to  FIGS. 2 through 4 , characteristics curve  25  is made up of three straight segments. However, especially first range  25 . 1  and second range  25 . 2  are not linear in many injectors from series production, so that curved or non-linear segments of characteristics curve  25  may occur as well, which are also able to be handled by the method according to the present invention. 
     According to the present invention, it is now provided to determine first point of inflection WP 1  and second point of inflection WP 2  at regular intervals, such as after one hundred operating hours of the injector, for example, and to record associated actuation periods T 1  and T 2  and the associated times of flight FD WP1  and FD WP2 . As an alternative to recording points of inflection WP 1  and WP 2 , it is also possible to record the boundary between partial travel range TH and transition range ÜB by determining a local maximum of characteristics curve  25 . In analogous manner, the boundary between transition range ÜB and full travel range VH is able to be recorded by determining a local minimum, and specified. 
     Whether points of inflection or local extreme values are utilized for delimiting the different ranges may be decided as a function of the characteristics curve of the injector. 
       FIG. 4  shows an exemplary embodiment of the method according to the present invention, for which a hysteresis is provided in the suppression of transition range ÜB, so that the switch from first segment  25 . 1  of the characteristics curve to third segment  25 . 3  of the characteristics curve takes place less often, which results in a more stable method. 
     Starting from small actuation periods t A  that are much smaller than T 1 , segment  25 . 1  of the characteristics curve is utilized for calculating time of flight FD. This is done until actuation period t A  approaches value T 1 . To be more precise, time of flight FD resulting from the actuation period is checked as to whether the desired time of flight required for achieving a predefined injection quantity is smaller than time of flight FD WP1  at the first point of inflection, minus a first minimum distance ΔFD 1 . First minimum distance ΔFD 1  is plotted in  FIG. 4 . This switch from first segment  25 . 1  to third segment  25 . 3  of the characteristics curve with increasing actuation period t A  is indicated by a first arrow  27  in  FIG. 4 . For injection quantities that are increasing further, actuation period t A  is then calculated with the aid of third segment  25 . 3  of characteristics curve  25 . 
     If the injection quantity is to be reduced, this naturally leads to shortened actuation periods t A . Since the method in this state is based on third segment  25 . 3  of the characteristics curve, actuation period t A  drifts towards smaller values in the direction of T 2  with increasingly smaller injection quantities. T 2  is the actuation period which results if second point of inflection WP 2  of the characteristic curve is actuated. As soon as actuation period t A  or time of flight FD resulting therefrom is less than time of flight FD WP2  at the second point of inflection, plus a second minimum distance ΔFD ,2 , another switch takes place to first segment  25 . 1  of the characteristics curve. This switch is indicated by second arrow  29 . Since first arrow  27  and second arrow  29  are set apart from each other in the direction of the Y-axis, this results in a hysteresis of the method, or a hysteresis in the switchover or the change from one segment of the characteristics curve to the other segment of the characteristics curve, which increases the stability of the method. Since first minimum distance ΔFD, 1  and second minimum distance ΔFD, 2  depend on first point of inflection WP 1  or second point of inflection WP 2  in each case, the hysteresis is automatically adapted also by the renewed determination of points of inflection WP 1  and WP 2 , so that this hysteresis function, too, is active across the entire service life of the internal combustion engine, regardless of the drift of characteristics curve  25 . 
       FIG. 5  shows an exemplary embodiment of the method according to the present invention, in the form of a circuit diagram. In a first function block  31 , the so-called pilot control of the injector is implemented. In a first decision block  33 , it is queried whether first point of inflection WP 1  and/or second point of inflection WP 2 , or a first local maximum and a second local minimum, are present. If this query is answered in the negative, transition range ÜB of the characteristics curve is measured in a second function block  35 . 
     This is done by actuating injector  10  using different actuation periods t A , and recording the associated times of flight FD. The detection of the times of flight may be performed according to a method known from the related art. For instance, the nodes of the characteristics curve may be recorded in normal operation and in an expanded useful range of the characteristics curve or in a special injection mode. 
     Nodes of a current characteristics curve  25  result from the detection of the times of flight at actuating periods of different lengths. As soon as a sufficient number of nodes has been detected, the new current characteristics curve formed in this manner may be checked as to where first point of inflection WP 1  or a local maximum, and second point of inflection WP 2  or a local minimum are to be found. If first point of inflection WP 1,neu  and second point of inflection WP 2,neu  differ markedly from the previously stored points of inflection, then drift of characteristics curve  25  has taken place and the new values for the point of inflection are stored and the method according to the present invention is implemented on the basis of the newly stored points of inflection. When the points of deflection have been detected, so that the query in branching  33  is able to be answered by “yes”, it is queried in a second query block  37  whether cyclical remeasuring of characteristics curve  25  and the determination of the points of inflection or the transition range is required. If this query is answered by “yes”, branching to second function block  35  takes place in the method, the characteristics curve is remeasured again and transition range ÜB is determined as a function of newly determined points of inflection WP 1  and WP 2 . 
     If the query in second branching block  37  is negative, transition range ÜB in the characteristics curve is skipped and a monotonous characteristics curve is composed of ranges  25 . 1  and  25 . 3  of characteristics curve  25 . With the aid of this monotonous characteristics curve  25 , as it is shown in  FIG. 3 , for instance, injector  10  can now be actuated and exceedingly high metering accuracy be achieved across the entire operating range of the injector. A special advantage of the method according to the present invention is that drift of the injector is detectable as well and accordingly, by a modified/adapted definition of transition range ÜB, its suppression takes place. This makes the metering accuracy virtually constant across the entire service life of the internal combustion engine.