Abstract:
A method for controlling an injection system of an internal combustion engine having at least one injector, the fuel metering being divided into a first partial injection and at least one second partial injection, and a control signal which determines the fuel quantity to be injected with the aid of the at least one injector being corrected as a function of a pressure wave influencing the at least two partial injections, the pressure wave correction being performed on the basis of a periodic model which models the quantity wave as a sum of periodic functions.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for controlling an injection system of an internal combustion engine having at least one injector and to a corresponding control unit. 
   BACKGROUND INFORMATION 
   In fuel injection systems, of self-igniting engines in particular, the fuel quantities injected by injectors into the combustion chambers are divided into a plurality of partial injections. The partial injections usually follow one another in a rapid succession and may include one or more pilot injection(s) applied before a main injection. The time interval between two partial injections is implemented by the pause time between two electric trigger pulses of the injectors. The partial injections make improved mixture preparation and thus lower exhaust gas emissions of the engine, lower noise development during combustion, and higher mechanical power output of the engine possible. 
   In the case of the above-mentioned partial injections, the accuracy of the injected quantities is of great importance. However, each injection causes a brief drop in the fuel pressure in a fuel line connecting a high-pressure accumulator, known as a rail, to the corresponding injector. Such a pressure drop results in a fuel pressure wave between the rail and the injector after the end of the injector triggering; the effect of this wave on the injected quantity of the subsequent partial injections diminishes with an increasing time interval between the particular successive injections. This pressure wave effect intensifies with increasing lift frequency of the nozzle needle of the injector, so that taking it into account, also in future injector systems in particular, in which high-speed piezoelectric actuators are used as injection actuators for nozzle needle control in the particular injector, becomes increasingly important. 
   Since the above-described pressure wave phenomenon is of a highly systematic nature, and although it essentially depends on the time interval between the corresponding injection(s), the injected fuel quantity, the hydraulic fuel pressure, and the fuel temperature in the rail, compensation via an appropriate control function in the engine control unit may be implemented. In a method described in German Patent Application No. DE 101 23 035 for minimizing the pressure wave effect, the effect on the injected quantity of the particular injector is measured and the results of this measurement are taken into account in presetting the control data of the injector, specifically based on a previously empirically, i.e., experimentally, determined fuel quantity wave as a function of the time interval between the partial injections involved. The measured effect of the quantity on a subsequent injection is stored in characteristic maps, and the effect of the quantity is then compensated during the operation of the engine by appropriately modifying the duration of the energized state of the actuator which effects the subsequent injection. 
   The characteristic map is filled with data experimentally by measurements on a hydraulic test bench. The quantities influenced are ascertained in the form of “quantity waves” as a function of the interval between the corresponding injections and used for filling the characteristic map with the aid of a special algorithm. The excess or reduced quantities thus ascertained are stored in the above-mentioned characteristic maps and compensated during the operation of a control program of the engine by making the appropriate deductions in a quantity path of the engine control. 
   In the above-mentioned pressure wave correction, in principle a number of input and output quantities must be taken into account, the exact relationship between these quantities being extremely complex, since there are mutual dependencies such as interactions between the input quantities in particular. For this reason, considerable simplifications are necessary in the pressure wave correction to map the pressure wave phenomenon using the fewest possible characteristics maps; therefore, when mapping the pressure wave system, a considerable portion of the correction accuracy that would be possible in principle is lost. 
   It is therefore desirable to improve a method of the type mentioned above in such a way that a more accurate pressure wave correction than in the related art is made possible, which takes into account the largest possible number of input and/or output quantities in the pressure wave correction, omitting the fewest possible factors considered negligible, while using the least possible technical complexity at the same time. 
   SUMMARY OF THE INVENTION 
   The present invention is based on the idea of performing the pressure wave correction on the basis of a model which takes into account the empirically found fact that it is possible to represent the quantity waves as a continuously oscillating system. The basic idea of the present invention is that the quantity wave is modeled as a sum of a plurality of periodic functions. A great advantage of the method according to the present invention for controlling an injection system of an internal combustion engine is its simple and easily reproducible pressure wave correction structure, which makes it possible to considerably improve the correction accuracy compared to the methods known from the related art. In principle, the pressure waves may be modeled using the most diverse periodic functions. 
   In an advantageous embodiment of the present invention, the periodic functions are sine functions. 
   The periodic functions are preferably decaying periodic functions, i.e., sine functions that decay over time, for example. The parameters of the sine function, in particular its frequency, amplitude, damping, zero point displacement, and the like are advantageously determined as a function of the pressure and/or the quantity of the first partial injection and/or the quantity of the at least second partial injection, these functions being determined by adaptation to tests or simulations. The sine function parameters are advantageously stored in a memory of a control unit, which ensures that they are promptly accessible during the operation of the engine. 
   The quantity of a partial injection following a preceding partial injection which triggers a pressure wave is corrected. This makes a direct pressure wave correction possible. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a common rail injection system which is known from the related art and is suitable for use in the present invention. 
       FIG. 2  schematically shows a longitudinal partial section through a fuel injector of an injection system depicted in  FIG. 1 . 
       FIG. 3  shows an injection sequence, known per se, having a main injection and a pilot injection using appropriate triggering signals of an injection actuator, in particular for illustrating the pressure wave effect. 
       FIG. 4  schematically shows the quantity wave plotted against time and a function of the quantity wave, adapted using the method of the present invention, plotted against time. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows the components of a high-pressure based fuel injection system necessary for understanding the present invention using the example of a common rail (CR) injection system. A fuel reservoir is labeled with the numeral  1 . Fuel reservoir  1  is connected to a second filter  15  for pumping fuel via a first filter  5  and a presupply pump  10 . From second filter  15  the fuel is pumped to a high-pressure pump  25  via a line. The connecting line between second filter  15  and high-pressure pump  25  is also connected to the reservoir  1  via a connecting line having a low-pressure limiting valve  45 . High-pressure pump  25  is connected to a rail  30 . Rail  30  is also known as a (high-pressure) accumulator and is in turn connected in a pressure-conducting manner to different injectors  31  via fuel lines. Rail  30  is connectable to fuel reservoir  1  via a pressure release valve  35 . Pressure release valve  35  is controllable by a coil  36 . 
   The lines between the discharge of high-pressure pump  25  and the inlet of pressure release valve  35  are referred to as a “high-pressure area.” The fuel is under high pressure in this area. The pressure in the high-pressure area is detected with the aid of a sensor  40 . In contrast, the lines between fuel reservoir  1  and high-pressure pump  25  are referred to as a “low-pressure area.” A controller  60  sends trigger signal AP to high-pressure pump  25 , trigger signals A to each injector  31 , and/or a trigger signal AV to pressure release valve  35 . Controller  60  processes different signals of various sensors  65 , which characterize the operating state of the engine and/or of the motor vehicle propelled by this engine. Such an operating state is, for example, speed N of the engine. 
   The injection system depicted in  FIG. 1  operates as follows. The fuel stored in fuel reservoir  1  is pumped by presupply pump  10  through first filter  5  and second filter  15 . If the pressure in the above-mentioned low-pressure area increases to inadmissibly high levels, low-pressure limiting valve  45  opens and clears the connection between the discharge of presupply pump  10  and reservoir  1 . High-pressure pump  25  pumps fuel quantity QI from the low-pressure area into the high-pressure area. In doing so, high pressure pump  25  builds up a very high pressure in rail  30 . Normally, maximum pressure values of approximately 30 bar to 100 bar are achieved for injection systems of externally ignited engines and 1000 bar to 2000 bar for self-igniting engines. The fuel may thus be metered to the individual combustion chambers (cylinders) of the engine under high pressure using injectors  31 . Pressure P rail  in the rail, i.e., in the entire high-pressure area, is detected by sensor  40 . The pressure in the high-pressure area is regulated using controllable high-pressure pump  25  and/or pressure release valve  35 . Electric fuel pumps are normally used as presupply pump  10 . For pumping higher quantities, which are required for utility vehicles in particular, a plurality of presupply pumps connected in parallel may also be used. 
     FIG. 2  shows a piezoelectrically driven injector  101  described in German Patent No. DE 100 02 270 in partial section. Injector  101  has a piezoelectric unit  104  for operating a valve element  103  axially movable in a bore  113  of a valve body  107 . Injector  101  also has an adjusting piston  109  next to piezoelectric unit  104  and an operating piston  114  next to a valve closing element  115 . A hydraulic chamber  116  operating as a hydraulic transmission is situated between pistons  109 ,  114 . Valve closing element  115  cooperates with at least one valve seat  118 ,  119  and separates a low-pressure area  120  from a high-pressure area  121 . An electric control unit  112 , shown only schematically, delivers the trigger voltage for piezoelectric unit  104  as a function of the prevailing pressure level in high-pressure area  121 . An outflow throttle  130  and an inflow throttle  131  are additionally situated in high-pressure area  121  of injector  101 . The outflow/inflow adjustment ratio of these two throttles  130 ,  131  is set with the aid of a control valve  132 . 
     FIG. 3  shows typical trigger signal curves for an injector shown in  FIGS. 1 and 2  in the case of a main injection  200  and a preceding pilot injection  205 . The five signal curves shown represent different triggering states over time, in which the time interval (electrical pause time) between the two trigger signals  200 ,  205 , viewed from above downward, is reduced stepwise to a minimum value delta_t_min. Let us now assume that the time interval resulting from the calibration, delta_t_start, is selected in such a way that a pressure wave in the rail caused by pilot injection  205  has decayed again by the time main injection  200  is triggered. Such values are known beforehand in the form of empirical values. Let us furthermore assume that time difference delta_t_min between the injections represented by the lowermost curve corresponds to a minimum time interval in which the pressure wave caused by pilot injection  205  already results in a measurable change in a performance quantity, preferably in a change in the torque of the engine. 
   Of course, the two injections depicted in  FIG. 3  are only for illustration purposes, and therefore the method according to the present invention is also applicable to the calibration of a plurality of injections over time; even individual successive pilot injections may be influenced as described here because of the pressure waves. 
   The above-mentioned pressure wave effect may be explained with reference to  FIG. 3  as follows. If pilot injection ‘VE’  205  is separated from main injection ‘HE’  200  by a sufficiently long time interval, i.e., in this case by the interval delta_t_start, the pressure wave triggered by it has already decayed by the time of main injection  200  and therefore no longer has any effect on the fuel quantity injected during the main injection. Because of the wave velocity, which is, as is known, pressure-dependent, this time interval is essentially a function of the instantaneous pressure in the rail, among other things. An empirically ascertained suitable starting value for delta_t_start is &gt;2 ms. If the above-mentioned time interval is now varied by keeping the start of the main injection triggering constant but moving the time of the pilot injection closer to the main injection, the main injection quantity will be influenced starting at a certain time interval since, because of the pressure wave, the pressure, in particular in the area of the injector nozzle needle shown in  FIG. 2  at the time of and during opening of the nozzle needle, is either increased due to a wave crest or reduced due to a wave valley. This results in a quantity effect or torque effect, which may be sensed via a speed signal of the engine, for example. Alternatively, the quantity effect may also be sensed, as is known, via a lambda sensor or its controller. 
   The pressure wave correction according to the present invention is performed by the following steps:
         a. In a system simulation, the quantity waves are determined for a certain number of combinations of pilot injections, main injections, and rail pressures;   b. the quantity waves are adjusted by a sum of two sine functions (see  FIG. 4 , where the quantity wave in the 800 bar rail pressure and a function thus adjusted plotted against time are depicted);   c. the parameters of the sine function, i.e., for example, the frequency, amplitude, damping, and zero point displacement, for example, may be almost fully represented as a function of the pressure and/or of the pilot injection quantity and/or the main injection quantity, for example; these functions are also adjusted;   d. the functions ascertained in points b. and c., and possibly other non-correlatable quantities, are stored in the memory of control unit  60 ;   e. the quantity is then corrected in the control unit as follows: The requested main injection quantity, pilot injection quantity, time difference, and rail pressure are used to determine the actual quantity. The quantity request is corrected accordingly. To achieve higher accuracy, this procedure may be iteratively repeated.