DE102023211701B4 - Method, computing unit and computer program for adapting the control duration of an injector - Google Patents

Abstract

Method for adapting a control duration of at least one injector for an internal combustion engine, comprising the steps:
1) Setting a predetermined operating point of the injector;
2) Controlling the injector with a predetermined control duration (ti _DEM );
3) Determination of an actual injection quantity (Q _REAL );
4) Determination of a control duration difference (Δti _DEM , Δti) based on the actual injection quantity (Q _REAL );
5) Filtering the determined control duration difference (Δti _DEM , Δti) and determining a new control duration (ti _INJ ) based on the previous control duration (ti _DEM , ti _INJ ) and the filtered control duration difference (Δti _FIL );
6) Control the injector with the new control duration (ti _INJ );
7) Repeating steps 3) to 6) until at least one predetermined termination criterion is reached, wherein in step 5) at least one filter parameter (150) for filtering the determined control duration difference (Δti _DEM , Δti) is set depending on a current control duration difference (Δti) and/or a number of repetitions (LCC1 - LCC3) and/or the predetermined operating point (OP ₁ - OP ₃ ).

Description

The present invention relates to a method for adapting a control duration of at least one injector for an internal combustion engine as well as a computing unit and a computer program for carrying out the method.

Background of the invention

Current combustion engines can utilize micro- and multiple-injection systems to reduce emissions and increase efficiency. Injectors used for such injection systems must deliver the required injection quantity with high precision. For this reason, the injection quantity of each injector can be calibrated during production, and the calibrated injection quantity can be kept constant throughout its service life by adapting the injector's control duration. The control duration can be adapted while the injector is installed and the combustion engine is operating, particularly when the engine is in overrun mode.

DE 10 2017 117 677 A1 shows a method for calibrating an injector in a common rail injection system, in which fuel is injected into a cylinder assigned to the injector during overrun of an internal combustion engine, and a pressure drop in a closed high-pressure section of the common rail injection system is determined at several points in time. From the determined pressure drop, a withdrawal quantity of the fuel withdrawn from the high-pressure section is determined, from which in turn a quantity of fuel injected into the combustion chamber is determined. Based on the determined injection quantity, the actuation duration of the injector can be corrected. Such a calibration can be performed continuously during overrun phases of a vehicle until a stable correction value is reached. However, depending on the driving profile (number of overrun phases), this can require a considerable distance to be driven.

Current emissions legislation now requires calibration/adaptation of the injectors of an injection system within the first 1000 miles of a vehicle with the aim of avoiding violations of emission limits and abnormalities in terms of idle quality and noise.

From the document DE 102 35 105 A1 A method is known that relates to the operation of an internal combustion engine in which fuel is injected into a combustion chamber via an injection valve. Cylinder equalization compensates for the effects of different injection valves in different cylinders. Two parameter sets are provided for cylinder equalization: a first parameter set for normal operation and a second parameter set for abnormal operation, e.g., after manufacturing or repair.

The DE 102 53 739 B3 The disclosed method for speed control of an internal combustion engine calculates a first and a second filtered actual speed from an actual speed using two filters, and from these two control deviations. During dynamic state changes (e.g., load changes), a speed controller calculates a power-determining signal (e.g., injection quantity) from both control deviations, with the second control deviation being decisive. This increases the dynamics of a control loop and enables the use of clutches with a lower natural frequency. A second filter is software-based and can be integrated into existing engine control software.

From the disclosure document DE10 2009 047 898 A1 A fuel injection control system is known that detects combustion timing in a cylinder and calculates a target combustion timing. A correlation map in a memory links physical quantities with a combustion timing sensitivity and a control gain. The control gain is determined from a map using a detected physical quantity. A command injection timing is calculated based on the control gain and sent to an injector.

The DE 10 2019 200 903 A1 discloses a method for determining a correction value for fuel metering of an injector. This involves determining a relationship between a pressure difference in a high-pressure accumulator during an injection process and an injection duration. Two correction values are calculated: a first with a slow learning rate and a second with a fast learning rate. If both values deviate, an error is detected, and the learning rate of the first value is increased. The second value is used to detect injector replacement or installation and is determined after the engine is started.

Disclosure of the invention

According to the invention, a method for adapting the actuation duration of at least one injector for an internal combustion engine, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

The present invention makes it possible to accelerate the control duration adaptation and to adapt new part variations of the injectors within a short driving distance in such a way that a control duration-injection quantity ratio is substantially correct.

In the method according to the invention, a predetermined operating point of the injector is set in a first step. This can occur in particular when no power is required from the internal combustion engine, for example because a vehicle driven by the internal combustion engine is in overrun mode. The predetermined operating point is characterized in particular by an injection pressure of the injector. In other words, to set a predetermined operating point of the injector during overrun mode of a vehicle, a predetermined injection pressure can be set in a high-pressure accumulator of an injection system, such as a so-called rail or common rail.

Subsequently, in a second step, the injector is activated with a predetermined activation duration. It is also possible for multiple injectors of an injection system to be activated sequentially with the same predetermined activation duration.

In a third step, an actual injection quantity is determined. According to one embodiment, the injection quantity can be determined based on an injection pressure drop by controlling the injector. The actual injection quantity can be determined in a known manner from an injection pressure drop or fuel pressure drop in the injector or in a high-pressure accumulator of the injection system. For this purpose, the high-pressure accumulator can be closed during the injection(s), i.e., no fuel is pumped into the high-pressure accumulator during the injection(s), and no fuel is withdrawn from it except for the injection quantity.

Based on the determined injection quantity, the control duration difference is determined in a fourth step. According to one embodiment, this can be done by first determining an injection quantity difference between a target injection quantity and the actual injection quantity, and then, based on the determined injection quantity difference, determining the control duration difference. A relationship between the injection quantity and the control duration can be derived from a characteristic curve or characteristic map of the injector, which can be stored, for example, in a control unit of the internal combustion engine.

Alternatively, the control duration difference can be determined by first determining a target control duration of the actual injection quantity, i.e. a control duration associated with the actual injection quantity, e.g. from the injector characteristic curve or the injector characteristic map, and then determining the control duration difference based on a difference between the determined target control duration and the predetermined control duration.

Furthermore, in a fifth step, the determined control duration difference is filtered, and a new control duration is determined based on the previous control duration and the filtered control duration difference. This can be done, for example, by adding the filtered control duration and the previous control duration. Subsequently, in a sixth step, the injector is controlled with the new control duration.

The third to sixth steps are repeated in a further seventh step until at least one predetermined termination criterion is met. In the fifth step, at least one filter parameter is set for filtering the determined control duration difference depending on a current control duration difference, a number of repetitions, and/or the predetermined operating point.

The control duration difference can be filtered, for example, using a PT ₁ , PT ₂ , ..., PT _n filter or a moving average filter. When using a PT _i filter (i = 1, .., n), the filter parameter can be a filter time constant, for example. When using an average filter, this can be a number of determined control duration differences over which the average is calculated.

According to one embodiment, the predetermined termination criterion is met when a predetermined injection quantity difference is reached or undershot. In particular, several predetermined injection quantity differences can be defined depending on the predetermined operating point. Instead of the injection quantity difference, one or more predetermined control duration differences can also be defined as the termination criterion. Alternatively or additionally, the predetermined termination criterion can be met when a predetermined number of repetitions of steps 3 to 6 (adaptation runs) is reached or exceeded.

According to one embodiment, a first filter parameter is set if the number of repetitions is less than or equal to a predetermined value. If the number of repetitions is greater than the predetermined value, a second filter parameter can be set that is greater than the first filter parameter. For example, the first filter parameter can be set for the first ten repetitions of steps 3 to 6, and the second filter parameter can be set from the eleventh repetition onwards. As a result, the determined control duration difference is initially filtered less intensively, and the newly determined control duration changes more significantly, as long as the number of repetitions does not exceed the predetermined value. In this way, the control duration or the control duration difference can be quickly approximated to a target value range. If the control duration/control duration difference is then in the target value range after the predetermined number of repetitions has been exceeded, the determined control duration difference can be filtered more intensively in order to hit the target value as accurately as possible.

Alternatively or additionally, the first filter parameter is adjusted if a current control duration difference is greater than at least a predetermined threshold. The predetermined threshold can, for example, correspond to an average variation exhibited by the injectors in their new condition due to manufacturing tolerances. This allows injectors with a higher manufacturing tolerance to be adapted with the first filter time constant even after exceeding the predetermined number of repetitions, thus reaching the target range more quickly. Furthermore, this criterion enables faster adaptation of larger injection quantity deviations that can occur over the service life of an injector due to wear or similar factors.

According to one embodiment, a plurality of operating points can be set, and the adaptation of the control duration can be performed using steps 2 to 7 described above at each of the set operating points. In particular, the injection pressure of the injector can be set differently at each operating point. For example, the adaptation can take place at three different injection pressures, e.g., at 400 bar, 900 bar, and 1400 bar. Any other injection pressure, particularly those at which very small or multiple injections occur during engine operation, is also possible. Furthermore, the adaptation of the control duration can take place multiple times at each of the operating points.

According to one embodiment, the operating points can be adjusted according to a predetermined sequence. The sequence can be determined depending on the injection pressure, in particular from low to high injection pressure. This allows the injector's injection quantity to be adjusted first at operating points where significant injection quantity deviations exist, allowing potential emissions impairments to be eliminated early on.

According to one embodiment, the predetermined sequence of operating points can be varied depending on the current control duration difference and/or the number of repetitions at the respective operating points. If, for example, after a first adaptation of the control duration according to steps 1 to 7 described above, the remaining deviation at all operating points is, for example, higher at 1400 bar than at 900 bar, the predetermined sequence can be deviated from and, during a further adaptation, the operating point at 1400 bar can be set before the operating point at 900 bar. The same applies, for example, if the number of repetitions of steps 2 to 5 is, for example, lower at 900 bar than at 400 bar. In this case, during a further adaptation, the operating point at 900 bar can first be set before the operating point at 400 bar. This ensures that the injection quantity is adjusted evenly across all operating points.

According to one embodiment, a value of the first filter parameter and a value of the second filter parameter can be adjusted based on the respective operating point. In other words, for example, both the smaller/shorter and the larger/longer filter time constant can be additionally adapted to the conditions of the respective operating point.

Using the process steps described above, the time required to reach a steady-state control duration correction can be significantly reduced, and it can be ensured that operating points with significant deviations in the injection quantity are given priority. This allows the requirements of current emissions legislation to be met.

A computing unit according to the invention, e.g. a control unit of the internal combustion engine, is configured, in particular in terms of programming, to carry out a method according to the invention.

The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for other tasks and is therefore already present. Suitable data carriers for providing the computer program are, in particular, magnetic, Optical and electrical storage devices, such as hard disks, flash memory, EEPROMs, DVDs, etc. It is also possible to download a program via computer networks (Internet, intranet, etc.).

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

The invention is illustrated schematically in the drawings using exemplary embodiments and is described in detail below with reference to the drawings.

Character description

• The 1a and 1b each schematically show a block diagram of a control scheme for adapting a control duration of at least one injector according to an embodiment of the invention.
• The 2a to 2d show schematically an adaptation of a control duration of six different injectors in three different operating points, which are determined by means of the control schemes from the 1a and 1b can be carried out.

Detailed description of the drawing

1a shows a schematic block diagram of a control loop or control scheme for adapting a control duration of an injector according to an embodiment of the invention, as it can be implemented in particular in a control unit of an internal combustion engine.

The control loop comprises several function blocks 100 to 140 and several operators A1 to A3, wherein a function block 140 has a filter whose filter parameters 150 can be varied. The filter can be, for example, a _PT1 filter with a variable filter time constant T _FIL as filter parameter 150. The filter parameter 150 is set depending on a current control duration difference and/or a number of adaptation repetitions and/or depending on a predetermined operating point at which the adaptation is performed.

In a first functional block 100, a target activation duration (predetermined activation duration) ti _DEM is determined based on input variables injection pressure p _INJ and target injection quantity Q _DEM , wherein the aforementioned input variables define the operating point. For example, the injection pressure p _lNJ can be 400 bar, 900 bar, or 1400 bar, and the target injection quantity Q _DEM can be 2 mm ³ . In particular, the adaptation can take place during overrun of a vehicle with an internal combustion engine (not shown).

To determine the target control duration ti _DEM , an injector characteristic curve or an injector characteristic map can be stored in the function block 100, for example, on the basis of which the target control duration ti _DEM can be determined as a function of the injection pressure p _INJ and the target injection quantity Q _DEM . At the start of the adaptation, the injector (not shown) is controlled with the target control duration as the injector control duration ti _INJ and an injection pressure drop Δp _INJ in a high-pressure accumulator (not shown) of the injection system is determined in a function block 110. It is also possible for several injectors of an injection system (not shown) to be controlled one after the other with the target control duration ti _DEM and for an injection pressure drop to be determined. For this purpose, a fuel pressure in the high-pressure accumulator can be measured during the injection(s) or before and after the injection(s) using a pressure sensor, such as a so-called rail pressure sensor (not shown), and the measurement signal p _MEAS can be fed to function block 110. Based on the determined pressure drop Δp _INJ , an actual injection quantity Q _REAL is then determined in a further function block 120, in particular knowing the volume of the high-pressure accumulator and the temperature of the fuel. For this purpose, for example, a general volume-pressure relationship of the fuel can be used, the parameters of which can be stored in function block 120.

The determined actual injection quantity Q _REAL is subtracted from the target injection quantity Q _DEM using an operator A2, and the injection quantity difference ΔQ _REAL is fed to a subsequent function block 130 as an input variable. This function block can, in turn, store the injector characteristic curve or the injector characteristic map, which can be used to determine a control duration difference Δti _DEM based on the measured injection pressure p _MEAS and the injection quantity difference ΔQ _DEM .

The thus determined control duration difference Δti _DEM is subsequently filtered in the filter 140 and the filtered control duration difference Δti _FIL is fed back and added to the target control duration ti _DEM by means of an operator A1. The injector is then controlled with the new control duration ti _INJ and in the function block 110 the The injection pressure drop Δp _INJ or the pressure drop in the high-pressure accumulator is determined, from which the actual injection quantity Q _REAL is subsequently determined in block 120, the injection quantity difference ΔQ _DEM is determined by A2, and the control duration difference Δti _DEM is determined in block 130. The latter is added to the filtered control duration difference Δti _FIL by means of an operator A3 and fed to the filter 140 as the new control duration difference Δti _FIL .

This process is repeated until a predetermined termination criterion is reached. For example, the predetermined termination criterion can be reached when a predetermined injection quantity difference ΔQ _DEM is reached or undershot. Alternatively or additionally, the predetermined termination criterion can be reached when a predetermined number of repetitions of the described process is reached or exceeded.

The filter parameter of filter 140 can vary, for example, depending on the number of repetitions. For example, a first filter parameter 150-1 can be set if the number of repetitions is less than or equal to a predetermined value (e.g., 10 repetitions). If the number of repetitions is greater than the predetermined value, a second filter parameter 150-2 can be set that is greater than the first filter parameter 150-1. As a result, the determined control duration difference Δti is initially filtered less strongly, and the newly determined control duration ti _INJ changes more significantly as long as the number of repetitions does not exceed the predetermined value. In this way, a rapid approximation of the control duration ti _INJ or the control duration difference Δti to a target value range can be achieved. If the control duration ti _INJ /control duration difference Δti is then in the target value range after exceeding the predetermined number of repetitions, the determined control duration difference Δti can be filtered more strongly in order to hit the target value as accurately as possible.

The 1b The control loop shown differs from the one in 1a shown in that the actual injection quantity Q _REAL determined in function block 120 is not subtracted from the target injection quantity Q _DEM , but is fed directly to the following function block 130, in which a target activation duration ti _REAL of the actual injection quantity Q REAL, i.e. a activation duration ti _REAL associated with the actual injection quantity Q _REAL , is determined using the injector characteristic curve or the injector characteristic map. In this case, in addition to the actual injection quantity Q _{REAL ,} the measured injection pressure p _MEAS can also be taken into account. The activation duration difference Δti _DEM is then determined using an operator A2' by subtracting the determined target activation duration ti _REAL of the actual injection quantity Q _REAL from the target activation duration ti _DEM . All other functions of the 1b The control loop shown is identical to that shown in 1a described.

The 2a to 2d show schematically an adaptation of a control duration of six different injectors in three different operating points, which are derived from the control loops described above. 1a and 1b can be carried out.

This shows 2a the injection pressure p _lNJ over a time t, and the 2b to 2d show a curve of the control duration difference Δti _1-6 for six injectors of an injection system at three different operating points OP ₁ - OP ₃ , which are defined by the injection pressure p _lNJ and each set twice (OP _{1_1} to OP _{2_2} ). In addition, 2b to 2d a counter for the number of repetitions LCC1 - LCC3 of the adaptation runs as well as a time window LF1 - LF3 are shown, in which the number of repetitions is below a predetermined value of 10 and the filtering of the control duration difference Δti _1-6 is carried out with a first, shorter filter parameter 150-1 than during the remaining time period.

In the first operating point OP ₁ , the injection pressure is 400 bar, in the second operating point OP ₂ , the injection pressure is 900 bar, and in the third operating point OP ₃ , the injection pressure is 1400 bar. The illustrated control duration adaptation of the six injectors begins in the operating point OP _{1_1} with the lowest injection pressure of 400 bar ( 2c ), in which the injectors show the highest deviations (see scaling of the vertical axes in the 2b to 2d ).

During the time window LF1, the control duration difference Δti _1-6 is adapted using the first filter parameter, resulting in significant changes in the control duration difference Δti _1-6 of all six injectors during this time window LF1. After ten adaptation runs, the second filter parameter 150-2 is set, which results in only small changes in the control duration differences Δti _1-6 , so that after a few more repetitions, the corresponding termination criterion—namely, the undershooting of a predetermined injection quantity difference—is reached.

According to a predetermined sequence of operating points depending on the injection pressure p _lNJ, the second operating point OP _{2_1} is subsequently set with an injection pressure of 900 bar ( 2b) . Also in this operating point OP _{2_1} , the filtering of the control duration differences Δti _1-6 for the first ten adaptation runs is carried out with the first filter parameter 150-1 (see time window LF2), the value of which can be adapted to the second operating point OP ₂ with the higher injection pressure p _INJ and can therefore differ from a value of the first filter parameter 150-1 in the first operating point OP _1. However, in the operating point OP _{2_1} , after leaving the time window LF2, a larger number of repetitions with a second filter parameter 150-2 take place in order to bring the control duration differences Δti _1-6 as close as possible to a target value. The second filter parameter 150-2 can also differ in the second operating point OP ₂ from that in the first operating point OP _1. In this case, for example, the number of repetitions LCC2 can be used as a termination criterion.

Subsequently, the third operating point OP _{3_1} is set with an injection pressure p _INJ of 1400 bar ( 2d ) and the adaptation during the first ten runs is carried out with a first filter parameter 150-1 (see time window LF3), which in turn can differ from the first filter parameters 150-1 in the first and second operating points OP ₁ , OP ₂ .

After leaving the time window LF3, the adaptation is repeated until the control duration differences Δti _1-6 or the injection quantity differences ΔQ _DEM fall below a predetermined value. This value can be lower than the one used as the termination criterion in the first operating point OP ₁ , since smaller deviations occur at higher pressures. It is clear that in the third operating point OP _{3_1} , after approximately ten further adaptation runs with the second filter parameter 150-2, only very small changes in the control duration differences Δti _1-6 occur.

Thereafter, according to the predetermined sequence of the operating points, an adaptation is carried out again in the first operating point OP _{1_2} at 400 bar injection pressure, wherein the second filter parameter 150-2 is set to filter the control duration difference Δti _1-6 , since during the previous termination of the adaptation runs in the first operating point at t = t _{SOP1_1} , no injector had a noticeable control duration difference Δti _1-6 above a threshold value, which would require a rapid adaptation. Contrary to the predetermined sequence, however, after the second setting of the first operating point OP _{1_2} , the third operating point OP _{3_1} is set, since during the first setting of the second operating point OP _{2_1} a higher number of adaptation runs had already been carried out than during the first setting of the third operating point OP _{3_1} (LCC2=22 > LCC3=20 when the adaptation is aborted in the operating point OP _{1_2} at t = t _{SOP1_2} ). Here too, due to the lack of noticeable control duration differences Δti _1-6 , the second filter parameter 150-2 is set and further adaptation runs are carried out in which the control duration differences Δti _1-6 of the individual injectors no longer change significantly. The second operating point OP _{2_2} is then set again and here too further adaptation runs are carried out with the second filter parameter 150-2. Since the control duration difference Δti _x of an individual injector changes even more significantly (change Δti _x marked with an arrow), the adaptation remains active longer in the second operating point OP _{2_2} than in all previous operating point settings OP _{1_1} to OP _{3_2} .

The presented examples clearly demonstrate that the adaptation method presented can significantly shorten the time required to achieve a steady-state control duration correction. Furthermore, it can ensure that operating points with significant deviations in the injection quantity are given priority, thus meeting the requirements of current emissions legislation.

Claims (15)

Method for adapting an actuation duration of at least one injector for an internal combustion engine, comprising the steps: 1) setting a predetermined operating point of the injector; 2) actuating the injector with a predetermined actuation duration (ti _DEM ); 3) determining an actual injection quantity (Q _REAL ); 4) determining an actuation duration difference (Δti _DEM , Δti) based on the actual injection quantity (Q _REAL ); 5) filtering the determined actuation duration difference (Δti _DEM , Δti) and determining a new actuation duration (ti _INJ ) based on the previous actuation duration (ti _DEM , ti _INJ ) and the filtered actuation duration difference (Δti _FIL ); 6) actuating the injector with the new actuation duration (ti _INJ ); 7) Repeating steps 3) to 6) until at least one predetermined termination criterion is reached, wherein in step 5) at least one filter parameter (150) for filtering the determined control duration difference (Δti _DEM , Δti) is set depending on a current control duration difference (Δti) and/or a number of repetitions (LCC1 - LCC3) and/or the predetermined operating point (OP ₁ - OP ₃ ). Procedure according to Claim 1 , wherein determining the control duration difference (Δti _DEM , Δti) based on the actual injection quantity (Q _REAL ) comprises: determining an injection quantity difference (ΔQ _DEM ) between a target injection quantity (Q _DEM ) and the actual injection quantity (Q _REAL ); and determining the control duration difference (Δti _DEM , Δti) based on the injection quantity difference (ΔQ _DEM ). Procedure according to Claim 1 , wherein determining the control duration difference (Δti _DEM , Δti) based on the actual injection quantity (Q _REAL ) comprises: determining a target control duration (ti _REAL ) of the actual injection quantity (Q _REAL ); and determining the control duration difference (Δti _DEM , Δti) based on a difference between the target control duration (ti _REAL ) of the actual injection quantity (Q _REAL ) and the predetermined control duration (ti _DEM ). Method according to one of the preceding claims, wherein in step 5) a first filter parameter (150) is set if the number of repetitions (LCC1 - LCC3) is less than or equal to a predetermined value, and a second filter parameter (150) is set if the number of repetitions (LCC1 - LCC3) is greater than the predetermined value, wherein the second filter parameter (150) is greater than the first filter parameter (150). Procedure according to Claim 4 , wherein the first filter parameter (150) is set when a current control duration difference (Δti) is greater than at least one predetermined threshold value. Method according to one of the preceding claims, wherein the predetermined termination criterion is reached when a predetermined injection quantity difference is reached or undershot and/or a predetermined number of repetitions (LCC1 - LCC3) is reached or exceeded. Method according to one of the preceding claims, wherein a plurality of operating points (OP ₁ - OP ₃ ) are set and steps 2) to 7) are carried out in each of the set operating points (OP ₁ - OP ₃ ). Procedure according to Claim 7 , whereby the operating points (OP ₁ - OP ₃ ) are set according to a predetermined sequence. Procedure according to Claim 8 , wherein the predetermined sequence of the operating points (OP ₁ - OP ₃ ) is varied depending on the current control duration difference (Δti) and/or the number of repetitions (LCC1 - LCC3) in the respective operating points (OP ₁ - OP ₃ ). Method according to one of the Claims 7 until 9 , wherein a value of the first filter parameter (150) and a value of the second filter parameter (150) are set based on the respective operating point (OP ₁ - OP ₃ ). Method according to one of the Claims 7 until 10 , whereby an injection pressure (p _INJ ) of the injector is set differently at each operating point (OP ₁ - OP ₃ ). Method according to one of the preceding claims, wherein the injection quantity difference is determined based on an injection pressure drop (Δp _INJ ) by the control of the injector. Computing unit comprising a processor configured to carry out the method according to any one of the preceding claims. Computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to Claim 1 until 12 to execute. Computer-readable data carrier on which the computer program is Claim 14 is stored.