CN108071510B

CN108071510B - Method and device for calibrating the injection quantity of a partial injection in an injection system of an internal combustion engine

Info

Publication number: CN108071510B
Application number: CN201711106762.XA
Authority: CN
Inventors: L.恩帕赫
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2016-11-11
Filing date: 2017-11-10
Publication date: 2022-11-25
Anticipated expiration: 2037-11-10
Also published as: CN108071510A; DE102016222196A1

Abstract

The invention relates to a method and a device for calibrating the injection quantity of at least one partial injection of fuel, which is provided in addition to a main injection, in an injection system of an internal combustion engine having at least two combustion chambers, wherein, in particular, a single injection having only the main injection (546, 547, 548, 549) and a multiple injection having at least one additional partial injection (510, 520, 530, 540) are alternately carried out in all combustion chambers (550) of the internal combustion engine (635), and a correction value for the injection quantity is determined (620, 640) for the at least one partial injection (510, 520, 530, 540) by evaluating a rotational speed oscillation (555) in the internal combustion engine (635) caused by the multiple injection.

Description

Method and device for calibrating the injection quantity of a partial injection in an injection system of an internal combustion engine

Technical Field

The invention relates to a method and a device for calibrating the injection quantity of a partial injection in an injection system of an internal combustion engine. The invention also relates to a computer program, a machine-readable data carrier for storing the computer program, and an electronic control device, by means of which the method according to the invention can be carried out.

Background

In modern fuel injection systems, in particular of self-igniting internal combustion engines, the fuel quantity injected into the combustion chamber by means of an injector is divided into partial injections, which are arranged close in time and consist, for example, of one or more pilot injections applied before a main injection. The time interval between two partial injections is defined here by the pause time (Pausenzeit) of the injector between two electrical control pulses that follow one another in time, by the crankshaft angle, or by a combination of these two variables.

The partial injections lead to improved mixture preparation and thus to lower exhaust emissions of the internal combustion engine (BKM), a reduction in the development of noise generated during combustion and an increase in the mechanical output of the BKM. In order to optimize both noise and exhaust gas, the injection quantity of the pilot injection is selected to be very small, in part in the vicinity of the minimum fuel quantity that can be indicated by the injector in question.

In order to be able to ensure accurate compliance with the pre-injection quantity over the operating time range of the injector in the event of a drift in the injector behavior, the following functions are necessary: this function enables the pilot injection quantity to be calibrated while the BKM is still running. From DE 10 2008 043 A1, a method and a device for calibrating the injection quantity of a pilot injection in an injection system of a BKM are therefore known, in which method correction values for the pilot injection into the individual cylinders of the BKM are determined by excitation of the injection pattern and by changes in the rotational speed oscillations caused by the injection pattern.

Calibration of the pilot injection Quantity, in particular in the field of Commercial Vehicles (CV), is also known here by evaluating the rotational speed using a so-called Zero Quantity Calibration ("Zero Fuel Quantity Calibration Low Idle" = ZFL) in Idle operation. The ZFL is determined by various tolerance effects, which are technically difficult to achieve.

In addition, a function ZML ("M", for example, a Monitor) is present, which is not a calibration of the pilot injection quantity, but rather a monitoring according to on-board diagnostics (OBD) specified in the united states, wherein individual injectors, whose injection quantity in the current injection cycle exceeds a predefined exhaust gas or emission limit, must be able to be identified only in two test cycles. In the current U.S. project, in addition, a Multi-cylinder fault (Multi-Zylinder-Fehler) must be reliably detected by means of the "Multi ZML" function, i.e., all injectors in the system must be adjusted (vertimemen) in the same way with respect to the new partial injection quantity (Neuteil-einsipitzmingen), wherein the detection of the exceeding of the exhaust gas limit must furthermore be carried out in only two test cycles. However, according to the prior art, the cylinders are also checked individually here, as in the ZFL. Since, in the identification of a multi-cylinder fault, all injectors can be faulty in the worst case, the amount of the individual injectors that can be tolerated is significantly reduced compared to the ZFL. With the aid of complex algorithms, the average quantity shift in the injection system is inferred from the information of the individual cylinders.

Disclosure of Invention

The invention is based on the following idea: for the calibration of the injection quantities of the partial injections referred to here, the injection patterns referred to are alternated simultaneously at all combustion chambers of the BKM for the purpose of the OBD referred to.

In the method according to the invention for calibrating the injection quantity of fuel of at least one partial injection provided in addition to a main injection of fuel in an injection system of a BKM having at least two combustion chambers, it is therefore provided, in particular, that a single injection having only a main injection and a multiple injection having at least one partial injection provided in addition are carried out alternately at all combustion chambers of the BKM, and that a correction value "tiVEcorr" for the injection quantity is obtained for the at least one partial injection by evaluating the rotational speed oscillations caused by the multiple injection in the BKM.

In the method according to the invention, provision can furthermore be made for single injections with only a main injection and multiple injections with additional at least one partial injection to be carried out alternately at least twice at all combustion chambers of the BKM, wherein in each of the at least two multiple injections the injection quantity of the fuel of the at least one partial injection is varied and the correction value "tiVEcorr" is determined for the at least one partial injection by evaluating the rotational speed oscillations of the BKM resulting from the at least two multiple injections.

The method according to the invention has the following advantages: the ZML measurement mentioned at the beginning can be carried out simultaneously at a plurality of, preferably all, combustion chambers or cylinders of the BKM, and in particular the multi-cylinder faults mentioned at the beginning can thus also be detected in order to meet the more stringent OBD requirements mentioned thereby.

In addition, the "multi ZML" function mentioned can be operated significantly more reliably, since on the one hand the multi-cylinder fault detection is accelerated and on the other hand the signal/noise ratio is significantly improved. The improvement in the signal/noise ratio results in particular from the following: the method according to the invention makes it possible to compensate at least partially for the influence of tolerances of individual cylinders (hermonisttetln) and to significantly increase the signal strength of the measurement signals compared to the above-mentioned ZML functions, which are each carried out at a single injector.

According to a further aspect of the method according to the invention, it can also be provided that the amplitude and/or phase of the rotational speed oscillations are minimized or set to a minimum. In addition, it can be provided that the actuation duration of the at least one partial injection is changed for so long that a certain, preferably empirically predeterminable, frequency influence on the rotational speed signal is minimized or set to zero.

According to another aspect, it can be provided that the switching between single injection performed at all combustion chambers of the BKM and multiple injection performed at all combustion chambers of the BKM is performed at a fraction of a camshaft frequency of 0.5.

In the device according to the invention for calibrating the injection quantity of at least one partial injection of fuel in an injection system of a BKM having at least two combustion chambers, a computer or a control unit is provided, by means of which a single injection having only a main injection and a multiple injection having at least one partial injection provided in addition to the main injection are carried out alternately at all combustion chambers of the BKM, and by means of which a correction value for the injection quantity can be determined for the at least one partial injection by evaluating a rotational speed oscillation caused by the multiple injection in the BKM.

The computing means or the control means can have a regulator for regulating the rotational speed oscillation to a minimum value, to be precise as a function of the acquired correction value.

The invention can be used with the advantages mentioned to calibrate the injection quantity of a partial injection of fuel in an injection system, in particular of a BKM for a motor vehicle. It should be noted here that the injection process referred to here can also comprise at least one pilot injection and/or post injection in addition to the main injection. The partial injection and the respectively metered injection quantity of the main injection are in particular dependent on the actuation duration with which the injection valve associated with the combustion chamber is actuated by the actuation signal. In this case, the injection nozzle of the injection valve is opened and the fuel injection quantity resulting from the actuation duration is injected into a combustion chamber, which is embodied, for example, as a cylinder.

The computer program according to the invention is provided for carrying out each step of the method, in particular when the computer program is run on a computing or control device. The computer program enables the method according to the invention to be carried out on an electronic control unit without structural changes having to be made on this electronic control unit. For this purpose, a data carrier is provided which can be read by a machine and on which a computer program according to the invention is stored. By running the computer program according to the invention on an electronic control unit, an electronic control unit according to the invention is obtained, which is provided for controlling the injection system referred to here by means of the method according to the invention.

Further advantages and embodiments of the invention emerge from the description and the enclosed drawing.

Drawings

Fig. 1 shows a schematic representation of a fuel metering system of an internal combustion engine according to the prior art, in which the present invention can be used.

Fig. 2 shows a calculated detail of the actuation duration of the electrically actuated valve shown in fig. 1 according to the prior art.

Fig. 3 shows a typical injection profile with a pilot injection and a main injection, by means of which a rearrangement (umlagergeng) of the injection quantities between the pilot injection and the main injection according to the invention is described.

Fig. 4a, b schematically show the injection profile typically occurring in the aforementioned zero quantity calibration (ZFL) in idle mode and the corresponding rotational speed, to be precise at the correct pilot injection quantity (fig. 4 a) and at an offset pilot injection quantity (fig. 4 b).

Fig. 5 schematically shows the injection curves and the rotational speeds that occur in the injection model according to the invention corresponding to fig. 4a and 4 b.

Fig. 6 shows a preferred embodiment of the calibration or correction function according to the invention according to a combined flow chart/block diagram.

Detailed Description

DE 10 2008 043 A1 discloses a method for calibrating the injection quantity of a pilot injection in an injection system of a BKM for an internal combustion engine, wherein a correction value for a partial injection into the respective cylinder of the BKM is determined by excitation of an injection model and by a change in the rotational speed oscillations caused by the injection model. The control algorithm is used to "rearrange" the set pilot injection quantity between the pilot injection and the main injection for the cylinder with a camshaft frequency of half, one third, one fourth, one fifth, etc., i.e., the cylinder is alternately loaded with a single injection and a multiple injection, preferably a dual injection consisting of the pilot injection (VE) and the main injection (HE), and the correction value for the pilot injection or partial injection is determined by a change or adjustment of the rotational speed oscillations of the BKM caused by the injection model. The following technical effects are based on here: in the event that the pilot injection quantity is not yet or only incompletely calibrated, oscillations of the camshaft frequency occur with half, two thirds (drei halber) etc. of the rotational speed signal, which oscillations can preferably be set to a minimum by means of a regulation.

The actuation duration of the partial injection, preferably the pilot injection, is preferably changed in the method mentioned so long as the influence on the rotational speed signal, which is determined and preferably can be predetermined empirically, is minimized or set to zero. The rotational speed oscillations mentioned can be detected not only by means of a rotational speed signal but also by means of other inputs that are customary to the person skilled in the art, such as a Lambda signal, a knock signal, an ion flow signal or the like. The minimization of the rotational speed oscillations is preferably achieved either by means of the amplitude and/or the phase of the rotational speed oscillations mentioned or by means of a combination of these two variables and is preferably obtained by a value and a phase of the rotational speed information converted into a frequency range.

Fig. 1 shows a block diagram of the main elements of such a fuel metering system of an internal combustion engine (BKM) 10. The BKM10 receives a specific quantity of fuel metered at a specific point in time from the fuel metering unit 30. Various sensors 40 detect the measured values 15 that characterize the operating state of the BKM and transmit these measured values to the control instrument 20. In addition, the various output signals 25 of the other sensors 45 are transmitted to the control device 20. Starting from these measured values 15 and the further quantities 25, the control device 20 calculates a control pulse 35, which is applied to the fuel metering unit 30.

Fig. 2 shows a likewise known device for controlling the metering of the fuel mentioned, wherein the elements already described in fig. 1 are designated by corresponding reference numerals. The signal arrival of the sensor 45 and of other sensors not shown reaches the predetermining element 110. This quantity presetting element 110 calculates a fuel quantity QKW corresponding to the driver's wish. This quantity signal QKW arrives at the first junction 115, on the second input of which the output signal QKM of the second synchronization element 155 is applied. The output signal of the first junction 115 reaches the second junction 130, which second junction 130 in turn loads the control duration calculation element 140. The signal QKO of the zero correction element 142 is applied to a second input of the second junction 130. At the two

junction points

115 and 130, the quantity signals are preferably additively combined. The actuation duration calculation element 140 calculates an actuation signal for loading the fuel metering unit 30, starting from the output signal of the junction 130. It should be noted that the calibration value of the zero correction element 142 as a time value which is already present or stored in [ ms ] can also be added after the actuation duration calculation element 140. The actuation duration calculation element calculates an actuation duration, which is applied to the electrically actuated valve.

On the sensing wheel (Geberad) 120, different markers are arranged, which are scanned by a sensor 125. In the exemplary embodiment shown, the sensor wheel is a so-called segment wheel (Segmentrad) which has a number of markings corresponding to the number of cylinders, which in the exemplary embodiment shown is four. This sensor wheel is preferably arranged on the crankshaft of the BKM, not shown. This means that a number of pulses is generated per engine revolution, which number corresponds to double the number of cylinders. The sensor 125 sends a corresponding number of pulses to the first synchronization element 150. The first synchronization element 150 loads the first, second, third and

fourth regulators

171, 172, 173 and 174. The number of regulators corresponds to the number of cylinders. The output signals of the four regulators then arrive at the mentioned second synchronization element 155.

Fig. 3 schematically shows a typical injection pattern consisting of a pilot injection and a main injection. This injection pattern is repeated after two revolutions of the camshaft. The upper graph shows the injector manipulation, wherein the flow regulation (stromregilung) is omitted. The following diagram then shows the fuel flow through the injector nozzle, resulting from the mentioned actuation, delayed in time. The area under the respective curve of the fuel flow corresponds here to the respective injected fuel quantity. It should be noted that the rearranged injection model of the pilot injection relative to the main injection, which is described here, or the changeover between the respective single injection and the multiple injection, is merely exemplary, and that in principle other injection models can also be envisaged by means of which the mentioned rotational speed oscillations can be excited. Thus, instead of a pilot injection, the post-injection can be rearranged correspondingly or a more complex "rearrangement model" can be produced in which more than one partial injection is rearranged.

In the embodiment shown in fig. 3, the fuel amount 300 set for the pre-injection is shifted to the fuel amount 305 to be added to the main injection 310. However, it should be understood that other forms of rearrangement are possible, such as rearranging the amount of fuel pre-injected with a post-injection that is temporally separate from the main injection. Since no rotational speed oscillations occur only in the following cases: in which the amounts of

fuel

300, 305 that are physically set (absetzen) during the periodic rearrangement are identical. However, as already explained with reference to fig. 1, it is possible for: the electrically actuated valves of the injectors measure different fuel quantities at the same actuation signal. Thus, no automatic guarantee is provided that the two

fuel quantities

300 and 305 are identical when the actuation duration is identical. If the injector is offset or if no calibration method is learned, the

fuel quantities

300, 305 are of course different and the rotational speed oscillations described above occur. The adjustment of the calibration method therefore aims at setting these rotational speed oscillations to a minimum or detecting phase jumps (phasejumping). The control variable for this is a corrective intervention for the actuation duration of pilot injection 300.

The injection patterns that are applied for the four-cylinder BKM assumed for the invention for the four cylinders "1" to "4" and for two

successive camshaft rotations

410, 415, respectively, are schematically shown in the

upper parts

400, 500 of fig. 4 and 5, respectively. The rotational speed curves or correspondingly approximated rotational speed oscillations obtained in the case of the respective injection model are shown in the

lower part

405, 505 of each of fig. 4 and 5.

According to the prior art illustrated in fig. 4a, in this case, during a measurement cycle comprising two

camshaft rotations

410, 415, only for a single injector of the injection system, to be precise of the injector currently arranged at the second cylinder "2", an injection pattern of

additional pilot injections

425, 445 is alternated, which injection pattern has

main injections

420, 430, 435, etc. or 440, 450, 455, etc. In the assumed case in fig. 4a, i.e. when the pilot injection quantity and ZFL application for the calibration are correctly set, in each case the

same speed profile

460, 465 or 470, 475 or the like is obtained in both injection patterns for a complete camshaft revolution, wherein the speed profile, in particular when actuating the second cylinder "2", corresponds to the remaining speed profiles in the manner shown. Therefore, substantially the same torque is generated in all the cylinders "1" to "4". Therefore, a uniform rotational speed curve is currently obtained for the effective frequency (Nutzfrequenz) "0.5 × f _ camshaft", and thus no rotational speed oscillations occur. The injection quantities and the corresponding resulting torques are compared at an effective frequency corresponding to half the camshaft frequency, since these differ when switching between the two injection models.

In contrast, fig. 4b shows an injection situation with

main injections

480, 490, 491, etc. or 492, 494, 495, etc. and

pilot injections

485, 493, wherein a rotational speed oscillation 498 with a total amplitude according to the double arrow 499 results as a result of the relatively increased

rotational speeds

496, 497.

Such rotational speed oscillations 498 can have different causes. As a result, the

pilot injection quantities

485, 493 can be offset due to the operating duration of the respective injectors, or there can be an intentional manipulation of the pilot injection quantities for the purpose of simulating an OBD exhaust gas limit within the framework of an OBD detection algorithm. The reason can also be an adjustment of the injection characteristic field (verimmung) for calculating the fuel quantity to be injected as a function of the actuation duration of the respective injector, or a ripple (Welligkeit) of the corresponding characteristic curve for the individual injector to be investigated accordingly.

Fig. 5 now shows an exemplary injection situation, in which the injection pattern is alternated at all four cylinders "1" to "4" of the four-cylinder BKM at the same time, corresponding to the method according to the invention. Thus, in the region 550, which already includes two camshaft revolutions,

pilot injections

510, 520, 530, 540 are additionally applied in all

main injections

515, 525, 535, 545, whereas in the second camshaft revolution only

main injections

546, 547, 548, 549 are applied, i.e. no pilot injection is applied. In the first half 550, the relatively increased

rotational speeds

551, 552, 553, 554 shown in the lower part of fig. 5 are thus obtained, and therefore a rotational speed oscillation 555 with a significantly increased overall amplitude 560 (which is labeled 499 in fig. 4 b) in relation to fig. 4b is obtained.

It should be noted that the method shown in fig. 5 is not applicable to a conventional ZFL in which the pre-injection amount is adjusted or calibrated individually for the cylinder. More precisely, in the injection method according to the invention, an average image of all injectors in the current injection system is obtained. However, such an average image is now necessary when the multi-cylinder fault is currently addressed for identification.

Further, it should also be noted that an invention significantly shortens the duration for executing the identification algorithm by measuring all cylinders or injectors simultaneously. On the other hand, as already mentioned, the recognition quality that can be achieved in principle with the ZFL based on statistically distributed, individual cylinder characteristic curve ripples is also significantly improved. Since such fault contribution values are at least partially reduced or statistically compensated by measuring all cylinders simultaneously. Finally, with the proposed method, the measurement signals of the individual cylinders are advantageously added without the noise being intensified.

Fig. 6 shows an embodiment of the method or device according to the invention by means of a combined flow chart/block diagram. On the two dashed

lines

600, 605, at the inlet of the "quantity-actuation duration-conversion 610", there are nominal quantities qHEsoll and qVEsoll for the main injection and the pilot injection. These nominal quantities are provided in a known manner by a "torque-to-quantity conversion" based on the nominal torque (not shown). Additionally, on the data path 615, depicted by a dashed line, a main injection of the correction function ZFX620 described below is provided with a sculpted (afgepr 228gte) stimulation qsense at half the camshaft frequency and with a corresponding value qHEstim.

For two camshaft revolutions, as already shown in fig. 5, a multiple injection with a pilot injection with an actuation duration tiVE and a main injection with an actuation duration tiHE is carried out for the first half of the camshaft revolution. In the exemplary embodiment, only a single injection with a main injection with a control duration tie is now carried out for the respectively subsequent second half-camshaft rotation by means of the illustrated switch or changeover switch 625, which is, for example, camshaft-controlled. It should be noted in this case that the mentioned rearrangement of the pilot injection by means of the switch 625 is merely exemplary and can also be implemented in a manner otherwise customary to the person skilled in the art.

The mentioned "quantity-actuation duration-conversion 610" calculates the actuation durations tiVE and tiHE for the pilot injection and the main injection from the values qVEstim and qHEstim. Then, at the schematically illustrated injector 630 of the current four cylinders of BKM, a value tiVEcorr is applied for the actuation duration of this pilot injection (including the corresponding regulating intervention of the calibration function) and an actuation duration value tiHE is applied for the main injection, respectively. These control values are converted, by means of the injector 630, into an actual value qVEphys for the pre-injection quantity to be injected and an actual value qHEphys for the main injection quantity to be injected. Based on these actual values, BKM635, which is likewise only schematically indicated, generates in a known manner during operation a rotational movement of a crankshaft thereof (not shown) which has a defined rotational speed n.

The amplitude and/or phase of the rotational speed oscillations occurring, for example, are detected for the rotational movement of the crankshaft, for example, on the basis of a conventional crankshaft signal. Based on an empirically predefined frequency value for the speed oscillation, the actuation duration of the pilot injection is changed in a known manner for as long as the influence of the predefined frequency value on the speed oscillation is minimized or adjusted to the value zero as possible. A corresponding correction value for the actuation duration of this pilot injection is then derived therefrom. It should be noted that instead of the speed signal, other already mentioned input variables can be used as a basis here.

The amplitude and/or phase of the occurring rotational speed oscillations is evaluated in this exemplary embodiment by means of a spectral analysis 640 at half the camshaft frequency and, alternatively, also at a frequency different from the fundamental frequency of the oscillations. The subsequent flow control of the correction function ZFX620 calculates the logic signal stVEstim for stimulating the injection model at half the camshaft frequency and calculates from the phase and/or the amplitude the actuating intervention tiveltim of the correction function ZFX620, that is to say the corresponding actuating intervention for compensating the assumed injector offset. The control intervention tiVEtrim and the current value tiVE for the actuation duration of the pilot injection are added at a junction 645 in order to thus supply the corresponding injector with a corrected value tiVEcorr for the actuation duration of the pilot injection.

It is understood that the inventive concept can be used not only for calibrating a pilot injection, but also for calibrating a partial injection which is also carried out in addition to a main injection, that is to say for example also for a post-injection or the like.

The method described can be implemented in the form of a control program for an electronic control unit for controlling an internal combustion engine or in the form of one or more corresponding Electronic Control Units (ECUs).

Claims

1. Method for calibrating the injection quantity of at least one partial injection of fuel provided in addition to a main injection in an injection system of an internal combustion engine having at least two combustion chambers, characterized in that a single injection having only a main injection and a multiple injection having an additional at least one partial injection are carried out alternately at all combustion chambers (550) of the internal combustion engine (635), and a correction value for the injection quantity is acquired (620, 640) for the at least one partial injection by evaluating a rotational speed oscillation (555) caused by the multiple injection in the internal combustion engine (635),

wherein for two consecutive camshaft revolutions, multiple injections are performed in all combustion chambers in one camshaft revolution and only a single injection is applied in all combustion chambers in the other camshaft revolution.

2. A method according to claim 1, characterized in that a single injection with only a main injection and a multiple injection with at least one partial injection arranged in addition to the main injection are performed alternately at least twice at all combustion chambers (550) of the internal combustion engine (635), wherein in at least two multiple injections the injection quantity of the fuel of the at least one partial injection is changed separately and a correction value is obtained (620, 640) for the at least one partial injection by evaluating a rotational speed oscillation (555) caused by the at least two multiple injections in the internal combustion engine (635).

3. Method according to claim 1 or 2, characterized in that the amplitude and/or phase of the rotational speed oscillation is minimized or adjusted (645) to a minimum.

4. The method according to claim 3, characterized in that the duration of the manipulation of the at least one partial injection is changed so long until the influence on the determined frequency of the speed signal is minimized or adjusted (645) to zero.

5. The method of claim 1, wherein switching (625) between a single injection performed at all combustion chambers (550) of the internal combustion engine (635) and a multiple injection performed at all combustion chambers (550) of the internal combustion engine (635) is performed at a fraction of a camshaft frequency of 0.5.

6. The method according to claim 1, characterized in that a multi-cylinder fault is identified by evaluating the rotational speed oscillations (555) caused by the multiple injections in the internal combustion engine (635).

7. Apparatus for calibrating an injection quantity of at least one partial injection of fuel provided in addition to a main injection in an injection system of an internal combustion engine having at least two combustion chambers, characterized by a calculation or control means for alternately performing a single injection with only a main injection and a multiple injection with at least one additional partial injection at all combustion chambers (550) of the internal combustion engine (635) and for acquiring (620, 640) a correction value for the injection quantity for the at least one partial injection by evaluating a rotational speed oscillation (555) caused by the multiple injection in the internal combustion engine (635),

8. An apparatus according to claim 7, characterized by an adjuster (645) for adjusting the rotational speed oscillation (555) to a minimum value by means of the acquired correction value.

9. A machine-readable data carrier, on which a computer program is stored, which is arranged to perform each step of the method according to any one of claims 1 to 6.

10. Electronic control unit, which is provided for controlling an injection system of an internal combustion engine having at least two combustion chambers by means of a method according to one of claims 1 to 6.