CN107084062B

CN107084062B - Method and device for operating an internal combustion engine with dual fuel injection

Info

Publication number: CN107084062B
Application number: CN201710073946.4A
Authority: CN
Inventors: C.旺德林; R.埃克尔; T.库恩; T.霍尔曼; U.舒尔茨
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2016-02-12
Filing date: 2017-02-10
Publication date: 2022-03-04
Anticipated expiration: 2037-02-10
Also published as: JP2017141834A; CN107084062A; DE102016202168A1

Abstract

The invention relates to a method and a device for operating an internal combustion engine having a dual intake-pipe-based and direct fuel distribution, wherein the amount of fuel respectively required for the intake-pipe-based and direct fuel distribution is calculated by means of a quantity distribution, and the internal combustion engine further has a variable compression of a fuel/air mixture in at least one combustion chamber of the internal combustion engine, wherein, in particular, it is provided that the current operating conditions of the internal combustion engine, including the currently existing compression (405), are detected (405, 410), and a value for the quantity distribution is determined (420) which is dependent on the detected operating conditions.

Description

Method and device for operating an internal combustion engine with dual fuel injection

Technical Field

The invention relates to a method and a device for operating an internal combustion engine having a dual fuel metering and a variable compression of the fuel/air mixture in the combustion chamber. A computer program, a machine-readable data carrier for storing the computer program, and an electronic control unit, by means of which the method according to the invention can be carried out, are also subject matter of the present invention.

Background

In the dual fuel distribution, the intake pipe injection and the direct injection are coupled or operated in parallel in the fuel distribution of the internal combustion engine. It is known from practice that such internal combustion engines can be designed as dual systems, in which, in hybrid operation, fuel can be supplied in divided amounts to the cylinders of the internal combustion engine in parallel by means of intake manifold injection (SRE) and by means of direct fuel injection (BDE). The apportioned quantity describes the division of the fuel into a quantity of fuel which can be injected by means of the intake manifold and delivered to the cylinders and a further quantity of fuel which can be injected directly by means of the fuel.

DE 102010039434 a1, for example, describes determining the distribution of the internal combustion engine in the described hybrid operation taking into account operating points, such as load and/or rotational speed. This hybrid operation with the respectively specifically implemented apportionment therefore allows optimum operation of the internal combustion engine for different operating conditions. Optimal mixture formation and combustion is achieved by taking advantage of both injection regimes. The BDE is therefore more advantageous when the internal combustion engine is running dynamically or at full load, since the known self-ignition of the combustion chamber charge (so-called "knocking") can thereby be avoided. On the other hand, in the case of SRE, the exhaust gas load with particles and/or Hydrocarbons (HC) is advantageously reduced during part-load operation of the internal combustion engine.

Knock control is used to suppress or avoid the knock mentioned above by incrementally shifting the ignition time back when knock is detected and then returning to the previous ignition time. The earliest possible ignition time is determined by means of a standard fuel and stored in a characteristic map. The time profile of the combustion can thereby be thermodynamically optimized.

DE 10258872 a1 also discloses an internal combustion engine without a so-called dual fuel metering, but with variable compression or variable compression ratio of the fuel/air mixture in the combustion chamber or cylinder, in which the position of the crankshaft can be variably adjusted by means of eccentric rings which can be rotated relative to the motor housing by means of an adjusting mechanism, so that the so-called "compression volume" and thus also the compression ratio can be varied. As an alternative to the adjusting mechanism, the compression ratio can also be changed by tilting the motor block relative to the crankshaft bearing arrangement or by tilting the cylinder head relative to the motor block or by raising or lowering the cylinder head relative to the motor block.

Variable compression can also be achieved by camshaft adjustment by means of a camshaft phase adjuster (phase adjustment) or by cam switching by means of a variable valve drive. The compression can also be varied by a variation of the closing moment of an inlet valve arranged at the cylinder of the combustion engine.

The variable compression described makes it possible to increase the thermodynamic efficiency during combustion in the partial load range, thereby achieving the energy consumption advantage and the reduction of CO2 emissions. However, as the compression ratio increases, the compression end temperature also increases, wherein the knocking tendency increases again as the compression end temperature increases. Thus, the maximum possible compression ratio is limited by the tendency of the fuel to knock. As the speed increases and the load decreases, the tendency for knocking decreases and therefore a higher compression can be selected.

Disclosure of Invention

The invention relates to a method and a corresponding device for fuel distribution in the double fuel distribution of an internal combustion engine, which has the described variable compression (puffer) of a fuel/air mixture in the combustion chamber of the internal combustion engine. The present invention is based on the recognition or technical effect that the described fuel injection system with a direct fuel distribution (BDE) in an operating range in which knocking is limited, which operating range has an earlier and therefore more favorable fuel mass conversion rate for the fuel, can be operated as a fuel injection system with an intake-pipe-based fuel distribution (SRE), since in BDE operation the combustion center of gravity, which can be adjusted or controlled by the ignition angle or by the ignition timing, can be selected earlier than in SRE.

The invention is based on the recognition that the type of fuel metering used, i.e. the BDE operation or the SRE operation or both in combination with the metering, has a great influence on the knock behavior during combustion and therefore provides a great margin for reducing fuel consumption and reducing exhaust emissions caused by combustion.

The real technical reason for this effect is that the knocking tendency in BDE operation can be reduced compared to SRE operation in that the fuel undergoes an increased mixture cooling and therefore a faster cooling by the direct conversion of the fuel into the combustion chamber and by the increased turbulence generated by the kinetic energy of the injected fuel particles or fuel droplets brought in by the BDE operation. This effect is advantageous thermodynamically, above all during operation of the internal combustion engine at higher loads, and in particular enables a reduction in fuel consumption.

The method according to the invention is based on the solution that a suitable or even optimum value for the quantity distribution is determined depending on the current operating point of the internal combustion engine, the current compression ratio epsilon (epothilone), the ambient conditions and/or the early state of the preceding or final combustion. The stated possible advantages in terms of mixture preparation in SRE operation are combined with the stated advantages of higher knock insensitivity of BDE operation. The early cases of the last combustion also include current and/or previous transient behavior or behavior of the internal combustion engine or of the injection system during transient operation, that is to say, for example, a sudden torque request or a sudden load change as a result of the driver's intention.

The method according to the invention particularly proposes that the quantity distribution of the dosed fuel between the intake-pipe-based fuel dosing (SRE) and the direct fuel dosing (BDE) is adjusted as a function of the current variable compression or displacement. Preferably, in the presence of higher compression and simultaneously higher load, the quantity distribution is shifted in the direction of the relatively higher BDE metering and the ignition angle is additionally adjusted further forward. The technical effect described here is based on the fact that the knock limit in BDE operation can be shifted forward, as a result of which the efficiency of combustion can be advantageously increased in the present knock-limiting operation. In this case, the ignition angle is preferably not adjusted forward, but rather an inefficient rearward adjustment of the ignition angle for reducing knocking can be dispensed with, as a result of which the motor can be operated in an optimum operating point.

It is to be noted here that the technical details in the realization of the variable compression ratio are not important in the present invention, and that the method according to the invention can therefore be used in all technically possible implementations of variable compression with the advantages described here.

In particular, the method according to the invention provides that the current compression or compressor pressure is detected (for example, read by a motor controller) and the current load state of the internal combustion engine is detected (for example, also read by the controller). If both such detected values of compression and load exceed empirically predeterminable thresholds, the metering is changed in the direction of the relatively high BDE fuel distribution.

As an alternative to a discrete calculation or adaptation of the quantity distribution based on the threshold value, a continuous calculation or adaptation of the quantity distribution, more precisely in relation to the compression ratio, can also be carried out. The respective values, for example the optimum values for the quantity distribution, can be stored in the respective characteristic map, specifically with regard to the compression, the rotational speed, the load, etc.

In the case of the described comparison of the compression and the load detected for the operation of the internal combustion engine, relevant environmental conditions, such as the motor and/or the intake air temperature, can additionally be taken into account, since these operating parameters have a significant influence on the knock behavior of the internal combustion engine.

Furthermore, the method according to the invention can be used for individual fault analysis of the cylinders. If, for example, an irregular change in the compression occurs in all cylinders at the same time, it can be concluded that, for example, the adjusting mechanism for variable compression mentioned at the outset has become defective. On the other hand, it can be concluded that, when an irregular deviation of the compression ratio epsilon occurs in only one cylinder, for example, the gas exchange valves and/or piston rings are not sealed at the relevant cylinder.

In the method according to the invention, it can also be provided that the knock characteristic of the internal combustion engine is preferably stored in a characteristic map, a reference characteristic map, an application reference characteristic map, etc., in relation to different operating states, quantity distributions, indicated early states of combustion and/or (variable) compression values of the internal combustion engine or of the injection system. The values stored there can be learned in a manner known per se by suitable algorithms during operation of the internal combustion engine. In the event of the described fault, the throughput allocation or the corresponding allocation factor ("division factor") can thus react quickly when the knock limit is taken into account or counteract the corresponding fault state.

Alternatively or additionally, the compression ratio epsilon currently present during operation can be directly derived by simply comparing the knock limit currently determined during continuous operation of the internal combustion engine with the (reference) values stored in the characteristic map. The idea is based on the idea that the compression ratio can be determined by the change in the throughput allocation or the corresponding allocation factor.

The values of the knock limit and the division factor in the operating point can be stored in the characteristic diagram in a compression-dependent manner. If one now determines the knock limit by means of a change in the division factor, the current or actually existing compression can be deduced by comparing the expected knock limit with the knock limit determined in this way.

With the described method for determining the compression ratio epsilon, a diagnostic function for the described variable compression can be implemented, which is either performed cyclically, for example once per operating cycle, or only when required, for example when there is a combustion problem such as knocking or combustion interruption.

The proposed adaptation or shifting of the distribution factor thus enables the method according to the invention to operate the internal combustion engine with the double fuel distribution system described here less prone to knocking than in the prior art. The combination of the advantages of variable compression and variable fuel distribution results overall in lower consumption, lower exhaust emissions and better driving or operating comfort. As already mentioned, SRE operation contributes here to better mixture preparation and BDE operation contributes to higher knock insensitivity.

The calculation according to the invention for the two fuel quantities for the intake manifold-based fuel metering and for the direct fuel metering is preferably carried out for each cylinder of the internal combustion engine, to be precise continuously or in succession.

The invention can be used in particular in the dual fuel injection system of an internal combustion engine of a motor vehicle. Furthermore, it is also possible to use internal combustion engines with such dual fuel injection in industrial fields, for example in chemical engineering.

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 computer or a controller. This makes it possible to carry out the method according to the invention on an electronic control unit without structural changes having to be made to this electronic control unit. For this purpose, a machine-readable data carrier is provided, on which a computer program according to the invention is stored.

The electronic control unit according to the invention is obtained by running the computer program according to the invention on an electronic control unit, which is provided to control the double fuel metering in an internal combustion engine with variable compression by means of the method according to the invention.

Other advantages and design aspects of the invention will appear from the description and the accompanying drawings.

The features mentioned above and those yet to be explained below can of course be used not only in the respective combinations described, but also in other combinations or alone without departing from the framework of the invention.

Drawings

Fig. 1 shows a schematic illustration of a dual fuel injection system for a four-cylinder internal combustion engine according to the prior art;

FIG. 2 shows schematically the time course of the fuel injection during the fuel intake pipe injection according to the prior art;

FIG. 3 shows schematically the time course of the fuel injection during the direct fuel injection according to the prior art;

fig. 4 shows an embodiment of the method according to the invention by means of a flow chart;

fig. 5 shows an exemplary embodiment of a (reference) characteristic diagram according to the invention, which contains data which have been determined beforehand as to the knock characteristic of the internal combustion engine as a function of different operating states of the internal combustion engine or of the injection system.

Detailed Description

The internal combustion engine shown in fig. 1 has four cylinder blocks 11, which are covered by a cylinder head 12. In each cylinder 11, a cylinder head 12 delimits, together with a reciprocating piston, not shown here, guided in the cylinder 11, a combustion chamber 13 having an inlet controlled by a feed valve. The inlet forms a junction of the feed passage through the head 12.

The fuel injection device shown comprises an air flow path 18 for conveying combustion air to the combustion chamber 13 of the cylinder 11, which has flow channels 17 leading to the individual feed channels, which are separated from one another at the end side. Further, a first group of fuel injection valves 19 that directly inject fuel into each of the one combustion chambers 13 of the cylinder 11 and a second group of fuel injection valves 20 that inject fuel into the flow passage 17 are provided.

The first group of fuel injection valves 19, which are directly injected into the cylinder 11, are supplied from a high-pressure fuel pump 21, and the second group of fuel injection valves 20, which are injected into the flow passage 17, are supplied from a low-pressure fuel pump 22. The low-pressure fuel pump, which is usually arranged in a fuel tank 23, delivers fuel from the fuel tank 23 on the one hand to the second group of fuel injection valves 20 and on the other hand to the high-pressure fuel pump 21. The injection times and injection durations of the

fuel injection valves

19, 20 are controlled by an electronic control unit integrated in the engine controller as a function of the operating point of the internal combustion engine, wherein the fuel injection is essentially carried out by the first group of fuel injection valves 19, and the second group of fuel injection valves 20 is used only in a supplementary manner in order to improve the inadequacy of the direct fuel injection by the first group of fuel injection valves 19 in a specific operating range and to utilize an additional degree of freedom or injection strategy.

The fuel injection valves 20 of the second group are designed as multi-beam injection valves which simultaneously inject or inject at least two separate fuel beams which are angularly offset from one another and are arranged in the air flow path 18 in such a way that the injected

fuel beams

24, 25, which usually have the form of a cone spray, reach different flow channels. In this internal combustion engine, two dual-

beam injection valves

26, 27 are provided, which are arranged in the air flow path 18 in such a way that: so that one dual beam injection valve 26 injects into the flow channel 17 leading to the first and second cylinders 11 and the second dual beam injection valve 27 injects into the flow channel 17 leading to the third and fourth cylinders 11. For this purpose, the flow channels 17 are designed such that a filling point for the dual-

beam injection valve

26 or 27 is present between two directly adjacent flow channels 17.

It is to be noted that, in the internal combustion engine shown in fig. 1, in most cases, one (shown) fuel injection valve 19 and one (only two shown here) dual-

beam injection valve

26, 27 are arranged at each of the four cylinders 11.

It is also known that, when the fuel intake manifold of the internal combustion engine mentioned here is injected, an air-fuel mixture is produced outside the combustion chamber in the intake manifold. The injection valves each inject fuel before the intake valve, wherein the mixture flows in the intake system through the open intake valve into the combustion chamber. The fuel supply is effected by means of a fuel delivery module which delivers a required fuel quantity with a defined pressure from a tank to the injection valve. The air control unit is responsible for providing the correct air mass for the internal combustion engine in each operating point. An injection valve arranged at the fuel dispenser doses the desired amount of fuel into the air flow precisely. The motor controller regulates the respective required air-fuel mixture on the basis of the torque as a central reference variable. Effective exhaust gas purification is achieved by lambda regulation, by means of which the stoichiometric air/fuel ratio (lambda = 1) is always regulated.

Accordingly, when the fuel is injected directly, an air-fuel mixture is formed directly in the combustion chamber. Here, fresh air flows in through the intake valve, wherein fuel is injected into this air stream with a high pressure (typically 200 bar). This achieves an optimum swirl of the air-fuel mixture and better cooling of the combustion chamber.

It is also known that in a four-stroke internal combustion engine (gasoline motor), the working cycle comprises the processes of suction, compression, work, exhaust, wherein each cylinder moves up and down twice and is stationary in two top dead centers (OT) and two bottom dead centers (UT) here. The crankshaft thus executes two revolutions in one working cycle, whereas the camshaft executes one revolution. The ignition of the gas-fuel mixture introduced into the cylinder takes place at the top dead center, where the mixture is just compressed. Ignition top dead center (ZOT) is mentioned here. Accordingly, there is also an overlap top dead center (Ü OT) at which both the intake and exhaust valves are opened at the transition from exhaust to intake.

Ignition is therefore carried out directly after start-up at least in the cylinder at all top dead centers (OT), wherein at a specific top dead center, in particular at every second top dead center, a respective shift of the ignition time takes place at a crankshaft angle of 720 °. Depending on whether the air-fuel mixture is actually ignited at top dead center (OT), at which the ignition time shift is performed, or at a crank angle shifted by 360 °, a reduction of the physical work performed in each cylinder can be ascertained.

In fig. 2, the y-direction represents intake pipe injection at different rotational speeds of the internal combustion engine over a crank angle (KW) measured in units of "degrees". The four-stroke combustion cycle on the gasoline motor principle is known to comprise a crank angle between a first bottom dead center (UT 1), a first top dead center (OT), another bottom dead center (UT 2) and another top dead center (ZOT),

at this crank angle, the air-fuel mixture present in the combustion chamber is ignited.

The reference marks of the times are predetermined to be very different for the two injection paths. In the case of intake manifold injection (SRE), as schematically shown in fig. 2, with injection 200 at only four different rotational speeds n = 1000, 2000, 4000 and 7000U/min, for example, a constant time delay portion 205 provided before the end 210 of the injection cycle 225 is taken into account, since the injection valves are arranged outside the respective combustion chambers of the internal combustion engine in the case of SRE and the fuel therefore has to first enter the combustion chambers from the injection point. As can be seen in fig. 2, this additional time requirement is not changed when the rotational speed of the internal combustion engine changes or increases. The injection is therefore triggered correspondingly earlier, for example at 7000U/min even before UT1, which follows the ignition in the preceding ZOT 220, in time, so that a constant time requirement 205 is provided at all rotational speeds. The injection window for the total time of the injection cycle shown corresponds to the bracket 225 drawn as already mentioned. The next ZOT following the previous ZOT 220 is labeled with reference numeral 215.

In contrast, in the case of gasoline direct injection (BDE), in each injection 300, a (specific) angle marking is empirically predetermined as a reference marking, as is schematically shown in fig. 3. This means that, in contrast to SRE, a constant time share is not taken into account in BDE, as is shown, for example, by the trend 305 of the end of each injection. In this case, therefore, injection can take place close to the ignition event of ZOT 315 and therefore correspondingly at a later time. In the present example, the end 310 of the injection cycle 325 shown here is followed by the firing at the next ZOT 315. The time of ignition prior to this ZOT 315 occurs at the previous ZOT 320.

The internal combustion engine mentioned here also comprises the variable compression described in said DE 10258872 a1, in which compression the crankshaft

The eccentric ring is mounted in the motor housing by means of the eccentric ring, which is itself mounted in a rotatable manner in a bearing in the motor housing. The eccentric ring can be rotated in a controlled manner by means of an adjusting mechanism, whereby the position of the crankshaft relative to the motor body changes. The position of the piston in the cylinder of the internal combustion engine and the position of the piston in the cylinder, the so-called top dead center (OT), change as a result, and the compression volume VC contained in the piston top dead center position by the piston changes as a result. Since the bottom dead center position of the piston changes accordingly, the stroke volume VH does not change when the crankshaft position changes relative to the motor body. Therefore, a change in the compression volume when the stroke volume VH is constant implies a change in the compression ratio = (VH + VC)/VC.

By means of the variable compression, the thermodynamic efficiency can be increased during combustion in the partial load range. As a result, consumption advantages and reduction of CO2 emissions may be achieved. The higher the compression ratio, the higher the compression end temperature, wherein the risk of ignition (knocking) of the combustion chamber charge itself in a gasoline motor increases as the compression end temperature increases, so the maximum possible compression ratio is limited by the knocking tendency of the fuel.

In the dual system for fuel metering mentioned here, the two portions described, namely the SRE portion and the BDE portion, are combined as is known in the form of a system or system components. In particular, the total fuel mass to be provided or metered needs to be correctly distributed. The total fuel mass KMges for a cylinder is composed of:

，

wherein KM_SREReferring to the relative fuel quality of the SRE path and KM_BDERefers to the relative fuel mass of the BDE path. The corresponding procedure for calculating or assigning the fuel mass required for injection in such a dual system is explained next with the aid of the flow chart shown in fig. 4.

The following formula is carried out in this exemplary embodiment continuously or successively for all cylinders of the internal combustion engine, specifically for the i-th cylinder in the present case.

After the start-up 400 of the program, the current values of the compression 405 and the load 410 are detected. Further combustion-determining variables, in particular the currently present ambient conditions, such as air pressure and/or air temperature, are additionally detected and taken into account in the next step. In step 415, it is checked whether the detected compression 405 and the at least one further detected variable 410, which is currently the load, exceed a threshold value, which is empirically predefined in the preparation phase in each case. These thresholds are determined by: so that said knocking at the time of combustion is effectively avoided. If this condition is not met, a jump is made back to the beginning of the program.

It is noted here that as an alternative to the discrete matching described, a continuous matching may also be performed.

However, if condition 415 is met, the fuel quantity distribution is shifted 420 toward a higher BDE injection, i.e., the arithmetic ratio KM derived from the two variables mentioned above_BDE / KM_SREI.e. the so-called "allocation factor", is increased by a value which is also empirically predetermined. It can additionally be provided that the ignition time or the ignition angle of the internal combustion engine does not have to be moved 425 backwards.

The BDE injection described 440 and the 445 SRE injection for the remaining fuel quantity are then carried out for the current i-th cylinder on the basis of the thus changed distribution factor or on the basis of the apportioned fuel quantity of the BDE path. Followed by the next, i.e., currently executing 450 the illustrated program 405- > 445 for the (i + 1) th cylinder.

It is to be noted that the exact value of the quantity apportionment, the exact threshold value of the change in the ignition point of time, and the actual value are not important at present, since these values are each associated with the respective internal combustion engine or the fuel metering used.

Fig. 5 shows an exemplary embodiment of a characteristic diagram according to the invention, in which data 505 relating to the knock behavior of the internal combustion engine, measured by tests or determined empirically, are stored beforehand as a function of different operating states 500 of the internal combustion engine or of the injection system. The operating state comprises in this example the value KM of said quantity allocation_BDE / KM_SREThe compression value epsilon and the value of the early state of the combustion, more precisely the piston temperature T in the present example_kIn this example in units of "° C". It is to be noted here that the piston heats up only after a few seconds at the earliest and even after a few minutes when there is a load jump from a low motor load to a high motor load. In this case, at the beginning of a high load situation, the still cooler piston is wetted with fuel, which is therefore gasified too slowly, which in turn leads to an increase in the particle emissions and/or HC/CO emissions, to be precise for a period of time which is as long as: until the piston is sufficiently heated.

It is also to be noted that the values of these variables stored in the characteristic map can also be learned by suitable algorithms during operation of the internal combustion engine.

In the event of a fault as shown in fig. 4, the respective fault situation can thus be countered quickly by targeted variation of the distribution factor assigned to the quantity. The currently existing compression ratio epsilon can also be derived by comparing the knock limit currently determined during operation of the internal combustion engine with a reference value stored in the characteristic map.

The described method 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 operating an internal combustion engine having a dual fuel dosing, wherein an intake pipe-based fuel dosing (20) or a direct fuel dosing (19) or a combination of an intake pipe-based fuel dosing (20) and a direct fuel dosing (19) is used, wherein the amount of fuel respectively required in the intake pipe-based fuel dosing and in the direct fuel dosing is calculated by means of a dosing, and the internal combustion engine further has a variable compression of a fuel/air mixture in at least one combustion chamber of the internal combustion engine, characterized in that a current operating condition of the internal combustion engine, including the currently existing compression (405), is detected (405, 410), and a value of the mentioned dosing in relation to the detected operating condition is determined (420), wherein, the variables characterizing the knock behavior of the internal combustion engine are stored in a characteristic map in relation to different operating states of the internal combustion engine or of an injection system of the internal combustion engine, to the quantity distribution, to the early state of the combustion and to variable compression values, wherein a compression ratio epsilon currently present in the operation of the internal combustion engine is deduced by comparing the knock limit currently determined in the operation of the internal combustion engine with the knock value stored in the characteristic map.

2. A method according to claim 1, in which said operating conditions of the internal combustion engine, in addition to the current compression ratio epsilon, also comprise the current operating point of the internal combustion engine and/or the current load and/or ambient conditions and/or earlier circumstances concerning the combustion in the combustion chambers of the internal combustion engine.

3. A method according to claim 2, wherein said early condition of said combustion comprises current and/or previous transient characteristics of said internal combustion engine or of said fuel dosing portion.

4. A method according to any of claims 1 to 3, characterized in that upon detection of compression exceeding a predeterminable first threshold value and upon additional detection of a load exceeding a predeterminable second threshold value, the dose is moved (420) discretely or continuously in the direction of the elevated BDE fuel dosing.

5. The method according to claim 4, characterized in that the current ignition angle or ignition time of the internal combustion engine is not adjusted backwards (425).

6. A method as set forth in claim 1, characterized in that the values of the knock limit and of the division factor in the operating points of the internal combustion engine are stored in the characteristic map in relation to the compression, the knock limit being determined by a change in the division factor, and the compression actually occurring being deduced by comparing the expected knock limit with the knock limit thus determined.

7. Method according to claim 1 or 6, characterized in that a diagnostic function for variable compression is implemented by determining the compression ratio ε, which diagnostic function is executed cyclically or when needed.

8. Method according to any one of claims 1 to 3, characterized in that a calculation of two fuel quantities for an intake pipe-based fuel distribution and for a direct fuel distribution is performed for each cylinder of the internal combustion engine.

9. Method according to claim 8, characterized in that the calculation of the two fuel quantities for the intake-pipe-based fuel distribution and for the direct fuel distribution for each cylinder of the internal combustion engine is performed continuously or successively.

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

11. An electronic control unit which is provided for controlling the dual fuel metering by means of the method according to any one of claims 1 to 9.