Method for operating an internal combustion engine and computing unit

Abstract

Method for operating an internal combustion engine (10) with at least two cylinders (11-14) in at least two operating modes, wherein in a first operating mode all cylinders (11-14) are fired and in a second operating mode only some of the cylinders (11-14) are fired, wherein a lambda sum actual value of the internal combustion engine (10) is set to a desired lambda sum setpoint value by means of a lambda controller (52) by adjusting the amount of fuel and / or air supplied to the fired cylinders (11-14), wherein when switching from the first to the second operating mode the lambda controller (52) is subjected to a pilot control value which counteracts a shift in the lambda sum actual value which would occur without the application due to the switching.

Description

The present invention relates to a method for operating an internal combustion engine and to a computing unit for carrying it out.Prior ArtIn spark-ignition engines for so-called homogeneous operation, the air / fuel ratio is set by lambda control in such a way that the mean value of the lambda values of all cylinders is λ=1.0. This enables low-exhaust-gas operation with conventional three-way catalytic converters which are known to have their greatest effectiveness in the case of stoichiometric combustionDue to metering tolerances and cylinder-specific air / filling differences, e.g. due to system tolerances, the lambda values in the individual cylinders of an internal combustion engine may deviate from one another despite identical actuation. The lambda value measured in the exhaust gas, which is also referred to below as the lambda sum actual value and is composed of the contributions of the respective individual cylinders, can therefore assume the setpoint value 1.0, although the lambda individual actual values fluctuate about this average value.For example, Zyl results for the cylinders. 1 to Zyl. 4 of a four-cylinder engine with lambda individual actual values λ Zyl. 1= 1,1, λ Zyl. 2= 0,9, λ Zyl. 3= 1,2 and λ Zyl. 4= 0,8 a lambda sum actual value λ Zyl. 1 ... 4= 1,0. A corresponding deviation of individual cylinders from the mean value (i.e. a imbalance, in relation to the individual cylinders) is referred to as cylinder imbalance in the context of this application.Cylinder imbalance has a number of disadvantages. The trimming of the cylinder-specific lambda value initially leads directly to an increase in the fuel consumption. If the trim exceeds a certain threshold value, the emissions may also deteriorate. In this case, the so-called sturgeiness of the exhaust gas, i.e. the formation of flow filaments in the exhaust gas mass flow, for example on account of different cylinder charges, additionally plays a role. It is desirable and in part demanded by legislators to be able to detect such exhaust gas degradations and / or to be able to compensate by suitable control strategies.From the prior art, different methods for detecting or regulating out cylinder imbalance in homogeneous operation are known.On the one hand, the signal of the lambda probe can be analyzed, wherein this oxygen probe is broken down into individual, cylinder-related values and evaluated. This is described, for example, in WO 96 / 35 048 A1. However, the usefulness of this method is highly dependent on the geometry of the exhaust line and places high demands on the engine and exhaust line design, which often cannot be fulfilled.Speed-based methods, on the other hand, provide for the detection of quantity errors of the engine in lean operation (λ>1). In this case, all cylinders are simultaneously transferred to lean operation and a cylinder-specific feature relating to rough running is evaluated. In contrast to homogeneous operation, in lean operation, the engine torque is linearly correlated with the injection quantity. In order to ensure exhaust gas-neutral operation and to obtain a total lambda of λ=1.0, a late, torque-ineffective postinjection is initiated. The method is therefore not suitable for intake pipe engines. Corresponding methods and further aspects thereof are disclosed and explained, for example, in DE 195 27 218 A1, DE 43 19 677 A1, DE 10 2004 010 412 A1, DE 197 33 958 A1, EP 0 929 794 B1 and DE 10 2006 026 390 A1.All of the above-mentioned methods require a so-called full engine operation in which all cylinders are fired. However, modern engine concepts provide for individual cylinders to be completely deactivated in order to save fuel in the low partial load range, so that combustion no longer takes place therein. Such operation is also referred to as half-engine operation.Therefore, an optimum lambda setting in half-engine or partial engine operation is also desirable.Disclosure of the InventionAgainst this background, the present invention proposes a method for operating an internal combustion engine and a computing unit for carrying it out having the features of the independent patent claims. Advantageous embodiments are the subject matter of the respective dependent claims and of the following description.The measures proposed according to the invention are used within the scope of a method for operating an internal combustion engine having at least two cylinders, which is operated in at least two operating modes. In a first operating mode, all cylinders of the internal combustion engine are fired to generate a corresponding engine torque, i.e., in the usual engine operation, e.g., a four-stroke operation, a fuel / air mixture is applied to and actively ignited or subjected to a self-ignition method. In at least one further operating mode, only a part of the cylinders, optionally also only a single cylinder, are fired.In all operating modes, a lambda sum actual value of the internal combustion engine is set to a lambda sum setpoint value by means of a lambda regulator by adapting the quantity of fuel and / or air supplied to the cylinders (in total) that are respectively fired. This is referred to as global lambda control and is sufficiently known in the prior art. As already partially explained above, the lambda actual value indicates the stoichiometric ratios present in the internal combustion engine. By increasing the respectively supplied fuel quantity with respect to the present air, a corresponding fuel / air mixture can be "enriched", i.e. the lambda value can be shifted to values <1, and conversely, by reducing the supplied fuel quantity, a fuel / air mixture can be "leaned", i.e. the lambda value can be shifted accordingly into a range >1.If a partial engine operation, i.e. a further operating mode in which only a part of the cylinders is fired, is now to be realized, a change in the lambda sum actual value can occur when switching from the first to the second operating mode as a result of the omission of individual cylinders. As explained above, each individual lambda actual value does not necessarily have to correspond to the actual lambda sum value, e.g. if there are metering tolerances and / or cylinder-specific air / filling differences. The individual lambda actual values can therefore deviate from the actual lambda sum value which they form in their entirety. If no additional measures are now taken, a mixture which is too lean or too rich-that is to say an excessively high or excessively low total lambda actual value-can therefore be present directly after the changeover. Such a lambda error worsens the emission behavior of the internal combustion engine and must be corrected.According to the invention, it is therefore provided that, when switching from the first operating mode (the full engine operation) into another operating mode (the partial engine operation), the lambda controller (or its controller output) is supplied with a pilot control value which counteracts a displacement of the lambda sum actual value and preferably compensates it to the greatest possible extent. If the lambda sum actual value is thus increased by a deactivation of a part of the cylinders, a pilot control value can be applied to a lambda controller (or its controller output), which pilot control value correspondingly increases the fuel quantity supplied to the engine overall (and / or decreases the air quantity), so that the desired lambda sum target value (of, for example, the desired lambda sum target value again decreases. 1). Conversely, if a changeover from the first operating mode to the other operating mode would result in a reduction of the lambda value, a pilot control value can be applied to the lambda controller (or its controller output), which pilot control value reduces the fuel quantity (and / or increases the air quantity).The present invention therefore brings about an advantageous pilot control of a global lambda control, with which a shift of the lambda sum actual value can be counteracted. A suitable pilot control value can be determined, for example, by observing a change in the lambda controller output during the changeover. The change is stored as a suitable pilot control value.A particular advantage of the measures according to the invention can be seen in the fact that the controlled variable (i.e. the lambda sum actual value) remains virtually unchanged as a result of the additional intervention at the output of the lambda regulator mentioned. Lambda correction is disadvantageous during operation because the control variables of the lambda controller are frequently monitored by motor vehicle systems and unfavorable adaptation values can lead to (here false positive) error entries in error memories.In a particularly preferred embodiment, in addition to the global lambda control, a cylinder-specific lambda control is also provided. In the cylinder-specific lambda control, a correction factor for correcting the quantity of fuel and / or air supplied to the respective cylinder is determined for each fired cylinder in order also to set a lambda individual actual value of the respective cylinder to a desired lambda individual setpoint value, usually essentially λ=1. In this embodiment, the invention can develop particular advantages. In the case of the certain cylinder-specific lambda controls, the correction factors are namely determined in such a way that they result on average 100% (so that the resulting correction of the overall system by the cylinder-specific adaptation values takes place lambda-neutral; it is precisely no displacement of the lambda sum actual value that is desired). Therefore, it will usually occur that during the changeover the correction factors of the remaining cylinders do not result in 100% and are tracked accordingly. This tracking, however, changes the lambda sum actual value and causes an intervention of the lambda regulator, which is to be avoided.The cylinder-specific lambda control is advantageously carried out when the lambda sum actual value is set to the lambda sum setpoint value by the lambda controller of the internal combustion engine and identical nominal fuel quantities are supplied to the cylinders in each case. In this context, the cylinder-specific deviations can be determined best. The nominal fuel quantities do not necessarily have to correspond to the supplied actual fuel quantities, for example when deviations in the injection behavior of the injection valves are present, nozzle needles are occupied or the like. A setpoint-actual value discrepancy can be a source of cylinder-specific lambda deviations, but does not have to represent the only source. In the first operating mode, i.e. when all cylinders are fired, the fuel quantity supplied to each fired cylinder is adjusted on the basis of the respectively determined characteristic variable by means of an adaptation value (local, i.e. cylinder-specific lambda control) superimposed on the (global) lambda control in such a way that each individual lambda actual value corresponds at least largely to the actual lambda sum value. If an individual cylinder has a lambda value of >1, as explained above, the corresponding fuel quantity is increased accordingly by the adaptation value (and / or the air quantity is reduced). Conversely, the fuel quantity is reduced (and / or the air quantity is increased) if the cylinder-specific lambda value is <1. In other words, the individual lambda values are adapted to the actual lambda sum value in a cylinder-specific manner.The required correction, i.e. corresponding pilot control values for the lambda controller, which are used during the changeover, can be determined, for example, by the internal combustion engine (e.g. in the context of a test operating mode) being transferred from the first operating mode into the other and a shift of the lambda sum actual value effected as a result being determined. The determined shift of the lambda sum actual value can later be taken over directly into a corresponding pilot control within the scope of a control operation.It can also be advantageous to determine the pilot control value by determining an expected shift of the lambda sum actual value on the basis of the correction factors (adaptation values) of the cylinders fired in each case. For this purpose, for example, a characteristic curve can be used in which corresponding deviations of the total adaptation values are converted into lambda correction values. Alternatively or additionally, a correction can be determined from a reaction or actuating variable change of the lambda regulator after (initially uncorrected) switching to the partial or half engine operation.A computing unit according to the invention, e.g. a control unit of a motor vehicle or of an internal combustion engine, is configured, in particular by programming, to carry out a method according to the invention.The implementation of the method in the form of software is also advantageous since this causes particularly low costs, in particular if an executing control device is also used for further tasks and is therefore present in any case. Suitable data carriers for providing the computer program are, in particular, floppy disks, hard disks, flash memories, EEPROMs, CD-ROMs, DVDs, and others. Download of a program via computer networks (Internet, intranet, etc.) is also possible.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.The invention is schematically illustrated in the drawing on the basis of an exemplary embodiment and is described in detail below with reference to the drawing.Brief Description of the DrawingsFIG. 1 shows an internal combustion engine in which aspects according to the invention can be realized, in plan view. FIG. 2 shows an internal combustion engine in which aspects according to the invention can be realized, in a side view. FIG. 3 shows a method in which aspects according to the invention can be realized, in schematic representation.Embodiment(s) of the InventionFIG. 1 schematically shows a detail of a motor vehicle having an internal combustion engine 10 with a fuel system 20, an air supply system 30 and an exhaust system 40, and a computing unit 50 as a control unit for controlling the latter in plan view. The internal combustion engine 10 is preferably designed as a spark ignition engine with direct fuel injection. In the exemplary embodiment shown, the internal combustion engine 10 comprises four cylinders 11, 12, 13, 14, but any other number of cylinders is also possible. Fuel is provided by fuel system 20 and injected via corresponding injectors 21 into cylinders 11, 12, 13, 14, respectively.Air is supplied to the cylinders 11, 12, 13, 14 via the supply air system 30, wherein an inlet valve 31 is provided for each of the cylinders 11, 12, 13, 14. Combustion exhaust gas is expelled from the cylinders 11, 12, 13, 14 via exhaust valves 41 and discharged via the exhaust system 40. In the exhaust system 40, a catalytic converter 42 is provided which converts carbon monoxide and nitrogen oxides, among other things, and is advantageously designed as a three-way catalytic converter.The control unit 50 is operatively connected to actuating elements of the internal combustion engine 10, the fuel system 20, the supply air system 30 and / or the exhaust system 40, in order to actuate the latter in a suitable manner. In detail, the control unit 50 controls, for example, the injection valves 21, the intake valves 31, the exhaust valves 41 and further actuators. In particular, control unit 50 is designed to preset a defined fuel quantity by means of injection valves 21. The control unit 50 can have a lambda controller 52 designed as part of the control unit 50. The control device 50 is designed by programming for carrying out a method according to the invention.Furthermore, suitable sensors, such as in particular a lambda probe 51, which is arranged in the exhaust system 40 upstream of the catalytic converter 42, and also temperature and / or pressure sensors, not shown, are provided in order to detect corresponding engine states, so that the operation of the internal combustion engine 10 can be realized as a function thereof by means of the control unit 50. Lambda probe 51 is configured to detect an oxygen content in exhaust system 40 and transmits this oxygen content or a corresponding value derived therefrom, for example, to lambda controller 52 implemented in control unit 50.The control unit 50 controls the internal combustion engine by means of control commands O or by transmitting corresponding parameters in order to provide a drive torque. For this purpose, control unit 50 receives inputs I that include, for example, external requirements, such as a driver's desired torque, an accelerator pedal position, and the like, with which a drive torque desired can be specified externally. Furthermore, the control device 50 receives from the sensors mentioned corresponding information about engine states as inputs I, for example a rotational speed, pressures and temperatures in the air supply system 30 and / or in the exhaust system 40.In full engine operation, all cylinders 11, 12, 13, 14 of internal combustion engine 10 are active and are fired in a predefined sequence, for example, according to a well-known four-stroke operation, which is not explained in more detail here.Depending on the desired operating state, for example the specification of a driver's desired torque or an operating state of the internal combustion engine 10, such as an idling operation, for example, the control unit 50 can decide that the drive torque should be provided by only one or only one part of the cylinders 11, 12, 13, 14 in each case. In this case, at least one of the cylinders 11, 12, 13, 14 is deactivated and the entire drive torque is still provided only by a fired cylinder 11, 12, 13, 14 or a corresponding set of fired cylinders 11, 12, 13, 14. A corresponding partial deactivation is referred to as partial motor operation. If half of the cylinders 11, 12, 13, 14 are deactivated, this is referred to as half engine operation. Half-engine operation represents the standard of partial engine operation, since it loads the mechanics of internal combustion engine 10 least. In this case, it can also be provided to switch from one set of cylinders 11, 12, 13, 14 to another set of cylinders 11, 12, 13, 14, so that, for example, cylinders 11 and 13 are fired in a first operating mode and cylinders 12 and 14 are fired in a second operating mode.FIG. 2 shows an alternative illustration of the detail from FIG. 1 in a side view, wherein elements corresponding to FIG. 1 are not explained again for the sake of clarity. The illustration of a series of components, in particular of the fuel system 20, of the supply air system 30 and of the exhaust system 40, has been omitted here.In the cylinders 11, 12, 13, 14, pistons 11', 12', 13', 14' are arranged. The gas forces acting on the pistons 11', 12', 13', 14' when the corresponding cylinder 11, 12, 13, 14 is fired are transmitted via piston rods 11'', 12'', 13'', 14'' assigned to the piston rods to a crankshaft 15. In the case of a cylinder imbalance explained above, for example with different fuel quantities, the gas forces acting on the pistons 11', 12', 13', 14' vary and thus also the uniformity of the rotational movement of the crankshaft 15.To determine the rough running, a master wheel 16 is coupled to the crankshaft 15 in a rotationally fixed manner. The rotary motion of the encoder wheel 16 is formed, for example, in a signal 53' of a rotary angle sensor 53. The control unit 50 or a correspondingly provided evaluation module 54 evaluates the signal 53' and determines cylinder-specific values therefrom.The encoder wheel 16, which can be seen in FIG. 2 in a side view, has markings 16' distributed over its circumference. These markings 16' can be, for example, ferromagnetic projections, the flanks of which generate steep flanks in the signal 53' when passing an inductive sensor used as a rotational speed sensor 53. The encoder wheel 16 can be divided into segments. Each segment may have a predetermined number of markings 16'. By counting the signal edges, the control device 50 establishes the beginning and end of a corresponding segment in each case and determines segment times in which the segments pass the rotational speed sensor 53.In FIG. 3, a method according to a particularly preferred embodiment of the invention is schematically illustrated and denoted overall by 100.In a first method step 1, an internal combustion engine, such as the internal combustion engine 10 of FIGS. 1 and 2, runs in a full engine operation. A lambda controller here continuously adjusts a lambda sum actual value of the internal combustion engine 10 to a lambda sum nominal value, for example the value 1.0 for achieving homogeneous operation. By means of a suitable method, for example by means of a rough running method, cylinder-specific lambda deviations are determined and in each case compensated by adaptation values superimposed in accordance with the lambda control, i.e. individual lambda actual values of the individual cylinders are also controlled to an individual lambda setpoint value, for example the value 1.0.In a second method step 2, a control device, for example the control unit 50 of FIGS. 1 and 2, receives a corresponding control signal 50' requesting partial or half-engine operation. If a corresponding signal 50' is obtained, the lambda regulator 52 is controlled with a pilot signal 52', which counteracts an intervention of the lambda regulator 52. For this purpose, in particular the output signal of the lambda regulator is acted upon, in particular multiplicatively, with a pilot control value. As explained above, the pilot control value can be stored in a corresponding control device. It can be determined by detecting and storing the reaction of the lambda regulator after a first changeover, for example. During later changeover processes, the stored result is then switched as a pilot control value to the lambda controller output, so that the lambda controller itself no longer has to react.In a third method step 3, the internal combustion engine is running in half-engine operation, the corresponding pilot control for lambda control 52 being active and this thereby keeping a total lambda at the desired setpoint value. The method may be executed cyclically and, as illustrated by the dashed arrow 110, return to step 1, i.e. the internal combustion engine 10 may be transferred again to a full engine operation.For example, in a four-cylinder engine in full engine operation, the following actual injection quantities (EM) for the individual cylinders Cyl_ 1 to Cyl_ 4 could result at a setpoint injection quantity of 100 %: EM_Cyl_ 1=100%, EM_Cyl_ 2=100%, EM_Cyl_ 3=100%, EM_Cyl_ 4=140%.The individual lambda actual values behave indirectly proportional to the injection quantity with sufficient accuracy, resulting in a lambda sum actual value of approximately 0.93.The global lambda control could reduce the injection quantity over all cylinders: in order to achieve a lambda sum actual value of 1.The individual lambda values are now approximately:The cylinder-specific lambda control could determine the following correction factors (FAK) with mean value=1:This would result in full engine operation:Deactivation of cylinders 3 and 4, for example, would result in a change in the correction factors to obtain the mean=1: and correspondingly in a global mitigation:Here, the lambda controller would start and enrich the mixture, in the present example by a factor of 1.08. This is to be avoided for the reasons already explained above. Moreover, this enrichment would have to be corrected again during a later switch back into full engine operation. Both interventions of the lambda regulator are actually unnecessary.In the implementation of the invention, pilot control within the global lambda control is now carried out in parallel with the changeover to half-engine operation. As a result of this intervention, the leaning on account of the adjustment of the correction factors is compensated for by a simultaneous enrichment by the pilot-controlled lambda controller (in the above example, the global lambda controller output would be pilot-controlled, for example, by a factor of 1.08). The pilot control of the lambda regulator can be established, for example, by previous application. As explained, a corresponding correction of the lambda controller is stored for each rich or lean shift, which can occur when the adaptation values switch to half engine operation. Alternatively, it is also conceivable to measure the lambda change after switching to half-engine operation and to permanently adapt the correction of the lambda controller resulting therefrom for pilot control.A particular advantage of the proposed method is that possibly present "visual defects" (systematic errors) of the lambda probe in half-engine operation are also taken into account in the pilot control. As a result, after an adaptation during the first changeover to partial engine operation, a corresponding value is learned, so that no further reaction of the lambda regulator is necessary during future changeovers.

Claims

Method for operating an internal combustion engine (10) having at least two cylinders (11 - 14) in at least two operating modes, wherein in a first operating mode all cylinders (11 - 14) are fired and in a second operating mode only a part of the cylinders (11 - 14) are fired, wherein a lambda sum actual value of the internal combustion engine (10) is set by means of a lambda regulator (52) by adapting the quantity of fuel and / or air supplied to the fired cylinders (11 - 14) to a desired lambda sum target value, wherein, when switching from the first to the second operating mode, the lambda regulator (52) is acted upon by a pilot control value which counteracts a displacement of the lambda sum actual value which takes place without the action on account of the switching.Method according to Claim 1, wherein, in the at least two operating modes, an individual correction factor for correcting the quantity of fuel and / or air supplied to the respective cylinder (11 - 14) is determined for each cylinder (11 - 14) in order to set an individual lambda actual value of the respective cylinder (11 - 14) to a desired individual lambda setpoint value.Method according to Claim 2, wherein the individual lambda actual values are set to the same individual lambda setpoint value.Method according to Claim 2 or 3, wherein the pilot control value is determined by determining an expected shift in the lambda sum actual value on the basis of a change in the correction factors of the cylinders (11-14) fired in the second operating mode during the changeover.Method (100) according to one of the preceding claims, wherein the pilot control value is determined by the internal combustion engine (10) being switched from the first operating mode into the second operating mode and a shift of the lambda sum actual value effected thereby being determined.Method (100) according to one of the preceding claims, wherein the internal combustion engine (10) is operated in the first operating mode or in the second operating mode as a function of load.Method (100) according to Claim 6, in which the second operating mode comprises a half-engine operation in which exactly half of the cylinders (11-14) of the internal combustion engine (10) are not fired.Computing unit (50) which is configured to carry out a method according to one of the preceding claims.

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