Method and device for operating an internal combustion engine

A method and a device for operating an internal combustion engine make it possible to shift the combustion limit. In this context, a predefined output variable of the internal combustion engine such as the torque, for example, is realized in at least one operating state of the internal combustion engine by at least a retardation of an ignition angle. It is checked whether, due to the retardation of the ignition angle, a variable like, for example, a misfiring, characteristic for the combustibility of the air/fuel mixture in a combustion chamber of the internal combustion engine, exceeds a predefined limiting value in terms of a deterioration of the combustibility, and in this case, at least one actuator of the internal combustion engine, different from an actuator for setting the ignition angle, such as the exhaust-gas recirculation valve or EGR valve, for instance, is controlled along the lines of improving the combustibility of the air/fuel mixture.

FIELD OF THE INVENTION

The present invention relates to a method and a device for operating an internal combustion engine.

BACKGROUND INFORMATION

For a torque-neutral switchover of the internal combustion engine to a new engine operating mode having a smaller adjustable torque range, e.g., for a switchover from full-engine operation to half-engine operation in which only half the cylinders of the internal combustion engine are activated compared to full-engine operation, it is already known to build up the charge of the internal combustion engine accordingly prior to the switchover to the new engine operating mode, in order to be able to compensate for the jump in torque at the instant of the switchover. During the charge buildup, the additional torque share above the charge buildup for the combustion must be offset, in order to constantly be able to adjust a desired setpoint torque at the output of the combustion engine during the switchover.

This is accomplished via a retardation of the ignition angle up to a combustion limit of the internal combustion engine. The retardation of the ignition angle means, on one hand, a decrease in engine efficiency, which is associated with higher fuel consumption, but on the other hand, in comparison to an injection blank-out, for example, does not cause a deterioration of the exhaust-gas quality.

SUMMARY

In contrast, the method of example embodiments of the present invention and the device of example embodiments of the present invention for operating an internal combustion engine having the features described herein have the advantage that it is checked whether, due to the retardation of the ignition angle, a variable characteristic for the combustibility of the air/fuel mixture in a combustion chamber of the internal combustion engine exceeds a predefined limiting value in terms of a deterioration of the combustibility, and that if so, at least one actuator of the internal combustion engine, different from an actuator for setting the ignition angle, is driven along the lines of improving the combustibility of the air/fuel mixture. In this manner, the combustion limit for the ignition angle may be shifted, so that a larger range is available for the retardation of the ignition angle, and therefore the setting range for setting the predetermined output variable by retardation of the ignition angle is enlarged. In addition, in this manner, the use of a manipulated variable other than the retardation of the ignition angle to realize the predefined output variable of the internal combustion engine may be limited as far as possible, if not prevented completely. This is advantageous primarily when the further manipulated variable has a negative influence on the exhaust-gas quality compared to the retardation of the ignition angle. For example, this is the case when an injection blank-out is selected as a further manipulated variable, which results in a worsening of the exhaust-gas quality compared to a retardation of the ignition angle.

It is particularly advantageous if an uneven running is selected as a variable characteristic for the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine. The uneven running of the internal combustion engine may be determined easily and with little expenditure, and in addition, is a reliable measure for the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine.

A further advantage is yielded if, as actuator in the sense of improving the combustibility of the air/fuel mixture, an exhaust-gas recirculation valve is driven in such a way that an exhaust-gas recirculation rate is reduced. In this manner, the portion of an external exhaust-gas recirculation in the combustion-chamber charge may be reduced, and therefore the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine may be increased, so that a greater retardation of the ignition angle is allowed without having to put up with an unwelcome poor combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine.

A corresponding advantage results if, as actuator in the sense of improving the combustibility of the air/fuel mixture, at least one intake valve and/or at least one exhaust valve of at least one cylinder of the internal combustion engine is controlled in its lift and/or in its phase in such a way that an internal residual exhaust-gas rate is reduced. The combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine is increased in this manner, as well, and a greater range is allowed for the retardation of the ignition angle without having to put up with an unwanted poor combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine.

An especially simple and low-cost possibility for reducing the internal residual exhaust-gas rate is to control the intake and exhaust valves of at least one cylinder of the internal combustion engine in such a way that a valve-overlap phase of the intake and exhaust valves of the at least one cylinder of the internal combustion engine is reduced. In this manner, less exhaust gas arrives at the combustion chamber of the internal combustion engine via its exhaust valve or exhaust valves.

Furthermore, it is advantageous if, as actuator along the lines of improving the combustibility of the air/fuel mixture, a swirl control valve is driven in such a way that a movement of the charge supplied to the combustion chamber is increased. In this way, the air/fuel mixture in the combustion chamber of the internal combustion engine burns through better and faster, so that the range for the retardation of the ignition angle may be increased in this manner, as well, without having to put up with an unwanted adverse effect on the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine.

The movement of the charge fed to the combustion chamber may also be increased particularly easily if, as actuator in the sense of improving the combustibility of the air/fuel mixture, an intake valve of at least one cylinder of the internal combustion engine is controlled with respect to its opening instant in such a way that a movement of the charge fed to the combustion chamber is increased. This also promotes a better and faster burn-through of the air/fuel mixture in the combustion chamber of the internal combustion engine, and therefore a larger range for the retardation of the ignition angle, without an unwanted adverse effect on the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine. The same advantage results if, as actuator along the lines of improving the combustibility of the air/fuel mixture, an intake valve of at least one cylinder of the internal combustion engine is controlled with respect to its valve lift in such a way that a movement of the charge fed to the combustion chamber is increased.

In addition, it is advantageous if, in the case in which the exceedance of the predefined limiting value by the variable characteristic for the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine cannot be neutralized even by controlling the at least one actuator of the internal combustion engine, different from the actuator for setting the ignition angle, along the lines of improving the combustibility of the air/fuel mixture, the retardation of the ignition angle is limited with the reaching of the predefined limiting value by the variable characteristic for the combustibility of the air/fuel mixture in the combustion chamber of the internal combustion engine, and the predefined output variable of the internal combustion engine is additionally realized by a manipulated variable different from the retardation of the ignition angle, preferably an injection blank-out. This ensures that a maximum possible range for the retardation of the ignition angle is utilized for realizing the predefined output variable of the internal combustion engine by retarding the ignition angle, and when the retardation of the ignition angle has reached the maximum retardation of this range, a manipulated variable different from the retardation of the ignition angle is additionally drawn upon for realizing the predefined output variable, so that the use of this manipulated variable, different from the retardation of the ignition angle, for realizing the predefined output variable of the internal combustion engine is reduced to the greatest extent possible.

Particularly advantageously, the retardation of the ignition angle described is used when a switchover between a first operation of the internal combustion engine with a first number of activated cylinders and a second operation of the internal combustion engine with a second number of activated cylinders, preferably a switchover between full-engine operation and half-engine operation, is selected as the at least one operating state. In this manner, a compensation for the jump in torque at the instant of the switchover may be realized for this operating state, the compensation being based completely or to the greatest extent possible on a retardation of the ignition angle.

A further advantage is yielded if the retardation of the ignition angle compensates for an increase of a cylinder charge in the at least one operating state in terms of maintaining the output variable of the internal combustion engine constant. In this manner, a reserve for the setting of the output variable of the internal combustion engine may be built up by the retardation of the ignition angle, which may then be called on as quickly as possible by readjusting the ignition angle in the advance direction.

An exemplary embodiment of the present invention is represented in the drawing and elucidated in greater detail in the following description.

DETAILED DESCRIPTION

InFIG. 1, reference numeral1denotes an internal combustion engine in the form of an Otto engine. Internal combustion engine1includes one or more cylinders, of which one is represented by way of example inFIG. 1and denoted by reference numeral35. Hereinafter, cylinder35is considered as example without restricting the generality. Fresh air is supplied to cylinder35via an air inlet70. The quantity of fresh air supplied and therefore the charge of a combustion chamber5of cylinder35may be influenced by a throttle valve75in air inlet70. To that end, throttle valve75is controlled by an engine management40, for instance, as a function of the degree of actuation of an accelerator pedal of a vehicle powered by internal combustion engine1. Additionally or alternatively, the degree of opening of throttle valve75may also be varied as a function of demands of further control systems on an output variable of internal combustion engine1, e.g., as a function of the demand of an antilock system, an electronic stability program, a cruise control, an idle speed control, etc. The direction of flow in air inlet70is denoted by arrows inFIG. 1. Downstream of throttle valve75, an exhaust-gas recirculation line90opens through into air inlet70, which downstream of throttle valve75, changes into the part of air inlet70designated as intake manifold145. Exhaust-gas recirculation line90connects an exhaust branch95of internal combustion engine1to intake manifold145. Disposed in exhaust-gas recirculation line90is an exhaust-gas recirculation valve30, which is controlled by engine management40for adjusting a degree of opening as a function of a predefined exhaust-gas recirculation rate. Downstream of the introduction of exhaust-gas recirculation line90into intake manifold145, a swirl control valve10is disposed in intake manifold145. The degree of opening of swirl control valve10is likewise controlled by engine management40, namely, for example, in such a way that, as in the manner described in DE 10 2004 011 589, the gas supplied to combustion chamber5via air inlet70and exhaust-gas recirculation line90is set into a predefined movement within combustion chamber5. In this manner, for example, the combustion in combustion chamber5may be optimized, particulate emissions may be reduced and the exhaust-gas temperature may be lowered. Downstream of swirl control valve10, the gas in intake manifold145is then drawn into combustion chamber5via an intake valve15of cylinder35. In this context, for example, the opening instant, phase and lift of intake valve15are controlled by a camshaft. Alternatively, this control may also be accomplished in fully variable manner on the part of engine management40if, for example, an electrohydraulic valve control EHVS or an electromagnetic valve control EMVS is provided. Fuel is injected directly into combustion chamber5via a fuel injector80. Alternatively, fuel injector80may also be disposed in air inlet70or in intake manifold145in order to realize a manifold injection that is central or cylinder-specific. Fuel injector80is likewise controlled by engine management40for setting a desired injection quantity and a desired injection time, e.g., in order to set a predefined air/fuel mixture ratio. The air/fuel mixture located in combustion chamber5is ignited by a spark plug85whose ignition angle or moment of ignition is likewise set by engine management40, in order to realize a predefined efficiency of the combustion. The exhaust gas built up during the combustion of the air/fuel mixture in combustion chamber5is discharged into exhaust branch95via an exhaust valve20whose opening instant, phase and lift are likewise controlled by engine management40using corresponding means as for the control of intake valve15. A crankshaft-angle sensor100is situated in the area of cylinder35and detects the instantaneous crankshaft angle of the crankshaft driven by cylinder35or by the cylinders of internal combustion engine1, and passes it on as a continuous signal to engine management40. Engine management40is supplied with further input variables150, e.g., in the form of the degree of actuation of the accelerator pedal described above and/or the demands of further control systems. For example, input variables150may also include one or more signals which characterize performance quantities of the internal combustion engine, and from which, in a manner familiar to one skilled in the art, engine management40derives a measure for the instantaneous torque output by internal combustion engine1.

FIG. 2shows a functional diagram for the device according to example embodiments of the present invention which, for example, may be implemented in the form of hardware and/or software in engine management40. In this context, the signal from crankshaft-angle sensor100is supplied to a first evaluation unit115. From the crankshaft-angle signal of crankshaft-angle sensor100supplied continuously over time, first evaluation unit115ascertains the crankshaft angular acceleration of consecutive working cycles of internal combustion engine1, and ascertains the differences of the crankshaft angular acceleration of consecutive working cycles of internal combustion engine1as uneven-running value. The uneven-running values ascertained by first evaluation unit115are transmitted to a checking unit60and compared there to a predefined limiting value, which is supplied to checking unit60from a limiting-value memory120. For example, the limiting value for the uneven running is applied on a test stand in such a way that, in response to an exceedance of the predefined limiting value by the uneven-running values of first evaluation unit115in the sense of a deterioration of the combustibility, the combustibility of the air/fuel mixture in combustion chamber5is assessed as no longer adequate for maintaining a comfortable engine operation. If the uneven-running signal, supplied to checking unit60by first evaluation unit115, having the instantaneously ascertained uneven-running value exceeds the predefined limiting value in the sense of a deterioration of the combustibility, then checking unit60provides a set signal at its output, otherwise a reset signal. The output of checking unit60is supplied on one hand to a time-delay element125, and on the other hand, to an AND gate130. Time-delay element125delays the output signal of checking unit60by a predetermined time. The output signal of time-delay element125, with the output signal of checking unit60delayed by the predetermined time, is supplied to AND gate130, as well.

AND gate130delivers a set signal at its output when its two input signals are set simultaneously, otherwise, AND gate130provides a reset signal at its output. Furthermore, a control unit65is provided which is supplied with the output signal of checking unit60and which drives swirl control valve10, intake valve15, exhaust valve20and exhaust-gas recirculation valve30along the lines of improving the combustibility of the air/fuel mixture in combustion chamber5when the output signal of checking unit60is set. Otherwise, control unit65drives swirl control valve10, intake valve15, exhaust valve20and exhaust-gas recirculation valve30in conventional manner as described above.

In addition, a second evaluation unit116is provided which ascertains a setpoint value for an output variable of internal combustion engine1as a function of one or more demands. These demands may stem from further control systems of internal combustion engine1like, for example, an antilock system, an electronic stability program, a cruise control, an idle speed control and so forth, as well as the degree of actuation of an accelerator pedal of a vehicle powered by internal combustion engine1. Second evaluation unit116coordinates these demands in a manner familiar to one skilled in the art, in order to form a resultant setpoint value for the output variable of internal combustion engine1and to convert to, in each instance, a predefined output variable of internal combustion engine1for various control paths. For example, the output variable may be a torque or a power output of internal combustion engine1. In the example according toFIG. 2, an accelerator-pedal module110is shown by way of example, which generates a demand for the output variable of internal combustion engine1as a function of the degree of actuation of the accelerator pedal, and transmits it to second evaluation unit116. From this, second evaluation unit116derives a predefined output variable for the ignition path, a predefined output variable for the air path and, if indicated, a predefined output variable for the injection path, as well. The predefined output variable of internal combustion engine1for the ignition path is supplied to a first implementation unit45. The predefined output variable of internal combustion engine1for the air path is supplied to a second implementation unit50. The predefined output variable of internal combustion engine1for the injection path is supplied to a third implementation unit55. As a function of the predefined output variable supplied, implementation unit45generates an ignition angle to be set or a retard shift of the ignition angle to be set, and drives an actuator25for setting this ignition angle. As a function of the predefined output variable of internal combustion engine1supplied, second implementation unit50ascertains a required degree of opening of throttle valve75and drives it accordingly. As a function of the predefined output variable supplied, third implementation unit55ascertains a required injection time and quantity and drives fuel injector80accordingly to realize the predefined injection time and quantity. The predefined output variable of the internal combustion engine for the ignition-angle path may involve a predefined ignition-angle efficiency. The predefined output variable of the internal combustion engine for the air path may involve a predefined value for the charge of combustion chamber5of internal combustion engine1. The predefined output variable of internal combustion engine1for the injection path may involve a predefined injection quantity and injection time for realizing a predefined air/fuel mixture ratio. All predefined output variables for the three control paths indicated share in common that they are ascertained by second evaluation unit116in such a way that the demand of accelerator-pedal module110on the output variable of internal combustion engine1, or, correspondingly, the resultant demand on the output variable of internal combustion engine1in the case of several different demands on the output variable of internal combustion engine1by different control systems, is realized in the form of a desired torque or a desired power output.

The signal of crankshaft-angle sensor100is supplied to a third evaluation unit117, as well as the signal of an ascertainment unit105, which furnishes the instantaneous value of the output variable of internal combustion engine1as a continuous signal to third evaluation unit117. Third evaluation unit117ascertains the instantaneous engine speed of internal combustion engine1from the crankshaft-angle signal of crankshaft-angle sensor100by differentiation. As a function of the ascertained engine speed and the supplied instantaneous value of the output variable of internal combustion engine1, third evaluation unit117checks whether a switchover of the operation of internal combustion engine1with a first number of activated cylinders to an operation of internal combustion engine1with a second number of activated cylinders is possible, the first number of cylinders being greater than the second number of activated cylinders. For example, third evaluation unit117checks whether a switchover from full-engine operation in which all cylinders of internal combustion engine1are activated, to half-engine operation in which only half the cylinders of internal combustion engine1are activated is possible. This is the case when both the instantaneous engine speed and the instantaneous value for the output variable of internal combustion engine1each lie in a predefined range. This range may be determined in a manner familiar to one skilled in the art. If third evaluation unit117determines that the described switchover between the operating modes of internal combustion engine1with the different number of activated cylinders is possible, it then prompts second implementation unit50to increase the charge of internal combustion engine1along the lines of building up a reserve for the output variable of internal combustion engine1. Moreover, third evaluation unit117prompts first implementation unit45to retard the ignition angle, in order to compensate for the charge increase in view of its influence on the output variable of internal combustion engine1. At the instant of the switchover between the operating modes of internal combustion engine1using the different number of activated cylinders, the built-up reserve for the output variable of internal combustion engine1may then be called upon by advancing the ignition angle, so that a jump in torque of internal combustion engine1at the instant of the switchover is avoided. The prompt by third evaluation unit117for the ignition-timing retard may be supplied to first implementation unit45via a first controlled switch135, which is controlled as a function of the output signal of checking unit60. If the output signal of checking unit60is set, then first controlled switch135is opened; otherwise, thus in the case of a reset output signal of checking unit60, first controlled switch135is closed. This means that the retard shift of the ignition angle is limited to the value which exists upon determination of an exceedance of the limiting value for the uneven-running values in the sense of a deterioration of the combustibility. At the same time, by the set output signal of checking unit60, control unit65is prompted to drive actuators10,15,20,30along the lines of improving the combustibility of the air/fuel mixture. If the uneven-running signal thereupon drops below the predefined limiting value in the sense of an improvement of the combustibility, then the output signal of checking unit60is reset, and therefore first controlled switch135is closed again in order to permit a further retard shift of the ignition angle up to the instant at which the uneven-running signal again exceeds the predefined limiting value, and therefore a further retard of the ignition angle must be discontinued up until a possible improvement again in the combustibility of the air/fuel mixture due to suitable driving of actuators10,15,20,30. In addition, third evaluation unit117requests from third implementation unit55a blank-out of injections as soon as a second predefined switch140is closed, with the aim of compensating, in torque-neutral fashion, for the increase in charge necessary for the reserve to be built up. In this context, second controlled switch140is driven by the output signal of AND gate130. Thus, if the output signal of AND gate130is set, then second controlled switch140is closed and the injection blank-out is activated; otherwise, thus in response to the reset output signal of AND gate130, second controlled switch140is opened and the injection blank-out is therefore prevented. For example, the predefined time of time-delay element125may be suitably applied on a test stand in such a way that it corresponds at most to the time in which, calculated from the instant of the setting of the output signal of checking unit60, an improvement in the combustibility of the air/fuel mixture due to the driving of actuators10,15,20,30may safely be expected. If the expected improvement in the combustibility of the air/fuel mixture does not occur within this predefined time, then the improvement in the combustibility of the air/fuel mixture can no longer be realized by the driving of actuators10,15,20,30, so that the increase in charge must now also be offset by the blank-out of injections, and therefore second controlled switch140must be closed. However, if the combustibility of the air/fuel mixture improves within the predefined time, then by the setting of the output signal of time-delay element125, the output signal of checking unit60is already reset again, so that the output signal of AND gate130remains reset, and second controlled switch140remains open, and therefore no injection blank-out takes place.

FIG. 3shows a flow chart for an exemplary sequence of the method according to example embodiments of the present invention.

With the start of the program from full-engine operation, at a program point200, the signals of crankshaft-angle sensor100, accelerator-pedal module110and possibly further control systems as well as ascertainment unit105are acquired, and a counter is set to zero. The program subsequently branches to a program point205.

At program point205, from the signals of accelerator-pedal module110and possibly further control systems, second evaluation unit116ascertains a setpoint value for the output variable of internal combustion engine1. The program subsequently branches to a program point210.

At program point210, based on the signals from crankshaft-angle sensor100and ascertainment unit105, third evaluation unit117checks whether a switchover from full-engine operation to half-engine operation is possible. If so, the program branches to a program point215, otherwise back to program point200.

At program point215, from the signal of crankshaft-angle sensor100, first evaluation unit115ascertains the instantaneous uneven-running value and transmits it to checking unit60. The program subsequently branches to a program point220.

At program point220, checking unit60checks whether the instantaneous uneven-running value exceeds the predefined limiting value in terms of a deterioration of combustibility. If so, the program branches to a program point225, otherwise, to a program point250.

At program point225, the output signal of checking unit60is set, and with that, first controlled switch135is opened, and therefore a further retard shift of the ignition angle is stopped; the ignition angle is thus retained at its current position. Moreover, at program point225, actuators10,15,20,30are driven by control unit65along the lines of improving the combustibility of the air/fuel mixture, based on the set output signal of checking unit60. The counter is thereupon incremented by one. The program subsequently branches to a program point230.

At program point230, it is checked whether the value of the counter reaches or exceeds a predefined threshold value. If so, the program branches to a program point235, otherwise, back to program point215in order, at program point215, to ascertain the uneven-running value then current.

The function of the counter in the flow chart according toFIG. 3corresponds to the function of time-delay element125in the functional diagram according toFIG. 2. Therefore, the function of the predefined threshold value for the counter in the flow chart according toFIG. 3corresponds to the predefined time for time-delay element125according toFIG. 2and is applied accordingly. At program point235, and therefore with the expiration of the predefined time, the output signal of time-delay element125is set, and on the basis of the output signal of checking unit60which is likewise set, the output signal of AND gate130is set, and thus the injection blank-out is enabled by closure of second controlled switch140. Consequently, at program point235, the charge is increased by a predefined differential value by suitable control on the part of second implementation unit50, and this increase in charge is offset in its effect on the output variable of internal combustion engine1by suitable control of the injection quantity and injection time of fuel injector80by third implementation unit55. The program subsequently branches to a program point240.

At program point240, second implementation unit50checks whether it has already completely realized the specification of third evaluation unit117with respect to the charge increase. If so, the program branches to a program point245, otherwise, to a program point270.

At program point245, the switchover to half-engine operation may then be implemented by deactivating half the cylinders of internal combustion engine1, that is, by cutoff, and therefore, during the half-engine operation, permanent closure of all intake and exhaust valves of the cylinders to be deactivated. The program is subsequently exited. At program point270, second implementation unit50checks whether a maximum possible charge of combustion chamber5of internal combustion engine1was reached. If so, the program is exited and no switchover is brought about to half-engine operation; otherwise, the program branches back to program point235and the charge is further incremented as described above, the increase in charge thereby produced being offset by suitable injection blank-out in view of the effect on the output variable of internal combustion-engine1.

At program point250, second implementation unit50likewise increases the charge in the manner described by a predefined increment, in this case, the effect on the output variable of internal combustion engine1caused by this being offset by suitable retardation of the ignition angle. The program subsequently branches to a program point255.

At program point255, second implementation unit50checks, in the manner previously described, whether the specification of third evaluation unit117for the increase in charge was completely realized. If so, the program branches to a program point265, otherwise, to a program point260.

At program point265, a switchover to half-engine operation is brought about in the manner described. The program is subsequently exited.

At program point260, second implementation unit50checks whether the maximum possible charge was already reached. If this is the case, the program is exited and no switchover to half-engine operation is carried out; otherwise, the program branches back to program point215in order, at program point215, to ascertain the instantaneous uneven-running value existing then.

FIG. 4shows an example for the switchover from full-engine operation to half-engine operation. For this purpose, the time characteristic of charge rl is shown inFIG. 4a), and the time characteristic of ignition-angle efficiency η and of torque M of internal combustion engine1are shown inFIG. 4b). In so doing, inFIG. 4a), the characteristic of the setpoint value for charge rl is represented with a broken line, and the characteristic for the actual value of the charge is represented in the form of a solid line. The solid line with thin line thickness inFIG. 4b) denotes the time characteristic of ignition-angle efficiency η, whereas the thick solid line inFIG. 4b) denotes the time characteristic of torque M of internal combustion engine1. At an instant t=0, internal combustion engine1is in full-engine operation. Up to a first instant t0>0, the setpoint value and the actual value for charge rl both assume the same first charge value rl1. Up to first instant t0, ignition-angle efficiency η lies at a first ignition-angle-efficiency value η1. At first instant t0, it is determined at program point210that the intention is to switch over from full-engine operation to half-engine operation. Therefore, from first instant t0up to a following second instant t1, the switchover to half-engine operation is prepared. To that end, at first instant t0, the setpoint value for charge rl is increased abruptly from first charge value rl1to a second charge value rl2. First of all, because of the delay as a result of the intake-manifold dynamics, the actual value for charge rl cannot abruptly match the sudden change in the charge setpoint value, and also, on the basis of the exemplary embodiment described, new setpoint value rl2is tracked, for example, only successively as described, in order to offset the described compensation for the charge increase by the lowering of the ignition-angle efficiency and possibly by the blank-out of injections as a function of the instantaneous uneven-running measurements. In so doing, the goal is to realize a predefined, constant, desired torque M1before, during and after the switchover from full-engine operation to half-engine operation, so that the actual torque of the internal combustion engine corresponds constantly to desired torque M1before, during and after the switchover from full-engine operation to half-engine operation, as well, as shown inFIG. 4b). To compensate for the increase in charge as of first instant t0, as shown inFIG. 4b), from first instant t0, ignition-angle efficiency η is lowered, and specifically, in such a way that the effect of the charge increase on the torque of internal combustion engine1is precisely offset by the lowering of the ignition-angle efficiency, so that internal combustion engine1constantly outputs torque M1. As described, the decrease in ignition-angle efficiency η is brought about by the retardation of the ignition angle. In this context, at a third instant t2between first instant t0and second instant t1, a first ignition angle zw1is reached, and for the first time, an exceedance of the predefined limiting value by the instantaneous uneven-running value is detected. First ignition angle zw1therefore designates the reaching of the combustion limit at third instant t2. A further retardation of the ignition angle beyond first ignition angle zw1would therefore lead to an unacceptable deterioration in the running smoothness of internal combustion engine1. Therefore, at third instant t2, the charge movement in combustion chamber5is increased by the measures described, and the residual exhaust-gas rate, thus the portion of residual exhaust gas in the gas mixture in combustion chamber5, is reduced, in order to shift the combustion limit in the direction of an even more retarded ignition angle in comparison to first ignition angle zw1. If this is no longer possible, then the desired increase in charge must be offset by injection blank-out. However, in the example according toFIG. 4b), the combustion limit is able to be shifted to a second ignition angle zw2which is later than first ignition angle zw1. Therefore, ignition-angle efficiency η may also be reduced beyond third instant t2. At second instant t1, the actual value of cylinder charge rl reaches setpoint value rl2of the cylinder charge without the ignition angle having to be shifted up to second ignition angle zw2at second instant t1. Thus, at second instant t1, the ignition angle reaches a value between first ignition angle zw1and second ignition angle zw2. Therefore, at second instant t1, the conditions are present for the switchover to half-engine operation which, in order to adjust the same desired torque M1as in full-engine operation, requires double the charge. That is why second charge value rl2is also twice as great as first charge value rl1. Therefore, for the switchover from full-engine operation to half-engine operation at second instant t1, ignition-angle efficiency η is increased again abruptly or in stepped fashion from value η2, reached at second instant t1, to first ignition-angle efficiency η1by suitable advance of the ignition angle, so that as of second instant t1, the half-engine operation is realized with second charge value rl2, and therefore with the still constant torque M1.

Thus, the period of time between first instant t0and second instant t1is necessary to prepare for the switchover from full-engine operation to half-engine operation.

In the subject matter of example embodiments of the present invention, an actuator10,15,20,30of internal combustion engine1that is different from actuator25for setting the ignition angle is driven to improve the combustibility of the air/fuel mixture. In principle, the combustibility of the air/fuel mixture in combustion chamber5of internal combustion engine1may be improved by two different measures. First of all, by decreasing the residual exhaust-gas rate in the gas mixture in combustion chamber5, and secondly, in addition or alternatively, by increasing the charge movement of the gas mixture in combustion chamber5. Several different possibilities are available for reducing the residual exhaust-gas rate, which may be implemented individually or in any combination. The residual exhaust-gas rate is a function of the external exhaust-gas recirculation and the internal exhaust-gas recirculation. Therefore, the combustibility of the air/fuel mixture may be improved if exhaust-gas recirculation valve30is controlled in such a way that the external exhaust-gas recirculation rate is reduced. The internal residual exhaust-gas rate, which, on the basis of the internal exhaust-gas recirculation, results as a function of the setting of intake and exhaust valves15,20of respective cylinder35, may be reduced to improve the combustibility of the air/fuel mixture by suitably controlling at least one intake valve15and/or at least one exhaust valve20of the at least one cylinder35in its lift and/or its phase. For example, such a suitable control involves reducing a valve-overlap phase of intake and exhaust valves15,20of the at least one cylinder35of internal combustion engine1. The valve-overlap phase is that time range or angular range in which both the intake and exhaust valves of cylinder35are open, so that residual exhaust gas from exhaust branch95may be drawn into combustion chamber5via the corresponding exhaust valve during an induction stroke of cylinder35. For example, a suitable control of intake and/or exhaust valves15,20of the at least one cylinder35may be realized by a variable camshaft timing control or by a fully variable valve gear on the basis, for instance, of an electrohydraulic valve control and/or an electromagnetic valve control. In addition or as an alternative to the lowering of the residual exhaust-gas rate in combustion chamber5of internal combustion engine1, one or more measures for increasing the movement of the charge supplied to combustion chamber5may be implemented in any combination in order to improve the combustibility of the air/fuel mixture, and consequently to postpone the combustion limit, i.e., the latest possible ignition angle for acceptable uneven-running values. For instance, the movement of the charge supplied to combustion chamber5may be increased by transferring swirl control valve10, if present, to a turned-on, closed or nearly closed position. Due to the specific position, a swirl may be generated for the air stream admitted into combustion chamber5. Additionally or alternatively, such a swirl generation may also be produced by suitable control of at least one intake valve15of the at least one cylinder35of internal combustion engine1with regard to the opening instant of the intake valve and/or with regard to the lift of intake valve15, so that the movement of the charge supplied to combustion chamber5may be increased in this manner, as well, and with that, the combustibility of the air/fuel mixture may be improved, and consequently, the combustion limit may be postponed in the manner described. For the control of intake valve15described, it is likewise advantageous if a variable camshaft timing control or a fully variable valve gear like, for example, in the case of an electrohydraulic or an electromagnetic valve control, is provided. If, in spite of these measures used individually or in any combination, a reduction of the instantaneous uneven-running value to below the predefined limiting value cannot be realized in the setting of the desired output variable of internal combustion engine1, then the retardation of the ignition angle is limited with the reaching of the predefined limiting value by the instantaneous uneven-running value, so that a further retard shift of the ignition angle is prevented because of the exceedance of the predefined limiting value by the instantaneous uneven-running value then occurring, and the predefined output variable of internal combustion engine1is then additionally realized by a variable different from the retardation of the ignition angle like, for example, an injection blank-out.

In the example described above, the switchover from full-engine operation to half-engine operation or, more generally, the switchover from a first operating state of internal combustion engine1with a first number of activated cylinders to a second operating state of internal combustion engine1with a second number of activated cylinders which is less than the first number of activated cylinders was described as the operating state in which a predefined output variable of internal combustion engine1is realized at least by a retardation of the ignition angle, the intention being to hold the output variable of internal combustion engine1constant during this switchover. In this context, a charge buildup necessary for the switchover to be carried out was offset by the retardation of the ignition angle. However, the present invention is not restricted to this special switchover operation of internal combustion engine1. Rather, it may be used quite generally in corresponding manner when a predefined output variable of internal combustion engine1, e.g., a torque or a power output in at least one operating state of internal combustion engine1is to be realized by at least a retardation of the ignition angle, and specifically, also without an increase in the charge of the internal combustion engine having to be offset by the retardation of the ignition angle. The case is also possible in which the predefined output variable of the internal combustion engine is to be reduced simply by a retardation of the ignition angle. In such a realization of the setting of a predefined output variable of the internal combustion engine by at least a retardation of the ignition angle, in the event the combustion limit of internal combustion engine1is reached by the retardation of the ignition angle, in principle, it involves taking measures by which the combustion limit is further postponed, and therefore a further retardation of the ignition angle is possible. This is permitted by an ongoing ascertainment of the variable characteristic for the combustibility of the air/fuel mixture in combustion chamber5of internal combustion engine1, and its comparison to the predefined limiting value. As soon as the variable characteristic for the combustibility exceeds the predefined limiting value in terms of a deterioration of the combustibility, the reaching of the combustion limit is detected and at least one actuator10,15,20,30of internal combustion engine1that is different from actuator25for setting the ignition angle is driven in the manner described along the lines of improving the combustibility of the air/fuel mixture, namely, for example, to reduce the residual exhaust-gas rate in the combustion chamber and/or to increase the charge movement. Therefore, owing to the subject matter of example embodiments of the present invention, by improving the combustibility of the air/fuel mixture in combustion chamber5, the combustion limit and therefore the latest possible ignition angle may be further shifted, so that a supplementary injection blank-out for realizing the predefined output variable of internal combustion engine1, e.g., to compensate for the charge buildup, may be prevented or at least carried out at a later instant and to a lesser extent. In this way, a deterioration in the exhaust-gas quality caused by the injection blank-out may be prevented or at least reduced.

For instance, a similar situation as in the switchover from full-engine operation to half-engine operation occurs when one or more cylinders of internal combustion engine1is/are operated in a first operating state with a first lift of the intake valves, and in a second operating state with a second lift of the intake valves, the first valve lift and the second valve lift differing from each other. If, for example, in this context, a switch is made from a larger valve lift to a smaller valve lift, then a situation results analogous to the switchover from full-engine operation to half-engine operation which, analogous to the manner described inFIGS. 2 and 3, is realized using a retardation of the ignition angle to compensate for a charge buildup, and possibly a shift of the combustion limit, if need be, additionally using an injection blank-out.