Method and device for operating an internal combustion engine

A method and a device for operating an internal combustion engine, in particular a motor vehicle having a plurality of cylinder banks are described, making it possible to eliminate fluctuations in a time trend in an output quantity of the internal combustion engine that may result due to differences in operation of different cylinder banks of the internal combustion engine. A characteristic value of the output quantity of the internal combustion engine is determined repeatedly over time. The thus formed time trend in the characteristic value for the output quantity of the internal combustion engine is filtered as a function of the operation of the cylinder banks.

FIELD OF THE INVENTION

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

BACKGROUND INFORMATION

Methods and devices for operating an internal combustion engine are already known. Such an internal combustion engine drives a motor vehicle, for example. Internal combustion engines having multiple cylinder banks are known. In such internal combustion engines, a value for the speed of the internal combustion engine is determined repeatedly over time.

SUMMARY OF THE INVENTION

The method and device according to the present invention for operating an internal combustion engine have the advantage over the related art that a characteristic value of an output quantity of the internal combustion engine is determined repeatedly over time and the time trend in the thus formed characteristic value for the output quantity of the internal combustion engine is filtered as a function of the operation of the cylinder banks. This makes it possible to eliminate unwanted fluctuations in the characteristic value of the output quantity of the internal combustion engine due to the mode of operation of the cylinder banks. It is possible in this way to operate functions of the internal combustion engine, which are provided with the characteristic value for the output quantity of the internal combustion engine in their time trend for further processing, independently of the aforementioned fluctuations and thus more conveniently. One might consider here, for example, an idling speed regulation as a function of the internal combustion engine.

Fluctuations in the time trend in the characteristic value for the output quantity of the internal combustion engine on the basis of simultaneous operation of different cylinder banks of the internal combustion engine in different operating modes are thus transparent for the functions of the internal combustion engine to which the characteristic value for the output quantity of the internal combustion engine is supplied in filtered form in its time trend, depending on this operation of the cylinder banks, i.e., these fluctuations are eliminated by the filtering and thus no longer have any effect on the aforementioned functions of the internal combustion engine, at least no significant effect.

It is advantageous if the time trend in the characteristic value for the output quantity of the internal combustion engine is filtered as a function of a firing frequency. This is because the aforementioned fluctuations in the time trend in the characteristic value for the output quantity of the internal combustion engine may include a frequency component that depends on the firing frequency, based on the operation of the cylinder banks.

One such frequency component is, for example, the firing frequency divided by the number of cylinder banks of internal combustion engine1, so that a frequency component of the time trend in the characteristic value for the output quantity of the internal combustion engine is eliminated by filtering in an advantageous manner; this filtering corresponds to the firing frequency divided by the number of cylinder banks.

It is also advantageous if the time trend in the characteristic value for the output quantity of the internal combustion engine is averaged over a number of chronologically successive characteristic values for the output quantity of the internal combustion engine. This is a particularly simple and effective method of filtering to eliminate the unwanted fluctuations in the time trend in the characteristic value for the output quantity of the internal combustion engine occurring as a function of the operation of the cylinder banks.

Another advantage is achieved when the cylinder banks are fired in a manner different from that in a cyclic change and when the time trend in the characteristic value for the output quantity of the internal combustion engine is filtered as a function of a frequency of the camshaft revolution or a multiple of this frequency. This makes it possible to eliminate fluctuations in the time trend in the characteristic value for the output quantity of the internal combustion engine or at least to essentially eliminate them; such fluctuations result when the cylinder banks of internal combustion engine1are not fired cyclically in succession and may be undesired for the operation of the internal combustion engine or of the above-mentioned at least one function of the internal combustion engine that further processes the characteristic value of the output quantity of the internal combustion engine in its time trend.

In the simplest case, a value for the output quantity itself may be selected as the characteristic value for the output quantity of the internal combustion engine. This is adequately reliable in particular when the output quantity is detected once per firing period and the cylinder banks are operated at the same torque contribution.

However, if the output quantity is detected several times per firing period, the reliability of further processing of the characteristic value for the output quantity of the internal combustion engine is increased when an output quantity averaged over several values, in particular several chronologically directly successive values for the output quantity of the internal combustion engine, is formed as the characteristic value for the output quantity of the internal combustion engine.

It is adequately reliable for further processing of the characteristic value for the output quantity of the internal combustion engine if the value for the output quantity of the internal combustion engine is detected several times, in particular twice, per firing, and if the output quantity average is formed as the average over the values for the output quantity of the internal combustion engine detected per firing period. In this way, fluctuations in the detected values for the output quantity of the internal combustion engine which occur during a firing period are taken into account and at the same time are transparent for subsequent further processing of the detected values for the output quantity of the internal combustion engine but do not act there in the form of a fluctuation, so that the comfort, in particular the driving comfort, may be increased and/or is not impaired in an unwanted manner due to the functions of the internal combustion engine that further process the detected values for the output quantity of the internal combustion engine, e.g., in the form of the characteristic value for the output quantity of the internal combustion engine.

DETAILED DESCRIPTION

FIG. 1shows an internal combustion engine1, which drives a vehicle, for example. Internal combustion engine1may be designed, for example, as a gasoline engine or as a diesel engine. In the example according toFIG. 1, it includes two cylinder banks, namely a first cylinder bank5and a second cylinder bank10, each of two cylinder banks5,10including four cylinders in the example according toFIG. 1. In addition, sensor means30are arranged in the area of internal combustion engine1, each detecting a value for the output quantity of the internal combustion engine at discrete points in time. The output quantity of the internal combustion engine may be, for example, a torque, a power, an engine speed, or a quantity derived from the torque and/or power and/or speed. For example, it shall be assumed below that the output quantity of internal combustion engine1detected by sensor means30is the engine speed of internal combustion engine1. Sensor means30in this case are designed as engine speed sensors.

The values for the engine speed of internal combustion engine1detected at discrete points in time by engine speed sensor30in this way are sent to a device15according to the present invention and from there to a determination unit20. Determination unit20forms a characteristic value from the values for the engine speed of internal combustion engine1supplied at discrete points in time. To this end, determination unit20is controlled by a control unit35of device15. Control unit35also controls engine speed sensor30in that it specifies for engine speed sensor30the points in time at which engine speed sensor30should sample the engine speed of internal combustion engine1, i.e., detect a corresponding value for the engine speed of internal engine1. In the simplest case, control unit35then instructs engine speed sensor30to detect the engine speed once per firing period of internal combustion engine1. The firing period of internal combustion engine1is the period between two successive firings of internal combustion engine1. If only one cylinder of internal combustion engine1is operated, the firing period is the time interval between two successive firings of this cylinder. If multiple cylinders of internal combustion engine1are operated, two successive firings of internal combustion engine1may also take place in different cylinders of internal combustion engine1, which need not be in the same cylinder bank. In this case, the firing period is in general the period of time between two successive firings independently of the particular cylinder fired in each case, i.e., simply the time interval between two consecutive firings, it being irrelevant in which cylinder(s) these two consecutive firings occur.

In the simplest case, control unit35instructs engine speed sensor30to detect only a single value for the engine speed of internal combustion engine1during a firing period, the same crankshaft position optionally always being specified per firing period by control unit35for a plurality of detection points in time in an advantageous manner. Determination unit20then forms the characteristic value for the engine speed of internal combustion engine1in the form of the particular value for the engine speed itself, as detected by engine speed sensor30.

Alternatively, control unit35may instruct engine speed sensor30to detect the engine speed at several points in time per firing period. It is also possible to provide for these points in time for each firing period to be at the same crankshaft angles. In this case, determination unit20is prompted by control unit35to form a speed average for each firing period from all engine speed values received from engine speed sensor30for the corresponding firing period. During a firing period of internal combustion engine1, the engine speed changes as do the engine torque and engine power because a firing period includes multiple operating phases of the fired cylinder, one operating phase of which is a compression phase having a lower engine speed, lower engine torque and lower engine power and another operating phase is a decompression phase having a higher engine speed, a higher engine torque and a higher engine power. This yields a fluctuation in the time trend in the engine speed, the engine torque and the engine power per firing period. With suitable triggering of engine speed sensor30by control unit35at the top dead center of the piston and at bottom dead center of the just fired cylinder, engine speed sensor30detects the engine speed at one or more characteristic locations in its time trend during a firing period. It is assumed below as an example that control unit35prompts engine speed sensor30to detect the engine speed twice per firing period. In an advantageous manner, control unit35then prompts engine speed sensor30to detect the engine speed at a point in time and/or at a crankshaft angle at which the cylinder to be fired at that moment is in a compression phase and at a point in time or a crankshaft angle at which the just fired cylinder is in a decompression phase. In this way, engine speed sensor30detects a relative high point and a relative low point of the engine speed per ignition period. These values are relayed to determination unit20, which forms an average as the characteristic value for the engine speed of internal combustion engine1from the high points and low points of the engine speed received per firing period. The speed detection by engine speed sensor30takes place advantageously at equidistant points in time. If four cylinders of internal combustion engine1are operated, the firing interval between two immediately successively fired cylinders of internal combustion engine1equals 180° of the crankshaft angle, so that in the example described here, the time interval between two directly successive rotational speed measurements by engine speed sensor30is 90° of the crankshaft angle. If all eight cylinders of internal combustion engine1are in operation in the example according toFIG. 1, the firing interval between two cylinders of internal combustion engine1fired in direct succession equals 90° of the crankshaft angle in each case and the distance between two consecutive measurement points for rotational speed detection equals 45° of the crankshaft angle in each case. The relative maximum of the engine speed is measured in the compression phase before reaching the piston top dead center of the cylinder about to be fired and the relative minimum of the engine speed is measured in the decompression phase after piston top dead center of the just fired cylinder.

When it is stated that determination unit20forms the average (referring to the arithmetic mean here), alternatively this could also refer to the geometric mean or some other mean, of the engine speed values detected per firing period, this is advantageously to be understood to refer to the forming of a sliding average over the two most recently detected engine speed values, which may also fall in directly adjacent firing periods, but both include a relative high point and a relative low point of the engine speed. This sliding average over the most recently detected engine speeds by engine speed sensor30in each case yields a time trend in the thus formed characteristic value for the engine speed of internal combustion engine1.

It is provided according to the present invention that the thus formed time trend for the characteristic value of the engine speed of internal combustion engine1is filtered as a function of the operation of cylinder banks5,10. This filtering is implemented by filter25, which receives from determination unit20the characteristic value for the engine speed, e.g., in the form of the sliding average for the engine speed. Specifically during operation of internal combustion engine1having two cylinder banks5,10, fluctuations in speed and also fluctuations in the torque and power of internal combustion engine1are excited at half the firing frequency by different modes of operation of two cylinder banks5,10at the same time. The firing frequency here corresponds to the inverse of the firing period, i.e., the inverse of the time interval between two immediately successive firings of internal combustion engine1. This frequency component of half the firing frequency is also still present in the rotational speed average formed by determination unit20. To eliminate this fluctuation by filtering via filter25for functions of internal combustion engine1which are not extremely time-critical, for example another sliding average may be formed by filter25over the two speed averages formed last by determination unit20. Filtering via filter25should be omitted in the case of extremely time-critical functions of the internal combustion engine which are to be understood here as meaning that these functions must follow a change in the engine speed more rapidly than two firing periods. Such time-critical functions may be provided, e.g., by an idling regulator or a bucking damping function. Two cylinder banks5,10may be operated in different modes at the same time, e.g., in that first cylinder bank5burns an air/fuel mixture in the particular cylinders in the conventional manner, e.g., for driving the vehicle, while second cylinder bank10, is operated in regeneration mode to regenerate an NOx storage catalytic converter in an exhaust system of the internal combustion engine (not shown inFIG. 1) or for regeneration of a particulate filter in the case of a diesel engine also being operated in the exhaust line of the internal combustion engine. Different operating modes of the two cylinder banks5,10may also occur when one of the two cylinder banks5,10is operated in homogeneous mode and the other of two cylinder banks5,10is operated in partially homogeneous operation or in a shift mode. At the same time, different operating modes of the two cylinder banks5,10may also occur when only the cylinders of one of the two cylinder banks5,10are operated, but the cylinders of the other of the two cylinder banks5,10are shut down. In all cases in which the two cylinder banks5,10of internal combustion engine1are operated at the same time in such different modes, the two cylinder banks5,10usually yield different torque contributions and thus yield different engine speed characteristics over time. In this way, the fluctuation in speed described here occurs simultaneously in different operating modes at half the firing frequency in such operation of the two cylinder banks5,10. If both cylinder banks5,10are operated in the same mode at the same time, there is no fluctuation in the speed at half the firing frequency or only a negligible fluctuation, but instead the speed fluctuation having a firing frequency already eliminated by determination unit20occurs in the form of the sliding average of the speed.

In the case of an internal combustion engine having more than two cylinder banks and operation of the internal combustion engine in which at least two of the cylinder banks of internal combustion engine1are operated in different modes at the same time, a fluctuation in speed, a fluctuation in torque, and a fluctuation in power of internal combustion engine1occur at a frequency corresponding to the firing frequency divided by the number of cylinder banks of internal combustion engine1. This may be eliminated via filter25by averaging in filter25over the n speed averages most recently determined by determination unit20, where n corresponds to the number of cylinder banks of internal combustion engine1.

Control unit35controls filter25as a function of the operation of the cylinder banks of internal combustion engine1for suitable filtering. If both cylinder banks5,10in the example according toFIG. 1are operated at the same time in the same mode, control unit35triggers filter25so that the speed averages formed in determination unit20and sent on to filter25are not averaged further but instead are output as such by filter25. Alternatively, averaging may also be activated permanently by filter25. For the case when in more than two cylinder banks of internal combustion engine1all cylinder banks are operated at the same time in the same mode, control unit35instructs filter25to output the sliding speed averages received from determination unit20as such at its output and not to filter them further. In all the cases described here, control unit35receives information regarding the operating modes of the cylinder banks of combustion engine1in a manner known to those skilled in the art, and this information may already be known to control unit35due to the fact that it prompts these modes and triggers the corresponding cylinder banks of internal combustion engine1inFIG. 1in a manner not depicted here.

Due to the averaging in filter25as described here, in the case when the cylinder banks of internal combustion engine1are operated at the same time in different modes, the time trend in the characteristic value for the engine speed of internal combustion engine1formed by determination unit20in the form of the speed average is filtered as a function of the firing frequency, a frequency component of the time trend in the characteristic value for the engine speed of combustion engine1formed by determination unit20being eliminated by filtering in filter25, this frequency component corresponding to the firing frequency divided by the number of cylinder banks of internal combustion engine1. Filtering may be accomplished by filter25in the manner described here particularly easily by averaging the time trend in the characteristic value for the engine speed of internal combustion engine1formed by determination unit20over a number of chronologically successive characteristic values for the engine speed of internal combustion engine1, this number advantageously corresponding to the number of cylinder banks of internal combustion engine1. For such filtering by averaging, a suitably designed digital filter may be used in the manner known to those skilled in the art.

For the case in which the cylinder banks of internal combustion engine1are not fired cyclically, i.e., in which in the firing and/or ignition of the cylinders there is no cyclic alternation among the cylinder banks of internal combustion engine1, the result in the case of simultaneous different operation of at least two cylinder banks of internal combustion engine1is fluctuations in speed, torque, and power having camshaft frequency and integral multiples of this camshaft frequency up to a frequency component corresponding to the firing frequency divided by the number of cylinder banks of internal combustion engine1. The camshaft frequency is the inverse of the period of time for one revolution of the camshaft of internal combustion engine1. The camshaft frequency corresponds to half the crankshaft frequency because one revolution of the crankshaft corresponds to two revolutions of the crankshaft. In such a case, control unit35must cause filter25to eliminate these frequencies from the time trend in the characteristic value for the engine speed supplied by determination unit20. These fluctuations may also be eliminated by a suitably designed digital filter in the manner known to those skilled in the art.

Depending on the operation of internal combustion engine1in which the cylinder banks of internal combustion engine1are fired simultaneously in the same mode, simultaneously in different modes, in cyclic alternation, or are not fired in cyclic alternation, different filtering of the time trend received by determination unit20in filter25in the characteristic value for the speed of the internal combustion engine is necessary. To this end, control unit35is able to switch as a function of the operation of internal combustion engine1between different filter functions of filter25which perform the filtering required for the instantaneous operation of internal combustion engine1in the manner described previously. The filtered time trend in the characteristic value for the speed of internal combustion engine1is output by filter25and may then be sent to various other regulators, characteristic curves, and engine characteristic maps of the internal combustion engine (not shown inFIG. 1) which must rely on speed information.

FIG. 2shows a flow chart for an example of a sequence of the method according to the present invention. After the start of the program, which may coincide with the start of internal combustion engine1, for example, control unit35causes the detection of the engine speed of internal combustion engine1by engine speed sensor30at a point in time and/or a corresponding crankshaft angle suitable in the manner described above. It then branches to a program point105.

At program point105, control unit35checks whether internal combustion engine1is still running. If this is the case, the program branches to a program point115; otherwise the program is terminated.

At program point115, control unit35checks whether enough speed values have been detected by engine speed sensor30at the current point in time and relayed to determination unit20, which permits the formation of a characteristic value for the engine speed and the filtering of this characteristic value by filter25. In the case of the above-described filtering by averaging, 2n speed values detected by engine speed sensor30in succession are necessary for this filtering in the case of n cylinder banks of internal combustion engine1. If there are enough speed values, the program branches to a program point120; otherwise it branches back to program point100and engine speed sensor30is triggered by control unit35to determine the next speed value at the next specified point in time and/or the next crankshaft angle. The specified points in time and/or crankshaft angles for ascertaining the engine speed may be preselected in the manner described above to detect relative high and low points of the time trend in the engine speed via engine speed sensor30.

At program point120, control unit35causes determination unit20to form a speed average as the characteristic value for the speed of internal combustion engine1from the speed values detected last by engine speed sensor30during the duration of a firing period. In the example described above, the average over the two last speed measurements by engine speed sensor30is triggered by control unit35in determination unit20. These two speed values represent a relative high point and a relative low point of the time trend in the speed, as already explained, the measurement frequency and/or sampling frequency of engine speed sensor30in this example being selected to be equal to twice the firing frequency. As an alternative, it may then also be selected to be equal to the firing frequency, in which case only one measured value is available per firing period which corresponds at the same time to the characteristic value for the speed. However, the measurement frequency and/or sampling frequency may also be greater than twice the firing frequency, in which case more than two measured speed values are supplied by engine speed sensor30per firing period; determination unit20must then take the average of these measured values to form the speed average as the characteristic value for the speed of internal combustion engine1.

In the latter case, for example, one or more other crankshaft angles may also be preselected in addition to the crankshaft angle for the relative high point and the crankshaft angle for the relative low point of the time trend in the engine speed in such a way that engine speed sensor30should detect the engine speed for these crankshaft angles, this specification optionally being random. Nevertheless it is less complex if the crankshaft angles for which engine speed sensor30is to detect and/or sample the engine speed are equidistant. After program point120, the program branches to a program point122. At program point122, control unit35checks whether an extremely time-critical function (as described above) of internal combustion engine1utilizing the speed information at the output of filter25is active. If this is the case, the program branches to a program point134; otherwise it branches to a program point110. At program point134, control unit35causes filter25to output the sliding speed average received by determination unit20, unfiltered at its output. The program then branches back to program point100and engine speed sensor30is triggered by control unit35to detect the next speed value.

At program point110, control unit35checks whether firing of the cylinder banks of internal combustion engine1has been currently set in cyclic alternation as the operating mode of internal combustion engine1or the cylinder banks of internal combustion engine1. If this is the case, the program branches to a program point125; otherwise it branches to a program point135.

At program point125, control unit35checks whether all cylinder banks of internal combustion engine1are being operated in the same mode at the same time. If this is the case, the program branches to program point134; otherwise it branches to a program point130.

At program point130, control unit35causes filter25to average over the n sliding speed averages most recently received from determination unit20, where n denotes the number of cylinder banks of internal combustion engine1. The thus formed sliding average represents a filtered characteristic value for the speed of internal combustion engine1and is made available at the output of filter25for further processing. The program subsequently branches back to program point100.

At program point135, control unit35determines the spectral components of the sliding speed average determined by determination unit20in time from the sliding speed averages formed last by determination unit20over one camshaft revolution period. This may take place via a Fourier analysis, for example. The program subsequently branches to a program point140.

At program point140, control unit35causes filter25to eliminate fluctuations occurring according to Fourier analysis at program point135, optionally with camshaft frequency as the inverse of a camshaft revolution period and also at an integral multiple of this camshaft frequency and a frequency corresponding to the firing frequency divided by the number of cylinder banks of internal combustion engine1. The signal filtered accordingly is made available at the output of filter25for further processing. The program then branches back to program point100and engine speed sensor30is prompted to detect the next speed value by control unit35. Program step135may also be omitted, and then at program point140filter25may be prompted in general by control unit35to eliminate spectral components in the camshaft frequency, integral multiples of the camshaft frequency and a frequency corresponding to the firing frequency divided by the number of cylinder banks of internal combustion engine1from the time trend in the characteristic value for the speed of internal combustion engine1, this time trend having been received by determination unit20, and to make this available at the output of filter25.

The filter functions in filter25may also be linked to an estimation method, e.g., a linear or quadratic estimation method to compensate for the time lag caused by the filter functions.

In such an estimation method, instantaneous output value fKof filter25and one or more previous output values fK−1, fK−2of filter25are needed. The equation for a linear estimation method is as follows, for example:
y=a0*fK+a1*fK−1(1)

The equation for a quadratic estimation method is as follows, for example:
y=a0*fK+a1*fK−1+a2*fK−2(2)
Coefficients a0, a1, a2are determined in a method known to those skilled in the art as a function of the filter algorithm in filter25. At the output of the estimation method supplied with the output values of filter25, estimate y determined according to equation (1) or equation (2) is then available. The estimation method, shown by dashed lines inFIG. 1, is represented by reference numeral40as an optional module at the output of filter25and still belongs to device15.