Abstract:
A method for controlling the quantity of fuel and/or air to an internal combustion engine on a cylinder-by-cylinder basis is characterized in that a signal that is influenced by combustion or pertains to a quantity that influences the combustion and contains items of information from all cylinders, mutually offset in time, is analyzed by ascertaining vibration components in the frequency range caused by cylinder-specific differences and regulating these components separately for selected frequencies, and in that an amplitude regulator that determines the amplitude of a correction intervention measure and a phase regulator that determines the allocation of an intervention pattern with respect to the cylinders are provided for each frequency to be compensated.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for controlling the quantity of fuel and/or air to an internal combustion engine on a cylinder-by-cylinder basis.  
       BACKGROUND INFORMATION  
       [0002]     In internal combustion engines, in particular self-igniting internal combustion engines, the fuel injection quantity is controlled based on rotational speed on a cylinder-by-cylinder basis. Through this method, also known as quantity compensation regulation, injection quantity errors resulting in differences in torque and thus uneven rotational speeds are compensated. However, errors in air quantity resulting in lambda differences between individual cylinders for the same injection quantity cannot be detected and compensated by this method. Such errors in air quantity may, however, result in very large deviations in the exhaust-gas compositions.  
         [0003]     There are lambda-regulating systems for gasoline engines on a cylinder-by-cylinder basis but they are used only with nonsupercharged engines. These methods are based on an analysis in the time range with the help of an observer structure. One such method is described in European Published Patent Application No. 1 426 594, for example.  
         [0004]     German Published Patent Application No. 100 62 895 describes a method for individual lambda regulation in which a control deviation and a regulator are assigned to each cylinder of the internal combustion engine, each regulator specifying a cylinder-specific triggering signal based on the assigned control deviation. Cylinder-specific actual values are thus ascertained, based on a signal of a sensor situated in the exhaust system and compared with a setpoint value. Based on the comparison, triggering signals for controlling the quantity of fuel and/or air on a cylinder-by-cylinder basis are specified. This method is based essentially on a frequency analysis similar to the aforementioned quantity compensation regulation in diesel engines. A prerequisite for stable functioning of both of the methods mentioned above is a fixed phase relationship between the injection quantity of the cylinders and the measured lambda value. Both signals represent all cylinders. The injection quantity is allocated to each of the cylinders whereas the lambda value represents a continuous signal and is measured in a portion of the exhaust system through which exhaust gas of all cylinders to be analyzed flows. In the observer model mentioned above, an altered phase relationship may be compensated, e.g., by an altered dead time or by adjusting the allocation of the sampling values to the cylinders.  
         [0005]     The phase relationship may also be determined as a characteristics map e.g. via rotational speed-load. It is characteristic, however, that the phase relationship is determined in the calibration phase and the correlation is defined. The methods described above, however, fail to take into account the fact that the phase relationship of the analyzed signals also depends on other parameters. For example, changes in exhaust-gas recirculation rate, pressures and temperatures of the internal combustion engine and in particular the operating parameters of an exhaust turbocharger such as its rotational speed, scoop position and the like have a definite influence on the phase relationship in the signal to be analyzed, e.g., a lambda signal. It is problematical that most of these influences cannot be modeled with sufficient accuracy to minimize the risk of instability of the control circuit, so the previously known cylinder-specific lambda regulating methods are also limited to relatively few operating ranges.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the present invention is to improve upon a method for controlling the quantity of fuel and/or air to an internal combustion engine on a cylinder-by-cylinder basis to the extent that all possible influences on the phase relationship are taken into account and compensated, thus permitting stable control of the quantity of fuel and/or air to an internal combustion engine on a cylinder-by-cylinder basis, i.e., a lambda equalization on a cylinder-by-cylinder basis.  
         [0007]     The basic idea of the present invention is to ascertain vibration components in the frequency range caused by differences between individual cylinders and to compensate them separately for selected frequencies, to which end the following are provided per frequency to be compensated: an amplitude regulator that determines the amplitude of a correction intervention and a phase regulator that determines the allocation of an intervention pattern with regard to the cylinders.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1 , schematically shows an essentially known internal combustion engine in which the method according to the present invention is used.  
         [0009]      FIG. 2  schematically shows the method according to the present invention on the basis of the camshaft frequency.  
         [0010]      FIG. 3  schematically shows the calculation of the weighting factors for the intervention patterns. 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows an internal combustion engine  100 . Air is supplied to the engine through fresh air line  118 , compressor  115  and intake line  110 . The exhaust gases from the internal combustion engine enter through exhaust-gas line  120  and turbine  125  into exhaust pipe  128 . Turbine  125  drives compressor  115  via a shaft (not shown).  
         [0012]     A quantity-determining actuating device  150  is assigned to the internal combustion engine. Fuel is supplied to the internal combustion engine via this actuating device. In the process, an individual fuel quantity may be allotted to each cylinder. This is depicted in  FIG. 1  by the fact that a quantity-determining actuating element  151  through  154  is assigned to each cylinder. A control unit  160  applies triggering signals to the individual actuating elements  151  through  154 . Actuating elements  151  through  154  are, for example, solenoid valves or piezoelectric actuators, which control the fuel metering in the particular cylinder. It may be provided in this context that per cylinder one injector is provided as well as a distributor pump or another element determining the injected fuel quantity, which alternately meters fuel into the cylinders. Control unit  160  also acts upon another final controlling element  155  that influences the amount of fresh air supplied to internal combustion engine  100 . In a simplified specific embodiment, this final controlling element  155  may also be omitted. In addition, control unit  160  processes the output signals of various sensors  170  which for example characterize the ambient conditions, e.g., temperature and pressure values as well as the driver input.  
         [0013]     In addition, control unit  170  processes signals from sensors  180  that characterize the exhaust-gas composition or the pressure and/or temperature in the exhaust gas. These sensors  180  are preferably situated between the internal combustion engine and turbine  125 . Alternatively or additionally, a sensor  185  may also be situated downstream from the turbine in the exhaust-gas line. Sensors  150  and/or  185  preferably detect a signal characterizing the oxygen concentration in the exhaust gas. Alternatively and/or additionally, it may also be provided for the pressure in the exhaust-gas line to be analyzed upstream or downstream from turbine  125 .  
         [0014]     The system functions as follows. The fresh air is compressed by compressor  115  and enters internal combustion engine  100  via intake line  110 . Quantity-determining actuating device  150  meters fuel into internal combustion engine  100 . A cylinder-specific fuel quantity is supplied to each cylinder as a function of the triggering signal of control unit  160 . Via the exhaust-gas line, the exhaust gases enter turbine  125 , drive the turbine and then reach the environment via exhaust-gas line  128 . Turbine  125  drives compressor  115  via a shaft (not shown).  
         [0015]     Based on the various input signals, the driver input in particular, control unit  160  calculates the triggering signals for acting upon actuating elements  151  through  154 . A preferred specific embodiment additionally final controlling element  155 , which controls the air supply to the internal combustion engine. This may be, for example, an exhaust-gas recirculation system that determines the quantity of recirculated exhaust gas. In a particularly preferred specific embodiment the quantity of air supplied to the individual cylinder is influenced. This may be implemented by valve control of the inlet and outlet valves, for example.  
         [0016]     Ascertaining the triggering signals for actuating elements  151  through  155  will be explained now in greater detail in conjunction with  FIGS. 2 and 3 .  
         [0017]     The lambda signal ascertained by sensor  180  is analyzed in the frequency range. The relevant frequencies are the camshaft frequency (NW) and its harmonics up to half the ignition frequency, e.g., for a four-cylinder engine NW, 2NW=KW (crankshaft frequency). In contrast with generally known methods, emerging e.g., from DE 100 62 895 A1, the method described below determines, in addition to the amplitude of these frequencies, also their phase. These may be ascertained using a fast Fourier transform, for example. Alternatively, the signal may also be bandpass filtered. For this purpose, the phase value is easily ascertained, e.g., from the passages through zero. Since there need not be any fixed correlation between the phase changes at the various frequencies, a separate regulator for coordinating internal combustion engine  100  is used for each frequency, as explained in greater detail below.  
         [0018]      FIG. 2  shows as an example how a cylinder-specific detuning at a certain frequency F may be depicted as a point A F  in the complex plane, length l F  representing the complex amplitude of the vibration and angle φ F  representing the phase offset between injection of one cylinder and the effect on the output signal detected by sensor  180 . The basic idea of the present invention is to create a regulator divided into a phase regulator and an amplitude regulator for each frequency.  
         [0019]     The task of the phase regulator is to determine the correct intervention pattern, i.e., the distribution of the intervention of the amplitude regulator to the individual cylinders. Since only differences between individual cylinders are to be compensated, the sum of the interventions must always equal zero for each frequency.  
         [0020]      FIG. 3  shows the allocation for camshaft frequency NW and crankshaft frequency KW in a four-cylinder engine as an example of the method according to the present invention. A periodic mean-free function, e.g., a sine function, is used as the basic function, containing one period for the NW frequency and more periods accordingly for its harmonics.  
         [0021]     Injection pattern G is obtained for each frequency F from the basic function on the basis of the angle assignment for the individual cylinders, the separation of the cylinders with respect to one another being fixed 2π/number of cylinders, but the absolute starting angle of the assignment being arbitrary, e.g., 0 for cylinder  1 . Weighting factors of the injection patterns are ascertained as follows:  
         g   NW     =     [       g     NW   ,     Cy   ⁢           ⁢   11         ,     g     NW   ,     Cy   ⁢           ⁢   12         ,     g     NW   ,     Cy   ⁢           ⁢   13         ,     g     NW   ,     Cy   ⁢           ⁢   14           ]         
         g   NW     =     [       g     KW   ,     Cy   ⁢           ⁢   11         ,     g     KW   ,     Cy   ⁢           ⁢   12         ,     g     KW   ,     Cy   ⁢           ⁢   13         ,     g     KW   ,     Cy   ⁢           ⁢   14           ]         
     or     
           g   NW     =     [       sin   ⁡     (     Δ   ⁢           ⁢     Φ   NW       )       ;     sin   ⁡     (       π   2     +     ΔΦ   NW       )       ;     sin   ⁡     (     π   +     Φ   NW       )       ;     sin   ⁡     (         3   ·   π     2     +     ΔΦ   NW       )         ]       ;       
           g   KW     =     [       sin   ⁡     (     Δ   ⁢           ⁢     Φ   NW       )       ;     sin   ⁡     (       2   ·     π   2       +     ΔΦ   KW       )       ;     sin   ⁡     (       2   ·   π     +     Φ   KW       )       ;     sin   ⁡     (       2   ·       3   ·   π     2       +     ΔΦ   KW       )         ]       ;       
 
 where ΔΦ is an angle offset for the shift in the injection pattern as determined by the phase regulator. Based on a cylinder-specific initial detuning of the signal to be analyzed at a frequency F having amplitude  1  ( FIG. 2 , point A) and an initial setting of intervention pattern g F , the amplitude regulator attempts to compensate the vibration via a quantity intervention Δme F . If the intervention pattern is not correct, however, i.e., the phase regulator is not tuned in a stable manner, a change results in the complex plane to A F ′. Both regulators may be active at the same time for this purpose. This results in a phase change Δφ F  and an amplitude change Δl F . A positive Δφ F  means a larger phase offset between the intervention quantity and the output quantity. 
 
         [0022]     The object of the phase regulator is to prevent phase changes Δφ F  between the input signal and output signal. The absolute value of phase φ F  is not important, however. If φ F  changes due to an intervention into the injection quantities to φ F ′, the phase regulator then attempts to keep the phase constant at φ F ′. For this purpose, the phase regulator adjusts the intervention pattern through intervention into phase offset Δφ F  in such a way that the previous phase change is counteracted. If an intervention having a certain intervention pattern into the injection quantity does not result in a phase shift, i.e., Δφ F =0, but only results in an amplitude change, then intervention pattern g F  into the different cylinders corresponds to the ratio of the actual detuning of the cylinders with respect to one another. The amplitude regulator may then coordinate the cylinders via the magnitude of intervention Δme F , i.e., it may then compensate the vibration. Point A F ′ then migrates in the complex plane directly to the origin, i.e., the cylinders are coordinated. Even if the phase cannot be kept entirely constant, the amplitude regulator ensures a reduction in complex amplitude.  
         [0023]     The intervention into the injection quantity of the cylinder Δme Cyl.i  is thus obtained from 
 
Δ me   Cyl.i   =Δme   NW   ·g   NW,Cyl.i   +Δme   KW   ·g   KW,Cyl.i . 
 
         [0024]     For example, a PI regulator may be used for this regulating operation. To stabilize the regulating operation at the origin, the intervention quantity of the amplitude regulator may be selected as a function of the distance from the zero point or, in the case of a small amplitude, i.e., when the value falls below a shutdown threshold, the amplitude regulator, like the phase regulator, may be shut down entirely. It is reactivated on exceeding an activation threshold. By superimposing the regulators for the different frequencies, the internal combustion engine is coordinated on the whole. This regulator is insensitive to further phase shifts, e.g., due to signal filtering.  
         [0025]     It should be emphasized that the method described above may be used in addition to a lambda compensation regulating method with all systems in which a joint output signal is analyzed, which has influences from various input quantities that are separated by a phase offset. The above method is especially suitable for regulating non-phase-stable systems. Thus, for example, the regulator may also be used for regulating air quantity if air interventions are possible on a cylinder-by-cylinder basis. The regulating method described above also has the great advantage that the regulator may be used as a self-learning regulator for phase-stable systems for reducing the need for calibration, e.g., for regulating rotational speed as an alternative to known quantity compensation regulating methods.