Abstract:
Devices and methods for knock detection and knock control are described, in which, in addition to the customary component parts, the knock-detection circuit has a filter with switchable filter characteristic. The switchover of the filter characteristic, i.e. the shift, for example, of the mid-frequency of the filter, is carried out taking into consideration specifiable quantities or parameters, for instance, as a function of rotational speed. During the switchover of the filter characteristic, i.e. the shift of the mid-frequency, problems could occur in the knock detection; that is why measures are proposed which carry out the knock detection according to a special knock-detection algorithm during a switchover phase lasting a specifiable time.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for knock detection for an internal combustion engine and a corresponding device. 
   BACKGROUND INFORMATION 
   It is known that, for an internal combustion engine having a knock control, a knock-detection method is carried out in which the structure-borne noise picked up by the knock sensor or knock sensors and transformed into an electrical signal is processed in a signal-processing device. The signal-processing device, which, for example, is in the controller of the internal combustion engine, carries out a digital or analog signal conditioning. An essential component of the signal conditioning and of the associated evaluation circuit is a band-pass filter. In this context, the characteristic of the band-pass filter, thus its mid-frequency, quality, etc., must be planned such that the energetic point of concentration of the knock frequencies lies in the passband, while if possible, the frequencies of interference noises should lie outside of the passband. The interference noises are thereby essentially suppressed. 
   In modern engines, i.e. internal combustion engines, having a plurality of auxiliary systems and actuators, the frequencies of the interference noises may exhibit a strong dependence on rotational speed. Therefore, to be able to ensure good knock detection over the entire rotational-speed range, a speed-dependent filter characteristic is used for such engines or internal combustion engines. A device which carries out the above-described knock detection is described in the publication European Published Patent Application No. 0 576 650. 
   The device for knock detection known from European Published Patent Application No. 0 576 650 includes in its signal-processing branch, inter alia, a band-pass filter whose mid-frequency is alterable as a function of the engine speed. The mid-frequency in that case is such that the portions of the signal caused by the knocking are filtered as little as possible, while the background or interference signals are filtered out as well as possible. Since the frequencies are able to shift as a function of the rotational speed, a band-pass filter having alterable mid-frequency is used. In the known device, knocking is detected in the customary manner when an integrated measurand, which is formed from knock-typical portions of the signal, differs in a specifiable manner from a value dependent on the background signals. 
   With a change of the filter characteristic, generally the filtered signal, which represents a measure for the noise of the combustion, changes as well. Since the knock detection is carried out by forming the relationship virkr of the noise, i.e. the so-called knock integral ikr of the instantaneous combustion, and the noise averaged over several preceding combustions of the same cylinder, the so-called reference level rkr, problems may occur upon switching of the filter characteristic. If the value of virkr=ikr/rkr(old) exceeds knock-detection threshold ke, then knocking is detected, i.e., knocking is detected at ikr&gt;rkr(old)*ke. 
   The reference level is usually calculated recursively during knock-free operation according to the following formula or a similar formula and is described, for example, in German Published Patent Application No. 195 456 49. The following applies:
 
 rkr (new)=(1−1 /KRFTP )* rkr (old)+1 /KRFTP*ikr.  
 
   In this context, KRFTP is designated as the so-called compensation factor. 
   If a change of the filter characteristic takes place from one combustion to the next, i.e. if, for example, the mid-frequency of the filter is adapted to new or changed circumstances, then the instantaneous combustion noise, i.e. the knock integral ikr, experiences a possibly perceptible change, while the reference level rkr experiences this change, that is to say, is corrected only slowly. If the change of ikr is in particular a distinct increase, a so-called erroneous detection of knocking may occur, that is to say, a non-knocking combustion is mistakenly recognized as knocking. Consequently, changes of the ignition angle are caused which are actually unnecessary. As a result of this first erroneous detection, further erroneous detections may occur, since the knock integral ikr measured for a knocking combustion is not completely taken into account in the reference level rkr, but rather is immediately divided by the factor ke in order to avoid an increase of rkr due to the supposed knocking noise. The calculation is carried out according to the formula:
 
 rkr (new)=(1−1 /KRFTP )* rkr (old)+1 /KRFTP *( ikr/ke )
 
   Conversely, given a perceptible decrease of the combustion noise, the knock detection becomes deaf for a certain time, that is to say, knocking combustions are possibly not detected. Therefore, the change in the filter characteristic leads to a time-limited uncertainty in the knock detection. In addition, each erroneous detection of knocking leads to an unnecessary ignition retard, and therefore to corresponding loss of power and efficiency. On the other hand, the non-detection of knocking combustions causes increased engine wear. 
   SUMMARY OF THE INVENTION 
   An object of the present intention is to increase reliability of knock detection during the switchover phase of the filter characteristic. This objective is achieved by a method for knock detection having the features of the present invention. These features include an algorithm for calculating a reference level, that is defined such that the problem of erroneous detection or non-detection in the switchover phase of the filter characteristic is minimized, and at the same time, detection of actual knocking is still made possible. In this context, according to the present invention, two measures are proposed for the specifiable duration of the switchover phase, which on one hand lead to a reference-level compensation by selection of a smaller reference-level compensation factor KRFTP. Consequently, changes in the basic noise of the internal combustion engine, i.e. the engine, are detected more quickly. On the other hand, in response to a switchover of the filter characteristic which leads to a greater basic noise, the knock-detection threshold ke is corrected by a factor FKEFMU 1 &gt;1, i.e., the knock detection becomes more insensitive in order to avoid erroneous detections. 
   Correspondingly, in response to a switchover which leads to a smaller basic noise, a correction is carried out with FKEFMU 2 &lt;1. A reliable knock detection thereby continues to be ensured. 
   The method and the device of the present invention have the advantage that erroneous detection of knocking is reliably avoided even during the switchover phase of the filter, thus during the change of the filter characteristic. At the same time, knocking is reliably detected in this phase, as well. This advantage is achieved in that, during the switchover phase, a modified knock-detection method is carried out in which the knock-detection threshold is switched to be more insensitive and/or the reference-level compensation is modified. 
   Further advantages of the present invention include, advantageously, for the switchover phase, a specifiable time duration whose expiration is determined, for example, by a counter. 
   The values for the knock-detection thresholds and/or for the reference-level compensation which are necessary for the modified knock detection are advantageously formed with the aid of suitable factors that are applied to the customary values. 
   Further advantages of the design approaches can be seen in that different distances of the knock sensors to the individual cylinders are taken into account, since frequency shifts of the signals, which develop because of a combustion in the cylinders, and of the interference signals occur on the basis of the different distances of the knock sensors to the individual cylinders. Moreover, it is taken into account that a frequency shift of the signals from the combustion and of the interference signals takes place at different rotational speeds, so that here as well, the optimal frequency range is observed, and the knock detection may be carried out in an optimal manner, accordingly. 
   It is particularly advantageous that, given a shift of the mid-frequency of the band-pass filter, the passband is altered in a cylinder-individual manner and/or as a function of the rotational speed. Likewise, a simple change of the passband of the band-pass filter is implemented by shifting the upper and/or lower cut-off frequency of the passband. An advantageous quick readout of the mid-frequency or of the lower or the upper cut-off frequency of the passband is achieved, in that the mid-frequency and/or the lower cut-off frequency and/or the upper cut-off frequency is read from a specific characteristic map, the values of the characteristic map being allocated to values of the rotational speed and/or to the respective cylinder. 
   It is also advantageous to adapt the knock-detection threshold and/or a cylinder-individual reference value for the integrated knock signal as quickly as possible in response to the change of the passband of the band-pass filter. To that end, given a change of the passband, it is advantageous in each case to multiply both the knock-detection threshold and a compensation factor of the cylinder-individual reference value of the integrated knock signal by a correction factor, so that the knock detection is quickly adapted to the altered integrated knock signals to be expected. 
   The method for knock detection according to the present invention is advantageously executed in the processor of a controller used for controlling the internal combustion engine, the controller naturally including all means such as memory, connections, etc., which are necessary for the calculations, and receiving all necessary information, and initiating the necessary countermeasures for knock prevention after knocking has been detected. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a first system of the present invention for knock detection. 
       FIG. 2  shows a flow diagram according to the present invention. 
       FIG. 3  shows a second system of the present invention, in which the method of the present invention according to  FIG. 2  is executed. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a device of the present invention, described below, for detecting knock in an internal combustion engine. This device of the present invention is able to carry out the method according to the invention for knock detection for an internal combustion engine. 
   With the aid of knock sensors  1   a ,  1   b  through  1   n , it is possible to detect signals which develop because of the combustion taking place in the cylinders of the internal combustion engine (not shown here). The knock sensors are able to detect signals both from the combustion chamber of the cylinders, as well as signals near the cylinders, it being possible for the knock sensors to be arranged both in the combustion chamber and outside of the combustion chamber. Such sensors are, for example, pressure sensors for the combustion-chamber pressure, an ionic-current detector, an acceleration sensor, optical sensors, microphones or piezoceramic sensors which are mounted, for instance, in the cylinder-head bolt, the bolt on the crankshaft main bearing, the spark plug, in the cylinder-head gasket or on the engine block. In most internal combustion engines manufactured today, a plurality of knock sensors of the same type are provided, it also being possible, however, to provide only one knock sensor. A combination of various types of knock sensors is likewise conceivable. 
   The vibrations detected by knock sensors  1   a ,  1   b , . . .  1   n  are emitted as electrical signals and routed to an evaluation circuit  20 . Provided first of all in evaluation circuit  20  is a multiplexer  21 , to which the signals from individual knock sensors  11 ,  12 , . . . ,  1   n  are routed. In this context, the signal of a specific, definable knock sensor  1   a ,  1   b , . . .  1   n  is selected by multiplexer  21  depending on the specific cylinder in which a combustion is currently taking place, and thus a knock signal may also be expected. This selection is controlled by control unit  31  of microcomputer  30 , the control unit being connected to multiplexer  21 . The output signal of multiplexer  21  is subsequently routed in evaluation circuit  20  to an amplifier  23 , and is amplified there in accordance with the requirements of the further evaluation. This amplified signal is subsequently passed on to a band-pass filter  25  which selects a specific frequency band from the amplified signal. In so doing, band-pass filter  25  selects a frequency band in which frequencies lie that are characteristic for the knocking. This band-pass filtering is able to effectively blank out interference signals which lie in a different range. 
   The signal filtered by the band-pass is routed from band-pass filter  25  to rectifier  27  and is rectified in rectifier  27 . The rectified signal is subsequently integrated by an integrator  29 , so that at this point, a signal is available which is characteristic for the intensity of the knocking in a specific cylinder of the internal combustion engine. In this context, the integration of the signal transmitted from the rectifier is only carried out during a specific timing window, the timing window, also called a measuring window, including a time span which is characteristic for the appearance of knock signals. Analogous to the selection of the frequency band, by the selection of the timing window, interference signals may be blanked out. The timing window is predefined by control unit  31  of microcomputer  30 , control unit  31  being connected to integrator  29 . 
   The integrated signal obtained in evaluation circuit  20 , in the following also known as knock signal, is subsequently passed on to a microcomputer  30 , the signal first being converted by an analog-digital converter (A/D converter)  33  into a digital signal. The digital signal is routed to a knock-detection unit  35  likewise contained in microcomputer  30 . In knock-detection unit  35 , the digitalized knock signal is compared to a knock-detection threshold value. Knock-detection unit  35  obtains the knock-detection threshold from a memory unit  37  likewise contained in microcomputer  30 . In one simple exemplary embodiment, knocking is recognized by knock-detection unit  35  when the digitalized knock signal exceeds the knock-detection threshold. If the knock-detection threshold is not exceeded, then knock-detection unit  35  recognizes that no knocking has occurred. 
   In a further preferred exemplary embodiment, instantaneous digitalized knock signal U INT,inst  is compared to a reference value U REF,old  of the respective active cylinder. The comparison includes ascertaining a relative knock intensity RKI which results as the quotient of the instantaneous knock signal and the cylinder-individual reference value: 
           RKI   =       U     INT   ,   inst         U     REF   ,   old               
Relative knock intensity RKI is subsequently compared in knock-detection unit  35  to a knock-detection threshold value. In this exemplary embodiment as well, the knock-detection threshold value is again made available by memory unit  37 . In one preferred exemplary embodiment, memory unit  37  provides a cylinder-individual knock-detection threshold value for the active cylinder.
 
   In a further preferred exemplary embodiment, cylinder-individual reference value U REF,old , which is needed for calculating relative knock intensity RKI, is continuously adapted to the instantaneous operating state of the internal combustion engine, This is carried out using a compensation factor N, with whose aid a new cylinder-individual reference value U REF,new  is calculated, in which instantaneous knock signal U INT,inst  is taken into consideration. New, cylinder-individual reference value U REF,new  is preferably calculated using the following equation: 
   
     
       
         
           
             U 
             
               REF 
               , 
               new 
             
           
           = 
           
             
               
                 
                   N 
                   - 
                   1 
                 
                 N 
               
               × 
               
                 U 
                 
                   REF 
                   , 
                   old 
                 
               
             
             + 
             
               
                 1 
                 N 
               
               × 
               
                 U 
                 
                   INT 
                   , 
                   inst 
                 
               
             
           
         
       
     
   
   In order to recognize in which cylinder of the internal combustion engine a combustion is taking place at the moment, i.e. knocking may occur, provision is made in the internal combustion engine for a cylinder-detection unit  40  which makes it possible to recognize in which cylinder a combustion is taking place at the moment. The cylinder detection and cylinder allocation are preferably carried out with the aid of crankshaft detectors or camshaft detectors. The information about the cylinder in which a combustion process is currently taking place, ascertained by cylinder-detection unit  40 , is passed on both to knock-detection unit  35  and to memory unit  37 , and to a control unit  31 , likewise contained in microcomputer  30 , for the evaluation circuit. In knock-detection unit  35 , the information about the active cylinder is needed to provide the cylinder-individual reference value for calculating relative knock intensity RKI. In memory unit  37 , the information about the active cylinder is used to pass on the knock-detection threshold corresponding to the active cylinder, to knock-detection unit  35 . 
   In the device for knock detection according to the present invention, it is possible, with the aid of control unit  31  for the evaluation circuit, to control band-pass filter  25  in such a way that the passband of the band-pass filter is altered. To that end, control unit  31  for the evaluation circuit is connected to band-pass filter  25  of evaluation circuit  20 . In this context, in a first exemplary embodiment of the invention, the passband of band-pass filter  25  may be altered in a cylinder-individual manner. This is useful, since the knock sensors are located at different distances from the cylinders of the internal combustion engine. Moreover, the cylinders may have structural differences, so that the frequency range characteristic for knocking processes is different depending on the cylinder. In this context, control unit  31  receives the information about the active cylinder from cylinder-detection unit  40 . 
   In a further preferred exemplary embodiment of the invention, an engine-speed sensor  50  is also provided which measures the instantaneous speed of the internal combustion engine. Preferably, sensors are used for measuring the speed which are mounted on the crankshaft. The information about the engine speed is relayed by engine-speed sensor  50  to control unit  31  for the evaluation circuit. Control unit  31  uses this information and changes the passband of band-pass filter  25  as a function of the engine speed. The passband of band-pass filter  25  is changed analogously to the preceding exemplary embodiment, via a connection between control unit  31  and band-pass filter  25 . 
   To change the passband of band-pass filter  25 , in one preferred exemplary embodiment, the mid-frequency of band-pass filter  25  may be changed by control unit  31 . In another preferred exemplary embodiment, the lower cut-off frequency of the band-pass filter and/or the upper cut-off frequency of the band-pass filter may be changed. In this context, the resonant frequency of the band-pass filter is designated as mid-frequency. As lower or lower cut-off frequency of the band-pass filter, that frequency is designated below or above which the signal is so greatly attenuated that it has a negligible intensity there. For example, the frequency at which the attenuation is 3 dB may be utilized for this purpose: However, these cut-off frequencies may also be defined differently depending on the type of filter. In one particularly preferred exemplary embodiment, the mid-frequency and/or the lower cut-off frequency and/or the upper cut-off frequency is/are contained in memory unit  37 , in each case in a characteristic map, the values being allocated to ranges of the rotational speed and/or to the cylinder number. For readout of the mid-frequency and/or the lower cut-off frequency and/or the upper cut-off frequency, memory unit  37  is connected to control unit  31 . 
   In another preferred exemplary embodiment of the invention, in response to a change of the passband of the band-pass filter, the knock detection is adapted rapidly to this change. To that end, in a first exemplary embodiment, in response to change of the passband of the band-pass filter, the knock-detection threshold contained in memory unit  37  is multiplied by a threshold-value correction factor which is likewise contained in memory unit  37 . The knock-detection threshold thus corrected is then compared to relative knock intensity RKI in knock-detection unit  35 . The correction of the knock-detection threshold is advantageous, since with a change of the frequency range determined during the knock detection, an altered knock signal U INT,inst , and thus an altered relative knock intensity RKI, is ascertained. This change is first equalized after several steps by the adaptation of cylinder-individual reference value U REF,old . In one preferred exemplary embodiment, such a threshold-value correction factor may be predefined in a manner that it is established in the application; in a further exemplary embodiment, a cylinder-individual threshold-value correction factor may also be provided. In this context, in one preferred exemplary embodiment, the cylinder-individual knock-detection threshold contained in memory unit  37  is additionally a function of load and rotational speed, and is preferably from a characteristic map, contained in memory unit  37 , which is a function of load, rotational speed and/or cylinder number. 
   In a further preferred exemplary embodiment, likewise provided for compensation factor N in memory unit  37  is a compensation correction factor which is multiplied by compensation factor N. A compensation factor N CORR  corrected in this way is used in response to a change of the passband of the band-pass filter instead of compensation factor N, in order to calculate a cylinder-individual reference value U REF,new . With the aid of corrected compensation factor N CORR , it is possible when calculating a new cylinder-individual reference value to take the instantaneous integrated knock signal more strongly into account during the calculation. The knock detection is thus adapted quickly to the altered conditions due to the change of the passband of the band-pass filter. In one preferred exemplary embodiment, the compensation correction factor is cylinder-individual and may be established in the application. 
     FIG. 2  shows a further exemplary embodiment of the invention, in which the following method according to the invention is executed, for example, in the computer of the controller of an internal combustion engine: 
   If, when working with a knock-detection device which has a switchable filter, particularly a band-pass filter, and is known, for example from the European Patent 0 576 560 B1, a change of the filter characteristic is ascertained in step SCH 1 , then in step SCH 2 , a working-cycle counter is set to a starting value that may be established in the application, and reference-level compensation factor KRFTP is set to (smaller) value KRFTPO. The definition of the reference-level compensation factor may be gathered from the indicated publication. In this context, the working-cycle counter determines the duration of the switchover phase. In step SCH 3 , it is determined whether the change in the filter characteristic will lead to an increase or decrease of the basic engine noise, i.e. the basic noise of the internal combustion engine. In steps SCH 4 . 1  and SCH 4 . 2 , respectively, correction factor FK for knock-detection threshold KE is set to a value FKEFMU 1 &gt;1 or FKEFMU 2 &lt;1, accordingly. 
   In steps SCH 5  and SCH 6 , the working-cycle counter is decremented, if necessary. After the switchover phase has ended, if it is recognized in step SCH 7  that the working-cycle counter is at zero, i.e. has reached the value 0, the measures are canceled, that is to say, knock-detection threshold correction factor FK is set to 1, and reference-level compensation factor KRFTP is set to its normal value KRFTP 1 . The program thereupon begins anew. 
   If no change of the filter characteristic is recognized in step SCH 1 , in step SCH 9  it is checked whether a switchover phase is triggered. If it is determined in step SCH 9  that a switchover phase is active, thus, that a switchover phase was triggered in one of the preceding run-throughs and the working-cycle counter is &gt;0, the working-cycle counter is decremented with step SCH 5 . If, on the other hand, it is recognized in step SCH 9  that the working-cycle counter is not active, the program begins once more with step SCH 1 . 
   The meanings of the abbreviations in  FIG. 1  are: 
   AS: Working cycle 
   FK: Correction factor for the knock-detection threshold 
   FKEFMU 1 : Correction factor for the knock-detection threshold in the case of increasing engine noise, greater than or equal to 1 
   FKEFMU 2 : Correction factor for the knock-detection threshold in the case of decreasing engine noise, greater than or equal to 1. 
   KRFTP: Reference-level compensation factor 
   KRFTPO: Reference-level compensation factor in the case of non-steady engine noise (switchover phase) 
   KRFTP 1 : Reference-level compensation factor in the case of steady engine noise (without switchover phase) 
   The duration of the switchover phase may also be realized, for example, by a combustion counter and timer, instead of by a working-cycle counter. KRFTPO may also be determined, for instance, from the multiplication of KRFTP by a correction factor. 
     FIG. 2  shows an example for a knock-detection device for an internal combustion engine, with which the method of the present invention may also be carried out. Going into particulars,  10  designates a knock sensor that is allocated to a cylinder of the internal combustion engine and detects the noise in the cylinder or the noise of the internal combustion engine, and supplies an output signal as a function of this noise. The detected noise is conveyed via a controllable amplifier  11  and a filter  12  to a demodulating circuit  13 . 
   The rectifier of demodulating circuit  13  is connected to an integrator  15 . Due to the control of the amplification factor, the reference level of the output signal remains largely constant and independent of the engine speed. During a crankshaft-synchronous measuring window, which is formed by a controller  16  as a function of the output signal of an engine speed sensor  17 , integrator  15  forms measuring signal ikr. The measuring signal is compared in a comparator  18  to knock threshold KS predefined by the controller, and the output signal of comparator  19  is used as knock-detection signal. For the knock control, an output signal is conveyed from controller  16  to driver stage  20   a  for triggering the ignition in the respective cylinder. 
   The characteristic of filter  12  is alterable. For example, a band-pass filter may be switched over, for instance, by suitable drive pulses from controller  16 , so that predefinable frequencies lie in the passband. The switchover, i.e. the frequency change, may be triggered, for example, by suitable drive pulses from the microprocessor of controller  16 . Since, in particular, the background noise to be blanked out is dependent in its frequency on the rotational speed, the filter characteristic is changed or switched over as a function of the rotational speed. 
   The filter device, the rectifier and the integrator as well as further components, if desired, may be realized digital and/or analog, thus also as a combination of analog and digital elements, and integrated in controller  16  of an internal combustion engine, e.g. also in the processor of the controller. The necessary method steps are likewise executed, for example, in controller  16 . Controller  16 , e.g. the customary controller of an internal combustion engine, has suitable processor and storage means for carrying out the methods of the present invention.