Apparatus for generating control signal for controlling operation of internal combustion engine
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control apparatus for generating control signals for controlling operation of an internal combustion engine by effecting a timing control on the basis of identified reference positions corresponding to individual engine cylinders, respectively. More particularly, the present invention is concerned with a control apparatus for an internal combustion engine which can rapidly perform the cylinder identification to be reflected onto the timing control with a simplified structure while deriving a reference position signal relating to a crank shaft with high reliability to thereby ensure an enhanced accuracy for the timing control and which apparatus is capable of executing a backup control even in the case where the angular position signal indicating angular position of the crank shaft can not be obtained (i.e., even when a failure occurs in a first signal series mentioned hereinafter). 
     2. Description of Related Art 
     In general, in a control system for an internal combustion engine (hereinafter also referred to simply as the engine), there are employed a reference position signal and a cylinder identifying signal generated in synchronism with rotation of the engine for the purpose of controlling such parameters as the ignition timing, amount or quantity of fuel to be injected (hereinafter also referred to as the fuel injection quantity) and others. Usually, the signal generator for generating these signals is mounted on a cam shaft of the engine and structured such that one-to-one correspondence to the engine cylinders can be established for thereby detecting indirectly rotation or angular positions of a crank shaft. 
     For having better understanding of the present invention, technical background thereof will be descried in some detail. FIG. 8 is a perspective view showing a mechanical assembly of a rotation signal generator employed in a hitherto known engine control system, and FIG. 9 is a circuit diagram showing an electric signal processing circuit provided in association with the assembly shown in FIG. 8. For more particulars, reference may be made to Japanese Unexamined Patent Application Publication No. 68252/1994 (JP-A-6-68252). In this conjunction, it is assumed that the internal combustion engine now of concern has six cylinders. 
     Referring to FIG. 8, a cam shaft 1 is rotated at a speed equal to a half of the rotation speed (rpm) of a crank shaft (not shown) so that the control timings for all the six cylinders can fall within a single rotation of the cam shaft 1. 
     More specifically, a rotating disk 2 fixedly secured to the cam shaft 1 so as to corotate therewith is formed with a series of radial slits 3a in an outer peripheral portion of the rotating disk 2 with equal angular distance therebetween for generating an angular position signal POS composed of a series of pulses generated at every predetermined angle in the course of rotation of the rotating disk 2. Additionally, the rotating disk 2 is formed with a number of windows 3b (six windows in this case) for generating reference position signals REF in one-to-one correspondence to the engine cylinders, respectively. 
     A light emission diode (LED) 4a is disposed fixedly at a position facing a circular array of the slits 3a, while a light emission diode (LED) 4b is fixedly disposed at a position facing a circular array of the windows 3b. Further, photodiodes 5a and 5b are fixedly disposed in opposition to the light emission diodes 4a and 4b, respectively, with the rotating disk 2 being interposed therebetween, wherein the light emission diode 4a and photodiode 5a cooperate to constitute a first photocoupler while the light emission diode 4b and the photodiode 5b constituting a second photocoupler. 
     Now, referring to FIG. 9, amplifier circuits 6a and 6b are connected to output terminals of the photodiodes 5a and 5b, respectively, while connected to the output terminals of the amplifier circuits 6a and 6b are output transistors 7a and 7b, respectively. 
     As can be appreciated from the above, the rotating disk 2, the photocouplers 4a; 5a and 4b; 5b, the amplifier circuits 6a and 6b and the output transistors 7a and 7b constitute a rotation signal generator 8 for generating the angular position signal POS and the reference position signal REF. 
     FIG. 10 is a block diagram showing an engine control system known heretofore. Referring to the figure, the angular position signal POS and the reference position signal REF outputted from the rotation signal generator 8 are supplied to a microcomputer 10 by way of an interface circuit 9 to be arithmetically processed for controlling the ignition timing, the fuel injection quantity and others. 
     FIG. 11 is a waveform diagram for illustrating the angular position signal POS and the reference position signal REF outputted from the rotation signal generator 8. 
     Referring to the figure, the angular position signal POS is comprised of a series of pulses generated in correspondence to the slits 3a, respectively, formed in the rotating disk 2, wherein each of the pulses of the angular position signal POS is generated, for example, at every crank angle of 1°. Thus, the angular position signal POS can be used for determining or measuring the angular position of the crank shaft. On the other hand, the reference position signal REF has a pulse sequence repeated upon every rotation of the crank shaft for a crank angle of 720°. More specifically, the pulse sequence of the reference position signal REF includes six pulses each rising up at a predetermined crank angle in correspondence to each of the engine cylinders, wherein the six pulses have respective pulse widths which differ from one to another engine cylinder so that they can be used as the cylinder identifying signals, respectively. Needless to say, the pulses of the reference position signal REF having the different pulse widths or durations can be generated, for example, by changing correspondingly the circumferential length of the windows 3b, respectively. 
     The conventional engine control system implemented in such structure as described above by reference to FIGS. 8 to 10 can discriminatively identify the individual engine cylinders and the reference positions (reference crank angles) on the basis of the angular position signal POS and the reference position signal REF for effectuating optimal control of the ignition timing, the fuel injection quantity and others in dependence on the engine operation states. 
     However, because the cam shaft 1 is driven from the crank shaft (not shown) by way of a transmission mechanism such as a transmission belt/pulley mechanism (not shown either), there may arise a phase difference in rotation between the cam shaft 1 and the crank shaft, although it depends on the engine operation state. As a result of this, the angular positions indicated by the angular position signal POS and the reference position signal REF generated by the rotation signal generator 8 may undesirably be deviated or offset from the actual crank angle. Accordingly, when the engine operation control is performed on the basis of the signals suffering the phase deviation, the control of the ignition timing and other will naturally be accompanied with corresponding deviation or error, making it difficult or impossible to obtain the engine operation performance as intended. 
     To cope with the problem mentioned above, there has already been proposed such an apparatus which is so implemented as to generate the angular position signal POS and the reference position signal REF with high accuracy in association with the crank shaft while generating only the cylinder identifying signals bearing one-to-one correspondence to the individual engine cylinders, respectively, in association with the cam shaft 1, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 68252/1994 (JP-A-6-68252). 
     However, the engine control system disclosed in the above publication suffers shortcomings in that the sensor as well as peripheral devices thereof provided in association with the crank shaft for generating the angular position signal POS and the reference position signal REF is much complicated and expensive and that a great difficulty is encountered in realizing a backup control in the case where either one of the angular position signal POS or the reference position signal REF becomes unavailable due to occurrence of abnormality or fault in the sensors provided in association with the crank shaft, incurring possibly shutdown of the engine operation. 
     As is apparent from the foregoing, the engine control system known heretofore suffers a problem that the detection accuracy of the angular position signal POS and the reference position signal REF is impaired when the rotation signal generator 8 is provided in association with the cam shaft 1 because of possibility of the phase difference in rotation between the rotation signal generator 8 and the crank shaft, as a result of which deviation or error is involved in the control of the ignition timing and other, presenting a great obstacle in realizing the performance as intended. 
     On the other hand, in the case of the engine control system such as disclosed in Japanese Unexamined Patent Application Publication No. 68252/1994 (JP-A-6-68252) where the angular position signal POS and the reference position signal REF are generated by the sensor device provided in association with the crank shaft, while the cylinder identifying signal is generated by the means provided in association with-the cam shaft, there arises problems that the sensor and peripheral devices provided in association with the crank shaft are much complicated and that the backup control can not be carried in the case where the angular position signal POS or the reference position signal REF becomes unavailable. 
     SUMMARY OF THE INVENTION 
     In the light of the state of the art described above, it is an object of the present invention to provide an engine control system or apparatus which is capable of performing rapidly engine cylinder identification which is to be reflected to the timing control of the engine with a relatively simplified structure. 
     Another object of the present invention is to provide an engine control system or apparatus which can allow the reference position signal REF to be acquired with high reliability in association with the crank shaft to thereby enhance the accuracy of the timing control involved in the control of engine operation. 
     A further object of the present invention is to provide an engine control system or apparatus which is capable of performing a backup control even in the case where the angular position signal is not available (e.g. due to occurrence of error or unavailability of a first signal series mentioned hereinafter). 
     In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention an apparatus for generating a control signal for controlling operation of an internal combustion engine, which apparatus includes a first signal detector for generating a first signal series relating to rotation of a crank shaft of the internal combustion engine, a second signal detector for generating a second signal series relating to rotation of a cam shaft driven with a speed reduction ratio of &#34;1/2&#34; relative to the crank shaft, and a control means for controlling at least one parameter involved in operation of the internal combustion engine on the basis of at least one of the first and second signal series. The first signal series includes an angular position signal generated at every first predetermined angular position in synchronism with the rotation of the crank shaft and a reference position signal generated at every second predetermined angular position corresponding to a reference position of each of the engine cylinders. The second signal series contains a cylinder identifying signal, wherein a pulse form of the cylinder identifying signal at least for a given one of the engine cylinders differs from those for the other engine cylinders and wherein a pulse form of the cylinder identifying signal for a group of the engine cylinders which can simultaneously be controlled differs from the other engine cylinder group. The control means which may be constituted by a microcomputer includes a reference position signal detecting means for detecting the reference position signal on the basis of the first signal series, a cylinder group identifying means for identifying the cylinder group on the basis of at least the second signal series, a cylinder identifying means for discriminatively identifying each of the engine cylinders on the basis of at least the second signal series, a control timing arithmetic means for arithmetically determining control timings for controlling the parameter or parameters on the basis of the results of the cylinder identification performed by the cylinder identifying means and the second signal series, and an abnormality decision means for generating and outputting an abnormality decision signal to the cylinder identifying means and the control timing arithmetic means upon detection of a failure in the first signal series. 
     By providing the first detector for detecting the first signal series containing the reference position signal and the angular position signal in association with the crank shaft as described above, it is possible to enhance the accuracy of timings for controlling the operation of the internal combustion engine. Furthermore, by providing the second detector for detecting the second signal series in association with the cam shaft, the cylinder identification can easily and reliably be realized. Besides, by combination of the cylinder identifying signal, the reference position signal and the angular position signal, the cylinder identification which is to be reflected onto the timing control of the internal combustion engine can be carried out rapidly. Moreover, with the aid of the cylinder identifying signals corresponding to the individual cylinders, the internal combustion engine performance can be ensured at least to a necessary minimum even in the case where the first signal series is unavailable. 
     In a preferred mode for carrying out the invention, the cylinder identifying signal of the second signal series for identifying the given one cylinder group may be formed by pulses having a phase overlapping that of the reference position signal. The cylinder group identifying means may identify the cylinder group on the basis of a signal level of the second signal series at a time point at which the reference position signal is detected. 
     Owing to the arrangement in which the phase of the cylinder identifying signal (second signal series) is overlapped to that of the reference position signal, the cylinder group can rapidly be identified on the basis of the cylinder identification signal level upon detection of the reference position signal. 
     In another preferred mode for carrying out the invention, the control timing arithmetic means may be so arranged as to arithmetically determine the control timings for the parameter or parameters by counting pulses of the angular position signal. 
     By virtue of the arrangement mentioned above, the control timing can arithmetically be determined with high accuracy by counting the angular position signal pulses. 
     In yet another preferred mode for carrying out the invention, the reference position signal may be formed by a signal which corresponds to a low level interval of the first signal series during which the angular position signal is not generated continuously. In that case, a terminal end of the low level interval or the reference position signal may be so selected as to correspond to the reference position of each of the engine cylinders. 
     By providing the low or &#34;L&#34; interval in the first signal series with the reference position for each of the individual cylinders being set at the time point at which generation of the succeeding angular signal is started, the reference position signal can be obtained with high accuracy notwithstanding of simplified hardware structure. 
     In still another preferred mode for carrying out the invention, the reference position signal may be formed by pulses which are inserted in the angular position signal and which have a signal level differing from that of the pulses forming the angular position signal. 
     By inserting in the first signal series the pulses of level differing from the former for identifying the reference positions of the individual engine cylinders, it is possible to derive rapidly and accurately the reference position signal with simplified structure. 
     In a further preferred mode for carrying out the invention, the cylinder identifying signal may contain a pulse for identifying the given one cylinder, the pulse having a pulse width differing from those of the other pulses for identifying the other engine cylinders. 
     By setting the pulse width of the cylinder identifying signal for the given or specific engine cylinder so as to be different from those for the other cylinders, the engine cylinder identification can easily be accomplished. 
     In a yet further preferred mode for carrying out the invention, the cylinder identifying signal may contain an additional pulse generated within a predetermined angle relative to the cylinder identifying signal pulse for identifying the given one engine cylinder. 
     By generating the additional pulse in the vicinity of the cylinder identifying signal pulse for identifying the specific or given one cylinder, the cylinder identification can be carried out easily and rapidly. 
     In a still further preferred mode for carrying out the invention, the cylinder identifying means may be so implemented as to measure a time interval during which the cylinder identifying signal is generated on the basis of a count value of the angular position signal, to thereby identify discriminatively the individual engine cylinders from one another on the basis of the results of the measurement. 
     By measuring the duration of the interval during which the cylinder identifying signal is generated by counting the angular position signal pulses, as mentioned above, the cylinder identification can be realized with high reliability. 
     In a further preferred mode for carrying out the invention, the cylinder identifying means may be so arranged as to identify the individual engine cylinders on the basis of ratios of time intervals during which the cylinder identifying signals are generated, respectively. 
     By arithmetically determining the duty ratio of the cylinder identifying signal pulse, as mentioned above, the cylinder identification can be realized with high accuracy even when the first signal series can not be obtained, whereby the backup control can be realized with high accuracy and reliability. 
     The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the course of the description which follows, reference is made to the drawings, in which: 
     FIG. 1 is a functional block diagram showing schematically a general arrangement of an engine control apparatus according to a first embodiment of the invention; 
     FIG. 2 is a view showing schematically structures of first and second signal detectors employed in the engine control apparatus according to the first embodiment of the invention; 
     FIG. 3 is a fragmental perspective view showing exaggeratedly the first signal detector shown in FIG. 2; 
     FIG. 4 is a waveform diagram for illustrating, by way of an example, the first and second signal series generated in the engine control apparatus according to the first embodiment of the present invention; 
     FIG. 5 is a waveform diagram for illustrating operation of an engine control apparatus according to a second embodiment of the invention; 
     FIG. 6 is a waveform diagram for illustrating operation of an engine control apparatus according to a third embodiment of the invention; 
     FIG. 7 is a waveform diagram for illustrating operation of an engine control apparatus according to a fourth embodiment of the invention; 
     FIG. 8 is a perspective view showing a mechanical assembly of a rotation signal generator employed in a hitherto known engine control apparatus; 
     FIG. 9 is a circuit diagram showing an electric signal processing circuit of the rotation signal generator employed in the hitherto known engine control apparatus; 
     FIG. 10 is a block diagram showing a structure of the engine control apparatus known heretofore; and 
     FIG. 11 is a waveform diagram for illustrating operation of the hitherto known engine control apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described in detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the several views. 
     EMBODIMENT 1 
     Now, the engine control system or apparatus according to a first embodiment of the present invention will be described by reference to FIGS. 1 to 4, wherein FIG. 1 is a functional block diagram showing schematically a general arrangement of the engine control system or apparatus according to the first embodiment of the invention, FIG. 2 is a view showing schematically structures of signal detectors employed in the engine control system shown in FIG. 1, FIG. 3 is a fragmental perspective view showing exaggeratedly a first signal detector, and FIG. 4 is a waveform diagram for illustrating, by way of an example, first and second signal series generated in the engine control apparatus according to the first embodiment of the invention. 
     Referring to the figures and in particular to FIG. 2, a cam shaft 1 is rotated in synchronism with a crank shaft 11 of an internal combustion engine by way of a transmission mechanism such as a belt drive mechanism or the like with a speed reduction ratio of &#34;1/2&#34; relative to the crank shaft 11. 
     A first signal detector generally denoted by 81 is designed to output a first signal series POSR which is associated with the rotation of the crank shaft 11. More particularly, the first signal detector 81 is comprised of a rotating disk 12 mounted integrally on the crank shaft 11 for corotarion therewith, a plurality of projections (or teeth) 81a formed in the rotating disk 12 around an outer peripheral edge thereof with a predetermined angular distance or pitch (e.g. for every crank angle within a range of 1° to 10°) and a sensor 81b which may be constituted by an electromagnetic pickup device, Hall element, magnetoresistance type sensor device or the like. In the case of the structure shown in FIGS. 2 and 3, it is assumed, only by way of an example, that the sensor 81b is constituted by an electromagnetic pickup device. 
     The first signal series POSR includes angular position signal pulses generated by the sensor 81b at every first predetermined angular position of the crank shaft 11 in synchronism with the rotation thereof and reference position signals generated at every second predetermined angle (e.g. at every crank angle of 180°) which corresponds to the reference position of each cylinder of the internal combustion engine. More specifically, the angular position signal includes a series of pulses which are generated in correspondence to the individual projections 81a which are formed in succession around the outer peripheral edge of the rotating disk 12, wherein there are provided in the circumferential row of the projections 81a a predetermined number of non-toothed portions or segments 80 in which the projections or teeth 81a are absent over predetermined angular ranges each of ten to several ten degrees in terms of crank angle and in which the pulses of the angular position signal are not generated. In this conjunction, it should be mentioned that the terminal end of the non-toothed portion or segment 80 (corresponding to the start position of generation of the succeeding pulse train of the angular position signal) corresponds to the reference position of each of the engine cylinders. 
     On the other hand, provided in association with the cam shaft 1 is a second signal detector 82 for generating a second signal series SGC, wherein the second signal detector 82 is constituted by a rotating disk 2 mounted integrally on the cam shaft 1 for corotation therewith, a predetermined number of projections or teeth 82a formed in the rotating disk 2 around the outer peripheral edge in one-to-one correspondence to the engine cylinders, respectively, and a sensor 82b which may be constituted by an electromagnetic pickup device. Parenthetically, it is assumed, only by way of example, that the internal combustion engine now under consideration incorporates four cylinders. Accordingly, the number of the projections 82a is equal to four (refer to FIG. 2). 
     The second signal series SGC is composed of cylinder identifying signal pulses which are generated in correspondence to the individual engine cylinders, respectively. For a specific group of the engine cylinders (the engine cylinders #1 and #4 which can be controlled simultaneously), the second signal series SGC overlaps the reference position signal contained in the first signal series POSR in respect to the phase and assumes a high or &#34;H&#34; level at the reference position θR (refer to FIG. 4, PW1 and PW4). At this juncture, it should be mentioned that at least one of the cylinder identifying signal pulses which corresponds to a specific one of the engine cylinders (e.g. the cylinder #1) has a pulse duration or width PW1 which is longer than the pulse widths PW2 to PW4 of the other cylinder identifying signal pulses. 
     The first signal series POSR and the second signal series SGC are supplied to a microcomputer 100 by way of an interface circuit 90, as shown in FIG. 1. 
     The microcomputer 100 constitutes a control means for controlling parameters involved in the operation of the internal combustion engine. To this end, the microcomputer 100 is comprised of a reference position signal detecting means 101 for detecting a reference position signal from the first signal series POSR, a cylinder group identifying means 102 for discriminatively identifying a cylinder group including the engine cylinders which can be controlled simultaneously on the basis of the signal level of the second signal series SGC at a time point at which the reference position signal is detected, a cylinder identifying means 103 for identifying the individual cylinders on the basis of the temporal duration (pulse width) ratio of the cylinder identifying signal pulses of the second signal series SGC, a control timing arithmetic means 104 for arithmetically determining or calculating control timings for the engine operation parameters (such as ignition timing and others) by counting the angular position signal pulses contained in the first signal series POSR for thereby generating a parameter control timing signal, and an abnormality decision means 105 for outputting an abnormality decision signal E to the cylinder identifying means 103, the cylinder group identifying means 102 and the control timing arithmetic means 104 upon detection of occurrence of a failure in the first signal series POSR, as can be seen in FIG. 1. 
     The cylinder identifying means 103 is so designed as to identify the engine cylinders on the basis of at least the second signal series SGC, while the control timing arithmetic means 104 is so arranged as to arithmetically determine the control timing for the control parameter P on the basis of at least the result of the engine cylinder identification performed by the cylinder identifying means 103 and/or the second signal series SGC. 
     More specifically, when the engine system operates normally, the cylinder identifying means 103 measures the time intervals during which the cylinder identifying signal pulses contained in the second signal series SGC are generated, by counting the angular position signal pulses contained in the first signal series POSR during the corresponding time intervals, respectively, to thereby identify discriminatively the individual engine cylinders on the basis of the results of the measurement, as will be described later on. On the other hand, upon occurrence of abnormality such as occurrence of unavailability or absence of the first signal series POSR, the cylinder identifying means 103 responds to the abnormality decision signal E issued by the abnormality decision means 105 to thereby discriminatively identify the individual engine cylinders on the basis of the result of the calculation of the ratio of the temporal duration of the cylinder identifying signal pulse (e.g. the duty ratio between the duration of &#34;H&#34; level and that of &#34;L&#34; level adjacent to each other) by using only the second signal series SGC. In this manner, a backup control can be realized. 
     Similarly, the control timing arithmetic means 104 is so designed as to arithmetically determine or calculate the control timings for the engine operation parameter by counting the angular position signal pulses by making use of the reference position signal contained in the first signal series POSR as well as the cylinder identifying signal contained in the second signal series SGC so long as the engine operation is normal. By contrast, upon occurrence of abnormality (i.e., in the state in which the first signal series POSR can not be obtained), the control timing arithmetic means 104 responds to the abnormality decision signal E issued by the abnormality decision means 105 to thereby realize the backup control by regarding the falling edge timing of the cylinder identifying signal pulse as the reference position by relying on only the second signal series SGC. 
     Parenthetically, so long as the engine operation is normal, the control timing arithmetic means 104 arithmetically determines the control parameters P such as the ignition timing, the fuel injection quantity and others by reference to data stored in the form of a map in a memory (not shown) on the basis of operation state signals D supplied from a variety of sensors (not shown), to thereby control the individual engine cylinders in accordance with the control parameters P as determined. 
     Now, referring to FIG. 4, description will be made of operation of the engine control system implemented in the structure shown in FIGS. 1 to 3 according to the first embodiment of the present invention. 
     As mentioned previously, the rotating disk 12 having the projections or teeth 81a formed over every first predetermined angle around the outer peripheral edge is mounted on the crank shaft 11 with the sensor 81b being disposed in opposition to the projections 81a to thereby constitute the first signal detector 81 for generating the first signal series POSR which contains the angular position signal pulse corresponding to the projections 81a, respectively, and the reference position signal pulses corresponding to the non-toothed segments 80, respectively. 
     In this conjunction, it should be recalled that the row of the projections 81a is partially provided with the non-toothed portions or segments 80 (e.g. at two locations angularly distanced by 180° in crank angle in the case of the four-cylinder engine) so that the first signal series POSR includes not only the angular position signal pulses but also the reference position signal pulses. 
     The non-toothed segments 80 are detected by the sensor 81b which transforms the presence/absence of the projections or teeth 81a into the first signal series POSR (electric signal) to be inputted to the reference position signal detecting means 101 incorporated in the microcomputer 100, wherein the non-toothed segments 80 are detected or identified by the reference position signal detecting means 101 by comparing the intervals at which the angular position signal pulses and the reference position signal pulses are periodically generated, respectively. 
     As mentioned previously, the first signal series POSR (refer to FIG. 4) generated in correspondence to the projections 81a formed in the rotating disk 12 mounted on the crank shaft 11 contains the angular position signals constituted by the pulses generated upon every predetermined angle (e.g. at every crank angle of 1°) and the reference position signal which is constituted by the pulses each equivalent to the interval or period τ of &#34;L&#34; level during which the angular position signal can not be obtained over a predetermined angle corresponding to the non-toothed segment 80. In other words, the reference position signal originates in the non-toothed segment 80. The position at which the interval τ of &#34;L&#34; level is terminated (i.e., the position at which generation of the succeeding angular position signal is started) represents the reference position θR which is employed in the arithmetic determination of the control timings for the individual cylinders as executed by the control timing arithmetic means 104. 
     On the other hand, the second signal series SGC generated in correspondence to the projections 82a formed in the rotating disk 2 mounted on the cam shaft 1 contains the cylinder identifying signal pulses, wherein the pulse corresponding to a specific cylinder (e.g. the cylinder #1) is so set as to have the pulse width PW1 which is longer than the other engine cylinders by forming the projection 82a corresponding to the specific cylinder longer than those for the other cylinders. 
     Furthermore, each of the pulses for the engine cylinders #1 and #4 identified by the pulse widths PW1 and PW4, respectively, is at high or &#34;H&#34; level over the interval which covers the &#34;L&#34;-level interval or period τ of the first signal series POSR, while the pulses corresponding to the cylinders #3 and #2 identified by the pulse widths PW3 and PW2, respectively, assume &#34;H&#34; level immediately in succession to the reference position θR indicated by the first signal series POSR. 
     Consequently, the level of the second signal series SGC for the engine cylinders 190 1 and #4 which can be controlled simultaneously assumes &#34;H&#34; level at the reference position θR indicated by the first signal series POSR. On the other hand, the second signal series SGC for the engine cylinders #3 and #2 which can also simultaneously be controlled assumes the &#34;L&#34; level at the reference position θR indicated by the first signal series POSR. (See FIG. 4.) 
     By referencing the first signal series POSR and the second signal series SGC described above, the cylinder group identifying means 102 can discriminatively identify the group of the engine cylinders which can simultaneously be controlled on the basis of the signal level of the second signal series SGC (cylinder identifying signal) at the time point when the reference position θR is detected by the reference position signal detecting means 101. 
     More specifically, when the second signal series SGC is at &#34;H&#34; level at the reference position θR, the cylinder group identifying means 102 identifies the engine cylinder #1 or #4, while when the second signal series SGC is at &#34;L&#34; level at the reference position θR, the cylinder group identifying means 102 identifies the engine cylinder #3 or #2. 
     In this way, the group of the engine cylinders which can be fired on a group basis can rapidly be identified by the cylinder group identifying means 102, whereby the internal combustion engine control performance or controllability as required at the minimum can be ensured. 
     Of course, so long as the first signal series POSR and the second signal series SGC are obtained without failure, the cylinder identifying means 103 can discriminatively identify the specific engine cylinder as well as the other cylinders by measuring the pulse width of the second signal series SGC while counting the number of the angular position signal pulses contained in the first signal series POSR. 
     On the other hand, unless the first signal series POSR can be obtained normally due to a failure or defect of the sensor 81b provided in association with the crank shaft 11 (i.e., when the first signal series POSR continues to remain at a constant level or exhibits an abnormal pulse width), the abnormality decision means 105 generates the abnormality decision signal E which is then inputted to the cylinder group identifying means 102, the cylinder identifying means 103 and the control timing arithmetic means 104, as can be seen in FIG. 1. 
     In response, the cylinder identifying means 103 performs the engine cylinder identification on the basis of only the second signal series SGC, to thereby permit the backup control of the control parameters of the internal combustion engine. 
     In more concrete, the cylinder identifying means 103 performs calculation and comparison of the ratios between the &#34;H&#34;-level durations and the &#34;L&#34;-level durations of the pulses contained in the second signal series SGC sequentially to thereby identify the specific engine cylinder on the basis of the pulse having the greatest pulse width PW1 during which the second signal series SGC is at &#34;H&#34; level and then identifying the other cylinders successively. 
     In this conjunction, it should be appreciated that by setting the timings at which the individual pulses of the second signal series SGC fall as the ignition timings for the individual cylinders, there can be ensured the internal combustion engine control performance required at least to a necessary minimum for the engine control. 
     As will now be appreciated, by providing the first signal detector 81 for detecting the first signal series POSR containing the angular position signal and the reference position signal in association with the crank shaft 11, there takes place no phase difference due to interposition of the transmission mechanism such as the belt drive mechanism. Thus, the crank angle and the reference position θR can be detected with high accuracy, which in turn means that the ignition timings as well as the fuel injection quantity can be controlled with high reliability. 
     Furthermore, because of inter-phase overlap between the pulses of the cylinder identifying signal and the reference position signal, it is possible to identify the engine cylinders on a group-by-group basis by referencing the signal level of the second signal series SGC upon every detection of the reference position θR, whereby the group of the engine cylinders which can simultaneously be controlled can be detected rapidly and easily. Thus, the ignition timing control and the fuel injection control can be carried out rapidly and properly in particular upon starting of the engine operation. 
     Additionally, even in the case where the first signal series POSR can not be obtained due to a fault of the first signal detector 81 or for any other reason, the backup function for the engine cylinder identification as well as for the reference position identification can be realized on the basis of the duty cycle of the pulses contained in the second signal series SGC, as a result of which the ignition timing control as well as the fuel injection control can continuously be sustained by the backup control. 
     EMBODIMENT 2 
     In the case of the engine operation control system according to the first embodiment of the invention described above, the pulse contained in the second signal series SGC and identifying a specific engine cylinder is so set as to have a longer pulse width than the pulses for the other engine cylinders. However, such arrangement may equally be adopted that an additional pulse is generated in addition to the specific cylinder identifying signal pulse in the vicinity thereof within a predetermined angular range. 
     FIG. 5 is a waveform diagram for illustrating operation of the engine control system according to a second embodiment of the invention in which an additional pulse Ps is generated in the vicinity of the specific engine cylinder identifying signal pulse. 
     Referring to FIG. 5, the specific engine cylinder can discriminatively be identified by detecting the additional pulse Ps generated within a predetermined angular range in the vicinity of the intrinsic engine cylinder identifying signal pulse for the specific cylinder. 
     More specifically, so long as the first signal series POSR and the second signal series SGC are generated normally, it is possible to detect the additional pulse Ps within a predetermined angular range relative to the intrinsic engine cylinder identifying signal pulse by counting the angular position signal pulses contained in the first signal series POSR. On the other hand, when the first signal series POSR can not be obtained, existence of the additional pulse Ps within the predetermined angular range can discriminatively be detected through comparison of the duty ratios of the pulses contained in the second signal series SGC. 
     EMBODIMENT 3 
     In the case of the engine control system according to the second embodiment of the invention, the additional pulse Ps is generated relative to the pulse having the extended pulse width or duration PW1 designating the specific engine cylinder. In this conjunction, it is to be noted that the pulses for the individual engine cylinders may have a same pulse width when the additional pulse is added for identifying the specific engine cylinder as mentioned above. 
     Besides, the number of the additional pulses Ps is never limited to one pulse but may be selected rather arbitrarily. Alternatively, for the cylinder identifying signal pulses for the engine cylinder #4 which can be controlled simultaneously with the specific engine cylinder (cylinder #1), different number of the additional pulses Ps differing from that for the specific engine cylinder (#1) may be additionally generated for the discriminative cylinder identification. 
     FIG. 6 is a waveform diagram for illustrating operation of the engine control system according to a third embodiment of the invention in which the additional pulse Ps are generated with the pulse width of the second signal series SGC being set to a same value. 
     In the case of the engine control system according to the instant embodiment of the invention, two additional pulses Ps are generated in the vicinity of the cylinder identifying signal pulse for the specific engine cylinder #1, while one additional pulse Ps is additionally generated in the vicinity of the cylinder identifying signal pulse for the engine cylinder #4. 
     In this case, the engine cylinder #4 constituting the cylinder group together with the engine cylinder #1 can rapidly be identified on the basis of the number of the added pulses Ps. 
     Further, even in the case where the first signal series POSR cannot be obtained, it is possible to identify the individual engine cylinders by determining the number of the additional pulses Ps through the arithmetic determination of the duty ratios of the pulses contained in the second signal series SGC. 
     In this way, it is possible to perform in continuation the desired backup control by utilizing as the control timings the falling time points of the pulses or pulse groups (including the additional pulse Ps) contained in the second signal series SGC, wherein the falling time points mentioned above coincide with one another for the individual engine cylinders, respectively. 
     EMBODIMENT 4 
     In the engine control apparatus according to the preceding embodiments of the invention, the &#34;L&#34; level interval or period τ during which the angular position signal is not continuously generated is used as the reference position signal contained in the first signal series POSR. To this end, however, the pulses of different levels contained in the angular position signal generated continuously may be used. 
     FIG. 7 is a waveform diagram for illustrating operation of the engine control apparatus according to a fourth embodiment of the present invention in which a pulse PH having the level differing from that of the other angular position signal pulses is generated, wherein the position at which the pulse PH having the different level (higher level) corresponds to the reference position θR. 
     In the case of the instant embodiment of the invention, the projections 81a (refer to FIG. 3) formed around the outer peripheral edge of the rotating disk 12 mounted on the crank shaft 11 can be provided continuously without any interruptions or non-toothed portions 80. Furthermore, permanent magnets (not shown) can be provided at positions corresponding to the reference positions of the individual engine cylinders (at every 180° in crank angle in the case of the four-cylinder engine) in place of the projections 81a, respectively. 
     By mounting the permanent magnets on the outer peripheral edge of the rotating disk 12 at the reference angular positions, respectively, as described above, a large pulse PH makes appearance in the first signal series POSR at every reference position θR, which allows the reference position θR to be detected easily. 
     Furthermore, by using the pulse PH having the level or amplitude which differs from that of the first signal series POSR, as shown in FIG. 7, detection of the reference position θR can be performed rapidly, because it is no more required to wait for the termination of the interval τ of &#34;L&#34; level (see FIGS. 4 to 6). 
     MODIFICATIONS 
     Many features and advantages of the present invention are apparent form the detailed description and thus it is intended by the appended claims to cover all such features and advantages of the system which fall within the true spirit and scope of the invention. Further, since numerous modifications and combinations will readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation illustrated and described. 
     By way of example, in the case of the preceding embodiments, phase of the cylinder identifying signal for the specific engine cylinder and that of the reference position signal are overlapped each other so that the cylinder group can be discriminatively identified on the basis of the level of the cylinder identifying signal at the reference position. However, similar effect can be achieved by setting the pulse width (e.g. PW1) of the cylinder identifying signal for the specific cylinder group so that it differs from the cylinder identifying signals for the other engine cylinder group or alternatively by adding appropriately the additional pulse Ps so that the engine cylinder group of concern can be identified. In that case, it will be unnecessary to overlap the phase of the cylinder identifying signal with that of the reference position signal. 
     Accordingly, all suitable modifications and equivalents may be resorted to, falling within the spirit and scope of the invention.