Abstract:
The present invention suppresses the worsening of stability due to a variation in EGR amounts between cylinders in a spark ignition engine. An engine control device for controlling a spark ignition engine equipped with an EGR means for recirculating exhaust gas in a combustion chamber and an air-fuel-ratio detection means for detecting the air-fuel ratio in each cylinder, the engine control device being characterized by being equipped with a means for changing the parameters for ignition control of a rich cylinder, when the air-fuel ratio of cylinders varies and there are richer cylinders and leaner cylinders relative to a prescribed air-fuel ratio during the execution of exhaust gas recirculation by the EGR means.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a control device of an engine, and particularly to an ignition control device of a spark ignition engine. 
       BACKGROUND ART 
       [0002]    As a background technique in this related art, there is disclosed JP 10-73068 A (PTL 1). In the publication, there is disclosed “an ignition timing control device of an internal combustion engine equipped with an ignition timing control means to control an ignition timing of the internal combustion engine, comprising: an air-fuel-ratio detection means which is provided in an exhaust system of the engine; and an air-fuel ratio estimation means which sets an observer to observe an inner state on the basis of a model describing a behavior of the exhaust system of the engine and receives an output of the air-fuel-ratio detection means to estimate an air-fuel ratio of each cylinder, wherein the ignition timing control means controls the ignition timing of each cylinder of the engine according to an estimated air-fuel ratio of each cylinder.” (see [Claim  1 ]). In addition, there is disclosed JP 2010-242630 A (PTL 2). In the publication, there are disclosed “a control device of a multi-cylinder engine, comprising: a supplying air adjustment means that adjusts an air amount to be supplied into a cylinder; an EGR passage through which part of an exhaust is recirculated to an intake system; an EGR gas amount adjustment means which adjusts an EGR gas amount recirculating to the intake system through the EGR passage; a combustion state estimation means which detects or estimates a combustion state of an air-fuel mixture; an air-fuel-ratio detection means which detects an air-fuel ratio of the exhaust; and a control means which controls the supplying air adjustment means and the EGR gas amount adjustment means, wherein the control means performs the EGR at the time of a heavy load operation in which a load of the engine is equal to or more than a predetermined value, and obtains a variation degree of the combustion state between the cylinders and/or a variation degree of the air-fuel ratio between the cylinders on the basis of at least one of an estimation result of the combustion state estimation means and a detection result of the air-fuel-ratio detection means, and controls an EGR rate on the basis of the variation degree,” and “the control device of the multi-cylinder engine in which the control means controls also an ignition timing in addition to the EGR rate (see [Claim  2 ] and [Claim  3 ]). 
       CITATION LIST 
     Patent Literature 
       [0003]    PTL 1: JP 10-73068 A 
         [0004]    PTL 2: JP 2010-242630 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    PTL 1 fails in taking that a combustion performance is degraded due to the variation in EGR amount among the cylinders into consideration. 
         [0006]    In addition, it is not possible in PTL 2 that a cylinder causing a variation in EGR amount is specified to suppress the combustion stability from being degraded in every cylinder. 
         [0007]    The invention has been made in view of the above problems, and an object thereof is to provide a spark ignition engine in which a cylinder causing a degradation of a combustion stability is specified when a variation in the EGR amount occurs among the cylinders, and suppresses the subject cylinder from being degraded in the combustion stability. 
       Solution to Problem 
       [0008]    In order to solve the above problems, the configurations disclosed in claims are employed for example. 
       Advantageous Effects of Invention 
       [0009]    According to the invention, a cylinder having an excessive EGR amount is specified at the time when the EGR amount varies between the cylinders, and a parameter related to an ignition control of the subject cylinder is corrected to suppress the stability of the subject cylinder from being degraded. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a diagram of an engine control system in first, third, fourth, and fifth embodiments. 
           [0011]      FIG. 2  is a diagram illustrating an inner configuration of a control unit in the first, third, fourth, and fifth embodiments. 
           [0012]      FIG. 3  is a block diagram illustrating the entire control in the first and fifth embodiments. 
           [0013]      FIG. 4  is a block diagram of a rich cylinder detection unit in the first, second, and fifth embodiments. 
           [0014]      FIG. 5  is a block diagram of an ignition timing calculation unit in the first, third, and fourth embodiments. 
           [0015]      FIG. 6  is a diagram of the engine control system in the second embodiment. 
           [0016]      FIG. 7  is a diagram illustrating an inner configuration of the control unit in the second embodiment. 
           [0017]      FIG. 8  is a block diagram illustrating the entire control in the second embodiment. 
           [0018]      FIG. 9  is a block diagram of a conduction time calculation unit in the second embodiment. 
           [0019]      FIG. 10  is a block diagram illustrating the entire control in the third embodiment. 
           [0020]      FIG. 11  is a block diagram of a two-rotational component calculation unit in the third and fourth embodiments. 
           [0021]      FIG. 12  is a block diagram of a two-rotational component phase calculation unit in the third and fourth embodiments. 
           [0022]      FIG. 13  is a block diagram of a rich cylinder detection unit in the third embodiment. 
           [0023]      FIG. 14  is a block diagram illustrating the entire control in the fourth embodiment. 
           [0024]      FIG. 15  is a block diagram of an individual-cylinder rotational variation calculation unit in the fourth embodiment. 
           [0025]      FIG. 16  is a block diagram of an unstable cylinder detection unit in the fourth embodiment. 
           [0026]      FIG. 17  is a block diagram of an excessive-EGR cylinder detection unit in the fourth embodiment. 
           [0027]      FIG. 18  is a block diagram of an ignition timing calculation unit in the fifth embodiment. 
           [0028]      FIG. 19  is profiles of EGR amount, G/F, A/F, and instability of a specific cylinder at the time when a failure occurs in an EGR control system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Hereinafter, embodiments will be described using the drawings. 
       First Embodiment 
       [0030]    In this embodiment, the description will be given about an example in which a cylinder having the richest air-fuel ratio is detected using an air-fuel ratio sensor of an exhaust manifold integrated part at the time of executing the external EGR, and an ignition timing of the subject cylinder is advanced. 
         [0031]      FIG. 1  is a system diagram illustrating this embodiment. In a multi-cylinder engine  9  (herein, four cylinders), the air from the outside passes through an air cleaner  1 , and flows into the cylinder via an intake manifold  4  and a collector  5 . An intake amount of air is adjusted by an electronic throttle  3 . The intake amount of air is detected by an air flow sensor  2 . In addition, an intake air temperature is detected by an intake air temperature sensor  29 . In a crank angle sensor  15 , a signal is output every rotation angle of 10° of a crank shaft and every combustion period. A water temperature sensor  14  detects a coolant temperature of the engine  9 . In addition, an accelerator opening sensor  13  detects a stepping amount of an accelerator  6 , and thus detects a demand torque of a driver. 
         [0032]    The signals of the accelerator opening sensor  13 , the air flow sensor  2 , the intake air temperature sensor  29 , and a throttle opening sensor  17  attached to the electronic throttle  3 , the crank angle sensor  15 , the water temperature sensor  14  are transmitted to a control unit  16  described below. An operation state of the engine  9  is obtained from these sensor outputs, and an air amount, a fuel injection amount, an ignition timing, and a main operation amount of the engine  9  of an EGR amount are optimally calculated. 
         [0033]    A target air amount calculated in the control unit  16  is converted from a target throttle opening to an electronic throttle drive signal, and transmitted to the electronic throttle  3 . The fuel injection amount is converted into a value opening pulse signal, and transmitted to a fuel injection valve (injector)  7 . In addition, a drive signal to be ignited at the ignition timing calculated by the control unit  16  is transmitted to an ignition plug  8 . In addition, a drive signal is transmitted to an EGR valve  19  such that a target EGR amount calculated by the control unit  16  is executed. 
         [0034]    The injected fuel is mixed with the air from the intake manifold, and flows into the cylinder of the engine  9  so as to form an air-fuel mixture. The air-fuel mixture is exploded by a spark generated from the ignition plug  8  at a predetermined ignition timing. The piston is pushed down by the combustion pressure to be power of the engine  9 . The exhaust after the explosion is sent to a three-way catalyst  11  through an exhaust manifold  10 . Part of the exhaust is recirculated to the intake side through an exhaust recirculating pipe  18 . The amount of recirculation is controlled by the EGR valve  19 . 
         [0035]    The air-fuel ratio sensor  12  of the upstream of the catalyst is attached between the engine  9  and the three-way catalyst  11 . The O2 sensor  20  of the downstream of the catalyst is attached on the downstream side of the three-way catalyst  11 . 
         [0036]      FIG. 2  is a diagram illustrating an inner configuration of the control unit  16 . In an ECU  16 , the output values of the respective sensors such as the air flow sensor  2 , the air-fuel ratio sensor  12  of the upstream of the catalyst, the accelerator opening sensor  13 , the water temperature sensor  14 , the crank angle sensor  15 , the throttle opening sensor  17 , the O2 sensor  20  of the downstream of the catalyst, the intake air temperature sensor  29 , and a speed sensor are input. These output values are subjected to a signal processing such as noise removal by an input circuit  24  and then transmitted to an input/output port  25 . The values of the input port is managed by a RAM  23 , and subjected to a calculation processing in a CPU  21 . A control program describing the content of the calculation processing is written in a ROM  22  in advance. The values indicating the respective actuator operation amounts calculated according to the control program are stored in the RAM  23 , and then transmitted to the input/output port  25 . An operation signal of the ignition plug  8  is set to an ON/OFF signal which is turned on at the time when the primary coil in an ignition output circuit is energized, and turned off at the time when the coil is de-energized. The ignition timing is a time to be turned off from on. A signal which is set to the output port for the ignition plug  8  is amplified for an ignition output circuit  26  to have energy enough for the combustion, and supplied to the ignition plug  8 . In addition, a drive signal of the fuel injection valve  7  is set to an ON/OFF signal which is turned on at the time of opening and turned off at the time of closing. The drive signal is amplified for a fuel injection valve drive circuit  27  to have energy enough to open the fuel injection valve  7 , and transmitted to the fuel injection valve  7 . A drive signal to realize a target opening of the electronic throttle  3  is transmitted to the electronic throttle  3  through an electronic throttle drive circuit  28 . A drive signal to realize a target opening of the EGR valve  19  is transmitted to the EGR valve  19  through an EGR valve drive circuit  30 . 
         [0037]    Hereinafter, the description will be given about the control program written in the ROM  22 .  FIG. 3  is a block diagram illustrating the entire control, which is configured by the following calculation units.
       Rich cylinder detection unit ( FIG. 4 )   Ignition timing calculation unit ( FIG. 5 )       
 
         [0040]    The “rich cylinder detection unit” calculates the number (Cyl_R) of a cylinder of which the air-fuel ratio is richest. The “ignition timing calculation unit” calculates the ignition timings (Adv1 to Adv4) of the first to fourth cylinders. The ignition timing of the cylinder indicated by Cyl_R is advanced by a predetermined amount. Hereinafter, the details of the respective calculation units will be described. 
       &lt;Rich Cylinder Detection Unit (FIG.  4 )&gt; 
       [0041]    In this calculation unit, Cyl_R (the number of the rich cylinder (excessive EGR)) is calculated. The details are illustrated in  FIG. 4 . 
         [0042]    When an external EGR amount is equal to or more than a predetermined amount, this process is performed. The external EGR control will not be described here in detail since there are a number of well-known techniques.
       The engine rotation angle (from a reference angle) is set to Deg_R when Rabf satisfies Rabf&lt;M_Rabf and is minimized during a period of the engine rotates twice. The reference angle may be a predetermined angle such as an intake TDC of a specific cylinder.   A value obtained with reference to a map M_Deg_R_Cyl1 using TP (torque equivalent value) and Ne (rotation speed) is set to Deg_R_Cyl1 (a threshold value of the engine rotation angle at the time of being rich).   When Deg_R_Cyl1≦Deg_R&lt;Deg_R_Cyl1+180, Cyl_R=1   When Deg_R_Cyl1+180≦Deg_R&lt;Deg_R_Cyl1+360, Cyl_R=3   When Deg_R_Cyl1+360≦Deg_R&lt;Deg_R_Cyl1+540, Cyl_R=4   When Deg_R_Cyl1+540≦Deg_R&lt;Deg_R_Cyl1+720, Cyl_R=2       
 
         [0049]    M_Rabf is a target air-fuel ratio in an air-fuel ratio feedback control or an average air-fuel ratio of all the cylinders. 
         [0050]    The calculation method of TP is a well-known technique, and obtained by the following formula using Qa (air amount) and Ne (rotation speed) for example. 
         [0000]        K×Qa /( Ne ×Cyl)
 
         [0051]    Herein, K is a calibration coefficient for the conversion to the torque equivalent value, and Cyl is the number of cylinders of the engine. The map M_Deg_R_Cyl1 is determined according to the engine specification, a position where the air-fuel ratio sensor is attached, and characteristics. The map may be determined from a prototype test. 
       &lt;Ignition Timing Calculation Unit (FIG.  5 )&gt; 
       [0052]    In this calculation unit, Adv1 to Adv4 (ignition timings of the first to fourth cylinders) are calculated. The details are illustrated in  FIG. 5 .
       A value obtained with reference to a map M_Adv0_using TP (torque equivalent value) and Ne (rotation speed) is set to Adv0 (reference ignition timing).   When Cyl_R=1
           Adv1=Adv0+Adv_hos   Adv2=Adv3=Adv4=Adv0   
           When Cyl_R=2
           Adv2=Adv0+Adv_hos   Adv1=Adv3=Adv4=Adv0   
           When Cyl_R=3
           Adv3=Adv0+Adv_hos   Adv1=Adv2=Adv4=Adv0   
           When Cyl_R=4
           Adv4=Adv0+Adv_hos   Adv1=Adv2=Adv3=Adv0   
               
 
         [0066]    Further, since a minimum value of Rabf (air-fuel ratio) obtained by the rich cylinder detection unit is a rich degree, Adv_hos (advance angle correction amount) may be changed according to the rich degree (according to an excessive EGR amount). 
       Second Embodiment 
       [0067]    In this embodiment, the description will be given about an example in which a cylinder having the richest air-fuel ratio is detected using the air-fuel ratio sensor of the exhaust manifold integrated part at the time of performing the inner EGR, and an ignition energy of the subject cylinder is increased. 
         [0068]      FIG. 6  is a system diagram illustrating this embodiment. There is provided no external EGR system compared to the first embodiment ( FIG. 1 ). In addition, there are provided an intake valve  31  and an exhaust valve  32  of which the opening/closing timing is controllable. A drive signal is transmitted to the intake valve  31  and the exhaust valve  32  to realize a target opening/closing timing of the intake valve and the exhaust valve which is calculated by the control unit  16 . The other configurations are the same as those of the first embodiment ( FIG. 1 ), and thus the details will not be described. 
         [0069]      FIG. 7  illustrates the inner configuration of the control unit  16 , in which an intake valve drive circuit  33  and an exhaust valve drive circuit  34  are added to the first embodiment ( FIG. 2 ). The other configurations are the same as those of the first embodiment ( FIG. 1 ), and thus the details will not be described. 
         [0070]    Hereinafter, the description will be given about the control program written in the ROM  22 .  FIG. 8  is a block diagram illustrating the entire control which is configured by the following calculation units.
       Rich cylinder detection unit ( FIG. 4 )   Conduction time calculation unit ( FIG. 9 )       
 
         [0073]    The “rich cylinder detection unit” calculates the number (Cyl_R) of a cylinder of which the air-fuel ratio is richest. The “conduction calculation unit” calculates conduction times (Dwell1 to Dwell4) toward ignition coils of the first to fourth cylinders. The conduction time toward the ignition coil of the cylinder indicated by Cyl_R is lengthened by a predetermined amount. Hereinafter, the details of the respective calculation units will be described. 
       &lt;Rich Cylinder Detection Unit (FIG.  4 )&gt; 
       [0074]    In this calculation unit, Cyl_R (the number of rich cylinder (excessive EGR)) is calculated. While being illustrated in  FIG. 3 , the details are the same as those of the first embodiment, and thus will not be described. Further, this process is performed when an inner EGR amount is equal to or more than a predetermined amount. Since there are a lot of well-known techniques about the inner EGR control, the details thereof will not be described. 
       &lt;Conduction Time Calculation Unit (FIG.  9 )&gt; 
       [0075]    This calculation unit calculates Dwell1 to Dwell4 (the conduction times of the first to fourth cylinders). The details are illustrated in  FIG. 9 .
       A value obtained with reference to a map M_Dwell0 using TP (torque equivalent value) and Ne (rotation speed) is set to Dwell0 (reference conduction time).   When Cyl_R=1
           Dwell1=Dwell0+Dwell_hos   Dwell2=Dwell3=Dwell4=Dwell0   
           When Cyl_R=2
           Dwell2=Dwell0+Dwell_hos   Dwell1=Dwell3=Dwell4=Dwell0   
           When Cyl_R=3
           Dwell3=Dwell0+Dwell_hos   Dwell1=Dwell2=Dwell4=Dwell0   
           When Cyl_R=4
           Dwell4=Dwell0+Dwell_hos   Dwell1=Dwell2=Dwell3=Dwell0   
               
 
         [0089]    Further, since a minimum value of Rabf (air-fuel ratio) obtained by the rich cylinder detection unit is a rich degree, Dwell_hos (advance angle correction amount) may be changed according to the rich degree (according to an excessive EGR amount). 
       Third Embodiment 
       [0090]    In this embodiment, the description will be given about an example in which a cylinder having the richest air-fuel ratio is detected using a two-rotational frequency component of the engine in the signals of the air-fuel ratio sensors of the exhaust manifold integrated part at the time of executing the external EGR, and the ignition timing of the subject cylinder is advanced. 
         [0091]      FIG. 1  is a system diagram illustrating this embodiment. The configurations are the same as those of the first embodiment ( FIG. 1 ), and thus the details will not be described. 
         [0092]      FIG. 2  is a diagram illustrating the inner configuration of the control unit  16  which is the same as that of the first embodiment ( FIG. 2 ), and thus the details will not be described. 
         [0093]    Hereinafter, the description will be given about the control program written in the ROM  22 .  FIG. 10  is a block diagram illustrating the entire control which is configured by the following calculation units.
       Two-rotational component calculation unit ( FIG. 11 )   Two-rotational component phase calculation unit ( FIG. 12 )   Rich cylinder detection unit ( FIG. 13 )   Ignition timing calculation unit ( FIG. 5 )       
 
         [0098]    The “two-rotational component calculation unit” calculates a real part (R_2rev) and an imaginary part (I_2rev) of the two-rotational component of the signal (Rabf) of the air-fuel ratio sensor  12 . The “two-rotational component phase calculation unit” calculates a two-rotational component phase (Phase) from R_2rev and I_2rev. The “rich cylinder detection unit” calculates the number (Cyl_R) of a cylinder of which the air-fuel ratio is richest. The “ignition timing calculation unit” calculates the ignition timings (Adv1 to Adv4) of the first to fourth cylinders. The ignition timing of the cylinder indicated by Cyl_R is advanced by a predetermined amount. Hereinafter, the details of the respective calculation units will be described. 
       &lt;Two-Rotational Component Calculation Unit (FIG.  11 )&gt; 
       [0099]    In this calculation unit, the real part (R_2rev) and the imaginary part (I_2rev) of the two-rotational component of the signal (Rabf) of the air-fuel ratio sensor  12  are calculated. The details are illustrated in  FIG. 11 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed. The external EGR control will not be described here in detail since there are a number of well-known techniques. 
         [0100]    A difference between a current value and a previous value of Rabf is calculated, and set to dRabf (a differential value of signals of the air-fuel ratio sensor of the upstream of the catalyst). The process (discrete Fourier transform) surrounded by a dotted line in the drawing is performed on dRabf. The current value, the previous value, a first value before the previous value, and a second value before the first value of C_R×dRabf are added and set to R_2rev. In addition, the current value, the previous value, a first value before the previous value, and a second value before the first value of C_I×dRabf are added and set to I_2rev. Herein, C_R and C_I are calculated according to CYLCNT (cylinder No.) as follows. CYLCNT is updated when a piston position of an N-th cylinder is at a predetermined value. Herein, it is assumed that the updating is performed at 110 deg before the compression TDC of the subject cylinder.
       When CYLCNT=1, C_R=1   When CYLCNT=3 or 2, C_R=0   When CYLCNT=4, C_R=−1
 
In addition,
   When CYLCNT=1 or 4, C_I=0   When CYLCNT=3, C_I=1   When CYLCNT=2, C_I=−1       
 
         [0107]    When a difference between the current value and the previous value of Rabf is subjected to the Fourier transform, it is possible to detect a cylinder having a relatively rich air-fuel ratio among all the cylinders (a cylinder having an air-fuel ratio richer than an average value of all the cylinders). 
       &lt;Two-Rotational Component Phase Calculation Unit (FIG.  12 )&gt; 
       [0108]    In this calculation unit, the two-rotational component phase (Phase) is calculated. The details are illustrated in  FIG. 12 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed. Phase0 (a reference value of the two-rotational component phase) is obtained from R_2rev and I_2rev as follows. 
         [0000]      arc tan 2( I _2rev/ R _2rev)×(180/π)
 
         [0000]    Herein, arc tan 2 calculates an arc tangent value (=phase) corresponding to four quadrants (−180 to 180 deg). 
         [0109]    Further, Phase (two-rotational component phase) having a phase range of 0 to 360 deg is obtained. Specifically, Phase=Phase0+360 is set at the time of Phase0&lt;0, and Phase=Phase0 at the other time. 
       &lt;Rich Cylinder Detection Unit (FIG.  13 )&gt; 
       [0110]    In this calculation unit, Cyl_R (the number of the rich cylinder (excessive EGR)) is calculated using Phase. The details are illustrated in  FIG. 13 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed.
       A value obtained with reference to a map M_K_Phase1 using TP (torque equivalent value) and Ne (rotation speed) is set to K_Phase1 (threshold value of the phase at the time of being rich).   When K_Phase1≦Phase&lt;K_Phase1+90, Cyl_R=1   When K_Phase1+90≦Phase&lt;K_Phase1+180, Cyl_R=3   When K_Phase1+180≦Phase&lt;K_Phase1+270, Cyl_R=4   When K_Phase1+270≦Phase&lt;K_Phase1+360, Cyl_R=2       
 
         [0116]    The map M_K_Phase1 is determined according to the engine specification, a position where the air-fuel ratio sensor is attached, and characteristics. The map may be determined from a prototype test. 
       &lt;Ignition Timing Calculation Unit (FIG.  5 )&gt; 
       [0117]    In this calculation unit, Adv1 to Adv4 (ignition timings of the first to fourth cylinders) are calculated. While being illustrated in  FIG. 5 , the details are the same as those of the first embodiment, and thus will not be described. 
       Fourth Embodiment 
       [0118]    In this embodiment, the description will be given about an example in which a cylinder having an excessive EGR amount is detected using both the air-fuel ratio sensor of the exhaust manifold integrated part and the crank angle sensor at the time of executing the external EGR, and the ignition timing of the subject cylinder is advanced. 
         [0119]      FIG. 1  is a system diagram illustrating this embodiment. The configurations are the same as those of the first embodiment ( FIG. 1 ), and thus the details will not be described. 
         [0120]      FIG. 2  is a diagram illustrating the inner configuration of the control unit  16  which is the same as that of the first embodiment ( FIG. 2 ), and thus the details will not be described. 
         [0121]    Hereinafter, the description will be given about the control program written in the ROM  22 .  FIG. 14  is a block diagram illustrating the entire control which is configured by the following calculation units.
       Two-rotational component calculation unit ( FIG. 11 )   Two-rotational component phase calculation unit ( FIG. 12 )   Individual-cylinder rotational variation calculation unit ( FIG. 15 )   Unstable cylinder detection unit ( FIG. 16 )   Excessive-EGR cylinder detection unit ( FIG. 17 )   Ignition timing calculation unit ( FIG. 5 )       
 
         [0128]    The “two-rotational component calculation unit” calculates a real part (R_2rev) and an imaginary part (I_2rev) of the two-rotational component of the signal (Rabf) of the air-fuel ratio sensor  12 . The “two-rotational component phase calculation unit” calculates a two-rotational component phase (Phase) from R_2rev and I_2rev. The “individual-cylinder rotational variation calculation unit” calculates rotational variations (dNe_1, dNe_2, dNe_3, and dNe_4) of the cylinders from a signal of the crank angle sensor  15 . The “unstable cylinder detection unit” calculates a cylinder (instability flags of the first to fourth cylinders) (f_dNe_1 to f_dNe_4) which becomes unstable by the excessive EGR amount from dNe_1, dNe_2, dNe_3, and dNe_4. The “excessive-EGR cylinder detection unit” calculates the number (Cyl_R) of the cylinder having a rich air-fuel ratio due to the most excessive EGR amount. The “ignition timing calculation unit” calculates the ignition timings (Adv1 to Adv4) of the first to fourth cylinders. The ignition timing of the cylinder indicated by Cyl_R is advanced by a predetermined amount. Hereinafter, the details of the respective calculation units will be described. Hereinafter, the details of the respective calculation units will be described. 
       &lt;Two-Rotational Component Calculation Unit (FIG.  11 )&gt; 
       [0129]    In this calculation unit, the real part (R_2rev) and the imaginary part (I_2rev) of the two-rotational component of the signal (Rabf) of the air-fuel ratio sensor  12  are calculated. While being illustrated in  FIG. 11 , the details are the same as those of the third embodiment, and thus will not be described. 
       &lt;Two-Rotational Component Phase Calculation Unit (FIG.  12 )&gt; 
       [0130]    In this calculation unit, the two-rotational component phase (Phase) is calculated. While being illustrated in  FIG. 12 , the details are the same as those of the third embodiment, and thus will not be described. 
       &lt;Individual-Cylinder Rotational Variation Calculation Unit (FIG.  15 )&gt; 
       [0131]    In this calculation unit, the rotational variations (dNe_1, dNe_2, dNe_3, and dNe_4) of the respective cylinder are calculated. The details are illustrated in  FIG. 15 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed.
       A difference between the current value and the previous value of Ne (rotation speed) is calculated, and set to dNe (rotational variation).   When CYLCNT=1,
           dNe_1=dNe_1 (previous value), dNe_2=dNe_2 (previous value)   dNe_3=dNe_3 (previous value), dNe_4=dNe   
           When CYLCNT=2,
           dNe_1=dNe_1 (previous value), dNe_2=dNe_2 (previous value)   dNe_3=dNe, dNe_4=dNe_4 (previous value)   
           When CYLCNT=3,
           dNe_1=dNe_1 (previous value), dNe_2=dNe   dNe_3=dNe_3 (previous value), dNe_4=dNe_4 (previous value)   
           When CYLCNT=4,
           dNe_1=dNe, dNe_2=dNe_2 (previous value)   dNe_3=dNe_3 (previous value), dNe_4=dNe_4 (previous value)   
               
 
       Herein, 
       [0000]    
       
         
           
             dNe_1: Rotational variation of the first cylinder 
             dNe_2: Rotational variation of the second cylinder 
             dNe_3: Rotational variation of the third cylinder 
             dNe_4: Rotational variation of the fourth cylinder 
           
         
       
     
       &lt;Unstable Cylinder Detection Unit (FIG.  16 )&gt; 
       [0149]    In this calculation unit, the cylinders f_dNe_1 to f_dNe_4 (instability flags of the first to fourth cylinders) of which the stability is degraded due to the excessive EGR amount are detected using dNe_1, dNe_2, dNe_3, and dNe_4. The details are illustrated in  FIG. 16 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed.
       The cylinders are initialized to f_dNe_1=0, f_dNe_2=0, f_dNe_3=0, and f_dNe_4=0.   When dNe_1 k dNe, f_dNe_1=1   When dNe_2 k dNe, f_dNe_2=1   When dNe_3 k dNe, f_dNe_3=1   When dNe_4 k dNe, f_dNe_4=1       
 
       &lt;Excessive-EGR Cylinder Detection Unit (FIG.  17 )&gt; 
       [0155]    In this calculation unit, Cyl_R (the number of the cylinder having the excessive EGR amount) is calculated using Phase and f_dNe_1 to f_dNe_4. The details are illustrated in  FIG. 17 . When an external EGR amount is equal to or more than a predetermined amount, this process is performed.
       A value obtained with reference to a map M_K_Phase1 using TP (torque equivalent value) and Ne (rotation speed) is set to K_Phase1 (threshold value of the phase at the time of being rich).   When K_Phase1≦Phase&lt;K_Phase1+90 and f_dNe_1=1, Cyl_R=1   When K_Phase1+90≦Phase&lt;K_Phase1+180 and f_dNe_3=1, Cyl_R=3   When K_Phase1+180≦Phase&lt;K_Phase1+270 and f_dNe_4=1, Cyl_R=4   When K_Phase1+270≦Phase&lt;K_Phase1+360 and f_dNe_2=1, Cyl_R=2       
 
         [0161]    The map M_K_Phase1 is determined according to the engine specification, a position where the air-fuel ratio sensor is attached, and characteristics. The map may be determined from a prototype test. 
       &lt;Ignition Timing Calculation Unit (FIG.  5 )&gt; 
       [0162]    In this calculation unit, Adv1 to Adv4 (ignition timings of the first to fourth cylinders) are calculated. While being illustrated in  FIG. 5 , the details are the same as those of the first embodiment, and thus will not be described. 
       Fifth Embodiment 
       [0163]    In this embodiment, the description will be given about an example in which a cylinder having the richest air-fuel ratio is detected using an air-fuel ratio sensor of an exhaust manifold integrated part at the time of executing the external EGR, the ignition timing of the subject cylinder is advanced, and the ignition timings of the other cylinders are lagged. 
         [0164]      FIG. 1  is a system diagram illustrating this embodiment. The other configurations are the same as those of the first embodiment ( FIG. 1 ), and thus the details will not be described. 
         [0165]      FIG. 2  is a diagram illustrating the inner configuration of the control unit  16  which is the same as that of the first embodiment ( FIG. 2 ), and thus the details will not be described. 
         [0166]    Hereinafter, the description will be given about the control program written in the ROM  22 .  FIG. 3  is a block diagram illustrating the entire control, which is configured by the following calculation units.
       Rich cylinder detection unit ( FIG. 4 )   Ignition timing calculation unit ( FIG. 5 )       
 
         [0169]    The “rich cylinder detection unit” calculates the number (Cyl_R) of a cylinder of which the air-fuel ratio is richest. The “ignition timing calculation unit” calculates the ignition timings (Adv1 to Adv4) of the first to fourth cylinders. The ignition timing of the cylinder indicated by Cyl_R is advanced by a predetermined amount. The ignition timings of the other cylinders are lagged by a predetermined amount. Hereinafter, the details of the respective calculation units will be described. 
       &lt;Rich Cylinder Detection Unit (FIG.  4 )&gt; 
       [0170]    In this calculation unit, Cyl_R (the number of rich cylinder (excessive EGR)) is calculated. While being illustrated in  FIG. 4 , the details are the same as those of the first embodiment, and thus will not be described. 
       &lt;Ignition Timing Calculation Unit (FIG.  18 )&gt; 
       [0171]    In this calculation unit, Adv1 to Adv4 (ignition timings of the first to fourth cylinders) are calculated. The details are illustrated in  FIG. 18 .
       A value obtained with reference to a map M_Adv0 using TP (torque equivalent value) and Ne (rotation speed) is set to Adv0 (reference ignition timing).   When Cyl_R=1
           Adv1=Adv0+Adv_hos   Adv2=Adv3=Adv4=Adv0−Adv_hos_r   
           When Cyl_R=2
           Adv2=Adv0+Adv_hos   Adv1=Adv3=Adv4=Adv0−Adv_hos_r   
           When Cyl_R=3
           Adv3=Adv0+Adv_hos   Adv1=Adv2=Adv4=Adv0−Adv_hos_r   
           When Cyl_R=4
           Adv4=Adv0+Adv_hos   Adv1=Adv2=Adv3=Adv0−Adv_hos_r   
               
 
         [0185]    Further, since a minimum value of Rabf (air-fuel ratio) obtained by the rich cylinder detection unit is a rich degree, Adv_hos (advance angle correction amount) and Adv_hos_r (lag angle correction amount) may be changed according to the rich degree (according to an excessive EGR amount). In addition, the fuel injection amount of the cylinder other than the cylinder having the richest air-fuel ratio may be corrected to be increased. 
         [0186]    The effects of the configuration of the invention are as follows. An engine control device of the invention controls, for example, a spark ignition engine which includes an EGR means to recirculate an exhaust to a combustion chamber, and an air-fuel-ratio detection means to detect an air-fuel ratio of each cylinder. The engine control device includes a means which corrects the ignition timing of the rich cylinder to be advanced or increases the ignition energy of the rich cylinder when the exhaust is recirculated by the EGR means and when the air-fuel ratio varies in the respective cylinders to be a rich air-fuel ratio and to be a lean air-fuel ratio compared to a predetermined value. 
         [0187]    In other words, in a case where the EGR is performed to recirculate the exhaust to the combustion chamber, the EGR amount (exhaust recirculating amount) varies among the cylinders by failure and aging of the EGR control system. Herein, examples of the failure and the aging may include clogging, damage of components, and degradation. It may be considered a state where the variation in EGR amount occurs among the cylinders by an error of the EGR control system. As illustrated in  FIG. 19 , when a cylinder has an excessive EGR amount due to the variation in EGR amount among the cylinders, G/F (a ratio of a gas amount in the cylinder and a fuel amount) becomes large, and the stability is degraded. On the other hand, the cylinder having an excessive EGR amount has a less air amount compare to the other cylinders. The air-fuel ratio control in the related art uniformly corrects the fuel injection amounts of all the cylinders such that the air-fuel ratio of the exhaust manifold integrated part becomes a target air-fuel ratio. Therefore, a cylinder which contains a less air amount but has the excessive EGR amount becomes relatively rich compared to the other cylinders. 
         [0188]    In a case where only the fuel injection amount of the subject cylinder is reduced, the richness of the air-fuel ratio of the subject cylinder is suppressed, but the air-fuel ratio becomes lean and thus the combustion stability is degraded still more. Since the cylinder having a rich air-fuel ratio due to the excessive EGR amount is lagged in the combustion speed, it is possible to be improved to be stable by advancing the ignition timing. In addition, since the cylinder having a rich air-fuel ratio due to the excessive EGR is lagged in the combustion speed, it is possible to be improved to be stable by increasing the ignition energy. Herein, as a method of increasing the ignition energy, a method of lengthening a spark ignition period and a method of reinforcing the current flowing to the ignition coil are considered. 
         [0189]    In addition, in the engine control device of the invention, the EGR means is at least one of an external EGR means which recirculates the exhaust toward the intake side and an internal EGR means which controls the exhaust to be left in the combustion chamber or directly return. 
         [0190]    In other words, the EGR means which recirculates the exhaust to the combustion chamber is at least one of the external EGR means which recirculates the exhaust toward the intake side using a recirculation pipe, and the internal EGR means which controls, for example, an opening/closing period and a lift amount of intake and exhaust valves such that the exhaust is left in the combustion chamber or directly returns. 
         [0191]    In addition, in the engine control device of the invention, the predetermined air-fuel ratio is a target air-fuel ratio or an average air-fuel ratio of all the cylinders in the air-fuel ratio feedback control. 
         [0192]    In other words, due to a variation of the EGR amount among the cylinders, the air-fuel ratio varies between a rich side and a lean side around the target air-fuel ratio in the air-fuel ratio feedback control or an air-fuel ratio of all the cylinders which is substantially equivalent to the target air-fuel ratio. Therefore, the rich cylinder is determined as a cylinder having an excessive EGR amount on the basis of the target air-fuel ratio in the air-fuel ratio feedback control or the average air-fuel ratio of all the cylinders. 
         [0193]    In addition, the engine control device of the invention includes at least a means which detects a combustion stability of the rich cylinder. When the combustion stability of the rich cylinder departs from a predetermined range, the engine control device corrects the ignition timing of the rich cylinder to be advanced or increases the ignition energy of the rich cylinder. 
         [0194]    In other words, as described above, when the cylinder having the excessive EGR amount becomes rich, the combustion stability is simultaneously degraded. At least the combustion stability of the rich cylinder is detected for each cylinder. When the richness and the combustion stability depart from a predetermined range, the subject cylinder is determined as stable due to the excessive EGR amount. Further, a combustion pressure, torque, and an angular acceleration are considered as the combustion stability. 
         [0195]    In addition, the engine control device of the invention includes at least a means which detects an angular acceleration of the rich cylinder. When the angular acceleration of the rich cylinder is equal to or less than a predetermined value, the engine control device corrects the ignition timing of the rich cylinder to be advanced or increases the ignition energy of the rich cylinder. 
         [0196]    In other words, as described above, when the cylinder having the excessive EGR amount becomes rich, the combustion stability is simultaneously degraded. The degradation of the combustion stability is detected by the angular acceleration of each cylinder. When the richness and the angular acceleration are equal to or less than a predetermined value, the subject cylinder is determined to have the excessive EGR amount. 
         [0197]    In addition, the engine control device of the invention corrects the ignition timing of a cylinder other than the rich cylinder to be lagged, or corrects the fuel injection amount to be increased. 
         [0198]    In other words, while the cylinder having the excessive EGR amount becomes rich, the cylinder having a less EGR amount is increased in the air amount in proportion to being rich so as to become lean. Since a knock limit is shifted toward a lagged angle side due to the less EGR amount, the ignition timing of the subject cylinder is lagged. Alternatively, the leaning is released by correcting the fuel injection amount to be increased. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  air cleaner 
           2  air flow sensor 
           3  electronic throttle 
           4  intake manifold 
           5  collector 
           6  accelerator 
           7  fuel injection valve 
           8  ignition plug 
           9  engine 
           10  exhaust manifold 
           11  three-way catalyst 
           12  air-fuel ratio sensor of upstream of catalyst 
           13  accelerator opening sensor 
           14  water temperature sensor 
           15  crank angle sensor 
           16  control unit 
           17  throttle opening sensor 
           18  exhaust recirculating pipe 
           19  EGR valve 
           20  O2 sensor of downstream of catalyst 
           21  CPU 
           22  ROM 
           23  RAM 
           24  input circuit 
           25  input/output port 
           26  ignition output circuit 
           27  fuel injection valve drive circuit 
           28  electronic throttle drive circuit 
           29  intake air temperature sensor 
           30  EGR valve drive circuit 
           31  intake valve 
           32  exhaust valve 
           33  intake valve drive circuit 
           34  exhaust valve drive circuit