JP4710716B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention is applied to an internal combustion engine provided with an air-fuel ratio sensor disposed in an exhaust passage, and based on an output value of the air-fuel ratio sensor, an air-fuel ratio (hereinafter, “ The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that controls “air-fuel ratio”.

  Conventionally, as this type of air-fuel ratio control device, for example, the one disclosed in Patent Document 1 is known. In the air-fuel ratio control apparatus of this internal combustion engine (hereinafter, also simply referred to as “engine”), an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor are respectively provided upstream and downstream of the catalyst disposed in the exhaust passage. The air-fuel ratio control is executed as follows.

  Based on the engine operating condition (air flow meter output value, operating speed, etc.), the amount of air (in-cylinder intake air amount) sucked into the combustion chamber during the intake stroke is defined as the reference air-fuel ratio (= theoretical air-fuel ratio). The divided value (basic fuel injection amount) is determined by table search. Deviation between the output value of the downstream air-fuel ratio sensor and the target value of this output value (a value corresponding to the target air-fuel ratio (= theoretical air-fuel ratio)) is proportionally / integrated / differentiated (PID process) to perform downstream feedback correction A quantity is calculated. The deviation (corresponding to deviation) between the value obtained by correcting the output value of the upstream air-fuel ratio sensor with this downstream feedback correction amount and the target value of this output value (value corresponding to the target air-fuel ratio) is proportional / integrated ( PI processing) and the upstream feedback correction amount is calculated. The command fuel injection amount is calculated by adding the upstream feedback correction amount to the basic fuel injection amount (that is, by feedback correcting the basic fuel injection amount). By instructing the injector to inject the fuel of the command fuel injection amount, feedback control is performed so that the air-fuel ratio matches the target air-fuel ratio (= theoretical air-fuel ratio).

  Generally, the difference between the basic fuel injection amount determined by the above table search and the true fuel amount required to make the air-fuel ratio the target air-fuel ratio (table error), used to obtain the basic fuel injection amount. The difference between the intake air flow rate measured by the air flow meter and the actual air flow rate (variation of the air flow meter), the difference between the command fuel injection amount commanded to the injector and the amount of fuel actually injected (injector (Variation) and the like (hereinafter collectively referred to as “error of basic fuel injection amount”) inevitably occur.

  The upstream / downstream feedback correction amount (hereinafter also simply referred to as “feedback correction amount”) includes a value of an integral term (I term), that is, a deviation integral that is updated by sequentially integrating the deviation. The value is multiplied by the feedback gain. As a result, even when the “error in the basic fuel injection amount” occurs, the “error in the basic fuel injection amount” is compensated by the deviation integral value (accordingly, the value of the integral term) by executing the feedback control described above. As a result, the air-fuel ratio can be matched and converged with the target air-fuel ratio.

  When an abnormality occurs in the air-fuel ratio control system during the execution of the feedback control described above (for example, when an abnormality occurs in an air flow meter, an injector, an air-fuel ratio sensor, etc.), the deviation continues to be maintained at a large value. As a result, the feedback correction value can gradually increase as the deviation integral value (and hence the value of the integral term) gradually increases. When the feedback correction value becomes excessively large, the correction amount with respect to the basic fuel injection amount becomes large, and problems such as the air-fuel ratio corresponding to the command fuel injection amount deviate from the combustible region may occur.

  From the above, a value (feedback guard value) that should not exceed the correction amount for the basic fuel injection amount is set, and when the feedback correction amount (or deviation integrated value) exceeds the feedback guard value, the feedback correction amount (or It is preferable to perform a guard process that limits the deviation integral value) to the feedback guard value.

For example, Patent Document 2 shows an example in which guard processing is performed on the downstream feedback correction amount. Patent Document 3 shows an example in which guard processing is performed on the deviation integral value of the integral term included in the upstream feedback correction amount.
JP-A-7-197837 JP 2005-36742 A Japanese Patent Laid-Open No. 2004-60613

  By the way, a technique is known in which a target air-fuel ratio is set to a different air-fuel ratio from a reference air-fuel ratio (theoretical air-fuel ratio) in accordance with the engine operating state. Thus, when the target air-fuel ratio is changed according to the engine operating state, the feedforward correction amount is acquired according to the deviation of the target air-fuel ratio from the reference air-fuel ratio, and the basic fuel injection amount is set as the feedback correction amount. In many cases, the command fuel injection amount is calculated by correcting with the feedforward correction amount. In other words, in addition to the above-mentioned “feedback correction of the basic fuel injection amount by the feedback correction amount” (hereinafter also simply referred to as “feedback correction”), “feedforward correction of the basic fuel injection amount by the feedforward correction amount”. (Hereinafter, simply referred to as “feedforward correction”) is often performed.

  As described above, when “feedforward correction” is performed in addition to “feedback correction”, the “correction amount for the basic fuel injection amount” is used as the feedback guard value, as in the case where only the above “feedback correction” is performed. Consider the case where “a value that should not exceed” is used. In this case, the total correction amount for the basic fuel injection amount based on the feedback correction amount and the feedforward correction amount is at most the feedforward correction amount, rather than the “value that should not exceed the correction amount for the basic fuel injection amount”. Can be bigger. That is, even when the feedback correction amount is subjected to guard processing, problems such as the air-fuel ratio deviating from the combustible region may occur.

  Accordingly, an object of the present invention is to provide an internal combustion engine engine that can set the guard value of the feedback correction amount to an appropriate value when “feedforward correction” is performed on the basic fuel injection amount in addition to “feedback correction”. An object of the present invention is to provide a fuel ratio control device.

  The air-fuel ratio control apparatus according to the present invention is applied to an internal combustion engine including an air-fuel ratio sensor (upstream air-fuel ratio sensor and / or downstream air-fuel ratio sensor) and fuel injection means (injector).

  In the air-fuel ratio control apparatus according to the present invention, a value (basic fuel) obtained by dividing the in-cylinder intake air amount by the reference air-fuel ratio (for example, the theoretical air-fuel ratio) based on the operating state of the internal combustion engine by the basic fuel injection amount acquisition means. Injection amount) is acquired.

  The target air-fuel ratio acquisition means acquires a target air-fuel ratio that changes according to the operating state (accelerator operation amount, operating speed, etc.) of the internal combustion engine based on the operating state.

  A feedforward correction amount for correcting the basic fuel injection amount in accordance with a shift of the target air-fuel ratio from the reference air-fuel ratio is acquired by the feedforward correction amount acquisition means. The feedforward correction amount may be a value added (subtracted) to the basic fuel injection amount or a value multiplied by the basic fuel injection amount.

  A feedback correction amount acquisition unit acquires a feedback correction amount for correcting the basic fuel injection amount based on the output value of the air-fuel ratio sensor. Here, the feedback correction amount is, for example, a deviation integral value itself that is updated by sequentially integrating a value corresponding to a deviation between a value based on an output value of the air-fuel ratio sensor and a value corresponding to the target air-fuel ratio. It may be a value obtained by PID processing or the like on the “value corresponding to the deviation”. The “value based on the output value of the air-fuel ratio sensor” refers to, for example, the output value of the upstream air-fuel ratio sensor itself, the output value of the downstream air-fuel ratio sensor itself, and the output value of the upstream air-fuel ratio sensor. The value is corrected based on the output value. The “value corresponding to the deviation” is, for example, a deviation between the output value of the air-fuel ratio sensor and the output value equivalent to the target air-fuel ratio, a deviation between the air-fuel ratio detected by the air-fuel ratio sensor and the target air-fuel ratio, or the like. This feedback correction amount may also be a value added (subtracted) to the basic fuel injection amount or a value multiplied by the basic fuel injection amount.

  When the feedback correction amount exceeds the feedback guard value, guard processing is performed by the guard processing execution means to limit the feedback correction amount to the feedback guard value.

  The command fuel injection amount is calculated by correcting the basic fuel injection amount based on the feed-forward correction amount and the guard-corrected feedback correction amount by the command fuel injection amount calculation means. Then, the air-fuel ratio control means performs feedback control so that the air-fuel ratio matches the target air-fuel ratio by instructing the fuel injection means to inject fuel of the command fuel injection amount. Thus, in the air-fuel ratio control apparatus according to the present invention, “feed forward correction” is performed in addition to “feedback correction”.

  The air-fuel ratio control apparatus according to the present invention is characterized in that the guard processing execution means is configured to change the feedback guard value according to the feedforward correction amount.

  According to this, the feedback guard value can be determined in consideration of the feedforward correction amount (and further in consideration of the total correction amount for the basic fuel injection amount based on the feedback correction amount and the feedforward correction amount). Therefore, regardless of the value of the feedforward correction amount, it is possible to prevent problems such as the air-fuel ratio deviating from the combustible region.

  More specifically, the guard processing execution means determines the feedback guard value based on a total guard value that is a value that should not exceed a correction amount for the basic fuel injection amount and the feedforward correction amount. It is suitable to be configured.

  According to this, for example, the feedback guard value is the total guard value based on the feedback correction amount and the feedforward correction amount, and the total correction value (= the value that the correction amount for the basic fuel injection amount should not exceed) ]), It can be determined to be equal to (or less than) the feedback correction amount corresponding to the case. Therefore, there is a problem that the air-fuel ratio deviates from the combustible region while setting the feedback guard value as large as possible (that is, while ensuring the difference between the upper limit guard value and the lower limit guard value (guard width) as large as possible). This can be prevented more reliably.

  When the air-fuel ratio sensor is a limiting current type oxygen concentration sensor disposed in the exhaust passage upstream of the catalyst, the guard processing execution means changes the total guard value according to the target air-fuel ratio. It is preferable to be configured as described above. More preferably, the total guard value is set to a smaller value as the target air-fuel ratio becomes farther from the reference air-fuel ratio (theoretical air-fuel ratio).

  In general, an air-fuel ratio (detected air-fuel ratio) detected by a limiting current type oxygen concentration sensor has a characteristic that an error increases as the distance from the stoichiometric air-fuel ratio increases. Therefore, as the target air-fuel ratio is further away from the stoichiometric air-fuel ratio, the degree to which the actual air-fuel ratio that is controlled to match the target air-fuel ratio shifts away from the stoichiometric air-fuel ratio with respect to the detected air-fuel ratio is likely to increase ( Details will be described later).

  This means that problems such as the air-fuel ratio deviating from the combustible region are more likely to occur as the target air-fuel ratio is farther from the stoichiometric air-fuel ratio. On the other hand, as the total guard value is reduced, problems such as the air-fuel ratio deviating from the combustible region are less likely to occur. The above configuration is based on such knowledge. According to this, it is possible to prevent problems such as the air-fuel ratio deviating from the combustible region stably regardless of the target air-fuel ratio.

  The total guard value is set to a smaller value as the target air-fuel ratio is separated from the reference air-fuel ratio (theoretical air-fuel ratio). The feedback guard value is increased as the target air-fuel ratio is separated from the reference air-fuel ratio (theoretical air-fuel ratio). It means that it is set to a smaller value (the guard width is set to a smaller value).

  Further, when the air-fuel ratio sensor is a limiting current type oxygen concentration sensor disposed in an exhaust passage upstream of the catalyst, the guard processing execution means obtains a deterioration degree for acquiring a deterioration state of the air-fuel ratio sensor. It is also preferable that an acquisition unit is provided and the total guard value is changed in accordance with a deterioration state of the air-fuel ratio sensor. More preferably, the total guard value is set to a smaller value as the degree of deterioration of the air-fuel ratio sensor increases.

  In general, the detected air-fuel ratio obtained by the limiting current type oxygen concentration sensor is such that the greater the degree of deterioration of the oxygen concentration sensor, the greater the degree to which the actual air-fuel ratio deviates from the stoichiometric air-fuel ratio with respect to the detected air-fuel ratio. (Details will be described later). This means that the greater the degree of deterioration of the oxygen concentration sensor, the more easily problems such as the air-fuel ratio deviating from the combustible region. The above configuration is based on this viewpoint. According to this, it is possible to prevent problems such as the air-fuel ratio deviating from the combustible region stably regardless of the degree of deterioration of the air-fuel ratio sensor.

  Note that the larger the degree of deterioration of the air-fuel ratio sensor, the smaller the total guard value is set. The larger the degree of deterioration of the air-fuel ratio sensor, the smaller the feedback guard value is set (the guard width is larger). It is set to a smaller value).

  Hereinafter, embodiments of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 shows a schematic configuration of a system in which the air-fuel ratio control apparatus according to the first embodiment is applied to a spark ignition type multi-cylinder (four-cylinder) internal combustion engine 10. The internal combustion engine 10 includes a cylinder block unit 20 including a cylinder block, a cylinder block lower case, an oil pan, and the like, a cylinder head unit 30 fixed on the cylinder block unit 20, and a gasoline mixture in the cylinder block unit 20. And an exhaust system 50 for releasing exhaust gas from the cylinder block 20 to the outside.

  The cylinder block unit 20 includes a cylinder 21, a piston 22, a connecting rod 23, and a crankshaft 24. The piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 through the connecting rod 23, whereby the crankshaft 24 rotates. The heads of the cylinder 21 and the piston 22 form a combustion chamber 25 together with the cylinder head portion 30.

  The cylinder head portion 30 includes an intake port 31 that communicates with the combustion chamber 25, an intake valve 32 that opens and closes the intake port 31, an intake cam shaft that drives the intake valve 32, and continuously changes the phase angle of the intake cam shaft. A variable intake timing device 33, an actuator 33a of the variable intake timing device 33, an exhaust port 34 communicating with the combustion chamber 25, an exhaust valve 35 for opening and closing the exhaust port 34, an exhaust camshaft 36 for driving the exhaust valve 35, an ignition plug 37, An igniter 38 including an ignition coil that generates a high voltage to be applied to the spark plug 37 and an injector (fuel injection means) 39 for injecting fuel into the intake port 31 are provided.

  The intake system 40 includes an intake manifold 41 including an intake manifold that communicates with the intake port 31 and forms an intake passage together with the intake port 31, an air filter 42 provided at an end of the intake pipe 41, and an intake pipe 41. A throttle valve 43 having a variable opening cross-sectional area of the passage, and a throttle valve actuator 43a made of a DC motor constituting throttle valve driving means are provided.

  The exhaust system 50 includes an exhaust manifold 51 that communicates with the exhaust port 34, and an exhaust pipe (exhaust pipe) that is connected to the exhaust manifold 51 (actually, a collection portion of the exhaust manifolds 51 that communicate with each exhaust port 34). ) 52, an upstream side three-way catalyst 53 (upstream side catalytic converter, hereinafter referred to as “first catalyst 53”) disposed (intervened) in the exhaust pipe 52, and downstream of the first catalyst 53. A downstream three-way catalyst 54 (hereinafter referred to as a “second catalyst 54”) disposed (intervened) in the exhaust pipe 52 is provided. The exhaust port 34, the exhaust manifold 51, and the exhaust pipe 52 constitute an exhaust passage.

  On the other hand, this system includes a hot-wire air flow meter 61, a throttle position sensor 62, a cam position sensor 63, a crank position sensor 64, a water temperature sensor 65, an exhaust passage upstream of the first catalyst 53 (in this example, each of the above exhaust manifolds). The air-fuel ratio sensor 66 (hereinafter referred to as “upstream air-fuel ratio sensor 66”) disposed in the collecting portion 51), the exhaust downstream of the first catalyst 53 and upstream of the second catalyst 54. An air-fuel ratio sensor 67 (hereinafter referred to as “downstream air-fuel ratio sensor 67”) disposed in the passage and an accelerator opening sensor 68 are provided.

  The hot-wire air flow meter 61 detects the mass flow rate per unit time of the intake air flowing through the intake pipe 41 and outputs a signal representing the mass flow rate Ga. The throttle position sensor 62 detects the opening of the throttle valve 43 and outputs a signal representing the throttle valve opening TA. The cam position sensor 63 generates a signal (G2 signal) having one pulse every time the intake camshaft rotates 90 ° (that is, every time the crankshaft 24 rotates 180 °). The crank position sensor 64 has a narrow pulse every time the crankshaft 24 rotates 10 ° and outputs a signal having a wide pulse every time the crankshaft 24 rotates 360 °. This signal represents the operating speed NE. The water temperature sensor 65 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The upstream air-fuel ratio sensor 66 is a limiting current type oxygen concentration sensor, and as indicated by a solid line in FIG. 2, an output value Vabyfs that is a voltage corresponding to a current output according to the air-fuel ratio A / F. Is output. In particular, when the air-fuel ratio is the stoichiometric air-fuel ratio (reference air-fuel ratio), the output value Vabyfs becomes the upstream target value Vstoich. As is apparent from FIG. 2, the upstream air-fuel ratio sensor 66 can accurately detect a wide range of air-fuel ratio A / F.

  The downstream air-fuel ratio sensor 67 is an electromotive force type (concentration cell type) oxygen concentration sensor, and outputs an output value Voxs, which is a voltage that changes suddenly in the vicinity of the theoretical air-fuel ratio, as shown in FIG. ing. More specifically, the downstream air-fuel ratio sensor 67 is approximately 0.1 (V) when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and is approximately 0.1 when the air-fuel ratio is richer than the stoichiometric air-fuel ratio. When 9 (V) and the air-fuel ratio is the stoichiometric air-fuel ratio, a voltage of 0.5 (V) is output. The accelerator opening sensor 68 detects the operation amount of the accelerator pedal 81 operated by the driver, and outputs a signal representing the operation amount Accp of the accelerator pedal 81.

  The electric control device 70 includes a CPU 71 connected by a bus, a routine (program) executed by the CPU 71, a table (look-up table, map), a ROM 72 in which constants and the like are stored in advance, and the CPU 71 temporarily stores data as necessary. The microcomputer is composed of a RAM 73 for storing data, a backup RAM 74 for storing data while the power is turned on, and holding the stored data while the power is shut off, an interface 75 including an AD converter, and the like. The interface 75 is connected to the sensors 61 to 68, supplies signals from the sensors 61 to 68 to the CPU 71, and in response to instructions from the CPU 71, the actuator 33a, the igniter 38, the injector 39, and the throttle of the variable intake timing device 33. A drive signal is sent to the valve actuator 43a.

(Outline of air-fuel ratio control)
Next, an outline of the air-fuel ratio control performed by the air-fuel ratio control apparatus (hereinafter referred to as “this apparatus”) configured as described above will be described.

  In this apparatus, the output value of the downstream air-fuel ratio sensor 67 becomes the downstream target value Voxsref (in principle, 0.5 (V), see FIG. 3) corresponding to the theoretical air-fuel ratio (reference air-fuel ratio). As described above, the output value Vabyfs of the upstream air-fuel ratio sensor 66 (that is, the air-fuel ratio upstream of the first catalyst 53) and the output value Voxs of the downstream-side air-fuel ratio sensor 67 (that is, the air-fuel ratio downstream of the first catalyst 53). The air-fuel ratio is controlled accordingly.

  More specifically, as shown in FIG. 4 which is a functional block diagram, the present apparatus is configured to include the functional blocks A1 to A15. Hereinafter, each functional block will be described with reference to FIG. Hereinafter, “feedback” may be referred to as “FB”, and “feed forward” may be referred to as “FF”.

  The in-cylinder intake air amount calculation means A1 includes an intake air flow rate Ga measured by the air flow meter 61, an operating speed NE obtained based on the output of the crank position sensor 64, and a table MapMc stored in the ROM 72. Based on this, the in-cylinder intake air amount Mc (k) that is the current intake air amount of the cylinder that reaches the intake stroke is obtained. Here, the subscript (k) indicates a value for the current intake stroke (hereinafter, the same applies to other physical quantities). The in-cylinder intake air amount Mc is stored in the RAM 73 while corresponding to the intake stroke of each cylinder.

<Setting of upstream target air-fuel ratio>
The upstream target air-fuel ratio setting means A2 determines the upstream target air-fuel ratio abyfr (k) (the “target air-fuel ratio”) based on the operating speed NE that is the operating state of the internal combustion engine 10, the accelerator pedal operation amount Accp, and the like. decide. The upstream target air-fuel ratio abyfr (k) is basically set to the stoichiometric air-fuel ratio, but is also set to an air-fuel ratio other than the stoichiometric air-fuel ratio depending on the operating speed NE and the accelerator pedal operation amount Accp. . The upstream target air-fuel ratio abyfr is stored in the RAM 73 while corresponding to the intake stroke of each cylinder. The upstream target air-fuel ratio setting means A2 corresponds to the “target air-fuel ratio acquisition means”.

<Determination of basic fuel injection amount>
The basic fuel injection amount determining means A3 calculates the basic fuel injection amount Fbase by dividing the in-cylinder intake air amount Mc (k) by the stoichiometric air-fuel ratio stoich. The basic fuel injection amount determination means A3 corresponds to the “basic fuel injection amount acquisition means”.

<Calculation of FF correction amount>
The FF correction amount calculating means A4 calculates the FF correction amount for correcting the basic fuel injection amount Fbase according to the deviation of the upstream target air-fuel ratio abyfr (k) from the stoichiometric air-fuel ratio stoich according to the following equation (1). DFF (equivalent to the “feedforward correction amount”) is obtained.

DFF = (Mc (k) ・ (stoich−abyfr (k))) / (stoich ・ abyfr (k)) (1)

  This FF correction amount DFF is used to set the air / fuel ratio to the stoichiometric air / fuel ratio stoich from the fuel amount (= Mc (k) / abyfr (k)) for setting the air / fuel ratio to the upstream target air / fuel ratio abyfr (k). Equal to the value obtained by subtracting the fuel amount (= Mc (k) / stoich). The FF correction amount DFF becomes a positive value when the upstream target air-fuel ratio abyfr (k) is richer than the theoretical air-fuel ratio (a richer value becomes larger), and the upstream target air-fuel ratio abyfr (k) is When leaner than the stoichiometric air-fuel ratio, it becomes a negative value (the leaner the value, the larger the absolute value). The FF correction amount calculation means A4 corresponds to the “feedforward correction amount acquisition means”.

<Calculation of command fuel injection amount>
The command fuel injection amount calculation means A5 adds the FF correction amount DFF and an FB correction amount DFB (corresponding to the “feedback correction amount”) described later (corresponding to the “feedback correction amount”) to the basic fuel injection amount Fbase. Thus, the command fuel injection amount Fi is obtained based on the following equation (2). The command fuel injection amount calculation means A5 corresponds to the “command fuel injection amount calculation means”.

Fi = Fbase + DFF + DFB (2)

  In this way, the present apparatus provides a fuel injection instruction for the command fuel injection amount Fi obtained by correcting the basic fuel injection amount Fbase based on the FF correction amount DFF and the (FB-processed) FB correction amount DFB. Is performed on the injector 39 for the cylinder that reaches this intake stroke. The means for instructing fuel injection in this way corresponds to the “air-fuel ratio control means”.

<Acquisition of sub feedback correction amount>
Similar to the upstream target air-fuel ratio setting means A2 described above, the downstream target value setting means A6 determines the downstream target value Voxsref based on the operating speed NE that is the operating state of the internal combustion engine 10, the accelerator pedal operation amount Accp, and the like. decide. In this example, the downstream target value Voxsref is set so that the air-fuel ratio corresponding to the downstream target value Voxsref always matches the upstream target air-fuel ratio abyfr (k).

  Based on the following equation (3), the output deviation amount calculation means A7 calculates the downstream air-fuel ratio sensor 67 from the downstream target value Voxsref at the current time (specifically, the current Fi injection instruction start time). The output deviation amount DVoxs is obtained by subtracting the output value Voxs.

DVoxs = Voxsref−Voxs (3)

  The PID controller A8 obtains the sub feedback correction amount Vafsfb based on the following equation (4) by performing proportional / integral / differential processing (PID processing) on the output deviation amount DVoxs. In the following equation (4), Kp is a preset proportional gain (constant value), Ki is a preset integral gain (constant value), and Kd is a preset differential gain (constant value).

Vafsfb ＝ Kp ・ DVoxs ＋ Ki ・ SDVoxs ＋ Kd ・ DDVoxs (4)

  SDVoxs is a time integral value of the output deviation amount DVoxs, and DDVoxs is a time differential value of the output deviation amount DVoxs. Here, since the PID controller A8 includes the integral term Ki · SDVoxs, it is guaranteed that the output deviation amount DVoxs becomes zero in a steady state. In other words, the steady deviation between the downstream target value Voxsref and the output value Voxs of the downstream air-fuel ratio sensor 67 becomes zero.

  In this manner, the present apparatus obtains the sub feedback correction amount Vafsfb based on the output value Voxs so that the steady-state deviation between the downstream target value Voxsref and the output value Voxs of the downstream air-fuel ratio sensor 67 becomes zero. This sub-feedback correction amount Vafsfb is used to acquire the control air-fuel ratio abyfs, as will be described later.

<Acquisition of control air-fuel ratio>
The control air-fuel ratio equivalent output value calculating means A9 adds the downstream feedback correction amount Vafsfb to the current output value Vabyfs of the upstream air-fuel ratio sensor 66, so that the control air-fuel ratio equivalent output value (Vabyfs + Vafsfb) Ask for.

  The table conversion means A10 includes the control air-fuel ratio equivalent output value (Vabyfs + Vafsfb), the upstream air-fuel ratio sensor output value Vabyfs and the air-fuel ratio A / F indicated by the solid line in FIG. The current control air-fuel ratio abyfs (k) is determined based on the table Mapabyfs that defines the relationship between Thereby, the control air-fuel ratio abyfs (k) differs from the air-fuel ratio (detected air-fuel ratio) obtained from the output value Vabyfs of the upstream air-fuel ratio sensor 66 by an amount corresponding to the sub feedback correction amount Vafsfb. Become.

<Calculation of FB correction amount>
The upstream target air-fuel ratio delaying means A11 is the upstream target air-fuel ratio abyfr obtained for each intake stroke by the upstream target air-fuel ratio setting means A2 and stored in the RAM 73. The fuel ratio abyfr (kN) is read from the RAM 73. Here, the number N of strokes corresponds to the sum of “time related to stroke delay”, “time related to transport delay”, and “time related to response delay” (hereinafter referred to as “dead time L”). Is a number.

  The “time related to the stroke delay” is the time from the fuel injection instruction until the exhaust gas based on the combustion of the fuel injected by this injection instruction is discharged from the combustion chamber 25 to the exhaust passage via the exhaust valve 35. . “Time related to transport delay” is the time from when exhaust gas is discharged to the exhaust passage via the exhaust valve 35 until it reaches the upstream air-fuel ratio sensor 66 (detection unit thereof). “Time related to response delay” is the time until the air-fuel ratio of the exhaust gas that has reached the upstream air-fuel ratio sensor 66 (detection unit thereof) appears as the output value Vabyfs of the upstream air-fuel ratio sensor 66.

  The air-fuel ratio deviation calculating means A12 calculates the air-fuel ratio deviation by subtracting the upstream target air-fuel ratio abyfr (kN) N strokes before the current stroke from the current control air-fuel ratio abyfs (k) based on the following equation (5). Obtain the fuel ratio deviation DAF. Here, considering that the output value Vabyfs of the upstream air-fuel ratio sensor 66 represents the air-fuel ratio of the exhaust gas based on the combustion of the fuel injected by the injection instruction before the dead time L from the present time, this air-fuel ratio deviation DAF Is an amount representing the excess or deficiency of the fuel supplied into the cylinder at a time point N strokes before the present time.

DAF ＝ abyfs (k) −abyfr (k-N) (5)

  The PI controller A13 performs proportional / integral processing (PI processing) on the air-fuel ratio deviation DAF, so that an FB for compensating for the excess or deficiency in the fuel supply amount N strokes before the current stroke based on the following equation (6). A correction amount DFB (value before guard processing) is obtained.

DFB = (Gp / DAF + Gi / SDAF) / KFB (6)

  In the above equation (6), Gp is a proportional gain (constant value), and Gi is an integral gain (constant value). SDAF is a time integral value of the air-fuel ratio deviation DAF. The coefficient KFB is preferably variable depending on the operating speed NE, the in-cylinder intake air amount Mc, and the like, but is set to “1” in this example. The PI controller A13 corresponds to the “feedback correction amount acquisition unit”.

<Guard processing>
The guard processing execution means A14 determines the FB correction amount when the FB correction amount DFB obtained by the above equation (6) falls below the FB lower limit guard value Lgrdfb (<0, feedback guard value) set as described later. When DFB is limited to the FB lower limit guard value Lgrdfb and the FB correction amount DFB obtained by the above equation (6) exceeds the FB upper limit guard value Ugrdfb (> 0, feedback guard value) set as described later Then, processing for limiting the FB correction amount DFB to the FB upper limit guard value Ugrdfb (hereinafter referred to as “guard processing”) is performed. Hereinafter, a method for setting the FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb by the guard processing execution means A14 will be described with reference to FIG.

  In order to set the FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb, in this example, the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal are set as shown by a thick solid line in FIG. The total upper guard value Ugrdtotal is a value corresponding to the lean limit value of the air-fuel ratio range corresponding to the combustible region (or the air-fuel ratio rich by a predetermined amount from the limit value), and the total lower guard value Lgrdtotal is the combustible region. Is a value corresponding to the rich limit value of the air-fuel ratio range corresponding to (or an air-fuel ratio leaner by a predetermined amount than the limit value). That is, the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal correspond to the “value that the correction amount for the basic fuel injection amount (in this example, the FF correction amount DFF + FB correction amount DFB) should not exceed”. In other words, there is a relationship “Lgrdtotal ≦ (DFF + DFB) ≦ Ugrdtotal”.

  The total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal are based on the in-cylinder intake air amount Mc (k), the upstream target air-fuel ratio abyfr (k), and the deterioration index value α, based on Mc (k), They are determined using a table MapUgrdtotal and a table MapLgardtotal, each with abyfr (k) and α as arguments. Here, the deterioration index value α is a value representing the degree of deterioration of the upstream air-fuel ratio sensor 66 acquired by the deterioration degree acquiring means A15 described later, and is larger as the degree of deterioration of the upstream air-fuel ratio sensor 66 is larger. Set to a value.

  The absolute values of the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal are determined so as to be proportional to the in-cylinder intake air amount Mc (k). This is based on the fact that the FB correction amount DFB is a value added to the basic fuel injection amount Fbase (that is, not a multiplied value).

  As shown in FIG. 5, the absolute values of the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal are set to smaller values as the upstream target air-fuel ratio abyfr (k) becomes farther from the stoichiometric air-fuel ratio stoich. This is based on the following reason.

  As shown by a two-dot chain line in FIG. 2, the output value Vabyfs of the upstream air-fuel ratio sensor 66, which is a limiting current type oxygen concentration sensor, has a characteristic that the error tends to increase as the corresponding air-fuel ratio becomes far from the stoichiometric air-fuel ratio. Have. In other words, the detected air-fuel ratio detected by the upstream air-fuel ratio sensor 66 has a characteristic that the error increases as the distance from the stoichiometric air-fuel ratio increases. That is, the farther the upstream target air-fuel ratio abyfr (k) is from the stoichiometric air-fuel ratio stoich, the greater the degree to which the actual air-fuel ratio controlled to match the upstream target air-fuel ratio abyfr (k) deviates from the detected air-fuel ratio. Become. In addition, when the error of the upstream air-fuel ratio sensor 66 is generated in a direction in which the change gradient of the output value Vabyfs with respect to the change of the air-fuel ratio becomes smaller, the actual air-fuel ratio is the stoichiometric air-fuel ratio with respect to the detected air-fuel ratio. Shift away from the direction.

  From the above, as the upstream target air-fuel ratio abyfr (k) is further away from the stoichiometric air-fuel ratio stoich, there is a possibility that the actual air-fuel ratio is largely shifted in the direction away from the stoichiometric air-fuel ratio with respect to the detected air-fuel ratio. This means that the more the upstream target air-fuel ratio abyfr (k) is away from the stoichiometric air-fuel ratio stoich, the higher the possibility that problems such as the air-fuel ratio deviate from the combustible region will occur. On the other hand, as the absolute value of the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal is reduced, problems such as the air-fuel ratio deviating from the combustible region are less likely to occur.

  Therefore, in this device, the total upper limit guard value Ugrdtotal and the total lower limit are prevented in order to prevent problems such as stable departure of the air / fuel ratio from the combustible region regardless of the upstream target air / fuel ratio abyfr (k). The absolute value of the guard value Lgrdtotal is set to a smaller value as the upstream target air-fuel ratio abyfr (k) becomes farther from the stoichiometric air-fuel ratio stoich.

  Further, the absolute values of the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal are larger as the degree of deterioration of the upstream air-fuel ratio sensor 66 is larger as shown by the arrow in FIG. A larger value is set to a smaller value. This is based on the following reason.

  As shown by a broken line in FIG. 2, the change gradient of the output value Vabyfs of the upstream air-fuel ratio sensor 66 with respect to the air-fuel ratio change has a characteristic that becomes smaller as the degree of deterioration of the upstream air-fuel ratio sensor 66 increases. Therefore, as the degree of deterioration of the upstream air-fuel ratio sensor 66 increases, the degree to which the actual air-fuel ratio deviates from the stoichiometric air-fuel ratio with respect to the detected air-fuel ratio increases. This means that the higher the degree of deterioration of the upstream air-fuel ratio sensor 66, the higher the possibility that a problem such as the air-fuel ratio deviates from the combustible region will occur.

  Therefore, in this apparatus, in order to prevent problems such as the air-fuel ratio deviating from the combustible region stably regardless of the degree of deterioration of the upstream air-fuel ratio sensor 66, the total upper limit guard value Ugrdtotal and the total lower limit The absolute value of the guard value Lgrdtotal is set to a smaller value as the deterioration index value α is larger.

  The FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb are expressed by the following equation (7) using the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal set as described above and the FF correction amount DFF. , (8), respectively.

Ugrdfb ＝ Ugrdtotal−DFF (7)
Lgrdfb = Lgrdtotal−DFF (8)

  That is, the FB upper limit guard value Ugrdfb is determined to be equal to the corresponding FB correction amount DFB when the sum of the FB correction amount DFB and the FF correction amount DFF matches the total upper limit guard value Ugrdtotal, and the FB lower limit guard value Lgrdfb is The sum of the FB correction amount DFB and the FF correction amount DFF is determined to be equal to the FB correction amount DFB corresponding to the case where the sum is equal to the total lower limit guard value Lgrdtotal.

  For example, as shown in FIG. 5, when the upstream target air-fuel ratio abyfr (k) is the value AF1 (rich air-fuel ratio) (when the FF correction amount DFF is the value F1 (positive value)), the FB upper limit guard The value Ugrdfb is a value U1 (positive value), and the FB lower limit guard value Lgrdfb is a value (−L1) (negative value). Similarly, when the upstream target air-fuel ratio abyfr (k) is the value AF2 (lean air-fuel ratio) (when the FF correction amount DFF is the value (−F2) (negative value)), the FB upper limit guard value Ugrdfb is The value U2 (a positive value) is obtained, and the FB lower limit guard value Lgrdfb is a value (−L2) (a negative value).

  The FB correction amount DFB obtained by the above equation (6) is subjected to guard processing using the FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb set in this way. The guard-processed FB correction amount DFB is used when the command fuel injection amount Fi is obtained by the command fuel injection amount calculation means A5 as described above.

  As a result, the difference between the FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb (ie, the guard width) is secured as large as possible, and the air-fuel ratio deviates from the combustible region regardless of the value of the FF correction amount DFF. Problems can be reliably prevented from occurring. This guard process execution means A14 corresponds to the “guard process execution means”.

<Acquisition of deterioration level of upstream air-fuel ratio sensor>
As described above, the deterioration degree acquisition unit A15 acquires and updates the deterioration index value α indicating the deterioration degree of the upstream air-fuel ratio sensor 66 at every predetermined timing. In this example, the deterioration index value α is obtained by measuring the actual change gradient with respect to the change in the air-fuel ratio of the output value Vabyfs of the upstream air-fuel ratio sensor 66, and the above-described reference value (the solid line gradient shown in FIG. 2). The value is determined in accordance with the actually measured decrease amount of the change gradient. That is, the deterioration index value α is determined to be larger as the amount of decrease in the actual change gradient with respect to the change in the air-fuel ratio of the output value Vabyfs of the upstream air-fuel ratio sensor 66 is larger. The deterioration level acquisition unit A15 corresponds to the “deterioration level acquisition unit”.

  As described above, in order to compensate for the excess and deficiency of the fuel supplied into the cylinder at the time N strokes before the present time, the present control air-fuel ratio abyfs (k) The air-fuel ratio is feedback controlled so as to coincide with the upstream target air-fuel ratio abyfr (kN) before the stroke.

  In addition, the control air-fuel ratio abyfs is an air-fuel ratio obtained by correcting the detected air-fuel ratio obtained from the output value Vabyfs of the upstream air-fuel ratio sensor 66 by an amount corresponding to the sub feedback correction amount Vafsfb, as described above. Therefore, the control air-fuel ratio abyfs also changes according to the output deviation amount DVoxs. As a result, the air-fuel ratio is feedback-controlled so that the output value Voxs of the downstream air-fuel ratio sensor 67 matches the downstream target value Voxsref.

  In addition, since the PI controller A13 includes the integral term Gi · SDAF, it is guaranteed that the air-fuel ratio deviation DAF becomes zero in a steady state. In other words, the steady deviation between the upstream target air-fuel ratio abyfr (k−N) and the control air-fuel ratio abyfs (k) becomes zero. This guarantees that in the steady state, the control air-fuel ratio abyfs matches the upstream target air-fuel ratio abyfr, and therefore the upstream and downstream air-fuel ratios of the first catalyst 53 match the upstream target air-fuel ratio abyfr. Means that

  In the steady state, the proportional term Gp · DAF becomes zero when the air-fuel ratio deviation DAF becomes zero, so the FB correction value DFB becomes equal to the value of the integral term Gi · SDAF. The value of the integral term Gi · SDAF is a value corresponding to the “error of the basic fuel injection amount”. Thereby, the “error of the basic fuel injection amount” can be compensated. The above is the outline of the air-fuel ratio control performed by the present apparatus.

(Actual operation)
Next, the actual operation of this apparatus will be described. Hereinafter, for convenience of explanation, “MapX (a1, a2,...)” Represents a table for obtaining a value X having a1, a2,. Further, when the value of the argument is a detection value of the sensor, the current value is used.

<Air-fuel ratio control>
The CPU 71 performs a routine for calculating the FF correction amount DFF, the command fuel injection amount Fi, and the fuel injection instruction shown in the flowchart of FIG. 6. The CPU 71 performs a predetermined crank angle before each intake top dead center ( For example, it is repeatedly executed every time BTDC 90 ° CA). Therefore, when the crank angle of an arbitrary cylinder reaches the predetermined crank angle, the CPU 71 starts the process from step 600 and proceeds to step 605, and the cylinder (hereinafter referred to as the cylinder) that reaches the current intake stroke based on the table MapMc (NE, Ga). This is sometimes referred to as a “fuel injection cylinder.”) This in-cylinder intake air amount Mc (k) is estimated and determined.

  Next, the CPU 71 proceeds to step 610 to determine the basic fuel injection amount Fbase by dividing the in-cylinder intake air amount Mc (k) by the stoichiometric air-fuel ratio stoich. Next, the CPU 71 proceeds to step 615 to determine the current upstream target air-fuel ratio abyfr (k) based on the table Mapabyfr (NE, Accp).

  Subsequently, the CPU 71 proceeds to step 620, and obtains the FF correction amount DFF based on the in-cylinder intake air amount Mc (k), the upstream target air-fuel ratio abyfr (k), and the above equation (1). Next, the CPU 71 proceeds to step 625, and the basic fuel injection amount Fbase and the FF correction amount DFF are obtained by the routine described later (at the time of the previous fuel injection) according to the above equation (2). The command fuel injection amount Fi is determined by adding the latest (guard processed) FB correction amount DFB.

  Then, the CPU 71 proceeds to step 630 and instructs to inject the fuel of the command fuel injection amount Fi, and then the CPU 71 proceeds to step 695 to end the present routine once. Thus, the fuel injection cylinder is instructed to inject the fuel of the command fuel injection amount Fi after the basic fuel injection amount Fbase is FF corrected and FB corrected.

<Calculation of FB correction amount>
Next, the operation for calculating the FB correction amount DFB (guard processing) will be described. The CPU 71 performs the routine shown by the flowchart in FIG. 7 with the fuel injection start timing (fuel injection start time) for the fuel injection cylinder. Each time it arrives, it is executed repeatedly. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the process from step 700 and proceeds to step 705 to determine whether or not the feedback condition is satisfied. The feedback condition is, for example, that the engine coolant temperature THW is equal to or higher than the first predetermined temperature, the upstream air-fuel ratio sensor 66 is normal (including being in an active state), and the in-cylinder intake air amount Mc It is established when (k) (or load) is below a predetermined value.

  Now, assuming that the feedback condition is satisfied, the CPU 71 determines “Yes” in step 705 and proceeds to step 710, and the number of strokes based on the table MapN (Mc (k), NE). N is determined. This is based on the fact that the stroke number N decreases as the in-cylinder intake air amount Mc (k) increases or the operating speed NE increases.

  Next, the CPU 71 proceeds to step 715 and corresponds to the combined air-fuel ratio which is the sum of the current output value Vabyfs of the upstream air-fuel ratio sensor 66 and the latest value of the sub-feedback correction amount Vafsfb obtained in a routine described later. By converting the output value (Vabyfs + Vafsfb) based on the table Mapabyfs (Vabyfs + Vafsfb), the control air-fuel ratio abyfs (k) at this time (this time) is obtained (see FIG. 2).

  Next, the CPU 71 proceeds to step 720, obtains the air-fuel ratio deviation DAF by subtracting the upstream target air-fuel ratio abyfr (kN) from the control air-fuel ratio abyfs (k) according to the above equation (5), and in the subsequent step 725 The FB correction amount DFB is obtained based on the above equation (6).

  Next, the CPU 71 proceeds to step 730 to determine the total upper limit guard value Ugrdtotal based on the table MapUgrdtotal (Mc (k), abyfr (k), α), and the table MapLgrdtotal (Mc (k), abyfr (k) ), α), the total lower limit guard value Lgrdtotal is determined.

  Next, the CPU 71 proceeds to step 735 and obtains the FB upper limit guard value Ugrdfb based on the total upper limit guard value Ugrdtotal, the FF correction amount DFF obtained in the previous step 620, and the above equation (7). The FB lower limit guard value Lgrdfb is obtained based on the total lower limit guard value Lgrdtotal, the FF correction amount DFF, and the above equation (8).

  Subsequently, the CPU 71 proceeds to step 740 to perform the “guard processing” (FB lower limit guard value Lgrdfb ≦ DFB ≦ FB upper limit guard value Ugrdfb) on the FB correction amount DFB obtained in step 725, and in subsequent step 745. The air-fuel ratio deviation DAF obtained in the above step 720 is added to the integrated value SDAF of the air-fuel ratio deviation DAF at that time to obtain a new integrated value SDAF of the air-fuel ratio deviation. Then, the routine proceeds to step 795 and this routine is temporarily executed. finish.

  Thus, the FB correction amount DFB subjected to the guard processing is obtained, and the air-fuel ratio feedback control is executed by reflecting the FB correction amount DFB subjected to the guard processing to the command fuel injection amount Fi from step 625 of FIG. Is done.

  On the other hand, if the feedback condition is not satisfied at the time of the determination in step 705, the CPU 71 determines “No” in step 705, proceeds to step 750, sets the value of the FB correction amount DFB to “0”, Thereafter, the routine proceeds to step 795 to end the present routine tentatively. Thus, when the feedback condition is not satisfied, the FB correction amount DFB is set to “0”, and the FB correction of the basic fuel injection amount Fbase is not performed.

<Calculation of sub feedback correction amount>
Next, the operation when calculating the sub feedback correction amount Vafsfb will be described. The CPU 71 performs the routine shown by the flowchart in FIG. 8 every time the fuel injection start timing (fuel injection start time) arrives for the fuel injection cylinder. It is designed to be executed repeatedly.

  Therefore, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts processing from step 800 and proceeds to step 805 to determine whether or not the sub feedback condition is satisfied. The sub feedback condition is satisfied, for example, when the engine coolant temperature THW is equal to or higher than a second predetermined temperature higher than the first predetermined temperature, in addition to the main feedback condition of step 705 described above.

  Now, assuming that the sub-feedback condition is satisfied, the CPU 71 determines “Yes” in step 805 and proceeds to step 810 to calculate the current value from the downstream target value Voxsref according to the above equation (3). The output deviation amount DVoxs is obtained by subtracting the output value Voxs of the downstream air-fuel ratio sensor 67. Next, the CPU 71 proceeds to step 815 to obtain a differential value DDVoxs of the output deviation amount DVoxs based on the following equation (9).

DDVoxs = (DVoxs−DVoxs1) / Δt (9)

  In the above equation (9), DVoxs1 is the previous value of the output deviation amount DVoxs updated in step 830, which will be described later, during the previous execution of this routine. Δt is the time from the time when this routine was executed last time to the time when this routine was executed this time.

  Next, the CPU 71 proceeds to step 820 to obtain the sub feedback correction amount Vafsfb based on the above equation (4).

  Subsequently, the CPU 71 proceeds to step 825, adds the output deviation amount DVoxs obtained in step 810 to the integral value SDVoxs of the output deviation amount at that time, and obtains a new integrated value SDVoxs of the output deviation amount. In step 830, the previous value DVoxs1 of the output deviation amount DVoxs is set to a value equal to the output deviation amount DVoxs obtained in step 810, and then the routine proceeds to step 895 to end the present routine tentatively.

  Thus, the sub feedback correction amount Vafsfb is obtained. This sub-feedback correction amount Vafsfb is used to obtain the control air-fuel ratio abyfs at step 715 when the routine shown in FIG. 7 is executed next time.

  On the other hand, if the sub feedback condition is not satisfied at the time of determination in step 805, the CPU 71 determines “No” in step 805 and proceeds to step 835 to set the value of the sub feedback correction amount Vafsfb to “0”. Then, the process proceeds to step 895 to end this routine once. Thus, when the sub-feedback condition is not satisfied, the sub-feedback correction amount Vafsfb is set to “0”, and the air-fuel ratio feedback control based on the sub-feedback control is not performed.

  As described above, according to the first embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the basic fuel injection amount Fbase (unit: g) corresponding to the stoichiometric air-fuel ratio stoich is set to the target air-fuel ratio abyfr. FF correction amount DFF (unit: g) obtained according to the deviation from the theoretical air-fuel ratio and guard-processed FB correction amount DFB (unit: g) obtained based on the output value Vabyfs of the upstream air-fuel ratio sensor 66 ) To determine the commanded fuel injection amount Fi. In the guard processing for the FB correction amount DFB, guard processing is performed with the FB upper limit card value Ugrdfb (positive value, unit: g) and the FB lower limit guard value Lgrdfb (negative value, unit: g) as upper and lower limits, respectively. The FB upper limit guard value Ugrdfb is the value obtained by subtracting the FF correction amount DFF from the upper limit value (total upper limit guard value Ugrdtotal (positive value, unit: g)) that should not exceed the total correction amount (DFF + DFB) for the basic fuel injection amount ( Ugrdtotal-DFF), and the FB lower limit guard value Lgrdfb is from the lower limit value (total lower limit guard value Lgrdtotal (negative value, unit: g)) that should not fall below the total correction amount (DFF + DFB) for the basic fuel injection amount. A value obtained by subtracting the FF correction amount DFF (Lgrdtotal-DFF) is set.

  As a result, the difference between the FB upper limit guard value Ugrdfb and the FB lower limit guard value Lgrdfb (ie, the guard width) is secured as large as possible, and the air-fuel ratio deviates from the combustible region regardless of the value of the FF correction amount DFF. Problems can be reliably prevented from occurring.

(Second Embodiment)
Next, an air-fuel ratio control apparatus according to a second embodiment of the present invention will be described. FIG. 9 is a functional block diagram of the second embodiment. As shown in FIG. 9, in the second embodiment, the basic fuel injection amount Fbase (unit: g) is changed to the FF correction factor KFF (unit: g) obtained in accordance with the deviation of the target air-fuel ratio abyfr from the stoichiometric air-fuel ratio stoich. % / 100) plus 1 (KFF + 1) and the output value Vabyfs of the upstream air-fuel ratio sensor 66, the guard processed FB correction factor KFB (unit:% / 100) 4 is different from the first embodiment in which the functional block diagram is shown in FIG. 4 in that the command fuel injection amount Fi is determined by multiplying the value (KFB + 1) plus “1”. Hereinafter, such differences will be described through an actual operation in the second embodiment.

  The CPU 71 of the second embodiment directly executes the routine of FIG. 8 among the routines of FIGS. 6 to 8 executed by the CPU 71 of the first embodiment, and replaces the routines of FIGS. Each of the routines shown in the flowchart of FIG. 11 is executed. Hereinafter, in the routines of FIGS. 10 and 11, the same steps as those in the previous routine are denoted by the same step numbers as those in the previous routine, so that the description thereof will be substituted.

  FIG. 10 is a routine corresponding to FIG. The routine of FIG. 10 differs from the routine of FIG. 6 only in that steps 620 and 625 of FIG. 6 are replaced with steps 1005 and 1010, respectively.

  In step 1005, an FF correction factor KFF (the above-mentioned "" for correcting the basic fuel injection amount Fbase according to the deviation of the upstream target air-fuel ratio abyfr (k) from the stoichiometric air-fuel ratio stoich according to the following equation (10): Equivalent to “feedforward correction amount”).

KFF = (stoich−abyfr (k)) / stoich (10)

  This FF correction factor KFF is equal to the ratio of the deviation amount of the upstream target air-fuel ratio abyfr (k) from the stoichiometric air-fuel ratio stoich to the stoichiometric air-fuel ratio stoich. Similar to the FF correction amount DFF in the first embodiment, the FF correction factor KFF is a positive value when the upstream target air-fuel ratio abyfr (k) is richer than the stoichiometric air-fuel ratio (a larger value is richer). When the upstream target air-fuel ratio abyfr (k) is leaner than the stoichiometric air-fuel ratio, it becomes a negative value (the leaner the value, the larger the absolute value).

  In step 1010, the command fuel injection amount Fi is obtained according to the following equation (11). Here, the value obtained in step 610 in FIG. 10 is used as Fbase, and the value obtained in step 1005 in FIG. 10 is used as KFF. As the FB correction factor KFB, a value (latest value) obtained by a routine of FIG. 11 described later is used.

Fi = Fbase ・ (KFF + 1) ・ (KFB + 1) (11)

  FIG. 11 is a routine corresponding to FIG. The routine of FIG. 11 differs from the routine of FIG. 7 only in that steps 725 to 740 and 750 in FIG. 7 are replaced with steps 1105 to 1120 and 1125, respectively.

  In step 1105, the FB correction factor KFB (unit:% / 100) corresponding to the FB correction amount DFB (unit: g) in the first embodiment uses the air-fuel ratio deviation DAF obtained in step 720 as the proportional gain Gp1, It is obtained by PI processing using the integral gain Gi1.

  In step 1110, the total upper limit guard value Ugrdtotal1 (positive value, unit:% / 100) corresponding to the total upper limit guard value Ugrdtotal (unit: g) in the first embodiment is represented by the table MapUgrdtotal1 (abyfr (k), α). And the total lower limit guard value Lgrdtotal1 (negative value, unit:% / 100) corresponding to the total lower limit guard value Lgrdtotal (unit: g) in the first embodiment is the table MapLgrdtotal1 (abyfr (k ), α). Here, there is a relationship of “(Lgrdtotal1 + 1) ≦ ((KFF + 1) · (KFB + 1)) ≦ (Ugrdtotal1 + 1)”. This relationship corresponds to the relationship “Lgrdtotal ≦ (DFF + DFB) ≦ Ugrdtotal” in the first embodiment.

  The in-cylinder intake air amount Mc (k) is not included as an argument in the tables MapUgrdtotal1 and MapLgrdtotal1 because the value obtained by adding “1” to the FB correction factor KFB (KFB + 1) is the basic fuel injection amount Fbase. Since it is a value to be multiplied, it is based on the fact that the FB correction factor KFB is not influenced by the value of the cylinder intake air amount Mc itself.

  As a result, like the total upper limit guard value Ugrdtotal and the total lower limit guard value Lgrdtotal in the first embodiment, the absolute values of the total upper limit guard value Ugrdtotal1 and the total lower limit guard value Lgrdtotal1 are the upstream target air-fuel ratio abyfr (k) Is set to a smaller value as the distance from the stoichiometric air-fuel ratio stoich increases, and is set to a smaller value as the degree of deterioration of the upstream air-fuel ratio sensor 66 increases (thus, as the deterioration index value α increases).

  In step 1115, an FB upper limit guard value Ugrdfb1 (unit:% / 100) corresponding to the FB upper limit guard value Ugrdfb (unit: g) in the first embodiment is obtained according to the following equation (12), and the first implementation described above. The FB lower limit guard value Lgrdfb1 (unit:% / 100) corresponding to the FB lower limit guard value Lgrdfb (unit: g) in the embodiment is obtained according to the following equation (13).

Ugrdfb1 = (Ugrdtotal1 + 1) / (KFF + 1) −1 (12)
Lgrdfb1 = (Lgrdtotal1 + 1) / (KFF + 1) −1 (13)

  The above expression (12) is a part of the above-mentioned relationship of “(Lgrdtotal1 + 1) ≦ ((KFF + 1) · (KFB + 1)) ≦ (Ugrdtotal1 + 1)” (((KFF + In (1) · (KFB + 1)) ≦ (Ugrdtotal1 + 1) ”, KFB is replaced with Ugrdfb1, and an equation in which the inequality sign is replaced with an equal sign is solved for Ugrdfb1. Similarly, the above equation (13) solves for Lgrdfb1 by replacing KFB with Lgrdfb1 and replacing the inequality sign with an equal sign in “(Lgrdtotal1 + 1) ≦ ((KFF + 1) · (KFB + 1))” Can be obtained.

  In step 1120, guard processing (FB lower limit guard value Lgrdfb1 ≦ KFB ≦ FB upper limit guard value Ugrdfb1) is performed on the FB correction factor KFB obtained in step 1105. In step 1125, the FB correction rate KFB is set to “0” instead of the FB correction amount DFB.

  As described above, according to the second embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the basic fuel injection amount Fbase (unit: g) corresponding to the stoichiometric air-fuel ratio stoich is set to the target air-fuel ratio abyfr. Based on the value (KFF + 1) obtained by adding “1” to the FF correction factor KFF (unit:% / 100) obtained according to the deviation from the theoretical air-fuel ratio, and the output value Vabyfs of the upstream air-fuel ratio sensor 66 The commanded fuel injection amount Fi is determined by multiplying the value (KFB + 1) obtained by adding “1” to the guard processed FB correction factor KFB (unit:% / 100) obtained in this way. In the guard processing of the FB correction factor KFB, guard processing is performed with the FB upper limit card value Ugrdfb1 (unit:% / 100) and the FB lower limit guard value Lgrdfb1 (unit:% / 100) as upper and lower limits, respectively. The FB upper limit guard value Ugrdfb1 is calculated using the upper limit value (Ugrdtotal1 + 1) that should not exceed the total correction amount ((KFF + 1) · (KFB + 1)) for the basic fuel injection amount and the FF correction factor KFF ( The FB lower limit guard value Lgrdfb1 is set according to the equation (12). The lower limit value (Lgrdtotal1 + 1) and the FF correction should not be less than the total correction amount ((KFF + 1) · (KFB + 1)) for the basic fuel injection amount. It is set according to the above equation (13) using the rate KFF.

  Thereby, 2nd Embodiment also has the same effect as the said 1st Embodiment. That is, there is a problem that the difference between the FB upper limit guard value Ugrdfb1 and the FB lower limit guard value Lgrdfb1 (ie, the guard width) is ensured as large as possible and the air-fuel ratio deviates from the combustible region regardless of the value of the FF correction factor KFF. Can be reliably prevented from occurring.

  The present invention is not limited to the first and second embodiments, and various modifications can be employed within the scope of the present invention. For example, in the first and second embodiments, the feedback guard value is set to a smaller value by setting the total guard value to a smaller value as the target air-fuel ratio becomes farther from the reference air-fuel ratio (theoretical air-fuel ratio). (The guard width is set to a smaller value), but the total guard value is not changed according to the target air-fuel ratio, and the target air-fuel ratio becomes far from the reference air-fuel ratio (theoretical air-fuel ratio). The calculated feedback guard value may be corrected to a smaller value (the guard width is corrected to a smaller value).

  In the first and second embodiments, the feedback guard value is set to a smaller value by setting the total guard value to a smaller value as the degree of deterioration of the upstream air-fuel ratio sensor 66 increases. (The guard width is set to a smaller value.) However, the total guard value is not changed according to the deterioration degree of the upstream air-fuel ratio sensor 66, and the deterioration degree of the upstream air-fuel ratio sensor 66 increases. The calculated feedback guard value may be corrected to a smaller value (the guard width is corrected to a smaller value).

  In the first and second embodiments, the sub feedback control based on the output value Voxs of the downstream air-fuel ratio sensor 67 is executed. However, the sub feedback control may not be executed.

1 is a schematic diagram of an internal combustion engine to which an air-fuel ratio control apparatus according to a first embodiment of the present invention is applied. 2 is a graph showing the relationship between the output voltage of the upstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. 2 is a graph showing the relationship between the output voltage of the downstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. FIG. 2 is a functional block diagram when the air-fuel ratio control device shown in FIG. 1 executes air-fuel ratio control. It is a figure for demonstrating the setting method of the upper / lower limit guard value of the feedback correction amount by the air fuel ratio control apparatus shown in FIG. 2 is a flowchart showing a routine for performing a feedforward correction amount, a fuel injection amount calculation, and an injection instruction executed by a CPU shown in FIG. 1. FIG. 3 is a flowchart showing a routine for calculating a feedback correction amount executed by a CPU shown in FIG. 1. FIG. 4 is a flowchart showing a routine for calculating a sub feedback correction amount executed by a CPU shown in FIG. 1. It is a functional block diagram at the time of the air fuel ratio control apparatus which concerns on 2nd Embodiment of this invention performing air fuel ratio control. It is the flowchart which showed the routine for performing the feedforward correction factor which the CPU of the air fuel ratio control apparatus which concerns on 2nd Embodiment of this invention performs, the calculation of the fuel injection quantity, and the injection instruction | indication. It is the flowchart which showed the routine for calculating the feedback correction factor which CPU of the air fuel ratio control device which relates to 2nd execution form of this invention executes.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 25 ... Combustion chamber, 39 ... Injector, 52 ... Exhaust pipe (exhaust pipe), 53 ... Three way catalyst (1st catalyst), 66 ... Upstream air-fuel ratio sensor, 67 ... Downstream air-fuel ratio sensor, 70 ... electric control device, 71 ... CPU

Claims (9)

An air-fuel ratio sensor that is disposed in the exhaust passage of the internal combustion engine and outputs a value corresponding to the air-fuel ratio of the gas in the exhaust passage;
Fuel injection means for injecting fuel in response to an instruction to inject fuel of a command fuel injection amount;
An air-fuel ratio control device for an internal combustion engine applied to an internal combustion engine comprising:
Basic fuel injection amount acquisition means for acquiring a basic fuel injection amount that is a value obtained by dividing the amount of air taken into the combustion chamber of the internal combustion engine in the intake stroke by a reference air-fuel ratio;
Target air-fuel ratio acquisition means for acquiring a target air-fuel ratio that changes according to the operating state of the internal combustion engine based on the operating state;
Feedforward correction amount acquisition means for acquiring a feedforward correction amount for correcting the basic fuel injection amount in accordance with a shift of the target air-fuel ratio from the reference air-fuel ratio;
Feedback correction amount acquisition means for acquiring a feedback correction amount for correcting the basic fuel injection amount based on an output value of the air-fuel ratio sensor;
Guard process execution means for performing a guard process for limiting the feedback correction amount to the feedback guard value when the feedback correction amount exceeds a feedback guard value;
Command fuel injection amount calculating means for calculating the command fuel injection amount by correcting the basic fuel injection amount based on the feedforward correction amount and the feedback correction amount subjected to the guard process;
Air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber to coincide with the target air-fuel ratio by instructing the fuel injection means to inject fuel of the command fuel injection amount; ,
In an air-fuel ratio control apparatus for an internal combustion engine comprising:
The guard processing execution means includes
An air-fuel ratio control apparatus for an internal combustion engine configured to determine the feedback guard value based on a total guard value that is a value that should not exceed a correction amount for the basic fuel injection amount and the feedforward correction amount . The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The air-fuel ratio sensor is a limiting current type oxygen concentration sensor disposed in the exhaust passage upstream of the catalyst disposed in the exhaust passage,
The guard processing execution means includes
An air-fuel ratio control apparatus for an internal combustion engine configured to change the total guard value according to the target air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 2 ,
The guard processing execution means includes
An air-fuel ratio control apparatus for an internal combustion engine configured to set the total guard value to a smaller value as the target air-fuel ratio becomes farther from the reference air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The air-fuel ratio sensor is a limiting current type oxygen concentration sensor disposed in the exhaust passage upstream of the catalyst disposed in the exhaust passage,
The guard processing execution means includes
A deterioration degree acquisition means for acquiring a deterioration state of the air-fuel ratio sensor;
An air-fuel ratio control apparatus for an internal combustion engine configured to change the total guard value according to a deterioration state of the air-fuel ratio sensor. The air-fuel ratio control apparatus for an internal combustion engine according to claim 4 ,
The guard processing execution means includes
An air-fuel ratio control apparatus for an internal combustion engine configured to set the total guard value to a smaller value as the degree of deterioration of the air-fuel ratio sensor increases. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The air-fuel ratio sensor is a limiting current type oxygen concentration sensor disposed in the exhaust passage upstream of the catalyst disposed in the exhaust passage,
The guard processing execution means includes
A deterioration degree acquisition means for acquiring a deterioration state of the air-fuel ratio sensor;
An air-fuel ratio control apparatus for an internal combustion engine configured to change the total guard value according to the target air-fuel ratio and a deterioration state of the air-fuel ratio sensor. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The guard processing execution means includes
An air-fuel ratio control apparatus for an internal combustion engine configured to further change the feedback guard value according to the target air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The guard processing execution means includes
A deterioration degree acquisition means for acquiring a deterioration state of the air-fuel ratio sensor;
An air-fuel ratio control apparatus for an internal combustion engine configured to further change the feedback guard value according to a deterioration state of the air-fuel ratio sensor. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 ,
The guard processing execution means includes
A deterioration degree acquisition means for acquiring a deterioration state of the air-fuel ratio sensor;
An air-fuel ratio control apparatus for an internal combustion engine configured to further change the feedback guard value in accordance with the target air-fuel ratio and a deterioration state of the air-fuel ratio sensor.

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