JP4305358B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention relates to an upstream air-fuel ratio sensor disposed in an exhaust passage on the upstream side and a downstream side of a three-way catalyst (hereinafter also simply referred to as “catalyst”) in an exhaust passage of an internal combustion engine, and a downstream The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that feedback-controls the air-fuel ratio of an air-fuel mixture supplied to the engine based on output values of a side air-fuel ratio sensor. Hereinafter, the internal combustion engine may be simply referred to as “engine”.

  Conventionally, this type of air-fuel ratio control apparatus is widely known (see, for example, Patent Document 1). This air-fuel ratio control device (exhaust gas purification device) is applied to an internal combustion engine having a fuel injection valve (hereinafter referred to as “port injection valve”) that injects fuel into an intake port upstream of the intake valve. Thus, the air-fuel ratio of the air-fuel mixture supplied to the engine is controlled as follows.

  That is, this device is based on the difference between the output value of the downstream air-fuel ratio sensor and the downstream target value that is the target value of the output of the sensor (specifically, the sub-feedback controller proportionally integrates the difference. A sub feedback correction amount is calculated by performing differentiation processing (PID processing). Further, this device is based on the difference between the value obtained by correcting the output value of the upstream air-fuel ratio sensor by the calculated sub feedback correction amount and the upstream target value that is the target value of the output of the upstream air-fuel ratio sensor. Specifically, the main feedback correction amount is calculated by performing proportional / integration processing (PI processing) on the difference with the main feedback controller.

This device corrects the amount of fuel injected from the port injection valve (hereinafter referred to as “port injection amount”) by the calculated main feedback correction amount, thereby emptying the air-fuel mixture supplied to the engine. The fuel ratio is feedback controlled.
JP 2004-183585 A

  The disclosed apparatus is configured such that the value for calculating the main feed correction amount (that is, the value subjected to PI processing) is directly changed according to the value of the sub feedback correction amount. In other words, the main feedback controller that calculates the main feedback correction amount and the sub feedback controller that calculates the sub feedback correction amount are arranged in series.

  Therefore, one of the main feedback control constant (proportional gain and integral gain) used for the main feedback controller and the sub feedback control constant (proportional gain, integral gain and derivative gain) used for the sub-feedback controller. The value on the other side has a strong influence on the adaptation.

  In other words, the respective feedback control constants cannot be matched independently for each feedback control loop (closed loop). Therefore, it is difficult to adapt the feedback control constants, and there is a problem that much labor is required when performing the adaptation.

  In order to cope with such a problem, it is considered preferable to arrange the main feedback controller and the sub feedback controller in parallel with respect to correction of the fuel injection amount. In this case, specifically, the sub feedback correction amount is calculated based on the difference between the downstream air-fuel ratio sensor output value and the downstream target value by the sub-feedback controller, and the upstream air-fuel ratio sensor is calculated by the main feedback controller. A main feedback correction amount is calculated based on the difference between the output value and the upstream target value. Then, the port injection amount is directly corrected independently by the calculated main feedback correction amount and the sub feedback correction amount, whereby the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback controlled.

  As a result, the feedback control constants can be matched independently of each other for each feedback control loop, and the labor for matching the feedback control constants can be reduced.

  By the way, normally, when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio, the catalyst stores the oxygen depleted from the nitrogen oxide by reducing nitrogen oxide (NOx) in the exhaust gas and flows in the same. When the air-fuel ratio of the exhaust gas is a rich air-fuel ratio, it has a function of oxidizing unburned components such as HC and CO in the exhaust gas with the stored oxygen (hereinafter referred to as “oxygen storage function”). Yes. With such an oxygen storage function, a relatively high frequency component in the air-fuel ratio fluctuation of the exhaust gas upstream of the catalyst, and a relatively low frequency in the air-fuel ratio fluctuation have a relatively small amplitude (deviation from the theoretical air-fuel ratio). Small low-frequency components tend to be hardly absorbed and thus hardly appear as air-fuel ratio fluctuations in exhaust gas downstream of the catalyst.

  On the other hand, a low-frequency component having a relatively low frequency and a relatively large amplitude in the air-fuel ratio fluctuation of the exhaust gas upstream of the catalyst (hereinafter referred to as a “catalyst-passing low-frequency component”) is completely in the oxygen storage function of the catalyst. Passes through the catalyst without being absorbed, and as a result, tends to appear as an air-fuel ratio fluctuation of exhaust gas downstream of the catalyst with a slight delay. In other words, the catalyst having an oxygen storage function functions as a “dead time element” in feedback control for the low-frequency component passing through the catalyst.

  In this way, the catalyst functions as a “dead time element” for the low-frequency component passing through the catalyst, whereby the output value of the upstream air-fuel ratio sensor and the output value of the downstream air-fuel ratio sensor are compared with the stoichiometric air-fuel ratio. There is a case in which the value indicates an air-fuel ratio shifted in opposite directions. In this case, the engine air-fuel ratio control (main feedback control) based on the main feedback correction amount and the engine air-fuel ratio control (sub feedback control) based on the sub feedback correction amount interfere with each other. Air-fuel ratio control cannot be performed.

  Such interference between the main feedback control and the sub feedback control can be avoided by setting the control frequency band of the main feedback control and the control frequency band of the sub feedback control regarding the air-fuel ratio fluctuation of the exhaust gas so as not to overlap each other. For this purpose, it is considered effective to remove the frequency band related to the catalyst-passing low frequency component from the control frequency band of the main feedback control.

  Furthermore, in this case, the removal of high frequency components due to noise inevitably appearing in the output value of the downstream air-fuel ratio sensor ensures the overlap of the control frequency band between the main feedback control and the sub feedback control. Therefore, it is considered preferable to avoid this problem.

  From the above, the present applicant has already proposed the following air-fuel ratio control device (exhaust gas purification device) in Japanese Patent Application No. 2004-13225. This device uses a high-pass filter and a low-pass filter having a common cutoff frequency ω 1 whose frequency-gain characteristics are shown in FIG. This cutoff frequency ω1 is set, for example, in the vicinity of the maximum value (upper limit value) of the frequency band of the catalyst-passing low frequency component.

  The proposed apparatus calculates a sub feedback correction amount by a sub feedback controller based on a value obtained by low-pass filtering the difference between the downstream air-fuel ratio sensor output value and the downstream target value. As a result, the high frequency component equal to or higher than the cutoff frequency ω1 in the fluctuation of the downstream air-fuel ratio sensor output value is attenuated, so that the control frequency band of the sub feedback control can be equal to or lower than the cutoff frequency ω1.

  Further, the proposed apparatus calculates a main feedback correction amount by a main feedback controller based on a value obtained by subjecting a difference value between the upstream air-fuel ratio sensor output value and the upstream target value to a high-pass filter process. As a result, the low frequency component below the cut-off frequency ω1 in the fluctuation of the upstream air-fuel ratio sensor output value is attenuated, so the control frequency band of the main feedback control can be a band above the cut-off frequency ω1.

  The proposed device feedback-controls the air-fuel ratio of the air-fuel mixture supplied to the engine by directly correcting the port injection amount independently of the calculated main feedback correction amount and sub-feedback correction amount. To do.

  Thereby, as mentioned above, the labor at the time of adapting each feedback control constant can be reduced. In addition, since the control frequency band of the main feedback control and the control frequency band of the sub feedback control can be set so as not to overlap each other, the interference between the main feedback control and the sub feedback control described above can be avoided, and as a result, good Engine air-fuel ratio control can be achieved.

Incidentally, in recent years, in order to achieve improvement in combustion efficiency and reduction in fuel consumption, a fuel injection valve that directly injects fuel into the combustion chamber (hereinafter referred to as “in-cylinder injection valve”) is provided as a main fuel injection valve. In order to ensure startability at the time of starting the engine, an internal combustion engine having the port injection valve as a fuel injection valve has been developed (see, for example, Patent Document 2). Hereinafter, a system including two fuel injection valves, i.e., the in-cylinder injection valve and the port injection valve, is referred to as a “dual injection system”. The amount of fuel injected from the in-cylinder injection valve is referred to as “in-cylinder injection amount”.
JP 2004-60474 A

  When the proposed air-fuel ratio control apparatus is applied to an internal combustion engine having such a dual injection system, the fuel injection amount correction target based on the main feedback correction amount and the fuel injection amount based on the sub feedback correction amount are controlled. The problem is how to set the correction target from among the port injection amount and the in-cylinder injection amount. The above Patent Document 2 neither discloses nor suggests how to set the correction target of the fuel injection amount.

  Further research by the present applicant revealed that depending on how the fuel injection amount correction target is set, a sudden output fluctuation (that is, torque fluctuation) occurs in the engine. It has been found that drivability may deteriorate.

  Accordingly, the present invention has been made to solve the above-described problems, and an object of the present invention is an air-fuel ratio control apparatus applied to an internal combustion engine having a dual injection system, which is upstream and downstream of a catalyst. Another object of the present invention is to provide a device capable of avoiding mutual interference between air-fuel ratio feedback controls based on air-fuel ratio sensors respectively disposed in the exhaust passages and suppressing abrupt engine torque fluctuations.

The air-fuel ratio control apparatus according to the present invention is applied to an internal combustion engine having a dual injection system. This device calculates an upstream feedback correction amount (by an upstream feedback controller) based on a value that is based on an output value of the upstream air-fuel ratio sensor and that has been subjected to a high-pass filter process. The air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is feedback-controlled by correcting only the amount of fuel (port injection amount) injected by the port injection means (port injection valve) by the side feedback correction amount. A downstream feedback correction amount is calculated (by the downstream feedback controller) based on the upstream feedback control means and a value based on the output value of the downstream air-fuel ratio sensor , and the cylinder is calculated based on the calculated downstream feedback correction amount. The amount of fuel injected by the internal injection means (the in-cylinder injection valve) (the in-cylinder injection) ) Alone, or by correcting the amount of fuel injected by the amount and the port injection means of fuel injected by the in-cylinder injection means, the feedback control of the air-fuel ratio of the mixture supplied to the internal combustion engine And downstream feedback control means. Hereinafter, the in-cylinder injection amount and the port injection amount may be collectively referred to as “fuel injection amount”.

  According to this, the fuel injection amount is directly corrected independently by the upstream feedback correction amount calculated by the upstream feedback control unit and the downstream feedback correction amount calculated by the downstream feedback control unit. As a result, as in the proposed apparatus, the labor for adapting each feedback control constant can be reduced.

  Further, the upstream feedback controller is input with a value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filter processing. Here, the cutoff frequency of the high-pass filter in the high-pass filter processing should generally be determined in light of the degree of the oxygen storage function of the catalyst, and preferably, as described above, the frequency of the low-frequency component passing through the catalyst. It is set near the maximum value (upper limit) of the band.

  Thereby, as described above, the control frequency band of the upstream feedback control and the control frequency band of the downstream feedback control can be set so as not to overlap each other. Interference with side feedback control can be avoided.

  Here, the “value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filtering” is, for example, the output value (or detected air-fuel ratio) of the upstream air-fuel ratio sensor, or This is a value after high-pass filter processing is performed on a value corresponding to the degree of difference between the output value of the upstream air-fuel ratio sensor and the upstream target value that is the target value of the output of the sensor.

  The “value based on the output value of the downstream air-fuel ratio sensor” corresponds to, for example, the degree of difference between the output value of the downstream air-fuel ratio sensor and the downstream target value that is the target value of the output of the sensor. Value.

  The “upstream target value” and the “downstream target value” are preferably set to values corresponding to the theoretical air-fuel ratio, or these values are set so that the corresponding air-fuel ratios are equal to each other. It is preferable.

  The “value according to the degree of difference between the sensor output value and the target value” is, for example, a deviation between the sensor output value and the target value, or a detected air-fuel ratio (actual air-fuel ratio) corresponding to the sensor output value. And the target air-fuel ratio corresponding to the target value, the cylinder intake air amount divided by the detected air-fuel ratio corresponding to the sensor output value, the actual cylinder fuel supply amount and the cylinder intake air amount It is a deviation from the target in-cylinder fuel supply amount that is a value divided by the target air-fuel ratio corresponding to the target value, and is not limited to these.

  According to the above configuration, the air-fuel ratio control when the air-fuel ratio of the exhaust gas suddenly changes or fluctuates at a high frequency equal to or higher than the cutoff frequency of the high-pass filter in the high-pass filter processing, such as when the engine is in a transient operation state. (In other words, compensation for a sudden change in the air-fuel ratio in the transient operation state) is quickly performed by upstream feedback control.

  Further, the air-fuel ratio control for the steady air-fuel ratio fluctuation in the frequency band (generally, a relatively low frequency band) of the low-passage component passing through the catalyst that can appear as the air-fuel ratio fluctuation downstream of the catalyst is ensured by the downstream feedback control. Can be achieved.

  Further, the air-fuel ratio control apparatus according to the present invention is characterized in that only the port injection amount is corrected by the upstream feedback correction amount (that is, the in-cylinder injection amount is not corrected by the upstream feedback correction amount). is there.

  The total amount of fuel directly injected into the combustion chamber by the in-cylinder injection valve can contribute to the immediately following combustion. Therefore, when the in-cylinder injection amount fluctuates, all the influences of the fluctuation can appear as engine output fluctuations (that is, torque fluctuations). Therefore, when the air-fuel ratio feedback control is executed by correcting the in-cylinder injection amount with the feedback correction amount, if the feedback correction amount fluctuates abruptly, the in-cylinder injection amount fluctuates abruptly, causing a sudden torque to the engine. Fluctuations occur, and as a result, drivability tends to deteriorate.

  Here, the higher the fluctuation frequency of the feedback correction amount, the greater the change speed with respect to the same amplitude. Accordingly, abrupt fluctuations are likely to occur in the upstream feedback correction amount that can fluctuate at a high frequency that is at least equal to or higher than the cut-off frequency of the high-pass filter (for example, near the maximum value of the frequency band of the low-frequency component passing through the catalyst). Therefore, it is not preferable to correct the in-cylinder injection amount by the upstream feedback correction amount because it leads to a rapid torque fluctuation of the engine.

  On the other hand, as described above, part of the fuel injected by the port injection valve into the intake passage (intake port) upstream of the intake valve is referred to as an intake port and an intake valve (hereinafter referred to as “intake port etc.”). Once). Therefore, even if the port injection amount fluctuates rapidly, the fluctuation of the amount of fuel actually sucked into the combustion chamber (hereinafter referred to as “in-cylinder intake fuel amount”) is attenuated. Sudden torque fluctuation is unlikely to occur. Therefore, even if the port injection amount is corrected by the upstream feedback correction amount in which sudden fluctuation is likely to occur, rapid engine torque fluctuation is unlikely to occur.

  From the above, if the upstream feedback control is performed by correcting only the port injection amount without correcting the in-cylinder injection amount by the upstream feedback correction amount as in the above configuration, the upstream feedback control is performed. Sudden engine torque fluctuations can be suppressed.

On the other hand, the downstream feedback correction amount can change only at a relatively low frequency, which is a frequency within the frequency band of the low-frequency component passing through the catalyst, so that the rate of change is relatively small. Therefore, it is difficult for a rapid fluctuation to occur in the downstream feedback correction amount. Therefore, even if the downstream feedback control is performed by correcting only the in-cylinder injection amount or the port injection amount and the in-cylinder injection amount by the downstream feedback correction amount as in the above configuration, the downstream feedback control is performed. Sudden engine torque fluctuations are less likely to occur.

  As a result, the air-fuel ratio control apparatus according to the present invention suppresses both the sudden engine torque fluctuation based on the upstream feedback control and the sudden engine torque fluctuation based on the downstream feedback control. Therefore, deterioration of drivability can be suppressed.

As described above, it is preferable that the downstream feedback control is performed by correcting at least the in-cylinder injection amount by the downstream feedback correction amount. The reason is as follows. That is, as described above, the entire amount of the fuel injected by the in-cylinder injection valve can contribute to the combustion immediately after. Therefore, if the air-fuel ratio feedback control is executed by correcting the in-cylinder injection amount by the feedback correction amount, all of the influence of the feedback correction amount can be immediately reflected in the air-fuel ratio of the exhaust gas. Sex can be secured.

  On the other hand, as described above, part of the fuel injected by the port injection valve temporarily adheres to the intake port. Therefore, when the air-fuel ratio feedback control is executed by correcting the port injection amount with the feedback correction amount, not all of the influence of the feedback correction amount is immediately reflected in the air-fuel ratio of the exhaust gas. As a result, the responsiveness of the air-fuel ratio feedback control becomes lower than when the in-cylinder injection amount is corrected by the feedback correction amount.

  From the above, as described above, when configured so that at least the in-cylinder injection amount is corrected by the downstream feedback correction amount, high responsiveness can be secured in the downstream feedback control. As a result, it is possible to further suppress the emission amount of emissions discharged from the catalyst while suppressing deterioration of drivability.

  In the air-fuel ratio control apparatus according to the present invention, the downstream feedback control means determines the downstream feedback correction amount based on a value that is based on an output value of the downstream air-fuel ratio sensor and is subjected to low-pass filter processing. Calculated and at least the in-cylinder injection amount is corrected by the calculated downstream feedback correction amount, and the ratio of the in-cylinder injection amount to the sum of the in-cylinder injection amount and the port injection amount. When a cylinder injection ratio changing means for changing a cylinder injection ratio according to the operating state of the internal combustion engine is provided, a filter in the low-pass filter processing by the downstream feedback control means according to the cylinder injection ratio It is preferable to further include low-pass filter characteristic changing means for changing the characteristic. Here, examples of the “filter characteristics” include a cut-off frequency and a gain.

  In general, an internal combustion engine having a dual injection system is configured such that the in-cylinder injection ratio can be changed in accordance with the operating state of the engine (for example, engine speed, in-cylinder intake air amount, cooling water temperature, etc.). Often done. In general, the rate of change in the variation in the fuel injection amount corrected by the feedback correction amount with respect to the variation in the same feedback correction amount increases as the fuel injection amount itself increases. In other words, even if there is no sudden fluctuation in the feedback correction amount, a sudden engine torque fluctuation is more likely to occur as the in-cylinder injection amount itself corrected by the feedback correction quantity increases.

  Therefore, even when the in-cylinder injection amount is corrected by the downstream feedback correction amount that is unlikely to cause rapid fluctuation as described above, the engine becomes more rapid as the in-cylinder injection amount increases due to the increase in the in-cylinder injection ratio. Torque fluctuations are likely to occur. In this case, in order to suppress the occurrence of such abrupt engine torque fluctuation, the change speed can be further reduced by lowering the frequency in the fluctuation of the downstream feedback correction amount as the in-cylinder injection ratio increases. Can be considered. For this purpose, the cutoff frequency of the low-pass filter used for calculation of the downstream feedback correction amount is set to the in-cylinder injection ratio (for example, a value equal to or lower than the maximum value of the frequency band of the catalyst-passing low frequency component). As the value increases, it may be set to a lower value.

  The present invention is based on such knowledge. That is, if the filter characteristic in the low-pass filter process is changed according to the in-cylinder injection ratio as in the above configuration, for example, the cut-off frequency of the low-pass filter is lowered as the in-cylinder injection ratio increases. Can be set to a value. As a result, stable and rapid engine torque fluctuations can be suppressed regardless of the in-cylinder injection ratio.

  In this case, the gain of the low-pass filter may be set to a lower value as the in-cylinder injection ratio increases. If the gain of the low-pass filter used for calculation of the downstream feedback correction amount is reduced, the fluctuation range of the downstream feedback correction amount is reduced, so that the changing speed is also reduced. Therefore, this also makes it possible to suppress the occurrence of stable and rapid engine torque fluctuations regardless of the in-cylinder injection ratio.

  Here, the “value based on the output value of the downstream air-fuel ratio sensor and subjected to the low-pass filter process” is, for example, the difference between the output value of the downstream air-fuel ratio sensor and the downstream target value. The value after the low-pass filter processing is performed on the value corresponding to the degree, or the output value (or the detected air-fuel ratio) of the downstream air-fuel ratio sensor and the downstream target value (or the downstream target value). It is a value corresponding to the degree of difference from the air / fuel ratio.

  In the air-fuel ratio control apparatus according to the present invention, the downstream feedback control means is a low-pass filter process based on a value that is based on an output value of the downstream air-fuel ratio sensor and that has undergone a low-pass filter process. Calculates the downstream downstream feedback correction amount and calculates the downstream feedback correction amount after the bandpass filter processing based on the value that is based on the output value of the downstream air-fuel ratio sensor and is subjected to the bandpass filter processing. By correcting only the in-cylinder injection amount by the calculated downstream feedback correction amount after the low-pass filter processing, and correcting only the port injection amount by the calculated downstream feedback correction amount after the band-pass filter processing. Feedback control of the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine It may be made.

  As described above, in the case where the cutoff frequency of the low-pass filter used for calculation of the downstream feedback correction amount is reduced in accordance with the increase of the in-cylinder injection ratio, the upstream feedback correction amount is calculated. Consider a case where the cutoff frequency of the high-pass filter used is always kept constant (for example, the maximum value of the frequency band of the low-frequency component passing through the catalyst). Note that the variation range of the cut-off frequency of the low-pass filter is a minimum value ωmin to a maximum value ωmax (the maximum value ωmax is set to the maximum value of the frequency band of the catalyst-passing low frequency component, for example).

  In this case, when the in-cylinder injection ratio increases, the cut-off frequency of the low-pass filter becomes lower than the cut-off frequency of the high-pass filter, so that the frequency band that is not included in any control frequency band of the air-fuel ratio feedback control (Hereinafter referred to as “non-controlled frequency band”) may occur in the control frequency band of the downstream feedback control. The occurrence of such a non-control-target frequency band is generally considered undesirable for maintaining reliable air-fuel ratio control.

  Here, the downstream feedback correction amount calculated based on the frequency component within the fluctuation range (ωmin to ωmax) of the cutoff frequency of the low-pass filter in the fluctuation of “value based on the output value of the downstream air-fuel ratio sensor”. If the configuration is such that the in-cylinder injection amount is corrected, as described above, an abrupt engine torque fluctuation is likely to occur due to an increase in the in-cylinder injection ratio. On the other hand, the port injection amount is corrected by the downstream feedback correction amount calculated based on the frequency component (ωmin to ωmax) in the fluctuation of the “value based on the output value of the downstream air-fuel ratio sensor”. Then, regardless of the in-cylinder injection ratio, as described above, sudden torque fluctuations of the engine are unlikely to occur due to the fuel adhering to the intake port or the like.

  Therefore, for example, a bandpass filter is newly prepared in which the minimum value ωmin is set to the low frequency side cutoff frequency and the maximum value ωmax is set to the high frequency side cutoff frequency. Port injection based on the downstream feedback correction amount (ie, downstream feedback correction amount after the bandpass filter processing) based on the value that is “the value based on the output value of the sensor” and that has been filtered by the bandpass filter prepared in the same manner By configuring so as to correct the amount, it is possible to surely suppress the occurrence of a sudden engine torque fluctuation without generating an uncontrolled frequency band in the control frequency band of the downstream feedback control.

  Further, the change rate of the frequency component less than the minimum value ωmin in the fluctuation of the “value based on the output value of the downstream air-fuel ratio sensor” is sufficiently small. In other words, the in-cylinder injection amount is corrected by the downstream feedback correction amount calculated based on the frequency component less than the minimum value ωmin in the variation of the “value based on the output value of the downstream air-fuel ratio sensor”. Even in this case, an increase in the in-cylinder injection ratio does not easily cause a sudden engine torque fluctuation.

  Accordingly, the cut-off frequency of the low-pass filter is set to the minimum value ωmin, and the “low-pass air-fuel ratio sensor output value” as in the above-described configuration, which is filtered by the low-pass filter. If the configuration is such that the in-cylinder injection amount is corrected by the downstream feedback correction amount based on the value (that is, the downstream feedback correction amount after the low-pass filter processing), the occurrence of abrupt engine torque fluctuation is suppressed, and High responsiveness can be secured in downstream feedback control based on frequency components less than the minimum value ωmin.

  As described above, by dividing the control frequency band of the downstream feedback control into the high frequency band related to the correction of the port injection amount and the low frequency band related to the correction of the in-cylinder injection amount, the downstream feedback control is performed. While ensuring a certain level of high responsiveness in the control, it is possible to reliably suppress the occurrence of abrupt engine torque fluctuations without generating an uncontrolled frequency band in the control frequency band of the downstream feedback control.

  In this case, the cutoff frequency of the high-pass filter processing and the cutoff frequency of the high-frequency side of the band-pass filter processing are the same, and the cutoff frequency of the low-frequency side of the band-pass filter processing and the cutoff frequency of the low-pass filter processing Are preferably the same.

  According to this, the control frequency band of the upstream feedback control related to the correction of the port injection amount, the control frequency band of the downstream feedback control related to the correction of the port injection amount, and the downstream feedback related to the correction of the in-cylinder injection amount. The control frequency band of control is continuous, and the non-control target frequency band is completely eliminated. That is, the air-fuel ratio control can be reliably executed by any one of the air-fuel ratio feedback control with respect to the fluctuation of the air-fuel ratio at any frequency, and as a result, the emission amount of emissions discharged from the catalyst can be further stabilized. Can be suppressed.

  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 equipped with a dual injection system. 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 communicating with the combustion chamber 25, an intake valve 32 that opens and closes the intake port 31, an intake camshaft that drives the intake valve 32, and continuously changes the phase angle of the intake camshaft. The variable intake timing device 33, the actuator 33 a of the variable intake timing device 33, the exhaust port 34 communicating with the combustion chamber 25, the exhaust valve 35 that opens and closes the exhaust port 34, the exhaust camshaft 36 that drives the exhaust valve 35, and the spark plug 37 , An igniter 38 including an ignition coil that generates a high voltage to be applied to the spark plug 37, a port injection valve 39P for injecting fuel into the intake port 31, and an in-cylinder injection valve 39C for injecting fuel directly into the combustion chamber 25. Yes.

  The intake system 40 is provided in an intake pipe 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 the intake pipe 41. A throttle valve 43 for changing the opening cross-sectional area of the intake 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 three-way catalyst 53 (also referred to as an upstream catalytic converter or a start catalytic converter) disposed (interposed) in the exhaust pipe 52, hereinafter referred to as a “first catalyst 53”. ), And a three-way catalyst 54 on the downstream side of the exhaust pipe 52 downstream of the first catalyst 53 (because it is disposed below the floor of the vehicle, an under-floor catalytic converter) However, it is hereinafter referred to as “second catalyst 54”). 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 outputs a voltage Vg corresponding to the mass flow rate per unit time of the intake air flowing through the intake pipe 41. The relationship between the output Vg of the air flow meter 61 and the measured intake air amount (flow rate) Ga is as shown in FIG. 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 outputs a signal having a narrow pulse every time the crankshaft 24 rotates 10 °, and a wide pulse every time the crankshaft 24 rotates 360 °. This signal represents the engine 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 outputs a current corresponding to the air-fuel ratio A / F as shown in FIG. 3, and an output value vabyfs which is a voltage corresponding to this current. In particular, when the air-fuel ratio is the stoichiometric air-fuel ratio, the output value vabyfs becomes the value vstoich. As is apparent from FIG. 3, the upstream air-fuel ratio sensor 66 can accurately detect the air-fuel ratio A / F over a wide range.

  The downstream air-fuel ratio sensor 67 is an electromotive force type (concentration cell type) oxygen concentration sensor, and outputs an output value Voxs that is a voltage that suddenly changes 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 an 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. This is a microcomputer comprising 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 an instruction from the CPU 71, the actuator 33a, the igniter 38, the port injection valve 39P of the variable intake timing device 33, Drive signals are sent to the in-cylinder injection valve 39C and the throttle valve actuator 43a.

(Outline of air-fuel ratio feedback control)
Next, an outline of feedback control of the air-fuel ratio of the air-fuel mixture supplied to the engine (hereinafter sometimes simply referred to as “engine air-fuel ratio”) performed by the air-fuel ratio control apparatus configured as described above will be described. To do.

  A three-way catalyst such as the first and second catalysts 53 and 54 (hereinafter sometimes simply referred to as “catalyst”) is configured such that when the air-fuel ratio of the gas flowing into the catalyst is the stoichiometric air-fuel ratio, HC, It oxidizes CO and reduces NOx, purifying these harmful components with high efficiency. Further, the catalyst has the above-described oxygen storage function (oxygen storage / release function) for storing and releasing oxygen, and even if the air-fuel ratio shifts from the stoichiometric air-fuel ratio to a certain extent by this oxygen storage / release function, HC, CO, and NOx can be purified. That is, when the air-fuel ratio of the engine becomes lean and the gas flowing into the catalyst contains a large amount of NOx, the catalyst takes oxygen molecules from NOx and occludes the oxygen molecules and reduces the NOx. To purify. Further, when the air-fuel ratio of the engine becomes rich and the gas flowing into the catalyst contains a large amount of HC and CO, the catalyst gives (releases) the oxygen molecules stored therein to oxidize, thereby Purifies HC and CO.

  Therefore, in order to efficiently purify a large amount of HC and CO into which the catalyst flows continuously, the catalyst must store a large amount of oxygen, and conversely, a large amount of NOx that flows in continuously. In order to purify efficiently, the catalyst must be in a state where it can sufficiently store oxygen. From the above, the purification capacity of the catalyst depends on the maximum oxygen amount (maximum oxygen storage amount) that the catalyst can store.

  On the other hand, three-way catalysts such as the first and second catalysts 53 and 54 are deteriorated by poisoning due to lead or sulfur contained in the fuel, or heat applied to the catalyst, and the maximum oxygen storage amount gradually decreases accordingly. Come on. Even when the maximum oxygen storage amount is reduced in this way, the air-fuel ratio of the gas discharged from the catalyst (hence, the air-fuel ratio of the gas flowing into the catalyst) is used to continuously suppress the emission emission amount. However, it is necessary to control so as to be very close to the stoichiometric air-fuel ratio.

  Therefore, in the air-fuel ratio control apparatus of the present embodiment, the output values of the upstream and downstream air-fuel ratio sensors 66 and 67 match the corresponding sensor target values (in principle, values corresponding to the theoretical air-fuel ratio). Further, the upstream air-fuel ratio sensor output value vabyfs (that is, the air-fuel ratio upstream of the first catalyst) and the downstream air-fuel ratio sensor output value Voxs (that is, the air-fuel ratio downstream of the first catalyst and upstream of the second catalyst). Feedback control of engine air-fuel ratio.

  More specifically, this air-fuel ratio control device (hereinafter also referred to as “this device”) includes each of elements A1 to A17 as shown in FIG. 5 which is a functional block diagram thereof. It consists of Hereinafter, each element will be described with reference to FIG.

<Determination of basic fuel injection amount>
First, the determination of the basic fuel injection amount Fbase will be described. The upstream target air-fuel ratio setting means A1 corresponds to the target value (upstream target value) of the upstream air-fuel ratio sensor output based on the engine speed NE that is the operating state of the internal combustion engine 10, the throttle valve opening TA, and the like. An upstream target air-fuel ratio abyfr (k) to be determined is determined. Here, the subscript (k) indicates a value for the current intake stroke (hereinafter, the same applies to other physical quantities). The upstream target air-fuel ratio abyfr (k) is set to the stoichiometric air-fuel ratio except for special cases after the warm-up of the internal combustion engine 10, for example. Further, the upstream target air-fuel ratio abyfr is stored in the RAM 73 while corresponding to the intake stroke of each cylinder.

  The basic fuel injection amount determining means A2 is obtained based on a predetermined table using as arguments the intake air flow rate Ga measured by the air flow meter 61 and the engine rotational speed NE obtained based on the output of the crank position sensor 63. By dividing the in-cylinder intake air amount Mc (k), which is the amount of intake air of the cylinder that reaches this intake stroke, by the upstream target air-fuel ratio abyfr (k) set by the upstream target air-fuel ratio setting means A1 The basic fuel injection amount Fbase is obtained. That is, the basic fuel injection amount Fbase is the fuel injection amount from the port injection valve 39P and the in-cylinder injection valve 39C for the current intake stroke required for setting the air-fuel ratio of the engine to the upstream target air-fuel ratio abyfr (k). Total amount. As described above, the present apparatus obtains the basic fuel injection amount Fbase by using the upstream target air-fuel ratio setting means A1 and the basic fuel injection amount determination means A2.

<Calculation of in-cylinder injection amount and port injection amount>
Next, calculation of the in-cylinder injection amount Fic and the port injection amount Fip will be described. The in-cylinder injection ratio determining means (in-cylinder injection ratio changing means) A3 is a predetermined parameter that uses as arguments the engine speed NE that is the operating state of the internal combustion engine 10, the in-cylinder intake air amount Mc, and the cooling water temperature THW. Based on the table, the ratio of in-cylinder injection amount Fic to the sum of in-cylinder injection amount Fic and port injection amount Fip (more precisely, to the sum of basic in-cylinder injection amount Fbasec described later and basic port injection amount Fbasep described later) The in-cylinder injection ratio R, which is the ratio of the basic in-cylinder injection amount Fbasec), is determined. Thereby, the in-cylinder injection ratio R is changed according to the operating state of the engine.

  The basic in-cylinder injection amount determination means A4 multiplies the basic fuel injection amount Fbase obtained by the basic fuel injection amount determination means A2 by the determined in-cylinder injection ratio R, so that the basic cylinder injection amount Fbase is determined according to the following equation (1). The internal injection amount Fbasec is determined.

Fbasec = Fbase ・ R (1)

  Similarly, the basic port injection amount determination means A5 determines the basic port injection amount Fbasep according to the following equation (2) by multiplying the basic fuel injection amount Fbase by a value (1-R).

Fbasep = Fbase · (1−R) (2)

  The in-cylinder injection amount calculation unit A6 multiplies the basic in-cylinder injection amount Fbasec obtained by the basic in-cylinder injection amount determination unit A4 by a sub-feedback correction coefficient KFisub (downstream feedback correction amount) to be described later. Based on the equation (3), the (final) in-cylinder injection amount Fic is obtained.

Fic ＝ Fbasec ・ KFisub (3)

  Similarly, the port injection amount calculation means A7 adds the sub-feedback correction coefficient KFisub (downstream feedback correction amount) and a main feedback correction coefficient KFimain described later to the basic port injection amount Fbasep obtained by the basic port injection amount determination means A5. By multiplying (upstream feedback correction amount), the (final) port injection amount Fip is obtained based on the following equation (4).

Fip = Fbasep / KFisub / KFimain (4)

  In this way, the present apparatus reaches the intake stroke of the fuel of the in-cylinder injection amount Fic obtained by correcting the basic in-cylinder injection amount Fbasec by the sub-feedback correction coefficient KFisub by the in-cylinder injection amount calculation means A6. The cylinder is injected by a cylinder injection valve 39C. In addition, the port injection amount Fipp, which is obtained by independently correcting the basic port injection amount Fbasep by the main feedback correction coefficient KFimain and the sub feedback correction coefficient KFisub by the port injection amount calculation means A7, reaches the current intake stroke. The cylinder is injected by the port injection valve 39P.

<Sub feedback control>
Next, sub feedback control as downstream feedback control will be described. The downstream target value setting means A8 is similar to the upstream target air-fuel ratio setting means A1 described above, and is based on the engine speed NE, which is the operating state of the internal combustion engine 10, the throttle valve opening TA, and the like. The downstream target value Voxsref, which is the output target value, is determined. The downstream target value Voxsref is set to 0.5 (V), which is a value corresponding to the stoichiometric air-fuel ratio except for special cases, for example, after the warm-up of the internal combustion engine 10 is finished (see FIG. 4). reference.). In this example, the downstream target value Voxsref is set so that the downstream target 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 (5), the output deviation amount calculation means A9 calculates the current output value of the downstream air-fuel ratio sensor 67 from the current downstream target value Voxsref set by the downstream target value setting means A8. The output deviation amount DVoxs is obtained by subtracting Voxs. This output deviation amount DVoxs corresponds to “a value corresponding to the degree of difference between the output value Voxs of the downstream air-fuel ratio sensor 67 and the downstream target value Voxsref”.

DVoxs = Voxsref-Voxs (5)

  The low-pass filter A10 (LPF) is a primary filter (soft filter, digital filter) as shown in the following formula (6) in which the characteristics are expressed using the Laplace operator s. In the following formula (6), τlpf is a time constant of the low-pass filter A10, and Glpf is a gain at the input signal frequency “0” of the low-pass filter A10 (hereinafter simply referred to as “gain”. In this example, Glpf = 1). It is. The frequency-gain characteristic of the low-pass filter A10 is as shown in FIG. As shown in FIG. 6, the low-pass filter A10 passes the high-frequency component by attenuating the high-frequency component equal to or higher than the cutoff frequency ωlpf (= 1 / τlpf) in the fluctuation of the input value (input signal). Is substantially prohibited. The cut-off frequency ωlpf of the low-pass filter A10 is changed by LPF characteristic changing means A17 described later.

Glpf ・ (1 / (1 + τlpf ・ s)) (6)

  The low-pass filter A10 is a value after the value of the output deviation amount DVoxs obtained by the output deviation amount calculating means A9 is input and the value of the output deviation amount DVoxs is subjected to low-pass filter processing according to the above equation (6). Output the output deviation DVoxslow after passing the low-pass filter. Therefore, the output deviation amount DVoxslow after passing through the low-pass filter is “a value after low-pass filter processing of a value corresponding to the degree of difference between the output value Voxs of the downstream air-fuel ratio sensor 67 and the downstream target value Voxsref”.

  The PID controller A11 (downstream feedback controller) performs proportional / integral / differential processing (PID processing) on the output deviation amount DVoxslow after passing through the low-pass filter, which is an output value of the low-pass filter A10, based on the following equation (7). A sub feedback correction coefficient KFisub (> 0) is obtained as a downstream feedback correction amount.

KFisub = (Kp · DVoxslow + Ki · SDVoxslow + Kd · DDVoxslow) + 1 (7)

  In the above equation (7), Kp is a preset proportional gain (proportional constant), Ki is a preset integral gain (integral constant), and Kd is a preset differential gain (differential constant). SDVoxslow is a time integral value of the output deviation amount DVoxslow after passing through the low-pass filter, and DDVoxslow is a time differential value of the output deviation amount DVoxslow after passing through the low-pass filter.

  In this way, the present apparatus outputs the output deviation after passing through the low-pass filter, which is a value after low-pass filtering the value corresponding to the degree of difference between the output value Voxs of the downstream air-fuel ratio sensor 67 and the downstream target value Voxsref. The sub feedback correction coefficient KFisub is obtained by inputting the quantity DVoxslow (downstream feedback input value) to the PID controller A11. Then, the present device multiplies the basic in-cylinder injection amount Fbasec and the basic port injection amount Fbasep by the same sub feedback correction coefficient KFisub, respectively, to thereby perform main feedback control (to be described later) (by the main feedback correction coefficient KFimain). The sub-feedback control is executed by correcting the basic in-cylinder injection amount Fbasec and the basic port injection amount Fbasep independently of the correction of the basic port injection amount Fbasep.

  For example, when the average (steady) air-fuel ratio of the engine is lean, the output value Voxs of the downstream-side air-fuel ratio sensor 67 constantly shows a value corresponding to the air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. The value of the output deviation amount DVoxs obtained by the output deviation amount calculating means A9 is constantly a positive value (see FIG. 4). Accordingly, the output deviation amount DVoxslow after passing through the low-pass filter is also a positive value, and the sub-feedback correction coefficient KFisub obtained by the PID controller A11 is a value larger than “1”. As a result, the total amount of the in-cylinder injection amount Fic and the port injection amount Fip respectively obtained by the in-cylinder injection amount calculation means A6 and the port injection amount calculation means A7 becomes larger than the basic fuel injection amount Fbase. The air-fuel ratio is controlled to be rich.

  On the contrary, when the steady air-fuel ratio of the engine is rich, the downstream air-fuel ratio sensor output value Voxs constantly shows a value corresponding to the air-fuel ratio that is richer than the theoretical air-fuel ratio, and the output deviation amount DVoxs (Therefore, since the output deviation amount DVoxslow after passing through the low-pass filter) becomes a negative value, the sub feedback correction coefficient KFisub becomes a value (> 0) smaller than “1”. Thus, the total amount of the in-cylinder injection amount Fic and the port injection amount Fip is controlled to be smaller than the basic fuel injection amount Fbase, and the air-fuel ratio of the engine becomes lean.

  Further, since the PID controller A11 executes the integration process (that is, the integral (I) term “Ki · SDVoxslow” is used), the output deviation amount DVoxs is set to “0” when the engine is in the steady operation state. Guaranteed to be. In other words, the steady deviation of the output value Voxs of the downstream air-fuel ratio sensor 67 from the downstream target value Voxsref becomes zero. In the steady operation state, the output deviation amount DVoxs becomes “0” so that the proportional term Kp · DVoxslow and the differential term Kd · DDVoxslow become “0”. The value obtained by adding “1” to the value of. By multiplying this value by the basic in-cylinder injection amount Fbasec and the basic port injection amount Fbasep, respectively, an error (instructed fuel injection amount and actual fuel injection amount) of the in-cylinder injection valve 39C and the port injection valve 39P. ) And an error of the air flow meter 61 (difference between the intake air flow rate measurement value Ga and the actual intake air flow rate) are compensated for, while the air-fuel ratio downstream of the first catalyst 53 in the steady operation state (and hence the air-fuel ratio of the engine). ) Converges to a target air-fuel ratio (that is, theoretical air-fuel ratio in principle) corresponding to the downstream target value Voxsref. As described above, the in-cylinder injection amount calculation means A6, the port injection amount calculation means A7, the downstream target value setting means A8, the output deviation amount calculation means A9, the low-pass filter A10, and the PID controller A11 correspond to the downstream feedback control means.

<Main feedback control>
Next, main feedback control as upstream feedback control will be described. The table conversion means A12 converts the output value vabyfs of the upstream air-fuel ratio sensor 66 and the table that defines the relationship between the upstream air-fuel ratio sensor output value vabyfs and the air-fuel ratio A / F shown in FIG. Based on this, the detected air-fuel ratio abyfs at the present time by the upstream air-fuel ratio sensor 66 is obtained.

  The target air-fuel ratio delay unit A13 is N strokes before the N stroke (N intake strokes) from the present time among the upstream target air-fuel ratio abyfr obtained for each intake stroke by the upstream target air-fuel ratio setting unit A1 and stored in the RAM 73. The upstream target air-fuel ratio abyfr for the cylinder that has reached the intake stroke is read from the RAM 73 and set as the upstream target air-fuel ratio abyfr (kN). Here, the value N differs depending on the displacement of the internal combustion engine 10, the distance from the combustion chamber 25 to the upstream air-fuel ratio sensor 66, and the like.

  As described above, the upstream target air-fuel ratio abyfr (kN) N strokes before the current stroke is used until the air-fuel mixture burned in the combustion chamber 25 reaches the upstream air-fuel ratio sensor 66. This is because time L corresponding to N stroke is required.

  The upstream air-fuel ratio deviation calculating means A14 calculates the current air-fuel ratio abyfs obtained by the table conversion means A12 based on the following equation (8) from the present time set by the target air-fuel ratio delay means A13 to N The upstream air-fuel ratio deviation Dabyf is obtained by subtracting the upstream target air-fuel ratio abyfr (kN) before the stroke. The upstream air-fuel ratio deviation Dabyf is an amount that represents the difference between the actual air-fuel ratio and the target air-fuel ratio at the time point before N strokes, and is “the output value vabyfs of the upstream air-fuel ratio sensor 66 and the upstream target value. Corresponds to the value according to the degree of difference.

Dabyf ＝ abyfs−abyfr (k-N) (8)

  The high-pass filter A15 (HPF) is a primary filter (soft filter, digital filter) as shown in the following equation (9) in which the characteristics are expressed using the Laplace operator s. In the following equation (9), τhpf is a time constant of the high-pass filter A15, and Ghpf is a gain at the input signal frequency “∞” of the high-pass filter A15 (hereinafter simply referred to as “gain”. In this example, Ghpf = 1) It is. The frequency-gain characteristic of the high-pass filter A15 is as shown in FIG. As shown in FIG. 6, the high-pass filter A15 passes the low-frequency component by attenuating the low-frequency component below the cutoff frequency ωhpf (= 1 / τhpf) in the fluctuation of the input value (input signal). Is substantially prohibited. In this example, the cutoff frequency ωhpf of the high-pass filter A15 is set to the maximum value (value ωcut described later) of the frequency band of the catalyst-passing low frequency component.

Ghpf ・ (1-1 / (1 + τhpf ・ s)) (9)

  The high-pass filter A15 receives the value of the upstream air-fuel ratio deviation Dabyf obtained by the upstream air-fuel ratio deviation calculating means A14, and calculates the value of the upstream air-fuel ratio deviation Dabyf according to the above equation (9). An upstream air-fuel ratio deviation Dabyfhi after passing the high-pass filter, which is a value after processing, is output. Therefore, the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter is “a value after high-pass filter processing is performed on a value corresponding to the degree of difference between the output value vabyfs of the upstream air-fuel ratio sensor 66 and the upstream target value”. .

  The PI controller A16 (upstream feedback controller) performs proportional / integration processing (PI processing) on the upstream air-fuel ratio deviation Dabyfhi after passing through the highpass filter, which is the output value of the highpass filter A15, based on the following equation (10). A main feedback correction coefficient KFimain (> 0) is obtained.

KFimain = (Gp / Dabyfhi + Gi / SDabyfhi) +1 (10)

  In the above equation (10), Gp is a preset proportional gain (proportional constant), and Gi is an integral gain (integral constant). SDabyfhi is a time integral value of the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter.

  In this way, the present apparatus connects the main feedback control circuit and the sub feedback control circuit in parallel with respect to the correction of the basic fuel injection amount Fbase (more precisely, the correction of the basic port injection amount Fbasep). In addition, the present apparatus is a high-pass filter that is a value obtained by performing high-pass filtering on a value corresponding to the degree of difference between the output value vabyfs of the upstream air-fuel ratio sensor 66 and the upstream target value corresponding to the upstream target air-fuel ratio abyfr. A main feedback correction coefficient KFimain is obtained by inputting an upstream air-fuel ratio deviation Dabyfhi (upstream feedback input value) to the PI controller A16 after passing through the filter, and the basic port injection amount Fbasep is multiplied by the main feedback correction coefficient KFimain. Thus, the main feedback control is executed by correcting the basic port injection amount Fbasep independently of the correction of the basic in-cylinder injection amount Fbasec and the basic port injection amount Fbasep by the sub feedback control.

  For example, when the air-fuel ratio of the engine suddenly changes and becomes lean, as can be understood from FIG. 3, the detected air-fuel ratio abyfs based on the upstream air-fuel ratio sensor output value is the upstream side set by the upstream target air-fuel ratio setting means A1. The target air-fuel ratio abyfr is also a large value. For this reason, the upstream air-fuel ratio deviation Dabyf obtained by the upstream air-fuel ratio deviation calculating means A14 has a positive value. Here, the signal indicating the upstream air-fuel ratio deviation Dabyf due to a sudden change in the air-fuel ratio of the engine includes a high frequency component equal to or higher than the cut-off frequency ωhpf. Such a high frequency component can pass through the high pass filter A15. Therefore, the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter is also a positive value. As a result, the main feedback correction coefficient KFimain calculated by the PI controller A16 becomes a value larger than “1”. Thus, the total amount of the in-cylinder injection amount Fic and the port injection amount Fip obtained by the in-cylinder injection amount calculation means A6 and the port injection amount calculation means A7 is larger than the basic fuel injection amount Fbase, The engine air / fuel ratio is controlled to be rich.

  On the contrary, when the air-fuel ratio of the engine suddenly changes and becomes rich, the detected air-fuel ratio abyfs becomes a value smaller than the upstream target air-fuel ratio abyfr. For this reason, the upstream air-fuel ratio deviation Dabyf is a negative value. Also in this case, the signal indicating the upstream side air-fuel ratio deviation Dabyf includes a high frequency component equal to or higher than the cutoff frequency ωhpf that can pass through the high pass filter A15. Accordingly, the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter also takes a negative value. As a result, the main feedback correction coefficient KFimain becomes a value (> 0) smaller than “1”. Thus, the total amount of the in-cylinder injection amount Fic and the port injection amount Fip is controlled to be smaller than the basic fuel injection amount Fbase and the engine air-fuel ratio becomes lean.

  The port injection amount calculation means A7, table conversion means A12, target air-fuel ratio delay means A13, upstream air-fuel ratio deviation calculation means A14, high-pass filter A15, and PI controller A16 correspond to upstream feedback control means.

<Avoidance of mutual interference between air-fuel ratio feedback control>
Generally, when the input signal to the high-pass filter shifts from a steady state to a transient state (for example, when the input signal changes in a stepped manner), the high-pass filter is the same as the high-pass filter when the input signal shifts to the transient state. All frequency components in the input signal are substantially passed over a period corresponding to the time constant of the filter. In other words, a frequency component lower than the cut-off frequency of the high pass filter is substantially passed.

  Therefore, it is preferable that the time constant of the high-pass filter is as small as possible from the viewpoint of prohibiting the passage of frequency components lower than the cutoff frequency as much as possible. Here, the time constant of the high pass filter is inversely proportional to the cutoff frequency of the high pass filter. From the above point of view, it is preferable that the cutoff frequency ωhpf of the high-pass filter A15 is as large as possible.

  On the other hand, as described above, there is an upper limit in the frequency band of the catalyst-passing low frequency component that can pass through the three-way catalyst such as the first catalyst 53. That is, the frequency component exceeding the maximum value of the frequency band of the low-frequency component passing through the catalyst in the air-fuel ratio fluctuation of the exhaust gas cannot pass through the catalyst. Therefore, when the cutoff frequency ωhpf of the high-pass filter A15 is set to a value exceeding the maximum value of the frequency band of the low-frequency component passing through the catalyst with respect to the first catalyst 53, the above-described non-control-target frequency band is substantially generated.

  Therefore, it is considered that the cut-off frequency ωhpf of the high-pass filter A15 is preferably set to the maximum value (hereinafter referred to as “value ωcut”) of the low-frequency component passing through the catalyst with respect to the first catalyst 53. From the above, in this example, the cutoff frequency ωhpf of the high-pass filter A15 is set to the value ωcut (constant). The value ωcut can be acquired through an experiment using the first catalyst 53 or the like.

  Since the value Dabyfhi input to the PI controller A16, which is the upstream feedback controller, contains only a high frequency component equal to or higher than the cut-off frequency ωhpf (= ωcut) of the high-pass filter A15, the main feedback control is controlled. The frequency band is a band higher than ωhpf.

  On the other hand, the value DVoxslow input to the PID controller A11 which is the downstream feedback controller includes only a low frequency component equal to or lower than the cut-off frequency ωlpf of the low-pass filter A10. Therefore, the control frequency band of the sub feedback control is ωlpf The bandwidth is as follows.

  Here, the apparatus sets the cutoff frequency ωlpf to a value equal to or less than the value ωcut as will be described later. Thereby, the control frequency bands of the main feedback control and the sub feedback control are set so as not to overlap each other, thereby avoiding mutual interference between the two air-fuel ratio feedback controls.

  Further, when the air-fuel ratio of the exhaust gas suddenly changes or fluctuates at a high frequency equal to or higher than the cut-off frequency ωhpf of the high-pass filter A15, such as when the engine is in a transient operation state, the upstream-side air-fuel ratio sensor The detected air-fuel ratio abyfs based on the output value includes a high frequency component having a cutoff frequency ωhpf or higher. This high frequency component passes through the high pass filter A15. Therefore, in the present apparatus, the air-fuel ratio control (compensation) for the sudden change of the air-fuel ratio in the transient operation state can be performed quickly and reliably by the main feedback control.

  Further, when a steady air-fuel ratio fluctuation occurs at a relatively low frequency equal to or lower than the cut-off frequency ωlpf of the low-pass filter A10 that can appear as an air-fuel ratio fluctuation downstream of the first catalyst 53, the steady-state Due to the air-fuel ratio fluctuation, the output value Voxs of the downstream air-fuel ratio sensor 67 includes a low frequency component having a cutoff frequency ωlpf or less. This low frequency component passes through the low pass filter A10. Therefore, in this apparatus, air-fuel ratio control with respect to such steady air-fuel ratio fluctuation can be reliably achieved by sub-feedback control.

<Suppression of sudden engine torque fluctuation caused by main feedback control>
Since the main feedback correction coefficient KFimain, which is the output of the PI controller A16, can fluctuate at a high frequency equal to or higher than the cut-off frequency ωhpf (= ωcut) of the high-pass filter A15, the rate of change can be large. Accordingly, the main feedback correction coefficient KFimain may change rapidly. Here, as described above, if the configuration is such that the in-cylinder injection amount Fic (more precisely, the basic in-cylinder injection amount Fbasec) is corrected by the main feedback correction coefficient KFimain that can cause such a rapid fluctuation, the engine suddenly increases. Torque fluctuations can occur.

  On the other hand, in this example, the port feedback amount Fip (to be precise, the basics) is controlled by the main feedback correction coefficient KFimain so that sudden torque fluctuation does not occur in the engine even if the main feedback correction coefficient KFimain suddenly changes. Main feedback control is executed by correcting only the port injection amount Fbasep). As a result, sudden torque fluctuations of the engine due to the main feedback control are suppressed.

<Suppression of sudden engine torque fluctuation caused by sub-feedback control>
The sub feedback correction coefficient KFisub, which is the output of the PID controller A11, can change only at a relatively low frequency that is a frequency equal to or lower than the value ωcut, so that the rate of change is relatively small. Accordingly, the sub feedback correction coefficient KFisub is unlikely to change rapidly. Therefore, in this example, not only the port injection amount Fip (more precisely, the basic port injection amount Fbasep) but also the in-cylinder injection amount Fic (more precisely, the basic in-cylinder injection amount Fbasec) is obtained by the sub feedback correction coefficient KFisub. Sub-feedback control is performed by correcting.

  However, as described above, even when the in-cylinder injection amount Fic is corrected by the sub-feedback correction coefficient KFisub that is unlikely to cause rapid fluctuations, the in-cylinder injection amount Ric increases to increase the in-cylinder injection amount Fic. There is a tendency that sudden torque fluctuations of the engine are likely to occur.

  In this case, as the in-cylinder injection ratio R increases, the change rate of the sub feedback correction coefficient KFisub can be further reduced by lowering the maximum frequency (that is, the cutoff frequency ωlpf) in the fluctuation of the sub feedback correction coefficient KFisub. It is thought that sudden torque fluctuations due to such sub-feedback control can be suppressed.

  From the above, this apparatus (specifically, the LPF characteristic changing means A17) is sequentially determined by the in-cylinder injection ratio determining means A3 according to the relationship between the in-cylinder injection ratio R and the cutoff frequency ωlpf shown in FIG. The cut-off frequency ωlpf of the low-pass filter A10 is sequentially changed according to the in-cylinder injection ratio R being changed. Thereby, the cutoff frequency ωlpf is set to a smaller value within the range of the value ω0 to the value ωcut as the in-cylinder injection ratio R increases (see FIG. 6).

  The relationship between the in-cylinder injection ratio R and the cut-off frequency ωlpf shown in FIG. 7 is, for example, a cut-off frequency at which rapid engine torque fluctuations do not occur when the in-cylinder injection ratio R is fixed to a certain value. The experiment for obtaining the maximum value of ωlpf can be obtained by repeatedly performing the in-cylinder injection ratio R while gradually changing it within the range of “0” to “1”.

  As described above, the cut-off frequency ωlpf of the low-pass filter A10 is changed by the LPF characteristic changing unit A17, so that sudden engine torque fluctuation caused by the sub-feedback control is suppressed. As described above, the LPF characteristic changing unit A17 corresponds to the low-pass filter characteristic changing unit. The above is the outline of the feedback control of the air-fuel ratio of the engine performed by this apparatus.

(Actual operation)
Next, the actual operation of the air / fuel ratio control apparatus according to the first embodiment will be described.
<Air-fuel ratio feedback control>
The CPU 71 performs a routine for calculating the in-cylinder injection amount Fic and the port injection amount Fip shown in the flowchart of FIG. 8 and instructing fuel injection. A predetermined crank angle (for example, the crank angle of each cylinder before the intake top dead center) , BTDC 90 ° CA) every time.

  Therefore, when the crank angle of an arbitrary cylinder reaches the predetermined crank angle, the CPU 71 starts processing from step 800 and proceeds to step 805, and the intake air flow rate Ga measured by the air flow meter 61 and the crank position sensor 64. The in-cylinder intake air amount Mc, which is the intake air amount of the cylinder that reaches the current intake stroke, is obtained based on the engine speed NE obtained based on the output and the table MapMc using Ga and NE as arguments.

  Next, the CPU 71 proceeds to step 810 and divides the obtained in-cylinder intake air flow rate Mc by the current upstream target air-fuel ratio abyfr (k), so that the engine air-fuel ratio becomes the same as the upstream target air-fuel ratio abyfr. A basic fuel injection amount Fbase for obtaining (k) is obtained.

  Next, the CPU 71 proceeds to step 815 to determine the engine speed NE, the in-cylinder intake air amount Mc obtained above, the cooling water temperature THW obtained from the water temperature sensor 65, and the table MapR using NE, Mc, THW as arguments. Based on this, the in-cylinder injection ratio R is obtained.

  Subsequently, the CPU 71 proceeds to step 820, and based on the obtained basic fuel injection amount Fbase, the obtained in-cylinder injection ratio R, and the above formulas (1) and (2), the basic in-cylinder injection amount. Fbasec and basic port injection amount Fbasep are obtained.

  Next, the CPU 71 proceeds to step 825, and multiplies the obtained in-cylinder injection amount Fbasec by the sub-feedback correction coefficient KFisub calculated in the routine described later to obtain the in-cylinder injection amount Fic according to the above equation (3). calculate.

  Then, the CPU 71 proceeds to step 830 and multiplies the obtained basic port injection amount Fbasep by the sub feedback correction coefficient KFisub and the main feedback correction coefficient KFimain calculated in the routine described later, respectively (4). The port injection amount Fip is calculated according to the equation.

  Then, the CPU 71 proceeds to step 835 to give an instruction for injecting the fuel of the in-cylinder injection amount Fic at a predetermined time to the in-cylinder injection valve 39C of the cylinder that reaches the current intake stroke, and the port injection amount Fip. An instruction for injecting the fuel at a predetermined time is given to the port injection valve 39P of the cylinder that reaches the current intake stroke, and then the routine proceeds to step 895 to end this routine once.

  As described above, the fuel injection amount after being independently corrected by the main feedback control and the sub feedback control, that is, the fuel of the in-cylinder injection amount Fic and the port injection amount Fip, respectively, is injected into the cylinders that reach the intake stroke. Is done.

<Calculation of main feedback correction factor>
Next, the operation when calculating the main feedback correction coefficient KFiup in the main feedback control will be described. The CPU 71 executes a routine (corresponding to the upstream feedback control means) shown in the flowchart of FIG. 9 for a predetermined time (for example, 8 msec). It is executed repeatedly every time. Therefore, when the predetermined timing comes, the CPU 71 starts processing from step 900 and proceeds to step 905 to determine whether or not the main feedback control condition is satisfied.

  The main feedback control condition is, for example, that the engine coolant temperature THW is equal to or higher than a first predetermined temperature, the intake air amount (load) per one rotation of the engine is equal to or lower than a predetermined value, and the upstream air-fuel ratio sensor 66 Holds when is fully active.

  Now, assuming that the main feedback control condition is satisfied, the CPU 71 determines “Yes” in step 905 and proceeds to step 910 to display the current output value vabyfs of the upstream air-fuel ratio sensor 66. The detected air-fuel ratio abyfs at the present time is obtained by performing conversion based on the table shown in FIG.

  Subsequently, the CPU 71 proceeds to step 915 and subtracts the upstream target air-fuel ratio abyfr (k−N) N strokes before the current stroke from the obtained detected air-fuel ratio abyfs from the current time according to the above equation (8). An upstream air-fuel ratio deviation Dabyf representing the difference between the actual air-fuel ratio before the N stroke and the target air-fuel ratio is obtained.

  Next, the CPU 71 proceeds to step 920, where the upstream air-fuel ratio deviation Dabyf is subjected to high-pass filtering with a high-pass filter A15 having a cutoff frequency ωhpf (= ωcut constant) to obtain the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter. In step 925, the main feedback correction coefficient KFimain is obtained according to the above equation (10). Here, as SDabyfhi, the latest value obtained in the previous execution of this routine in step 930 is used.

  That is, when the CPU 71 proceeds to step 930, it adds the upstream air-fuel ratio deviation Dabyfhi after passing the high-pass filter obtained in step 920 to the integrated value SDabyfhi of the upstream air-fuel ratio deviation after passing the high-pass filter at that time, and a new The integrated value SDabyfhi of the upstream air-fuel ratio deviation after passing through the high-pass filter is obtained.

  Then, the CPU 71 proceeds to step 995 to end the present routine tentatively. Thus, the main feedback correction coefficient KFimain is obtained, and this main feedback correction coefficient KFimain is reflected only in the port injection amount Fip in step 830 of FIG. 8 described above, whereby the air-fuel ratio control of the engine based on the main feedback control described above. Is executed.

  On the other hand, if the main feedback control condition is not satisfied at the time of determination in step 905, the CPU 71 determines “No” in step 905 and proceeds to step 935 to set the value of the main feedback correction coefficient KFimain to “1”. In step 940, the integrated value SDabyfhi of the upstream air-fuel ratio deviation after passing through the high-pass filter is reset to “0”. Then, the process proceeds to step 995, and this routine is temporarily terminated. As described above, when the main feedback control condition is not satisfied, the value of the main feedback correction coefficient KFimain is set to “1”, and the correction of the air-fuel ratio of the engine based on the main feedback control is not performed.

<Calculation of sub feedback correction factor>
Next, the operation for calculating the sub-feedback correction coefficient KFisub in the sub-feedback control will be described. The CPU 71 executes a routine shown in the flowchart of FIG. 10 (corresponding to the downstream feedback control means) for a predetermined time (for example, 8 msec). It is executed repeatedly every time. Therefore, when the predetermined timing is reached, the CPU 71 starts processing from step 1000 and proceeds to step 1005 to determine whether or not the sub feedback control condition is satisfied.

  The sub feedback control condition is, for example, that the cooling water temperature THW of the engine is equal to or higher than a second predetermined temperature higher than the first predetermined temperature, and the intake air amount (load) per one rotation of the engine is equal to or lower than a predetermined value. This is established when the downstream air-fuel ratio sensor 67 is in a fully active state.

  Now, assuming that the sub-feedback control condition is satisfied, the CPU 71 determines “Yes” in step 1005 and proceeds to step 1010. According to the above equation (5), the current downstream target value The output deviation amount DVoxs is obtained by subtracting the current output value Voxs of the downstream air-fuel ratio sensor 67 from Voxsref.

  Next, the CPU 71 proceeds to step 1015, and the function funcωlpf (R) representing the relationship between the in-cylinder injection ratio R and the cutoff frequency ωlpf shown in FIG. Based on the in-cylinder injection ratio R, the cutoff frequency ωlpf of the low-pass filter A10 to be set at the present time is obtained.

  Subsequently, the CPU 71 proceeds to step 1020 to low-pass filter the output deviation amount DVoxs with the obtained low-pass filter A10 having the cut-off frequency ωlpf to obtain an output deviation amount DVoxslow after passing through the low-pass filter.

  Next, the CPU 71 proceeds to step 1025 to obtain a differential value DDVoxslow of the output deviation amount DVoxslow after passing through the low-pass filter based on the following equation (11).

DDVoxslow = (DVoxslow-DVoxslow1) / Δt (11)

  In the above equation (11), DVoxslow1 is the previous value of the output deviation amount DVoxslow after passing through the low-pass filter set (updated) in step 1040 described later at the time of the previous execution of this routine. Δt is the execution interval time (predetermined time) of this routine.

  Next, the CPU 71 proceeds to step 1030, obtains the sub-feedback control coefficient KFisub according to the above equation (7), and then proceeds to step 1035 to set the integrated value SDVoxslow of the output deviation amount after passing through the low-pass filter at that time to the above step 1020. Is added to the output deviation amount DVoxslow after passing through the low-pass filter to obtain a new integrated value SDVoxslow after passing through the low-pass filter, and in step 1040, the output after passing through the low-pass filter obtained in step 1020 is obtained. After setting the deviation amount DVoxslow as the previous value DVoxslow1 of the output deviation amount DVoxslow after passing through the low-pass filter, the routine proceeds to step 1095 to end the present routine tentatively.

  Thus, the sub feedback control coefficient KFisub is obtained, and the sub feedback control coefficient KFisub is reflected in the in-cylinder injection amount Fic and the port injection amount Fip in steps 825 and 830 of FIG. The engine air-fuel ratio control based on the control is executed.

  On the other hand, if the sub-feedback control condition is not satisfied at the time of determination in step 1005, the CPU 71 determines “No” in step 1005 and proceeds to step 1045 to set the value of the sub-feedback control coefficient KFisub to “1”. In step 1050, the integrated value SDVoxslow of the output deviation after passing through the low-pass filter is reset to “0”. Then, the process proceeds to step 1095 to end the present routine tentatively. Thus, when the sub-feedback control condition is not satisfied, the sub-feedback control coefficient KFisub is set to “1”, and the correction of the air-fuel ratio of the engine based on the sub-feedback control is not performed.

  As described above, according to the first embodiment of the air-fuel ratio control device for an internal combustion engine according to the present invention, the air-fuel ratio based on the output value vabyfs of the upstream-side air-fuel ratio sensor 66 disposed upstream of the first catalyst 53. In upstream feedback control (main feedback control) as fuel ratio control, a value (upstream air-fuel ratio deviation Dabyf) based on the upstream air-fuel ratio sensor output value vabyfs is high-passed by a high-pass filter A15 (cutoff frequency: ωhpf = ωcut constant). The main feedback correction coefficient KFimain is obtained by performing proportional / integration processing (PI processing) on the value after the filter processing by the upstream feedback controller (PI controller A16).

  In downstream feedback control (sub-feedback control), which is air-fuel ratio control based on the output value Voxs of the downstream air-fuel ratio sensor 67 disposed downstream of the first catalyst 53 and upstream of the second catalyst 54, The value after the low-pass filter processing of the value (output deviation amount DVoxs) based on the downstream air-fuel ratio sensor output value Voxs by the low-pass filter A10 (cut-off frequency: ω0 ≦ ωlpf ≦ ωcut) is the downstream feedback controller (PID controller A11). The sub-feedback correction coefficient KFisub is obtained by performing proportional / integral / derivative processing (PID processing).

  Then, the main feedback control and the sub feedback control are executed by correcting the in-cylinder injection amount Fic and the port injection amount Fip independently of each other by the main feedback correction coefficient KFimain and the sub feedback correction coefficient KFisub.

  Thereby, the control frequency bands of the main feedback control and the sub feedback control are respectively a high frequency band (ωcut or more) and a low frequency band (ωcut or less), and can be set so as not to overlap each other. Thereby, since mutual interference between the main feedback control and the sub feedback control is avoided, good air-fuel ratio control can be achieved, and as a result, emission emission can be stably suppressed.

  Thereby, the main feedback control can achieve air-fuel ratio control for high-frequency air-fuel ratio fluctuations (including high-frequency fluctuations due to disturbances, etc.) when the engine is in a transient operation state. The sub-feedback control can achieve air-fuel ratio control with respect to steady air-fuel ratio fluctuation at a relatively low frequency that can appear as air-fuel ratio fluctuation downstream of the first catalyst 53. Further, in the sub-feedback control, since the integration process is executed by the PID controller A11, it is guaranteed that the steady deviation of the air-fuel ratio downstream of the first catalyst from the target air-fuel ratio (theoretical air-fuel ratio) becomes “0”. The As a result, it is possible to effectively reduce the emission emission amount in the steady operation state.

  In addition, the main feedback correction coefficient KFimain allows the port injection amount Fip (to be precise, so that sudden torque fluctuation does not occur in the engine even if the main feedback correction coefficient KFimain is subject to sudden fluctuations. The main feedback control is executed by correcting only the basic port injection amount Fbasep). In other words, the in-cylinder injection amount Fic is not corrected by the main feedback correction coefficient KFimain. As a result, sudden torque fluctuations of the engine due to the main feedback control are suppressed.

  Furthermore, sub-feedback control is executed by correcting both the in-cylinder injection amount Fic and the port injection amount Fip with a sub-feedback correction coefficient KFisub that is less likely to cause rapid fluctuations. At this time, the cutoff frequency ωlpf of the low-pass filter A10 is set to ω0 ≦ ωlpf in order to suppress the rapid engine torque fluctuation caused by the increase of the in-cylinder injection ratio R and the in-cylinder injection amount Fic. Within the range of ≦ ωcut, the in-cylinder injection ratio R is set so as to become smaller. As a result, regardless of the in-cylinder injection ratio R, the occurrence of a sudden engine torque fluctuation caused by the sub feedback control is suppressed.

  The present invention is not limited to the first embodiment, and various modifications can be employed within the scope of the present invention. For example, in the first embodiment, sub-feedback control is executed by correcting both the in-cylinder injection amount Fic and the port injection amount Fip using the sub-feedback correction coefficient KFisub. The sub feedback control may be executed by correcting only the internal injection amount Fic.

  Further, the sub feedback control may be executed by correcting only the port injection amount Fip with the sub feedback correction coefficient KFisub. In this case, the cut-off frequency ωlpf of the low-pass filter A10 is preferably maintained constant at the value ωcut. This is because abrupt engine torque fluctuation caused by the sub-feedback control hardly occurs regardless of the in-cylinder injection ratio R, and it is not necessary to change the cutoff frequency ωlpf. As a result, the non-control-target frequency band is completely eliminated, and the emission amount of emissions discharged from the catalyst can be suppressed more stably.

  In the first embodiment, the cutoff frequency ωlpf of the low-pass filter A10 is set so as to decrease as the in-cylinder injection ratio R increases. However, the cutoff frequency ωlpf is constant (for example, the above value). ωcut), and the gain Glpf of the low-pass filter A10 may be set so as to decrease as the in-cylinder injection ratio R increases. As a result, the fluctuation range of the sub feedback correction coefficient KFisub becomes smaller as the in-cylinder injection ratio R becomes larger, so that sudden engine torque fluctuation caused by the sub feedback control is suppressed regardless of the in-cylinder injection ratio R. The

(Second Embodiment)
Next, an air-fuel ratio control apparatus according to the second embodiment will be described. In the second embodiment, as shown in FIG. 11 which is a functional block diagram thereof, the point that the LPF characteristic changing means A17 is omitted from the first embodiment whose functional block diagram is shown in FIG. This is mainly different from the first embodiment in that a pass filter A18 and a PID controller A19 are newly added. Hereinafter, the description will be focused on the difference.

  In the second embodiment, the LPF characteristic changing means A17 is omitted, and as shown in FIG. 12, the cutoff frequency ωlpf of the low-pass filter A10 is the minimum value of the fluctuation range of the cutoff frequency ωlpf in the first embodiment. (Ie, the value ω0 shown in FIGS. 6 and 7). Further, as shown in FIG. 12, the cutoff frequency ωhpf of the high-pass filter A15 is fixed to the value ωcut as in the first embodiment.

  On the other hand, the second embodiment includes a bandpass filter A18 (BPF). The bandpass filter A18 is a primary filter (soft filter, digital filter) as shown in the following equation (12) in which the characteristics are expressed using the Laplace operator s. The frequency-gain characteristic of the bandpass filter A18 is as shown in FIG. As shown in FIG. 12, the band-pass filter A18 has a lower cutoff frequency ω0 (= 1 / τlpf) equal to or lower than the cutoff frequency ωlpf of the low-pass filter A10 in the fluctuation of the input value (input signal). The low frequency component is attenuated and the high frequency component equal to or higher than the cutoff frequency ωcut (= 1 / τhpf) on the high frequency side equal to the cutoff frequency ωhpf of the high-pass filter A15 is attenuated. As a result, the bandpass filter A18 has a medium frequency component not lower than the cut-off frequency ω0 on the low frequency side and not higher than the cut-off frequency ωcut on the high frequency side (that is, within the fluctuation range of the cut-off frequency ωlpf in the first embodiment). Only the frequency component) is allowed to pass.

1 / (1 + τhpf · s) + (1-1 / (1 + τlpf · s)) (12)

  The bandpass filter A18 receives the value of the output deviation amount DVoxs obtained by the output deviation amount calculation means A9, and is a value after bandpass filtering the value of the output deviation amount DVoxs according to the above equation (12). The output deviation amount DVoxsba after passing through the band pass filter is output. Accordingly, the output deviation amount DVoxsba after passing through the bandpass filter is “a value based on the output value of the downstream air-fuel ratio sensor 67 and subjected to the bandpass filter process”, and is “the downstream air-fuel ratio sensor 67. Is a value obtained after band-pass filtering is performed on a value corresponding to the degree of difference between the output value Voxs of the output and the downstream target value Voxsref.

  The second embodiment also includes a PID controller A19. The PID controller A19 performs proportional / integral / differential processing (PID processing) on the output deviation amount DVoxsba after passing through the bandpass filter, which is an output value of the bandpass filter A18, based on the same formula as the above formula (7). Sub-feedback correction coefficient KFisubba (> 0) after band-pass filter processing as a downstream feedback correction amount after pass filter processing is obtained. In the first embodiment, the output value of the PID controller A11 is simply referred to as “sub-feedback correction coefficient KFisub”. However, in the second embodiment, the output value of the PID controller A11 is referred to as the “bandpass filter”. In order to distinguish it from the “sub-feedback correction coefficient KFisubba after processing”, it will be called “sub-feedback correction coefficient KFisublow after low-pass filter processing”.

  Further, in the first embodiment, the in-cylinder injection amount Fic (more precisely, the basic in-cylinder injection amount Fbasec) and the port injection amount Fip (more precisely, the basic port injection amount) are obtained by the sub-feedback correction coefficient KFisublow after low-pass filter processing. In this second embodiment, as shown in FIG. 11, the in-cylinder injection amount Fic (more precisely, the basic in-cylinder injection amount Fbasec) is obtained by the sub-feedback correction coefficient KFisublow after the low-pass filter processing, as shown in FIG. ) Is corrected, and only the port injection amount Fic (more precisely, the basic port injection amount Fbasep) is corrected by the sub-feedback correction coefficient KFisubba after the band-pass filter processing.

  In the second embodiment, as shown in FIG. 11, only the port injection amount Fip (more precisely, the basic port injection amount Fbasep) is corrected by the main feedback correction coefficient KFimain, as in the first embodiment.

  That is, in the second embodiment, as shown in FIG. 12, the control frequency band of sub-feedback control (ωcut or less) is a high frequency band (ω0 or more and ωcut or less; BPF sharing band) related to the correction of the port injection amount Fip. ) And a low frequency side band (less than ω0, LPF sharing band) related to the correction of the in-cylinder injection amount Fic.

  The entire frequency band (HPF shared band) equal to or higher than ωcut which is the control frequency band of the main feedback control and the BPF shared band (namely, frequency band equal to or higher than ω0) in the sub feedback control are related to the correction of the port injection amount Fip. Only the LPF sharing band in the feedback control (that is, the frequency band equal to or less than ω0) is related to the correction of the in-cylinder injection amount Fic.

  Based on this difference, the CPU 71 of the second embodiment is only a routine for calculating the main feedback correction coefficient KFimain shown in FIG. 9 among the routines shown in FIGS. 8 to 10 executed by the CPU 71 of the first embodiment. , The routine for calculating the in-cylinder injection amount Fic and the port injection amount Fip shown in the flowchart of FIG. 13 corresponding to the routine of FIG. 8 instead of the routine of FIG. 8, and the routine of FIG. The routine for calculating the sub-feedback correction coefficient KFisublow after the low-pass filter processing shown in the flowchart of FIG. 14 corresponding to this routine is executed. Furthermore, the CPU 71 of the second embodiment additionally executes a routine for calculating a post-bandpass filter-processed sub-feedback correction coefficient KFisubba shown in the flowchart of FIG.

  Of the steps included in the routine shown in FIG. 13, the same steps as those in FIG. 8 are given the same step numbers as those in FIG. Similarly, among the steps included in the routines shown in FIGS. 14 and 15, the same steps as those in FIG. 10 are given the same step numbers as those in FIG.

  The routines shown in FIG. 13 and FIG. 14 are very similar to the routines shown in FIG. 8 and FIG. 10, respectively, and the routine shown in FIG. 15 is very similar to the routine shown in FIG. The detailed description of each routine shown in FIGS. 13 to 15 is omitted.

  As described above, according to the second embodiment, the fluctuation of the air-fuel ratio in the HPF sharing band and the BPF sharing band (that is, the frequency band of ω 0 or more) shown in FIG. 12, which is the control frequency band related to the correction of the port injection amount Fip, is Appears as fluctuations in the port injection amount Fip. Therefore, even if the port injection amount Fip fluctuates suddenly due to a sudden air-fuel ratio fluctuation, as described above, the fluctuation of the in-cylinder intake fuel amount is attenuated due to the fuel adhering to the intake port, etc. As a result, sudden torque fluctuations are unlikely to occur in the engine.

  Furthermore, the air-fuel ratio fluctuation in the LPF sharing band (that is, the frequency band below ω 0) shown in FIG. 12, which is a control frequency band for correcting the in-cylinder injection amount Fic, is abrupt because the rate of change is sufficiently small. It cannot be changed. Therefore, a sudden torque fluctuation does not occur in the engine due to the air-fuel ratio fluctuation in the LPF sharing band. From the above, it is possible to reliably suppress the occurrence of sudden torque fluctuations in the engine.

  Furthermore, as shown in FIG. 12, the HPF shared band, the BPF shared band, and the LPF shared band are continuous, and the non-controlled frequency band is completely eliminated. That is, the air-fuel ratio control can be reliably executed by any one of the air-fuel ratio feedback control with respect to the fluctuation of the air-fuel ratio at any frequency, and as a result, the emission amount of emissions discharged from the catalyst can be further stabilized. Can be suppressed.

  The present invention is not limited to the above embodiments, and various modifications can be adopted within the scope of the present invention. For example, in each of the above embodiments, as “a value based on the degree of difference between the output value of the upstream air-fuel ratio sensor and the upstream target value” which is “a value based on the output value of the upstream air-fuel ratio sensor”, The difference between the detected air-fuel ratio abyfs of the upstream air-fuel ratio sensor 66 and the upstream target air-fuel ratio abyfr is used, but the upstream target corresponding to the upstream air-fuel ratio sensor output value vabyfs itself and the upstream target air-fuel ratio abyfr. Or the value obtained by dividing the in-cylinder intake air amount Mc by the target air-fuel ratio abyfr, which is the value obtained by dividing the in-cylinder intake air amount Mc by the detected air-fuel ratio abyfs. The difference from the target in-cylinder fuel supply amount may be used.

  Similarly, in each of the above embodiments, “a value based on the degree of difference between the output value of the downstream air-fuel ratio sensor and the downstream target value”, which is “a value based on the output value of the downstream air-fuel ratio sensor”. The difference between the downstream air-fuel ratio sensor output value Voxs itself and the downstream target value Voxsref is used. The difference between the detected air-fuel ratio of the downstream air-fuel ratio sensor 67 (see FIG. 4) and the target air-fuel ratio abyfr is used. Differences may be used.

  In each of the above embodiments, the main feedback control is executed by multiplying the basic port injection amount Fbasep by the main feedback correction coefficient KFimain (> 0) as the upstream feedback correction amount. The main feedback control may be executed by adding a main feedback correction amount that can take positive and negative values corresponding to KFimain to the basic port injection amount Fbasep.

  Similarly, in each of the above embodiments, the basic in-cylinder injection amount Fbasec or the basic port injection amount Fbasep is multiplied by the sub feedback correction coefficient KFisub (or KFisublow, KFisubba) (> 0) as the downstream feedback correction amount. The sub-feedback control is executed at the same time, but the sub-feedback correction amount that can take a positive or negative value corresponding to the sub-feedback correction coefficient KFisub is added to the basic in-cylinder injection amount Fbasec or the basic port injection amount Fbasep. Feedback control may be executed.

  In each of the above embodiments, the sub-feedback correction coefficient KFisub (or KFisublow) is based on the value DVoxslow after low-pass filtering the difference DVoxs between the output value Voxs of the downstream air-fuel ratio sensor 67 and the downstream target value Voxsref. , KFisubba), the sub-feedback correction coefficient KFisub (or, alternatively, the output value Voxs of the downstream air-fuel ratio sensor 67 is low-pass filtered and based on the difference between the downstream target value Voxsref and the downstream target value Voxsref. , KFisublow, KFisubba) may be calculated.

  In each of the above embodiments, a primary filter is used as the filter (high-pass filter A15, low-pass filter A10). However, when it is necessary to more clearly divide each band shared by each filter, these filters are used. A second or higher order filter may be used as the filter.

  Further, in each of the above embodiments, the upstream air-fuel ratio deviation Dabyf as “a value corresponding to the degree of difference between the output value of the upstream air-fuel ratio sensor and the upstream target value” is input to the high-pass filter A15. Although the feedback correction coefficient KFimain is calculated (see FIGS. 5 and 11), the detected air-fuel ratio abyfs of the upstream air-fuel ratio sensor 66 is directly applied to the high-pass filter A15 instead of the upstream air-fuel ratio deviation Dabyf. The main feedback correction coefficient KFimain may be calculated by inputting.

  In each of the above embodiments, the upstream air-fuel ratio deviation Dabyf inputted to the high-pass filter A15 subtracts the upstream target air-fuel ratio abyfr (in principle, constant at the theoretical air-fuel ratio) from the detected air-fuel ratio abyfs of the upstream air-fuel ratio sensor 66. Value. Therefore, the signal indicating the upstream air-fuel ratio deviation Dabyf is a signal having the same waveform that increases or decreases with the same timing and the same amplitude while the central value of the fluctuation is different from the signal indicating the detected air-fuel ratio abyfs.

  Therefore, the signal indicating the upstream air-fuel ratio deviation Dabyfhi after passing through the high-pass filter and having the high frequency component equal to or higher than the cut-off frequency ωhpf after passing through the high-pass filter A15 is obtained when the detected air-fuel ratio abyfs is input to the high-pass filter A15. It becomes a signal having exactly the same value as the signal indicating the output value of the high-pass filter A15. From the above, it is possible to obtain the same operation and effect as the above embodiments.

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. It is the graph which showed the relationship between the output voltage of the air flow meter shown in FIG. 1, and the measured intake air flow rate. 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 feedback control. It is the figure which each showed the frequency-gain characteristic about the high-pass filter shown in FIG. 5, and a low-pass filter. It is the graph which showed the relationship between the cylinder injection ratio and the cut-off frequency of a low-pass filter represented by the function which CPU shown in FIG. 1 refers. 3 is a flowchart showing a routine for calculating an in-cylinder injection amount and a port injection amount executed by a CPU shown in FIG. 1. 2 is a flowchart showing a routine for calculating a main feedback correction coefficient executed by a CPU shown in FIG. 1. 3 is a flowchart showing a routine for calculating a sub-feedback correction coefficient executed by a CPU shown in FIG. 1. It is a functional block diagram when the air-fuel ratio control device according to the second embodiment of the present invention executes air-fuel ratio feedback control. It is the figure which each showed the frequency-gain characteristic about the high pass filter shown in FIG. 11, a low pass filter, and a band pass filter. It is the flowchart which showed the routine for calculation of the cylinder injection quantity and the port injection quantity which CPU of the air fuel ratio control device which relates to 2nd execution form of this invention executes. It is the flowchart which showed the routine for calculating the sub feedback correction coefficient which CPU of the air fuel ratio control device which relates to 2nd execution form of this invention executes. It is the flowchart which showed the routine for calculating the sub feedback correction coefficient which CPU of the air fuel ratio control device which relates to 2nd execution form of this invention executes. It is the figure which each showed the frequency-gain characteristic about the high pass filter and low pass filter which the conventional air fuel ratio control apparatus uses.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 25 ... Combustion chamber, 39C ... In-cylinder injection valve, 39P ... Port injection valve, 52 ... Exhaust pipe (exhaust pipe), 53 ... Three-way catalyst (first catalyst), 66 ... Upstream air-fuel ratio sensor , 67 ... downstream air-fuel ratio sensor, 70 ... electric control device, 71 ... CPU, A10 ... low-pass filter, A15 ... high-pass filter, A17 ... LPF characteristic changing means, A18 ... band-pass filter

Claims (5)

A catalyst disposed in an exhaust passage of the internal combustion engine;
An upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the catalyst;
A downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst;
Port injection means for injecting fuel into the intake passage upstream of the intake valve;
In-cylinder injection means for injecting fuel into the combustion chamber;
An air-fuel ratio control device for an internal combustion engine applied to an internal combustion engine comprising:
An upstream feedback correction amount is calculated based on a value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filtering, and is calculated by the port injection means based on the calculated upstream feedback correction amount. Upstream-side feedback control means for feedback-controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting only the amount of injected fuel;
Wherein calculating the downstream-side feedback correction amount based on a value based on the output value of the downstream air-fuel ratio sensor, the downstream-side feedback correction amount the calculated, only the amount of fuel injected by the in-cylinder injection means, or , Downstream feedback for feedback control of the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting the amount of fuel injected by the in-cylinder injection unit and the amount of fuel injected by the port injection unit Control means;
An air-fuel ratio control apparatus for an internal combustion engine comprising: A catalyst disposed in an exhaust passage of the internal combustion engine;
An upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the catalyst;
A downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst;
Port injection means for injecting fuel into the intake passage upstream of the intake valve;
In-cylinder injection means for injecting fuel into the combustion chamber;
An air-fuel ratio control device for an internal combustion engine applied to an internal combustion engine comprising:
An upstream feedback correction amount is calculated based on a value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filtering, and is calculated by the port injection means based on the calculated upstream feedback correction amount. Upstream-side feedback control means for feedback-controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting only the amount of injected fuel;
A downstream feedback correction amount is calculated based on a value based on the output value of the downstream air-fuel ratio sensor, and the amount of fuel injected by the in-cylinder injection means by the calculated downstream feedback correction amount, and / or Downstream feedback control means for feedback controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting the amount of fuel injected by the port injection means;
In an air-fuel ratio control apparatus for an internal combustion engine comprising:
The downstream feedback control means includes
A downstream feedback correction amount is calculated based on a value based on an output value of the downstream air-fuel ratio sensor and subjected to low-pass filter processing, and at least the in-cylinder injection is performed based on the calculated downstream feedback correction amount. Configured to correct the amount of fuel injected by the means,
The in-cylinder injection ratio, which is the ratio of the amount of fuel injected by the in-cylinder injection unit to the sum of the amount of fuel injected by the in-cylinder injection unit and the amount of fuel injected by the port injection unit, In-cylinder injection ratio changing means for changing according to the operating state of the engine,
Low-pass filter characteristic changing means for changing filter characteristics in the low-pass filter processing by the downstream feedback control means according to the in-cylinder injection ratio;
An air-fuel ratio control apparatus for an internal combustion engine, further comprising: The air-fuel ratio control apparatus for an internal combustion engine according to claim 2,
The low-pass filter characteristic changing means is
An air-fuel ratio control apparatus for an internal combustion engine configured to change a gain of a low-pass filter and / or a cutoff frequency as the filter characteristic. A catalyst disposed in an exhaust passage of the internal combustion engine;
An upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the catalyst;
A downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst;
Port injection means for injecting fuel into the intake passage upstream of the intake valve;
In-cylinder injection means for injecting fuel into the combustion chamber;
An air-fuel ratio control device for an internal combustion engine applied to an internal combustion engine comprising:
An upstream feedback correction amount is calculated based on a value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filtering, and is calculated by the port injection means based on the calculated upstream feedback correction amount. Upstream-side feedback control means for feedback-controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting only the amount of injected fuel;
A downstream feedback correction amount is calculated based on a value based on the output value of the downstream air-fuel ratio sensor, and the amount of fuel injected by the in-cylinder injection means by the calculated downstream feedback correction amount, and / or Downstream feedback control means for feedback controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine by correcting the amount of fuel injected by the port injection means;
In an air-fuel ratio control apparatus for an internal combustion engine comprising:
The downstream feedback control means includes
A downstream feedback correction amount after low-pass filter processing is calculated based on a value based on an output value of the downstream air-fuel ratio sensor and subjected to low-pass filter processing, and an output value of the downstream air-fuel ratio sensor is calculated. A downstream feedback correction amount after the bandpass filter processing is calculated based on the value based on the bandpass filter processing,
Only the amount of fuel injected by the in-cylinder injection unit is corrected by the calculated downstream feedback correction amount after the low-pass filter processing, and the port injection is corrected by the calculated downstream feedback correction amount after the band-pass filter processing. An air-fuel ratio control apparatus for an internal combustion engine configured to feedback control the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine by correcting only the amount of fuel injected by the means. An air-fuel ratio control apparatus for an internal combustion engine according to claim 4,
The cutoff frequency of the high-pass filter processing by the upstream feedback control means is the same as the cutoff frequency of the high-frequency side of the bandpass filter processing by the downstream feedback control means,
An air-fuel ratio control apparatus for an internal combustion engine, wherein a cut-off frequency on a low frequency side of the band-pass filter processing by the downstream feedback control means is the same as a cut-off frequency of the low-pass filter processing by the downstream feedback control means.

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