JP2007077869A - Air fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly compensate an error in a base fuel injection quantity without obstructing quick compensation responding to sudden change of air fuel ratio by air fuel ratio control, in the air fuel ratio control based on a value after high pass filtering process of output value of an air fuel ratio sensor upstream of a catalyst. <P>SOLUTION: In an air fuel ratio control device, Fbaset=Fi(k-MB) X abyfs(k)/abyfr(k) is defined by a relation that a product of multiplication of command fuel injection quantity Fi(K-MB) and detected air fuel ratio abyfs(k) is equal to a product of target air fuel ratio abyfr(k) and target bas fuel injection quantity Fbaset for making actual air fuel ratio of an engine to target air fuel ratio abyfr(k) under a hypothesis that cylinder intake air quantity is fixed, and base fuel injection quantity Fbaseb(k) before correction is corrected by base fuel injection quantity correction coefficient KF=Fbaset/Fbaseb(k). Time constant of low pass filtering process in calculation of the KF, is kept small for a predetermined period of time after occurrence of large disturbance in an air fuel ratio control system based on start and stop or the like of purge. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention includes an air-fuel ratio sensor in at least an upstream exhaust passage of a catalyst (three-way catalyst) disposed in an exhaust passage of an internal combustion engine, and feeds back the air-fuel ratio of the engine based on the output value of the air-fuel ratio sensor. The present invention relates to an air-fuel ratio control device for an internal combustion engine to be controlled.

  Conventionally, this type of air-fuel ratio control apparatus is widely known. As such a device, an upstream air-fuel ratio sensor and a downstream side of an exhaust passage upstream and downstream of a catalyst disposed in an exhaust passage of an internal combustion engine (hereinafter also simply referred to as “engine”), respectively. Some are equipped with a side air-fuel ratio sensor. In this case, the apparatus is based on the difference between the output value of the downstream air-fuel ratio sensor and a predetermined downstream target value (for example, by performing the proportional / integral / differential process (PID process) on the difference) and the sub feedback correction amount. And a main feedback correction amount is calculated based on a difference between the output value of the upstream air-fuel ratio sensor and a predetermined upstream target value (for example, proportional / integral processing (PI processing) of the difference).

  Then, the apparatus relates to the amount of fuel (basic fuel injection amount) for obtaining the target air-fuel ratio, which is acquired based on the operating state of the engine (for example, accelerator opening, operating speed, etc.). A command fuel injection amount obtained by correction based on the amount and the sub feedback correction amount, and an air-fuel mixture supplied to the engine by instructing the injector to inject the fuel of the command fuel injection amount The air-fuel ratio (hereinafter sometimes simply referred to as “air-fuel ratio”) is feedback controlled.

  By the way, the catalyst (three-way catalyst) normally stores oxygen depleted from the nitrogen oxide by reducing nitrogen oxide (NOx) in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio. In addition, when the air-fuel ratio of the inflowing exhaust gas is a rich air-fuel ratio, it has a so-called oxygen storage function that oxidizes unburned components such as HC and CO in the exhaust gas by the stored oxygen.

  Therefore, the high-frequency component having a relatively high frequency in the fluctuation of the air-fuel ratio of the exhaust gas upstream of the catalyst and the low-frequency component having a relatively low frequency and a relatively small amplitude (amount of deviation from the theoretical air-fuel ratio) are oxygen contained in the catalyst. Since it can be completely absorbed by the occlusion function, it does not appear as a change in the air-fuel ratio of the 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 fluctuation of the air-fuel ratio of the exhaust gas upstream of the catalyst is not completely absorbed by the oxygen storage function of the catalyst, and the exhaust gas downstream of the catalyst is slightly delayed. Appears as a change in fuel ratio. As a result, the output value of the upstream air-fuel ratio sensor and the output value of the downstream air-fuel ratio sensor may be values indicating the air-fuel ratio shifted in the opposite directions with respect to the theoretical air-fuel ratio. In this case, the engine air-fuel ratio control based on the main feedback control (main feedback correction amount) and the engine air-fuel ratio control based on the sub feedback control (sub feedback correction amount) interfere with each other. The fuel ratio cannot be controlled.

  From the above, among the frequency components in the fluctuation of the output value of the upstream side air-fuel ratio sensor, the frequency components that can appear as fluctuations in the air-fuel ratio downstream of the catalyst (that is, low frequency components below a predetermined frequency) have been cut. If the output value of the subsequent upstream air-fuel ratio sensor is used for main feedback control, it is possible to avoid the occurrence of interference in the air-fuel ratio control of the engine.

Based on such knowledge, the engine control device (air-fuel ratio control device) described in Patent Document 1 below has a value obtained by performing high-pass filtering on the output value of the upstream air-fuel ratio sensor and the output of the downstream air-fuel ratio sensor. The air-fuel ratio control is executed based on the value (in this example, the value after the low-pass filter process is performed on the output value). According to this, it is possible to avoid the above-mentioned interference of the air-fuel ratio control of the engine, and to control the air-fuel ratio (steady state) with respect to the air-fuel ratio fluctuation below a predetermined frequency that can appear as the air-fuel ratio fluctuation downstream of the catalyst. Effective air-fuel ratio control) can be reliably performed by sub-feedback control.
JP-A-5-187297

  In addition, the high frequency component above the predetermined frequency in the fluctuation of the output value of the upstream air-fuel ratio sensor passes through the high pass filter and thus appears as a value after the high pass filter process. Accordingly, when the internal combustion engine is in a transient operation state and the air-fuel ratio of the exhaust gas greatly fluctuates at a high frequency equal to or higher than the predetermined frequency, air-fuel ratio control (ie, transient Compensation for a sudden change in the air-fuel ratio in the operating state) can be performed quickly and reliably by the main feedback control.

  By the way, generally, the difference between the air flow rate in the intake passage measured by the air flow meter used to obtain the basic fuel injection amount and the actual air flow rate, the command fuel injection amount for the injector that injects the fuel, and the actual fuel flow Differences from the injection amount (hereinafter collectively referred to as “error of basic fuel injection amount”) inevitably occur. In order to converge the air-fuel ratio to the target air-fuel ratio while compensating for such an error in the basic fuel injection amount (specifically, the steady deviation between the output value of the air-fuel ratio sensor and the predetermined target value is set to “0”) In at least one of the main feedback control and the sub feedback control, a process for calculating a feedback correction amount based on a time integral value of the deviation between the output value of the air-fuel ratio sensor and the predetermined target value (that is, integration) Process (I process)) needs to be executed.

  However, the high-pass filter process is a process that achieves the same function as the differential process (D process). Therefore, in the apparatus described in the above document, even if the main feedback control executes a process including the integration process (for example, a proportional / integration process (PI process)), the main feedback control substantially includes the main feedback control. The above integration process cannot be executed. Therefore, in this case, the integration process needs to be executed in the sub feedback control.

  However, the change in the air-fuel ratio of the air-fuel mixture supplied to the engine due to the influence of the oxygen storage function of the catalyst described above appears as a change in the air-fuel ratio of the exhaust gas downstream of the catalyst with a slight delay. Therefore, when the error in the basic fuel injection amount suddenly increases, the error in the basic fuel injection amount cannot be quickly compensated only by the sub-feedback control, and as a result, the emission amount increases. There is a problem that happens.

  For this reason, in addition to the main feedback control and the sub feedback control, it is necessary to provide means for correcting the basic fuel injection amount in order to quickly compensate for the error in the basic fuel injection amount. In this case, in order to realize stable correction of the basic fuel injection amount (basic fuel injection amount correction), a value used for the basic fuel injection amount correction (hereinafter referred to as “basic fuel injection amount correction value”). It is necessary to perform low-pass filter processing. It is preferable that the time constant of the low-pass filter process is as large as possible within a range that does not impair the quickness in compensating for the error in the basic fuel injection amount.

  On the other hand, if the time constant of the low-pass filter process is always set to a large value, the engine that causes an abrupt change of the actual air-fuel ratio from the target air-fuel ratio (hereinafter sometimes simply referred to as “abrupt change of the air-fuel ratio”). If there is a change in the operating state (i.e., when a large disturbance occurs in the air-fuel ratio control system), the response delay of the low-pass filter processing is large, so the basic fuel injection amount correction value is temporarily appropriate. Cases can arise where the values differ significantly. In this case, there is a problem that the rapid compensation corresponding to the sudden change in the air-fuel ratio by the main feedback control described above is hindered, and as a result, the emission emission amount temporarily increases.

  Therefore, an object of the present invention is to provide an internal combustion engine capable of suppressing an increase in emission emission amount even when a large disturbance occurs in the air-fuel ratio control system while quickly compensating for an error in the basic fuel injection amount. An object is to provide an air-fuel ratio control apparatus for an engine.

  An air-fuel ratio control apparatus according to the present invention includes a catalyst disposed in an exhaust passage of an internal combustion engine, an upstream air-fuel ratio sensor disposed in the exhaust passage upstream of the catalyst, and fuel according to an instruction. The present invention is applied to an internal combustion engine provided with fuel injection means for injecting.

  The air-fuel ratio control apparatus according to the present invention comprises basic fuel injection amount acquisition means for acquiring a basic fuel injection amount that is an amount of fuel for obtaining a target air-fuel ratio based on an operating state of the internal combustion engine, and the upstream side air-fuel ratio acquisition means. Feedback correction amount calculation means for calculating a feedback correction amount (hereinafter referred to as “upstream feedback correction amount”) based on a value based on an output value of the fuel ratio sensor and subjected to high-pass filter processing; The amount of fuel actually injected by the fuel injection means when receiving a fuel injection instruction of the fuel injection amount is necessary for the actual air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be the target air-fuel ratio The basic fuel injection amount is corrected with a value for correcting the basic fuel injection amount so as to be a low-pass filter (ie, the basic fuel injection amount correction value). Fuel injection amount correction means, command fuel injection amount calculation means for calculating a command fuel injection amount by correcting the corrected basic fuel injection amount with the feedback correction amount, and an instruction for fuel injection of the command fuel injection amount Air-fuel ratio control means for feedback-controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine.

  According to this, the basic fuel injection amount correction unit corrects the basic fuel injection amount acquired by the basic fuel injection amount acquisition unit with the basic fuel injection amount correction value. In other words, apart from the upstream feedback control, an error in the basic fuel injection amount can be quickly compensated. As described above, the time constant of the low-pass filter processing applied to the basic fuel injection amount correction value is set as large as possible within a range that does not impair the quickness in the compensation of the basic fuel injection amount error. It is preferred that

  The air-fuel ratio control apparatus for an internal combustion engine according to the present invention is characterized in that the basic fuel injection amount correction means causes a shift of an actual air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine from the target air-fuel ratio. The time constant of the low-pass filter processing is made small over a predetermined period after the change of the operating state of the internal combustion engine.

  According to this, the time constant of the low-pass filter process is reduced for a predetermined period after the time when a large disturbance occurs in the air-fuel ratio control system, and the response delay of the low-pass filter process is reduced. Therefore, by setting the predetermined period so as to coincide with a period in which a large disturbance in the air-fuel ratio can occur, the above-described “case where the basic fuel injection amount correction value is significantly different from an appropriate value” is unlikely to occur. Become. As a result, the rapid compensation for the sudden change in the air-fuel ratio by the upstream side feedback control is hardly hindered, and the temporary increase in the emission amount can be suppressed.

  Further, after the predetermined period has elapsed (that is, after the air-fuel ratio has converged to the target air-fuel ratio), the time constant of the low-pass filter process can be returned to the above-mentioned large value again. Thereby, the stable correction of the basic fuel injection amount by the basic fuel injection amount correction value is started again.

  Here, the low-pass filter may be an analog filter or a digital filter. When a digital filter is used as the low-pass filter, the basic fuel injection amount correction means changes the value related to the responsiveness (for example, a dull control constant described later) used in the low-pass filter process. Even if the time constant is configured to be small, the low-pass filter processing is performed by shortening the cycle (hereinafter referred to as “calculation cycle”) for performing the low-pass filter processing on the basic fuel injection amount correction value. The time constant may be reduced. These are based on the fact that the time constant of the digital filter is proportional to the product of the dull control constant described later and the calculation cycle.

  In the air-fuel ratio control apparatus according to the present invention, the downstream feedback correction amount is calculated based on the value based on the output value of the downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst. The command fuel injection amount calculation unit further includes a feedback correction amount calculation unit, wherein the command fuel injection amount calculation unit corrects the corrected basic fuel injection amount with the upstream feedback correction amount and the downstream feedback correction amount. It may be configured to calculate the quantity. In this case, the value based on the output value of the downstream air-fuel ratio sensor may be a low-pass filtered value.

  In the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the basic fuel injection amount correction means uses the output value of the upstream air-fuel ratio sensor, the target air-fuel ratio as the basic fuel injection amount correction value. It is preferable that a parameter value calculated based on the command fuel injection amount and the basic fuel injection amount is used.

  In general, the product of the fuel injection amount and the air-fuel ratio (and hence the air-fuel ratio of exhaust gas) is constant under the assumption that the amount of cylinder intake air taken into the cylinder (combustion chamber) is constant. Therefore, the product of the command fuel injection amount and the air-fuel ratio (hereinafter sometimes referred to as “detected air-fuel ratio”) corresponding to the output value of the upstream air-fuel ratio sensor is the product of the air-fuel mixture supplied to the engine. A basic fuel injection amount (injection command value to the fuel injection means, which may be referred to as a “target basic fuel injection amount” hereinafter) necessary for making the actual air-fuel ratio the target air-fuel ratio, and the target air-fuel ratio The relationship that is equal to the product of is established.

  Therefore, the target basic fuel injection amount can be calculated based on the known values of the command fuel injection amount, the detected air-fuel ratio, the target air-fuel ratio, and the relationship. If the target basic fuel injection amount can be calculated, the basic fuel injection amount is calculated based on a comparison result between the target basic fuel injection amount and a known basic fuel injection amount (that is, the value itself acquired by the basic fuel injection amount acquisition means). A parameter value (for example, a correction coefficient) for correcting the injection amount can be calculated.

  The parameter value that can be calculated in this way is necessary for the amount of fuel actually injected by the fuel injection means to receive the actual air-fuel ratio as the target air-fuel ratio when receiving the fuel injection instruction of the basic fuel injection amount. This is a value for correcting the basic fuel injection amount so as to be an appropriate amount (that is, a value for making the basic fuel injection amount coincide with the target basic fuel injection amount).

  Accordingly, if the basic fuel injection amount is corrected using the parameter value as in the above configuration, the basic fuel injection amount can be matched with the target basic fuel injection amount with a simple calculation and with high accuracy. Can be corrected.

  By the way, in general, in an internal combustion engine, liquid fuel evaporates in a fuel tank and fuel gas (hereinafter sometimes referred to as “evaporative gas”) is generated, so that the evaporated gas is released into the atmosphere. In order to prevent this, an evaporative fuel processing mechanism is provided for introducing the evaporation gas into the intake passage of the engine (hereinafter also referred to as “purge”).

  The evaporative fuel processing mechanism includes a purge control valve that controls purge, and controls the purge control valve based on the operating state of the engine to thereby introduce an evaporative gas introduced into the intake passage (for example, a purge rate). ) To control. Here, purge rate = evaporation gas flow rate / (evaporation gas flow rate + air flow meter detected air flow rate): the unit is mass%. This sudden change in the state of introduction of the evaporation gas can be a large disturbance in the air-fuel ratio control system.

  Therefore, the internal combustion engine introduces a fuel gas introduction path for introducing a fuel gas based on the fuel evaporated in the fuel tank into the intake passage of the internal combustion engine, and an introduction state of the fuel gas introduced into the intake passage of the internal combustion engine. And a purge control valve that is controlled based on the operating state, the basic fuel injection amount correcting means is a fuel gas introduced into the intake passage when the operating state of the internal combustion engine changes. It is preferable to use a point in time when the introduction state of the gas is changed by the control of the purge control valve.

  According to this, the time constant of the low-pass filter is reduced only for a predetermined period after the sudden change in the state of introduction of the vapor gas (for example, a sudden change in the purge rate) that can be a large disturbance in the air-fuel ratio control system. Therefore, it is possible to reliably suppress a temporary increase in the amount of emission due to a sudden change in the state of introduction of the evaporation gas.

  As the “time when the fuel gas introduction state is changed by the control of the purge control valve”, for example, when the introduction of the fuel gas is started by the control of the purge control valve (purge control start time) and / or is stopped. The time point when the purge control is completed (the time point when the purge control ends).

  Furthermore, the fuel gas introduction state (for example, the purge rate) is often controlled to change according to the air flow rate in the intake passage. In this case, the time when the air flow rate in the intake passage changes during the period in which the introduction of the fuel gas is continued can also be “the time when the fuel gas introduction state is changed by the control of the purge control valve”.

  The internal combustion engine introduces a fuel gas introduction path for introducing fuel gas based on the fuel evaporated in the fuel tank into the intake passage of the internal combustion engine, and an introduction state of the fuel gas introduced into the intake passage of the internal combustion engine. In the case of further comprising a purge control valve that is controlled based on the operating state, the basic fuel injection amount acquisition means is configured to acquire the basic fuel injection amount in consideration of the amount of fuel in the fuel gas. May be.

  The air-fuel ratio control apparatus according to the present invention further includes a transport delay time acquisition means for acquiring a transport delay time that is a time until the fuel gas reaches the combustion chamber of the internal combustion engine from the purge control valve. The basic fuel injection amount correction means starts the predetermined period from the time when the transportation delay time has elapsed from the time when the introduction state of the fuel gas introduced into the intake passage is changed by the control of the purge control valve. It is preferable to be configured as described above.

  At the time of purging, the evaporated gas flows into the intake passage via the purge control valve, and reaches the cylinder of the engine by moving through the intake passage. That is, a predetermined time (that is, the transportation delay time) is required for the evaporation gas to reach the cylinder of the engine from the purge control valve.

  Therefore, the change in the fuel gas introduction state appears as a disturbance in the air-fuel ratio control system after the transportation delay time has elapsed. In other words, the predetermined period in which the time constant of the low-pass filter is reduced may be started from the time when the transportation delay time has elapsed from the time when the fuel gas introduction state has changed.

  The above configuration is based on such knowledge. According to this, the predetermined period can be shortened as compared with the case where the predetermined period starts immediately after the fuel gas introduction state changes. In other words, the period during which the time constant of the low-pass filter is maintained as large as possible can be lengthened. As a result, the period during which stable correction of the basic fuel injection amount is realized can be lengthened. it can.

  In this case, it is preferable that the transportation delay time acquisition unit is configured to change the transportation delay time in accordance with an operating state of the engine. The transport delay time varies depending on the operating state of the engine, such as the in-cylinder intake air amount. Therefore, according to the above configuration, since the transport delay time can be accurately acquired regardless of the operating state of the internal combustion engine, the start timing of the predetermined period can be set more appropriately.

  Embodiments of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention (hereinafter simply referred to as “air-fuel ratio control apparatus” in the following description) (embodiments considered to be the best at the time of filing of the present application) This will be described with reference to the drawings.

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

  The cylinder block unit 20 includes a cylinder 21, a piston 22, a connecting rod 23, and a crankshaft 24. In the cylinder block portion 20, the piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 via the connecting rod 23, whereby the crankshaft 24 rotates. Yes. 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 And an igniter 38 including an ignition coil that generates a high voltage to be applied to the spark plug 37, and an injector (fuel injection means) 39 for injecting fuel into the intake port 31.

  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 with the intake port 31, an air filter 42 provided at an end of the intake pipe 41, and the intake pipe 41. Throttle valve 43 for varying the opening cross-sectional area of the intake passage, throttle valve actuator 43a comprising a DC motor constituting throttle valve driving means, swirl control valve (hereinafter referred to as "SCV") 44, and DC motor. SCV actuator 44a, fuel tank 45 for storing liquid fuel, canister 46 capable of storing a predetermined amount of fuel gas, vapor collection tube 47 for introducing fuel gas evaporated in fuel tank 45 to canister 46, canister 46 Purge flow path for introducing the fuel gas desorbed from the intake pipe 41 It comprises 8, and the purge control valve 49 is a normally closed switching valve interposed in the purge flow path 48 (the fuel gas introduction path). Here, the intake port 31 and the intake pipe 41 constitute an intake passage.

  The canister 46 is a well-known charcoal canister, and has a tank port 46a connected to the vapor collecting pipe 47, a purge port 46b connected to the purge flow path 48, and an atmospheric port 46c. The adsorbent 46d for adsorbing the fuel gas is housed.

  The fuel gas generated in the fuel tank 45 due to heating by sunlight or the like while the engine is stopped is in the flow of the evaporated gas from the fuel tank 45, the vapor collection pipe 47, the canister 46, and the purge control valve 49 of the purge passage 48. The closed space formed by the upstream side (canister 46 side) is filled. The fuel gas filled in the closed space is occluded in the canister 46 by being adsorbed by the adsorbent 46d.

  Note that the atmospheric port 46c has an external air flow so that when the purge control valve 49 is opened, a negative pressure generated in the intake pipe 41 causes a gas flow toward the intake pipe 41 in the purge flow path 48. Is an air communication hole for introducing the gas into the canister 46.

  In addition, the canister 46 has a direct gas flow path from the tank port 46a to the purge port 46b. The gas flow path allows the fuel gas generated in the fuel tank 45 to flow into the canister 46 from the tank port 46a when the purge control valve 49 is opened while the internal combustion engine 10 is in operation. It can be discharged from the purge port 46b as it is without being adsorbed by the adsorbent 46d later.

  That is, in this system, when the purge control valve 49 is opened, the tank vapor channel and the canister purge channel are formed simultaneously. Here, the “tank vapor flow path” is a gas flow path from the fuel tank 45 to the tank port 46a through the vapor collection pipe 47, and does not pass through the inside of the adsorbent 46d from the tank port 46a. A gas flow from the fuel tank 45 to the intake pipe 41, which connects the gas flow path directly from the body to the purge port 46b and the gas flow path from the purge port 46b to the intake pipe 41 through the purge flow path 48. Say the road. The “canister purge flow path” refers to a gas flow path from the atmospheric port 46 c to the purge port 46 b through the inside of the adsorbent 46 d, and from the purge port 46 b to the intake pipe 41 through the purge flow path 48. .

  The purge control valve 49 is configured by a well-known VSV (vacuum switching valve) (in the following description of the present embodiment, the purge control valve 49 is abbreviated as “VSV 49”). The VSV 49 is opened when a purge execution condition described later is satisfied. When the VSV 49 is opened, the first gas flow (tank vapor) passing through the tank vapor passage and the second gas flow (canister) passing through the canister purge passage due to the negative pressure generated in the intake pipe 41. The vapor gas mixed with the purge gas) can be sucked into the intake pipe 41 through the purge control valve 49.

  The target purge rate PGR, which is the target value of the purge rate (= (evaporation gas flow rate / (evaporation gas flow rate + air flow meter detected air flow rate)): unit is mass%), is measured by an air flow meter 61 described later. On the basis of the air amount (flow rate) Ga and the table MapPGR (Ga) that defines the relationship between the intake air flow rate Ga and the target purge rate PGR, it is determined so as to increase as the intake air flow rate Ga increases. ing. Then, the VSV 49 is duty controlled in accordance with an instruction from the electric control device 70 described later, so that the actual purge rate is controlled to coincide with the target purge rate PGR.

  The exhaust system 50 includes an exhaust manifold 51 that communicates with the exhaust port 34, and an exhaust pipe (exhaust pipe) that is connected to the exhaust manifold 51 (actually, a collection portion of the exhaust manifolds 51 that communicate with each exhaust port 34). ) 52, an upstream side catalytic device 53 (also referred to as an upstream side catalytic converter or a start catalytic converter) disposed (intervened) in the exhaust pipe 52, hereinafter referred to as “first catalyst 53”). And a downstream side catalyst device 54 disposed (interposed) in the exhaust pipe 52 downstream of the first catalyst 53 (which is also referred to as an under-floor catalytic converter because it is disposed below the vehicle floor, (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 an 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 exhaust manifolds 51 is An air-fuel ratio sensor 66 (hereinafter referred to as an “upstream air-fuel ratio sensor 66”) disposed in the aggregated portion), an exhaust passage downstream of the first catalyst 53 and upstream of the second catalyst 54. An air-fuel ratio sensor 67 (hereinafter referred to as a “downstream air-fuel ratio sensor 67”), an accelerator opening sensor 68, and a fuel temperature that is a temperature sensor for detecting the fuel temperature in the fuel tank 45. A sensor 69 is provided.

  The air flow meter 61 is configured by a known hot-wire air flow meter, and 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 has a narrow pulse every time the crankshaft 24 rotates 10 ° and outputs a signal having a wide pulse every time the crankshaft 24 rotates 360 °. This signal represents the operating speed NE. The water temperature sensor 65 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The upstream air-fuel ratio sensor 66 is a limiting current type oxygen concentration sensor, and 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 upstream target 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 indicating the operation amount Accp of the accelerator pedal 81.

  The system further includes an electrical control device 70. The electric control device 70 includes a CPU 71 connected to each other by a bus, a routine (program) executed by the CPU 71, a table (look-up table, map), a ROM 72 in which parameters 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 69, supplies signals from the sensors 61 to 69 to the CPU 71, and in response to instructions from the CPU 71, the actuator 33a, the igniter 38, the injector 39, and the throttle of the variable intake timing device 33. Drive signals are sent to the valve actuator 43a, the SCV actuator 44a, and the VSV 49.

(Outline of air-fuel ratio control)
Next, an outline of the air-fuel ratio control of the engine performed by the air-fuel ratio control apparatus configured as described above will be described.

  As is well known, the first catalyst 53 (the same applies to the second catalyst 54) is configured by arranging a so-called three-way catalyst in a metal casing, and the gas flowing into the first catalyst 53. When the air-fuel ratio is substantially the stoichiometric air-fuel ratio, the three-way catalyst oxidizes HC and CO and reduces NOx, thereby purifying these harmful components with high efficiency. The three-way catalyst provided in the first catalyst 53 has a function of storing / releasing oxygen (oxygen storage function, oxygen storage / release function), and the oxygen storage / release function allows the air-fuel ratio to be the stoichiometric air-fuel ratio. HC, CO, and NOx can be purified even if they deviate to a certain extent. That is, the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber (hereinafter sometimes referred to as “air-fuel ratio”) is lean and the gas flowing into the first catalyst 53 contains a large amount of NOx. The three-way catalyst provided in the first catalyst 53 deprives the NOx of oxygen molecules, occludes the oxygen molecules and reduces the NOx, thereby purifying the NOx. Further, when the air-fuel ratio of the engine becomes rich and the gas flowing into the first catalyst 53 contains a large amount of HC and CO, the three-way catalyst releases the stored oxygen molecules and gives them to the HC and CO. This oxidizes HC and CO, thereby purifying HC and CO.

  Therefore, in order to efficiently purify a large amount of HC and CO into which the first catalyst 53 continuously flows, the three-way catalyst provided in the first catalyst 53 must store a large amount of oxygen. On the contrary, in order to efficiently purify a large amount of NOx that continuously flows in, the three-way catalyst must be in a state where it can sufficiently store oxygen. From the above, the purification capacity of the first catalyst 53 depends on the maximum oxygen amount (maximum oxygen storage amount) that the three-way catalyst can store.

  On the other hand, the three-way catalyst provided in the first catalyst 53 is deteriorated by poisoning due to lead or sulfur contained in the fuel, or heat applied to the catalyst, and accordingly, the maximum oxygen storage amount gradually decreases. As a result, the range of the catalyst window is narrowed. Even when the maximum oxygen storage amount is reduced and the catalyst window is narrowed, the air-fuel ratio of the gas discharged from the first catalyst 53 (accordingly, to suppress the emission emission amount continuously) The average air-fuel ratio of the gas flowing into the first catalyst 53 needs to be controlled with high precision so as to be very close to the stoichiometric air-fuel ratio.

  In view of this, the air-fuel ratio control apparatus of the present embodiment has an upstream feedback (hereinafter referred to as main feedback) using the output value of the upstream air-fuel ratio sensor 66 and a downstream feedback using the output value of the downstream air-fuel ratio sensor 67. Two air-fuel ratio feedback controls (hereinafter referred to as sub-feedback) are performed. In addition, errors in the basic fuel injection amount that are difficult to compensate quickly and sufficiently only by these feedback controls are compensated by correcting the basic fuel injection amount.

  More specifically, this air-fuel ratio control device (hereinafter also referred to as “the present device”) has functions A1 to A19 as shown in FIG. 5 and FIG. 8 which are functional block diagrams. It is composed of blocks. Hereinafter, each functional block will be described with reference to the drawings.

<Calculation of basic fuel injection amount>
First, the in-cylinder intake air amount calculation means A1 is a table MapMc stored in the ROM 72 and the intake air flow rate Ga measured by the air flow meter 61, the operating speed NE obtained based on the output of the crank position sensor 64, and the ROM 72. Based on the above, the in-cylinder intake air amount Mc (k), which is the current intake air amount of the cylinder that reaches the intake stroke, is obtained. Here, the subscript (k) indicates a value for the current intake stroke (hereinafter, the same applies to other physical quantities).

  The upstream target air-fuel ratio setting means A2 is an upstream target air-fuel ratio abyfr (k) corresponding to a predetermined upstream target value based on the operating speed NE that is the operating state of the internal combustion engine 10, the throttle valve opening TA, and the like. To decide. 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 non-purge basic fuel injection amount calculating means A3 uses the upstream target air amount set by the upstream target air-fuel ratio setting means A2 based on the in-cylinder intake air amount Mc (k) obtained by the in-cylinder intake air amount calculation means A1. By dividing by the fuel ratio abyfr (k), the current non-purge basic fuel injection amount Fbaseb0 (k) for obtaining the upstream target air-fuel ratio abyfr (k) at the non-purging time is obtained.

  The pre-correction basic fuel injection amount calculation means A4 is obtained by a purge correction coefficient setting means A16, which will be described later, to the current non-purge basic fuel injection quantity Fbaseb0 (k) obtained by the non-purge basic fuel injection quantity calculation means A3. The purge correction coefficient KP is multiplied by the purge correction coefficient KP (k−MA) before the MA stroke (MA intake strokes) from the present time read by the purge correction coefficient delay means A17. The basic fuel injection amount Fbaseb (k) before correction in consideration of the amount of the fuel is obtained. The purge correction coefficient setting means A16 and the purge correction coefficient delay means A17 will be described in detail later. The non-purge basic fuel injection amount calculating means A3 and the uncorrected basic fuel injection amount calculating means A4 correspond to the basic fuel injection amount acquiring means.

  The corrected basic fuel injection amount calculating means A5 adds a basic fuel injection amount correction coefficient setting means A18 described later to the current uncorrected basic fuel injection amount Fbaseb (k) obtained by the uncorrected basic fuel injection amount calculating means A4. The corrected basic fuel injection amount Fbase is obtained by multiplying the obtained basic fuel injection amount correction coefficient KF (already in the previous intake stroke). Here, a low-pass filter time constant (to be referred to simply as a “time constant”) τ used in the basic fuel injection amount correction coefficient setting means A18 is set by the blunt control constant setting means A19. It is determined based on the dull control constant n. The basic fuel injection amount correction coefficient setting means A18 and the blunt control constant setting means A19 will be described in detail later.

  As described above, the present apparatus includes the cylinder intake air amount calculation means A1, the upstream target air-fuel ratio setting means A2, the non-purge basic fuel injection amount calculation means A3, the uncorrected basic fuel injection amount calculation means A4, and the corrected basic fuel quantity. The corrected basic fuel injection amount using the fuel injection amount calculating means A5, purge correction coefficient setting means A16, purge correction coefficient delay means A17, basic fuel injection amount correction coefficient setting means A18, and blunt control constant setting means A19 Find Fbase. As will be described in detail later, this corrected basic fuel injection amount Fbase compensates for the error in the basic fuel injection amount obtained in the previous intake stroke, and calculates the actual air-fuel ratio in the current intake stroke This is the fuel injection amount (before the main and sub-feedback) to be instructed to the injector 39 (the target basic fuel injection amount) in order to make it coincide with the side target air-fuel ratio abyfr (k).

<Calculation of command fuel injection amount>
The command fuel injection amount calculation means A6 adds a main feedback correction amount DFi_main and a sub feedback correction amount DFi_sub, which will be described later, to the corrected basic fuel injection amount Fbase, and based on the following equation (1), Find the quantity Fi (k). The command fuel injection amount Fi (k) is stored in the RAM 73 while corresponding to the intake stroke of each cylinder. The command fuel injection amount calculation means A6 corresponds to the command fuel injection amount calculation means.
Fi (k) = Fbase + DFi_main + DFi_sub (1)

  In this apparatus, the command fuel injection obtained by correcting the corrected basic fuel injection amount Fbase based on the main feedback correction amount DFi_main and the sub feedback correction amount DFi_sub by the command fuel injection amount calculation means A6 in this way. An instruction to inject the fuel of the amount Fi (k) is given to the injector 39 for the cylinder that reaches the current intake stroke. The means for instructing fuel injection in this way corresponds to the air-fuel ratio control means.

<Sub feedback control>
First, the downstream target value setting means A7, like the above-described upstream target air-fuel ratio setting means A2, is based on the operating speed NE that is the operating state of the internal combustion engine 10, the throttle valve opening TA, and the like. A downstream target value (predetermined downstream target value) Voxs_ref corresponding to the fuel ratio is determined. The downstream target value Voxs_ref is set to 0.5 (V), which is a value corresponding to the stoichiometric air-fuel ratio except for special cases, after the warm-up of the internal combustion engine 10, for example (see FIG. 4). reference.). In this example, the downstream target value Voxs_ref is set so that the downstream target air-fuel ratio corresponding to the downstream target value Voxs_ref always matches the upstream target air-fuel ratio abyfr (k).

The output deviation amount calculation means A8 is based on the following equation (2), at the current time set by the downstream target value setting means A7 (specifically, the current Fi (k) injection instruction start time). The output deviation amount DVoxs is obtained by subtracting the current output value Voxs of the downstream air-fuel ratio sensor 67 from the downstream target value Voxs_ref.
DVoxs = Voxs_ref−Voxs (2)

The low-pass filter A9 is a first-order filter as shown in the following formula (3) in which the characteristic is expressed using the Laplace operator s. In the following formula (3), T is a time constant. The low-pass filter A9 substantially prohibits the passage of high frequency components having a frequency (1 / T) or higher. The low-pass filter A9 receives the value of the output deviation amount DVoxs obtained by the output deviation amount calculating means A8 and is a value after low-pass filtering the value of the output deviation amount DVoxs according to the following equation (3). Output the output deviation DVoxs_low after passing a certain low-pass filter.
1 / (1 + T ・ s) (3)

The PID controller A10 performs proportional / integral / differential processing (PID processing) on the output deviation amount DVoxs_low after passing through the low-pass filter, which is an output value of the low-pass filter A9, so that the sub feedback correction amount DFi_sub is calculated based on the following equation (4) Ask.
DFi_sub = Kp · DVoxs_low + Ki · SDVoxs_low + Kd · DDVoxs_low (4)

  In the equation (4), 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). SDVoxs_low is a time integral value of the output deviation amount DVoxs_low after passing through the low-pass filter, and DDVoxs_low is a time differential value of the output deviation amount DVoxs_low after passing through the low-pass filter.

  In this way, this apparatus is based on the output deviation amount DVoxs (actually, the output deviation amount DVoxs_low after passing through the low-pass filter) that is the deviation between the downstream target value Voxs_ref and the output value Voxs of the downstream air-fuel ratio sensor 67. Sub-feedback correction amount DFi_sub, and adding the sub-feedback correction amount DFi_sub to the corrected basic fuel injection amount Fbase, thereby correcting the basic fuel injection by the main feedback control described later (by the main feedback correction amount DFi_main). The corrected basic fuel injection amount Fbase is corrected independently of the correction of the amount Fbase.

  For example, if the output value Voxs of the downstream air-fuel ratio sensor 67 indicates a value corresponding to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio because the average air-fuel ratio is lean, the output deviation amount calculating means A8 obtains it. Since the output deviation amount DVoxs obtained is a positive value (see FIG. 4), the sub feedback correction amount DFi_sub obtained by the PID controller A10 is a positive value. Thus, the command fuel injection amount Fi (k) obtained by the command fuel injection amount calculation means A6 is controlled to be larger than the corrected basic fuel injection amount Fbase so that the air-fuel ratio becomes rich.

  Conversely, if the output value Voxs of the downstream air-fuel ratio sensor 67 shows a value corresponding to the air-fuel ratio that is richer than the theoretical air-fuel ratio because the average air-fuel ratio is rich, the output deviation amount calculation means A8 Since the obtained output deviation amount DVoxs has a negative value, the sub feedback correction amount DFi_sub obtained by the PID controller A10 has a negative value. Thereby, the command fuel injection amount Fi (k) obtained by the command fuel injection amount calculation means A6 is controlled to be smaller than the corrected basic fuel injection amount Fbase and the air-fuel ratio becomes lean.

  Further, since the PID controller A10 includes the integral term Ki · SDVoxs_low, it is guaranteed that the output deviation amount DVoxs becomes zero in a steady state. In other words, the steady deviation between the downstream target value Voxs_ref and the output value Voxs of the downstream air-fuel ratio sensor 67 becomes zero. In the steady state, since the output deviation amount DVoxs becomes zero, the proportional terms Kp / DVoxs_low and the differential terms Kd / DDVoxs_low both become zero, so the sub feedback correction amount DFi_sub is equal to the value of the integral term Ki / SDVoxs_low. . This value is a value based on a time integral value of the deviation between the output value Voxs of the downstream air-fuel ratio sensor 67 and the downstream target value Voxs_ref.

  By executing such integration processing in the PID controller A10, the above-described error in the basic fuel injection amount can be compensated, and the air-fuel ratio (and hence the air-fuel ratio) downstream of the first catalyst 53 in the steady state is the downstream. It is possible to converge to the downstream target air-fuel ratio (that is, the theoretical air-fuel ratio) corresponding to the side target value Voxs_ref.

<Main feedback control>
As described above, the first catalyst 53 has an oxygen storage function. Accordingly, a relatively high frequency component (for example, the frequency (1 / T) or more) in the fluctuation of the air-fuel ratio of the exhaust gas upstream of the first catalyst 53 and a relatively low frequency (for example, the above-described frequency) A low frequency component having a frequency (1 / T) or less and a relatively small amplitude (amount of deviation from the theoretical air-fuel ratio) is completely absorbed by the oxygen storage function of the first catalyst 53, whereby the first catalyst It does not appear as fluctuations in the air-fuel ratio of the exhaust gas downstream of 53. Therefore, for example, when the internal combustion engine 10 is in a transient operation state and the air-fuel ratio of the exhaust gas greatly fluctuates at a high frequency equal to or higher than the frequency (1 / T), the fluctuation of the air-fuel ratio is detected by the downstream air-fuel ratio sensor. Since the output value Voxs of 67 does not appear, the air-fuel ratio control for the air-fuel ratio fluctuation of the same frequency (1 / T) or more (that is, compensation for the sudden change of the air-fuel ratio in the transient operation state, etc.) should be executed by the sub-feedback control. I can't. Therefore, in order to reliably perform compensation for a sudden change in the air-fuel ratio in a transient operation state or the like, it is necessary to perform main feedback control that is air-fuel ratio control based on the output value Vabyfs of the upstream air-fuel ratio sensor 66.

  On the other hand, a low frequency component having a relatively low frequency (for example, equal to or less than the frequency (1 / T)) in the fluctuation of the air-fuel ratio of the exhaust gas upstream of the first catalyst 53 is a first catalyst. The oxygen storage function of 53 does not completely absorb, and appears as a change in the air-fuel ratio of the exhaust gas downstream of the first catalyst 53 with a slight delay. As a result, there is a case where the output value Vabyfs of the upstream air-fuel ratio sensor 66 and the output value Voxs of the downstream air-fuel ratio sensor 67 become values indicating the air-fuel ratio shifted in the opposite directions with respect to the theoretical air-fuel ratio. . Therefore, in this case, if the engine air-fuel ratio control based on the main feedback control (main feedback correction amount DFi_main described later) and the sub-feedback control (accordingly, the sub-feedback correction amount DFi_sub) are performed simultaneously. Since the two air-fuel ratio controls interfere with each other, good engine air-fuel ratio control cannot be performed.

  From the above, a predetermined frequency (in this example, a frequency component that can appear as a fluctuation in the air-fuel ratio downstream of the first catalyst 53 among the frequency components in the fluctuation in the output value Vabyfs of the upstream air-fuel ratio sensor 66 (in this example). If the output value Vabyfs of the upstream air-fuel ratio sensor 66 after cutting the low frequency component below the frequency (1 / T) is used for the main feedback control, the interference of the air-fuel ratio control of the engine occurs. Can be avoided, and compensation for sudden changes in the air-fuel ratio in the transient operation state can be reliably performed.

  Therefore, as shown in FIG. 5 described above, the present system is configured to include the functional blocks A11 to A15. Hereinafter, each functional block will be described with reference to FIG.

<< Calculation of main feedback correction amount >>
First, the table conversion means A11 is a table that defines the output value Vabyfs of the upstream air-fuel ratio sensor 66 and the relationship between the upstream air-fuel ratio sensor output value Vabyfs and the air-fuel ratio A / F shown in FIG. Based on the above, the current detected air-fuel ratio abyfs (k) at the present time detected by the upstream air-fuel ratio sensor 66 (specifically, the current Fi (k) injection instruction start time) is obtained.

  The target air-fuel ratio delay means A12 is the target before the MB stroke (MB intake strokes) from the present time out of the target air-fuel ratio abyfr obtained for each intake stroke by the upstream target air-fuel ratio setting means A2 and stored in the RAM 73. The air-fuel ratio abyfr is read from the RAM 73 and set as the target air-fuel ratio abyfr (k−MB). This value MB will be described later.

The upstream air-fuel ratio deviation calculating means A13 calculates the target air-fuel ratio abyfr (MB stroke before the MB stroke from the present time obtained by the target air-fuel ratio delay means A12 from the detected air-fuel ratio abyfs (k) this time based on the following equation (5). k−MB) is subtracted to obtain the upstream air-fuel ratio deviation DAF before the MB stroke from the present time.
DAF ＝ abyfs (k) −abyfr (k−MB) (5)

  Thus, in order to obtain the upstream air-fuel ratio deviation DAF before the MB stroke from the present time, the target air-fuel ratio abyfr (k−MB) before the MB stroke from the current time is subtracted from the current detected air-fuel ratio abyfs (k). This is because a delay time LB (details will be described later) corresponding to the MB stroke is required until the air-fuel mixture burned in the combustion chamber 25 reaches the upstream air-fuel ratio sensor 66. This upstream air-fuel ratio deviation DAF is a value corresponding to the excess or deficiency of the fuel supplied into the cylinder before the MB stroke.

The high-pass filter A14 is a first-order filter as shown in the following formula (6) in which the characteristics are expressed using the Laplace operator s. In the following equation (6), T is the same time constant as the time constant T of the low-pass filter A9. The high-pass filter A14 substantially prohibits the passage of low-frequency components having a frequency (1 / T) or less.
1-1 / (1 + T · s) (6)

  The high-pass filter A14 inputs the value of the upstream air-fuel ratio deviation DAF obtained by the upstream air-fuel ratio deviation calculating means A13 and performs high-pass filter processing on the value of the upstream air-fuel ratio deviation DAF according to the equation (6). The upstream air-fuel ratio deviation DAFhi is output after passing through the high-pass filter, which is the later value. Therefore, the upstream air-fuel ratio deviation DAFhi after passing through the high-pass filter is a value after high-pass filtering is performed on a value based on the deviation between the output value Vabyfs of the upstream air-fuel ratio sensor 66 and the upstream target value.

The P controller A15 performs proportional processing (P processing) on the upstream side air-fuel ratio deviation DAFhi that has passed through the high-pass filter, which is the output value of the high-pass filter A14, so that the fuel supply amount before the MB stroke is calculated based on the following equation (7). A main feedback correction amount DFi_main for compensating for excess or deficiency (excess or deficiency of only a high frequency component equal to or higher than the frequency (1 / T)) is obtained.
DFi_main = Gphi ・ DAFhi (7)

  In Equation (7), Gphi is a preset proportional gain (proportional constant). The main feedback correction amount DFi_main is used when the command fuel injection amount Fi (k) is obtained by the command fuel injection amount calculation means A6 as described above.

  In this way, the present apparatus connects the main feedback control circuit and the sub feedback control circuit in parallel to the internal combustion engine 10. Then, this apparatus passes the high-pass filter, which is a value after high-pass filter processing of a value based on the deviation between the upstream target value corresponding to the upstream target air-fuel ratio abyfr and the output value Vabyfs of the upstream air-fuel ratio sensor 66. Based on the upstream side air-fuel ratio deviation DAFhi, a main feedback correction amount DFi_main is obtained, and by adding the main feedback correction amount DFi_main to the corrected basic fuel injection amount Fbase, the above-described sub feedback control (sub feedback correction amount DFi_sub The corrected basic fuel injection amount Fbase is corrected independently of the correction of the corrected basic fuel injection amount Fbase.

  For example, when the air-fuel ratio suddenly changes and becomes lean, the current detected air-fuel ratio abyfs (k) obtained by the table conversion means A11 is upstream before the MB stroke from the current time set by the upstream target air-fuel ratio setting means A2. It is obtained as a leaner value (a larger value) than the side target air-fuel ratio abyfr (k−MB). For this reason, the upstream air-fuel ratio deviation DAF is obtained as a large positive value. Further, since the signal indicating the upstream air-fuel ratio deviation DAF due to a sudden change in the air-fuel ratio has a high frequency component equal to or higher than the frequency (1 / T), the upstream side after passing through the high-pass filter A14 after passing through the high-pass filter A14. The air-fuel ratio deviation DAFhi is also a large positive value. Therefore, the main feedback correction amount DFi_main is a large positive value. Thus, the command fuel injection amount Fi (k) obtained by the command fuel injection amount calculation means A6 is controlled to be larger than the corrected basic fuel injection amount Fbase and the air-fuel ratio becomes rich.

  In contrast, when the air-fuel ratio suddenly changes and becomes rich, the detected air-fuel ratio abyfs (k) this time is richer (smaller value) than the upstream target air-fuel ratio abyfr (k-MB) before the MB stroke from the present time. As required. Therefore, the upstream air-fuel ratio deviation DAF is obtained as a negative value. Further, since the signal indicating the upstream air-fuel ratio deviation DAF has a high frequency component equal to or higher than the frequency (1 / T) due to a sudden change in the air-fuel ratio, the upstream air-fuel ratio deviation DAFhi after passing through the high-pass filter is also a negative value. It becomes. Therefore, the main feedback correction amount DFi_main is a negative value. As a result, the command fuel injection amount Fi (k) is controlled to be smaller than the corrected basic fuel injection amount Fbase so that the air-fuel ratio becomes lean. The table conversion unit A11, the target air-fuel ratio delay unit A12, the upstream air-fuel ratio deviation calculation unit A13, the high-pass filter A14, and the P controller A15 correspond to the feedback correction amount calculation unit.

  In this way, steady air-fuel ratio control with respect to air-fuel ratio fluctuations below a frequency (1 / T) that can appear as air-fuel ratio fluctuations downstream of the first catalyst 53 can be reliably performed by sub-feedback control. At the same time, since the low frequency component equal to or lower than the same frequency (1 / T) cannot pass through the high pass filter A14 and is not input to the P controller A15, it is possible to avoid the above-described interference in the air / fuel ratio control of the engine. Further, since the high frequency component equal to or higher than the frequency (1 / T) in the fluctuation of the air-fuel ratio (and hence the fluctuation of the output value Vabyfs of the upstream air-fuel ratio sensor 66) passes through the high-pass filter A14, the air-fuel ratio in the transient operation state Compensation for a sudden change of can be quickly and reliably performed by the main feedback control.

<Purge correction coefficient setting>
The purge correction coefficient setting means A16 is a purge execution condition (in this example, during engine operation, (a) after warming up the engine, (b) during air-fuel ratio feedback control, (c) the current cylinder in the current intake stroke Non-purge basic fuel in consideration of the amount of fuel contained in the evaporative gas when the current KFmem (m) renewal in the region of the internal intake air amount Mc (end of details will be described later) is established and purging is in progress A purge correction coefficient KP for correcting (decreasing) the injection amount Fbaseb0 is set. Hereinafter, the purge correction coefficient KP setting method by the purge correction coefficient setting means A16 will be described.

As described above, the purge correction coefficient KP is set so that the value obtained by multiplying the non-purge basic fuel injection amount Fbaseb0 (k) by the purge correction coefficient KP is equal to the pre-correction basic fuel injection amount Fbaseb (k). Therefore, the purge correction coefficient KP can be expressed by the following equation (8).
KP = Fbaseb (k) / Fbaseb0 (k) (8)

Here, the pre-correction basic fuel injection amount Fbaseb (k) is the amount of fuel contained in the evaporative gas sucked into the cylinder in the current intake stroke from the non-purge basic fuel injection amount Fbaseb0 (k) (hereinafter referred to as “purge”). It is equal to the amount obtained by subtracting the excess fuel amount ΔFbase ”(= Fbaseb0 (k) −ΔFbase). Further, as described above, the non-purge basic fuel injection amount Fbaseb0 (k) is obtained by dividing the cylinder intake air amount Mc (k) by the upstream target air-fuel ratio abyfr (k). Therefore, using these relationships, the purge correction coefficient KP can be expressed by the following equation (9).
KP = 1− (ΔFbase · abyfr (k) / Mc (k)) (9)

The purge surplus fuel amount ΔFbase is represented by the following equation (10): the amount of evaporation gas sucked into the cylinder in this intake stroke (hereinafter referred to as “evaporation gas amount Geva”) and the evaporation gas concentration Ceva. Expressed as a product. The method for obtaining the evaporation gas concentration Ceva will be described later. On the other hand, the actual purge rate (= target purge rate PGR) in the current intake stroke can be expressed by the following equation (11) when the engine 10 is in a steady operation state. The following equation (12) is derived by substituting the equation obtained by eliminating the evaporative gas amount Geva from the following equation (10) and the following equation (11) and solving for ΔFbase into the equation (9).
ΔFbase ＝ Geva ・ Ceva ・ ・ ・ (10)
PGR = Geva / (Geva + Mc (k)) (11)
KP = 1- (PGR / (1-PGR)) ・ Ceva ・ abyfr (k) (12)

  The purge correction coefficient setting means A16 sets the purge correction coefficient KP (k) according to the equation (12). The purge correction coefficient KP is stored in the RAM 73 while corresponding to the intake stroke of each cylinder. According to the equation (12), when the purge is not executed, the value of the target purge rate PGR is set to “0”, so the purge correction coefficient KP becomes “1” (that is, the non-purge basic fuel injection) The amount Fbaseb0 is not corrected for reduction, and the base fuel injection amount Fbaseb before correction coincides with the non-purge basic fuel injection amount Fbaseb0.) On the other hand, when purging is performed, the purge correction coefficient KP is greater than “0” and equal to or less than “1”. As a result, the uncorrected basic fuel injection amount Fbaseb is not purged by the purge surplus fuel amount ΔFbase. The value is reduced from the basic fuel injection amount Fbaseb0.

  The purge correction coefficient delay means A17 calculates the value before the MA stroke (MA intake strokes) from the current time out of the purge correction coefficient KP obtained for each intake stroke by the purge correction coefficient setting means A16 and stored in the RAM 73. And this value is set as a purge correction coefficient KP (k−MA) for multiplying the current non-purge basic fuel injection amount Fbaseb0 (k).

  As described above, the purge correction coefficient KP (k-MA) before the MA stroke from the present time is used as the purge correction coefficient KP for multiplying the current non-purge basic fuel injection amount Fbaseb0. This is because a time corresponding to the MA stroke (transport delay time LA) is required to reach 25. This transport delay time LA (and hence the stroke number MA) tends to become shorter as the cylinder intake air amount Mc increases. Accordingly, the stroke number MA is based on the in-cylinder intake air amount Mc (k) and the table MapMA (Mc) that defines the relationship between the in-cylinder intake air amount Mc and the stroke number MA shown in the graph of FIG. Can be obtained. The above is the method for setting the purge correction coefficient KP. The purge correction coefficient delay means A17 corresponds to the transport delay time acquisition means.

<Setting of basic fuel injection correction factor>
As described above, by performing the integration process in the PID controller A10, an error in the basic fuel injection amount can be compensated for in the sub-feedback control. However, since the change in the air-fuel ratio appears as a change in the air-fuel ratio of the exhaust gas downstream of the first catalyst 53 due to the influence of the oxygen storage function of the first catalyst 53 described above, an error in the basic fuel injection amount suddenly occurs. In the case of an increase, there is a problem that an error in basic fuel injection cannot be immediately compensated only by the sub-feedback control, and as a result, the emission emission amount may temporarily increase.

  In addition, since the integration process is not executed in the P controller A15, an error in the basic fuel injection amount cannot be compensated for in the main feedback control. From the above, it is necessary to immediately compensate the basic fuel injection amount without using the main feedback control and the sub feedback control.

  For this purpose, the corrected basic fuel injection amount Fbase, which is a value other than the main feedback correction amount DFi_main and the sub feedback correction amount DFi_sub among the values that determine the command fuel injection amount Fi, sets the actual air-fuel ratio to the target air-fuel ratio. A value corrected so as to coincide with (approach) the amount of fuel to be instructed to be injected into the injector 39 of the cylinder that reaches the intake stroke to obtain abyfr (hereinafter referred to as “target basic fuel injection amount Fbaset”) Need to be.

  For this purpose, as can be understood from FIG. 5, the basic fuel injection amount correction coefficient KF set by the basic fuel injection amount correction coefficient setting means A18 described above is multiplied by the current uncorrected basic fuel injection amount Fbaseb (k). The basic fuel injection amount correction coefficient KF needs to be set so that the value matches (approaches) the target basic fuel injection amount Fbaset. Hereinafter, a method for setting the basic fuel injection amount correction coefficient KF by the basic fuel injection amount correction coefficient setting means A18 will be described.

In general, under the assumption that the in-cylinder intake air amount sucked into the combustion chamber is constant, the fuel injection amount and the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber (and hence the air-fuel ratio of the exhaust gas) The product is constant. Therefore, under such an assumption, in general, the product of the command fuel injection amount Fi and the air-fuel ratio abyfs detected by the upstream air-fuel ratio sensor 66 is the actual air-fuel ratio of the air-fuel mixture supplied to the combustion chamber. A relationship is established that is equal to the product of the target basic fuel injection amount Fbaset required for setting the target air-fuel ratio abyfr (k) and the target air-fuel ratio abyfr (k). Therefore, the target basic fuel injection amount Fbaset can be generally expressed according to the following equation (13).
Fbaset = (abyfs / abyfr (k)) ・ Fi (13)

Here, as described above, the value obtained by multiplying the basic fuel injection amount Fbaseb (k) before correction by the basic fuel injection amount correction coefficient KF is equal to the target basic fuel injection amount Fbaset obtained according to the equation (13). Thus, the correction coefficient KF can be set according to the following equation (14).
KF = Fbaset / Fbaseb (k) (14)

  By the way, fuel injection (injection instruction) is executed during the intake stroke (or before the intake stroke), and the injected fuel enters the combustion chamber at a time near the compression top dead center that arrives thereafter. Can be ignited (burned). As a result, the generated exhaust gas is discharged from the combustion chamber to the exhaust passage via the exhaust valve, and then moves in the exhaust passage to reach the upstream air-fuel ratio sensor (detection unit thereof). Furthermore, a predetermined time is required until a change in the air-fuel ratio of the exhaust gas that has reached the detection unit of the upstream air-fuel ratio sensor appears as a change in the output value of the sensor.

  From the above, from the fuel injection instruction until the air-fuel ratio of the exhaust gas based on the combustion of the fuel injected by the injection instruction appears as the output value Vabyfs of the upstream air-fuel ratio sensor 66, the delay related to the combustion stroke ( A delay time LB expressed as a sum of a delay related to the movement of exhaust gas in the exhaust passage (a transport delay) and a delay related to the response of the upstream air-fuel ratio sensor (response delay) is required. In other words, the air-fuel ratio abyfs detected by the upstream air-fuel ratio sensor 66 is a value representing the air-fuel ratio of the exhaust gas generated based on the fuel injection instruction executed before the delay time LB.

  Therefore, when the target basic fuel injection amount Fbaset is calculated according to the above equation (13), the current detected air-fuel ratio abyfs (k) is used as the detected air-fuel ratio abyfs, while the command fuel injection amount Fi Specifically, the command fuel injection amount related to the fuel injection instruction executed before the MB stroke (MB intake strokes) corresponding to the delay time LB from the current Fi (k) injection instruction start point). It is preferable that the command fuel injection amount Fi (k-MB) before the MB stroke is used from a certain present time.

  Then, the above-described stroke delay and the time related to the transport delay become shorter as the in-cylinder intake air amount Mc increases, and the time related to the transport delay tends to become shorter as the operating speed NE increases. . Accordingly, the delay time LB (and therefore the stroke number MB) is determined by the in-cylinder intake air amount Mc (k), the operation speed NE, and the operation speed NE and in-cylinder intake air amount Mc shown in the graph of FIG. , Based on the table MapMB (Mc, NE) that defines the relationship with the stroke number MB.

  Further, when the engine is in a transient operation state, the detected air-fuel ratio abyfs, the command fuel injection amount Fi, and the pre-correction basic fuel injection amount Fbaseb can vary greatly at a high frequency equal to or higher than a predetermined frequency. In such a case, there is a possibility that the relationship shown in the equation (13) and the equation (14) cannot be maintained. Therefore, it is preferable to use a low-pass filter process in order to realize stable correction of the basic fuel injection amount. From the above, the basic fuel injection amount correction coefficient setting means A18 includes the functional blocks A18a to A18d as shown in FIG. 8 which is a functional block diagram thereof.

  The command fuel injection amount delay means A18a is based on the above-described table MapMB (Mc, NE) stored in the ROM 72, the current in-cylinder intake air amount Mc (k), and the current operation speed NE. The stroke number MB is obtained. The command fuel injection amount delay means A18a reads from the RAM 73 the value before the MB stroke from the current time out of the command fuel injection quantity Fi obtained for each intake stroke by the command fuel injection amount calculation means A6 and stored in the RAM 73. This is set as the command fuel injection amount Fi (k-MB).

The target basic fuel injection amount calculation unit A18b is obtained by dividing the value of the detected air-fuel ratio abyfs (k) by the current target air-fuel ratio abyfr (k) according to the following equation (15) corresponding to the above (13). The target basic fuel injection amount Fbaset is obtained by multiplying the value by the value of the command fuel injection amount Fi (k−MB).
Fbaset = (abyfs (k) / abyfr (k)) ・ Fi (k−MB) (15)

The basic fuel injection amount correction coefficient setting unit A18c converts the target basic fuel injection amount Fbaset obtained by the target basic fuel injection amount calculation unit A18b according to the following equation (16) corresponding to the above (14) to the basic fuel injection amount before correction. By dividing by Fbaseb (k), the basic fuel injection amount correction coefficient KFb before the low-pass filter processing is obtained.
KFb = Fbaset / Fbaseb (k) (16)

The low-pass filter A18d is a first-order filter as shown in the following equation (17) that expresses the characteristics using the Laplace operator s. In the following equation (17), τ is a time constant. The low-pass filter A18d is a digital filter, and the time constant τ is changed in accordance with a dull control constant n (value related to responsiveness) changed / set by a dull control constant setting means A19 described later. It has become.
1 / (1 + τ ・ s) (17)

The low-pass filter A18d receives the value of the basic fuel injection amount correction coefficient KFb before the low-pass filter processing obtained by the basic fuel injection amount correction coefficient setting unit A18c as an input, and converts the KFb value into the low-pass filter according to the following equation (18). A basic fuel injection amount correction coefficient KF (k), which is a value after processing, is output. In the following equation (18), KF (k−1) is the previous output value. n is the dull control constant, and is set to a value of “1” or more. Since the time constant τ is proportional to the product of the dull control constant n and the calculation cycle, the time constant τ increases as the value of the dull control constant n increases.
KF (k) = ((n−1) / n) · KF (k−1) + (1 / n) · KFb (18)

  The basic fuel injection amount correction coefficient setting method is as described above. The corrected basic fuel injection amount calculation means A5 and the basic fuel injection amount correction coefficient setting means A18 correspond to the basic fuel injection amount correction means.

<Storage processing of basic fuel injection amount correction coefficient>
In “when the output value Vabyfs of the upstream air-fuel ratio sensor 66 does not become a normal value” such as during warm-up operation of the internal combustion engine, the detected air-fuel ratio abyfs does not accurately represent the air-fuel ratio of the exhaust gas. In such a case, the value of the basic fuel injection amount correction coefficient KF calculated according to the equations (15) to (18) using the value of the detected air-fuel ratio abyfs is also the basic fuel injection amount Fbaseb (k) before correction. Is not a value for accurately correcting the target fuel injection amount Fbaset. Therefore, in such a case, the basic fuel injection amount correction coefficient KF calculated according to the above equations (15) to (18) should not be used for correcting the pre-correction basic fuel injection amount Fbaseb (k).

  Therefore, the present apparatus is limited to “when the output value Vabyfs of the upstream air-fuel ratio sensor 66 becomes a normal value (specifically, when a main feedback condition described later is satisfied)”. The base fuel injection amount Fbaseb (k) before correction is corrected using the basic fuel injection amount correction coefficient KF calculated according to equation (18), and the calculated value of the base fuel injection amount correction coefficient KF is sequentially applied. The data is stored and updated in the backup RAM 74.

  Here, when the intake air flow rate Ga increases (and therefore the in-cylinder intake air amount Mc also increases), an error in the basic fuel injection amount becomes an in-cylinder intake air amount Mc. Accordingly, there is a tendency that the value of the basic fuel injection amount correction coefficient KF increases in accordance with the in-cylinder intake air amount Mc. Therefore, as shown in FIG. 9, the present apparatus divides the possible range of the cylinder intake air amount Mc into a plurality of (in this example, four) classifications. Then, every time a new basic fuel injection amount correction coefficient KF is calculated, this system selects the class to which the current in-cylinder intake air amount Mc (k) belongs, and the basic fuel corresponding to the selected class. The value of the injection amount correction coefficient KFmem (m) (m: 1, 2, 3, 4) is updated and stored as the calculated new basic fuel injection amount correction coefficient KF.

  On the other hand, in the case where “the output value Vabyfs of the upstream side air-fuel ratio sensor 66 does not become a normal value (specifically, when a main feedback condition described later is not satisfied)” Selects the category to which the quantity Mc (k) belongs, and corresponds to the selected category of the basic fuel injection amount correction coefficient KFmem (m) (m: 1, 2, 3, 4) stored in the backup RAM 74 Is set as the basic fuel injection amount correction coefficient memory value KFmem.

  Then, instead of the basic fuel injection amount correction coefficient KF (k) calculated according to the above equations (15) to (18), the basic fuel injection amount correction coefficient stored value KFmem is used and the basic fuel injection amount before correction Fbaseb ( k) is corrected. As a result, even when “the output value Vabyfs of the upstream air-fuel ratio sensor 66 does not become a normal value”, the basic fuel injection amount Fbaseb (k) before correction is made to coincide with the target basic fuel injection amount Fbaset with a certain degree of accuracy. As a result, the error of the basic fuel injection amount is compensated to some extent.

  However, when purge is being performed, the value of the basic fuel injection amount correction coefficient KF in a state where disturbance to the air-fuel ratio control system due to the purge has entered is “the output value Vabyfs of the upstream air-fuel ratio sensor 66 is a normal value. Should not be used as a correction value for Accordingly, the basic fuel injection amount correction coefficient KF is stored and updated in the backup RAM 74 only when the purge is not performed.

<Acquisition of evaporative gas concentration>
As described above, the evaporation gas concentration Ceva needs to be acquired in order to calculate the purge correction coefficient KP. This apparatus acquires the evaporation gas concentration Ceva according to the following equation (19). The following equation (19) is obtained by eliminating the evaporation gas amount Geva from the equations (10) and (11) and solving for the evaporation gas concentration Ceva. Hereinafter, a method for obtaining the purge surplus fuel amount ΔFbase will be described.
Ceva = ((1-PGR) / (Mc (k) · PGR)) · ΔFbase (19)

When the cylinder intake air amount Mc (k) is constant, the target basic fuel injection amount Fbaset (= KF (k) · KP (k−MA) · Fbaseb0 (k)) between the non-purge and the purge time Is equal to the pure purge surplus fuel amount ΔFbase from which the influence of the error of the basic fuel injection amount is excluded. When the cylinder intake air amount Mc (k) is constant, the non-purge basic fuel injection amount Fbaseb0 (k) is constant. Accordingly, considering that the purge correction coefficient KP (k−MA) = 1 at the time of non-purge, and assuming that the KF value at the time of non-purge is KFnonpurge and the KF value at the time of purge is KFpurge, the excess fuel amount ΔFbase Can be expressed by the following equation (20).
ΔFbase = (KFnonpurge-KFpurge · KP (k-MA)) · Fbaseb0 (k) (20)

  Therefore, in steady operation during purge, the value of KFpurge corresponds to the current basic fuel injection amount correction coefficient KF (k), and the value of KFnonpurge corresponds to the current in-cylinder intake air amount Mc (k) By setting the basic fuel injection amount correction coefficient stored value KFmem as described above, the purge surplus fuel amount ΔFbase is obtained in accordance with the equation (20). The above is the method for obtaining the evaporation gas concentration Ceva. The evaporation gas concentration Ceva is updated at each timing described later. The latest value of the evaporated gas concentration Ceva updated in this way (hereinafter referred to as “evaporated gas concentration learning value”) is used in the equation (12), and the purge correction coefficient setting means A16 performs purge correction. Set the coefficient KP (k).

<Setting / changing the low-pass filter time constant>
In order to realize stable correction of the basic fuel injection amount by the basic fuel injection amount correction coefficient KF, the time constant τ of the low-pass filter A18d in the basic fuel injection amount correction coefficient setting means A18 is an error of the basic fuel injection amount. It is preferable to set a value as large as possible (hereinafter referred to as “value τ1”) within a range that does not impede rapidity in compensation.

  By the way, the evaporative gas concentration learning value described above always includes an estimation error. Therefore, the purge correction coefficient KP set according to the above equation (12) using the evaporative gas concentration learning value includes an error due to the evaporative gas concentration learned value error at the time of purging. This becomes a large disturbance in the air-fuel ratio control system when the purge is started.

  Further, the purge correction coefficient KP set according to the above equation (12) also changes stepwise by the target purge rate PGR changing stepwise by the start / stop of purge. On the other hand, the actual purge rate of the gas sucked into the combustion chamber 25 is changed in a step shape due to the influence of dispersion of the evaporation gas in the process in which the purged evaporated gas flows from the VSV 49 to the combustion chamber 25 in the intake passage. Follow the purge rate PGR with a certain delay.

  Therefore, the purge correction coefficient KP set according to the above equation (12) using the target purge rate PGR includes an error caused by the dispersion of the evaporation gas in the intake passage within each short period after the start and stop of the purge. It is included. This becomes a large disturbance in the air-fuel ratio control system when the purge is started / stopped.

  Even when a large disturbance occurs in the air-fuel ratio control system based on the error of the purge correction coefficient KP as described above, if the time constant τ of the low-pass filter A18d is maintained at the value τ1, the response delay of the low-pass filter A18d is large. For this reason, there may occur a case where the basic fuel injection amount correction coefficient KF is significantly different from an appropriate value temporarily. In this case, the rapid compensation for the sudden change of the air-fuel ratio by the main feedback control is hindered, and as a result, a situation may occur in which the air-fuel ratio is largely shifted from the target air-fuel ratio abyfr temporarily.

  Therefore, the blunting control constant setting means A19 normally sets the blunting control constant n used in the equation (18) to a value n0 corresponding to the value τ1, and when the purge is started / stopped. Thereafter, it is temporarily reduced. Hereinafter, this will be described with reference to FIG.

  FIG. 10 shows an example of changes in various variables when the purge is started at time t1 and the purge that has been continued after time t1 is stopped at time t4 when the intake air flow rate Ga is constant. It is a time chart. XPG shown in (B) is set to “1” when the purge execution condition described above is satisfied (that is, during purge), and when the purge execution condition is not satisfied (that is, when not purged). Is a purge execution flag set to “0”.

  In this example, before the time t1 when the purge is started, the air-fuel ratio A / F is maintained at the target air-fuel ratio abyfr as shown in (G), and the basic fuel injection amount as shown in (F). The correction coefficient KF is maintained at the value KF0. The deviation amount of the value KF0 with respect to “1” corresponds to the error amount of the basic fuel injection amount. In this example, it is assumed that the evaporative gas concentration learning value includes an error that is shifted in a direction larger than the true value.

  In this case, as shown in (C), the purge correction coefficient KP (k) is maintained at “1” in conjunction with the value of the flag XPG before the time t1 and after the time t4, which are not purged. Between times t1 to t4, which are hours, is set according to the above equation (12) using the evaporative gas concentration learning value and the target purge rate PGR (≠ 0, constant) determined from the intake air flow rate Ga (constant) Value KP0 (0 <KP0 <1) is maintained.

  Therefore, as indicated by a solid line in (D), the purge correction coefficient KP (k−MA) used for the calculation of the pre-correction basic fuel injection amount Fbaseb (and hence the calculation of the command fuel injection amount Fi) At time t2 when the transport delay time L1 corresponding to the MA stroke has elapsed from the start time t1, the transport delay corresponding to the MA stroke is changed from “1” to the value KP0 in a stepped manner at time t2. At time t5 when the time L2 has elapsed, the value KP0 changes to “1” stepwise.

  On the other hand, the alternate long and short dash line in (D) shows a purge correction coefficient (hereinafter referred to as “purge correction coefficient”) set according to the above equation (12) using the true vapor gas concentration value and the actual purge rate of the gas sucked into the combustion chamber 25 , Referred to as “necessary purge correction coefficient”). This necessary purge correction coefficient is the air-fuel ratio at the time of purge, excluding the influence of the error of the purge correction coefficient KP caused by the above-mentioned “error of the evaporated gas concentration learning value” and “dispersion of the evaporated gas in the intake passage”. This is the true purge correction coefficient that should be multiplied by the non-purge basic fuel injection amount Fbaseb0 to obtain the target air-fuel ratio abyfr.

  As indicated by the alternate long and short dash line in (D), the required purge correction coefficient is a purge correction coefficient that decreases in a step-like manner due to the above-described dispersion of the evaporation gas in the intake passage in the short period after time t2. Follow KP (k-MA) with a certain delay. In addition, since the evaporative gas concentration learning value is deviated in a direction larger than the true evaporative gas concentration value, the necessary purge correction coefficient is changed from “1” to a value KP1 (0 <KP1) larger than the value KP0 after time t2. Converge to <1).

  As a result, immediately after time t2, the purge correction coefficient KP (k−MA) becomes smaller than the necessary purge correction coefficient, and an error occurs in the purge correction coefficient KP in a direction that makes the air-fuel ratio leaner than the target air-fuel ratio abyfr. To do. This is a large disturbance in the air-fuel ratio control system.

  As a result, as shown in (G), the air-fuel ratio A / F maintained at the target air-fuel ratio abyfr until time t2 starts shifting in the lean direction immediately after time t2. As a result, as shown in (F), the basic fuel injection amount correction coefficient KF maintained at the value KF0 until the time t2 starts to change toward the value KF1 (> value KF0) immediately after the time t2. . The change of the basic fuel injection amount correction coefficient KF from the value KF0 to the value KF1 is based on an error in the purge correction coefficient KP (deviation of the value KP0 from the value KP1) due to an error in the evaporated gas concentration learning value. .

  Here, consider the case where the dull control constant n is always maintained at the value n0 as indicated by the broken line in (E). In this case, since the response delay of the low-pass filter A18d is large, the basic fuel injection amount correction coefficient KF prevents the rapid compensation for the sudden change of the air-fuel ratio by the main feedback control as shown by the broken line in (F). Slowly converge toward the value KF1. As a result, as indicated by a broken line in (G), the air-fuel ratio A / F temporarily shifts greatly in the lean direction immediately after time t2.

  On the other hand, the dull control constant setting means A19 dulls the control constant n from the value n0 to the value n1 (<n0) as indicated by the solid line in FIG. As a result, the time constant τ of the low-pass filter A18d is reduced and the response delay is reduced. Therefore, as shown by the solid line in FIG. It converges relatively rapidly toward the value KF1 with almost no impediment to rapid compensation. As a result, as indicated by a solid line in (G), the degree to which the air-fuel ratio A / F is temporarily largely shifted in the lean direction immediately after time t2 can be reduced.

  Similarly, within the short period after time t5, as indicated by the alternate long and short dash line in (D), the necessary purge correction coefficient increased in a step-like manner due to the above-described dispersion of the evaporation gas in the intake passage. The purge correction coefficient KP (k−MA) is followed with a certain delay. As a result, immediately after time t5, the purge correction coefficient KP (k−MA) becomes larger than the required purge correction coefficient, and an error occurs in the purge correction coefficient KP in a direction that makes the air-fuel ratio richer than the target air-fuel ratio abyfr. To do. This is a large disturbance in the air-fuel ratio control system.

  As a result, as shown in (G), the air-fuel ratio A / F maintained at the target air-fuel ratio abyfr until time t5 starts shifting in the rich direction immediately after time t5. As a result, as shown in (F), the basic fuel injection amount correction coefficient KF maintained at the value KF1 until time t5 starts to change toward the value KF0 immediately after time t5.

  In this case, the dull control constant setting means A19 dulls the control constant n from the value n0 to the value n2 (= n1) as indicated by the solid line in FIG. As a result, the basic fuel injection amount correction coefficient KF and the air-fuel ratio A / F change as indicated by solid lines in (F) and (G).

  As a result, as compared to the case where the dull control constant n is maintained at the value n0, which is indicated by a broken line in (E), (F), and (G), For the same reason, it is possible to reduce the extent to which the air-fuel ratio A / F is temporarily largely shifted in the rich direction immediately after time t5.

  In this way, the blunting control constant setting means A19 normally sets the blunting control constant n to the value n0 that “the time constant τ of the low-pass filter A18d becomes the value τ1”, and when the purge is started. The dull control constant n is made smaller than the value n0 only for a predetermined period α1 from the time when the transportation delay time L1 has elapsed from the time point and for a predetermined period α2 from the time when the transportation delay time L2 has elapsed since the purge was stopped. Thus, the time constant τ of the low-pass filter A18d is made smaller than the value τ1.

  The predetermined periods α1 and α2 are set to the time required for the air-fuel ratio A / F to converge to the target air-fuel ratio abyfr after the air-fuel ratio A / F is disturbed. In this example, the dull control is performed. The constant n is set to three times the time constant τ of the low-pass filter A18d when the value is n1 (= n2). The above is the outline of the air-fuel ratio control by this apparatus.

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

<Air-fuel ratio feedback control>
The CPU 71 performs a routine for calculating the command fuel injection amount Fi shown in the flowchart of FIG. 11 and instructing fuel injection. The CPU 71 performs a predetermined crank angle before each intake top dead center (for example, BTDC 90 ° CA). ) Is repeated every time. Accordingly, when the crank angle of an arbitrary cylinder reaches the predetermined crank angle, the CPU 71 starts processing of the “Fi calculation / injection” routine 1100.

  First, in step 1105, based on the table MapMc (NE, Ga), the current in-cylinder intake air that has been sucked into the cylinder that will reach the current intake stroke (hereinafter also referred to as “fuel injection cylinder”). The quantity Mc (k) is estimated and determined. Next, the CPU 71 proceeds to step 1110, and divides the in-cylinder intake air amount Mc (k) by the current target air-fuel ratio abyfr (k), so that the current non-purge basic fuel injection amount Fbaseb0 (k) To decide.

  Next, the CPU 71 proceeds to step 1115 to determine the stroke number MA based on the table MapMA (Mc (k)) (see FIG. 6). Subsequently, the CPU 71 proceeds to step 1120, and among the purge correction coefficients KP calculated in a routine to be described later and stored in the RAM 73 for each intake stroke, the purge correction coefficient KP (k−MA) before the MA stroke from the present time. Is multiplied by the non-purge basic fuel injection amount Fbaseb0 (k) determined in the previous step 1110, to obtain the current pre-correction basic fuel injection amount Fbaseb (k).

  Next, the CPU 71 proceeds to step 1125 to determine whether or not the main feedback condition is satisfied. Here, the main feedback condition is, for example, that the coolant temperature THW of the engine is equal to or higher than the first predetermined temperature and the upstream air-fuel ratio sensor 66 is normal (including that the engine is in an active state). This is established when the intake air amount (load) per rotation is a predetermined value or less. That is, the fact that the main feedback condition is satisfied corresponds to the above-described “when the output value Vabyfs of the upstream air-fuel ratio sensor 66 becomes a normal value”.

  When the main feedback condition is satisfied (step 1125 = “Yes”), the CPU 71 proceeds to step 1130, and the basic fuel injection obtained in the routine described later is set as the uncorrected basic fuel injection amount Fbaseb (k). The corrected basic fuel injection amount Fbase is determined by multiplying the latest value of the amount correction coefficient KF (k).

  On the other hand, when the main feedback condition is not satisfied (step 1125 = “No”), the CPU 71 proceeds to step 1135, and the basic fuel injection amount correction coefficient storage value KFmem is corrected to the basic fuel injection amount correction stored in the backup RAM 74. Of the coefficients KFmem (m) (m: 1, 2, 3, 4), the value of KFmem (m) selected from the value of the in-cylinder intake air amount Mc (k) is determined. Subsequently, the CPU 71 proceeds to step 1140 to determine a corrected basic fuel injection amount Fbase by multiplying the uncorrected basic fuel injection amount Fbaseb (k) by the basic fuel injection amount correction coefficient storage value KFmem.

  Next, the CPU 71 proceeds to step 1145 to obtain the corrected basic fuel injection amount Fbase obtained as described above according to the equation (1) by a routine described later (at the time of the previous fuel injection). By adding the latest main feedback correction amount DFi_main and the latest sub-feedback correction amount DFi_sub obtained in the routine described later (at the time of the previous fuel injection), the current command fuel injection amount Fi (k) is obtained. decide. The value of the commanded fuel injection amount Fi (k) is stored in the RAM 73 while corresponding to the intake stroke of each cylinder as described above.

  Subsequently, the CPU 71 proceeds to step 1150 to instruct to inject the fuel of the command fuel injection amount Fi (k), and then proceeds to step 1195 to end the present routine tentatively. As described above, the uncorrected basic fuel injection amount Fbaseb (k) is corrected to coincide with the target basic fuel injection amount Fbaset described above, and the corrected basic fuel injection amount Fbaseb (k) (that is, the correction) The fuel injection cylinder is instructed to inject the fuel of the command fuel injection amount Fi (k) after the post-basic fuel injection amount Fbase) is subjected to the main feedback correction and the sub feedback correction.

<Calculation of main feedback correction amount>
Next, the operation when calculating the main feedback correction amount DFi_main in the main feedback control will be described. The CPU 71 executes the “calculation of the main feedback correction amount DFi_main” routine 1200 shown in the flowchart of FIG. 12 for the fuel injection cylinder. Each time the fuel injection start time (injection instruction start time) arrives, it is repeatedly executed. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the processing of the routine 1200, and first, at step 1205, determines whether or not the main feedback condition is satisfied. This main feedback condition is the same as the main feedback condition in step 1125 of FIG.

  Now, if the description is continued assuming that the main feedback condition is satisfied, the CPU 71 determines “Yes” in step 1205 and proceeds to step 1210, and based on the table Mapabyfs (Vabyfs) (see FIG. 3), The current detected air-fuel ratio abyfs (k) is obtained.

  Next, the CPU 71 proceeds to step 1215 to determine the stroke number MB based on the table MapMB (Mc (k), NE) (see FIG. 7). Next, the CPU 71 proceeds to step 1220, and subtracts abyfr (k-MB), which is the target air-fuel ratio before the MB stroke (MB intake strokes) from the present time, from the obtained detected air-fuel ratio abyfs (k). Obtain the side air-fuel ratio deviation DAF.

  Subsequently, the CPU 71 proceeds to step 1225, where the upstream air-fuel ratio deviation DAF is subjected to high-pass filter processing by the high-pass filter A14, and the upstream air-fuel ratio deviation DAFhi is obtained after passing through the high-pass filter. Next, the CPU 71 proceeds to step 1230 to obtain the main feedback correction amount DFi_main according to the equation (7), and then proceeds to step 1295 to end the present routine tentatively.

  Thus, the main feedback correction amount DFi_main is obtained, and this main feedback correction amount DFi_main is reflected in the command fuel injection amount Fi (k) in step 1145 of FIG. Air-fuel ratio control is executed.

  On the other hand, if the main feedback condition is not satisfied at the time of determination in step 1205, the CPU 71 determines “No” in step 1205 and proceeds to step 1235 to set the value of the main feedback correction amount DFi_main to “0”. Thereafter, the routine proceeds to step 1295 to end the present routine tentatively. Thus, when the main feedback condition is not satisfied, the main feedback correction amount DFi_main is set to “0” and the air-fuel ratio correction based on the main feedback control is not performed.

<Calculation of sub feedback correction amount>
Next, the operation when calculating the sub-feedback correction amount DFi_sub in the sub-feedback control will be described. The CPU 71 executes the “calculation of the sub-feedback correction amount DFi_sub” routine 1300 shown in the flowchart of FIG. 13 for the fuel injection cylinder. Each time the fuel injection start time (injection instruction start time) arrives, it is repeatedly executed. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the processing of the routine 1300. First, in step 1305, it is determined whether or not the sub feedback condition is satisfied. The sub feedback condition is satisfied, for example, when the engine coolant temperature THW is equal to or higher than a second predetermined temperature higher than the first predetermined temperature, in addition to the main feedback condition in step 1125 (and step 1205) described above.

  Now, assuming that the sub-feedback condition is satisfied, the CPU 71 determines “Yes” in step 1305 and proceeds to step 1310, and from the downstream target value Voxs_ref according to the equation (2), The output deviation amount DVoxs is obtained by subtracting the output value Voxs of the downstream air-fuel ratio sensor 67.

Next, the CPU 71 proceeds to step 1315 to low-pass filter the output deviation amount DVoxs with the low-pass filter A9 to obtain an output deviation amount DVoxs_low after passing through the low-pass filter. In the following step 1320, the following equation (21) is obtained. Based on this, a differential value DDVoxs_low of the output deviation amount DVoxs_low after passing through the low-pass filter is obtained.
DDVoxs_low = (DVoxs_low−DVoxs_low1) / dt (21)

  In the equation (21), DVoxs_low1 is the previous value of the output deviation amount DVoxs_low after passing through the low-pass filter set (updated) in step 1335 described later at the time of the previous execution of this routine. Further, dt is the time from the time when this routine is executed last time to the time when this routine is executed this time.

  Next, the CPU 71 proceeds to step 1325, obtains the sub feedback correction amount DFi_sub according to the equation (4), and then proceeds to step 1330 to set the integrated value SDVoxs_low of the output deviation amount after passing through the low-pass filter at that time to the step 1315. Is added to the output deviation amount DVoxs_low after passing through the low-pass filter to obtain a new integrated value SDVoxs_low of the output deviation amount after passing through the low-pass filter. Subsequently, in step 1335, the output after passing through the low-pass filter is obtained in step 1315. After setting the deviation amount DVoxs_low as the previous value DVoxs_low1 of the output deviation amount DVoxs_low after passing through the low-pass filter, the routine proceeds to step 1395 to end the present routine tentatively.

  Thus, the sub feedback correction amount DFi_sub is obtained, and this sub feedback correction amount DFi_sub is reflected in the command fuel injection amount Fi (k) in step 1145 of FIG. Air-fuel ratio control is executed.

  On the other hand, if the sub feedback condition is not satisfied at the time of determination in step 1305, the CPU 71 determines “No” in step 1305 and proceeds to step 1340 to set the value of the sub feedback correction amount DFi_sub to “0”. Thereafter, the routine proceeds to step 1395 to end the present routine tentatively. Thus, when the sub-feedback condition is not satisfied, the sub-feedback correction amount DFi_sub is set to “0”, and the air-fuel ratio is not corrected based on the sub-feedback control.

<Setting of blunt control constant>
Next, the operation when setting / changing the blunting control constant n will be described. The CPU 71 executes the “setting blunting control constant n” routine 1400 shown in the flowchart of FIG. It is repeatedly executed every time (injection instruction start time) comes. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the processing of the routine 1400. First, in step 1402, it is determined whether or not the main feedback condition is satisfied. This main feedback condition is the same as the main feedback condition in step 1125 of FIG. 11 (and step 1205 of FIG. 12).

  Now, if the description is continued assuming that the main feedback condition is satisfied and the purge execution condition is satisfied (see time t1 in FIG. 10), the CPU 71 determines “Yes” in step 1402 and determines whether or not step 1404 is satisfied. Then, it is determined whether the purge execution condition is satisfied. The current time is immediately after the purge execution condition is satisfied. Accordingly, the CPU 71 makes a “Yes” determination at step 1404 to proceed to step 1406 to change the value of the purge execution flag XPG maintained at “0” to “1”.

  Next, the CPU 71 proceeds to step 1408 to determine whether or not the value of the purge execution flag XPG has changed from “0” to “1”. Since the current time is immediately after the value of the purge execution flag XPG is changed from “0” to “1”, the CPU 71 determines “Yes” in step 1408 and proceeds to step 1410 to transfer the transport delay L1 (see FIG. The stroke number M1 corresponding to 10) is set to the stroke number MA determined in step 1115 of FIG.

  Next, the CPU 71 proceeds to step 1412 to set the value of the counter z for counting the number of strokes from the purge start time to “0”. Subsequently, the CPU 71 proceeds to step 1414 to determine whether or not the counter z is equal to or greater than the stroke number M1.

  Since the counter z is currently “0”, the CPU 71 determines “No” in step 1414 and proceeds to step 1416 to set the dull control constant n to the value n0. Then, the CPU 71 proceeds to step 1495 to end the present routine tentatively. Thereafter, the CPU 71 determines “No” every time it proceeds to step 1408, proceeds to step 1418, and increments the counter z by “1”. That is, as long as the counter z incremented in step 1418 does not reach the stroke number M1, the CPU 71 repeatedly executes the processing of steps 1402 to 1408, 1418, 1414, and 1416. As a result, the blunt control constant n is maintained at the value n0 (see times t1 to t2 in FIG. 10).

  When the counter z reaches the stroke number M1 (see time t2 in FIG. 10), the CPU 71 determines “Yes” when it proceeds to step 1414, proceeds to step 1420, and sets the dull control constant n to the value n1 (< Set to value n0).

  Next, the CPU 71 proceeds to step 1422 to acquire the elapsed time TIME1 from the time when the counter z reaches the stroke number M1. The elapsed time TIME1 is measured by a timer (not shown) built in the electric control device 70. Next, the CPU 71 proceeds to step 1424 to determine whether or not the elapsed time TIME1 is shorter than the predetermined period α1.

  At this time, since the counter z has just reached the stroke number M1, the CPU 71 makes a “Yes” determination at step 1424 to proceed to step 1495 to end the present routine tentatively. Thereafter, as long as the elapsed time TIME1 is shorter than the predetermined period α1, the CPU 71 repeatedly executes the processing of steps 1402 to 1408, 1418, and 1414 to 1424. As a result, the blunt control constant n is maintained at the value n1 (see times t2 to t3 in FIG. 10).

  When the elapsed time TIME1 reaches the predetermined period α1 (see time t3 in FIG. 10), the CPU 71 determines “No” when it proceeds to step 1424, proceeds to 1416 described above, and dulls the control constant n by the value n1. Instead of this, the value n0 is set. Thereafter, as long as the purge execution condition is satisfied, the CPU 71 repeatedly executes the processing of steps 1402 to 1408, 1418, 1414 to 1424, and 1416. As a result, the blunt control constant n is maintained at the value n0 (see times t3 to t4 in FIG. 10).

  Next, a case where the purge execution condition is no longer satisfied in this state (see time t4 in FIG. 10) will be described. In this case, the CPU 71 determines “No” when it proceeds to step 1404, and executes the processing after step 1426.

  Here, the processing of step 1426 to step 1436 corresponds to the above-described step 1406 to step 1416, respectively, and thus detailed description thereof will be omitted. As a result, the dull control constant n is a value n0 until the counter z reaches the stroke number M2 corresponding to the transport delay time L2 (see FIG. 10), as in the case where the purge execution condition is satisfied. (Refer to times t4 to t5 in FIG. 10), and is maintained at the value n2 (= n1) from when the counter z reaches the stroke number M2 until the predetermined period α2 elapses (see FIG. 10). After the time point when the predetermined period α2 has elapsed (see times t5 to t6), the value is returned to the value n0 (see after time t5 in FIG. 10 and before time t1).

  On the other hand, if the main feedback condition is not satisfied at the time of determination in step 1402, the CPU 71 determines “No” in step 1402 and immediately proceeds to step 1495 to end the present routine tentatively. Thus, when the main feedback condition is not satisfied, the value of the dull control constant n is maintained at the value set at the previous execution of this routine. The dull control constant n set in this way is used for calculation of a basic fuel injection amount correction coefficient KF described later.

<Calculation of purge correction factor>
Next, the operation for calculating the purge correction coefficient KP will be described. The CPU 71 executes a “purge correction coefficient KP calculation” routine 1500 shown in the flowchart of FIG. It is repeatedly executed every time the start time) arrives. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the processing of the routine 1500. First, in step 1505, it is determined whether or not the value of the purge execution flag XPG is “1”. .

  First, the case where the value of the purge execution flag XPG is “0” (that is, when the purge execution condition is not satisfied; see before time t1 and after time t4 in FIG. 10) will be described. In this case, the CPU 71 determines “No” in step 1505 and proceeds to step 1510 to set the target purge rate PGR to “0”. As a result, the VSV 49 is maintained in the closed state.

  Next, the CPU 71 proceeds to step 1515 to set the purge correction coefficient KP (k) to “1”, and then proceeds to step 1595 to end the present routine tentatively. Thus, when the purge execution condition is not satisfied, the purge correction coefficient KP (k) is maintained at “1”. As described above, the purge correction coefficient KP (k) value (= 1) is stored in the RAM 73 while corresponding to the intake stroke of each cylinder.

  Next, a case where the value of the purge execution flag XPG is “1” (that is, when the purge execution condition is satisfied; see times t1 to t4 in FIG. 10) will be described. In this case, the CPU 71 makes a “No” determination at step 1505 to proceed to step 1520 to obtain the target purge rate PGR based on the table MapPGR (Ga). Thus, the duty of the VSV 49 is controlled so that the actual purge rate matches the target purge rate PGR (≠ 0) corresponding to the current intake air flow rate Ga, and as a result, the purge is executed.

  Subsequently, the CPU 71 proceeds to step 1525 to determine whether or not an evaporative gas concentration learning value update condition is satisfied. As described above, the evaporative gas concentration learning value needs to be updated in a steady operation state in which the value of the basic fuel injection amount correction coefficient KF is stable during purge execution. In this example, the evaporative gas concentration learning value update condition is that the engine 10 is in a predetermined steady operation state (for example, the fluctuation range of the operating speed NE and the accelerator pedal operation amount Accp is not more than a predetermined value over a predetermined period up to the present time. The purge correction coefficient KP (k−MA) ≠ 1 and the elapsed time TIME1 (see step 1422 in FIG. 14) reaches the time αz (>> α1) (in FIG. 10). And at the time when the time αz has elapsed from the time t2 (that is, at a certain time from the time t3 to the time t4). Here, the time αz is a time required until the basic fuel injection amount correction coefficient KF sufficiently converges after the disturbance of the basic fuel injection amount correction coefficient KF occurs.

  If it is determined in step 1525 that the evaporative gas concentration learning value update condition is not satisfied, the CPU 71 makes a “No” determination in step 1525 to immediately proceed to step 1545 to obtain the target purge obtained in the previous step 1520. The purge correction coefficient based on the rate PGR, the latest value of the evaporative gas concentration Ceva updated in step 1540 (evaporative gas concentration learned value), the target air-fuel ratio abyfr (k), and the equation (12) Calculate KP (k). As described above, the purge correction coefficient KP (k) value (0 <KP (k) ≦ 1) is stored in the RAM 73 while corresponding to the intake stroke of each cylinder.

  On the other hand, if it is determined in step 1525 that the evaporative gas concentration learning value update condition is satisfied, the CPU 71 determines “Yes” in step 1525 and proceeds to step 1530 to store the basic fuel injection amount correction coefficient stored value KFmem in a later-described manner. Of the basic fuel injection amount correction coefficient KFmem (m) (m: 1, 2, 3, 4) stored in the backup RAM 74 by execution of the routine to perform in-cylinder intake estimated and determined in step 1105 of FIG. A value selected from the value of the air amount Mc (k) is set.

  Next, the CPU 71 proceeds to step 1535 to store the basic fuel injection amount correction coefficient stored value KFmem, the latest value of the basic fuel injection amount correction coefficient KF (k) calculated in the routine described later, and the RAM 73. The purge correction coefficient KP (k−MA) before the MA stroke from the present time, the non-purge basic fuel injection amount Fbaseb0 (k) determined in step 1110 in FIG. 11, and the above equation (20). Based on the equation, the purge surplus fuel amount ΔFbase is obtained.

  Then, the CPU 71 proceeds to step 1540 to update the evaporation gas concentration Ceva based on the target purge rate PGR, the in-cylinder intake air amount Mc (k), the purge surplus fuel amount ΔFbase, and the equation (19). After the evaporative gas concentration learning value is updated, the process proceeds to step 1545 described above.

  Thus, the purge correction coefficient KP (k) is calculated and stored, and the purge correction coefficient KP (k−MA) before the MA stroke from the current time stored in the RAM 73 is used in step 1120 of FIG. To go. In addition, the evaporative gas concentration learned value used for calculating the purge correction coefficient KP (k) is updated only once when the evaporative gas concentration learned value update condition is satisfied during one purge execution period.

<Calculation and storage of basic fuel injection amount correction coefficient>
Next, the operation for calculating the basic fuel injection amount correction coefficient KF will be described. The CPU 71 executes a “calculation of the basic fuel injection amount correction coefficient KF” routine 1600 shown in the flowchart of FIG. It is repeatedly executed every time (in this example, 8 msec). Accordingly, when the predetermined timing is reached, the CPU 71 starts the processing of the routine 1600. First, in step 1605, it is determined whether or not the main feedback condition is satisfied. Immediately, the routine is terminated once. In this case, calculation of the basic fuel injection amount correction coefficient KF and storage processing of the value of the basic fuel injection amount correction coefficient KF in the backup RAM 74 are not executed. This main feedback condition is the same as the main feedback condition in steps 1125, 1205 and 1402.

  Now, assuming that the main feedback condition is satisfied, the CPU 71 determines “Yes” in step 1605 and proceeds to step 1610 to determine the number of strokes based on the table MapMB (Mc (k), NE). Determine the MB.

  Next, the CPU 71 proceeds to step 1615, where the latest value of the detected air-fuel ratio abyfs (k) obtained in step 1210 in FIG. 12 and the target air-fuel ratio abyfr (k) used in step 1110 in FIG. ), The command fuel injection amount Fi (k-MB) before the MB stroke from the current time stored in the RAM 73, and the above-mentioned equation (15), the target basic fuel injection amount Fbaset is obtained.

  Next, the CPU 71 proceeds to step 1620, in which the target basic fuel injection amount Fbaset, the latest value of the pre-correction basic fuel injection amount Fbaseb (k) obtained in step 1120 of FIG. The basic fuel injection amount correction coefficient KFb before the low-pass filter processing is obtained based on

  Subsequently, the CPU 71 proceeds to step 1625, in which the basic fuel injection amount correction coefficient KF (k−1) and the blunt control constant n set in any of steps 1416, 1420, 1436, 1440 in FIG. Of the basic fuel injection amount correction coefficient KFb before the low-pass filter processing and the basic fuel injection amount correction coefficient KFb before the low-pass filter processing based on the latest value of A fuel injection amount correction coefficient KF (k) is obtained. Here, as the basic fuel injection amount correction coefficient KF (k−1), the latest value already updated in step 1640 described later at the time of the previous execution of this routine is used.

  Next, the CPU 71 proceeds to step 1630 to determine whether or not the KF value storage condition is satisfied. The storage process of the basic fuel injection amount correction coefficient KF value is preferably executed in a steady operation state in which the value of the basic fuel injection amount correction coefficient KF is stable during non-purge execution. In this example, the KF value storage condition is that the engine 10 is in a predetermined steady operation state (for example, a state where the fluctuation range of the operation speed NE and the accelerator pedal operation amount Accp is not more than a predetermined value over a predetermined period up to the present time). When the purge correction coefficient KP (k−MA) = 1 and the elapsed time TIME2 (see step 1442 in FIG. 14) is equal to or longer than the predetermined period α2 (in FIG. 10, after time t6). It is established by.

  Assuming that the KF value storage condition is satisfied, the CPU 71 determines “Yes” in step 1630 and proceeds to step 1635 to select from the latest value of the in-cylinder intake air amount Mc (k). The value of KFmem (m) (m: 1,2,3,4) is updated to the value of the basic fuel injection amount correction coefficient KF (k) obtained in the previous step 1625, and the updated KFmem (m ) Is stored in the corresponding memory of the backup RAM 74, and the process proceeds to step 1640. On the other hand, if the CPU 71 determines “No” in step 1630, it proceeds to step 1640 immediately without executing the processing of step 1635.

  When the CPU 71 proceeds to step 1640, the CPU 71 updates the value of KF (k−1) to the value of KF (k) obtained in the previous step 1625 during the current execution of this routine, and proceeds to step 1695. The routine is temporarily terminated.

  Thereby, when the main feedback condition is satisfied, the calculation (update) of the basic fuel injection amount correction coefficient KF (k) is executed every elapse of a predetermined calculation cycle Δt0. Then, in step 1130 of the routine shown in FIG. 11, the latest value of the basic fuel injection amount correction coefficient KF (k) calculated (updated) as described above is used, so that the basic fuel injection amount before correction is used. Fbaseb (k) is corrected. In addition, when the KF value storage condition is satisfied, the storage process of the value of the basic fuel injection amount correction coefficient KF (k) to the backup RAM 74 is executed.

  As described above, according to the first embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the command fuel injection is performed on the assumption that the in-cylinder intake air amount sucked into the combustion chamber is constant. The product of the amount Fi (k−MB) and the air / fuel ratio abyfs (k) detected by the upstream air / fuel ratio sensor 66 is the amount of fuel to be instructed for injection in order to set the actual air / fuel ratio to the target air / fuel ratio abyfr (k). The target basic fuel injection amount Fbaset (= (abyfs (k) / abyfr (k)) · Fi (k−MB) is equal to the product of the target basic fuel injection amount Fbaset and the target air-fuel ratio abyfr (k). )) Is required. A value KFb (= Fbaset / Fbaseb (k)) obtained by dividing the target basic fuel injection amount Fbaset by the uncorrected basic fuel injection amount Fbaseb (k) is low-pass filtered by a low-pass filter A18d having a time constant τ which is a digital filter. A basic fuel injection amount correction coefficient KF (k) is obtained (see FIG. 8).

  The base fuel injection amount Fbaseb (k) before correction is corrected by multiplying the base fuel injection amount Fbaseb (k) before correction by the base fuel injection amount correction coefficient KF (k) (that is, the base fuel injection after correction is corrected). The injection amount Fbase is determined). Accordingly, the corrected basic fuel injection amount Fbase is determined so as to coincide with (approach) the amount of fuel to be instructed to make the air-fuel ratio the target air-fuel ratio abyfr. Apart from feedback control, errors in the basic fuel injection amount are quickly compensated.

  In addition, the dull control constant n proportional to the time constant τ of the low-pass filter A18d is normally “a value τ1 that is as large as possible within a range in which the time constant τ does not impede rapidity in compensating for the error in the basic fuel injection amount. Is set to the value n0. On the other hand, the dull control constant n is a period during which a large disturbance in the air-fuel ratio can occur. The predetermined period α1 from the time when the transportation delay time L1 has elapsed from the time when the purge is started, and the time when the purge is stopped From the point of time when the transport delay time L2 elapses, the value is made smaller than the value n0 for a predetermined period α2, thereby making the time constant τ smaller than the value τ1 (see FIG. 10). As a result, normally, when the basic fuel injection amount is stably corrected by the basic fuel injection amount correction coefficient KF (k), a large turbulence in the air-fuel ratio can occur as the purge starts and stops. As a result, it is difficult for the rapid compensation for the sudden change in the air-fuel ratio by the main feedback control to be hindered, and it is possible to suppress the temporary increase in the emission amount.

(Second Embodiment)
Next, an air-fuel ratio control apparatus according to a second embodiment of the present invention will be described. In the second embodiment, the control constant n is blunted for a predetermined period after the time when there is a change in the operating state of the engine 10 that causes the actual air-fuel ratio to deviate from the target air-fuel ratio abyfr. Although it is the same as the first embodiment in that the time constant τ of the low-pass filter A18d is reduced, the time when the intake air flow rate Ga suddenly changes during the purge is used as the time when the operating state of the engine 10 has changed. This is different from the first embodiment.

  As described above, the target purge rate PGR is determined so as to increase as the intake air flow rate increases. Therefore, when the intake air flow rate Ga changes suddenly during the purge, the target purge rate PGR changes in a step shape, and as a result, the purge correction coefficient KP set according to the above equation (12) also changes in a step shape. As a result, the target purge rate in which the actual purge rate of the gas sucked into the combustion chamber 25 changes in a step-like manner due to the influence of the dispersion of the evaporation gas in the intake passage, etc., just after the start / stop of the purge described above. Follow rate PGR with some delay.

  Therefore, the purge correction coefficient KP includes an error due to the dispersion of the evaporation gas in the intake passage within a short period after the sudden change in the intake air flow rate Ga. This is a large disturbance in the air-fuel ratio control system. It becomes. Hereinafter, this will be described with reference to FIG. 17 corresponding to FIG.

  FIG. 17 shows that the purge air is being executed and the intake air flow rate Ga maintained at the value Ga0 before the time t11 is changed from the value Ga0 to the value Ga1 (> Ga0) at the time t11. FIG. 6 is a time chart showing an example of changes in various variables and the like when rapidly increasing to a value Ga1 thereafter.

  In this example, before the time t11 when the intake air flow rate Ga suddenly increases, the air-fuel ratio A / F is maintained at the target air-fuel ratio abyfr as shown in (G), and the basic fuel is shown in (F). The injection amount correction coefficient KF is maintained at the value KF2. The deviation amount of the value KF2 with respect to “1” corresponds to the error amount of the basic fuel injection amount. In this example, it is assumed that the evaporative gas concentration learning value does not include an error.

  In this case, as shown in (C), the purge correction coefficient KP (k) set in accordance with the above equation (12) using the target purge rate PGR determined based on the intake air flow rate Ga is equal to or before time t11. Is maintained at the value KP2 corresponding to the value Ga0, and is maintained at the value KP3 corresponding to the value Ga1 after the time t11.

  Accordingly, as indicated by a solid line in (D), the purge correction coefficient KP (k−MA) used for calculation of the pre-correction basic fuel injection amount Fbaseb (and hence the calculation of the command fuel injection amount Fi) is the intake air. At time t12 when the transport delay time L3 corresponding to the MA stroke elapses from time t11 when the flow rate Ga suddenly increases, the value decreases from the value KP2 to the value KP3 stepwise.

  On the other hand, the alternate long and short dash line in (D) represents the change in the necessary purge correction coefficient, as in FIG. This necessary purge correction coefficient has a certain delay with respect to the purge correction coefficient KP (k−MA) decreased in a step-like manner due to the above-described dispersion of the evaporation gas in the intake passage in a short period after time t12. Follow. In this example, since the evaporative gas concentration learning value does not include an error, the required purge correction coefficient converges to the purge correction coefficient KP (k−MA) value itself (= KP3).

  As a result, for a short period immediately after time t12, the purge correction coefficient KP (k−MA) is smaller than the necessary purge correction coefficient, and the purge correction coefficient KP tends to make the air-fuel ratio leaner than the target air-fuel ratio abyfr. An error occurs. This is a large disturbance in the air-fuel ratio control system.

  As a result, as shown in (G), the air-fuel ratio A / F maintained at the target air-fuel ratio abyfr until time t12 starts shifting in the lean direction immediately after time t12. As a result, as shown in (F), the basic fuel injection amount correction coefficient KF, which has been maintained at the value KF2 until time t12, starts to change in a direction immediately after time t12.

  In this case, the dull control constant setting means A19 of the second embodiment dulls the control constant n from the value n0 to the value n3 (<n0) from the time t12 for a predetermined period α3 as shown by a solid line in FIG. Switch to. As a result, the basic fuel injection amount correction coefficient KF and the air-fuel ratio A / F change as indicated by solid lines in (F) and (G).

  As a result, immediately after time t2 in FIG. 10 described above, as compared to “when the dull control constant n is maintained at the value n0” indicated by broken lines in (E), (F), and (G). For the same reason as described above, it is possible to reduce the extent to which the air-fuel ratio A / F temporarily shifts greatly in the lean direction immediately after time t12.

  Thus, the dull control constant setting means A19 of the second embodiment is performing the purge for a predetermined period α3 from the time when the transportation delay time L3 has elapsed from the time when the intake air flow rate Ga suddenly changes. The time constant τ of the low-pass filter A18d is made smaller than the value τ1 by making the control constant n smaller than the value n0.

  The predetermined period α3 is set to a time required until the air-fuel ratio A / F converges to the target air-fuel ratio abyfr after the disturbance of the air-fuel ratio A / F occurs, similar to the predetermined periods α1 and α2. In this example, the dull control constant n is set to three times the time constant τ of the low-pass filter A18d when the value is n3.

(Actual operation of the second embodiment)
Next, actual operation of the air-fuel ratio control apparatus according to the second embodiment will be described. The CPU 71 of the second embodiment executes routines other than the routine shown in FIG. 14 among the routines shown in FIGS. 11 to 16 executed by the CPU 71 of the first embodiment. Instead, the routine shown in the flowchart of FIG. 18 is executed. Hereinafter, the routine shown in FIG. 18 unique to the second embodiment will be described.

  The CPU 71 repeatedly executes the “set blunt control constant n” routine shown in FIG. 18 every time the fuel injection start time (injection instruction start time) arrives for the fuel injection cylinder. Accordingly, when the fuel injection start timing comes for the fuel injection cylinder, the CPU 71 starts the processing of the routine 1800, and first, at step 1805, it is determined whether or not the main feedback condition is satisfied. This main feedback condition is the same as the condition of step 1402 in FIG.

  The description will be continued assuming that the main feedback condition is satisfied, purge is being executed, and the intake air flow rate Ga does not change abruptly (see before time t11 in FIG. 17). XAT shown in the steps described later is an air transient determination flag that is changed from “0” to “1” when it is determined that the intake air flow rate Ga has suddenly changed.

  In this case, the CPU 71 determines “Yes” in step 1805 and proceeds to step 1810 to determine whether or not the air transient determination flag XAT is “1”. At present, the intake air flow rate Ga does not change suddenly, and the air transient determination flag XAT is maintained at “0”. Accordingly, the CPU 71 makes a “No” determination at step 1810 to proceed to step 1815, and from the intake air flow rate Ga (k) measured by the air flow meter 61 at the present time, the intake air measured at the previous execution of this routine. An intake air flow rate deviation ΔGa is obtained by reducing the flow rate Ga (k−1).

  Next, the CPU 71 proceeds to step 1820 to determine whether or not the purge execution flag XPG is “1”. If “No” is determined, the CPU 71 proceeds to step 1825 to slow down the control constant n to the value n0. Then, the routine proceeds to step 1895 to end the present routine tentatively. At this time, since the value of the purge execution flag XPG is “1”, the CPU 71 determines “Yes” in step 1820 and proceeds to step 1830, where the absolute value of the intake air flow rate deviation ΔGa is larger than the predetermined value β. It is determined whether or not the intake air flow rate Ga has changed suddenly. At present, the intake air flow rate Ga does not change suddenly, and the absolute value of the intake air flow rate deviation ΔGa is equal to or less than the predetermined value β. Therefore, the CPU 71 makes a “No” determination at step 1830 to proceed to step 1825 described above. Thereafter, as long as the condition of step 1830 is not satisfied, the CPU 71 repeatedly executes the processing of steps 1805, 1810-1820, 1830, 1825. As a result, the dull control constant n is maintained at the value n0 (see time t11 and earlier in FIG. 17).

  Next, a case where the intake air flow rate Ga changes suddenly (that is, when the absolute value of the intake air flow rate deviation ΔGa becomes larger than the predetermined value β will be described with reference to time t11 in FIG. 17). In this case, when the CPU 71 proceeds to step 1830, the CPU 71 determines “Yes”, proceeds to step 1835, changes the value of the air transient determination flag XAT from “0” to “1”, and performs subsequent processing after step 1840. Execute. Here, the processing of step 1840 to step 1880 respectively corresponds to step 1408 to step 1424 of FIG. 14 described above, and detailed description thereof will be omitted. Thereafter, since the value of the air transient determination flag XAT is “1”, the CPU 71 determines “Yes” every time it proceeds to Step 1810, and immediately executes Step 1840 and subsequent steps without executing the processing of Step 1815 to Step 1835. The process will be executed.

  As a result, as in the case of FIG. 14 described above, the blunting control constant n is maintained at the value n0 until the counter z reaches the stroke number M3 corresponding to the transport delay time L3 (see FIG. 17). (Refer to times t11 to t12 in FIG. 17), the value n3 is maintained from the time when the counter z reaches the stroke number M3 until the predetermined period α3 elapses (see times t12 to t13 in FIG. 17). After the elapse of the predetermined period α3, the value is returned to the value n0 (see after the time t13 in FIG. 17). When the predetermined period α3 has elapsed from the time when the counter z reaches the number of strokes M3, the CPU 71 makes a “No” determination at step 1880 and proceeds to step 1885 before proceeding to step 1860 to determine air transient. The value of the flag XAT is changed from “1” to “0”.

  On the other hand, if the main feedback condition is not satisfied at the time of determination in step 1805, the CPU 71 determines “No” in step 1805 and immediately proceeds to step 1895 to end this routine once. Thus, when the main feedback condition is not satisfied, the value of the dull control constant n is maintained at the value set at the previous execution of this routine. The dull control constant n set in this way is used for the calculation of the basic fuel injection amount correction coefficient KF shown in FIG.

  As described above, according to the second embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the blunt control constant n proportional to the time constant τ of the low-pass filter A18d causes a large disturbance in the air-fuel ratio. The time constant τ is made smaller than the value n0 by a predetermined period α3 from the time when the transportation delay time L3 has elapsed from the time when the intake air flow rate Ga suddenly changes during the purge, which is a possible period. (See FIG. 17). As a result, normally, the stable basic fuel injection amount correction by the basic fuel injection amount correction coefficient KF (k) is realized, and the air-fuel ratio is greatly disturbed due to the sudden change in the intake air flow rate Ga during the purge. If it can occur, the rapid compensation for the sudden change in the air-fuel ratio by the main feedback control becomes difficult to be inhibited, and the temporary increase in the emission amount can be suppressed.

(Third embodiment)
Next, an air-fuel ratio control apparatus according to a third embodiment of the present invention will be described. As shown in FIG. 19 which is the functional block diagram of the third embodiment, the calculation cycle is replaced with the dull control constant setting means A19 with respect to the first embodiment whose functional block diagram is shown in FIG. The difference is that the setting means A20 is used.

  More specifically, as described above, the time constant τ of the low-pass filter A18d is the period during which the low-pass filter processing is performed on the dull control constant n and the basic fuel injection amount correction coefficient KF by the low-pass filter A18d ( That is, it is proportional to the product of the execution cycle of the “calculation of basic fuel injection amount correction coefficient KF” routine and the calculation cycle Δt).

  In the first embodiment, the execution cycle of the “calculation of basic fuel injection amount correction coefficient KF” routine shown in FIG. 16, which is the calculation cycle Δt, is maintained at the calculation cycle Δt0, and the value of the dull control constant n is set to the value n0. The time constant τ of the low-pass filter A18d is changed from the value τ1.

  In contrast, in the third embodiment, the value of the dull control constant n is maintained at the value n0, and the execution cycle of the “calculation of basic fuel injection amount correction coefficient KF” routine, which is the calculation cycle Δt, is calculated from the calculation cycle Δt0. By changing, the time constant τ of the low-pass filter A18d is changed from the value τ1.

  Based on this difference, the CPU 71 of the third embodiment executes the routines shown in FIGS. 11 to 13 and FIG. 15 among the routines shown in FIGS. 11 to 16 executed by the CPU 71 of the first embodiment. At the same time, the routines shown in the flowcharts of FIGS. 20 and 21 corresponding to the routines of FIGS. 14 and 16 are executed instead of the routines of FIGS. 20 and 21, the same steps as those shown in FIGS. 14 and 16 are denoted by the same step numbers as those in FIGS. 14 and 16. Hereinafter, the routines shown in FIGS. 20 and 21 unique to the third embodiment will be described in order.

  The “setting calculation period Δt” routine shown in FIG. 20 is a routine for setting / changing the calculation period Δt, which is the execution period of the “calculation of basic fuel injection amount correction coefficient KF” routine shown in FIG. is there. In the routine shown in FIG. 20, the calculation cycle Δt is set to the calculation cycle Δt0 in steps 2016 and 2036, the calculation cycle Δt is set to the calculation cycle Δt1 (<Δt0) in step 2020, and step 2040. 14 is the same as the routine shown in FIG. 14 except that the calculation cycle Δt is set to the calculation cycle Δt2 (= Δt1). Therefore, detailed description thereof is omitted here.

  The “calculation of basic fuel injection amount correction coefficient KF” routine shown in FIG. 21 is repeatedly executed at every elapse of the calculation period Δt set in any of the steps 2016, 2020, 2036, and 2040 in FIG. Is done. This routine is the same as the routine shown in FIG. 16 except that in step 2125, the value n0 (constant) is used as the dull control constant n used in the equation (18). Then, the detailed explanation is omitted.

  As described above, according to the third embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the calculation cycle Δt proportional to the time constant τ of the low-pass filter A18d is normally “the time constant τ is the basic fuel. The calculation period Δt0 is set to the largest possible value τ1 within a range that does not impair the rapidity in compensating the injection amount error. On the other hand, the calculation cycle Δt is a period during which large disturbances in the air-fuel ratio can occur, and the transportation period from the point in time when the transportation delay time L1 elapses from the time when the purge is started and the time point when the purge is stopped. Only the predetermined period α2 from the time when the delay time L2 has elapsed is made shorter than the calculation cycle Δt0, thereby making the time constant τ smaller than the value τ1 (see FIG. 10). As a result, the same effect as the first embodiment is obtained.

(Fourth embodiment)
Next, an air-fuel ratio control apparatus according to a fourth embodiment of the present invention will be described. In the fourth embodiment, as in the third embodiment, instead of changing the value of the dull control constant n from the value n0, the “calculation of the basic fuel injection amount correction coefficient KF” routine that is the calculation period Δt is executed. Is different from the second embodiment in that the time constant τ of the low-pass filter A18d is changed from the value τ1 by changing the execution period of? From the calculation period? T0.

  Based on this difference, the CPU 71 of the fourth embodiment is the same as the routines shown in FIGS. 11 to 13, FIG. 15, FIG. 16, and FIG. 18 executed by the CPU 71 of the second embodiment. 15 and the routine shown in FIG. 15 are executed, the routine shown in FIG. 21 executed by the CPU 71 of the third embodiment is executed instead of the routine shown in FIG. 16, and the routine shown in FIG. The routine shown by the flowchart in FIG. 22 is executed. In FIG. 22, steps that are the same as the steps shown in FIG. 18 are given the same step numbers as the step numbers in FIG. Hereinafter, the routine shown in FIG. 22 unique to the fourth embodiment will be described.

  The “setting calculation period Δt” routine shown in FIG. 22 is a routine for setting / changing the calculation period Δt, which is the execution period of the “calculation of basic fuel injection amount correction coefficient KF” routine shown in FIG. The routine shown in FIG. 22 is the same except that the calculation cycle Δt is set to the calculation cycle Δt0 in steps 2225 and 2260, and the calculation cycle Δt is set to the calculation cycle Δt3 (<Δt0) in step 2270. Since it is the same as the routine shown in FIG. 18, detailed description thereof is omitted here.

  As described above, according to the fourth embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, the calculation cycle Δt proportional to the time constant τ of the low-pass filter A18d is normally the same as in the third embodiment. The calculation cycle Δt0 is set. On the other hand, the calculation period Δt is a period during which a large disturbance in the air-fuel ratio can occur. The calculation period Δt0 is only a predetermined period α3 from the time when the transportation delay time L3 has elapsed from the time when the intake air flow rate Ga suddenly changes during the purge. Thus, the time constant τ is made smaller than the value τ1 (see FIG. 17). As a result, the same effects as those of the second embodiment are obtained.

  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 the first embodiment, the values n1 and n2 used as the dull control constant n are equal in steps 1420 and 1440 in FIG. 14, but the values n1 and n2 are made different. May be. Similarly, in the third embodiment, in steps 2020 and 2040 in FIG. 20, the calculation period Δt1 used as the calculation period Δt and the calculation period Δt2 are set to the same value. However, the calculation period Δt1 and the calculation period Δt2 are You may comprise so that it may differ.

  In the first and third embodiments, the predetermined period α1, which is a period during which the time constant τ of the low-pass filter A18d is reduced from the value τ1, is constant, but the purge correction coefficient KP (k) at the start of the purge. The predetermined period α1 may be changed according to the fluctuation range. Similarly, in the first and third embodiments, the predetermined period α2 may be changed according to the fluctuation range of the purge correction coefficient KP (k) at the purge stop time. Furthermore, in the second and fourth embodiments, the predetermined period α3 may be changed according to the fluctuation range of the purge correction coefficient KP (k) at the time when the intake air flow rate Ga suddenly changes.

  Further, in each of the above embodiments, the low-pass filter A18d is operated for a predetermined period from the time when the purge is started / stopped, or when the transport delay time LA elapses from the time when the intake air flow rate Ga suddenly changes during the purge. Although the time constant τ is configured to be smaller than the value τ1, the time constant of the low-pass filter A18d is set for a predetermined period from the time when the purge is started / stopped and the time when the intake air flow rate Ga suddenly changes during the purge. You may comprise so that (tau) may be made small from value (tau) 1.

  In each of the above embodiments, the stroke number MA corresponding to the transport delay time LA is obtained based on the in-cylinder intake air amount Mc, and the passage of the transport delay time LA is determined by the number of strokes reaching MA times. However, the transport delay time LA itself may be obtained based on the operating state of the engine 10 (for example, the in-cylinder intake air amount Mc and the operating speed NE).

  In the second and fourth embodiments, the predetermined value in the previous period compared with the intake air flow rate deviation ΔGa when determining whether or not the intake air flow rate Ga has suddenly changed in step 1830 of FIG. 18 (FIG. 22). Although β is a constant value, the predetermined value β may be changed according to the operating state of the engine 10 (for example, the evaporative gas concentration learning value).

  Further, in each of the above embodiments, the value MA (a value corresponding to the transport delay time LA) for the purge correction coefficient KP (k−MA) before the MA stroke from the present time used for calculating the basic fuel injection amount Fbaseb before correction. ) Is obtained based on the table MapMA (Mc (k)) shown in FIG. 6, but the value MA may be a predetermined constant value.

  Further, in each of the above-described embodiments, the value MB (the delay time LB) of the command fuel injection amount Fi (k−MB) before the MB stroke from the current time point used when obtaining the basic fuel injection amount correction coefficient KF. (Value) is calculated based on the table MapMB (Mc (k), NE) shown in FIG. 7, but the value MB may be set to a predetermined constant value.

  Further, in each of the above embodiments, as shown in FIG. 8 (routines of FIGS. 16 and 21), the low-pass filter A18d provided in the basic fuel injection amount correction coefficient setting means A18 detects the detected air-fuel ratio abyfs (k ), The command fuel injection amount Fi (k−MB), and the pre-correction basic fuel injection amount Fbaseb (k) itself, the basic fuel injection amount correction coefficient KFb (= (abyfs (k) · Fi ( k−MB)) / (abyfr (k) · Fbaseb (k))), and the basic fuel injection amount correction coefficient KFb before the low-pass filter processing is low-pass filtered by the low-pass filter A18d, thereby obtaining the basic fuel injection amount correction coefficient. Although KF is obtained, instead of this, the detected air-fuel ratio abyfs (k), the command fuel injection amount Fi (k-MB), and the uncorrected basic fuel injection amount Fbaseb (k) are respectively reduced by the low-pass filter A18d. Each value after low pass filter processing separately It may be used to obtain the basic fuel injection amount correction coefficient KF.

  In each of the above embodiments, as shown in FIG. 8, the current detected air-fuel ratio abyfs (k), the commanded fuel injection amount Fi (k-MB) before the MB stroke from the present time, the current target air-fuel ratio abyfr ( k), and the basic fuel injection amount correction coefficient KF is obtained based on the current pre-correction basic fuel injection amount Fbaseb (k). The detected air-fuel ratio abyfs (k) this time, the command fuel before the MB stroke from the present time Basic fuel injection based on the injection amount Fi (k-MB), the target air-fuel ratio abyfr (k-MB) before the MB stroke from the current time, and the basic fuel injection amount Fbaseb (k-MB) before the MB stroke from the current time before the correction You may comprise so that the quantity correction coefficient KF may be calculated | required.

  In the above embodiment, in the main feedback control, a value obtained by subtracting the target air-fuel ratio abyfr (k−MB) before the MB stroke from the current time from the current air-fuel ratio abyfs (k) detected by the upstream air-fuel ratio sensor 66. The main feedback correction amount DFi_main is obtained based on this, but the value obtained by dividing the in-cylinder intake air amount Mc (k−MB) before the MB stroke from the current time by the target air-fuel ratio abyfr (k−MB) before the MB stroke from the current time. From the target in-cylinder fuel supply amount Fcr (k−MB) before the MB stroke from the present time, the in-cylinder intake air amount Mc (k−MB) before the MB stroke from the present time is detected as the current detected air-fuel ratio abyfs (k) The main feedback correction amount DFi_main is obtained based on the in-cylinder fuel supply amount deviation DFc, which is a value obtained by subtracting the actual in-cylinder fuel supply amount Fc (k−MB) before the MB stroke from the present time, which is a value divided by. Configure Good.

  In each of the above embodiments, the evaporated fuel processing mechanism for purging is provided. However, the present invention is also applied to an internal combustion engine that is not provided with the evaporated fuel processing mechanism. In this case, for example, the time constant τ of the low-pass filter A18d is decreased from the value τ1 over a predetermined period from the time when it is determined that the intake air flow rate Ga has suddenly changed (for example, when the condition of Step 1830 in FIG. 18 is satisfied). You may comprise.

  As a result, even if a large disturbance occurs in the air-fuel ratio control system based on the intake air flow rate Ga measured by the air flow meter 61 following the true intake air flow rate that changes stepwise with a response delay. , Temporary increase in emission emissions can be suppressed.

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. 3 is a graph showing a table that defines the relationship between the in-cylinder intake air amount and the number of strokes corresponding to the transport delay time, which is referred to by the CPU shown in FIG. 1. 3 is a graph showing a table that defines a relationship between an operation speed and an in-cylinder intake air amount, and a stroke number corresponding to a delay time, which is referred to by the CPU shown in FIG. 1. FIG. 6 is a functional block diagram when a basic fuel injection amount correction coefficient setting unit shown in FIG. 5 sets a basic fuel injection amount correction coefficient. It is the figure which showed a mode that the calculated basic fuel injection amount correction coefficient was classified according to the cylinder intake air amount, and was memorize | stored in the memory of backup RAM. 2 is a time chart showing an example of changes in various variables when the air-fuel ratio control device shown in FIG. 1 executes air-fuel ratio feedback control. FIG. 3 is a flowchart showing a routine for calculating a command fuel injection amount executed by a CPU shown in FIG. 1 and performing an injection instruction. FIG. 3 is a flowchart showing a routine for calculating a main feedback correction amount executed by a CPU shown in FIG. 1. 4 is a flowchart showing a routine for calculating a sub feedback correction amount executed by a CPU shown in FIG. 1. It is the flowchart which showed the routine for setting the blunting control constant which CPU shown in FIG. 1 performs. 2 is a flowchart showing a routine for calculating a purge correction coefficient executed by a CPU shown in FIG. 1. 2 is a flowchart showing a routine for calculating a basic fuel injection amount correction coefficient executed by a CPU shown in FIG. 1. It is the time chart which showed an example of change of various variables etc. when the air fuel ratio control device concerning a 2nd embodiment of the present invention performs air fuel ratio feedback control. It is the flowchart which showed the routine for setting the blunt control constant which CPU of the air fuel ratio control device which relates to 2nd execution form of this invention executes. 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 flowchart which showed the routine for setting the calculation period which CPU of the air fuel ratio control device which relates to 3rd execution form of this invention executes. It is the flowchart which showed the routine for calculating the basic fuel injection quantity correction coefficient which CPU of the air fuel ratio control device which relates to 3rd execution form of this invention executes. It is the flowchart which showed the routine for setting the calculation period which CPU of the air fuel ratio control device which relates to 4th execution form of this invention executes.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 25 ... Combustion chamber, 39 ... Injector, 48 ... Purge flow path, 49 ... Purge control valve (VSV), 51 ... Exhaust manifold, 53 ... Three-way catalyst (first catalyst), 66 ... Empty upstream Fuel ratio sensor, 67 ... downstream air-fuel ratio sensor, 70 ... electric control device, 71 ... CPU, 74 ... backup RAM

Claims (8)

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;
Fuel injection means for injecting fuel in response to an instruction;
Applied to an internal combustion engine with
Basic fuel injection amount acquisition means for acquiring a basic fuel injection amount that is an amount of fuel for obtaining a target air-fuel ratio based on an operating state of the internal combustion engine;
Feedback correction amount calculation means for calculating a feedback correction amount based on a value based on the output value of the upstream air-fuel ratio sensor and subjected to high-pass filter processing;
The amount of fuel actually injected by the fuel injection means when receiving an instruction to inject fuel of the basic fuel injection amount is set so that the actual air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is the target air-fuel ratio. A basic fuel injection amount correcting means for correcting the basic fuel injection amount with a value for correcting the basic fuel injection amount and a value subjected to low-pass filtering,
Command fuel injection amount calculating means for calculating a command fuel injection amount by correcting the corrected basic fuel injection amount with the feedback correction amount;
An air-fuel ratio control means for performing feedback control of an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine by instructing the fuel injection means to inject fuel of the command fuel injection amount;
In an air-fuel ratio control apparatus for an internal combustion engine comprising:
The basic fuel injection amount correction means includes
When the low-pass filter process is performed over a predetermined period after a change in the operating state of the internal combustion engine that causes a deviation of the actual air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine from the target air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine configured to reduce a constant. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The basic fuel injection amount correcting means is a value for correcting the basic fuel injection amount,
An air-fuel ratio of the internal combustion engine configured to use parameter values calculated based on the output value of the upstream air-fuel ratio sensor, the target air-fuel ratio, the command fuel injection amount, and the basic fuel injection amount Control device. An air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2,
The internal combustion engine
A fuel gas introduction path for introducing a fuel gas based on the fuel evaporated in the fuel tank into the intake passage of the internal combustion engine;
A purge control valve that controls an introduction state of the fuel gas introduced into the intake passage based on an operation state of the internal combustion engine;
The basic fuel injection amount correction means, as a time when the operating state of the internal combustion engine has changed,
An air-fuel ratio control apparatus for an internal combustion engine configured to use a time point when an introduction state of fuel gas introduced into the intake passage is changed by control of the purge control valve. The air-fuel ratio control apparatus for an internal combustion engine according to claim 3,
The basic fuel injection amount correction means includes
As the time when the introduction state of the fuel gas introduced into the intake passage is changed by the control of the purge control valve,
An air-fuel ratio control apparatus for an internal combustion engine configured to use a time point at which introduction of the fuel gas is started and / or stopped by control of the purge control valve. In the air-fuel ratio control device for an internal combustion engine according to claim 3 or 4,
The basic fuel injection amount correction means includes
As the time when the introduction state of the fuel gas introduced into the intake passage is changed by the control of the purge control valve,
An air-fuel ratio control apparatus for an internal combustion engine configured to use a time point when an air flow rate in the intake passage changes during a period in which the introduction of the fuel gas is continued. An air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 3 to 5,
Further comprising transport delay time acquisition means for acquiring a transport delay time which is a time until the fuel gas reaches the combustion chamber of the internal combustion engine from the purge control valve;
The basic fuel injection amount correction means includes
Air-fuel ratio control of an internal combustion engine configured to start the predetermined period from the time when the transportation delay time has elapsed from the time when the introduction state of the fuel gas introduced into the intake passage is changed by the control of the purge control valve apparatus. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 6,
The basic fuel injection amount correction means uses a digital filter as a low-pass filter for performing the low-pass filter process, and reduces a time constant of the low-pass filter process by changing a value related to responsiveness used in the low-pass filter process. An air-fuel ratio control apparatus for an internal combustion engine configured to do so. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 6,
The basic fuel injection amount correcting means uses a digital filter as a low-pass filter for performing the low-pass filter processing, and shortens a cycle for performing the low-pass filter processing on a value for correcting the basic fuel injection amount. An air-fuel ratio control apparatus for an internal combustion engine configured to reduce a time constant of the low-pass filter process.

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