JP2015206273A - Internal combustion engine air-fuel ratio control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an internal combustion engine air-fuel ratio control system that can suppress deterioration in drivability and exhaust gas quality.SOLUTION: The internal combustion engine air-fuel ratio control system reduces an increment of a feedback control constant at a timing that an output value of an upstream-side O2 sensor 110 provided upstream of a catalyst 109 cleaning an exhaust gas to detect an air-fuel ratio in an upstream-side exhaust gas becomes a predetermined voltage or more and reduces a decrement of the feedback control constant at a timing that the output value of the upstream-side O2 sensor 110 becomes a predetermined voltage or lower.

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an air-fuel ratio control apparatus for an internal combustion engine provided with air-fuel ratio feedback control means for periodically oscillating an air-fuel ratio supplied to the internal combustion engine in a rich direction and a lean direction. It is about.

  Generally, a three-way catalyst (hereinafter referred to as a catalyst) that simultaneously purifies harmful components HC, CO, and NOx in exhaust gas is installed in an exhaust passage of an internal combustion engine. In this type of catalyst, the purification rate is high for any of the harmful components HC, CO, and NOx in the vicinity of the theoretical air-fuel ratio. Therefore, in an internal combustion engine control device, an O2 sensor is usually provided on the upstream side of the catalyst, the fuel injection amount is adjusted so that the air-fuel ratio is close to the theoretical air-fuel ratio, and the air-fuel ratio is feedback-controlled.

  Further, the catalyst is added with an oxygen storage capability that acts like a primary filter, and absorbs a temporary fluctuation of the upstream air-fuel ratio (corresponding to the output value of the upstream O2 sensor) from the stoichiometric air-fuel ratio. It is like that. That is, when the upstream air-fuel ratio (hereinafter referred to as upstream A / F) is leaner than the stoichiometric air-fuel ratio, the catalyst takes in and accumulates oxygen in the exhaust gas, and conversely, Releases the oxygen accumulated in the catalyst. As a result, the temporary fluctuation of the upstream A / F is filtered in the catalyst and hardly appears in the air-fuel ratio downstream of the catalyst (hereinafter referred to as the downstream A / F).

  Further, the maximum value of the oxygen storage amount of the catalyst is determined by the amount of the substance having the catalyst storage capacity attached at the time of production of the catalyst, and the oxygen storage amount is the maximum oxygen storage amount or the minimum oxygen storage amount (= 0) of the catalyst. When the value reaches, the fluctuation in the upstream side A / F can no longer be absorbed, so the air-fuel ratio in the catalyst deviates from the stoichiometric air-fuel ratio, and the purification capacity of the catalyst decreases. At this time, since the air-fuel ratio on the downstream side of the catalyst greatly deviates from the stoichiometric air-fuel ratio, it can be detected that the oxygen storage amount of the catalyst has reached the maximum value or the minimum value (= 0).

  In addition, the catalyst is exposed to high-temperature exhaust gas, but it is designed so that the purification function of the catalyst does not rapidly deteriorate under use conditions normally considered in an internal combustion engine for a vehicle. However, the oxygen storage capacity of the catalyst may be significantly reduced due to some cause during use (for example, in-catalyst combustion that occurs when misfire occurs). When it reaches kilos, the oxygen storage capacity gradually decreases due to aging.

  Further, as is apparent from the timing charts of FIGS. 30 and 31 showing the behaviors at the time of normal catalyst and deteriorated catalyst, when the catalyst deteriorates and the oxygen storage amount OSC decreases, the output of the upstream O2 sensor. It is known that fluctuations in the value V1 (upstream A / F) cannot be absorbed by the catalyst, and fluctuations in the output value V2 (downstream A / F) of the downstream O2 sensor increase. FIG. 30 shows each behavior at the time of a normal catalyst, FIG. 31 shows each behavior at the time of a deteriorated catalyst, in which (a) is an upstream A / F and (b) is an output of an upstream O2 sensor. Values V1 and (c) indicate the estimated oxygen storage amount, and (d) indicates the behavior of the output value V2 of the downstream O2 sensor.

  Therefore, conventionally, an air-fuel ratio control apparatus for an internal combustion engine that diagnoses catalyst deterioration by comparing the fluctuation of the output value V1 of the upstream O2 sensor and the fluctuation of the output value V2 of the downstream O2 sensor has been proposed. (For example, refer to Patent Document 1).

  Further, in recent years, exhaust gas regulations have been strengthened from the improvement of the awareness of the global environment, and it has been demanded to detect more minor catalyst deterioration (decrease in maximum oxygen storage amount). Moreover, since the heat resistance of substances having oxygen storage capacity is improving year by year, the amount added to the catalyst can be increased, and the maximum oxygen storage amount of the catalyst that needs to be detected for deterioration has increased. Yes.

  Therefore, as shown in the timing chart of FIG. 32, the vibration width of the oxygen storage amount of the catalyst is increased by increasing the period and the vibration width of the air-fuel ratio vibration in the rich direction and the lean direction of the output value V1 of the upstream O2 sensor. An air-fuel ratio control device for an internal combustion engine that detects a slight deterioration of the catalyst by increasing the engine is also proposed. In FIG. 32, (a) is the upstream A / F, (b) is the output value V1 of the upstream O2 sensor, (c) is the estimated oxygen storage amount, and (d) is the output value V2 of the downstream O2 sensor. The behavior is shown. (For example, refer to Patent Document 2 and Patent Document 3).

  Further, in the air-fuel ratio control device for an internal combustion engine disclosed in Patent Documents 1 to 3, it is necessary to greatly change the fluctuation range or cycle of the air-fuel ratio oscillation in accordance with the change in the maximum oxygen storage amount. There are problems that various performances such as air-fuel ratio controllability, exhaust gas performance, air-fuel ratio feedback performance, and torque fluctuation increase are deteriorated. As a means for solving the problems, an average air-fuel ratio oscillation means is provided. In order to oscillate the average air-fuel ratio of the air-fuel ratio periodically oscillated by the air-fuel ratio feedback control means in the rich direction and the lean direction in a predetermined cycle, the predetermined rich period of the average air-fuel ratio and the predetermined lean period are alternately The setting means and the average air-fuel ratio are ripened so as to have a predetermined vibration width in the rich direction during the rich period and a predetermined vibration width in the lean direction during the lean period. A means for setting the feedback control constant corresponding to the period and the lean period is provided, and the vibration width of the oxygen storage amount is changed without changing the setting of the period or the vibration width of the air-fuel ratio vibration that emphasizes air-fuel ratio feedback performance and torque fluctuation. An air-fuel ratio control apparatus for an internal combustion engine that can be freely changed to adapt to catalyst deterioration has been proposed (see, for example, Patent Document 4).

Japanese Patent Publication No. 6-39932 JP-A-6-26330 JP 7-259600 A JP 2008-157133 A

  In a conventional air-fuel ratio control apparatus for an internal combustion engine, for example, as described in Patent Document 4, control is performed with a predetermined vibration width in the rich direction during the rich period, and control is performed with a predetermined vibration width in the lean direction during the lean period. In this case, since the air-fuel ratio correction coefficient is continuously added or subtracted until the predetermined period is reached, the control is performed in a region where the upstream A / F greatly deviates from the theoretical air-fuel ratio, and drivability and exhaust gas deterioration are suppressed. There was a problem that could not be done.

  The present invention has been made in view of the above problems, and an object thereof is to obtain an air-fuel ratio control device for an internal combustion engine that can suppress drivability and deterioration of exhaust gas.

  An air-fuel ratio control apparatus for an internal combustion engine according to the present invention is provided on an upstream side of a catalyst that is installed in an exhaust system of the internal combustion engine and purifies exhaust gas from the internal combustion engine, and detects an air-fuel ratio in upstream exhaust gas Detecting means; operating condition detecting means for detecting operating conditions of the internal combustion engine; and air-fuel ratio supplied to the internal combustion engine in accordance with an output value of the air-fuel ratio detecting means and a predetermined feedback control constant of the operating condition detecting means And an air-fuel ratio feedback control means for periodically oscillating the air-fuel ratio in the rich direction and the lean direction, and the air-fuel ratio feedback control means for the internal combustion engine, When the output value of the air-fuel ratio detecting means becomes equal to or higher than a predetermined voltage, the increase amount of the feedback control constant is reduced, and the output value of the air-fuel ratio detecting means is predetermined. It is intended to reduce the decrease of the feedback control constants at the timing when the voltage less.

  According to the present invention, it is possible to suppress control in a region greatly deviating from the theoretical air-fuel ratio of the upstream side A / F during feedback control, and it is possible to suppress deterioration of drivability and exhaust gas.

1 is a configuration diagram schematically showing an air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 of the present invention. FIG. FIG. 2 is a functional block diagram showing a basic configuration of a control circuit shown in FIG. 1. 3 is a flowchart showing a calculation processing operation by a first air-fuel ratio feedback control means shown in FIG. 3 is a flowchart showing a calculation processing operation by a first air-fuel ratio feedback control means shown in FIG. 3 is a timing chart for supplementarily explaining the operation of the first air-fuel ratio feedback control means shown in FIG. It is explanatory drawing which shows the control area | region of the general target air fuel ratio variably set according to an operating condition. It is a flowchart which shows the arithmetic processing operation | movement of the average air fuel ratio oscillation means shown in FIG. It is a flowchart which shows the arithmetic processing operation | movement of the average air fuel ratio oscillation means shown in FIG. It is explanatory drawing which shows the output characteristic of a downstream O2 sensor at the time of using a general (lambda) type | mold sensor. It is explanatory drawing which shows the hysteresis width of a general rich / lean determination threshold value. It is explanatory drawing which shows the characteristic of the vibration period of the rich direction set according to the intake air amount by the air-fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the characteristic of the vibration width of the rich direction set according to the intake air amount by the air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 of the present invention. It is explanatory drawing which shows the characteristic of the vibration period of a lean direction set according to the intake air amount by the air fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the characteristic of the vibration width of the lean direction set according to the intake air intake amount by the air fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the period correction coefficient and vibration width correction coefficient which are set according to the frequency | count of vibration by the air-fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention with a table. 3 is a timing chart for supplementarily explaining the operation of the average air-fuel ratio oscillation means shown in FIG. It is explanatory drawing which shows the other example of the period correction coefficient set according to the frequency | count of a vibration by the air fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention, and a vibration width correction coefficient with a table. FIG. 16 is a timing chart for supplementarily explaining the operation of the average air-fuel ratio oscillating means based on the period correction coefficient and the vibration width correction coefficient of FIG. 15. FIG. 3 is a timing chart for supplementarily explaining the operation of the average air-fuel ratio oscillation means in FIG. 2. FIG. 3 is a flowchart showing a control constant setting calculation process by an average air-fuel ratio oscillating means shown in FIG. It is a flowchart which shows the arithmetic processing operation by the maximum oxygen storage amount calculating means shown in FIG. It is explanatory drawing which shows the one-dimensional map of the temperature correction coefficient set according to the catalyst temperature by the air fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the one-dimensional map of the deterioration correction coefficient set according to the catalyst deterioration degree by the air-fuel ratio control apparatus of the internal combustion engine according to Embodiment 1 of the present invention. FIG. 3 is a flowchart showing a calculation processing operation of a catalyst deterioration degree by a maximum oxygen storage amount calculation unit shown in FIG. 2. FIG. 3 is a timing chart for supplementarily explaining the operation of the catalyst deterioration diagnosis unit shown in FIG. It is a flowchart which shows the arithmetic processing operation | movement of the catalyst deterioration diagnostic means shown in FIG. It is a flowchart which shows the arithmetic processing operation | movement of the catalyst deterioration diagnostic means shown in FIG. 3 is a timing chart for supplementarily explaining the operation of the catalyst deterioration diagnosis unit shown in FIG. 3 is a flowchart showing a calculation processing operation by a second air-fuel ratio feedback control means shown in FIG. It is a figure which shows the one-dimensional map of the update amount for the integral calculation of the target average air fuel ratio set according to a deviation by the air fuel ratio control apparatus of the internal combustion engine which concerns on Embodiment 1 of this invention. FIG. 6 is a timing chart showing a processing operation for a slightly deteriorated catalyst by a conventional air-fuel ratio control device for an internal combustion engine, for explaining the effect of the first embodiment of the present invention. FIG. 3 is a timing chart showing a processing operation for a slightly deteriorated catalyst for explaining the effect of the air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 of the present invention; It is a timing chart which shows the processing operation at the time of a normal catalyst by the air-fuel ratio control apparatus of the conventional internal combustion engine. It is a timing chart which shows the processing operation at the time of the deterioration catalyst by the air-fuel ratio control apparatus of the conventional internal combustion engine. It is a timing chart which shows the processing operation at the time of the light degradation catalyst by the air-fuel ratio control apparatus of the conventional internal combustion engine.

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

Embodiment 1 FIG.
FIG. 1 is a block diagram showing the entire air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 of the present invention, together with peripheral devices. In FIG. 1, an internal combustion engine 101 is provided with an intake pipe 105 having an air cleaner 102, a throttle valve 103, and a surge tank 104 as an intake system.

  The intake pipe 105 includes an air flow sensor 106 for detecting the intake air amount Qa, an injector 107 for injecting fuel, a throttle sensor 117 for detecting the throttle opening θ of the throttle valve 103, and an idle switch 118 for detecting idling. And are provided. The idle switch 118 generates an idle signal DL that is turned on at an idling opening (the throttle opening θ is in a fully closed state).

  The internal combustion engine 101 is provided with an exhaust pipe 108 as an exhaust system. In the exhaust pipe 108, a three-way catalyst (hereinafter referred to as catalyst) 109 for purifying harmful components in the exhaust gas is disposed, and is an air-fuel ratio detecting means disposed on the upstream side of the catalyst 109. An O2 sensor (hereinafter referred to as an upstream O2 sensor) 110 and a λO2 sensor (hereinafter referred to as a downstream O2 sensor) 111 which is an air-fuel ratio detecting means disposed on the downstream side of the catalyst 109 are provided. Here, a case where a λO2 sensor is used as the upstream O2 sensor 110 will be described as an example.

  The internal combustion engine control unit (hereinafter referred to as ECU) 112 is constituted by a microcomputer, and includes a central processing unit (hereinafter referred to as CPU) 113, a read only memory (hereinafter referred to as ROM) 114, and random access. A memory (hereinafter referred to as RAM) 115, an input / output interface (hereinafter referred to as I / O) 116, a drive circuit 122, and a backup RAM 124 are included.

  The internal combustion engine 101 includes a coolant temperature sensor 119 for detecting the coolant temperature WT, a crank angle sensor 120 for generating a crank angle signal CA corresponding to the crank angle position, and a cam angle for generating a cam angle signal corresponding to the cam angle position. A sensor 121 is provided.

  The water temperature sensor 119, the crank angle sensor 120, and the cam angle sensor 121 are other sensor means (air flow sensor 106, upstream O2 sensor 110, downstream O2 sensor 111, throttle sensor 117, idle switch 118, etc.) The operation state detection means which detects the operation state of the internal combustion engine 101 is comprised, and each detection signal is input into ECU112 as operation state information.

  In the internal combustion engine 101 shown in FIG. 1, the intake air cleaned by the air cleaner 102 is controlled to an intake amount corresponding to the load by the throttle valve 103, and is sent to each cylinder of the internal combustion engine 101 via the surge tank 104 and the intake pipe 105. Inhaled. At this time, the intake air amount Qa to the internal combustion engine 101 is detected by the air flow sensor 106. Further, fuel for each cylinder of the internal combustion engine 101 is injected into the intake pipe 105 through the injector 107. The intake air amount Qa may be estimated based on the operating point of the internal combustion engine 101 and the pressure in the surge tank 104 instead of being detected by the air flow sensor 106.

  The air-fuel mixture (air and fuel) sucked into each cylinder of the internal combustion engine 101 passes through a combustion stroke to become exhaust gas, and the exhaust gas passes through the catalyst 109 disposed in the exhaust pipe 108, thereby causing harmful components. Purified and exhausted into the atmosphere.

  At this time, the upstream O2 sensor 110 provided on the upstream side of the catalyst 109 detects the oxygen concentration λO2 in the exhaust gas to detect whether the atmosphere of the exhaust gas is rich or lean. Further, the downstream O2 sensor 111 provided on the downstream side of the catalyst 109 detects the oxygen concentration λO2 in the exhaust gas. The upstream O2 sensor 110 and the downstream O2 sensor 111 generate electrical signals (voltage signals) corresponding to the air-fuel ratio in the exhaust gas as output values V1 and V2.

  In the ECU 112, various operating state information (intake air amount Qa, throttle opening θ, idle signal DL, cooling water temperature WT, air-fuel ratio A / F, oxygen concentration λO2, crank angle signal CA, cam angle from cam angle sensor 121) Signal etc.) is taken into the CPU 113 via the I / O 116.

  The ECU 112 constitutes an air-fuel ratio feedback control system, and the oxygen concentration λO2 upstream of the catalyst and the oxygen concentration λO2 downstream of the catalyst from the sensors 110 and 111 disposed before and after the catalyst 109 (upstream and downstream). Based on the above, a drive signal for the injector 107 is generated, and a required amount of fuel is injected.

  The ECU 112 functions as a device for detecting the deterioration of the catalyst 109 as well as a device for optimally controlling the internal combustion engine 101. The drive circuit 122 in the ECU 112 drives not only the injector 107 but also various actuators related to the internal combustion engine 101, such as an ISC valve (not shown).

  That is, the ECU 112 performs various controls such as ignition timing control and idle speed control in addition to air-fuel ratio control, and detects a failure of various components that cause exhaust gas deterioration as a self-diagnosis function. In accordance with the detected information, the warning lamp 123 is turned on via the I / O 116.

  The ROM 114 stores not only a control routine for the oxygen change amount of the catalyst 109 but also a control program such as a routine for detecting the deterioration of the catalyst 109, and further stores a map necessary for these control processes. Yes.

  FIG. 2 is a functional block diagram showing a basic configuration of the ECU 112 in FIG. 1, and each means in FIG. As described above, the ECU 112 has the output value V1 of the upstream O2 sensor 110 (the air-fuel ratio in the upstream exhaust gas of the catalyst 109) and the output value V2 of the downstream O2 sensor 111 (the downstream exhaust gas of the catalyst 109). And the detection information from other various sensors.

  In FIG. 2, the ECU 112 includes a first air-fuel ratio feedback control means 201, a second air-fuel ratio feedback control means 202, an average air-fuel ratio oscillation means 203, a maximum oxygen storage amount calculation means 204, and a catalyst deterioration oscillation means. 205, and the output value V1 of the upstream O2 sensor 110 is input to the first air-fuel ratio feedback control means 201.

  The output value V2 of the downstream O2 sensor 111 is input to the second air-fuel ratio feedback control means 202, the average air-fuel ratio oscillation means 203, and the catalyst deterioration oscillation means 205, and the maximum oxygen storage amount calculation means 204 receives other values. Detection information from various sensors is input.

  The first air-fuel ratio feedback control means 201 controls the drive circuit 122 of the injector 107 in accordance with the output value V1 of the upstream O2 sensor 110 and a predetermined feedback control constant (hereinafter referred to as a control constant). The air-fuel ratio supplied to the internal combustion engine 101 is adjusted to periodically oscillate the air-fuel ratio in the rich direction and the lean direction.

  The first air-fuel ratio feedback control means 201 includes control gain changing means 207 that changes the control gain of the first air-fuel ratio feedback control means 201, and the control gain changing means 207 is an output of the upstream O2 sensor 110. The control gain is changed according to the value V1.

  The first air-fuel ratio feedback control means 201 includes a control period measurement means 208 that measures the control period of the first air-fuel ratio feedback control means 201. The control period measuring unit 208 is a period from when the air-fuel ratio oscillation shifts from the lean period to the rich period until the control constant increase amount is switched to a small value, or when the air-fuel ratio oscillation shifts from the rich period to the lean period. Then, the period from when the decrease amount of the control constant is switched to a small value is measured.

  Based on the oxygen storage amount of the catalyst 109 (the estimated oxygen storage amount OSC described later), the average air-fuel ratio oscillation means 203 oscillates the average air-fuel ratio that averages the periodically oscillating air-fuel ratio in the rich and lean directions. As described above, the control constant used in the first air-fuel ratio feedback control means 201 is manipulated.

Specifically, the average air-fuel ratio oscillating means 203 sets a control constant according to the target average air-fuel ratio AFAVEobj with respect to the average air-fuel ratio, and periodically oscillates the target average air-fuel ratio AFAVEobj in the rich direction and the lean direction.

  Further, for example, the average air-fuel ratio oscillating means 203 is set according to the operating condition of the internal combustion engine 101 when the vibration width ΔOSC of the oxygen storage amount of the catalyst 109 is within the range of the maximum oxygen storage amount OSCmax of the catalyst 109. The vibration width or vibration cycle of the average air-fuel ratio is set according to the operating conditions of the internal combustion engine 101 so that the predetermined vibration width is obtained.

  Alternatively, the average air-fuel ratio oscillating means 203 can be used for a deteriorated catalyst in which the vibration width ΔOSC of the oxygen storage amount of the catalyst 109 is within the range of the maximum oxygen storage amount OSCmax before the catalyst 109 deteriorates and the deterioration diagnosis is necessary. The oscillation width or oscillation cycle of the average air-fuel ratio is set according to the operating conditions of the internal combustion engine 101 so that it is outside the range of the maximum oxygen storage amount.

  The average air-fuel ratio oscillating means 203 sets the initial vibration period at the start of the average air-fuel ratio vibration to half of the finally set vibration period, and sets the initial vibration width at the start of the average air-fuel ratio vibration, Set to half the final vibration width. The average air-fuel ratio oscillating means 203 stops executing the average air-fuel ratio oscillating process during a predetermined period during the transient operation of the internal combustion engine 101 or after the transient operation.

  The average air-fuel ratio oscillating means 203 oscillates the average air-fuel ratio in a rich direction and a lean direction at a predetermined cycle, and when the average air-fuel ratio is set in the rich direction, the output value V2 of the downstream O2 sensor 111 is rich. When the direction is reversed, the setting cycle of the average air-fuel ratio in the rich direction is terminated, and the average air-fuel ratio is forcibly reversed in the lean direction. Further, when the average air-fuel ratio is set in the lean direction and the output value V2 of the downstream O2 sensor 111 is reversed in the lean direction when the average air-fuel ratio is set in the lean direction, the average air-fuel ratio oscillating means 203 moves toward the lean direction of the average air-fuel ratio. Is completed, and the average air-fuel ratio is forcibly reversed in the rich direction.

  The average air-fuel ratio oscillation means 203 vibrates the average air-fuel ratio in the rich direction and the lean direction based on the estimated oxygen storage amount OSC, and when the average air-fuel ratio is set in the rich direction, the downstream O2 sensor When the output value V2 of 111 is reversed in the rich direction, the estimated oxygen storage amount OSC is reset to the lower limit value of the vibration range of the oxygen storage amount of the catalyst 109, and the average air-fuel ratio is forcibly reversed in the lean direction. .

Further, when the average air-fuel ratio is set in the lean direction and the output value V2 of the downstream O2 sensor 111 is reversed in the lean direction when the average air-fuel ratio is set in the lean direction, the average air-fuel ratio oscillating means 203 While resetting to the upper limit value of the vibration range of the oxygen storage amount of 109, the average air-fuel ratio is forcibly reversed in the rich direction.
Further, the average air-fuel ratio oscillating means 203 determines the average air-fuel ratio so that the vibration width ΔOSC of the oxygen storage amount of the catalyst 109 changes between when the catalyst 109 is diagnosed by the catalyst deterioration diagnosing means 205 and when it is not deteriorated. Change the vibration width or vibration cycle.

  The second air-fuel ratio feedback control unit 202 corrects the oscillation center (central air-fuel ratio) AFCNT of the average air-fuel ratio that is oscillated by the average air-fuel ratio oscillation unit 203 based on the output value V2 of the downstream O2 sensor 111.

  The second air-fuel ratio feedback control means 202 includes control gain changing means 206 that changes the control gain of the second air-fuel ratio feedback control means 202, and the control gain changing means 206 is controlled by the average air-fuel ratio oscillation means 203. While the average air-fuel ratio oscillation process is being executed, the control gain is changed.

  The catalyst deterioration diagnosis unit 205 diagnoses the presence or absence of deterioration of the catalyst 109 based on the maximum oxygen storage amount OSCmax calculated by the maximum oxygen storage amount calculation unit 204. Further, the catalyst deterioration diagnosis unit 205 diagnoses the deterioration of the catalyst 109 based on at least the output value V2 of the downstream O2 sensor 111 during the execution of the average air-fuel ratio vibration process by the average air-fuel ratio vibration unit 203. A diagnosis result by the catalyst deterioration diagnosis unit 205 is input to an alarm driving unit such as an alarm lamp 123.

  Next, referring to the flowcharts of FIGS. 3A and 3B, the arithmetic processing operation by the first air-fuel ratio feedback control means 201 in FIG. 2 and the control gain of the first air-fuel ratio feedback control means 201 in FIG. An arithmetic processing operation by the changing unit 207 and an arithmetic processing operation by the control period measuring unit 208 of the first air-fuel ratio feedback control unit 201 in FIG. 2 will be described. 3A and 3B shows a calculation control procedure of the fuel correction coefficient FAF based on the output value V1 of the upstream O2 sensor 110. The first air-fuel ratio feedback is performed every predetermined period (for example, 5 msec). It is executed by the control means 201.

  In FIG. 3A and FIG. 3B, the symbols “Y” and “N” of the branching units from each determination process indicate “YES” and “NO”, respectively. First, the output value V1 of the upstream O2 sensor 110 is A / D converted and captured (step 401), and it is determined whether or not the air-fuel ratio feedback F / B (closed loop) condition by the upstream O2 sensor 110 is satisfied. (Step 402).

  At this time, air-fuel ratio control conditions other than the stoichiometric air-fuel ratio control (for example, during engine start-up, during enrichment control at low water temperature, during enrichment control of high load power increase, during lean control for improving fuel efficiency, start The closed loop condition is determined to be not established in any case such as when the subsequent lean control or fuel cut is established, or when the upstream O2 sensor 110 is in an inactive state or a failure, and in other cases, It is determined that the closed loop condition is satisfied.

  If it is determined in step 402 that the closed-loop condition is not satisfied (ie, NO), the fuel correction coefficient FAF is set to “1.0” (step 441), and the delay counter CDLY is reset to “0” (step) 442), the counter TAF that measures the control period from when the delayed air-fuel ratio flag F1 is inverted until the output value V1 of the upstream O2 sensor 110 becomes equal to or higher than a predetermined voltage or lower than a predetermined voltage is reset to “0” ( Step 443). The fuel correction coefficient FAF may be a value immediately before the end of the closed loop control or a learned value (a stored value in the backup RAM 124 in the ECU 112).

  Subsequently, it is determined whether or not the output value V1 of the upstream O2 sensor 110 is equal to or lower than the comparison voltage VR1 (lean) (step 444), and the upstream A / F is in a lean state (V1 ≦ VR1) (ie, YES ), The pre-delay air-fuel ratio flag F0 is set to “0” (lean) (step 445), and the post-delay air-fuel ratio flag F1 is set to “0” (lean) (step 446). Then, the process routine of FIG. 3 is exited (step 449). The comparison voltage VR1 is set to a lean determination reference voltage (for example, about 0.45 V).

  If it is determined in step 444 that V1> VR1 (ie, NO), the upstream A / F is in a rich state, so the pre-delayed air-fuel ratio flag F0 is set to “1” (rich) (step 447). Then, the delayed air-fuel ratio flag F1 is set to “1” (rich) (step 448), and the processing routine of FIGS. 3A and 3B is exited (step 449). By the above steps 441 to 448, the initial value when the air-fuel ratio closed loop condition is not established is set.

  On the other hand, if it is determined in step 402 that the closed loop (feedback) condition is satisfied (that is, YES), the output value V1 of the upstream O2 sensor 110 is subsequently equal to or lower than the comparison voltage VR1 (for example, 0.45 V). That is, it is determined whether the upstream A / F of the catalyst 109 is rich or lean with respect to the comparison voltage VR1 (step 403).

  If it is determined in step 403 that V1 ≦ VR1 (ie, YES), it is determined that the upstream A / F is in a lean state, and subsequently, it is determined whether or not the delay counter CDLY is equal to or greater than the maximum value TDR. (Step 404). The maximum value TDR corresponds to a “rich delay period” for holding a determination that the engine is in the lean state even if the output value V1 of the upstream O2 sensor 110 changes from lean to rich, and is a positive value. Defined by

  If it is determined in step 404 that CDLY ≧ TDR (that is, YES), the delay counter CDLY is reset to “0” (step 405), and the pre-delay air-fuel ratio flag F0 is set to “0” (lean). (Step 406), the process proceeds to step 416. Step 416 will be described later.

  If it is determined in step 404 that CDLY <TDR (ie, NO), it is subsequently determined whether or not the pre-delay air-fuel ratio flag F0 is “0” (lean) (step 407). If it is determined that = 0 (lean) (that is, YES), the delay counter CDLY is decremented by “1” (step 408), and the process proceeds to step 416. If it is determined in step 407 that F0 = 1 (rich) (ie, NO), the delay counter CDLY is incremented by “1” (step 409), and the process proceeds to step 416.

  On the other hand, if it is determined in step 403 that V1> VR1 (that is, NO), it is considered that the upstream A / F is in a rich state, and then whether or not the delay counter CDLY is equal to or smaller than the minimum value TDL is determined. Determination is made (step 410). Note that the minimum value TDL corresponds to a “lean delay period” for maintaining a determination that the upstream side O2 sensor 110 is in a rich state even if the output value V1 of the upstream O2 sensor 110 changes from rich to lean. Defined by

  If it is determined in step 410 that CDLY ≦ TDL (that is, YES), the delay counter CDLY is reset to “0” (step 411), and the pre-delay air-fuel ratio flag F0 is set to “1” (rich). (Step 412), the process proceeds to step 416.

  If it is determined in step 410 that CDLY> TDL (that is, NO), it is subsequently determined whether or not the pre-delay air-fuel ratio flag F0 is “0” (lean) (step 413). If it is determined that = 0 (that is, YES), the delay counter CDLY is subtracted by “1” (step 414), and the process proceeds to step 416.

  If it is determined in step 413 that F0 = 1 (rich) (that is, NO), the delay counter CDLY is incremented by “1” (step 415), and the process proceeds to step 416.

  In step 416, it is determined whether or not the delay counter CDLY is equal to or smaller than the minimum value TDL. If it is determined that CDLY> TDL (that is, NO), the process proceeds to step 419. Step 419 will be described later.

  If it is determined in step 416 that CDLY ≦ TDL (that is, YES), the delay counter CDLY is set to the minimum value TDL (step 417), and the post-delay air-fuel ratio flag F1 is set to “0” (lean). (Step 418). That is, when the delay counter CDLY reaches the minimum value TDL, guarding is performed with the minimum value TDL, and the post-delay air-fuel ratio flag F1 is set to “0” (lean).

Subsequently, it is determined whether or not the delay counter CDLY is equal to or greater than the maximum value TDR (step 419). If it is determined that CDLY <TDR (ie, NO), the process proceeds to step 422.
Step 422 will be described later.

  If it is determined in step 419 that CDLY ≧ TDR (ie, YES), the delay counter CDLY is set to the maximum value TDR (step 420), and the post-delay air-fuel ratio flag F1 is set to “1” (rich). After setting (step 421), the process proceeds to step 422. That is, when the delay counter CDLY reaches the maximum value TDR, the delay counter CDLY is guarded with the maximum value TDR, and the post-delay air-fuel ratio flag F1 is set to “1” (rich).

  In step 422, prior to execution of the fuel correction coefficient FAF skip increase / decrease process (or integration process), first, the sign of the post-delay air-fuel ratio flag F1 is inverted to determine whether or not the post-delay air-fuel ratio has been inverted. Judgment is made based on whether or not

  In step 422, if it is determined that the sign (air-fuel ratio) of the post-delay air-fuel ratio flag F1 is inverted (that is, YES), then the counter TAF is reset to “0” (step 423), and the rich It is determined whether the inversion from lean to lean or the inversion from lean to rich is based on whether the value of the post-delay air-fuel ratio flag F1 is “0” (step 424).

  If it is determined in step 424 that F1 = 0 (that is, YES), it is an inversion from rich to lean. Therefore, the fuel correction coefficient FAF is set to “FAF + RSR” and is increased by a constant RSR in a skipping manner (step 425). Go to step 436. Step 436 will be described later.

  If it is determined in step 424 that F1 = 1 (that is, NO), it is the reverse from lean to rich, so the fuel correction coefficient FAF is set to “FAF-RSL” and is decreased by a constant RSL in a skipping manner. (Step 426), the process proceeds to Step 436.

  On the other hand, if it is determined in step 422 that the sign (air-fuel ratio) of the post-delay air-fuel ratio flag F1 is not reversed (that is, NO), then the post-delay air-fuel ratio flag F1 is “0” (lean). (Step 427). If it is determined that F1 = 0 (that is, YES), it is in a lean state, and the process proceeds to step 428.

  In step 428, it is determined whether or not the output value V1 of the upstream O2 sensor 110 is equal to or lower than the predetermined voltage VR. If it is determined that V1 ≦ VR (that is, YES), the fuel correction coefficient FAF is set to “FAF + KIR”. Then, the constant KIR (<RSR) is increased (step 429), "1" is added to the counter TAF (430), and the process proceeds to step 436.

  On the other hand, if it is determined in step 428 that V1> VR (that is, NO), the fuel correction coefficient FAF is increased to “FAF + KIR2” using the constant KIR2 (<KIR) (step 431), and the process proceeds to step 436. .

  That is, in step 431, by using the constant KIR2 smaller than the constant KIR as the increase amount of the fuel correction coefficient FAF, the A / F control range after the fuel correction coefficient FAF has advanced to some extent in the rich direction can be suppressed. In addition, drivability and exhaust gas deterioration associated therewith can be suppressed. Note that the increase amount “KIR2” used in step 431 may be set to “0” so that the fuel correction coefficient FAF after the fuel correction coefficient FAF has advanced to some extent in the rich direction is not increased.

  Further, in step 428, by setting the predetermined voltage VR for switching the increase amount of the fuel correction coefficient FAF to the theoretical air-fuel ratio equivalent voltage VR3 of the upstream O2 sensor 110, when the catalyst upstream atmosphere is in a lean state, When the large increase amount “KIR” is set and the atmosphere upstream of the catalyst is rich, the small increase amount “KIR2” can be set. Combining with the operation in the lean state in step 432, which will be described later, makes it possible to reliably carry out the air-fuel ratio oscillation between the rich state and the lean state, and the A / F control range tends to increase due to acceleration / deceleration and the like. In this case, an effect of suppressing exhaust gas deterioration when the oxygen storage amount at the time of the deteriorated catalyst is reduced can be obtained.

  If it is determined in step 427 that F1 = 1 (that is, NO), the state is rich, and the process proceeds to step 432.

  In step 432, it is determined whether or not the output value V1 of the upstream O2 sensor 110 is equal to or higher than the predetermined voltage VL. If it is determined that V1 ≧ VL (that is, YES), the fuel correction coefficient FAF is set to “FAF− “KIL” is decreased by a constant KIL (<RSL) (step 433), “1” is added to the counter TAF (step 434), and the process proceeds to step 436.

  On the other hand, if it is determined in step 432 that V1 <VL (ie, NO), the fuel correction coefficient FAF is decreased to “FAF−KIL2” using the constant KIL2 (<KIL) (step 435), and step 436. Proceed to

  That is, in step 435, by using the constant KIL2 smaller than the constant KIL as the amount of decrease in the fuel correction coefficient FAF, the A / F control range after the fuel correction coefficient FAF has advanced to some extent in the lean direction can be suppressed. In addition, drivability and exhaust gas deterioration associated therewith can be suppressed. Note that the reduction amount “KIL2” used in step 435 may be set to “0” so that the fuel correction coefficient FAF after the fuel correction coefficient FAF has advanced to some extent in the lean direction is not reduced.

  In step 432, the predetermined voltage VL for switching the amount of decrease in the fuel correction coefficient FAF is set to the stoichiometric air-fuel ratio equivalent voltage VR3 of the upstream O2 sensor 110. When the large reduction amount “KIL” is set and the atmosphere upstream of the catalyst is lean, the small reduction amount “KIL2” can be set. By combining with the operation in the rich state in step 428, it becomes possible to reliably carry out the air-fuel ratio oscillation between the rich state and the lean state, and the A / F control range tends to increase due to acceleration / deceleration and the like. In this case, an effect of suppressing the deterioration of exhaust gas can be obtained when the oxygen storage amount at the time of the deteriorated catalyst is reduced.

  The integral constants KIR, KIR2, KIL, and KIL2 are set to sufficiently small values compared to the skip constants RSR and RSL. Accordingly, in steps 429 and 431, the fuel injection amount in the lean state (F1 = 0) is gradually increased, and in steps 433 and 435, the fuel injection amount in the rich state (F1 = 1) is gradually decreased. I will let you.

  In step 436, it is determined whether or not the counter TAF is smaller than the counter upper limit TAFH. If it is determined that TAF <TAFH (ie, YES), the process proceeds to step 437.

  On the other hand, if it is determined in step 436 that TAF ≧ TAFH (that is, NO), the process proceeds to step 441 to stop the execution of the air-fuel ratio oscillation process.

  In other words, when the behavior of the upstream O2 sensor 110 is delayed due to a delay in response due to deterioration of the O2 sensor and the period in which V1> VR in step 428 or the period in which V1 <VL is increased in step 432 is increased. Then, it is determined that the A / F control range of the fuel correction coefficient FAF has become too large, and the execution of the air-fuel ratio oscillation process is stopped. As an effect thereof, it is possible to reduce the possibility of erroneously detecting a normal catalyst as a deteriorated catalyst at the time of diagnosis of the deteriorated catalyst.

  In step 437, it is determined whether or not the fuel correction coefficient FAF is smaller than “0.8”. If it is determined that FAF <0.8 (that is, YES), the fuel correction coefficient FAF is set to “0.8”. (Step 438), and the process proceeds to step 439.

  On the other hand, if it is determined in step 437 that FAF ≧ 0.8 (that is, NO), it is subsequently determined whether or not the fuel correction coefficient FAF is larger than “1.2” (step 439). If it is determined that> 1.2 (that is, YES), the fuel correction coefficient FAF is set to “1.2” (step 440), and the processing routine of FIGS. 3A and 3B is exited (step 449). If it is determined in step 439 that FAF ≦ 1.2 (that is, NO), the processing routine of FIGS. 3A and 3B is immediately exited (step 449).

  That is, the fuel correction coefficient FAF calculated in steps 425, 426, 429, 431, 433, and 435 is guarded at “0.8” (minimum value) in steps 437 and 438, and in steps 439 and 440. , “1.2” (maximum value). As a result, when the fuel correction coefficient FAF is too large or too small for some reason, the air-fuel ratio of the internal combustion engine 101 is controlled with the maximum value (for example, 1.2) or the minimum value (for example, 0.8). To prevent over-rich or over-lean.

  3A and 3B is completed, and the fuel correction coefficient FAF calculated in steps 401 to 449 is stored in the RAM 115 in the ECU 112.

Next, the arithmetic processing operation shown in FIGS. 3A and 3B will be supplementarily described with reference to the timing chart of FIG.
4A is a calculation processing operation in a conventional air-fuel ratio control apparatus for an internal combustion engine (for example, Patent Document 4), and the timing chart in FIG. 4B is an internal combustion engine according to the first embodiment. In this air-fuel ratio control apparatus, the timing for switching the increase amount of the fuel correction coefficient FAF to KIR2 and the decrease amount to KIL2 is the timing when the output value V1 of the upstream O2 sensor 110 becomes the predetermined voltages VR and VL. The timing chart of 4 (c) shows the output value of the upstream O2 sensor 110 at the timing of switching the increase amount of the fuel correction coefficient FAF to KIR2 and the decrease amount to KIL2 in the air-fuel ratio control apparatus of the internal combustion engine according to the first embodiment. This is a timing when V1 becomes the stoichiometric air-fuel ratio equivalent voltage VR3 and the values of KIR2 and KIL2 are both "0".

  4A, 4B, and 4C, when the air-fuel ratio signal before the delay process (comparison result of the rich / lean determination) is obtained based on the output value V1 of the upstream O2 sensor 110, before the delay process. The pre-delay air-fuel ratio flag F0 that responds to the air-fuel ratio signal changes to a rich state or a lean state.

  The delay counter CDLY is counted up in response to the rich state of the pre-delay air-fuel ratio flag F0 (corresponding to the pre-delay air-fuel ratio signal) within the range between the maximum value TDR and the minimum value TDL. In response to the lean state, it is counted down. As a result, the post-delay air-fuel ratio flag F1 indicates the delayed air-fuel ratio signal.

  For example, even if the air-fuel ratio signal before delay processing (the comparison result of the output value V1) is inverted from lean to rich at time t1, the delayed air-fuel ratio signal (post-delay air-fuel ratio flag F1) is rich-delayed. It changes to rich at time t2 after being held lean for the period τDR.

  Similarly, at time t3, even if the air-fuel ratio signal before the delay process (upstream A / F) changes from rich to lean, the air-fuel ratio signal after delay process (after-delay air-fuel ratio flag F1) At time t4 after the period τDL is kept rich, it changes to lean.

  However, for example, after time t9 (after the start of the rich delay process), as shown at times t10 and t11, the air-fuel ratio signal (comparison result) before the delay process is inverted in a period shorter than the rich delay period τDR. However, during the delay process (time t9 to t12) until the delay counter CDLY reaches the rich delay period τDR, the pre-delay air-fuel ratio flag F0 is not inverted.

  That is, the pre-delay air-fuel ratio flag F0 is not affected by the fluctuation of the temporary comparison result (the air-fuel ratio signal after the delay process) caused by the minute fluctuation of the output value V1 of the upstream O2 sensor 110. Compared to the air-fuel ratio signal before processing), the waveform is stable. Thus, by executing the delay process, a stable pre-delay air-fuel ratio flag F0 and a post-delay air-fuel ratio signal (post-delay air-fuel ratio flag F1) are obtained, and based on the post-delay air-fuel ratio flag F1, An appropriate fuel correction factor FAF is obtained.

  In the waveform of the fuel correction coefficient FAF in FIG. 4A, the slopes in the increasing direction and the decreasing direction correspond to integration constants KIR and KIL, respectively, and the skip increase / decrease amounts correspond to RSR and RSL.

  In the waveform of the fuel correction coefficient FAF in FIG. 4B, the gradient in the increasing direction until the timing when the output value of the upstream O2 sensor 110 becomes VR corresponds to the integration constant KIR, and the output value of the upstream O2 sensor 110. The inclination in the increasing direction after the timing when becomes VR corresponds to the integral constant KIR2. Further, the inclination in the decreasing direction until the timing when the output value of the upstream O2 sensor 110 becomes VL corresponds to the integration constant KIL, and the inclination in the decreasing direction after the timing when the output value of the upstream O2 sensor 110 becomes VL. Corresponds to the integral constant KIL2. Further, during the period in which the integration constants KIR and KIL are used for the fuel correction coefficient FAF, the integration of the counter TAF is performed. When the counter TAF becomes equal to or greater than TAFH, the execution of the air-fuel ratio oscillation process is stopped.

  In the waveform of the fuel correction coefficient FAF in FIG. 4C, the gradient in the increasing direction until the timing when the output value of the upstream O2 sensor 110 becomes VR3 corresponds to the integral constant KIR, and the output value of the upstream O2 sensor 110 The gradient in the increasing direction after the timing when becomes VR3 corresponds to the integration constant KIR2 (“0” in FIG. 4C). Further, the inclination in the decreasing direction until the timing when the output value of the upstream O2 sensor 110 becomes VR3 corresponds to the integral constant KIL, and the inclination in the decreasing direction after the timing when the output value of the upstream O2 sensor 110 becomes VR3. Corresponds to an integral constant KIL2 (“0” in FIG. 4C). Further, during the period in which the integration constants KIR and KIL are used for the fuel correction coefficient FAF, the integration of the counter TAF is performed. When the counter TAF becomes equal to or greater than TAFH, the execution of the air-fuel ratio oscillation process is stopped.

Thereafter, the drive circuit 122 in the ECU 112 causes the air / fuel ratio to coincide with the target air / fuel ratio A / Fo in accordance with the fuel correction coefficient FAF calculated by the first air / fuel ratio feedback control means 201 and the basic fuel amount Qfuel0. In order to drive the injector 107 as described above, the fuel supply amount Qfuel supplied to the internal combustion engine 101 is adjusted as in the following equation (1).
Qfuel1 = Qfuel0 × FAF (1)

However, in the equation (1), the basic fuel amount Qfuel0 is calculated as the following equation (2) using the air amount Qacyl supplied to the internal combustion engine 101 and the target air-fuel ratio A / Fo.
Qfuel0 = Qacyl / (A / Fo) (2)

  In equation (2), the air amount Qacyl to the internal combustion engine 101 is calculated based on the intake air amount Qa detected from the air flow sensor 106. When the air flow sensor 106 is not used, the intake air amount Qa may be calculated based on an output signal of a pressure sensor (not shown) provided on the downstream side of the throttle valve in the intake passage 105. The calculation may be based on the rotational speed Ne or the throttle opening of the throttle valve.

  Further, the target air-fuel ratio A / Fo is set to a value that is set as a region in a two-dimensional map of the engine speed Ne and the load as shown in FIG. That is, the target air-fuel ratio A / Fo is reflected in a feed-forward manner as the target average air-fuel ratio calculated by the average air-fuel ratio oscillating means 203 during the theoretical air-fuel ratio (A / F≈14.53) control. Set to a value.

  As a result, the feedback follow-up delay when the target value changes can be improved, and the fuel correction coefficient FAF can be maintained at a value near the center of “1.0”. At this time, since learning control is performed to absorb the change with time and production variation of the components related to the first air-fuel ratio feedback control means 201 based on the fuel correction coefficient FAF, fuel correction is performed by feedforward correction. The more stable the coefficient FAF, the more accurate the learning control.

  Next, the average air-fuel ratio oscillation means in FIG. 2 will be described with reference to the explanatory diagrams in FIGS. 7 to 13 and 15 and the timing charts in FIGS. 14, 16 and 17 together with the flowcharts in FIGS. 6A and 6B. The arithmetic processing operation by 203 will be described. The arithmetic processing routines of FIGS. 6A and 6B are executed every predetermined period (for example, 5 msec).

  6A and 6B, first, rich / lean inversion of the output value V2 of the downstream O2 sensor 111 is determined (step 701). The downstream O2 sensor 111 is a λ-type sensor having a binary output characteristic. As shown in FIG. 7, the output value V2 (voltage value) is near the theoretical air-fuel ratio with respect to the air-fuel ratio change in the sensor atmosphere. It changes rapidly. The λ-type sensor having the characteristics shown in FIG. 7 has a very high detection resolution with respect to an air-fuel ratio in the vicinity of the theoretical air-fuel ratio and good detection accuracy.

  That is, in step 701, as shown in FIG. 8, it is determined whether the output value V2 of the downstream O2 sensor 111 is rich or lean with reference to the determination threshold value (one-dot chain line). It is determined whether the determination result is reversed.

  In step 701, when it is determined that the inversion from lean to rich is performed, the inversion flag FRO2 of the downstream O2 sensor 111 is set to “1” (a value indicating rich inversion), and the inversion from rich to lean is determined. Sometimes the inversion flag FRO2 is set to “2” (a value indicating lean inversion), and when no inversion is determined, the inversion flag FRO2 is set to “0” (a value indicating non-inversion).

  Note that the determination threshold value (dashed line) shown in FIG. 8 may simply be set to a predetermined voltage according to operating conditions such as the engine rotational speed Ne and the load, and the downstream side related to the second air-fuel ratio feedback control means 202. It may be set to a target voltage VR2 described later of the side O2 sensor 111. Since the output value V2 of the downstream O2 sensor 111 is controlled in the vicinity of the target voltage VR2, when the determination threshold is set to the target voltage VR2, the detection accuracy of the downstream O2 sensor 111 in the rich direction or the lean direction is detected. Will improve.

  In addition, a value obtained by performing filter processing (or averaging processing such as averaging) on the target voltage VR2 of the downstream O2 sensor 111 may be set as a determination threshold value. Thereby, even if the target voltage VR2 changes suddenly in a state where the output value V2 of the downstream O2 sensor 111 does not change, the possibility of erroneous determination of rich / lean inversion can be reduced.

  Further, a value obtained by performing filter processing (or smoothing processing such as averaging) on the output value V2 of the downstream O2 sensor 111 may be set as the determination threshold value. Thereby, even when the output value V2 of the downstream O2 sensor 111 is shifted in the rich direction or the lean direction while being shifted from the fixed threshold value, the rich / lean inversion can be reliably detected.

  Further, instead of the output value V2 compared with the determination threshold value, a value obtained by subjecting the output value V2 to filter processing (or averaging processing such as averaging) may be used. Thereby, it is possible to prevent erroneous determination due to the high frequency component of the output value V2.

  At this time, the filter process (or averaging process such as averaging) for the output value V2 of the downstream O2 sensor 111 is adjusted to reduce the influence of the fluctuation cycle of the output value V1 of the upstream O2 sensor 110. You may do it. Thus, even when the fluctuation of the output value V2 of the downstream O2 sensor 111 approaches the fluctuation of the output value V1 of the upstream O2 sensor 110 due to the significant deterioration of the catalyst 109, the rich / lean inversion determination is performed at a high frequency. The problem that the behavior of the control system becomes unstable can be avoided.

  Furthermore, as shown in FIG. 8, in the rich or lean determination, hysteresis (or dead zone) centered on the determination threshold is set between the determination threshold from rich to lean and the determination threshold from lean to rich. It may be set and the width of the hysteresis (or dead zone) may be adjusted. As a result, chattering of the determination result due to minute fluctuations in the output value V2 can be prevented, and the fluctuation range of the output value V2 for inversion determination can be adjusted.

  Returning to FIG. 6A and FIG. 6B, after step 701, the average air-fuel ratio oscillating means 203 determines whether the average air-fuel ratio oscillation condition is satisfied depending on whether or not the oscillation condition flag FPT is set to “1”. It is determined whether or not (step 702).

  The vibration conditions in step 702 include a state in which the catalyst 109 is stable and a state in which the internal combustion engine 101 is in an operating condition designed in advance. For example, the theoretical air-fuel ratio control by the first air-fuel ratio feedback control means 201 is performed. Is running, when the engine speed Ne, the load, the intake air amount Qa and other operating conditions are within predetermined ranges, or when a predetermined period of time has elapsed since the start of the internal combustion engine 101, the cooling water temperature THW is equal to or higher than the predetermined temperature. In this case, the determination is made in cases other than idle operation, cases other than transient operation, and cases other than a predetermined period after transient operation.

  The transient operation is a condition in which the oxygen storage amount of the catalyst 109 suddenly changes due to an increase in the air-fuel ratio fluctuation. During sudden acceleration / deceleration, fuel cut, rich control, lean control, the first empty When the control by the fuel ratio feedback control means 201 is stopped, when the control by the second air / fuel ratio feedback control means 202 is stopped, when the fuel correction coefficient FAF from the first air / fuel ratio feedback control means 201 changes greatly, for fault diagnosis This includes the forced driving of the actuator and the sudden change of the transpiration gas.

  Sudden acceleration / deceleration is determined, for example, because the amount of change in the throttle opening (or intake air amount Qa) per unit period indicates a predetermined value or more. Moreover, the sudden change of the transpiration gas introduction is determined from the fact that the amount of change per unit period of the valve opening for introducing the transpiration gas shows a predetermined value or more.

  Even after the transient operation, the vibration process is not executed until the predetermined period elapses because the influence of the fluctuation of the oxygen storage amount of the catalyst 109 remains. The predetermined period may be simply set, or the period until the cumulative intake air amount after the transient operation reaches the predetermined amount using the intake air amount Qa that is proportional to the change in the oxygen storage amount of the catalyst 109. May be set. By determining the passage of the predetermined period based on the intake air amount Qa, the vibration start timing can be appropriately set according to the behavior of the oxygen storage amount of the catalyst 109.

  If it is determined in step 702 that the vibration condition is satisfied and FPT = 1 (ie, YES), the process proceeds to step 703. If the vibration condition is not satisfied and it is determined that FPT = 0 (that is, NO), the process proceeds to step 723. Step 723 will be described later.

  When the vibration condition is satisfied, initial values for initial vibration after the vibration condition is satisfied are set in steps 703 to 705. First, it is determined whether or not the vibration is the first vibration depending on whether or not the vibration frequency PTN is “0” (step 703). If it is determined that PTN = 0 (that is, YES), the initial value is set as the initial value. The vibration direction flag FRL is set to “1” (rich direction) (step 704), the number of vibrations PTN is set to “1” (indicating that the initial vibration is being performed) (step 705), and the process proceeds to step 706.

  On the other hand, if it is determined in step 703 that PTN> 0 (that is, NO), the initial value setting process (steps 704 and 705) is not executed, and the process proceeds to step 706. In step 704, the initial value of the vibration direction flag FRL is set to “1” (rich direction), but may be set to “2” (lean direction).

  Next, in steps 706 to 708, the cycle Tj and the vibration width DAFj in the rich direction and the lean direction of the average air-fuel ratio vibration are set, respectively. First, it is determined whether or not the vibration direction is the rich direction based on whether or not the vibration direction flag FRL is “1” (step 706), and the rich direction (FRL = 1) (that is, YES) is determined. Then, the cycle Tr and the vibration width DAFr in the rich direction are set as the cycle Tj and the vibration width DAFj, respectively (step 707), and the process proceeds to step 709. Step 709 will be described later.

  In step 707, the rich direction period Tr and the rich direction vibration width DAFr are set so that the vibration width ΔOSC of the oxygen storage amount of the catalyst 109 becomes a predetermined amount, as shown in the explanatory diagrams of FIGS. It is set by a one-dimensional map corresponding to the intake air amount Qa.

  On the other hand, if it is determined in step 706 that the vibration direction is the lean direction (FRL = 2) (that is, NO), the lean direction period Tl and the vibration width DAFl are set as the period Tj and the vibration width DAFj, respectively ( The process proceeds to step 708) and step 709.

  In step 708, the lean period T1 and the average air-fuel ratio oscillation width DAF1 in the lean direction are the oxygen storage amount of the catalyst 109 as shown in the explanatory diagrams of FIGS. 11 and 12 similar to FIGS. Is set by a one-dimensional map corresponding to the intake air amount Qa so that the vibration width ΔOSC of the predetermined amount becomes a predetermined amount.

The vibration width ΔOSC of the oxygen storage amount is obtained by using the cycle Tj [sec], the absolute value of the vibration width DAFj, the intake air amount Qa [g / sec], and a predetermined coefficient KO2 for conversion into the oxygen storage amount. It is expressed as the following formula (3).
ΔOSC [g] = Tj × | DAFj | × Qa × KO2 (3)

Here, in order to set the vibration width ΔOSC to a predetermined amount, it is necessary to change the vibration width DAFj or the period Tj by changing the intake air amount Qa.
For example, when the vibration width DAFj is a fixed value, the period Tj is set to a value that is inversely proportional to the intake air amount Qa, and when the period Tj is a fixed value, the vibration width DAFj is inversely proportional to the intake air amount Qa. Set to the value you want.

  However, in practice, there are various restrictions on the setting range of the period Tj and the vibration width DAFj for the purpose of improving the purification characteristics of the catalyst 109, improving drivability, or improving responsiveness. Both of the vibration widths DAFj are variably set according to the intake air amount Qa so as to have a vibration width ΔOSC of a predetermined oxygen storage amount.

  Further, the period Tj (or the vibration width DAFj) in the rich direction and the lean direction of the average air-fuel ratio vibration may be set to be asymmetric with each other. For example, in order to improve the NOx purification characteristics of the catalyst 109 or to reduce torque reduction, the absolute value of the vibration width DAFj in the lean direction is set to be smaller than the absolute value of the vibration width DAFj in the rich direction. The period Tj in the lean direction may be set larger than the period Tj in the rich direction so that the width ΔOSC is constant.

  The vibration width ΔOSC of the oxygen storage amount is set so as to be within the range of the maximum oxygen storage amount OSCmax of the catalyst 109. The oxygen storage amount of the catalyst 109 is the maximum oxygen storage amount OSCmax and the minimum oxygen storage amount (= 0). ) In the range between. Further, by controlling the fuel correction coefficient FAF to suppress the vibration width using the voltage of the upstream O2 sensor 110, the fluctuation of the upstream A / F of the catalyst 109 is reliably absorbed by the change of the oxygen storage amount, and the catalyst Since the air-fuel ratio in 109 is kept near the stoichiometric air-fuel ratio, it is possible to prevent a significant deterioration in the purification rate of the catalyst 109.

  Further, even within the range of the maximum oxygen storage amount OSCmax, the vibration width ΔOSC of the oxygen storage amount is adjusted according to various conditions for the purpose of improving the purification characteristics of the catalyst 109 and diagnosing deterioration of the catalyst 109, and has a predetermined amount. Set to For example, the exhaust gas component from the internal combustion engine 101 and the temperature of the catalyst 109 change depending on the engine rotational speed Ne and the load, and the purification characteristics of the catalyst 109 also change. Therefore, the vibration amount ΔOSC of the oxygen storage amount is set to the engine rotational speed Ne. Or change according to the load. Thereby, the purification characteristics of the catalyst 109 can be further improved.

  Further, the vibration width ΔOSC of the oxygen storage amount at the time of deterioration diagnosis is within the range of the maximum oxygen storage amount OSCmax of the catalyst 109 before deterioration and is outside the range of the maximum oxygen storage amount of the catalyst that needs to be deteriorated. Is set. As a result, when a catalyst that requires deterioration diagnosis is used, the disturbance of the output value V2 of the downstream O2 sensor 111 is increased, so that deterioration determination accuracy in deterioration diagnosis is improved.

Returning to FIG. 6, in step 709, the average air-fuel ratio oscillation period Tj and vibration width DAFj set in steps 707 and 708 are determined according to the maximum oxygen storage amount OSCmax calculated by the maximum oxygen storage amount calculation means 204. Each is corrected adaptively. Specifically, the period Tj and the vibration width DAFj are corrected as shown in the following equations (4) and (5) using correction coefficients Kosct and Koscaf, respectively.
Tj = Tj (n−1) × Kosct (4)
DAFj = DAFj (n−1) × Koscaf (5)

In equations (4) and (5), (n−1) represents the previous value before correction. The correction coefficient Kosct for the cycle Tj and the correction coefficient Ko for the average air-fuel ratio oscillation width DAFj
Each scaf is set by a one-dimensional map corresponding to the maximum oxygen storage amount OSCmax.

  The correction coefficients Kosct and Koscaf are within the range of the maximum oxygen storage amount OSCmax after the change, and the oxygen storage amount OSCmax is reduced along with the decrease of the maximum oxygen storage amount OSCmax so that the vibration width ΔOSC of the oxygen storage amount is maintained. The vibration amount ΔOSC of the occlusion amount is set to decrease. Further, by suppressing the vibration width by using the voltage of the upstream O2 sensor 110 in the control of the fuel correction coefficient FAF, it is possible to prevent the vibration width ΔOSC of the oxygen storage amount from deviating from the maximum oxygen storage amount OSCmax and greatly shaking out. Thus, it is possible to prevent the exhaust gas from deteriorating significantly.

  Further, following step 709, the cycle Tj and the vibration width DAFj are multiplied by correction coefficients Kptnt and Kptnaf corresponding to the number of vibrations PTN after the start of the vibration of the average air-fuel ratio in the same manner as Expressions (4) and (5). And further corrected (step 710). Note that the correction coefficient Kptnt for the period Tj and the correction coefficient Kptnaf for the vibration width DAFj are set according to the table shown in FIGS. 13A and 13B according to the number of vibrations PTN, respectively.

  In FIG. 13A, only the value for the initial vibration (PTN = 1) is set to “0.5”, and the period correction coefficient Kptnt is set to “1.0” for the other number of vibrations PTN. The In FIG. 13B, the vibration width correction coefficient Kptnaf is all set to “1.0” regardless of the vibration frequency PTN.

  By setting the correction coefficients Kptnt and Kptnaf as shown in FIGS. 13A and 13B, as shown in the timing chart of FIG. 14, the vibration width ΔOSC of the oxygen storage amount is finally set only at the first vibration. Set to half of the value. Further, by suppressing the vibration width using the voltage of the upstream O2 sensor 110 in the control of the fuel correction coefficient FAF, the vibration width ΔOSC does not exceed the predetermined width.

  13 and 14 show the case where the period correction coefficient Kptnt for the initial vibration is set to “0.5”, but the vibration width correction coefficient Kptnaf for the initial vibration is set to “0.5”. Good. Further, the combination of the correction coefficients Kptnt and Kptnaf for the period and the vibration width may be set so that the vibration width ΔOSC of the oxygen storage amount at the first vibration is halved.

  Further, as shown in the explanatory diagram of FIG. 15 and the timing chart of FIG. 16, the correction coefficients Kptnt of the period and vibration width, so that the vibration width ΔOSC of the oxygen storage amount gradually increases as the number of vibrations PTN increases. Kptnaf may be set. As a result, sudden changes in the state of the catalyst 109 can be prevented, and poor followability of air-fuel ratio control (particularly, control by the second air-fuel ratio feedback control means 202) can be prevented.

  Returning to FIG. 6, following step 710, in steps 711 to 714, the oxygen storage amount of the catalyst 109 becomes the maximum oxygen storage amount OSCmax or the minimum oxygen storage amount due to the rich / lean inversion of the output value V <b> 2 of the downstream O2 sensor 111. When the fluctuation of (= 0) is detected, a process for forcibly reversing the oscillation direction of the average air-fuel ratio is executed.

  First, it is determined whether or not the vibration is in the rich direction based on whether or not the vibration direction flag FRL is “1” (step 711), and the vibration is in the rich direction (FRL = 1) (ie, YES). Then, the downstream A / F is rich inversion depending on whether or not the reverse flag FRO2 of the downstream O2 sensor 111 is “1” (the output value V2 of the downstream O2 sensor 111 is It is determined whether or not the reversal from lean to rich is indicated (step 712).

  If it is determined in step 712 that the downstream A / F is rich inversion (FRO2 = 1) (that is, YES), the period counter (timer counter) Tmr is reset to the period Tj so that the vibration is inverted ( The process proceeds to step 714) and step 715. If it is determined in step 712 that the downstream A / F is not rich inversion and FRO2 ≠ 1 (that is, NO), the process returns to step 715 without executing the reset process of the cycle counter Tmr (step 714). move on.

  On the other hand, if it is determined in step 711 that the vibration is in the lean direction (FRL = 2) (that is, NO), then whether or not the reverse flag FRO2 of the downstream O2 sensor 111 is “2”, It is determined whether or not the downstream A / F is lean reversal (the output value V2 of the downstream O2 sensor 111 indicates reversal from rich to lean) (step 713).

  If it is determined in step 713 that the downstream A / F is lean reversal (FRO2 = 2) (that is, YES), the process proceeds to a reset process (step 714) of the period counter Tmr so that the vibration is reversed. If it is determined in step 713 that FRO2 ≠ 1 and the downstream A / F is not lean-reversed (that is, NO), the reset process (step 714) of the cycle counter Tmr is not performed, and the step is performed. Proceed to 715.

  Here, with reference to the timing chart of FIG. 17, a behavior when the oxygen storage amount of the catalyst 109 is not fully shaken will be described. When the oxygen storage amount suddenly changes due to disturbance of the air-fuel ratio due to disturbance, the maximum oxygen storage amount OSCmax decreases due to deterioration of the catalyst 109 or a decrease in the catalyst temperature Tmpcat, or the like. It is caused when the timing of reversing the fuel ratio is delayed.

  As shown in FIG. 17, when a large disturbance of the air-fuel ratio in the lean direction occurs immediately before time t141, the estimated oxygen storage amount OSC of the catalyst 109 increases significantly and rapidly, and at time t141, the maximum oxygen storage amount The amount is greatly out of the amount OSCmax.

  At this time, if the forced inversion process is not executed, the value of the period counter Tmr does not reach the inversion period Tj as indicated by the dotted line waveform, so that the vibration in the lean direction (FRL = 2) is continued. In addition, since the oxygen occlusion amount is maintained over the period from time t141 to time t142, the air-fuel ratio in the catalyst 109 deviates from the stoichiometric air-fuel ratio, and the exhaust gas purification state is significantly deteriorated.

  On the other hand, when the forced inversion process is executed in step 714 of FIG. 6, the output value V2 of the downstream O2 sensor 111 is inverted at time t141, the inversion flag FRO2 is changed from “0” to “2”, and the catalyst 109 In response to this, the period counter Tmr is reset to the inversion period Tj, and the vibration is forcibly inverted in the rich direction, as shown in the solid line waveform. As a result, the oxygen storage amount can be recovered from the shake-off state, and the deterioration of the exhaust gas can be suppressed to the minimum.

  Next, following the forced reset process (step 714), in steps 715 to 721, a rich / lean cycle inversion process by a timer process is executed. First, the cycle counter Tmr is updated by incrementing a predetermined amount Dtmr (step 715), and it is determined whether or not the cycle counter Tmr exceeds the cycle Tj (step 716). The predetermined amount Dtmr is set to a calculation cycle of 5 msec.

  If it is determined in step 716 that Tmr> Tj (that is, YES), the inversion timing has been reached, so the period counter Tmr is reset to “0” (step 717) and the vibration frequency PTN is set to “1”. (Step 718). Subsequently, it is determined whether or not the current vibration direction is rich depending on whether or not the vibration direction flag FRL is “1” (step 719).

  If it is determined in step 719 that the current vibration direction is rich (FRL = 1) (that is, YES), the vibration direction flag FRL is set to “2” and reversed in the lean direction (step 720). Proceed to 722.

  If it is determined in step 719 that the current vibration direction is lean (FRL = 2) (that is, NO), the vibration direction flag FRL is set to “1” and reversed in the rich direction (step 721). Go to step 722.

  On the other hand, if it is determined in step 716 that Tmr ≦ Tj (i.e., NO), the inversion timing has not been reached, and thus the process proceeds to step 722 immediately without executing steps 717 to 721.

In step 722, the target average air-fuel ratio AFAVEobj when the vibration condition is satisfied is set. At this time, the target average air-fuel ratio AFAVEobj is obtained by adding the vibration width DAFj to the vibration center AFCNT (target average air-fuel ratio calculated by the second air-fuel ratio feedback control means 202) as shown in the following equation (6). Calculated.
AFAVEobj = AFCNT + DAFj (6)

  As described above, the state of the oxygen storage amount of the catalyst 109 is detected based on the output value V2 of the downstream O2 sensor 111, so that the maximum oxygen storage amount OSCmax or the minimum oxygen storage amount (= 0) is not shaken. Since the vibration center AFCNT of the average air-fuel ratio AFAVEobj can be adjusted, the control accuracy of the vibration processing of the oxygen storage amount can be further improved.

  Note that the vibration center AFCNT may be set to a predetermined value depending on operating conditions. Further, the purification state of the catalyst 109 may be changed by shifting the vibration center AFCNT in a lean direction or a rich direction depending on conditions. Further, the vibration processing may be used not only for the deterioration diagnosis of the catalyst 109 but also for the failure diagnosis of the sensor or the like.

  On the other hand, if it is determined in the first step 702 that the vibration condition is not satisfied (that is, NO), the vibration frequency PTN is reset to “0” (step 723), and the cycle counter Tmr is reset to “0” (step 723). Step 724), the target average air-fuel ratio AFAVEobj when the vibration condition is not satisfied is set to the vibration center AFCNT (Step 725).

  Finally, a control constant in the first air-fuel ratio feedback control means 201 is set so as to coincide with the target average air-fuel ratio AFAVEobj set in step 722 or 725 (step 726). The processing routine of FIGS. 6A and 6B ends and exits.

  Next, the final step 726 in FIG. 6B will be specifically described. First, the average air-fuel ratio operation process based on the control constant executed in step 726 will be described. The average air-fuel ratio is a control constant (rich / lean skip amount RSR, RSL, rich / lean integral constants KIR, KIR2, KIL, KIL2, rich / lean delay periods τDR, τDL, or And the comparison voltage VR1, the switching voltage VR between KIR and KIR2, the switching voltage VL between KIL and KIL2 with respect to the output value V1 of the upstream O2 sensor 110, and the like.

  For example, the average air-fuel ratio shifts to the rich side when the rich skip amount RSR is increased or the lean skip amount RSL is decreased, and the lean side when the lean skip amount RSR is increased or the rich skip amount RSR is decreased. Migrate to That is, the average air-fuel ratio can be controlled by changing the rich skip amount RSR and the lean skip amount RSL.

  The average air-fuel ratio shifts to the rich side when the rich integral constants KIR and KIR2 are increased or the lean integral constants KIL and KIL2 are decreased, and the lean integral constants KIL and KIL2 are increased or the rich integral constants KIR. When KIR2 is reduced, the shift is made to the lean side. That is, the average air-fuel ratio can be controlled by changing the rich integration constants KIR, KIR2 and the lean integration constants KIL, KIL2.

  Further, the average air-fuel ratio shifts to the rich side if the rich delay period τDR and lean delay period τDL are set to the relationship of “τDR> τDL”, and conversely, if the relationship of “τDL> τDR” is set, Move to the lean side. That is, the average air-fuel ratio can be controlled by changing the rich / lean delay periods τDR, τDL.

  Further, the average air-fuel ratio shifts to the rich side when the comparison voltage VR1 with respect to the output value V1 of the upstream O2 sensor 110 is increased, and shifts to the lean side when the comparison voltage VR1 is decreased. That is, the average air-fuel ratio can be controlled by changing the comparison voltage VR1. Thus, the upstream average air-fuel ratio can be controlled by changing the control constants (delay period, skip amount, integral gain, comparison voltage, etc.).

  Moreover, the controllability of the average air-fuel ratio can be improved by simultaneously operating two or more of the control constants. However, when two or more control constants are operated, the rich / lean operation direction of the average air-fuel ratio can be managed, but the operation amount may be difficult to manage due to nonlinear interaction. Therefore, in order to solve the problems caused by the operation of a plurality of control constants and to actively use the degree of freedom, a means for calculating the operation amount of the control constant from the target average air-fuel ratio is further provided. A method is also conceivable in which control constants are set in accordance with the management index of the fuel ratio, and the operation of the control constant is managed by the average air-fuel ratio.

  Further, in controlling the average air-fuel ratio for each control constant, for example, there are advantages and disadvantages regarding the control accuracy of the average air-fuel ratio, the operation width, or the control cycle, the air-fuel ratio oscillation width, etc., but the target average air-fuel ratio By finely setting each control constant according to the operating point, it is possible to take advantage of each advantage.

Next, control constant setting calculation processing by the average air-fuel ratio oscillation means 203 will be described with reference to FIG.
FIG. 18 is a flowchart schematically showing a control constant setting calculation process. The control constants in the first air-fuel ratio feedback control means 201 (each skip amount RSR, RSL, each integral constant KIR, An arithmetic routine for setting KIR2, KIL, KIL2, delay periods τDR, τDL, comparison voltage VR1, switching voltages VR, VL) is shown. The calculation routine of FIG. 18 is executed every predetermined period (for example, 5 msec).

  In FIG. 18, first, the rich skip amount RSR is calculated using a one-dimensional map corresponding to the target average air-fuel ratio AFAVEobj (step 1501). Note that the value of the one-dimensional map is set in advance based on desktop calculation or experiment, and a set value (map search result) corresponding to the input value of the target average air-fuel ratio AFAVEob is output as the rich skip amount RSR.

  Further, the one-dimensional map in step 1501 is provided for each operating condition of the internal combustion engine 101, and the map search is performed by switching the one-dimensional map according to the change in the operating condition. The operating conditions include conditions related to responsiveness and characteristics of the configuration of the first air-fuel ratio feedback control means 201 (for example, engine speed Ne, load, idle state, cooling water temperature THW, exhaust temperature, upstream O2 sensor) Temperature, EGR valve opening, etc.). Further, for example, the operation condition can be set as an operation region divided by a predetermined rotation speed, load, and cooling water temperature.

  Furthermore, the calculation map of the rich skip amount RSR is not necessarily a one-dimensional map, and may be any means that represents the relationship between the input value and the output value. For example, instead of the one-dimensional map, an arbitrary approximate expression may be used, and a high-dimensional map or higher-order function corresponding to more input values may be used.

  Thereafter, the skip amount RSL is calculated according to the target average air-fuel ratio AFAVEobj in the same processing method as in step 1501 (step 1502), the integration constant KIR is calculated according to the target average air-fuel ratio AFAVEobj (step 1503), and the target An integration constant KIR2 is calculated according to the average air-fuel ratio AFAVEobj (step 1504), an integration constant KIL is calculated according to the target average air-fuel ratio AFAVEobj (step 1505), and an integration constant KIL2 is calculated according to the target average air-fuel ratio AFAVEobj. (Step 1506), the delay period τDR is calculated according to the target average air-fuel ratio AFAVEobj (step 1507), the delay period τDL is calculated according to the target average air-fuel ratio AFAVEobj (step 1508), and the target average air-fuel ratio AFAVEobj is set. According to the comparison voltage VR1 is calculated (step 1509), the switching voltage VR is calculated according to the target average air-fuel ratio AFAVEobj (step 1510), the switching voltage VL is calculated according to the target average air-fuel ratio AFAVEobj (step 1511), and FIG. The processing routine ends.

  Thus, according to the target average air-fuel ratio AFAVEobj, the control constants (each skip amount RSR, RSL, each integral constant KIR, KIR2, KIL, KIL2, each delay period τDR, τDL, comparison voltage VR1, switching voltage VR, VL) ) Are respectively calculated.

  As described above, the set values on the calculation maps in steps 1501 to 1511 are calculated in advance so that the actual average air-fuel ratio upstream of the catalyst 109 matches the target average air-fuel ratio AFAVEobj which is the input value. Or it is set based on experimental values. Further, the actual average air-fuel ratio is set to match the target average air-fuel ratio AFAVEobj regardless of the operating conditions by changing the set value of the control constant according to the operating conditions.

  Next, the processing operation by the maximum oxygen storage amount calculating means 204 will be described with reference to the explanatory diagrams of FIGS. 20 and 21 together with the flowchart of FIG. The calculation routine of FIG. 19 is executed every predetermined period (for example, 5 msec).

  In FIG. 19, first, an initial value OSCmax0 of the maximum oxygen storage amount of the catalyst 109 is set (step 1601). As the initial value OSCmax0, a maximum oxygen storage amount of a newly designed catalyst may be set. Further, as the initial value OSCmax0, the maximum oxygen storage amount of the durable catalyst after traveling a predetermined distance determined by the exhaust gas regulation may be set. In this case, the initial value OSCmax0 that reliably satisfies the requirements of the exhaust gas regulation. Can be set. Further, as the initial value OSCmax0, the maximum oxygen storage amount in the steady state may be set based on the operating conditions (engine speed Ne, load, intake air amount Qa, etc.) of the internal combustion engine 101. Will improve.

  Subsequently, the catalyst temperature Tmpcat is calculated (step 1602). The catalyst temperature Tmpcat may be obtained directly by measurement by attaching a temperature sensor to the catalyst 109 or by arranging temperature sensors upstream and downstream of the catalyst 109.

  Further, the catalyst temperature Tmpcat may be obtained by estimation calculation from other operating condition information. For example, the catalyst temperature Tmpcat is estimated and calculated as a value in the steady state by reading out the value in the steady state set for each operating condition (engine speed Ne, load, intake air amount Qa, etc.) by map calculation. be able to. Further, the behavior of the internal combustion engine 101 during the transition can be estimated by applying a filter process to the steady-state catalyst temperature Tmpcat.

  Furthermore, the initial catalyst temperature Tmpcat0 at the time of start can be estimated from the coolant temperature THW at the time of start or the interval between the previous stop and the current start. As a result, not only the transient temperature behavior from the start of the internal combustion engine 101 to the activation of the catalyst 109 until the steady state is reached, but also the transient temperature behavior due to fluctuations in operating conditions can be obtained.

  Next, following step 1602, a temperature correction coefficient Ktmpcat for the maximum oxygen storage amount OSCmax is calculated from a one-dimensional map (see FIG. 20) set according to the catalyst temperature Tmpcat (step 1603).

  As shown in FIG. 20, the temperature correction coefficient Ktmpcat is set to a small value so that the maximum oxygen storage amount OSCmax decreases as the catalyst temperature Tmpcat decreases. Further, since the oxygen storage function of the catalyst 109 has a characteristic that it is rapidly activated in a temperature range of about 300 ° C. to 400 ° C., the temperature correction coefficient Ktmpcat is set in consideration of the temperature characteristic of the catalyst 109.

  Subsequently, the catalyst deterioration degree Catdet is adaptively calculated with respect to the output value V2 of the downstream O2 sensor 111 (step 1604). The degree of catalyst deterioration Catdet increases as the degree of deterioration of the catalyst 109 increases.

  Subsequently, a deterioration correction coefficient Kcatdet for the maximum oxygen storage amount is calculated from a one-dimensional map (see FIG. 21) set in accordance with the catalyst deterioration degree Catdet (step 1605). As shown in FIG. 21, the deterioration correction coefficient Kcatdet is set to a small value so as to decrease the maximum oxygen storage amount OSCmax as the catalyst deterioration degree Catdet increases.

Finally, the initial value OSCmax0 of the maximum oxygen storage amount is corrected based on the temperature correction coefficient Ktmpcat and the deterioration correction coefficient Kcatdet, and the maximum oxygen storage amount OSCmax is calculated as in the following equation (7) (step 1606).
OSCmax = OSCmax0 × Ktmpcat × Kcatdet (7)

  According to the equation (7), the maximum oxygen storage amount OSCmax that changes in accordance with various conditions such as a change in the catalyst temperature Tmpcat according to not only changes in various operating conditions but also during the activation of the catalyst 109 and during transition, and deterioration of the catalyst 109. And the control accuracy of the vibration processing of the oxygen storage amount of the catalyst 109 can be improved.

  Next, the catalyst deterioration degree calculation process (step 1604) in FIG. 19 by the maximum oxygen storage amount calculation means 204 will be described in more detail with reference to the flowchart of FIG. The calculation routine of FIG. 22 is executed every predetermined period (for example, 5 msec).

In FIG. 22, first, it is determined whether an initialization condition for the catalyst deterioration degree Catdet is satisfied (step 1901). If it is determined that the initialization condition is satisfied (that is, YES), the catalyst deterioration is performed. The degree Catdet is reset to “0” (no deterioration state) (step 1902), and the process proceeds to step 1903. If it is determined in step 1901 that the initialization condition is not satisfied (that is, NO), the process proceeds to step 1903 without executing step 1902.

  The catalyst deterioration degree Catdet is recorded and held in the backup RAM 124 (or EEPROM or the like) in the ECU 112 so that it is not reset when the internal combustion engine 101 is stopped, but it is the first after the battery is removed or the EEPROM is initialized. When the power is turned on, the initialization condition is established.

  Further, when the calculation of the catalyst deterioration degree Catdet becomes impossible (when a sensor failure such as the downstream O2 sensor 111 is detected), the recalculation condition of the catalyst deterioration degree Catdet is satisfied, or the external Even when there is a reset request by communication from a device (not shown), it is determined in step 1901 that the initialization condition is satisfied.

  Subsequently, the rich / lean inversion determination process of the output value V2 of the downstream O2 sensor 111 is performed (step 1903). The determination process in step 1903 is performed in the same manner as the determination process in step 701 in FIG.

  That is, when the output value V2 of the downstream O2 sensor 111 is inverted from lean to rich, the inversion flag FRO2det of the downstream O2 sensor 111 is set to “1”, and when inverted from rich to lean, it is inverted. The flag FRO2det is set to “2”, and if not inverted, the inversion flag FRO2det is set to “0”. Note that the hysteresis setting range or dead band setting range and the degree of the smoothing processing of the output value V2 shown in FIG. 8 may be set differently from the case of the average air-fuel ratio oscillating means 203.

  Next, following step 1903, it is determined whether or not an update condition for the catalyst deterioration degree Catdet is satisfied (step 1904), and it is determined that an update condition for the catalyst deterioration degree Catdet is satisfied (that is, YES). If so, the process proceeds to step 1905 and subsequent steps. If it is determined in step 1904 that the update condition is not satisfied (that is, NO), steps 1905 to 1910 are not executed, and the processing routine of FIG. 22 is terminated.

  The conditions for updating the catalyst deterioration degree Catdet are satisfied under conditions where it can be determined that the catalyst 109 is sufficiently active and under conditions where vibration processing of the average air-fuel ratio is being executed. Further, the active state of the catalyst 109 may be determined directly from the catalyst temperature Tmpcat, such as an elapsed period after the start of the internal combustion engine 101, an integrated intake air amount after the start, an engine rotational speed Ne, a load, or the like. You may determine based on predetermined driving | running conditions. Further, the active state of the catalyst 109 may be determined based on whether or not the number of vibrations PTN of the vibration processing of the average air-fuel ratio has reached a predetermined number or more.

  Subsequently, in steps 1905 to 1909, based on the rich / lean inversion of the output value V2 of the downstream O2 sensor 111, the oxygen storage amount of the catalyst 109 is changed from the maximum oxygen storage amount OSCmax or the minimum oxygen storage amount (= 0). Is detected, and the catalyst deterioration degree Catdet is increased or decreased.

  First, based on whether or not the vibration direction flag FRL is “1”, it is determined whether or not the vibration is in the rich direction (step 1905), and it is determined that the vibration is in the rich direction (FRL = 1) (that is, YES). If so, the process proceeds to step 1906. If it is determined in step 1905 that vibration is in the lean direction (FRL = 2) (that is, NO), the process proceeds to step 1907.

  In step 1906, which is executed when it is determined in step 1905 that FRL = 1 (that is, YES), rich inversion (downstream) is performed depending on whether or not the inversion flag FRO2det of the downstream O2 sensor 111 is “1”. The output value V2 of the side O2 sensor 111 is inverted from lean to rich).

If it is determined in step 1906 that rich inversion (FRO2det = 1) (that is, YES), the catalyst deterioration degree Catdet is increased by a predetermined set value XdetH (step 1908) and updated as in the following equation (8). Calculate and proceed to step 1910.
Catdet = Catdet + XdetH (8)

  On the other hand, in step 1907, which is executed when it is determined in step 1905 that FRL = 2 (that is, NO), lean inversion is performed depending on whether or not the inversion flag FRO2det of the downstream O2 sensor 111 is “2”. It is determined whether or not the output value V2 of the downstream O2 sensor 111 is inverted from rich to lean.

  If it is determined in step 1907 that lean reversal (FRO2det = 2) (that is, YES) is reached, the process proceeds to step 1908, and the catalyst deterioration degree Catdet is increased by a predetermined set value XdetH as shown in the equation (8).

On the other hand, if it is determined in step 1906 that lean reversal (FRO2det = 2) (that is, NO), or if it is determined in step 1907 that rich reversal (FRO2det = 1) (that is, NO), The catalyst deterioration degree Catdet is decreased by a predetermined set value XdetL (step 1909), the update calculation is performed as in the following equation (9), and the process proceeds to step 1910.
Catdet = Catdet−XdetL (9)

  The predetermined set values XdetH and XdetL in the equations (8) and (9) are set in consideration of the oscillation cycle of the average air-fuel ratio, and the intake air amount is inversely proportional to the intake air amount Qa. It is set according to Qa or according to operating conditions.

Finally, in step 1910, the upper and lower limit restriction processing of the catalyst deterioration degree Catdet is performed using the following equation (10) so as to be a value within the range between the upper limit value MXdet and the lower limit value MNdet, and FIG. The processing routine ends.
MNdet ≦ Catdet ≦ MXdet (10)

  Next, the processing operation of the catalyst deterioration diagnosis unit 205 will be described with reference to FIGS. 23, 24A, and 24B. FIG. 23 is a timing chart showing the behavior of the catalyst 109 when it is deteriorated, and FIGS. 24A and 24B are flowcharts showing the processing operation of the catalyst deterioration diagnosis means 203. 24A and 24B are executed every predetermined period (for example, 5 msec).

  In FIG. 23, when the maximum oxygen storage amount OSCmax decreases due to the deterioration of the catalyst 109, and the vibration width of the oxygen storage amount due to the vibration processing of the average air-fuel ratio starts to swing from the reduced maximum oxygen storage amount OSCmax, the downstream O2 As the rich / lean inversion of the output value V2 of the sensor 111 increases, the catalyst deterioration degree Catdet increases.

24A and 24B, first, it is determined whether or not an initialization condition for the deterioration diagnosis of the catalyst 109 is satisfied (step 2101), and it is determined that the initialization condition is satisfied (that is, YES). For example, the diagnosis number Nratio is reset to “0” (step 2102), the integrated value Roasm of the inversion number ratio Roa is reset to “0” (step 2103), and the deterioration diagnosis result Fcatj is set to “0” (unjudged state). (Step 2104), the inversion frequency ratio average value Roave is reset to “0” (step 2105), and then it is determined whether or not the deterioration diagnosis condition is satisfied (step 2106).

  If it is determined in step 2101 that the initialization condition is not satisfied (that is, NO), the process proceeds to step 2106 without executing steps 2102 to 2105.

  The information of the catalyst deterioration diagnosis means 205 (catalyst deterioration degree Catdet and the like) is recorded and held in the backup RAM 124 (or EEPROM or the like) so that it is not reset when the internal combustion engine 101 is stopped, but the battery is removed. When the power is turned on for the first time after initialization or after initialization of the EEPROM, the initialization condition of step 2101 is established.

  Further, when the calculation of the catalyst deterioration degree Catdet becomes impossible (when a failure of the downstream O2 sensor 111 or the like is detected), the recalculation condition of the catalyst deterioration degree Catdet is satisfied, or an external device (not shown) Also in the case where there is a reset request by communication from), it is determined in step 2101 that the initialization condition is satisfied.

  If it is determined in step 2106 that the deterioration diagnosis condition is satisfied (that is, YES), then it is determined whether or not the target average air-fuel ratio has been reversed between rich / lean (step 2107) and reversed. If it is determined (that is, YES), the average air-fuel ratio inversion number Naf is increased by “1” (step 2108), and the process proceeds to step 2109.

  If it is determined in step 2107 that the target average air-fuel ratio is not reversed (that is, NO), step 2108 is not executed and the routine proceeds to step 2109.

  Note that the inversion determination of the target average air-fuel ratio in step 2107 is performed based on whether or not the vibration direction flag FRL has changed to “1” (rich) or “2” (lean). That is, the reversal of the target average air-fuel ratio can be determined by storing the vibration direction flag FRL at the previous calculation and comparing it with the vibration direction flag FRL at the current calculation.

  On the other hand, if it is determined in step 2106 that the deterioration diagnosis condition is not satisfied (that is, NO), the average air-fuel ratio inversion number Naf is reset to “0” (step 2132), and the downstream O2 inversion number Nro2 is set. It is reset to “0” (step 2133), the delay determination flag Frsdly is reset to “0” (indicating non-execution of delay processing described later) (step 2134), and the process proceeds to step 2127. Step 2127 will be described later.

  Note that the deterioration diagnosis condition in step 2106 is the same as the update condition of the catalyst deterioration degree Catdet described above (step 1904 in FIG. 22) under the conditions under which it can be determined that the catalyst 109 is sufficiently active and the average air-fuel ratio. It is established under conditions where vibration processing is being executed. Further, the active state of the catalyst 109 may be determined directly from the catalyst temperature Tmpcat, such as an elapsed period after the start of the internal combustion engine 101, an integrated intake air amount after the start, an engine rotational speed Ne, a load, or the like. You may determine based on predetermined driving | running conditions.

  Further, the active state of the catalyst 109 may be determined based on whether or not the number of vibrations PTN of the vibration processing of the average air-fuel ratio has reached a predetermined number or more. This prevents misdiagnosis caused by diagnosing an unstable state after the start of vibration, and can start diagnosis from the point of time when the vibration starts stably, improving diagnostic accuracy Can do.

  Returning to Step 2108, subsequently, the processing for determining the rich / lean inversion of the output value V2 of the downstream O2 sensor 111 is executed (Step 701 in FIG. 6A, Step 1903 in FIG. 22) (Step S108). 2109).

  When it is determined in step 2109 that the output value V2 is inverted from lean to rich, the inversion flag FRO2rv of the downstream O2 sensor 111 is set to “1”, and the inversion from rich to lean is determined. The inversion flag FRO2rv is set to “2”, and when the inversion is not determined, the inversion flag FRO2rv is set to “0”.

  At this time, as in the case of the above-described step 1903 (see FIG. 22), the hysteresis setting range or dead band setting range shown in FIG. You may set so that it may differ from the case of.

  Steps 2106 to 2109 detect that the oxygen storage amount of the catalyst 109 has been swung from the maximum oxygen storage amount OSCmax or the minimum oxygen storage amount (= 0) by rich / lean inversion of the output value V2 of the downstream O2 sensor 111. In response to this, the catalyst deterioration degree Catdet is increased or decreased.

  Next, it is determined whether or not the output value V2 (downstream A / F) is inverted depending on whether or not the inversion flag FRO2rv is “1” or “2” (step 2110), and is inverted (FRO2rv = 1 or FRO2rv = 2) (ie, YES), the downstream O2 inversion number Nro2 is increased by “1” (step 2111).

  Subsequently, it is determined whether or not the update condition of the determination reference value Xroa for deterioration diagnosis is satisfied based on whether or not the average air-fuel ratio inversion frequency Naf is equal to or greater than the update condition determination value Xnaf (step 2112). If it is determined that the update condition of the reference value Xroa is satisfied (Naf ≧ Xnaf) (ie, YES), the determination is made by setting the average air-fuel ratio inversion number Naf as the determination average air-fuel ratio inversion number Nafj. The average air-fuel ratio inversion frequency Nafj for use is updated (step 2113).

  Further, as a preparation for calculating the next determination reference value Xroa, the average air-fuel ratio inversion number Naf is reset to “0” (step 2114) and until the output value V2 fluctuates after the average air-fuel ratio fluctuates. Is set to “1” (indicating that the delay process is in progress) (step 2115), and whether the delay process is in progress depends on whether or not the delay determination flag Frsdly is “1”. It is determined whether or not (step 2116).

  On the other hand, if it is determined in step 2112 that the update condition for the determination reference value Xroa is not satisfied (Naf <Xnaf) (that is, NO), the process proceeds to step 2116 without executing steps 2213 to 2115.

If it is determined in step 2116 that the delay process is being performed (Frsdly = 1) (that is, YES), the delay timer Trsdly is increased and updated by a predetermined value DTrsdly as shown in the following equation (11) (step 2117), the process proceeds to Step 2119.
Trsdly = Trsdly + DTrsdly (11)

  In Expression (11), the predetermined value DTrsdly for timer update is set to, for example, a calculation cycle of 5 msec.

  If it is determined in step 2116 that the delay process is not in progress (Frsdly = 0) (ie, NO), the delay timer Trsdly is reset to “0” (step 2118), and the process proceeds to step 2119.

  In step 2119, it is determined whether or not the delay period has elapsed based on whether or not the delay timer Trsdly is greater than the predetermined determination value Xrsdly, and the delay period has not elapsed (Trsdly ≦ Xrsdly) (ie, NO). If it is determined, step 2127 follows. Step 2127 will be described later.

  If it is determined in step 2119 that the delay period has elapsed (Trsdly> Xrsdly) (that is, YES), the update condition for the deterioration diagnosis determination information based on the output value V2 of the downstream O2 sensor 111 is satisfied. Therefore, the following update process (steps 2120 to 2126) is executed.

  The predetermined determination value Xrsdly is set in consideration of a period delay until the output value V2 of the downstream O2 sensor 111 of the catalyst 109 fluctuates due to fluctuations in the average air-fuel ratio. This period delay includes a period delay from when the fuel is injected from the injector 107 until the air-fuel mixture actually moves to the installation position of the downstream O2 sensor 111, and a period delay due to the oxygen storage action of the catalyst 109. Generally, it is inversely proportional to the intake air amount Qa. Accordingly, the predetermined determination value Xrsdly is set by a one-dimensional map corresponding to the intake air amount Qa, for example.

  Further, although the delay timer Trsdly (timer operation) is used for the determination of the update condition in Step 2119, the delay determination flag Frsdly is set to “1” without using the delay timer Trsdly (during delay processing). ) Is calculated, and it may be determined that the update condition is satisfied when the integrated amount of the intake air amount Qa is larger than a predetermined amount.

  In the deterioration diagnosis determination information update process subsequent to step 2119, first, the determination downstream O2 inversion number Nro2j is set as the downstream O2 inversion number Nro2j, thereby updating the determination downstream O2 inversion number Nro2j (step 2119). 2120).

  In preparation for calculating the next determination reference value Xroa, the downstream O2 inversion number Nro2 is reset to “0” (step 2121), and the delay determination flag Frsdly is reset to “0” (step 2122). ), And the delay processing ends.

Subsequently, since the determination average air-fuel ratio inversion number Nafj and the corresponding determination downstream O2 inversion number Nro2j are aligned, the determination average air-fuel ratio inversion number Nafj is expressed by the following equation (12). And the inversion number ratio Roa with the determination downstream O2 inversion number Nro2j is updated (step 2123).
Roa = Nro2j / Nafj (12)

Subsequently, in order to update the average value Roave of the inversion number ratio Roa, first, the inversion number ratio Roa is added to the previous integrated value Roasm to update the integrated value Roasm (step 2124), and the diagnosis number Nratio is calculated. After incrementing by “1” (step 2125), the inversion frequency ratio average value Roave is updated as shown in the following equation (13) (step 2126).
Roave = Roasm / Nratio (13)

  Next, it is determined whether or not the deterioration diagnosis process is not executed depending on whether or not the deterioration diagnosis result Fcatj is “0” (step 2127), and the diagnosis process has been executed (Fcatj = 1 or Fcatj). = 2) (ie, NO), the processing routine of FIG. 24 is terminated.

  If it is determined that the diagnosis process has not been executed (Fcatj = 0) (that is, YES), then the diagnosis condition is satisfied depending on whether the diagnosis count Nratio matches the diagnosis execution count Xnr. It is determined whether or not (step 2128), and if it is determined that the diagnosis condition is not satisfied (Nratio ≠ Xnr) (that is, NO), the processing routine of FIG. 24 is terminated.

  If it is determined in step 2128 that the diagnosis condition is satisfied (Nratio = Xnr) (that is, YES), the deterioration diagnosis process of the catalyst 109 is executed, and the inversion number ratio average value Roave is determined as the determination reference value Xroa. Whether or not the catalyst has deteriorated is determined based on whether or not it is above (step 2129).

  If it is determined in step 2129 that the catalyst 109 is in a deteriorated state (Roaave ≧ Xroa) (that is, YES), the deterioration diagnosis result Fcatj is set to “2” (indicating deterioration) (step 2130). The process routine of 24 is ended.

  In Step 2129, if it is determined that the catalyst 109 is in a normal state (Roaave <Xroa) (ie, NO), the deterioration diagnosis result Fcatj is set to “1” (indicating normal) (Step 2131). Then, the processing routine of FIG. 24 is terminated.

The determination reference value Xroa is adjusted to a value that can detect a decrease state of the maximum oxygen storage amount OSCmax of the catalyst that needs to be diagnosed for deterioration.
Further, by setting the oxygen storage amount due to the oscillation of the average air-fuel ratio to a value larger than the maximum oxygen storage amount OSCmax of the catalyst that needs to be deteriorated, it is possible to reliably detect the catalyst that requires the deterioration diagnosis.

  Further, the downstream O2 inversion frequency Nro2 (the inversion frequency of the output value V2 of the downstream O2 sensor 111) is determined based on a comparison with the oscillation frequency of the oxygen storage amount (the oscillation frequency PTN of the average air-fuel ratio oscillation process). Accordingly, it is possible to prevent deterioration in the determination accuracy of the deterioration diagnosis due to the vibration cycle that changes depending on the operation condition and operation pattern of the internal combustion engine 101.

  Here, the deterioration of the catalyst is diagnosed by using the average number of inversions Roave. However, when the catalyst deterioration degree Catdet calculated by the maximum oxygen storage amount calculating means 204 indicates a predetermined value or more, the catalyst 109 is deteriorated. You may judge.

  Next, the behavior in the catalyst deterioration diagnosis will be described with reference to the timing chart of FIG. FIG. 25 shows the behavior of each parameter when the maximum oxygen storage amount OSCmax decreases due to the deterioration of the catalyst 109 and the vibration width of the oxygen storage amount starts to swing out.

  In FIG. 25, even if it is determined that the output value V2 of the downstream O2 sensor 111 is reversed, the reason why the average air-fuel ratio is not reversed is that the hysteresis width of the catalyst deterioration diagnosis unit 205 is that of the average air-fuel ratio oscillation unit 203. This is because it is set narrower than the hysteresis width.

  First, at time t221, when the average air-fuel ratio (see the vibration direction flag FRL) is reversed from rich to lean, the average air-fuel ratio inversion number Naf reaches the update condition determination value Xnaf, and the delay timer Trsdly starts to increase.

  Subsequently, the influence of reversal from rich to lean at time t221 due to the above-described movement delay of the air-fuel mixture and oxygen storage action starts to appear around time t222 with a time delay, and at time t222, the output of the downstream O2 sensor 111 is output. The value V2 is richly inverted.

  On the other hand, the delay timer Trsdly reaches the predetermined determination value Xrsdly at time t223, and updates the determination downstream O2 inversion number Nro2j. As described above, by providing the delay timer Trsdly in consideration of the delay of the control system, it is possible to accurately detect the fluctuation of the output value V2 of the downstream O2 sensor 111 corresponding to the oscillation of the average air-fuel ratio.

  Next, calculation processing by the second air-fuel ratio feedback control means 202 will be described with reference to the flowchart of FIG. 26 and the explanatory diagram of FIG. The processing routine of FIG. 26 shows a procedure for calculating the vibration center AFCNT of the average air-fuel ratio vibration based on the output value V2 of the downstream O2 sensor 111, and is executed every predetermined period (for example, 5 msec).

  In FIG. 26, the second air-fuel ratio feedback control means 202 first reads the output value V2 of the downstream O2 sensor 111 and performs filter processing (or smoothing processing such as averaging processing) (step 2301). Control based on the output value V2flt after processing is enabled.

  Subsequently, it is determined whether or not the feedback region (the closed loop condition is satisfied) by the downstream O2 sensor 111 (step 2302). In step 2302, for example, air-fuel ratio control conditions other than the stoichiometric air-fuel ratio control (during the start of the internal combustion engine 101, during the enrichment control when the cooling water temperature THW is low, during the enrichment control when the high load power is increased. The closed-loop condition is determined not to be established when the downstream O2 sensor 111 is in an inactive state or at a failure, etc., during leaning control when improving fuel efficiency, during leaning control after starting, during fuel cut, etc.) In other cases, it is determined that the closed loop condition is satisfied.

  Whether the downstream O2 sensor 111 is active / inactive is determined whether or not a predetermined period has elapsed since the start, or whether or not the level of the output value V2 of the downstream O2 sensor 111 has once crossed the predetermined voltage. It can be determined by.

If it is determined in step 2302 that the closed-loop condition is not satisfied (that is, NO), the initial value AFCNT0 of the oscillation center (central air-fuel ratio) of the average air-fuel ratio oscillation and the integral calculation value (hereinafter referred to as an integral value). Using AFI, the vibration center AFCNT of the average air-fuel ratio vibration is obtained as in the following equation (14) (step 2313), and the processing routine of FIG.
AFCNT = AFCNT0 + AFI (14)

  In the equation (14), the initial value AFCNT0 is set to “14.53”, for example. Further, the integral value AFI is a value immediately before the end of the closed loop control, and is held in the backup RAM 106 in the control circuit 10. The initial value AFCNT0 and the integral value AFI are set values that are held for each operation condition (for example, an operation region divided by the engine rotational speed Ne, load, and coolant temperature THW), and are respectively stored in the backup RAM 106. Is held in.

  On the other hand, if it is determined in step 2302 that the closed loop condition is satisfied (that is, YES), the target value VR2 of the output value V2 of the downstream O2 sensor 111 is set (step 2303).

  The target value VR2 may be set to a predetermined output value (for example, around 0.45 V) of the downstream O2 sensor 111 corresponding to the purification window of the catalyst 109 near the theoretical air-fuel ratio, or the NOx purification rate of the catalyst 109 May be set to a higher voltage (for example, near 0.75 V), a lower voltage (for example, near 0.2 V), etc., to increase the purification rate of CO, HC, etc. It may be variably changed according to operating conditions.

  When the target value VR2 is changed according to the operating conditions, a smoothing process (for example, first-order lag filter process) is performed on the target value VR2 in order to mitigate fluctuations in the air-fuel ratio due to step changes at the time of change. May be applied.

  Next, following step 2303, a deviation ΔV2 (= VR2−V2flt) between the target value VR2 of the output value V2 and the output value V2flt after filtering is calculated (step 2304), and the deviation ΔV2 is “0”. In order to set such a vibration center AFCNT, PI control processing (proportional calculation and integral calculation) corresponding to the deviation ΔV2 is performed (steps 2305 to 2311).

  For example, if the output value V2 of the downstream O2 sensor 111 is smaller than the target value VR2 and is on the lean side, the upstream target average air-fuel ratio AFAVEobj is set to the rich side, and the output value of the downstream O2 sensor 111 is set. The operation is performed so that V2 returns to the target value VR2.

The target average air-fuel ratio AFAVEobj on the upstream side of the catalyst 109 is obtained by an ordinary PI controller using an initial value AFAVE0 of the target average air-fuel ratio, an integral operation amount Σ {Ki2 (ΔV2)} based on the integral gain Ki2, and a proportional gain. Using the proportional manipulated variable Kp2 (ΔV2) based on Kp2, the calculation is performed as in the following equation (15).
AFAVEobj = AFAVE0 + Σ {Ki2 (ΔV2)} + Kp2 (ΔV2) (15)

  In equation (15), the initial value AFAVE0 is a value corresponding to the theoretical air-fuel ratio set for each operating condition, and is set to “14.53”, for example. Further, the integral calculation based on the integral gain Ki2 generates an output while integrating the deviation ΔV2 and operates relatively slowly, so that the output value V2 of the downstream O2 sensor 111 caused by the characteristic variation of the upstream O2 sensor 110 is calculated. There is an effect of eliminating the steady deviation.

  As the integral gain Ki2 is set larger, the absolute value of the integral operation amount Σ {Ki2 (ΔV2)} becomes larger and the control effect for eliminating the deviation becomes larger. However, if the integral gain Ki2 is set too large, the phase delay becomes larger. Since the control system becomes unstable and hunting occurs, an appropriate gain setting is required.

  On the other hand, since the proportional calculation based on the proportional gain Kp2 generates an output proportional to the deviation ΔV2 and exhibits quick response, it has an effect of quickly returning the deviation. The larger the proportional gain Kp2 is set, the larger the absolute value of the proportional manipulated variable Kp2 (ΔV2) (for example, “Kp2 · ΔV2”), and the return speed becomes faster. Since it becomes unstable and causes hunting, an appropriate gain setting is required.

  In the PI control process, first, it is determined whether or not an update condition for the integral value AFI is satisfied (step 2305). The update condition of the integral value AFI is satisfied during the transient operation and when it is outside the predetermined period after the transient operation.

  For example, during transient operation, the upstream side A / F is greatly disturbed, and similarly the downstream side A / F is also disturbed. Therefore, if an integral operation is performed in this state, an incorrect value is integrated. Since it operates slowly, the control performance deteriorates by holding the wrong value for a while after the transient operation.

  Therefore, during transient operation, the integration calculation is temporarily stopped and the integration value AFI is held to prevent the erroneous integration calculation as described above. In addition, even after the transient operation, the influence remains for a while due to the delay of the controlled object, so that the update of the integral value AFI is prohibited even in a predetermined period after the transient operation. In particular, since the delay of the catalyst 109 is large, the predetermined period after the transient operation may be set as the period until the integrated intake air amount after the transient operation reaches a predetermined value. This is because the speed until the state of the catalyst 109 returns from the influence of the transient operation depends on the O 2 occlusion action of the catalyst 109 and is proportional to the intake air amount Qa.

  The transient operation includes sudden acceleration / deceleration, fuel cut, enrichment control, lean control, stop of control by the second air-fuel ratio feedback control unit 202, stop of control by the first air-fuel ratio feedback control unit 201, Including sudden changes in transpiration gas introduction. Abrupt acceleration / deceleration is determined when the amount of change in the throttle opening per unit period exceeds a predetermined value, or when the amount of change in the intake air amount Qa per unit period indicates a predetermined value or more. Moreover, the sudden change of the transpiration gas introduction is determined when the amount of change per unit period of the valve opening for introducing the transpiration gas is greater than a predetermined value.

  If it is determined in step 2305 that the update condition for the integral value AFI is satisfied (that is, YES), the update amount Ki2 (ΔV2) based on the integral gain Ki2 is added to the previous integral value AFI to integrate the integral value AFI. The value AFI is updated (step 2306), and the process proceeds to step 2308.

  The integrated value AFI is held in the backup RAM 106 for each operating condition as described above. The update amount Ki2 (ΔV2) may be simply set as “Ki2 · ΔV2”, or may be variably set to a value corresponding to the deviation ΔV2 using a one-dimensional map as shown in FIG. 27 (so-called variable gain). Setting).

  Further, the characteristic fluctuation of the upstream O2 sensor 110 compensated by the integral value AFI changes according to the operating conditions such as the exhaust temperature or the exhaust pressure, so the integrated value AFI is updated and set every time the operating conditions change. Are stored in the backup RAM 106 and are switched for each operating condition. Further, since the integral value AFI is held in the backup RAM 106, it is reset every time the internal combustion engine 101 is stopped or restarted, and a decrease in control performance can be avoided.

On the other hand, if it is determined in step 2305 that the update condition of the integral value AFI is not satisfied (that is, NO), the previous integral value AFI is set as it is, and the integral value AFI is not updated (step 2307). ), Go to Step 2308. In step 2308, the upper and lower limit limiting processing of the integrated value AFI is performed using the minimum value AFImin and the maximum value AFImax of the integrated value AFI so as to satisfy the following expression (16).
AFImin <AFI <AFImax (16)

  The minimum value AFImin and the maximum value AFImax are set to appropriate limit values that can compensate for the characteristic fluctuation range (which can be grasped in advance) of the upstream O2 sensor 110. Thereby, an excessive air-fuel ratio operation can be avoided.

  Subsequently, a proportional calculation process is performed, and a proportional operation amount Kp2 (ΔV2) is set as a proportional calculation value (hereinafter referred to as a proportional value) AFP (step 2309). The proportional value Kp2 (ΔV2) may be simply set as “Kp2 · ΔV2”. Like the update value Ki2 (ΔV2) of the integral value AFI, a deviation ΔV2 is obtained using a one-dimensional map as shown in FIG. Depending on, variable setting (variable gain setting) may be performed.

  Further, the integral gain Ki2 and the proportional gain Kp2 may be set and changed in accordance with the presence / absence of the average air / fuel ratio oscillation process by the average air / fuel ratio oscillation means 203 or the average air / fuel ratio oscillation width. In this case, when the fluctuation in the output value V2 of the downstream O2 sensor 111 is increased by the average air-fuel ratio oscillation means 203, the average air-fuel ratio feedback control means 202 controls the average air-fuel ratio so as to suppress the fluctuation in the output value V2. Since the fuel ratio is manipulated, the mean air-fuel ratio oscillating means 203 and the second air-fuel ratio control means 202 influence each other. That is, the integral gain Ki2 and the proportional gain Kp2 are changed during the average air-fuel ratio oscillation process, and are set in consideration of mutual influences.

  The integral gain Ki2 and the proportional gain Kp2 are the maximum oxygen storage amount OSCmax calculated by the maximum oxygen storage amount calculation means 204, the catalyst temperature Tmpcat, the catalyst deterioration degree Catdet, or the deterioration determination result by the catalyst deterioration diagnosis means 205. Depending on the setting, the setting may be changed. In this case, by changing the integral gain Ki2 and the proportional gain Kp2, it is possible to set an appropriate gain according to the change in the maximum oxygen storage amount OSCmax of the catalyst 109.

  Further, during a predetermined period after the transient operation under the transient operation condition (the update condition of the integral value AFI is not established), the absolute value of the proportional gain Kp2 is set to be large so that the return speed of the purification state of the catalyst 109 deteriorated by the disturbance is increased. Speed up. On the other hand, after the lapse of a predetermined period after the transient operation, the absolute value of the proportional gain Kp2 is set to a small value to avoid the deterioration of drivability due to the excessive operation amount of the target air-fuel ratio A / Fo.

  The predetermined period after the transient operation in the proportional calculation may be controlled to the period until the integrated air amount after the transient operation reaches a predetermined value, as in the case of the integral calculation. This is because the speed until the state of the catalyst 109 returns from the influence of the transient operation depends on the O 2 occlusion action of the catalyst 109 and is proportional to the intake air amount Qa.

  Therefore, by setting the absolute value of the proportional gain Kp2 large during a predetermined period after the transient operation, the deterioration of the purification state of the catalyst 109 due to the transient operation can be quickly recovered, and the drivability during the normal operation is also deteriorated. It can be avoided.

Next, following step 2309, in order to prevent an excessive air-fuel ratio operation, the proportional value AFP is set to satisfy the following expression (17) using the minimum value AFPmin and the maximum value AFPmax of the proportional value AFP. Upper / lower limit processing is performed (step 2310).
AFPmin <AFP <AFPmax (17)

Subsequently, the integral value AFI obtained in steps 2306 to 2308 and the proportional value AFP obtained in steps 2309 and 2310 are summed, and the vibration center AFCNT is calculated by the following equation (18) (step 2311).
AFCNT = AFAVE0 + AFP + AFI (18)

  The vibration center AFCNT formed by the sum of the PI calculation values as in Expression (18) corresponds to the above-described Expression (15) for obtaining the target average air-fuel ratio AFAVEobj on the upstream side of the catalyst 109.

Finally, in order to avoid an excessive air / fuel ratio operation, the minimum value AFCNTmin and the maximum value AFCNTmax of the vibration center AFCNT (corresponding to the target average air / fuel ratio AFAVEobj) are used so that the following equation (19) is satisfied. The upper and lower limit limiting processing for the center AFCNT (target average air-fuel ratio AFAVEobj) is performed (step 2312), and the processing routine of FIG.
AFCNTmin <AFCNTobj <AFCNTmax (19)

  As described above, the air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 of the present invention is provided on the upstream side of the catalyst 12 and detects the air-fuel ratio in the upstream exhaust gas, and the upstream O2 sensor 110. First air-fuel ratio feedback control means for adjusting the air-fuel ratio supplied to the internal combustion engine 101 in accordance with the output value V1 of the upstream O2 sensor 110 and the control constant, and causing the air-fuel ratio to periodically oscillate in the rich direction and the lean direction. 201, and when the air-fuel ratio supplied to the internal combustion engine 101 is adjusted by the first air-fuel ratio feedback control means 201, control is performed at a timing when the output value V1 of the upstream O2 sensor 110 becomes a predetermined voltage or more. The amount of increase of the constant is reduced, and the amount of decrease of the control constant is reduced at the timing when the constant voltage or less is reached.

  As a result, the air-fuel ratio supplied to the internal combustion engine 101 is not greatly deviated from the stoichiometric air-fuel ratio, and drivability and exhaust gas deterioration can be prevented.

  Further, when adjusting the air-fuel ratio supplied to the internal combustion engine 101, the increase amount of the control constant is reduced at the timing when the output value V1 of the upstream O2 sensor 110 becomes equal to or higher than the theoretical air-fuel ratio equivalent voltage VR3, and the upstream O2 sensor. The control constant is switched at the timing when the air-fuel ratio supplied to the internal combustion engine 101 changes to rich or lean by reducing the decrease amount of the control constant when the output value V1 of 110 becomes equal to or lower than the theoretical air-fuel ratio equivalent voltage VR3. be able to. As a result, the air-fuel ratio can be reliably oscillated between the rich state and the lean state, and when the A / F control width tends to increase due to acceleration / deceleration or the like, or the oxygen storage during the deteriorated catalyst The effect of suppressing exhaust gas deterioration when the amount is reduced can be obtained.

  Further, when adjusting the air-fuel ratio supplied to the internal combustion engine 101, the period during which the output value V1 of the upstream O2 sensor 110 reaches the predetermined voltage VR or VL is measured, and the A / F control width of the fuel correction coefficient FAF is large. If the period becomes too long, the execution of the air-fuel ratio oscillation process is stopped using the longer period, thereby reducing the possibility of erroneously detecting a normal catalyst as a deteriorated catalyst at the time of diagnosis of catalyst deterioration.

  Next, the effects of the air-fuel ratio control apparatus for an internal combustion engine according to Embodiment 1 will be further described with reference to the timing charts of FIGS. FIG. 28 shows the behavior of the conventional air-fuel ratio control device for an internal combustion engine (for example, Patent Document 4) at the time of the deteriorated catalyst, and FIG. 29 shows the behavior of the air-fuel ratio control device for the internal combustion engine according to Embodiment 1 at the time of the deteriorated catalyst. Show.

  In the uppermost upstream A / F in FIG. 28 and FIG. 29, the solid line waveform indicates the fluctuation of the actual air-fuel ratio, and the dotted line waveform indicates the fluctuation of the average air-fuel ratio. The vibration width of the actual air-fuel ratio in FIG. 28 shows a state greatly deviating from the theoretical air-fuel ratio as compared with the case of FIG. Even in the control that can be expected to suppress the A / F oscillation width by using the average air-fuel ratio, it is necessary to change the average air-fuel ratio in order to adapt to the lightly deteriorated catalyst. It cannot be suppressed from coming off greatly. Therefore, as seen in the behavior of the output value V2 of the downstream O2 sensor, the fluctuation range of the actual air-fuel ratio is large, so that the exhaust gas fluctuation occurs and the exhaust gas deteriorates. Further, as seen in the behavior of the engine rotation, the fluctuation of the actual air-fuel ratio width is large, so that the fluctuation of the rotation occurs and the drivability is deteriorated. In the first embodiment, the case where the average air-fuel ratio is used has been described. However, the same problem occurs even when the average air-fuel ratio is considered not to be larger than when the average air-fuel ratio is used. Therefore, the same effects as described above are obtained.

On the other hand, as in the first embodiment, the increase or decrease of the fuel correction coefficient FAF used for the upstream A / F air-fuel ratio oscillation is reduced according to the voltage of the upstream O2 sensor 110, thereby reducing the theoretical sky. There will be no significant deviation from the fuel ratio. Therefore, as seen in the behavior of the output value V2 of the downstream O2 sensor 111, the fluctuation range of the actual air-fuel ratio becomes small, so that the exhaust gas fluctuation can be suppressed and the deterioration of the exhaust gas can be suppressed. Further, as seen in the behavior of engine rotation, the fluctuation range of the actual air-fuel ratio becomes small, so that the fluctuation in rotation can be suppressed and the deterioration in drivability can be suppressed.

  Although the embodiments of the present invention have been described in detail above, the present invention can be appropriately modified or omitted within the scope of the present invention. For example, in the first embodiment, a λ-type sensor is used as the downstream O2 sensor 111. However, the downstream O2 sensor 111 may be any other sensor as long as it can detect the purification state of the catalyst 109 located on the upstream side. However, even if a linear O2 sensor, NOx sensor, HC sensor, CO sensor, or the like is used, the purification state of the catalyst 109 can be controlled, so that the same effect as described above can be obtained.

  DESCRIPTION OF SYMBOLS 101 Internal combustion engine, 102 Air cleaner, 103 Throttle valve, 104 Surge tank, 105 Intake pipe, 106 Air flow sensor, 107 Injector, 108 Exhaust pipe (exhaust system), 109 Three-way catalyst, 110 Upstream O2 sensor, 111 Downstream O2 sensor , 112 Internal combustion engine control unit (ECU), 113 Central processing unit (CPU), 114 Read only memory (ROM), 115 Random access memory (RAM), 116 Input / output interface (I / O), 117 Throttle sensor, 118 Idle Switch, 119 water temperature sensor, 120 crank angle sensor, 121 cam angle sensor, 122 drive circuit, 123 warning lamp, 124 backup RAM, 201 first air-fuel ratio feedback control means, 202 second Air-fuel ratio feedback control means, 203 average air-fuel ratio vibration means, 204 maximum oxygen storage amount calculation means, 205 catalyst deterioration vibration means, 206, 207 control gain change means, 208 control period measurement means, V1 upstream O2 sensor output value, V2 Output value of downstream O2 sensor, WT cooling water temperature, CA crank angle signal, DL idle signal, θ throttle opening, Qa Intake air amount.

An air-fuel ratio control apparatus for an internal combustion engine according to the present invention is provided on an upstream side of a catalyst that is installed in an exhaust system of the internal combustion engine and purifies exhaust gas from the internal combustion engine, and detects an air-fuel ratio in upstream exhaust gas Detecting means; operating condition detecting means for detecting operating conditions of the internal combustion engine; and air-fuel ratio supplied to the internal combustion engine in accordance with an output value of the air-fuel ratio detecting means and a predetermined feedback control constant of the operating condition detecting means The air-fuel ratio feedback control means for periodically oscillating the air-fuel ratio in the rich direction and the lean direction, catalyst deterioration diagnosis means for diagnosing the presence or absence of deterioration of the catalyst, and oxygen storage amount of the catalyst And the control used by the air-fuel ratio feedback control means so that the average air-fuel ratio obtained by averaging the periodically oscillating air-fuel ratio oscillates in the rich direction and the lean direction. A air-fuel ratio control apparatus for an internal combustion engine having a mean air-fuel ratio oscillation means for manipulating the number, the, the air-fuel ratio feedback control means, at the timing when the output value of the air-fuel ratio detecting means is equal to or more than a predetermined voltage to reduce the increase amount of the feedback control constants, the output value of the air-fuel ratio detecting means to reduce the amount of decrease in the feedback control constants at the timing when a predetermined voltage or less, the average air fuel ratio oscillation means, the catalyst deterioration The vibration width or vibration cycle of the average air-fuel ratio is changed so that the vibration width of the oxygen storage amount of the catalyst changes between when the catalyst is diagnosed for deterioration and when it is not deteriorated .

In FIG. 2, the ECU 112 includes a first air-fuel ratio feedback control means 201, a second air-fuel ratio feedback control means 202, an average air-fuel ratio oscillation means 203, a maximum oxygen storage amount calculation means 204, and a catalyst deterioration diagnosis means. 205, and the output value V1 of the upstream O2 sensor 110 is input to the first air-fuel ratio feedback control means 201.

The output value V2 of the downstream O2 sensor 111 is input to the second air-fuel ratio feedback control means 202, the average air-fuel ratio oscillation means 203, and the catalyst deterioration diagnosis means 205, and the maximum oxygen storage amount calculation means 204 receives other values. Detection information from various sensors is input.

The catalyst deterioration diagnosis unit 205 diagnoses the presence or absence of deterioration of the catalyst 109 based on the maximum oxygen storage amount OSCmax calculated by the maximum oxygen storage amount calculation unit 204. Further, the catalyst deterioration diagnosis unit 205 diagnoses the deterioration of the catalyst 109 based on at least the output value V2 of the downstream O2 sensor 111 during the execution of the average air-fuel ratio vibration process by the average air-fuel ratio vibration unit 203. A diagnosis result by the catalyst deterioration diagnosis unit 205 is input to an alarm driving unit such as a warning lamp 123.

In equation (2), the air amount Qacyl to the internal combustion engine 101 is calculated based on the intake air amount Qa detected from the air flow sensor 106. When the air flow sensor 106 is not used, the intake air amount Qa may be calculated based on an output signal of a pressure sensor (not shown) provided on the downstream side of the throttle valve in the intake pipe 105. The calculation may be based on the rotational speed Ne or the throttle opening of the throttle valve.

Further, the integral gain Ki2 and the proportional gain Kp2 may be set and changed in accordance with the presence / absence of the average air / fuel ratio oscillation process by the average air / fuel ratio oscillation means 203 or the average air / fuel ratio oscillation width. In this case, when the fluctuation in the output value V2 of the downstream O2 sensor 111 is increased by the average air-fuel ratio oscillation means 203, the average air-fuel ratio feedback control means 202 controls the average air-fuel ratio so as to suppress the fluctuation in the output value V2. Since the fuel ratio is manipulated, the mean air-fuel ratio oscillation means 203 and the second air-fuel ratio feedback control means 202 influence each other. That is, the integral gain Ki2 and the proportional gain Kp2 are changed during the average air-fuel ratio oscillation process, and are set in consideration of mutual influences.

DESCRIPTION OF SYMBOLS 101 Internal combustion engine, 102 Air cleaner, 103 Throttle valve, 104 Surge tank, 105 Intake pipe, 106 Air flow sensor, 107 Injector, 108 Exhaust pipe (exhaust system), 109 Three-way catalyst, 110 Upstream O2 sensor, 111 Downstream O2 sensor , 112 Internal combustion engine control unit (ECU), 113 Central processing unit (CPU), 114 Read only memory (ROM), 115 Random access memory (RAM), 116 Input / output interface (I / O), 117 Throttle sensor, 118 Idle Switch, 119 water temperature sensor, 120 crank angle sensor, 121 cam angle sensor, 122 drive circuit, 123 warning lamp, 124 backup RAM, 201 first air-fuel ratio feedback control means, 202 second air-fuel ratio feedback Click control means 203 average air fuel ratio oscillation means, 204 maximum oxygen storage amount calculation means, 205 a catalyst deterioration diagnosis means, 206 and 207 control gain changing means 208 control period measuring means, V1 output value of the upstream O2 sensor, V2 downstream Side O2 sensor output value, WT cooling water temperature, CA crank angle signal, DL idle signal, θ throttle opening, Qa intake air amount.

Claims (3)

An air-fuel ratio detecting means provided on the upstream side of a catalyst installed in an exhaust system of the internal combustion engine to purify exhaust gas from the internal combustion engine, and detecting an air-fuel ratio in the upstream exhaust gas;
An operating state detecting means for detecting an operating condition of the internal combustion engine;
The air-fuel ratio supplied to the internal combustion engine is adjusted according to the output value of the air-fuel ratio detection means and a predetermined feedback control constant of the operating state detection means, and the air-fuel ratio is periodically oscillated in the rich direction and the lean direction. An air-fuel ratio feedback control means,
An air-fuel ratio control apparatus for an internal combustion engine comprising:
The air-fuel ratio feedback control means includes
The amount of increase in the feedback control constant is reduced at a timing when the output value of the air-fuel ratio detection means becomes equal to or higher than a predetermined voltage, and the feedback control constant is reached at a timing when the output value of the air-fuel ratio detection means becomes lower than a predetermined voltage. An air-fuel ratio control apparatus for an internal combustion engine, characterized in that the amount of decrease is reduced.   2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the predetermined voltage is set to a voltage corresponding to a theoretical air-fuel ratio of the air-fuel ratio detection means. A control period from when the air-fuel ratio oscillation shifts from the lean direction to the rich direction until the feedback control constant increases, or the air-fuel ratio oscillation shifts from the rich direction to the lean direction. And a control period measuring means for measuring a control period until the amount of decrease in the feedback control constant is reduced,
The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2, wherein when the control period is longer than a predetermined period, the execution of the air-fuel ratio oscillation process by the air-fuel ratio feedback control means is stopped.