JPH0674071A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To improve the emission characteristics of exhaust gas by adjusting an air-fuel ratio of air-fuel mixture constantly to a desired value even when an exhaust gas passage is switched. CONSTITUTION:In feedback control of an air-fuel ratio, an output value from an LAF sensor is read and when it is decided that an engine is in a fundamental mode, a target air-fuel ratio factor KCMD is determined at S21-S23 according to whether fuel is under cut. When a BPV is OFF, the KCMD is not corrected and an air-fuel ratio is set to a corrected target value KAMDM at S24 and S25. When the BPV is ON, KCMD is calculated by effecting correction and limit check of the KCMDM at S24-S27. Based on the calculated KCMDM value, feedback control of the air-fuel ratio is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for an internal combustion engine, and particularly to a first exhaust gas concentration sensor having an output characteristic which is substantially proportional to the exhaust gas concentration, and an output signal inverted in the vicinity of a target air-fuel ratio. An air-fuel ratio control device for an internal combustion engine, comprising: a second exhaust gas concentration sensor, and feedback-controlling an output value of the first exhaust gas concentration sensor to a corrected target air-fuel ratio corrected based on the output value of the second exhaust gas concentration sensor. Regarding

[0002]

2. Description of the Related Art Conventionally, the exhaust passage of an internal combustion engine has two
It is known that individual catalyst devices (first and second catalyst devices) are arranged in series. According to this internal combustion engine, by arranging the first catalytic device having a relatively small volume in the vicinity of the engine and promoting activation of the catalytic device at the time of cold start, exhaust gas is efficiently emitted at the time of cold start of the engine. It is possible to purify. However, in this device, since the first catalyst device is arranged in the exhaust passage in the vicinity of the engine, the first catalyst device is exposed to high temperature exhaust gas for a long time after the engine is warmed up. Therefore, there is a drawback that the catalyst deteriorates severely and the life of the catalyst is short.

Therefore, in order to solve such a drawback, a bypass passage bypassing the first catalyst device and an exhaust gas from the engine are provided in the exhaust passage of the internal combustion engine.
An exhaust gas purifying device for an internal combustion engine, which has a switching valve for switching to the catalyst device side or the bypass passage side, has already been proposed (for example, Japanese Utility Model Laid-Open No. 52-135713).

According to this prior art, the exhaust gas can be efficiently purified through the first catalyst device at the time of cold start, while the switching valve is moved to the bypass passage side after the engine is warmed up. By switching, the exhaust gas is purified only by the second catalyst device, and the durability of the first catalyst device can be improved.

On the other hand, a wide range oxygen concentration sensor (hereinafter referred to as "LAF sensor") having an output characteristic substantially proportional to the exhaust gas concentration is disposed upstream of the catalyst device disposed in the exhaust passage of the internal combustion engine. On the other hand, an oxygen concentration sensor (hereinafter, referred to as “O2 sensor”) whose output signal is inverted in the vicinity of the target air-fuel ratio (theoretical air-fuel ratio) is arranged on the downstream side of the catalyst device, and based on the output value of the O2 sensor. An air-fuel ratio control device for an internal combustion engine is known in which the target air-fuel ratio is always the stoichiometric air-fuel ratio by performing feedback control of the output value of the LAF sensor to the corrected target air-fuel ratio corrected by correcting the target air-fuel ratio. (For example, Japanese Patent Laid-Open No. 2-674
43 publication).

[0006]

By the way, the above-mentioned 2
When the switching valve and the bypass passage described above are added to the air-fuel ratio control device including the oxygen concentration sensors (LAF sensor and O2 sensor), the target air-fuel ratio is always set to the stoichiometric air-fuel ratio regardless of the engine warm-up state. It is considered that it becomes possible to set the value and further improve the emission characteristics of the exhaust gas.

However, in this case, when the exhaust gas from the engine is switched to the bypass passage side by the switching valve, the so-called O 2 storage effect by the first catalytic device disappears, so that the downstream side of the first catalytic device is removed. The output value of the O2 sensor directly reflects the air-fuel ratio characteristic of the exhaust gas from the engine, which shortens the inversion cycle between the rich side and the lean side. Therefore, the O2 on the downstream side of the first catalyst device
If the target air-fuel ratio is corrected based on the output value of the sensor and the output value of the LAF sensor is feedback-controlled to the corrected target air-fuel ratio thus corrected, the convergence of the air-fuel ratio may decrease, and the emission characteristics of the exhaust gas may be reduced. However, there is a problem that it will cause the deterioration of.

The present invention has been made in view of the above-mentioned problems, and can improve the emission characteristics of exhaust gas by always setting the air-fuel ratio of the air-fuel mixture to the desired air-fuel ratio regardless of the switching state of the switching valve. An object is to provide an air-fuel ratio control device for an engine.

[0009]

In order to achieve the above object, an air-fuel ratio control system for an internal combustion engine according to the present invention comprises a first catalytic device arranged in an exhaust passage near the internal combustion engine,
A second catalyst device arranged in the exhaust passage downstream of the first catalyst device, a bypass passage bypassing the first catalyst device, and exhaust gas from the engine for the first catalyst device. Switching means for switching to either the device side or the bypass passage side; and a first exhaust gas concentration sensor arranged in the exhaust passage upstream of the switching means and having an output characteristic substantially proportional to the exhaust gas concentration. An operating state detecting means for detecting an operating state of the engine, a target air-fuel ratio calculating means for calculating a target air-fuel ratio based on a detection result of the operating state detecting means, the first catalyst device and the second catalyst. A second exhaust gas concentration sensor whose output signal is inverted in the vicinity of the target air-fuel ratio interposed between the device and a correction for correcting the target air-fuel ratio based on the output value of the second exhaust gas concentration sensor. Means and the first discharge And a control means for feedback-controlling the air-fuel ratio of the air-fuel mixture detected by the concentration sensor to the target air-fuel ratio corrected by the correction means, wherein the switching means switches the exhaust gas passage to the bypass passage side. It is characterized in that a prohibiting means for prohibiting correction of the target air-fuel ratio by the correcting means is sometimes provided.

[0010]

According to the above construction, when the exhaust gas from the engine passes through the bypass passage side, the correction of the target air-fuel ratio based on the output value of the second exhaust gas concentration sensor is prohibited, and the first
The air-fuel ratio of the air-fuel mixture detected by the exhaust concentration sensor of
Feedback control is performed to the target air-fuel ratio calculated by the target air-fuel ratio calculating means.

[0011]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is an overall configuration diagram showing an embodiment of an air-fuel ratio control system for an internal combustion engine according to the present invention.

In the figure, reference numeral 1 denotes a DOHC in-line 4-cylinder internal combustion engine (hereinafter simply referred to as "engine") in which each cylinder is provided with an intake valve and an exhaust valve (not shown). A throttle body 3 is provided in the middle of an intake pipe 2 of the engine 1, and a throttle valve 3'is arranged inside thereof. Further, a throttle valve opening degree (θTH) sensor 4 is connected to the throttle valve 3 ', and an electronic control unit (hereinafter referred to as "ECU") 5 outputs an electric signal according to the opening degree of the throttle valve 3'. Supply to.

The fuel injection valve 6 is provided for each cylinder in the middle of the intake pipe 2 between the engine 1 and the throttle valve 3 ', and is connected to a fuel pump (not shown) and EC.
It is electrically connected to U5, and the valve opening time of fuel injection is controlled by a signal from the ECU 5.

A branch pipe 7 is provided downstream of the throttle valve 3'of the intake pipe 2, and an absolute pressure (PBA) sensor 8 is attached to the tip of the branch pipe 7. The PBA sensor 8 is electrically connected to the ECU 5, and the intake pipe 2
The absolute pressure PBA therein is converted into an electric signal by the PBA sensor 8 and supplied to the ECU 5.

An intake air temperature (TA) sensor 9 is mounted on the pipe wall of the intake pipe 2 on the downstream side of the branch pipe 7, and the intake air temperature TA detected by the TA sensor 9 is converted into an electric signal and the ECU 5 is operated. Is supplied to.

An engine water temperature (TW) sensor 10 composed of a thermistor or the like is attached to the peripheral wall of the cylinder filled with the cooling water of the cylinder block of the engine 1, and the engine cooling water temperature TW detected by the TW sensor 10 is converted into an electric signal. It is converted and supplied to the ECU 5.

An engine speed (NE) sensor 11 and a cylinder discrimination (CYL) sensor 12 are mounted around a cam shaft or a crank shaft (not shown) of the engine 1.

The NE sensor 11 outputs a signal pulse (hereinafter referred to as "TDC signal pulse") at a predetermined crank angle position every 180 degrees rotation of the crankshaft of the engine 1, and CY
The L sensor 12 outputs a TDC signal pulse at a predetermined crank angle position of a specific cylinder, and each of these TDC signal pulses is supplied to the ECU 5.

The spark plug 13 of each cylinder of the engine 1 is
It is electrically connected to the ECU 5, and the ignition timing is controlled by the ECU 5.

Exhaust pipe (exhaust passage) 14 of the engine 1
A LAF sensor 15 is provided near the engine. The LAF sensor 15 has a pair of upper and lower battery elements and an oxygen pump element attached to a predetermined position of a sensor element made of zirconia solid electrolyte (ZrO ₂ ) or the like, and the sensor element is electrically connected to an amplifier circuit. Has been done. Then, the LAF sensor 15 outputs an electric signal substantially proportional to the oxygen concentration in the exhaust gas passing through the inside of the sensor element, and supplies the electric signal to the ECU 5.

Exhaust pipe 14 downstream of the LAF sensor 15
In the middle of the process, the first catalytic device 16 and the second catalytic device 1
7 are arranged in series.

The first catalyst device 16 is provided for activating the first catalyst device 16 promptly when the engine 1 is cold-started to improve the exhaust gas emission characteristics when the engine 1 is cold-started. The capacity is smaller than that of the second catalyst device 17. Then, by these first and second catalyst devices 16 and 17, HC and C in the exhaust gas are
The harmful components such as O and NOx are purified.

An O 2 sensor 18 is arranged in the exhaust pipe 14 between the first catalytic device 16 and the second catalytic device 17.

The sensor element of the O2 sensor 18 is the above L
Similar to AF sensor 15, zirconia solid electrolyte (Zr
O ₂ ), the electromotive force of which is abruptly changed before and after the stoichiometric air-fuel ratio, and the output signal is inverted from the lean signal to the rich signal or from the rich signal to the lean signal at the stoichiometric air-fuel ratio. That is, the O2 sensor 18
Output signal becomes high level on the exhaust gas rich side and becomes low level on the lean side, and the output signal is supplied to the ECU 5.

An exhaust communication passage (hereinafter referred to as "bypass passage") 14a that bypasses the first catalyst device 16 side.
Is provided so as to branch from the exhaust pipe 14 on the downstream side of the LAF sensor 15. That is, the bypass passage 14a
Has one end connected to the exhaust pipe 14 on the upstream side of the first catalyst device 16 and the other end connected to the exhaust pipe 14 on the downstream side of the first catalyst device 16.

In the exhaust pipe 14 at the branch point between the upstream side of the first catalyst device 16 and the bypass passage 14a, the exhaust gas from the engine is supplied to the first catalyst device 16 side and the bypass passage 14.
A switching valve (bypass valve, hereinafter referred to as “BPV”) 19 as switching means for switching to the a side is provided, and the BPV 19 further has an electric actuator 20.
(For example, a solenoid valve, a motor, etc.) are connected.

The electric actuator 20 is connected to the ECU 5 and is driven by a signal from the ECU 5. That is, the BPV 19 is switched and driven by the drive of the electric actuator 20, so that the exhaust gas from the engine can be switched between the first catalytic device 16 side and the bypass passage 14a side.

When the electric actuator 20 is not in operation (hereinafter referred to as "BPV OFF"), the BPV 19 is deenergized so that exhaust gas from the engine is directed to the bypass passage 14a side, and when the electric actuator 20 is in operation (hereinafter referred to as "BPV OFF"). ,
"BPV ON") is urged to direct the exhaust gas to the first catalytic device 16 side. That is,
The BPV 19 is set at the position shown by the solid line in FIG. 1 when the BPV is OFF, and at the position shown by the broken line in FIG. 1 when the BPV is ON.

Further, at the branch point between the first catalyst device 16 side and the bypass passage 14a side, which of the first catalyst device 16 side and the bypass passage 14a side is the exhaust gas passage of the BPV 19 switched to? A BPV position detection sensor (hereinafter referred to as “BP sensor”) 21 for detecting the position is provided, and the BP sensor 21 detects the position of the BPV 19 and outputs an electric signal thereof to the ECU 5. Since the BPV 19 is driven by the electric actuator 20 according to a signal from the ECU 5, the position detection of the BPV 19 may be replaced with the drive signal of the electric actuator 20. The BP sensor 21 is provided for the BPV 19 and the electric actuator 2.
Even if 0 is delayed due to deterioration, etc., B
This is to reliably detect the position of the PV 19 and improve controllability.

Further, the atmospheric pressure (PA) sensor 22 is arranged at a proper position of the engine 1, detects the atmospheric pressure PA, and supplies its electric signal to the ECU 5.

Thus, the ECU 5 shapes the input signal waveforms from the various sensors described above, corrects the voltage level to a predetermined level, and converts the analog signal value into a digital signal value. A central processing circuit (hereinafter referred to as "CPU") 5b, a storage means 5c including a ROM and a RAM for storing various calculation programs executed by the CPU 5b and various maps and calculation results to be described later, and the fuel injection valve. 6, an output circuit 5d for supplying a drive signal to the ignition plug 13 is provided.

The CPU 5b determines various engine operating states such as a feedback control operating region and an open loop control operating region according to the oxygen concentration in the exhaust gas based on the above various engine parameter signals, and determines the engine operating state. Accordingly, the fuel injection time TOUT of the fuel injection valve 6 is calculated in synchronism with the TDC signal pulse in accordance with the equation (1) in the basic mode and in accordance with the equation (2) in the starting mode, and the result is calculated. Storage means 5c (RA
M).

TOUT = TiM × KCMDM × KLAF × K1 + K2 (1) TOUT = TiCR × K3 + K4 (2) where TiM is the basic fuel injection time in the basic mode, specifically the engine speed NE and the intake pipe This is the basic fuel injection time set according to the absolute pressure PBA,
The TiM map for determining the iM value is stored in the storage means 5c.
It is stored in (ROM).

TiCR is the basic fuel injection time in the start mode, and is set in accordance with the engine speed NE and the intake pipe absolute pressure PBA, like the TiM value, and the TiCR map for determining the TiCR value is stored. Means 5c (RO
M).

KCMDM is a corrected target air-fuel ratio coefficient, which is an air-fuel ratio correction value set based on the target air-fuel ratio coefficient KCMD calculated based on the operating state of the engine and the output value of the O2 sensor 18 as described later. It is set according to ΔKCMD.

KLAF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 15 matches the target air-fuel ratio during air-fuel ratio feedback control, and depends on the engine operating state during open-loop control. It is set to a predetermined value.

K1, K2, K3, and K4 are correction coefficients and correction variables calculated according to various engine parameter signals, respectively, and various parameters such as fuel consumption characteristics and acceleration characteristics according to the operating state of the engine for each cylinder. It is set to a predetermined value that optimizes the characteristics.

Next, the air-fuel ratio feedback control method of the present invention executed by the CPU 5b will be described in detail.

FIG. 2 is a flow chart showing the main routine of the air-fuel ratio feedback control.

First, in step S1, the LAF sensor 15
Read the output value from. Next, it is determined whether the engine is in the starting mode (step S2). Here, whether or not the engine is in the starting mode is determined by, for example, whether or not a starter switch of an engine (not shown) is on and the engine speed is equal to or lower than a predetermined starting speed (cranking speed).

Then, the answer in step S2 is affirmative (YE
In S), that is, in the start mode, the engine is at a low water temperature, and the KTWLAF map that is a function of the engine cooling water temperature TW and the intake pipe absolute pressure PBA is searched to retrieve the target air-fuel ratio coefficient at the low water temperature. KTWLAF is calculated (step S3), and the KTWLAF value is set to the target air-fuel ratio coefficient KCMD (step S4). Next, the flag FLAFFB is set to "0" to stop the feedback control of the air-fuel ratio (step S5), and the air-fuel ratio correction coefficient K
The LAF and its integral term (I term) KLAFI are set to 1.0 (steps S6 and S7), and this program ends.

On the other hand, when the answer to step S2 is negative (NO), that is, in the basic mode, the corrected target air-fuel ratio coefficient KCMDM is calculated based on the flowchart of FIG. 3 described later (step S8), and then the flag FACT is set. It is determined whether or not the value is "1" to determine whether or not the LAF sensor 15 is activated (step S9). Where LA
The activation determination of the F sensor 15 is performed by a LAF sensor activation determination routine (not shown) which is processed in the background. For example, the output voltage VOU of the LAF sensor 15 is determined.
When the difference between T and its center voltage VCENT is smaller than a predetermined value (for example, 0.4 V), it is determined that “LAF sensor 15 has been activated”.

Then, the answer to step S9 is negative (NO).
If so, the process proceeds to step S5, while if the answer to step S9 is affirmative (YES), that is, if the activation of the LAF sensor 15 has been completed, the process proceeds to step S10.
Equivalent ratio KACT of the air-fuel ratio detected by the F sensor 15
(14.7 / (A / F)) (hereinafter “detected air-fuel ratio coefficient”)
Is calculated). Here, the detected air-fuel ratio coefficient KAC
In view of the fact that the exhaust pressure fluctuates due to fluctuations in the intake pipe absolute pressure PBA, the engine speed NE, and the atmospheric pressure PA, T is
It is calculated to a value corrected according to these operating parameters, and is calculated by executing a KACT calculation routine (not shown).

Next, in step S11, a feedback processing routine is executed and this program ends. That is, when the predetermined feedback condition is not satisfied, the flag FLAFFB is set to "0" to inhibit the feedback control, while when the predetermined feedback condition is satisfied, the flag FLAFFB is set to "1" to correct the air-fuel ratio. The coefficient KLAF is calculated, the execution of the feedback control is instructed, and the program ends.

FIG. 3 is a flow chart of the KCMDM calculation routine executed in step S8 (FIG. 2), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, it is determined whether or not the engine 1 is in the fuel cut (fuel supply is stopped) (step S21).
Whether the engine is in the fuel cut state depends on the engine speed N.
It is determined based on E and the valve opening degree θTH of the throttle valve 3 ', and is specifically determined by executing a fuel cut determination routine (not shown).

Then, the answer to step S21 is negative (N
O), that is, in the basic mode, step S
In step 22, the target air-fuel ratio coefficient KCMD is calculated. The target air-fuel ratio coefficient KCMD is usually a KCMD map (not shown) in which a map value KCMD is given in a matrix according to the engine speed NE and the intake pipe absolute pressure PBA.
It is read from the KCMD, but is corrected as appropriate when the vehicle starts, when the water temperature is low, or when the vehicle is operating under a predetermined high load.
By executing a calculation routine (not shown), a value suitable for these operating states is set.

On the other hand, the answer in step S21 is affirmative (YE
In the case of S), the target air-fuel ratio coefficient KCMD is set to a predetermined value KCM.
Set to DFC (for example, 1.0) (step S2
3) and proceeds to step S24.

In step S24, "BPV OFF"
Or not. This determination is made according to the output signal of the BP sensor 21, and the answer to step S24 is affirmative (Y
In the case of (ES), that is, in the case of "BPV OFF", the target air-fuel ratio coefficient KCMD obtained in step S22 or step S23 is directly used as the corrected target air-fuel ratio coefficient KCMDM (step S25), and this program is ended and the main routine ( Return to FIG. 2).

On the other hand, the answer to step S24 is negative (NO).
If it is, that is, if it is "BPV ON", the process proceeds to step S26 and O2 processing is performed. That is, as will be described later, the corrected target air-fuel ratio coefficient KCMDM is calculated by correcting the target air-fuel ratio coefficient KCMD based on the output value from the O2 sensor 18 under predetermined requirements.

Then, in step S27, a limit check is performed on the corrected target air-fuel ratio coefficient KCMDM, the program is terminated, and the process returns to the main routine (FIG. 2). That is, the magnitude relationship between the KCMDM value calculated in step S26 and the predetermined upper and lower limit values KCMDMH and KCMDML is compared, and when the KCMDM value is larger than the upper limit value KCMDMH, the KCMDM value is set to the upper limit value KCMDMH, Is smaller than the lower limit KCMDML, the KCMDM value is set to the lower limit KCMDML.

Therefore, FIG. 4 shows the above-mentioned step S26.
It is a flowchart of the O2 processing routine executed in (FIG. 3), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S31, it is determined whether or not the flag FO2 is "1", and it is determined whether or not the O2 sensor 18 is activated. Whether or not the O2 sensor 18 is activated is determined by executing an O2 sensor activation determination routine (not shown).

Then, the answer to step S31 is negative (N
O), that is, when it is determined that the O2 sensor 18 is not activated yet, the process proceeds to step S32, the timer tmRX is set to a predetermined value T2 (for example, 0.25 sec), and then the flag FVREF is set to "0". It is determined whether or not
Initial value VRRE of the target correction value VREF of the O2 sensor 18
It is determined whether or not F (hereinafter, referred to as "initial correction value") is already set (step S33).

Then, in the first loop, step S3
Since the answer to 3 is affirmative (YES), the routine proceeds to step S34, where VREF value is set according to the atmospheric pressure PA.
The RREF table (not shown) is searched for the corrected initial value V
Calculate RREF.

Next, at step S35, the integral term (I term) VREFI (n- of the target correction value in the previous loop is obtained.
1) is set to the correction initial value VRREF, the program is terminated, and the process returns to the main routine (FIG. 2). That is, the target correction value VREFI (n-1) of the I term is initialized and the process returns to the main routine (FIG. 2). still,
When step S33 is executed after the next loop,
Since the correction initial value setting of the target correction value has already been made as described above, the answer is negative (NO), and step S3
This program ends without executing steps 4 and 35.

The answer to step S31 is affirmative (Y
ES), it is determined that the O2 sensor 18 has been activated, the process proceeds to step S36, and the timer tmRX is set.
Is determined to be "0". Then, when the answer is negative (NO), the process proceeds to step S33, while when the answer at step S36 is affirmative (YES), the O2 sensor 1
It is determined that the activation of No. 8 is completed and the process proceeds to step S37, and whether the target air-fuel ratio coefficient KCMD set in step S22 or S23 (FIG. 3) is larger than a predetermined lower limit value KCMDZL (for example, 0.98). To determine.

When the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture is set to the lean burn state, and while this program is terminated, when the answer is affirmative (YES). In step S38, the target air-fuel ratio coefficient KCMD is the predetermined upper limit value KCMDZH (for example,
1.13) Determine whether it is smaller than. When the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture is set to fuel rich, and while this program ends, when the answer is affirmative (YES), This is the case where the air-fuel ratio should be set to the theoretical air-fuel ratio (A / F = 14.7), and the routine proceeds to step S39, where it is judged if the engine is under fuel cut. And the answer is affirmative (Y
In the case of ES), this program is terminated and returns to the main routine (FIG. 2), while in the case of negative (NO), it is judged whether or not the fuel cut state was in the previous loop (step S40). . If the answer is affirmative (YES), after setting the counter NAFC to a predetermined value N1 (for example, 4) (step S41),
The counter value N1 of the counter NAFC is decremented by "1" (step S42), and this program ends.

On the other hand, the answer to step S40 is negative (NO).
When it becomes, the process proceeds to step S43, and the counter NAF
It is determined whether C is "0". Then, when the answer is negative (NO), the count value of the counter NAFC is decremented by "1" (step S42), and the program ends, while when the answer is affirmative (YES), the fuel cut is performed. After exiting the state, it is determined that stable fuel supply is being performed, and the routine proceeds to step S44, where O2
After executing the feedback processing routine, this program is terminated and the process returns to the main routine (FIG. 2).

FIG. 5 is a flow chart of the O2 feedback processing routine executed in step S44 (FIG. 4), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S61, the thinning-out variable N
It is determined whether the IVR is "0". This decimation variable NI
VR is a variable that is subtracted each time a TDC signal pulse is generated by the number of thinned TDCs NI set according to the engine operating state, as will be described later. Since it is initially "0", the answer in step S61 is The determination is affirmative (YES), and the process proceeds to step S62.

When the answer to step S61 is negative (NO) in the subsequent loop, the process proceeds to step S63, and the value obtained by subtracting the thinning-out TDC number NI (for example, 1) from the thinning-out variable NIVR is a new thinning-out variable. After setting to NIVR, the process proceeds to step S72 described later.

Therefore, in step S62, 02
It is determined whether the output voltage VO2 of the sensor 18 is smaller than a predetermined lower limit value VL (for example, 0.3V). And
When the answer is affirmative (YES), it is determined that the air-fuel ratio of the air-fuel mixture is deviating from the stoichiometric air-fuel ratio (target air-fuel ratio coefficient KCMD = 1.0) to the lean side, and while proceeding to step S65, When the answer is negative (NO), the process proceeds to step S64, where the output voltage VO2 of the 02 sensor 18 is the predetermined upper limit value VH.
(For example, 0.8) is determined. When the answer is affirmative (YES), it is determined that the air-fuel ratio of the air-fuel mixture has deviated from the stoichiometric air-fuel ratio to the rich side, and the routine proceeds to step S65.

Next, in step S65, a KVP map, a KVI map, a KVD map, and a NIVR map (not shown) are searched to change speed of the O2 feedback control, that is, a proportional term (P term) coefficient KVP, an integral term ( I
The term) coefficient KVI, the differential term (D term) coefficient KVD, and the thinning-out number NIVR are calculated.

Next, in step S66, the thinning-out variable NIV is used.
After setting R to the NIVR value calculated in step S65, the process proceeds to step S67, and step S34 is performed.
Deviation Δ between the corrected initial value VRREF calculated in (FIG. 4) and the output voltage VO2 of the 02 sensor 18 in the current loop
Calculate V (n).

Next, in step S68, equations (3)-
Based on (5), the target correction values VREFP (n), VREFI (n), V of each correction term, that is, P term, I term, D term,
After calculating REFD (n), these correction terms are added to calculate the target correction value VREF (n) in the O2 feedback based on Expression (6).

VREFP (n) = ΔV (n) × KVP (3) VREFI (n) = VREF + ΔV (n) × KVI (4) VREFD (n) = (ΔV (n) -ΔV (n-1) ) × KVD (5) VREF (n) = VREFP (n) + VREFI (n) + VREFD (n) (6) Next, in step S69, a VREF (n) limit check subroutine (not shown) is executed. To do. After the VREF (n) limit check is completed in step S69, the process proceeds to step S70, and the air-fuel ratio correction value ΔKCMD
Is calculated by searching the ΔKCMD table (not shown).

Next, at step S71, the corrected target air-fuel ratio coefficient KCMDM (= theoretical air-fuel ratio) is calculated by adding the air-fuel ratio correction value ΔKCMD to the target air-fuel ratio coefficient KCMD calculated at step S22 (FIG. 3). , This program ends.

The answer to step S64 is negative (N
O), that is, the output voltage VO2 of the 02 sensor 18 is V
When L ≦ VO2 ≦ VH, the O2 feedback control is prohibited, and the ΔV value (deviation between the correction initial value VRREF and the output value of the 02 sensor 18) in steps S72 to S74,
The present value is set equal to the previous value for each of the target correction value VREF and the air-fuel ratio correction value ΔKCMD, and the program ends. Thus, when the air-fuel ratio of the air-fuel mixture can maintain the stoichiometric air-fuel ratio, unnecessary O2 feedback control is prohibited, and good controllability can be maintained. That is, the air-fuel ratio of the air-fuel mixture can be maintained stable.

As described above in detail, in the present embodiment, in the KCMDM calculation routine of FIG. 3, the correction of the target air-fuel ratio coefficient KCMD is executed or prohibited in accordance with the setting state of the BPV 19 (BPV ON or BPV OFF). Therefore, the feedback control in the main routine of FIG. 2 makes it possible to improve the emission characteristics of the exhaust gas by always setting the air-fuel ratio of the air-fuel mixture to the desired air-fuel ratio.

[0072]

As described above in detail, according to the present invention, in the air-fuel ratio control system for an internal combustion engine, when the exhaust gas passage is on the bypass passage side, the target exhaust air amount based on the output of the second exhaust gas concentration sensor is used. Since the correction of the fuel ratio is prohibited, the air-fuel ratio of the air-fuel mixture can always be set to a desired value, and the emission characteristic of exhaust gas can be improved.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention.

FIG. 2 is a flowchart showing a main routine of air-fuel ratio feedback control of an internal combustion engine according to the present invention.

FIG. 3 is a flowchart of a KCMDM calculation routine.

FIG. 4 is a flowchart of an O2 processing routine.

FIG. 5 is a flowchart of an O2 feedback control routine.

[Explanation of symbols]

1 engine 5 ECU (target air-fuel ratio calculation means, correction means, control means, prohibition means) 8 PBA sensor (operating state detection means) 11 NE sensor (operating state detection means) 14 exhaust pipe (exhaust passage) 14a bypass passage 15 LAF Sensor (first exhaust gas concentration sensor) 16 First catalyst device 17 Second catalyst device 18 O2 sensor (second exhaust gas concentration sensor) 19 Switching valve (switching means)

Claims (1)

[Claims] 1. A first catalyst device provided in an exhaust passage near an internal combustion engine, a second catalyst device provided in the exhaust passage downstream of the first catalyst device, and A bypass passage for bypassing the first catalyst device, a switching means for switching the exhaust gas from the engine to one of the first catalyst device side and the bypass passage side, and the exhaust gas upstream of the switching means. A first exhaust gas concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration arranged in the passage, an operating state detecting means for detecting an operating state of the engine, and a target based on the detection result of the operating state detecting means. A target air-fuel ratio calculating means for calculating an air-fuel ratio and a second exhaust gas concentration at which an output signal is inverted in the vicinity of the target air-fuel ratio interposed between the first catalytic device and the second catalytic device. The sensor and the second exhaust concentration sensor Correction means for correcting the target air-fuel ratio based on the output value of the sensor, and control for feedback-controlling the air-fuel ratio of the air-fuel mixture detected by the first exhaust gas concentration sensor to the target air-fuel ratio corrected by the correction means. And a prohibiting means for prohibiting the correction of the target air-fuel ratio by the correcting means when the exhaust gas passage is switched to the bypass passage side by the switching means. Air-fuel ratio control device.