JP2692307B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

BACKGROUND OF THE INVENTION [FIELD OF THE INVENTION The present invention (herein, oxygen concentration sensor (O ₂ sensor)) air-fuel ratio sensor on the downstream side of the catalytic converter is provided, the catalyst downstream of the O ₂ sensor The present invention relates to an air-fuel ratio control device for an internal combustion engine, which performs air-fuel ratio feedback control according to.

[Conventional technology]

O The air-fuel ratio feedback control using the _second sensor, and a single O ₂ sensor system based on single O ₂ sensor, upstream of the catalyst, and the double O ₂ sensor system based on two O ₂ sensor provided downstream There, further, as a single O ₂ sensor system, of a type provided with a O ₂ sensor in the catalyst upstream, and the O ₂ sensor is of the type provided downstream of the catalyst.

In a single O ₂ sensor system in which an O ₂ sensor is installed upstream of the catalyst, the O ₂ sensor is installed at a location in the exhaust system that is as close to the combustion chamber as possible, that is, at the exhaust manifold manifold upstream of the catalytic converter. Gas non-equilibrium (non-uniformity), for example, even though the air-fuel ratio is rich
For O ₂ is present, inverting timing or deviation of the O ₂ sensor,
Further, in a multi-cylinder engine, there is a problem that the O ₂ sensor cannot detect the average air-fuel ratio due to the influence of the air-fuel ratio variation among the cylinders, and as a result, the air-fuel ratio control accuracy is low.

On the other hand, in the single O ₂ sensor system having a O ₂ sensor downstream of the catalyst, but is eliminated for non-detection of non-equilibrium level and the average air-fuel ratio of the exhaust gas, the position of the O ₂ sensor becomes longer than the exhaust valve The responsiveness of the O ₂ sensor is low due to the capacity and purification performance of the catalyst (size of the O ₂ storage effect, etc.), and therefore the responsiveness of the air-fuel ratio feedback control system deteriorates. There is a problem that the emission cannot be achieved and the emission becomes worse.

Further, in the double O ₂ sensor system in which O ₂ sensors are provided upstream and downstream of the catalyst, the air-fuel ratio feedback control by the downstream O ₂ sensor is performed in addition to the air-fuel ratio feedback control by the upstream O ₂ sensor. For example, the average air-fuel ratio is detected by the downstream O ₂ sensor, and the result is reflected in the value such as the skip control constant of the air-fuel ratio feedback control by the upstream O ₂ sensor to perform the overall air-fuel ratio control. Therefore, as long as the downstream O ₂ sensor maintains a stable output characteristic, good exhaust emission is guaranteed. However, in the double O ₂ sensor system, two O ₂ sensors are required, so the cost is high, and if the air-fuel ratio feedback control cycle by the upstream O ₂ sensor decreases due to aging, etc., the performance of the catalyst is still sufficient. There is a problem that it can not be demonstrated to the full.

Therefore, the present applicant has already in a single O ₂ sensor system having a O ₂ sensor downstream of the catalyst, the downstream O ₂ the center value of a predetermined amplitude and a predetermined frequency of the forced self excitation control waveform (forced oscillation waveform) It has been proposed to change it according to the output of the sensor (see Japanese Patent Laid-Open No. 1-66441). That is, as shown in FIG. ₂ , the output V of the downstream O ₂ sensor
_{When OX} changes, the center value (coarse adjustment term) AF _c of the forced self-excited control waveform AF _s is changed according to the output V _OX of the downstream O ₂ sensor. In this case, when the output V _OX of the downstream O ₂ sensor is lean, the coarse adjustment term AF _c is gradually increased, while
When the output V _OX of the downstream O ₂ sensor is rich, the coarse adjustment term AF _c is gradually reduced. That is, the coarse adjustment term AF _c is integration-controlled. This is because when the forced self-excitation control waveform fluctuates near the stoichiometric air-fuel ratio (λ = 1) as shown in Fig. 3 (AF _s = A
For F _so ), the catalyst can maximize its purification performance, but even if the forced self-excited control waveform fluctuates with the air-fuel ratio on the rich side (λ <1) or the air-fuel ratio on the lean side (λ> 1) (AF _s1 , A
F _s2 ) Purification performance of catalyst cannot be exhibited. Therefore, the coarse adjustment term AF _c (integral term) is introduced in order to bring the forced self-excited control waveform AF _s1 or AF _s2 close to AF _so that the purification performance of the catalyst can be exhibited.

[Problems to be solved by the invention]

However, the above-described conventional device has a problem that high-precision air-fuel ratio control cannot be performed in a vehicle in which the O ₂ storage effect often cannot be exhibited.
For example, the air-fuel ratio of the catalyst entering gas is deviated from the stoichiometric air-fuel ratio, yet the three-way catalyst this shift long lasting to O ₂
Storage amount significantly varies relative to the O ₂ storage amount in the normal state, O ₂ if the storage effect can not be exhibited controls the central value of the forced self excitation control waveform by simply integral control is a conventional device described above Therefore, the convergence of the control air-fuel ratio to the theoretical air-fuel ratio is poor, and as a result, there is a problem that the purification performance of the catalyst cannot be exhibited and the emission is deteriorated.

Therefore, the object of the present invention is to monitor the amount of O ₂ storage to sufficiently exert the purification performance of the catalyst, and to prevent the deterioration of the air-fuel ratio control accuracy due to the O ₂ storage effect to prevent the deterioration of emissions. It is to provide a fuel ratio feedback control system.

[Means for solving the problem]

The means for solving the above-mentioned problem is shown in FIG. That is, a catalyst downstream air-fuel ratio sensor for detecting the air-fuel ratio of the engine is provided in the exhaust passage downstream of the three-way catalyst provided in the exhaust passage of the internal combustion engine. The coarse adjustment term calculation means calculates a coarse adjustment term AF _c that gradually changes to the lean side when the output V _OX of the air-fuel ratio sensor is rich and gradually changes to the rich side when the output V _OX of the air-fuel ratio sensor is lean. Calculate
The forced self-excited control waveform generating means generates the forced self-excited control waveform AFs having a predetermined amplitude ΔAFs, a predetermined frequency (corresponding to the period T), and a predetermined rich ratio and lean ratio determined by a predetermined duty ratio. Further, the duty ratio varying means to increase the lean ratio of forced self excitation control waveform when the output V _OX of the air-fuel ratio sensor is rich, the output V _OX of the air-fuel ratio sensor is rich ratio of forced self excitation control waveform when the lean The duty ratio of the forced self-excited control waveform is made variable so as to be increased. The air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the rough adjustment term AF _c and the forced self-excited control waveform AF _s .

[Action]

According to the above means, the air-fuel ratio control is performed in the vicinity of the logical air-fuel ratio by the rough adjustment term, so the purification performance of the catalyst can be maintained high, and the forced self-excitation control is performed according to the O ₂ storage amount of the three-way catalyst. Since the waveform rich and lean duty ratios are variable, it is possible to reduce the deviation of the air-fuel ratio control amount due to the O ₂ storage effect without increasing the fluctuation range of the air-fuel ratio control value.

〔Example〕

FIG. 4 is an overall schematic diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention. In FIG. 4, an air flow meter 3 is provided in an intake passage 2 of an engine main body 1. The air flow meter 3 directly measures the amount of intake air, and has a built-in potentiometer to generate an analog voltage output signal proportional to the amount of intake air. This output signal is output from the A / D converter 1 with built-in multiplexer in the control circuit 10.
Supplied to 01. The distributor 4 has, for example, a crank angle sensor 5 that generates a reference position detection pulse signal every 720 ° converted into a crank angle, and a reference position detection pulse signal every 30 ° converted into a crank angle. A crank angle sensor 6 for generating is provided. The pulse signals of the crank angle sensors 5 and 6 are supplied to an input / output interface 102 of the control circuit 10, and the output of the crank angle sensor 6 is supplied to an interrupt terminal of the CPU 103.

Further, the intake passage 2 is provided with a fuel injection valve 7 for supplying pressurized fuel from a fuel supply system to an intake port for each cylinder.

The water jacket 8 of the cylinder block of the engine body 1 is provided with a water temperature sensor 9 for detecting the temperature of the cooling water. The water temperature sensor 9 detects the temperature TH of the cooling water.
Generates an analog voltage electric signal corresponding to W. This output is also supplied to the A / D converter 101.

The exhaust system downstream of the exhaust manifold 11, a catalytic converter 12 is provided to accommodate three toxic components HC in the exhaust gas, CO, a three-way catalyst for simultaneously purifying NO _x.

An O ₂ sensor 14 is provided on the exhaust pipe 13 downstream of the catalytic converter 12.
Is provided. The O ₂ sensor 14 generates an electric signal according to the oxygen component concentration in the exhaust gas. That is, the O ₂ sensor
14 is an A / D converter 101 with a control circuit 10 that outputs different output voltages depending on whether the air-fuel ratio is lean or rich with respect to the theoretical air-fuel ratio.
Occurs. The control circuit 10 is configured as, for example, a microcomputer, and includes a ROM 104, a RAM 105, a backup RAM 106, a clock generation circuit 107, and the like in addition to the A / D converter 101, the input / output interface 102, and the CPU 103.

Further, the throttle valve 15 in the intake passage 2 is provided with an idle switch 16 for detecting whether or not the throttle valve 15 is fully closed. This output signal is supplied to the input / output interface 102 of the control circuit 10. .

Reference numeral 17 denotes a secondary air introduction intake valve for supplying secondary air to the exhaust pipe 11 at the time of deceleration or idling to reduce HC and CO emissions.

In the control circuit 10, the down counter 108, the flip-flop 109, and the drive circuit 110
Is to control the That is, when the fuel injection amount TAU is calculated in a routine described later, the fuel injection amount TAU is preset in the down counter 108 and the flip-flop 109 is also set. As a result, the drive circuit 110 starts energizing the fuel injection valve 7. On the other hand, when the down counter 108 counts a clock signal (not shown) and its borrow-out terminal finally becomes "1" level, the flip-flop 109 is set and the drive circuit 110 sets the fuel injection valve 7 Stop energizing. That is, the fuel injection valve 7 is biased by the above-mentioned fuel injection amount TAU, and therefore, the amount of fuel corresponding to the fuel injection amount TAU is sent to the combustion chamber of the engine body 1.

Note that the CPU 103 generates an interrupt when the A / D conversion of the A / D converter 101 ends, when the input / output interface 102 receives the pulse signal of the crank angle sensor 6,
When an interrupt signal from 7 is received, and so on.

The intake air amount data Q and the cooling water temperature data THW of the air flow meter 3 are fetched by an A / D conversion routine executed every predetermined time and stored in a predetermined area of the RAM 105.
That is, the data Q and THW in the RAM 105 are updated every predetermined time. Further, the rotation speed data Ne is calculated by the interrupt of the crank angle sensor 6 every 30 ° CA and RA
It is stored in a predetermined area of M105.

FIG. 5 shows a routine for calculating the fine adjustment term AF _f, which is executed every predetermined time, for example, 4 ms. Step 5
At 01, it is determined whether or not the air-fuel ratio feedback condition is satisfied. For example, if the cooling water temperature is a predetermined value, for example 70
When the temperature is below ℃, during engine start, during post-start increase, during warm-up increase,
When the throttle valve 16 is fully closed (LL = "1") during power increase and fuel cut, secondary air is introduced based on the rotation speed Ne, vehicle speed, idle switch 16 signal LL, cooling water temperature THW, etc. The air-fuel ratio feedback condition is not satisfied when the O ₂ sensor 14 is not activated, the air-fuel ratio feedback condition is satisfied, and the air-fuel ratio feedback condition is satisfied in other cases. When the air-fuel ratio feedback condition is not satisfied, the process directly proceeds to step 513. If the air-fuel ratio feedback condition is satisfied, the routine proceeds to step 502. In step 502, the output V _OX of the O ₂ sensor 14 is A / D converted and captured, and in step 503, it is compared with a reference voltage V _R, for example, 0.45V. As a result, if V _OX ≦ V _R (lean), step 504
Set the air-fuel ratio flag XOX to "0" (lean) at step 5
At 05, it is determined whether or not the previous air-fuel ratio flag XOXO is "1" (rich). As a result, only when the flag XOX is inverted from "1" (rich) to "0" (lean), the fine adjustment term AF _f is changed to ΔAF _f (constant value) in step 507 as shown in FIG.
And Then, it proceeds to step 512. On the other hand, step 5
At 03, V _OX> V _R the air-fuel ratio flag XOX in if (rich) Step 508 "1", and (rich) at step 509 the previous air fuel ratio flag XOXO "0" or (lean) It is determined whether or not. As a result, the flag XOX changes from "0" (lean) to "1".
Only when inverted to (rich), as shown in FIG.
In step 511, the fine adjustment term AF _f is set to −ΔAF _f (constant value). Then, it proceeds to step 512.

At step 512, the routine shown in FIG.
₂ Clear the counter CNT for calculating the inversion cycle of the output V _OX of the sensor 14.

 Then, in step 513, this routine ends.

Thus, according to the routine of FIG. 5, as shown in FIG. 6, the fine adjustment term AF _f of the skipped waveform is calculated every time the output of the O ₂ sensor 14 is inverted. That is, the self-excited control waveform is obtained from the output of the O ₂ sensor 14 itself. In other words,
The control of the fine adjustment term AF _f corresponds to the skip control.

FIG. 7 shows a routine for calculating the rough adjustment term AF _c, which is executed every predetermined time, for example, every 64 ms. Steps
In step 701, it is determined whether or not the air-fuel ratio feedback condition is satisfied, as in step 501 in FIG. As a result, steps 702 to 7 are performed only when the air-fuel ratio feedback condition is satisfied.
The flow of 07 is executed. That is, in step 702, it is determined whether or not the counter CNT has reached the constant value KCNT. The counter CNT is the O ₂ sensor 1 as described above.
4 output Cleared at every inversion of V _OX . Therefore,
At the beginning, go from step 702 to step 703
CNT is incremented by +1 and the process proceeds to step 708. When the counter CNT reaches KCNT, that is, time KCNT × 64 ms
After that, the flow in Step 702 is Steps 704 to 70.
Go to 7.

In step 704, the counter CNT is cleared and the step
At 705, it is determined whether the present catalyst downstream air-fuel ratio is lean (“0”) or rich (“1”) by the air-fuel ratio flag XOX. As a result, if it is lean, the coarse adjustment term AF _c is increased by ΔAF _c (constant value) in step 706, while if it is rich, it is decreased by ΔAF _{c in} step 707. Then, it proceeds to step 708.

The value ΔAF _c is smaller than the skip amount ΔAF _f used in steps 507 and 511 in FIG. That is, ΔAF _c <ΔAF _f . Therefore, as shown in FIG. 8, when the air-fuel ratio is lean (XOX = “0”), the rough adjustment term AF _c is gradually increased by ΔAF _c, and when the air-fuel ratio is rich (XOX = “0”).
“1”), the coarse adjustment term AF _c is gradually reduced by ΔAF _c .
That is, the control of the rough adjustment term AF _c corresponds to the integral control. It is also possible to introduce a skip control for each inversion of the air-fuel ratio into the rough adjustment term AF _c to improve the convergence of the air-fuel ratio.

The execution or non-execution of the rough adjustment term calculation routine of FIG. 7 depends on the execution or non-execution of the fine adjustment term calculation routine of FIG. 5, respectively. That is, when the catalyst downstream air-fuel ratio deviates from the stoichiometric air-fuel ratio, either V _OX ≤V _R (lean) or V _OX > V _R (rich) is held, and therefore the routine of FIG. Fine adjustment term by
AF _f is held in either ΔAF _f or −ΔAF _f , and as a result, the counter CNT is not cleared in step 512. On the other hand, in this case, the rough adjustment term AF _c according to the routine of FIG. 7 is gradually increased or decreased every KCNT × 64 ms. That is, the coarse adjustment term AF _c is controlled rather than the fine adjustment term AF _f .

Conversely, if the catalyst downstream air-fuel ratio converges to the stoichiometric air-fuel ratio, reversal of the output V _OX of the O ₂ sensor 14 is performed miscellaneous frequent, that is, the inversion cycle of the output V _OX of the O ₂ sensor 14 is short As a result, the fine adjustment term AF _f is frequently repeated between ΔAF _f and −ΔAF _f . In this case, the counter CNT is cleared by step 512 in FIG. 5 before reaching KCNT, so that the flow in step 702 in FIG. 7 always proceeds to step 703.
That is, an increase or decrease of the coarse adjustment section AF _c is not, therefore, the value control is prohibited coarse adjustment section AF _c is held, only the control of the fine adjustment term AF _f is performed.

FIG. 9 is a routine for generating the self-excited oscillation term (forced oscillation term) AF _s, which is executed every predetermined time, for example, every 4 ms. In step 901, similarly to step 501 of FIG. 5, it is determined whether or not the air-fuel ratio feedback condition is satisfied.
As a result, if the air-fuel ratio feedback condition is not satisfied, the process directly proceeds to step 918, and only when the air-fuel ratio feedback condition is satisfied, the process proceeds to step 902.

In step 902, the output V _OX of the O ₂ sensor 14 is A / D converted and belongs to one of the three regions shown in FIG. 10, that is, V _OX ≦ V ₁ V ₁ <V _OX <V ₂ V _OX ≧ Determine if it belongs to V ₂ . As a result, the rich rich time T _R at step 904 if V _OX ≦ V _1, at T _R ← T ₁ (e.g. 750ms equivalent value) and, V ₁ <V _OX <If V ₂ Step 905 time
The T _R, T _R ← T ₂ and (e.g. 500ms equivalent value), V _OX ≧ V ₂ a long if the rich time at step 906 T
_{Let R} be T _R ← T ₃ (for example, a value equivalent to 250 ms). Here, T ₁ > T ₂ > T ₃ . Then step 90
At 7, the lean time T _L is set to T _L ← T−T _R , where T is a constant value, for example, 1000 ms. That is, since the output V _OX of the O ₂ sensor 14 indirectly represents the O ₂ storage amount of the three-way catalyst, as shown in FIG.
_{If OX} ≤ V ₁ , it is considered that the O ₂ storage amount of the three-way catalyst is large, and the rich duty ratio T _R / T of the self-oscillation term AF _s is increased. If V ₁ <V _OX <V ₂ , Considering that the O ₂ storage amount of the source catalyst is medium, the rich duty ratio T _R / T of the self-excited oscillation term AF _s is set to medium, and if V _OX ≧ V ₂ , the O ₂ storage amount of the three-way catalyst is small. As a result, the rich duty ratio T _R / T of the self-excited oscillation term AF _s is reduced, thereby improving the convergence of the air-fuel ratio.

The flow of steps 908 to 917 is based on the rich time T _R and the lean time T _L described above, and the forced self-excited control waveform AF shown in FIG.
generate _s . That is, in step 908, the counter CNTS
Is incremented by 1 and the flag XSIC is added in step 909.
Determines whether is "1". As a result, if XSIC = "1", the counter CNTS is determined whether the host vehicle has reached the rich period T _R to step 910, if the counter CNTS has reached the rich period T _R only, the counter at step 911 CNTS is cleared, and in step 912 the self-oscillation term AF _s is inverted to −ΔAF _s , and in step 913 the flag XSIC is inverted to “0”. Similarly, if XSIC = "0" at step 909, the counter CNTS is determined whether the host vehicle has reached the lean time T _L in step 914, if the counter CNTS has reached the lean time T _L only, The counter CNTS is cleared in step 915, the self-excited oscillation term AF _s is inverted to ΔAF _s in step 916, and the flag XSIC is inverted to “1” in step 917.

 Then, in step 918, this routine ends.

FIG. 12 is an injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° CA. In step 1201, intake air amount data Q and rotation speed data are read from RAM 105.
Ne is read to calculate the basic injection amount TAUP. For example, TAUP
← α · Q / Ne (α is a constant). In step 1202, the final injection amount TAU is calculated by TAU ← TAUP · (AF _f + AF _c + AF _s + β) +
Calculate with γ. Here, β and γ are correction amounts determined by other operation state parameters. Then step 12
In 03, the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection. Then, in step 1204, this routine ends. As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is reset by the borrow-out signal of the down counter 108, and the fuel injection ends.

As described above, the duty ratio of the self-excited oscillation term AF _s is made variable according to the three divided regions of the output V _OX of the O ₂ sensor 14, but the number of divided regions can be other numbers.

Moreover, the O ₂ storage section AF the O ₂ storage amount above
_By using _CCRO together, the convergence of the air-fuel ratio can be improved.

FIG. 13 is a routine for calculating the O ₂ storage term AF _CCRO, which is executed every predetermined time, for example, every 16 ms. In step 1301, the same as step 501 in FIG. 5,
It is determined whether or not the air-fuel ratio feedback condition is satisfied. As a result, if the air-fuel ratio feedback condition is not satisfied, the process directly proceeds to step 1311, and only when the air-fuel ratio feedback condition is satisfied, the process proceeds to step 1302. In step 1302, the output V _OX of the O ₂ sensor 14 is A / D converted and captured, and in step 1303
Determine V _OX . That is, as shown in FIG.
Divide between 1.0V into 7 divisions, that is, O to OX 1 OX 1 to OX 2 OX 2 to OX 3 OX 3 to OX 4 OX 4 to OX 5 OX 5 to OX 6 OX 6 to 1.0V divided into 7 and Determine which of these regions _OX is in. That is, if 0 ≦ V _OX <OX 1, step 1
At 304, the O ₂ storage term AF _CCRO is set to AF _CCRO ← f ₁ , and if OX 1 ≤ V _OX <OX 2, then at step 1305, O
_{2 Set the} storage term AF _CCRO to AF _CCRO ← f ₂ , and if OX 2 ≤ V _OX <OX 3, set O in step 1306.
_{2 Set the} storage term AF _CCRO to AF _CCRO ← f ₃ , and if OX 3 ≤ V _OX <OX 4, set O in step 1307.
₂ Storage term AF _CCRO is set to AF _CCRO ← 0, and if OX 4 ≤ V _OX <OX 5, then at step 1308, O
_{2 Set the} storage term AF _CCRO to AF _CCRO ← f ₄ , and if OX 5 ≤ V _OX <OX 6, set O in step 1309.
_{2 Set the} storage term AF _CCRO to AF _CCRO ← f ₅ , and if OX 6 ≤ V _OX ≤ 1.0 V, in step 1310, set O
_{2 Set the} storage term AF _CCRO to AF _CCRO ← f ₆ .

Then, in step 1311, the routine of FIG. 13 ends.

Thus, since the O ₂ sensor 14 is located downstream of the catalyst,
The output V _OX of the O ₂ sensor 14 can further monitor the O ₂ storage amount of the three-way catalyst. Therefore, in addition to varying the duty ratio of the forced self-oscillation term AF _s according to this amount, the O ₂ storage _{Compute the} term AF _CCRO . _If only the O ₂ storage term AF _CCRO is made variable according to the O ₂ storage amount, the amplitude of the O ₂ storage term AF _CCRO must be increased, and as a result, the air-fuel ratio fluctuates greatly. Although the air-fuel ratio convergence and drivability deteriorate, the amplitude of the O ₂ storage term AF _CCRO can be reduced by changing the duty ratio of the forced self-excited oscillation term AF _s. The deterioration of the ability can be reduced.

FIG. 15 shows an injection amount calculation routine when the O ₂ storage term AF _CCRO is added, and step 1501 is provided instead of step 1202 of FIG. That is, the final injection amount TA
U is TAU ← TAUP · (AF _f + AF _c + AF _s + AF _CCRO + β) + γ.

Note that f ₁ , f ₂ , and f ₃ in steps 1304, 1305, and 1306 in FIG. 13 are f ₁ · d _tL , f ₂ · d _tL , and f ₃ · d, respectively.
_tL (d _tL is the lean duration), and steps 1308,1309,1
F ₄ , f ₅ , and f ₆ in 310 are f ₄ · d _tR and f ₅ ·, respectively.
It may be d _tR , f ₆ · d _tR (d _tR is rich duration time).
Also, steps 1304, 1305, 1306, 1308, 1309, 131 in FIG.
It may be a value corresponding to the change rate of the load at 0, for example, the intake air amount Q / Ne per one rotation.

Further, although the fine adjustment term AF _f is introduced in the above-described embodiment, the air-fuel ratio control can be performed by introducing only the coarse adjustment term AF _c and the duty self-oscillation waveform AF _{s with a} variable duty ratio. In this case, when the air-fuel ratio deviates from the vicinity of the stoichiometric air-fuel ratio, the forced self-excited oscillation waveform AF _s takes the place of the fine adjustment term AF _f .

Further, instead of an air flow meter, a Karman vortex sensor, a heat wire sensor, or the like can be used as the intake air amount sensor.

Further, in the above-described embodiment, the fuel injection amount is calculated according to the intake air amount and the engine speed, but the fuel injection amount is calculated according to the intake air pressure and the engine speed, or the throttle valve opening and the engine speed. The fuel injection amount may be calculated.

Further, in the above-described embodiment, the internal combustion engine in which the fuel injection valve controls the fuel injection amount to the intake system is described. However, the present invention can be applied to a carburetor-type internal combustion engine. For example, the air-fuel ratio is controlled by adjusting the intake air amount of the engine using an electric air control valve (EACV).
An electric bleed air control valve is used to adjust the air bleed amount of the carburetor to control the air-fuel ratio by introducing air into the main passage and the slow passage, and the 2 o'clock air amount sent to the exhaust system of the engine. The present invention can be applied to things that are adjusted. In this case, the basic fuel injection amount corresponding to the basic injection amount TAUP in step 1201 is determined by the carburetor itself, that is, the intake pipe negative pressure according to the intake air amount and the rotational speed of the engine. At the final fuel injection amount TAU
Is calculated.

Furthermore, in the above-described embodiment, the O ₂ sensor is used as the air-fuel ratio sensor, but a CO sensor, a lean mixture sensor, or the like may be used. In particular, when a TiO ₂ sensor is used as the air-fuel ratio sensor, control response is improved and overcorrection due to the output of the downstream side air-fuel ratio sensor can be prevented.

Further, although the above-described embodiment is constituted by a microcomputer, that is, a digital circuit, it may be constituted by an analog circuit.

〔The invention's effect〕

As described above, according to the present invention, since the duty ratio of the forced self-excited oscillation term is variable according to the O ₂ storage amount, the convergence of the control air-fuel ratio can be improved and the deterioration of emission can be prevented. , The control frequency is kept high,
Therefore, the purification performance of the catalyst can be maximized.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing the basic configuration of the present invention, FIG. 2 is a timing diagram for explaining the problems to be solved by the present invention, and FIG. 3 shows the relationship between the forced self-excited control waveform and the catalyst cleaning function. The graph shown in FIG. 4 is an overall schematic view showing one embodiment of the air-fuel ratio control system for an internal combustion engine according to the present invention, FIG. 5, FIG. 7, FIG. 9, FIG. 12, FIG. 13, FIG. FIG. 6 is a flow chart for explaining the operation of the control circuit of FIG. 4, FIG. 6 is a timing chart supplementarily explaining the flow chart of FIG. 5, and FIG. 8 is a timing diagram supplementarily explaining the flow chart of FIG. 10 is a diagram for supplementary explanation of the flowchart of FIG. 9, FIG. 11 is a timing diagram for supplementary explanation of the flowchart of FIG. 9, and FIG. 14 is a timing diagram for supplementary explanation of the flowchart of FIG.

Claims (1)

(57) [Claims] 1. A three-way catalyst (12) provided in an exhaust passage of an internal combustion engine, and a catalyst downstream air-fuel ratio sensor (for detecting an air-fuel ratio of the engine provided in an exhaust passage downstream of the three-way catalyst ( 14) and a coarse adjustment that calculates a coarse adjustment term (AFc) that gradually changes to the lean side when the output of the air-fuel ratio sensor is rich and gradually changes to the rich side when the output of the air-fuel ratio sensor is lean. Self-excited control for generating a forced self-excited control waveform (AFs) having a term calculation means, a predetermined amplitude (ΔAFs), a predetermined frequency (period T), and a predetermined rich ratio and lean ratio determined by a predetermined duty ratio. When the output of the air-fuel ratio sensor is rich, the lean ratio in the forced self-excited control waveform is increased, and when the output of the air-fuel ratio sensor is lean, the rich ratio in the forced self-excited control waveform is generated. Duty ratio varying means for varying the duty ratio in the forced self-excited control waveform so as to increase the air-fuel ratio adjustment for adjusting the air-fuel ratio of the engine in accordance with the rough adjustment term and the forced self-excited control waveform. An air-fuel ratio control device for an internal combustion engine, comprising: