JP2692309B2 - 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 applicant has already determined that in a single O ₂ sensor system in which an O ₂ sensor is provided downstream of the catalyst, the center value of the forced self-excitation control waveform (forced oscillation waveform) having a predetermined amplitude and a predetermined frequency is set to the downstream O ₂ 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 _ox of the downstream O ₂ sensor
When is changed, 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
In F _s0 ), the catalyst can maximize its purification performance, but even if the forced self-excited control waveform fluctuates with the rich air-fuel ratio (λ <1) or lean air-fuel ratio (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 _s0 so that the purification performance of the catalyst can be exhibited.

[Problems to be solved by the invention]

However, in the conventional device described above, the catalyst
If the air-fuel ratio feedback control is performed in the idle state where the O ₂ storage effect can be fully exerted, the convergence of the theoretical air-fuel ratio of the control air-fuel ratio becomes poor, or the air-fuel ratio becomes overcorrected, and the emission deteriorates. There is a problem of inviting.

That is, as shown in FIG. 4, even if the air-fuel ratio upstream of the catalyst changes as shown by the dotted line, the air-fuel ratio downstream of the catalyst is delayed as shown by the solid line due to the O ₂ storage effect of the catalyst. Moreover, the duration of this O ₂ storage effect is longer when idle than when running. Moreover, considering the gas transportation time between the catalyst inlet and the catalyst outlet, the air-fuel ratio downstream of the catalyst is delayed as shown by the chain line. Moreover, the gas transportation time is longer when the vehicle is idle than when traveling.

Further, as shown in FIG. 5, the purge time (responsiveness) of the O ₂ sensor depends on the gas flow rate. Note that FIG. 5 shows the case where the gas flow rate is small, ignoring the O ₂ storage effect of the catalyst. That is, as indicated by the diagonal lines, when the gas flow rate is small, both the change from rich to lean and the change from lean to rich in the output of the O ₂ sensor are delayed,
In the worst case, the output V _ox of the O ₂ sensor may not be inverted as shown by arrows X ₁ , X ₂ , and X ₃ . The purge time of such an O ₂ sensor also becomes longer during idling than during traveling.

Therefore, when the control air-fuel ratio deviates greatly from the stoichiometric air-fuel ratio during idling due to the above-mentioned O ₂ storage effect, gas transportation time, purge time of the O ₂ sensor, etc., the above coarse adjustment term AF _{When c} is introduced, it takes time for the control air-fuel ratio to converge to the stoichiometric air-fuel ratio, resulting in deterioration of emission. It is also possible to increase the update speed of the coarse adjustment term AF _c described above to increase the convergence speed of the control air-fuel ratio to the theoretical air-fuel ratio, but in this case, the change speed of the control air-fuel ratio is large. As a result, the control air-fuel ratio is overcorrected and the emission is deteriorated.

The rough adjustment term AF _c
It is conceivable to increase the update speed of the engine and then decrease it (see Japanese Patent Laid-Open No. 63-97847), but the control air-fuel ratio converges to the stoichiometric air-fuel ratio and the output of the O ₂ sensor fluctuates near the stoichiometric air-fuel ratio In that case, overcorrection due to the large update speed still occurs, and the control air-fuel ratio does not converge to the theoretical air-fuel ratio.

Therefore, an object of the present invention is to provide a single air-fuel ratio sensor system having a downstream side air-fuel ratio sensor in which convergence of the control air-fuel ratio is improved without overcorrecting even during idling.

[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 idle state determination means determines whether the engine is in the idle state. As a result, when the engine is in the idle state, the post-reversal time measuring means measures the post-reversal time of the output V _ox of the air-fuel ratio sensor, and the coarse adjustment term computing means calculates the post-reversal time less than the predetermined time (CNTF ≠ 0), the first depending on the output V _ox of the air-fuel ratio sensor
Of the update rate delta _1, when after the reversal time is greater than the predetermined time by (CNTF = 0) the air-fuel ratio first update rate delta ₁ is smaller than the second update rate delta ₂ in accordance with the output V _ox of the sensor, sky A coarse adjustment term AF _c that gradually changes to the lean side when the output V _{ox of the} 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 is calculated.
On the other hand, when the engine is idle, the inversion period determination means measures the inversion period T of the air-fuel ratio sensor, the inversion period T is determined whether or not the predetermined period T ₁ is less than or, as a result, the air-fuel ratio sensor When the inversion period T is smaller than the predetermined period T ₁ , the prohibiting means prohibits the calculation of the rough adjustment term AF _c . Then, the air-fuel ratio adjusting means adjusts the air-fuel ratio of the engine according to the rough adjustment term AF _c .

[Action]

According to the above-described means, as shown in FIG. 6, a large coarse adjustment section AF _cf the control air-fuel ratio of the update rate close up near the stoichiometric air-fuel ratio, then a small update rate coarse adjustment section AF _cs
Since the control air-fuel ratio is set to the stoichiometric air-fuel ratio, the control air-fuel ratio becomes the stoichiometric air-fuel ratio without being overcorrected. Moreover,
When the control air-fuel ratio becomes the stoichiometric air-fuel ratio and the inversion cycle T of the air-fuel ratio sensor becomes short, updating of the rough adjustment term AF _c is prohibited, so that the control air-fuel ratio is less likely to deviate from the theoretical air-fuel ratio. Therefore, the convergence of the control air-fuel ratio to the theoretical air-fuel ratio is improved.

〔Example〕

FIG. 7 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. 7, an air flow meter 3 is provided in an intake passage 2 of an engine 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 generates an output voltage that differs depending on whether the air-fuel ratio is leaner or richer than the theoretical air-fuel ratio, and the A / D of the control circuit 10
Supply to the converter 101. The control circuit 10 is configured as, for example, a microcomputer, and in addition to the A / D converter 101, the input / output interface 102, the CPU 103, the ROM 104, the RAM 105,
A backup RAM 106, a clock generation circuit 107, etc. are provided.

Further, the throttle valve 15 in the intake passage 2 has an idle switch 16 for detecting whether the throttle valve 15 is fully closed.
The 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. 8 shows the fine adjustment term AF _f and the inversion period T of the O ₂ sensor 14.
Is a routine for calculating
Executed every 4ms. In step 801, it is determined whether or not the air-fuel ratio feedback condition is satisfied. For example, when the cooling water temperature is below a predetermined value, for example, 70 ° C, the throttle valve 16 is fully closed (LL = "1") during engine start, during start-up, during warm-up, during power increase, during fuel cut. At this time, when the secondary air is introduced based on the rotation speed Ne, the vehicle speed, the signal LL of the idle switch 16, the cooling water temperature THW, etc., O
_{2 When the} sensor 14 is not activated, the air-fuel ratio feedback condition is not satisfied, and in other cases, the air-fuel ratio feedback condition is satisfied. When the air-fuel ratio feedback condition is not satisfied, the process proceeds to step 831 via steps 829 and 830. That is, in step 829, the update prohibition flag XT 1 of the idle coarse adjustment term AF _c1 described later is reset and initialized, and in step 830, the counter CNTF for controlling the update speed of the coarse adjustment term AF _c1 during idle is set. Clear, that is, set the update speed to a small state, and step
Continue to 831.

In step 801, if the air-fuel ratio feedback condition is satisfied, the process proceeds to steps 802 to 828. Here, steps 802 to 811 are a flow for calculating the fine adjustment term AF _f , steps 812 to 821 are a flow for calculating the inversion period T of the O ₂ sensor 14 during idling, and steps 822 to 828 are inversion periods. It is a flow for determining prohibition and execution of the calculation of a rough adjustment term AF _c1 at the time of idling, which will be described later, according to T.

First, steps 802-181 will be described. Steps
In the 802, the output V _OX of the O ₂ sensor 14 is A / D converted and captured,
In step 803, the reference voltage V _R is compared with 0.45 V, for example.
Whether a result, the air-fuel ratio flag XOX step 804 if V _OX ≦ V _R (lean) "0" and (lean), the previous air fuel ratio flag X0X0 at step 805 is "1" (rich) or Determine whether. As a result, only when the flag XOX is inverted from “1” (rich) to “0” (lean), the fine adjustment term AF _f is set to ΔAF _f (constant value) in step 807 as shown in FIG. . Then, the process proceeds to step 816. On the other hand, step 803
If V _OX > V _R (rich), the air-fuel ratio flag XOX is set to “1” (rich) in step 808, and it is determined in step 809 whether the previous air-fuel ratio flag XOXO is “0” (lean). Determine. As a result, the flag XOX changes from "0" (lean) to "1".
Only when inverted to (rich), as shown in FIG.
In step 811, the fine adjustment term AF _f is set to −ΔAF _f (constant value). Then, the process proceeds to step 819.

Thus, according to steps 802 to 811 of FIG. 8, as shown in FIG. 9, the fine adjustment term AF _f of the skipped waveform is calculated each 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.

Next, steps 812 to 821 will be described. Steps
If the flag XOX is not inverted at 805, step 812
At, it is determined whether or not the idle switch 16 is on (LL = “1”), that is, whether or not it is idle. As a result, the lean duration counter CNTL is counted up by +1 in step 813 only when the engine is idle to measure the lean duration counter CNTL. Similarly, if the flag XOX is not inverted in step 809, it is determined in step 814 whether the idle switch 16 is on (LL = "1"), that is, whether or not it is idle. As a result, the rich duration counter CNTR is set in step 815 only when idle.
The rich duration counter CNTR is measured by incrementing by 1.

If the vehicle is traveling (LL = “0”) in steps 812 and 814, the process proceeds to step 831 via steps 829 and 830.

Also in step 816, it is determined whether or not the idle switch 16 is on (LL = “1”), that is, whether or not it is idle. As a result, the inversion period T of the O ₂ sensor 14 is calculated by T ← CNTL + CNTR in step 817 only in the idle state, the counter CNTL is cleared in step 818, and the process proceeds to step 822. Similarly, in step 819,
It is determined whether or not the idle switch 16 is on (LL = “1”), that is, whether or not it is idle. As a result, the reversal period T of the O ₂ sensor 14 is determined in step 820 only when the engine is idle.
Is calculated by T ← CNTR + CNTL, the counter CNTR is cleared in step 821, and the process proceeds to step 822.

Next, steps 822 to 828 will be described. Steps
In 822, the inversion period T of the O ₂ sensor 14 calculated in step 817 or 820 is compared with a predetermined period T _1, and as a result,
If T <T ₁ , the process proceeds to steps 823 to 826 to prohibit the update of the idle coarse adjustment term AF _c1 . Conversely, if T ≧ T ₁ , the process proceeds to steps 827 and 828 to proceed to the idle coarse adjustment term AF _c1. To be updated. That is, in step 823, it is determined whether or not the update flag XT1 is “0”, and when XT 1 = “0”, the value at the time of stopping the update of the coarse adjustment term AF _c1 for idle is set to AF _c1 ← ( AF _c1max + AF _c1min ) / 2 as the intermediate value between the previous maximum and minimum values. Then, in step 825, the flag XT 1 is set (XT 1 =
If “1”) and T <T ₁ , step 824 is executed only once, and thereafter, the idle coarse adjustment term AF _c1 is fixed to the intermediate value. Next, at step 826, the update speed counter CNTF of the idle coarse adjustment term AF _c1 is cleared. On the other hand, in step 822,
If T ≧ T ₁ , in step 827, the update prohibition flag XT 1 is set to “0” to enable the idle coarse adjustment term AF _c1 to be updated, and in step 828, the idle coarse adjustment term AF _c1 is updated. CNTF is set to a predetermined value, for example, 8. That is, O ₂
The update speed counter CNTF is set to 8 each time the output of the sensor 14 is inverted, whereby the update speed of the idle coarse adjustment term AF _c1 is increased by, for example, 8 × 512 ms by the routine shown in FIG. 11 described later. .

 Then, in step 831, this routine ends.

FIG. 10 shows a routine for processing for transition from running to idle time, which is executed every predetermined time, for example, every 64 ms. This routine is O ₂ depending on the intake air amount state immediately before shifting to idle.
If the purge time (responsiveness) of the sensor 14 is estimated and the purge time is long, it is for starting the update of the idle coarse adjustment term AF _c1 immediately after shifting to the idle state and for setting the update rate to be large. is there. That is, in the routine of FIG. 8, immediately after shifting from the running state to the idle state,
Until the first inversion of the O ₂ sensor 14, XT 1 = “0” (step 82
According to 9), CNTF = 0 (according to step 820) is held, therefore, update speed sized control idling rough adjustment term AF _c1 is being prohibited, O ₂ sensor 14 in the running state by the routine of FIG. 10 If the purge time of
Immediately after shifting from the running state to the idle state, the update of the idle coarse adjustment term AF _c1 is started, and the update speed can be increased.

In step 1001, it is determined whether or not the vehicle is running (LL = "0"). As a result, if the vehicle is running (LL = "0"), step 10
The process proceeds to 02 to 1007, and if idle (LL = "1"), proceeds to step 1008.

Steps 1002 to 1007 will be described. Step 1002
Then, the intake air amount data Q is read from the RAM 105, and Q ≦ Q ₀
It is determined whether or not (constant value). As a result, low load (Q ≦
If it is Q ₀ ), the routine proceeds to step 1003, where the counter CNTQ indicating the purge time of the O ₂ sensor 14 is incremented by +1. On the other hand, if the load is high, the routine proceeds to step 1004, where the counter CNTQ
Count down 1 Then, in step 1005, CN
The purge time of the O ₂ sensor 14 is estimated by determining whether TQ> A (constant value). As a result, if the purge time of the O ₂ sensor 14 is long (CNTQ> A), the update speed counter CNTF is set to 8. On the other hand, if the purge time of the O ₂ sensor 14 is short (CNTQ ≦ A), update at step 1007. The speed counter CNTF is held at 0, and the process proceeds to step 1009.

When the engine switches from the running state to the idle state in the above state, the flow in step 1001 proceeds to step 1009.

As described above, according to the routine of FIG. 10, it is possible to update the coarse adjustment term AF _c1 during idle by XT 1 = “0” immediately after the engine shifts to the idle state, and the purge time of the O ₂ sensor 14 can be increased. When is large, the update speed counter CNTF is set to 8 in order to increase the update speed of AF _c1 .

FIG. 11 shows a routine for calculating the rough adjustment terms AF _c0 and AF _c1, which is executed every predetermined time, for example, every 64 ms. Here, AF _c1 is a rough adjustment term during idling as described above, and AF _c0 is a rough adjustment term during running. Step 11
05 to 1112 are the update flows of the coarse adjustment term AF _c1 at idle when the update speed is high, and the flows of steps 1113 to 1117 are the update flows of the coarse adjustment term AF _c1 at idle when the update speed is low, and steps 1118 to 11 Reference numeral 1120 is a flow for updating the coarse adjustment term AF _c0 during driving.

That is, in step 1101, step 801 in FIG.
Similarly, it is determined whether or not the air-fuel ratio feedback control condition is satisfied. However, it is assumed that this condition includes a condition of whether or not a predetermined time has elapsed after the fuel cut is returned. As a result,
Only when the air-fuel ratio feedback control condition is satisfied, the process proceeds to step 1102, and when it is not satisfied, the process directly proceeds to step 1121.

In step 1102, the idle switch 16 is turned on (LL =
It is determined whether the vehicle is idle (LL = "1") or running (LL = "0") depending on whether it is "1"). When idle, step 1103
And when traveling, proceed to step 1118.

In step 1103, it is determined by the update prohibition flag XT 1 _whether the coarse adjustment term AF _c1 during idling is updated or prohibited. In addition,
As described above, the update prohibition flag XT 1 is cleared by the routine of FIG. 8 when the inversion period T after the first inversion of the O ₂ sensor 14 is long (T ≧ T ₁ ). As a result, the update prohibition flag
Only when XT 1 is “0”, proceed to step 1104, while XT 1
When = "1", the process directly proceeds to step 1121.

In step 1104, the update speed counter CNTF indicates a high update speed state (CNTF ≠ 0) or a low update speed state (CNTF =
0) is determined. As a result, if the update speed is high (see AF _{cf in} FIG. 6), proceed to steps 1105 to 1112,
If the update speed is low (see AF _{cs in} FIG. 6), the process proceeds to steps 1113 to 1117.

The steps 1105-1112 will be described. Step 1105
Now, the counter CNTF is decremented by 1 and step 11
It is guarded with 0 at 06,1107. That is, steps 1108 to 1
112 is executed for a predetermined period (equivalent to CNTF = 8, in this case, 64 ms × 8). In step 1108, it is judged from the air-fuel ratio flag XOX whether the present catalyst downstream air-fuel ratio is rich (“1”) or lean (“0”). If the result is rich, in step 1109 the coarse adjustment AF during idle is performed.
_c1 is decreased by Δ ₁ (constant value), and the minimum value AF _c1min is updated in step 1110. On the other hand, if it is lean, it is increased by Δ _{1 in} step 1111 and the maximum value is obtained in step 1112.
Update AF _c1max . Then, the process proceeds to step 1121. The minimum value AF _c1min and the maximum value AF _c1max are the steps in FIG.
It is used for the fixed value calculation of the idle coarse adjustment term AF _c1 in 824.

Next, steps 1113 to 1117 will be described. In step 1113, it is determined by the air-fuel ratio flag XOX whether the present catalyst downstream air-fuel ratio is rich (“1”) or lean (“0”).
If the result is rich, the coarse adjustment term AF _c1 during idling is decreased by Δ ₂ (constant value) in step 1114, and the minimum value AF _c1min is updated in step 1115. On the other hand, if it is lean, it is increased by Δ _{2 in} step 1116, and step 11
The maximum value AF _c1max is updated at 12. And step 112
Proceed to 1. The constant value Δ ₂ has a relationship of Δ ₁ > Δ ₂ .

In this way, steps 1109 and 1111 and steps 1114 and 1116
Then, the update speed of the coarse adjustment term AF _c1 for idle is set to Δ ₁ and Δ
By switching to ₂ , there is a difference in update speed.

Steps 1118 to 1120 will be described. Step 1118
Then, it is determined by the air-fuel ratio flag XOX whether the present catalyst downstream air-fuel ratio is rich (“1”) or lean (“0”). As a result, if rich, the idle coarse adjustment term AF _c1 is decreased by Δ ₀ (constant value) in step 1119, while if lean, it is increased by Δ _{0 in} step 1120. Then, the process proceeds to step 1121. Here, the constant value Δ ₀ has a relationship of Δ ₁ > Δ ₀ Δ ₂ , and any of the values Δ ₀ , Δ ₁ , and Δ ₂ is the eighth value.
Skip amount ΔA used in steps 807 and 811 in the figure
Small compared to F _f . Therefore, if the air-fuel ratio is lean (XOX = "0"), the coarse adjustment terms AF _c0 ,, AF _c1 are both gradually increased, and if the air-fuel ratio is rich (XOX = "1"), the coarse adjustment is performed. Both terms AF _c0 and AF _c1 are gradually reduced. That is, the control of the rough adjustment term corresponds to the integral control. In particular, the idle coarse adjustment term AF _c1 is, as shown in FIG. 12, the air-fuel ratio flag XO.
After the reversal of X (including the case where the purge time of the O ₂ sensor 14 is long even immediately after idling), the update speed is increased for a predetermined period (CNTF = equivalent value of 8 times).

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 801 in FIG. 8,
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 O ₂ storage term AF _CCRO is _set to 0 in step 1315 and step 1316 is _executed.
And proceeds to step 1302 only when the air-fuel ratio feedback condition is satisfied. In step 1302, the output of the O ₂ sensor 14
A / D conversion of V _OX is taken in, and in step 1303, it is determined whether the engine is idle (LL = "1") or running (LL = "0"), and the flow of steps 1304 to 1307 and steps 1308 to 1314 are determined. It divides into the flow. As a result, the amplitude of the O ₂ storage term AF _CCRO is made smaller at the time of idling than at the time of traveling to reduce the disturbance of the air-fuel ratio.

Steps 1304 to 1307 will be described. Step 1304
Then, the output V _OX of the O ₂ sensor 14 is determined. Here, the region of V _OX is divided into three. That is, it is divided into 0 to V ₁ V _{1 to} V ₂ V _{2 to} 1.0 V in three divisions, and it is determined which of these regions V _OX is in. That is, if 0 ≦ V _OX ≦ V ₁ , step 130
In step 5, set O ₂ storage term AF _CCRO to AF _CCRO ← AF _CCROP (constant value), and if V ₁ <V _OX ≤ V ₂ , in step 1306, set O ₂ storage term AF _CCRO to AF _CCRO If V ₂ <V _OX ≦ 1.0V as ← 0, then at step 1307, O
₂ Storage item AF _CCRO is set as AF _CCRO ← _{−AF CCROP} .

Steps 1308 to 1314 will be described. Step 1308
Then, the output V _OX of the O ₂ sensor 14 is determined. Here again, the V _OX region is divided into three. As a result, if 0 ≦ V _OX <V ₁ ,
In step 1309, the integral term AF of the O ₂ storage term
_CCROi is updated by AF _CCROi ← AF _CCROi + δ (constant value), and in step 1310, O ₂ storage item AF
_{Set CCRO} as AF _CCRO ← AF _CCROP + AF _CCROi . If V ₁ <V _OX ≤ V ₂ , then in step 1311, O ₂
The integral term AF _CCROi of the storage term is _set to AF _CCROi ← 0, and the O ₂ storage term AF _CCRO is set to AF _CCRO ← 0 in step 1312. Further, if V ₂ <V _OX ≤ 1.0V, step 131
At 3, the integral term AF _CCROi of the O ₂ storage term is updated by AF _CCROi ← AF _CCROi + δ, and at step 1314, the O ₂ storage term AF
_{Let CCRO} be AF _CCRO ← AF _CCROP − AF _CCROi .

Then, in step 1316, 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 monitor the O ₂ storage amount of the three-way catalyst, and therefore the O ₂ storage term AF _CCRO is calculated according to this amount, but at this time, the output of the O ₂ sensor 14
Since the duration of the same region of V _OX also affects the amount of O ₂ storage in the catalyst, the integral amount AF _CCROi is introduced.

The number of divisions in FIG. 13 can be a number other than three.

FIG. 14A 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 1401, similarly to step 801 in FIG. 8, 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 1419, and only when the air-fuel ratio feedback condition is satisfied, the process proceeds to step 1402. In step 1402, it is determined whether the engine is idle (LL = "1") or running (LL = "0"). As a result, steps 1403 to 1410 are determined when idle.
And in the flow it generates a self-excited Fuko AF _s, generates a self-excited Fuko AF _s in the flow of steps 1411-1418 during running.

That is, in step 1403, the counter CNTS is set to T /
Determine if 2 has been reached. That is, the counter CNTS
Is incremented by 1 in step 1410, and CN
Proceed to steps 1404 to 1409 every TS = T / 2. Step 1404
Then, the counter CNTS is cleared, and in step 1405, it is determined whether or not the self-excited oscillation flag XSIC is “0”. If XSIC = “0”, the self-excited oscillation term AF _s is set to −ΔAF _{s in} step 1406. (Constant value), the flag XSIC is inverted to "1" in step 1407. As a result, when the counter CNTS reaches T / 2 again, the flow of step 1405 proceeds to steps 1408 and 1409. The self-oscillation term AF _s is set to ΔAF _{s in} step 1408, and the flag XSIC is inverted to “0” in step 1409.

On the other hand, when traveling (LL = "0"), the same as step 14
Self-oscillation term AF _s is generated by the flow from 11 to 1418. In this case, each step 1411 to 1418 corresponds to steps 1403 to 1410, and in this case, the amplitude of the self-excited oscillation term AF _s is ΔAF _s ′.
(> ΔAF _s ) and the period is T ′ (<T). That is, as shown in FIG. 14B, at the time of idling, the amplitude of the self-excited oscillation waveform AF _s is made small and the period is made large so as to reduce the disturbance of the air-fuel ratio.

FIG. 15 shows an injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° CA. In step 1501, the intake air amount data Q and the rotation speed data are read from the RAM 105.
Ne is read to calculate the basic injection amount TAUP. For example, TAUP
← α · Q / Ne (α is a constant). In step 1502, it is determined whether the engine is idling (LL = "1") or running (LL = "0"). As a result, if it is idling, in step 1503 the coarse adjustment term AF _{c is set} to idling. For the coarse adjustment term AF _c1 for driving, on the other hand, if it is during traveling, the coarse adjustment term AF _c ) is set as the coarse adjustment term for driving AF _{co in} step 1504. Next, in step 1506,
Set the final injection amount TAU as TAU ← TAUP ・ (AF _f ＋ AF _c ＋ AF _CCRO ＋ AF
_{It is} calculated by _s + β) + γ. Here, β and γ are correction amounts determined by other operation state parameters. Then
In step 1506, 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 1507, 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.

16 and 17 are modified examples of the routines of FIGS. 8 and 11, respectively, and show a second embodiment of the present invention. That is, in the second embodiment, the inversion period T of the O ₂ sensor 14 is smaller than the predetermined period T _{0 in} the control of the running coarse adjustment term AF _co , as in the control of the idle coarse adjustment term AF _c1. If it becomes smaller, updating of the coarse adjustment term AF _c0 for traveling is prohibited to reduce the disturbance of the air-fuel ratio during traveling.

In FIG. 16, steps 816, 819, 812, 81 in FIG.
4 is deleted, and even when idle (LL = "1") while running (LL =
"0") also calculates the inversion period T of the O ₂ sensor 14. Also, the update prohibition flag XT 0 of the rough adjustment term AF _c0 for driving
Is introduced and steps 1601 to 1607 are added. That is, if the air-fuel ratio feedback control condition is not satisfied, the update prohibition flag XT 1 is initialized to “0” in step 1607.

Further, when the air-fuel ratio feedback control condition is satisfied, the reversal cycle T of the O ₂ sensor 14 is calculated both during idling and during traveling, and the routine proceeds to step 1601. As a result, when idle (LL
= “1”), the flow of steps 822 to 827 is executed as in the case of FIG. On the other hand, when driving (LL
= “0”), the flow proceeds to steps 1602-1606.

Steps 1602-1606 correspond to steps 822-825,827. That is, in step 1602, the inversion period T of the O ₂ sensor 14 calculated in step 817 or 820 is compared with a predetermined period T ₀ (<T ₁ ), and if T <T _{0 as a} result, steps 1603 to Proceeding to 1605, updating of the coarse adjustment term AF _c0 for traveling is prohibited. Conversely, if T ≧ T ₀ , the procedure proceeds to step 1606 to update the coarse adjustment term AF _c0 for traveling. That is, in step 1603, it is determined whether or not the update flag XT 0 is “0”, and when XT 0 = “0”, the value at the time of stopping the update of the rough adjustment term AF _c0 for traveling is set to AF _c0 ← (AF _c0max + AF _c0min ) / 2 as the intermediate value between the maximum value and the minimum value immediately before. Then, in step 1605, the flag XT 0 is set (XT 0 =
If “1”) and T <T ₀ , step 1603 is executed only once, so that after that, the rough adjustment term AF for driving is executed.
Fix _c0 to the above intermediate value. On the other hand, step
If T ≧ T ₀ in 1602, the update prohibition flag XT 0 is set to “0” in step 1606, and the running coarse adjustment term AF _c0 can be updated.

In FIG. 17, steps 1701 to 1703 are added to the routine of FIG. That is, when traveling, it is determined whether the update prohibition flag XT 0 calculated by the routine of FIG. 16 is reset or set. If the prohibition flag XT 0 is “1”, the process directly proceeds to step 1121 and prohibits the update of the running coarse adjustment term AF _c0 . On the other hand,
If the prohibition flag XT 0 is “0”, steps 1118 to 1120,170
Continue to 2,1703. That is, at step 1118, the present catalyst downstream air-fuel ratio is rich (“1”) due to the air-fuel ratio flag XOX.
Or lean (“0”). If the result is rich, the running coarse adjustment term AF _C0 is decreased by Δ ₀ in step 1119, and the minimum value AF _c0min is updated in step 1702. On the other hand, if it is lean, it is incremented by Δ _{0 in} step 1120, and the maximum value AF _c0max is updated in step 1703.
Then, the process proceeds to step 1121. The minimum value AF _c0min ,
The maximum value AF _c0max is used for the fixed value calculation of the running coarse adjustment term AF _c0 in step 1604 of FIG.

Thus, according to the routines of FIGS. 16 and 17, O ₂
When the reversal period T of the sensor 14 becomes small, updating of the coarse adjustment term AF _c0 for traveling is prohibited, and the control air-fuel ratio is stabilized.

It should be noted that in the above-described embodiment, the coarse adjustment terms AF _c1 and AF _{c0 are used.}
At the time of updating, the rich side update speed and the lean side update speed are made the same, but it is also possible to make them asymmetric. For example, steps 1109 and 1111 in FIG.
It is also possible to make the update rate Δ ₁ in the different. This is because the output characteristic of the O ₂ sensor 14 is different between the change from rich to lean and the change from lean to rich. For example, as shown in FIG. 5, when the air-fuel ratio changes from lean to rich, the output V _OX of the O ₂ sensor 14 changes slowly, and conversely, when the air-fuel ratio changes from rich to lean. In some cases, the output V _OX of the O ₂ sensor 14 may change abruptly. In such a case, the update rate delta ₁ of the update rate delta ₁ of Figure 11 in step 1111 step 1109
This is because if the setting is made larger, the convergence of the control air-fuel ratio is improved.

Further, in the above-mentioned embodiment, the fine adjustment term AF _f , the O ₂ storage term AF _CCRO , and the forced self-excitation term AF _s are introduced, but the introduction of only the coarse adjustment term AF _c enables the air-fuel ratio control. .

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 1501 is determined by the carburetor itself, that is, the intake pipe negative pressure according to the intake air amount and the engine speed, and step 1505 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. Especially, as an air-fuel ratio sensor, TiO ₂
When the sensor is used, the 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, at the time of idling, the update speed of the coarse adjustment term is increased only for a predetermined period after the output of the air-fuel ratio sensor is inverted, and thereafter, it is decreased. Since the update of the coarse adjustment term is prohibited after the control air-fuel ratio has fully converged, the disturbance of the air-fuel ratio is reduced, and as a result, the deterioration of the emission can be prevented and therefore the purification performance of the catalyst can be improved. It can be demonstrated to the maximum.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing a basic configuration of the present invention, FIG. 2 is a timing diagram for explaining a conventional technique, FIG. 3 is a graph showing a relationship between a forced self-excited control waveform and a catalyst cleaning function, and FIG. 5 and 5 are timing charts for explaining the problem to be solved by the present invention, FIG. 6 is a timing chart for explaining the operation of the present invention, and FIG. 7 is a diagram of the air-fuel ratio control apparatus for the internal combustion period according to the present invention Overall schematic view showing one embodiment, FIG. 8, FIG. 10, FIG. 11, FIG. 13, FIG. 14A, FIG.
16A, 16B, and 17 are flowcharts for explaining the operation of the control circuit in FIG. 7, FIG. 9 is a timing diagram for supplementarily explaining the flowchart in FIG. 8, and FIG. 12 is FIG. FIG. 14B is a timing diagram for supplementary explanation of the flowchart of FIG. 14, FIG. 14B is a diagram for supplementary explanation of the flowchart of FIG. 14A, and FIG. 16 is a diagram showing a combination of FIGS. 16A and 16B.

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), an idle state determination unit that determines whether the engine is in an idle state, and a post-reversal time measurement unit that measures the post-reversal time of the output of the air-fuel ratio sensor when the engine is in the idle state, When the post-reversal time is shorter than a predetermined time, the first update speed is set according to the output of the air-fuel ratio sensor, and when the post-reversal time is larger than the predetermined time, the first update speed is set according to the output of the air-fuel ratio sensor. Second less than update rate
The coarse adjustment term (AF) 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 due to the update speed of
_c ) coarse adjustment term calculation means, and when the engine is idle, a reversal cycle determination means that measures the reversal cycle of the air-fuel ratio sensor and determines whether or not the reversal cycle is shorter than a predetermined cycle. And prohibiting means for prohibiting calculation of the rough adjustment term (AF _c ) when the reversal cycle of the air-fuel ratio sensor is shorter than a predetermined cycle, and an air-fuel ratio adjusting the air-fuel ratio of the engine according to the rough adjustment term. An air-fuel ratio control device for an internal combustion engine, comprising: adjusting means.