JP2600767B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides an air-fuel ratio sensor (in this specification, an oxygen concentration sensor (O ₂ sensor)) on the upstream and downstream sides of a catalytic converter. O ₂ relates to an air-fuel ratio control apparatus for an internal combustion engine which performs air-fuel ratio feedback control by the O ₂ sensor downstream in addition to the air-fuel ratio feedback control by the sensor.

[Conventional technology]

In simple air-fuel ratio feedback control (single O ₂ sensor system), an O ₂ sensor that detects oxygen concentration is provided at a point in the exhaust system as close as possible to the combustion chamber, that is, at the gathering portion of the exhaust manifold upstream of the catalytic converter. In addition, the variation in the output characteristics of the O ₂ sensor hinders the improvement of the air-fuel ratio control accuracy. In order to compensate for such variations in the output characteristics of the O ₂ sensor and variations in components such as the fuel injection valve, aging or changes over time, a second O ₂ sensor is provided downstream of the catalytic converter, and the upstream O ₂ sensor is used. Downstream O ₂ in addition to air-fuel ratio feedback control
Double O ₂ with air-fuel ratio feedback control by sensor
Sensor systems have already been proposed (see:
No. -48756). In this double O ₂ sensor system,
O ₂ sensor provided downstream of the catalytic converter, compared with the upstream O ₂ sensor, but has a low response speed,
There is an advantage that the variation in output characteristics is small for the following reasons.

(1) Since the exhaust gas temperature is low downstream of the catalytic converter, there is little thermal effect.

(2) Downstream of the catalytic converter, various poisons are trapped in the catalyst, so that the amount of poisoning of the downstream O ₂ sensor is small.

(3) The exhaust gas is sufficiently mixed downstream of the catalytic converter, and the oxygen concentration in the exhaust gas is close to the equilibrium state.

Therefore, as described above, the air-fuel ratio feedback control based on the outputs of the two O ₂ sensors (double O ₂ sensor system), the variations in the output characteristic of the upstream O ₂ sensor can be absorbed by the downstream O ₂ sensor. In fact, as shown in FIG. 2, when the output characteristics of the O ₂ sensor deteriorate in the single O ₂ sensor system, the exhaust emission characteristics are directly affected, whereas in the double O ₂ sensor system, the upstream side Even if the output characteristics of the O ₂ sensor deteriorate, the exhaust emission characteristics do not deteriorate. That is, in the double O ₂ sensor system, as long as the downstream O ₂ sensor maintains stable output characteristics, good exhaust emissions are guaranteed.

[Problems to be solved by the invention]

However, even when such a double O ₂ sensor system is employed, the exhaust emission greatly changes when the three-way catalyst is new and deteriorated. That is, the three-way catalyst stores oxygen in the exhaust gas, so-called O ₂ has a storage function, this O ₂ storage function of oxygen deprived from the NO _x when the lean air-fuel ratio in the oxygen three-way catalyst When stored, the oxygen stored in the three-way catalyst at a rich air-fuel ratio is consumed to oxidize HC and CO. Therefore, the Repeating alternating with lean air-fuel ratio and the rich air-fuel ratio by feedback control of the air-fuel ratio HC This O ₂ storage function, CO, NO _x is purified.

Incidentally in this case, three-way catalyst air-fuel ratio average value a third view of the W ₂ in a relatively wide if it is kept in a window eta _0% or more purification rate shown in the out of new is obtained. On the other hand, by using the three-way catalyst for a long time, O ₂
When the storage function weakens, the average value of the air-fuel ratio becomes
To be kept within the narrow window shown by W ₁ in FIG η
A purification rate of ₀ % or more cannot be obtained. Thus, η ₀ %
The width of the window in which the above purification rate is obtained varies depending on the strength of the O ₂ storage function, but as can be seen from FIG. 3, the purification rate becomes highest when the average value of the air-fuel ratio is the stoichiometric air-fuel ratio. When the average value of the air-fuel ratio deviates from the stoichiometric air-fuel ratio, it is necessary to return the average value of the air-fuel ratio to near the stoichiometric air-fuel ratio as soon as possible.

Incidentally, in the double O ₂ sensor system, the average value of the air-fuel ratio is controlled based on the output signal of the downstream O ₂ sensor.
Specifically, for example, the average value of the air-fuel ratio becomes lean,
When this is detected by the downstream O ₂ sensor, that is, when the downstream O ₂ sensor issues a lean signal indicating that the average value of the air-fuel ratio is lean, the average value of the air-fuel ratio is gradually shifted to the rich side. .

In this case, until the average value of the air-fuel ratio when the three-way catalyst was new deviates from the window W _2, i.e. the downstream O ₂ sensor to the average value of the air fuel ratio is considerably richer continues to generate a lean signal, average of the air-fuel ratio for the first time the downstream O ₂ sensor outside window W ₂ generates a rich signal. When the downstream O ₂ sensor generates a rich signal, the average value of the air-fuel ratio is shifted toward the lean side, but at this time, the average value of the air-fuel ratio is considerably shifted toward the rich side. To return the average value to near the stoichiometric air-fuel ratio, the average value of the air-fuel ratio must be greatly changed.

On the other hand, when the O ₂ storage function is weakened, a downstream O ₂ sensor rich signal is generated when the average value of the air-fuel ratio is out of the window W _1, that is, when the air-fuel ratio is not so rich. I do. Therefore, at this time, in order to return the average value of the air-fuel ratio to near the stoichiometric air-fuel ratio, the average value of the air-fuel ratio may be slightly changed. That is, the average value of the air-fuel ratio to be changed when the output signal of the downstream O ₂ sensor switches from the lean signal to the rich signal must be changed according to the degree of richness of the average value of the air-fuel ratio at that time. Will be.

However the rate of change of the degree of the rich average of the air-fuel ratio is different is accompanied thereto from lean to rich of the output signal of the downstream O ₂ sensor has been found to vary. That is, if the degree of richness of the average value of the air-fuel ratio is high when the window deviates, the O ₂ molecules on the downstream O ₂ sensor are rapidly consumed by a large amount of unburned HC and CO, so that the downstream O ₂ The output signal of the sensor changes rapidly from a lean signal to a rich signal. O ₂ and low degree of rich average of the air-fuel ratio on the downstream O ₂ sensor when outside the window contrast
Since the molecules are slowly consumed by a small amount of unburned HC and CO, the output signal of the downstream O ₂ sensor slowly changes from a lean signal to a rich signal.

Incidentally in this case, the change rate of the lean of the output signal of the downstream O ₂ sensor to the rich can be represented by the duration of the output of the downstream O ₂ sensor takes a value within a range of near stoichiometric air-fuel ratio. That is, the duration is shorter when the rate of change of the downstream O ₂ sensor output signal is high, the duration becomes longer when the rate of change of the downstream O ₂ Princesa output signal is slow. Therefore, when this duration is short, the average value of the air-fuel ratio is largely changed to the lean side,
When the duration is long, the average value of the air-fuel ratio can be returned to near the stoichiometric air-fuel ratio by slightly changing the average value of the air-fuel ratio to the lean side.

The present invention has been made based on this finding, it is an object is to maintain HC, CO, a good cleaning action of NO _x.

[Means for solving the problem]

The means for solving the above-mentioned problem is shown in FIG. That is, the upstream exhaust passage of the three-way catalyst CC _R0 provided in an exhaust passage of an internal combustion engine, it is provided upstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine, also the three-way catalyst CC _R0
A downstream air-fuel ratio sensor for detecting an air-fuel ratio of the engine is provided in an exhaust passage on the downstream side of the engine. Control constant computing means for computing an air-fuel ratio feedback control constant example skip amounts RSR, the RSL in accordance with the output V ₂ of the downstream air-fuel ratio sensor.
Inversion discriminating means discriminates whether the output V ₂ of the downstream air-fuel ratio sensor is reversed to the rich signal or from the lean signal to a lean signal from the rich signal, the result, the output V ₂ of the downstream air-fuel ratio sensor At each reversal, the skip means skips the air-fuel ratio feedback control constant in the control constant calculation means by a predetermined skip amount RSSKP. The stoichiometric air-fuel ratio vicinity duration measuring means outputs the output V ₂ of the downstream air-fuel ratio sensor.
When the value changes from lean to rich or from rich to lean, the duration CNTAV that takes a value within a predetermined range near the stoichiometric air-fuel ratio is measured. As a result, the skip amount calculating means determines the predetermined time according to the duration CNTAV. Calculate the skip amount RSSKP. Then, the air-fuel ratio correction amount calculating means calculates an air-fuel ratio correction amount FAF in accordance with the output V ₁ and air-fuel ratio feedback control constant of the upstream air-fuel ratio sensor, the air-fuel ratio adjusting means in accordance with the air-fuel ratio correction amount FAF It adjusts the air-fuel ratio of the engine.

(Operation)

The operation of the above means will be described with reference to FIGS.

FIG. 4 (A), as (B), the air-fuel ratio A / F of the catalyst upstream changes, the output V ₁ of the upstream-side air-fuel ratio sensor changes, the air-fuel ratio correction amount FAF is FIG. 4 (C ). In this case, the air-fuel ratio correction amount FAF, for example, the upstream air-fuel ratio sensor output V ₁ of the rich integration constant KIR in accordance with the lean, varies KIL, the output V ₁ of the upstream-side air-fuel ratio sensor
In response to the inversion from rich to lean or from lean to rich.

On the other hand, the air-fuel ratio A / F of the catalyst downstream changes to or slow as shown in FIG. 5 (A), as a result, the output V ₂ of the downstream air-fuel ratio sensor changes as shown in FIG. 5 (B) . As a result,
As shown in FIG. 5 (C), the control constant computing means downstream air-fuel ratio sensor output V ₂ rich, changing the air-fuel ratio feedback control constant example Suippu amount RSR in integration constant ΔRS in accordance with the lean. Further, the inversion time from the downstream-side air-fuel ratio rich output V ₂ of the sensor from lean to rich or lean, to skip the air-fuel ratio feedback control constant only Suippu amount RSSKP, to correct the excessive correction of the air-fuel ratio. In this case, the present invention further a skip amount RSSKP output V ₂ of the downstream air-fuel ratio sensor is varied in accordance with the stoichiometric air-fuel ratio duration remaining in the vicinity of CNT (or a smoothed value CNTAV). That is, if the duration CNT (or CNTAV) is large, the skip amount RSSKP is reduced assuming that the average air-fuel ratio (catalyst upstream air-fuel ratio) is close to the stoichiometric air-fuel ratio, and if the duration CNT (or CNTAV) is small, Assuming that the average air-fuel ratio (air-fuel ratio upstream of the catalyst) is out of the stoichiometric air-fuel ratio, the skip amount RSSKP is increased.

〔Example〕

FIG. 6 is an overall schematic diagram showing an embodiment of an internal combustion ratio control device for an internal combustion engine according to the present invention. In FIG. 6, 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 a crank angle sensor 5 whose axis generates, for example, a reference position detection pulse signal every 720 ° in terms of crank angle, and a reference position detection pulse signal in every 30 degrees which is converted into crank angle. A generated crank angle sensor 6 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.

The exhaust manifold 11 includes a catalytic converter 12
A first O ₂ sensor 13 is provided on the upstream side, and a second O ₂ sensor 15 is provided on the exhaust pipe 14 downstream of the catalytic converter 12.

The O ₂ sensors 13 and 15 generate an electric signal corresponding to the concentration of the oxygen component in the exhaust gas. That is, the O ₂ sensors 13 and 15 generate different output voltages to the A / D converter 101 of the control circuit 10 depending on whether the air-fuel ratio is leaner or richer than the stoichiometric air-fuel ratio. The control circuit 10 is configured as, for example, a microcomputer, and includes an A / D converter 101, an input / output interface
102, CPU103, ROM104, RAM105, Backup RAM1
06, a clock generation circuit 107 and the like are provided.

An idle switch 17 for detecting whether or not the throttle valve 16 is fully closed is provided at the throttle valve 16 of the intake passage 2.
The output signal is supplied to the input / output interface 102 of the control circuit 10.

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 the carry-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 energized by the above-described fuel injection amount TAU, so that an 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. The rotation speed data Ne is calculated by an interruption of the crank angle sensor 6 at every 30 ° CA, and is stored in the RAM 1.
05 is stored in a predetermined area.

Figure 7 is a first air-fuel ratio feedback control routine for calculating the air-fuel ratio correction factor FAF in accordance with the output of the upstream O ₂ sensor 13 is executed at a predetermined time, for example, 4ms each.

In step 701, by the upstream O ₂ sensor 13 air-fuel ratio of the closed loop (feedback) condition is determined whether or not satisfied. For example, when the cooling water temperature is equal to or lower than a predetermined value, during engine start, during increase after start, during warm-up, during power increase,
During OTP boost for the catalyst overheat prevention, when the output signal of the upstream O ₂ sensor 13 is not also inverted once, also a closed loop condition any fuel cut secondary is is not satisfied, otherwise it is a closed loop condition is satisfied. When the closed loop condition is not satisfied, the process proceeds to step 727. Note that the air-fuel ratio correction coefficient FAF is 1.
It may be initialized to 0. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 702.

In step 702, V ₁ is determined whether or not the comparison voltage V _R1 for example 0.45V or less uptake, at step 703 the output V ₁ of the upstream O ₂ sensor 13 converts A / D, that is, the air-fuel ratio It is determined whether the rich is lean, that is, the air-fuel ratio is lean (V ₁
If ≦ V _R1) a, delay counter CD at step 704
It is determined whether or not LY is positive. If CDLY> 0, step 705 is executed.
To set CDLY to 0, and proceed to step 706. In step 706, the delay counter CDLY is decremented by one, and in steps 707 and 70
At 8, guard the delay counter CDLY with the minimum value TDL.
In this case, when the delay counter CDLY reaches the minimum value TDL, the first air-fuel ratio flag F1 is set to “0” (lean) in step 709. The minimum value TDL is set to the upstream side O.
₂ A lean delay state for maintaining the determination that the state is the rich state even when there is a change from rich to lean in the output of the sensor 13, and is defined by a negative value. On the other hand, if rich (V ₁ > V _R1 ), it is determined in step 710 whether or not the delay counter CDLY is negative. If CDLY <0, CDLY is set to 0 in step 711 and the process proceeds to step 712. In step 712, the delay counter CDLY is incremented by 1, and in step 7
Guard the delay counter CDLY with the maximum value TDR at 13,714. In this case, when the duty counter CDLY reaches the maximum value TDR, the first air-fuel ratio flag F
1 is set to “1” (rich). Note that the maximum value TDR is the upstream O ₂
This is a rich delay time for maintaining the determination of the lean state even when the output of the sensor 13 changes from lean to rich, and is defined as a positive value.

In step 716, it is determined whether or not the sign of the first air-fuel ratio flag F1 has been inverted, that is, whether or not the air-fuel ratio after the delay processing has been inverted. If the air / fuel ratio has been inverted, it is determined in step 717 whether the inversion is from rich to lean or from lean to rich, based on the value of the first air-fuel ratio flag F1. In the case of inversion from rich to lean, in step 718, FAF ← FAF + RSR is increased in a skipping manner. On the other hand, in the case of inversion from lean to rich, in step 719, FAF ← FAF−RSL is skipped. Decrease. That is, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted at step 716, the integration processing is performed at steps 720, 721, and 722. That is, in step 720, it is determined whether or not F1 = "0". If F1 = "0" (lean), the FAF is determined in step 721.
← FAF + KIR. On the other hand, if F1 = “1” (rich), at step 722, FAF ← FAF−KIL. Where the integration constant
KIR, KIL is set sufficiently smaller than the skip amounts RSR, RSL, that is, KIR (KIL) <RSR (RSL). Therefore, step 721 gradually increases the fuel injection amount in the lean state (F1 = "0"), and step 722 sets the rich state (F1 =
In “1”), the fuel injection amount is gradually reduced.

The air-fuel ratio correction coefficient FAF calculated in Steps 718, 719, 721, 722 is guarded at a minimum value, for example, 0.8 at Steps 723, 724, and at a maximum value, for example, at Steps 725, 726.
Guarded at 1.2. Thus, if the air-fuel ratio correction coefficient FAF becomes too large or too small for some reason, the air-fuel ratio of the engine is controlled with that value to prevent over-rich or over-lean.

The FAF calculated as described above is stored in the RAM 105, and this routine ends in step 727.

FIG. 8 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. Rich As shown in Figure 8 by the output of the upstream O ₂ sensor 13 (A), when the air-fuel ratio A / F is obtained, the delay counter CDLY is
As shown in FIG. 8 (B), the count is incremented in the rich state and is counted down in the lean state. As a result, as shown in FIG. 8 (C), a delayed air-fuel ratio signal A / F '(corresponding to the flag F1) is formed. For example, the air-fuel ratio signal A / F at the time t ₁ is changed from lean to rich, the air-fuel ratio signal A / F which is delayed processed 'is at time t ₂ after being held lean only the rich delay time TDR Change rich. Also the air-fuel ratio signal A / F from the rich at time t ₃ is changed to the lean delay processed air-fuel ratio signal A / F 'is the time t ₄ after being held rich only equivalent lean delay time (-TDL) It changes to lean at. But the air-fuel ratio signal A / F
There Invert at time t _5, a short period of rich delay time TDR as the t _6, t _7, takes time delay counter CDLY reaches the maximum value TDR, a result, the delay processing at time t ₈ The subsequent air-fuel ratio signal A / F 'is inverted. That is, the air-fuel ratio signal A / F 'after the delay processing is more stable than the air-fuel ratio signal A / F before the delay processing. In this way, the air-fuel ratio correction coefficient FAF shown in FIG. 8D is obtained based on the stable air-fuel ratio signal A / F 'after the delay processing.

Next, a description will be given of the second air-fuel ratio feedback control by the downstream O ₂ sensor 15. As the second air-fuel ratio feedback control, the skip amounts RSR, RSL, the integration constants KIR, KIL, the delay times TDR, TDL, or the output V ₁ of the upstream O ₂ sensor 13 as the first air-fuel ratio feedback control constant are determined. There are a system that makes the comparison voltage _VR1 variable, and a system that introduces the second air-fuel ratio correction air number FAF2. The system that introduces the second air-fuel ratio correction coefficient FAF2 is a system in which the skip amount, the integration constant, the delay time, and the like, which are the air-fuel ratio feedback control constants of the second air-fuel ratio correction coefficient FAF2, are variable.

For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and if the lean skip amount RSL is reduced, the control air-fuel ratio can be shifted to the rich side, while if the lean skip amount RSL is increased, , The control air-fuel ratio can be shifted to the lean side, and the rich skip
Even if the RSR is reduced, the control air-fuel ratio can be shifted to the lean side. Accordingly, the air-fuel ratio can be controlled by correcting the rich skip amount RSR and the lean skip amount RSL in accordance with the output of the downstream O ₂ sensor 15. Also, the rich integration constant KIR
The control air-fuel ratio can be shifted to the rich side by increasing the control air-fuel ratio, and the control air-fuel ratio can be shifted to the rich side even if the lean integration constant KIL is reduced. The control air-fuel ratio can be shifted to the lean side even if the rich integration constant KIR is reduced. Accordingly, the air-fuel ratio can be controlled by correcting the rich integration constant KIR and the lean integration constant KIL in accordance with the output of the downstream O ₂ sensor 15. By increasing the rich delay time TDR, the control air-fuel ratio can be shifted to the rich side.
Even if the lean delay time (-TDL) is reduced, the control air-fuel ratio can be shifted to the lean side, and conversely, the lean delay time (-TDL)
The control air-fuel ratio can be shifted to the lean side by increasing the control air-fuel ratio, and the control air-fuel ratio can be shifted to the lean side even if the rich delay time (TDR) is reduced. That is, the air-fuel ratio can be controlled by correcting the delay time TDR, the TDL in accordance with the output of the downstream O ₂ sensor 15. Furthermore, if the comparison voltage V _R1 is increased, the control air-fuel ratio can be shifted to the rich side, and the comparison voltage V _R1 can be shifted.
By reducing _R1 , the control air-fuel ratio can be shifted to the lean side.
Therefore, according to the output of the downstream O ₂ sensor 15, the comparison voltage V _R1
Is corrected, the air-fuel ratio can be controlled.

Making the step amount, the integration constant, the delay time, and the comparison voltage variable by the downstream O ₂ sensor has advantages respectively. For example, the delay time allows very fine adjustment of the air-fuel ratio, and the skip amount can be controlled with good response without lengthening the air-fuel ratio feedback cycle unlike the delay time. Therefore, these variable amounts can be used in combination of two or more.

Next, a description will be given double O ₂ sensor system varies the step amount of the air-fuel ratio feedback control constant.

First, the calculation of the skip amount RSR (RSL) and the skip amount RSSKP will be described with reference to FIGS. 9 and 10A (FIG. 10B).

Figure 9 is a 16ms routine for calculating the duration CNT to output V ₂ of the downstream O ₂ sensor 15 from staying in the vicinity of the stoichiometric air-fuel ratio. Note that the routine may be a 4 ms to 64 ms routine. In steps 901 to 905, step 11 of the routine of FIG.
Similar to 01-1105, closed loop conditions by the downstream O ₂ sensor 15 is determined or not. For example, when the cooling water temperature THW is equal to or lower than a predetermined value (for example, 70 ° C.) (step 902), the throttle valve 16 is fully closed (LL = “LL”), in addition to the satisfaction of the closed loop condition by the downstream O ₂ sensor 13 (step 901). 1 ") when (step 903), when the light load (Q / Ne <X ₁₎ (step 904), when the downstream O ₂ sensor 15 is not activated (step 905) and the closed loop condition is not satisfied Yes, otherwise the closed loop condition is satisfied. If the condition is not the closed loop condition, the process proceeds to step 912 to clear the duration counter CNT.

At step 906, the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and taken in. At steps 907 and 908, the downstream O ₂ sensor 1
Within a predetermined range output V ₂ is near stoichiometric air-fuel ratio of 5, for example [V _R2
-0.05 V, it is determined whether V _R2 + 0.05 V] or. Where V _R2
Is a comparison voltage corresponding to the stoichiometric air-fuel ratio of the output V ₂ of the downstream O ₂ sensor 15 and is set to, for example, 0.55 V, and the output characteristics of the upstream and downstream of the catalytic converter 12 due to the influence of raw gas are different and Although it is set higher than the comparison voltage V _R1 of the output of the upstream O ₂ sensor 13 in consideration of the difference in the deterioration rate and the like, this setting may be arbitrary. As a result, V _R2 −
If _{0.05V ≦ V 2 ≦ V R2 +} 0.05V, +1 increases the counter CNT at step 909, on the other hand, step if V _2> V _R2 + 0.05V or V ₂ <V _R2 -0.05V 910 It is determined whether or not CNT is 0. If CNT is not 0, step 9
In step 11, the average value is updated in CNTAV, and in step 912, the counter CNT is cleared, and the flow advances to step 913. If CNT = 0 in step 910, the flow directly proceeds to step 913.

Thus, according to the routine of FIG. 9, the downstream O ₂ under air-fuel ratio feedback condition is satisfied by the output V ₂ of the sensor 15, the output V ₂ of the downstream O ₂ sensor 15 is near stoichiometric air-fuel ratio range [V _R2 -0.05V, V _R2 + 0.05V] a is duration CNTAV is calculated. The range width 0.1V is updated as appropriate,
Instead of the counter CNT smoothed value CNTAV as the duration,
The average value can be used, or the value of the CNT of the counter can be used directly.

FIG. 10A shows the calculation of the skip amount RSSKP of the skip amount RSR according to the duration CNTAV calculated in FIG.
It is a routine. That is, in step 1001, the duration CNTAV is 0 or not, and in step 1002, the duration CNTAV is
Whether less than C _1, Step 1003 the duration CNTAV it is determined whether or not less than C _2. Note that 0 <C ₁ <C ₂ . As a result, if CNTAV = 0 at step 1004 and RSSKP ← RSSKP _{0, 0} <if CNTAV <C _1, and RSSKP ← RSSKP ₁ at step 1005, step if C ₁ ≦ CNTAV <C ₂ as RSSKP ← RSSKP ₂ at 1006, and RSSKP ← RSSKP ₃ at step 1007 if CNTAV ≧ C _2. However, the _{RSSKP 0> RSSKP 1> RSSKP 2} > RSSKP 3.

 Then, in step 1008, this routine ends.

FIG. 10B shows a modification of FIG. 10A. That is, it is determined in step 1011 whether or not the duration CNTAV is 0, and CNTAV =
If 0, the skip amount RSSKP is set to a relatively large value PSSKP ₀ .

In step 1012, determines whether CNTAV> C _R. As a result, CNTAV> C if _{R ΔSKP} (constant value) to skip amount RSSKP at step 1013 only reduced, on the other hand,
0 <a CNTAV ≦ C if _R skip amount RSSKP increased by DerutaSKP. Here, C _R is, for example, suitable values 1s, therefore, the skip amount RSSKP is feedback controlled so duration CNTAV is C _R.

 Then, this routine ends in step 1016.

Figure 11 is the skip amount based on the output of the downstream O ₂ sensor 15 RSR, a second air-fuel ratio feedback control routine for calculating the RSL, is executed at predetermined time, for example 512ms. At step 1101 to 1105, to determine whether the closed loop condition by the downstream O ₂ sensor 15 as in step 901 to 905 of FIG. 9 described above. If the condition is not the closed loop condition, the process proceeds directly to step 1119.

Steps 1106 to 1118 if the closed loop condition is satisfied
Proceed to. That is, in step 1106, the downstream O ₂ sensor
Captures the output V ₂ of the 15 converts A / D, V ₂ at step 1107
Is lower than or equal to the comparison voltage _VR2 , that is, whether the air-fuel ratio is rich or lean. As a result, if the air-fuel ratio is lean (V ₂ ≦ VR ₂ ), the second air-fuel ratio flag F2 is set to “0” (lean) in step 1108, while the air-fuel ratio is rich (V ₂ > VR _2). if), a second air-fuel ratio flag F ₂ in step 1109 "1" and (rich). Note that step
At 1107, when the second air-fuel ratio flag F2 is “0”, V _R2 ← _VR20 + 0.05V, and when F2 is “1”,
U _R2 ← V _R20 −0.05V (V _R20 is a constant value).

In step 1110, it is determined whether or not the sign of the second air-fuel ratio flag F2 has been inverted, that is, whether or not the air-fuel ratio downstream of the catalyst has been inverted. If the air-fuel ratio is reversed,
In step 1111, the value of the second air-fuel ratio flag F2 is
It is determined whether the inversion is from rich to lean or from lean to rich. If it's a rich to lean reversal,
In step 1112, RSR ← RSR + RSSKP is increased in a skipping manner. Conversely, if the transition is from lean to rich, in step 1113, RSR ← RSR−RSSKP is reduced in a skipping manner. That is, skip processing is performed.

If the sign of the second air-fuel ratio flag F2 is not inverted in step 1110, the integration process is performed in steps 1114, 1115, and 1116. That is, in step 1114, it is determined whether or not F2 = "0", and if F2 = "0" (lean), then in step 1115
RSR ← RSR + ΔRS. On the other hand, if F2 = “1” (rich), in step 1116, RSR ← RSR−ΔRS. Here, the integration constant ΔRS is set smaller than the skip amount RSSKP. That is, ΔRS <RSSKP. Therefore, step 1
115 gradually shifts the control of the fuel injection amount to the rich side in the lean state (F2 = "0"), and step 1116 gradually shifts the control of the fuel injection amount to the lean side in the rich state (F2 = "1"). Migrate.

In step 1117, the skip amount RSR calculated in steps 1112, 1113, 1115, and 1116 is guarded with the minimum value MIN and the maximum value MAX. Note that the minimum value MIN, for example, 2.5 is a value at a level at which the transient tracking performance is not impaired, and the maximum value MAX
For example, 7.5 is a level value at which drivability does not deteriorate due to air-fuel ratio fluctuation. And step 11
At 18, the lean skip amount RSL is calculated by RSL ← 10% −RSR.

 Then, in step 1119, this routine ends.

Thus, when the output V ₂ of the downstream O ₂ sensor 15 is inverted, the rich skip amount RSR and the lean skip amount R
SL is skipped by the skip amount RSSKP, while the downstream O
If the output V ₂ of the ₂ sensor 15 is not inverted, the rich skip amount RSR and the lean skip amount RSL is integrated controlled by integration constant .DELTA.Rs. Moreover, the skip amount RSSKP are variable depending on the duration CNTAV the output V ₂ of the downstream O ₂ sensor 15 is retained within a predetermined range of the near stoichiometric air-fuel ratio.

FIG. 12 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360 ° CA. In step 1201, the intake air data Q and the rotation speed data Ne are read from the RAM 105.
To calculate the basic injection amount TAUP. For example, TAUP ←
Let α · Q / Ne (α is a constant). RAM 105 in step 1202
Further, the cooling water temperature data THW is read out, and the warm-up increase value FWL is interpolated and calculated based on the one-dimensional map stored in the ROM 104. In step 1203, the final injection amount TAU is calculated as TAU ← TAUP / FAF /
It is calculated by (FWL + β) + γ. Here, β and γ are correction amounts determined by other operation state parameters. Next, in step 1204, the injection amount TAU is
And the flip-flop 109 is set to start fuel injection. Then, in step 1205, 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 carry-out signal of the down counter 108, and the fuel injection ends.

FIG. 13 is a timing chart for explaining the effect of the present invention. That is, in the period I, when the rich skip amount RSR is controlled by the integration constant ΔRS according to the output V ₂ of the downstream O ₂ sensor 15 as in the related art, the vicinity of the stoichiometric air-fuel ratio [V
_R2 -0.05 V, if the duration CNT is paddle short of V _R2 + 0.05 V], the catalyst downstream (average) air-fuel ratio A / F is deviated from the stoichiometric air-fuel ratio, as a result, HC, CO, NO _x emissions Worsens. In such a case, the introduction of skip control the skip amount RSSKPA the rich skip amount RSR every inversion of the output V ₂ of the downstream O ₂ sensor 15, as shown in the period II, the catalyst downstream air-fuel ratio A / F The deviation from the stoichiometric air-fuel ratio is reduced, so that deterioration of the emission is prevented. In this case,
Near stoichiometric air-fuel ratio [V _R2 -0.05V, V _R2 + 0.05V] duration of C
NT increases. Further, when the skip amount RSSKP of the rich skip amount RSR is increased to RSSKPB, as shown in period III, the deviation from the stoichiometric air-fuel ratio of the downstream A / F of the catalyst is further reduced, so that deterioration of the emission is further prevented. If the skip amount RSSKP of the rich skip amount RSR is too large, on the contrary, the fluctuation of the air-fuel ratio becomes large, and the drivability is deteriorated. Accordingly, the present invention range of the output V ₂ of the downstream O ₂ sensor 15 is a variable control of the skip amount RSSKP of such rich skip amount RSR [V _R2 -0.0
5 V, V _R2 +0.05 V].

In a single O ₂ sensor system in which an O ₂ sensor is provided only downstream of the catalyst to perform air-fuel ratio feedback control, RSR and RSL of the second air-fuel ratio feedback routine are replaced with the first air-fuel ratio feedback routine described above. FAF
It should be calculated as

Also, the first air-fuel ratio feedback control is performed every 4 ms,
In addition, the second air-fuel ratio feedback control is performed every 512 ms because the air-fuel ratio feedback control mainly performs control using the upstream O ₂ sensor with good response, and performs control using the downstream O ₂ sensor with poor response. It is to do so.

Further, the double O ₂ for correcting other control constant in the air-fuel ratio feedback control by the upstream O ₂ sensor, for example the delay time, the integration constant, or the like by the output of the downstream O ₂ sensor
The present invention can be applied to a sensor system and also to a double O ₂ sensor system that introduces a second air-fuel ratio correction coefficient. In addition, two of the skip amount, the delay time, and the integration constant
The controllability can be improved by controlling the two at the same time. Further, either one of the skip amounts RSR and RSL is fixed and only the other is variable, or one of the delay times TDR and TDL is fixed and only the other is variable, or the rich integration constant KIR and lean It is also possible to fix one of the integration constants KIL and make the other variable.

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).
Electric bleed air control valve adjusts 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 controls the amount of secondary air sent to the exhaust system of the engine. The present invention can be applied to a device to be 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, determined in accordance with the intake pipe negative pressure according to the intake air amount and the engine speed, and step 1203. At the final fuel injection amount TAU
The corresponding supply air amount 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.

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, the skip control is introduced into the air-fuel ratio control amount calculated based on the output of the downstream air ratio sensor, for example, the skip amount RSR (RSL).
Since the skip amount RSSKP is made dependent on the time during which the output of the downstream air-fuel ratio sensor exists near the stoichiometric air-fuel ratio, it is possible to prevent the deterioration of the emission, the fuel consumption, the driver's billy, and the like.

[Brief description of the drawings]

FIG. 1 is an overall block diagram for explaining the structure of the present invention, FIG. 2 is an exhaust emission characteristic diagram for explaining a single O ₂ sensor system and a double O ₂ sensor system, and FIG. 3 is a three-way catalyst O _2. 4 and 5 are timing diagrams for explaining the operation of the present invention, FIG. 6 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, 7, 9, 10A, 10B, 11, and 12 are flow charts for explaining the operation of the control circuit of FIG. 5, and FIG. 8 is a supplementary flow chart of FIG. FIG. 13 is a timing chart for explaining the effect of the present invention. 1 ...... engine body, 3 ...... air flow meter, 4 ...... distributor, 5,6 ...... crank angle sensor, 10 ...... control circuit, 12 ...... catalytic converter, 13 ...... upstream O ₂ sensor, 15 ...... downstream O ₂ sensor, 17 ...... idle switch.

Claims (1)

(57) [Claims] A three-way catalyst provided in an exhaust passage of an internal combustion engine; and an upstream air-fuel ratio sensor provided in an exhaust passage upstream of the three-way catalyst for detecting an air-fuel ratio of the engine. 13), a downstream air-fuel ratio sensor (15) provided in an exhaust passage downstream of the three-way catalyst and detecting an air-fuel ratio of the engine, and an air-fuel ratio feedback according to an output of the downstream air-fuel ratio sensor. Control constant calculating means for calculating a control constant; inversion determining means for determining whether the output of the downstream air-fuel ratio sensor has been inverted from a rich signal to a lean signal or from a lean signal to a rich signal; A skip unit for skipping the air-fuel ratio feedback control constant in the control constant calculation unit by a predetermined skip amount every time the output of the fuel ratio sensor is inverted, and the output of the downstream air-fuel ratio sensor may be from lean to rich. A stoichiometric air-fuel ratio proximity duration measuring means for measuring a duration that takes a value within a predetermined range near the stoichiometric air-fuel ratio when inverting from rich to lean; a skip amount for calculating the predetermined skip amount according to the duration. Calculation means; air-fuel ratio correction amount calculation means for calculating an air-fuel ratio correction amount according to the output of the upstream air-fuel ratio sensor and the air-fuel ratio feedback control constant of the control constant calculation means; And an air-fuel ratio adjusting means for adjusting the air-fuel ratio of the engine.