JP2623667B2 - 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. in addition to the air-fuel ratio feedback control by the O ₂ sensor, 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.

[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. Such O ₂ component variation variation and the fuel injection valve and the output characteristics of the sensor, to compensate for time or secular change, a second O ₂ sensor disposed downstream of the catalytic converter, by the upstream O ₂ sensor 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:
-234241). 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, good exhaust emissions are guaranteed as long as the downstream O ₂ sensor maintains stable output characteristics.

In the double O ₂ sensor system described above, the downstream O ₂
In the air-fuel ratio feedback control execution by the sensor, the air-fuel ratio correction amount FAF based on the output of the upstream O ₂ sensor
Control constants for example the rich skip amount RSR, there is a system for variably controlled based lean skip amount RSL in the output of the downstream O ₂ sensor, by the output of the downstream O ₂ sensor by inactivation or the like of the downstream O ₂ sensor When stopping the variable control of the control constant, the upstream O is used by using the value stored in the backup RAM when the control constant is variably controlled.
_The air-fuel ratio feedback control was performed only by the outputs of the _two sensors (see JP-A-61-192828). Further, in a system in which the control constant is variably controlled, an upper limit value and a lower limit value are provided so as not to impair transient followability and to prevent deterioration of drivability due to air-fuel ratio fluctuation. Constants are guarded (see JP-A-61-234241).

[Problems to be solved by the invention]

However, for example, in an internal combustion engine equipped with a fuel vapor emission prevention device, when the temperature in the fuel tank is high and a large amount of vapor is generated from the fuel in the fuel tank, the vapor in the tank is purged by the purge system. It is drawn into the combustion chamber through the canister, and thus the air-fuel ratio becomes very rich. If the condition persists, the air-fuel ratio correction amount FAF based on the output of the upstream O ₂ sensor is corrected to the lean side,
It will stick to the lower limit provided for the prevention of Ovaleen. Moreover, in this case, the air-fuel ratio is not completely correct the air-fuel ratio is just air-fuel ratio correction quantity FAF, so that the control constants RSR based on downstream O ₂ sensor, RSL is also over-corrected to the lean side. After the engine is stopped in such a state,
When the vehicle is brought into a nearly steady running state again, the backup RAM
Over-corrected control constant RSR on the lean side stored in
Since the air-fuel ratio correction amount FAF is calculated using the RSL, the rate of increase of the air-fuel ratio correction amount FAF is small, and since the fuel temperature in the tank is reduced due to the stop of the engine and the amount of steam is small, the downstream O ₂ sensor holds the lean output during the interim pleasure, increase of the NO _x emissions, there is a problem that leads to deterioration of the drivability.

Also, for example, in an internal combustion engine that performs high altitude learning correction, if the vehicle travels from high altitude to low altitude without learning, the amount of intake air decreases due to a decrease in atmospheric pressure, and therefore the air-fuel ratio becomes very rich, and skipping occurs. The amounts RSR, RSL are overcorrected to the lean side. Moving to lowland in such a state, the intake air amount than the increase in the atmospheric pressure increases, the downstream O ₂ sensor during the lean output for some time holds, there is a similar problem.

As described above, if the air-fuel ratio correction amount FAF based on the output of the upstream O ₂ sensor is guarded at the allowable upper and lower limits by some factor, the output of the upstream O ₂ sensor originally intended for the double O ₂ sensor system There is a problem that the function of controlling the desired air-fuel ratio by detecting the deviation of the average air-fuel ratio due to the result of the air-fuel ratio feedback cannot be exhibited, and, as described above, if the air-fuel ratio is biased toward lean or rich, It is determined that the O ₂ sensor output has been activated upon the output inversion from lean to rich or rich to lean, and the downstream O ₂
In the case of starting the air-fuel ratio feedback based on the output of the sensor, there is a problem that the start of the feedback is delayed.

Therefore, the present applicant has already when the air-fuel ratio correction amount FAF has reached its allowable width, the air-fuel ratio feedback control by the downstream O ₂ sensor is proposing to stop (see Japanese Patent Application No. Sho 62 −35820). In this case, for example, as shown in FIG. 3, when the vehicle travels from lowland to highland without learning, the control air-fuel ratio is rich,
air-fuel ratio correction quantity FAF at t ₁ is sticking to the lower limit (i.e., smaller than the lower limit value is the minimum value of the variation of FAF), this result, updating of the air-fuel ratio correction quantity FAF is stopped, the air-fuel ratio correction amount Until the FAF is safely stuck to the lower limit (ie, F
During the period (t _{1 to} t ₂ ), the air-fuel ratio correction amount FAF departs from the lower limit and depends on the output of the downstream O ₂ sensor until the maximum value as well as the minimum value of the AF fluctuation becomes smaller than the lower limit value. The air-fuel ratio feedback condition is satisfied. As a result, the control constant is still updated by the output of the downstream O ₂ sensor, for example, the rich skip amount RSR is reduced, and the lean skip amount RSR is reduced.
SL increases. Similarly, the air-fuel ratio correction amount FAF period of start disengaged from the lower limit value (t ₃ ~t ₄₎ even in the downstream O ₂ air-fuel ratio feedback control conditions according to the output of the downstream O ₂ sensor is met The control constant is updated by the output of the sensor. As described above, during the period when the air-fuel ratio correction amount FAF starts to be applied to the upper limit value and the lower limit value and during the start of departure, the control constant is erroneously controlled, and again, the emission deteriorates, the drivability deteriorates, the fuel consumption deteriorates, and the catalyst It causes the generation of offensive odor.

Accordingly, an object of the present invention is to provide a downstream air-fuel ratio when the air-fuel ratio correction amount FAF is stuck to an upper limit value or a lower limit value due to some factors such as a case where the vehicle travels from a lowland to a highland or a highland to a lowland without learning. It is an object of the present invention to prevent erroneous control of the air-fuel ratio feedback control based on the output of a sensor, thereby preventing deterioration of emission, drivability, fuel consumption, generation of unusual odor of catalyst exhaust, and the like.

[Means and actions for solving the problem]

The means for solving the above-mentioned problem is shown in FIG. That is, the upstream side of the three-way catalyst CC _R0 provided in an exhaust passage of an internal combustion engine, the exhaust passage downstream, upstream of detecting the air-fuel ratio of the engine, the downstream air-fuel ratio sensor is provided. The air-fuel ratio correction amount calculating means calculates the air-fuel ratio correction amount FAF according to the upstream and downstream air-fuel ratio sensor outputs V ₁ , V ₂ , and the guard means converts the calculated air-fuel ratio correction amount to a predetermined allowable width, for example. Guard at 0.8-1.2, air-fuel ratio adjustment means air-fuel ratio correction amount
Adjust the engine air-fuel ratio according to the FAF. On the other hand, the allowable width reaching determination means determines whether or not the air-fuel ratio correction amount FAF calculated by the air-fuel ratio correction amount calculation means has reached an upper limit or a lower limit of a predetermined allowable width. If it has reached, the output of the downstream air-fuel ratio sensor will be
Stop operation of the air-fuel ratio correction amount by V _2. The within-permissible-width return determining means determines whether or not the air-fuel ratio correction amount calculated by the air-fuel ratio correction amount calculating means has returned to a state controlled within a predetermined allowable width, that is, the calculated air-fuel ratio correction amount. to determine whether the maximum and minimum values of the amount of variation were both falls within the allowable range, resumed operation of the air-fuel ratio correction amount FAF by the output V ₂ of the downstream air-fuel ratio sensor at the return to the state Is what you do.

(Operation)

According to the above-described means, when the air-fuel ratio correction amount FAF reaches the upper limit or the lower limit of the allowable range, the air-fuel ratio feedback control based on the output of the downstream air-fuel ratio sensor stops, but the air-fuel ratio correction amount FAF It is not restarted until it completely departs from the upper limit value and the lower limit value of the allowable range, that is, until both the maximum value and the minimum value of the fluctuation of the FAF fall within the allowable range. Accordingly, in the period t ₁ ~t ₄ of FIG. 3, the air-fuel ratio feedback control by the output of the downstream air-fuel ratio sensor is stopped.

〔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 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 has 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 sensor 1
3, 15 are different output voltages depending on whether the air-fuel ratio is leaner or richer than the stoichiometric air-fuel ratio.
Occurs at 01. The control circuit 10 is configured as, for example, a microcomputer, and is provided with 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.

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.

The CPU 103 generates an interrupt when the input / output interface 102 receives the pulse signal of the crank angle sensor 6 after the A / D conversion of the A / D converter 101 is completed.
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 an interruption of the crank angle sensor 6 at every 30 ° CA, and RA
It is stored in a predetermined area of M105.

 Hereinafter, the operation of the control circuit of FIG. 4 will be described.

Figure 5 is a first air-fuel ratio feedback control routine for calculating the air-fuel ratio correction coefficient FAF based on the output of the upstream O ₂ sensor 13 is executed at a predetermined time, for example, 4ms each.

In step 501, the air-fuel ratio of the closed loop by the upstream O ₂ sensor 13 (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. If the closed loop condition is not satisfied, the process proceeds directly to step 535. That is, the air-fuel ratio correction coefficient FAF is set immediately before the end of the closed loop control. On the other hand, if the closed loop condition is satisfied, the process proceeds to step 502.

In step 502, V ₁ is determined whether or not the comparison voltage V _R1 for example 0.45V or less uptake, at step 503 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 air-fuel ratio is lean (V ₁
If _≤VR1 , then in step 504 the delay counter CD
It is determined whether or not LY is positive. If CDLY> 0, step 505 is performed.
The value of CDLY is set to 0, and the process proceeds to step 506. Step 506
Then, the count value of the delay counter CDLY is decremented by one, and step 507,
At 508, the delay counter CDLY is guarded by the minimum value TDL. In this case, when the delay counter CDLY reaches the minimum value TDL, in step 509, the first air-fuel ratio flag F1 is set to "0" (lean). Note that TDL is a lean delay state for holding the judgment that the rich state even if the change from rich to lean in the output of the upstream O ₂ sensor 13 is defined by a negative value You. On the other hand, if rich (V ₁ > V _R1 ), the delay count is
It is determined whether or not CDLY is negative. If CDLY <0, step 51 is executed.
In step 1, CDLY is set to 0, and the process proceeds to step 512. Step 512
Then, add 1 to the delay counter CDLY, and go to steps 513 and 51.
At 4, guard the delay counter CDLY with the maximum value TDR.
In this case, when the delay counter CDLY reaches the maximum value TDR, the first air-fuel ratio flag F1 is set to "1" (rich) in step 515. Incidentally, a maximum value TDR upstream O ₂ rich delay time for holding the judgment that even if a change in Ritchihe from lean a lean state at the output of the sensor 13, is defined by a positive value .

In step 516, 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 is inverted, it is determined in step 517 whether the air-fuel ratio is inverted from rich to lean or from lean to rich, based on the value of the first air-fuel ratio flag F1. If inversion from rich to lean, at step 518, the air-fuel ratio feedback stop flag SFBL by the output V ₂ of the downstream O ₂ sensor 15, it is determined whether or not "1", when SFBL = "1" Only, proceed to steps 519,520. In step 519, it is determined whether or not the air-fuel ratio correction coefficient FAF has deviated from the lower limit value 0.8 of the guard processing, and FAF> 0.8
If so, in step 520, the flag SFBL is reset.
In step 521, FAF ← FAF + RSR and skipping to increase, conversely, if the inversion from lean to rich, the air-fuel ratio feedback stop flag SFBL by the output V ₂ of the downstream O ₂ sensor 15 in step 522 " 1 "or not, SFBL
Only when “1”, the process proceeds to steps 523 and 524. Step 5
At 23, it is determined whether or not the air-fuel ratio correction coefficient FAF has deviated from the upper limit value 1.2 of the guard processing. If FAF <1.2, the flag SFBR is reset at step 524. At step 525, FAF is reduced in a skipping manner as FAF-FAF-RSL. That is, in steps 521 and 525, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 has not been inverted at step 516, the integration processing is performed at steps 526, 527 and 528. That is, at step 526, it is determined whether or not F1 = "0". If F1 = "0" (lean), the FAF is determined at step 527.
← FAF + KIR. On the other hand, if F1 = “1” (rich), in step 528, FAF ← FAF−KIL. Where the integration constant
KIR, KIL is set sufficiently smaller than the skip amounts RSR, RSL, that is, KIR (KIL) <RRS (RSL). Accordingly, in step 527, in the lean state (F1 = "0"), the fuel injection amount is gradually increased to shift to the rich side, and step 52
Numeral 8 indicates a rich state (F1 = “1”), in which the fuel injection amount is gradually reduced to shift to the lean side.

The air-fuel ratio correction coefficient FAF calculated in steps 521, 525, 527, 528 is guarded by the lower limit value 0.8 in steps 529, 530 and guarded by the upper limit value 1.2 in steps 532, 533. As a result, the air-fuel ratio correction coefficient FAF
Is too large or too small,
The value is used to control the air-fuel ratio of the engine to prevent over-rich or over-lean. In this case, the air-fuel ratio correction coefficient FAF
Reaches the lower limit value 0.8, the air-fuel ratio stop flag SFBL is set in step 531, while the air-fuel ratio correction coefficient FA
When F reaches the upper limit value 1.2, the air-fuel ratio stop flag SFBR is set in step 534.

The FAF calculated as described above is stored in the backup RAM 106, and this routine ends in step 535.

That is, as shown in FIG. 6A, the air-fuel ratio correction coefficient FAF
Flag reaches the lower limit value of 0.8 for guard processing.
It is set, thereby to stop the air-fuel ratio feedback control by the output V ₂ of the downstream O ₂ sensor 15 which will be described later, the lower limit value of the air-fuel ratio correction coefficient FAF in the rich skip process immediately before
When the vehicle departs from 0.8, that is, when the minimum value of the fluctuation of the air-fuel ratio correction coefficient FAF becomes larger than the lower limit value 0.8, the flag S
FBL is reset and restarts the air-fuel ratio feedback control by the output V ₂ of the downstream O ₂ sensor 15. Similarly, as shown in FIG. 6B, the air-fuel ratio positive coefficient FAF is the upper limit value of the guard process.
Flag SFBR is set when it reaches 1.2, thereby to stop the air-fuel ratio feedback control by the output V ₂ of the downstream O ₂ sensor 15 which will be described later, the upper limit air-fuel ratio correction factor FAF in lean skip process immediately before 1.2 , That is, when the maximum value of the variation of the air-fuel ratio correction coefficient FAF becomes smaller than the upper limit value 1.2, the flag SFBR is reset, and the
O ₂ resumes the air-fuel ratio feedback control by the output V ₂ of the sensor 15. The air-fuel ratio feedback control by the output V ₂ of the downstream O ₂ sensor 15, as described later, the stop flag SF
If any one of BL and SFBR is set, the operation is stopped. That is, when the maximum or minimum value of the variation of the air-fuel ratio correction coefficient FAF is out of the allowable range, while out of the allowable range, it stops the air-fuel ratio feedback control based on the output V ₂ of the downstream O ₂ sensor 15 Continued, air-fuel ratio correction coefficient
Only when the FAF returns to the state where it is controlled within the allowable range (that is, when both the maximum value and the minimum value of the variation of the air-fuel ratio correction coefficient FAF are within the allowable range), the downstream O ₂ sensor
Air-fuel ratio feedback control is restarted based on the output V ₂ of 15. As shown by the dotted lines in FIGS. 6A and 6B, the reset of the stop flags SFBL and SFBR can be stabilized by delaying a predetermined time after the conditions of steps 519 and 523 are satisfied.

FIG. 7 is a timing chart for supplementarily explaining the operation according to the flowchart of FIG. Rich as shown in FIG. 7 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. 7 (B), the count is performed in a rich state and the count is reduced in a lean state. As a result, as shown in FIG. 7 (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 is changed from lean to rich at time t _1, the air-fuel ratio signal A / F which is delayed processed 'at time t ₂ after being held lean only the rich delay time TDR Change richly. Also the air-fuel ratio signal A / F from the rich at time t ₃ is changed to the lean air-fuel ratio signal A / F which is delayed processed '
Changes to lean at time t ₄ after being held rich only equivalent lean delay time (-TDL). But the air-fuel ratio signal
If A / F is reversed at time t _5, a short period of rich delay time TDR as the t _6, t _7, it takes time delay counter CDLY reaches the maximum value TDR, the result, at time t ₈ Thus, the air-fuel ratio signal A / F 'after the delay processing 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. Thus, the air-fuel ratio correction coefficient FAF shown in FIG. 7D 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 V _R1 variable, and a system that introduces the second air-fuel ratio correction coefficient FAF2.

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. If the rich delay time TDR is set large or the lean delay time (-TDR) is set small, the control air-fuel ratio can shift to the rich side. Conversely, if the lean delay time (-TDL) is set large or the lean delay time If (TDR) is set small, the control air-fuel ratio can shift to the lean side. 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, it shifts the control air-fuel ratio by increasing the comparison voltage V _R1 to the rich side, also, possible shifts the control air-fuel ratio by decreasing the reference voltage V _R1 to the lean side. Accordingly, the air-fuel ratio can be controlled by correcting the reference voltage V _R1 in accordance with the output of the downstream O ₂ sensor 15.

These skip amount, integration constant, the delay time, to a variable comparison voltage by the downstream O ₂ sensor has an advantage in, 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 in which the skip amounts as an air-fuel ratio feedback control constant to a variable.

Figure 8 is 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. In step 801 to 806, it is determined whether or not the closed loop condition by the downstream O ₂ sensor 15. For example, downstream O ₂
Failure of closed loop condition by sensor 13 (step 801)
In addition, when the cooling water temperature THW is equal to or lower than a predetermined value (for example, 70 ° C.) (Step 802), when the downstream O ₂ sensor 15 is not activated (Step 803), and when the engine is in the light load region (Q / Ne). ≦ X ₁ ) (step 804), etc., the closed loop condition is not satisfied. In steps 805 and 806, it is determined whether or not the stop flags SFBL and SFBR calculated in the routine of FIG. 5 are set ("1"). If at least one is set, the closed loop is not established. In other cases, the closed loop condition is satisfied. If the closed loop is established, the process proceeds to step 807, and if the closed loop condition is not established, the process proceeds to step 813.

In step 807, the output V ₂ of the downstream O ₂ sensor 15 is A / D converted and incorporated, and in step 708, V ₂ is compared with the comparison voltage V _R2, for example.
It is determined whether it is 0.55V or less. Incidentally, than the comparison voltage V _R1 of the output of the comparison voltage V _R2 upstream O ₂ sensor 13 upstream of the catalytic converter 12, it and the degradation rate output characteristics due to the influence of the raw gas is different downstream in consideration of different like Although set high, this setting may be arbitrary. That is, it is determined whether the air-fuel ratio is rich or lean. This results in a step
If V ₂ ≦ VR ₂ (lean) at 808, the process proceeds to step 809, while if V ₂ > VR ₂ (rich), the process proceeds to step 810.

In step 809, the rich skip amount RSR is read from the RAM 105 and RSR ← RSR + ΔRS (constant value), that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side, while V ₂ > V _R2 (rich ) When step 8
At 10, read the rich skip amount RSR from the RAM 105, and
RSR−ΔRS, that is, the air-fuel ratio is shifted to the lean side by reducing the rich skip amount RSR. Step 811,
Guard processing of the RSR calculated as described above is performed. For example, guard is performed with a maximum value MAX = 7.5% and a minimum value MIN = 2.5%. Note that the minimum value MIN is a value at which the transient followability is not deteriorated, and the maximum value MAX is a level at which the drivability does not deteriorate due to the air-fuel ratio fluctuation.

In step 812, the lean skip amount RSL is calculated by RSL ← 10% −RSR. That is, RSR + RSL = 10%. Then, in step 813, this routine ends.

FIG. 9 shows an injection amount calculation routine, which is executed at every predetermined crank angle, for example, every 360 ° CA. In step 901
Intake air amount data Q and rotation speed data Ne from RAM 105
To calculate the basic injection amount TAUP. For example, TAUP ←
Let α · Q / Ne (α is a constant). RAM 105 in step 902
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 903, the final injection amount TAU is set 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, at step 904, the injection amount TAU is
And the flip-flop 109 is set to start fuel injection. Then, in step 905, this routine ends.

As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is set by the carry-out signal of the down counter 108, and the fuel injection ends.

FIG. 10 shows a modification of FIG. 5, and FIG. 11 shows a modification of FIG.

In FIG. 10, steps 1001 to 1015 are the same as steps 501 to 515 in FIG.

In step 1016, it is determined whether or not the sign of the first air-fuel ratio flag F1 has been inverted. That is, it is determined whether or not the air-fuel ratio after the delay processing has been inverted. If the air-fuel ratio is inverted, the skip counter CSKP is incremented by one in step 1017. Note that the skip counter CSKP is for counting the number of skips of the air-fuel ratio correction coefficient FAF after the air-fuel ratio correction coefficient FAF reaches the lower limit value 0.8 or the upper limit value 1.2. Next, in step 1018, whether the inversion from rich to lean is determined based on the value of the first air-fuel ratio flag F1 or not.
Determine whether the transition is from lean to rich. If the transition is from rich to lean, at step 1019, FAF ← FAF +
RSR is increased in a skipping manner, and conversely, if it is an inversion from lean to rich, in step 1020, it is reduced in a skipping manner as FAF ← FAF-RSL. That is, skip processing is performed.

If the sign of the first air-fuel ratio flag F1 is not inverted at step 1016, the integration process is performed at steps 1021, 1022, and 1023. That is, at step 1020, it is determined whether or not F1 = "0", and if F1 = "0" (lean), at step 1022
FAF ← FAF + KIR. On the other hand, if F1 = “1” (rich), in step 1023, FAF ← FAF−KIL.

The air-fuel ratio correction coefficient FAF calculated in steps 1019, 1020, 1022, and 1023 is guarded by the lower limit value 0.8 in steps 1024 and 1025, and is guarded by the upper limit value 1.2 in steps 1027 and 1028. In this case, the air-fuel ratio correction coefficient FAF is
When the value reaches 0.8, the skip counter CSKP is reset in step 1026. On the other hand, when the air-fuel ratio correction coefficient FAF reaches the upper limit value 1.2, the skip counter CSKP is reset in step 1029.

The FAF calculated as described above is stored in the backup RAM 106, and this routine ends in step 1030.

That is, the air-fuel ratio correction coefficient FAF is the lower limit value of the guard processing.
Skip counter when reaching 0.8 or upper limit 1.2
Clear CSKP, to stop the air-fuel ratio feedback control by the output V ₂ of the downstream O ₂ sensor 15 which will be described later. This suspension
It continues until the value of the skip counter CSKP reaches a predetermined value a. This corresponds to step 805 in the routine of FIG.
This is performed by replacing 806 with step 1101 in FIG. That is, from the time when the value of the air-fuel ratio correction coefficient finally reaches the upper limit value or the lower limit value (CSKP = 0), if the air-fuel ratio correction coefficient FAF is inverted a predetermined number of times, the maximum value of the fluctuation of the air-fuel ratio correction coefficient FAF, It is considered that the minimum value no longer reaches the upper limit value or the lower limit value, and it can be determined that the air-fuel ratio correction coefficient FAF is controlled within a predetermined allowable range. Therefore,
In this embodiment, the air-fuel ratio correction coefficient FAF is the lower limit value of the guard processing.
It is determined by the number of FAF skips that control is performed within the range of 0.8 and the upper limit 1.2, and the output V _{2 of the} downstream O ₂ sensor 15 is determined.
Restarts the air-fuel ratio feedback control.

The air-fuel ratio correction coefficient FAF is lower than the guard processing lower limit of 0.8,
The fact that control is performed within the range of the upper limit value 1.2 indicates that the average value FAFAV of the air-fuel ratio correction coefficient FAF, for example, the average value FAFAV of the immediately preceding skip value shown in FIG. 12 = (FAF immediately before the rich skip + FAF immediately before the lean skip) / 2 , And it can be determined that FAFAV is within a predetermined range, for example, 0.9 to 1.1. In this case, it is determined in step 519 of FIG. 5 whether or not FAFAV> 0.9, and in step 523, it is determined whether or not FAFAV <1.1.

In addition, the air-fuel ratio correction coefficient FAF has a lower limit value of 0.8 for the guard processing,
May be the duty ratio of the rich output time of the upstream O ₂ sensor 13 and the lean output time that is controlled within the range between the upper limit value 1.2 is determined by in a predetermined range.

Also, the first air-fuel ratio feedback control is performed every 4 ms,
Further, the second air-fuel ratio feedback control is performed for each 512ms, the air-fuel ratio feedback control mainly performs control by the upstream O ₂ sensor good response, the control by the upstream O ₂ sensor 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 air bleed amount of carburetor and controls air-fuel ratio by introducing air into main passage and slow passage. It 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 901 is determined by the carburetor itself, that is, determined in accordance with the intake pipe negative pressure corresponding to the intake air amount and the engine speed, and step 903. Then, the supply air amount corresponding to 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.

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 air-fuel ratio correction amount FAF sticks to the upper and lower limits of the guard processing due to some factor such as when the vehicle travels from lowland to highland or vice versa without learning. Erroneous control of the air-fuel ratio feedback control due to the output of the sensor can be more reliably prevented, and therefore, deterioration of emission, drivability, fuel consumption, catalyst exhaust odor, and the like can be prevented.

[Brief description of the drawings]

FIG. 1 is an overall block diagram for explaining the configuration 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. FIG. 4 is an overall schematic diagram showing an embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention, FIG. 5, FIG. 8, FIG. 9, FIG. FIG. 11 is a flow chart for explaining the operation of the control circuit of FIG. 4, FIG. 6A, FIG. 6B, FIG. 7 is a timing chart for supplementing the flow chart of FIG. 5, and FIG. FIG. 11 is a timing chart showing another embodiment. 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 according to outputs of the upstream and downstream air-fuel ratio sensors. Air-fuel ratio correction amount calculating means for calculating the air-fuel ratio correction amount; guard means for guarding the calculated air-fuel ratio correction amount within a predetermined allowable range; adjusting the air-fuel ratio of the engine according to the air-fuel ratio correction amount An air-fuel ratio adjusting unit that determines whether the air-fuel ratio correction amount has reached an upper limit value or a lower limit value of the predetermined allowable width, and determines whether the air-fuel ratio correction amount is the upper limit value or the lower limit of the predetermined allowable width. The air-fuel ratio based on the output of the downstream air-fuel ratio sensor Allowable width reaching determination means for stopping the calculation of the correction amount; determining whether or not the air-fuel ratio correction amount has returned to a state controlled within the predetermined allowable width; An air-fuel ratio control device for an internal combustion engine, comprising: an in-permissible-width return determination unit that restarts the calculation of the air-fuel ratio correction amount based on the output of the side air-fuel ratio sensor.