JPH0610739A - Air-fuel ratio controller of engine - Google Patents

Abstract

PURPOSE:To improve accuracy in control by correcting a target air-fuel ratio to the leaner side than before, and carrying out feedback-control in response to the target lean air-fuel ratio, when a combustion variation quantity detected in an opening loop control system is smaller than the standard value for example. CONSTITUTION:A fuel injection valve 3 arranged near a throttle chamber 1 is drive-controlled by a microcomputer 13 based on a detection signal at least from an air-fuel ratio sensor 9, in this air-fuel ratio controller. Meanwhile, a microcomputer 13 is provided with an closed loop control system which performs feedback-control so that a real air-fuel ratio becomes a target air-fuel ratio, and an open loop control system which temporarily stops the feedback- control. In this case, when a combustion variation quantity detected in the open loop control system is more than a standard value, a target air-fuel ratio is corrected to the leaner side than before. On the other hand, when the combustion variation quantity is less than the standard value, the target air-fuel ratio is corrected to the leaner side. The feedback-control is carried out in response to the corrected target lean air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control system.

[0002]

2. Description of the Related Art In recent years, in the automobile field, in order to reduce fuel consumption and prevent exhaust pollution, the air-fuel ratio (A / F) of the engine is set to an allowable target lean air-fuel ratio, and as lean as possible. Practical use of a lean burn system that operates the engine in a state close to the combustion limit (however, the target lean air-fuel ratio is set to a point where the air-fuel ratio is smaller than the lean combustion limit point in order to maintain combustion stability) Is being done.

As such a lean burn system, for example, an air-fuel ratio sensor (A / F sensor) is provided in the exhaust passage to detect an actual air-fuel ratio, and this detection signal is input to a micro computer to obtain a target lean air-fuel ratio. There is one that performs closed loop control (feedback control) so that the fuel ratio is achieved.

Further, in the conventional air-fuel ratio control technique, a technique which replaces the feedback control for controlling the actual air-fuel ratio to the target lean air-fuel ratio by using the A / F sensor as described above is used for lean air-fuel ratio operation. Various techniques have been proposed for achieving stable combustion and operation.

For example, in Japanese Unexamined Patent Publication No. 52-17127, when the engine is operated in the lean air-fuel ratio region near the misfire limit in the exhaust gas, fluctuations in air-fuel ratio, distribution, changes in atomization, and outside air temperature. Taking into account the fact that combustion conditions deteriorate due to iso-external conditions and misfires occur, the hydrocarbon (HC) concentration in the exhaust gas of the engine is detected, and the amplitude of this detected electric signal is preset. There is disclosed a technique in which a sign of misfire is detected when the value is larger than the reference value and the substantial air-fuel ratio of the air-fuel mixture is corrected to the rich side.

In addition, for example, JP-A-60-27748.
As disclosed in Japanese Patent Application Laid-Open No. 60-13938, etc., a roughness sensor detects a combustion fluctuation state (for example, torque fluctuation, cylinder pressure fluctuation, rotational speed fluctuation, etc.) of an engine,
Lean limit feedback control for controlling the air-fuel ratio so that the combustion fluctuation amount reaches an allowable set value, and JP-A-60-1356.
As disclosed in Japanese Patent Laid-Open No. 34-34, the air-fuel ratio is intentionally changed to a lean side to such an extent that the drivability is not impaired for a short period while the engine is operating, and the combustion fluctuation amount at this time is used to determine the engine A control technique or the like has been proposed in which a change in the lean combustion limit point (misfire limit point) is detected and the control air-fuel ratio is corrected according to the change.

[0007]

In the lean burn system, in order to improve the control accuracy, the lean air-fuel ratio feedback control described at the beginning of the above-mentioned prior art,
That is, an ideal technique is to provide an A / F sensor in the exhaust passage to detect the actual air-fuel ratio and perform feedback control so that this actual air-fuel ratio becomes the target lean air-fuel ratio. It is desirable to always keep the target lean air-fuel ratio at the optimum value, taking into account the change in the combustion limit point.

However, the above-mentioned prior art does not give any technical consideration to this point.

That is, in Japanese Patent Laid-Open No. 52-17127, there is a special case in which the air-fuel ratio is controlled in the open mode in the normal combustion state and the air-fuel mixture is rich-corrected from the HC detection signal only when the combustion state deteriorates. Only the feedback control technique is adopted, and further, the lean correction when the lean combustion limit point is shifted to the lean side is impossible, and in JP-A-60-27748, the target lean is disclosed. Although the lean air-fuel ratio feedback control that defines the air-fuel ratio is adopted, if the combustion fluctuation amount becomes abnormal, the lean air-fuel ratio feedback control causes lean air-fuel ratio limit feedback (the combustion fluctuation amount becomes a predetermined allowable combustion fluctuation amount). The above-mentioned lean air-fuel ratio is disclosed in Japanese Patent Laid-Open No. 60-13938. The lean air-fuel ratio limit feedback uses only the field feedback, and the lean air-fuel ratio limit feedback captures the combustion fluctuation amount by any of the parameters of torque fluctuation detected by the roughness sensor, cylinder pressure fluctuation, and engine speed fluctuation to determine the air-fuel ratio. Is a feedback control, and is a mode different from the lean air-fuel ratio feedback control using the air-fuel ratio sensor (control in which the actual air-fuel ratio becomes the target lean air-fuel ratio). Feedback control using this roughness sensor is
Parameters such as torque fluctuation and in-cylinder pressure are greatly affected by external factors such as engine inertia (piston, rotor, etc.), vehicle body inertia, vehicle body vibration system, etc. Therefore, only combustion fluctuation is separated from the above external factors. It is difficult to detect. Therefore, the feedback control accuracy is likely to be sacrificed because of the influence of the external factors.

Further, in Japanese Patent Laid-Open No. Sho 60-135634, the air-fuel ratio is intentionally changed to a lean side so that the drivability is not impaired only for a short period during the operation of the engine, and the combustion fluctuation amount at this time is used to determine the engine. The change in the lean combustion limit point of the above is detected, and the control air-fuel ratio is corrected according to this change.However, the air-fuel ratio control itself is an open-loop method, and it is expected that the feedback control desired by the applicant of this application will be further improved in accuracy. I can't.

The present invention has been made in view of the above points, and an object thereof is to improve the accuracy of lean air-fuel ratio feedback control.

[0012]

In order to achieve the above object, the present invention detects an actual air-fuel ratio from exhaust gas of an engine using an air-fuel ratio sensor, and this actual air-fuel ratio becomes a target lean air-fuel ratio. Thus, a closed loop control system for performing feedback control, and an open loop control system for temporarily interrupting the feedback control to detect a combustion fluctuation amount by open loop control are provided, and the combustion fluctuation amount detected by the open loop control system is set in advance. When it is larger than the set reference value, the target lean air-fuel ratio is corrected to the rich side until now, and when the combustion fluctuation amount is smaller than the reference value, the target lean air-fuel ratio is corrected to the lean side until now. There is proposed an air-fuel ratio control device which is set so as to execute the subsequent feedback control with a corrected target lean air-fuel ratio.

[0013]

According to the present invention, during operation of the lean burn system, basically, the closed loop control system causes the actual air-fuel ratio detected by the air-fuel ratio sensor to be the target lean air-fuel ratio (the target lean air-fuel ratio is usually Near the lean combustion limit, set the air-fuel ratio to some extent below the lean combustion limit point,
The lean air-fuel ratio control is provided with a margin so that stable lean combustion can be obtained). Since the optimum value of the target lean air-fuel ratio will change due to the lean combustion limit point (misfire limit point) accompanying the change with time of the engine, etc., is the feedback control temporarily interrupted to change the lean combustion limit point? The confirmation mode is set. This mode is performed by arbitrarily setting an air-fuel ratio point capable of knowing the change of the lean combustion limit point by the open loop control system and detecting the combustion fluctuation amount of this air-fuel ratio point.

The change of the lean combustion limit point by such an open loop control system is restricted by the feedback control which gives the target lean air-fuel ratio in the closed loop control system, and the above-mentioned arbitrary set air-fuel ratio point is restrained. This is because it is not possible to accurately capture the change in the lean combustion limit point. In this way, the change in the combustion fluctuation amount captured by the open loop can be accurately detected, and therefore the change in the lean combustion limit point can also be accurately known from the combustion fluctuation amount, and the change in the lean combustion limit point can be detected. Therefore, the optimum value of the target lean air-fuel ratio can be corrected accurately. Specifically, when the combustion fluctuation amount detected by the open loop control system is larger than a preset reference value, the target lean air-fuel ratio becomes richer than ever, assuming that the lean combustion limit point has shifted to the rich side. Fix on the side,
When the combustion fluctuation amount is smaller than the reference value, it is assumed that the lean combustion limit point has shifted to the lean side, and the target lean air-fuel ratio is corrected to the lean side.

Since the corrected target lean air-fuel ratio is used, the lean air-fuel ratio feedback control using the air-fuel ratio sensor (the air-fuel ratio sensor is more reliable than the roughness sensor) is executed again thereafter. Always realize the optimum lean air-fuel ratio control.

The detection of the combustion fluctuation amount by the open loop control can be grasped from the output of the air-fuel ratio sensor, for example. This is because when the air-fuel ratio does not exceed the lean combustion limit (misfire limit), there is almost no fluctuation component (pulsation component that changes with time) contained in the sensor output, and the air-fuel ratio exceeds the lean combustion limit point. The larger the combustion fluctuation amount, the larger the fluctuation component in the sensor output.
This is because as the combustion fluctuation of the engine increases, the oxygen concentration in the exhaust gas pulsation correspondingly increases correspondingly when the cylinder is exhausted, and this becomes a fluctuation component of the sensor output. Further, instead of the air-fuel ratio sensor, it is also possible to detect from engine torque fluctuations, cylinder pressure fluctuations, engine speed fluctuations, and the like. Even if the open loop control is performed temporarily by capturing such a phenomenon with the roughness sensor, the subsequent lean air-fuel ratio feedback control is performed by the closed loop control system using the air-fuel ratio sensor provided in the exhaust pipe. The accuracy of lean air-fuel ratio feedback control is not reduced.

[0017]

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

FIG. 1 is a block diagram showing a control system of an automobile engine to which the present invention is applied. In the figure, 1 is a throttle valve, 2 is a throttle valve, 3 is a fuel injection valve, 4 is an ignition plug. 5 is a combustion chamber in the cylinder, 6 is a water temperature sensor for detecting the temperature of the cooling water of the engine, 7 is a crank angle sensor, and 8 is a crank angle sensor.
Is an exhaust pipe, 9 is an air-fuel ratio sensor (hereinafter abbreviated as A / F sensor), 10 is a control circuit for the A / F sensor 9, 11 is a heater drive circuit, 12 is a sensitive coil, and 13 is air-fuel ratio control. The microcomputer 13 is a closed loop control system for performing lean air-fuel ratio feedback control of the present embodiment described below by this microcomputer 13, and the target lean air-fuel ratio is corrected by temporarily interrupting the lean air-fuel ratio feedback (however, correction is performed). An open loop control system for performing

In the vehicle engine control system according to this embodiment, the A / F (target A / F) to be controlled by the engine speed, load, water temperature, etc., is the microcomputer 1.
3 and the injection valve 3 is output based on this determined value. Then, the air-fuel mixture formed in the throttle chamber 1 enters the combustion chamber 5 and is ignited by the ignition plug 4, and then the exhaust gas flows into the exhaust pipe 8. At this time,
The A / F sensor 9 outputs an output signal V _out proportional to the oxygen concentration in the exhaust gas to detect the actual A / F, inputs the signal to the micro computer 13, and the micro computer 13 outputs the target A / F. Between the actual A / F and the actual A / F, and based on this deviation, air-fuel ratio feedback control is performed so that the target A / F is achieved using a closed loop control (P / I control) system.

Here, the lean A / F feed control is set near the lean combustion limit (misfire limit) point with a margin to some extent where the air-fuel ratio is smaller than the lean combustion limit point. The reason will be described based on FIG. FIG. 4 shows the A / F of the engine, the HC gas in the exhaust gas, the fuel consumption F and the A / F.
The output characteristic V _out of the F sensor 9 is shown. As shown in the figure, HC, which is a harmful component in the exhaust gas, and fuel consumption F
Decreases most when the A / F becomes lean and approaches the misfire area (hatched area), but increases sharply when entering the misfire area. Therefore, in order to reduce exhaust pollution and reduce fuel consumption, it is preferable to set the target lean A / F near the lean combustion limit point with some allowance. A target lean A / F is set.

The A / F sensor 9 of this embodiment is made of a solid electrolyte such as zirconium oxide and can output an output signal proportional to the oxygen concentration in the exhaust gas.
As shown in (1), the oxygen concentration in the exhaust gas increases as the A / F increases, and the output V _out of the A / F sensor 9 also increases in proportion to this. The output characteristic of this A / F sensor detects the actual A / F in the exhaust gas as described above. Since the A / F sensor 9 must be heated to a high temperature due to the characteristics of the solid electrolyte used, the heater drive circuit 11 is provided.

The lean A / F feedback control is generally performed during a partial load operation after warming up, but the target lean A / F feedback control is performed.
/ F does not guarantee stable operation if it is always kept constant, and even if the lean A / F is kept constant, the limit (lean combustion limit) point of the misfire region changes as the engine changes over time. Therefore, it is necessary to correct the target lean A / F in response to such a change in the lean combustion limit point.

FIG. 2 (a) shows a change in the lean combustion limit point with the aging of the engine, and the hatched portion is the misfire region. The characteristic line in the figure represents the change in the sensor output V _out with respect to the excess air ratio λ = actual air-fuel ratio / theoretical air-fuel ratio (14.7), and the initial lean combustion limit point A is λ _{A in} terms of excess air ratio. First, assuming that the excess air ratio λ ₀ (λ ₀ <λ _A ) before this λ _A is the target lean A / F of the lean A / F feedback control, the A / F sensor 9 corresponding to λ ₀ The fuel injection amount (in other words, the supply air-fuel ratio) is feedback-controlled so that the actual A / F sensor output becomes V _out , 0 with the output value V _out , 0 of _V.sub.out 0 as the sensor output target value.

Here, when the lean combustion limit point is changed to point B (λ _B ) which is closer to the latch than before due to the change with time of the engine, the excess air ratio becomes λ ₀ > λ _B , so the target output of the closed loop control. If the value V _out , 0 is left as it is, a misfire will occur. On the other hand, when the lean combustion limit changes to the leaner C point (λ _C ), the excess air ratio becomes λ ₀ <λ _C , so it is possible to operate with a lean air-fuel ratio larger than the current λ _0. Becomes

In the present embodiment, in consideration of the above change in the lean combustion limit, the sensor output target value which is the reference of the lean A / F feedback control, that is, the target lean A / F (excess air ratio) is set as follows. Fix it. Here, prior to explaining such target value correction, the output variation characteristic of the A / F sensor 9 will be described. Fig. 5 shows changes in A / F and A
The relationship between the output fluctuation width ΔV _out of the / F sensor 9 and the fluctuation width ΔHC of the HC is shown, and the actual output signal of the A / F sensor 9 is shown at the top of the graph.
As shown in the figure, the variation amount ΔHC of the exhaust amount of the exhausted HC gas is almost constant until the A / F (the A / F is at the position of 18.8 in this example) which is almost at the limit of the misfire area. However, when a misfire occurs in at least one of the cylinders of the engine, the amount of HC discharged fluctuates with time. Such a phenomenon is because the amount of HC exhausted from the misfiring cylinder increases in a pulsating manner, and the variation amount ΔHC thereof is, as is clear from the graph, the magnitude of A / F in the misfire region, that is, It is proportional to the degree of misfire (combustion fluctuation). Also, this HC
As the fluctuation range ΔHC of the discharge amount increases, the output fluctuation range ΔV _out of the A / F sensor 9 also increases as shown in the waveform of the sensor signal in FIG. 5 and this waveform (output fluctuation range) graph. This is because when the discharge amount of HC gas fluctuates with time due to misfire, the oxygen concentration in the exhaust gas also fluctuates with time in proportion to A / F corresponding to this fluctuation ΔHC.
This is because the variation amount ΔV _out of the output signal of the / F sensor 9 becomes large.

As described above, when the misfire starts due to the output characteristics of the A / F sensor 9, not only the output level V _out of the A / F sensor 9 but also the temporal fluctuation degree ΔV of the output level.
_Out also increases in proportion to the degree of misfire.

In this embodiment, such an A / F sensor 9 is used.
The demold the output signal V _out from the fluctuation component [Delta] V _{o ut,} detects a change in the lean combustion limit point weight combustion variation in other words the variation [Delta] V _out catches a phenomenon remarkable, the lean combustion limit engine If the change occurs due to the change over time, the target lean A / F is corrected so as to be able to cope with this.

Specific means for correcting the target lean A / F will be described below.

FIG. 6A shows the signal V _out of the A / F sensor 9.
2 shows an example of a circuit for extracting the fluctuation component ΔV _out from the circuit. This circuit performs full-wave rectification on the fluctuation signal ΔV _out . Here, the fluctuation component Δ as shown in FIG.
If you enter a sensor signal V _out including V _out, Katsuhito and alternating current component (fluctuation component) DC component by capacitors 14 delta
V _out is taken out. Next, the circuit constituted by the amplifiers 15 and 16 extracts the half-wave above the dotted line in FIG. 6B, and the circuit including the amplifier 17 extracts the half-wave below the dotted line. Further, the half-waves extracted by the circuit including the amplifier 18 are combined and rectified, and an analog value proportional to ΔV _out can be output (FIG. 6).
(C)). The detection signal ΔV _out extracted in this way
Is input to the microcomputer 13 shown in FIG.
In order to obtain an analog value proportional to ΔV _out , digital processing by a microcomputer is possible, but considering the processing time and the capacity of the computer, it is preferable to perform analog processing beforehand as described above. The microcomputer 13 obtains the detection signal ΔV _out in the lean A / F setting (correction) mode, and corrects the target lean A / F as follows based on the detection signal ΔV _out .

FIGS. 2 (a) and 2 (b) are explanatory views for making this correction. In FIG. 2 (a), the misfire of the engine over time with respect to the excess air ratio (A / F is represented by the excess air ratio) is misfired. The change in the region is shown, and FIG. 6B shows the change in the sensor output change amount ΔV _out characteristic line of the engine time change with respect to the excess air ratio. Here, it is assumed that the misfire limit region in the initial engine state is at point A (air excess ratio λ _A ), and initially the air excess ratio λ ₀ before this λ _A is the initial target lean A / F.
Then, the sensor output V _out , 0 corresponding to this λ ₀ is used as a target closed loop control value, and the output of the A / F sensor is V _out ,
It is assumed that the microcomputer 13 controls the fuel injection amount so that it becomes zero.

Further, the reference value for correcting the target lean A / F with the aging of the engine is set to the sensor output fluctuation ΔV _out = ε (see FIG. 2B) at the lean combustion limit point. Under such conditions, when the lean combustion limit point A shifts to point B on the latch side in FIG. 2 (a) due to the change with time of the engine, the sensor output as shown by the one-dot chain line B'in FIG. 2 (b). The rise of fluctuations is accelerated. Therefore, based on the sensor output value V _out , A at the initial lean combustion limit point A,
At that time, the initial misfire reference sensor output fluctuation value ε is compared with the engine output change amount ΔV _out1 after the engine has changed over time, so that the degree of misfire after the engine can be determined. In this example, ΔV _out1 is the reference sensor output fluctuation value ε.
Since it is larger, it is necessary to correct the sensor output target value corresponding to the target lean A / F by α corresponding to ΔV _out1 −ε. Corrected excess air ratio in this case (target lean A / F
Value) λ _m is expressed by equation (1).

[0032]

## EQU1 ## λ _m = λ ₀ −K ₁ α (1) where K ₁ is a correction coefficient and λ ₀ is the original excess air ratio. By making such a correction, as shown in FIG. 2A, a new target lean A / F, that is, λ _m , can be set in front of the misfire limit point B after the engine aging. Then, in this case, V _out , m corresponding to λ _m is obtained based on the sensor output characteristic, and after correction, lean A / F feedback control is performed using this V _out , m as the target sensor output.

On the contrary, when the misfire limit area A shifts to the lean side point C in FIG. 2 (a), the broken line C'in FIG. 2 (b).
As shown in, the rise of the sensor output fluctuation is delayed. In this case, the reference sensor output fluctuation value ε and the sensor output fluctuation amount ΔV _out2 due to engine aging at the initial misfire limit point A.
By comparing with, it is possible to judge the degree of misfire after engine aging. Then, in this example, ΔV _out2 is smaller than ε, so it is determined that the lean side can be operated, and
It suffices to correct the corrected air excess ratio λ _n, which is the control target, to be increased by β corresponding to ε-ΔV _out2 . The corrected excess air ratio λ _{n in} this case is expressed by the equation (2).

[0034]

## EQU2 ## λ _n = λ ₀ + K ₂ β (2) K ₂ is a correction coefficient. Then, by this correction, a new target A / F, λ _n is set before the lean combustion limit point C after the engine aging. Further, V _out , n corresponding to λ _n is calculated based on a predetermined sensor output characteristic, and after correction, this V _out , n is used as a sensor output target value, and lean A /
F feedback control is performed.

After the target lean A / F (sensor output target value) has been corrected, if the misfire limit region changes again due to the change with time of the engine, the same correction as described above is made, but in this case, B Alternatively, the point C is regarded as the lean combustion limit point, and ε at this time and the points B and C after the engine changes with time.
The target excess air ratio may be corrected based on α or β corresponding to the difference in ΔV _{out at} the point.

A flow chart for performing the above control is shown in FIG. As shown in the figure, when the target lean A / F is corrected, it is first necessary to determine whether the lean A / F operation range is the same as that in the partial load operation by determining the engine speed and the accelerator opening degree. Judge based on (load). Next, when a mode for detecting a change in the lean combustion limit (lean limit mode) is entered, the closed loop control of the lean A / F is temporarily suspended and the open loop control is performed. The reason is that if the closed loop control for keeping the target lean air-fuel ratio is left as it is, the influence of the feedback control operation will be exerted on the sensor signal, and the output V _out and the component of ΔV _out due to the misfire at the point A cannot be extracted. .

That is, in the lean limit detection mode, the sensor output value Δ at the air-fuel ratio λ _A point is set after the open loop is formed.
V _out (ΔV _out1 or ΔV _{out2 in} this example) is detected,
As described above, the difference between ΔV _out and ε is obtained, and when ΔV _out > ε, λ ₀ is corrected by α (= ΔV _out −ε), and ΔV _out <
In the case of ε, λ ₀ is corrected by β (= ε-ΔV _out ). Then, based on the output of the A / F sensor corresponding to the corrected excess air ratios λ _m and λ _n , the A / F is closed-loop controlled again to change the engine over time and change the lean combustion limit point. Corresponding to this, it becomes possible to perform lean combustion operation just before the lean combustion limit point.

The above operation is shown in FIG. 7 by an A / F control map. The horizontal axis of Fig. 7 represents the engine load state,
The vertical axis represents the excess air ratio λ that is the control target. As shown in this map, except that the throttle is fully opened to λ <1.0 and the switch is on the latch side, almost lean operation is performed, and particularly during partial load operation in which combustion is stable, the lean combustion operation mode is set. Here, when it is determined that the detected sensor fluctuation amount ΔV _out after the lean combustion limit point changes is ΔV _out > ε,
Since it is supposed that misfire will occur in the current A / F control,
As described above, the excess air ratio λ ₀ is changed to the latch side λ _m by K ₁ α. On the contrary, when ΔV _out is determined to be ΔV _out <ε, λ ₀ is changed to the lean side λ _n by K ₂ β because there is a margin to reach the misfire area. Such correction is performed in all the load regions in the lean limit region, and the A / F control map is rewritten in this way.

According to the present embodiment, in the lean burn system, the lean A / F feedback control is basically performed by the closed loop control system using the highly reliable air-fuel ratio sensor, and this is temporarily interrupted. Correct the target lean A / F by the open loop control system, and then again
Since the lean A / F feedback control is executed by the corrected target lean A / F, the optimum lean air-fuel ratio control is always realized. In addition, lean A / F correction can be performed using an existing A / F sensor, which is advantageous in terms of cost.

FIG. 8 is a flow chart showing a second embodiment of the present invention. This embodiment is basically based on the sensor output fluctuation amount ΔV _out to be detected as in the first embodiment described above. Although the excess air ratio is corrected, the difference is that the correction value is set to a predetermined constant value α'in both cases of ΔV _out > ε and ΔV _out <ε. That is, when ΔV _out > ε, λ _m = λ ₀ −α ′ and the excess air ratio λ is reduced by a constant value. When ΔV _out <ε, λ _n = λ ₀ + α ′ can be set to increase the excess air ratio by a certain value. According to such a control method, the control can be easily performed, and the lean A / F can be corrected even if ε is sufficiently small by making α ′ have a sufficient width.

FIG. 9 shows another example (third embodiment) of rewriting the target lean A / F map. In this example, when the sensor output fluctuation ΔV _out after the change of the misfire limit region is detected, the target lean A is detected. / F value, λ '

[0042]

## EQU3 ## λ '= λ ₀ · f (ΔV _out ) ... It is corrected by the equation (3). Here, f ([Delta] V _out), when [Delta] V _out becomes large (3) left side of the target lean A / F, lambda 'decreases, [Delta] V _out is the lambda reduced' to set the like increases relationship. For example,

[0043]

Equation 4] rewritten _{λ '= λ 0 (K 1} + K 2 / ΔV out) ......... using the relation such as (4) lambda'. in this case,
K ₂ serves as a reference value for knowing the magnitude of ΔV _out .

FIGS. 10A, 10B and 11 show the fourth embodiment of the present invention. In this embodiment, misfire is detected for each cylinder and the target lean A / F is corrected for each cylinder. The principle is shown in FIG. 10 (a). (A) of the figure (a) is a cylinder discrimination reference signal that the crank outputs every two revolutions, (b) is a delay time t _d until the exhaust reaches the A / F sensor 9, such as a flow delay of the exhaust. This is the cylinder discrimination reference signal p ₁ . _Let the time between pulses be t _r . (C) is the output signal of the A / F sensor 9, and the sensor output fluctuation value ΔV _out becomes large in the portion corresponding to the cylinder in which the misfire has occurred. The signal of (d) is obtained by converting this increasing time into a pulse signal p ₂ by a comparator or the like. Then,
If the signal (d) is generated with a delay of t _C from the cylinder discrimination reference signal (b) (t _C is represented as a time difference between the generation periods of the cylinder discrimination signal p ₁ and the sensor signal p ₂ ),
By determining t _C / t _r , it is possible to detect which cylinder is currently misfiring. Incidentally, as shown in FIG. 10 (b), the delay time t _d so changed by the engine rotational speed N, and the like, it is necessary to store in advance microcomputer relationship delay time t _d with respect to the engine rotational speed in the .

FIG. 11 is a flow chart showing a method for correcting the target A / F for each cylinder based on such a principle. In the correction method of FIG. 11, first, t _C for n times
The average value m of is calculated. That is, t _C for n times is r (= t _C
+ R '), the average value m is m = r / n.
Then, the ratio Q = m / t _r between m and the time t _r between pulses is obtained. In addition, reference numerical values ε ₁ , ε ₂ , ε ₃ , which notify a misfiring cylinder by comparing with Q in a storage area of a microcomputer or the like,
ε ₄ is recorded. For example, in case of 4 cylinders, Q ≦
When ε ₁ , it is judged that the first cylinder has misfired, and ε ₁ <
The second cylinder when the Q ≦ epsilon ₂ is, the third cylinder when the epsilon ₂ <Q ≦ epsilon ₃ is, when epsilon ₃ <Q is determined that the fourth cylinder is misfiring. Then, the target lean A / F λ'is corrected for each cylinder in which misfire has occurred. In the flow chart of FIG. 11, λ ₁ , λ ₂ , λ ₃ and λ ₄ are target lean A / F before correction of each cylinder, λ ′ ₁ , λ ′ ₂ , λ ′ ₃ and λ ′ ₄ Is the corrected target lean A / F.

In all of the above embodiments, the misfire is detected by the fluctuation of the signal of the A / F sensor, and the target lean A / F is corrected based on this fluctuation ΔV _out . Shows a target lean A / F correction method different from this.

Embodiments of FIGS. 12 (a), 12 (b) and 13
Is the electricity of the heater 20 for heating the A / F sensor 9.
Detect the amount and correct the A / F control target value based on this value
It shows a principle diagram to be attempted. Figure 12 (a)
Is the A / F and the heater current value I _H , HC relationship
It was When A / F is increased, misfire begins and HC
As more is discharged, the degree of misfire progresses and the combustion temperature
Exhaust temperature around the A / F sensor is low due to the low temperature.
Become On the other hand, this kind of heater 20 keeps the sensor temperature constant.
Since it is set so that the function to keep it constant will work,
When the air temperature becomes low, the heater current I_HTo increase. this
For I_H Is detected, the combustion temperature decreases due to misfire,
That is, the degree of misfire can be detected.

FIG. 12B shows an actual heater control circuit for detecting I _{H. As} shown in FIG. 12, the solid electrolyte of the heater 9 is controlled at a predetermined timing controlled by the timing control circuit 21. Electrodes 22a, 2 on both sides of 20
A constant current Ic is made to flow by the constant current circuit 23 between 2b. At this time, a voltage proportional to the internal resistance r of the solid electrolyte 20 is synchronously taken in by the sample hold circuit 24, compared with a reference value V _ref, and I _H is controlled to control the internal resistance r to be constant. To do. Therefore, exhaust temperature is lowered, the temperature of the sensor is reduced, by increasing the I _H, to keep the sensor temperature raised heater temperature constant. That is,
The exhaust temperature can be detected by I _H. Here, in order to detect I _H , V _H is measured via the resistor R, and the degree of misfire can be determined by the degree of V _H.

FIG. 13 shows the target lean A / based on the above V _H.
The flow chart for correcting F is shown. As shown in this flow chart, the detection signal V _H is read in the lean limit detection mode in the lean operation range. Next, the engine speed N is read, and according to the value of the speed N,
A reference value ε to be compared with V _H is determined (ε ₁ to ε _n ). The reason why ε is changed depending on the engine speed N is that the degree of the exhaust temperature is different depending on N. That is, the reference value ε is set variously from ε ₁ to ε _n according to the change of the engine speed N, and the reference value ε corresponding to the current engine speed is set.
If V _H is V _H ≧ ε, the control target lean A
/ F (λ ')

[0050]

## EQU5 ## Corrected by (5) λ '= λ ₀ -α (5), and when V _H is V _H <ε,

[0051]

## EQU6 ## Correction is performed by λ '= λ ₀ + α (6).

[0052]

As described above, according to the present invention, in the lean burn system, basically, the closed loop control system using the highly reliable air-fuel ratio sensor is used to perform the lean air-fuel ratio feedback control with high accuracy and temporarily. The target lean air-fuel ratio is corrected accurately by the open loop control system, and thereafter the lean air-fuel ratio feedback control is executed again by the corrected target lean air-fuel ratio, so that the optimum lean air-fuel ratio is always maintained. Realize air-fuel ratio control.

[Brief description of drawings]

FIG. 1 is a diagram of an automobile engine control system to which the present invention is applied.

FIG. 2 is an air excess ratio-sensor output characteristic diagram for explaining the first embodiment in the A / F control of the present invention.

FIG. 3 is a flow chart showing the A / F control of the first embodiment.

FIG. 4 is a characteristic diagram showing the relationship between the output of the A / F sensor used in the first embodiment, fuel consumption, and HC gas.

FIG. 5 shows the sensor output fluctuation component of the A / F sensor and A / F.
Characteristic diagram showing the relationship with F

6 (a), (b), and (c) are explanatory diagrams of a circuit and a waveform for extracting an output fluctuation component of the A / F sensor.

FIG. 7 is a diagram showing a state before and after correction of the A / F control map of the first embodiment.

FIG. 8 is a flow chart showing a second embodiment of the present invention.

FIG. 9 is a flow chart showing a third embodiment of the present invention.

10 (a) and 10 (b) are operation waveform diagrams and characteristic diagrams showing a fourth embodiment of the present invention.

FIG. 11 is a flow chart of the fourth embodiment.

FIG. 12 is an explanatory diagram showing a fifth embodiment of the present invention.

FIG. 13 is an explanatory diagram showing a sixth embodiment of the present invention.

[Explanation of symbols]

1 ... Slot torch chamber, 2 ... Throttle valve, 3 ... Fuel injection valve, 5 ... Engine (combustion chamber), 8 ... Exhaust passage, 9 ... A /
F sensor, 13 ... Micro computer (closed loop control system, open loop control system), 14-18 ... Sensor output fluctuation amount detection circuit.

Claims (1)

[Claims] 1. A closed loop control system for detecting an actual air-fuel ratio from exhaust gas of an engine using an air-fuel ratio sensor and performing feedback control so that the actual air-fuel ratio becomes a target lean air-fuel ratio, and the feedback control is temporarily performed. An open loop control system for detecting the combustion fluctuation amount by interrupting open loop control is provided, and when the combustion fluctuation amount detected by the open loop control system is larger than a preset reference value, the target lean air-fuel ratio is more than ever. When the combustion fluctuation amount is smaller than the reference value, the target lean air-fuel ratio is corrected to a leaner side than ever before, and the feedback control is performed thereafter based on the corrected target lean air-fuel ratio. An air-fuel ratio control device for an engine, characterized by being set as follows.