JPS61104138A - Control device for air-fuel ratio of engine - Google Patents

Abstract

PURPOSE:To prevent the occurrence of a fluctuation in torque and to improve stability of an engine, by providing a desired value fixing means which fixes a desired value to a value responding to that in the vicinity of a theoretical air-fuel ratio when the temperature of an air-fuel sensor is decreased to lower than a given value. CONSTITUTION:A comparing means 16 is provided for comparing the output of an air-fuel ratio sensor 8 with a desired value set by a desired value setting means 15. An air-fuel ratio control means 17 controls the air-fuel ratio of air-fuel mixture to a desired value. A desired value fixing means 18 is provided for receiving the output of a temperature detecting means 10, serving to detect the temperature of the air-fuel ratio sensor 8, and fixing a desired value from the desired value setting means 15 to a value responding to that in the vicinity of a theoretical air-fuel ratio when temperature is below a given value. This enables prevention of a fluctuation in torque and stability of an engine through prevention of a fluctuation in an air-fuel ratio.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an air-fuel ratio Bi control device for an engine, and in particular to an air-fuel ratio Bi control device using an air-fuel ratio sensor whose output changes linearly in accordance with 11 degrees of oxygen in exhaust gas. The present invention relates to an engine in which the air-fuel ratio of an engine is feedback-controlled to a predetermined value.

(Prior Art) It is widely known that the air-fuel ratio of an engine is detected based on the oxygen concentration in exhaust gas of the engine, and the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback-controlled to a predetermined value.

In this case, the air-fuel ratio sensor that indirectly detects the air-fuel ratio by detecting the oxygen m IJi [in the exhaust gas] outputs (electromotive force) in steps at the oxygen concentration corresponding to the stoichiometric air-fuel ratio. change in shape. There is a so-called λ sensor. This λ sensor is suitable for controlling the air-fuel ratio to the stoichiometric air-fuel ratio due to its output characteristics, but it is also suitable for controlling the air-fuel ratio to the stoichiometric air-fuel ratio when high output is required, such as during acceleration or high-load operation. When setting the air-fuel ratio to be richer than the stoichiometric air-fuel ratio, or when setting the air-fuel ratio to the stoichiometric air-fuel ratio J: leaner to improve fuel efficiency during steady high-speed driving, only the magnitude relative to the stoichiometric air-fuel ratio is determined as described above. Therefore, it is not possible to accurately detect air-fuel ratios that deviate from the stoichiometric air-fuel ratio to the lean or rich side.
It is not suitable for 211.

Therefore, the present applicant proposed an air-fuel ratio sensor to replace the above-mentioned λ sensor, which has an output that changes linearly according to the degree of oxygen in the exhaust gas, as shown in Japanese Patent Laid-Open No. 59-100854. The air-fuel ratio can be detected continuously from the rich region to the lean region.

We have proposed so-called wide-range air-fuel ratio sensors, which make it possible to control the air-fuel ratio to any value. That is, this wide-range air-fuel ratio sensor has porous electrodes formed on both sides of an MNN ion-containing solid electrolyte, and a semi-catalyst mainly composed of Pt etc. as the porous electrode on the side that contacts the gas to be measured (exhaust gas). 5n02, which oxidizes HC and generates co, near the three-phase point consisting of the electrode, solid electrolyte, and gas to be measured.
It is formed by the presence of gold a oxide such as.

(Problems to be Solved by the Invention) However, due to its structure, the wide-range air-fuel ratio sensor as described above has a structure in which the catalytic activity of the porous electrode such as PT varies depending on the exhaust gas temperature, that is, the temperature of the wide-range air-fuel ratio sensor itself. Since the temperature is different, the electromotive force characteristics change due to temperature changes. (This also applies to the λ sensor mentioned above, but since the λ sensor only determines whether the air-fuel ratio is large or small from the stoichiometric air-fuel ratio, there is no problem even if the electromotive force characteristics change slightly. .)
In particular, when the wide-range air-fuel ratio sensor's ls1 degree <exhaust gas temperature) is low (for example, 350'C or less), the output characteristic approaches that of the λ sensor, and on the lean or rich side of the stoichiometric air-fuel ratio, the air-fuel ratio The electromotive force gradient of the fuel ratio sensor becomes extremely small. Therefore, when setting the air-fuel ratio to be leaner or richer than the stoichiometric air-fuel ratio, the electromotive force should be used to determine the air-fuel ratio. Accurate detection is not possible, and fluctuations in the air-fuel ratio are greatly amplified.
The accuracy of air-fuel ratio control deteriorates significantly, which causes torque fluctuations.

The present invention has been made in view of these points, and its purpose is to measure the temperature of the air-fuel ratio sensor (exhaust gas temperature).
is below a predetermined degree, the target air-fuel ratio is forcibly fixed near the stoichiometric air-fuel ratio where the electromotive force gradient of the air-fuel ratio sensor is large, thereby preventing torque fluctuations due to deterioration of air-fuel ratio control accuracy. It is in.

(Means for solving the problem) In order to achieve the above object, the solving means of the present invention is as follows:
As shown in the figure, an air-fuel ratio sensor 8 is provided in the exhaust passage of the engine, and its output changes depending on the oxygen level in the exhaust gas. a target value setting means 15 for setting a target value of the fuel ratio sensor 8; and a comparison means 1 for comparing the output of the air-fuel ratio sensor 8 with 11 standards set by the target value setting means 15.
6, and an air-fuel ratio control means 17 which receives the output of the comparison means 16 and controls the air-fuel ratio of the air-fuel mixture supplied to the engine to the above-mentioned IIA value.

In addition to this, a temperature detection means 10 for detecting the temperature of the air-fuel ratio sensor 8, and receiving the output of the temperature detection means 10,
When the temperature is below a predetermined value, the target Im setting means 15
The configuration includes target value fixing means 18 for fixing the target value from 1 to a value corresponding to around the stoichiometric air-fuel ratio.

(Function) With the above configuration, in the present invention, oxygen in the exhaust gas! 1
The output changes linearly depending on the temperature.

When feedback controlling the air-fuel ratio to a set value using a so-called wide-range air-fuel ratio sensor, when the tA degree of the air-fuel ratio sensor is below a predetermined value, the target value of the air-fuel ratio sensor is fixed to a value corresponding to around the stoichiometric air-fuel ratio. As a result, the electromotive force gradient of the air-fuel ratio sensor near the stoichiometric air-fuel ratio is large even at low temperatures, so the mvi degree is improved while suppressing fluctuations in the air-fuel ratio]! I! It will be controlled near the stoichiometric fuel ratio.

(Example) Hereinafter, an example of the present invention will be described based on the drawings from FIG. 2 onwards.

FIG. 2 shows a schematic configuration of an air-fuel ratio control system for an engine according to a practical example of the present invention, where 1 is an engine, 2 is an intake passage for supplying intake air to the engine 1, and 3 is an engine 1.
This is an exhaust passage for discharging exhaust gas from. The intake passage 2 has an intake air amount of 1 to be supplied to the engine 1.
A 1 liter valve 4 is disposed at the throttle valve 4, and a fuel injection valve 5 for injecting and supplying fuel to the engine 1 is disposed in the intake passage 2 downstream of the throttle valve 4.

Further, downstream of the throttle valve 4 in the intake passage 2, an air flow sensor 6 for detecting an intake air layer and an intake temperature sensor 7 for detecting the temperature of intake air are provided. On the other hand, in the exhaust passage 3, there is an air-fuel ratio sensor 8 that detects the air-fuel ratio based on 15 degrees of oxygen in the exhaust gas, and an air-fuel ratio sensor 8 that detects the air-fuel ratio based on 15 degrees of oxygen in the exhaust gas.
+-10) An HC sensor 9 for detecting 111111 and an exhaust temperature sensor 1o as fA degree detection means for detecting the temperature of the air-fuel ratio sensor 8 based on the exhaust gas temperature are provided, and each output of these sensors 6 to 10 is is input to the air-fuel ratio controller 11 that controls the fuel injection valve 5 υJtll. Further, 12 is a spark plug, 13 is an ignition coil, and 14 is an igniter 'C. An ignition signal from the igniter 14 is inputted to the air-fuel ratio controller 11 as an engine rotational speed signal or the like.

As mentioned above, the air-fuel ratio sensor 8 has porous electrodes formed on both sides of an oxygen ion conductive solid electrolyte, and the gas to be measured (
A porous electrode such as Pt having semi-catalytic performance is used as the porous electrode on the side that comes into contact with the exhaust gas), and near the three-phase point composed of the electrode, solid electrolyte, and the gas to be measured (exhaust gas), Oxidize HC and generate TCO 3n 02
, ln 203, Ni O, Qo 30a, Cn
It is formed by the presence of gold 7+i M compounds such as O, and its electromotive force characteristics are as shown in Figure 3. ! The so-called wide-range air-fuel ratio sensor has the basic characteristic of being able to continuously detect the air-fuel ratio from the rich region to the lean region by linearly changing the electromotive force as its output for each element.

It is. As shown in FIG. 4, the electromotive force characteristic of this air-fuel ratio sensor 8 has a temperature characteristic that changes depending on the S temperature of the air-fuel ratio sensor 8 (exhaust gas temperature), and as the temperature decreases, the stoichiometric air-fuel ratio On the lean side, the electromotive force increases, and on the rich side, the electromotive force decreases. When the temperature drops below a predetermined value (for example, 350'C), the electromotive force decreases overall, and the electromotive force decreases beyond the stoichiometric air-fuel ratio. Changes in steps. The characteristics are similar to the electromotive force characteristics of a so-called λ sensor. In addition, the electromotive force characteristic of the air-fuel ratio sensor 8 has an 801 m characteristic that changes depending on the degree of HCl in the exhaust gas, and the electromotive force increases as HCI m increases on the lean side of the stoichiometric air-fuel ratio (
Note that on the rich side, there is almost no change in the electromotive force even though HCl is originally high at 11 degrees).

Next, the operation of the air-fuel ratio controller 11 will be explained with reference to the flowchart shown in FIG. 5. After resetting, in step S1, a zone flag F zone (
+1 Q n on the lean side, 1111+ on the rich side, and set the delay flags Fl and F on the lean side and rich side to distinguish whether the fuel injection is delayed or not.
r (II Q 11 when not in delay, "'1°゛" during delay) together with "OII", and feedback factor j that determines the relationship between engine speed and injection time.
l Cfb is initialized to ``1pa,'' and in step S2, a basic timer that determines a constant cycle for calculating the engine speed, etc. is reset, and then in the next step S3.
The basic timer is allowed to elapse for a certain period of time Ti, and when the certain period of time Ti has elapsed, the basic timer is reset again in step S4. Note that this basic timer is a counter that counts up the time from the moment it is reset.

Next, in step S5, the engine rotation speed Ne is calculated based on the ignition pulse signal from the igniter 14, and in step Se, the intake air flow ff1Ue is calculated based on the signals from the L aflow sensor 6 and the intake air temperature sensor 7.
Zuru.

Next, in step S7, the electromotive force VS signal as an output signal from the air-fuel ratio sensor 8 and the HC signal from the 1-1c sensor 9 are
The 8a degree signal and the exhaust gas temperature signal from the exhaust temperature sensor 10 (air-fuel ratio sensor salt e, (No. 8) are input.

Roughly speaking, it is determined in step S8 whether the exhaust gas temperature is below a predetermined value (for example, 350° C.), and when the exhaust gas temperature is above a predetermined value (NO), the target air-fuel ratio is set in the next step S9. By inputting the HCll111m and the exhaust gas temperature into a data table as shown in FIG. 6, the slice level center line Vre4 as the target value of the air-fuel ratio sensor 8 corresponding to the target air-fuel ratio is determined, and the slice level center line Vre4 as the target value is calculated. Slice level center 111Vref
Lean side and rich side dead band width VhQ, V
Find hr.

Here, the target air-fuel ratio is set according to the engine operating state using the engine speed and engine load. For example, during high-load operation, the target air-fuel ratio A/F is set to the stoichiometric air-fuel ratio (
A/F=14.7) is set to be richer than the stoichiometric air-fuel ratio, and during high-speed steady driving, the air-fuel ratio is set to be leaner than the stoichiometric air-fuel ratio. 1. In the data table shown in Fig. 6 above, the slice level median value V ref corresponding to the exhaust gas concentration and the 1-IC section is included for each target air-fuel ratio, and the slice level median value V ref is included in the data table in FIG. On the other hand, on the rich side of the stoichiometric air-fuel ratio (A/F-14,7), y rat increases as the temperature rises, and on the lean side, as the temperature rises, V ref decreases, and the theoretical Regarding the air-fuel ratio, V ref is almost constant as the temperature changes.

In addition, for 1-IC concentration change, the stoichiometric air-fuel ratio (A/F-1
4, 7) On the lean side, as the HCC variation increases, v
raf increases, and at the stoichiometric air-fuel ratio and richer side than H (41m), V ref remains almost constant against the change in H (41m).
Dead band width (that is, hysteresis width) VhlVh
r is set to eliminate the influence of noise on the output (electromotive force) of the air-fuel ratio sensor 8, and changes according to the slice level center I71 V ref, that is, the target air-fuel ratio, and reaches a maximum near the stoichiometric air-fuel ratio. -11 It becomes smaller as the air-fuel ratio becomes leaner or richer than the stoichiometric air-fuel ratio.

Thereafter, in the following step S+a-830, the air-fuel ratio is set to a target with a predetermined dead zone based on the correspondence between the output characteristics of the air-fuel ratio sensor 8 and the average fuel injection amount from the fuel injection valve 5 as shown in FIG. Feedback control is executed to adjust the air-fuel ratio. That is, first, in order to determine the dead zone (hysteresis) of the target electromotive force of the air-fuel ratio sensor 8 for noise resistance, the zone flag F is set in step S1e.
Determine whether the zone is “OII” or “1P”, 70n
When e=Q is on the lean side, the lean side dead zone width vh 11 is calculated in step 5IIT with respect to the slice level median value Vrcf obtained in step S9.
When one=1, on the rich side, step S!
] Based on the rich-side dead band width Vhr for the slice level median value ref found in step S+2't', the slice level median value V'ref is set as ■ref-17hr,
The process then proceeds to step S13. And step S
13, the actually measured electromotive force VS from the air-fuel ratio sensor 8 and the above step S.

Or the slice level median value v'ref determined in S12
Compare and determine the size.

When the determination in step S+3'r is that Vs≧V'ref, the zone flag F zone is determined in step S14, and when Fzone-1 is on the rich side, it is determined that the air-fuel ratio is richer than the target value. Step S
+s to make the air-fuel ratio leaner, that is, to reduce the fuel injection amount, the feedback coefficient Cfb is changed to Cfb-Cr (Cr:
(integration constant), and in step S+s, the fuel injection time τ is calculated using the formula K - Cfb - Ue /Ne, and in step S3
Return to

After that, as the fuel injection amount decreases in step S16, the air-fuel ratio becomes lean as shown in FIG. 7, and when the determination in step S13 becomes Vs <V'rer,
In step S17, the zone flag F zone is determined, and since it is still on the rich side with F zone = 1,
In the next step S +a, lean side delay flag Fil
is 1′.

If FG-0 is NO, it is determined that the change has occurred from the rich side to the lean side, and step S +q is performed.
After setting the delay flag Fl to "1", step S
? Reset the delay timer with e (This delay timer, like the basic timer mentioned above, is a timer that counts up the time from the moment it is reset.) Then, when F9 = 1 (YES) and the delay is in progress, the next In step S21, the delay timer is set to 1 when the predetermined delay is reached.
mtd! It is determined whether Q has elapsed or not, and if it has not elapsed, step S I5 is performed to prevent the influence of noise.
Then, the feedback coefficient Cfb is maintained at Cfb-(:, r, and the process returns to Step S3 while decreasing the fuel injection m in Step S16. On the other hand, when the delay time tdR has elapsed, the zone flag Fz' is set in Step S22. on
After e is set to "OII" and the delay flag F9 is set to "0"', the feedback coefficient Cfb is set to (jb+cs 9 (
C3It: proportionality constant), the fuel injection tJJffl is increased in step S+s, and the process returns to step S3.

Next, even with this increase in the fuel injection amount, the determination in step S13 is still VS < In order to enrich the feedback coefficient Cfb, Cfb+(,Q
(C1: integral constant), the fuel injection amount is further increased in step S16, and the process returns to step S3.

Thereafter, due to this increase in fuel injection amount, step S13
The determination in step S1 is VS, ≧v'ref.
Since the judgment in step 4 is that the zone flag l: zoba -0 is on the lean side, it is judged in step S25 whether the rich side delay flag Fr is on the lean side. After determining that it has been reversed to the rich side, the delay flag F is set to 1 in step S2s, and then the delay timer is reset in step 82F. Then, during the delay when F=1 is YES, the delay timer determines whether or not a predetermined gay lay time tdr has elapsed in the next step 82B, and if it has not elapsed, a step is executed to prevent the influence of noise. The program moves to S24, keeps the feedback coefficient Cfb at Qfb+01, and returns to step S3 while increasing the fuel injection tJJm at step S+g.On the other hand, when the delay time tdr has elapsed, the zone flag Fzone is set at step 329.
'1'.

and after setting the delay flag l”r to “0'',
In step Si, the air-fuel ratio is
t feedback coefficient Cfb@Cfb −Csr (C
sr: proportionality constant), the fuel injection amount is decreased in step S+s, and the process returns to step S3. The bond, step S +s determination is Vs≧y'rer, step SI4
The determination is F zonO-1, and the same operation as above is repeated thereafter.

As shown in FIG. 8, the injection timing of the fuel injection valve 5 is interrupted during the main flow of the air-fuel ratio controller 11 by the rise of the ignition pulse from the igniter 14, and the injection timer is first set to the fuel injection time τ. (Note that this injection timer is a counter that counts down the set time and generates an interjected injection end signal at the moment it reaches zero).
The current to the fuel injection valve 5 is turned ON to start fuel injection. Then, the fuel injection is terminated by being interlaced with the +C9A termination interlaced signal from the injection timer, as shown in FIG. 9, and the current to the fuel injection valve 5 is turned off. On the other hand, if the exhaust gas temperature is below the predetermined value (YES in step S8), the next step S
At 3+, the exhaust gas temperature and l-I C temperature are
The slice level median value V ref corresponding to the vicinity of the stoichiometric air-fuel ratio at that time is fixed without changing pjpl, and the dead band width Vh 9 is fixed correspondingly to Vhre+FII stoichiometric air-fuel ratio. After that, steps 810-830 are performed in the same way as when the exhaust gas intake is equal to or higher than the predetermined value.
In this step, feedback control is executed to bring the air-fuel ratio to the target air-fuel ratio (stoichiometric air-fuel ratio). That is, in this control, as shown in FIG.
), which approaches the electromotive force characteristic of a so-called λ sensor, which changes in a step-like manner as the boundary of slice level 1.
+1IVref is where the electromotive force gradient of the air-fuel ratio sensor 8 is large, and the air-fuel ratio is controlled to be close to the stoichiometric air-fuel ratio with good control accuracy.

Therefore, in the operation flow of the air-fuel ratio controller 11, in step Ss, the target value (slice level median value Vref) of the air-fuel ratio sensor 8 corresponding to the preset air-fuel ratio of the air-fuel mixture is set. Setting means 15
It consists of Further, in step S13, the output of the air-fuel ratio sensor 8 (electromotive force VS) and the target value 52
The target value set by 5 (slice level median value v'
ref). Furthermore, in steps 814 to S30, the comparison means 1
6, the air-fuel ratio control means 17 controls the air-fuel ratio of the air-fuel mixture supplied to the engine 1 by controlling the fuel injection amount of the fuel injection valve 5. . Also, steps S8 and 831. When the temperature of the air-fuel ratio sensor 8 is below a predetermined value, the exhaust gas temperature and H
The target value fixing means 18 is configured to fix the slice level center 1fflVrc4 as a target value from the target value setting means 15, its dead zone width Vh intersection, and Vhr to a value corresponding to the vicinity of the stoichiometric air-fuel ratio without considering the C concentration. ing.

Therefore, in the above embodiment, the air-fuel ratio is detected by the air-fuel ratio sensor 8 whose output (electromotive force) changes depending on the m degrees of oxygen in the exhaust gas of the engine 1, and the air-fuel ratio is The ratio m* of the air-fuel ratio sensor 8 corresponding to the set air-fuel ratio is compared, and the injection of fuel F4I+# from the fuel++ntJ4 valve 5 is controlled according to the deviation, thereby supplying the fuel to the engine 1. The air-fuel ratio of the air-fuel mixture is feedback-controlled to the target value.

In this case, 1liil! of the air-fuel ratio sensor 8! 1 (exhaust gas conduction) and l-I Cm v in the exhaust gas, the electromotive force characteristics of the air-fuel ratio sensor 8 change as shown in FIG. The median value) is corrected according to the above-mentioned temperature and ffHCl degree, and corresponds to the change in the electromotive force characteristic, so that the @ degree of the air-fuel ratio sensor 8 and the HC'a in the exhaust gas
The deviation of the air-fuel ratio with respect to tx is compensated for, and air-fuel ratio control using the wide-range air-fuel ratio sensor 8 can be performed accurately.

Further, when the exhaust gas temperature is below a predetermined value, the electromotive force characteristic of the air-fuel ratio sensor 8 changes in a stepwise manner with the stoichiometric air-fuel ratio as a boundary. By being fixed at a value corresponding to the stoichiometric air-fuel ratio, the target value becomes a location where the electromotive force gradient of the air-fuel ratio sensor 8 is large, suppressing fluctuations in the air-fuel ratio, and bringing the air-fuel ratio close to the stoichiometric air-fuel ratio. can be well controlled,
It is possible to prevent torque fluctuations from occurring. Moreover, since the air-fuel ratio is brought to near the stoichiometric air-fuel ratio,
For example, at low engine speeds and low loads when the exhaust gas temperature is low, the operating state of the engine 1 can be prevented from becoming unstable by making the air-fuel ratio leaner.

In addition, in the above embodiment, in the fuel injection method, the fuel Fl
Although the air-fuel ratio was controlled by the injection suspension ill 'In,
In the carburetor system, the air-fuel ratios i and IIH may be determined by controlling the amount of upper bleed.

(Effects of the Invention) As explained above, according to the present invention, the air-fuel ratio of the engine is feedback-controlled to the set air-fuel ratio using an air-fuel ratio sensor whose output changes according to the oxygen concentration in the exhaust gas of the engine. In this case, when the temperature of the air-fuel ratio sensor (exhaust gas temperature) is below a predetermined value, the target value of the air-fuel ratio sensor is fixed at a value corresponding to around the stoichiometric air-fuel ratio, so fluctuations in the air-fuel ratio are suppressed ( The air-fuel ratio can be precisely controlled to near the stoichiometric air-fuel ratio, thereby preventing torque fluctuations and improving engine stability during low-speed, low-load operation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the present invention. 2 to 9 illustrate embodiments of the present invention, FIG. 2 is a schematic diagram of the air-fuel ratio control system of the engine, and FIGS. 3 and 4 show the electromotive force characteristics of the air-fuel ratio sensor, respectively. Characteristic diagram showing basic characteristics and temperature characteristics, Figure 5 is a flowchart diagram showing the operation of the air-fuel ratio controller, Figure 6 is a diagram showing an example of a data table, Figure 7 is the output characteristic of the air-fuel ratio sensor and average fuel injection FIG. 8 and FIG. 9 are explanatory diagrams showing the correspondence relationship with m, and are diagrams showing interwoven processing at the start and end of fuel injection, respectively. DESCRIPTION OF SYMBOLS 1... Engine, 3... Exhaust passage, 5... 5tij valve for fuel, 8... Air-fuel ratio sensor, 10... Exhaust waterfall sensor, 11... Air-fuel ratio controller, 15... Target value setting means, 16... Comparison means, 17... Air-fuel ratio 1-control means, 18... Target value fixing means. Figure 3 Air-fuel ratio (A/F) Electromotive force (V) Figure 4 Air-fuel (A/F)

Claims (1)

[Claims] (1) An air-fuel ratio sensor that is installed in the exhaust passage of the engine and whose output changes linearly according to the oxygen concentration in the exhaust gas, and an air-fuel ratio sensor that corresponds to a preset air-fuel ratio of the air-fuel mixture. target value setting means for setting a target value; comparison means for comparing the output of the air-fuel ratio sensor with the target value set by the target value setting means; and an air-fuel mixture that receives the output of the comparison means and supplies to the engine. an air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel ratio to the target value; a temperature detection means for detecting the temperature of the air-fuel ratio sensor; 1. An air-fuel ratio control device for an engine, comprising target value fixing means for fixing a target value from the means to a value corresponding to near the stoichiometric air-fuel ratio.