JPH0448933B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an air-fuel ratio control device for an engine.
In particular, the present invention relates to an engine in which the air-fuel ratio of an engine is feedback-controlled to a predetermined value using an air-fuel ratio sensor whose output changes approximately linearly in accordance with the oxygen concentration in exhaust gas.

(Prior Art) It is widely known that the air-fuel ratio of the engine is detected based on the oxygen concentration in the 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, which indirectly detects the air-fuel ratio by detecting the oxygen concentration in the exhaust gas, outputs (electromotive force) in steps at the oxygen concentration corresponding to the stoichiometric air-fuel ratio. There are so-called λ sensors that vary. This λ sensor is suitable for controlling the air-fuel ratio to the stoichiometric air-fuel ratio due to its output characteristics, but when high output is required, such as during acceleration or high-load operation, the air-fuel ratio is lower than the stoichiometric air-fuel ratio. When setting the air-fuel ratio to be richer, or when setting the air-fuel ratio leaner than the stoichiometric air-fuel ratio to improve fuel efficiency during steady high-speed driving, only the magnitude relative to the stoichiometric air-fuel ratio is determined as described above. , it is not possible to accurately detect air-fuel ratios that deviate from the stoichiometric air-fuel ratio to the lean or rich side, and it is not suitable for controlling the air-fuel ratio to an arbitrary value.

Therefore, the present applicant developed an air-fuel ratio sensor to replace the above-mentioned λ sensor, as shown in Japanese Patent Application Laid-Open No. 59-100854, in which the output changes almost linearly according to the oxygen concentration in the exhaust gas. We have proposed a so-called wide-range air-fuel ratio sensor that can continuously detect the fuel ratio from a linear region to a lean region, making it possible to control the air-fuel ratio to an arbitrary value. In other words, this wide-range air-fuel ratio sensor has porous electrodes formed on both sides of an oxygen ion conductive solid electrolyte, and the porous electrode on the side that comes into contact with the gas to be measured (exhaust gas) has _Pt etc. as its main component. In addition to using a material with semi-catalytic performance, a metal oxide such as SnO ₂ that oxidizes HC to generate CO is present near the three-phase point consisting of the electrode, solid electrolyte, and gas to be measured. That's what happens.

(Problem to be Solved by the Invention) By the way, when the air-fuel ratio of the engine is feedback-controlled to a predetermined value according to the operating state of the engine with a predetermined control gain using the wide-range air-fuel ratio sensor as described above, the wide-range air-fuel ratio The output (electromotive force) characteristics of the sensor are such that the electromotive force gradient (slope) is maximum at the stoichiometric air-fuel ratio (A/F = 14.7), and the electromotive force gradient increases as you move toward the lean side and rich side from this stoichiometric air-fuel ratio. As shown in Figure 3, the control response is good near the stoichiometric air-fuel ratio where the electromotive force gradient is large, but when the electromotive force is leaner or richer than the stoichiometric air-fuel ratio, Since the slope is gentle, there is a problem in that the air-fuel ratio fluctuates greatly and the control response is poor.

In addition, looking at the detection response of the wide-range air-fuel ratio sensor itself, it is found that it differs between richer and leaner sides than the stoichiometric air-fuel ratio, and on the rich side, unburned components in the exhaust gas
Since the ratio of HC and CO is high, when HC and CO are adsorbed and desorbed by the wide range air-fuel ratio sensor, the desorption is not done quickly and it takes time, so the detection response is poor compared to the lean side. (See FIG. 4), there is a problem in that the accuracy of air-fuel ratio control is reduced.

The present invention has been made in view of the above points, and its purpose is to perform feedback control of the air-fuel ratio to a target air-fuel ratio according to the engine operating state with a predetermined control gain using a wide-range air-fuel ratio sensor. By changing this target gain in accordance with the electromotive force gradient characteristic of the air-fuel ratio sensor with respect to the target air-fuel ratio, control responsiveness is improved and the air-fuel ratio can be accurately feedback-controlled over the entire operating range.

(Means for Solving the Problems) In order to achieve the above object, the solving means of the present invention is provided in the exhaust passage of an engine, as shown in FIG. an air-fuel ratio sensor 8 whose output changes almost linearly;
Operating state detecting means 15 for detecting the operating state of the engine; and receiving the output of the operating state detecting means 15;
A target air-fuel ratio setting means 16 sets a target value of the air-fuel ratio of the air-fuel mixture to be supplied to the engine according to the operating state of the engine, and receives the output of the target air-fuel ratio setting means 16 and the output of the air-fuel ratio sensor 8. , a comparison means 18 for comparing both outputs, and an air-fuel ratio control means 19 for receiving the output of the comparison means 18 and controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to the target air-fuel ratio with a predetermined control gain. This is the basic structure.
In addition, a control gain changing means 17 is provided for changing the control gain of the air-fuel ratio control means 19 according to the electromotive force gradient characteristic of the air-fuel ratio sensor with respect to the target air-fuel ratio from the target air-fuel ratio setting means 16. That is. Here, in the control gain changing means 17, it is preferable that the control constant in the region where the target air-fuel ratio is richer than the stoichiometric air-fuel ratio is set smaller than the control constant in the region where the target air-fuel ratio is leaner.

(Function) With the above configuration, the present invention uses a so-called wide-range air-fuel ratio sensor whose output changes almost linearly according to the oxygen concentration in exhaust gas to control the air-fuel ratio to the engine operating state with a predetermined control gain. When performing feedback control to the target air-fuel ratio according to the target air-fuel ratio, the control gain in the feedback control is changed according to the target air-fuel ratio, and the electromotive force gradient is large near the stoichiometric air-fuel ratio where the control response is good, for example, the stoichiometric air-fuel ratio. It decreases as the electromotive force gradient becomes gentler and the control response becomes worse, preferably on the rich side where the air-fuel ratio sensor itself has poor detection response. The control constant on the lean side becomes smaller than the control constant on the lean side. As a result, the control gain at that time corresponds well to the control response or detection response of the air-fuel ratio sensor at each target air-fuel ratio depending on the engine operating state, and the control gain at that time corresponds well to the control response or detection response of the air-fuel ratio sensor at each target air-fuel ratio depending on the engine operating state. It becomes possible to accurately feedback control the air-fuel ratio to the target air-fuel ratio in the operating range.

(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 engine air-fuel ratio control system according to an embodiment of the present invention, in which 1 is an engine, 2 is an intake passage for supplying intake air to the engine 1, and 3 is an exhaust gas from the engine 1. This is an exhaust passage for discharging. A throttle valve 4 for controlling the amount of intake air supplied to the engine 1 is disposed in the intake passage 2, and a fuel injection valve 5 for injecting fuel to the engine 1 is disposed in the intake passage 2 downstream of the throttle valve 4. It is arranged.

Further, upstream of the throttle valve 4 in the intake passage 2, an air flow sensor 6 for detecting the amount of intake air and an intake temperature sensor 7 for detecting the temperature of intake air are provided. On the other hand, the exhaust passage 3 includes an air-fuel ratio sensor 8 that detects the air-fuel ratio based on the oxygen concentration in the exhaust gas, an HC sensor 9 that detects the hydrocarbon (HC) concentration in the exhaust gas, and an air-fuel ratio sensor 9 that detects the air-fuel ratio based on the exhaust gas temperature. An exhaust temperature sensor 10 is provided to detect the temperature of the sensor 8, and the outputs of these sensors 6 to 10 are input to an air-fuel ratio controller 11 that controls the fuel injection valve 5. Also, 12 is a spark plug, 13 is an ignition coil, 14
is an igniter, and the 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 porous electrode on the side that comes into contact with the gas to be measured (exhaust gas) is made of semi-catalytic material such as Pt. In addition, near the three-phase point consisting of the electrode, solid electrolyte, and gas to be measured (exhaust gas),
SnO ₂ , In ₂ O ₃ , which oxidizes HC to generate CO
It is made up of metal oxides such as NiO, Co ₃ O ₄ and CnO, and its electromotive force characteristics vary depending on the oxygen concentration in the exhaust gas, as shown in Figure 3. It is a so-called wide-range air-fuel ratio sensor that has a basic characteristic of linearly changing the air-fuel ratio and continuously detecting the air-fuel ratio from a rich region to a lean region. Although the air-fuel ratio sensor 8 has a nearly linear electromotive force characteristic, a closer look shows that the electromotive force gradient (slope) near the stoichiometric air-fuel ratio (A/F = 14.7) as shown in Figure 3. At maximum, the electromotive force gradient becomes gentler as it goes richer or leaner than the stoichiometric air-fuel ratio, so if the control gain in feedback control is constant, good control response is achieved near the stoichiometric air-fuel ratio. On the other hand,
On the richer or leaner side, control responsiveness tends to deteriorate. Moreover, as shown in Fig. 4, when looking at the electromotive force oscillation frequency of the air-fuel ratio sensor 8 during feedback when the control gain is constant, that is, the control frequency indicating detection response, it is richer than the stoichiometric air-fuel ratio. On the side, HC in the exhaust gas,
Since the proportion of CO is high, when HC and CO adsorb and desorb to the air-fuel ratio sensor 8, the desorption is not done quickly and takes time, so the detection response is poor compared to the lean side. Show characteristics. Also,
The electromotive force characteristic of this air-fuel ratio sensor 8 has a temperature characteristic that changes depending on the temperature of the air-fuel ratio sensor 8 (exhaust gas temperature), and as the temperature becomes higher, the electromotive force increases on the lean side of the stoichiometric air-fuel ratio. On the rich side, the electromotive force increases. Further, the electromotive force characteristic of the air-fuel ratio sensor 8 has an HC concentration characteristic that changes depending on the HC concentration in the exhaust gas, and the electromotive force increases as the HC concentration increases on the lean side of the stoichiometric air-fuel ratio ( In addition, since the HC concentration is originally high on the rich side, almost no change in electromotive force occurs).

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 _S11 , a zone flag Fzone (lean side "0" on the rich side and "1" on the rich side) to "0", and the delay flags Fl, Fr on the lean side and rich side to distinguish whether or not fuel injection is in delay mode (when not in delay mode,
The feedback coefficient Cfb, which determines the relationship between the engine speed and the injection time, is initially set to "1". Furthermore, in step _S2 , the basic timer that determines a fixed cycle for calculating engine speed, etc. is reset, and in the next step _S3 , the basic timer waits for a certain period of time Ti to elapse, and when the certain period of time Ti has elapsed, the basic timer is reset. In step _S4 , the basic timer is reset again. 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 speed Ne is determined by the ignition pulse signal from the igniter 14.
Calculate the airflow sensor ₆ in step S6.
The intake air flow rate Ue is calculated based on the signal from the intake air temperature sensor 7, and the operating state of the engine 1 is detected.

Next, in step _S7 , the electromotive force Vs signal as an output signal from the air-fuel ratio sensor 8, the HC concentration signal from the HC sensor 9, and the exhaust gas temperature signal from the exhaust temperature sensor 10 (air-fuel ratio sensor temperature signal) are input. Thereafter, in step _S8 , the target air-fuel ratio, HC concentration, and exhaust gas temperature according to the engine operating state are input into a data table as shown in FIG. 6, and the target air-fuel ratio sensor 8 corresponding to the target air-fuel ratio is determined. The slice level median value Vref as a value is determined, and the dead band widths Vhl and Vhr on the lean side and the rich side with respect to the slice level median value Vref as the target value are determined. Furthermore, the steps
In _S9 , integral constants Cl, Cr, proportional constants Csl, Csr, and delay times tdl, tdr in feedback control, which will be described later, are determined based on the target air-fuel ratio.

Here, the target air-fuel ratio is set depending on the engine operating state, for example, based on engine speed and engine load. For example, during high-load operation, the target air-fuel ratio A/F is richer than the stoichiometric air-fuel ratio (A/F = 14.7). In addition, during high-speed steady driving, the air-fuel ratio is set to be leaner than the stoichiometric air-fuel ratio. The data table in Figure 6 above also includes slice level median value Vref according to exhaust gas temperature and HC concentration for each target air-fuel ratio.
is written, and for the exhaust gas temperature, Vref increases as the temperature rises on the rich side with the stoichiometric air-fuel ratio (A/F = 14.7) as the boundary, and as the temperature rises on the lean side. Vref decreases, and at the stoichiometric air-fuel ratio, Vref remains almost constant with respect to temperature changes. In addition, for the HC concentration, the stoichiometric air-fuel ratio (A/
On the leaner side than F=14.7), Vref increases as the HC concentration increases, and at the stoichiometric air-fuel ratio and on the richer side, Vref remains almost constant with respect to changes in HC concentration. Furthermore, the dead band widths (that is, hysteresis widths) Vhl and Vhr for the median slice level Vref are set to eliminate the influence of noise on the output (electromotive force) of the air-fuel ratio sensor 8, and correspond to the target air-fuel ratio. The slice level changes according to the median slice level Vref, and is maximum at the stoichiometric air-fuel ratio and decreases as the air-fuel ratio becomes leaner or richer than the stoichiometric air-fuel ratio.

In addition, the integral constants Cl and Cr, which are one of the control constants for the target air-fuel ratio, are obtained from the map shown in Fig. 7, and Cl and Cr are maximum near the stoichiometric air-fuel ratio (A/F = 14.7). The air-fuel ratio is set to become smaller as it goes richer or leaner than the stoichiometric air-fuel ratio, and is smaller on the rich side than on the lean side of the stoichiometric air-fuel ratio. In addition, the proportionality constants Csl and Csr for the target air-fuel ratio are obtained from the map shown in Fig. 8, and have the same characteristics as the above-mentioned integral constants Cl and Cr, that is, they are maximum near the stoichiometric air-fuel ratio and are richer than the stoichiometric air-fuel ratio. Alternatively, it is set so that it becomes smaller as it goes to the lean side, and is smaller on the rich side than on the lean side than the stoichiometric air-fuel ratio. Furthermore, the delay time for the target air-fuel ratio
tdl and tdr are determined by the MAKUP shown in FIG. 9, and are set to have the same characteristics as the integral constants Cl and Cr and the proportionality constants Csl and Csr.

After that, in the following steps _S10 to _S30 ,
As shown in FIG. 10, 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 is used to provide feedback so that the air-fuel ratio can reach the target air-fuel ratio within a predetermined dead zone and a predetermined control gain. Control is executed. That is, in order to determine the dead zone (hysteresis) of the target electromotive force of the air-fuel ratio sensor 8 for noise resistance, first, in step _S10 , it is determined whether the zone flag Fzone is "0" or "1".
When Fzone = 0 on the lean side, perform step S ₈ above.
Based on the lean side dead zone width Vhl for the slice level median value Vref found in step _S11 , the slice level median value V′ref is set to Vref + Chl, and Fzone=
If it is on the rich side of 1, the slice level median value V'ref is set to Vref - Vhr in step _S12 using the rich side dead zone width Vhr for the slice level median value Vref obtained in step _S8 , and the process proceeds to step _S13 . Then, in step _S13 , the actually measured electromotive force Vs from the air-fuel ratio sensor 8 and the above step
Median slice level determined by S ₁₁ or S ₁₂
Compare and determine the size with V′ref.

If the determination in step _S13 is that Vs≧V'ref, the zone flag Fzone is determined in step _S14 , and if Fzone=1, which is on the rich side, it is determined that the air-fuel ratio is richer than the target air-fuel ratio. Then, in step _S15 , the feedback coefficient Cfb is set to Cfb-Cr in order to make the air-fuel ratio leaner, that is _, to reduce the fuel injection amount, based on the integral constant Cr corresponding to the target air-fuel ratio determined in step S9. At _{step 16} , the fuel injection time τ is calculated using the formula K.Cfb.Ue/Ne, and the process returns to step _S3 .

After that, as the fuel injection amount decreases in step _S16 , the air-fuel ratio becomes lean as shown in FIG. 10, and when the determination in step _S13 becomes Vs<V'ref,
In step _S17 , the zone flag Fzone is determined.
Since it is still on the rich side with Fzone = 1, it is determined in the next step _S18 whether the lean side delay flag Fl is "1" or not. Decide and step
After setting the delay flag Fl to "1" in _S19 , the delay timer is reset in step _S20 (in addition,
This delay timer is similar to the basic timer mentioned above.
(This is a timer that counts up the time from the moment it is reset.) Then, during the delay of YES with Fl=1, in the next step _S21 , the delay timer determines whether a predetermined delay time tdl has elapsed according to the target air-fuel ratio obtained in the above step _S9 , If the fuel injection amount has not elapsed yet, the process moves to step _S15 to maintain the feedback coefficient Cfb at Cfb-Cr in order to prevent the influence of noise, and returns to step _S3 with the fuel injection amount reduced in step _S16 . on the other hand,
When the delay time tdl has elapsed, the zone flag Fzone is set to "0" and the delay flag is set to "0" in step _S22 .
After setting Fl to “0”, in step _S23 ,
In order to enrich the air-fuel ratio based on the proportionality constant Csl corresponding to the target air-fuel ratio obtained in step _S9 above, the feedback coefficient Cfb is set as Cfb + Csl, and the step
The fuel injection amount is increased in _S16 and the process returns to step _S3 .

Next, even with this increase in the fuel injection amount, the determination in step _S13 is still that Vs<V'ref, so in step _S17 it is determined that the zone flag Fzone is on the lean side of 0, and in step _S24 , In order to further enrich the air-fuel ratio, the feedback coefficient Cfb is set to Cfb+Cl based on the integral constant Cl corresponding to the target air-fuel ratio obtained in step _S9 , and the fuel injection amount is further increased in step _S16 , and the process returns to step _S3 . .

After that, this increase in fuel injection amount causes the step
The determination in S ₁₃ is Vs≧V′ref, but step S ₁₄
Since the judgment is on the lean side with the zone flag Fzone = 0, it is determined in step _S25 whether the rich side delay flag Fr is "1" or not, and if it is NO with Fr = 0, it is reversed from the lean side to the rich side. Judging that this is the case, set the day delay flag Fr to “1” in step _S26 .
After that, the day delay timer is reset in step _S27 . Then, during the delay of YES with Fr=1, in the next step _S28 , the delay timer determines whether or not a predetermined delay time tdr corresponding to the target air-fuel ratio obtained in the above step _S9 has elapsed. If the feedback coefficient Cfb has not elapsed, the process moves to step _S24 to prevent the influence of noise.
+Cl and returns to step _S3 while increasing the fuel injection amount in step _S16 . On the other hand, when the delay time tdr has elapsed, the zone flag is set in step _S29 .
After Fzone is set to "1" and the delay flag Fr is set to "0", in step _S30 , a proportional constant is determined according to the target air-fuel ratio obtained in step _S9 above.
In order to make the air-fuel ratio lean based on Csr, the feedback coefficient Cfb is set to Cfb - Csr, and step _S16
Decrease the fuel injection amount and return to step _S3 . Thereafter, the determination in step _S13 is that Vs≧V'ref, and the determination in step _S14 is that Fzone=1, and the same operation as above is repeated thereafter.

Note that the injection timing of the fuel injection valve 5 is the 11th injection timing.
As shown in the figure, the rise of the ignition pulse from the igniter 14 interrupts the inflow of the air-fuel ratio controller 11, and first the injection timer is set to the fuel injection time τ (note that this injection timer is (This is a counter that counts down the time that has elapsed and generates an injection end interrupt signal, which will be described later, at the moment it reaches zero.) After that, the current to the fuel injection valve 5 is turned on to start fuel injection. Then, the end of the fuel injection is interrupted by the injection end interrupt signal from the injection timer, as shown in FIG. 12, and the current to the fuel injection valve 5 is turned off.

Therefore, in the operation flow of the air-fuel ratio controller 11, steps _S5 and _S6 constitute the operating state detection means 15 that detects the operating state of the engine 1. Furthermore, step _S8 constitutes a target air-fuel ratio setting means 16 which receives the output of the operating state detecting means 15 and sets a target value of the air-fuel mixture to be supplied to the engine 1 in accordance with the engine operating state. Furthermore, in step _S9 , the integral constants Cl, Cr, proportionality constants Csl, Csl,
The control gain is changed by changing Csr and delay times tdl and tdr, and the control gain is increased when the target air-fuel ratio is near the stoichiometric air-fuel ratio, and is decreased as the target air-fuel ratio becomes leaner or richer. The control gain changing means 17 is configured such that the air-fuel ratio is smaller on the rich side than on the lean side. Also, in step _S13 , the air-fuel ratio sensor 8
output (electromotive force Vs) and target air-fuel ratio setting means 16
(slice level median value V'ref as a target air-fuel ratio). Further, in steps _S14 to _S30 , the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is adjusted to the target level with a predetermined control gain by receiving the output of the comparing means 18 and controlling the fuel injection amount of the fuel injection valve 5. It constitutes an air-fuel ratio control means 19 that controls the air-fuel ratio.

Therefore, in the above embodiment, the air-fuel ratio is detected by the air-fuel ratio sensor 8 whose output (electromotive force) changes almost linearly according to the oxygen concentration in the exhaust gas of the engine 1; The output of the engine is compared with a target value (slice level median value) corresponding to the target air-fuel ratio set according to the engine operating state, and the amount of fuel injected from the fuel injection valve 5 is controlled according to the deviation. As a result, the air-fuel ratio of the air-fuel mixture supplied to the engine 1 is feedback-controlled to the target air-fuel ratio with a predetermined control gain.

In this case, the air-fuel ratio sensor 8 has an electromotive force characteristic that changes almost linearly as shown in FIG. As the air-fuel ratio becomes leaner or richer than the stoichiometric air-fuel ratio, the electromotive force gradient becomes gentler and the control response becomes worse. It has a characteristic that the detection response on the lean side is worse than the detection response on the lean side.
On the other hand, the control gains (integral constants Cl, Cr, proportionality constants Csl, Csr,
delay time tdl, tdr), control gain changing means 1
7, the target air-fuel ratio is changed according to the target air-fuel ratio, and the target air-fuel ratio is maximum near the stoichiometric air-fuel ratio (A/F = 14.7), and decreases as it goes leaner and richer than the stoichiometric air-fuel ratio. By setting the fuel ratio on the rich side to be smaller than on the lean side, the control gain at that time becomes smaller with respect to the control response and detection response of the air-fuel ratio sensor 8 at each target air-fuel ratio according to the engine operating state. As a result, the air-fuel ratio can be accurately and accurately feedback-controlled to the target air-fuel ratio in the entire operating range without causing hunting, etc., and the air-fuel ratio can be controlled accurately and stably. can.

In the above embodiment, the air-fuel ratio is controlled by controlling the fuel injection amount in the fuel injection method, but the air-fuel ratio may be controlled by controlling the air bleed amount in the carburetor method.

(Effects of the Invention) As explained above, according to the present invention, the air-fuel ratio of the engine is controlled by a predetermined control gain using an air-fuel ratio sensor whose output changes almost linearly according to the oxygen concentration in engine exhaust gas. When feedback control is performed to the target air-fuel ratio according to the engine operating state, the control gain is changed according to the electromotive force gradient characteristic of the air-fuel ratio sensor with respect to the target air-fuel ratio, and the control gain is controlled at each target air-fuel ratio according to the engine operating state. Since the responsiveness of the air-fuel ratio sensor and the control gain at that time are made to correspond well, the air-fuel ratio can be accurately feedback controlled in the entire operating range, and the air-fuel ratio can be controlled stably and accurately. be able to.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the present invention. 2 to 12 illustrate embodiments of the present invention, FIG. 2 is a schematic configuration diagram of an engine air-fuel ratio control system, FIG. 3 is a characteristic diagram showing the electromotive force characteristics of the air-fuel ratio sensor, and FIG. The figure is a characteristic diagram showing control frequency (responsiveness) characteristics with respect to the electromotive force gradient of the air-fuel ratio sensor, 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, 7 is a diagram showing a map of the integral constant with respect to the target air-fuel ratio, FIG. 8 is a diagram showing a map of the proportionality constant with respect to the target air-fuel ratio, FIG. 9 is a diagram showing a map of the delay time with respect to the target air-fuel ratio, and FIG.
Figures 11 and 12 are explanatory diagrams showing the correspondence between the output characteristics of the air-fuel ratio sensor and the average fuel injection amount.
The figures are diagrams showing interrupt processing at the start and end of fuel injection, respectively. DESCRIPTION OF SYMBOLS 1... Engine, 3... Exhaust passage, 5... Fuel injection valve, 8... Air-fuel ratio sensor, 11... Air-fuel ratio controller, 15... Operating state detection means, 16...
...Target air-fuel ratio setting means, 17... Control gain changing means, 18... Comparing means, 19... Air-fuel ratio control means.

Claims (1)

[Scope of Claims] 1. An air-fuel ratio sensor that is provided in the exhaust passage of the engine and whose output changes approximately linearly according to the oxygen concentration in the exhaust gas, and an operating state detection means that detects the operating state of the engine. , target air-fuel ratio setting means for receiving the output of the operating state detection means and setting a target value of the air-fuel ratio of the air-fuel mixture to be supplied to the engine according to the operating state of the engine; and the output of the target air-fuel ratio setting means and the above. a comparison means that receives the output of the air-fuel ratio sensor and compares both outputs; and an air-fuel ratio that receives the output of the comparison means and controls the air-fuel ratio of the air-fuel mixture supplied to the engine to the target air-fuel ratio with a predetermined control gain. The present invention is characterized by comprising a control means and a control gain changing means for changing the control gain of the air-fuel ratio control means in accordance with the characteristics of the electromotive force gradient of the air-fuel ratio sensor with respect to the target air-fuel ratio of the target air-fuel ratio setting means. Air-fuel ratio control device for engines. 2. The control gain changing means controls the air-fuel ratio of the engine according to claim 1, wherein the target air-fuel ratio is set to be smaller than the stoichiometric air-fuel ratio, and the control constant in the rich region is smaller than the control constant in the lean region. Device.