JPH0319377B2 - - 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 the engine is feedback-controlled to a predetermined value using an air-fuel ratio sensor whose output varies linearly in accordance with the oxygen concentration in exhaust gas and whose output gradient varies depending on the output value.

(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, as an air-fuel ratio sensor that replaces the above-mentioned λ sensor, as shown in Japanese Patent Application Laid-Open No. 59-100854, the output changes linearly according to the oxygen concentration in the exhaust gas, and the air-fuel ratio is enriched. A so-called wide-range air-fuel ratio sensor has been proposed that can continuously detect the air-fuel ratio from the lean region to the lean region, and makes 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-conducting solid electrolyte, and a semi-porous electrode mainly made of Pt etc. as the porous electrode on the side that comes into contact with the gas to be measured (exhaust gas). In addition to using a material with 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. It is what it is.

(Problems to be Solved by the Invention) By the way, when feedback controlling the air-fuel ratio of the engine to a predetermined value using an air-fuel ratio sensor having linear output characteristics such as the above-mentioned wide-range air-fuel ratio sensor, it is necessary to take into account the influence of noise. By setting a dead band (hysteresis) of a predetermined width at the target value of the air-fuel ratio sensor corresponding to the target air-fuel ratio, and controlling the air-fuel ratio to the target value with the predetermined dead band width, that is, within a predetermined range, noise resistance is achieved. It is possible to increase the sexiness (see Figure 6).

However, even with an air-fuel ratio sensor having such a linear output characteristic, the output characteristic is not exactly linear but only approximately linear, and the output gradient differs depending on the output value. For example, in the case of the above-mentioned wide range air-fuel ratio sensor, its output (electromotive force) characteristic is an electromotive force gradient (slope) at the stoichiometric air-fuel ratio (A/F = 14.7).
is the maximum, and the electromotive force gradient becomes gentler toward the lean side and rich side from this stoichiometric air-fuel ratio (see Fig. 3). Therefore,
In the case of this wide-range air-fuel ratio sensor, if the width of the dead zone is set large for noise countermeasures based on the vicinity of the stoichiometric air-fuel ratio where the electromotive force gradient is large, the electromotive force gradient will be gentler on the lean or rich side than the stoichiometric air-fuel ratio. For this reason, there is a problem in that the dead zone increases fluctuations in the air-fuel ratio, reducing the accuracy of air-fuel ratio control.

The present invention has been made in view of the above, and its purpose is to control the air-fuel ratio within a predetermined dead band width using an air-fuel ratio sensor whose output changes linearly and whose output gradient varies depending on the output value. When controlling the air-fuel ratio to the target air-fuel ratio, that is, within a predetermined range, by changing this dead band width according to the output gradient of the output value from the air-fuel ratio sensor, noise resistance can be improved without reducing the accuracy of air-fuel ratio control. It's about improving.

(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 varies linearly and whose output gradient varies depending on the output value, and target value setting means for setting a target value of the air-fuel ratio sensor 8 corresponding to a preset air-fuel ratio of the air-fuel mixture. 15, a comparison means 17 for comparing the output of the air-fuel ratio sensor 8 and the target value set by the target value setting means 15, and a comparison means 17 that receives the output of the comparison means 17 and determines the air-fuel ratio of the air-fuel mixture to be supplied to the engine. The basic configuration includes an air-fuel ratio control means 18 that controls the above-mentioned target value with a predetermined dead band width. In addition, when the output gradient of the output value from the air-fuel ratio sensor 8 is small, compared to when it is large, the air-fuel ratio control means 18
This configuration includes a control range changing means 16 that changes the dead zone width so as to reduce the width of the dead zone.

(Function) With the above configuration, the present invention uses an air-fuel ratio sensor whose output changes linearly according to the oxygen concentration in the exhaust gas and whose output gradient varies depending on the output value to control the air-fuel ratio within a predetermined range. In the case of feedback control, the width of the dead band of the target value (target electromotive force) of the air-fuel ratio sensor corresponding to the target air-fuel ratio is compared when the output gradient is small and when it is large according to the output gradient of the output value from the air-fuel ratio sensor. It will be changed to become smaller. For example, in the case of the above-mentioned wide range air-fuel ratio sensor, the dead band width is maximum near the stoichiometric air-fuel ratio where the electromotive force gradient is maximum, and as it goes leaner or richer than the stoichiometric air-fuel ratio, that is, the electromotive force gradient becomes smaller as it becomes more gradual. As a result, when the width of the dead zone is large, the output gradient is large, so that the air-fuel ratio does not fluctuate much due to the dead zone, and the noise resistance is significantly improved. On the other hand, when the dead band width is small, the output gradient is small, so while ensuring noise resistance to some extent, the increase in air-fuel ratio fluctuations due to the dead band is suppressed, and a decrease in control accuracy is prevented.

(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 oxygen concentration in the exhaust gas. 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.

The air-fuel ratio sensor 8 has porous electrodes formed on both sides of an oxygen ion-conducting solid electrolyte as in the past, and the porous electrode on the side that comes into contact with the gas to be measured (exhaust gas) has a semi-catalytic performance 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 produce CO;
It is made by the presence of metal oxides such as NiO, Co ₃ O ₄ and CnO, and its electromotive force characteristics are such that the output electromotive force is linear according to 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 the basic characteristics of being able to continuously detect the air-fuel ratio from the rich region to the lean region. Alternatively, the output gradient becomes smaller as it goes to the richer side. Moreover, the superpower characteristic of the 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, it occurs on the leaner side than the stoichiometric air-fuel ratio. The power decreases and the electromotive force increases on the rich side. In addition, the electromotive force of the air-fuel ratio sensor 8 varies depending on the HC concentration in the exhaust gas.
It has concentration characteristics and is leaner than the stoichiometric air-fuel ratio.
As the HC concentration increases, the electromotive force increases (note that since the HC concentration is originally high on the rich side, there is almost no change in the electromotive force).

Next, the operation of the air-fuel ratio controller 11 will be explained with reference to _the flowchart shown in FIG.
Fzone (“0” on the lean side, “1” on the rich side) is set to “0”, and delay flags Fl and Fr (not in delay) on the lean side and rich side are used to distinguish whether or not fuel injection is in delay. Initial setting is ``0'' during the injection period and ``1'' during the delay period), and a 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 period for calculating engine speed, etc. is reset, and in the next step _S3 , the basic timer is reset for a fixed period of time.
Wait for Ti to elapse, and when Ti elapses for a certain period of time, 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 speed Ne is determined by the ignition pulse signal from the igniter 14.
Calculate the airflow sensor ₆ in step S6.
And the intake air flow rate Ue is calculated based on the signal from the intake temperature sensor 7.

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. Later, in step _S8 , the target air-fuel ratio, HC
By inputting the concentration and exhaust gas temperature into a data table as shown in FIG. 5, the slice level median value Vref as the target value of the air-fuel ratio sensor 8 corresponding to the target air-fuel ratio is determined, and the slice level value Vref as the target value is calculated. Determine the dead band widths Vhl and Vhr on the lean side and rich side with respect to the level median value Vref.

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 5 above also includes slice level median value Vref according to exhaust gas concentration 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 with respect to the slice level median value Vref are set to eliminate the influence of noise on the output (electromotive force) of the air-fuel ratio sensor 8, as shown in FIG. And this dead band width
Noise is dealt with by ignoring changes in electromotive force within Vhl and Vhr. And this dead band width Vhl,
Vhr is determined by the map shown in Figure 7, and changes according to the slice level median value Vref corresponding to the target air-fuel ratio, and is maximum at the slice level median value Vref corresponding to the stoichiometric air-fuel ratio, which is ₀ , and is lower than the stoichiometric air-fuel ratio. It becomes smaller as the slice level median value corresponds to the lean side or the rich side.

After that, in the following steps _S9 to _S29 ,
Feedback control is executed to adjust the air-fuel ratio to the target air-fuel ratio 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. Ru. 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 _S9 , it is determined whether the zone flag Fzone is "0" or "1", and when Fzone=0, When on the lean side, the lean side dead zone width Vhl is calculated based on the slice level median value Vref obtained in step _S8 above.
In step S ₁₀ , the slice level median value V′ref is set to Vref
+Vhl and when on the rich side with Fzone=1,
Median slice level obtained in step _S8 above
Based on the rich side dead zone width Vhr with respect to Vref, the slice level median value V′refVref−Vhr is determined in step _S11 .
, respectively, proceed to step _S12 . and,
In step _S12 , the electromotive force Vs actually measured from the air-fuel ratio sensor 8 is compared with the slice level median value V'ref determined in step _S10 or _S11 .

If the determination in step _S12 is that Vs≧V'ref, the zone flag Fzone is determined in step _S13 , and if Fzone=1, which is on the rich side, it is determined that the air-fuel ratio is richer than the target value. step
In step _S14 , the feedback coefficient Cfb is set to Cfb-Cr (Cr: integral constant) to lean the air-fuel ratio, that is, to reduce the fuel injection amount, and in step _S15 , the fuel injection time τ is calculated from the formula K, Cfb, Ue/Ne. Then return to step _S3 .

After that, as the fuel injection amount decreases in step _S15 , the air-fuel ratio becomes lean as shown in FIG. 8, and when the determination in step _S12 becomes Vs<V'ref,
In step _S16 , the zone flag Fzone is determined,
Since it is still on the rich side with Fzone = 1, it is determined in the next step _S17 whether the lean side delay flag Fl is "1" or not. Decide and step
After setting the delay flag Fl to "1" in _S18 , the delay timer is reset in step _S19 (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 _S20 , the delay timer determines whether or not the predetermined delay time tdl has elapsed, and if it has not elapsed, the delay timer determines whether or not the predetermined delay time tdl has elapsed, and if it has not elapsed, the delay timer determines whether or not the predetermined delay time tdl has elapsed. The program moves to step _S14 , maintains the feedback coefficient Cfb at Cfb-Cr, and returns to step _S3 with the fuel injection amount reduced in step _S15 . On the other hand, when the delay time tdl has elapsed, step _S21
After setting the zone flag Fzone to "0" and the delay flag Fl to "0", in step _S22 , the feedback coefficient is set to enrich the air-fuel ratio.
Cfb is set to Cfb+Csl (Csl: proportionality constant), the fuel injection amount is increased in step _S15 , and the process returns to step _S3 .

Next, even with this increase in the fuel injection amount, the determination in step _S12 is still that Vs<V'ref, so in step _S16 it is determined that the zone flag Fzone is on the lean side of 0, and in step _S23 To enrich the air-fuel ratio, change the feedback coefficient Cfb to Cfb + Cl.
(Cl: integral constant), the fuel injection amount is further increased in step _S15 , and the process returns to step _S3 .

After that, this increase in fuel injection amount causes the step
The determination in S ₁₂ is Vs≧V′ref, but step S ₁₃
Since the judgment is on the lean side with the zone flag Fzone = 0, it is determined in step _S24 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 delay flag Fr to “1” in step S ₂₅ .
After that, the day delay timer is reset in step _S26 . Then, during the delay of YES with Fr=1, in the next step _S27 , the delay timer determines whether or not a predetermined delay time tdr has elapsed, and if it has not elapsed, a step is executed to prevent the influence of noise. The process moves to _S23 , and the feedback coefficient Cfb is maintained at Cfb+Cl, and the process returns to step _S3 while increasing the fuel injection amount in step _S15 . on the other hand,
When the delay time tdr has elapsed, the zone flag Fzone is set to "1" and the delay flag Fr is set to "0" in step _S28 , and then the feedback coefficient Cfb is set in order to make the air-fuel ratio leaner in step _S29 .
As Cfb−Csr (Csr: constant of proportionality), step
At _S15 , the fuel injection amount is decreased and the process returns to step _S3 .
Thereafter, the determination in step _S12 is that Vs≧V'ref, and the determination in step _S13 is that Fzone=1, and the same operation as above is repeated thereafter.

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, as shown in FIG. After setting the injection timer to the time τ (note that this injection timer is a counter that counts down the set time and generates a subsequent injection end interrupt signal at the moment it reaches zero), the current to the fuel injection valve 5 is Turn on and start fuel injection. Then, the end of fuel injection is interrupted by the injection end interrupt signal from the injection timer, as shown in FIG. 10, and the current to the fuel injection valve 5 is turned off.

Therefore, in the operation flow of the air-fuel ratio controller 11, in step S8, the target value setting is performed to set 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. It constitutes means 15, and changes dead band widths Vhl and Vhr on the lean side and rich side according to the target value (slice level median value Vref corresponding to the target air-fuel ratio) from the target value setting means 15, The control range changing means 16 is configured such that the output gradient is maximized when the output gradient is near the stoichiometric air-fuel ratio, and is made smaller as the output gradient becomes leaner and richer. Further, in step _S12 , the output of the air-fuel ratio sensor 8 (electromotive force Vs) and the target value set by the target value setting means 15 (slice level median value
It constitutes a comparison means 17 for comparing with V'ref). Furthermore, in steps _S13 to _S29 , the air-fuel ratio of the mixture supplied to the engine 1 is adjusted to a predetermined dead band width Vhl, Vhr by receiving the output of the comparison means ₁₇ and controlling the fuel injection amount of the fuel injection valve 5. This constitutes an air-fuel ratio control means 18 that controls the above target value.

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 enzyme concentration in the exhaust gas of the engine 1, and the output of the air-fuel ratio sensor 8 and The target value of the air-fuel ratio sensor 8 (having predetermined dead band widths Vhl and Vhr) corresponding to a preset air-fuel ratio is compared, 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 within a predetermined range.

In this case, the electromotive force characteristic of the air-fuel ratio sensor 8 is
As shown in the figure, it changes linearly, but the electromotive force gradient (slope) is maximum near the stoichiometric air-fuel ratio (A/F = 14.7), and becomes gentler as it gets leaner and richer than the stoichiometric air-fuel ratio. It has the characteristics of On the other hand, in order to prevent the influence of noise on the electromotive force of the air-fuel ratio sensor 8, dead band widths Vhl and Vhr are set for the target value (slice level median value Vref) of the air-fuel ratio sensor 8 corresponding to the target air-fuel ratio.
However, the dead band widths Vhl and Vhr are changed to the above-mentioned slice level median value by the control range changing means 16.
Vref, that is, 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 becomes smaller as it goes leaner and richer than the stoichiometric air-fuel ratio. Near the stoichiometric air-fuel ratio where the electromotive force gradient is large, the dead band width is large.
The noise resistance can be significantly improved by the dead band widths Vhl and Vhr without causing much variation in the air-fuel ratio due to Vhl and Vhr. On the other hand, on the lean side or rich side of the stoichiometric air-fuel ratio, the dead band width Vhl is small,
While ensuring a certain degree of noise resistance with VHR,
Increase in fluctuations in the air-fuel ratio due to the dead band widths Vhl and Vhr is suppressed, and a decrease in the accuracy of air-fuel ratio control is prevented. Therefore, the air-fuel ratio control can be performed accurately and stably with improved noise resistance without reducing its accuracy.

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, an air-fuel ratio sensor whose output changes depending on the oxygen concentration in the exhaust gas of the engine and whose output gradient differs depending on the output value is used to When performing feedback control of the air-fuel ratio to an air-fuel ratio within a predetermined range, the dead band width of the target value of the air-fuel ratio sensor corresponding to the target air-fuel ratio is changed according to the magnitude of the output gradient of the air-fuel ratio sensor.
Since the output gradient is made smaller when it is small compared to when it is large, noise resistance can be improved without reducing the accuracy of air-fuel ratio control, and the above-mentioned air-fuel ratio control can be performed stably and accurately. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the present invention. 2 to 10 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. Figure 5 is a flowchart showing the operation of the air-fuel ratio controller, Figure 5 is a diagram showing an example of a data table, Figure 6 is an explanatory diagram showing the influence of noise on the electromotive force of the air-fuel ratio sensor, and Figure 7 is the slice level. A diagram showing a map of the dead band width with respect to the median value, Figure 8 is an explanatory diagram showing the correspondence between the output characteristics of the air-fuel ratio sensor and the average fuel injection amount, and Figures 9 and 10 are the graphs at the start and end of fuel injection, respectively. It is a figure which shows interrupt processing at the time. 1... Engine, 3... Exhaust passage, 5... Fuel injection valve, 8... Air-fuel ratio sensor, 9... HC sensor, 10... Exhaust temperature sensor, 11... Air-fuel ratio controller, 15... Target value Setting means, 16... Control range changing means, 17... Comparing means, 18... Air-fuel ratio control means.

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 whose output slope varies depending on the output value, and an air-fuel ratio of the air-fuel mixture that is set in advance. a target value setting means for setting a target value of the air-fuel ratio sensor corresponding to the target value; a comparison means for comparing the output of the air-fuel ratio sensor with the target value set by the target value setting means; air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to the target value within a predetermined dead band width; 1. An air-fuel ratio control device for an engine, comprising: control range changing means for changing a dead band width of the air-fuel ratio controlling means to reduce a dead band width of the air-fuel ratio controlling means.