JPH0437244Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a feedback type carburetor that can maintain the air-fuel ratio at a target value through feedback control.

[Prior Art] It is well known that a carburetor is used to vaporize and atomize fuel in intake air to create an air-fuel mixture. By changing the flow rate of fuel supplied from the carburetor into the intake air, the air-fuel ratio of the air-fuel mixture is precisely controlled by feedback control, improving engine output, fuel efficiency, or purifying exhaust gas. A so-called feedback type vaporizer has been proposed to improve performance (for example, see Japanese Patent Laid-Open No. 57-56652). In such a feedback type carburetor, the exhaust passage is
While the air-fuel ratio of the air-fuel mixture is detected by installing an O ₂ sensor (the air-fuel ratio can be calculated from the O ₂ concentration in the exhaust gas), a A bypass fuel passage is provided for passing fuel by bypassing the interposed main jet, and a bypass valve for opening and closing the bypass fuel passage is provided, and the bypass valve is installed in accordance with the deviation of the air-fuel ratio from the target value (for example, the stoichiometric air-fuel ratio). Feedback control is performed to maintain the air-fuel ratio at a target value by opening and closing valves.

However, in conventional feedback type carburetors that maintain the air-fuel ratio at a predetermined target value, when the intake air temperature is high, the density of the intake air is low, so the amount of intake air (weight) is small even if the throttle opening is the same as at low temperatures. Therefore, if an attempt is made to maintain the air-fuel ratio at the target value, the bypass valve will shift to the closed side compared to when the temperature is low, and the flow rate of fuel flowing in the bypass fuel passage will decrease, and especially when the engine is idling, the flow rate will be extremely low. To explain this, for example, when the bypass valve is a duty solenoid type and the duty ratio is changed, the target value is set to the stoichiometric air-fuel ratio of 14.7 and the air-fuel ratio is feedback controlled. When the temperature is relatively low, the seventh
As shown in Figure a, when the duty ratio is 100%, that is, when the bypass valve is fully closed, the straight line G ₁
As shown by the line G3, the air-fuel ratio is 18. On the other hand, when the duty ratio is 0%, that is, the bypass valve is fully opened, the air-fuel ratio is 13, as shown by the straight line _G3 .
At this time, the air-fuel ratio is set to the target value of 14.7 as shown by curve _G2 .
If an attempt is made to maintain the air-fuel ratio (the stoichiometric air-fuel ratio), the duty ratio will be 40%, and since the bypass valve is opened appropriately, a corresponding amount of fuel will flow into the bypass fuel passage. The output waveform of the signal applied to the duty solenoid of the bypass valve in this case is shown in FIG. 6a. On the other hand, when the intake air temperature is relatively high (about 65 to 75℃), the intake air density is low and the intake air amount (weight)
As a result, as shown in Figure 7b, when the duty ratio is 100%, that is, when the bypass valve is fully closed, the air-fuel ratio is 15, as shown by the straight line _G4 , which is much richer than at the low temperature mentioned above. ,on the other hand,
When the duty ratio is 0%, that is, the bypass valve is fully open, the _air -fuel ratio is
It becomes 10. At this time, if an attempt is made to maintain the air-fuel ratio at the target value of 14.7 as shown by curve _G5 , the duty ratio will be 70%, the bypass valve will shift to the closed side significantly compared to when the temperature is low, and the fuel flowing in the bypass fuel passage will Especially when idle, it becomes very low. The output waveform of the signal applied to the duty solenoid of the bypass valve in this case is shown in FIG. 6b.

As described above, in the conventional feedback type carburetor, when the intake air temperature is high, especially when the engine is idling, the flow rate of fuel in the bypass fuel passage is extremely reduced. On the other hand, the temperature inside the engine compartment is quite high due to heat radiation from the engine, and the bypass fuel passage is constantly heated, so as mentioned above, the flow rate of fuel flowing through the bypass fuel passage during idling becomes extremely low. When this happens, the fuel in the bypass passage reaches a high temperature and eventually boils, causing a mixture of fuel and vaporized fuel vapor in the bypass fuel passage.As a result, the amount of fuel supplied fluctuates, causing feedback in the air-fuel ratio. There were problems with control being disrupted and idling becoming unstable.

[Purpose of the invention] The present invention was made in view of the above-mentioned conventional problems.
It is an object of the present invention to provide an air-fuel ratio control device for a feedback type carburetor that can prevent fuel from vaporizing in a bypass fuel passage of the carburetor and maintain idle stability when intake air temperature is high.

[Structure of the invention] In order to achieve the above object, the present invention includes an air-fuel ratio detection means, a bypass valve that opens and closes a bypass fuel passage that bypasses the main jet, and a signal from the air-fuel ratio detection means as input information. A control means that performs feedback control to open the bypass valve when the detected air-fuel ratio is larger than the target value, and close the bypass valve when the detected air-fuel ratio is smaller than the target value to maintain the air-fuel ratio at the target value. In an engine air-fuel ratio control device equipped with
Means is provided for fixing the closing amount of the bypass valve to a fixed value smaller than the predetermined value when the closing amount of the bypass valve becomes equal to or higher than a predetermined value by the control means during idling. The present invention provides an air-fuel ratio control device for an engine having characteristics.

[Effects of the invention] According to the invention, when the intake air temperature rises and the amount of bypass valve closing exceeds a predetermined value during idling, the air-fuel ratio feedback control is stopped and the bypass valve closes. The valve closing amount is fixed to a fixed value smaller than the predetermined value. Therefore, even during idling when the intake air temperature is high, the flow rate of fuel in the bypass fuel passage does not decrease excessively, and vaporization of fuel in the bypass fuel passage is prevented. Therefore, fuel supply is stabilized and idle stability is improved.

[Example] Hereinafter, embodiments of the present invention will be described in detail.

As shown in FIG. 1, when the intake valve 1 is opened, the engine E sucks intake air (air mixture) into the combustion chamber 3 from the intake port 2, and compresses this intake air (air mixture) with the piston 4. After that, the combustion gas is ignited by a spark plug (not shown), and when the exhaust valve 5 is opened, the combustion gas is discharged from the exhaust port 6 to the exhaust passage 7. A series of steps are continuously repeated. The resulting reciprocating movement of the piston 4 is transmitted to the crankshaft (not shown) via the connecting rod 8.
The basic configuration is that the rotational motion is converted into rotational motion and extracted as the output of the engine E. In the exhaust passage 7, O ₂ is used to detect the O ₂ (oxygen) concentration in the exhaust gas in order to calculate the air-fuel ratio of the air-fuel mixture starting from the upstream.
Sensor 11 and CO (carbon monoxide) in exhaust gas,
Exhaust purification device 12 filled with a three-way catalyst for removing HC (hydrocarbons) and NO _x (nitrogen oxides)
and is provided.

On the other hand, in order to supply intake air to the combustion chamber 3, an intake passage 13 communicating with the intake port 2 is provided, and an air cleaner 14 for removing floating dust in the intake air is interposed near the air inlet of this intake passage 13. has been done. In order to vaporize and atomize fuel and supply it to the intake air in the intake passage 13, a two-barrel type feedback type is used in which the amount of fuel supplied is feedback-controlled by a control circuit 20, as will be explained in detail later. A vaporizer 15 is provided. The intake passage 13 is located at a branch 16 slightly upstream of the position where the carburetor 15 is disposed, and is connected to the two-barrel type carburetor 15 so that the intake passage 13 always takes in air regardless of the engine speed or the engine load. It is branched into a primary intake passage 17p through which intake air passes, and a secondary intake passage 17s through which intake air passes only in a predetermined high rotation/high load range. These primary intake passages 17p and secondary intake passages 17
s, the intake passages 13 are gathered together again at a gathering portion 18 slightly downstream of the position where the carburetor 15 is disposed. A butterfly type choke valve 19 is installed at a position slightly downstream of the branch portion 16 of the primary intake passage 17p, and this choke valve 19 is used when a rich air-fuel mixture is required, such as when starting the engine or when the engine is cold. In order to increase the intake negative pressure and promote fuel suction from the carburetor 15, it is almost closed. Furthermore, a primary throttle valve 21 that opens and closes in response to depression of an accelerator pedal (not shown) is provided downstream of the choke valve 19 in the primary intake passage 17p. On the other hand, in the secondary intake passage 17s, a secondary throttle valve 22 that opens and closes in conjunction with the primary throttle valve 21 via a link mechanism (not shown) is interposed at a position corresponding to the primary throttle valve 21. It is set up.

In addition, in order to suppress excessive rise in combustion temperature and reduce NO _x in exhaust gas, a part of exhaust gas is recirculated to the intake system, so-called EGR.
An EGR passage 23 is provided to communicate the position of the exhaust passage 7 upstream of the O ₂ sensor 11 and the position of the gathering part 18 of the intake passage 13.
An EGR valve 24 for adjusting the EGR amount is provided.

Incidentally, fuel stored in a fuel tank 25 is supplied to the carburetor 15 through a fuel pipe 26 by a fuel pump 27.
A fuel filter 28 is provided in the fuel supply passage 26 at a position slightly downstream of the fuel pump 27 to remove dust from the fuel. Then, a connector 29 provided downstream of the fuel filter 28 of the fuel pipe 26 and the fuel tank 25 are communicated with each other.
A fuel recirculation passage 31 is provided for returning excess fuel not supplied to the carburetor 15 to the fuel tank 25. In addition, in order to recover the evaporated fuel in the fuel tank 25, a evaporated fuel recirculation passage 32 is provided to communicate the upper space of the fuel tank 25 with a position slightly upstream of the branch part 16 of the intake passage 13. A canister 33 filled with an adsorbent for adsorbing fuel vapor contained in air leaking from the fuel tank 25 to the intake passage 13 is interposed in the evaporated fuel recirculation passage 32 .

The feedback type carburetor 15 will be explained in detail below with reference to FIG. Note that the carburetor 15 is a two-barrel type and has a structure that supplies fuel to both the primary intake passage 17p and the secondary intake passage 17s. Since the effects are almost the same, only the primary side will be explained.

As shown in FIG. 2, the carburetor 15 is configured such that the fuel stored in the float chamber 35 is passed through a fuel passage 3 in which a main jet 36 is interposed to adjust the amount of fuel supplied.
7 into the primary intake passage 17p, the air-fuel mixture is vaporized and atomized and sucked out by the intake negative pressure to form an air-fuel mixture in the primary intake passage 17p. During idling and at a predetermined low speed and low load (slow time), the flow rate of the fuel is throttled at the main jet 36, and then the flow rate is further throttled at the slow jet 38, and then passes through the fuel passage 37 into the primary intake passage 17p. However, at idle, fuel is mainly supplied from the idle port 39 to the primary intake passage 1.
On the other hand, during slow operation, fuel is mainly supplied to the primary intake passage 17p from the slow port 41 having a slightly vertical opening. In addition, in order to adjust the engine speed at idle, a mixture adjust screw 42 is provided for adjusting the width of the opening of the idle port 39. In order to promote the vaporization and atomization of the fuel in the intake air or to reduce the viscous resistance of the fuel in the fuel passage 37, air is mixed into the fuel flowing in the fuel passage 37 to form a so-called emulsion. A first slow air bleed 43 and a second slow air bleed 44 to be formed are provided, and these first slow air bleeds 4
3 and the second slow air bleed 44 are supplied with slightly high pressure air from the vicinity of the choke valve 19 in the primary intake passage 17p through the first air bleed passage 45.

Further, at a predetermined high speed and high load, although not shown, a negative pressure is formed by a small bench lily 46 branching off from a position in the fuel passage 37 downstream of the main jet 36 and upstream of the slow jet 38. Fuel is supplied to the primary intake passage 17p through a branch fuel passage communicating with a predetermined position within the primary intake passage 17p. At this time, in order to stop the supply of fuel to the idle port 39 and the slow port 41, a slow cut solenoid valve 47 provided downstream of the slow jet 38 in the fuel passage 37 is closed.

By the way, in order to perform feedback control of the air-fuel ratio by changing the amount of fuel supplied from the carburetor 15 to the primary intake passage 17p, the main jet 36 is bypassed and the float chamber 35 and the fuel passage 37 are located downstream of the main jet 36. A bypass fuel passage 49 is provided which communicates with the main jet 51. A correction main jet 51 is interposed in the bypass fuel passage 49, and a solenoid valve 52 for opening and closing the correction main jet 51 is provided. Note that this solenoid valve 52 corresponds to the bypass valve described in the claims of the utility model registration of this application. The solenoid valve 52 is duty-controlled by the control circuit 20 (see FIG. 1), and when the air-fuel ratio becomes leaner than the target value, the correction main jet 51 is opened, and on the other hand, when the air-fuel ratio becomes richer than the target value. When this occurs, the correction main jet 51 is closed, thereby adjusting the amount of fuel supplied and controlling the air-fuel ratio by feedback. Further, in order to mix air into the fuel flowing in the bypass fuel passage 49 to form an emulsion, a correction slow air bleed 53 is provided, and a second air bleed passage 54 and a correction slow air bleed 53 are provided. Air is supplied through the third air bleed passage 55.

The control circuit 20 (see Fig. 1) calculates the air-fuel ratio using input information such as the O ₂ concentration in the exhaust gas (which allows the air-fuel ratio to be calculated) detected by the O ₂ sensor 11 (see Fig. 1). The control method of the control circuit 20 will be described below with reference to the control flowchart shown in FIG. 3, which is designed to perform feedback control.

When the control is started, in step S1, the engine speed, intake pressure, throttle opening, _O2 concentration, etc. are read, and the operating state of the engine E is ascertained.

Next, in step S2, the operating state of the engine E is at the time of starting which requires a rich air-fuel mixture, and in order to determine whether starting increase correction is required, it is determined whether the engine speed is 500 rpm or less. are compared. As a result of the comparison, the engine speed is
If the engine speed is below 500 rpm (YES), the engine speed has not reached the idle speed (for example, 700 rpm) and the engine E is starting, so in order to correct the starting fuel increase, Control proceeds to step S101.

In step S101, in order to make the air-fuel mixture as rich as possible, the air-fuel ratio feedback control is stopped, the duty ratio is set to a fixed value of 0%, the solenoid valve 52 is fully opened, and fuel is supplied through the bypass fuel passage 49. be carried out to the maximum extent possible.
Here, as shown in FIG. 5, the solenoid valve 52 is configured such that one step-shaped output waveform is applied from the control circuit 20 every 50 msec for one cycle, and the output waveform is applied within each cycle of 50 msec. The on-state duration time of the step signal can be set arbitrarily from 0 to 50 msec. At this time, the duty ratio is expressed as a ratio (%) to the 50 msec duration of the signal's ON state; when the duty ratio is 100%, the solenoid valve 52 is fully closed, and when the duty ratio is 0%, the solenoid valve 52 is fully open. It is becoming like this. And solenoid valve 5
2 is opened and closed according to the duty ratio of a signal applied from the control circuit 20, and controls the flow rate of fuel supplied through the bypass fuel passage 49.

Thereafter, control proceeds to step S15, where it is compared whether the ignition switch is on. As a result of the comparison, if the ignition switch is on (YES), engine E continues to operate, and control returns to step S1 and continues. On the other hand, if the ignition switch is off (NO), the engine E has stopped operating and the control is ended.

As a result of the comparison in step S2, if the engine speed exceeds 500 rpm (NO), the engine E is not being started and there is no need to perform the starting increase correction, so the control proceeds to step S3.

In a series of steps from step S3 to step S6, the operating state of the engine E is in the deceleration range and the feedback control becomes unstable, so the feedback control is stopped and the deceleration is controlled at a relatively lean fixed duty ratio. It is determined whether or not it is necessary to perform correction. During deceleration, engine E is at high revolutions, the throttle opening is extremely small, and the output shaft of engine E is dynamically connected to the wheels, so it is determined whether all of these conditions are met. Ru. In step S3, it is compared whether the engine rotation speed is 1400 rpm or more, that is, whether there is a corresponding rotation speed during deceleration.
In step S4, a comparison is made to see whether the throttle opening is the idle opening, that is, whether the throttle opening is extremely small.
The comparison is made to determine whether the transmission and the clutch are transmitting power, that is, whether the output shaft and the wheels are dynamically connected. As a result of a series of comparisons from step S3 to step S6, if all steps are YES, the operating condition of engine E matches the operating condition during deceleration described above, and therefore falls within the deceleration region, and correction during deceleration is performed. To do this, control proceeds to step S104.

In step S104, air-fuel ratio feedback control is stopped. This is because during deceleration, misfires occur frequently and dilution increases, making it impossible to accurately calculate the air-fuel ratio from the _O2 concentration in the exhaust gas, making it impossible to obtain accurate control information and disrupting feedback control. It is from. In order to improve fuel efficiency or prevent the three-way catalyst from deteriorating due to HC, the duty ratio is set to a fixed value of 50%, which is relatively lean. Thereafter, the control proceeds to step S15, and depending on whether the ignition switch is turned on or off, the control returns to step S1 and continues or is terminated.

On the other hand, if the result of the series of comparisons from step S3 to step S6 is NO in at least one step, the operating state of the engine E does not fall within the deceleration range, so no deceleration correction is performed, and the control proceeds to step S7. You can proceed to

In a series of steps from step S7 to step S9, it is determined whether or not a high load increase correction to enrich the air-fuel ratio is required because the operating state of the engine E is in a high load range and requires high output. Ru.

In step S7, a comparison is made to see if the intake pressure is -120 mmHg or higher. As a result of the comparison, the intake pressure is -
If it is 120mmHg or more (YES), the throttle valve is wide open and the intake pressure is high, and the operating condition of engine E is within the specified high load range. If it gets too hot or gets too hot while driving uphill,
Control proceeds to step S8 to determine whether a maximally rich mixture is required.

In step S8, a comparison is made to determine whether the vehicle in question is an autotransmission vehicle (ATX vehicle). As a result of the comparison, if the car in question is an ATX car (YES), the gear ratio will be automatically selected, so it will not be possible to downshift to obtain strong output, so it will not be possible to downshift to obtain a powerful output, so it will not be possible to downshift to obtain powerful output. In order to supply the air-fuel mixture and secure the output, the control is unconditionally advanced to step S101 where the richest air-fuel mixture is supplied. In this case, the control method from step S101 onwards is based on the result of the comparison at step S2.
This is the same as when the determination is YES and the control proceeds to step S101, so the explanation will be omitted.

On the other hand, as a result of the comparison in step S8, the car in question is
If the vehicle is not an ATX vehicle (NO), the control proceeds to step S9 to determine whether the vehicle is in a low rotation/high load range that requires the richest possible air-fuel mixture, such as when climbing a hill.

In step S9, a comparison is made to see if the engine speed is 3800 rpm or higher. As a result of the comparison, if the engine speed is less than 3800 rpm (NO), Engine E is in the low/medium speed and high load range, such as when climbing a hill, and requires the richest possible air-fuel mixture to obtain high output. Since the machine is in the operating state, control proceeds to step S101, and YES is answered in step S8.
In the same way as when it is determined that

On the other hand, as a result of the comparison in step S9, if the engine speed is 3800 rpm or more (YES), the operating state of engine E is in the high speed/high load range such as during high speed operation, and the engine E is in the high speed/high load range such as during high speed operation. Since no air-fuel mixture is required, control proceeds to step S102 to perform normal high-load increase correction.

In step S102, since a fairly rich air-fuel mixture is required, the air-fuel ratio feedback control is stopped and fuel is supplied with the duty ratio set at a fixed value of 30%. After this, the control goes to step S15.
Then, depending on whether the ignition switch is turned on or off, the control returns to step S1 and continues or is terminated.

As a result of comparison at step S7, the intake pressure is -120mm.
If it is less than Hg (NO), the operating state of the engine E is not in the high load range, and no high load increase correction is performed, control proceeds to step S10, and the engine E
The comparison is made to see if the rotation speed is 4000 rpm or more. As a result of the comparison, if the engine speed is 4000 rpm or more (YES), the operating state of engine E is in the high speed range that requires the richest mixture, so the feedback control is stopped and the fixed duty ratio ( 0%), the control proceeds to step S101. The control method from step S101 onwards is the same as in step S2 above.
This is the same as when the determination is YES and the control proceeds to step S101, so the explanation will be omitted.

As a result of the comparison in step S10, if the engine speed is less than 4000 rpm (NO), the high speed increase correction is not performed and the control proceeds to step S11.

In steps S11 to S12, the engine E
It is determined whether the operating state of the engine is in a medium rotation/medium load range that requires an air-fuel mixture considerably richer than the stoichiometric air-fuel ratio. At step S11, the intake pressure is -150mm
A comparison is made to see if it is greater than or equal to Hg. As a result of the comparison,
If the intake pressure is -150 mmHg or more (YES), the operating state of the engine E is in the medium load range, and the control proceeds to step S12 to further determine whether the engine is in the medium rotation range. In step S12, it is compared whether the engine rotation speed is 1700 rpm or more. As a result of comparison, the engine speed is 1700 rpm
If this is the case (YES), the operating state of the engine E is in the medium rotation/medium load range, so the control proceeds to step S102 in order to stop the feedback control and perform medium load increase correction at a fixed duty ratio of 30%. You can proceed. The control method from step S102 onward is the same as when the determination in step S9 is YES and the control proceeds to step S102, so the explanation will be omitted.

As a result of the comparison in step S11, the intake pressure is -150
less than mmHg (NO) or step S12
As a result of the comparison, if the engine speed is less than 1700 rpm (NO), the operating state of engine E does not correspond to the medium speed/medium load range, so the medium load increase correction is not performed and the control proceeds to step S13. Then, it is determined whether the duty ratio is higher than a predetermined value (70%) due to an increase in the intake air temperature.

In step S13, it is compared whether the duty ratio is 70% or more. As a result of the comparison, if the duty ratio is 70% or more (YES), the opening degree of the solenoid valve 52 is very small, and the bypass fuel passage 4
Since the flow rate of fuel flowing through the bypass fuel passage 4 decreases, especially at idle, the flow rate decreases extremely. Therefore, if feedback control is performed so that the air-fuel ratio matches the target value, the fuel within the air-fuel ratio of the bypass fuel passage 4 will vaporize. , the fuel supply amount becomes unstable and idle stability worsens, so feedback control is stopped and
In order to control with a richer fixed duty ratio,
Control proceeds to step S103.

In step S103, the duty ratio is fixed at 40% in order to ensure a fuel flow rate that can substantially prevent fuel vaporization within the bypass fuel passage 49. This substantially prevents fuel vaporization in the bypass fuel passage 49 caused by feedback control of the high-temperature air-fuel ratio of the intake air, thereby ensuring idling stability. Thereafter, the control proceeds to step S15, and returns to step S1 to continue or terminate depending on whether the ignition switch is turned on or off.

As a result of the comparison in step S13, the duty ratio is
If less than 70% (NO), bypass fuel passage 4
Since fuel vaporization does not substantially occur within the fuel cell 9, the control proceeds to step S14 to perform feedback control.

In step S14, it is compared whether the throttle opening is the idle opening. As a result of the comparison, if the idle opening is (YES), the control is set to step.
The process proceeds to S105, where a target value for idling that is slightly richer than the stoichiometric air-fuel ratio is set, and feedback control is performed. This is because when idling, the amount of intake air decreases and the mixed gas is diluted by dilution gas, so unless the air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio, for example around 13, and feedback is not performed, combustibility will decrease. This is because it will get worse. On the other hand, as a result of the comparison in step S14, if the throttle opening is not the idle opening (NO),
The control proceeds to step S105, where the target value is set to the stoichiometric air-fuel ratio of 14.7, and feedback control is performed.

When feedback control of the air-fuel ratio is performed in this way, for example, when the target value is 14.7, the air-fuel ratio becomes the target value 14.7, as shown in FIG. 4a.
It exhibits characteristics such that the average value almost matches the target value of 14.7, while oscillating slightly up and down around . At this time, the O ₂ sensor 11 detects an output voltage as shown in FIG. 4b, and the control circuit 20 uses this output voltage as input information to operate the solenoid valve 52 with a duty ratio characteristic as shown in FIG. 4c. Feedback control. At this time, as shown in Figure 4c,
The control characteristic is such that the integral term is expressed by I and the proportional term is expressed by P.The integral term I adjusts the duty ratio to maintain the air-fuel ratio at the target value, and the point in time when the increase or decrease in the duty ratio is reversed. In this case, the proportional term P prevents the control amount from going too far (so-called overshoot).

Thereafter, the control proceeds to step S15, and depending on whether the ignition switch is turned on or off, the control returns to step S1 and continues, or is terminated.

[Brief explanation of the drawing]

FIG. 1 is a system configuration diagram of an engine fuel system showing an embodiment of the present invention. Figure 2 shows the first
FIG. 2 is a cross-sectional explanatory diagram showing the configuration of the carburetor shown in the figure.
FIG. 3 is a control flowchart showing a control method by the control circuit shown in FIG. Figure 4a,
Figures b and c are diagrams showing the characteristics of the air-fuel ratio, the O ₂ sensor output voltage, and the duty ratio when the air-fuel ratio is feedback-controlled by the respective control circuits. FIG. 5 is a diagram showing the characteristics of the signal applied to the solenoid valve from the control circuit. Figure 6 a and b are
FIG. 7 is a diagram showing signals applied to the solenoid valve from the control circuit at low temperature and high temperature, respectively, during feedback control. FIGS. 7a and 7b are diagrams showing the relationship between the duty ratio and the air-fuel ratio at low temperatures and high temperatures, respectively. E...Engine, 11... _O2 sensor, 15...
... Carburizer, 20 ... Control circuit, 36 ... Main jet, 37 ... Fuel passage, 38 ... Slow jet, 49 ... Bypass fuel passage, 51 ... Main jet for correction, 52 ... Solenoid valve (bypass valve).

Claims (1)

[Scope of Claim for Utility Model Registration] An air-fuel ratio detection means, a bypass valve that opens and closes a bypass fuel passage that bypasses the main jet;
Using the signal from the air-fuel ratio detection means as input information, the bypass valve is opened when the detected air-fuel ratio is larger than the target value, and the bypass valve is closed when the detected air-fuel ratio is smaller than the target value. In an engine air-fuel ratio control device provided with a control means for performing feedback control to maintain the air-fuel ratio at a target value, when the amount of closing of the bypass valve becomes equal to or greater than a predetermined value by the control means during idling, the above-mentioned An air-fuel ratio control device for an engine, comprising means for fixing a closing amount of a bypass valve to a fixed value smaller than the predetermined value.