JPH06264802A - Lean burn engine system and its auxiliary air controller - Google Patents

Abstract

(57) [Summary] [Purpose] A large-capacity auxiliary air controller was adopted in a lean-burn engine that operates between stoichiometric air-fuel ratio and lean air-fuel ratio by switching auxiliary air amount and fuel amount. To ensure safety in case of failure. [Structure] The flow control valve 1 for controlling the amount of auxiliary air is provided with a stopper 26 for limiting the stroke amount of a valve rod 27 having valve bodies 27a, 27b, and the amount of advance / retreat of the stopper 26 in the direction of the valve rod 27 is set. The adjustment is made by the opening degree of the throttle valve 9. That is, the smaller the opening of the throttle valve 9, the smaller the maximum stroke amount of the valve rod 27, and the larger the opening of the throttle valve 9, the larger the maximum stroke amount . As a result, even if the flow rate control valve 1 fails when the opening of the throttle valve 9 is large, or the control unit fails and the flow rate control valve 1 is in the fully open state, the assistance depends on the accelerator depression amount. since the maximum value of the air amount is set, it does not run out of control.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lean burn engine equipped with an auxiliary air control device for correcting a torque step generated when an air-fuel ratio is switched, and more particularly to safety when the auxiliary air control device or control unit fails. TECHNICAL FIELD The present invention relates to a lean burn engine and a supplementary air control device for the lean burn engine, which are suitable for ensuring the performance.

[0002]

2. Description of the Related Art A lean-burn engine is operated at a theoretical air-fuel ratio (14.7) and a lean air-fuel ratio (2
It is desirable to selectively use the operation according to 2 to 25) according to the operating condition. For example, magazine “Automotive Engineering” Vol.41, No.1
In the prior art described on pages 0 and 74 to 80, the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio or in the opposite direction by increasing or decreasing the fuel injection amount to suppress the torque step before and after the switching. The operating range of the intermediate air-fuel ratio is provided between them, or the operating range is transiently gradually changed over time.

[0003]

In the above-mentioned prior art, it is necessary to provide an intermediate air-fuel ratio between the stoichiometric air-fuel ratio which produces the largest amount of NOx emissions and the lean air-fuel ratio so as not to cause a Tokule step. is there. FIG. 8A is a graph showing the relationship between the air-fuel ratio (A / F) and the NOx generation amount, and FIG. 8B is a graph showing the relationship between the air-fuel ratio and the NOx purification rate. Since the combustion temperature is high near the stoichiometric air-fuel ratio, the amount of NOx generated is large, but this NOx can be reduced by the action of the three-way catalyst. In addition, the lean air-fuel ratio is not an air-fuel ratio at which the three-way catalyst works efficiently, so NO
Although the purification rate of x is small, there is no problem in terms of emission because the original amount of NOx generated itself is small. However, at an air-fuel ratio intermediate between the stoichiometric air-fuel ratio and the lean air-fuel ratio,
The amount of NOx generated is high, and the three-way catalyst does not work.
There is a problem that the amount of Ox emission increases.

Switching between the stoichiometric air-fuel ratio and the lean air-fuel ratio
If the auxiliary air is used in addition to the fuel injection amount, the problem of torque step difference can be solved and the operation at the intermediate air-fuel ratio becomes unnecessary, so that the NOx emission amount can be suppressed. For example, as shown in FIG. 10, when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio, if the air-fuel ratio is switched only by reducing the fuel injection amount, the point A shifts to the point B. ΔT occurs. Therefore, it becomes necessary to gradually shift using an intermediate air-fuel ratio,
As mentioned above, NOx emission becomes a problem.

On the other hand, when the intake air flow rate is increased by ΔQ, the point moves from the point A to the point C, but no torque step occurs even if the air-fuel ratio becomes thin. In this case, since the required amount of auxiliary air is large, there is a risk of runaway if the flow control valve or control unit fails when the amount of auxiliary air is increased. Therefore, it cannot be applied to an actual vehicle unless some measures are taken.

It is an object of the present invention to provide a lean burn engine and an auxiliary air control device for the lean burn engine which does not run out of control when switching between the lean burn air-fuel ratio operation and the stoichiometric air fuel ratio operation using the auxiliary air amount. .

[0007]

The above object is to provide a bypass passage bypassing a throttle valve, a flow control valve for controlling the amount of auxiliary air flowing through the bypass passage, and a theoretical air-fuel ratio control for controlling the flow control valve. When switching to leaner than the fuel ratio, the valve opening amount of the flow control valve is increased to increase the auxiliary air amount, and when switching from the lean to the rich air-fuel ratio, the valve opening amount of the flow control valve is decreased to increase the auxiliary air amount. In a lean burn engine system including a control unit for reducing the amount of air flow, when the valve opening amount is large according to the valve opening amount of the throttle valve and a limiting means for limiting the maximum value of the auxiliary air amount passing through the flow control valve. Is provided with variable means for varying the maximum value to a large value when the opening amount of the throttle valve is small and the maximum value to a small value,
To be achieved.

[0008]

When a large amount of auxiliary air is supplied to the engine and the engine is operated at a lean air-fuel ratio, it is assumed that, for example, a failure occurs in which the flow control valve remains open. If a failure occurs when the throttle valve opening is large and a large amount of auxiliary air is supplied to the engine in addition to the amount of air passing through the throttle valve, the engine will run out of control. But,
In the present invention, when the throttle valve opening is large, the maximum value of the auxiliary air amount is limited to a small value, so there is no danger of runaway. Further, when the throttle valve opening is small, its maximum value is set to a large value so that a large amount of auxiliary air can be supplied to the engine, but there is no problem because the throttle valve opening itself is small.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a configuration diagram of a lean burn engine according to an embodiment of the present invention. Combustion chamber 14 of engine 13
Communicate with the intake pipe 10 via the intake valve 12 and communicate with the exhaust pipe 19 via the exhaust valve 18. An air cleaner 6 is attached to the uppermost stream of the intake pipe 10, and a fuel injection valve 16 is attached near the intake valve 12 on the downstream side. A swirl control valve 11 is attached to the intake pipe 10 slightly upstream of the fuel injection valve 16, and a throttle valve 9 is provided at a substantially central portion of the intake pipe 10. The throttle valve 9 is provided at the portion of the intake pipe 10 where the throttle valve 9 is arranged.
There is provided an auxiliary air bypass passage 15 that detours around, and a flow rate control valve 1 that controls the amount of auxiliary air that passes through the passage 15. The exhaust pipe 19 is provided with a three-way catalyst 20 that purifies the exhaust gas discharged from the combustion chamber 14. A spark plug 17 is attached to the combustion chamber of the engine 13. The spark plug 17 is connected to an ignition system (not shown) which is improved so that sparks are discharged more strongly and longer than before in order to stably burn the air-fuel mixture in the combustion chamber 14 at the lean burn air-fuel ratio.

Engine control unit (ECU)
Reference numeral 7 denotes an air flow sensor 8 for detecting the intake air amount, a throttle valve opening sensor 23, an O ₂ sensor 21, and other engine water temperature sensor, engine speed sensor, intake air temperature sensor, oil temperature sensor, etc. The detection signal is taken in to determine the operating state of the engine, the fuel injection amount from the fuel injection valve 16, the swirl control valve 11, and the flow control valve 1
Control the amount of auxiliary air.

FIG. 9 is an A / F map of the lean burn engine. The ECU 7 stores the A / s stored in the built-in ROM.
It is judged from the F map and various sensors which region the operating state of the engine is now. For example, when the vehicle is in an acceleration or deceleration state and it is necessary to operate at the stoichiometric air-fuel ratio of A / F = 14.7, feedback control is performed by the detection value of the O ₂ sensor 21 to keep the air-fuel ratio at the stoichiometric air-fuel ratio. When it is determined that the running state becomes the constant speed running state and the lean burn air-fuel ratio operation is started, the feedback control based on the detection value of the O ₂ sensor 21 is terminated and the auxiliary air amount and the fuel injection amount are controlled. The lean-burn air-fuel ratio of A / F = 22 to 25 is set as the air-fuel ratio, and the swirl control valve 11 is controlled so that lean fuel is easily combusted, and swirl is generated in the air-fuel mixture.

FIG. 5 shows an electric circuit of a control device for controlling on / off of the flow control valve 1 shown in FIG. A valve body (not shown) of the flow rate control valve turns on / off the bypass passage in response to an on / off current flowing through the coil 1. A protective diode 2 is provided in parallel with the coil 1, and a transistor 3, a battery 4, and a sensor 5 represented by a variable resistor are connected in series with the flow control valve (coil) 1 and the diode 2. The transistor 3 is driven by the duty voltage signal from the control unit 7, and the valve element is driven accordingly. The sensor 5 corresponds to the air flow sensor 8 and the throttle valve opening sensor 23 of FIG. By setting the sensor 5 in the drive circuit of the flow control valve 1 and adjusting the resistance value thereof, the maximum current flowing through the flow control valve 1 is limited according to the engine operating state. The resistance value of the sensor 5 has a maximum value at the time of idling with the least load, and decreases with an increase in the load, and when switching the air-fuel ratio in each operating state, the maximum flow rate with a slight margin in the required auxiliary air flow rate. To be set.

FIG. 3 is a characteristic diagram of the embodiment shown in FIG. E is a voltage applied to the flow control valve 1, and shows a characteristic that the resistance of the sensor 5 is the smallest when the load is E1 at the time of E1 and a characteristic that the load is lighter as the subscript increases. Figure 5
In the case of the above configuration, since the variable resistor is used for the sensor 5, the change causes the characteristics to move substantially in parallel. In the case of this embodiment,
Since the maximum amount of auxiliary air is limited by the electric circuit,
It is not necessary to limit the number of sensors 5 for detecting the driving state to one,
Using various types of sensors, it is possible to finely control the maximum value by dividing the conditions. In this embodiment, the same current is supplied to both the flow control valve 1 and the sensor 5,
The circuit may be configured with a hardware configuration in which another current flows by using an amplifier circuit or the like.

Next, the flow rate control valve and its maximum flow rate limiting structure will be described with reference to FIGS. 1, 6 and 7. FIG. 1 is a configuration diagram of a flow control valve, and FIG. 6 is an enlarged view of the valve portion. When receiving the ON / OFF duty signal from the ECU 7, the flow control valve 1 excites the coil when the signal is ON and moves the valve rod 27 to the right against the spring 30. As a result, the valve bodies 27a and 27b provided on the valve rod 27 open the valve holes 31 and 32, respectively, and take in air from the throttle valve upstream side opening (IN) of the bypass passage, which is then introduced into the valve holes 31 and 32. It is divided into 32 parts and is bypassed to the throttle valve downstream side (OUT) of the bypass passage. During the off period of the signal, the force of the spring 30 causes the valve bodies 27a and 27b to move into the valve holes 31 and 3, respectively.
2 is closed and the auxiliary air is shut off. By controlling the duty ratio of such an on / off signal, the amount of auxiliary air is controlled.

A stopper 26 is provided on the spring 30 side of the valve rod 27.
Is provided. The stopper 26 is configured to advance and retreat in the valve rod 27 direction by rotating, and when the shaft of the throttle valve 9 rotates in the valve closing direction, the stopper 26 also interlocks with this via the gears 24 and 25. It rotates so that the distance L between the end of the valve rod 27 and the stopper 26 becomes longer. That is, the smaller the throttle valve opening, the valve rod 27
Is longer, and the larger the throttle valve opening, the shorter the stroke of the valve rod 27 is.

The maximum auxiliary air flow rate is the stopper 26
As shown in FIG. 7, the area of the measuring portion (valve hole when the valve is opened) changes and is determined depending on the position of. Therefore, the characteristic of the auxiliary air flow rate according to this embodiment is as shown in FIG. The stroke limit length L becomes longer as the throttle opening is smaller, and cuts the flow rate at high duty. In the case of the present embodiment, as compared with the embodiment of the electric circuit of FIG. 5, there is an advantage that the relationship of the air flow rate with respect to the duty does not change at a low duty which is not cut, and the control matching becomes easy.

In this embodiment, the gears 24 and 25 are used as the mechanical elements interlocking with the throttle valve 9, but the same effect can be expected by using levers, links, cams or the like instead. It is also possible to use an intake negative pressure that changes with the opening degree of the throttle valve 9 to mechanically limit the maximum flow rate of the flow rate control valve 1.

[0018]

According to the present invention, the maximum value of the auxiliary air amount is limited in accordance with the operating state of the engine, so that even if the control unit or the like fails while the auxiliary air amount control valve is open. Since the maximum auxiliary air flow rate is limited, runaway can be avoided. Therefore, the lean burn engine can achieve both good fuel economy, emission and drivability. Needless to say, the large capacity auxiliary air device for lean burn can also perform auxiliary air control other than lean burn torque control such as idle speed control, electric load, air conditioner load control, fast idle control, etc. .

[Brief description of drawings]

FIG. 1 is a configuration diagram of a flow rate control valve that controls a flow rate of auxiliary air according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a lean burn engine system according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram of the auxiliary air control circuit shown in FIG.

FIG. 4 is a characteristic diagram of the flow control valve shown in FIG.

FIG. 5 is a conceptual diagram of an auxiliary air control circuit according to another embodiment of the present invention.

6 is a partially enlarged view of FIG.

7 is a cross-sectional view taken along the line AA of FIG.

FIG. 8 is a graph (a) showing the relationship between the air-fuel ratio (A / F) and the NOx amount, the air-fuel ratio (A / F) and the NO by the three-way catalyst.
It is a graph (b) showing the relationship with the X purification rate.

FIG. 9 is an A / F map diagram of a lean burn engine.

FIG. 10 is an explanatory diagram of air-fuel ratio switching.

[Explanation of symbols]

1 ... Flow control valve, 5 ... Sensor, 8 ... Air flow sensor,
9 ... Throttle valve, 15 ... Bypass passage, 16 ... Fuel injection valve, 20 ... Three way catalyst, 21 ... O ₂ sensor, 23 ... Throttle sensor, 24, 25 ... Gear, 26 ... Stopper, 2
7 ... Valve rod, 27a, 27b ... Valve body.

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[Procedure amendment]

[Submission date] April 15, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 6

[Correction method] Change

[Correction content]

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 8

[Correction method] Change

[Correction content]

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 9

[Correction method] Change

[Correction content]

[Procedure Amendment 5]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 10

[Correction method] Change

[Correction content]

[Procedure correction 6]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

[0003]

In the above-mentioned prior art, it is necessary to provide an intermediate air-fuel ratio between the stoichiometric air-fuel ratio and the lean air-fuel ratio, which generate the largest amount of NOx emissions, so as not to cause a torque step. is there. FIG. 8A is a graph showing the relationship between the air-fuel ratio (A / F) and the NOx generation amount, and FIG. 8B is a graph showing the relationship between the air-fuel ratio and the NOx purification rate. Since the combustion temperature is high near the stoichiometric air-fuel ratio, the amount of NOx generated is large, but this NOx can be reduced by the action of the three-way catalyst. In addition, the lean air-fuel ratio is not an air-fuel ratio at which the three-way catalyst works efficiently, so NO
Although the purification rate of x is small, there is no problem in terms of emission because the original amount of NOx generated itself is small. However, at an air-fuel ratio intermediate between the stoichiometric air-fuel ratio and the lean air-fuel ratio,
High degree to which there is an amount of NOx generated, yet does not work is a three-way catalyst, there is a problem that the emissions of NOx becomes many.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

[0007]

The above object is to provide a bypass passage bypassing a throttle valve, a flow control valve for controlling the amount of auxiliary air flowing through the bypass passage, and a theoretical air-fuel ratio control for controlling the flow control valve. the size of the valve opening amount is increased to auxiliary air amount of the flow control valve when switching leaner than ratio
In a lean-burn engine system and a control unit to reduce the less to the auxiliary air quantity valve opening degree of the flow control valve when switching from Ku Shi lean air-fuel ratio of the rich direction, the auxiliary air quantity passing through the flow control valve Limiting means for limiting the maximum value of the throttle valve, and the maximum value when the opening amount is large according to the opening amount of the throttle valve.
This is achieved by providing a variable means for varying the maximum value to a small value when the opening amount of the throttle valve is small to a large value.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

[0008]

When a large amount of auxiliary air is supplied to the engine and the engine is operated at a lean air-fuel ratio, it is assumed that, for example, a failure occurs in which the flow control valve remains open. If a failure occurs when the throttle valve opening is small and a large amount of auxiliary air is supplied to the engine in addition to the amount of air passing through the throttle valve, the engine will run out of control. But,
In the present invention, when the throttle valve opening is small , the maximum value of the auxiliary air amount is limited to a small value, so there is no danger of runaway . Although when throttling valve opening is large is set to a large value and the maximum value so as to supply a large amount of auxiliary air to the engine, the throttle valve opening itself large
There is no problem because I am fine.

[Procedure Amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

FIG. 9 is an A / F map of the lean burn engine. The ECU 7 stores the A / s stored in the built-in ROM.
It is judged from the F map and various sensors which region the operating state of the engine is now. For example, medium to high load constant speed running
When the vehicle is in the running state and it is necessary to operate at the stoichiometric air-fuel ratio of A / F = 14.7, feedback control is performed by the detection value of the O ₂ sensor 21 to keep the air-fuel ratio at the stoichiometric air-fuel ratio. When it is determined that the running state is a light load constant-speed running state and the lean-burn air-fuel ratio operation is started, feedback control based on the detection value of the O ₂ sensor 21 is terminated,
The air-fuel ratio is controlled by A / F = by controlling the amount of auxiliary air and the amount of fuel injection.
The lean burn air-fuel ratio is set to 22 to 25, and the swirl control valve 11 is controlled so that the lean fuel is easily combusted, and swirl is generated in the air-fuel mixture.

[Procedure Amendment 10]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

FIG. 3 is a characteristic diagram of the embodiment shown in FIG. E is a voltage applied to the flow control valve 1, and shows a characteristic that the resistance of the sensor 5 is the smallest when the load is E1 at the time of E1 and a characteristic that the load is lighter as the subscript increases. Real
In the embodiment, as shown in FIG.
The auxiliary air flow rate at 100%, that is, its maximum value is small
I am trying to make it small. In the case of the configuration of FIG. 5, a variable resistor is used for the sensor 5, and the change causes the characteristics to move in parallel. In the case of the present embodiment, since the maximum value of the auxiliary air amount is limited by the electric circuit, it is not necessary to limit the number of sensors 5 for detecting the operating state to one, and various types of sensors are used
It is possible to finely control the maximum value by dividing the conditions.
In this embodiment, the same current is supplied to both the flow control valve 1 and the sensor 5, but it is also possible to use an amplifier circuit or the like to form a circuit having a hardware structure in which another current flows.

[Procedure Amendment 11]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

A stopper 26 is provided on the spring 30 side of the valve rod 27.
Is provided. The stopper 26 is configured to advance and retreat in the valve rod 27 direction by rotating, and when the shaft of the throttle valve 9 rotates in the valve closing direction, the stopper 26 also interlocks with this via the gears 24 and 25. When the stopper 2 is rotated, the maximum stroke amount between the end of the valve rod 27 and the stopper 26 is
Only projecting amount of 6 shorter, it has become so that is constrained. That is, the smaller the throttle valve opening, the shorter the stroke of the valve rod 27, and the larger the throttle valve opening,
The stroke of the valve rod 27 is restricted to be long .

[Procedure Amendment 12]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

The maximum auxiliary air flow rate is the stopper 26
As shown in FIG. 7, the area of the measuring portion (valve hole when the valve is opened) changes and is determined depending on the position of. Therefore, the characteristic of the auxiliary air flow rate according to this embodiment is as shown in FIG. The stroke limit length L becomes longer as the throttle opening becomes smaller, and the flow rate at high duty is cut off.
Similarly, the lighter the load, the smaller the maximum auxiliary air flow rate should be.
That it has been in. In the case of the present embodiment, as compared with the embodiment of the electric circuit of FIG. 5 whose characteristics are shown in FIG . 3, the relationship of the air flow rate with respect to the duty does not change at a low duty without cutting,
There is an advantage that control matching becomes easy.

[Procedure Amendment 13]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 6

[Correction method] Change

[Correction content]

[Figure 6]

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area F02M 23/04 Z

Claims (10)

[Claims] 1. A bypass passage bypassing a throttle valve, a flow control valve for controlling the amount of auxiliary air flowing through the bypass passage, and a flow control valve for controlling the air-fuel ratio to leaner than the theoretical air-fuel ratio. And a control unit for increasing the amount of opening of the flow control valve to increase the amount of auxiliary air and reducing the amount of opening of the flow control valve to reduce the amount of auxiliary air when switching from lean to rich air-fuel ratio. In the lean burn engine system, limiting means for limiting the maximum value of the auxiliary air amount passing through the flow control valve, and the maximum value to a small value when the valve opening amount is large according to the valve opening amount of the throttle valve. A lean burn engine system comprising: variable means for varying the maximum value to a large value when the opening amount of the throttle valve is small. 2. The variable limiter according to claim 1, wherein the limiter is a variable resistor that limits a value of a current flowing through an electric circuit that drives the flow control valve, and the value of the variable resistor is changed according to a throttle valve opening. A lean burn engine system characterized by its construction. 3. The lean burn engine system according to claim 2, wherein a sensor for detecting a change in throttle valve opening is used as the variable resistance. 4. The lean burn engine system according to claim 2, wherein a sensor for detecting a change in intake air flow rate is used as the variable resistance. 5. The means for limiting the stroke amount of a flow control valve body according to claim 1, wherein:
The lean burn engine system is characterized in that a mechanical element that adjusts the restricting means in association with a throttle valve opening degree to change a stroke amount is used as the changing means. 6. A bypass passage bypassing the throttle valve, a flow control valve for controlling the amount of auxiliary air flowing through the bypass passage, and a flow control valve for controlling the air-fuel ratio to leaner than the theoretical air-fuel ratio. And a control unit for increasing the amount of opening of the flow control valve to increase the amount of auxiliary air and reducing the amount of opening of the flow control valve to reduce the amount of auxiliary air when switching from lean to rich air-fuel ratio. In the lean burn engine system, a limiting means for limiting the maximum value of the auxiliary air amount passing through the flow rate control valve, and when the engine load is detected and the load becomes large, the maximum value is made small and the engine load is made small. A lean burn engine system is provided with a variable means for varying the maximum value to a large value when the maximum value is reached. 7. The lean burn engine system according to claim 6, wherein the engine load is detected by negative pressure in the intake pipe. 8. A flow control valve for adjusting an amount of auxiliary air flowing through a bypass passage bypassing a throttle valve, and the flow control valve for operating the engine at a lean air-fuel ratio as compared to when operating the engine at a stoichiometric air-fuel ratio. The auxiliary air amount control device for a lean burn engine, comprising: a control device for adjusting the amount of the auxiliary air amount to increase the amount of auxiliary air; Means for varying the full-open amount to a small value when the engine load is small, and a means for varying the full-open amount to a large value. 9. An air-fuel ratio is feedback-controlled to a stoichiometric air-fuel ratio by using a detected value of an oxygen sensor, and when the engine operating state enters a lean burn operating region, the feedback control is interrupted and auxiliary air is supplied to the engine. In a lean burn engine system that operates with a lean air-fuel ratio, the maximum value of the auxiliary air amount is limited, and the maximum value is changed to a smaller value as the engine load is larger. Air amount control method. 10. A flow control valve having a valve body attached to a bypass passage bypassing a throttle valve for turning on / off the bypass passage, a stopper provided in a stroke direction of the valve body for limiting the stroke amount, and a throttle valve. An auxiliary air amount control device comprising: a mechanical element that restricts the stroke amount as the throttle valve opening increases as the stopper advances and retracts in the stroke direction in response to the opening of the shaft.