JPH06241094A - Idling speed control device of engine - Google Patents

Abstract

PURPOSE:To provide an idling speed control device of simple structure, which can enhance the accuracy in idling speed control while the fuel consumption performance and the emission performance are enhanced. CONSTITUTION:A lean burn engine CE has a control unit C which makes feedback control of the idling speed. When the engine speed deviation at idling exceeds the specified value, it is shrunk quickly by adjusting the degree of opening of an electric throttle valve 25, and when below the specified value, the idling speed is feedback controlled accurately by adjusting the air-fuel ratio, i.e., the fuel injecting amount, and it is possible to enhance the idling stability while the fuel consumption performance and emission performance are enhanced. Because no bypass intake passage, ISC valve, etc., are provided, the engine CE can be structured simple, and the manufacturing cost be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine idle speed control device.

[0002]

2. Description of the Related Art Generally, an automobile engine is provided with an idle speed control device for feedback-controlling an engine speed at idle, that is, an idle speed so as to follow a target idle speed. As a control device, a type in which the engine torque is adjusted by changing the intake air amount to control the idle speed has been widely used. In an engine equipped with such a conventional idle speed control device, the air-fuel ratio is generally close to the theoretical air-fuel ratio (A / F = 1) in order to improve idle stability during idling.
4.7, that is, the excess air ratio λ = 1) is set.

In a normal engine, the intake air amount is adjusted by a throttle valve, but it is difficult for a normal throttle valve to make a delicate adjustment of the intake air amount which is required during idling. If the accuracy of controlling the intake air amount of the throttle valve is increased to such an extent that the intake air amount during idling can be adjusted, the throttle valve or its control mechanism becomes complicated, resulting in a significant cost increase.

Therefore, when controlling the idle speed by adjusting the intake air amount, usually, a bypass intake passage for bypassing the throttle valve is provided and an ISC valve (valve intake amount adjusting valve) is provided in the bypass intake passage. The intake air amount is adjusted by this ISC valve to feedback control the idle speed.

[0005]

However, when the bypass passage and the ISC valve are provided in this way, there is a problem that the number of parts increases and the cost increases.
Therefore, focusing on the fact that the engine torque changes when the ignition timing is advanced or retarded, an idle speed control device has been proposed that controls the idle speed by adjusting the ignition timing. However, such an idle speed control device has a problem that fuel efficiency deteriorates when the ignition timing is retarded.

That is, as shown in FIG. 4, the range in which the ignition timing can be changed in order to change the engine torque (F / B range) is a state where the engine torque is maximum (M.B.T. ) and the curve G ₁ showing the, the area limit (engine torque that can retard the ignition timing is sandwiched by the curve G ₃ showing a minimum). Note that, in FIG. 4, the curve G ₂ indicates the center position of the F / B range. Here, the F / B range changes according to the engine load, but in FIG. 4, the load indicated by A indicates the idle state.

FIG. 5 shows the characteristic (curve G ₄ ) of the fuel consumption rate be (fuel consumption rate) with respect to the ignition timing when the engine load is A, that is, when the engine is idle. As is clear from FIG. 5, the fuel rate be becomes larger (worse) as the ignition timing is retarded. Therefore, when the ignition timing is set to the most retarded side of the F / B range during the feedback control of the idle speed, the normal state (M.
The fuel rate be deteriorates by the amount indicated by d as compared with B.T.).

Further, there has been proposed an idle speed control device which controls the idle speed by adjusting the engine torque by changing the air-fuel ratio during idling.
(See, for example, JP-A-64-41638). However, when the idle speed is controlled by the air-fuel ratio in this way, when the idle speed is higher than the target idle speed, the air-fuel ratio is changed to the theoretical air-fuel ratio (A / F = 14.7, that is, λ).
= 1), but generally the NOx emission rate becomes maximum in the air-fuel ratio range slightly leaner than the theoretical air-fuel ratio (A / F is around 16), so emission of NOx There is a problem that it becomes worse.

Therefore, in a predetermined low output region, the air-fuel ratio is
In the so-called lean burn engine, which is set to the lean side of the air-fuel ratio range where the NOx generation rate is maximum, the air-fuel ratio at idle is the air-fuel ratio on the lean side of the air-fuel ratio area where the NOx generation rate is maximum There has been proposed an idle speed control device in which the idle speed is controlled by adjusting (see, for example, Japanese Patent Laid-Open No.
215945). However, in such an idle speed control device, since the range of the air-fuel ratio that can be changed at the time of idling is narrow, there is a problem that responsiveness deteriorates when the deviation of the idle speed from the target idle speed is large. Also, when the idling speed becomes significantly lower than the target idling speed, the air-fuel ratio must be changed to the rich side significantly. In such a case, the air-fuel ratio is in the air-fuel ratio range where the NOx generation rate is high. There is a problem of reaching.

The present invention has been made in order to solve the above-mentioned conventional problems, and is simple and low-cost in which the control accuracy of idle speed control can be improved while improving the fuel consumption performance and the emission performance. It is an object of the present invention to provide an idle speed control device of the above.

[0011]

In order to achieve the above object, as shown in the configuration of FIG. 1, in the first invention, the air-fuel ratio is set leaner than the stoichiometric air-fuel ratio during idling, and In an engine idle speed control device that is feedback-controlled so that the engine speed follows the target idle speed, the intake air amount is changed when the deviation of the idle speed from the target idle speed is relatively large. The idle speed of the engine is controlled by feedback control of the idle speed, and when the deviation is relatively small, the idle speed control means a for feedback controlling the idle speed by changing the air-fuel ratio is provided. Provide a numerical control device.

According to a second aspect of the present invention, in the engine idle speed control device according to the first aspect of the present invention, auxiliary air adjustment capable of adjusting the idle speed by changing the throttle opening to change the intake air amount. Means b and
A fuel injection amount adjusting means c capable of adjusting the air-fuel ratio by changing the fuel injection amount is provided in the air-fuel ratio region on the lean side of the air-fuel ratio region where the NOx generation rate is the maximum, and idle speed control is provided. When the deviation of the idle speed from the target idle speed is equal to or more than a predetermined value, the means a controls the idle speed by feedback control by adjusting the intake air amount via the auxiliary air adjusting means b.
When the deviation is less than the predetermined value, the idle speed control device for an engine is characterized in that the idle speed is feedback-controlled by adjusting the air-fuel ratio through the fuel injection amount adjusting means c. provide.

According to a third aspect of the present invention, in the engine idle speed control device according to the first aspect of the present invention, the auxiliary air adjustment capable of adjusting the idle speed by changing the throttle opening to change the intake air amount. Means b and
A fuel injection amount adjusting means c capable of adjusting the air-fuel ratio by changing the fuel injection amount is provided in the air-fuel ratio region on the lean side of the air-fuel ratio region where the NOx generation rate is the maximum, and idle speed control is provided. When the means a has a deviation of the idling speed from the target idling speed, first, the idle speed is feedback-controlled by adjusting the intake air amount through the auxiliary air adjusting means b, and then the deviation is equal to or less than a predetermined value. Fuel injection amount adjusting means c
(EN) An idle speed control device for an engine, wherein the idle speed is feedback-controlled by adjusting an air-fuel ratio via the.

[0014]

EXAMPLES Examples of the present invention will be specifically described below.
As shown in FIG. 2, each cylinder (1
(Only one is shown), basically, when the first and second intake valves 1 and 2 are opened, the first and second independent intake passages are connected via the first and second intake ports 3 and 4. The air-fuel mixture is sucked into the combustion chamber 7 from 5, 6 and is compressed by the piston 8 to be ignited and combusted by the spark plug 9, so that the first and second exhaust valves 10, 11
When the engine is opened, the combustion gas is discharged to the first and second exhaust ports 1
The process of discharging to the first and second independent exhaust passages 14 and 15 via 2 and 13 is repeated. The engine CE has a rich zone in which the air-fuel ratio A / F is set to approximately the theoretical air-fuel ratio (A / F = 14.7, that is, an excess air ratio λ = 1), and the air-fuel ratio is lean (A / F = 1).
9 to 24) and a lean burn engine in which rich / lean zone switching is performed according to the operating state.

By the continuous repetition of such a process, the piston 8 reciprocates in the axial direction of the cylinder bore, and the reciprocating motion of the piston 8 is performed via the connecting rod 16, the crank pin 17, and the crank arm 18. The rotary motion is converted and transmitted to the crankshaft 19. A high voltage is applied to the spark plug 9 from the direct ignition coil 20 at a predetermined timing, and the high voltage causes a spark discharge in the electrode portion of the spark plug 9. The first and second independent exhaust passages 14 and 15 are gathered downstream in one common exhaust passage 41, and a catalytic converter 42 for purifying exhaust gas is provided in the common exhaust passage 41. Common exhaust passage 4
1, a linear O ₂ sensor 63 for detecting the O ₂ concentration in the exhaust gas (that is, the air-fuel ratio) and first and second exhaust temperature sensors 64, 65 for detecting the exhaust gas temperature are provided in advance. Also, the cooling water temperature (engine temperature)
A water temperature sensor 66 for detecting the water temperature is provided.

In order to supply air to the combustion chamber 7 of each cylinder, a single common intake passage 21 whose upstream end is open to the atmosphere.
The common intake passage 21 is provided with an air cleaner 22 for removing dust in the intake air, an air flow sensor 23 for detecting the intake air amount, and a control unit C in order from the upstream side in the flow direction of the air. An electric throttle valve 25 that is opened and closed by a servo motor 24 according to the applied signal is interposed. The downstream end of the common intake passage 21 is connected to a surge tank 26 that stabilizes the flow of intake air, and the surge tank 26 is provided with the first and second independent intake passages 5 for each cylinder.
The upstream ends of 6 are connected. In addition, this intake system,
A boost sensor 61 that detects an intake negative pressure in the surge tank 26 and an intake temperature sensor 62 that detects an intake air temperature.
And a throttle opening sensor 6 for detecting the throttle opening
7 are provided.

In the engine CE, the intake air intake amount for controlling the idle speed is adjusted by the electric throttle valve 25 as will be described later, so that the bypass intake air bypassing the throttle valve is used. No ISC valve is provided to control the passage or bypass intake amount. Therefore, the number of parts is reduced and the manufacturing cost of the engine CE is reduced.

A fuel injection valve 31 for injecting fuel (gasoline) into the air in the first independent intake passage 5 to form a mixture is provided for the first independent intake passage 5. The control unit C controls the fuel injection amount of the fuel injection valve 31 so that the air-fuel ratio A / F of the air-fuel mixture follows the target air-fuel ratio set according to the operating state (engine speed etc.) using the air-fuel ratio map. Is controlled by.

An open / close valve 32 is provided in the second independent intake passage 6.
The on-off valve 32 is opened and closed by an actuator 33 composed of a negative pressure responsive diaphragm device. Specifically, by the three-way valve 35,
Whether the negative pressure in the surge tank 26 or the atmospheric pressure is introduced into the pressure chamber of the actuator 33 through the negative pressure passage 34 is switched, and when the negative pressure is introduced into the pressure chamber, the actuator 33 opens and closes the valve. When the valve 32 is closed and the atmospheric pressure is introduced into the pressure chamber, the opening / closing valve 32 is opened. The on-off valve 32 is closed in a low load region or the like, at which time air is supplied to the combustion chamber 7 only from the first intake port 3, swirl is generated in the combustion chamber 7, and the air-fuel mixture is stratified in the combustion chamber 7. And the ignitability and combustibility of the air-fuel mixture are improved. The on-off valve 32 is opened in a high load region or the like, and at this time, air is supplied from both intake ports 3 and 4 into the combustion chamber 7 so that intake charging efficiency is improved.

A blow-by gas passage 37 equipped with a PCV valve 36 is provided for introducing blow-by gas generated in the engine (crank chamber) into the surge tank 26.
Is provided. Further, a fresh air introduction passage 38 for supplying a part of the air in the common intake passage 21 into the engine is provided in order to accelerate the discharge of the blow-by gas.

The fuel supply system for supplying fuel (gasoline) to the fuel injection valve 31 will be described below. In this fuel supply system, the fuel in the fuel tank 45 is the fuel filter 4
After being sucked into the fuel pump 47 through 6, the fuel pump 47 is discharged at a predetermined discharge pressure and is supplied to the fuel injection valve 31 through a fuel supply passage 48 in which a filter 49 is interposed. Excess fuel that is not injected by the fuel injection valve 31 is returned to the fuel tank 45 through a fuel return passage 51 in which a pressure regulator 52 that controls the fuel pressure is provided. In order to eliminate the influence of the intake negative pressure, the intake negative pressure in the surge tank 26 is introduced through the negative pressure introducing passage 53.

The air containing the gasoline vapor in the fuel tank 45 is released to the surge tank 26 through the in-tank air release passage 54 in order to prevent the diffusion of the gasoline vapor into the atmosphere. And
In the in-tank air release passage 54, a separator 55 for separating the gasoline mist in the released air, a two-way valve 56, and a gasoline vapor in the released air in order from the upstream side in the flow direction of the air. A canister 57 for adsorbing the toner, an orifice 58, and a duty solenoid valve 59 are interposed.

In the engine CE, the control unit C equipped with a microcomputer detects the intake air amount detected by the air flow sensor 23, the intake negative pressure detected by the boost sensor 61, and the intake temperature sensor 62. Intake air temperature,
O ₂ concentration (air-fuel ratio) detected by the linear O ₂ sensor 63, exhaust gas temperature detected by the first and second exhaust temperature sensors 64, 65, cooling water temperature (engine temperature) detected by the water temperature sensor 66 , Throttle opening sensor 6
Throttle opening and rotation speed sensor detected by 7
(Not shown) engine speed detected by the accelerator, accelerator opening sensor (not shown) detected accelerator opening, accelerator switch (not shown) accelerator switch signal or the like output from the control information, Various controls are performed.

However, since the general control of the engine CE is not the subject of the present invention, its explanation is omitted, and in the following, according to the flow chart shown in FIG.
Only the idle speed control according to the gist of the present invention will be described with reference to FIG. 2 as appropriate. The control unit C is a comprehensive control device including the “idle speed control means”, the “auxiliary air adjusting means”, and the “fuel injection amount adjusting means” described in the claims.

In this idle speed control, basically, the deviation of the idle speed from the target idle speed is determined.
When the rotational speed deviation is comparatively large, feedback control of the idle rotational speed is first performed by adjusting the opening of the electric throttle valve 25, and when the rotational speed deviation is comparatively small, the air-fuel ratio, that is, the fuel injection. The idle speed is feedback-controlled by adjusting the amount.

That is, when the intake air amount is adjusted by the electric throttle valve 25 to control the idling speed, the electric throttle valve 25 is almost closed in the idle region, so the intake air amount should be finely adjusted. However, the control accuracy is not so high. However, the range of the intake air amount that can be changed, that is, the range of the engine torque that can be changed is very wide (it can be considered as 0 to ∞ when controlling the idle speed). Even when the deviation is large, it is possible to reduce the rotation speed deviation rapidly (good responsiveness). Therefore, when the rotational speed deviation is large, the idle rotational speed is feedback-controlled by adjusting the opening of the electric throttle valve 25 in order to quickly reduce the rotational speed deviation.

On the other hand, when controlling the idle speed by adjusting the air-fuel ratio (fuel injection amount) in the lean zone,
Since the fuel injection amount of the fuel injection valve 31 can be accurately controlled, the control accuracy of the idle control becomes extremely high.
However, in the lean zone, the range of air-fuel ratio that can be changed, that is, the range of engine torque that can be changed is relatively narrow. However, if the air-fuel ratio goes too far to the lean side, engine stall occurs, and if it goes too far to the rich side, it reaches the air-fuel ratio range where the NOx generation rate becomes high. Therefore, when the rotation speed deviation is large, it is difficult to reduce it rapidly (with good responsiveness). Therefore, only when the rotation speed deviation is small, the idle rotation speed is feedback-controlled by adjusting the air-fuel ratio (fuel injection amount) in order to precisely control the idle rotation speed.

Specifically, when the idle speed control is started, first, at step # 1, the engine speed Ne and
The throttle opening TVO, the intake air amount Ce, the accelerator switch signal, and the air-fuel ratio A / F are read. Next, in step # 2, it is compared and determined whether or not the engine CE is in the idle state. If it is determined that the engine CE is not in the idle state (NO), there is no need to control the idle speed, so Skip all steps and step # 1
Return to. It The comparison / judgment as to whether or not the vehicle is in the idle state is performed based on the accelerator switch signal.

On the other hand, if it is determined in step # 2 that the engine is in the idle state (YES), the target air-fuel ratio AF ₀ is calculated in step # 3. Here, the target air-fuel ratio AF ₀ is
It is set according to the engine speed Ne and the intake air amount Ce by searching the air-fuel ratio map by a generally known ordinary method. As described above, in the engine CE, the fuel injection amount of the fuel injection valve 31 is controlled by the control unit C so that the air-fuel ratio follows the target air-fuel ratio.

Next, in step # 4, it is compared and judged whether or not the target air-fuel ratio AF ₀ is lean, that is, whether the engine CE is in the lean zone and is operated at a lean air-fuel ratio. . As described above, the target air-fuel ratio in the lean zone is far to the lean side (A
/ F = 19 to 24), fuel efficiency and emission performance are significantly improved. If it is determined that the engine is not lean (NO), the idle speed control cannot be performed by adjusting the air-fuel ratio, so all the subsequent steps are skipped and the process returns to step # 1.

On the other hand, if it is judged to be lean in step # 4 (YES), the current engine speed Ne is read in step # 5 and this current engine speed Ne is read.
Is stored as the idle speed Ne ₁ , and subsequently, in step # 6, the target engine speed, that is, the target idle speed Ne ₀ is calculated. The target idle speed Ne ₀ is
It is set by a well-known ordinary method according to the operating state of the engine CE (for example, engine temperature, intake air temperature, drive state of auxiliary machinery, etc.).

Next, at step # 7, the idle speed N
It is compared and determined whether or not the deviation between e ₁ and the target idle speed Ne _{0, that} is, the absolute value ΔNe (ΔNe = | Ne ₀ −Ne ₁ |) of the speed deviation is larger than a predetermined value K ₀ . Where K ₀
Is set to a value that becomes a boundary at which the idle speed control by adjusting the air-fuel ratio becomes difficult when ΔNe becomes larger than this.

When it is determined in step # 7 that ΔNe> K ₀ (YES), that is, when the rotation speed deviation is relatively large, steps # 8 to # 8 are executed to quickly reduce the rotation speed deviation. In step # 10, the electric throttle valve 2
By adjusting the opening degree of 5, the idle speed is feedback-controlled. That is, in step # 8, it is compared and determined whether or not the actual idle speed Ne ₁ is larger than the target idle speed Ne _{0, and} if it is determined that Ne ₁ ≦ Ne ₀ (NO), idle Since the rotation speed is less than or equal to the target idle rotation speed, the electric throttle valve 25 is controlled to the open side in step # 9, and the idle rotation speed is increased. On the other hand, when it is determined that Ne ₁ > Ne ₀ ,
(YES), the idle speed exceeds the target idle speed, so in step # 10, the electric throttle valve 25
Is controlled to the closed side, and the idle speed is reduced. After that, the process is returned to step # 5 and the control is continued. Note that steps # 8 to # 10 are repeated until ΔNe ≦ K ₀ , that is, until the rotation speed deviation is reduced to a predetermined value or less.

By the way, in step # 7, ΔNe
When it is determined that ≦ K ₀ (NO), that is, when the rotation speed deviation is relatively small, in order to accurately control the idle rotation speed, in steps # 11 to # 14,
The idle speed is feedback-controlled by adjusting the air-fuel ratio, that is, the fuel injection amount of the fuel injection valve 31. That is, in step # 11, it is compared and determined whether the actual idle speed Ne ₁ is larger than the target idle speed Ne _{0, and} if it is determined that Ne ₁ > Ne ₀ ,
(YES), the idle speed exceeds the target idle speed, so in step # 12, the air-fuel ratio is controlled to the lean side with a predetermined small width, and the idle speed is reduced.
On the other hand, if it is determined that Ne ₁ ≦ Ne ₀ (NO), the idle speed is less than or equal to the target idle speed, so in step # 13, the air-fuel ratio is controlled to the rich side with a predetermined small width. The idle speed is increased.

Thereafter, in step # 14, the deviation of the number of revolutions is
Whether or not (Ne ₀ −Ne ₁ ) becomes 0, that is, whether or not the actual idle speed matches the target idle speed is compared and determined. If Ne ₀ −Ne ₁ ≠ 0, NO), the process returns to step # 11, and steps # 11 to # 1
4 is repeated. On the other hand, if Ne ₀ −Ne ₁ = 0 (Y
ES), and returns to step # 1. If ΔNe is originally small and is equal to or smaller than K ₀ , the idle speed is directly feedback-controlled by adjusting the air-fuel ratio in steps # 11 to # 14 without going through steps # 8 to # 10. Become.

As described above, according to the present embodiment, by effectively utilizing the electric throttle valve 25, the idling speed is set as a target at the time of idling with a simple structure without providing the bypass intake passage or the ISC valve. It is possible to quickly and accurately follow the idling speed, and to improve the idling stability. In addition, fuel efficiency performance and emission performance are improved.

[0037]

According to the first aspect of the invention, when the deviation of the idling speed from the target idling speed (rotational speed deviation) is large, the intake air amount adjustment capable of setting a large variation range of the engine torque. Since the idle speed is feedback-controlled by, the control response is enhanced and the speed deviation is reduced rapidly. On the other hand, when the deviation of the rotation speed is small, the idle rotation speed is feedback-controlled by the air-fuel ratio adjustment capable of precisely controlling the engine torque, so that the control accuracy of the idle rotation speed control is improved. For this reason, it is possible to make the idle speed quickly and accurately follow the target idle speed while improving fuel efficiency and emission performance during idling.
Idle stability can be increased.

According to the second invention, basically, the same operation and effect as those of the first invention can be obtained. Further, when the deviation of the rotational speed is large, the idle rotational speed control is performed by adjusting the throttle valve opening, so that it is not necessary to provide the bypass intake passage or the ISC valve, and the structure of the engine is simplified, and the manufacturing thereof is simplified. Cost is reduced.

According to the third invention, basically, the same operation and effect as those of the first invention can be obtained. Further, when there is a rotation speed deviation, first the idle speed control is performed by adjusting the throttle valve opening degree, and then the idle speed control is performed by adjusting the air-fuel ratio. Responsiveness and accuracy are further enhanced. Moreover, since it is not necessary to provide the bypass intake passage or the ISC valve, the structure of the engine is simplified and the manufacturing cost thereof is reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of first to third inventions corresponding to claims 1 to 3.

FIG. 2 is a system configuration diagram of an engine including an idle speed control device according to the present invention.

FIG. 3 is a flowchart showing a control method of idle speed control.

FIG. 4 is a diagram showing a range in which the ignition timing can be changed in the conventional idle speed control by adjusting the ignition timing.

FIG. 5 is a diagram showing a characteristic of a fuel consumption rate with respect to ignition timing in a conventional engine.

[Explanation of symbols]

CE ... Engine C ... Control unit 24 ... Servo motor 25 ... Electric throttle valve 31 ... Fuel injection valve 63 ... Linear O ₂ sensor

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Takagi 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Motor Corporation

Claims (3)

[Claims] 1. An idle speed control device for an engine, wherein an air-fuel ratio is set leaner than a stoichiometric air-fuel ratio during idling and feedback control is performed so that the idle speed follows a target idle speed. , When the deviation of the idle speed from the target idle speed is relatively large, the idle speed is feedback-controlled by changing the intake air amount, and when the deviation is relatively small, the idle speed is changed by changing the air-fuel ratio. An idle speed control device for an engine, comprising an idle speed control means for feedback control. 2. The engine idle speed control device according to claim 1, wherein auxiliary air adjusting means is capable of adjusting the idle speed by changing the intake air amount by changing the throttle opening. A fuel injection amount adjusting means capable of adjusting the air-fuel ratio by changing the fuel injection amount is provided in the air-fuel ratio range on the lean side of the air-fuel ratio range where the NOx generation rate becomes maximum, and the idle speed control means However, when the deviation of the idle speed from the target idle speed is equal to or more than a predetermined value, the idle speed is feedback-controlled by adjusting the intake air amount through the auxiliary air adjusting means, while the deviation is less than the predetermined value. Sometimes, the idle speed is feedback controlled by adjusting the air-fuel ratio via the fuel injection amount adjusting means. Idling speed control system for an engine according to claim Rukoto. 3. The engine idle speed control device according to claim 1, wherein the auxiliary air adjusting means is capable of adjusting the idle speed by changing the intake air amount by changing the throttle opening. A fuel injection amount adjusting means capable of adjusting the air-fuel ratio by changing the fuel injection amount is provided in the air-fuel ratio range on the lean side of the air-fuel ratio range where the NOx generation rate becomes maximum, and the idle speed control means However, when there is a deviation of the idle speed from the target idle speed, feedback control of the idle speed is first performed by adjusting the intake air amount through the auxiliary air adjusting means, and then the deviation becomes less than or equal to the predetermined value. Since then, the idle speed is feedback-controlled by adjusting the air-fuel ratio via the fuel injection amount adjusting means. DOO idling speed control system for an engine according to claim.