JPH05302543A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To improve operability and an exhaust gas characteristic while improving the fuel consumption by setting specified air-fuel ratio in accordance with an exhaust gas recirculation condition, in an engine wherein air-fuel ratio of a mixture is controlled to the specified air-fuel ratio in a lean side from the theoretical air-fuel ratio at the time of specified operation. CONSTITUTION:In the case of calculating fuel injection time of a fuel injection valve 6 by an ECU5, first in accordance with an engine speed NE and an intake pipe absolute pressure PBA, basic fuel injection time is calculated. In accordance with an engine operative condition, an objective air-fuel ratio coefficient of representing objective air-fuel ratio, air-fuel ratio correction coefficient, other correction coefficient and correction variable are respectively calculated, and the basic fuel injection time is corrected by these correction coefficients and variables, to obtain the final fuel injection time. Here is determined objective air-fuel ratio based on a basic value of its coefficient set to a value larger in accordance with increasing an engine load, and further this objective air-fuel ratio is set, for instance, to 14.7 or 22 at EGR time and to an intermediate value between these values, for instance, 18 at EGR on time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to a device for controlling the air-fuel ratio of an air-fuel mixture supplied to an engine having an exhaust gas recirculation mechanism to a predetermined air-fuel ratio leaner than the theoretical air-fuel ratio. Regarding

[0002]

2. Description of the Related Art An air-fuel ratio (hereinafter referred to as "supply air-fuel ratio") of an air-fuel mixture supplied to an internal combustion engine equipped with an exhaust gas recirculation mechanism is controlled to a stoichiometric air-fuel ratio, and exhaust gas recirculation (hereinafter referred to as "EGR").
It is conventionally known that the exhaust gas characteristics are improved.

Further, in the case of controlling the supply air-fuel ratio to a predetermined air-fuel ratio leaner than the stoichiometric air-fuel ratio, there is also known a method of reducing the EGR amount or stopping the EGR in order to stabilize combustion and improve operability. It is (actual public Sho 61-23631
Publication).

[0004]

However, according to the method of controlling the supply air-fuel ratio to the stoichiometric air-fuel ratio and performing the EGR, the exhaust gas characteristics are improved, but there is still room for improvement in terms of fuel consumption. ..

Further, if the EGR amount is reduced or stopped when the supply air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, the improvement of the exhaust gas characteristic, which is the original purpose of performing EGR, becomes insufficient.

The present invention has been made in view of the above points, and an object thereof is to provide an air-fuel ratio control device capable of improving fuel efficiency and satisfying the requirements of drivability and exhaust gas characteristics. To do.

[0007]

To achieve the above object, the present invention sets the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to a predetermined air-fuel ratio leaner than the theoretical air-fuel ratio when the engine is in a predetermined operating state. In an air-fuel ratio control device for an internal combustion engine having lean control means for controlling and exhaust gas recirculation means for recirculating a part of the exhaust gas of the engine to an intake system, a lean for setting the predetermined air-fuel ratio according to the exhaust gas recirculation state. An air-fuel ratio setting means is provided.

Further, it is desirable that the exhaust gas recirculation state is whether or not exhaust gas recirculation is performed.

[0009]

The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled to a predetermined air-fuel ratio leaner than the stoichiometric air-fuel ratio, and the predetermined air-fuel ratio is set according to the exhaust gas recirculation state.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter simply referred to as “engine”) equipped with an exhaust gas recirculation mechanism according to an embodiment of the present invention and a control system therefor. A throttle valve 3 is provided in the middle of the pipe 2. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 and outputs an electric signal according to the opening of the throttle valve 3 to output an electronic control unit for engine control (hereinafter referred to as “ECU”) 5
Supply to.

The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) in the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). The valve opening time of fuel injection is controlled by a signal from the ECU 5 that is electrically connected to the ECU 5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 7 is provided immediately downstream of the throttle valve 3, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 7 is supplied to the ECU 5. .. Further, an intake air temperature (TA) sensor 8 is attached downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies it to the ECU 5.

The engine water temperature (TW) sensor 9 mounted on the main body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal and supplies it to the ECU 5. The engine speed (NE) sensor 10 and the cylinder discrimination (CYL) sensor 11 are mounted around the cam shaft or crank shaft (not shown) of the engine 1. The engine speed sensor 10 outputs a pulse (hereinafter referred to as a "TDC signal pulse") at a predetermined crank angle position every 180 degrees rotation of the crank shaft of the engine 1, and the cylinder discrimination sensor 11 outputs a predetermined crank angle of a specific cylinder. A signal pulse is output at the position, and each of these signal pulses is supplied to the ECU 5.

The three-way catalyst 14 is arranged in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO and NOx in the exhaust gas. An oxygen concentration sensor (hereinafter referred to as “LAF sensor”) 12 as an exhaust gas concentration detector is mounted on the exhaust pipe 13 upstream of the three-way catalyst 14, and detects the oxygen concentration in the exhaust gas to detect the detected value. Is output and supplied to the ECU 5. The oxygen concentration sensor 12 is a linear type that outputs a signal proportional to the oxygen concentration.

The ECU 5 is further connected to a vehicle speed sensor 31 for detecting the speed of a vehicle equipped with the engine 1 and a clutch sensor 32 for detecting the on / off of the clutch, and the detection signals of these sensors are supplied to the ECU 5. It

Next, the exhaust gas recirculation mechanism 20 will be described.

The exhaust gas recirculation path 21 of this mechanism 20 has one end 2
1a communicates with the upstream side of the three-way catalyst 14 of the exhaust pipe 13, and the other end 21b communicates with the downstream side of the throttle valve 3 of the intake pipe 2.
An exhaust gas recirculation valve 22 for controlling the exhaust gas recirculation amount and a volume chamber 21C are provided in the exhaust gas recirculation passage 21. The exhaust gas recirculation valve 22 is a solenoid valve having a solenoid 22a, which is connected to the ECU 5.
The valve opening is configured to be linearly changed by a control signal from the ECU 5. The exhaust gas recirculation valve 22 has a lift sensor 23 that detects the valve opening degree.
Is provided, and the detection signal thereof is supplied to the ECU 5.

The ECU 5 determines the engine operating state based on the engine parameter signals from the above-mentioned various sensors and the like, and the valve opening degree of the exhaust gas recirculation valve 22 set according to the intake pipe absolute pressure PBA and the engine speed NE. Command value LCMD
And the exhaust gas recirculation valve 22 detected by the lift sensor 23
A control signal is supplied to the solenoid 22a so as to make the deviation from the actual valve opening value LACT of 0.

The ECU 5 shapes the input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and the like, a central processing circuit (hereinafter referred to as a central processing circuit). 5b, referred to as a "CPU", a storage means 5c for storing various calculation programs executed by the CPU 5b and calculation results, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, and the like.

The CPU 5b discriminates various engine operating states such as a feedback control operating region to the stoichiometric air-fuel ratio by the oxygen concentration sensor 12 and an open loop control operating region based on the above various engine parameter signals, and at the same time, the engine operating state. According to the fuel injection time TOUT of the fuel injection valve 6 and the valve opening command value LCMD of the exhaust gas recirculation valve.
Is calculated.

Fuel injection by the fuel injection valve 6 is performed in synchronization with the TDC signal pulse, and the fuel injection time TOUT is calculated by the following equation (1).

TOUT = TI × KCMD × KLAF × K1 + K2 (1) where TI is the basic fuel amount, specifically, the engine speed N
The basic fuel injection time is determined according to E and the intake pipe absolute pressure PBA, and a TI map for determining this TI value is stored in the storage means 5c.

KCMD is a target air-fuel ratio coefficient which is set according to the engine operating condition and represents a target air-fuel ratio.

KLAF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 12 matches the target air-fuel ratio during the air-fuel ratio feedback control, and depends on the engine operating state during the open loop control. It is set to a predetermined value.

K1 and K2 are other correction factors and correction variables calculated according to various engine parameter signals, respectively, so that various characteristics such as fuel consumption characteristics and engine acceleration characteristics can be optimized according to the engine operating state. Is set to any value.

The CPU 5b outputs a signal for driving the fuel injection valve 6 and the exhaust gas recirculation valve 22 through the output circuit 5d based on the result calculated as described above.

FIG. 2 is a flow chart of a main routine for calculating the target air-fuel ratio coefficient KCMD and the air-fuel ratio correction coefficient KLAF, and this routine is executed in synchronization with each generation of the TDC signal pulse.

In step S1, it is determined whether or not the mode is the starting mode. If the answer is affirmative (YES), the KLAF value is set to the value 1.0 and the present routine is terminated. Step S
When the answer to 1 is negative (NO), that is, when the engine is not in the starting mode, the KCMD calculation subroutine shown in FIG. 3 is executed (step S2) to determine whether or not the LAF sensor 12 is in an active state (a state in which oxygen concentration can be detected). Is determined (step S3). When the answer is negative (NO), the routine proceeds to step S6, and when the answer is positive (YES), the equivalence ratio KACT representing the detected air-fuel ratio is calculated based on the output of the LAF sensor 12 (step S4). The air-fuel ratio correction coefficient KLAF is calculated so that the target air-fuel ratio coefficient KCMD and the detected air-fuel ratio KACT match (step S
5) Then, this routine ends.

According to the main routine of FIG. 2, when the engine is not in the starting mode and the LAF sensor 12 is in the active state, the detected air-fuel ratio (KACT) is the target air-fuel ratio (KCM).
The KLAF value is calculated so as to match D).

FIG. 3 is a flow chart of a subroutine for calculating the target air-fuel ratio coefficient KCMD executed in step S2 of FIG.

In step S11, the clutch sensor 32
It is determined whether or not the shift change is in progress based on the output of, and when the answer is affirmative (YES), the current value KC of KCMD is KC.
MD (N) is set to the previous value KCMD (N-1) and this routine ends. The answer to step S11 is negative (N
O), that is, when the shift change is not in progress, it is determined whether or not the fuel cut is in progress (step S13). When the answer is negative (NO), that is, when the fuel cut is not in progress, the basic value KBSM of the target air-fuel ratio coefficient is calculated by the subroutine shown in FIG. 4 (step S14),
When the answer to step S13 is affirmative (YES), that is, during fuel cut, the basic value KBSM is set to the predetermined value KCMDFC for fuel cut (step S15).
This KCMDFC value is the KC immediately after the fuel cut.
It is used as the previous value KCMD (N-1) of the MD value.

In step S16, step S14 or S
The basic value KBSM calculated in 15 is set as the current value KCMD (N) of the target air-fuel ratio coefficient (step S16), and further, FIG.
Perform the KCMD limit processing shown in (step S1
7) Then, this routine ends.

According to the subroutine of FIG. 3, when the shift change is not in progress and the fuel cut is not in progress,
KB calculated by the subroutine of FIG. 4 described below
The SM value is made the target air-fuel ratio coefficient KCMD (step S
14, S16).

FIG. 4 shows that in step S14 of FIG.
5 is a flowchart of a subroutine for calculating a basic value KBSM of a target air-fuel ratio coefficient.

In step S21, it is determined whether the engine water temperature TW is lower than a predetermined water temperature TWLEAN (eg, 65 ° C.), and the answer is affirmative (YES), that is, TW <TWL.
When EAN, search the KTWLAF table,
The low water temperature target air-fuel ratio coefficient KTWLAF is calculated (step S23), the KBSM value is set to the KTWLAF value (step S24), and the process proceeds to step S25. KTWLAF
The table is set to a value corresponding to the air-fuel ratio on the rich side of the stoichiometric air-fuel ratio, and KT increases as the engine water temperature TW increases.
The WLAF value is set to decrease.

When the answer to step S21 is negative (NO), that is, when TW ≧ TWLEAN, the KBSM value is calculated by the subroutine shown in FIG. 5 (step S2).
2), go to step S25.

In step S25, it is determined whether or not the engine is in the idle state. When the answer is affirmative (YES), the KBSM value is set to the predetermined value KBSMI for idle.
It is set to DL (for example, a value corresponding to A / F = 14.7) (step S26), and this routine is ended.

When the answer to step S25 is negative (NO), that is, when the vehicle is not in the idle state, the vehicle speed V is the predetermined vehicle speed VPL.
It is determined whether it is lower than AF (for example, 20 km / h) (step S27). This answer is negative (NO), that is, V ≧
When VPLAF, this routine is immediately terminated, and when affirmative (YES), that is, when V <VPLAF, KBS
The M value is a predetermined value KBSWLF for low vehicle speed (for example, A / F =
(A value equivalent to 14.7) (step S28), and this routine ends.

According to the subroutine shown in FIG. 4, when the engine water temperature TW is equal to or higher than the predetermined water temperature TWLEAN and the vehicle speed V is equal to or higher than the predetermined vehicle speed VPLAF, step S2 is performed.
2, the KBSM value calculated by the subroutine of FIG. 5 described below is adopted.

In step S31 of FIG. 5, KB is determined according to the detected engine speed NE and the intake pipe absolute pressure PBA.
Search the SM map. The KBSM map has predetermined values KBSM1 to KBSM5 (for example, 20H and 30 respectively, corresponding to regions A to E set according to the engine speed NE and the intake pipe absolute pressure PBA as shown in FIG. 7, for example.
H, 40H, 60H, 80H and H are hexadecimal numbers), and the map is set to A / B for KBSM1.
A value corresponding to F = 22, a value corresponding to A / F = 20 for KBSM2, a value corresponding to A / F = 18 for KBSM3, a value corresponding to A / F = 16 for KBSM4, and an A / F = 14.7 for KBSM5.
It is a considerable value.

In the following step S32, an FKBSM for setting the lean burn permission determination flag FKBSMJG and the EGR target A / F permission determination flag FKBSMJGE.
The setting subroutine (FIG. 6) is executed.

In step S51 of FIG. 6, it is determined whether or not the KBSM value retrieved in step S31 of FIG. 5 is larger than the EGR target A / F permission determination threshold value KBSMJGE, and the answer is affirmative (YES). , Ie KBSM> KBS
At MJGE EGR target A / F permission determination flag FK
When BSMJGE = 1 (step S53), the answer is negative (NO), that is, FK when KBSM ≦ KBSMJGE.
BSMJGE = 0 is set (step S52). here,
FKBSMJGE = 1 indicates that the EGR target A / F can be selected. The threshold value KBSMJGE is
For example, it is set to 3FH (a value (on the lean side) slightly smaller than the value (40H) corresponding to A / F = 18). Note that hysteresis may be added to KBSMJGE to prevent hunting in determination.

In a succeeding step S54, it is determined whether or not the vehicle speed V is higher than a predetermined vehicle speed VKBSM (for example, 80 km / h). If the answer is negative (NO), that is, V≤VKBSM, the lean burn permission judgment is performed. The threshold value KBSMJG is set to the low vehicle speed threshold value KBSMJG1, and the answer is affirmative (YES), that is, when V> VKBSM, the KBSMJG value is set to the high vehicle speed threshold value KBSMJG2. here,
Hysteresis is added to the thresholds KBSMJG1 and KBSMJG2, and the threshold KBSM for low vehicle speed is set.
JG1 is, for example, 21H / 5FH (the smaller value is 21
H, the larger value is 5FH), and the high vehicle speed threshold KBSMJG2 is set to 21H / 7FH, for example. 21
H is a value slightly larger (rich side) than A / F = 22 equivalent, 5FH is slightly smaller (lean side) than A / F = 16 equivalent, and 7FH is A / F = 1
This value is slightly smaller (on the lean side) than the value equivalent to 4.7.

In the following step S57, it is determined whether or not the KBSM value is larger than the lean burn permission determination threshold value KBSMJG determined in step S55 or S56.
The answer is affirmative (YES), that is, KBSM> KBSMJG
When it is, it is determined that it is outside the lean burn area, and the lean burn permission determination flag FKBSMJG = 0 is set. On the other hand, when the answer is negative (NO), that is, when KBSM ≦ KBSMJG, it is determined that the lean burn area is set and FKBSMJG = 1.
And

According to the subroutine of FIG. 6, the EGR target A / F permission judgment flag FKBSMJGE is set to a value 0 in the areas A and B of FIG. 7 and set to a value 1 in the areas C, D and E. .. Further, the lean burn permission determination flag FKBSMJG is set to a value 0 in the areas B, C, D and E and set to a value 1 in the area A when the KBSM value is decreasing, regardless of whether the vehicle speed is high or low. Increased value and high vehicle speed (V> VKBSM)
, The value is set to 0 in the area E, and the areas A, B,
The value is set to 1 in C and D, the KBSM value increases, and the vehicle speed is low (V ≦ VKBSM)
, The value is set to 0 in the areas D and E, and the area A,
The value is set to 1 in B and C.

Returning to FIG. 5, in step S33, it is determined whether or not the LAF feedback flag FLAFFB which is set to 1 when the engine is in the operating state in which the feedback control should be executed based on the output of the LAF sensor 12 is 1 If the answer is negative (NO), that is, FLA
When FFB = 0, a target A / F permission flag FEGRAF for EGR, which will be described later, is set to 0 (step S3).
4) and proceeds to step S41.

When the answer to step S33 is affirmative (YES), that is, when FLAFFB = 1, it is determined whether or not the EGR on flag FEGR set to a value 1 when the EGR is being executed is a value 1 (step S35). Is affirmative (YES)
When EGR target E / F target A / F permission determination flag FKBS
It is determined whether MJGE has a value of 1 (step S36).
Both the answers in steps S35 and S36 are affirmative (YE
S), that is, when FEGR = 1 and FKBSMJGE = 1, the target A / F permission flag for EGR FEGRAF is set to the value 1 (step S38), while step S3
If the answer of 5 or S36 is negative (NO), that is, if FEGR = 0 or FKBSMJGE = 0, the flag FEGRA is set.
F is set to the value 0 (step S37). FEGRAF
= 1 indicates that the EGR target A / F (= 18) can be adopted.

In a succeeding step S39, it is determined whether or not the lean burn permission determination flag FKBSMJG has a value of 1, and the answer is affirmative (YES), that is, FKBSMJG = 1, and when the lean burn permission region is set, the EGR target is set. A
It is determined whether the / F permission flag FEGRAF has a value of 1 (step S40). When the answer to step S40 is negative (NO), that is, when FEGRAF = 0, the EGR target A / F cannot be adopted, so the KBSM value is A / F =
While the predetermined value KBSML corresponding to 22 is set (step S43), the answer in step S40 is affirmative (YES), that is, F.
When EGRAF = 1, the target A / F for EGR can be adopted, so the KBSM value is a predetermined value K equivalent to A / F = 18.
The BSME is set (step S44), and this routine is finished.

When the answer to step S39 is negative (NO), that is, when FKBSMJG = 0 and it is outside the lean burn region, the KBSM value is a predetermined value K corresponding to A / F = 14.7.
It is determined whether BSM0 or more (step S41). When this answer is affirmative (YES), that is, when KBSM ≧ KBSM0, this routine is immediately terminated. Therefore KB
As the SM value, the value retrieved in step S31 is directly used.

When the answer to step S41 is negative (NO), that is, when KBSM <KBSM0, the target A / F for EGR is set.
It is determined whether the permission flag FEGRAF has a value of 1 (step S42). The answer is affirmative (YES), that is, FEGR
When AF = 1, the process proceeds to step S44 and KBS
While setting M = KBSME (value corresponding to A / F = 18) (step S44), the answer is negative (NO), that is, FE.
When GRAF = 0, KBSM = KBSM0 is set (step S45), and this routine ends.

The setting of the KBSM value by the subroutine of FIG. 5 will be described with reference to FIGS. 6 and 7. In regions A and E of FIG. 7, KBSM = KBSM, regardless of the vehicle speed and the on / off (execution / stop) of EGR.
1L and KBSM = KBSM0.

High vehicle speed (V> VKBSM) and EGR
When the engine load (PBA) changes from low load to high load when off, the KBSM value (map search value) is the lean burn permission determination threshold value KBSMJGH (7FH) as shown by the solid line in FIG. K until
BSM = KBSML and KBSM = KBSM0 when the KBSMJGH value is exceeded. Therefore, KBSM = KBSML in the areas B, C, and D of FIG.

When the vehicle speed is high and EGR is off,
When the engine load decreases, the KBSM value (map search value) is the lean burn permission determination threshold value KBSMJG.
KBSM = KBSM0 until L (21H) is reached, and when it becomes smaller than the KBSMJGL value, KBSM = K
It is called BSML. Therefore, the areas B, C, and D in FIG.
, KBSM = KBSM0.

When the vehicle speed is high and EGR is on,
When the engine load increases, as shown by the broken line in FIG. 8, the KBSM value (map search value) becomes equal to the EGR target A.
/ F permission determination threshold value KBSMJGE (3FH) is reached, KBSM = KBSML, and KBSMJG
When the EH value is exceeded, KBSM = KBSME is set, and when the KBSMJGH value (7FH) is exceeded, KBSM = KB.
SM0. Therefore, KBSM = KBSML in the area B of FIG. 7, and KB in the areas C and D.
SM = KBSME.

When the engine load decreases when the vehicle speed is high and EGR is on, the KBSM value (map search value) is the lean burn permission determination threshold value KBSMJG.
Until H (7FH) is reached, KBSM = KBSM0, which is smaller than the KBSMJGH value, and EGR target A
/ F permission determination threshold value KBSMJGE (3FH) is reached, KBSM = KBSME, KBSMJG
When it becomes smaller than the E value, KBSM = KBSML.
Therefore, in the regions C and D of FIG. 7, KBSM = KBSME, and in the region B, KBSM = KBSML, similarly to when the load is increased.

When the vehicle speed is low, the lean burn permission determination threshold value KBSMJGH = 5FH (step S55 in FIG. 6).
In response to OFF, KBSM = KBSME / KBSM0, and the region where lean burn is permitted becomes narrower than that at high vehicle speed. The other points are the same.

As described above, according to this embodiment, the EGR
When it is on, the target air-fuel ratio coefficient KCMD (= KBSM) is A in the area C (also in the area D when the vehicle speed is high).
/ F = 18 (KBSME), and the air-fuel ratio correction coefficient KL is set so that the detected air-fuel ratio KACT matches this.
Since the AF is determined, it is possible to improve the fuel efficiency by making the air-fuel ratio lean, and to improve the engine drivability by reducing the torque shock caused by the change in the target air-fuel ratio and satisfy the requirements of the exhaust gas characteristics.

FIG. 9 is a flow chart of a subroutine of the KCMD limit process executed in step S17 of FIG.

In step S61, the change amount DKCMD of the KCMD value is calculated as the current calculated value KCMD (N) and the previous calculated value K.
Difference from CMD (N-1) (KCMD (N) -KCMD
(N-1)), and then the previously calculated value KCMD
It is determined whether (N-1) is larger than a predetermined value KCMD0 corresponding to the theoretical air-fuel ratio (step S62). When the answer is negative (NO), that is, when KCMD (N−1) ≦ KCMD0, the increase variable DKC1 corresponding to the changing speed of the target air-fuel ratio in the rich direction is set to the lean side increase predetermined value DKCM1 (step S63). ), The decrease variable DKC2 corresponding to the change speed of the target air-fuel ratio in the lean direction is set to the lean-side decrease predetermined value DKC2M (step S64),
It proceeds to step S67.

When the answer to step S62 is affirmative (YES), that is, when KCMD (N-1)> KCMD0, the increasing variable DKC1 is set to the rich side increasing predetermined value DKC1M2 (step S65), and the decreasing variable DKC2 is set to the rich side. Decrease predetermined value DKC2H is set and step S67
Proceed to.

In step S67, it is determined whether or not the change amount DKCMD of the KCMD value is a negative value, and when the answer is affirmative (YES), that is, when the KCMD value changes in the decreasing direction, the change amount DKCMD is changed. It is determined whether or not the absolute value is smaller than the decrease variable DKC2 (step S7).
0). The answer to step S70 is negative (NO), that is, | DK
When CMD | ≧ DKC2, the current value KCMD (N)
To (KCMD (N-1) + DKC2) (step S71), while the answer to step S70 is affirmative (YE
If S), the process immediately proceeds to step S72.

If the answer to step S67 is negative (NO),
That is, when DKCMD ≧ 0 and the KCMD value changes in the increasing direction, it is determined whether or not the absolute value of the change amount DKCMD is smaller than the increasing variable DKC1 (step S
68). The answer to step S68 is negative (NO), that is, | DK
When CMD | ≧ DKC1, the current value KCMD (N)
To (KCMD (N-1) + DKC1) (step S69), while the answer to step S68 is affirmative (YE
If S), the process immediately proceeds to step S72.

According to steps S67 to S71, the KCM
When the absolute value of the change amount DKCMD of the D value is larger than the increase variable DKC1 or the decrease variable DKC2, the current value KCM
Set D (N) to DKC1 value or DKC2 value and previous value KCMD
By changing to the value calculated by (N-1), the KCMD value is prevented from changing abruptly and the drivability is deteriorated.

In steps S72 to S75, a KCMD value limit check is performed. That is, the KCMD value is compared with the predetermined upper and lower limit values KCMLMH and KCMLML (steps S72 and S73), and the KCMD value is the upper limit value KCMLMH.
When it is larger, the KCMD value is set to the upper limit value (step S75), and the KCMD value is set to the lower limit value KCMLML.
When it is smaller, the KCMD value is set to the lower limit value thereof (step S74), and this routine is ended.

FIG. 10 shows the valve opening command value LC of the exhaust gas recirculation valve.
7 is a flowchart of a routine for calculating MD.

In steps S81 to S87, it is determined whether or not the engine is in an operating state in which EGR can be performed. That is, whether or not the engine is in the start mode (step S8)
1) Whether or not feedback control based on the LAF sensor output is in progress (step S82), whether or not fuel cut is in progress (step S83), and whether the engine speed NE is lower than a predetermined speed NHEC (eg, 4,500 rpm). Whether or not (step S84), the engine water temperature TW is equal to the predetermined water temperature T
Whether it is higher than WE1 (for example, 75 ° C.) (step S8)
5), the absolute pressure PBA in the intake pipe is a predetermined upper and lower limit value PBAEC
H, PBAECL (eg 500 mmHg and 2 respectively
Whether it is within the range of 00 mmHg) (steps S86, S
87) is determined. As a result, steps S81 and S8
Answer 3 is negative (NO), and steps S82 and S84-
When all the answers in S87 are affirmative (YES), EGR
When it is determined that the execution is possible, steps S88 to S90 are executed, while the answer to step S81 or S83 is affirmative (YE
S), or if the answer to any of steps S82 and S84 to S87 is negative (NO), it is determined that EGR cannot be executed, and the LCMD value is set to 0 (step S91).
The value of the flag FEGR is set to 0 (step S92).

In step S88, the engine speed NE
And the map value LMAP of the lift amount of the exhaust gas recirculation valve is calculated according to the intake pipe absolute pressure PBA, and then the LCMD value is calculated by the following equation (2).

LCMD = LMAP × KLCMD (2) Here, KLCMD is a correction coefficient set according to the target air-fuel ratio coefficient KCMD as shown in FIG. Figure 1
1, KCMD1 is, for example, a value corresponding to A / F = 14.7, KCMD2 is, for example, a value corresponding to A / F = 16, KC
MD3 has a value corresponding to A / F = 18, and KCMD4 has a value corresponding to A / F = 22.

In this way, the LCMD value is corrected according to the target air-fuel ratio (KCMD), and the smaller the KCMD value (the closer the target air-fuel ratio is set to the lean side), the smaller the value of the KCMD is. When changing the target air-fuel ratio from a value corresponding to A / F = 22 to a value corresponding to A / F = 14.7, for example, an appropriate amount of exhaust gas recirculation can be performed, so the change speed is relatively slow. However, deterioration of exhaust gas characteristics can be prevented. As a result, it is possible to prevent a rapid change in the engine output torque due to the target air-fuel ratio changing speed being too fast. Further, in the vicinity of A / F = 22, exhaust gas recirculation is performed to the same extent as when A / F = 16 to 18, so that it is possible to prevent the occurrence of problems such as unstable combustion.

In the above-described embodiment, the target air-fuel ratio is A / F = 22 and A / F depending on whether EGR is on or off.
Although it is determined whether to adopt an intermediate value of F = 14.7, the target air-fuel ratio may be determined according to the EGR rate as shown in FIG. 12, for example.

Further, as in the present embodiment, only when the EGR is on, the supply air-fuel ratio is made lean to about A / F = 18 in a predetermined engine operating state (operating state corresponding to the region C in FIG. 7). The method described above is applicable not only to feedback control of the air-fuel ratio but also to open loop control.

[0073]

As described above in detail, according to the present invention, the air-fuel ratio of the air-fuel mixture supplied to the engine is controlled to a predetermined air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, and the predetermined air-fuel ratio is exhaust gas recirculation. Since it is set according to the state, it is possible to improve fuel efficiency and satisfy the requirements of drivability and exhaust gas characteristics.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an internal combustion engine and its control device according to an embodiment of the present invention.

FIG. 2 is a flowchart of a main routine for calculating a target air-fuel ratio coefficient (KCMD) and an air-fuel ratio correction coefficient (KLAF).

FIG. 3 is a flowchart of a subroutine for calculating a target air-fuel ratio coefficient (KCMD).

FIG. 4 is a flowchart of a subroutine for calculating a basic value (KBSM) of a target air-fuel ratio coefficient.

FIG. 5 is a flowchart of a subroutine for calculating a basic value (KBSM) of a target air-fuel ratio coefficient.

FIG. 6 is a flowchart of a subroutine for setting flags used in the routine of FIG.

FIG. 7 is a diagram showing a map for calculating a basic value (KBSM) of a target air-fuel ratio coefficient.

FIG. 8 is a diagram for explaining the operation of the subroutine of FIG.

FIG. 9 is a flowchart of a subroutine for performing target air-fuel ratio coefficient (KCMD) limit processing.

FIG. 10 is a flowchart of a routine for calculating a valve opening command value (LCMD) of an exhaust gas recirculation valve.

FIG. 11 is a diagram showing a table for calculating a correction coefficient KLCMD for a valve opening degree command value of an exhaust gas recirculation valve according to a target air-fuel ratio coefficient (KCMD).

FIG. 12 is a diagram showing a table for setting a target air-fuel ratio according to an exhaust gas recirculation rate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Internal combustion engine 5 Electronic control unit (ECU) 7 Absolute pressure sensor in intake pipe 10 Engine speed sensor 12 Oxygen concentration sensor 20 Exhaust gas recirculation mechanism

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location F02D 45/00 F 7536-3G F02M 25/07 550 R

Claims (2)

[Claims] 1. A lean control means for controlling an air-fuel ratio of an air-fuel mixture supplied to the engine to a predetermined air-fuel ratio leaner than a stoichiometric air-fuel ratio when the internal combustion engine is in a predetermined operating state, and exhaust gas of the engine. In an air-fuel ratio control apparatus for an internal combustion engine having an exhaust gas recirculation means for recirculating a portion to an intake system, a lean air-fuel ratio setting means for setting the predetermined air-fuel ratio according to an exhaust gas recirculation state is provided. Air-fuel ratio controller. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas recirculation state is whether or not exhaust gas recirculation is performed.