JPH06229296A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent a change of an air-fuel ratio in the lean direction and improve follow-up performance by setting an air-fuel ratio correction factor in a prescribed value when a condition where a purge control valve of an evaporation fuel processing device is opened and a drain shut valve is closed is transferred to a condition where the purge control valve is closed. CONSTITUTION:An internal combustion engine 1 has an evaporation fuel processing device. That is, the first control valve 30 is interposed in a purge passage 27 to connect a canister 25 and an intake air system 2 of the internal combustion engine 1 to each other, and the second control valve 26 is added to an intake air port of the canister 25. An ECU(electronic control unit) 5 controls an air-fuel ratio of air-fuel mixture according to an air-fuel ratio correction factor determined according to output of an exhaust air concentration sensor 32 in an exhaust air system 12 of the internal combustion engine 1. In this case, when a condition where the first control valve 30 is opened and the second control valve 26 is closed is transferred to a condition where the first control valve is closed, the air-fuel ratio correction factor is set in a prescribed value. Thereby, the air-fuel ratio can be prevented from changing in the lean direction just after transfer.

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 system for an internal combustion engine having a fuel vapor treatment system.

[0002]

2. Description of the Related Art The applicant of the present application has already proposed a method for determining an abnormality in an evaporative fuel processing system in which fuel vapor generated in a fuel tank is temporarily stored in a canister and is appropriately purged into an intake system of an internal combustion engine. (Japanese Patent Application No. 3-36)
0630).

[0003]

According to the method proposed above, in order to bring the inside of the fuel tank into a negative pressure state, purging is performed with the atmosphere inlet of the canister closed, and then the purging is stopped. (The purge control valve provided in the purge passage that connects the canister and the intake pipe is closed) to detect the amount of pressure variation in the fuel tank, and the abnormality determination is performed based on that amount of variation. At the time of execution, rich fuel vapor that is not diluted with air is supplied to the intake pipe, and when the purge is stopped, the supply is suddenly cut off.

Even during execution of this abnormality determination, feedback control of the air-fuel ratio is performed based on the output of the exhaust gas concentration sensor (oxygen concentration sensor) provided in the exhaust system of the engine, but the followability is particularly poor when purging is stopped. However, the exhaust gas characteristics may be temporarily deteriorated.

The present invention has been made to solve this problem, and prevents fluctuations in the air-fuel ratio of the air-fuel mixture supplied to the engine even when the abnormality of the evaporative fuel processing system is judged, thereby achieving excellent exhaust gas. An object is to provide an air-fuel ratio control device that can maintain the characteristics.

[0006]

To achieve the above object, the present invention provides a fuel tank, a canister having an intake port communicating with the atmosphere, and a charge passage connecting the canister and the fuel tank. Evaporative fuel provided with a purge passage connecting the canister and an intake system of an internal combustion engine, a first control valve interposed in the purge passage, and a second control valve opening and closing an intake port of the canister. Air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture to be supplied to the engine by using an air-fuel ratio correction coefficient determined according to the output of an exhaust gas concentration sensor provided in the exhaust system of the engine. In an air-fuel ratio control device for an internal combustion engine equipped with,
When the state in which the first control valve is opened and the second control valve is closed is changed to the state in which the first control valve is closed, the air-fuel ratio correction coefficient is set to A transient control means for setting a predetermined value is provided.

Further, it is desirable that the transient control means sets the air-fuel ratio correction coefficient to a predetermined value after a lapse of a predetermined time from the transition time point.

Further, in order to achieve the same object, the present invention replaces the transient control means with the first control valve being opened and the second control valve being closed. When the first control valve shifts to the closed state, a transient control means for temporarily increasing the degree of correction by the air-fuel ratio correction coefficient compared to before the shift is provided.

Further, it is preferable that the transient control means temporarily changes the correction degree after a lapse of a predetermined time from the transition time point.

[0010]

When the first control valve is opened and the second control valve is closed and the first control valve is closed, the air-fuel ratio correction coefficient is set to a predetermined value. Set to the value.

Further, it is desirable that the setting to the predetermined value is performed after a predetermined time has elapsed from the transition point.

At the time of the transition, instead of setting the air-fuel ratio correction coefficient to a predetermined value, the degree of correction by the coefficient is temporarily set higher than that before the transition.

It is desirable to change the correction degree after a predetermined time has elapsed from the transition point.

[0014]

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

FIG. 1 is an overall configuration diagram of an evaporated fuel processing apparatus for an internal combustion engine according to an embodiment of the present invention.

In the figure, reference numeral 1 denotes an internal combustion engine having four cylinders (hereinafter, simply referred to as "engine"), and a throttle valve 3 is arranged in the intake pipe 2 of the engine 1. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 and outputs an electric signal corresponding to the opening of the throttle valve 3 to an electronic control unit (hereinafter referred to as “ECU”) 5. Supply.

The fuel injection valve 6 is provided for each cylinder in the middle of the intake pipe 2 and slightly upstream of an intake valve (not shown) between the engine 1 and the throttle valve 3. Further, each fuel injection valve 6 is connected to a fuel tank 9 via a fuel supply pipe 7, and a fuel pump 8 is provided in the middle of the fuel supply pipe 7. The fuel injection valve 6 is electrically connected to the ECU 5, and a signal from the ECU 5 controls the valve opening timing of the fuel injection.

The intake pipe absolute pressure (PB) for detecting the intake pipe absolute pressure PBA is provided on the downstream side of the throttle valve 3 of the intake pipe 2.
BA) sensor 13 and intake air temperature (T
A) The sensors 14 are mounted, and the detection signals of these sensors are supplied to the ECU 5.

An engine water temperature (TW) sensor 15 composed of a thermistor or the like is attached to the cylinder peripheral wall filled with cooling water of the cylinder block of the engine 1, and the engine cooling water temperature TW detected by the TW sensor 15 is converted into an electric signal. It is converted and supplied to the ECU 5.

An engine speed (NE) sensor 16 is provided around a cam shaft or crank shaft (not shown) of the engine 1.
Is attached.

The NE sensor 16 outputs a signal pulse (hereinafter referred to as "TDC signal pulse") at a predetermined crank angle position for each 180-degree rotation of the crankshaft of the engine 1, and the TDC signal pulse is supplied to the ECU 5. .

An O ₂ sensor 32 as an exhaust gas concentration sensor is mounted in the middle of the exhaust pipe 12, detects the oxygen concentration in the exhaust gas, outputs a signal corresponding to the detected value VO2, and supplies it to the ECU 5. To do. O ₂ sensor 32 of the exhaust pipe 12
A three-way catalyst 33, which is an exhaust gas purification device, is provided downstream of the.

The ECU 5 is also connected to a vehicle speed sensor 17 for detecting the traveling speed VP of the vehicle equipped with the engine 1, a battery voltage sensor 18 for detecting the battery voltage VB, and an atmospheric pressure sensor 19 for detecting the atmospheric pressure PA. The detection signals of these sensors are supplied to the ECU 5.

Next, the evaporative emission control system (hereinafter referred to as "emission control system") 31 including the fuel tank 9, the charge passage 20, the canister 25, the purge passage 27 and the like will be described.

The fuel tank 9 has a fuel amount sensor 10 for detecting the amount of fuel in the tank and a pressure PTNK in the tank.
A tank internal pressure sensor 11 for detecting the above is provided, and detection signals of these sensors are supplied to the ECU 5. The output signal VFUEL of the fuel amount sensor 10 has a voltage value that decreases as the fuel amount increases.

The fuel tank 9 is connected to the canister 25 via the charge passage 20, and the charge passage 20 has first to third branch portions 20a to 20c. A one-way valve 21 and a puff loss valve 22 are provided in the first branch portion 20a. The one-way valve 21 is configured to be opened only when the tank pressure PTNK becomes higher than the atmospheric pressure by about 12 to 13 mmHg. Pafloss valve 22
Is an electromagnetic valve that is opened during purging described later and closed when the engine is stopped, and its operation is controlled by the ECU 5.

A two-way valve 23 is provided at the second branch portion 20b. The two-way valve 23 is configured to open when the tank internal pressure PTNK is higher than the atmospheric pressure by about 20 mmHg and when the tank internal pressure PTNK is lower than the pressure on the canister 25 side of the two-way valve 23 by a predetermined pressure. ing.

A bypass valve 24 is provided at the third branch portion 20c.
Is provided. The bypass valve 24 is a solenoid valve that is normally closed and is opened / closed during execution of abnormality determination, which will be described later, and its operation is controlled by the ECU 5.

The canister 25 contains activated carbon that adsorbs fuel vapor and has an intake port (not shown) that communicates with the atmosphere via the passage 26a. A drain shutoff valve 26 is provided in the middle of the passage 26a. The drain shutoff valve 26 is an electromagnetic valve that is normally held in an open state and is temporarily closed during execution of abnormality determination, which will be described later, and its operation is controlled by the ECU 5.

The canister 25 is connected to a downstream side of the throttle valve 3 of the intake pipe 2 via a purge passage 27, and the purge passage 27 has first and second branch portions 27a and 2a.
7b. The first branch portion 27a is provided with a jet orifice 28 and a jet purge control valve 29, and the second branch portion 27b is provided with a purge control valve 30. The jet purge control valve 29 is an electromagnetic valve for controlling a small flow rate of the purge fuel mixture that cannot be accurately controlled by the purge control valve 30, and the purge control valve 30 is
The electromagnetic valve is configured so that the flow rate can be continuously controlled by changing the on-off duty ratio of the control signal, and the operation of these electromagnetic valves 29, 30 is controlled by the ECU 5.

The ECU 5 shapes the input signal waveforms from the various sensors described above to correct the voltage level to a predetermined level,
An input circuit having a function of converting an analog signal value into a digital signal value, and a central processing circuit (hereinafter referred to as "CPU").
Storage means for storing a calculation program executed by the CPU, a calculation result, etc., the fuel injection valve 6, the PAPROS valve 22, the bypass valve 24, and the jet purge control 29.
And an output circuit for supplying a drive signal to the purge control valve 30.

The ECU 5 determines various engine operating states such as a feedback control operating region and an open loop control operating region according to the oxygen concentration in the exhaust gas based on the above-mentioned various engine parameter signals, and also determines the engine operating state according to the engine operating state. , The fuel injection time Tout of the fuel injection valve 6 synchronized with the TDC signal pulse based on the following equation (1).
Is calculated.

Tout = Ti × K ₁ × KO 2 + K ₂ (1) Here, Ti is a reference value of the injection time Tout of the fuel injection valve 6, and is set according to the engine speed NE and the intake pipe absolute pressure PBA. Read from the Ti map.

KO2 is an air-fuel ratio correction coefficient and is set in accordance with the oxygen concentration in the exhaust gas detected by the O ₂ sensor 32 during feedback control, and in a plurality of open loop control operating regions where no feedback control is performed. It is a coefficient set to a value according to each operation area.

K ₁ and K ₂ are other correction coefficients and correction variables calculated according to various engine parameter signals, respectively, and various characteristics such as fuel consumption characteristics and engine acceleration characteristics according to engine operating conditions can be optimized. It is set to a predetermined value as shown.

In this embodiment, the ECU 5 constitutes part of the air-fuel ratio control means and the transient control means.

FIG. 2 is a view showing the puff loss valve 22 according to the present embodiment.
It is a figure which shows the operation pattern of bypass valve 24, drain shut valve 26, purge control valve 30, and jet purge control valve 29, and change of tank internal pressure PTNK at that time, and the abnormality judging method in this example with reference to the figure. The outline of is explained. The tank pressure PTNK is the atmospheric pressure (PAT
It is indicated by the differential pressure with respect to M).

First, in the purge mode during normal operation (FIG. 2, A), the puff loss valve 22 and the drain shut valve 2 are
6, purge control valve 30, and jet purge control valve 29
Is opened and the bypass valve 24 is closed. At this time, the fuel vapor generated in the fuel tank 9 flows into the canister 25 through the charge passage 20 and is temporarily stored in the canister 25. Then, air is introduced from the passage 26, and the fuel vapor flowing into the canister 25 is supplied to the intake pipe 2 together with the air via the purge passage 27.

Next, the following preconditions (abnormality judgment permission conditions)
When is satisfied, each solenoid valve is operated as shown in B to I of FIG. 2 and the abnormality determination of the emission suppression system 31 is performed.

First, an atmosphere opening process (FIG. 2, B) is performed.
That is, the puff loss valve 22, the drain shut valve 26, and the jet purge control valve 29 are kept open, the bypass valve 24 is opened, the purge control valve 30 is closed, and the inside of the fuel tank 9 is opened to the atmosphere. To do. As a result, if PTNK = + 4 mmHg during normal operation, for example, during period B, PTNK = 0 mmHg (= PATM).

Next, a temporary positive pressure check (FIG. 2, C) is performed. That is, the puff loss valve 22 and the bypass valve 24 are closed, and the other valves maintain the previous state. In this state, the fuel vapor generated in the fuel tank causes the tank internal pressure PTN.
Since K increases by, for example, about 2 mmHg, the variation amount (PVARIA) of the tank internal pressure at this time is measured.

Next, a pressure reduction process (FIG. 2, D) is performed. That is, the bypass valve 24 and the purge control valve 30 are opened, the drain shut valve 26 is closed, and the other valves maintain the previous state. In this state, due to the negative pressure of the intake pipe 2,
The emission suppression system 31 is depressurized. In this pressure reducing process, the tank internal pressure PTNK is set to a predetermined pressure PLVL (for example, -15 mmH).
Repeat until g).

Next, an overshoot check is performed (FIG. 2, E). That is, the bypass valve 24, the purge control valve 30, and the jet purge control valve 29 are closed, and the states before the other valves are maintained. In this state, the exhaust suppression system 31 is shut off from the intake pipe 2, but if the exhaust suppression system 31 is normal, the tank internal pressure PTNK further decreases. Immediately after the decompression process is completed, the pressure in the purge passage 27 and the canister 25 is lower than the tank internal pressure PTNK, and this is because of the influence. When there is a leak, the tank internal pressure PTNK increases as shown by the broken line.

Next, a first leak check is performed (FIG. 2, F). That is, the state of each valve is maintained at the previous state, the pressure PMIN at the time when the tank internal pressure PTNK changes from the decrease to the increase is measured, and from that time, a predetermined time (tLEA
K) The pressure PEND after the lapse of time is measured, and the fluctuation amount PVARIB of the tank internal pressure is calculated. At this time, the bypass valve 24
If there is more leakage on the fuel tank side, the second fluctuation amount PVARIB becomes larger (see the broken line in FIG. 2 (f)).

Next, a second leak check is performed (FIG. 2, G). That is, the bypass valve 24 is closed, the other valves are maintained in the previous state, and the tank internal pressure (PCANI) after a predetermined time (tLEAK2) has elapsed is measured. In this state,
If there is no leak on the canister side of the bypass valve 24, the tank internal pressure PTNK decreases (solid line and broken line in FIG. 2 (f)), but rises if there is leak (dashed line in FIG. 2).

Next, a pressure canceling process is performed (FIG. 2,
H). That is, the drain shut valve 26 and the jet purge control valve 29 are opened, the other valves are maintained in the previous states, and the pressure in the discharge suppression system 31 is set to approximately atmospheric pressure.

Next, the positive pressure for correction is checked (FIG. 2,
I). That is, the bypass valve 24 is closed, the other valves are maintained in the previous state, and the fluctuation amount PVARIC of the tank internal pressure due to the fuel vapor generated in the fuel tank is calculated.

Next, the puff loss valve 22 and the purge control valve 30
Is opened, the other valves maintain the previous state, and the normal purge mode (FIG. 2, J) is entered.

FIGS. 3 and 4 are flowcharts of a program (overall configuration) for executing the above-described abnormality determination processing. This program is executed every predetermined time (for example, 80 msec).
To be executed.

In step S1, it is determined whether or not a precondition for permitting execution (monitoring) of abnormality determination is satisfied,
If the precondition is not satisfied, the process proceeds to step S2, and the normal purge mode is set (FIG. 2, A). That is, the puff loss valve 22, the drain shut valve 26, the purge control valve 30.
Also, the jet purge control valve 29 is opened and the bypass valve 24 is closed. At this time, a predetermined time tA
While setting TMOP (for example, 12 seconds) and starting it (step S3), tank internal pressure PTNK
Flag FPMIN which is set to value 1 when is minimum
Is set to 0 (step S4).

When the answer to step S1 is affirmative (YES), that is, when the previous condition is satisfied, it is judged whether or not the count value of the tATMOP timer is 0 (step S).
5) At first, since tATMOP> 0, a predetermined time tTP is set in the tTP timer that measures the time of the temporary positive pressure check.
(For example, 0.3 seconds) is set and started (step S6), and the atmosphere opening process (FIG. 2, B) is performed (step S7).

After that, when tATMOP = 0, the process proceeds to step S8, the tank internal pressure PTNIC is measured, and the measured tank internal pressure PTNIC is used as the tank pressure PATM after opening to the atmosphere. In addition, PA
The TM value is updated only immediately after the atmospheric release process is completed. Next, it is determined whether or not the count value of the tTP timer is 0 (step S9), and at first tTP> 0. Therefore, the tPRG timer for measuring the time of the pressure reducing process is set to the predetermined time tP.
RG (for example, 24 seconds) is set and started (step S10), and a temporary positive pressure check (FIG. 2, C) is performed. The predetermined time tPRG is set to a time sufficient to reduce the tank internal pressure PTNK to the predetermined pressure PLVL when the device is normal.

After that, when tTP = 0, step S
In step 12, it is determined whether the flag FPLVL is 1 or not. The flag FPLVL has a value of 1 when the tank internal pressure PTNK drops to a predetermined pressure (when the pressure reducing process ends).
Is a flag set to. Initially the flag FPLVL =
Since it is 0, the process proceeds to step S13 of FIG. 4, the minimum tank internal pressure PMIN is initialized, and a tOS timer for measuring the time of the overshoot check has a predetermined time tO.
S (for example, 0.3 seconds) is set and started (step S14). Then, it is determined whether or not the count value of the tPRG timer is 0 (step S15),
Since tPRG> 0 at the beginning, a predetermined time tCA is set in the tCANCEL timer that measures the time of the pressure cancellation process.
The NCEL (for example, 16 seconds) is set and started (step S16), and the pressure reducing process (FIG. 2, D) is performed (step S17).

When tPRG = 0 during the depressurization process, it means that the depressurization process has not been completed within the predetermined time tPRG, so it is determined that leakage may occur. The process proceeds to step S29 without performing the first and second leak checks.

When the answer to step S12 is affirmative (YES), that is, when the pressure reducing process is completed (when FPLVL = 1), the process proceeds to step S18, and it is determined whether the flag FPMIN is 1 or not. . Since FPMIN = 0 at first, the process proceeds to step S19, and it is determined whether or not the count value of the timer tOS is 0. First tOS>
Since it is 0, the tLEAK timer that measures the time of the first leak check has a predetermined time tLEAK (for example, 16 LEAK).
Seconds) to start this (step S2
0), overshoot check (FIG. 2, E) is performed (step S21).

After that, when the tank internal pressure PTNK becomes the minimum (when FPMIN = 1) or tOS = 0, the process proceeds to step S22 in FIG. 4 and whether the count value of the timer tLEAK is 0 or not. Determine whether or not. At first, since tLEAK> 0, the timer tLEAK2 for measuring the time of the second leak check is set to a predetermined time tLEAK2 (for example, 10 seconds) and started (step S23), and the first leak check (Fig. 2, F) is performed (step S24).

After that, when tLEAK = 0, the process proceeds to step S25, where the count value of the tLEAK2 timer is 0.
Or not. Initially, tLEAK2> 0, so the routine proceeds to step S26, where it is determined whether or not the tank internal pressure PTNK is higher than a predetermined lower limit value PLIM (for example, -20 mmHg). Normally, PTNK> PLIM is established, so tCANCEL is the same as in step S16.
A predetermined time tCANCEL is set in the timer and started (step S27), and a second leak check is performed (step S28).

During the second leak check, PTNK≤PL
When it becomes IM or when tLEAK2 = 0, the count value of the tLEAK2 timer is set to 0 (step S29), and it is determined whether or not the count value of the tCANCEL timer is 0 (step S30). Since tCANCEL> 0 at first, a predetermined time tTP2 (for example, 16 seconds) is set to the timer tTP2 that measures the time for checking the positive pressure for correction, and this is started (step S31), and the pressure canceling process (FIG. 2, H) is performed (step S32).

After that, when tCANCEL = 0, the process proceeds to step S33, and it is determined whether or not the count value of tTP2 is 0. At first, tTP2> 0, so a positive pressure check for correction (FIG. 2, I) is performed, and then tTP2
When = 0, a determination process described later is performed (step S3
5).

5 and 6 are flow charts of a program for determining whether or not the precondition in step S1 of FIG. 3 is satisfied.

In step S41 of FIG. 5, it is determined whether or not monitoring (execution of abnormality determination) is permitted by the monitor execution control. Specifically, whether or not a failure of a sensor necessary for determining the precondition or an oxygen concentration sensor (not shown) or the like is detected, whether or not an engine operating state in which purging from the canister should be prohibited, and FIG. It is determined whether or not the abnormality determination processing by the program of 4 is completed, and the monitor is not permitted when the sensor failure is detected, the purge is prohibited, or the abnormality determination processing is completed. In that case, the process proceeds to step S56 in FIG. 6 and the tank internal pressure PTNK is set to the initial pressure PCON, and the precondition is not satisfied (step S5).
7).

When monitoring is permitted by the monitor execution control, whether or not the engine water temperature TWI at engine start is equal to or lower than a predetermined water temperature TWEVPST (for example, 20 ° C.) (step S43), the catalytic converter provided in the exhaust system of the engine is determined. Whether or not the deterioration determination process (not shown) is completed (step S43), the tank internal pressure PT
Whether NK is greater than or equal to a predetermined lower limit value PLIM (step S4
4), the throttle valve opening θTH is a predetermined upper and lower limit value θTHP
It is determined whether or not it is within the range of CHKH, θTHPCHKL (for example, 10 degrees, 5 degrees) (step S45), and all the answers of these steps S42 to S45 are affirmative (YE
If S), the process proceeds to step S46. On the other hand, TWI
> TWEVPST is satisfied, and catalytic converter deterioration determination processing is not completed, PTNK <PLIM
When is satisfied or when the throttle valve opening degree θTH is not within the range of the predetermined upper and lower limit values, the process proceeds to step S56.

Here, the starting water temperature TWI is the predetermined water temperature TW.
If it is higher than EVPST, the precondition is not satisfied,
This is because it is sufficient to execute the abnormality determination when the engine has not been operated for a long time (for example, once / day). Further, when the deterioration determination of the catalytic converter is not completed, this abnormality determination is prioritized, and therefore this abnormality determination is not performed. Further, when the tank internal pressure PTNK is abnormally reduced (PTNK <PLIM), the precondition is not satisfied in order to prevent overnegative pressure. Further, according to step S45, when the throttle valve opening θTH is out of the predetermined range, the abnormality determination is stopped even after the first leak check is started. This is to eliminate the influence of disturbances such as turning of the vehicle and braking.

In step S46, it is determined whether or not the flag FPLVL is 1, and if FPLVL = 1, then
The process immediately proceeds to step S55 and the precondition is satisfied. FPL
VL = 1 indicates that the depressurization process is completed,
After the depressurization process is completed, the determination in steps S47 to S54 is not performed.

When FPLVL = 0, it is first determined whether or not the monitor permission determination based on the fuel level (fuel amount in the fuel tank) is made (step S4).
7). Specifically, this determination is performed by the program shown in FIG.

In step S61 of FIG. 7, the battery voltage VB and the output VFUEL of the fuel amount sensor 10 are read, and then the average value VFUELAVE of the VFUEL value is calculated by the following formula (step S62).

VFUELAVE = CFUEL × VFUEL / A1 + (A1-CFUEL) × VFUELAVE / A1 where VFULEAVE on the right side is the average value calculated up to the previous time, A1 is a constant, and CFUEL is set to a value between 1 and A1. This is the smoothing coefficient.

In the following step S63, VFUELAV
The lower limit value LMTF and the upper limit value LMTE of the E value are determined according to the battery voltage VB. Specifically, as shown in FIG. 8, VFULELE corresponding to the predetermined voltages VBL and VBH.
0, VFUELF0 and VFUELE1, VFUELF
VB-LMTE table and VB-LM set to 1
The TF table is searched according to the battery voltage VB, and interpolation calculation is performed to calculate the LMTE value and the LMTF value.
As a result, the upper limit value LMTE is set as shown by the solid line I and the lower limit value LMTF is set as shown by the solid line II.

Returning to FIG. 7, in step S64, VFUE
It is determined whether the LAVE value is larger than the lower limit value LMTF or smaller than the upper limit value LMTE in step S65,
When LMTF <VFUELAVE <LMTE, the monitor is permitted (step S66), while VFUEL is set.
When AVE ≧ LMTE or VFUELAVE ≦ LMTF holds, the monitor is not permitted (monitor prohibited) (step S67). Therefore, in the shaded area in FIG.
Monitor is prohibited.

Note that the VFUEL (VFUELAVE) value means that the larger the value, the smaller the fuel amount (see FIG. 9). Therefore, when VFUELAVE ≧ LMTE, the fuel tank is in a substantially empty state (EMPTY). In response to VFUELAVE ≦ LMTF, the fuel tank corresponds to a substantially full state (FULL), and monitoring is prohibited in these states.

The upper and lower limit values LMTE and LMTF are set according to the battery voltage VB because the fuel sensor output VF is set.
This is because the UEL changes depending on the battery voltage VB. Returning to FIG. 5, in step S48, it is determined whether the intake air temperature TA is within predetermined upper and lower limit values TAPCHKH and TAPCHKL (for example, 90 ° C. and 70 ° C.), and whether the engine water temperature TW is the predetermined upper and lower limit values TWPCHKH and TWPCHKL.
Whether the engine speed NE is within a range (for example, 90 ° C., 50 ° C.), the engine speed NE is a predetermined upper and lower limit value NEPCHKH, NEP.
Whether or not it is within the range of CHKL (for example, 4000 rpm, 2000 rpm), the intake pipe absolute pressure PBA is a predetermined upper and lower limit value PBAPCHKH, PBAPCHKL (for example, 6
Whether the vehicle speed VP is within a range of 10 mmHg, 350 mmHg), the vehicle speed VP is a predetermined upper and lower limit value VPCHKH, VPCHK.
It is determined whether or not it is within the range of L (for example, 61 km / h, 53 km / h), and either the TA value, the TW value, the NE value, the PBA value, or the VP value is outside the predetermined upper and lower limit values. If so, the process proceeds to step S56. On the other hand, if all the values are within the predetermined upper and lower limits, the process proceeds to step S49.

In subsequent steps S49 to S54, the differential pressure PBG (= PA-P between the atmospheric pressure PA and the intake pressure absolute pressure PBA).
BA) is equal to or higher than a predetermined pressure PBGLM (for example, +80 mmHg), or whether the vehicle speed VP is substantially constant (for example, ± 0.8 k).
Whether the vehicle speed fluctuation within m / h has continued for 2 seconds), the initial pressure PCON set in step S56 is the predetermined pressure PCL.
Whether or not it is below IMH (for example +10 mmHg), the tank internal pressure PATM at the end of opening to the atmosphere is the predetermined pressure PATMLMH.
Whether or not (for example, +5 mmHg) or less, the first fluctuation amount PV
ARIA is a predetermined value PVARIALMH (eg +0.1
25 mmHg) or less, whether the integrated flow rate QPAIRT is a predetermined value QPAIRTLIM (for example, about 30 to 40 l) or more, and when all the answers in steps S49 to S54 are affirmative (YES), it is determined that the precondition is satisfied (YES). Step S55). On the other hand, when the answer to any of the steps S49 to S54 is negative (NO), the above step S56.
Proceed to.

Here, in step S49, the differential pressure PBG
The reason why the precondition is not satisfied when is smaller than the predetermined pressure PBGLM is in consideration of the case where the differential pressure PBG decreases in the highland and the pressure reducing process cannot be performed sufficiently. When the initial pressure PCON is higher than the predetermined pressure PLIMH (step S51), the tank internal pressure PATM after opening to the atmosphere is the predetermined PA.
When it is higher than TMLMH (step S52) and when the first variation amount PVARIA is greater than the predetermined value PVARIALMH (step S53), the amount of fuel vapor generated is large, the accuracy of abnormality determination is reduced, and the air-fuel ratio variation due to purge is reduced. Since it becomes larger, the precondition is not met.
In addition, the integrated flow rate QPAIRT in step S54 is the opening of the purge control valve 30 and the differential pressure PBG (= PA-PBA).
The purge flow rate calculated according to the above is integrated from the time when the engine is started, and this QPAIRT value is a predetermined value QPA
When it is smaller than IRTLIM, the fuel vapor amount (canister charge amount) accumulated (charged) in the canister 25 is large as shown in FIG. 10, and the air-fuel ratio fluctuation increases due to the abnormality determination, so the precondition is not satisfied. I am trying. In FIG. 10, the canister charge amount QCFUL when QPAIRT = 0 corresponds to the charge amount when the canister 25 is in the fully charged state (state where the charge amount is maximum), and the charge amount when QPAIRT = QPAIRTLIM is greater than the QCLIM value. It never grows. Therefore, by performing the abnormality determination only when QPAIRT ≧ QPAIRTLIM, it is possible to improve the accuracy of the abnormality determination and prevent large air-fuel ratio fluctuations.

Next, the atmosphere opening process (step S7), the temporary positive pressure check (step S11), the pressure reducing process (step S17), the overshoot check (step S27), and the first leak check (step S7) of FIGS. S2
4), second leak check (step S28), pressure cancellation process (step S32), correction positive pressure check (step S34), and determination process (step S35).
Will be described with reference to FIGS.

(1) Atmosphere opening process (FIG. 2, B) As shown in FIG. 11, the bypass valve 24 and the puff loss valve 22 are used.
And the drain shutoff valve 26 is opened (step S7).
1) The purge control valve is closed (step S72), and the jet purge control valve 29 is opened (step S73). Here, the reason why the jet purge control valve 29 is opened is that it is desirable to continue a small amount of purge, and even if the valve is opened, the tank internal pressure PTNIC is hardly affected. Note that this point is the same during the temporary positive pressure check, the pressure cancellation, and the correction positive pressure check, which will be described later.

(2) Temporary positive pressure check (FIG. 2, C) As shown in FIG. 12, the bypass valve 24 and the puff loss valve 2
2 is closed and the other valves maintain the previous state (step S8).
1-S83), the tank pressure PTNK is measured and PCLS
(Step S84), the first variation PV is calculated by the following equation.
The ARIA is calculated (step S85).

PVARIA = (PCLS-PATM) / tTP In a succeeding step S86, it is determined whether or not PVARIA has a negative value, and when it has a negative value, PVARIA = 0.
(Step S87).

The process of FIG. 12 is performed by steps S8 and S of FIG.
Since it is executed via 9, the initial calculated value of the PVARIA value becomes 0, but the calculated value at the time when the predetermined time tTP has elapsed is the fluctuation of the tank internal pressure PTNK per unit time during the temporary positive pressure check. Indicates the amount. Normally, the PVARIA value becomes a positive value due to the generation of fuel vapor, but when it becomes a negative value, the value is set to 0 in steps S86 and S87.

(3) Pressure reduction process (FIG. 2, C) As shown in FIG. 13, the bypass valve 24 and the purge control valve 30 are opened, the drain shut valve 26 is closed, and the other valves are in the previous state. Is maintained (steps S91 to S9
3), the tank internal pressure PTNK is a predetermined pressure PLVL (for example, −
15 mmHg) is determined (step S
94). Initially PTNK> PLVL, and PTNK ≦
When it becomes PLVL, the flag FPLVL is set to the value 1 to indicate that the depressurization process is completed (step S9).
5).

(4) Overshoot Check (FIG. 2, E) As shown in FIG. 14, the bypass valve 24 and the purge control valve 3
0 and the jet purge control valve 29 are closed, the other valves maintain the previous state (steps S101 to S103), and it is determined whether the tank internal pressure PTNK is higher than the minimum tank internal pressure PMIN (step S104). Figure 4 for PMIN value
Since the value is set to a sufficiently large value in step S13 of, the PTNK ≦ PMIN is initially satisfied, and the PMIN value is updated to the PTNK value at that time (step S10).
6). When the tank internal pressure PTNK is decreasing, this updating is executed, and when the tank pressure PTNK is rising, PTNK> PMIN is satisfied, and the flag FPMIN is set to the value 1 (step S105).

By this processing, the PMIN value becomes the minimum tank internal pressure after the pressure reducing processing is completed. If there is a leak in the emission control system 31 and the PTNK value changes as shown by the broken line in FIG. 2, PMIN = PLVL.

(5) First Leak Check (FIG. 2, F) As shown in FIG. 15, each valve maintains the same state as in the overshoot check (steps S111 to S113),
The tank internal pressure PTNK is measured as PEND (step S114), and the second fluctuation amount PVARIB is calculated by the following equation (step S115).

PVARIB = (PEND-PMIN) / tLEAK The PVARIB value is, when a predetermined time tLEAK has elapsed,
The fluctuation amount of the tank internal pressure PTNK per unit time during the first leak check is shown.

In the following step S116, it is determined whether or not the PVARIB value is a negative value, and if it is a negative value, PVARIB = 0 is set (step S117).

(6) Second Leak Check (FIG. 2, G) As shown in FIG. 16, the bypass valve 24 is opened and the other valves are maintained in the previous state (steps S121 to S12).
3), the tank internal pressure PTNK is measured and set to PCANI (step S124).

Since this processing is executed for the predetermined time tLEAK2, the PCANI value becomes the tank internal pressure after tLEAK2 has elapsed from the end of the first leak check.

(7) Pressure Canceling Process (FIG. 2, H) As shown in FIG. 17, the drain shut valve 26 and the jet purge control valve 29 are opened, and the other valves are maintained in the previous state (step S131 to S133), tank internal pressure PT
NK is measured and set as PATM2 (step S13)
4).

Since this process is executed for the predetermined time tCANCEL, the PATM2 value becomes the tank internal pressure after the elapse of tCANCEL from the end of the second leak check.
The PATM2 value should be tCANCEL so that it becomes approximately atmospheric pressure.
A value is set, and a third variation amount PVARI described later
Used to calculate C.

(8) Positive pressure check for correction (FIG. 2, I) As shown in FIG. 18, the bypass valve 24 is opened and the other valves are maintained in the previous states (steps S141 to S14).
3), the tank internal pressure PTNK is measured and set to PCLS2 (step S144), and the third variation amount PVA is calculated by the following equation.
RIC is calculated (step S145).

PVARIC = (PCLS2-PATM2) / tTP2 The PVARIC value indicates the amount of fluctuation in the tank internal pressure per unit time during the correction positive pressure check when the predetermined time tTP2 has elapsed.

In the following step S146, PVARIC
It is determined whether or not the value is a negative value, and when the value is a negative value, PVARIC = 0 is set (step S147).

(9) Judgment processing This is executed by the program shown in FIG.

In step S151, the flag FPLVL is set.
Is 1 or not, that is, the depressurizing process is performed for a predetermined time tPRG
If it is determined that FPLVL = 0 and the depressurization process is not completed, the third variation amount PVA is determined.
It is determined whether the RIC is equal to or less than the predetermined value PVARIC0 (step S152). As a result, PVARIC> PVA
When RIC0 is established, the process proceeds to step S160 without determining whether it is abnormal or normal, and the flag FEVPCHK is set to the value 1 to indicate that the determination process is completed.
On the other hand, when PVARIC ≦ PVARIC0 is satisfied, the tank internal pressure PT is maintained even if the bypass valve 24 is closed even though the pressure reducing process is not completed within the predetermined time tPRG.
Since it means that NK does not rise, the bypass valve 24
It is further determined that the fuel tank side is leaking, and the flag FEVPNGT is set to the value 1 to indicate this (step S155), and the process proceeds to step S160.

The answer to step S151 is affirmative (YES),
That is, FPLVL = 1 and the depressurization process is performed for a predetermined time tPR.
When completed within G, the difference between the second variation PVARIB and the third variation PVARIC ΔPVARI (= PV
It is determined whether ARIB-PVARIC) is a negative value (step S153). As a result, if the ΔPVARI value is negative, the process immediately proceeds to step S156, where ΔPVARI is
When the value is 0 or a positive value, it is determined whether or not the value is PVARI 0 or less (step S154). △ PVARI>
When PVAR0 is established, the third variation amount PVA
The second variation PVARIB is a predetermined value PV as compared with RIC.
Since ARI is greater than or equal to 0, it is determined that it corresponds to the case shown by the broken line in FIG. 2, and the process proceeds to step S155. Since the bypass valve 24 and the puff loss valve 22 are closed during the first leak check (FIG. 2, F), in this case, it is considered that the leak is occurring on the fuel tank side of the bypass valve 24. Because it will be done.

The answer to step S154 is affirmative (YES),
That is, when ΔPVARI ≦ PVARI0 is satisfied,
Proceeding to step S156, the tank internal pressure PEND at the start of the second leak check and the tank internal pressure PCANI at the end of the second leak check.
It is determined whether or not the difference ΔP2 (= PCANI-PEND) from the value is a negative value (step S156). As a result, △ P2
When the value is a negative value, it is determined to be normal, the flag FEVPOK is set to the value 1 to indicate this (step S159), and the process proceeds to step S160.

When the answer to step S156 is negative (NO), that is, when ΔP2 is 0 or positive, it is determined whether or not it is less than or equal to the predetermined value P0 (step S157), and when it is less than or equal to the predetermined value P0, it is determined as normal. Then, the process proceeds to step S159.

When the answer to step S157 is negative (NO), that is, when ΔP2> P0 is satisfied, it corresponds to the case shown by the alternate long and short dash line in FIG. 2, and it is determined that leakage from the bypass valve 24 on the canister side has occurred. Then, the flag FEVPNGC is set to the value 1 to indicate this (step S15).
8) and proceeds to step S160. The reason for making this determination is as follows. That is, during the first leak check, the bypass valve 24 is closed, and the pressure on the canister side is kept lower than the tank internal pressure by the valve 24 if there is no leak. Therefore, when the bypass 24 is opened and the second leak check is performed, the tank internal pressure is reduced under normal conditions (solid line in FIG. 2). However, if it rises as shown by the alternate long and short dash line in FIG. 2, it is considered that the pressure on the canister side from the bypass valve 24 rose due to leakage during the first leak check. Therefore, the determination is made as described above.

FIG. 20 is a flow chart of the calculation process of the air-fuel ratio correction coefficient KO2 in the steady operating state of the engine for executing the above-mentioned abnormality determination, and this process is executed every predetermined time (for example, 5 msec). It

First, in step S161, FIG.
Leak check KO2 initialization processing is performed.

In step S171 of FIG. 21 (a), it is determined whether or not the purge cut is being performed at the time of the leak check, that is, the drain shut valve 26 is closed and the purge control valve 30 is opened from the purge execution state (pressure reduction processing). It is determined whether or not the purge cut state (overshoot check / first leak check) in which the purge control valve 30 and the jet purge control valve 29 are closed is entered. On the other hand, while the processing is ended, if the purge cutting is being performed at the time of the leak check, it is determined whether the previous purge cutting was also performed (step S172).

If the previous purge cut was also in progress, this processing is immediately terminated, and if the previous purge cut was not in progress, that is, immediately after the purge execution state was changed to the cut state, the KO2 value is slightly changed from 1.0. Large predetermined value KO2P
Set to C (for example, 1.05) (step S17
3) Then, this process ends.

According to the process shown in FIG. 21A, when the purge state at the time of leak check (the purge state with the drain shut valve 26 closed) is changed to the purge cut state, KO2 = KO2PC, Otherwise,
No initial value is set.

Returning to FIG. 20, in step S162, O
_{2 It} is determined whether or not the sensor output VO2 is inverted, that is, whether it is changed from a high level to a low level or vice versa, and when it is not inverted, KO is performed by integral control (I term control).
A binary value is calculated (step S164). Specifically, V
When the O2 value is larger than the reference value VREF, the KO2 value is calculated by the following expression (2) and the KO2 value is gradually decreased, while when the VO2 value is smaller than the reference value VREF, the KO2 value is calculated by the following expression (3). Then, the KO2 value is gradually increased (see FIG. 22 (a)).

KO2 = KO2-I (2) KO2 = KO2 + I (3) Here, I is an integral term set according to the engine speed NE, for example.

The answer to step S162 is affirmative (YES),
That is, when the O ₂ sensor output VO2 is inverted, the KO2 value is calculated by proportional control (P term control) (step S
163). Specifically, when the VO2 value changes from a state smaller than the reference value VREF to a state larger than the reference value VREF, the KO2 value is calculated by the following equation (4) and the KO2 value is skipped in the decreasing direction, while the VO2 value is reduced to the reference value VREF. When the state changes from the larger state to the smaller state, the KO2 value is calculated by the following equation (5) and the KO2 value is skipped in the increasing direction (see FIG. 22A).

KO2 = KO2-PL (4) KO2 = KO2 + PR (5) Here, PL and PR are proportional terms set according to the engine speed, for example.

FIG. 22A is a diagram showing the transition of the KO2 value calculated by the procedure shown in FIGS. Time t1 is a time point when the depressurization process (purge is executed with the drain shut valve 26 closed) shifts to the overshoot check. As is clear from this figure, during the depressurization process, the KO2 value gradually decreases due to the effect of the enrichment of the air-fuel ratio by the purge, KO2 = KO2PC is set at time t1, and then normal feedback control is continued.

FIG. 10C shows the transition of the KO2 value in the case of the conventional KO2 calculation method, which shows K after the time t1.
Although the increase of the O2 value is delayed and the air-fuel ratio becomes lean, the exhaust gas characteristics are deteriorated. However, according to the present embodiment, the air-fuel ratio can be promptly set to an appropriate value, and such an adverse effect can be prevented.

FIG. 21B is a flow chart of another embodiment of the leak check KO2 initialization process, and steps S171 to S173 are the same as the corresponding steps of FIG. 21A.

In FIG. 21B, step S172
If the answer is negative (NO), that is, immediately after shifting to the purge cut, the tEVPPC timer is set to the predetermined time tEVPPC.
(For example, 1.0 second) is set and started (step S174), and this processing ends.

When the purge cut was being performed last time,
It is determined whether or not the count value of the tEVPPC timer is 0 (step S175). At first, since tEVPPC> 0, this processing is immediately terminated, and then tEVPPC
When = 0, the KO2 value is set to the predetermined value K02PC at the time of shifting the purge cut (step S173), and this processing is ended.

According to this processing, KO2 = K after the elapse of the predetermined time tEVPPC from the time of shifting to the purge cut state.
Since it is O2PC, the KO2 value is initialized when the fuel vapor in the purge passage 27 (between the purge control valve 30 and the intake pipe 2) is inhaled, and the fluctuation of the air-fuel ratio is further reduced. can do.

The KO2 value is set to the predetermined value K as described above.
Instead of setting to O2PC, the proportional term P of the equation (5)
Immediately after the transition to purge cut, R may be changed to a value larger than the value being purged and then gradually decreased to the value being purged. As a result, as shown in FIG. 22B, the KO2 value can be quickly increased and the fluctuation of the air-fuel ratio can be prevented.

Also in this case, the value of the proportional term PR is changed to a larger value after a lapse of a predetermined time from the time of shifting to the purge cut in consideration of the time until the fuel vapor in the purge passage is taken in. However, it may be gradually reduced thereafter.

[0115]

As described in detail above, according to the air-fuel ratio control apparatus of the first aspect, the first control valve is opened and the second control valve is closed. When the control valve of (1) shifts to the closed state, the air-fuel ratio correction coefficient is set to a predetermined value, so the air-fuel ratio is prevented from fluctuating in the lean direction immediately after the shift and the followability is improved. Good exhaust gas characteristics can be maintained.

According to the air-fuel ratio control apparatus of the second aspect, since the setting of the air-fuel ratio correction coefficient to the predetermined value is performed after a predetermined time has elapsed from the transition point, the engine is switched from the first control valve to the engine at the transition point. The air-fuel ratio can be corrected at an appropriate timing in consideration of the time taken for the purge fuel remaining in the purge passage leading to the intake system of FIG.

According to the air-fuel ratio control device of the third aspect, at the time of the transition, the degree of correction by the air-fuel ratio correction coefficient is temporarily made larger than that before the transition. Therefore, the same effect as that of the device of the first aspect is obtained. Play.

According to the air-fuel ratio control device of the fourth aspect, since the change of the correction degree by the air-fuel ratio correction coefficient is performed after a lapse of a predetermined time from the transition time point, the same effect as the device of the second aspect is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of an evaporated fuel processing apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an operating state of a valve provided in an evaporated fuel processing apparatus and a transition of a fuel tank internal pressure (PTNK).

FIG. 3 is a flowchart of a program (overall configuration) that executes abnormality determination processing.

FIG. 4 is a flowchart of a program (overall configuration) that executes abnormality determination processing.

FIG. 5 is a flowchart of a program for determining whether or not a precondition is satisfied.

FIG. 6 is a flowchart of a program for determining whether or not a precondition is satisfied.

FIG. 7 is a flowchart of a program for determining whether or not to permit abnormality determination based on the amount of fuel in the fuel tank.

8 is a predetermined value (LMTF, LMTE) used for determination in the program of FIG. 6 according to the battery voltage (VB).
It is a figure which shows the table for determining.

FIG. 9 is a diagram showing a relationship between a fuel amount (fuel level) and a fuel amount sensor output (VFUEL).

FIG. 10 is a diagram showing a relationship between an integrated flow rate (QPAIRT) and a canister charge amount.

FIG. 11 is a flowchart of a program for performing an atmosphere opening process.

FIG. 12 is a flowchart of a program for performing a temporary positive pressure check.

FIG. 13 is a flowchart of a program for performing depressurization processing.

FIG. 14 is a flowchart of a program for performing overshoot check.

FIG. 15 is a flowchart of a program for performing a first leak check.

FIG. 16 is a flowchart of a program for performing a second leak check.

FIG. 17 is a flowchart of a program for performing pressure cancellation processing.

FIG. 18 is a flowchart of a program for performing a correction positive pressure check.

FIG. 19 is a flowchart of a program that performs determination processing.

FIG. 20 is a flowchart of a process for calculating an air-fuel ratio correction coefficient (KO2).

21 is a flowchart of a subroutine for initializing an air-fuel ratio correction coefficient value at the time of a leak check in the process of FIG.

FIG. 22 is a diagram showing changes in the value of an air-fuel ratio correction coefficient (KO2).

[Explanation of symbols]

 1 Internal Combustion Engine 2 Intake Pipe 5 Electronic Control Unit (ECU) 9 Fuel Tank 11 Fuel Tank Internal Pressure Sensor 20 Charge Passage 24 Bypass Valve 25 Canister 26 Drain Shut Valve 27 Purge Passage 30 Purge Control Valve

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masayoshi Yamanaka 1-4-1 Chuo, Wako, Saitama Prefecture

Claims (4)

[Claims] 1. A fuel tank, a canister having an intake port communicating with the atmosphere, a charge passage connecting the canister and the fuel tank, and a purge passage connecting the canister and an intake system of an internal combustion engine. And an evaporative fuel treatment device that includes a first control valve interposed in the purge passage and a second control valve that opens and closes the intake port of the canister, and is provided in the exhaust system of the engine. The air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine using the air-fuel ratio correction coefficient determined according to the output of the exhaust concentration sensor. When the control valve is opened and the second control valve is closed and the first control valve is closed, the air-fuel ratio correction coefficient is set to a predetermined value. Set An air-fuel ratio control device for an internal combustion engine, characterized in that a transient control means is provided. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the transient control means sets the air-fuel ratio correction coefficient to a predetermined value after a lapse of a predetermined time from the transition time point. 3. A fuel tank, a canister having an intake port communicating with the atmosphere, a charge passage connecting the canister and the fuel tank, and a purge passage connecting the canister and an intake system of an internal combustion engine. And an evaporative fuel treatment device that includes a first control valve interposed in the purge passage and a second control valve that opens and closes the intake port of the canister, and is provided in the exhaust system of the engine. The air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine using the air-fuel ratio correction coefficient determined according to the output of the exhaust concentration sensor. When the control valve is opened and the second control valve is closed, and the first control valve is closed, the correction degree by the air-fuel ratio correction coefficient is changed. An air-fuel ratio control device for an internal combustion engine, characterized in that a transient control means for temporarily increasing the value before the transition is provided. 4. The air-fuel ratio control apparatus for an internal combustion engine according to claim 3, wherein the transient control means temporarily changes the correction degree after a lapse of a predetermined time from the transition time point.