JPH0763089A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PURPOSE:To restrain the discharge of nitrogen oxides from increasing at a time when a target air-fuel ratio is switched from a theoretical air-fuel ratio to the lean one. CONSTITUTION:After a fuel cut is over, such fuel injection control as making a theoretical air-fuel ratio turn to the target air-fuel ratio is continued (P18), and for duration till the accumulated value TQan (P11, P12) of an intake air quantity Qan measured in suchlike theoretical air-fuel ratio control becomes more than the specified comparative value SLV (P16), this theoretical air-fuel ratio control is continued even if those of lean air-fuel ratio control conditions (P14, P15) are materialized.

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 specifically, it is equipped with an exhaust gas purification catalyst having an oxygen storage effect, and the air-fuel ratio of the engine intake air-fuel mixture is the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio. TECHNICAL FIELD The present invention relates to a technique for suppressing NOx emissions when switching from a stoichiometric air-fuel ratio to a lean air-fuel ratio in an internal combustion engine that is controlled to switch to a leaner air-fuel ratio.

[0002]

2. Description of the Related Art Conventionally, the air-fuel ratio of an engine intake air-fuel mixture is
According to the engine operating conditions, the stoichiometric air-fuel ratio and an air-fuel ratio significantly leaner than the stoichiometric air-fuel ratio (for example, air-fuel ratio = 21 to 23) (hereinafter, simply referred to as lean air-fuel ratio) are controlled to be switched. Also known is an air-fuel ratio control device (see Japanese Patent Laid-Open No. 63-12849).

[0003]

By the way, in the air-fuel ratio control device disclosed in Japanese Patent Laid-Open No. 63-12849, the air-fuel ratio switching is controlled so that the lean air-fuel ratio switching is not performed excessively. However, basically, the air-fuel ratio is switched according to the engine load and engine speed, and the amount of oxygen storage in a three-way catalyst that has an oxygen storage effect, such as a three-way catalyst impregnated with cerium oxide. The air-fuel ratio is not switched in correlation with.

Therefore, for example, when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio immediately after fuel cut, a large amount of NOx may be emitted. That is, if the fuel cut is continued, a large amount of oxygen is contained in the exhaust gas during the fuel cut, so that 2CeO ₂ ← Ce ₂ O ₃ +1.
The / 2O ₂ reaction proceeds, and a large amount of oxygen is adsorbed on the catalyst.

Even if fuel injection with the stoichiometric air-fuel ratio as the target air-fuel ratio is restarted in the state where the oxygen storage amount becomes large in this way, if the lean air-fuel ratio is immediately switched to, the fuel cut is performed. The air-fuel ratio switching will be executed before the oxygen adsorbed in a large amount inside is sufficiently reduced. If the amount of oxygen storage in the catalyst is large, the amount of oxygen storage will be saturated soon after the NOx is purified with switching to the lean air-fuel ratio, and NOx generated during switching the air-fuel ratio cannot be normally purified. .

A large amount of oxygen is stored in the three-way catalyst even during the normal lean air-fuel ratio control. However, from this state, the stoichiometric air-fuel ratio is temporarily controlled, and then the lean air-fuel ratio is restored. Even when switching to the fuel ratio, as in the case of switching from the stoichiometric air-fuel ratio after the fuel cut to the lean air-fuel ratio, switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio should be performed in a situation where much oxygen is adsorbed. become,
Similarly, a large amount of NOx will be emitted at the time of switching.

That is, when the air-fuel ratio is switched to the lean air-fuel ratio immediately after the fuel cut, or when the stoichiometric air-fuel ratio control is temporarily performed and then the lean control is returned to, the stoichiometric air-fuel ratio is continuously controlled. Compared with this, since the oxygen storage amount in the three-way catalyst is large, the oxygen storage amount reaches the saturation amount earlier than usual, and the NOx conversion performance is reduced accordingly, and the theoretical air-fuel ratio is switched to the lean air-fuel ratio. Therefore, the amount of NOx emission at that time increases.

[0008] Here, Japanese Patent Laid-Open No. 4-81539 discloses an air-fuel ratio control device that sets the air-fuel ratio richer than the stoichiometric air-fuel ratio for a predetermined time when the air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio. It is disclosed. That is, the technique disclosed in JP-A-4-81539 discloses a lean air-fuel ratio control state in which the air-fuel ratio is made rich according to the oxygen storage amount when switching from the lean air-fuel ratio to the rich air-fuel ratio. The oxygen stored in the catalyst is consumed at an early stage to ensure conversion performance by the catalyst immediately after switching to the stoichiometric air-fuel ratio.

However, the above-mentioned prior art is an air-fuel ratio control technique when the air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio, and in anticipation of switching to the lean air-fuel ratio after switching to the stoichiometric air-fuel ratio. Since the control is not performed, the above problems that occur when the air-fuel ratio is switched from the stoichiometric air-fuel ratio to the lean air-fuel ratio cannot be solved.

That is, when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio, it is necessary to keep the oxygen storage amount in the catalyst low in advance, but in the above-mentioned prior art, the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio. However, it is not possible to reliably avoid switching to the lean air-fuel ratio when the oxygen storage amount is comparatively large, and it is not possible to reliably prevent the NOx emission amount during the switching. It cannot be stably kept low.

The present invention has been made in view of the above problems, and by controlling the switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio in correlation with the oxygen storage amount in the catalyst,
N when switching from stoichiometric air-fuel ratio to lean air-fuel ratio
It is an object of the present invention to ensure that the Ox emission amount can be kept low.

[0012]

Therefore, an air-fuel ratio control system for an internal combustion engine according to the present invention is constructed as shown in FIG. In FIG. 1, the exhaust purification catalyst is a catalyst that is provided in the exhaust passage of the engine and has an oxygen storage effect.

The air-fuel ratio switching control means controls the air-fuel ratio of the engine intake air-fuel mixture to be switched to the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio according to the engine operating conditions. The air-fuel ratio switching permission means switches the stoichiometric air-fuel ratio from the stoichiometric air-fuel ratio to the lean air-fuel ratio by a period during which the stoichiometric air-fuel ratio is continuously controlled.
Only when the predetermined time period or more which correlates with the amount of oxygen storage in the exhaust purification catalyst, is permitted.

Here, the period may be detected by any one of the cumulative value of the engine intake air amount, the cumulative value of the engine speed, and the time. Further, the predetermined period in the air-fuel ratio switching permission means is such that the cumulative value of the engine intake air amount, the cumulative value of the engine speed, the control in the lean air-fuel ratio control state or the fuel supply stopped state before the stoichiometric air-fuel ratio is controlled. It is preferable to variably set it according to any one of the durations.

Furthermore, when the predetermined period in the air-fuel ratio switching permission means is in the fuel supply stopped state before the stoichiometric air-fuel ratio is controlled, it depends on the engine speed at which the stop of the fuel supply is started. You may make it variably set.

[0016]

According to the air-fuel ratio control device having such a configuration, if the period during which the stoichiometric air-fuel ratio is continuously controlled is not longer than the predetermined period that correlates with the oxygen storage amount in the exhaust purification catalyst, switching to the lean air-fuel ratio is performed. Not done Therefore, for example, when the lean air-fuel ratio control state shifts to the stoichiometric air-fuel ratio, or when the fuel supply stopped state shifts to the stoichiometric air-fuel ratio, the exhaust purification catalyst is theoretically adsorbed with a large amount of oxygen. When shifting to the air-fuel ratio control, the stoichiometric air-fuel ratio control for a predetermined period or more can surely reduce the large amount of adsorbed oxygen, and then the lean air-fuel ratio can be shifted.

Therefore, when the stoichiometric air-fuel ratio is changed to the lean air-fuel ratio, it is possible to prevent the oxygen storage amount in the exhaust purification catalyst from reaching a saturated state at an early stage. It is possible to secure the conversion performance of the reduced NOx. As described above, the period during which the control at the stoichiometric air-fuel ratio is forced needs to be correlated with the oxygen storage amount. Therefore, the period is the cumulative value of the engine intake air amount correlated with the change in the oxygen storage amount, the engine It is detected by either the cumulative value of rotation speed or time.

Further, the oxygen storage amount of the exhaust purification catalyst does not always become a constant value in the lean air-fuel ratio control state before controlling to the stoichiometric air-fuel ratio or in the fuel supply stopped state. Therefore, any one of the cumulative value of the engine intake air amount, the cumulative value of the engine speed, and the control continuation time that correlates with the oxygen storage amount in the lean air-fuel ratio control state before performing the theoretical air-fuel ratio control or the fuel supply stop state Based on the above, the predetermined period for which the stoichiometric air-fuel ratio control is forced is variably set, and the stoichiometric air-fuel ratio control is forced only for the period corresponding to the actual oxygen storage amount.

Further, when the fuel supply is stopped before the stoichiometric air-fuel ratio control, the oxygen storage amount during the fuel supply stop can be estimated by the rotation speed at which the fuel supply stop is started. Therefore, the predetermined period for forcing the stoichiometric air-fuel ratio control may be set according to the rotational speed.

[0020]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 showing the system configuration of the embodiment, air is sucked into each cylinder of the V-type internal combustion engine 1 through an air cleaner 2, a throttle valve 3 and an intake manifold 4. An electromagnetic fuel injection valve 5 is provided at each branch of the intake manifold 4.

Exhaust gas from the engine 1 is discharged to the exhaust manifold 6
After being collected for each bank by a and 6b, they are guided to the muffler 8 by exhaust pipes 7a and 7b, respectively. Catalytic converters (exhaust gas purification catalysts) 9a and 9b are provided in the exhaust pipes 7a and 7b, respectively. The catalytic converters 9a, 9b are configured to reduce NOx, HC, and
It has both a three-way catalytic action for purifying CO at a high conversion rate at the same time and a lean NOx catalytic action for reducing (converting) NOx in an oxygen excess state (lean air-fuel ratio combustion state). It is a catalyst that has an oxygen storage effect by being impregnated.

The control unit 10 has a built-in microcomputer, calculates a fuel injection amount Ti (injection pulse width) by the fuel injection valve 5 based on detection signals from various sensors, and calculates the fuel injection amount Ti. The fuel supply to the engine 1 is electronically controlled by controlling the open drive of the fuel injection valve 5 based on The engine of this embodiment is a so-called lean burn engine that burns at an air-fuel ratio (for example, 22) that is significantly leaner than the stoichiometric air-fuel ratio in a predetermined operating range.
Except for the lean combustion operation region, combustion is performed with the stoichiometric air-fuel ratio as the target air-fuel ratio.

Further, in this embodiment, the control unit 10 controls the fuel injection valve 5 under a predetermined deceleration operation condition.
The fuel injection (fuel supply) is forcibly stopped (hereinafter, simply referred to as fuel cut). As the various sensors, an air flow meter 11 that detects the intake air amount Qa of the engine 1 on the upstream side of the throttle valve 3, a crank angle sensor 12 that extracts an engine rotation signal from a cam shaft, and a cooling water temperature Tw of the engine 1 are detected. It is turned on at the fully closed position of the water temperature sensor 13, the oxygen sensors 14a and 14b, which are respectively provided in the collecting portion of the exhaust manifolds 6a and 6b to detect the oxygen concentration in the exhaust for each bank, and the throttle valve 3.
Idle switch 15 is provided.

Further, exhaust gas temperature sensors 16a and 16b are provided on the downstream sides of the catalytic converters 9a and 9b, respectively. Reference numeral 17 is a control valve for adjusting the intake air amount during idling, and adjusts the amount of auxiliary air supplied to the engine 1 via a bypass passage 18 that bypasses the throttle valve 3.

Here, the structure of the fuel injection control by the control unit 10 is simplified and shown in the block diagram of FIG. As shown in FIG. 3, the control unit
Reference numeral 10 denotes a basic fuel injection amount calculation means A for calculating a basic fuel injection amount Tp based on detection signals of the crank angle sensor 12 and the air flow meter 11, a basic fuel injection amount Tp (engine load representative value), and a crank angle sensor 12. Lean determination means B for determining a condition for controlling to a lean air-fuel ratio based on the output (rotation speed), the output (rotation speed) of the crank angle sensor 12 and the ON / OFF signal of the idle switch 15 for determining the fuel cut condition. The fuel injection amount i is finally calculated based on the output from the fuel cut determination means C, the basic fuel injection amount calculation means A, the lean determination means B, the fuel cut determination means C and the detection signals of the oxygen sensors 14a and 14b. It has a function as the injection amount calculation means D. Then, based on the fuel injection amount Ti calculated by the injection amount calculation means D, the fuel injection valve 5
Are driven and controlled.

The function as the air-fuel ratio switching control means and the air-fuel ratio switching permission means is provided in software by the control unit 10 as shown in the flow chart described later, and in the block diagram shown in FIG. The determination unit B and the injection amount calculation unit D constitute an air-fuel ratio switching control unit, and the function as the air-fuel ratio switching permission unit is included in the injection amount calculation unit D.

Next, the state of fuel injection control (air-fuel ratio control) performed according to the configuration shown in FIG. 3 will be described in detail with reference to the flow chart of FIG. In addition, the program shown in the flow chart of FIG.
An interrupt is executed every s). In the flowchart of FIG. 4, at P1, the air flow meter 11
The intake air amount Qan of the engine 1 is detected based on the detection signal of. The subscript n in the label name of the intake air amount data is a value indicating the sampling order.

Next, at P2, the engine speed Ne is detected based on the detection signal of the crank angle sensor 12. Then, at P3, the basic injection amount (basic injection pulse width) T at the fuel injection valve 5 is based on the engine speed Ne and the intake air amount Qan detected at P1 and P2, respectively.
p = K × Qa / Ne (K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5) is calculated. The basic injection amount Tp calculated in P3 is calculated as a stoichiometric air-fuel ratio equivalent amount.

At the next P4, the idle switch 15 is turned on.
・ Read the OFF signal. Then, at P5, it is determined whether or not the idle switch 15 is ON (the throttle valve 3 is fully closed). Here, if it is determined that the idle switch 15 is ON, then the routine proceeds to P6, where the engine speed Ne is the predetermined speed (for example,
Determine whether it exceeds 1200 rpm).

When the idle switch 15 is ON and the rotation speed Ne is higher than the predetermined rotation speed, it is determined that the deceleration operation condition for fuel cut is set, and the routine proceeds to P7. In P7, the fuel cut flag F _FC indicating the execution state of the fuel cut is discriminated, and when 1 is not set in the fuel cut flag F _FC , that is, when the fuel cut condition is satisfied for the first time, the process proceeds to P8. Then, after the fuel cut flag F _FC is set to 1, the process proceeds to P9.

If it is determined at P7 that the fuel cut flag F _FC has already been set to 1, the process jumps to P8 and proceeds to P9. At P9, the pulse width Ts corresponding to the invalid injection time corresponding to the battery voltage corresponding to the power supply voltage of the fuel injection valve 5 is set as the final fuel injection amount Ti, and the fuel injection by the fuel injection valve 5 is substantially performed. Not be done.

On the other hand, at least one of P5 and P6,
When it is determined that the fuel cut conditions are not met
Goes to P10. At P10, the fuel cut flag F_FC
Is determined. The fuel cut flag F _FCAs mentioned above
Is set to 1 during fuel cut, and as described later
With the flag that is reset to 0 when the fuel cut is completed
Because there is, the fuel cut flag F at P10_FCIs set to 1
If it is determined that the
It can be considered that it is the first time.

At P10, the fuel cut flag F _{FC is set} to 1
When it is determined that is set, the program proceeds to P11 to calculate the cumulative intake air amount TQan after the fuel injection is restarted, and the cumulative intake air amount TQan is calculated.
Is reset to 0. On the other hand, at P10, the fuel cut flag F
When it is determined that _FC is set to 0, P12
Then, the update calculation of the cumulative intake air amount TQan is performed according to the following equation.

TQan = TQan _-1 + Qan In the above formula, TQan _-1 is the cumulative intake air amount up to the previous time, and the intake air amount Qan detected at P1 this time is added to the previous value to cut the fuel cut. The intake air amount Qan detected at P1 is accumulated after the injection is restarted.

When the processing of P11 or P12 is finished, P13
To the fuel cut flag F. _FCReset to 0
It At P14, it is determined whether the rotation speed Ne is within a predetermined range.
It is determined that the rotation speed Ne is within a predetermined range.
Then, proceeding to P15, the basic fuel injection that represents the engine load
It is determined whether or not the injection amount Tp is within a predetermined range.

The rotation speed Ne and the basic fuel injection amount T
If it is determined that p is within the predetermined range,
In the present embodiment, it is determined that the operation range is controlled to the lean air-fuel ratio, but in the next P16, the cumulative intake air amount TQan is compared with the predetermined comparison value SLV.
Then, the cumulative intake air amount TQan is the predetermined comparison value SLV.
In the case of the following, the process proceeds to P18, and the fuel injection amount Ti (= Tp + Ts) with the theoretical air-fuel ratio as the target air-fuel ratio is calculated based on the basic fuel injection amount Tp and the invalid injection time Ts.

That is, even if the conditions of the revolution speed Ne and the basic fuel injection amount Tp are satisfied as the lean air-fuel ratio control condition,
When the cumulative intake air amount TQan after the fuel cut is finished and the fuel injection is restarted is equal to or smaller than the predetermined comparison value SLV, the lean air-fuel ratio control is not performed, but the stoichiometric air-fuel ratio is controlled. Even when it is determined at P14 and P15 that the lean air-fuel ratio control condition is not satisfied, P18
Then, the process proceeds to and the fuel injection amount Ti equivalent to the theoretical air-fuel ratio is calculated.

On the other hand, if the cumulative intake air amount TQan exceeds the predetermined comparison value SLV, the process proceeds to P17,
By converting the basic fuel injection amount Tp into a value equivalent to the target lean air-fuel ratio (= 22), the injection amount Ti equivalent to the target lean air-fuel ratio
(= Tp × 14.6 / 22 + Ts) is calculated. In this way, in this embodiment, even after the fuel is cut, the intake air amount after the fuel injection with the target air-fuel ratio as the stoichiometric air-fuel ratio is restarted is accumulated at a predetermined value or more, even if the lean air-fuel ratio control condition is satisfied. Even if it is established, it will be controlled to the stoichiometric air-fuel ratio.

During the fuel cut, a large amount of oxygen is contained in the exhaust gas, and a large amount of oxygen is adsorbed in the catalytic converters 9a and 9b having an oxygen storage effect. Then, when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio immediately after the fuel cut is finished and fuel injection with the stoichiometric air-fuel ratio as the target air-fuel ratio is restarted, compared to when the stoichiometric air-fuel ratio is continuously controlled. With a large amount of oxygen adsorbed on the catalyst, switching to the lean air-fuel ratio is performed. In this case, when the air-fuel ratio is switched to the lean air-fuel ratio, the oxygen storage amount is saturated at an early stage, so that the NOx conversion performance in the three-way catalytic action cannot be exerted when the air-fuel ratio is switched, and a large amount of NO is generated.
There is a fear that x will be discharged.

On the other hand, in this embodiment, after the fuel cut, the cumulative intake air amount TQan predicted to be correlated with the oxygen storage amount in the catalyst is the predetermined comparison value SLV.
Until it exceeds the limit, the stoichiometric air-fuel ratio is used for control even if the conditions for shifting to the lean air-fuel ratio are satisfied.Therefore, the oxygen storage amount that has become excessive due to fuel cut is stabilized in the normal stoichiometric air-fuel ratio control state. After lowering to the level, switching to the lean air-fuel ratio will be permitted. Therefore, at the time of switching to the lean air-fuel ratio, a time lag until the oxygen storage amount in the catalyst becomes saturated can be secured, and therefore, the N-way due to the three-way catalytic action at the time of switching can be secured.
It is possible to secure the Ox conversion performance and avoid an increase in the NOx emission amount (see FIG. 5).

The comparison value SLV used to determine the cumulative intake air amount TQan in the above embodiment may be a fixed value,
When the oxygen storage amount during fuel cut is small, it is possible to permit the shift to the lean air-fuel ratio earlier, so it is preferable to variably set according to the parameter that correlates with the oxygen storage amount during fuel cut. .

Specifically, as shown in FIG. 6, for example, the cumulative intake air amount TQ is determined according to the starting rotational speed of fuel cut.
The comparison value SLV of an can be variably set. That is, when the fuel cut start rotation speed is high, the fuel cut is performed for that long, and the oxygen storage amount increases accordingly. Therefore, the higher the fuel cut start rotation speed, the comparative value SLV becomes. Set higher.

Generally, the oxygen storage amount during fuel cut increases proportionally as the starting speed of fuel cut increases, but at a certain speed or higher, a saturation amount determined by the weight of cerium oxide in the catalyst is reached. I will end up. In addition to the above fuel cut start speed, the cumulative intake air amount and cumulative speed during fuel cut, and the continuous control time for fuel cut are also correlated with the oxygen storage amount during fuel cut. The comparison value SLV may be variably set by a parameter.

Therefore, the detection of the period during which the control with the stoichiometric air-fuel ratio is forced after the fuel cut is detected is not limited to the above-mentioned cumulative intake air amount, but the cumulative rotational speed and time, which are parameters that are similarly correlated with the change in the oxygen storage amount. The configuration may be such that it is detected based on For example, in the case of forcibly controlling the stoichiometric air-fuel ratio for a predetermined time after the end of fuel cut,
The P11 and P12 parts in the flow chart of FIG. 4 are used to reset and count up the timer value, and in P16,
The counted timer value may be compared with a predetermined time, and the lean air-fuel ratio may be changed only when the predetermined time or more has elapsed.

Here, if the predetermined time is set to a fixed value or is set variably in accordance with the starting rotational speed of fuel cut, the cumulative air amount during fuel cutting, the cumulative rotational speed, and the fuel cut time. good. Further, when the configuration is such that the stoichiometric air-fuel ratio is forcibly controlled until the cumulative rotation speed becomes equal to or higher than the predetermined rotation speed after the fuel cut is completed, the cumulative intake air amount of the portions P11 and P12 in the flowchart of FIG. 4 is accumulated. Replacing with the engine speed, in P16, the engine speed accumulated after fuel cut is compared with a predetermined engine speed, and the lean air-fuel ratio is set only when the engine speed is higher than the engine speed. It is sufficient to make a transition.

Here again, the predetermined cumulative rotational speed is a fixed value, or is set to a variable speed according to the starting rotational speed of the fuel cut, the cumulative air amount during the fuel cut, the cumulative rotational speed, and the fuel cut time. Just do it. Further, in the above-mentioned embodiment, the control is shown for the case where the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio immediately after the fuel cut, but the control state at the lean air-fuel ratio is changed to the stoichiometric air-fuel ratio, and the lean air-fuel ratio is changed again. Also in the case of switching, the period for controlling to the stoichiometric air-fuel ratio is sufficiently secured to reduce a large amount of adsorbed oxygen in the immediately preceding lean air-fuel ratio control state (fuel cut state in the first embodiment), and then to the lean air-fuel ratio. It is desirable to perform the above switching, and an embodiment corresponding to such lean->rich-> lean switching will be described with reference to the flowchart of FIG.

In the flowchart of FIG. 7, at P21, the intake air amount Qa of the engine 1 is detected based on the detection signal of the air flow meter 11. Next, at P22, the engine speed Ne is detected based on the detection signal of the crank angle sensor 12. Then, at P23, the basic injection amount (basic injection pulse width) Tp = K × Qa / Ne at the fuel injection valve 5 based on the engine speed Ne and the intake air amount Qan detected at P21 and P22, respectively. (K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5.) is calculated. The basic injection amount Tp calculated in P23 is calculated as the theoretical air-fuel ratio equivalent amount.

At the next P24, it is judged whether or not the rotation speed Ne is within a predetermined range. If it is judged that the rotation speed Ne is within a predetermined range, the routine proceeds to P25 to represent the engine load. It is determined whether or not the basic fuel injection amount Tp is within a predetermined range. If it is determined that the rotation speed Ne and the basic fuel injection amount Tp are within the predetermined ranges, respectively, it is determined that the operation range in which the lean air-fuel ratio is controlled in the present embodiment is applied. When it is determined that at least one of the basic fuel injection amount Tp is not within the predetermined range, it is determined that the operation range is controlled to the stoichiometric air-fuel ratio.

In the case of the stoichiometric air-fuel ratio control range,
Go to P26. At P26, the stoichiometric flag Fs indicating the stoichiometric air-fuel ratio control state is determined. This stoichiometric flag Fs
As will be described later, 0 is set during lean air-fuel ratio control. Therefore, if it is determined at P26 that the stoichiometric flag Fs is set to 0, the lean air-fuel ratio control state is changed to the stoichiometric air-fuel ratio control. This is equivalent to the first transition.

When it is the first time to shift to the stoichiometric air-fuel ratio control, that is, when the stoichiometric flag Fs = 0, the routine proceeds to P27, where a predetermined time ST _S for forcibly continuing the current stoichiometric air-fuel ratio control is It is variably set according to the duration (lean time) _TL of the immediately preceding lean air-fuel ratio control state.
The measurement of the lean time T _L will be described later. At the next P28, the theoretical air-fuel ratio time T _St is reset to zero in order to measure the time during which the theoretical air-fuel ratio control is continued, and further,
At P29, the stoichiometric flag Fs is set to 1.

Then, the process proceeds to P31 and is calculated in P23.
Fuel injection equivalent to the theoretical air-fuel ratio using the basic fuel injection amount Tp
The injection amount Ti (= Tp + Ts) is calculated. In P29 above
By setting the Toiki Flag Fs to 1, next time
If it is the stoichiometric air-fuel ratio control condition, from P26 to P
30, the theoretical air-fuel ratio time T _StThe update of
From P to the calculation of the fuel injection amount Ti of P31.

When it is determined from the above stoichiometric air-fuel ratio control state that the lean air-fuel ratio control condition has been reached by the determination of P24 and P25, the routine proceeds to P32, where the stoichiometric flag Fs is determined. Since the stoichiometric flag Fs is set to 1 in the stoichiometric air-fuel ratio control state, when the lean air-fuel ratio control condition is satisfied in the stoichiometric air-fuel ratio control state, the stoichiometric flag Fs is determined to be 1 in P32. It will be.

When it is determined at P32 that the stoichiometric flag Fs is 1, the routine proceeds to P33, where the stoichiometric air-fuel ratio time T _St measured at P28 and P30 and the lean time _TL at P27 are determined. The set predetermined time ST _S is compared.
Here, when the duration time T _St for controlling to the stoichiometric air-fuel ratio is shorter than the comparison time ST _S set according to the duration time T _L of the lean air-fuel ratio control state before the theoretical control state, Although the condition for shifting to the lean air-fuel ratio is satisfied, the routine proceeds to P26, where the stoichiometric air-fuel ratio control is continued.

That is, in the lean air-fuel ratio control state, a large amount of oxygen is adsorbed on the catalyst, and even if the air-fuel ratio is shifted to the theoretical air-fuel ratio thereafter, the control time at the theoretical air-fuel ratio is controlled to the lean air-fuel ratio. If the time is short, that is, the amount of oxygen adsorbed in the lean air-fuel ratio control state, switching to the lean air-fuel ratio is performed while the amount of oxygen adsorbed in the catalyst is large. In this case, the amount of oxygen storage in the catalyst becomes saturated at an early stage, and NOx cannot be sufficiently purified when the air-fuel ratio is switched.

Therefore, even if the lean air-fuel ratio control condition is satisfied, the stoichiometric air-fuel ratio is not immediately switched to the lean air-fuel ratio, and it is necessary to reduce the oxygen storage amount with respect to the time when the lean air-fuel ratio is controlled. After continuously controlling to the stoichiometric air-fuel ratio for a sufficient time, in other words, the shift to the lean air-fuel ratio is permitted after the oxygen storage amount in the catalyst reaches a stable level corresponding to the stoichiometric air-fuel ratio control state. (See FIG. 8).

Therefore, until the theoretical air-fuel ratio control continuation time T _St exceeds the comparison time ST _S at P33,
When the stoichiometric air-fuel ratio control is forcibly continued and the comparison time ST _S is exceeded, the routine proceeds to P34 and thereafter to start lean air-fuel ratio control. At P34, since the lean air-fuel ratio control is actually started, the stoichiometric flag Fs is reset to zero, and further,
At the next P35, the lean time T _L is reset to zero in order to newly measure the lean time T _L.

At P37, the basic fuel injection amount Tp calculated at P23 is converted into an amount corresponding to the target lean air-fuel ratio (= 22), so that the injection amount Ti (corresponding to the target lean air-fuel ratio is = Tp × 14.6 / 22 + Ts) is calculated.
Since the stoichiometric flag Fs is reset to zero in P34, if the lean air-fuel ratio control condition is satisfied next time, P32
From P to P36, the lean time T _L is counted up, and then the injection amount Ti corresponding to the lean air-fuel ratio is calculated in P37.

In the above embodiment, the theoretical air-fuel ratio control time and the lean air-fuel ratio control time are measured respectively, and the theoretical air-fuel ratio time T _St exceeds the comparison time ST _S set according to the lean time T _L. Until this happens, switching to the lean air-fuel ratio was prohibited.However, instead of the parameter of the above time, the parameters of the cumulative intake air amount and cumulative rotational speed during lean air-fuel ratio control are used to calculate the theoretical air-fuel ratio. It is also possible to variably set the period for which the fuel ratio control is forced (the period detected by the cumulative intake air amount or the cumulative rotation speed) and detect the variably set period.

Further, for simplicity, the theoretical air-fuel ratio time T
The transition to the lean air-fuel ratio may be regulated by comparing the cumulative intake air amount, the cumulative rotation speed and the fixed value when _St or the stoichiometric air-fuel ratio is controlled. In the above embodiment, the case where the state before the stoichiometric air-fuel ratio control state is the fuel cut state and the case where it is the lean air-fuel ratio control state are described separately, but it is configured to execute these controls in combination. It is preferable.

[0060]

As described above, according to the present invention,
In an engine in which the air-fuel ratio of the engine intake air-fuel mixture is controlled by switching between the stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the period during which the stoichiometric air-fuel ratio is continuously controlled is the oxygen storage in the exhaust purification catalyst. The amount of oxygen storage in the exhaust gas purification catalyst is higher than usual because the switch from the stoichiometric air-fuel ratio to the lean air-fuel ratio is allowed only when it is over a predetermined period that correlates with the amount. Therefore, it is possible to avoid switching to the lean air-fuel ratio, and thus it is possible to secure a time lag until the oxygen storage amount is saturated when switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio. There is an effect that the NOx emission amount can be reliably suppressed to a low level.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention.

FIG. 2 is a schematic diagram showing a system configuration of an embodiment.

FIG. 3 is a block diagram showing a control configuration of the embodiment.

FIG. 4 is a flowchart showing the air-fuel ratio control of the first embodiment.

FIG. 5 is a time chart showing the control characteristics of the first embodiment.

FIG. 6 is a diagram showing characteristics of comparative values in the first embodiment.

FIG. 7 is a flowchart showing air-fuel ratio control of the second embodiment.

FIG. 8 is a time chart showing the control characteristics of the second embodiment.

[Explanation of symbols]

 1 Internal Combustion Engine 5 Fuel Injection Valve 9a, 9b Catalytic Converter (Exhaust Gas Purification Catalyst) 10 Control Unit 11 Air Flow Meter 12 Crank Angle Sensor 13 Water Temperature Sensor

Claims (4)

[Claims] 1. An exhaust purification catalyst having an oxygen storage effect, which is provided in an exhaust passage of an engine, and a stoichiometric air-fuel ratio of an engine intake air-fuel mixture depending on engine operating conditions and an air leaner than the stoichiometric air-fuel ratio. The air-fuel ratio switching control means for switching control to the fuel ratio, and switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio by the air-fuel ratio switching control means, the period during which the stoichiometric air-fuel ratio is continuously controlled is in the exhaust purification catalyst. An air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio switching permission unit that permits only when a predetermined period of time or more that is correlated with the oxygen storage amount is exceeded. 2. The empty space of the internal combustion engine according to claim 1, wherein the period is detected by any one of a cumulative value of an engine intake air amount, a cumulative value of an engine speed, and a time. Fuel ratio control device. 3. The cumulative value of the engine intake air amount and the cumulative engine speed in the lean air-fuel ratio control state before the stoichiometric air-fuel ratio is controlled or the fuel supply is stopped during the predetermined period in the air-fuel ratio switching permission means. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control device is variably set according to any one of a value and a control duration time. 4. When the predetermined period of time in the air-fuel ratio switching permission means is in a fuel supply stopped state before the stoichiometric air-fuel ratio is controlled, the predetermined period is dependent on the engine speed at which the stop of the fuel supply is started. 3. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control device is variably set.