JP2913282B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal combustion engine having a three-way catalyst capable of changing a target air-fuel ratio between stoichiometric and lean and purifying exhaust gas, and more particularly to an air-fuel ratio control method thereof.

[0002]

2. Description of the Related Art A three-way catalyst generally used as an exhaust gas purifying catalyst for an internal combustion engine has a problem that the NOx purifying effect cannot be sufficiently exhibited when the air-fuel ratio is lean. Measures such as reducing the lean operation area at the expense of the effect were taken. Further, since the NOx emission amount of the internal combustion engine becomes maximum at the stoichiometric and lean intermediate air-fuel ratio (A / F = 15 to 17), the air-fuel ratio is stepped between the stoichiometric and lean so as to jump over the intermediate air-fuel ratio. I was switching.

In recent years, NOx storage type exhaust gas purifying catalysts have also appeared, but this exhaust gas purifying catalyst has a limit in NOx storage amount, and it is estimated that the NOx storage amount reached the limit during lean operation. In this case, the air-fuel ratio is temporarily changed to a state richer than the stoichiometric state, and the stored NOx
Has been discharged.

[0004]

However, the conventional N
The Ox storage type exhaust gas purifying catalyst needs to frequently perform the above-mentioned stored NOx emission control during the lean operation due to the limit of the NOx storage amount, and there are problems such as deterioration of drivability and complicated control. Was. Moreover, the NOx storage type exhaust gas purifying catalyst is not yet sufficiently heat-resistant and oxidation-poisoning-resistant, and its use is greatly restricted.

On the other hand, in an internal combustion engine equipped with a three-way catalyst, when the air-fuel ratio is switched stepwise from lean to stoichiometric, a lean atmosphere remains in the three-way catalyst for a while, and excess O ₂ is stored. Therefore, there is a problem that the exhaust gas purifying function is not exhibited for a while even after the inside of the three-way catalyst becomes a rich atmosphere, and NOx is discharged to the atmosphere during that time. This problem occurs not only at the time of return from lean to stoichiometric by fuel injection amount control, but also at the time of return from lean to stoichiometric by fuel cut control.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to prevent an increase in NOx emission when changing a target air-fuel ratio of an internal combustion engine having a three-way catalyst from lean to stoichiometric. With the goal.

[0007]

In order to achieve the above object, according to the present invention, there is provided a three-way catalyst capable of changing a target air-fuel ratio between stoichiometric and lean, and purifying exhaust gas. in an internal combustion engine having, when changing the target air-fuel ratio from lean to stoichiometric changes to the stoichiometric target air-fuel ratio through a further-rich than the stoichiometric, or
Target sky in a richer state than the stoichiometric
The fuel ratio and the duration of this state are
Duration, the load of the internal combustion engine during the lean operation and
It is characterized in that it is determined based on the rotation speed .

[0008]

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described based on embodiments of the present invention shown in the accompanying drawings.

1 to 7 show an embodiment of the present invention. FIG. 1 is an overall configuration diagram of an air-fuel ratio control device for an internal combustion engine, FIG. 2 is a first partial view of a main routine flowchart, and FIG.
FIG. 4 is a flowchart of a subroutine of Step S5 of the main routine, FIG. 5 is a diagram showing a map for calculating the enrichment target time and the enrichment target air-fuel ratio, and FIG. Lean →
FIG. 7 is a time chart showing the change of the target air-fuel ratio at the time of the stoichiometric switching, and FIG. 7 is a graph showing the NOx emission amount at the time of the lean-to-stoichiometric switching.

[0011] As shown in FIG. 1, four cylinder internal combustion engine E (hereinafter, simply referred to as engine E) an intake passage 1 of is connected through an intake manifold 2 into four cylinders 3 ₁ to 3 _4. The intake passage 1 is provided with a throttle valve 4 which is connected to an accelerator pedal (not shown) and opens and closes.
This throttle valve 4 is connected to the throttle opening θ
A signal from the throttle opening sensor 5 for detecting TH is input to the electronic control unit U. The intake passage 1 is provided with an intake air amount sensor 6 for detecting an intake air amount Qair, and a signal from the intake air amount sensor 6 is input to the electronic control unit U. The engine E is provided with an engine speed sensor 7 that detects the engine speed Ne based on the rotation of a crankshaft (not shown). A signal from the engine speed sensor 7 is input to the electronic control unit U. . The intake manifold 2 has four cylinders 3
_Four fuel injectors 8 ₁ to 8 corresponding to _{1 to} 34 respectively
₄ are provided. Each fuel injection valve 8 _{1-8 4} is connected to an electronic control unit U controls the fuel injection amount Ti. A three-way catalyst 11 for purifying exhaust gas is provided in an exhaust passage 10 connected to the engine E via an exhaust manifold 9.

The electronic control unit U searches the map for the target air-fuel ratio A / F based on the throttle opening θTH detected by the throttle opening sensor 5 and the engine speed Ne detected by the engine speed sensor 7. In the normal operation range of the engine E, the target air-fuel ratio A / F is set to stoichiometric, that is, A / F = 14.7 which is the stoichiometric air-fuel ratio. On the other hand, in a specific operation region such as when the engine E is decelerating, the target air-fuel ratio A / F is significantly leaned in order to reduce fuel consumption, and the target air-fuel ratio A / F is set to, for example, A / F = 23. You. The switching timing of the target air-fuel ratio A / F from lean to stoichiometric and the switching timing from stoichiometric to lean are determined by the intake air amount Qair detected by the intake air amount sensor 6 and the engine speed Ne detected by the engine speed sensor 7. It is determined based on a map as a parameter.

The electronic control unit U has a target air-fuel ratio A
When / F is the stoichiometric air-fuel ratio, the fuel injection according to the air intake amount Qair detected by the intake air amount sensor 6 and the engine speed Ne detected by the engine speed sensor 7 so as to obtain the stoichiometric air-fuel ratio. setting the fuel injection amount Ti of the valve 8 _{1-8 4.} On the other hand, when the target air-fuel ratio A / F is made leaner than the stoichiometric air-fuel ratio, the fuel injection amount Ti is set so as to obtain the leaned target air-fuel ratio A / F.

Next, when switching from the lean state or the fuel cut state to the stoichiometric state, the target air-fuel ratio A /
The control of F will be further described with reference to the flowcharts of FIGS.

First, in step S1, the throttle opening θT
H and the basic target air-fuel ratio A based on the engine speed Ne.
/ F is searched by map. Since the enrichment flag described later is initially reset to "0" in step S2, the process proceeds to step S3. In step S3, the target air-fuel ratio A /
If F is the last lean, the process proceeds to step S4,
When the stoichiometric switching command is output for the first time in step S4, that is, when the switching from lean to stoichiometric is instructed, in step S5, the continuation time of the lean operating state up to that time and the average load of the engine E during that time (ie, , Intake air amount Qair) and the engine speed Ne.
The enrichment target time and the enrichment target air-fuel ratio are calculated.

Here, based on FIGS. 4 and 5, the enrichment target time TMrichF and the enrichment target air-fuel ratio KA
A method of calculating FrichF will be described.

In step S51 of the flowchart shown in FIG. 4, the engine speed Ne detected by the engine speed sensor 7 is read, and the air intake amount Qair detected by the intake air amount sensor 6 is read in step S52. At this time, the air intake amount Qair is changed to the engine speed N.
e and the intake negative pressure Pb. In the following step S53, the enrichment target time correction coefficient Kt is determined based on the duration of the lean operation state from the map of FIG.
rich and enrichment target air-fuel ratio correction coefficient Kkrich
Search for.

Further, in step S54, a basic enrichment target time TMrich is retrieved from the map of FIG. 5B based on the air intake amount Qair.
A basic enrichment target air-fuel ratio KAFrich is retrieved from the map (C) based on the air intake amount Qair. In step S56, the final enrichment target time TMri
chF is calculated by multiplying the basic enrichment target time TMrich by the enrichment target time correction coefficient Ktrich, and the final enrichment target air-fuel ratio KA
FrichF is set to the above-mentioned enriched target air-fuel ratio KAFric.
h is multiplied by the enrichment target air-fuel ratio correction coefficient Kkrich.

The value of the target enrichment time and the value of the target enrichment air-fuel ratio thus retrieved are determined by the following equation: the lean continuation time is long, the engine speed Ne is high, and the air intake amount Q
The greater the air, that is, the three-way catalyst 11 during the lean operation,
The higher the amount of O ₂ stored within, the higher the O ₂ .

Returning to the flowcharts of FIGS. 2 and 3, not the previous lean operation at step S3, the previous fuel cut operation at step S7, the fuel cut stop command is output for the first time at step S8, and the step S3 is executed.
When the stoichiometric switching command is output for the first time in step 9, that is, when switching from fuel cut to stoichiometric is instructed, the continuation time of the fuel cut operating state up to that time is searched in step S10. In step S11, the enrichment target time and the enrichment target air-fuel ratio are calculated based on the fuel cut continuation time searched in step S10. The method of calculating the enrichment target time and the enrichment target air-fuel ratio in the steps S10 and S11 is substantially the same as that in the step S5, and the lean continuation time in the step S53 in the flowchart of FIG. It can be implemented by changing.

Until the fuel cut stop command is output in step S8, the fuel cut processing is continued in step S12.

Thus, when switching from lean to stoichiometric,
Alternatively, when the enrichment target time and the enrichment target air-fuel ratio are calculated at the time of switching from fuel cut to stoichiometric, the enrichment target time counter is set in step S13, and the enrichment flag is set to "1" in step S14. .

In step S15, the target air-fuel ratio A / F
Is switched from the basic target air-fuel ratio retrieved in step S1 to the enrichment target air-fuel ratio determined in step S5 or step S11. That is, at the time of switching from lean to stoichiometric, the lean air-fuel ratio is switched from the basic air-fuel ratio at the time of leaning to the rich target air-fuel ratio determined at step S5. It is switched to the target air-fuel ratio. Subsequently, in step S16, the enrichment target time counter is counted down. When the enrichment target time has elapsed in step S17, the enrichment flag is reset to "0" in step S18.

In step S19, the final target
Fuel injection valve 8 according to air-fuel ratio A / F ₁~ 8_FourFuel from
The injection amount Ti is determined, and a point corresponding to the fuel injection amount Ti is determined.
The fire timing and the fuel injection timing are determined.

The above operation will be described with reference to the time chart of FIG. The time chart of FIG. 6 shows a case where the target air-fuel ratio A / F is switched from lean to stoichiometric. The target air-fuel ratio A / F during lean operation is set to A / F = 23, which is the basic target air-fuel ratio. . When the lean-to-stoichiometric switching command is output, the lean continuation time and the intake air amount Qai
The target air-fuel ratio A / F during the stoichiometric operation is calculated until the target enrichment time elapses, based on the target enrichment time and the target enrichment air-fuel ratio retrieved based on r and the engine speed Ne. The target air-fuel ratio is set to be richer than /F=14.7. Then, after the enrichment target time has elapsed, the operation shifts to the stoichiometric operation, and the target air-fuel ratio A / F is A, which is the basic target air-fuel ratio during the stoichiometric operation.
/F=14.7.

As described above, when the target air-fuel ratio A / F is switched from lean (or fuel cut) to stoichiometric, the target air-fuel ratio A / F is kept richer than stoichiometric for a predetermined time, and then the stoichiometric operation is performed. By doing so, NOx discharged from the three-way catalyst 11 can be reduced. That is, since the excess O ₂ is stored in the three-way catalyst 11 during the lean operation, the NOx purification function is not exhibited for a while after switching from the lean operation to the stoichiometric operation. At times, the target air-fuel ratio A / F is temporarily made richer than the stoichiometric ratio so that the O ₂ stored in the three-way catalyst 11 is stored.
Can be quickly discharged, and the NOx purification ability of the three-way catalyst 11 can be restored to prevent NOx from being discharged to the atmosphere.

Further, the conventional three-way catalyst 11 can be used without using a NOx occlusion type exhaust gas purifying catalyst which is not yet sufficiently resistant to heat deterioration and oxidation poisoning as the exhaust gas purifying catalyst.
Can be used as it is, which is advantageous in terms of durability and cost.

FIG. 7 shows the state at the time of switching from lean to stoichiometric.
The graph shows the NOx emission amount when the above-described enrichment control is performed (shown by a solid line) and when it is not performed (shown by a broken line).
It can be seen that the NOx emission is significantly reduced by performing the enrichment control.

Although the embodiments of the present invention have been described in detail, various design changes can be made in the present invention without departing from the gist thereof.

[0030] For example, in the embodiment four cylinders 3 ₁ to 3 ₄
Of it is switched the target air-fuel ratio A / F at the same time, may be switched the target air-fuel ratio A / F of the four cylinders 3 ₁ to 3 ₄ from the output of the air-fuel ratio switching command sequentially at predetermined intervals, By doing so, torque shock at the time of switching the air-fuel ratio can be reduced. Note that the lean state of the target air-fuel ratio in the present invention includes a fuel cut state.

[0031]

As described above, according to the first aspect of the present invention, when the target air-fuel ratio is changed from lean to stoichiometric, the target air-fuel ratio is changed to stoichiometric through a state richer than stoichiometric. Therefore, during the lean operation, excess O ₂ stored in the three-way catalyst can be quickly discharged to restore the NOx purification capability of the three-way catalyst, and NOx can be effectively prevented from being discharged to the atmosphere. In addition, the conventional three-way catalyst can be used as it is without using a NOx storage type exhaust gas purification catalyst, which is advantageous in terms of durability and cost. In addition, more than stoichiometric
The target air-fuel ratio in the
And the duration of the previous lean operation and during the lean operation
Is determined based on the load and speed of the internal combustion engine
Therefore, estimate the amount of O ₂ storage in the three-way catalyst accurately.
Emit the amount of reducing substance corresponding to the O ₂ storage amount
As a result, the NOx purification ability of the three-way catalyst can be confirmed.
You can really recover.

[0032]

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an air-fuel ratio control device for an internal combustion engine.

FIG. 2 is a first branch diagram of a flowchart of a main routine.

FIG. 3 is a second partial diagram of a flowchart of a main routine.

FIG. 4 is a flowchart of a subroutine of step S5 of the main routine.

FIG. 5 is a diagram showing a map for calculating a rich target time and a rich target air-fuel ratio.

FIG. 6 is a time chart showing a change in a target air-fuel ratio at the time of switching from lean to stoichiometric;

FIG. 7 is a graph showing NOx emissions at the time of switching from lean to stoichiometric;

[Explanation of symbols]

E internal combustion engine U electronic control unit 1 intake passage 3 _{1 to} 3 ₄ cylinders 8 ₁ to 8 ₄ fuel injection valve 10 exhaust passage 11 three-way catalyst

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naohiro Yonekura 1-4-1 Chuo, Wako-shi, Saitama Pref. Inside the Honda R & D Co., Ltd. (72) Inventor Hiroshi Asano 1-4-1 Chuo, Wako-shi, Saitama Pref. (56) References JP-A-62-162746 (JP, A) JP-A-8-177567 (JP, A) JP-A-9-53496 (JP, A) JP-A-8-296471 ( JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) F02D 41/00-41/40

Claims (1)

(57) [Claims] In an internal combustion engine having a three-way catalyst for purifying exhaust gas, the target air-fuel ratio can be changed from lean to stoichiometric when the target air-fuel ratio is changed from lean to stoichiometric. Change to stoichiometry through a state richer than stoichiometric , and set a target in a state richer than stoichiometric
The air-fuel ratio and the duration of the state are
Duration and load and load of the internal combustion engine during the lean operation.
An air-fuel ratio control method for an internal combustion engine, characterized in that the air-fuel ratio is determined on the basis of an engine speed and a rotation speed .