JP2005299588A - Air fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable suppression of deterioration of a catalyst inlet part due to reaction heat while maintaining purifying capability of an exhaust gas purifying catalyst high in an air fuel ratio control device for an internal combustion engine. <P>SOLUTION: When an exhaust gas air fuel ratio at downstream of an exhaust gas purifying catalyst gets rich, firstly, the air fuel ratio is switched to a rich initial target air fuel ratio AO1 on the stoichiometric side more than a rich final target air fuel ratio A1, and thereafter changed from the rich initial target air-fuel ratio AO1 to the rich final target air fuel ratio A1. When an exhaust gas air fuel ratio gets lean, firstly, the air fuel ratio is switched to a lean initial target air fuel ratio A02 on the stoichiometric side more than a lean final target air fuel ratio A2, and thereafter it is changed from the lean initial target air fuel ratio A02 to the lean final target air fuel ratio A2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an air-fuel ratio control apparatus that switches a target air-fuel ratio alternately between lean and rich based on an exhaust air-fuel ratio downstream of a catalyst.

  An exhaust purification catalyst (three-way catalyst) is disposed in the exhaust passage of the internal combustion engine. The purification capability of the exhaust purification catalyst is exhibited when the ambient atmosphere is in the vicinity of the theoretical air-fuel ratio, and NOx, CO, and HC that are harmful components in the exhaust gas are simultaneously purified. The exhaust purification catalyst has an oxygen storage / release function. When the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst fluctuates between lean and rich, the oxygen stored in the exhaust purification catalyst is removed. Changes in the air-fuel ratio of the surrounding atmosphere are suppressed by releasing it into the phase or taking in and storing oxygen in the gas phase.

The purification ability of the exhaust purification catalyst can be maintained high by activating the catalyst noble metal by repeated oxygen storage / release. As a means for repeatedly storing / releasing oxygen to / from the exhaust purification catalyst, for example, a technique described in Patent Document 1 is known. The prior art described in Patent Document 1 switches the target air-fuel ratio alternately between lean and rich in a rectangular shape in accordance with a change in the output signal of an oxygen sensor arranged downstream of the exhaust purification catalyst. . According to such switching of the target air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst can be alternately reversed between lean and rich, thereby efficiently storing oxygen in the exhaust purification catalyst. / Can be released.
JP 2001-221086 A Japanese Patent Laid-Open No. 10-246139 Japanese Patent Application Laid-Open No. 6-221955 Japanese Patent Laid-Open No. 5-141287

  By the way, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is rich, the stored oxygen is released from the exhaust purification catalyst. This oxygen release phenomenon is the exhaust purification catalyst through which the exhaust gas first passes. Starting from the entrance side. Similarly, when the air-fuel ratio of the inflowing exhaust gas is reversed to lean, oxygen is stored in the exhaust purification catalyst from the inlet side. As described above, since the oxygen storage / release starts from the inlet side of the exhaust purification catalyst, the change in the air-fuel ratio of the ambient atmosphere is smaller on the outlet side of the exhaust purification catalyst than on the inlet side.

  In order to maximize the purification capability of the exhaust purification catalyst, it is necessary to effectively store and release oxygen throughout the entire area from the inlet to the outlet of the exhaust purification catalyst. Thus, it is necessary to greatly change the air-fuel ratio even on the outlet side where the change in the air-fuel ratio of the surrounding atmosphere is small. For this purpose, for example, it is considered effective to increase the range of change in the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, that is, the range of leaning and enriching with respect to stoichiometry.

  However, when increasing the change width of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, there are the following problems. In the exhaust purification catalyst, oxygen is occluded / released in accordance with the deviation of the air-fuel ratio of the ambient atmosphere from the stoichiometry. At this time, reaction heat is generated due to the oxygen occlusion / release reaction. Since this reaction heat increases as the air-fuel ratio change range of the ambient atmosphere increases, increasing the air-fuel ratio change range of the exhaust gas flowing into the exhaust purification catalyst increases the catalyst temperature of the exhaust purification catalyst, particularly the ambient atmosphere. There is a possibility that the temperature of the inlet portion where the air-fuel ratio variation is large exceeds the heat-resistant temperature, leading to deterioration of the exhaust purification catalyst.

  The present invention has been made to solve the above-described problems, and is an internal combustion engine that can suppress deterioration of a catalyst inlet due to reaction heat while maintaining a high purification capacity of an exhaust purification catalyst. An object is to provide an air-fuel ratio control device.

In order to achieve the above object, a first invention is an air-fuel ratio control device for an internal combustion engine that controls an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be a target air-fuel ratio,
An exhaust gas sensor disposed downstream of the exhaust purification catalyst in the exhaust passage of the internal combustion engine;
When the output signal of the exhaust gas sensor changes to a rich side from a predetermined rich determination value, the target air-fuel ratio is switched to the initial target air-fuel ratio at the rich time set to the lean side from the stoichiometric, and then the rich-time The initial target air-fuel ratio is changed to the rich final target air-fuel ratio that is set to the lean side further than the rich initial target air-fuel ratio, and the output signal of the exhaust gas sensor changes to the lean side from the predetermined lean determination value In this case, after the target air-fuel ratio is switched to the lean initial target air-fuel ratio set to the rich side from the stoichiometry, the lean initial target air-fuel ratio is set to a richer side than the lean initial target air-fuel ratio. And a target air-fuel ratio switching means for changing to the lean final target air-fuel ratio.

According to a second invention, in the first invention, there is provided an exhaust gas amount estimating means for estimating an exhaust gas amount flowing into the exhaust purification catalyst,
The target air-fuel ratio switching means is configured to change the target air-fuel ratio from the rich initial target air-fuel ratio to the rich final target air-fuel ratio based on the exhaust gas amount after switching the target air-fuel ratio to the rich initial target air-fuel ratio. The target is changed from the lean initial target air-fuel ratio to the lean final target air-fuel ratio based on the exhaust gas amount after changing the target air-fuel ratio to the lean initial target air-fuel ratio. It is characterized by changing the air-fuel ratio.

  According to a third invention, in the first or second invention, the target air-fuel ratio switching means is configured to change from the rich initial target air-fuel ratio to the rich final target air-fuel ratio, and from the lean initial target air-fuel ratio. The target air-fuel ratio is gradually changed to the lean final target air-fuel ratio.

  In a fourth aspect based on the first or second aspect, the target air-fuel ratio switching means is configured to change from the rich initial target air-fuel ratio to the rich final target air-fuel ratio, and from the lean initial target air-fuel ratio. The target air-fuel ratio is changed stepwise to the lean final target air-fuel ratio.

  According to the first to fourth aspects, when the target air-fuel ratio is switched from the rich side to the lean side or from the lean side to the rich side, the target air-fuel ratio is once switched to the initial target air-fuel ratio with a small air-fuel ratio difference from the stoichiometric ratio. Since the final target air-fuel ratio is set to the lean side or the rich side, the exhaust purification catalyst accompanying the lean / rich switching of the target air-fuel ratio, particularly the abrupt air-fuel ratio of the ambient atmosphere at its inlet The change can be suppressed, and the exhaust purification catalyst can be prevented from deteriorating due to excessive reaction heat accompanying the change in the air-fuel ratio.

  In particular, according to the second aspect of the present invention, the exhaust gas can be switched to the final target air-fuel ratio after a certain amount of exhaust gas has passed through the exhaust purification catalyst. It can be surely suppressed.

Embodiment 1.
The first embodiment of the present invention will be described below with reference to FIGS.

  FIG. 1 is a diagram for explaining the overall configuration of an internal combustion combustor system incorporating an air-fuel ratio control apparatus according to Embodiment 1 of the present invention. As shown in the figure, an exhaust passage 4 is connected to the internal combustion engine 2. An exhaust purification catalyst (three-way catalyst) 6 for purifying harmful components (NOx, CO, HC) in the exhaust gas is disposed in the exhaust passage 4. An A / F sensor 8 is attached upstream of the exhaust purification catalyst 6 in the exhaust passage 4, and an oxygen sensor 10 is attached downstream of the exhaust purification catalyst 6. The A / F sensor 8 is an exhaust gas sensor that outputs a signal according to the air-fuel ratio of the exhaust gas, and the oxygen sensor 10 is an exhaust gas sensor that outputs a signal according to the oxygen concentration in the exhaust gas.

  In the internal combustion engine system, an ECU (Electronic Control Unit) 20 is provided as a control device that comprehensively controls the operation of the entire system. The A / F sensor 8 and the oxygen sensor 10 are connected to the ECU 20 and supply an output signal corresponding to the detection information to the ECU 20. The ECU 20 feedback-controls the air-fuel ratio of the internal combustion engine 2 based on output signals from the A / F sensor 8 and the oxygen sensor 10.

  The air-fuel ratio feedback control that can be executed by the ECU 20 includes the following two methods. The first feedback control method is a method of adjusting the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 becomes the target air-fuel ratio based on the output signal from the A / F sensor 8 upstream of the catalyst. is there. The second feedback control method is a method of adjusting the fuel injection amount so that the air-fuel ratio of the exhaust gas after purification by the exhaust purification catalyst 6 becomes the target air-fuel ratio based on the output signal from the oxygen sensor 10 downstream of the catalyst. . The present embodiment is characterized by the latter second feedback control method. Hereinafter, the content of the air-fuel ratio feedback control executed by the ECU 20 in the present embodiment will be described.

  FIG. 2 is a time chart for explaining the outline of the air-fuel ratio feedback control executed by the ECU 20 in the present embodiment. The upper stage in the time chart of FIG. 2 is the target air-fuel ratio, and the lower stage is the output signal of the oxygen sensor 10. As shown in the figure, the ECU 20 is configured to change the target air-fuel ratio alternately between the rich side and the lean side around the stoichiometric air-fuel ratio (stoichiometric). The target air-fuel ratio switching timing is determined based on the output signal of the oxygen sensor 10, and when the output signal of the oxygen sensor 10 changes to a richer side than the predetermined rich determination output Vr, as shown in the figure, the target air-fuel ratio is changed. Is switched from the rich side to the lean side. Further, when the output signal of the oxygen sensor 10 changes to the lean side from the predetermined lean determination output Vl, the target air-fuel ratio is switched from the lean side to the rich side.

In the present embodiment, when the output signal of the oxygen sensor 10 changes to the rich side with respect to the rich determination output Vr, the target air-fuel ratio is first switched to the rich initial target air-fuel ratio A01. The rich initial target air-fuel ratio A01 is set on the stoichiometric side with respect to the rich final target air-fuel ratio A1, which is the final target air-fuel ratio. After switching to the rich initial target air-fuel ratio A01, the target air-fuel ratio is gradually changed from the rich initial target air-fuel ratio A01 to the rich final target air-fuel ratio A1. The following equation (1) is a calculation equation for calculating the set value of the target air-fuel ratio in this gradual change period.
Target air-fuel ratio = A1-k1 * (gaA- [Delta] ga) / gaA * (A1-14.6) (1)
In the above equation (1), k1 is a predetermined coefficient that takes a value between 0 and 1, Δga is the accumulated amount of exhaust gas that has flowed into the exhaust purification catalyst 6 after the lean / rich switching of the target air-fuel ratio, and gaA is the accumulated value. This is the judgment value of the exhaust gas amount. The exhaust gas amount can be estimated from the intake air amount measured by an air flow meter (not shown) provided in the intake passage.

According to the above equation (1), when the accumulated exhaust gas amount Δga reaches the determination value gaA, the target air-fuel ratio becomes the rich target air-fuel ratio A1. After the integrated exhaust gas amount Δga exceeds the determination value gaA, the target air-fuel ratio is as shown in the following equation (2) until the output signal of the oxygen sensor 10 changes to the lean side from the lean determination output Vl. The final target air-fuel ratio A1 is maintained at the rich time.
Target air-fuel ratio = A1 (2)

Since the integrated exhaust gas amount Δga is zero immediately after the target air-fuel ratio is switched to the rich initial target air-fuel ratio A01, according to the above equation (1), the rich initial target air-fuel ratio A01 is It can be represented by (3).
Initial target air-fuel ratio at rich A01 = A1-k1 × (A1-14.6) (3)

On the other hand, when the output signal of the oxygen sensor 10 changes to the lean side from the lean determination output Vl, the target air-fuel ratio is first switched to the lean initial target air-fuel ratio A02. The lean initial target air-fuel ratio A02 is set on the stoichiometric side with respect to the lean final target air-fuel ratio A2, which is the final target air-fuel ratio. After switching to the lean initial target air-fuel ratio A02, the target air-fuel ratio is gradually changed from the lean initial target air-fuel ratio A02 to the lean final target air-fuel ratio A2. The following formula (4) is a calculation formula for calculating the set value of the target air-fuel ratio in this gradual change period.
Target air-fuel ratio = A2 + k2 x (gaB-Δga) / gaB x (14.6-A2) (4)
In the above equation (4), k2 is a predetermined coefficient that takes a value between 0 and 1, Δga is the accumulated exhaust gas amount that has flowed into the exhaust purification catalyst 6 after the lean / rich switching of the target air-fuel ratio, and gaB is the accumulated value. This is the judgment value of the exhaust gas amount. Note that the determination value gaB may be the same value as or different from the determination value gaA.

According to the above equation (4), when the cumulative exhaust gas amount Δga reaches the determination value gaB, the target air-fuel ratio becomes the rich target air-fuel ratio A2. After the integrated exhaust gas amount Δga exceeds the determination value gaB, the target air-fuel ratio becomes equal to the following expression (5) until the output signal of the oxygen sensor 10 changes to the rich side with respect to the rich determination output Vr. The final target air-fuel ratio A2 is maintained during lean.
Target air-fuel ratio = A2 (5)

Since the integrated exhaust gas amount Δga is zero immediately after the target air-fuel ratio is switched to the lean initial target air-fuel ratio A02, according to the above equation (4), the lean initial target air-fuel ratio A02 is It can be represented by (6).
Lean initial target air-fuel ratio A02 = A2 + k2 × (14.6−A2) (6)

  The target air-fuel ratio calculation process described above is performed in the air-fuel ratio control routine shown in the flowchart of FIG. In the routine shown in FIG. 3, first, it is determined whether or not the start condition of the air / fuel feedback control is satisfied (step 100). The start conditions are, for example, that the oxygen sensor 10 is at an activation temperature, the exhaust purification catalyst 6 is at an activation temperature, and the like.

  After the determination condition in step 100 is satisfied, air / fuel feedback control is started. At the start of air-fuel feedback control, the target air-fuel ratio is set to a predetermined initial value A0 (step 102). The initial value A0 is set slightly richer than stoichiometric. By setting the target air-fuel ratio to be rich, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 also becomes richer than stoichiometric, and the exhausted catalyst 6 releases the stored oxygen. Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a depleted state, and the output signal of the oxygen sensor 10 shows a rich output.

  In step 104, it is determined whether or not the output signal V of the oxygen sensor 10 is greater than the rich determination output Vr. As a result of the determination, when the output signal V of the oxygen sensor 10 exceeds the rich determination output Vr, the target air-fuel ratio is switched to the lean side, and the above formula (1) or (2) is selected according to the accumulated exhaust gas amount Δga. The value is set according to (step 106). By switching the target air-fuel ratio to the lean side rather than the stoichiometry, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is also reversed to the lean side. However, since the rich initial target air-fuel ratio A01, which is the target air-fuel ratio immediately after switching, is set to the stoichiometric side with respect to the rich final target air-fuel ratio A1, which is the final target air-fuel ratio, the rich target air-fuel ratio A sudden change in the air-fuel ratio of the exhaust gas accompanying the change from lean to lean is suppressed. Further, since the rich initial target air-fuel ratio A01 is gradually switched from the rich initial target air-fuel ratio A1 in accordance with the passage of a certain amount of exhaust gas according to the determination value gaA, the rich final target air-fuel ratio A1. A sudden change in the air-fuel ratio associated with switching to A1 is suppressed. If it is determined in step 104 that the output signal V is equal to or less than the rich determination output Vl, the process proceeds to step 110 described later.

  In the next step 108, it is determined whether or not the continuation condition for the air / fuel feedback control is satisfied. The air-fuel feedback control may be temporarily interrupted under predetermined operating conditions, for example, when a fuel cut is performed. In such a case, the determination condition in step 108 is not satisfied, and the air / fuel feedback control is stopped. When the air-fuel feedback control is continued, the determination at step 110 is performed.

  In step 110, it is determined whether the output signal V of the oxygen sensor 10 is smaller than the lean determination output Vl. Until the determination condition of step 110 is satisfied, the processing of step 106 to step 110 is repeated. Meanwhile, the target air-fuel ratio is gradually made lean from the rich initial target air-fuel ratio A01 to the rich target final air-fuel ratio A1, and after the integrated exhaust gas amount Δga exceeds the determination value gaA, the rich final target air-fuel ratio A1. Maintained.

  By maintaining the target air-fuel ratio at the final target air-fuel ratio A1 when rich, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is kept leaner than stoichiometric, and the exhaust purification catalyst 6 stores oxygen. . Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a saturated state, and the output signal of the oxygen sensor 10 shows a lean output. As a result of determination in step 110, when the output signal V of the oxygen sensor 10 falls below the lean determination output Vl, the target air-fuel ratio is switched to the rich side, and the above equation (4) or equation is changed according to the integrated exhaust gas amount Δga. It is set according to (5) (step 112).

  When the target air-fuel ratio is switched to the rich side with respect to the stoichiometry, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is also reversed to the rich side. However, since the lean initial target air-fuel ratio A02, which is the target air-fuel ratio immediately after switching, is set to the stoichiometric side with respect to the lean final target air-fuel ratio A2, which is the final target air-fuel ratio, the lean target air-fuel ratio A sudden change in the air-fuel ratio of the exhaust gas accompanying switching from to rich is suppressed. Further, the lean initial target air-fuel ratio A02 is gradually switched to the lean final target air-fuel ratio A2 in accordance with the passage of a certain amount of exhaust gas according to the determination value gaB. A sudden change in the air-fuel ratio accompanying switching to the final target air-fuel ratio A2 is suppressed.

  In the next step 114, as in step 108, it is determined whether or not the continuation condition for the air-fuel feedback control is satisfied. When the air-fuel feedback control is continued, the determination at step 116 is performed.

  In step 116, it is determined whether or not the output signal V of the oxygen sensor 10 is greater than the rich determination output Vr. Until the determination condition of step 116 is satisfied, the processing of step 112 to step 116 is repeated. Meanwhile, the target air-fuel ratio is gradually enriched from the lean initial target air-fuel ratio A02 to the lean final target air-fuel ratio A2, and after the integrated exhaust gas amount Δga exceeds the determination value gaB, the lean final target air-fuel ratio A2 Maintained.

  By maintaining the target air-fuel ratio at the lean final target air-fuel ratio A2, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is kept richer than stoichiometric, and the exhaust purification catalyst 6 releases oxygen. . Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a depleted state, and the output signal of the oxygen sensor 10 shows a rich output. If the result of determination in step 116 is that the output signal V of the oxygen sensor 10 exceeds the rich determination output Vr, the routine proceeds to step 106 where the target air-fuel ratio is switched to the lean side. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is reversed to the lean side by the processing of step 106.

  Thereafter, the processing from step 106 to step 116 is repeatedly executed until the continuation condition of the air / fuel feedback control is not satisfied in step 108 or step 114.

  By executing the air-fuel ratio control apparatus routine described above, the target air-fuel ratio is made lean to the final target air-fuel ratios A1 and A2 without abruptly changing the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6. It can be enriched. As a result, the exhaust purification catalyst 6 is effectively occluded / released oxygen while effectively preventing deterioration of the inlet portion of the exhaust purification catalyst 6 due to excessive reaction heat caused by a sudden change in the air-fuel ratio. It becomes possible to maintain the purification capability of the exhaust purification catalyst 6 high.

Embodiment 2.
Hereinafter, Embodiment 2 of the present invention will be described with reference to FIGS. 4 and 5.

  The air-fuel ratio control apparatus according to the present embodiment is shown in the time chart of FIG. 4 and the flowchart of FIG. 5 instead of the air-fuel ratio control shown in the time chart of FIG. 2 and the flowchart of FIG. This can be realized by executing air-fuel ratio control.

  FIG. 4 is a time chart for explaining the outline of the air-fuel ratio feedback control executed by the ECU 20 in the present embodiment. The upper stage in the time chart of FIG. 4 is the target air-fuel ratio, and the lower stage is the output signal of the oxygen sensor 10. As in the first embodiment, when the output signal of the oxygen sensor 10 changes to the rich side with respect to the rich determination output Vr, the ECU 20 switches the target air-fuel ratio from the rich side to the lean side, and the output signal of the oxygen sensor 10 becomes lean. When the determination output Vl changes to the lean side, the target air-fuel ratio is switched from the lean side to the rich side.

  In the present embodiment, when the output signal of the oxygen sensor 10 changes to the rich side with respect to the rich determination output Vr, the target air-fuel ratio is first switched to the rich initial target air-fuel ratio A01, and then the rich initial target. The air-fuel ratio A01 is switched stepwise from the rich target air-fuel ratio A1. The rich initial target air-fuel ratio A01 is expressed by the above equation (3) as in the embodiment. The target air-fuel ratio is switched from the rich initial target air-fuel ratio A01 to the rich final target air-fuel ratio A1 when the integrated exhaust gas amount Δga reaches the determination value gaA. The definitions of the accumulated exhaust gas amount Δga and the determination value gaA are the same as those in the first embodiment.

  On the other hand, when the output signal of the oxygen sensor 10 changes to the lean side from the lean determination output Vl, the target air-fuel ratio is first switched to the lean initial target air-fuel ratio A02, and then from the lean initial target air-fuel ratio A02. It is switched to the final target air-fuel ratio A2 at the time of lean stepwise. The lean target initial air-fuel ratio A02 is expressed by the above equation (6) as in the embodiment. The target air-fuel ratio is switched from the lean initial target air-fuel ratio A02 to the lean final target air-fuel ratio A2 when the integrated exhaust gas amount Δga reaches the determination value gaB. The definition of the determination value gaB is the same as that in the first embodiment.

  The target air-fuel ratio calculation process described above is performed in the air-fuel ratio control routine shown in the flowchart of FIG. In the routine shown in FIG. 5, it is first determined whether or not the start condition of the air / fuel feedback control is satisfied (step 200). The contents of the start condition are as described in the first embodiment.

  After the determination condition of step 200 is satisfied, air / fuel feedback control is started. At the start of air-fuel feedback control, the target air-fuel ratio is set to a predetermined initial value A0 (step 202). The initial value A0 is set slightly richer than stoichiometric. By setting the target air-fuel ratio to be rich, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 also becomes richer than stoichiometric, and the exhausted catalyst 6 releases the stored oxygen. Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a depleted state, and the output signal of the oxygen sensor 10 shows a rich output.

  In step 204, it is determined whether or not the output signal V of the oxygen sensor 10 is greater than the rich determination output Vr. As a result of the determination, when the output signal V of the oxygen sensor 10 exceeds the rich determination output Vr, the target air-fuel ratio is switched to the rich initial target air-fuel ratio A01 (step 206). Since the rich initial target air-fuel ratio A01 is set to the lean side with respect to the stoichiometry, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is reversed to the lean side. However, since the rich initial target air-fuel ratio A01 is set to the stoichiometric side with respect to the rich target final air-fuel ratio A1, which is the final target air-fuel ratio, the exhaust gas accompanying the switching of the target air-fuel ratio from rich to lean A sudden change in the air-fuel ratio of the gas is suppressed. If it is determined in step 204 that the output signal V is equal to or less than the rich determination output Vr, the process proceeds to step 218 described later.

  In the next step 208, it is determined whether or not the continuation condition for the air / fuel feedback control is satisfied. The contents of the continuation condition are as described in the first embodiment. When the air-fuel feedback control is continued, the determination at step 210 is performed.

  In step 210, it is determined whether or not the output signal V of the oxygen sensor 10 is greater than the lean determination output Vl. If the output signal V is still larger than the lean determination output Vl, the integrated exhaust gas amount Δga after the target air-fuel ratio is switched to the rich initial target air-fuel ratio A01 is calculated, and the integrated exhaust gas amount Δga and the determination value gaA are calculated. Are compared (step 212). As a result of the comparison, the processing from step 206 to step 212 is repeated until the accumulated exhaust gas amount Δga exceeds the determination value gaA. Meanwhile, the target air-fuel ratio is maintained at the rich initial target air-fuel ratio A01. If it is determined in step 210 that the output signal V is equal to or lower than the lean determination output Vl, the process proceeds to step 218 described later.

  When the cumulative exhaust gas amount Δga exceeds the determination value gaA in the determination in step 212, the target air-fuel ratio is switched from the rich initial target air-fuel ratio A01 to the rich final target air-fuel ratio A1 (step 214). Since the rich target air-fuel ratio A1 is set to be leaner than the rich initial target air-fuel ratio A01, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is further leaned. Since the switching is performed after passage of a certain amount of exhaust gas according to the determination value gaA, a rapid change in the air-fuel ratio accompanying switching to the final target air-fuel ratio A1 when the target air-fuel ratio is rich is suppressed.

  In the next step 216, as in step 208, it is determined whether the continuation condition for the air / fuel feedback control is satisfied. When the air-fuel feedback control is continued, the determination at step 218 is performed.

  In step 218, it is determined whether the output signal V of the oxygen sensor 10 is smaller than the lean determination output Vl. Until the determination condition of step 218 is satisfied, the processing of step 214 to step 218 is repeated. Meanwhile, the target air-fuel ratio is maintained at the final rich target air-fuel ratio A1.

  By maintaining the target air-fuel ratio at the final target air-fuel ratio A1 when rich, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is kept leaner than stoichiometric, and the exhaust purification catalyst 6 stores oxygen. . Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a saturated state, and the output signal of the oxygen sensor 10 shows a lean output. As a result of the determination in step 218, when the output signal V of the oxygen sensor 10 falls below the lean determination output Vl, the target air-fuel ratio is switched to the lean initial target air-fuel ratio A02 (step 220).

  Since the lean initial target air-fuel ratio A02 is set to be richer than stoichiometric, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is reversed to the rich side. However, since the lean initial target air-fuel ratio A02 is set on the stoichiometric side with respect to the final lean target air-fuel ratio A2, which is the final target air-fuel ratio, the exhaust gas accompanying the change of the target air-fuel ratio from lean to rich A sudden change in the air-fuel ratio of the gas is suppressed.

  In the next step 222, as in step 208, it is determined whether or not the continuation condition for the air-fuel feedback control is satisfied. When the air-fuel feedback control is continued, the determination at step 224 is performed.

  In step 224, it is determined whether the output signal V of the oxygen sensor 10 is smaller than the rich determination output Vr. When the output signal V is still smaller than the rich determination output Vr, the integrated exhaust gas amount Δga after switching the target air-fuel ratio to the lean initial target air-fuel ratio A02 is calculated, and the integrated exhaust gas amount Δga and the determination value gaB are calculated. Are compared (step 226). As a result of the comparison, the processing from step 220 to step 226 is repeated until the accumulated exhaust gas amount Δga exceeds the determination value gaB. Meanwhile, the target air-fuel ratio is maintained at the lean initial target air-fuel ratio A02. If it is determined in step 224 that the output signal V is equal to or greater than the lean determination output Vr, the process proceeds to step 232 described later.

  In the determination at step 226, when the accumulated exhaust gas amount Δga exceeds the determination value gaB, the target air-fuel ratio is switched from the lean initial target air-fuel ratio A02 to the lean final target air-fuel ratio A2 (step 228). Since the lean final target air-fuel ratio A2 is set to be richer than the lean initial target air-fuel ratio A02, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is further enriched. Since the switching is performed after the passage of a certain amount of exhaust gas according to the determination value gaB, a rapid change in the air-fuel ratio accompanying the switching of the target air-fuel ratio to the final final target air-fuel ratio A2 is suppressed.

  In the next step 230, as in step 208, it is determined whether or not the continuation condition for the air-fuel feedback control is satisfied. If the air / fuel feedback control is continued, the determination in step 232 is performed.

  In step 232, it is determined whether the output signal V of the oxygen sensor 10 is greater than the rich determination output Vr. Until the determination condition of step 232 is satisfied, the processing of step 228 to step 232 is repeated. Meanwhile, the target air-fuel ratio is maintained at the lean final target air-fuel ratio A2.

  By maintaining the target air-fuel ratio at the lean final target air-fuel ratio A2, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is kept richer than stoichiometric, and the exhaust purification catalyst 6 releases oxygen. . Eventually, the stored oxygen amount of the exhaust purification catalyst 6 approaches a depleted state, and the output signal of the oxygen sensor 10 shows a rich output. If the result of determination in step 232 is that the output signal V of the oxygen sensor 10 exceeds the rich determination output Vr, the routine proceeds to step 206 where the target air-fuel ratio is switched to the rich initial target air-fuel ratio A01. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6 is reversed to the lean side by the process of step 206.

  Thereafter, the processing from step 206 to step 232 is repeatedly executed until the continuation condition of the air-fuel feedback control is not satisfied in step 208, 216, 222, or 230.

  By executing the air-fuel ratio control apparatus routine described above, the target air-fuel ratio is set to the final target air-fuel ratio without abruptly changing the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 6, as in the first embodiment. A1 and A2 can be made leaner or richer. As a result, the exhaust purification catalyst 6 is effectively occluded / released oxygen while effectively preventing deterioration of the inlet portion of the exhaust purification catalyst 6 due to excessive reaction heat caused by a sudden change in the air-fuel ratio. It becomes possible to maintain the purification capability of the exhaust purification catalyst 6 high.

Others.
While the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, an oxygen sensor is used as an exhaust gas sensor disposed on the downstream side of the exhaust purification catalyst 6, but an A / F sensor may be used.

  Further, the target air-fuel ratio is gradually changed from the rich initial target air-fuel ratio A01 to the rich final target air-fuel ratio A1 or from the lean initial target air-fuel ratio A02 to the lean final target air-fuel ratio A2. May be.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining a configuration of an internal combustion engine system to which an air-fuel ratio control apparatus as Embodiment 1 of the present invention is applied. 3 is a time chart for explaining an overview of air-fuel ratio feedback control executed in Embodiment 1 of the present invention. It is a flowchart which shows the air fuel ratio control routine performed in Embodiment 1 of this invention. It is a time chart for demonstrating the outline | summary of the air fuel ratio feedback control performed in Embodiment 2 of this invention. It is a flowchart which shows the air fuel ratio control routine performed in Embodiment 2 of this invention.

Explanation of symbols

2 Internal combustion engine 4 Exhaust passage 6 Exhaust purification catalyst 8 A / F sensor 10 Oxygen sensor 20 ECU

Claims (4)

An air-fuel ratio control apparatus for an internal combustion engine that controls an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be a target air-fuel ratio,
An exhaust gas sensor disposed downstream of the exhaust purification catalyst in the exhaust passage of the internal combustion engine;
When the output signal of the exhaust gas sensor changes to a rich side from a predetermined rich determination value, the target air-fuel ratio is switched to the initial target air-fuel ratio at the rich time set to the lean side from the stoichiometric, and then the rich-time The initial target air-fuel ratio is changed to the rich final target air-fuel ratio that is set to the lean side further than the rich initial target air-fuel ratio, and the output signal of the exhaust gas sensor changes to the lean side from the predetermined lean determination value In this case, after the target air-fuel ratio is switched to the lean initial target air-fuel ratio set to the rich side from the stoichiometry, the lean initial target air-fuel ratio is set to a richer side than the lean initial target air-fuel ratio. Target air-fuel ratio switching means for changing to the final target air-fuel ratio at the time of lean,
An air-fuel ratio control apparatus for an internal combustion engine, comprising: An exhaust gas amount estimating means for estimating an exhaust gas amount flowing into the exhaust purification catalyst,
The target air-fuel ratio switching means is configured to change the target air-fuel ratio from the rich initial target air-fuel ratio to the rich final target air-fuel ratio based on the exhaust gas amount after switching the target air-fuel ratio to the rich initial target air-fuel ratio. The target is changed from the lean initial target air-fuel ratio to the lean final target air-fuel ratio based on the exhaust gas amount after changing the target air-fuel ratio to the lean initial target air-fuel ratio. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio is changed.   The target air-fuel ratio switching means gradually changes the target air-fuel ratio from the rich initial target air-fuel ratio to the rich final target air-fuel ratio, and from the lean initial target air-fuel ratio to the lean final target air-fuel ratio. 3. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio control apparatus is changed.   The target air-fuel ratio switching means steps the target air-fuel ratio from the rich initial target air-fuel ratio to the rich final target air-fuel ratio, and from the lean initial target air-fuel ratio to the lean final target air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2, wherein

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