Air–fuel ratio control method and device for internal combustion engine

Abstract

This air-fuel ratio control device comprises a front air-fuel ratio sensor (12) and a rear air-fuel ratio sensor (13) consisting of a linear air-fuel ratio sensor on the upstream side and the downstream side of a three-way catalyst (11), and performs feedback control of a fuel injection amount so that the upstream-side exhaust air-fuel ratio detected by the front air-fuel ratio sensor (12) conforms with a target air-fuel ratio. When the downstream-side exhaust air-fuel ratio by the rear air-fuel ratio sensor (13) is within a theoretical air-fuel ratio window between a first lean slice level (LSL1) and a first rich slice level (RSL1), the target air-fuel ratio is set on the basis of the oxygen storage estimation value of the catalyst. When the downstream-side exhaust air-fuel ratio is outside the theoretical air-fuel ratio window, a target air-fuel ratio on a rich side or a lean side is set from a map using the downstream-side exhaust air-fuel ratio as a parameter.

Description

Method and device for controlling air-fuel ratio of an internal combustion engine

[0001] The present invention relates to air-fuel ratio control for an internal combustion engine equipped with a three-way catalyst, and more particularly to air-fuel ratio control for an internal combustion engine equipped with wide-range air-fuel ratio sensors both upstream and downstream of the three-way catalyst.

[0002] A known configuration includes wide-range air-fuel ratio sensors (linear air-fuel ratio sensors) that generate an output according to the exhaust air-fuel ratio, both upstream and downstream of a three-way catalyst in the exhaust passage. Patent Document 1 discloses an air-fuel ratio control technology based on such a configuration.

[0003] In Patent Document 1, the oxygen storage amount of the three-way catalyst is estimated using the exhaust air-fuel ratio at the inlet side of the three-way catalyst detected by a front air-fuel ratio sensor located upstream of the three-way catalyst and the exhaust air-fuel ratio at the outlet side of the three-way catalyst detected by a rear air-fuel ratio sensor located downstream of the three-way catalyst. The difference between this estimated oxygen storage amount and a target oxygen storage amount is converted into a target air-fuel ratio, and the fuel injection amount is controlled to conform to this target air-fuel ratio. In other words, the target air-fuel ratio is set so that the estimated oxygen storage amount becomes the target oxygen storage amount, and feedback control is performed so that the exhaust air-fuel ratio detected by the front air-fuel ratio sensor becomes this target air-fuel ratio.

[0004] In such conventional air-fuel ratio control, the oxygen storage amount of the three-way catalyst is appropriately maintained, thereby achieving good exhaust purification performance with both oxidation and reduction. However, due to factors such as changes in operating conditions, the oxygen storage amount of the three-way catalyst may become saturated (i.e., 100%) or conversely become zero. In such cases, it takes time for the actual oxygen storage amount to return to the target oxygen storage amount, and during that time, exhaust purification performance decreases.

[0005] Japanese Patent Application Laid-Open No. 2000-008921

[0006] The present invention provides an air-fuel ratio control method for an internal combustion engine, which is provided with a front air-fuel ratio sensor and a rear air-fuel ratio sensor that generate outputs according to the exhaust air-fuel ratio on the upstream and downstream sides of a three-way catalyst in the exhaust passage, respectively, and which feedback-controls the fuel injection amount so that the upstream exhaust air-fuel ratio detected by the front air-fuel ratio sensor is in line with a target air-fuel ratio, the method comprising: determining an estimated oxygen storage value for the three-way catalyst using the output of the front air-fuel ratio sensor and the output of the rear air-fuel ratio sensor; comparing the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor with a first lean slice level and a first rich slice level that determine a stoichiometric air-fuel ratio window of the three-way catalyst; setting the target air-fuel ratio based on the estimated oxygen storage value if the downstream exhaust air-fuel ratio is within the stoichiometric air-fuel ratio window; and setting the target air-fuel ratio based on the downstream exhaust air-fuel ratio if the downstream exhaust air-fuel ratio is outside the stoichiometric air-fuel ratio window.

[0007] According to this type of air-fuel ratio control, when the downstream exhaust air-fuel ratio deviates for some reason from a state in which the downstream exhaust air-fuel ratio is maintained within the stoichiometric air-fuel ratio window by injection amount control based on the estimated oxygen storage value to a target air-fuel ratio setting based on the downstream exhaust air-fuel ratio, the target air-fuel ratio setting is switched to a target air-fuel ratio setting based on the downstream exhaust air-fuel ratio. In other words, if the downstream exhaust air-fuel ratio is lean, the target air-fuel ratio becomes relatively rich, and if the downstream exhaust air-fuel ratio is rich, the target air-fuel ratio becomes relatively lean.

[0008] Therefore, the downstream exhaust air-fuel ratio returns to within the stoichiometric air-fuel ratio window in a relatively short time, and the target air-fuel ratio setting based on the estimated oxygen storage value is restored.

[0009] Therefore, the exhaust gas purification performance of the three-way catalyst is improved overall.

[0010] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments and is not to be construed as limiting the invention.

[0011] An embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] 1 is an explanatory diagram showing a schematic system configuration of an internal combustion engine 1 equipped with an air-fuel ratio control device according to the present invention. The internal combustion engine 1 is a spark-ignition internal combustion engine, a so-called gasoline engine, and is equipped with an ignition plug 4 at the center of the ceiling surface of a combustion chamber 2 formed by a piston 3, and an intake valve 5 and an exhaust valve 6 located opposite each other across the ignition plug 4.

[0013] A fuel injection valve 9 is disposed in an intake passage 7 connected to the combustion chamber 2 via the intake valve 5 so as to inject fuel toward the intake valve 5, and a throttle valve 8 for adjusting the amount of intake air and an air flow meter 15 for measuring this amount of intake air are provided upstream of the fuel injection valve 9. Note that the present invention may also be configured as a direct injection type in which fuel is injected directly into the combustion chamber 2 by the fuel injection valve.

[0014] A catalytic converter 11 consisting of a three-way catalyst for purifying exhaust gas is disposed in an exhaust passage 10 connected to the combustion chamber 2 via the exhaust valve 6. This catalytic converter 11 is attached, for example, to the outlet of an exhaust manifold that constitutes part of the exhaust passage 10. A second catalytic converter (not shown) may be provided downstream of the catalytic converter 11. The second catalytic converter is disposed, for example, under the floor of the vehicle.

[0015] A front air-fuel ratio sensor 12 is disposed upstream of the catalytic converter 11, and a rear air-fuel ratio sensor 13 is disposed downstream of the catalytic converter 11. Both the front air-fuel ratio sensor 12 and the rear air-fuel ratio sensor 13 are so-called wide-range air-fuel ratio sensors (linear air-fuel ratio sensors) that generate an output according to the exhaust air-fuel ratio. The front air-fuel ratio sensor 12 detects the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 and flowing into the catalytic converter 11, i.e., the exhaust air-fuel ratio on the inlet side of the catalytic converter 11 (this is called the upstream exhaust air-fuel ratio). The rear air-fuel ratio sensor 13 detects the air-fuel ratio of the exhaust gas that has passed through the catalytic converter 11, i.e., the exhaust air-fuel ratio on the outlet side of the catalytic converter 11 (this is called the downstream exhaust air-fuel ratio).

[0016] In addition to the detection signals from the front air-fuel ratio sensor 12 and rear air-fuel ratio sensor 13, the engine controller 20 also receives detection signals from a number of sensors, such as the detection signal from the air flow meter 15, the crank angle sensor 14 for detecting the engine speed, the water temperature sensor 16 for detecting the temperature of the cooling water flowing through the water jacket, an accelerator pedal position sensor (not shown), etc. Based on these detection signals, the engine controller 20 optimally controls the fuel injection amount and injection timing of the fuel injection valve 9, the ignition timing of the spark plug 4, the opening of the throttle valve 8, etc.

[0017] Next, we will explain the air-fuel ratio control, which is a key aspect of the present invention. As is well known, the three-way catalyst in the catalytic converter 11 can efficiently oxidize HC and CO and reduce NOx when the atmosphere near the catalyst is within a relatively narrow air-fuel ratio range (called the stoichiometric air-fuel ratio window) centered around the stoichiometric air-fuel ratio (e.g., approximately 14.7). Furthermore, the three-way catalyst has an oxygen storage capacity that stores and releases oxygen, and this oxygen storage capacity serves to maintain the atmosphere near the stoichiometric air-fuel ratio. If the oxygen storage amount is saturated, no more oxygen can be stored. Conversely, if the oxygen storage amount is zero, no oxygen can be released. Therefore, it is generally desirable to maintain the oxygen storage amount of the three-way catalyst at a certain intermediate target value.

[0018] In this embodiment, the air-fuel ratio control has the following five control modes (first to fifth control modes) for setting the target air-fuel ratio, which are switched depending on the conditions. Here, the target air-fuel ratio is a target value of the air-fuel ratio indicated by the exhaust gas discharged from the internal combustion engine 1 (i.e., the upstream exhaust air-fuel ratio). The amount of fuel injected by the fuel injection valve 9 is feedback-controlled using an appropriate method, such as PID control, so that the upstream exhaust air-fuel ratio detected by the front air-fuel ratio sensor 12 conforms to the target air-fuel ratio. Feedback control of the fuel injection amount is well known, so a description thereof will be omitted.

[0019] The first control mode is a basic control mode. In this first control mode, the oxygen storage amount of the three-way catalyst is estimated using the outputs of the front air-fuel ratio sensor 12 and the rear air-fuel ratio sensor 13, and the target air-fuel ratio is set based on this estimated oxygen storage amount. In one embodiment, the oxygen storage amount is estimated based mainly on the output of the front air-fuel ratio sensor 12 (i.e., the upstream exhaust air-fuel ratio) and the gas volume (intake air amount). The oxygen storage amount is added if the air-fuel ratio is lean, and subtracted if the air-fuel ratio is rich, thereby calculating an estimated oxygen storage amount. The output of the rear air-fuel ratio sensor 13 is used to calibrate this estimated oxygen storage amount. When the output of the rear air-fuel ratio sensor 13 is leaner than a first lean slice level LSL1 (described later), the integrated estimated oxygen storage amount is calibrated to 100%. When the output of the rear air-fuel ratio sensor 13 is richer than a first rich slice level RSL1 (described later), the integrated estimated oxygen storage amount is calibrated to 0%. As with the oxygen storage estimation in Patent Document 1, oxygen storage estimation may be performed from the oxygen concentration on the inlet side indicated by the output of the front air-fuel ratio sensor 12 and the oxygen concentration on the outlet side indicated by the output of the rear air-fuel ratio sensor 13.

[0020] In one embodiment, a target oxygen storage amount is set, for example, by map search based on the operating conditions (mainly the rotational speed and load) of the internal combustion engine 1, and a target air-fuel ratio is calculated so that the estimated oxygen storage value approaches this target oxygen storage amount. For example, the target air-fuel ratio is determined by an appropriate feedback control method according to the deviation between the target oxygen storage amount and the estimated oxygen storage value. Alternatively, for simplicity, the target oxygen storage amount may be a constant value regardless of the operating conditions.

[0021] The first control mode is selected when the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13 is within the stoichiometric air-fuel ratio window of the three-way catalyst. The stoichiometric air-fuel ratio window is determined by a first lean slice level LSL1 and a first rich slice level RSL1, which are preset based on the oxidation / reduction capabilities of the three-way catalyst. If the downstream exhaust air-fuel ratio, which is the exhaust air-fuel ratio after passing through the three-way catalyst (i.e., the output of the rear air-fuel ratio sensor 13), is between the first lean slice level LSL1 and the first rich slice level RSL1, the atmosphere near the catalyst is considered to be within the stoichiometric air-fuel ratio window, and air-fuel ratio control using the first control mode is selected. In the first control mode, the oxygen storage amount of the three-way catalyst is maintained near the target oxygen storage amount, effectively utilizing the oxygen storage capacity to efficiently oxidize HC and CO and reduce NOx.

[0022] The second control mode is selected when the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13 is leaner than the first lean slice level LSL1. The downstream exhaust air-fuel ratio being leaner than the first lean slice level LSL1 means that the atmosphere near the catalyst is lean. In the second control mode, the target air-fuel ratio is set richer than the stoichiometric air-fuel ratio based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13, with the degree of richness corresponding to the downstream exhaust air-fuel ratio. In one embodiment, a map is provided in which target air-fuel ratios are assigned using the downstream exhaust air-fuel ratio and the gas volume passing through the catalytic converter 11 as parameters, and the target air-fuel ratio is determined by referring to this map. The gas volume is calculated based on the intake air amount. Basically, the leaner the downstream exhaust air-fuel ratio, the richer the target air-fuel ratio, and the larger the gas volume, the richer the target air-fuel ratio. Alternatively, for simplicity, the target air-fuel ratio may be determined from the downstream exhaust air-fuel ratio without taking the gas volume into consideration. The degree to which the target air-fuel ratio is rich may be greater than the degree to which the target air-fuel ratio is rich given in the first control mode.

[0023] The second control mode is started when the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level LSL1, and then ends when the downstream exhaust air-fuel ratio becomes richer than the first lean slice level LSL1 (i.e., when it returns to within the stoichiometric air-fuel ratio window). By using this second control mode, when the downstream exhaust air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio window for some reason, it is possible to return it to within the stoichiometric air-fuel ratio window in a relatively short time, and to minimize, for example, deterioration of NOx.

[0024] The third control mode is a control mode selected when the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13 is richer than the first rich slice level RSL1. The downstream exhaust air-fuel ratio being richer than the first rich slice level RSL1 means that the atmosphere near the catalyst is rich. In the third control mode, the target air-fuel ratio is set leaner than the stoichiometric air-fuel ratio based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13, with the degree of leanness corresponding to the downstream exhaust air-fuel ratio. In one embodiment, a map is provided in which target air-fuel ratios are assigned using the downstream exhaust air-fuel ratio and the gas volume passing through the catalytic converter 11 as parameters. The target air-fuel ratio is determined by referencing this map. Basically, the richer the downstream exhaust air-fuel ratio, the leaner the target air-fuel ratio becomes, and the larger the gas volume, the leaner the target air-fuel ratio becomes. Simply put, the target air-fuel ratio may be determined from the downstream exhaust air-fuel ratio without considering the gas volume. The degree to which the target air-fuel ratio is lean can be greater than the degree to which the target air-fuel ratio is leaned in the first control mode.

[0025] The third control mode is started when the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1, and then ends when the downstream exhaust air-fuel ratio becomes leaner than the first rich slice level RSL1 (i.e., when it returns to within the stoichiometric air-fuel ratio window). By using this third control mode, when the downstream exhaust air-fuel ratio becomes richer than the stoichiometric air-fuel ratio window for some reason, it is possible to return it to within the stoichiometric air-fuel ratio window in a relatively short time, and to minimize deterioration of, for example, HC and CO.

[0026] The fourth control mode is a control mode selected when the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13 becomes leaner than the second lean slice level LSL2. The second lean slice level LSL2 is a threshold value set even leaner than the first lean slice level LSL1, and is set, for example, corresponding to an air-fuel ratio at which NOx emissions rapidly deteriorate. In the fourth control mode, the target air-fuel ratio is set richer than the stoichiometric air-fuel ratio based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13, with a degree of richness corresponding to the downstream exhaust air-fuel ratio. In one embodiment, a map is provided in which target air-fuel ratios are assigned using the downstream exhaust air-fuel ratio and the gas volume passing through the catalytic converter 11 as parameters, and the target air-fuel ratio is determined by referring to this map. Basically, the leaner the downstream exhaust air-fuel ratio, the richer the target air-fuel ratio, and the larger the gas volume, the richer the target air-fuel ratio. For simplicity, the target air-fuel ratio may be determined from the downstream exhaust air-fuel ratio without taking the gas volume into consideration.

[0027] Here, the degree to which the target air-fuel ratio is rich can be greater than the degree to which the target air-fuel ratio is rich in the second control mode. For example, when the downstream exhaust air-fuel ratio is the same lean value in the second control mode and the fourth control mode, the target air-fuel ratio in the fourth control mode is relatively richer than the target air-fuel ratio in the second control mode.

[0028] The fourth control mode is initiated when the downstream exhaust air-fuel ratio becomes leaner than the second lean slice level LSL2, and then terminates when the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1 (i.e., richer than the stoichiometric air-fuel ratio window). In other words, the fourth control mode continues until the atmosphere near the catalyst becomes rich. By using this fourth control mode, when the downstream exhaust air-fuel ratio becomes significantly lean due to some factor, the lean state can be escaped in a relatively short time, and the deterioration of NOx can be suppressed. Furthermore, by continuing the fourth control mode until the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1, oxygen can be completely removed from the three-way catalyst that has been exposed to excess oxygen, improving the catalyst's conversion capability.

[0029] The fifth control mode is a control mode selected when the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13 becomes leaner than the second rich slice level RSL2. The second rich slice level RSL2 is a threshold value set even richer than the first rich slice level RSL1. In the fifth control mode, the target air-fuel ratio is set leaner than the stoichiometric air-fuel ratio based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13, with the degree of leanness corresponding to the downstream exhaust air-fuel ratio. In one embodiment, a map is provided in which target air-fuel ratios are assigned using the downstream exhaust air-fuel ratio and the gas volume passing through the catalytic converter 11 as parameters. The target air-fuel ratio is determined by referencing this map. Basically, the richer the downstream exhaust air-fuel ratio, the leaner the target air-fuel ratio becomes, and the larger the gas volume, the leaner the target air-fuel ratio becomes. For simplicity, the target air-fuel ratio may be determined from the downstream exhaust air-fuel ratio without considering the gas volume.

[0030] Here, the degree to which the target air-fuel ratio is lean can be greater than the degree to which the target air-fuel ratio is leaned in the third control mode. For example, when the downstream exhaust air-fuel ratio is the same rich value in the third control mode and the fifth control mode, the target air-fuel ratio in the fifth control mode will be relatively leaner than the target air-fuel ratio in the third control mode.

[0031] The fifth control mode is started when the downstream exhaust air-fuel ratio becomes richer than the second rich slice level RSL2, and then ends when the downstream exhaust air-fuel ratio becomes leaner than the second rich slice level RSL2. In other words, the fifth control mode is in effect while the downstream exhaust air-fuel ratio is richer than the second rich slice level RSL2. By using this fifth control mode, when the downstream exhaust air-fuel ratio becomes significantly rich due to some factor, it becomes possible to escape from the rich state in a relatively short time.

[0032] Next, an example of the operation of one embodiment will be described with reference to the time chart shown in Fig. 3. The time chart in Fig. 3 shows, from top to bottom, (a) control mode, (b) target air-fuel ratio, (c) output of the rear air-fuel ratio sensor 13 (i.e., downstream exhaust air-fuel ratio), and (d) estimated oxygen storage value. Note that for (b) target air-fuel ratio and (c) output of the rear air-fuel ratio sensor 13, the upper side of the figure is lean and the lower side is rich. Furthermore, the line ABF0 in the (b) target air-fuel ratio column indicates the stoichiometric air-fuel ratio.

[0033] Between times t1 and t2, air-fuel ratio control is performed in the first control mode, and the target air-fuel ratio is calculated based on the estimated oxygen storage value. During this time, the output of the rear air-fuel ratio sensor 13, i.e., the downstream exhaust air-fuel ratio, is within the stoichiometric air-fuel ratio window determined by the first lean slice level LSL1 and the first rich slice level RSL1, and both the oxidation and reduction actions of the three-way catalyst are achieved.

[0034] At time t2, the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level LSL1 due to some factor, and the control mode shifts from the first control mode to the second control mode. In the second control mode, the target air-fuel ratio is basically set to the rich side based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. This causes the downstream exhaust air-fuel ratio to return to the stoichiometric air-fuel ratio window in a relatively short time. At time t3, the downstream exhaust air-fuel ratio becomes richer than the first lean slice level LSL1, and the second control mode ends, and the control mode shifts to the first control mode.

[0035] The estimated oxygen storage value is calibrated to 100% when the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level LSL1 at time t2, and this value of 100% is maintained while the downstream exhaust air-fuel ratio is leaner than the first lean slice level LSL1. When the control mode shifts to the first control mode at time t3, the estimation process for the estimated oxygen storage value resumes.

[0036] Between times t3 and t4, the air-fuel ratio control is performed in the first control mode.

[0037] At time t4, the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1 due to some factor, and the control mode shifts from the first control mode to the third control mode. In the third control mode, the target air-fuel ratio is basically set to the lean side based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. This causes the downstream exhaust air-fuel ratio to return to the stoichiometric air-fuel ratio window in a relatively short time. At time t5, the downstream exhaust air-fuel ratio becomes leaner than the first rich slice level RSL1, and the third control mode ends, and the control mode shifts to the first control mode.

[0038] The estimated oxygen storage value is calibrated to 0% because the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1 at time t4, and this value of 0% is maintained while the downstream exhaust air-fuel ratio is richer than the first rich slice level RSL1. When the control mode shifts to the first control mode at time t5, the estimation process for the estimated oxygen storage value resumes.

[0039] Between times t5 and t6, the air-fuel ratio control is performed in the first control mode.

[0040] At time t6, the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level LSL1 due to some factor, so the control mode shifts from the first control mode to the second control mode. In the second control mode, the target air-fuel ratio is basically set to the rich side based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13.

[0041] Here, in the example of FIG. 3, the lean tendency continues, and at time t7, the downstream exhaust air-fuel ratio becomes leaner than the second lean slice level LSL2.

[0042] As a result, at time t7, the control mode transitions from the second control mode to the fourth control mode. In the fourth control mode, the target air-fuel ratio is set to the rich side basically based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. In the fourth control mode, the target air-fuel ratio becomes relatively richer than in the second control mode. The fourth control mode continues until, at time t8, the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1. At time t8, the control mode transitions from the fourth control mode to the third control mode.

[0043] The estimated oxygen storage value is calibrated to 100% when the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level LSL1 at time t6, and this value of 100% is maintained while the downstream exhaust air-fuel ratio is leaner than the first lean slice level LSL1. When the downstream exhaust air-fuel ratio becomes richer than the first lean slice level LSL1 (immediately before time t8 in the illustrated example), the estimation process for the estimated oxygen storage value resumes, but when the downstream exhaust air-fuel ratio becomes richer than the first rich slice level RSL1 at time t8, the estimated oxygen storage value is calibrated to 0%.

[0044] In the third control mode, the target air-fuel ratio is set leaner based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. After the third control mode starts at time t8, in the illustrated example, the downstream exhaust air-fuel ratio becomes richer than the second rich slice level RSL2 at time t9. Therefore, the control mode shifts from the third control mode to the fifth control mode. In the fifth control mode, the target air-fuel ratio is set leaner based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. In the fifth control mode, the target air-fuel ratio becomes leaner relatively than in the third control mode. Then, at time t10, the downstream exhaust air-fuel ratio becomes leaner than the second rich slice level RSL2, causing the control mode to shift again from the fifth control mode to the third control mode.

[0045] In the fifth control mode, the degree of leanness becomes large, so that the rich state can be escaped in a relatively short time. In the illustrated example, the third control mode ends at time t11, and the control mode transitions to the first control mode.

[0046] As described above, according to the above embodiment, if the downstream exhaust air-fuel ratio falls outside the stoichiometric air-fuel ratio window for some reason while air-fuel ratio control is being performed in the first control mode, the second to fifth control modes can be executed to quickly return to the first control mode, shortening the time during which the downstream exhaust air-fuel ratio falls outside the stoichiometric air-fuel ratio window, thereby improving the overall exhaust purification performance of the three-way catalyst.

[0047] FIG. 2 is a functional block diagram of an air-fuel ratio control device according to one embodiment. Each block is implemented by the engine controller 20. The air-fuel ratio control device includes a downstream exhaust air-fuel ratio determination unit 31, a first target air-fuel ratio calculation unit 32, a second target air-fuel ratio calculation unit 33, a final target air-fuel ratio setting unit 34, and a fuel injection amount calculation unit 35. The downstream exhaust air-fuel ratio determination unit 31 compares the output of the rear air-fuel ratio sensor 13 with a first lean slice level LSL1, a first rich slice level RSL1, a second lean slice level LSL2, and a second rich slice level RSL2, and outputs the comparison result. The first target air-fuel ratio calculation unit 32 uses the output of the front air-fuel ratio sensor 12, the intake air amount, and the output of the calibration rear air-fuel ratio sensor 13 to determine an estimated oxygen storage value for the three-way catalyst, and calculates a first target air-fuel ratio based on this estimated oxygen storage value. A second target air-fuel ratio calculation unit 33 calculates a second target air-fuel ratio, for example by the aforementioned map search, based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor 13. A final target air-fuel ratio setting unit 34 outputs either the first target air-fuel ratio or the second target air-fuel ratio as a final target air-fuel ratio, depending on the selection condition for each of the aforementioned control modes based on a comparison between the downstream exhaust air-fuel ratio and each slice level. A fuel injection amount calculation unit 35 calculates a target fuel injection amount for the fuel injector based on this final target air-fuel ratio.

[0048] Although one embodiment of the present invention has been described above in detail, the present invention is not limited to the above embodiment and various modifications are possible. For example, in the above embodiment, the target air-fuel ratio in the second to fifth control modes is set using a map that uses the downstream exhaust air-fuel ratio as a parameter, but the target air-fuel ratio may be calculated by an appropriate calculation so that the downstream exhaust air-fuel ratio converges within the stoichiometric air-fuel ratio window.

Claims

1. An air-fuel ratio control method for an internal combustion engine, which is provided with a front air-fuel ratio sensor and a rear air-fuel ratio sensor that generate outputs according to the exhaust air-fuel ratio on the upstream and downstream sides of a three-way catalyst in the exhaust passage, respectively, and which feedback-controls the fuel injection amount so that the upstream exhaust air-fuel ratio detected by the front air-fuel ratio sensor is in line with a target air-fuel ratio, the method comprising: determining an oxygen storage estimate for the three-way catalyst using the output of the front air-fuel ratio sensor and the output of the rear air-fuel ratio sensor; comparing the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor with a first lean slice level and a first rich slice level that determine a stoichiometric air-fuel ratio window for the three-way catalyst; setting the target air-fuel ratio based on the oxygen storage estimate if the downstream exhaust air-fuel ratio is within the stoichiometric air-fuel ratio window; and setting the target air-fuel ratio based on the downstream exhaust air-fuel ratio if the downstream exhaust air-fuel ratio is outside the stoichiometric air-fuel ratio window.

2. The air-fuel ratio control method for an internal combustion engine according to claim 1, wherein, when the downstream exhaust air-fuel ratio becomes richer than the first rich slice level, the target air-fuel ratio is set to be leaner than the stoichiometric air-fuel ratio with a degree of leanness corresponding to the downstream exhaust air-fuel ratio until the downstream exhaust air-fuel ratio returns to within the stoichiometric air-fuel ratio window.

3. The air-fuel ratio control method for an internal combustion engine according to claim 2, wherein the target air-fuel ratio is set using a map having parameters of the downstream exhaust air-fuel ratio and the volume of gas passing through the three-way catalyst.

4. An air-fuel ratio control method for an internal combustion engine as set forth in claim 1, wherein, when the downstream exhaust air-fuel ratio becomes leaner than the first lean slice level, the target air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio with a degree of richness according to the downstream exhaust air-fuel ratio until the downstream exhaust air-fuel ratio returns to within the stoichiometric air-fuel ratio window.

5. The air-fuel ratio control method for an internal combustion engine according to claim 4, wherein the target air-fuel ratio is set using a map having parameters of the downstream exhaust air-fuel ratio and the volume of gas passing through the three-way catalyst.

6. An air-fuel ratio control method for an internal combustion engine as described in claim 1, wherein a second lean slice level is set to be leaner than the first lean slice level, and when the downstream exhaust air-fuel ratio becomes leaner than the second lean slice level, the target air-fuel ratio is set to be richer than the theoretical air-fuel ratio with a degree of richness corresponding to the downstream exhaust air-fuel ratio until the downstream exhaust air-fuel ratio becomes richer than the first rich slice level.

7. The air-fuel ratio control method for an internal combustion engine according to claim 6, wherein the target air-fuel ratio is set using a map having parameters of the downstream exhaust air-fuel ratio and the volume of gas passing through the three-way catalyst.

8. An air-fuel ratio control device for an internal combustion engine, comprising: a front air-fuel ratio sensor and a rear air-fuel ratio sensor, respectively provided upstream and downstream of a three-way catalyst in an exhaust passage, for generating an output according to the exhaust air-fuel ratio; and a controller for feedback controlling a fuel injection amount so that the upstream exhaust air-fuel ratio detected by the front air-fuel ratio sensor coincides with a target air-fuel ratio, wherein the controller comprises: a first target air-fuel ratio calculation section that uses the output of the front air-fuel ratio sensor and the output of the rear air-fuel ratio sensor to determine an estimated oxygen storage value for the three-way catalyst and calculates a first target air-fuel ratio based on this estimated oxygen storage value; and a second target air-fuel ratio calculation section that calculates a second target air-fuel ratio based on the downstream exhaust air-fuel ratio detected by the rear air-fuel ratio sensor. an air-fuel ratio control device for an internal combustion engine, comprising: a final target air-fuel ratio setting unit that compares the downstream exhaust air-fuel ratio with a first lean slice level and a first rich slice level that determine a stoichiometric air-fuel ratio window of a three-way catalyst, and sets the target air-fuel ratio to the first target air-fuel ratio if the downstream exhaust air-fuel ratio is within the stoichiometric air-fuel ratio window, and sets the target air-fuel ratio to the second target air-fuel ratio if the downstream exhaust air-fuel ratio is outside the stoichiometric air-fuel ratio window.

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