JP2013064384A - Air-fuel ratio control system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-fuel ratio control system capable of further improving emission, even when responsiveness of a downstream side air-fuel ratio sensor is not excellent.SOLUTION: When an output value Voxs of the downstream side air-fuel ratio sensor arranged downstream of a catalyst reduces and the size |ΔVoxs| of a variation ΔVoxs per predetermined time of the output value Voxs becomes a lean determining threshold value ΔVLeanth or more, the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich. When the output value Voxs increases and the size |ΔVoxs| of the variation ΔVoxs becomes a rich determining threshold value ΔVRichth or more, the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean. As responsiveness of a change in the output value Voxs of the downstream side air-fuel ratio sensor 56 to a change in the air-fuel ratio of catalyst outflow gas becomes not excellent, the lean determining threshold value is changed so that the size of the lean determining threshold value becomes small, and the rich determining threshold value is changed so that the size of the rich determining threshold value becomes small.

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine having a three-way catalyst in an exhaust passage.

  Conventionally, in order to purify exhaust gas discharged from an internal combustion engine, a “three-way catalyst having an oxygen storage function” is disposed in an exhaust passage of the engine. As is well known, when a gas flowing into the three-way catalyst (catalyst inflow gas) contains excessive oxygen, the three-way catalyst occludes the oxygen and purifies NOx. When the catalyst inflow gas contains excessive unburned substances, the three-way catalyst releases the stored oxygen and purifies the unburned substances. Hereinafter, the three-way catalyst is also simply referred to as “catalyst”.

  A conventional air-fuel ratio control device (conventional device) includes a downstream air-fuel ratio sensor disposed in the engine exhaust passage and downstream of the catalyst. The conventional device substantially determines the state of the catalyst (oxygen storage state) based on the output value of the downstream air-fuel ratio sensor, and the air-fuel ratio of the air-fuel mixture supplied to the engine based on the determined state of the catalyst (That is, the air-fuel ratio of the catalyst inflow gas) is changed. The air-fuel ratio of the air-fuel mixture supplied to the engine is also referred to as “engine air-fuel ratio”.

  More specifically, the downstream air-fuel ratio sensor outputs the output value Voxs shown in FIG. This downstream air-fuel ratio sensor is also called a concentration cell type oxygen concentration sensor.

  As shown in FIG. 2, the output value Voxs of the downstream air-fuel ratio sensor is obtained when the air-fuel ratio of the gas flowing out from the catalyst (catalyst outflow gas) is smaller than the stoichiometric air-fuel ratio (the air side richer than the stoichiometric air-fuel ratio). In the case of the fuel ratio), that is, when the catalyst outflow gas does not contain excessive oxygen, the maximum value Vmax (for example, 1 V) or a value near the maximum value Vmax is obtained. In this case, the state of the catalyst can generally be determined to be an oxygen-deficient state (almost no oxygen is stored in the catalyst).

  The output value Voxs of the downstream side air-fuel ratio sensor is obtained when the air-fuel ratio of the catalyst outflow gas is larger than the stoichiometric air-fuel ratio (when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio), that is, excess oxygen in the catalyst outflow gas. Is included, it becomes a minimum value Vmin (for example, 0 V) or a value in the vicinity of the minimum value Vmin. In this case, it can be determined that the state of the catalyst is generally an oxygen-excess state (the amount of oxygen stored in the catalyst is close to the maximum oxygen storage amount Cmax).

  Therefore, the conventional device has an output value Voxs and a determination value Vth (target value) set to “median value Vmid = (Vmax + Vmin) / 2 which is an average value (central value) of maximum value Vmax and minimum value Vmin”. ) And the catalyst state is substantially determined, and the air-fuel ratio of the engine is controlled substantially based on the determination result.

  That is, in the conventional apparatus, when the output value Voxs of the downstream air-fuel ratio sensor is larger than the determination value Vth, the air-fuel ratio of the catalyst inflow gas becomes an air-fuel ratio (lean air-fuel ratio) larger than the stoichiometric air-fuel ratio (catalyst). The air / fuel ratio of the engine is controlled so that excess oxygen flows into the engine. Further, when the output value Voxs of the downstream air-fuel ratio sensor is smaller than the determination value Vth, the conventional device is configured so that the air-fuel ratio of the catalyst inflow gas becomes an air-fuel ratio (rich air-fuel ratio) smaller than the stoichiometric air-fuel ratio (catalyst). The air-fuel ratio of the engine is controlled so that excessive unburned material flows into the engine (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 2005-171982

  However, in practice, when the air-fuel ratio of the catalyst inflow gas is a lean air-fuel ratio, when the output value Voxs of the downstream air-fuel ratio sensor starts to decrease clearly, the catalyst outflow gas contains a large amount of excess oxygen. Included. Therefore, the oxygen storage amount of the catalyst is close to the maximum oxygen storage amount Cmax, and maintaining the air-fuel ratio of the catalyst inflow gas at the lean air-fuel ratio in this case is when the output value Voxs is larger than the determination value Vth. However, it is not preferable in that the possibility that a large amount of NOx is discharged increases.

  Similarly, when the air-fuel ratio of the catalyst inflow gas is a rich air-fuel ratio, when the output value Voxs of the downstream air-fuel ratio sensor starts to increase obviously, the catalyst outflow gas contains a large amount of excess unburned matter. include. Therefore, the oxygen storage amount of the catalyst is close to “0”. In this case, maintaining the air-fuel ratio of the catalyst inflow gas at the rich air-fuel ratio is even when the output value Voxs is smaller than the determination value Vth. However, it is not preferable in that the possibility that a large amount of unburned material is discharged increases.

Based on such knowledge, the air-fuel ratio control apparatus according to the present invention (hereinafter referred to as “the present invention apparatus”) performs the air-fuel ratio control described below.
(1) The device of the present invention obtains a change amount per predetermined time of the output value Voxs of the downstream air-fuel ratio sensor (hereinafter also referred to as “output change amount ΔVoxs”).
(2) When the target air-fuel ratio is set to the target lean air-fuel ratio (that is, when the air-fuel ratio of the catalyst inflow gas is the lean air-fuel ratio), the device according to the present invention reduces the output value Voxs and outputs When the magnitude of the change amount ΔVoxs becomes equal to or greater than a predetermined lean determination threshold, it is determined that the state of the catalyst is approaching the oxygen excess state, and the target air-fuel ratio is set to the target rich air-fuel ratio. That is, the air-fuel ratio of the catalyst inflow gas is set to a rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio.
(3) When the target air-fuel ratio is set to the target rich air-fuel ratio (that is, when the air-fuel ratio of the catalyst inflow gas is the rich air-fuel ratio), the device of the present invention increases the output value Voxs and outputs When the magnitude of the change amount ΔVoxs becomes equal to or greater than a predetermined rich determination threshold, it is determined that the state of the catalyst is approaching an oxygen-deficient state, and the target air-fuel ratio is set to the target lean air-fuel ratio. That is, the air-fuel ratio of the catalyst inflow gas is set to a lean air-fuel ratio that is larger than the stoichiometric air-fuel ratio.

  However, changes in the output value Voxs with respect to changes in the air-fuel ratio of exhaust gas reaching the downstream air-fuel ratio sensor are different among the individual sensors due to manufacturing variations (individual differences between sensors) of the downstream air-fuel ratio sensor. To do. That is, the response of the output value Voxs to the change in the air-fuel ratio of the exhaust gas that reaches the downstream air-fuel ratio sensor (also referred to as “responsiveness of the output value of the downstream air-fuel ratio sensor” or “responsiveness of the downstream air-fuel ratio sensor”). Is not constant between sensors. This difference in responsiveness is thought to be due to the fact that the thickness, porosity, etc. of the porous protective layer (layer where the exhaust gas reaches first) provided in the downstream air-fuel ratio sensor are not constant among the sensors. .

  FIG. 4 shows (A) the output value Voxs of the downstream air-fuel ratio sensor, (B) the output change amount ΔVoxs (the slope of the output value Voxs, the time differential value of the output value Voxs) that is the change amount per predetermined time of the output value Voxs. d (Voxs) / dt)), (C) upstream target air-fuel ratio abyfr corresponding to the target air-fuel ratio, and (D) nitrogen oxide (NOx) emission amount. . In FIG. 4, the solid line represents each value for the downstream air-fuel ratio sensor whose responsiveness is the reference responsiveness, the broken line represents each value for the downstream air-fuel ratio sensor whose responsiveness is better than the standard responsiveness, and the alternate long and short dash line Indicates each value for the downstream air-fuel ratio sensor whose responsiveness is not better than the standard responsiveness.

  In the example shown in FIG. 4, the target air-fuel ratio before the time t0 is the target lean air-fuel ratio afLean, and oxygen starts to flow out from the catalyst in the vicinity of the time t0, so as shown in FIG. In addition, the output value Voxs starts to decrease from time t0. However, the output value Voxs of the downstream air-fuel ratio sensor with good responsiveness indicated by the broken line decreases relatively quickly, whereas the output value of the downstream air-fuel ratio sensor with poor responsiveness indicated by the one-dot chain line Voxs decrease relatively slowly.

  Therefore, as shown in FIG. 4B, when the responsiveness of the downstream air-fuel ratio sensor is good, the output change amount ΔVoxs is the lean determination threshold value (the first lean determination threshold value in FIG. 4) at time t1. On the other hand, when the responsiveness of the downstream air-fuel ratio sensor is not good, the output change amount ΔVoxs reaches the lean determination threshold at “time t3 after time t1”. For this reason, when the responsiveness of the downstream air-fuel ratio sensor is not good, the time when the target air-fuel ratio is set to the target rich air-fuel ratio afRich is delayed (see time t3 with respect to time t1 in FIG. 4C).

  That is, when the responsiveness of the downstream side air-fuel ratio sensor is not good, the change in the output value ΔVoxs is also delayed because the change in the output value Voxs is delayed with respect to the change in the air-fuel ratio of the exhaust gas. Therefore, when the response of the downstream air-fuel ratio sensor is not good, the determination that the state of the catalyst is approaching the oxygen excess state is delayed, and the time during which the target air-fuel ratio is set to the target lean air-fuel ratio becomes longer. In this case, since the time during which the air-fuel ratio of the catalyst inflow gas is the lean air-fuel ratio becomes longer than necessary, the amount of NOx emission increases (see the dashed line and the dashed line in FIG. 4D).

  The present invention has been made to cope with such a problem, and an object of the present invention is to control the air-fuel ratio of the internal combustion engine that can further improve the emission even when the response of the downstream air-fuel ratio sensor is not good. To provide an apparatus.

More specifically, one aspect of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention is:
A three-way catalyst disposed in the exhaust passage of the internal combustion engine;
A downstream air-fuel ratio sensor that is a concentration cell type oxygen concentration sensor disposed downstream of the catalyst in the exhaust passage;
Air-fuel ratio control means;
Target air-fuel ratio setting means;
Responsiveness index value acquisition means;
Threshold changing means;
Is provided.

  The air-fuel ratio control means has the same air-fuel ratio when the target value of the air-fuel ratio of the engine (that is, the target air-fuel ratio) is set to a predetermined target lean air-fuel ratio (an air-fuel ratio larger than the theoretical air-fuel ratio). The air-fuel ratio of the engine is controlled to match the target lean air-fuel ratio. Further, the air-fuel ratio control means is configured so that the air-fuel ratio of the engine matches the target rich air-fuel ratio when the target air-fuel ratio is set to a predetermined target rich air-fuel ratio (an air-fuel ratio smaller than the theoretical air-fuel ratio). Control the air / fuel ratio of the engine.

  When the target air-fuel ratio is set to the target lean air-fuel ratio and the output value of the downstream air-fuel ratio sensor is decreasing, the target air-fuel ratio setting means is a change amount per predetermined time of the output value. The target air-fuel ratio is set to the target rich air-fuel ratio when the magnitude of a certain output change amount exceeds a predetermined lean determination threshold value. Further, the target air-fuel ratio setting means is configured such that when the target air-fuel ratio is set to the target rich air-fuel ratio and the output value increases, the magnitude of the output change amount is equal to or greater than a predetermined rich determination threshold value. The target air-fuel ratio is set to the target lean air-fuel ratio.

  The responsiveness index value acquisition means displays a “responsiveness index value” indicating “responsiveness of the output value of the downstream air / fuel ratio sensor to a change in the air / fuel ratio of exhaust gas reaching the downstream air / fuel ratio sensor” on the downstream side Acquired based on the output value of the air-fuel ratio sensor. The responsiveness index value can be obtained by various methods as will be described later.

  The threshold value changing means sets the lean determination threshold value based on the acquired responsiveness index value so that the lean determination threshold value becomes smaller as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. And change.

  According to this, when the responsiveness of the downstream air-fuel ratio sensor is not good, the magnitude of the lean determination threshold value becomes small (see the second lean determination threshold value, arrow in FIG. 4B). Therefore, when the responsiveness of the downstream air-fuel ratio sensor is not good, the magnitude of the output change amount ΔVoxs is set to “a shorter time than the lean judgment threshold whose magnitude is changed smaller than when the lean judgment threshold is constant. To reach. Therefore, even when the response of the downstream air-fuel ratio sensor is not good, the device according to the present invention sets the target air-fuel ratio to the target lean air-fuel ratio and the output value of the downstream air-fuel ratio sensor decreases. At this time, it can be detected without delay that the state of the catalyst is approaching the oxygen-excess state. Therefore, the target air-fuel ratio can be changed to the target rich air-fuel ratio without delay. As a result, the lean air-fuel ratio gas containing a large amount of NOx can be prevented from excessively flowing into the catalyst, so that the amount of NOx emission can be reduced.

In one aspect of the device of the present invention,
Based on the acquired responsiveness index value, the threshold value changing unit is configured so that the richness determination threshold value decreases as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. (Refer to the second rich determination threshold value and arrow in FIG. 5B).

  According to this, when the responsiveness of the downstream side air-fuel ratio sensor is not good, the rich determination threshold value becomes small. Therefore, when the responsiveness of the downstream air-fuel ratio sensor is not good, the magnitude of the output change amount ΔVoxs is set to “the shorter time than the rich judgment threshold whose magnitude is changed smaller than when the rich judgment threshold is constant. To reach. Therefore, even when the downstream air-fuel ratio sensor is not responsive, the device according to the present invention sets the target air-fuel ratio to the target rich air-fuel ratio and the output value of the downstream air-fuel ratio sensor increases. At this time, it can be detected without delay that the state of the catalyst is approaching an oxygen-deficient state. Therefore, the target air-fuel ratio can be changed to the target lean air-fuel ratio without delay. As a result, it is possible to avoid a rich air-fuel ratio gas containing a large amount of unburned substances (HC, CO, etc.) from flowing excessively into the catalyst, so that the amount of unburned substances discharged can be reduced.

Furthermore, in one aspect of the apparatus of the present invention, the threshold value changing means is
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the acquired responsiveness index value,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the lean determination threshold is set to a first lean determination threshold;
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the lean determination threshold is set to a second lean having a magnitude smaller than the first lean determination threshold. Set to the judgment threshold
(See FIG. 4).

Similarly, in the aspect of the apparatus of the present invention, the threshold value changing means includes:
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the acquired responsiveness index value,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the rich determination threshold is set to a first rich determination threshold;
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the rich determination threshold is set to a second rich having a size smaller than the first rich determination threshold. Set to the judgment threshold
Configured as follows.

  Furthermore, in another aspect of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, instead of (or in addition to) changing the lean determination threshold and / or the rich determination threshold based on the responsiveness index value, the target rich The air-fuel ratio and / or the target lean air-fuel ratio is changed based on the response index value.

  More specifically, the other aspect includes the three-way catalyst, the downstream air-fuel ratio sensor, the air-fuel ratio control means, and the responsiveness index value acquisition means. Further, the other aspect includes a target air-fuel ratio setting means described below.

  When the target air-fuel ratio is set to the target lean air-fuel ratio and the output value of the downstream air-fuel ratio sensor is decreasing, the target air-fuel ratio setting means is a change amount per predetermined time of the output value. The target air-fuel ratio is set to the target rich air-fuel ratio when the magnitude of a certain output change amount exceeds a predetermined lean determination threshold value. Further, the target air-fuel ratio setting means is configured such that when the target air-fuel ratio is set to the target rich air-fuel ratio and the output value increases, the magnitude of the output change amount is equal to or greater than a predetermined rich determination threshold value. The target air-fuel ratio is set to the target lean air-fuel ratio.

Further, the target air-fuel ratio setting means includes:
The target rich air-fuel ratio is changed based on the acquired responsiveness index value so that the target rich air-fuel ratio becomes smaller as the responsiveness of the output value of the downstream side air-fuel ratio sensor is not good. The

  According to this, even if the timing of setting the target air-fuel ratio to the target rich air-fuel ratio is delayed due to the poor response of the output value of the downstream air-fuel ratio sensor, the air-fuel ratio of the subsequent catalyst inflow gas is delayed. Becomes “richer (smaller) air-fuel ratio” than “the air-fuel ratio of the catalyst inflow gas when the response value of the output value of the downstream air-fuel ratio sensor is good”. As a result, even if the response of the output value of the downstream air-fuel ratio sensor is not good and the timing for setting the target air-fuel ratio to the target rich air-fuel ratio is delayed, the oxygen storage amount of the catalyst can be quickly reduced. Can do. That is, the period in which the catalyst state is close to the oxygen excess state can be shortened. As a result, the amount of NOx emission can be reduced.

Furthermore, the target air-fuel ratio setting means in another aspect of the present invention includes:
The target lean air-fuel ratio is configured to be changed based on the acquired responsiveness index value so that the target lean air-fuel ratio increases as the responsiveness of the output value of the downstream side air-fuel ratio sensor is not good. The

  According to this, even if the timing for setting the target air-fuel ratio to the target lean air-fuel ratio is delayed due to the poor response of the output value of the downstream air-fuel ratio sensor, the air-fuel ratio of the subsequent catalyst inflow gas is delayed. Is “a leaner (larger) air-fuel ratio” than “the air-fuel ratio of the catalyst inflow gas when the response of the output value of the downstream air-fuel ratio sensor is good”. As a result, even if the response of the output value of the downstream air-fuel ratio sensor is not good and the timing for setting the target air-fuel ratio to the target lean air-fuel ratio is delayed, the oxygen storage amount of the catalyst can be increased quickly. Can do. That is, it is possible to shorten a period in which the state of the catalyst is close to an oxygen-deficient state. As a result, the amount of unburned material discharged can be reduced.

Further, the target air-fuel ratio setting means includes:
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the acquired responsiveness index value,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the target rich air-fuel ratio is set to a predetermined first target rich air-fuel ratio,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the target rich air-fuel ratio is changed to a second target rich air-fuel ratio that is smaller than the first target rich air-fuel ratio. Configured to set.

  According to this, even if the timing of setting the target air-fuel ratio to the target rich air-fuel ratio is delayed due to the poor response of the output value of the downstream air-fuel ratio sensor, the air-fuel ratio of the subsequent catalyst inflow gas is delayed. Can be set to “a richer (smaller) air-fuel ratio corresponding to the second target rich air-fuel ratio”. As a result, the amount of oxygen stored in the catalyst can be quickly reduced, so that the period in which the state of the catalyst is close to the oxygen excess state can be shortened. As a result, the amount of NOx emission can be reduced.

In this case, the target air-fuel ratio setting means is
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, when the target air-fuel ratio is switched from the target lean air-fuel ratio to the target rich air-fuel ratio, the target The air-fuel ratio is set to the second target rich air-fuel ratio, and after a predetermined time has elapsed, the target air-fuel ratio is larger than the second target rich air-fuel ratio and smaller than the stoichiometric air-fuel ratio. It can be configured to set the air-fuel ratio. The predetermined time is “a time from when the target air-fuel ratio is set to the second target rich air-fuel ratio until the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes a maximum value (the output change amount ΔVoxs is a minimum value). Or “until the output change amount ΔVoxs becomes a minimum value after the target air-fuel ratio is set to the second target rich air-fuel ratio, and then the output change amount ΔVoxs becomes a predetermined value or more. Time ". Further, the third target rich air-fuel ratio may be the same as or different from the first target rich air-fuel ratio.

  According to this, when the timing for setting the target air-fuel ratio to the target rich air-fuel ratio is delayed due to the poor response of the output value of the downstream air-fuel ratio sensor, the target rich air-fuel ratio is initially set to “second”. “Target rich air-fuel ratio” is set, and then “third target rich air-fuel ratio” is set. Therefore, even if the timing of setting the target rich air-fuel ratio is delayed and the catalyst state approaches the oxygen excess state, the catalyst state can be immediately moved away from the oxygen excess state. Further, once the target air-fuel ratio is set to the third target rich air-fuel ratio after moving away from the excessive oxygen state, conversely, excess unburned material flows into the catalyst (that is, the catalyst state is suddenly changed). Approaching an oxygen-deficient state). Therefore, it is possible to avoid an increase in the amount of unburned material discharged.

Similarly, the target air-fuel ratio setting means includes
It is determined based on the responsiveness index value whether the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the predetermined responsiveness threshold,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the target lean air-fuel ratio is set to a predetermined first target lean air-fuel ratio,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the target rich air-fuel ratio is changed to a second target lean air-fuel ratio that is larger than the first target lean air-fuel ratio. Configured to set.

  According to this, even if the timing for setting the target air-fuel ratio to the target lean air-fuel ratio is delayed due to the poor response of the output value of the downstream air-fuel ratio sensor, the air-fuel ratio of the subsequent catalyst inflow gas is delayed. Can be set to “a leaner (larger) air-fuel ratio corresponding to the second target lean air-fuel ratio”. As a result, the oxygen storage amount of the catalyst can be increased rapidly, so that the period in which the state of the catalyst is close to the oxygen-deficient state can be shortened. As a result, the amount of unburned material discharged can be reduced.

Even in this case, the target air-fuel ratio setting means is
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, when the target air-fuel ratio is switched from the target rich air-fuel ratio to the target lean air-fuel ratio, the target The air-fuel ratio is set to the second target lean air-fuel ratio, and after a predetermined time has elapsed, the target air-fuel ratio is smaller than the second target lean air-fuel ratio and larger than the stoichiometric air-fuel ratio. It can be configured to set the air-fuel ratio. The predetermined time is “a time until the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes a maximum value after the target air-fuel ratio is set to the second target lean air-fuel ratio (the output change amount ΔVoxs is a maximum value). Until the target air-fuel ratio is set to the second target rich air-fuel ratio, the output change amount ΔVoxs becomes a maximum value, and then the output change amount ΔVoxs becomes a predetermined value or less. Time ". Further, the third target lean air-fuel ratio may be the same as or different from the first target lean air-fuel ratio.

  According to this, when the timing for setting the target air-fuel ratio to the target lean air-fuel ratio is delayed due to poor response of the output value of the downstream air-fuel ratio sensor, the target lean air-fuel ratio is initially set to “second”. The “target lean air-fuel ratio” is set, and then the “third target lean air-fuel ratio” is set. Therefore, even if the timing of setting the target lean air-fuel ratio is delayed, even if the state of the catalyst approaches an oxygen-deficient state, the state of the catalyst can be immediately moved away from the oxygen-deficient state. Furthermore, once the target air-fuel ratio is set to the third target lean air-fuel ratio after moving away from the oxygen-deficient state, on the contrary, excess oxygen flows into the catalyst (that is, the catalyst is in excess of oxygen all at once). Can be avoided. Therefore, it is possible to avoid an increase in the NOx emission amount.

  Incidentally, the minimum value of the output value that first appears after the target air-fuel ratio is changed from the target lean air-fuel ratio to the target rich air-fuel ratio becomes so small that the responsiveness of the downstream air-fuel ratio sensor is not good (see FIG. 4). (See q4, q5, and q6.) Therefore, this local minimum value can be adopted as the responsiveness index value.

  Similarly, the maximum value of the output value that first appears after the target air-fuel ratio is changed from the target rich air-fuel ratio to the target lean air-fuel ratio becomes so large that the response of the downstream air-fuel ratio sensor is not good (FIG. 5). (See q4, q5 and q6.) Therefore, this maximum value can be adopted as the responsiveness index value.

Therefore, the responsiveness index value acquisition means is
The minimum value of the output value of the downstream air-fuel ratio sensor that appears first after the target air-fuel ratio is changed from the target lean air-fuel ratio to the target rich air-fuel ratio, or
A maximum value of the output value of the downstream air-fuel ratio sensor that appears first after the target air-fuel ratio of the engine is changed from the target rich air-fuel ratio to the target lean air-fuel ratio;
As the responsiveness index value.

  According to this, the responsiveness index value can be acquired with a simple configuration and during feedback of the air-fuel ratio based on the output value of the downstream air-fuel ratio sensor (based on the output change amount).

  Further, the minimum value of the output value that first appears after the target air-fuel ratio is changed from the target lean air-fuel ratio to the target rich air-fuel ratio (see q4, q5, and q6 in FIG. 4), and the target air-fuel ratio The difference between the maximum output values (see q1, q2, and q3 in FIG. 4) that appeared immediately before switching to the target rich air-fuel ratio (the output value difference between q1 and q4 in FIG. 4, q2 and q5) Output value difference and q3 and q6 output value difference) become so large that the responsiveness of the downstream air-fuel ratio sensor is not good. Therefore, this difference (the magnitude of the difference) can be adopted as the responsiveness index value.

  Similarly, the maximum value of the output value that first appears after the target air-fuel ratio is changed from the target rich air-fuel ratio to the target lean air-fuel ratio (see q4, q5, and q6 in FIG. 5), and the target air-fuel ratio. (See q1, q2, and q3 in FIG. 5) and the difference between the minimum output values (see q1, q2, and q3 in FIG. 5), q2 and the output value difference between q1 and q4 in FIG. The difference in output value from q5 and the difference in output value from q3 and q6) become so large that the responsiveness of the downstream air-fuel ratio sensor is not good. Therefore, this difference (the magnitude of the difference) can be adopted as the responsiveness index value.

Therefore, the responsiveness index value acquisition means is
The maximum value of the output value of the downstream air-fuel ratio sensor when the target air-fuel ratio is set to the target lean air-fuel ratio, and then the target air-fuel ratio changes from the target lean air-fuel ratio to the target rich air-fuel ratio. The magnitude of the difference between the output value of the downstream air-fuel ratio sensor first appearing after being changed, or
The minimum value of the output value of the downstream air-fuel ratio sensor when the target air-fuel ratio is set to the target rich air-fuel ratio, and then the target air-fuel ratio changes from the target rich air-fuel ratio to the target lean air-fuel ratio. The maximum value of the output value of the downstream air-fuel ratio sensor that appears first after being changed,
As the responsiveness index value.

  According to this, the responsiveness index value can be acquired with a simple configuration and during feedback of the air-fuel ratio based on the output value of the downstream air-fuel ratio sensor (based on the output change amount).

The responsiveness index value acquisition means includes
After the fuel cut operation state is completed and the target air-fuel ratio is changed to a predetermined target rich air-fuel ratio after the fuel cut operation state where the engine operation state stops the fuel supply to the engine, the downstream side Time until the output value of the air-fuel ratio sensor reaches a predetermined value (increases), or
When the engine operating state is set to an increased rich air / fuel ratio that is smaller than the stoichiometric air / fuel ratio regardless of the output value of the downstream air / fuel ratio sensor, the increased operating state is terminated. A time until the output value of the downstream air-fuel ratio sensor reaches a predetermined value (decreases) after the target air-fuel ratio is changed to a predetermined target lean air-fuel ratio;
May be acquired as the responsiveness index value.
These times become shorter as the responsiveness of the output value of the downstream side air-fuel ratio sensor is higher, and become longer as it is lower.

  Furthermore, the time acquired in this case may be corrected according to the maximum oxygen storage amount Cmax of the catalyst calculated separately, and the corrected value may be adopted as the responsiveness index value.

Furthermore, the responsiveness index value acquisition means includes:
Regardless of the output value of the downstream air-fuel ratio sensor, after maintaining the target air-fuel ratio at a constant target rich air-fuel ratio and confirming that the output value has reached the maximum value Vmax, the target air-fuel ratio is set to the target rich air-fuel ratio. Is equivalent to an intermediate air-fuel ratio between the target rich air-fuel ratio and the target lean air-fuel ratio (or an air-fuel ratio obtained by internally dividing them by a predetermined ratio). The time until the value or the value corresponding to the target lean air-fuel ratio is reached may be acquired as the responsiveness index value.

Similarly, the responsiveness index value acquisition means includes
Regardless of the output value of the downstream air-fuel ratio sensor, after maintaining the target air-fuel ratio at a constant target lean air-fuel ratio and confirming that the output value has reached the minimum value Vmin, the target air-fuel ratio is set to the target lean air-fuel ratio. Corresponds to an intermediate air-fuel ratio between the target lean air-fuel ratio and the target rich air-fuel ratio (or an air-fuel ratio obtained by internally dividing the air-fuel ratio by a predetermined ratio). The time until the value or the value corresponding to the target rich air-fuel ratio is reached may be acquired as the responsiveness index value.

1 is a schematic diagram of an internal combustion engine to which an air-fuel ratio control apparatus (first control apparatus) for an internal combustion engine according to a first embodiment of the present invention is applied. 2 is a graph showing the relationship between the output value (output voltage) of the downstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. 3A is a schematic cross-sectional view of the element portion of the downstream air-fuel ratio sensor shown in FIG. 1, and FIG. 3B is an enlarged schematic cross-sectional view of the element portion. It is a time chart for demonstrating the action | operation of a 1st control apparatus. It is a time chart for demonstrating the action | operation of a 1st control apparatus. It is the flowchart which showed the routine which CPU of a 1st control apparatus performs. It is the flowchart which showed the routine which CPU of a 1st control apparatus performs. It is the flowchart which showed the routine which CPU of a 1st control apparatus performs. It is the flowchart which showed the routine which CPU of a 1st control apparatus performs. It is the flowchart which showed the routine which CPU of a 1st control apparatus performs. It is a table which CPU which concerns on the modification of a 1st control apparatus refers. It is a table which CPU which concerns on the modification of a 1st control apparatus refers. It is a time chart for demonstrating the action | operation of the air fuel ratio control apparatus (2nd control apparatus) of the internal combustion engine which concerns on 2nd Embodiment of this invention. It is a time chart for demonstrating the action | operation of a 2nd control apparatus. It is the flowchart which showed the routine which CPU of a 2nd control apparatus performs. It is a table which CPU which concerns on the modification of a 2nd control apparatus refers. It is a time chart for demonstrating the action | operation of the air fuel ratio control apparatus (3rd control apparatus) of the internal combustion engine which concerns on 3rd Embodiment of this invention. It is a time chart for demonstrating the action | operation of the modification of a 3rd control apparatus. It is a time chart for demonstrating the action | operation of the air fuel ratio control apparatus (4th control apparatus) of the internal combustion engine which concerns on 4th Embodiment of this invention. It is a time chart for demonstrating the action | operation of the modification of a 4th control apparatus. It is a time chart for demonstrating the action | operation of another modification of a 3rd control apparatus. It is a time chart for demonstrating the action | operation of another modification of a 4th control apparatus.

  Hereinafter, an air-fuel ratio control apparatus for an internal combustion engine according to each embodiment of the present invention will be described with reference to the drawings.

<First Embodiment>
(Constitution)
FIG. 1 shows a schematic configuration of an internal combustion engine 10 to which an air-fuel ratio control apparatus (hereinafter also referred to as “first control apparatus”) according to a first embodiment of the present invention is applied. The engine 10 is a four-cycle / spark ignition type / multi-cylinder (four cylinders in this example) / gasoline fuel engine. The engine 10 includes a main body 20, an intake system 30, and an exhaust system 40.

  The main body portion 20 includes a cylinder block portion and a cylinder head portion. The main body portion 20 includes a plurality (four) of combustion chambers (first cylinder # 1 to fourth cylinder # 4) 21 including a piston top surface, a cylinder wall surface, and a lower surface of the cylinder head portion.

  In the cylinder head portion, an intake port 22 for supplying “a mixture of air and fuel” to each combustion chamber (each cylinder) 21, and an exhaust gas (burned gas) from each combustion chamber 21 are discharged. An exhaust port 23 is formed. The intake port 22 is opened and closed by an unillustrated intake valve, and the exhaust port 23 is opened and closed by an unillustrated exhaust valve.

  A plurality (four) of spark plugs 24 are fixed to the cylinder head portion. Each spark plug 24 is disposed such that its spark generating part is exposed at the center of each combustion chamber 21 and in the vicinity of the lower surface of the cylinder head part. Each spark plug 24 generates an ignition spark from the spark generating portion in response to the ignition signal.

  A plurality (four) of fuel injection valves (injectors) 25 are further fixed to the cylinder head portion. One fuel injection valve 25 is provided for each intake port 22 (that is, one for each cylinder). In response to the injection instruction signal, the fuel injection valve 25 injects “the fuel of the indicated injection amount included in the injection instruction signal” into the corresponding intake port 22.

  Further, an intake valve control device 26 is provided in the cylinder head portion. The intake valve control device 26 has a known configuration that adjusts and controls the relative rotation angle (phase angle) between an intake camshaft (not shown) and an intake cam (not shown) by hydraulic pressure. The intake valve control device 26 operates based on an instruction signal (drive signal), and can change the valve opening timing (intake valve opening timing) of the intake valve.

  The intake system 30 includes an intake manifold 31, an intake pipe 32, an air filter 33, a throttle valve 34, and a throttle valve actuator 34a.

  The intake manifold 31 includes a plurality of branch portions connected to each intake port 22 and a surge tank portion in which the branch portions are gathered. The intake pipe 32 is connected to the surge tank portion. The intake manifold 31, the intake pipe 32, and the plurality of intake ports 22 constitute an intake passage. The air filter 33 is provided at the end of the intake pipe 32. The throttle valve 34 is rotatably attached to the intake pipe 32 at a position between the air filter 33 and the intake manifold 31. The throttle valve 34 changes the opening cross-sectional area of the intake passage formed by the intake pipe 32 by rotating. The throttle valve actuator 34a is formed of a DC motor, and rotates the throttle valve 34 in response to an instruction signal (drive signal).

  The exhaust system 40 includes an exhaust manifold 41, an exhaust pipe (exhaust pipe) 42, an upstream catalyst 43, and a downstream catalyst 44.

  The exhaust manifold 41 includes a plurality of branch portions 41a connected to each exhaust port 23, and a collection portion (exhaust collection portion) 41b in which the branch portions 41a are gathered. The exhaust pipe 42 is connected to a collective portion 41 b of the exhaust manifold 41. The exhaust manifold 41, the exhaust pipe 42, and the plurality of exhaust ports 23 constitute a passage through which exhaust gas passes. In the present specification, a passage formed by the collecting portion 41b of the exhaust manifold 41 and the exhaust pipe 42 is referred to as an “exhaust passage” for convenience.

The upstream catalyst 43 supports “a noble metal (one or more kinds of palladium Pd, platinum Pt, rhodium Rd, etc.)” and “ceria (CeO2) as an oxygen storage material” on a support made of ceramic. Thus, it is a three-way catalyst having an oxygen storage / release function (oxygen storage function). The upstream catalyst 43 is disposed (intervened) in the exhaust pipe 42. When the upstream catalyst 43 reaches a predetermined activation temperature, it exhibits a “catalytic function for simultaneously purifying unburned substances (HC, CO, H _2, etc.) and nitrogen oxides (NOx)” and “oxygen storage function”. . The upstream catalyst 43 is also referred to as a start catalytic converter (SC) or a first catalyst.

  The downstream catalyst 44 is a three-way catalyst similar to the upstream catalyst 43. The downstream catalyst 44 is disposed (intervened) in the exhaust pipe 42 downstream of the upstream catalyst 43. Since the downstream side catalyst 44 is disposed below the floor of the vehicle, it is also referred to as an under-floor catalytic converter (UFC) or a second catalyst. In the present specification, when the term “catalyst” is simply used, the “catalyst” means the upstream catalyst 43.

  The present control device includes a hot-wire air flow meter 51, a throttle position sensor 52, an engine speed sensor 53, a water temperature sensor 54, an upstream air-fuel ratio sensor 55, a downstream air-fuel ratio sensor 56, and an accelerator opening sensor 57.

  The hot-wire air flow meter 51 detects the mass flow rate of the intake air flowing through the intake pipe 32 and outputs a signal representing the mass flow rate (intake air amount per unit time of the engine 10) Ga.

  The throttle position sensor 52 detects the opening degree of the throttle valve 34 and outputs a signal representing the throttle valve opening degree TA.

  The engine rotational speed sensor 53 outputs a signal having a narrow pulse every time the intake camshaft rotates 5 ° and a wide pulse every time the intake camshaft rotates 360 °. A signal output from the engine rotational speed sensor 53 is converted into a signal representing the engine rotational speed NE by an electric control device 60 described later. Further, the electric control device 60 acquires the crank angle (absolute crank angle) of the engine 10 based on signals from the engine rotation speed sensor 53 and a crank angle sensor (not shown).

  The water temperature sensor 54 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The upstream air-fuel ratio sensor 55 is disposed in either the exhaust manifold 41 or the exhaust pipe 42 (that is, the exhaust passage) at a position between the collecting portion 41 b of the exhaust manifold 41 and the upstream catalyst 43. The upstream air-fuel ratio sensor 55 is disclosed in, for example, “limit current type wide area air-fuel ratio including a diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. Sensor ".

  The upstream air-fuel ratio sensor 55 corresponds to the air-fuel ratio of the exhaust gas flowing through the position where the upstream air-fuel ratio sensor 55 is disposed (the air-fuel ratio of the “catalyst inflow gas” that is the gas flowing into the catalyst 43, the upstream air-fuel ratio abyfs). Output the output value Vabyfs. The output value Vabyfs increases as the air-fuel ratio of the catalyst inflow gas increases (that is, as the air-fuel ratio of the catalyst inflow gas becomes leaner).

  The electric control device 60 stores an air-fuel ratio conversion table (map) Mapabyfs that defines the relationship between the output value Vabyfs and the upstream air-fuel ratio abyfs. The electric control device 60 detects the actual upstream air-fuel ratio abyfs (obtains the detected upstream air-fuel ratio abyfs) by applying the output value Vabyfs to the air-fuel ratio conversion table Mapabyfs.

The downstream air-fuel ratio sensor 56 is disposed in the exhaust pipe 42 (that is, the exhaust passage) at a position between the upstream catalyst 43 and the downstream catalyst 44. The downstream air-fuel ratio sensor 56 is a well-known concentration cell type oxygen concentration sensor (O ₂ sensor). The downstream air-fuel ratio sensor 56 outputs the output value Voxs shown in FIG.

  The downstream air-fuel ratio sensor 56 includes a test tubular element portion shown in FIG. As shown in FIG. 3B, the element portion includes a solid electrolyte layer 56a, an exhaust gas side electrode layer 56b formed outside the solid electrolyte layer 56a, and an air chamber AR (inside the solid electrolyte layer 56a). An atmosphere-side electrode layer (inner electrode) 56c formed inside the solid electrolyte layer 56a so as to face the exhaust gas-side electrode layer (outer electrode) 56b across the solid electrolyte layer 56a, and an exhaust gas-side electrode layer The coating layer 56d covering 56b, the catalyst layer 56e covering the coating layer 56d, and the exhaust gas Ex covering the catalyst layer 56e and contacting with the exhaust gas Ex (exhaust gas Ex flowing into the protective cover through a through hole of a protective cover not shown) A protective layer (trap layer) 56f (disposed to be exposed therein).

  The solid electrolyte layer 56a and the like may be plate-shaped. Gas flowing out from the catalyst 43 (catalyst outflow gas) passes through the through hole of the protective cover and flows into the protective cover, and then passes through the protective layer 56f, the catalyst layer 56e, and the coating layer 56d to form the exhaust gas side electrode. Reach layer 56b.

  The output value Voxs of the downstream side air-fuel ratio sensor 56 is determined in accordance with “the difference between the oxygen partial pressure in the exhaust gas side electrode layer (outer electrode) 56b and the oxygen partial pressure in the atmosphere side electrode layer (inner electrode) 56c”. It is an electromotive force that causes the migration of oxygen ions in the solid electrolyte layer 56a.

  More specifically, the output value Voxs is excessive when the air-fuel ratio of the catalyst outflow gas is smaller than the stoichiometric air-fuel ratio (when the air-fuel ratio is richer than the stoichiometric air-fuel ratio). When oxygen is not included, the maximum value Vmax or a value near the maximum value Vmax (for example, about 0.9 to 1.0 V) is obtained. In this case, it can be determined that the state of the catalyst 43 is generally an oxygen-deficient state (almost no oxygen is stored in the catalyst).

  On the other hand, when the air-fuel ratio of the catalyst outflow gas is larger than the stoichiometric air-fuel ratio (when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio), that is, the catalyst outflow gas contains excessive oxygen. If it is, the minimum value Vmin or a value in the vicinity of the minimum value Vmin (for example, about 0 to 0.1 V) is obtained. In this case, it can be determined that the state of the catalyst 43 is generally an oxygen-excess state (the amount of oxygen stored in the catalyst is close to the maximum oxygen storage amount Cmax).

  Further, the output value Voxs rapidly decreases from the maximum value Vmax to the minimum value Vmin when the air-fuel ratio of the catalyst outflow gas changes from the rich air-fuel ratio to the lean air-fuel ratio with respect to the stoichiometric air-fuel ratio. . Conversely, the output value Voxs rapidly increases from the minimum value Vmin to the maximum value Vmax when the air-fuel ratio of the catalyst outflow gas changes from the lean air-fuel ratio to the rich air-fuel ratio with respect to the stoichiometric air-fuel ratio. . The average value of the minimum value Vmin and the maximum value Vmax is referred to as a median value Vmid (= (Vmax + Vmin) / 2) or a stoichiometric air-fuel ratio equivalent voltage Vst.

  Incidentally, as described above, the catalyst outflow gas passes through the protective layer 56f, the catalyst layer 56e, and the coating layer 56d, and reaches the exhaust gas side electrode layer 56b. Therefore, when these layers (particularly, the thickness and porosity of the protective layer 56f) vary in production, the responsiveness of the change in the output value Voxs to the change in the air-fuel ratio (oxygen partial pressure) of the catalyst effluent gas varies. The downstream air-fuel ratio sensor 56 is different.

  Referring to FIG. 1 again, the accelerator opening sensor 57 detects the operation amount of the accelerator pedal AP operated by the driver, and outputs a signal representing the operation amount Accp of the accelerator pedal AP.

  The electric control device 60 is an electronic circuit including a “well-known microcomputer” including “an interface including a CPU, a ROM, a RAM, a backup RAM, and an AD converter”.

  The backup RAM included in the electric control device 60 is a battery mounted on the vehicle regardless of the position of an ignition key switch (not shown) of the vehicle on which the engine 10 is mounted (any one of an off position, a start position, an on position, etc.). It is supposed to receive power supply from. When receiving power from the battery, the backup RAM stores data according to an instruction from the CPU (data is written) and holds (stores) the data so that the data can be read. The backup RAM cannot retain data when the power supply from the battery is interrupted, for example, when the battery is removed from the vehicle. In other words, the data held so far is lost (destroyed).

  The interface of the electric control device 60 is connected to the sensors 51 to 57 so as to supply signals from the sensors 51 to 57 to the CPU. Further, the interface sends an instruction signal (drive signal) or the like to the ignition plug 24 of each cylinder, the fuel injection valve 25 of each cylinder, the intake valve control device 26, the throttle valve actuator 34a, etc. in accordance with an instruction from the CPU. It is like that. The electric control device 60 sends an instruction signal to the throttle valve actuator 34a so that the throttle valve opening TA increases as the acquired accelerator pedal operation amount Accp increases.

(Outline of air-fuel ratio feedback control by the first controller)
The first controller determines the state of the catalyst 43 (oxygen storage state, catalyst state) as described below. The catalyst 43 is in a state where the catalyst 43 stores oxygen to a value close to its maximum oxygen storage amount (that is, an oxygen excess state, a lean state), and a state where the catalyst 43 hardly stores oxygen (ie, It is determined that the state is any one of an oxygen deficient state, a reduced state, and a rich state. In order to determine the state of the catalyst 43, the first controller uses an output change amount ΔVoxs that is a change amount per predetermined time (unit time) of the output value Voxs of the downstream air-fuel ratio sensor 56. The output change amount ΔVoxs is a value corresponding to the differential value dVoxs / dt, and is also the slope of the output value Voxs.

  Further, the first control device calculates “the air-fuel ratio of the exhaust gas (catalyst inflow gas) flowing into the catalyst 43 (and hence the air-fuel ratio of the air-fuel mixture supplied to the engine 10)” based on the determined state of the catalyst 43. Feedback control. More specifically, according to the state of the catalyst 43, the first control device sets the “target air-fuel ratio (upstream target air-fuel ratio abyfr) that is the target value of the air-fuel ratio of the engine” to the target rich air-fuel ratio afRich and the target lean. Set to any one of air-fuel ratio afLean. For example, the target rich air-fuel ratio afRich is 14.2, the theoretical air-fuel ratio stoich is 14.5, and the target lean air-fuel ratio afLean is 14.7.

<When the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean>
In the example shown in FIG. 4, the target air-fuel ratio abyfr before time t0 is the target lean air-fuel ratio afLean. Therefore, since excess oxygen is contained in the catalyst inflow gas, the state of the catalyst 43 approaches the oxygen excess state, and as a result, oxygen begins to flow out of the catalyst 43 abruptly at or immediately before time t0.

  Now, it is assumed that the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is the reference response (design value response). In this case, as indicated by the solid line in FIG. 4A, the output value Voxs starts decreasing at time t0. In this case, the degree of reduction is moderate.

  In the first control device, when the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean and the output value Voxs is decreasing, the magnitude | ΔVoxs | of the output change amount ΔVoxs is “a positive predetermined value” When it becomes larger than the “lean determination threshold value ΔVLeanth”, it is determined that the state of the catalyst 43 is approaching the oxygen excess state, and the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich. Note that the lean determination threshold value ΔVLeanth is initially set to the first lean determination threshold value ΔVthL1. As indicated by the solid line in FIG. 4B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the lean determination threshold value ΔVLeanth at time t2. Thereby, as indicated by a solid line in FIG. 4C, the first control device sets the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich at time t2.

  Further, it is assumed that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is better (higher responsiveness) than the reference responsiveness. Also in this case, as indicated by a broken line in FIG. 4A, the output value Voxs starts to decrease at time t0. However, the degree of decrease in the output value Voxs is large. That is, the output value Voxs decreases rapidly. Therefore, in this case, as indicated by a broken line in FIG. 4B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the lean determination threshold value ΔVLeanth at “time t1 before time t2”. As a result, the first control device sets the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich at time t1, as indicated by a broken line in FIG.

  In contrast, it is assumed that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not better (lower responsiveness) than the reference responsiveness. Also in this case, the output value Voxs starts to decrease at time t0, as indicated by the one-dot chain line in FIG. However, the degree of decrease is small. That is, the output value Voxs decreases gently. Therefore, in this case, as indicated by a one-dot chain line in FIG. 4B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the lean determination threshold value ΔVLeanth at “time t3 after time t2”. . As a result, the first controller sets the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich at time t3, as indicated by the alternate long and short dash line in FIG. As a result, since the exhaust gas having a lean air-fuel ratio excessively flows into the catalyst 43, the NOx emission amount increases as shown by the one-dot chain line in FIG.

  Thereafter, the output value Voxs continues to decrease and then begins to increase. That is, the output value Voxs takes the minimum value Voxsmin. The minimum value Voxsmin becomes so small that the responsiveness of the downstream air-fuel ratio sensor 56 is not good (see q4, q5, and q6 in FIG. 4A). This is because excessive oxygen flows into the catalyst 43 as the responsiveness of the downstream side air-fuel ratio sensor 56 is not good. In other words, the timing for setting the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich is delayed. It is. Furthermore, even if unburned substances start to flow out from the catalyst 43, the detection of the output value Voxs of the downstream air-fuel ratio sensor 56 with poor responsiveness is delayed.

  In addition, the maximum value Voxsmax (see q1, q2, and q3 in FIG. 4A) that appears before the minimum value Voxsmin, and the minimum value Voxsmin (q4, q5, and 4), that is, the output value difference between q1 and q4 in FIG. 4, the output value difference between q2 and q5, and the output value between q3 and q6. The magnitude of the difference becomes so great that the response of the downstream air-fuel ratio sensor is not good. Therefore, the first control device acquires the magnitude DV (= first responsiveness index value DV1) of the output value difference as the “responsiveness index value of the output value Voxs of the downstream air-fuel ratio sensor 56”.

  Furthermore, the first control device determines whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the magnitude DV of the output value difference. More specifically, the first control device determines whether or not the magnitude DV of the output value difference is equal to or greater than “first threshold output value difference DV1th corresponding to the responsiveness threshold value”, and the output value difference. Is smaller than the first threshold output value difference DV1th, it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold, and the output value difference magnitude DV is the first threshold value DV. When the threshold output value difference DV1th is 1 or more, it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than a predetermined responsiveness threshold.

  When the first control device determines that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the predetermined responsiveness threshold, the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is within an allowable range. As a result, the lean determination threshold value ΔVLeanth is maintained at the first lean determination threshold value ΔVthL1. On the other hand, when the first control apparatus determines that the response of the output value of the downstream air-fuel ratio sensor is lower than the predetermined response threshold, the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is assumed. The lean determination threshold value ΔVLeanth is changed to “second lean determination threshold value ΔVthL2 having a size smaller than the first lean determination threshold value ΔVthL1”. For example, the second lean determination threshold value ΔVthL2 is a value obtained by subtracting a predetermined positive value α from the first lean determination threshold value ΔVthL1. However, the second lean determination threshold value ΔVthL2 is also a positive value.

  Thereby, when the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is not better than expected, the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean and the output value Voxs is decreased. The output change amount ΔVoxs reaches the lean determination threshold value ΔVLeanth (= second lean determination threshold value ΔVthL2) at an early timing. Therefore, it is possible to avoid the occurrence of “a situation where excessive oxygen flows into the catalyst 43 and the timing for setting the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich is delayed”. As a result, the amount of NOx emission can be reduced.

<When the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich>
Even when the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich, the first control device performs the same operation as when the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean.

  In the example shown in FIG. 5, the target air-fuel ratio before time t0 is the target rich air-fuel ratio afRich. Therefore, since the catalyst inflow gas contains excessive unburned substances, the state of the catalyst 43 approaches an oxygen-deficient state, and as a result, unburned substances suddenly move from the catalyst 43 at time t0 or immediately before. It begins to flow out. In FIG. 5, as in FIG. 4, the solid line represents each value for the downstream air-fuel ratio sensor whose response is the standard response, and the broken line represents the downstream air-fuel ratio sensor whose response is better than the standard response. Each of the values and the one-dot chain line indicate the values of the downstream air-fuel ratio sensor whose responsiveness is not better than the standard responsiveness.

  Now, it is assumed that the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is the reference response (design value response). In this case, as indicated by a solid line in FIG. 5A, the output value Voxs starts increasing at time t0. In this case, the degree of increase is moderate.

  In the first control device, when the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich and the output value Voxs increases, the magnitude | ΔVoxs | of the output change amount ΔVoxs is “a positive predetermined value”. When it becomes larger than the “rich determination threshold ΔVRichth”, it is determined that the state of the catalyst 43 is approaching an oxygen-deficient state, and the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean. Note that the rich determination threshold value ΔVRichth is initially set to the first rich determination threshold value ΔVthR1. Therefore, as indicated by a solid line in FIG. 5B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the rich determination threshold value ΔVRichth at time t2. Therefore, the first control device sets the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean at time t2, as indicated by the solid line in FIG.

  Further, it is assumed that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is better (higher responsiveness) than the reference responsiveness. Also in this case, as indicated by the broken line in FIG. 5A, the output value Voxs starts increasing at time t0. However, the degree of increase in the output value Voxs is large. That is, the output value Voxs increases rapidly. Therefore, in this case, as indicated by the broken line in FIG. 5B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the rich determination threshold value ΔVRichth at “time t1 before time t2.” As a result, the first control apparatus sets the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean at time t1, as indicated by a broken line in FIG.

  In contrast, it is assumed that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not better (lower responsiveness) than the reference responsiveness. Also in this case, as indicated by the alternate long and short dash line in FIG. 5A, the output value Voxs starts increasing at time t0. However, the degree of increase is small. That is, the output value Voxs increases gently. Therefore, in this case, as indicated by a one-dot chain line in FIG. 5B, the magnitude | ΔVoxs | of the output change amount ΔVoxs reaches the rich determination threshold value ΔVRichth at “time t3 after time t2.” . As a result, the first control device sets the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean at time t3, as indicated by the alternate long and short dash line in FIG. As a result, the rich air-fuel ratio exhaust gas excessively flows into the catalyst 43, so that the amount of unburned substances (for example, HC) increases as indicated by the one-dot chain line in FIG.

  Thereafter, the output value Voxs continues to increase and then begins to decrease. That is, the output value Voxs takes the maximum value Voxsmax. This maximum value Voxsmax increases as the response of the downstream air-fuel ratio sensor 56 is not good (see q4, q5, and q6 in FIG. 5A). This is because excessive unburned matter flows into the catalyst 43 as the response of the downstream air-fuel ratio sensor 56 is not good. In other words, the timing at which the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean. This is to delay. Furthermore, even if oxygen begins to flow out from the catalyst 43, the detection of the output value Voxs of the downstream air-fuel ratio sensor 56 with poor responsiveness is delayed.

  Further, the minimum value Voxsmin (see q1, q2, and q3 in FIG. 5A) that appears before the maximum value Voxsmax and the maximum value Voxsmax (q4, q5, and q6 in FIG. 5A). 5), that is, the output value difference between q1 and q4 in FIG. 5, the output value difference between q2 and q5, and the output value difference between q3 and q6. Is so large that the responsiveness of the downstream air-fuel ratio sensor is not good. Therefore, the first control device acquires the magnitude DV (= second responsiveness index value DV2) of the output value difference as the “responsiveness index value of the output value Voxs of the downstream air-fuel ratio sensor 56”.

  Furthermore, the first control device determines whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the magnitude DV of the output value difference. More specifically, the first control device determines whether or not the magnitude DV of the output value difference is equal to or greater than the “second threshold output value difference DV2th corresponding to the responsiveness threshold”, and the output value difference Is smaller than the second threshold output value difference DV2th, it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold, and the output value difference DV is the first threshold value DV. When the threshold output value difference DV2th is equal to or greater than 2, it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than a predetermined responsiveness threshold.

  When the first control device determines that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the predetermined responsiveness threshold, the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is within an allowable range. As a result, the rich determination threshold value ΔVRichth is maintained at the first rich determination threshold value ΔVthR1. On the other hand, when the first control apparatus determines that the response of the output value of the downstream air-fuel ratio sensor is lower than the predetermined response threshold, the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is assumed. The rich determination threshold value ΔVRichth is changed to “second rich determination threshold value ΔVthR2 having a size smaller than the first rich determination threshold value ΔVthR1”. For example, the second rich determination threshold value ΔVthR2 is a value obtained by subtracting a predetermined positive value β from the first rich determination threshold value ΔVthR1. However, the second rich determination threshold value ΔVthR2 is also a positive value.

  The first rich determination threshold value ΔVthR1 may be the same as or different from the first lean determination threshold value ΔVthL1. Furthermore, the predetermined value β may be the same as or different from the predetermined value α.

  Thus, when the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is not better than expected, the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich and the output value Voxs is increased. The output change amount ΔVoxs reaches the rich determination threshold value ΔVRichth (= second rich determination threshold value ΔVthR2) at an early timing. Therefore, it is possible to avoid the occurrence of “a situation in which excessive unburned matter flows into the catalyst 43 and the timing for setting the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean is delayed”. As a result, the amount of unburned material discharged can be reduced.

(Operation)
Next, the actual operation of the first control device will be described.

<Fuel injection control>
The CPU of the first control device repeatedly executes the fuel injection control routine shown in FIG. 6 for each cylinder every time the crank angle of any cylinder reaches a predetermined crank angle before the intake top dead center. It has become. The predetermined crank angle is, for example, BTDC 90 ° CA (90 ° crank angle before intake top dead center). A cylinder whose crank angle coincides with the predetermined crank angle is also referred to as a “fuel injection cylinder”. The CPU calculates the commanded fuel injection amount (final fuel injection amount) Fi and instructs fuel injection by this fuel injection control routine.

  When the crank angle of an arbitrary cylinder matches the predetermined crank angle before the intake top dead center, the CPU starts the process from step 600, and in step 605, determines whether or not the value of the fuel cut flag XFC is “0”. judge. The value of the fuel cut flag XFC is set to “1” when the fuel cut start condition is satisfied, and is “0” when the fuel cut end condition is satisfied when the value of the fuel cut flag XFC is “1”. Set to The value of the fuel cut flag XFC is further set to “0” in the initial routine. The initial routine is a routine executed by the CPU when the ignition key switch of the vehicle on which the engine 10 is mounted is changed from OFF to ON.

  The fuel cut start condition is satisfied when the throttle valve opening degree TA is “0” and the engine rotational speed NE is equal to or higher than the fuel cut rotational threshold speed NEFC. The fuel cut end condition is satisfied when the throttle valve opening degree TA is not “0” or when the engine speed NE is equal to or lower than the fuel cut end rotation threshold speed NERT. The fuel cut end rotation threshold speed NERT is smaller than the fuel cut rotation threshold speed NEFC.

  Assume that the value of the fuel cut flag XFC is “0”. In this case, the CPU makes a “Yes” determination at step 605 to proceed to step 610, where “the engine rotation speed obtained by the intake air amount Ga measured by the air flow meter 51 and the signal of the engine rotation speed sensor 53 is acquired. Based on the speed NE and the look-up table MapMc (Ga, NE), “the amount of air taken into the fuel injection cylinder (ie, the in-cylinder intake air amount) Mc)” is acquired. The in-cylinder intake air amount Mc may be calculated by a well-known air model (a model constructed according to a physical law simulating the behavior of air in the intake passage).

Next, the CPU proceeds to step 615 to determine whether or not the value of the feedback control flag XFB is “1”. The value of the feedback control flag XFB is set to “1” when the feedback control condition is satisfied, and is set to “0” when the feedback control condition is not satisfied. The feedback control condition is satisfied when, for example, all the following conditions are satisfied.
(A1) The upstream air-fuel ratio sensor 55 is activated.
(A2) The downstream air-fuel ratio sensor 56 is activated.
(A3) The engine load KL is less than or equal to the threshold KLth.
(A4) The value of the fuel cut flag XFC is “0”.

  At this time, if the value of the feedback control flag XFB is not “1”, the CPU makes a “No” determination at step 615 to proceed to step 620 to set the target air-fuel ratio abyfr (upstream target air-fuel ratio abyfr) to the theoretical sky. The fuel ratio is set to stoich (for example, 14.5).

  Next, the CPU sequentially performs the processing from step 625 to step 640 described below, and proceeds to step 695 to end the present routine tentatively.

  Step 625: The CPU calculates the basic fuel injection amount Fbase by dividing the in-cylinder intake air amount Mc by the target air-fuel ratio abyfr. The basic fuel injection amount Fbase is a feed-forward amount of the fuel injection amount necessary for making the air-fuel ratio of the engine coincide with the target air-fuel ratio abyfr.

  Step 630: The CPU reads the main feedback amount KFmain separately calculated by a routine not shown. The main feedback amount KFmain is calculated based on known PID control so that the upstream air-fuel ratio abyfs matches the target air-fuel ratio abyfr. Specifically, the main feedback amount DFmain is increased when the upstream air-fuel ratio abyfs is larger (lean) than the target air-fuel ratio abyfr, and the upstream air-fuel ratio abyfs is smaller than the target air-fuel ratio abyfr (rich). Is reduced). The main feedback amount KFmain is set to “1” when the value of the feedback control flag XFB is “0”. Further, the main feedback amount KFmain may always be set to “1”. That is, feedback control using the main feedback amount KFmain is not essential in this embodiment.

  Step 635: The CPU calculates the command fuel injection amount Fi by correcting the basic fuel injection amount Fbase with the main feedback amount KFmain. More specifically, the CPU calculates the command fuel injection amount Fi by multiplying the basic fuel injection amount Fbase by the main feedback amount KFmain.

  Step 640: The CPU sends an injection instruction signal for injecting “the fuel of the indicated fuel injection amount Fi” from the “fuel injection valve 25 provided corresponding to the fuel injection cylinder” to the fuel injection valve 25. To do.

  As a result, an amount of fuel required to make the air-fuel ratio of the engine coincide with the target air-fuel ratio abyfr is injected from the fuel injection valve 25 of the fuel injection cylinder. That is, Step 625 to Step 640 constitute the command fuel injection amount control means and the air fuel ratio control means that “control the command fuel injection amount Fi so that the air fuel ratio of the engine matches the target air fuel ratio abyfr”.

  On the other hand, if the value of the feedback control flag XFB is “1” at the time when the CPU performs the process of step 615, the CPU determines “Yes” in step 615 and proceeds to step 645 to display the catalyst rich state display. It is determined whether or not the value of the flag XCCRORich is “1”. As will be described later, when it is determined that the state of the catalyst 43 is an oxygen-deficient state (catalyst rich state), the value of the catalyst rich state display flag XCCRORich is set to “1”. Further, when it is determined that the state of the catalyst 43 is an oxygen excess state (catalyst lean state), the value of the catalyst rich state display flag XCCRORich is set to “0”. The value of the catalyst rich state display flag XCCRORich is set by a routine described later.

  At this time, if the value of the catalyst rich state display flag XCCRORich is “1”, the CPU makes a “Yes” determination at step 645 to proceed to step 650 to set the target air-fuel ratio abyfr to “a predetermined target lean air-fuel ratio afLean. To "". The target lean air-fuel ratio afLean is a predetermined air-fuel ratio (lean air-fuel ratio) that is larger than the stoichiometric air-fuel ratio stoich. In this case, the target lean air-fuel ratio afLean is the first target lean air-fuel ratio afLean1, for example, 14.7. Thereafter, the CPU proceeds to step 625 and subsequent steps. Accordingly, the air-fuel ratio of the engine is made to coincide with the target lean air-fuel ratio afLean.

  On the other hand, if the value of the catalyst rich state display flag XCCRORich is “0” at the time when the CPU executes the process of step 645, the CPU makes a “No” determination at step 645 to proceed to step 655, The target air-fuel ratio abyfr is set to “predetermined target rich air-fuel ratio afRich”. The target rich air-fuel ratio afRich is a predetermined air-fuel ratio (rich air-fuel ratio) that is smaller than the stoichiometric air-fuel ratio stoich. In this case, the target rich air-fuel ratio afRich is the first target rich air-fuel ratio afRich1, for example, 14.2. Thereafter, the CPU proceeds to step 625 and subsequent steps. Accordingly, the air-fuel ratio of the engine is made to coincide with the target rich air-fuel ratio afRich.

  On the other hand, if the value of the fuel cut flag XFC is “1” at the time when the CPU executes the process of step 605, the CPU makes a “No” determination at step 605 to directly proceed to step 695, and this routine Is temporarily terminated. In this case, since fuel injection by the process of step 640 is not executed, fuel cut control (fuel supply stop control) is executed. That is, the operating state of the engine 10 is a fuel cut operating state.

<Catalyst State (Oxygen Storage State of Catalyst 43) Determination>
The CPU repeatedly executes the “catalyst state determination routine” shown in the flowchart of FIG. 7 every elapse of a predetermined time ts. Therefore, when the predetermined timing is reached, the CPU starts the process from step 700 and proceeds to step 705. From the “current output value Voxs of the downstream air-fuel ratio sensor 56” to the “previous output value of the downstream air-fuel ratio sensor 56”. By subtracting “Voxsold”, a change amount ΔVoxs of the output value Voxs per predetermined time ts (unit time) (also simply referred to as “output change amount ΔVoxs”) is calculated.

  Next, the CPU proceeds to step 710 to store the current output value Voxs as “previous output value Voxsold”. Thus, the previous output value Voxsold of the downstream air-fuel ratio sensor 56 is the output value Voxs at the time point a predetermined time ts before the current time (the output value Voxs when this routine was executed last time).

  Next, the CPU proceeds to step 715, where the output value Voxs is larger than “a value obtained by adding a minute positive value γ to the minimum value Vmin (Vmin + γ)” and “a minute positive value from the maximum value Vmax”. It is determined whether or not it is smaller than a value obtained by subtracting γ (Vmax−γ). That is, the CPU determines whether or not “the state of the catalyst 43 is neither an obvious oxygen excess state (Voxs <Vmin + γ) nor an obvious oxygen deficiency state (Voxs> Vmax−γ)”. The value (Vmin + γ) is very close to the minimum value Vmin and is therefore smaller than the median value Vmid. The value (Vmax−γ) is very close to the maximum value Vmax and is therefore larger than the median value Vmid.

Assume that the following conditions are all satisfied.
(Condition 1) The output value Voxs is between the value (Vmin + γ) and the value (Vmax−γ).
(Condition 2) The target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean.
(Condition 3) The output change amount ΔVoxs is negative (the output value Voxs is decreasing).
(Condition 4) Immediately after the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes equal to or greater than the lean determination threshold value ΔVLeanth.

  In this case, the CPU makes a “Yes” determination at step 715 to proceed to step 720 to determine whether or not the target air-fuel ratio abyfr is the target lean air-fuel ratio afLean. According to the above assumption, the CPU makes a “Yes” determination at step 720 to proceed to step 725 to determine whether or not the output change amount ΔVoxs is negative (less than “0”). That is, the CPU determines in step 725 whether or not the output value Voxs is decreasing.

  Based on the above assumption, the CPU makes a “Yes” determination at step 725 to proceed to step 730, where the magnitude | ΔVoxs | (the absolute value of the output change amount ΔVoxs) of the output change amount ΔVoxs is greater than or equal to the lean determination threshold value ΔVLeanth. It is determined whether or not there is.

  Based on the above assumption, the CPU makes a “Yes” determination at step 730 to proceed to step 735 to set the value of the catalyst rich state display flag XCCRORich to “0”. That is, the CPU determines that the state of the catalyst 43 is “oxygen-excess state (catalyst lean state) or approaching an oxygen-excess state”. Thereafter, the CPU proceeds to step 740.

  In step 740, the CPU determines whether or not the output value Voxs is equal to or less than “a value obtained by adding a minute positive value γ to the minimum value Vmin (Vmin + γ)”. According to the above assumption, the CPU makes a “No” determination at step 740 to proceed to step 745, where the output value Voxs is equal to or greater than “a value obtained by subtracting a minute positive value γ from the maximum value Vmax (Vmax−γ)”. It is determined whether or not. Based on the above assumption, the CPU makes a “No” determination at step 745 to directly proceed to step 795 to end the present routine tentatively.

  As described above, when all of the above conditions 1 to 4 are satisfied, the value of the catalyst rich state display flag XCCRORich is set to “0”. Therefore, the CPU makes a “No” determination at step 645 of FIG. 6 to proceed to step 655, and thus the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich.

  On the other hand, if the condition 2 is not satisfied among the above conditions 1 to 4 (that is, the target air-fuel ratio abyfr is not set to the target lean air-fuel ratio afLean), the CPU makes a “No” determination at step 720. Then, the process proceeds to step 750. As will be described later, in this case, the value of the catalyst rich state display flag XCCRORich is not changed to “0” (see step 735).

  Further, if the condition 3 is not satisfied among the conditions 1 to 4 (that is, the output change amount ΔVoxs is “0” or more, in other words, the output value Voxs is increased), the CPU In Step 725, it is determined as “No”, and the process proceeds to Step 740, Step 745, and Step 795. Therefore, in this case, the value of the catalyst rich state display flag XCCRORich is not changed (maintained to “1”).

  Further, if the condition 4 is not satisfied among the above conditions 1 to 4 (that is, the magnitude of the output change amount ΔVoxs | ΔVoxs | is less than the lean determination threshold value ΔVLeanth), the CPU determines “No And go to Step 740, Step 745, and Step 795. Therefore, also in this case, the value of the catalyst rich state display flag XCCRORich is not changed (maintained to “1”).

Next, it is assumed that the following conditions are all satisfied.
(Condition 1) The output value Voxs is between the value (Vmin + γ) and the value (Vmax−γ).
(Condition 5) The target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich.
(Condition 6) The output change amount ΔVoxs is positive (the output value Voxs is increasing).
(Condition 7) Immediately after the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes equal to or greater than the rich determination threshold value ΔVRichth.

  In this case, the CPU makes a “Yes” determination at step 715 following step 705 and step 710 to proceed to step 720, determines “No” at step 720, and proceeds to step 750. In step 750, the CPU determines whether the target air-fuel ratio abyfr is the target rich air-fuel ratio afRich.

  According to the above assumption, the CPU makes a “Yes” determination at step 750 to proceed to step 755 to determine whether or not the output change amount ΔVoxs is positive (greater than “0”). That is, the CPU determines in step 755 whether or not the output value Voxs has increased.

  Based on the above assumption, the CPU makes a “Yes” determination at step 755 to proceed to step 760 to determine whether or not the magnitude | ΔVoxs | of the output change amount ΔVoxs is equal to or greater than the rich determination threshold value ΔVRichth.

  Based on the above assumption, the CPU makes a “Yes” determination at step 760 to proceed to step 765 to set the value of the catalyst rich state display flag XCCRORich to “1”. That is, the CPU determines that the state of the catalyst 43 is “oxygen-deficient state (catalyst-rich state) or approaching an oxygen-deficient state”. Thereafter, the CPU proceeds to step 740. Further, according to the above assumption, the CPU makes a “No” determination at both steps 740 and 745 to proceed to step 795 to end the present routine tentatively.

  As described above, when all of the condition 1 and the conditions 5 to 7 are satisfied, the value of the catalyst rich state display flag XCCRORich is set to “1”. Accordingly, the CPU makes a “Yes” determination at step 645 of FIG. 6 to proceed to step 650, so that the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean.

  On the other hand, if the condition 5 is not satisfied among the condition 1 and the conditions 5 to 7 (that is, the target air-fuel ratio abyfr is not set to the target rich air-fuel ratio afRich), the CPU “ It is determined as “No”, and the process proceeds to Step 740, Step 745, and Step 795. Therefore, in this case, the value of the catalyst rich state display flag XCCRORich is not changed (maintained to “0”).

  Furthermore, if the condition 6 is not satisfied among the condition 1 and the conditions 5 to 7 (that is, the output change amount ΔVoxs is less than “0”, in other words, the output value Voxs is decreased). The CPU makes a “No” determination at step 755 to proceed to step 740, step 745, and step 795. Therefore, also in this case, the value of the catalyst rich state display flag XCCRORich is not changed (maintained to “0”).

  Further, if the condition 7 is not satisfied among the condition 1 and the conditions 5 to 7 (that is, if the magnitude | ΔVoxs | of the output change amount ΔVoxs is less than the rich determination threshold ΔVRichth), the CPU performs step 760. At step 740, step 745 and step 795 are executed. Therefore, also in this case, the value of the catalyst rich state display flag XCCRORich is not changed (maintained to “0”).

Next, it is assumed that the following conditions are satisfied.
(Condition 8) The output value Voxs is equal to or less than the value (Vmin + γ).

  In this case, the CPU makes a “No” determination at step 715 following step 705 and step 710 to directly proceed to step 740. In this case, the state of the catalyst 43 is clearly an oxygen excess state. Therefore, the CPU makes a “Yes” determination at step 740, proceeds to step 770, sets the catalyst rich state display flag XCCRORich to “0”, proceeds to step 795, and ends this routine once.

  As a result, since the value of the catalyst rich state display flag XCCRORich is set to “0”, the CPU makes a “No” determination at step 645 in FIG. 6 to proceed to step 655, so that the target air-fuel ratio abyfr is the target rich The air-fuel ratio is set to afRich.

Next, it is assumed that the following conditions are satisfied.
(Condition 9) The output value Voxs is greater than or equal to the value (Vmax−γ).

  In this case, the CPU makes a “No” determination at step 715 following step 705 and step 710 to directly proceed to step 740, determines “No” at step 740, and proceeds to step 745. In this case, the state of the catalyst 43 is clearly an oxygen-deficient state. Therefore, the CPU makes a “Yes” determination at step 745, proceeds to step 775, sets the catalyst rich state display flag XCCRORich to “1”, proceeds to step 795, and ends this routine once.

  As a result, since the value of the catalyst rich state display flag XCCRORich is set to “1”, the CPU makes a “Yes” determination at step 645 of FIG. 6 to proceed to step 650, so that the target air-fuel ratio abyfr is the target lean. The air-fuel ratio is set to afLean.

  The CPU can be configured not to execute the routine shown in FIG. 7 when the value of the feedback control flag XFB is “0”. Also, step 715, step 740, step 745, step 770, and step 775 can be omitted.

<Acquisition of maximum value Voxsmax and minimum value Voxsmin>
The CPU repeatedly executes the “maximum value / minimum value acquisition routine” shown in the flowchart of FIG. 8 every elapse of a predetermined time. Therefore, when the predetermined timing comes, the CPU starts the process from step 800 and proceeds to step 810 to determine whether or not the value of the feedback control flag XFB is “1”. At this time, if the value of the feedback control flag XFB is not “1”, the CPU directly proceeds from step 810 to step 895 to end the present routine tentatively.

  On the other hand, if the value of the feedback control flag XFB is “1”, the CPU makes a “Yes” determination at step 810 to proceed to step 820 to determine whether the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich. Determine whether.

  Assume that the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich. In this case, the CPU makes a “Yes” determination at step 820 to proceed to step 830 to determine “whether or not the current time point is immediately after the sign of the output change amount ΔVoxs changes from negative to positive”. Thus, it is determined whether or not the output value Voxs has passed the minimum value. If the output value Voxs does not pass the minimum value, the CPU proceeds directly from step 830 to step 895 to end the present routine tentatively.

  On the other hand, if the current time is “immediately after the output value Voxs passes the minimum value”, the CPU makes a “Yes” determination at step 830 to proceed to step 840 to set the previous output value Voxsold as the minimum value Voxsmin. get. Thereafter, the CPU proceeds to step 895 to end the present routine tentatively.

  On the other hand, when the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean, the CPU makes a “No” determination at step 820 to proceed to step 850, “The current change sign ΔVoxs is correct. Whether or not the output value Voxs has passed the maximum value is determined by determining whether or not it is immediately after changing from negative to negative. If the output value Voxs does not pass the maximum value, the CPU directly proceeds from step 850 to step 895 to end the present routine tentatively.

  On the other hand, if the current time is “immediately after the output value Voxs passes the maximum value”, the CPU makes a “Yes” determination at step 850 to proceed to step 860 to set the previous output value Voxsold as the maximum value Voxsmax. get. Thereafter, the CPU proceeds to step 895 to end the present routine tentatively. As described above, the maximum value Voxsmax and the minimum value Voxsmin are updated.

<Change of lean determination threshold value ΔVLeanth>
The CPU repeatedly executes the “lean determination threshold value changing routine” shown in the flowchart of FIG. 9 every elapse of a predetermined time. Therefore, at a predetermined timing, the CPU starts the process from step 900 and proceeds to step 910 to determine whether or not the current time is “a time immediately after the minimum value Voxsmin is updated”. At this time, if the current time is not “the time immediately after the minimum value Voxsmin is updated”, the CPU makes a “No” determination at step 910 to directly proceed to step 995 to end the present routine tentatively.

  On the other hand, if the current time is “the time immediately after the minimum value Voxsmin is updated”, the CPU makes a “Yes” determination at step 910 to proceed to step 920 to obtain the latest minimum value Voxsmin from the latest maximum value Voxsmax. The reduced value is acquired as the output value difference DV.

  Next, the CPU proceeds to step 930 to determine whether or not the output value difference magnitude DV is greater than or equal to the first threshold output value difference DV1th. As described above, the magnitude DV of the output value difference decreases as the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is better, and increases as it is not better. Therefore, the magnitude DV of the output value difference is a response index value of the output value Voxs of the downstream air-fuel ratio sensor 56. In other words, the CPU substantially determines in step 930 whether or not the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the predetermined responsiveness threshold value.

  At this time, if the magnitude DV of the output value difference is less than the first threshold output value difference DV1th (that is, the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than a predetermined responsiveness threshold). The CPU makes a “No” determination at step 930 to proceed to step 940 to set the lean determination threshold value ΔVLeanth to “the first lean determination threshold value ΔVthL1 that is a predetermined positive value”. Thereafter, the CPU proceeds to step 995 to end the present routine tentatively.

  On the other hand, if the output value difference magnitude DV is equal to or greater than the first threshold output value difference DV1th (ie, if the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than a predetermined responsiveness threshold). ), The CPU makes a “Yes” determination at step 930 to proceed to step 950, and sets the lean determination threshold value ΔVLeanth to “a second predetermined lean determination threshold value ΔVthL2”. Thereafter, the CPU proceeds to step 995 to end the present routine tentatively. The second lean determination threshold value ΔVthL2 is also a value obtained by subtracting a predetermined positive value α from the first lean determination threshold value ΔVthL1, and is a positive value having a magnitude smaller than the magnitude of the first lean determination threshold value ΔVthL1. As described above, the lean determination threshold value ΔVLeanth is changed based on the responsiveness index value (the magnitude of the output value difference DV) indicating the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56.

  As shown in FIG. 11, the CPU increases the output value difference magnitude DV (first output value difference magnitude DV1) (that is, the responsiveness of the output value Voxs decreases). The lean determination threshold value ΔVLeanth may be continuously changed so that the lean determination threshold value ΔVLeanth becomes smaller.

<Change of rich determination threshold value ΔVRichth>
The CPU repeatedly executes the “rich determination threshold change routine” shown in the flowchart of FIG. 10 every elapse of a predetermined time. Accordingly, when the predetermined timing comes, the CPU starts processing from step 1000 and proceeds to step 1010 to determine whether or not the current time is “a time immediately after the maximum value Voxsmax is updated”. At this time, if the current time is not “the time immediately after the maximum value Voxsmax is updated”, the CPU makes a “No” determination at step 1010 to directly proceed to step 1095 to end the present routine tentatively.

  On the other hand, if the current time is “the time immediately after the maximum value Voxsmax is updated”, the CPU makes a “Yes” determination at step 1010 to proceed to step 1020 to calculate the latest minimum value Voxsmin from the latest maximum value Voxsmax. The reduced value is acquired as the output value difference DV.

  Next, the CPU proceeds to step 1030 to determine whether or not the output value difference magnitude DV is equal to or greater than the second threshold output value difference DV2th. As described above, the magnitude DV of the output value difference decreases as the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is better, and increases as it is not better. Therefore, the magnitude DV of the output value difference is a response index value of the output value Voxs of the downstream air-fuel ratio sensor 56. In other words, the CPU substantially determines in step 1030 whether or not the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the predetermined responsiveness threshold value.

  At this time, if the magnitude DV of the output value difference is less than the second threshold output value difference DV2th (that is, the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than a predetermined responsiveness threshold). The CPU makes a “No” determination at step 1030 to proceed to step 1040 to set the rich determination threshold value ΔVRichth to “a first rich determination threshold value ΔVthR1 that is a predetermined positive value”. Thereafter, the CPU proceeds to step 1095 to end the present routine tentatively.

  On the other hand, when the magnitude DV of the output value difference is greater than or equal to the second threshold output value difference DV2th (that is, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the predetermined responsiveness threshold). ), The CPU makes a “Yes” determination at step 1030 to proceed to step 1050, and sets the rich determination threshold value ΔVRichth to “a second rich determination threshold value ΔVthR2 that is a predetermined positive value”. Thereafter, the CPU proceeds to step 1095 to end the present routine tentatively. The second rich determination threshold value ΔVthR2 is also a value obtained by subtracting a predetermined positive value β from the first rich determination threshold value ΔVthR1, and is a positive value having a magnitude smaller than the magnitude of the first rich determination threshold value ΔVthR1.

  As shown in FIG. 12, the CPU increases as the output value difference magnitude DV (second output value difference magnitude DV2) increases (that is, as the responsiveness of the output value Voxs decreases). The rich determination threshold value ΔVRichth may be continuously changed so that the rich determination threshold value ΔVRichth becomes smaller.

As described above, the first control device
When the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean, the air-fuel ratio of the engine is controlled so that the engine air-fuel ratio matches the target lean air-fuel ratio afLean, and the target air-fuel ratio abyfr is the target rich Air-fuel ratio control means (see the routine of FIG. 6) for controlling the air-fuel ratio of the engine so that the air-fuel ratio of the engine matches the target rich air-fuel ratio afRich when the air-fuel ratio is set to afRich;
When the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean and the output value Voxs of the downstream air-fuel ratio sensor 56 is decreased (output change amount ΔVoxs <0), the magnitude of the output change amount ΔVoxs | ΔVoxs When | is equal to or greater than a predetermined lean determination threshold ΔVLeanth, the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich, and the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich and the output value Voxs Is increased (output change amount ΔVoxs> 0), the magnitude of the output change amount ΔVoxs | ΔVoxs | the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean when the value exceeds a predetermined rich determination threshold ΔVRichth Target air-fuel ratio setting means (see the routine of FIG. 7),
A responsiveness index value (output value difference magnitude DV) representing the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 to the change in the air-fuel ratio of the exhaust gas reaching the downstream air-fuel ratio sensor 56 is output as Voxs. Responsiveness index value acquisition means (see the routine of FIG. 8, step 920 of FIG. 9, step 1020 of FIG. 10) acquired based on
Threshold value for changing the lean determination threshold value ΔVLeanth based on the acquired responsiveness index value so that the lean determination threshold value ΔVLeanth becomes smaller as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. Changing means (see step 930 to step 950 in FIG. 9 and FIG. 11);
Is provided.

  Further, the threshold value changing means sets the rich determination threshold value ΔVRichth so that the rich determination threshold value ΔVRichth becomes smaller as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. It changes based on the index value (see step 1030 to step 1050 in FIG. 10 and FIG. 11).

Furthermore, the threshold value changing means of the first control device is:
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold value, and the response of the output value of the downstream air-fuel ratio sensor is determined. When it is determined that the responsiveness is higher than the responsiveness threshold, the lean determination threshold ΔVLeanth is set to the first lean determination threshold ΔVthL1, and the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold. Is determined to be lower, the lean determination threshold value is set to a second lean determination threshold value ΔVthL2 having a size smaller than the first lean determination threshold value (the routine of FIG. 9 is set). reference.).

Furthermore, the threshold value changing means of the first control device is:
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold value, and the response of the output value of the downstream air-fuel ratio sensor is determined. When it is determined that the response is higher than the responsiveness threshold, the rich determination threshold ΔVRichth is set to the first rich determination threshold ΔVthR1, and the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold. 10 is configured to set the rich determination threshold ΔVRichth to a second rich determination threshold ΔVthR2 having a size smaller than the first rich determination threshold (the routine of FIG. 10). See).

  Therefore, even when the output value Voxs of the downstream air-fuel ratio sensor 56 is low, the first control device determines the state of the catalyst 43 without delay and delays the target air-fuel ratio abyfr to an appropriate air-fuel ratio. It can be set without. Therefore, the NOx emission amount and the unburned matter emission amount can be reduced.

  Note that the first control device may change only one of the lean determination threshold value ΔVLeanth and the rich determination threshold value ΔVRichth based on the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56.

Second Embodiment
Next, an air-fuel ratio control apparatus (hereinafter also referred to as “second control apparatus”) according to a second embodiment of the present invention will be described.

  Instead of changing the lean determination threshold value ΔVLeanth and / or the rich determination threshold value ΔVRichth based on the responsiveness index value, the second control device uses the target rich air / fuel ratio afRich and / or the target lean air / fuel ratio afLean based on the responsiveness index value. However, it is different from the first control device only in that it is changed. Hereinafter, this difference will be mainly described.

(Outline of air-fuel ratio feedback control by the second controller)
As shown in FIG. 13, the second control device, like the first control device, has an output change amount ΔVoxs that is negative and an output change amount when the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean. When the magnitude | ΔVoxs | of ΔVoxs becomes equal to or greater than the lean determination threshold ΔVLeanth, the target air-fuel ratio abyfr is changed to the target rich air-fuel ratio afRich.

  The second control device sets the target rich air-fuel ratio afRich to a smaller (richer) air-fuel ratio so that the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good. More specifically, as shown by the broken line and the solid line in FIG. 13, when the output value Voxs of the downstream air-fuel ratio sensor 56 has good response (when the response is higher than the response index value). The second control device sets the target rich air-fuel ratio afRich to the first target rich air-fuel ratio afRich1 as in the first control device.

  On the other hand, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good as shown by the one-dot chain line in FIGS. 13A and 13B (the responsiveness is lower than the responsiveness index value). ), The second control device sets the target rich air-fuel ratio afRich to “the second target rich that is smaller (rich) than the first target rich air-fuel ratio afRich1” as indicated by a two-dot chain line in FIG. Set to “air-fuel ratio afRich2”.

  According to this, even if the timing of setting the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the subsequent catalyst inflow The air-fuel ratio of the gas is “richer (smaller) than“ the air-fuel ratio when the response value of the output value Voxs of the downstream air-fuel ratio sensor 56 is good (the air-fuel ratio corresponding to the first target rich air-fuel ratio afRich1) ”. ) Air-fuel ratio (the air-fuel ratio corresponding to the second target rich air-fuel ratio afRich2) ”. As a result, even if the timing of setting the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the oxygen storage amount of the catalyst 43 Can be quickly reduced. That is, the period in which the state of the catalyst 43 is close to the oxygen excess state can be shortened. As a result, the amount of NOx emission can be reduced.

  Further, as shown in FIG. 14, the second control device, like the first control device, has the output change amount ΔVoxs positive and the output when the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich. When the magnitude | ΔVoxs | of the change amount ΔVoxs becomes equal to or larger than the rich determination threshold ΔVRichth, the target air-fuel ratio abyfr is changed to the target lean air-fuel ratio afLean.

  The second control device sets the target lean air-fuel ratio afLean to a larger (more lean) air-fuel ratio so that the response of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good. More specifically, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is good as shown by the broken line and the solid line in FIG. 14 (when the responsiveness is higher than the responsiveness index value). The second control device sets the target lean air-fuel ratio afLean to the first target lean air-fuel ratio afLean1 as in the first control device.

  On the other hand, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good as shown by the one-dot chain line in FIGS. 14A and 14B (the responsiveness is lower than the responsiveness index value). The second control device sets the target lean air-fuel ratio afLean to “a second target lean that is larger (lean) than the first target lean air-fuel ratio afLean1”, as indicated by a two-dot chain line in FIG. Air-fuel ratio afLean2 ”is set.

  According to this, even if the timing for setting the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the subsequent catalyst inflow The air-fuel ratio of the gas is “lean (larger) than“ the air-fuel ratio when the output value Voxs of the downstream air-fuel ratio sensor 56 is good (the air-fuel ratio corresponding to the first target lean air-fuel ratio afLean1) ”. ) Air-fuel ratio (the air-fuel ratio corresponding to the second target lean air-fuel ratio afLean2) ". As a result, even if the timing of setting the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the oxygen storage amount of the catalyst 43 Can be quickly increased. That is, the period when the state of the catalyst 43 is close to the oxygen-deficient state can be shortened. As a result, the amount of unburned material discharged can be reduced.

(Operation)
Next, the actual operation of the second control device will be described. Similar to the CPU of the first control device, the CPU of the second control device executes the routines shown in FIGS. Further, the CPU of the second control device executes a “target rich air-fuel ratio / target lean air-fuel ratio setting routine” shown in FIG. Hereinafter, the routine shown in FIG. 15 will be described. In the second control device, the rich determination threshold value ΔVRichth is set (fixed) to the first rich determination threshold value ΔVRichth1, and the lean determination threshold value ΔVLeanth is set (fixed) to the first lean determination threshold value ΔVLeanth1.

<Setting of target rich air-fuel ratio afRich and target lean air-fuel ratio afLean>
The CPU repeatedly executes the routine shown by the flowchart in FIG. 15 every elapse of a predetermined time. Therefore, at the predetermined timing, the CPU starts processing from step 1500 and proceeds to step 1505, where the value of the post-setting completion flag XafCNG (hereinafter referred to as “post-completion flag XafCNG”) is “0”. It is determined whether or not.

  The post-completion flag XafCNG is set to “0” in the above-described initial routine. Further, the value of the post-completion flag XafCNG is set to “1” in step 1545 described later.

  Now, it is assumed that the value of the post-completion flag XafCNG is “0” after the current operation of the engine 10 is started. In this case, the CPU makes a “Yes” determination at step 1505 to proceed to step 1510 to set the target rich air-fuel ratio afRich to the first target rich air-fuel ratio afRich1 (eg, 14.2). Next, the CPU proceeds to step 1515 to set the target lean air-fuel ratio afLean to the first target lean air-fuel ratio afLean1 (for example, 14.7).

  Next, the CPU proceeds to step 1520 to determine whether both the maximum value Voxsmax and the minimum value Voxsmin have been acquired by the routine of FIG. 8 when the value of the feedback control flag XFB is “1” after the engine 10 is started this time. judge. At this time, if at least one of the maximum value Voxsmax and the minimum value Voxsmin has not been acquired, the CPU makes a “No” determination at step 1520 to directly proceed to step 1595 to end the present routine tentatively.

  On the other hand, if both the local maximum value Voxsmax and the local minimum value Voxsmin are acquired, the CPU makes a “Yes” determination at step 1520 to proceed to step 1525 where the output value obtained by subtracting the local minimum value Voxsmin from the local maximum value Voxsmax. The difference DV (responsiveness index value) is acquired.

  Next, the CPU proceeds to step 1530 to determine whether or not the output value difference magnitude DV is larger than the threshold output value difference DVth. As described above, the magnitude DV of the output value difference decreases as the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is better, and increases as it is not better. Therefore, in step 1530, the CPU substantially determines whether or not the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than a predetermined responsiveness threshold value.

  At this time, if the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is good, and therefore the magnitude DV of the output value difference is equal to or less than the threshold output value difference DVth, the CPU determines “No” in step 1530. The process proceeds to step 1535, where the target rich air-fuel ratio afRich is confirmed and set to the first target rich air-fuel ratio afRich1. Further, the CPU proceeds to step 1540 to confirmly set the target lean air-fuel ratio afLean to the first target lean air-fuel ratio afLean1.

  Next, the CPU proceeds to step 1545 to set the value of the post-completion flag XafCNG to “1”, and proceeds to step 1595 to end the present routine tentatively. As a result, if it is determined that the response value of the output value Voxs of the downstream air-fuel ratio sensor 56 is good (higher than the response threshold value) after the current operation of the engine 10 is started, the target rich air-fuel ratio afRich is The first target rich air-fuel ratio afRich1 continues to be set, and the target lean air-fuel ratio afLean continues to be set to the first target lean air-fuel ratio afLean1.

  On the other hand, at the time when the CPU executes the processing of step 1530, the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good, and therefore the magnitude DV of the output value difference is greater than the threshold output value difference DVth. If YES, the CPU makes a “Yes” determination at step 1530 to proceed to step 1550 to set the target rich air-fuel ratio afRich to “the second target rich air-fuel ratio smaller (richer) than the first target rich air-fuel ratio afRich1. afRich2 ”. The second target rich air-fuel ratio afRich2 is also a value obtained by subtracting the positive value dafr from the first target rich air-fuel ratio afRich1, and is 13.9, for example.

  Note that, as shown in FIG. 16, the CPU increases the target value as the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 deteriorates (ie, as the output value difference DV increases). The rich air-fuel ratio afRich may be continuously reduced.

  Next, the CPU proceeds to step 1555 to set the target lean air-fuel ratio afLean to “a second target lean air-fuel ratio afLean2 that is larger (more lean) than the first target lean air-fuel ratio afLean1”. The second target lean air-fuel ratio afLean2 is a value obtained by adding a positive value dafl to the first target lean air-fuel ratio afLean1, for example, 15.0.

  Note that, as shown in FIG. 16, the CPU increases the target value as the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 deteriorates (ie, as the output value difference DV increases). The lean air-fuel ratio afLean may be continuously increased.

  Next, the CPU sets the value of the post-completion flag XafCNG to “1” in step 1545, proceeds to step 1595, and once ends this routine. As a result, if it is determined that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good (lower than the responsiveness threshold) after the start of the current operation of the engine 10, the target rich air-fuel ratio afRich is The target rich air-fuel ratio afRich1 is set, and the target lean air-fuel ratio afLean is set to the second target lean air-fuel ratio afLean2.

  When the value of the post-completion flag XafCNG is set to “1” by the process of step 1545, the CPU determines “No” at the time when the CPU executes the process of step 1505, and step 1595. Go directly to, and end this routine once. Therefore, the change (setting) of the target rich air-fuel ratio afRich and the target lean air-fuel ratio afLean is executed only once after the engine 10 is started until the operation is completed. In other words, the output value difference magnitude DV, which is a responsiveness index value, is obtained when the target rich air-fuel ratio afRich is the first target rich air-fuel ratio afRich1 and the target lean air-fuel ratio afLean is the first target lean air-fuel ratio afLean. Get as long as.

As described above, the second control device includes the same air-fuel ratio control means as the first control device. Furthermore, the second control device
When the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean and the output value Voxs of the downstream air-fuel ratio sensor 56 is decreased (output change amount ΔVoxs <0), the magnitude of the output change amount ΔVoxs | ΔVoxs | When the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich and the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich, the output value Voxs increases. If the output change amount ΔVoxs> 0, the magnitude of the output change amount ΔVoxs | ΔVoxs | the target air-fuel ratio setting means for setting the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean when the value becomes equal to or greater than the predetermined rich determination threshold ΔVRichth (See routine in FIG. 7);
A responsiveness index value indicating the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 to the change in the air-fuel ratio of the exhaust gas reaching the output value Voxs of the downstream air-fuel ratio sensor 56 (the magnitude of the output value difference DV ) Based on the output value Voxs (refer to the routine of FIG. 8 and steps 1520 and 1525 of FIG. 15);
Is provided.

In addition, the target air-fuel ratio setting means of the second controller is
The target rich air-fuel ratio afRich is acquired so that the target rich air-fuel ratio afRich becomes smaller as the responsiveness of the output value Voxs of the downstream side air-fuel ratio sensor 56 becomes less favorable (the smaller the output value difference DV). The response index value (the magnitude DV of the output value difference DV) is changed (see Step 1530, Step 1535, Step 1550 and FIG. 16 in FIG. 15).

More specifically, the target air-fuel ratio setting means is
It is determined based on the acquired responsiveness index value (output value difference magnitude DV) whether or not the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than a predetermined responsiveness threshold value ( Step 1530 in FIG.
When it is determined that the responsiveness of the output value Voxs of the downstream side air-fuel ratio sensor 56 is higher than the responsiveness threshold, the target rich air-fuel ratio afRich is set to a predetermined first target rich air-fuel ratio afRich1 (FIG. 15 steps 1535),
When it is determined that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold, the target rich air-fuel ratio afRich is set to a second target that is smaller than the first target rich air-fuel ratio afRich. The rich air-fuel ratio is set to afRich2 (step 1550 in FIG. 15).

  According to this, even if the timing of setting the target air-fuel ratio abyfr to the target rich air-fuel ratio afRich is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the subsequent catalyst inflow The air-fuel ratio of the gas can be set to “a richer (smaller) air-fuel ratio corresponding to the second target rich air-fuel ratio afRich2”. As a result, the oxygen storage amount of the catalyst 43 can be quickly reduced, so that the period in which the state of the catalyst 43 is close to the oxygen excess state can be shortened. As a result, the amount of NOx emission can be reduced.

Further, the target air-fuel ratio setting means is
The target lean air-fuel ratio afLean is acquired so that the target lean air-fuel ratio afLean increases as the response of the output value Voxs of the downstream side air-fuel ratio sensor 56 becomes less favorable (the smaller the output value difference DV). The response index value (the magnitude DV of the output value difference DV) is changed (see step 1530, step 1540, step 1555 and FIG. 16 in FIG. 15).

More specifically, the target air-fuel ratio setting means is
It is determined based on the acquired responsiveness index value (output value difference magnitude DV) whether or not the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than a predetermined responsiveness threshold value ( Step 1530 in FIG.
When it is determined that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than the responsiveness threshold, the target lean air-fuel ratio afLean is set to a predetermined first target lean air-fuel ratio afLean1 (FIG. 15 steps 1540),
When it is determined that the responsiveness of the output value Voxs of the downstream side air-fuel ratio sensor 56 is lower than the responsiveness threshold, the target lean air-fuel ratio afLean is set to a second target that is smaller than the first target lean air-fuel ratio afLean. The lean air-fuel ratio is set to afLean2 (step 1555 in FIG. 15).

  According to this, even if the timing for setting the target air-fuel ratio abyfr to the target lean air-fuel ratio afLean is delayed due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56, the subsequent catalyst inflow The air-fuel ratio of the gas can be set to “a leaner (larger) air-fuel ratio corresponding to the second target lean air-fuel ratio afLean2.” As a result, the oxygen storage amount of the catalyst 43 can be quickly increased, so that the period in which the state of the catalyst 43 is close to an oxygen-deficient state can be shortened. As a result, the amount of unburned material discharged can be reduced.

  The second control device always maintains the target rich air-fuel ratio afRich at the first target rich air-fuel ratio afRich1 regardless of the responsiveness index value, and changes only the target lean air-fuel ratio afLean according to the responsiveness index value. Also good. Further, the second control device always maintains the target lean air-fuel ratio afLean at the first target lean air-fuel ratio afLean1 regardless of the responsiveness index value, and changes only the target rich air-fuel ratio afRich according to the responsiveness index value. Also good.

<Third Embodiment>
Next, an air-fuel ratio control apparatus (hereinafter also referred to as “third control apparatus”) according to a third embodiment of the present invention will be described.

  Instead of changing the lean determination threshold value ΔVLeanth and / or the rich determination threshold value ΔVRichth based on the responsiveness index value, the third control device does not change the target rich if the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good. The air-fuel ratio afRich is controlled in a manner different from that of the second control device. Hereinafter, the difference between the second control device and the third control device will be mainly described.

  As shown by a solid line in FIG. 17, the third control device determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than the responsiveness threshold (that is, the output value difference). When the magnitude DV is smaller than the threshold output value difference DVth), the value of the catalyst rich state display flag XCCRORich changes from “1” to “0” (see time t1 in FIG. 17). abyfr is set to the first target rich air-fuel ratio afRich1 (for example, 14.2). As long as the value of the catalyst rich state display flag XCCRORich is “0”, the target air-fuel ratio abyfr is maintained at the first target rich air-fuel ratio afRich1. When the value of the catalyst rich state display flag XCCRORich is “1”, the target air-fuel ratio abyfr is set to the first target lean air-fuel ratio afLean1 (for example, 14.7).

  Furthermore, as shown by the two-dot chain line in FIG. 17, the third control device determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold (that is, When the output value difference magnitude DV is equal to or greater than the threshold output value difference DVth), the value of the catalyst rich state display flag XCCRORich changes from “1” to “0” (see time t1 in FIG. 17). First, the target air-fuel ratio abyfr is set to “the second target rich air-fuel ratio afRich2 (for example, 13.9) smaller than the first target rich air-fuel ratio afRich1”.

  In addition, the third control device actually outputs the output change amount ΔVoxs at time t2 when a predetermined time has elapsed from the time when the value of the catalyst rich state display flag XCCRORich changes from “1” to “0”. (See p1 in FIG. 17), the target air-fuel ratio abyfr is set to “the third target rich air-fuel ratio afRich3 that is a rich air-fuel ratio larger than the second target rich air-fuel ratio afRich2”. Set. The third target rich air-fuel ratio afRich3 may be an air-fuel ratio between the second target rich air-fuel ratio afRich2 and the stoichiometric air-fuel ratio stoich (for example, 14.1). Therefore, the third target rich air-fuel ratio afRich3 may be the same as or different from the first target rich air-fuel ratio afRich1.

  Note that, as shown in FIG. 18, the third control device (modified example of the third control device) determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold. When the value of the catalyst rich state display flag XCCRORich changes from “1” to “0” (that is, when the output value difference magnitude DV is equal to or greater than the threshold output value difference DVth) (time in FIG. 18). First, the target air-fuel ratio abyfr is set to “the second target rich air-fuel ratio afRich2 (eg, 13.9) smaller than the first target rich air-fuel ratio afRich”.

  Further, a modified example of the third control device is a time t2 ′ at which a predetermined time has elapsed from the time (time t1) when the value of the catalyst rich state display flag XCCRORich changes from “1” to “0” (actually). At the time when the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes the predetermined value ΔVthLc after the output change amount ΔVoxs passes the minimum value (p1) (see p2 in FIG. 18)). The fuel ratio abyfr is set to “a third target rich air-fuel ratio afRich3 that is a rich air-fuel ratio larger than the second target rich air-fuel ratio afRich2.”

  According to the third control device and the modification of the third control device, the target air-fuel ratio abyfr is set to the target rich air-fuel ratio afRich due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56. When the timing to perform is delayed, the target air-fuel ratio abyfr is initially set to “second target rich air-fuel ratio afRich2”, and then set to “third target rich air-fuel ratio afRich3”. Therefore, even if the timing of changing from the target lean air-fuel ratio afLean to the target rich air-fuel ratio afRich is delayed and the state of the catalyst 43 approaches an oxygen-excess state, a gas with a high richness can be caused to flow into the catalyst 43. Therefore, the state of the catalyst 43 can be immediately moved away from the oxygen excess state. Furthermore, once the target air-fuel ratio abyfr is set to the third target rich air-fuel ratio afRich3 once it has moved away from the oxygen excess state, conversely, excess unburned material flows into the catalyst 43 (that is, the catalyst 43 Can be prevented from approaching an oxygen-deficient state at once. Therefore, it is possible to avoid an increase in the amount of unburned material discharged.

  Note that the third controller separately acquires the maximum oxygen storage amount Cmax (the degree of catalyst deterioration) of the catalyst 43 based on a well-known method, and the maximum oxygen storage amount Cmax is equal to or greater than the threshold maximum oxygen storage amount Cmaxth. Is set to the target rich air-fuel ratio afRich as in the modification of the third control device, and when the maximum oxygen storage amount Cmax is less than the threshold maximum oxygen storage amount Cmaxth, the target rich air-fuel ratio afRich is set as in the third control device. May be set. According to this, when the maximum oxygen storage amount Cmax is small, it is possible to avoid an excessive unburned material from flowing into the catalyst 43.

<Fourth embodiment>
Next, an air-fuel ratio control apparatus (hereinafter also referred to as “fourth control apparatus”) according to a fourth embodiment of the present invention will be described.

  Instead of changing the lean determination threshold value ΔVLeanth and / or the rich determination threshold value ΔVRichth based on the responsiveness index value, the fourth control device, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is not good, The air-fuel ratio afLean is controlled in a manner different from that of the second control device. Hereinafter, the difference between the second control device and the fourth control device will be mainly described.

  As shown by a solid line in FIG. 19, the fourth control device determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is higher than the responsiveness threshold (that is, the output value difference). When the magnitude DV is smaller than the threshold output value difference DVth), the value of the catalyst rich state display flag XCCRORich changes from “0” to “1” (see time t1 in FIG. 19). abyfr is set to the first target lean air-fuel ratio afLean1 (for example, 14.7). As long as the value of the catalyst rich state display flag XCCRORich is “1”, the target air-fuel ratio abyfr is maintained at the first target lean air-fuel ratio afLean1. When the value of the catalyst rich state display flag XCCRORich is “0”, the target air-fuel ratio abyfr is set to the first target rich air-fuel ratio afRich1 (for example, 14.2).

  Furthermore, as shown by the two-dot chain line in FIG. 19, the fourth control device determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold (that is, When the value of the output value difference DV is equal to or greater than the threshold output value difference DVth), the value of the catalyst rich state display flag XCCRORich changes from “0” to “1” (see time t1 in FIG. 19). First, the target air-fuel ratio abyfr is set to “the second target lean air-fuel ratio afLean2 (for example, 15.1) larger than the first target lean air-fuel ratio afLean1”.

  In addition, the fourth control device actually outputs the output change amount ΔVoxs at a time t2 when a predetermined time has elapsed since the value of the catalyst rich state display flag XCCRORich changed from “0” to “1”. (See p1 in FIG. 19), the target air-fuel ratio abyfr is set to “a third target lean air-fuel ratio afLean3 that is a lean air-fuel ratio smaller than the second target lean air-fuel ratio afLean2.” Set. The third target lean air-fuel ratio afLean3 may be an air-fuel ratio between the second target lean air-fuel ratio afLean2 and the stoichiometric air-fuel ratio stoich (for example, 14.8). Therefore, the third target lean air-fuel ratio afLean3 may be the same as or different from the first target lean air-fuel ratio afLean1.

  Note that, as shown in FIG. 20, the fourth control device (modified example of the fourth control device) determines that the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold. When the value of the catalyst rich state display flag XCCRORich changes from “0” to “1” (that is, when the output value difference magnitude DV is equal to or greater than the threshold output value difference DVth) (time in FIG. 20) First, the target air-fuel ratio abyfr is set to “the second target lean air-fuel ratio afLean2 (eg, 15.1) smaller than the first target lean air-fuel ratio afLean”).

  Further, a modified example of the fourth control device is a time t2 ′ at which a predetermined time has passed since the value of the catalyst rich state display flag XCCRORich changed from “0” to “1” (time t1) (actual When the output change amount ΔVoxs passes through the maximum value (p1) and the magnitude | ΔVoxs | of the output change amount ΔVoxs becomes a predetermined value ΔVthRc (see p2 in FIG. 20), the target air-fuel ratio abyfr is set. The third target lean air-fuel ratio afLean3, which is a lean air-fuel ratio smaller than the second target lean air-fuel ratio afLean2, is set.

  According to the fourth control device and the modification of the fourth control device, the target air-fuel ratio abyfr is set to the target lean air-fuel ratio afLean due to the poor response of the output value Voxs of the downstream air-fuel ratio sensor 56. When the timing to perform is delayed, the target air-fuel ratio abyfr is initially set to “second target lean air-fuel ratio afLean2”, and then set to “third target lean air-fuel ratio afLean3”. Therefore, even if the timing of changing from the target rich air-fuel ratio afRich to the target lean air-fuel ratio afLean is delayed and the state of the catalyst 43 approaches an oxygen-deficient state, a gas with a large lean degree can be caused to flow into the catalyst 43. Therefore, the state of the catalyst 43 can be immediately moved away from the oxygen-deficient state. Further, once the target air-fuel ratio abyfr is set to the third target lean air-fuel ratio afLean3 after having moved away from the oxygen-deficient state, conversely, excess oxygen flows into the catalyst 43 (that is, the state of the catalyst 43 ) Can be avoided. Therefore, it is possible to avoid an increase in the NOx emission amount.

  The fourth control device separately acquires the maximum oxygen storage amount Cmax (the degree of catalyst deterioration) of the catalyst 43 based on a known method, and the maximum oxygen storage amount Cmax is equal to or greater than the threshold maximum oxygen storage amount Cmaxth. Is set to the target lean air-fuel ratio afLean as in the modification of the fourth control device, and when the maximum oxygen storage amount Cmax is less than the threshold maximum oxygen storage amount Cmaxth, the target lean air-fuel ratio afLean is set as in the fourth control device. May be set. According to this, when the maximum oxygen storage amount Cmax is small, excess oxygen and NOx can be prevented from flowing into the catalyst 43.

  The fourth control device can be used in combination with the third control device and / or a modification of the third control device. Similarly, the modification example of the fourth control device can be used in combination with the modification example of the third control device and / or the third control device.

  As described above, the air-fuel ratio control apparatus according to each embodiment of the present invention reduces the amount of NOx and / or unburned matter regardless of the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56. can do.

  The present invention is not limited to the above embodiment, and various modifications can be employed within the scope of the present invention.

  For example, when the responsiveness of the output value Voxs of the downstream side air-fuel ratio sensor 56 is lower than the responsiveness threshold, the target rich air-fuel ratio afRich may be changed as shown in FIG. That is, as indicated by the one-dot chain line in FIG. 21, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold, the value of the catalyst rich state display flag XCCRORich is changed from “1” to “0”. The target rich air-fuel ratio afRich may be set to the second target rich air-fuel ratio afRich2 immediately after the change to "," and then gradually increased after a predetermined time has elapsed. Note that when the target rich air-fuel ratio afRich is gradually increased to reach “a value smaller than the stoichiometric air-fuel ratio stoich by a certain value (fourth target rich air-fuel ratio afRich4)”, the value of the catalyst rich state display flag XCCRORich is If the value does not change from “0” to “1”, the target air-fuel ratio abyfr is maintained at the fourth target rich air-fuel ratio afRich4.

  Alternatively, as indicated by a two-dot chain line in FIG. 21, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold, the value of the catalyst rich state display flag XCCRORich is changed from “1” to “1”. The target rich air-fuel ratio afRich may be set to the second target rich air-fuel ratio afRich2 immediately after changing to “0” and then gradually increased. Also in this case, when the target rich air-fuel ratio afRich is gradually increased to reach “a value smaller than the stoichiometric air-fuel ratio stoich by a certain value (fourth target rich air-fuel ratio afRich4)”, the catalyst rich state display flag XCCRORich Is not changed from “0” to “1”, the target air-fuel ratio abyfr is maintained at the fourth target rich air-fuel ratio afRich4.

  Similarly, when the responsiveness of the output value Voxs of the downstream side air-fuel ratio sensor 56 is lower than the responsiveness threshold, the target rich air-fuel ratio afRich may be changed as shown in FIG. That is, as indicated by the one-dot chain line in FIG. 22, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold, the value of the catalyst rich state display flag XCCRORich is changed from “0” to “1”. The target lean air-fuel ratio afLean may be set to the second target lean air-fuel ratio afLean2 immediately after the change to “,” and thereafter gradually decreased after a predetermined time has elapsed. Note that when the target lean air-fuel ratio afLean is gradually decreased and reaches a value that is larger than the stoichiometric air-fuel ratio stoich by a certain value (fourth target lean air-fuel ratio afLean4), the value of the catalyst rich state display flag XCCRORich is If it has not changed from “1” to “0”, the target air-fuel ratio abyfr is maintained at the fourth target lean air-fuel ratio afLean4.

  Alternatively, as indicated by a two-dot chain line in FIG. 22, when the responsiveness of the output value Voxs of the downstream air-fuel ratio sensor 56 is lower than the responsiveness threshold, the value of the catalyst rich state display flag XCCRORich is changed from “0” to “ Immediately after changing to “1”, the target rich air-fuel ratio afRich may be set to the second target lean air-fuel ratio afLean2, and thereafter may be gradually decreased. Even in this case, the value of the catalyst rich state display flag XCCRORich is reached when the target lean air-fuel ratio afLean is gradually decreased and reaches a value larger than the stoichiometric air-fuel ratio stoich by a certain value (fourth target lean air-fuel ratio afLean4). Is not changed from “1” to “0”, the target air-fuel ratio abyfr is maintained at the fourth target lean air-fuel ratio afLean4.

  Further, each control device stops the air-fuel ratio feedback control based on the output value Voxs of the downstream air-fuel ratio sensor 56 in order to promote engine warm-up, prevent overheating of the catalyst, etc., and set the target air-fuel ratio abyfr. Control for maintaining a constant target rich air-fuel ratio (increase control) may be executed. In this case, after the completion of such increase control, each control device sets the target air-fuel ratio abyfr to a constant target lean air-fuel ratio afLean, and the output value Voxs of the downstream air-fuel ratio sensor 56 is predetermined from the maximum value Vmax. May be acquired as the response index value of the output value Voxs of the downstream air-fuel ratio sensor 56 (eg, the median value Vmid or half of the median value Vmid). This time becomes shorter as the responsiveness of the output value of the downstream air-fuel ratio sensor becomes higher, and becomes longer as the response value becomes lower.

  Further, each control device sets the target air-fuel ratio abyfr to a constant target rich air-fuel ratio afRich after completion of the fuel cut operation for stopping fuel injection, and the output value Voxs of the downstream air-fuel ratio sensor 56 is predetermined from the minimum value Vmin. May be acquired as the responsiveness index value of the output value Voxs of the downstream air-fuel ratio sensor 56 (e.g., the median value Vmid or 2/3 of the maximum value Vmax). This time becomes shorter as the responsiveness of the output value of the downstream air-fuel ratio sensor becomes higher, and becomes longer as the response value becomes lower.

  Further, each control device estimates the warm-up state (temperature) of the output value Voxs of the downstream air-fuel ratio sensor 56 from the exhaust gas temperature, corrects the responsiveness index value based on the estimated sensor temperature, and is corrected. The “lean determination threshold value ΔVLeanth, rich determination threshold value ΔVRichth, second target rich air-fuel ratio afRich, second target lean air-fuel ratio afLean, etc.” may be changed based on the responsiveness index value.

  In addition, the third control device increases the time during which the target air-fuel ratio abyfr is maintained at the second target rich air-fuel ratio afRich2 as the maximum oxygen storage amount Cmax of the catalyst 43 separately obtained by a well-known method increases, or You may set so that it may become long, so that the temperature of the catalyst 43 estimated based on exhaust gas temperature etc. by the well-known method becomes high.

  Similarly, the fourth control device increases the time during which the target air-fuel ratio abyfr is maintained at the second target lean air-fuel ratio afLean2 as the maximum oxygen storage amount Cmax of the catalyst 43 increases or the temperature of the catalyst 43 increases. You may set so that.

Furthermore, each air-fuel ratio control device
Regardless of the output value Voxs of the downstream air-fuel ratio sensor 56, the target air-fuel ratio is maintained at a constant target rich air-fuel ratio, and after confirming that the output value Voxs has reached the maximum value Vmax, the target air-fuel ratio is set to the target value. When changing from a rich air-fuel ratio to a constant target lean air-fuel ratio, an intermediate air-fuel ratio between the target rich air-fuel ratio and the target lean air-fuel ratio (or an air-fuel ratio obtained by dividing the air-fuel ratio by a predetermined ratio) May be acquired as the responsiveness index value. The time until reaching a value corresponding to (for example, the median value Vmid) or a value corresponding to the target lean air-fuel ratio may be obtained. This time becomes shorter as the responsiveness of the output value of the downstream air-fuel ratio sensor becomes higher, and becomes longer as the response value becomes lower.

Similarly, each air-fuel ratio control device
Regardless of the output value Voxs of the downstream side air-fuel ratio sensor 56, the target air-fuel ratio is maintained at a constant target lean air-fuel ratio, and after confirming that the output value Voxs has reached the minimum value Vmin, the target air-fuel ratio is set to the target value. When changing from a lean air-fuel ratio to a constant target rich air-fuel ratio, an intermediate air-fuel ratio between the target lean air-fuel ratio and the target rich air-fuel ratio (or an air-fuel ratio obtained by internally dividing them by a predetermined ratio, ) (For example, the median value Vmid) or a time required to reach a value corresponding to the target rich air-fuel ratio may be acquired as the responsiveness index value. This time becomes shorter as the responsiveness of the output value of the downstream air-fuel ratio sensor becomes higher, and becomes longer as the response value becomes lower.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 22 ... Intake port, 23 ... Exhaust port, 25 ... Fuel injection valve, 41 ... Exhaust manifold, 41b ... Collecting part (exhaust collecting part), 42 ... Exhaust pipe, 43 ... Upstream catalyst (catalyst), 44 ... downstream catalyst, 55 ... upstream air-fuel ratio sensor, 56 ... downstream air-fuel ratio sensor, 60 ... electric control device.

Claims (12)

A three-way catalyst disposed in the exhaust passage of the internal combustion engine;
A downstream air-fuel ratio sensor that is a concentration cell type oxygen concentration sensor disposed downstream of the catalyst in the exhaust passage;
When the target air-fuel ratio of the engine that is the air-fuel ratio of the air-fuel mixture supplied to the engine is set to a predetermined target lean air-fuel ratio that is an air-fuel ratio larger than the stoichiometric air-fuel ratio, the air-fuel ratio of the engine is the same. When the air-fuel ratio of the engine is controlled so as to match the target lean air-fuel ratio, and the target air-fuel ratio is set to a predetermined target rich air-fuel ratio that is an air-fuel ratio smaller than the stoichiometric air-fuel ratio, the air-fuel ratio of the engine Air-fuel ratio control means for controlling the air-fuel ratio of the engine so that the fuel ratio matches the target rich air-fuel ratio;
When the target air-fuel ratio is set to the target lean air-fuel ratio and the output value of the downstream air-fuel ratio sensor is decreasing, the magnitude of the output change amount that is the change amount per predetermined time of the output value is When the target air-fuel ratio is set to the target rich air-fuel ratio when the predetermined lean determination threshold value is exceeded, and the output value increases when the target air-fuel ratio is set to the target rich air-fuel ratio A target air-fuel ratio setting means for setting the target air-fuel ratio to the target lean air-fuel ratio when the magnitude of the output change amount is equal to or greater than a predetermined rich determination threshold value;
An air-fuel ratio control apparatus for an internal combustion engine comprising:
Responsiveness index value acquisition means for acquiring a responsiveness index value representing the responsiveness of the output value of the downstream side air-fuel ratio sensor to the change in the air-fuel ratio of exhaust gas reaching the downstream side air-fuel ratio sensor, based on the output value; ,
Threshold change means for changing the lean determination threshold based on the acquired responsiveness index value so that the lean determination threshold becomes smaller as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. When,
An air-fuel ratio control device. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The threshold value changing means is
The rich determination threshold value is changed based on the acquired responsiveness index value so that the rich determination threshold value becomes smaller as the responsiveness of the output value of the downstream air-fuel ratio sensor is not good. Air-fuel ratio control device. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2,
The threshold value changing means is
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold value, and the response of the output value of the downstream air-fuel ratio sensor is determined. When it is determined that the responsiveness is higher than the responsiveness threshold, the lean determination threshold is set to the first lean determination threshold, and the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold. An air-fuel ratio control device configured to set the lean determination threshold value to a second lean determination threshold value having a magnitude smaller than the first lean determination threshold value. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2,
The threshold value changing means is
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold value, and the response of the output value of the downstream air-fuel ratio sensor is determined. When the response is determined to be higher than the responsiveness threshold, the rich determination threshold is set to the first rich determination threshold, and the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold. The air-fuel ratio control apparatus is configured to set the rich determination threshold value to a second rich determination threshold value that is smaller than the first rich determination threshold value. A three-way catalyst disposed in the exhaust passage of the internal combustion engine;
A downstream air-fuel ratio sensor that is a concentration cell type oxygen concentration sensor disposed downstream of the catalyst in the exhaust passage;
When the target air-fuel ratio of the engine that is the air-fuel ratio of the air-fuel mixture supplied to the engine is set to a predetermined target lean air-fuel ratio that is an air-fuel ratio larger than the stoichiometric air-fuel ratio, the air-fuel ratio of the engine is the same. When the air-fuel ratio of the engine is controlled so as to match the target lean air-fuel ratio, and the target air-fuel ratio is set to a predetermined target rich air-fuel ratio that is an air-fuel ratio smaller than the stoichiometric air-fuel ratio, the air-fuel ratio of the engine Air-fuel ratio control means for controlling the air-fuel ratio of the engine so that the fuel ratio matches the target rich air-fuel ratio;
When the target air-fuel ratio is set to the target lean air-fuel ratio and the output value of the downstream air-fuel ratio sensor is decreasing, the magnitude of the output change amount that is the change amount per predetermined time of the output value is When the target air-fuel ratio is set to the target rich air-fuel ratio when the predetermined lean determination threshold value is exceeded, and the output value increases when the target air-fuel ratio is set to the target rich air-fuel ratio A target air-fuel ratio setting means for setting the target air-fuel ratio to the target lean air-fuel ratio when the magnitude of the output change amount is equal to or greater than a predetermined rich determination threshold value;
An air-fuel ratio control apparatus for an internal combustion engine comprising:
Responsiveness index value acquisition means for acquiring a responsiveness index value representing the responsiveness of the output value of the downstream side air-fuel ratio sensor to the change in the air-fuel ratio of exhaust gas reaching the downstream side air-fuel ratio sensor based on the output value Prepared,
The target air-fuel ratio setting means is
The target rich air-fuel ratio is changed based on the acquired responsiveness index value so that the target rich air-fuel ratio becomes smaller as the responsiveness of the output value of the downstream side air-fuel ratio sensor is not good. Air-fuel ratio control device. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5,
The target air-fuel ratio setting means is
The target lean air-fuel ratio is configured to be changed based on the acquired responsiveness index value so that the target lean air-fuel ratio increases as the responsiveness of the output value of the downstream side air-fuel ratio sensor is not good. Air-fuel ratio control device. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5,
The target air-fuel ratio setting means is
It is determined whether or not the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than a predetermined responsiveness threshold based on the acquired responsiveness index value,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the target rich air-fuel ratio is set to a predetermined first target rich air-fuel ratio,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the target rich air-fuel ratio is changed to a second target rich air-fuel ratio that is smaller than the first target rich air-fuel ratio. An air-fuel ratio control device configured to set. The air-fuel ratio control apparatus for an internal combustion engine according to claim 7,
The target air-fuel ratio setting means is
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, when the target air-fuel ratio is switched from the target lean air-fuel ratio to the target rich air-fuel ratio, the target The air-fuel ratio is set to the second target rich air-fuel ratio, and after a predetermined time has elapsed, the target air-fuel ratio is larger than the second target rich air-fuel ratio and smaller than the stoichiometric air-fuel ratio. An air-fuel ratio control device configured to set an air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 6,
The target air-fuel ratio setting means is
It is determined based on the responsiveness index value whether the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the predetermined responsiveness threshold,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is higher than the responsiveness threshold, the target lean air-fuel ratio is set to a predetermined first target lean air-fuel ratio,
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, the target rich air-fuel ratio is changed to a second target lean air-fuel ratio that is larger than the first target lean air-fuel ratio. An air-fuel ratio control device configured to set. The air-fuel ratio control apparatus for an internal combustion engine according to claim 9,
The target air-fuel ratio setting means is
When it is determined that the responsiveness of the output value of the downstream air-fuel ratio sensor is lower than the responsiveness threshold, when the target air-fuel ratio is switched from the target rich air-fuel ratio to the target lean air-fuel ratio, the target The air-fuel ratio is set to the second target lean air-fuel ratio, and after a predetermined time has elapsed, the target air-fuel ratio is smaller than the second target lean air-fuel ratio and larger than the stoichiometric air-fuel ratio. An air-fuel ratio control device configured to set an air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 10,
The responsiveness index value acquisition means
The minimum value of the output value of the downstream air-fuel ratio sensor that appears first after the target air-fuel ratio is changed from the target lean air-fuel ratio to the target rich air-fuel ratio, or
A maximum value of the output value of the downstream air-fuel ratio sensor that appears first after the target air-fuel ratio is changed from the target rich air-fuel ratio to the target lean air-fuel ratio;
Is an air-fuel ratio control device configured to obtain the responsiveness index value. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 10,
The responsiveness index value acquisition means
The maximum value of the output value of the downstream air-fuel ratio sensor when the target air-fuel ratio is set to the target lean air-fuel ratio, and then the target air-fuel ratio changes from the target lean air-fuel ratio to the target rich air-fuel ratio. The magnitude of the difference between the output value of the downstream air-fuel ratio sensor first appearing after being changed, or
The minimum value of the output value of the downstream air-fuel ratio sensor when the target air-fuel ratio is set to the target rich air-fuel ratio, and then the target air-fuel ratio changes from the target rich air-fuel ratio to the target lean air-fuel ratio. The maximum value of the output value of the downstream air-fuel ratio sensor that appears first after being changed,
Is an air-fuel ratio control device configured to obtain the responsiveness index value.

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