JP5310444B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air fuel ratio control device keeping an actual air fuel ratio at a catalyst demand air fuel ratio by preventing excessive correction of air fuel ratio feedback control by a catalyst downstream air fuel ratio sensor when a catalyst is deteriorated. <P>SOLUTION: Since change speed &Delta;Voxs of output value Voxs of the air fuel ratio sensor 56 at a downstream side of the catalyst 43 indicates a state of the catalyst (oxygen storage state), air fuel ratio of catalyst flow-in gas can be made in accordance with catalyst flow-in gas demanded air fuel ratio by controlling the air fuel ratio of the catalyst flow-in gas based on the change speed &Delta;Voxs of the output value Voxs. When the output value Voxs of the downstream side air fuel ratio sensor 56 increases and an absolute value ¾&Delta;Voxs¾ of the change speed of the output value Voxs is not less than a first change speed threshold value &Delta;V1th, the air fuel ratio of the catalyst flow-in gas is set to first lean air fuel ratio. When the output value Voxs decreases and the absolute value ¾&Delta;Voxs¾ of the change speed of the output value Voxs is not less than a second change speed threshold value &Delta;V2th, the air fuel ratio of the catalyst flow-in gas is set to first rich air fuel ratio. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

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 is disposed in the exhaust passage of the engine. As is well known, the three-way catalyst has an “oxygen storage function” for storing or releasing oxygen in accordance with the components of the gas flowing into the three-way catalyst. Hereinafter, the three-way catalyst is also simply referred to as “catalyst”, and the gas flowing into the catalyst is also referred to as “catalyst inflow gas”.

  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 apparatus obtains a “basic fuel injection amount for making the air-fuel ratio of the air-fuel mixture supplied to the engine coincide with the stoichiometric air-fuel ratio” based on the air amount sucked into the cylinder, and at least the basic fuel injection amount is downstream. Correction is made based on the output value of the side air-fuel ratio sensor.

  More specifically, the downstream air-fuel ratio sensor outputs the output value Voxs shown in FIG. The downstream air-fuel ratio sensor is also called a concentration cell type oxygen concentration sensor. 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 (hereinafter also referred to as “catalyst outflow gas”) is smaller than the stoichiometric air-fuel ratio (richer than the stoichiometric air-fuel ratio). In other words, when the catalyst outflow gas does not contain excessive oxygen, the maximum output value Vmax or a value near the maximum output value Vmax is obtained. “When the catalyst effluent gas does not contain excess oxygen” means that “unburned material and oxygen” in the catalyst effluent gas are combined, resulting in insufficient oxygen and unburned material remaining (catalyst effluent gas). The catalyst effluent gas contains less oxygen than is necessary to oxidize all of the unburned material in the catalyst), and when no oxygen is present in the catalyst effluent gas, That is.

  Further, the output value Voxs of the downstream side air-fuel ratio sensor is excessive 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, excessive in the catalyst outflow gas. When oxygen is contained, the minimum output value Vmin or a value near the minimum output value Vmin is obtained. “When the catalyst effluent gas contains excess oxygen” means that the unburned material disappears and oxygen remains as a result of the combination of “unburned material and oxygen” in the catalyst effluent gas (catalyst effluent gas). The catalyst effluent gas contains more oxygen than is necessary to oxidize all the unburned material in the catalyst) and the catalyst effluent gas contains no unburned material That's it.

  Thus, if the catalyst outflow gas contains excessive oxygen, the output value Voxs becomes the minimum output value Vmin (for example, 0 to 0.1 V). If the catalyst outflow gas does not contain excessive oxygen, the output value Voxs is output. The value Voxs is a maximum output value Vmax (for example, 0.9 to 1.0 V). Therefore, when the output value Voxs coincides with “the central value Vmid of the maximum output value Vmax and the minimum output value Vmin (that is, the median value Vmid = (Vmax + Vmin) / 2)”, the air-fuel ratio of the catalyst outflow gas is It is thought to be consistent with the theoretical air / fuel ratio.

  In the conventional apparatus, the air-fuel ratio feedback is made so that the output value Voxs of the downstream air-fuel ratio sensor matches the “downstream target value Voxsref set to a value corresponding to the theoretical air-fuel ratio (ie, the median value Vmid)”. The amount is calculated based on proportional / integral control (PI control) or the like. This air-fuel ratio feedback amount is also referred to as a “sub-feedback amount” for convenience. The conventional apparatus controls the air-fuel ratio of the air-fuel mixture supplied to the engine by correcting the “basic fuel injection amount for obtaining the theoretical air-fuel ratio” with the “sub-feedback amount”, and thereby the catalyst inflow gas. The air-fuel ratio is controlled.

  On the other hand, as the catalyst deteriorates, the maximum value of the oxygen storage amount that can be stored by the catalyst (maximum oxygen storage amount Cmax) decreases. Therefore, as the catalyst deteriorates, the time until the change in the air-fuel ratio of the catalyst inflow gas appears as the change in the air-fuel ratio of the catalyst outflow gas becomes shorter. As a result, as the catalyst deteriorates, the amplitude and fluctuation frequency of the output value Voxs increase. Therefore, the conventional apparatus obtains the trajectory length of the output value Voxs and determines whether or not the catalyst has deteriorated based on the trajectory length (see, for example, Patent Document 1).

Japanese Patent No. 2570930

  FIG. 24 is a time chart showing the state of air-fuel ratio control by the “conventional device described above” and the “air-fuel ratio control device according to the present invention” by a broken line and a solid line, respectively. In the example shown in FIG. 24, at time t0, the output value Voxs of the downstream air-fuel ratio sensor changes from a value smaller than the median value Vmid to a value greater than the median value Vmid. As described above, the conventional apparatus sets the downstream target value Voxsref to the median value Vmid.

  Accordingly, since the output value Voxs after time t0 is larger than the median value Vmid, the sub feedback amount calculated by the conventional device is a value for reducing (decreasing the amount of fuel) the basic fuel injection amount. As a result, the air-fuel ratio of the catalyst inflow gas is controlled to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. Hereinafter, the air-fuel ratio leaner than the stoichiometric air-fuel ratio is also simply referred to as “lean air-fuel ratio”.

  As a result, excessive oxygen is contained in the catalyst inflow gas, so that the amount of oxygen stored in the catalyst (hereinafter also referred to as “oxygen storage amount OSA”) increases. When the oxygen storage amount OSA of the catalyst is relatively small, the catalyst can efficiently store oxygen. Therefore, when the oxygen storage amount OSA at time t0 is relatively small, most of the excess oxygen contained in the catalyst inflow gas is stored in the catalyst after time t0. As a result, since the state in which the catalyst outflow gas contains almost no oxygen continues, the output value Voxs of the downstream air-fuel ratio sensor continues to increase toward the maximum output value Vmax.

  Thereafter, when the oxygen storage amount OSA of the catalyst reaches the predetermined upper limit value CHi at time t1, the catalyst cannot store oxygen efficiently. Therefore, a relatively large amount of oxygen starts to be contained in the catalyst outflow gas. As a result, the output value Voxs of the downstream air-fuel ratio sensor starts to decrease toward the minimum output value Vmin from time t2, which is the time immediately after time t1.

  However, since the output value Voxs is larger than the median value Vmid (the downstream target value Voxsref of the conventional device) from the time t2 to the subsequent time t5, the sub feedback amount by the conventional device is a value that decreases the basic fuel injection amount. Continue to be. As a result, the oxygen storage amount OSA continues to increase after time t2, and reaches the maximum oxygen storage amount Cmax at time t4 prior to time t5.

  At this time, the air-fuel ratio of the catalyst inflow gas is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the air-fuel mixture supplied to the engine is also an air-fuel ratio leaner than the stoichiometric air-fuel ratio. For this reason, the catalyst inflow gas contains a large amount of NOx (nitrogen oxide). However, since the oxygen storage amount OSA reaches the maximum oxygen storage amount Cmax, the catalyst cannot sufficiently purify NOx. As a result, during the period from time t4 to time t5, a relatively large amount of NOx may be discharged downstream of the catalyst.

  As described above, the conventional apparatus may perform “fuel injection amount reduction correction” that is unnecessary for the exhaust gas purifying action by the catalyst (see the air-fuel ratio hatching portion of the catalyst inflow gas in FIG. 24). In other words, according to the conventional apparatus, the air-fuel ratio of the catalyst inflow gas is also referred to as “the air-fuel ratio required to maintain the catalyst exhaust purification efficiency at a good value (hereinafter referred to as“ catalyst inflow gas required air-fuel ratio ”). The air-fuel ratio on the leaner side than “)” may be controlled.

  On the other hand, when the output value Voxs of the downstream air-fuel ratio sensor becomes smaller than the “downstream target value Voxsref set to the median value Vmid”, the sub feedback amount calculated by the conventional device increases the basic fuel injection amount (increase correction) ). Thereby, the air-fuel ratio of the catalyst inflow gas is controlled to be richer than the stoichiometric air-fuel ratio. Hereinafter, the air-fuel ratio richer than the stoichiometric air-fuel ratio is also simply referred to as “rich air-fuel ratio”.

As a result, the catalyst inflow gas contains excessive unburned substances (CO, HC, H _2, etc.), so oxygen stored in the catalyst is used for purification of the unburned substances. Therefore, the oxygen storage amount OSA decreases. However, according to the conventional apparatus, the oxygen storage amount OSA is in the vicinity of the maximum oxygen storage amount Cmax immediately after the output value Voxs of the downstream air-fuel ratio sensor becomes smaller than the downstream target value Voxsref. (See immediately after time t5 in FIG. 24). Therefore, although it is a very small amount, oxygen contained in the catalyst inflow gas flows out as it is downstream of the catalyst. Further, an amount of unburned matter sufficient to completely consume oxygen remaining in the vicinity of the downstream air-fuel ratio sensor or in the diffusion resistance layer of the downstream air-fuel ratio sensor does not flow downstream of the catalyst. As a result, the output value Voxs of the downstream air-fuel ratio sensor maintains a value smaller than the downstream target value Voxsref.

  Thereafter, when the oxygen storage amount OSA of the catalyst decreases to a predetermined lower limit value CLo (<CHi), the catalyst starts to efficiently store a small amount of oxygen contained in the catalyst inflow gas and unburned matter contained in the catalyst inflow gas. Can not be completely purified. Therefore, oxygen is not included in the catalyst outflow gas, and a relatively large amount of unburned material starts to be included. Oxygen remaining in the vicinity of the downstream air-fuel ratio sensor or in the diffusion resistance layer of the downstream air-fuel ratio sensor is consumed by the unburned matter. As a result, the output value Voxs of the downstream air-fuel ratio sensor starts to increase toward the maximum output value Vmax.

  However, since the output value Voxs is smaller than the downstream target value Voxsref (median value Vmid) for a while from that time, the sub-feedback amount by the conventional device continues to be a value that increases the basic fuel injection amount. As a result, the oxygen storage amount OSA of the catalyst continues to decrease and reaches “0”.

  At this time, the air-fuel ratio of the catalyst inflow gas is richer than the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the air-fuel mixture supplied to the engine is also richer than the stoichiometric air-fuel ratio. For this reason, a large amount of unburned matter is contained in the catalyst inflow gas. Furthermore, since the oxygen storage amount OSA has reached “0”, the catalyst cannot sufficiently purify the unburned matter. As a result, a large amount of unburned matter may be discharged downstream of the catalyst.

  As described above, the conventional apparatus may perform “increase correction of the fuel injection amount” that is unnecessary for the exhaust gas purification action by the catalyst. In other words, according to the conventional apparatus, the air-fuel ratio of the catalyst inflow gas may be controlled to a richer air-fuel ratio than the “catalyst inflow gas required air-fuel ratio”.

  Therefore, the present inventor controls the emission by controlling the “air-fuel ratio of the air-fuel mixture supplied to the engine” so that the actual air-fuel ratio of the catalyst inflow gas matches the “catalyst inflow gas required air-fuel ratio” as much as possible. An air-fuel ratio control apparatus for an internal combustion engine that can further improve the above is developed. Hereinafter, the air-fuel ratio control apparatus under development is referred to as “the present invention apparatus”. According to the apparatus of the present invention, even if the maximum oxygen storage amount Cmax of the catalyst in a new state is reduced by reducing the amount of the noble metal supported by the catalyst in advance in order to reduce the cost of the catalyst, it can be maintained over a long period of time. “Avoiding worsening of emissions” is possible.

  Here, the outline of the air-fuel ratio control of the device of the present invention will be described. Since the change rate ΔVoxs of the output value Voxs of the downstream air-fuel ratio sensor represents the state of the catalyst (oxygen occlusion state), the present invention device determines the air-fuel ratio of the catalyst inflow gas (ie, Based on the knowledge that the air-fuel ratio of the catalyst inflow gas can be matched with the “catalyst inflow gas required air-fuel ratio” by controlling the “air-fuel ratio of the air-fuel mixture supplied to the engine”. The change rate ΔVoxs of the output value Voxs is a value corresponding to a change amount of the output value Voxs per unit time, that is, a time differential value (dVoxs / dt) of the output value Voxs.

  Hereinafter, the reason why the change rate ΔVoxs of the output value Voxs of the downstream side air-fuel ratio sensor “represents the state of the catalyst” will be described with respect to each case.

(1) A lean air-fuel ratio combustion gas is added to a catalyst (a catalyst in an oxygen-deficient state, an oxygen-deficient catalyst) in which the oxygen storage amount OSA is equal to or lower than the lower limit value CLo (that is, a predetermined value close to “0”). When (exhaust gas) is supplied.

In this case, as schematically shown in FIG. 4, the catalyst inflow gas as the combustion gas contains “a trace amount of unburned matter (HC, etc.)” and “a large amount and excess oxygen (O ₂ )”. ing. Oxygen is stored in the catalyst 43 by being combined with the oxygen storage material in the catalyst 43. The unburned matter is combined with “oxygen in the catalyst inflow gas or oxygen remaining in the catalyst 43”. In this way, oxygen contained in the catalyst inflow gas is occluded or consumed in the catalyst 43, so that there is almost no oxygen in the catalyst outflow gas. As a result, the output value Voxs of the downstream air-fuel ratio sensor becomes a value near the maximum output value Vmax.

  However, the amount of oxygen contained in the lean air-fuel ratio catalyst inflow gas is much larger than the amount of oxygen contained in the rich air-fuel ratio catalyst inflow gas. When the catalyst inflow gas flows in, oxygen leaks out from the catalyst although there is a trace amount. Therefore, the output value Voxs of the downstream air-fuel ratio sensor becomes a value slightly lower than the maximum output value Vmax (however, a value much larger than the median value Vmid) (see point P1 in FIG. 3).

(2) When the oxygen storage amount OSA becomes equal to or higher than the upper limit value CHi (that is, a predetermined value close to the maximum oxygen storage amount Cmax) by continuing to supply the lean air-fuel ratio combustion gas to the catalyst.

  In this case, as schematically shown in FIG. 5, the catalyst inflow gas which is the combustion gas contains “a trace amount of unburned matter” and “a large amount and excess oxygen”. In this state, since the remaining capacity of the catalyst 43 for storing oxygen is small, a part of the oxygen in the catalyst inflow gas is stored in the catalyst 43, but most of the remaining oxygen flows out downstream of the catalyst 43. start. The unburned matter is combined with “oxygen stored in the catalyst 43”. Thus, the catalyst effluent gas begins to contain excess oxygen. Therefore, the output value Voxs of the downstream air-fuel ratio sensor starts to decrease rapidly toward the vicinity of the minimum output value Vmin, and then reaches the minimum output value Vmin.

  As understood from the above explanation, when the output value Voxs of the downstream air-fuel ratio sensor starts to decrease when the combustion gas having the air-fuel ratio leaner than the stoichiometric air-fuel ratio is supplied to the catalyst, The oxygen storage amount OSA is considerably large. Therefore, in this state, it is not appropriate to continue to supply “a gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio” to the catalyst. In other words, when the output value Voxs of the downstream air-fuel ratio sensor decreases relatively quickly (that is, when the output value Voxs decreases and the absolute value of the change rate ΔVoxs of the output value Voxs is greater than the predetermined threshold value) ) Even if the output value Voxs is larger than the median value Vmid, the “catalyst inflow gas required air-fuel ratio” is the stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio.

(3) When the rich air-fuel ratio combustion gas is supplied to the catalyst in which the oxygen storage amount OSA is equal to or higher than the above-described upper limit value CHi (the catalyst in the oxygen excess state, the oxygen excess catalyst).

  In this case, as schematically shown in FIG. 6, the catalyst inflow gas that is the combustion gas contains “a large amount and excessive unburnt substances” and “a small amount of oxygen”. Since the unburned matter is combined with “oxygen stored in the catalyst 43”, it does not leak downstream of the catalyst 43. On the other hand, a small amount of oxygen in the catalyst inflow gas passes through the catalyst 43 without being occluded by the catalyst 43 and flows out downstream of the catalyst 43. As a result, the output value Voxs of the downstream air-fuel ratio sensor is a value in the vicinity of the minimum output value Vmin and is a value slightly increased from the minimum output value Vmin (however, a value considerably smaller than the median value Vmid) ( (See point P2 in FIG. 3).

(4) When the oxygen storage amount OSA becomes equal to or lower than the above-described lower limit value CLo (that is, a predetermined value close to “0”) by continuously supplying the rich air-fuel ratio combustion gas to the catalyst.

  In this case, as schematically shown in FIG. 7, the catalyst inflow gas that is the combustion gas contains “a large amount and an excessive amount of unburned matter” and “a small amount of oxygen”. At this time, since the “remaining capacity of the catalyst 43” that gives the unoccluded oxygen to the unburned material is small at this time, a part of the unburned material in the catalyst inflow gas is “to the catalyst 43. Although it is combined with “occluded oxygen” and the other part is combined with “oxygen in the catalyst inflow gas”, most of the remainder begins to flow downstream of the catalyst 43. Further, oxygen in the catalyst inflow gas is occluded by the catalyst 43. Therefore, the catalyst outflow gas does not contain oxygen and starts to contain unburned substances. Therefore, the output value Voxs of the downstream air-fuel ratio sensor increases rapidly toward the vicinity of the maximum output value Vmax, and then reaches the maximum output value Vmax.

  As understood from the above description, when the combustion value of the air-fuel ratio richer than the stoichiometric air-fuel ratio is supplied to the catalyst, when the output value Voxs of the downstream air-fuel ratio sensor starts increasing, The oxygen storage amount OSA is considerably small. Therefore, in this state, it is not appropriate to continue to supply “a gas having an air-fuel ratio richer than the theoretical air-fuel ratio” to the catalyst. In other words, when the output value Voxs of the downstream air-fuel ratio sensor increases relatively quickly (that is, the output value Voxs increases and the absolute value of the change rate ΔVoxs of the output value Voxs is larger than the predetermined threshold value). ), Even if the output value Voxs is smaller than the median value Vmid, the “catalyst inflow gas required air-fuel ratio” is the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  However, when the output value Voxs is a value in the vicinity of the maximum output value Vmax, the state of the catalyst 43 is in an oxygen-deficient state (a state where the oxygen storage amount OSA is substantially “0”) regardless of the change rate ΔVoxs. It is believed that there is. Accordingly, the “catalyst inflow gas required air-fuel ratio” in that case is the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  Similarly, when the output value Voxs is a value in the vicinity of the minimum output value Vmin, the state of the catalyst 43 is in an oxygen excess state (the oxygen storage amount OSA is substantially the maximum oxygen storage amount Cmax regardless of the change rate ΔVoxs). State). Accordingly, the “catalyst inflow gas required air-fuel ratio” in that case is the stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio.

  The apparatus of the present invention has been made based on such knowledge, and is a concentration cell type oxygen concentration sensor (downstream) that outputs an output value Voxs corresponding to the concentration of oxygen (oxygen partial pressure) contained in the catalyst outflow gas. Side air-fuel ratio sensor), and air-fuel ratio control means for controlling the “air-fuel ratio of the air-fuel mixture supplied to the engine (air-fuel ratio of the engine)” based on the output value Voxs and the change speed ΔVoxs of the oxygen concentration sensor, Prepare.

More specifically, the air-fuel ratio control means includes
(A1) When the output value Voxs is between “predetermined low side threshold value” and “predetermined high side threshold value greater than the low side threshold value”, the catalyst inflow gas empties in the first period described below. The air-fuel ratio of the engine is controlled so that the fuel ratio becomes a predetermined first lean air-fuel ratio leaner than the stoichiometric air-fuel ratio.

The start time point of the first period is a time point when the absolute value | ΔVoxs | of the change rate of the output value when the output value Voxs increases is equal to or greater than a predetermined first change rate threshold value.
The end point of the first period is a time point when the absolute value | ΔVoxs | of the change rate of the output value when the output value Voxs decreases is equal to or greater than a predetermined second change rate threshold value, and the output value. One of the time points when Voxs becomes equal to or higher than the high-side threshold.

Further, the air-fuel ratio control means includes:
(A2) When the output value Voxs is between “the low threshold value” and “the high threshold value”, the air-fuel ratio of the catalyst inflow gas is richer than the stoichiometric air-fuel ratio in the second period described below. The air-fuel ratio of the engine is controlled so as to be a predetermined first rich air-fuel ratio.

The start time of the second period is a time when the absolute value | ΔVoxs | of the change rate of the output value when the output value Voxs is decreasing becomes equal to or greater than the second change rate threshold.
The end point of the second period is the time when the absolute value | ΔVoxs | of the change rate of the output value when the output value Voxs increases is equal to or greater than the first change rate threshold, and the output value Voxs Is the time when the value becomes equal to or lower than the low-side threshold.

  The low threshold value is a predetermined value between the median value Vmid and the minimum output value Vmin, and the high threshold value is a predetermined value between the median value Vmid and the maximum output value Vmax. Furthermore, the first change speed threshold and the second change speed threshold are both positive predetermined values, and both may be the same or different.

  As described above, when the output value Voxs of the downstream air-fuel ratio sensor (oxygen concentration sensor) increases relatively quickly, even when the output value of the downstream air-fuel ratio sensor is smaller than the median value Vmid. However, the oxygen storage amount OSA of the catalyst is not close to the maximum oxygen storage amount Cmax, but rather decreases to a value close to “0”. Therefore, when the output value of the downstream air-fuel ratio sensor increases relatively quickly (more specifically, the change rate ΔVoxs of the output value of the downstream air-fuel ratio sensor is positive and its absolute value | ΔVoxs | Is larger than the first change speed threshold), the required air-fuel ratio of the catalyst inflow gas is an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  Therefore, according to the configuration (A1), “the air-fuel ratio of the catalyst inflow gas” is set to “the air-fuel ratio leaner than the stoichiometric air-fuel ratio” before the oxygen storage amount OSA reaches “0”. Thus, the oxygen storage amount OSA can be started to increase (see the solid line after time t7 in FIG. 24). That is, the device according to the present invention does not perform unnecessary increase correction of the fuel injection amount unlike the conventional device, so that a large amount of unburned material can be avoided.

  In addition, as described above, when the output value Voxs of the downstream air-fuel ratio sensor (oxygen concentration sensor) decreases relatively quickly, even when the output value of the downstream air-fuel ratio sensor is larger than the median value Vmid. Even so, the oxygen storage amount OSA of the catalyst is not an amount in the vicinity of “0”, but rather increases to a value close to the maximum oxygen storage amount Cmax. Therefore, when the output value of the downstream air-fuel ratio sensor is decreasing relatively quickly (more specifically, the change rate ΔVoxs of the output value of the downstream air-fuel ratio sensor is negative and its absolute value | ΔVoxs | Is greater than the second change speed threshold), the required air-fuel ratio for catalyst inflow gas is an air-fuel ratio richer than the stoichiometric air-fuel ratio.

  Therefore, according to the configuration (A2), “the air-fuel ratio of the catalyst inflow gas” is set to “the air-fuel ratio richer than the stoichiometric air-fuel ratio” before the oxygen storage amount OSA reaches the maximum oxygen storage amount Cmax. Thus, the oxygen storage amount OSA can be started to decrease (see the solid line after time t3 in FIG. 24). That is, the device of the present invention does not perform unnecessary fuel injection amount reduction correction unlike the conventional device, so that a large amount of NOx can be prevented from being discharged downstream of the catalyst.

  Incidentally, as described above, the high-side threshold is set to a value between the median value Vmid and the maximum output value Vmax. For example, the high-side threshold is when “the air-fuel ratio of the catalyst inflow gas” is “the air-fuel ratio leaner than the stoichiometric air-fuel ratio” and the oxygen storage amount OSA of the catalyst is increasing, The output value Voxs (upper limit value Vjogen) when the “outflow gas air-fuel ratio” is “theoretical air-fuel ratio” or a value closer to the median value Vmid than the upper limit value Vjogen is set.

  When the oxygen storage amount OSA of the catalyst is “0” or substantially “0” (when the catalyst is in an oxygen-deficient state), oxygen does not flow downstream of the catalyst (see FIG. 7) or oxygen Only a very small amount flows out downstream of the catalyst (see FIG. 4). Therefore, when the catalyst is in an oxygen-deficient state, the output value Voxs is the maximum output value Vmax or a value near the maximum output value Vmax, and thus the output value Voxs is equal to or higher than the high-side threshold value.

  Therefore, in such a case (when the output value Voxs is larger than the high-side threshold value), it is better not to set “the air-fuel ratio of the catalyst inflow gas” to “the air-fuel ratio richer than the stoichiometric air-fuel ratio”. In other words, the “catalyst inflow gas required air-fuel ratio” in this case is the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

Therefore, the air-fuel ratio control means further includes:
(B) When the output value Voxs is equal to or higher than the high-side threshold,
The air-fuel ratio of the engine is controlled so that the air-fuel ratio of the catalyst inflow gas becomes the second lean air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  According to this, for example, in the case where the oxygen storage amount OSA reaches “0” by performing fuel increase for preventing catalyst overheating and securing output, the oxygen storage amount OSA is reached when the increase is no longer necessary. Can be quickly increased. Further, during the control of the air-fuel ratio according to the above (A1) and (A2), even when the output value Voxs becomes equal to or higher than the high-side threshold, the oxygen storage amount OSA is rapidly increased to reduce the output value Voxs. It can return between the high side threshold and the low side threshold.

  On the other hand, the low threshold value is set to a value between the median value Vmid and the minimum output value Vmin. For example, the low-side threshold value is a case where “the air-fuel ratio of the catalyst inflow gas” is “the air-fuel ratio richer than the stoichiometric air-fuel ratio” and the oxygen storage amount OSA of the catalyst is decreased. The output value Voxs (lower limit value Vkagen) when the “outflow gas air-fuel ratio” is “theoretical air-fuel ratio” or a value closer to the median value Vmid than the lower limit value Vkagen is set.

  When the oxygen storage amount OSA of the catalyst is the maximum oxygen storage amount Cmax or substantially the maximum oxygen storage amount Cmax (when the catalyst is in an oxygen-excess state), whether oxygen flows in a large amount downstream of the catalyst (see FIG. 5). See)) or a small amount of oxygen flows downstream of the catalyst (see FIG. 6). Therefore, when the catalyst is in an oxygen excess state, the output value Voxs is a minimum output value Vmin or a value in the vicinity of the minimum output value Vmin, and therefore the output value Voxs is equal to or lower than the low threshold value.

  Therefore, in such a case (when the output value Voxs is smaller than the low-side threshold value), it is better not to set “the air-fuel ratio of the catalyst inflow gas” to “the air-fuel ratio leaner than the stoichiometric air-fuel ratio”. In other words, the “catalyst inflow gas required air-fuel ratio” in this case is the stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio.

Therefore, the air-fuel ratio control means further includes:
(C) When the output value Voxs is equal to or lower than the low-side threshold, the air-fuel ratio of the engine is controlled so that the air-fuel ratio of the catalyst inflow gas becomes a second rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio. It is configured.

  According to this, for example, when the fuel cut operation is performed, the oxygen storage amount OSA reaches the maximum oxygen storage amount Cmax, and thereafter, when the fuel cut operation is terminated, the oxygen storage amount OSA can be quickly reduced. it can. Further, during the control of the air-fuel ratio according to the above (A1) and (A2), even when the output value Voxs becomes equal to or lower than the low threshold value, the oxygen storage amount OSA is rapidly reduced to reduce the output value Voxs. It can return between the high side threshold and the low side threshold.

  As shown in FIG. 8A, the device of the present invention having such an air-fuel ratio control means, when the degree of deterioration of the catalyst is small, excludes “normal” immediately after the end of the fuel cut and immediately after the increase in the catalyst overheating prevention. During feedback control, the air-fuel ratio of the engine can be controlled so that the output value Voxs changes slowly between the high side threshold and the low side threshold. That is, according to the apparatus of the present invention, the air-fuel ratio of the engine can be controlled so that the oxygen storage amount OSA of the catalyst does not reach “0” or the maximum oxygen storage amount Cmax, so that the emission can be maintained well. it can.

  Furthermore, as shown in FIG. 8 (B), the device of the present invention increases the degree of deterioration of the catalyst to “the degree of deterioration to be determined as a deteriorated catalyst” (the degree of deterioration is medium). However, during normal feedback control, the engine air-fuel ratio can be controlled so that the output value Voxs changes relatively slowly between the high-side threshold and the low-side threshold. That is, according to the device of the present invention, even if the catalyst is somewhat deteriorated, the air-fuel ratio of the engine can be controlled so that the oxygen storage amount OSA of the catalyst does not reach “0” or the maximum oxygen storage amount Cmax. As a result, the emission can be maintained well.

  By the way, when the catalyst is further deteriorated (that is, when the degree of deterioration of the catalyst is extremely large and the maximum oxygen storage amount Cmax is extremely small), the change in the air-fuel ratio of the catalyst inflow gas immediately causes the empty of the catalyst outflow gas. Appears as a change in fuel ratio. For this reason, for example, when the change rate ΔVoxs of the output value Voxs is positive and the absolute value | ΔVoxs | is equal to or greater than the first change rate threshold, the air-fuel ratio of the catalyst inflow gas becomes the first lean according to the configuration (A1). Immediately after setting the air-fuel ratio, the change rate ΔVoxs of the output value Voxs is negative and its absolute value becomes equal to or greater than the second change rate threshold. Therefore, according to the configuration (A2), the air-fuel ratio of the catalyst inflow gas is immediately set to the first rich air-fuel ratio. Immediately thereafter, the change rate ΔVoxs of the output value Voxs is positive and the absolute value | ΔVoxs | It becomes 1 change speed threshold value or more. As a result, the air-fuel ratio of the catalyst inflow gas is set again to the first lean air-fuel ratio.

  As described above, when the deterioration degree of the catalyst becomes extremely large, the above-described feedback control of the air-fuel ratio fails, and the output value Voxs has an extremely large amplitude and fluctuation frequency as shown in FIG. It changes to become.

  Therefore, the relationship between the degree of catalyst deterioration and the “catalyst deterioration index value indicating the degree of catalyst deterioration” calculated based on the output value Voxs changes as shown by the broken line in FIG. The catalyst deterioration index value may be a value that increases as the amplitude and fluctuation frequency of the output value Voxs increase (in the example shown in FIG. 9, a trajectory ratio described later, a trajectory length of the output value Voxs, etc.). The value may be a value that decreases as the amplitude and fluctuation frequency of the output value Voxs increase (for example, the reciprocal of the locus length of the output value Voxs).

  In FIG. 9, a catalyst having a degree of deterioration larger than a predetermined value Y is “a catalyst to be determined to be deteriorated (deteriorated catalyst)”, and a catalyst having a degree of deterioration smaller than a predetermined value Y is determined to be “not deteriorated”. "Catalyst catalyst (non-degraded catalyst)". As can be seen from the broken line in FIG. 9, according to the device of the present invention, feedback control does not fail even with a deteriorated catalyst (see FIG. 8B). Even if it increases in the vicinity of, the amount of increase in the catalyst deterioration index value is very small. Therefore, if the determination as to whether or not the catalyst is deteriorated is made by comparing the catalyst deterioration index value with the deterioration determination reference value R1, it is difficult to make the determination accurately.

Therefore, when a predetermined catalyst deterioration determination execution condition is satisfied, the catalyst deterioration index value is acquired based on the output value Voxs, and whether or not the catalyst has deteriorated based on the acquired catalyst deterioration index value is determined. In the device of the present invention comprising a catalyst deterioration determination means for determining,
The air-fuel ratio control means includes
When the catalyst deterioration determination execution condition is satisfied,
Setting the first change speed threshold to a value smaller than the case where the catalyst deterioration determination execution condition is not satisfied,
Setting the second change speed threshold to a value smaller than the case where the catalyst deterioration determination execution condition is not satisfied,
The air-fuel ratio of the catalyst inflow gas in the first period is a predetermined third lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio instead of the first lean air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled so that the absolute value of the difference from the fuel ratio becomes a third lean air-fuel ratio that is larger than the absolute value of the difference between the first lean air-fuel ratio and the stoichiometric air-fuel ratio. And
and,
The air-fuel ratio of the catalyst inflow gas in the second period is a predetermined third rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio instead of the first rich air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled so that the absolute value of the difference from the fuel ratio becomes a third rich air-fuel ratio that is larger than the absolute value of the difference between the first rich air-fuel ratio and the stoichiometric air-fuel ratio. To do.

  According to this, when the catalyst deterioration determination execution condition is satisfied, the output value Voxs increases and the absolute value | ΔVoxs | of the change rate ΔVoxs is equal to or greater than “a value smaller than the first change rate threshold value during normal feedback”. When this happens, the catalyst inflow gas is changed to “a third lean air-fuel ratio that is leaner than the first lean air-fuel ratio”. That is, the air-fuel ratio of the catalyst inflow gas is changed so that a larger amount of oxygen is supplied to the catalyst at an earlier timing than when the catalyst deterioration determination execution condition is not satisfied.

  Furthermore, when the catalyst deterioration determination execution condition is satisfied, the output value Voxs decreases and the absolute value | ΔVoxs | of the change rate ΔVoxs is equal to or greater than “a value smaller than the second change rate threshold value during normal feedback”. Then, the catalyst inflow gas is changed to “a third rich air-fuel ratio that is richer than the first rich air-fuel ratio”. That is, the air-fuel ratio of the catalyst inflow gas is changed so that a larger amount of unburned material is supplied to the catalyst at an earlier timing than when the catalyst deterioration determination execution condition is not satisfied.

  As a result, even if the degree of deterioration of the catalyst is not very large, when the degree of deterioration of the catalyst is close to a predetermined value Y (deterioration degree that should be determined that the catalyst has deteriorated), the catalyst becomes “the air-fuel ratio of the catalyst inflow gas. Cannot change enough. Therefore, when the catalyst deterioration determination execution condition is satisfied, the device of the present invention immediately changes the catalyst outflow gas from the time before the catalyst deterioration degree becomes extremely large (that is, the output value Voxs). Change). In other words, when the catalyst deterioration determination execution condition is satisfied, the device according to the present invention determines that the output value Voxs is between the high side threshold and the low side threshold when the deterioration degree of the catalyst is close to the predetermined value Y. The above-described feedback control for changing the oxygen storage amount OSA between a value larger than 0 and a value smaller than the maximum oxygen storage amount Cmax can be broken.

  As a result, when the degree of deterioration of the catalyst reaches the vicinity of the predetermined value Y, the output value Voxs is the case where the degree of deterioration of the catalyst is extremely large in normal feedback control as shown in FIG. (See (C) of FIG. 8), the amplitude and the variation frequency change so as to be very large.

  Therefore, according to the device of the present invention, when performing the catalyst deterioration determination, as shown by the solid line in FIG. 9, the catalyst deterioration index value rapidly changes as the catalyst deterioration degree reaches the vicinity of the predetermined value Y ( Increase or decrease). Therefore, the device of the present invention can accurately determine whether or not the catalyst has deteriorated, for example, by comparing the catalyst deterioration index value with the deterioration determination reference value R2. As described above, the apparatus of the present invention can maintain good emission even when the catalyst is somewhat deteriorated during normal times (except when determining catalyst deterioration), and accurately determine the catalyst deterioration when determining catalyst deterioration. Can do.

Further, the air-fuel ratio control means includes:
After the time when it is determined that the catalyst is deteriorated, at least one of the first rich air-fuel ratio and the second rich air-fuel ratio is a value before the time when it is determined that the catalyst is deteriorated. Becomes an air-fuel ratio closer to the theoretical air-fuel ratio,
After the time when it is determined that the catalyst is deteriorated, at least one of the first lean air-fuel ratio and the second lean air-fuel ratio is greater than the value at the time before it is determined that the catalyst is deteriorated. So that the air-fuel ratio is closer to the theoretical air-fuel ratio.
It is preferable that the air-fuel ratio of the air-fuel mixture supplied to the engine is controlled.

  According to this, when it is determined that the catalyst has deteriorated, the “deviation from the theoretical air-fuel ratio” of the air-fuel ratio of the catalyst inflow gas becomes smaller than before it is determined that the catalyst has deteriorated. That is, after the time when it is determined that the catalyst has deteriorated, the magnitude of the correction amount of the air-fuel ratio in the normal feedback control becomes small. As a result, excess oxygen or excess unburned material is unlikely to flow into the deteriorated catalyst, and normal feedback control is unlikely to fail. Therefore, the device of the present invention can maintain the emission well even when the catalyst is deteriorated.

Further, the air-fuel ratio control means includes:
The first lean air-fuel ratio is a constant value;
The third lean air-fuel ratio becomes a value that increases as the absolute value of the change speed of the output value increases.
The first rich air-fuel ratio is a constant value;
The third rich air-fuel ratio becomes a value that decreases as the absolute value of the change rate of the output value increases.
It is preferable to control the air-fuel ratio of the air-fuel mixture supplied to the engine.

  As the catalyst deteriorates (as the maximum oxygen storage amount Cmax decreases), the change in the air-fuel ratio of the catalyst outflow gas (and hence the change in the output value Voxs) more sensitively follows the change in the air-fuel ratio of the catalyst inflow gas. Therefore, as the catalyst deteriorates, the absolute value of the change rate ΔVoxs of the output value Voxs tends to increase. Therefore, if the third lean air-fuel ratio and the third rich air-fuel ratio are changed in accordance with the absolute value | ΔVoxs | of the change speed of the output value Voxs as in the above configuration, the third lean air-fuel ratio and the third rich air-fuel ratio are changed. The air-fuel ratio becomes an air-fuel ratio having a larger deviation from the theoretical air-fuel ratio as the catalyst deteriorates. As a result, according to the above configuration, the above-described air-fuel ratio feedback control is likely to fail as the catalyst deteriorates. In other words, when the degree of deterioration of the catalyst approaches “the degree of deterioration to determine that the catalyst has deteriorated”, the change in the catalyst deterioration index value can be rapidly increased.

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 voltage of the upstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. 2 is a graph showing the relationship between the output voltage of the downstream air-fuel ratio sensor shown in FIG. 1 and the air-fuel ratio. It is the conceptual diagram which showed the effect | action of the catalyst when the gas of the lean air fuel ratio (the air fuel ratio leaner than the theoretical air fuel ratio) flows into the catalyst in an oxygen deficient state. It is the conceptual diagram which showed the effect | action of the catalyst when the gas of a lean air fuel ratio flows in into the catalyst in an oxygen excess state. FIG. 5 is a conceptual diagram showing the operation of the catalyst when a rich air-fuel ratio (air-fuel ratio richer than the theoretical air-fuel ratio) gas flows into the catalyst in an oxygen-excess state. It is the conceptual diagram which showed the effect | action of the catalyst when the gas of rich air fuel ratio flows in into the catalyst in an oxygen-deficient state. 7 is a time chart showing the output value of the downstream air-fuel ratio sensor and the air-fuel ratio of the catalyst inflow gas when the air-fuel ratio is feedback controlled by the first control device (other than during catalyst deterioration determination execution). 6 is a graph showing the relationship between the degree of catalyst deterioration and the catalyst deterioration index value (trajectory ratio) calculated based on the output value of the downstream air-fuel ratio sensor. 7 is a time chart showing the output value of the downstream air-fuel ratio sensor and the air-fuel ratio of the catalyst inflow gas when the air-fuel ratio is feedback controlled by the first controller (during execution of catalyst deterioration determination). 6 is a time chart showing an output value of a downstream air-fuel ratio sensor and an air-fuel ratio of catalyst inflow gas when the air-fuel ratio is feedback controlled by the first control device. 6 is a time chart showing an output value of a downstream air-fuel ratio sensor and an air-fuel ratio of catalyst inflow gas when the air-fuel ratio is feedback controlled by the first control device. 6 is a time chart showing an output value of a downstream air-fuel ratio sensor and an air-fuel ratio of catalyst inflow gas when the air-fuel ratio is feedback controlled by the first control device. It is the flowchart which showed the "routine for catalyst state judgment" which CPU of the 1st control device performs. It is the flowchart which showed "the routine for other catalyst state determination" which CPU of a 1st control apparatus performs. 3 is a schematic flowchart showing an “air-fuel ratio control routine” executed by the CPU of the first control device. It is the flowchart which showed the "catalyst degradation judgment routine" which CPU of the 1st control device performs. It is the flowchart which showed the "fuel injection control routine" which CPU of a 1st control apparatus performs. 7 is a flowchart showing a “routine for acquiring a change rate of an output value of a downstream air-fuel ratio sensor” executed by a CPU of a first control device. It is the flowchart which showed the "main feedback amount calculation routine" which CPU of a 1st control apparatus performs. It is the flowchart which showed the "main feedback amount restriction | limiting routine" which CPU of a 1st control apparatus performs. It is the flowchart which showed the "sub feedback amount calculation routine" which CPU of a 1st control apparatus performs. 7 is a flowchart showing a part of a “sub feedback amount calculation routine” executed by a CPU of an air-fuel ratio control apparatus (second control apparatus) for an internal combustion engine according to a second embodiment of the present invention. It is a time chart for demonstrating the action | operation of the conventional air fuel ratio control apparatus and the air fuel ratio control apparatus by this invention.

  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. One end of 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 other 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 side catalyst 43 supports “noble metal as a catalyst substance” and “ceria (CeO 2) as an oxygen storage material” on a support made of ceramic, and has an oxygen storage / release function (oxygen storage function). The original catalyst. 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 first 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 ".

  As shown in FIG. 2, the upstream air-fuel ratio sensor 55 is an air-fuel ratio of exhaust gas flowing through the position where the upstream air-fuel ratio sensor 55 is disposed (the air-fuel ratio of “catalyst inflow gas” which is a gas flowing into the catalyst 43, The output value Vabyfs corresponding to the detected upstream air-fuel ratio abyfs) is output. 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 the air-fuel ratio conversion table (map) Mapabyfs shown in FIG. 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.

  Referring again to FIG. 1, 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 (O2 sensor). The downstream air-fuel ratio sensor 56 is exposed to, for example, a solid electrolyte layer, an exhaust gas side electrode layer formed outside the solid electrolyte layer, and an air chamber (inside the solid electrolyte layer) and sandwiches the solid electrolyte chamber layer. An atmosphere side electrode layer formed inside the solid electrolyte layer so as to face the exhaust gas side electrode layer, and a diffusion resistance layer covering the exhaust gas side electrode layer and in contact with the exhaust gas (disposed to be exposed to the exhaust gas) And comprising. The solid electrolyte layer may be a test tube or a plate. The downstream air-fuel ratio sensor 56 is responsive to the air-fuel ratio (downstream air-fuel ratio afdown) of the exhaust gas flowing through the position where the downstream air-fuel ratio sensor 56 is disposed (that is, the “catalyst outflow gas” that is the gas flowing out from the catalyst 43). The output value Voxs is output.

  As shown in FIG. 3, the output value Voxs of the downstream air-fuel ratio sensor 56 is such that the air-fuel ratio of the catalyst outflow gas (detected gas) is richer than the stoichiometric air-fuel ratio, When the oxygen partial pressure of the gas after oxidation equilibrium is small, the maximum output value Vmax (for example, about 0.9 to 1.0 V) is obtained. That is, the downstream air-fuel ratio sensor 56 outputs the maximum output value Vmax when the catalyst outflow gas does not contain excessive oxygen.

  The output value Voxs is the minimum output value min (for example, when the air-fuel ratio of the catalyst outflow gas is leaner than the stoichiometric air-fuel ratio and the oxygen partial pressure of the gas after oxidation equilibrium of the catalyst outflow gas is large) , About 0 to 0.1 V). That is, the downstream air-fuel ratio sensor 56 outputs the minimum output value Vmin when the catalyst outflow gas contains excessive oxygen.

  Further, this output value Voxs rapidly increases from the maximum output value Vmax to the minimum output 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. Decrease. Conversely, the output value Voxs rapidly increases from the minimum output value Vmin to the maximum output 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. Increase. The average value of the minimum output value Vmin and the maximum output value Vmax is referred to as a median value Vmid (= (Vmax + Vmin) / 2).

  The accelerator opening sensor 57 shown in FIG. 1 detects the operation amount of the accelerator pedal AP operated by the driver, and outputs a signal indicating the operation amount Accp of the accelerator pedal AP.

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

(Overview of determination of catalyst state and air-fuel ratio feedback control by the first controller)
The first control device determines the state of the catalyst (oxygen storage state) as described below, and based on the determined state of the catalyst, the air-fuel ratio of the catalyst inflow gas (therefore, the air-fuel mixture supplied to the engine is empty). (Fuel ratio) is feedback controlled. In this control, the first control device at least outputs the output value Voxs of the downstream air-fuel ratio sensor 56 and the change speed ΔVoxs of the output value Voxs (a value corresponding to a time differential value of the output value Voxs, an output value Voxs per unit time). Change amount). Further, the first control device uses the low threshold value VLth, the high threshold value VHth, the first change speed threshold value ΔV1th, and the second change speed threshold value ΔV2th.

The low threshold value VLth is a predetermined value between the median value Vmid and the minimum output value Vmin.
The high threshold value VHth is a predetermined value between the median value Vmid and the maximum output value Vmax.
The first change speed threshold ΔV1th and the second change speed threshold ΔV2th are both positive predetermined values. The first change speed threshold value ΔV1th and the second change speed threshold value ΔV2th may be the same or different from each other.

<Decision 1>
(A1) When the output value Voxs of the downstream air-fuel ratio sensor 56 is between the low-side threshold value VLth and the high-side threshold value VHth, the first control device displays “the output value when the output value Voxs increases” The absolute value of the change rate ΔVoxs of the output value Voxs when the output value Voxs has decreased since the absolute value | ΔVoxs | of the change rate ΔVoxs of the Voxs becomes equal to or greater than the first change rate threshold value ΔV1th. It is determined that the state of the catalyst 43 is “not in an oxygen excess state (lean negation flag XNOTlean = 1)” until the time “|” becomes equal to or greater than the second change speed threshold value ΔV2th. Then, during this period, the first control device controls the air-fuel ratio of the engine so that the air-fuel ratio of the catalyst inflow gas becomes the lean air-fuel ratio (see times t2 to t4 and times t6 to t8 in FIG. 11). The lean air-fuel ratio at this time is also referred to as “first lean air-fuel ratio” for convenience.

(A2) When the output value Voxs of the downstream air-fuel ratio sensor 56 is between the low-side threshold VLth and the high-side threshold VHth, the first control device The absolute value of the change rate ΔVoxs of the output value Voxs when the output value Voxs increases from the time when the absolute value | ΔVoxs | of the change rate ΔVoxs of the Voxs becomes equal to or greater than the second change rate threshold value ΔV2th | ΔVoxs It is determined that the state of the catalyst 43 is “not in an oxygen-deficient state (rich negative flag XNOTrich = 1)” for a period until “|” becomes the first change speed threshold value ΔV1th. Then, during this period, the first control device controls the air-fuel ratio of the engine so that the air-fuel ratio of the catalyst inflow gas becomes the rich air-fuel ratio (see times t4 to t6 in FIG. 11). The rich air-fuel ratio at this time is also referred to as “first rich air-fuel ratio” for convenience.

<Decision 2>
(B) When the output value Voxs is equal to or higher than the high-side threshold value VHth, the first control device determines that the state of the catalyst 43 is an oxygen-deficient state (rich flag XCCROrich = 1), and the air-fuel ratio of the catalyst inflow gas is The air / fuel ratio of the engine is controlled so as to be a lean air / fuel ratio. The lean air-fuel ratio at this time is also referred to as “second lean air-fuel ratio” for convenience.

(C) When the output value Voxs is equal to or lower than the low threshold value VLth, the first control device determines that the state of the catalyst 43 is an excess oxygen state (lean flag XCCROlean = 1), and the air-fuel ratio of the catalyst inflow gas is The air-fuel ratio of the engine is controlled so as to be a rich air-fuel ratio. The rich air-fuel ratio at this time is also referred to as “second rich air-fuel ratio” for convenience.

(D) The first control device is a case where the output value Voxs of the downstream air-fuel ratio sensor 56 is between the low-side threshold value VLth and the high-side threshold value VHth, and the catalyst is obtained by the above (A1) and (A2). If the output value Voxs is smaller than the median value Vmid, the catalyst is tentatively determined to be in an oxygen excess state (provisional lean flag XZlean = 1). Then, the first control device controls the air-fuel ratio of the engine so that the air-fuel ratio of the catalyst inflow gas becomes the first rich air-fuel ratio or the second rich air-fuel ratio. In such a case, for example, the output value Voxs changes from a value smaller than the lower threshold VLth to a value larger than the lower threshold VLth, and the absolute value | ΔVoxs | Occurs when the change speed threshold value ΔV1th is not exceeded.

(E) The first control device is a case where the output value Voxs of the downstream air-fuel ratio sensor 56 is between the low-side threshold value VLth and the high-side threshold value VHth, and the catalyst is obtained by the above (A1) and (A2). If the output value Voxs is greater than the median value Vmid, the catalyst is tentatively determined to be in an oxygen-deficient state (provisional rich flag XZrich = 1). The first control device controls the air-fuel ratio of the engine so that the air-fuel ratio of the catalyst inflow gas becomes the first lean air-fuel ratio or the second lean air-fuel ratio. In such a case, for example, the output value Voxs changes from a value larger than the high-side threshold value VHth to a value smaller than the high-side threshold value VHth, and the absolute value | ΔVoxs | Occurs when the change speed threshold value ΔV2th is not exceeded.

  According to these controls, for example, as shown in FIG. 12, when the fuel cut operation is executed before time t2, the oxygen storage amount OSA reaches the maximum oxygen storage amount Cmax, and then the fuel cut operation is performed. Since the output value Voxs is equal to or lower than the low-side threshold VLth when the process is completed (see time t2 in FIG. 12), the air-fuel ratio of the catalyst inflow gas is set to the rich air-fuel ratio (second rich air-fuel ratio). Thereafter, the absolute value | ΔVoxs | of the change rate ΔVoxs is changed at a time point (see time t5 in FIG. 12) where the oxygen storage amount OSA starts to decrease and becomes a small value but does not reach “0”. Since the first change speed threshold value ΔV1th is exceeded, the air-fuel ratio of the catalyst inflow gas is set to the lean air-fuel ratio (first lean air-fuel ratio).

  Further, for example, as shown in FIG. 13, when the fuel increase control for preventing catalyst overheating is executed before time t1, the oxygen storage amount OSA reaches “0”, and then the fuel increase control is terminated. In this case, since the output value Voxs is equal to or higher than the high-side threshold value VHth (see time t1 in FIG. 13), the air-fuel ratio of the catalyst inflow gas is set to the lean air-fuel ratio (second lean air-fuel ratio). Thereafter, the absolute value of the change rate ΔVoxs | ΔVoxs at a time (see time t3 in FIG. 13) at which the oxygen storage amount OSA starts to increase and becomes a large value but does not reach the maximum oxygen storage amount Cmax. Since | exceeds the second change speed threshold value ΔV2th, the air-fuel ratio of the catalyst inflow gas is set to the rich air-fuel ratio (first rich air-fuel ratio).

  The first lean air-fuel ratio and the second lean air-fuel ratio may be the same air-fuel ratio or different air-fuel ratios. Furthermore, these may be constant values or values that change. For example, the first lean air-fuel ratio increases as the absolute value | ΔVoxs | of the change speed ΔVoxs increases during the period in which the change speed ΔVoxs increases. The air-fuel ratio may be a constant value during the period in which the change rate ΔVoxs is decreasing and increasing. The second lean air-fuel ratio may be a value in which “the absolute value of the deviation between the second lean air-fuel ratio and the theoretical air-fuel ratio” increases as the absolute value of the difference between the output value Voxs and the median value Vmid increases.

  The first rich air-fuel ratio and the second rich air-fuel ratio may be the same air-fuel ratio or different air-fuel ratios. Furthermore, these may be constant values or values that change. For example, the first rich air-fuel ratio is such that, as the absolute value | ΔVoxs | of the change speed ΔVoxs increases during the period in which the change speed ΔVoxs decreases, the “absolute value of the deviation between the first rich air-fuel ratio and the stoichiometric air-fuel ratio” increases. The air-fuel ratio may be a constant value during a period in which the change rate ΔVoxs increases and increases. The second rich air-fuel ratio may be a value in which “the absolute value of the deviation between the second rich air-fuel ratio and the theoretical air-fuel ratio” increases as the absolute value of the difference between the output value Voxs and the median value Vmid increases.

(Operation)
The CPU of the first control device executes the routine shown by the flowchart in FIG. 14 every time a predetermined time elapses in order to determine the catalyst state described above. Changes in the output value Voxs of the downstream air-fuel ratio sensor 56, the flags, the air-fuel ratio of the catalyst inflow gas, and the like described below are illustrated in FIGS.

  When the predetermined timing is reached, the CPU starts processing from step 1400 in FIG. 14 and proceeds to step 1405 to determine whether or not the output value Voxs of the downstream air-fuel ratio sensor 56 is equal to or higher than the high-side threshold value VHth. The CPU proceeds to step 1410 if the output value Voxs is greater than or equal to the high threshold value VHth, sets the value of the rich flag XCCROrich to “1”, and if the output value Voxs is less than the high threshold value VHth, proceeds to step 1415. Go ahead and set the value of the rich flag XCCROrich to “0”. Note that the processing of step 1405 and step 1410 is a step of realizing the function of (B) of the determination 2 described above.

  Next, in step 1420, the CPU determines whether or not the output value Voxs of the downstream air-fuel ratio sensor 56 is equal to or lower than the low threshold value VLth. The CPU proceeds to step 1425 if the output value Voxs is equal to or lower than the low threshold value VLth, sets the value of the lean flag XCCROlean to “1”, and proceeds to step 1430 if the output value Voxs is greater than the low threshold value VLth. Go ahead and set the value of the lean flag XCCROlean to "0". Note that the processing of step 1420 and step 1425 is a step of realizing the function of (C) of determination 2 described above.

  Next, the CPU proceeds to step 1435 to determine whether or not the output value Voxs is a value between the low side threshold VLth and the high side threshold VHth. At this time, if the output value Voxs is not a value between the low threshold value VLth and the high threshold value VHth, the CPU makes a “No” determination at step 1435 to directly proceed to step 1495 to end the present routine tentatively. .

  On the other hand, if the output value Voxs is a value between the low-side threshold VLth and the high-side threshold VHth, the CPU makes a “Yes” determination at step 1435 to proceed to step 1440, where the catalyst deterioration determination execution condition is It is determined whether it is established.

This catalyst deterioration determination condition is satisfied, for example, when all of the following determination conditions are satisfied. The catalyst deterioration determination condition may be a condition that is satisfied when any one or more of the following conditions are satisfied.
Determination condition 1: The catalyst deterioration determination is not executed after the engine 10 is started this time.
Determination condition 2: The load of the engine 10 (for example, the filling rate KL or the cylinder intake air amount Mc) is within a predetermined range.
Determination condition 3: The engine speed NE is within a predetermined range.
Determination condition 4: The coolant temperature THW is within a predetermined range.

  Now, it is assumed that the catalyst deterioration determination condition is not satisfied. In this case, the CPU makes a “No” determination at step 1440 to proceed to step 1445 to set the first normal threshold value ΔV1mid to the first change speed threshold value ΔV1th and the second normal threshold value ΔV2mid to the second change speed threshold value ΔV2th. Set. The first normal threshold value ΔV1mid and the second normal threshold value ΔV2mid are set such that the air-fuel ratio feedback control described above does not fail until the degree of deterioration of the catalyst exceeds the “deterioration degree to be determined as a deteriorated catalyst” and becomes extremely large. The positive predetermined value is set (selected) in advance. That is, if the first change speed threshold value ΔV1th and the second change speed threshold value ΔV2th are respectively set to the first normal threshold value ΔV1mid and the second normal threshold value ΔV2mid, a catalyst deterioration index value (trajectory ratio) described later is shown by a broken line in FIG. It will change.

  On the other hand, if the catalyst deterioration determination condition is satisfied at the time when the CPU performs the process of step 1440, the CPU makes a “Yes” determination at step 1440 to proceed to step 1450, where the first change speed threshold ΔV1th is A first determination threshold value ΔV1small is set, and a second determination threshold value ΔV2small is set as the second change speed threshold value ΔV2th.

The first determination threshold value ΔV1small is a positive predetermined value smaller than the first normal threshold value ΔV1mid.
The second determination threshold value ΔV2small is a positive predetermined value smaller than the second normal threshold value ΔV2mid.
The first determination threshold value ΔV1small and the second determination threshold value ΔV2small are such that when the degree of deterioration of the catalyst is in the vicinity of the “degree of deterioration of the catalyst to be determined to be a deteriorated catalyst”, the air-fuel ratio feedback control described above is performed. The value is set (selected) in advance so as to start to fail. That is, if the first change speed threshold value ΔV1th and the second change speed threshold value ΔV2th are respectively set to the first determination threshold value ΔV1small and the second determination threshold value ΔV2small, the catalyst deterioration index value (trajectory ratio) described later is a solid line in FIG. It changes as shown in.

  Next, the CPU proceeds to step 1455 to determine whether or not the change rate ΔVoxs of the output value Voxs of the downstream air-fuel ratio sensor 56 is “0” or more. That is, the CPU determines whether or not the output value Voxs is increasing. The change speed ΔVoxs is calculated from “the output value Voxs at the time when the predetermined time ts has passed (current output value Voxs)” to “the output value Voxs before the predetermined time ts has passed, every time the minute predetermined time ts has passed. It is calculated separately by subtracting (previous output value Voxsold) (see FIG. 19).

  At this time, if the change rate ΔVoxs of the output value Voxs is equal to or greater than “0”, the CPU proceeds to step 1460 and “the absolute value | ΔVoxs | of the change rate ΔVoxs of the output value Voxs” is It is determined whether or not it is equal to or greater than the first change speed threshold value ΔV1th set.

  If the absolute value | ΔVoxs | of the change speed ΔVoxs is equal to or greater than the first change speed threshold value ΔV1th, the CPU makes a “Yes” determination at step 1460 to proceed to step 1465 and set the value of the lean negative flag XNOTlean to “1”. In step 1470, the rich negative flag XNOTrich is set to "0".

  On the other hand, if the absolute value | ΔVoxs | of the change speed ΔVoxs is less than the first change speed threshold value ΔV1th at the time when the CPU performs the process of step 1460, the CPU determines “No” in step 1460 and step 1495. Go directly to, and end this routine once.

  Furthermore, when the CPU performs the processing of step 1455, if the change rate ΔVoxs of the output value Voxs of the downstream air-fuel ratio sensor 56 is less than “0” (that is, the output value Voxs is decreasing), the CPU Proceeding to step 1475, it is determined whether or not “absolute value | ΔVoxs | of change speed ΔVoxs of output value Voxs” is equal to or greater than second change speed threshold value ΔV2th set in step 1445 or 1450 described above.

  If the absolute value | ΔVoxs | of the change rate ΔVoxs is equal to or greater than the second change rate threshold value ΔV2th, the CPU makes a “Yes” determination at step 1475 to proceed to step 1480 and set the value of the lean negative flag XNOTlean to “0”. In step 1485, the rich negative flag XNOTrich is set to "1".

  On the other hand, if the absolute value | ΔVoxs | of the change rate ΔVoxs is less than the second change rate threshold value ΔV2th at the time when the CPU performs the process of step 1475, the CPU makes a “No” determination at step 1475 to determine the step 1495. Go directly to, and end this routine once. Note that the processing from step 1435 to step 1485 is a step of realizing the functions of (A1) and (A2) of the determination 1 described above.

  Further, the CPU executes a routine shown by a flowchart in FIG. 15 every time a predetermined time elapses. Therefore, at a predetermined timing, the CPU starts processing from step 1500 in FIG. 15 and proceeds to step 1505 to determine whether or not the output value Voxs is a value between the low side threshold VLth and the high side threshold VHth. judge. At this time, if the output value Voxs is not a value between the low threshold value VLth and the high threshold value VHth, the CPU makes a “No” determination at step 1505 to directly proceed to step 1595 to end the present routine tentatively. To do.

  On the other hand, if the output value Voxs is a value between the low-side threshold VLth and the high-side threshold VHth at the time when the CPU performs the processing of step 1505, the CPU determines “Yes” in the step 1505. Proceeding to step 1510, it is determined whether or not all of the rich flag XCCROrich, the lean flag XCCROlean, the rich negative flag XNOTrich, and the lean negative flag XNOTlean are “0”. That is, the CPU determines whether or not the output value Voxs is a value between the low side threshold VLth and the high side threshold VHth and the catalyst state is not determined.

  At this time, if the rich flag XCCROrich, the lean flag XCCROlean, the rich negative flag XNOTrich, and the lean negative flag XNOTlean are all “0”, the CPU makes a “Yes” determination at step 1510 to proceed to step 1515 and proceed downstream. It is determined whether or not the output value Voxs of the side air-fuel ratio sensor 56 is equal to or greater than the median value Vmid. If the output value Voxs is equal to or greater than the median value Vmid, the CPU proceeds to step 1520 to set the value of the provisional rich flag XZrich to “1” and at step 1525 to set the value of the provisional lean flag XZlean to “0”. Set to.

  On the other hand, if the output value Voxs of the downstream air-fuel ratio sensor 56 is smaller than the median value Vmid at the time when the CPU performs the process of step 1515, the CPU proceeds to step 1530 and sets the value of the temporary rich flag XZrich to “0”. At the same time, in step 1535, the value of the temporary lean flag XZlean is set to “1”.

  Furthermore, when any of the rich flag XCCROrich, the lean flag XCCROlean, the rich negative flag XNOTrich, and the lean negative flag XNOTlean is “1” when the CPU executes the process of step 1510, the CPU proceeds to step 1540. Then, the value of the temporary rich flag XZrich is set to “0”, and the value of the temporary lean flag XZlean is set to “0” in step 1545. Note that the routine of FIG. 15 is a step of realizing the functions of (D) and (E) in the above-described determination 2.

  In addition, the CPU executes the “air-fuel ratio control routine” shown in the flowchart of FIG. 16 every time a predetermined time elapses. In practice, the CPU realizes the functions of the routine shown in FIG. 16 by executing the routines shown in FIGS.

  When the predetermined timing is reached, the CPU starts processing from step 1600 in FIG. 16 and proceeds to step 1610 to determine whether or not the catalyst deterioration determination execution condition is satisfied at the present time.

  If the catalyst deterioration determination execution condition is not satisfied, the CPU proceeds to step 1620 to control the air-fuel ratio of the engine (accordingly, the air-fuel ratio of the catalyst inflow gas) as described below according to the value of each flag. .

(1) When the value of the lean negative flag XNOTlean is “1”, the CPU controls the air / fuel ratio of the engine to the first lean air / fuel ratio (a constant air / fuel ratio).
(2) When one of the values of the rich flag XCCROrich and the temporary rich flag XZrich is “1”, the CPU controls the air / fuel ratio of the engine to the second lean air / fuel ratio (a constant air / fuel ratio).
(3) When the value of the rich negative flag XNOTrich is “1”, the CPU controls the air-fuel ratio of the engine to the first rich air-fuel ratio (a constant air-fuel ratio).
(4) When one of the values of the lean flag XCCROlean and the temporary lean flag XZlean is “1”, the CPU controls the air / fuel ratio of the engine to the second rich air / fuel ratio (a constant air / fuel ratio).

  The first lean air-fuel ratio and the second lean air-fuel ratio may be different or the same. The first lean air-fuel ratio and the second lean air-fuel ratio may be variable. For example, the first lean air-fuel ratio may be a lean air-fuel ratio that is further away from the theoretical air-fuel ratio as the absolute value | ΔVoxs | of the change rate ΔVoxs of the output value Voxs is larger. The second lean air-fuel ratio may be a lean air-fuel ratio proportional to the difference between the output value Voxs and the median value Vmid.

  The first rich air-fuel ratio and the second rich air-fuel ratio may be different or the same. The first rich air fuel ratio and the second rich air fuel ratio may be variable. For example, the first rich air-fuel ratio may be a rich air-fuel ratio that is further away from the theoretical air-fuel ratio as the absolute value | ΔVoxs | of the change rate ΔVoxs of the output value Voxs is larger. The second rich air-fuel ratio may be a rich air-fuel ratio proportional to the difference between the output value Voxs and the median value Vmid.

  On the other hand, if the catalyst deterioration determination execution condition is satisfied at the time when the CPU executes the process of step 1610, the CPU proceeds to step 1630 and determines the air-fuel ratio of the engine as described below according to the value of each flag. (Thus, the air-fuel ratio of the catalyst inflow gas) is controlled.

(5) When the value of the lean negative flag XNOTlean is “1”, the CPU controls the air / fuel ratio of the engine to the third lean air / fuel ratio. The third lean air-fuel ratio is a lean air-fuel ratio in which the difference from the stoichiometric air-fuel ratio increases as the absolute value | ΔVoxs | of the change speed ΔVoxs increases. The third lean air-fuel ratio is a lean air-fuel ratio in which "the absolute value of the difference between the third lean air-fuel ratio and the stoichiometric air-fuel ratio" is larger than "the absolute value of the difference between the first lean air-fuel ratio and the stoichiometric air-fuel ratio". Is set as follows. Note that the third lean air-fuel ratio may be a constant air-fuel ratio.

(6) When one of the values of the rich flag XCCROrich and the temporary rich flag XZrich is “1”, the CPU controls the air / fuel ratio of the engine to the fourth lean air / fuel ratio. The fourth lean air-fuel ratio is a lean air-fuel ratio equal to or higher than the second lean air-fuel ratio. That is, the absolute value of the difference between the fourth lean air-fuel ratio and the stoichiometric air-fuel ratio is greater than or equal to the absolute value of the difference between the second lean air-fuel ratio and the stoichiometric air-fuel ratio. The fourth lean air-fuel ratio is a constant air-fuel ratio. The fourth lean air-fuel ratio may be a lean air-fuel ratio that changes in proportion to the difference between the output value Voxs and the median value Vmid. In this example, the fourth lean air-fuel ratio and the second lean air-fuel ratio are the same air-fuel ratio.

(7) When the value of the rich negative flag XNOTrich is “1”, the CPU controls the air-fuel ratio of the engine to the third rich air-fuel ratio. The third rich air-fuel ratio is a rich air-fuel ratio in which the difference from the theoretical air-fuel ratio increases as the absolute value | ΔVoxs | of the change speed ΔVoxs increases. The third rich air-fuel ratio is such that “the absolute value of the difference between the three rich air-fuel ratio and the stoichiometric air-fuel ratio” is larger than the “absolute value of the difference between the first rich air-fuel ratio and the stoichiometric air-fuel ratio”. Set to Note that the third rich air-fuel ratio may be a constant air-fuel ratio.

(8) When one of the values of the lean flag XCCROlean and the temporary lean flag XZlean is “1”, the CPU controls the air / fuel ratio of the engine to the fourth rich air / fuel ratio. The fourth rich air-fuel ratio is a rich air-fuel ratio equal to or lower than the second rich air-fuel ratio. That is, the absolute value of the difference between the fourth rich air-fuel ratio and the stoichiometric air-fuel ratio is greater than or equal to the absolute value of the difference between the second rich air-fuel ratio and the stoichiometric air-fuel ratio. The fourth rich air-fuel ratio is a constant air-fuel ratio. The fourth rich air-fuel ratio may be a rich air-fuel ratio that changes in proportion to the difference between the output value Voxs and the median value Vmid. In this example, the fourth rich air-fuel ratio and the second rich air-fuel ratio are the same air-fuel ratio.

  Further, the CPU executes a “catalyst deterioration determination routine” shown by a flowchart in FIG. 17 every time the predetermined time ts elapses. Therefore, when the predetermined timing comes, the CPU starts the process from step 1700 in FIG. 17 and proceeds to step 1705 to determine whether or not the above-described catalyst deterioration determination execution condition is satisfied.

  At this time, if the catalyst deterioration determination execution condition is not satisfied, the CPU makes a “No” determination at step 1705 to directly proceed to step 1795 to end the present routine tentatively.

  On the other hand, if the catalyst deterioration determination execution condition is satisfied when the CPU executes the process of step 1705, the CPU determines “Yes” in step 1705 and proceeds to step 1710 to determine the catalyst deterioration determination. It is determined whether or not the condition is satisfied for a predetermined time or more.

  At this time, if the state in which the catalyst deterioration determination condition is satisfied has not elapsed for a predetermined time or more, the CPU determines “No” in step 1710, sequentially performs the processing of steps 1715 to 1745 described below, Proceeding to step 1795, the present routine is temporarily ended.

  Step 1715: The CPU obtains the absolute value of the difference between the current output value Voxs of the downstream air-fuel ratio sensor 56 and the previous output value Voxsolds as a unit time trajectory length dVoxs. Here, the previous output value Voxsolds is the output value Voxs when this routine was executed last time (that is, at a time point before the predetermined time ts). The previous output value Voxsolds is set to the median value Vmid when the catalyst deterioration determination execution condition is not satisfied.

Step 1720: The CPU stores the current output value Voxs as the previous output value Voxsolds.
Step 1725: The CPU updates the downstream air-fuel ratio sensor output value trajectory length LVoxs by adding the intra-unit time trajectory length dVoxs obtained in step 1715 to the current downstream air-fuel ratio sensor output value trajectory length LVoxs. To do. The downstream air-fuel ratio sensor output value locus length LVoxs is set to “0” when the catalyst deterioration determination execution condition is not satisfied.

Step 1730: The CPU obtains the detected upstream air-fuel ratio abyfs by applying the output value Vabyfs of the upstream air-fuel ratio sensor 55 to the table Mapabyfs shown in FIG.
Step 1735: The CPU obtains the absolute value of the difference between the current detected upstream air-fuel ratio abyfs acquired in step 1730 and the previous detected upstream air-fuel ratio abyfsold as a unit time trajectory length dabyfs. Here, the previous detected upstream air-fuel ratio abyfsold is the detected upstream air-fuel ratio abyfsold when this routine was executed last time (that is, at a time point before the predetermined time ts). The previous detected upstream air-fuel ratio abyfsold is set to a value corresponding to the theoretical air-fuel ratio when the catalyst deterioration determination execution condition is not satisfied.

Step 1740: The CPU stores the current detected upstream air-fuel ratio abyfs as the previous detected upstream air-fuel ratio abyfsold.
Step 1745: The CPU adds upstream trajectory length dabyfs obtained in Step 1735 to the upstream upstream air-fuel ratio sensor output value trajectory length (detected upstream air-fuel ratio trajectory length) Labyfs, thereby obtaining The fuel ratio sensor output value trajectory length Labyfs is updated. The upstream air-fuel ratio sensor output value trajectory length Labyfs is set to “0” when the catalyst deterioration determination execution condition is not satisfied.

  Thereafter, if the catalyst deterioration determination execution condition continues to be satisfied, an elapsed time after the catalyst deterioration determination execution condition is satisfied exceeds a predetermined time. At this time, when the CPU executes the processing of step 1710, the CPU makes a “Yes” determination at step 1710 to proceed to step 1750, and the downstream air-fuel ratio sensor output value locus length LVoxs is determined as the upstream air-fuel ratio sensor output value. It is determined whether or not the value divided by the locus length Labyfs (the catalyst deterioration index value LVoxs / Labyfs, also referred to as the locus ratio) is equal to or greater than the deterioration determination reference value Rth.

  At this time, if the catalyst deterioration index value LVoxs / Labyfs is greater than or equal to the deterioration determination reference value Rth, the CPU determines that the catalyst 43 has deteriorated and proceeds to step 1755 to set the value of the catalyst deterioration detection flag Xrekka to “1”. Set to. The value of the catalyst deterioration detection flag Xrekka is stored in the backup RAM. Thereafter, the CPU sets the value of the catalyst deterioration determination execution flag XHJ to “1” in step 1760, and proceeds to step 1795 to end the present routine tentatively.

  On the other hand, if the catalyst deterioration index value LVoxs / Labyfs is smaller than the deterioration determination reference value Rth at the time when the CPU performs the processing of step 1750, the CPU determines that the catalyst 43 has not deteriorated. Then, the CPU proceeds directly from step 1750 to step 1760, and then proceeds to step 1795 to end the present routine tentatively. The first controller performs the catalyst deterioration determination as described above.

In this example, the catalyst deterioration index value is a value (trajectory ratio) obtained by dividing the downstream air-fuel ratio sensor output value trajectory length LVoxs by the upstream air-fuel ratio sensor output value trajectory length Labyfs. The value is not particularly limited as long as the amplitude and fluctuation frequency of the output value Voxs of the sensor 56 increases or the value decreases as the amplitude and fluctuation frequency of the output value Voxs of the downstream air-fuel ratio sensor 56 increases. Examples of such catalyst deterioration index values include the following values.
-Downstream air-fuel ratio sensor output value trajectory length LVoxs
The frequency of the output value Voxs of the downstream air-fuel ratio sensor 56 The area surrounded by the output value Voxs of the downstream air-fuel ratio sensor 56 and the median value Vmid The reciprocal of the downstream air-fuel ratio sensor output value trajectory length LVoxs The reversal cycle of the output value Voxs of the fuel ratio sensor 56, the reciprocal of the area surrounded by the output value Voxs of the downstream air-fuel ratio sensor 56 and the median value Vmid, the upstream air-fuel ratio sensor output value trajectory length Labyfs, and the downstream air-fuel ratio sensor output Value divided by value locus length LVoxs

(Actual detailed operation)
Next, the actual detailed operation of the CPU will be described.
<Fuel injection control>
The CPU performs the routine for calculating the final fuel injection amount Fi and the injection instruction shown in the flowchart of FIG. 18 according to a predetermined crank angle (for example, BTDC 90 ° CA) before the intake top dead center of each cylinder. Each time it becomes, it is executed repeatedly. Therefore, when the crank angle of an arbitrary cylinder reaches the predetermined crank angle, the CPU starts the process from step 1800, sequentially performs the processes of steps 1810 to 1860 described below, and proceeds to step 1895.

  Step 1810: The CPU obtains (estimates / determines) the in-cylinder intake air amount Mc (k) to be drawn into the “cylinder that reaches the current intake stroke” based on the table MapMc (Ga, NE). The cylinder that reaches this intake stroke is also referred to as a “fuel injection cylinder”. Ga is the intake air amount measured by the air flow meter 51. NE is an engine speed that is separately required. The in-cylinder intake air amount Mc (k) is stored in the RAM while corresponding to the intake stroke of each cylinder. The CPU may estimate the in-cylinder intake air amount Mc (k) using a known “air model”.

  Step 1820: The CPU divides the in-cylinder intake air amount Mc (k) by the upstream target air-fuel ratio abyfr to thereby obtain a basic fuel injection amount Fbase for making the engine air-fuel ratio coincide with the upstream target air-fuel ratio abyfr. Ask. In this case, the upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich (14.7 in this example). Accordingly, the basic fuel injection amount Fbase is a feedforward amount for making the air-fuel ratio of the engine coincide with the stoichiometric air-fuel ratio.

  Step 1830: The CPU calculates a final fuel injection amount (indicated injection amount) Fi by correcting the basic fuel injection amount Fbase based on the main feedback amount DFmain and the sub feedback amount KFsub. That is, the CPU obtains the final fuel injection amount Fi by adding the main feedback amount DFmain to a value obtained by multiplying the basic fuel injection amount Fbase by the sub feedback amount KFsub. The main feedback amount DFmain and the sub feedback amount KFsub are correction amounts for correcting the basic fuel injection amount Fbase, and are also referred to as “air-fuel ratio correction amounts” individually or in combination.

  Step 1840: The CPU determines whether or not a fuel cut (fuel supply cutoff) condition is satisfied. The fuel cut condition (FC condition) is satisfied, for example, when the accelerator pedal operation amount Accp or the throttle valve opening degree TA is “0” and the engine speed NE is equal to or higher than the fuel cut speed NEFC. Further, the fuel cut condition is that the accelerator pedal operation amount Accp or the throttle valve opening TA is not “0” during the fuel cut (when the fuel cut condition is established), or the engine speed NE is the fuel cut return speed NEFK. It is not established when: The fuel cut return rotational speed NEFK is smaller than the fuel cut rotational speed NEFC.

  When the fuel cut condition is satisfied, the CPU makes a “Yes” determination at step 1840 to proceed to step 1850, sets the final fuel injection amount Fi to “0”, and then proceeds to step 1860. On the other hand, when the fuel cut condition is not established, the CPU makes a “No” determination at step 1840 to directly proceed to step 1860.

  Step 1860: The CPU issues an injection instruction to the fuel injection valve 25 so that the fuel of the final fuel injection amount (instructed injection amount) Fi is injected from the fuel injection valve 25 for the fuel injection cylinder. Accordingly, when the fuel cut condition is satisfied, the final fuel injection amount Fi is “0”, so that fuel injection is not executed.

<Acquisition of change rate ΔVoxs of output value of downstream air-fuel ratio sensor>
The CPU executes the “downstream air-fuel ratio sensor output value change rate acquisition routine” shown in the flowchart of FIG. 19 every time the predetermined time ts elapses. Accordingly, when the predetermined timing comes, the CPU starts the process from step 1900 of FIG. 19 and proceeds to step 1910, from the “output value Voxs of the downstream air-fuel ratio sensor 56 at the present time” to the “output value Voxs before the predetermined time ts”. The value obtained by subtracting the previous output value Voxsold is obtained as “the change rate ΔVoxs of the output value Voxs of the downstream air-fuel ratio sensor 56”.

  Next, the CPU proceeds to step 1920 to store the current output value Voxs of the downstream air-fuel ratio sensor 56 as the previous output value Voxsold. Thereafter, the CPU proceeds to step 1995 to end the present routine tentatively.

<Calculation of main feedback amount>
The CPU executes a “main feedback amount calculation routine” shown by a flowchart in FIG. 20 every time a predetermined time elapses. Therefore, when the predetermined timing comes, the CPU starts the process from step 2000 in FIG. 20 and proceeds to step 2005 to determine whether or not the “main feedback control condition (upstream air-fuel ratio feedback control condition)” is satisfied. To do.

The main feedback control condition is satisfied when all of the following conditions are satisfied.
(A-1) The upstream air-fuel ratio sensor 55 is activated.
(A-2) The engine load (load factor) KL is equal to or less than the threshold KLth.
(A-3) Fuel cut is not in progress.

  Note that the load factor KL is obtained by an expression of KL = (Mc (k) / (ρ · L / 4)) · 100%. In this equation, Mc (k) is the in-cylinder intake air amount, ρ is the air density (unit is (g / l)), L is the displacement of the engine 10 (unit is (l)), and “4” is The number of cylinders of the engine 10. Note that the accelerator pedal operation amount Accp may be used as the engine load instead of the load factor KL.

  The description will be continued assuming that the main feedback control condition is satisfied. In this case, the CPU makes a “Yes” determination at step 2005 to sequentially perform the processing from step 2010 to step 2035 described below, and proceeds to step 2095 to end the present routine tentatively.

Step 2010: The CPU obtains the detected upstream air-fuel ratio abyfs by applying the output value Vabyfs of the upstream air-fuel ratio sensor 55 to the table Mapabyfs shown in FIG.
Step 2015: The CPU obtains “in-cylinder fuel supply amount Fc (k−N)” which is “the amount of fuel actually supplied to the combustion chamber 21 at a time point N cycles before the current time point”. That is, the CPU divides “the in-cylinder intake air amount Mc (k−N) at a point N cycles before the current point (ie, N · 720 ° crank angle)” by “the detected upstream air-fuel ratio abyfs”. Thus, the in-cylinder fuel supply amount Fc (k−N) is obtained. Thus, in order to obtain the in-cylinder fuel supply amount Fc (k−N), the in-cylinder intake air amount Mc (k−N) N strokes before the current stroke is divided by the detected upstream air-fuel ratio abyfs. This is because “a time corresponding to the N stroke” is required until “the exhaust gas generated by the combustion of the air-fuel mixture in the combustion chamber 21” reaches the upstream air-fuel ratio sensor 55.

  Step 2020: The CPU divides the in-cylinder intake air amount Mc (k−N) N strokes before the current time by the upstream target air-fuel ratio abyfr (theoretical air-fuel ratio in this example), thereby obtaining the target cylinder. The internal fuel supply amount Fcr (k−N) is obtained. The target in-cylinder fuel supply amount Fcr (k−N) is the amount of fuel that should have been supplied to the combustion chamber 21 at a time point N cycles before the current time point.

  Step 2025: The CPU obtains an in-cylinder fuel supply amount deviation DFc by subtracting the in-cylinder fuel supply amount Fc (k−N) from the target in-cylinder fuel supply amount Fcr (k−N). This in-cylinder fuel supply amount deviation DFc is an amount representing the excess or deficiency of the fuel supplied into the cylinder at the time point before the N stroke.

  Step 2030: The CPU obtains a main feedback amount DFmain by multiplying the in-cylinder fuel supply amount deviation DFc by a preset proportional gain Gp. Thus, the “main feedback amount DFmain” for making the detected upstream air-fuel ratio abyfs coincide with the upstream target air-fuel ratio abyfr is calculated.

  Step 2035: The CPU corrects (limits) the main feedback amount DFmain according to the “catalyst inflow gas required air-fuel ratio” by executing the routine shown in FIG. The routine shown in FIG. 21 will be described later.

  As described above, the main feedback amount DFmain is obtained, and this main feedback amount DFmain is reflected in the final fuel injection amount Fi by the process of step 1830 in FIG. The CPU adds “the integral term Gi · SDFc obtained by multiplying the integral value SDFc of the in-cylinder fuel supply amount deviation DFc by the integral gain Gi” to “the proportional term Gp · DFc”, thereby adding the main feedback amount DFmain. You may ask for.

  On the other hand, if the main feedback control condition is not satisfied at the time of determination in step 2005 of FIG. 20, the CPU determines “No” in step 2005 and proceeds to step 2040 to set the value of the main feedback amount DFmain to “0”. To "". Thereafter, the CPU proceeds to step 2095 to end the present routine tentatively. Thus, when the main feedback control condition is not satisfied, the main feedback amount DFmain is set to “0”. Accordingly, the basic fuel injection amount Fbase is not corrected by the main feedback amount DFmain.

<Restriction of main feedback amount>
Furthermore, as described above, when the CPU proceeds to step 2035 in FIG. 20, the CPU executes a “main feedback amount restriction (correction) routine” shown in the flowchart in FIG.

  Therefore, when the predetermined timing comes, the CPU starts processing from step 2100 in FIG. 21 and proceeds to step 2110 to determine whether or not the main feedback amount DFmain is positive. That is, in step 2110, the CPU sets “a value by which the main feedback amount DFmain increases and corrects the basic fuel injection amount Fbase” (“the air / fuel ratio of the catalyst inflow gas equal to the air / fuel ratio of the engine”) It is determined whether or not the value to be corrected to the air-fuel ratio of ")".

  At this time, if the value of the main feedback amount DFmain is positive (that is, if the main feedback amount DFmain is a value for shifting the air-fuel ratio of the catalyst inflow gas to the rich air-fuel ratio), the CPU at step 2110 It determines with "Yes" and progresses to step 2120, and it is determined whether at least one of the rich flag XCCROrich, the lean negative flag XNOTlean, and the temporary rich flag XZrich is "1". In other words, in step 2120, the CPU determines whether the state of the catalyst 43 is either “oxygen-deficient state” or “non-oxygen-excess state”, and whether the catalyst inflow gas required air-fuel ratio is a lean air-fuel ratio. Determine.

  At this time, if at least one of the rich flag XCCROrich, the lean negative flag XNOTlean, and the provisional rich flag XZrich is “1”, the CPU makes a “Yes” determination at step 2120 to proceed to step 2130, and the main feedback amount The value of DFmain is set to “0”. Thus, the main feedback amount DFmain does not correct the air / fuel ratio of the catalyst inflow gas to an air / fuel ratio (rich air / fuel ratio) different from the “catalyst inflow gas required air / fuel ratio (in this case, lean air / fuel ratio)”. Limited (corrected).

  In step 2130, the CPU may set a value obtained by multiplying the main feedback amount DFmain by a positive coefficient smaller than “1” as the final main feedback amount DFmain. That is, the CPU may reduce the size of the main feedback amount DFmain at step 2130.

  On the other hand, when the CPU proceeds to step 2120, if all of the rich flag XCCROrich, the lean negative flag XNOTlean, and the provisional rich flag XZrich are “0”, the CPU determines “No” in step 2120, The process directly proceeds to step 2195 to end the present routine tentatively.

  On the other hand, when the CPU proceeds to step 2110, if the value of the main feedback amount DFmain is negative (or 0) (that is, the main feedback amount DFmain attempts to shift the air-fuel ratio of the catalyst inflow gas to the lean air-fuel ratio). If it is a value, the CPU makes a “No” determination at step 2110 to proceed to step 2140 to determine whether at least one of the lean flag XCCROlean, the rich negative flag XNOTrich, and the temporary lean flag XZlean is “1”. Determine whether. In other words, in step 2140, the CPU determines whether the state of the catalyst 43 is either “oxygen-excess state” or “non-oxygen state”, and whether the catalyst inflow gas required air-fuel ratio is a rich air-fuel ratio. Determine.

  At this time, if at least one of the lean flag XCCROlean, the rich negative flag XNOTrich, and the provisional lean flag XZlean is “1”, the CPU makes a “Yes” determination at step 2140 to proceed to step 2150, and the main feedback amount The value of DFmain is set to “0”. Thus, the main feedback amount DFmain does not correct the air / fuel ratio of the catalyst inflow gas to an air / fuel ratio (lean air / fuel ratio) different from the “catalyst inflow gas required air / fuel ratio (in this case, rich air / fuel ratio)”. Limited (corrected).

  In step 2150, the CPU may set a value obtained by multiplying the main feedback amount DFmain by a positive coefficient smaller than “1” as the final main feedback amount DFmain. That is, the CPU may decrease the size of the main feedback amount DFmain at step 2150.

  On the other hand, when the CPU proceeds to step 2140, if all of the lean flag XCCROlean, the rich negative flag XNOTrich, and the temporary lean flag XZlean are “0”, the CPU determines “No” in step 2140, The process directly proceeds to step 2195 to end the present routine tentatively.

<Calculation of sub feedback amount>
The CPU executes a “sub feedback amount calculation routine” shown by a flowchart in FIG. 22 every time a predetermined time elapses. Therefore, at the predetermined timing, the CPU starts the process from step 2200 in FIG. 22 and proceeds to step 2205 to determine whether or not the “sub feedback control condition (downstream air-fuel ratio feedback control condition)” is satisfied. To do.

The sub-feedback control condition is satisfied when all of the following conditions are satisfied.
(B-1) The main feedback control condition is satisfied.
(B-2) The downstream air-fuel ratio sensor 56 is activated.
(B-3) The upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich.

  The description will be continued assuming that the sub-feedback control condition is satisfied and further assuming that the catalyst deterioration determination execution condition is not satisfied. In this case, the CPU makes a “Yes” determination at step 2205 to proceed to step 2210 to determine whether at least one of the rich flag XCCROrich and the provisional rich flag XZrich is “1”. In other words, the CPU determines in step 2210 whether the state of the catalyst 43 is “oxygen-deficient state” and the catalyst inflow gas required air-fuel ratio is a lean air-fuel ratio.

  At this time, if both the rich flag XCCROrich and the temporary rich flag XZrich are “0”, the CPU makes a “No” determination at step 2210 to directly proceed to step 2220.

  On the other hand, if at least one of the rich flag XCCROrich and the temporary rich flag XZrich is “1”, the CPU makes a “Yes” determination at step 2210 to proceed to step 2215 to set the value of the sub feedback amount KFsub. Set to a predetermined value A2. The predetermined value A2 is a constant value larger than “0” and smaller than “1”, and realizes an air-fuel ratio corresponding to the second lean air-fuel ratio (equal to the fourth lean air-fuel ratio in this example). Value. Thereafter, the CPU proceeds to step 2220.

  In step 2220, the CPU determines whether or not the value of the lean negative flag XNOTlean is “1”. At this time, if the value of the lean negative flag XNOTlean is “0”, the CPU makes a “No” determination at step 2220 to proceed directly to step 2240.

  On the other hand, if the value of the lean negative flag XNOTlean is “1”, the CPU makes a “Yes” determination at step 2220 to proceed to step 2225 to determine whether or not the catalyst deterioration determination execution condition is satisfied. To do. According to the above assumption, the catalyst deterioration determination execution condition is not satisfied. Therefore, the CPU makes a “No” determination at step 2225 to proceed to step 2230 to set the value of the sub feedback amount KFsub to the predetermined value A1. The predetermined value A1 is a constant value larger than “0” and smaller than “1”, and is a value that realizes an air-fuel ratio corresponding to the first lean air-fuel ratio. Thereafter, the CPU proceeds to step 2240. The predetermined value A1 and the predetermined value A2 may be equal.

  In step 2240, the CPU determines whether at least one of the lean flag XCCROlean and the temporary lean flag XZlean is “1”. In other words, the CPU determines in step 2240 whether or not the state of the catalyst 43 is “oxygen excess state” and the catalyst inflow gas required air-fuel ratio is a rich air-fuel ratio.

  At this time, if both the lean flag XCCROlean and the temporary lean flag XZlean are “0”, the CPU makes a “No” determination at step 2240 to directly proceed to step 2250.

  On the other hand, if at least one of the lean flag XCCROlean and the temporary lean flag XZlean is “1”, the CPU makes a “Yes” determination at step 2240 to proceed to step 2245 to set the value of the sub feedback amount KFsub. Set to a predetermined value B2. This predetermined value B2 is a constant value larger than “1”, and is a value that realizes an air-fuel ratio corresponding to the second rich air-fuel ratio (equal to the fourth rich air-fuel ratio in this example). Thereafter, the CPU proceeds to step 2250.

  In step 2250, the CPU determines whether the value of the rich negative flag XNOTrich is “1”. At this time, if the value of the rich negative flag XNOTrich is “0”, the CPU makes a “No” determination at step 2250 to directly proceed to step 2295 to end the present routine tentatively.

  On the other hand, if the value of the rich negative flag XNOTrich is “1”, the CPU makes a “Yes” determination at step 2250 to proceed to step 2255 to determine whether or not the catalyst deterioration determination execution condition is satisfied. To do. According to the above assumption, the catalyst deterioration determination execution condition is not satisfied. Therefore, the CPU makes a “No” determination at step 2255 to proceed to step 2260 to set the value of the sub feedback amount KFsub to the predetermined value B1. The predetermined value B1 is a constant value larger than “1”, and is a value that realizes an air-fuel ratio corresponding to the first rich air-fuel ratio. Thereafter, the CPU proceeds to step 2295 to end the present routine tentatively. The predetermined value B1 and the predetermined value B2 may be equal.

  The sub-feedback amount KFsub thus obtained is multiplied by the basic fuel injection amount Fbase for obtaining the stoichiometric air-fuel ratio by the processing of step 1830 in FIG. Therefore, when the sub-feedback amount KFsub is larger than “1”, the air-fuel ratio of the engine (and hence the air-fuel ratio of the catalyst inflow gas) is “the richer the difference between the stoichiometric air-fuel ratio is larger as the sub-feedback amount KFsub is larger. The air / fuel ratio is controlled. When the sub-feedback amount KFsub is smaller than “1”, the air-fuel ratio of the engine is controlled to “a lean air-fuel ratio in which the difference from the stoichiometric air-fuel ratio is larger as the sub-feedback amount KFsub is smaller”.

  Next, the description will be continued assuming that the sub-feedback control condition is satisfied, and further assuming that the catalyst deterioration determination execution condition is satisfied. In this case, except for the processing described below, the CPU performs the same processing as when the sub feedback control condition is satisfied and the catalyst deterioration determination execution condition is not satisfied.

  That is, when the CPU proceeds to step 2225, the CPU makes a “Yes” determination at step 2225 to proceed to step 2235, where the sub-feedback amount KFsub is set to “the absolute value of change speed ΔVoxs | A value (1−kd · | ΔVoxs |) ”obtained by subtracting a value obtained by multiplying by“ 1 ”from“ 1 ”is set.

  However, the value (1-kd · | ΔVoxs |) is limited to be larger than “0” and smaller than the predetermined value A1. The value (1-kd · | ΔVoxs |) decreases as the absolute value | ΔVoxs | of the change rate ΔVoxs increases. Therefore, according to the sub feedback amount KFsub set in step 2235, the basic fuel injection amount Fbase is corrected to decrease more as the absolute value | ΔVoxs | of the change speed ΔVoxs increases. The lean air-fuel ratio (the air-fuel ratio having a larger difference from the theoretical air-fuel ratio) is corrected. This value (1-kd · | ΔVoxs |) is a value that realizes an air-fuel ratio corresponding to the third lean air-fuel ratio. Thereafter, the CPU proceeds to step 2240.

  Further, when the CPU proceeds to step 2255, the CPU makes a “Yes” determination at step 2255 to proceed to step 2265, in which the sub feedback amount KFsub is set to “the absolute value of change speed ΔVoxs | Is set to a value (1 + kd · | ΔVoxs |) ”obtained by adding a value obtained by multiplying a constant of (1) to“ 1 ”.

  However, the value (1 + kd · | ΔVoxs |) is limited to be larger than the predetermined value B1. The value (1 + kd · | ΔVoxs |) increases as the absolute value | ΔVoxs | of the change rate ΔVoxs increases. Therefore, according to the sub-feedback amount KFsub set in step 2265, the basic fuel injection amount Fbase is corrected to increase as the absolute value | ΔVoxs | of the change speed ΔVoxs increases. The air-fuel ratio is corrected to “rich air-fuel ratio (rich air-fuel ratio having a larger difference from the theoretical air-fuel ratio)”. This value (1 + kd · | ΔVoxs |) is a value that realizes an air-fuel ratio corresponding to the third rich air-fuel ratio. Thereafter, the CPU proceeds to step 2295 to end the present routine tentatively.

  The sub-feedback amount KFsub thus obtained is multiplied by the basic fuel injection amount Fbase for obtaining the stoichiometric air-fuel ratio by the processing of step 1830 in FIG. .

On the other hand, if the sub feedback control condition is not satisfied at the time when the CPU performs the process of step 2205, the CPU makes a “No” determination at step 2205 to proceed to step 2260, and set the sub feedback amount KFsub to “1”. .0 ". Thereafter, the CPU proceeds to step 2295 to end the present routine tentatively. As a result, when the sub feedback control condition is not satisfied, the basic fuel injection amount Fbase is not corrected by the sub feedback amount KFsub.

  As described above, in the first control device, when the catalyst deterioration determination execution condition is satisfied, the first change speed threshold value ΔV1th and the second change speed threshold value ΔV2th are not satisfied. In this case, the first change speed threshold value ΔV1th and the second change speed threshold value ΔV2th are set to smaller values (see step 1440 to step 1445 in FIG. 14).

  Further, the first control device sets the third lean air-fuel ratio and the third rich air-fuel ratio to values having a larger difference from the stoichiometric air-fuel ratio than the first lean air-fuel ratio and the first rich air-fuel ratio, respectively. (See step 1630 in FIG. 16, step 2245 and step 2265 in FIG. 22, etc.).

  As a result, the first control device determines that the catalyst deterioration index value of the catalyst 43 is determined if the degree of deterioration of the catalyst 43 is close to “the degree of deterioration of the catalyst to be determined to be deteriorated” at the time of catalyst deterioration determination. The air-fuel ratio of the engine can be controlled so as to change more greatly as the degree of deterioration progresses. Therefore, the catalyst deterioration determination can be executed with higher accuracy.

  Note that the CPU may set the values of the lean negative flag XNOTlean and the rich flag to “0” in step 1415 and / or step 1430 of FIG.

Second Embodiment
Next, an air-fuel ratio control apparatus (hereinafter also referred to as “second control apparatus”) for an internal combustion engine according to a second embodiment of the present invention will be described. After the time point when it is determined that the catalyst 43 has deteriorated, the second control device determines the magnitude of the air-fuel ratio correction width based on the sub-feedback amount KFsub (air-fuel ratio correction amount) (actually, “1” and sub-feedback). It differs from the first control device only in that the absolute value of the difference from the amount KFsub is made smaller than when it is not determined that the catalyst 43 has deteriorated. Therefore, the following description will be made with this difference as the center.

  The CPU of the second control device includes routines executed by the CPU of the first control device (the routines shown in FIGS. 14, 15, and 17 to 21 except for the routine shown in FIG. 22) and the routine of FIG. The “sub-feedback amount calculation routine” in which the steps shown in FIG. 23 are added is executed every time a predetermined time elapses. Therefore, hereinafter, the description will focus on differences from the first control device in the sub feedback amount calculation routine.

  After the processing of step 2260 or step 2265 in FIG. 22, the CPU proceeds to step 2310 in FIG. 23 (see “1” in the circle in the figure). Then, the CPU indicates that the value of the catalyst deterioration detection flag Xrekka is “1”. It is determined whether or not there is. That is, the CPU determines whether or not it is determined that the catalyst 43 has deteriorated.

  At this time, if the value of the catalyst deterioration detection flag Xrekka is “0”, the CPU makes a “No” determination at step 2310 to directly proceed to step 2295 of FIG. 22 to end the present routine temporarily (◯ in the figure). (See item 2).)

  On the other hand, if the value of the catalyst deterioration detection flag Xrekka is “1” at the time when the CPU executes the process of step 2310, the CPU makes a “Yes” determination at step 2310 to proceed to step 2320. A value (1 + km · (KFsub−) obtained by adding “1” to a value obtained by subtracting “1” from the sub-feedback amount KFsub calculated up to that point (KFsub−1) ”and a constant km. 1)) is set as the final sub-feedback amount KFsub. The constant km is a value larger than “0” and smaller than “1” (for example, km = 0.9).

  That is, the CPU brings the sub feedback amount KFsub closer to “1” after the time when it is determined that the catalyst 43 has deteriorated. For example, when the sub feedback amount KFsub obtained up to that point is 1 + α, the finally obtained sub feedback amount KFsub is 1 + km · α. This is because the sub-feedback amount KFsub obtained up to that point is a value (= 1 + α1, α1> 0) that corrects the air / fuel ratio of the engine to a predetermined rich air / fuel ratio AR1. The feedback amount KFsub is a value (1 + km · α1) that realizes the rich air-fuel ratio AR2 that is closer to the stoichiometric air-fuel ratio than the rich air-fuel ratio AR1. Similarly, when the sub-feedback amount KFsub obtained up to that point is a value that corrects the air-fuel ratio of the engine to a predetermined lean air-fuel ratio AL1 (= 1−α2, α2> 0), it is finally obtained. The sub feedback amount KFsub is a value (1-km · α2) that realizes a lean air-fuel ratio AL2 that is closer to the theoretical air-fuel ratio than the lean air-fuel ratio AL1.

  As a result, the basic fuel injection amount Fbase is corrected by a smaller amount when the value of the catalyst deterioration detection flag Xrekka is “1” than when the value of the catalyst deterioration detection flag Xrekka is “0”.

  The second control device executes the routine of FIG. 22 that does not add the routine of FIG. 23, and when the value of the catalyst deterioration detection flag Xrekka is “1”, the predetermined control used in step 2230 of FIG. The value A1 may be corrected to a value closer to “1” than when the value of the catalyst deterioration detection flag Xrekka is “0”. That is, the second control device may set the first lean air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio after the time when it is determined that the catalyst 43 has deteriorated. Further, the second control device executes the routine of FIG. 22 that does not add the routine of FIG. 23, and when the value of the catalyst deterioration detection flag Xrekka is “1”, the predetermined control used in step 2215 of FIG. The value A2 may be corrected to a value closer to “1” than when the value of the catalyst deterioration detection flag Xrekka is “0”. That is, the second control device may set the second lean air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio after the time point when it is determined that the catalyst 43 has deteriorated.

  Similarly, the second control device executes the routine of FIG. 22 in which the routine of FIG. 23 is not added, and when the value of the catalyst deterioration detection flag Xrekka is “1”, it is used in step 2260 of FIG. The predetermined value B1 may be corrected to a value closer to “1” than when the value of the catalyst deterioration detection flag Xrekka is “0”. That is, the second control device may set the first rich air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio after the time point when it is determined that the catalyst 43 has deteriorated. Further, the second control device executes the routine of FIG. 22 in which the routine of FIG. 23 is not added, and when the value of the catalyst deterioration detection flag Xrekka is “1”, the predetermined control used in step 2245 of FIG. The value B2 may be corrected to a value closer to “1” than when the value of the catalyst deterioration detection flag Xrekka is “0”. That is, the second control device may set the second rich air-fuel ratio to an air-fuel ratio closer to the stoichiometric air-fuel ratio after the time point when it is determined that the catalyst 43 has deteriorated.

  According to this second control apparatus, when it is determined that the catalyst 43 has deteriorated, the “deviation from the stoichiometric air-fuel ratio” of the air-fuel ratio of the catalyst inflow gas compared to before the determination that the catalyst 43 has deteriorated. Becomes smaller. That is, after the determination that the catalyst 43 has deteriorated, the magnitude of the correction amount of the air-fuel ratio in the normal feedback control (the absolute value of the difference between 1 and the sub feedback amount KFsub) decreases. As a result, excess oxygen or excess unburned material is unlikely to flow into the deteriorated catalyst, and normal feedback control is unlikely to fail. Therefore, the second control device can maintain the emission well even when the catalyst 43 deteriorates.

  As described above, the control device according to each embodiment of the present invention can maintain the emission satisfactorily and perform the catalyst deterioration determination with high accuracy. In addition, this invention is not limited to the said embodiment, A various modified example is employable within the scope of the present invention. For example, the above embodiment uses the temporary rich flag XZrich and the temporary lean flag XZlean, but after the rich flag XCCROrich is set to “1”, the rich negative flag XNOTrich is set to “1” or The value of the rich flag XCCROrich may be maintained at “1” until the lean flag XCCROlean is set to “1”. Similarly, after the lean flag XCCROlean is set to “1”, the value of the lean flag XCCROlean is set to “1” until the lean negative flag XNOTlean is set to “1” or the rich flag XCCROrich is set to “1”. 1 "may be maintained.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 41 ... Exhaust manifold, 41b ... Collecting part, 42 ... Exhaust pipe, 43 ... Upstream catalyst (three-way catalyst), 55 ... Upstream air-fuel ratio sensor, 56 ... Downstream air-fuel ratio sensor (oxygen concentration sensor) ), 60 ... electric control device.

Claims (3)

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 that is disposed downstream of the catalyst in the exhaust passage and outputs an output value corresponding to the concentration of oxygen contained in the gas flowing out of the catalyst; ,
Air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine so as to change the air-fuel ratio of the catalyst inflow gas which is a gas flowing into the catalyst based on the output value;
When a predetermined catalyst deterioration determination execution condition is satisfied, a value that increases or decreases as the amplitude and fluctuation frequency of the output value increases as the catalyst deterioration index value indicating the degree of deterioration of the catalyst based on the output value. Catalyst deterioration determination means for determining whether or not the catalyst has deteriorated based on the acquired catalyst deterioration index value;
In an air-fuel ratio control apparatus for an internal combustion engine comprising:
The air-fuel ratio control means includes
When the output value is between a predetermined low-side threshold and a predetermined high-side threshold greater than the low-side threshold, the absolute value of the change rate of the output value when the output value is increasing is predetermined The time when the absolute value of the change speed of the output value when the output value has decreased from the time when the output value becomes equal to or higher than the first change speed threshold or the output value becomes higher than the predetermined second change speed threshold. The output value is decreased so that the air-fuel ratio of the catalyst inflow gas becomes a predetermined first lean air-fuel ratio leaner than the stoichiometric air-fuel ratio in the first period until the point when the value becomes greater than or equal to the side threshold value. The absolute value of the change rate of the output value when the output value increases from the time when the absolute value of the change rate of the output value becomes equal to or greater than the second change rate threshold. Or when the output value is greater than or equal to the lower threshold As the air-fuel ratio of the catalyst inflow gas in the second period to the point made is first rich air-fuel ratio richer predetermined than the stoichiometric air-fuel ratio,
When the output value is equal to or higher than the high-side threshold, the air-fuel ratio of the catalyst inflow gas becomes a predetermined second lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio.
When the output value is equal to or lower than the low-side threshold, the air-fuel ratio of the catalyst inflow gas becomes a predetermined second rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio.
While controlling the air-fuel ratio of the air-fuel mixture supplied to the engine,
When the catalyst deterioration determination execution condition is satisfied,
Setting the first change speed threshold to a value smaller than the case where the catalyst deterioration determination execution condition is not satisfied,
Setting the second change speed threshold to a value smaller than the case where the catalyst deterioration determination execution condition is not satisfied,
The air-fuel ratio of the catalyst inflow gas in the first period is a predetermined third lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio instead of the first lean air-fuel ratio, and the third lean air-fuel ratio and the stoichiometric air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled so that the absolute value of the difference from the fuel ratio becomes a third lean air-fuel ratio that is larger than the absolute value of the difference between the first lean air-fuel ratio and the stoichiometric air-fuel ratio. And
The air-fuel ratio of the catalyst inflow gas in the second period is a predetermined third rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio instead of the first rich air-fuel ratio. The air-fuel ratio of the air-fuel mixture supplied to the engine is controlled so that the absolute value of the difference from the fuel ratio becomes a third rich air-fuel ratio that is larger than the absolute value of the difference between the first rich air-fuel ratio and the stoichiometric air-fuel ratio. To
An air-fuel ratio control device configured as described above. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The air-fuel ratio control means includes
After the time when it is determined that the catalyst is deteriorated, at least one of the first rich air-fuel ratio and the second rich air-fuel ratio is a value before the time when it is determined that the catalyst is deteriorated. Becomes an air-fuel ratio closer to the theoretical air-fuel ratio,
After the time when it is determined that the catalyst is deteriorated, at least one of the first lean air-fuel ratio and the second lean air-fuel ratio is greater than the value at the time before it is determined that the catalyst is deteriorated. So that the air-fuel ratio is closer to the theoretical air-fuel ratio.
An air-fuel ratio control device for controlling an air-fuel ratio of an air-fuel mixture supplied to the engine. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The air-fuel ratio control means includes
The first lean air-fuel ratio is a constant value;
The third lean air-fuel ratio becomes a value that increases as the absolute value of the change speed of the output value increases.
The first rich air-fuel ratio is a constant value;
The third rich air-fuel ratio becomes a value that decreases as the absolute value of the change rate of the output value increases.
An air-fuel ratio control device for controlling an air-fuel ratio of an air-fuel mixture supplied to the engine.

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