JP2006125252A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air-fuel ratio control device of an internal combustion engine capable of preventing an emission from being deteriorated owing to the deviation in the output value of a downstream side sensor to detect the air-fuel ratio state at the downstream side of an exhaust gas purifying catalyst. <P>SOLUTION: When the output value of an oxygen sensor is deviated to the lean side than the output value equivalent to a theoretical air-fuel ratio, it is judged that a lean stack abnormal occurs in the oxygen sensor (S100). When it is judged that the lean stack abnormal occurs in the oxygen sensor, an upper limit guard value Gmax to restrict the maximum value of a sub feedback correction amount SFB is changed into a smaller value (S110). On the other hand, when the output value of the oxygen sensor is deviated to the rich side than the output value equivalent to the theoretical air-fuel ratio, it is judged that the rich stack abnormal occurs in the oxygen sensor (S120). When it is judged that the rich stack abnormal occurs in the oxygen sensor, a lower limit guard Gmin to restrict the minimum value of a sub feedback correction value SFB is changed into a larger value (S130). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention includes sensors for detecting an air-fuel ratio state of exhaust on the upstream side and downstream side of an exhaust purification catalyst of an internal combustion engine, respectively, and performs air-fuel ratio control based on the output value of the upstream sensor, as well as the downstream sensor The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that executes correction for the air-fuel ratio control with a correction amount set based on the output value of the engine.

  In an internal combustion engine, exhaust components are purified by an exhaust purification catalyst provided in an exhaust passage. The purification of exhaust components by the catalyst is efficiently performed when the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine is within a predetermined range. Therefore, a sensor for detecting the oxygen concentration of the exhaust gas is provided on the upstream side of the catalyst, the air-fuel ratio of the air-fuel mixture is detected based on the output signal of the upstream sensor, and the fuel is set so that the detected air-fuel ratio becomes the target air-fuel ratio. Air-fuel ratio control is performed in which an air-fuel ratio correction amount with respect to the injection amount is obtained to increase or decrease the fuel injection amount.

Further, a sensor for detecting the oxygen concentration of the exhaust gas is also provided on the downstream side of the catalyst so that the exhaust gas purification by the catalyst is always properly performed under such air-fuel ratio controlled conditions. A device that performs so-called air-fuel ratio sub-feedback control that calculates a correction amount for the air-fuel ratio correction amount based on a signal is known (for example, Patent Document 1).
JP-A-7-197837

  By the way, if the correction amount calculated by the sub-feedback control becomes excessive, there is a risk of adversely affecting air-fuel ratio control, engine operation, and the like. It is desirable that the value be set within a limit value range such as a value.

  Here, when the output value of the downstream sensor periodically changes to the rich side or the lean side centering on the output value equivalent to the theoretical air-fuel ratio, the fuel injection amount increase correction or reduction correction by the correction amount is performed. Since it is repeated periodically, it is unlikely that the fuel injection amount will be excessively increased or decreased.

  On the other hand, the downstream sensor has a rich stack abnormality in which the output value is biased to the rich side rather than the output value equivalent to the theoretical air-fuel ratio regardless of the actual air-fuel ratio downstream of the catalyst, and the output value is also There may be a lean stack abnormality such that the output value corresponding to the fuel ratio is deviated to the lean side. In this way, when the output value of the downstream sensor does not depend on the actual air-fuel ratio and deviates from the output value equivalent to the theoretical air-fuel ratio, before the calculated correction amount reaches the limit value, Alternatively, when the fuel injection amount is reached, the fuel injection amount is excessively increased or decreased, and the exhaust gas cannot be sufficiently purified by the catalyst, which may deteriorate the emission.

  The present invention has been made in view of such circumstances, and its purpose is to suppress the deterioration of emissions caused by deviation of the output value of the downstream sensor that detects the air-fuel ratio state on the downstream side of the exhaust purification catalyst. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine that can be used.

In the following, means for achieving the above object and its effects are described.
The invention according to claim 1 includes sensors for detecting the air-fuel ratio state of the exhaust on the upstream side and the downstream side of the exhaust purification catalyst of the internal combustion engine, respectively, and through the fuel injection amount increase / decrease correction based on the output value of the upstream sensor. In the air-fuel ratio control apparatus for an internal combustion engine that executes air-fuel ratio control and executes correction for increase / decrease correction of the fuel injection amount by a correction amount set based on the output value of the downstream sensor, the output of the downstream sensor Determining means for determining whether or not the value is deviated from the output value corresponding to the stoichiometric air-fuel ratio, and when the determining means determines that the output value of the downstream sensor is deviated, the correction amount And a changing means for changing a preset limit value.

  According to this configuration, when the output value of the downstream sensor is biased, the limit value that regulates the correction amount is changed. Therefore, excessive increase correction or decrease correction of the fuel injection amount by the correction amount is limited. Accordingly, it is possible to suppress the deterioration of the emission even when the output value of the downstream sensor for detecting the air-fuel ratio downstream of the exhaust purification catalyst is deviated.

  According to a second aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to the first aspect, the determination means is configured such that the output value of the downstream sensor is more lean than the output value corresponding to the theoretical air-fuel ratio. The downstream sensor determines that there is a lean stack abnormality, and when the determination means determines that the downstream sensor has a lean stack abnormality, the changing means is the limit value and the correction The gist is to change the upper limit value that regulates the maximum amount to a smaller value.

  According to this configuration, when the output value of the downstream sensor is biased toward the lean side, the upper limit value that regulates the maximum value of the correction amount is changed to a smaller value. For this reason, even when the correction amount increases in the direction of increasing the fuel injection amount, the maximum value becomes a smaller value than usual, and excessive increase correction of the fuel injection amount is suppressed. Therefore, it is possible to suppress the deterioration of the emission when the lean stack abnormality occurs in the downstream sensor.

  In determining the lean stack abnormality of the downstream sensor, as described in the third aspect of the invention, when the upstream sensor indicates that the upstream sensor is rich, the determination means outputs the output value of the downstream sensor. According to the configuration in which the downstream sensor determines that there is a lean stack abnormality when the output value is leaner than the output value corresponding to the theoretical air-fuel ratio, or according to the invention according to claim 4, The means measures the frequency at which the output value of the downstream sensor becomes an output value on the lean side of the output value equivalent to the theoretical air-fuel ratio when the upstream sensor indicates rich, and the measured frequency is It is possible to adopt a configuration in which it is determined that there is a lean stack abnormality in the downstream sensor when there are more than the reference values.

  According to a fifth aspect of the present invention, in the air-fuel ratio control apparatus for an internal combustion engine according to any one of the first to fourth aspects, the determination means outputs an output whose output value of the downstream sensor corresponds to the theoretical air-fuel ratio. When the downstream sensor is biased to the rich side than the value, it is determined that the downstream sensor has a rich stack abnormality, and when the determination means determines that the downstream sensor has a rich stack abnormality, the changing means The gist is to change the lower limit value that is the limit value and restricts the minimum value of the correction amount to a larger value.

  According to this configuration, when the output value of the downstream sensor is biased toward the rich side, the lower limit value that regulates the minimum value of the correction amount is changed to a larger value. Therefore, even in a situation where the correction amount becomes smaller in the direction of decreasing the fuel injection amount, the minimum value becomes a larger value than usual, and excessive reduction correction of the fuel injection amount is suppressed. Accordingly, it is possible to suppress the deterioration of the emission when the rich stack abnormality occurs in the downstream sensor.

  When determining the rich stack abnormality of the downstream sensor, as described in the sixth aspect of the present invention, the determination means determines that the output value of the downstream sensor is low when the upstream sensor indicates lean. According to the configuration in which the downstream sensor determines that there is a rich stack abnormality when the output value is richer than the output value corresponding to the theoretical air-fuel ratio, or according to the invention according to claim 7, the determination The means measures the frequency at which the output value of the downstream sensor becomes a richer output value than the output value equivalent to the theoretical air-fuel ratio when the upstream sensor indicates lean, and the measured frequency is It is possible to adopt a configuration in which it is determined that there is a rich stack abnormality in the downstream sensor when there is more than the reference value.

Hereinafter, an embodiment of an air-fuel ratio control apparatus for an internal combustion engine according to the present invention will be described with reference to FIGS.
FIG. 1 shows a schematic configuration of a vehicle-mounted internal combustion engine to which an air-fuel ratio control apparatus for an internal combustion engine according to the present invention is applied, and its peripheral configuration.

  As shown in FIG. 1, the intake passage 11 of the internal combustion engine 10 is provided with a throttle valve 15 having a variable passage area, and the amount of air taken in through the air cleaner 14 is adjusted by opening degree control. Yes. The amount of air sucked here (intake air amount) is detected by the air flow meter 16. The air sucked into the intake passage 11 is mixed with fuel injected from an injector 17 provided downstream of the throttle valve 15 and then sent to the combustion chamber of the internal combustion engine 10 where it is burned.

  On the other hand, an exhaust gas purification catalyst 18 for purifying components in the exhaust gas is provided in the exhaust gas passage 13 through which exhaust gas generated by combustion in the combustion chamber is sent. The catalyst 18 has a function of purifying the exhaust gas by oxidizing HC and CO in the exhaust gas and reducing NOx in the exhaust gas in a state where combustion is performed in the vicinity of the theoretical air-fuel ratio.

An air-fuel ratio sensor 19 is provided upstream of the catalyst 18. An oxygen sensor 20 is provided on the downstream side of the catalyst 18.
The air-fuel ratio sensor 19 is a known limiting current oxygen sensor. This limiting current type oxygen sensor is a sensor that provides an output current according to the oxygen concentration in the exhaust gas by providing a ceramic layer called a diffusion rate limiting layer in the detection part of the concentration cell type oxygen sensor, and the oxygen concentration in the exhaust gas When the air-fuel ratio closely related to the stoichiometric air-fuel ratio is the stoichiometric air-fuel ratio, the output current becomes “0”. Further, the output current increases in the negative direction as the air-fuel ratio becomes rich, and the output current increases in the positive direction as the air-fuel ratio becomes lean. Therefore, based on the output of the air-fuel ratio sensor 19, the degree of leanness or the degree of richness can be detected for the air-fuel ratio state upstream of the catalyst 18. The air-fuel ratio sensor 19 constitutes the upstream sensor.

  The oxygen sensor 20 is a well-known concentration cell type oxygen sensor. As for the output characteristics of this concentration cell type oxygen sensor, an output of about 1 V is obtained when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, and an output of about 0 V is obtained when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. . Further, the output voltage changes greatly in the vicinity of the theoretical air-fuel ratio. Therefore, based on the output of the oxygen sensor 20, it is possible to detect whether the air-fuel ratio state downstream of the catalyst 18 is lean or rich. The oxygen sensor 20 constitutes the downstream sensor.

  The oxygen sensor 20 is provided on the downstream side of the catalyst 18 in order to monitor the state of the exhaust gas purification action of the catalyst 18. That is, when the reduction action at the catalyst 18 is promoted and oxygen is released into the exhaust gas, the output of the oxygen sensor 20 becomes lean. On the other hand, when the oxidizing action in the catalyst 18 is promoted and oxygen in the exhaust is consumed, the output of the oxygen sensor 20 becomes rich. Based on the detection result of the air-fuel ratio by the oxygen sensor 20, the state of the exhaust gas purification action is monitored.

  The catalyst 18 efficiently removes all of the main harmful components (HC, CO, NOx) in the exhaust gas by an oxidation-reduction reaction only when the air-fuel ratio of the air-fuel mixture to be burned is in a narrow range (window) near the stoichiometric air-fuel ratio. Purify. In order for such a catalyst 18 to function effectively, strict air-fuel ratio control is required in which the air-fuel ratio of the air-fuel mixture is adjusted to the center of the window.

  Such control of the air-fuel ratio is performed by the electronic control unit 22. The electronic control unit 22 includes various sensors such as the air flow meter 16, the air-fuel ratio sensor 19, the oxygen sensor 20, an accelerator sensor that detects the depression amount of the accelerator pedal, or a rotational speed sensor that detects the engine rotational speed. Detection signal is input. The throttle valve 15 and the injector 17 are driven and controlled in accordance with the operating conditions of the internal combustion engine 10 and the vehicle ascertained from the detection signals of the sensors, thereby controlling the air-fuel ratio as described above. The outline of air-fuel ratio control by such an electronic control unit 22 is as follows.

  First, the electronic control unit 22 obtains a required amount of intake air amount that is grasped according to the amount of depression of the accelerator pedal and the detection result of the engine speed, and the throttle valve 15 of the throttle valve 15 is obtained so as to obtain the intake air amount accordingly. Adjust the opening. On the other hand, an amount of fuel sufficient to obtain the theoretical air-fuel ratio is obtained from the actual measured value of the intake air amount detected by the air flow meter 16, and the fuel injection amount from the injector 17 is adjusted accordingly. Thereby, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber can be brought close to the stoichiometric air-fuel ratio to some extent. However, that alone is not sufficient for the required highly accurate air-fuel ratio control.

  Therefore, the electronic control unit 22 grasps the actual measurement value of the air-fuel ratio upstream of the catalyst 18 based on the detection result of the air-fuel ratio sensor 19, and determines the difference between the actual measurement value and the target air-fuel ratio, that is, the theoretical air-fuel ratio. Based on this, an air-fuel ratio feedback correction amount is calculated. Based on the air-fuel ratio feedback correction amount, the fuel injection amount of the injector 17 is corrected to increase or decrease. This air-fuel ratio feedback control ensures the required accuracy of air-fuel ratio control.

  Further, the electronic control unit 22 estimates the oxygen storage state or oxygen release state of the catalyst 18 from the detection result of the oxygen sensor 20, and performs further correction on the air-fuel ratio feedback correction amount based on this estimation. In this correction process, the sub feedback correction amount SFB calculated based on the output of the oxygen sensor 20 is corrected to increase or decrease, and the air / fuel ratio feedback correction amount is further corrected by the sub feedback correction amount SFB.

  Specifically, while the output of the oxygen sensor 20 is rich, the sub-feedback correction amount SFB is decreased by a constant amount so that the air-fuel ratio upstream of the catalyst 18 gradually changes to the lean side. Will be increased. Then, by correcting the air-fuel ratio feedback correction amount with the sub feedback correction amount SFB increased to the minus side in this way, the fuel injection amount is further corrected to decrease by an amount corresponding to the sub feedback correction amount SFB. .

  On the other hand, while the output of the oxygen sensor 20 indicates lean, the sub-feedback correction amount SFB is increased by a certain amount to the plus side so that the air-fuel ratio upstream of the catalyst 18 gradually changes to the rich side. The Then, by correcting the air-fuel ratio feedback correction amount with the sub feedback correction amount SFB increased to the plus side in this way, the fuel injection amount is further increased and corrected by an amount corresponding to the sub feedback correction amount SFB. .

By such sub-feedback control, the purification action of the catalyst 18 is effectively utilized.
By the way, if the sub feedback correction amount SFB calculated by the sub feedback control becomes excessive, there is a risk of adversely affecting air-fuel ratio control, engine operation, and the like. Therefore, in the present embodiment, a preset limit value is provided that restricts the sub feedback correction amount SFB. More specifically, an upper limit guard value Gmax that restricts the maximum value of the sub-feedback correction amount SFB to limit the increase correction of the fuel injection amount, and a sub-feedback correction amount SFB to limit the decrease correction of the fuel injection amount. And a lower limit guard value Gmin for restricting the minimum value of. The sub feedback correction amount SFB is set within a range of limit values such as the upper limit guard value Gmax and the lower limit guard value Gmin.

  Here, when the air-fuel ratio upstream of the catalyst 18 periodically changes from the stoichiometric air-fuel ratio to the rich side or the lean side, the output value of the oxygen sensor 20 as shown in FIG. Is periodically changed to the rich side or the lean side around the target value PV corresponding to the theoretical air-fuel ratio, the fuel injection amount increase / decrease correction by the sub feedback correction amount SFB is periodically repeated. Therefore, it is unlikely that the fuel injection amount is excessively increased or decreased.

  On the other hand, as shown in FIG. 2B, the output value of the oxygen sensor 20 is biased to the lean side from the output value corresponding to the theoretical air-fuel ratio, regardless of the actual air-fuel ratio downstream of the catalyst 18. When a sensor abnormality occurs, that is, when a lean stack abnormality occurs in the oxygen sensor 20, the following problems may occur. That is, even if the air-fuel ratio on the downstream side of the catalyst 18 is “rich”, the output value of the oxygen sensor 20 indicates “lean”. The correction amount SFB is set to the side for increasing the fuel injection amount. Therefore, before the sub feedback correction amount SFB reaches the upper limit guard value Gmax, or when it reaches the upper limit guard value Gmax, an excessive fuel injection amount correction is made, and the exhaust gas purification by the catalyst 18 may not be sufficiently performed. There is.

  Further, as shown in FIG. 2C, the output value of the oxygen sensor 20 is biased to the rich side from the output value corresponding to the theoretical air-fuel ratio, regardless of the actual air-fuel ratio downstream of the catalyst 18. When a sensor abnormality occurs, that is, when a rich stack abnormality occurs in the oxygen sensor 20, the following problems may occur. That is, even if the air-fuel ratio on the downstream side of the catalyst 18 is “lean”, the output value of the oxygen sensor 20 indicates “rich”. The correction amount SFB is set to the side on which the fuel injection amount is corrected to decrease. Therefore, before or when the sub feedback correction amount SFB reaches the lower limit guard value Gmin, an excessive fuel injection amount reduction correction is performed, and the exhaust gas may not be sufficiently purified by the catalyst 18. There is.

  As described above, when there is an abnormality in which the output value of the oxygen sensor 20 is deviated to the rich side or the lean side from the output value corresponding to the stoichiometric air-fuel ratio, exhaust purification by the catalyst 18 can be sufficiently performed. There is a risk that emissions will be worsened due to the inability to do so.

  Therefore, in this embodiment, the guard value changing process described below is executed. In this process, first, it is determined whether or not the output value of the oxygen sensor 20 is deviated from the output value corresponding to the theoretical air-fuel ratio. When it is determined that the output value of the oxygen sensor 20 is deviated, it is a value that regulates the sub-feedback correction amount SFB and is a preset limit value (the above-described upper limit guard value Gmax and lower limit guard value Gmin). Thus, the deterioration of the emission due to the deviation of the output value of the oxygen sensor 20 is suppressed.

  FIG. 3 shows a processing procedure for the guard value changing process executed by the electronic control unit 22. This process is repeatedly executed at predetermined intervals when an execution condition set as appropriate is satisfied.

  When this process is started, it is first determined whether or not the oxygen sensor 20 is in a lean stack abnormality (S100). Here, when the output value of the air-fuel ratio sensor 19 is rich, and the sensor output value SV of the oxygen sensor 20 is an output value leaner than the target value PV, the sensor output of the oxygen sensor 20 It is determined that the value SV is biased leaner than the output value corresponding to the theoretical air-fuel ratio, that is, the lean stack abnormality has occurred in the oxygen sensor 20. Note that the target value PV is an output value equivalent to the theoretical air-fuel ratio (for example, 0.55 V), and the exhaust air empty so that the average value of the sensor output value SV of the oxygen sensor 20 becomes the target value PV. The fuel ratio is controlled.

When it is determined that the oxygen sensor 20 has a lean stack abnormality (S100: YES), the lean stack abnormality flag LF having an initial value of “0” is changed to “1”.
Then, the upper limit guard value Gmax of the sub feedback correction amount SFB is changed to a smaller value (S110). More specifically, the preset upper limit guard value Gmax is changed to a value that is smaller by the upper limit guard change amount A. This upper limit guard change amount A is a change amount that can suppress an excessive increase correction of the fuel injection amount and thereby suppress the deterioration of the emission even when the lean stack abnormality occurs in the oxygen sensor 20. ing. The upper limit guard change amount A is a fixed value set in advance, and is variably set according to the degree of deviation between the sensor output value SV and the target value PV, or according to the engine operating state. Also good.

In this way, when the upper limit guard value Gmax is changed to a smaller value, this process is temporarily terminated.
When it is determined in step S100 that the lean stack abnormality has not occurred in the oxygen sensor 20 (S100: NO), it is determined whether or not the oxygen sensor 20 has a rich stack abnormality (S120). Here, when the output value of the air-fuel ratio sensor 19 indicates lean, and the sensor output value SV of the oxygen sensor 20 is richer than the target value PV, the sensor of the oxygen sensor 20 It is determined that the output value SV is biased to the rich side with respect to the output value corresponding to the stoichiometric air-fuel ratio, that is, that the rich stack abnormality has occurred in the oxygen sensor 20.

When it is determined that the oxygen sensor 20 has a rich stack abnormality (S120: YES), the rich stack abnormality flag RF having an initial value “0” is changed to “1”.
Then, the lower limit guard value Gmin of the sub feedback correction amount SFB is changed to a larger value (S130). More specifically, the preset lower limit guard value Gmin is changed to a value that is larger by the lower limit guard change amount B. This lower limit guard change amount B is a change amount that can suppress excessive decrease correction of the fuel injection amount and thereby suppress emission deterioration even when a rich stack abnormality has occurred in the oxygen sensor 20. ing. The lower limit guard change amount B is also set to a preset fixed value, and is variably set according to the degree of deviation between the sensor output value SV and the target value PV, or according to the engine operating state. Also good.

Thus, when the lower limit guard value Gmin is changed to a larger value, the present process is temporarily terminated.
When it is determined in step S120 that the rich stack abnormality has not occurred in the oxygen sensor 20 (S120: NO), the upper limit guard value Gmax and the lower limit guard value Gmin are not changed, and this process is temporarily performed. Is terminated.

  Incidentally, in the above-described series of processes, the lean stack abnormality of the oxygen sensor 20 is determined and then the rich stack abnormality of the sensor is determined. However, after the rich stack abnormality is determined, the lean stack abnormality is determined. Also good. Moreover, the process of step S100 and step S120 comprises the said determination means, and the process of step S110 and step S130 comprises the said change means.

Next, a change mode of the limit value by the guard value changing process described above is shown.
FIG. 4 shows how the upper limit guard value Gmax is changed when a lean stack abnormality has occurred in the oxygen sensor 20. When the lean stack abnormality occurs in the oxygen sensor 20, the output value of the oxygen sensor 20 shows "lean" even if the air-fuel ratio downstream of the catalyst 18 is "rich". The sub-feedback correction amount SFB that should be set to the side that corrects the injection amount to be reduced is set to the side that corrects the fuel injection amount to be increased, and the value gradually increases. At this time, if the upper limit guard value Gmax remains a preset value as indicated by a one-dot chain line in FIG. 4, the sub feedback correction amount SFB is the upper limit as indicated by a broken line in FIG. It is increased until it reaches the guard value Gmax (increased to the plus side). For this reason, before the sub-feedback correction amount SFB reaches or exceeds the upper limit guard value Gmax, an excessive increase in fuel injection amount is corrected, and exhaust gas purification by the catalyst 18 cannot be sufficiently performed. There is a fear.

  On the other hand, in this embodiment, when it is determined that a lean stack abnormality has occurred (time t1), the upper limit guard value Gmax is changed to a value that is smaller by the upper limit guard change amount A. Therefore, even when the sub-feedback correction amount SFB is increased in the direction in which the fuel injection amount is increased, the maximum value is smaller than usual, and excessive increase correction of the fuel injection amount is suppressed. Therefore, it is possible to suppress the deterioration of the emission when the lean stack abnormality occurs in the oxygen sensor 20.

  FIG. 5 shows how the lower limit guard value Gmin is changed when a rich stack abnormality has occurred in the oxygen sensor 20. When the rich stack abnormality occurs in the oxygen sensor 20, the output value of the oxygen sensor 20 shows "rich" even if the air-fuel ratio on the downstream side of the catalyst 18 is "lean". The sub-feedback correction amount SFB that should be set to the side that corrects the injection amount is set to the side that corrects the fuel injection amount to decrease, and the value gradually decreases. At this time, if the lower limit guard value Gmin remains at a preset value as shown by a one-dot chain line in FIG. 5, the sub feedback correction amount SFB is lower than the lower limit as shown by a broken line in FIG. It is decreased until it reaches the guard value Gmin (increased to the minus side). For this reason, before the sub feedback correction amount SFB reaches or reaches the lower limit guard value Gmin, excessive fuel injection amount reduction correction is performed, and exhaust gas purification by the catalyst 18 cannot be sufficiently performed. There is a fear.

  On the other hand, in this embodiment, when it is determined that a rich stack abnormality has occurred (time t1), the lower limit guard value Gmin is changed to a value that is larger by the lower limit guard change amount B. Therefore, even when the sub-feedback correction amount SFB is decreased in the direction of reducing the fuel injection amount, the minimum value thereof is larger than usual, and excessive reduction correction of the fuel injection amount is suppressed. Accordingly, it is possible to suppress the deterioration of the emission when the rich stack abnormality occurs in the oxygen sensor 20.

As described above, according to the present embodiment, the following effects can be obtained.
(1) It is determined whether or not the sensor output value SV of the oxygen sensor 20 is deviated from the output value (target value PV) corresponding to the theoretical air-fuel ratio, and the sensor output value SV of the oxygen sensor 20 is deviated. When the determination is made, a limit value set in advance and a value that regulates the sub feedback correction amount SFB is changed. Therefore, excessive increase correction or decrease correction of the fuel injection amount by the sub feedback correction amount SFB is limited. Therefore, the deterioration of the emission can be suppressed even when the sensor output value SV of the oxygen sensor 20 that detects the air-fuel ratio state on the downstream side of the catalyst 18 is deviated.

  (2) When the sensor output value SV of the oxygen sensor 20 is biased leaner than the output value (target value PV) corresponding to the theoretical air-fuel ratio, it is determined that the oxygen sensor 20 has a lean stack abnormality. Yes. More specifically, if the sensor output value SV of the oxygen sensor 20 is leaner than the output value equivalent to the stoichiometric air-fuel ratio when the air-fuel ratio sensor 19 is rich, the oxygen sensor At 20, it is determined that there is a lean stack abnormality. When it is determined that there is a lean stack abnormality in the oxygen sensor 20, the upper limit guard value Gmax that restricts the maximum value of the sub feedback correction amount SFB, which is the limit value, is changed to a smaller value. Therefore, even in a situation where the sub feedback correction amount SFB increases in the direction of increasing the fuel injection amount, the maximum value becomes a smaller value than usual, and excessive increase correction of the fuel injection amount is suppressed. Therefore, it is possible to suppress the deterioration of the emission when the lean stack abnormality occurs in the oxygen sensor 20.

  (3) When the sensor output value SV of the oxygen sensor 20 is biased to the rich side from the output value (target value PV) corresponding to the theoretical air-fuel ratio, it is determined that the oxygen sensor 20 has a rich stack abnormality. Yes. More specifically, when the air-fuel ratio sensor 19 indicates lean, the sensor output value SV of the oxygen sensor 20 is richer than the stoichiometric air-fuel ratio output value. At 20, it is determined that there is a rich stack abnormality. When it is determined that there is a rich stack abnormality in the oxygen sensor 20, the lower limit guard value Gmin that restricts the minimum value of the sub feedback correction amount SFB, which is the limit value, is changed to a larger value. Therefore, even in a situation where the sub feedback correction amount SFB decreases in the direction of decreasing the fuel injection amount, the minimum value becomes a larger value than usual, and excessive decrease correction of the fuel injection amount is suppressed. Accordingly, it is possible to suppress the deterioration of the emission when the rich stack abnormality occurs in the oxygen sensor 20.

In addition, the said embodiment can also be changed and implemented as follows.
In the above embodiment, the lean stack abnormality and the rich stack abnormality of the oxygen sensor 20 are determined, and the upper limit guard value Gmax and the lower limit guard value Gmin are changed according to the determination result. Only the abnormality may be determined, and only the upper limit guard value Gmax may be changed according to the determination result. Alternatively, only the rich stack abnormality of the oxygen sensor 20 may be determined, and only the lower limit guard value Gmin may be changed according to the determination result.

  In the above embodiment, the frequency at which the sensor output value SV of the oxygen sensor 20 becomes the leaner output value than the output value (target value PV) corresponding to the theoretical air-fuel ratio when the air-fuel ratio sensor 19 indicates richness. If the measured frequency is greater than a reference value set as appropriate, the oxygen sensor 20 may determine that there is a lean stack abnormality. In this case, the upper limit guard change amount A may be variably set according to the measured frequency. For example, the upper limit guard value Gmax is changed to a smaller value as the measured frequency increases. The upper guard change amount A may be variably set.

  In the above embodiment, the frequency at which the sensor output value SV of the oxygen sensor 20 becomes richer than the output value equivalent to the theoretical air-fuel ratio (target value PV) when the air-fuel ratio sensor 19 indicates lean. If the measured frequency is greater than a reference value set as appropriate, the oxygen sensor 20 may be determined to have a rich stack abnormality. In this case, the lower limit guard change amount B may be variably set according to the measured frequency. For example, the lower limit guard value Gmin is changed to a larger value as the measured frequency increases. The lower limit guard change amount B may be variably set.

  The present invention can be similarly implemented even when the air-fuel ratio sensor 19 is the oxygen sensor 20 as described above. The present invention can be implemented based on the same principle even when the oxygen sensor 20 is the air-fuel ratio sensor 19 as described above.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the internal combustion engine to which this is applied, and its periphery structure about one Embodiment of the air-fuel ratio control apparatus of the internal combustion engine concerning this invention. (A) is a schematic diagram which shows a sensor output value when an oxygen sensor is functioning normally. (B) is a schematic diagram showing a sensor output value when a lean stack abnormality occurs in the oxygen sensor. (C) is a schematic diagram showing a sensor output value when a rich stack abnormality occurs in the oxygen sensor. The flowchart which shows the procedure about the guard value change process in the embodiment. The schematic diagram which shows the change aspect of an upper limit guard value when the lean stack abnormality has arisen in the oxygen sensor 20. FIG. The schematic diagram which shows the change aspect of a lower limit guard value when the rich stack abnormality has arisen in the oxygen sensor 20. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 11 ... Intake passage, 13 ... Exhaust passage, 14 ... Air cleaner, 15 ... Throttle valve, 16 ... Air flow meter, 17 ... Injector, 18 ... Catalyst, 19 ... Air-fuel ratio sensor, 20 ... Oxygen sensor, 22 ... Electronic control device.

Claims (7)

Sensors for detecting the air-fuel ratio of exhaust gas are provided upstream and downstream of the exhaust purification catalyst of the internal combustion engine, respectively, and the air-fuel ratio control is executed through correction of increase / decrease of the fuel injection amount based on the output value of the upstream sensor, and the downstream In an air-fuel ratio control apparatus for an internal combustion engine that performs correction for increase / decrease correction of the fuel injection amount by a correction amount set based on an output value of a side sensor,
Determining means for determining whether or not an output value of the downstream sensor is deviated from an output value equivalent to the theoretical air-fuel ratio;
And changing means for changing the preset limit value, which is a value that regulates the correction amount, when the judging means determines that the output value of the downstream sensor is biased. An air-fuel ratio control device for an internal combustion engine. The determination means determines that there is a lean stack abnormality in the downstream sensor when the output value of the downstream sensor is biased leaner than the output value corresponding to the theoretical air-fuel ratio,
When the determination unit determines that there is a lean stack abnormality in the downstream sensor, the changing unit changes the upper limit value, which is the limit value and regulates the maximum value of the correction amount, to a smaller value. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1. When the upstream sensor indicates a rich state, the determination means determines whether the downstream sensor outputs an output value that is leaner than an output value equivalent to the theoretical air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 2, wherein it is determined that there is a lean stack abnormality. The determination means measures the frequency at which the output value of the downstream sensor becomes an output value on the lean side of the output value corresponding to the theoretical air-fuel ratio when the upstream sensor indicates rich, and the measurement is performed. The air-fuel ratio control apparatus for an internal combustion engine according to claim 2, wherein when the frequency is higher than a reference value, the downstream sensor determines that there is a lean stack abnormality. The determination means determines that the downstream sensor has a rich stack abnormality when the output value of the downstream sensor is biased to the rich side with respect to the output value corresponding to the theoretical air-fuel ratio,
When the determination unit determines that there is a rich stack abnormality in the downstream sensor, the changing unit changes the lower limit value that restricts the minimum value of the correction amount to the larger value as the limit value. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 4. When the upstream sensor indicates lean, the determination means determines that the downstream sensor has a richer output value than the theoretical air-fuel ratio. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein it is determined that there is a rich stack abnormality. The determination means measures the frequency at which the output value of the downstream sensor becomes an output value richer than the output value equivalent to the theoretical air-fuel ratio when the upstream sensor indicates lean, and the measurement is performed. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein when the frequency is higher than a reference value, it is determined that the downstream sensor has a rich stack abnormality.

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