JP2007198259A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To release almost all the oxygen adsorbed in a three-way catalyst device by an O<SB>2</SB>storage capacity by enriching control immediately after fuel cut in an air-fuel ratio control device of an internal combustion engine controlling a combustion air-fuel ratio to a desired air-fuel ratio according to the output of an air-fuel ratio sensor on the upstream side of the three-way catalyst device. <P>SOLUTION: The output of the air-fuel ratio sensor on the upstream side of the three-way catalyst device is corrected by a differential term due to a variation in output of an oxygen sensor on the downstream side of the three-way catalyst device. The correction of the output of the air-fuel ratio sensor by a differential term is prohibited or suppressed (step 102) until the oxygen adsorbed into the three-way catalyst device by the O<SB>2</SB>storage capacity is determined to be almost all released by the enriching control immediately after the fuel cut (step 105). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.

It has been proposed that an air-fuel ratio sensor whose output varies linearly according to the air-fuel ratio of exhaust gas is disposed in the engine exhaust system, and the combustion air-fuel ratio is controlled to a desired air-fuel ratio based on the output of this air-fuel ratio sensor. Generally, a three-way catalyst device for purifying exhaust gas is arranged in the engine exhaust system. Since the three-way catalyst device purifies exhaust gas well when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio, in general, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst device is theoretically When the air-fuel ratio is leaner, the excess oxygen is stored, and when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst device is richer than the stoichiometric air-fuel ratio, the insufficient oxygen is released, It has an O ₂ storage capability in which the air-fuel ratio of the exhaust gas is close to the theoretical air-fuel ratio.

Thereby, the air-fuel ratio sensor for detecting the combustion air-fuel ratio is arranged upstream of the three-way catalyst device so that the air-fuel ratio of the detected exhaust gas is not influenced by the O ₂ storage capacity of the three-way catalyst device. Yes. By the way, in order to make the air-fuel ratio control by the air-fuel ratio sensor accurate, an oxygen sensor whose output changes suddenly when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio is disposed downstream of the three-way catalyst device. The deviation of the output of the air-fuel ratio sensor to the rich side or the lean side is corrected based on the output of the oxygen sensor whose change is moderated by the O ₂ storage capability.

  The correction of the output of the air-fuel ratio sensor based on the output of the oxygen sensor is generally performed by the reference output corresponding to the theoretical air-fuel ratio of the oxygen sensor and the actual output of the oxygen sensor in the operation where the target air-fuel ratio is the stoichiometric air-fuel ratio. The proportional term based on the deviation and the integral term based on the integrated value of the deviation are used. The integral term corrects the recent trending deviation of the output of the air-fuel ratio sensor, and the proportional term corrects the current deviation of the output of the air-fuel ratio sensor corrected by the integral term. For example, the integral term is updated so as to match the current state of the air-fuel ratio sensor with the update time as the time when the deviation of the oxygen sensor for the set number of times is calculated.

By the way, in an internal combustion engine, in order to reduce fuel consumption, fuel cut is frequently performed every time the engine is decelerated. When the fuel cut is performed, air flows into the three-way catalyst device as exhaust gas, and a large amount of oxygen in the air is occluded in the three-way catalyst device by the O ₂ storage capability. In order to enable good purification of exhaust gas even when the air-fuel ratio of the exhaust gas becomes leaner or richer than the stoichiometric air-fuel ratio, the three-way catalyst device has a maximum storable oxygen with O ₂ storage capacity It is preferable to store a desired amount of oxygen corresponding to about half of the amount. Thus, in general, immediately after the fuel cut, the combustion air-fuel ratio is made rich in order to release the oxygen stored in excess of the desired amount during the fuel cut so that only the desired amount of oxygen is stored. The enrichment control is performed.

  During fuel cut and enrichment control, the deviation of the output of the oxygen sensor with respect to the theoretical air-fuel ratio has a large absolute value as a matter of course and becomes a meaningless value. Air-fuel ratio control is not required during fuel cut, but air-fuel ratio control is required to make the combustion air-fuel ratio the desired rich air-fuel ratio during enrichment control. It has been proposed to correct the output of the fuel ratio sensor and prohibit the correction of the output of the air fuel ratio sensor by the proportional term (see, for example, Patent Document 1).

JP2005-61356 JP 2000-105834 A

By the way, it has also been proposed to further correct the output of the air-fuel ratio sensor by a differential term corresponding to the magnitude of the output change of the oxygen sensor. In the above-described background art, oxygen stored to about half of the maximum storable oxygen amount of O ₂ storage capacity is released by the enrichment control. During this enrichment control, the output of the oxygen sensor is lean. It does not change greatly with the value indicating. Thereby, although there is no description regarding the differential term, even if the correction by the differential term is performed, since the change in the output of the oxygen sensor is small, the absolute value of the differential term is also small, and there is no particular problem.

However, in the enrichment control, when all of the oxygen stored in the three-way catalyst device is released by the O ₂ storage capability, the output of the oxygen sensor at the end of the enrichment control is reduced from the value indicating lean. If the output of the air-fuel ratio sensor is corrected by the differential term at this time, the combustion air-fuel ratio is greatly corrected to the lean side, and the stored oxygen cannot be released sufficiently. is there.

Accordingly, an object of the present invention is to provide an enrichment control immediately after a fuel cut in an air-fuel ratio control apparatus for an internal combustion engine that controls the combustion air-fuel ratio to a desired air-fuel ratio based on the output of the air-fuel ratio sensor upstream of the three-way catalyst device. It is possible to release almost all the oxygen stored in the three-way catalyst device by the O ₂ storage capability.

An air-fuel ratio control apparatus for an internal combustion engine according to claim 1 of the present invention is provided on the upstream side of a three-way catalyst device of an engine exhaust system, and an air-fuel ratio sensor whose output changes in accordance with the air-fuel ratio of exhaust gas, An oxygen sensor disposed downstream of the three-way catalyst device of the engine exhaust system and having an output that changes abruptly when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio, and based on the output of the air-fuel ratio sensor In an air-fuel ratio control apparatus for an internal combustion engine that controls a combustion air-fuel ratio to a desired air-fuel ratio, the output of the air-fuel ratio sensor is corrected by a differential term based on a change in the output of the oxygen sensor, and O in rich control immediately after a fuel cut. until the oxygen occluded in the three way catalytic converter by ₂ storage capability is determined to be substantially all released, to prohibit or suppress the correction of the output of the air-fuel ratio sensor according to the differential term And butterflies.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 2 according to the present invention is the air-fuel ratio control apparatus for internal combustion engine according to claim 1, wherein the period determined based on the deterioration state of the three-way catalyst apparatus is enriched. When the control is started, it is determined that almost all of the oxygen stored in the three-way catalyst device is released by the O ₂ storage capability in the enrichment control immediately after the fuel cut.

The air-fuel ratio control apparatus for an internal combustion engine according to claim 3 according to the present invention is the air-fuel ratio control apparatus for internal combustion engine according to claim 1, wherein the output of the oxygen sensor has a value corresponding to the set rich air-fuel ratio. In the enrichment control immediately after the fuel cut, it is determined that almost all of the oxygen stored in the three-way catalyst device is released by the O ₂ storage capability.

According to the air-fuel ratio control device for an internal combustion engine according to claim 1 of the present invention, it is determined that almost all of the oxygen stored in the three-way catalyst device has been released by the O ₂ storage capability in the enrichment control immediately after the fuel cut. Until then, the correction of the output of the air-fuel ratio sensor by the differential term is prohibited or suppressed, so that at the end of the enrichment control, the output of the oxygen sensor suddenly changes from a value indicating lean to a value indicating rich. Even at this time, since the correction of the output of the air-fuel ratio sensor by the differential term is prohibited or suppressed at this time, the combustion air-fuel ratio is not greatly corrected to the lean side, and the rich control is performed. Almost all of the stored oxygen in the three-way catalyst device can be released.

According to the air-fuel ratio control apparatus for an internal combustion engine according to claim 2 of the present invention, the period determined based on the deterioration state of the three-way catalyst apparatus is rich in the air-fuel ratio control apparatus for the internal combustion engine according to claim 1. When the control is started, it is determined that almost all of the oxygen stored in the three-way catalyst device has been released by the O ₂ storage capability in the enrichment control immediately after the fuel cut. Even if the O ₂ storage capacity changes depending on the state, it can be reliably determined that almost all of the oxygen stored in the three-way catalyst device has been released.

According to the air-fuel ratio control apparatus for an internal combustion engine according to claim 3 of the present invention, in the air-fuel ratio control apparatus for internal combustion engine according to claim 1, the output of the oxygen sensor is a value corresponding to the set rich air-fuel ratio. since the time, it is adapted to determine the oxygen occluded in the three-way catalytic converter is nearly all released by the O ₂ storage capability in enriching control immediately after the fuel cut, O ₂ due to the deterioration state of the three-way catalytic converter Even if the storage capacity changes, it can be reliably determined that almost all of the oxygen stored in the three-way catalyst device has been released.

  FIG. 1 is a schematic diagram showing an engine exhaust system, in which 1 is a three-way catalyst device, 2 is an air-fuel ratio sensor disposed upstream of the three-way catalyst device 1, and 3 is a three-way catalyst device 1. It is an oxygen sensor arranged on the downstream side. The air-fuel ratio sensor 2 and the oxygen sensor 3 both have an output voltage that changes according to the oxygen concentration in the exhaust gas, and the air-fuel ratio sensor 2 changes its output linearly corresponding to the air-fuel ratio of the exhaust gas. It is a linear output type, and the oxygen sensor 3 is a step output type in which the output changes abruptly when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio.

  The air-fuel ratio control apparatus according to the present invention feedback-controls the combustion air-fuel ratio to a desired air-fuel ratio based on the output of the air-fuel ratio sensor 2. Since the air-fuel ratio sensor 2 is arranged upstream of the three-way catalyst device 1 and is always exposed to unpurified exhaust gas, the output reliability is not so high, and the output goes to the rich side or the lean side. It may shift. Thereby, it is arrange | positioned in the downstream of the three-way catalyst apparatus 1, and generally detects whether the air-fuel ratio of exhaust gas is rich or lean, without being exposed to unpurified exhaust gas. The output of the air-fuel ratio sensor is corrected based on the output of the oxygen sensor 3 that is used for this purpose and has high output reliability.

The output V of the air-fuel ratio sensor 2 used for the feedback control of the combustion air-fuel ratio is corrected as described below to an output V ′ during operation in which the target combustion air-fuel ratio is the stoichiometric air-fuel ratio.
V ′ = V + P + I + D
Here, P is a proportional term obtained by multiplying a deviation d between a reference output (for example, 0.5 volts) with respect to the theoretical air-fuel ratio and an actual output of the oxygen sensor 3 by a predetermined gain Pg, and I is a deviation d of the set number of times. Is an integral term obtained by multiplying the integrated value by a predetermined gain Ig, and D is a differential term obtained by multiplying the output change v _i -vi _-1 of the oxygen sensor 3 by the predetermined gain Dg. In this way, the integral term I corrects the current tendency shift of the output of the air-fuel ratio sensor 2, and the proportional term P corrects the current shift of the output of the air-fuel ratio sensor 2 corrected by the integral term I. Therefore, the differential term D corrects the output of the air-fuel ratio sensor 2 so as to eliminate the change in the output of the oxygen sensor 3. Based on the output V ′ of the air / fuel ratio sensor 2 corrected in this way, the air / fuel ratio of the exhaust gas is accurately estimated, and the intake air amount detected by the air flow meter is adjusted so that the combustion air / fuel ratio becomes the stoichiometric air / fuel ratio. Thus, the fuel injection amount is feedback corrected.

The present air-fuel ratio control apparatus controls the combustion air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio (stoichiometric). However, as shown in the time chart of FIG. 2, in order to reduce fuel consumption during engine deceleration, etc. When the fuel cut F / C is performed, of course, the fuel injection is stopped, so that the air-fuel ratio control is not performed. In the fuel cut F / C, air containing a large amount of oxygen flows into the three-way catalyst device 1 as exhaust gas, and therefore the three-way catalyst device 1 has oxygen up to the maximum storable oxygen amount of O ₂ storage capacity. Will be occluded. Thus, as it is, when the combustion air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio after the fuel cut, the three-way catalyst device 1 cannot store excess oxygen well, and the NO _x purification capability is improved. It will decline.

  Therefore, immediately after the fuel cut F / C, the combustion air-fuel ratio is controlled to the desired rich air-fuel ratio, and almost all of the oxygen stored in the three-way catalyst device 1 is released during the fuel cut, so that the combustion air-fuel ratio becomes lean. It is sometimes preferable to prepare. When the enrichment control immediately after the fuel cut F / C is completed, the stoichiometric control with the target combustion air-fuel ratio as the stoichiometric air-fuel ratio is started.

  The deviation d between the reference output of the oxygen sensor 3 with respect to the stoichiometric air-fuel ratio and the actual output of the oxygen sensor 3 cannot be calculated during the fuel cut and during the enrichment control immediately thereafter. In the control, the output of the air-fuel ratio sensor 2 cannot be corrected by the proportional term P. Further, during the enrichment control, it is preferable to control the combustion air-fuel ratio to the desired rich air-fuel ratio by correcting the output of the air-fuel ratio sensor 2 with the integral term I.

  In addition, at the beginning of the enrichment control immediately after the fuel cut, the output of the oxygen sensor 3 changes to a value indicating lean (near 0 volts) and does not change greatly, so the absolute value of the differential term D becomes small, and the differential term There is no particular problem even if correction by D is performed. However, at the end of the enrichment control, the stored oxygen amount of the three-way catalyst device 1 becomes small, and the output of the oxygen sensor 3 is a value indicating the stoichiometric air-fuel ratio (0.5) from a value indicating lean (near 0 volts). If the correction by the differential term D is performed at this time, the differential term D significantly corrects the output of the air-fuel ratio sensor 2 to the lean side. . As a result, the combustion air-fuel ratio is suddenly changed from the desired rich air-fuel ratio to the lean air-fuel ratio, so that the oxygen stored in the three-way catalyst device 1 during the fuel cut cannot be sufficiently released, and the combustion air-fuel ratio A relatively large torque shock is generated with a significant change in.

  This air-fuel ratio control apparatus is configured to release almost all of the oxygen stored in the three-way catalyst apparatus by the enrichment control according to the first flowchart shown in FIG. First, in step 101, it is determined whether or not enrichment control is being performed immediately after fuel cut. When this determination is negative, the routine proceeds to step 105, where it is determined whether the differential term correction described below is prohibited or not. If the normal stoichiometric control is being performed, this determination is negative and the process ends as it is. To do. On the other hand, when the enrichment control is being performed immediately after the fuel cut, the determination in step 101 is affirmed, and in step 102, the correction of the output of the air-fuel ratio sensor 2 by the differential term D is prohibited. Next, at step 103, it is determined whether or not the output of the oxygen sensor 3 has reached a value (0.5 volts) corresponding to the stoichiometric air-fuel ratio. When this determination is denied, the processing is terminated as it is. However, when the determination at step 103 is affirmed, it is assumed that almost all of the oxygen stored in the three-way catalyst device 1 is released by the enrichment control. The stoichiometric control is completed and the target air-fuel ratio is set to the stoichiometric air-fuel ratio. At the same time, correction of the output of the air-fuel ratio sensor 2 by the proportional term P is also started.

  Next, at step 105, it is determined whether correction of the output of the air-fuel ratio sensor 2 by the differential term D is prohibited. Immediately after completion of the enrichment control, correction by the differential term D is prohibited in step 102, so the determination in step 105 is affirmed and the routine proceeds to step 106. In step 106, it is determined whether or not the output of the oxygen sensor 3 has reached a value (0.6 volts) corresponding to an air / fuel ratio slightly richer than the stoichiometric air / fuel ratio. When this determination is denied, the process is terminated as it is. At this time, the process passes through steps 101 and 105, and the determination becomes step 106 again. This flow is repeated until the determination of step 106 is affirmed. If the determination in step 106 is affirmed, in step 107, the prohibition of correction of the output of the air-fuel ratio sensor 2 by the differential term D is canceled. As a result, the combustion air-fuel ratio is maintained at the rich air-fuel ratio in the enrichment control until almost all of the oxygen stored in the three-way catalyst device 1 is released, and oxygen release is prevented from becoming insufficient. Of course, if the output of the oxygen sensor 3 reaches a value (0.5 volts) corresponding to the stoichiometric air-fuel ratio, almost all of the oxygen stored in the three-way catalyst device 1 is released, and at this time, the differential term D The prohibition of correction may be canceled.

  As shown in FIG. 2, when the output of the oxygen sensor 3 reaches a value (0.5 volts) corresponding to the stoichiometric air-fuel ratio, the enrichment control is completed and the stoichiometric control that sets the target air-fuel ratio to the stoichiometric air-fuel ratio is started. However, at the same time, the exhaust gas near the stoichiometric air-fuel ratio does not flow into the three-way catalyst device 1, and the output of the oxygen sensor 3 is the theoretical air-fuel ratio exhaust gas just before the rich control is completed. It changes to a value that is slightly richer than the fuel ratio (approximately 0.6 volts), and then the output of the oxygen sensor 3 is in the vicinity of a value that indicates the stoichiometric air-fuel ratio when exhaust gas of the stoichiometric air-fuel ratio flows into the three-way catalyst device It changes in.

  As a result, when the correction of the output of the air-fuel ratio sensor 2 by the differential term D is started simultaneously with the completion of the enrichment control (when the output of the oxygen sensor 3 becomes 0.5 volts), the output of the subsequent oxygen sensor 3 Due to the change to the rich side, the output of the air-fuel ratio sensor 2 is greatly corrected to the lean side, and a relatively large torque shock occurs due to a significant change in the combustion air-fuel ratio. Accordingly, in this flowchart, the prohibition of correction by the differential term D is not canceled until the output of the oxygen sensor 3 reaches a value (0.6 volts) corresponding to the set rich air-fuel ratio that is slightly richer than the theoretical air-fuel ratio. I have to.

FIG. 4 shows a second flowchart implemented in place of the first flowchart of FIG. Only the differences from the first flowchart will be described below. In the second flowchart, when the enrichment control is started, in step 203, based on the map shown in FIG. 5, the correction prohibition period t1 based on the differential term D is set according to the current degree of deterioration of the three-way catalyst device 1. It has become. This correction prohibition period t1 is the time from when the enrichment control is started until the output of the oxygen sensor 3 reaches a value (0.6 volts) corresponding to the set rich air-fuel ratio that is slightly richer than the theoretical air-fuel ratio. This time is shortened because the O ₂ storage capacity decreases as the three-way catalyst device 1 deteriorates.

  In step 207, it is determined whether or not the elapsed time t from the start of the enrichment control has reached the correction prohibition period t1, and when this determination is affirmative, the output of the air-fuel ratio sensor 2 by the differential term D is determined. The prohibition of correction is cancelled. Thereby, the same effect as the first flowchart of FIG. 3 can be obtained.

  In the first and second flowcharts described above, the enrichment control is completed when the output of the oxygen sensor 3 reaches a value corresponding to the stoichiometric air-fuel ratio. Of course, depending on the degree of deterioration of the three-way catalyst device 1 Then, the enrichment control period may be set, and the enrichment control may be completed when this period has elapsed since the start of the enrichment control. The enrichment control period determined by the degree of deterioration of the three-way catalyst device 1 is slightly shorter than the above-described correction prohibition period t1 determined by the same degree of deterioration.

  In the first and second flowcharts, instead of prohibiting correction of the output of the air-fuel ratio sensor by the differential term D, for example, the absolute value of the differential term D calculated by making the gain Dg of the differential term D very small Also, the correction of the output of the air-fuel ratio sensor by the differential term D may be suppressed.

It is the schematic which shows the exhaust system of the internal combustion engine which the air-fuel ratio control apparatus by this invention controls. It is a time chart which shows the change of a target air fuel ratio and an oxygen sensor output. It is a 1st flowchart implemented by the air fuel ratio control apparatus by this invention. It is a 2nd flowchart implemented by the air fuel ratio control apparatus by this invention. It is a map for determining a differential term correction prohibition period.

Explanation of symbols

1 Three-way catalyst device 2 Air-fuel ratio sensor 3 Oxygen sensor

Claims (3)

An air-fuel ratio sensor disposed upstream of the engine exhaust system three-way catalyst device and having an output varying in accordance with the air-fuel ratio of the exhaust gas, and an exhaust gas disposed downstream of the engine exhaust system three-way catalyst device An air-fuel ratio control apparatus for an internal combustion engine, comprising: an oxygen sensor whose output changes abruptly when the air-fuel ratio of gas is close to the theoretical air-fuel ratio, and controlling the combustion air-fuel ratio to a desired air-fuel ratio based on the output of the air-fuel ratio sensor The output of the air-fuel ratio sensor is corrected by a differential term based on the output change of the oxygen sensor, and almost all of the oxygen stored in the three-way catalyst device is released by the O ₂ storage capability in the enrichment control immediately after the fuel cut. The air-fuel ratio control apparatus for an internal combustion engine is characterized by prohibiting or suppressing the correction of the output of the air-fuel ratio sensor by the differential term until it is determined that it has been. When the period determined based on the deterioration state of the three-way catalyst device has elapsed since the start of the enrichment control, almost all of the oxygen stored in the three-way catalyst device by the O ₂ storage capability in the enrichment control immediately after the fuel cut is performed. 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio control apparatus determines that it has been released. When the output of the oxygen sensor becomes a value corresponding to the set rich air-fuel ratio, it is determined that substantially all of the oxygen stored in the three-way catalyst device has been released by the O ₂ storage capability in the enrichment control immediately after the fuel cut. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein:

Images