JPH1162666A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the NOx reductive processing performance of the NOx absorption catalyst used in an internal combustion engine. SOLUTION: A control is performed such that the rich level of air-fuel ratio is increased to maximum level when the NOx reductive processing control conditions of a NOx absorption catalyst are met, followed by a gradual decrement, and at this time, the exhaust gas air-fuel ratio downstream of the NOx absorption catalyst is detected and the time t1 of being maintained near the stoichiometry in the initial period is measured, and the maximum rich level is subjected to a learning correction on the basis of the time t1, and the time t2 of being maintained in the rich condition thereafter is measured, and the rich level decremental speed is subjected to learning correction on the basis of the time t2. This establishes a good NOx reductive processing performance while both the NOx emission amount and HC and CO contents are held below the respective reference values.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to a technology for controlling an internal combustion engine having an NOx storage catalyst in an exhaust passage to an optimum air-fuel ratio required for purifying NOx. About.

[0002]

2. Description of the Related Art Conventionally, NOx in exhaust gas is stored when the exhaust air-fuel ratio is lean, and the stored NOx is stored when the exhaust air-fuel ratio is stoichiometric or rich.
An engine equipped with a NOx storage catalyst (NOx storage type three-way catalyst) that releases x and performs a reduction process is known (Japanese Patent Application Laid-Open No. 7-1995).
No. 139397).

The NOx storage catalyst stores NOx during lean combustion and reduces the amount of NOx exhausted to the atmosphere. However, when the stored amount exceeds the maximum amount, the NOx exhausted from the engine becomes catalytic. It will be discharged to the atmosphere as it is without occlusion. Therefore, when it is estimated that the NOx storage amount of the NOx storage catalyst has reached the maximum amount based on the load, rotation, air-fuel ratio, and the like,
The target air-fuel ratio of the combustion air-fuel mixture is forcibly and temporarily switched to rich (hereinafter referred to as rich spike), so that the NOx stored in the NOx storage catalyst is released and reduced.

[0004]

Conventionally, the above-described rich spike has been uniformly performed with a constant characteristic. However, this method is not suitable for the deterioration of the NOx storage catalyst with the lapse of time or the variation or the change of the NOx emission characteristic from the engine with the lapse of time. NO
Since the amount of NOx stored in the x storage catalyst changes, good control could not be performed.

More specifically, the NOx desorption characteristics of the NOx storage catalyst will be described. When rich exhaust is supplied by rich spikes, the NOx stored in the upstream portion of the catalyst is released at once, and thereafter the downstream portion of the catalyst is discharged. The part has a characteristic that it gradually becomes rich and gradually desorbs. For this reason, control is performed such that the rich level at the start of the control is increased in accordance with the desorption characteristics, and then the rich level is reduced and the rich state is continued for a predetermined time.

[0006] However, if the rich level that is increased at the start of the control is too small, NOx is desorbed from the NOx storage catalyst, but the reducing action of the catalyst is sufficient because the NOx reducing materials such as HC and CO are insufficient. Therefore, NOx is directly discharged. Especially the N that had been occluded until then
When the amount of Ox is large, a large amount of NOx is discharged (see FIG. 10). Conversely, if the rich level is too high,
Although NOx discharged at the beginning can be sufficiently reduced, excessive HC and CO are discharged.

If the time for which the rich state is continued is too short, the NOx occluded in the entire catalyst remains on the catalyst without being sufficiently reduced, and the amount of NOx that can be occluded during the next lean operation decreases, and the NOx The occlusion performance is reduced. Conversely, if the rich state continuation time is too long, the exhaust gas continues to be supplied even after the NOx of the entire catalyst is reduced, thereby increasing the amount of HC and CO emissions. In particular, NO
If the x amount is small, a large amount of HC and CO will be discharged (see FIG. 10).

The present invention has been made in view of the above problems, and even if the characteristics of the NOx storage catalyst or the exhaust emission characteristics of the engine change, the rich spike characteristic which is always optimal for NOx purification in accordance with the change in the characteristics. , And NO
It is an object of the present invention to provide an exhaust gas purification device capable of suppressing HC and CO emissions while efficiently purifying x.

[0009]

Therefore, according to the first aspect of the present invention, when the exhaust air-fuel ratio is lean, the NO
NO that performs occlusion and releases the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich.
In an exhaust purification system for an internal combustion engine which is provided with an x-storage catalyst and performs control to reduce the NOx stored in the NOx storage catalyst by temporarily making the air-fuel ratio of the combustion mixture rich, the NOx storage is performed during the NOx reduction process control. The exhaust air-fuel ratio downstream of the catalyst is detected, and based on the time during which the exhaust air-fuel ratio is maintained in the vicinity of the stoichiometric air-fuel ratio and the time during which the exhaust air-fuel ratio is maintained in a rich state thereafter, each is required during NOx reduction processing control. The rich level and the rich continuation time of the combustion mixture are learned and corrected.

According to this configuration, when the NOx reduction processing control for temporarily making the air-fuel ratio of the combustion air-fuel mixture rich is performed,
Initially, the time during which the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio reflects the rich level in the rich spike. Since the maintained time reflects the rich continuation time in the rich spike, learning of the rich continuation time is performed based on the time.

As a result, the H level can be reduced while suppressing the NOx emission at the start of the rich spike by learning the rich level.
The emissions of C and CO are also suppressed, and the amount of NOx stored in the entire NOx storage catalyst can be sufficiently reduced while the emission of HC and CO is suppressed by learning the rich continuation time. In the invention according to claim 2, as shown in FIG. 1, NOx in the exhaust gas is stored when the exhaust air-fuel ratio is lean, and the NOx is stored when the exhaust air-fuel ratio is stoichiometric air-fuel ratio or rich. A NOx storage catalyst for releasing and reducing NOx is provided, and NOx reduction processing control means for performing control for temporarily reducing the air-fuel ratio of the combustion air-fuel mixture to reduce NOx stored in the NOx storage catalyst is provided. In the exhaust gas purifying apparatus for an internal combustion engine, during the NOx reduction processing control, exhaust air-fuel ratio detection means for detecting an exhaust air-fuel ratio downstream of the NOx storage catalyst, and the detected exhaust air-fuel ratio is maintained near a stoichiometric air-fuel ratio. Based on the time being
Rich level learning means for learning and correcting the rich level of the combustion mixture at the time of the NOx reduction processing control; and a time period during which the detected exhaust air-fuel ratio is maintained in a rich state after the state near the stoichiometric air-fuel ratio. And a rich duration learning means for learning and correcting the rich duration of the combustion air-fuel mixture during the NOx reduction control.

[0012] With this configuration, when the NOx reduction processing control means performs control to temporarily make the air-fuel ratio of the combustion air-fuel mixture rich, the rich exhaust air-fuel ratio detection means uses NO.
While detecting the exhaust air-fuel ratio downstream of the x-storage catalyst, the rich-level learning means performs rich-level learning correction based on the time during which the rich air-fuel ratio is maintained near the stoichiometric air-fuel ratio, and the rich continuation time learning means is maintained in the rich state. Learning of rich continuation time based on the time being performed.

[0013] Accordingly, the learning of the rich level suppresses the NOx emission at the start of the rich spike while reducing the NO.
The emissions of C and CO are also suppressed, and the amount of NOx stored in the entire NOx storage catalyst can be sufficiently reduced while the emission of HC and CO is suppressed by learning the rich continuation time. According to a third aspect of the present invention, in the NOx reduction process control, the rich level of the air-fuel ratio of the combustion air-fuel mixture is maximized at the start of the control, and thereafter the control is gradually reduced. The correction is a learning correction of a maximum rich level given at the start of the control, and the learning correction of the rich continuation time is a learning correction of a decreasing speed of the rich level.

According to this configuration, the maximum rich level given at the start of the NOx reduction processing control is learned and corrected by rich level learning, and then the speed at which the rich level is reduced is learned and corrected by learning the rich continuation time. As a result, the maximum rich level is learned and corrected to an appropriate value, so that the NOx emissions at the start of the rich spike are suppressed and the HC and CO emissions are also suppressed. Is learned and corrected to an appropriate value, the amount of NOx stored in the entire NOx storage catalyst can be sufficiently reduced while suppressing the emission of HC and CO.

Further, the invention according to claim 4 is characterized in that the NOx
Is a control in which the rich level of the air-fuel ratio of the combustion air-fuel mixture is maximized at the start of the control, then reduced to a predetermined level and continued for a predetermined time, and the learning correction of the rich level is given at the start of the control. It is a learning correction of a maximum rich level, and the learning correction of the rich continuation time is a learning correction of a time continuing at the predetermined level.

According to this configuration, the maximum rich level given at the start of the NOx reduction processing control is learned and corrected by rich level learning, and the time that continues at a predetermined level after that is learned and corrected by learning of the rich continuation time. As a result, the maximum rich level is learned and corrected to an appropriate value, so that the amount of HC and CO emissions is suppressed while the amount of NOx emitted at the start of the rich spike is suppressed. Since the time is learned and corrected to an appropriate value, the amount of NOx stored in the entire NOx storage catalyst can be sufficiently reduced while suppressing the emission of HC and CO.

Further, the invention according to claim 5 is characterized in that the NOx
The detection of the exhaust air-fuel ratio downstream of the storage catalyst is to continuously detect the exhaust air-fuel ratio. The time during which the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio and the time during which the exhaust air-fuel ratio is maintained in a rich state thereafter Are detected by comparing a slice level set corresponding to each with the detected exhaust air-fuel ratio.

According to this configuration, by comparing the detected exhaust air-fuel ratio with the slice level set for detecting the state near the stoichiometric air-fuel ratio, the state near the stoichiometric air-fuel ratio is detected and maintained in this state. By measuring the time during which the rich state is detected and comparing it with the slice level set for rich state detection, the time during which the state is maintained in this state while detecting the rich state is measured and the sustain time is measured. Is detected.

Thus, the stoichiometric air-fuel ratio maintaining time and the rich state maintaining time can be detected with high accuracy.
In the invention according to claim 6, the detection of the exhaust air-fuel ratio downstream of the NOx storage catalyst detects on-off whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio. The time that is maintained in the vicinity of the stoichiometric air-fuel ratio is detected by the time until the detected exhaust air-fuel ratio reverses from lean to rich or the time until it reaches the rich peak value, and the time during which the rich state is maintained. The detection is performed based on the duration during which the detected exhaust air-fuel ratio is rich.

According to this configuration, the time required for the detected exhaust air-fuel ratio to reverse from lean to rich or to reach the rich peak value is measured to detect the stoichiometric air-fuel ratio vicinity maintaining time, and the exhaust air-fuel ratio is detected. The rich maintenance time is detected by measuring the time during which the detected value is maintained rich.
With this, the inexpensive stoichiometric air-fuel ratio sensor is used to detect the stoichiometric air-fuel ratio vicinity maintaining time and the rich maintaining time,
You can learn rich levels and rich durations.

Further, the invention according to claim 7 is characterized in that the learning correction of the rich level and the rich continuation time is performed for each region according to the operating state of the engine. According to such a configuration, the learning correction of the rich level and the rich continuation time is performed for each region according to the operating state of the engine at that time.

Thus, since the learning is performed separately for each region according to the NOx storage amount in the NOx storage catalyst which differs depending on the operation state of the engine, highly accurate learning can be performed. Also,
The invention according to claim 8 is characterized in that the area according to the operating state of the engine is an area divided by a lean continuation time before learning correction.

According to this configuration, the learning correction of the rich level and the rich continuation time is performed for each area divided by the lean continuation time before the learning correction. Thereby, since the learning is performed separately for each region according to the NOx storage amount in the NOx storage catalyst that differs depending on the lean continuation time, highly accurate learning can be performed. The invention according to claim 9 is characterized in that the region according to the operating state of the engine is a region divided by the NOx storage amount in the NOx storage catalyst estimated based on the engine operating state before learning correction. And

According to this configuration, the learning correction of the rich level and the rich continuation time is performed for each area divided by the NOx storage amount in the NOx storage catalyst estimated based on the engine operation state before the learning correction. As a result, NO to the NOx storage catalyst that differs depending on the engine operating state before the learning correction is performed.
Since the learning is performed separately for each of the estimated NOx storage amount regions while estimating the x storage amount with high accuracy, more accurate learning can be performed.

Further, the invention according to claim 10 is characterized in that upper and lower limits are set for the rich level and the rich continuation time. According to such a configuration, the rich level and the rich continuation time are limited by the upper and lower limits.

As a result, it is possible to perform the NOx reduction process while avoiding the influence on the operation of the engine such as misfire and torque fluctuation.

[0027]

Embodiments of the present invention will be described below. FIG. 2 is a diagram showing a system configuration of the internal combustion engine according to the first embodiment. In the engine 1, air measured by a throttle valve 2 is sucked, and the fuel injected from a fuel injection valve 3 and the fuel The mixture with the intake air forms an air-fuel mixture.

The fuel injection valve 3 may be one that injects fuel into the intake port portion or one that directly injects fuel into the combustion chamber. The air-fuel mixture is ignited and burned by spark ignition by the spark plug 4, and the combustion exhaust gas is purified by the NOx storage catalyst 5 provided in the exhaust gas passage 9 and then discharged to the atmosphere.

The NOx storage catalyst 5 stores NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and stores the NOx when the exhaust air-fuel ratio is stoichiometric or rich.
This is a catalyst (NOx storage type three-way catalyst) that releases x and performs a reduction treatment in the three-way catalyst layer. In addition, a light-off catalyst 21 composed of a three-way catalyst is provided in the exhaust passage portion upstream of the NOx storage catalyst 5 to enhance the exhaust gas purifying performance at the time of starting by promoting the activity with a small capacity.

The control unit 6 for controlling the injection timing and injection amount of the fuel injection valve 3 and the ignition timing of the ignition plug 4 includes a microcomputer, and performs arithmetic processing based on detection signals from various sensors. A fuel injection signal (injection pulse signal) is output to the fuel injection valve 3, and an ignition plug 4 (power transistor) is output.
, An ignition signal is output.

In the calculation of the fuel injection signal, the target air-fuel ratio is determined according to the operating conditions, and the fuel injection amount (injection pulse width) is calculated so that a mixture of the target air-fuel ratio is formed. An air-fuel ratio that is leaner than the stoichiometric air-fuel ratio is set as the target air-fuel ratio. The various sensors include an air flow meter 7 for detecting an intake air flow rate of the engine 1, a throttle sensor 8 for detecting an opening degree of the throttle valve 2, and an exhaust gas which is disposed in an exhaust passage 9 upstream of the NOx storage catalyst 5. First to detect air-fuel ratio
An air-fuel ratio sensor 10, a second air-fuel ratio sensor 11 disposed in the exhaust passage 9 on the downstream side of the NOx storage catalyst 5 for detecting the exhaust air-fuel ratio and performing the NOx reduction processing control according to the present invention are provided. In addition, the control unit 6 includes a rotation signal from a crank angle sensor (not shown) and a water temperature sensor 12.
And the like are input.

The first air-fuel ratio sensor 10 is a sensor for detecting the exhaust air-fuel ratio based on the oxygen concentration in the exhaust gas, and may be a stoichiometric sensor for detecting only the stoichiometric air-fuel ratio. A wide area air-fuel ratio sensor that can detect the fuel ratio over a wide area may be used. In the present embodiment, the second air-fuel ratio sensor 11 is a wide-range air-fuel ratio sensor capable of detecting the exhaust air-fuel ratio in a wide range.

The control unit 6 is usually
An air-fuel ratio feedback correction coefficient α for correcting the fuel injection amount is set by, for example, proportional differential control so that the exhaust air-fuel ratio detected by the first air-fuel ratio sensor 10 approaches the target air-fuel ratio. On the other hand, during the NOx reduction processing control, the NOx storage catalyst 5 is used by using the second air-fuel ratio sensor 11.
The learning correction of the control is performed while detecting the downstream exhaust air-fuel ratio, and the state of the control corresponding to the learning correction is shown in the flowchart of FIG. The characteristics of the control according to the flowchart are shown in the time chart of FIG.

In the flowchart of FIG. 3, first, in step 1 (indicated as S1 in the figure, the same applies hereinafter), a flag FRS indicating the satisfaction of the NOx reduction processing control condition is set.
Is determined to determine whether the control condition is satisfied. The NOx storage catalyst 5 stores NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich. It is preferable to perform the main control for enriching the air-fuel ratio when the air-fuel ratio is switched from lean to the stoichiometric air-fuel ratio or rich because there is no uncomfortable feeling in driving. Therefore, when the condition is satisfied, the flag FR
S is set to 1 and the flag FRS is reset to 0 when the enrichment control ends.

However, the switching of the target air-fuel ratio from lean to the stoichiometric air-fuel ratio or rich is performed according to operating conditions (acceleration, changes in load / rotation), and conditions under which the lean air-fuel ratio is originally set as the target air-fuel ratio. Even if it is below, when it is estimated that the NOx storage amount in the NOx storage catalyst 5 has reached the limit amount, the setting is such that the rich control is temporarily performed. This includes switching to rich control.

When the target air-fuel ratio is switched from lean to the stoichiometric air-fuel ratio or rich and the flag FRS is set to 1, the routine proceeds to step 2, where each variable is initialized when the control is started. For example, a flag FS1 for determining the end of the stoichiometric vicinity maintenance time by the second air-fuel ratio sensor 11 described later and a flag FS2 for starting the measurement of the rich state maintenance time thereafter are reset to 0, respectively.

In step 3, the rich level learning value LRsk (i) and the rich continuation time learning value LIα (i) stored in the RAM of the microcomputer constituting the control 6 obtained by this control are referred to. Here, the rich level learning value LRsk (i) is the maximum rich level learning value given at the start of this control. Further, the rich continuation time learning value LIα (i) is learning of the time to continue the rich,
In the present embodiment, the differential control for decreasing the rich level from the maximum rich level is performed, and the rich state maintaining time is determined by the decreasing speed, that is, the differential value. Therefore, the differential value is a learning value obtained by learning the differential value.

The learning is performed at the time of starting the learning.
The process is performed for each area divided according to the amount of NOx stored in the x storage catalyst 5. For example, the lean continuation time before the learning is measured, and learning is performed for each continuation time region i. By doing so, the learning accuracy is improved. Alternatively, the integrated NOx emission value may be estimated from the operating state before learning to estimate the NOx storage amount, and the estimation may be performed for each area divided according to the estimated NOx storage amount. The accuracy is further improved.

In step 4, the rich level basic amount KNa (i) and the rich continuation time basic amount Iα (i) also used in this control are referred to from the ROM. The rich level basic amount KNa (i) and the rich continuation time basic amount Iα (i) are also set for each region classified according to the lean continuation time for performing the learning and the estimated NOx occlusion amount. Refer to the set one. FIG. 5 shows specific characteristics of these basic amounts KNa (i) and Iα (i). Rich level basic amount KNa
(i) sets the basic amount KNa (i) to a large value because the larger the lean continuation time or the estimated NOx storage amount i, the larger the NOx storage amount and the larger the amount of NOx desorbed at the beginning. The upper limit is set in consideration of the flammability such as generation of odor. In addition, the rich maintaining time basic amount Iα (i) is set such that the rich continuation time for completely reducing the stored NOx amount decreases as the lean continuation time or the estimated NOx storage amount i increases and the NOx storage amount increases. Since it is necessary to prolong it, the basic quantity Iα (i), which is a differential value in the decreasing direction, is set to a small value.

In step 5, the rich level control amount αsk is calculated by the following equation based on the reference basic amount and the learning value.
And the rich continuation time control amount Iαsk. αsk = KNa (i) + LRsk (i) Iαsk = Iα (i) + LIα (i) Further, the calculated rich level control amount αsk and rich continuation time control amount Iαsk are compared with the set upper and lower limit values, respectively. It is limited by these upper and lower limits. As a result, while avoiding the influence on operation such as misfire and torque fluctuation,
NOx reduction processing control can be performed.

In step 6, the final rich control amount αsk finally calculated is output. This rich control amount αsk
Is set as an air-fuel ratio correction coefficient by which the basic fuel injection amount Tp is multiplied. When the control first proceeds from step 5 to step 6, the rich level control amount αsk calculated in step 5 is output as the maximum initial value.

In step 7, the current rich control amount αsk
Is the equivalent value during stoichiometric (stoichiometric air-fuel ratio) control10
It is determined whether it exceeds 0%. Rich control amount αsk
Is greater than 100%, the routine proceeds to step 8, where the value obtained by subtracting the latest rich duration control amount Iαsk currently calculated from the rich control amount αsk as shown in the following equation is used.
After updating as the rich control amount αsk to be output next time, the process proceeds to step 9.

Αsk = αsk−Iαsk If it is determined in step 7 that the rich control amount αsk is 100% or less, step 8 is jumped to step 9. In step 9, it is determined whether or not the value of the flag FS1 for determining the end of the stoichiometric vicinity maintaining time measurement is 0, that is, the NOx detected by the second air-fuel ratio sensor 11.
It is determined whether or not the state in which the exhaust air-fuel ratio downstream of the storage catalyst 5 is in the vicinity of the stoichiometric state has not ended.

If FS1 = 0, that is, if it is determined that the stoichiometric vicinity state has not ended, the routine proceeds to step 10, where the output value VRO2 of the second air-fuel ratio sensor 11 is set to the first value.
It is determined whether the slice level has fallen below the slice level S1. Here, the value of the first slice level S1 is set to a value slightly leaner than a value corresponding to stoichiometry. In step 10, the output value VRO2 is equal to the first slice level S1.
If it is determined that the above is the case, the exhaust air-fuel ratio downstream of the NOx storage catalyst 5 has not yet been enriched to near stoichiometric. In this case, the process returns to step 6 without measuring the stoichiometric continuous duration. The same control is repeated.

If it is determined in step 10 that the output value VRO2 has fallen below the first slice level S1, the process proceeds to step 11, in which the value CT1 of the counter for measuring the stoichiometric neighborhood duration is counted by a predetermined amount Td. Up. That is, in this step, the time during which the exhaust air-fuel ratio downstream of the NOx storage catalyst 5 is near the stoichiometric ratio is measured.

Next, at step 12, it is determined whether or not the output value VRO2 of the second air-fuel ratio sensor 11 has fallen below the second slice level S2. Here, the value of the second slice level S2 is set to a value slightly richer than a value corresponding to stoichiometry. In step 12, the output value VRO2
Is determined to be equal to or higher than the second slice level S2, it is determined that the stoichiometric vicinity state is maintained, and the process returns to step 6 to repeat the same control as described above.

In step 12, the output value VRO2 is
If it is determined that the slice level has fallen below the slice level S2, N
It is determined that the exhaust air-fuel ratio downstream of the Ox storage catalyst 5 has deviated from the near stoichiometric state to the rich side, and the routine proceeds to step 13. In step 13, the value of the measurement end determination flag FS1 is set to 1 in order to end the measurement of the stoichiometric neighborhood maintaining time.

In step 14, the value of the flag FS2 for starting measurement of the time during which the exhaust air-fuel ratio downstream of the NOx storage catalyst 5 is maintained in a rich state is set to 1, and
The current count value CT1 of the counter that counts the stoichiometric near continuation time is set as the stoichiometric near continuation time T1. In step 15, the value CT2 of the counter for measuring the rich state detection time is counted up by a predetermined amount Td. That is, in this step, the time during which the exhaust air-fuel ratio downstream of the NOx storage catalyst 5 is maintained in a rich state is measured.

In step 16, during the rich state measurement in which the flag FS2 is set to 1, the output VRO2 of the second air-fuel ratio sensor 11 has exceeded the second slice level S2, that is, the downstream of the NOx storage catalyst 5 It is determined whether the exhaust air-fuel ratio has deviated from the rich state. While the exhaust air-fuel ratio is maintained in the rich state, the control returns to step 6 to repeat the control for gradually bringing the air-fuel ratio closer to the lean direction. Since the measurement of the stoichiometric vicinity maintaining time has been completed and the flag FS1 = 1 has been set, the process jumps from step 9 to step 15.

By making the air-fuel ratio closer to the lean side as described above, when it is determined that the output VRO2 of the second air-fuel ratio sensor 11 has become equal to or lower than the second slice level S2, that is, that it has deviated from the rich state, Proceeding to step 17, the current value C of the counter for measuring the rich state maintaining time
T2 is set as the rich state maintaining time T2. Then, in step 18, the stoichiometric vicinity maintaining time T1 obtained as described above and the rich state maintaining time T
2, the learning of the rich level and the learning of the duration of the rich state are performed. Specifically, the rich level learning value LRsk (i) and the rich continuation time learning value Iα (i) are calculated and updated as follows.

First, as for the rich level learning value LRsk (i), as shown in FIG. 6, the NOx emission increases as the stoichiometric neighborhood maintaining time t1 increases, and the HC increases as the stoichiometric vicinity maintaining time t1 decreases.
Since the amount of (and CO) emissions increases, a time range of the stoichiometric vicinity maintaining time t1 in which both of them are equal to or less than the reference value is set. When the stoichiometric neighborhood maintaining time t1 is in the above-mentioned time range, the learning value LRsk (i) is maintained at the current state. The learning value LRsk (i) is increased so as to increase the learning value LRsk (i). If the learning value LRsk (i) is smaller than the time range, the learning value LRsk (i) is decreased so as to decrease the rich level so that t1 increases.

Here, when the rich level is increased in the case where the stoichiometric proximity maintaining time t1 is long, the stoichiometric proximity maintaining time t1 decreases because the catalyst atmosphere is not completely reduced when the t1 is large. When the rich level is increased in order to increase the amount of HC as a reducing agent, the time for maintaining the vicinity of stoichiometry is shortened.

The above learning is performed by the following equation using the learning correction value HLRsk (i) set as shown in FIG. 7, for example. LRsk (i) = LRsk (i) + HLRsk (i) On the other hand, the rich duration learning value Iα (i) is shown in FIG.
When the rich state maintaining time t2 becomes longer as shown in FIG.
As the C (and CO) emission increases and decreases, the NOx emission increases, so that the time range of the rich state maintaining time t2 in which both of them fall below the reference value is set as described above. When the rich state maintaining time t2 is within the above-mentioned time range, the learning value LIα (i) is maintained at the current level.
If the rich state maintaining time t2 is longer than the time range, the learning value LIα (i) is increased so as to increase the rate of reduction of the rich level so that t2 is decreased. If the rich state maintaining time t2 is smaller than the time range, t2 is increased. Thus, the learning value LIα (i) is reduced so as to reduce the rate of decrease of the rich level.

The above learning is performed by the following equation using the learning correction value HLIα (i) set as shown in FIG. 9, for example. LIα (i) = LIα (i) + HLIα (i) After the learning value is updated, the process proceeds to step 19, and since the current NOx reduction processing control has been completed, the flag FRS is maintained until the next NOx reduction processing control start condition is satisfied. Is reset to 0.

As described above, by learning the rich level and the rich continuation time, as shown in FIG.
Both x emission and HC (and CO) emission can be suppressed below the standard. In the above embodiment, the control method in which the rich level at the start of the rich spike is maximized, and thereafter the rich level is gradually reduced is shown. However, as the rich spike method, as shown in FIG. There is also a method in which the rich level initially maximized is reduced stepwise (or rapidly) to a predetermined level and maintained at the predetermined level for a predetermined time. When the present invention is applied to this method, learning similar to that of the above-described embodiment is performed for the initial rich level, and the rich continuation time is set to the predetermined level according to the detected rich state maintaining time. What is necessary is to learn so that the duration may be decreased or increased. In addition, as shown in FIG. 12, there is also a spike method in which the initially increased rich level is continued for a predetermined period of time, and then gradually reduced to a predetermined level and then gradually reduced. , The duration of the initial large rich level is learned and corrected, and the rich state maintaining time is measured to learn and correct the level deceleration speed after the predetermined level.

In the above embodiment, the detection of the exhaust air-fuel ratio downstream of the NOx storage catalyst can be continuously detected by the wide-range sensor, so that highly accurate learning can be performed based on the detected value. The present invention can also be implemented using a stoichiometric sensor (so-called O2 sensor) that detects the fuel ratio in a rich, lean, on / off manner with respect to the stoichiometric air-fuel ratio.

In the embodiment using the stoichiometric sensor, the state near the stoichiometric sensor cannot be directly detected. For example, the time until the output value of the stoichiometric sensor changes from lean to rich after the start of the rich spike, Alternatively, the time until the peak value in the rich direction becomes the stoichiometric neighborhood maintaining time can be estimated. Therefore, the rich level may be learned based on the measured value of the time. However, since there is a time delay before the exhaust of the rich spiked air-fuel mixture reaches the NOx storage catalyst, accuracy can be improved by performing a correction by subtracting the delay from the measurement time, and the delay correction can be further reduced. If the setting is made in accordance with the engine operating state (mainly the rotational speed), the accuracy can be further improved, and learning can be performed using an inexpensive stoichiometric sensor with an accuracy close to that of using a wide-area sensor.

In the learning of the rich maintenance time, the time from when the output of the stoichiometric sensor is richly inverted to when the output is maintained rich (until it is again inverted to lean) is estimated as the rich state maintenance time. Therefore, the rich continuation time may be learned based on the measured value of the time.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an exhaust gas purifying apparatus according to the second embodiment.

FIG. 2 is a system configuration diagram of the internal combustion engine according to the first embodiment.

FIG. 3 is a flowchart illustrating a state of NOx reduction processing control according to the first embodiment.

FIG. 4 is a time chart showing characteristics of rich spike control according to the first embodiment.

FIG. 5 is a diagram showing characteristics of a rich level basic amount and a rich continuation time basic amount in the first embodiment.

FIG. 6 is a view showing an exhaust emission characteristic according to a stoichiometric vicinity maintaining time t1 in the first embodiment.

FIG. 7 is a diagram showing characteristics of a learning correction amount of a rich level with respect to a stoichiometric neighborhood maintaining time t1 in the first embodiment.

FIG. 8 is a view showing an exhaust emission characteristic according to a rich state maintaining time t2 in the first embodiment.

FIG. 9 shows characteristics of a rich level learning correction amount with respect to a rich state maintaining time t2 in the first embodiment.

FIG. 10 is a time chart showing exhaust emission characteristics in the first embodiment in comparison with a conventional example without learning.

FIG. 11 is a diagram illustrating another example of the rich spike characteristic.

FIG. 12 is a diagram illustrating still another example of the rich spike characteristic.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Throttle valve 3 Fuel injection valve 4 Spark plug 5 NOx storage catalyst 6 Control unit 7 Air flow meter 8 Throttle sensor 9 Exhaust passage 10 First air-fuel ratio sensor 11 Second air-fuel ratio sensor

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 F01N 3/24 R F02D 41/04 305 F02D 41/04 305Z 45/00 368 45/00 368Z

Claims (10)

[Claims] 1. A NOx storage catalyst for storing NOx in exhaust gas when the exhaust air-fuel ratio is lean, and releasing and reducing the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich. An exhaust purification device for an internal combustion engine, which performs a control for reducing the NOx stored in the NOx storage catalyst by temporarily making the air-fuel ratio of the combustion air-fuel mixture rich, wherein the exhaust gas downstream of the NOx storage catalyst during the NOx reduction process control is provided. The air-fuel ratio is detected, and based on the time during which the exhaust air-fuel ratio is maintained in the vicinity of the stoichiometric air-fuel ratio and the time during which the exhaust air-fuel ratio is maintained in the rich state thereafter, the combustion air-fuel ratio required during the NOx reduction process control is determined. An exhaust gas purification device for an internal combustion engine, wherein a rich level and a rich continuation time are learned and corrected. 2. A NOx storage catalyst which stores NOx in exhaust gas when the exhaust air-fuel ratio is lean and releases the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich to perform a reduction process. And the air-fuel ratio of the combustion air-fuel mixture is temporarily made rich to reduce the NO stored in the NOx storage catalyst.
an exhaust air purification device for an internal combustion engine having NOx reduction processing control means for performing control for reducing x, wherein during the NOx reduction processing control, exhaust air-fuel ratio detection means for detecting an exhaust air-fuel ratio downstream of the NOx storage catalyst; Rich level learning means for learning and correcting the rich level of the combustion air-fuel mixture during the NOx reduction processing control based on the time during which the detected exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio; The determined exhaust air-fuel ratio is determined based on a time period in which the exhaust air-fuel ratio is maintained in a rich state after the state near the stoichiometric air-fuel ratio.
An exhaust purification device for an internal combustion engine, comprising: a rich duration learning unit that learns and corrects a rich duration of a combustion mixture during x reduction processing control. 3. The control of the NOx reduction process is a control in which the rich level of the air-fuel ratio of the combustion air-fuel mixture is maximized at the start of the control, and thereafter, the rich level is gradually decreased. Learning correction of the maximum rich level given at the start, the learning correction of the rich continuation time,
3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the correction is a learning correction of the rate of decrease of the rich level. 4. The NOx reduction process control is a control in which the rich level of the air-fuel ratio of the combustion air-fuel mixture is maximized at the start of the control, and thereafter, is reduced to a predetermined level and continues for a predetermined time.
The learning correction of the rich level is a learning correction of a maximum rich level given at the start of the control, and the learning correction of the rich continuation time is a learning correction of a time continuing at the predetermined level. An exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2. 5. The detection of the exhaust air-fuel ratio downstream of the NOx storage catalyst is performed by continuously detecting the exhaust air-fuel ratio, and the time during which the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio and the subsequent rich air-fuel ratio are determined. The time during which the state is maintained is detected by comparing a slice level set corresponding to each of the states with the detected exhaust air-fuel ratio. An exhaust gas purification device for an internal combustion engine according to one of the above aspects. 6. The exhaust air-fuel ratio downstream of the NOx storage catalyst is detected on / off whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio, and the exhaust air-fuel ratio is close to the stoichiometric air-fuel ratio. The maintained time is detected by the time until the detected exhaust air-fuel ratio reverses from lean to rich or the time until it reaches a rich peak value, and the detection of the time during which the rich state is maintained is detected. The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the detection is performed based on a duration during which the exhaust air-fuel ratio is rich. 7. The internal combustion engine according to claim 1, wherein the learning correction of the rich level and the rich continuation time is performed for each region according to the operating state of the engine. Engine exhaust purification device. 8. The exhaust gas purifying apparatus for an internal combustion engine according to claim 7, wherein the region according to the operating state of the engine is a region divided by a lean duration before learning correction. 9. The area according to the operating state of the engine is an area divided by the NOx storage amount in the NOx storage catalyst estimated based on the engine operating state before learning correction. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. 10. The method according to claim 1, wherein upper and lower limits are set for the rich level and the rich continuation time.
The exhaust gas purification apparatus for an internal combustion engine according to any one of the above.