JPH09112308A - Air-fuel ratio controller for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To feed unburned HC without excess and deficiency for reduction of NOx for improved exhaust emission and fuel consumption by making rich state control while obtaining NOx amount attracted during lean driving and the accumulated amount of unburned HC when a rich condition is made in a switching process from lean to strike condition. SOLUTION: This controller is provided with a means 56 for calculating NOx attracted amount attracted in an NOx attracting catalyst during lean driving, a means 57 for setting the rich degree of air-fuel ratio according to the NOx attracted amount, a means 58 for calculating the generating amount of HC according to the rich degree, and a means 59 for calculating the accumulated supply amount of HC from the driving condition and the rich degree after the air-fuel ratio is switched from a lean condition. The rich correction control of the air-fuel ratio is conducted so as to meet the amount necessary for the accumulated supply amount of HC to release or reduce NOx attracted in the NOx attraction catalyst.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lean burn type air-fuel ratio control system for an internal combustion engine.

[0002]

2. Description of the Related Art In order to improve the fuel efficiency of an internal combustion engine, the air-fuel ratio is mainly set to a lean air-fuel ratio which is thinner than stoichiometry.
There is a so-called lean burn engine that achieves efficient combustion even with a lean air-fuel ratio by improving the intake system, the shape of the combustion chamber, the ignition structure, and the like.

By the way, in this lean burn engine, the amount of NOx produced is originally small,
In order to further improve the exhaust emission, there is a device in which an NOx adsorption catalyst is attached to the exhaust passage (Japanese Patent Laid-Open No. 6-242242)
-66185 gazette).

In the lean burn engine, the oxygen concentration in the exhaust gas is high and NOx cannot be reduced by the three-way catalyst. The NOx adsorption catalyst has a property of releasing NOx adsorbed in a lean atmosphere in a rich atmosphere,
Therefore, a three-way catalyst is provided downstream of this NOx adsorbing catalyst, and when the air-fuel ratio is switched from lean to stoichiometric depending on operating conditions, the air-fuel ratio is temporarily made rich.
NOx adsorbed on the x-adsorption catalyst is released, and reduction processing is performed by the downstream three-way catalyst. In this way, HC and CO during lean operation are oxidized by the three-way catalyst while NOx
The NOx adsorbed on the adsorption catalyst can be reduced at the time of switching to the stoichiometric operation, and the exhaust emission of the lean burn engine can be greatly improved.

The rich degree of the air-fuel ratio required to release the NOx adsorbed on the NOx adsorbing catalyst is determined based on the amount of NOx adsorbed up to that time. Based on the estimated value of the amount, it is set so that more unburned components (HC) than the amount required to desorb and reduce the adsorbed NOx are generated, and excess HC etc. is oxidized by the three-way catalyst. By doing so, it is ensured that NOx can be reduced.

This is because when switching the air-fuel ratio, it is difficult to supply unburned HC without excess or deficiency corresponding to the amount of NOx adsorbed, and if less than necessary unburned HC can be supplied, N
This is because it becomes impossible to reliably reduce the total amount of Ox.

[0007]

However, if the air-fuel ratio is made rich enough to generate excess unburned HC in this way, it is unavoidable that fuel consumption deteriorates. Also,
Since reducing the adsorbed NOx depends on the total amount of unburned HC supplied by the enrichment, for example, when operating conditions such as fuel cut overlap during the enrichment, HC is not generated during this period. The required HC may be insufficient.

The present invention has been proposed in order to solve such a problem, and is the amount of NOx adsorbed during lean operation and the cumulative amount of unburned HC when enriched in the process of switching from lean to stoichiometric. The purpose of the present invention is to supply HC without excess or deficiency for the reduction of NOx by controlling the enrichment while seeking, and to improve exhaust emission and fuel efficiency.

[0009]

As shown in FIG. 15, a first invention is an NOx adsorption catalyst 51 installed in an exhaust system for adsorbing NOx in a lean atmosphere and desorbing NOx adsorbed in a rich atmosphere, A three-way catalyst 52 installed in the exhaust system downstream thereof to reduce NOx and oxidize HC and CO
And an internal combustion engine equipped with an air-fuel ratio switching means 53 for switching the air-fuel ratio between lean and stoichiometric according to operating conditions, and an air-fuel ratio transient correction means 54 for temporarily making rich when the air-fuel ratio is switched from lean to stoichiometric. In the air-fuel ratio control device for the engine, a means 55 for detecting the operating state, a means 56 for calculating the NOx adsorption amount adsorbed on the NOx adsorption catalyst during lean operation, and a degree of enrichment of the air-fuel ratio according to the NOx adsorption amount. And a means 58 for calculating the amount of generated HC according to the degree of enrichment, and a cumulative supply amount of HC from the operating state after the air-fuel ratio is switched from lean and the amount of generated HC. Means 59, and means 60 for performing rich correction control of the air-fuel ratio so that the cumulative supply amount of HC coincides with the amount required to desorb and reduce the NOx adsorbed on the NOx adsorption catalyst.

According to a second aspect of the present invention, in the first aspect of the present invention, the air-fuel ratio enrichment setting means subtracts a predetermined amount from the enrichment degree set at the start of enrichment by a predetermined amount every predetermined unit time. The cumulative supply amount in one feedback cycle is set so as to correspond to the NOx adsorption amount.

In a third aspect based on the second aspect, when the air-fuel ratio enrichment setting means has a degree of enrichment of the air-fuel ratio exceeding a predetermined value, the first enrichment control is limited to this predetermined value. The rich correction control means repeats the enrichment control until the cumulative supply amount of HC corresponds to the remaining NOx adsorption amount.

[0012]

In the first aspect of the present invention, when the air-fuel ratio is switched from lean to stoichiometric, the NOx adsorbed on the NOx adsorbing catalyst is desorbed by temporarily controlling to rich, and the downstream three-way catalyst is used. The reduction is performed in a rich atmosphere, and this separation and reduction is the H supplied to the exhaust gas.
It depends on the total amount of C.

If a fuel cut or the like occurs after the air-fuel ratio is switched, the supply of HC is interrupted during that time and, of course, NO.
The departure of x is not done either. However, in the present invention, the supply amount of HC when the air-fuel ratio is made rich is calculated as the cumulative supply amount while judging the operating state after the start of the rich control and the degree of enrichment, and the operating state changes midway and the HC Is interrupted or the degree of enrichment is changed, this period is not included in the cumulative amount of HC, and the cumulative amount is calculated corresponding to the actual amount of generated HC. Moreover, since the degree of enrichment of the air-fuel ratio is set corresponding to the amount of NOx adsorbed during lean operation, when the amount of NOx adsorbed is small, the air-fuel ratio will not be unnecessarily rich.

As a result, it is possible to supply HC without excess or deficiency corresponding to the amount of NOx adsorbed at the time of enrichment, to reliably desorb NOx adsorbed on the NOx adsorbing catalyst, and to carry out reduction treatment. Unnecessarily increase the degree of enrichment,
Exhaust emissions can be improved while preventing deterioration of fuel efficiency and drivability without increasing the rich period.

In the second aspect of the invention, since the enrichment of the air-fuel ratio is basically limited to one cycle of feedback control,
The impact on the drivability when shifting from lean to stoichi can be contained in such a short time.

In the third aspect of the invention, when the required enrichment degree of the air-fuel ratio exceeds a predetermined value, the required amount of HC is supplied by performing the enrichment in a plurality of times, so that the air-fuel ratio is extremely rich. It is possible to avoid deterioration of drivability due to torque shock.

[0017]

Embodiments of the present invention will be described below.

In FIG. 1, 1 is an engine main body, 2 is an intake passage, 3 is an exhaust passage, a throttle valve 4 for controlling an intake air amount is provided in the intake passage 2, and intake air of each cylinder further downstream thereof is provided. A swirl control valve 5 for generating swirl in intake air flowing into the cylinder during lean operation of the air-fuel ratio is provided in the port portion. Reference numeral 6 is a fuel injector for supplying fuel to each intake port portion.

A catalyst device 7 is provided in the exhaust passage 3, and the catalyst device 7 adsorbs NOx when the air-fuel ratio is lean and releases the adsorbed NOx when rich, and a stoichiometric or rich air-fuel ratio. A three-way catalyst 7B, which sometimes reduces NOx and oxidizes HC and CO when stoichiometric or lean, is arranged in series.

A control unit 1 for controlling the amount of fuel injected from the fuel injector 6 in order to make the air-fuel ratio lean or stoichiometric depending on the operating conditions, or to temporarily enrich the air-fuel ratio when switching from lean to stoichiometric.
0 is provided. The control unit 10 simultaneously controls the opening degree of the swirl control valve 5, the ignition timing of the spark plug 11, the opening degree of the ACC valve 12 that bypasses the throttle valve 4 and flows auxiliary air, and further operates the fuel pump 13 and exhausts the gas. The operation of the warning light 14 at high temperature is controlled.

In order to perform these controls according to the operating state, various signals for detecting the operating state are input to the control unit 10. Therefore, the air flow meter 21 for measuring the intake air amount and the throttle valve 4 are measured. A throttle sensor (which also serves as an idle switch) 22 for detecting the opening, a rotation speed sensor 23 for detecting the engine speed, an air-fuel ratio sensor (O ₂ sensor) 24 for detecting the oxygen concentration in the exhaust gas, and the temperature of the catalyst device 7. Outputs such as a temperature sensor 25 for detecting the temperature, a neutral sensor 26 for detecting the neutral position of the transmission, a water temperature sensor 27 for detecting the engine cooling water temperature, a knock sensor 28 for detecting the knocking state of the engine, and a vehicle speed sensor 29 for detecting the vehicle speed. The circuits are connected.

As will be described later, in the control unit 10, the intake swirl is controlled while controlling the fuel injection amount so that the air-fuel ratio becomes lean in a predetermined operating region centered on low and medium speeds and medium load regions. The opening of the swirl control valve 5 is narrowed so as to occur, and the ignition timing is appropriately controlled to perform lean operation with good fuel economy. In other areas, the air-fuel ratio is stoichiometric based on the output of the O ₂ sensor 24. Feedback control is applied to meet the requirements of engine stability and output. And NO during lean operation
By adsorbing NOx on the x adsorption catalyst 7A and temporarily changing the air-fuel ratio to rich when switching from lean operation to stoichiometric operation, by appropriately controlling the enrichment degree and period as described later, The NOx separated from the NOx adsorption catalyst 7A is reliably reduced in the downstream three-way catalyst 7B.

This control will be specifically described with reference to the flowchart.

First, FIG. 2 is a flow chart for lean operation by making the air-fuel ratio lean, and it is judged from the output of the idle switch whether or not the idle switch is ON. If the idle switch is ON, lean operation is performed. Therefore, the routine proceeds to step 16 and the lean operation permission flag is set to FLEAN = 0.

On the other hand, when the idle switch is not ON, various operating conditions are judged in step 3 and subsequent steps, and when the conditions are satisfied, the lean operation permission flag FLEAN = 1 is set in step 15 and The lean operation is permitted, but in other cases, the process proceeds to step 16 and the lean operation is not performed.

That is, in steps 3 and 4, the cooling water temperature T
W is detected and it is within a predetermined temperature range, that is, T
It is determined whether or not WL≤TW≤TWH, the fuel injection pulse width TP (engine load) is detected in steps 5 and 6, and it is determined whether or not this is within a predetermined range, that is, TPL≤TP≤TPH, and step 7 , 8 detects the engine speed NE, and this is within a predetermined range, that is, NEL ≦ N.
It is determined whether E ≦ NEH, and steps 9 and 10 are performed.
Then, the throttle opening TVO is detected, and it is judged whether it is equal to or smaller than a predetermined opening, that is, TVO ≦ TVOH. In steps 11 and 12, the vehicle speed VSP is detected, and this is a predetermined value or more, that is, VSP ≧ VSPL. It is determined whether or not the acceleration of the vehicle is ΔV in steps 13 and 14.
SP is obtained by differentiating the vehicle speed, and it is determined whether this is a predetermined value or less, that is, ΔVSP ≦ DVH, and when the respective conditions are satisfied, the routine proceeds to step 15 and lean operation is performed.

The corresponding relationship is shown in FIGS. 3 to 5 with respect to the region in which the lean operation is performed under each condition of the cooling water temperature, the load, and the vehicle speed.

When the lean operating condition is determined in this manner, the fuel-air ratio (the stoichiometry is set to 1 and the air-fuel ratio to the stoichiometric air-fuel ratio which becomes larger than 1 when stoichiometric is set to 1 is followed according to the flow chart shown in FIG. The correction coefficient DML of the comparison value) is calculated.

In step 21, it is judged whether or not the flag FLEAN = 1, and when the lean operation is permitted, in step 22, the target fuel-air ratio TDML is assigned to the lean fuel-air ratio by the engine load and the number of revolutions. Ask from the map. If the lean operation is not permitted, the target fuel-air ratio TDML is similarly obtained from the stoichiometric fuel-air ratio map in step 27. However, in this case, the target fuel-air ratio may be richer than stoichiometric when the engine is under a high load, etc., depending on operating conditions.

After confirming that FLEAN = 1 in step 23, TDML obtained from the lean fuel-air ratio map
Based on the above, the fuel-air ratio correction coefficient DML is calculated as follows (step 24).

DML = Max (DML−ΔDML, TDML) (1) where ΔDML is a correction amount per time, and as shown in FIG. 7, the changing speed Δ of the throttle opening at that time.
Varies with TVO.

When FLEAN = 1 is not set in step 23, the fuel-air ratio correction coefficient DML is calculated as follows based on TDML obtained from the stoichiometric fuel-air ratio map in step 28.

DML = Min (DML−ΔDML, TDML) (2) Next, if DML = 1.0 in step 25, the control shifts to feedback control so that the air-fuel ratio becomes stoichiometric, and otherwise. Is the air-fuel ratio correction coefficient α
Clamp to α = 1.0, and open control is performed so that the air-fuel ratio is based on the fuel-air ratio correction coefficient.

In this way, the air-fuel ratio is controlled according to the operating conditions. Next, according to the flowchart of FIG. 8, the NO that is adsorbed by the NOx adsorption catalyst after shifting to the lean operation.
x amount and NOx released from the catalyst in an operation other than lean
The operation of calculating the NOx adsorption amount up to the present from the amount will be described.

In step 31, it is judged whether or not the lean operation is performed, based on whether or not the fuel-air ratio correction coefficient DML <1.0.
If lean, in step 32, the NOx adsorption rate (ratio) DABSR to the catalyst is calculated as follows.

DABSR = DABSR0 # × TP / TP0 × Q / Q0 × (ABSFC # −ABSTC) / ABSFC # (3) However, ABSTC means the amount of NOx adsorbed, and the larger the load TP, the more The larger the intake air amount Q, the higher the adsorption speed.

In step 33, the NOx adsorption amount ABSTC
Is calculated by adding the current adsorption amount (adsorption rate) DABSR to the previous value as in the following equation.

ABSTC = ABSTC (old) + DABSR (4) On the other hand, when lean operation is not performed in step 31, the amount of NOx released from the catalyst is calculated. To judge. Unburned H for reducing NOx during fuel cut
Since C is not emitted and NOx is not emitted, NO
The process ends without calculating the x adsorption amount. When the fuel is not being cut, in step 35, the desorption rate DPRGR of NOx from the catalyst is calculated as follows.

DPRGR = DPRGR0 # × TP / TP0 × Q / Q0 × ABSTC / AB SFC # (5) Note that this desorption also increases as the load TP and the intake air amount Q increase. Then, in step 36, the NOx adsorption amount ABSTC is calculated by the following equation.

ABSTC = ABSTC (old) -DPRGR (6) In this way, when the NOx amount adsorbed by the NOx adsorption catalyst to date is calculated, when the lean operation is switched to the stoichiometric operation, the adsorption is performed. The air-fuel ratio is temporarily rich-shifted in order to separate and reduce the generated NOx. The operation of calculating the air-fuel ratio rich correction coefficient ALPHA at this time will be described with reference to the flowchart of FIG. 9. It should be noted that this flow is executed every one revolution of the engine.

First, in step 41, DML <100%
If it is in the lean operation, the routine proceeds to step 57 and thereafter, the rich shift execution flag FRSFT = 1 is set, and the cumulative supply amount MH of unburned HC is set.
While clearing to C = 0, the rich correction coefficient ALPHA is clamped at step 58, and the enrichment degree (LRPL) addition flag FPLAD at the time of rich shift is added.
Set D = 1 and end the process.

On the other hand, when the lean operation is not being performed, the routine proceeds to step 41, where even during the stoichiometric operation,
It is determined whether or not there are operating conditions for clamping the ALPHA at the time of full-open acceleration or deceleration, and if so, the process proceeds to step 58 and thereafter to clamp the ALPHA.

If the clamp conditions are not met, step 42
It is determined whether or not the rich shift execution flag FRSFT = 1, and if FRSFT = 0, the routine proceeds to step 60 to carry out normal air-fuel ratio feedback control, and the integral part i of the feedback control constant and the proportional part PL, Read PR from map.

On the other hand, when FRSFT = 1,
The rich shift LRPL addition flag FPLADD is checked. When FPLADD = 1, the routine proceeds from step 45 to step 49 to calculate the degree of enrichment, and FPLADD
When = 0, the cumulative supply amount of HC based on the enrichment is calculated, and therefore the routine proceeds from step 51 to step 56.

First, at step 45, the RNOX is read out by referring to the table for the NOx amount desorbed from the catalyst based on the previous cumulative supply amount MHC of HC.

In this case, MHC is 0 at the first rich shift at the time of shifting from lean to stoichiometric, and the process here is restarted after the rich shift process is interrupted midway due to fuel cut or the like. Moreover, it is important when accurately calculating the NOx release amount.

In step 46, the NOx adsorbed amount ABSTC calculated from the NOx adsorbed amount ABSTC obtained during lean operation is used.
Is subtracted, and this is set as the current NOx adsorption amount ABSTC. The supply amount TMHC of the target HC required to release this ABSTC is calculated as TMHC = RHN # × ABS
It is calculated as TC (however, RHN # is a constant) (step 47). Then, at step 48, the enrichment degree LRPL of the air-fuel ratio is obtained by table lookup based on the target supply amount TMHC.

In this case, the larger the TMHC, the larger the degree of enrichment LRPL. However, if it becomes too large, the torque fluctuation due to the abrupt change of the air-fuel ratio becomes large, so that the value is larger than the allowable torque fluctuation range. Not set to. In addition, when the HC supply amount is insufficient in one rich shift, the rich shift may be repeated a plurality of times.

When the NOx adsorption amount is small and the target supply amount TMHC is small, LRPL = 0 is set in order to avoid unnecessary air-fuel ratio fluctuations, and rich shift is not performed.

When the enrichment degree LRPL is obtained in this way, the LRPL addition flag during rich shift is cleared to FPLADD = 0 in step 49, and in step 50 A
Transition to LPHA feedback control.

When FPLADD = 0 in step 44, first, in order to obtain the cumulative supply amount of HC, step 5 is executed.
At 1, the i (integration) part of the air-fuel ratio control constant is set to the i part LRI # during the rich shift. Next, HC cumulative supply amount MH
C is calculated by the following formula.

MHC = MHC + QA × K0 × (ALPHA-1.0) (7) The HC supply amount per unit time is the enrichment degree of ALP.
As a representative of the deviation of HA, this is multiplied by a unit-matching constant K0, and further obtained as a product of the intake air amount QA, and this is added to the MHC up to the previous time.
The cumulative supply amount MHC as an integrated value is calculated (step 52).

Next, in step 53, it is checked whether or not one cycle of ALPHA feedback control has elapsed. When one cycle has elapsed, the cumulative supply amount is compared with the target supply amount in step 54. If MHC ≧ TMHC, rich shift is performed. The execution flag FRSFT is cleared and the NOx adsorption amount ABST
C is cleared and the processing of this time is ended.

However, the cumulative supply amount MHC of HC is TMH
If less than C, perform rich shift again,
At the time of rich shift, set the LRPL addition flag FPLADD to F
The above routine is repeated with PLADD = 1.

With the above control, the following effects can be obtained.

FIG. 10 shows the air-fuel ratio feedback correction coefficient ALPHA when shifting from lean operation to stoichiometric operation.
Represents the movement of.

ALPHA is fixed at 1.0 during lean operation. The oxygen sensor cannot accurately detect the air-fuel ratio during lean operation, and the air-fuel ratio is open-controlled during this period. When changing from the lean operation range to the stoichiometric operation range, the air-fuel ratio is enriched by one cycle before setting ALPHA to a value for normal feedback control.
Air-fuel ratio rich correction coefficient LRPL (proportional amount) and LRI
(Integrated amount) is obtained.

LRPL is N adsorbed on the NOx adsorption catalyst.
This is a value for enriching the air-fuel ratio to leave Ox, and the cumulative supply amount of HC while ALPHA set based on this is greater than 1.0 is NOx immediately before the rich shift. NO adsorbed on the adsorption catalyst
The adsorbed amount of x is set to be a necessary and sufficient amount for releasing and reducing the adsorbed amount.

The LRI is ALPH for each engine revolution until ALPHA reaches the lower limit set value during normal control.
Subtracted from A, and when one cycle elapses in this way, feedback control starts.

By the way, as shown in FIG. 11, when the throttle opening decreases during lean operation and the engine load TP becomes smaller than the lean lower limit load, control is switched to return the air-fuel ratio to stoichiometric. However, if the throttle opening is further reduced and the vehicle speed at this time is relatively high, fuel cut is performed.

When the engine shifts from lean to stoichiometric, the ALPHA is rich-shifted as described above to temporarily enrich the air-fuel ratio. However, if the fuel is cut off in the middle of this, HC is not supplied and the catalyst Adsorbed on N
Ox will not come off.

In this case, however, the calculation of the amount of detachment from the catalyst is stopped, the enrichment of ALPHA is also stopped, and the amount is clamped to a predetermined value. Then, when the fuel cut is completed and the air-fuel ratio is returned to stoichiometric, the adsorption amount of NOx remaining in the catalyst is calculated, and H corresponding to this is calculated.
The cumulative supply amount of C is calculated, and the air-fuel ratio is enriched again as necessary.

Thus, even if the ALPHA rich correction is interrupted during the rich shift, the accumulated amount of HC and NO
Depending on the amount of remaining adsorbed x, enrichment is performed again after the fuel cut is completed, and the desorption and reduction of NOx are reliably executed.

Next, another embodiment of the present invention shown in FIG. 12 will be described.

In this example, even if the air-fuel ratio feedback correction coefficient ALPHA is in the non-clamping condition and FRSFT = 1, it is checked in step 43A whether or not the idle switch is ON, and the idle switch is ON during idle operation. Differs from the flowchart of FIG. 9 only in that the rich shift is prohibited.

That is, when the rich shift is performed while the vehicle is stopped at idle, the engine speed fluctuation is easily felt, and when the rich shift is performed during deceleration when the throttle is fully closed, an acceleration feeling due to an increase in torque occurs. To prevent this, when the idle switch is ON, the rich shift is prohibited.

In the other embodiment of FIG. 13, the burden of calculation of the cumulative supply amount of HC is reduced as compared with FIG.

A description will be given focusing on the parts different from FIG.
When the lean operation is being performed (DML <1.0) in step 61, the routine proceeds to step 76, where the rich shift execution flag FRSFT = 1 and the AL during rich shift is set.
PHA peak value αRSFT is 100% (1.0)
And αSTP, which is the value of ALPHA at the time of interruption of the rich shift, is also set to 100%.

On the other hand, in step 64, LR at the time of rich shift
It is determined whether or not the PL addition flag FPLADD = 1, and if so, the HC supply degree RMHC is determined by the following equation in step 65.

RMHC = (αRSFT−αSTP) / αRSFT (8) This RMHC represents the degree of progress of the rich shift process, and is the target HC supply amount of the HC amount supplied until the enrichment process is interrupted. RMH
The larger C is, the larger the cumulative supply amount of HC is, and when the maximum value is 1, the required amount of HC is supplied.

In step 66, the NOx release amount RNOX is read out by a table lookup based on this RMHC, and the NOx adsorption amount ABS currently adsorbed on the catalyst.
TC is calculated as ABSTC = ABSTC-RNOX (step 67).

Then, the enrichment degree LRPL is read out by table lookup based on this ABSTC, and the ALPHA peak value αRSFT at the time of rich shift is obtained by the following equation.

ΑRSFT = LRPL + ALPEEK # (9) Note that ALPEEK # is a fixed value obtained from the experiment,
These relationships are set as shown in FIG. 14, for example. In this way, when αRSFT is calculated, FPLA
Reset DD to 0.

On the other hand, if the rich shift is started,
In step 72, the integral i = LRI # is set, and αS
Set TP = ALPHA and wait until one feedback cycle elapses (steps 73 and 74).

After one cycle, FRSFT =
0, ABSTC = 0, and the rich shift ends.

In this example, as the RMHC, the ratio of the actually supplied HC to the target supply amount of HC is obtained, and this is used as a substitute for the cumulative supply amount of HC, and the remaining amount is calculated based on this RMHC. Since the necessary degree of enrichment is calculated, in the case of FIG.
The burden of calculating the cumulative supply amount can be eliminated.

[0077]

According to the first aspect of the present invention, when the air-fuel ratio is switched from lean to stoichiometric, NOx adsorbed by the NOx adsorbing catalyst is desorbed, and the reduction treatment is performed by the three-way catalyst. In enriching the air-fuel ratio, the total amount of HC generated with the enrichment is accumulated.
Since the rich correction control is performed so as to correspond to the Ox adsorption amount, there is no excess or deficiency in the HC corresponding to the NOx adsorption amount.
The NOx adsorbed on the NOx adsorbing catalyst can be surely desorbed and reduced, so that the degree of enrichment is not increased unnecessarily and the rich period is not lengthened. Exhaust emissions can be improved while preventing deterioration of performance.

According to the second aspect of the invention, since the enrichment of the air-fuel ratio is basically limited to one cycle of the feedback control, the influence on the drivability at the time of shifting from lean to stoichiometric is shortened by that much. Therefore, it is possible to maintain good driving performance.

According to the third aspect of the present invention, when the required enrichment degree of the air-fuel ratio exceeds the predetermined value, the enrichment is performed in a plurality of times, so that drivability such as torque shock due to the extreme enrichment of the air-fuel ratio is achieved. You can avoid the deterioration.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a flow chart of lean operation control.

FIG. 3 is an explanatory diagram showing a lean operation region based on a relationship with a cooling water temperature.

FIG. 4 is an explanatory diagram showing a lean operation region based on a relationship between a rotation speed and a load.

FIG. 5 is an explanatory diagram showing a lean operation region based on a relationship with a vehicle speed.

FIG. 6 is a flowchart of calculation control of a fuel-air ratio correction coefficient.

FIG. 7 is an explanatory diagram showing how the fuel-air ratio changes.

FIG. 8 is a flowchart of calculation control of an NOx adsorption amount.

FIG. 9 is a flowchart of air-fuel ratio enrichment control.

FIG. 10 is an explanatory diagram showing the operation of air-fuel ratio rich control.

FIG. 11 is an explanatory diagram showing the movement of air-fuel ratio rich control at the time of fuel cut.

FIG. 12 is a flow chart of air-fuel ratio enrichment control according to another embodiment of the present invention.

FIG. 13 is a flowchart of air-fuel ratio enrichment control according to still another embodiment.

FIG. 14 is an explanatory diagram similarly showing the operation of rich control of the air-fuel ratio.

FIG. 15 is a configuration diagram of the present invention.

[Explanation of symbols]

 55 operating state detection means 56 NOx adsorption amount calculation means 57 enrichment degree setting means 58 HC generation amount calculation means 59 HC cumulative supply amount calculation means 60 air-fuel ratio rich correction control means

Claims (3)

[Claims] 1. A NOx adsorbing catalyst installed in an exhaust system for adsorbing NOx in a lean atmosphere and desorbing NOx adsorbed in a rich atmosphere, and a NOx reduction and HC, CO installed in an exhaust system downstream thereof.
A three-way catalyst that oxidizes the fuel, an air-fuel ratio switching device that switches the air-fuel ratio between lean and stoichiometric depending on the operating conditions, and a transient correction device for the air-fuel ratio that temporarily becomes rich when switching the air-fuel ratio from lean to stoichiometric. In an air-fuel ratio control device for an internal combustion engine, including: a means for detecting an operating state; a means for calculating the NOx adsorption amount adsorbed by the NOx adsorption catalyst during lean operation; and a means for calculating the air-fuel ratio according to the NOx adsorption amount. Means for setting the degree of enrichment, means for calculating the amount of HC generated according to the degree of enrichment, and calculation of the cumulative supply amount of HC from the operating state after the air-fuel ratio is switched from lean and the amount of HC generated And the cumulative supply amount of this HC is the N adsorbed on the NOx adsorption catalyst.
An air-fuel ratio control device for an internal combustion engine, comprising: means for performing rich-correction control of an air-fuel ratio so as to match an amount required to separate and reduce Ox. 2. An air-fuel ratio enrichment setting means subtracts a predetermined amount for each predetermined unit time from the enrichment degree set at the start of enrichment, and the cumulative supply amount in one feedback cycle of air-fuel ratio control is NOx. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control device is set so as to correspond to the adsorption amount. 3. When the air-fuel ratio enrichment setting means exceeds the predetermined value, the first enrichment control sets the enrichment degree within the predetermined value.
The rich correction control means uses NO when the cumulative supply amount of HC remains.
The air-fuel ratio control device for an internal combustion engine according to claim 2, wherein the enrichment control is repeated until it corresponds to the x adsorption amount.