JP3222685B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art When an internal combustion engine is in a predetermined operation state, an air-fuel ratio is controlled to a target value (for example, 22) on a fuel lean side (lean side) with respect to a stoichiometric air-fuel ratio (14.7) so that fuel consumption of the engine is improved. An air-fuel ratio control method for improving characteristics and the like is known. In such a lean air-fuel ratio control method, there is a problem that nitrogen oxides (NOx) cannot be sufficiently purified by the conventional three-way catalyst device.

In order to solve this problem, NOx exhausted from an engine in an oxygen-rich state (oxidizing atmosphere)
By utilizing the property of adsorbing NOx and reducing the adsorbed NOx in a hydrocarbon (HC) excess state (reducing atmosphere)
A so-called NOx catalyst that reduces the amount of Ox emission is known. With this NOx catalyst, NOx is used during lean air-fuel ratio control.
However, if lean combustion operation is performed continuously, the amount of catalyst adsorbed is limited, and when the amount of adsorption reaches a saturation amount, most of the NOx emitted from the engine is discharged to the atmosphere. become. Therefore, before the adsorption amount of the NOx catalyst reaches saturation, the air-fuel ratio must be switched to the rich air-fuel ratio control for controlling the stoichiometric air-fuel ratio or a value close to the stoichiometric air-fuel ratio, and the reduction of NOx must be started in the reducing atmosphere (rich state). However, the timing of switching from the lean combustion operation to the rich combustion operation becomes a problem.

As a timing for switching from the lean combustion operation to the rich combustion operation, a method of measuring the elapsed time from the start of the lean air-fuel ratio control and switching to the rich air-fuel ratio control when a predetermined time has elapsed is disclosed in Japanese Unexamined Patent Application Publication No. Hei. 5-1332
No. 60 is known. In this air-fuel ratio control method, the NO adsorbed on the catalyst by the rich air-fuel ratio control
When the reduction of x is completed, the control is returned to the lean air-fuel ratio control again,
The NOx amount is reduced by alternately repeating the lean combustion operation and the rich combustion operation.

[0005]

In the air-fuel ratio control method disclosed in the above-mentioned publication, it is necessary to forcibly switch to the rich combustion operation after a predetermined time has elapsed during the lean combustion operation. As fuel efficiency deteriorates, engine torque fluctuations (fluctuations similar to acceleration shocks) occur. Particularly, when the latter torque fluctuations occur frequently, there is a problem that the driving feeling deteriorates. Further, the amount of HC emission increases due to the enrichment of the air-fuel ratio, and it is not preferable to frequently switch the air-fuel ratio forcibly from the viewpoint of HC reduction, and the frequency of switching from the lean combustion operation to the rich combustion operation is not preferable. It is desirable to reduce

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. It is an object of the present invention to always maintain a nitrogen oxide (NOx) emission amount at a predetermined value or less and to perform a rich combustion operation. Accordingly, it is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine in which the fuel consumption is deteriorated due to the fuel consumption and the engine torque fluctuation is minimized.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, an exhaust purification catalyst device is provided in an exhaust passage of an internal combustion engine, and the catalyst device has a leaner air-fuel ratio than a stoichiometric air-fuel ratio. Nitrogen oxidation by adsorbing nitrogen oxides in exhaust gas during lean combustion operation at a low air-fuel ratio and reducing the adsorbed nitrogen oxides during rich combustion operation at a stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio An air-fuel ratio control device for an internal combustion engine for reducing the emission of substances, a nitrogen oxide emission estimating means for estimating an emission of nitrogen oxide emitted from the exhaust purification catalyst device, and a nitrogen oxide emission estimation When the nitrogen oxide emission amount estimated by the means exceeds a predetermined value, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is changed to switch from the lean combustion operation to the rich combustion operation. Characterized in that it comprises a means.

According to a second aspect of the present invention, in the nitrogen oxide emission estimating means, an engine emission estimating means for estimating an emission amount of nitrogen oxide discharged from the internal combustion engine to an exhaust passage, and the exhaust purification catalyst Adsorbing rate estimating means for estimating the adsorbing rate of nitrogen oxides to the apparatus, wherein the nitrogen oxide emission amount estimated by the engine emission amount estimating means and the nitrogen oxide estimated by the adsorbing rate estimating means are provided. And the amount of nitrogen oxides discharged from the exhaust purification catalyst device is estimated based on the adsorption rate of the exhaust gas.

According to a third aspect of the present invention, the nitrogen oxide emission estimation means further includes a purification rate estimation means for estimating a purification rate of nitrogen oxides by the exhaust purification catalyst device, and the engine emission estimation means. The exhaust purification catalyst device based on the nitrogen oxide emission amount estimated by the above, the nitrogen oxide adsorption rate estimated by the adsorption rate estimation means, and the nitrogen oxide purification rate estimated by the purification rate estimation means. It is characterized by estimating the emission amount of nitrogen oxides discharged from.

Further, a preferred embodiment according to claim 2 or 3
Then , the engine emission estimating means discharges the exhaust gas from the internal combustion engine to the exhaust passage based on the exhaust concentration of the nitrogen oxides exhausted from the internal combustion engine and the intake air amount information related to the amount of air taken in by the internal combustion engine. It is better to estimate the nitrogen oxide emissions. At this time, in the engine exhaust amount estimating means, the emission concentration of the nitrogen oxide exhausted from the internal combustion engine, it is to estimate based on the air-fuel ratio information on an air-fuel ratio of the mixture supplied to the internal combustion engine.

Further, in the engine exhaust amount estimating means, it is to correct the emission estimates of the nitrogen oxides discharged into the exhaust passage from the internal combustion engine depending on the ignition timing
Ku, in the engine exhaust amount estimating means, it is preferable to correct in accordance with the amount of exhaust gas refluxed emissions estimates engine nitrogen oxides emitted to the exhaust passage from the internal combustion engine.

Further, in a preferred embodiment of the present invention ,
In the adsorption rate estimating means, based on a total emission amount of nitrogen oxides discharged from the internal combustion engine to the exhaust purification catalyst device during a period from the start of the lean combustion operation to the switching to the rich combustion operation. It is better to estimate the nitrogen oxide adsorption rate.

[0013]

According to the air-fuel ratio control device of the first aspect, the amount of nitrogen oxides emitted from the exhaust purification catalyst device provided in the exhaust passage of the internal combustion engine is estimated by the emission amount estimating means. Is done. When the estimated value of the nitrogen oxide emission exceeds a predetermined value, the operating state is changed to the lean combustion operation with an air-fuel ratio leaner than the stoichiometric air-fuel ratio by the switching means for switching from the lean combustion operation to the rich combustion operation. Thus, the operation is switched to the stoichiometric air-fuel ratio or the rich combustion operation with an air-fuel ratio that is higher than the stoichiometric air-fuel ratio. Since the exhaust gas during the rich combustion operation contains a large amount of hydrocarbons, the hydrocarbons reduce the nitrogen oxides adsorbed on the exhaust purification catalyst device during the lean combustion operation, and the exhaust purification catalyst device Purification capacity will be restored, and the emission of nitrogen oxides will always be kept low.

According to the air-fuel ratio control device of the second aspect, the emission amount of nitrogen oxides discharged from the exhaust purification catalyst device is determined by the emission amount of nitrogen oxides estimated by the engine emission estimation means and the adsorption rate estimation. It can be determined well based on the nitrogen oxide adsorption rate estimated by the means. According to the air-fuel ratio control device of the third aspect, the emission amount of nitrogen oxides emitted from the exhaust purification catalyst device is estimated by the nitrogen oxide emission amount estimated by the engine emission amount estimation unit and the adsorption rate estimation unit. It can be satisfactorily obtained based on the obtained nitrogen oxide adsorption rate and the nitrogen oxide purification rate estimated by the purification rate estimating means.

[0015] According to a preferred embodiment of the air-fuel ratio control apparatus according to claim 2 or 3, the nitrogen oxides emissions are discharged into the exhaust passage from the internal combustion engine, emission of nitrogen oxides discharged from an internal combustion engine It is well determined based on the concentration and the intake air amount information related to the amount of air taken in by the internal combustion engine. At this time , the emission concentration of nitrogen oxides discharged from the internal combustion engine can be satisfactorily obtained based on air-fuel ratio information on the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine.

Further, emissions estimates of nitrogen oxides discharged into the exhaust passage from the internal combustion engine, emissions of nitrogen oxides is suitably corrected in accordance with the ignition timing, and is discharged from the inner combustion engine into the exhaust passage The estimated value is suitably corrected according to the amount of exhaust gas recirculated to the engine.

Further, in the air-fuel ratio control apparatus according to the second aspect,
According to a preferred aspect , the nitrogen oxide adsorption rate is determined by the total amount of nitrogen oxides discharged from the internal combustion engine to the exhaust purification catalyst device between the start of the lean combustion operation and the switching to the rich combustion operation. Satisfactorily based on

[0018]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 2 is a schematic configuration diagram showing an air-fuel ratio control device according to the present invention. In the figure, reference numeral 1 denotes an automobile engine, for example, an in-line 6-cylinder gasoline engine (hereinafter, referred to as an in-line engine).
This is simply referred to as an engine), and the combustion chamber, the intake system, the ignition system, and the like are designed to be capable of lean combustion. Engine 1
, An air cleaner 5, an intake port 4 having a fuel injection valve 3 attached to each cylinder.
An intake pipe 9 having an air flow sensor 6 for detecting an intake air amount Af, a throttle valve 7, an ISC valve 8, and the like is connected. As the air flow sensor 6, a Karman vortex air flow sensor or the like is suitably used. Further, an exhaust pipe 14 to which an air-fuel ratio sensor (such as a linear air-fuel ratio sensor) 12 for detecting an excess air ratio λ (air-fuel ratio information) is attached via an exhaust manifold 11 to the exhaust port 10.
The exhaust pipe 14 is connected to the exhaust purification catalyst device 1.
A muffler (not shown) is connected via 3.

The exhaust purification catalyst device 13 includes a NOx catalyst 13
a and a three-way catalyst 13b.
The x catalyst 13a is disposed upstream of the three-way catalyst 13b. The NOx catalyst 13a has a function of adsorbing NOx (nitrogen oxide) in an oxidizing atmosphere and reducing NOx to N ₂ (nitrogen) or the like in a reducing atmosphere in which HC (hydrocarbon) is present. As the NOx catalyst 13a, for example, a catalyst made of Pt and an alkali rare earth such as lanthanum or cerium is used. On the other hand, the three-way catalyst 13b has a function of oxidizing HC and CO (carbon monoxide) and reducing NOx. The reduction of NOx by the three-way catalyst 13b is based on the stoichiometric air-fuel ratio (14.
7) The maximum is promoted in the vicinity.

The engine 1 is provided with an ignition plug 16 for igniting a mixed gas of air and fuel supplied from the intake port 2 to the combustion chamber 15. Also, reference numeral 18
Is a crank angle sensor that detects a crank angle synchronizing signal θCR from an encoder linked to the camshaft, 19 is a throttle sensor that detects the opening degree θTH of the throttle valve 7, 20 is a water temperature sensor that detects a cooling water temperature TW,
Reference numeral 21 denotes an atmospheric pressure sensor that detects the atmospheric pressure Pa, and reference numeral 2 denotes
Reference numeral 2 denotes an intake air temperature sensor that detects the intake air temperature Ta. still,
The engine rotation speed Ne is calculated from the time interval of generation of the crank angle synchronization signal θCR detected by the crank angle sensor 18.

In the passenger compartment, an input / output device (not shown) and a storage device (ROM, RA
M, nonvolatile RAM, etc.), a central processing unit (CPU), an ECU (electronic control unit) 2 including a timer counter, etc.
3 are provided to perform comprehensive control of the air-fuel ratio control device including the engine 1. The various sensors described above are connected to the input side of the ECU 23, and detection information from these sensors is input. On the other hand, on the output side,
The above-described fuel injection valve 3, the ignition unit 24, and the like are connected, and output optimal values such as a fuel injection amount and an ignition timing calculated based on detection information from various sensors. The ignition unit 24 outputs a high voltage to the ignition plug 16 of each cylinder according to a command from the ECU 23.

Next, the operation of the air-fuel ratio control device configured as described above will be described with reference to FIG. 1 and FIGS. FIG. 1 is a block diagram showing a system configuration of the air-fuel ratio control device. The flowchart shown in FIG.
The air-fuel ratio control procedure executed by the CU 23 is executed every time a crank angle synchronizing signal θCR supplied from the crank angle sensor 18 is generated (for example, every crank angle 120 ° CA). In the air-fuel ratio control, when the NOx adsorption capacity of the NOx catalyst 13a becomes substantially saturated during the lean combustion operation in the oxidizing atmosphere, the mode is switched to the rich combustion operation, and the NOx catalyst 13a is placed in the reducing atmosphere for a predetermined time, N
The control for restoring the adsorption capacity of the Ox catalyst 13a is repeatedly performed. Here, the lean combustion operation is
The fuel injection amount from the fuel injection valve 3 is kept substantially constant, the throttle valve 7 and / or the ISC valve 8 are opened to increase the intake air amount, and the air-fuel ratio is larger than the stoichiometric air-fuel ratio (14.7). The operation of operating the engine 1 by burning the fuel-lean mixture having the specified value. On the other hand, the rich combustion operation refers to an operation of burning an air-fuel mixture having an air-fuel ratio of a stoichiometric air-fuel ratio (14.7) or a value lower than the stoichiometric air-fuel ratio. Contains more HC and CO than at the time. Therefore, the exhaust gas during the rich combustion operation is a reducing atmosphere, and NOx can be reduced. Here, the rich combustion operation includes an operation in which the air-fuel ratio is feedback-controlled to substantially the stoichiometric air-fuel ratio.

The ECU 23 first executes step S10 in FIG.
Then, it is determined whether or not the lean combustion operation condition is satisfied. The lean combustion operation conditions include, for example, that the engine is in a warm-up state, is operating in a predetermined operation range determined by the engine rotation speed Ne and the engine load, and is not in an operation state to accelerate or decelerate. is necessary.

If the decision result in the step S10 is No (negative)
If the lean combustion operation condition is not satisfied at step S12, the process proceeds to step S12 to execute the rich combustion operation control.
On the other hand, if the determination result of step S10 is affirmative (Yes), the process proceeds to step S14, and the calculation of the NOx emission amount is executed. In this step, the amount of NOx discharged from the exhaust purification catalyst device 13 is estimated based on the detection information of various sensors, and the subroutine shown in FIGS. 4 and 5 is executed. Hereinafter, the NOx emission amount calculation procedure will be described with reference to the flowchart of the subroutine.

Steps S20 to S26 in FIG. 4 are steps for obtaining the amount of NOx exhausted from the engine QNO.
First, in step S20, the ECU 23 functions as the engine exhausted NOx concentration estimating means 31 shown in the block diagram of FIG. 1 and is stored in the ECU 23, and the engine according to the excess air ratio λ is obtained from the map shown in the graph of FIG. Emission N
Read the Ox concentration value DN. Since this read value is not an actual measured value but a value read from a map set in a previous experiment, it is treated as an estimated value.

FIG. 6 shows the relationship between the excess air ratio λ (air-fuel ratio information) and the concentration DN of the engine exhaust NOx, and the value 1.0 of the excess air ratio λ on the horizontal axis is the stoichiometric air-fuel ratio (14.7). It corresponds to. When the excess air ratio λ is greater than 1.0,
It means that the air-fuel ratio is lean, and conversely, a range smaller than 1.0 means that the air-fuel ratio is rich. The excess air ratio λ may be an actual measurement value by the air-fuel ratio sensor 12 or may be a target value set according to the engine operating state. The fuel ratio or the equivalent ratio may be used.

In this map, the engine emission NOx
The concentration DN takes the maximum value when the value of the excess air ratio λ is slightly larger than 1.0, that is, when the air-fuel ratio becomes slightly leaner than the stoichiometric air-fuel ratio. The density DN changes with a substantially constant gradient with respect to the excess air ratio λ before and after the value of becomes the maximum value.

When the reading of the engine exhaust NOx concentration value DN is completed, step S22 is executed. Step S22 is a step of calculating various correction coefficient values. In this step, the ECU 23 functions as, for example, the retard correction means 32 of FIG.
The correction coefficient value KIg depending on the ignition timing or the like is read from a map as shown in the graph of FIG.

Normally, when the ignition timing changes to the retard side, the combustion slows down and the combustion temperature does not become too high, so that the amount of NOx emitted from the engine decreases. Therefore, NO
The x emission amount needs to be corrected by the ignition timing, and a correction coefficient KIg corresponding to the ignition timing is set, and the engine emission NO.
The x amount QNO is corrected to an appropriate value. In this map, the correction coefficient value KIg takes a reference value of 1.0 when the ignition timing is a predetermined value on the advance side, and decreases as the ignition timing changes to the retard side, so that the NOx generation amount becomes small. Is set to

The calculation of the correction coefficient in step S22 includes, in addition to the ignition timing, the EGR amount, the intake air temperature,
There are humidity and the like, and these correction coefficient values may be calculated as needed. In the next step S24, the ECU 23
Functions as the intake air amount calculating means 33 in FIG. 1, and detects the detected value Af from the air flow sensor 6 and the engine speed N
Based on e, the intake air amount Qa per cylinder, that is, from the time of the previous measurement (before the crank angle of 120 ° CA) to the time of the current measurement, is obtained. Here, the air flow sensor 6
Is affected by the atmospheric pressure and the intake air temperature, the detected value Af is corrected by the detection signals Pa and Ta from the atmospheric pressure sensor 21 and the intake air temperature sensor 22. Note that the intake air amount Qa can also be obtained from the engine rotation speed Ne and the intake pressure Pb, and the calculation method is not particularly limited.

Step S26 is a step for obtaining an engine exhaust NOx amount QNO for each detection of the crank angle synchronization signal θCR from the engine exhaust NOx concentration DN, the intake air amount Qa, the correction coefficient KIg, and the like obtained as described above. It is calculated by the following equation (1). In this case, the ECU 23
It functions as the engine exhaust NOx amount estimating means 30 of FIG.

QNO = k1.times.KIg.times.Qa.times.DN (1) Here, k1 is the other correction coefficient described above, and is set in the same manner as the correction coefficient KIg in accordance with, for example, the EGR amount and humidity. When the engine exhaust NOx amount QNO for each detection of the crank angle synchronization signal θCR is obtained as described above, the process proceeds to step S28. Step S28 is a step of determining whether or not a switch to the rich combustion operation to be described later has been performed and immediately after the rich combustion operation for a predetermined time t _R (for example, 3 seconds) has been performed. The determination result is Yes (Yes)
If it is immediately after the predetermined time t _R (3 seconds) has elapsed, the process proceeds to step S 32, but details will be described later. If the engine is in the lean combustion operation state, the determination result is N
o (No), the process proceeds to step S30.

In step S30, the engine exhaust NOx for each detection of the crank angle synchronizing signal θCR obtained in step S26.
The integrated value of the amount QNO, that is, the current accumulated value SQN (i +) of the engine exhaust NOx amount QNO passing through the exhaust purification catalyst device 13.
This is the step of calculating 1) from the following equation (2). {QNOdt} SQN (i + 1) = SQN (i) + QNO (2) Here, SQN (i) indicates the accumulated value when the routine was last executed.

Step S34 is the same as step S30 described above.
Based on the accumulated value SQN (i + 1) of the engine exhaust NOx amount obtained in the above, when the engine exhaust NOx passes through the exhaust purification catalyst device 13, the adsorption rate KNOX of the NOx adsorbed by the NOx catalyst 13a is obtained. . In this case, the ECU 2
3 functions as the adsorption rate estimating means 35 of FIG.
The storage unit 23 reads the adsorption rate knox from a map in which the accumulated value SQN (i + 1) and the adsorption rate knox have a relationship as shown in the graph of FIG.

In this map, the adsorption rate KNOX can be approximated by an exponential function as shown in the following equation (3).
When the cumulative value SQN (i + 1) is zero, the maximum value is 1.0, and as the cumulative value SQN (i + 1) increases, the predetermined value KN1
(For example, a value of 0.1). Knox {(1−KN1) × exp} (− k2) × SQN (i + 1)} + KN1 (3) where k2 is a correction coefficient (constant).

The above-mentioned estimated value of the adsorption rate KNOX may be obtained by calculation using this equation (3) without using a map. Also, the regression equation KNOX (i + 1) = 1- (1-KN1) × k2 × ｛(1-α) × SQN
(i) + α × QN (i)｝. Here, α is a constant. Step S3
In step 6, the ECU 23 functions as the purification rate estimating means 36 in FIG. 1 and reads the estimated value of the NOx purification rate KCAT by the three-way catalyst 13b of the exhaust purification catalyst device 13 from the map in FIG. As shown in the figure, the NOx purification rate KCAT by the three-way catalyst 13b becomes extremely large in a narrow range near the stoichiometric air-fuel ratio where the excess air ratio λ is 1.0, and reaches a maximum value at a λ value of 1.0. KC2 (for example, a value of 0.95). When the λ value deviates from a narrow range around 1.0 in the front and rear, the value of the purification rate KCAT rapidly decreases, and gradually approaches a predetermined value KC1 (for example, a value of 0 to 0.1). Normally, during the lean combustion operation, the excess air ratio λ often takes a value considerably larger than the value 1.0 (for example, the value 1.5). Therefore, the purification rate KCAT at this time is equal to the predetermined value KC1. You may consider it.

As described above, the NOx generated by the three-way catalyst 13b
Is effective only near the stoichiometric air-fuel ratio. From this, without using the map, a predetermined range (for example, 0.95) around the value 1.0 of the excess air ratio λ
≦ λ ≦ 1.05), the purification rate KCAT is 0.95
Alternatively, the value of the purification rate KCAT may be estimated as a value 0 outside a predetermined range around the value 1.0 (for example, λ <0.95, 1.05 <λ).

In step S38, the ECU 23 executes the process shown in FIG.
Functioning as the NOx emission rate calculating means 37, the NOx emission rate QNO based on the engine exhaust NOx amount QNO, the NOx adsorption rate KNOX of the NOx catalyst 13a, and the NOx purification rate KCAT of the three-way catalyst 13b. ｛(1-
KNOX) × (1-KCAT)} is obtained, and the value of the NOx emission rate is multiplied by the engine exhaust NOx amount QNO calculated by the engine exhaust NOx amount estimating means 30 shown in FIG. The catalyst exhaust NOx amount QNT for each detection of the angle synchronization signal θCR is calculated from the following equation (4). As a result, the amount of NOx emitted into the atmosphere is estimated to be substantially equal to the actually measured value.

QNT = QNO × {(1−KNOX) × (1−KCAT)} (4) This equation indicates that the purification rate KCAT of the three-way catalyst 13b is equal to the predetermined value KC1 as in the case of the lean combustion operation. In the maintained state, when the adsorption of NOx to the NOx catalyst 13a progresses and the NOx adsorption rate KNOX decreases, the overall NOx
This indicates that the purification capacity is reduced and the catalyst exhaust NOx amount QNT is increased.

As described above, the catalyst exhaust NOx amount QNT
After the estimation is completed, the process returns to the flowchart of FIG. 3 and executes step S16. In step S16, the ECU
23 functions as the comparator 38 of FIG. 1, and the estimated value QNT of the catalyst exhaust NOx amount obtained as described above is equal to the predetermined threshold Q
Determine if it is greater than NT0. The threshold value QNT0 is, for example,
It is set based on the NOx limit value defined by the law. When the determination result is No (No), it can be determined that the NOx amount discharged to the atmosphere is equal to or less than the allowable amount, and the process proceeds to Step S18 to perform the lean combustion operation control.

On the other hand, if the decision result in the step S16 is Yes.
If the result is (Affirmative) and the catalyst exhausted NOx amount QNT is larger than the threshold value QNT0, the adsorption capacity of the NOx catalyst 13a can be considered to be in a saturated state, and the process proceeds to step S12 described above, where rich combustion is performed as shown in FIG. Output the operation signal,
Perform rich combustion operation control. Thus, the NOx amount QNT
Is switched to the rich combustion operation at a time point when the pressure becomes larger than the threshold value QNT0, thereby increasing the amount of HC discharged from the engine 1 as compared with the lean combustion operation, causing HC and NOx to react with each other, and adsorbing the NOx catalyst 13a. NOx that has been reduced can be reduced and released into the atmosphere. This allows
The NOx catalyst 13a can again adsorb NOx.

According to the determination in step S16, step S1
2 is executed and the NOx adsorbed on the catalyst 13a starts to be reduced, and at the same time, the ECU 2
The timer counter of 3 starts counting time. After the rich combustion operation control is started, the routine is repeatedly executed, and when the determination result of step S28 in FIG. 5 is No (No), the predetermined time t _R that can be regarded as the completion of the NOx reduction yet. (For example, for 3 seconds), the process proceeds to step S30, and the engine discharge NOx
The cumulative value SQN (i + 1) of the quantity is continuously accumulated. At this time, since the catalyst exhaust NOx amount QNT does not become equal to or less than the threshold value QNT0, the determination result in step S16 of FIG. 3 maintains Yes (Yes), the rich combustion operation state is continued, and NOx
Will be sufficiently reduced.

After the start of the rich combustion operation control, a predetermined time t
_{When R} (3 seconds) elapses, the determination result of step S28 becomes Yes (Yes), and the process proceeds to step S32.
In step S32, since the predetermined time t _R (3 seconds) has elapsed, it is considered that NOx has been completely reduced from the NOx catalyst 13a, and a reset signal is output to the adsorption rate estimation means 35 in FIG. This is a step of resetting the accumulated value SQN (i + 1) of the NOx amount to zero.

As a result, the value of the NOx adsorption rate KNOX estimated in the next step S34 returns to 1.0, and in step S38, the estimated value of the catalyst exhausted NOx amount QNT calculated by the equation (4) Once becomes zero.
This routine is repeatedly executed, and the NOx adsorption rate KNOX
However, as shown in the map of FIG. 8, when the accumulated value SQN (i + 1) decreases again (step S34), the estimated value of the catalyst exhaust NOx amount QNT increases again. (Step S38).

FIG. 10 shows the case where the air-fuel ratio is controlled as described above,
Engine exhaust NOx that fluctuates by switching the operating state of engine 1 from lean combustion operation to rich combustion operation
FIG. 4 is a schematic diagram showing a temporal change of an amount QNO and a catalyst exhaust NOx amount QNT. In the figure, the engine exhaust NOx amount QNO shown by the dashed line and the catalyst exhaust NOx amount QNO shown by the solid line.
The difference from NT (shaded area) is the exhaust purification catalyst device 13
NOx adsorption amount or purification amount.

When the time t elapses and the NOx purification amount of the exhaust purification catalyst device 13 decreases, and the catalyst exhaust NOx amount QNT shown by the solid line reaches a predetermined value QNT0, the operating state becomes the lean combustion operation (A). To rich combustion operation, and the rich combustion operation is maintained for a predetermined time t _R (3 seconds). As a result, all the NOx adsorbed on the NOx catalyst 13a is reduced, and when the lean combustion operation (B) is started again after a lapse of a predetermined time t _R (3 seconds), the NOx adsorbing ability is restored. I have.

As described above, the switching from the lean combustion operation to the rich combustion operation is performed when the catalyst exhausted NOx amount QNT has reached the predetermined value QNT0. It can be kept below QNT0. Also, the duration t of a single lean burn operation, as in the duration t _L2 duration t _L1 and the lean-burn operation of the lean-burn operation shown in FIG. (A) (B), and that always matches Therefore, in the case of an operating condition in which the amount of NOx emission is small, the frequency of switching to the rich combustion operation can be reduced, and deterioration of fuel efficiency and torque fluctuation can be suppressed as much as possible. Further, the present invention can be applied to such an operation without any trouble even when the operation is performed such that the operation regions having different target air-fuel ratios change one after another.

In the above embodiment, in the rich combustion operation control for releasing the NOx adsorbed on the NOx catalyst 13a, the air-fuel ratio is set to substantially the stoichiometric air-fuel ratio.
It is not always necessary to control the stoichiometric air-fuel ratio, and the NOx catalyst 13a may be set to a reducing atmosphere in the presence of HC, and the air-fuel ratio may be set to a value richer than the stoichiometric air-fuel ratio. .

In the above-described embodiment, the switching between the lean combustion operation and the rich combustion operation is determined by the catalyst exhaust NOx amount QNT.
And the threshold value QNT0 by comparing the NOx amount (g / sec). However, the present invention is not limited to this.
(PPM) may be used. Further, in the above embodiment, the estimated values obtained from various maps are used, but actual measured values may be used instead of these estimated values.

[0050]

As is apparent from the above description, according to the air-fuel ratio control device of the first aspect of the present invention, an exhaust purification catalyst device is disposed in an exhaust passage of an internal combustion engine, and this catalyst device has:
Nitrogen oxides in exhaust gas are adsorbed during lean combustion operation with an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and the adsorbed nitrogen oxides are reduced during rich combustion operation with a stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio In the air-fuel ratio control device for an internal combustion engine, which reduces the amount of nitrogen oxides discharged, a means for estimating the amount of nitrogen oxides discharged from the exhaust purification catalyst device is provided. Switching means for changing from the lean combustion operation to the rich combustion operation by changing the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine when the nitrogen oxide emission amount estimated by the material emission amount estimation means exceeds a predetermined value. As a result, the emission amount of nitrogen oxides can always be maintained at a predetermined value or less, and unless the predetermined value is reached, the air-fuel ratio is not switched to the rich side. Because it can so, it is possible to suppress the deterioration of fuel consumption by the rich burn operation, the torque fluctuation of the engine to a minimum.

Further, according to the air-fuel ratio control device of the second aspect, the nitrogen oxide emission estimated by the engine emission estimation means and the nitrogen oxide adsorption rate estimated by the adsorption rate estimation means are determined. By estimating the emission amount of nitrogen oxides discharged from the exhaust purification catalyst device in this way, the emission amount of nitrogen oxides from the exhaust purification catalyst device can be satisfactorily and easily obtained.

According to the third aspect of the present invention, the nitrogen oxide emission estimated by the engine emission estimation means and the nitrogen oxide adsorption rate and purification rate estimated by the adsorption rate estimation means are determined. By estimating the amount of nitrogen oxides discharged from the exhaust purification catalyst device based on the nitrogen oxide purification rate estimated by the estimation means, the amount of nitrogen oxide emissions from the exhaust gas purification catalyst device can be more accurately determined. Can be sought.

According to a preferred embodiment of the air-fuel ratio control device according to the second or third aspect , the intake air amount information relating to the exhaust concentration of nitrogen oxides discharged from the internal combustion engine and the amount of air taken in by the internal combustion engine. It is preferable to estimate the amount of nitrogen oxide discharged from the internal combustion engine to the exhaust passage based on the above , so that the amount of nitrogen oxide discharged from the internal combustion engine can be satisfactorily and easily obtained.

At this time , it is preferable to estimate the concentration of nitrogen oxides discharged from the internal combustion engine based on air-fuel ratio information on the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine .
Thus, the nitrogen oxide emission concentration emitted from the internal combustion engine can be satisfactorily and easily determined. Also ,
Selfishness emissions estimates of the nitrogen oxide exhausted from the inner combustion engine into the exhaust passage is adjusted in response to the ignition timing,
As a result, the amount of nitrogen oxide emissions from the internal combustion engine can be determined more accurately.

[0055] In addition, the emissions estimates of the nitrogen oxide exhausted from the inner combustion engine into the exhaust passage is corrected in accordance with the amount of exhaust gas recirculated to the engine C., thereby <br/> internal combustion Nitrogen oxide emissions from the engine can be determined more accurately. Furthermore, according to a preferred embodiment of the air-fuel ratio control apparatus according to claim 2, nitrogen oxides discharged from an internal combustion engine to the exhaust purifying catalyst device during a period from the start of the lean burn operation until switched to rich burn operation It is better to estimate the nitrogen oxide adsorption rate based on the total emissions of
Thereby, the nitrogen oxide adsorption rate can be determined well and easily.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a system corresponding to a function of an air-fuel ratio control device to which an embodiment of the present invention is applied.

FIG. 2 is a schematic configuration diagram of an air-fuel ratio control device to which an embodiment of the present invention is applied.

FIG. 3 is a flowchart of an air-fuel ratio control routine executed by an electronic control unit (ECU) of FIG. 2;

FIG. 4 is a part of a flowchart of a NOx emission amount QNT calculation subroutine shown in FIG. 3;

FIG. 5 is a NOx emission amount Q following the flowchart of FIG. 4;
It is the remainder of the flowchart of the NT calculation subroutine.

FIG. 6 is a graph showing an example of a relationship between an excess air ratio λ and an engine exhaust NOx concentration DN set according to the excess air ratio λ.

FIG. 7 is a graph showing an example of a relationship between an ignition timing and a correction coefficient KIg set according to the ignition timing.

FIG. 8 shows a cumulative value SQN (i + 1) of the engine exhaust NOx amount,
5 is a graph showing an example of the relationship between the NOx catalyst adsorption rates KNOX set accordingly.

FIG. 9 is a graph showing an example of the relationship between the excess air ratio λ and the NOx purification rate KCAT of the three-way catalyst set according to the excess air ratio λ.

FIG. 10 shows engine exhaust NOx amount QNO and catalyst exhaust NOx
It is a graph which shows an example of a temporal change of quantity QNT.

[Explanation of symbols]

 Reference Signs List 1 engine 3 fuel injection valve 6 air flow sensor 12 air-fuel ratio sensor 13 exhaust purification catalyst device 13a NOx catalyst 13b three-way catalyst 16 spark plug 18 crank angle sensor 23 electronic control unit (ECU) 30 engine exhaust NOx amount estimating means 31 engine exhaust NOx Concentration estimating means 32 retard angle correcting means 33 intake air amount calculating means 35 adsorption rate estimating means 36 purification rate estimating means 37 NOx emission rate calculating means 38 comparator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI F02D 43/00 301 F02D 43/00 301E 301N 45/00 301 45/00 301G 366 366F F02P 5/15 F02P 5/15 Z (56 References JP-A-6-66129 (JP, A) JP-A-6-66185 (JP, A) JP-A-6-50139 (JP, A) JP-A-6-66135 (JP, A) International publication 93 / 25806 (WO, A1)

Claims (3)

(57) [Claims] An exhaust purification catalyst device is provided in an exhaust passage of an internal combustion engine, and the catalyst device adsorbs nitrogen oxides in exhaust gas during a lean combustion operation at an air-fuel ratio leaner than a stoichiometric air-fuel ratio. The exhaust gas purification catalyst according to the air-fuel ratio control device for an internal combustion engine, wherein the adsorbed nitrogen oxide is reduced during a rich combustion operation at a stoichiometric air-fuel ratio or at an air-fuel ratio richer than the stoichiometric air-fuel ratio to reduce the amount of nitrogen oxide emission. A nitrogen oxide emission amount estimating means for estimating the emission amount of nitrogen oxides emitted from the device; and an internal combustion engine when the nitrogen oxide emission amount estimated by the nitrogen oxide emission amount estimation means exceeds a predetermined value. Switching means for changing from the lean combustion operation to the rich combustion operation by changing the air-fuel ratio of the air-fuel mixture supplied to the engine. 2. The exhaust gas estimating means for estimating the amount of nitrogen oxide discharged from the internal combustion engine into an exhaust passage, and the nitrogen oxide emission to the exhaust purification catalyst device. Adsorbing rate estimating means for estimating the adsorbing rate of the substance, wherein the nitrogen oxide emission amount estimated by the engine emission amount estimating means and the nitrogen oxide adsorbing rate estimated by the adsorbing rate estimating means are calculated. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the amount of nitrogen oxide discharged from the exhaust purification catalyst device is estimated based on the exhaust gas. 3. The nitrogen oxide emission estimating means further includes a purification rate estimating means for estimating a purification rate of nitrogen oxides by the exhaust gas purifying catalyst device, and the nitrogen oxide estimated by the engine emission estimating means. Nitrogen discharged from the exhaust purification catalyst device based on the amount of oxide emission, the nitrogen oxide adsorption rate estimated by the adsorption rate estimation means, and the nitrogen oxide purification rate estimated by the purification rate estimation means. 3. The air-fuel ratio control device for an internal combustion engine according to claim 2, wherein the amount of oxide emission is estimated.