JP3509502B2 - Exhaust gas purification 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 exhaust gas purification control device for an internal combustion engine which performs lean air-fuel ratio operation leaner than the stoichiometric air-fuel ratio under required operating conditions.

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for an internal combustion engine operated at a lean air-fuel ratio, a NOx absorption / storage catalyst is arranged in an exhaust passage, and the NOx absorption / storage catalyst is used to operate at a lean air-fuel ratio. While absorbing the NOx discharged from the engine, after absorbing the NOx, it is operated at a rich air-fuel ratio to release the NOx absorbed by the NOx absorption / storage catalyst, and HC and CO contained in the exhaust gas. There is known a technique for purifying (reducing) by using.

Since this NOx absorption / storage catalyst has a fixed amount of NOx that can be absorbed, the NOx absorption amount is NOx.
It is necessary not to exceed the saturation amount of the absorption / storage catalyst.

Therefore, basically, the integrated value obtained by integrating the NOx emission amount of the engine estimated from the operating state and the total NOx emission amount estimated from the duration of lean air-fuel ratio operation are the NOx absorption / storage catalysts. When the saturation amount approaches, the air-fuel ratio is enriched in a spike manner, and the absorbed NOx is released and purified (JP-A-7-103033).
No.

Further, since the air-fuel ratio is generally made rich at the time of sudden acceleration where a high torque of the engine is required, the air-fuel ratio at this time is controlled to be richer,
There is one that improves acceleration feeling while releasing and purifying NOx (JP-A-6-58185).

[0006]

However, in such a conventional technique, attention is paid only to the NOx absorption amount, and the NOx absorption efficiency of the NOx absorption / storage catalyst, that is, the NOx absorption / storage catalyst is No consideration was given to the amount of NOx that can be absorbed per unit time.

When a large amount of NOx flows into the NOx absorption / storage catalyst in a short time, the NOx absorption reaction of the catalyst does not catch up and some of the NOx is absorbed even if the NOx absorption amount has not reached saturation. -It may pass through the occlusion type catalyst and flow out to the downstream side of the catalyst.

It has been found that the absorption efficiency of this NOx absorption / storage catalyst is affected by the amount of NOx absorbed already. Specifically, the absorption efficiency is high when the NOx absorption amount is small, and decreases as the NOx absorption amount approaches saturation.

Therefore, during lean air-fuel ratio operation, when the NOx concentration in the exhaust gas is low and the gas flow rate is low, such as during steady operation, even if the absorption efficiency of the NOx absorption / occlusion catalyst is somewhat reduced. , NOx can be absorbed without omission until the NOx absorption flow rate reaches saturation, but the absorption efficiency of the NOx absorption / storage catalyst decreases during acceleration operation in which the NOx concentration in the exhaust gas is high and the gas flow rate is high. In this state, even before the NOx absorption amount reaches saturation, the NOx flowing into the catalyst cannot be completely absorbed, and the amount of NOx passing through the catalyst increases.

In order to improve fuel economy, it is necessary to carry out lean air-fuel ratio operation under a wide range of operating conditions.
Therefore, during normal acceleration operation, operation is often performed with a lean air-fuel ratio, and in the case where rich enrichment is performed only during predetermined rapid acceleration as in JP-A-6-58185, the above problem may occur. Is big. In addition, JP-A-7-1
The problem described above is inevitable even in the case where the enrichment is performed when NOx approaches the saturation amount, such as 03033.

The present invention is a NOx absorption / storage catalyst N
An object of the present invention is to provide an exhaust gas purification control device capable of appropriately absorbing and purifying NOx by performing rich control under the conditions of the Ox absorption amount and the NOx emission amount of the engine based on the acceleration state.

[0012]

According to a first aspect of the present invention, lean air-fuel ratio operation leaner than the stoichiometric air-fuel ratio is performed under required operating conditions, while NOx is absorbed and reduced in an oxidizing atmosphere in an exhaust passage. N that releases and purifies NOx absorbed in the atmosphere
In an internal combustion engine equipped with an Ox absorption / storage catalyst, N
An NOx absorption amount calculation means for calculating the NOx absorption amount absorbed by the Ox absorption / storage catalyst, a judgment value setting means for setting a judgment value according to the NOx absorption amount calculated by the NOx absorption amount calculation means, and an engine. When the acceleration state detection means for detecting the acceleration state of the, and the acceleration state detected by the acceleration state detection means exceeds the judgment value set by the judgment value setting means, the air-fuel ratio of the air-fuel mixture supplied to the engine, And an air-fuel ratio switching means for switching to a predetermined rich air-fuel ratio in which the exhaust gas becomes a reducing atmosphere .
Depending on the catalyst temperature detection means that detects the temperature and the catalyst temperature
And a correction unit that corrects the determination value .

In a second aspect based on the first aspect, the determination value setting means sets the determination value smaller as the NOx absorption amount of the NOx absorption / storage catalyst increases.

[0014]

The third invention is based on the first and second inventions.
Te has a catalyst deterioration detecting means for detecting a deterioration degree of the NOx absorption-storage catalyst, and a correction means for correcting small the determination value as the catalyst deterioration degree is large.

[0016]

According to the first and second aspects of the invention, the acceleration state is such that the current NOx absorption / storage catalyst NOx absorption efficiency cannot sufficiently absorb the NOx flowing in with the acceleration. In the acceleration state, the air-fuel ratio of the air-fuel mixture supplied to the engine is switched to a predetermined rich air-fuel ratio in which the exhaust gas becomes the reducing atmosphere. Therefore, NOx absorbed by the NOx absorption / storage catalyst is released and purified, and at the same time, the NOx absorption efficiency of the NOx absorption / storage catalyst is restored to the maximum to reduce the NOx spillage as much as possible. You can

When the acceleration state is such that the current NOx absorption / storage catalyst NOx absorption efficiency remains the same, but the inflowing NOx can be sufficiently absorbed during the acceleration, the rich air-fuel ratio can be increased. Since no switching is performed, lean driving can be performed in a wide range, and drivability and fuel efficiency can be improved.

[0018] Then, with respect to the change in the NOx absorption efficiency depending on the temperature of the NOx absorption / storage catalyst, the NOx absorption,
The release and purification can be performed accurately.

According to the third aspect of the present invention, even if the NOx absorption / storage catalyst is deteriorated, it is possible to properly absorb, release, and purify NOx.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.
However, the underlying technology will be explained first.

In FIG. 1, 10 is an engine main body, and 11
Indicates an intake passage (intake pipe), 12 indicates an exhaust passage (exhaust pipe), and the engine body 10 is provided with a fuel injection valve 13 for injecting fuel directly into the combustion chamber.

From the fuel injection valve 13, fuel is injected in the latter half of the compression stroke, such as in the low and medium load regions of the engine, and a combustible mixture layer is formed only near the spark plug 14 at the compression top dead center. Stratified combustion of an air-fuel mixture with an ultra-lean air-fuel ratio exceeding A / F = 40 is performed, fuel is injected in the intake stroke in the engine high load range, etc., and fuel and air are premixed in the entire combustion chamber, It is designed to perform homogeneous combustion in a mixture near the air-fuel ratio.

A NOx absorption / storage catalyst 15 is arranged in the exhaust passage 12. The NOx absorption / occlusion catalyst 15 is composed of a carrier such as alumina, a noble metal such as platinum Pt, potassium K, sodium Na, lithium Li, and cesium Cs.
Alkali metals such as, barium Ba, calcium Ca
Alkaline earth such as, lanthanum La, yttrium Y
And at least one of the rare earths such as

As this NOx absorption / storage catalyst 15,
Taking platinum Pt and barium Ba as an example,
NOx exhausted from the engine during operation at a lean air-fuel ratio (exhaust gas in an oxidizing atmosphere) is combined with oxygen O ₂ on platinum Pt and is absorbed by barium Ba in the form of barium nitrate. In this state, when operating at a rich air-fuel ratio higher than the theoretical air-fuel ratio (exhaust gas is a reducing atmosphere), the absorbed NO
x is released and reduced by the action of HC and CO contained in the exhaust gas.

NOx absorption / storage catalyst 1 in the exhaust passage 12
The first and second air-fuel ratio sensors 16 and 17, which generate outputs proportional to the oxygen concentration in the exhaust gas, are provided on the upstream side and the downstream side of the NOx 5 on the NOx absorption / storage catalyst 15, respectively.
A catalyst temperature sensor 18 for detecting the temperature of the x absorption / storage catalyst 15 is installed, and these outputs are input to the control unit 20.

As means for detecting the operating conditions of the engine, a rotational speed sensor (crank angle sensor) 21 for detecting the rotational speed of the engine and a crank angle, and an intake air amount (load) of the engine.
An intake sensor 22 for detecting the temperature, a throttle opening sensor 23 for detecting the throttle opening, a water temperature sensor 24 for detecting the cooling water temperature of the engine, etc. are provided, and these signals are also input to the control unit 20.

Based on these sensor outputs and signals, the control unit 20 causes the fuel injection amount of the fuel injection valve 13 to perform lean operation (stratified combustion operation) and stoichiometric operation (homogeneous combustion operation) according to operating conditions. The injection timing is controlled, and during lean operation, the NOx emission amount of the engine based on the acceleration state, the NOx absorption / storage catalyst 1
On the condition of the NOx absorption amount of 5 or the like, a predetermined enrichment control is performed so that the exhaust gas temporarily becomes a reducing atmosphere, that is, a rich spike control that temporarily enriches the fuel injection amount from the fuel injection valve 13 is performed. .

Next, the rich spike control will be described with reference to the flowcharts of FIGS. It should be noted that these flows are executed at a predetermined control cycle.

In the main routine of FIG. 2, in step 1, the engine speed, crank angle, intake air amount of the engine,
Read throttle opening TVO, engine coolant temperature, etc.
At this time, the displacement ΔTV of the throttle opening per unit time
Calculate O.

In step 2, the fuel injection valve 13 is controlled based on these signals.

In this case, the fuel injection amount and the injection timing of the fuel injection valve 13 are set so that the lean air-fuel ratio lean operation is performed in the engine low / medium load region and the stoichiometric air-fuel ratio stoichiometric operation is performed in the engine high load region. To control.

On the other hand, when a command for permitting rich spikes (described later) is issued during acceleration, the fuel of the fuel injection valve 13 is controlled so that a rich spike with a predetermined rich spike amount R1 and rich spike time T1 is performed at the beginning of the acceleration. Injection quantity,
Control the injection timing.

The rich spike amount R1 and the rich spike time T1 are set to such values that the maximum amount of NOx that can be absorbed by the NOx absorption / storage catalyst 15 can be released and purified.

FIG. 3 is a routine for setting a threshold value (determination value) TVO ₁ for permitting rich spike.

As shown in FIG. 3, in step 11, the elapsed time (lean operating time) from the time when the previous rich spike is performed is counted, and in step 12, the NOx absorption / storage catalyst is calculated based on the elapsed time. The NOx absorption amount of 15 is calculated.

In this case, it has been confirmed that the NOx absorption amount of the NOx absorption / storage catalyst 15 increases with the characteristic as shown in FIG. 5 with respect to the elapsed time.
The NOx absorption amount of the x absorption / storage catalyst 15 is obtained.

The NOx emission amount of the engine is estimated and integrated from the operating state, and NOx of the NOx absorption / storage catalyst 15 is calculated.
It is also possible to obtain the amount of absorption.

Then, in step 13, NOx absorption /
The threshold value TVO ₁ based on the NOx absorption amount of the storage catalyst 15
To calculate.

In this case, when the NOx absorption amount of the NOx absorption / storage catalyst 15 is small as shown in FIG. 6, the threshold value TVO is set.
₁ is increased and the threshold TVO ₁ is decreased as the NOx absorption amount of the NOx absorption / storage catalyst 15 increases.

FIG. 4 shows a rich spike determination routine for deciding whether or not to perform a rich spike during acceleration.

As shown in FIG. 4, in step 21, the operating state is taken in. In this case, the data of the displacement ΔTVO of the throttle opening and the injection timing of the fuel injection valve 13 calculated in the main routine are acquired.

In step 22, it is judged from the injection timing of the fuel injection valve 13 whether the lean operation or the stoichiometric operation is in progress.

During stoichiometric operation, the routine proceeds to step 23, where rich spike is not performed.

If the vehicle is in lean operation, step 24
Then, the displacement ΔTVO of the throttle opening is compared with the threshold TVO ₁ calculated in the threshold setting routine of FIG.

The displacement ΔTVO of the throttle opening is the threshold TV.
When it is O ₁ or less, the routine proceeds to step 25, where the rich spike is not performed.

When the displacement ΔTVO of the throttle opening is larger than the threshold value TVO _{1, the} routine proceeds to step 26,
Issue a rich spike permit.

With such a structure, when normal acceleration is performed during lean operation, the amount of NOx discharged from the engine increases in accordance with the magnitude of the acceleration and flows into the NOx absorption / storage catalyst 15. The amount of NOx increases, but if the displacement ΔTVO of the throttle opening exceeds the threshold value TVO ₁ at this time, a rich spike that temporarily enriches the fuel injection amount from the fuel injection valve 13 is generated at the initial stage of the acceleration. Done.

The threshold value TVO ₁ is set to a large value when the NOx absorption amount is small and a small value when the NOx absorption amount is large, based on the NOx absorption amount of the NOx absorption / storage catalyst 15. Therefore, when the NOx absorption / storage catalyst 15 has a small amount of NOx absorption and the absorption efficiency is high, and the acceleration is large, the NOx absorption / storage catalyst 15 is large.
When the NOx absorption amount is large and the absorption efficiency is low, the rich spike is performed since the acceleration is small.

That is, when it is determined that the acceleration state is such that the current NOx absorption / storage catalyst 15 NOx absorption efficiency cannot sufficiently absorb the inflowing NOx due to the acceleration ( When the inflowing NOx amount is relatively large with respect to the NOx absorption efficiency), the rich spike is performed in the initial stage of acceleration as shown in FIG.

As a result, the NOx absorption / storage catalyst 15 is formed.
At the same time that the NOx absorbed in is released and purified,
The NOx absorption / storage catalyst 15 is restored to the maximum NOx absorption efficiency to prepare for the subsequent inflow of NOx. Therefore, the amount of NOx missed can be reduced as much as possible.

On the other hand, when it is determined that the acceleration state is such that the current NOx absorption / storage catalyst 15 has the same NOx absorption efficiency as it is, but is capable of sufficiently absorbing the inflowing NOx. The rich spike is not performed (when the inflowing NOx amount is relatively small with respect to the NOx absorption efficiency).

Therefore, the frequency of enrichment can be sufficiently reduced.

As a result, it is possible to sufficiently reduce NOx while maintaining a wide range of lean operation, maintain good drivability, and improve fuel efficiency.

The rich spike amount R1 and the rich spike time T1 are determined by the NOx absorption / storage catalyst 15 NOx.
It is advisable to calculate and set R1 and T1 that can supply HC and the like necessary and sufficient for NOx release and purification, depending on the amount of absorption.

[0055] Figure 8, Figure 9 shows the implementation of the embodiment of the present invention, the threshold TVO ₁ NOx absorption-storage catalyst 15
Is corrected according to the temperature of NOx and the degree of deterioration of the NOx absorption / storage catalyst 15.

In this case, in FIG. 8, the threshold value TVO ₁ is calculated on the basis of the NOx absorption amount of the NOx absorption / storage catalyst 15 as in the above-mentioned base technology (steps 41 to 43).
At the same time, in steps 44 and 45, the correction value TVO ₂ is calculated from the temperature of the NOx absorption / storage catalyst 15 and the correction value TVO ₃ is calculated from the deterioration degree of the NOx absorption / storage catalyst 15.

The correction value TVO _{2 with} respect to the temperature of the NOx absorption / storage catalyst 15 is N of the NOx absorption / storage catalyst 15.
There is a temperature at which the absorption efficiency of Ox changes depending on the temperature and the absorption efficiency becomes the best, and the absorption efficiency decreases as the distance from the temperature increases. Therefore, based on the characteristics, the correction value is set from the data set as shown in FIG. Ask for TVO ₂ . That is, the correction value TVO ₂ is increased when the NOx absorption efficiency is high, and the correction value TVO ₂ is decreased when the NOx absorption efficiency is low.

The temperature of the NOx absorption / storage catalyst 15 is detected by the catalyst temperature sensor 18. However, if the catalyst temperature sensor 18 is not provided, it may be estimated from the operating condition.

The degree of deterioration of the NOx absorption / storage catalyst 15 is
It can be estimated from the total operating time. Further, as is known, the degree of deterioration of the NOx absorption / storage catalyst 15 can be estimated based on the output states of the air-fuel ratio sensors 16 and 17 before and after the NOx absorption / storage catalyst 15. NOx absorption
Since the NOx absorption efficiency decreases as the degree of deterioration of the storage catalyst 15 increases, the correction value TVO _{3 is set} to be smaller when the estimated degree of deterioration is larger as shown in FIG. 11, and the estimated degree of deterioration is smaller. , Increase the correction value TVO ₃ .

Then, in step 46, step 43
The threshold value TVO ₁ calculated in step ₂ is multiplied by the correction value TVO ₂ for the temperature of the NOx absorption / storage catalyst 15 and the correction value TVO ₃ for the deterioration degree of the NOx absorption / storage catalyst 15 to obtain the threshold TVO ₁ .

In FIG. 9, similar to the above-described base technology , data on the operating state, the throttle opening displacement ΔTVO, and the injection timing of the fuel injection valve 13 are fetched (steps 51 and 5).
With 2), while in lean operation, step 54
Then, the displacement ΔTVO of the throttle opening is compared with the threshold TVO ₁ obtained in the threshold setting routine of FIG.

When the displacement ΔTVO of the throttle opening is larger than the threshold value TVO _{1, the} routine proceeds to step 56,
Issue a rich spike permit.

In this way, when the NOx absorption / storage catalyst 15 has a high NOx absorption efficiency at a temperature, the NOx absorption / storage catalyst 15 absorbs NOx.
When the storage catalyst 15 is not deteriorated, it is possible to sufficiently absorb the NOx that flows in with the acceleration and reduce enrichment.

Further, the NO of the NOx absorption / storage catalyst 15 is
NOx absorption / storage catalyst 1 at a temperature at which x absorption efficiency is low
When 5 is deteriorated, the NOx absorbed and stored in the NOx absorption / storage catalyst 15 can be appropriately released and purified by the enrichment.

Therefore, NOx can be absorbed, released and purified more accurately.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an apparatus .

FIG. 2 is a flowchart showing control contents.

FIG. 3 is a flowchart showing control contents.

FIG. 4 is a flowchart showing control contents.

FIG. 5 is a characteristic diagram showing a relationship between elapsed time and NOx absorption amount.

FIG. 6 is a characteristic diagram showing a relationship between a NOx absorption amount and a threshold value.

FIG. 7 is a timing chart of rich spike control.

8 is a flow chart showing the control contents of the implementation forms.

9 is a flow chart showing the control contents of the implementation forms.

FIG. 10 is a flowchart showing the relationship between catalyst temperature and correction value.

FIG. 11 is a flowchart showing the relationship between catalyst deterioration degree and correction value.

[Explanation of symbols]

10 engine body 11 Intake pipe 12 Exhaust pipe 13 Fuel injection valve 14 Spark plug 15 NOx absorption / storage catalyst 16,17 Air-fuel ratio sensor 18 Catalyst temperature sensor 20 control unit 21 Rotation speed sensor (crank angle sensor) 22 Intake sensor 23 Throttle opening sensor 24 Cooling water temperature sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI F02D 41/04 305 F02D 41/04 305A 45/00 312 45/00 312E (56) Reference JP-A-7-103016 (JP, A) JP-A-8-200049 (JP, A) JP-A-6-272540 (JP, A) JP-A-6-280550 (JP, A) JP-A-9-112308 (JP, A) JP-A-3 -253744 (JP, A) JP-A-6-58185 (JP, A) JP-A-7-1003033 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/10 305 F01N 3/08 F01N 3/20 F02D 41/04 305 F02D 45/00 312

Claims (3)

(57) [Claims] 1. A lean air-fuel ratio operation that is leaner than the stoichiometric air-fuel ratio is performed under required operating conditions, while NOx absorbed in an oxidizing atmosphere and NO absorbed in a reducing atmosphere in the exhaust passage.
In an internal combustion engine having a NOx absorption / storage catalyst for releasing and purifying x, NOx absorption calculation means for calculating the NOx absorption quantity absorbed by the NOx absorption / storage catalyst, and calculation by the NOx absorption quantity calculation means The determination value setting means for setting the determination value according to the NOx absorption amount, the acceleration state detecting means for detecting the acceleration state of the engine, and the acceleration state detected by the acceleration state detecting means are set by the determination value setting means. And an air-fuel ratio switching means for switching the air-fuel ratio of the air-fuel mixture supplied to the engine to a predetermined rich air-fuel ratio in which the exhaust gas becomes a reducing atmosphere when the NOx absorption / storage catalyst is exceeded. Temperature detection to detect the temperature of
Means and a correction means for correcting the judgment value according to the catalyst temperature
An exhaust gas purification control device for an internal combustion engine, which has a step . 2. The exhaust gas purification control device for an internal combustion engine according to claim 1, wherein the determination value setting means sets the determination value smaller as the NOx absorption amount of the NOx absorption / storage catalyst increases. 3. A degree of deterioration of a NOx absorption / storage catalyst is detected.
The catalyst deterioration detecting means for
The exhaust gas purification control device for an internal combustion engine according to claim 1 or 2, further comprising a correction unit that corrects the determination value to be small .