JP3509482B2 - Exhaust gas purification 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 purifying apparatus for an internal combustion engine, and more particularly to a cylinder injection type internal combustion engine provided with a fuel injection valve for directly injecting fuel into a combustion chamber,
The present invention relates to a technique for reducing NOx absorbed by a NOx absorption catalyst.

[0002]

2. Description of the Related Art Conventionally, when the exhaust air-fuel ratio is lean, it absorbs NOx in the exhaust gas, and when the exhaust air-fuel ratio is stoichiometric or rich, the absorbed NOx is absorbed.
A cylinder injection type internal combustion engine equipped with a NOx absorption catalyst (NOx absorption type three-way catalyst) that releases x and performs reduction processing is known (see Japanese Patent Laid-Open No. 9-32619).

In the method disclosed in Japanese Patent Laid-Open No. 9-32619, when sulfur adheres to the NOx absorption catalyst, the purifying performance decreases, so that when the adhered amount of sulfur reaches a predetermined amount, While the fuel injection is being performed, the fuel is injected in the exhaust stroke, and the fuel injected in the exhaust stroke is burned in the exhaust passage and the NOx absorption catalyst to remove the sulfur.

[0004]

In the exhaust stroke injection, however, there is a problem that even if the sulfur adhering to the NOx absorption catalyst can be removed, the NOx reduction process cannot be performed. That is, the fuel injection in the exhaust stroke is performed during lean combustion, and the fuel (HC) is NOx in the excess oxygen state.
Since it will flow into the absorption catalyst, NOx will not be desorbed. Even if the NOx absorption catalyst has an oxidizing effect, the fuel burns in the front part of the catalyst and only the rear part of the catalyst becomes a reducing atmosphere, so the NOx absorbed upstream is left unpurified. I will end up.

Furthermore, since the NOx absorption catalyst has lower heat resistance than the ordinary three-way catalyst, NO of the fuel injected in the exhaust stroke is
If the atmosphere in the catalyst is made to be a reducing atmosphere by combustion in the x absorption catalyst, there is a problem that the temperature of the catalyst rises and the catalyst deteriorates. The present invention has been made in view of the above problems, and NO is obtained by fuel injection in the expansion to exhaust stroke without changing the combustion air-fuel ratio.
The NOx absorbed by the x absorption catalyst can be purified, and N
An object of the present invention is to provide an exhaust gas purification device that can avoid thermal deterioration of an Ox absorption catalyst.

[0006]

Therefore, the invention according to claim 1 is provided with a fuel injection valve for directly injecting fuel into a combustion chamber of an internal combustion engine, and can perform a combustion operation at a lean air-fuel ratio rather than a stoichiometric air-fuel ratio. An exhaust purification device for an internal combustion engine, which absorbs NOx in the exhaust when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio, and when the exhaust air-fuel ratio is richer than the theoretical air-fuel ratio or the theoretical air-fuel ratio. An NOx absorption catalyst that releases the absorbed NOx and performs a reduction process, and an oxidation catalyst that is interposed in an exhaust passage on the upstream side of the NOx absorption catalyst, are expanded by the fuel injection valve during lean combustion to exhaust gas. By injecting fuel in the stroke, the exhaust air-fuel ratio at the inlet of the NOx absorption catalyst is made rich, and the NOx absorbed by the NOx absorption catalyst is reduced , and
The amount of fuel injected during the expansion-exhaust stroke is calculated as
Depending on the burnt air-fuel ratio and the NOx absorption amount in the NOx absorption catalyst
The configuration is set as follows.

According to this structure, the fuel injected during the expansion-exhaust stroke is discharged into the exhaust passage together with the lean combustion exhaust, and first flows into the oxidation catalyst. Then, oxygen is consumed by the combustion of the fuel by the oxidizing action of the oxidation catalyst, the exhaust air-fuel ratio becomes rich at the oxidation catalyst outlet,
Exhaust gas having such a rich air-fuel ratio flows into the NOx absorption catalyst, and NOx desorption and reduction processing in the NOx absorption catalyst is performed. Also, intake air amount, combustion air-fuel ratio, NOx absorption
Depending on the amount, the inside of the NOx absorption catalyst is optimal for NOx reduction processing
For fuel injection during expansion to exhaust stroke to create a perfect atmosphere
Injection amount is set.

Generally, an oxidation catalyst (a three-way catalyst is also possible)
Has a higher heat resistance than the NOx absorption catalyst, and deterioration of the oxidation catalyst due to combustion of the fuel injected in the expansion to exhaust stroke is sufficiently small.

According to the second aspect of the present invention, the fuel amount for consuming oxygen in the exhaust gas by the oxidation reaction in the oxidation catalyst is calculated based on the intake air amount and the combustion air-fuel ratio, while the NOx absorption catalyst is calculated. The amount of fuel for reducing NOx is calculated on the basis of the amount of NOx absorbed in the above, and the sum of these is set as the amount of fuel to be injected in the expansion-exhaust stroke.

According to this structure, the amount of fuel for consuming excess oxygen in the exhaust gas by combustion in the oxidation catalyst can be determined from the amount of intake air and the combustion air-fuel ratio, and the NOx reduction process is performed simultaneously with oxidation. Since the presence of HC is indispensable, the amount of fuel required for the NOx reduction process can be determined from the NOx absorption amount, and the sum of the amount of fuel for consuming the excess oxygen and the amount of fuel required for the reduction process can be determined. Are injected in the expansion-exhaust stroke.

According to a third aspect of the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine, comprising a fuel injection valve for directly injecting fuel into a combustion chamber of the internal combustion engine, and capable of performing a combustion operation at a lean air-fuel ratio rather than a stoichiometric air-fuel ratio. Therefore, the configuration is as shown in FIG.
In FIG. 1, the NOx absorption catalyst absorbs NOx in the exhaust when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or is richer than the stoichiometric air-fuel ratio, The oxidation catalyst is a catalyst that releases the absorbed NOx and performs a reduction process, and the oxidation catalyst is provided in the exhaust passage on the upstream side of the NOx absorption catalyst.

On the other hand, the NOx absorption amount predicting means is the NOx
Predict the amount of NOx absorbed by the x absorption catalyst. Then, the exhaust stroke injection control means causes the fuel injection valve to perform fuel injection during expansion to the exhaust stroke when the NOx absorption amount predicted by the NOx absorption amount prediction means during the lean combustion operation becomes a predetermined value or more. Is performed for a predetermined period.

Here, the exhaust stroke injection quantity computing means sets the fuel injection quantity by the exhaust stroke injection control means to the intake air quantity of the engine, the combustion air-fuel ratio, and the NOx absorption quantity predicted by the NOx absorption quantity prediction means. Calculate based on According to this configuration, when the NOx amount absorbed by the NOx absorption catalyst becomes equal to or larger than the predetermined value during lean combustion, in addition to the normal fuel injection (intake stroke injection or compression stroke injection) for forming the lean combustion mixture, Fuel is injected even during the expansion-exhaust stroke.

The fuel injected into the combustion chamber in the expansion stroke or exhaust stroke is discharged from the combustion chamber together with the combustion exhaust gas and flows into the oxidation catalyst, where it is burned by the oxidation catalyst action and consumes excess oxygen due to lean combustion. To do. The fuel (HC) remaining without being burned by the oxidation catalyst is NOx along with lean exhaust.
N that had flowed into the absorption catalyst and was absorbed by the NOx absorption catalyst
Ox is desorbed under lean exhaust gas and is reduced at the same time as the fuel (HC) is oxidized.

The fuel injection amount in the expansion-exhaust stroke is calculated on the basis of the intake air amount and combustion air-fuel ratio of the engine, which indicates the excess oxygen amount, and the NOx absorption amount to be reduced. In the invention according to claim 4 , the exhaust stroke injection amount calculation means is
Based on the intake air amount and the combustion air-fuel ratio, the fuel amount for consuming oxygen in the exhaust gas by the oxidation reaction in the oxidation catalyst is calculated, while the NOx reduction process is performed based on the NOx absorption amount in the NOx absorption catalyst. The fuel amount is calculated and the sum of these is calculated as the fuel amount injected during the expansion-exhaust stroke.

According to such a configuration, the amount of fuel for consuming excess oxygen by combustion in the oxidation catalyst is calculated from the amount of intake air and the combustion air-fuel ratio, and the HC which is simultaneously oxidized in the NOx reduction process is calculated. Therefore, the fuel amount required for the NOx reduction process (fuel amount to be oxidized) is calculated from the NOx absorption amount, and the sum of these is set as the fuel amount to be injected in the expansion-exhaust stroke.

According to the fifth aspect of the invention, the exhaust stroke injection amount calculation means is based on the intake air amount of the engine and the combustion air-fuel ratio, and the fuel injection amount corresponding to the stoichiometric air-fuel ratio combustion and the combustion air at that time. A difference from the fuel injection amount at the fuel ratio is calculated, and this difference is set as a fuel amount for consuming oxygen in the exhaust gas by the oxidation reaction in the oxidation catalyst. According to such a configuration, from the fuel injection amount when performing stoichiometric air-fuel ratio combustion,
Since the difference from the fuel injection amount in lean combustion at that time is the shortage of the fuel that causes excess oxygen, the difference is injected in the expansion-exhaust stroke and once discharged as excess oxygen to the exhaust passage. Oxygen is consumed by combustion of the fuel injected in the expansion to exhaust stroke injection in the oxidation catalyst.

According to a sixth aspect of the present invention, there is provided an exhaust air-fuel ratio detecting means for detecting an exhaust air-fuel ratio in an exhaust passage downstream of the oxidation catalyst and upstream of the NOx absorption catalyst, and the exhaust air-fuel ratio detecting means. And an injection quantity feedback means for feedback-correcting the fuel injection quantity from expansion to exhaust stroke by the exhaust stroke injection control means based on the detected exhaust air-fuel ratio.

According to this structure, when fuel injection is performed in the expansion to exhaust stroke to consume excess oxygen by lean combustion, the exhaust air-fuel ratio is detected on the downstream side of the oxidation catalyst,
It is confirmed whether or not the excess oxygen is actually consumed, and the fuel amount in the injection from the expansion to the exhaust stroke is feedback-corrected.

[0020]

According to the invention described in claim 1, NOx can be desorbed and reduced by using the NOx absorption catalyst as a reducing atmosphere by injection in the expansion-exhaust stroke during lean combustion, and the expansion-exhaust stroke. co the fuel by injection in can avoid thermal degradation of the NOx absorbent catalyst so that combustion in the oxidation catalyst
To expand the NOx absorption catalyst to a reducing atmosphere ~
The injection amount in the exhaust stroke can be set appropriately and NOx is incomplete.
Can prevent the exhaust gas and fuel consumption from deteriorating.
Has the effect of

According to the second aspect of the present invention, the amount of fuel required for consumption of excess oxygen and the amount of fuel required for reduction treatment are respectively obtained, and the sum of these is injected in the expansion-exhaust stroke. As a result, there is an effect that the excess oxygen state can be reliably eliminated, and the deoxidized NOx can be reduced.

According to the third aspect of the invention, when the NOx absorbed by the NOx absorption catalyst exceeds a predetermined value,
With the lean combustion operation as it is, NOx can be purified by forming a reducing atmosphere in the NOx absorption catalyst by fuel injection in the expansion to exhaust stroke, and the excess oxygen state due to lean combustion is eliminated by combustion in the upstream oxidation catalyst. Therefore, there is an effect that thermal deterioration of the NOx absorption catalyst can be avoided.

According to the invention described in claim 4 , the amount of fuel required for consumption of excess oxygen by combustion in the oxidation catalyst and NOx
By injecting the amount of fuel required for the reduction process in the absorption catalyst in the expansion-exhaust stroke, the excess oxygen state is reliably eliminated to desorb NOx, and the deoxidized NOx is reduced. The effect is that it can be performed reliably.

According to the fifth aspect of the present invention, there is an effect that the amount of fuel for eliminating the oxygen excess state in the lean combustion state by the combustion of the fuel in the oxidation catalyst can be simply and surely obtained. According to the invention described in claim 6, there is an effect that the fuel amount capable of eliminating the excess oxygen state by the combustion of the fuel in the oxidation catalyst can be injected in the expansion-exhaust stroke without excess or deficiency.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 2 is a diagram showing a system configuration of an internal combustion engine according to the embodiment. The internal combustion engine 1 includes an air cleaner 2
The air that has passed through is throttled by the throttle valve 3 and sucked. Each cylinder of the engine 1 is provided with a fuel injection valve 4 that directly injects fuel into the combustion chamber, and the fuel injection by the fuel injection valve 4 forms a mixture in the combustion chamber.

Here, a homogeneous lean combustion operation by fuel injection in the intake stroke and / or a stratified lean combustion operation by fuel injection in the compression stroke are performed, and the control unit 5 incorporating a microcomputer is the target. The fuel injection amount and injection timing of the fuel injection valve 4 are controlled to form an air-fuel mixture. The air-fuel mixture formed by the fuel injection from the fuel injection valve 4 is ignited and combusted by spark ignition by the spark plug 6, and the combustion exhaust gas is an exhaust gas in which an oxidation catalyst 7 and a NOx absorption catalyst 8 are interposed in order from the upstream side. It is discharged into the atmosphere via the passage 9.

The oxidation catalyst 7 is a catalyst having an oxidizing action, and may be a three-way catalyst. Further, the NOx absorption catalyst 8 absorbs NOx in the exhaust when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or is richer than the stoichiometric air-fuel ratio, It is a NOx absorption type three-way catalyst that releases absorbed NOx and performs reduction processing.

The action of absorbing and releasing NOx in the NOx absorption catalyst 8 will be described by taking as an example the case where platinum Pt and barium Ba are carried on a carrier. The same applies to the case of using other noble metals, alkali metals, alkaline earths, and rare earths. When the exhaust air-fuel ratio is lean, FIG.
As shown in (A), oxygen O ₂ in the exhaust gas adheres to the surface of platinum Pt in the O ₂₋ form. On the other hand, NO in the exhaust gas reacts with O ₂₋ on the surface of platinum Pt to become NO ₂ (2NO +
O ₂ → 2NO ₂ ). Then, a part of the generated NO ₂ is further oxidized on the platinum Pt and is absorbed in the absorbing substance (barium) in the form of nitrate ion NO _{3 −} .

On the other hand, when the oxygen concentration in the exhaust gas is reduced by the switching to stoichiometric combustion or rich combustion to reduce the amount of NO ₂ produced, the reaction proceeds in the reverse direction (NO ₃ → NO ₂ ) and Nitrate ion NO _3-of is released from the absorbent in the form of NO ₂ . The released NO ₂ reacts with CO or HC and is reduced to N ₂ . Detection signals from various sensors are input to the control unit 5 for fuel injection control.

As the various sensors, an air flow meter 10 for detecting the intake air flow rate Qa of the engine 1, a throttle sensor 11 for detecting the opening TVO of the throttle valve 3, and a water temperature sensor 12 for detecting the cooling water temperature TW of the engine 1. A crank angle sensor 13 that detects a crank angle, an oxygen sensor 14 that is installed in the exhaust passage 9 on the upstream side of the oxidation catalyst 7 and that detects the oxygen concentration in the exhaust gas, and the like are provided. Based on the detection signal from the crank angle sensor 13, the engine speed N
e (rpm) is calculated.

The control unit 5 is shown in FIG.
The fuel injection by the fuel injection valve 4 is controlled as shown in the control block diagram of FIG. The lean / stoichi switching means A sets the combustion air-fuel ratio to lean and stoichiometric (theoretical air-fuel ratio).
And the stoichiometric air-fuel ratio control means B or the lean air-fuel ratio control means C controls the fuel injection amount by the fuel injection valve 4 in accordance with the switching to form a combustion mixture.

On the other hand, during lean combustion operation in which fuel injection is controlled by the lean air-fuel ratio control means C, the NOx absorption amount prediction means D predicts the amount of NOx absorbed by the NOx absorption catalyst 8, and based on the prediction result. The secondary injection permission means E makes a determination of permission of secondary injection (expansion to exhaust stroke injection), and the injection amount calculation means F (exhaust stroke injection control means, exhaust stroke injection amount calculation means) uses the secondary injection. The injection amount in (expansion-exhaust stroke injection) is calculated, and fuel injection is performed by the fuel injection valve 4 in the expansion-exhaust stroke in addition to normal fuel injection.

Next, each means shown in FIG. 4 will be described in detail with reference to the flowcharts of FIGS. The flowchart in FIG. 5 shows a routine (lean / stoichiometric switching means A) for making a lean burn permission determination. First, in step 1 (denoted as S1 in the figure, the same applies hereinafter), the engine speed Ne is It is determined whether the slice level is lower than SLNE. If the slice level is lower than SLNE, the process proceeds to step 2.

In step 2, it is judged whether the engine load is less than the slice level SLTP. Here, the engine load can be represented by a basic injection amount Tp (cylinder intake air amount) corresponding to the theoretical air-fuel ratio calculated based on the intake air flow rate Qa and the engine speed Ne. Step two
When it is determined that the engine load is less than the slice level SLTP, the process further proceeds to step 3, and it is determined whether the throttle opening change rate ΔTVO is less than the slice level SLTVO. It is determined whether or not the opening is stable and in a steady state.

When the throttle opening change rate ΔTVO is less than the slice level SLTVO, the routine proceeds to step 4, where it is judged if the water temperature TW exceeds the slice level SLTW. And the water temperature TW is the slice level S
When it exceeds LTW, that is, when it is in the steady and completely warm state of the low load and low rotation region, the routine proceeds to step 5,
The permission of lean combustion is determined, and the flag FLGLEN is set to 1.

On the other hand, when any one of the lean permission conditions of steps 1 to 4 is not satisfied, the routine proceeds to step 6, where it is judged that lean combustion is not permitted and 0 is set to the flag FLGLEN. In step 7, SIGNO, which is a predicted value of the NOx absorption amount in the NOx absorption catalyst 8, is set to 0. When the theoretical air-fuel ratio (or rich) combustion operation is performed instead of the lean combustion operation, NOx is successively reduced in the NOx absorption catalyst 8 and NOx is not absorbed. Set SIGNO to 0.

The flow chart of FIG. 6 shows the state of the injection amount Ti control at stoichiometric air-fuel ratio combustion (Stoichiometric air-fuel ratio control means B). In step 11, the intake air flow rate Qa and the engine speed Ne are detected. Read the value. In step 12, the basic injection amount Tp is calculated as Tp = k × Qa / Ne (k is a constant).

In step 13, the rich lean of the actual air-fuel ratio with respect to the theoretical air-fuel ratio is judged based on the signal of the oxygen sensor 14, and based on the rich lean judgment result,
The air-fuel ratio feedback correction coefficient α is, for example, proportionally / integrally controlled. In step 14, the basic injection amount Tp is corrected by the air-fuel ratio feedback correction coefficient α or the like to calculate the final fuel injection amount Ti.

Ti = Tp × α The control unit 5 outputs an injection pulse signal having a pulse width corresponding to the fuel injection amount Ti to the fuel injection valve 4 at the injection timing adjusted to the intake stroke of each cylinder, and the fuel is injected. The fuel injection by the injection valve 4 is controlled to perform stoichiometric air-fuel ratio (stoichiometric) combustion operation.

On the other hand, the flowchart of FIG. 7 shows the state of the injection amount Ti control (lean air-fuel ratio control means C) at the time of lean combustion. In step 21, the intake air flow rate Qa and the engine speed Ne are detected. The value is read, and in step 22, the basic injection amount Tp equivalent to the theoretical air-fuel ratio is calculated. Then, in step 23, the fuel injection amount Ti during lean combustion is calculated based on the target air-fuel ratio at that time and the basic injection amount Tp corresponding to the theoretical air-fuel ratio.

Ti = Tp × 14.7 / target air-fuel ratio control unit 5 controls the compression stroke of each cylinder when stratified lean combustion is performed, and the intake stroke of each cylinder when homogeneous lean combustion is performed. , An injection pulse signal having a pulse width corresponding to the fuel injection amount Ti is output to the fuel injection valve 4 to perform lean combustion operation.

Here, during lean combustion operation, NOx is absorbed by the NOx absorption catalyst 8 as described above, and the NOx absorption amount for this NOx absorption catalyst 8 is as shown in the flow chart of FIG. Estimated (NO
x absorption amount predicting means D). The flow chart of FIG. 8 is 1se
It is executed every c, and in step 31,
Whether the lean combustion state is set or not is determined by the determination of the flag FLGLEN.

When the flag FLGLEN is set to 1 and the condition is that lean combustion is permitted, the routine proceeds to step 32, where N absorbed by the NOx absorbing catalyst 8 is admitted.
Whether or not the expansion-exhaust stroke injection for Ox reduction processing is permitted is determined based on the flag FLGRS. The expansion-exhaust stroke injection and the flag FLGRS
Will be described in detail later.

When the flag FLGRS is set to 0 and the expansion-exhaust stroke injection for the NOx reduction process is not performed, in other words, the lean combustion in which NOx is absorbed by the NOx absorption catalyst 8 is performed. If it is during operation, go to step 33. In step 33, the engine speed N
e and the basic injection amount Tp representing the engine load are read.

In step 34, the map in which the NOx emission amount NOG per second is stored in advance with the engine speed Ne and the engine load as parameters is referred to, and the current engine speed N
The emission amount NOG corresponding to e and the engine load is read. In step 35, the emission amount NO read in step 34
G is added to the absorption amount SIGNO until then, and the addition result is set as a new absorption amount SIGNO. That is, during the lean combustion operation, the NOx emission amount estimated from the engine speed Ne and the engine load is sequentially integrated to predict the NOx absorption amount for the NOx absorption catalyst 8.

The method of predicting the NOx absorption amount is not limited to the above, and when the combustion air-fuel ratio changes under conditions other than engine speed and engine load, the combustion air-fuel ratio is included in the parameters to absorb NOx. Just predict the quantity. The flowchart of FIG. 9 shows the control (secondary injection permission means E) for performing desorption / reduction processing of NOx during lean combustion operation based on the absorption amount SIGNO.

In step 41, the estimated absorption amount S
It is determined whether the IGNO exceeds the slice level SLSNO. Then, the NO in the NOx absorption catalyst 8
When the x absorption amount SIGNO exceeds the slice level SLSNO, the routine proceeds to step 42, where the NOx processing time N
It is determined whether or not OTIME (expansion for NOx purification-duration of exhaust stroke injection) is less than the slice level SLNOTM.

When the NOx processing time NOTTIME is less than the slice level SLNOTM, the routine proceeds to step 43, in order to execute the processing for desorbing / reducing the NOx absorbed by the NOx absorption catalyst 8, the flag FLGRS.
Set 1 to. On the other hand, NOx processing time NOTIME
Is above the slice level SLNOTM,
NO increased to exceed the slice level SLSNO
It is considered that the desorption / reduction processing of the x absorption amount SIGNO has been completed, and the NOx absorption amount SIGNO is reset to zero in step 44, and then the routine proceeds to step 45, where the flag F
Set 0 to LGRS.

Even when it is determined in step 41 that the NOx absorption amount SIGNO is less than or equal to the slice level SLSNO, it is judged that the desorption / reduction processing of the absorbed NOx is not necessary, and the routine proceeds to step 45. The NOx absorption catalyst 8
In this case, if NOx is absorbed during lean combustion operation and the operation switches to stoichiometric air-fuel ratio combustion or rich combustion, the NOx absorbed up to that point will be desorbed / reduced, but lean combustion operation If there is no opportunity to continue desorption / reduction of NOx, the NOx absorption amount will exceed the maximum absorption amount, and NOx discharged from the engine 1 will be directly discharged to the atmosphere. I will end up.

Therefore, the NOx absorption amount SI during lean combustion
When the GNO exceeds the allowable amount SLSNO, it is necessary to forcibly desorb and reduce the absorbed NOx as the reducing atmosphere in the atmosphere of the NOx absorption catalyst 8. In the present invention, the upstream side of the NOx absorption catalyst 8 Oxidation catalyst 7
The expansion and the fuel injection in the exhaust stroke are combined to create the reducing atmosphere while the lean combustion operation is performed.

The flow chart of FIG. 10 shows the state of control of the expansion-exhaust stroke injection (injection amount calculation means F: exhaust stroke injection control means, exhaust stroke injection amount calculation means). It is determined whether 1 is set in the flag FLGRS. The flag FLGRS
If 1 is set to, go to step 52,
The fuel injection amount TIO2 to be injected from the expansion to the exhaust stroke of each cylinder in order to consume excess oxygen is calculated as TIO2 = Tp × (1-14.7 / target air-fuel ratio).

The basic injection amount Tp is a basic injection amount corresponding to the theoretical air-fuel ratio and represents the cylinder intake air amount, and the target air-fuel ratio is the combustion air-fuel ratio at that time. The fuel injection amount TIO2 is the basic injection amount Tp corresponding to the theoretical air-fuel ratio.
It becomes a value obtained by subtracting the basic injection amount Tp ′ corresponding to lean combustion from this, and since the amount of fuel used for combustion is small by this TIO2, exhaust of excess oxygen is exhausted. Since the fuel injection valve 4 injects fuel directly into the combustion chamber, if fuel is injected in the expansion-exhaust stroke, the fuel will flow out to the exhaust system together with the exhaust gas, and the fuel discharged from this combustion chamber will be Oxidation of the oxidation catalyst 7 burns and consumes oxygen. Therefore, if the fuel is injected by the fuel amount TIO2 during the expansion to the exhaust stroke, the fuel is burned in the oxidation catalyst 7, and excess oxygen is consumed.

In step 53, the NOx absorption amount SIG is previously set.
A table that stores the fuel amount TINO for NOx reduction according to NO is referred to, and the fuel amount TINO corresponding to the absorption amount SIGNO at that time is read. And step
At 54, the sum of TIO2 and TINO is set to the fuel injection amount TIRS in the final expansion-exhaust stroke. In addition to the fuel injection used for combustion in the intake stroke or the compression stroke, the control unit 5 outputs an injection pulse signal having a pulse width corresponding to the fuel injection amount TIRS in each cylinder from the expansion stroke to the exhaust stroke of each cylinder. Output to 4.

Here, when the predetermined time SLNOTM determined in step 2 of the flow chart of FIG. 9 has elapsed from the start of the exhaust stroke injection by the fuel injection amount TIRS, the NOx absorption amount SIGNO is reset to zero and the flag FLGRS. By setting 1 to, the process proceeds from step 51 to step 55. Then, in step 55, the fuel injection amount TIRS is set to 0, so that the fuel injection in the expansion-exhaust stroke is stopped and only the injection in the intake or compression stroke for normal lean combustion is performed. Will be done.

By injecting fuel in the expansion-exhaust stroke by the fuel amount TIO2, the excess oxygen state is eliminated, and N
Although it is possible to make the air-fuel ratio of the exhaust gas flowing into the Ox absorption catalyst 8 rich, it is necessary to simultaneously perform the oxidation process of HC for the reduction process of NOx. Therefore, in order to secure the HC that reaches the NOx absorption catalyst 8 without being burned by the oxidation catalyst 7 and is oxidized at the same time as the NOx reduction process, the fuel amount TINO corresponding to the NOx absorption amount is added to the fuel amount TIO2. It is what makes it jet.

If the expansion-exhaust stroke injection is performed by the fuel injection amount TIRS as described above, the excess oxygen state is eliminated by the combustion in the oxidation catalyst 7 and the exhaust air-fuel ratio becomes rich, and in the NOx absorption catalyst 8 on the downstream side. The NOx is desorbed, and further, the NOx reduction process is performed along with the oxidation reaction of the fuel (HC) flowing into the NOx absorption catalyst 8 without being burned by the oxidation catalyst 7.

As described above, the fuel supplied by the injection (secondary injection) from the expansion to the exhaust stroke is burned by the oxidation catalyst 7, but generally the oxidation catalyst (compared to the NOx absorption catalyst 8 ( Since the three-way catalyst 7 has high heat resistance, it does not cause catalyst deterioration. Further, as described above, the amount of fuel required to eliminate the excess oxygen state and the desorbed NO
If the configuration is such that the amount of fuel to be injected in the expansion-exhaust stroke is determined based on the amount of fuel (HC) required to reduce x, the desorption and reduction processes of NOx can be performed with certainty. It is possible to avoid deterioration of fuel efficiency.

By the way, in order to surely make the air-fuel ratio of the exhaust gas flowing into the NOx absorption catalyst 8 rich by the fuel injection from the expansion to the exhaust stroke, as shown in FIG. 11, the NOx absorption is performed on the downstream side of the oxidation catalyst 7. In the exhaust passage 9 on the upstream side of the catalyst 8,
An air-fuel ratio sensor 15 (exhaust air-fuel ratio detecting means) that can detect the exhaust air-fuel ratio in a wide range based on the oxygen concentration in the exhaust is provided, and the fuel injection amount TIRS is feedback-controlled based on the detection result by the air-fuel ratio sensor 15. Alternatively, the injection amount feedback means may be used.

Specifically, the exhaust air-fuel ratio detected by the air-fuel ratio sensor 15 is compared with the target air-fuel ratio of the exhaust gas flowing into the NOx absorption catalyst 8, and the actual air-fuel ratio is leaner than the target. Sometimes, the injection amount is corrected to increase, and conversely, when it is rich, the injection amount is corrected to decrease. With such a configuration, even if there are various variation factors, NO
By making the exhaust air-fuel ratio at the inlet of the x absorption catalyst 8 rich, it is possible to reliably desorb and reduce the NOx absorbed by the NOx absorption catalyst 8.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an exhaust emission control device according to a third aspect of the invention.

FIG. 2 is a system configuration diagram of an internal combustion engine in the embodiment.

FIG. 3 is a diagram for explaining a NOx absorption / release action of a NOx absorption catalyst.

FIG. 4 is a basic fuel control block diagram in the above embodiment.

FIG. 5 is a flowchart showing lean / stoichi switching control.

FIG. 6 is a flowchart showing fuel injection control during stoichiometric combustion.

FIG. 7 is a flowchart showing fuel injection control during lean combustion.

FIG. 8 is a flowchart showing a predictive control of the NOx absorption amount.

FIG. 9 is a flowchart showing expansion-exhaust stroke injection permission control.

FIG. 10 is a flowchart showing an injection amount calculation of expansion-exhaust stroke injection.

FIG. 11 is a system configuration diagram of an internal combustion engine according to the second embodiment.

[Explanation of symbols]

1 Internal combustion engine 2 air cleaner 3 Throttle valve 4 Fuel injection valve 5 control unit 6 spark plugs 7 Oxidation catalyst 8 NOx absorption catalyst 9 exhaust passage 10 Air flow meter 11 Throttle sensor 12 Water temperature sensor 13 Crank angle sensor 14 Oxygen sensor 15 Air-fuel ratio sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI F01N 3/24 F01N 3/24 R F02D 41/02 325 F02D 41/02 325A 41/04 305 41/04 305Z 41/14 310 41 / 14 310A (56) Reference JP-A-8-200045 (JP, A) JP-A-9-32619 (JP, A) JP-A-8-312408 (JP, A) International Publication 96/22457 (WO, A1) ) (58) Fields surveyed (Int.Cl. ⁷ , DB name) F02D 41/34 F01N 3/08 F01N 3/24 F02D 41/02 325 F02D 41/04 305 F02D 41/14 310

Claims (6)

(57) [Claims] 1. An exhaust gas purification apparatus for an internal combustion engine, comprising: a fuel injection valve for directly injecting fuel into a combustion chamber of the internal combustion engine; and capable of performing a combustion operation at a lean air-fuel ratio rather than a stoichiometric air-fuel ratio. Is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas is absorbed, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, the absorbed NOx is released to perform reduction processing. The NOx absorption catalyst is provided with a catalyst and an oxidation catalyst that is interposed in the exhaust passage on the upstream side of the NOx absorption catalyst, and injects fuel during the expansion-exhaust stroke by the fuel injection valve during lean combustion. of the exhaust air-fuel ratio at the inlet to the rich, performs reduction processing of the NOx absorbed in the NOx absorbent catalyst, before
The amount of fuel injected during the expansion-exhaust stroke is defined as the intake air amount,
Depending on the combustion air-fuel ratio and the NOx absorption amount in the NOx absorption catalyst,
An exhaust gas purification device for an internal combustion engine, which is characterized in that it is set in advance. 2. A fuel amount for consuming oxygen in exhaust gas by an oxidation reaction in the oxidation catalyst is calculated on the basis of the intake air amount and a combustion air-fuel ratio, and based on a NOx absorption amount in the NOx absorption catalyst. the NOx calculates the amount of fuel quantity reduction treatment Te, exhaust purification system of an internal combustion engine according to claim 1, characterized in that the amount of fuel injected these total expansion-exhaust stroke. 3. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a fuel injection valve for directly injecting fuel into a combustion chamber of the internal combustion engine and capable of performing a combustion operation at a lean air-fuel ratio rather than a stoichiometric air-fuel ratio. Is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas is absorbed, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, the absorbed NOx is released to perform reduction processing. A catalyst, an oxidation catalyst installed in an exhaust passage upstream of the NOx absorption catalyst, a NOx absorption amount prediction means for predicting the amount of NOx absorbed in the NOx absorption catalyst, and the NOx absorption during lean combustion operation. Exhaust stroke injection control means for causing the fuel injection valve to perform fuel injection only during a predetermined period during the expansion-exhaust stroke when the NOx absorption amount predicted by the amount predicting means exceeds a predetermined value; An exhaust stroke injection quantity computing means for computing the fuel injection quantity by the exhaust stroke injection control means based on the intake air quantity of the engine, the combustion air-fuel ratio, and the NOx absorption quantity predicted by the NOx absorption quantity prediction means. An exhaust gas purification device for an internal combustion engine, which is configured. 4. The exhaust stroke injection amount calculation means calculates a fuel amount for consuming oxygen in exhaust gas by an oxidation reaction in the oxidation catalyst on the basis of the intake air amount and a combustion air-fuel ratio, while 4. The internal combustion engine according to claim 3 , wherein the amount of fuel for reducing NOx is calculated based on the amount of NOx absorbed in the NOx absorption catalyst, and the sum of these is calculated as the amount of fuel injected during the expansion-exhaust stroke. Exhaust gas purification device for engines. 5. The exhaust stroke injection amount calculation means, based on the intake air amount of the engine and the combustion air-fuel ratio, the fuel injection amount corresponding to the stoichiometric air-fuel ratio combustion and the fuel injection amount at the combustion air-fuel ratio at that time. 5. The exhaust gas purification apparatus for an internal combustion engine according to claim 4 , wherein a difference between the exhaust gas and the exhaust gas is calculated, and the difference is used as a fuel amount for consuming oxygen in the exhaust gas by the oxidation reaction in the oxidation catalyst. 6. Exhaust air-fuel ratio detection means for detecting an exhaust air-fuel ratio in an exhaust passage downstream of the oxidation catalyst and upstream of the NOx absorption catalyst, and exhaust air-fuel ratio detected by the exhaust air-fuel ratio detection means. according to any one of claims 3-5, characterized in that a, the injection quantity feedback unit for feedback correction of the fuel injection quantity in the expansion-exhaust stroke by the exhaust stroke injection control means based on Exhaust gas purification device for internal combustion engine.