JP2001342870A - Exhaust emission control device for cylinder injection type internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for a cylinder injection type internal combustion engine capable of preventing CO, THC spike and NOx spike by surely balancing reactions of emission and reduction in purging NOx in addition to shortening of NOx purging period. SOLUTION: When NOx of NOx catalyst should be emitted, stratified combustion is performed with a stoichiometric or slightly rich air-fuel ratio (14.42 in the figure) by injecting fuel in compression stroke, and a large quantity of CO, HC is thereby produced near a spark plug by incomplete combustion, while a large quantity of O2 of a lean air-fuel ratio is made to remain around there. The reactions of emission and reduction of NOx with CO, HC on the NOx catalyst and oxidation of CO, HC are mixed by feeding CO, HC and O2 to the NOx catalyst, and an amount of CO, HC contributing to the emission and reduction of NOx is decreased. Emission velocity of NOx is thereby decreased to surely reduce NOx, and excessive CO, HC to emitted NOx are oxidized with O2 to purify.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust purification system for a direct injection internal combustion engine (hereinafter referred to as an engine), and more particularly to a technique for removing NOx stored in NOx storage means.

[0002]

[Related Background Art] In a lean-burn engine for the purpose of improving fuel efficiency, the air-fuel ratio is controlled to be leaner than the stoichiometric (stoichiometric air-fuel ratio) to perform combustion in an oxygen-excess atmosphere. NOx in exhaust gas due to its characteristics
(Nitrogen oxides) cannot be sufficiently purified. Therefore, this type of engine is provided with a storage-type NOx catalyst that can purify NOx even in an oxygen-excess atmosphere.

A storage NOx catalyst converts NOx in exhaust gas into nitrate X-NO in an oxidizing atmosphere having a low concentration of reducing components.
Occluded as _3, the occluded NOx CO (carbon monoxide) and HC characteristic (hydrocarbon) or the like is reduced to N ₂ (nitrogen) in a reducing atmosphere present in a large amount (simultaneously carbonates X-C
O ₃ is produced). With this type of storage NOx catalyst, N
Ox is stored to prevent emission to the atmosphere, and periodically (when the NOx storage amount reaches a saturation amount) the air-fuel ratio is controlled to the stoichiometric or rich side to control the CO of the exhaust gas flowing into the NOx catalyst. , HC concentration, thereby increasing NO
NOx from the x catalyst is released (NOx purge) and reduced
We are trying to regenerate the catalyst.

At the time of this NOx purging, the fuel consumption is deteriorated as compared with the normal lean operation. Therefore, it is desirable to shorten the NOx purging period, and at the same time, it is desirable to suppress the torque change at the time of transition to the NOx purging operation. As a technique focusing on such a point, for example, Japanese Patent Laid-Open No. 7-1
JP-A-39388 can be mentioned.
This exhaust gas purifying apparatus is intended for a general engine that injects fuel into an intake pipe, and performs feedback control by setting a target air-fuel ratio slightly richer than stoichiometric during NOx purging.

[0005]

As is well known, NOx purging consists of a process for releasing NOx from the NOx catalyst and a process for reducing the released NOx.
It is necessary to advance these reactions of release and reduction while balancing them, and when the reduction rate is lower than the release rate, NOx is not sufficiently reduced and NOx spikes are generated,
Conversely, when the reduction rate is high, excess CO and HC cause C
O, THC spikes occur. When the reaction rate of the release reduction is increased by enrichment as in the technique described in the above-mentioned publication, the equilibrium state of both reactions becomes very unstable, and the balance is broken by trivial factors.

For example, the air-fuel ratio and the purge time at the time of NOx purging are determined in advance by a bench test in consideration of the characteristics of the NOx catalyst itself or the characteristics of the three-way catalyst when there is a three-way catalyst on the upstream side. However, when the amount of CO and HC supplied to the NOx catalyst fluctuates due to the deterioration of the NOx catalyst or the deterioration of the three-way catalyst on the upstream side, N
The state of release and reduction of Ox changes, and the balance between the two easily collapses. Needless to say, since the influence of the catalyst deterioration cannot be dealt with by the prior matching, the technique described in the above-mentioned publication discloses NOx spike, CO, THC and the like.
There was a problem of generating spikes.

According to the present invention, in-cylinder injection in which the NOx purge period is shortened, and the discharge and reduction reactions during NOx purge are surely balanced to prevent CO, THC spikes and NOx spikes. It is an object of the present invention to provide an exhaust gas purification device for an internal combustion engine.

[0008]

In order to achieve the above object, according to the present invention, there is provided an exhaust passage for an in-cylinder injection type internal combustion engine, and the NO in the exhaust gas when the exhaust air-fuel ratio is lean.
NOx storage means for storing x, releasing and reducing the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich;
control means for operating the internal combustion engine at a stoichiometric or slightly rich air-fuel ratio by injecting fuel in the compression stroke when NOx stored in the x storage means is to be released and reduced.

Therefore, stoichiometric or slightly rich air-fuel
When stratified combustion is performed by compression stroke injection at a ratio
Extremely rich air-fuel ratio locally in the vicinity of the lag
O _TwoFuel is incomplete, resulting in incomplete combustion.
While a large amount of CO and HC is generated,
Large amount of O due to lean air-fuel ratio_TwoRemain and these C
O, HC and O_TwoIs supplied to the NOx storage means in the state where
Is done.

Then, the CO, supplied in this manner,
The HC and O _2, because the on NOx storage unit emission reduction of NOx in the O ₂ presence is performed, CO, due to the HC N
Ox release and reduction reactions and CO and HC oxidation reactions are mixed. As a result, the amounts of CO and HC contributing to the release and reduction of NOx are reduced, so that the release speed of NOx is reduced, NOx is reliably reduced, NOx spikes are prevented, and excess NOx is released. C
O and HC are oxidized and purified by O _2, thereby preventing CO and THC spikes.

The present invention is preferably configured such that the control means performs fuel injection in an expansion stroke and / or an exhaust stroke in addition to a compression stroke while maintaining a stoichiometric or slightly rich air-fuel ratio. It is desirable to do. This allows
Sub-injection is performed in the expansion stroke and the exhaust stroke, and NOx is reliably reduced by the injected fuel to prevent NOx spikes.

Preferably, the present invention provides the control means,
It is desirable that after the execution of the compression stroke injection, the intake stroke injection be executed while maintaining the air-fuel ratio. NOx
The emission characteristics are such that the initial emission amount is large and the NOx emission reaction is slowed down thereafter.Therefore, by switching to the intake stroke injection that is closer to complete combustion, the emission reduction is given priority while reducing fuel consumption. Can be continued.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an exhaust gas purifying apparatus for a direct injection internal combustion engine embodying the present invention will be described. FIG. 1 is an overall configuration diagram showing an exhaust gas purifying apparatus for a direct injection internal combustion engine according to an embodiment. As shown in FIG.
Is configured as an in-cylinder injection spark ignition type in-line four-cylinder gasoline engine. The cylinder head of the engine 1 is provided with an electromagnetic fuel injection valve 3 together with an ignition plug 2 for each cylinder, and fuel is directly injected from the fuel injection valve 3 into the combustion chamber. An intake port 4a is formed in the cylinder head in a substantially upright direction for each cylinder, and these intake ports 4a are connected to the intake manifold 4
And the throttle valve 5 is connected to an intake passage (not shown).

Further, exhaust ports 6a are formed in the cylinder head in a substantially horizontal direction.
Is connected to an exhaust passage 7 via an exhaust manifold 6. A proximity three-way catalyst 8 is provided in the exhaust passage 7, and a storage type N as NOx storage means is provided downstream thereof.
An Ox catalyst 9 is provided, and further downstream thereof is a three-way catalyst 1.
0 is provided. The components of the close three-way catalyst 8 are adjusted so that the oxygen storage function is significantly inferior to that of a normal three-way catalyst. The NOx catalyst 9 has platinum (Pt), rhodium (Rh) or the like as a noble metal, an alkali metal such as barium (Ba) or an alkaline earth metal as a storage material, and stores NOx in an oxidizing atmosphere. It has a function of releasing NOx mainly in a reducing atmosphere where CO and HC are present. The NOx catalyst 9
Released from the NOx is reduced to N ₂ or the like by the NOx catalyst 9 or the three-way catalyst 10.

On the other hand, an ECU (electronic control unit) 11 as a control means including an input / output device, a storage device (ROM, RAM, nonvolatile RAM, etc.), a central processing unit (CPU), a timer counter and the like is provided in the vehicle interior. Is installed. The input side of the ECU 11 has a throttle valve 5
Throttle sensor 12 for detecting the opening degree θth of the engine, a rotational speed sensor 13 for detecting the rotational speed Ne of the engine 1, NO
NOx sensor 14 for detecting the NOx concentration in the exhaust gas passing through the x catalyst, and O ₂ sensor 15 for detecting the concentration of oxygen contained in exhaust gas discharged is connected from the engine 1, the detection of these sensors Information is entered. On the output side of the ECU 11, the above-described ignition plug 2,
The fuel injection valve 3 and the like are connected.

The ECU 11 performs comprehensive control of the engine 1 based on information input from each sensor. The fuel injection amount control is set in advance from a target average effective pressure Pe (representing an engine load) obtained from an accelerator opening and an engine speed Ne detected by an accelerator sensor (not shown) and the engine speed Ne. The optimum fuel injection mode and target air-fuel ratio are determined according to the map. For example, in a low-load low-speed range such as during idling or low-speed running, a compression stroke lean mode in which the target air-fuel ratio is set to the lean side and fuel injection is performed in the compression stroke is selected, and the target average effective pressure Pe And engine speed Ne
As the fuel pressure increases, the S-F / B mode in which the fuel injection is executed in the intake stroke and the air-fuel ratio is controlled stoichiometrically based on the signal from the O ₂ sensor 15, and the target air-fuel ratio is set to the rich side and the intake The mode is sequentially switched to the intake stroke rich mode in which fuel injection is performed in the stroke.

In the present embodiment, the storage NOx catalyst 3
When executing the NOx purging process for releasing and reducing the NOx stored in the fuel cell 0a, as one type of fuel injection mode, an intake process purge mode in which fuel injection is set to an intake process to purge NOx, and fuel injection is compressed. Set the process to N
A compression stroke purge mode for purging Ox is executed. Hereinafter, the details of the NOx purge process performed using both the intake process purge mode and the compression stroke purge mode will be described.

The ECU 11 executes the NOx purge routine shown in FIG. 2 at a predetermined control interval, and first determines in step S2 whether or not a NOx purge period in which the NOx purge should be executed. For example, when the NOx concentration detected by the NOx sensor 14 is equal to or higher than a predetermined value and the NOx catalyst 9
When leakage of NOx from the NOx catalyst 9 is estimated, or when the integrated time of the lean operation in the compression stroke lean mode is equal to or more than a predetermined value, and when it is estimated that the occlusion amount of the NOx catalyst 9 has reached the vicinity of the limit, or acceleration or the like The NOx purge control is started, for example, when it is started, and if it is during the control period, it is determined that it is during the NOx purge period. If the determination in step S2 is NO (negative), the routine ends as it is. Therefore, in this case, according to the operating state of the engine 1, the normal fuel injection mode (compression stroke lean mode, S
-F / B mode, intake stroke rich mode) are executed.

Then, the amount of occlusion of the NOx catalyst 9 gradually increases due to the continuation of the lean operation, and YES is determined in step S2.
If a positive determination is made, the ECU 11 proceeds to step S4. In step S4, the average equivalent ratio of the exhaust gas at the time of NOx purge is set. The average equivalent ratio at this time is, for example, the engine rotation speed Ne, the target average effective pressure Pe, the volume efficiency, the traveling distance of the vehicle, the total flow rate of the exhaust gas passing through the NOx catalyst, the temperature of the NOx catalyst, the exhaust gas temperature, the exhaust gas equivalent ratio,
It is set based on at least one or more requirements among the requirements such as the cooling water temperature. In this embodiment, the equivalent ratio is 1.00.
Set within the range of ~ 1.03. This is equivalent to 1.
If it is less than 00, O ₂ is excessive and NOx releasing action cannot be expected. If it exceeds 1.03, CO and HC are excessive and CO and TH
From the viewpoint of causing the C spike, it may be set outside this range.

In a succeeding step S6, it is determined whether or not the compression mode is in progress. This compression mode period is NO
NOx as a period during which the release reaction of x proceeds relatively rapidly
This is a period initially set during the purge period. In the present embodiment, the length of the compression mode period is set based on the same requirement as the above-described equivalent ratio, but may be a fixed value set in advance. The ECU 11 makes a determination of YES at the beginning of the process of step S6, proceeds to step S8 to execute the compression process purge mode, and thereafter ends the routine.

The compression step purge mode is a mode peculiar to the present invention, and controls the air-fuel ratio by feedback or open loop by compression stroke injection so as to achieve the average equivalent ratio of exhaust gas set in step S4. .
Since the average equivalent ratio is set in the range of 1.00 to 1.03 as described above, the air-fuel ratio is maintained at the stoichiometric or slightly rich side as a result of the control. At this time, feedback or open loop control may be performed while performing perturbation for periodically changing the air-fuel ratio with a predetermined amplitude so that the average value of the equivalence ratio becomes the average equivalence ratio.

When the compression mode period ends and the determination in step S6 is NO, the ECU 11 proceeds to step S6.
The routine proceeds to step 10, where the intake stroke purge mode is executed, and thereafter, the routine ends. In the intake stroke purge mode, the air-fuel ratio is controlled by the intake stroke injection so as to achieve the same average equivalent ratio as in the above-described compression stroke purge mode. At the end of the compression mode period, since the release reaction of the NOx catalyst 9 becomes slower with the decrease in the NOx storage amount, the reduction in fuel consumption is prioritized over the speeding up of the purge process, and the combustion is close to complete combustion. The intake stroke purge mode is executed. This intake stroke purge mode is continued until the end of the NOx purge period.
If the determination in step S2 becomes NO due to the end of the x purge period, the control shifts to control based on the normal fuel injection mode.

Next, while comparing the compression stroke purge mode, which is a control unique to the present invention, with the intake stroke purge mode, NO
The difference in the purge status of x will be described. As is well known, in the normal compression step lean mode, the fuel injected in the compression step is guided to the vicinity of the ignition plug 2, and the ignitable mixture near the stoichiometry is concentrated around the ignition plug 2, while leaning around the ignition plug 2. An air-fuel mixture having a low air-fuel ratio is present, thereby performing stratified combustion to enable operation with an extremely lean overall air-fuel ratio. On the other hand, in the compression process purge mode in which the overall air-fuel ratio is set near the stoichiometric ratio, the air-fuel ratio locally becomes extremely rich in the vicinity of the ignition plug 2 and O ₂ becomes insufficient, so that the fuel becomes insufficient. While complete combustion causes a large amount of CO and HC to be generated, a large amount of O ₂ is generated in the surroundings due to a lean air-fuel ratio.
In the state where O, HC and O ₂ coexist, the three-way catalyst 8 and the N on the downstream side thereof pass through the exhaust passage 7 without reacting on the way.
It is supplied to the Ox catalyst 9.

FIG. 3 shows the CO concentration and the O ₂ concentration in the exhaust gas flowing into the NOx catalyst 9 in comparison between the compression stroke injection and the intake stroke injection, and as shown in FIG. In the compression stroke injection, the CO concentration and the O ₂ concentration are remarkably high in all the air-fuel ratio regions including the stoichiometric ratio. That is, at the time of NOx purging, similar to the air-fuel ratio in the compression stroke purge mode of the present embodiment, Japanese Patent Laid-Open No.
Although the air-fuel ratio is controlled in the vicinity of stoichiometry also in the prior art described in 139388, the exhaust gas composition is completely different, for example, stoichiometry in the figure (14.42 in this example).
In the case of (1), in the related art in which the intake stroke injection is performed (similarly in the intake stroke purge mode of the present embodiment), only a small amount of CO and HC is supplied, and almost no O ₂ is supplied. In the compression stroke purge mode of this embodiment, a large amount of CO, HC and O ₂ is supplied.

The CO, supplied in this manner,
Due to HC and O ₂ , N _{2 is} present on the NOx catalyst 9 in the presence of O _2.
Ox is released and reduced. However, since CO and HC contributing to NOx release and reduction easily react with O ₂ , NOx release and reduction reaction by CO and HC and CO and H
The oxidation reaction of C can be mixed. As a result, the amounts of CO and HC contributing to the release and reduction of NOx are reduced by that amount, so that the release speed of NOx is reduced, and the NOx spike in which the NOx is reliably reduced and the NOx emission temporarily increases is reduced. It is prevented beforehand. Further, since CO and HC which become excessive with respect to the released NOx are oxidized and purified by O ₂ , CO and THC spikes in which CO and HC emissions temporarily increase are also prevented beforehand.

Further, since the NOx purging process is performed with a sufficient amount of CO and HC as compared with the prior art described in the publication, the NOx stored in the NOx catalyst 9 is quickly released and reduced. As a result, according to the exhaust gas purifying apparatus for a direct injection internal combustion engine of the present embodiment, the NOx purge period is greatly shortened, and the NOx catalyst 9 is deteriorated or the upstream three-way catalyst 8 is deteriorated. Accordingly, even if the amounts of CO and HC supplied to the NOx catalyst 9 fluctuate with the above, the release of NOx and the reduction reaction can be reliably balanced to prevent CO, THC spikes and NOx spikes from occurring. .

The description of the embodiment is finished above, but aspects of the present invention are not limited to this embodiment. For example, in the above-described embodiment, the mode is switched to the intake stroke purge mode in the middle of the NOx purge period. However, this switching is not necessarily performed. May be continued to complete the purging process.

Further, in the above-described embodiment, fuel injection is performed only in the compression stroke in the compression stroke purge mode. However, when NOx spikes are generated even when the compression stroke purge mode is executed, sub-injection is performed. CO and HC may be supplemented. Specifically, when the occurrence of a NOx spike is estimated from the detection value of the NOx sensor 14, or when the operating range of the NOx spike, which is obtained in advance from a bench test, is reached, as a sub-injection in addition to the compression stroke, Fuel injection is performed in the expansion stroke and the exhaust stroke to increase the supply of CO and HC, thereby preventing NOx spikes.

[0029]

As described above in detail, according to the exhaust gas purifying apparatus for a direct injection type internal combustion engine of the present invention, the NOx purge period is shortened, and the NOx emission and the reduction reaction are surely balanced. Thus, CO, THC spikes and NOx spikes can be prevented.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an exhaust purification device of a direct injection internal combustion engine of an embodiment.

FIG. 2 is a flowchart illustrating a NOx purge routine executed by an ECU.

FIG. 3 is an explanatory diagram comparing CO concentration and O ₂ concentration in exhaust gas flowing into a NOx catalyst between a compression stroke injection and an intake stroke injection.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 engine (in-cylinder injection internal combustion engine) 7 exhaust passage 9 storage NOx catalyst (NOx storage means) 11 ECU (control means)

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Koujiro Okada 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation F-term (reference) 3G091 AA12 AA17 AA24 AA28 AB03 AB06 AB09 BA14 BA15 BA19 BA33 CB02 CB03 DA01 DA02 DB06 DB10 DC01 EA01 EA07 EA16 EA17 EA18 EA21 EA30 EA33 EA34 EA38 FA17 FB10 FB11 FB12 FC02 GB02Y GB03Y GB05W GB06W HA03 HA08 HA12 HA18 HA47 3G301 HA04 JA25 JA26 LB04 MA01 MA19 NE13 NE01 PDZ

Claims (1)

[Claims] 1. An exhaust passage for an in-cylinder injection type internal combustion engine, which stores NOx in exhaust gas when the exhaust air-fuel ratio is lean, and stores the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich. NOx storage means for releasing and reducing, and control means for operating the internal combustion engine at a stoichiometric or slightly rich air-fuel ratio by injecting fuel in a compression stroke when NOx stored in the NOx storing means is to be released and reduced. An exhaust gas purification apparatus for a direct injection internal combustion engine, comprising: