JP2000204932A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely reduce and control exhaust emission when NOx absorbed during a lean operation is discharged by rich spike. SOLUTION: A first catalyst 3 for emission control which can absorbs NOx is disposed in an exhaust passage 2 of internal combustion engine 1, a second catalyst 4 whose oxygen absorbing function is small is disposed more downstream than the first catalyst 3 in the exhaust passage 2, and a third catalyst 5 whose oxygen absorbing function is large is disposed more downstream than the second catalyst 4 in the exhaust passage 2. When an NOx absorbing amount reaches a certain value, an air fuel ratio is forced to be excessively rich, and desorption of absorbed NOx is carried out. Most of NOx desorbed and discharged is subjected to emission control on the first catalyst 3 for emission control, and the NOx which is not reduced in this step is reduced on the second catalyst 4. Since the oxygen absorbing function of the second catalyst 4 is made small, NOx is efficiently reduced independent of affection of discharge of oxygen. Further, NOx is surely reduced by being excessively rich state. Excess reduction component which is not used at the time of reducing of NOx is subjected to emission control on the third catalyst 5 having the oxygen absorbing function.

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 an exhaust gas purifying apparatus for efficiently purifying nitrogen oxides (NOx) which are largely discharged due to lean combustion.

[0002]

2. Description of the Related Art In recent years, a so-called lean-burn engine that can be operated even at an air-fuel ratio higher than the stoichiometric air-fuel ratio has attracted attention from the viewpoint of improving fuel efficiency and reducing carbon dioxide emissions. In this type of internal combustion engine, exhaust gas discharged at the time of lean combustion has a high oxygen content, and a generally used three-way catalyst does not sufficiently purify NOx. Therefore, development of a technology for efficiently removing NOx during lean operation is desired.

As one of them, Japanese Patent No. 2640092 discloses a NOx absorbent that absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio and releases NOx when the inflowing exhaust gas has a rich air-fuel ratio. Purification devices used have been proposed. In this system, NOx exhausted from the engine during lean operation is absorbed by a NOx absorbent, and at a predetermined time, an operation is performed in which the air-fuel ratio is temporarily made rich,
The NOx absorbed by the NOx absorbent is released. Then, the released NOx is reduced and purified by the catalyst component carried on the NOx absorbent.

[0004] Here, when the reducing power of the catalyst component carried on the NOx absorbent is weak, the amount of NOx that flows out of the NOx absorbent downstream without being reduced on the NOx absorbent increases. It is also disclosed that a catalyst capable of reducing NOx, for example, a three-way catalyst is disposed downstream of the absorbent.

Japanese Patent Application Laid-Open No. 6-66185 discloses that
An exhaust gas purification device in which a three-way catalyst having an oxygen absorption capacity (oxygen storage capacity) is arranged downstream of the NOx absorbent, and when releasing NOx from the NOx absorbent, the air-fuel ratio is made excessively rich. Is disclosed. This is N
By providing the reducing component in excess of the amount of the reducing component necessary for reducing the NOx absorbed in the Ox absorbent, the released NOx can be more reliably reduced and purified on the NOx absorbent. Therefore, a surplus excess component not used for the reduction of NOx is purified by the three-way catalyst on the downstream side.

[0006]

Generally, a three-way catalyst is
In addition to a catalyst metal such as platinum, a co-catalyst component such as ceria is supported, and the co-catalyst component enhances the oxygen absorption capacity. By having this oxygen absorption capacity,
The three-way catalyst can exhibit good exhaust gas purification performance even for exhaust gas whose air-fuel ratio deviates to some extent from the stoichiometric air-fuel ratio.

However, if a three-way catalyst having an oxygen absorbing ability is disposed downstream of the NOx absorbent as in the above-mentioned Japanese Patent No. 2600492, the lean combustion in which the NOx absorbent absorbs NOx is performed. During this time, when oxygen is stored in the three-way catalyst and the air-fuel ratio is made rich to release NOx, the stored oxygen is released, resulting in a lean atmosphere in the three-way catalyst. , NOx cannot be efficiently reduced.

In the technique disclosed in Japanese Patent Application Laid-Open No. 6-66185, when the NOx is released from the NOx absorbent, the air-fuel ratio is made excessively rich. However, actually, the reducing component is excessively supplied. In the early stage of NOx release, some NOx
x is discharged without being reduced on the NOx absorbent. FIG. 7 explains this phenomenon. Assuming that the air-fuel ratio of the internal combustion engine is made rich from an lean combustion state for an appropriate period L1 as shown by the solid line in FIG. Release of NOx and oxygen absorbed in the agent is started. Therefore, the air-fuel ratio at the outlet side of the NOx absorbent is temporarily (L in the figure) as indicated by the broken line in (A).
During (2), the stoichiometric air-fuel ratio is maintained, and then the air-fuel ratio becomes rich. During this section L2, the average air-fuel ratio inside the NOx absorbent is almost the stoichiometric air-fuel ratio. However, since a large amount of NOx and oxygen are desorbed in the NOx absorbent,
The active site of the NOx absorbent is partially lean, so that the NOx is not sufficiently reduced.
Further, in the section L2, the air-fuel ratio on the downstream side of the NOx absorbent is substantially the stoichiometric air-fuel ratio, and the exhaust corresponding to the stoichiometric air-fuel ratio flows into the downstream three-way catalyst. However, if the three-way catalyst is given sufficient oxygen absorption capacity, the catalytically active site partially becomes a lean atmosphere due to the oxygen that has been absorbed as described above, and the reduction capacity is reduced. As a result, NOx cannot be sufficiently reduced. Therefore, as shown in FIG.
In this case, some NOx flows out.

[0009]

In view of the above, the present invention has made the catalyst disposed on the downstream side particularly low in oxygen absorption capacity to reduce the released NOx.

That is, the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect has a function of absorbing and releasing NOx according to the air-fuel ratio of exhaust gas flowing into an exhaust passage of the internal combustion engine.
And a second catalyst having a reduced oxygen absorbing ability is provided downstream of the exhaust purification catalyst.

In this case, NOx discharged while the internal combustion engine is performing lean combustion is absorbed by the first exhaust gas purifying catalyst. The absorbed NOx is released when the air-fuel ratio of the internal combustion engine is made rich, and is reduced on the first exhaust gas purification catalyst. At this time, some of the NOx not reduced by the first exhaust gas purifying catalyst is:
It is reduced by the downstream second catalyst. Since the second catalyst has a low oxygen absorbing ability, the second catalyst is efficiently reduced without partially becoming a lean atmosphere due to the release of the oxygen accumulated during the lean combustion.

According to a second aspect of the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine, wherein the first exhaust gas purifying catalyst has a function of absorbing and releasing NOx in accordance with an air-fuel ratio of exhaust gas flowing into an exhaust passage of the internal combustion engine. A second catalyst located downstream of the exhaust gas purification catalyst, and a third catalyst located further downstream of the second catalyst. The oxygen absorption capacity of the second catalyst is It is characterized in that it is smaller than the oxygen absorption capacity of the third catalyst.

According to a third aspect of the invention, which is a further embodiment of the second aspect, the third catalyst has a sufficient oxygen absorbing ability, and the second catalyst has an oxygen absorbing ability. It is characterized in that the absorption capacity is made as small as possible.

As described above, in the configuration in which the third catalyst having a high oxygen absorbing ability is provided further downstream of the second catalyst, the first catalyst is provided.
Excess reduction components not used for NOx reduction in the exhaust gas purification catalyst and the second catalyst can be purified by the third catalyst having a high oxygen absorption capacity.

[0015] Catalyst metals such as platinum, rhodium and palladium generally used as catalysts themselves have some oxygen absorbing ability. By adding a co-catalyst component such as cerium, the oxygen absorbing ability can be further enhanced. According to a fourth aspect of the present invention, which further embodies the first to third aspects of the present invention, the second catalyst does not carry a promoter component for enhancing the oxygen absorbing ability. In other words, even in this case, the oxygen absorbing ability does not become completely zero, but only the oxygen absorbing ability of the catalytic metal itself such as platinum.

The invention according to claim 5 further comprises air-fuel ratio enrichment means for positively enriching the air-fuel ratio in order to desorb and purify NOx from the first exhaust gas purification catalyst. The air-fuel ratio at the time of is set to an excessively rich air-fuel ratio so as to provide a reduction component equal to or greater than a reduction component necessary for desorbing and purifying NOx from the first exhaust gas purification catalyst. .

Further, according to a sixth aspect of the present invention, which is a further embodiment of the fifth aspect, the reducing component necessary for the NOx desorption purification is desorbed from the first exhaust gas purification catalyst with enrichment. It is characterized in that the concentration of released NOx is predicted and determined in accordance with this concentration.

As described above, the reduction of NOx released from the first exhaust gas purification catalyst can be performed by the first exhaust gas purification catalyst and the second exhaust gas purification catalyst by setting the air-fuel ratio to be excessively rich at the time of NOx desorption purification. 2 can be performed reliably.

[0019]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, a conventional three-way catalyst having an oxygen absorbing ability is arranged downstream of the first exhaust gas purifying catalyst capable of absorbing NOx. In comparison, a part of the NOx which is released when the air-fuel ratio is made rich and is not completely reduced on the first exhaust gas purifying catalyst is more reliably reduced by the second catalyst on the downstream side. Can be.

Further, according to the second and third aspects of the present invention, the excess reduction component not used for NOx reduction in the first exhaust gas purifying catalyst and the second catalyst is removed by the first exhaust gas purifying catalyst and the second catalyst having a high oxygen absorption capacity. The catalyst can be purified by the catalyst of No. 3.

According to the fifth and sixth aspects of the present invention, NOx can be more reliably reduced by excessively providing a reducing component.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows the structure of an exhaust gas purifying apparatus according to the present invention. The exhaust gas purifying apparatus absorbs NOx according to the air-fuel ratio of exhaust gas flowing into an exhaust passage 2 of an internal combustion engine 1 comprising a spark ignition gasoline engine. A first exhaust gas purifying catalyst 3 having a releasing action is interposed, and a second catalyst 4 having a reduced oxygen absorption capacity is interposed downstream of the first exhaust gas purifying catalyst 3. Further, a third catalyst 5 having a sufficiently large oxygen absorbing capacity is interposed downstream of the second catalyst 4.

The first exhaust gas purifying catalyst 3 is composed of, for example, platinum Pt,
Based on a catalyst supporting a noble metal such as palladium Pd or rhodium Rh, at least one component selected from an alkaline earth metal represented by barium Ba or an alkali metal represented by cesium Cs is supported thereon. When the exhaust air-fuel ratio is lean, it absorbs NOx in the exhaust, and when the exhaust air-fuel ratio is rich, it releases NOx absorbed by reducing components (HC, CO, H2, etc.) in the exhaust. At the same time as reducing and purifying.

Further, the second catalyst 4 and the third catalyst 5
Can be used, for example, on a honeycomb carrier coated with alumina, platinum Pt, palladium Pd, rhodium Rh
And the like, and the third catalyst 5
Further, in order to enhance the oxygen absorption capacity, for example, cerium Ce is supported as a promoter component. With this cerium, the third catalyst 5 absorbs oxygen in the exhaust gas when the exhaust air-fuel ratio is lean, and reduces, ie, absorbs, the inflowing reducing component with the oxygen that has been absorbed in the rich condition. It exerts an oxygen absorption capacity (oxygen storage capacity) that releases the oxygen according to the inflowing reducing component. Further, in the second catalyst 4, a co-catalyst component such as cerium having an oxygen absorbing ability is reduced as much as possible, and desirably, the use of the co-catalyst component is zero.

On the other hand, a fuel injection valve 7 for injecting fuel toward an intake port of each cylinder is provided in an intake passage 6 of the internal combustion engine 1, and a throttle valve 11, an air flow meter 8, and an air cleaner 12 are sequentially arranged. It is interposed. The air flow meter 8 detects an intake air amount Qa of the internal combustion engine 1, and a detection signal is input to the engine control unit 10. Also,
Crank angle sensor 9 for detecting the crank angle of internal combustion engine 1
And an accelerator opening sensor 13 for detecting the accelerator opening of the vehicle, and these detection signals are also input to the engine control unit 10. The engine control unit 10 calculates the engine speed Ne and the engine load L from these signals, calculates the basic fuel injection pulse width Tp, and adds various corrections such as the target air-fuel ratio to the final value. The fuel injection pulse width Ti is determined and output to the fuel injection valve 7.

Next, a specific control flow will be described with reference to the flowchart of FIG.

The routine shown in FIG. 2 is repeatedly executed in the engine control unit 10, for example, every 10 ms. First, in step 1, it is determined whether or not the rich spike flag FRS is "1". As described later, the rich spike flag FRS indicates that the first exhaust gas purifying catalyst 3 absorbs a predetermined amount or more of NOx,
When it is determined that the removal and purification of NOx is necessary by giving a rich air-fuel ratio, the value becomes "1", and while this is "1", the target air-fuel ratio is set to the rich air-fuel ratio. Therefore, when this flag FRS is 0, it indicates that operation at a lean air-fuel ratio is possible.

If it is determined in step 1 that FRS ≠ 1, the routine proceeds to step 2, where the target air-fuel ratio TFBYA is obtained from the load L and the engine speed Ne by referring to the map shown in FIG. Note that the value of the target air-fuel ratio TFBYA is shown as an equivalent ratio in this embodiment, and TFBYA =
1 corresponds to the stoichiometric air-fuel ratio. In the next step 3, it is determined from the value of the target air-fuel ratio TFBYA whether or not the operation is lean. Specifically, if the value of TFBYA is less than 1.0, it is determined that the operation is lean.

In the case of the lean operation, the NOx exhausted from the internal combustion engine 1 is absorbed by the first exhaust gas purifying catalyst 3, so that the routine proceeds to steps 4 to 7, where the NOx absorption amount is estimated and rich NO spikes are performed. The necessity of the NOx desorption reduction process is determined. That is, if it is determined in step 3 that the operation is the lean operation, the process proceeds to step 4, and the NOx emission concentration CNOx is calculated from the load L and the engine speed Ne by using FIG.
Ask by referring to the map. The map of FIG. 4 is obtained in advance by an experiment. Then step 5
And the N in the first exhaust gas purifying catalyst 3 at that time.
The Ox absorption amount TNOx is calculated. This is obtained by multiplying the NOx emission concentration CNOx by the intake air amount Qa and the constant K1.
The minute increase of the NOx absorption amount TNOx is obtained, and is added to the previous NOx absorption amount TNOx, thereby sequentially obtaining it. Then, in the next step 6, it is determined whether or not the NOx absorption amount TNOx exceeds a predetermined upper limit value TNOx0. This upper limit value TNOx0 is set to a certain ratio (for example, 1/2) of the saturated NOx absorption amount experimentally obtained in advance for the first exhaust gas purifying catalyst 3. When the catalyst deteriorates, the saturated NOx absorption amount also decreases. Therefore, the deterioration of the catalyst may be estimated by appropriate means, and the upper limit value TNOx0 may be reduced and corrected according to the degree of the deterioration. In step 6, the NOx absorption amount TNOx is changed to the upper limit value TNOx0.
If it is determined that the value exceeds the threshold value, the process proceeds to step 7, and the above-described rich spike flag FRS is set to "1".

When the rich spike flag FRS becomes "1", the process proceeds from step 1 to step 8 and thereafter, and a process for desorbing and removing NOx by rich spike is performed. First, in step 8, the catalyst temperature Tcat of the first exhaust gas purification catalyst 3 is calculated from the load L and the engine speed Ne.
The estimation is performed with reference to the map shown in FIG. Note that it is of course possible to provide a catalyst temperature sensor for direct detection. Next, the routine proceeds to step 9, where the NOx emission concentration RNOx due to the rich spike is estimated. Specifically, the catalyst temperature Tcat and the NOx absorption amount TNOx at that time (which sequentially decreases with release as described later) are used as parameters.
The estimation is made with reference to the map of FIG. 6 obtained in advance by experiments. In step 10, the air-fuel ratio TFBYANOx required for desorption purification of NOx with this NOx release concentration RNOx is calculated as follows:
It is calculated by the following equation.

TFBYANOx = 1 + K3 · RNOx where 1 is the stoichiometric air-fuel ratio, and “K3 · RNO
“x” is the air-fuel ratio required for purifying the released NOx. K3 is a constant for converting the NOx concentration into an air-fuel ratio.

Next, in step 11, the required air-fuel ratio TFBYA is set so as to supply an excessive amount of the reducing component in consideration of the efficiency used for the NOx desorption purification of the supplied reducing component.
NOx is corrected to obtain a final target air-fuel ratio TFBYA. Specifically, the target air-fuel ratio TFBYANOx is multiplied by a constant coefficient (for example, 1.2) larger than 1 to obtain the target air-fuel ratio TFBYANO.

In step 12, the NOx absorption amount TNOx that gradually decreases with the release of NOx is determined. Specifically, the NOx release concentration RNOx is multiplied by the intake air amount Qa and the constant K1 to obtain a small decrease in the NOx absorption amount TNOx, and this is calculated as the previous NOx absorption amount TNOx.
Is obtained sequentially by subtracting

In step 13, this NOx absorption amount TN
It is determined whether Ox has become smaller than 0, and if it has become smaller, the NOx absorption amount TNOx is reset to 0 in step 14 and the flag FRS is set to 0 in step 15.
To end the rich spike.

As described above, the value of the air-fuel ratio TFBYA during the lean operation and during the rich spike is determined. The actual fuel injection amount (the injection pulse width applied to the fuel injection valve 7) Ti Is the basic fuel injection amount Tp calculated from the intake air amount Qa and the engine speed Ne,
The air-fuel ratio TFBYA is multiplied and the invalid injection pulse width Ts is added to obtain the following equation.

Ti = Tp · TFBYA + Ts As described above, in the above embodiment, NOx
When the absorption amount TNOx reaches a certain value, the air-fuel ratio is forcibly enriched, and the absorbed NOx is desorbed.
Most of the desorbed and released NOx is purified on the first exhaust gas purification catalyst 3. Further, the first exhaust gas purifying catalyst 3
NOx not reduced by the second catalyst 4 is reduced in the second catalyst 4 on the downstream side. Here, since the second catalyst 4 has a very small oxygen absorbing ability, the atmosphere does not locally become a lean atmosphere due to the release of oxygen, and the NOx flowing into the second catalyst 4 is efficiently reduced. . Moreover, as described above, the air-fuel ratio is set so as to be greater than the required reduction component with respect to the concentration RNOx of the released NOx, so that the NOx can be reduced more reliably. Excess reduction components not used in the reduction of NOx in the first exhaust gas purification catalyst 3 and the second catalyst 4 are sufficiently purified by the third catalyst 5 having an oxygen absorbing ability.

In the above embodiment, the fuel is injected toward the intake port. However, the present invention can be similarly applied to a direct injection type internal combustion engine which injects fuel directly into the combustion chamber. It is.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing an embodiment of an exhaust gas purification device according to the present invention.

FIG. 2 is a flowchart showing a flow of the control.

FIG. 3 shows an air-fuel ratio T with respect to a load L and an engine speed Ne.
FIG. 4 is a characteristic diagram showing a map of FBYA.

FIG. 4 is a characteristic diagram showing a map of a NOx emission concentration CNOx with respect to a load L and an engine speed Ne.

FIG. 5 is a characteristic diagram showing a map of a catalyst temperature Tcat with respect to a load L and an engine speed Ne.

FIG. 6 shows a catalyst temperature Tcat and a NOx absorption amount TNOx.
FIG. 4 is a characteristic diagram showing a map of the NOx release concentration RNOx with respect to FIG.

FIG. 7 is a characteristic diagram showing a conventional air-fuel ratio change (A) and NOx emission concentration (B) due to a rich spike.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 3 ... 1st exhaust gas purification catalyst 4 ... 2nd catalyst 5 ... 3rd catalyst 7 ... Fuel injection valve

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Naganuma required Nissan Motor Co., Ltd. 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Prefecture (72) Inventor Takashi Aoyama 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. Terms (Reference) 3G091 AA02 AA12 AA17 AA23 AA24 AB03 AB06 BA14 BA33 CB02 DA04 DB06 DB10 DB13 DC06 EA01 EA05 EA07 EA18 FB10 FB11 FB12 FC02 GA06 GA19 GB01X GB02W GB03W GB05W GB06W GB07W GB10HA3GB01 HA08HA10HAB08HA11 MA01 MA11 NA04 NA08 NC02 ND42 NE01 NE06 NE13 NE14 NE15 PA01A PA01B PD12A PD12B PE01A PE01B PE03A PE03B PF03A PF03B

Claims (6)

[Claims] A first exhaust purification catalyst having an action of absorbing and releasing NOx in accordance with an air-fuel ratio of exhaust gas flowing into an exhaust passage of an internal combustion engine is provided, and a first exhaust purification catalyst is provided downstream of the exhaust purification catalyst. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a second catalyst having a reduced oxygen absorption capacity. 2. A first exhaust purification catalyst having a function of absorbing and releasing NOx in accordance with an air-fuel ratio of exhaust gas flowing into an exhaust passage of an internal combustion engine, and a first exhaust purification catalyst located downstream of the first exhaust purification catalyst. A second catalyst, and a third catalyst located further downstream of the second catalyst, wherein the oxygen absorption capacity of the second catalyst is smaller than the oxygen absorption capacity of the third catalyst. An exhaust purification device for an internal combustion engine. 3. The method according to claim 1, wherein the third catalyst has a sufficient oxygen absorbing capacity, and the second catalyst has a reduced oxygen absorbing capacity as much as possible. Item 3. An exhaust gas purification device for an internal combustion engine according to Item 2. 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the second catalyst does not carry a promoter component for enhancing oxygen absorption capacity. . 5. An air-fuel ratio enriching means for positively enriching an air-fuel ratio to desorb and purify NOx from the first exhaust gas purifying catalyst, wherein the air-fuel ratio at the time of the enrichment is: The air-fuel ratio is set to an excessively rich air-fuel ratio so as to provide a reduction component equal to or greater than a reduction component necessary for desorbing and purifying NOx from the first exhaust gas purification catalyst. An exhaust purification device for an internal combustion engine according to any one of the above. 6. The reduction component required for NOx desorption purification is determined by predicting the concentration of NOx desorbed from the first exhaust purification catalyst with enrichment and determining the concentration corresponding to the concentration. The exhaust gas purifying apparatus for an internal combustion engine according to claim 5, characterized in that: