JP3716585B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine including a three-way catalyst and a NOx storage catalyst.
[0002]
[Prior art]
As an exhaust gas purification device for an internal combustion engine, a three-way catalyst capable of efficiently treating HC, CO, and NOx in a stoichiometric operation and an NOx is absorbed in a stoichiometric or rich operation, and NOx is released in a stoichiometric or rich operation. And a NOx occlusion catalyst that performs a reduction treatment by reacting with NOx (see, for example, JP-A-8-270440).
[0003]
By the way, the three-way catalyst is used as a countermeasure against the cold start of the internal combustion engine (to ensure the catalyst conversion efficiency even at the cold start and to reduce the HC), so that it is relatively upstream (for example, the exhaust gas) The direct injection gasoline engine that injects gasoline fuel directly into the combustion chamber is good in thermal efficiency and lowers the exhaust gas temperature, and thus needs to be arranged in this way.
[0004]
On the other hand, since the NOx storage catalyst has poor heat resistance, such as thermal degradation of the absorbent material at high loads, it is necessary to arrange it as a so-called underfloor catalyst relatively downstream of the exhaust passage.
Therefore, it is necessary to arrange a three-way catalyst upstream of the exhaust passage and a NOx storage catalyst downstream.
[0005]
[Problems to be solved by the invention]
However, when the three-way catalyst is disposed upstream of the exhaust passage and the NOx storage catalyst is disposed downstream, the released NOx is reduced when NOx is released from the NOx storage catalyst during stoichiometric or rich operation. However, there is a problem that the HC necessary for the purification is purified by the upstream three-way catalyst, and the NOx reduction treatment by the NOx storage catalyst becomes insufficient.
[0006]
In view of such conventional problems, the present invention is intended to ensure sufficient NOx reduction treatment performance when releasing NOx from the NOx storage catalyst downstream of the three-way catalyst during stoichiometric or rich operation. Objective.
[0007]
[Means for Solving the Problems]
For this reason, in the invention according to claim 1, in the internal combustion engine provided with the three-way catalyst in the exhaust passage, and on the downstream side thereof, the NOx storage catalyst that absorbs NOx during the lean operation and releases NOx during the stoichiometric or rich operation. The cell density on the exhaust passage center side of the catalyst carrier of the three-way catalyst is smaller than the cell density on the exhaust passage peripheral side (outer peripheral side).
[0008]
By doing in this way, the following operations are obtained.
If the cell density on the center side of the catalyst carrier of the three-way catalyst is made smaller than the cell density on the peripheral side, the catalyst surface area on the center side can be reduced and the conversion efficiency on the center side can be lowered.
On the other hand, in an internal combustion engine capable of switching between lean operation and stoichiometric or rich operation, switching to stoichiometric or rich operation is performed when the air flow rate (and hence the exhaust flow rate) is relatively high, such as during acceleration.
[0009]
When the exhaust flow rate is small, the flow rate difference between the center side and the peripheral side of the catalyst is small and the exhaust gas flows uniformly. However, when the exhaust flow rate is large, the flow rate at the center side of the catalyst is higher than the flow rate at the peripheral side. It becomes very large and the ratio of flowing through the center increases.
Therefore, by reducing the cell density on the center side of the three-way catalyst and reducing the conversion efficiency on the center side, the exhaust flow rate becomes large and the ratio of flowing through the center side of the three-way catalyst increases. During the stoichiometric or rich operation, the conversion efficiency of the three-way catalyst can be substantially reduced.
[0010]
For this reason, when NOx is released from the NOx storage catalyst during stoichiometric or rich operation, a certain amount of HC is supplied from the three-way catalyst having reduced upstream conversion efficiency, and it is possible to ensure NOx reduction treatment performance. Become.
In addition, since the cell density on the catalyst center side where a large amount of exhaust flows during acceleration or the like is small, the pressure loss of the exhaust system is reduced, and the output can be improved.
[0011]
In the invention according to claim 2, the catalyst carrier of the three-way catalyst is formed by alternately laminating flat plates and corrugated plates, and the wave pitch of the corrugated plates is on the exhaust passage peripheral side with respect to the central portion of the exhaust passage. and larger, wherein the varying cell density.
The invention according to claim 3 is characterized in that the cell density is changed in two steps on the exhaust passage center side and the exhaust passage periphery side.
[0012]
The invention according to claim 4 is characterized in that the cell density is continuously changed from the exhaust passage center side to the exhaust passage periphery side.
[0013]
【The invention's effect】
According to the first aspect of the present invention, when NOx is released from the NOx storage catalyst on the downstream side of the three-way catalyst during stoichiometric or rich operation, the conversion efficiency of the three-way catalyst is reduced to reduce the HC required for NOx reduction. Can be supplied, and an effect that sufficient NOx reduction treatment performance can be secured is obtained. Moreover, the effect that the pressure loss of the exhaust system can be reduced and the output can be improved is also obtained.
[0014]
According to the second aspect of the present invention, when the cell density is changed, an effect can be easily obtained by changing the wave pitch of the corrugated plate.
According to the third aspect of the present invention, the effect of being easily implemented can be obtained by changing the cell density in two stages.
According to the invention which concerns on Claim 4, the effect that NOx reduction process performance etc. can be improved further by changing a cell density continuously is acquired.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a system diagram of an exhaust system of an automobile internal combustion engine to which the present invention is applied.
The internal combustion engine 1 is, for example, a direct injection gasoline engine that directly injects gasoline fuel into a combustion chamber, and can be switched at least between a lean operation and a stoichiometric operation by controlling an air-fuel ratio. Specifically, as shown in an example of a running pattern in FIG. 4, the lean operation is performed during steady and deceleration, and the operation is switched to the stoichiometric operation during acceleration.
[0016]
A three-way catalyst 3 is disposed in the exhaust passage 2 of the internal combustion engine 1 on the relatively upstream side (for example, the exhaust manifold outlet). The three-way catalyst 3 can efficiently process HC, CO, and NOx during stoichiometric operation.
A NOx occlusion catalyst 4 is disposed relatively downstream (under the floor position) of the exhaust passage 2.
[0017]
The NOx occlusion catalyst 4 absorbs NOx during lean operation, reduces NOx during stoichiometric operation and reacts with HC to reduce the amount of HC. When doing so, the conversion rate improves as the amount of HC increases.
That is, FIG. 5 is a diagram showing a saturated absorption amount when NOx is absorbed by the NOx storage catalyst under a certain flow rate and exhaust gas having HC and NOx concentrations, and the smaller the HC / NOx, That is, the smaller the amount of HC, the better the absorption. FIG. 6 is a graph showing a tendency when the HC concentration to be introduced is changed with respect to the NOx conversion rate at which the absorbed NOx can be converted for a certain period of time. As the HC concentration increases, the NOx conversion rate increases. It is shown that.
[0018]
Here, as shown in FIG. 2 or FIG. 3, for example, the metal carrier for supporting the catalyst of the three-way catalyst 3 includes a flat plate 11 and a corrugated plate 12 made of a metal foil material having a constant thickness (for example, 30 to 50 μm). Are alternately stacked and wound concentrically, and a cell (small space) is formed between adjacent flat plates 11 and 11 for each wave pitch of the corrugated plate 12.
[0019]
Then, the wave pitch of the corrugated plate 12 is made larger at the exhaust passage center side than at the exhaust passage periphery side, the cell density is changed, and the cell density on the center side of the metal carrier of the three-way catalyst 3 is changed to the peripheral side. The cell density is smaller (coarse).
In this case, as shown in FIG. 2, the cell density may be changed in two steps at the central side and the peripheral side, or the cell density is continuously changed from the central side to the peripheral side as shown in FIG. You may make it change to.
[0020]
Further, it is appropriate that the cell density on the center side is 50 to 200 cpsi, and the cell density on the peripheral side is about 400 to 1000 cpsi (cpsi = cell per square inch).
In addition, the metal carrier has a concentric winding shape and an S-shaped winding shape as in this example, and may be applied to an S-shaped winding shape.
[0021]
Next, the operation will be described.
If the cell density on the center side of the metal carrier of the three-way catalyst 3 is made smaller than the cell density on the peripheral side, the catalyst surface area on the center side can be reduced and the conversion efficiency on the center side can be lowered.
On the other hand, switching to the stoichiometric operation is performed when the air flow rate (and hence the exhaust flow rate) is relatively high, such as during acceleration.
[0022]
As shown in FIG. 7, when the exhaust flow rate is small, the flow rate difference between the central side and the peripheral side of the catalyst is small and the exhaust flows uniformly, but when the exhaust flow rate is large, The flow rate on the center side of the catalyst becomes extremely larger than the flow rate on the peripheral side, and the ratio of flowing through the center side increases.
Therefore, by reducing the cell density on the center side of the three-way catalyst 3 and reducing the conversion efficiency on the center side, the exhaust flow rate becomes large, and the ratio of flowing through the center side of the three-way catalyst 3 is increased. At the time of increasing stoichiometric operation, the conversion efficiency of the three-way catalyst 3 can be substantially reduced.
[0023]
For this reason, when NOx is released from the NOx occlusion catalyst 4 during the stoichiometric operation, a certain amount of HC is supplied from the three-way catalyst 3, and the NOx reduction treatment performance can be ensured.
That is, when the exhaust flow rate is relatively small and the flow velocity distribution in the catalyst is uniform as in lean operation, the three-way catalyst 3 converts HC as much as possible to improve the absorption performance of the downstream NOx storage catalyst 4. When the exhaust gas flow rate is relatively large and the central flow rate of the catalyst is high, such as during stoichiometric operation, the HC is supplied to the downstream NOx storage catalyst 4 without being converted by the three-way catalyst 3 as much as possible, and NOx reduction is performed. It promotes the reaction.
[0024]
Further, since the cell density on the catalyst center side where a large amount of exhaust gas flows is small during acceleration in which stoichiometric operation is performed, the pressure loss in the exhaust system is reduced and the output can be improved.
When changing the cell density, it is possible to change the distance between the flat plates 11 and change the wave height of the corrugated sheet 12 that enters between them. However, it is better to change the wave pitch of the corrugated sheet 12. It is relatively easy to implement.
[0025]
Further, when a metal carrier is used, it can be energized, so that it can be configured as a catalyst with a heater (EHC).
[Brief description of the drawings]
FIG. 1 is a system diagram of an exhaust system of an internal combustion engine showing an embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line AA in FIG. 1 showing an example of a cell structure of a three-way catalyst. 1 is a cross-sectional view taken along the line AA in FIG. 1 showing another example of the cell structure. FIG. 4 is a diagram showing an example of a running pattern. FIG. 5 is a diagram showing NOx absorption characteristics of the NOx storage catalyst. Diagram showing characteristics [Fig. 7] Diagram showing flow velocity distribution in catalyst [Explanation of symbols]
1 Internal combustion engine 2 Exhaust passage 3 Three-way catalyst 4 NOx storage catalyst 11 Flat plate 12 Corrugated plate

Claims (4)

In an exhaust gas purification apparatus for an internal combustion engine comprising a three-way catalyst in an exhaust passage and having a NOx occlusion catalyst that absorbs NOx during lean operation and releases NOx during stoichiometric or rich operation on the downstream side thereof,
An exhaust gas purification apparatus for an internal combustion engine, characterized in that the cell density on the exhaust passage center side of the catalyst carrier of the three-way catalyst is smaller than the cell density on the exhaust passage periphery side. The catalyst carrier of the three-way catalyst is formed by alternately laminating flat plates and corrugated plates, and the wave density of the corrugated plates is larger on the central side of the exhaust passage than on the peripheral side of the exhaust passage, so that the cell density is increased. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is changed. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the cell density is changed in two stages at the exhaust passage center side and the exhaust passage periphery side. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the cell density is continuously changed from the exhaust passage center side to the exhaust passage periphery side.

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