JP3449174B2 - 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 purification device for an internal combustion engine, and more particularly to a technique for improving the NOx purification rate in an internal combustion engine having an NOx absorption catalyst in an exhaust passage.

[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.
An engine provided with a NOx absorption catalyst (NOx absorption reduction type three-way catalyst) that releases x and performs reduction processing is known.

Further, in the exhaust gas purifying apparatus having the NOx absorbing catalyst, there is also known a structure in which a three-way catalyst is provided on each of the upstream side and the downstream side of the NOx absorbing catalyst (Japanese Patent Laid-Open No. 8-270440, etc.). reference).

[0004]

In the above NOx absorption catalyst, when the air-fuel ratio of the combustion mixture is switched from lean to stoichiometric or rich and the exhaust air-fuel ratio begins to become rich,
The NOx absorbed during the lean combustion until then is desorbed, and this desorbed NOx is reduced by the three-way layer of the NOx absorption catalyst or the three-way catalyst behind.

However, due to the oxygen storage capacity of the catalyst, a large amount of oxygen is accumulated in the NOx absorption catalyst and the three-way catalyst arranged upstream of the NOx absorption catalyst during lean combustion. Even if the air-fuel ratio is switched by desorption of NOx, the exhaust air-fuel ratio inside the catalyst and downstream of the catalyst does not easily become rich, and further, HC, which is necessary for NOx reduction,
Since CO is oxidized by the oxygen accumulated during the lean combustion, it is not possible to efficiently reduce NOx in the NOx absorption catalyst or the three-way catalyst arranged downstream of the NOx absorption catalyst. was there.

Here, the inside of the catalyst is enriched at an early stage so that N
In order to improve the Ox purification rate, it is necessary to give a large rich spike, but in this case, the fuel consumption deteriorates and
The problem arises that the amount of HC and CO emissions is increased. The oxygen storage capacity of the catalyst mainly changes depending on the amount of ceria Ce supported on the catalyst, but conventionally, the supported amount of ceria per unit volume is approximately the same for each catalyst, and the catalyst capacity based on the required purification performance is The amount of ceria supported on each catalyst is determined by the setting, and the above NO
x It was not set in consideration of the effect of oxygen that is simultaneously desorbed during desorption.

The present invention has been made in view of the above circumstances, and in an exhaust emission control device including at least a NOx absorption catalyst and a three-way catalyst arranged downstream thereof, the air-fuel ratio is switched from lean to stoichiometric or rich. NOx instead
The purpose is to improve the processing capacity of NOx when desorbed.

[0008]

Therefore, according to the invention described in claim 1, NO in exhaust gas when the exhaust air-fuel ratio is lean.
NO that absorbs x and releases the absorbed NOx when the exhaust air-fuel ratio is stoichiometric or rich
In an exhaust gas purification apparatus for an internal combustion engine, which comprises at least an x-absorbing catalyst and a three-way catalyst arranged on the downstream side of the NOx absorbing catalyst, the amount of components contributing to the oxygen storage capacity carried by the three-way catalyst is On the other hand, the total amount of the components carried by the catalyst on the upstream side of the three-way catalyst is set to about 1.2 times or less.

According to this structure, when the catalyst is not arranged upstream of the NOx absorption catalyst, the amount of the component that contributes to the oxygen storage capacity of the NOx absorption catalyst is
It is set to be approximately 1.2 times or less than the amount carried by the three-way catalyst arranged on the downstream side of the NOx absorption catalyst.
When the catalyst is arranged upstream of the NOx absorption catalyst,
The total amount of the carried amount of the NOx absorption catalyst and the carried amount of the catalyst arranged on the upstream side of the NOx absorption catalyst is NO.
The amount is set to be about 1.2 times or less than the amount supported by the three-way catalyst arranged on the downstream side of the x absorption catalyst.

On the other hand, the invention according to claim 2 absorbs NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich. NOx absorption catalyst for reduction treatment by
In an exhaust gas purification apparatus for an internal combustion engine, which comprises at least a three-way catalyst arranged on the downstream side of an Ox absorption catalyst, a catalyst on the upstream side of the three-way catalyst is higher than the oxygen storage capacity of the three-way catalyst. It is characterized by having a low oxygen storage capacity.

According to this structure, when the catalyst is not arranged upstream of the NOx absorption catalyst, the oxygen storage capacity of the NOx absorption catalyst is higher than that of the three-way catalyst arranged downstream of the NOx absorption catalyst. Set low, also
When the catalyst is arranged upstream of the NOx absorption catalyst,
The sum of the oxygen storage capacities of the NOx absorption catalyst and the catalyst arranged upstream of the NOx absorption catalyst is NOx.
It is set lower than the three-way catalyst arranged on the downstream side of the absorption catalyst.

According to a third aspect of the present invention, in the structure of the second aspect, the catalyst on the upstream side of the three-way catalyst with respect to the amount of the component that contributes to the oxygen storage capacity carried by the three-way catalyst. The total amount of the above-mentioned components carried by is set to about 0.6 times or less. According to this configuration, the setting for lowering the oxygen storage capacity of the catalyst on the upstream side of the three-way catalyst relative to the oxygen storage capacity of the three-way catalyst is defined by the ratio of the component amounts that contribute to the oxygen storage capacity. And a particularly preferable range is specified as approximately 0.6 times or less.

According to the invention of claim 4, claim 1 or 3
In the above configuration, the total amount of the components carried by the catalyst on the upstream side of the three-way catalyst is set to approximately 0.2 times or more the amount of the component contributing to the oxygen storage capacity carried by the three-way catalyst. . According to such a configuration, in the configuration that regulates the maximum value of the ratio of the component amount that contributes to the oxygen storage capacity, the minimum ratio is set to approximately 0.2 times or more, and the oxygen storage capacity of the NOx absorption catalyst and the catalyst upstream of the NOx absorption catalyst is Specify the minimum required level.

According to the fifth aspect of the invention, the amount of ceria Ce is set as a component that contributes to the oxygen storage capacity. According to this configuration, the ceria C carried by the NOx absorption catalyst and the catalyst upstream of the NOx absorption catalyst is carried out.
The amount of e is regulated on the basis of the amount of ceria Ce carried by the three-way catalyst on the downstream side of the NOx absorption catalyst.

According to the sixth aspect of the invention, the three-way catalyst is arranged upstream of the NOx absorption catalyst. With this configuration, the catalysts are arranged in this order from the upstream side to the three-way catalyst, the NOx absorption catalyst, and the three-way catalyst, and the oxygen storage in the upstream side three-way catalyst and the NOx absorption catalyst is based on the downstream side three-way catalyst. Capacity (the amount of components (ceria) that contribute to oxygen storage capacity) is limited.

In a seventh aspect of the present invention, the NOx absorption catalyst contains more barium Ba than the three-way catalyst, so that the barium Ba is used as a NOx absorbent to absorb NOx. According to this structure, even if the NOx absorption catalyst and the downstream three-way catalyst both carry barium Ba, the NOx absorption catalyst carries more barium Ba for the purpose of NOx absorption. The catalyst is distinguished from a normal three-way catalyst, and the oxygen storage capacity (the amount of the component (ceria) that contributes to the oxygen storage capacity) of each catalyst is determined based on this distinction.

[0017]

According to the first aspect of the present invention, when the lean combustion state is switched to stoichiometric or rich, the amount of oxygen desorbed from the NOx absorption catalyst and the catalyst arranged upstream of the NOx absorption catalyst is increased. There is an effect that the amount can be limited to a necessary and sufficient amount, and the NOx absorption catalyst and the downstream three-way catalyst can be enriched at an early stage to efficiently perform the NOx reduction process.

According to the second aspect of the present invention, lean combustion is achieved by lowering the oxygen storage capacity of the catalyst including the NOx reduction catalyst on the upstream side of the oxygen storage capacity of the three-way catalyst on the downstream side of the NOx absorption catalyst. When NOx is desorbed by switching from the state to stoichiometric or rich, the amount of oxygen desorbed at the same time can be surely limited, so that the reduction process of NOx can be efficiently performed. .

According to the third aspect of the present invention, the amount of oxygen desorbed from the NOx absorption catalyst and the catalyst arranged upstream of the NOx absorption catalyst when the lean combustion state is switched to stoichiometric or rich. There is an effect that the NOx reduction treatment can be performed most efficiently by reducing the NOx reduction most effectively. According to the invention described in claim 4, there is an effect that the exhaust gas purification performance during normal stoichiometry can be secured while limiting the amount of oxygen desorbed at the same time when NOx is desorbed.

According to the fifth aspect of the present invention, by adjusting the amount of ceria supported on the catalyst, it is possible to set the oxygen storage capacity for efficiently performing the NOx reduction process. According to the sixth aspect of the present invention, the three-way catalyst is also provided on the upstream side of the NOx reduction catalyst, so that the exhaust purification at the start can be promoted and the NO
There is an effect that the reduction process of the NOx absorbed by the x absorption catalyst can be efficiently performed.

According to the seventh aspect of the present invention, by including more barium than a normal three-way catalyst, the barium is used as a NOx absorbent, and a NOx absorption catalyst that absorbs NOx during lean combustion is provided. The NOx
There is an effect that the reduction process of NOx absorbed by the absorption catalyst can be efficiently performed.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a system configuration of an internal combustion engine in an embodiment, in which the engine 1 sucks air measured by a throttle valve 2 and injects fuel injected from a fuel injection valve 3 and the intake air. Are mixed to form an air-fuel mixture.

The fuel injection valve 3 may be one that injects fuel into the intake port portion or one that directly injects fuel into the combustion chamber. The air-fuel mixture is ignited and burned by spark ignition by the spark plug 4, and the combustion exhaust gas is purified by a three-way catalyst (light-off catalyst) 6, a NOx absorption catalyst 7, and a three-way catalyst 8 which are provided in the exhaust passage 5. Are released into the atmosphere.

The three-way catalysts 6 and 8 are both catalysts that simultaneously purify NOx, CO, and HC in a stoichiometric (theoretical air-fuel ratio) atmosphere, and the three-way catalyst 6 is directly below the exhaust manifold for the purpose of purifying exhaust gas at the time of starting. The three-way catalyst 8 is arranged in the underfloor portion of the vehicle together with the NOx absorption catalyst 7. The NOx absorption catalyst 7 absorbs NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich to form a three-way catalyst layer. It is a catalyst (NOx absorption reduction type three-way catalyst) for reduction treatment.

The NOx absorption catalyst 7 and the three-way catalyst 6, 8
All carry barium Ba, but the NOx absorption catalyst 7
In order to use barium Ba as a NOx absorbent, the amount of barium Ba supported on the three-way catalyst 8 is about twice as much as the amount of barium Ba supported on the three-way catalyst 8. The NOx absorption catalyst is distinguished from an ordinary three-way catalyst by. However, the NOx absorbent is not limited to barium Ba.

The control unit 9 for controlling the injection timing / injection amount by the fuel injection valve 3 and the ignition timing by the spark plug 4 is constituted by including a microcomputer and is operated by arithmetic processing based on detection signals from various sensors. , A fuel injection signal (injection pulse signal) is output to the fuel injection valve 3, and a spark plug 4 (power transistor)
An ignition signal is output to.

In the calculation of the fuel injection signal, the target air-fuel ratio is determined according to the operating condition, and the fuel injection amount (injection pulse width) is calculated so that the air-fuel mixture having the target air-fuel ratio is formed. As the target air-fuel ratio, a lean air-fuel ratio is set in addition to the stoichiometric air-fuel ratio (stoichiometric ratio) and the rich air-fuel ratio. In the present application, both rich and lean are based on the theoretical air-fuel ratio.

As the various sensors, an air flow meter 10 for detecting the intake air flow rate of the engine 1, a throttle sensor 11 for detecting the opening of the throttle valve 2 and the like are provided, and the control unit 9 has a crank angle (not shown). A rotation signal or the like from the sensor is input. The NOx
The absorption catalyst 7 absorbs NOx during lean combustion,
When the target air-fuel ratio is switched to the stoichiometric air-fuel ratio (stoichiometric) or rich, NOx absorbed up to that time is desorbed, and the desorbed NOx is removed from the three-way catalyst layer and the downstream side of the NOx absorption catalyst 7. Reduction treatment is performed by the three-way catalyst 8.

Here, if the lean combustion state continues, N
Since the amount of NOx absorbed by the Ox absorption catalyst 7 reaches the limit, NOx cannot be absorbed any more, and NOx discharged from the engine 1 is discharged as it is. Therefore, from the load, rotation, air-fuel ratio, etc. The amount of NOx absorbed is estimated, and when it is estimated that the limit has been reached, the air-fuel ratio is forcibly controlled temporarily to the stoichiometric air-fuel ratio or rich.

The NOx absorption catalyst 7 and the three-way catalysts 6 and 8 each carry ceria Ce as a component that contributes to the oxygen storage capacity, and accumulate a large amount of oxygen during lean combustion. However, it is not limited to the configuration in which ceria Ce is supported as a component that contributes to the oxygen storage capacity. When the air-fuel ratio is switched from lean to the theoretical air-fuel ratio (stoichiometric) or rich, the accumulated oxygen is desorbed, and the oxygen desorbed in the NOx absorption catalyst 7 and the three-way catalyst 8 is removed. Existence and three-way catalyst 6
And N that reduces the desorbed NOx due to the presence of oxygen desorbed from the NOx absorption catalyst 7 and flowing downstream.
The atmosphere of the Ox absorption catalyst 7 and the three-way catalyst 8 does not easily become rich, and NOx cannot be reduced efficiently.

Therefore, in this embodiment, each of the catalysts 6 to 8 is
Ceria, a component that contributes to the oxygen storage capacity of
The amount per unit volume of is set as follows. The amount of conventional ceria Ce per unit volume is 6
8 to 8 are substantially the same, the amount of ceria in each of the catalysts 6 to 8 is determined by the catalyst capacity, and the sum of the ceria amount A of the three-way catalyst 6 and the ceria amount B of the NOx absorption catalyst 7 is three. The amount C of ceria in the original catalyst 8 was about 2.4 times.

With respect to the above setting, the ceria amount C of the three-way catalyst 8
The total amount A + B of ceria in the three-way catalyst 6 and the NOx absorption catalyst 7 is reduced while keeping the above condition as it is. Ratio ((A + B)
It has been experimentally confirmed that the NOx emission amount at the time of switching from lean combustion to stoichiometric (or rich) decreases as / C) is decreased, as shown in FIG.

The reduction of the amount of ceria A + B is realized by reducing the amount of ceria per unit volume. This is because the three-way catalyst 6 and the N
The oxygen storage capacity of the Ox absorption catalyst 7 decreases,
Even if the lean combustion is switched to stoichiometric (or rich), the amount of oxygen desorbed is small, so the NOx absorption catalyst 7
Also, the atmosphere in the three-way catalyst 8 becomes rich relatively quickly and becomes a reducing atmosphere, so that NOx can be reduced efficiently.

The NOx emission amount when switching from lean combustion to stoichiometric (or rich) is the above ratio (A +
B) / C decreases as B / C decreases (see FIG. 2), but as shown in FIG. 3, if the ceria amount A + B is decreased too much, the NOx purification rate in the normal stoichiometric combustion state decreases and N
Since the Ox emission amount is increased, and in order to suppress the NOx emission amount in the stoichiometric range within the allowable value, the lower limit of (A + B) / C of the ratio needs to be approximately 0.2.

On the other hand, as shown in FIG. 4, in a predetermined traveling mode (for example, in the 10-15 mode), the amount of NOx discharged during lean-to-stoichi switching and the amount of NOx discharged in the stoichiometric state are included. The total amount of NOx emissions is
If the ratio (A + B) / C is reduced from the conventional value of about 2.4 to about 1.2, it will be reduced sufficiently (for example, half the amount of the conventional amount), and the ratio (A / B) / C will be within the range of about 0.2 to 1.2 times. By setting A + B) / C, the total Nx is increased without increasing the NOx emission amount during normal stoichiometric combustion.
Ox emissions can be effectively reduced.

When the ratio (A + B) / C is further reduced from about 1.2, as shown in FIG.
Although the amount of NOx discharged at the time of desorption of Ox decreases, the amount of NOx does not decrease further even if it is about 0.6 or less. Therefore, the most efficient NOx purification can be achieved by the ratio (A + B) /
This is the case where C is set within the range of approximately 0.2 to 0.6. As described above, when the ratio (A + B) / C is set within the range of approximately 0.2 to 0.6, the oxygen storage capacity of the three-way catalyst 6 and the NOx absorption catalyst 8 is higher than the oxygen storage capacity of the three-way catalyst 8. This means that the total sum of abilities has been lowered.

In the above embodiment, the three-way catalyst 6 is provided on the upstream side of the NOx absorption catalyst 7, but the three-way catalyst 6 may not be provided. In this case, B /
The ratio of C is within the range of about 0.2 to 1.2, preferably about 0.2.
The amount of ceria per unit volume in the NOx absorption catalyst 7 may be adjusted so that it falls within the range of to 0.6.

[Brief description of drawings]

FIG. 1 is a system configuration diagram showing an internal combustion engine of an embodiment.

FIG. 2 is a diagram showing a correlation between a NOx emission amount and a ceria amount ratio when switching from lean to stoichiometric.

FIG. 3 is a diagram showing a correlation between a NOx emission amount and a ceria amount ratio during stoichiometric combustion.

FIG. 4 is a diagram showing a correlation between a NOx emission amount and a ceria amount ratio in a predetermined traveling mode.

[Explanation of symbols]

1 Internal combustion engine 2 Throttle valve 3 Fuel injection valve 4 Spark plug 5 exhaust passage 6 three-way catalyst 7 NOx absorption catalyst 8 three-way catalyst 9 Control unit 10 Air flow meter 11 Throttle sensor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI F01N 3/24 F01N 3/24 E B01D 53/36 101B 104A (56) Reference JP-A-8-270440 (JP, A) Kaihei 8-319822 (JP, A) JP-A 8-57315 (JP, A) Actual Kaihei 4-34425 (JP, U) International Publication 93/08383 (WO, A1) (58) Fields investigated (Int .Cl. ⁷ , DB name) F01N 3/08-3/28 B01D 53/94 B01J 23/04 B01J 23/10

Claims (7)

(57) [Claims] 1. A NOx absorption catalyst that absorbs NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the exhaust air-fuel ratio is the theoretical air-fuel ratio or is rich. In an exhaust gas purification device for an internal combustion engine, which comprises at least a three-way catalyst arranged on the downstream side of the NOx absorption catalyst, the amount of a component contributing to the oxygen storage capacity carried by the three-way catalyst is An exhaust emission control device for an internal combustion engine, wherein the total amount of the components carried by the catalyst upstream of the three-way catalyst is set to approximately 1.2 times or less. 2. A NOx absorption catalyst that absorbs NOx in the exhaust when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or is rich. In an exhaust gas purification device for an internal combustion engine, which comprises at least a three-way catalyst arranged on the downstream side of the NOx absorption catalyst, the oxygen storage capacity of the three-way catalyst is more upstream than the three-way catalyst. An exhaust emission control device for an internal combustion engine, characterized in that the oxygen storage capacity of the catalyst is set low. 3. The total amount of the components carried by the catalyst upstream of the three-way catalyst is set to about 0.6 times or less the amount of the components contributing to the oxygen storage capacity carried by the three-way catalyst. The exhaust gas purification device for an internal combustion engine according to claim 2, wherein 4. The total amount of the components carried by the catalyst on the upstream side of the three-way catalyst is set to approximately 0.2 times or more the amount of the components contributing to the oxygen storage capacity carried by the three-way catalyst. An exhaust emission control device for an internal combustion engine according to claim 1 or 3, characterized in that. 5. The exhaust gas purification device for an internal combustion engine according to claim 1, 3 or 4, wherein the amount of ceria Ce is set as a component that contributes to the oxygen storage capacity. 6. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein a three-way catalyst is also arranged on the upstream side of the NOx absorption catalyst. 7. The NOx absorption catalyst contains more barium Ba than the three-way catalyst, so that the barium Ba is NO.
The exhaust gas purification device for an internal combustion engine according to any one of claims 1 to 6, wherein NOx is absorbed as the x absorbent.