JPH112114A - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

(57) [PROBLEMS] To efficiently reduce NOx absorbed by a NOx absorption catalyst during lean combustion when the air-fuel ratio is switched to a stoichiometric air-fuel ratio or rich. A three-way catalyst is arranged on each of an upstream side and a downstream side of a NOx absorption catalyst. Then, assuming that the amount of ceria carried by the NOx absorption catalyst is B, the amount of ceria carried by the upstream three-way catalyst is A, and the amount of ceria carried by the downstream three-way catalyst is C, A + B) / C is approximately 0.2 to 1.2
, More preferably in the range of about 0.2 to 0.6, the amount of ceria per unit volume in the NOx absorption catalyst and the upstream three-way catalyst is adjusted. (A + B) /
By limiting the maximum value of C, the oxygen storage capacity is limited, the amount of oxygen released simultaneously with the release of NOx is reduced, and NOx is efficiently reduced. In addition, (A)
By limiting the minimum value of + B) / C, it is possible to avoid a decrease in the NOx purification ability at the stoichiometric air-fuel ratio.

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 technique for improving a NOx purifying rate in an internal combustion engine having a NOx absorbing catalyst in an exhaust passage.

[0002]

2. Description of the Related Art Conventionally, NOx in exhaust gas has been absorbed when the exhaust air-fuel ratio is lean, and the absorbed NOx has been absorbed when the exhaust air-fuel ratio is stoichiometric or rich.
2. Description of the Related Art There is known an engine equipped with a NOx absorption catalyst (NOx absorption reduction type three-way catalyst) that releases x and performs a reduction process.

[0003] Further, in an exhaust gas purifying apparatus provided with the above-mentioned NOx absorption catalyst, a configuration in which a three-way catalyst is provided on the upstream side and the downstream side of the NOx absorption catalyst, respectively, is also known (Japanese Patent Application Laid-Open No. Hei 8-270440). reference).

[0004]

In the above NOx absorption catalyst, when the air-fuel ratio of the combustion air-fuel mixture is switched from lean to stoichiometric or rich, and the exhaust air-fuel ratio starts to become rich,
The NOx absorbed during the lean combustion up to that time is desorbed, and the desorbed NOx is reduced by the three-way layer of the NOx absorption catalyst or the three-way catalyst at the rear.

[0005] However, a large amount of oxygen is accumulated in the NOx absorption catalyst or the three-way catalyst disposed upstream of the NOx absorption catalyst during lean combustion due to the oxygen storage capacity of the catalyst. Even when the air-fuel ratio is switched due to desorption of the exhaust gas, the exhaust air-fuel ratio in the catalyst and downstream of the catalyst does not easily become rich, and furthermore, HC,
Since CO is oxidized by the oxygen accumulated during the lean combustion, the NOx reduction catalyst or the three-way catalyst disposed downstream of the NOx absorption catalyst cannot efficiently reduce NOx. was there.

Here, the inside of the catalyst is enriched at an early
In order to improve the Ox purification rate, it is necessary to give a large rich spike, but in this case, the fuel efficiency deteriorates, and
There is a problem that the emission amount of HC and CO is increased. The oxygen storage capacity of the catalyst varies depending on the amount of ceria Ce mainly supported on the catalyst. Conventionally, the amount of ceria supported per unit volume is substantially the same for each catalyst, and the catalyst capacity based on the required purification performance is reduced. The ceria carrying amount of each catalyst is determined by the setting, and the above NO
The setting was not made in consideration of the influence of oxygen desorbed at the time of x desorption.

The present invention has been made in view of the above circumstances, and in an exhaust gas purification apparatus including at least a NOx absorption catalyst and a three-way catalyst disposed downstream thereof, the air-fuel ratio is switched from lean to stoichiometric or rich. NOx instead
It is an object of the present invention to improve the processing capacity of NOx when NO is desorbed.

[0008]

Therefore, according to the first aspect of the present invention, when the exhaust air-fuel ratio is lean, the NO
NO that absorbs x 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 apparatus for an internal combustion engine comprising at least an x-absorption catalyst and a three-way catalyst disposed downstream of the NOx absorption catalyst, the amount of a component contributing to the oxygen storage capacity carried by the three-way catalyst is reduced. On the other hand, the total amount of the components carried by the catalyst upstream of the three-way catalyst is set to be approximately 1.2 times or less.

According to such a configuration, when the catalyst is not arranged on the upstream side of the NOx absorption catalyst, the amount of the component contributing to the oxygen storage capacity carried by the NOx absorption catalyst becomes:
The amount is set to be approximately 1.2 times or less the amount carried by the three-way catalyst disposed downstream of the NOx absorption catalyst, and
When the catalyst is arranged on the upstream side of the NOx absorption catalyst,
The total amount of the amount supported on the NOx absorption catalyst and the amount supported on the catalyst disposed upstream of the NOx absorption catalyst is NO.
The amount is set to be approximately 1.2 times or less the amount carried by the three-way catalyst disposed downstream of the x-absorption catalyst.

On the other hand, the invention according to claim 2 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 stoichiometric or rich. A NOx absorption catalyst to be reduced by
In an exhaust gas purification apparatus for an internal combustion engine comprising at least a three-way catalyst disposed downstream of the Ox absorption catalyst, a catalyst upstream of the three-way catalyst may be more upstream than the oxygen storage capacity of the three-way catalyst. The low oxygen storage capacity is set.

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

According to a third aspect of the present invention, in the configuration of the second aspect, a catalyst upstream of the three-way catalyst with respect to an amount of a component contributing to the oxygen storage capacity carried by the three-way catalyst. Is set so that the total amount of the components carried by is set to approximately 0.6 times or less. According to this configuration, the setting for lowering the oxygen storage capacity of the catalyst upstream of the three-way catalyst than 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 described in claim 4, claim 1 or 3
, The total amount of the components carried by the catalyst upstream of the three-way catalyst is set to be about 0.2 times or more the amount of the components contributing to the oxygen storage capacity carried by the three-way catalyst. . According to this configuration, in the configuration that regulates the maximum value of the ratio of the components contributing to the oxygen storage capacity, the minimum ratio is set to about 0.2 times or more, and the oxygen storage capacity of the NOx absorption catalyst and the catalyst on the upstream side of the NOx absorption catalyst is reduced. Specify the minimum required level.

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

In the invention according to claim 6, a three-way catalyst is disposed upstream of the NOx absorption catalyst. According to this configuration, the three-way catalyst, the NOx absorption catalyst, and the three-way catalyst are arranged in this order from the upstream side, and the oxygen storage in the upstream three-way catalyst and the NOx absorption catalyst is based on the downstream three-way catalyst. Capacity (the amount of component (ceria) that contributes to oxygen storage capacity) is limited.

In the invention according to claim 7, the NOx absorption catalyst contains barium Ba more than the three-way catalyst, so that NOx is absorbed using barium Ba as a NOx absorbent. According to such a configuration, even when both the NOx absorption catalyst and the downstream three-way catalyst carry barium Ba, the NOx absorption catalyst carries more barium Ba for the purpose of NOx absorption. The catalyst is distinguished from the ordinary three-way catalyst, and the oxygen storage capacity (the amount of the component (ceria) contributing to the oxygen storage capacity) of each catalyst is determined based on the distinction.

[0017]

According to the first aspect of the present invention, when the state is switched from the lean combustion state to the stoichiometric or rich state, the amount of oxygen released from the NOx absorption catalyst and the catalyst disposed upstream of the NOx absorption catalyst is reduced. The amount can be limited to a necessary and sufficient amount, and the NOx absorption catalyst and the three-way catalyst on the downstream side can be enriched early so that the NOx reduction process can be efficiently performed.

According to the second aspect of the present invention, the oxygen storage capacity of the catalyst including the NOx reduction catalyst on the upstream side is made lower than the oxygen storage capacity of the three-way catalyst on the downstream side of the NOx absorption catalyst, whereby lean combustion is performed. When the state is switched to stoichiometric or rich and NOx is desorbed, the amount of oxygen desorbed at the same time can be reliably limited, so that there is an effect that the NOx reduction process can be performed efficiently. .

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

According to the fifth aspect of the present invention, by adjusting the amount of ceria carried on the catalyst, there is an effect that the oxygen storage capacity for efficiently performing the NOx reduction treatment can be set. According to the sixth aspect of the present invention, the three-way catalyst is provided also on the upstream side of the NOx reduction catalyst, so that the exhaust gas purification at the time of starting is promoted, and
There is an effect that the reduction treatment of NOx absorbed by the x-absorption catalyst can be efficiently performed.

According to the seventh aspect of the present invention, there is provided a configuration including a NOx absorption catalyst that contains barium in an amount larger than that of a normal three-way catalyst and absorbs NOx during lean combustion by using the barium as a NOx absorbent. The NOx
There is an effect that the reduction treatment of NOx absorbed by the absorption catalyst can be efficiently performed.

[0022]

Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a system configuration of an internal combustion engine according to an embodiment. In the engine 1, air measured by a throttle valve 2 is sucked, and fuel injected from a fuel injection valve 3 and the intake air are mixed. Are mixed to form an air-fuel mixture.

The fuel injection valve 3 may be one that injects fuel into the intake port 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 the three-way catalyst (light-off catalyst) 6, the NOx absorption catalyst 7, and the three-way catalyst 8 provided in the exhaust passage 5. Is released into the atmosphere.

The three-way catalysts 6 and 8 are both catalysts that simultaneously purify NOx, CO and HC in a stoichiometric (stoichiometric air-fuel ratio) atmosphere, and the three-way catalyst 6 is located immediately below an exhaust manifold for the purpose of purifying exhaust gas during startup. And the three-way catalyst 8 is arranged together with the NOx absorption catalyst 7 in the underfloor portion of the vehicle. The NOx absorption catalyst 7 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 stoichiometric or rich, and the three-way catalyst layer This is a catalyst for reduction treatment (NOx absorption reduction type three-way catalyst).

The NOx absorption catalyst 7 and the three-way catalysts 6, 8
All carry barium Ba, but the NOx absorption catalyst 7
Carries barium Ba per unit volume approximately twice as much as the amount of barium Ba supported on the three-way catalyst 8 in order to use barium Ba as the NOx absorbent. Thus, the catalyst is classified into a NOx absorption catalyst and a normal three-way catalyst. However, the NOx absorbent is not limited to barium Ba.

The control unit 9 for controlling the injection timing / injection amount of the fuel injection valve 3 and the ignition timing of the ignition plug 4 includes a microcomputer, and performs 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 an ignition plug 4 (power transistor) is output.
, An ignition signal is output.

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

The various sensors include 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. 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, the NOx absorbed so far is desorbed, and the desorbed NOx is removed from the three-way catalyst layer of the NOx absorption catalyst 7 and the downstream side. The 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 and NOx can no longer be absorbed, and NOx discharged from the engine 1 is directly discharged, the load, rotation, air-fuel ratio, etc. The NOx absorption amount 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, 8 each carry ceria Ce as a component that contributes to the oxygen storage capacity, and accumulates a large amount of oxygen during lean combustion. However, the configuration is not limited to a configuration that supports ceria Ce as a component that contributes to the oxygen storage capacity. When the air-fuel ratio is switched from lean to the stoichiometric air-fuel ratio (stoichiometric) or rich, the stored oxygen is desorbed, and the oxygen desorbed in the NOx absorption catalyst 7 and the three-way catalyst 8 is removed. Presence and three-way catalyst 6
And a process for reducing the desorbed NOx by the presence of oxygen desorbed from the NOx absorption catalyst 7 and flowing downstream.
The atmospheres of the Ox absorption catalyst 7 and the three-way catalyst 8 are not easily enriched, so that NOx cannot be efficiently reduced.

Therefore, in the present embodiment, each of the catalysts 6 to 8
Ce, a component that contributes to the oxygen storage capacity of
Is set per unit volume as follows. The amount of the conventional ceria Ce per unit volume is 6 for each catalyst.
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 total of the ceria amount A of the three-way catalyst 6 and the ceria amount B of the NOx absorption catalyst 7 is 3 The amount of ceria in the original catalyst 8 was about 2.4 times the amount C.

With respect to the above setting, the ceria amount C of the three-way catalyst 8
Is reduced, the total amount A + B of ceria in the three-way catalyst 6 and the NOx absorption catalyst 7 is reduced. Ratio ((A + B)
As shown in FIG. 2, it was experimentally confirmed that the NOx emission when switching from lean combustion to stoichiometric (or rich) was reduced as / C) was decreased.

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

The NOx emission amount when switching from lean combustion to stoichiometric (or rich) is determined by the ratio (A +
B) As C / C decreases (see FIG. 2), as shown in FIG. 3, when the ceria amount A + B is excessively reduced, the NOx purification rate in the normal stoichiometric combustion state decreases and N
Since the amount of Ox emission increases, the lower limit of the ratio (A + B) / C needs to be set to about 0.2 in order to suppress the amount of NOx emission in the stoichiometric range within an allowable value.

On the other hand, as shown in FIG. 4, in a predetermined traveling mode (for example, in the 10-15 mode), it includes the NOx amount discharged when switching from lean to stoichiometric and the NOx amount discharged in the stoichiometric state. The total amount of NOx emissions is
If the ratio (A + B) / C is reduced from approximately 2.4 to approximately 1.2 in the related art, the ratio can be sufficiently reduced (for example, halved with respect to the conventional amount). If (A + B) / C is set, the total amount of Nx can be reduced without increasing the amount of NOx emission during normal stoichiometric combustion.
Ox emissions can be effectively reduced.

As the ratio (A + B) / C is further reduced from approximately 1.2, as shown in FIG.
Although the amount of NOx emission at the time of Ox desorption progresses, even if it is approximately 0.6 or less, the NOx amount does not decrease any more. Therefore, NOx can be purified most efficiently by the ratio (A + B) /
This is the case where C is set within a range of approximately 0.2 to 0.6. As described above, when the ratio (A + B) / C is set in 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 more than the oxygen storage capacity of the three-way catalyst 8. This means that the sum of the abilities has been lowered.

In the above-described embodiment, the three-way catalyst 6 is provided upstream of the NOx absorption catalyst 7. However, the three-way catalyst 6 may not be provided.
The proportion of C is in 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 as to fall within the range of 0.6.

[Brief description of the drawings]

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

FIG. 2 is a graph showing a correlation between a NOx emission amount and a ceria amount ratio at the time of switching from lean to stoichiometric.

FIG. 3 is a graph 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]

 DESCRIPTION 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

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 F01N 3/24 E B01D 53/36 101B 104A

Claims (7)

[Claims] 1. An NOx absorption catalyst for absorbing NOx in exhaust gas when the exhaust air-fuel ratio is lean, and releasing and reducing the absorbed NOx when the exhaust air-fuel ratio is stoichiometric or rich. An exhaust gas purification device for an internal combustion engine comprising at least a three-way catalyst disposed downstream of the NOx absorption catalyst, wherein the three-way catalyst carries an oxygen storage capacity, An exhaust gas purification apparatus 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 about 1.2 times or less. 2. An NOx absorption catalyst for absorbing NOx in exhaust gas when the exhaust air-fuel ratio is lean, and releasing and reducing the absorbed NOx when the exhaust air-fuel ratio is stoichiometric or rich. An exhaust gas purification device for an internal combustion engine comprising at least a three-way catalyst disposed downstream of the NOx absorption catalyst, wherein the oxygen storage capacity of the three-way catalyst is more upstream than the three-way catalyst. An exhaust gas purification device for an internal combustion engine, wherein an oxygen storage capacity of a catalyst is set low. 3. The total amount of the components carried by the catalyst upstream of the three-way catalyst is set to approximately 0.6 times or less the amount of the components contributing to the oxygen storage capacity carried by the three-way catalyst. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein: 4. The total amount of the components carried by the catalyst upstream of the three-way catalyst is set to be about 0.2 times or more the amount of the components contributing to the oxygen storage capacity carried by the three-way catalyst. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 3, wherein: 5. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein an amount of ceria Ce is set as a component contributing to the oxygen storage capacity. 6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein a three-way catalyst is disposed upstream of the NOx absorption catalyst. 7. The NOx absorption catalyst contains more barium Ba than the three-way catalyst, so that barium Ba is reduced to NO.
The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 1 to 6, wherein NOx is absorbed as an x absorbent.