JPH08270440A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE: To reduce the discharge quantity of NOx into the outside air in a wide temperature area from the low temperature area of exhaust gas to the high temperature area in a lean operation area as an exhaust emission control device for an engine mainly operated with the mixture of a lean air-fuel ratio. CONSTITUTION: An exhaust emission control device for an internal combustion engine is provided with an NOx adsorbing catalyst 5 having the ability to adsorb NOx in exhaust gas in a lean operation area and to release the adsorbed NOx in a stoichiometric air-fuel ratio operation area and formed in such a way that the NOx adsorption rate becomes high when the inlet gas temperature is in a temperature area of 250-350 deg.c, a three-way catalyst 6 disposed downstream the adsorbing catalyst 5, and an electric heater catalyst 4 (EHC) disposed downstream of the adsorbing catalyst 5 and formed by providing the three-way catalyst 6 additionally with a heater that can heat the three-way catalyst 6.

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 apparatus for an internal combustion engine, which is mainly operated with a lean air-fuel mixture such as a lean-burn engine of an automobile, and is particularly operated with a lean air-fuel mixture. The present invention relates to a measure for reducing the amount of NOx (nitrogen oxide) emitted to the outside air generated in a region (hereinafter referred to as a lean operation region).

[0002]

2. Description of the Related Art Conventionally, for example, Japanese Patent Laid-Open No. 5-13322.
No. 2 publication discloses a catalyst for NOx purification (for example, transition metal such as copper ion-exchanged zeolite) for reducing and purifying NOx in exhaust gas in the presence of HC (hydrocarbon) in a lean operating region in an exhaust system of a lean burn engine. (Zeolite) is disposed downstream of the NOx purification catalyst, HC, C with respect to the exhaust gas in a region where the mixture is operated at a stoichiometric air-fuel ratio (hereinafter referred to as a stoichiometric air-fuel ratio operating region).
A catalyst with a heater (hereinafter referred to as a three-way catalyst having a three-way ability of O (carbon monoxide) and NOx purification ability (hereinafter, simply referred to as three-way ability)) is provided with an electric heater capable of heating the three-way catalyst. , EHC).

In this conventional example, when the engine is started, the heater of the EHC is energized when the engine is started, so that the catalyst of the EHC is activated within a short period of time (for example, 40 seconds). In the case of original catalyst, it is generally 300 ℃
Exhaust gas purification can be performed by raising the temperature above).

On the other hand, for example, in Japanese Unexamined Patent Publication (Kokai) No. 5-302508, NOx in the exhaust gas is adsorbed in the exhaust system of the lean burn engine in the lean operation region, while it is adsorbed in the stoichiometric air-fuel ratio operation region. Released NOx
In the case where the Ox adsorption catalyst is arranged, a technique is proposed in which the NOx purification catalyst having the NOx purification ability in the lean operation region is arranged on the upstream side of the NOx adsorption catalyst in the same manner as the above case. Has been done.

In this proposed example, a portion of NOx in the lean operation region is purified by the NOx purification catalyst so that only the remaining NOx is adsorbed by the NOx adsorption catalyst. Has been done. That is, NO
In order to release the NOx adsorbed on the x adsorption catalyst, it is necessary to switch the lean operation region to the stoichiometric air-fuel ratio operation region. Therefore, if the amount of NOx to be adsorbed by the NOx adsorbing catalyst can be reduced, the frequency of switching to the stoichiometric air-fuel ratio operation region can be reduced, so that the fuel consumption rate can be improved accordingly.

[0006]

However, in the former conventional example, even in the lean operation region, when the NOx purification catalyst does not reach the activation temperature range, at low temperature, the NOx in the exhaust gas is used for NOx purification. Since it is not purified by the catalyst, there is a problem that it is discharged to the outside air in an unpurified state.

On the other hand, in the latter conventional example, the amount of NOx that needs to be adsorbed on the NOx adsorbing catalyst is certainly reduced, but the NOx adsorbing catalyst is even in the lean operating region. NOx is not adsorbed at an adsorption rate of 100%, and the NOx adsorption rate has a fluctuation range of about 20 to 80%. Under these circumstances, if only the fuel consumption rate is improved while the NOx adsorption capacity of the NOx adsorption catalyst cannot be fully utilized, N
Part of the NOx that has flowed into the Ox adsorbing catalyst is not adsorbed by the NOx adsorbing catalyst, and there is a fear that it will be discharged to the outside air without being purified. Therefore, if it is possible to improve both the fuel consumption rate and the NOx emission amount, it is very delicate and complicated that the lean operation region and the stoichiometric air-fuel ratio operation region are switched according to the fluctuation of the NOx adsorption ratio. Various control systems are required.

The present invention has been made in view of the above points, and an object thereof is to provide an EHC and NOx adsorption catalyst in an exhaust gas purification apparatus for an internal combustion engine which is mainly operated with a lean air-fuel mixture. By properly combining them and enabling the NOx adsorption rate of the NOx adsorbing catalyst to be maintained at a high level, the NOx adsorbed to the outside air in a wide temperature range from a low temperature range to a high temperature range of the exhaust gas in the lean operation range. It is to be able to reduce the amount of emissions.

[0009]

In order to achieve the above object, the invention of claim 1 is based on the finding that the fluctuation of the NOx adsorption rate of the NOx adsorption catalyst depends on the temperature. The temperature of the inlet gas of the NOx adsorbing catalyst can be controlled by the heating temperature of the heater, so that the NOx adsorbing rate of the NOx adsorbing catalyst can be maintained at a high level.

Specifically, according to the present invention, the NOx in the exhaust gas is arranged in the exhaust system of the internal combustion engine to adsorb NOx in the exhaust gas in the lean operation region, while adsorbing NOx in the stoichiometric air-fuel ratio operation region. NOx adsorption having at least NOx adsorption capacity for releasing NOx and having a higher NOx adsorption rate when the inlet gas temperature is in a predetermined temperature range than in a temperature range other than the predetermined temperature range. Catalyst, an exhaust purification catalyst disposed downstream of the NOx adsorption catalyst and having at least a ternary capacity for exhaust gas in the stoichiometric air-fuel ratio operating region, and an upstream side of the NOx adsorption catalyst. The catalyst for purification of exhaust gas having at least three-way capacity for exhaust gas in the theoretical air-fuel ratio operation region,
A catalyst with a heater provided with a heater capable of heating the exhaust gas purification catalyst is provided.

According to a second aspect of the present invention, in the above first aspect of the invention, there is provided temperature detecting means for detecting an inlet gas temperature of the NOx adsorbing catalyst, and the inlet gas detected by the temperature detecting means in a lean operation region. Control means for controlling the heating temperature of the heater-equipped catalyst by the heater so that the temperature is maintained in a predetermined temperature range.

According to a third aspect of the present invention, in the above-mentioned second aspect, the NOx adsorption catalyst has an inlet gas temperature of 2
When the NOx adsorption rate is higher in the temperature range of 50 to 350 ° C than in the temperature range other than the temperature range, the control means controls the inlet of the NOx adsorption catalyst. It is assumed that the heating temperature of the heater-equipped catalyst is controlled so that the gas temperature is maintained in the temperature range of 250 to 350 ° C.

[0013]

With the above structure, in the invention of claim 1, when the exhaust gas temperature is in the low temperature range in the exhaust system of the internal combustion engine, the exhaust gas is heated by the heater of the EHC and the N
The NOx adsorbing catalyst and the exhaust gas purifying catalyst pass through the Ox adsorbing catalyst and the EHC exhaust gas purifying catalyst in this order, whereby the NOx adsorbing catalyst and the exhaust gas purifying catalyst are heated to an active temperature range in a short time (for example, to 300 ° C. in 40 seconds). ). Then, in the stoichiometric air-fuel ratio operation region, the exhaust purification catalyst purifies HC, CO and NOx in the exhaust gas. On the other hand, in the lean operation region, after the NOx in the exhaust gas is adsorbed by the NOx adsorbing catalyst, the HC in the exhaust gas is exhausted.
And CO are purified by the exhaust purification catalyst.

At this time, the NOx adsorption rate in the NOx adsorption catalyst changes depending on the heating temperature of the EHC heater. That is, when the inlet gas temperature of the NOx adsorption catalyst is in the predetermined temperature range, the NOx adsorption rate is higher than in the temperature range other than the predetermined temperature range. Therefore,
By heating the inlet gas temperature of the NOx adsorbing catalyst to a predetermined temperature range by the EHC heater, NO
The NOx adsorption rate of the x adsorption catalyst is maintained at a high level. Therefore, NOx is efficiently adsorbed by the NOx adsorbing catalyst in a wide temperature range from a low temperature region of the exhaust gas to a high temperature region, and the NOx emission amount in the lean operation region is reduced accordingly. It will be.

In the invention of claim 2, the inlet gas temperature of the NOx adsorbing catalyst is detected by the temperature detecting means, and the control means controls the heater of the heater-equipped catalyst so that the inlet gas temperature falls within a predetermined temperature range. The heating temperature is controlled. As a result, the NOx adsorption rate of the NOx adsorption catalyst is maintained at a high level. Therefore, the action of the invention of claim 1 is carried out specifically.

In the invention of claim 3, the inlet gas temperature of the NOx adsorbing catalyst is maintained in a temperature range of 250 to 350 ° C., so that the NOx adsorbing rate of the NOx adsorbing catalyst is maintained at a high level. . Therefore, the action of the invention of claim 1 is carried out more specifically.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An exhaust gas purifying apparatus for an automobile lean burn engine according to an embodiment of the present invention will be described below with reference to the drawings.

In the above exhaust gas purifying apparatus, as shown in FIG. 1, one catalyst case 3 is arranged in the middle of an exhaust passage 2 which is an exhaust system of an engine 1 as an internal combustion engine. Inside, from the upstream side to the downstream side, E
The HC 4, the NOx adsorption catalyst 5, and the three-way catalyst 6 as an exhaust gas purification catalyst are sequentially arranged.

Although not shown, the EHC 4 has a three-way catalyst as an exhaust gas purification catalyst having a three-way capacity in the stoichiometric air-fuel ratio operation region and a heater capable of heating the three-way catalyst. ing. Further, the NOx adsorption catalyst 5 is
While adsorbing NOx in the exhaust gas in the lean operating region, the adsorbed NOx in the stoichiometric air-fuel ratio operating region
NOx adsorbing ability to release NOx and ternary ability for exhaust gas in the stoichiometric air-fuel ratio operating range, and when the inlet gas temperature is in a predetermined temperature range and is in a temperature range other than the predetermined temperature range The NOx adsorption rate is higher than that. The three-way catalyst 6 has the same catalytic ability as the three-way catalyst of the EHC 4 and has a larger capacity than the three-way catalyst.

At a position on the outlet side of the engine 1 in the exhaust passage 2, an O ₂ sensor 7 for detecting the concentration of O ₂ in the exhaust gas for checking the air-fuel ratio of the air-fuel mixture supplied to the engine 1. Are arranged. In addition, the above NOx
On the inlet side of the adsorption catalyst 5, a temperature sensor 8 is arranged as a temperature detection means for detecting the inlet gas temperature of the NOx adsorption catalyst 5. Further, a temperature sensor 9 for detecting the heating temperature of the heater of the EHC 4 is arranged at the heater portion of the EHC 4 in order to confirm the operation of the heater. Each detection signal of these sensors 7 to 9 is connected to the CPU 10 of the automobile as a control means so as to be inputtable. The CPU 10 controls energization of the heater of the EHC 4 based on each detection signal.

Specifically, the heater of the EHC 4 is basically energized at the same time when the ignition switch is turned on when the engine 1 of the automobile is started to start temperature rise and reach a predetermined temperature. Then, the energization is automatically stopped. Its heating performance is, for example, 2
The heating temperature of 300 ° C. is reached when about 40 seconds have passed from the time when the energization was started in the atmosphere of 5 ° C.

The NOx adsorbing catalyst 5 is a catalyst powder in which Rh (rhodium) as an active species is supported on MFI type zeolite as a base material, and 2 g per liter of the carrier is made of a cordierite honeycomb carrier. It is to be carried in a ratio so that the capacity is 0.5 liter. This NOx adsorption catalyst 5
In the lean operation area, as shown in FIG.
NOx is adsorbed in the active temperature range of 00 to 400 ° C, but about 70% to 80% in the temperature range of 250 to 350 ° C.
The NOx adsorption rate is high in the range exceeding 3%,
It has a NOx adsorption characteristic that reaches a peak at 00 ° C. .

The three-way catalyst 6 has alumina (aluminum oxide) and ceria (cerium oxide) as base materials,
A catalyst powder supporting Pt (platinum) and Rh as active species is carried on a cordierite honeycomb carrier at a rate of 1.6 g per liter of the carrier. At that time, Pt and Rh are Pt: Rh
= 5: 1. The capacity is 1.0
It is a liter.

The CPU 10 controls the heating temperature by the heater of the EHC 4 so that the inlet gas temperature of the NOx adsorption catalyst 5 is maintained in the temperature range of 250 to 350 ° C. That is, here, EHC
The energization is not stopped when the heating temperature of the heater of No. 4 reaches 300 ° C., for example, the energization is stopped when the inlet gas temperature reaches 300 ° C., and thereafter, the inlet gas temperature is 300 ± 50 ° C. Energization and stopping thereof are repeated to maintain the temperature range.

Here, in the above exhaust purification device, CP
The operation when the heater of the EHC 4 is not controlled by U10 and when it is performed will be described in the cold state.

First, when the heater control is not performed,
As shown in FIG. 3, during the period from the start to the time when the oil temperature and the water temperature rise to a predetermined temperature, the engine 1 keeps the theoretical air-fuel ratio (λ
= 1) The operation is performed in the region or the rich region. On the other hand, the heater of the EHC 4 is energized (operated) at the same time as the engine 1 is started, the temperature is raised to about 300 ° C. when 40 seconds have passed from that time, and then a predetermined time (for example, 10
The power supply is stopped (does not operate) when (seconds) has elapsed. Then, the three-way catalyst of the EHC 4 and the three-way catalyst 6 (TWC) on the downstream side start exhaust gas purification. In this stoichiometric air-fuel ratio operation region, the NOx adsorption catalyst 5 functions (operates) as a three-way catalyst for exhaust gas.

After the oil temperature and the water temperature of the engine 1 have sufficiently risen, the engine 1 is mainly operated in the lean region. In this lean operation region, the three-way catalyst 6 is
Almost no purification of Ox (partial operation). On the other hand, the NOx adsorbing catalyst 5 is used for the NOx in the exhaust gas in the lean operation region.
x is adsorbed, but since the power supply to the heater of the EHC 4 is stopped at an early point, the inlet gas temperature of the NOx adsorbing catalyst 5 rises sufficiently in the stage immediately after switching to the lean operation region. Therefore, the NOx adsorption rate remains low (partial operation). Eventually, the temperature of the exhaust gas itself rises, and the inlet gas temperature of the NOx adsorption catalyst 5 rises to 2
Peak at 00 to 400 ° C, preferably 300 ° C 2
When it enters the temperature range of 50 to 350 ° C., the NOx adsorption catalyst 5 adsorbs (operates) NOx at a high adsorption rate.

Then, when the engine 1 returns to the stoichiometric air-fuel ratio operation region, NOx is released from the NOx adsorbing catalyst 5 and is purified by the three-way catalyst 6. That is, in this case, NO
It cannot be said that the NOx adsorption ability of the x adsorption catalyst 5 is fully utilized. It should be noted that, regardless of the presence or absence of heater control, when the inlet gas temperature exceeds 350 to 400 ° C., the NOx adsorption rate of the NOx adsorption catalyst 5 decreases (partially operates). Since the load of No. 1 is large and the air-fuel ratio is switched to the stoichiometric air-fuel ratio operating region, the exhaust gas at that time is the three-way catalyst of EHC4 and the three-way catalyst 6 of the downstream side.
Purified by. Therefore, the NOx emission amount to the outside air does not increase due to the decrease in the NOx adsorption rate.

Next, in the case where the heater control is performed, as shown in FIG. 4, even after the temperature is raised to 300 ° C. after the cold start, the inlet gas temperature of the NOx adsorption catalyst 5 is 300 ± 50.
To reach and maintain the temperature range of ℃, E
The energization (operation) to the heater of HC4 is repeated. That is, the heater control is continued so that the NOx adsorption rate of the NOx adsorption catalyst 5 is always at the maximum level.

Then, as shown in FIG. 5, when the lean control region and the stoichiometric air-fuel ratio drive region are switched in accordance with the vehicle speed while the heater control is continued (operation),
The NOx adsorption catalyst 5 releases NOx adsorbed in the lean operation region in the stoichiometric air-fuel ratio operation region, and the released NOx is purified (operated) by the three-way catalyst 6.

Therefore, according to this embodiment, the EHC4
NOx by controlling the heating temperature by the heater of
Since the inlet gas temperature of the adsorption catalyst 5 can be maintained in the temperature range of 250 to 350 ° C., the NOx adsorption rate of the NOx adsorption catalyst 5 can be maintained at a high level of 70% or more. Therefore, NOx is efficiently adsorbed by the NOx adsorbing catalyst 5 in a wide temperature range from a low temperature region of the exhaust gas to a high temperature region, and the NOx generated in the lean operation region is discharged to the outside by that much. The amount can be reduced.

In the above embodiment, the NOx adsorption catalyst 5 is used.
As one of the preferred examples of the above, there is mentioned one composed of zeolite as a base material carrying Rh as an active species and also having ternary ability for exhaust gas in the theoretical air-fuel ratio operation region. The NOx adsorbing catalyst is not limited to the above-mentioned one as long as it has at least the NOx adsorbing ability in the lean operation region.

In the above embodiment, the exhaust purification catalyst for EHC 4 and the exhaust purification catalyst on the downstream side of the NOx adsorption catalyst 5 are three-way catalysts, respectively. An example is described in which alumina and ceria as base materials are made to carry Pt and Rh as active species, and the exhaust purification catalyst is as follows.
It is not necessary to limit to the three-way catalyst as long as it has at least three-way capacity in the stoichiometric air-fuel ratio operation range. For example, instead of the three-way catalyst, the three-way capacity can be added to the NOx purification capacity in the lean operation range. A NOx purifying catalyst (so-called lean burn three-way catalyst) may be used together. As an example of the material structure of such a NOx purification catalyst, zeolite is mainly used as a base material, and Pt and Ir are used as active species.
Examples include those using (iridium) and Rh, or those using Pt and Rh in a ratio of Pt / Rh ≦ 1/20. A cordierite honeycomb carrier is suitable as the carrier.

Further, in the above embodiment, EHC4, NOx
Although the adsorption catalyst 5 and the three-way catalyst 6 constitute an exhaust gas purification device, other exhaust gas purification members such as a NOx purification catalyst may be additionally arranged. still,
When a NOx purification catalyst such as a transition metal zeolite that decomposes and purifies NOx in the presence of HC is added, it can be arranged on the upstream side of the EHC 4, the downstream side of the three-way catalyst 6, or the like. From the viewpoint of ensuring the amount of HC to be supplied to the purification catalyst, it is desirable to arrange it on the upstream side of EHC4.

Further, in the above embodiment, as for the control by the CPU 10, only the heater control of the EHC 4 was explained, but as shown by the broken line arrow in FIG. Output to a fuel injection device, an ignition device, etc.,
As a result, control for switching the engine 1 from the lean operation region to the stoichiometric air-fuel ratio operation region can also be performed in accordance with the NOx adsorption amount of the NOx adsorption catalyst 5.

-Specific Example- An experiment carried out to examine the exhaust gas purifying ability of the exhaust gas purifying apparatus will now be described. In the experiment, in a vehicle that travels in the lean region of the engine during acceleration and at a constant speed, most of the exhaust passage on the downstream side from the exhaust manifold of the engine to the downstream side by about 0.5 mm. The system of the present invention (shown in FIG. 1) in which the EHC 4, the NOx adsorption catalyst 5, and the three-way catalyst 6 of the embodiment are arranged in this order was taken as an example of the present invention.

The above vehicle was equipped with an in-line four-cylinder DOHC engine having a displacement of 1500 cc. The lean limit of the engine is A / F at air-fuel ratio A / F.
= 27. The vehicle weight is 1300 kg.

Comparative Example 1 for comparison with the above-mentioned example of the present invention
As a comparative example 2, only a three-way catalyst having a three-way capacity in the stoichiometric air-fuel ratio operating region and an EHC-only one which is the same as the case of the present invention example are prepared. Exhaust gas purification rate (Y1 purification rate) when operating only in the stoichiometric air-fuel ratio region when cooling at ambient temperature, and exhaust gas purification rate (FTP purification rate) when operating almost all regions except the idle operation in the lean region I examined each. The results are also shown in Table 1 below.

[0039]

[Table 1]

As can be seen from Table 1 above, in the example of the present invention,
The NOx purification rate is improved in both the stoichiometric air-fuel ratio operation region and the lean operation region. This is because NOx generated in the lean operating region is adsorbed by the NOx adsorbing catalyst 5, and the NOx is purified by the three-way catalyst 6 in the stoichiometric air-fuel ratio operating region to discharge NOx to the outside air. This is probably because the amount was controlled.
Further, in the case of the stoichiometric air-fuel ratio operation, the NOx adsorption catalyst 5 is a three-way catalyst, so that the capacity is increased in cooperation with the downstream three-way catalyst 6, and the gas space is correspondingly increased. It is considered that the decrease in speed and the improvement in the NOx purification rate also contributed.

Next, in the above system of the present invention, EHC
Inventive Example 1 (Table 1), in which the heater control of No. 4 is not performed, that is, when the heater is energized at the time of starting and the energization is stopped when the heating temperature reaches 300 ° C.
Inventive example), after the heating temperature by the heater reaches 300 ° C., the inlet gas temperature of the NOx adsorption catalyst 5 is 250 to
In the case of controlling the temperature to be 350 ° C., the same heater control as in the case of the invention example 2 and the invention example 2 is performed, and the NOx adsorption catalyst 5 adsorbs NOx and releases the adsorbed NOx. The case where the air-fuel ratio control in which the air-fuel ratio is switched between lean and stoichiometric air-fuel ratio is performed is defined as Invention Example 3, and the same exhaust gas purification rates as described above were investigated for Invention Examples 1 to 3. The results are also shown in Table 2 below.

[0042]

[Table 2]

As can be seen from Table 2, Invention Examples 1 and 2
In contrast, in the invention example 2, the NOx purification rate is improved. This is because the heater control of the EHC 4 is performed so that the NOx of the NOx adsorption catalyst 5 is reduced regardless of the temperature change of the exhaust gas.
It is considered that this is because the adsorption rate can be maintained at a high level. Note that the NOx adsorbing catalyst 5 may be arranged upstream of the EHC 4 as long as it is located upstream of the three-way catalyst 6, but in that case, NOx controlled by the heater is used. The adsorption properties cannot be fully utilized.

Finally, as shown in FIG. 6 (a), in addition to the NOx purifying ability of decomposing and purifying NOx in the presence of HC, HC is adsorbed in the low temperature range while the adsorbed HC is adsorbed in the high temperature range.
The catalyst 11 for purifying NOx (for example, transition metal zeolite such as copper ion-exchanged zeolite) adapted to have the ability to adsorb HC is released from the other catalyst case 3a upstream of the catalyst case 3b in which the system of the present invention is housed. Inventive Example 1 arranged inside, and as shown in FIG. 6 (b), Inventive Example 2 in which the same NOx purification catalyst 11 is arranged on the downstream side of the three-way catalyst 6 of the system of the present invention; Three-way catalyst 6 of Inventive Example 1
NOx as an exhaust gas purification catalyst that has both the purification performance equivalent to that of the three-way catalyst 6 and the NOx purification performance in the lean operation region.
Inventive Example 3 in which the purifying catalyst 6 '(three-way catalyst for lean burn described in the above description of the Examples) is replaced is prepared, and for these Inventive Examples 1 to 3, the same exhaust gas purification rate as described above is prepared. I investigated each. At that time, for comparison, a comparative example (same as comparative example 2 in Table 1 above) having only the same EHC as in the case of the system of the present invention was prepared, and this comparative example was also examined. The results are also shown in Table 3 below.

[0045]

[Table 3]

As can be seen from Table 3 above, Invention Examples 1 and 2
In comparison, in Invention Example 1 in which the NOx purification catalyst 11 is arranged on the upstream side of the system of the present invention, almost all purification rates are improved. On the other hand, when the NOx purifying catalyst 11 is arranged on the downstream side of the system of the present invention as in Inventive Example 2,
Since the HC necessary for NOx decomposition is purified by EHC4, the NOx purification efficiency of the NOx purification catalyst 11 is slightly reduced. Further, comparing invention examples 1 and 3, it can be seen that even if the three-way catalyst 6 is replaced by the NOx purification catalyst 6 ′, purification performance comparable to that of the invention example 1 can be obtained.

[0047]

As described above, according to the invention of claim 1, as an exhaust purification system for an internal combustion engine, NOx in the exhaust gas is adsorbed in a lean operation region while the adsorption is performed in a stoichiometric air-fuel ratio operation region. Has at least a NOx adsorbing capacity for releasing NOx, and when the inlet gas temperature is in a predetermined temperature range, N is higher than that in a temperature range other than the predetermined temperature range.
A NOx adsorption catalyst having a high Ox adsorption rate, an exhaust gas purification catalyst disposed at a downstream side of the NOx adsorption catalyst and having at least three-way capacity in the theoretical air-fuel ratio operation region, and the NOx adsorption catalyst. A catalyst with a heater, which is arranged on the upstream side of the exhaust catalyst and has at least a heater capable of heating the exhaust purification catalyst, in addition to the exhaust purification catalyst having at least ternary capacity in the stoichiometric air-fuel ratio operation region. Because I chose
The heater of the EHC can heat the inlet gas temperature of the NOx adsorption catalyst to the above predetermined temperature range.
The NOx adsorption rate of the NOx adsorption catalyst can be stably maintained at a high level. Therefore, NOx adsorption by the NOx adsorption catalyst can be efficiently performed in a wide temperature range from a low temperature region of exhaust gas to a high temperature region, and the NOx generated in the lean operation region is correspondingly increased.
It is possible to reduce the amount of exhausted to the outside air.

According to the second aspect of the present invention, the temperature detecting means for detecting the inlet gas temperature of the NOx adsorbing catalyst and the inlet gas temperature detected by the temperature detecting means in the lean operation region are predetermined. Since the control means for controlling the heating temperature of the heater-equipped catalyst by the heater so as to be maintained in the temperature range is provided, the inlet gas temperature of the NOx adsorption catalyst is maintained in the predetermined temperature range of the NOx adsorption catalyst. can do. Therefore, the effect according to the invention of claim 1 can be specifically obtained.

According to the third aspect of the invention, the predetermined temperature range of the inlet gas temperature of the NOx adsorption catalyst is 250 to 350.
When the temperature is ℃, the inlet gas temperature can be maintained in the temperature range of 250 to 350 ° C. by the control means, so that the effect of the invention of claim 1 can be efficiently obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an exhaust emission control device according to an embodiment of the present invention.

FIG. 2 is a NOx adsorption characteristic diagram showing a change in NOx adsorption rate of a NOx adsorption catalyst in relation to an inlet gas temperature.

FIG. 3 is a timing chart showing the operation of the exhaust gas purification device during cooling when EHC heater control is not performed together with the temperature change of exhaust gas.

FIG. 4 is a diagram corresponding to FIG. 3, showing the operation of the exhaust gas purification device during cooling when EHC heater control is performed together with the temperature change of exhaust gas.

FIG. 5 is a timing chart showing the operation of the exhaust emission control device when the EHC heater control is performed together with the vehicle speed change.

FIG. 6 is a schematic view showing each modification of the present embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 engine (internal combustion engine) 2 exhaust passage (exhaust system) 4 EHC (catalyst with heater) 5 NOx adsorption catalyst 6 three-way catalyst (exhaust gas purification catalyst) 6'NOx purification catalyst (exhaust gas purification catalyst) 8 temperature sensor (Temperature detection means) 10 CPU (control means)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F01N 3/24 ZAB F01N 3/24 ZABR

Claims (3)

[Claims] 1. An NOx adsorbing ability, which is arranged in an exhaust system of an internal combustion engine and adsorbs NOx in exhaust gas in a lean operating region, and releases the adsorbed NOx in a stoichiometric air-fuel ratio operating region. And a NOx adsorption catalyst having a higher NOx adsorption rate when the inlet gas temperature is in a predetermined temperature range than in a temperature range other than the predetermined temperature range, and the NOx adsorption catalyst An exhaust gas purification catalyst having at least a ternary capacity of HC, CO, and NOx purification capacity for exhaust gas in the theoretical air-fuel ratio operation region, and an upstream side of the NOx adsorption catalyst. An exhaust gas purification catalyst having at least a ternary function of purifying HC, CO and NOx with respect to exhaust gas in an air-fuel ratio operation region is provided with a heater capable of heating the exhaust gas purification catalyst. Exhaust purification system of an internal combustion engine, characterized in that it comprises a chromatography data with the catalyst. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein a temperature detecting means for detecting an inlet gas temperature of the NOx adsorbing catalyst, and an inlet gas detected by the temperature detecting means in a lean operation region. An exhaust emission control device for an internal combustion engine, comprising: a control unit that controls a heating temperature of a heater-equipped catalyst by a heater so that the temperature is maintained in a predetermined temperature range. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the NOx adsorption catalyst has an inlet gas temperature of 250 to 35.
When the temperature is 0 ° C., the NOx adsorption rate is higher than when it is in a temperature range other than the temperature range, and the control means is such that the inlet gas temperature of the NOx adsorption catalyst is 2
An exhaust emission control device for an internal combustion engine, characterized in that the heating temperature of the heater-equipped catalyst is controlled so as to be maintained in a temperature range of 50 to 350 ° C.