JP3055270B2 - 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 having a NOx catalyst and capable of suppressing the emission of O.

[0002]

2. Description of the Related Art Japanese Patent Laying-Open No. 1-39145 discloses that a Cu / zeolite catalyst in which Cu is ion-exchanged and supported on zeolite is disposed in an exhaust system of an internal combustion engine capable of lean combustion, and an oxidation catalyst or An exhaust purification device for an internal combustion engine having a three-way catalyst is disclosed. In such an exhaust gas purifying apparatus for an internal combustion engine, in a steady operation state after warm-up, when the air-fuel ratio is lean, the presence of HC and N
Ox is purified and HC, CO is oxidized or ternary catalyst.
Is purified.

[0003]

However, the conventional apparatus has the following problems. That is, in order for the Cu / zeolite catalyst to purify NOx at a high purification rate, the exhaust gas velocity passing through the catalyst, that is, the space velocity SV, must be small, and for that purpose, the catalyst capacity must be large. However, when the capacity of the Cu / zeolite catalyst is increased, the calorific value of the exhaust gas is reduced at the time of cold such as when starting the engine.
It is consumed for warming up the zeolite catalyst, and the warming-up of the downstream oxidation catalyst or the three-way catalyst is delayed, so that there is a problem that HC and CO emissions are directly discharged to the outside air without being purified. Since the generation of CO is relatively small in the lean burn state, the problem of HC emission is particularly large.

An object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine in which a NOx purifying catalyst comprising a zeolite carrying a transition catalyst such as Cu in an exhaust system is provided. It is to control.

[0005]

According to the present invention, there is provided, in accordance with the present invention, a heater in an exhaust system of an internal combustion engine capable of lean combustion.
Not provided, a transition metal / zeolite catalyst carrying a transition metal by ion exchange was provided, and a catalyst having an oxidizing ability including a start catalyst with a heater was provided downstream of the transition metal / zeolite catalyst. This is achieved by an exhaust gas purification device for an internal combustion engine.

[0006]

[Effect] The transition metal / zeolite catalyst is used for cold HC
Has the effect of adsorbing HC on the catalyst surface, and therefore adsorbs HC immediately after the start of the engine. Even if the transition metal / zeolite catalyst is saturated thereafter, the downstream catalyst is warmed up early by turning on the heater, so that the HC is purified and the emission amount is reduced. As the temperature of the transition metal / zeolite catalyst rises, HC desorbs. Conventionally, the warm-up of the downstream catalyst is slow, and a large amount of HC may be discharged at this point. However, in the present invention, the downstream catalyst is also heated by the heater before the transition metal / zeolite catalyst reaches the HC desorption temperature. HC is purified by the downstream catalyst, and the emission of HC is suppressed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings. As shown in FIGS. 1 and 2, a transition metal / zeolite catalyst 4 in which a transition metal such as Cu is ion-exchanged and supported on zeolite is disposed in an exhaust system 2 of an internal combustion engine capable of lean combustion. . On the downstream side of the transition metal / zeolite catalyst 4 in the exhaust system 2, catalysts 6 and 8 (oxidation catalyst or reduction catalyst) having oxidizing ability are provided. The catalysts 6 and 8 having an oxidizing ability include an upstream start catalyst 6 and a downstream catalyst 8. Of the catalysts 6 and 8 having oxidizing ability, at least the start catalyst 6 is provided with a heater. These catalysts 4, 6, 8 are arranged in series.
The heater-equipped start catalyst 6 and the downstream catalyst 8 may be disposed apart from each other as shown in FIG. 1 or may be disposed in contact with each other as shown in FIG.

[0008] The catalyst 6 has a smaller capacity than the downstream catalyst 8 so that the temperature of the catalyst can be raised quickly when the heater is turned on, and functions as a start catalyst at the time of cold. The downstream catalyst 8 functions as a catalyst for normal operation after warm-up.

The arrangement order of the transition metal / zeolite catalyst 4 and the start catalyst 6 is the same as the order of the catalysts 4 and 6. If the order is reversed, the start catalyst 6 is discharged from the internal combustion engine.
Since C is oxidized, the transition metal / zeolite catalyst 4
HC in the exhaust gas flowing into
This is because the NOx reducing action in the zeolite catalyst 4 decreases. More specifically, the transition metal / zeolite catalyst 4
The NOx reduction mechanism is determined to be such that active species generated by partial oxidation of HC react with NOx to reduce NOx. Therefore, in order for the transition metal / zeolite catalyst 4 to exhibit a high NOx purification rate, it is necessary to supply a required amount of HC. Therefore, the catalysts 6 and 8 are arranged downstream of the catalyst 4 because the supply of the catalyst is reduced.

The transition metal / zeolite catalyst 4 exhibits a high NOx removal rate at low space velocities, but exhibits a high NOx removal rate at high space velocities.
x Purification rate decreases. In order to reduce the space velocity in the transition metal / zeolite catalyst 4, the capacity of the transition metal / zeolite catalyst 4 must be increased and the flow rate of the exhaust gas in the catalyst 4 must be reduced. When the capacity and volume of the metal / zeolite catalyst 4 are increased, the heat capacity increases, so that it takes time for the transition metal / zeolite catalyst 4 to reach the activation temperature immediately after starting the engine, and the transition metal / zeolite catalyst 4 Exhaust gas is deprived of energy by the temperature rise, and the temperature of the exhaust gas becomes low and exits the transition metal / zeolite catalyst 4, so that the temperature rise of the downstream catalysts 6, 8 and the arrival at the activation temperature are delayed. For this reason, at the time of a cold immediately after the start, a large amount of HC discharged from the engine immediately after the start tends to pass through the catalyst that has not yet reached the activation temperature and be discharged to the outside air.

In order to suppress such discharge of HC into the outside air, at least the start catalyst 6 of the downstream catalysts 6, 8 is provided with a heater. The heater is turned on only at the time of cold, and heats the start catalyst 6 so as to reach the activation temperature early. When the start catalyst 6 is a three-way catalyst, the activation start temperature is about 250 ° C.

The oxidation catalyst or three-way catalyst 8 disposed downstream of the start catalyst 6 is made of, for example, a Pt / alumina catalyst in which Pt is supported on an alumina carrier, and has a larger capacity than the start catalyst 6. After the engine is warmed up, HC and CO can be sufficiently purified. The Pt / alumina catalyst 8 takes a certain amount of time even without the help of a heater, but becomes higher than the activation temperature at the exhaust gas temperature.

The arrangement of the transition metal / zeolite catalyst 4, the start catalyst with heater 6, and the Pt / alumina catalyst 8 has the following meaning. The transition metal / zeolite catalyst 4 is
In a low temperature range of about 250 ° C. or less, the crystalline structure has a property of adsorbing HC. This low-temperature HC adsorption characteristic continues until the transition metal / zeolite catalyst 4 is saturated, and the HC adsorption time lasts several hours. However, when the temperature becomes higher than about 250 ° C., the transition metal / zeolite catalyst 4 has a property of releasing HC.

Therefore, when a large amount of HC is discharged at the time of cold immediately after the start of the engine, the transition metal / zeolite catalyst 4 is used even if the starting catalyst 6 is not yet warmed up to the activation temperature even when the heater is turned on. Is also a low temperature, most of HC is transition metal / zeolite catalyst 4
And the discharge to the outside is suppressed. When the transition metal / zeolite catalyst 4 is heated to a temperature equal to or higher than the HC desorption start temperature, the desorption of HC starts, and the desorbed HC flows downstream. By that time, the start catalyst with heater 6 (the example in FIG. 2) In this case, since the temperature of the catalysts 6 and 8) has been raised to the activation temperature or more by turning on the heater, the released HC is reduced to the start catalyst 6
(Catalysts 6 and 8 in the example of FIG. 2) are oxidized and the emission to the outside is suppressed. That is, by disposing the heater-equipped catalyst 6 (catalysts 6 and 8 in the example of FIG. 2) downstream of the transition metal / zeolite catalyst 4, it is possible to purify the desorbed HC during cold.

When the time further elapses, not only the catalyst 6 but also the most downstream catalyst 8 becomes higher than the activation temperature. At that time, the catalyst 6 and the catalyst 8 remove HC and CO that could not be completely purified by the upstream catalyst 4. Is done. In the example of FIG. 1, the reason why the start catalyst 6 is located upstream of the large-capacity catalyst 8 is to speed up the warm-up of the start catalyst 6. This is to prevent the warm-up of the start catalyst 6 from being delayed.

Further, since the heater of the start catalyst 6 with heater (catalysts 6 and 8 in the example of FIG. 2) only needs to be turned on at the time of cold, it is turned on when the ignition switch is turned on and is turned off after a predetermined time. It is only necessary to control the on / off of the heater so that the heater is turned on when the timer is turned on. That is, a switch may be provided in a circuit connecting the heater and the power supply battery, and this switch may be turned on when the timer is turned on. Alternatively, the water temperature is set to a predetermined value (for example, 60 °
C) The heater may be turned on for the following periods. The structure of the heater-equipped catalyst 6 (catalysts 6 and 8 in the example of FIG. 2) may be such that a nichrome wire heater or the like is arranged on the upstream side of the catalyst, or that the catalyst itself is made of a spirally wound metal plate having resistance. It may be composed of a material coated with a catalyst such as Pt supported thereon.

Further, since the transition metal / zeolite catalyst 4 exhibits thermal degradation at a high temperature of about 600 ° C. or more, the exhaust system 2
Of these, it is desirable to provide it at a site remote from the engine. Therefore, it is desired to be arranged under the floor of the vehicle.
For this reason, the start catalyst 6 and the Pt / alumina catalyst 8 are also located away from the engine, and the warm-up is slow. Therefore, in order to suppress HC and CO emissions during cold, the start catalyst 6 (in the example of FIG. The catalysts 6, 8) need to be equipped with a heater.

As is clear from the above description, the operation of the transition metal / zeolite catalyst 4 is H
Since it has the property of adsorbing C on the catalyst surface, it adsorbs HC immediately after the start of the engine. Therefore, release of HC to the outside air is suppressed.

After a while, the transition metal / zeolite catalyst 4 becomes higher than the HC desorption start temperature and desorbs HC.
The start catalyst 6 (catalysts 6 and 8 in the example of FIG. 2) is warmed up early by the heater, and the separated HC is removed from the start catalyst 6
(The catalysts 6 and 8 in the example of FIG. 2) are purified, and emission to the outside air is suppressed. Conventionally, since there is no heater-equipped start catalyst 6 and the large-capacity catalyst 8 has not yet reached the activation temperature, a large amount of HC may be released at this point. Does not occur. When the temperature further rises, the large-capacity catalyst 8 also reaches the activation temperature or higher, and even if the heater of the start catalyst 6 is turned off,
HC is sufficiently purified by the catalyst 8.

Regarding the purification of NOx, the transition metal / zeolite catalyst 4 causes the NOx purification activation temperature (about 2
Until the temperature reaches 50 ° C.), NOx is hardly purified, but when cold at the time of starting the engine, the amount of NOx emitted from the engine itself is small, so that there is no problem in NOx emission regulation.

When the exhaust gas temperature rises and the normal operation is started,
Since the transition metal / zeolite catalyst 4 is at or above the NOx purification activation temperature, NOx is highly reduced and purified in the air-fuel ratio lean region in the presence of HC. At this time, since there is no oxidation catalyst or three-way catalyst on the upstream side, sufficient HC
The quantity is supplied.

Even in an engine capable of lean combustion, the air-fuel ratio is made stoichiometric or slightly rich at the time of output or the like. At that time, however, the transition metal / NOx that exhibits NOx purifying ability only in the exhaust gas in the air-fuel ratio lean region is used. Although the zeolite catalyst 4 can hardly purify NOx, the three-way catalysts 6 and 8 on the downstream side become stoichiometric and have a good NOx purifying ability.
And 8 purify NOx. Therefore, in all operating states, the amount of NOx emission is reduced.

[0023]

According to the present invention, no heater is provided.
Downstream of the transition metal was supported by ion-exchange the transition metal / zeolite catalyst, including the start catalyst with the heater,
Since a catalyst having an oxidation ability (an oxidation catalyst or a three-way catalyst) is provided, it is possible to suppress the discharge of HC emissions to the outside air during the cold by turning on the heater during the cold. Also, turn on the heater and turn on the heater.
HC between until per catalyst becomes active temperature, discharged into the outside air is suppressed by the HC adsorption action of the transition metal / zeolite catalyst at low temperatures, separation of when the transition metal / zeolite catalyst has reached the HC leaving temperature There is no problem because HC is purified by the heater-equipped catalyst that is heated early.

[Brief description of the drawings]

FIG. 1 is a sectional view of an exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention.

FIG. 2 is a sectional view of an exhaust gas purifying apparatus for an internal combustion engine according to another embodiment of the present invention.

[Explanation of symbols]

2 Exhaust system 4 Transition metal / zeolite catalyst 6 Start catalyst with heater (catalyst having oxidizing ability, for example, three-way catalyst) 8 Pt / alumina catalyst (neither with heater nor with heater)

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁷ Identification code FI F01N 3/24 B01D 53/36 102H (72) Inventor Tetsu Iguchi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56 JP-A-5-31359 (JP, A) JP-A-5-31326 (JP, A) JP-A-6-106029 (JP, A) JP-A-4-358525 (JP, A) 6-218235 (JP, A) JP-A 4-129829 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) F01N 3/28 301 B01D 53/94 B01J 23/72 F01N 3 / 20 F01N 3/24

Claims (1)

(57) [Claims] To 1. A exhaust system of a lean burn internal combustion engine capable arsenide
And a transition metal / zeolite catalyst having a transition metal supported thereon by ion exchange of a transition metal, and including a start catalyst with a heater downstream of the transition metal / zeolite catalyst.
No exhaust gas control apparatus being characterized in that disposed a catalyst having an oxidizing ability.