JPH1026014A - Emission control system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an NOX control factor by a method wherein the temperature of a lean NOX catalyst is always maintained at a value within a controlling temperature window. SOLUTION: In an emission control system for an internal combustion engine formed such that a lean NOX catalyst is arranged at an exhaust system and an HC addition means is arranged upper stream from the lean NOX catalyst, a second catalyst carrier having heat capacity higher than that of the carrier of the lean NOX catalyst is arranged upper stream from an HC addition means. The second catalyst carrier forms an oxidation catalyst and a second HC addition means is arranged upper stream from the second catalyst carrier. When the temperature of exhaust gas flowing in the lean NOX catalyst is low, by operating the second HC addition means, the second catalyst carrier is heated. Or, the second catalyst carrier may form a high temperature type leans NOX catalyst and the downstream lean NOX catalyst may form a low temperature type lean NOX catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a lean N
The present invention relates to an exhaust gas purification device for an internal combustion engine having an O _x catalyst and an HC addition means provided upstream of the lean NO _x catalyst.

[0002]

2. Description of the Related Art In recent years, there has been a remarkable progress in the technology of purifying automobile exhaust gas. The harmful components in the exhaust gas include HC (hydrocarbon), CO (carbon monoxide) and NO _x (nitrogen oxide).
Is mentioned. In many engines, the exhaust gas purification rate is improved by achieving combustion at a stoichiometric air-fuel ratio by feedback control using an O ₂ sensor and using a three-way catalyst. The three-way catalyst removes H in the exhaust gas.
C, is intended to promote the reduction of the oxidized and NO _x in the CO at the same time, shows a high purification rate when the air-fuel ratio of the mixture burned is near the stoichiometric air-fuel ratio.

[0003] However, burning a mixture having a stoichiometric air-fuel ratio is not always the best from the viewpoint of fuel economy. Therefore, a lean burn (lean burn) engine has been developed as a gasoline engine, and the application range of a diesel engine is expanding. In a diesel engine or a lean burn engine, since the lean mixture is burned, the amount of unreacted O ₂ in the exhaust gas is large, and the reduction of NO _x is insufficient by the ordinary three-way catalyst.

Therefore, a lean NO _x catalyst capable of reducing and purifying NO _{x in} exhaust gas in a lean state, that is, a state in which O ₂ is excessively used, is used. As the lean NO _x catalyst, a zeolite-based catalyst supporting a transition metal or a noble metal is often used. Although in _the NO _x purification by lean NO _x catalyst requires the presence of a reducing agent such as HC, since it is insufficient in the amount of reducing agent present in the exhaust gas, the reducing agent upstream of the lean NO _x catalyst Is often provided. JP-4-214919 discloses relates to a exhaust purification device provided with a HC addition means on the upstream side of the lean NO _x catalyst with placing lean NO _x catalyst in an exhaust system, in particular, a lean NO _x catalyst A technique is disclosed in which a plurality of units including HC adding means are arranged in series, and HC is supplied to a lean NO _x catalyst within a predetermined temperature range, thereby improving the NO _x purification rate.

[0005]

THE INVENTION Problems to be Solved] lean NO _x catalyst is N
The temperature range in which O _x can be reduced and purified, that is, the temperature window of the lean NO _x catalyst is a narrow range (from a ° C. to b ° C.) as shown in FIG. Therefore, when the temperature of the exhaust gas flowing into the catalyst changes abruptly as shown in FIG. 2A, as in the case of actual running of the vehicle, the temperature of the exhaust gas falls within the temperature window of the lean NO _x catalyst. short are time, as a result, as shown in FIG. 2 (B), NO _x purification rate is low.

[0006] The techniques disclosed in JP-A 4-214919 discloses that the is provided with a plurality of units consisting of a lean NO _x catalyst and the HC addition means, of which HC addition the temperature corresponding to the catalyst is within the above window Although it is intended to activate the means, it is not always possible to maintain a high purification rate at all times because the catalyst temperature of all the units may not be within the window.

[0007] In view of such circumstances, an object of the present invention is to dispose a lean NO _x catalyst in an exhaust system and to install the lean NO _x catalyst in the exhaust system.
In O _x exhaust gas purification apparatus for an internal combustion engine provided with a HC addition means on the upstream side of the catalyst, by to be able to maintain the temperature of the lean NO _x catalyst always purification temperature window, of the NO _x purification rate The goal is to improve. In addition, an object of the present invention is to contribute to prevention of air pollution.

[0008]

It was devised to achieve the above object, according to the solution to ## according to the first aspect of the present invention, the with the exhaust gas purifying apparatus for an internal combustion engine, placing the lean NO _x catalyst in an exhaust system in the exhaust purification system of an internal combustion engine provided with a HC addition means on the upstream side of the lean NO _x catalyst, the lean NO _x catalyst carrier (dispersing a substance that acts as a catalyst on the upstream side of the HC addition means, to hold Material), a second catalyst carrier having a larger heat capacity is provided.

Further, the exhaust gas purification apparatus according to a second aspect of the present invention is the exhaust gas purification apparatus according to the first aspect,
The second catalyst carrier is used as an oxidation catalyst and the second
The catalyst comprises a second HC addition means on the upstream side of the carrier, the lean NO _x catalyst flows the second catalyst support by allowed to operate the second HC addition means when the temperature of the exhaust gas is low Is heated.

[0010] The exhaust gas purification apparatus according to a third aspect of the present invention is the exhaust gas purification apparatus according to the first aspect,
Wherein with the second catalyst support purification rate at the high temperature wide range with high high-temperature type lean NO _x catalyst, the downstream side of said lean NO _x catalyst cold narrow high purification rate in the range low-temperature lean NO _x catalyst It is characterized by having.

In the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect of the present invention, which is configured as described above, even if the temperature of the exhaust gas changes suddenly during the transient operation, the second exhaust gas purifying method is performed.
The heat capacity of the catalyst carrier can be kept the temperature of the lean NO _x catalyst on the downstream side at all times within the high temperature range of the purification rate. Further, in the exhaust gas purification apparatus according to the second aspect of the present invention, even when activation of the lean NO _x catalyst may be delayed during a cold start, HC is added to the upstream side of the second catalyst carrier. As a result, the heat of the oxidation reaction can be generated in the second catalyst carrier, so that such a delay in activation can be prevented. Further, in the exhaust gas purification device according to a third aspect of the present invention, disadvantage purification temperature range of low temperature lean NO _x catalyst on the downstream side narrower, the upstream side to achieve a high purification rate at elevated temperatures of a wide range of it is eliminated by being combined with a high-temperature type lean NO _x catalyst, as a result, it is possible to improve _the NO _x purification rate in a wide temperature range from low temperatures to high temperatures.

[0012]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 3 is a block diagram of the exhaust gas purifying apparatus for an internal combustion engine according to the first embodiment of the present invention. Reference numeral 1 denotes a diesel engine, reference numeral 2 denotes a distribution type fuel injection pump, and reference numerals 3a, 3b and 3c denote parts constituting the exhaust passage 3. Two catalytic converters 4 and 5 are arranged in the exhaust passage 3. The upstream catalytic converter 4 contains an oxidation catalyst (for example, a noble metal catalyst), and a carrier having a large heat capacity such as cordierite is used as a carrier for supporting the oxidation catalyst. On the other hand, the downstream catalytic converter 5 contains a lean NO _x catalyst, and the lean NO _x catalyst is supported on a carrier having a small heat capacity such as metal.

Further, in order to add the fuel i.e. diesel oil (HC) as a reducing agent in the exhaust gas, upstream of the exhaust passage downstream catalytic converter 5 accommodating the lean NO _x catalyst 3
b and the fuel injection pump 2 are communicated with each other by HC supply passages 6a, 6b and 6c.
A downstream HC addition control valve 7 is provided between b and 6c. Similarly, the exhaust passage 3a and the fuel injection pump 2 are connected to the H so that HC can be added to the exhaust passage 3a on the upstream side of the upstream catalytic converter 4 containing the oxidation catalyst.
C supply passages 6a, 6d and 6e and an upstream HC between the HC supply passages 6d and 6e.
An addition control valve 8 is provided.

A secondary air introduction passage 9 and a secondary air introduction pump 10 are provided so that secondary air can be introduced into the exhaust passage 3b. Further, an exhaust gas temperature sensor 11 is provided to detect the temperature of the exhaust gas flowing into the downstream catalytic converter 5. As will be described later, the control unit 12 of the exhaust gas purification apparatus controls the downstream HC addition control valve 7, the upstream HC addition control valve 8, and the secondary HC addition control valve in accordance with the output signal (exhaust gas temperature flowing into the catalyst) of the exhaust gas temperature sensor 11. It outputs a signal for controlling the air introduction pump 10.

As described above, since the upstream catalyst carrier has a large heat capacity such as cordierite, the fluctuation range of the temperature of the exhaust gas flowing into the upstream catalyst carrier is limited as shown in FIG. Even if it is large, the fluctuation range of the temperature of the exhaust gas flowing out therefrom becomes small. That is, the temperature of the variation range of the exhaust gas flowing out from the upstream side catalyst carrier becomes possible to within the desired range, in the present embodiment, the exhaust gas into _the NO _x purification temperature window of the lean NO _x catalyst on the downstream side The heat capacity of the carrier is adjusted by changing the size of the carrier so that the temperature enters.

[0017] Incidentally, when starting the engine or the like, when the upstream catalyst carrier is cold, because the heat capacity of the carrier is large, it takes a time until the effluent gas temperature reaches _the NO _x purification temperature window. In that case, the upstream side H
The C addition control valve 8 is opened, and the HC supply passages 6a, 6d and 6
The control is performed so that HC is added to the exhaust passage 3a via e. Then, HC is burned in the oxidation catalyst 4,
Since the catalyst carrier is heated early by the combustion heat, the above-mentioned problem of the cold start can be easily solved. in this way,
The upstream HC addition means and the oxidation catalyst carried on the catalyst carrier are provided as heating means, and can be omitted when there is no need to consider such a problem of cold start. That is, at this time, the supported substance does not have to be present on the catalyst carrier of the upstream catalytic converter 4.

Meanwhile, when a high-load operation of the uphill or the like is continued, stable at high temperatures than _the NO _x purification temperature window temperature of the exhaust gas flowing out of the upstream catalyst carrier heat capacity large is the downstream lean NO _x catalyst There is a risk that it will. In this case, by turning on the secondary air introduction pump 10, the secondary air passes through the passage 9 and the exhaust passage 3b
Control to be introduced to. As described above, the secondary air introduction passage 9 and the secondary air introduction pump 10 function as cooling means, and when there is no problem of such a high temperature state, they can be omitted.

In order to realize the above control, the control unit 12 of the exhaust gas purifying apparatus controls the downstream side HC addition control valve 7 according to the lean NO _x catalyst inflowing exhaust gas temperature T obtained from the output of the exhaust gas temperature sensor 11. The upstream HC addition control valve 8 and the secondary air introduction pump 10 are controlled as follows. That is, when the temperature T is within the NO _x purification temperature window (see FIG. 1), that is, when a ≦ T ≦ b, the downstream side HC addition control valve 7 is opened to achieve reduction and purification of exhaust gas. Further, when the temperature T is lower than _the NO _x purification temperature window, i.e. T <when a, open the upstream HC addition control valve 8, to promote HC oxidation in the oxidation catalyst, early warm-up due to the combustion heat Plan. Further, when the temperature T is higher than _the NO _x purification temperature window, i.e. when b <T is a secondary air introduction pump 10 is turned on to cool the exhaust gas.

By arranging the heat capacity member upstream of the lean NO _x catalyst in this manner, even in the operating region where the temperature of the exhaust gas initially flowing into the exhaust passage 3a greatly fluctuates, the lean NO _x catalyst Temperature of the exhaust gas flowing into the NO _x purification window can be suppressed, and a stable and high NO _x purification rate can always be maintained. Incidentally, as in this embodiment, the lean NO _x catalyst, as the temperature of the whole becomes uniform, it is preferably supported on a small carrier heat capacity, such as metal.

Next, a second embodiment of the present invention will be described. FIG. 5 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to the second embodiment. Components that are the same as the components in FIG. 3 are given the same reference numerals, and descriptions thereof will be omitted. As shown in FIG. 5, the second embodiment is obtained by modifying the upstream-side catalytic converter 4 in the first embodiment. That is, the upstream-side catalytic converter 4 includes two components, and the downstream-side component 4b includes the first component.
An oxidation catalyst is supported on a carrier made of cordierite as in the embodiment, but the upstream component 4a
Is a three-way catalyst with an electric heater. Battery 13
Is supplied to the electric heater by the control unit 12. According to this configuration, the temperature T becomes N
The warm-up when the temperature is lower than the O _x purification temperature window can be performed more quickly than in the first embodiment.

FIG. 6 shows the first embodiment and the second embodiment described above.
When control is performed according to the form, the exhaust
Gas-gas temperature change (A) is lean NO_xJust before the catalyst
Is adjusted to such a temperature change (B₁, B_Two)
And the resulting NO_xChange in purification rate
(C₁, C_Two) And the lean NO_xCatalyst and HC addition
When performing control using the conventional configuration with only stages
(B₀, C₀FIG. Shown in this figure
As described above, according to the present invention, a stable high NO _x
Purification rates are achieved. Note that in the second embodiment, the electric
Quick start-up of purification by using a three-way catalyst with a motor
It has become.

Finally, a third embodiment of the present invention will be described. FIG. 7 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to the third embodiment. Components that are the same as the components in FIG. 3 are given the same reference numerals, and descriptions thereof will be omitted. Also in the third embodiment, similarly to the first and second embodiments, a catalyst carrier having a large heat capacity such as cordierite is used as the catalyst carrier of the upstream catalytic converter 4 and the catalyst carrier of the downstream catalytic converter 5 is used. Is used, such as metal having a small heat capacity. Therefore, similarly to the first and second embodiments, the temperature of the exhaust gas flowing into the downstream catalyst can be maintained in a temperature range where the purification rate is high.

Meanwhile, in the third embodiment, together with the upstream side catalyst carrier high temperature lean NO _x catalyst is supported, on the downstream side catalyst carrier low temperature lean NO _x catalyst. Here, examples of the low-temperature lean NO _x catalyst has a noble metal catalyst such as Pt, as shown in FIG. 8, NO at low temperature narrow range (from c ° C to d ° C)
Purify _x . On the other hand, examples of the high-temperature lean NO _x catalyst, Cu, Co, there is a base metal catalyst such as Ag, as shown in the figure, e ° C from the hot wide range (d ° C
) To purify NO _x .

Therefore, in this configuration, unlike the first and second embodiments, the upstream catalytic converter can also contribute to NO _x purification. Figure 9 is a diagram illustrating a change in the catalyst inlet exhaust gas temperature for the upstream side of the high-temperature lean NO _x catalyst (solid line) and the downstream side of the low temperature lean NO _x catalyst (dashed line). As shown in this figure, in the low temperature lean NO _x catalyst on the downstream side, the change in exhaust gas temperature has been allowed to decay (from c ° C to d ° C) temperature window shown in FIG. 8 in Has been maintained. The disadvantage purification temperature range of low temperature lean NO _x catalyst on the downstream side is narrow, e ° from the hot wide range (d ° C C
High temperature lean N that achieves high purification rate
It is eliminated by being combined with the O _x catalyst, so that a wide temperature range from low to high (c ° C to e)
° C), the NO _x purification rate can be improved.

Although the embodiments of the present invention have been described above, the present invention is of course not limited to these embodiments, and it will be easy for those skilled in the art to devise various embodiments.

[0027]

As described above, according to the present invention,
In the exhaust purification system of an internal combustion engine provided with a HC addition means on the upstream side of the lean NO _x catalyst with placing lean NO _x catalyst in an exhaust system, the temperature of the lean NO _x catalyst is maintained constantly purification temperature window NO
_{x The} purification rate is greatly improved. As a result, the present invention contributes to prevention of air pollution.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature characteristic of a NO _x purification rate by a lean NO _x catalyst.

FIG. 2 shows a flow of a catalyst according to a change in time when the vehicle is running.
Inlet and exhaust gas temperature (A) and NO _xWhat is the purification rate (B)
FIG. 7 is a diagram illustrating an example of how it changes.

FIG. 3 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a state in which exhaust gas passes through a catalyst carrier having a large heat capacity, thereby alleviating temperature fluctuations.

FIG. 5 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to a second embodiment of the present invention.

[6] When performing the control according to the first embodiment and the second embodiment, the exhaust gas temperature changes immediately after the exhaust passage flows (A) is adjusted to any temperature change in the lean NO _x catalyst just before (B ₁ , B ₂ )
Changes in the resulting NO _x purification rate (C ₁ , C ₂ )
FIG. 3 is a diagram showing a comparison with a case where control is performed by a conventional configuration including only a lean NO _x catalyst and HC adding means (B ₀ , C ₀ ).

FIG. 7 is a configuration diagram of an exhaust gas purification device for an internal combustion engine according to a third embodiment of the present invention.

FIG. 8 is a diagram showing temperature characteristics of a NO _x purification rate by a low-temperature lean NO _x catalyst and temperature characteristics of a NO _x purification rate by a high-temperature lean NO _x catalyst.

[9] high temperature lean NO _x catalyst on the upstream side of the change in catalyst inlet exhaust gas temperature (solid line) and the downstream low temperature lean N
It is a diagram illustrating the O _x catalyst (dashed line).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Diesel engine 2 ... Distribution type fuel injection pump 3a, 3b, 3c ... Exhaust passage 4 ... Upstream catalytic converter 5 ... Downstream catalytic converter 6a, 6b, 6c, 6d, 6e ... HC supply passage 7 ... Downstream HC addition Control valve 8 ... Upstream side HC addition control valve 9 ... Secondary air introduction passage 10 ... Secondary air introduction pump 11 ... Exhaust temperature sensor 12 ... Control unit 13 ... Battery

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiaki Tanaka 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (3)

[Claims] 1. Lean NO in the exhaust system_xWhen the catalyst is placed
Both are lean NO _xProvide HC addition means upstream of the catalyst.
In the exhaust gas purifying apparatus for an internal combustion engine, the HC
Lean NO upstream of the stage_xGreater heat than catalyst support
A second catalyst carrier having a capacity is provided.
An exhaust gas purification device for an internal combustion engine. 2. The exhaust gas flowing into the lean NO _x catalyst has a low temperature, the second catalyst carrier being an oxidation catalyst, and a second HC adding means being provided upstream of the second catalyst carrier. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the second catalyst carrier is heated by operating the second HC adding means. 3. A with purification rate of the second catalyst support in a wide range of high temperature and high high-temperature type lean NO _x catalyst,
It said lean NO _x catalyst on the downstream side, characterized in that the purification rate at low temperatures narrow range has higher low-temperature lean NO _x catalyst, an exhaust purification system of an internal combustion engine according to claim 1.