JPH0598954A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

PURPOSE:To provide an exhaust emission control system for internal combustion engine which ca purify NOX even in the lean region of fuel air ratio at all times irrespective of the presence of HC. CONSTITUTION:An NOX absorbing member 2 is provided in an exhaust gas passage 6 and an NOX decomposition catalyst 4 is provided at its downstream, and a bypass passage 8 which bypasses them is provided. A bypass valve 10 is provided in branch section of both passages 6, 8, and the bypass valve 10 of NOX absorbing member 2 is controlled by a bypass valve control means 12 in such a way that the amount of exhaust gas on the exhaust gas passage 6 side is restricted by a bypass valve control means 12 when temperature of exhaust gas is high. NOX is absorbed by the NOX absorbing member 2 at the time of normal operation, and NOX which is discharged by the NOX absorbing member 2 when temperature of exhaust gas is high is decomposed by the NOX decomposition catalyst 4. The NOX decomposition catalyst 4 decomposes NOX even in the lean region without HC if decomposition speed is low space speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention purifies NOx using a combination of a NOx absorbent and a NOx decomposing catalyst, even in an internal combustion engine system that emits almost no HC, or in an internal combustion engine system that always operates in a lean air-fuel ratio range. The present invention relates to an exhaust emission control device for an internal combustion engine.

[0002]

2. Description of the Related Art A lean burn engine that burns in a lean region of an air-fuel ratio has been developed and partially put into practical use in order to improve fuel efficiency and reduce CO ₂ emission all at once.

The problem with the lean burn engine is that the three-way catalyst has almost no NOx purification capability in the exhaust gas in the lean range of the air-fuel ratio, so a system capable of purifying NOx in the exhaust gas in the lean range of the air-fuel ratio is required. It is to develop.

As a catalyst capable of purifying NOx even in a lean air-fuel ratio range, Japanese Patent Laid-Open No. 1-139145 proposes a so-called lean NOx catalyst in which a transition metal (Cu) is ion-exchanged and supported on zeolite. This catalyst is
NOx can be reduced in the presence of HC.

Further, as a system for purifying NOx of a lean burn engine, the applicant of the present invention has previously described Ba-C.
An exhaust gas purification device has been proposed which is composed of a combination of a uO-based NOx absorbent and a three-way catalyst. In this system, NOx is stored in the NOx absorbent at low temperature in the lean range of the air-fuel ratio, and when the air-fuel ratio becomes stoichiometric and the exhaust gas temperature becomes high, NOx is released to the NOx absorbent, and this release The NOx thus produced was reduced by the downstream three-way catalyst by utilizing the fact that the air-fuel ratio was stoichiometric.

[0006]

[Problems to be Solved by the Invention] However, lean NOx
The catalyst requires HC to reduce NOx, so HC
In the case of a system that does not generate much, for example, in the case of a diesel engine, it is necessary to generate HC by partially sacrificing combustion, and it is not possible to sufficiently bring out the effect of improving fuel efficiency of lean burn. There is.

In addition, in a system of a combination of a NOx absorbent and a three-way catalyst, in an engine which is always operated in a lean region of the air-fuel ratio, such as a diesel engine, the three-way catalyst has an opportunity to perform NOx purification. I ca n’t hold it,
After the NOx absorbent is saturated, there is a problem that NOx flows out as it is.

It is an object of the present invention to provide an exhaust gas purification apparatus for an internal combustion engine which can purify NOx even in an engine that emits almost no HC and in an engine that always operates in a lean air-fuel ratio range.

[0009]

According to the present invention, the above object is achieved by the following exhaust gas purification apparatus for an internal combustion engine. That is, the NOx absorbent installed in the exhaust passage of the internal combustion engine, which absorbs NOx at a predetermined temperature or lower and releases the absorbed NOx at a temperature higher than the predetermined temperature, and the NOx absorbent.
NOx decomposing catalyst downstream from the x absorbent, a bypass passage bypassing the NOx absorbing material and the NOx decomposing catalyst, an exhaust gas amount flowing to the NOx absorbent and the NOx decomposing catalyst side, and an exhaust gas amount flowing to the bypass passage. And a bypass valve for controlling the bypass valve so as to throttle the amount of exhaust gas flowing to the NOx absorbent and the NOx decomposition catalyst side for a predetermined time when the exhaust gas temperature exceeds a predetermined temperature. An exhaust emission control device for an internal combustion engine, comprising: a control unit.

[0010]

For example, the Ba-Cu-O-based NOx absorbent is
It occludes NOx for a relatively long time below a specified temperature (about 500 ° C), and when it exceeds the specified temperature, it will occlude in a short time.
To release. Further, the NOx decomposing catalyst decomposes NOx into N ₂ and O ₂ when the space velocity is low even in the absence of HC.

When the NOx absorbent is placed under the floor of the vehicle, the exhaust gas from the engine during normal operation becomes 500 ° C. or less due to heat dissipation in the exhaust pipe on the way.
NOx is absorbed by the NOx absorbent, and its release to the outside is suppressed. When the exhaust gas temperature rises, such as during acceleration, the NOx absorbent exceeds a predetermined temperature, releases the NOx stored up to that point, and regenerates itself in a short time. NO released
x reaches the NOx decomposition catalyst on the downstream side, where N ₂ and O ₂
As it is decomposed into, the release to the outside is suppressed.

The NOx decomposition catalyst is Cu-ZSM-5, P
It is composed of t / Al ₂ O ₃ or Co / Al ₂ O _3, etc., and is N under low space velocity conditions even without HC (reducing agent).
Decomposes Ox. Since the space velocity SV of a diesel engine is high and the NOx decomposing catalyst has a weak NOx decomposing ability as it is, a bypass valve is used to reduce the amount of exhaust gas flowing to the NOx absorbent and the NOx decomposing catalyst side at high exhaust temperature to reduce the SV. In this case, the exhaust gas flowing to the side of the bypass passage is discharged without purifying NOx, but such a state lasts only for a short time, for example, 10 seconds, which causes no problem.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an 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 FIG. 1, an NOx absorbent 2 and a NOx decomposition catalyst 4 are arranged downstream of the NOx absorbent 2 in an exhaust passage 6 of an internal combustion engine (for example, a diesel engine). A bypass passage 8 is provided in the exhaust passage 6 so as to bypass the NOx absorbent 2 and the NOx decomposition catalyst 4. A bypass valve 10 is provided at a branch point between the exhaust passage 6 and the bypass passage 8 to change the ratio of the amount of exhaust gas flowing to the NOx absorbent 2 and the NOx decomposition catalyst 4 side to the amount of exhaust gas flowing to the bypass passage 8. .. The bypass valve 10 is controlled by the bypass valve control means 12 so that the exhaust gas temperature T is the predetermined temperature T.
When it exceeds ₀ , NO for a predetermined time, for example, 10 seconds.
The amount of exhaust gas flowing to the x absorber 2 and the NOx decomposition catalyst 4 side is throttled.

FIG. 2 shows a concrete system diagram. 1
Reference numeral 4 is a control device, which is composed of a microcomputer. The bypass valve control means 12 comprises means set in the program stored in the control device 14, that is, in the programs of FIGS. The control device 14 is a central processor unit (CPU) 14 that executes calculations.
a, read only memory (R
OM) 14b, random access memory (RAM) 14bc that is a memory for temporary storage, A / D converter 14d that converts an analog amount into a digital amount, an input interface 14e, an output interface 14
f. A timer 14g, which will be described later, is further connected to the control device 14.

An exhaust temperature sensor 16 is installed upstream of the NOx absorbent 2 in the exhaust passage 6, and its output is A / D.
It is input to the converter 14d and used as the exhaust temperature T in the calculation. Further, the calculation command of the CPU 14a is sent to the actuator of the bypass valve 10 via the output interface 14f, and the bypass valve 10
Is controlled.

In the above apparatus, the NOx absorbent 2 is disposed in the exhaust passage portion where the exhaust gas temperature is about 500 ° C or less during normal operation and the exhaust gas temperature exceeds 500 ° C during acceleration or the like. Such an exhaust passage portion is usually an underfloor portion of the vehicle 20, which is separated from the engine 18, as shown in FIG. In FIG. 5, 22 indicates a muffler.

The NOx absorbent 2 has a predetermined temperature (for example, 5
It is made of a solid material that absorbs NOx at a temperature of less than 00 ° C and releases the absorbed NOx at a temperature higher than the above-mentioned predetermined temperature. As a NOx absorbent having such properties, a composite oxide of alkaline earth and copper ( Ba-Cu-O system), for example, Mn
There are O ₂ and BaCuO ₂ . Ba-Cu-O-based NOx
At about 500 ° C. or below, the absorber 2 turns NO into NO ₂ type and can stably absorb it into the NO x absorber crystal structure. The Ba-Cu-O-based NOx absorbent 2 is about 500 °
At temperatures in excess of C, the NO ₂ lost absorption capacity, the NO ₂ which has been absorbed far, far rapidly than the absorption step has a property of releasing. For example, what is absorbed over several hours is released in about 10 seconds.

The above Ba-Cu-O-based NOx absorbent 2
Is produced, for example, as follows. Production Example 1 Platinum-containing copper nitrate and Ba nitrate were mixed in an equimolar ratio and calcined in air at 650 ° C. for 4 hours. This was immersed in platinum octanitrate to support platinum. Then, it was baked in a nitrogen stream at 500 ° C. Production Example 2 Copper nitrate containing cerium and Ba nitrate were mixed in an equimolar ratio, and the mixture was baked at 650 ° C. for 4 hours in air. This was immersed in a cerium nitrate solution to carry cerium. Then, it was fired in an air stream at 500 ° C. Monolithization 100 parts of the powder of the above fired product, 100 parts of silica sol,
A slurry was prepared by mixing 10 parts of partial water, and a monolith made of cordierite was immersed therein. The plugging cell was clogged with air and dried at 250 ° C. A series of operations was repeated several times. Then, it was baked at 500 ° C. to obtain a NOx absorbent 2.

Further, FIG. 6 shows that Ba--Cu--O (Ba: C).
u = 1: 13) Shows the characteristics of NO concentration and temperature in the outlet gas when NOx is supplied to the NOx absorbent.
However, the space velocity is 6000 / hour in the absence of oxygen. When tested in the presence of oxygen, the NOx emission characteristics of FIG. 6 shift to the right at 50-100 ° C. From this, B
The a-Cu-O-based NOx absorbent is NO at a temperature exceeding about 500 ° C in exhaust gas with excess oxygen in the lean air-fuel ratio range.
It can be seen that x is released and NOx is absorbed below about 500 ° C. Further, when the air-fuel ratio is stoichiometric or rich, the NOx releasing characteristic shifts to the low temperature side due to the near non-oxygen coexistence, and the NOx releasing is promoted.

The NOx decomposition catalyst 4 is Cu-ZSM.
-5, that is, a catalyst in which a transition metal such as Cu is ion-exchanged and supported on zeolite ZSM-5, or alumina Al
_It is composed of a catalyst in which platinum Pt is supported on ₂ O ₃ , a catalyst in which cobalt Co is supported on alumina, and the like, each of which contains NOx at a low space velocity and no hydrocarbon HC,
_It can be decomposed into ₂ and O ₂ . On the contrary, if the amount of HC is too large, the poisoning of HC will occur and the NOx decomposition efficiency will decrease, and at high space velocity, the NOx decomposition efficiency will be poor. Therefore, the NOx absorbent 2
When releasing NOx in a high temperature range, the exhaust gas flow flowing per unit area of the NOx decomposing catalyst 4 must be low speed (low space velocity) in order to decompose the released NOx with high efficiency. For this reason, the bypass valve control means 12 throttles the exhaust gas flowing to the NOx absorbent 2 and the NOx decomposition catalyst 4 side when the exhaust gas temperature is high.
The bypass valve 10 is controlled.

3 and 4 show that NOx is NO during normal operation.
The NOx absorbent 2 absorbs N when the exhaust temperature is high.
A flow chart of a program for controlling the bypass valve 10 so as to release Ox and throttle the exhaust gas flow to cause the NOx decomposition catalyst 4 to decompose NOx is shown. These programs are stored in the ROM 14b and are read by the CPU 14a to execute the calculation. The flowcharts of FIGS. 3 and 4 are interrupted at regular time intervals.

In the flow of FIG. 3, step 102
Then, it is determined whether the bypass flag operation flag Fdb is 1 or not. Fdb = 1 corresponds to the case where the bypass flag 10 throttles the amount of exhaust gas flowing to the NOx absorbent 2 and the NOx decomposition catalyst 4 side and causes the exhaust gas to flow also to the bypass passage 8 side, and Fdb = 0 indicates the total amount of exhaust gas. Is flowing to the NOx absorbent 2 and the NOx decomposition catalyst 4 side.

If it is determined in step 102 that Fdb = 1, the bypass valve 10 is already in operation,
Since it is not necessary to perform the bypass valve operation through steps 104 to 110, the process directly returns. If it is determined in step 102 that Fdb = 1 is not satisfied, the process proceeds to step 104, and it is determined whether the rotation integration flag Fdg is 1. Fdg = 1 means that the integrated time SNe after the NOx absorbent 2 is regenerated does not exceed the predetermined time SNeO,
It means that the NOx absorbent 2 is not saturated, and Fdg
= 0 means that the NOx absorbent 2 is saturated or is about to be saturated.

When it is judged in step 104 that Fdg = 1, it is not necessary to regenerate the NOx absorbent 2 since no time has passed since the NOx absorbent 2 was released with NOx, and the process is returned as it is. If it is determined that Fdg = 1 is not established, the routine proceeds to step 106, where it is determined whether the exhaust temperature T is higher than the predetermined temperature T ₀ . Where T ₀ is
When the NOx absorption catalyst 2 is in the exhaust of lean air-fuel ratio, NOx
It is the temperature at the boundary between the temperature range that absorbs NOx and the temperature range that releases NOx, and is usually near 500 ° C.

When it is determined in step 104 that T is equal to or less than T ₀ , the NOx absorbent 2 is not at a temperature at which NOx can be released, and therefore, the routine returns as is, and when it is determined that T exceeds T ₀ , In order to control the bypass valve 10 so that the NOx decomposition catalyst 4 can decompose NOx released by the NOx absorbent 2 with high efficiency, steps 108 and 110 are performed.
Go to.

In step 108, the bypass valve 10 is operated when passing through this route, so the bypass valve operation flag Fdb is set to 1. Then, step 1
10, the bypass valve 10 is operated and the timer 14g is turned on. The operation of the bypass valve 10 means that the bypass valve 10 is operated so as to reduce the amount of exhaust gas flowing to the NOx absorbent 2 and the NOx decomposition catalyst 4 side.

The timer 14g clears the cumulative engine speed SNe to 0 and the bypass valve operation flag Fdb to 0 about 10 seconds after being turned on. Therefore, the timer 14g continues the operation of the bypass valve 10 for about 10 seconds, during which the NOx of the NOx absorption catalyst 2 is NOx.
The emission is completed, and Fdb is cleared to 0 10 seconds after the ON state when the NOx emission is supposed to be completed. In the above,
The steps 106 and 110 and the timer 14g constitute the bypass valve control means 12.

The routine of FIG. 3 is interrupted by the routine of FIG. The routine of FIG. 4 is the same as the routine of FIG.
When the absorbent 2 is regenerated, the bypass valve 10 is kept until the NOx absorbent 2 is saturated even if the exhaust temperature T exceeds T _0.
Is a routine for preventing the exhaust gas from flowing through the bypass passage 8 to the outside without being purified of NOx for a long time.

In step 202, it is determined whether Fdb = 1. About 10 seconds after turning on the timer 14g, Fdb = 0
Will be cleared to Fdb =
Since it is not 1, the routine proceeds to step 204, where the usage time after regeneration of the NOx absorbent 2 is accumulated for each interruption to the routine. When it is determined in step 202 that Fdb = 1, the NOx absorbent 2 is on the verge of saturation, and therefore the routine directly returns and enters the next interruption to the routine of FIG.

When it is determined at step 202 that Fdb = 1 and the routine proceeds to step 204, the engine revolution integrated SNe is accumulated, and at step 206 it is determined whether SNe exceeds a predetermined engine revolution integrated SNeO. SNe
Is less than SNeO, it may be considered that the NOx absorbent 2 is not yet saturated, so the routine proceeds to step 212 and Fd
When g = 1, the preparation for the calculation in FIG. 3 is performed. Step 202
If it is determined that SNe has exceeded SNeO, the process proceeds to steps 208 and 210 to set Fdb = 1 and Fdg = 1 in preparation for the calculation of FIG.

Next, the operation will be described. During normal operation, the entire amount of exhaust gas flows to the NOx absorbent 2 and the NOx decomposition catalyst 4 side. At this time, the exhaust gas temperature is NOx
NOx in the exhaust gas is absorbed by the NOx absorbent 2 because it is 500 ° C or less at the inlet of the absorbent 2. Although the NOx absorption capacity of the NOx absorbent 2 depends on the volume of the NOx absorbent, it is usually such that it can absorb NOx continuously for several hours. During several hours of continuous operation, the accelerating condition appears repeatedly, and at that time, the exhaust gas temperature is 5 at the NOx absorbent 2 inlet.
Over 00 ° C. In this high temperature state, the NOx absorbent 2 releases NOx.
Try to play. That is, when releasing NOx, NOx
The bypass valve 10 restricts the exhaust gas flow flowing through the NOx decomposing catalyst 4 so that the decomposing catalyst 4 has a highly efficient NOx decomposing ability, so as to have a low space velocity. By this, N
The NOx released from the Ox absorbent 2 is the NOx decomposition catalyst 4
At high efficiency, it is decomposed into N ₂ and O ₂ .

Regeneration of the NOx absorbent 2, that is, absorbed NO
Full release of x is achieved by flowing exhaust gas above 500 ° C for about 10 seconds. During this time, a part of the exhaust gas is caused to flow into the bypass passage 8 and the exhaust gas passing through the bypass passage is discharged to the outside without decomposing NOx.
This state is as short as about 10 seconds, so due to NOx emission regulations,
It doesn't matter.

The absorption of NOx by the NOx absorbent 2 and the decomposition of NOx by the NOx decomposition catalyst 4 at high exhaust gas temperature can be performed without the presence of HC, and the internal combustion is always operated in the lean range. It can also work in engines, for example diesel engines. Therefore, it can also be used for NOx purification of an exhaust system of an internal combustion engine to which a conventional lean NOx catalyst or a combination system of a NOx absorbent and a three-way catalyst cannot be applied.

[0034]

According to the present invention, the NOx absorbent and the NOx decomposition catalyst are provided downstream of the NOx absorbent in the exhaust passage of the internal combustion engine, and
A bypass passage that bypasses both the x-absorbent and the NOx-decomposing catalyst, and a bypass valve that changes the ratio of the flow rate to both passages are provided, and the exhaust gas flowing to the NOx-absorbing material and the NOx-decomposing catalyst at high exhaust temperature is provided by the bypass valve control means. Since it is throttled, it effectively purifies NOx regardless of whether the amount of HC in the exhaust gas is or not, and whether it is always operated in the lean range or is repeatedly operated in the lean range or stoichiometry. be able to.

[Brief description of drawings]

FIG. 1 is a system diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention.

FIG. 2 is a system diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention, which is more detailed than FIG.

FIG. 3 is a flowchart of regeneration control of the NOx absorbent used in FIG.

FIG. 4 is a flowchart of a routine for controlling various flags so as not to enter NOx regeneration step 106-110 until a certain time SNeO has elapsed after the regeneration of the NOx absorbent is completed.

5 is a schematic side view of a state in which the exhaust emission control device for the internal combustion engine of FIG. 1 is mounted on a vehicle.

FIG. 6 is a NOx absorption / release / exhaust temperature characteristic diagram of the NOx absorbent used in FIG. 1.

[Explanation of symbols]

 2 NOx absorbent 4 NOx decomposition catalyst 6 exhaust passage 8 bypass passage 10 bypass valve 12 bypass valve control means 14 control device 16 exhaust temperature sensor 18 internal combustion engine 20 vehicle 22 muffler

Claims (1)

[Claims] 1. An NOx absorbent installed in an exhaust passage of an internal combustion engine, which absorbs NOx at a predetermined temperature or lower and releases the absorbed NOx at a temperature higher than the predetermined temperature, and the NOx.
NOx decomposition catalyst downstream from the x absorbent, a bypass passage bypassing the NOx absorbent and the NOx decomposition catalyst, an exhaust gas amount flowing to the NOx absorbent and the NOx decomposition catalyst side, and an exhaust gas amount flowing to the bypass passage. And a bypass valve that controls the bypass valve so that when the exhaust gas temperature exceeds a predetermined temperature, the amount of exhaust gas flowing to the NOx absorbent and the NOx decomposition catalyst side is throttled for a predetermined time. An exhaust emission control device for an internal combustion engine, comprising: a control means.