JPH0610656A - Exhaust emission control system of engine - Google Patents

Abstract

PURPOSE:To prevent the thermal deterioration caused by the excessive temperature rise of the catalyst without using a catalyst temperature sensor or even when the sensor is failured in an exhaust emission control system where a bypass passage to bypass an NOx purifying catalyst is provided and the exhaust gas flows in the bypass passage when the temperature of the catalyst is high. CONSTITUTION:The running condition of an engine is detected by an engine load sensor 6 and an engine speed sensor 7, a judgement is made on the shifting tendency of the temperature of the catalyst 1 based on the running condition of the engine, and the opening/closing of a bypass valve 5 is controlled by a valve controlling means 10 based on the results of the judgement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine exhaust gas purifying apparatus.

[0002]

2. Description of the Related Art As a catalyst for purifying engine exhaust gas,
Oxidation of CO (carbon monoxide) and HC (hydrocarbons), N
A three-way catalyst that simultaneously performs reduction of Ox (nitrogen oxide) is generally known. This three-way catalyst is, for example, γ-alumina on which Pt (platinum) and Rh (rhodium) are supported, and when the air-fuel ratio (A / F) of the engine is controlled to near the theoretical air-fuel ratio of 14.7. , High purification efficiency can be obtained.

On the other hand, in the field of automobiles, there is a demand for increasing the air-fuel ratio to reduce the fuel consumption of the engine. For this reason, a lean-burn engine has been developed.
In other words, this engine is designed to improve the atomization of the air-fuel mixture so that stable combustion can be realized even with a lean air-fuel mixture.When the engine temperature is low, the air-fuel ratio is close to the stoichiometric air-fuel ratio. After the temperature is set and the engine temperature rises and the combustion stability of the air-fuel mixture becomes high, the air-fuel ratio is normally set by switching to the lean side. In that case, since the exhaust gas contains excess oxygen, even if CO and HC can be oxidized and purified by the three-way catalyst, NOx
It becomes impossible to reduce and purify.

On the other hand, in recent years, studies have been conducted on zeolite (crystalline aluminosilicate) -based NOx purification catalysts in which a transition metal is supported by ion exchange.
In the case of this catalyst, a reducing agent (for example, CO, HC, etc.) that directly or coexists with NOx even in a lean atmosphere
Can be decomposed into N ₂ and O ₂ . For example, Japanese Patent Application Laid-Open No. 1-139145 describes an exhaust gas purifying apparatus in which a NOx purifying catalyst and a three-way catalyst are arranged in series so that the former is on the upstream side and the latter is on the downstream side. is there.

Further, in Japanese Patent Laid-Open No. 57-210116, two catalysts are arranged in series and a bypass passage bypassing the upstream catalyst is provided, and when the exhaust gas temperature is high, the exhaust gas is passed through the bypass passage. There is a description of an exhaust gas purifying device that prevents thermal deterioration of the upstream side catalyst at high exhaust gas temperature by flowing the exhaust gas.

[0006]

Therefore, the inventor of the present invention
A combination of the conventional technology of arranging the NOx purification catalyst and the three-way catalyst in series and the conventional technology of providing the bypass passage was examined. That is, N on the upstream side
A bypass passage bypassing the Ox purification catalyst is provided and a bypass valve is placed there. When the temperature of the NOx purification catalyst is suitable for purification of NOx components, the exhaust gas is caused to flow through the catalyst, otherwise the exhaust gas is bypassed. The idea is to run it down the passage. According to this, the NOx component can be purified while preventing the NOx purification catalyst from being thermally deteriorated, and both the NOx purification catalyst and the three-way catalyst can be effectively used for purification of the exhaust gas.

However, in the case of such a bypass system,
The temperature of the NOx purification catalyst (hereinafter, unless otherwise specified,
It requires a temperature sensor for detecting the catalyst temperature). Then, even if the bypass valve is opened after the catalyst temperature is detected to be high by the temperature sensor, the catalyst temperature continues to rise for a while, and the NOx purification catalyst is likely to be thermally deteriorated. The reason why the temperature of the NOx purification catalyst rises even after the bypass valve is opened is that the HC component that has been accumulated in the catalyst without being purified up to that point is burned.

To surely prevent such heat deterioration,
The temperature threshold value for opening the bypass valve may be set to a low value. However, even if the catalyst temperature does not rise above that value, the bypass valve opens and the NOx purification catalyst cannot be used efficiently.

When the temperature sensor malfunctions and shows a temperature lower than the actual temperature, the NOx purification catalyst is thermally deteriorated. On the contrary, when the temperature sensor shows a higher temperature than the actual temperature, the catalyst can be used. Even if you are in a state, you cannot use it. Further, in order to solve the problem of thermal deterioration of the catalyst, it is conceivable to incorporate a control for opening the bypass valve when the temperature sensor fails, but a sensor failure detection means is required, and the control system is It gets complicated.

[0010]

Means for Solving the Problem and Its Action The present invention is
With respect to such a problem, a means for determining the tendency of the temperature transition of the NOx purification catalyst from the operating state of the engine and performing the opening / closing control of the bypass valve based on the determination result is provided.

-Regarding the invention according to claim 1- That is, the means is provided in the exhaust system of the engine and is capable of purifying NOx in the exhaust gas at a lean air-fuel ratio.
An Ox purifying catalyst, a bypass passage provided in the exhaust system for bypassing the NOx purifying catalyst and discharging exhaust gas, a three-way catalyst for purifying exhaust gas discharged through the bypass passage, and the bypass passage Based on at least one of a load valve and a rotational speed of the engine detected by the operating state detecting means and an operating state detecting means for detecting at least one of the load and the rotational speed of the engine. Then, the engine exhaust gas purifying apparatus is provided with: a valve control unit that determines the transition tendency of the temperature of the NOx purification catalyst and controls the opening and closing of the bypass valve based on the determination result.

Regarding the determination of the transition tendency of the temperature of the NOx purification catalyst, for example, when the engine is operating under a high load, it is determined that the catalyst temperature tends to increase because the exhaust gas temperature increases. You can Also,
Since the SV (space velocity) of the exhaust gas in the catalyst becomes low when the engine is operating under a low load or a low rotation speed, N
It is easy to think that the temperature of the Ox purification catalyst will not rise, but this is not always the case.

That is, although the catalytic reaction in the NOx purification catalyst depends on SV and is difficult to react at high SV,
At low SV, the reaction easily proceeds even if the catalyst temperature is low or the exhaust gas temperature is low. Therefore, for example, when the operating state of the engine is in the operating region of high load or high rotation, the amount of exhaust gas is large and therefore the SV is high, so NOx
HC components that could not be purified are accumulated in the purification catalyst. Then, the accumulated HC component may be combusted when the SV is lowered due to the shift to the low load or low rotation operation region, thereby heating the NOx purification catalyst. Therefore, it is possible to determine that the catalyst temperature rises even in the low load or low rotation operating state.

If the catalyst temperature tends to rise excessively on the basis of the result of the determination, the bypass valve is opened to prevent the exhaust gas from flowing into the NOx purification catalyst, and its heat deterioration is caused. Can be prevented. Therefore, it is not necessary to separately detect the temperature of the catalyst. Furthermore, since the transition tendency of the catalyst temperature is grasped, it is possible to open and close the appropriate bypass valve in advance, which delays the opening of the bypass valve and causes thermal deterioration of the NOx purification catalyst. The valve is not opened to prevent the catalyst from being effectively used.

Of course, in carrying out the present invention, NOx
It does not prevent that the temperature of the catalyst is separately detected by the sensor in order to reliably prevent the heat deterioration of the purification catalyst and the opening / closing control of the bypass valve is performed using the detection result and the determination result. Even in that case, even if the temperature sensor fails, it is possible to grasp the transition tendency of the catalyst temperature and execute appropriate opening / closing control of the bypass valve, which is advantageous.

As to the invention described in claim 2, as described above, the engine operating state in which the catalyst temperature is likely to be excessively raised is a low load or low rotation operating region, and the catalyst temperature is in the operating region. Is determined to increase above a predetermined value, and the bypass valve is controlled to be opened. By doing so, it is possible to prevent thermal deterioration of the NOx purification catalyst.

-Regarding the invention as set forth in claim 3, the catalyst for NOx purification is provided when the operating state of the engine shifts from a high-load or high-rotation operating region to a predetermined low-load or low-rotation operating region. It is a suitable means to judge that the temperature of the above rises above a predetermined value and control the bypass valve to open. The reason why the catalyst temperature rises when the operating state of the engine shifts from the high load or high rotation operating region to the predetermined low load or low rotation operating region is as described above, and therefore, in that case, the bypass temperature is increased. When the valve is opened, it is possible to prevent thermal deterioration of the NOx purification catalyst.

With regard to the invention described in claim 4, the bypass valve is opened immediately when the operating condition of the engine is in the above-mentioned low load or low rotation operating region, or when the engine enters the operating region. However, it is a more preferable means to control the bypass valve to be open when the state in the operation region continues for a predetermined time or longer. This is because if the state in such an operating region is for a short time, the catalyst temperature does not increase so much, and if the state continues for a predetermined time or longer, it is highly probable that the catalyst temperature tends to excessively rise. Because you can

-Regarding the invention according to claim 5 (relationship with the air-fuel ratio of the engine) -an air-fuel ratio sensor for detecting the air-fuel ratio of the engine is provided, and the lean air-fuel ratio, that is, lean is detected by this air-fuel ratio sensor. It is meaningful to perform the above-mentioned opening / closing control of the bypass valve when the vehicle is in the open state.

That is, since the exhaust gas temperature is usually low in the lean case, the opening / closing control of the bypass valve is performed based on the air-fuel ratio of the engine (the bypass valve is opened only in the lean case). This is because it is impossible to reliably prevent thermal deterioration of the NOx purification catalyst by itself. In other words, even if the engine is lean, depending on the operating condition of the engine, for example, the catalyst temperature may excessively rise in the above-mentioned low load or low rotation operating range. If controlled to, heat deterioration of the NOx purification catalyst can be prevented.

-Regarding the invention according to claim 6-When the air-fuel ratio detected by the air-fuel ratio sensor is not a lean air-fuel ratio, that is, when the air-fuel ratio is rich (theoretical air-fuel ratio or rich air-fuel ratio), it is lean. More than once
It is a suitable means to expand the predetermined low load or low rotation operating range to the high load side or the high rotation side to control the opening / closing of the bypass valve. This is because the exhaust gas temperature is high when the air-fuel ratio is rich, which makes the temperature increase of the NOx purification catalyst more remarkable.

-Catalyst- As the above-mentioned NOx purification catalyst, a catalyst in which a metal-containing silicate having micropores such as zeolite is supported with a metal active species is preferable. As this metal-containing silicate, zeolite is preferable, but instead of this,
For example, Al and F are used as the metal forming the crystal skeleton.
A metal-containing silicate formed by combining with another metal (semi-metal) such as e, Ce, Mn, Tb, Cu, B, and P, or A
Non-aluminosilicates not containing 1 can also be used, and these are effective in obtaining heat resistance. From the viewpoint of improving heat resistance, H type is preferable to Na type, and H type zeolite is particularly preferable. As the above-mentioned zeolite, ZSM-5 is suitable, but A type, X
It may be a mold, a Y-shape, or the like.

A transition metal is preferable as the metal active species, and Cu is particularly preferable. Of course, other transition metals such as Co, Cr, Ni, Fe, and Mn may be used, or a noble metal may be used. Further, this transition metal is preferably carried on the metal-containing silicate by ion exchange,
It is also possible to employ an impregnation method or a dry solidification method in which water is removed by heating to support the zeolite, and a coprecipitation method in which coexisting ions are carried in a solution state.

Regarding the NOx purification catalyst,
It may have two upper and lower catalyst layers on the carrier. For example, a silicate-based catalyst in which a metal-containing silicate having two or more kinds of metals as the skeleton-constituting elements of the crystal is loaded with a metal active species is used as an upper layer, and γ-alumina or zeolite is effective for a metal active species, particularly an oxidation reaction. A catalyst in which various active species are supported is preferably arranged in the lower layer. According to this, even when the catalyst temperature is low, the temperature of the upper layer can be increased even if the combustion reaction of HC in the lower layer proceeds smoothly, whereby NOx can be purified even when the exhaust gas temperature is low. Because you can.

As the three-way catalyst, a porous carrier on which a noble metal such as Pt, Pd or Rh is supported is preferable, and ceria or the like may be added. Further, it is preferable that the three-way catalyst is arranged on the downstream side of the downstream end of the bypass passage in the exhaust system, and in this case, in the exhaust gas that passes through the NOx purification catalyst and flows rearward thereof. It is possible to purify HC and CO.

The NOx purification catalyst and the three-way catalyst may be pellet type when used, but a monolith carrier may be used, and cordierite is suitable as the carrier in that case. Also, other inorganic porous materials can be used.

[0027]

Therefore, according to each of the above means (the inventions of claims 1 to 6), the transition tendency of the catalyst temperature is determined based on the operating state of the engine, and the opening / closing control of the bypass valve is performed based on the determination result. Therefore, the exhaust gas can be purified by effectively utilizing the NOx purification catalyst without thermally degrading without using the catalyst temperature sensor.

According to the present invention, it is determined that the catalyst temperature rises above a predetermined value when the engine operating condition is in the low load or low rotation operating region (the invention of claim 2). This is advantageous in preventing the thermal deterioration of the.

It is determined that the catalyst temperature rises above a predetermined value when the operating state of the engine shifts from a high load or high rotation operating region to a low load or low rotation operating region (claim 3). According to the invention), the bypass valve can be opened when the catalyst temperature is extremely likely to rise above a predetermined value, and thermal deterioration of the NOx purification catalyst can be prevented while reducing unnecessary valve control. can do.

When the temperature in the NOx purifying catalyst is judged to rise above a predetermined value when the state in the low load or low rotation operating range continues for a predetermined time or longer (invention of claim 4) This is advantageous in that wasteful valve control can be further reduced.

Further, according to the above-mentioned valve control when the air-fuel ratio is lean (the invention of claim 5), the bypass valve is opened when the air-fuel ratio is not lean and the bypass valve is closed when the air-fuel ratio is rich. It is possible to prevent thermal deterioration of the NOx purification catalyst while adopting a simple basic control.

When the air-fuel ratio is rich, the low-load or low-rotation operating range is expanded to the high-load side or the high-rotation side to perform the valve control (the invention of claim 6). Valve control can be performed in consideration of the gas temperature, which is advantageous in preventing thermal deterioration of the NOx purification catalyst.

[0033]

EXAMPLES Examples of the present invention will be described below.

-Structure of Exhaust Gas Purification Device- Fig. 1 shows the structure of the device of the embodiment. In the figure, 1 is a NOx purifying catalyst provided in an exhaust passage 2 of an engine body 11, 3 is a bypass passage bypassing the NOx purifying catalyst 1, 4 is a downstream end of the bypass passage 3 (the exhaust passage 1 and This is a three-way catalyst that is provided downstream of the portion where it joins the bypass passage 3. A bypass valve 5 is provided at the downstream end of the bypass passage 3 to switch the flow of exhaust gas between the NOx purification catalyst 1 side and the bypass passage 3 side. The bypass valve 5 includes a load sensor 6 that detects an engine load, a rotation sensor 7 that detects an engine speed, an air-fuel ratio sensor (O ₂ sensor) 8 that detects an air-fuel ratio of the engine, and the NOx purification catalyst 1. The drive is controlled by the valve control means 10 based on the output of the temperature sensor 9 for detecting the temperature.

-Valve Control 1- The valve control means 10 detects the engine load detected by the load sensor 6 and the rotation sensor 7 when the air-fuel ratio lean is detected by the air-fuel ratio sensor 8. The transition tendency of the temperature of the NOx purification catalyst 1 is determined from the operating region of the engine on the basis of the engine speed, and the opening / closing control of the bypass valve 5 is performed based on the determination result. The operating region for the above determination is shown in FIG. 2, and is divided into a B region where the SV of the exhaust gas in the NOx purification catalyst 1 is low and an A region where the SV is high. The area demarcation line shown by the solid line is when the air-fuel ratio is lean, and the area demarcation line shown by the chain line is when the air-fuel ratio is rich. B for rich
The area is larger than when it is lean.

The specific contents of the valve control are shown in the control flow in FIG. When the air-fuel ratio of the engine is not lean, the B region is expanded (S1 → S2).
Then, when the operating state of the engine is in the B range, the opening / closing control of the bypass valve 5 is performed by the catalyst temperature T. That is, when the temperature T is lower than the predetermined value To (the temperature at which the NOx purification catalyst 1 is likely to be thermally deteriorated), the bypass valve 5 is closed (S3 → S9 → S1).
0), when the catalyst temperature T is higher than the predetermined value To, the bypass valve 5 is opened (S9 → S7). When the bypass valve 5 is opened in this way, the predetermined time t
_It is monitored whether or not the catalyst temperature T becomes lower than the predetermined value To every _two times, and if it becomes lower than the predetermined value To, the valve 5
Is closed (S7 → S8 → S9 → S10).

When the operating condition of the engine is in the A region, the shift to the B region is monitored (S3 → S4). If the operating region remains A, the bypass valve 5 is closed according to the catalyst temperature T when it is lower than the predetermined value To,
When it is equal to or larger than the predetermined value To, it is controlled to open (S4 → S1).
1 → S12, S13).

Thus, when the operating state shifts from the A region to the B region, the timer t ₁ is set, and when the state of the B region continues for a predetermined time t ₁ , the timer is reset and the bypass valve is set. 5 is opened (S4 → S5 →
S6 → S7). In this way, when the bypass valve 5 is opened, the catalyst temperature T is monitored every predetermined time t ₂ as in the previous case (S7 → S8 → S9 → S10). When returning to the area A before the predetermined time t ₁ has elapsed,
The bypass valve 5 is controlled to open / close according to the catalyst temperature T (S6 → S11 → S12, S13).

In summary, one of the points of the above valve control is that, when the operating state of the engine shifts from the A region to the B region and the state in the B region continues for a predetermined time t ₁ , the catalyst temperature is high or low. Instead, the bypass valve 5 is opened.

That is, when the engine operating condition is in the A region, the bypass valve 5 is closed unless the catalyst temperature is high (S4 → S11 → S12), so the exhaust gas is directed toward the NOx purification catalyst 1. Flowing. However, when the operating state of the engine is in the A range, the exhaust gas amount is large and N
Since the SV in the Ox purification catalyst 1 is high, the HC component in the exhaust gas is not completely purified, and the unburned HC component is accumulated in the catalyst 1.

FIG. 4 shows an actual vehicle exhaust gas (NOx: 1000 ppm, HC: 2500 pp) with A / F = 20 to 23 when the NOx purification catalyst 1 is Cu ion-exchanged zeolite.
m, inlet gas temperature: 350 ° C.) shows the relationship between SV, NOx purification rate, and HC combustion rate. SV =
It is understood that unburned HC is produced when it exceeds 50,000 hr ^-1 .

In such a state, when the operating state of the engine is in the B range, the SV is lowered, so that all HC in the exhaust gas is combusted and purified by the catalyst 1, and
The previously accumulated HC will also be burned. In this case, since the SV is low, the heat radiation effect due to the gas blow-through is small, and therefore the temperature of the NOx purification catalyst 1 increases.

The inlet gas temperature and the catalyst temperature T of the NOx purifying catalyst 1 in each engine operating state in the conventional mode in which the bypass valve 5 is closed, and the inlet gas temperature and the catalyst temperature T when the operating state is changed. Is as shown in Table 1. As the NOx purification catalyst, Na-type Z
Cu ion-exchanged zeolite in which Cu was carried on SM-5 by ion exchange was used. In addition, the three-way catalyst 4 is γ-
It is made by supporting Pt and Rh on alumina.

[0044]

[Table 1]

Test No. Nos. 1 and 5 belong to the B range and are in the low load and low speed operation state. 2 to 4 are operating states of medium load or high rotation and belong to the A region. The exhaust gas temperature is high and the catalyst temperature T is also high in the region A, but both the inlet gas temperature and the catalyst temperature T are low in the region B where the load is low and the rotation speed is low. On the other hand, No. Nos. 6 to 9 are cases where the operating state was shifted from the A range to the B range, and although the inlet gas temperature was low, the catalyst temperature T was higher than that in the case of operating in the A range. This result supports the fact that the catalyst temperature T rises due to the decrease in SV that accompanies the change in the operating state.

On the other hand, in the above-mentioned valve control of the present invention, after the operating state shifts from the A region to the B region, this B control is performed.
Bypass valve 5 when the state of the region continues for a predetermined time t ₁
As a result, the catalyst temperature is prevented from further increasing and thermal deterioration of the NOx purification catalyst 1 is prevented. Table 2 shows the previous test No. 6 to 9 show the catalyst temperature T in the valve control of the embodiment. It can be seen that the catalyst temperature T is lower than in the case of Table 1.

[0047]

[Table 2]

In this case, NOx is reduced to the predetermined time t _1.
In the state where the unburned HC is saturated and accumulated in the purification catalyst 1, the temperature rises from the predetermined temperature To to the allowable temperature when the temperature rises at the highest speed in the operating state of the region B.

When the bypass valve 5 is opened as described above, the bypass valve 5 is controlled to be closed if the catalyst temperature is lower than the predetermined temperature To when the predetermined time t ₂ has elapsed. The valve 5 is not unnecessarily left open.

Further, when the air-fuel ratio is rich, the exhaust gas temperature rises, so that the B region is expanded (S1 → S) from the viewpoint of reliably preventing thermal deterioration of the NOx purification catalyst 1.
2) The bypass valve 5 is easily opened. But,
When rich, there is no problem because the exhaust gas is smoothly purified by the three-way catalyst 4.

On the contrary, it is not preferable that a large amount of exhaust gas flows to the three-way catalyst 4 when the air-fuel ratio is lean, but in the case of the above valve control, the B region is narrow in the lean side toward the lower engine speed. Therefore, even when the bypass valve 5 is opened, the amount of exhaust gas flowing through the three-way catalyst 4 is small, which is advantageous in preventing deterioration of exhaust gas.

-Valve Control 2 (Estimation of Catalyst Temperature) -In the above-mentioned valve control 1, the catalyst temperature sensor 9 was used as an auxiliary. Next, an example of control in which no temperature sensor is used will be described.

FIG. 5 shows a map of the exhaust gas temperature at the inlet of the NOx purification catalyst 1 when the engine is operated at A / F = 23. The inlet gas temperature is the engine speed and the average effective pressure. Is basically determined by The valve control means 10 has the above-mentioned inlet gas temperature map, and also has maps similarly prepared for the HC concentration, CO concentration and O ₂ concentration in the exhaust gas.

For the amount of change ΔT in catalyst temperature T, the following formula is empirically established. ΔT = heat transfer amount from exhaust gas + heat amount obtained by combustion of HC Note that the heat transfer amount from the exhaust gas is a value in consideration of the heat radiation amount.

In this case, when the engine load becomes low, the inlet gas temperature becomes lower than the catalyst temperature, and the SV at that time is lowered.
If the value is slightly high, the cooling effect of the NOx purification catalyst 1 is produced, so that the increasing rate of the catalyst temperature T becomes slow as shown in FIG. Further, the longer the running in the operating range where the HC component in the exhaust gas cannot be completely burned and purified, the more the amount of unburned HC adsorbed to the catalyst 1 increases, so that as shown in FIG.
The rising speed of is faster and the reached temperature is higher. The time (minutes) shown in FIG. 7 is 2500 rpm, -400 mm.
It is the running time in Hg.

In view of the above points, the valve control means 10 calculates ΔT based on the inlet gas temperature, exhaust gas flow rate, HC concentration, etc., and inputs and integrates ΔT according to the transition of the engine operating state. The catalyst temperature T at that time is estimated by going forward.

Regarding the setting of the predetermined time t ₁ for monitoring the duration of the B area after the A area → the B area, the unburned H
The accumulated amount of C is obtained from the transition of the engine operating state, and the inlet gas temperature and the SV when the region A to the region B is changed.
From the above, the rate of increase of the catalyst temperature T is obtained, and the time until the temperature reaches the allowable temperature is set to the predetermined time t ₁ . Others are the same as the valve control 1 described above.

Therefore, in the valve control 2, N
A sensor for detecting the temperature of the Ox purification catalyst 1 is not necessary, and thermal deterioration of the catalyst 1 can be prevented depending on the engine operating state (engine load and engine speed) and the air-fuel ratio.

-NOx Purifying Catalyst- Next, a preferred example of the NOx purifying catalyst 1 will be described.
In this catalyst example, as shown in FIG. 8, a cordierite monolith honeycomb carrier 15 is provided with two catalyst layers 16 and 17. In this case, the lower catalyst layer 16 is an HC combustion catalyst in which γ-alumina or zeolite carries metal active species, particularly active species effective for oxidation reaction, and the upper catalyst layer 17
Is a silicate-based catalyst in which a metal active species is supported on a composite metal-containing silicate having two or more kinds of metals as crystal skeleton-constituting elements.

Suitable examples of the HC combustion catalyst for the lower catalyst layer 16 are noble metal-supported zeolite catalysts in which PSM or Rh is supported on ZSM-5 and transition metals / γ-alumina in which Cu or the like is supported on γ-alumina. System catalyst, upper catalyst layer 1
The following are suitable as the silicate-based catalyst of No. 7.

Cu ²⁺ / Al-Ga Silicate (Pentacyl Type) Cu ²⁺ / Al-Mn Silicate (Pentacyl Type) Cu ²⁺ / Al-B Silicate (Pentacyl Type) Cu ²⁺ / Al-Tb Silicate (Pentacyl Type) ) Co ²⁺ / Al-Ga silicate (pentasil type) Ni ²⁺ / Al-Ga silicate (pentasil type) The Cu ²⁺ / Al-Ga silicate (pentasil type) is a skeleton structure in which the kind of silicate is a crystal. It means that it has Al and Ga in addition to Si and O as elements, and it also means that it has a pentasil-type crystal structure. Further, (Cu ²⁺ /) means that Cu ²⁺ is supported by ion exchange. The same applies to other silicates.

(Catalyst Example 1) Upper catalyst layer: Cu ²⁺ / Al-Ga silicate (pentasil type) Lower catalyst layer: Cu / γ-alumina With the above layer structure, the NOx purification characteristics were examined under the following measurement conditions.

Simulated exhaust gas composition; [NO] = 2000 ppm, [HC] = 6000 ppm
C, [O ₂ ] = 8%, [CO ₂ ] = 10%, [CO] = 0.2%, [H ₂ ] = 650 ppm space velocity; SV = 25000 hr ⁻¹

The results are shown in FIG. 9 together with the Cu ²⁺ / Al-Ga silicate (pentasil type) alone and Cu / γ-alumina alone. In the case of catalyst example 1, C
Compared with the u ²⁺ / Al-Ga silicate (pentasil type) alone, the catalyst activation temperature range is expanded to the low temperature side without causing a large decrease in the maximum purification rate. This is because the lower catalyst layer promotes combustion of HC, and as a result, the upper catalyst layer is heated and activated from a state where the inlet gas temperature is low.

(Catalyst example 2) Upper catalyst layer: Cu ²⁺ / Al-Ga silicate (pentasil type) Lower catalyst layer: Pt / ZSM-5 NOx under the same conditions as in the above catalyst example 1 as the above layer structure. The purification characteristics were investigated. The results are Cu ²⁺ / Al-Ga silicate (pentasil type) alone and Pt / ZSM
It is shown in FIG. 10 with -5 alone. The expansion of the catalytic activity temperature range to the low temperature side of this example is smaller than that of Catalyst Example 1, but the temperature range is still larger than that of Cu ²⁺ / Al-Ga silicate (pentasil type) alone. Expanded, lower catalyst layer (Pt / ZSM-5)
The effect of expanding the active temperature range is observed.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an engine exhaust gas purifying apparatus according to an embodiment.

FIG. 2 is a map diagram showing an engine operating region.

[Fig. 3] Flow chart of valve control

FIG. 4 is a graph showing the relationship between SV, NOx purification rate, and HC combustion rate.

FIG. 5 is a map diagram of the inlet gas temperature of the NOx purification catalyst.

FIG. 6 is a graph showing the difference in catalyst temperature increase rate depending on SV.

FIG. 7 is a graph showing a difference in catalyst temperature rising speed depending on operating time.

FIG. 8 is a sectional view of a NOx purification catalyst.

FIG. 9 is a graph showing the NOx purification characteristics of NOx purification catalyst example 1.

FIG. 10 is a graph showing the NOx purification characteristics of NOx purification catalyst example 2.

[Explanation of symbols]

 1 NOx purification catalyst 2 exhaust passage 3 bypass passage 4 three-way catalyst 5 bypass valve 6 load sensor 7 rotation sensor 8 air-fuel ratio sensor 10 valve control means 11 engine body

Claims (6)

[Claims] 1. An NOx purification catalyst, which is provided in an exhaust system of an engine and is capable of purifying NOx in exhaust gas at a lean air-fuel ratio, and an exhaust gas which bypasses the NOx purification catalyst, which is provided in the exhaust system. A bypass passage for discharging, a three-way catalyst for purifying exhaust gas discharged through the bypass passage, a bypass valve for opening and closing the bypass passage, and an operating state for detecting at least one of a load and a rotational speed of the engine The transition tendency of the temperature of the NOx purification catalyst is determined based on at least one of the load of the engine and the rotational speed detected by the detection unit and the operating state detection unit, and the bypass based on the determination result. An exhaust gas purifying apparatus for an engine, comprising: a valve control unit that controls opening and closing of a valve. 2. The valve control means, when the operating state of the engine is in a predetermined low load or low rotation operating range,
The exhaust gas purifying apparatus for an engine according to claim 1, wherein it is determined that the temperature of the NOx purifying catalyst rises above a predetermined value, and the bypass valve is controlled to open. 3. The temperature of the NOx purification catalyst when the operating condition of the engine shifts from a high load or high rotation operating region to a predetermined low load or low rotation operating region. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the bypass valve is controlled to be opened when it is determined that is increased above a predetermined value. 4. The valve control means determines that the temperature of the NOx purification catalyst rises to a predetermined value or more when the engine is in a predetermined low load or low speed operation region for a predetermined time. The exhaust gas purifying apparatus for an engine according to claim 2 or 3, wherein the bypass valve is controlled to be opened. 5. An air-fuel ratio sensor for detecting an air-fuel ratio of an engine is provided, and the valve control means controls the opening / closing of the bypass valve when the lean air-fuel ratio is detected by the air-fuel ratio sensor. An exhaust gas purifying apparatus for an engine according to claim 4. 6. An air-fuel ratio sensor for detecting an air-fuel ratio of an engine is provided, and the valve control means has a lean air-fuel ratio when the air-fuel ratio detected by the air-fuel ratio sensor is not a lean air-fuel ratio, The engine exhaust according to any one of claims 2 to 4, wherein the predetermined low load or low rotation operating range is expanded to a high load side or a high rotation side to perform opening / closing control of the bypass valve. Gas purification device.