JPH05340236A - Exhaust emission control device for engine - Google Patents

Abstract

PURPOSE:To improve the exhaust emission control rate and raise the temperature of an ONx purifying catalyst speedily, during the cold time when the engine temperature is low and a three-component catalyst does not effectively operate. CONSTITUTION:A bypass passage 3 which discharges exhaust gas, making a detour around an NOx purifying catalyst 1 and a three-component catalyst 4 which purifies the exhaust gas discharged through the bypass passage 3 are installed in an engine exhaust system, and when the air-fuel ratio is over a prescribed value or less than a prescribed value and the engine temperature is below a prescribed value, the exhaust gas is allowed to flow to the NOx purifying catalyst 1 side.

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 be 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. Therefore, a lean-burn engine has been developed.
In other words, this engine is designed to realize stable combustion even with a lean air-fuel mixture by improving the atomization of the air-fuel mixture.When the engine temperature is low, the air-fuel ratio is kept close to the stoichiometric air-fuel ratio. After the temperature is set, the engine temperature rises and the combustion stability of the air-fuel mixture becomes high, and then 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, JP-A-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 catalyst on the upstream side is provided. 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 technique of arranging the NOx purification catalyst and the three-way catalyst in series and the conventional technique of providing a bypass passage was examined. That is, N on the upstream side
A bypass passage bypassing the Ox purifying catalyst is provided to allow exhaust gas to flow through the bypass passage when the engine is operated on the rich side, and exhaust gas N when the engine is operated on the lean side.
The idea is to let it flow to the Ox purification catalyst.

According to this, when the stoichiometric air-fuel ratio is adopted when the engine temperature is cold, the combustion stability of the engine is easily obtained, and the exhaust gas directly flows from the bypass passage to the three-way catalyst. The original catalyst can purify it. On the other hand, even in a high load / high speed operation range of an engine where the exhaust gas temperature is high, the exhaust gas is caused to flow through the bypass passage by operating the engine at the stoichiometric air-fuel ratio.
x The purification catalyst can be protected from heat. And
When the air-fuel ratio is lean, the NOx purification catalyst causes N
Ox emissions can be effectively reduced. That is, it becomes possible to purify the exhaust gas of the engine even when the air-fuel ratio fluctuates while preventing thermal deterioration of the NOx purification catalyst on the upstream side.

However, when the engine temperature is low, the exhaust gas temperature is usually low, and therefore, even if the exhaust gas flows directly from the bypass passage to the three-way catalyst, the three-way catalyst is exhausted. There is a problem that it does not work effectively for purification of. Particularly, when the three-way catalyst is arranged on the downstream side of the NOx purification catalyst, the temperature of the three-way catalyst increases as the distance from the engine increases, and during that time, HC, CO,
The desired purification cannot be expected for all NOx. Further, even after the combustion state of the engine becomes stable, the air-fuel ratio is switched to the lean side and the exhaust gas is made to flow to the NOx purification catalyst, but at that time, the temperature of the NOx purification catalyst is still low. Therefore, there is a problem that it does not work effectively for purification of NOx.

[0009]

Means for Solving the Problem and Its Action The present invention is
For such a problem, not only the switching of the exhaust gas flow between the NOx purification catalyst side and the bypass passage side is performed based not only on the air-fuel ratio, but also on the basis of the engine temperature. To do.

That is, the means for solving the above-mentioned problems are as follows: a NOx purification catalyst provided in the exhaust system of the engine for purifying NOx in the exhaust gas at a lean air-fuel ratio; and a NOx purification catalyst provided in the exhaust system. A bypass passage that bypasses the catalyst to discharge the exhaust gas, a three-way catalyst that purifies the exhaust gas discharged through the bypass passage, and a flow of the exhaust gas between the NOx purification catalyst side and the bypass passage side. By-pass valve for switching between, the air-fuel ratio detection means for detecting the air-fuel ratio of the engine, a temperature sensor for detecting the temperature of the engine, the air-fuel ratio detected by the air-fuel ratio detection means is a predetermined value or more, And the air-fuel ratio detected by the air-fuel ratio detecting means is less than the predetermined value and the engine temperature detected by the temperature sensor is not more than the predetermined value. In some case, the exhaust gas is the N
An exhaust gas purifying apparatus for an engine, comprising: valve control means for controlling switching of the bypass valve so that the bypass valve flows to the Ox purifying catalyst side.

In such means, when the air-fuel ratio of the engine is a predetermined value or more in a so-called lean atmosphere (lean air-fuel ratio), the exhaust gas flows to the NOx purification catalyst side,
NOx is efficiently purified by the catalyst. And
In a so-called rich atmosphere (theoretical air-fuel ratio or rich air-fuel ratio) where the air-fuel ratio of the engine is less than the above-mentioned predetermined value, the exhaust gas basically flows into the bypass passage and is purified by the three-way catalyst. When the temperature is low, the NOx purification catalyst flows even in a rich atmosphere.

This means that even in a rich atmosphere, when the engine temperature is low, the temperature of the three-way catalyst is low, and therefore even if exhaust gas flows through the three-way catalyst, its purification cannot be sufficiently achieved. , The NOx purifying catalyst purifies the NOx purifying catalyst, and simultaneously raises the temperature of the NOx purifying catalyst at an early stage. In this case, the early temperature rise of the catalyst can be achieved not only by the heat of the exhaust gas, but also by the heat of the catalytic reaction. Then, due to this early temperature increase, the purification efficiency of the NOx purification catalyst is increased.

As the NOx purification catalyst, a metal-containing silicate having micropores such as zeolite on which a transition metal is supported is preferable. As the metal-containing silicate, zeolite is preferable, but instead of this, for example, as a metal forming the skeleton of the crystal,
A metal-containing silicate formed by combining Al with another metal (semi-metal) such as Fe, Ce, Mn, Tb, Cu, B, and P, or a non-aluminosilicate not containing Al can be adopted, and these are heat-resistant. It is effective in gaining sex. Also, from the viewpoint of improving heat resistance, H
A type is preferable, and an H-type zeolite is particularly preferable.
As the above zeolite, ZSM-5 is preferable,
It may be A type, X type, Y type or the like.

Although Cu is preferable as the transition metal,
Other transition metals such as Co, Cr, Ni, Fe, Mn, etc. can also be used. Further, it is preferable that the transition metal is supported on the metal-containing silicate by ion exchange, but the transition metal is supported by the impregnation method or a dry solidification method in which moisture is removed and supported on zeolite, and coexisting ions are supported in a solution state. A coprecipitation method or the like can also be adopted.

Regarding the NOx purification catalyst,
It is preferable to provide a low temperature active catalyst portion supporting a noble metal as an active species on the downstream side thereof. That is,
The noble metal active species promotes the oxidation of HC, which causes N
It has the effect of smoothly reducing Ox. Therefore, the NOx purification efficiency can be increased even when the engine temperature is low. Further, in the above rich atmosphere and when the engine temperature is low, the downstream portion of the NOx purification catalyst acts as a three-way catalyst due to the presence of the noble metal, so that HC, CO, and NOx can be purified. ..

The three-way catalyst is preferably a porous carrier on which a noble metal such as Pt, Pd or Rh is supported, 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 above-mentioned NOx purifying catalyst and three-way catalyst may be of pellet type in use, but a monolith carrier may be used, and cordierite is suitable as a carrier in that case. Also, other inorganic porous materials can be used.

[0018]

As described above, according to the exhaust gas purifying apparatus for an engine, the bypass passage for bypassing the NOx purifying catalyst to exhaust the exhaust gas to the exhaust system of the engine and the exhaust gas exhausted through the bypass passage are provided. A three-way catalyst for purification is provided, and exhaust gas flows to the NOx purification catalyst side when the air-fuel ratio is equal to or higher than a predetermined value and when the air-fuel ratio is lower than the predetermined value and the engine temperature is equal to or lower than the predetermined value. Therefore, when the engine temperature is low and the three-way catalyst does not work effectively, it is possible to purify the exhaust gas by the NOx purification catalyst and to increase the temperature of the NOx purification catalyst at an early stage to purify the NOx. The rate can be increased.

Further, according to the catalyst provided with the low temperature active catalyst portion carrying the noble metal as the active species at the downstream side portion of the NOx purification catalyst, the NOx purification efficiency is enhanced in the lean atmosphere and when the engine temperature is low. In addition, even in a rich atmosphere, when the engine temperature is low, the downstream portion of the NOx purification catalyst can be used as a three-way catalyst to purify HC, CO, and NOx.

According to the three-way catalyst arranged on the downstream side of the downstream end of the bypass passage in the exhaust system, not only the exhaust gas flowing through the bypass passage can be purified,
It becomes possible to purify HC and CO in the exhaust gas that passes through the NOx purifying catalyst and flows behind the NOx purifying catalyst.

[0021]

EXAMPLES Examples of the present invention will be described below.

-Regarding the Structure of the Exhaust Gas Purification Device- Fig. 1 shows the structure of the device of the embodiment. In the figure, 1 is NOx provided in the exhaust passage 2 of the engine.
The purification catalyst, 3 is a bypass passage bypassing the NOx purification catalyst 1, and 4 is the downstream end of the bypass passage 3 (the exhaust passage 1 described above).
Is a three-way catalyst provided downstream from the confluence of the bypass passage 3). The bypass passage 3 is provided with a bypass valve 5 for switching the flow of exhaust gas between the NOx purification catalyst 1 side and the bypass passage 3 side. The drive of the bypass valve 5 is controlled by the valve control means 8 based on the outputs of the air-fuel ratio detecting means 6 for detecting the air-fuel ratio of the engine and the temperature sensor 7 for detecting the engine temperature (cooling water temperature). It is.

-Catalyst- The NOx purifying catalyst 1 is a catalyst capable of purifying NOx in exhaust gas in both a rich atmosphere and a lean atmosphere. As shown in FIG. 1a, a medium temperature active catalyst portion 1b and a low temperature active catalyst portion 1c are sequentially provided from the upstream side. The high temperature active catalyst portion 1a is
A catalyst material comprising Co having a specific surface area of 300 m ² / g and Co supported by ion exchange (ion exchange rate: 8
0%), and the medium temperature active catalyst part 1b is a catalyst material (ion exchange rate 100%) in which Cu is supported by ion exchange on zeolite having a specific surface area of 300 m ² / g.
%). Also, the low temperature active catalyst part 1
c is γ-alumina having a specific surface area of 280 m ² / g and Rh supported so that 3.5 g per 1 liter of the catalyst is supported. The three-way catalyst 4 has a specific surface area of 80
Pt and Rh are supported on m ² / g of γ-alumina at a ratio of 5: 1 so that the total amount of Pt and Rh is 1.6 g per liter of the catalyst.

-Valve Control- The valve control means 8 controls the bypass valve 5 according to the flow shown in FIG. That is, the bypass valve 5 is initially opened, and the engine air-fuel ratio controlled by the air-fuel ratio control main routine is the theoretical air-fuel ratio (λ = 1).
If the engine temperature Ts is equal to or higher than the predetermined value Tm in the range from to the rich side, the bypass valve 5 is opened (steps S1 to S5). On the other hand, when it is determined to be lean in step S3, and when the engine temperature Ts is less than the predetermined value Tm even when it is determined to be rich, the bypass valve 5 is closed (step S6).

The predetermined value Tm is determined by the air-fuel ratio on the lean side (for example, A / F =) in the air-fuel ratio control main routine.
22) is the engine temperature (high engine combustion stability) when controlled. In the air-fuel ratio control main routine, basically, when the engine temperature is lower than the predetermined value Tm and when the temperature of the NOx purification catalyst 1 detected by the catalyst temperature sensor (not shown) is the predetermined value Tc.
When the above is the case, the air-fuel ratio is controlled to λ = 1 or to the rich side, and otherwise the lean side is controlled.

Table 1 shows the control contents of the bypass valve 5.

[Table 1]

-Regarding the test and its results-In the FTP-75 mode, that is, with the air-fuel ratio when the engine is cold set to λ = 1 and the air-fuel ratio after warm-up is A / F = 22, that is,
LA-4 mode test method (CV
The exhaust gas purification rate was measured by S-C / H). For comparison, in the same catalyst arrangement as in FIG. 1, the exhaust gas purification rate was measured in the mode in which the bypass valve 5 is opened during engine cold and the bypass valve 5 is closed after warming up. The results are shown in Table 2.

[0028]

[Table 2]

In the case of the embodiment, the purification rate of any of HC, CO and NOx is higher than that of the comparative example. The reason for this is that, in the case of the comparative example, the purification of exhaust gas is performed only by the three-way catalyst 4 which is not so effective even when the engine temperature is low, whereas in the case of the example, the catalyst closer to the engine at that time is used. It is recognized that the NOx purification catalyst 1 whose temperature easily rises contributes to the above purification. At this time, the NOx purification catalyst is the low temperature active catalyst portion 1c at the downstream portion.
Is thought to work as a three-way catalyst.

Another reason is that the NOx purification catalyst 1 is used from the time when the engine temperature is low as described above.
It is considered that the temperature of the catalyst was raised early, and HC was burned and NOx was purified with high efficiency.

[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 side view showing a partially cutaway NOx purification catalyst of the embodiment.

FIG. 3 is a flow chart of control of a bypass valve.

[Explanation of symbols]

 1 NOx Purifying Catalyst 1c Low Temperature Active Catalyst Section 2 Exhaust Passage 3 Bypass Passage 4 Three-Way Catalyst 5 Bypass Valve 6 Air-Fuel Ratio Detection Means 7 Temperature Sensor 8 Valve Control Means

Front Page Continuation (72) Inventor Shuji Iwakuni 3-1, Shinchi Fuchu-cho, Aki-gun, Hiroshima Mazda Motor Corporation

Claims (3)

[Claims] 1. An NOx purification catalyst that 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 that bypasses the NOx purification catalyst that 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 switching the flow of the exhaust gas between the NOx purifying catalyst side and the bypass passage side, Air-fuel ratio detecting means for detecting the air-fuel ratio of the engine, a temperature sensor for detecting the temperature of the engine, when the air-fuel ratio detected by the air-fuel ratio detecting means is a predetermined value or more, and detected by the air-fuel ratio detecting means When the air-fuel ratio is less than the predetermined value and the engine temperature detected by the temperature sensor is less than the predetermined value, the exhaust gas is Exhaust gas purifying device for an engine, characterized by comprising a valve control means for controlling the switching of the bypass valve to flow to the Ox purifying catalyst side. 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the NOx purifying catalyst is provided with a low temperature active catalyst portion supporting a noble metal as an active species at a downstream side portion thereof. 3. The exhaust gas purifying apparatus for an engine according to claim 2, wherein the three-way catalyst is provided downstream of a downstream end of the bypass passage in the exhaust system.