JP3113065B2 - Exhaust gas purification device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

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

[0003] On the other hand, in the field of automobiles, there is a demand to increase the air-fuel ratio to reduce the fuel consumption of the engine. To this end, a so-called lean burn engine of a lean burn system has been developed. This engine aims to improve the atomization of the air-fuel mixture and achieve stable combustion even with a lean air-fuel mixture.At low engine temperatures, the air-fuel ratio is set near the stoichiometric air-fuel ratio. Normally, the air-fuel ratio is switched and set to a lean side after the engine temperature rises and the combustion stability of the mixture increases. In this case, since the exhaust gas becomes excessive in oxygen, even if CO and HC can be oxidized and purified by the three-way catalyst, NOx
Cannot be reduced and purified.

On the other hand, in recent years, research on a zeolite (crystalline aluminosilicate) -based NOx purification catalyst in which a transition metal is supported by ion exchange has been advanced.
In the case of this catalyst, even in a lean atmosphere, NOx is directly or coexisting with a reducing agent (eg, CO, HC, etc.).
Can be decomposed into N ₂ and O ₂ .

For example, Japanese Patent Application Laid-Open No. 1-171625 discloses that an exhaust passage is branched in the middle, a NOx purification catalyst is provided on one side, a three-way catalyst is provided on the other side, and a passage switching valve is provided at a branch portion of the exhaust passage. When the temperature is low, the exhaust gas is caused to flow through the NOx purifying catalyst based on the exhaust gas temperature, and the three components of HC, CO and NOx, particularly NOx
On the other hand, there is a description of an exhaust gas purifying apparatus in which exhaust gas is caused to flow through a three-way catalyst in order to prevent thermal deterioration of the NOx purifying catalyst when exhaust gas temperature rises while purifying exhaust gas.

[0006]

However, in the exhaust gas purifying apparatus as described above, unburned gas, carbon, or the like in the exhaust gas adheres to a valve provided at a branch portion of the exhaust passage, or a mechanical device of the valve. Due to a target or electrical failure, the valve may be stuck at an intermediate opening such that the exhaust gas flows to both the NOx purification catalyst side and the three-way catalyst. In particular, in the case where the apparatus is provided with valve control means having a control state of setting the valve to an intermediate opening degree, the possibility is high, and in that case, the desired effect cannot be obtained.

That is, if the bypass valve is stuck at an intermediate opening degree despite the high exhaust gas temperature, NO
x High-temperature exhaust gas flows through the purification catalyst, which causes early deterioration of the catalyst. Further, if the air-fuel ratio is controlled to the lean side and there is sticking at the intermediate opening, a part of the exhaust gas flows to the three-way catalyst, resulting in insufficient purification of the NOx component.

[0008]

SUMMARY OF THE INVENTION The present invention provides
In order to cope with such a problem, the air-fuel ratio control of the engine is corrected when the valve is stuck at an intermediate opening as described above.

That is, means for solving the above problems include a NOx purification catalyst provided in an exhaust system of an engine and capable of purifying NOx in 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 and discharges exhaust gas; a three-way catalyst that purifies exhaust gas discharged through the bypass passage; a bypass valve that opens and closes the bypass passage; and an air-fuel ratio of the engine Air-fuel ratio control means for controlling based on the operating state; failure detection means for detecting the fixation of the bypass valve at an intermediate opening; and when the fixation of the bypass valve at the intermediate opening is detected by the failure detection means. And an air-fuel ratio correcting means for correcting the air-fuel ratio control by the air-fuel ratio control means.

In such a means, when the bypass valve is stuck at an intermediate opening, for example, from the viewpoint of preventing thermal deterioration of the NOx purifying catalyst, the air-fuel ratio is corrected to a lean side to correct the high temperature. Exhaust gas can be prevented from flowing to the NOx purification catalyst. Also,
From the viewpoint of purification of exhaust gas, when the air-fuel ratio is lean, by correcting this in the rich direction, that is, to a stoichiometric air-fuel ratio suitable for the three-way catalyst, the three-way catalyst can be effectively used. By working, the purification rate of exhaust gas can be increased.

-Regarding opening / closing control of the bypass valve-As the opening / closing control of the bypass valve, the bypass valve is closed (bypass path is closed) when the air-fuel ratio controlled by the air-fuel ratio control means is lean. Otherwise, the valve is opened when the temperature of the exhaust gas is low, the valve is closed when the temperature of the exhaust gas is low, and the valve is opened when the temperature of the exhaust gas is high. Any of the control modes can be adopted. The bypass valve may be controlled not only at two positions, that is, fully closed and fully opened, but also at an intermediate opening. This means is particularly effective when the bypass valve is sometimes controlled to the intermediate opening degree. In that case, the bypass valve is likely to be fixed at the intermediate opening degree.

Means for executing such intermediate opening degree control include a temperature sensor for detecting the temperature of the NOx purifying catalyst, and a method for controlling the temperature of the NOx purifying catalyst based on the temperature detected by the temperature sensor. And a valve control means for controlling the bypass valve to have an intermediate opening degree. According to this, when the temperature of the NOx purification catalyst is low, a part of the exhaust gas flows to the NOx purification catalyst, and the catalyst is heated by using the heat of catalytic reaction or the catalyst is heated to a predetermined temperature. It becomes possible to keep the temperature or higher.

That is, the catalytic reaction in the NOx purifying catalyst depends on the space velocity (SV) of the exhaust gas, and although it is difficult to react at a high SV, it reacts relatively well at a low SV even if the catalyst temperature is low. At the intermediate opening as described above, the SV of the exhaust gas flowing through the NOx purification catalyst is low,
The combustion reaction is smoothly performed and the catalyst is heated by the reaction heat, and at the same time, the purification of the NOx component also proceeds. As described above, even when the air-fuel ratio is switched from, for example, the stoichiometric air-fuel ratio to lean as a result of the heating of the NOx purification catalyst, and the entire amount of the exhaust gas flows to the NOx purification catalyst 1, it is immediately detected. It is possible to obtain the NOx purification rate of the period.

-Regarding the air-fuel ratio correcting means-The air-fuel ratio correcting means is configured to set the air-fuel ratio to a stoichiometric air-fuel ratio when the fixed opening of the bypass valve detected by the failure detecting means is equal to or more than a predetermined opening. It is preferable to make correction so that A large fixed opening degree of the bypass valve means that the amount of exhaust gas flowing to the three-way catalyst through the bypass passage is large. In this case, the fact that the air-fuel ratio becomes the stoichiometric air-fuel ratio means that the three-way catalyst becomes the stoichiometric air-fuel ratio. This is because the source catalyst can be effectively used for purifying the exhaust gas. Further, even if the air-fuel ratio is set to the stoichiometric air-fuel ratio, there is no problem because the NOx purification catalyst can purify NOx even if the stoichiometric air-fuel ratio is used.

In this case, when the fixed opening is smaller than the predetermined opening, the amount of exhaust gas flowing through the NOx purifying catalyst increases accordingly, so that the air-fuel ratio can be directly controlled based on the operating state of the engine. it can. That is, since the NOx purifying catalyst itself can purify the exhaust gas regardless of whether the air-fuel ratio is rich or lean, there is no need to particularly limit the air-fuel ratio control. It is preferable to allow the control to reduce the fuel consumption of the engine.

The air-fuel ratio correction means reduces the air-fuel ratio by controlling the air-fuel ratio by the air-fuel ratio control means when the failure detection means detects the fixation of the bypass valve at an intermediate opening. That is, the correction may be performed in the rich direction. This is because this is advantageous in that the three-way catalyst is effectively used for purifying the exhaust gas, and the air-fuel ratio control itself can be adapted to the operating state of the engine, and the fuel consumption can be reduced. It is.

-NOx Purifying Catalyst- As the NOx purifying catalyst, a catalyst in which a transition metal is supported on a metal-containing silicate having micropores such as zeolite is preferable. As the metal-containing silicate, zeolite is preferable. Alternatively, for example, Al and Fe, as metals forming a crystal skeleton,
Metal-containing silicates in combination with other metals (semimetals) such as Ce, Mn, Tb, Cu, B, P, and Al
Non-aluminosilicates containing no can be used, and these are effective in obtaining heat resistance. From the viewpoint of improving the heat resistance, H-type is more preferable than Na-type, and H-type zeolite is particularly preferable. As the above-mentioned zeolite, ZSM-5 is preferable, but A-type, X-type
Type, Y type or the like.

As the transition metal, Cu is preferable.
Other transition metals such as Co, Cr, Ni, Fe, Mn and the like can also be used. It is preferable that the transition metal is supported on the metal-containing silicate by ion exchange. However, it is preferable that the transition metal be supported on zeolite by removing water by an impregnation method or heating. A coprecipitation method or the like can be employed.

-Regarding the three-way catalyst-As the three-way catalyst, noble metals, for example, Pt,
Those supporting Pd, Rh or the like are suitable, and ceria or the like can be further added. Further, it is preferable that the three-way catalyst is disposed both in the bypass passage in the exhaust system and in a passage downstream of the downstream end thereof. In this case, the three-way catalyst passes through the NOx purification catalyst and is rearward thereof. HC and CO in the flowing exhaust gas can be purified.

The NOx purifying catalyst and the three-way catalyst can be used in the form of a pellet when used, but a monolithic carrier can also be used. In this case, cordierite is suitable as the carrier. Alternatively, another inorganic porous body can be used.

[0021]

According to the exhaust gas purifying apparatus, therefore, there is provided a failure detecting means for detecting the fixation of the bypass valve at the intermediate opening, and the failure detecting means detects the fixation of the bypass valve at the intermediate opening. The control of the air-fuel ratio by the air-fuel ratio control means is corrected when it is performed, so that the exhaust gas temperature is reduced by correcting the air-fuel ratio to the lean side to prevent thermal deterioration of the NOx purification catalyst. , The air-fuel ratio is corrected to be the stoichiometric air-fuel ratio suitable for the three-way catalyst,
For example, the purification rate of exhaust gas can be increased.

Further, since such a failure detecting means and an air-fuel ratio correcting means are provided, a temperature sensor for detecting the temperature of the NOx purifying catalyst is interposed between the bypass valve and the temperature sensor when the temperature is lower than a predetermined value. Valve control means for controlling the opening degree, and when the temperature of the NOx purification catalyst is low, a part of the exhaust gas is caused to flow through the NOx purification catalyst to warm or keep the catalyst warm. It is assured that thermal deterioration of the NOx purifying catalyst or deterioration of exhaust gas at the time of fixing the intermediate opening degree is prevented.

The air-fuel ratio correcting means corrects the air-fuel ratio to a stoichiometric air-fuel ratio when the fixed opening of the bypass valve detected by the failure detecting means is equal to or greater than a predetermined opening. When employed, the three-way catalyst can be effectively used for purifying the exhaust gas, and the emission of the unpurified exhaust gas can be suppressed.

Further, as the air-fuel ratio correction means, when the malfunction detection means detects the fixation of the bypass valve at an intermediate opening, the air-fuel ratio is controlled by the air-fuel ratio control means to lower the air-fuel ratio. When such correction is employed, the three-way catalyst can be effectively used for purifying exhaust gas while reducing the fuel efficiency by controlling the air-fuel ratio in accordance with the operating state of the engine.

[0025]

Embodiments of the present invention will be described below.

-Overall Configuration- FIG. 1 shows the configuration of the apparatus of the embodiment. In FIG. 1, reference numeral 1 denotes NOx provided in an exhaust passage 2 of the engine.
The purification catalyst 3 is a bypass passage for exhausting exhaust gas by bypassing the NOx purification catalyst 1, 4 is a first three-way catalyst provided in the bypass passage 3, and 5 is a downstream end of the bypass passage 3 This is a second three-way catalyst provided downstream of the junction between the exhaust passage 2 and the bypass passage 3). A branch portion between the exhaust passage 2 and the bypass passage 3 is provided with a vane-type bypass valve 6 for adjusting a flow amount of exhaust gas between the NOx purification catalyst 1 and the bypass passage 3. .

The operation of the bypass valve 6 is controlled by valve control means 7 based on the air-fuel ratio of the engine and the temperature of the NOx purifying catalyst 1. The air-fuel ratio of the engine is obtained by the air-fuel ratio control means 8 for controlling the fuel injection valve of the engine based on the engine temperature (cooling water temperature), the engine speed and the engine load.
The temperature of the x-purifying catalyst 1 is obtained by a temperature sensor 9 for detecting the gas temperature at the catalyst inlet. The bypass valve 6 is provided with a failure detecting means 11 for detecting a failure of the valve 6. As a countermeasure against the failure, the air-fuel ratio control means 8 controls the air-fuel ratio according to the failure state of the bypass valve 6. An air-fuel ratio correction unit 12 for correcting the fuel ratio control is provided.

-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. Cu is carried on zeolite by ion exchange. Catalyst material (ion exchange rate 133
%). The supported amount was 150 g per liter of the catalyst (a honeycomb carrier made of cordierite).

The three-way catalysts 4 and 5 are composed of Pt and Rh.
At a ratio of 3: 1 with a catalyst material supported on γ-alumina such that the total amount of both becomes 1.6 g per liter of the catalyst.

-Valve Control- The valve control means 7 controls the bypass valve 6 according to the flow shown in FIG. That is, the bypass valve 6 is initially fully opened (the bypass passage 3 is opened), and if the engine air-fuel ratio controlled by the air-fuel ratio control main routine is lean, the bypass valve 6 is closed (steps S1 to S1). S4). When the engine air-fuel ratio is λ = 1, the control angle is calculated by comparing the temperature (inlet gas temperature) Ts of the NOx purifying catalyst 1 with a temperature judgment reference value m of a map stored in advance. Then, the bypass valve 6 is opened so as to reach the calculated angle (step S).
3, S5 to S7). The control angle is determined by the determination reference value m.
The opening degree of the bypass valve 6 is set to decrease as the (temperature) decreases.

Therefore, according to the valve control, if the air-fuel ratio is lean, the bypass valve 6 is fully closed, and the entire amount of exhaust gas flows to the second three-way catalyst 5 via the NOx purification catalyst 1. Therefore, the NOx component in the exhaust gas is NO
x is efficiently purified by the purification catalyst 1 and
Other unpurified components in the exhaust gas are also purified by the second three-way catalyst 5.

When the air-fuel ratio is not lean, that is, when λ = 1, the bypass valve 6 is basically opened, but the temperature of the NOx purifying catalyst 1 is reduced as in the case of cold engine. Is low, the bypass valve 6 has an intermediate opening according to the temperature. Therefore, not all of the exhaust gas flows to the first three-way catalyst 4 but part of the exhaust gas flows to the NOx purification catalyst 1. As a result, the NOx purifying catalyst 1 is heated or kept warm.

That is, at the intermediate opening as described above, NOx
The SV of the exhaust gas flowing through the purification catalyst 1 is low, the HC combustion reaction is performed smoothly, and the reaction heat heats the catalyst. At the same time, the purification of the NOx component also proceeds. As described above, the NOx purification catalyst 1
= 1, the air-fuel ratio is switched to lean, and the total amount of exhaust gas is reduced by the NOx purifying catalyst 1.
Thus, the desired NOx purification rate can be obtained immediately even when the flow starts flowing.

The following test was conducted to confirm the above points. That is, for each example in which the opening of the bypass valve 6 is fixed to 0 degree (fully closed), 30 degrees, 60 degrees, and 90 degrees (fully opened), the purification rate of the exhaust gas at the cold start in the LA-4 mode, the catalyst inlet Gas temperature and catalyst temperature were measured. However, each of the three-way catalysts was only the second three-way catalyst 5. The results are as shown in Table 1.

[0035]

[Table 1]

Although there is not much difference between the examples in terms of HC and CO purification rates, there is a difference in NOx purification rates. An opening (30
(60 °), the NOx purification rate is high because when the engine is fully closed, the total amount of exhaust gas is NOx even when the engine is cold (λ = 1).
Since it flows to the second three-way catalyst 5 via the purification catalyst 1, the temperature of the second three-way catalyst 5 cannot be raised at an early stage, and the purification of the NOx component is performed even though λ = 1. It is recognized as a result of having become dependent on. In addition, N
The low Ox purification rate is considered to be the result of the need to purify the NOx component by the second three-way catalyst 5 even after the engine temperature rises and the air-fuel ratio becomes lean.

However, regarding the inlet gas temperature of the NOx purifying catalyst 1 (250 seconds after the start of the test) and the temperature of the catalyst 1, the smaller the opening of the bypass valve 6, the higher the temperature. This seems at first seemingly contradictory to the earlier explanation that the lower the SV, the more the catalyst 1 is heated. However, when the total amount of the exhaust gas is not so large, such a result is obtained. do not do. In any case, in order to raise the temperature of the NOx purifying catalyst 1 at an early stage, it can be said that flowing exhaust gas to the catalyst 1 from an early stage is effective.

Based on the above test results, Table 2 below shows the exhaust gas purification rate at the cold start in the LA-4 mode with respect to the embodiment having the above-described intermediate valve opening control and the comparative example (empty). (A control of switching between full opening and full closing of the valve based on the fuel ratio). However, in each of the examples and the comparative examples, only the second three-way catalyst 5 was used as the three-way catalyst.

[0039]

[Table 2]

For each of HC, CO and NOx,
The embodiment shows a high purification rate, particularly a high NOx purification rate, from which the effect of the valve control having the intermediate opening degree is recognized.

-Regarding air-fuel ratio control and correction thereof-The air-fuel ratio control means 8 determines the air-fuel ratio based on the engine temperature when the engine is cold when the temperature is less than a predetermined value. When the temperature is equal to or higher than a predetermined value, a lean air-fuel ratio (A / F = 22) is set, and a stoichiometric air-fuel ratio is set based on the engine speed and the engine load when the engine is running at a high speed and a high load. I do.

Failure detection means 1 for correcting air-fuel ratio control
1 is provided with a valve position sensor for detecting the opening degree of the bypass valve 6, and the opening degree detected by the sensor is compared with the opening degree set by the valve control means 7. Sometimes bypass valve 6
Is determined to be stuck at a specific opening and malfunctioning.

When the failure detecting means 11 detects a failure of the bypass valve 6, the air-fuel ratio correcting means 12 fixes the opening degree θ of the bypass valve 6, as shown in FIG.
Is equal to or greater than the predetermined opening degree θo, a correction command is issued to the air-fuel ratio control means 8 so that the air-fuel ratio is set to λ = 1. When the fully closed angle is 0 degree and the fully opened angle is 90 degrees in FIG.

FIG. 4 shows the flow of the air-fuel ratio correction control. If a failure of the bypass valve 6 is detected and the fixed opening θ is equal to or larger than the predetermined opening θo, the air-fuel ratio becomes λ = 1. (Steps S11 to S13). If there is no failure, and if there is a failure, but the fixed opening θ is less than the predetermined opening θo, the air-fuel ratio control is continued by the air-fuel ratio control main routine (steps S11, S12, S1).
4).

Therefore, according to the air-fuel ratio correction control,
When a predetermined amount or more of exhaust gas is discharged through only the three-way catalysts 4 and 5 due to the failure of the bypass valve 6, the air-fuel ratio of the engine is forcibly set to λ = 1. . Therefore, the exhaust gas has properties suitable for purification by the three-way catalysts 4 and 5, and is efficiently purified by the three-way catalysts 4 and 5. In this case, a part of the exhaust gas flows to the NOx purifying catalyst 1, but this catalyst 1 can purify even the exhaust gas having the air-fuel ratio of λ = 1, and there is no problem.

If the fixed opening degree θ due to the failure of the bypass valve 6 is smaller than the predetermined opening degree θo, the normal air-fuel ratio control is continued. The amount of exhaust gas discharged only through the catalysts 4 and 5 is small and does not matter. Rather, the fuel efficiency can be reduced by executing the normal air-fuel ratio control.

-Effect of SV on Valve Control- In the valve control described above, it has been described that the catalytic reaction easily proceeds at a low SV and the catalyst can be heated. This will be described in more detail.

That is, in the exhaust gas purifying apparatus shown in FIG. 5, reference numeral 1 denotes N which is provided in the exhaust passage 2 of the engine 10.
The Ox purification catalyst 3, a bypass passage bypassing the NOx purification catalyst 1, a three-way catalyst 5 provided downstream of the downstream end of the bypass passage 3, and a bypass valve 16 provided in the bypass passage 3. is there. Table 3 shows the exhaust gas temperature when a vehicle equipped with a lean burn engine was driven at a constant speed. In addition, NOx of Cu ion exchanged zeolite catalyst
The purification characteristics (SV = 55000h ^-1 ) are as shown in FIG.

[0049]

[Table 3]

The vehicle is cold started and 40k
FIG. 7 shows the exhaust gas temperature, the temperature at the downstream portion of the catalyst 1, and the NOx purification rate when traveling at a constant speed of m / h. The bypass valve 16 is switched from open to closed for a predetermined time after the cold start, and thereafter, when the exhaust gas temperature reaches a predetermined value, the air-fuel ratio is changed to λ.
= 1 was switched to lean.

FIG. 7 shows that although the temperature of the catalyst 1 was rising due to the closing of the bypass valve 16, the temperature of the catalyst did not rise to 300 ° C. or more. This indicates that the NOx purification rate is extremely low as expected from FIG.

Therefore, when the bypass valve 16 was controlled not to be completely closed but to an intermediate opening degree and then to be fully closed, the result shown in FIG. 8 was obtained. That is, the catalyst temperature has risen to 300 ° C. or higher, and therefore a high NOx purification rate has been obtained.

The reason for this is that the SV can be suppressed to a low level because the bypass valve 16 is not fully closed at first but at an intermediate opening degree, so that the catalytic reaction in the NOx purifying catalyst 1 proceeds smoothly. That is, the catalyst temperature was raised by the heat reaction.

FIG. 9 shows the relationship between the SV and the NOx purification rate. That is, at a high SV (120,000 h ^-1 ), the catalyst exhibits no substantial NOx purification activity unless the catalyst temperature becomes 400 ° C. or higher. However, as the SV becomes lower, the temperature at which the NOx purification activity becomes lower becomes less than 25000 h ^-1. It can be understood that NOx purification activity is exhibited even at about 300 ° C. From this, it can be said that the results of FIGS. 7 and 8 are supported.

-Others- The reason why the first three-way catalyst 4 is disposed in the bypass passage 3 in the above embodiment is that only the second three-way catalyst 5 is used.
This is because the catalyst temperature is so far from the engine body that an early rise in the catalyst temperature cannot be expected, and the purification rates of all HC, CO, and NOx in a cold state are low. That is, for the embodiment having the first three-way catalyst 4 and the comparative example not having the first three-way catalyst 4, the air-fuel ratio when the engine is cold is λ = 1, and the air-fuel ratio after warm-up is A / A.
With F = 22, the exhaust gas purification rate (total) from the start of the engine to a predetermined time after warm-up was measured. The results are as shown in Table 4.

[0056]

[Table 4]

In the case of the embodiment, the purification rate of each of HC, CO and NOx is higher than that of the comparative example. The reason is that in the case of the embodiment, the first three-way catalyst 4 is located near the engine body.
It is recognized that the first three-way catalyst 4 was heated relatively quickly and contributed to purification of exhaust gas.

[Brief description of the drawings]

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

FIG. 2 is a flowchart of valve control.

FIG. 3 is a diagram showing a relationship between an opening degree of a bypass valve and control of an air-fuel ratio.

FIG. 4 is a flowchart of air-fuel ratio correction control;

FIG. 5 is a configuration diagram showing another example of the arrangement of the catalyst in the exhaust gas purification device.

FIG. 6 is a characteristic diagram showing a relationship between a catalyst inlet gas temperature and a NOx purification rate.

FIG. 7 is a diagram showing changes in exhaust gas temperature, catalyst downstream temperature, and NOx purification rate by a conventional valve control method.

FIG. 8 shows the exhaust gas temperature by the valve control method of the present invention,
Diagram showing changes in catalyst downstream temperature and NOx purification rate

FIG. 9 is a characteristic diagram showing a relationship between a NOx purification rate and SV.

[Description of Signs] 1 NOx purification catalyst 2 exhaust passage 3 bypass passage 4 first three-way catalyst 5 second three-way catalyst 16, 6 bypass valve 7 valve control means 8 air-fuel ratio control means 9 catalyst temperature sensor 10 engine body 11 Failure detection means 12 Air-fuel ratio correction means

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Toshishi Kamioka 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Co., Ltd. (72) Hideharu Iwakuni 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Inside (72) Inventor Makoto Kyogoku 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Corporation (56) References JP-A-3-225013 (JP, A) JP-A-1-171625 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) F01N 3/20 F01N 3/24 F02D 41/00-41/40

Claims (4)

(57) [Claims] 1. A NOx purification catalyst provided in an exhaust system of an engine and capable of purifying NOx in exhaust gas at a lean air-fuel ratio, and an exhaust gas bypassing the NOx purification catalyst 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 air for controlling an air-fuel ratio of the engine based on an operation state of the engine. Fuel ratio control means; failure detection means for detecting fixation of the bypass valve at an intermediate opening; and air-fuel ratio control means when the fixation of the bypass valve at the intermediate opening is detected by the failure detection means. An exhaust gas purifying apparatus comprising: an air-fuel ratio correcting unit that corrects a control of a fuel ratio. 2. A temperature sensor for detecting the temperature of the NOx purifying catalyst, and based on the temperature detected by the temperature sensor, the bypass valve is set to an intermediate opening when the temperature is equal to or lower than a predetermined value. The exhaust gas purifying apparatus for an engine according to claim 1, further comprising a valve control means for controlling. 3. The air-fuel ratio corrector corrects the air-fuel ratio to a stoichiometric air-fuel ratio when the fixed opening of the bypass valve detected by the failure detector is equal to or greater than a predetermined opening. The exhaust gas purifying apparatus according to claim 1 or 2. 4. The air-fuel ratio correction means decreases the air-fuel ratio by controlling the air-fuel ratio by the air-fuel ratio control means when the failure detection means detects that the bypass valve is stuck at an intermediate opening. The exhaust gas purifying apparatus according to claim 1 or 2, wherein the correction is performed as follows.