JP2001003732A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the temperature of each exhaust emission control agent without reducing emission, in an exhaust emission control device for an internal combustion provided with a bypass passage bypassing an NOx absorbing agent to avoid so-called SOx poisoning. SOLUTION: In this device, a switching means 28 is controlled in a manner to be situated in a switching position where when stoichiometic control is executed during waiting after starting of an internal combustion engine 1 and when lean control of an air-fuel ratio of exhaust gas is executed, exhaust gas is caused to flow in an NOx absorbent 20. Further, control is executed such that the switching means is situated in a switching position where when stoichiometic control of an air-fuel ratio of exhaust gas is effected during high load running of the internal combustion engine 1, exhaust gas is caused to flow in an exhaust bypass passage 26.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly to an exhaust gas purification device provided with a bypass passage bypassing a NOx catalyst in order to avoid so-called SOx poisoning. The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, which can heat an exhaust catalyst and raise the temperature without the need.

[0002]

2. Description of the Related Art A three-way catalyst is known as a catalyst for purifying exhaust gas discharged from an internal combustion engine capable of lean combustion. This three-way catalyst can purify CO, HC, and NOx. However, the three-way catalyst exhibits an extremely high purification rate of any of CO, HC and NOx during stoichiometric air-fuel ratio control, but has a low NOx purification ability during lean control. Therefore, conventionally, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx
And when the oxygen concentration of the exhaust gas decreases and reaches a stoichiometric or rich state, the absorbed NOx
A so-called occlusion reduction type or selective reduction type NOx catalyst is used to release and reduce NOx.

[0003] Further, since sulfur is contained in the fuel and the lubricating oil of the engine, SOx is contained in the exhaust gas. However, the NOx catalyst is the same as that used to absorb the NOx. SOx is absorbed by the mechanism. in this case,
Since the SOx absorbed by the NOx catalyst forms a stable sulfate over time, the NOx from the NOx catalyst
SOx is not emitted under the conditions when releasing NOx, and NOx
There is a problem that the amount of SOx in the catalyst increases, and as a result, the absorption amount of NOx decreases, and the purification efficiency of NOx decreases.

[0004] Therefore, in order to avoid such a problem, there has been conventionally proposed a technique in which an SOx absorbent is disposed upstream of a NOx catalyst in an exhaust gas passage.
The SOx absorbent, if the air-fuel ratio of the exhaust gas is lean state, as well as absorb SOx, oxygen concentration in the exhaust gas is lowered, the SOx when a stoichiometric or rich state SO ₂ It is configured to emit as.

However, when the SOx absorbent is disposed in the exhaust gas passage on the upstream side of the NOx catalyst, the concentration of oxygen in the exhaust gas is reduced in order to release NOx from the NOx catalyst. When a rich state is reached, SOx is also released from the SOx absorbent, and the released SOx is discharged from N 2
Absorbed by the Ox catalyst, the same problem as described above occurs in the NOx catalyst, and a situation occurs in which the NOx purification efficiency is reduced.

Therefore, in order to avoid such a so-called SOx poisoning of the NOx catalyst, for example, SO
A technique has been proposed in which an exhaust bypass passage for connecting a portion between the x-absorbing material and the NOx catalyst and a portion downstream of the NOx catalyst and a switching means are provided (Japanese Patent No. 2605580).
issue). According to this technique, when the oxygen concentration of the exhaust gas decreases, the air-fuel ratio reaches a stoichiometric or rich state, and SOx is released from the SOx absorbent, the switching means is operated to discharge the SOx from the SOx absorbent. The exhaust gas containing the SOx flows through a bypass passage bypassing the NOx catalyst, and is configured to be able to avoid SOx poisoning of the NOx catalyst.

In this case, as described above, the H contained in the exhaust gas flowing from the SOx absorbent through the NOx catalyst bypasses the NOx catalyst.
It is desirable that a three-way catalyst be provided downstream of the bypass passage in order to purify C, CO, and NOx.

By the way, the NO in the exhaust gas as described above
A catalyst capable of purifying x or SOx can purify SOx or NOx only when heated to a predetermined temperature or higher. That is, in a catalyst capable of purifying NOx, NOx is released and reduced when the temperature is raised to 150 ° C. or higher, and a temperature of 700 ° C. or higher is required for purifying SOx. Therefore, in order to purify the exhaust gas efficiently, it is necessary to raise the temperature of these catalysts to a predetermined temperature or more at the early stage of the engine start.

In this case, these catalysts are heated and heated by using the heat of the exhaust gas of the engine. When the engine is warmed up at the time of starting the engine, the air-fuel ratio control is a stoichiometric control. Since the SOx absorbent releases SOx at the time of stoichiometric control, the switching means is connected to SOx
In order to avoid poisoning, the exhaust gas is disposed at a position where the exhaust gas flows into the bypass passage so that the exhaust gas does not flow into the NOx catalyst. As a result, the high-temperature exhaust gas at the time of the stoichiometric control flows into the three-way catalyst via the bypass passage. Therefore, the three-way catalyst is heated and heated by the high-temperature exhaust gas during the stoichiometric control during the warm-up.

Thereafter, the air-fuel ratio control of the engine is performed when it is determined that the warm-up operation has been completed and that the heating of the three-way catalyst has been completed at a temperature higher than the temperature at which the catalyst is activated. Fuel ratio control shifts to lean control. When the air-fuel ratio control of the engine shifts to the lean control,
Since the NOx catalyst is in a state capable of storing NOx, the switching means switches the NOx from the position where the bypass passage is opened.
The position is switched to a position where exhaust gas flows into the catalyst, and as a result, the exhaust gas flows into the NOx catalyst.

In this case, since the exhaust gas has an air-fuel ratio under lean control, it is lower than the exhaust gas temperature under the stoichiometric control. As a result, it takes a longer time to heat the NOx catalyst and raise the temperature than when the three-way catalyst is activated by the exhaust gas during the stoichiometric control. Therefore, there is a problem that it takes a long time until the NOx catalyst reaches a predetermined temperature and is activated, so that emission of the exhaust purification material is not good.

[0012]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to purify exhaust gas of an internal combustion engine provided with a bypass passage bypassing a NOx absorbent and a three-way catalyst in order to avoid so-called SOx poisoning. The point of the apparatus is to raise the temperature of each exhaust gas purifying material without lowering the emission.

A second object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine in which a three-way catalyst is arranged in series downstream of the NOx absorbent in addition to the object of the first invention. It is in.

A third object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine which can heat a three-way catalyst more efficiently, in addition to the object of the first invention.

[0015]

According to the first aspect of the present invention, there is provided an exhaust passage provided in an exhaust passage of an internal combustion engine, the exhaust gas flowing from the internal combustion engine capable of lean combustion. N when the air-fuel ratio of the engine is lean
A NOx absorber that absorbs Ox and releases the absorbed NOx when the oxygen concentration of the exhaust gas flowing in is low;
If the air-fuel ratio of the inflowing exhaust gas is in a lean state, it is provided upstream of the NOx absorbent in the exhaust passage.
When the oxygen concentration of the exhaust gas that flows in while absorbing x is low, the SOx absorbent that releases the absorbed SOx and the exhaust gas that is provided in the middle of the exhaust passage and contains the SOx that is exhausted from the SOx absorbent are included. An exhaust bypass passage configured to flow around the NOx absorbent;
A three-way catalyst capable of purifying CO and NOx, and an exhaust gas flowing from the internal combustion engine through an exhaust passage via the SOx absorbent can be selectively introduced into either the NOx absorbent or the exhaust bypass passage. In an exhaust gas purification apparatus for an internal combustion engine including a switching unit, the switching unit includes:
At the time of stoichiometric control at the time of warm-up after the start of the internal combustion engine and when the air-fuel ratio control of the exhaust gas is controlled to be lean, the exhaust gas is controlled to be arranged at a switching position for causing the exhaust gas to flow into the NOx absorbent. In addition, when the air-fuel ratio control of the exhaust gas is performed under the high load stoichiometric control when the internal combustion engine is under a high load, the exhaust gas is controlled to be located at a switching position where the exhaust gas can flow into the exhaust bypass passage. .

Here, the "NOx absorbent" corresponds to, for example, a storage reduction type NOx catalyst. NOx storage reduction catalyst absorbs NOx in the exhaust gas air-fuel ratio flows in the case of a lean state, reduced by releasing the absorbed NOx to N ² when the oxygen concentration of the exhaust gas flowing drops Catalyst.

Therefore, according to the first aspect of the present invention,
When the internal combustion engine is warmed up after the start of the internal combustion engine, the air-fuel ratio is controlled to a stoichiometric or rich state, and the switching unit reduces the exhaust gas from the internal combustion engine to N.
It is arranged at the switching position where it flows into the Ox absorbent side, and N
The exhaust passage communicating with the Ox absorbent side is open, and the exhaust bypass passage is closed.
In this case, when the warm-up operation of the internal combustion engine is performed by the stoichiometric or rich air-fuel ratio control, the ratio of fuel to oxygen in the exhaust gas is high,
The combustion temperature of the exhaust gas is high. Therefore, the exhaust gas having a high combustion temperature is supplied to the NOx absorbent side to heat and raise the temperature of the NOx absorbent.

In this case, the exhaust gas is supplied to the NOx absorbent through the SOx absorbent. The SOx absorbed by the SOx absorbent is smaller than the NOx absorbed by the NOx absorbent. Stable and difficult to decompose, S
It is understood from experiments that decomposition of Ox does not release SOx unless the temperature of the SOx absorbent is further increased. That is, in the state after the start of the internal combustion engine, even if the air-fuel ratio control in the stoichiometric or rich state is performed, the temperature at which SOx can be released from the SOx absorbent has not yet reached. S from absorbent
The release rate of Ox is extremely low, and SOx
Is hardly released.

Therefore, as described above, S
Even when the exhaust gas is supplied via the Ox absorbent, since the exhaust gas contains almost no SOx, there is no possibility that the NOx absorbent absorbs the SOx. It is heated and heated without SOx poisoning.

Thereafter, when it is determined that the temperature of the NOx absorbent has been sufficiently raised and activated, the air-fuel ratio control of the engine shifts to a lean state. Thus, even when the air-fuel ratio control of the engine shifts to the lean state, the arrangement of the switching means is not changed, and the exhaust bypass passage is still closed. As a result, S
Exhaust gas exhausted from the engine via the Ox absorbent is supplied to the NOx absorbent. Therefore, in this state, NOx
The absorbent absorbs NOx in the exhaust gas flowing into the absorbent.

Further, for example, when the accelerator opening reaches 85% or more and the internal combustion engine reaches a high load state, the air-fuel ratio control shifts to a stoichiometric state. In the case of such a high-load stoichiometric state, switching control is performed in which the switching means is disposed at a switching position at which exhaust gas can flow into the exhaust bypass passage. The exhaust passage communicating with the exhaust port is closed. As a result, the exhaust gas is supplied to the three-way catalyst via the exhaust bypass which bypasses the NOx absorbent.

As a result, the exhaust gas having a high combustion temperature at the time of such high load stoichiometry flows into the three-way catalyst, and the three-way catalyst is heated and heated efficiently in a short time. At this time, the unburned HC, CO, and NOx in the inflowing exhaust gas are purified by the three-way catalyst.

Therefore, according to the first aspect of the present invention,
At the time of warm-up at the time of engine start, while the air-fuel ratio control is in the stoichiometric control state, high-temperature exhaust gas is supplied to the NOx absorbent to heat and raise the temperature of the NOx absorbent.
Preparations can be made for the x catalyst to be able to absorb NOx sufficiently. Therefore, after that, when the air-fuel ratio control shifts to the lean control, the NOx absorbent can surely absorb NOx. Further, when the internal combustion engine becomes in a high load state during acceleration or the like and shifts again to the stoichiometric control, the switching means operates to supply exhaust gas having a high combustion temperature to the three-way catalyst, The catalyst can be heated and heated in a short time.

Therefore, according to the first aspect of the present invention,
The NOx absorbent can be heated and heated by using stoichiometric control during warm-up operation after the internal combustion engine is started, and the three-way catalyst can be heated for a short time by using higher temperature exhaust gas during high load stoichiometric control. Can be efficiently heated. As a result, it is possible to heat and raise the temperature of the NOx absorbent and the three-way catalyst without lowering the emission and avoiding SOx poisoning of the NOx catalyst.

According to the second aspect of the present invention, the three-way catalyst is provided on the exhaust passage downstream of the NOx absorbent in the exhaust passage, and the exhaust bypass passage is connected to the NOx absorbent. A portion between the SOx absorbent and a portion between the NOx absorbent and the three-way catalyst are connected to each other.

Therefore, according to the second aspect of the present invention,
When the air-fuel ratio control is in a stoichiometric state at the time of warm-up after the start of the internal combustion engine, the exhaust gas flowing out of the SOx absorbent is supplied to the NOx absorbent without the exhaust gas passing through the exhaust bypass passage. The switching position is set, the exhaust passage communicating with the NOx absorbent is open, and the exhaust bypass passage is closed.

As a result, the high-temperature exhaust gas discharged from the SOx absorbent is supplied to the NOx absorbent. in this case,
As described above, SOx absorbed by the SOx absorbent is stable and less likely to be decomposed than NOx absorbed by the NOx absorbent, and SOx is decomposed if the temperature of the SOx absorbent is higher. At this stage, SOx is hardly released since it is not generated unless otherwise. Also,
Unburned HC, CO, and NOx in the exhaust gas are purified by the three-way catalyst.

Therefore, as described above, S
Even when the exhaust gas is supplied via the Ox absorbent, since the exhaust gas contains almost no SOx, there is no possibility that the NOx absorbent absorbs the SOx. Heated and heated without poisoning.

Thereafter, the warm-up of the internal combustion engine is completed, and NOx
When it is determined that the temperature of the catalyst has been sufficiently raised and activated, the air-fuel ratio control of the internal combustion engine shifts to a lean control state. Thus, even when the air-fuel ratio control of the internal combustion engine shifts to the lean control state, the arrangement of the switching means does not change, and the exhaust gas discharged from the internal combustion engine via the SOx absorbent is exhaust gas. It is not supplied to the bypass passage and is supplied to the NOx absorbent. Therefore, in this state, the NOx absorbent absorbs NOx in the exhaust gas flowing into the NOx absorbent.

Next, when the internal combustion engine reaches a high load state, the air-fuel ratio control also returns to the stoichiometric state, the switching means is controlled to switch, the exhaust bypass passage is opened, and NOx is reduced. The exhaust passage communicating with the absorbent is closed. As a result, the exhaust gas is supplied to the three-way catalyst via the bypass passage bypassing the NOx absorbent. Therefore, the exhaust gas having a high combustion temperature at the time of high load stoichiometry flows into the three-way catalyst, and heats and raises the temperature of the three-way catalyst.

In this case, since the internal combustion engine is under a high load state, the exhaust gas discharged is also at a high temperature, and the temperature of the three-way catalyst is increased in a short time by the high-temperature heating. At this time,
Unburned CO, HC, and NOx contained in the exhaust gas are purified by the three-way catalyst.

According to the third aspect of the present invention, the exhaust bypass passage is provided by connecting a portion between the NOx absorbent and the SOx absorbent and a downstream portion of the NOx absorbent. The three-way catalyst is provided in the exhaust bypass passage.

Therefore, according to the third aspect of the present invention,
As in the case of the second aspect of the invention, in the case of the air-fuel ratio control in the stoichiometric state at the time of warm-up after the start of the internal combustion engine, the switching means closes the bypass passage and communicates with the NOx absorbent. The exhaust passage is located at a control position where the exhaust gas is opened.
The SOx is supplied to the absorbent side and the SOx is not released.
The Ox absorber is heated and heated. Thereafter, if the NOx absorbent is heated sufficiently to promote the chemical reaction,
Although the air-fuel ratio control of the internal combustion engine is switched to the lean control, the switching means does not operate and the switching position at which the exhaust gas is supplied to the NOx absorbent is maintained, so that the exhaust bypass passage is closed as described above. I have. Therefore, in this state, the NOx absorbent absorbs NOx in the exhaust gas.

Thereafter, when the internal combustion engine is under a high load,
The state again shifts to the stoichiometric control state. In this case, the switching means switches to a switching position in which the exhaust gas is supplied to the exhaust bypass passage side and the exhaust passage communicating with the NOx absorbent is closed.

As a result, the exhaust gas is supplied to the three-way catalyst provided in the exhaust bypass passage via the exhaust bypass passage. In this case, since the internal combustion engine is under a high load state, the exhaust gas discharged is also at a high temperature, and the temperature of the three-way catalyst is raised in a short time by high-temperature heating. Further, unburned HC, CO, and NOx in the inflowing exhaust gas are purified by the three-way catalyst.

In the present invention, unlike the invention of the second aspect, the three-way catalyst is provided on the exhaust bypass passage, not on the downstream side of the NOx absorbent in the exhaust passage. , The NOx absorbent is arranged in parallel with the NOx absorbent, so that the NOx absorbent and the three-way catalyst are connected and arranged in a serial positional relationship as in the case of the invention according to claim 2. In addition, it is possible to arrange the three-way catalyst closer to the internal combustion engine body in terms of distance. As a result, it is possible to supply the exhaust gas having a higher temperature to the three-way catalyst than in the case of the invention described in claim 2, and it is possible to heat and raise the temperature of the three-way catalyst more efficiently.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described in detail based on an embodiment shown in the accompanying drawings.

FIG. 1 is a diagram showing a schematic configuration when the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes an engine body, reference numeral 2 denotes a piston, reference numeral 3 denotes a combustion chamber, reference numeral 4 denotes a spark plug, reference numeral 5 denotes an intake valve, reference numeral 6 denotes an intake port, reference numeral 7 denotes an exhaust valve, reference numeral 8 denotes an exhaust port. Are shown respectively.

The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. The surge tank 10 is connected to an air cleaner 14 via an intake duct 12 and an air flow meter 13, and a throttle valve 15 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 16.
The outlet of the casing 18 is connected via an exhaust pipe 19 to a casing 21 containing a storage-reduction type NOx catalyst 20. The storage reduction type NOx catalyst 20 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter referred to as exhaust air-fuel ratio) is lean, and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low. And is reduced to N ₂ .

In the present embodiment, the three-way catalyst 43 is disposed on the exhaust passage downstream of the storage-reduction type NOx catalyst 20 as the NOx absorbent. That is, a casing 42 containing a three-way catalyst 43 is provided downstream of the exhaust pipe 44 of the casing 21. The three-way catalyst 43 is configured so that the air-fuel ratio control can purify HC, CO, and NOx during stoichiometry.

In the present embodiment, a portion between the NOx storage reduction catalyst 20 and the SOx catalyst 17 and a portion between the NOx storage reduction catalyst 20 and the three-way catalyst 43 are defined as follows. The connection is provided. That is, the inlet 21a of the casing 21 and the casing 2 of the NOx catalyst 20
An exhaust pipe 44 provided between the first catalyst 43 and the casing 42 of the three-way catalyst 43 is connected by a bypass pipe 26 that bypasses the NOx catalyst 20, and an inlet of the casing 21 which is a branch of the bypass pipe 26. An exhaust switching valve 28 as switching means, in which a valve body is operated by an actuator 27, is provided at 21a.

The exhaust switching valve 28 is connected to the actuator 27
As a result, as shown by the solid line in FIG.
1 is closed, and the inlet to the downstream NOx catalyst 20 is fully opened; and, as shown by the broken line in FIG. 1, the inlet to the downstream NOx catalyst 20 is closed and the bypass pipe 26 is closed. Is configured to operate by selecting one of the bypass open positions for fully opening the inlet portion of the motor. The casing 42 is connected to a muffler (not shown) via an exhaust pipe 48.

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, and a CPU (Central Processor) by a bidirectional bus 31. Unit) 3
4, an input port 35 and an output port 36 are provided. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and this output voltage is input to an input port 35 via an AD converter 37.

On the other hand, a temperature sensor 23 for generating an output voltage proportional to the temperature of the exhaust gas flowing out of the upstream NOx catalyst 17 is attached to the exhaust pipe 19 downstream of the upstream NOx catalyst 17. The output voltage of 23 is input to the input port 35 via the AD converter 38. The input port 35 is connected to a rotation speed sensor 41 that generates an output pulse indicating the engine rotation speed. Output port 36
Are connected to the ignition plug 4, the fuel injection valve 11, and the actuator 27 via corresponding drive circuits 39, respectively.

In this gasoline engine, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
2 is stored. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.
When it becomes 0, the air-fuel ratio of the mixture supplied to the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the gasoline engine according to the present embodiment, the lean air-fuel ratio control is performed with the value of the correction coefficient K being smaller than 1.0 in the low engine load operation region of the engine, and the engine high load operation region is performed. During a warm-up operation, an acceleration, and a constant speed operation at 120 km / h or more when the engine is started, the value of the correction coefficient K is set to 1.0, and the stoichiometric control is performed. Is set to a value larger than 1.0 and the rich air-fuel ratio control is performed.

In the internal combustion engine, normally, the low-medium load operation is most frequently performed. Therefore, the value of the correction coefficient K is made smaller than 1.0 during most of the operation period, and the lean mixture is burned. become.

FIG. 3 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from this figure, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer. The concentration of oxygen O _{2 in} the discharged exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in casing 21
The catalyst 20 has, for example, alumina as a carrier, on which potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, yttrium Y At least one selected from such rare earths and a noble metal such as platinum Pt are supported.

When an NOx storage reduction catalyst such as the NOx catalyst 20 is disposed in the exhaust passage of the engine, the NOx storage reduction catalyst becomes NOx when the air-fuel ratio of the inflowing exhaust gas (hereinafter referred to as exhaust air-fuel ratio) is lean. And absorbs and releases NOx when the oxygen concentration in the inflowing exhaust gas decreases. Here, the exhaust air-fuel ratio refers to the air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage and the NOx storage reduction catalyst.
The ratio of

When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx storage reduction catalyst, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber. Therefore, in this case, the NOx storage reduction catalyst absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber is lean, and absorbs NOx when the oxygen concentration in the mixture supplied to the combustion chamber decreases. Will be released.

The detailed mechanism of the NOx absorption / release operation by the NOx storage reduction catalyst is not clear in some parts. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.

That is, when the air-fuel ratio of the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases,
As shown in FIG. 4A, oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO contained in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the generated NO ₂ is oxidized on platinum Pt, absorbed in the NOx storage reduction catalyst, and combined with barium oxide BaO, as shown in FIG. 4 (A). nitrate ions NO ₃ ^- storage reduction NO in the form of
x Diffusion into the catalyst. In this way, NOx is absorbed in the NOx storage reduction catalyst 20.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt, and as long as the NOx absorption capacity of the NOx storage reduction catalyst is not saturated, NO ₂ is absorbed in the NOx storage reduction catalyst. As a result, nitrate ions NO ₃ ^- are generated.

On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the NO ₂ generation amount decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), and the nitric acid in the NOx storage reduction catalyst becomes negative. The ions NO ₃ ^- are stored and reduced N in the form of NO ₂ or NO.
Released from the Ox catalyst. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx storage reduction catalyst. As shown in FIG. 3, when the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, when the degree of leanness of the inflowing exhaust gas decreases, NOx is reduced from the NOx storage reduction catalyst. Will be released.

On the other hand, at this time, if the mixture supplied to the combustion chamber becomes stoichiometric or rich, a large amount of unburned HC and CO is discharged from the engine as shown in FIG. Is the oxygen O ₂ ^- or O ^2- on platinum Pt
And oxidize.

[0059] Further, the NOx storage reduction catalyst is NO ₂ or NO is released to the air-fuel ratio of exhaust gas oxygen concentration of the inflowing exhaust gas becomes stoichiometric or rich to extremely lowered, the NO ₂ or NO, FIG. As shown in FIG. 4 (B), it reacts with unburned HC and CO to be reduced to N ₂ .

[0060] That is, HC in the inflowing exhaust gas, CO, first oxygen O ₂ on the platinum Pt ^- or O ^2- immediately be reacted with oxidized, then the platinum Pt on the oxygen O ₂ ^- or O ^2- If HC and CO still remain after consumption, this H
C, NOx discharged from the released NOx, the engine from the NOx storage reduction catalyst by CO is made to reduction to N _2.

In this way, NO _{2 is} deposited on the surface of platinum Pt.
Alternatively, when NO is no longer present, NO ₂ or NO is released one after another from the NOx storage reduction catalyst, and is further reduced to N ₂ . Therefore, if the exhaust air-fuel ratio is made stoichiometric or rich, the storage reduction type N
NOx will be released from the Ox catalyst.

As described above, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx storage reduction catalyst, and when the exhaust air-fuel ratio is made stoichiometric or rich, NOx is stored in the NOx storage reduction catalyst.
Released in a short time from the Ox catalyst is reduced to N _2. Therefore, emission of NOx into the atmosphere can be prevented.

During the full load operation, the mixture supplied to the combustion chamber is made rich. At the time of high load operation, during warm-up operation when starting the engine, during acceleration, and at 120 km / h.
During the above constant speed operation, the air-fuel ratio is calculated based on the theoretical air-fuel ratio (stoichiometric)
When the air-fuel mixture is lean during low-medium load operation, NOx in the exhaust gas is absorbed by the NOx storage reduction catalyst during low-medium load operation, and the NOx storage reduction catalyst is used during full load operation and high load operation. NOx is released from the catalyst and reduced. However, if the frequency of full-load operation or high-load operation is low and the frequency of low-medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and the NOx absorption capacity (NOx absorption Capacity) is saturated and NOx cannot be absorbed.

Therefore, in such a case, when the lean air-fuel mixture is being burned, that is, when the medium-low load operation is being performed, the stoichiometric or spike-like (short-time) operation is performed in a relatively short cycle. In some cases, a method of controlling the air-fuel ratio of the air-fuel mixture so that the rich air-fuel mixture is burned and releasing and reducing NOx in a short cycle may be employed.

As described above, due to the absorption and release of NOx, the exhaust air-fuel ratio (in this embodiment, the air-fuel ratio of the air-fuel mixture) changes in a relatively short cycle between "lean" and "spike-like stoichiometric or rich (hereinafter, referred to as" rich "). This is referred to as a rich-spike control) is referred to as a lean-rich spike control, and the present embodiment also employs a lean-rich spike control. In this application, the lean-rich spike control is included in the lean air-fuel ratio control.

On the other hand, the fuel contains sulfur (S).
When the sulfur in the fuel burns, SO_TwoAnd SO_ThreeSuch as sulfur
Oxides (SOx) are generated, and the NOx storage reduction catalyst is exhausted.
These SOx in the gas are also absorbed. NOx storage reduction catalyst
SOx absorption mechanism is the same as NOx absorption mechanism
It is believed that there is. That is, the NOx absorption mechanism is explained.
Pt and barium B on the carrier as described
If the case where a is carried out is explained as an example,
Thus, when the exhaust air-fuel ratio is lean, the oxygen O_TwoIs O_Two
^-Or O^2-Pt surface of NOx storage reduction catalyst in the form of
And SOx in the inflowing exhaust gas (for example, S
O_Two) Is oxidized on the surface of platinum Pt to form SO _ThreeBecomes

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, absorbed in the NOx storage reduction catalyst, and combined with the barium oxide BaO, and the sulfate ion SO 3
_{4 The} sulfate BaS diffuses into the NOx storage reduction catalyst in the form of ^2-
Generate O ₄ . This sulfate BaSO ₄ is stable and hard to decompose, and is not decomposed even if the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich for a short time by the lean-rich spike control described above, and is not decomposed into the NOx storage reduction catalyst. Will remain. Therefore, the storage reduction type N
When the amount of BaSO ₄ generated in the Ox catalyst increases, the amount of BaO that can participate in the absorption of the NOx storage reduction catalyst decreases, and N
Ox absorption capacity is reduced. This is SOx poisoning.

Therefore, in the exhaust gas purifying apparatus according to the present embodiment, when the air-fuel ratio of the inflowing exhaust gas is lean, the SOSOx is absorbed and the inflow of SOSOx is prevented so that the SOx does not flow into the NOx absorbent 20. The SOx catalyst 17 having a function of a three-way catalyst that releases the absorbed SOx when the air-fuel ratio of the exhaust gas to be exhausted becomes rich is disposed upstream of the NOx catalyst 20. When the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 is lean, the SOx catalyst 17 absorbs NOx together with the SOx. However, when the air-fuel ratio of the exhaust gas flowing into the SOX catalyst 17 is made rich, not only the absorbed NOX but also the absorbed NOX is absorbed. It also releases SOX.

As described above, in the NOx catalyst 20, SOx
Is absorbed to form stable sulfate BaSO4,
As a result, even when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 is made rich, SOx is not released from the NOx catalyst 20. Therefore, when the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 is made rich, the SOx absorbent 1
In order to release SOx from 6, the absorbed SOx must be present in the absorbent in the form of sulfate ions SO4 ^2- , or even if sulfate BaSO4 is produced, sulfate BaSO4 is It is necessary to be present in the absorbent in an unstable state. As the SOx catalyst 17 which enables this, a transition metal such as copper Cu, iron Fe, manganese Mn and nickel Ni, sodium Na, titanium Ti and lithium L on a support made of alumina
An absorbent carrying at least one selected from i can be used.

[0070] absorbed by the air-fuel ratio while SO ₂ contained in the exhaust gas when the lean is oxidized at the surface of the absorbent ions SO4 ^2-sulfate form of the exhaust gas flowing into the SOx catalyst 17 in the SOx catalyst 17 Absorbed in the agent and then diffused into the absorbent. In this case, when allowed to carry platinum Pt on the support of the SOx catalyst 17 SO ₂ is likely to adhere on the platinum Pt in SO3 ^2-form, thus to SO2 is absorbed in the sulfate ions SO4 ^2-form It becomes easy to be absorbed in the agent. Therefore, in order to promote the absorption of SO2, it is preferable to carry platinum Pt on the carrier of the SOx catalyst 17. As described above, when the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 becomes lean, SOx is absorbed by the SOx catalyst 17, and therefore, the NOx absorbent 1 provided downstream of the SOx catalyst 17
9 absorbs only NOx.

[0071] On the other hand, the SOx absorbed in the SOx catalyst 17 as described above sulfate ions SO4 ^{2 -} or are diffused in the absorbent in the form of, or sulfate in an unstable state BaSO
It is 4. Therefore, when the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 becomes rich, the SOx absorbed by the SOx absorbent 16 is released from the SOx absorbent 16.

Next, the action of releasing NOx from the NOx absorbent 19 and the action of releasing SOx from the SOx catalyst 17 will be described with reference to FIG. FIG. 5A shows SOx absorbent 1
7 and the temperature T of the NOx absorbent 20 and the SOx catalyst 17 when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 20 is made rich, the NOx release rate f (T) from the NOx absorbent 19 and the SOx catalyst 17 SOx release rate g
FIG. 5B shows the correction coefficient Kt for the basic fuel injection time TP (the stoichiometric air-fuel ratio when Kt = 1.0, rich when Kt> 1.0, and Kt <1.0 And the NOx release rate f (Kt) from the NOx absorbent 19
And SOx release rate g (Kt) from SOx absorbent 16
The relationship is shown.

In the case of the NOx absorbent 20, if the temperature of the NOx catalyst 20 is approximately 150 ° C. or more, the NO2 on the platinum Pt surface
Is no longer present, the reaction immediately (NO3-→ NO
In the direction of 2), NOx is immediately released from the absorbent. Therefore, as shown in FIG. 5 (A), even if the temperature of the NOx catalyst 19 is considerably low, the NOx release rate f (T) becomes considerably high. That is, NOx is produced at a rather high speed.
It will be released from the catalyst 20. Note that FIG.
As shown in (2), the higher the temperature T of the NOx absorbent 19, the higher the NOx release rate f (T) and the correction coefficient K
The NOx release rate f (Kt) increases as the value of t increases, that is, as the degree of richness of the exhaust gas air-fuel ratio increases.

On the other hand, the SOx absorbed by the SOx catalyst 17 is changed to the NOx absorbed by the NOx absorbent 19.
This SOx is difficult to disassemble because it is more stable than
Does not sufficiently occur unless the temperature T of the SOx catalyst 17 exceeds the temperature To determined by the type of the SOx catalyst 17.
Therefore, as shown in FIG. 5A, when the temperature T of the SOx catalyst 17 is lower than To, the SOx release rate g (T)
Is extremely low, that is, almost SO
When x is not released and the temperature T of the SOx catalyst 17 exceeds To, the action of releasing SOx from the SOx catalyst 17 is substantially started. As for SOx, if the temperature T of the SOx catalyst 17 exceeds To, as shown in FIG.
As the temperature T of the Ox catalyst 17 increases, the SOx release rate g increases.
(T) increases, and as the value of the correction coefficient Kt increases, as shown in FIG. 5B, the SOx release rate g (K
t) increases.

FIG. 6A shows the NOx catalyst 20 and SOx
NO when the temperature T of the catalyst 17 is lower than To (FIG. 5)
The accumulated N from the NOx catalyst 20 when the air-fuel ratio of the exhaust gas flowing into the x catalyst 20 and the SOx catalyst 17 is made rich.
6B shows the Ox release amount and the cumulative SOx release amount from the SOx catalyst 17, and the solid line in FIG. 6B shows the NOx catalyst when the temperature T of the NOx catalyst 20 and the SOx catalyst 17 is higher than To (FIG. 5). 20 and the accumulated NOx release from the NOx catalyst 20 and the accumulated SOx from the SOx catalyst 17 when the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 is made rich.
And the amount of release.

When the temperature T of the SOx catalyst 17 is lower than To, SOx is hardly released as shown in FIG. 5 (A).
When the air-fuel ratio of the exhaust gas flowing into the Ox absorbent 16 is made rich, NOx is rapidly released from the NOx catalyst 20, but almost no SOx is released from the SOx catalyst 17, as shown in FIG.

On the other hand, when the temperature T of the SOx catalyst 17 becomes higher than To, the release operation of SOx is performed as shown in FIG.
When the air-fuel ratio of the exhaust gas flowing into the x catalyst 17 is made rich, both NOx and SOx are released as shown by the solid line in FIG. In this case, NOx is released from the NOx catalyst 20 within a short time, but the SOx catalyst 1
7 has a low decomposition rate of SOx,
It is released only slowly from the Ox catalyst 17. In addition,
Even in this case, if the temperature T of the SOx catalyst 17 becomes higher, FIG.
As can be seen from (A), the SOx release rate g (T) increases, so that SOx becomes S as indicated by the broken line in FIG. 6 (B).
It is released relatively quickly from the Ox catalyst 17.

In FIG. 6B, NO indicated by a solid line
x release amount is copper Cu, iron Fe,
6 shows the amount of NOx released from the SOx catalyst 17 supporting a transition metal such as nickel Ni, sodium Na, or lithium Li. In the case of the SOx catalyst 17 supporting, for example, titania TiO2 on a support made of alumina, FIG.
As shown by the broken line in (B), SOx is the SOx catalyst 1
7 release relatively fast. Thus, the SOx catalyst 1
The release speed of SOx from 7 varies depending on the type of SOx catalyst 17 and also varies depending on the temperature T of SOx catalyst 17.

By the way, as described above, the SOx catalyst 17
When the air-fuel ratio of the exhaust gas flowing into the SOx catalyst 17 and the NOx catalyst 20 is made rich when the temperature T is higher than To, SOx is released from the SOx catalyst 17 and NOx
NOx is released from the catalyst 20. At this time, if the exhaust gas flowing out of the SOx catalyst 17 is allowed to flow into the NOx catalyst 20, the SO
x is absorbed by the NOx catalyst 20, and the NOx catalyst 2
0 means so-called SOx poisoning, and S
There is no point in providing the Ox catalyst 17. Therefore, in the present invention, when SOx released from the SOx absorbent 16 is to be released from the SOx absorbent 16 in order to prevent the SOx released from the SOx absorbent 16 from being absorbed by the NOx absorbent 19, the exhaust gas flowing out of the SOx catalyst 17 is used. The bypass passage 2
6.

That is, in the embodiment according to the present invention, when the air-fuel ratio is in the stoichiometric or rich control state during the warm-up operation after the start of the internal combustion engine, and when the air-fuel ratio control is completed after the warm-up operation is completed. Is in the lean control state, the exhaust gas switching valve 28 is configured to be held at the bypass closed position indicated by the solid line in FIG. Therefore, at this time, SOx
The exhaust gas flowing out of the catalyst 17 flows into the NOx catalyst 20. Therefore, at this time, SOx in the exhaust gas is SOx
Since NOx is absorbed by the catalyst 17, only NOx is absorbed in the NOx catalyst 20.

Thereafter, as in the case where the engine is accelerating or the like, a high load state is reached and the stoichiometric control state is reached, the exhaust switching valve 28 is switched to the bypass open position shown by the broken line in FIG. When the air-fuel mixture supplied into the combustion chamber 3 becomes stoichiometric or rich, the SOx catalyst 17
However, at this time, the exhaust gas flowing out of the SOx catalyst 17 does not flow into the NOx catalyst 20, but flows into the bypass passage 26.

When the high load state of the engine is terminated and the air-fuel ratio control returns to the lean state, the exhaust switching valve 28 is simultaneously switched to the bypass closed position shown by the solid line in FIG. Thus, in this embodiment, the SO
When SOx is being released from the x catalyst 17, SOx
The exhaust gas flowing out of the catalyst 17 flows into the NOx catalyst 20.

At this time, unburned HC, CO
And NOx are discharged. However, since the SOx catalyst 17 has a three-way catalyst function as described above, these unburned H
Since C and CO are purified by the SOx catalyst 17, and NOx is purified by the NOx catalyst 20,
There is no danger of releasing large amounts of unburned HC, CO and NOx into the atmosphere.

Further, in the present embodiment, as described above, the three-way catalyst 43 as the three-way catalyst
The catalyst 20 is disposed on the exhaust passage downstream of the exhaust passage. That is, the casing 21 of the NOx catalyst 20
A casing 42 containing a three-way catalyst 43 is provided downstream of the exhaust pipe 44.

The three-way catalyst 43 is provided for purifying unburned HC, CO and NOx contained in the exhaust gas flowing down through the SOx catalyst 17 during the stoichiometric control or the rich control as described above. I have.

Hereinafter, the operation of the exhaust gas purifying apparatus for an internal combustion engine according to this embodiment will be described with reference to the flow chart of FIG. 7 showing the relationship between the air-fuel ratio control and the exhaust gas switching valve 28. A description will be given based on a routine. This flowchart is stored in the ROM 32 of the ECU 30, and all the processes in each step of the flowchart are executed by the CPU 34 of the ECU 30.

<Step 101> First, the ECU 30
Whether the engine has been started in step 101,
Judge. In this case, if it is determined that the engine has been started, the process proceeds to step 102. If it is determined that the engine has not been started, the following routine is not executed.

<Step 102> If it is determined in step 101 that the engine has been started, in step 102, the exhaust gas switching valve 28
It is determined whether or not the exhaust pipe 19 communicating with the exhaust pipe 19 is open. If it is determined that the exhaust gas switching valve 28 has opened the exhaust pipe 19, the process proceeds to step 104, where the engine is warmed up by stoichiometric or rich control. That is, the exhaust pipe 19 through which the exhaust switching valve 28 finally communicates with the NOx catalyst 20 when the engine is stopped last time.
Is released, the warm-up operation is performed in step 104 as it is.

On the other hand, if it is determined in step 102 that the exhaust gas switching valve 28 has closed the exhaust pipe 19 communicating with the NOx catalyst 20, the actuator 27 is driven in step 103 to switch the exhaust gas. By operating the valve 28, the exhaust pipe 19 communicating with the NOx catalyst 20 is opened. The operation of the exhaust switching valve 28 brings the SOx catalyst 17 and the NOx catalyst 20 into communication with each other via the exhaust pipe 19. When it is confirmed that the exhaust gas switching valve 28 has been operated and the exhaust pipe 19 has been opened as described above, the routine proceeds to step 104, where the engine is warmed up by stoichiometric or rich control.

<Step 104> In step 104, the engine is warmed up when the air-fuel ratio control of the engine is stoichiometric or rich. Whether or not this processing has been performed is determined by detecting the exhaust gas temperature with the temperature sensor 23 and using this temperature data. When the stoichiometric control or the rich control is performed, the exhaust gas having a high combustion temperature flows out of the engine body 1 through the exhaust manifold 16 because the exhaust gas has a high fuel content relative to oxygen. The SOx catalyst 17 flows into the SOx catalyst 17 in the casing 18 and is heated and heated. Further, the high-temperature exhaust gas is supplied through the exhaust pipe 19 to the NO.
The NOx catalyst 20 flows into the x catalyst 20 and is heated and heated.

In this case, although the stoichiometric or rich control is performed as described above, the SOx in the SOx catalyst 17 is still low because the temperature of the exhaust gas of the engine has not reached the temperature at which SOx is released even though the exhaust temperature is high. It is not released, and the NOx catalyst 20 does not absorb SOx in the exhaust gas. <Step 105> The stoichiometric or rich state of the engine air-fuel ratio control in step 104 is continued for a predetermined time, and the temperature of the NOx catalyst 20 is reduced by a temperature sensor (not shown) provided near the outlet of the NOx catalyst 20. If it is determined that the temperature has risen to the predetermined temperature or more and the NOx catalyst 20 has been sufficiently activated, and it is determined that the warm-up operation has been completed from the operating state of the engine, the next step 106
Proceed to. In this case, if it is determined that the temperature of the NOx catalyst 20 has not been sufficiently heated and raised, or if the warm-up operation of the engine 1 is not yet sufficient, the determination is NO.
Step 10 until the temperature of the x-catalyst 20 rises sufficiently and the operating state of the engine reaches a state where the warm-up operation becomes unnecessary.
In 4, warm-up by stoichiometric and rich control is continued. <Step 106> When the temperature of the NOx catalyst 20 rises to a predetermined temperature or higher and the operating state of the engine 1 is stabilized, the air-fuel ratio control of the engine shifts from stoichiometric or rich control to lean control. . In this state, the exhaust gas having a high oxygen content with respect to the fuel is supplied to the SOx catalyst 17 via the exhaust manifold 16, and is supplied to the NOx catalyst 20 via the exhaust pipe 19. It is also supplied to the three-way catalyst 43 via 44. In this case, in the SOx catalyst 17, HC, C
O and SOx are absorbed and the NOx catalyst 20
Ox is absorbed. <Step 107> Next, when it is determined that the accelerator opening is equal to or more than the predetermined amount and the engine is in a high load state, such as during acceleration, the engine air-fuel ratio control is switched from lean control to stoichiometric. Move to the control state. <Step 108> If it is determined that the engine air-fuel ratio control is in the stoichiometric control state, an operation signal is issued to the actuator 27, and the actuator 27 drives the exhaust switching valve 28 to open the bypass passage 26 and The exhaust pipe 19 is closed. In this case, the exhaust gas having a higher air-fuel ratio under the stoichiometric control is supplied to the three-way catalyst 43 through the bypass passage 26 and the exhaust pipe 44 to heat and raise the temperature of the three-way catalyst 43. As a result, the three-way catalyst 43 is heated and heated in a short time by the high-temperature exhaust gas to be activated. Unburned HC, CO, and NOx in the exhaust gas are purified by the three-way catalyst 43.

As described above, in the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment, the NOx catalyst 20
Even when the three-way catalyst 43 is heated and activated by the high-temperature exhaust gas to be activated and the NOx catalyst 20 is supplied with the exhaust gas under the stoichiometric or rich control at the time of starting the engine, no SOx poisoning occurs. Does not occur.

In this case, the exhaust gas under the stoichiometric control at the time of warm-up when the engine is started is not supplied to the NOx catalyst 20 side as in the present embodiment, but a bypass passage is provided as in the prior art. The exhaust switching valve 28 is disposed so as to supply the exhaust gas to the NOx catalyst 20 side, and thereafter, when shifting to the lean control, the exhaust switching valve 28 is disposed at a switching position such that the exhaust gas flows into the NOx catalyst 20 side. When the switching control is performed as described above, the three-way catalyst 43 as the three-way catalyst is heated and heated by the exhaust gas under the stoichiometric control at the time of the warm-up.
When the warm-up of the internal combustion engine is completed, it is determined that the three-way catalyst 43 has been sufficiently heated and the temperature has been raised, the air-fuel ratio control is switched to the lean control, and the exhaust gas flows into the NOx catalyst 20 side. The exhaust gas flowing into the NOx catalyst 20 is under lean control, and is lower than the exhaust gas temperature under stoichiometric control.

Therefore, the NOx catalyst 20 has a smaller value than when each absorbent is heated and heated in the procedure as in the present embodiment.
Activation takes a long time, and as a result, the emission of the NOx catalyst is reduced. As a result, in the invention according to the present embodiment, the NOx catalyst 20 can be heated and heated using the stoichiometric control during the warm-up operation after the internal combustion engine is started, and the three-way catalyst as the three-way catalyst can be used. The source catalyst 43 can be efficiently heated in a short time by using the exhaust gas at a higher temperature during the high-load stoichiometric control. Therefore, it is possible to heat and raise the temperature of the NOx catalyst 20 and the three-way catalyst 43 as the SOx absorbent without lowering the emission and avoiding the SOx poisoning of the NOx catalyst 20. It works.

Next, another embodiment of the present invention will be described. As shown in FIG. 8, in the present embodiment, the bypass passage 26 has a portion between the NOx catalyst 20 and the SOx catalyst 17 and a portion 4 on the downstream side of the NOx catalyst 20.
5 and the three-way catalyst 49.
Is provided in the bypass passage 26.

That is, a casing 50 is provided at an intermediate portion of the bypass passage 26, and a three-way catalyst 49 is incorporated in the casing 50. Therefore, in the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment, similarly to the case of the above-described embodiment, in the case of air-fuel ratio control in a stoichiometric or rich state at the time of warm-up after engine start, The switching means 28 is disposed at a control position indicated by a solid line in FIG. 8 and closes the bypass passage 26 and opens the exhaust pipe 19 communicating with the NOx catalyst 20. Is supplied to the NOx catalyst 20, and the NOx catalyst 20 is heated and heated in a state where SOx has not yet been released from the SOx catalyst 17.

Thereafter, when it is determined that the NOx catalyst 20 is sufficiently heated to promote the chemical reaction and that the warm-up operation is completed, the air-fuel ratio control of the engine is switched to the lean control. However, the switching means 28 does not operate, and the exhaust pipe 19 is opened as in the previous period, and the exhaust gas is supplied to the NOx catalyst 20 to maintain the switching position indicated by the solid line. 26 is closed. Therefore, in this state, the NOx catalyst 20 absorbs NOx in the exhaust gas.

Then, when the engine is under heavy load,
Again, the state shifts to the stoichiometric control state. In this case, the switching means 28 is driven by the actuator 27 to close the exhaust pipe 19, and exhaust gas is supplied to the bypass passage 26 side as shown by a broken line in FIG. At the same time, the switching position is switched to a switching position at which the exhaust pipe 46 communicating with the NOx catalyst 20 is closed. As a result, the exhaust gas is supplied to the three-way catalyst 49 provided in the bypass passage 26 via the bypass passage 26.

In this case, the exhaust gas discharged from the high load state of the engine is also hot, and the three-way catalyst 4
Since 9 is heated by high temperature, the temperature is raised in a short time. As described above, unburned HC contained in exhaust gas,
CO and NOx are purified by the three-way catalyst 49.

The control procedure as the operation in the present embodiment is performed by the same routine as the flowchart in the first embodiment. In the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment, unlike the above-described embodiment, the SOx catalyst 49 is not located further downstream of the NOx catalyst 20 in the exhaust passage but on the bypass passage 26. , Since the NOx catalyst 20 and the three-way catalyst 49 are arranged in parallel with the NOx catalyst 20 as in the above-described embodiment, In comparison with the three-way catalyst, a three-way catalyst 49 is arranged closer to the engine.

As a result, the exhaust gas having a higher temperature can be supplied to the three-way catalyst 49 as a three-way catalyst than in the case of the exhaust gas purifying apparatus of the internal combustion engine according to the embodiment, and the three-way catalyst can be more efficiently supplied. The three-way catalyst 49 can be heated and heated.

[0102]

According to the first aspect of the present invention, in an exhaust gas purifying apparatus for an internal combustion engine provided with a bypass passage that bypasses a NOx absorbent in order to avoid so-called SOx poisoning, emission is not reduced. The temperature of each exhaust gas purifying material can be raised.

According to the second aspect of the present invention, the first aspect is provided.
Another object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine in which a three-way catalyst is arranged in series downstream of a NOx absorbent in addition to the effects of the invention described above.

According to the third aspect of the present invention, a first aspect is provided.
In addition to the effects of the invention described above, it is possible to provide an exhaust gas purification device for an internal combustion engine that can more efficiently heat a three-way catalyst.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention, in which a three-way catalyst as a three-way catalyst is located downstream of an NOx catalyst as a NOx absorbent in an exhaust passage. FIG. 3 is a diagram showing a case in which they are arranged in series.

FIG. 2 is a diagram showing an example of a map of a basic fuel injection time.

FIG. 3 Unburned HC and C of exhaust gas discharged from the engine
It is a graph which shows the density | concentration of O and oxygen schematically.

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of a storage reduction type NOx catalyst.

FIG. 5 is a graph showing a NOx release rate and a SOx release rate.

FIG. 6 is a graph showing the cumulative release amounts of NOx and SOx.

FIG. 7 is a flowchart showing a control routine relating to the relationship between air-fuel ratio control and the exhaust gas switching valve in the exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention.

FIG. 8 is a schematic configuration diagram showing an exhaust gas purifying apparatus for an internal combustion engine according to another embodiment of the present invention, wherein a NOx catalyst as a NOx absorbent and a three-way catalyst as a three-way catalyst are:
It is a figure showing the case where it is arranged in parallel on the exhaust passage.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine main body 2 Combustion chamber 4 Spark plug 11 Fuel injection valve 16, 18, 22, 45 Exhaust pipe (exhaust passage) 17 SOx absorbent (SOx catalyst) 20 NOx absorbent (NOx catalyst) 23 Temperature sensor 26 Exhaust bypass passage ( (Bypass passage) 28 switching means (exhaust switching valve) 30 ECU 43, 49 three-way catalyst

Continued on the front page F term (reference) 3G091 AA02 AA12 AA13 AA17 AA23 AA28 AB01 AB03 AB06 AB09 AB11 BA11 BA14 BA15 BA19 BA20 BA32 BA33 CA01 CA12 CA13 CB02 DA01 DA02 DA03 DA04 DB06 DB10 DC01 EA01 EA05 EA07 FA12 FA02 FA02 FA02 FA02 FA02 FA14 FA17 FA18 FB02 FB10 FB11 FB12 FC02 FC07 GB01X GB02W GB03W GB04W GB05W GB06W GB10X GB16X HA07 HA08 HA12 HA18 HA36 HA37 HB03 3G301 HA01 HA06 HA15 HA18 JA15 JA25 JA26 JA33 JB09 LA01 LB02 MA01 NE01 NE04 NE06 NA06 PA01B PA11A PD11A PD11B PE01A PE01B

Claims (3)

[Claims] When the air-fuel ratio of exhaust gas flowing from an internal combustion engine that is provided in an exhaust passage of an internal combustion engine and is capable of lean combustion is in a lean state, NOx is absorbed and the oxygen concentration of the flowing exhaust gas is reduced. A NOx absorbent that releases the absorbed NOx when it is low, and a NOx absorbent that is provided upstream of the exhaust passage of the NOx absorbent and that has a lean air-fuel ratio when the inflowing exhaust gas is lean.
x and a SOx absorber that releases the absorbed SOx when the oxygen concentration of the exhaust gas that flows in is low, and an exhaust gas that is provided in the middle of the exhaust passage and contains SOx that is discharged from the SOx absorbent. An exhaust bypass passage configured to flow around the NOx absorbent, and H contained in the exhaust gas flowing from the SOx absorbent.
A three-way catalyst capable of purifying C, CO and NOx; and an exhaust gas flowing through the exhaust passage from the internal combustion engine via the SOx absorbent, selectively flowing into either the NOx absorbent or the exhaust bypass passage. An exhaust gas purifying apparatus for an internal combustion engine, the exhaust gas purifying apparatus comprising: a switching unit configured to perform stoichiometric control during warm-up after the internal combustion engine is started, and to perform lean control on exhaust gas air-fuel ratio control. When the internal combustion engine is under high load, the exhaust gas is bypassed when the stoichiometric control is performed when the internal combustion engine is under a high load. An exhaust gas purifying apparatus for an internal combustion engine, which is controlled to be arranged at a switching position where the exhaust gas can flow into a passage. 2. The three-way catalyst is provided on the exhaust passage downstream of the NOx absorbent in the exhaust passage, and the exhaust bypass passage is provided in the exhaust passage with the NOx absorbent.
The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein a portion between the x-absorbent material and a portion between the NOx absorbent and the three-way catalyst are connected. 3. The exhaust bypass passage is provided so as to connect a portion between the NOx absorbent and the SOx absorbent and a downstream portion of the NOx absorbent, and the three-way catalyst is provided with the exhaust gas. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is provided in a bypass passage.