JP2000345829A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a HC release to an atmosphere at the time of low temperature starting and to utilize the HC released from a HC adsorbing agent for a NOx purification by an occlusion reduction type NOx catalyst. SOLUTION: In this emission control device, an occlusion reduction type NOx catalyst 20; an exhaust passages 43, 44 bypassing a main exhaust passage at a more upstream side than the occlusion reduction type NOx catalyst; a HC adsorbing agent 21 provided on the exhaust passage 43; and a control valve 27 for controlling an exhaust gas flow so as to flow the exhaust gas to the exhaust passages 43, 44 and a HC-adsorbent 21 when the HC adsorbing agent 21 is at a HC-adsorption temperature area and an oxygen concentration of the exhaust gas is reduced so as to release a NOx from the occlusion reduction type NOx catalyst 20 on the main exhaust passage connected to a lean burnable internal combustion engine.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing nitrogen oxides (NO
The present invention relates to an exhaust gas purification device capable of purifying x).

[0002]

2. Description of the Related Art Currently, lean NOx catalysts capable of purifying NOx from exhaust gas exhausted from an internal combustion engine capable of lean combustion include a selective reduction type NOx catalyst and a storage reduction type Nx catalyst.
There are two types of catalysts called Ox catalysts.

[0003] The selective reduction type NOx catalyst is a catalyst that reduces or decomposes NOx in the presence of hydrocarbons in an oxygen-excess atmosphere. This selective reduction type NOx catalyst is obtained by ion exchange of a transition metal such as Cu to zeolite. And a catalyst in which a noble metal is supported on zeolite or alumina.

On the other hand, the NOx storage reduction catalyst, the catalyst air-fuel ratio of the inflowing exhaust gas is absorbed NOx when the lean, the oxygen concentration in the inflowing exhaust gas is reduced to an N ₂ release NOx absorbed to decrease This storage-reduction type NOx catalyst uses, for example, alumina as a carrier and, on the carrier, for example, potassium K, sodium Na, lithium Li, cesium C
alkali metals such as s, barium Ba, calcium C
At least one selected from alkaline earths such as a, lanthanum La, and rare earths such as yttrium Y, and a noble metal such as platinum Pt are supported.

[0005] The air-fuel ratio of the exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the lean NOx catalyst. These lean NOx catalysts have an active temperature range, and the lean NOx catalyst does not function at all or hardly if it is not within the active temperature range. Therefore, low-temperature exhaust gas containing unburned HC discharged at the time of a cold start of the internal combustion engine or the like is lean NO.
Even when flowing through the x catalyst, the exhaust gas is not purified, and HC may be released to the atmosphere.

Therefore, an HC adsorbent is provided in the exhaust passage upstream of the lean NOx catalyst, and a bypass passage is provided for bypassing the HC adsorbent. Exhaust gas is transferred to the HC adsorbent or the bypass passage according to the operation state of the internal combustion engine. It was thought that shed.

For example, in an exhaust gas purifying apparatus disclosed in Japanese Patent Application Laid-Open No. 7-11947, an HC adsorbent and a selective reduction type NOx catalyst are provided in the exhaust passage of an internal combustion engine in order from the upstream, and the HC adsorbent is bypassed. A bypass passage is provided to allow exhaust gas to flow through the selective reduction type NOx catalyst, and an exhaust path switching valve is provided at a start end of the bypass passage. The exhaust gas is supplied when the internal combustion engine is started and the exhaust gas temperature reaches a predetermined temperature. In the exhaust gas by flowing
C is adsorbed, and when the exhaust gas temperature becomes equal to or higher than a predetermined temperature, the exhaust gas is caused to flow through the bypass passage. Further, when the internal combustion engine is operated at a lean air-fuel ratio and the exhaust gas temperature is in a predetermined high temperature range, the exhaust gas is exhausted. Gas flows to HC adsorbent
The HC adsorbed by the adsorbent is released, and the HC is passed to a selective reduction type NOx catalyst and used for NOx purification.

[0008]

In the case of the conventional exhaust gas purifying apparatus, since the lean NOx catalyst is a selective reduction type NOx catalyst, the HC adsorbed by the HC adsorbent is released during the lean operation of the internal combustion engine. By doing so, the released HC can be used for NOx purification.
This technique cannot be directly applied to an NOx storage reduction catalyst using an Ox catalyst.

[0009] In the NOx storage reduction catalyst, the HC released from the HC adsorbent can be used for NOx purification only when the NOx absorbed in the NOx storage reduction catalyst is released and reduced. That is, this is because the air-fuel ratio of the exhaust gas is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio, and not when the NOx storage-reduction catalyst absorbs NOx, that is, when the air-fuel ratio of the exhaust gas is made lean.

SUMMARY OF THE INVENTION The present invention has been made in view of such problems of the prior art, and an object to be solved by the present invention is to reduce the emission of HC into the atmosphere at the time of a cold start and to adsorb HC. NOx stored and reduced from HC release
An object of the present invention is to make it available for NOx purification by a catalyst.

[0011]

The present invention has the following features to attain the object mentioned above. The present invention relates to (a)
NOx is provided in the main exhaust passage communicating with the internal combustion engine capable of lean combustion, and when the air-fuel ratio of the inflowing exhaust gas is lean.
A NOx storage reduction catalyst for reducing absorbed NOx is released into N ₂ when there is a low oxygen concentration in the exhaust gas to absorb flow to bypass the main exhaust passage upstream than (ii) the storage-reduction type NOx catalyst (C) an HC adsorbent provided in the bypass passage for adsorbing HC at a low temperature and releasing HC at a high temperature; and (d) when the HC adsorbent is in the HC adsorption temperature range and when the occlusion is performed. Exhaust flow control means for controlling exhaust gas to be introduced into the bypass passage when the oxygen concentration of the exhaust gas is reduced so as to release NOx from the reduced NOx catalyst. Device.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the HC adsorbent is H
When the temperature is in the C adsorption temperature range, the exhaust flow control means is controlled so that the exhaust gas flows to the HC adsorbent. As a result, HC in the exhaust gas is adsorbed by the HC adsorbent, so that the emission of HC into the atmosphere during the cold start can be reduced.

After the HC adsorption by the HC adsorbent is completed, when the oxygen concentration of the exhaust gas is reduced in order to release NOx from the NOx storage reduction catalyst, the exhaust gas is synchronized with the HC adsorbent. The exhaust flow control means is controlled so as to flow through the exhaust gas. At this time, since the HC adsorbent is in the HC release temperature range, when exhaust gas flows through the HC adsorbent, the HC adsorbed by the HC adsorbent is desorbed and introduced into the NOx storage reduction catalyst together with the exhaust gas. Is done. As a result, HC desorbed from the HC adsorbent can be used as a part of a reducing agent required for releasing and reducing NOx in the NOx storage reduction catalyst.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, examples of the internal combustion engine capable of lean burn include a direct-injection-type lean-burn gasoline engine and a diesel engine. In the case of a lean burn gasoline engine, the air-fuel ratio control of the exhaust gas can be realized by the air-fuel ratio control of the air-fuel mixture supplied to the combustion chamber. In the case of a diesel engine, the air-fuel ratio of exhaust gas is controlled by performing a so-called sub-injection of injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke, or reducing the fuel in a main exhaust passage upstream of a NOx storage reduction catalyst. This can be realized by supplying an agent.

Here, the air-fuel ratio of the exhaust gas refers to a ratio of air and fuel (hydrocarbon) supplied into the main exhaust passage upstream of the engine intake passage and the NOx storage reduction catalyst. In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the storage reduction type NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, barium Ba, An example in which at least one selected from alkaline earths such as calcium Ca, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt are supported. In addition, zeolite and the like can be exemplified as the HC adsorbent.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS. [First Embodiment] FIG. 1 is a diagram showing a schematic configuration in a case where the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes the engine body, and reference numeral 2
Is a piston, reference numeral 3 is a combustion chamber, reference numeral 4 is a spark plug, reference numeral 5
Denotes an intake valve, 6 denotes an intake port, 7 denotes an exhaust valve, and 8 denotes an exhaust port.

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.
Is connected to a casing 18 containing a three-way catalyst 17 via an exhaust pipe 19. An outlet of the casing 18 is connected to an upstream end of a casing 40 containing a storage-reduction type NOx catalyst 20 and an HC adsorbent 21 through an exhaust pipe 19. Connected, casing 4
The downstream side of 0 is connected via an exhaust pipe 22 to a casing 24 containing a three-way catalyst 23, and the casing 24 is connected via an exhaust pipe 25 to a muffler (not shown). Hereinafter, the NOx storage reduction catalyst 20 is abbreviated as the NOx catalyst 20.

The internal structure of the casing 40 will be described. Inside the casing 40, a middle pipe 41 and an inner pipe 42 are provided concentrically with the casing 40 and with a radial gap therebetween. The middle pipe 41 has a small diameter portion on the upstream side and a large diameter portion on the downstream side.
Are extended to near the corresponding ends of the casing 40 and are opened in the casing 40. The inner pipe 42 is installed so as to penetrate the downstream end of the casing 40, and the exhaust pipe 22 is connected to the downstream end of the inner pipe 42. Inner pipe 42
Is inserted into the large-diameter portion of the middle pipe 41 and is set open in the middle pipe 41. The HC adsorbent 2 is placed in an annular space formed between the casing 40 and the large-diameter portion of the middle pipe 41.
1 is provided inside the inner pipe 42.
A catalyst 20 is provided.

A control valve (exhaust flow control means) 27 whose valve is actuated by an actuator 26 is provided at the small diameter portion of the middle pipe 41. The control valve 27 is controlled by an actuator 26 between a fully closed position where the small diameter portion of the middle pipe 41 is closed as shown in FIG. 1 and FIG. 2 and a fully open position where the small diameter portion is opened as shown in FIG. It can be stopped at the opening. The flow of the exhaust gas can be controlled by controlling the opening of the control valve 27.

As shown in FIG. 3, when the control valve 27 is in the fully open position, the exhaust gas flowing into the casing 40 from the upstream end of the casing 40 flows from the upstream end of the middle pipe 41 to the middle pipe 41. Into the inner pipe 42 through the small diameter portion of the middle pipe 41, and flows from the downstream end of the inner pipe 42 to the exhaust pipe 22 through the NOx catalyst 20. That is, at this time, almost no exhaust gas flows through the HC adsorbent 21. In this way, when the control valve 27 is fully opened, the flow path of the exhaust gas, that is, the exhaust pipe 16, the casing 18, the exhaust pipe 19, the casing 4
0 upstream end, small diameter part of middle pipe 41, inner pipe 4
2. The exhaust pipe 22, the casing 24, and the exhaust pipe 25 constitute a main exhaust passage.

On the other hand, as shown in FIG. 2, when the control valve 27 is in the fully closed position, the exhaust gas flowing into the casing 40 from the upstream end of the casing 40 flows between the casing 40 and the middle pipe 41. After entering the exhaust passage 43 having an annular cross-section and passing through the HC adsorbent 21, the flow direction of the exhaust gas is reversed so that the middle pipe 41 and the inner pipe 4
After passing through the exhaust passage 44 having an annular cross-section formed between the NOx catalyst 2 and the exhaust passage 44, the flow direction of the exhaust gas is reversed again, enters the inner pipe 42, and passes through the NOx catalyst 20. From the downstream end of the inner pipe 42 to the exhaust pipe 22.
Flows to Thus, the exhaust passages 43, 44
A bypass passage that bypasses the main exhaust passage upstream of the catalyst 20 is configured.

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 28 for generating an output voltage proportional to the temperature of the exhaust gas passing through the three-way catalyst 17 is mounted in the exhaust pipe 19 downstream of the three-way catalyst 17. The voltage is input to the input port 35 via the AD converter 38. The input port 35 is connected to a rotation speed sensor 29 that generates an output pulse representing the engine rotation speed. The output port 36 is connected to the ignition plug 4 and the fuel injection valve 11 via a corresponding drive circuit 39, respectively.
It is connected to an actuator 26.

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 to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, the air-fuel ratio becomes a lean air-fuel ratio.
If> 1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, a rich air-fuel ratio.

In the gasoline engine of this embodiment,
In the engine low-medium load operation region, the value of the correction coefficient K is set to a value smaller than 1.0, and the lean air-fuel ratio control is performed, and the engine high load operation region, the warm-up operation when starting the engine, the acceleration,
And at a constant speed operation of 120 km / h or more, the value of the correction coefficient K is set to 1.0 and the stoichiometric air-fuel ratio control is performed. Then, rich air-fuel ratio control is performed.

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

FIG. 5 schematically shows the concentration 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.

The NOx catalyst 20 housed in the inner pipe 42 uses, for example, alumina as a carrier, and on the carrier, for example, alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, barium Ba, and calcium Ca. And at least one selected from the group consisting of alkaline earths, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt. The ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx catalyst 20 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 (hereinafter referred to as the exhaust air-fuel ratio). The NOx catalyst 20 absorbs NOx when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases.
Performs the absorption and release action.

When no fuel (hydrocarbon) or air is supplied into the exhaust passage upstream of the NOx catalyst 20, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx catalyst 2
When the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is lean, 0 absorbs NOx, and releases the absorbed NOx when the oxygen concentration of the air-fuel mixture supplied into the combustion chamber 3 decreases.

It is considered that the NOx absorption / release operation of the NOx catalyst 20 is performed by a mechanism as shown in FIG. In the following, this mechanism will be described by using platinum P
The case where t and barium Ba are supported will be described as an example, but other noble metals, alkali metals, alkaline earths,
The same mechanism is obtained even when rare earth elements are used.

First, when the air-fuel ratio of the inflowing exhaust gas becomes considerably lean, the oxygen concentration of the inflowing exhaust gas greatly increases, and as shown in FIG. 6A, the oxygen O ₂ becomes O ₂ ⁻ or O ^2−. On the surface of platinum Pt. 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 absorbed in the NOx catalyst 20 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. NO ₃ ^- diffuses into the NOx catalyst 20 in the form of. In this way, NOx is absorbed in the NOx catalyst 20.

As long as the oxygen concentration of the inflowing exhaust gas is high, platinum P
As long as NO ₂ is generated on the surface of t and the NOx absorption capacity of the NOx catalyst 20 is not saturated, NO ₂ is absorbed in the NOx catalyst 20 and nitrate ions NO ₃ ⁻ are generated.

On the other hand, when the oxygen concentration of the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the opposite direction (N
O ₃ ⁻ → NO ₂ ), and nitrate ions N in the NOx catalyst 20
O ₃ ^- is released from the main catalyst 20 in the form of NO ₂ or NO. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx catalyst 20.
As shown in FIG. 3, when the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration of the inflowing exhaust gas decreases. Therefore, when the degree of leanness of the inflowing exhaust gas decreases, NOx decreases.
NOx is released from the catalyst 20.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made stoichiometric or rich and the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, as shown in FIG. Unburned HC and CO are discharged, and these unburned HC and CO are separated from oxygen O ₂ ^- or O ^2- on platinum Pt.
And oxidize.

When the exhaust air-fuel ratio reaches the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration of the inflowing exhaust gas extremely decreases, so that NO ₂ or NO is released from the NOx catalyst 20, and this NO ₂ or NO As shown in FIG. 6 (B), it reacts with unburned HC and CO to be reduced to N ₂ .

[0038] 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 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 from the NOx catalyst 20 one after another, and is further reduced to N ₂ . Therefore, if the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio, the NOx catalyst 2
NOx will be released from zero.

As described above, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 20, and when the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio, the NOx becomes NOx catalyst 2.
0 is released in a short time from being reduced to N _2. Therefore, emission of NOx into the atmosphere can be prevented.

By the way, in this embodiment, as described above, the mixture supplied to the combustion chamber 3 is set to the rich air-fuel ratio at the time of full load operation, and at the time of high load operation and warm-up operation at the time of engine start. When the vehicle is accelerating, and when operating at a constant speed of 120 km / h or more, the air-fuel mixture is set to the stoichiometric air-fuel ratio. When the engine is operated at low to medium load, the air-fuel mixture is set to the lean air-fuel ratio. The NOx is absorbed by the NOx catalyst 20, and is released and reduced from the NOx catalyst 20 during the full load operation and the high load operation. However, if the frequency of full load operation or high load operation is low, the frequency of low / medium load operation is high, and the operation time is long, NO
x release / reduction cannot be performed in time,
The absorption capacity of Ox is saturated and NOx cannot be absorbed.

Therefore, in this embodiment, when the lean air-fuel mixture is being burned, that is, when the medium-low load operation is being performed, the stoichiometric or rich stoichiometric or rich operation is performed in a relatively short cycle. The air-fuel ratio of the air-fuel mixture is controlled so that the air-fuel mixture is burned, and NOx is released and reduced in a short cycle. As described above, the exhaust air-fuel ratio (in this embodiment, the air-fuel ratio of the air-fuel mixture) is used to absorb and release NOx.
In the following description, it is referred to as "lean-rich spike control" in which "lean" and "spike-like stoichiometric air-fuel ratio or rich air-fuel ratio" are alternately repeated in a relatively short cycle.

As described above, in this gasoline engine, the engine is operated at the stoichiometric air-fuel ratio at the time of warm-up operation at the time of engine start, and it is still NO at the time of engine start.
Since the x catalyst 20 has not reached the active temperature range (for example, 250 ° C. to 550 ° C.), it is difficult for the NOx catalyst 20 to purify exhaust gas during the warm-up operation of the engine. Therefore, in the gasoline engine, by providing the three-way catalyst 17 in the main exhaust passage upstream of the NOx catalyst 20, the three-way catalyst 17 is provided during the warm-up operation based on the stoichiometric air-fuel ratio.
It purifies the exhaust gas.

However, even if the three-way catalyst 17 has an activation temperature range (for example, 250 ° C. or higher) and the three-way catalyst 17 is disposed immediately downstream of the engine body 1, immediately after the engine is started, the The source catalyst 17 has not reached the activation temperature range. As described above, when the three-way catalyst 17 is in the inactive state, the exhaust gas purification by the three-way catalyst 17 is difficult to be performed, and the exhaust gas including the unburned HC passes through the three-way catalyst 17 without being purified.

Therefore, in this gasoline engine, while the three-way catalyst 17 is inactive, the exhaust gas that has passed through the three-way catalyst 17 is introduced into the HC adsorbent 21 to remove unburned HC in the exhaust gas. It is adsorbed by the material 21, thereby reducing the emission of HC into the atmosphere when the engine is cold. Furthermore, after the three-way catalyst 17 is activated, the introduction of exhaust gas into the HC adsorbent 21 is basically stopped, and lean-rich spike control of the air-fuel ratio is performed after the completion of warm-up of the engine. When the exhaust gas is introduced into the HC adsorbent 21 in synchronization with the timing of the rich spike, the H
C is desorbed, and the desorbed HC is introduced into the NOx catalyst 20 so as to be used as a part of a reducing agent necessary for purifying exhaust gas in the NOx catalyst 20. Hereinafter, this will be described in detail.

First, when the engine is in the warm-up operation state and the three-way catalyst 17 has not reached the activation temperature range,
The ECU 30 sets the control valve 27 to the fully closed position as shown in FIGS. In this embodiment, whether the three-way catalyst 17 is in the activation temperature range is determined by using the exhaust gas temperature at the outlet of the three-way catalyst 17 detected by the temperature sensor 28 as the catalyst temperature of the three-way catalyst 17. And the ECU 30
Do.

When the control valve 27 is in the fully closed position, as described above, the exhaust gas flowing from the exhaust pipe 19 into the casing 40 is discharged from the exhaust passage 43 → the HC adsorbent 21 → the exhaust passage 44 → the inner pipe 42. → NOx catalyst 20 → flow through exhaust pipe 22. Here, until the three-way catalyst 17 reaches the activation temperature range, the HC adsorbent 21 remains in the HC adsorption temperature range (for example, 1
00 ° C. or less) so that the temperature does not reach the HC desorption temperature, the heat radiation from the three-way catalyst 17 to the HC adsorbent 21 is taken into consideration, and the installation positions and dimensions of the exhaust pipe 19 and the casing 40 are determined in advance. (Length, diameter) etc. are set. With this setting, the exhaust gas that has hardly been purified by the three-way catalyst 17 and has passed through the three-way catalyst 17 will be replaced with unburned HC in the exhaust gas when passing through the HC adsorbent 21. The concentration of HC in the exhaust gas is reduced by being adsorbed by the adsorbent 21,
The exhaust gas having a reduced HC concentration flows through the NOx catalyst 20 to the exhaust pipe 22. As a result, the emission of unburned HC to the atmosphere during a cold start of the engine can be significantly reduced. When this operating state is continued, the temperature of the three-way catalyst 17 gradually increases.

If it is determined that the three-way catalyst 17 has reached the activation temperature range during the warm-up operation of the engine, the ECU 30
Moves the control valve 27 to the fully open position as shown in FIG. When the control valve 17 is set to the fully open position, as described above, the exhaust gas flowing into the casing 40 from the exhaust pipe 19 flows through the path of the middle pipe 41 → the inner pipe 42 → the NOx catalyst 20 → the exhaust pipe 22, Almost no water flows into the HC adsorbent 21. At this time, since the engine is still warming up and operating at the stoichiometric air-fuel ratio, the exhaust gas discharged from the engine body 1 is purified when passing through the three-way catalyst 17, and the NOx catalyst 20 is removed. The gas flows through the exhaust pipe 22 through the exhaust pipe 22.

Thereafter, when the ECU 30 determines that the warm-up of the engine has been completed, the engine body 1 is switched to the air-fuel ratio control after the warm-up. After the engine is warmed up, the engine body 1 is
The rich air-fuel ratio control, the stoichiometric air-fuel ratio control, and the lean / rich stoichiometric control are executed. After the completion of the warm-up of the engine, the control valve 27 is held at the fully open position in principle, and the exhaust gas is not flown to the HC adsorbent 21.

After the engine warm-up is completed, the NOx catalyst 20
Has also reached the activation temperature range. Therefore, during lean operation in the lean-rich spike control, the N
Ox is absorbed by the NOx catalyst 20, and is absorbed by the NOx catalyst 20 during the stoichiometric operation, the rich operation, and the rich spike operation in the lean-rich spike control.
Ox is reduced and purified to be discharged N _2.

The three-way catalyst 23 disposed downstream of the NOx catalyst 20 purifies gas components (mainly HC and CO) that could not be purified by the NOx catalyst 20 during lean operation in lean-rich spike control. The gas component (mainly NOx) that cannot be purified by the NOx catalyst 20 during the stoichiometric operation, the rich operation, or the rich spike operation in the lean / rich spike control is purified.

As described above, after the completion of the warm-up of the engine, the exhaust gas is not flown to the HC adsorbent 21 in principle.
In the following case, the operation of the control valve 27 is controlled so that the exhaust gas flows to the HC adsorbent 21 even after the engine is warmed up.

(1) In the case of performing HC desorption processing The first case in which exhaust gas flows into the HC adsorbent 21 after the engine warm-up is completed, the HC adsorbed by the HC adsorbent 21 when the engine is started is used for the HC adsorbent. 21 and this is N
This is a case where the Ox catalyst 20 is used as a part of a reducing agent necessary for releasing and reducing NOx.

In this case, the control valve 27 is brought into the fully closed position or a partially closed position at a predetermined valve opening in synchronization with the rich spike in the lean / rich spike control. 27 in the fully open position.

If the control valve 27 is set to the fully closed position or the partially closed position in synchronization with the rich spike in the lean / rich spike control, all or a part of the exhaust gas becomes HC during the execution of the rich spike. It flows to the adsorbent 21. After the completion of engine warm-up, the HC adsorbent 21 has also reached the HC desorption temperature. Therefore, HC
When the exhaust gas flows into the adsorbent 21, the HC adsorbed by the HC adsorbent 21 is desorbed and flows into the NOx catalyst 20 together with the exhaust gas. As a result, the HC desorbed from the HC adsorbent 21 is also reduced together with the reducing components in the exhaust gas.
Will be utilized for the reduction of NOx.

While the HC desorbed from the HC adsorbent 21 is used as a part of the reducing agent,
The amount of the reducing agent introduced into the NOx catalyst 20 by the rich spike corresponds to the amount of HC desorbed from the HC adsorbent 21 and flows into the NOx catalyst 20 during the normal time (the rich spike when the control valve 27 is fully opened). Correction control is performed so as to be less than (h).

The reason why the control valve 27 is returned to the fully open position at the time of lean in the lean / rich spike control is that the HC adsorbent 2
This is because it is not desired to introduce the HC desorbed from 1 into the NOx catalyst 20 at the time of lean operation. That is, even if HC desorbed from the HC adsorbent 21 is introduced into the NOx catalyst 20 at the time of lean operation,
This HC cannot be used effectively, and H
HC desorbed from C adsorbent 21 together with exhaust gas is NO
This is because when introduced into the x catalyst 20, the air-fuel ratio of the exhaust gas is reduced, and there is a possibility that the NOx absorbing action of the NOx catalyst 20 may be reduced.

Then, at the time of the rich spike, the HC
The control valve 27 is controlled to open and close so that the exhaust gas flows into the adsorbent 21, and the process of desorbing HC from the HC adsorbent 21, that is, the HC desorption process, is performed by the HC adsorbed by the HC adsorbent 21.
Is repeated, that is, until the desorption of HC is completed. After the HC desorption of the HC adsorbent 21 is completed, the control valve 27 is again held at the fully open position, and the exhaust gas is transferred to the HC adsorbent 2.
Do not introduce it into 1.

Here, the completion of the HC desorption of the HC adsorbent 21 may be determined by estimating the completion of the HC desorption when the number of times of opening and closing of the control valve 27 reaches a predetermined number set in advance. Alternatively, an air-fuel ratio sensor may be provided upstream and downstream of the casing 40, and when there is no difference between the outputs of the air-fuel ratio sensors during a rich spike, it may be determined that HC desorption is complete.

(2) High-speed operation Next, the second case in which exhaust gas flows into the HC adsorbent 21 after the completion of warm-up of the engine is when the vehicle is operating at high speed. In this case, when the vehicle is running at a high speed equal to or higher than a predetermined speed, the control valve 27 is set to the fully closed position, and the entire amount of the exhaust gas flows to the HC adsorbent 21.

At high speed operation, the temperature of the exhaust gas becomes high. When this high-temperature exhaust gas flows through the NOx catalyst 20 with the control valve 27 fully opened, the NOx catalyst 20 comes off to a higher temperature side than its active temperature range. As a result, the NOx purification rate may decrease. In such a case, when the control valve 27 is set to the fully closed position and the exhaust gas is caused to flow through the path passing through the HC adsorbent 21, the flow path length from the engine body 1 to the NOx catalyst 20 becomes longer, and the heat release amount also increases. Therefore, the temperature of the exhaust gas flowing into the NOx catalyst 20 can be lowered, and the NOx catalyst 20
Can be kept in the activation temperature range. This makes it possible to improve exhaust gas purification in a high-speed range. Further, by flowing the high-temperature exhaust gas to the HC adsorbent 21, HC remaining in the HC adsorbent 21 can be completely desorbed even when the above-described HC desorption process is performed, and the next HC adsorbent 21 at engine start
Can be recovered to the maximum.

[Second Embodiment] FIG. 7 is a diagram showing a main part of an exhaust gas purifying apparatus for an internal combustion engine according to a second embodiment of the present invention. In the first embodiment described above, NO
Although the x catalyst 20 and the HC adsorbent 21 are housed in one casing 40 to form a unit, in the second embodiment, the NOx catalyst 20 and the HC adsorbent 21 are housed in separate casings, respectively. They are arranged separately.

That is, in the second embodiment, the three-way catalyst 1
One end of an exhaust pipe 19 is connected to a casing 18 containing the casing 7, one end of a casing 50 containing the HC adsorbent 21 is connected to the other end of the exhaust pipe 19, and the other end of the casing 50 is connected via an exhaust pipe 51. Is connected to one end of a casing 52 containing the NOx catalyst 20, and the other end of the casing 52 is connected to one end of a casing 24 containing the three-way catalyst 23 via an exhaust pipe 22, and the other end of the casing 24 is connected to an exhaust pipe. 25.

Inside the casing 50, a middle pipe 53 is installed concentrically, with a gap in the radial direction with respect to the casing 50, and with both ends open. An exhaust passage 54 having an annular cross section is formed between the casing 50 and the middle pipe 53, and the HC adsorbent 21 is provided in the exhaust passage 54. At the downstream end of the middle pipe 53, there is provided a control valve 27 whose valve body is operated by an actuator (not shown). In this embodiment, the middle pipe 5
3 forms a part of the main exhaust passage, and the exhaust passage 54
A bypass passage that bypasses the main exhaust passage upstream of the Ox catalyst 20 is formed.

Also in the case of the second embodiment, the control valve 2
When the valve 7 is in the fully open position, most of the exhaust gas flows from the exhaust pipe 19 to the exhaust pipe 51 through the middle pipe 53, and hardly flows to the exhaust passage 54 and the HC adsorbent 21. Also,
When the control valve 27 is in the fully closed position or the partially closed position,
All or a part of the exhaust gas flowing into the casing 50 from the exhaust pipe 19 can flow to the exhaust passage 54 and the HC adsorbent 21. In the second embodiment configured as described above, the same operation and effect as in the first embodiment can be achieved.
The effect can be obtained.

[0066]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the HC in the exhaust gas is adsorbed by the HC adsorbent at a low temperature to thereby reduce the emission of HC into the atmosphere. The HC adsorbed by the storage and reduction type N
There is an excellent effect that the Ox catalyst can be used as a reducing agent required for NOx purification.

[Brief description of the drawings]

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

FIG. 2 is an enlarged view of a main part of the exhaust gas purification device according to the first embodiment, showing a state in which a control valve is in a fully closed position.

FIG. 3 is an enlarged view of a main part of the exhaust gas purification device according to the first embodiment, showing a state where a control valve is at a fully open position.

FIG. 4 is a diagram showing an example of a map of a basic fuel injection time.

FIG. 5 shows unburned HC in exhaust gas discharged from the engine,
FIG. 3 is a diagram schematically showing the concentrations of CO and oxygen.

FIG. 6 is a diagram for explaining the NOx absorbing / releasing action of a storage reduction type NOx catalyst.

FIG. 7 is a diagram illustrating a main configuration of a second embodiment of the exhaust gas purification apparatus for an internal combustion engine according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body (internal combustion engine) 3 Combustion chamber 4 Spark plug 11 Fuel injection valve 16, 19, 22, 25, 51 Exhaust pipe (main exhaust passage) 17 Three-way catalyst 20 Storage-reduction type NOx catalyst 21 HC adsorbent 23 Three-way Catalyst 27 Control valve (exhaust flow control means) 28 Temperature sensor 30 ECU 40 Casing 42 Inner pipe (main exhaust passage) 43, 44, 54 Exhaust passage (bypass passage) 53 Medium pipe (main exhaust passage)

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hiroshi Tanaka 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 3G091 AA02 AA12 AA13 AA17 AA18 AA23 AA24 AB03 AB06 AB10 BA14 BA15 CA13 CA18 CA26 CB02 DA04 DB06 DC01 EA01 EA05 EA17 EA34 FA02 FA04 FA11 FA13 FA14 FA17 FB02 FB03 FB06 FB07 FB10 FB11 FB12 GB02W GB03W GB04W GB05W GB06W GB09Y GB10X HA09 HA10 HA12 HA19 HA20 HA36 HA37 HB03

Claims (1)

[Claims] 1. A main exhaust passage connected to an internal combustion engine capable of lean combustion absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio and has a low oxygen concentration in the inflowing exhaust gas. a NOx storage reduction catalyst absorbed NOx to release reduced to N ₂ in a bypass passage bypassing the main exhaust passage upstream than (ii) the storage-reduction type NOx catalyst is provided in the bypass passage (c) HC adsorbent that adsorbs HC at low temperature and releases HC at high temperature,
(D) When the HC adsorbent is in the HC adsorption temperature range and when reducing the oxygen concentration of the exhaust gas to release NOx from the NOx storage reduction catalyst, control is performed to introduce exhaust gas into the bypass passage. An exhaust gas purifying apparatus for an internal combustion engine, comprising: an exhaust flow control unit.