JP2830652B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly, to an exhaust gas purifying apparatus for an internal combustion engine which burns at a lean air-fuel ratio in most areas such as a diesel engine and a lean burn gasoline engine. The present invention relates to an exhaust gas purification device that efficiently removes NOx.

[0002]

2. Description of the Related Art An example of a method for purifying NOx in exhaust gas of an internal combustion engine performing combustion at a lean air-fuel ratio is disclosed in Japanese Patent Application Laid-Open No. 62-10 / 1987.
There is one described in Japanese Patent No. 6826. According to the method disclosed in the publication, a catalyst that oxidizes and absorbs NOx in the presence of oxygen is disposed in an exhaust passage of a diesel engine, and when the NOx absorption efficiency of the catalyst is reduced, the inflow of exhaust gas to the catalyst is cut off and the reducing agent Is introduced to release the absorbed NOx and to purify by reduction.

[0003]

The above-mentioned Japanese Patent Application Laid-Open No. 62-10 / 1987.
Japanese Patent No. 6826 discloses that the exhaust gas containing a large amount of oxygen is blocked from flowing into the catalyst and a reducing agent is introduced into the catalyst to lower the oxygen concentration in the catalyst portion to release NOx from the catalyst and perform reduction purification. I have. However, in the method of the above publication,
When releasing and purifying NOx from the catalyst, it is necessary to shut off the inflow of exhaust gas to the catalyst. To release and purify NOx during operation of the engine, a plurality of NOx absorption catalysts are arranged in parallel in the exhaust system. Switching sequentially to shut off the exhaust inflow, release,
It is necessary to purify. For this reason, in the above method, a plurality of exhaust systems are required, and there is a possibility that the device becomes complicated and the mountability to a vehicle is deteriorated.

On the other hand, if NOx is released and purified from the catalyst only by introducing a reducing agent without interrupting the flow of exhaust gas into the catalyst, the above-mentioned complicated exhaust system can be prevented, but a large amount of oxygen is contained. In order to maintain the catalyst in a reducing atmosphere under the flow of exhaust gas, in addition to the amount required for the release and reduction of NOx released from the catalyst, a large amount of oxygen is used to generate a reducing atmosphere by consuming oxygen in the inflowing exhaust gas. Requires a reducing agent. For this reason, the consumption of the reducing agent increases, and a problem arises that fuel efficiency deteriorates, especially when fuel oil is used as the reducing agent.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an exhaust gas purifying apparatus capable of removing NOx in exhaust gas having a lean air-fuel ratio without complicating the structure of an exhaust system and without increasing the consumption of a reducing agent. It is intended to provide.

[0006]

According to the present invention, there is provided an exhaust purification system for an internal combustion engine which burns at a lean air-fuel ratio in at least most of the operating range, wherein the exhaust air-fuel ratio is reduced in an exhaust passage of the engine. A NOx absorbent that absorbs NOx at the time of release and releases the absorbed NOx when the exhaust oxygen concentration is lowered is arranged, and a reducing agent is introduced into the exhaust under predetermined operating conditions to lower the exhaust oxygen concentration to reduce the exhaust oxygen concentration. in the exhaust purifying apparatus reduces and purifies the released NOx with the release of NOx from the absorbent, the _the NO _X absorbent to consume oxygen in the exhaust gas flowing into the NOx absorbent when the reducing agent introduced
Exhaust purification system of an internal combustion engine, characterized in that the provided oxygen consumption means for reducing the reducing agent consumption during NO _X emission from is provided.

[0007]

When the reducing agent is introduced into the exhaust gas, the oxygen consuming means consumes the oxygen in the exhaust gas flowing into the NOx absorbent in advance, thereby reducing the exhaust oxygen concentration. For this reason, the inflow of oxygen into the NOx absorbent is reduced, and the amount of the reducing agent required to generate the reducing atmosphere is reduced even when the inflow of exhaust gas is continued.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example in which the exhaust emission control device of the present invention is applied to a diesel engine will be described. FIG. 1 is a diagram showing a first embodiment of the present invention. In FIG. 1, 1 is a diesel engine main body, 2 is an engine intake pipe, 3 is an engine exhaust pipe, and 4 is a fuel injection valve for injecting fuel into an engine combustion chamber.

In this embodiment, the intake pipe 2 is provided with a shutter valve 6. The shutter valve 6 is, for example, a butterfly valve having a small resistance when fully opened, and is kept fully open during normal operation. The shutter valve 6 is closed when NOx is released from the absorbent, which will be described later. Reduce the amount of air that is generated. 7 is EC described later
An appropriate type of actuator, such as a solenoid or a negative pressure actuator, that drives the shutter valve 6 to open and close in response to a signal from U20.

On the other hand, the engine exhaust pipe 3 has a particulate filter 11 for collecting fine particles (diesel particulate) such as carbon and soluble organic components (SOF) contained in the exhaust gas, and NOx contained in a casing.
An absorbent 15 is provided. In addition, what is indicated by 20 in the figure is an electronic control unit (ECU) of the engine 1. E
The CU 20 connects the CPU 21, the RAM 22, the ROM 23, the input port 24, and the output port 25 to each other through a bidirectional bus 26.
A basic computer control such as control of the amount of fuel injected from the fuel injection valve 4 is performed. The ECU 20 also controls the release of NOx from a NOx absorbent 15, which will be described later, and the regeneration of the particulate filter 11. For these controls, an input port 24 of the ECU 20 provides an engine speed signal, an accelerator opening signal, and the like. The temperature signals are input from sensors (not shown), and an exhaust temperature signal is input from an exhaust temperature sensor 17 provided at the entrance of the particulate filter 11. The output port 25 of the ECU 20 is connected to the fuel injection valve 4 and the shutter valve actuator 7, and controls the fuel injection amount, the opening / closing operation of the shutter valve 6, and the like.

The particulate filter 11 is usually formed of a heat-resistant filter material such as ceramics, and filters and collects particulates in exhaust gas. Therefore, the collected particulates accumulate in the filter 11 over time, so that it is necessary to periodically regenerate and remove the collected particulates. Normally, regeneration of the filter is performed by heating the particulates collected by the filter to an ignition temperature (about 600 ° C.) or higher and burning it. As mentioned above, particulates are carbon or SO
Since it is composed of F or the like, depending on the conditions, a reducing gas such as CO is generated by combustion. In the present embodiment, as described later, at the time of the NOx releasing operation from the NOx absorbent, the exhaust valve temperature is raised to the particulate ignition temperature or higher by closing the shutter valve 6 and increasing the fuel injection amount so that the filter 11 is operated. Burn the collected particulates. As a result, the oxygen in the exhaust gas is consumed by the particulate combustion, the exhaust oxygen concentration is reduced, and the particulate combustion generates a reducing gas such as CO or CO ₂ , which is supplied to the NOx absorbent.

The NOx absorbent 15 housed in the casing uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, barium Ba, and calcium Ca. And at least one selected from rare earths such as alkaline earth, lanthanum La and yttrium Y, and a noble metal such as platinum Pt.

The NOx absorbent 15 absorbs NOx when the inflowing exhaust air-fuel ratio is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases.
x has a function of absorbing and releasing. Here, the above-mentioned exhaust air-fuel ratio is
It refers to the ratio of the total amount of air supplied to the exhaust passage, the engine combustion chamber, the intake passage and the like upstream of the NOx absorbent 15 to the total fuel. Therefore, when no fuel or air is supplied to the exhaust passage on the upstream side of the NOx absorbent 15, the exhaust air-fuel ratio becomes equal to the air-fuel ratio in combustion in the engine combustion chamber. In this embodiment, since a diesel engine is used, the exhaust air-fuel ratio is lean during normal operation, and the NOx absorbent 15 absorbs NOx in the exhaust.
Further, when the oxygen concentration in the exhaust gas is reduced by an operation described later, the NOx absorbent 15 releases the absorbed NOx.

The details of the NOx absorbing / releasing action of the NOx absorbent 15 are 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 inflow exhaust gas becomes considerably lean, the oxygen concentration in the inflow exhaust gas greatly increases, and as shown in FIG. 2 (A), the oxygen O ₂ is converted into O ₂ ^{− in} the form of O ₂ ^{− on} the surface of the platinum Pt. Adheres to On the other hand, NO in the inflowing exhaust gas is platinum Pt.
On the surface of the reacts with the O ₂ ^- and becomes NO ₂ (2NO + O
₂ → 2NO ₂ ). Next, a part of the produced NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt, and is absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. 2A while being combined with barium oxide BaO. Diffuses into agent. In this manner, NOx is absorbed in the NOx absorbent 15.

As long as the oxygen concentration in the inflow exhaust gas is high, platinum Pt
NO ₂ is produced on the surface of, as long as NO ₂ to NOx absorption ability of the absorbent is not saturated is absorbed nitrate ions NO ₃ in the absorbent ^- is produced. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ion NO ₃ ⁻ in the absorbent is converted. It is released from the absorbent in the form of NO _2. That is, when the oxygen concentration in the inflow exhaust gas decreases, NOx is released from the NOx absorbent 15.

On the other hand, at this time, unburned HC, C
These ingredients when the reducing component is present in the O such oxygen O ₂ on the platinum Pt ^- is oxidized by reacting with. The NO ₂ released from the absorbent 15 due to the decrease in the oxygen concentration in the exhaust gas is shown in FIG.
As shown in (B), it is reduced by reacting with unburned HC and CO. In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Therefore, when the unburned HC and CO components in the inflowing exhaust gas increase, NOx is released from the NOx absorbent 15 and reduced in a short time.

[0018] That is, the unburned HC in the inflowing exhaust gas, CO, first O ₂ on the platinum Pt ^- immediately reacts been oxidized, and then the platinum Pt on the O ₂ ^- are still unburned HC be consumed , CO remains, the unburned HC and CO reduce NOx released from the absorbent and NOx discharged from the engine. Therefore, during engine operation, NOx
The amount of reducing agent required to release and reduce
The amount needed to sufficiently reduce the oxygen concentration in the exhaust,
(2) O ₂ on the platinum Pt ^- amount needed to consume and, (3) the sum of the amount required to reduce all the NOx discharged from the entire NOx and engine released from the absorbent 15 Becomes By the way, since the exhaust gas of a diesel engine contains a large amount of oxygen, the amount of the reducing agent ((1) above) for sufficiently lowering the oxygen concentration in the exhaust gas is extremely large, and the consumption amount of the reducing agent is large. More. In this embodiment, by performing the following operation, the oxygen in the exhaust gas is consumed in advance to lower the oxygen concentration in the exhaust gas, thereby reducing the amount of the reducing agent required for the above (1).

FIG. 3 shows a flowchart of the operation of releasing and reducing the NOx absorbent 15 in the exhaust gas purifying apparatus of the embodiment shown in FIG. This release / reduction operation is controlled by the ECU 20. FIG. 3, step 31 shows a determination as to whether or not conditions for executing the NOx release / reduction operation of the NOx absorbent 15 are satisfied. The execution conditions are (1) that the engine exhaust temperature is equal to or higher than a predetermined value; (2) that the vehicle is in deceleration operation;

In the above (1), the exhaust gas temperature is input to the ECU 20 from the exhaust gas temperature sensor 17 of the exhaust pipe 3. Here, the above-mentioned predetermined value of the exhaust gas temperature is such that the temperature of the NOx absorbent 15 is NO.
The exhaust temperature is equal to or higher than the activation temperature for x release and reduction. Further, the determination (2) that the vehicle is being decelerated is made based on the fact that the engine speed is equal to or higher than the predetermined value and the accelerator opening is fully closed. The reason why the NOx release / reduction operation is performed only during the vehicle deceleration operation is to prevent the occurrence of torque shock because it is necessary to close the shutter valve 6 and reduce the engine intake during release / reduction as described later. is there.

If the conditions for executing the discharge and reduction are satisfied, the particulate filter 11 is regenerated in steps 32 and 33. That is, in step 32, the shutter valve 6 is closed to reduce the engine intake air amount, and in step 33, the fuel injection amount is increased.
The temperature of the exhaust gas is raised to the particulate ignition temperature (about 600 ° C.) or higher.

Thus, the particulate filter 11
Then, the combustion of the collected exhaust fine particles is started. Although the exhaust air-fuel ratio during normal operation of the diesel engine is significantly lean, the exhaust air-fuel ratio shifts to the rich side by reducing the intake air amount due to the closing of the shutter valve and increasing the fuel injection amount. Components such as unburned HC and CO therein greatly increase. In addition, since the exhaust particulates such as carbon collected by the particulate filter 11 start burning as described above, the oxygen in the exhaust is consumed by the combustion, and the oxygen concentration in the exhaust further decreases. Is generated, the reducing components such as unburned HC and CO in the exhaust further increase, and the conditions of NOx release and reduction in the NOx absorbent 15 are established.

The NOx absorbent 15 has a characteristic that the higher the temperature is, the easier it is to release the absorbed NOx, and the NOx absorbent 15 generally has an ignition temperature of the particulate filter 11 of 600 ° C.
Above, the NOx release speed becomes extremely high. Therefore, the release and reduction of NOx from the NOx absorbent 15 can be completed in a short time by performing the NOx release and reduction from the NOx absorbent 15 at the same time as the combustion regeneration of the particulate filter 11 as in the present embodiment. .

It should be noted that if the discharge reduction condition is not satisfied in step 31, or if the discharge reduction operation is being performed, step 3
If condition 1 is no longer satisfied, step 34,
At 35, the shutter valve is opened and the fuel injection increase is stopped, and the engine returns to the normal operation state. As described above, in the present embodiment, the NOx release and reduction by the NOx absorbent 15 are started by the regeneration operation of the particulate filter 11, thereby (1) the oxygen concentration in the exhaust gas flowing into the NOx absorbent. At the same time, the reduction and (2) the increase of the reducing components such as CO and HC in the exhaust gas and (3) the reduction of the NOx emission reduction time due to the increase in the exhaust gas temperature are achieved.

In the above-described embodiment, the particulate ignition is performed only by the exhaust gas temperature rise due to the closing of the shutter valve 6 and the increase of the fuel injection amount. However, the particulate filter 11 is provided with an electric heater for ignition. Alternatively, the ignition may be promoted in combination with the exhaust gas heating. Next, FIG. 4 shows a second embodiment of the present invention. 4, the same reference numerals as those in FIG. 1 denote the same elements. This embodiment is different from the embodiment of FIG. 1 in that a burner 12 using fuel oil is provided at the entrance of a particulate filter 11. That is, in the present embodiment, when the particulate filter 11 is regenerated, the temperature of the exhaust gas is increased by closing the shutter valve 6 and burning the fuel oil with the burner 12, and the fuel injection amount to the engine is not increased. According to the present embodiment, even when the amount of exhaust particulates collected by the particulate filter 11 is small and the exhaust gas temperature in the particulate filter 11 is not sufficient, the amount of fuel supplied to the burner 12 is increased to increase the exhaust gas. Since the temperature and oxygen consumption can be achieved, the NOx absorbent 15
There is an advantage that the timing of performing the NOx release / reduction operation in the above is not affected by the amount of exhaust particulate collected by the particulate filter 11. In this embodiment, unburned HC and CO generated from the parner 12 act as NOx reducing agents as in the embodiment shown in FIG.

FIG. 5 shows a third embodiment of the present invention. In this embodiment, the particulate filter of the embodiment shown in FIGS. 1 and 4 is not provided, and a burner 12 using fuel oil and a glow heater 13 for ignition are arranged in an exhaust passage. NOx release and reduction at 15 are performed. That is, in the present embodiment, all the effects of the oxygen consumption and the generation of the reducing component due to the combustion in the particulate filter 11 and the exhaust gas temperature rise are performed only by closing the shutter valve 6 and burning the burner 12. This makes NO
There is an advantage that the timing of the NOx release / reduction operation in the x absorbent 15 can be freely set.

FIG. 6 shows a fourth embodiment of the present invention.
In this embodiment, a small-capacity oxidation catalyst 28 is disposed on the exhaust pipe on the upstream side of the NOx absorbent 15, preferably immediately downstream of the exhaust manifold. As the catalyst 28, a metal carrier having a small heat capacity is used, and a product having a low activation temperature, for example, a high Pt-supported product is suitable. In the present embodiment, the oxygen consumption and the exhaust gas temperature are increased by the oxidation catalyst 28.

That is, when the execution condition of the NOx release / reduction operation with the NOx absorbent 15 is satisfied, the shutter valve 6 of the intake pipe 2 is closed, the fuel injection amount of the engine is increased, and the air-fuel ratio is greatly increased. Move to As a result, components such as unburned HC and CO in the exhaust gas increase, and the catalyst 2
At 8, an oxidation reaction occurs, oxygen in the exhaust gas is consumed, and the exhaust gas temperature rises due to reaction heat. Further, since the catalyst 28 has a small capacity, only a part of the unburned HC and CO in the exhaust gas generated by the increase in the fuel injection amount can be consumed, and most of the remaining unburned HC and CO components are removed from the catalyst. N after passing 28
It reaches the Ox absorbent 15 and is used as a reducing agent for the released NOx.

In this embodiment, an oxidation catalyst is used as the catalyst 28. However, as described above, the NOx absorbent has a function as an oxidation catalyst. Can also be used. By arranging a small-capacity NOx absorbent instead of the catalyst 28 as described above, the NOx treatment capacity can be increased as a whole together with the NOx absorbent arranged downstream.

In each of the above-mentioned embodiments, the shutter valve 6 of the intake pipe 2 is closed to reduce the amount of intake air during NOx release and reduction. For example, in these cases, exhaust gas recirculation (EGR) is performed. When used together, it is more effective in reducing the amount of oxygen in the exhaust gas. Although the above-described embodiment has been described only for a diesel engine, the present invention is also applicable to a gasoline engine (lean burn engine) that performs lean combustion.

[0031]

As described above, according to the present invention, NOx
Means for consuming oxygen in the exhaust gas are arranged upstream of the absorbent, and the concentration of oxygen in the exhaust gas is reduced in advance during the NOx release and reduction operations, thereby preventing the required amount of the reducing agent from increasing. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram for explaining a NOx absorption / release operation.

FIG. 3 is a flowchart illustrating a NOx releasing and reducing operation in the embodiment of FIG. 1;

FIG. 4 is a diagram showing a second embodiment of the present invention.

FIG. 5 is a diagram showing a third embodiment of the present invention.

FIG. 6 is a diagram showing a fourth embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Diesel engine 2 ... Intake pipe 3 ... Exhaust pipe 6 ... Intake shutter valve 11 ... Particulate filter 12 ... Burner 15 ... NOx absorbent 17 ... Exhaust temperature sensor 20 ... Electronic control unit 28 ... Oxidation catalyst

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) F01N 3/08-3/28

Claims (1)

(57) [Claims] An exhaust purification system for an internal combustion engine that performs combustion at a lean air-fuel ratio in at least most of an operating region, wherein NOx is absorbed in an exhaust passage of the engine when the exhaust air-fuel ratio is lean, and exhaust gas oxygen is exhausted. A NOx absorbent that releases the absorbed NOx when the concentration is reduced is arranged, and a reducing agent is introduced into the exhaust gas under predetermined operating conditions to lower the exhaust oxygen concentration to release NOx from the absorbent and release the NOx. NO
In the exhaust purifying apparatus reduces and purifies x, when NO _X released from the _the NO _X absorbent to consume oxygen in the exhaust gas flowing into the NOx absorbent when the reducing agent introduced
An exhaust gas purifying apparatus for an internal combustion engine, comprising an oxygen consuming means for reducing the amount of reducing agent consumed .