JP2000265828A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, capable of purifying NOx in a range of operation areas as wider as possible as compared with that in the past. SOLUTION: An NOx absorption and reduction catalyst 3 capable of occluding NOx when the air-fuel ratio of exhaust gas is lean, and of emitting and reducing occluded NOx when the oxygen concentration in exhaust gas is lowered, is provided for the exhaust passage of a lean combustion type internal combustion engine, and the engine is also provided with an urea selective reducing catalyst 4 capable of letting reduction take place while urea is being added, and it is so devised that exhaust emission control is performed by two catalysts while they are mutually compensating for exhaust emission control roughly in the whole range of operation areas for the internal combustion engine.

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 apparatus for purifying NOx and the like in exhaust gas discharged from a lean burn type internal combustion engine.

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for a lean-burn internal combustion engine, for example, an apparatus described in Japanese Patent No. 2605580 is known.

This device absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is reduced.
This is a device in which an Ox absorbent is disposed in an exhaust passage, and performs a rich spike control for injecting fuel in an internal combustion engine to generate unburned gas (reducing agent) in order to reduce the oxygen concentration of the exhaust gas. This means that the reducing agent for purifying NOx is supplied to the NOx absorbent via the internal combustion engine.

[0004]

In the exhaust gas purifying apparatus for supplying the reducing agent from the internal combustion engine as described above, when the internal combustion engine is of the lean burn type, the exhaust gas purifying apparatus is in a lean state even at a high speed and a high load. Although there is a request to drive,
In such an operating state, the reducing agent cannot be supplied to the NOx absorbent (NOx catalyst) due to rich spikes.

In order to make the air-fuel ratio rich, it is necessary to reduce the intake air amount by reducing the throttle. However, when the internal combustion engine is operated at a high speed and a high load and lean combustion, such a rich condition is required. If it is formed, the combustion of fuel will be hindered, and smoke will be generated.

Therefore, it is only necessary to give up lean combustion and perform stoichiometric combustion (stoichiometric air-fuel ratio). However, it is not possible to improve the fuel efficiency by lean combustion using an internal combustion engine of a lean burn type.

[0007] It is also conceivable to provide a selective reduction catalyst using HC or H as a reducing agent. However, such a selective reduction catalyst using HC or H as a reducing agent has a problem in a high rotation and high load state.
NOx purification rate is low due to high temperature.

[0008] From such a fact, NOx in the entire operation range.
Was difficult to purify. SUMMARY OF THE INVENTION The present invention has been made in view of such a point, and it is an object of the present invention to provide an exhaust gas purifying apparatus for an internal combustion engine which can purify NOx in an operation range as wide as possible as compared with the related art.

[0009]

The present invention adopts the following means in order to solve the above-mentioned problems. That is, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention stores NOx in the exhaust passage of the lean burn type internal combustion engine when the air-fuel ratio of the exhaust gas discharged from the internal combustion engine is lean, and reduces the oxygen concentration in the exhaust gas. A NOx storage-reduction type catalyst that releases and reduces the stored NOx when the NOx reduction occurs, and an ammonia compound selective reduction catalyst that performs selective reduction by adding an ammonia compound.

The internal combustion engine to which the present invention is applied is a lean burn type diesel engine or gasoline engine, and includes a direct injection type engine. In the NOx storage reduction type catalyst, when the internal combustion engine is operated under high speed and high load lean combustion, NOx absorbed by the NOx absorbent is not reduced by the catalyst and the NOx absorbent is not reduced. Impossible. However, under high-rotation, high-load operating conditions, the ammonia compound selective reduction catalyst functions to purify NOx. Therefore, compared with the case where only the NOx storage reduction type catalyst is provided, the operating range in which NOx can be purified is widened.

The present invention has been made on the premise that the reducing agent is supplied to the NOx storage-reduction catalyst via the internal combustion engine. However, this device is of a type for supplying the reducing agent to an exhaust passage connected to the internal combustion engine. However, there is no problem in applying the present invention.

Here, there is provided an operating state detecting means for detecting an operating state of the internal combustion engine, and the N is determined according to the operating state of the internal combustion engine detected by the operating state detecting means.
If one of the Ox storage reduction type catalyst and the ammonia compound selective reduction catalyst is provided with a switching means for switching the exhaust gas flow, an appropriate catalyst for NOx purification can be selected according to the operation state.

For example, when the operating state detected by the operating state detecting means is equal to or less than a predetermined high rotation high load value, the NOx storage reduction type catalyst is selected, and the detected operating state exceeds the predetermined high rotation high load value. Then, an ammonia compound selective reduction catalyst is selected.

The operating state to be detected is an operating area in which the NOx occlusion reduction type catalyst cannot be reduced. In this case, it is determined that reduction cannot be performed in a predetermined high-speed high-load operating state. Exhaust gas purification is performed. Therefore, in order to detect such an operation state,
In addition to one or both of the engine speed and the engine load, parameters that can directly or indirectly indicate the non-reducible region of the NOx storage reduction catalyst, such as the intake air amount and the throttle opening, can be used.

Here, the NOx storage reduction catalyst and the ammonia compound selective reduction catalyst can be arranged in series in an exhaust passage. In this case, the ammonia compound selective reduction catalyst may be disposed downstream of the NOx storage reduction catalyst in the exhaust passage, or the ammonia compound selective reduction catalyst may be disposed upstream of the NOx storage reduction catalyst in the exhaust passage. It may be arranged on the side.

In the case where the exhaust gas is arranged in series as described above, the exhaust gas is provided by bypassing the catalyst arranged on the upstream side and guiding the exhaust gas to the catalyst on the downstream side. The gas flow may be switched.

It is also possible to arrange the NOx storage reduction type catalyst and the ammonia compound selective reduction catalyst in parallel in an exhaust passage.

In the case of the parallel arrangement, the exhaust passage is branched into a first exhaust passage and a second exhaust passage which are parallel to each other,
Disposing the NOx storage reduction catalyst in a first exhaust passage;
The ammonia compound selective reduction catalyst is disposed in a second exhaust passage, and a switching valve is disposed at the branch point between the first exhaust passage and the second exhaust passage as the switching means, and is switched according to an operating condition. Either the first exhaust passage or the second exhaust passage can be selected by switching using a valve.

With the above-described structure, the NOx storage reduction catalyst and the ammonia compound selective reduction catalyst function in such a manner that they complement each other according to the operating conditions. Therefore, exhaust gas purification can be performed in as wide a range of operation as possible as compared with the case of using only one exhaust gas purification catalyst.

Further, the amount of added ammonia compound for estimating the amount of ammonia compound to be added to the ammonia compound selective reduction catalyst from the amount of NOx in exhaust gas flowing into the ammonia compound selective reduction catalyst and the amount of intake air of the internal combustion engine. When the determining means is provided in the exhaust gas purifying apparatus having the above-described configurations, the amount of the ammonia compound to be added can be easily determined.

Further, an ammonia compound detecting means for detecting an ammonia compound flowing out of the ammonia compound selective reduction catalyst, and an ammonia compound amount to be added is adjusted to an appropriate addition amount based on the ammonia compound detection amount detected by the ammonia compound detecting means. If the control means for correcting is provided, the amount of added ammonia compound can be controlled more accurately, and more effective exhaust gas purification can be performed. In addition, urea, ammonium carbamate, etc. are mentioned as an ammonia compound as a reducing agent used in an ammonia compound selective reduction catalyst. The components of the present invention described above can be combined with each other as much as possible.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. In the following example, urea is used as the ammonia compound.

<Embodiment 1> An example shown in FIG. 1 is an exhaust pipe 2 of a lean-burn gasoline engine 1 which is a direct injection type.
In this example, the NOx occlusion reduction catalyst 3 and the urea selective reduction catalyst 4 as an ammonia compound selective reduction catalyst are arranged in series. The urea selective reduction catalyst 4 is arranged downstream of the NOx occlusion reduction catalyst 3. is there.

In the engine 1, 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, they are stored in the ROM in advance in the form of a map.

The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder.
If it is 1.0, the air-fuel mixture supplied to the engine cylinder has a stoichiometric air-fuel ratio. On the other hand, 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,
When K> 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, becomes rich.

In the engine 1, in the engine low-medium load operation region, the value of the correction coefficient K is set to a value smaller than 1.0 to perform the lean air-fuel ratio control. During warm-up operation, acceleration, and at a constant speed operation of, for example, 120 km / h or more, 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 rich air-fuel ratio control is performed.

In an internal combustion engine, low-medium load operation is usually performed most frequently. 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. Will be.

The NOx storage-reduction catalyst 3 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, and an alkaline earth such as barium Ba and calcium Ca. And at least one selected from rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt. Engine intake passage and NOx
The ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the NOx storage reduction catalyst 3
When referred to as the air-fuel ratio of the inflowing exhaust gas, the NOx storage reduction catalyst 3 absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio, and absorbs the NOx when the oxygen concentration in the inflowing exhaust gas decreases. Release.

When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx storage reduction catalyst 3, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber. Therefore, in this case, NOx
The storage reduction catalyst 3 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is lean, and absorbs NOx when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber decreases.
Release and reduce x.

NOx absorption and reduction in the NOx storage reduction catalyst 3
It is believed that the reduction is performed by a mechanism as shown in FIG. This mechanism is 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.

First, when the exhaust gas becomes considerably lean, the oxygen concentration in the exhaust gas greatly increases.
Oxygen O ₂ is O ₂ as shown in ^- or platinum Pt in O ^2- in the form
Adheres to the surface of Next, NO contained in the exhaust gas
O ₂ on the surface of the platinum Pt ^- or reacts with O ^2-, the _{NO 2 (2NO + O 2 →} 2NO 2).

Thereafter, as long as the NOx absorption capacity of the NOx storage reduction catalyst 3 is not saturated, the generated NO ₂ is absorbed into the catalyst while being oxidized on the platinum Pt and combined with the barium oxide BaO, as shown in FIG. nitrate ions NO as shown in a) ₃ ^- diffuses in the NOx storage reduction catalyst 3 in the form of. In this way, NOx is absorbed in the NOx storage reduction type catalyst 3.

On the other hand, when the oxygen concentration in the exhaust gas decreases, the amount of produced NO ₂ decreases, and the nitrate ion NO ₃ ⁻ in the NOx storage reduction catalyst 3 is reduced by the reverse reaction. Is released from the NOx storage reduction catalyst 3 in the form of NO ₂ or NO.

That is, NOx is released from the NOx storage reduction catalyst 3 when the oxygen concentration in the exhaust gas decreases. As shown in FIG. 3, if the lean degree of the inflowing exhaust gas is reduced, the oxygen concentration in the inflowing exhaust gas is reduced. Even when the fuel ratio is lean, NOx is released from the NOx storage reduction catalyst 3.

On the other hand, at this time, when the air-fuel mixture supplied to the combustion chamber is made stoichiometric or rich and the air-fuel ratio of the exhaust gas becomes stoichiometric or rich, as shown in FIG. Emitted from 1.
These unburned HC and CO are immediately oxidized by reacting with oxygen O ₂ ^- or O ^2- on platinum Pt.

When the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the oxygen concentration in the exhaust gas is extremely reduced, so that the NOx storage reduction catalyst 3 releases NO ₂ or NO. This NO ₂ or NO is shown in FIG.
As shown in (B), it is reduced by reacting with unburned HC and CO. When NO ₂ or NO on the platinum Pt disappears in this way, NO ₂ or NO is released from the catalyst one after another. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx storage reduction catalyst 3 within a short time. Platinum Pt on the O ₂ ^- or O ^2- unburned be consumed HC, any remaining CO is, NOx released from the NOx storage reduction catalyst 3 also, even NOx discharged from the engine 1, the It is reduced by unburned HC and CO.

Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed in the NOx storage reduction catalyst 3 is released within a short time, and the released NOx is also released.
NOx can be prevented from being discharged into the atmosphere due to reduction of x.

Further, since the NOx storage-reduction catalyst 3 has the function of a reduction catalyst, the NOx released from the NOx storage-reduction catalyst 3 is reduced even if the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio. You. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NOx is gradually released from the NOx storage-reduction catalyst 3, so that all the NOx absorbed by the NOx storage-reduction catalyst 3 is released. Takes a long time.

If the air-fuel ratio of the inflowing exhaust gas is made lower, the NOx is released from the NOx storage reduction catalyst 3 even if the air-fuel ratio of the inflowing exhaust gas is lean. Therefore, in order to release NOx from the NOx storage reduction catalyst 3, the oxygen concentration in the inflowing exhaust gas should be reduced.

Next, the urea selective reduction catalyst 4
The selective reduction is performed by adding urea to the catalyst. The NOx catalyst mentioned here can be exemplified by a zeolite catalyst containing an oxide of a transition element and / or a rare earth oxide in the fourth, fifth and sixth cycles. Particularly preferred is a catalyst in which Ti and V are supported on Al ₂ O ₃ .

When an aqueous urea solution is added to the catalyst,
At a predetermined exhaust temperature, nitrogen oxides in the exhaust are reduced according to the following reaction formula.

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ ^X + CO ₂ (1) 4NH ₃ ^X + 4NO + O ₂ → 4N ₂ + 6H ₂ O (2)

In order to make the NOx occlusion reduction type catalyst 3 and the urea selective reduction catalyst 4 function complementarily to each other as described above, in this embodiment, the downstream side of the NOx occlusion reduction type catalyst 3 and the upstream side of the urea selective reduction catalyst 4 On the side, a NOx sensor 5 and a urea addition control valve 6 are provided. The NOx sensor 5 is located immediately downstream of the NOx storage reduction catalyst 3 and upstream of the urea addition control valve 6. Further, a catalyst input gas temperature sensor 7 is disposed immediately upstream of the urea selective reduction catalyst 4, and
An ammonia sensor 8 is arranged downstream of the urea selective reduction catalyst 4.

The NOx sensor 5, the urea addition control valve 6, the catalyst-containing gas temperature sensor 7, and the ammonia sensor 8 are each electrically connected to a control unit (ECU) 9 composed of a computer. Further, a speed sensor for detecting the engine speed is provided, and this sensor is also provided by the control device (E
CU) 9.

Based on the information from these sensors and the like, the state of each catalyst and the operating state of the internal combustion engine are detected. Operating state detecting means 10 for detecting the operating state of the internal combustion engine from data input from these sensors and the like is implemented on a computer of the control device (ECU) 9.
Further, in response to the detected operating state, the urea addition control valve 6 issues a urea addition command and controls the addition amount.
U) 9 is implemented on the computer. When urea is added by the urea addition amount control means 11, a reducing agent indicator 12 for indicating to the driver that urea is being added is provided on a meter panel 13 or the like.

The NOx sensor 5 detects the NOx concentration in the exhaust gas passing through the NOx occlusion reduction type catalyst 3, that is, the NOx concentration in the gas entering the urea selective reduction catalyst 4. The urea addition amount control means 11 obtains the NOx amount discharged from the internal combustion engine based on the NOx concentration detected by the NOx sensor 5 and the air amount detected by an air flow meter (not shown), and selects urea from the obtained NOx amount. It has an added urea amount determining means for estimating the amount of urea to be added to the reduction catalyst 4, and instructs to add an amount of urea according to the estimated value.

Here, in order to estimate the amount of urea, a map in which the relationship between the detected amount of NOx and the amount of urea to be added may be stored in advance in the ROM. It should be noted that, instead of the NOx sensor 5, the accelerator opening, the fuel injection amount, the engine speed, the EGR amount by the EGR control device, etc.
The x amount may be estimated. Further, the intake air amount of the internal combustion engine is other means instead of detection with an air flow meter, for example,
It may be based on the throttle opening.

The catalyst-input gas temperature sensor 7 functions as a catalyst temperature detecting means for detecting the temperature of the exhaust gas flowing into the urea selective reduction catalyst 4.
Can be determined. When the temperature of the gas entering the catalyst is low, the purifying ability of the catalyst is low, so the urea addition amount control means 11 reduces the urea addition amount.
The relationship between the temperature of gas entering the catalyst (catalyst temperature) and the amount of urea added is stored in the ROM in advance as a map.

The ammonia sensor 8 is used for correcting the amount of urea added. That is, detecting ammonia by the ammonia sensor 8 downstream of the urea selective reduction catalyst 4 means that the added urea is too much more than the NOx amount. Therefore, a feedback control unit is provided which feeds back the detected amount of ammonia detected by the ammonia sensor 8 to the urea addition amount control unit 11 and corrects the amount of urea to be added to an appropriate target value. This control means is also realized on a computer of the control device (ECU) 9 as a part of the urea addition amount control means 11.

Hereinafter, the exhaust gas purification control according to this embodiment will be described. When the internal combustion engine is operated, fuel is burned in the cylinder to discharge exhaust gas, and the discharged gas flows in the exhaust pipe 2, and the NOx storage reduction catalyst 3 and the urea selective reduction catalyst 4 Through the muffler (not shown)
Released to the atmosphere.

In this embodiment, the urea selective reduction catalyst 4 functions in the high-load and high-speed engine operation region beyond the purification region of the NOx storage reduction type catalyst 3, thereby purifying exhaust gas in the widest possible range. Do.

First, when the engine speed is not in the predetermined high-load, high-speed region, the NOx storage-reduction catalyst 3 repeats the storage and reduction of NOx according to the above-described principle, thereby purifying the exhaust gas. That is, when NOx is released from the NOx storage reduction catalyst 3, the air-fuel ratio of the inflowing exhaust gas is made rich,
The released NOx is reduced in the NOx storage reduction type catalyst 3.

On the other hand, if the air-fuel ratio is made rich by reducing the throttle and reducing the intake air amount when the internal combustion engine has reached a predetermined high load and high speed, the intake air amount (oxygen amount)
As a result, the fuel becomes unburned and so-called smoke is generated. Therefore, in a predetermined high-load and high-speed operation region, the NOx storage reduction catalyst 3
Cannot be reduced.

Therefore, in response to the detection of the operating state by the operating state detecting means 10, the urea addition amount control means 11 issues a urea addition command to the urea addition control valve 6. An aqueous urea solution is injected from the urea addition control valve 6, whereby the exhaust gas is purified according to the above-described principle.

During this time, the NOx sensor 5 detects the concentration of NOx that has flowed through without being purified by the NOx storage-reduction catalyst 3, and upon receiving this detection value, the urea addition amount control means 11 of the control unit (ECU) 9 The NOx amount discharged from the internal combustion engine is determined from the NOx concentration and the intake air amount to the internal combustion engine, and the urea amount to be added to the urea selective reduction catalyst 4 is estimated from the obtained value by the added urea amount determining means. The urea addition control valve 6 is commanded to add urea in an amount according to the value.

The temperature of the gas entering the urea selective reduction catalyst 4 is measured by the catalyst entering gas temperature sensor 7, and the urea addition amount control means 11 of the control unit (ECU) 9 Increase or decrease the amount of urea added.

Further, the ammonia sensor 8 detects the concentration of ammonia in the exhaust gas that has passed through the urea selective reduction catalyst 4.
The feedback control means in (1) corrects the added urea amount in a direction in which the ammonia concentration in the exhaust gas passing through the urea selective reduction catalyst 4 decreases.

Thus, the NOx storage reduction catalyst 3
The exhaust gas purification is performed by the NOx storage reduction catalyst 3 and the urea selective reduction catalyst 4. The complementary relationship between the NOx storage reduction catalyst 3 and the urea selective reduction catalyst 4 is shown in FIG. 4A shows a purifiable region by the NOx storage reduction catalyst 3, and FIG. 4B shows a purifiable region by the urea selective reduction catalyst 4. As shown in FIG.

FIG. 5 shows the relationship between the exhaust gas temperature and the purification rate in the NOx occlusion reduction type catalyst 3 and the urea selective reduction catalyst 4. FIG. 5A shows a purifiable region by the NOx storage reduction catalyst 3, and FIG. 4B shows a purifiable region by the urea selective reduction catalyst 4. As shown in FIG. As is clear from FIG. 5, it is understood that the NOx occlusion reduction type catalyst 3 functions in a region where the exhaust gas temperature is low, and the urea selective reduction catalyst 4 functions in a region where the exhaust gas temperature is high.

In the present embodiment, the positions of the NOx storage reduction catalyst 3 and the urea selective reduction catalyst 4 may be exchanged.

<Embodiment 2> Next, another embodiment will be described with reference to FIG. FIG. 6 shows a configuration in which a start cat 21 is further provided in the configuration shown in the first embodiment. The start cat 21 is a NOx catalyst provided in the exhaust pipe 2 as close as possible to the internal combustion engine, and purifies NOx emitted from the internal combustion engine before the NOx storage reduction catalyst 3 is warmed up at the time of engine start. Start cat 21
Is disposed in a portion near the internal combustion engine in front of the NOx storage reduction catalyst 3, so that it is quickly heated by the exhaust gas when the engine is started, and the temperature is raised to the purification region.

<Embodiment 3> As shown in FIG. 7, the exhaust gas purifying apparatus shown in this embodiment uses the NOx storage reduction catalyst 3 arranged on the upstream side in addition to the configuration shown in FIG. It has a bypass 31 that bypasses and guides exhaust gas to the downstream catalyst.

At a branch point between the exhaust pipe 2 and the bypass passage 31, a switching valve 32 is provided as switching means for switching the exhaust gas flow by opening and closing the bypass passage 31.

The switching valve 32 is connected to the control device (EC
U) The solenoid valve is electrically connected to and controlled by 9.
Then, the switching valve 32 is set to N
When it is determined that the operating range is such that the Ox storage reduction catalyst 3 functions, the bypass passage 31 is closed to allow the exhaust gas to flow to the NOx storage reduction catalyst 3. When the internal combustion engine is in a high-load, high-speed operation state, this state is detected, the switching valve 32 opens the bypass passage 31, and the N
The exhaust gas passage to the Ox storage reduction catalyst 3 is closed so that the exhaust gas flows directly from the bypass passage 31 into the urea selective reduction catalyst 4.

Therefore, when the operation state of the internal combustion engine detected by the operation state detection means 10 is not high load and high speed, the switching valve 32 closes the bypass passage 31 by a command from the control device, and N
When the exhaust gas flows into the Ox storage reduction type catalyst 3 and the operating state of the internal combustion engine detected by the operating state detecting means 10 is high load and high speed, the switching valve 32 opens the bypass passage 31 by a command from the control device. The exhaust gas flow path to the NOx storage reduction catalyst 3 is closed, and the exhaust gas flows from the bypass path 31 directly to the urea selective reduction catalyst 4. The urea addition amount control means 11 issues a urea addition command to the urea addition control valve 6 in response to the detection of the operation state by the operation state detection means 10. An aqueous urea solution is injected from the urea addition control valve 6, whereby the exhaust gas is purified according to the above-described principle.

<Embodiment 4> As shown in FIG. 8, in the exhaust gas purifying apparatus shown in this embodiment, the exhaust passage is branched into a first exhaust passage 41 and a second exhaust passage 42 which are parallel to each other. The first exhaust passage 41 has a configuration in which a NOx storage reduction catalyst is disposed, and the second exhaust passage 42 has a urea selective reduction catalyst 4 disposed therein.

A switching valve 32 is used as the switching means.
Is disposed at a branch point between the first exhaust passage 41 and the second exhaust passage 42. As in the configuration shown in FIG. 1, a NOx sensor 5 is provided on the upstream side of the urea selective reduction catalyst 4, and a catalyst input gas temperature sensor 7 is provided on the upstream side of the urea selective reduction catalyst 4. In addition, an ammonia sensor 8 is provided downstream of the urea selective reduction catalyst 4. Further, a urea addition control valve 6 is provided immediately upstream of the urea selective reduction catalyst 4.

The switching valve 32 is connected to the control device (EC
U) The solenoid valve is electrically connected to and controlled by 9.
Then, the switching valve 32 is set to N
When it is determined that the operating range is such that the Ox storage reduction catalyst 3 functions, the first exhaust passage 41 is selected so that the exhaust gas flows to the NOx storage reduction catalyst 3. Exhaust gas purification by the NOx storage reduction catalyst 3 is as described above.

When the internal combustion engine is in a high-load, high-speed operation state, this state is detected and the switching valve 32 is switched to the second state.
And the exhaust gas flows into the urea selective reduction catalyst 4. When the urea selective reduction catalyst 4 is selected, the urea addition amount control means 11 issues a urea addition command to the urea addition control valve 6. An aqueous urea solution is injected from the urea addition control valve 6, whereby the exhaust gas is purified according to the above-described principle.

[Other Embodiments] In the above embodiment, the present invention is applied to the gasoline engine 1, but it goes without saying that the present invention can be applied to a diesel engine. In the case of a diesel engine, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric condition, so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 3 is very lean under a normal engine operating condition. , NOx is absorbed, but NOx is hardly released.

Therefore, in a diesel engine, for example, an exhaust gas recirculation device (so-called EGR device) is introduced, and a large amount of exhaust gas recirculation gas is introduced into the combustion chamber.
The air-fuel ratio of the exhaust gas can be made stoichiometric or rich to release the NOx absorbed by the catalyst.

[0072]

According to the exhaust gas purifying apparatus for an internal combustion engine of the present invention, since the NOx storage reduction type catalyst and the ammonia compound selective reduction catalyst which complement each other in the exhaust gas purifying area are provided, the operating range is as wide as possible. Exhaust gas purification can be performed.

Here, operating state detecting means for detecting the operating state of the internal combustion engine is provided, and according to the operating state of the internal combustion engine,
By switching the exhaust gas flow to one of the NOx storage reduction type catalyst and the ammonia compound selective reduction catalyst by the switching means, an optimum catalyst can be selected according to the operating state.

Further, the amount of added ammonia compound is determined from the amount of NOx in the exhaust gas flowing into the ammonia compound selective reduction catalyst and the amount of intake air of the internal combustion engine by the means for determining the amount of added ammonia compound.
The amount of ammonia compound to be added to the ammonia compound selective reduction catalyst can be easily determined.

In addition, the ammonia compound flowing out of the ammonia compound selective reduction catalyst is detected by the ammonia compound detection means, and the amount of the ammonia compound to be added is corrected to the appropriate addition amount by the control means based on the detected ammonia compound detection amount. By doing so, the amount of added ammonia compound can be controlled more accurately, and more effective exhaust gas purification can be performed.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

FIG. 2 is a diagram for explaining the NOx absorbing / releasing action of a storage reduction type NOx catalyst.

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

FIG. 4 is a graph showing an exhaust gas purification region using a catalyst in relation to an engine speed and an engine load.

FIG. 5 is a diagram showing a relationship between an exhaust gas temperature and an exhaust gas purification rate.

FIG. 6 is a diagram showing a second embodiment.

FIG. 7 is a diagram showing a third embodiment.

FIG. 8 is a diagram showing a fourth embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust pipe 3 Storage-reduction catalyst 4 Ammonia compound (urea) selective reduction catalyst 5 NOx sensor 6 Ammonia compound addition control valve (reducing agent supply means) 7 Catalyst gas temperature sensor 8 Ammonia sensor (ammonia compound detection means) 9 Control device (ECU) 10 Operating state detection means 11 Ammonia compound (urea) addition amount control means (Additional ammonia compound (urea) amount determination means) 12 Reducing agent indicator 13 Meter panel etc. 21 Start cat 31 Bypass path 32 Switching valve (switching) Means) 41 First exhaust passage 42 Second exhaust passage

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/08 F01N 3/20 NB 3/20 3/24 E B01D 53/36 ZABE 3/24 101A F Terms (reference) 3G091 AA02 AA11 AA12 AA17 AA18 AA24 AA28 AB05 AB06 BA03 BA14 BA33 CA12 CA13 CA16 CA17 CA18 CB02 CB07 CB08 DA01 DA02 DA03 DB06 DB10 DC03 EA01 EA03 EA05 EA07 EA33 FA01 FA04 FA09 FA12 FA13 FB03 GB02W GB03W GB04W GB05W GB06W GB09X GB10X GB16X HA08 HA11 HA12 HA36 HA37 HA38 HB02 HB05 4D048 AA06 AB02 AC04 BA03Y BA07Y BA11Y BA15X BA23Y BA30X BA39X CA01 CC25 CC26 DA01 DA02 DA06 DA08 DA09 DA10 EA04

Claims (11)

[Claims] 1. An exhaust passage of a lean-burn internal combustion engine stores NOx when the air-fuel ratio of exhaust gas discharged from the internal combustion engine is lean, and releases the stored NOx when the oxygen concentration in the exhaust gas decreases. An exhaust gas purification device for an internal combustion engine, comprising: a NOx storage reduction catalyst to be reduced; and an ammonia compound selective reduction catalyst that performs selective reduction by adding an ammonia compound. 2. An operating condition detecting means for detecting an operating condition of the internal combustion engine, wherein the NOx storage-reduction catalyst and the ammonia compound selective reducing catalyst are connected in accordance with the operating condition of the internal combustion engine detected by the operating condition detecting device. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, further comprising a switching means for switching an exhaust gas flow in one of the exhaust gas purifying apparatuses. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said NOx storage reduction catalyst and said ammonia compound selective reduction catalyst are arranged in series in an exhaust passage. 4. The ammonia compound selective reduction catalyst,
4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the exhaust gas purifying device is disposed downstream of the NOx storage reduction catalyst. 5. The ammonia compound selective reduction catalyst,
4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the exhaust gas purifying device is disposed upstream of the NOx storage reduction catalyst. 6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the NOx storage reduction catalyst and the ammonia compound selective reduction catalyst are arranged in parallel in an exhaust passage. 7. A bypass passage for bypassing a catalyst disposed on an upstream side and guiding exhaust gas to a catalyst on a downstream side, wherein the switching means switches an exhaust gas flow by opening and closing the bypass passage. The exhaust gas purifying apparatus according to claim 3, characterized in that: 8. An exhaust passage is branched into a first exhaust passage and a second exhaust passage which are parallel to each other, the NOx storage reduction catalyst is disposed in the first exhaust passage, and the NOx storage reduction catalyst is disposed in the second exhaust passage. 7. The exhaust gas of an internal combustion engine according to claim 6, wherein an ammonia compound selective reduction catalyst is disposed, and a switching valve is disposed at a branch point between the first exhaust passage and the second exhaust passage as the switching means. Purification device. 9. The amount of added ammonia compound for estimating the amount of ammonia compound to be added to the ammonia compound selective reduction catalyst from the amount of NOx in exhaust gas flowing into the ammonia compound selective reduction catalyst and the amount of intake air of the internal combustion engine. The exhaust gas purification device for an internal combustion engine according to any one of claims 1 to 8, further comprising a determination unit. 10. An ammonia compound detecting means for detecting an ammonia compound flowing out from said ammonia compound selective reduction catalyst, and an ammonia compound amount to be added is adjusted to an appropriate addition amount based on the ammonia compound detection amount detected by said ammonia compound detecting means. The exhaust gas purifying apparatus for an internal combustion engine according to claim 9, further comprising control means for correcting the exhaust gas. 11. A catalyst temperature detecting means for detecting a temperature state of an ammonia compound selective reduction catalyst, and an ammonia compound addition amount controlling means for increasing / decreasing an ammonia compound addition amount to the ammonia compound selective reduction catalyst according to the detected catalyst temperature. The exhaust gas purification device for an internal combustion engine according to any one of claims 1 to 10, further comprising: