JP2002188429A - Exhaust gas purifying device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce consumption of reducing agent by properly setting the selection condition of NOx catalyst, in an exhaust gas purifying device of an internal combustion engine equipped with a selective reduction type NOx catalyst and an occlusive reduction type NOx catalyst. SOLUTION: The exhaust gas purifying device of internal combustion engine is equipped with the selective reduction type NOx catalyst for selectively reducing nitrogen oxide 37, occlusive reduction type NOx catalyst emitting and reducing the nitrogen oxide 20, means for feeding NOx catalyst reducing agent 38, 28 and the means for feeding occlusive reduction type NOx catalyst reducing agent 28 stops feeding the reducing agent, when the quantity of the reducing agent fed into the occlusive reduction type NOx catalyst 20 exceeds the threshold and the means for feeding the selective reduction type NOx catalyst feeds the reducing agent to the selective reduction type NOx catalyst 37.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

2. Description of the Related Art In recent years, lean NOx catalysts such as a selective reduction type NOx catalyst and a storage reduction type NOx catalyst have been used as one of means for reducing nitrogen oxides (NOx) contained in exhaust gas of an internal combustion engine capable of lean combustion. It has been known.

A selective reduction type NOx catalyst is a catalyst that reduces or decomposes nitrogen oxides (NOx) when a reducing agent is present in an oxygen-excess atmosphere. As such a catalyst, zeolite is used as a carrier and Cu or the like is used. And a catalyst supporting titanium oxide / vanadium, a catalyst supporting a noble metal using zeolite or alumina as a carrier, and the like.

In order to purify nitrogen oxides (NOx) using this selective reduction type NOx catalyst, an appropriate amount of a reducing agent is required. Techniques using hydrocarbon (HC), ammonia-derived compounds, and the like as the reducing agent have been developed.

[0005] On the other hand, when the oxygen concentration of the inflowing exhaust gas is high, the NOx storage-reduction type NOx catalyst removes nitrogen oxides (NO
x), and when the oxygen concentration of the inflowing exhaust gas is reduced and a reducing agent is present, the nitrogen oxides (NO
It is a catalyst that reduces to nitrogen (N ₂ ) while releasing x).

When the storage reduction type NOx catalyst is disposed in the exhaust system of the internal combustion engine, when the internal combustion engine is operated in a lean burn operation and the air-fuel ratio of the exhaust becomes high, the nitrogen oxides (NO
x) is stored in the NOx storage-reduction catalyst,
When the air-fuel ratio of the exhaust gas flowing into the x-catalyst becomes low, nitrogen oxides (NOx) stored in the NOx storage-reduction catalyst
Is reduced to nitrogen (N ₂ ) while being released.

However, since the NOx storage capacity of the NOx storage reduction catalyst is limited, when the internal combustion engine is operated for lean combustion for a long period of time, the NOx storage capacity of the NOx storage reduction catalyst is saturated, and the NOx in the exhaust gas is saturated. Oxides (NOx) are released to the atmosphere without being removed by the NOx storage reduction catalyst.

Therefore, when the NOx storage reduction catalyst is applied to a lean burn internal combustion engine, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst before the NOx absorption capacity of the NOx storage reduction catalyst is saturated. It is necessary to release and reduce the nitrogen oxides (NOx) absorbed in the NOx storage reduction catalyst by executing a so-called fuel addition control to lower the NOx.

As described above, a lean NOx catalyst such as a selective reduction type NOx catalyst or a storage reduction type NOx catalyst can uniformly purify nitrogen oxides (NOx) in the exhaust gas in the presence of a reducing agent. When purifying nitrogen oxides (NOx) in exhaust gas using a lean NOx catalyst, it is necessary to supply an appropriate amount of a reducing agent to the lean NOx catalyst.

[0010] The selective reduction type NOx catalyst and the storage reduction type NOx catalyst have different nitrogen oxide purification rates depending on the operation range of the internal combustion engine. Therefore, a technique for purifying nitrogen oxides in a wide operating range by combining these lean NOx catalysts has been disclosed. For example, in JP-A-11-65692,
Selective reduction NO using ammonia-derived compound as reducing agent
The x catalyst and the NOx storage reduction catalyst are combined. The reason why a compound derived from ammonia was selected as the reducing agent of the selective reduction type NOx catalyst is that the purification rate of nitrogen oxides (NOx) is high even under high rotation and high load conditions.

[0011] In the exhaust gas purification device configured as described above,
The reducing agent is supplied to the selective reduction type NOx catalyst in the high rotation and high load region, and the reducing agent is supplied to the occlusion reduction type NOx catalyst in the other operation regions to purify nitrogen oxides (NOx).

[0012]

The operation range of the internal combustion engine having a high nitrogen oxide purification rate is completely different between a selective reduction type NOx catalyst using a compound derived from ammonia as a reducing agent and an occlusion reduction type NOx catalyst. There is an area that partially overlaps instead of the area.

In this partially overlapping operation region, a predetermined nitrogen oxide (NOx) is obtained regardless of which lean NOx catalyst is selected.
Although a purification rate can be obtained, according to JP-A-11-65692, the NOx catalyst is selected based on the operating state of the internal combustion engine, such as the load, the rotation speed, or the catalyst bed temperature of the internal combustion engine.

However, if a storage-reduction type NOx catalyst is used in the partially overlapping region, a large amount of reducing agent is required,
When the fuel of the internal combustion engine is used as the reducing agent, fuel efficiency is deteriorated. In such a case, it is preferable to use a selective reduction type NOx catalyst.

Therefore, it is important to appropriately set the conditions for selecting the lean NOx catalyst in the partially overlapping region.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a selective reduction type NOx using an ammonia-derived compound as a reducing agent.
In an exhaust gas purifying apparatus for an internal combustion engine including a catalyst and an NOx storage reduction catalyst, the selection condition of a NOx catalyst for purifying nitrogen oxides (NOx) is appropriately set, thereby reducing the consumption of a reducing agent. It is in.

[0017]

Means for Solving the Problems To achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention employs the following means.

That is, a selective reduction type NOx catalyst provided in an exhaust passage of an internal combustion engine for selectively reducing nitrogen oxides in the presence of a reducing agent derived from ammonia, and an exhaust gas provided in an exhaust passage of an internal combustion engine in an oxygen-excess atmosphere. A storage-reduction NOx catalyst that releases and reduces the stored nitrogen oxides when the nitrogen oxides therein are stored and the oxygen concentration decreases, and wherein the amount of the reducing agent supplied to the storage-reduction NOx catalyst is a threshold. The NOx storage reduction type NOx catalyst reducing agent supply means stops supplying the reducing agent and the selective reduction type NOx catalyst reducing agent supply means supplies a reducing agent derived from ammonia to the selective reduction type NOx catalyst. It is characterized by.

In the exhaust gas purifying apparatus for an internal combustion engine thus configured, nitrogen oxides (NOx) contained in the exhaust gas are
It is absorbed by the NOx storage reduction catalyst provided in the exhaust passage.

When it becomes necessary to supply a reducing agent to the NOx storage reduction catalyst, the reducing agent supply means for the NOx storage reduction catalyst supplies the reducing agent to the exhaust passage upstream of the NOx catalyst. .

The reducing agent supplied to the exhaust passage flows into the NOx storage reduction catalyst together with the exhaust gas flowing from the upstream of the exhaust passage. The NOx storage reduction catalyst uses a reducing agent to reduce and purify harmful gas components in the exhaust gas.

When it becomes necessary to supply a reducing agent to the selective reduction type NOx catalyst, the selective reduction type NOx catalyst reducing agent supply means supplies a compound derived from ammonia as a reducing agent into the exhaust gas.

The reducing agent supplied to the exhaust passage flows into the selective reduction type NOx catalyst together with the exhaust gas flowing from the upstream of the exhaust passage. Then, the selective reduction type NOx catalyst reduces and purifies the harmful gas components in the exhaust using the reducing agent.

Which of the lean NOx catalysts to supply the reducing agent to purify the nitrogen oxides (NOx) depends on the amount of the reducing agent supplied to the NOx storage reduction catalyst by the NOx storage reduction catalyst supply means. It is determined by whether or not a threshold is exceeded.

That is, if the amount of the reducing agent supplied to the NOx storage reduction catalyst by the NOx storage reduction catalyst supply means exceeds the threshold value, the fuel consumption deteriorates. Supply.

When the amount of the reducing agent supplied to the NOx storage reduction catalyst by the NOx storage reduction catalyst supply means is equal to or less than the threshold value, the required amount of the reducing agent is small and does not cause deterioration of fuel efficiency. A reducing agent is supplied to the reduced NOx catalyst.

By selecting a lean NOx catalyst for supplying the reducing agent under such conditions, it is possible to reduce the consumption of the reducing agent.

As the reducing agent derived from ammonia,
Examples include urea and ammonium carbamate.

In the present invention, the storage-reduction type NOx
The catalyst reducing agent supply means may be a sub-injection for injecting the fuel again at a time when the engine output does not become the engine output after the fuel for the engine output is injected into the internal combustion engine.

An EGR device for recirculating a part of the exhaust gas to the intake system of the internal combustion engine is provided.
The catalyst reducing agent supply means may perform low-temperature combustion in which fuel is burned by adding more EGR gas than the amount of EGR gas at which the generation amount of particulate matter reaches a peak.

Further, the storage-reduction type NOx catalyst reducing agent supply means may add a reducing agent through a reducing agent addition nozzle which adds the reducing agent to an exhaust passage of the internal combustion engine.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings. Here, a case where the exhaust gas purification apparatus according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which the exhaust gas purifying apparatus according to the present invention is applied and an intake / exhaust system thereof.

The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2.

The internal combustion engine 1 has a fuel injection valve 3 for directly injecting fuel into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulation chamber (common rail) 4 for accumulating fuel up to a predetermined pressure. The common rail 4 is provided with a common rail pressure sensor 4a that outputs an electric signal corresponding to the pressure of the fuel in the common rail 4.

The common rail 4 is in communication with a fuel pump 6 via a fuel supply pipe 5. This fuel pump 6
Is a pump that operates using the rotational torque of the output shaft (crankshaft) of the internal combustion engine 1 as a drive source. A pump pulley 6 a attached to the input shaft of the fuel pump 6 is connected to the output shaft (crankshaft) of the internal combustion engine 1. It is connected to the attached crank pulley 1a via a belt 7.

In the fuel injection system configured as described above, when the rotational torque of the crankshaft is transmitted to the input shaft of the fuel pump 6, the fuel pump 6 is transmitted from the crankshaft to the input shaft of the fuel pump 6. The fuel is discharged at a pressure corresponding to the rotating torque.

The fuel discharged from the fuel pump 6 is
The fuel is supplied to the common rail 4 via the fuel supply pipe 5, accumulated in the common rail 4 to a predetermined pressure, and distributed to the fuel injection valves 3 of each cylinder 2. When a drive current is applied to the fuel injection valve 3, the fuel injection valve 3 opens, and as a result, fuel is injected from the fuel injection valve 3 into the cylinder 2.

Next, an intake branch pipe 8 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 8 communicates with a combustion chamber of each cylinder 2 via an intake port (not shown). .

The intake branch pipe 8 is connected to an intake pipe 9.
This intake pipe 9 is connected to an air cleaner box 10. An air flow meter 11 that outputs an electric signal corresponding to a mass of the intake air flowing through the intake pipe 9 is provided in an intake pipe 9 downstream of the air cleaner box 10 and a temperature corresponding to the temperature of the intake air flowing through the intake pipe 9. And an intake air temperature sensor 12 that outputs a detected electric signal.

An intake throttle valve 13 for adjusting the flow rate of intake air flowing through the intake pipe 9 is provided at a position immediately upstream of the intake branch pipe 8 in the intake pipe 9. The intake throttle valve 13 is provided with an intake throttle actuator 14 that is configured by a stepper motor or the like and drives the intake throttle valve 13 to open and close.

The intake pipe 9 located between the air flow meter 11 and the intake throttle valve 13 is provided with a compressor housing 15a of a centrifugal supercharger (turbocharger) 15 which operates using heat energy of exhaust gas as a driving source. And
An intercooler 16 for cooling intake air, which has been compressed in the compressor housing 15a and has become high temperature, is provided in the intake pipe 9 downstream of the compressor housing 15a.
Is provided.

In the intake system configured as described above, the intake air flowing into the air cleaner box 10 passes through the intake pipe 9 after dust and the like in the intake air are removed by an air cleaner (not shown) in the air cleaner box 10. And flows into the compressor housing 15a.

The intake air flowing into the compressor housing 15a is compressed by rotation of a compressor wheel provided in the compressor housing 15a. The intake air that has been compressed in the compressor housing 15a and has become high temperature is cooled by the intercooler 16, and then flows into the intake branch pipe 8 with the flow rate adjusted by the intake throttle valve 13 as necessary. The intake air flowing into the intake branch pipe 8 is distributed to the combustion chamber of each cylinder 2 via each branch pipe, and is burned using the fuel injected from the fuel injection valve 3 of each cylinder 2 as an ignition source.

On the other hand, an exhaust branch pipe 18 is connected to the internal combustion engine 1, and each branch pipe of the exhaust branch pipe 18 communicates with a combustion chamber of each cylinder 2 via an exhaust port (not shown).

The exhaust branch pipe 18 is connected to the centrifugal turbocharger 15.
Of the turbine housing 15b. The turbine housing 15b is connected to an exhaust pipe 19, and the exhaust pipe 19 is connected downstream to a muffler (not shown).

In the middle of the exhaust pipe 19, a storage-reduction type N
Particulate filter 20 carrying an Ox catalyst and a selective reduction type NOx catalyst 3 using urea as a reducing agent downstream thereof
7 are arranged. Exhaust pipe 1 upstream of filter 20
An exhaust temperature sensor 24 that outputs an electric signal corresponding to the temperature of exhaust flowing through the exhaust pipe 19 is attached to the exhaust pipe 9.

Downstream of the filter 20 and the selective reduction type N
An exhaust pipe 19 upstream of the Ox catalyst 37 has a selective reduction type NOx
A NOx sensor 39 that outputs an electric signal corresponding to the NOx concentration flowing into the catalyst 37 and an air-fuel ratio sensor 23 that outputs an electric signal corresponding to the air-fuel ratio of the exhaust gas flowing through the exhaust pipe 19 are attached. Downstream of the NOx sensor 39 and the air-fuel ratio sensor 23 and the selective reduction type NOx catalyst 3
A urea addition control valve 38 for adding urea as a reducing agent to the selective reduction type NOx catalyst 37 is provided upstream of the NOx catalyst 7.

An exhaust throttle valve 21 for adjusting the flow rate of exhaust gas flowing through the exhaust pipe 19 is provided downstream of the selective reduction type NOx catalyst 37. This exhaust throttle valve 21
The exhaust throttle valve 21 includes a stepper motor and the like.
An exhaust throttle actuator 22 for driving the opening and closing of the valve is mounted.

In the exhaust system configured as described above, the air-fuel mixture (burned gas) burned in each cylinder 2 of the internal combustion engine 1 is discharged to the exhaust branch pipe 18 through the exhaust port, and then to the exhaust branch pipe 18. To the turbine housing 15b of the centrifugal supercharger 15
Flows into The exhaust gas flowing into the turbine housing 15b rotates a turbine wheel rotatably supported in the turbine housing 15b by using thermal energy of the exhaust gas. At this time, the rotational torque of the turbine wheel is transmitted to the compressor wheel of the compressor housing 15a described above.

The exhaust gas discharged from the turbine housing 15b flows into the filter 20 and the selective reduction type NOx catalyst 37 through the exhaust pipe 19, and the exhaust gas flows into the filter 20 and the selective reduction type NOx catalyst 37. Curated Matter (Particulate Matter, unless otherwise noted "PM"
That. ) Is collected and harmful gas components are removed or purified. The exhaust gas whose PM has been collected by the filter 20 and the harmful gas component has been purified by the filter 20 and the selective reduction type NOx catalyst 37 is adjusted through an exhaust throttle valve 21 as needed, and then discharged through the muffler. Released during.

The exhaust branch pipe 18 and the intake branch pipe 8 are connected via an exhaust gas recirculation passage (EGR passage) 25 for recirculating a part of the exhaust gas flowing through the exhaust branch pipe 18 to the intake branch pipe 8. Are in communication. In the middle of the EGR passage 25, a solenoid valve or the like is provided.
A flow control valve (EGR valve) 26 for changing the flow rate of exhaust gas (hereinafter, referred to as EGR gas) flowing through the passage 25 is provided.

In the EGR passage 25, an EGR valve 26
A portion upstream of the EGR passage 25
An EGR cooler 27 for cooling the GR gas is provided.

In the exhaust gas recirculation mechanism configured as described above, when the EGR valve 26 is opened, the EGR passage 25 becomes conductive, and a part of the exhaust gas flowing through the exhaust branch pipe 18 is partially discharged. And is guided to the intake branch pipe 8 through the EGR cooler 27.

At this time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing through the EGR passage 25 and a predetermined refrigerant, and the EGR gas is cooled.

The EGR gas recirculated from the exhaust branch pipe 18 to the intake branch pipe 8 through the EGR passage 25 is guided to the combustion chamber of each cylinder 2 while mixing with fresh air flowing from the upstream of the intake branch pipe 8. Then, the fuel injected from the fuel injection valve 3 is burned using the ignition source.

Here, the EGR gas contains an inert gas component such as water (H ₂ O) or carbon dioxide (CO ₂ ) which does not burn itself and has endothermic properties, such as water (H ₂ O) and carbon dioxide (CO ₂ ). Therefore, if EGR gas is contained in the air-fuel mixture,
The combustion temperature of the air-fuel mixture is lowered, so that nitrogen oxides (NO
x) is suppressed.

Further, when the EGR gas is cooled in the EGR cooler 27, the temperature of the EGR gas itself decreases and the volume of the EGR gas decreases, so that when the EGR gas is supplied into the combustion chamber, The ambient temperature of the air does not unnecessarily rise, and the amount of fresh air (volume of fresh air) supplied into the combustion chamber does not unnecessarily decrease.

Next, the selective reduction type N in this embodiment
The Ox catalyst 37 will be described.

The selective reduction type NOx catalyst 37 performs selective reduction by adding urea as a reducing agent to the NOx catalyst.

Examples of the selective reduction type NOx catalyst include a catalyst in which a transition metal such as Cu is supported by ion exchange using zeolite as a carrier, a catalyst in which titania / vanadium is supported, and a catalyst in which noble metal is supported in zeolite or alumina as a carrier. Can be illustrated. When urea is added on this catalyst, the following reaction occurs.

2NO + 2O ₂ + 2CO (NH ₂ ) ₂ → 3N ₂
+ 2CO ₂ + 4H ₂ O or 6NO + 2CO (NH ₂ ) ₂ → 5N ₂ + 2CO ₂ + 4H ₂ O According to this reaction formula, NOx contained in exhaust gas is reduced and purified.

Next, the filter 20 according to the present embodiment will be described.

FIG. 4 shows the structure of the filter 20. In addition,
4A shows a cross section of the filter 20 in the horizontal direction, and FIG. 4B shows a cross section of the filter 20 in the vertical direction. As shown in FIGS. 4A and 4B, the filter 20 is a so-called wall flow type including a plurality of exhaust passages 50 and 51 extending parallel to each other. These exhaust passages are constituted by an exhaust inflow passage 50 whose downstream end is closed by a plug 52 and an exhaust outflow passage 51 whose upstream end is closed by a plug 53. FIG.
In FIG. 5A, the hatched portion indicates the plug 53. Accordingly, the exhaust inflow passages 50 and the exhaust outflow passages 51 are alternately arranged with the thin partition walls 54 interposed therebetween. In other words, the exhaust inflow passage 50 and the exhaust outflow passage 51 are arranged such that each exhaust inflow passage 50 is surrounded by the four exhaust outflow passages 51 and each exhaust outflow passage 51 is surrounded by the four exhaust inflow passages 50.

The filter 20 is formed of a porous material such as cordierite, so that the exhaust gas flowing into the exhaust inflow passage 50 flows through the surrounding partition wall 54 as shown by an arrow in FIG. Then, it flows out into the adjacent exhaust outflow passage 51.

In the embodiment according to the present invention, each exhaust inflow passage 5
0 and the peripheral wall surface of each exhaust outflow passage 51, that is, each partition wall 54
A layer of a carrier made of, for example, alumina is formed on both side surfaces and on the inner wall surface of the pores in the partition wall 54, and the storage reduction type NOx catalyst is carried on this carrier.

Next, the operation of the NOx storage reduction catalyst carried on the filter 20 according to the present embodiment will be described.

The filter 20 uses, for example, alumina as a carrier and places potassium (K) and sodium (N) on the carrier.
a) Alkali metals such as lithium (Li) or cesium (Cs), alkaline earths such as barium (Ba) or calcium (Ca), and rare earths such as lanthanum (La) or yttrium (Y). And a noble metal such as platinum (Pt). In the present embodiment, a description will be given of an example of an occlusion reduction type NOx catalyst constituted by supporting barium (Ba) and platinum (Pt) on a carrier made of alumina.

The NOx catalyst configured as described above is
When the oxygen concentration of exhaust gas flowing into the Ox catalyst is high, nitrogen oxide (NOx) in the exhaust gas is absorbed.

On the other hand, when the oxygen concentration of the exhaust gas flowing into the NOx catalyst decreases, the NOx catalyst releases the absorbed nitrogen oxides (NOx). At this time, if reducing components such as hydrocarbons (HC) and carbon monoxide (CO) are present in the exhaust gas, the NOx catalyst converts the nitrogen oxides (NOx) released from the NOx catalyst into nitrogen (N ₂ ) can be reduced.

Although the NOx absorption / release action of the NOx catalyst has not been clarified in some parts, it is considered that the action is performed by the following mechanism.

First, in the NOx catalyst, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst becomes a lean air-fuel ratio and the oxygen concentration in the exhaust increases, as shown in FIG. (O ₂₎ is O ₂ ^- or O ^2- in the form deposited on the surface of the platinum (Pt). Nitric oxide (NO) in the exhaust reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum (Pt) to form nitrogen dioxide (NO ₂ ) (2NO + O ₂ → 2NO ₂ ).
Nitrogen dioxide (NO ₂ ) is further oxidized on the surface of platinum (Pt) and absorbed by the NOx catalyst in the form of nitrate ions (NO ₃ ⁻ ). Incidentally, nitrate ions (N
O ₃ ⁻ ) combines with barium oxide (BaO) to form barium nitrate (Ba (NO ₃ ) ₂ ).

As described above, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is the lean air-fuel ratio, nitrogen oxides (NOx) in the exhaust gas are absorbed by the NOx catalyst as nitrate ions (NO ₃ ⁻ ).

The NOx absorbing action as described above is based on the fact that the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio and the NOx catalyst
It is continued as long as the Ox absorption capacity is not saturated. Therefore, N
When the air-fuel ratio of the exhaust gas flowing into the Ox catalyst is a lean air-fuel ratio, unless the NOx absorption capacity of the NOx catalyst is saturated,
Nitrogen oxides (NOx) in the exhaust are absorbed by the NOx catalyst,
Nitrogen oxides (NOx) are removed from the exhaust gas.

On the other hand, in the NOx catalyst, when the oxygen concentration of the exhaust gas flowing into the NOx catalyst decreases, platinum (P
Since the amount of generated nitrogen dioxide (NO ₂ ) on the surface of t) is reduced, nitrate ions (NO ₃ ⁻ ) bonded to barium oxide (BaO) are conversely changed to nitrogen dioxide (NO ₂ ) and nitric oxide. It becomes (NO) and separates from the NOx catalyst.

At this time, if reducing components such as hydrocarbons (HC) and carbon monoxide (CO) are present in the exhaust gas, those reducing components are converted to oxygen (O ₂ ^- or O ₂ ) on platinum (Pt). ^2- )
Reacts to form active species. This active species
Nitrogen dioxide (NO ₂ ) and nitric oxide (NO) released from the NOx catalyst are reduced to nitrogen (N ₂ ).

Accordingly, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio and the oxygen concentration in the exhaust decreases and the concentration of the reducing agent increases, the NO
The nitrogen oxides (NOx) absorbed by the x catalyst are released and reduced, whereby the NOx absorption capacity of the NOx catalyst is regenerated.

When the internal combustion engine 1 is operating in the lean burn operation, the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 becomes a lean atmosphere, and the oxygen concentration of the exhaust gas becomes high.
Nitrogen oxides (NOx) contained in the exhaust gas are absorbed by the NOx catalyst. However, if the lean burn operation of the internal combustion engine 1 is continued for a long time, the NOx absorption capacity of the NOx catalyst is saturated, and Of nitrogen oxides (NOx) are released to the atmosphere without being removed by the NOx catalyst.

In particular, in a diesel engine such as the internal combustion engine 1, a mixture having a lean air-fuel ratio is burned in most of the operating region, and the air-fuel ratio of exhaust gas becomes a lean air-fuel ratio in most of the operating region. Therefore, the Nx of the NOx catalyst
Ox absorption capacity is easily saturated.

Therefore, when the internal combustion engine 1 is operating in the lean burn operation, the concentration of oxygen in the exhaust gas flowing into the NOx catalyst is reduced and the concentration of the reducing agent is increased before the NOx absorption capacity of the NOx catalyst is saturated. It is necessary to release and reduce nitrogen oxides (NOx) absorbed by the NOx catalyst.

Therefore, there is provided a reducing agent supply mechanism for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage upstream of the filter 20, and the fuel is added from the reducing agent supply mechanism to the exhaust gas. The oxygen concentration of the exhaust gas flowing into the filter 20 is reduced and the concentration of the reducing agent is increased.

As shown in FIG. 1, the reducing agent supply mechanism is attached to the cylinder head of the internal combustion engine 1 so that its injection hole faces the exhaust branch pipe 18, and fuel at a predetermined valve opening pressure or higher is applied. A reducing agent injection valve 28 that opens and injects fuel when the fuel is discharged; a reducing agent supply passage 29 that guides the fuel discharged from the fuel pump 6 to the reducing agent injection valve 28; The reducing agent supply path 2
Flow regulating valve 30 for regulating the flow rate of fuel flowing through
A shutoff valve 31 provided in the reducing agent supply passage 29 upstream of the flow regulating valve 30 to block the flow of fuel in the reducing agent supply passage 29; and a reducing agent supply passage upstream of the flow regulating valve 30. A reducing agent pressure sensor 32 that is attached to and outputs an electric signal corresponding to the pressure in the reducing agent supply passage 29;
And

The reducing agent injection valve 28 is configured such that the injection hole of the reducing agent injection valve 28 is located downstream of the connection portion of the exhaust branch pipe 18 with the EGR passage 25 and the four branch pipes of the exhaust branch pipe 18 are connected to each other. It is preferable that the projection is protruded to the exhaust port of the cylinder 2 closest to the collecting portion and is attached to the cylinder head so as to face the collecting portion of the exhaust branch pipe 18.

This prevents the reducing agent (unburned fuel component) injected from the reducing agent injection valve 28 from flowing into the EGR passage 25, and also allows the reducing agent to be centrifuged without stagnation in the exhaust branch pipe 18. This is to reach the turbine housing 15b of the supercharger.

In the example shown in FIG. 1, the first (# 1) cylinder 2 of the four cylinders 2 of the internal combustion engine 1 is located closest to the gathering portion of the exhaust branch pipe 18, so that the first (# 1) cylinder 2 1) Although the reducing agent injection valve 28 is attached to the exhaust port of the cylinder 2, when the cylinder 2 other than the first (# 1) cylinder 2 is located closest to the gathering portion of the exhaust branch pipe 18, The reducing agent injection valve 28 is attached to the exhaust port of the cylinder 2.

The reducing agent injection valve 28 is designed to penetrate a water jacket (not shown) formed in the cylinder head or to be mounted close to the water jacket, and use cooling water flowing through the water jacket. The reducing agent injection valve 28 may be cooled.

In such a reducing agent supply mechanism, when the flow control valve 30 is opened, the high-pressure fuel discharged from the fuel pump 6 is supplied through the reducing agent supply passage 29 to the reducing agent injection valve 28.
Is applied. When the pressure of the fuel applied to the reducing agent injection valve 28 reaches or exceeds the valve opening pressure, the reducing agent injection valve 2
8 is opened, and fuel as a reducing agent is injected into the exhaust branch pipe 18.

The reducing agent injected from the reducing agent injection valve 28 into the exhaust branch pipe 18 flows into the turbine housing 15b together with the exhaust gas flowing from the upstream of the exhaust branch pipe 18.
The exhaust gas and the reducing agent that have flowed into the turbine housing 15b are agitated and uniformly mixed by the rotation of the turbine wheel to form an exhaust gas having a rich air-fuel ratio.

The rich air-fuel ratio exhaust gas thus formed flows into the filter 20 from the turbine housing 15b via the exhaust pipe 19, and releases nitrogen oxides (NOx) absorbed by the filter 20. It will be reduced to nitrogen (N ₂ ).

Thereafter, when the flow control valve 30 is closed and the supply of the reducing agent from the fuel pump 6 to the reducing agent injection valve 28 is interrupted, the pressure of the fuel applied to the reducing agent injection valve 28 is increased. As a result, the reducing agent injection valve 28 closes, and the addition of the reducing agent into the exhaust branch pipe 18 is stopped.

The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 35 for controlling the internal combustion engine 1. The ECU 35 is a unit that controls the operating state of the internal combustion engine 1 according to the operating conditions of the internal combustion engine 1 and the driver's requirements.

The ECU 35 has a common rail pressure sensor 4
a, air flow meter 11, intake air temperature sensor 12, intake pipe pressure sensor 17, air-fuel ratio sensor 23, exhaust gas temperature sensor 24, NOx sensor 39, reducing agent pressure sensor 32, crank position sensor 33, water temperature sensor 34, accelerator opening sensor Various sensors such as 36 are connected via electric wiring, and output signals of the various sensors described above are input to the ECU 35.

On the other hand, the ECU 35 includes a fuel injection valve 3, an intake throttle actuator 14, an exhaust throttle actuator 22, an EGR valve 26, a flow control valve 30, a shutoff valve 31,
Etc. are connected via electric wiring, and
35 can be controlled.

Here, as shown in FIG. 3, the ECU 35 is connected to a C
A PU 351, a ROM 352, a RAM 353, a backup RAM 354, an input port 356, and an output port 357.
And an A / D converter (A / D) 355 connected to.

The input port 356 inputs the output signals of a sensor that outputs a digital signal, such as the crank position sensor 33, and converts those output signals to C.
The data is transmitted to the PU 351 and the RAM 353.

The input port 356 is connected to the common rail pressure sensor 4a, the air flow meter 11, the intake air temperature sensor 1
2. Intake pipe pressure sensor 17, air-fuel ratio sensor 23, exhaust temperature sensor 24, reducing agent pressure sensor 32, water temperature sensor 3.
4. A / A of a sensor that outputs a signal in the form of an analog signal, such as an accelerator opening sensor 36, a NOx sensor 39, etc.
D355, and outputs those signals to the CPU 3
51 and the RAM 353.

The output port 357 is connected to the fuel injection valve 3,
Intake throttle actuator 14, Exhaust throttle actuator 22, EGR valve 26, flow control valve 30, shut-off valve 3
1, connected to the urea addition control valve 38 and the like via electric wiring, and outputs a control signal output from the CPU 351 to the fuel injection valve 3, the intake throttle actuator 14, the exhaust throttle actuator 22, the EGR valve 26, the flow rate Regulating valve 3
0, which is transmitted to the shutoff valve 31 and the urea addition control valve 38.

The ROM 352 includes a fuel injection control routine for controlling the fuel injection valve 3, an intake throttle control routine for controlling the intake throttle valve 13, an exhaust throttle control routine for controlling the exhaust throttle valve 21, and EGR. An EGR control routine for controlling the valve 26, a PM combustion control routine for burning and removing PM trapped in the filter 20, and a nitrogen oxide (N) absorbed by the NOx storage reduction catalyst.
A storage NOx purification control routine for purifying Ox), a selective reduction NOx purification control routine for supplying a reducing agent to the selective reduction NOx catalyst 37 to reduce and purify nitrogen oxides (NOx), and an oxide of the filter 20 And an application program such as a NOx catalyst selection control routine for selecting a NOx catalyst to be used by the occlusion reduction type NOx catalyst and the selective reduction type NOx catalyst 37.

The ROM 352 stores various control maps in addition to the application programs described above. The control map includes, for example, a fuel injection amount control map indicating a relationship between an operation state of the internal combustion engine 1 and a basic fuel injection amount (basic fuel injection time), and a relation between the operation state of the internal combustion engine 1 and the basic fuel injection timing. Fuel injection timing control map,
An intake throttle valve opening control map showing the relationship between the operating state of the internal combustion engine 1 and the target opening of the intake throttle valve 13, and the exhaust throttle showing the relationship between the operating state of the internal combustion engine 1 and the target opening of the exhaust throttle valve 21. Valve opening control map, operating state of internal combustion engine 1 and EGR
EGR valve opening control map showing the relationship with the target opening of the valve 26, reducing agent addition control showing the relationship between the operating state of the internal combustion engine 1 and the target adding amount of the reducing agent (or the target air-fuel ratio of the exhaust). A flow control valve control map showing a relationship between a target addition amount of the reducing agent and a valve opening time of the flow control valve 30, and a urea addition showing a relationship between the target addition amount of urea and the valve opening time of the urea addition control valve 38. There are a control valve control map, a NOx catalyst selection control map showing the relationship between the amount of reducing agent supplied to the NOx storage reduction catalyst and the NOx catalyst to be used, and the like.

The RAM 353 stores an output signal from each sensor, a calculation result of the CPU 351 and the like. The calculation result is, for example, an engine speed calculated based on a time interval at which the crank position sensor 33 outputs a pulse signal. These data are rewritten to the latest data each time the crank position sensor 33 outputs a pulse signal.

The backup RAM 354 is a nonvolatile memory capable of storing data even after the operation of the internal combustion engine 1 is stopped.

The CPU 351 operates according to an application program stored in the ROM 352 to control fuel injection valve control, intake throttle control, exhaust throttle control, E
GR control, PM combustion control, stored NOx purification control, poisoning elimination control, selective reduction NOx purification control, NOx catalyst selection control, and the like are executed.

For example, in the fuel injection valve control, the CPU 35
1 first determines the amount of fuel injected from the fuel injection valve 3 and then determines the timing for injecting fuel from the fuel injection valve 3.

When determining the fuel injection amount, the CPU 35
1 reads the engine speed and the output signal (accelerator opening) of the accelerator opening sensor 36 stored in the RAM 353. The CPU 351 accesses the fuel injection amount control map and calculates a basic fuel injection amount (basic fuel injection time) corresponding to the engine speed and the accelerator opening. C
The PU 351 corrects the basic fuel injection time based on output signal values of the air flow meter 11, the intake air temperature sensor 12, the water temperature sensor 34, and the like, and determines a final fuel injection time.

When determining the fuel injection timing, the CPU 3
51 accesses a fuel injection start timing control map and calculates a basic fuel injection timing corresponding to the engine speed and the accelerator opening. The CPU 351 corrects the basic fuel injection timing by using output signal values of the air flow meter 11, the intake air temperature sensor 12, the water temperature sensor 34, and the like as parameters, and determines the final fuel injection timing.

When the fuel injection time and the fuel injection timing are determined, the CPU 351 compares the fuel injection timing with the output signal of the crank position sensor 33, and the output signal of the crank position sensor 33 starts the fuel injection. At the time coincident with the timing, application of drive power to the fuel injection valve 3 is started. The CPU 351 stops applying the driving power to the fuel injection valve 3 when the elapsed time from the start of the application of the driving power to the fuel injection valve 3 reaches the fuel injection time.

In the fuel injection control, when the operating state of the internal combustion engine 1 is an idle operating state, the CPU 351
Is a target idle speed of the internal combustion engine 1 using parameters such as the output signal value of the water temperature sensor 34 and the operating state of accessories that operate using the rotational force of a crankshaft, such as a compressor of a vehicle interior air conditioner. Is calculated. And
The CPU 351 performs feedback control of the fuel injection amount so that the actual idle speed matches the target idle speed.

In the intake throttle control, the CPU 351
Reads the engine speed and the accelerator opening stored in the RAM 353, for example. The CPU 351 accesses the intake throttle valve opening control map, and calculates a target intake throttle valve opening corresponding to the engine speed and the accelerator opening. C
The PU 351 applies drive power corresponding to the target intake throttle valve opening to the intake throttle actuator 14. At this time, the CPU 351 detects the actual opening of the intake throttle valve 13 and feeds back the intake throttle actuator 14 based on the difference between the actual opening of the intake throttle valve 13 and the target intake throttle valve opening. You may make it control.

In the exhaust throttle control, the CPU 351
For example, when the internal combustion engine 1 is in a warm-up operation state after a cold start, or when a vehicle interior heater is in an operating state, the exhaust throttle actuator is driven to close the exhaust throttle valve 21 in the valve closing direction. 22 is controlled.

In this case, the load on the internal combustion engine 1 increases, and the fuel injection amount is correspondingly increased. As a result, the calorific value of the internal combustion engine 1 increases, the warm-up of the internal combustion engine 1 is promoted, and the heat source of the vehicle interior heater is secured.

In the EGR control, the CPU 351
The engine speed stored in the RAM 353, the output signal of the water temperature sensor 34 (cooling water temperature), the accelerator opening sensor 3
The output signal of 6 (accelerator opening) and the like are read, and it is determined whether or not the execution condition of the EGR control is satisfied.

The above-mentioned EGR control execution conditions are as follows: the coolant temperature is equal to or higher than a predetermined temperature, the internal combustion engine 1 has been continuously operated for a predetermined time or more from the start, and the change amount of the accelerator opening is a positive value. Conditions such as certain conditions can be exemplified.

When it is determined that the EGR control execution condition described above is satisfied, the CPU 351 accesses the EGR valve opening control map using the engine speed and the accelerator opening as parameters, and And a target EGR valve opening corresponding to the accelerator opening. CPU
351 applies the drive power corresponding to the target EGR valve opening to the EGR valve 26. On the other hand, EGR as described above
If it is determined that the control execution condition is not satisfied,
U351 controls to keep the EGR valve 26 in the fully closed state.

In the EGR control, the CPU 351
EGR valve 2 using intake air amount of internal combustion engine 1 as a parameter
The so-called EGR valve feedback control for performing feedback control of the opening degree of the valve 6 may be performed.

In the EGR valve feedback control, for example, the CPU 351 determines the target intake air amount of the internal combustion engine 1 using the accelerator opening, the engine speed and the like as parameters. At this time, the relationship between the accelerator opening, the engine speed, and the target intake air amount may be mapped in advance, and the target intake air amount may be calculated from the map, the accelerator opening, and the engine speed. .

When the target intake air amount is determined by the above procedure, the CPU 351 reads out the output signal value (actual intake air amount) of the air flow meter 11 stored in the RAM 353, and reads the actual intake air amount and the target intake air amount. Compare with air volume.

When the actual intake air amount is smaller than the target intake air amount, the CPU 351 closes the EGR valve 26 by a predetermined amount. In this case, the amount of EGR gas flowing into the intake branch pipe 8 from the EGR passage 25 decreases, and accordingly, the amount of EGR gas drawn into the cylinder 2 of the internal combustion engine 1 decreases. As a result, the cylinder 2 of the internal combustion engine 1
The amount of fresh air sucked in increases by the amount of the decrease in the EGR gas.

On the other hand, when the actual intake air amount is larger than the target intake air amount, the CPU 351 opens the EGR valve 26 by a predetermined amount. In this case, the amount of EGR gas flowing into the intake branch pipe 8 from the EGR passage 25 increases, and accordingly, the amount of EGR gas drawn into the cylinder 2 of the internal combustion engine 1 increases. As a result, the amount of fresh air sucked into the cylinder 2 of the internal combustion engine 1 decreases by an amount corresponding to the increase in the EGR gas.

Next, the PM combustion control will be described.

In the PM combustion control, the CPU 351 executes a fuel addition control for adding fuel to the exhaust gas flowing into the filter 20.

In the fuel addition control, the CPU 351 determines whether or not the fuel addition control execution condition is satisfied at predetermined intervals. As the fuel addition control execution conditions, for example, the filter 20 is in an active state, the output signal value (exhaust gas temperature) of the exhaust gas temperature sensor 24 is equal to or lower than a predetermined upper limit, or the poisoning elimination control is executed. For example, the conditions such as whether or not there is not.

When it is determined that the fuel addition control execution condition described above is satisfied, the CPU 351 controls the flow control valve 30 so that the reducing agent injection valve 28 injects the fuel as the reducing agent. The air-fuel ratio of the exhaust gas flowing into the filter 20 is temporarily set to a predetermined target rich air-fuel ratio.

As a result, the air-fuel ratio of the exhaust gas flowing into the filter 20 alternates between lean and rich in a relatively short cycle. Then, the fuel flowing into the filter 20 is burned by the catalyst, and the heat causes the PM to ignite and burn.

Next, in the stored NOx purification control, the CPU 3
When the operating state of the internal combustion engine 1 is low-speed low-load, low-temperature combustion that makes the EGR gas ratio larger than usual is performed, and when operating conditions other than low-speed low-load, exhaust gas flowing into the NOx storage reduction catalyst is performed. The fuel addition control is executed to set the air-fuel ratio to a rich air-fuel ratio in a spike-like (short-time) manner at a relatively short cycle.

In the storage NOx purification control, the CPU 351
Determines whether or not the storage NOx purification control execution condition is satisfied at predetermined intervals. The storage NOx purification control execution conditions include, for example, the storage reduction type NOx shown in FIG.
Conditions such as whether the output signal value (exhaust gas temperature) of the exhaust gas temperature sensor 24 is within the temperature window of the catalyst, whether the poisoning elimination control is not executed, and the like can be exemplified. here,
FIG. 5 shows the selective reduction type NOx catalyst 37 and the storage reduction type NOx.
It is a figure showing the bed temperature of a catalyst and NOx purification rate. In the figure, the temperature region where the NOx purification rate is high is called a temperature window.

Further, in order to select whether to reduce NOx by low-temperature combustion or fuel addition control, the CPU 351 stores a map in which the engine speed and the accelerator opening stored in the ROM 352 in advance are used as parameters. Access and select low temperature combustion or fuel addition control.

Here, low-temperature combustion will be described.

Conventionally, EGR has been used to suppress the generation of NOx in an internal combustion engine. Since the EGR gas has a relatively high specific heat ratio and therefore can absorb a large amount of heat, the combustion temperature in the cylinder 2 decreases as the proportion of the EGR gas in the intake air increases. When the combustion temperature decreases, the amount of generated NOx also decreases.
The higher the gas ratio, the lower the NOx emission can be.

However, when the EGR gas ratio is increased, the amount of soot generation starts to increase rapidly at a certain ratio or more. Normal EGR control has a lower EG than soot starts to increase sharply.
It is performed at the R gas ratio.

However, when the EGR gas ratio is further increased, the soot sharply increases as described above. However, there is a peak in the amount of generated soot, and the EGR gas ratio further exceeds this peak. With a higher level, the soot will then start to decrease sharply, and will almost never occur.

This is because when the temperature of the fuel and its surrounding gas during combustion in the combustion chamber is lower than a certain temperature, the growth of hydrocarbons (HC) stops at a stage before reaching soot,
This is because when the temperature of the fuel and its surrounding gas becomes higher than a certain temperature, hydrocarbons (HC) grow into soot at a stretch.

Therefore, if the combustion during combustion in the combustion chamber and the gas temperature around the combustion are suppressed to a temperature at which the growth of hydrocarbons (HC) stops halfway, soot is not generated.
In this case, the endothermic effect of the gas around the fuel when the fuel burns greatly affects the temperature of the fuel and the gas around the fuel, and the amount of heat absorbed by the gas around the fuel, that is, EGR, depends on the amount of heat generated during the combustion of the fuel. By adjusting the gas ratio, it is possible to suppress the generation of soot.

The EGR gas ratio when performing low-temperature combustion is as follows:
ROM that is obtained in advance by experiments etc. and mapped
352. EGR based on this map
Perform feedback control.

On the other hand, hydrocarbons (HC) whose growth has stopped halfway before reaching soot can be purified by combustion using a NOx absorbent or the like.

As described above, low-temperature combustion is based on purifying hydrocarbons (HC) whose growth has stopped halfway before reaching soot with a NOx absorbent or the like. Therefore NOx
When the absorbent is not activated, the hydrocarbon (H
C) cannot be used for low-temperature combustion because it is released to the atmosphere without purification.

The fuel and the surrounding gas temperature during combustion in the cylinder 2 can be controlled to be lower than the temperature at which the growth of hydrocarbons (HC) stops halfway because of the relatively small engine load with a small amount of heat generated by combustion. Only when is low.

Therefore, in the present embodiment, the low temperature is used only when the internal combustion engine 1 is operated at a low rotation speed and a low load and is within the temperature window of the NOx storage reduction catalyst supported on the filter 20. Combustion control was performed.

Whether the temperature is within the temperature window region can be determined based on the output signal of the exhaust gas temperature sensor 24.

As described above, in low-temperature combustion, hydrocarbon (HC) as a reducing agent can be supplied to the NOx storage reduction catalyst while suppressing emission of PM represented by soot, and NOx can be reduced and purified. .

When it is determined that the fuel addition control execution condition is satisfied, the CPU 351 controls the flow control valve 30 so that the reducing agent injection valve 28 injects the fuel as the reducing agent in a spike manner. The storage reduction type NO
x The air-fuel ratio of the exhaust gas flowing into the catalyst is temporarily set to a predetermined target rich air-fuel ratio.

More specifically, the CPU 351
Engine speed, accelerator opening sensor 3 stored in
6 output signal (accelerator opening), air flow meter 11
The output signal value (intake air amount), fuel injection amount, and the like are read out. Further, the CPU 351 accesses the reducing agent addition amount control map of the ROM 352 using the engine speed, the accelerator opening, the intake air amount, and the fuel injection amount as parameters, and sets the air-fuel ratio of the exhaust gas to a preset target rich air-fuel ratio. The addition amount (target addition amount) of the reducing agent required for obtaining the fuel ratio is calculated.

Subsequently, the CPU 351 accesses the flow control valve control map of the ROM 352 using the target addition amount as a parameter, and controls the flow control valve necessary for injecting the target addition amount of the reducing agent from the reducing agent injection valve 28. The valve opening time of 30 (target valve opening time) is calculated.

When the target opening time of the flow control valve 30 is calculated, the CPU 351 opens the flow control valve 30.
In this case, since the high-pressure fuel discharged from the fuel pump 6 is supplied to the reducing agent injection valve 28 via the reducing agent supply path 29, the pressure of the fuel applied to the reducing agent injection valve 28 is equal to or higher than the valve opening pressure. , And the reducing agent injection valve 28 opens.

The CPU 351 closes the flow control valve 30 when the target valve opening time has elapsed since the time when the flow control valve 30 was opened. In this case, since the supply of the reducing agent from the fuel pump 6 to the reducing agent injection valve 28 is shut off, the pressure of the fuel applied to the reducing agent injection valve 28 becomes lower than the valve opening pressure, and the reducing agent injection valve 28 is closed. Give a valve.

When the flow control valve 30 is opened for the target opening time in this manner, the target amount of fuel is supplied to the reducing agent injection valve 2.
8, the fuel is injected into the exhaust branch pipe 18. Then, the reducing agent injected from the reducing agent injection valve 28 mixes with the exhaust gas flowing from the upstream of the exhaust branch pipe 18 to form an air-fuel mixture having a target rich air-fuel ratio and flows into the NOx storage reduction catalyst.

As a result, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst alternates between "lean" and "spike-like target rich air-fuel ratio" alternately in a relatively short cycle. Thus, the NOx storage reduction catalyst alternately repeats the absorption, release and reduction of nitrogen oxides (NOx) in a short period.

In the selective reduction NOx purification control, the CPU 35
1 executes urea addition control for adding urea as a reducing agent to exhaust gas flowing into the selective reduction type NOx catalyst 37.

In the urea addition control, the CPU 351 determines whether or not the urea addition control execution condition is satisfied at predetermined intervals. Examples of the urea addition control execution condition include a condition such as whether the output signal value (exhaust gas temperature) of the exhaust gas temperature sensor 24 is within the temperature window of the selective reduction type NOx catalyst shown in FIG.

If it is determined that the urea addition control execution condition described above is satisfied, the CPU 351 controls the urea addition control valve 38 to inject urea as a reducing agent from the urea addition control valve 38. By the selective reduction type N
Urea is added to the exhaust gas flowing into the Ox catalyst 37.

More specifically, the CPU 351
Engine speed, accelerator opening sensor 3 stored in
6 output signal (accelerator opening), air flow meter 11
, The output signal value (NOx concentration) of the NOx sensor 39, and the like. Further, the CPU 35
1 accesses the urea addition amount control map of the ROM 352 using the above-described engine speed, accelerator opening, intake air amount, and NOx concentration as parameters, and calculates the required urea addition amount (target urea addition amount). .

Subsequently, the CPU 351 accesses the urea addition control valve control map of the ROM 352 using the target urea addition amount as a parameter, and supplies the urea necessary for injecting the target urea addition amount urea from the urea addition control valve 38. The valve opening time (target valve opening time) of the addition control valve 38 is calculated.

When the target opening time of the urea addition control valve 38 is calculated, the CPU 351 opens the urea addition control valve 38.

The CPU 351 closes the urea addition control valve 38 when the target valve opening time has elapsed since the time when the urea addition control valve 38 was opened.

As described above, when the urea addition control valve 38 is opened for the target valve opening time, the urea of the target urea addition amount is injected from the urea addition control valve 38 into the exhaust pipe 19. Then, the urea injected from the urea addition control valve 38 is mixed with the exhaust gas flowing from the upstream of the exhaust pipe 19 and flows into the selective reduction type NOx catalyst 37 to perform the selective reduction of NOx.

Next, the NOx catalyst selection control according to the present invention will be described.

As is apparent from FIG. 5, there is a region where the temperature windows of the selective reduction type NOx catalyst 37 and the storage reduction type NOx catalyst partially overlap. In the region where the temperature windows overlap, a high NOx purification rate can be obtained regardless of which of the selective reduction NOx catalyst 37 and the storage reduction NOx catalyst is selected.

However, when the storage reduction type NOx catalyst is selected in this region, the amount of fuel added as a reducing agent increases as the internal combustion engine 1 enters a state of high rotation and high load, and the deterioration of fuel efficiency is reduced. Will be invited. Therefore, in the present invention, in the temperature region where the temperature windows overlap, an amount of fuel exceeding a predetermined threshold value is stored and reduced.
When added to the catalyst, the catalyst for purifying NOx is switched to the selective reduction type NOx catalyst 37. That is, the addition of fuel to the NOx storage reduction catalyst is stopped, and urea as a reducing agent is added to the NOx selective reduction catalyst 37.

Specifically, it is determined whether or not the stored NOx purification control is being executed and the addition amount (target addition amount) of the reducing agent calculated to execute the fuel addition control exceeds a threshold value.

In order to make this determination, CPU 351
By accessing the M353 and the ROM 352, the target addition amount of the reducing agent and its threshold value are read and compared.

When the low-temperature combustion control is being performed, the difference between the fuel injection amount in the cylinder 2 at that time and the fuel injection amount in the cylinder 2 when the low-temperature combustion is not performed is determined by the target addition of the reducing agent. The quantity is compared with a threshold.

If the target addition amount of the reducing agent exceeds the threshold value, the control for purifying stored NOx is stopped, and the control for purifying NOx is selectively started.

At this time, the control is performed on the premise that the bed temperature of the selective reduction type NOx catalyst 37 is within the temperature window, but the output signal value (exhaust temperature) of the exhaust temperature sensor 24 is selectively reduced for confirmation. The condition that the temperature is within the temperature window of the type NOx catalyst 37 may be added.

When the occlusion NOx purification control is being performed in the region where the temperature windows overlap, and low-temperature combustion is not being performed, that is, when the fuel addition control is being performed, the selective reduction NOx purification control is not performed. It may be performed. This is because low-temperature combustion requires less fuel consumption than fuel addition control.

In the present embodiment, instead of the fuel addition control, fuel is injected again at a time when the fuel for engine output is not the engine output after the main injection of the fuel for engine output into the cylinder 2 of the internal combustion engine 1. Sub injection may be performed.

The reason for this is that if an attempt is made to shift the air-fuel ratio toward the rich air-fuel ratio by only the main injection, a problem such as smoke may occur. Further, when the amount of the main injection is increased, the combustion of the fuel becomes the engine output, so that the torque fluctuates and the operating state deteriorates. Therefore, the sub-injection is performed in an expansion stroke or the like that hardly affects the engine output after the main injection, and the air-fuel ratio is shifted to the rich air-fuel ratio side.

The fuel injected by the sub-injection burns in the cylinder 2 and lowers the oxygen concentration of the gas in the cylinder 2. Cylinder 2
The gas combusted therein becomes exhaust gas, passes through the exhaust pipe 19, reaches the NOx storage reduction catalyst, and supplies hydrocarbon (HC) as a reducing agent to the NOx storage reduction catalyst.

If the relationship between the accelerator opening, the engine speed, and the sub-injection amount or the sub-injection timing is mapped in advance and stored in the ROM 352, the sub injection amount and the injection timing can be obtained by comparing the map with the accelerator opening. And the engine speed. Further, the cooling water temperature of the internal combustion engine 1 may be added as a parameter.

[0168]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the NOx storage-reduction type NOx catalyst and the selective reduction type NOx catalyst are provided. N based on agent requirements
Since the Ox catalyst is selected, the consumption of the reducing agent can be reduced.

Accordingly, when fuel is used as the reducing agent, the fuel consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which an exhaust gas purification device for an internal combustion engine according to the present invention is applied, and an intake and exhaust system thereof.

FIG. 2A is a diagram illustrating a NOx absorption mechanism of a storage reduction type NOx catalyst. (B) is a diagram for explaining the NOx release mechanism of the NOx storage reduction catalyst.

FIG. 3 is a block diagram showing an internal configuration of an ECU.

FIG. 4A is a diagram showing a cross section of a particulate filter in a lateral direction. (B) is a figure which shows the longitudinal direction cross section of a particulate filter.

FIG. 5 is a diagram showing a bed temperature and a NOx purification rate of a selective reduction type NOx catalyst and a storage reduction type NOx catalyst.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Cylinder 3 ... Fuel injection valve 4 ... Common rail 5 ... Fuel supply pipe 6 ... Fuel pump 18 ... Exhaust branch pipe 19 ... exhaust pipe 20 ... particulate filter 21 ... exhaust throttle valve 23 ... air-fuel ratio sensor 25 ... EGR passage 26 ... EGR valve 27 ... EGR cooler 28 ... reducing agent Injection valve 29 ... Reducing agent supply path 30 ... Flow regulating valve 31 ... Shutoff valve 32 ... Reducing agent pressure sensor 33 ... Crank position sensor 34 ... Water temperature sensor 35 ... ECU 36 ... accelerator opening sensor 37 ... selective reduction type NOx catalyst 38 ... urea addition control valve 39 ... NOx sensor 351 ... CPU 352 ... ROM 353 ... RAM 354 ... backup RAM 50 ... Exhaust inflow passages 51 ... exhaust outflow passages 54 ... partition wall

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) F01N 3/24 F01N 3/24 E 3/28 301C 3/28 301 F02M 25/07 A F02M 25/07 B B01D 53 / 36 101Z (72) Inventor Shinya Hirota 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Koichiro Nakaya 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 3G062 AA01 AA03 AA05 AA06 BA02 BA04 BA05 BA06 CA01 CA02 CA03 CA07 CA10 EA12 EB15 ED08 FA08 FA13 GA01 GA02 GA06 GA08 GA09 GA12 GA14 GA17 3G090 AA03 BA01 DA01 DA09 DA10 DA11 DA12 DA14 DA18 DA20 EA02 A05 A10 A06 A05 A06 A07 A10 A05 A10 A05 A10 A11 AB13 BA00 BA03 BA11 BA14 BA15 BA33 CA13 CA17 CA18 CB02 CB03 CB07 CB08 DA04 DA08 DB10 DC01 EA00 EA01 EA05 EA06 EA07 EA15 EA16 EA17 EA31 EA33 EA34 FA02 FA04 FA06 FA12 FB02 FB10 FB11 GB02 GB03 FC04 GB04 GB05W GB06 W GB09X GB10X GB16X GB17X HA08 HA14 HA16 HA36 HA37 HB05 HB06 4D048 AA06 AB02 AC03 BA03X BA14X BA15X BA18X BA30X CC27 DA01 DA02 DA08 DA10

Claims (4)

[Claims] 1. A selective reduction NOx catalyst provided in an exhaust passage of an internal combustion engine for selectively reducing nitrogen oxides in the presence of a reducing agent derived from ammonia, and an exhaust gas provided in an exhaust passage of the internal combustion engine in an oxygen-excess atmosphere. A storage-reduction NOx catalyst that releases and reduces the stored nitrogen oxides when the nitrogen oxides therein are stored and the oxygen concentration decreases, and a selective reduction-type NOx catalyst that supplies a reducing agent to the selective reduction-type NOx catalyst Means for supplying a reducing agent to the NOx storage reduction catalyst, and a reducing agent supply means for storing NOx catalyst when the amount of the reducing agent supplied to the NOx storage reduction catalyst exceeds a threshold value. The storage reduction type NOx catalyst reducing agent supply means stops supplying the reducing agent, and the selective reduction type NOx catalyst reducing agent supply means supplies a reducing agent derived from ammonia to the selective reduction type NOx catalyst. Exhaust purification system of an internal combustion engine that. 2. The storage-reduction type NOx catalyst-reducing agent supply means is a sub-injection for injecting fuel again at a time when the fuel for engine output does not become the engine output after the main injection of the fuel for engine output to the internal combustion engine. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein: 3. An EGR device for recirculating a part of exhaust gas to an intake system of an internal combustion engine, wherein said storage-reduction type NOx catalyst reducing agent supply means has a peak value of an amount of particulate matter generated.
2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the combustion is performed at a low temperature by adding more EGR gas than the GR gas amount to burn the fuel. 4. The internal combustion engine according to claim 1, wherein said storage agent-type NOx catalyst reducing agent supply means adds a reducing agent to the exhaust passage of an internal combustion engine by a reducing agent addition nozzle for adding the reducing agent. Engine exhaust purification device.