JP2003049681A - Exhaust purification device for internal combustion engine - Google Patents

Abstract

(57) Abstract: An object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine in which a pre-catalyst having an oxygen storage capacity is arranged upstream of a NOx catalyst without reducing the purifying performance of the NOx catalyst. It is an object to improve the purification ability of a catalyst. According to the present invention, a pre-catalyst having an oxygen storage capacity is disposed upstream of a NOx catalyst, and an air-fuel ratio reduction control is performed to consume stored oxygen of the pre-catalyst immediately before execution of rich spike control. The exhaust gas purifying device for an internal combustion engine that is executed is characterized in that lean spike control for temporarily increasing the exhaust air-fuel ratio is performed during execution of rich spike control.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly, to an exhaust gas for an internal combustion engine in which an NOx catalyst is provided in an exhaust system of the internal combustion engine and a front catalyst having an oxygen storage capacity is arranged upstream of the NOx catalyst. Purification device

[0002]

2. Description of the Related Art In an internal combustion engine mounted on an automobile, a lean burn internal combustion engine capable of burning an air-fuel mixture having a higher air-fuel ratio than the stoichiometric air-fuel ratio (oxygen-excessive air-fuel mixture) is used in order to reduce fuel consumption. Development is in progress. Correspondingly, nitrogen oxides (NOx) emitted from the lean-burn internal combustion engine
There is a demand for a technology for efficiently purifying the exhaust gas and a technology for improving exhaust emission at the time of starting the lean-burn internal combustion engine and / or immediately after the starting.

In response to such demands, conventionally, a NOx catalyst is arranged in the exhaust passage of a lean-burn internal combustion engine, and a front catalyst having a smaller heat capacity than the NOx catalyst is arranged in the exhaust passage upstream of the NOx catalyst. Is proposed.

As an example of the above technique, there is known an exhaust emission control device for a lean burn internal combustion engine as disclosed in Japanese Patent Laid-Open No. 2000-27677.

In the exhaust gas purification apparatus for a lean burn internal combustion engine described in the above publication, an NOx storage reduction catalyst is arranged in an exhaust passage of the lean combustion internal combustion engine, and exhaust gas upstream of the NOx storage reduction catalyst is disposed. In an exhaust emission control device in which a three-way catalyst having an oxygen storage capacity is arranged in a passage, an exhaust air-fuel ratio is made lower than an air-fuel ratio at the time of rich spike operation in a predetermined period immediately after the start of rich spike operation.

The lean-burn type internal combustion engine exhaust gas purification apparatus thus constructed is configured so that the air-fuel ratio of the exhaust gas is made lower than that at the time of rich spike operation immediately before the rich spike operation is substantially started. To immediately release all of the oxygen stored in the air-fuel ratio and to switch the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst from the lean air-fuel ratio to the desired rich spike air-fuel ratio or stoichiometric air-fuel ratio without delay. Is.

[0007]

By the way, according to the conventional technique as described above, when the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is switched from the lean air-fuel ratio to the predetermined rich air-fuel ratio or the theoretical air-fuel ratio. Since all the stored oxygen of the three-way catalyst is released, the oxidizing ability of the three-way catalyst during the rich spike operation is reduced, and the purification rate of hydrocarbons (HC) and carbon monoxide (CO) may be reduced. is there.

Further, when the internal combustion engine is operated at the stoichiometric air-fuel ratio after the rich spike operation is completed, there is no opportunity to store oxygen in the three-way catalyst, so hydrocarbons (HC) in the exhaust gas are discharged.
It may be difficult to sufficiently oxidize and purify carbon monoxide (CO).

The present invention has been made in view of the above-mentioned various problems, and an NOx catalyst provided in an exhaust passage of an internal combustion engine and an oxygen storage provided in an exhaust passage upstream of the NOx catalyst. It is an object of the present invention to provide a technique capable of improving the purification performance of a pre-catalyst without deteriorating the purification performance of an NOx catalyst in an exhaust gas purification apparatus for an internal combustion engine equipped with a pre-catalyst having capability. .

[0010]

The present invention adopts the following means in order to solve the above problems. That is, an exhaust gas purification apparatus for an internal combustion engine according to the present invention is provided with an internal combustion engine capable of combusting an air-fuel mixture in an oxygen excess state, an exhaust passage of the internal combustion engine, and nitrogen oxidation in the exhaust gas in the presence of a reducing agent. A NOx catalyst for purifying substances, a pre-catalyst arranged upstream of the NOx catalyst in the exhaust passage and having an oxygen storage capacity, and an exhaust gas when it is necessary to purify nitrogen oxides by the NOx catalyst. A rich spike control executing means for reducing the fuel ratio to a first rich air-fuel ratio lower than the stoichiometric air-fuel ratio, and an exhaust air-fuel ratio from the first rich air-fuel ratio before the rich spike control executing means executes the rich spike control. The air-fuel ratio reduction control executing means for decreasing the air-fuel ratio to the second low rich air-fuel ratio, and the exhaust air temporarily during the execution period of the rich spike control by the rich spike control executing means. It includes a lean spike control execution means for increasing the ratio, a.

In this exhaust gas purifying apparatus for an internal combustion engine, a precatalyst having an oxygen storage capacity is arranged upstream of the NOx catalyst, and immediately before the rich spike control is executed, the air-fuel ratio lowering control is performed so as to consume the stored oxygen of the precatalyst. The greatest feature of the exhaust gas purification apparatus for an internal combustion engine in which is performed is to temporarily increase the exhaust air-fuel ratio during the execution period of the rich spike control.

In such an exhaust gas purification apparatus for an internal combustion engine, NO
When purifying nitrogen oxides (NOx) by the x catalyst, first, the air-fuel ratio reduction control execution means sets the exhaust air-fuel ratio to the second air-fuel ratio lower than the air-fuel ratio (first rich air-fuel ratio) during the rich spike control. Reduce to rich air-fuel ratio. At this time, the oxygen stored in the precatalyst is immediately consumed.

Then, the rich spike control execution means reduces the exhaust air-fuel ratio to the first rich air-fuel ratio. At this point, since all of the oxygen stored in the precatalyst has already been consumed, oxygen is not released from the precatalyst and the air-fuel ratio of the exhaust gas discharged from the precatalyst is the first rich air-fuel ratio. It comes to almost match the fuel ratio. As a result, the exhaust gas of the first rich air-fuel ratio will flow into the NOx catalyst, and the NOx catalyst will be able to suitably purify nitrogen oxides (NOx).

On the other hand, the lean spike control execution means temporarily increases the air-fuel ratio of the exhaust gas during the rich spike control execution period.

In this case, since the exhaust gas having a high air-fuel ratio, that is, the exhaust gas having a high oxygen concentration, temporarily flows into the precatalyst, the oxygen storage capacity of the precatalyst comes to function.
As a result, the oxidizing ability of the precatalyst is activated, and hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas are oxidized and purified by the precatalyst.

The lean spike control by the lean spike control executing means may be executed only once during the execution period of the rich spike control, or may be executed a plurality of times at predetermined time intervals. Good. Further, the lean spike control execution means preferably increases the air-fuel ratio of the exhaust gas within a range in which the amount of oxygen contained in the exhaust gas does not exceed the oxygen storage capacity of the precatalyst. This is because if the amount of oxygen in the exhaust exceeds the oxygen storage capacity of the precatalyst due to the execution of lean spike control, excess oxygen that cannot be stored in the precatalyst will flow out from the precatalyst, and the NOx catalyst will be exhausted. This is because there is a risk that the air-fuel ratio of the exhaust gas flowing into the engine will become unnecessarily high.

Next, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention may employ 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 is an internal combustion engine capable of burning an air-fuel mixture in an oxygen excess state, and is disposed in an exhaust passage of the internal combustion engine, and nitrogen oxidation in the exhaust gas in the presence of a reducing agent is performed. A NOx catalyst for purifying substances, and an exhaust passage upstream of the NOx catalyst,
A pre-catalyst having an oxygen storage capacity, a rich spike control execution means for reducing an exhaust air-fuel ratio below a stoichiometric air-fuel ratio when purifying nitrogen oxides by the NOx catalyst, and a rich spike control by the rich spike control execution means. When the internal combustion engine is operated at an air-fuel ratio equal to or less than the stoichiometric air-fuel ratio after execution of the control, lean spike control execution means for temporarily increasing the exhaust air-fuel ratio above the theoretical air-fuel ratio when the execution of the rich spike control is completed. And may be provided.

This exhaust gas purification device for an internal combustion engine is an exhaust gas purification device for an internal combustion engine in which a front catalyst having an oxygen storage capacity is arranged upstream of a NOx catalyst, and when the execution of the rich spike control is completed, the internal combustion engine is When operating at the stoichiometric air-fuel ratio, the greatest feature is that the exhaust air-fuel ratio is temporarily set to the lean air-fuel ratio when the execution of the rich spike control ends.

In such an exhaust emission control system for an internal combustion engine, when the internal combustion engine is operated at the stoichiometric air-fuel ratio after the rich spike control executing means executes the rich spike control, the lean spike control executing means executes the rich spike control. At the end, the exhaust air-fuel ratio is temporarily made higher than the theoretical air-fuel ratio.

When the internal combustion engine is operated at the stoichiometric air-fuel ratio after the rich spike control is executed, there is no opportunity to store oxygen in the precatalyst, so the internal combustion engine is operated at the stoichiometric air-fuel ratio. There is a possibility that the oxidizing ability of the pre-catalyst at this time is reduced, in the exhaust gas purification device of the internal combustion engine according to the present invention, since the exhaust air-fuel ratio is made higher than the theoretical air-fuel ratio at the time when the execution of the rich spike control is completed. At that time, oxygen is stored in the precatalyst, and even if the internal combustion engine is subsequently operated at the stoichiometric air-fuel ratio, the oxidizing ability of the precatalyst does not decrease.

The lean spike control execution means preferably increases the air-fuel ratio of the exhaust gas within a range in which the amount of oxygen contained in the exhaust gas does not exceed the oxygen storage capacity of the precatalyst. This is because if the amount of oxygen in the exhaust exceeds the oxygen storage capacity of the precatalyst due to the execution of lean spike control, excess oxygen that cannot be stored in the precatalyst will flow out from the precatalyst, and the NOx catalyst will be exhausted. This is because there is a risk that the air-fuel ratio of the exhaust gas flowing into the engine will become unnecessarily high.

Further, the NOx catalyst of the present invention stores nitrogen oxides (NOx) in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio and keeps the air-fuel ratio of the inflowing exhaust gas below the stoichiometric air-fuel ratio. Occlusion-reduction type NOx catalysts that reduce the stored nitrogen oxides (NOx) to nitrogen (N ₂ ) at the same time, and when the inflowing exhaust gas has a high oxygen concentration and a reducing agent is present in the exhaust gas. Examples thereof include a selective reduction type NOx catalyst that reduces or decomposes nitrogen oxides (NOx).

As a method of executing the rich spike control in the present invention, in the case of a direct injection type internal combustion engine, a method of injecting a fuel that does not contribute to combustion from a fuel injection valve of a cylinder in an expansion stroke or an exhaust stroke, A method of attaching a fuel addition nozzle to the exhaust port or the exhaust passage to add fuel into the exhaust gas can be exemplified.

Further, as a method for executing the lean spike control during the execution period of the rich spike control, a method for stopping the injection or addition of the fuel from the fuel injection valve, the fuel addition nozzle or the like as described above or during the exhaust is performed. A method of adding secondary air can be exemplified, and a method of executing lean spike control at the end of execution of rich spike control can be exemplified by a method of supplying secondary air into exhaust gas.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Specific embodiments of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings.

<First Embodiment> First, a first embodiment of an exhaust purification system for an internal combustion engine according to the present invention will be described with reference to FIGS.

FIG. 1 is a diagram showing a schematic structure of an internal combustion engine to which the present invention is applied and an intake / exhaust system thereof. The internal combustion engine 1 shown in FIG. 1 includes a plurality of cylinders 21 and each cylinder 21
This is a four-cycle cylinder injection gasoline engine equipped with a fuel injection valve 32 that directly injects fuel.

The internal combustion engine 1 comprises a cylinder block 1b having a plurality of cylinders 21 and a cooling water passage 1c formed therein, and a cylinder head 1a fixed to the upper portion of the cylinder block 1b.

A crankshaft 23, which is an engine output shaft, is rotatably supported on the cylinder block 1b,
The crankshaft 23 is connected to a piston 22 slidably mounted in each cylinder 21 via a connecting rod.

Above the piston 22, the piston 2
A combustion chamber 24 surrounded by the top surface of No. 2 and the wall surface of the cylinder head 1a is formed. A spark plug 25 is attached to the cylinder head 1a so as to face the combustion chamber 24,
An igniter 25a for applying a drive current to the spark plug 25 is connected to the spark plug 25.

In the cylinder head 1a, the open ends of the two intake ports 26 and the open ends of the two exhaust ports 27 are formed so as to face the combustion chamber 24, and their injection holes face the combustion chamber 24. A fuel injection valve 32 is attached.

A secondary air injection nozzle 53 is attached to the cylinder head 1a so that its injection hole can be seen in the exhaust port 27. The secondary air injection nozzle 53 is connected to an air pump (not shown) and supplies the secondary air supplied from the air pump into the exhaust port 27.

Each open end of the intake port 26 is adapted to be opened and closed by an intake valve 28 which is supported by the cylinder head 1a so as to be able to move back and forth.
The intake side camshaft 30 rotatably supported by the cylinder head 1a drives the cylinder head 1a to move back and forth.

Each opening end of the exhaust port 27 is opened and closed by an exhaust valve 29 which is supported by the cylinder head 1a so as to be movable back and forth. The exhaust valves 29 are rotatably supported by the cylinder head 1a. The exhaust side camshaft 31 is driven to move back and forth.

The intake side camshaft 30 and the exhaust side camshaft 31 are connected to the crankshaft 23 via a timing belt (not shown).

Two intake ports 2 communicating with each cylinder 21
The intake port 26 of one of the
a straight port having a linear flow path from the opening end formed on the outer wall to the opening end facing the combustion chamber 24, the other intake port 26 is formed from the opening end on the outer wall of the cylinder head 1a. It is configured by a helical port having a flow path formed so as to swirl toward the open end of the combustion chamber 24 in a plane perpendicular to the axial direction of the cylinder 21.

Each intake port 26 described above communicates with each branch pipe of the intake branch pipe 33 attached to the cylinder head 1a.

In the intake branch pipe 33, a branch pipe communicating with the straight port of the two intake ports 26 corresponding to each cylinder 21 is provided with a swirl control valve 37 for adjusting the flow rate in the branch pipe. There is.

The swirl control valve 37 is composed of a stepper motor or the like, and is used to open / close the swirl control valve 37 according to the magnitude of the applied current.
CV actuator 37a and SCV that outputs an electrical signal corresponding to the opening degree of the swirl control valve 37
A position sensor 37b is attached.

The intake branch pipe 33 is connected to a surge tank 34, and the surge tank 34 has the intake pipe 3 connected thereto.
5 is connected. The intake pipe 35 is connected to an air cleaner 36 for removing dust and the like in the intake air.

The intake pipe 35 is provided with a throttle valve 39 for adjusting the flow rate of fresh air flowing through the intake pipe 35. The throttle valve 39 is composed of a stepper motor or the like, and the throttle valve 3 is connected according to the magnitude of the applied current.
A throttle actuator 40 for opening and closing 9;
A throttle position sensor 41 that outputs an electric signal corresponding to the opening of the throttle valve 39 is attached.

The throttle valve 39 has an accelerator lever (not shown) which rotates in conjunction with the accelerator pedal 42.
An accelerator position sensor 43 that outputs an electric signal corresponding to the rotational position of the accelerator lever (the depression amount of the accelerator pedal 42) is attached to the accelerator lever.
Is attached.

Intake pipe 3 upstream of the throttle valve 39
An air flow meter 4 for outputting an electric signal corresponding to the mass of fresh air (intake air mass) flowing in the intake pipe 35
4 is attached.

On the other hand, each exhaust port 27 of the internal combustion engine 1
Communicate with each branch pipe of the exhaust branch pipe 45 attached to the cylinder head 1a. The exhaust branch pipe 45 is connected to a three-way catalyst 46 as a pre-catalyst according to the present invention.

The above-mentioned three-way catalyst 46 includes, for example, a ceramic carrier made of cordierite formed in a lattice shape having a plurality of through holes extending in the exhaust flow direction, and a catalyst layer coated on the surface of the ceramic carrier. It consists of

The catalyst layer is formed, for example, by supporting a platinum-rhodium (Pt-Rh) -based noble metal catalyst substance on the surface of porous alumina (Al ₂ O ₃ ) having a large number of pores. .

Furthermore, in the catalyst layer of the three-way catalyst 46, a metal component such as cerium (Ce) is supported in addition to the noble metal catalyst substance. In this case, the three-way catalyst 46 is the three-way catalyst 4
When the air-fuel ratio of the exhaust flowing into 6 is higher than the theoretical air-fuel ratio (that is, when the exhaust air-fuel ratio is the lean air-fuel ratio),
By utilizing the fact that cerium combines with oxygen (O ₂ ) in the exhaust gas to form cerium oxide (ceria), oxygen (O ₂ ) is stored, and the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 46 is theoretical. When the air-fuel ratio is lower than the air-fuel ratio (that is, when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or the rich air-fuel ratio), oxygen (O ₂ ) and metal cerium (Ce) are decomposed into oxygen ( It has a so-called oxygen storage capacity (OSC), which releases O ₂ ).

Three way catalyst 46 constructed as described above
Is activated at a temperature equal to or higher than a predetermined activation temperature (for example, 300 °), and if the air-fuel ratio of the inflowing exhaust gas is within a predetermined range (catalyst purification window) near the stoichiometric air-fuel ratio, hydrocarbons contained in the exhaust gas ( HC) and carbon monoxide (CO) react with oxygen (O ₂ ) in the exhaust gas to produce water (H ₂ O) and carbon dioxide (C).
At the same time as it is oxidized to O ₂ ), nitrogen oxides (NO
x) is exhausted hydrocarbon (HC) and carbon monoxide (C
O) to react with water (H ₂ O), carbon dioxide (CO ₂ ),
Reduce to nitrogen (N ₂ ).

An exhaust pipe 47 is connected to the above-mentioned three-way catalyst 46, and the exhaust pipe 47 is connected downstream to a muffler (not shown). A storage-reduction type NOx catalyst 48 as a NOx catalyst according to the present invention is arranged in the middle of the exhaust pipe 47.

The storage reduction type NOx catalyst 48 has, for example, a flow path in which the upstream end is open and the downstream end is closed, and the upstream end is closed and the downstream end is closed. It is composed of a carrier in which the channels open to each other are alternately arranged so as to form a honeycomb shape, and the NOx absorbent loaded on the wall surface of each channel.

The carrier is made of, for example, a porous ceramic. Examples of the NOx absorbent include alkali metals such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), and lanthanum. An example is one that includes at least one selected from rare earths such as (La) and yttrium (Y), and a noble metal such as platinum (Pt).

When the air-fuel ratio of the exhaust gas flowing into the storage-reduction type NOx catalyst 48 is a lean air-fuel ratio, the NOx catalyst 48 of the storage reduction type constructed as described above produces nitrogen oxides (NOx) in the exhaust gas. When the oxygen concentration of the exhaust gas, which is absorbed by the NOx absorbent and flows into the NOx storage reduction catalyst 48, decreases, the nitrogen oxides (NO
x) is released. At that time, if a reducing agent is present in the NOx storage reduction catalyst 48, the nitrogen oxide (NOx) released from the NOx absorbent reacts with the reducing agent to reduce and purify nitrogen (N ₂ ). To be done.

A first electric signal is output to the exhaust branch pipe 45 in accordance with the air-fuel ratio of the exhaust gas flowing in the exhaust branch pipe 45, in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 46.
The air-fuel ratio sensor 49a is attached to the storage reduction type N.
The exhaust pipe 47 upstream of the Ox catalyst 48 is connected to the exhaust pipe 47.
The air-fuel ratio of the exhaust gas flowing inside, in other words, the storage reduction type N
A second air-fuel ratio sensor 49b that outputs an electric signal corresponding to the air-fuel ratio of the exhaust flowing into the Ox catalyst 48 is attached.

Further, the internal combustion engine 1 has a crankshaft 2
A crank position sensor 51 consisting of a timing rotor 51a attached to the end of No. 3 and an electromagnetic pickup 51b attached to a cylinder block 1b near the timing rotor 51a, and a cooling water passage 1c formed inside the internal combustion engine 1 A water temperature sensor 52 attached to the cylinder block 1b to detect the temperature of the cooling water is provided.

An electronic control unit (ECU, hereinafter referred to as ECU) 20 for controlling the operating state of the internal combustion engine 1 is installed in the internal combustion engine 1 thus constructed.

The ECU 20 includes an SCV position sensor 37b, a throttle position sensor 41, an accelerator position sensor 43, an air flow meter 44, and a first
Various sensors such as the air-fuel ratio sensor 49a, the second air-fuel ratio sensor 49b, the crank position sensor 51, and the water temperature sensor 52 are connected via electrical wiring, and the output signal of each sensor is input to the ECU 20. There is.

The ECU 20 includes an igniter 25a,
Fuel injection valve 32, SCV actuator 37a, throttle actuator 40, secondary air injection nozzle 53
Etc. are connected via electrical wiring, and the ECU 20 uses the output signal values of various sensors as parameters to igniter 25a,
Fuel injection valve 32, SCV actuator 37a, throttle actuator 40, secondary air injection nozzle 53
Can be controlled.

Here, as shown in FIG. 5, the ECU 20 connects the CPs connected to each other by the bidirectional bus 200.
A U 201, a ROM 202, a RAM 203, a backup RAM 204, an input port 205, an output port 206, and an A / D converter (A / D) 207 connected to the input port 205.

The input port 205 inputs the output signals of a sensor that outputs a digital signal format signal such as the crank position sensor 51, and outputs those output signals to C
It is transmitted to the PU 201 or the RAM 203.

The input port 205 includes an SCV position sensor 37b, a throttle position sensor 41, an accelerator position sensor 43, an air flow meter 44,
Output signals of sensors that output analog signal format signals such as the first air-fuel ratio sensor 49a, the second air-fuel ratio sensor 49b, and the water temperature sensor 52 are input via the A / D 207, and those output signals are output to the CPU 201 and the RAM 203. Send to.

The output port 206 is connected to the CPU 20.
The control signal output from the igniter 25a, the fuel injection valve 32, the SCV actuator 37a, the throttle actuator 40, or the secondary air injection nozzle 5
Send to 3.

The ROM 202 includes a fuel injection amount control routine for determining the fuel injection amount, a fuel injection timing control routine for determining the fuel injection timing, an ignition timing control routine for determining the ignition timing, and a swirl control valve. (SCV) SCV for controlling the opening of 37
In addition to known application programs such as a control routine and a throttle control routine for controlling the opening of the throttle valve 39, a rich spike control routine which is the gist of the present invention is stored.

The ROM 202 stores various control maps in addition to the application programs described above. The control map described above is, for example, a fuel injection amount control map showing the relationship between the operating state of the internal combustion engine 1 and the fuel injection amount, a fuel injection timing control map showing the relationship between the operating state of the internal combustion engine 1 and the fuel injection timing, Ignition timing control map showing relationship between operating state of internal combustion engine 1 and ignition timing, internal combustion engine 1
Operating status and swirl control valve (SCV) 3
7 is an SCV opening control map showing the relationship with the opening of No. 7 and a throttle opening control map showing the relationship between the operating state of the internal combustion engine 1 and the throttle valve 39.

The RAM 203 stores the output signal of each sensor, the calculation result of the CPU 201, and the like. The calculation result is, for example, the engine speed calculated based on the output signal of the crank position sensor 51. RAM
The various data stored in 203 are rewritten to the latest data each time the crank position sensor 51 outputs a signal.

The backup RAM 204 is a non-volatile memory that retains data even after the operation of the internal combustion engine 1 is stopped.

The CPU 201 operates according to an application program stored in the ROM 202 and executes various controls such as fuel injection control, ignition control, SCV control, throttle control, rich spike control and the like.

For example, the CPU 201 determines the load of the internal combustion engine 1 using the output signal value of the crank position sensor 51, the accelerator position sensor 43, the air flow meter 44 or the like as a parameter.

The CPU 201 controls the SCV actuator 37a, the throttle actuator 40, the fuel injection valve 32, and the spark plug 25 in accordance with the load of the internal combustion engine 1 to perform stratified charge combustion operation with a lean mixture. And a homogeneous lean combustion operation using a lean air-fuel mixture and a homogeneous combustion operation using an air-fuel mixture having a stoichiometric ratio or less.

Here, when the internal combustion engine 1 is in the stratified charge combustion operation state or the homogeneous lean combustion operation state, in other words, when the internal combustion engine 1 is in the lean burn operation state, the internal combustion engine 1
Since the air-fuel ratio of the exhaust gas discharged from the engine becomes a lean air-fuel ratio, the three-way catalyst 46 cannot sufficiently purify nitrogen oxides (NOx) in the exhaust gas. On the other hand, in the present embodiment, the NOx storage reduction catalyst 4 is provided downstream of the three-way catalyst 46.
Since NO. 8 is arranged, the nitrogen oxides (NOx) that could not be purified by the three-way catalyst 46 are stored and reduced NOx catalyst 48.
It is stored in and is not released into the atmosphere.

By the way, the N of the NOx storage reduction catalyst 48 is
Since the Ox storage capacity is limited, if the lean burn operation of the internal combustion engine 1 is continued for a long time, the NOx storage reduction catalyst 4
The NOx storage capacity of No. 8 is saturated, and the nitrogen oxide (NOx) in the exhaust gas is not removed by the storage reduction type NOx catalyst 48 and is released into the atmosphere.

On the other hand, the CPU 201 determines that the internal combustion engine 1
During the lean-burn operation, the rich spike control is executed to set the air-fuel ratio of the exhaust gas to a predetermined rich air-fuel ratio (hereinafter referred to as the first rich air-fuel ratio) every predetermined time.

As a specific method of executing the rich spike control, a method of injecting a fuel (auxiliary fuel) which does not contribute to the fuel from the fuel injection valve 32 of the cylinder 21 in the expansion stroke or the exhaust stroke, the exhaust port of the internal combustion engine 1 It is possible to exemplify a method in which a dedicated fuel injection valve is attached to the exhaust passage and the fuel is injected from the fuel injection valve. However, in the present embodiment, from the fuel injection valve 32 of the cylinder 21 during the exhaust stroke, A method of injecting fuel will be described as an example.

At this time, the CPU 201 controls the amount of auxiliary fuel injected from the fuel injection valve 32 so that the output signal value of the first air-fuel ratio sensor 49a becomes the desired first rich air-fuel ratio, so-called rich. The spike feedback control may be executed.

When the rich spike control is executed by such a method, the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 alternates between the lean air-fuel ratio and the first rich air-fuel ratio in a short cycle. . In this case, the lean air-fuel ratio exhaust gas and the first rich air-fuel ratio exhaust gas flow into the NOx storage reduction catalyst 48 alternately in a short cycle.

As a result, the NOx storage reduction catalyst 48 alternately repeats occlusion and release / reduction of nitrogen oxide (NOx), so that the lean burn operation of the internal combustion engine 1 is continued for a long period of time. Even if there is NOx storage reduction catalyst 48
The NOx storage capacity of is never saturated.

Further, since the three-way catalyst 46 having an oxygen storage capacity is arranged in the exhaust passage upstream of the NOx storage reduction catalyst 48, the rich spike control is executed to make the exhaust air-fuel ratio the first rich air-fuel ratio. When the fuel ratio is set, the oxygen stored in the three-way catalyst 46 is released from the three-way catalyst 46.

When the stored oxygen is released from the three-way catalyst 46 as described above, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 46, in other words, the air-fuel ratio of the exhaust gas flowing into the storage reduction type NOx catalyst 48 becomes The air-fuel ratio becomes higher than the first rich air-fuel ratio. Therefore, the NOx storage reduction catalyst 48 is
There is a risk that all of the stored nitrogen oxides (NOx) cannot be reduced / purified.

Therefore, immediately before executing the rich spike control, the CPU 201 executes the air-fuel ratio reduction control for decreasing the exhaust air-fuel ratio to the second rich air-fuel ratio lower than the first rich air-fuel ratio. As a method of executing this air-fuel ratio reduction control, the same method as the rich spike control described above can be exemplified. However, at that time, the fuel injection valve 3
The amount of sub fuel injected from 2 is made larger than the amount of sub fuel at the time of executing the rich spike control.

When the air-fuel ratio reduction control as described above is executed immediately before the execution of the rich spike control, the exhaust gas of the second rich air-fuel ratio flows into the three-way catalyst 46. At that time, the second rich air-fuel ratio is set lower than the first rich air-fuel ratio related to the normal rich spike control, so that all of the oxygen stored in the three-way catalyst 46 is promptly released.

When the rich spike control is executed subsequent to the air-fuel ratio lowering control, the exhaust gas of the first rich air-fuel ratio flows into the three-way catalyst 46. At that time, since oxygen is not stored in the three-way catalyst 46, the three-way catalyst 46
The air-fuel ratio of the exhaust gas flowing out from the air-fuel ratio substantially matches the first rich air-fuel ratio.

As a result, the exhaust gas of the first rich air-fuel ratio will flow into the NOx storage reduction catalyst 48.
The nitrogen oxides (NOx) stored in the storage-reduction type NOx catalyst 48 are appropriately reduced and purified.

On the other hand, when the air-fuel ratio lowering control and the rich spike control as described above are continuously executed, the three-way catalyst is activated during the period from the start of the execution of the air-fuel ratio lowering control to the end of the execution of the rich spike control. Since the oxygen storage capacity of 46 does not function, hydrocarbons (H
C) and carbon monoxide (CO) may have a reduced oxidizing ability, which may deteriorate exhaust emission.

On the other hand, the CPU 201 executes lean spike control for temporarily increasing the air-fuel ratio of the exhaust gas during the rich spike control execution period.

As a method of executing lean spike control,
A method of temporarily reducing the amount of auxiliary fuel injected from the fuel injection valve 32 during execution of the rich spike control, a method of temporarily injecting secondary air from the secondary air injection nozzle 53 during execution of the rich spike control, and the like are available. It can be illustrated.

Specifically, during execution of the rich spike control, the CPU 201 causes the secondary air injection nozzle 53 to inject secondary air at predetermined time intervals, as shown in FIGS.

At this time, the target center air-fuel ratio of the rich spike feedback control may be set higher than the first rich air-fuel ratio and lower than the theoretical air-fuel ratio, as shown in FIG. 3, or FIG. The stoichiometric air-fuel ratio may be set as shown in FIG. However, the amount of secondary air injected from the secondary air injection nozzle 53 at one time is set so that the amount of oxygen contained in the exhaust gas after the secondary air is supplied does not exceed the oxygen storage capacity of the three-way catalyst 46. Preferably. This is because when the amount of oxygen contained in the exhaust gas after the supply of the secondary air exceeds the oxygen storage capacity of the three-way catalyst 46, the three-way catalyst 46.
This is because oxygen that cannot be stored due to the three-way catalyst 46 flows out, and the air-fuel ratio of the exhaust gas that flows into the NOx storage reduction catalyst 48 may become higher than the desired first rich air-fuel ratio.

When such lean spike control is executed during the execution period of the rich spike control, the three-way catalyst 46
Since the exhaust gas in the oxygen excess state flows into the three-way catalyst 46, it becomes possible for the three-way catalyst 46 to store oxygen. As a result, the oxidation capacity of the three-way catalyst 46 is activated, and it becomes possible to oxidize and purify hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas.

The rich spike control in the present embodiment will be specifically described below. The CPU 201 executes the rich spike control in a predetermined cycle under the condition that the internal combustion engine 1 is in the lean burn operation. At that time, C
PU201 will perform rich spike control according to the rich spike control routine as shown in FIG.

The rich spike control routine described above is
This is a routine stored in advance in the ROM 202, and C
This is a routine that is repeatedly executed by the PU 201 every predetermined time (for example, every time the crank position sensor 51 outputs a pulse signal).

In the rich spike control routine shown in FIG. 5, the CPU 201 first determines in S501 whether the internal combustion engine 1 is in the lean burn operation, specifically, the internal combustion engine 1 is in the stratified charge combustion operation or the homogeneous lean burn operation. Is determined.

When it is determined in S501 that the internal combustion engine 1 is not in the lean burn operation, in other words,
When it is determined that the internal combustion engine 1 is in the homogeneous combustion operation with the air-fuel mixture having the stoichiometric ratio or less, the CPU 201 determines that it is not necessary to execute the rich spike control, and ends the execution of this routine.

On the other hand, if it is determined in S501 that the internal combustion engine 1 is in the lean burn operation, the CPU 2
In step 01, the process proceeds to step S502, and it is determined whether the rich spike control execution condition is satisfied.

Examples of the rich spike control execution conditions described above include conditions in which the NOx storage reduction catalyst 48 is in an active state, the elapsed time from the previous execution of the rich spike control is a predetermined time or more, and the like. be able to.

When it is determined that the rich spike control execution condition as described above is satisfied, the CPU 201
Advances to S503, and executes air-fuel ratio reduction control to reduce the air-fuel ratio of exhaust gas to the above-described second rich air-fuel ratio. Specifically, the CPU 201 controls the cylinder 2 in the exhaust stroke.
The auxiliary fuel is injected from the fuel injection valve 32 of No. 1.

In S504, the CPU 201 activates the air-fuel ratio reduction control counter. The air-fuel ratio reduction control counter is a counter that measures the elapsed time from the start of execution of the air-fuel ratio reduction control, and is configured by, for example, a register incorporated in the CPU 201.

In S505, the CPU 201 determines that the value of the air-fuel ratio reduction control counter is the predetermined air-fuel ratio reduction control execution time:
It is determined whether T1 or more. The air-fuel ratio reduction control execution time: T1 is the time required for the three-way catalyst 46 to release all of the stored oxygen under the condition that the exhaust gas having the second rich air-fuel ratio flows into the three-way catalyst 46. is there.

If it is determined in S505 that the value of the air-fuel ratio reduction control counter is less than the air-fuel ratio reduction control execution time: T1, the CPU 201 determines that the three-way catalyst 46
Considers that all of the stored oxygen has not yet been released, and continues the air-fuel ratio reduction control.

On the other hand, when it is determined in S505 that the value of the air-fuel ratio reduction control counter is the air-fuel ratio reduction control execution time: T1 or more, the CPU 201 causes the three-way catalyst 46 to release all the stored oxygen. S5
Proceed to 06.

In S506, the CPU 201 shifts the control from the air-fuel ratio lowering control to the rich spike control. Specifically, the CPU 201 reduces the injection amount of the auxiliary fuel so as to increase the air-fuel ratio of the exhaust gas from the second rich air-fuel ratio to the first rich air-fuel ratio.

In S507, the CPU 201 activates the rich spike control counter and the lean spike control counter. The rich spike control counter is a counter that counts the elapsed time from the start of execution of the rich spike control. On the other hand, the lean spike control counter is a counter that counts the elapsed time from the end of the previous execution of the lean spike control, but if it is activated for the first time after the execution of the rich spike control is performed, the rich spike control is performed. Time elapsed from the start.

In S508, the CPU 201 determines whether or not the value of the lean spike control counter is equal to or greater than a predetermined lean spike control execution interval: T2.

In step S508, the value of the lean spike control counter is the lean spike control execution interval: T2.
When it is determined that it is less than, the CPU 201 re-executes the processing from S508.

On the other hand, if it is determined in S508 that the value of the lean spike control counter is greater than or equal to the lean spike control execution interval: T2, the CPU 201
Advances to S509 to execute lean spike control.
Specifically, the CPU 201 controls the secondary air injection nozzle 53.
To inject a predetermined amount of secondary air. In this case, since the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 temporarily increases, the oxygen contained in the exhaust gas is stored in the three-way catalyst 46,
Alternatively, in the three-way catalyst 46, hydrocarbons (H
C) and carbon monoxide (CO) are oxidized.

When the execution of the lean spike control as described above is completed, the CPU 201 proceeds to S510 and restarts the lean spike control counter. In this case, the lean spike control counter measures the elapsed time from the end of execution of the lean spike control.

In S511, the CPU 201 determines whether or not the value of the rich spike control counter has reached a predetermined rich spike control execution time: T3 or more.

In S511, the value of the rich spike control counter is the rich spike control execution time: T3.
When it is determined that it is less than, the CPU 201 repeatedly executes the above-described processing of S508 and thereafter. In this case, while the rich spike control continues to be executed,
The lean spike control is repeatedly executed at the lean spike control execution interval T2.

On the other hand, if it is determined in S511 that the value of the rich spike control counter is the rich spike control execution time: T3 or more, the CPU 201
Advances to S512 and ends the execution of the rich spike control. That is, the CPU 201 stops the injection of the sub fuel from the fuel injection valve 32.

In S513, the CPU 201 resets all the values of the air-fuel ratio reduction control counter, the rich spike control counter, and the lean spike control counter described above.

As described above with reference to FIGS. 3 and 4, the CPU 201 executes the rich spike control routine, so that the air-fuel ratio of the exhaust gas increases at predetermined intervals during the rich spike control execution period. Will be raised.

As a result, the oxygen storage capacity of the three-way catalyst 46 can be made to function even during the execution of the rich spike control, and the oxidation capacity of the three-way catalyst 46 is activated so that the hydrocarbon (HC) in the exhaust gas can be activated. It is possible to oxidize and purify carbon monoxide (CO).

Furthermore, the amount of secondary air supplied to the exhaust gas by one lean spike control is such that the amount of oxygen contained in the exhaust gas after the secondary air is supplied does not exceed the oxygen storage capacity of the three-way catalyst 46. Therefore, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 48 does not become unnecessarily high, and the nitrogen oxides (N
The purification rate of Ox) does not unnecessarily decrease.

Therefore, according to the exhaust purification system of the internal combustion engine of the present embodiment, during the rich spike control execution period, hydrocarbons (HC) and monoxide are reduced without reducing the purification rate of nitrogen oxides (NOx). It is possible to improve the purification rate of carbon (CO).

<Second Embodiment> Next, a second embodiment of the exhaust purification system for an internal combustion engine according to the present invention will be described with reference to FIGS. 6 to 7. Here, the configuration different from that of the first embodiment described above will be described, and the description of the same configuration will be omitted.

The difference between this embodiment and the above-described first embodiment is that the first embodiment described above executes lean spike control during the execution period of rich spike control. In the embodiment, in the case where the internal combustion engine 1 is operated with the air-fuel mixture of the stoichiometric air-fuel ratio after the execution of the rich spike control, the lean spike control is executed when the execution of the rich spike control is completed.

Here, immediately after the exhaust air-fuel ratio lowering control and the rich spike control are executed, the internal combustion engine 1 is operated with the air-fuel mixture having the theoretical air-fuel ratio (hereinafter, the internal combustion engine 1 is operated with the air-fuel mixture having the theoretical air-fuel ratio). This is called stoichiometric operation),
During the period from the execution start time of the exhaust air-fuel ratio reduction control to the end time of the stoichiometric operation of the internal combustion engine 1, the three-way catalyst 4
The oxygen storage capacity of No. 6 does not function, and the purification rate of hydrocarbons (HC) and carbon monoxide (CO) during stoichiometric operation may decrease.

Therefore, in the exhaust emission control system for an internal combustion engine according to the present embodiment, when the internal combustion engine 1 is stoichiometrically operated after the completion of the rich spike control, as shown in FIG. The lean spike control is executed when the execution is completed.

As a concrete execution method of the lean spike control, a method of injecting secondary air from the secondary air injection nozzle 53 can be exemplified as in the first embodiment described above.

At this time, the amount of secondary air injected from the secondary air injection nozzle 53 is set so that the amount of oxygen contained in the exhaust gas after the secondary air is supplied does not exceed the oxygen storage capacity of the three-way catalyst 46. It is preferable that the amount of oxygen contained in the exhaust gas after the secondary air is supplied is less than or equal to the oxygen storage capacity and half or more of the oxygen storage capacity.

This is because when the amount of oxygen contained in the exhaust gas after the secondary air supply becomes excessively small, the three-way catalyst 46 removes hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas during stoichiometric operation. This is because there is a risk that it will not be able to be sufficiently oxidized and purified.

As described above, in the case where the internal combustion engine 1 is stoichiometrically operated after the completion of the execution of the rich spike control,
When the lean spike control is executed at the time when the execution of the rich spike control is completed, the three-way catalyst 46 can store a sufficient amount of oxygen before the internal combustion engine 1 is operated in the stoichiometric state, so that the stoichiometric operation is performed. Hydrocarbons (H
It is possible to improve the purification rate of C) and carbon monoxide (CO).

The rich spike control in the present embodiment will be specifically described below. When executing the rich spike control, the CPU 201 executes a rich spike control routine as shown in FIG. 7. This rich spike control routine is a routine stored in the ROM 202 in advance, and is a routine that is repeatedly executed by the CPU 201 every predetermined time (for example, every time the crank position sensor 51 outputs a pulse signal).

In the rich spike control routine shown in FIG. 7, the CPU 201 first determines in S701 whether the internal combustion engine 1 is in the lean burn operation.

If it is determined in S701 that the internal combustion engine 1 is not in the lean burn operation, the CPU 201
Considers that it is not necessary to execute the rich spike control, and ends the execution of this routine.

On the other hand, if it is determined in S701 that the internal combustion engine 1 is in the lean burn operation, the CPU 2
In step 01, the process proceeds to step S702, and it is determined whether the rich spike control execution condition is satisfied.

If it is determined in S702 that the rich spike control execution condition is satisfied, the CPU 2
01 proceeds to S703, where the air-fuel ratio of the exhaust is set to the second value described above.
The air-fuel ratio reduction control is executed to reduce the rich air-fuel ratio.

In S704, the CPU 201 activates the air-fuel ratio reduction control counter.

In S705, the CPU 201 determines the value of the air-fuel ratio reduction control counter is the predetermined air-fuel ratio reduction control execution time:
It is determined whether T1 or more.

When it is determined in S705 that the value of the air-fuel ratio reduction control counter is less than the air-fuel ratio reduction control execution time: T1, the CPU 201 determines that the three-way catalyst 46
Considers that all of the stored oxygen has not yet been released, and continues the air-fuel ratio reduction control.

On the other hand, if it is determined in S705 that the value of the air-fuel ratio reduction control counter is greater than or equal to the air-fuel ratio reduction control execution time: T1, the CPU 201 causes the three-way catalyst 46 to release all of the stored oxygen. S7
Proceed to 06.

In S706, the CPU 201 shifts the control from the air-fuel ratio lowering control to the rich spike control.

In S707, the CPU 201 activates the rich spike control counter.

In S708, the CPU 201 determines whether or not the value of the rich spike control counter has reached a predetermined rich spike control execution time: T3 or more.

In step S708, the value of the rich spike control counter is the rich spike control execution time: T3.
When it is determined that the value is less than the above, the CPU 201 continues the execution of the rich spike control until the value of the rich spike control counter becomes equal to or more than the rich spike control execution time: T3.

In step S708, the value of the rich spike control counter is the rich spike control execution time: T3.
If it is determined that the above is the case, the CPU 201, S7
In step 09, the execution of the rich spike control is completed. That is, the CPU 201 stops the injection of the sub fuel from the fuel injection valve 32.

In S710, the CPU 201 resets all the values of the air-fuel ratio lowering control counter and the rich spike control counter described above.

At S711, the CPU 201 determines whether or not the stoichiometric operation condition of the internal combustion engine 1 is satisfied. As a condition for operating the internal combustion engine 1 in a stoichiometric operation, for example, the output signal value (accelerator opening) of the accelerator position sensor 43 is a predetermined opening or more, and the output signal value (intake air amount) of the air flow meter 44 is a predetermined amount or more. Is,
Conditions such as the engine speed being equal to or higher than a predetermined speed can be exemplified.

If it is determined in S711 that the stoichiometric operation condition of the internal combustion engine 1 is not satisfied, the CP
U201 ends the execution of this routine.

On the other hand, if it is determined in S711 that the stoichiometric operation condition of the internal combustion engine 1 is satisfied,
The CPU 201 proceeds to S712 and executes lean spike control. Specifically, the CPU 201 causes the secondary air injection nozzle 53 to inject a predetermined amount of secondary air. In this case, since the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 is temporarily increased, oxygen contained in the exhaust gas is stored in the three-way catalyst 46, and the oxidation ability of the three-way catalyst 46 is activated. .

When the internal combustion engine 1 is stoichiometrically operated after such lean spike control is executed, hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas are suitably oxidized by the three-way catalyst 46. And will be purified.

Therefore, according to the exhaust purification system of the internal combustion engine of the present embodiment, when the internal combustion engine 1 is stoichiometrically operated after the execution of the rich spike control, the NOx purification rate during the execution of the rich spike control is reduced. Without this, it is possible to improve the HC purification rate and the CO purification rate during stoichiometric operation.

[0141]

According to the present invention, in a device for purifying exhaust gas of an internal combustion engine in which a precatalyst having an oxygen storage capacity is arranged upstream of a NOx catalyst, the precatalyst of the precatalyst can be treated without lowering the purifying capacity of the NOx catalyst. It becomes possible to improve the purification ability.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine according to a first embodiment.

FIG. 2 is a block diagram showing the internal configuration of the ECU.

FIG. 3 is a diagram (1) showing a state of an exhaust air-fuel ratio during execution of rich spike control.

FIG. 4 is a diagram (2) showing a state of an exhaust air-fuel ratio during execution of rich spike control.

FIG. 5 is a diagram showing a rich spike control routine according to the first embodiment.

FIG. 6 is a diagram (3) showing a state of an exhaust air-fuel ratio during execution of rich spike control.

FIG. 7 is a diagram showing a rich spike control routine in the second embodiment.

[Explanation of symbols]

1 ... Internal combustion engine 20 ... ECU 21 ... cylinder 32 ... Fuel injection valve 46 ... Three-way catalyst 47 ... Exhaust pipe 48: NOx storage reduction catalyst 53 ... Secondary air injection nozzle

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/28 301 F02D 43/00 301H F02D 43/00 301 301T 45/00 301G 45/00 301 B01D 53 / 36 103B 101B F-term (reference) 3G084 AA04 BA09 BA13 BA24 DA10 EB12 EB16 EC01 FA07 FA10 FA20 FA26 FA33 FA38 3G091 AA02 AA12 AA17 AB02 AB05 BA01 BA11 BA14 BA17 BA27 CA18 CB02 DA04 DC01 EA01 EA03 EA05 EA07 EA16 EA34 FB06 FB10 FB11 FB12 GA06 HA10 HA36 HA37 HA42 3G301 HA01 HA15 HA17 JA25 LB04 LC01 MA01 MA12 MA26 NA01 NA08 NC01 NC02 ND02 ND22 NE02 NE07 NE13 NE14 NE15 PA01Z PA11Z PD09Z PE01Z PE03Z PE08Z PF03Z PF15Z 4D048 AA06 AA13 AA18 AB46 DA04 CC32 DA02 CC02

Claims (4)

[Claims] 1. An internal combustion engine capable of burning an oxygen-rich mixture, an NOx catalyst provided in an exhaust passage of the internal combustion engine for purifying nitrogen oxides in the exhaust in the presence of a reducing agent, When it is necessary to purify nitrogen oxides by the pre-catalyst which is arranged upstream of the NOx catalyst in the exhaust passage and has an oxygen storage capacity, and the NOx catalyst needs to purify nitrogen oxides, the exhaust air-fuel ratio lower than the stoichiometric air-fuel ratio Rich spike control executing means for reducing the rich air-fuel ratio to the second rich air-fuel ratio, and before the rich spike control executing means executes the rich spike control, the exhaust air-fuel ratio is reduced to a second rich air-fuel ratio lower than the first rich air-fuel ratio. An air-fuel ratio lowering control executing means for increasing the exhaust air-fuel ratio during the execution of the rich spike control by the rich spike control executing means. Exhaust purification system of an internal combustion engine, characterized in that it comprises a means. 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the lean spike control execution means increases the exhaust air-fuel ratio at every predetermined time during the execution period of the rich spike control. 3. An internal combustion engine capable of burning an oxygen-rich mixture, an NOx catalyst disposed in an exhaust passage of the internal combustion engine for purifying nitrogen oxides in the exhaust in the presence of a reducing agent, A pre-catalyst, which is arranged in the exhaust passage upstream of the NOx catalyst and has an oxygen storage capacity, and a rich spike control execution means for making the exhaust air-fuel ratio lower than the theoretical air-fuel ratio when purifying nitrogen oxides by the NOx catalyst. When the internal combustion engine is operated at the stoichiometric air-fuel ratio after the execution of the rich spike control by the rich spike control execution means, the exhaust air-fuel ratio is temporarily changed from the stoichiometric air-fuel ratio at the end of the execution of the rich spike control. An exhaust emission control device for an internal combustion engine, comprising: a lean spike control executing means for increasing the exhaust gas. 4. The lean spike control execution means increases the air-fuel ratio of exhaust gas within a range in which the amount of oxygen contained in the exhaust gas does not exceed the oxygen storage capacity of the precatalyst. An exhaust emission control device for an internal combustion engine according to claim 3.