JP2002070541A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of sufficiently purifying a separated HC. SOLUTION: This exhaust emission control device is equipped with an HC adsorption catalyst 22 having a ternary catalyst and an HC adsorbent, installed in an exhaust pipe 70; an ECU 30 acting as a desorption decision means deciding the desorption of an HC from the HC adsorbent of the HC adsorption catalyst 22; the ECU 30 acting as an air fuel ratio control means controlling the air fuel ratio in a lean state when the desorption of an HC from the HC adsorbent is decided with the desorption decision means; and the ECU 30 acting as an exhaust gas-containing oxygen concentration keeping means keeping the oxygen concentration contained in the exhaust gas flowing through the exhaust pipe 70 in the specified value.

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 purifying exhaust gas of an internal combustion engine, and more particularly to a so-called H-type exhaust gas purifying apparatus.
The present invention relates to an exhaust gas purification device for an internal combustion engine having a C adsorption catalyst in an engine exhaust passage.

[0002]

2. Description of the Related Art For example, Japanese Unexamined Patent Publication No. 11-82111 discloses an HC for adsorbing hydrocarbons (hereinafter referred to as "HC" unless otherwise specified) as an exhaust purification device for an internal combustion engine.
An HC adsorption catalyst comprising a three-way catalyst layer coated on an adsorbent is disclosed.

[0003] The HC adsorbing catalyst adsorbs HC onto the HC adsorbing material at a cold time, and removes the adsorbed HC into H
It is a catalyst for desorbing from the C adsorbent and oxidizing the desorbed HC (hereinafter referred to as “desorbed HC”) in a three-way catalyst layer to convert it into carbon dioxide or water and purify the same.

[0004]

However, according to the technology described in the above publication, an HC adsorption catalyst is used for the purpose of oxidizing carbon monoxide (hereinafter referred to as "CO") and HC in exhaust gas components. The air-fuel ratio is switched to lean so that sufficient oxygen can be supplied to the three-way catalyst.

However, various organic compounds are contained in the hydrocarbon system contained in the exhaust gas, and when the concentration of oxygen increases, oxidation tends to be difficult (combustibility deteriorates). And specific organic compounds such as low-molecular paraffins such as methane, ethane and propane (hereinafter referred to as “specific organic compounds” unless otherwise specified).

On the other hand, the catalyst has a characteristic of purifying harmful components in exhaust gas when its temperature rises to a certain temperature range. The catalyst temperature when the catalyst is activated so that the purification rate of the exhaust gas by the catalyst becomes 50% or more is referred to as the 50% purification temperature of the catalyst by the oxidation reaction. Does not show up. Therefore, in this specification, 50% purification temperature of the catalyst by the oxidation reaction is hereinafter referred to as “catalyst activation temperature”.
Notation.

It is conceivable that simply increasing the oxygen concentration in the exhaust gas for the purpose of accelerating the oxidation reaction of CO and HC by the three-way catalyst makes it difficult for the specific organic compound to raise the temperature of the catalyst. Therefore, in this case, the three-way catalyst hardly becomes the catalyst activation temperature and thus becomes difficult to be activated, so that there is a possibility that an adverse effect that the oxidation reaction of CO or HC becomes difficult to be promoted.

However, in the conventional exhaust gas purifying apparatus for an internal combustion engine using an HC adsorbing catalyst, there is no technology for purifying desorbed HC in consideration of the presence of the specific organic compound such as paraffin. It has been considered that HC purification is not sufficiently performed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine that can sufficiently purify desorbed HC.

[0010]

In order to solve the above-mentioned problems, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention employs the following means.

(1) The exhaust gas purifying apparatus for an internal combustion engine of the present invention comprises:
An HC adsorption catalyst provided in the engine exhaust passage and having a three-way catalyst and an HC adsorbent; a desorption judgment means for judging that HC is desorbed from the HC adsorbent of the HC adsorption catalyst; and a desorption judgment. Means for controlling the air-fuel ratio to lean when it is determined that HC has desorbed from the HC adsorbent by the means, and the air-fuel ratio control means for controlling the air-fuel ratio to be lean when the desorption determining means determines that HC has been desorbed. By air-fuel ratio control,
Exhaust gas-containing oxygen concentration maintaining means for maintaining the concentration of oxygen contained in the exhaust gas flowing through the engine exhaust passage at a predetermined value.

Here, an ECU for controlling the entire internal combustion engine is provided.
And the constituent elements of the present invention will be described.

The ECU comprises a digital computer, as is well known, interconnected by a bidirectional bus.
It comprises a CPU as a central processing controller, a ROM as a read-only memory, a RAM as a random access memory, an input port, an output port and the like.

The input port is electrically connected to various sensors attached to the internal combustion engine or the vehicle. When output signals of these various sensors enter the ECU through the input ports, the parameters related to these various sensors are temporarily changed. Is stored in the RAM.

Then, the CPU sends R through the bidirectional bus.
The parameters stored in the AM are called up as necessary, and the CPU performs necessary arithmetic processing based on these parameters. As a result of the arithmetic processing, various components of the internal combustion engine function via the output port. I do.

The "desorption determining means" includes an application program set to determine that HC is desorbed from the HC adsorbent. The application program is executed by the CPU, and the attribute of the CPU is in the ECU. Therefore, the ECU can be regarded as a detachment determination unit.

The "air-fuel ratio control means" includes an application program which is set so as to control the air-fuel ratio lean when the desorption determining means determines that HC has been desorbed from the HC adsorbent. The application program is executed by the CPU, and the attribute of the CPU is in the ECU. Therefore, the ECU can be referred to as air-fuel ratio control means.

As the "exhaust gas-containing oxygen concentration maintaining means", the exhaust gas flowing through the engine exhaust passage is controlled by the air-fuel ratio control means when the desorption determining means determines that HC has been desorbed. An application program set so as to maintain the concentration of oxygen contained at a predetermined value can be given. The application program is executed by the CPU, and the attribute of the CPU is in the ECU. Therefore, the ECU can be regarded as an exhaust gas-containing oxygen concentration maintaining means.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, when the desorption determining means determines that HC has desorbed, the air flows through the engine exhaust passage by the air-fuel ratio control by the air-fuel ratio control means. The concentration of oxygen contained in the exhaust gas is maintained at the predetermined value by the exhaust gas-containing oxygen concentration maintaining means. (2) Therefore, the organic compound, which tends to be less susceptible to oxidation due to the increase in the concentration of oxygen contained in the exhaust gas component, can undergo the oxidation reaction even when the concentration of oxygen contained in the exhaust gas increases. Value, it is possible to prevent the specific organic compound contained in the exhaust gas component from causing an oxidation reaction, that is, the concentration of oxygen contained in the exhaust gas flowing through the engine exhaust passage is the predetermined value. Until the specific organic compound undergoes an oxidation reaction, the activation of the three-way catalyst of the HC adsorption catalyst is hardly suppressed. As a result, even when the concentration of oxygen contained in the exhaust gas increases, it is difficult to prevent the three-way catalyst from reaching the activation temperature, and thus the three-way catalyst sufficiently purifies HC desorbed from the HC adsorbent. be able to. (3) The predetermined value can be maintained by the exhaust gas-containing oxygen concentration maintaining means by switching the air-fuel ratio to rich or lean by the air-fuel ratio control means.

Here, the concept of rich includes a so-called stoichiometric air-fuel ratio (stoichiometric).

Since the predetermined value is maintained by switching to rich or lean compared to the case where the predetermined value is maintained only with rich or lean only, the maintenance adjustment is easier. (4) The maintenance of the predetermined value by the exhaust gas-containing oxygen concentration maintaining means is desirably performed by setting the air-fuel ratio to a lean upper limit by the air-fuel ratio control means. In this manner, an excessive increase in the oxygen concentration can be effectively suppressed. Therefore, it is more difficult for the three-way catalyst to be at the activation temperature, so that the three-way catalyst can sufficiently purify HC desorbed from the HC adsorbent.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows an example in which the exhaust gas purifying apparatus of the present invention is applied to a diesel engine which is a compression ignition type internal combustion engine.

Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston,
5 is a combustion chamber, 6 is an electric control type fuel injection valve, 7 is an intake valve,
8 indicates an intake port, 9 indicates an exhaust valve, and 10 indicates an exhaust port.

The intake port 8 is connected to a surge tank 12 via a corresponding intake branch 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13. The intake pipe 90 to which the compressor 15 is attached is provided with an air cleaner and an air flow meter (not shown).

A step motor 16 is provided in the intake duct 13.
And a cooling device 18 for cooling intake air flowing through the intake duct 13 is provided around the intake duct 13. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 18,
The intake air is cooled by the engine cooling water.

On the other hand, the exhaust port 10 is connected to a turbine 21 of the turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the turbine 21 is connected to an exhaust purification device A provided in an exhaust pipe 70 which is an engine exhaust passage. Connected. Further, a fuel addition nozzle (not shown), which is a fuel supply means, is attached to the exhaust port 10. Therefore, the fuel addition nozzle is located upstream of the exhaust gas purification device A in the exhaust pipe 70.

The exhaust gas purifying apparatus A encloses an HC adsorption catalyst 22 in which a three-way catalyst (not shown) is coated on an HC adsorbent for adsorbing HC to form a three-way catalyst layer in a case body 23. It is.

The exhaust manifold 19 and the surge tank 12 are connected to each other via an EGR passage 24 which is a component of an exhaust gas recirculation device (hereinafter, referred to as EGR). The EGR passage 24 is electrically controlled EG
It has an R control valve 25. In addition, a cooling device 26 for cooling the EGR gas flowing through the EGR passage 24 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 26, and the EGR gas is cooled by the engine cooling water.

On the other hand, the fuel injection valve 6 is connected to a common rail 27 as a fuel reservoir via a fuel supply pipe 6a.

The fuel is supplied into the common rail 27 by an electric control type variable discharge fuel pump 28. The fuel supplied to the common rail 27 is supplied to the fuel supply pipe 6.
The fuel is supplied to the fuel injection valve 6 via a. Common rail 2
7, a fuel pressure sensor 29 for detecting the fuel pressure in the common rail 27 is attached. Based on the output signal of the fuel pressure sensor 29, the target fuel in which the fuel pressure in the common rail 27 is the most reasonable fuel pressure The discharge amount of the fuel pump 28 is controlled so as to be a pressure.

The amount of fuel injected from the fuel injection valve 6 is stored in a ROM 32 of an electronic control unit (ECU) 30 described below in the form of a map as a function of the amount of depression of the accelerator pedal 40 and the engine speed. A required torque corresponding to the amount of depression of the accelerator pedal 40 and the engine speed is obtained from a required torque calculation map (not shown) stored in the memory, and is calculated based on the required torque.

The air-fuel ratio required by the engine (hereinafter referred to as "engine required air-fuel ratio") varies depending on the engine speed and the load. The ECU 30 controls the air-fuel ratio so that various performances such as drivability and the like are optimized.
Therefore, the ECU 30 can be referred to as air-fuel ratio control means. In performing the air-fuel ratio control, the engine fuel, for example, light oil, is supplied into the combustion chamber 5 using the fuel injection valve 6 so as to match the required engine air-fuel ratio.

The ECU 30 comprises a digital computer and ROMs connected to each other by a bidirectional bus 31.
(Read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34,
An input port 35 and an output port 36 are provided. RO
M32 stores various application programs necessary for operating various components of the internal combustion engine and various maps required for executing these application programs. When executing the air-fuel ratio control means, a corresponding appropriate application program (not shown) is used.

The output signal of the fuel pressure sensor 29 corresponds to A
The data is input to the input port 35 via the D converter 37.

The exhaust gas purifying apparatus A is provided with a temperature sensor 79 for detecting the temperature of the catalyst to detect the temperature at which HC is desorbed from the HC adsorbent of the HC adsorption catalyst 22. It is also conceivable to attach an A / F sensor before and after the exhaust gas purification device A to substitute for the temperature sensor 79.

Further, an oxygen sensor (or an air-fuel ratio sensor) 83 is mounted on the upstream side of the catalyst temperature sensor 79, and the oxygen sensor 83 detects the oxygen concentration in the exhaust gas entering the exhaust gas purification device A.

The output signals of the temperature sensor 79 and the oxygen sensor 83 enter the input port 35 via the AD converter 37.

Further, in the exhaust pipe 70 downstream of the exhaust gas purifying device A, a catalytic converter and an exhaust throttle valve (both not shown) for accommodating, for example, an occlusion-reduction type NOx catalyst are mounted in the case body.

The accelerator pedal 40 includes an accelerator pedal 4
A load sensor 41 that generates an output voltage proportional to the amount of depression of 0 is connected, and the output voltage of the load sensor 41 enters the input port 35 via the corresponding AD converter 37.
Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse each time the crankshaft (not shown) rotates, for example, 30 °.

On the other hand, the output port 36 is connected to the fuel injection valve 6, the throttle valve driving step motor 16, the EGR control valve 25, and the fuel pump 28 via the corresponding drive circuit 38.
Connected to

Next, a desorption HC purification control execution routine for purifying HC desorbed from the HC adsorbent of the exhaust gas purification apparatus A according to the present embodiment with a three-way catalyst is realized using the flowchart of FIG. Application program for the following.

The program executes step 1 described below.
01 and step 102. Further, a program including these steps is stored in the ROM 32 of the ECU 30, and is called up as needed. The processing in each of the above steps is all performed by the CPU 34 of the ECU 30.
by. Note that the symbol S is used, and for example, if it is step 101, it is abbreviated as S101.

In step S101, the catalyst temperature of the exhaust gas purification device A is detected by the catalyst temperature sensor 79, and the detected value is used to determine the HC.
It is determined whether or not the catalyst temperature has reached the desorption temperature (hereinafter, referred to as “HC desorption temperature”). Whether or not the catalyst temperature has reached the HC desorption temperature depends on the size of the catalyst to be used and the type of internal combustion engine to which the present apparatus A is applied. The desorption start temperature of HC is stored in the ROM, and it is determined whether or not the HC adsorption catalyst 22 has reached the HC desorption temperature. An appropriate map may be used for storage in the ROM.

If an affirmative determination is made in S101, S102
If the determination is negative, this process is repeated until the temperature reaches the HC desorption temperature.

This S101 is performed in the HC adsorption catalyst 22.
Since it is determined that HC is desorbed from the HC adsorbent, this step S101 can be referred to as HC desorption determination means.
Since the attributes of the ROM 32 are stored in the ECU 32, the ECU 30 is hereinafter referred to as a detachment determination unit.

In S102, A / F vibration is repeated. The ECU 30 serving as the desorption determining means removes H from the HC adsorbent.
The air-fuel ratio control by the ECU 30 is executed based on an application program set to control the air-fuel ratio lean when it is determined that C has desorbed.

The A / F oscillation means that the exhaust pipe 70 is controlled by the air-fuel ratio control by the ECU 30 as the air-fuel ratio control means when the desorption judgment means determines that HC has been desorbed.
The oxygen sensor 83 detects the oxygen concentration in the exhaust gas entering the exhaust gas purification device A in the exhaust pipe 70 so that the concentration of oxygen contained in the exhaust gas flowing through the exhaust gas can be maintained at a predetermined value.
ECU as the air-fuel ratio control means based on the detected value
Switching the air-fuel ratio to rich (including stoichiometric) or lean by 30. Here, the "predetermined value"
It is a limit value at which an organic compound, which tends to be hardly oxidized due to an increase in the concentration of oxygen contained in the exhaust gas component, can undergo an oxidation reaction even when the concentration of oxygen contained in the exhaust gas increases.

Since the concentration of oxygen contained in the exhaust gas can be maintained at the predetermined value by the A / F oscillation, S102 is referred to as an exhaust gas-containing oxygen concentration maintaining means. And this S1
02 is stored in the ROM 32,
Since the attribute of the OM 32 is in the ECU 30, the ECU 30
Reference numeral 30 denotes exhaust gas-containing oxygen concentration maintaining means.

After performing the processing of S102, this program is repeated as necessary.

Preferably, the maintaining of the predetermined value by the exhaust gas-containing oxygen concentration maintaining means sets an air-fuel ratio lean upper limit by the air-fuel ratio control means.

In short, S102 is a process for creating a state in which both oxygen and a substance serving as a reducing agent are sufficient. For example, stoichiometric and lean repetitive processing, processing to insert a rich spike or stoichiometric spike when the engine combustion is in a lean combustion state, and processing to insert a lean spike when the engine combustion is in a stoichiometric combustion state can be mentioned. By performing these processes, combustion of the desorbed HC is promoted, and oxygen poisoning described below can be avoided.

Oxygen poisoning is a phenomenon in which the surface of the catalyst is covered with oxygen when excessive oxygen is present, and the original function of the catalyst, which promotes a chemical reaction, does not work well.

By the way, various organic compounds are contained in the hydrocarbon system contained in the exhaust gas. When the oxygen concentration is increased, the organic compounds tend to be easily oxidized (combustibility is improved). While there are organic compounds, as described above, organic compounds (specific organic compounds) such as methane, ethane, propane, and other low-molecular paraffins are also included. That is, there are an organic compound whose combustion is promoted by an increase in oxygen concentration and an organic compound whose combustion is inhibited. Therefore, an appropriate map capable of suitably performing the A / F vibration in consideration of the reaction of these organic compounds is obtained in advance by an experiment or the like, and the optimum A / F vibration according to the engine operating state is applied from the map. It preferably purifies HC.

What has been described above is a diesel engine to which the exhaust gas purifying apparatus for an internal combustion engine according to this embodiment is applied.

Next, the operation and effect of this embodiment will be described.

The ECU 30 serving as a desorption determining means controls H
When it is determined that C has desorbed, the exhaust pipe 70 is controlled by the air-fuel ratio control by the ECU 30 as the air-fuel ratio control means.
The concentration of oxygen contained in the exhaust gas flowing through the exhaust gas is maintained at the predetermined value by the ECU 30 serving as an exhaust gas-containing oxygen concentration maintaining means.

The specific value is such that the specific organic compound contained in the exhaust gas component, which tends to be hardly oxidized due to an increase in the oxygen concentration, can undergo an oxidation reaction even when the concentration of the oxygen contained in the exhaust gas increases. This is the limit. Therefore, it is possible to prevent the specific organic compound contained in the exhaust gas component from causing an oxidation reaction, that is, until the concentration of oxygen contained in the exhaust gas flowing through the exhaust pipe 70 reaches the predetermined value. Since the specific organic compound causes an oxidation reaction, the activation of the three-way catalyst of the HC adsorption catalyst 22 is hardly suppressed. As a result, even if the concentration of oxygen contained in the exhaust gas increases, it is difficult to prevent the three-way catalyst from reaching the activation temperature, so that the three-way catalyst sufficiently purifies HC desorbed from the HC adsorbent. Can be.

The maintenance of the predetermined value by the ECU 30 as the exhaust gas-containing oxygen concentration maintaining means can be performed by switching the air-fuel ratio between rich and lean by the ECU 30 as the air-fuel ratio control means. Thus, it is easier to maintain the predetermined value than when the predetermined value is maintained, and the maintenance adjustment is easier.

Further, the predetermined value is maintained by the ECU 30 as the exhaust gas-containing oxygen concentration maintaining means by setting the lean upper limit value of the air-fuel ratio by the ECU 30 as the air-fuel ratio controlling means. Can be suppressed. Therefore, it is more difficult for the three-way catalyst to be at the activation temperature, and HC desorbed from the HC adsorbent can be sufficiently purified by the three-way catalyst.

When the exhaust air-fuel ratio is lean, the E
It is also conceivable to make the combustion of the engine lean using the GR27.

In addition, the HC adsorption catalyst 22 may have a structure including a zeolite such as ZSM5 which is excellent in HC adsorption performance.

Further, in order to more efficiently adsorb HC having a relatively high boiling point close to raw fuel in cold weather and HC having a high boiling point discharged from a diesel engine, a base material supporting a catalyst is known. A wall flow structure may be applied. In this case, the high-boiling point HC has an increased chance of contact with the catalyst, and exhibits a higher HC adsorption effect than a known monolithic catalyst.

Further, when a catalyst having a wall flow structure is used, a particulate matter (Particulate Matter) such as a diesel engine or a gasoline direct injection engine is used.
tter: Particulate matter, hereinafter referred to as “PM”. In the case where the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment is applied to an engine having emissions of (1), a diesel engine particulate filter (hereinafter, referred to as “DP”) that collects and burns PM to purify it.
F ". ) Etc.
A simpler structure can be achieved, and an improvement in the exhaust gas purification action can be expected.

In determining the desorption temperature of HC,
It is also conceivable to realize the above control by estimating based on the duration from the start instead of using the temperature sensor. This eliminates the need for a temperature sensor, so that the number of components can be reduced and a simpler structure can be achieved.

[0066]

According to the exhaust gas purifying apparatus for an internal combustion engine of the present invention,
Purification of the desorbed HC can be sufficiently performed.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine to which an exhaust gas purification device for an internal combustion engine of the present invention is applied.

FIG. 2 is a flowchart for explaining an application program for realizing a desorbed HC purification control execution routine for purifying HC desorbed from the HC adsorbent of the exhaust purification device according to the embodiment with a three-way catalyst;

[Description of Signs] A Exhaust gas purifier 1 Engine body 2 Cylinder block 3 Cylinder head 4 Piston 5 Combustion chamber 6 Electric control type fuel injection valve 6a Fuel supply pipe 7 Intake valve 8 Intake port 9 Exhaust valve 10 Exhaust port 11 Intake branch pipe DESCRIPTION OF SYMBOLS 12 Surge tank 13 Intake duct 14 Exhaust turbocharger 15 Compressor 16 Step motor 17 Throttle valve 18 Cooling device 19 Exhaust manifold 20 Exhaust pipe 21 Turbine 22 HC adsorption catalyst 23 Case body 24 EGR passage 25 Electric control type EGR control valve 26 Cooling device 27 Common rail 28 fuel pump 29 fuel pressure sensor 30 ECU (desorption determining means, air-fuel ratio control means, exhaust gas content concentration maintaining means) 31 bidirectional bus 32 ROM 33 RAM 34 CPU 35 input port 36 output port 37 AD Exchanger 38 drive circuit 40 accelerator pedal 41 a load sensor 42 crank angle sensor 70 an exhaust pipe (exhaust passage) 79 exhaust temperature sensor 83 oxygen sensor (inlet side oxygen concentration detecting means) 90 intake pipe

 ──────────────────────────────────────────────────続 き Continued from the front page F term (reference) PE03Z PF03Z

Claims (4)

[Claims] An HC adsorbing catalyst provided in an engine exhaust passage and having a three-way catalyst and an HC adsorbent; a desorption determining means for judging that HC is desorbed from the HC adsorbent of the HC adsorbing catalyst; Air-fuel ratio control means for controlling the air-fuel ratio to lean when it is determined by the desorption determining means that HC has desorbed from the HC adsorbent; and when the desorption determining means determines that HC has been desorbed, An exhaust gas purifying apparatus for an internal combustion engine, comprising: exhaust gas-containing oxygen concentration maintaining means for maintaining the concentration of oxygen contained in exhaust gas flowing through the engine exhaust passage at a predetermined value by air-fuel ratio control by air-fuel ratio control means. 2. The predetermined value is a limit at which an organic compound, which tends to be hardly oxidized due to an increase in the concentration of oxygen contained in an exhaust gas component, can undergo an oxidation reaction even when the concentration of oxygen contained in the exhaust gas increases. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the value is a value. 3. The internal combustion engine according to claim 2, wherein said predetermined value is maintained by said exhaust gas-containing oxygen concentration maintaining means by switching an air-fuel ratio between rich and lean by said air-fuel ratio control means. Exhaust purification equipment. 4. The internal combustion engine according to claim 3, wherein said predetermined value is maintained by said exhaust gas-containing oxygen concentration maintaining means by setting a lean upper limit to an air-fuel ratio by said air-fuel ratio control means. Engine exhaust purification device.