US20090049825A1

US20090049825A1 - Exhaust Gas Purification Device For Internal Combustion Engine

Info

Publication number: US20090049825A1
Application number: US12/224,997
Authority: US
Inventors: Nobumoto Ohashi
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2006-04-27
Filing date: 2007-04-26
Publication date: 2009-02-26
Also published as: JP2007297918A; EP2013455A1; WO2007126140A1; CN101427010A

Abstract

A NOx absorbent is arranged in an exhaust passage of an internal combustion engine, and a fuel supply valve (28) is arranged in the exhaust passage upstream of the NOx absorbent. If the temperature of the NOx absorbent is lower than a predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from a basic lean air-fuel ratio to and maintained at a lean air-fuel ratio with a lower leanness for a predetermined lean time, and is then switched to a rich air-fuel ratio. If the temperature of the NOx absorbent is higher than the predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of the exhaust gas flowing through the NOx absorbent is switched to the rich air-fuel ratio without being switched to the lean air-fuel ratio with a lower leanness.

Description

FIELD OF THE INVENTION

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

BACKGROUND ART

There is known an internal combustion engine wherein a NOx absorbent is arranged in the exhaust passage of the engine in which an NOx absorbent absorbs NOx contained in the exhaust gas therein when the air-fuel ratio of the exhaust gas is lean and releases absorbed NOx therefrom when the air-fuel ratio of the exhaust gas is switched to rich, wherein a fuel supply valve is arranged in the exhaust passage upstream of the NOx absorbent, and wherein fuel is supplied from the fuel supply valve to the NOx absorbent to make the air-fuel ratio of exhaust gas flowing through the NOx absorbent temporarily rich, when the NOx must be released from the NOx absorbent (see Japanese Unexamined Patent Publication No. 11-62666, for example). In the engine, NOx generated when combustion is carried out under a lean air-fuel ratio is absorbed in the NOx absorbent. On the other hand, when the NOx absorption capacity has reached a saturated state, the air-fuel ratio is temporarily made rich to release NOx from the NOx absorbent and reduce the NOx.
However, for example if an engine is idled for a long time, the temperature of the NOx absorbent is lowered since the temperature of exhaust gas inflowing through the NOx absorbent at this time is low. When the temperature of the NOx absorbent is low as mentioned above, the release rate of NOx from the NOx absorbent is low. Therefore, if the air-fuel ratio of exhaust gas is simply switched to rich, it may be impossible to obtain an adequate release of NOx from the NOx absorbent.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to provide an exhaust gas purification device for an internal combustion engine, which is capable of obtaining an adequate release of NOx from an NOx absorbent even when the temperature of the NOx absorbent is low.
According to the present invention, there is provided an exhaust gas purification device for an internal combustion engine having an exhaust passage, combustion being carried out under a basic lean air-fuel ratio, comprising: a NOx absorbent arranged in the exhaust passage, the NOx absorbent absorbing NOx contained in exhaust gas therein when the air-fuel ratio of exhaust gas is lean and releasing absorbed NOx therefrom when the air-fuel ratio of exhaust gas is switched to rich; and control means for controlling the air-fuel ratio of exhaust gas flowing through the NOx absorbent, wherein, when NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from the basic lean air-fuel ratio to and maintained at a lean air-fuel ratio with a lower leanness for a predetermined lean time, and is then switched to a rich air-fuel ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of an internal combustion engine of a compression ignition type;

FIG. 2 is a sectional side view of a NOx storing catalyst;

FIGS. 3A and 3B are sectional views of a surface part of a catalyst carrier;

FIGS. 4A and 4B are views of the structure of a particulate filter;

FIG. 5 is a time chart explaining a NOx release control;

FIG. 6 is a map illustrating the amount of NOx adsorbed per unit time dNOx;

FIGS. 7A and 7B are time charts illustrating variations of the air-fuel ratio of flowing exhaust gas AFEG;

FIG. 8 is a map illustrating a predetermined temperature TcS;

FIGS. 9A to 9D are maps illustrating lean time tL, respectively; and

FIG. 10 is a flowchart for executing the NOx release control.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates a case where the present invention is applied to an internal combustion engine with a compression type ignition. Alternatively, the present invention may also be applied to an internal combustion engine with a spark type ignition.
Referring to FIG. 1, numeral 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electrically-controlled fuel injector for injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to an outlet of a compressor 7 a of a turbocharger 7. The inlet of the compressor 7 a is connected via an air flow meter 8 to an air cleaner 9. An electrically-controlled throttle valve 10 is arranged in the intake duct 6. Further, a cooling device 11 is arranged around the intake duct 6 for cooling intake air flowing through the intake duct 6. In the embodiment shown in FIG. 1, engine cooling water is guided into the cooling device 11 and cools intake air. On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7 b of the exhaust turbocharger 7, while the outlet of the exhaust turbine 7 b is connected to an exhaust aftertreatment system 20.
The exhaust manifold 5 and the intake manifold 4 are interconnected through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage 12. The EGR passage 12 is provided with an electrically-controlled EGR control valve 13. Further, a cooling device 14 is arranged around the EGR passage 12 for cooling EGR gas flowing through the EGR passage 12. In the embodiment shown in FIG. 1, engine cooling water is guided into the cooling device 14 and cools the EGR gas. Each fuel injector 3 is connected through a fuel feed tube 15 to a common rail 16. This common rail 16 is supplied with fuel from an electrically-controlled type variable discharge fuel pump 20. Fuel supplied into the common rail 16 is supplied through each fuel feed tube 15 to the fuel injector 3.
The exhaust aftertreatment system 20 comprises an exhaust pipe 21 connected to an outlet of the exhaust turbine 7 b, a catalytic converter 22 connected to the exhaust pipe 21, and an exhaust pipe 23 connected to the catalytic converter 22. A NOx storing catalyst 24 and a particulate filter 25 are arranged in the catalytic converter 22 in order, starting from the upstream side. In addition, a temperature sensor 26 for detecting the temperature of exhaust gas discharged from the catalytic converter 22 and an air-fuel ratio sensor 27 for detecting the air-fuel ratio of exhaust gas discharged from the catalytic converter 22 are arranged in the exhaust pipe 23. The temperature of exhaust gas discharged from the catalytic converter 22 represents the temperature of the NOx storing catalyst 24 and the particulate filter 25.
On the other hand, the exhaust manifold 5 is provided with a fuel supply valve 28. The fuel supply valve 28 is supplied with fuel from the common rail 16, the fuel is fed from the fuel supply valve 28 to the exhaust manifold 5. In the embodiment according to the present invention, fuel is comprised of light oil. The fuel supply valve 28 may be arranged in the exhaust pipe 21, alternatively.
An electronic control unit 30 is comprised of a digital computer provided with read only memory (ROM) 32, random access memory (RAM) 33, a microprocessor (CPU) 34, an input port 35, and an output port 36, all connected to each other by a bidirectional bus 31. The output signals of the air flow meter 8, the temperature sensor 26 and the air-fuel ratio sensor 27 are input through corresponding AD converters 37 to the input port 35. Further, connected to the accelerator pedal 39 is a load sensor 40 generating output voltage proportional to the amount of the depression L of an accelerator pedal 39. Outputted voltage of the load sensor 40 is input through a corresponding AD converter 37 to the input port 35. Furthermore, connected to the input port 35 is a crank angle sensor 41 generating an output pulse each time the crankshaft turns, for example, by 15 degrees. The CPU 34 calculates engine speed N based on the output pulse from the crank angle sensor 41. On the other hand, the output port 36 is connected through corresponding drive circuits 38 to the fuel injectors 3, driver for the throttle valve 10, EGR control valve 13, fuel pump 20, and fuel supply valve 28.
FIG. 2 shows the structure of the NOx storing catalyst 24. In the embodiment shown in FIG. 2, the NOx storing catalyst 24 is formed of a honeycomb structure and is provided with a plurality of exhaust gas passages 61 separated from each other by partitions 60. The opposite surfaces of the partitions 60 carry a catalyst carrier comprised of, for example, alumina. FIGS. 3A and 3B schematically show the cross-section of the surface part of this catalyst carrier 65. As shown in FIGS. 3A and 3B, the catalyst carrier 65 carries a precious metal catalyst 66 diffused on its surface. Further, the catalyst carrier 65 is formed with a layer of a NOx absorbent 67 on its surface.
In the embodiment according to the present invention, platinum Pt is used as the precious metal catalyst 66. As the ingredient for forming the NOx absorbent 67, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Ca, or another alkali earth, lanthanum La, yttrium Y, or another rare earth may be used.
The ratio of air and fuel (hydrocarbons) supplied to the engine intake passage, combustion chambers 2, and exhaust passage upstream of the NOx storing catalyst 24 is referred to as an air-fuel ratio of the exhaust gas. The NOx absorbent 67 performs NOx absorption and release action of absorbing the NOx when the air-fuel ratio of the exhaust gas is lean and releasing the absorbed NOx when the oxygen concentration in the exhaust gas falls.
That is, if in the case of using barium Ba as the ingredient forming the NOx absorbent 67, when the air-fuel ratio of exhaust gas is lean, that is, when the oxygen concentration in exhaust gas is high, the NO contained in the exhaust gas is oxidized on the platinum Pt 66 such as shown in FIG. 3A to become NO₂, and is then absorbed in the NOx absorbent 67 and diffused in the NOx absorbent 67 in the form of nitric acid ions NO₃ ⁻ while bonding with the barium carbonate BaCO₃. In this way, NOx is absorbed in the NOx absorbent 67. If the oxygen concentration in the exhaust gas is high, NO₂is produced on the surface of the platinum Pt 66. If the NOx absorbing capability of the NOx absorbent 67 is not saturated, the NO₂is absorbed in the NOx absorbent 67 and nitric acid ions NO₃ ⁻ are produced.
In contrast, when the air-fuel ratio of the exhaust gas is made rich or a stoichiometric air-fuel ratio, since the oxygen concentration in the exhaust gas falls, the reaction proceeds in the reverse direction (NO₃ ⁻->NO₂), and therefore nitric acid ions NO₃ ⁻ in the NOx absorbent 67 are released from the NOx absorbent 67 in the form of NO₂. The released NOx is then reduced to unburned hydrocarbons or CO that is included in exhaust gas.
In the engine shown in FIG. 1, combustion under a lean air-fuel ratio is continued, and the air-fuel ratio of the exhaust gas flowing through the NOx absorbent 67 is thus maintained lean so long as the fuel supply from the fuel supply valve 28 is stopped. The NOx included in exhaust gas is absorbed into the NOx absorbent 67 at this stage. However, if combustion under a lean air-fuel ratio is continued, the NOx absorbing capability of the NOx absorbent 67 will end up becoming saturated, and therefore NOx will no longer be able to be absorbed by the NOx absorbent 67. Therefore, in the embodiment according to the present invention, before the absorbing capability of the NOx absorbent 67 becomes saturated, fuel is supplied from the fuel supply valve 28 so as to temporarily make the air-fuel ratio of the exhaust gas rich, and thereby release NOx from the NOx absorbent 67.
FIGS. 4A and 4B show the structure of the particulate filter 25. Note that FIG. 4A is a front view of the particulate filter 25, while FIG. 4B is a side sectional view of the particulate filter 25. As shown in FIGS. 4A and 4B, the particulate filter 25 forms a honeycomb structure and is provided with a plurality of exhaust passages 70 and 71 extending parallel with each other. These exhaust passages are comprised of exhaust gas inflow passages 70 with downstream ends sealed by plugs 72 and exhaust gas outflow passages 71 with upstream ends sealed by plugs 73. Note that the hatched portions in FIG. 4A show plugs 73. Therefore, the exhaust gas inflow passages 70 and exhaust gas outflow passages 71 are arranged alternately through thin wall partitions 74. In other words, the exhaust gas inflow passages 70 and exhaust gas outflow passages 71 are arranged so that each exhaust gas inflow passage 70 is surrounded by four exhaust gas outflow passages 71, and each exhaust gas outflow passage 71 is surrounded by four exhaust gas inflow passages 70.
The particulate filter 25 is formed from a porous material such as cordierite. Therefore, exhaust gas flowing into the exhaust gas inflow passages 70 flows out into the adjoining exhaust gas outflow passages 71 through the surrounding partitions 74 as shown by the arrows in FIG. 4B.
In the embodiment according to the present invention, the peripheral walls of the exhaust gas inflow passages 70 and exhaust gas outflow passages 71, that is, the opposite surfaces of the partitions 74 and the inside walls of the micropores of the partitions 74 also carry a catalyst carrier comprised of, for example, alumina. As shown in FIGS. 3A and 3B, the catalyst carrier 65 carries a precious metal catalyst 66 diffused on its surface. Further, the catalyst carrier 65 is formed with a layer of the NOx absorbent 67 on its surface.
Therefore, combustion under a lean air-fuel ratio is carried out, NOx contained in the exhaust gas is also absorbed in the NOx absorbent 67 carried on the particulate filter 25. The thus absorbed NOx is released and reduced by supplying fuel from the fuel supply valve 28.
On the other hand, the particulate matter contained in the exhaust gas is trapped on the particulate filter 25 and successively oxidized. However, if the amount of the particulate matter trapped becomes greater than the amount of the particulate matter oxidized, the particulate matter will gradually be deposited on the particulate filter 25. In this case, if the amount of particulate matter deposited increases, engine output may be decreased. Therefore, it is necessary to remove the deposited particulate matter when the amount of particulate matter deposited increases. In this case, if raising the temperature of the particulate filter 25 under an excess of air to about 600° C., the deposited particulate matter is oxidized and removed.
In the embodiment according to the present invention, when the amount of the particulate matter deposited on the particulate filter 25 exceeds an allowable amount, fuel is supplied from the fuel supply valve 28 while the air-fuel ratio of the exhaust gas flowing in the particulate filter 25 is maintained lean, and then raising the temperature of the particulate filter 25 by the oxidation heat of the thus supplied fuel, and thereby oxidizing and removing the deposited particulate matter.
Note that the NOx storing catalyst 24 may be omitted in FIG. 1. In addition, in FIG. 1, a particulate filter that does not carry NOx absorbent 67 may be used as a particulate filter 25.
In the embodiment according to the present invention, whenever a cumulative amount ΣNOx of NOx absorbed in the NOx absorbent 67 exceeds an allowable amount MAX as indicated by X in FIG. 5, fuel is supplied from the fuel supply valve 28 in the form of successive pulses, and thereby the air-fuel ratio of the exhaust gas flowing through the NOx absorbent 67, which is carried on the NOx storing catalyst 24 and the particulate filter 25, is switched to rich temporarily. As a result, NOx is released from the NOx absorbent 67 and is reduced. Alternatively, fuel may be supplied to the NOx absorbent 67 by injecting additional fuel from the fuel injectors 3 during the power or exhaust stroke.
In this case, in the embodiment according to the present invention, the amount of NOx dNOx absorbed in the NOx absorbent 67 per unit of time is stored in ROM 32 in advance in the form of a map as shown in FIG. 6 as a function of the required torque TQ and engine speed N. The cumulative NOx amount ΣNOx is calculated by a cumulation of the NOx amount of dNOx.
However, as mentioned at the beginning of this specification, when the temperature of the NOx absorbent 67 is low, it may be impossible to obtain an adequate release of NOx from the NOx absorbent if the air-fuel ratio of the exhaust gas is simply switched to rich.
Therefore, in the embodiment according to the present invention, the temperature Tc of the NOx absorbent 67 is first detected, and the air-fuel ratio of the exhaust gas flowing to the NOx absorbent 67 is switched to a rich air-fuel ratio or is changed depending on the absorbent temperature Tc. This will be explained with reference to FIGS. 7A and 7B.
FIG. 7A shows a case where the temperature Tc of the NOx absorbent 67 is lower than a predetermined temperature TcS. As shown in FIG. 7A, fuel supply from the fuel supply valve 28 is not carried out until the timing indicated by X, that is, until the cumulative NOx amount ΣNOx exceeds the allowable amount MAX and NOx must be released from the NOx absorbent 67 (see FIG. 5). At this time, the air-fuel ratio AFEG of exhaust gas flowing through the NOx absorbent 67 is maintained at a lean air-fuel ratio. If the lean air-fuel ratio at this time is a basic lean air-fuel ratio AFLB, the basic air-fuel ratio AFLB then conforms to the air-fuel ratio in the combustion chambers 2, in the engine shown in FIG. 1.
When NOx must be released from the NOx absorbent 67 as indicated by X in FIG. 7A, fuel from the fuel supply valve 28 is switched to start the air-fuel ratio of the inflowing exhaust gas AFEG from the basic lean air-fuel ratio AFLB to a lean air-fuel ratio with a lower leanness AFLL. When the air-fuel ratio of the inflowing exhaust gas AFEG is maintained at a lean air-fuel ratio with a lower leanness AFLL for a lean time tL, it is followed by the air-fuel ratio of the inflowing exhaust gas AFEG being switched to a rich air-fuel ratio AFR. When the air-fuel ratio of the inflowing exhaust gas AFEG is maintained at a rich air-fuel ratio AFR for a rich time tR, the fuel supply is then stopped and the air-fuel ratio of the inflowing exhaust gas AFEG is returned to a basic lean air-fuel ratio AFLB.
When the air-fuel ratio of the inflowing exhaust gas AFEG is switched to and maintained at the lean air-fuel ratio with a lower leanness AFLL, the amount of unburned HC and CO contained in the exhaust gas is increased, compared to when the air-fuel ratio of the inflowing exhaust gas AFEG is a basic lean air-fuel ratio AFLB. The increased amount of unburned HC and CO will be oxidized in the NOx absorbent 67 under the presence of excess oxygen, and thus the temperature Tc of the NOx absorbent 67 increases rapidly. Therefore, the air-fuel ratio of inflowing exhaust gas AFEG is switched to the rich air-fuel ratio AFR after the temperature Tc of the NOx absorbent 67 is high, and an adequate NOx release from the NOx absorbent 67 is accordingly obtained.
In addition, in the embodiment according to the present invention, the air-fuel ratio of the inflowing exhaust gas AFEG is returned from the rich air-fuel ratio AFR back to the basic lean air-fuel ratio AFLB, and is maintained at the basic lean air-fuel ratio AFLB until the NOx must be released from the NOx absorbent 67 again as shown in FIG. 5. In other words, fuel from the fuel supply valve 28 is stopped when the air-fuel ratio of the inflowing exhaust gas AFEG is returned back to the basic lean air-fuel ratio AFLB until the cumulative NOx amount ΣNOx exceeds the allowable amount MAX again. This ensures that an increment in the temperature of the NOx absorbent 67 is carried out only when it is necessary, and that supplied fuel is used effectively for NOx release and reduction. Note that NOx is well absorbed in the NOx absorbent 67 even when the temperature Tc of the NOx absorbent 67 is lower than the predetermined temperature TcS.
In contrast, if the temperature Tc of the NOx absorbent 67 is higher than the predetermined temperature TcS when the NOx must be released from the NOx absorbent 67, as indicated by X in FIG. 7B, the air-fuel ratio of the inflowing exhaust gas AFEG is immediately switched to the rich air-fuel ratio AFR, without being switched to the lean air-fuel ratio with a lower leanness AFLL. When the air-fuel ratio of the inflowing exhaust gas AFEG is maintained at the rich air-fuel ratio AFR for the rich time tR, the fuel is stopped and the air-fuel ratio of the inflowing exhaust gas AFEG is returned to the basic lean air-fuel ratio AFLB. That is, in this case, it is not necessary for the temperature Tc of the NOx absorbent 67 to be increased.
As can be understood from the above explanation, the predetermined temperature TcS is a temperature required for a good release of NOx from the NOx absorbent 67. The temperature necessary for a good release of NOx from the NOx absorbent 67 will vary depending on the degree of deterioration of the NOx absorbent 67. Therefore, in the embodiment according to the present invention, the degree of the deterioration DET of the NOx absorbent 67 is first detected, and the predetermined temperature TcS is then determined depending on the degree of deterioration DET of the NOx absorbent 67. Specifically, the predetermined temperature TcS is set higher as the degree of deterioration DET becomes higher, as shown in FIG. 8. The predetermined temperature TcS is stored in ROM 32 in advance, in the form of a map as shown in FIG. 8. Note that there are many procedures for obtaining the degree of deterioration DET of the NOx absorbent 67. For example, the degree of deterioration DET of the NOx absorbent 67 may be judged to be higher as the increment of the temperature Tc of the NOx absorbent 67 obtained when fuel is supplied from the fuel supply valve 28 to the NOx absorbent 67 is smaller.
On the other hand, TcY indicated in FIG. 7A is the temperature Tc of the NOx absorbent 67 when the lean time tL has elapsed from when the air-fuel ratio of inflowing exhaust gas AFEG is switched to the lean air-fuel ratio with a lower leanness AFLL. If the temperature TcY conforms approximately to the predetermined temperature TcS mentioned above, an adequate NOx release will be obtained while the amount of fuel from the fuel supply valve 28 is kept low. Therefore, the lean time tL is the amount of time required to increase the temperature Tc of the NOx absorbent 67 to approximately the predetermined temperature TcS when the air-fuel ratio of the inflowing exhaust gas AFEG is maintained at the lean air-fuel ratio with a lower leanness AFLL.
In this case, the lean time tL becomes longer as the temperature Tc of the NOx absorbent 67 becomes lower as shown in FIG. 9A, as the amount of intake air Ga becomes larger as shown in FIG. 9B, and as the degree of deterioration DET of the NOx absorbent 67 becomes higher as shown in FIG. 9C. In the embodiment according to the present invention, the lean time tL is stored in ROM 32 in advance, in the form of a map shown in FIG. 9D, as a function of the temperature Tc and the degree of deterioration DET of the NOx absorbent 67 and the amount of intake air Ga. Here, the amount of intake air Ga represents the amount of exhaust gas flowing through the NOx absorbent 67.
Note that, when fuel supply from the fuel supply valve 28 is carried out, the air-fuel ratio of the inflowing exhaust gas AFEG is made leaner by reducing the number of fuel supply pulses per unit time, and is made richer by increasing the number fuel pulses per unit time.
FIG. 10 shows a routine of the NOx release control.
Referring to FIG. 10, the routine proceeds to step 100 where the amount of NOx ΣNOx absorbed in the NOx absorbent 67 is calculated. Specifically, in the embodiment according to the present invention, the amount of NOx dNOx adsorbed in the NOx absorbent 67 per unit time is calculated using the map shown in FIG. 6, and is then added to the absorbed NOx amount ΣNOx. In the following step 101, it is determined whether the absorbed NOx amount ΣNOx exceeds the allowable amount MAX. When the amount is ΣNOx≦MAX, the processing cycle is ended. In contrast, when the amount is ΣNOx>MAX, the routine proceeds to step 102 where the predetermined temperature TcS is calculated using the map shown in FIG. 8. In the following step 103, it is determined whether the temperature Tc of the NOx absorbent 67 is lower than the predetermined temperature TcS. When the amount is Tc<TcS, the routine proceeds to step 104, where the lean time tL is calculated using the map shown in FIG. 9D. In the following step 105, the fuel supply valve 28 supplies fuel to maintain the air-fuel ratio of inflowing exhaust gas AFEG at the lean air-fuel ratio with a lower leanness AFLL for the lean time tL. Then, the routine proceeds to step 106. In contrast, when the amount is Tc≧TcS, the routine jumps from step 103 to step 106. In step 106, the fuel supply valve 28 supplies fuel to maintain the air-fuel ratio of the inflowing exhaust gas AFEG at the rich air-fuel ratio AFR for the rich time tR. In the following step 107, the absorbed NOx amount ΣNOx is returned to zero.

Claims

1. An exhaust gas purification device for an internal combustion engine having an exhaust passage, combustion being carried out under a basic lean air-fuel ratio, comprising:

a NOx absorbent arranged in the exhaust passage, the NOx absorbent absorbing NOx contained in exhaust gas therein when the air-fuel ratio of exhaust gas is lean and releasing absorbed NOx therefrom when the air-fuel ratio of exhaust gas is switched to rich; and

control means for controlling the air-fuel ratio of exhaust gas flowing through the NOx absorbent,

wherein, when NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from the basic lean air-fuel ratio to and maintained at a lean air-fuel ratio with a lower leanness for a predetermined lean time, and is then switched to a rich air-fuel ratio.

2. An exhaust gas purification device for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas flowing through the NOx absorbent is returned to and maintained at the basic lean air-fuel ratio until the NOx must be released from the NOx absorbent once more.

3. An exhaust gas purification device for an internal combustion engine according to claim 1, further comprising means for obtaining a temperature of the NOx absorbent, wherein, if the temperature of the NOx absorbent is lower than a predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from the basic lean air-fuel ratio to and maintained at the lean air-fuel ratio with a lower leanness for the predetermined lean time, and is then switched to the rich air-fuel ratio, if the temperature of the NOx absorbent is higher than the predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is switched to the rich air-fuel ratio without being switched to the lean air-fuel ratio with a lower leanness.

4. An exhaust gas purification device for an internal combustion engine according to claim 3, further comprising means for obtaining a degree of deterioration of the NOx absorbent, wherein the predetermined temperature when the degree of deterioration of the NOx absorbent is high is set higher than that when the degree of deterioration is low.

5. An exhaust gas purification device for an internal combustion engine according to claim 1, further comprising means for obtaining a temperature of the NOx absorbent, wherein the lean time is set in accordance with the obtained temperature of the NOx absorbent.

6. An exhaust gas purification device for an internal combustion engine according to claim 1, further comprising means for obtaining an amount of the exhaust gas flowing through the NOx absorbent, wherein the lean time is set in accordance with the amount obtained in the exhaust gas.

7. An exhaust gas purification device for an internal combustion engine according to claim 1, further comprising means for obtaining a degree of deterioration of the NOx absorbent, wherein the lean time is set in accordance with the obtained degree of deterioration.

8. An exhaust gas purification device for an internal combustion engine according to claim 1, wherein the lean time is set to make the temperature of the NOx absorbent or the increment thereof, obtained by maintaining the air-fuel ratio of the exhaust gas inflowing the NOx absorbent at the lean air-fuel ratio with a lower leanness for the lean time, equal to a target value.