Exhaust Gas Purification Device For Internal Combustion Engine
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.
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The present invention relates to an exhaust gas purification device for an internal combustion engine.
BACKGROUND ARTThere 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 INVENTIONIt 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.
Referring to
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
The exhaust aftertreatment system 20 comprises an exhaust pipe 21 connected to an outlet of the exhaust turbine 7b, 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.
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
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
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
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
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
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
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
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
When NOx must be released from the NOx absorbent 67 as indicated by X in
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
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
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
On the other hand, TcY indicated in
In this case, the lean time tL becomes longer as the temperature Tc of the NOx absorbent 67 becomes lower as shown in
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.
Referring to
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: 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.
- 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,
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.
Type: Application
Filed: Apr 26, 2007
Publication Date: Feb 26, 2009
Applicant: TOYOTA JIDOSHA KABUSHIKI KAISHA (Toyota-shi, Aichi)
Inventor: Nobumoto Ohashi (Susono-shi)
Application Number: 12/224,997
International Classification: F01N 9/00 (20060101);