Exhaust purification device of an internal combustion engine

In an internal combustion engine, a pair of NOx storing catalysts are arranged in series in an engine exhaust passage. When causing the upstream NOx storing catalyst to release NOx and store the released NOx in the downstream NOx storing catalyst, the oxygen concentration in the exhaust gas is temporarily reduced in a state where the exhaust gas is maintained at a lean air-fuel ratio. As opposed to this, when releasing NOx from the upstream NOx storing catalyst and the downstream NOx storing catalyst and reducing the NOx, the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich.

TECHNICAL FIELD

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

BACKGROUND ART

Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOxstoring catalyst which stores NOxcontained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases stored NOxwhen the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich (see for example Japanese Patent Publication (A) No. 2004-108176). In this internal combustion engine, if the NOxstorage ability of the NOxstoring catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich whereby NOxis released from the NOxstoring catalyst and reduced.

In this way, in the past, when releasing NOxfrom an NOxstoring catalyst, the air-fuel ratio of the NOxexhaust gas is made the stoichiometric air-fuel ratio or rich to release the NOxwhich is then reduced in the NOxstoring catalyst. However, depending on the case, there are also cases when it is preferable for the NOxreleased from the NOxstoring catalyst to be exhausted from the NOxstoring catalyst without being reduced.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device of an internal combustion engine reducing or not reducing NOxreleased from an NOxstoring catalyst according to need.

According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging in series in an engine exhaust passage at least a pair of NOxstoring catalysts which store NOxcontained in an exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and release stored NOxwhen an oxygen concentration of the inflowing exhaust gas drops, the oxygen concentration of the exhaust gas being temporarily reduced while the exhaust gas is maintained at a lean air-fuel ratio when releasing NOxfrom an upstream NOxstoring catalyst and storing the released NOxin a downstream NOxstoring catalyst, the air-fuel ratio of the exhaust gas being temporarily switched from lean to rich when releasing NOxfrom the upstream NOxstoring catalyst and the downstream NOxstoring catalyst and reducing NOx.

In the present invention, by moving the NOxstored in the upstream NOxstoring catalyst, which has a comparatively high catalyst temperature and a high NOxstorage ability, to the downstream NOxstoring catalyst according to need, the NOxstorage ability of the upstream NOxstoring catalyst can be restored.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1shows an overview of a compression ignition type internal combustion engine.

Referring toFIG. 1,1indicates an engine body,2a combustion chamber of each cylinder,3an electronically controlled fuel injector for injecting fuel into each combustion chamber2,4an intake manifold, and5an exhaust manifold. The intake manifold4is connected through an intake duct6to the outlet of a compressor7aof an exhaust turbocharger7. The inlet of the compressor7ais connected to an air cleaner9via an intake air amount detector8. Inside the intake duct6is arranged a throttle valve10driven by a step motor. Further, around the intake duct6is arranged a cooling device11for cooling the intake air flowing through the inside of the intake duct6. In the embodiment shown inFIG. 1, the engine cooling water is guided into the cooling device11. The engine cooling water cools the intake air.

On the other hand, the exhaust manifold5is connected to the inlet of an exhaust turbine7bof the exhaust turbocharger7. The outlet of the exhaust turbine7bis connected to the inlet of an upstream NOxstoring catalyst12, and the outlet of the upstream NOxstoring catalyst12is connected to the inlet of a downstream NOxstoring catalyst14via an exhaust pipe13. Note that another NOxstoring catalyst may be further arranged downstream of the downstream NOxstoring catalyst14. On the other hand, a reducing agent feed valve15for feeding a reducing agent comprised of a hydrocarbon into the exhaust gas flowing within the exhaust manifold5is attached to the exhaust manifold5.

The exhaust manifold5and the intake manifold4are interconnected through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage16, and an electronically controlled EGR control valve17is arranged in the EGR passage16. Further, around the EGR passage16is arranged a cooling device18for cooling the EGR gas flowing through the inside of the EGR passage16. In the embodiment shown inFIG. 1, the engine cooling water is guided into the cooling device18. The engine cooling water cools the EGR gas. On the other hand, each fuel injector3is connected through a fuel feed tube19to a common rail20. This common rail20is supplied with fuel from an electronically controlled variable discharge fuel pump21. The fuel supplied into the common rail20is supplied through each fuel feed tube19to the fuel injector3.

An electronic control unit30is comprised of a digital computer provided with a read only memory (ROM)32, a random access memory (RAM)33, a microprocessor (CPU)34, an input port35, and an output port36all connected to each other by a bidirectional bus31. A temperature sensor22for detecting the temperature of the exhaust gas flowing within the exhaust pipe13is arranged in the exhaust pipe13. When the temperature of the exhaust gas flowing within the exhaust pipe13changes, the temperatures of the upstream NOxstoring catalyst12and the downstream NOxstoring catalyst14accordingly change and, thus, the temperature of the exhaust gas flowing within the exhaust pipe13represents the temperatures of a pair of the NOxstoring catalysts12,14arranged in series. As shown inFIG. 1, the output signals of the temperature sensor22and the intake air amount detector8are input through corresponding AD converters37to the input port35.

An accelerator pedal40has a load sensor41generating an output voltage proportional to the amount of depression L of the accelerator pedal40connected to it. The output voltage of the load sensor41is input through a corresponding AD converter37to the input port35. Further, the input port35has a crank angle sensor42generating an output pulse each time the crankshaft turns for example by 15 degrees connected to it. On the other hand, the output port36is connected through corresponding drive circuits38to the fuel injectors3, the step motor for driving the throttle valve10, reducing agent feed valve15, EGR control valve17, and fuel pump21.

First, explaining the NOxstoring catalysts12,14shown inFIG. 1, a catalyst carrier comprised of for example alumina is carried on the NOxstoring catalysts12,14, andFIG. 2schematically show the cross-section of the surface part of this catalyst carrier45. As shown inFIG. 2, the catalyst carrier45carries a precious metal catalyst46diffused on its surface. Further, the catalyst carrier45is formed with a layer of an NOxabsorbent47on its surface.

In this embodiment of the present invention, platinum Pt is used as the precious metal catalyst46. As the ingredient forming the NOxabsorbent47, 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 is used.

If the ratio of the air and fuel (hydrocarbons) supplied to the engine intake passage, combustion chambers2, and exhaust passage upstream of the NOxstoring catalyst12is referred to as the “air-fuel ratio of the exhaust gas”, the NOxabsorbent47performs an NOxabsorption and release action of storing the NOxwhen the air-fuel ratio of the exhaust gas is lean and releasing the stored NOxwhen the oxygen concentration in the exhaust gas falls.

That is, if explaining this taking as an example the case of using barium Ba as the ingredient forming the NOxabsorbent47, when the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas is oxidized on the platinum Pt46such as shown inFIG. 2to become NO2, then is absorbed in the NOxabsorbent47and diffuses in the NOxabsorbent47in the form of nitric acid ions NO3−while bonding with the barium oxide BaO. In this way, the NOxis absorbed in the NOxabsorbent47. So long as the oxygen concentration in the exhaust gas is high, NO2is produced on the surface of the platinum Pt46. So long as the NOxabsorbing capability of the NOxabsorbent47is not saturated, the NO2is absorbed in the NOxabsorbent47and nitric acid ions NO3−are produced.

As opposed to this, if a reducing agent is fed from the reducing agent feed valve15, the air-fuel ratio of the exhaust gas becomes small. At this time, since oxygen contained in the exhaust gas is consumed to oxidize the reducing agent, the oxygen concentration in the exhaust gas drops. If the oxygen concentration in the exhaust gas drops, the reaction proceeds in the reverse direction (NO3−→NO2or NO) as shown inFIGS. 3(A), (B) and therefore the nitric acid ions NO3−in the NOxabsorbent47are released from the NOxabsorbent47in the form of NO2or NO. Namely, if the oxygen concentration in the inflow exhaust gas drops, NOxis released from the NOxabsorbent47.

However, when lowering the air-fuel ratio of the exhaust gas while maintaining a lean air-fuel ratio of the exhaust gas and reducing the oxygen concentration of the exhaust gas, the exhaust gas contains a large amount of oxygen O2in addition to a reducing agent HC as shown inFIG. 3(A). If a large amount of oxygen O2is contained in the exhaust gas in this way, the NO2or the NO released from the NOxabsorbent47will not be reduced any further.

On the other hand, if the exhaust gas is made a rich air-fuel ratio, almost all of the oxygen in the exhaust gas ends up being consumed, therefore, as shown inFIG. 3(B), almost none of the oxygen O2remains in the exhaust gas and therefore the reducing agent HC or CO is present in the exhaust gas. If there is a reducing agent HC or CO present in a state where there is no oxygen O2remaining, the NO2or NO released from the NOxabsorbent47will be reduced down to N2. Accordingly, at this time, NOxis not exhausted from the NOxstoring catalyst.

When the air-fuel ratio of the exhaust gas is lean as mentioned above, that is, when combustion is performed under a lean air-fuel ratio, the NOxin the exhaust gas is absorbed in the NOxabsorbent47. In this case, since the upstream NOxstoring catalyst12has a higher temperature in comparison to the downstream NOxstoring catalyst14, the upstream NOxstoring catalyst12has a higher NOxstorage ability, accordingly, NOxis absorbed more easily in the NOxabsorbent47of the upstream NOxstoring catalyst12. However, if combustion is continuously performed under a lean air-fuel ratio, the NOxabsorption ability of the NOxabsorbent47eventually ends up becoming saturated, and, therefore, the NOxabsorbent47becomes unable to absorb NOxany longer Therefore, in the embodiment according to the present invention, before the NOxabsorbent47becomes saturated in absorption ability, a reducing agent is fed from the reducing agent feed valve15so as to temporarily make the air-fuel ratio of the exhaust gas rich and thereby make the NOxabsorbent47release NOx.

In this regard, when the temperature of the NOxstoring catalyst is low and the catalyst46is not activated sufficiently, if a large amount of reducing agent HC is fed from the reducing agent feed valve15to make the air-fuel ratio of the exhaust gas rich, a problem occurs in that a considerable amount of reducing agent HC is exhausted into the atmosphere because the reducing agent HC is not reduced sufficiently. Therefore, in the present invention, when the temperatures of the NOxstoring catalysts12and14are low and the catalyst46is not activated sufficiently, only an amount of reducing agent that is able to be oxidized is fed. At this time, the oxygen concentration in the exhaust gas drops while the exhaust gas is maintained at a lean air-fuel ratio, therefore, as seen fromFIG. 3(A), NO2or NO is exhausted from the upstream NOxstoring catalyst12and this NO2or NO is stored in the downstream NOxstoring catalyst14.

Next, this will be explained while referring toFIG. 4. Note that, inFIG. 4, TC is the temperature detected by the temperature sensor22, that is, indicates a temperature representing the temperature of the pair of NOxstoring catalysts12and14. Further, ΣNOX1indicates the stored NOxamount stored in the upstream NOxstoring catalyst12, ΣNOX2indicates the stored NOxamount stored in the downstream NOxstoring catalyst14, and A/F indicates the air-fuel ratio of the inflowing gas of the upstream NOxstoring catalyst12which changes according to the feed of reducing agent HC from the reducing agent feed valve15.

In the embodiment according to the present invention, the stored NOxamounts ΣNOX1and ΣNOX2are calculated according to the NOxamount NOXA exhausted per unit time from the combustion chamber2, the NOxstorage speed of the upstream NOxstoring catalyst12, etc. Here, the NOxamount NOXA exhausted per unit time from the combustion chamber2is stored as a function of the required torque TQ and engine speed N in the form of a map as shown inFIG. 5(A)in advance in the ROM32. If this exhausted NOxamount NOXA is smaller than the NOxstorage speed of the upstream NOxstoring catalyst12, that is, the maximum NOxstorage amount MAX that the upstream NOxstoring catalyst12is able to store per unit time, the exhausted NOxamount NOXA becomes the NOxamount NOX1stored per unit time in the upstream NOxstoring catalyst12. By cumulatively adding these NOxamounts NOX1, the stored NOxamount ΣNOX1is obtained.

As opposed to this, if the exhausted NOxamount NOXA is larger than the NOxstorage speed of the upstream NOxstoring catalyst12, that is, the maximum NOxstorage amount MAX that the upstream NOxstoring catalyst12is able to store per unit time, this maximum NOxstorage amount MAX becomes the NOxamount NOX1stored per unit time in the upstream NOxstoring catalyst12. At this time, the surplus NOxamount (NOXA−MAX) that was not stored in the upstream NOxstoring catalyst12becomes the NOxamount NOX2stored per unit time in the downstream NOxstoring catalyst14. By cumulatively adding these NOxamounts NOX2, the stored NOxamount ΣNOX2is obtained.

Note that, the maximum NOxstorage amount MAX that the upstream NOxstoring catalyst12can store per unit time is a function of the bed temperature of the upstream NOxstoring catalyst12, the storage NOxamount ΣNOX1, and the exhaust gas amount, that is, the intake air amount, etc. The maximum NOxstorage amount MAX is stored as a function of these in advance in the ROM32. Note that,FIG. 5(B)shows, as an example, the relationship of the maximum NOxstorage amount MAX and the bed temperature T of the upstream NOxstoring catalyst12.

As shown inFIG. 4, the air-fuel ratio A/F continues to be in a lean state until the time t1. In this period, the NOxstorage amounts ΣNOX1and ΣNOX2gradually increase. Next, at the time t1, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12reaches the allowable value X1. At this time, if the representative temperature TC representative of the NOxstoring catalysts12and14is lower than a predetermined set temperature TX, as shown inFIG. 4, the air-fuel ratio A/F of the exhaust gas is lowered under a lean air-fuel ratio. At this time, NOxis released from the upstream NOxstoring catalyst12and this released NOxis stored in the downstream NOxstoring catalyst14. Accordingly, at this time, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12drops, and the NOX storage amount ΣNOX2of the downstream NOxstoring catalyst14increases.

Next, assume that, at the time t2, the NOxstorage amount ΣNOX2of the downstream NOxstoring catalyst14reaches the allowable value X2. At this time, if assuming the representative temperature TC of the NOxstoring catalysts12and14is higher than the predetermined set temperature TX, as shown inFIG. 4, the air-fuel ratio A/F of the exhaust gas is switched from lean to rich. At this time, NOxis released from the upstream NOxstoring catalyst12and the downstream NOxstoring catalyst14, and the released NOxis reduced. Accordingly, at this time, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12and the NOxstorage amount ΣNOX2of the downstream NOxstoring catalyst14are both reduced.

Next, assume that, at the time t3, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12reaches the allowable value X1. At this time, if the representative temperature TC of the NOxstoring catalysts12and14is higher than the predetermined set temperature TX, the air-fuel ratio A/F of the exhaust gas is switched from lean to rich as shown inFIG. 4. At this time, NOxis released from the upstream NOxstoring catalyst12and the downstream NOxstoring catalyst14, and the released NOxis reduced. Accordingly, at this time, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12and the NOxstorage amount ΣNOX2of the downstream NOxstoring catalyst14are both reduced.

Next, assume that, at the time t4, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12once again reaches the allowable value X1. At this time, if the representative temperature TC of the NOxstoring catalysts12and14is lower than the predetermined set temperature TX, as shown inFIG. 4, the air-fuel ratio A/F of the exhaust gas is lowered under a lean air-fuel ratio. At this time, NOxis released from the upstream NOxstoring catalyst12, and this released NOxis stored in the downstream NOxstoring catalyst14. Accordingly, at this time, the NOxstorage amount ΣNOX1of the upstream NOxstoring catalyst12drops, and the NOxstorage amount ΣNOX2of the downstream NOxstoring catalyst14increases.

FIG. 6shows the purification processing routine of the NOx. This routine is executed by interruption every predetermined time period.

Referring toFIG. 6, first, at step50, the exhausted NOxamount NOXA exhausted per unit time is calculated from the map shown inFIG. 5(A). Next, at step51, it is judged if this exhausted NOxamount NOXA is smaller than the maximum NOxstorage amount MAX. When NOXA≦MAX, the routine proceeds to step52, where the NOxamount NOX1stored per unit time in the upstream NOxstoring catalyst12is made the exhausted NOxamount NOXA. Next, the routine proceeds to step55.

As opposed to this, when it is judged at step51that NOXA>MAX, the routine proceeds to step53, where the NOxamount NOX1stored per unit time in the upstream NOxstoring catalyst12is made the maximum NOxstorage amount MAX. Next, the routine proceeds to step54, where the NOxamount NOX2stored per unit time in the downstream NOxstoring catalyst14is made the exhausted NOxamount NOXA minus the maximum NOxstorage amount MAX (NOXA−MAX). Next, the routine proceeds to step55. At step55, NOX1is added to the NOxamount ΣNOX1stored in the upstream NOxstoring catalyst12, next at step56, NOX2is added to the NOxamount ΣNOX2stored in the downstream NOxstoring catalyst14.

Next, at step57, it is judged if the stored NOxamount ΣNOX1of the upstream NOxstoring catalyst12exceeds the allowable value X1. When ΣNOX1>X1, the routine proceeds to step58, where it is judged if the representative temperature TC of the NOxstoring catalysts12and14is higher than the predetermined set value TX. When TC≦TX, the routine proceeds to step59, where lean spike processing where the amount is lowered under a lean air-fuel ratio is performed. Next, at step60, ΣNOX2is added to ΣNOX1·α(0<α≦1), next, at step61, ΣNOX1is made ΣNOX1·(1−α). As opposed to this, when it is judged at step58that TC>TX, the routine proceeds to step64, where rich spike processing where the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich is performed. Next, at step65, ΣNOX1and ΣNOX2are cleared.

On the other hand, when it is judged at step57that ΣNOX1≦X1, the routine proceeds to step62, where it is judged if the stored NOxamount ΣNOX2of the downstream NOxstoring catalyst14exceeds the allowable value X2. When ΣNOX2>X2, the routine proceeds to step63, where it is judged if the representative temperature TC of the NOxstoring catalysts12and14is higher than the predetermined set temperature TX. When TC≦TX, the processing cycle is ended. As opposed to this, when TC>TX, the routine proceeds to step64, where rich spike processing where the air-fuel ratio of the exhaust gas is switched from lean to rich is performed.

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