# Patent

## Title
Exhaust purification device of internal combustion engine

## Abstract

An internal combustion engine in an engine exhaust passage of which an NOx adsorption catalyst (12) adsorbing NOx contained in exhaust gas at the time of a low temperature and desorbing the adsorbed NOx when the temperature rises and an NOx storage catalyst (14) are arranged. When the NOx storage catalyst (14) can store NOx, the temperature of the NOx adsorption catalyst (12) is made to forcibly rise to a target temperature at which the amount of NOx which the NOx storage catalyst (14) can store is desorbed, and the NOx desorbed from the NOx adsorption catalyst (12) is stored in the NOx storage catalyst (14).

## Background
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 NOx storage catalyst storing NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOx produced at the time of combustion under a lean air-fuel ratio is stored in the NOx storage catalyst to thereby prevent the NOx from being exhausted into the atmosphere.
However, the NOx storage ability of this NOx storage catalyst falls the lower the temperature of the NOx storage catalyst. Therefore, when the temperature of the NOx storage catalyst is low, the NOx ends up being exhausted into the atmosphere. Accordingly, there is known an internal combustion engine arranging in an engine exhaust passage upstream of the NOx storage catalyst an NOx adsorption catalyst adsorbing the NOx contained in the exhaust gas at the time of a low temperature and desorbing the adsorbed NOx when the temperature rises (for example, see Patent Literature 1).
In this internal combustion engine, when the temperature of the NOx adsorption catalyst is low such as for example at the time of engine startup, the NOx exhausted from the engine is adsorbed at the NOx adsorption catalyst. When the temperature of NOx adsorption catalyst rises, the NOx is desorbed from the NOx adsorption catalyst and the desorbed NOx is stored in the NOx storage catalyst.
CITATION LIST
Patent Literature


    Patent Literature 1: Japanese Patent Publication (A) No. 2007-245050


SUMMARY OF INVENTION
Technical Problem
However, if leaving the action of desorption of NOx from the NOx adsorption catalyst to the circumstances in this way, for example, when the temperature of the NOx adsorption catalyst will not easily rise, the NOx adsorption catalyst will end up becoming saturated in its NOx adsorption ability and therefore the problem will arise of the NOx in the exhaust gas no longer being able to be adsorbed at the NOx adsorption catalyst.
An object of the present invention is to provide an exhaust purification device of an internal combustion engine able to avoid saturation of the NOx adsorption ability of an NOx adsorption catalyst.
Solution to Problem
According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging in an engine exhaust passage an NOx storage catalyst storing NOx contained in an exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and releasing stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich and arranging upstream of the NOx storage catalyst in the engine exhaust passage an NOx adsorption catalyst adsorbing NOx contained in exhaust gas when at a low temperature and desorbing an adsorbed NOx when a temperature rises, wherein when the NOx storage catalyst is able to store NOx, a temperature of the NOx adsorption catalyst is forcibly made to rise to a target temperature at which an amount of NOx which the NOx storage catalyst can store is desorbed, and the NOx desorbed from the NOx adsorption catalyst is made to be stored in the NOx storage catalyst.
Advantageous Effects of Invention
The temperature of the NOx adsorption catalyst is forcibly raised and NOx is made to desorb from the NOx adsorption catalyst, so saturation of the NOx adsorption ability of the NOx adsorption catalyst can be avoided.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is an overview of a compression ignition type internal combustion engine.
FIG. 2 is a cross-sectional view of a surface part of a catalyst carrier of an NOx storage catalyst.
FIG. 3 is a view showing an NOx adsorption rate and NOx storage rate.
FIG. 4 is a view showing a map of an exhaust NOx amount NOXA.
FIG. 5 is a view showing an NOx desorption rate and NOx storage rate.
FIG. 6 is a flow chart for exhaust purification processing.
FIG. 7 is a flow chart for temperature raising control.
FIG. 8 is an overview of another embodiment of a compression ignition type internal combustion engine.
FIG. 9 is a flow chart of temperature raising control.



DESCRIPTION OF EMBODIMENTS
FIG. 1 shows an overview of a compression ignition type internal combustion engine.
Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically 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 the outlet of a compressor 7a of an exhaust turbocharger 7. The inlet of the compressor 7a is connected to an air cleaner 9 via an intake air amount detector 8. Inside the intake duct 6 is arranged a throttle valve 10 driven by a step motor. Further, around the intake duct 6 is arranged a cooling device 11 for cooling the intake air flowing through the inside of the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 11. The engine cooling water cools the intake air. On the other hand, the exhaust manifold 5 is connected to the inlet of an exhaust turbine 7b of the exhaust turbocharger 7, while the outlet of the exhaust turbine 7b is connected to the inlet of an NOx adsorption catalyst 12. Further, the outlet of the NOx adsorption catalyst 12 is connected through an exhaust pipe 13 to an NOx storage catalyst 14.
The exhaust manifold 5 and the intake manifold 4 are interconnected through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage 15. The EGR passage 15 is provided with an electronically controlled EGR control valve 16. Further, around the EGR passage 15 is arranged a cooling device 17 for cooling the EGR gas flowing through the inside of the EGR passage 15. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 17. The engine cooling water cools the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel feed tube 18 to a common rail 19. This common rail 19 is supplied with fuel from an electronically controlled variable discharge fuel pump 20. The fuel supplied into the common rail 19 is supplied through each fuel feed tube 18 to the fuel injector 3.
An electronic control unit 30 is 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 port 35, and an output port 36 all connected to each other by a bidirectional bus 31. The NOx adsorption catalyst 12 is provided with a temperature sensor 21 for detecting the temperature of the NOx adsorption catalyst 12. The NOx storage catalyst 14 is provided with a temperature sensor 22 for detecting the temperature of the NOx storage catalyst 14. The output signals of the temperature sensors 21 and 22 and the intake air amount detector 8 are input through corresponding AD converters 37 to the input port 35.
An accelerator pedal 40 has a load sensor 41 generating an output voltage proportional to the amount of depression L of the accelerator pedal 40 connected to it. The output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35. Further, the input port 35 has a crank angle sensor 42 generating an output pulse each time the crankshaft turns for example by 15 degrees connected to it. On the other hand, the output port 36 is connected through corresponding drive circuits 38 to the fuel injectors 3, step motor for driving throttle valve 10 EGR control valve 16, and fuel pump 20.
First, explaining the NOx storage catalyst 14 shown in FIG. 1, a catalyst carrier 45 comprised of for example alumina is carried on the substrate of the NOx storage catalyst 14. FIG. 2 schematically shows the cross-section of the surface part of this catalyst carrier 45. As shown in FIG. 2 the catalyst carrier 45 carries a precious metal catalyst 46 diffused on its surface. Further, the catalyst carrier 45 is formed with a layer of an NOx absorbent 47 on its surface.
In this embodiment of the present invention, platinum Pt is used as the precious metal catalyst 46. As the ingredient forming the NOx absorbent 47, 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.
If the ratio of the air and fuel (hydrocarbons) supplied to the engine intake passage, combustion chambers 2, and exhaust passage upstream of the NOx storage catalyst 14 is referred to as the “air-fuel ratio of the exhaust gas”, the NOx absorbent 47 performs an NOx absorption and release action of storing the NOx when the air-fuel ratio of the exhaust gas is lean and releasing the stored NOx when 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 NOx absorbent 47, 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 Pt 46 such as shown in FIG. 2 to become NO2, then is absorbed in the NOx absorbent 47 and diffuses in the NOx absorbent 47 in the form of nitric acid ions NO3− while bonding with the barium oxide BaO. In this way, the NOx is absorbed in the NOx absorbent 47. So long as the oxygen concentration in the exhaust gas is high, NO2 is produced on the surface of the platinum Pt 46. So long as the NOx absorbing capability of the NOx absorbent 47 is not saturated, the NO2 is absorbed in the NOx absorbent 47 and nitric acid ions NO3− are produced.
As opposed to this, if the air-fuel ratio of the exhaust gas is made rich or the stoichiometric air-fuel ratio, since the oxygen concentration in the exhaust gas falls, the reaction proceeds in the reverse direction (NO3−→NO2) and therefore the nitric acid ions NO3− in the NOx absorbent 47 are released from the NOx absorbent 47 in the form of NO2. Next, the released NOx is reduced by the unburned hydrocarbons or CO included in the exhaust gas.
In this way, when the air-fuel ratio of the exhaust gas is lean, that is, when burning fuel under a lean air-fuel ratio, the NOx in the exhaust gas is absorbed in the NOx absorbent 47. However, if continuing to burn fuel under a lean air-fuel ratio, during that time the NOx absorbing capability of the NOx absorbent 47 will end up becoming saturated and therefore NOx will end up no longer being able to be absorbed by the NOx absorbent 47. Therefore, in this embodiment according to the present invention, before the absorbing capability of the NOx absorbent 47 becomes saturated, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby release the NOx from the NOx absorbent 47.
Now then, as explained above, when combustion is performed under a lean air-fuel ratio, the NOx in the exhaust gas is absorbed in the NOx absorbent 47. In other words, when combustion is performed under a lean air-fuel ratio, the NOx in the exhaust gas is stored in the NO storage catalyst 14. However, in this case, the NOx storage rate showing the ratio of the amount of NOx stored in the NOx storage catalyst 14 to the amount of NOx in the exhaust gas changes in accordance with the temperature of the NOx storage catalyst 14. The change in this NOx storage rate RB with respect to the temperature TC of the NOx storage catalyst 14 is shown in FIG. 3.
In the example shown in FIG. 3, the NOx storage rate RB peaks when the temperature TC of the NOx storage catalyst 14 is about 300° C. to about 500° C. in range. As opposed to this, if the temperature TC of the NOx storage catalyst 14 falls, the oxidizing power of the precious metal 46 against NO falls along with this, so the NOx storage rate RB falls the lower the temperature TC of the NOx storage catalyst 14 becomes. In the example shown in FIG. 3, the NOx storage rate RB is made to fall if the temperature TC of the NOx storage catalyst 14 becomes about 300° C. or less. This NOx storage rate RB becomes zero when the temperature TC of the NOx storage catalyst 14 becomes about 130° C.
The NOx storage rate RB shown in FIG. 3 is one example. Depending on the type of the NOx storage catalyst 14, there are also NOx storage catalysts where the peak of the NOx storage rate RB extends to a further lower temperature. However, no matter what kind of NOx storage catalyst 14 is used, if the temperature TC of the NOx storage catalyst 14 becomes low, the NOx storage rate RB will fall and the NOx contained in the exhaust gas will no longer be able to be sufficiently stored.
Therefore, in the present invention, upstream of the NOx storage catalyst 14 in the engine exhaust passage, an NOx adsorption catalyst 12 adsorbing NOx contained in the exhaust gas when at a low temperature and desorbing the adsorbed NOx when the temperature rises is arranged. This NOx adsorption catalyst 12 is formed from a material containing for example cerium Ce which can sufficiently adsorb NOx even at room temperature, that is, 15° C. to 25° C. or so. In this NOx adsorption catalyst 12 as well, the NOx adsorption rate showing the ratio of the amount of NOx adsorbed at the NOx adsorption catalyst 12 to the amount of NOx in the exhaust gas changes in accordance with the temperature of the NOx adsorption catalyst 12. The change in this NOx adsorption rate RA with respect to the temperature TC of the NOx adsorption catalyst 12 is shown in FIG. 3.
In the example shown in FIG. 3, the NOx adsorption rate RA peaks when the temperature TC of the NOx adsorption catalyst 12 is at room temperature, that is, is 15° C. to 25° C. or so. The NOx adsorption rate RA is reduced as the temperature TC of the NOx adsorption catalyst 12 becomes higher. In the example shown in FIG. 3, when the temperature TC of the NOx adsorption catalyst 12 rises to about 170° C., the NOx adsorption rate RA becomes zero. From FIG. 3, it is learned that when the temperature TC of the NOx adsorption catalyst 12 is low, the NOx adsorption rate RA is high and, therefore, at this time, the NOx contained in the exhaust gas can be adsorbed well at the NOx adsorption catalyst 12.
On the other hand, FIG. 5 shows the NOx desorption rate DA showing the amount of NOx desorbed from the NOx adsorption catalyst 12 per unit time as a function of the temperature TC of the NOx adsorption catalyst 12. Further, it shows the NOx storage rate DB showing the amount of NOx stored in the NOx storage catalyst 14 per unit time as a function of the temperature TC of the NOx storage catalyst 14. Further, FIG. 5 shows curves showing the NOx desorption rate DA when the NOx adsorption amount of the NOx adsorption catalyst 12 is W1, N2, and W3. In this case, the NOx adsorption amount becomes greater from W1 toward W3. Further, FIG. 5 shows curves showing the NOx storage rate DB when the NOx storage amount of the NOx storage catalyst 14 is Q1, Q2, and Q3. In this case, the NOx storage amount becomes greater from Q1 toward Q3.
The NOx adsorption catalyst 12 performs an NOx adsorption action and an NOx desorption action before and after a certain catalyst temperature TC. In this embodiment of the present invention, this catalyst temperature TC is about 170° C. Therefore, as shown in FIG. 3, if the catalyst temperature TC rises to about 170° C., the NOx adsorption rate RA becomes zero. As shown in FIG. 5, if the catalyst temperature TC becomes higher than 170° C., the NOx desorption rate DA increases exponentially along with the rise of the catalyst temperature TC. Note that, the greater the NOx adsorption amount W, the more the desorbed amount of NOx increases as well, so at the same catalyst temperature TC, the greater the NOx adsorption amount W, the higher the NOx desorption rate DA.
On the other hand, the NOx storage rate DB shown in FIG. 5 changes with respect to the temperature TC of the NOx storage catalyst 14 in the same way as the NOx storage rate RB shown in FIG. 3. That is, when the temperature TC of the NOx storage catalyst 14 becomes about 130° C. or more, the NOx storage rate DB starts to rise. When the temperature TC of the NOx storage catalyst 14 rises to about 300° C., the NOx storage rate DB peaks. Note that, the more the NOx storage amount Q increases, the harder it is for the NOx to be stored, so at the same catalyst temperature TC, the more the NOx storage amount Q increases, the lower the NOx storage rate DB becomes.
Now then, when, like at the time of engine startup or the time of engine low load operation, the temperature TC of the NOx adsorption catalyst 12 is low, the NOx adsorption catalyst 12 continues to adsorb NOx. In this case, when the NOx adsorption catalyst 12 adsorbs a certain extent of NOx, it is preferable to make the NOx adsorption catalyst 12 desorb the NOx. That is to say, if making it desorb the NOx, when next the temperature TC of the NOx adsorption catalyst 12 becomes low, it can sufficiently adsorb the NOx. However, in this case, even if desorbing the NOx, if making the NOx adsorption catalyst 12 desorb the NOx when the NOx storage catalyst 14 does not have an NOx storage ability, this NOx will end up being released into the atmosphere without being stored at the NOx storage catalyst 14.
Therefore, in the present invention, when the NOx storage catalyst 14 can store NOx, the temperature TC of the NOx adsorption catalyst 12 is forcibly made to rise to the target temperature at which an amount of NOx which the NOx storage catalyst 14 can store is desorbed, and the NOx desorbed from the NOx adsorption catalyst 12 is made to be stored in the NOx storage catalyst 14.
However, in this case, if the amount of desorption of NOx from the NOx adsorption catalyst 12 is greater than the amount of NOx which can be stored at the NOx storage catalyst 14, part of the desorbed NOx will be released into the atmosphere without being stored at the NOx storage catalyst 14. In the present invention, to prevent NOx from being released into the atmosphere in this way, the NOx adsorption catalyst 12 is made to desorb the NOx so that the amount of desorption of NOx from the NOx adsorption catalyst 12 becomes less than the amount of NOx which can be stored at the NOx storage catalyst 14.
That is, in the present invention, the target temperature TO of the NOx adsorption catalyst 12 to be raised to for making the NOx adsorption catalyst 12 desorb the NOx is made the temperature at which the amount of desorption of NOx from the NOx adsorption catalyst 12 becomes less than the amount of NOx able to be stored at the NOx storage catalyst 14. In other words, in the present invention, the target temperature TO of the NOx adsorption catalyst 12 to be raised to is made the temperature at which the NOx desorption rate DA becomes less than the NOx storage rate DB.
Next, the method of finding this target temperature TO will be explained with reference to FIG. 5. FIG. 5 shows the case of raising the temperature of the NOx adsorption catalyst 12 to make it desorb the NOx when the NOx adsorption amount is W3, the NOx storage amount is Q1, and the NOx storage rate DB exceeds the predetermined NOx storage rate DB0, that is, when the NOx storage catalyst 14 is in the state shown by the black dot b1 in FIG. 5. In the example shown in FIG. 5, at this time, the NOx adsorption catalyst 12 is in the state shown by the black dot a1 of FIG. 5. Further, in this example, at this time, the temperature TC of the NOx adsorption catalyst 12 becomes higher than the temperature TC of the NOx storage catalyst 14.
Now then, the NOx storage rate DB when the NOx storage catalyst 14 is in the state shown by the black dot b1 is expressed by the horizontal line DB1 passing through the black dot b1. The catalyst temperature T1 at the intersection of this horizontal line DB1 and the solid line W3 shows the temperature when the NOx desorption rate DA of the NOx adsorption catalyst 12 when the NOx adsorption amount is W3 becomes equal to the NOx storage rate DB1. That is, when the NOx storage catalyst 14 is in the state shown by the black dot b1 and the temperature TC of the NOx adsorption catalyst 12 becomes T1 when the NOx adsorption amount of the NOx adsorption catalyst 12 is W3, the NOx desorption rate DA is no longer equal to the NOx storage rate DB1. At this time, theoretically, the entire NOx desorbed from the NOx adsorption catalyst 12 is adsorbed at the NOx storage catalyst 14.
In this case, if the temperature TC of the NOx adsorption catalyst 12 becomes higher than T1, the NOx desorption rate DA will become higher than even the NOx storage rate DB1 and the amount of desorbed NOx will become greater than the amount of adsorbed NOx. Therefore, the temperature TC of the NOx adsorption catalyst 12 cannot be made higher than T1. As opposed to this, when the temperature TC of the NOx adsorption catalyst 12 is lower than T1, the NOx desorption rate DA becomes lower than even the NOx storage rate DB1 and thus at this time, the desorbed NOx is reliably stored in the NOx storage catalyst 14.
Now then, in the example shown in FIG. 5, as explained above, when the NOx storage catalyst 14 is in the state shown by the black dot b1, the NOx adsorption catalyst 12 is in the state shown by the black dot a1. At this time, the NOx desorption rate DA becomes considerably lower than the NOx storage rate DB1. At this time, if raising the temperature TC of the NOx adsorption catalyst 12 to close to T1 within a range not exceeding T1, the NOx desorption rate DA will become higher in a range not exceeding the NOx storage rate DB1 and thus it will become possible to rapidly desorb NOx from the NOx adsorption catalyst 12.
Thus, in the present invention, the temperature TC of the NOx adsorption catalyst 12 is made to rise toward a temperature somewhat lower than T1. That is, the target temperature TO of the NOx adsorption catalyst 12 to be raised to is made a temperature somewhat lower than T1. Note that, the temperature raising action of the NOx adsorption catalyst 12 is for example obtained by delaying the timing of injection of fuel from the fuel injector 3 to raise the exhaust gas temperature or by feeding additional fuel into the combustion chamber at the time of the expansion stroke so as to raise the exhaust gas temperature.
The state of the NOx adsorption catalyst 12 and the state of the NOx storage catalyst 14 when a certain time has elapsed from the start of the temperature raising action are respectively shown by the black dots a2 and b2. When a certain time has elapsed from the start of the temperature raising action, as shown in FIG. 5, the temperature TC of the NOx adsorption catalyst 12 rises to the temperature shown by the black dot a2 and, due to the desorption of NOx, the NOx adsorption amount falls to W2′. On the other hand, the temperature TC of the NOx storage catalyst 14 rises to the temperature shown by the black dot b2 and, due to the storage of the desorbed NOx and NOx exhausted from the engine, the NOx storage amount increases to Q1′.
When the NOx adsorption catalyst 12 and NOx storage catalyst 14 become the states respectively shown by the black dots a2 and b2, the target temperature TO of the NOx adsorption catalyst 12 is updated. That is, the catalyst temperature T2 at the intersection between the horizontal line DB2 passing through the black dot b2 and the solid line W2′ passing through the black dot a2 is found and a temperature somewhat lower than this catalyst temperature T2 is made the new target temperature TO. When the new target temperature TO is determined, the temperature TC of the NOx adsorption catalyst 12 is made to rise toward this new target temperature TO. FIG. 5 shows the state of the NOx adsorption catalyst 12 and the state of the NOx storage catalyst 14 when a certain time elapses from the start of the temperature raising action toward this new target temperature TO by the black dots a3 and b3.
When the NOx adsorption catalyst 12 and NOx storage catalyst 14 become the states shown by the blacks dot a3 and b3, the target temperature TO of the NOx adsorption catalyst 12 is again updated. That is, the catalyst temperature T3 at the intersection between the horizontal line DB3 passing through the black dot b3 and the solid line W2 passing through the black dot a3 is found and a temperature somewhat lower than this catalyst temperature T3 is made the new target temperature TO. When the new target temperature TO is determined, the temperature TC of the NOx adsorption catalyst 12 is made to rise toward this new target temperature TO. FIG. 5 shows the state of the NOx adsorption catalyst 12 and the state of the NOx storage catalyst 14 when a certain time elapses from the start of the temperature raising action toward this new target temperature TO by the black dots a4 and b4.
When the NOx adsorption catalyst 12 and NOx storage catalyst 14 become the states shown by the blacks dot a4 and b4, the target temperature TO of the NOx adsorption catalyst 12 is again updated and a temperature somewhat lower than the catalyst temperature T4 is made the new target temperature TO. In this way, the target temperature TO is repeatedly updated during the action of desorption of NOx from the NOx adsorption catalyst 12 and thereby NOx is made to quickly be desorbed from the NOx adsorption catalyst 12.
Note that, in the embodiments of the present invention, the NOx desorption rate DA from the NOx adsorption catalyst 12 shown in FIG. 5 is stored as a function of the NOx adsorption amount W adsorbed at the NOx adsorption catalyst 12 and the temperature TC of the NOx adsorption catalyst 12 in advance in the ROM 32, while the NOx storage rate DB to the NOx storage catalyst 14 is stored as a function of the NOx storage amount Q stored in the NOx storage catalyst 14 and the temperature TC of the NOx storage catalyst 14 in advance in the ROM 32.
FIG. 6 and FIG. 7 show a routine for executing exhaust purification processing including the temperature raising control shown in FIG. 5. This routine is executed by interruption every certain time interval.
Referring to FIG. 6, first, at step 100, the amount NOXA of NOx exhausted from the engine per unit time is calculated. This exhausted NOx amount NOXA is stored as a function of the required torque TQ and engine speed N in the form of a map shown in FIG. 4 in advance in the ROM 32. At the next step 101, the NOx adsorption rate RA is calculated from the relationship shown in FIG. 3, while at the next step 102, the NOx desorption rate DA is calculated from the relationship shown in FIG. 5.
At the next step 103, the NOx adsorption amount W of NOx adsorbed at the NOx adsorption catalyst 12 is calculated based on the following formula:

W←W+NOXA·RA−DA 

That is, the amount of adsorption of NOx at the NOx adsorption catalyst 12 per unit time becomes NOXA·RA and the amount of desorption of NOx from the NOx adsorption catalyst 12 is DA, so the NOx adsorption amount W at the NOx adsorption catalyst 12 is expressed by the above formula.
At the next step 104, the NOx storage rate RRB is calculated from the relationship shown in FIG. 3, while at the next step 105, the NOx storage amount Q stored at the NOx storage catalyst 14 is calculated by the following formula:

Q←Q+[NOXA·(1−RA)+DA]·RB 

That is, the amount of NOx passing through the NOx adsorption catalyst 12 per unit time is NOXA·(1−RA) and the amount of desorption of NOx desorbed from the NOx adsorption catalyst 12 per unit time is DA, so the amount of NOx stored in the NOx storage catalyst 14 per unit time becomes [NOXA·(1−RA)+DA]·RB. Therefore, the NOx storage amount Q to the NOx storage catalyst 14 is expressed by the above formula.
At the next step 106, it is determined if a temperature raising flag showing that temperature raising control for making the NOx adsorption catalyst 12 desorb the NOx should be executed is set. When the temperature raising flag is not set, the routine proceeds to step 107 where it is determined if the NOx adsorption amount W is smaller than a predetermined lower limit MIN. When the NOx adsorption amount W is small and W<MIN, it is meaningless even if temperature raising control is performed, so the routine jumps to step 110.
At step 110, rich control is performed to temporarily make the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 14 rich so as to make the NOx storage catalyst 14 release the NOx. This rich control is performed when the NOx storage amount Q exceeds a predetermined allowable value and the NOx storage catalyst 14 is sufficiently activated. When this rich control is performed, the NOx storage amount Q is made zero.
On the other hand, when it is determined at step 107 that W≧MIN, the routine proceeds to step 108 where it is determined if the NOx storage rate DB determined from the NOx storage amount Q and the temperature TC of the NOx storage catalyst 14 has exceeded the predetermined NOx storage rate DB0 shown in FIG. 5. When DB>DB0, the routine proceeds to step 111 where the temperature raising flag is set, then at the next step 112, temperature raising control is performed. If the temperature raising flag is set, after that, the routine jumps from step 1006 to step 112.
On the other hand, when it is determined at step 108 that DB≦DB0, the routine proceeds to step 109 where it is determined if the NOx adsorption amount W has exceeded an allowable maximum limit value MAX. When it is determined that W>MAX, the routine proceeds to step 111 where the temperature raising flag is set. That is, when the NOx adsorption amount W approaches the saturated adsorption amount, temperature raising control is started even when the NOx) storage rate DB is low.
The temperature raising control at step 112 is shown in FIG. 7. Referring to FIG. 7, at step 120, the NOx storage rate DB is calculated from the NOx storage amount Q and the temperature TC of the NOx storage catalyst 14. At the next step 121, the catalyst temperatures Ti represented by T1, T2, T3, and T4 in FIG. 5 are calculated from the NOx storage rate DB and the NOx adsorption amount W. At the next step 122, a small fixed value α is subtracted from the catalyst temperature Ti to calculate the target temperature TO. At the next step 123, the basic injection amount Qf0 of the additional fuel injected into the combustion chamber 2 at the time of the expansion stroke for example for raising the temperature of the NOx adsorption catalyst 12 is calculated. This basic injection amount Qf0 is stored as a function of the temperature difference of the current temperature TC of the NOx adsorption catalyst 12 and the target temperature TC in advance in the ROM 32.
At the next step 124, it is determined if the temperature TC of the NOx adsorption catalyst 12 is lower than the target temperature TO. When TC<TO, the routine proceeds to step 125 where a fixed amount β is added to the correction amount ΔQf for the basic injection amount Qf0, then the routine proceeds to the next step 127. As opposed to this, when TC≧TO, the routine proceeds to step 126 where the fixed value β is subtracted from the correction amount ΔQf, then the routine proceeds to the next step 127. At the step 127, the correction amount ΔQf is added to the basic injection amount Qf0 so as to calculate the final injection amount Qf of the additional fuel. This injection amount Qf is used for the action of injection of the additional fuel. At the next step 128, it is determined if the NOx adsorption amount W has become zero. When it is determined that W=0, the routine proceeds to step 129 where the temperature raising flag is reset.
FIG. 8 shows another embodiment.
In this embodiment, upstream of the NOx adsorption catalyst 12 in the engine exhaust passage, an SOx trap catalyst 23 able to trap SOx contained in the exhaust gas is arranged. This SOx trap catalyst 23 has attached to it a temperature sensor 24 for detecting the temperature of the SOx trap catalyst 23.
That is, exhaust gas contains SOx. When this SOx is adsorbed at the NOx adsorption catalyst 12, the NOx adsorption catalyst 12 falls in NOx adsorption ability. Therefore, in this embodiment, to prevent the SOx from flowing into the NOx adsorption catalyst 12, an SOx trap catalyst 23 is arranged upstream of the NOx adsorption catalyst 12. This SOx trap catalyst 23 usually continues to trap SOx. SOx is never released from the SOx trap catalyst 23.
However, when the temperature TS of the SOx trap catalyst 23 rises, SOx is released from the SOx trap catalyst 23. The catalyst temperature TSOX where SOx is released in this way, while differing depending on the type of the SOx trap catalyst 23, is usually 500° C. or more. In this regard, when making the NOx adsorption catalyst 12 desorb NOx by raising the temperature of the exhaust gas temperature and thereby raising the temperature of the NOx adsorption catalyst 12, the SOx trap catalyst 23 is also simultaneously raised in temperature.
Therefore, in this case, when the temperature TC of the NOx adsorption catalyst 12 is made to rise toward the target temperature TO, if the temperature TS of the SOx trap catalyst 23 becomes higher than the temperature TSOX at which the SOx is released, SOx is released from the SOx trap catalyst 23. Therefore, in this embodiment, the target temperature TO is determined so that the temperature TS of the SOx trap catalyst 23 becomes less than the temperature TSOX at which the SOx is released when the exhaust gas temperature is made to rise so that the NOx adsorption catalyst 12 desorbs NOx.
FIG. 9 shows a temperature raising control routine for working this embodiment. This routine just has the part K surrounded by the broken lines added between steps 123 and 124 of the routine shown in FIG. 7. Aside from this part K, the routine is the same as that shown in FIG. 7, so only the part K surrounded by the broken line will be explained below. Referring to the part K surrounded by the broken line, at step 123a, it is determined if the temperature TS of the SOx trap catalyst 23 is higher than the temperature TSOX at which SOx is released. When TS≦TSOX, the routine proceeds to step 124. As opposed to this, when TS>TSOX, the routine proceeds to step 123b where the target temperature TO is reduced by exactly a predetermined temperature ΔTO so that TS<TSOX.
REFERENCE SIGNS LIST


    
    
        4 . . . intake manifold
        5 . . . exhaust manifold
        7 . . . exhaust turbocharger
        12 . . . NOx adsorption catalyst
        14 . . . NOx storage catalyst

## Claims

1. An exhaust purification device of an internal combustion engine, comprising:
an NOx storage catalyst disposed in an engine exhaust passage, the NOx storage catalyst storing NOx contained in an exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and releasing stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich;
an NOx adsorption catalyst disposed upstream of the NOx storage catalyst in the engine exhaust passage that adsorbs NOx contained in the exhaust gas when the NOx adsorption catalyst is at a low temperature and that desorbs adsorbed NOx when the temperature of the NOx adsorption catalyst rises; and
an electronic control unit (ECU) programmed to execute control of the exhaust purification device such that when the NOx storage catalyst is active and able to store NOx, the ECU executes programmed instructions to control a temperature of the NOx adsorption catalyst such that the temperature of the NOx adsorption catalyst is forcibly made to rise to a target temperature at which an amount of NOx which the NOx storage catalyst can store is desorbed, and the NOx desorbed from the NOx adsorption catalyst is made to be stored in the NOx storage catalyst.

2. The exhaust purification device of an internal combustion engine as claimed in claim 1, wherein said target temperature is made a temperature where an NOx desorption rate from the NOx adsorption catalyst becomes less than an NOx storage rate to the NOx storage catalyst.

3. The exhaust purification device of an internal combustion engine as claimed in claim 2, wherein said NOx desorption rate from the NOx adsorption catalyst is stored in advance as a function of an NOx adsorption amount adsorbed at the NOx adsorption catalyst and the temperature of the NOx adsorption catalyst,
wherein said NOx storage rate at the NOx storage catalyst is stored in advance as a function of an NOx storage amount stored at the NOx storage catalyst and the temperature of the NOx storage catalyst, and
wherein said target temperature is determined based on the NOx adsorption amount adsorbed at the NOx adsorption catalyst, the NOx storage amount stored at the NOx storage catalyst, and the temperature of the NOx storage catalyst.

4. The exhaust purification device of an internal combustion engine as claimed in claim 2, wherein said target temperature is repeatedly updated during an action of desorption of NOx from the NOx adsorption catalyst.

5. The exhaust purification device of an internal combustion engine as claimed in claim 1, further comprising an SOx trap catalyst disposed upstream of said NOx adsorption catalyst in the engine exhaust passage, the SOx trap catalyst trapping SOx contained in exhaust gas, and
wherein when NOx should be desorbed from the NOx adsorption catalyst, the ECU executes programmed instructions to control a temperature of the exhaust gas flowing into the SOx trap catalyst such that the temperature of the exhaust gas flowing into the SOx trap catalyst is raised so as to raise the temperature of the NOx adsorption catalyst, and at this time, the target temperature is determined so that a temperature of the SOx trap catalyst becomes less than a temperature where SOx is released.

6. The exhaust purification device of an internal combustion engine as claimed in claim 1, further comprising a fuel injector,
wherein an additional fuel amount is injected via the fuel injector into a combustion chamber at a time of an expansion stroke to raise the temperature of the NOx adsorption catalyst.

7. The exhaust purification device of an internal combustion engine as claimed in claim 1, further comprising a temperature sensor that detects the temperature of the NOx adsorption catalyst.

8. The exhaust purification device of an internal combustion engine as claimed in claim 1, wherein the ECU executes programmed instructions to determine whether to execute temperature raising control for making the NOx adsorption catalyst desorb the NOx.