WO2008102915A1

WO2008102915A1 - Exhaust purification device of internal combustion engine

Info

Publication number: WO2008102915A1
Application number: PCT/JP2008/053349
Authority: WO
Inventors: Kohei Yoshida; Shinya Hirota; Takamitsu Asanuma; Hiromasa Nishioka; Hiroshi Otsuki
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2007-02-23
Filing date: 2008-02-20
Publication date: 2008-08-28
Also published as: JP2008208739A

Abstract

An engine wherein a plurality of NOx storing catalysts (11, 12) are arranged in parallel in an engine exhaust passage, reducing agent feed valves (14, 15) for supplying reducing agent to the NOx storing catalysts (11, 12) are individually provided for the NOx storing catalysts (11, 12), and, when NOx should be released from the NOx storing catalysts (11, 12), the upstream-most NOx storing catalyst (11) is fed with the reducing agent required for consuming the oxygen contained in the exhaust gas in addition to the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst (11) from the first reducing agent feed valve (14).

Description

DESCRIPTION

EXHAUST PURIFICATION DEVICE OF INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD

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

Known in the art is an internal combustion engine arranging lean NOx catalysts able to reduce the NOx contained in exhaust gas in the presence of hydrocarbons, that is, HC, in series in an engine exhaust passage, individually providing each lean NOx catalyst with an HC feed device for feeding HC to each lean NOx catalyst, and feeding the HC necessary for reducing NOx in each lean

NOx catalyst from each corresponding HC feed device (see Japanese Patent Publication (A) No. 10-54223) . In this internal combustion engine, NOx is reduced well in each lean NOx catalyst . However, when arranging a pair of NOx storing catalysts storing the NOx contained in the 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 the stoichiometric air-fuel ratio or rich in the engine exhaust passage, even if feeding the reducing agent required for reducing the NOx stored in each NOx storing catalyst to each NOx storing catalyst by the same method as the above- mentioned internal combustion engine, it is not possible to obtain a good reduction action of NOx in each NOx storing catalyst.

That is, when arranging a pair of NOx storing catalysts in the engine exhaust passage, there is a method of feeding suitable reducing agents corresponding to this.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device of internal combustion engine designed to optimally feed a reducing agent when arranging a plurality of NOx storing catalysts in an engine exhaust passage. According to the present invention, there is provided an exhaust purification device of an internal combustion engine, wherein a plurality of NOx storing catalysts, storing NOx contained in exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when an air-fuel ratio of inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, is arranged in series in an engine exhaust passage, each NOx storing catalyst is provided with a reducing agent feed device for feeding a reducing agent to each NOx storing catalyst, and a larger amount of reducing agent than the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst is fed from the corresponding reducing agent feed device to the upstream-most NOx storing catalyst positioned at the upstream-most side so as to consume the oxygen contained in the exhaust gas when NOx should be released from the NOx storing catalysts.

BRIEF DESCRIPTION OF THE 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 storing catalyst.

FIG. 3 is a time chart showing control of the feed of a reducing agent. FIG. 4 is a flow chart for calculating a stored NOx amount etc.

FIG. 5 is a view showing a map of the stored NOx amount etc.

FIG. 6 is a view showing amounts of reducing agents. FIG. 7 is a flowchart for controlling the feed of a reducing agent.

BEST MODE FOR CARRYING OUT THE INVENTION 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 electronic control type 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 7a of the exhaust turbocharger 7, while an inlet of the compressor 7a is connected through an intake air detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 driven by a step motor is arranged. Further, around the intake duct 6, a cooling device 11 for cooling the intake air flowing through the inside of the intake duct 6 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided to the cooling device 11 where the engine cooling water cools the intake air.

On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7b of the exhaust turbocharger 7, while an outlet of the exhaust turbine 7b is connected to the serially arranged plurality of NOx storing catalysts 11 and 12. That is, the outlet of the exhaust turbine 7b is connected to the inlet of the upstream-most NOx storing catalyst 11 positioned at the upstream-most side, while the outlet of the upstream-most NOx storing catalyst 11 is connected through the exhaust pipe 13 to the inlet of a downstream side NOx storing catalyst 12 positioned downstream of the upstream-most NOx storing catalyst 11. In the embodiment shown in FIG. 1, only one downstream side NOx storing catalyst 12 is arranged, but a plurality of downstream side NOx catalysts 12 may also be arranged in series. As shown in FIG. 1, a first reducing agent feed valve 14 for feeding a reducing agent comprised of hydrocarbons to the upstream-most NO_x storing catalyst 11 is arranged in the exhaust manifold 5, while a second reducing agent feed valve 14 for feeding a reducing agent comprised of hydrocarbons to the downstream side NO_x storing catalyst 12 is arranged in the exhaust pipe 13. That is, reducing agent feed devices having reducing agent feed values 14 and 15 for feeding reducing agents to the NOx storing catalysts 11 and 12 are separately provided for the NOx storing catalysts 11 and 12.

The exhaust manifold 5 and the intake manifold 4 are interconnected to each other through an exhaust gas recirculation (hereinafter referred to as "EGR") passage 16. Inside the EGR passage 16, an electronic control type EGR control valve 17 is arranged. Further, around the EGR passage 16, a cooling device 18 for cooling the EGR gas flowing through the EGR passage 16 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 18 where the engine cooling water cools the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel feed pipe 19 to a common rail 20. This common rail 20 is supplied with fuel from an electronic control type variable discharge fuel pump 21. The fuel supplied to the common rail 20 is fed through each fuel feed pipe 19 to each fuel injector 3.

The electronic control unit 30 is comprised of a digital computer provided with a ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35, and output port 36 all connected to each other through a bi-directional bus 31. The upstream- most NOx storing catalyst 11 has attached to it a temperature sensor 22 for detecting the temperature of the upstream-most NOx storing catalyst 11. The output signal of the intake air detector 8 and the output signal of the temperature sensor 22 are input through the corresponding AD converters 37 to the input port 35. An accelerator pedal 40 is connected to a load sensor 41 generating an output voltage proportional to the amount of depression L of the accelerator pedal 40. The output voltage of the load sensor 41 is input through the corresponding AD converter 37 to the input port 35. Further, the input port 35 is connected to a crank angle sensor 42 generating an output pulse each time the crankshaft rotates by for example 15°. On the other hand, the output port 36 is connected through the corresponding drive circuit 38 to a fuel injector 3, a step motor for driving the throttle valve 10, the reducing agent feed valves 14 and 15, the EGR control valve 17, and the fuel pump 21.

Explaining first the NOx storing catalysts 11 and 12 shown in FIG. 1, these NOx storing catalysts 11 and 12 are carried on three dimensional mesh structure monolith carriers or pellet carriers. FIG. 2 schematically shows a surface part of a catalyst carrier 45 comprised of for example alumina. As shown in FIG. 2, the surface of the catalyst carrier 45 carries a precious metal catalyst 46 diffused thereon. Further, the surface of the catalyst carrier 45 is formed with a layer of an NO_x absorbent 47. In an embodiment of the present invention, platinum Pt is used as the precious metal catalyst 46. As the ingredient forming the NO_x absorbent 47, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth, lanthanum La, yttrium Y, or another such rare earth is used.

If the ratio of the air and fuel (hydrocarbons) supplied into the engine intake passage, combustion chambers 2, and exhaust passage upstream of the NOx storing catalysts 11 and 12 is called the "air-fuel ratio of the exhaust gas", the NO_x absorbent 47 carries out an NO_x absorption and release action such that the NOx absorbent 47 absorbs the NOx when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the exhaust gas falls. That is, explaining this taking as an example the case of using barium Ba as the ingredient forming the NO_x absorbent 47, when the air-fuel ratio of the air-fuel ratio 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 and becomes NO₂ as shown in FIG. 2, next, is absorbed in the NO_x absorbent 47, bonds with barium oxide BaO, and is diffused in the form of sulfuric acid ions NO₃ ^" in the NO_x absorbent 47. In this way, the NO_x is absorbed in the NO_x absorbent 47. So long as the oxygen concentration in the exhaust gas is high, the NO₂ is formed on the surface of the platinum Pt 46. So long as the NO_x absorption ability of the NO_x absorbent 47 does not become saturated, the NO₂ is absorbed in the NO_x absorbent 47 and sulfuric acid ions NO₃ ^" are formed. As opposed to this, if feeding reducing agents from the reducing agent feed valves 14 and 15 to make the air- fuel ratio of the exhaust gas rich or the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the reverse direction (NO₃ ^~-»NO₂) and therefore the sulfuric acid ions NO₃ ^" in the NO_x absorbent 47 are released in the form of NO₂ from the NO_x absorbent 47. Next, the released NO_x is reduced by the unburnt HC and CO contained in the exhaust gas. In this way, when the air-fuel ratio of the exhaust gas is lean, that is, when the fuel is burned under a lean air-fuel ratio, the NOx in the exhaust gas is absorbed in the NOx absorbent 47. However, when the fuel continues to be burned under a lean air-fuel ratio, the NOx absorption ability of the NOx absorbent 47 ends up becoming saturated and therefore the NOx absorbent 47 can no longer absorb the NOx. Therefore, in the embodiment according to the present invention, before the absorption ability of the NO_x absorbent 47 becomes saturated, reducing agents are fed from the reducing agent feed valves 14 and 15 to temporarily make the air-fuel ratio of the exhaust gas rich and thereby make the NO_x absorbent 47 release the NO_x.

Note that if the exhaust gas contains SO_x, that is, SO₂, and this SO2 flows into the NOx storing catalysts 11 and 12, this SO₂ is oxidized at the platinum Pt 46 and becomes SO₃. Next, this SO₃ is absorbed in the NO_x absorbent 47, bonds with the barium oxide BaO, diffuses in the form of sulfuric acid ions SO₄ ^2~in the NO_x absorbent 47, and forms stable sulfate BaSO₄. However, the NO_x absorbent 47 has a strong basicity, so the sulfate BaSO₄ is stable and hard to break down. By just making the air- fuel ratio of the exhaust gas rich, the sulfate BaSO₄ remains as it is without breaking down. Therefore, in the NO_x absorbent 47, the sulfate BaSO₄ increases along with the elapse of time, therefore the amount of NOx which the NOx absorbent 47 can absorb falls along with the elapse of time. That is, the NOx storing catalysts 11 and 12 deteriorate.

Next, the action of the NOx storing catalyst on the NOx storing catalysts 11 and 12 and the action of release and reduction of NOx from the NOx storing catalysts 11 and 12 will be explained with reference to FIG. 3.

In FIG. 3, ∑NOXl shows the stored NOx amount stored in the upstream-most NOx storing catalyst 11, while ΣN0X2 shows the stored NOx amount stored in the downstream side NOx storing catalyst 12. These stored NOx amounts ∑NOXl and ΣN0X2 are calculated from the NOx amount exhausted per unit time from the engine. Further, in FIG. 3, ∑SOXl shows the stored SOx amount stored in the upstream-most NOx storing catalyst 11. The stored SOx amount ∑SOXl is also calculated from the amount of SOx exhausted from the engine per unit time.

Therefore, first, the method of calculation of the amount of stored NOx will be explained with reference to FIG. 4. Note that the routine for calculation of the amount of stored NOx shown in FIG. 4 etc. is executed by interruption every set time. Referring to FIG. 4, first, at step 50, the exhaust NOx amount NOXA exhausted from the engine per unit time is calculated. This exhaust NO_x 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. 5(A) in advance in the ROM 32. Next, at step 51, the exhaust SOx amount SOXA exhausted from the engine per unit time is calculated. This exhaust SO_x amount SOXA is also stored as a function of the required torque TQ and engine speed N in the form of a map shown in FIG. 5(B) in advance in the ROM 32. The SOx exhausted from the engine is almost all stored in the upstream-most NOx storing catalyst 11. Therefore, at step 52, the SOx amount SOXA calculated from FIG. 5 (B) is added to the stored SOx amount ∑SOXl stored in the upstream-most NOx storing catalyst 11.

Next, at step 53, the rate of storage of NOx in the upstream-most NOx storing catalyst 11, that is, the NOx amount QNOX able to be stored in the upstream-most NOx storing catalyst 11 per unit time, is calculated. The greater the stored NOx amount ∑NOXl or the greater the stored SOx amount ∑SOXl, the harder it is for the NO_x to be stored in the upstream-most NOx storing catalyst 11, so the greater the stored NOx amount ∑NOXl and the greater the stored SOx amount ∑SOXl, the smaller the storable NO_x amount QNOX. Further, the faster the flow rate of the exhaust gas, that is, the greater the amount of intake air, the smaller the storable NO_x amount QNOX. Further, this changes in accordance with the temperature of the upstream-most NOx storing catalyst 11. The storable NO_x amount QNOX is stored as a function of the stored NOx amount ∑NOXl, stored SOx amount ∑SOXl, amount of intake air, and temperature of the upstream-most NOx storing catalyst 11 in advance in the ROM 32. The storable NOx amount QNOX is calculated from this stored relationship.

Next, at step 54, it is judged if the exhaust NOx amount NOXA from the engine is larger than the storable NO_x amount QNOX. If NOXA>QNOX, only QNOX is stored, so at this time, the routine proceeds to step 55 where QNOX is made the stored NOx amount NOXl stored in the upstream- most NOx storing catalyst 11 per unit time. Next, the routine proceeds to step 57. As opposed to this, when it is judged at step 54 that NOXA≤QNOX, all of the exhaust NO_x amount NOXA is stored in the upstream-most NOx storing catalyst 11, so at this time, the routine proceeds to step 56, where the exhaust NO_x amount NOXA is made the stored NOx amount NOXl. Next, the routine proceeds to step 57.

At step 57, the stored NOx amount NOXl is added to the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11. Next, at step 58, by subtracting the stored NOx amount NOXl from the exhaust NO_x amount NOXA, the stored NOx amount N0X2 stored in downstream side NOx storing catalyst 12 per unit time is calculated. Next, at step 59, the stored NOx amount N0X2 is added to the stored NOx amount ΣN0X2 stored in the downstream side NOx storing catalyst 12. In this way, the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11 and the stored NOx amount ΣN0X2 stored in the downstream side NOx storing catalyst 12 are calculated. Returning again to FIG. 3, in the embodiment according to the present invention, when the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11 reaches the allowable value NXl or the stored NOx amount ΣN0X2 stored in the downstream side NOx storing catalyst 12 reaches the allowable value NX2, the NOx release actions from the NOx storing catalysts 11 and 12 are performed. Note that in the example shown in FIG. 3, the case is shown where the NOx release action from each of the NOx storing catalysts 11 and 12 is performed by the stored NOx amount ∑NOXl stored in the upstream- most NOx storing catalyst 11 reaching the allowable value NXl .

As shown in FIG. 3, when ∑NOXl reaches NXl, as shown by the reducing agent feed 1, a reducing agent is fed from the first reducing agent feed valve 14, whereby the air-fuel ratio (A/F) 1 of the exhaust gas flowing into the upstream-most NOx storing catalyst 11 is temporarily- switched from lean to rich. At this time, the stored NOx is released from the upstream-most NOx storing catalyst 11 and reduced. Note that in this way, when NOx should be released from the upstream-most NOx storing catalyst 11, to consume the oxygen contained in the exhaust gas, a greater amount of reducing agent is fed from the first reducing agent feed valve 14 to the upstream-most NOx storing catalyst 11 than the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst 11. That is, when making the air-fuel ratio of the exhaust gas flowing into the upstream-most NOx storing catalyst 11 rich so as to release NOx from the upstream-most NOx storing catalyst 11, macroscopically speaking, the amount of reducing agent required for reducing the air-fuel ratio of the exhaust gas from lean to the stoichiometric air-fuel ratio, that is, required for consuming the oxygen contained in the exhaust gas, and the amount of reducing agent required for lowering the air-fuel ratio of the exhaust gas from the stoichiometric air-fuel ratio to rich, that is, required for releasing and reducing the NOx stored in the upstream-most NOx storing catalyst 11, are fed. That is, to make the upstream-most NOx storing catalyst 11 release the NO_x, first it is necessary to consume the oxygen contained in the exhaust gas flowing into the upstream-most NOx storing catalyst 11. Therefore, in the present invention, as explained above, when releasing NOx from the upstream-most NOx storing catalyst 11, the upstream-most NOx storing catalyst 11 is fed with the reducing agent required for consuming the oxygen contained in the exhaust gas in addition to the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst 11 from the first reducing agent feed valve 14. Next, when the part of the exhaust gas flow where the air-fuel ratio becomes rich due to the feed of reducing agent from the first reducing agent feed valve 14 reaches the reducing agent injection region of the second reducing agent feed valve 15, the reducing agent is fed from the second reducing agent feed valve 15 as shown by the reducing agent feed 2 and the part of the exhaust gas flow containing the reducing agent fed from the second reducing agent feed valve 15 flows into the downstream side NOx storing catalyst 12. The oxygen in the exhaust gas at this part of the exhaust gas flow is already consumed by the reducing agent fed from the first reducing agent feed valve 14, so it is sufficient to feed the amount of reducing agent required for releasing and reducing the NOx stored in the downstream side NOx storing catalyst 12 from the second reducing agent feed valve 15. Therefore, in the present invention, taking as an example the case where a plurality of downstream side NOx storing catalysts 12 are arranged downstream of the upstream-most NOx storing catalyst 11, when NOx should be released from each downstream side NOx storing catalyst 12, the reducing agent required for reducing the NOx stored in each downstream side NOx storing catalyst 12 is fed from the corresponding second reducing agent feed valve 15. Now, in this embodiment of the present invention, as explained above, the timing of feed of the reducing agent from the second reducing agent feed valve 15 is delayed from the timing of feed of the reducing agent from the first reducing agent feed valve 14 by exactly the Δt time as shown in FIG. 3 so that the reducing agent is fed from the second reducing agent feed valve 15 into the part of the exhaust gas flow where the air-fuel ratio falls due to the feed of reducing agent from the first reducing agent feed valve 14. This reducing agent feed delay time Δt becomes shorter the faster the flow rate of the exhaust gas, that is, the greater the amount of intake air. This reducing agent feed delay time Δt is stored as a function of the amount of intake air in advance in the ROM 32.

As shown in FIG. 3, the SOx amount ∑SOXl stored in the upstream-most NOx storing catalyst 11 gradually increases along with the elapse of time. That is, the upstream-most NOx storing catalyst 11 gradually deteriorates along with the elapse of time. FIG. 6 shows the change in the ratio of feed of reducing agent changing in accordance with the degree of deterioration of the upstream-most NOx storing catalyst 11 in the same operating state. Note that in FIG. 6, QO shows the amount of reducing agent required for consuming the oxygen in the exhaust gas flowing into the upstream-most NOx storing catalyst 11, Ql shows the amount of reducing agent required for releasing and reducing the NOx stored in the upstream-most NOx storing catalyst 11, and Q2 shows the amount of reducing agent required for releasing and reducing the NOx stored in the downstream side NOx storing catalyst 12. The figures in FIG. 6 show the amounts of reducing agents QO, Ql, and Q2 when the total amount of reducing agent fed is 100. No. 1, No. 2, and No. 3 show the cases of differing degrees of deterioration of the upstream- most NOx storing catalyst 11. In the case shown by No. 1 where the degree of deterioration of the upstream-most NOx storing catalyst 11 is lowest, QO is 80, Ql is 15, and Q2 is 5, Q0+Q1 is fed from the first reducing agent feed valve 14, and Q2 is fed from the second reducing agent feed valve 15. On the other hand, if the deterioration of the upstream-most NOx storing catalyst 11 advances, the NOx amount stored in the upstream-most NOx storing catalyst 11 is reduced, so as shown by No. 2, Ql becomes 5 and Q2 becomes 15. That is, the amount of NOx stored in the downstream side NOx storing catalyst 12 increases over the amount of NOx stored in the upstream-most NOx storing catalyst 11. Note that in the case of this No. 2, QO is the same as the case of No. 1.

On the other hand, when the deterioration of the upstream-most NOx storing catalyst 11 further advances and the amount of NOx stored in the upstream-most NOx storing catalyst 11 becomes zero, as shown by No. 3, Ql becomes 0 and Q2 becomes 20. That is, all of the NOx contained in the exhaust gas is stored in the downstream side NOx storing catalyst 13. Note that in the case of this No.^" 3 as well, QO becomes the same as the case of No. 1 and No. 2.

That is, when the inside of the upstream-most NOx storing catalyst 11 deteriorates and is filled by the stored SO_x, the layer of the NO_x absorbent 47 becomes weaker in basicity and the oxidation ability by the platinum 46 is improved. Therefore, even in the case of No. 3, QO is fed from the first reducing agent feed valve 14 so as to consume well the oxygen contained in the exhaust gas due to the reducing agent fed utilizing the improved oxidation performance.

In this way, in the embodiment according to the present invention, the degree of deterioration of the upstream-most NOx storing catalyst 11 is estimated from the stored SOx amount ∑SOXl, and the amounts of reducing agent fed from the corresponding reducing agent feed valves 14 and 15 to the upstream-most NOx storing catalyst 11 and downstream side NOx storing catalyst 12 are adjusted in accordance with the degree of deterioration estimated. In this case, the higher the degree of deterioration of the upstream-most NOx storing catalyst 11, the greater the amount of reducing agent fed to the downstream side NOx storing catalyst 12 from the second reducing agent feed valve.

Next, the control routine for the feed of the reducing agent will be explained with reference to FIG. 7. If referring to FIG. 7, first, at step 60, it is judged if the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11 has exceeded the allowable value NXl. When ∑NOX1>NX1, the routine jumps to step 62, while when ∑NOXl≤NXl, the routine proceeds to step 61. At step 61, it is judged if the stored NOx amount ΣN0X2 stored in the downstream side NOx storing catalyst 12 has exceeded the allowable value NX2. When ∑NOX2>NX2, the routine proceeds to step 62, while when ∑NOX2<NX2, the processing cycle is ended. That is, when the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11 has exceeded the allowable value NXl or the stored NOx amount ΣNOX2 stored in the downstream side NOx storing catalyst 12 has exceeded the allowable value NX2, the routine proceeds to step 62. At step 62, the amount of reducing agent Ql required for releasing and reducing the stored NOx is calculated based on the calculated value of the stored NOx amount ∑NOXl stored in the upstream-most NOx storing catalyst 11. Next, at step 63, the amount of reducing agent Q2 required for releasing and reducing the stored NOx is calculated based on the calculated amount of the stored NOx amount ΣN0X2 stored in the stored downstream side NOx storing catalyst 12. Next, at step 64, the amount of reducing agent QO required for consuming the excess oxygen from the amount of intake air and amount of fuel injection, that is, the oxygen in the exhaust gas flowing into the upstream-most NOx storing catalyst 11, is calculated. Next, at step 65, the reducing agent feed delay time Δt is calculated. Next, at step 66, the sum of QO and Ql is supplied from the first reducing agent feed valve 14. Next, at step 67, it is waited until the reducing agent feed delay time Δt has elapsed. When the reducing agent feed delay time Δt has elapsed, the routine proceeds to step 68, where Q2 is fed from the second reducing agent feed valve 15. Next, at step 69, ∑NOXl and ΣNOX2 are cleared.

Claims

1. An exhaust purification device of internal combustion engine, wherein a plurality of NOx storing catalysts, storing NOx contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean and releasing stored NOx when an air-fuel ratio of inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, is arranged in series in an engine exhaust passage, each NOx storing catalyst is provided with a reducing agent feed device for feeding a reducing agent to each NOx storing catalyst, and a larger amount of reducing agent than the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst is fed from the corresponding reducing agent feed device to the upstream-most NOx storing catalyst positioned at the upstream-most side so as to consume the oxygen contained in the exhaust gas when NOx should be released from the NOx storing catalysts.

2. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein when

NOx should be released from the NOx storing catalysts, said upstream-most NOx storing catalyst is fed with the reducing agent required for consuming the oxygen contained in the exhaust gas in addition to the reducing agent required for reducing the NOx stored in the upstream-most NOx storing catalyst from the corresponding reducing agent feed device, and each downstream side NOx storing catalyst positioned at the downstream side of the upstream-most NOx storing catalyst is fed with the reducing agent required for reducing the NOx stored in each downstream side NOx storing catalyst from the corresponding reducing agent feed device.

3. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein a degree of deterioration of said upstream-most NOx storing catalyst is estimated and an amount of reducing agent fed from the corresponding reducing agent feed device to the upstream-most NOx storing catalyst and each downstream side NOx storing catalyst positioned at the downstream side of the upstream-most NOx storing catalyst is adjusted in accordance with the estimated degree of deterioration.

4. An exhaust purification device of an internal combustion engine as set forth in claim 3, wherein the higher the degree of said upstream-most NOx storing catalyst, the greater the amount of the reducing agent fed from the corresponding reducing agent feed device to said downstream side NOx storing catalyst.

5. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein the timing of feed of the reducing agent to the downstream side NOx storing catalyst is delayed from the timing of feed of the reducing agent to the upstream-most NOx storing catalyst so that the reducing agent to be fed to the downstream side NOx storing catalyst is fed to a part of the exhaust gas flow where the air-fuel ratio falls due to the feed of reducing agent from the upstream-most NOx storing catalyst.