JP2007046493A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the discharge of unburnt HC and CO at the time of discharging NO<SB>x</SB>from an NO<SB>x</SB>occlusion reduction catalyst. <P>SOLUTION: In the exhaust emission control device, an engine exhaust passage is branched into a pair of branch passages 12, 13 to arrange NO<SB>x</SB>occlusion reduction catalysts 14, 15 in respective branch passage 12, 13 as shown in the Figure 1. When the difference between NO<SB>x</SB>discharge completion timings of the NO<SB>x</SB>occlusion reduction catalysts 14, 15 is determined to exist, the air-fuel ratio of exhaust gas flowing in the respective NO<SB>x</SB>occlusion reduction catalysts 14, 15 when controlling NO<SB>x</SB>discharge from the NO<SB>x</SB>occlusion reduction catalysts 14, 15 is first maintained in a rich condition and subsequently in a theoretical air-fuel ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

In the normal lean air-fuel ratio of an internal combustion engine the combustion under performed, the air-fuel ratio is stoichiometric or rich exhaust gas air-fuel ratio of the inflowing exhaust gas when the lean occluding flow of NO _x in the exhaust gas comes to the _the NO _x storage reduction catalyst for releasing occluded NO _x arranged in the engine exhaust passage, arranging the oxygen concentration sensor which generates an output corresponding to the air-fuel ratio in _the NO _x storage reduction catalyst downstream of the engine exhaust passage and, when it should be released NO _x from _the NO _x storage reduction catalyst internal combustion engine which is adapted to the air-fuel ratio of the exhaust gas flowing into the NO _x storage-reduction catalyst switched from lean to rich it is known by supplying the reducing agent (For example, refer to Patent Document 1).

In this case, the air-fuel ratio of the exhaust gas during the flowing out from _the NO _x storage reduction catalyst NO _x or O ₂ from _the NO _x storage-reduction catalyst is continuously released and is just only lean, the NO _{x storage-and-reduction} catalyst When the completion timing of the NO _x releasing action approaches, the air-fuel ratio of the exhaust gas flowing out from the NO _x storage reduction catalyst begins to become rich. Thus after the releasing action of the NO _x is started in this internal combustion engine, when the air-fuel ratio of the exhaust gas flowing out from _the NO _x storage-reduction catalyst that began rich is detected by the oxygen concentration sensor, shortly NO _x Is determined to be completed, and the air-fuel ratio of the exhaust gas flowing into the NO _x storage reduction catalyst is switched from rich to lean.

By the way, in this case, if the amount of reducing agent supplied to release NO _x , for example, the amount of fuel is too large compared to the amount of NO _x to be reduced, a large amount of unburned HC and CO is discharged into the atmosphere. to become, _the nO _x storage-reduction catalyst gradually nO to the reducing agent, i.e., the amount of fuel can not be sufficiently release the nO _x from the too small compared to _the amount of nO _x to be reduced _the nO _x storage-reduction catalyst with respect to this Cannot occlude _x . Therefore, in this internal combustion engine is the air-fuel ratio of the exhaust gas flowing into the NO _x storage reduction catalyst when the releasing action of the NO _x from _the NO _x storage-reduction catalyst is completed from rich to switch to a lean, to reduction whereby An amount of reducing agent corresponding to the amount of NO _x , that is, fuel, is supplied.
JP 2003-56379 A

By the way the structure of the vehicle, may need to place the respective NO _{x storage-and-reduction} catalyst by branching the engine exhaust passage to the pair of branch passages in each branch passage, in this case of the NO _{x storage-and-reduction} catalyst In many cases, there is a difference in the amount of NO _x stored in each NO _x storage-reduction catalyst due to the difference in NO _x and O ₂ storage capacities and the like and the difference in the amount of exhaust gas flowing into each branch passage. However, when a difference in amount of NO _x occluded in this way in the NO _{x storage-and-reduction} catalyst occurs, the air-fuel ratio of the exhaust gas flowing into the NO _x storage-reduction catalyst so as to release the NO _x from these _the NO _x storage-reduction catalyst When rich, the exhaust gas air-fuel ratio cannot be switched from rich to lean at the time when NO _x release from both NO _x storage reduction catalysts is completed. That is, in this case, the amount of reducing agent supplied to at least one NO _x storage reduction catalyst is excessive or insufficient, and thus a large amount of unburned HC and CO is released into the atmosphere. , or _the NO _x storage-reduction NO _x and O ₂ storage capacity of the catalyst occurs the problem of reduction.

According to the present invention in order to solve the above problems, people husband splits the engine exhaust passage into a plurality of branch passages in each branch passage, the air-fuel ratio of the inflowing exhaust gas to NO _x in the exhaust gas when the lean occluded air-fuel ratio of the inflowing exhaust gas is arranged _the NO _x storage reduction catalyst release and reduce NO _x occluding becomes the stoichiometric air-fuel ratio or rich, the release completion time of the NO _x in between the NO _{x storage-and-reduction} catalyst determined that when it is determined that there is no difference to maintain the air-fuel ratio of the exhaust gas flowing into the nO _{x storage-and-reduction} catalyst until completion from the time _the nO _x releasing control start from _the nO _x storage reduction catalyst rich, there is a difference way from _the NO _x releasing control of _the NO _x releasing control period while maintaining the air-fuel ratio of the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst from the time of _the NO _x releasing control start to the middle of _the NO _x releasing control period to rich when it is Complete Until so as to maintain the air-fuel ratio of the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst substantially stoichiometric air-fuel ratio at the time.

A large amount of unburned HC and CO is not discharged into the outside air when NO _{x is} released, and the stored NO _x can be released well.

FIG. 1 shows a V-type 6-cylinder engine having a first bank 1 and a second bank 2.
As shown in FIG. 1, the first cylinder group constituting the first bank 1 has three cylinders 5 including a fuel injection valve 3 and a spark plug 4, and constitutes the second bank 2. The second cylinder group also has three cylinders 5 each having a fuel injection valve 3 and a spark plug 4. These cylinders 5 are supplied with intake air via a common intake passage 6.

A first exhaust passage 7 is connected to the first cylinder group, and a three-way catalyst 8 is disposed in the first exhaust passage 7. A second exhaust passage 9 is connected to the second cylinder group, and a three-way catalyst 10 is also disposed in the second exhaust passage 9. The first exhaust passage 7 and the second exhaust passage 9 are joined to a common exhaust passage 11. A plurality of branch passages are branched from the common exhaust passage 11, and in the example shown in FIG. 1, a pair of branch passages 12 and 13 are branched, and NO _x storage reduction catalysts 14 and 15 are respectively provided in the branch passages 12 and 13. Be placed. The downstream ends of the branch passages 12 and 13 are joined to a common exhaust passage 16, and an oxygen concentration sensor 17 is disposed in the common exhaust passage 16.

  The electronic control unit 20 comprises a digital computer and is connected to each other by a bidirectional bus 21. A ROM (read only memory) 22, a RAM (random access memory) 23, a CPU (microprocessor) 24, an input port 25 and an output port 26 are connected. It comprises. The output signal of the oxygen concentration sensor 17 is input to the input port 25 via the corresponding AD converter 27. A load sensor 31 that generates an output voltage proportional to the amount of depression of the accelerator pedal 30 is connected to the accelerator pedal 30, and the output voltage of the load sensor 31 is input to the input port 25 via the corresponding AD converter 27. Further, a crank angle sensor 32 that generates an output pulse every time the crankshaft rotates, for example, 30 ° is connected to the input port 25. On the other hand, the output port 26 is connected to each fuel injection valve 3 and each spark plug 4 via a corresponding drive circuit 28.

The NO _x storage reduction catalysts 14 and 15 have a base having a three-dimensional network structure, for example, and a catalyst carrier made of alumina, for example, is supported on the base. 2A and 2B schematically show a cross section of the surface portion of the catalyst carrier 40. FIG. As shown in FIGS. 2A and 2B, a noble metal catalyst 41 is dispersedly supported on the surface of the catalyst carrier 40, and a layer of NO _x absorbent 42 is further formed on the surface of the catalyst carrier 40. Is formed.

In the embodiment according to the present invention, platinum Pt is used as the noble metal catalyst 41, and the constituents of the NO _x absorbent 42 are, for example, alkali metals such as potassium K, sodium Na, cesium Cs, barium Ba, calcium Ca. At least one selected from alkaline earths such as these, lanthanum La, and rare earths such as yttrium Y is used.

When the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber, and the exhaust passage upstream of the NO _x storage reduction catalysts 14 and 15 is referred to as the air-fuel ratio of the exhaust gas, the NO _x absorbent 42 is exhausted. air-fuel ratio of the gas is occluded or absorbed NO _x when the lean performs absorbing and releasing action of the NO _x to be emitted when the oxygen concentration in the exhaust gas falls occluded or absorbed NO _x.
That is, the case where barium Ba is used as a component constituting the NO _x absorbent 42 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, it is contained in the exhaust gas. As shown in FIG. 2 (A), NO is oxidized on platinum Pt 41 to become NO ₂ , and then absorbed into the NO _x absorbent 42 and combined with barium oxide BaO in the form of nitrate ions NO ₃ ^−. _{x Diffuses in the} absorbent 42. In this way, NO _x is absorbed into the NO _x absorbent 42. Oxygen concentration in the exhaust _gas, NO ₂ is produced on the surface as long as the platinum Pt41 high, _the NO _x absorbent 42 of the NO _x absorbing capability as long as NO ₂ not to saturate been absorbed in the NO _x absorbent 42 nitrate ions NO ₃ ^- is generated.

On the other hand, when the air-fuel ratio of the exhaust gas is made rich or stoichiometric, the oxidation concentration in the exhaust gas decreases, so that the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ). (B) as shown in _the NO _x absorbent in the 42 nitrate ions NO ₃ ^- is released from the NO _x absorbent 42 in the form of NO _2. Next, the released NO _x is reduced by unburned HC and CO contained in the exhaust gas.
Thus, when the air-fuel ratio of the exhaust gas is lean, that is, when combustion is performed under the lean air-fuel ratio, NO _x in the exhaust gas is absorbed into the NO _x absorbent 42. However becomes saturated the absorption of NO _x capacity of the NO _x absorbent 42 during the combustion of the fuel under a lean air-fuel ratio is continued, no longer able to absorb NO _x by _the NO _x absorbent 42 and thus End up. Therefore, in this embodiment of the present invention to temporarily make the air by the air-fuel ratio of the mixture in the combustion chamber before the absorbing capability of the NO _x absorbent 42 is saturated in rich, whereby NO _x NO _x is released from the absorbent 42.

  FIG. 3 shows the relationship between the output voltage E (V) of the oxygen concentration sensor 17 and the air-fuel ratio of the exhaust gas. As can be seen from FIG. 3, the output voltage E (V) of this oxygen concentration sensor is about 0.5 (V) when the air-fuel ratio is the stoichiometric air-fuel ratio, and about 0.9 (V) when the air-fuel ratio becomes rich. When the air-fuel ratio becomes lean, it becomes about 0.1 (V).

FIG. 4 shows only the internal combustion engine shown in FIG. 1, and the measurement points A, B, and C of the exhaust gas air-fuel ratio shown in FIGS. 5 to 7 are displayed in FIG. That is, the air-fuel ratio of the exhaust gas flowing out from the NO _x storage reduction catalyst 14 is detected at point A, the air-fuel ratio of the exhaust gas flowing out from the NO _x storage reduction catalyst 15 is detected at point B, and each NO _x is detected at point C. The air-fuel ratio of the exhaust gas that has flowed out of and mixed with the storage reduction catalysts 14 and 15 is detected. The air-fuel ratio of the exhaust gas is detected by an oxygen concentration sensor having output characteristics as shown in FIG.

FIG. 5 (A), the shows the change in the fuel-rich signal and the air-fuel ratio when (B) is being made good _the NO _x releasing control. Note that the rich signal shown in FIGS. 5A and 5B is a drive signal applied to the fuel injection valve 3 to increase the amount of injected fuel in order to make the air-fuel ratio in the combustion chamber rich. Also, the air-fuel ratio A / F shown in FIG. 5A indicates the air-fuel ratio in the combustion chamber, and the output of the O ₂ sensor shown in FIG. 5B is the points A, B, C in FIG. 5 shows the change in the output voltage of the oxygen concentration sensor at the point. In FIG. 5B, unburned HC and CO are the amounts of unburned HC and CO in the exhaust gas at points A, B and C in FIG. Represents. The same applies to FIG. 6 and FIG.

In the embodiment according to the present invention, when a rich signal is generated to release NO _x from the NO _x storage reduction catalysts 14 and 15, the air-fuel ratio in the combustion chamber is made rich while the rich signal is generated. The In this case, as shown in FIG. 5A, the richness of the air-fuel ratio in the combustion chamber is increased in the first half and decreased in the second half.

That is, the three-way catalysts 8 and 10 and the NO _x storage reduction catalysts 14 and 15 both have a function of storing excess oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, that is, an O ₂ storage function. Therefore, usually, a large amount of oxygen is stored in the three-way catalysts 8 and 10 and the NO _x storage reduction catalysts 14 and 15. In this state, when the air-fuel ratio in the combustion chamber is made rich to release NO _x from the NO _x storage reduction catalyst, a large amount of unburned HC and CO contained in the exhaust gas at this time are stored in a large amount. Oxidized by oxygen. In this way, the degree of richness is large in the first half of the period in which the rich signal is generated so that the air-fuel ratio of the exhaust gas is maintained rich even if the unburned HC and CO are oxidized by a large amount of stored oxygen. Is done.

While the air-fuel ratio of the exhaust gas flowing in the rich signal is issued to _the NO _x storage-reduction catalyst 15 becomes rich, it releasing action of the NO _x from _the NO _x storage-reduction catalyst 14, 15 by is performed, The air-fuel ratio of the exhaust gas flowing out from the NO _x storage reduction catalysts 14 and 15 is slightly lean. At this time, no unburned HC and CO are discharged from the NO _x storage reduction catalysts 14 and 15.

Then, when _the amount of NO _x is reduced, which is occluded in _the NO _x storage-reduction catalyst 14, i.e., the completion of the releasing action of the NO _x increases gradually the output voltage of the near Nuisance and the oxygen concentration sensor, _the NO _x storage-reduction catalyst 14 and which is not little NO _x occluded in the 15, i.e. the output voltage of the oxygen concentration sensor when the releasing action is completed of the NO _x increases rapidly. At this time, generation of the rich signal is stopped and the air-fuel ratio of the exhaust gas is switched from rich to lean. Accordingly, as shown in FIG. 5B, only a small amount of unburned HC and CO are discharged from the NO _x storage reduction catalysts 14 and 15.

FIG. 5B shows the case where the NO _x storage and reduction catalyst 14 and the NO _x storage and reduction catalyst 15 simultaneously complete the NO _x release action. In this case, the timing at which the generation of the rich signal is stopped, that is, the timing at which the air-fuel ratio is switched from rich to lean is set immediately before the NO _x releasing action from both the NO _x storage reduction catalysts 14 and 15 is completed. Specifically, the rich to lean switching timing is calculated by calculating the amount of NO _x stored in the NO _x storage reduction catalysts 14 and 15 and calculating the required rich time from the calculated amount of NO _x. Or when the output voltage of the oxygen concentration sensor exceeds the set level is the timing for switching from rich to lean.

However in some cases, or to a difference between the NO _x and O ₂ absorption capacity of _the NO _x storage NO _x and O ₂ absorption capacity of the reduction catalyst 14 and _the NO _x storage-reduction catalyst 15 caused by aging, or _the NO _x storage Since there is a difference between the amount of exhaust gas flowing into the reduction catalyst 14 and the amount of exhaust gas flowing into the NO _x storage reduction catalyst 15, there may be a difference in the storage amount of NO _x between the NO _x storage reduction catalysts 14 and 15. There are many. In addition, when the amount of oxygen stored in the NO _x storage reduction catalysts 14 and 15 is large, the amount of reducing agent used for NO _x reduction decreases, and the NO _x storage reduction catalysts 14 and 15 store. the amount of reducing agent used for reduction of when the oxygen storage amount which can be less in NO _x increases. Accordingly, the resulting difference in release completion time of the NO _x in between _the NO _x storage-reduction catalyst 14, 15 by the difference of the difference and the oxygen storage amount of the NO _x storage amount between _the NO _x storage-reduction catalyst 14, 15.

FIG 6 (B) is changed and unburnt HC in the output voltage of the oxygen concentration sensor when a difference in release completion time of the NO _x occurs in between thus _the NO _x storage-reduction catalyst 14, 15, CO emissions It shows a change. In this case as well, as can be seen from FIG. 6A, the richness of the air-fuel ratio in the combustion chamber when the rich signal is generated is changed in the same manner as in FIG. 5A. Further, as can be seen from the change in the output voltage of the oxygen concentration sensor at points A and B in FIG. 6B, FIG. 6B shows that the air-fuel ratio increases when the NO _x release of the NO _x storage reduction catalyst 14 is completed. is switched from rich to lean, and towards _the NO _x releasing completion time of the NO _x storage-reduction catalyst 15 in comparison with _the NO _x releasing completion time of the NO _x storage-reduction catalyst 14 indicates the case earlier.

If this towards _the NO _x releasing completion time of the NO _x storage-reduction catalyst 15 in comparison with _the NO _x releasing completion time of the NO _x storage-reduction catalyst 14 is fast so, as shown in the point B shown in FIG. 6 (B) In addition, the amount of unburned HC and CO discharged from the NO _x storage reduction catalyst 15 increases. At this time, the output voltage of the oxygen concentration sensor at the point C becomes a value such as an average value of the sum of the output voltages of the oxygen concentration sensors at the points A and B, and there is a specific fluctuation with a step until the peak value is reached. It becomes a pattern.

Thus, from the fluctuation pattern of the output voltage of the oxygen concentration sensor at the points A and B, or the fluctuation pattern of the output voltage of the oxygen concentration sensor at the point C, in other words, the exhaust gas flowing out from the NO _x storage reduction catalysts 14 and 15. It can be determined whether or not there is a difference in the completion timing of NO _x release from the fluctuation pattern when the NO _x release control of the gas air-fuel ratio is completed.

In this case, in the embodiment according to the present invention, as shown in FIG. 1, only one oxygen concentration sensor 17 is provided in the common exhaust passage 16, and based on only the output voltage of the oxygen concentration sensor 17, that is, the oxygen concentration at the point C. difference in completion time _the nO _x releasing on the basis of only the output voltage of the sensor so that it is determined whether or not there. In this case, when the NO _x release control is completed, the output voltage of the oxygen concentration sensor 17 reaches the peak value after being maintained at an intermediate value that is substantially half of the peak value as shown at point C in FIG. 6B. At this time, it is determined that there is a difference in the NO _x release completion timing when the time maintained at the intermediate value is equal to or longer than a predetermined set time.

Note that to take an intermediate value before the output voltage of the oxygen concentration sensor 17 reaches a peak value as shown in point C shown in FIG. 6 (B) when there is a difference in _the NO _x releasing completion timing, when the releasing action of the NO _x has been completed in the NO _x NO _{x storage-and-reduction} catalysts towards slow releasing action completion time of, NO _x releasing action of _the NO _x storage-reduction catalyst 14 in the example shown in FIG. 6 (B) is completed When this happens, it is necessary to switch the air-fuel ratio from rich to lean. In this case, for example, when the output voltage of the oxygen concentration sensor at point C exceeds the set level, the switching timing from rich to lean is set.

Now, when it is determined that there is a difference in _the NO _x releasing completion time of each NO _{x storage-and-reduction} catalyst 14, 15 according to the present invention Figure 7 NO from _the NO _x releasing control at the start, as shown in (A) _x The air-fuel ratio in the combustion chamber is kept rich until the middle of the release control period, and the air-fuel ratio in the combustion chamber is maintained at the stoichiometric or almost stoichiometric air-fuel ratio from the middle of the NO _x release control period to the completion of the NO _x release control. The In other words, _the NO _x storage-reduction NO _x each _the NO _x storage halfway when it is determined that there is a difference in the release completion time from when _the NO _x releasing control start _the NO _x releasing control period reduction catalyst 14 of the catalyst 14, 15 The air-fuel ratio of the exhaust gas flowing into the exhaust gas is maintained rich, and the air-fuel ratio of the exhaust gas flowing into the NO _x storage reduction catalysts 14 and 15 is stoichiometrically from the middle of the NO _x release control period until the completion of the NO _x release control. The fuel ratio is maintained at or approximately the stoichiometric air-fuel ratio. If it does in this way, as shown in Drawing 7 (B), the quantity of unburned HC and CO in A point and B point will decrease.

That is, when the air-fuel ratio of the exhaust gas flowing into each NO _x storage reduction catalyst 14, 15 is switched from rich to the stoichiometric air fuel ratio or substantially the stoichiometric air fuel ratio, NO _x remains in the NO _x storage reduction catalyst 14, 15. In this case, the residual NO _x is released from the NO _x storage reduction catalysts 14 and 15 and is reduced, although it is slower than when the air-fuel ratio of the exhaust gas is rich. At this time, unburned HC and CO are not discharged from the NO _x storage reduction catalysts 14 and 15.

Then _the NO _x storage-reduction when releasing action of the NO _x from the catalyst 14, 15 is completed _the NO _x storage-reduction air-fuel ratio of the exhaust gas flowing out from the catalyst 14, 15 the stoichiometric air-fuel ratio or 7 to substantially the stoichiometric air-fuel ratio As shown at points A and B in (A), the output voltage of the oxygen concentration sensor increases. At this time, since the air-fuel ratio of the exhaust gas flowing out from the NO _x storage reduction catalysts 14 and 15 does not become rich, the amounts of unburned HC and CO discharged from the NO _x storage reduction catalysts 14 and 15 at this time are small. Figure fuel ratio when the fuel-rich signal is issued when this way there is a difference in releasing action completion time of the NO _x in between _the NO _x storage-reduction catalyst 14, 15 from the rich air-fuel ratio shown in FIG. 6 (A) 7 By switching to the combination of the rich air-fuel ratio and the stoichiometric air-fuel ratio shown in (A), the amount of unburned HC and CO emissions can be suppressed.

As described above, when the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio or substantially the stoichiometric air-fuel ratio, NO _x is slowly released from the NO _x storage reduction catalysts 14 and 15, and therefore in this case NO _x is released. It takes time to complete. Therefore, as shown in FIG. 7A, when the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio, the generation period of the rich signal, that is, compared with the cases shown in FIGS. 5A and 6A, that is, The period of NO _x release control is lengthened.

Note that, in FIG. 7A, even if the timing for returning the air-fuel ratio from the stoichiometric air-fuel ratio to lean is slightly delayed, the emission amount of unburned HC and CO as well as NO _x does not increase. Therefore, the timing for returning the air-fuel ratio from the stoichiometric air-fuel ratio to lean can be set relatively freely.

FIG. 8 shows the NO _x release control routine.
Whether it is the time from _the NO _x storage-reduction catalyst 14, 15 to the release of NO _x at first, at step 50 and reference to FIG. 8 is determined. When it is not time to release NO _x , the processing cycle is completed. On the other hand, when it is time to release NO _x , the routine proceeds to step 51, where it is judged if the normal rich control shown in FIG. When normal rich control is to be performed, the routine proceeds to step 52, where the normal rich control shown in FIG. 5A is performed.

Next, at step 53, the output voltage of the oxygen concentration sensor 17 is read. Next, at step 54, is there a difference in the NO _x release completion timing between the NO _x storage reduction catalysts 14 and 15 from the fluctuation pattern of the output voltage of the oxygen concentration sensor 17? It is determined whether or not. When it is determined that there is no difference in the NO _x release completion timing, the routine proceeds to step 55 where it is determined that the normal rich control shown in FIG. In contrast, it is determined that when it is determined that there is a difference in _the NO _x releasing completion time should be changed to the rich control shown in FIG. 7 (A) rich control proceeds to step 56. When it is determined that the rich control should be changed, the routine proceeds from step 51 to step 57 when it is determined in step 50 that NO _x should be released, and the rich shown in FIG. Control is performed.

1 is an overall view of an internal combustion engine. It is a diagram for explaining the absorbing and releasing action of NO _x. It is a figure which shows the output voltage E of an oxygen concentration sensor. It is a figure which shows the internal combustion engine shown in FIG. It is a figure which shows the fluctuation pattern etc. of an air fuel ratio when a rich signal is emitted. It is a figure which shows the fluctuation pattern etc. of an air fuel ratio when a rich signal is emitted. It is a figure which shows the fluctuation pattern etc. of an air fuel ratio when a rich signal is emitted. 3 is a flowchart for performing NO _x release control.

Explanation of symbols

7 First exhaust passage 9 Second exhaust passage 12, 13 Branch passage 14, 15 NO _x storage reduction catalyst 17 Oxygen concentration sensor

Claims (5)

The engine exhaust passage is branched into a plurality of branch passages, and when the air-fuel ratio of the exhaust gas flowing into each branch passage is lean, the air-fuel ratio of the exhaust gas that stores NO _x in the exhaust gas and flows in is the stoichiometric air-fuel ratio. or becomes rich to release occluded nO _x arranged _the nO _x storage-reduction catalyst that reduces, _the nO _x storage reduction catalyst when it is determined that there is no difference in the release completion time of the nO _x in between the nO _{x storage-and-reduction} catalyst From the start of NO _x release control to the completion of NO _x , the air-fuel ratio of the exhaust gas flowing into each NO _x storage reduction catalyst is maintained rich, and when it is determined that there is a difference, the NO _x release control starts from the start of NO _x the _the NO _x releasing control the way from _the NO _x releasing control completion until each NO _{x storage-and-reduction} catalyst period while maintaining the air-fuel ratio of the exhaust gas up to the middle of the _x releasing control period flowing into the NO _{x storage-and-reduction} catalyst in a rich Inflow An exhaust gas purification apparatus for an internal combustion engine in which the air-fuel ratio of exhaust gas to be maintained is maintained at a substantially stoichiometric air-fuel ratio. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the NO _x release control period is made longer when it is determined that there is the difference than when it is determined that there is no difference. The exhaust emission control device for an internal combustion engine according to claim 1, wherein whether or not there is the difference is determined from a fluctuation pattern of the air-fuel ratio of the exhaust gas flowing out from each NO _x storage reduction catalyst when the NO _x release control is completed. Each branch passage is joined to one common exhaust passage, and an oxygen concentration sensor is arranged in the common exhaust passage. From the fluctuation pattern of the output value of the oxygen concentration sensor when the NO _x release control is completed. The exhaust emission control device for an internal combustion engine according to claim 1, wherein whether or not there is the difference is determined. In the case when the difference is not at all reach the peak value output value of the oxygen concentration sensor when the _the NO _x releasing control completion rises sharply, the output value of the oxygen concentration sensor when _the NO _x releasing control completion almost to peak value The exhaust of the internal combustion engine according to claim 4, wherein the difference is determined when the peak value is reached after being maintained at an intermediate value that is half and the time that the intermediate value is maintained at this time is equal to or longer than a set time. Purification equipment.

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