JP4224984B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
When a plurality of cylinders are divided into a pair of cylinder groups, and the air-fuel ratio of the exhaust gas flowing into the downstream exhaust passage formed by joining a pair of upstream exhaust passages connected to these cylinder groups is lean store up NO _X in the exhaust gas flowing into the air-fuel ratio of the inflowing exhaust gas is stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when the reduced NO _X the amount will place the NO _X catalyst of decreasing, in order to raise the temperature of the NO _X catalyst is burned in retaining the air-fuel ratio of the mixture burned in one cylinder group to a lean and the other cylinder group the air-fuel ratio of the mixture holds rich exhaust gas control apparatus that the average air-fuel ratio of the entire exhaust gas was set to the stoichiometric air-fuel ratio flowing into this time the NO _X catalyst is known (JP-a 8-61052 gazette).
[0003]
In this case, the exhaust gas discharged from one cylinder group contains a large amount of oxygen, and the exhaust gas discharged from the other cylinder group contains a large amount of unburned HC and CO. . Therefore, these oxygen and unburned HC, the CO reacts in the NO _X catalyst Italian the NO _X catalyst, the temperature of the NO _X catalyst is raised.
[0004]
[Problems to be solved by the invention]
The exhaust gas discharged from one cylinder group and the exhaust gas discharged from the other cylinder group both flow into the downstream exhaust passage and merge. However, not limited to these exhaust gases are well mixed to stirring until reaching the NO _X catalyst, there is a risk of rather flowing into oxygen and unburned HC, the NO _X catalyst while CO is unevenly distributed. In this case, unburned HC to be supplied to the NO _X catalyst, will not be able to effectively use the CO for the temperature rise of the NO _X catalyst, further, a long time the temperature of the NO _X catalyst to raise up to the target temperature It will also require. The above publication does not suggest anything about this problem.
[0005]
SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can increase the temperature of a catalyst effectively and quickly.
[0006]
[Means for Solving the Problems]
In order to solve the above-described problem, according to a first aspect of the invention, a plurality of cylinders are divided into a pair of cylinder groups, and a downstream exhaust gas formed by joining a pair of upstream exhaust passages connected to the cylinder groups. In order to dispose a catalyst having oxidizing ability in the passage and raise the temperature of the catalyst, the air-fuel ratio of the exhaust gas flowing out from one upstream exhaust passage into the downstream exhaust passage is kept lean and the other The air-fuel ratio of the exhaust gas flowing out from the upstream exhaust passage into the downstream exhaust passage is kept rich, and the average air-fuel ratio of the entire exhaust gas flowing into the downstream exhaust passage at this time is the stoichiometric air fuel ratio or slightly richer In the exhaust gas purification apparatus for an internal combustion engine, the exhaust gas from one upstream exhaust passage and the exhaust gas from the other upstream exhaust passage are mixed in the downstream exhaust passage upstream of the catalyst. Means And location, the mixing means and the downstream exhaust passage between the catalyst formed from a pair of branch passages extending in parallel to each other, are arranged additional catalyst having respective oxidation ability in these branch passages.
[0009]
Further, in the first aspect, according to a second aspect of the invention, the mixing means, the exhaust gas is formed from the turning device for guiding the exhaust gas to pivot about substantially the center axis of the downstream side exhaust passage.
[0010]
Further, in the first aspect according to the third invention, the combustion in the internal combustion engine are adapted to be continuously performed under a lean air-fuel ratio, the air-fuel ratio of the exhaust gas to the catalyst and flows stored but stored the NO _X in the exhaust gas flowing at the time of the lean air-fuel ratio of the inflowing exhaust gas by reducing NO _X are stored as containing a reducing agent in the exhaust gas when the reduced the amount of and NO _X is formed from NO _X catalyst decreases.
[0011]
Further, in the first aspect according to the fourth invention, it is formed from the three-way catalyst the additional catalyst.
[0012]
In the present specification, the ratio of the air supplied to the exhaust passage upstream of a certain position of the exhaust passage, the combustion chamber, and the intake passage and the hydrocarbon HC and carbon monoxide CO is determined by the ratio of the exhaust gas at that position. This is called the air-fuel ratio.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a case where the present invention is applied to a spark ignition type internal combustion engine. However, the present invention can also be applied to a compression ignition type internal combustion engine.
[0014]
Referring to FIG. 1, the engine main body 1 includes a plurality of, for example, six cylinders. These cylinders are, for example, a first cylinder group 1a composed of, for example, three cylinders aligned on one side of the central axis of the engine body 1, and a first cylinder composed of, for example, three cylinders aligned on the other side of the central axis of the engine body 1. It is divided into two cylinder groups 1b. The first and second cylinder groups 1a and 1b are respectively connected to the corresponding branch portions 3a and 3b of the Y-tube 3 via the corresponding exhaust manifolds 2a and 2b, respectively. Connected to the junction 4 c of the Y-tube 4. The inlets of the casing 5 are respectively connected to the pair of branch portions 4 a and 4 b of the Y-shaped tube 4, and the outlets of the casing 5 are connected to the inlet of the casing 8 via the Y-shaped tube 6 and the exhaust pipe 7. An exhaust pipe 9 is connected to the outlet of the casing 8. In FIG. 1, reference numeral 10 indicates an electromagnetically controlled fuel injection valve disposed in the combustion chamber of each cylinder.
[0015]
In the casing 8 is accommodated catalyst 11 having an oxidizing ability, in the embodiment according to the present invention the catalyst 11 is formed from NO _X catalyst. The NO _X catalyst 11 has, for example, alumina as a carrier, and on this carrier, for example, alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y, etc. And at least one selected from rare earths such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir. On the other hand, an additional catalyst 12 having oxidation ability is accommodated in the casing 5. In embodiments according to the present invention, the additional catalyst 12 is each formed from a three-way catalyst.
[0016]
Referring further to FIG. 1, for example, in the junction 4 c of the Y-tube 4 from the merge of the branches 3 a and 3 b of the Y-tube 3 to the branch of the branches 4 a and 4 b of the Y-tube 4. A swivel device 13 is arranged. As shown in FIG. 2, the turning device 13 includes an outer ring 13 a fixed on the inner wall surface of the Y-tube 4, an inner ring 13 b disposed substantially at the center of the outer ring 13 a, and an outer ring spaced apart from each other in the circumferential direction. 13a and a plurality of blades 13c supported by the inner ring 13b. In FIG. 2, the hatched portion indicates the blade 13c.
[0017]
These blades 13c extend spirally along the central axis of the junction 4c of the Y-shaped tube 4, so that the exhaust gas swirls around the substantially central axis of the junction 4c of the Y-shaped tube 4 by the swiveling device 13. To be guided. In the embodiment according to the present invention, these blades 13c cannot be rotated, but the blades 13c can be rotatably supported as long as the exhaust gas swirls.
[0018]
On the other hand, the first and second cylinder groups 1a and 1b are connected to a common intake duct (not shown) via corresponding surge tanks (not shown).
[0019]
The electronic control unit 40 is composed of a digital computer, and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 are connected. It comprises. The exhaust pipe 7 is attached air-fuel ratio sensor 49r for detecting the average air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11, the exhaust pipe 9 detects the temperature of the exhaust gas flowing out from the NO _X catalyst 11 A temperature sensor 49t is attached. The temperature of the exhaust gases represents the temperature of the NO _X catalyst 11. The output voltages of these sensors 49r and 49t are input to the input port 45 via the corresponding AD converters 47, respectively. A load sensor 51 that generates an output voltage proportional to the amount of depression of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. The Further, the input port 45 is connected with a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 10 via a corresponding drive circuit 48.
[0020]
The NO _X catalyst 11 stores NO _X when the average air-fuel ratio of the inflowing exhaust gas is lean, and stores NO _X when the reducing gas is contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas decreases. Accumulate and reduce to reduce the amount of NO _X stored by reducing _X.
[0021]
It not fully elucidated detailed mechanism of accumulation reducing action of the NO _X catalyst. However, the mechanism currently considered can be briefly described as follows, taking as an example the case where platinum Pt and barium Ba are supported on a support.
[0022]
That is, the oxygen concentration in the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO _X catalyst flows become considerably leaner than the stoichiometric air-fuel ratio is increased by a large margin, the oxygen O ₂ is O ₂ ^- or O ^2- of It adheres to the surface of platinum Pt in the form. On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become NO ₂ (NO + O ₂ → NO ₂ ). Next, a part of the generated NO ₂ is further oxidized on the platinum Pt and absorbed into the NO _X catalyst and combined with barium oxide BaO, and diffuses in the NO _X catalyst in the form of nitrate ions NO ₃ ⁻ . In this way, NO _X is stored in the NO _X catalyst.
[0023]
On the other hand, when the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst becomes rich or stoichiometric, the oxygen concentration in the exhaust gas decreases, the amount of NO ₂ generated decreases, and the reaction proceeds in the reverse direction (NO ₃ ^- → proceeds to NO), NO _X catalyst in the nitrate ions NO ₃ and thus ^- are released from the NO _X catalyst in the form of NO. When the exhaust gas contains a reducing agent, that is, HC and CO, the released NO _X reacts with these HC and CO and is reduced. When NO _X no longer exists on the surface of platinum Pt in this way, NO _X is released from the NO _X catalyst to the next and reduced, and the amount of NO _X stored in the NO _X catalyst gradually increases. Decrease.
[0024]
Incidentally, stored without any NO _X to form a nitrate, it can be reduced without any NO _X to release NO _X.
[0025]
Internal combustion engine shown in FIG. 1 are continuously performed combustion under a lean air-fuel ratio, therefore the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is maintained lean. As a result, NO _X in the exhaust gas is stored in the NO _X catalyst 11.
[0026]
The accumulated NO _X amount in the NO _X catalyst 11 gradually increases with time. Therefore, in this embodiment of the present invention, for example, when the accumulated amount of NO _X in the NO _X catalyst 11 exceeds the allowable amount, so as to temporarily switch to make the air flowing into the NO _X catalyst 11 Yes. In this way, reduces the accumulated amount of NO _X in the NO _X catalyst 11, NO _X does not flow out from the NO _X catalyst 11 at this time.
[0027]
However, the exhaust gas contains sulfur in the form of SO _X , and not only NO _X but also SO _X is stored in the NO _X catalyst 11. The accumulation mechanism of SO _{X in} the NO _X catalyst 11 is considered to be the same as the accumulation mechanism of NO _X. That is, when briefly described as an example the case of carrying platinum Pt and barium Ba on the carrier, the oxygen O ₂ as the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 described above when the lean O ₂ ^- Alternatively, it is attached to the surface of platinum Pt in the form of O ²⁻ , and SO ₂ in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become SO ₃ . Next, the generated SO ₃ is further oxidized on the platinum Pt while being absorbed into the NO _X catalyst 11 and combined with barium oxide BaO, and diffuses into the NO _X catalyst 11 in the form of sulfate ions SO ₄ ⁻ . This sulfate ion SO ₄ ⁻ is then combined with barium ion Ba ⁺ to produce sulfate BaSO ₄ .
[0028]
This sulfate BaSO ₄ is not easily decomposed, and the amount of sulfate BaSO ₄ in the NO _X catalyst 11 does not decrease even if the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is simply made rich. Therefore, increasing the amount of sulfate BaSO ₄ in the NO _X catalyst 11 over time, the amount of resulting NO _X catalyst 11 can stored NO _X is decreased.
[0029]
However, if the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is made to be the stoichiometric air fuel ratio or slightly rich while maintaining the temperature of the NO _X catalyst 11 at, for example, 550 ° C. or higher, the sulfate BaSO ₄ in the NO _X catalyst 11 Is decomposed and released from the NO _X catalyst 11 in the form of SO ₃ . The released SO ₃ reacts with HC and CO and is reduced to SO ₂ when a reducing agent, that is, HC and CO is contained in the exhaust gas. In this way, the amount of SO _X stored in the NO _X catalyst 11 gradually decreases, and at this time, SO _X does not flow out from the NO _X catalyst 11 in the form of SO ₃ .
[0030]
In the embodiment according to the present invention, therefore, for example, when the accumulated SO _X amount in the NO _X catalyst 11 exceeds the allowable amount, to reduce the accumulation SO _X amount in the NO _X catalyst 11, the temperature of the NO _X catalyst 11 while maintaining the elevated and then 550 ° C. or higher to 550 ° C., and the average air ratio of the exhaust gas flowing into the NO _X catalyst 11 to hold the stoichiometric air fuel ratio or just slightly rich.
[0031]
Specifically, while the average air-fuel ratio of the entire exhaust gas flowing into the three-way catalyst 12 is made to be the stoichiometric air-fuel ratio or slightly richer, the exhaust gas exhaust flowing through the branch portion 3a of the Y-shaped tube 3 is exhausted. The fuel ratio is kept lean, and the air-fuel ratio of the exhaust gas flowing in the branch portion 3b of the Y-shaped tube 3 is temporarily switched to rich. As a result, an exhaust gas containing a relatively large amount of oxygen and an exhaust gas containing a relatively large amount of HC and CO are formed.
[0032]
These exhaust gases merge with each other in the merge portion 3c of the Y-shaped tube 3 and in the merge portion 4c of the Y-shaped tube 4, and then flow into the three-way catalyst 12. As a result, they are included in the exhaust gas in the three-way catalyst 12. A large amount of HC and CO burned in the presence of a large amount of oxygen. Therefore, the temperature increase of the exhaust gas flowing out from the three-way catalyst 12, the temperature of the NO _X catalyst 11 is raised by flowing into the NO _X catalyst 11 is then exhaust gas at this high temperature.
[0033]
In this case, the exhaust gas is swirled by the swirler 13 before flowing into the three-way catalyst 12, so that the oxygen from the branch part 3a and the HC and CO from the branch part 3b are well stirred and mixed. ing. Therefore, HC supplied to the three-way catalyst 12, CO has become easily burned in the three-way catalyst 12, that is, be effectively utilized to increase the temperature of the NO _X catalysts 11. In addition, the time required to raise the temperature of the NO _X catalyst 11 to the target temperature can be shortened, and further, the amount of HC and CO flowing out from the three-way catalyst 12 can be reduced.
[0034]
There are various methods for switching the air-fuel ratio of the exhaust gas flowing through the branch portion 3b of the Y-shaped tube 3 to be rich. That is, for example, a method of switching the air-fuel ratio of the air-fuel mixture combusted in the second cylinder group 1b to be rich, or an exhaust gas while maintaining the air-fuel ratio of the air-fuel mixture combusted in the second cylinder group 1b lean. There is a method of supplying additional fuel into the manifold 2b or the branch portion 3b, or supplying additional fuel from the fuel injection valve 10 into the combustion chamber of the second cylinder group 1b in the explosion process or the exhaust stroke.
[0035]
FIG. 3 schematically shows the outlets of the branches 3a and 3b of the Y-shaped tube 3 and the inlets of the branches 4a and 4b of the Y-shaped tube 4 in the embodiment shown in FIG. A straight line L3 connecting the outlet centers of the branch portions 3a and 3b of the tube 3 and a straight line L4 connecting the outlet centers of the branches 4a and 4b of the Y tube 4 are substantially parallel to each other. In FIG. 3, CC indicates a merging chamber formed by the merging portion 3 c of the Y-shaped tube 3 and the merging portion 4 c of the Y-shaped tube 4.
[0036]
The exhaust gases flowing through the branch portions 3a and 3b of the Y-shaped tube 3 respectively flow out into the merge chamber CC and merge, and then flow into the branch portions 4a and 4b of the Y-shaped tube 4. As a result, the exhaust gas flowing out from the branch portion 3a of the Y-tube 3 is distributed to the branch portions 4a and 4b of the Y-tube 4, and the exhaust gas flowing out from the branch portion 3b is distributed to the branch portions 4a and 4b. It means to be distributed.
[0037]
In this case, the volume of the merge chamber CC is relatively small, and mixing of exhaust gas here cannot be expected. Therefore, if the swivel device 13 is not provided, the exhaust gas flowing out from the branch portions 3a and 3b of the Y-shaped tube 3 is in a direction determined according to the shape of the branch portions 3a and 3b, for example. It will proceed with inertia. For this reason, the case where most of the exhaust gas flowing out from, for example, the branch portion 3a of the Y-tube 3 flows into the branch portion 4a of the Y-tube 4, and the remaining small portion flows into the branch portion 4b. In some cases, most of the exhaust gas flows into the branch portion 4b and a small part of the remaining portion flows into the branch portion 4a. The same can be said for the exhaust gas flowing out from the branch part 3b. As a result, one of the three-way catalysts 12 is supplied with excess HC and CO, and the other three-way catalyst 12 is supplied with excess oxygen.
[0038]
Therefore, in the embodiment according to the present invention, the swirling device 13 for swirling and mixing the exhaust gas is disposed in the merge chamber CC. As a result, the exhaust gas flowing out from the branches 3 a and 3 b of the Y-shaped tube 3 is sufficiently mixed and then flows into the branches 4 a and 4 b of the Y-shaped tube 4. Therefore, in this case, the exhaust gas flowing out from the branch portion 3a of the Y-tube 3 flows into the branches 4a and 4b of the Y-tube 4 almost evenly and flows out from the branch portion 3b of the Y-tube 3. The exhaust gas flows into the branches 4a and 4b of the Y-shaped tube 4 almost evenly. For this reason, excess or deficiency does not occur in HC, CO and oxygen supplied to each three-way catalyst 12.
[0039]
FIG. 4 shows a case where a straight line L3 connecting the outlet centers of the branch portions 3a and 3b of the Y-shaped tube 3 and a straight line L4 connecting the outlet centers of the branch portions 4a and 4b of the Y-shaped tube 4 are substantially perpendicular to each other. In this case, the present invention can be applied. If the swivel device 13 is not provided in this case, excess HC and CO may be locally supplied to a part of each three-way catalyst 12 and excess oxygen may be locally supplied to another part. There is. In this case, if the swirling device 13 is provided, such a problem can be solved, that is, by providing the swirling device 13, HC, CO, and oxygen can be supplied to the entire three-way catalyst 12. In other words, the entire three-way catalyst 12 can be used effectively.
[0040]
This embodiment that has been described up to the exhaust passage between the turning device 13 and the NO _X catalyst 11 is formed from a pair of branch passages each other extending in parallel, it is disposed, respectively a three-way catalyst 12 in those branch passages. In this way, the capacity of each three-way catalyst 12 can be reduced, and therefore the increase in engine back pressure can be suppressed. However, the exhaust passage between the turning device 13 and the NO _X catalyst 11 is formed from a single exhaust passage, also possible to apply the present invention in the case of arranging the single three-way catalyst in this single exhaust passage Can do. Alternatively, even if between the turning device 13 and the NO _X catalyst 11 is not arranged a three-way catalyst can be applied to the present invention. In this case, oxygen from the branch part 3 a and HC and CO from the branch part 3 b react with each other in the NO _X catalyst 11.
[0041]
【The invention's effect】
The temperature of the catalyst can be increased effectively and quickly.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
FIG. 2 is a front view of the turning device.
FIG. 3 is a view schematically showing an outlet and an inlet of Y-shaped tubes 3 and 4;
FIG. 4 is a diagram schematically showing an outlet and an inlet of a Y-tube 3, 4 in another embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1a ... 1st cylinder group 1b ... 2nd cylinder group 3, 4 ... Y-shaped pipe 11 ... NO _X catalyst 12 ... Three-way catalyst 13 ... Swivel device

Claims (4)

A plurality of cylinders are divided into a pair of cylinder groups, and a catalyst having oxidizing ability is disposed in a downstream exhaust passage formed by joining a pair of upstream exhaust passages connected to the cylinder groups. Exhaust gas that maintains the air-fuel ratio of the exhaust gas flowing out from one upstream exhaust passage into the downstream exhaust passage in a lean manner and flows out from the other upstream exhaust passage into the downstream exhaust passage in order to increase the temperature In the exhaust gas purification apparatus for an internal combustion engine, the catalyst is maintained at a rich air-fuel ratio, and at this time, the average air-fuel ratio of the entire exhaust gas flowing into the downstream exhaust passage is made the stoichiometric air-fuel ratio or slightly richer. A mixing means for mixing the exhaust gas from one upstream exhaust passage and the exhaust gas from the other upstream exhaust passage is disposed in the upstream downstream exhaust passage, and the downstream side between the mixing means and the catalyst Exhaust Was formed from a pair of branch passages extending in parallel with each other, an exhaust purifying apparatus for an internal combustion engine arranged an additional catalyst having respective oxidation ability of these branch passage. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the mixing means is formed of a swirling device that guides the exhaust gas so that the exhaust gas swirls around a substantially central axis of the downstream exhaust passage . In the internal combustion engine, combustion is continuously performed under a lean air-fuel ratio, and the catalyst stores NO _X in the exhaust gas flowing in when the air-fuel ratio of the exhaust gas flowing in is lean. , the air-fuel ratio of the inflowing exhaust gas formed from NO _X catalyst amount of the NO _X which the NO _X that accumulated to contain a reducing agent in the exhaust gas are stored by reduction decreases upon reduction The exhaust emission control device for an internal combustion engine according to claim 1. The exhaust purification device for an internal combustion engine according to claim 1, wherein the additional catalyst is formed of a three-way catalyst .

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