JP3578107B2 - Exhaust gas purification method - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification method.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in a diesel engine, a particulate filter is disposed in an engine exhaust passage to remove particulates contained in exhaust gas, and the particulate filter once collects particulates in the exhaust gas. The particulate filter collected above is ignited and burned to regenerate the particulate filter. However, the particulate matter collected on the particulate filter does not ignite and burn unless the temperature becomes higher than about 600 ° C., whereas the exhaust gas temperature of a diesel engine is usually much lower than 600 ° C. Therefore, it is difficult to ignite and burn the fine particles collected on the particulate filter by the heat of the exhaust gas. Therefore, a technique for igniting and burning the collected fine particles even at a relatively low temperature is known (for example, see Japanese Patent Publication No. 7-106290).
[0003]
On the other hand, when the exhaust gas is lean on the particulate filter, NO_xNO when the oxygen concentration decreases_xReleases NO_xNO with loading and releasing agent_xA technique for purifying both particles and fine particles is also known (JP-A-6-272541).
[0004]
[Problems to be solved by the invention]
This NO_xThe absorbing and releasing agent is poisoned by the sulfur component contained in the exhaust gas, and its NO_xAbsorption / release capacity and oxidation capacity may be reduced. Therefore NO_xNO of absorption / release agent_xNO to maintain high absorption and release capacity_xIt is necessary to remove the sulfur component poisoning the absorbent. NO_xSince the sulfur component poisoning the absorbing and releasing agent is desorbed when the temperature of the particulate filter becomes relatively high and the surrounding oxygen concentration decreases, NO_xIn order to remove the sulfur component from the absorbing / releasing agent, the temperature of the particulate filter may be increased and the concentration of the surrounding oxygen may be decreased. However, if the sulfur component is NO_xSince the temperature at which it can be desorbed from the absorbent is higher than the ignition temperature of the fine particles, NO_xWhen the temperature of the particulate filter is increased in order to remove the sulfur component from the absorbing / releasing agent, fine particles may be burned at once on the surface of the particulate filter. In this case, the temperature of the particulate filter rapidly rises, and the particulate filter is melted by the heat of combustion of the fine particles._xThe absorption / release agent may be thermally degraded.
[0005]
In view of such a problem, an object of the present invention is to solve the problem that the particulate filter is melted or burned out by the heat of combustion of the fine particles deposited on the surface of the particulate filter._xAn object of the present invention is to remove a sulfur component from a particulate filter while preventing a heat absorbing / releasing agent from being thermally deteriorated.
[0006]
[Means for Solving the Problems]
In the first invention, in order to achieve the above object, when excess oxygen is present around the particulate filter having an oxidizing function, NO_xNO that is taken in and held and held when the oxygen concentration in the surroundings decreases_xReleases NO_xCarrying an absorbing / releasing agent;_xWhen the amount of fine particles deposited on the particulate filter is smaller than a predetermined amount when the absorbing and releasing agent is poisoned by the sulfur component, the air-fuel ratio of the exhaust gas flowing into the particulate filter is set to the first degree. When the amount of fine particles deposited on the particulate filter is larger than the predetermined amount, the air-fuel ratio of the exhaust gas flowing into the particulate filter is set to a second degree larger than the first degree. Be rich. Here, from the relationship between the rich degree and the air-fuel ratio of the exhaust gas, the larger the rich degree, the smaller the air-fuel ratio of the exhaust gas.
[0007]
According to a second aspect of the present invention, in the first aspect, the poisoning recovery control is started with the air-fuel ratio of the exhaust gas flowing into the particulate filter as a predetermined air-fuel ratio, and based on the temperature change of the particulate filter during the poisoning recovery. The amount of fine particles deposited on the particulate filter is detected.
In a third aspect, in the first aspect, a noble metal catalyst is supported on the particulate filter.
[0008]
According to a fourth aspect, in the third aspect, the present invention provides the active oxygen releasing agent according to the third aspect, which takes in oxygen and retains oxygen when there is excess oxygen in the surroundings, and releases the retained oxygen in the form of active oxygen when the surrounding oxygen concentration decreases. Carry on the particulate filter, release the active oxygen from the active oxygen release agent when the fine particles adhere to the particulate filter, oxidize the fine particles attached to the particulate filter by the released active oxygen I have.
[0009]
In a fifth aspect based on the fourth aspect, the active oxygen releasing agent comprises an alkali metal, an alkaline earth metal, a rare earth, a transition metal, or a carbon group element.
In a sixth aspect based on the fifth aspect, the alkali metal and the alkaline earth metal are made of a metal having a higher ionization tendency than calcium.
According to a seventh aspect, in the fourth aspect, the air-fuel ratio of a part or the whole of the exhaust gas is temporarily made rich to oxidize the fine particles adhering to the particulate filter.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described with reference to the illustrated embodiments. FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. The present invention can also be applied to a spark ignition type internal combustion engine.
Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, 9 Denotes an exhaust valve, and 10 denotes an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13. A mass flow meter 13a for detecting the mass flow rate of the air to be taken in is attached to the intake pipe 13b on the upstream side of the compressor 15. A throttle valve 17 driven by a step motor 16 is arranged in the intake duct 13, and a cooling device 18 for cooling intake air flowing through the intake duct 13 is arranged around the intake duct 13. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 18 and the intake air is cooled by the engine cooling water. On the other hand, the exhaust port 10 is connected to an exhaust turbine 21 of the exhaust turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the exhaust turbine 21 is connected to a casing 23 having a built-in particulate filter 22 through an exhaust pipe 20a. Be linked.
[0011]
The exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter, EGR) passage 24, and an electrically controlled EGR control valve 25 is disposed in the EGR passage 24. A cooling device 26 for cooling the EGR gas flowing in the EGR passage 24 is disposed around the EGR passage 24. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 26, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 27, via a fuel supply pipe 6a. Fuel is supplied into the common rail 27 from a fuel pump 28 of an electrically controlled variable discharge amount, and the fuel supplied into the common rail 27 is supplied to the fuel injection valve 6 through each fuel supply pipe 6a. A fuel pressure sensor 29 for detecting the fuel pressure in the common rail 27 is attached to the common rail 27, and the fuel pump 28 is controlled so that the fuel pressure in the common rail 27 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 29. Is controlled.
[0012]
The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31 such as a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35 and an output port 36. Is provided. The output signal of the fuel pressure sensor 29 is input to the input port 35 via the corresponding AD converter 37. A temperature sensor 39 for detecting the temperature of the particulate filter 22 is attached to the particulate filter 22, and an output signal of the temperature sensor 39 is input to an input port 35 via a corresponding AD converter 37. The output signal of the mass flow meter 13a is input to the input port 35 via the corresponding AD converter 37. A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. . Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, by 30 °. On the other hand, the output port 36 is connected to the fuel injection valve 6, the throttle valve driving step motor 16, the EGR control valve 25, and the fuel pump 28 via the corresponding drive circuit 38.
[0013]
FIG. 2 shows the structure of the particulate filter 22. 2A is a front view of the particulate filter 22, and FIG. 2B is a side cross-sectional view of the particulate filter 22. As shown in FIGS. 2A and 2B, the particulate filter 22 has a honeycomb structure and includes a plurality of exhaust passages 50 and 51 extending in parallel with each other. These exhaust passages are constituted by an exhaust gas inflow passage 50 whose downstream end is closed by a plug 52 and an exhaust gas outflow passage 51 whose upstream end is closed by a plug 53.
[0014]
In FIG. 2A, a hatched portion indicates a plug 53. Therefore, the exhaust gas inflow passages 50 and the exhaust gas outflow passages 51 are alternately arranged via the thin partition walls 54. In other words, the exhaust gas inflow passages 50 and the exhaust gas outflow passages 51 are each surrounded by four exhaust gas outflow passages 51, and each exhaust gas outflow passage 51 is surrounded by the four exhaust gas inflow passages 50. It is arranged so that.
[0015]
The particulate filter 22 is formed of, for example, a porous material such as cordierite. Therefore, the exhaust gas that has flowed into the exhaust gas inflow passage 50 is in the surrounding partition wall 54 as indicated by an arrow in FIG. And flows out into the adjacent exhaust gas outflow passage 51.
In the embodiment of the present invention, the entire peripheral wall surface of each exhaust gas inflow passage 50 and each exhaust gas outflow passage 51, that is, on both side surfaces of each partition wall 54, the outer end surface of the plug 53 and the inner end surfaces of the plugs 52, 53 A support layer made of, for example, alumina is formed over the support.On the support, a noble metal catalyst and oxygen are taken in when excess oxygen is present in the surroundings to retain oxygen, and when the surrounding oxygen concentration decreases, the retained oxygen is removed. An active oxygen releasing agent that releases in the form of active oxygen is carried.
[0016]
In the embodiment of the present invention, platinum Pt is used as a noble metal catalyst, and alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, rubidium Rb, barium Ba, calcium Ca, strontium Sr are used as active oxygen releasing agents. At least one selected from the group consisting of alkaline earth metals such as, lanthanum La, yttrium Y, rare earths such as cerium Ce, transition metals such as iron Fe, and carbon group elements such as tin Sn is used.
[0017]
As the active oxygen releasing agent, it is preferable to use an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, that is, potassium K, lithium Li, cesium Cs, rubidium Rb, barium Ba, and strontium Sr.
Next, the action of removing particulates in exhaust gas by the particulate filter 22 will be described by taking as an example a case where platinum Pt and potassium K are carried on a carrier, but other noble metals, alkali metals, alkaline earth metals, rare earths, transition metals Even when a carbon group element is used, a similar fine particle removing action is performed.
[0018]
In a compression ignition type internal combustion engine as shown in FIG. 1, combustion takes place under excess air, and thus the exhaust gas contains a large amount of excess air. That is, if the ratio of air and fuel supplied into the intake passage and the combustion chamber 5 is referred to as the air-fuel ratio of the exhaust gas, the air-fuel ratio of the exhaust gas in the compression ignition type internal combustion engine as shown in FIG. I have. Since NO is generated in the combustion chamber 5, NO is contained in the exhaust gas. The fuel also contains sulfur S, which reacts with oxygen in the combustion chamber 5 to₂  It becomes. Therefore, SO2 is contained in the exhaust gas.₂  It is included. Thus, excess oxygen, NO and SO₂  Will flow into the exhaust gas inflow passage 50 of the particulate filter 22.
[0019]
FIGS. 3A and 3B schematically show enlarged views of the surface of the carrier layer formed on the inner peripheral surface of the exhaust gas inflow passage 50. 3A and 3B, reference numeral 60 denotes platinum Pt particles, and reference numeral 61 denotes an active oxygen releasing agent containing potassium K.
As described above, since a large amount of excess oxygen is contained in the exhaust gas, when the exhaust gas flows into the exhaust gas inflow passage 50 of the particulate filter 22, as shown in FIG.₂  Is O₂ ⁻Or O^2-On the surface of platinum Pt. On the other hand, NO in the exhaust gas becomes O 2 on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂  (2NO + O₂  → 2NO₂  ). NO generated next₂  Is absorbed in the active oxygen releasing agent 61 while being oxidized on the platinum Pt, and combined with potassium K to form nitrate ions NO as shown in FIG.₃ ⁻Is diffused into the active oxygen releasing agent 61 in the form of potassium nitrate KNO₃  Generate
[0020]
On the other hand, as described above, SO2 is contained in the exhaust gas.₂  Is also included in this SO₂  Is also absorbed into the active oxygen releasing agent 61 by the same mechanism as NO. That is, as described above, oxygen O₂  Is O₂ ⁻Or O^2-Is attached to the surface of platinum Pt in the form of₂  Is O on the surface of platinum Pt₂ ⁻Or O^2-Reacts with SO₃  It becomes. Then the generated SO₃  Is absorbed in the active oxygen releasing agent 61 while being further oxidized on platinum Pt, and combined with potassium K to form sulfate ions SO.₄ ^2-In the active oxygen releasing agent 61 in the form of potassium sulfate K₂SO₄  Generate Thus, potassium nitrate KNO is contained in the active oxygen releasing agent 61.₃  And potassium sulfate K₂SO₄Is generated.
[0021]
On the other hand, fine particles mainly composed of carbon C are generated in the combustion chamber 5, and therefore, these fine particles are contained in the exhaust gas. These fine particles contained in the exhaust gas are generated when the exhaust gas flows in the exhaust gas inflow passage 50 of the particulate filter 22 or when the exhaust gas flows from the exhaust gas inflow passage 50 to the exhaust gas outflow passage 51 in FIG. As shown by 62 in B), it contacts and adheres to the surface of the carrier layer, for example, the surface of the active oxygen releasing agent 61.
[0022]
When the fine particles 62 adhere to the surface of the active oxygen releasing agent 61 as described above, the oxygen concentration at the contact surface between the fine particles 62 and the active oxygen releasing agent 61 decreases. When the oxygen concentration decreases, a concentration difference occurs between the active oxygen releasing agent 61 having a high oxygen concentration and the oxygen inside the active oxygen releasing agent 61. Try to move. As a result, potassium nitrate KNO formed in the active oxygen releasing agent 61₃  Is decomposed into potassium K, oxygen O and NO, and oxygen O is directed to the contact surface between the fine particles 62 and the active oxygen releasing agent 61, while NO is released from the active oxygen releasing agent 61 to the outside. The NO released to the outside is oxidized on platinum Pt on the downstream side, and is again absorbed in the active oxygen releasing agent 61.
[0023]
At this time, potassium sulfate K formed in the active oxygen releasing agent 61 is used.₂SO₄  Also potassium K, oxygen O and SO₂  Oxygen O is directed to the contact surface between the fine particles 62 and the active oxygen releasing agent 61, while₂  Is released from the active oxygen releasing agent 61 to the outside. SO released outside₂  Is oxidized on platinum Pt on the downstream side, and is again absorbed in the active oxygen releasing agent 61. However, potassium sulfate K₂SO₄  Is potassium sulfate K₂SO₄  Is potassium nitrate KNO₃  It is harder to release active oxygen than it is.
The active oxygen releasing agent 61 is NO as described above._xTo nitrate ion NO₃ ⁻When it is absorbed in the form of, it generates and releases active oxygen in the process of reacting with oxygen. Similarly, the active oxygen releasing agent 61 is made of SO 2 as described above.₂  The sulfate ion SO₄ ^2-When it is absorbed in the form of, it generates and releases active oxygen in the process of reacting with oxygen.
[0024]
By the way, oxygen O toward the contact surface between the fine particles 62 and the active oxygen releasing agent 61 is potassium nitrate KNO₃  And potassium sulfate K₂SO₄  Is oxygen decomposed from such a compound. Oxygen O decomposed from the compound has high energy and extremely high activity. Therefore, the oxygen that goes to the contact surface between the fine particles 62 and the active oxygen releasing agent 61 is active oxygen O. Similarly, NO in the active oxygen releasing agent 61_xReaction process with oxygen and SO₂  Oxygen generated in the reaction process between oxygen and oxygen is also active oxygen. When the active oxygen O comes into contact with the fine particles 62, the fine particles 62 are oxidized within a short time (several seconds to several tens of minutes) without emitting a bright flame, and the fine particles 62 are completely disappeared. Therefore, the fine particles 62 hardly accumulate on the particulate filter 22. That is, the active oxygen releasing agent 61 is an oxidizing substance for oxidizing the fine particles.
[0025]
When the particulates accumulated in a layer on the particulate filter 22 are burned as in the related art, the particulate filter 22 glows red and burns with a flame. Such combustion with a flame cannot be sustained unless it is at a high temperature, so that the temperature of the particulate filter 22 must be maintained at a high temperature in order to sustain the combustion with such a flame.
[0026]
On the other hand, in the present invention, the fine particles 62 are oxidized without emitting a bright flame as described above, and at this time, the surface of the particulate filter 22 does not glow. In other words, in other words, in the present invention, the fine particles 62 are oxidized and removed at a considerably lower temperature than in the prior art. Therefore, the action of removing fine particles 62 that do not emit a luminous flame by oxidation according to the present invention is completely different from the action of removing fine particles by conventional combustion accompanied by a flame.
[0027]
Incidentally, the platinum Pt and the active oxygen releasing agent 61 are activated as the temperature of the particulate filter 22 increases, so that the amount of oxidizable and removable fine particles that can be oxidized and removed on the particulate filter 22 without emitting a luminous flame per unit time is limited to the particle size. The temperature increases as the temperature of the curated filter 22 increases.
[0028]
The solid line in FIG. 5 indicates the amount G of oxidizable and removable fine particles that can be oxidized and removed without emitting a bright flame per unit time. In FIG. 5, the horizontal axis represents the temperature TF of the particulate filter 22. When the amount of fine particles discharged from the combustion chamber 5 per unit time is referred to as a discharged fine particle amount M, when the discharged fine particle amount M is smaller than the oxidizable and removable fine particles G, that is, when the discharged fine particle amount M is in the region I in FIG. When all of the fine particles discharged from the filter contact the particulate filter 22, they are oxidized and removed on the particulate filter 22 in a short time (several seconds to several tens of minutes) without emitting a bright flame.
[0029]
On the other hand, when the amount M of discharged fine particles is larger than the amount G of fine particles that can be removed by oxidation, that is, in the region II of FIG. 5, the amount of active oxygen is insufficient to oxidize all the fine particles. FIGS. 4A to 4C show how the fine particles are oxidized in such a case.
That is, when the amount of active oxygen is insufficient to oxidize all the fine particles, only a part of the fine particles 62 is oxidized when the fine particles 62 adhere to the active oxygen releasing agent 61 as shown in FIG. The fine particles that have not been sufficiently oxidized remain on the carrier layer. Next, when the state of the shortage of the active oxygen amount continues, the fine particles which were not oxidized one after another remain on the carrier layer, and as a result, as shown in FIG. It becomes covered with the residual fine particle portion 63.
[0030]
When the surface of the carrier layer is covered with the residual fine particle portion 63, NO, SO₂  4 and the active oxygen releasing agent 61 no longer releases the active oxygen, so that the residual fine particle portion 63 remains without being oxidized. Thus, as shown in FIG. Another fine particle 64 is deposited one after another on 63. That is, the fine particles are deposited in a layered manner. When the fine particles are deposited in a stacked manner in this manner, the fine particles 64 are no longer oxidized by the active oxygen O, and therefore, further fine particles are deposited on the fine particles 64 one after another. That is, when the state in which the amount M of discharged fine particles is larger than the amount G of fine particles that can be removed by oxidation continues, the fine particles are deposited on the particulate filter 22 in a layered manner. Unless the temperature of 22 is increased, the deposited fine particles cannot be ignited and burned.
[0031]
As described above, in the region I of FIG. 5, the fine particles are oxidized within a short time without emitting a bright flame on the particulate filter 22, and in the region II of FIG. I do. Therefore, in order to prevent the fine particles from depositing on the particulate filter 22 in a stacked state, it is necessary to always keep the discharged fine particle amount M smaller than the oxidizable and removable fine particle amount G.
[0032]
As can be seen from FIG. 5, the particulate filter 22 used in the embodiment of the present invention can oxidize the fine particles even when the temperature TF of the particulate filter 22 is considerably low, and therefore the compression filter shown in FIG. In the ignition type internal combustion engine, it is possible to maintain the amount M of discharged fine particles and the temperature TF of the particulate filter 22 so that the amount M of discharged fine particles is always smaller than the amount G of fine particles that can be removed by oxidation. Therefore, in the first embodiment of the present invention, the amount M of discharged particulates and the temperature TF of the particulate filter 22 are maintained such that the amount M of discharged particulates is always smaller than the amount G of particulates that can be removed by oxidation.
[0033]
If the amount M of discharged fine particles is always smaller than the amount G of fine particles that can be removed by oxidation, no fine particles are deposited on the particulate filter 22, and thus the back pressure hardly increases. Therefore, the engine output hardly decreases.
On the other hand, as described above, once the fine particles are deposited on the particulate filter 22 in a layered manner, even if the amount M of discharged fine particles becomes smaller than the amount G of fine particles that can be removed by oxidation, it is difficult to oxidize the fine particles with active oxygen O. Have difficulty. However, when the unoxidized fine particle portion starts to remain, that is, when the amount of exhaust fine particles M becomes smaller than the amount G of oxidizable and removable fine particles when the fine particles are deposited only below a certain limit, the residual fine particle portion becomes active oxygen O. It is oxidized and removed without emitting bright flame. Therefore, in the second embodiment, even if the amount M of discharged fine particles is normally smaller than the amount G of fine particles removable by oxidation, and the amount M of discharged fine particles is temporarily larger than the amount G of fine particles removable by oxidation, FIG. As shown in (1), the amount of fine particles of a certain amount or less that can be oxidized and removed when the amount of discharged fine particles M becomes smaller than the amount of fine particles G that can be removed by oxidation so that the surface of the carrier layer is not covered with the residual fine particle portion 63. The amount M of discharged particulates and the temperature TF of the particulate filter 22 are maintained so that only the particulate filter 22 is laminated.
[0034]
In particular, immediately after the engine is started, the temperature TF of the particulate filter 22 is low. Therefore, at this time, the amount M of discharged particulates is larger than the amount G of particulates that can be removed by oxidation. Therefore, it is considered that the second embodiment is more suitable for actual driving.
On the other hand, even if the amount M of discharged particulates and the temperature TF of the particulate filter 22 are controlled so that the first embodiment or the second embodiment can be executed, the particulates accumulate on the particulate filter 22 in a stacked manner. May be. In such a case, the particulates deposited on the particulate filter 22 can be oxidized without emitting a bright flame by temporarily making the air-fuel ratio of a part or the whole of the exhaust gas rich.
[0035]
That is, if the state in which the air-fuel ratio of the exhaust gas is lean continues for a certain period of time, a large amount of oxygen will adhere to platinum Pt, and the catalytic action of platinum Pt will decrease. However, when the air-fuel ratio of the exhaust gas is made rich to lower the oxygen concentration in the exhaust gas, oxygen is removed from the platinum Pt, and thus the catalytic action of the platinum Pt is restored. Thus, when the air-fuel ratio of the exhaust gas is made rich, the active oxygen O is easily released from the active oxygen releasing agent 61 to the outside at a stretch. Thus, the deposited fine particles are transformed into a state that is easily oxidized by the active oxygen O released at a stretch, and the fine particles are burnt and removed by the active oxygen without emitting a bright flame. Thus, when the air-fuel ratio of the exhaust gas is made rich, the amount G of the oxidizable and removable fine particles increases as a whole. In this case, the air-fuel ratio of the exhaust gas may be made rich when the particulates are deposited on the particulate filter 22 in a stacked manner, or the exhaust gas may be periodically exhausted regardless of whether the particulates are deposited in the stacked manner. The air-fuel ratio of the gas may be made rich.
[0036]
As a method for enriching the air-fuel ratio of the exhaust gas, for example, when the engine load is relatively low, the throttle valve 17 is opened so that the EGR rate (EGR gas amount / (intake air amount + EGR gas amount)) becomes 65% or more. The degree and the opening of the EGR control valve 25 are controlled, and at this time, a method of controlling the injection amount so that the average air-fuel ratio in the combustion chamber 5 becomes rich can be used.
[0037]
FIG. 6 shows an example of the operation control routine of the internal combustion engine described above.
Referring to FIG. 6, first, in step 100, it is determined whether or not the average air-fuel ratio in the combustion chamber 5 should be made rich. When it is not necessary to make the average air-fuel ratio in the combustion chamber 5 rich, the opening of the throttle valve 17 is controlled in step 101 so that the amount M of discharged particulates becomes smaller than the amount G of particulates that can be removed by oxidation. The opening of the control valve 25 is controlled, and in step 103, the fuel injection amount is controlled.
[0038]
On the other hand, when it is determined in step 100 that the average air-fuel ratio in the combustion chamber 5 should be made rich, the opening of the throttle valve 17 is controlled in step 104 so that the EGR rate becomes 65% or more. In step, the opening degree of the EGR control valve 25 is controlled, and the fuel injection amount is controlled in step 106 so that the average air-fuel ratio in the combustion chamber 5 becomes rich.
[0039]
Incidentally, the fuel and the lubricating oil contain calcium Ca, and therefore, the calcium Ca is contained in the exhaust gas. This calcium Ca is SO₃  In the presence of calcium sulfate CaSO₄  Generate This calcium sulfate CaSO₄  Is a solid and does not thermally decompose at high temperatures. Therefore, calcium sulfate CaSO₄  Is produced, this calcium sulfate CaSO₄  As a result, the pores of the particulate filter 22 are closed, and as a result, it becomes difficult for the exhaust gas to flow through the particulate filter 22.
[0040]
In this case, when an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, such as potassium K, is used as the active oxygen releasing agent 61, SO diffuses into the active oxygen releasing agent 61.₃  Combines with potassium K to form potassium sulfate K₂SO₄  And calcium Ca is SO₃  The exhaust gas flows through the partition wall 54 of the particulate filter 22 into the exhaust gas outlet passage 51 without being combined with the exhaust gas. Therefore, the pores of the particulate filter 22 are not clogged. Therefore, as described above, as the active oxygen releasing agent 61, an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, that is, potassium K, lithium Li, cesium Cs, rubidium Rb, barium Ba, and strontium Sr may be used. It will be preferable.
[0041]
The present invention can also be applied to a case where only a noble metal such as platinum Pt is carried on a carrier layer formed on both side surfaces of the particulate filter 22. However, in this case, the solid line indicating the amount of fine particles G that can be removed by oxidation moves slightly to the right as compared with the solid line shown in FIG. In this case, NO held on the surface of platinum Pt₂  Or SO₃  Releases active oxygen.
[0042]
NO as an active oxygen releasing agent₂  Or SO₃  Is adsorbed and held, and these adsorbed NO₂  Or SO₃  It is also possible to use a catalyst capable of releasing active oxygen from the catalyst.
By the way, as described above, the sulfur component S in the exhaust gas is SO₂  Included in the form. This sulfur component S is adsorbed on the platinum Pt of the particulate filter 22 or is absorbed by the active oxygen releasing agent 61. Such sulfur component S is desorbed from platinum Pt and the active oxygen releasing agent 61 if the temperature of the particulate filter 22 is high to some extent. However, when the temperature of the particulate filter 22 is relatively low, the sulfur component S remains adsorbed on the platinum Pt and remains absorbed by the active oxygen releasing agent 61. In this case, the oxidizing ability of the platinum Pt is reduced, and the active oxygen releasing agent 61 can absorb NO._xAmount (NO_xAbsorption capacity) is also reduced. Therefore, the oxidizing ability of platinum Pt is maintained at a high level and the active oxygen releasing agent 61 can absorb NO._xIn order to maintain a large amount of sulfur, it is necessary to desorb the sulfur component S adsorbed on the platinum Pt or absorbed by the active oxygen releasing agent 61. For this purpose, the temperature of the particulate filter 22 may be raised to some extent and the oxygen concentration around the platinum Pt and the active oxygen releasing agent 61 may be reduced. However, the temperature at which the sulfur component S can be desorbed (hereinafter, the desorption temperature) is higher than the temperature at which the fine particles start igniting and burning (hereinafter, the fine particle ignition temperature), so that the particulate filter 22 is required to desorb the sulfur component. When the temperature is raised to the desorption temperature, if particulates are deposited on the particulate filter 22, the particulates burn at a stretch, the temperature of the particulate filter 22 rises rapidly, and the particulate filter 22 is melted, NO even if it does not lead to melting_xThe absorption / release agent is thermally degraded (hereinafter, referred to as thermal degradation including erosion).
[0043]
Therefore, in the present invention, when the sulfur component is to be desorbed, it is determined whether or not a large amount of fine particles is deposited so as to cause thermal deterioration of the particulate filter 22. When the particulate filter 22 is not deposited, the temperature of the particulate filter 22 is raised to the desorption temperature without performing a process for preventing the particulate filter 22 from being thermally degraded, while fine particles cause the particulate filter 22 to be thermally degraded. When a large amount of gas is accumulated, exhaust gas having an air-fuel ratio with a rich degree larger than the minimum degree required to raise the temperature of the particulate filter 22 to the desorption temperature is caused to flow into the particulate filter 22. To
[0044]
When exhaust gas having an air-fuel ratio having a large rich degree flows into the particulate filter 22, a large amount of HC is supplied to the particulate filter, and the HC reacts with oxygen in the exhaust gas in the particulate filter 22. Therefore, the oxygen concentration in the particulate filter becomes low. Therefore, the amount of oxygen for burning the deposited fine particles is reduced, and it is possible to prevent the deposited fine particles from burning and burning at once. Further, since the HC reacts with oxygen in the particulate filter 22, the temperature of the particulate filter 22 is increased, and the concentration difference between the oxygen concentration in the active oxygen releasing agent 61 and the surface of platinum Pt and the oxygen concentration in the surroundings. Therefore, the sulfur component S can be easily desorbed. Thus, according to the present invention, it is possible to recover the poisoning of the particulate filter due to the sulfur component while suppressing the thermal deterioration of the particulate filter.
[0045]
Next, a specific process of recovering the particulate filter poisoning by the sulfur component S of the present invention will be described with reference to a flowchart of FIG. First, in step 200, it is determined whether or not the accumulated sulfur component amount As is larger than a predetermined amount AsTH (As> AsTH). Here, as shown in FIG. 8, the accumulated sulfur component amount As is a function of the engine speed Ne and the engine required load L, and the sulfur component amount Asmm estimated to be deposited on the particulate filter 22 per unit time is determined by an experiment or the like. It is obtained in advance in the form of a map, and is calculated by integrating the sulfur component amount Asmm. Further, the predetermined amount AsTH decreases the oxidizing ability of platinum Pt to an unacceptable limit or the NO of active oxygen releasing agent 61_xThe amount of the sulfur component is set so as to lower the absorption capacity beyond an allowable limit. When it is determined in step 200 that As> AsTH, it is determined that the poisoning by the sulfur component should be recovered, and the process proceeds to step 201. On the other hand, when it is determined in step 200 that As ≦ AsTH, it is determined that there is no need to recover the poisoning due to the sulfur component, and the routine ends.
In step 201, it is determined whether or not the amount Apm of accumulated particulates is greater than a predetermined amount ApmTH (Apm> ApmTH). Here, as shown in FIG. 9, the accumulated particle amount Apm is a function of the engine rotation speed Ne and the required engine load L, and the particle amount Apmm estimated to be accumulated on the particulate filter 22 per unit time is obtained by experiments or the like. In advance, and is calculated by integrating the particle amount Apmm. Also, the predetermined amount ApmTH causes the particulate filter 22 to thermally degrade when the temperature of the particulate filter 22 is increased in order to desorb the sulfur component S without executing the thermal degradation prevention processing of the particulate filter 22. It is set to the amount of fine particles that would cause When it is determined in step 201 that Apm ≦ ApmTH, it is determined that the particulate filter 22 does not thermally degrade even if the process for recovering the poisoning due to the sulfur component is performed without performing the thermal degradation prevention process, Proceeding to step 203, a rich process I for making the exhaust gas air-fuel ratio richer than the stoichiometric air-fuel ratio is executed, and then proceeding to step 204. Here, the degree of richness of the air-fuel ratio of the exhaust gas is such a degree that the exhaust gas contains substantially no excessive or insufficient amount of HC to raise the temperature of the particulate filter 22 to the sulfur component desorption temperature. Therefore, the temperature of the particulate filter 22 is raised to the sulfur component desorption temperature by the rich process I in step 203, and thus the sulfur component is removed from the particulate filter 22. On the other hand, when it is determined that Apm> ApmTH, if the process for recovering the poisoning due to the sulfur component is performed without performing the thermal deterioration prevention process, it is determined that the particulate filter 22 is thermally deteriorated, and the process proceeds to step 202. Then, a rich process II for making the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio is executed, and the routine proceeds to step 204. Here, the rich degree of the exhaust air-fuel ratio is set to be larger than the rich degree in the rich processing I. According to this, as described above, the sulfur component is removed from the particulate filter while suppressing burning of the deposited fine particles at once.
[0046]
In step 204, the accumulated sulfur component amount As is reset to zero, but a process of subtracting the sulfur component amount estimated to be removed for each rich process from the accumulated sulfur component amount As may be executed.
In order to make the air-fuel ratio of the exhaust gas richer than the stoichiometric air-fuel ratio, a small amount of fuel is injected preliminary just before fuel injection for engine driving, or a small amount is injected during the intake stroke before fuel injection for engine driving. Or HC may be directly injected into the engine exhaust passage upstream of the particulate filter 22. Of course, even if these processes are combined, the air-fuel ratio of the exhaust gas can be made richer than the stoichiometric air-fuel ratio.
[0047]
Further, instead of executing the rich process I in step 203, the internal combustion engine is caused to execute low-temperature combustion, which will be described later, to increase the temperature of the exhaust gas, and thus increase the temperature of the particulate filter to the sulfur desorption temperature. You may.
Next, low-temperature combustion will be described. It is known that when the EGR rate is increased, the amount of generated fine particles gradually increases and reaches a peak, and when the EGR rate is further increased, the generated amount of fine particles rapidly decreases. This will be described with reference to FIG. 10 showing the relationship between the EGR rate and smoke when the degree of cooling of the EGR gas is changed. In FIG. 10, a curve A shows a case where the EGR gas is cooled strongly and the EGR gas temperature is maintained at about 90 ° C., a curve B shows a case where the EGR gas is cooled by a small cooling device, and a curve C shows a case where the EGR gas is cooled. Shows a case where cooling is not forcibly performed.
[0048]
As shown by the curve A in FIG. 10, when the EGR gas is cooled strongly, the generation amount of fine particles reaches a peak when the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. In this case, almost no fine particles are generated. On the other hand, as shown by the curve B in FIG. 10, when the EGR gas is slightly cooled, the generation amount of the fine particles reaches a peak when the EGR rate is slightly higher than 50%, and in this case, the EGR rate is increased to about 65% or more. In this case, almost no fine particles are generated. As shown by the curve C in FIG. 10, when the EGR gas is not forcibly cooled, the generation amount of fine particles reaches a peak when the EGR rate is around 55%. In this case, the EGR rate is reduced to about 70%. By doing so, almost no fine particles are generated. As described above, when the EGR rate is set to 55% or more, the generation of the fine particles is stopped because the temperature of the fuel and the surrounding gas during the fuel combustion does not become so high due to the endothermic effect of the EGR gas, and the low temperature combustion is performed. This is because hydrocarbons do not grow to soot.
[0049]
This low-temperature combustion suppresses the generation of fine particles regardless of the air-fuel ratio while reducing NO._xHas the characteristic that the amount of generation of can be reduced. That is, when the air-fuel ratio is made richer than the stoichiometric air-fuel ratio, the fuel becomes excessive, but the combustion temperature is suppressed to a low temperature, so that the excess fuel does not grow to soot, thus generating fine particles. There is no. Also at this time NO_xOnly a very small amount is generated. On the other hand, when the average air-fuel ratio is leaner than the stoichiometric air-fuel ratio, or when the air-fuel ratio is set to the stoichiometric air-fuel ratio, a small amount of soot is generated if the combustion temperature increases, but the combustion temperature decreases to a lower temperature under low-temperature combustion. No fine particles are generated at all, and NO_xOnly a very small amount is generated.
[0050]
On the other hand, when this low-temperature combustion is performed, the temperature of the fuel and the surrounding gas is lowered, but the temperature of the exhaust gas is raised. This will be described with reference to FIG. The solid line in FIG. 11A shows the relationship between the average gas temperature Tg in the combustion chamber 5 and the crank angle when low-temperature combustion is performed, and the broken line in FIG. 5 shows the relationship between the average gas temperature Tg in the combustion chamber 5 and the crank angle. The solid line in FIG. 11B shows the relationship between the fuel and ambient gas temperature Tf and the crank angle when low-temperature combustion is performed, and the broken line in FIG. 11B shows the normal combustion when normal combustion is performed. The relationship between the temperature of the fuel and the surrounding gas temperature Tf and the crank angle is shown.
[0051]
When the low-temperature combustion is performed, the amount of the EGR gas is larger than when the normal combustion is performed. Therefore, as shown in FIG. 11A, before the compression top dead center, that is, during the compression stroke, a solid line is used. The average gas temperature Tg at the time of the low-temperature combustion shown is higher than the average gas temperature Tg at the time of the normal combustion shown by the broken line. At this time, as shown in FIG. 11B, the temperature of the fuel and its surrounding gas Tf is substantially equal to the average gas temperature Tg.
[0052]
Next, combustion starts near the compression top dead center. In this case, when low-temperature combustion is being performed, the temperature of the fuel and its surrounding gas Tf does not increase so much as indicated by the solid line in FIG. On the other hand, when normal combustion is performed, a large amount of oxygen exists around the fuel, so that the temperature of the fuel and its surrounding gas Tf becomes extremely high as shown by the broken line in FIG. 11B. . Thus, when normal combustion is performed, the temperature of the fuel and its surrounding gas Tf becomes considerably higher than in the case where low-temperature combustion is performed, but the temperature of most of the other gases is low-temperature combustion. 11A is lower than in the case where normal combustion is performed, and therefore, as shown in FIG. 11A, the average gas temperature in the combustion chamber 5 near the compression top dead center is shown in FIG. Tg is higher when low-temperature combustion is performed than when normal combustion is performed. As a result, as shown in FIG. 11 (A), the average gas temperature in the combustion chamber after the completion of the combustion is higher when the low-temperature combustion is performed than when the normal combustion is performed. Thus, when low-temperature combustion is performed, the temperature of the exhaust gas increases.
[0053]
Next, another sulfur component poisoning recovery control of the present invention will be described. In this embodiment, when the sulfur component poisoning is to be recovered, the air-fuel ratio of the exhaust gas is once set to a predetermined air-fuel ratio (for example, the rich processing I of the above-described embodiment is executed), and the temperature rise of the particulate filter 22 at this time is performed. The rate is monitored (accordingly, the amount of fine particles deposited on the particulate filter can be substantially estimated). When the temperature rise rate is relatively large, the particulate filter 22 may be thermally degraded. Then, the rich process I is switched to the rich process II of the above-described embodiment. According to this, the thermal deterioration of the particulate filter can be prevented.
[0054]
Next, the sulfur component poisoning recovery control will be described in detail with reference to the flowchart of FIG. First, in step 300, it is determined whether or not the accumulated sulfur component amount As is larger than a predetermined amount AsTH (As> AsTH), similarly to step 200 in FIG. When it is determined in step 300 that As> AsTH, it is determined that sulfur component poisoning should be recovered, and the rich process I is executed in step 301. The rich processing I here is the same as the rich processing I of the embodiment shown in FIG. Next, at step 302, it is determined whether or not the temperature rise rate Rtf of the particulate filter is larger than a predetermined rise rate RtfTH (Rtf> RtfTH). Here, the predetermined increase rate RtfTH is set to an increase rate that may cause the deposited fine particles to emit flame and burn at a stretch. If the deposited particulates start burning when the rich processing I is executed in step 301, the temperature of the particulate filter 22 starts to rise rapidly. Therefore, in this case, it is determined in step 302 that Rtf> RtfTH. At this time, the process proceeds to step 303, where the rich process II is executed, and the process proceeds to step 304. Here, the rich processing II is the same as the rich processing II of the embodiment shown in FIG. On the other hand, if the accumulated particulates do not start burning when the rich processing I is performed in step 301, the temperature of the particulate filter 22 does not rise so rapidly. Therefore, in this case, it is determined in step 302 that Rtf ≦ RtfTH. At this time, rich processing I is executed in step 305, and the process proceeds to step 304.
[0055]
In step 304, the accumulated sulfur component amount As is set to zero, and the routine ends. When it is determined in step 300 that As ≦ AsTH, the routine ends without executing the sulfur component poisoning recovery process.
In step 302, the condition for executing the rich process II is that the rate of temperature rise of the particulate filter 22 is greater than a predetermined rate of rise. In addition to this, the temperature of the particulate filter 22 is set to a predetermined temperature ( For example, the temperature may be higher than a temperature at which fine particles can be oxidized or a fine particle ignition temperature. In step 301, instead of executing the rich process I, the temperature of the particulate filter is increased while keeping the air-fuel ratio of the exhaust gas flowing into the particulate filter 22 leaner than the stoichiometric air-fuel ratio. The amount of particulates deposited on the particulate filter 22 is estimated from the temperature change of the particulate filter 22. When the amount of deposited particulates is large to a certain extent, the air-fuel ratio of the exhaust gas is maintained leaner than the stoichiometric air-fuel ratio, and May be oxidized and removed, and then the rich processing I may be executed. Alternatively, instead of executing the rich processing I in step 301, the temperature of the particulate filter 22 is increased while keeping the air-fuel ratio of the exhaust gas flowing into the particulate filter 22 leaner than the stoichiometric air-fuel ratio. The amount of fine particles deposited on the particulate filter may be estimated from the temperature change of the filter 22, and the rich processing II may be executed when the amount of the deposited fine particles is large to some extent. Of course, in step 301, the air-fuel ratio of the exhaust gas may be set to a rich degree smaller than the rich degree in the rich processing I.
[0056]
【The invention's effect】
According to the present invention, NO due to sulfur component_xWhen a relatively large amount of fine particles is deposited on the particulate filter when the poisoning of the absorbent is recovered, the air-fuel ratio of the exhaust gas flowing into the particulate filter is made relatively large and rich. If the exhaust gas having the air-fuel ratio having a relatively large rich degree flows into the particulate filter as described above, the hydrocarbons in the exhaust gas react with oxygen in the particulate filter, so that the particulates deposited on the particulate filter burn. You can't do that. For this reason, a rapid rise in the temperature of the particulate filter due to the heat of combustion of the fine particles is suppressed, and thus the thermal deterioration of the particulate filter can be suppressed. Also, the temperature of the particulate filter is raised by the reaction between hydrocarbon and oxygen, and NO_xSince the oxygen concentration around the absorbing and releasing agent decreases, NO_xThe poisoning of the absorption / release agent can be restored.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
FIG. 2 is a diagram showing a particulate filter.
FIG. 3 is a diagram for explaining the oxidizing action of fine particles.
FIG. 4 is a diagram for explaining a deposition action of fine particles.
FIG. 5 is a diagram showing the relationship between the amount of fine particles that can be removed by oxidation and the temperature of a particulate filter.
FIG. 6 is a flowchart for controlling operation of the engine.
FIG. 7 is a flowchart of sulfur component poisoning recovery control.
FIG. 8 is a map showing the amount of deposited sulfur components per unit time.
FIG. 9 is a map showing the amount of deposited fine particles per unit time.
FIG. 10 is a diagram showing the relationship between the EGR rate and the amount of fine particles.
11A is a diagram showing a relationship between a crank angle and an average gas temperature in a combustion chamber, and FIG. 11B is a diagram showing a relationship between a crank angle and a gas temperature around fuel.
FIG. 12 is a flowchart of another sulfur component poisoning recovery control.
[Explanation of symbols]
5. Combustion chamber
6 ... Fuel injection valve
22 ... Particulate filter
25 ... EGR control valve

Claims (7)

On the particulate filter has an oxidation function, the NO _x absorbing polishes that releases NO _x which the oxygen concentration in the surrounding and capture, hold NO _x when the excess oxygen present around holds the reduced carrying, the air-fuel ratio of the exhaust gas flowing into the particulate filter when less than the amount the amount of particulates reaches a predetermined deposited on the particulate filter at the time said NO _x absorbing polishes are poisoned by sulfur components first When the amount of fine particles deposited on the particulate filter is larger than the predetermined amount, the air-fuel ratio of the exhaust gas flowing into the particulate filter is set to be larger than the first degree. The exhaust gas purification method is designed to make the degree of the exhaust gas rich. The poisoning recovery control is started with the air-fuel ratio of the exhaust gas flowing into the particulate filter as a predetermined air-fuel ratio, and the amount of fine particles deposited on the particulate filter based on the temperature change of the particulate filter during the poisoning recovery. The exhaust gas purifying method according to claim 1, wherein the exhaust gas is detected. The exhaust gas purifying method according to claim 1, wherein a noble metal catalyst is supported on the particulate filter. When there is excess oxygen in the surroundings, the active oxygen releasing agent that takes in oxygen to retain oxygen and releases the retained oxygen in the form of active oxygen when the surrounding oxygen concentration decreases is carried on the particulate filter, and the particulate filter 4. The exhaust gas purifying method according to claim 3, wherein when the fine particles adhere to the surface, active oxygen is released from the active oxygen releasing agent, and the released active oxygen oxidizes the fine particles adhered to the particulate filter. . The exhaust gas purifying method according to claim 4, wherein the active oxygen releasing agent comprises an alkali metal, an alkaline earth metal, a rare earth, a transition metal, or a carbon group element. The exhaust gas purification method according to claim 5, wherein the alkali metal and the alkaline earth metal are made of a metal having a higher ionization tendency than calcium. 5. The exhaust gas purifying method according to claim 4, wherein the air-fuel ratio of a part or the whole of the exhaust gas is temporarily made rich to oxidize the fine particles adhered on the particulate filter.

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