JP2002106385A - Exhaust gas purification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a particulate filter from eroded or thermally deteriorated by combustion heat of particles accumulated on the surface of the particulate filter. SOLUTION: The particulate filter having oxidation function is made to carry NOX absorbing and emitting agent thereon. In a case where the quantity of the accumulated particles is less than a prescribed quantity when the NOX absorbing and emitting agent is poisoned by a sulfur component, the air-fuel ratio of exhaust gas is set to a first rate richness. On the other hand, where the quantity of the accumulated particles is more than the prescribed value, the air-fuel ratio of the exhaust gas is set to a second rate richness larger than the first rate richness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for purifying exhaust gas.

[0002]

2. Description of the Related Art Conventionally, in a diesel engine, a particulate filter is disposed in an engine exhaust passage in order to remove fine particles contained in exhaust gas, and the fine particles in the exhaust gas are once collected by the particulate filter. In addition, the particulate filter is regenerated by igniting and burning the fine particles collected on 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 the 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, on the particulate filter, the exhaust gas is absorbed NO _x when the lean, the NO _x concentration of oxygen carrying the NO _x absorbing polishes that releases NO _x which had been absorbed to decrease A technique for purifying both fine particles is also known (JP-A-6-272541).

[0004]

SUMMARY OF THE INVENTION_xAbsorption and release agent
Poisoned by sulfur components contained in exhaust gas,
O_xAbsorption / release capacity and oxidation capacity may be reduced. this
NO_xNO of absorption and release agent _xTo maintain high absorption and release capacity
NO for_xRemoves sulfur components poisoning sorbents
Need to be NO_xSulfur component poisoning the absorbent
Means that the temperature of the particulate filter becomes relatively high and
NO if the oxygen concentration in the surrounding area decreases_xAbsorption and release
Particulates to remove sulfur components from powder
Raise the filter temperature and lower the ambient oxygen concentration.
You can let it down. However, if the sulfur component is NO_xAbsorption and release
The temperature at which the agent can be desorbed is the ignition temperature of the fine particles
Higher than NO_xRemove sulfur components from absorption and release agents
And raised the temperature of the particulate filter
Particles on the surface of the particulate filter
May burn. In this case, particulate
The temperature of the filter rises sharply,
The particulate filter will not melt or
NO_xThe absorption / release agent may be thermally degraded.

In view of these problems, it is an object of the present invention to prevent the particulate filter from being melted down by the heat of combustion of the fine particles deposited on the surface of the particulate filter, or the particulate filter while preventing the NO _x absorbent from being thermally degraded. To remove sulfur components from coal.

[0006]

According to a first aspect of the present invention, in order to achieve the above object, when excess oxygen is present on the surroundings of a particulate filter having an oxidizing function, NO _x is taken in and retained and the surrounding oxygen is removed. carrying NO _x absorbing polishes that releases NO _x held concentration is lowered, the N
When the amount of fine particles deposited on the particulate filter is smaller than a predetermined amount when the O _x absorbing / 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 value. 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. The degree is 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.

In the second invention, in the first invention,
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. Is detected. In a third aspect, in the first aspect, the noble metal catalyst is supported on the particulate filter.

[0008] In the fourth invention, in the third invention,
When excess oxygen exists 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 Active particles are released from the active oxygen releasing agent when the fine particles adhere to the upper surface, and the released fine particles oxidize the fine particles attached to the particulate filter.

In the fifth invention, in the fourth invention,
The active oxygen releasing agent comprises an alkali metal or alkaline earth metal or rare earth or transition metal or 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]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below 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 be applied to a spark ignition type internal combustion engine. Referring to FIG.
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 is an exhaust valve, 1
0 indicates each 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 flowmeter 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 the exhaust pipe 20.
Through a, it is connected to a casing 23 containing a particulate filter 22.

The exhaust manifold 19 and the surge tank 12 are connected to each other through an exhaust gas recirculation (hereinafter, EGR) passage 24, and an electrically controlled EGR passage is provided in the EGR passage 24.
A control valve 25 is arranged. 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, engine cooling water is guided into the cooling device 26, and the engine cooling water cools the EGR gas. 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.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35, 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 set to a corresponding AD signal.
The signal is input to the input port 35 via the converter 37. The output signal of the mass flow meter 13a is output to the corresponding AD converter 37.
Through the input port 35. 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 EG via a corresponding drive circuit 38.
It is connected to the R control valve 25 and the fuel pump 28.

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 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.

In FIG. 2A, a hatched portion indicates a plug 53. Therefore, the exhaust gas inflow passage 50 and the exhaust gas outflow passage 51 are formed of thin partition walls 54.
Are alternately arranged via In other words, the exhaust gas inflow passage 50 and the exhaust gas outflow passage 51 are each surrounded by the 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.

The particulate filter 22 is formed of a porous material such as cordierite, so that the exhaust gas flowing into the exhaust gas inflow passage 50 is supplied to the surrounding filter as indicated by arrows in FIG. Partition wall 54
It flows out into the adjacent exhaust gas outflow passage 51 through the inside. In the embodiment of the present invention, the peripheral wall surface of each exhaust gas inflow passage 50 and each exhaust gas outflow passage 51, that is, each partition 54
A layer of a carrier made of, for example, alumina is formed over the entire surface on both side surfaces of the plug, the outer end face of the plug 53 and the inner end faces of the plugs 52 and 53, and a noble metal catalyst and excess oxygen And an active oxygen releasing agent that takes in oxygen to retain oxygen when it is present and releases the retained oxygen in the form of active oxygen when the surrounding oxygen concentration decreases.

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, and rubidium Rb, barium Ba, and calcium Ca are used as active oxygen releasing agents. Alkaline earth metals such as strontium Sr, lanthanum La, yttrium Y, cerium Ce
At least one selected from the group consisting of rare earths such as, transition metals such as iron (Fe), and carbon group elements such as tin (Sn) is used.

The active oxygen releasing agent is calcium C
It is preferable to use an alkali metal or alkaline earth metal having a higher ionization tendency than a, that is, potassium K, lithium Li, cesium Cs, rubidium Rb, barium Ba, and strontium Sr. Next, the action of removing particulates in the 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,
A similar effect of removing fine particles can be obtained by using an alkaline earth metal, a rare earth, a transition metal, or a carbon group element.

In a compression ignition type internal combustion engine as shown in FIG. 1, combustion takes place under excess air, so that the exhaust gas contains a large amount of excess air. That is, if the ratio of air and fuel supplied to 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.
It is included. Further, the fuel contains sulfur S, which reacts with oxygen in the combustion chamber 5 to become SO ₂ . Therefore, SO ₂ is contained in the exhaust gas. Therefore, exhaust gas containing excess oxygen, NO and SO ₂ flows into the exhaust gas inflow passage 50 of the particulate filter 22.

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. FIG. 3A and 3B, reference numeral 60 denotes platinum Pt particles, and reference numeral 61 denotes an active oxygen releasing agent containing potassium K. When the exhaust gas because the exhaust gas as described above contains a large amount of excess oxygen flows into the exhaust gas inflow passages 50 of the particulate filter 22 3 These oxygen O ₂ as shown in (A) There O ₂ ^- it is attached to or O the surface of the platinum Pt in ^2-form. On the other hand, NO in the exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become NO ₂ (2N
O + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed in the active oxygen releasing agent 61 while being oxidized on the platinum Pt, and is combined with potassium K to form nitrate ion NO ₃ ⁻ as shown in FIG. And diffuses into the active oxygen releasing agent 61 to generate potassium nitrate KNO ₃ .

On the other hand, as described above, the exhaust gas contains SO
₂ is also included, this SO ₂ is absorbed in the active oxygen release agent 61 by a mechanism similar to the NO. That oxygen O ₂ as described above O ₂ ^- or platinum P in O ^2- in the form
The SO ₂ in the exhaust gas is platinum P
the SO ₃ reacts or O ^2- and ^- O ₂ at the surface of t. Next, a part of the generated SO ₃ is absorbed in the active oxygen releasing agent 61 while being further oxidized on the platinum Pt, and the potassium K
While being bonded to the active oxygen releasing agent 61, it diffuses into the active oxygen releasing agent 61 in the form of sulfate ions SO ₄ ^2- to generate potassium sulfate K ₂ SO ₄ . Thus, potassium nitrate KNO ₃ and potassium sulfate K ₂ SO ₄ are generated in the active oxygen releasing agent 61.

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.

As described above, the fine particles 62 form the active oxygen releasing agent 6
1 and the fine particles 62 and the active oxygen releasing agent 6
At the contact surface with No. 1, the oxygen concentration decreases. When the oxygen concentration decreases, a difference in concentration occurs between the active oxygen releasing agent 61 having a high oxygen concentration and the oxygen in the active oxygen releasing agent 61 is directed toward the contact surface between the fine particles 62 and the active oxygen releasing agent 61. Try to move. As a result, potassium nitrate KNO ₃ formed in the active oxygen releasing agent 61 becomes potassium K and oxygen O
And NO are decomposed, 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.

At this time, potassium sulfate K ₂ SO ₄ formed in the active oxygen releasing agent 61 also contains potassium K and oxygen O.
And SO _2, and oxygen O is directed to the contact surface between the fine particles 62 and the active oxygen releasing agent 61, while SO ₂ is released from the active oxygen releasing agent 61 to the outside. The SO ₂ released to the outside is oxidized on platinum Pt on the downstream side, and is again absorbed in the active oxygen releasing agent 61. However, potassium sulfate K ₂ SO ₄ is stable and hard to decompose, so that potassium sulfate K ₂ SO ₄ is harder to release active oxygen than potassium nitrate KNO ₃ . Also, as described above, the active oxygen releasing agent 61 generates and releases active oxygen in the process of reacting with oxygen even when absorbing NO _x in the form of nitrate ions NO ₃ ⁻ . Similarly, the active oxygen release agent 61 to the SO ₂ as described above to produce an active oxygen in the reaction process with oxygen even when absorbed by sulfate ions SO ₄ ^2-form release.

The fine particles 62 and the active oxygen releasing agent 61
The oxygen O heading toward the contact surface is oxygen decomposed from compounds such as potassium nitrate KNO ₃ and potassium sulfate K ₂ SO ₄ . 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, active oxygen releasing agent 61
The oxygen generated in the reaction process between NO _x and oxygen or the reaction process between SO ₂ and oxygen in the above 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 eliminated. 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.

A conventional particulate filter 2
When the fine particles deposited in a stack on the combustor 2 are burned, 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 maintain the combustion with such a flame.

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 red. 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 by oxidation of the fine particles 62 that do not emit a bright flame according to the present invention is completely different from the action of removing fine particles by conventional combustion accompanied by a flame.

Meanwhile, platinum Pt and active oxygen releasing agent 6
1 is activated as the temperature of the particulate filter 22 increases, so that the amount of oxidizable particles that can be oxidized and removed on the particulate filter 22 without emitting a luminous flame per unit time is equal to the amount of the particulate filter 2.
2 increases as the temperature increases.

The solid line in FIG. 5 shows the amount of fine particles G which can be oxidized and removed without emitting a bright flame per unit time. In FIG. 5, the horizontal axis indicates 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.

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 in 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. In other words, when the amount of active oxygen is insufficient to oxidize all the fine particles, the fine particles 62 are used as the active oxygen releasing agent 6 as shown in FIG.
When the fine particles 62 adhere to the surface of the carrier 1, only a part of the fine particles 62 is oxidized, and the finely oxidized fine particles 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.

When the surface of the carrier layer is covered with the residual fine particles 63, the oxidation of NO and SO ₂ by platinum Pt and the release of active oxygen by the active oxygen releasing agent 61 are not performed. The fine particles 64 remain on the remaining fine particle portion 63 one after another, as shown in FIG. 4C. That is, the fine particles are deposited in a layered manner. When the fine particles are deposited in a layered manner in this way, the fine particles 6
No. 4 is no longer oxidized by the active oxygen O, and further fine particles are deposited on the fine particles 64 one after another. That is, if 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.

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. 5, the fine particles are laminated on the particulate filter 22. Deposit in a shape. Therefore, in order to prevent the fine particles from depositing on the particulate filter 22 in a layered manner, the amount M of discharged fine particles must always be smaller than the amount G of fine particles that can be removed by oxidation.

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 if the temperature TF of the particulate filter 22 is considerably low. In the illustrated compression ignition type internal combustion engine, it is possible to maintain the amount M of discharged particulate and the temperature TF of the particulate filter 22 so that the amount M of discharged particulate is always smaller than the amount G of particulate 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
Is always kept smaller than the amount G of particles that can be removed by oxidation.

The amount M of discharged fine particles is the amount G of fine particles that can be removed by oxidation.
If the amount is smaller than usual, 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 in a layered manner on the particulate filter 22, even if the amount M of discharged fine particles is 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 a bright flame. Therefore, in the second embodiment, even if the amount M of discharged fine particles is usually 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 M of discharged fine particles is smaller than the amount G of fine particles that can be removed by oxidation so that the surface of the carrier layer is not covered by the residual fine particle portion 63. Only 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 stacked on the particulate filter 22.

In particular, immediately after the start of the engine, the temperature TF of the particulate filter 22 is low. Therefore, at this time, the amount M of the discharged fine particles is larger than the amount G of the fine particles removable by oxidation. Therefore, it is considered that the second embodiment is more suitable for actual driving. On the other hand, the amount M of discharged particulates and the temperature TF of the particulate filter 22 are set so that the first embodiment or the second embodiment can be executed.
Is controlled even if the particulate filter 22 is controlled.
Fine particles may be deposited on the upper surface in a stacked state. In such a case, the air-fuel ratio of part or all of the exhaust gas is temporarily made rich to thereby increase the particulate filter 22.
The fine particles deposited thereon can be oxidized without emitting a bright flame.

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 the catalytic action of the platinum Pt is restored. Accordingly, 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 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 accumulate on the particulate filter 22 in a stacked manner, or the exhaust gas may be periodically exhausted regardless of whether the particulates are accumulated in a stacked manner. The air-fuel ratio of the gas may be made rich.

As a method of making the air-fuel ratio of the exhaust gas rich, for example, when the engine load is relatively low, the EGR rate (EG
The opening degree of the throttle valve 17 and the opening degree of the EGR control valve 25 are controlled so that the R gas amount / (intake air amount + EGR gas amount) becomes 65% or more. At this time, the average air-fuel ratio in the combustion chamber 5 is reduced. A method of controlling the injection amount so as to be rich can be used.

FIG. 6 shows an example of the operation control routine of the internal combustion engine described above. Referring to FIG. 6, first, at 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.

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 105, the opening of the EGR control valve 25 is controlled, and in step 106, the fuel injection amount is controlled so that the average air-fuel ratio in the combustion chamber 5 becomes rich.

Incidentally, the fuel and the lubricating oil contain calcium Ca, and therefore, calcium Ca is contained in the exhaust gas. This calcium Ca produces calcium sulfate CaSO ₄ when SO ₃ is present. This calcium sulfate CaSO ₄ is solid and does not thermally decompose even at high temperatures. Therefore, when the calcium sulfate CaSO ₄ is generated, the pores of the particulate filter 22 are blocked by the calcium sulfate CaSO ₄ , and as a result, it becomes difficult for the exhaust gas to flow through the particulate filter 22.

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 ₃ diffused into the active oxygen releasing agent 61 binds to potassium K. To form potassium sulfate K ₂ SO ₄ , and calcium Ca binds to particulate filter 2 without binding to SO _3.
After passing through the second partition 54, it flows out into the exhaust gas outflow passage 51. 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 C
s, rubidium Rb, barium Ba, strontium S
It will be preferable to use r.

The present invention also relates to a particulate filter 2.
The present invention can also be applied to a case where only a noble metal such as platinum Pt is supported on a carrier layer formed on both side surfaces of the substrate 2. 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 side as compared with the solid line shown in FIG.
In this case, active oxygen is released from NO ₂ or SO ₃ held on the surface of platinum Pt.

As an active oxygen releasing agent, NO ₂ or S
O ₃ is adsorbed and held, and the adsorbed NO ₂ or SO _2
A catalyst capable of releasing active oxygen from ₃ can also be used. By the way, as described above, the sulfur component S is contained in the exhaust gas.
Is contained in the form of SO ₂ . 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. If the temperature of the particulate filter 22 is high to some extent, the sulfur component S is released from the platinum Pt and the active oxygen releasing agent 61. 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 platinum Pt decreases, and the amount of NO _x that can be absorbed by active oxygen releasing agent 61 (NO _x absorbing ability) also decreases. Therefore, in order to maintain the oxidizing ability of platinum Pt high and to maintain a large amount of NO _x that can be absorbed by the active oxygen releasing agent 61, platinum Pt must be used.
It is necessary to desorb the sulfur component S that is adsorbed on t 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 the particulates are deposited on the particulate filter 22, the particulates are burned at once, and the particulate filter 22 is burned.
Temperature rises rapidly and the particulate filter 22
Is melted, or even if not melted, the NO _x absorbing / desorbing agent is thermally deteriorated (hereinafter, also referred to as thermal deterioration including erosion).

Therefore, in the present invention, it is determined whether or not a large amount of fine particles are deposited so as to cause thermal degradation of the particulate filter 22 when the sulfur component is to be desorbed, and the fine particles cause thermal degradation of the particulate filter 22. When the deposit is not so large, the particulate filter 2
2, the temperature of the particulate filter 22 is raised to the desorption temperature without performing the processing for preventing the thermal deterioration of
On the other hand, when the particulates are deposited in such a large amount as to cause thermal degradation of the particulate filter 22, the air having a rich degree larger than the minimum necessary for raising the temperature of the particulate filter 22 to the desorption temperature is obtained. Exhaust gas having a fuel ratio is caused to flow into the particulate filter 22.

As described above, when the 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.
Since C reacts with oxygen in the exhaust gas, the oxygen concentration in the particulate filter decreases. For this reason, the amount of oxygen for burning the deposited fine particles is reduced, and it is possible to prevent the deposited fine particles from burning a flame 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 inside of the active oxygen releasing agent 61 and the platinum P
Since there is a concentration difference between the oxygen concentration at the t surface and the oxygen concentration around the t surface, the sulfur component S is 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.

Next, a specific process of the present invention for recovering the particulate filter from poisoning by the sulfur component S will be described with reference to the flowchart of FIG.
First, in step 200, the accumulated sulfur component amount As is larger than a predetermined amount AsTH (As> AsTH).
Is determined. Here, the accumulated sulfur component amount As is shown in FIG.
As shown in FIG. 3, the particulate filter 2 per unit time is a function of the engine speed Ne and the required engine load L.
2 is obtained in advance in the form of a map by experiments or the like, and the sulfur component amount A
It is calculated by integrating smm. Further, the predetermined amount AsTH is set to an amount of a sulfur component that reduces the oxidizing ability of platinum Pt to an allowable limit or lowers the NO _x absorption capacity of the active oxygen releasing agent 61 to an allowable limit. You. In step 200, As> AsT
When it is determined to be H, 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 the deposited fine particles is larger than a predetermined amount ApmTH (Apm> ApmTH).
Here, as shown in FIG. 9, the accumulated particle amount Apm is a function of the engine 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 determined by experiment or the like. The particle size is calculated in advance by integrating the particle amount Apmm. Further, 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 performing 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 air-fuel ratio of the exhaust gas 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 processing 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.

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 at 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 preliminarily injected immediately 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.

Further, in step 203, instead of performing the rich processing I, the internal combustion engine is caused to perform low-temperature combustion as described later to increase the temperature of the exhaust gas, and thus the temperature of the particulate filter is increased to the sulfur desorption temperature. You may make it. Next, low-temperature combustion will be described. EG
It is known that when the R 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. Regarding this, E when the degree of cooling of the EGR gas is changed
This will be described with reference to FIG. 10 showing the relationship between the GR rate and smoke. In FIG. 10, a curve A indicates a case where the EGR gas is strongly cooled to maintain the EGR gas temperature at approximately 90 ° C., a curve B indicates a case where the EGR gas is cooled by a small cooling device, and a curve C indicates the EGR gas. Indicates a case where cooling is not forcibly performed.

As shown by the curve A in FIG. 10, when the EGR gas is strongly cooled, the amount of generated fine particles reaches a peak when the EGR rate is slightly lower than 50%.
In this case, if the EGR rate is set to about 55% or more, almost no fine particles are generated. On the other hand, when the EGR gas is slightly cooled as shown by the curve B in FIG.
When the rate is slightly higher than 50%, the generation amount of the fine particles reaches a peak. In this case, if the EGR rate is set to about 65% or more, almost no fine particles are generated. Further, when the EGR gas is not forcibly cooled as shown by the curve C in FIG. 10, the generation amount of the fine particles reaches a peak when the EGR rate is around 55%.
If the R ratio is set to approximately 70% or more, 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 is not 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] with the feature that this low temperature combustion can reduce the generation amount of the NO _x while suppressing the generation of particles regardless of the air-fuel ratio. That is, if 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. The only occurs very little even at this time NO _x. 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 under low-temperature combustion, the combustion temperature decreases to a lower temperature. because it is suppressed particles does not occur at all, nO _x is also only an extremely small amount of generated.

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. FIG.
1 (A) shows the relationship between the average gas temperature Tg in the combustion chamber 5 and the crank angle when the low-temperature combustion is performed, and FIG.
The broken line 1 (A) indicates the combustion chamber 5 when normal combustion is performed.
2 shows the relationship between the average gas temperature Tg 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 fuel and the surrounding gas temperature Tf and the crank angle is shown.

When low-temperature combustion is performed, the amount of EGR gas is larger than when normal combustion is performed. Therefore, as shown in FIG. 11A, before the compression top dead center, that is, during the compression stroke, Is that the average gas temperature Tg at the time of low-temperature combustion shown by the solid line is higher than the average gas temperature Tg at the time of normal combustion shown by the broken line. At this time, as shown in FIG. 11B, the temperature of the fuel and the surrounding gas Tf are substantially equal to the average gas temperature Tg.

Next, combustion starts near the compression top dead center. In this case, when low-temperature combustion is being performed, as shown by the solid line in FIG. No. 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. . When normal combustion is performed in this manner, the temperature of the fuel and the surrounding gas Tf is 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. 11A, 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.

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 process I of the above-described embodiment is executed).
At this time, the temperature rise rate of the particulate filter 22 is monitored (according to this, it is possible to substantially estimate the amount of fine particles deposited on the particulate filter), and this temperature rise rate is relatively large. Sometimes, it is determined that there is a possibility that the particulate filter 22 is thermally degraded, and the rich processing I is performed in the rich processing II of the above-described embodiment.
Switch to. According to this, the thermal deterioration of the particulate filter can be prevented.

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 in step 301, the rich process I is executed. 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, a predetermined rise rate RtfT
H is set to a rising rate that may cause the deposited fine particles to emit a flame and burn at once. Assuming that 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, and in this case, the process proceeds to step 303 to perform the rich process I.
I is executed, and the routine proceeds to step 304. The rich processing II here 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 executed 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, the rich process I is executed in step 305, and the process proceeds to step 304.

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 increase. In addition, 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 the particulates deposited on the particulate filter 22 is estimated from the temperature change of the particulate filter 22. When the amount of the particulates is large to some extent, the air-fuel ratio of the exhaust gas is kept leaner than the stoichiometric air-fuel ratio while the accumulated particulates are maintained. May be oxidized and removed, and then the rich processing I may be performed. 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]

If the particles are relatively large amount of deposited particulate filter when recovering the poisoning of the NO _x absorbing and polishes by sulfur component according to the present invention is flowing into the particulate filter exhaust The air-fuel ratio of the gas is relatively large and rich. If the exhaust gas with 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 fine particles deposited on the particulate filter burn. You will not be able to do it. 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. The temperature of the particulate filter by the reaction between the hydrocarbon and oxygen are raised, and NO _x absorbing polishes NO _x by sulfur component because the oxygen concentration around the drops
The 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 view for explaining an oxidizing action of fine particles.

FIG. 4 is a diagram for explaining a deposition action of fine particles.

FIG. 5 is a graph 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.

FIG. 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

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F01N 3/00 F01N 3/08 A 4D058 3/02 301 3/10 A 3/08 3/20 E 3 / 10 3/28 301C 3/20 301P 3/28 301 B01D 53/36 104A ZAB 104B 101B (72) Inventor Nobuya Hirota 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Kazuhiro Ito Aichi Toyota Motor Co., Ltd., Toyota City, Toyota Prefecture (72) Inventor Takamitsu Asanuma 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Koichi Kimura 1, Toyota Town, Toyota City, Aichi Prefecture F-term (reference) in Toyota Motor Corporation 3G090 AA01 AA02 AA03 BA01 CA04 DA13 DA18 DA20 EA05 EA06 3G091 AA10 AA11 AA17 AA18 AB01 AB09 AB13 BA01 BA08 BA11 BA14 BA39 EA01 EA02 EA05 EA07 EA18 EA19 FB10 FB12 GA06 GB01W GB02W GB03W GB04W GB05W GB06W GB17X GB17Y HA14 3G301 HA01 HA02 HA11 HA13 JA21 JA24 JA25 JA33 LA01 LB11 LB13 MA01 NAZ PD01 NE13 PD01 AA01 BA05 BC07 CA01 CB04 4D048 AA06 AA14 AA18 AB01 AB07 BA30X BA31Y BA32Y BA33Y BA34Y CC41 DA01 DA02 DA20 EA04 4D058 JA32 JB06 MA41 SA08 TA06 UA01

Claims (7)

[Claims] To 1. A on the particulate filter having an oxidation function, NO _x absorption release that releases NO _x which the oxygen concentration in the surrounding and holding the NO _x takes in the excess of oxygen present around holds to decrease agent carrying the exhaust gas the NO _x absorbing polishes is flowing into the particulate filter when less than the amount the amount of particulates reaches a predetermined deposited on the particulate filter in when poisoned by sulfur components 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 the first degree. An exhaust gas purifying method in which the second degree is richer than the second degree. 2. 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 accumulated on the particulate filter based on a temperature change of the particulate filter during the poisoning recovery. 2. The exhaust gas purifying method according to claim 1, wherein the amount of the fine particles is detected. 3. The exhaust gas purifying method according to claim 1, wherein a noble metal catalyst is supported on the particulate filter. 4. An active oxygen releasing agent, which takes in oxygen and retains oxygen when there is an excess of oxygen in the surroundings and releases the retained oxygen in the form of active oxygen when the concentration of surrounding oxygen decreases, is carried on the particulate filter. 4. The method according to claim 3, wherein when the fine particles adhere to the particulate filter, the active oxygen is released from the active oxygen releasing agent, and the released active oxygen oxidizes the fine particles adhered to the particulate filter. Exhaust gas purification method. 5. 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. 6. 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. 7. The 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 adhering to the particulate filter.
The exhaust gas purification method according to any one of the above.