JP4192532B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
Conventionally, NO in the exhaust gas flowing in when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the internal combustion engine where combustion is continuously performed under a lean air-fuel ratio is lean_XNO when storing the reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in decreases_XNO is being reduced and stored_XNO decreases in quantity_XPlace catalyst and NO_XNO in the catalyst_XNO_XNO stored in the catalyst_XTo reduce the amount of NO_X2. Description of the Related Art An exhaust gas purification apparatus for an internal combustion engine is known in which a rich spike that temporarily switches the air-fuel ratio of exhaust gas flowing into a catalyst to a rich air-fuel ratio is performed (see, for example, Patent Document 1).
[0003]
NO_XNO in the catalyst_XNot only sulfur, eg SO_XIs stored in the form of sulfate, NO in the form of sulfate_XSO stored in the catalyst_XIn order to reduce the amount of NO_XMaintain the temperature of the catalyst at, for example, 550 ° C. or higher and NO_XIt is also known that it is necessary to switch the air-fuel ratio of the exhaust gas flowing into the catalyst to the stoichiometric air-fuel ratio or rich (see, for example, Patent Document 1). According to this, NO_XEven if the temperature of the catalyst is high, the air-fuel ratio of the inflowing exhaust gas is lean, or even if the air-fuel ratio of the inflowing exhaust gas is rich, NO_XIf the temperature of the catalyst is low, NO_XFrom catalyst to SO_XWill not be discharged.
[0004]
[Patent Document 1]
JP 2002-155724 A
[0005]
[Problems to be solved by the invention]
However, according to the present inventor, NO_XNO even if the average air-fuel ratio of the exhaust gas flowing into the catalyst is kept lean_XWhen the temperature of the catalyst increases, NO_XSO in exhaust gas flowing out of catalyst_XConcentration is NO_XSO in the exhaust gas flowing into the catalyst_XIt has been confirmed that the concentration is temporarily higher than the concentration. This is NO_XNO at higher catalyst temperatures_XSO stored in the catalyst_XIs discharged and this SO_XNO without forming sulfate_XIt means that it is stored in the catalyst.
[0006]
However, if the air-fuel ratio of the inflowing exhaust gas is lean at this time, the stored SO is formed without forming sulfate.₂Is NO_XSulfate SO in the catalyst₃Oxidized to sulfate SO₃NO in the form of_XThere is a risk of being discharged from the catalyst. NO_XFrom catalyst to SO₂Even if it is discharged as it is, NO_XWhen a catalyst having oxidizing ability is disposed downstream of the catalyst, the sulfate SO is contained in the auxiliary catalyst.₃There is a risk of oxidation.
[0007]
Accordingly, an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can reduce the amount of sulfate discharged into the atmosphere.
[0008]
[Means for Solving the Problems]
In order to solve the above-described problem, according to a first aspect of the present invention, an exhaust gas flowing into an internal combustion engine in which combustion is continuously performed under a lean air-fuel ratio flows when the air-fuel ratio of the exhaust gas flowing in is lean. NO in exhaust gas_XNO when storing the reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in decreases_XNO is being reduced and stored_XNO decreases in quantity_XPlace the catalyst and the NO_XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XTo reduce the amount of NO_XIn an exhaust gas purification apparatus for an internal combustion engine configured to perform a rich spike that temporarily switches the air-fuel ratio of exhaust gas flowing into the catalyst to a rich air-fuel ratio, NO is formed without forming sulfate._XThe amount of sulfur stored in the catalyst is determined and NO is formed without forming the sulfate._XWhen the amount of sulfur stored in the catalyst is large, the NO is smaller than when the amount is small._XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XThe rich air-fuel ratio in the rich spike for reducing the amount of air is reduced.
[0009]
In order to solve the above problem, according to the second invention, when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the internal combustion engine in which combustion is continuously performed under the lean air-fuel ratio is lean In exhaust gas flowing into_XNO when storing the reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in decreases_XNO is being reduced and stored_XNO decreases in quantity_XPlace the catalyst and the NO_XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XTo reduce the amount of NO_XIn an exhaust gas purification apparatus for an internal combustion engine that performs a rich spike that switches an air-fuel ratio of exhaust gas flowing into a catalyst to a rich air-fuel ratio for a rich holding time, NO is formed without forming sulfate._XThe amount of sulfur stored in the catalyst is determined and NO is formed without forming the sulfate._XWhen the amount of sulfur stored in the catalyst is large, the NO is smaller than when the amount is small._XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XThe rich holding time in the rich spike for reducing the amount of the amount is increased.
[0010]
According to a third aspect of the invention for solving the above-described problem, when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the internal combustion engine in which combustion is continuously performed under the lean air-fuel ratio is lean. In exhaust gas flowing into_XNO when storing the reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in decreases_XNO is being reduced and stored_XNO decreases in quantity_XPlace the catalyst and the NO_XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XTo reduce the amount of NO_XIn an exhaust gas purification apparatus for an internal combustion engine configured to perform a rich spike that temporarily switches the air-fuel ratio of exhaust gas flowing into the catalyst to a rich air-fuel ratio, NO is formed without forming sulfate._XThe amount of sulfur stored in the catalyst is determined and NO is formed without forming the sulfate._XWhen the amount of sulfur stored in the catalyst is large, the NO is smaller than when the amount is small._XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XThe time interval of the rich spike for reducing the amount of the signal is shortened.
[0011]
According to the fourth invention, in any one of the first to third inventions, NO_XThe amount of sulfur flowing into the catalyst is determined, and the NO_XNO based on the amount of sulfur flowing into the catalyst without forming the sulfate._XThe amount of sulfur stored in the catalyst is determined.
[0012]
According to the fifth invention, in any one of the first to third inventions, NO_XNO stored in the catalyst_XThe amount of NO_XWhen the amount of is more than the allowable amount, NO_XNO in the catalyst_XTo reduce the NO_XNO stored in the catalyst_XRich spikes are performed to reduce the amount.
[0013]
According to a sixth aspect, in any one of the first to third aspects, the NO_XA catalyst is supported on a particulate filter that collects particulates in the inflowing exhaust gas.
[0014]
According to a seventh aspect, in any one of the first to third aspects, the NO_XAn auxiliary catalyst having oxidation ability is arranged in the exhaust passage downstream of the catalyst.
[0015]
In this specification, the ratio of the air supplied into the exhaust passage upstream of a certain position of the exhaust passage, the combustion chamber, and the intake passage and the reducing agent such as hydrocarbon HC and carbon monoxide CO This is called the air-fuel ratio of the exhaust gas at the position.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
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.
[0017]
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 Is an exhaust valve, and 10 is 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 throttle valve 17 driven by a step motor 16 is disposed in the intake duct 13, and a cooling device 18 for cooling intake air flowing through the intake duct 13 is disposed around the intake duct 13. In the embodiment according to the present invention, the throttle opening degree is maintained at the maximum opening degree in almost all operation regions, and is made smaller than the maximum opening degree when the required load L becomes considerably small, and small when the required load L becomes zero. To be.
[0018]
On the other hand, the exhaust port 10 is connected to the inlet of the exhaust turbine 21 of the exhaust turbocharger 14 via the exhaust manifold 19 and the exhaust pipe 20, and the outlet of the exhaust turbine 21 is connected to the casing 22a via the exhaust pipe 20a. A particulate filter 22b for collecting particulates in the exhaust gas is accommodated in the casing 22a, and NO is applied to the particulate filter 22b as will be described later._XA catalyst 22 is supported. The catalytic converter 22 is connected to a casing 23a via an exhaust pipe 20b, and an auxiliary catalyst 23 having oxidation ability is accommodated in the casing 23a. In this case, NO_XThe air-fuel ratio of the exhaust gas flowing into the catalyst 22 matches the air-fuel ratio of the exhaust gas flowing into the particulate filter 22b, and NO_XThe temperature of the catalyst 22 also matches the temperature of the particulate filter 22b.
[0019]
Still referring to FIG. 1, the exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 24, and an electrically controlled EGR control valve 25 is provided in the EGR passage 24. Be placed. A cooling device 26 for cooling the EGR gas flowing in the EGR passage 24 is disposed around the EGR passage 24.
[0020]
On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 27, through a fuel supply pipe 6a. Fuel is supplied into the common rail 27 from an electrically controlled fuel pump 28 with variable discharge amount, and the fuel supplied into the common rail 27 is supplied to the fuel injection valve 6 via 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 a fuel pump 28 is set so that the fuel pressure in the common rail 27 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 29. The discharge amount is controlled.
[0021]
The electronic control unit 40 is composed of a digital computer, and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 It comprises. The output signal of the fuel pressure sensor 29 is input to the input port 45 via the corresponding AD converter 47. A pressure sensor 49 for detecting the pressure in the exhaust pipe 20 a, that is, the engine back pressure, is attached to the exhaust pipe 20 a upstream of the particulate filter 22 b, and the output voltage of the pressure sensor 49 passes through the corresponding AD converter 47. Input to the input port 45. NO_XNO in the exhaust pipe 20b downstream of the catalyst 22_XA temperature sensor 50 for detecting the temperature of the exhaust gas flowing out from the catalyst 22 is attached, and the output voltage of the temperature sensor 50 is input to the input port 45 via the corresponding AD converter 47. The temperature of this exhaust gas is NO_XThe temperature of the catalyst 22 is shown. A load sensor 52 that generates an output voltage proportional to the amount of depression of the accelerator pedal 51 is connected to the accelerator pedal 51, and the output voltage of the load sensor 52 is input to the input port 45 via the corresponding AD converter 47. Further, the input port 45 is connected to a crank angle sensor 53 that generates an output pulse every time the crankshaft rotates, for example, 30 °. The CPU 44 calculates the engine speed N based on the output pulse from the crank angle sensor 53. On the other hand, the output port 46 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 corresponding drive circuits 48.
[0022]
NO on the partition walls of the particulate filter 22a, for example, on both side surfaces of the partition walls and the inner wall surfaces of the pores._XEach catalyst 22 is supported. This NO_XThe catalyst 22 has, for example, alumina as a carrier, and an alkali metal such as potassium K, sodium Na, lithium Li and cesium Cs, an alkaline earth such as barium Ba and calcium Ca, lanthanum La and yttrium Y on the carrier. At least one selected from rare earths and a noble metal such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir are supported.
[0023]
NO_XThe catalyst is NO when the average air-fuel ratio of the exhaust gas flowing in is lean._XNO when storing the reducing agent in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in decreases_XNO is being reduced and stored_XAccumulation and reduction action to reduce the amount of.
[0024]
NO_XThe detailed mechanism of the accumulation and reduction action of the catalyst has not been fully clarified. However, the mechanism currently considered can be briefly described as follows, taking as an example the case where platinum Pt and barium Ba are supported on a carrier.
[0025]
That is, NO_XWhen the air-fuel ratio of the exhaust gas flowing into the catalyst becomes considerably leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas greatly increases and oxygen O₂Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas adheres to the surface of platinum Pt and O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(NO + O₂→ NO₂+ O^*Where O^*Is active oxygen). Then the generated NO₂Part of the NO is being oxidized further on platinum Pt_XNitrate ion NO while being absorbed in the catalyst and combined with barium oxide BaO₃ ⁻NO in the form of_XIt diffuses into the catalyst. In this way NO_XIs NO_XStored in the catalyst.
[0026]
In contrast, NO_XWhen the air-fuel ratio of the exhaust gas flowing into the catalyst becomes rich or stoichiometric, the oxygen concentration in the exhaust gas decreases and NO₂Production amount decreases and the reaction proceeds in the reverse direction (NO₃ ⁻→ NO + 2O^*) And thus NO_XNitrate ion NO in the catalyst₃ ⁻NO in the form of NO_XReleased from the catalyst. This released NO_XWhen the exhaust gas contains a reducing agent, that is, HC or CO, it reacts with these HC and CO to be reduced. In this way, NO on the surface of platinum Pt._XNO when no longer exists_XNO from catalyst to next_XIs released and reduced, NO_XNO stored in the catalyst_XThe amount of is gradually reduced.
[0027]
NO without forming nitrate_XStore NO_XNO without releasing_XIt is also considered possible to reduce In addition, active oxygen O^*If you pay attention to, NO_XThe catalyst is NO_XWith the accumulation and release of oxygen^*It can also be regarded as an active oxygen generating catalyst that generates
[0028]
On the other hand, in the embodiment according to the present invention, the auxiliary catalyst 23 is formed of a noble metal catalyst containing noble metal such as platinum Pt without containing alkali metal, alkaline earth, and rare earth. However, the auxiliary catalyst 23 is the NO described above._XYou may form from a catalyst.
[0029]
The internal combustion engine shown in FIG. 1 continues to burn under a lean air-fuel ratio, and therefore NO._XThe air-fuel ratio of the exhaust gas flowing into the catalyst 22 is maintained lean. As a result, NO in the exhaust gas_XIs NO_XIt is stored in the catalyst 22.
[0030]
NO over time_XAccumulated NO in catalyst 22_XThe amount increases gradually. Therefore, in an embodiment according to the present invention, NO_XAccumulated NO in catalyst 22_XDetermine the amount, NO_XAccumulated NO in catalyst 22_XAmount is NO_XWhen the allowable amount is exceeded, NO_XNO stored in catalyst 22_XNO_XAccumulated NO in catalyst 22_XNO to reduce the amount_XA rich spike for temporarily switching the air-fuel ratio of the exhaust gas flowing into the catalyst 22 to rich is performed. In the following, NO_XNO stored in catalyst 22_XNO_XAccumulated NO in catalyst 22_XRich spikes to reduce the amount, simply NO_XIt will be called a rich spike for.
[0031]
Specifically, NO per unit time_XNO stored in catalyst 22_XThe amount of NO is per unit time_XNO flowing into the catalyst 22_XDepending on the amount of NO per unit time_XNO flowing into the catalyst 22_XThe amount of NO discharged from the internal combustion engine per unit time_XAmount of NO_XDepends on the quantity dQN. Emission NO_XThe quantity dQN depends on the engine operating state, for example, the required load L and the engine speed, that is, the exhaust NO._XEmission NO as the amount dQN increases and the engine speed N increases_XThe quantity dQN increases.
[0032]
Therefore, discharge NO_XAn integrated value QN obtained by integrating the amount dQN multiplied by the processing cycle time Δt is NO_XNO stored in catalyst 22_XRepresents the amount of. In the following, this integrated value QN is set to NO._XIt will be called a count value. Discharge NO_XThe quantity dQN is obtained in advance by experiments, and is stored in advance in the ROM 42 as a function of the required load L and the engine speed N in the form of a map shown in FIG.
[0033]
As indicated by arrow X in FIG._XThe count value QN is NO as described above._XNO equivalent to the allowable amount_XIf it exceeds the allowable value QNA, NO_XThe air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is switched to the rich air-fuel ratio AFR, held at the rich air-fuel ratio AFR for the rich holding time tR, and then returned to lean. In this way NO_XA rich spike for is done. If the air-fuel AFN of the inflowing exhaust gas is returned to lean, NO_XThe count value QN is cleared (QN = 0).
[0034]
As is clear from FIG. 3, NO_XCount value QN is NO_XNO each time the allowable value QNA is exceeded_XA rich spike for is done. In this case, NO_XIf the allowable value QNA is reduced, the time interval INT of the rich spike is shortened and NO._XIncreasing the allowable value QNA increases the time interval INT. Therefore, NO_XThe allowable value QNA represents the rich spike time interval INT.
[0035]
NO_XThere are various methods for the rich spike for temporarily switching the air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 to rich. For example, a method for temporarily switching the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 to rich. Or NO_XThere is a method of temporarily injecting additional fuel or reducing agent into the exhaust passage upstream of the catalyst 22. Here, in order to switch the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 to rich, for example, the air-fuel ratio of the air-fuel mixture combusted in the combustion chamber 5 can be switched to rich, or the compression top dead center Additional fuel may be injected during the expansion stroke or the exhaust stroke in addition to the main fuel performed in the vicinity. In the embodiment according to the present invention, the rich spike is performed by temporarily injecting additional fuel during the expansion stroke or the exhaust stroke.
[0036]
By the way, sulfur content is SO._XIs included in the form of NO_XNO in the catalyst 22_XNot only SO_XCan also be stored. This SO_XNO_XThe accumulation mechanism in the catalyst 22 is NO._XThis is considered to be the same as the accumulation mechanism. That is, when the case where platinum Pt and barium Ba are supported on a carrier is simply described as an example, NO_XWhen the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is lean, as described above, the oxygen O₂Is O₂ ⁻Or O^2-Attached to the surface of platinum Pt in the form of SO₂Adheres to the surface of platinum Pt and O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with SO₃It becomes. The generated SO₃NO is being oxidized on platinum Pt_XWhile being absorbed into the catalyst 22 and combined with barium oxide BaO, sulfate ions SO₄ ⁻NO in the form of_XIt diffuses into the catalyst 22. This sulfate ion SO₄ ⁻Then barium ion Ba⁺Combined with sulfate BaSO₄Is generated.
[0037]
This sulfate BaSO₄Is difficult to decompose, NO_XEven if the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is simply rich, NO_XSulfate BaSO in catalyst 22₄The amount of does not decrease. For this reason, as time passes, NO_XSulfate BaSO in catalyst 22₄The amount of NO increases, resulting in NO_XNO that the catalyst 22 can store_XThe amount of will decrease.
[0038]
However, NO_XWhile maintaining the temperature of the catalyst 22 at, for example, 550 ° C. or higher, NO_XWhen the average air-fuel ratio of the exhaust gas flowing into the catalyst 22 is made the stoichiometric air-fuel ratio or rich, NO_XSulfate BaSO in catalyst 22₄Decomposes into SO₃NO in the form of_XReleased from the catalyst 22. This released SO₃If exhaust gas contains a reducing agent, ie, HC or CO, it reacts with HC and CO to react with SO.₂To be reduced. In this way NO_XSulfate BaSO in the catalyst 22₄SO stored in the form of_XThe amount of NO decreases gradually, at this time NO_XFrom catalyst 22 to SO_XIs SO₃It will not leak out.
[0039]
Thus, in an embodiment according to the present invention, NO in the form of sulfate._XSO stored in catalyst 22_XThis amount of SO_XThe amount of SO determined in advance_XWhen the allowable amount is exceeded, NO in the form of sulfate_XSO stored in catalyst 22_XTo reduce the amount of NO_XWhile maintaining the temperature of the catalyst 22 at, for example, 550 ° C. or higher, NO_XA rich spike is performed to slightly enrich the average air-fuel ratio of the exhaust gas flowing into the catalyst 22.
[0040]
Specifically, NO in the form of sulfate._XSO stored in catalyst 22_XThe amount of NO is per unit time_XSO flowing into the catalyst 22_XDepending on the integrated value of the amount of NO per unit time_XSO flowing into the catalyst 22_XThis amount depends on the amount of fuel injected from the fuel injection valve 6. Therefore, the integrated value QS obtained by integrating the total amount QF of the fuel injected from the fuel injection valve 6 within a predetermined time and the additional fuel is NO in the form of sulfate._XSO stored in catalyst 22_XRepresents the amount of. In the following, this integrated value QS is set as SO._XIt will be called a count value.
[0041]
This SO_XThe count value QS is equal to the SO described above._XSO equivalent to the allowable amount_XWhen it exceeds the allowable value QSA, NO in the form of sulfate._XSO stored in catalyst 22_XTo reduce the amount of NO_XRich spike is performed while the temperature of the catalyst 22 is raised to, for example, 550 ° C. and held. NO_XThere are various methods for raising the temperature of the catalyst 22, for example, NO._XAn electric heater is arranged at the upstream end of the catalyst 22, and NO is generated by the electric heater._XCatalyst 22 or NO_XA method of heating the exhaust gas flowing into the catalyst 22, NO_XNO is obtained by secondarily injecting fuel into the exhaust passage upstream of the catalyst 22 and burning the fuel._XNO2 by heating the catalyst 22 or increasing the temperature of exhaust gas discharged from the internal combustion engine._XThere is a method of increasing the temperature of the catalyst 22. Here, in order to increase the temperature of the exhaust gas discharged from the internal combustion engine, for example, the main fuel can be injected at a retarded angle, or in addition to the main fuel, it can be added during the expansion stroke or the exhaust stroke. Fuel can also be injected. In an embodiment according to the invention, NO fuel is injected by injecting additional fuel during the expansion stroke or during the exhaust stroke._XThe temperature of the catalyst 22 is raised to 550 ° C. or higher and maintained.
[0042]
On the other hand, fine particles mainly composed of carbon solid contained in the exhaust gas are collected on the particulate filter 22b. As described above, the internal combustion engine shown in FIG. 1 is continuously combusted under a lean air-fuel ratio, and NO_XSince the catalyst 22 has an oxidizing ability, if the temperature of the particulate filter 22b is maintained at a temperature at which particulates can be oxidized, for example, 250 ° C. or more, the particulates are oxidized and removed on the particulate filter 22b. The
[0043]
In this case, the above-mentioned NO_XNO of catalyst 22_XAccording to the accumulation and reduction mechanism of NO_XNO in the catalyst 22_XNO is also stored when_XActive oxygen is also generated when is released. This active oxygen is oxygen O₂Therefore, the particulates deposited on the particulate filter 22b are rapidly oxidized. That is, NO on the particulate filter 22b._XWhen the catalyst 22 is supported, the particulates deposited on the particulate filter 22b are oxidized regardless of whether the air-fuel ratio of the exhaust gas flowing into the particulate filter 22b is lean or rich. In this way, the fine particles are continuously oxidized.
[0044]
However, if the temperature of the particulate filter 22b is not maintained at a temperature that can oxidize the particulates or if the amount of particulates flowing into the particulate filter 22b per unit time becomes considerably large, the particulates that accumulate on the particulate filter 22b. Gradually increases, and the pressure loss of the particulate filter 22b increases.
[0045]
Therefore, in the embodiment according to the present invention, for example, when the amount of particulates deposited on the particulate filter 22b exceeds the particulate allowable amount, the particulate filter 22b while maintaining the air-fuel ratio AFN of the exhaust gas flowing into the particulate filter 22b lean. The fine particle oxidation action is performed to raise and maintain the temperature of, for example, 600 ° C. or higher. When this fine particle oxidation action is performed, the fine particles deposited on the particulate filter 22b are ignited and burned and removed. In the embodiment according to the present invention, when the engine back pressure detected by the pressure sensor 49 exceeds an allowable value, it is determined that the amount of accumulated particulates on the particulate filter 22b exceeds the allowable amount of fine particles.
[0046]
However, as I mentioned at the beginning, NO_XNO even if the average air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is kept lean_XWhen the temperature of the catalyst 22 increases, NO_XSO in the exhaust gas flowing out from the catalyst 22_XConcentration is NO_XSO in the exhaust gas flowing into the catalyst 22_XIt has been confirmed that the concentration is temporarily higher than the concentration. This is NO_XWhen the temperature of the catalyst 22 increases, NO_XSO stored in catalyst 22_XIs discharged and this SO_XNO without forming sulfate_XIt means that it is stored in the catalyst 22.
[0047]
This SO_XWhat is NO_XWhether it is stored in the catalyst 22 is not necessarily clear, but is considered to be stored as follows. That is, as described above, NO_XSO in the exhaust gas flowing into the catalyst 22₂Is first stored, for example, in the form of sulfate after depositing on the platinum Pt surface. NO in the form of sulfate_XSO stored in catalyst 22_XWhen the amount of SO is small, SO deposited on the platinum Pt surface₂Form sulfates relatively easily. However, SO stored in sulfate form_XAs the amount of SO increases, the SO adhering to the platinum Pt surface₂Becomes difficult to form sulfate, SO₂It continues to adhere on the platinum Pt surface. In this way SO_XCan be stored without forming sulfate. Like this, NO_XSO stored in the form of sulfate in the catalyst 22_XIf so, SO can be stored without forming sulfate._XThere will be.
[0048]
For example, the particulate oxidation action is started and NO_XWhen the temperature of the catalyst 22 is, for example, 300 ° C. or higher, the stored SO is formed without forming a sulfate.₂Is NO_XIt is discharged from the catalyst 22. NO at this time_XSince the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is maintained lean, this SO₂Is NO_XSulfate SO in the catalyst 22 or in the auxiliary catalyst 23₃There is a risk of oxidation.
[0049]
On the other hand, NO without forming sulfate in this way_XSO stored in catalyst 22_XIs NO_XWhen the air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 becomes rich, NO_XEven if the temperature of the catalyst 22 is relatively low, the SO₂NO in the form of_XIt is discharged from the catalyst 22. NO_XWhen the air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is rich, the air-fuel ratio of the exhaust gas flowing into the auxiliary catalyst 23 is also rich, and therefore NO._XSO discharged from the catalyst 22₂Is sulphate SO even in the auxiliary catalyst 23.₃Not oxidized until.
[0050]
Therefore, NO without forming sulfate_XSO stored in catalyst 22_XIf rich spikes are periodically performed to reduce the amount of sulfate SO released into the atmosphere₃The amount of can be reduced. However, it is not preferable to perform the rich spike only for this purpose from the viewpoint of the fuel consumption rate.
[0051]
Therefore, in an embodiment according to the present invention, NO_XWith rich spike for NO without forming sulfate_XSO stored in catalyst 22_XTry to reduce the amount of. That is, NO_XNO in exhaust gas when performing rich spike for_XNO in the catalyst 22_XNO_XAccumulated NO in catalyst 22_XFor example, the amount of reducing agent required to bring the amount to almost zero, for example, unburned HC and CO as well as NO without forming sulfate_XSO stored in catalyst 22_XNO is included so that the amount of reducing agent is included to reduce the amount of target to a target value, for example, approximately zero._XA rich spike for is done.
[0052]
In other words, NO_XRich spike for NO and NO without forming sulfate_XSO stored in catalyst 22_XIn other words, the rich spike for reducing the amount is performed in synchronization. Or NO_XRich spike for NO_XAccumulated NO in catalyst 22_XConsidering that it is performed according to the amount, NO without forming sulfate_XSO stored in catalyst 22_XRich spikes to reduce the amount of NO_XAccumulated NO in catalyst 22_XIt can be said that it is done according to the amount.
[0053]
NO_XRich spikes for are relatively frequent, so NO without forming sulfate_XSO stored in catalyst 22_XThe amount can be kept small. As a result, regardless of the timing of particulate oxidation of the particulate filter 22b, the sulfate SO discharged into the atmosphere at this time₃The amount of can be reduced. NO_XWhen the air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is rich, the air-fuel ratio of the exhaust gas flowing into the auxiliary catalyst 23 is also rich._XIs stored in the auxiliary catalyst 23 without forming sulfate, this SO_XIs NO_XWith a rich spike for SO₂Is discharged from the auxiliary catalyst 23.
[0054]
In this case, NO without forming sulfate_XSO stored in catalyst 22_XWhen the amount of NO is large, NO is not formed without the formation of sulfate compared to when it is small._XSO stored in catalyst 22_XThe amount of reducing agent required to reduce the amount of is increased.
[0055]
Thus, in an embodiment according to the present invention, NO is formed without forming sulfate._XSO stored in catalyst 22_XThis amount of SO_XWhen the amount of NO is large, it is NO compared to when it is small_XNO per unit time by rich spike for_XThe amount of reducing agent flowing into the catalyst 22 is increased.
[0056]
NO without forming sulfate_XSO stored in catalyst 22_XIt is difficult to directly determine the amount. However, NO in the form of sulfate_XSO stored in catalyst 22_XRepresents the amount of SO_XWhen the count value QS is small, the sulfate is easily formed as described above, so the SO that can be stored without forming the sulfate._XThe amount of SO_XAs the count value QS increases, it is difficult to form sulfate, so SO that can be stored without forming sulfate._XThe amount of
[0057]
Then, SO_XThe count value QS is NO in the form of sulfate._XSO stored in catalyst 22_XNot only represents the amount of NO, but also NO without forming sulfate_XSO stored in catalyst 22_XIt means that the amount of. In addition, SO_XCount value QS is NO_XSO flowing into the catalyst 22_XConsidering that it is required based on the amount of NO, NO without forming sulfate_XSO stored in catalyst 22_XNo amount of NO_XSO flowing into the catalyst 22_XIt will be determined based on the amount of.
[0058]
In addition, in the first embodiment according to the present invention, as shown in FIG._XWhen the count value QS is large, the NO is smaller than when the count value QS is small._XTherefore, the rich air-fuel ratio AFR in the rich spike is reduced. In this way, SO_XWhen the count value QS increases, NO per unit time_XThe amount of reducing agent flowing into the catalyst 22 increases.
[0059]
FIG. 5 shows a rich spike control routine of an embodiment according to the present invention. This routine is executed by interruption every predetermined processing cycle time Δt. Referring to FIG. 5, first, in step 100, the total value QF of the fuel amount injected from the fuel injection valve 6 from the previous processing cycle to the current processing cycle is calculated as SO_XIt is added to the count value QS (QS = QS + QF). In the following step 101, SO_XCount value QS is SO_XIt is determined whether or not it is larger than the allowable value QSA. When QS ≦ QSA, the process proceeds to step 102 and NO, which will be described later._XProcessing is performed. On the other hand, when QS> QSA, the routine proceeds to step 103 where NO in the form of sulfate._XSO stored in catalyst 22_XTo reduce the amount of NO_XFor example, the rich spike is performed for a certain time while the temperature of the catalyst 22 is maintained at, for example, 550 ° C. or more. In the following step 104, SO_XCount value QS and NO_XEach count value QN is cleared.
[0060]
FIG. 6 shows NO according to the first embodiment described above._XA processing routine is shown. This routine is executed in step 102 of FIG. Referring to FIG. 6, first, at step 120, the exhaust NO._XThe quantity dQN is calculated from the map of FIG. In the following step 121, NO_XDQN · Δt is added to the count value QN (QN = QN + dQN · Δt). In the following step 122, NO_XCount value QN is NO_XIt is determined whether or not it is larger than the allowable value QNA. When QN ≦ QNA, the processing cycle is terminated. When QN> QNA, the routine proceeds to step 123, where the rich air-fuel ratio AFR is calculated from the map of FIG. In the following step 124, NO_XA rich spike for is done. In this case, NO_XWith a time interval INT represented by the tolerance QNA, NO_XThe air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is switched to the rich air-fuel ratio AFR for the rich holding time tR. In this embodiment, the rich retention time tR and NO_XTolerance QNA is SO_XCount value SO_XIt is a constant value not depending on. In the following step 125, NO_XThe count value QN is cleared.
[0061]
On the other hand, in the second embodiment according to the present invention, as shown in FIG._XWhen the count value QS is large, the NO is smaller than when the count value QS is small._XFor this reason, the rich holding time tR in the rich spike is increased. Even in this way, SO_XWhen the count value QS increases, NO_XNO per unit time by rich spike for_XThe amount of reducing agent flowing into the catalyst 22 increases.
[0062]
FIG. 8 shows the NO according to the second embodiment described above._XA processing routine is shown. This routine is executed in step 102 of FIG. Referring to FIG. 8, first, in step 130, NO per unit time._XNO flowing into the catalyst 22_XThe amount dQN is calculated from the map of FIG. In the following step 131, NO_XDQN · Δt is added to the count value QN (QN = QN + dQN · Δt). In the following step 132, NO_XCount value QN is NO_XIt is determined whether or not it is larger than the allowable value QNA. When QN ≦ QNA, the processing cycle is terminated, and when QN> QNA, the routine proceeds to step 133, where the rich retention time tR is calculated from the map of FIG. In the following step 134, NO_XA rich spike for is done. In this case, NO_XWith a time interval INT represented by the tolerance QNA, NO_XThe air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is switched to the rich air-fuel ratio AFR for the rich holding time tR. In this embodiment, the rich air-fuel ratio AFR and NO_XTolerance QNA is SO_XCount value SO_XIt is a constant value not depending on. In the following step 135, NO_XThe count value QN is cleared.
[0063]
Furthermore, in a third embodiment according to the present invention, SO_XWhen the count value QS is large, the NO is smaller than when the count value QS is small._XThe allowable value QNA should be reduced, ie NO_XThe time interval INT of the rich spike for the purpose is shortened. In this case as well, SO_XWhen the count value QS increases, NO_XNO per unit time by rich spike for_XThe amount of reducing agent flowing into the catalyst 22 increases.
[0064]
FIG. 10 shows NO according to the third embodiment described above._XA processing routine is shown. This routine is executed in step 102 of FIG. Referring to FIG. 10, first in step 140, NO per unit time._XNO flowing into the catalyst 22_XThe amount dQN is calculated from the map of FIG. In the following step 141, NO_XDQN · Δt is added to the count value QN (QN = QN + dQN · Δt). In the following step 142, NO from the map of FIG._XAn allowable value QNA is calculated. In the following step 143, NO_XCount value QN is NO_XIt is determined whether or not it is larger than the allowable value QNA. When QN ≦ QNA, the processing cycle is terminated. When QN> QNA, the process proceeds to step 144._XA rich spike for is done. In this case, NO_XWith a time interval INT represented by the tolerance QNA, NO_XThe air-fuel ratio AFN of the exhaust gas flowing into the catalyst 22 is switched to the rich air-fuel ratio AFR for the rich holding time tR. In this embodiment, the rich air-fuel ratio AFR and the rich holding time tR are SO._XCount value SO_XIt is a constant value not depending on. In the following step 145, NO_XThe count value QN is cleared.
[0065]
FIG. 11 shows NO_XNO when the rich spike for_XFIG. 11A shows the case of the first embodiment of the present invention, and FIG. 11B shows the case of the second embodiment of the present invention. FIG. 11C shows the case of the third embodiment according to the present invention. In FIG. 11, the solid line is SO._XA broken line indicates a case where the count value QS is large, and a broken line indicates a case where the QS is small.
[0066]
In each of the embodiments according to the present invention described so far, NO_XThe catalyst 22 is supported on the particulate filter 22b. However, NO_XThe catalyst 22 may be formed separately from the particulate filter 22b and disposed in the exhaust passage downstream of the particulate filter 22b. Further, as in each of the embodiments according to the present invention described above, by controlling the air-fuel ratio and temperature of the exhaust gas discharged from the internal combustion engine, NO._XIf the air-fuel ratio and temperature of the exhaust gas flowing into the catalyst 22 or the particulate filter 22b are controlled, NO_XThe catalyst 22 may be disposed in the exhaust passage upstream of the particulate filter 22b.
[0067]
【The invention's effect】
The amount of sulfate discharged into the atmosphere can be reduced.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
[Figure 2] Emission NO_XIt is a diagram which shows quantity dQN.
FIG. 3 is a diagram for explaining a rich spike.
FIG. 4 is a diagram showing a rich air-fuel ratio AFR.
FIG. 5 is a flowchart showing a rich spike control routine of an embodiment according to the present invention.
FIG. 6 shows NO of the first embodiment according to the present invention._XIt is a flowchart which shows a process routine.
FIG. 7 is a diagram showing a rich retention time tR.
FIG. 8 shows NO of the second embodiment according to the present invention._XIt is a flowchart which shows a process routine.
FIG. 9 NO_XIt is a diagram which shows tolerance value QNA.
FIG. 10 shows NO of the third embodiment according to the present invention._XIt is a flowchart which shows a process routine.
FIG. 11: NO_XNO when the rich spike for_XIt is a figure which shows the air fuel ratio of the inflow exhaust gas into a catalyst.
[Explanation of symbols]
1 ... Engine body
20a, 20b ... exhaust pipe
22 ... NO_Xcatalyst
22a ... Particulate filter
23 ... Auxiliary catalyst

Claims (7)

In the exhaust passage of the internal combustion engine in which combustion is continuously performed under a lean air-fuel ratio, NO _X in the exhaust gas flowing in when the air-fuel ratio of the inflowing exhaust gas is lean is stored, and the inflowing exhaust gas A NO _X catalyst that reduces the amount of NO _X stored by reducing the stored NO _X when the reducing agent is contained in the exhaust gas when the air-fuel ratio is reduced is disposed in the NO _X catalyst. to reduce the amount of the NO _X that the reduction of of the NO _X is stored in the NO _X catalyst, rich spike to switch temporarily rich air-fuel ratio of the exhaust gas flowing into the NO _X catalyst in the exhaust purification system of an internal combustion engine to perform, determine the amount of sulfur that is accumulated in the NO _X catalyst without forming sulfates, accumulated in the NO _X catalyst without forming the sulfate When there is a lot of sulfur Than when less, NO _X and NO _X in the catalyst was reduced internal combustion engine so as to reduce the rich air-fuel ratio in the rich spike for reducing the amount of the NO _X that is stored in the NO _X catalyst Exhaust purification equipment. In the exhaust passage of the internal combustion engine in which combustion is continuously performed under a lean air-fuel ratio, NO _X in the exhaust gas flowing in when the air-fuel ratio of the inflowing exhaust gas is lean is stored, and the inflowing exhaust gas A NO _X catalyst that reduces the amount of NO _X stored by reducing the stored NO _X when the reducing agent is contained in the exhaust gas when the air-fuel ratio is reduced is disposed in the NO _X catalyst. to reduce the amount of the NO _X that the reduction of of the NO _X is stored in the NO _X catalyst, rich spike switching the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst rich air-fuel ratio by the rich retention time in the exhaust purification system of an internal combustion engine to perform the, determine the amount of sulfur that is accumulated in the NO _X catalyst without forming sulfates, stored in the NO _X catalyst without forming the sulfate The amount of sulfur Than when smaller when large and so as to lengthen the rich retention time in the rich spike for reducing the NO _X in the NO _X catalyst reduces the amount of the NO _X that is stored in the NO _X catalyst combustion Engine exhaust purification system. In the exhaust passage of the internal combustion engine in which combustion is continuously performed under a lean air-fuel ratio, NO _X in the exhaust gas flowing in when the air-fuel ratio of the inflowing exhaust gas is lean is stored, and the inflowing exhaust gas A NO _X catalyst that reduces the amount of NO _X stored by reducing the stored NO _X when the reducing agent is contained in the exhaust gas when the air-fuel ratio is reduced is disposed in the NO _X catalyst. to reduce the amount of the NO _X that the reduction of of the NO _X is stored in the NO _X catalyst, rich spike to switch temporarily rich air-fuel ratio of the exhaust gas flowing into the NO _X catalyst in the exhaust purification system of an internal combustion engine to perform, determine the amount of sulfur that is accumulated in the NO _X catalyst without forming sulfates, accumulated in the NO _X catalyst without forming the sulfate When there is a lot of sulfur Than when less, reducing the NO _X in the NO _X catalyst of an internal combustion engine so as to shorten the time interval of rich spike for reducing the amount of the NO _X that is stored in the NO _X catalyst Exhaust purification device. Determine the amount of sulfur flowing into the NO _X catalyst, to determine the amount of sulfur that is accumulated in the NO _X catalyst without forming the amount the sulfate based on the sulfur flowing into the NO _X catalyst The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3. Determine the amount of the NO _X that is stored in the NO _X catalyst, wherein when the amount of the NO _X becomes more than the allowable amount of predetermined, NO _X and NO _X in the catalyst is reduced the NO _X catalyst an exhaust purification system of an internal combustion engine according to any one of claims 1 to perform the amount rich spike for reducing the are stored NO _X to 3 within. The NO _X catalyst, exhaust gas control apparatus according to any one of the particulates in the exhaust gas flowing from claim 1, which is carried on a particulate filter for trapping to 3. An exhaust purification system of an internal combustion engine according to any one of up to 3 claim 1 in which the auxiliary catalyst is arranged having oxidizing ability in the exhaust passage of the NO _X catalyst downstream.

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