JP2004211676A - Exhaust gas cleaner for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaner having a smaller outflow amount of N<SB>2</SB>O. <P>SOLUTION: The exhaust gas cleaner comprises an exhaust gas purifying catalyst 23 arranged on an exhaust passage in the internal combustion engine, a N<SB>2</SB>O outflow amount estimating means for estimating the outflow amount of N<SB>2</SB>O from the exhaust gas purifying catalyst, and a N<SB>2</SB>O outflow suppressing means for reducing the outflow amount of N<SB>2</SB>O. When the outflow amount of N<SB>2</SB>O estimated by the N<SB>2</SB>O outflow amount estimating means is a preset amount or larger, the outflow amount of N<SB>2</SB>O from the exhaust gas purifying catalyst is reduced by the N<SB>2</SB>O outflow suppressing means. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification device for an internal combustion engine.
[0002]
[Prior art]
NO in exhaust gas of internal combustion engine_xAnd this NO_xConventionally, various exhaust purification catalysts have been developed in order to remove the exhaust gas. Many of such exhaust gas purifying catalysts are mounted downstream of the exhaust gas of an internal combustion engine and include NO contained in exhaust gas flowing into the exhaust gas purifying catalyst._xBy reducing NO_x(For example, Patent Document 1).
[0003]
[Patent Document 1]
JP-A-6-212961
[0004]
[Problems to be solved by the invention]
However, with the exhaust purification catalyst as described above, NO_xIs reduced to N₂And O₂Not only N₂O may occur. This N₂O is NO_xIt is necessary to suppress release into the atmosphere in the same manner as described above.
[0005]
Therefore, the object of the present invention is N₂An object of the present invention is to provide an exhaust gas purification device with a small amount of O outflow.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, according to a first aspect, an exhaust purification catalyst disposed on an exhaust passage of an internal combustion engine and N₂N for estimating O outflow₂O outflow amount estimation means and the N₂N to reduce O outflow₂O outflow suppressing means,₂O N estimated by the outflow estimation means₂If the outflow amount of O is equal to or more than a predetermined amount, the N₂N outflow from the exhaust purification catalyst by the O outflow suppression means₂The O outflow was reduced.
According to the first invention, the N in the exhaust gas flowing downstream from the exhaust purification catalyst to the exhaust gas₂N which is the amount of O₂The O outflow amount is always suppressed to a predetermined amount or less. In particular, if the above-mentioned predetermined amount is set to almost zero, almost no N₂O is not released.
[0007]
According to a second aspect, in the first aspect, the N₂The O outflow amount estimating means is configured to determine whether or not the exhaust gas purifying catalyst is based on at least one of the temperature of the exhaust gas purifying catalyst, the oxygen concentration in the exhaust gas flowing into the exhaust gas purifying catalyst, and the degree of deterioration of the exhaust gas purifying catalyst. N₂Estimate O outflow.
When the temperature of the exhaust purification catalyst is high, N₂When the outflow of O is small and the catalyst temperature is low, N₂Assuming that the amount of O outflow is large, and when the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst is high, N₂When the O outflow is large and the oxygen concentration is low, N₂Assuming that the amount of O outflow is small, and when the degree of deterioration of the exhaust purification catalyst is high, N₂O When the amount of outflow is large and the degree of deterioration is low, N₂O₂The O outflow is estimated. Therefore, if the catalyst temperature increases, the oxygen concentration increases, or if the degree of catalyst deterioration increases, N₂The outflow amount of O may exceed a predetermined amount.₂Decrease O outflow.
Further, the degree of deterioration of the exhaust purification catalyst refers to the degree to which the activity of a catalyst such as platinum carried on the exhaust purification catalyst has decreased (particularly, the degree of reduction in oxidizing ability), or to the degree of deterioration held by the exhaust purification catalyst. Oxygen, NO_xThis means the degree of reduction in the purification performance of the catalyst which does not recover even if the processing for desorbing the sulfur component or the processing for oxidizing and removing the fine particles adhering to the exhaust purification catalyst is executed.
[0008]
According to a third aspect, in the first or second aspect, the above N₂The O outflow suppressing means sets the temperature of the exhaust purification catalyst to N₂The temperature is raised to a temperature or higher at which the O outflow amount becomes smaller than the predetermined amount.
[0009]
In a fourth aspect based on the first aspect, the exhaust purification catalyst is characterized in that when the air-fuel ratio of the inflowing exhaust gas is lean, NO in the exhaust gas is reduced._xAnd when the air-fuel ratio of the inflowing exhaust gas is substantially the stoichiometric air-fuel ratio or rich,_xNO to leave_xA catalyst, the NO_xThe air-fuel ratio of the exhaust gas flowing into the catalyst is set to substantially the stoichiometric air-fuel ratio or_xNO held by catalyst_xNO to leave_xNO for executing withdrawal processing_xThe apparatus further includes a detaching unit.
According to the fourth invention, NO_xWhen the air-fuel ratio of the exhaust gas into which the catalyst flows is lean, NO in the exhaust gas_xIn this case, NO_xN from catalyst₂The O outflow is almost zero. Conversely, NO_xThe catalyst holds NO when the air-fuel ratio of the inflowing exhaust gas is almost the stoichiometric air-fuel ratio or rich._xNO to remove_xIn an internal combustion engine that is almost lean except when the departure process is executed, NO_xNO only when withdrawal process is executed_xNO depending on conditions such as catalyst temperature_xN from catalyst₂O is let out.
[0010]
In a fifth aspect, in the fourth aspect, the above N₂The O outflow amount estimating means calculates the temperature of the exhaust purification catalyst, the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst, the degree of deterioration of the exhaust purification catalyst,_xNO retained in the catalyst_xQuantity and NO_xNO based on at least one of the sulfur component amounts held in the catalyst_xN from catalyst₂Estimate O outflow.
Note that NO_xNO retained in the catalyst_xN when the amount is small₂Low O outflow and NO_xN when the amount is large₂O outflow increases, and NO_xWhen the amount of sulfur component retained in the catalyst is small, N₂When the O outflow is small and the sulfur content is large, N₂O As the amount of outflow increases, N₂The O outflow is estimated. Therefore, NO_xNO retained in the catalyst_xIf the amount is large, and NO_xWhen the amount of sulfur component retained in the catalyst is large, N₂The outflow amount of O may exceed a predetermined amount.₂Decrease O outflow.
[0011]
In a sixth aspect, in the fourth or the fifth aspect, the above N₂The O outflow amount estimating means is NO_xNO before executing withdrawal process_xNO when the withdrawal process is executed_xN from catalyst₂O The amount of outflow is estimated, and the N₂The O outflow suppressing means uses the estimated N₂When the O outflow amount is equal to or more than the predetermined amount, N₂NO until the outflow amount of O is estimated to be smaller than the predetermined amount._xProhibit withdrawal processing.
According to the sixth invention, NO_xNO by separation means_xNO before executing withdrawal process_xN when the withdrawal process is executed₂O outflow is estimated and estimated N₂NO based on O outflow_xSince it is determined whether or not to perform the withdrawal process, NO_xEven if the withdrawal process is executed, N₂O outflow does not exceed a predetermined amount. According to the sixth aspect, the estimated N₂When the O outflow amount is equal to or more than the predetermined amount, for example, NO_xNO if the withdrawal process is executed_xNO from catalyst_xEven if it is possible to₂NO until the outflow amount of O becomes smaller than the predetermined amount_xSince the withdrawal process is not performed, NO in the present invention_xThe execution timing of the withdrawal process is N₂NO ignoring O outflow_xNO of catalyst_xNO according to the holding amount etc._xNO when executing withdrawal processing_xThis is different from the execution timing of the withdrawal process.
[0012]
According to a seventh aspect, in the sixth aspect, the N₂The O outflow suppressing means further includes the estimated N₂NO to a temperature at which the amount of O outflow is smaller than the predetermined amount_xAfter raising the temperature of the catalyst, the NO_xNO by separation means_xExecute withdrawal processing.
[0013]
In an eighth aspect based on the seventh aspect, the NO._xWhen the temperature of the catalyst rises, the NO_xThis is carried out by a reducing agent mixing means for causing a reducing agent to be contained in the exhaust gas flowing into the catalyst._xNO until the withdrawal process is executed_xThe reducing agent is contained so that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes substantially the stoichiometric air-fuel ratio or lean.
According to the eighth invention, NO_xUntil the withdrawal process is executed, that is, the estimated N₂NO to a temperature at which the amount of O outflow is smaller than the predetermined amount_xNO until the catalyst is heated_xThe air-fuel ratio of the exhaust gas flowing into the catalyst is not made rich, and therefore NO_xNO retained in the catalyst_xWill not be removed. On the other hand, NO_xSince the reducing agent is mixed into the exhaust gas flowing into the catalyst, this reducing agent_xOn the exhaust passage upstream of the catalyst or NO_xGenerates heat by reacting with catalyst etc., NO_xThe temperature of the catalyst is raised. Thus, according to the eighth aspect, NO_xNO from catalyst_x, N₂NO without leaving O_xThe temperature of the catalyst can be raised.
In the eighth aspect, NO_xAlthough the reducing agent is contained such that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes substantially the stoichiometric air-fuel ratio or lean, it is preferable that the reducing agent be contained such that the air-fuel ratio of the exhaust gas becomes lean. This is NO_xEven if the air-fuel ratio of the exhaust gas flowing into the catalyst is almost the stoichiometric air-fuel ratio, a slight NO_xAnd N₂This is because O may be released.
The reducing agent mixing means includes at least one or both of a reducing agent adding device and an engine exhaust air-fuel ratio lowering means described later. Furthermore, examples of the reducing agent include fuel, hydrocarbons, carbon monoxide, hydrogen, etc., but can reduce the oxygen concentration in the exhaust gas and reduce NO_xNO released from the catalyst_xAny reducing agent may be used as long as it can reduce. Hereinafter, “reducing agent” in the present specification means such a reducing agent.
[0014]
In a ninth invention, in any one of the fourth to eighth inventions, NO_xNo release means_xA reducing agent mixing means for causing a reducing agent to be contained in the exhaust gas flowing into the catalyst;₂The O outflow suppressing means includes NO as a reducing agent contained in the exhaust gas._xN from catalyst₂The reducing agent mixing means selects such a reducing agent that the O outflow amount is smaller than the predetermined amount.
NO_xN from catalyst₂NO outflow is NO_xIt differs depending on the type of reducing agent used in the desorption treatment, and the ease of use differs depending on the type of reducing agent (for example, CO is less likely to be generated than HC). According to the ninth aspect, when each reducing agent is used, N₂The optimum reducing agent is selected in consideration of the outflow amount of O, the ease of use of the reducing agent, and the like.
[0015]
According to a tenth aspect, in any one of the fourth to seventh aspects, the NO_xA reducing agent adding device for adding a reducing agent to the exhaust gas passing through the engine exhaust passage upstream of the catalyst, and an engine exhaust air-fuel ratio for reducing an air-fuel ratio of the engine exhaust gas discharged from the internal combustion engine body to a predetermined range Means for reducing₂The O outflow suppressing means adds NO from the reducing agent adding device to the engine exhaust gas whose air-fuel ratio has been lowered by the engine exhaust air-fuel ratio reducing means._xPerformed withdrawal processing.
Generally, in an internal combustion engine such as a diesel internal combustion engine, in which the air-fuel ratio of the engine exhaust gas during normal operation (hereinafter, referred to as "engine exhaust air-fuel ratio") is almost lean, the engine exhaust air-fuel ratio is almost stoichiometric or rich. If this is attempted, the stability (drivability) of the operation of the internal combustion engine will be seriously deteriorated. On the other hand, NO by reducing the engine exhaust air-fuel ratio_xWhen the air-fuel ratio of the exhaust gas flowing into the catalyst is reduced, the oxygen concentration of the exhaust gas flowing into the catalyst can be reduced as compared with the case where the reducing agent is added from the reducing agent addition device, and the reducing agent having higher reactivity is obtained. And, in some cases, NO_xThe volume flow of exhaust gas through the catalyst can be reduced,₂O generation can be suppressed. Therefore, in the tenth invention, NO_xIn performing the departure process, the engine exhaust air-fuel ratio is reduced, and a reducing agent is further added from a reducing agent adding device.₂O generation can be suppressed.
In addition, "engine exhaust gas" means exhaust gas discharged from the combustion chamber of the internal combustion engine. The reduction of the engine exhaust air-fuel ratio by the engine exhaust air-fuel ratio reducing means is performed by, for example, the low-temperature combustion control, the post-injection control, the intake throttle control, and the like described in the embodiment.
[0016]
In an eleventh aspect based on the tenth aspect, the apparatus further comprises an exhaust gas recirculation passage for returning exhaust gas from the engine exhaust gas passage to the engine intake passage, wherein the exhaust gas recirculation passage includes an exhaust gas passing through the exhaust gas recirculation passage. A recirculation gas cooling device for cooling the recirculation gas cooling device, and a cooling device bypass passage for bypassing the recirculation gas cooling device are provided. The air-fuel ratio of the engine exhaust gas is reduced while returning the exhaust gas to the engine intake passage through the passage.
According to the eleventh aspect, since the exhaust gas returned to the engine exhaust passage (hereinafter, referred to as “recirculated gas”) does not pass through the recirculated gas cooling device, its temperature is high. For this reason, since high-temperature recirculated gas is mixed into the intake gas, the temperature of the intake gas flowing into the combustion chamber of the internal combustion engine is high, and the temperature of the exhaust gas discharged from the combustion chamber of the internal combustion engine after combustion is also high. . Therefore, when the engine exhaust air-fuel ratio is reduced by the engine exhaust air-fuel ratio lowering means, the temperature of the engine exhaust gas is also high at the same time._xHigh-temperature exhaust gas flows into the catalyst, and NO_xThe temperature of the catalyst is raised early.
[0017]
In the twelfth aspect, when the air-fuel ratio of the inflowing exhaust gas is lean, the NO_xAnd when the air-fuel ratio of the inflowing exhaust gas is substantially the stoichiometric air-fuel ratio or rich,_xNO to leave_xA catalyst is provided on the engine exhaust passage and the NO._xA reducing agent adding device for adding a reducing agent to the exhaust gas passing through the engine exhaust passage upstream of the catalyst, and an engine exhaust air-fuel ratio for reducing an air-fuel ratio of the engine exhaust gas discharged from the internal combustion engine body to a predetermined range In the exhaust gas purifying apparatus for an internal combustion engine having the reducing means, the reducing agent is added to the engine exhaust gas whose air-fuel ratio has been reduced by the engine exhaust air-fuel ratio reducing means, by adding a reducing agent from the reducing agent adding device._xThe air-fuel ratio of the exhaust gas flowing into the catalyst is made substantially stoichiometric air-fuel ratio or rich.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an exhaust emission control device of the present invention will be described with reference to the drawings. FIG. 1 shows a direct injection compression ignition type diesel internal combustion engine equipped with the exhaust gas purifying apparatus of the present invention. The exhaust gas purification device used in the present invention can be mounted on a spark ignition type internal combustion engine.
[0019]
Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, 9 Denotes an exhaust valve, and 10 denotes an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13.
[0020]
A throttle valve 17 driven by a throttle valve driving 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. You. In the internal combustion engine shown in FIG. 1, 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 an exhaust turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the exhaust turbine 21 is connected to a casing 24 containing an exhaust purification catalyst 23 via an exhaust pipe 22. Be linked. A temperature sensor 25 for detecting the temperature of the exhaust purification catalyst 23 is attached to the exhaust purification catalyst 23.
[0021]
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 26, and an electrically controlled EGR control valve 27 is disposed in the EGR passage 26. An EGR gas cooling device 28 for cooling the EGR gas flowing in the EGR passage 26 is disposed around the EGR passage 26. In the internal combustion engine shown in FIG. 1, engine cooling water is guided into the EGR gas cooling device 28, and the EGR gas is cooled by the engine cooling water.
[0022]
On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 29, via a fuel supply pipe 6a. Fuel is supplied into the common rail 29 from a fuel pump 30 of an electrically controlled variable discharge amount, and the fuel supplied into the common rail 29 is supplied to the fuel injection valve 6 through each fuel supply pipe 6a. A fuel pressure sensor 31 for detecting the fuel pressure in the common rail 29 is attached to the common rail 29, and the fuel pump 30 is controlled so that the fuel pressure in the common rail 29 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 31. Is controlled.
[0023]
The electronic control unit (ECU) 40 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 42, a RAM (Random Access Memory) 43, a CPU (Microprocessor) 44, an input port 45, An output port 46 is provided. Output signals of the temperature sensor 25 and the fuel pressure sensor 31 are input to the input port 45 via the corresponding AD converter 47.
[0024]
A load sensor 50 that generates an output voltage proportional to the amount of depression of the accelerator pedal 49 is connected to the accelerator pedal 49, and the output voltage of the load sensor 50 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 51 that generates an output pulse every time the crankshaft rotates, for example, by 30 °. 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 27, and the fuel pump 30 via the corresponding drive circuit 48.
[0025]
By the way, the exhaust gas purifying catalyst as described above has NO_xTo N₂And O₂To be reduced to Further, depending on the values of various factors such as the temperature of the exhaust purification catalyst, NO_xTo N₂Reduce to O. Therefore, the exhaust gas discharged through the exhaust gas purifying catalyst as described above and discharged into the atmosphere contains N 2₂And O₂Plus N₂O may be included. This N₂O is NO_xIt is necessary to suppress release into the atmosphere in the same manner as described above. That is, N flowing out of the exhaust purification catalyst to the exhaust downstream₂N which is the amount of O₂It is necessary to keep O outflow low. However, conventionally, the exhaust purification catalyst has₂The conditions under which O flows out are not accurately grasped, and therefore N₂The outflow of O could not be kept low. Therefore, from the conventional exhaust gas purification apparatus, N₂Exhaust gas containing a large amount of O was sometimes released into the atmosphere.
[0026]
On the other hand, the exhaust gas purification apparatus according to the first embodiment of the present invention includes N 2₂O amount or N in the exhaust gas currently flowing downstream of the exhaust purification catalyst 23₂O amount (hereinafter simply referred to as “N₂N) for estimating₂O outflow amount estimating means and N in the exhaust gas flowing downstream of the exhaust gas purifying catalyst 23.₂O amount (hereinafter “N₂O outflow))₂O outflow suppressing means. And this N₂O N estimated by the outflow estimation means₂If the O outflow is equal to or greater than the predetermined outflow, N₂The N outflow from the exhaust purification catalyst 23 is controlled by the O outflow suppressing means.₂The O outflow is reduced. In particular, in the case described above, N₂By operating the O outflow suppressing means, N from the exhaust purification catalyst 23 is reduced.₂It is preferable to make the O outflow almost zero.
[0027]
Therefore, according to the first embodiment of the present invention, N₂O Outflow is high or high N₂When estimated by the O outflow amount estimating means, N₂The outflow of O is kept low or nearly zero. Therefore, the N 2 in the exhaust gas discharged to the atmosphere after passing through the exhaust gas purification device of the present invention₂The amount of O can be kept low.
[0028]
Note that N₂The predetermined outflow amount relating to the O outflow amount is determined by changing the state of the internal combustion engine and the exhaust purification catalyst 23 to N from the exhaust purification catalyst 23.₂N from the exhaust gas purification catalyst 23 when the O outflow amount is reduced.₂O equal to the outflow or such N₂The amount is slightly larger than the O outflow amount. Alternatively, the predetermined outflow amount may be substantially zero. According to the exhaust gas purification apparatus of the present embodiment, N₂Since the O outflow amount is always maintained at or below the predetermined outflow amount, by setting the predetermined outflow amount in this manner, N₂O can hardly flow out.
[0029]
Next, N from the exhaust gas purification catalyst 23 in the exhaust gas purification device of the first embodiment.₂N to keep O outflow small₂The O outflow suppression control will be described. First, the N contained in the exhaust gas flowing downstream of the exhaust gas of the exhaust purification catalyst 23₂O outflow Q_n2oTo N₂It is estimated by the O outflow amount estimating means. In this case, N₂O N estimated by the outflow estimation means₂O outflow Q_n2oIs N flowing out of the exhaust purification catalyst 23 at the time of estimation.₂It may be the amount of O, or N which is estimated to flow out of the exhaust purification catalyst 23 after a unit time.₂The amount of O may be used. And N₂N estimated by O outflow estimation means₂O outflow Q_n2oIs the prescribed outflow Q_n2oaWith the above, N flowing out of the exhaust purification catalyst 23 to the exhaust gas downstream₂N to keep the amount of O small₂O outflow suppression processing is N₂This is executed by the O outflow suppressing means.
[0030]
Next, the exhaust emission control device of the first embodiment of the present invention will be described in more detail. In the first embodiment of the present invention, the ratio of air and fuel supplied to the exhaust passage, the combustion chamber 5, and the intake passage on the upstream side of the exhaust purification catalyst is referred to as the air-fuel ratio of the exhaust gas (or referred to as the exhaust air-fuel ratio). When the air-fuel ratio of the inflowing exhaust gas is lean, NO in the exhaust gas_xIs held when the air-fuel ratio of the inflowing exhaust gas is almost stoichiometric or rich, more specifically when the oxygen concentration in the inflowing exhaust gas is low._xNO to leave_xThe catalyst 23 is used as an exhaust purification catalyst. NO_xNO from catalyst 23_xIf a reducing agent such as fuel is present in the exhaust gas when_xNO released from the catalyst 23_xIs reduced.
[0031]
Such NO_xIn the catalyst 23, the NO in the exhaust gas flowing in_xCannot be held indefinitely and therefore NO_xNO on catalyst 23_xIs held above a certain amount, NO_xWhen the exhaust air-fuel ratio flowing into the catalyst 23 (hereinafter referred to as “inflow exhaust air-fuel ratio”) is substantially stoichiometric or rich, NO_xNO held by catalyst 23_xNO to leave_xNO for executing withdrawal processing_xNO by forcible means_xNO from catalyst 23_xWithdraw. In particular, as in this embodiment, in a diesel internal combustion engine, the exhaust air-fuel ratio (hereinafter, referred to as “engine exhaust air-fuel ratio”) exhausted from the engine body is often lean, and therefore NO_xNO only when separation processing is performed_xIn many cases, the air-fuel ratio of the exhaust gas flowing into the catalyst 23 becomes substantially the stoichiometric air-fuel ratio or rich. Therefore, NO_xNO when the withdrawal process is not executed_xNO in exhaust gas flowing into the catalyst_xIs almost NO_xNO because it is held by the catalyst_xN from catalyst₂O hardly escapes. On the other hand, NO_xNO when performing the withdrawal process_xNO from catalyst_xIs removed and its NO_xIs reduced. Therefore, in the exhaust gas purification apparatus of the present embodiment, N₂O is generated and NO_xIt is basically NO that flows out of the catalyst 23 downstream of the exhaust gas._xThis is only in the case of performing withdrawal processing.
[0032]
Therefore, in the exhaust gas purification apparatus according to the first embodiment of the present invention, NO_xNO even when the withdrawal process should be performed_xNO when withdrawal process is performed_xN from catalyst 23₂If the O outflow exceeds the predetermined outflow, N₂NO when estimated by the O outflow amount estimation means_xNO even after leaving process_xCatalyst 23 to N₂NO until O almost disappears_xProhibit execution of the withdrawal process. Alternatively, in the above case, NO_xN before the removal process₂Activate the O outflow suppression means and set NO_xNO even after leaving process_xCatalyst 23 to N₂NO after making almost no outflow of O_xExecute withdrawal processing. By doing so, NO_xNO during the withdrawal process_xCatalyst 23 to N₂O is prevented from leaking out, so that NO_xCatalyst 23 to N₂O is prevented from flowing out.
[0033]
By the way, NO_xIn the catalyst, NO_xN from catalyst₂O outflow is NO_xIt changes depending on the temperature of the catalyst. More specifically, as shown in FIG._xNO when catalyst temperature rises_xN from catalyst₂O outflow decreases, NO_xNO when catalyst temperature drops_xN from catalyst₂O outflow increases.
[0034]
Therefore, in the exhaust gas purification apparatus of the present embodiment, N₂If the temperature sensor 25 is used as the O_xTemperature T of catalyst 23_catIs detected. On the other hand, as shown in FIG._xN when performing withdrawal processing₂NO such that the O outflow becomes a predetermined outflow_xThe temperature of the catalyst 23 is set to a predetermined temperature T_aAsking. In this case, the NO detected by the temperature sensor 25_xWhen the temperature of the catalyst 23 is a predetermined temperature T_aHigher than N₂O outflow is the specified outflow Q_n2oaLess than the detected NO_xWhen the temperature of the catalyst 23 is a predetermined temperature T_aN if₂O outflow is the specified outflow Q_n2oaThis indicates that this is the case. Therefore, NO_xNO detected by the temperature sensor 25 before performing the separation process_xTemperature T of catalyst 23_catIs the predetermined temperature T_aNO if_xWhen the temperature of the catalyst 23 reaches a predetermined temperature T_aNO until higher than_xDo not perform withdrawal processing. Alternatively, in the above case, N₂O outflow suppression means is N₂NO by executing O outflow suppression processing_xWhen the temperature of the catalyst 23 is at least the predetermined temperature T_aAnd then preferably at a predetermined temperature T_aHigher temperature T_bNO after rising to_xExecute withdrawal processing.
[0035]
Therefore, N of this embodiment₂In the O outflow suppression control, NO_xNO from catalyst 23_x, For example, NO_xNO held in catalyst 23_xIs the amount of NO_xWhen the holding amount is equal to or more than the fixed holding amount, first, the temperature sensor 25_xTemperature T of catalyst 23_catIs detected. NO detected_xTemperature T of catalyst 23_catIs the predetermined temperature T_aIn the above case, NO_xWithdrawal processing is performed. On the other hand, the detected NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aNO if NO_xTemperature increase control for increasing the temperature of the catalyst 23 is executed and NO_xTemperature T of catalyst 23_catIs at least a predetermined temperature T_aUp to the above, and then NO_xNO for catalyst 23_xWithdrawal processing is performed.
[0036]
The predetermined temperature T_aMay be a predetermined value, or may be another factor (for example, type of reducing agent, NO_xRetention amount, sulfur component retention amount, oxygen concentration, NO_xIt may be a value that changes according to the degree of catalyst deterioration).
Also, generally, NO_xNo withdrawal processing_xThis is performed even when the temperature of the catalyst is relatively low. However, usually, the predetermined temperature T_aIs NO_xThe temperature at which the separation process can be executed (that is, NO_xIf the air-fuel ratio of the exhaust gas flowing into the catalyst is almost stoichiometric or rich, NO_xNO retained in the catalyst_xIs set higher than the lowest temperature of the_xThe catalyst temperature is NO_xNO when the temperature has dropped to the lowest temperature among the temperatures at which the separation process can be performed._xWhen the withdrawal process is executed, NO_xN from catalyst₂A lot of O flows out. Therefore, in the present invention, NO_xThe catalyst temperature is NO_xEven if the temperature is within the temperature range in which the separation process can be performed, the temperature is NO._xCatalyst temperature T_aNO if lower than_xThe withdrawal process is not executed.
[0037]
NO_xExamples of the method of raising the temperature of the catalyst 23 include delaying the timing of injecting fuel into the combustion chamber of the internal combustion engine, and injecting a small amount of fuel after injecting fuel for driving the engine into the combustion chamber 5 of the internal combustion engine. Or NO_xAn electric heater or a glow plug (not shown) is provided upstream of the catalyst 23 to operate the electric heater or the glow plug,_xA reducing agent adding device (see reference numeral 32 in FIG. 11) for adding fuel or reducing agent to the exhaust gas is provided upstream of the catalyst 23, and the fuel or reducing agent is added to the exhaust gas from the reducing agent adding device. (Hereinafter referred to as “reducing agent addition control”),_xA method of raising the temperature of the catalyst 23 is exemplified. When an ignition plug for igniting fuel is provided in the combustion chamber 5, the temperature of the exhaust gas can also be increased by delaying the ignition timing of the fuel by the ignition plug.
[0038]
NO_xThe only way to perform the release process is to change the opening of the throttle valve 17 to a small value and to make the air-fuel ratio of the intake gas flowing into the combustion chamber 5 of the internal combustion engine rich (hereinafter, referred to as "intake throttle control"). Instead, a small amount of fuel is injected after injecting fuel for driving the engine into the combustion chamber 5 of the internal combustion engine, and the fuel is discharged from the combustion chamber 5 without burning (hereinafter, referred to as “post injection control”). ), Adding a fuel or a reducing agent to the exhaust gas from the above-described reducing agent adding device,_xSupplying fuel or a reducing agent into the exhaust gas flowing into the catalyst 23 to make the exhaust air-fuel ratio substantially the stoichiometric air-fuel ratio or rich.
[0039]
Next, with reference to FIG. 3, N in the exhaust gas purification apparatus of the first embodiment of the present invention will be described.₂A control routine of the O outflow suppression control will be described. First, in step 121, NO_xIt is determined whether it is time to execute the withdrawal process. NO_xIf it is determined that it is not time to execute the withdrawal process, the control routine is ended. On the other hand, in step 121, NO_xIf it is determined that it is time to perform the withdrawal process, the process proceeds to step 122. In step 122, the temperature sensor 25_xTemperature T of catalyst 23_catIs detected. Next, in step 123, NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aIt is determined whether or not: NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aIf it is determined to be higher, the control routine is terminated. On the other hand, in step 123, NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aIf it is determined that it is below, the process proceeds to step 124. In step 124, NO_xThe control for raising the temperature of the catalyst 23 is executed, and the control routine is ended.
[0040]
Next, a modification of the first embodiment of the present invention will be described. In the modification of the first embodiment, NO_xTemperature increase processing of catalyst 23 and NO_xThe departure process is performed with the engine body 1 and NO_xThis is performed by a fuel addition device provided in the exhaust passage between the catalyst 23.
[0041]
NO as described above_xWhen the withdrawal process should be executed, that is, NO_xNO held in catalyst 23_xWhen the holding amount is equal to or more than the certain holding amount, NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aNO immediately in the above cases_xWithdrawal processing is executed and NO_xFuel is added to the exhaust gas from the fuel addition device such that the air-fuel ratio of the exhaust gas flowing into the catalyst 23 becomes substantially the stoichiometric air-fuel ratio or rich. On the other hand, NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aIf it is lower than_xAfter the temperature raising process of the catalyst 23 is performed, NO_xA withdrawal process is performed. Here, fuel is also added to the exhaust gas from the fuel addition device when performing the temperature raising process._xIt is added so that the exhaust air-fuel ratio flowing into the catalyst 23 remains lean.
[0042]
This situation is shown in FIG. FIG. 4 is NO_xThe air-fuel ratio of the exhaust gas flowing into the catalyst 23 (upper) and the NO_x4 is a time chart with a temperature (lower) of a catalyst 23. First, at time a, for example, NO_xNO held in catalyst 23_xWhen it is detected that the holding amount has exceeded a certain holding amount, NO_xIt is determined that it is time to perform the withdrawal process. NO at this time_xTemperature T of catalyst 23_catIs the predetermined temperature T_aLower than. Therefore, NO_xThe temperature raising process is performed before the separation process is performed. NO during execution of the heating process_xThe air-fuel ratio of the exhaust gas flowing into the catalyst 23 is reduced to a level that does not become substantially equal to the stoichiometric air-fuel ratio or rich._xThe temperature of the catalyst 23 rises. And NO_xTemperature T of catalyst 23_catIs the predetermined temperature T_aWhen it becomes above, NO_xWithdrawal processing is executed and NO_xThe air-fuel ratio of exhaust gas flowing into the catalyst 23 is reduced so as to be rich. Then NO_xNO retained in the catalyst_xIs almost completely removed_xThe release process is stopped, that is, the addition of fuel from the fuel addition device is stopped, and at the same time, NO_xThe temperature of the catalyst 23 also drops and returns.
[0043]
In the modification of the first embodiment, NO_xTemperature increase processing of catalyst 23 and NO_xThe release process is performed by adding fuel from the fuel addition device. However, NO such as changing the engine exhaust air-fuel ratio by the post injection control, intake throttle control, or the like._xAs long as the supply amount of the fuel or the reducing agent into the exhaust gas flowing into the catalyst 23 can be changed, the above-described temperature increase processing and NO_xA separation process may be performed.
[0044]
Next, an exhaust emission control device according to a second embodiment of the present invention will be described. The configuration in the second embodiment of the present invention is basically the same as the configuration in the first embodiment. In the second embodiment, NO_xWhen the air-fuel ratio of the exhaust gas flowing into the catalyst 23 is substantially the stoichiometric air-fuel ratio or rich,_xNO held by catalyst 23_xNO to leave_xNo release means_xThis is a reducing agent mixing means for mixing a reducing agent into the exhaust gas flowing into the catalyst 23. The reducing agent mixing means mixes various reducing agents into the exhaust gas to reduce NO._xThe air-fuel ratio of the exhaust gas flowing into the catalyst 23 can be made substantially the stoichiometric air-fuel ratio or rich.
[0045]
By the way, NO_xN from catalyst 23₂O outflow is NO_xNO when performing withdrawal process_xIt depends on the type of the reducing agent in the exhaust gas flowing into the catalyst 23. This is shown in FIG. In FIG. 5, the horizontal axis is NO as in FIG._xTemperature of catalyst 23, vertical axis is NO_xN from catalyst 23₂Shows the amount of O outflow.
[0046]
As can be seen from FIG. 5, when hydrocarbon is used as the reducing agent, that is, when fuel is used as the reducing agent, carbon monoxide (CO) and hydrogen (H₂) Compared with N₂The amount of O outflow is basically large, and when carbon monoxide is used as the reducing agent, N is smaller than when hydrogen is used as the reducing agent.₂O outflow is basically large. In addition, as described with reference to FIG._xAs the temperature of the catalyst 23 decreases, N₂O outflow increases and NO_xThe lower the temperature of the catalyst 23, the more N₂The difference in the amount of O outflow increases.
[0047]
Therefore, in the exhaust gas purification apparatus according to the second embodiment of the present invention, N₂The O outflow suppressing means includes NO as a reducing agent contained in the exhaust gas._xN from catalyst 23₂O outflow Q_n2oIs the predetermined outflow Q_n2oaThe reducing agent is selected by the reducing agent mixing means so as to reduce the amount of the reducing agent. For example, NO_xWhen hydrocarbons are mixed in the exhaust gas by the reducing agent mixing means to perform the separation process, NO_xThe temperature of the catalyst 23 is the first temperature T in FIG.₁NO if below_xN from catalyst 23₂O outflow Q_n2oIs the prescribed outflow Q_n2oaWill be exceeded. On the other hand, NO_xWhen carbon monoxide is mixed into the exhaust gas by the reducing agent mixing means for performing the separation process, NO_xThe temperature of the catalyst 23 is the first temperature T in FIG.₁NO even if_xN from catalyst 23₂O outflow Q_n2oIs the prescribed outflow Q_n2oaLess than NO_xThe temperature of the catalyst 23 is the second temperature T in FIG.₂NO until_xN flowing out of the catalyst 23₂The O outflow remains less than the predetermined outflow.
[0048]
Therefore, in the exhaust gas purification device of the second embodiment, NO_xThe NO detected by the temperature sensor 25 when performing the separation process_xWhen the temperature of the catalyst 23 is the first temperature T₁If it is higher, the reducing agent mixing means mixes any of hydrocarbons and carbon monoxide as a reducing agent into the exhaust gas,_xWhen the temperature of the catalyst 23 is the first temperature T₁Lower than the second temperature T₂If the pressure is higher than the above, the reducing agent mixing means mixes carbon monoxide as a reducing agent into the exhaust gas. Furthermore, NO_xNO when performing the withdrawal process_xWhen the temperature of the catalyst 23 is the second temperature T₂NO if_xWhen the temperature of the catalyst 23 is the second temperature T₂N even if₂O outflow is the specified outflow Q_n2oaHydrogen maintained below may be used as the reducing agent. NO_xBy selecting the reducing agent to be used when performing the separation process in this way, NO_xN flowing out of the catalyst 23 downstream of the exhaust gas₂O outflow is a predetermined value Q_n2oaIt can be suppressed below.
[0049]
If the reducing agent mixing means is NO_xAs a method of mixing the reducing agent into the exhaust gas flowing into the catalyst 23, NO_xProviding a reducing agent addition device (not shown) for adding each reducing agent to the exhaust gas flowing into the catalyst 23 may be mentioned. Furthermore, by simply enriching the air-fuel ratio of the intake gas flowing into the internal combustion engine, or by injecting a small amount of fuel after injecting fuel for driving the engine into the combustion chamber of the internal combustion engine, hydrocarbons are contained in the exhaust gas. Can be mixed. Also, if the combustion chamber of the internal combustion engine is filled with EGR gas and the gas in the combustion chamber is burned at a low temperature, the amount of carbon monoxide in the exhaust gas increases, so that carbon monoxide is mixed in the exhaust gas. Can be done.
[0050]
Here, if the reducing agent mixing means is NO_xMixing hydrocarbons into the exhaust gas flowing into the catalyst 23 can be easily performed without using a reducing agent addition device because the exhaust air-fuel ratio may be made rich. If the reducing agent mixing means is NO_xMixing carbon monoxide into the exhaust gas flowing into the catalyst 23 can be performed by low-temperature combustion without using a reducing agent addition device. On the other hand, if the reducing agent mixing means is NO_xMixing hydrogen into the exhaust gas flowing into the catalyst 23 is difficult to execute unless a reducing agent addition device is used. Therefore, a case where the reducing agent mixing means does not use a reducing agent adding device, that is, a case where only hydrocarbons and carbon monoxide can be mixed into exhaust gas will be described below.
[0051]
Next, referring to FIG. 6, N in the exhaust gas purifying apparatus of the second embodiment will be described.₂A control routine of the O outflow suppression control will be described. NO in step 141_xNO for catalyst 23_xIt is determined whether it is time to execute the withdrawal process. NO_xIf it is determined that it is not time to execute the withdrawal process, the control routine is ended. NO_xIf it is determined that it is time to execute the withdrawal process, the process proceeds to step 142. In step 142, NO is detected by the temperature sensor 25._xTemperature T of catalyst 23_catIs detected. Next, in step 143, NO_xTemperature T of catalyst 23_catIs the first temperature T₁It is determined whether it is higher. Temperature T_catIs the first temperature T₁If it is determined to be higher than the above, the process proceeds to step 144. In step 144, NO is determined using hydrocarbon as the reducing agent._xWithdrawal processing is performed, and the control routine is terminated. On the other hand, in step 143, the temperature T_catIs the first temperature T₁If it is determined that it is below, the process proceeds to step 145. In step 145, the temperature T_catIs the second temperature T₂It is determined whether the temperature is higher than the temperature T._catIs the second temperature T₂If it is determined that it is higher than this, the process proceeds to step 146. In step 146, NO is determined using carbon monoxide as the reducing agent._xWithdrawal processing is performed, and the control routine is terminated. On the other hand, in step 145, the temperature T_catIs the second temperature T₂If it is determined that the following conditions are satisfied, the control routine is terminated.
[0052]
In the above-described second embodiment, three reducing agents of hydrocarbon, carbon monoxide, and hydrogen have been described, but the oxygen concentration in the exhaust gas can be reduced and NO_xNO released from the catalyst_xAny substance generally used as a reducing agent may be used as the reducing agent mixed by the reducing agent mixing means as long as the reducing agent can be reduced.
[0053]
Next, an exhaust emission control device according to a third embodiment of the present invention will be described. The configuration in the third embodiment of the present invention is basically the same as the configuration in the first embodiment and the second embodiment. By the way, NO_xNO for catalyst 23_xNO when executing the withdrawal process_xN from catalyst 23₂O outflow is NO_xIt changes depending on the oxygen concentration in the exhaust gas flowing into the catalyst 23, and more specifically, as shown in FIG._xWhen the oxygen concentration in the exhaust gas flowing into the catalyst 23 increases, N₂O outflow increases, and conversely, NO_xWhen the oxygen concentration in the exhaust gas flowing into the catalyst 23 decreases, N₂O outflow is reduced.
[0054]
Here, the oxygen concentration and N₂The reason why the relationship with the O outflow amount is as described above will be briefly described. Here, NO_xThe case where the exhaust gas air-fuel ratio flowing into the catalyst 23 is rich will be described.
NO_xEven if the air-fuel ratio of the exhaust gas flowing into the catalyst 23 is rich, NO_x(For example, nitric oxide (NO)). This NO is dissociated into nitrogen (N) and oxygen (O) by the following reaction formula (1).
NO → N + O (1)
O thus dissociated is NO_xThe catalyst is held by the catalyst 23 and reacts with a reducing agent (eg, CO) also contained in the exhaust gas (reaction formula (2)). On the other hand, N dissociated as described above reacts with each other to form a nitrogen molecule (N₂(Formula (3)).
O + CO → CO₂      (2)
N + N → N₂          (3)
However, when the reaction of the above reaction formula (1) is slow and the dissociation of NO does not proceed, N and NO are converted into N by the following reaction formula (4).₂O is formed.
N + NO → N₂O (4)
Therefore, N₂In order to suppress the generation of O, it is necessary to promote the reaction of the above (1) so as not to cause the reaction of the above (4). Here, in order to promote the reaction of the above (1), it is required that the oxygen concentration in the exhaust gas be low from the viewpoint of chemical equilibrium. As described above, when the oxygen concentration of the exhaust gas is low, the reaction of the above reaction formula (1) is promoted, and as a result, N₂O is less likely to be generated, and conversely, if the oxygen concentration of the exhaust gas is high, the above-mentioned reaction (1) does not proceed, and as a result N₂O is easily generated.
[0055]
Therefore, in the exhaust gas purification apparatus according to the third embodiment of the present invention, the N₂The O outflow amount estimating means is NO_xNO based on the oxygen concentration in the exhaust gas flowing into the catalyst 23_xN flowing out downstream of the exhaust of the catalyst 23₂The amount of O is estimated. More specifically, N₂An oxygen concentration estimating means is provided as the O outflow amount estimating means._xNO before executing the withdrawal process_xNO when the withdrawal process is executed_xOxygen concentration C in exhaust gas expected to flow into catalyst 23_oIs estimated. On the other hand, as shown in FIG.₂The oxygen concentration at which the O outflow becomes the predetermined outflow is determined as the predetermined oxygen concentration C_oaIs calculated as
[0056]
In this case, the oxygen concentration estimated by the oxygen concentration detecting means is equal to the predetermined oxygen concentration C_oaLower than N₂O outflow is the specified outflow Q_n2oaIs smaller than the predetermined oxygen concentration C._oaIf so, N₂O outflow is the specified outflow Q_n2oaThis indicates that this is the case. Therefore, the oxygen concentration estimated by the oxygen concentration estimating means is equal to the predetermined oxygen concentration C_oaIf so, N₂N by O outflow control means₂The O outflow suppression processing is executed.
[0057]
In the exhaust gas purification apparatus of the present embodiment, N₂O outflow control means is N₂If the O outflow is greater than the predetermined outflow, ie, NO_xNO when performing the withdrawal process_xIf the oxygen concentration in the exhaust gas flowing into the catalyst 23 is equal to or higher than the predetermined oxygen concentration, NO_xThe oxygen concentration in the exhaust gas is reduced or substantially reduced to zero by oxygen concentration reducing means for reducing the oxygen concentration in the exhaust gas flowing into the catalyst 23. Thus, NO_xBy constantly suppressing the oxygen concentration of the exhaust gas flowing into the catalyst 23 to a predetermined oxygen concentration or less, the N 2₂The O outflow can be suppressed to a predetermined outflow or less.
[0058]
The predetermined oxygen concentration may be a predetermined value, or the predetermined oxygen concentration and the predetermined outflow Q_n2oaAnd other factors (NO_xTemperature of catalyst 23, type of reducing agent, NO_xRetention amount, sulfur component retention amount, NO_xThe relationship between the predetermined amount of outflow Q is stored in the ROM 42 of the ECU 40 in advance as a map._n2oaAlternatively, the predetermined oxygen concentration may be calculated from other factors.
[0059]
NO_xAs the oxygen concentration reducing means for reducing the oxygen concentration of the exhaust gas flowing into the catalyst 23, there is a method of filling the combustion chamber 5 of the internal combustion engine with EGR gas and burning the gas in the combustion chamber at a low temperature. By supplying fuel to the combustion chamber 5 so that the exhaust air-fuel ratio becomes substantially the stoichiometric air-fuel ratio or rich and burning the gas in the combustion chamber 5 at a low temperature as described above, most of the oxygen in the combustion chamber 5 is reduced. It is used for combustion and reduces the amount of oxygen in exhaust gas.
[0060]
Next, an exhaust emission control device according to a fourth embodiment of the present invention will be described. The configuration in the fourth embodiment of the present invention is basically the same as the configuration in the first to third embodiments. By the way, NO_xN flowing out of the catalyst 23₂NO outflow is NO_xNO held in catalyst 23_xIs the amount of NO_xIt changes according to the holding amount, and more specifically, as shown in FIG._xNO of catalyst 23_xWhen the holding amount increases, N₂O outflow increases, NO_xNO of catalyst 23_xWhen the holding amount decreases, N₂O outflow is reduced.
[0061]
Therefore, in the exhaust gas purification apparatus according to the fourth embodiment of the present invention, the N₂NO outflow estimation means_xNO of catalyst 23_xNO based on retained amount_xN from catalyst 23₂O outflow Q_noxIs estimated. More specifically, N₂NO as O outflow estimation means_xA holding amount estimating means is provided._xNO by holding amount estimation means_xNO of catalyst 23_xThe holding amount was estimated, and as shown in FIG.₂O outflow is equal to the predetermined outflow Q_n2oaLower runoff Q slightly less than_n2obNO such that_xNO of catalyst 23_xSet the holding amount to predetermined NO_xRetention Q_noxaDetected as
[0062]
In this case, NO_xNO estimated by the holding amount estimating means_xHold amount is predetermined NO_xRetention Q_noxaLess than N₂O outflow is lower outflow Q_n2obLess than the estimated NO_xHold amount is predetermined NO_xRetention Q_noxaIf so, N₂O outflow is lower outflow Q_n2obThis indicates that this is the case. Therefore, NO_xNO estimated by the holding amount estimating means_xRetention Q_noxIs predetermined NO_xRetention Q_noxaIf this is the case, N₂N by O outflow control means₂The O outflow suppression processing is executed.
[0063]
In the exhaust gas purification apparatus of the present embodiment, N₂O outflow suppressing means is N₂If the O outflow is equal to or greater than the lower outflow, ie, NO_xNO of catalyst 23_xHold amount is predetermined NO_xNO when exceeding the holding amount_xNO_xNO held in catalyst 23_xWithdraw. Thus, N₂O outflow is lower outflow Q_n2obNO if above_xBy performing the withdrawal process, N₂O outflow is equal to the predetermined outflow Q_n2oaNO before_xWithdrawal processing is performed, and thus N₂The O outflow can always be maintained at or below the predetermined outflow.
[0064]
Note that the predetermined NO_xThe holding amount may be a preset value or a predetermined NO._xRetention volume and predetermined outflow volume and other factors (NO_xTemperature of catalyst 23, type of reducing agent, sulfur component holding amount, oxygen concentration, NO_xThe degree of catalyst deterioration is stored in advance in the ROM 42 of the ECU 40 as a map._xThe holding amount may be calculated.
[0065]
NO_xIn a conventional exhaust gas purification device provided with a catalyst, NO_xNO of catalyst_xNO limit_xNO in the inflowing exhaust gas when it exceeds the holding amount (constant holding amount)_xBecause it becomes difficult to maintain_xNO of catalyst_xNO limit_xNO when exceeding the holding amount_xNO after departure process_xNO retained in the catalyst_xHad been removed. On the other hand, in the exhaust gas purification apparatus of the present invention, the predetermined NO_xNO limit_xNO_xNO of catalyst 23_xNO limit_xNO before reaching the holding amount_xNO held by catalyst 23_xHas been removed. That is, in the exhaust gas purification apparatus of the present embodiment, N₂NO with conventional exhaust gas purifying catalyst that does not monitor O outflow_xThe execution timing of the withdrawal process is different, and_xNO of catalyst 23_xNO from the viewpoint of the holding amount_xNO of catalyst 23_xNO before the purification capacity decreases_xNO held by the holding agent_xIs removed.
[0066]
NO_xNO of catalyst 23_xAs a method of estimating the holding amount, for example, NO_xNO in exhaust gas flowing into catalyst 23_xNO for detecting the amount_xA sensor (not shown) is provided and this NO_xNO detected by the sensor_xIntegrating the amount over time.
[0067]
Next, an exhaust emission control device according to a fifth embodiment of the present invention will be described. The configuration in the fifth embodiment of the present invention is basically the same as the configuration in the first to fourth embodiments. By the way, NO_xThe catalyst 23 is NO_xNO in exhaust gas flowing into catalyst 23_xNot only sulfur components but also NO_xWhen the air-fuel ratio of the exhaust gas flowing into the catalyst 23 is lean, the sulfur component contained in the exhaust gas flowing into the catalyst 23 is retained and NO_xWhen the air-fuel ratio of the exhaust gas flowing into the catalyst 23 is almost the stoichiometric air-fuel ratio or rich and NO_xWhen the temperature of the catalyst 23 is equal to or higher than the sulfur separation / desorption temperature, the retained sulfur component is released. Such NO_xIn the catalyst 23, NO_xN flowing out of the catalyst 23₂NO outflow is NO_xIt changes in accordance with the sulfur component holding amount, which is the amount of the sulfur component held in the catalyst 23, and more specifically, as shown in FIG._xWhen the sulfur component holding amount of the catalyst 23 increases, N₂O outflow increases, NO_xWhen the sulfur component holding amount of the catalyst 23 decreases, N₂O outflow is reduced.
[0068]
Therefore, in the exhaust gas purification apparatus according to the fifth embodiment of the present invention, the N₂NO outflow estimation means_xNO based on the sulfur component holding amount of the catalyst 23_xN from catalyst 23₂Estimate O outflow. More specifically, N₂Sulfur component holding amount estimating means is provided as O outflow amount estimating means._xSulfur component holding amount Q of catalyst 23_s, While N, as shown in FIG.₂O outflow is the specified outflow Q_n2oaLower runoff Q slightly less than_n2ocNO such that_xThe sulfur component holding amount of the catalyst 23 is changed to a predetermined sulfur component holding amount Q._saIs calculated as
[0069]
In this case, the sulfur component holding amount Q estimated by the sulfur component holding amount estimating means_sIs the predetermined sulfur component holding amount Q_saLess than N₂O outflow is lower outflow Q_n2ocOn the other hand, when the estimated sulfur component holding amount is equal to or more than the predetermined sulfur component holding amount, N₂O outflow is lower outflow Q_n2ocMore than Therefore, the sulfur component holding amount Q estimated by the sulfur component holding amount estimating means_sIs the predetermined sulfur component holding amount Q_saIf this is the case, N₂N by O outflow control means₂The O outflow suppression processing is executed.
[0070]
In the exhaust gas purification apparatus of the present embodiment, N₂O outflow control means is N₂If the O outflow is greater than the lower outflow, ie, NO_xNO when the sulfur component holding amount of the catalyst 23 becomes equal to or more than the predetermined sulfur component holding amount._xThe catalyst 23 is subjected to sulfur separation / de-treatment, and NO_xThe sulfur component held in the catalyst 23 is released. Thus, N₂O outflow is lower outflow Q_n2ocIf the above is the case, by performing the sulfur separation / de-treatment, the N₂O outflow is equal to the predetermined outflow Q_n2oaBefore being subjected to sulfur separation and de-treatment,₂The O outflow amount can be suppressed to the predetermined outflow amount or less.
[0071]
The predetermined sulfur component holding amount may be a predetermined value, or the predetermined sulfur component holding amount, the predetermined outflow amount, and other factors (NO_xTemperature of catalyst 23, type of reducing agent, NO_xNO of catalyst 23_xRetention amount, NO_xThe relationship with the degree of catalyst deterioration may be stored in advance in the ROM 42 of the ECU 40 as a map, and the predetermined sulfur component holding amount may be calculated from the predetermined outflow amount and other factors.
[0072]
NO_xIn a conventional exhaust gas purification device provided with a catalyst, NO_xNO in the inflowing exhaust gas when the sulfur component holding amount of the catalyst exceeds the limit sulfur component holding amount_xNO that can hold_xNO_xIf the sulfur component holding amount of the catalyst is equal to or more than the limit sulfur component holding amount, sulfur component separation and removal treatment is performed and NO_xThe sulfur component held in the catalyst was released. On the other hand, in the exhaust gas purification apparatus of the present invention, since the above-mentioned predetermined sulfur component holding amount is smaller than the limit sulfur component holding amount, NO_xNO before the sulfur component holding amount of the catalyst 23 reaches the limit sulfur component holding amount._xThe sulfur component held by the catalyst 23 is released. That is, in the exhaust gas purification apparatus of the present embodiment, N₂The execution timing of the sulfur separation / de-treatment is different from that of the conventional exhaust gas purification catalyst in which the amount of O_xNO from the viewpoint of the sulfur component holding amount of the catalyst 23_xNO of catalyst 23_xNO before the purification capacity decreases_xThe sulfur component held by the holding agent is released.
[0073]
NO_xThe sulfur separation and desorption process for desorbing the sulfur component held in the catalyst 23 is NO_xThe air-fuel ratio of the exhaust gas flowing into the catalyst 23 is made substantially equal to the stoichiometric air-fuel ratio or rich, and the NO._xThis is performed by raising the temperature of the catalyst 23 to a temperature equal to or higher than the sulfur separation / desorption temperature.
[0074]
Next, an exhaust emission control device according to a sixth embodiment of the present invention will be described. The configuration in the sixth embodiment of the present invention is basically the same as the configuration in the first to fifth embodiments. By the way, NO_xThe catalyst 23 deteriorates with time or when the temperature thereof becomes extremely high. Where NO_xDeterioration of the catalyst 23 means NO_xThe activity of the catalyst such as platinum supported on the catalyst 23 decreases, and NO_xNO held in catalyst 23_x, Sulfur components and fine particles (particulates), that is, NO_xDesorption treatment, sulfur separation and removal treatment, and a reduction in the purification performance of the exhaust purification catalyst that does not recover even if a filter regeneration process for oxidizing and removing particulates performed when a particulate filter is used as the exhaust purification catalyst means In particular, it means a decrease in the oxidizing ability of a catalyst such as platinum due to heat.
[0075]
NO_xIn the catalyst 23, NO_xWhen the catalyst 23 is hardly used and its deterioration degree is low, NO_xN₂O outflow is small, then NO_xAs the usage time of the catalyst 23 increases, the degree of its deterioration increases, and NO_xN when the removal process is performed₂O outflow increases. This is NO_xIf the degree of deterioration of the catalyst 23 is high, the reaction (O + CO → CO₂) Does not proceed, and as a result, similarly to the case where the oxygen concentration is high, the reaction (N + NO → N₂O) proceeds.
[0076]
Therefore, in the exhaust emission control device according to the sixth embodiment of the present invention, the above N₂NO outflow estimation means_xNO based on the degree of deterioration of the catalyst 23_xN from catalyst 23₂Estimate O outflow. More specifically, N₂A deterioration degree estimating means for estimating the degree of deterioration of the exhaust gas purification catalyst is provided as the O outflow amount estimating means. The deterioration degree estimating means is, for example, NO in the present embodiment._xTotal operating distance, total operating time or NO since the catalyst was installed in the vehicle in a new state_xThe degree of deterioration of the exhaust gas purification catalyst is estimated based on the total time during which the temperature of the catalyst has reached a high temperature equal to or higher than a predetermined temperature._xIt is considered that the degree of deterioration of the catalyst is low._xNO as the degree of catalyst deterioration increases_xEstimate the degree of catalyst deterioration.
[0077]
For example, when the degree of deterioration is estimated from the total operation time, the total operation time td and N₂O outflow Q_n2oIs obtained in advance and stored in the ROM 42 of the ECU 40 as a map.₂O outflow is equal to the predetermined outflow Q_n2oaThe total operation time such that_aIs calculated as
[0078]
In this case, the total operation time is the predetermined operation time td._aN shorter than₂O outflow is equal to the predetermined outflow Q_n2oaIs smaller than the predetermined operating time td._aN if above₂O outflow is the specified outflow Q_n2oaThis indicates that this is the case. Therefore, NO_xWhen executing the withdrawal process, the total operation time is equal to the predetermined operation time td._aIf so, N₂N by O outflow control means₂The O outflow suppression processing is executed. Where NO_xSince the deterioration of the catalyst cannot be recovered,₂NO as O outflow suppression processing_xThe deterioration of the catalyst cannot be recovered. Therefore, in the present embodiment, N₂N in the first to fifth embodiments described above as the O outflow suppressing process₂O outflow suppression processing is performed._xNO when executing withdrawal process_xThe catalyst temperature is NO_xNO after the temperature reaches or exceeds a predetermined temperature set in consideration of catalyst deterioration_xWithdrawal processing is performed or NO_xNO when executing withdrawal process_xThe oxygen concentration of the exhaust gas flowing into the catalyst is NO_xNO at or above a predetermined oxygen concentration set in consideration of catalyst deterioration_xOr withdrawal processing. The predetermined operation time td_aIs the other factor (NO_xTemperature of catalyst 23, oxygen concentration of inflowing exhaust gas, NO_xThe amount is calculated based on, for example, NO_xWhen the temperature of the catalyst 23 is high, the predetermined operation time td_aIs also set long.
[0079]
Further, in the exhaust gas purifying apparatus for an internal combustion engine of the present invention, NO_xThe degree of deterioration of the catalyst can be estimated. Such an example will be described below as a modification of the sixth embodiment. The configuration of the modification of the sixth embodiment is the same as the configuration of the sixth embodiment, except that NO in the modification._xNO on the exhaust gas upstream side of the catalyst_xN from catalyst 23₂N to detect O outflow₂O outflow detection device (N₂An O sensor (not shown) is provided.
[0080]
By the way, NO_xNO without considering deterioration of catalyst 23_xN in the exhaust gas flowing out of the catalyst 23₂When the O outflow amount is estimated, that is, when the NO_xTemperature of catalyst 23, NO_xOxygen concentration of exhaust gas flowing into catalyst 23, NO_xNO of catalyst 23_xFrom the retained amount and sulfur component retained amount,₂When the amount of O outflow is estimated, the estimated N₂O outflow is NO_xWhen the degree of deterioration of the catalyst 23 is low, N₂N detected by O sensor₂O is almost equal to the amount of outflow, but NO_xWhen the degree of deterioration of the catalyst 23 is high, N₂N detected by O sensor₂It is larger than the O outflow. And the estimated N₂O outflow and detected N₂O outflow (hereinafter simply referred to as “N₂The greater the difference in the amount of O outflow, the greater the NO_xThe degree of deterioration of the catalyst 23 is large. Therefore, the above N₂NO from the difference in O outflow_xThe degree of deterioration of the catalyst 23 can be estimated.
[0081]
NO_xWhen the degree of deterioration of the catalyst 23 changes, NO_xN flowing out of the catalyst 23₂O outflow changes. That is, NO_xTaking the temperature of the catalyst 23 as an example, NO_xThe higher the degree of deterioration of the catalyst 23, the more NO_xN when performing withdrawal processing₂The temperature at which the O outflow becomes equal to or lower than the predetermined outflow increases.
[0082]
Therefore, in a modification of the sixth embodiment, for example, the predetermined temperature T set in the first embodiment is set to a predetermined temperature T._aNO_xIt changes according to the degree of deterioration of the catalyst 23. As described above, the predetermined temperature T_aIs, for example, NO_xConcentration of oxygen flowing into catalyst 23, type of reducing agent, NO_xNO of catalyst 23_xIt is determined according to the retained amount and the retained amount of the sulfur component. Further, in the present modified example, the predetermined temperature T_aNO_xIt changes according to the degree of deterioration of the catalyst 23.
[0083]
More specifically, the N₂O outflow difference and correction temperature ΔT_aIs obtained in advance and stored in the ROM 42 of the ECU 40 as a map. Where N₂O outflow difference and correction temperature ΔT_aIs related to N₂When the difference in the amount of O outflow is small, the correction temperature ΔT_aIs also small, N₂When the difference in the amount of O outflow increases, the correction temperature ΔT_aIs also large. When used, N₂Based on the difference in the outflow amount of O, the correction temperature ΔT_aAnd the correction temperature ΔT_aNO_xThe predetermined temperature T determined based on a factor other than the degree of deterioration of the catalyst 23_aIs added to. Then, the actual catalyst temperature T_catIs the temperature (T_a+ ΔT_aNO if higher_xThe separation process is executed, and the actual catalyst temperature T_catIs the temperature (T_a+ ΔT_a) If below, the catalyst temperature T_catIs the temperature (T_a+ ΔT_aNO)_xExecute withdrawal processing.
[0084]
In the modification of the sixth embodiment, the degree of deterioration (that is, N₂The correction temperature is changed in accordance with the difference in₂The correction temperature may be changed by another method using the degree of deterioration, such as raising the correction temperature by about 10 ° C. when the difference between the O outflow amounts becomes equal to or more than the predetermined difference.
[0085]
Further, in the modified example of the sixth embodiment, NO_xNO based on the degree of catalyst deterioration_xNO during separation processing_xThe temperature of the catalyst is changed, but NO_xNO at the time of sulfur separation / detreatment based on the degree of catalyst deterioration_xNO detected as described above, such as changing the temperature of the catalyst_xThe degree of catalyst deterioration may be used for other purposes.
[0086]
By the way, as described above, NO_xNO of catalyst_xWhen the retained amount or sulfur component retained amount is large, NO_xAs in the case where the degree of catalyst deterioration is high, N₂O outflow increases. Therefore, NO_xNO of catalyst 23_xN without considering the retained amount and sulfur component retained amount₂NO when the O outflow is estimated_xNO of catalyst_xWhen the retained amount or sulfur component retained amount is large, NO_xDespite the low degree of catalyst degradation, N₂O outflow increases, and the above N₂The difference in the amount of O outflow increases, resulting in NO_xIt is estimated that the degree of deterioration of the catalyst is high.
[0087]
In this case, NO_xAfter performing the separation process or the sulfur separation / desorption process, N₂N by O sensor₂If the outflow amount of O is detected, NO_xNO of catalyst_xNO due to large retention amount and sulfur component retention amount_xIt is prevented that the degree of deterioration of the catalyst is estimated to be high. Therefore, in this modified example, NO_xNO of catalyst 23_xNO described above without considering the retained amount and the sulfur component retained amount_xTemperature of catalyst 23, NO_xN based on the oxygen concentration of the exhaust gas flowing into the catalyst 23, etc.₂NO when the O outflow is estimated_xAfter performing the separation process or the sulfur separation / desorption process, N₂N by O sensor₂The outflow amount of O may be detected.
[0088]
By the way, as can be seen from the first to sixth embodiments, N₂The amount of O outflow is determined by the six factors mentioned above, namely, NO_xThe temperature of the catalyst 23 and NO_xThe type of the reducing agent and the NO_xNO of catalyst 23_xHolding amount and NO_xSulfur component holding amount of catalyst 23 and NO_xNO when executing withdrawal process_xThe oxygen concentration of the exhaust gas flowing into the catalyst 23 and the NO concentration_xIt changes according to the degree of deterioration of the catalyst 23. That is, N₂O outflow is a function of these factors. Therefore, in the present invention, at least two of the first to sixth embodiments may be combined. A case where the above embodiments are combined in this manner will be described below as a seventh embodiment. In particular, in the first to sixth embodiments, N₂O outflow estimation means and N₂O outflow suppressing means, but in this embodiment, the N outflow of the first to sixth embodiments is₂N of at least one of O outflow estimation means₂O outflow amount estimating means and N of the first to fifth embodiments₂At least one of the optimum N₂It can be combined with O outflow suppressing means. In this case, for example, N₂O Outflow amount estimation means N from two of the above factors₂Calculate the O outflow amount. For example, N₂O outflow is the specified outflow Q_n2oaThe value of the factor in the case of is obtained experimentally in advance, and is stored in the ROM 42 of the ECU 40 as a map. Then, during the operation of the internal combustion engine, N₂O outflow is the specified outflow Q_n2oaIf it is estimated that the above is true, N₂N by O outflow control means₂Execute O suppression processing.
[0089]
In addition, N of the present embodiment₂The O outflow suppressing means is the same as that of the first to fifth embodiments.₂At least one optimal N among O suppression processes₂Select and execute O suppression processing. By the way, each N₂The O suppression process may or may not be suitable for the operating state of the internal combustion engine at that time, from the viewpoints of feasibility, fuel efficiency, and operational stability (drivability) of the internal combustion engine. Therefore, in this modified example, N₂The O outflow suppressing means is provided with the above-mentioned N so that the fuel efficiency, the stability of the operation of the internal combustion engine, and the like are most advantageous in accordance with the operation state of the internal combustion engine.₂Optimal N in O suppression processing₂Select and execute O suppression processing.
[0090]
Next, as an example of the seventh embodiment, an example of a combination of the first embodiment and the second embodiment will be described below with reference to FIG.
[0091]
As shown in the second embodiment, NO_xWhen a reducing agent adding device is provided to mix the reducing agent into the exhaust gas flowing into the catalyst 23, one or more reducing agents can be stably_xIt can flow into the catalyst 23. However, the amount of carbon monoxide in the exhaust gas is increased by filling the combustion chamber of the internal combustion engine with a large amount of EGR gas and burning the gas in the combustion chamber at a low temperature (hereinafter, referred to as “low temperature combustion control”). Thus, when carbon monoxide is used as the reducing agent, it is not always possible to increase the amount of carbon monoxide, and the possibility of the execution varies depending on the operation state of the internal combustion engine. Therefore, in this case, carbon monoxide cannot always be used as the reducing agent.
[0092]
Therefore, in this combination example, NO_xN when performing withdrawal processing₂O outflow is the specified outflow Q_n2oaAfter raising the temperature to a temperature maintained below, NO_xExecute withdrawal processing. For example, when it is possible to use both carbon monoxide and hydrocarbon as the reducing agent, NO_xNO immediately before executing the process_xWhen the temperature of the catalyst 23 is the second temperature T₂Higher than NO using carbon monoxide as the reducing agent_xImmediately perform withdrawal processing,_xNO immediately before executing withdrawal processing_xWhen the temperature of the catalyst 23 is the second temperature T₂NO if NO_xExecute the temperature raising process of the catalyst 23 and_xWhen the temperature of the catalyst 23 is at least the second temperature T₂And then use carbon monoxide as the reducing agent to reduce NO_xExecute withdrawal processing. On the other hand, when only hydrocarbons can be used as the reducing agent, NO_xNO immediately before executing withdrawal processing_xWhen the temperature of the catalyst 23 is the first temperature T₁NO if higher than_xExecute withdrawal processing, but NO_xNO immediately before executing withdrawal processing_xWhen the temperature of the catalyst 23 is the first temperature T₁NO if NO_xExecute the temperature raising process of the catalyst 23 and_xWhen the temperature of the catalyst 23 is at least the first temperature T₁To a higher temperature than NO_xExecute withdrawal processing. By doing so, NO_xWhile minimizing energy consumption by performing the temperature raising process of the catalyst 23,₂The amount of O outflow can always be suppressed to a predetermined amount or less.
[0093]
Next, with reference to FIG.₂A control routine of the O suppression control will be described. First, in step 161, NO_xTemperature T of catalyst 23_catIs detected. Next, at step 162, it is determined whether carbon monoxide can be used as a reducing agent. If it is determined in step 162 that carbon monoxide can be used, the process proceeds to step 163. In step 163, NO_xTemperature T of catalyst 23_catIs the second temperature T₂It is determined whether the temperature is below the second temperature T.₂If it is determined that it is higher than the above, the process proceeds to step 165. NO in step 163_xTemperature T of catalyst 23_catIs the second temperature T₂If it is determined that it is below, the process proceeds to step 164. In step 164, NO_xTemperature T of catalyst 23_catIs at least the second temperature T₂NO until it reaches_xThe process of raising the temperature of the catalyst 23 is performed, and the process proceeds to step 165. In step 165, NO is determined using carbon monoxide as the reducing agent._xThe withdrawal process is executed, and the control routine is ended.
[0094]
On the other hand, if it is determined in step 162 that carbon monoxide cannot be used as the reducing agent, the process proceeds to step 166. In step 166, NO_xTemperature T of catalyst 23_catIs the first temperature T₁It is determined whether or not:_xTemperature T of catalyst 23_catIs the first temperature T₁If it is determined that it is higher than the above, the process proceeds to step 165. At step 166, NO_xTemperature T of catalyst 23_catIs the first temperature T₁If it is determined that it is below, the process proceeds to step 167. In step 167, NO_xTemperature T of catalyst 23_catIs the first temperature T₁NO until it reaches_xThe process of raising the temperature of the catalyst 23 is performed, and the process proceeds to step 168. In step 168, NO is determined using hydrocarbon as the reducing agent._xWithdrawal processing is performed, and the control routine is completed.
[0095]
Next, an exhaust emission control device according to an eighth embodiment of the present invention will be described. The configuration of the exhaust emission control device of the eighth embodiment is basically the same as the configuration of the above-described embodiment._xN from catalyst 23₂N to detect O outflow₂O outflow detection device (N₂O sensor) is N₂It is provided as O outflow amount estimation means. In the exhaust gas purifying apparatus according to the eighth embodiment, N₂Actual N detected by O sensor₂O outflow and the predetermined outflow Q_n2oaAnd, accordingly, N₂The O departure process is executed. More specifically, NO_xNO for catalyst 23_xWhen the withdrawal process is to be executed, N₂N detected by O sensor₂O outflow, ie NO_xN from catalyst 23₂O outflow is the specified outflow Q_n2oaNO immediately if less than_xPerform the withdrawal process and find the detected N₂O outflow is the specified outflow Q_n2oaN if more than₂N by O outflow control means₂NO after executing O suppression processing_xExecute withdrawal processing.
[0096]
Next, an exhaust emission control device according to a ninth embodiment of the present invention will be described. The configuration of the exhaust gas purification apparatus of the ninth embodiment is basically the same as the exhaust gas purification apparatus of the above embodiment, as shown in FIG. 11, but includes a reducing agent addition device 32. Further, the exhaust gas purification apparatus of the ninth embodiment is particularly used in an internal combustion engine in which the exhaust air-fuel ratio during normal operation is almost lean, and a case where the exhaust gas purification apparatus is used in such an internal combustion engine will be described below.
[0097]
By the way, as described above, NO_xWhen performing the withdrawal process, NO_xThe higher the oxygen concentration of the exhaust gas flowing into the catalyst 23, the more NO_xNO released from catalyst 23_xIs N₂It is easily converted to O. Where NO_xIn the departure process, the engine exhaust air-fuel ratio is made rich and NO_xA comparison will be made between a case where the inflow exhaust air-fuel ratio to the catalyst 23 is made rich and a case where the inflow exhaust air-fuel ratio is made rich by adding a reducing agent from the reducing agent addition device. When making the engine exhaust air-fuel ratio rich, most of the oxygen reacts with fuel or the like in the combustion chamber 5, so that the oxygen concentration of the exhaust gas is low. On the other hand, when the reducing agent is added from the reducing agent adding device 32 to enrich the inflow exhaust air-fuel ratio, the reducing agent is added to the exhaust gas and then the exhaust gas becomes NO._xSince only part of the oxygen and the reducing agent react before reaching the catalyst 23, NO_xThe oxygen concentration of the exhaust gas when flowing into the catalyst 23 is higher than when the engine exhaust air-fuel ratio is made rich. Therefore, while lowering the oxygen concentration,_xIt is preferable to make the engine exhaust air-fuel ratio rich from the viewpoint of making the exhaust gas air-fuel ratio flowing into the catalyst 23 rich.
[0098]
On the other hand, the enrichment of the engine exhaust air-fuel ratio is performed by the above-described low-temperature combustion control, post-injection control, and intake throttle control (hereinafter collectively referred to as "engine exhaust air-fuel ratio lowering means"). Directly affects the combustion in the combustion chamber 5, the output of the engine and the like fluctuate, and the stability (drivability) of operation is deteriorated. On the other hand, when the reducing agent is added from the reducing agent adding device, the reducing agent is added to the exhaust gas completely exhausted from the engine body 1. Has almost no effect on the combustion of the engine, so that there is no fluctuation in the engine output or the like. Therefore, from the viewpoint of driving stability (drivability), it is preferable to add fuel from the fuel addition device to make the air-fuel ratio of the exhaust gas rich.
[0099]
Therefore, in the exhaust emission control device of the ninth embodiment, NO_xWhen performing the departure process, the engine exhaust air-fuel ratio is reduced by the engine exhaust air-fuel ratio lowering means, and the reducing agent is added from the reducing agent addition device at the same time to make the inflow exhaust air-fuel ratio rich. . More specifically, the engine exhaust air-fuel ratio is reduced within the lean range by the engine exhaust air-fuel ratio lowering means, that is, the engine exhaust air-fuel ratio is reduced to such an extent that the engine output or the like does not greatly change. Therefore, the exhaust gas discharged from the internal combustion engine has a relatively low oxygen concentration while having a lean air-fuel ratio. Then, the reducing agent is added from the reducing agent adding device so that the inflow exhaust gas becomes rich in the exhaust gas whose engine exhaust air-fuel ratio has been reduced in this way. Thus, in the exhaust emission control device of the present embodiment, NO_xAt the time of performing the departure process, the exhaust gas whose air-fuel ratio is made rich while keeping the oxygen concentration relatively low while preventing a large fluctuation in the engine output and the like is NO._xIt flows into the catalyst. For this reason, according to the exhaust gas purification apparatus of the present embodiment, NO_xAt the time of performing the separation process, compared with the case where the inflow exhaust air-fuel ratio is enriched only by the addition of the reducing agent from the reducing agent addition device, N₂O outflow can be suppressed.
[0100]
According to the exhaust gas control apparatus of the ninth embodiment, NO_xBy lowering the oxygen concentration of the exhaust gas flowing into the catalyst 23,₂In addition to suppressing the outflow of O, the following effects can be obtained.
[0101]
First, since the oxygen concentration is reduced as described above, NO_xDuring the separation process, the reaction of the reaction formula (5) is NO_xIt is less likely to be carried out with the catalyst 23 (eg, platinum). Therefore, NO_xSince the catalyst of the catalyst 23 is almost used only for promoting the reaction of the reaction formula (6), NO_xNO during separation process_xNO held in catalyst 23_xIs reduced and purified well.
H_nC_m+ O₂→ CO₂+ H₂O (5)
H_nC_m+ NO → CO₂+ H₂O + N₂        (6)
[0102]
Further, when fuel is added as a reducing agent added from the reducing agent adding device, that is, when HC having a large number of carbon atoms (hereinafter, referred to as “heavy HC”) is added, most of the heavy HC is NO as it is._xIt flows into the catalyst 23. On the other hand, when the engine exhaust air-fuel ratio is lowered by the engine exhaust air-fuel ratio lowering means, the fuel in the combustion chamber 5 of the internal combustion engine causes the fuel to have a low carbon number (hereinafter referred to as "light HC"). And CO, and even if a small amount of heavy HC is subsequently added from the reducing agent addition device, NO_xMost of the reducing agent in the exhaust gas flowing into the catalyst 23 is light HC or CO. Therefore, according to the present embodiment, the N described in the second embodiment is used.₂Exhaust gas containing a large amount of a reducing agent that can further suppress the amount of O_xIt can flow into the catalyst 23.
[0103]
Also, depending on the type of reducing agent, NO_xNO held in catalyst 23_xIs reduced (hereinafter referred to as “NO_x(Reduction rate)), and as shown in FIG._xHighest reduction rate, NO of heavy HC_xLowest reduction rate. Therefore, according to the exhaust gas purification apparatus of the present embodiment, NO_xExhaust gas containing much reducing agent with high reduction rate is NO_xIt can flow into the catalyst 23. In FIG. 12, C is used as the heavy HC.₁₀H₁₂NO_xThe reduction rate is C as light HC.₃H₆NO_xThe reduction rate is shown.
[0104]
Furthermore, when the engine exhaust air-fuel ratio is reduced by the intake throttle control or the low-temperature combustion control, the NOx is reduced compared to the case where the reducing agent is added by the reducing agent addition device 32 to reduce the inflow exhaust air-fuel ratio._xThe volume flow rate of the exhaust gas flowing into the catalyst 23 decreases. That is, when the intake throttle control is performed, the throttle valve 17 is throttled, so that the intake gas flowing into the combustion chamber 5 decreases, and as a result, the volume flow rate of the exhaust gas also decreases. Further, when performing low-temperature combustion control, the amount of exhaust gas mixed into the intake gas as EGR gas is increased, so that a large amount of exhaust gas discharged from the internal combustion engine flows into the EGR passage, and as a result, NO_xThe volume flow rate of the exhaust gas flowing into the catalyst is reduced.
[0105]
On the other hand, as shown in FIG. 13, when the volume flow rate of the exhaust gas is large, the reducing agent contained in the exhaust gas contains oxygen or NO._xNO without reacting with_xSince it often passes through the catalyst, NO_xWhen the reduction rate is low and the volume flow rate of the exhaust gas is small, the reducing agent contained in the exhaust gas is NO._xNO without reacting with_xLess passage through the catalyst, reducing agent and oxygen and NO_xIs easy to react with, so NO_xThe reduction rate is high. Therefore, when the engine exhaust air-fuel ratio is reduced by the intake throttle control or the low-temperature combustion control, the NOx is reduced compared with the case where the reducing agent is added by the reducing agent addition device 32 to reduce the inflow exhaust air-fuel ratio._xSince the volume flow rate of the exhaust gas flowing into the catalyst 23 decreases, the NO_xThe reduction rate increases. Thus, as in the present embodiment, the reducing agent is added from the reducing agent adding device 32 by adding the reducing agent by the reducing agent adding device 32 while lowering the engine exhaust air-fuel ratio by the intake throttle control or the low-temperature combustion control. Compared with the case where the inflow exhaust air-fuel ratio is lowered only by_xFrom the viewpoint that the volume flow rate of the exhaust gas flowing into the catalyst 23 decreases, the NO_xThe reduction rate can be increased.
[0106]
The exhaust emission control device according to the ninth embodiment will be described more specifically. In the following description, a case in which the ninth embodiment is combined with a modification of the first embodiment will be described. However, in the exhaust purification device of the ninth embodiment, the air-fuel ratio is reduced by the engine exhaust air-fuel ratio lowering unit. NO by adding a reducing agent to the exhaust gas of the engine from the reducing agent adding device._xNO procedure in any case if the withdrawal process is executed_xA withdrawal process may be performed.
[0107]
As shown in FIG. 14, in the internal combustion engine of the present embodiment, NO_xThe exhaust gas air-fuel ratio flowing into the catalyst 23 is lean. And NO_xNO of catalyst 23_xIf the holding amount is equal to or more than the certain holding amount, NO_xWhen the temperature of the catalyst 23 is lower than the predetermined temperature, NO_xAs a process of raising the temperature of the catalyst 23, the engine exhaust air-fuel ratio is reduced by post-injection control, which is engine exhaust air-fuel ratio lowering means. At this time, the engine exhaust air-fuel ratio is made lean near the stoichiometric air-fuel ratio. Then NO_xWhen the temperature of the catalyst 23 becomes equal to or higher than the predetermined temperature, the reducing agent is added from the reducing agent adding device 32 while the air-fuel ratio of the engine exhaust is lowered by the post-injection control (α in the figure). Make the fuel ratio rich (β in the figure). Then NO_xNO of catalyst 23_xWhen the held amount becomes substantially zero, the post injection control and the addition of the reducing agent from the reducing agent adding device 32 are stopped.
[0108]
Next, a modification of the exhaust emission control device according to the ninth embodiment of the present invention will be described. The configuration of the modification of the ninth embodiment is basically the same as the configuration of the exhaust gas purification device of the ninth embodiment. However, as shown in FIG. 11, a bypass passage 33 for bypassing the EGR gas cooling device 28 is provided. Provided. A flow control valve 34 is provided at a branch point from the EGR passage 26 to the bypass passage 33, and the flow control valve 34 adjusts a flow rate of the EGR gas flowing into the EGR gas cooling device 28 and the bypass passage 33. Also, in this modified example, NO_xWhen the departure process is executed, the engine exhaust air-fuel ratio is reduced by low-temperature combustion control, and the reducing agent is added from the reducing agent adding device 32, so that the inflow exhaust air-fuel ratio is made substantially equal to the stoichiometric air-fuel ratio or rich.
[0109]
Further, in this modified example, NO_xWhen the low-temperature combustion control is performed during the separation process, the flow control valve 34 is switched so that most of the EGR gas passes through the bypass passage 33 without passing through the EGR gas cooling device. Therefore, high-temperature uncooled EGR gas is mixed into the intake gas. Therefore, the intake gas containing the EGR gas flowing into the combustion chamber 5 has a relatively high temperature, and as a result, the exhaust gas discharged from the engine body 1 also has a high temperature.
[0110]
As described above, in this modified example, NO_xIn the execution of the separation process, since the exhaust gas discharged from the engine body 1 is at a high temperature, NO_xThe temperature of the catalyst 23 also increases. And, as described above, NO_xWhen the temperature of the catalyst 23 increases, NO_xN from catalyst 23₂O outflow is suppressed. Therefore, in this modified example, NO_xBy mixing the EGR gas bypassing the EGR gas cooling device into the intake gas during the execution of the desorption process, the NO_xThe temperature of the catalyst 23 can be raised.
[0111]
Note that in the above modified example, NO_xAlthough the EGR gas bypassing the EGR gas cooling device is mixed into the intake gas at the time of the departure process, it is necessary to increase the temperature of the exhaust gas when performing a process of lowering the engine exhaust air-fuel ratio such as a temperature raising process. The EGR gas may be mixed into the intake gas without cooling whenever necessary. In particular, during the temperature raising process, by performing such control, NO_xThe temperature of the catalyst 23 can be raised.
[0112]
Note that NO_xN from catalyst 23₂The amount of O outflow is determined by the above factor (NO_xTemperature of catalyst 23, type of reducing agent, NO_xRetention amount, sulfur component retention amount, oxygen concentration, NO_xIn addition to the degree of deterioration of the catalyst 23), NO_xIt changes according to the flow rate of the exhaust gas flowing into the catalyst 23, and if the flow rate of the exhaust gas flowing in increases, N₂If the amount of O flowing out increases and the flow rate of the exhaust gas flowing in decreases,₂O outflow is also reduced. Therefore, in each of the above embodiments, NO_xN from catalyst 23₂Predetermined outflow Q of O outflow_n2oaIs NO_xCorrected according to the flow rate of the exhaust gas flowing into the catalyst 23, NO_xIf the flow rate of the exhaust gas flowing into the catalyst 23 increases, the predetermined outflow amount Q_n2oaIs also increased, NO_xIf the flow rate of the exhaust gas flowing into the catalyst 23 decreases, the predetermined outflow amount Q_n2oaIs also reduced. Or NO_xN from catalyst 23₂O outflow, not N₂O selectivity (NO_xNO when reducing_xIs N₂O ratio, especially NO in the first to seventh embodiments_xNO released from the catalyst 23_xN out of₂N based on the rate of conversion to O)₂O suppression processing may be performed. In this case, N₂If the O selectivity is higher than the predetermined selectivity, N₂O suppression processing is performed.
[0113]
Further, in the above embodiment, NO_xNO for catalyst 23_xN before executing withdrawal processing₂O outflow was estimated, but of course, NO_xNO for catalyst 23_xWhile the withdrawal process is being performed, NO_xN flowing out of the catalyst 23 downstream of the exhaust gas₂O outflow Q_n2oAnd the estimated N₂O outflow Q_n2oIs the prescribed outflow Q_n2oaN if exceeds₂N by O outflow control means₂Control may be performed to reduce the O outflow amount. For example, as the oxygen concentration detecting means, O₂NO using sensor_xNO for catalyst 23_xNO while the withdrawal process is being performed_xThe oxygen concentration of the exhaust gas flowing into the catalyst 23 is changed to O₂The oxygen concentration detected by the sensor and the detected oxygen concentration is a predetermined oxygen concentration C_oaN if higher₂O outflow suppression processing may be executed.
[0114]
Further, in the above embodiment, the exhaust purification catalyst is NO_xAlthough a catalyst is used, NO is also used when the three-way catalyst and the inflow exhaust air-fuel ratio are lean._xNO that can be reduced_xIt may be a catalyst. In this case, N₂The O outflow amount estimating means calculates N based on the temperature of the catalyst and the oxygen concentration of the exhaust gas flowing into the catalyst.₂Estimate O runoff and N₂O outflow control means changes these two factors to N₂Suppress the outflow of O. Of course, NO_xNO like catalyst_xAny exhaust purification catalyst, such as a particulate filter, may be used as long as the catalyst has the above retention ability.
[0115]
In the above embodiment, N₂NO estimated by O outflow estimation means_xN when executing the withdrawal process₂If the O outflow is equal to or greater than the predetermined outflow, N₂N by O outflow control means₂NO after executing the O suppression process_xThe withdrawal process is executed, but N₂NO without executing O suppression processing_xEven if the withdrawal process is executed, N₂Wait until the O outflow is less than the predetermined outflow, then_xA withdrawal process may be performed. For example, NO_xNO when withdrawal process should be performed_xThe temperature of the catalyst 23 is the predetermined temperature T in FIG._aNO if NO_xWhen the temperature of the catalyst 23 is N₂The predetermined temperature T by processing other than the O suppression processing_aWaiting until it is over, NO_xExecute withdrawal processing.
[0116]
Finally, the NO of the present invention_xExhaust gas purification mechanism by catalyst 23, particularly NO in exhaust gas_xWith reference to FIG. 15, the holding / separating and reducing / purifying actions of will be described. NO_xThe catalyst 23 is at least one selected from alkali metals such as potassium K, sodium Na, lithium Li and cesium Cs, alkaline earths such as barium Ba and calcium Ca, lanthanum La and rare earths such as yttrium Y. NO_xIt consists of a retaining agent and a noble metal such as platinum (Pt).
[0117]
Here, this NO_xThe mechanism of the action of retention / desorption and reduction and purification of platinum will be described by taking platinum (Pt) and barium (Ba) as an example, but the same mechanism can be used with other noble metals, alkali metals, alkaline earths, and rare earths. It becomes. FIGS. 15A and 15B show NO._xFIG. 4 schematically shows an enlarged view of the surface of a carrier layer formed on the surface of the partition wall of the catalyst 23 and on the surface of the pores of the partition wall. 15A and 15B, reference numeral 60 denotes platinum particles, and reference numeral 61 denotes NO such as barium._x3 shows a carrier layer containing a retaining agent.
[0118]
When the inflow exhaust air-fuel ratio becomes considerably lean, the oxygen concentration in the exhaust gas greatly increases, and as shown in FIG.₂ ⁻Or O^2-Adheres to the surface of the platinum 60 in the form of On the other hand, NO in the inflowing exhaust gas becomes O 2 on the surface of the platinum 60.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). NO generated next₂Is partially oxidized on platinum 60 while NO_xAs shown in FIG. 15A, nitrate ions (NO) are absorbed in the retaining agent 61 and combined with barium oxide (BaO).₃ ⁻NO)_xIt diffuses into the holding agent 61. NO in this way_xIs NO_xIt is held by the holding agent 61.
[0119]
As long as the oxygen concentration in the inflowing exhaust gas is high, NO₂Is generated, and NO_xNO of holding agent 61_xNO unless retention capacity is saturated₂Is NO_xNitrate ions (NO₃ ⁻) Is generated. On the other hand, the oxygen concentration in the exhaust gas decreases and NO₂The reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus NO_xNitrate ions (NO₃ ⁻) Is NO₂NO in the form of_xReleased from the holding agent. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NO_xNO from holding agent 61_xWill be disengaged. If the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the exhaust gas decreases. Therefore, if the degree of leanness of the inflowing exhaust gas decreases, NO_xNO from holding agent 61_xWill be disengaged.
[0120]
On the other hand, if the inflow exhaust air-fuel ratio is reduced at this time, HC and CO₂ ⁻Or O^2-And oxidize. Further, when the inflow exhaust air-fuel ratio is reduced, the oxygen concentration in the exhaust gas is extremely reduced._xNO from holding agent 61₂Is released and this NO₂Is reduced and purified by reacting with unburned HC and CO as shown in FIG. In this way, NO on the surface of platinum 60₂NO when no longer exists_xNO from the holding agent 61 one after another₂Is detached. Therefore, when the inflow exhaust air-fuel ratio is reduced and the reducing agent is present, NO_xNO from holding agent 61_xIs separated and reduced and purified.
[0121]
In this specification, NO_xNO from catalyst_xAlternatively, when the sulfur component is desorbed, it is described that the inflow exhaust air-fuel ratio is made substantially stoichiometric air-fuel ratio or rich. However, since the oxygen concentration of the inflowing exhaust gas becomes lower than a predetermined oxygen concentration, NO_xNO from catalyst_xAnd sulfur components are easily separated. Therefore, the description of "making the air-fuel ratio of exhaust gas (exhaust air-fuel ratio) substantially stoichiometric air-fuel ratio or rich" in the above-described embodiment means "making the oxygen concentration of the inflowing exhaust gas less than or equal to a predetermined oxygen concentration". means.
[0122]
Further, in the present specification, the term “hold” refers to NO_xAbsorption "when NO is accumulated in the form of nitrate and NO_xNO₂The term "adsorption" is used to include both meanings of "adsorption". NO_xThe term "desorption" from the catalyst is used to include the meaning of "release" corresponding to "absorption" as well as "desorption" corresponding to "adsorption".
[0123]
【The invention's effect】
According to the present invention, NO_xN in exhaust gas flowing downstream from the catalyst to the exhaust₂N which is the amount of O₂Since the outflow amount of O is always suppressed to a predetermined amount or less, N₂An exhaust gas purification device with a small amount of O outflow is provided.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine provided with an exhaust purification device of the present invention.
FIG. 2 NO_xCatalyst temperature and N₂It is a figure which shows the relationship with O outflow.
FIG. 3 shows N in the first embodiment.₂It is a flowchart of a control routine of O outflow suppression control.
FIG. 4 shows a NO in a modification of the first embodiment._xExhaust air-fuel ratio flowing into catalyst and NO_xIt is a time chart with the temperature of a catalyst.
FIG. 5: NO using various reducing agents_xNO when the withdrawal process is executed_xCatalyst temperature and N₂It is a figure which shows the relationship with O outflow.
FIG. 6 shows N in the second embodiment.₂It is a flowchart of a control routine of O outflow suppression control.
FIG. 7_xOxygen concentration of catalyst and N₂It is a figure which shows the relationship with O outflow.
FIG. 8 NO_xNO of catalyst_xRetention amount and N₂It is a figure which shows the relationship with O outflow.
FIG. 9 NO_xSulfur component retention of catalyst and N₂It is a figure which shows the relationship with O outflow.
FIG. 10 shows N in the seventh embodiment.₂It is a flowchart of a control routine of O outflow suppression control.
FIG. 11 is a view similar to FIG. 1, illustrating a ninth embodiment;
FIG. 12: Type of reducing agent and NO_xIt is a figure showing a relation with a reduction rate.
FIG. 13 NO_xVolumetric flow rate of exhaust gas flowing into catalyst and NO_xIt is a figure showing a relation with a reduction rate.
FIG. 14 is a view similar to FIG. 4 in a ninth embodiment;
FIG. 15 NO_xNO in catalyst_xIt is a figure for explaining holding and separation of.
[Explanation of symbols]
5. Combustion chamber
23 ... exhaust purification catalyst, NO_xcatalyst
25 ... Temperature sensor
40 ... ECU

Claims (12)

An exhaust purification catalyst disposed on an exhaust passage of an internal combustion engine, N ₂ O outflow amount estimating means for estimating an N ₂ O outflow amount from the exhaust purification catalyst, and reducing the N ₂ O outflow amount ; and a the N _{2 O} outflow suppressing means, when N _{2 O} outflow amount estimated by the N _{2 O} outflow amount estimating means is equal to or greater than a predetermined amount, the exhaust gas purification catalyst by the N _{2 O} outflow suppressing means An exhaust gas purification device for an internal combustion engine, characterized in that the amount of N ₂ O flowing out of the exhaust gas is reduced. The N ₂ O outflow amount estimating means is configured to control the exhaust gas based on at least one of the temperature of the exhaust purification catalyst, the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst, and the degree of deterioration of the exhaust purification catalyst. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the amount of N ₂ O flowing out from the purifying catalyst is estimated. _3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the N ₂ O outflow suppressing unit raises the temperature of the exhaust gas purification catalyst to a temperature at which the N ₂ O outflow amount becomes smaller than the predetermined amount. 4. The exhaust purification catalyst, with the air-fuel ratio of the inflowing exhaust gas when the lean holding the NO _x in the exhaust gas, the air-fuel ratio of the exhaust gas flowing holds when almost the stoichiometric air-fuel ratio or rich NO _x a the NO _x catalyst for disengaging the to perform almost stoichiometric or NO _x withdrawal process the NO _x catalyst is disengaging the NO _x held in the rich air-fuel ratio of the exhaust gas flowing into the the NO _x catalyst an exhaust purification system of an internal combustion engine according to claim 1, further comprising a NO _x withdrawal means. The N _{2 O} outflow estimating means, the exhaust temperature of the purification catalyst, the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst, a deterioration degree of the exhaust purification catalyst, NO _x amount held in the NO _x catalyst and NO exhaust purification system of an internal combustion engine according to claim 4 for estimating the N _{2 O} outflow from NO _x catalyst based on at least one of the sulfur component _{amount x} catalyst held in. The _{N 2} O outflow amount estimation means estimates the _{N 2} O outflow from the NO _x catalyst in the case of executing the NO _x withdrawal process before performing the NO _x withdrawal process, the _{N 2} O outflow inhibition means 6. The NOx release process according to claim 4, wherein when the estimated N ₂ O outflow amount is equal to or more than the predetermined amount, the NO _x desorption process is prohibited until the N ₂ O outflow amount is estimated to be smaller than the predetermined amount. Exhaust purification device for internal combustion engine. The N _{2 O} outflow inhibition means further, NO _x released from the estimated N _{2 O} runoff was heated to the NO _x catalyst to a temperature that is less than the predetermined amount by the NO _x withdrawal means The exhaust gas purifying apparatus for an internal combustion engine according to claim 6, wherein the processing is performed. Heating of the the NO _x catalyst is performed by the reducing agent mixed means for containing a reducing agent in the exhaust gas flowing into the the NO _x catalyst, in the NO _x catalyst to the reducing agent-mixed unit executes the NO _x withdrawal process The exhaust gas purifying apparatus for an internal combustion engine according to claim 7, wherein the reducing agent is contained such that the air-fuel ratio of the inflowing exhaust gas becomes substantially the stoichiometric air-fuel ratio or lean. The NO _x desorbing means is a reducing agent mixing means for causing a reducing agent to be contained in exhaust gas flowing into the NO _x catalyst. The N ₂ O outflow suppressing means serves as a reducing agent to be contained in the exhaust gas from the NO _x catalyst. N 2 _O runoff exhaust purification system of an internal combustion engine according to any one of claims 4-8 to select the reducing agent-mixed means a reducing agent such as less than the predetermined amount. Engine to lower the reducing agent addition device for adding a reducing agent to the exhaust gas, the air-fuel ratio of the engine exhaust gas discharged from the internal combustion engine body within a predetermined range to pass through the engine exhaust passage in the exhaust upstream side of the the NO _x catalyst Exhaust air-fuel ratio lowering means, wherein the N ₂ O outflow suppressing means adds a reducing agent from the reducing agent adding device to the engine exhaust gas whose air-fuel ratio has been lowered by the engine exhaust air-fuel ratio lowering means. an exhaust purification system of an internal combustion engine according to any one of claims 4-7 which is adapted to perform the NO _x withdrawal process by. An exhaust gas recirculation passage for returning exhaust gas from the engine exhaust passage to the engine intake passage, wherein the exhaust gas recirculation passage is provided with a recirculation gas cooling device for cooling exhaust gas passing through the exhaust gas recirculation passage; A cooling device bypass passage for bypassing the recirculating gas cooling device, wherein the engine exhaust air-fuel ratio lowering means returns the exhaust gas to the engine intake passage through the cooling device bypass passage without passing through the recirculating gas cooling device. The exhaust gas purification device for an internal combustion engine according to claim 10, wherein the air-fuel ratio of the exhaust gas is reduced. With the air-fuel ratio of the inflowing exhaust gas when the lean holding the NO _x in the exhaust gas, NO _x catalyst for releasing the NO _x held when the air-fuel ratio of the inflowing exhaust gas of approximately stoichiometric or rich Check There is provided on the engine exhaust passage, a reducing agent addition device for adding a reducing agent to the exhaust gas passing through the engine exhaust passage in the exhaust upstream side of the the NO _x catalyst, the engine exhaust gas discharged from an internal combustion engine body Engine exhaust air-fuel ratio lowering means for lowering the fuel ratio within a predetermined range,
Be substantially the stoichiometric air-fuel ratio or rich air-fuel ratio of the exhaust gas flowing into the NO _x catalyst by the air-fuel ratio by the engine exhaust air-fuel ratio decrease means for adding a reducing agent from the reducing agent addition device to the engine exhaust gas is caused to decrease Exhaust gas purifying apparatus for an internal combustion engine.

Images