JP4868096B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

The engine exhaust passage, NO _X storage catalyst air-fuel ratio of the inflowing exhaust gas when the lean that releases NO _X air-fuel ratio of the exhaust gas which is occluded becomes rich for occluding NO _X contained in the exhaust gas inflow was placed, NO _X occluding catalyst upstream of the engine oxidation catalyst having an adsorbing function in the exhaust passage disposed, NO _X from occluding catalyst when releasing the NO _X is feeding hydrocarbons into the engine exhaust passage an oxidation catalyst upstream An internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is made rich is known (see, for example, Patent Document 1).
In this internal combustion engine, hydrocarbons supplied when NO _X is to be released from the NO _X storage catalyst are converted into gaseous hydrocarbons in the oxidation catalyst, and the gaseous hydrocarbons are sent to the NO _X storage catalyst. As a result, the NO _X released from the NO _X storing catalyst is made to satisfactorily reduced.

Patent No. 3969450

However, the NO _X storage catalyst has a problem that the NO _X purification rate decreases when the temperature becomes high.
An object of the present invention is to provide an exhaust purification device for an internal combustion engine that can obtain a high NO _x purification rate even when the temperature of the exhaust purification catalyst becomes high.

According to the present invention, a hydrocarbon supply valve for supplying hydrocarbons is disposed in the engine exhaust passage, hydrocarbons injected from the hydrocarbon supply valve and partially oxidized, and NO _X contained in the exhaust gas, An exhaust purification catalyst for reacting the gas is disposed in the engine exhaust passage downstream of the hydrocarbon feed valve, a noble metal catalyst is supported on the exhaust purification catalyst and a basic layer is formed. When the hydrocarbon is injected from the hydrocarbon supply valve at a predetermined supply interval while maintaining the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst lean, it has the property of reducing NO _X contained in the exhaust gas has a property that storage amount of the NO _X contained a longer in the exhaust gas than a predetermined supply interval supplied intervals hydrocarbons is increased, the exhaust gas flowing into the exhaust gas purifying catalyst at the time of engine operation A first NO _X removal method for purifying NO _X contained in the exhaust gas by injecting fuel ratio of the scan at a predetermined supply interval hydrocarbons from the hydrocarbon feed valve while maintaining lean exhaust A second NO _X purification method for purifying NO _X by changing the air-fuel ratio of the exhaust gas flowing into the purification catalyst from lean to rich at intervals longer than a predetermined supply interval according to the operating state of the engine An exhaust gas purification apparatus for an internal combustion engine that is selectively used is provided.

By selectively using the first NO _X purification method and the second NO _X purification method, a high NO _X purification rate can be obtained regardless of the operating state of the engine.

FIG. 1 is an overall view of a compression ignition type internal combustion engine.
FIG. 2 is a view schematically showing the surface portion of the catalyst carrier.
FIG. 3 is a diagram for explaining the oxidation reaction in the oxidation catalyst.
FIG. 4 is a diagram showing changes in the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst.
Figure 5 is a diagram illustrating a NO _X purification rate.
FIG. 6 is a diagram for explaining the oxidation-reduction reaction in the exhaust purification catalyst.
FIG. 7 is a diagram for explaining the oxidation-reduction reaction in the exhaust purification catalyst.
FIG. 8 is a diagram showing changes in the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst.
Figure 9 is a diagram illustrating a map of exhaust amount of NO _X NOXA.
FIG. 10 is a diagram showing the fuel injection timing.
Figure 11 is a diagram illustrating a NO _X purification rate.
FIG. 12 is a diagram showing a map of the injection amount of hydrocarbons.
FIG. 13 is a diagram showing the NO _X discharge speed and the like.
FIG. 14 is a time chart showing changes in the air-fuel ratio (A / F) in of the exhaust gas when switching from the second NO _X purification method to the first NO _X purification method.
FIG. 15 is a flowchart for performing NO _X purification control.
FIG. 16 is a diagram showing a flowchart and the like showing another embodiment of the NO _X purification method determining unit A shown in FIG.
FIG. 17 is a flowchart showing still another embodiment of the NO _X purification method determination unit A shown in FIG.
FIG. 18 is a flowchart showing still another embodiment of the NO _X purification method determination unit A shown in FIG.
FIG. 19 is a time chart showing changes in the air-fuel ratio (A / F) in of the exhaust gas when switching from the second NO _X purification method to the first NO _X purification method.
FIG. 20 is a time chart showing changes in the air-fuel ratio (A / F) in of the exhaust gas when switching from the second NO _X purification method to the first NO _X purification method.
FIG. 21 is a diagram showing an increase coefficient.
Figure 22 is a partially enlarged cross-sectional view of another catalyst for purifying NO _X.

  FIG. 1 shows an overall view of a compression ignition type internal combustion engine.
  Referring to FIG. 1, 1 is an engine body, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, 4 is an intake manifold, and 5 is an exhaust manifold. Respectively. The intake manifold 4 is connected to the outlet of the compressor 7 a of the exhaust turbocharger 7 via the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 9 via the intake air amount detector 8. A throttle valve 10 driven by a step motor is disposed in the intake duct 6, and a cooling device 11 for cooling intake air flowing through the intake duct 6 is disposed around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 11, and the intake air is cooled by the engine cooling water.
  On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7 b of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7 b is connected to the inlet of the hydrocarbon partial oxidation catalyst 13 that can partially oxidize hydrocarbon HC via the exhaust pipe 12. Connected. In the embodiment shown in FIG. 1, the hydrocarbon partial oxidation catalyst 13 is an oxidation catalyst. The outlet of the hydrocarbon partial oxidation catalyst, that is, the oxidation catalyst 13 is connected to the inlet of the exhaust purification catalyst 14, and the outlet of the exhaust purification catalyst 14 is connected to the particulate filter 15 for collecting fine particles contained in the exhaust gas. Is done. In the exhaust pipe 12 upstream of the oxidation catalyst 13, a hydrocarbon supply valve 16 is provided for supplying hydrocarbons composed of light oil and other fuels used as fuel for a compression ignition type internal combustion engine. In the embodiment shown in FIG. 1, light oil is used as the hydrocarbon supplied from the hydrocarbon supply valve 16. The present invention can also be applied to a spark ignition type internal combustion engine in which combustion is performed under a lean air-fuel ratio. In this case, the hydrocarbon supply valve 16 supplies hydrocarbons made of gasoline or other fuel used as fuel for the spark ignition type internal combustion engine.
  On the other hand, the exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 17, and an electronically controlled EGR control valve 18 is disposed in the EGR passage 17. A cooling device 19 for cooling the EGR gas flowing in the EGR passage 17 is disposed around the EGR passage 17. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 19, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a common rail 21 via a fuel supply pipe 20, and this common rail 21 is connected to a fuel tank 23 via an electronically controlled variable discharge pump 22. The fuel stored in the fuel tank 23 is supplied into the common rail 21 by the fuel pump 22, and the fuel supplied into the common rail 21 is supplied to the fuel injection valve 3 through each fuel supply pipe 20.
  The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. A temperature sensor 24 for detecting the temperature of the oxidation catalyst 13 is attached to the oxidation catalyst 13, and a temperature sensor 25 for detecting the temperature of the exhaust purification catalyst 14 is attached to the exhaust purification catalyst 14. The output signals of the temperature sensors 24 and 25 and the intake air amount detector 8 are input to the input port 35 via corresponding AD converters 37, respectively. A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. Is done. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 °. On the other hand, the output port 36 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 10, the hydrocarbon supply valve 16, the EGR control valve 18, and the fuel pump 22 through corresponding drive circuits 38.
  FIG. 2A schematically shows a surface portion of the catalyst carrier supported on the base of the oxidation catalyst 13. As shown in FIG. 2A, a catalyst 51 made of a noble metal such as platinum Pt or a transition metal such as silver Ag or copper Cu is supported on a catalyst carrier 50 made of alumina, for example.
  On the other hand, FIG. 2B schematically shows the surface portion of the catalyst carrier supported on the base of the exhaust purification catalyst 14. In this exhaust purification catalyst 14, as shown in FIG. 2B, noble metal catalysts 53 and 54 are supported on a catalyst support 52 made of alumina, for example, and further on this catalyst support 52 potassium K, sodium Na , Alkali metals such as cesium Cs, alkaline earth metals such as barium Ba, calcium Ca, rare earths such as lanthanoids and silver Ag, copper Cu, iron Fe, NO such as iridium In_XA basic layer 55 containing at least one selected from metals capable of donating electrons is formed. Since the exhaust gas flows along the catalyst carrier 52, it can be said that the noble metal catalysts 53 and 54 are supported on the exhaust gas flow surface of the exhaust purification catalyst 14. Further, since the surface of the basic layer 55 exhibits basicity, the surface of the basic layer 55 is referred to as a basic exhaust gas flow surface portion 56.
  In FIG. 2B, the noble metal catalyst 53 is made of platinum Pt, and the noble metal catalyst 54 is made of rhodium Rh. That is, the noble metal catalysts 53 and 54 supported on the catalyst carrier 52 are composed of platinum Pt and rhodium Rh. In addition to platinum Pt and rhodium Rh, palladium Pd can be further supported on the catalyst carrier 52 of the exhaust purification catalyst 14, or palladium Pd can be supported instead of rhodium Rh. That is, the noble metal catalysts 53 and 54 supported on the catalyst carrier 52 are composed of platinum Pt and at least one of rhodium Rh and palladium Pd.
  When hydrocarbons are injected into the exhaust gas from the hydrocarbon supply valve 16, the hydrocarbons are oxidized in the oxidation catalyst 13. In the present invention, at this time, the hydrocarbon is partially oxidized in the oxidation catalyst 13, and the exhausted catalyst 14 is used for NO by using the partially oxidized hydrocarbon._XTo purify. In this case, if the oxidizing power of the oxidation catalyst 13 is increased too much, the hydrocarbon is oxidized without being partially oxidized in the oxidation catalyst 13, and it is necessary to weaken the oxidizing power of the oxidation catalyst 13 in order to partially oxidize the hydrocarbon. . Therefore, in the embodiment according to the present invention, a catalyst having a small amount of noble metal catalyst supported, a catalyst supporting a base metal, or a catalyst having a small capacity is used as the oxidation catalyst 13.
  FIG. 3 schematically shows an oxidation reaction performed in the oxidation catalyst 13. As shown in FIG. 3, the hydrocarbon HC injected from the hydrocarbon feed valve 16 is converted into a radical hydrocarbon HC having a small number of carbons by the catalyst 51. At this time, some of the hydrocarbon HC is combined with NO to become a nitroso compound as shown in FIG. 3, and some of the hydrocarbon HC is NO.₂To form a nitro compound. These radical hydrocarbons HC and the like generated in the oxidation catalyst 13 are sent to the exhaust purification catalyst 14.
  Next, referring to FIG. 4 to FIG. 6, the first NO._XA purification method will be described.
  4 shows the change in the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14, and FIG. 5 shows the change of the exhaust gas flowing into the exhaust purification catalyst 14 as shown in FIG. NO by exhaust purification catalyst 14 when air-fuel ratio (A / F) in is changed_XThe purification rate is shown for each catalyst temperature TC of the exhaust purification catalyst 14.
  Now, the present inventor has determined NO over a long period of time._XIn the course of the research, as shown in FIG. 4, the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 is leaned at a certain time interval which will be described later. When it is intermittently lowered within the air-fuel ratio range, as shown in FIG. 5, the NO is extremely high even in a high temperature region of 400 ° C. or higher._XIt has been found that a purification rate can be obtained. Further, at this time, a large amount of reducing intermediates containing nitrogen and hydrocarbons are kept or adsorbed on the surface of the basic layer 55, that is, on the basic exhaust gas flow surface portion 56 of the exhaust purification catalyst 14. Reducing intermediate is high NO_XIt turns out that it plays a central role in obtaining the purification rate.
  Next, this will be described with reference to FIGS. 6 (A) and 6 (B). 6A and 6B schematically show the surface portion of the catalyst carrier 52 of the exhaust purification catalyst 14, and these FIGS. 6A and 6B are shown in FIG. FIG. 2 shows a reaction that is supposed to occur when the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 is intermittently lowered within the range of the lean air-fuel ratio.
  That is, as can be seen from FIG. 4, since the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is maintained lean, the exhaust gas flowing into the exhaust purification catalyst 14 is in an oxygen excess state. Therefore, the NO contained in the exhaust gas is oxidized on the platinum 53 as shown in FIG.₂It becomes. Then this NO₂Is further oxidized and stable nitrate ion NO₃ ⁻It becomes.
  On the other hand, nitrate NO₃ ⁻Is produced, this nitrate NO₃ ⁻Is pulled back in the direction of reduction by the hydrocarbon HC fed onto the surface of the basic layer 55, and oxygen is desorbed and unstable NO.₂ ^*It becomes. This unstable NO₂ ^*Is highly active, and this unstable NO₂ ^*Active NO₂ ^*Called. This active NO₂ ^*As shown in FIG. 6 (A), mainly radical hydrocarbons HC adhering to the surface of the basic layer 55 or rhodium Rh54, or mainly radical carbonization contained in the exhaust gas. Reacts with hydrogen HC on rhodium Rh54, thereby producing a reducing intermediate. This reducing intermediate is attached or adsorbed on the surface of the basic layer 55.
  Note that the first reducing intermediate produced at this time is the nitro compound R-NO.₂It is thought that. This nitro compound R-NO₂Is produced, it becomes a nitrile compound R-CN, but this nitrile compound R-CN can only survive for a moment in that state, so it immediately becomes an isocyanate compound R-NCO. When this isocyanate compound R-NCO is hydrolyzed, the amine compound R-NH₂It becomes. However, in this case, it is considered that a part of the isocyanate compound R-NCO is hydrolyzed. Therefore, as shown in FIG. 6A, most of the reducing intermediates retained or adsorbed on the surface of the basic layer 55 are isocyanate compound R-NCO and amine compound R-NH.₂It is thought that.
  On the other hand, the active NO generated as shown in FIG.₂ ^*Is a reducing intermediate R-NCO or R-NH on rhodium Rh54.₂Reacts with N₂, CO₂, H₂O, so NO_XWill be purified. That is, the reducing intermediate R-NCO or R-NH is formed on the basic layer 55.₂NO if not retained or adsorbed_XNo purification is performed. Therefore high NO_XGenerated active NO to obtain purification rate₂ ^*N₂, CO₂, H₂A sufficient amount of reducing intermediate R-NCO or R-NH to form O₂Must always be present on the basic layer 55, that is, on the basic exhaust gas flow surface portion 26.
  That is, as shown in FIGS. 6A and 6B, in order to oxidize NO on platinum Pt 53, the air-fuel ratio (A / F) in of the exhaust gas must be lean, and the generated active NO₂ ^*N₂, CO₂, H₂A sufficient amount of reducing intermediate R-NCO or R-NH on the surface of the basic layer 55 to form O₂I.e. reducing intermediates R-NCO and R-NH₂Therefore, it is necessary to provide a basic exhaust gas flow surface portion 26 in order to hold the air.
  Therefore, as shown in FIGS. 6A and 6B, NO contained in the exhaust gas._XReactive intermediates R-NCO and R-NH containing nitrogen and hydrocarbons by reacting with partially oxidized hydrocarbons₂In order to produce NO, noble metal catalysts 53 and 54 are supported on the exhaust gas flow surface of the exhaust purification catalyst 14, and the produced reducing intermediates R-NCO and R-NH₂Is maintained around the noble metal catalyst 53, 54, and a basic exhaust gas flow surface portion 26 is formed around the noble metal catalyst 53, 54. Reducing intermediates R-NCO and R-NH₂NO due to the reducing action of_XIs reduced. Therefore, this first NO_XIn the purification method, hydrocarbon HC is intermittently supplied from the hydrocarbon supply valve 16 at a predetermined supply interval while maintaining the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 lean. The predetermined supply interval of hydrogen HC is such that reducing intermediates R-NCO and R-NH are provided on the basic exhaust gas flow surface portion 56.₂This is the supply interval necessary to continue to exist.
  In this case, if the injection amount is too large or the injection interval becomes too short, the amount of hydrocarbons becomes excessive and a large amount of hydrocarbon HC is discharged from the exhaust purification catalyst 14, and the injection amount is too small, or the injection interval is short. If the length is too long, the reducing intermediate R-NCO or R-NH is formed on the basic exhaust gas flow surface portion 56.₂Will no longer exist. Therefore, what is important in this case is that excess hydrocarbons HC are not discharged from the exhaust purification catalyst 14 and the reducing intermediates R-NCO and R-NH are formed on the basic exhaust gas flow surface portion 26.₂Is to set the injection amount of hydrocarbons and the injection interval so as to continue to exist. Incidentally, in the example shown in FIG. 4, the injection interval is 3 seconds.
  Next, referring to FIG. 7 to FIG._XA purification method will be described. In the case shown in FIG. 4, when the supply interval of hydrocarbon HC is made longer than the above-described predetermined supply interval, hydrocarbon HC, reducing intermediate R-NCO, R-NH are formed on the surface of basic layer 55.₂Disappeared, and nitrate ion NO generated on platinum Pt53 at this time₃ ⁻Nitrate ion NO₃ ⁻The force to pull it back in the direction of reducing does not act. Therefore, at this time, nitrate ion NO₃ ⁻As shown in FIG. 7A, it diffuses into the basic layer 55 and becomes nitrate. That is, at this time, NO in the exhaust gas_XWill be absorbed in the basic layer 55 in the form of nitrate.
  On the other hand, FIG._XThis shows a case where the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is made the stoichiometric air-fuel ratio or rich when NO is absorbed in the basic layer 55 in the form of nitrate. In this case, the reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂), And thus the nitrates absorbed in the basic layer 55 are successively converted to nitrate ions NO.₃ ⁻And NO as shown in FIG.₂From the basic layer 55. Then released NO₂Is reduced by hydrocarbons HC and CO contained in the exhaust gas.
  FIG. 8 shows this NO_X2nd NO using the absorption and release action of_XThe purification method is shown. That is, this second NO_XIn the purification method, the occluded NO occluded in the basic layer 55 as shown in FIG._XWhen the amount ΣNOX exceeds a predetermined allowable amount MAX, the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 is temporarily made rich. When the air-fuel ratio (A / F) in of the exhaust gas is made rich, the NO absorbed in the basic layer 55 when the air-fuel ratio (A / F) in of the exhaust gas is lean_XAre released from the basic layer 55 at once and reduced. NO_XIs purified.
  Occlusion NO_XThe amount ΣNOX is, for example, NO discharged from the engine_XCalculated from the quantity. In the embodiment according to the present invention, the emission NO discharged from the engine per unit time_XThe amount NOXA is stored in advance in the ROM 32 as a function of the engine load L and the engine speed N in the form of a map as shown in FIG._XFrom NOXA to NO_XAn amount ΣNOX is calculated. The period in which the air-fuel ratio (A / F) in of the exhaust gas is made rich is much longer than the period in which the air-fuel ratio (A / F) in of the exhaust gas is lowered, as shown in FIG. The period during which the air-fuel ratio (A / F) in is made rich is usually 1 minute or longer.
  2nd NO_XIn the purification method, when the air-fuel ratio (A / F) in of the exhaust gas is lean, NO contained in the exhaust gas_XIs absorbed in the basic layer 55, so that the basic layer 55 is NO._XIt plays the role of an absorbent for temporarily absorbing. At this time, the basic layer 55 is NO._XTherefore, when the term occlusion is used as a term including both absorption and adsorption, the basic layer 55 is_XNO for temporary storage_XIt plays the role of a storage agent. That is, the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 2 and the exhaust passage upstream of the exhaust purification catalyst 14 is referred to as the air-fuel ratio of the exhaust gas._XIn the purification method, the exhaust purification catalyst 14 is NO when the air-fuel ratio of the exhaust gas is lean._XNO is stored when the oxygen concentration in the exhaust gas decreases._XNO release_XIt functions as a storage catalyst.
  In addition, this second NO_XIn the purification method, as shown in FIG. 10, the air-fuel ratio (A) of the exhaust gas flowing into the exhaust purification catalyst 14 by injecting additional fuel W in addition to the combustion fuel M from the fuel injection valve 3 into the combustion chamber 2. / F) in is made rich. The horizontal axis in FIG. 10 indicates the crank angle. This additional fuel W is injected when it burns but does not appear as engine output, that is, slightly before ATDC 90 ° after compression top dead center. Of course, the air-fuel ratio (A / F) in of the exhaust gas can be made rich by increasing the amount of hydrocarbons supplied from the hydrocarbon supply valve 16 in this case.
  FIG. 11 shows that the exhaust purification catalyst 14 is NO._XNO when functioning as a storage catalyst_XThe purification rate is shown. The horizontal axis in FIG. 11 indicates the catalyst temperature TC of the exhaust purification catalyst 14. Set the exhaust purification catalyst 14 to NO_XIn the case of functioning as an occlusion catalyst, as shown in FIG. 11, when the catalyst temperature TC is 300 ° C. to 400 ° C., extremely high NO_XA purification rate can be obtained, but NO when the catalyst temperature TC reaches 400 ° C or higher._XThe purification rate decreases.
  Thus, when the catalyst temperature TC reaches 400 ° C. or higher, NO_XThe purification rate decreases because when the catalyst temperature TC reaches 400 ° C. or higher, the nitrate is thermally decomposed and NO.₂This is because it is discharged from the exhaust purification catalyst 14 in the form of. That is, NO_XAs long as the catalyst temperature TC is high._XIt is difficult to obtain a purification rate. However, the first NO shown in FIGS. 4 to 6 (A) and (B)_XAs can be seen from FIGS. 6 (A) and 6 (B), in the purification method, nitrate is not produced or is produced in a very small amount even if it is produced, and thus is high even when the catalyst temperature TC is high as shown in FIG. NO_XA purification rate will be obtained.
  That is, the first NO shown in FIGS. 4 to 6 (A) and (B)._XThe purification method carries a noble metal catalyst and NO._XIn the case of using an exhaust purification catalyst having a basic layer capable of absorbing NO, NO hardly forms nitrates._XNew NO to purify_XIt can be said that it is a purification method. In fact, this first NO_XIf the purification method is used, the second NO_XCompared with the case where the purification method is used, the amount of nitrate detected from the basic layer 55 is extremely small.
  On the other hand, the first NO_XNO using the purification method_XNO in exhaust gas to purify_XEven when the concentration is low, it is necessary to supply a certain amount or more of hydrocarbons in a short cycle. Therefore, the exhaust gas NO_XNO when the concentration is low_XThe purification efficiency becomes worse. In contrast, the second NO_XIn the purification method, NO in exhaust gas_XWhen the concentration is low, storage NO_XSince the time until the amount ΣNOX reaches the allowable value MAX becomes longer, only the period for enriching the air-fuel ratio (A / F) in of the exhaust gas becomes longer._XPurification efficiency does not deteriorate. Therefore, NO in exhaust gas_XWhen the concentration is low, the first NO_XSecond NO over purification method_XIt can be said that it is preferable to use a purification method.
  That is, the first NO_XPurification method and second NO_XWhich of the purification methods should be used depends on the operating condition of the engine. Therefore, in the present invention, the noble metal catalysts 53 and 54 are supported on the exhaust purification catalyst 14 and the basic layer 55 is formed. The exhaust purification catalyst 14 is emptied of exhaust gas flowing into the exhaust purification catalyst 14. If hydrocarbons are injected from the hydrocarbon supply valve 16 at a predetermined supply interval while maintaining the fuel ratio lean, the NO contained in the exhaust gas_XNO and contained in the exhaust gas when the hydrocarbon supply interval is longer than the predetermined supply interval._XThis predetermined supply interval of hydrocarbons from the hydrocarbon supply valve 16 while maintaining the lean air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 during engine operation. NO contained in exhaust gas by injecting with_XFirst NO to purify_XNO by changing the purification method and changing the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 from lean to rich at an interval longer than this predetermined supply interval._XSecond NO to purify_XThe purification method is selectively used according to the operating state of the engine.
  Next, a representative embodiment according to the present invention will be described with reference to FIGS.
  FIG. 12A shows the hydrocarbon supply amount QE from the hydrocarbon supply valve 16, and FIG. 12B shows the additional fuel amount W supplied into the combustion chamber 2. The hydrocarbon supply amount QE is stored in advance in the ROM 32 in the form of a map as shown in FIG. 12A as a function of the engine load QE and the engine speed N, and the additional fuel amount W is also stored in the engine load QE and the engine. As a function of the rotational speed N, it is stored in advance in the ROM 32 in the form of a map as shown in FIG.
  FIG. 13A shows the storage NO discharged from the exhaust purification catalyst 14 when the air-fuel ratio (A / F) in of the exhaust gas is lean._XThe discharge speed NOXD is shown. As mentioned above, NO is stored in the form of nitrate._XIs exhausted by thermal decomposition when the temperature TC of the exhaust purification catalyst 14 rises._XExhalation speed NOXD, that is, NO exhaled per unit time_XThe amount NOXD increases rapidly when the temperature TC of the exhaust purification catalyst 14 exceeds the thermal decomposition start temperature of about 450 ° C.
  On the other hand, FIG. 13B shows the first NO._XNO by purification method_XNO stored in the exhaust purification catalyst 14 when the purification action is performed_XThe occlusion rate SX is shown. 1st NO_XNO by purification method_XWhen the purification action is being performed, the exhaust purification catalyst 14 is normally NO._XIs not occluded. However, when the exhaust gas flow rate is increased, that is, when the intake air amount GA is increased, the reaction time is shortened and the reaction is not sufficiently performed.₂ ^*NO absorbed by the basic layer 55 without becoming_XThe amount increases. Accordingly, as shown in FIG. 13B, when the intake air amount GA becomes larger than a certain value, NO_XThe storage rate SX begins to increase.
  Thus, the first NO_XNO by purification method_XEven when the purification action is being performed, the exhaust purification catalyst 14 has NO._XMay be occluded, and at this time NO is occluded per unit time._XThe amount is NO from the engine per unit time_XNO to amount NOXA_XThe value SX · NOXA is obtained by multiplying the storage rate SX. In the embodiment according to the present invention, the first NO is obtained by integrating SX · NOXA._XNO by purification method_XOcclusion NO that is occluded when the purification action is performed_XThe amount is calculated and the first NO_XThe second NO from the purification method_XWhen switched to the purification method, the first NO_XStorage NO calculated during the purification method_XNO occlusion based on quantity_XQuantity calculation is started.
  That is, in an exemplary embodiment according to the present invention, the first NO_XThe second NO from the purification method_XWhen switched to the purification method, the first NO_XNO calculated when the purification method was used_XStorage amount and second NO_XNO calculated after switching to purification method_XWhen the total amount ΣNOX exceeds a predetermined allowable value MAX when the stored amount is summed, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is temporarily made rich. In this case, the first NO_XOccupation NO when using purification method_XIf the amount is ignored, the second NO_XWhen switching to the purification method, the timing for enriching the air-fuel ratio (A / F) in of the exhaust gas is delayed, and therefore some NO._XWill be released into the atmosphere without being occluded. However, in the embodiment according to the present invention, the first NO_XNO when the purification method is used_XThe amount of occlusion is taken into account, and therefore the above-mentioned problems do not occur.
  On the other hand, the second NO_XNO from the purification method_XWhen switched to the purification method, NO is stored in the exhaust purification catalyst 14._XIs stored, when the temperature TC of the exhaust purification catalyst 14 rises due to the supply of hydrocarbons,_XIs NO_XIt is discharged from the purification catalyst 14. 1st NO_XNO by purification method_XWhen the purifying action is being performed, the NO exhaled in this way_XNO reduction action is performed, so NO_XWill be discharged into the atmosphere.
  However, if the air-fuel ratio (A / F) in of the exhaust gas is made rich, the stored NO remaining in the exhaust purification catalyst 14_XCan be reduced, and thus NO_XCan be prevented from being discharged into the atmosphere. Therefore, in the embodiment according to the present invention, as shown in FIG._XNO from the purification method_XWhen switched to the purification method, NO stored in the exhaust purification catalyst 14_XThe air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is made rich at a time in order to release and reduce the.
  In this case, the second NO in the embodiment shown in FIG._XNO from the purification method_XBy supplying additional fuel W into the combustion chamber 2 immediately before switching to the purification method, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is made rich. 14 shows the change in the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 and the storage NO stored in the exhaust purification catalyst 14._XThe change of the quantity ΣNOX is shown. As can be seen from FIG. 14, the first NO_XWhen the purification method is started, NO is stored_XThe quantity ΣNOX is zero, so NO_XIs prevented from being released into the atmosphere.
  On the other hand, the first NO_XNO by purification method_XThe purification action is not performed unless the oxidation catalyst 13 is activated. Therefore, in the embodiment according to the present invention, the first NO_XIn the purification method, the temperature TB of the oxidation catalyst 13 is the activation temperature TB.₀The temperature TB of the oxidation catalyst 13 is used only when the above becomes the activation temperature TB.₀1st NO when lower than_XUse of purification methods is prohibited. At this time, that is, the temperature TB of the oxidation catalyst 13 is the activation temperature TB.₀2nd NO when lower than_XA purification method is used.
  In a typical embodiment according to the present invention, the temperature TB of the oxidation catalyst 13 is the activation temperature TB.₀When the above is the first NO_XPurification method and second NO_XAny of the purification methods are used. In this case, the second NO_XFirst NO rather than using purification method_XIt is NO to use the purification method_XThe first NO when the purification efficiency is high_XA purification method is used and the first NO_X2nd NO rather than using purification method_XIt is NO to use the purification method_XWhen the purification efficiency is high, the second NO_XA purification method is used.
  FIG. 15 shows the NO for implementing an exemplary embodiment of the present invention._XThe purification control routine is shown. This routine is executed by interruption every predetermined time.
  Referring to FIG. 15, first, at step 60, the emission NO per unit time is calculated from the map shown in FIG._XThe quantity NOXA is calculated. Then the first NO_XUse purification method or second NO_XNO to decide whether to use purification method_XProceed to purification method determination unit A. This NO_XIn the purification method determining unit A, first, in step 61, the temperature TB of the oxidation catalyst 13 is changed to the activation temperature TB.₀It is determined whether or not this is the case. TB <TB₀When the second NO_XIt is determined that the purification method should be used.
  In contrast, TB ≧ TB₀At step 62, the routine proceeds to step 62 where the first NO_XNO when using purification method_XPurification efficiency F₁And second NO_XNO when using purification method_XPurification efficiency F₂And are calculated. This NO_XPurification efficiency F₁, F₂Is the unit NO_XIt represents the fuel or hydrocarbon consumption per unit time required to obtain the purification rate. In this case, NO_XPurification efficiency F₁Is the hydrocarbon feed rate QE and hydrocarbon injection interval shown in FIG. 12 (A) and the NO shown in FIG._XCalculated from the purification rate, NO_XPurification efficiency F₂Is the interval between the additional fuel amount W shown in FIG. 12 (B) and the timing at which the rich air-fuel ratio is made in FIG. 8 and the NO shown in FIG._XCalculated from the purification rate.
  Next, in step 63, NO_XPurification efficiency F₁Is NO_XPurification efficiency F₂Or higher is determined. F₁≧ F₂At the first NO_XIt is determined that the purification method should be used. On the other hand, F₁<F₂When the second NO_XIt is determined that the purification method should be used, and the process proceeds to step 64.
  Next, the second NO performed in step 64 to step 67_XThe purification method will be described first. First, at step 64, the exhaust NO shown in FIG._XOcclusion NO by adding the amount NOXA_XAn amount ΣNOX is calculated. Next, at step 65, NO is stored._XIt is determined whether or not the amount ΣNOX exceeds the allowable value MAX. When ΣNOX> MAX, the routine proceeds to step 66, where the additional fuel amount W is calculated from the map shown in FIG. 12B, and the additional fuel injection action is performed. Next, at step 67, ΣNOX is cleared.
  Next, the first NO performed in step 68 to step 74_XA purification method will be described. First, in step 68, the stored NO remaining in the exhaust purification catalyst 14 is stored._XOcclusion NO to process_XIt is determined whether or not processing is being performed. Occlusion NO_XWhen the process is not performed, the routine proceeds to step 69 where the second NO_XNO from the purification method_XIt is determined whether or not the decision to switch to the purification method has now been made. 2nd NO_XNO from the purification method_XWhen the decision to switch to the purification method is made now, the routine proceeds to step 70 and the storage NO._XIt is determined whether or not the amount ΣNOX is smaller than a predetermined small value MIN.
  When ΣNOX> MIN, the routine proceeds to step 71, where NO is stored._XProcessing is performed. In this embodiment, as shown in FIG._XNO from the purification method_XImmediately before switching to the purification method, the air-fuel ratio (A / F) in of the exhaust gas is temporarily made rich. Next, at step 72, ΣNOX is cleared. Storage NO_XWhen the process is started, NO is stored_XJump from step 68 to step 71 until processing is complete.
  On the other hand, in step 69, the second NO_XNO from the purification method_XWhen it is determined that the decision to switch to the purification method has not been made, the routine proceeds to step 73. Further, when it is determined in step 70 that ΣNOX <MIN, that is, almost NO._XWhen it is determined that is not occluded, the process also proceeds to step 73. In step 73, a hydrocarbon supply amount QE is calculated from the map shown in FIG. 12A, and a hydrocarbon injection process is performed. Next, at step 74, the first NO is calculated based on the following equation._XNO by purification method_XNO stored in the exhaust purification catalyst 14 during the purification action_XAn amount ΣNOX is calculated.
  ΣNOX ← ΣNOX + SX ・ NOXA-NOXD
  Here, SX / NOXA is NO stored per unit time as described above._XNOXD is the discharge speed shown in FIG. 13 (A). 1st NO_XThe second NO from the purification method_XWhen switching to the purification method, NOXA is added to ΣNOX calculated in step 74 in step 64.
  FIG. 16 shows another embodiment. In this embodiment, NO_XPurification efficiency F₁Than NO_XPurification efficiency F₂As shown by hatching in FIG. 16 (A), the operating range of the engine that is considered to be high is set in advance as a function of the engine load L and the engine speed N, and when the oxidation catalyst 13 is activated, FIG. NO according to (A)_XA purification method is determined.
  FIG. 16B shows NO in FIG._XAnother embodiment of the purification method determination unit A will be shown. Referring to FIG. 16B, in step 61, the temperature TB of the oxidation catalyst 13 is changed to the activation temperature TB.₀2nd NO when lower than_XIt is determined that the purification method should be used, and the process proceeds to step 64 in FIG. On the other hand, in step 61, TB ≧ TB₀When it is determined that the engine is operating, the routine proceeds to step 61a, where the operating state of the engine is indicated by hatching in FIG._XIt is determined whether or not the region where the purification method should be used. The engine operating state is the second NO_XWhen it is the region where the purification method is to be used, the process proceeds to step 64 in FIG. In contrast, the engine operating state is the second NO._XWhen it is determined that the region is not the region where the purification method should be used, the process proceeds to step 68 in FIG.
  FIG. 17 shows NO in FIG._XAnother example of the purification method determination unit A will be described. That is, the first NO_XNO when using purification method_XAs shown in FIG. 5, the purification rate is such that the temperature TC of the exhaust purification catalyst 14 is the limit temperature TC.₀It begins to decline rapidly when it becomes below. On the other hand, as shown in FIG._XNO when using purification method_XThe purification rate decreases relatively slowly when the temperature TC of the exhaust purification catalyst 14 decreases. Therefore, in this embodiment, the temperature TC of the exhaust purification catalyst 14 is the limit temperature TC.₀1st NO when higher than_XA purification method is used, and the temperature TC of the exhaust purification catalyst 14 is the limit temperature TC.₀2nd NO when lower than_XA purification method is used.
  That is, referring to FIG. 17, in step 61, the temperature TB of the oxidation catalyst 13 is changed to the activation temperature TB.₀2nd NO when lower than_XIt is determined that the purification method should be used, and the process proceeds to step 64 in FIG. On the other hand, in step 61, TB ≧ TB₀When it is determined that the temperature is TC, the routine proceeds to step 61a, where the temperature TC of the exhaust purification catalyst 14 becomes the limit temperature TC.₀Or higher is determined. TC <T₀In this case, the process proceeds to step 64 in FIG. In contrast, TC ≧ T₀At the first NO_XIt is determined that the purification method should be used, and the process proceeds to step 68 in FIG.
  18 shows NO in FIG._XAnother example of the purification method determination unit A will be described. That is, the first NO_XThe purification method is the second NO_XNO to be reduced compared to purification methods_XWhen the amount is large, that is, NO in the exhaust gas_XHigh NO when concentration D is high_XA purification rate can be obtained. Therefore, in this embodiment, NO in the exhaust gas._XDensity D is set value D₀First NO by whether or not_XUse purification method or 2nd NO_XIt is determined whether to use the purification method.
  That is, referring to FIG. 18, in step 61, the temperature TB of the oxidation catalyst 13 is changed to the activation temperature TB.₀2nd NO when lower than_XIt is determined that the purification method should be used, and the process proceeds to step 64 in FIG. On the other hand, in step 61, TB ≧ TB₀When it is determined that the temperature is TC, the routine proceeds to step 61a, where the temperature TC of the exhaust purification catalyst 14 becomes the limit temperature TC.₀Or higher is determined. TC <T₀In this case, the process proceeds to step 64 in FIG. In contrast, TC ≧ T₀In step 61b, for example, NO_XNO in exhaust gas detected by concentration sensor_XDensity D is set value D₀Or higher is determined. D <D₀In this case, the process proceeds to step 64 in FIG. In contrast, D ≧ D₀At the first NO_XIt is determined that the purification method should be used, and the process proceeds to step 68 in FIG.
  19 shows the storage NO performed in step 71 of FIG._XAnother embodiment of the process is shown. In this example, the second NO_XNO from the purification method_XImmediately after switching to the purification method, the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 is made rich. At this time, the air-fuel ratio (A / F) in of the exhaust gas is made rich by increasing the amount of hydrocarbons supplied from the hydrocarbon supply valve 16.
  That is, the second NO_XNO from the purification method_XNO occlusion when decision to switch to purification method is made_XWhen the amount ΣNOX is large, all the stored NO is obtained only by making the air-fuel ratio (A / F) in of the exhaust gas rich once by injecting additional fuel into the combustion chamber 2._XMay not be released and reduced. In such a case, as shown in FIG._XNO by purification method_XBy increasing the amount of hydrocarbons supplied when the purification action is started, the air-fuel ratio (A / F) in of the exhaust gas is made rich, whereby the total storage NO._XIs released and reduced.
  On the other hand, when additional fuel is injected into the combustion chamber 2, the temperature in the combustion chamber 2 increases. Therefore, during high load operation where the combustion temperature becomes high, it may not be possible to make the air-fuel ratio (A / F) in of the exhaust gas rich by injecting additional fuel into the combustion chamber 2. In such a case, the injection of additional fuel is stopped, and the amount of hydrocarbons supplied is increased to make the air-fuel ratio (A / F) in of the exhaust gas rich.
  FIG. 20 shows the storage NO performed in step 71 of FIG._XYet another embodiment of the process is shown. In this example, the second NO_XNO from the purification method_XAfter switching to the purification method, the first NO_XNO by purification method_XThe air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 14 after the purification action is started is made rich. In this embodiment, the storage NO._XThis was exhaled when NO was exhaled_XThe air-fuel ratio (A / F) in of the exhaust gas is made rich by supplying additional fuel into the combustion chamber 2 to reduce the amount of fuel or by increasing the amount of hydrocarbons supplied.
  On the other hand, when sulfur contained in the exhaust gas adheres to the surface of the noble metal, that is, when the noble metal causes sulfur poisoning, it is activated NO.₂ ^*Is difficult to generate. Therefore, even if noble metals cause sulfur poisoning, active NO₂ ^*It is preferable to increase the supply amount QE of hydrocarbons as the sulfur poisoning amount of the noble metal increases so that the production amount of hydrogen does not decrease. In the embodiment shown in FIG. 21, the active NO is increased even if the sulfur poisoning amount is increased.₂ ^*As the sulfur poisoning amount increases so as not to decrease the production amount of hydrocarbons, the increase coefficient with respect to the hydrocarbon supply amount QE is increased.
  FIG. 22 shows a case where the hydrocarbon partial oxidation catalyst 13 and the exhaust purification catalyst 14 shown in FIG. 1 are formed from one catalyst. This catalyst has, for example, a number of exhaust gas flow passages extending in the flow direction of the exhaust gas, and FIG. 22 shows an enlarged cross-sectional view of the surface portion of the inner peripheral wall 80 of the exhaust gas flow passage of this catalyst. As shown in FIG. 22, a lower coat layer 81 is formed on the surface of the inner peripheral wall 80 of the exhaust gas flow passage, and an upper coat layer 82 is formed on the lower coat layer 81. In the example shown in FIG. 22, both coat layers 81 and 82 are made of an aggregate of powder, and FIG. 22 shows an enlarged view of the powder constituting each coat layer 81 and 82. From the enlarged view of these powders, the upper coat layer 82 is composed of a hydrocarbon partial oxidation catalyst, for example, an oxidation catalyst shown in FIG. 2A, and the lower coat layer 81 is made of an exhaust purification catalyst shown in FIG. 2B. I understand that
  When the catalyst shown in FIG. 22 is used, the hydrocarbon HC contained in the exhaust gas diffuses into the upper coat layer 82 and is partially oxidized as shown in FIG. It diffuses into the lower coat layer 81. That is, in the example shown in FIG. 22 as well, the hydrocarbon partial oxidation catalyst and the exhaust purification catalyst are similar to the example shown in FIG. 1 in that the hydrocarbon partially oxidized in the hydrocarbon partial oxidation catalyst flows into the exhaust purification catalyst. Are arranged to be. On the other hand, the first NO in the catalyst shown in FIG._XIf the purification method is used, NO contained in the exhaust gas_XDiffuses into the lower coat layer 81 and becomes active NO₂ ^*It becomes. At this time, in the lower coat layer 81, the active NO₂ ^*From partially oxidized hydrocarbons to reducing intermediates R-NCO and R-NH₂Is generated and more active NO₂ ^*Is a reducing intermediate R-HCO or R-NH₂Reacts with N₂, CO₂, H₂O.
  On the other hand, as shown in FIG. 2B, noble metals 53 and 54 are supported on the catalyst carrier 52 of the exhaust purification catalyst 14. Can be reformed to hydrocarbon HC. In this case, if the hydrocarbons can be sufficiently reformed in the exhaust purification catalyst 14, that is, if the hydrocarbons can be sufficiently partially oxidized in the exhaust purification catalyst 14, the oxidation catalyst 13 upstream of the exhaust purification catalyst 14 as shown in FIG. Need not be placed. Therefore, in one embodiment according to the present invention, the oxidation catalyst 13 is not installed in the engine exhaust passage. Therefore, in this embodiment, the hydrocarbon injected from the hydrocarbon supply valve 16 is directly supplied to the exhaust purification catalyst 14.
  In this embodiment, the hydrocarbons injected from the hydrocarbon feed valve 16 are partially oxidized in the exhaust purification catalyst 14 and further contained in the exhaust gas in the exhaust purification catalyst 14._XFrom active NO₂ ^*Is generated. These active NOs are present in the exhaust purification catalyst 14.₂ ^*From partially oxidized hydrocarbons to reducing intermediates R-NCO and R-NH₂Is generated and more active NO₂ ^*Is a reducing intermediate R-NCO or R-NH₂Reacts with N₂, CO₂, H₂O. That is, in this embodiment, the hydrocarbons injected from the hydrocarbon feed valve 16 and partially oxidized hydrocarbons and NO contained in the exhaust gas are used._XIs disposed in the engine exhaust passage downstream of the hydrocarbon feed valve 16.

4 ... Intake manifold 5 ... Exhaust manifold 7 ... Exhaust turbocharger 12 ... Exhaust pipe 13 ... Oxidation catalyst 14 ... Exhaust purification catalyst 16 ... Hydrocarbon supply valve

Claims (18)

The hydrocarbon feed valve for feeding hydrocarbons is arranged in the engine exhaust passage, an exhaust for reacting NO _X contained the injected from the hydrocarbon feed valve and partially oxidized hydrocarbons in the exhaust gas A purification catalyst is disposed in an engine exhaust passage downstream of the hydrocarbon feed valve, a noble metal catalyst is supported on the exhaust purification catalyst, and a basic layer is formed. The exhaust purification catalyst is an exhaust purification catalyst. When the hydrocarbon is injected from the hydrocarbon supply valve at a predetermined supply interval while maintaining the lean air-fuel ratio of the exhaust gas flowing into the exhaust gas, it has the property of reducing NO _X contained in the exhaust gas, the feed interval has the property of absorbing the amount of NO _X contained in the exhaust gas to be longer than the supply interval defined the advance is increased, the air-fuel exhaust gas flowing into the exhaust purification catalyst at the time of engine operation A first NO _X removal method for purifying NO _X contained in the exhaust gas by the hydrocarbon from the hydrocarbon feed valve while maintaining lean injects a feed interval said predetermined, the exhaust purification catalyst A second NO _X purification method for purifying NO _X by switching the air-fuel ratio of the inflowing exhaust gas from lean to rich at intervals longer than the predetermined supply interval is selected according to the operating state of the engine An exhaust purification device for an internal combustion engine, which is used in a conventional manner.   In the engine exhaust passage downstream of the hydrocarbon feed valve, the exhaust purification catalyst and the hydrocarbon partial oxidation catalyst capable of partially oxidizing the hydrocarbon injected from the hydrocarbon feed valve were partially oxidized in the hydrocarbon partial oxidation catalyst. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein hydrocarbons are arranged to flow into the exhaust gas purification catalyst.   The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the hydrocarbon partial oxidation catalyst comprises an oxidation catalyst disposed in an engine exhaust passage upstream of the exhaust purification catalyst.   The exhaust purification device for an internal combustion engine according to claim 2, wherein an upper coat layer made of the hydrocarbon partial oxidation catalyst is formed on a lower coat layer made of the exhaust purification catalyst. In the first NO _X purification method, NO _X contained in the exhaust gas and the partially oxidized hydrocarbon react with each other by the noble metal catalyst to generate a reducing intermediate containing nitrogen and hydrocarbons. The reduced reducing intermediate is retained on the basic layer, NO _X is reduced by the reducing action of the reducing intermediate retained on the basic layer, and a predetermined supply interval of the hydrocarbon is The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the supply interval is necessary to keep the reducing intermediate on the surface portion of the basic exhaust gas flow. In the second NO _X purification method, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is lean, NO _X in the exhaust gas is absorbed in the basic layer and the exhaust gas flowing into the exhaust purification catalyst is emptied. fuel ratio exhaust gas control apparatus according to claim 1 that is reduced is released NO _X which is absorbed and becomes rich from the basic layer.   The exhaust purification device for an internal combustion engine according to claim 1, wherein the noble metal catalyst is composed of platinum Pt and at least one of rhodium Rh and palladium Pd. An exhaust purification system of an internal combustion engine according to claim 1, wherein the basic layer containing a metal capable of donating electrons to the alkali metal or alkaline earth metal or rare earth or NO _X. The first NO _X purification method is used only when the temperature of the oxidation catalyst is equal to or higher than the activation temperature, and the use of the first NO _X purification method is prohibited when the temperature of the oxidation catalyst is lower than the activation temperature. The exhaust emission control device for an internal combustion engine according to claim 1. The exhaust purification device for an internal combustion engine according to claim 9, wherein when the temperature of the oxidation catalyst is equal to or higher than the activation temperature, one of the first NO _X purification method and the second NO _X purification method is used. When than using the second of the NO _X purification method it is better to use the first NO _X removal method high NO _X purification efficiency is used the first NO _X removal method, than using first NO _X removal method even when the person using the second NO _X removal method high NO _X purification efficiency exhaust purification system of an internal combustion engine according to claim 10 in which the second of the NO _X purification method is used. The NO _X purification rate when the first NO _X purification method is used starts to decrease when the temperature of the exhaust purification catalyst falls below the limit temperature, and when the temperature of the exhaust purification catalyst is higher than the limit temperature, the first NO _X purification method is used, when the temperature of the exhaust purification catalyst is lower than the該限boundary temperature exhaust gas control apparatus according to claim 10 in which the second of the NO _X purification method is used. The exhaust purification device for an internal combustion engine according to claim 9, wherein the second NO _X purification method is used when the temperature of the oxidation catalyst is lower than the activation temperature. When switching from the second NO _X purification method to the first NO _X purification method, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is equalized in order to release NO _X stored in the exhaust purification catalyst and reduce it. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, which is sometimes rich. The exhaust gas purification apparatus for an internal combustion engine according to claim 14, wherein the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst is made rich immediately before switching from the second NO _X purification method to the first NO _X purification method. The exhaust gas purification apparatus for an internal combustion engine according to claim 15, wherein the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made rich immediately after switching from the second NO _X purification method to the first NO _X purification method. . After switching from the second NO _X purification method to the first NO _X purification method, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst after the NO _X purification action by the first NO _X purification method is started is The exhaust emission control device for an internal combustion engine according to claim 14, wherein the exhaust gas purification device is made rich. When the second NO _X purification method is used, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is temporarily reduced when the calculated NO _X storage amount in the exhaust purification catalyst exceeds a predetermined allowable value. When the first NO _X purification method is switched from the first NO _X purification method to the second NO _X purification method, the NO _X occlusion amount calculated when the first NO _X purification method is used and the second air-fuel ratio is temporarily of the exhaust gas flowing into the exhaust purification catalyst when the total value by summing the _the NO _X storage amount calculated after switching to the NO _X purification method exceeds a predetermined allowable value The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is made rich.

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