JP2006336589A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine capable of suppressing excessive rise of temperature of NOx absorption reduction catalyst by adding fuel and suppressing reduction of NOx removal rate due to high temperature of NOx absorption reduction catalyst after adding fuel. <P>SOLUTION: This exhaust emission control device of the internal combustion engine is provided with a fuel adding means 12, an HC absorption oxidation catalyst 42 arranged in an exhaust passage of the engine on the downstream side of the fuel adding means, and the NOx absorption reduction catalyst 43 arranged on the downstream side of the HC absorption oxidation catalyst. When fuel is added from the fuel adding means 12 by releasing NOx from the NOx absorption reduction catalyst 43 and reducing and purifying it, concentration of oxygen in exhaust gas exhausted from a combustion chamber 2 is controlled together with amount of fuel to be added to set air-fuel ratio of exhaust gas flowing into the NOx absorption reduction catalyst 43 to target air-fuel ratio decided in advance while preventing temperature of the NOx absorption reduction catalyst 43 from exceeding upper limit temperature decided in advance. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  When the air-fuel ratio of the exhaust gas flowing to purify NOx contained in the exhaust gas is lean, the NOx contained in the exhaust gas is occluded and the occluded NOx is released when the oxygen concentration in the inflowing exhaust gas decreases. An internal combustion engine in which an NOx occlusion reduction catalyst is arranged in an engine exhaust passage is known. In this internal combustion engine, NOx generated when combustion is performed under a lean air-fuel ratio is stored in the NOx storage reduction catalyst.

  By the way, when such a NOx occlusion reduction catalyst is used, it is necessary to release NOx from the NOx occlusion reduction catalyst before the NOx occlusion reduction capacity of the NOx occlusion reduction catalyst is saturated. In this case, the exhaust gas flowing into the NOx occlusion reduction catalyst must be released. If the air-fuel ratio is made rich, NOx can be released from the NOx storage reduction catalyst and the released NOx can be reduced. Therefore, in the conventional internal combustion engine, in order to release NOx from the NOx storage reduction catalyst, the air-fuel ratio in the combustion chamber is made rich, or fuel is supplied into the engine exhaust passage upstream of the NOx storage reduction catalyst to become the NOx storage reduction catalyst. The air-fuel ratio of the inflowing exhaust gas is made rich.

  On the other hand, an internal combustion engine in which an HC adsorption oxidation catalyst for adsorbing and oxidizing hydrocarbons contained in exhaust gas, that is, HC, is known in the engine exhaust passage upstream of the NOx storage reduction catalyst (for example, Patent Documents). 1). In this internal combustion engine, HC generated when combustion is performed at a lean air-fuel ratio is adsorbed by the HC adsorption oxidation catalyst, and NOx generated at this time is stored in the NOx storage reduction catalyst.

  By the way, in this internal combustion engine, when the temperature of the HC adsorption oxidation catalyst is near the activation temperature, that is, near 200 ° C., the oxidation reaction of the adsorbed HC becomes active, and as a result, oxygen in the exhaust gas is consumed rapidly. In addition, the oxygen concentration in the exhaust gas rapidly decreases. Therefore, at this time, if a small amount of fuel is additionally supplied, the air-fuel ratio of the exhaust gas can be made rich. Therefore, in this internal combustion engine, it is detected whether or not a sufficient amount of oxygen is consumed in the HC adsorption oxidation catalyst, and when the sufficient amount of oxygen is consumed in the HC adsorption oxidation catalyst, the exhaust gas air-fuel ratio is made rich. Thus, NOx is released from the NOx occlusion reduction catalyst.

JP 2003-97255 A JP 11-81991 A Japanese Patent Laid-Open No. 11-50894

  By the way, when NOx in the exhaust gas is occluded and removed using the NOx occlusion reduction catalyst, the ratio of NOx removed from the exhaust gas when the temperature of the NOx occlusion reduction catalyst is in a specific temperature range. The NOx removal rate becomes higher. Therefore, when removing NOx in the exhaust gas using the NOx storage reduction catalyst, it is desirable to maintain the temperature of the NOx storage reduction catalyst within the specific temperature range.

  On the other hand, in the internal combustion engine in which the HC adsorption oxidation catalyst is arranged in the engine exhaust passage upstream of the NOx storage reduction catalyst as described above, NOx is released from the NOx storage reduction catalyst and reduced (that is, flows into the NOx storage reduction catalyst). When fuel is added to the exhaust gas upstream of the NOx storage reduction catalyst (in order to make the air-fuel ratio of the exhaust gas rich), a part of the added fuel is oxidized in the HC adsorption oxidation catalyst and is exhausted in the exhaust gas. As a result, the temperature of the exhaust gas rises due to the oxidation reaction, and as a result, the temperature of the NOx storage reduction catalyst rises.

  This is convenient when the temperature of the NOx storage reduction catalyst is lower than the specific temperature range, but the temperature rises when the temperature of the NOx storage reduction catalyst is on the high temperature side within the specific temperature range. As a result, the temperature of the NOx occlusion reduction catalyst deviates from the above specified temperature range, and when NOx occlusion is removed by the NOx occlusion reduction catalyst after that, the NOx occlusion reduction catalyst decreases due to the high temperature of the NOx occlusion reduction catalyst. However, there is a possibility that the possibility of NOx being released to the outside increases.

  The present invention has been made in view of the above points, and an object of the present invention is to release an NOx from the NOx occlusion reduction catalyst by disposing an HC adsorption oxidation catalyst in the engine exhaust passage upstream of the NOx occlusion reduction catalyst. In the exhaust gas purification apparatus for an internal combustion engine in which the fuel is added to the exhaust gas upstream of the HC adsorption oxidation catalyst and part of the added fuel is oxidized in the HC adsorption oxidation catalyst, It is an object of the present invention to provide an exhaust emission control device for an internal combustion engine that suppresses excessive temperature rise of a NOx storage reduction catalyst when NOx is released from the storage reduction catalyst and reduced to be purified.

  The present invention provides an exhaust emission control device for an internal combustion engine described in each claim of the claims as means for solving the above-mentioned problems.

  The invention according to claim 1 is a fuel addition means for adding fuel to the exhaust gas, and is disposed in an engine exhaust passage downstream of the fuel addition means to adsorb hydrocarbons contained in the exhaust gas; When the air-fuel ratio of the HC adsorption oxidation catalyst that oxidizes and the exhaust gas that is disposed and flows in the engine exhaust passage downstream of the HC adsorption oxidation catalyst is lean, the NOx contained in the exhaust gas is occluded and flows into the exhaust gas that flows in A NOx occlusion reduction catalyst for releasing and reducing NOx occluded when the oxygen concentration is reduced, and reducing and purifying the released NOx if there is a reducing agent, and releasing and reducing NOx from the NOx occlusion reduction catalyst. When purification is to be performed, fuel is added from the fuel addition means, and a part of the added fuel is oxidized in the HC adsorption oxidation catalyst, and the exhaust gas flowing into the NOx storage reduction catalyst is emptied. In the exhaust gas purification apparatus for an internal combustion engine having a predetermined target air-fuel ratio, when NOx is released from the NOx occlusion reduction catalyst and reduced to be purified, fuel is added from the fuel addition means. Exhaust gas flowing into the NOx occlusion reduction catalyst while controlling the oxygen concentration in the exhaust gas discharged from the combustion chamber together with the amount of fuel to be added so that the temperature of the NOx occlusion reduction catalyst does not become higher than a predetermined upper limit temperature. An exhaust gas purification apparatus for an internal combustion engine, characterized in that the air-fuel ratio of the gas is set to the target air-fuel ratio.

  When NOx in the exhaust gas is occluded and removed using the NOx occlusion reduction catalyst, this is the ratio of NOx removed from the exhaust gas when the temperature of the NOx occlusion reduction catalyst is in a specific temperature range. The NOx removal rate increases. Therefore, when removing NOx in the exhaust gas using the NOx storage reduction catalyst, it is desirable to maintain the temperature of the NOx storage reduction catalyst within the specific temperature range.

  On the other hand, when the HC adsorption oxidation catalyst is disposed in the engine exhaust passage upstream of the NOx occlusion reduction catalyst as described above, NOx should be released from the NOx occlusion reduction catalyst and reduced and purified, upstream of the HC adsorption oxidation catalyst. In an exhaust gas purification apparatus for an internal combustion engine in which fuel is added to exhaust gas and a part of the added fuel is oxidized in the HC adsorption oxidation catalyst, a part of the added fuel is oxidized in the HC adsorption oxidation catalyst. Therefore, the oxygen concentration in the exhaust gas is reduced, and the temperature of the exhaust gas rises due to the oxidation reaction. As a result, the temperature of the NOx storage reduction catalyst rises.

  This is convenient when the temperature of the NOx storage reduction catalyst is lower than the specific temperature range, but the temperature rises when the temperature of the NOx storage reduction catalyst is on the high temperature side within the specific temperature range. As a result, the temperature of the NOx occlusion reduction catalyst deviates from the above specified temperature range, and when NOx occlusion is removed by the NOx occlusion reduction catalyst after that, the NOx occlusion reduction catalyst decreases due to the high temperature of the NOx occlusion reduction catalyst. However, there is a possibility that the possibility of NOx being released to the outside increases.

  On the other hand, in the invention described in claim 1, when adding fuel from the fuel addition means when NOx is released from the NOx storage reduction catalyst and reduced and purified, the amount of fuel to be added At the same time, the oxygen concentration in the exhaust gas discharged from the combustion chamber is controlled so that the temperature of the NOx storage reduction catalyst does not become higher than a predetermined upper limit temperature, and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is adjusted. The target air-fuel ratio is set. By doing so, it is possible to prevent the NOx storage reduction catalyst from being overheated when NOx is released from the NOx storage reduction catalyst and reduced and purified. As a result, when NOx in the exhaust gas is subsequently occluded and removed by the NOx occlusion reduction catalyst, the NOx occlusion reduction rate is suppressed from being lowered due to the high temperature of the NOx occlusion reduction catalyst. It is possible to prevent an increase in release.

  According to a second aspect of the invention, in the first aspect of the invention, when NOx is released from the NOx occlusion reduction catalyst and reduced and purified, the NOx occlusion reduction catalyst in the engine operating state at that time is used. When it is estimated that the temperature of the NOx occlusion reduction catalyst becomes higher than the upper limit temperature when an amount of fuel necessary to make the air-fuel ratio of the exhaust gas flowing into the target air-fuel ratio is added from the fuel addition means In order to make the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction catalyst the target air-fuel ratio, the oxygen concentration in the exhaust gas discharged from the combustion chamber must be added from the fuel addition means. A certain amount of fuel is being reduced.

  When the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst can be made rich by adding a smaller amount of fuel. That is, if the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced, the fuel that needs to be added from the fuel addition means in order to make the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst the target air-fuel ratio. The amount of can be reduced. When the amount of fuel added from the fuel addition means is reduced, the amount of fuel oxidized by the HC adsorption oxidation catalyst is also reduced, so that the temperature rise of the NOx storage reduction catalyst is suppressed.

  As described above, according to the second aspect of the invention, as in the first aspect of the invention, the NOx occlusion reduction catalyst is excessive when NOx is released from the NOx occlusion reduction catalyst and reduced and purified. The temperature rise is suppressed. As a result, when NOx in the exhaust gas is subsequently occluded and removed by the NOx occlusion reduction catalyst, the NOx occlusion reduction rate is suppressed from being lowered due to the high temperature of the NOx occlusion reduction catalyst, and the outside of the NOx It is possible to prevent an increase in release. Further, since the amount of fuel added from the fuel adding means is reduced, it is possible to suppress deterioration in fuel consumption.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, when NOx is released from the NOx occlusion reduction catalyst and reduced to be purified, the NOx occlusion reduction is performed in the engine operating state at that time. It is estimated that the temperature of the NOx occlusion reduction catalyst becomes higher than the upper limit temperature when an amount of fuel necessary to make the air-fuel ratio of the exhaust gas flowing into the catalyst the target air-fuel ratio is added from the fuel addition means. When the fuel is added from the fuel addition means, the amount of fuel estimated that the temperature of the NOx occlusion reduction catalyst is equal to or lower than the upper limit temperature is estimated to be the target added fuel amount and the target added fuel amount Exhaust gas discharged from the combustion chamber in which the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes the target air-fuel ratio when an amount of fuel is added from the fuel addition means The oxygen concentration in the exhaust gas is estimated to be the target oxygen concentration, the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced to the target oxygen concentration, and the fuel of the target added fuel amount is reduced from the fuel adding means. Is added.

  According to the invention of claim 3, as in the invention of claim 2, the NOx storage reduction catalyst is prevented from being overheated when NOx is released from the NOx storage reduction catalyst and reduced and purified. Is done. As a result, when NOx in the exhaust gas is subsequently stored and removed by the NOx storage reduction catalyst, the NOx removal rate is suppressed from decreasing, and the increase in the release of NOx to the outside can be prevented. it can. Further, similarly to the invention described in claim 2, it is possible to suppress deterioration of fuel consumption.

According to a fourth aspect of the present invention, in any one of the first to third aspects, the internal combustion engine includes an exhaust gas recirculation passage that communicates from the engine exhaust passage to the engine intake passage, and is discharged from the combustion chamber. When the oxygen concentration in the exhaust gas to be reduced is reduced, the exhaust gas recirculation is started through the exhaust recirculation passage, or the amount of exhaust gas recirculated through the exhaust recirculation passage is increased. It is supposed to be.
According to the fourth aspect of the invention, the oxygen concentration in the exhaust gas discharged from the combustion chamber can be easily reduced.

  According to a fifth aspect of the present invention, in the fourth aspect of the present invention, the internal combustion engine includes an exhaust turbocharger, and the exhaust recirculation passage is from an engine exhaust passage downstream of a turbine of the exhaust turbocharger. The exhaust turbocharger communicates with the engine intake passage upstream of the compressor.

  According to the fifth aspect of the present invention, when the exhaust gas recirculation passage communicates from the engine exhaust passage upstream of the exhaust turbocharger turbine to the engine intake passage downstream of the exhaust turbocharger compressor. The exhaust gas at a lower temperature than the NOx can be recirculated, so that the NOx storage reduction catalyst is more reliably suppressed from being overheated when NOx is released from the NOx storage reduction catalyst and reduced and purified. I can do it. Further, since a larger amount of exhaust gas can be recirculated, the oxygen concentration in the exhaust gas discharged from the combustion chamber can be further reduced.

  The invention according to claim 6 is the control according to claim 4 or 5, wherein the fuel injection pressure of the internal combustion engine is increased when the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced. At least one of control for advancing the fuel injection timing and control for enhancing the airflow of the in-cylinder inflow air is performed.

  Reducing the oxygen concentration in the exhaust gas exhausted from the combustion chamber by initiating exhaust gas recirculation or by increasing the amount of exhaust gas being recirculated reduces combustion and increases smoke and torque There is a risk of degradation. On the other hand, when the fuel injection pressure of the internal combustion engine is increased, atomization of the fuel is promoted and better combustion is performed. Further, when the fuel injection timing is advanced, a longer mixing time can be secured, so that the mixed state of fuel and air is improved and better combustion is performed. Further, if the airflow of the in-cylinder inflow air is strengthened, the mixing of fuel and air is promoted, the mixed state is improved, and better combustion is performed.

  As described above, according to the sixth aspect of the invention, exhaust gas is discharged from the combustion chamber by starting recirculation of exhaust gas or by increasing the amount of exhaust gas to be recirculated. When the oxygen concentration in the exhaust gas is reduced, it is possible to suppress an increase in smoke, a decrease in torque, or the like that may occur due to deterioration of combustion.

  In the invention described in each claim, when the HC adsorption oxidation catalyst is disposed in the engine exhaust passage upstream of the NOx storage reduction catalyst and NOx is released from the NOx storage reduction catalyst and should be reduced and purified, the HC adsorption oxidation In an exhaust gas purification apparatus for an internal combustion engine in which fuel is added to exhaust gas upstream of the catalyst and a part of the added fuel is oxidized in the HC adsorption oxidation catalyst, NOx is released from the NOx storage reduction catalyst and reduced. There is a common effect that the NOx occlusion reduction catalyst can be prevented from being excessively heated during purification. As a result, when NOx in the exhaust gas is subsequently occluded and removed by the NOx occlusion reduction catalyst, the NOx occlusion reduction rate is suppressed from being lowered due to the high temperature of the NOx occlusion reduction catalyst, and the outside of the NOx It is possible to prevent an increase in release.

FIG. 1 shows an overall view of a compression ignition type internal combustion engine to which the present invention is applied.
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 through the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 8. A throttle valve 9 driven by a step motor is arranged in the intake duct 6, and a cooling device (intercooler) 10 for cooling intake air flowing in the intake duct 6 is arranged around the intake duct 6. . In this embodiment, engine cooling water is guided into the intercooler 10 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 exhaust pipe 11. The exhaust manifold 5 is provided with a fuel addition valve 12 for adding fuel to the exhaust gas. In this embodiment, the fuel is made of light oil. As shown in FIG. 1, a post-treatment device 40 for purifying harmful components in exhaust gas, particularly NOx, is attached to the exhaust pipe 11. In this post-treatment device 40, an HC adsorption / oxidation catalyst 42 is disposed on the upstream side, and a NOx storage reduction catalyst 43 is disposed on the downstream side thereof.

  As shown in FIG. 1, in this internal combustion engine, the engine intake air upstream of the compressor 7a of the exhaust turbocharger 7 from the engine exhaust passage downstream of the aftertreatment device 40 (more specifically, downstream of the NOx storage reduction catalyst 43). An exhaust gas recirculation (hereinafter referred to as EGR) passage 13 communicating with the passage is provided, and an electronically controlled EGR control valve 14 is disposed in the EGR passage 13. A cooling device (EGR cooler) 15 for cooling the EGR gas flowing in the EGR passage 13 is disposed around the EGR passage 13. In the present embodiment, the engine cooling water is guided into the EGR cooler 15 and the EGR gas is cooled by the engine cooling water. In another embodiment, the EGR passage 13 may be provided so as to communicate from the exhaust manifold 5 as the engine exhaust passage to the intake manifold 4 as the engine intake passage.

  On the other hand, each fuel injection valve 3 is connected to a common rail 17 through a fuel supply pipe 16. Fuel is supplied into the common rail 17 from an electronically controlled fuel pump 18 with variable discharge amount, and the fuel supplied into the common rail 17 is supplied to the fuel injection valve 3 through each fuel supply pipe 16.

  The electronic control unit 20 comprises a digital computer and is connected to each other by a bidirectional bus 21. A ROM (read only memory) 22, a RAM (random access memory) 23, a CPU (microprocessor) 24, an input port 25 and an output port 26 are connected. It comprises. As shown in FIG. 1, a load sensor 31 that generates an output voltage proportional to the depression amount of the accelerator pedal 30 is connected to the accelerator pedal 30, and the output voltage of the load sensor 31 is input via a corresponding AD converter 27. Input to port 25. Further, a crank angle sensor 32 that generates an output pulse every time the crankshaft rotates, for example, 15 ° is connected to the input port 25. On the other hand, the output port 26 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 9, the fuel addition valve 12, the EGR control valve 14, and the fuel pump 18 through corresponding drive circuits 28.

  Next, the HC adsorption oxidation catalyst 42 shown in FIG. 1 will be described. FIG. 2 shows a side sectional view of the HC adsorption oxidation catalyst 42. As shown in FIG. 2, the HC adsorption oxidation catalyst 42 has a honeycomb structure and includes a plurality of exhaust gas flow passages 73 extending straight. In this embodiment, the HC adsorption oxidation catalyst 42 is made of a material having a large specific surface area having a pore structure such as zeolite, and the base of the HC adsorption oxidation catalyst 73 shown in FIG. 2 is mordenite which is a kind of zeolite. Consists of. The surface of the HC adsorption oxidation catalyst 42 has an uneven rough surface shape, and a large number of pores are formed on the surface having the rough surface shape, and a precious metal catalyst made of platinum Pt is dispersed and supported. ing. The HC adsorption oxidation catalyst 42 has an action of adsorbing and oxidizing hydrocarbons contained in the exhaust gas, that is, HC.

  Next, the NOx storage reduction catalyst 43 disposed on the downstream side of the HC adsorption oxidation catalyst 42 will be described. The NOx occlusion reduction catalyst 43 is supported on a monolithic carrier or pellet-like carrier having a three-dimensional network structure, or is supported on a particulate filter having a honeycomb structure. As described above, the NOx storage reduction catalyst 43 can be supported on various carriers. Hereinafter, the case where the NOx storage reduction catalyst 43 is supported on the particulate filter will be described.

  3A and 3B show the structure of the particulate filter 43a carrying the NOx storage reduction catalyst 43. FIG. 3A shows a front view of the particulate filter 43a, and FIG. 3B shows a side sectional view of the particulate filter 43a. As shown in FIGS. 3A and 3B, the particulate filter 43a has a honeycomb structure and includes a plurality of exhaust flow passages 60 and 61 extending in parallel with each other. These exhaust flow passages include an exhaust gas inflow passage 60 whose downstream end is closed by a plug 62 and an exhaust gas outflow passage 61 whose upstream end is closed by a plug 63. In addition, the hatched part in FIG. Therefore, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are alternately arranged via the thin partition walls 64. In other words, in the exhaust gas inflow passage 60 and the exhaust gas outflow passage 61, each exhaust gas inflow passage 60 is surrounded by four exhaust gas outflow passages 61, and each exhaust gas outflow passage 61 is surrounded by four exhaust gas inflow passages 60. Are arranged as follows.

  The particulate filter 43a is made of, for example, a porous material such as cordierite. Therefore, the exhaust gas flowing into the exhaust gas inflow passage 60 is contained in the surrounding partition wall 64 as indicated by an arrow in FIG. Through the exhaust gas outflow passage 61 adjacent thereto.

  When the NOx storage reduction catalyst 43 is thus supported on the particulate filter 43a, the peripheral wall surfaces of the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61, that is, on both side surfaces of the partition walls 64 and the partition walls 64 are used. A catalyst carrier made of alumina, for example, is supported on the inner wall surfaces of the pores, and FIGS. 4A and 4B schematically show a cross section of the surface portion of the catalyst carrier 70. As shown in FIGS. 4A and 4B, a noble metal catalyst 71 is dispersedly supported on the surface of the catalyst carrier 70, and a layer of NOx absorbent 72 is further provided on the surface of the catalyst carrier 70. Is formed.

  In this embodiment, platinum Pt is used as the noble metal catalyst 71, and the components constituting the NOx absorbent 72 are, for example, alkali metals such as potassium K, sodium Na, and cesium Cs, and alkalis such as barium Ba and calcium Ca. At least one selected from earth, lanthanum La, and rare earth such as yttrium Y is used.

  When the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 2 and the exhaust passage upstream of the NOx occlusion reduction catalyst 43 is referred to as the air-fuel ratio of the exhaust gas, the NOx absorbent 72 is exhausted from the exhaust gas. When the fuel ratio is lean, NOx is absorbed, and when the oxygen concentration in the exhaust gas decreases, the NOx is absorbed and released to release the absorbed NOx.

That is, the case where barium Ba is used as a component constituting the NOx absorbent 72 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas 4A is oxidized on platinum Pt 71 to become NO ₂ as shown in FIG. 4 (A), and then is absorbed into the NOx absorbent 72 and combined with barium oxide BaO to form a NOx absorbent in the form of nitrate ions NO ₃ ^−. It diffuses into 72. In this way, NOx is absorbed into the NOx absorbent 72. As long as the oxygen concentration in the exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt 71. As long as the NOx absorption capacity of the NOx absorbent 72 is not saturated, NO ₂ is absorbed in the NOx absorbent 72 and nitrate ions NO ₃ ⁻ are generated. Generated.

On the other hand, when the air-fuel ratio of the exhaust gas is rich or stoichiometric, the oxygen concentration in the exhaust gas decreases, so that the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ). As shown in (B), nitrate ions NO ₃ ⁻ in the NOx absorbent 72 are released from the NOx absorbent 72 in the form of NO ₂ . Next, the released NOx is reduced and purified by unburned HC and CO contained in the exhaust gas. That is, unburned HC and CO act as a reducing agent, and NOx is reduced to nitrogen N ₂ .

  Thus, when the air-fuel ratio of the exhaust gas is lean, that is, when combustion is performed under the lean air-fuel ratio, NOx in the exhaust gas is absorbed into the NOx absorbent 72. However, if combustion under a lean air-fuel ratio is continuously performed, the NOx absorbent capacity of the NOx absorbent 72 is saturated during that time, and therefore the NOx absorbent 72 cannot absorb NOx. Therefore, in this embodiment, the air-fuel ratio of the exhaust gas is temporarily made rich by adding fuel from the fuel addition valve 12 before the absorption capacity of the NOx absorbent 72 is saturated, thereby releasing NOx from the NOx absorbent 72. And reduce and purify.

  By the way, when the NOx occlusion reduction catalyst 43 including the NOx absorbent 72 as described above is used to occlude and remove NOx in the exhaust gas, the temperature of the NOx occlusion reduction catalyst 43 is in a specific temperature range. The NOx removal rate (= (NOx amount in inflowing exhaust gas−NOx amount in outflowing exhaust gas) / NOx amount in inflowing exhaust gas), which is the ratio of NOx removed from the exhaust gas, becomes high. This is because when the temperature of the NOx occlusion reduction catalyst 43 is within this specific temperature range, the entire process of NOx occlusion, release and reduction by the NOx occlusion reduction catalyst 43 is activated.

  FIG. 5 is an example of a graph showing the relationship between the temperature Tc of the NOx storage reduction catalyst 43 and the NOx removal rate L by the NOx storage reduction catalyst 43. According to this diagram, the NOx removal rate L by the NOx storage reduction catalyst 43 is shown. Becomes better when the temperature Tc of the NOx storage reduction catalyst 43 is in the temperature range X between the lower limit temperature Tck and the upper limit temperature Tcj. Here, the lower limit temperature Tck is a temperature between 250 ° C. and 300 ° C., and the upper limit temperature Tcj is a temperature between 350 ° C. and 400 ° C.

  When the NOx occlusion reduction catalyst 43 is used in this way, the NOx removal rate L increases in a specific temperature range X. Therefore, the NOx occlusion reduction catalyst 43 is used to remove NOx in the exhaust gas. In this case, it is desirable to maintain the temperature of the NOx storage reduction catalyst 43 within the specific temperature range X.

  On the other hand, when the HC adsorption oxidation catalyst 42 is disposed upstream of the NOx storage reduction catalyst 43 as in the present embodiment, NOx should be released and reduced from the NOx storage reduction catalyst 43 (that is, the NOx storage reduction catalyst). When the fuel is added to the exhaust gas from the fuel addition valve 12 provided upstream of the HC adsorption oxidation catalyst 42 (in order to make the air-fuel ratio of the exhaust gas flowing into the engine 43 rich), A part is oxidized in the HC adsorption oxidation catalyst 42. As a result, the oxygen concentration in the exhaust gas is reduced, and the temperature of the exhaust gas rises due to the oxidation reaction. As a result, the temperature Tc of the NOx storage reduction catalyst 43 rises.

  This is convenient when the temperature Tc of the NOx storage reduction catalyst 43 is lower than the specific temperature range X, that is, when it is lower than the lower limit temperature Tck. However, the temperature Tc of the NOx storage reduction catalyst 43 is, for example, the specific specification For example, when the temperature is higher in the temperature range X, the temperature Tc of the NOx occlusion reduction catalyst deviates from the specific temperature range X due to the temperature rise, and thereafter the air-fuel ratio of the exhaust gas becomes lean and the NOx occlusion is performed. When NOx is occluded and removed by the reduction catalyst 43, the NOx removal rate L may decrease due to the high temperature Tc of the NOx occlusion reduction catalyst 43, and the possibility that NOx is released to the outside may increase. is there.

  Therefore, in the present embodiment, special control as described below is performed to release the NOx from the NOx occlusion reduction catalyst 43 and reduce the NOx occlusion reduction catalyst 43 when it is purified by reduction. I try to suppress it. That is, when the fuel is added from the fuel addition valve 12 when NOx is released from the NOx occlusion reduction catalyst 43 and reduced and purified, it is discharged from the combustion chamber 2 together with the amount of fuel to be added. By controlling the oxygen concentration in the exhaust gas to be controlled so that the temperature Tc of the NOx storage reduction catalyst 43 does not become higher than the predetermined upper limit temperature Tcj, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 is targeted. The air-fuel ratio (that is, a predetermined rich air-fuel ratio) is set.

  Hereinafter, this control will be specifically described with reference to FIG. FIG. 6 is a flowchart showing a control routine for carrying out this control. This control routine is executed by the electronic control unit 20 by interruption every predetermined time.

  When this control routine starts, it is first determined in step 101 whether or not there is a request to release and reduce NOx, that is, whether or not it is time to release NOx from the NOx storage reduction catalyst 43 and reduce and reduce it. Is determined. That is, for example, when the combustion under the lean air-fuel ratio is continuously performed and the NOx absorption capacity of the NOx absorbent 72 is approaching saturation, it is determined that there is a request to release and reduce NOx. The

  When it is determined in step 101 that there is no request to release and reduce NOx, the present control routine ends. When it is determined that there is a request to release and reduce NOx, the process proceeds to step 103. In step 103, the exhaust gas temperature Tex in the engine operating state at that time is estimated. The exhaust gas temperature Tex is estimated using, for example, an exhaust temperature sensor (not shown) for measuring the exhaust gas temperature provided in the engine exhaust passage (for example, immediately upstream of the HC adsorption oxidation catalyst 42). However, in the present embodiment, a map for obtaining the exhaust gas temperature Tex from the engine operating state is created in advance and is performed based on the map. That is, for example, a map for obtaining the exhaust gas temperature Tex based on the engine speed and the required torque determined from the engine speed and the accelerator depression amount is created in advance and estimated based on the map.

  When the exhaust gas temperature Tex is estimated at step 103, the routine proceeds to step 105. In step 105, the fuel addition valve 12 is used to set the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 to a predetermined target air-fuel ratio (that is, a predetermined rich air-fuel ratio) in the engine operating state at that time. The amount of fuel Qar that needs to be added from is estimated. In the present embodiment, the estimation of the added fuel amount Qar is performed based on a map obtained in advance for obtaining the added fuel amount Qar from the engine operating state. That is, for example, a map capable of obtaining the added fuel amount Qar based on the engine speed and the required torque is prepared in advance and estimated based on the map.

  When the added fuel amount Qar is estimated at step 105, the routine proceeds to step 107. In step 107, the temperature Tcf of the NOx occlusion reduction catalyst 43 when the fuel of the added fuel amount Qar obtained in step 105 is added from the fuel addition valve 12 in the engine operating state at that time is estimated. In this embodiment, the temperature Tcf of the NOx occlusion reduction catalyst 43 is estimated in advance by creating a map for obtaining the temperature Tcf from the exhaust gas temperature, the amount of fuel added from the fuel addition valve 12, and the engine operating state. Aside from that, it is based on that. That is, based on this map, the temperature Tcf is estimated from the exhaust gas temperature Tex obtained in step 103, the added fuel amount Qar obtained in step 105, and the engine speed and required torque at that time. Is done.

  When the temperature Tcf of the NOx storage reduction catalyst 43 is estimated at step 107, the routine proceeds to step 109. In step 109, it is determined whether or not the temperature Tcf of the NOx storage reduction catalyst 43 estimated in step 107 is equal to or lower than a predetermined upper limit temperature Tcj. Here, the upper limit temperature Tcj used as a determination criterion is the upper limit temperature Tcj of the specific temperature range X where the NOx reduction purification rate L by the NOx occlusion reduction catalyst 43 described above with reference to FIG. .

  If it is determined in step 109 that the temperature Tcf of the NOx storage reduction catalyst 43 estimated in step 107 is equal to or lower than a predetermined upper limit temperature Tcj, the process proceeds to step 111. In step 111, fuel addition control for adding fuel from the fuel addition valve 12 is performed. The target added fuel amount at this time is the added fuel amount Qar obtained in step 105.

  That is, when it is determined in step 109 that the temperature Tcf of the NOx storage reduction catalyst 43 estimated in step 107 is equal to or lower than a predetermined upper limit temperature Tcj, the fuel of the added fuel amount Qar is supplied to the fuel addition valve 12. This is a case where the temperature Tcf of the NOx occlusion reduction catalyst 43 does not become higher than the upper limit temperature Tcj even when the NOx occlusion reduction catalyst 43 is added, and the temperature Tcf of the NOx occlusion reduction catalyst 43 does not deviate from the specific temperature range X. Therefore, in this case, by adding the fuel of the added fuel amount Qar obtained in step 105 as it is, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 is set to a predetermined target air-fuel ratio (that is, in advance The determined rich air-fuel ratio is reduced, and NOx is released from the NOx occlusion reduction catalyst 43 and is reduced and purified. When the fuel addition control is performed in step 111, this control routine ends.

  On the other hand, if it is determined in step 109 that the temperature Tcf of the NOx storage reduction catalyst 43 estimated in step 107 is higher than a predetermined upper limit temperature Tcj, the process proceeds to step 113. In step 113, the amount of fuel Qas estimated when the temperature of the NOx storage reduction catalyst 43 becomes equal to or lower than the upper limit temperature Tcj when added from the fuel addition valve 12 is estimated. In the present embodiment, the estimation of the added fuel amount Qas is performed based on a map obtained in advance for obtaining the added fuel amount Qas from the exhaust gas temperature and the engine operating state.

  Here, the added fuel amount Qas may be the amount of fuel estimated that the temperature of the NOx occlusion reduction catalyst 43 reaches the upper limit temperature Tcj when added from the fuel addition valve 12, or the NOx occlusion. The amount of fuel estimated that the temperature of the reduction catalyst 43 is lower than the upper limit temperature Tcj by a predetermined temperature (for example, 10 ° C.) may be used.

  The added fuel amount Qas is usually smaller than the added fuel amount Qar. That is, when it is determined in step 109 that the temperature Tcf of the NOx storage reduction catalyst 43 estimated in step 107 is higher than a predetermined upper limit temperature Tcj, fuel of the added fuel amount Qar is supplied from the fuel addition valve 12. When added, the temperature Tcf of the NOx occlusion reduction catalyst 43 becomes higher than the upper limit temperature Tcj, and the temperature Tcf of the NOx occlusion reduction catalyst 43 deviates from the specific temperature range X. Accordingly, in this case, the temperature increase of the NOx occlusion reduction catalyst 43 is suppressed by adding a fuel amount smaller than the added fuel amount Qar obtained in step 105 (that is, the added fuel amount Qas). Therefore, it is necessary to prevent the temperature Tc of the NOx storage reduction catalyst 43 from deviating from the specific temperature range X.

  When the added fuel amount Qas is estimated at step 113, the routine proceeds to step 115. In step 115, when the fuel of the added fuel amount Qas is added from the fuel addition valve 12, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 is the target air-fuel ratio (that is, a predetermined rich air-fuel ratio). The oxygen concentration Cos in the exhaust gas discharged from the combustion chamber 2 is estimated. That is, since it is necessary to reduce the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 in order to realize the target air-fuel ratio even if the amount of added fuel is reduced, the amount of added fuel is here. An oxygen concentration Cos corresponding to Qas is obtained. In the present embodiment, the estimation of the oxygen concentration Cos is performed based on a map in which the oxygen concentration Cos is obtained in advance from the added fuel amount Qas and the engine operating state.

  When the oxygen concentration Cos is estimated at step 115, the routine proceeds to step 117. In step 117, oxygen concentration reduction control for reducing the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is performed. The target oxygen concentration at this time is the oxygen concentration Cos obtained in step 115. In the present embodiment, the oxygen concentration reduction control more specifically starts the recirculation of exhaust gas through the EGR passage 13 or increases the amount of exhaust gas recirculated through the EGR passage 13. Is done by doing. That is, the EGR control valve 14 is controlled so that the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 becomes the oxygen concentration Cos.

  Following step 117, control proceeds to step 119. In step 119, fuel addition control is performed in which fuel is added from the fuel addition valve 12 in synchronization with the exhaust gas whose oxygen concentration has been reduced by the oxygen concentration reduction control in step 117. The target added fuel amount at this time is the added fuel amount Qas obtained in step 113.

  By performing the oxygen concentration reduction control in step 117 and the fuel addition control in step 119, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 is set to a predetermined target air-fuel ratio (that is, a predetermined rich air-fuel ratio). To be. As a result, NOx is released from the NOx occlusion reduction catalyst 43, and the released NOx is reduced and purified. At this time, the temperature Tc of the NOx storage reduction catalyst 43 is maintained within the specific temperature range X (that is, the NOx storage reduction catalyst 43 is suppressed from being excessively heated). When the control of step 117 and step 119 is performed, this control routine ends.

  As described above, when the control routine shown in FIG. 6 is performed, when NOx is released from the NOx occlusion reduction catalyst 43 and should be reduced and purified, the NOx occlusion is performed in the engine operating state at that time. The temperature Tc of the NOx occlusion reduction catalyst 43 becomes higher when the fuel addition valve 12 adds an amount of fuel Qar required to bring the air-fuel ratio of the exhaust gas flowing into the reduction catalyst 43 to a predetermined target air-fuel ratio. When it is estimated that the temperature is higher than the upper limit temperature Tcj, the amount of fuel Qas estimated that the temperature Tc of the NOx occlusion reduction catalyst 43 becomes lower than the upper limit temperature Tcj when added from the fuel addition valve 12 is estimated. To the target added fuel amount. At the same time, when fuel of the same target added fuel amount Qas is added from the fuel addition valve 12, the exhaust gas flowing into the NOx storage reduction catalyst 43 is discharged from the combustion chamber 2 where the air-fuel ratio becomes the target air-fuel ratio. The oxygen concentration Cos in the exhaust gas is estimated and set as the target oxygen concentration. Then, the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is reduced to the target oxygen concentration Cos, and the fuel of the target added fuel amount is added from the fuel addition valve 12.

  Further, when the control routine shown in FIG. 6 is performed, when NOx is released from the NOx storage reduction catalyst 43 and reduced to be purified, it flows into the NOx storage reduction catalyst 43 in the engine operating state at that time. When the amount Qar of fuel necessary for setting the air-fuel ratio of exhaust gas to be the target air-fuel ratio is added from the fuel addition valve 12, the temperature Tc of the NOx storage reduction catalyst 43 becomes higher than the upper limit temperature Tcj. In the case of estimation, the fuel addition is performed so that the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is reduced and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 becomes the target air-fuel ratio. It can be said that the amount of fuel that needs to be added from the valve 12 is reduced.

  When the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is reduced, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 can be made rich by adding a smaller amount of fuel. That is, if the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is reduced, it is necessary to add from the fuel addition valve 12 so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 43 becomes the target air-fuel ratio. It is possible to reduce the amount of fuel with a certain amount. When the amount of fuel added from the fuel addition valve 12 is reduced, the amount of fuel oxidized by the HC adsorption oxidation catalyst 42 is also reduced, so that the temperature rise of the NOx storage reduction catalyst 43 is suppressed.

  As described above, according to the present embodiment, it is possible to suppress the NOx storage reduction catalyst 43 from being overheated when NOx is released from the NOx storage reduction catalyst 43 and reduced and purified. As a result, when NOx in the exhaust gas is stored and removed by the NOx storage reduction catalyst 43 thereafter, the NOx removal rate L is suppressed from decreasing due to the high temperature Tc of the NOx storage reduction catalyst 43. , NOx emission to the outside can be prevented from increasing. Further, since the amount of fuel added from the fuel addition valve 12 is reduced, it is possible to suppress deterioration of fuel consumption.

  In the present embodiment, as described above, the EGR passage 13 communicates from the engine exhaust passage downstream of the turbine 7 b of the exhaust turbocharger 7 to the engine intake passage upstream of the compressor 7 a of the exhaust turbocharger 7. It is provided as follows. In this case, when the EGR passage 13 is provided so as to communicate from the engine exhaust passage upstream of the turbine of the exhaust turbocharger to the engine intake passage downstream of the compressor of the exhaust turbocharger, for example, the exhaust Compared to the case where the manifold 5 is provided so as to communicate with the intake manifold 4, low-temperature exhaust gas can be recirculated, and the recirculated exhaust gas passes through both the EGR cooler 15 and the intercooler 10. Therefore, when the above-described oxygen concentration reduction control is performed when NOx is released from the NOx storage reduction catalyst 43 and reduced and purified, the excessive temperature rise of the NOx storage reduction catalyst 43 is more reliably suppressed. I can do it. Further, since a larger amount of exhaust gas can be recirculated, the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 can be further reduced.

  In another embodiment of the present invention, when the oxygen concentration reduction control described above is performed, control for increasing the fuel injection pressure of the internal combustion engine, control for advancing the fuel injection timing, and in-cylinder inflow At least one of the controls for enhancing the airflow may be performed.

  That is, when the above-described oxygen concentration reduction control is performed, that is, by starting the exhaust gas recirculation, or by increasing the amount of exhaust gas to be recirculated, the oxygen in the exhaust gas discharged from the combustion chamber 2 When the concentration is reduced, there is a risk that combustion will deteriorate and smoke will increase or torque will decrease. On the other hand, when the fuel pump 18 is controlled to increase the fuel injection pressure of the internal combustion engine, the atomization of the fuel is promoted and better combustion is performed. Further, when the fuel injection timing is advanced, a longer mixing time can be secured, so that the mixed state of fuel and air is improved and better combustion is performed. Furthermore, when a swirl valve or the like is provided, if the swirl valve is controlled to enhance the airflow of in-cylinder inflow air, the mixing of fuel and air is promoted so that the mixing state is improved and better combustion is performed. become.

  For this reason, if the control for increasing the fuel injection pressure, the control for advancing the fuel injection timing, the control for strengthening the airflow of the in-cylinder inflow air, etc. are performed, the exhaust gas recirculation is started. If the oxygen concentration in the exhaust gas discharged from the combustion chamber 2 is reduced by increasing the amount of exhaust gas to be recirculated, the increase in smoke, torque reduction, etc. Can be suppressed.

FIG. 1 is an overall view of a compression ignition type internal combustion engine to which the present invention is applied. FIG. 2 is a side sectional view of the HC adsorption oxidation catalyst. FIG. 3 is a diagram illustrating the structure of the particulate filter. FIG. 4 is a cross-sectional view of the surface portion of the catalyst carrier of the NOx storage reduction catalyst. FIG. 5 is a graph showing the relationship between the temperature Tc of the NOx storage reduction catalyst and the NOx removal rate L by the NOx storage reduction catalyst. FIG. 6 is a flowchart showing a control routine of control executed in the embodiment of the present invention.

Explanation of symbols

2 Combustion chamber 3 Fuel injection valve 11 Exhaust pipe 12 Fuel addition valve 13 Exhaust gas recirculation passage (EGR passage)
42 HC adsorption oxidation catalyst 43 NOx storage reduction catalyst

Claims (6)

A fuel addition means for adding fuel to the exhaust gas, an HC adsorption oxidation catalyst disposed in an engine exhaust passage downstream of the fuel addition means to adsorb and oxidize hydrocarbons contained in the exhaust gas, and When the air-fuel ratio of the exhaust gas that is disposed in the engine exhaust passage downstream of the HC adsorption oxidation catalyst is lean, the NOx contained in the exhaust gas is occluded, and when the oxygen concentration in the inflowing exhaust gas decreases, the occluded NOx is removed. A NOx occlusion reduction catalyst that reduces and purifies the released NOx if there is a reducing agent that is released, and releases NOx from the NOx occlusion reduction catalyst and reduces and purifies the NOx from the fuel addition means. Fuel is added, a part of the added fuel is oxidized in the HC adsorption oxidation catalyst, and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is set to a predetermined target air fuel In the exhaust purification system of an internal combustion engine that,
When adding fuel from the fuel addition means when NOx is to be released from the NOx storage reduction catalyst and reduced and purified, oxygen concentration in the exhaust gas discharged from the combustion chamber together with the amount of fuel to be added And controlling the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst to be the target air-fuel ratio while controlling the NOx storage-reduction catalyst so that the temperature of the NOx storage-reduction catalyst does not become higher than a predetermined upper limit temperature. Engine exhaust purification system.   Necessary for setting the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst to the target air-fuel ratio when NOx is released from the NOx storage-reduction catalyst and is to be reduced and purified. When it is estimated that the temperature of the NOx occlusion reduction catalyst becomes higher than the upper limit temperature when a large amount of fuel is added from the fuel addition means, the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced. The amount of fuel that needs to be added from the fuel addition means to reduce the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst to the target air-fuel ratio is reduced. 2. An exhaust gas purification apparatus for an internal combustion engine according to 1.   Necessary for setting the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst to the target air-fuel ratio when NOx is released from the NOx storage-reduction catalyst and is to be reduced and purified. When it is estimated that the temperature of the NOx occlusion reduction catalyst becomes higher than the upper limit temperature when a large amount of fuel is added from the fuel addition means, the NOx occlusion reduction catalyst when added from the fuel addition means The amount of fuel that is estimated to be equal to or lower than the upper limit temperature is estimated to be the target added fuel amount, and when the fuel of the target added fuel amount is added from the fuel adding means, the NOx storage reduction catalyst The oxygen concentration in the exhaust gas discharged from the combustion chamber where the air-fuel ratio of the inflowing exhaust gas becomes the target air-fuel ratio is estimated to be the target oxygen concentration and discharged from the combustion chamber. Oxygen concentration in the exhaust gas fuel of the target fuel addition amount from the fuel adding means together with the reduced to the target oxygen concentration is added, the exhaust purification system of an internal combustion engine according to claim 1 or 2.   The internal combustion engine includes an exhaust gas recirculation passage that communicates from the engine exhaust passage to the engine intake passage. When the oxygen concentration in the exhaust gas discharged from the combustion chamber is reduced, the exhaust gas recirculation passage is provided. The exhaust gas purification of an internal combustion engine according to any one of claims 1 to 3, wherein recirculation of the exhaust gas passed through is started or an amount of exhaust gas recirculated through the exhaust gas recirculation passage is increased. apparatus.   The internal combustion engine includes an exhaust turbocharger, and the exhaust recirculation passage communicates from an engine exhaust passage downstream of the exhaust turbocharger turbine to an engine intake passage upstream of the exhaust turbocharger compressor. The exhaust emission control device for an internal combustion engine according to claim 4.   When reducing the oxygen concentration in the exhaust gas exhausted from the combustion chamber, control for increasing the fuel injection pressure of the internal combustion engine, control for advancing the fuel injection timing, The exhaust emission control device for an internal combustion engine according to claim 4 or 5, wherein at least one of the strengthening controls is performed.

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