JP3570306B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
A first catalyst and a second catalyst downstream from the engine exhaust passage are disposed in the engine exhaust passage, and after the engine is started, the auxiliary fuel is injected during the expansion stroke in addition to the main fuel to raise the exhaust gas temperature. An internal combustion engine in which the catalyst and the second catalyst are activated at an early stage is known (see Japanese Patent Application Laid-Open No. 10-212995).
[0003]
[Problems to be solved by the invention]
However, in this case, since the temperature of the first catalyst is higher than that of the second catalyst, if the exhaust gas temperature continues to rise until the second catalyst is activated, the first catalyst is overheated. There is a danger that the catalyst of the heat deteriorates. That is, when two catalysts are visible in this way, it is necessary to stop the exhaust gas rising action when the first catalyst reaches a predetermined temperature.
[0004]
[Means for Solving the Problems]
Therefore, in the first invention, the first catalyst is disposed in the engine exhaust passage, the second catalyst and the exhaust control valve are disposed in the engine exhaust passage downstream of the first catalyst, and the temperature of the first catalyst is increased. In addition to almost completely closing the exhaust control valve until it rises to a predetermined temperature, in addition to burning the main fuel injected into the combustion chamber under excess air to generate engine output The auxiliary fuel is additionally injected into the combustion chamber at a predetermined time during the expansion stroke or the exhaust stroke in which the auxiliary fuel can be combusted, and when the temperature of the first catalyst exceeds the predetermined temperature, the exhaust control valve is turned on. Fully open.
[0005]
In the second invention, in the first invention, the air-fuel ratio of the exhaust gas is made lean until the temperature of the first catalyst exceeds a predetermined temperature.
In the third invention, in the first invention, when the temperature of the first catalyst exceeds a predetermined temperature, the air-fuel ratio of the exhaust gas is maintained at the stoichiometric air-fuel ratio or slightly rich.
[0006]
In the fourth invention, in the third invention, the air-fuel ratio of the exhaust gas is maintained at the stoichiometric air-fuel ratio or slightly rich until the temperature of the second catalyst reaches a predetermined temperature.
According to a fifth aspect, in the third aspect, the first catalyst is a catalyst having an oxidation function, and the second catalyst is NO in the exhaust gas when the air-fuel ratio of the exhaust gas is lean._xNO is absorbed when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich._xNO release_xIt consists of an absorbent.
[0007]
In the sixth invention, in the fifth invention, NO_x An exhaust bypass passage that bypasses the absorbent is provided, and the first catalyst is SO_x Has a function of capturing SO, and SO from the first catalyst._x The exhaust gas air-fuel ratio is maintained at the stoichiometric air-fuel ratio or slightly rich until the release action of the exhaust gas is completed, and the exhaust gas is NO_x It bypasses the absorbent and flows through the exhaust bypass passage.
In the seventh invention, in the first invention, when the exhaust control valve is almost fully closed, the torque generated by the engine when the exhaust control valve is fully opened under the same engine operating condition is approached. The fuel injection amount is increased as compared with the case where the exhaust control valve is fully opened under the same engine operating state.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
1 and 2 show a case where the present invention is applied to a stratified combustion internal combustion engine. However, the present invention can also be applied to a spark ignition type internal combustion engine in which combustion is performed under a uniform lean air-fuel ratio and a diesel engine in which combustion is performed under excess air.
[0009]
Referring to FIG. 2, 1 is an engine main body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is a fuel injection valve disposed on the peripheral edge of the inner wall surface of the cylinder head 3, and 7 is An ignition plug disposed at the center of the inner wall surface of the cylinder head 3, 8 is an intake valve, 9 is an intake port, 10 is an exhaust valve, and 11 is an exhaust port.
Referring to FIGS. 1 and 2, the intake port 9 is connected to a surge tank 13 via a corresponding intake branch pipe 12, and the surge tank 13 is connected to an air cleaner 16 via an intake duct 14 and an air flow meter 15. A throttle valve 18 driven by a step motor 17 is disposed in the intake duct 14. On the other hand, the exhaust port 11 of each cylinder is connected to an exhaust manifold 19, and the outlet of the exhaust manifold 19 is connected to a catalytic converter 20 a that houses a first catalyst 20 having an oxidation function. The outlet of the catalytic converter 20a is connected to the catalytic converter 22a containing the second catalyst 22 via the exhaust pipe 21, and the exhaust pipe 23 is connected to the outlet of the catalytic converter 22a.
[0010]
In the embodiment shown in FIG. 1, the first catalyst 20 is a three-way catalyst, and the second catalyst 22 is NO._xIt consists of an absorbent. As shown in FIG. 1, an exhaust control valve 24 driven by an actuator 25 comprising a negative pressure diaphragm device or an electric motor is disposed in the exhaust pipe 23. This exhaust control valve can also be arranged in the exhaust pipe 21 as indicated by reference numeral 24 'in FIG.
[0011]
As shown in FIG. 1, the exhaust manifold 19 and the surge tank 13 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 26, and an electrically controlled EGR control valve 27 is disposed in the EGR passage 26. Is done. The fuel injection valve 6 is connected to a common fuel reservoir, so-called common rail 28. The fuel in the fuel tank 29 is supplied into the common rail 28 via an electrically controlled fuel pump 30 with variable discharge amount, and the fuel supplied in the common rail 28 is supplied to each fuel injection valve 6. A fuel pressure sensor 31 for detecting the fuel pressure in the common rail 28 is attached to the common rail 28, and a fuel pump 30 is set so that the fuel pressure in the common rail 28 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 31. The discharge amount is controlled.
[0012]
The electronic control unit 40 comprises a digital computer and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 are connected. It comprises. The air flow meter 15 generates an output voltage proportional to the amount of intake air, and this output voltage is input to the input port 45 via the corresponding AD converter 47. A water temperature sensor 32 for detecting the engine cooling water temperature is attached to the engine body 1, and the output signal of the water temperature sensor 32 is input to the input port 45 via the corresponding AD converter 47. Further, the output signal of the fuel pressure sensor 31 is input to the input port 45 via the corresponding AD converter 47.
[0013]
On the other hand, a temperature sensor 33 for detecting the temperature of the three-way catalyst 20 is attached to the catalytic converter 20a._xA temperature sensor 34 for detecting the temperature of the absorbent 22 is attached. The output signals of these temperature sensors 33 and 34 are input to the input port 45 via corresponding AD converters 47, respectively. An air-fuel ratio sensor 35 is disposed in the exhaust manifold 19, and an output signal from the air-fuel ratio sensor 35 is input to the input port 45 via the corresponding AD converter 47.
[0014]
A load sensor 51 that generates an output voltage proportional to the depression amount L of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. . The input port 45 is connected to a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 6, spark plug 7, throttle valve control step motor 17, exhaust control valve control actuator 25, EGR control valve 27, and fuel pump 30 through corresponding drive circuits 48. The
[0015]
FIG. 3 shows fuel injection amounts Q1, Q2, Q (= Q₁+ Q₂), Injection start timings θS1 and θS2, injection completion timings θE1 and θE2, and an average fuel ratio A / F in the combustion chamber 5. In FIG. 3, the horizontal axis L indicates the amount of depression of the accelerator pedal 50, that is, the required load.
As can be seen from FIG. 3, the required load L is L.₁If it is lower, the fuel injection Q2 is performed between θS2 and θE2 at the end of the compression stroke. At this time, the average air-fuel ratio A / F is considerably lean. Required load L is L₁And L₂During the period, the first fuel injection Q1 is performed between θS1 and θE1 at the beginning of the intake stroke, and then the second fuel injection Q2 is performed between θS2 and θE2 at the end of the compression stroke. Also at this time, the air-fuel ratio A / F is lean. Required load L is L₂If it is larger than that, the fuel injection Q1 is performed between θS1 and θE1 in the initial stage of the intake stroke. At this time, the average air-fuel ratio A / F is lean in the region where the required load L is low, the average air-fuel ratio A / F is made the stoichiometric air-fuel ratio when the required load L increases, and the average air-fuel ratio A / F becomes higher when the required load L becomes higher. The fuel ratio A / F is made rich. Note that only the required load L is the operation region in which the fuel injection Q2 is performed only at the end of the compression stroke, the operation region in which the fuel injections Q1 and Q2 are performed twice, and the operation region in which the fuel injection Q1 is performed only in the early stage of the intake stroke. Is actually determined by the required load L and the engine speed.
[0016]
FIG. 2 shows that the required load L is L₁It shows a case where the fuel injection Q2 is performed only when it is smaller than (FIG. 3), that is, at the end of the compression stroke. As shown in FIG. 2, a cavity 4a is formed on the top surface of the piston 4, and the required load L is L.₁When it is lower than that, fuel is injected from the fuel injection valve 6 toward the bottom wall surface of the cavity 4a at the end of the compression stroke. This fuel is guided by the peripheral wall surface of the cavity 4 a and travels toward the spark plug 7, whereby an air-fuel mixture G is formed around the spark plug 7. Next, the air-fuel mixture G is ignited by the spark plug 7.
[0017]
On the other hand, as described above, the required load L is L₁And L₂When it is between, fuel injection is performed twice. In this case, a lean air-fuel mixture is formed in the combustion chamber 5 by the first fuel injection Q1 performed at the beginning of the intake stroke. Next, an air-fuel mixture having an optimum concentration is formed around the spark plug 7 by the second fuel injection Q2 performed at the end of the compression stroke. The air-fuel mixture is ignited by the spark plug 7, and the lean air-fuel mixture is combusted by the ignition flame.
[0018]
On the other hand, the required load L is L₂If it is greater than that, a homogeneous mixture of lean, stoichiometric air-fuel ratio or rich air-fuel ratio is formed in the combustion chamber 5 as shown in FIG. 3, and this homogeneous mixture is ignited by the spark plug 7.
NO contained in catalytic converter 20a_xThe absorbent 22 has, for example, alumina as a carrier. On this carrier, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y, etc. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. Engine intake passage, combustion chamber 5 and NO_xIf the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the absorbent 22 is called the air-fuel ratio of the exhaust gas, this NO_xThe absorbent 22 is NO when the air-fuel ratio of the exhaust gas is lean._xNO is absorbed when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich._xNO release_xPerforms absorption and release action.
[0019]
This NO_xIf the absorbent 22 is placed in the engine exhaust passage, NO_xThe absorbent 22 is actually NO_xThe detailed mechanism of the absorption / release action is not clear. However, this absorption / release action is considered to be performed by the mechanism shown in FIG. Next, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on the support, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0020]
In the internal combustion engine shown in FIG. 1, combustion is normally performed with the air-fuel ratio in the combustion chamber 5 being lean. Thus, when combustion is performed with the air-fuel ratio being lean, the oxygen concentration in the exhaust gas is high. At this time, as shown in FIG.₂Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt in the form of On the other hand, NO in the inflowing exhaust gas is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). Then the generated NO₂As shown in FIG. 4 (A), a part of is absorbed in the absorbent while being oxidized on platinum Pt and combined with barium oxide BaO.₃ ⁻Diffuses into the absorbent in the form of In this way NO_xIs NO_xAbsorbed in the absorbent 22. As long as the oxygen concentration in the inflowing exhaust gas is high, NO on the surface of platinum Pt₂Is produced and NO in the absorbent_xNO unless absorption capacity is saturated₂Is absorbed into the absorbent and nitrate ion NO.₃ ⁻Is generated.
[0021]
On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas decreases, and as a result, NO on the surface of platinum Pt.₂The production amount of is reduced. NO₂When the production amount of NO decreases, the reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus nitrate ion NO in the absorbent₃ ⁻Is NO₂Is released from the absorbent in the form of NO at this time_xNO released from absorbent 22_xAs shown in FIG. 4 (B), it is reduced by reacting with a large amount of unburned HC and CO contained in the exhaust gas. In this way, NO on the surface of platinum Pt.₂NO from the absorbent to the next when no longer exists₂Is released. Therefore, when the air-fuel ratio of the exhaust gas is made rich, NO will be shortened in a short time._xNO from absorbent 22_xIs released, and this released NO_xNO in the atmosphere because_xWill not be discharged.
[0022]
In this case, even if the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, NO_xNO from absorbent 22_xIs released. However, if the exhaust gas air-fuel ratio is the stoichiometric air-fuel ratio, NO_xNO from absorbent 22_xNO is released only gradually_xTotal NO absorbed by absorbent 22_xIt takes a little longer time to release.
By the way NO_xNO of absorbent 22_xAbsorption capacity is limited, so NO_xNO of absorbent 22_xNO before absorption capacity saturates_xNO from absorbent 22_xNeed to be released. Therefore, in the embodiment according to the present invention, NO is used._xNO absorbed by absorbent 22_xWhen it is determined that the amount exceeds the allowable amount, the air-fuel ratio of the exhaust gas is temporarily made rich so that NO_xNO from absorbent 22_xTo be released.
[0023]
Now, for a while after the engine is started, the temperature in the combustion chamber 5 is low, and thus a large amount of unburned HC is generated. On the other hand, for a while after the engine is started, the three-way catalyst 20 and NO_xSince the temperature of the absorbent 22 is low, the oxidation reaction of unburned HC in the three-way catalyst 20 or the like cannot be expected. Thus, for a while after the engine is started, a large amount of unburned HC is discharged into the atmosphere. In this case, there are two methods for reducing unburned HC discharged into the atmosphere. One method is to promote the oxidation reaction of the unburned HC generated. Another method consists in activating the three-way catalyst 20 as soon as possible. In the present invention, a method capable of performing these simultaneously is used. First, this method will be described with reference to FIG. In FIG. 5, the horizontal axis indicates the crank angle, and BTDC and ATDC indicate before the top dead center and after the top dead center, respectively.
[0024]
FIG. 5A shows a case where it is not particularly necessary to reduce unburned HC by the method according to the present invention, and the required load L is L.₁The fuel injection timing when it is smaller than is shown. As shown in FIG. 5A, at this time, only the main fuel Qm is injected at the end of the compression stroke, and at this time, the exhaust control valve 24 is held in a fully opened state.
On the other hand, when it is necessary to reduce the unburned HC by the method according to the present invention, the exhaust control valve 24 is almost fully closed, and further, as shown in FIG. In addition to the injection of the main fuel Qm, during the expansion stroke, in the example shown in FIG. 5B, the auxiliary fuel Qa is additionally injected in the vicinity of 60 ° after compression top dead center (ATDC). In this case, after combustion of the main fuel Qm, the main fuel Qm is burned under excess air so that sufficient oxygen remains in the combustion chamber 5 to completely burn the sub fuel Qa. FIG. 5A and FIG. 5B show the fuel injection period when the engine load and the engine speed are the same. Therefore, when the period load and the period speed are the same, FIG. The injection amount of the main fuel Qm in the case shown in FIG. 5 (B) is increased compared to the injection amount of the main fuel Qm in the case shown in FIG. 5 (A).
[0025]
FIG.organAn example of the concentration (ppm) of unburned HC in the exhaust gas at each position of the exhaust passage is shown. In the example shown in FIG. 6, the black triangle indicates unburned in the exhaust gas at the outlet of the exhaust port 11 when the main fuel Qm is injected at the end of the compression stroke as shown in FIG. 5A with the exhaust control valve 24 fully opened. The concentration of HC (ppm) is shown. In this case, the concentration of unburned HC in the exhaust gas at the outlet of the exhaust port 11 becomes an extremely high value of 6000 ppm or more.
[0026]
On the other hand, in the example shown in FIG. 6, the black circles and solid lines indicate that the exhaust control valve 24 is almost fully closed, and the unburned gas in the exhaust gas when the main fuel Qm and the sub fuel Qa are injected as shown in FIG. 5B. The concentration of HC (ppm) is shown. In this case, the concentration of unburned HC in the exhaust gas at the outlet of the exhaust port 11 is 2000 ppm or less, and in the vicinity of the exhaust control valve 24, the concentration of unburned HC in the exhaust gas is reduced to 150 ppm. Therefore, in this case, it can be seen that the amount of unburned HC discharged into the atmosphere is greatly reduced.
[0027]
The reason why the unburned HC is reduced in the exhaust passage upstream of the exhaust control valve 24 is that the oxidation reaction of the unburned HC is promoted. However, as shown by the black triangle in FIG. 6, when the amount of unburned HC at the outlet of the exhaust port 11 is large, that is, when the amount of unburned HC generated in the combustion chamber 5 is large, unburned in the exhaust passage. Even if the oxidation reaction of HC is promoted, the amount of unburned HC discharged into the atmosphere is not reduced so much. That is, by promoting the oxidation reaction of unburned HC in the exhaust passage, the amount of unburned HC discharged into the atmosphere can be greatly reduced as shown by the black circle in FIG. This is when the concentration of unburned HC is low, that is, when the amount of unburned HC generated in the combustion chamber 5 is small.
[0028]
Thus, in order to reduce the amount of unburned HC discharged into the atmosphere, the amount of unburned HC generated in the combustion chamber 5 is reduced and the oxidation reaction of unburned HC in the exhaust passage is promoted. It is necessary to satisfy two requirements at the same time. First, the second requirement, that is, promoting the oxidation reaction of unburned HC in the exhaust passage will be described.
[0029]
According to the method of the present invention, when the amount of unburned HC discharged into the atmosphere is to be reduced, the exhaust control valve 24 is almost fully closed. As described above, when the exhaust control valve 24 is almost fully closed, the pressure in the exhaust port 11, the exhaust manifold 19, and the exhaust pipe 21 upstream of the exhaust control valve 24, that is, the back pressure becomes considerably high.
An increase in the back pressure means that when the exhaust gas is discharged from the combustion chamber 5 into the exhaust port 11, the pressure of the exhaust gas does not decrease so much, and therefore the temperature of the exhaust gas discharged from the combustion chamber 5 also decreases significantly. It means not to. Therefore, the temperature of the exhaust gas discharged into the exhaust port 11 is maintained at a considerably high temperature. On the other hand, a high back pressure means a high exhaust gas density, and a high exhaust gas density means that the exhaust gas flow velocity in the exhaust passage from the exhaust port 11 to the exhaust control valve 24 is high. Means slow. Accordingly, the exhaust gas discharged into the exhaust port 11 stays in the exhaust passage upstream of the exhaust control valve 24 for a long time at a high temperature.
[0030]
As described above, when the exhaust gas is retained in the exhaust passage upstream of the exhaust control valve 24 for a long time under a high temperature, the oxidation reaction of unburned HC is promoted during that time. In this case, according to experiments by the present inventor, in order to promote the oxidation reaction of unburned HC in the exhaust passage, the exhaust gas temperature at the outlet of the exhaust port 11 needs to be about 750 ° C. or higher, preferably 800 ° C. or higher. It has been found.
[0031]
Further, as the time during which the high-temperature exhaust gas stays in the exhaust passage upstream of the exhaust control valve 24 becomes longer, the reduction amount of unburned HC increases. This residence time becomes longer as the position of the exhaust control valve 24 is further away from the outlet of the exhaust port 11, so that the exhaust control valve 24 is separated from the outlet of the exhaust port 11 by a distance necessary to sufficiently reduce unburned HC. Need to be placed. If the exhaust control valve 24 is arranged at a distance necessary to sufficiently reduce unburned HC from the outlet of the exhaust port 11, the concentration of unburned HC is greatly reduced as shown by the solid line in FIG. According to experiments by the present inventors, it has been found that the distance from the outlet of the exhaust port 11 to the exhaust control valve 24 is preferably 1 meter or more in order to sufficiently reduce unburned HC.
[0032]
As described above, in order to promote the oxidation reaction of unburned HC in the exhaust passage, the exhaust gas temperature at the outlet of the exhaust port 11 needs to be about 750 ° C. or higher, preferably 800 ° C. or higher. Further, in order to reduce the amount of unburned HC discharged into the atmosphere, the first requirement described above must be satisfied. That is, it is necessary to reduce the amount of unburned HC generated in the combustion chamber 5. Therefore, in the present invention, in addition to the main fuel Qm for generating the engine output, the auxiliary fuel Qa is additionally injected after the injection of the main fuel Qm, and the auxiliary fuel Qa is burned in the combustion chamber 5.
[0033]
That is, when the auxiliary fuel Qa is burned in the combustion chamber 5, a large amount of unburned HC, which is the unburned main fuel Qm, is burned when the auxiliary fuel Qa is burned. Further, since the auxiliary fuel Qa is injected into the high-temperature gas, the auxiliary fuel Qa is burned well, so that unburned HC which is the unburned residue of the auxiliary fuel Qa is not generated so much. Thus, the amount of unburned HC finally generated in the combustion chamber 5 is considerably reduced.
[0034]
Further, when the auxiliary fuel Qa is burned in the combustion chamber 5, in addition to the heat generated by the combustion of the main fuel Qm and the auxiliary fuel Qa itself, the combustion heat of the unburned HC that is the unburned main fuel Qm is additionally generated. Therefore, the burnt gas temperature in the combustion chamber 5 becomes considerably high. In this way, the auxiliary fuel Qa is additionally injected in addition to the main fuel Qm to burn the auxiliary fuel Qa, thereby reducing the amount of unburned HC generated in the combustion chamber 5 and reducing the exhaust gas temperature at the outlet of the exhaust port 11 to 750. C. or higher, preferably 800.degree. C. or higher.
[0035]
Thus, in the method according to the present invention, the auxiliary fuel Qa needs to be combusted in the combustion chamber 5, and for that purpose, it is necessary that sufficient oxygen remains in the combustion chamber 5 when the auxiliary fuel Qa is burned. In addition, it is necessary to inject the auxiliary fuel Qa at a time when the injected auxiliary fuel Qa is burned well in the combustion chamber 5.
Therefore, in the method according to the present invention, the main fuel Qm is burned under excess air so that sufficient oxygen can remain in the combustion chamber 5 during the combustion of the auxiliary fuel Qa. Further, the injection timing at which the auxiliary fuel Qa injected in the stratified combustion internal combustion engine shown in FIG. 2 is burned well in the combustion chamber 5 is approximately 50 after compression top dead center (ATDC) indicated by an arrow Z in FIG. Therefore, in the stratified combustion internal combustion engine shown in FIG. 2, the auxiliary fuel Qa is injected after the compression top dead center (ATDC) in the expansion stroke of approximately 50 ° to approximately 90 °. . The secondary fuel Qa injected in the expansion stroke from approximately 50 ° to approximately 90 ° after the compression top dead center (ATDC) does not contribute to the generation of engine output.
[0036]
By the way, according to an experiment by the present inventor, in the stratified combustion internal combustion engine shown in FIG. 2, the unburned HC discharged into the atmosphere when the auxiliary fuel Qa is injected in the vicinity of 60 ° after compression top dead center (ATDC). The amount is the least. Therefore, in the embodiment according to the present invention, as shown in FIG. 5B, the injection timing of the auxiliary fuel Qa is approximately 60 ° after compression top dead center (ATDC).
[0037]
The optimal injection timing of the auxiliary fuel Qa varies depending on the engine type. For example, in a diesel engine, the optimal injection timing of the auxiliary fuel Qa is in the expansion stroke or in the exhaust stroke. Therefore, in the present invention, the fuel injection of the auxiliary fuel Qa is performed during the expansion stroke or the exhaust stroke. On the other hand, the burnt gas temperature in the combustion chamber 5 is affected by both the combustion heat of the main fuel Qm and the combustion heat of the auxiliary fuel Qa. That is, the burnt gas temperature in the combustion chamber 5 increases as the injection amount of the main fuel Qm increases, and increases as the injection amount of the auxiliary fuel Qa increases. Furthermore, the burnt gas temperature in the combustion chamber 5 is affected by the back pressure. That is, as the back pressure increases, the amount of burned gas remaining in the combustion chamber 5 increases because the burned gas does not easily flow out of the combustion chamber 5, and thus the exhaust control valve 24 is almost fully closed. The burnt gas temperature in the combustion chamber 5 is raised.
[0038]
By the way, when the exhaust control valve 24 is almost closed, and the back pressure becomes high, the generated torque of the engine decreases with respect to the optimum required generated torque. Therefore, in the embodiment according to the present invention, when the exhaust control valve 24 is almost fully closed as shown in FIG. 5 (B), the exhaust control is performed under the same engine operating state as shown in FIG. 5 (A). The injection amount of the main fuel Qm is increased as compared with the case where the exhaust control valve 24 is fully opened under the same engine operating state so as to approach the required generation torque of the engine when the valve 24 is fully opened. . In the embodiment according to the present invention, when the exhaust control valve 24 is almost fully closed, the engine generated when the exhaust control valve 24 is fully opened under the same engine operating state at that time. The main fuel Qm is increased so as to match the required generated torque.
[0039]
FIG. 7 shows a change in the main fuel Qm required to obtain the required generation torque of the engine with respect to the required load L. In FIG. 7, the solid line indicates the case where the exhaust control valve 24 is substantially fully closed, and the broken line indicates the case where the exhaust control valve 24 is fully opened.
On the other hand, FIG. 8 shows the relationship between the main fuel Qm and the sub fuel Qa required to bring the exhaust gas temperature at the outlet of the exhaust port 11 from about 750 ° C. to about 800 ° C. when the exhaust control valve 24 is almost fully closed. ing. As described above, even if the main fuel Qm is increased, the burnt gas temperature in the combustion chamber 5 is increased, and even if the sub fuel Qa is increased, the burnt gas temperature in the combustion chamber 5 is increased. Therefore, the relationship between the main fuel Qm and the sub fuel Qa required to change the exhaust gas temperature at the outlet of the exhaust port 11 from about 750 ° C. to about 800 ° C. is shown in FIG. Qa decreases, and if the main fuel Qm is decreased, the auxiliary fuel Qa increases.
[0040]
However, when the main fuel Qm and the auxiliary fuel Qa are increased by the same amount, the temperature increase in the combustion chamber 5 is much larger when the auxiliary fuel Qa is increased than when the main fuel Qm is increased. . Therefore, it can be said that it is preferable to raise the burnt gas temperature in the combustion chamber 5 by increasing the auxiliary fuel Qa from the viewpoint of reducing the fuel consumption.
[0041]
Therefore, in the embodiment according to the present invention, when the exhaust control valve 24 is almost fully closed, the main fuel Qm is increased by an amount necessary to increase the generated torque of the engine to the required generated torque, and mainly the combustion heat of the auxiliary fuel Qa. Thus, the burnt gas temperature in the combustion chamber 5 is raised.
In this way, when the exhaust control valve 24 is almost fully closed and the amount of the auxiliary fuel Qa required to bring the exhaust gas at the outlet of the exhaust port 11 to about 750 ° C. or higher, preferably about 800 ° C. or higher, is injected from the exhaust port 11. In the exhaust passage leading to the exhaust control valve 24, the concentration of unburned HC can be greatly reduced. At this time, in the exhaust passage from the exhaust port 11 to the exhaust control valve 24, as shown in FIG. p. In order to reduce the pressure to about m 2, the pressure in the exhaust passage upstream of the exhaust control valve 24 needs to be about 80 KPa or more with gauge pressure. At this time, the closing ratio of the exhaust passage sectional area by the exhaust control valve 24 is approximately 95% or more. Therefore, in the embodiment shown in FIG. 1, when the discharge amount of unburned gas to the atmosphere should be greatly reduced, the exhaust control is performed so that the closing ratio of the exhaust passage cross-sectional area by the exhaust control valve 24 becomes approximately 90% or more. The valve 24 is almost fully closed.
[0042]
Further, as described above, in the method according to the present invention, the exhaust gas temperature at the outlet of the exhaust port 11 becomes extremely high at 750 ° C. or higher, and therefore the exhaust gas temperature at the outlet of the exhaust manifold 19 becomes considerably high. Therefore, when the engine is started, the three-way catalyst 20 reaches the activation temperature in a short time.
By the way, in the method according to the present invention, unburned HC is reduced, but NO._x Cannot be reduced. In this case, NO_x NO to reduce_x NO due to absorbent 22_x Or the exhaust gas air-fuel ratio is maintained at the stoichiometric air-fuel ratio and the three-way catalyst 20_x There is one of the methods of reducing. But NO_x The absorbent 22threeSince the capacity is larger than that of the original catalyst 20 and it is located on the downstream side, the activation temperature is not easily reached. On the other hand, as described above, the three-way catalyst 20 reaches the activation temperature in a short time. Therefore, in the first embodiment according to the present invention, when the three-way catalyst 20 reaches the activation temperature, NO._x The air-fuel ratio is maintained at the stoichiometric air-fuel ratio until the absorbent 22 reaches the activation temperature.
[0043]
That is, in the first embodiment, as shown in FIG. 9, when the engine is started, the exhaust control valve 24 is almost fully closed and the auxiliary fuel Qa is injected. At this time, the average air-fuel ratio in the combustion chamber 5, that is, the air-fuel ratio of the exhaust gas is lean, and the amount of unburned HC discharged into the atmosphere is extremely small. On the other hand, when the exhaust control valve 24 is almost fully closed and the auxiliary fuel Qa is injected, the temperature of the three-way catalyst 20 rises rapidly because the exhaust gas temperature rises. Next, when the temperature T1 of the three-way catalyst 20 reaches the activation temperature, for example, 300 ° C., the exhaust control valve 24 is fully opened, the injection of the auxiliary fuel Qa is stopped, and the average air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio. Thus, the injection amount is feedback-controlled based on the output signal of the air-fuel ratio sensor 35.
[0044]
When the average air-fuel ratio in the combustion chamber 5 is maintained at the stoichiometric air-fuel ratio, unburned HC, CO and NO in the exhaust gas_xIs purified in the three-way catalyst 20 and thus discharged into the atmosphere, unburned HC, CO and NO_xIs greatly reduced. On the other hand, when the average air-fuel ratio in the combustion chamber 5 is maintained at the stoichiometric air-fuel ratio, the temperature T1 of the three-way catalyst 20 continues to rise due to the oxidation-reduction reaction heat, and NO_xThe temperature T2 of the absorbent 22 also gradually increases. Then NO_xWhen the temperature T2 of the absorbent 22 reaches an activation temperature, for example, 300 ° C., the normal operation state shown in FIG. 3 is switched to make the average air-fuel ratio in the combustion chamber 5 lean. NO_xWhen the absorbent 22 is activated, NO in the exhaust gas_xIs NO_xAbsorbed in the absorbent 22.
[0045]
Note that even if the average air-fuel ratio in the combustion chamber 5 is slightly rich, unburned HC, CO and NO_xIs purified by the three-way catalyst 20. Therefore, after the three-way catalyst 20 exceeds the activation temperature, NO_xUntil the absorbent 22 exceeds the activation temperature, the average air-fuel ratio in the combustion chamber 5 can be kept slightly rich.
FIG. 10 shows an operation control routine for executing the first embodiment.
[0046]
Referring to FIG. 10, first, at step 100, it is judged if a completion flag indicating that the catalyst temperature raising process or the like has been completed is set. When the completion flag is not set, the routine proceeds to step 101 where it is judged if the temperature T1 of the three-way catalyst 20 has exceeded the activation temperature, for example, 300 ° C. When T1 ≦ 300 ° C., the routine proceeds to step 102 where the exhaust control valve 24 is almost fully closed. Next, at step 103, injection control of the main fuel Qm is performed, and then at step 104, injection control of the auxiliary fuel Qa is performed.
[0047]
Next, when it is judged at step 101 that T1> 300 ° C., the routine proceeds to step 105, where the exhaust control valve 24 is fully opened, then at step 106, the average air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio or slightly richer. Be controlled. At this time, the auxiliary fuel Qa is not injected. Next, in step 107, NO_xIt is determined whether or not the temperature T2 of the absorbent 22 has exceeded an activation temperature, for example, 300 ° C. When T2 ≦ 300 ° C., the processing cycle is completed.
[0048]
On the other hand, when it is determined at step 107 that T2 ≧ 300 ° C., the routine proceeds to step 108 where the completion flag is set. When the completion flag is set, the routine proceeds from step 100 to step 109, where the normal injection control shown in FIG. 3 is started. Next, at step 110, NO_xNO absorbed by absorbent 22_xNO when the amount exceeds the allowable amount_xNO from absorbent 22_xNO to release_xRelease processing is performed.
[0049]
Next, a second embodiment will be described. In this second embodiment, after the engine is started, the exhaust control valve 24 is almost fully closed, the auxiliary fuel Qa is injected, and the temperature of the three-way catalyst 20 is the maximum allowable temperature within a range where the three-way catalyst 20 does not cause thermal deterioration. For example, the temperature is raised to about 800 ° C. Next, the exhaust control valve 24 is fully opened, the main fuel Qm is burned in a lean state, the auxiliary fuel Qa is injected during the expansion stroke, and the average air-fuel ratio in the combustion chamber 5, that is, the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. Based on the output signal of the air-fuel ratio sensor 35, the injection amount of the auxiliary fuel Qa is feedback-controlled so that When the auxiliary fuel Qa is injected to maintain the average air-fuel ratio in the combustion chamber 5 at the stoichiometric air-fuel ratio, a large amount of unburned HC is discharged from the combustion chamber 5 and oxidation of these large amounts of unburned HC on the three-way catalyst 20 is performed. The three-way catalyst 20 is maintained at a high temperature of, for example, 800 ° C. by the heat of reaction. As a result, NO_xThe absorbent 22 rapidly rises in temperature and reaches the activation temperature within a short time.
That is, also in the second embodiment, as shown in FIG. 11, when the engine is started, the exhaust control valve 24 is almost fully closed and the auxiliary fuel Qa is injected. At this time, the average air-fuel ratio in the combustion chamber 5, that is, the air-fuel ratio of the exhaust gas is lean, and the amount of unburned HC discharged into the atmosphere is extremely small. On the other hand, when the exhaust control valve 24 is almost fully closed and the auxiliary fuel Qa is injected, the temperature of the three-way catalyst 20 rises rapidly because the exhaust gas temperature rises. Next, when the temperature T1 of the three-way catalyst 20 reaches a temperature slightly lower than the allowable temperature 800 ° C., for example, 750 ° C., the exhaust control valve 24 is fully opened so that the average air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio. Based on the output signal of the air-fuel ratio sensor 35, the injection amount of the auxiliary fuel Qa is feedback-controlled.
[0050]
When the average air-fuel ratio in the combustion chamber 5 is maintained at the stoichiometric air-fuel ratio, unburned HC, CO and NO in the exhaust gas_xIs purified in the three-way catalyst 20 and thus discharged into the atmosphere, unburned HC, CO and NO_xIs greatly reduced. On the other hand, when the auxiliary fuel Qa is injected and the average air-fuel ratio in the combustion chamber 5 is maintained at the stoichiometric air-fuel ratio, the temperature T1 of the three-way catalyst 20 is maintained at a high temperature._xThe temperature T2 of the absorbent 22 rises rapidly. Then NO_xWhen the temperature T2 of the absorbent 22 reaches an activation temperature, for example, 300 ° C., the normal operation state shown in FIG. 3 is switched to make the average air-fuel ratio in the combustion chamber 5 lean. NO_xWhen the absorbent 22 is activated, NO in the exhaust gas_xIs NO_xAbsorbed in the absorbent 22.
[0051]
In this case as well, after the three-way catalyst 20 is raised to near the allowable temperature, the NO_xUntil the absorbent 22 exceeds the activation temperature, the average air-fuel ratio in the combustion chamber 5 can be kept slightly rich.
FIG. 12 shows an operation control routine for executing the second embodiment.
Referring to FIG. 12, first, at step 200, it is judged if a completion flag indicating that the catalyst temperature raising process or the like has been completed is set. When the completion flag is not set, the routine proceeds to step 201, where it is judged if the temperature T1 of the three-way catalyst 20 has exceeded a temperature 800 ° C.-α (α is a constant value) that is slightly lower than the allowable temperature. When T1 ≦ 800 ° C.−α, the routine proceeds to step 202 where the exhaust control valve 24 is almost fully closed. Next, at step 203, injection control of the main fuel Qm is performed, and then at step 204, injection control of the auxiliary fuel Qa is performed.
[0052]
Next, when it is judged at step 201 that T1> 800 ° C.−α, the routine proceeds to step 205, where the exhaust control valve 24 is fully opened, then at step 206, the average air-fuel ratio in the combustion chamber 5 is the stoichiometric air-fuel ratio or slightly lower. The injection amount of the auxiliary fuel Qa is controlled so as to be rich. Next, in step 207, NO_xIt is determined whether or not the temperature T2 of the absorbent 22 has exceeded an activation temperature, for example, 300 ° C. When T2 ≦ 300 ° C., the processing cycle is completed.
[0053]
On the other hand, when it is determined at step 207 that T2 ≧ 300 ° C., the routine proceeds to step 208 where the completion flag is set. When the completion flag is set, the routine proceeds from step 200 to step 209, where the normal injection control shown in FIG. 3 is started. Next, at step 210, NO_xNO absorbed by absorbent 22_xNO when the amount exceeds the allowable amount_xNO from absorbent 22_xNO to release_xRelease processing is performed.
[0054]
Next, a third embodiment will be described.
As described above, when the air-fuel ratio of the exhaust gas is lean, the NO in the exhaust gas_xIs NO_xAbsorbed by the absorbent 22. However, in the exhaust gas, SO_xIs included, NO_xAbsorbent 22 contains NO_xNot only SO_xIs also absorbed. This NO_xSO to absorbent 22_xNO absorption mechanism_xIt is considered that the absorption mechanism is the same.
[0055]
That is, NO_xThe case where platinum Pt and barium Ba are supported on a carrier as in the case of explaining the absorption mechanism of oxygen will be described as an example. As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, oxygen O₂Is O₂ ⁻Or O^2-Is attached to the surface of platinum Pt in the form of SO₂Is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with SO₃It becomes. The generated SO₃Part of the catalyst is further oxidized on platinum Pt and absorbed into the absorbent and combined with barium oxide BaO, while sulfate ion SO₄ ^2-Diffused into the absorbent in the form of a stable sulfate BaSO₄Is generated.
[0056]
However, this sulfate BaSO₄Is stable and difficult to decompose, even if the air-fuel ratio of the exhaust gas is simply rich, sulfate BaSO₄Remains undisassembled. Therefore NO_xIn the absorbent 22, sulfate BaSO as time passes.₄Will increase, so NO over time_xNO which the absorbent 22 can absorb_xThe amount will decrease.
[0057]
In this embodiment, therefore, SO in the exhaust gas is used._xCapture NO_xAbsorbent 22 with SO_xAs the first catalyst 20 in order to prevent the inflow of the exhaust gas, the SO 2 when the air-fuel ratio of the exhaust gas is lean is used._xWhen the catalyst temperature is higher than a predetermined temperature and the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich, the SO_xThe catalyst which releases | releases is used.
[0058]
A three-way catalyst can be used as such a first catalyst 20, or NO._xSO similar to absorbent 22_xIt is also possible to use an absorbent having an absorption capacity. In this case, when the temperature of the first catalyst 20 reaches a certain temperature, for example, 600 ° C. or higher, the sulfate BaSO is contained in the first catalyst 20.₄Therefore, if the air-fuel ratio of the exhaust gas is made rich at this time, the SO_xWill be released.
[0059]
Therefore, in the embodiment according to the present invention, the exhaust control valve 24 is almost fully closed when the engine is started, the temperature of the first catalyst 20 is increased to 600 ° C. or more by injecting the auxiliary fuel Qa, and then the exhaust control valve 24 is fully opened. The main fuel Qm is leancombustionThe auxiliary fuel Qa is injected during the expansion stroke so that the average air-fuel ratio in the combustion chamber 5, that is, the air-fuel ratio of the exhaust gas becomes slightly rich, based on the output signal of the air-fuel ratio sensor 35. The injection amount is feedback controlled, and during this time, the first catalyst 20 performs SO control._x To be released.
[0060]
It should be noted that the SO 2 from the first catalyst 20_xSO released when is released_xIs NO_xIt is necessary not to flow into the absorbent 22, and as a result, as shown in FIG._xAn exhaust bypass passage 37 that bypasses the absorbent 22 is provided, and NO._xExhaust control valves 24a and 24b that are actuated by actuators 25a and 25b are disposed in the main passage 36 and the exhaust bypass passage 37 upstream of the absorbent 22, respectively.
[0061]
That is, in this third embodiment, as shown in FIG. 14, when the engine is started, the exhaust control valves 24a and 24b are almost fully closed, and the auxiliary fuel Qa is injected. At this time, the average air-fuel ratio in the combustion chamber 5, that is, the air-fuel ratio of the exhaust gas is lean, and the amount of unburned HC discharged into the atmosphere is extremely small. On the other hand, when both the exhaust control valves 24a and 24b are almost fully closed and the auxiliary fuel Qa is injected, the exhaust gas temperature rises, and therefore the temperature T1 of the first catalyst 20 rises rapidly. Next, when the temperature T1 of the first catalyst 20 reaches approximately 600 ° C., the exhaust control valve 24a is fully closed and the exhaust control valve 24b is fully opened, so that the average air-fuel ratio in the combustion chamber 5 becomes slightly rich. In addition, the injection amount of the auxiliary fuel Qa is feedback controlled based on the output signal of the air-fuel ratio sensor 35. When the auxiliary fuel Qa is injected and the average air-fuel ratio in the combustion chamber 5 is kept slightly rich, the temperature T1 of the three-way catalyst 20 is maintained at 600 ° C. or higher by the oxidation-reduction reaction heat, and thus the first SO from catalyst 20_xThe release of.
[0062]
On the other hand, when the average air-fuel ratio in the combustion chamber 5 is kept slightly rich in this way, unburned HC, CO and NO in the exhaust gas_xIs purified in the first catalyst 20 and thus discharged into the atmosphere, unburned HC, CO and NO_xIs greatly reduced. At this time, the exhaust gas is NO._xSince it does not flow into the absorbent 22, NO as shown in FIG._xThe temperature of the absorbent 22 remains low.
[0063]
Next, the SO from the first catalyst 20_xWhen it is determined that the release of the fuel has been completed, the engine is switched to the normal operation state shown in FIG. At this time, the exhaust control valve 24a is fully opened, and the exhaust control valve 24b is fully closed. Therefore, the exhaust gas is NO_xInto the absorbent 22 and thus NO._xThe temperature of the absorbent 22 rises.
[0064]
In this embodiment as well, the SO 2 after the first catalyst 20 exceeds approximately 600 ° C._xIt is also possible to maintain the average air-fuel ratio in the combustion chamber 5 at the stoichiometric air-fuel ratio until the release of the fuel is completed.
FIG. 15 shows an operation control routine for executing the third embodiment.
Referring to FIG. 15, first, at step 300, it is judged if a completion flag indicating that the catalyst temperature raising process or the like has been completed is set. When the completion flag is not set, the routine proceeds to step 301, where it is judged if the temperature T1 of the first catalyst 20 has exceeded 600 ° C. When T1 ≦ 600 ° C., the routine proceeds to step 302 where both the exhaust control valves 24a, 24b are almost fully closed. Next, at step 303, injection control of the main fuel Qm is performed, and then at step 304, injection control of the auxiliary fuel Qa is performed.
[0065]
Next, when it is judged at step 301 that T1> 600 ° C., the routine proceeds to step 305, where the exhaust control valve 24a is fully closed and the exhaust control valve 24b is fully opened, then at step 306, the average empty in the combustion chamber 5 is reached. The injection amount of the auxiliary fuel Qa is controlled so that the fuel ratio becomes the stoichiometric air-fuel ratio or slightly rich. Next, at step 307, the SO from the first catalyst 20._xIt is determined whether or not the release of SO has been completed, and SO_xWhen the discharge is completed, the routine proceeds to step 308, where the completion flag is set.
[0066]
When the completion flag is set, the routine proceeds from step 300 to step 309, where the exhaust control valve 24a is fully opened and the exhaust control valve 24b is fully closed. Next, at step 310, the normal injection control shown in FIG. 3 is started. Next, at step 311, NO_xNO absorbed by absorbent 22_xNO when the amount exceeds the allowable amount_xNO from absorbent 22_xNO to release_xRelease processing is performed.
[0067]
【The invention's effect】
The first catalyst can be activated early without causing thermal degradation.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
FIG. 2 is a side sectional view of a combustion chamber.
FIG. 3 is a diagram showing an injection amount, an injection timing, and an air-fuel ratio.
FIG. 4 NO_xIt is a figure for demonstrating the absorption-and-release function of.
FIG. 5 is a diagram showing injection timing.
FIG. 6 is a diagram showing the concentration of unburned HC.
FIG. 7 is a diagram showing an injection amount of main fuel.
FIG. 8 is a diagram showing a relationship between an injection amount of main fuel and an injection amount of sub fuel.
FIG. 9 is a time chart showing changes in the air-fuel ratio and the like in the first embodiment.
FIG. 10 is a flowchart for operation control.
FIG. 11 is a time chart showing changes in air-fuel ratio and the like in the second embodiment.
FIG. 12 is a flowchart for operation control.
FIG. 13 is an overall view showing another embodiment of the internal combustion engine.
FIG. 14 is a time chart showing changes in air-fuel ratio and the like in the third embodiment.
FIG. 15 is a flowchart for operation control.
[Explanation of symbols]
6 ... Fuel injection valve
20 ... 1st catalyst
22 ... Second catalyst
24. Exhaust control valve

Claims (7)

A first catalyst is disposed in the engine exhaust passage, a second catalyst and an exhaust control valve are disposed in the engine exhaust passage downstream of the first catalyst, and the temperature of the first catalyst rises to a predetermined temperature. In the meantime, the exhaust control valve is almost fully closed, and in addition to burning the main fuel injected into the combustion chamber under excess air to generate engine output, the auxiliary fuel is burned by the auxiliary fuel. An internal combustion engine in which additional injection is performed into the combustion chamber at a predetermined time during an expansion stroke or an exhaust stroke, and the exhaust control valve is fully opened when the temperature of the first catalyst exceeds a predetermined temperature Exhaust purification equipment. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas is made lean until the temperature of the first catalyst exceeds a predetermined temperature. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein when the temperature of the first catalyst exceeds a predetermined temperature, the air-fuel ratio of the exhaust gas is maintained at the stoichiometric air-fuel ratio or slightly rich. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the air-fuel ratio of the exhaust gas is maintained at a stoichiometric air-fuel ratio or slightly rich until the temperature of the second catalyst reaches a predetermined temperature. The first catalyst is composed of a catalyst having an oxidation function, and the second catalyst absorbs NO _x in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, and the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich. an exhaust purification system of an internal combustion engine according to claim 3 consisting of _the NO _x absorbent to release the the absorbed NO _x. An exhaust bypass passage that bypasses the NO _x absorbent is provided, the first catalyst has a function of capturing SO _x, and the air-fuel ratio of the exhaust gas is calculated until the SO _x releasing action from the first catalyst is completed. air-fuel ratio or just slightly exhaust purification system of an internal combustion engine according to claim 5 maintained at the rich and the exhaust gas is to flow the exhaust bypass passage bypassing the _the NO _x absorbent. When the exhaust control valve is almost fully closed, the exhaust control is performed under the same engine operating condition so as to approach the engine generated torque when the exhaust control valve is fully opened under the same engine operating condition. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the fuel injection amount is increased as compared with a case where the valve is fully opened.

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