JP3632633B2 - 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]
NO in exhaust gas from internal combustion engines, especially diesel engines_XIs included, this NO_XNO in the engine exhaust system_XIt has been proposed to arrange an occlusion reduction catalyst device. NO_XThe storage reduction catalyst is NO when the oxygen concentration in the ambient atmosphere is high._XIs absorbed in the form of nitrate, and the absorbed NO is absorbed when the oxygen concentration in the surrounding atmosphere decreases._XIs reduced and purified. As a result, NO_XThe storage-reduction catalyst is NO from exhaust gas from diesel engines that are burned under excess air._XNO. By absorbing oxygen well and periodically reducing the oxygen concentration by setting the ambient atmosphere to the theoretical air-fuel ratio or rich air-fuel ratio,_XIs released and purified by reducing substances such as HC._XCan be purified well without being released into the atmosphere.
[0003]
By the way, the exhaust gas of the diesel engine includes particulates mainly composed of soot. This particulate also needs to be processed before being released into the atmosphere, and it has been proposed to arrange a particulate filter for collecting particulates in the engine exhaust system. The above-mentioned NO is added to such a particulate filter._XWhen the storage reduction catalyst is supported, NO_XIn addition to occlusion, particulates can be satisfactorily oxidized and removed. In this way, the particulate filter is NO._XIt is considered to be very effective to dispose a catalyst carrying a storage reduction catalyst in the engine exhaust system of a diesel engine.
[0004]
[Problems to be solved by the invention]
However, the structure of the particulate filter is generally a wall flow type in which the exhaust gas passes through the pores of the collection wall, and compared with a general catalyst device in which the exhaust gas flows along the partition wall carrying the catalyst. Thus, the catalyst carrying area on the surface of the collection wall that is mainly in contact with the exhaust gas is small, and in such a particulate filter, NO in the exhaust gas is reduced._XCan not be sufficiently purified.
[0005]
Therefore, the object of the present invention is NO_XIn an exhaust gas purification apparatus for an internal combustion engine having a particulate filter carrying a storage reduction catalyst, NO in exhaust gas_XIs to be sufficiently purified.
[0006]
[Means for Solving the Problems]
In the exhaust gas purification apparatus for an internal combustion engine according to claim 1 according to the present invention, when the ambient atmosphere is a lean air-fuel ratio, NO_XWhen the stoichiometric or rich air-fuel ratio is absorbed, NO_XNO is released and reduced and purified_XOcclusion reduction catalystOn the particulate collection wallA particulate filter carried and disposed in the engine exhaust system, and a NOx disposed in the engine exhaust system upstream of the particulate filter having an oxidation function_XWith purification catalyst deviceAnd the NO _X The purification catalyst device is NO when the ambient atmosphere is a lean air-fuel ratio. _X When the stoichiometric or rich air-fuel ratio is absorbed, NO _X NO is released and reduced and purified _X Control means for supporting the storage reduction catalyst and setting the inside of the particulate filter to the stoichiometric air-fuel ratio or rich air-fuel ratio is provided. The NO of the particulate filter _X NO from the storage reduction catalyst _X Is released to reduce and purify, and active oxygen is released to oxidize and remove the particulates collected on the particulate collection wall.It is characterized by that.
[0007]
An internal combustion engine exhaust gas purification apparatus according to a second aspect of the present invention is the internal combustion engine exhaust gas purification apparatus according to the first aspect, wherein the NO_XIt is characterized by comprising bypass means for allowing exhaust gas to bypass the particulate filter on the downstream side of the purification catalyst device.
[0008]
An internal combustion engine exhaust gas purification apparatus according to claim 3 of the present invention is the internal combustion engine exhaust gas purification apparatus according to claim 2, wherein the NO_XThe purification catalyst device is the NO_XSupports an occlusion reduction catalyst, said NO_XSO of purification catalyst equipment_XDuring the poisoning recovery, the bypass means is made to function so that the exhaust gas bypasses the particulate filter.
[0009]
According to a fourth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to a fourth aspect of the present invention._XThe purification catalyst device is the NO_XSupports an occlusion reduction catalyst, said NO_XSO of purification catalyst equipment_XImmediately after completion of the poisoning recovery, the exhaust means passes through the particulate filter without functioning the bypass means.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic longitudinal sectional view of a four-stroke diesel engine equipped with an exhaust emission control device according to the present invention, FIG. 2 is an enlarged longitudinal sectional view of a combustion chamber in the diesel engine of FIG. 1, and FIG. It is a bottom view of the cylinder head in the diesel engine of. Referring to FIGS. 1 to 3, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5a is a cavity formed on the top surface of the piston 4, and 5 is formed in the cavity 5a. The combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is a pair of intake valves, 8 is an intake port, 9 is a pair of exhaust valves, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to an air cleaner 14 via an intake duct 13. A throttle valve 16 driven by an electric motor 15 is disposed in the intake duct 13. On the other hand, the exhaust port 10 is connected to an exhaust pipe 18 via an exhaust manifold 17.
[0011]
As shown in FIG. 1, an air-fuel ratio sensor 21 is disposed in the exhaust manifold 17. The exhaust manifold 17 and the surge tank 12 are connected to each other via an EGR passage 22, and an electrically controlled EGR control valve 23 is disposed in the EGR passage 22. A cooling device 24 for cooling the EGR gas flowing in the EGR passage 22 is disposed around the EGR passage 22. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 24, and the EGR gas is cooled by the engine cooling water.
[0012]
On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 26, via a fuel supply pipe 25. Fuel is supplied into the common rail 26 from an electrically controlled fuel pump 27 with variable discharge amount, and the fuel supplied into the common rail 26 is supplied to the fuel injection valve 6 through each fuel supply pipe 25. A fuel pressure sensor 28 for detecting the fuel pressure in the common rail 26 is attached to the common rail 26, and a fuel pump 27 is set so that the fuel pressure in the common rail 26 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 28. The discharge amount is controlled.
[0013]
An electronic control unit 30 receives an output signal from the air-fuel ratio sensor 21 and an output signal from the fuel pressure sensor 28. Further, a load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and an output signal of the load sensor 41 is also input to the electronic control unit 30. For example, an output signal of a crank angle sensor 42 that generates an output pulse every 30 ° rotation is also input. Thus, the electronic control unit 30 operates the fuel injection valve 6, the electric motor 15, the EGR control valve 23, the fuel pump 27, and the switching valve 71a disposed in the exhaust pipe 18 based on various signals. The switching valve 71a will be described later.
[0014]
As shown in FIGS. 2 and 3, in the embodiment according to the present invention, the fuel injection valve 6 is composed of a hole nozzle having six nozzle ports, and the nozzle port of the fuel injection valve 6 is equiangularly slightly downward with respect to the horizontal plane. Fuel F is injected at intervals. As shown in FIG. 3, two of the six fuel sprays F are scattered along the lower surface of the valve body of each exhaust valve 9. 2 and 3 show the time when fuel injection is performed at the end of the compression stroke. At this time, the fuel spray F advances toward the inner peripheral surface of the cavity 5a, and is then ignited and combusted.
[0015]
FIG. 4 shows a case where additional fuel is injected from the fuel injection valve 6 when the lift amount of the exhaust valve 9 is maximum during the exhaust stroke. That is, as shown in FIG. 5, the main injection Qm is performed near the compression top dead center, and then, the additional fuel Qa is injected in the middle of the exhaust stroke. In this case, the fuel spray F traveling in the direction of the valve body of the exhaust valve 9 is directed between the rear surface of the umbrella portion of the exhaust valve 9 and the exhaust port 10. That is, in other words, of the six nozzle ports of the fuel injection valve 6, two of the nozzle ports discharge the fuel spray F when additional fuel Qa is injected while the exhaust valve 9 is open. It is formed so as to face between the rear surface of the umbrella portion of the valve 9 and the exhaust port 10. In the embodiment shown in FIG. 4, the fuel spray F collides with the back of the umbrella part of the exhaust valve 9 at this time, and the fuel spray F that collides with the back of the umbrella part of the exhaust valve 9 is reflected on the back of the umbrella part of the exhaust valve 9. To the exhaust port 10.
[0016]
Normally, no additional fuel Qa is injected, and only main injection Qm is performed. FIG. 6 shows the change in output torque when the air-fuel ratio A / F (horizontal axis in FIG. 6) is changed by changing the opening degree and EGR rate of the throttle valve 16 during engine low load operation, and smoke, HC , CO, NO_XThe experiment example which shows the change of the discharge amount of is shown. As can be seen from FIG. 6, in this experimental example, the EGR rate increases as the air-fuel ratio A / F decreases, and the EGR rate is 65% or more when the air-fuel ratio is less than the theoretical air-fuel ratio (≈14.6).
[0017]
As shown in FIG. 6, when the air-fuel ratio A / F is decreased by increasing the EGR rate, the EGR rate becomes around 40% and the amount of smoke generated increases when the air-fuel ratio A / F becomes about 30 To start. Next, when the EGR rate is further increased and the air-fuel ratio A / F is decreased, the amount of smoke generated increases rapidly and reaches a peak. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the smoke suddenly decreases, the EGR rate is increased to 65% or more, and when the air-fuel ratio A / F is near 15.0, the smoke becomes almost zero. . That is, almost no wrinkles occur. At this time, the output torque of the engine slightly decreases, and NO_XThe amount of generation is considerably low. On the other hand, the generation amount of HC and CO starts increasing at this time.
[0018]
FIG. 7A shows a change in combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is near 21 and the amount of smoke generated is the largest, and FIG. 7B shows that the air-fuel ratio A / F is 18. A change in the combustion pressure in the combustion chamber 5 when the amount of smoke generated in the vicinity is almost zero is shown. As can be seen by comparing FIG. 7A and FIG. 7B, the case of FIG. 7B where the amount of smoke generated is almost zero is shown in FIG. 7A where the amount of smoke generated is large. It can be seen that the combustion pressure is lower than the case.
[0019]
The following can be said from the experimental results shown in FIGS. That is, first, when the air-fuel ratio A / F is 15.0 or less and the amount of smoke generated is almost zero, as shown in FIG._XThe amount of generated is significantly reduced. NO_XThe reduction in the amount of generation means that the combustion temperature in the combustion chamber 5 has decreased, and therefore it can be said that the combustion temperature in the combustion chamber 5 is low when soot is hardly generated. The same can be said from FIG. That is, in the state shown in FIG. 7B where almost no soot is generated, the combustion pressure is low, and thus the combustion temperature in the combustion chamber 5 is low at this time.
[0020]
Second, when the amount of smoke generated, that is, the amount of soot is substantially zero, the HC and CO emissions increase as shown in FIG. This means that the hydrocarbons are discharged without growing to the soot. That is, linear hydrocarbons and aromatic hydrocarbons as shown in FIG. 8 contained in the fuel are thermally decomposed to form soot precursors when the temperature is raised in an oxygen-deficient state. A soot made of a solid in which carbon atoms are assembled is produced. In this case, the actual soot formation process is complicated, and it is not clear what form the soot precursor will take, but in any case the hydrocarbons shown in FIG. After that, it will grow up to heels. Therefore, as described above, when the generation amount of soot becomes almost zero, the emission amount of HC and CO increases as shown in FIG. 6. At this time, HC is a precursor of soot or a hydrocarbon in the previous state. .
[0021]
Summarizing these considerations based on the experimental results shown in FIGS. 6 and 7, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generated becomes almost zero, and at this time, the soot precursor or the previous state Hydrocarbons are discharged from the combustion chamber 5. As a result of repeated experimental research on this in detail, when the temperature of the fuel in the combustion chamber 5 and the gas surrounding it is below a certain temperature, the soot growth process stops midway, that is, soot It was found that soot was not generated at all, and soot was generated when the temperature of the fuel in the combustion chamber 5 and the surrounding temperature became below a certain temperature.
[0022]
By the way, when the hydrocarbon production process stops in the state of soot precursor, the temperature of the fuel and its surroundings, that is, the above-mentioned certain temperature changes depending on various factors such as the type of fuel, the air-fuel ratio and the compression ratio. I can't say how many times, but this certain temperature is NO_XTherefore, this certain temperature is NO._XIt can be defined to some extent from the generation amount of. That is, as the EGR rate increases, the temperature of the fuel during combustion and the surrounding gas decreases, and NO_XThe amount of generation decreases. At this time NO_XOf 10 p. p. When it becomes around m or less, wrinkles hardly occur. Therefore, the above mentioned temperature is NO_XOf 10 p. p. It almost corresponds to the temperature when it becomes around m or less.
[0023]
Once soot is produced, it cannot simply be purified by post-treatment using a catalyst having an oxidizing function. On the other hand, the soot precursor or the hydrocarbon in the previous state can be easily purified by post-treatment using a catalyst having an oxidation function. Like this, NO_XIt is extremely effective for purifying exhaust gas to reduce the generation amount of hydrocarbons and to discharge hydrocarbons from the combustion chamber 5 in the state of soot precursor or in front thereof.
[0024]
Now, in order to stop the growth of hydrocarbons in a state before soot is generated, it is necessary to suppress the temperature of the fuel and the surrounding gas in the combustion chamber 5 to a temperature lower than the temperature at which soot is generated. There is. In this case, in order to suppress the temperature of the fuel and the surrounding gas, it has been found that the endothermic action of the gas around the fuel when the fuel burns has a very large influence.
[0025]
That is, if there is only air around the fuel, the evaporated fuel immediately reacts with oxygen in the air and burns. In this case, the temperature of the air away from the fuel does not rise so much, and only the temperature around the fuel becomes extremely high locally. That is, at this time, the air away from the fuel hardly performs the endothermic action of the combustion heat of the fuel. In this case, since the combustion temperature becomes extremely high locally, the unburned hydrocarbons that have received this heat of combustion produce soot.
[0026]
On the other hand, the situation is slightly different when fuel is present in a mixed gas of a large amount of inert gas and a small amount of air. In this case, the evaporated fuel diffuses around and reacts with oxygen mixed in the inert gas and burns. In this case, since the combustion heat is absorbed by the surrounding inert gas, the combustion temperature does not rise so much. That is, the combustion temperature can be kept low. That is, the presence of the inert gas plays an important role in suppressing the combustion temperature, and the combustion temperature can be kept low by the endothermic action of the inert gas.
[0027]
In this case, in order to suppress the temperature of the fuel and the surrounding gas to a temperature lower than the temperature at which soot is generated, an amount of inert gas that can absorb a sufficient amount of heat is required. Therefore, if the amount of fuel increases, the amount of inert gas required increases accordingly. In this case, the greater the specific heat of the inert gas, the stronger the endothermic action. Therefore, the inert gas is preferably a gas having a large specific heat. This point, CO₂Since EGR gas has a relatively large specific heat, it can be said that it is preferable to use EGR gas as an inert gas.
[0028]
FIG. 9 shows the relationship between the EGR rate and smoke when EGR gas is used as the inert gas and the cooling degree of the EGR gas is changed. That is, in FIG. 9, curve A shows the case where EGR gas is cooled strongly and the EGR gas temperature is maintained at about 90 ° C., and curve B shows the case where EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0029]
As shown by curve A in FIG. 9, when the EGR gas is cooled strongly, soot generation peaks when the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. If you do, almost no wrinkles will occur. On the other hand, as shown by the curve B in FIG. 9, when the EGR gas is slightly cooled, the amount of soot generated peaks when the EGR rate is slightly higher than 50%. In this case, the EGR rate is almost 65% or more. If this is done, almost no wrinkles will occur.
[0030]
Further, as shown by the curve C in FIG. 9, when the EGR gas is not forcibly cooled, the amount of soot generated reaches a peak when the EGR rate is around 55%. In this case, the EGR rate is approximately 70%. If the percentage is exceeded, almost no wrinkles occur. FIG. 9 shows the amount of smoke generated when the engine load is relatively high. When the engine load is reduced, the EGR rate at which the amount of soot reaches a peak slightly decreases, and the EGR rate at which soot hardly occurs. The lower limit is also slightly reduced. Thus, the lower limit of the EGR rate at which soot hardly occurs varies depending on the degree of cooling of the EGR gas and the engine load.
[0031]
FIG. 10 shows the amount of mixed gas of EGR gas and air necessary for making the temperature of the fuel and the surrounding gas lower than the temperature at which soot is generated when EGR gas is used as the inert gas. And the ratio of the air in this gas mixture amount, and the ratio of the EGR gas in this gas mixture are shown. In FIG. 10, the vertical axis indicates the total intake gas amount sucked into the combustion chamber 5, and the chain line Y indicates the total intake gas amount that can be sucked into the combustion chamber 5 when supercharging is not performed. Yes. The horizontal axis indicates the required load, and Z1 indicates the low load operation region.
[0032]
Referring to FIG. 10, the ratio of air, that is, the amount of air in the mixed gas, indicates the amount of air necessary to completely burn the injected fuel. That is, in the case shown in FIG. 10, the ratio between the air amount and the injected fuel amount is the stoichiometric air-fuel ratio. On the other hand, the ratio of EGR gas in FIG. 10, that is, the amount of EGR gas in the mixed gas, is necessary to make the temperature of the fuel and its surrounding gas lower than the temperature at which soot is formed when the injected fuel is burned. The minimum amount of EGR gas is shown. This EGR gas amount is approximately 55% or more in terms of EGR rate, and is 70% or more in the embodiment shown in FIG. That is, if the total intake gas amount sucked into the combustion chamber 5 is a solid line X in FIG. 10, the ratio of the air amount and the EGR gas amount in the total intake gas amount X is a ratio as shown in FIG. The temperature of the fuel and the surrounding gas is lower than the temperature at which soot is produced, and so no soot is produced. Also, NO at this time_XThe amount generated is 10 p. p. around m or less, so NO_XThe amount of generated is extremely small.
[0033]
If the amount of fuel injection increases, the amount of heat generated when the fuel burns increases. Therefore, in order to maintain the temperature of the fuel and the surrounding gas at a temperature lower than the temperature at which soot is generated, the amount of heat absorbed by the EGR gas Must be increased. Therefore, as shown in FIG. 10, the amount of EGR gas must be increased as the amount of injected fuel increases. That is, the amount of EGR gas needs to increase as the required load increases.
[0034]
On the other hand, in the load region Z2 of FIG. 10, the total intake gas amount X necessary for preventing the generation of soot exceeds the total intake gas amount Y that can be sucked. Therefore, in this case, in order to supply the total intake gas amount X necessary for preventing the generation of soot into the combustion chamber 5, it is necessary to supercharge or pressurize both the EGR gas and the intake air, or the EGR gas. When EGR gas or the like is not supercharged or pressurized, the total intake gas amount X coincides with the total intake gas amount Y that can be sucked in the load region Z2. Therefore, in this case, in order to prevent the generation of soot, the amount of air is slightly decreased to increase the amount of EGR gas, and the fuel is burned under a rich air-fuel ratio.
[0035]
As described above, FIG. 10 shows the case where the fuel is burned under the stoichiometric air-fuel ratio, but even if the air amount is smaller than the air amount shown in FIG. 10 in the low load operation region Z1 shown in FIG. That is, even if the air-fuel ratio is rich, NO is generated while preventing the generation of soot._XOf 10 p. p. m or less, and even if the air amount is larger than the air amount shown in FIG. 10 in the low load region Z1 shown in FIG. 10, that is, the average value of the air-fuel ratio is 17 to 18. NO while preventing the occurrence of soot even if lean_XOf 10 p. p. It can be around m or less.
[0036]
That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but the combustion temperature is suppressed to a low temperature, so that the excess fuel does not grow to soot, and so no soot is generated. At this time, NO_XHowever, only a very small amount is generated. On the other hand, when the average air-fuel ratio is lean, or even when the air-fuel ratio is the stoichiometric air-fuel ratio, a small amount of soot is generated if the combustion temperature is high, but in the present invention the soot is suppressed to a low temperature, so Not generated at all. In addition, NO_XHowever, only a very small amount is generated.
[0037]
Thus, in the engine low load operation region Z1, regardless of the air-fuel ratio, that is, whether the air-fuel ratio is rich, the stoichiometric air-fuel ratio, or the average air-fuel ratio is lean, no soot is generated._XThe amount of generated is extremely small. Therefore, considering the improvement of the fuel consumption rate, it can be said that it is preferable to make the average air-fuel ratio lean at this time.
[0038]
By the way, in order to suppress the temperature of the fuel during combustion in the combustion chamber and the surrounding gas temperature to a temperature below the temperature at which hydrocarbon growth stops halfway, it is better when the engine load is relatively low and the amount of heat generated by combustion is small. preferable. Therefore, in the embodiment according to the present invention, when the engine load is relatively low, the temperature of the fuel during combustion and the surrounding gas temperature are suppressed to a temperature below the temperature at which hydrocarbon growth stops halfway, so that the first combustion, that is, low temperature combustion is performed. When the engine load is relatively high, the second combustion, that is, the combustion normally performed conventionally is performed. Here, the first combustion, that is, low temperature combustion, as is clear from the above description, the amount of inert gas in the combustion chamber is larger than the worst inert gas amount at which the amount of soot generation is maximum, and soot is almost generated. The second combustion, that is, the combustion that is normally performed in the past, is combustion in which the amount of inert gas in the combustion chamber is smaller than the worst inert gas amount that generates the largest amount of soot. Say.
[0039]
FIG. 11 shows a first operation region I in which the first combustion, that is, low-temperature combustion is performed, and a second combustion region II in which the second combustion, that is, combustion by the conventional combustion method is performed. In FIG. 11, the vertical axis L indicates the amount of depression of the accelerator pedal 40, that is, the required load, and the horizontal axis N indicates the engine speed. In FIG. 11, X (N) indicates the first boundary between the first operation region I and the second operation region II, and Y (N) indicates the first operation region I and the second operation region. A second boundary with region II is shown. The change determination of the operation region from the first operation region I to the second operation region II is performed based on the first boundary X (N), and the change from the second operation region II to the first operation region I is performed. The change determination of the operation region is performed based on the second boundary Y (N).
[0040]
That is, when the engine operating state is in the first operating region I and low-temperature combustion is being performed, if the required load L exceeds the first boundary X (N) that is a function of the engine speed N, the operating region is It is determined that the operation has shifted to the second operation region II, and combustion by the conventional combustion method is performed. Next, when the required load L becomes lower than the second boundary Y (N) that is a function of the engine speed N, it is determined that the operating region has shifted to the first operating region I, and low temperature combustion is performed again.
[0041]
FIG. 12 shows the output of the air-fuel ratio sensor 21. As shown in FIG. 12, the output current I of the air-fuel ratio sensor 21 changes according to the air-fuel ratio A / F. Therefore, the air-fuel ratio can be known from the output current I of the air-fuel ratio sensor 21. Next, operation control in the first operation region I and the second operation region II will be schematically described with reference to FIG.
[0042]
FIG. 13 shows the opening degree of the throttle valve 16, the opening degree of the EGR control valve 23, the EGR rate, the air-fuel ratio, the injection timing, and the injection amount with respect to the required load L. As shown in FIG. 13, in the first operation region I where the required load L is low, the opening degree of the throttle valve 16 is gradually increased from nearly fully closed to about half-open as the required load L increases, and the EGR control valve 23 As the required load L increases, the degree of opening is gradually increased from near full close to full open. Further, in the example shown in FIG. 13, in the first operating region I, the EGR rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio.
[0043]
In other words, the opening degree of the throttle valve 16 and the opening degree of the EGR control valve 23 are controlled so that the EGR rate becomes approximately 70% in the first operation region I and the air / fuel ratio becomes a slightly lean air / fuel ratio. The air-fuel ratio at this time is controlled to the target lean air-fuel ratio by correcting the opening degree of the EGR control valve 23 based on the output signal of the air-fuel ratio sensor 21. In the first operation region I, fuel injection is performed before the compression top dead center TDC. In this case, the injection start timing θS is delayed as the required load L is increased, and the injection completion timing θE is also delayed as the injection start timing θS is delayed.
[0044]
During the idling operation, the throttle valve 16 is closed to near full close. At this time, the EGR control valve 23 is also closed to close to full close. When the throttle valve 16 is closed to close to the fully closed state, the pressure in the combustion chamber 5 at the start of compression becomes low, so the compression pressure becomes small. When the compression pressure is reduced, the compression work by the piston 4 is reduced, so that the vibration of the engine body 1 is reduced. That is, during idling operation, the throttle valve 16 is closed to close to the fully closed state in order to suppress vibration of the engine body 1.
[0045]
On the other hand, when the engine operating region changes from the first operating region I to the second operating region II, the opening of the throttle valve 16 is increased stepwise from the half-open state to the fully-open direction. At this time, in the example shown in FIG. 13, the EGR rate is decreased in a step-like manner from approximately 70% to 40% or less, and the air-fuel ratio is increased in a step-like manner. That is, since the EGR rate exceeds the EGR rate range (FIG. 9) that generates a large amount of smoke, a large amount of smoke is generated when the engine operating region changes from the first operating region I to the second operating region II. There is no.
[0046]
Conventional combustion is performed in the second operation region II. In this combustion method, soot and NO_XHowever, when the engine operating region is changed from the first operating region I to the second operating region II, the injection amount is reduced stepwise as shown in FIG. I'm damned.
[0047]
In the second operation region II, the throttle valve 16 is held in a fully open state except for a part thereof, and the opening degree of the EGR control valve 23 is gradually reduced as the required load L increases. In this operation region II, the EGR rate decreases as the required load L increases, and the air-fuel ratio decreases as the required load L increases. However, the air-fuel ratio is made a lean air-fuel ratio even when the required load L increases. In the second operation region II, the injection start timing θS is set near the compression top dead center TDC.
[0048]
FIG. 14 shows the air-fuel ratio A / F in the first operating region I. In FIG. 14, the curves indicated by A / F = 15.5, A / F = 16, A / F = 17, and A / F = 18 have air-fuel ratios of 15.5, 16, 17, and 18, respectively. Time is shown, and the air-fuel ratio between the curves is determined by proportional distribution. As shown in FIG. 14, the air-fuel ratio is lean in the first operation region I, and in the first operation region I, the air-fuel ratio A / F is leaner as the required load L becomes lower.
[0049]
That is, the lower the required load L, the smaller the amount of heat generated by combustion. Therefore, low temperature combustion can be performed even if the EGR rate is reduced as the required load L is reduced. When the EGR rate is lowered, the air-fuel ratio increases, so as shown in FIG. 14, the air-fuel ratio A / F increases as the required load L decreases. As the air-fuel ratio A / F increases, the fuel consumption rate improves. Therefore, in order to make the air-fuel ratio as lean as possible, in this embodiment, the air-fuel ratio A / F increases as the required load L decreases.
[0050]
Note that the target opening ST of the throttle valve 16 required to set the air-fuel ratio to the target air-fuel ratio shown in FIG. 14 is a function of the required load L and the engine speed N as shown in FIG. The target opening degree SE of the EGR control valve 23 necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG. 14 is stored in the ROM 32 in advance as shown in FIG. And is stored in advance in the ROM 32 in the form of a map as a function of the engine speed N.
[0051]
FIG. 16 shows the target air-fuel ratio when the second combustion, that is, the normal combustion by the conventional combustion method is performed. In FIG. 16, the curves indicated by A / F = 24, A / F = 35, A / F = 45, and A / F = 60 indicate the target air-fuel ratios 24, 35, 45, and 60, respectively. The target opening ST of the throttle valve 16 necessary for setting the air-fuel ratio to the target air-fuel ratio is previously stored in the ROM 32 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. The target opening degree SE of the EGR control valve 23 necessary for setting the air-fuel ratio to the target air-fuel ratio is stored as a function of the required load L and the engine speed N as shown in FIG. It is stored in the ROM 32 in advance in the form of a map.
[0052]
Thus, in the diesel engine of the present embodiment, the first combustion, that is, low-temperature combustion, and the second combustion, that is, normal combustion are switched based on the depression amount L of the accelerator pedal 40 and the engine speed N. In each combustion, based on the depression amount L of the accelerator pedal 40 and the engine speed N, the opening control of the throttle valve 16 and the EGR valve is performed by the map shown in FIG.
[0053]
FIG. 18 is a plan view showing the exhaust purification apparatus of this embodiment, and FIG. 19 is a side view thereof. In the present exhaust purification apparatus, the switching unit 71 connected to the downstream side of the exhaust manifold 17 via the exhaust pipe 18, the particulate filter 70, and the first side connecting the one side of the particulate filter 70 and the switching unit 71. A connection part 72 a, a second connection part 72 b that connects the other side of the particulate filter 70 and the switching part 71, and an exhaust passage 73 on the downstream side of the switching part 71 are provided. The switching unit 71 includes a valve body 71 a that can block the exhaust flow in the switching unit 71. The valve body 71a is driven by a negative pressure actuator or a step motor. At the first cutoff position of the valve body 71a, the upstream side in the switching unit 71 is communicated with the first connection portion 72a and the downstream side in the switching unit 71 is communicated with the second connection portion 72b. As indicated by the arrow, the particulate filter 70 flows from one side to the other side.
[0054]
FIG. 20 shows a second blocking position of the valve body 71a. At this shut-off position, the upstream side in the switching unit 71 communicates with the second connection portion 72b and the downstream side in the switching unit 71 communicates with the first connection portion 72a, and the exhaust gas is as shown by arrows in FIG. The particulate filter 70 flows from the other side to the one side. Thus, by switching the valve body 71a from one of the first cutoff position and the second cutoff position to the other, the direction of the exhaust gas flowing into the particulate filter 70 can be reversed, that is, the exhaust of the particulate filter 70. It is possible to reverse the upstream side and the exhaust downstream side. FIG. 21 shows the open position of the valve body 71a between the first cutoff position and the second cutoff position. In this open position, the inside of the switching unit 71 is not shut off, and the exhaust gas flows bypassing the particulate filter 70 as indicated by arrows in FIG.
[0055]
FIG. 22 shows the structure of the particulate filter 70. In FIG. 22, (A) is a front view of the particulate filter 70, and (B) is a side sectional view. As shown in these drawings, the particulate filter 70 has an oblong front shape, and is, for example, a wall flow type having a honeycomb structure formed of a porous material such as cordierite, and has a large number of axes. It has a number of axial spaces subdivided by partition walls 54 extending in the direction. In the two adjacent axial spaces, one is closed on the exhaust downstream side by the plug 53 and the other is closed on the exhaust upstream side. In this way, one of the two adjacent axial spaces becomes the exhaust gas inflow passage 50 and the other becomes the outflow passage 51, and the exhaust gas always passes through the partition wall 54 as shown by an arrow in FIG. The particulates in the exhaust gas are very small compared to the size of the pores of the partition wall 54, but collide with the exhaust upstream surface of the partition wall 54 and the pore surface in the partition wall 54. Be collected. Thus, each partition wall 54 functions as a collecting wall for collecting particulates. In this particulate filter 70, NO or the like described below using alumina or the like on both side surfaces of each partition wall 54, and preferably on the pore surface of each partition wall._XAn absorbent and a noble metal catalyst such as platinum Pt are supported.
[0056]
NO supported on the partition wall 20a_XIn this embodiment, the absorbent is an alkaline metal such as potassium K, sodium Na, lithium Li, cesium Cs, or rubidium Rb, alkaline earth such as barium Ba, calcium Ca, or strontium Sr, lanthanum La, or yttrium Y. And at least one selected from such rare earths and transition metals. This NO_XThe absorbent is NO when the air-fuel ratio in the ambient atmosphere (the ratio of air to fuel, regardless of how much fuel is burned using oxygen in the air) is lean._XNO is absorbed when the air-fuel ratio becomes stoichiometric or rich._XNO release_XPerforms absorption and release action.
[0057]
This NO_XThe absorbent is actually NO_XThe detailed mechanism of this absorption / release action is not clear. However, it is considered that this absorption / release action is 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 particulate filter partition walls, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0058]
When combustion is performed with the air-fuel ratio being lean regardless of low-temperature combustion or normal combustion, 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. 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. 23 (A), a part of the Nitrate is absorbed in the absorbent while being oxidized on the platinum Pt and combined with the barium oxide BaO.₃ ⁻Diffuses into the absorbent in the form of In this way NO_XIs NO_XAbsorbed in the absorbent. NO on the surface of platinum Pt as long as the oxygen concentration in the vicinity atmosphere is high₂Is produced and NO in the absorbent_XUnless the absorption capacity is saturated, NO₂Is absorbed into the absorbent and nitrate ion NO.₃ ⁻Is generated.
[0059]
On the other hand, when the air-fuel ratio of the nearby atmosphere is made rich, the oxygen concentration 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 reverses (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 the absorbent_XAs shown in FIG. 23 (B), it is reduced by reacting with HC, CO, etc. contained in the nearby atmosphere. 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 nearby atmosphere is made rich, NO is quickly reached._XNO from absorbent_XIs released, and this released NO_XNO in the atmosphere because_XWill not be discharged.
[0060]
In this case, even if the air-fuel ratio of the nearby atmosphere is the stoichiometric air-fuel ratio, NO_XNO from absorbent_XIs released. However, in this case NO_XNO from absorbent_XNO is absorbed by the particulate filter because it is released only gradually._XIt takes a little longer time to release.
[0061]
By the way NO_XAbsorbent NO_XAbsorption capacity is limited, NO_XAbsorbent NO_XNO before absorption capacity saturates_XNO from absorbent_XNeed to be released. That is, NO absorbed in the particulate filter 70_XAmount is NO_XBefore reaching storage capacity, NO_XNeeds to be regenerated by reducing and purifying it._XThe amount needs to be estimated. Therefore, in this embodiment, NO per unit time when low temperature combustion is performed._XThe amount of absorption A is obtained in advance in the form of a map as shown in FIG. 24A as a function of the required load L and the engine speed N, and NO per unit time when normal combustion is performed._XAbsorption amount B is determined in advance in the form of a map as shown in FIG. 24B as a function of required load L and engine speed N, and NO per unit time is obtained._XNO absorbed in the particulate filter by integrating the absorption amounts A and B_XThe amount is estimated. Here, NO per unit time when low temperature combustion is performed_XThe amount of absorption A is, of course, NO when low-temperature combustion is performed at a rich air-fuel ratio._XWill be released, so it has a negative value. In this embodiment, this NO_XIn order to regenerate the particulate filter when the amount of absorption exceeds a predetermined allowable value, low-temperature combustion at the stoichiometric or rich air-fuel ratio is performed, or fuel is injected into the cylinder in the exhaust stroke As a result, the atmosphere in the vicinity of the particulate filter 70 is set to the stoichiometric air-fuel ratio or rich air-fuel ratio, and this state is maintained for at least the time until regeneration is completed (the shorter the air-fuel ratio in the vicinity atmosphere is, the shorter the state is). It has become.
[0062]
In this way, the particulate filter has NO_XWhen the absorbent is supported, the particulates collected on the collection wall can be satisfactorily oxidized and removed. This mechanism will be described with reference to FIG. As mentioned above, NO_XIs nitrate NO through platinum Pt60₃ ⁻NO in the form of_XIt is absorbed in the absorbent 61. This NO_XWhen the particulate 62 adheres to the absorbent, the particulate 62 and NO_XOn the contact surface with the absorbent 61, the oxygen concentration decreases. NO with high oxygen concentration when oxygen concentration decreases_XA concentration difference occurs between the absorbent 61 and NO._XOxygen in the absorbent 61 is particulate 62 and NO._XIt tries to move toward the contact surface with the absorbent 61. As a result, NO_XNitrate ion NO in absorbent 61₃ ⁻Is decomposed into oxygen O and NO, and oxygen O is converted into particulate 62 and NO._XHeading to the contact surface with the absorbent 61, NO is NO_XReleased from the absorbent 61 to the outside. NO released to the outside is oxidized on the platinum Pt on the downstream side, and again NO._XAbsorbed in the absorbent 61.
[0063]
On the other hand, the particulate 62 and NO_XThe oxygen O toward the contact surface with the absorbent 61 is nitrate, that is, oxygen decomposed from the compound. Oxygen O decomposed from the compound has high energy and has extremely high activity. Therefore, the particulate 62 and NO_XOxygen heading to the contact surface with the absorbent 61 is active oxygen O. When these active oxygen O comes into contact with the particulate 62, the particulate 62 is oxidized in a short time of several minutes to several tens of minutes without emitting a luminous flame. The active oxygen O that oxidizes the particulate 62 is NO._XIt is also released when NO is absorbed by the absorbent 61. That is, NO_XNO is repeatedly bonded and separated from oxygen atoms._XNitrate ion NO in the absorbent 61₃ ⁻The active oxygen is generated during this period. The particulate 62 is also oxidized by this active oxygen. Further, the particulates adhering to the particulate filter 70 are not only oxidized by the active oxygen O in this way, but also the particulates 62 are oxidized by oxygen in the exhaust gas.
[0064]
In this way, the particulate filter has NO_XAbsorbent and precious metal catalyst (hereinafter referred to as NO_X(Referred to as an occlusion reduction catalyst)_XIn addition to purification, it is effective to oxidize and remove the collected particulates so that the particulate filter is less likely to be clogged.
[0065]
However, the structure of the particulate filter is, as described above, a wall flow type in which exhaust gas passes through the pores of the collection wall, and a general catalyst device in which exhaust gas flows along the partition wall supporting the catalyst. In order to allow the same amount of exhaust gas to pass through in the comparison, the dimension between the collection walls must be larger than the dimension between the partition walls. Thereby, in the particulate filter, the exhaust gas is supported on the surface of the collection wall._XThere is less opportunity to contact the storage reduction catalyst than in the catalytic device. Further, when the exhaust gas passes through the pores of the collection wall, NO gas supported in the pores is collected._XIt is in contact with the storage reduction catalyst, but it is mainly NO supported on the surface of the collection wall._XIt contacts only the storage reduction catalyst. However, the catalyst support area on the surface of the collection wall is not so large due to the large number of pores. Thus, NO_XEven if the storage reduction catalyst is supported on the particulate filter, NO in the exhaust gas_XCan not be sufficiently purified.
[0066]
In order to solve this problem, in this embodiment, as shown in FIGS. 18 and 19, the exhaust pipe 18 on the upstream side of the switching unit 71 is provided with NO._XA purification catalyst device 74 is arranged. This NO_XThe purification catalyst device 74 is configured so that the NO in the particulate filter 70_XIt supplements purification and does not require a large capacity. Thus, NO_XPurification catalyst device 74 and NO_XNO in the exhaust gas by the particulate filter 70 having an occlusion reduction catalyst._XCan be sufficiently purified.
[0067]
NO_XAs the purification catalyst device 74, the above-described NO is applied to the honeycomb structure carrier._XOcclusion reduction catalyst or NO_XNO such as selective reduction catalyst_XWhat is necessary is just to carry | support the catalyst which can purify | clean.
[0068]
By the way, a soluble organic component SOF is contained in the exhaust gas, and this SOF has adhesiveness, and the particulates adhere to each other on the particulate filter and grow into a large lump. This facilitates clogging of the particulate filter by making it difficult to oxidize and remove the particulate in the particulate filter. As a result, NO_XNO carrying a catalyst having an oxidation function such as an occlusion reduction catalyst_XBy disposing the purification catalyst device 74 on the upstream side of the particulate filter 70, the SOF in the exhaust gas is burned out on the upstream side of the particulate filter, and the promotion of clogging of the particulate filter by the SOF can be prevented. .
[0069]
By the way, the fuel of the internal combustion engine contains sulfur, and the SO_XIs generated. SO_XNO to the particulate filter 70_XIt is absorbed in the form of sulfate by the same mechanism. This sulfate can also release active oxygen by the same mechanism as nitrate, but since sulfate is a stable substance, it is difficult to release from the particulate filter even if the surrounding atmosphere is rich air-fuel ratio. The occlusion amount gradually increases, remaining in the particulate filter. The amount of nitrate or sulfate that can be stored in the particulate filter is finite, and if the amount of sulfate stored in the particulate filter increases (hereinafter referred to as SO)._X(Referred to as poisoning), the amount of nitrate that can be stored decreases, and finally, NO_XCannot absorb.
[0070]
Therefore, in this embodiment, NO_XNO in the purification catalyst device 74_XNO is located on the upstream side of the sulfur gas in the exhaust gas, supporting the storage reduction catalyst_XActively absorbed by the purification catalyst device 74 and the SO of the particulate filter 70_XPreventing poisoning. NO associated with this_XSO of the purification catalyst device 74_XThe recovery from poisoning is performed by the following procedure.
[0071]
First, SO_XA determination is made as to whether it is time to recover from poisoning. For this determination, the fuel consumed so far is integrated, and when this integrated fuel amount reaches the set amount, the SO_XIt can be judged that it is time to recover from poisoning. NO_XAlso in the purification catalyst device, the same regeneration process as the particulate filter (NO_XIn this regeneration process, NO_XThe air-fuel ratio upstream of the exhaust gas from the purification catalyst device is made rich, but during regeneration, the reducing substances such as HC are released._XNO for use in reduction purification of NO_XThe air-fuel ratio downstream of the purification catalyst device is close to the stoichiometric air-fuel ratio. However, when playback is complete, NO_XThe air-fuel ratio on the downstream side of the purification catalyst device becomes almost equal to the upstream air-fuel ratio and becomes rich. If playback time is detected using this, SO_XThe recovery time of poisoning can be determined. Because SO needs recovery_XIf poisoning is in progress, NO during the regeneration period_XThis is because the amount of absorption is actually small and the regeneration time is short.
[0072]
SO_XDuring the poisoning regeneration period, the combustion air-fuel ratio is made lean so that the exhaust gas contains a relatively large amount of oxygen, and in-cylinder fuel injection or NO in the exhaust stroke_XBy injecting fuel into the engine exhaust system on the upstream side of the purification catalyst device, NO_XSupply sufficient oxygen and reducing substances such as unburned fuel to the purification catalyst device,_XThe reducing substance is sufficiently burned by the oxidation capability of the purification catalyst device.
[0073]
Thus, NO_XWhen the temperature of the purification catalyst device is raised to about 600 ° C., the stable sulfate can be obtained by reducing the oxygen concentration by setting the atmosphere near the stoichiometric or rich air-fuel ratio to decrease the oxygen concentration._XCan be released as NO_XWhen the temperature of the purification catalyst device is raised to 700 ° C or higher, the supported oxidation catalyst such as platinum Pt deteriorates due to sintering, and therefore NO._XIt is preferable to monitor the exhaust temperature etc. immediately downstream of the purification catalyst device to prevent this from occurring. This NO_XSO of purification catalyst equipment_XDuring the poisoning recovery process, the valve body 71a is in the open position in the switching unit 71, and NO_XSO released from the purification catalyst device_XIn order to bypass the particulate filter 70, the NO of the particulate filter 70_XIt is not absorbed by the absorbent. NO_XWhen the temperature of the purification catalyst device is set to a high temperature and the ambient atmosphere is set to a rich air-fuel ratio for a certain period,_XIt can be determined that the poisoning recovery process has been completed, and the combustion air-fuel ratio is returned to an air-fuel ratio suitable for normal operation.
[0074]
By the way, platinum Pt and NO supported on the particulate filter._XSince the absorbent 61 is activated as the temperature of the particulate filter increases, NO per unit time._XThe amount of active oxygen O released from the absorbent 61 increases as the temperature of the particulate filter increases. As a matter of course, the higher the temperature of the particulate itself, the easier it is to be removed by oxidation. Therefore, the amount of fine particles that can be removed by oxidation without emitting a luminous flame per unit time on the particulate filter increases as the temperature of the particulate filter increases. As a result, NO_XSO of purification catalyst equipment_XSimultaneously with the completion of the poisoning recovery process or immediately after the completion, the switching valve 71a of the switching unit 71 is set to one blocking position, and the NO is set to a high temperature (about 600 ° C.)._XIt is preferable that the exhaust gas heated through the purification catalyst device is guided to the particulate filter to raise the temperature of the particulate filter so that the particulates are easily removed by oxidation.
[0075]
The solid line in FIG. 26 indicates the amount G of fine particles that can be removed by oxidation without emitting a luminous flame per unit time. In FIG. 26, the horizontal axis indicates the temperature TF of the particulate filter. FIG. 26 shows the amount G of fine particles that can be removed by oxidation when the unit time is 1 second, that is, 1 second, but an arbitrary time such as 1 minute or 10 minutes is adopted as the unit time. be able to. For example, when 10 minutes is used as the unit time, the oxidizable and removable fine particle amount G per unit time represents the oxidizable and removable fine particle amount G per 10 minutes. Even in this case, the unit on the particulate filter 70 As shown in FIG. 26, the amount G of fine particles that can be removed by oxidation without emitting a luminous flame per hour increases as the temperature of the particulate filter 70 increases.
[0076]
Now, when the amount of particulate discharged from the combustion chamber per unit time is referred to as discharged particulate amount M, when the discharged particulate amount M is smaller than the oxidizable and removable particulate amount G, for example, the amount of discharged particulate per second. When M is less than the amount G of fine particles that can be removed by oxidation per second, or when the amount M of fine particles discharged per 10 minutes is smaller than the amount of fine particles G that can be removed by oxidation every 10 minutes, that is, in the region I in FIG. All the particulates discharged from the chamber are sequentially oxidized and removed within the short time without emitting a bright flame on the particulate filter 70.
[0077]
In contrast, when the amount M of discharged particulate is larger than the amount G of particulate that can be removed by oxidation, that is, in the region II of FIG. 26, the amount of active oxygen is insufficient to sequentially oxidize all the particulates. 27A to 27C show the state of particulate oxidation in such a case.
[0078]
That is, when the amount of active oxygen is insufficient to oxidize all the particulates, the particulate 62 is NO as shown in FIG._XWhen adhering to the absorbent 61, only a part of the particulate 62 is oxidized, and the part of the particulate that has not been sufficiently oxidized remains on the exhaust gas upstream side surface of the particulate filter. Next, when the state where the amount of active oxygen is deficient continues, the particulate portion that has not been oxidized from one to the next remains on the exhaust upstream surface. As a result, as shown in FIG. The exhaust upstream surface of the partition wall is covered with the residual particulate portion 63.
[0079]
Such residual particulate portion 63 gradually changes to a carbonaceous material that is difficult to oxidize, and when the exhaust upstream surface is covered with the residual particulate portion 63, NO and SO by platinum Pt.₂Oxidation of NO and NO_XThe action of releasing active oxygen by the absorbent 61 is suppressed. Accordingly, the residual particulate portion 63 can be gradually oxidized over time, but another particulate 64 is placed on the residual particulate portion 63 from the next to the next as shown in FIG. And deposit. That is, when the particulates are deposited in a laminated form, the particulates are platinum Pt and NO._XBecause of the distance from the absorbent, even particulates that are easily oxidized are not oxidized by active oxygen. Therefore, further particulates are deposited on the particulate 64 one after another. In other words, if the state in which the amount M of discharged particulates is larger than the amount G of particulates that can be removed by oxidation continues, particulates accumulate on the particulate filter in a stacked manner.
[0080]
In this way, in the region I in FIG. 26, the particulates are oxidized on the particulate filter within a short time without emitting a bright flame, and in the region II in FIG. 26, the particulates are deposited in a stacked manner on the particulate filter. To do. Therefore, if the relationship between the discharged fine particle amount M and the oxidizable and removable fine particle amount G is in the region I, the accumulation of particulates on the particulate filter can be prevented. As a result, the pressure loss of the exhaust gas flow in the particulate filter 70 is maintained at a substantially constant minimum pressure loss value without changing at all. Thus, a reduction in engine output can be kept to a minimum. However, this is not always realized, and if nothing is done, particulates may accumulate on the particulate filter.
[0081]
In the present embodiment, switching control of the valve body 71a is performed by the electronic control unit 30 according to the flowchart shown in FIG. 28 to prevent a large amount of particulates from accumulating on the particulate filter. This flowchart is repeated every predetermined time. First, in step 101, the travel distance integrated value A is calculated. In step 102, it is determined whether or not the travel distance integrated value A has reached the set travel distance As. When this determination is denied, the process is terminated as it is. When the determination is affirmative, the routine proceeds to step 103, where the accumulated travel distance A is reset to 0, and then at step 104, the valve body 71a is moved to the first cutoff position and the second cutoff position. Switching from one to the other reverses the exhaust upstream side and exhaust downstream side of the particulate filter.
[0082]
FIG. 29 is an enlarged cross-sectional view of the partition wall 54 of the particulate filter. While the vehicle travels the set travel distance As, the operation in the region II of FIG. 26 may be performed, and as shown by the grid in FIG. 29A, the partition wall 54 on which the exhaust gas mainly collides. The exhaust upstream surface of the exhaust gas and the exhaust gas flow facing surface in the pores collect and collide particulates as one of the collection surfaces._XThe particulates are oxidized and removed by the active oxygen released by the absorbent, but the oxidation removal is insufficient and the particulates may remain. At this time, the exhaust resistance of the particulate filter does not adversely affect the running of the vehicle. However, if particulates accumulate further, problems such as a significant decrease in engine output occur. In this flowchart, at this time, the exhaust upstream side and the exhaust downstream side of the particulate filter are reversed. As a result, no further particulate is deposited on the particulate remaining on one collecting surface of the partition wall 54, and the remaining particulate is gradually oxidized and removed by the active oxygen released from the one collecting surface. Is done. Further, the residual particulates are easily broken and fragmented by the exhaust gas flow in the reverse direction as shown in FIG. 29B, and flow mainly downstream in the pores.
[0083]
As a result, many of the finely divided particulates are dispersed in the pores of the partition walls and are supported on the inner surfaces of the pores of the partition walls._XThere is an increased chance of oxidation removal in direct contact with the absorbent. Thus, NO also enters the pores of the partition walls._XBy supporting the absorbent, the residual particulates can be remarkably easily oxidized and removed. Further, in addition to this oxidation removal, the other collecting surface of the partition wall 54 that has become upstream due to the backflow of the exhaust gas, that is, the exhaust upstream surface of the partition wall 54 where the exhaust gas mainly collides and the inside of the pores On the exhaust gas flow facing surface (which is on the opposite side of the one collecting surface), new particulates in the exhaust gas adhere to the NO._XIt is oxidized and removed by the active oxygen released from the absorbent. NO during these oxidation removals_XPart of the active oxygen released from the absorbent moves to the downstream side together with the exhaust gas, and oxidizes and removes the particulates still remaining due to the backflow of the exhaust gas.
[0084]
That is, the residual particulates on one collecting surface of the partition wall are not only used for the active oxygen released from this collecting surface but also for the oxidation removal of the particulates on the other collecting surface of the partition wall by the backflow of the exhaust gas. The remaining active oxygen used comes with the exhaust gas. As a result, even if particulates are accumulated to some extent on one collecting surface of the partition wall at the time of switching of the valve body, if the exhaust gas flows backward, the particulates accumulated on the residual particulates In addition to the arrival of active oxygen, no further particulates are deposited, so the deposited particulates are gradually oxidized and removed. Oxidation removal is possible. In this way, by alternately using the two collecting surfaces of the partition wall for collecting particulates, the particles on each collecting surface are compared with the case where the particulates are always collected on a single collecting surface. Since the amount of curate collected can be reduced and it is advantageous for the removal of particulates by oxidation, particulates do not accumulate on the particulate filter, and clogging of the particulate filter can be prevented.
[0085]
In this flowchart, the valve body is switched at every set travel distance, and the valve body is switched before the residual particulate on the particulate filter is changed to a carbonaceous material that is hardly oxidized. In addition, removing particulates by oxidation before a large amount of particulates accumulates prevents problems such as a large amount of deposited particulates igniting and burning at a time and the particulate filter melting due to a large amount of combustion heat. It also becomes. Moreover, even if a large amount of particulates accumulates on one collection surface of the particulate filter partition wall at the time of switching of the valve body due to some reason, if the valve body is switched, the accumulated particulates will be in the reverse direction. Particulate particulates that could not be oxidized and removed within the pores of the partition walls are discharged from the particulate filter because they are relatively easily destroyed and fragmented by the exhaust gas flow. The exhaust resistance of the filter is further increased and does not adversely affect the running of the vehicle, and new particulates can be collected by the other collecting surface of the particulate filter partition wall.
[0086]
Thus, if the valve body is switched for each set travel distance, it is possible to reliably prevent a large amount of particulate from accumulating on the particulate filter. The switching timing of the valve body for this purpose is not limited for each set travel distance, and may be set every set time or irregularly, for example.
[0087]
Further, by utilizing the fact that the differential pressure between the exhaust upstream side and the exhaust downstream side of the particulate filter 70 increases according to the amount of particulates remaining and deposited on the particulate filter, this differential pressure exceeds the set differential pressure. At this time, the valve body may be switched on the assumption that a certain amount of particulates is accumulated on the particulate filter. Specifically, the exhaust pressure on one side of the particulate filter 70, that is, the exhaust pressure in the first connection portion 72a (see FIG. 18) is detected by a pressure sensor disposed in the first connection portion 72a, and The exhaust pressure on the other side of the particulate filter, that is, the exhaust pressure in the second connection portion 72b (see FIG. 18) is detected by a pressure sensor disposed in the second connection portion 72b, and the differential pressure between these exhaust pressures. It is determined whether or not the absolute value of becomes greater than or equal to the set pressure difference. Here, the absolute value of the differential pressure is used because it is possible to grasp the increase in the differential pressure regardless of which of the first connection portion 72a and the second connection portion 72b is on the exhaust upstream side. Strictly speaking, this differential pressure also changes depending on the exhaust gas pressure exhausted from the cylinder. Therefore, it is preferable to determine the accumulation of particulates by specifying the engine operating state.
[0088]
In addition to this differential pressure, for example, the change of the electrical resistance value on the predetermined partition wall of the particulate filter is monitored, and when the electrical resistance value becomes lower than the set value due to the accumulation of particulates, the particulate filter The valve body may be switched assuming that a certain amount of particulates is accumulated on the top. Further, the valve body may be switched by utilizing the fact that the light transmittance is lowered or the light reflectance is lowered due to the accumulation of the particulates in the predetermined partition wall of the particulate filter. As described above, by directly determining the accumulation of particulates and switching the valve body, it is possible to prevent the engine output from greatly decreasing more reliably.
[0089]
Further, in order to prevent the accumulation of a large amount of particulates, it is possible to change NO without switching the valve body in this way._XSO of purification catalyst equipment_XIn the poisoning recovery process, the SO_XWhen the poisoning recovery process is completed, a blocking position opposite to the starting blocking position may be set.
[0090]
As described above, the present exhaust purification device can reverse the exhaust upstream side and the exhaust downstream side of the particulate filter with a very simple configuration. In addition, in the particulate filter, a large opening area is required to facilitate the inflow of exhaust gas. However, in the present exhaust purification device, vehicle mountability is deteriorated as shown in FIGS. In addition, a particulate filter having a large opening area can be used.
[0091]
Further, if the atmosphere in the vicinity of the particulate filter is set to a rich air-fuel ratio, that is, if the oxygen concentration in the vicinity atmosphere is lowered, NO._XActive oxygen O is released from the absorbent 61 to the outside at once. With the active oxygen O released at once, the deposited particulates are easily oxidized and easily removed by oxidation.
[0092]
On the other hand, if the ambient atmosphere is maintained at a lean air-fuel ratio, the surface of platinum Pt is covered with oxygen, and so-called oxygen poisoning of platinum Pt occurs. When such oxygen poisoning occurs, NO_XNO is reduced due to reduced oxidation_XThe absorption efficiency of NO and thus NO_XThe amount of active oxygen released from the absorbent 61 is reduced. However, when the air-fuel ratio is made rich, oxygen on the platinum Pt surface is consumed, so that oxygen poisoning is eliminated. Therefore, when the air-fuel ratio is switched again from rich to lean, NO._XNO to increase the oxidation effect on_XThe absorption efficiency of NO and thus NO_XThe amount of active oxygen released from the absorbent 61 is increased.
[0093]
Accordingly, when the air-fuel ratio is maintained lean, the oxygen poisoning of platinum Pt is eliminated every time the air-fuel ratio is temporarily switched from lean to rich, so that active oxygen release when the air-fuel ratio is lean is performed. The amount increases, and thus the oxidation action of the particulates on the particulate filter 70 can be promoted.
[0094]
Furthermore, the elimination of oxygen poisoning is, so to speak, the combustion of the reducing substance, so that the temperature of the particulate filter is raised with heat generation. As a result, the amount of fine particles that can be removed by oxidation in the particulate filter is improved, and the residual and deposited particulates can be easily removed by oxidation. If the air-fuel ratio of the exhaust gas is made rich immediately after switching between the exhaust upstream side and the exhaust downstream side of the particulate filter by the valve body 71a, the other collecting surface in the particulate filter partition wall where no particulates remain is obtained. Since the active oxygen is easily released as compared with one of the collecting surfaces, the remaining particulates on the one collecting surface can be more reliably oxidized and removed by a larger amount of the released active oxygen. Of course, the atmosphere in the vicinity may occasionally be set to a rich air-fuel ratio regardless of the switching of the valve body 71a, and thereby the particulates are difficult to remain and accumulate on the particulate filter.
[0095]
For example, the above-described low-temperature combustion may be performed as a method of making the near atmosphere a rich air-fuel ratio. Of course, when switching from normal combustion to low temperature combustion, or prior to that, the exhaust upstream side and the exhaust downstream side of the particulate filter may be switched. Further, the combustion air-fuel ratio may be simply made rich in order to make the vicinity atmosphere a rich air-fuel ratio. In addition to normal main fuel injection in the compression stroke, fuel may be injected into the cylinder (post-injection) in the exhaust stroke or expansion stroke by the engine fuel injection valve, or fuel in the cylinder in the intake stroke. May be jetted (bi-rubber jet). Of course, it is not always necessary to provide an interval between the post injection or the big rubber injection and the main fuel injection. It is also possible to supply fuel to the engine exhaust system. NO_XNOx carried in the purification catalyst device and the particulate filter_XNO from absorbent_XIn order to regenerate the exhaust gas, it is necessary to make the ambient atmosphere at least temporarily a rich air-fuel ratio, and this enrichment control is preferably performed after the upstream and downstream sides of the particulate filter are reversed.
[0096]
By the way, calcium Ca in the exhaust gas is SO.₃In the presence of calcium sulfate CaSO₄Is generated. This calcium sulfate CaSO₄Is hardly removed by oxidation and remains as ash on the particulate filter. Therefore, in order to prevent clogging of the particulate filter due to residual calcium sulfate, NO_XAs the absorbent 61, it is preferable to use an alkali metal or alkaline earth metal, such as potassium K, which has a higher ionization tendency than calcium Ca._XSO diffused in the absorbent 61₃Binds potassium K and potassium sulfate K₂SO₄Calcium Ca is SO₃It passes through the partition wall of the particulate filter without being combined with. Therefore, the particulate filter is not clogged by ash. Thus, as described above, NO_XAs the absorbent 61, it is preferable to use an alkali metal or alkaline earth metal having a higher ionization tendency than calcium Ca, that is, potassium K, lithium Li, cesium Cs, rubidium Rb, barium Ba, and strontium Sr.
[0097]
【The invention's effect】
As described above, according to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the NO._XWhen the stoichiometric or rich air-fuel ratio is absorbed, NO_XNO is released and reduced and purified_XOcclusion reduction catalystOn the particulate collection wallA particulate filter carried and disposed in the engine exhaust system, and a NOx disposed in the engine exhaust system upstream of the particulate filter having an oxidation function_XWith purification catalyst deviceNO _X The purification catalyst device is NO when the ambient atmosphere is a lean air-fuel ratio. _X When the stoichiometric or rich air-fuel ratio is absorbed, NO _X NO is released and reduced and purified _X Supports an occlusion reduction catalystBecause of this, NO is insufficient with just a particulate filter._XPurification, NO_XIn order for the purification catalyst device to compensate, NO in the exhaust gas_XCan be sufficiently purified. NO_XThe oxidation function of the purification catalyst device allows the SOF in the exhaust gas to be burned out on the upstream side of the particulate filter, preventing the particulate from growing into a large lump by the SOF on the particulate filter, and the particulate Filter clogging can be suppressed. Also,Control means for setting the inside of the particulate filter to the stoichiometric air-fuel ratio or rich air-fuel ratio is provided, and the control means sets the inside of the particulate filter to the stoichiometric air-fuel ratio or rich air-fuel ratio, so that the particulate filter NO. _X NO from the storage reduction catalyst _X It is possible to release and purify the particulates and to release the active oxygen to oxidize and remove the particulates collected on the particulate collection wall..
[Brief description of the drawings]
FIG. 1 is a schematic longitudinal sectional view of a diesel engine equipped with an exhaust emission control device according to the present invention.
FIG. 2 is an enlarged longitudinal sectional view of the combustion chamber of FIG.
FIG. 3 is a bottom view of the cylinder head of FIG. 1;
FIG. 4 is a side sectional view of a combustion chamber.
FIG. 5 is a diagram showing intake / exhaust valve lift and fuel injection.
FIG. 6 Smoke and NO_XIt is a figure which shows the generation amount of etc.
FIG. 7 is a diagram showing combustion pressure.
FIG. 8 is a diagram showing fuel molecules.
FIG. 9 is a diagram showing the relationship between the amount of smoke generated and the EGR rate.
FIG. 10 is a diagram showing the relationship between the amount of injected fuel and the amount of mixed gas.
FIG. 11 is a diagram showing a first operation region I and a second operation region II.
FIG. 12 is a diagram showing an output of an air-fuel ratio sensor.
FIG. 13 is a diagram showing an opening degree of a throttle valve and the like.
14 is a diagram showing an air-fuel ratio in a first operating region I. FIG.
FIG. 15 is a view showing a map of a target opening degree of a throttle valve or the like.
FIG. 16 is a diagram showing an air-fuel ratio in normal combustion.
FIG. 17 is a diagram showing a target opening degree of a throttle valve or the like.
FIG. 18 is a plan view of the vicinity of a switching unit and a particulate filter in an engine exhaust system.
FIG. 19 is a side view of FIG. 18;
FIG. 20 is a view showing another blocking position different from that of FIG. 18 of the valve body in the switching unit.
FIG. 21 is a diagram illustrating an open position of a valve body in a switching unit.
FIG. 22 is a diagram illustrating a structure of a particulate filter.
FIG. 23 NO_XIt is a figure for demonstrating the absorption-and-release function of.
FIG. 24: NO per unit time_XIt is a figure which shows the map of absorption amount.
FIG. 25 is a diagram for explaining the oxidizing action of particulates.
FIG. 26 is a diagram showing the relationship between the amount of fine particles that can be removed by oxidation and the temperature of the particulate filter.
FIG. 27 is a diagram for explaining the particulate deposition action.
FIG. 28 is a flowchart for preventing accumulation of a large amount of particulates on a particulate filter.
FIG. 29 is an enlarged cross-sectional view of a partition wall of a particulate filter.
[Explanation of symbols]
6 ... Fuel injection valve
16 ... Throttle valve
71 ... switching part
70 ... Particulate filter
74 ... NO_XPurification catalyst device

Claims (4)

Near atmosphere absorbs NO _X when the lean air-fuel ratio, the engine carries _the NO _X storage reduction catalyst to reduce and purify by releasing NO _X in collecting particulates wall when the stoichiometric air-fuel ratio or a rich air-fuel ratio exhaust A particulate filter disposed in the system, and a NO _x purification catalyst device having an oxidation function and disposed in the engine exhaust system upstream of the particulate filter , the NO _x purification catalyst device being in the vicinity atmosphere absorbs NO _X when the lean air-fuel ratio, stoichiometric air-fuel ratio or carrying _the NO _X storage reduction catalyst to reduce and purify by releasing NO _X when the rich air-fuel ratio, stoichiometric air-fuel ratio of the particulate in the filter Alternatively, a control means for making a rich air-fuel ratio is provided. An internal combustion engine characterized in that NO _x is released from the NO _x storage-reduction catalyst of a curate filter to reduce and purify, and active oxygen is released to oxidize and remove the particulates collected on the particulate collection wall. Exhaust purification equipment. An exhaust purification system of an internal combustion engine according to claim 1, in the downstream side exhaust gas, characterized by comprising a bypass means which make it possible to bypass the particulate filter of the NO _X purification catalyst device. The NO _X purification catalyst device carries the NO _X storage reduction catalyst, and during the SO _X poisoning recovery of the NO _X purification catalyst device, the bypass means functions so that exhaust gas bypasses the particulate filter. The exhaust emission control device for an internal combustion engine according to claim 2, wherein The NO _X purification catalyst device carries the NO _X storage reduction catalyst, and immediately after the SO _X poisoning recovery of the NO _X purification catalyst device is completed, the bypass means does not function and the exhaust gas passes through the particulate filter. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the exhaust gas purification apparatus is passed.

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