JP3772729B2 - 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]
Particulates mainly composed of soot are contained in the exhaust gas of internal combustion engines, particularly diesel engines. Since particulates are harmful substances, it has been proposed to arrange a filter for collecting particulates in the engine exhaust system before release into the atmosphere. Such a filter is required to burn off the collected particulates in order to prevent an increase in exhaust resistance due to clogging.
[0003]
In such filter regeneration, ignition and combustion occur when the particulates reach approximately 600 ° C., but the exhaust gas temperature of the diesel engine is considerably lower than 600 ° C. at normal times, and usually means such as heating the filter itself. is required.
[0004]
In Japanese Patent Publication No. 7-106290, if a platinum group metal and an alkaline earth metal oxide are supported on a filter, the particulates on the filter are about 400 ° C., which is the exhaust gas temperature of a diesel engine at normal times. It is disclosed that it burns continuously.
[0005]
However, even if this filter is used, the exhaust gas temperature is not always about 400 ° C, and a large amount of particulates may be released from the diesel engine depending on the operating state. Particulates that could not be burned out in time may gradually accumulate on the filter.
[0006]
In this filter, if particulates are accumulated to some extent, the ability to burn out the particulates is extremely reduced, so that the filter can no longer be regenerated by itself. In this way, simply placing this type of filter in the engine exhaust system may cause clogging relatively early, resulting in a significant reduction in engine output.
[0007]
In order to solve this problem, it is considered that this type of particulate filter is a wall flow type and the exhaust upstream side and the exhaust downstream side are reversed so that both sides of the collection wall are used alternately as the collection surface. It is done. By doing this, while collecting particulates on one side of the collection wall, the particulates collected on the other side of the collection wall can be burned out, preventing clogging of the particulate filter. It becomes possible to do.
[0008]
In order to enable reversal between the exhaust upstream side and the exhaust downstream side of the particulate filter, a switching unit is provided in the engine exhaust system, and the particulate filter is disposed downstream of the switching unit.
[0009]
By the way, NOx, which is a harmful substance in addition to particulates, is contained in exhaust gas from diesel engines._XIs included, this NO_XNO to purify_XThe purification catalyst device must be arranged in the engine exhaust system.
[0010]
In order to burn out the particulates well in the particulate filter described above, the temperature of the particulate filter is preferably higher. As a result, NO_XThe purification catalyst device is arranged on the upstream side of the particulate filter, and NO_XNO in purification catalyst equipment_XIt is conceivable to use the reaction heat of purification for raising the temperature of the particulate filter.
[0011]
[Problems to be solved by the invention]
However, simply NO_XEven if the purification catalyst device is arranged upstream of the particulate filter, NO_XSince the purification catalyst device is disposed upstream of the switching unit, NO_XThe reaction heat in the purification catalyst device flows into the particulate filter only through the switching unit. In practice, it is difficult to effectively raise the temperature of the particulate filter by the reaction heat.
[0012]
Accordingly, an object of the present invention is to provide a particulate filter, a switching unit for reversing the exhaust upstream side and the exhaust downstream side of the particulate filter,_XA purification catalyst device, NO_XAn object of the present invention is to provide an exhaust purification device for an internal combustion engine that can effectively use reaction heat in a purification catalyst device to raise the temperature of a particulate filter.
[0013]
[Means for Solving the Problems]
According to the present invention, the exhaust gas purification apparatus for an internal combustion engine according to claim 1 is provided with NO disposed in the exhaust passage._XPurification catalyst device and the NO_XA case penetrating the exhaust passage in the transverse direction on the downstream side of the purification catalyst device, a particulate filter disposed in the case for collecting particulates, and the exhaust passage on the downstream side of the penetration position of the case The exhaust gas that has passed around the case in the exhaust passage passes through the particulate filter in the case via the switching unit. The switching unit can reverse the exhaust upstream side and exhaust downstream side of the particulate filter, and the collected particulates are oxidized in the particulate filter.
[0014]
According to a second aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein the particulate filter carries an active oxygen release agent, and the active oxygen The active oxygen released from the releasing agent is characterized in that the particulates are oxidized.
[0015]
According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the second aspect, wherein the active oxygen release agent removes oxygen when excess oxygen is present in the surrounding area. In this case, oxygen is retained, and when the surrounding oxygen concentration is lowered, the retained oxygen is released in the form of active oxygen.
[0016]
According to a fourth aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the second aspect, wherein the active oxygen release agent is NO when excess oxygen is present in the surrounding area._XIn combination with oxygen and as the ambient oxygen concentration decreases, the combined NO_XAnd oxygen NO_XIt is characterized by being decomposed and released into active oxygen.
[0017]
An internal combustion engine exhaust gas purification apparatus according to claim 5 of the present invention is the internal combustion engine exhaust gas purification apparatus according to claim 1, wherein the NO_XThe purification catalyst device is NO when the ambient atmosphere is a lean air-fuel ratio._XWhen the stoichiometric or rich air-fuel ratio is absorbed, NO_XNO is released and reduced and purified_XIt is characterized by carrying an occlusion reduction catalyst.
[0018]
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 the exhaust manifold 17.
[0019]
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.
[0020]
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.
[0021]
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 a valve body 71 a in the switching unit 71 described later in detail based on various signals.
[0022]
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.
[0023]
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.
[0024]
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).
[0025]
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.
[0026]
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.
[0027]
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 therefore the combustion temperature in the combustion chamber 5 is low at this time.
[0028]
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. .
[0029]
Summarizing these considerations based on the experimental results shown in FIGS. 6 and 7, the amount of soot generated becomes almost zero when the combustion temperature in the combustion chamber 5 is low, 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.
[0030]
By the way, when the hydrocarbon generation 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 amount of occurrence. 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_xWhen the amount of generated is around 10 p.p.m or less, soot is hardly generated. Therefore, the above mentioned temperature is NO_xThis is almost the same as the temperature when the amount of generated is around 10 p.p.m or less.
[0031]
Once soot is produced, it cannot simply be purified by post-treatment using a catalyst having an oxidizing function. In contrast, soot precursors or hydrocarbons 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.
[0032]
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 gas around it, it has been found that the endothermic effect of the gas around the fuel when the fuel burns has a very large influence.
[0033]
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 increase 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.
[0034]
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.
[0035]
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.
[0036]
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.
[0037]
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.
[0038]
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.
[0039]
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. ing. The horizontal axis indicates the required load, and Z1 indicates the low load operation region.
[0040]
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, in FIG. 10, the ratio of EGR gas, that is, the amount of EGR gas in the mixed gas is used 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 EGR gas amount 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 around 10p.p.m or less, so NO_xThe amount of generated is extremely small.
[0041]
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.
[0042]
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.
[0043]
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._xCan be reduced to around 10 p.pm or less, and even if the amount of air in the low load region Z1 shown in FIG. 10 is larger than the amount of air shown in FIG. Even if the value is lean from 17 to 18, NO_xCan be reduced to around 10 p.p.m or less.
[0044]
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, even 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.
[0045]
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.
[0046]
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.
[0047]
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).
[0048]
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.
[0049]
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.
[0050]
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.
[0051]
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.
[0052]
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.
[0053]
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.
[0054]
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.
[0055]
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.
[0056]
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.
[0057]
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.
[0058]
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.
[0059]
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.
[0060]
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.
[0061]
18 is a plan view showing the exhaust emission control device, and FIG. 19 is a side view thereof. The exhaust purification apparatus includes a first expanded pipe portion 74 connected to the downstream side of the exhaust manifold 17 via the exhaust pipe 18, a second expanded pipe portion 75 connected to the downstream side of the first expanded pipe portion 74, A switching unit 71 connected to the downstream side of the double pipe expansion unit 75 and an exhaust pipe 73 on the downstream side of the switching unit 71 are provided. NO in the first expansion section 74_XA purification catalyst device 76 is arranged, and a particulate filter 70 is arranged in a case 77 penetrating the second expanded portion 75 in the transverse direction. Thus, as shown by the arrow in FIG._XAfter passing through the purification catalyst device 76, it passes through the case 77 in the second pipe expansion section 75 and reaches the switching section 71.
[0062]
One side opening of the case 77 is connected to the switching unit 71 by the first connection part 72a, and the other side opening of the case 77 is connected to the switching part 71 by the second connection part 72b. 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 a result, as described above, NO_XThe exhaust gas that has reached the switching unit 71 after passing through the purification catalyst device 76 passes through the particulate filter 70 from one side opening of the case 77 toward the other side opening, as indicated by an arrow in FIG.
[0063]
FIG. 20 shows a second blocking position of the valve body 71a. In this blocking position, the upstream side in the switching unit 71 is communicated with the second connection portion 72b and the downstream side in the switching unit 71 is communicated with the first connection portion 72a. As a result, NO_XThe exhaust gas that has reached the switching unit 71 after passing through the purification catalyst device 76 passes through the particulate filter 70 from the other side opening of the case 77 toward the one side opening, as indicated by an arrow in FIG. 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.
[0064]
Further, in the particulate filter, a large opening area is required for facilitating the inflow of exhaust gas. However, in this exhaust purification device, as shown in FIGS. A particulate filter having a large opening area can be used.
[0065]
FIG. 21 shows the structure of the particulate filter 70. 21A is a front view of the particulate filter 70, and FIG. 21B 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. Thus, 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 the arrow in FIG. 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 surface of the pores in the partition wall 54 and are collected. Is done. Thus, each partition wall 54 functions as a collecting wall for collecting particulates. In the present particulate filter 70, in order to oxidize and remove the collected particulates, alumina or the like is used on both side surfaces of the partition wall 54, and preferably on the pore surface in the partition wall 54, as described below. An active oxygen release agent and a noble metal catalyst are supported.
[0066]
The active oxygen release agent is an agent that promotes oxidation of particulates by releasing active oxygen. Preferably, oxygen is taken in and retained when excess oxygen is present in the surroundings, and the oxygen concentration in the surroundings is increased. When lowered, the retained oxygen is released in the form of active oxygen.
[0067]
As the noble metal catalyst, platinum Pt is usually used, and as the active oxygen release agent, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, and rubidium Rb, barium Ba, calcium Ca, and strontium Sr are used. At least one selected from alkaline earth metals, lanthanum La, rare earths such as yttrium Y, and transition metals is used.
[0068]
In this case, as the active oxygen release agent, 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 are used. preferable.
[0069]
Next, how the collected particulates are oxidized and removed by such a particulate filter carrying an active oxygen release agent will be described by taking platinum Pt and potassium K as an example. The same particulate removal action can be performed using other noble metals, alkali metals, alkaline earth metals, rare earths, and transition metals.
[0070]
In a diesel engine, combustion is usually performed under excess air, and therefore the exhaust gas contains a large amount of excess air. That is, when the ratio of air and fuel supplied to the intake passage and the combustion chamber is called the air-fuel ratio of the exhaust gas, the air-fuel ratio is lean. Further, since NO is generated in the combustion chamber, the exhaust gas contains NO. The fuel also contains sulfur S, which reacts with oxygen in the combustion chamber to react with SO.₂It becomes. Therefore, in the exhaust gas, SO₂It is included. Therefore excess oxygen, NO and SO₂The exhaust gas containing the gas flows into the exhaust upstream side of the particulate filter 70.
[0071]
22A and 22B schematically show enlarged views of the exhaust gas contact surface in the particulate filter 70. FIG. In FIGS. 22A and 22B, reference numeral 60 denotes platinum Pt particles, and reference numeral 61 denotes an active oxygen release agent containing potassium K.
[0072]
As described above, since a large amount of excess oxygen is contained in the exhaust gas, when the exhaust gas comes into contact with the exhaust gas contact surface of the particulate filter, as shown in FIG.₂Is O₂ ^-Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO in the exhaust gas is O on the surface of platinum Pt.₂ ^-Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). Then the generated NO₂Part of the oxygen is absorbed on the active oxygen release agent 61 while being oxidized on platinum Pt, and is combined with potassium K, as shown in FIG._Three ^-Diffused into the active oxygen release agent 61 in the form of potassium nitrate KNO_ThreeIs generated. Thus, in this embodiment, NO contained in the exhaust gas._xCan be absorbed by the particulate filter 70, and the amount released into the atmosphere can be reduced.
[0073]
On the other hand, as described above, the exhaust gas contains SO.₂Is also included, this SO₂Is absorbed into the active oxygen release agent 61 by the same mechanism as NO. That is, as described above, 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 SO_ThreeIt becomes. The generated SO_ThreeIs partly oxidized on the platinum Pt while being absorbed into the active oxygen release agent 61 and combined with potassium K._Four ^2-Diffused into the active oxygen release agent 61 in the form of potassium sulfate K₂SO_FourIs generated. In this way, potassium nitrate KNO is contained in the active oxygen release catalyst 61._ThreeAnd potassium sulfate K₂SO_FourIs generated.
[0074]
Particulates in the exhaust gas adhere to the surface of the active oxygen release agent 61 carried on the particulate filter, as indicated by 62 in FIG. At this time, the oxygen concentration at the contact surface between the particulate 62 and the active oxygen release agent 61 decreases. When the oxygen concentration is lowered, a difference in concentration occurs between the active oxygen release agent 61 having a high oxygen concentration. Try to move towards. As a result, potassium nitrate KNO formed in the active oxygen release agent 61_ThreeIs decomposed into potassium K, oxygen O, and NO, oxygen O is directed to the contact surface between the particulate 62 and the active oxygen release agent 61, and NO is released from the active oxygen release agent 61 to the outside. NO released to the outside is oxidized on the platinum Pt on the downstream side, and is again absorbed in the active oxygen release agent 61.
[0075]
On the other hand, potassium sulfate K formed in the active oxygen release agent 61 at this time₂SO_FourAlso potassium K, oxygen O and SO₂The oxygen O is directed to the contact surface between the particulate 62 and the active oxygen release agent 61, and SO₂Is released from the active oxygen release agent 61 to the outside. SO released to the outside₂Is oxidized on the platinum Pt on the downstream side and absorbed again into the active oxygen release agent 61. However, potassium sulfate K₂SO_FourIs stabilized, potassium nitrate KNO_ThreeIt is difficult to release active oxygen compared to.
[0076]
On the other hand, the oxygen O toward the contact surface between the particulate 62 and the active oxygen release agent 61 is potassium nitrate KNO._ThreeAnd potassium sulfate K₂SO_FourIt is oxygen decomposed from a compound such as Oxygen O decomposed from the compound has high energy and has extremely high activity. Therefore, the oxygen directed toward the contact surface between the particulate 62 and the active oxygen release agent 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. Further, the active oxygen O that oxidizes the particulate 62 is transferred to the active oxygen release agent 61 by NO and SO.₂It is also released when is absorbed. That is, NO_XNitrate ions NO in the active oxygen release agent 61 while repeatedly binding and separating oxygen atoms_Three ^-The active oxygen is generated during this period. The particulate 62 is also oxidized by this active oxygen. Further, the particulates 62 adhering to the particulate filter 70 in this manner are oxidized by the active oxygen O, but these particulates 62 are also oxidized by oxygen in the exhaust gas.
[0077]
By the way, since platinum Pt and the active oxygen release agent 61 are activated as the temperature of the particulate filter increases, the amount of active oxygen O released from the active oxygen release agent 61 per unit time increases as the temperature of the particulate filter increases. To do. 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.
[0078]
The solid line in FIG. 23 indicates the amount G of fine particles that can be removed by oxidation without emitting a bright flame per unit time. In FIG. 23, the horizontal axis indicates the temperature TF of the particulate filter. FIG. 23 shows the amount G of fine particles that can be oxidized and removed per second when the unit time is 1 second. As this unit time, an arbitrary time such as 1 minute or 10 minutes is adopted. be able to. For example, when 10 minutes is used as the unit time, the amount G of fine particles that can be removed by oxidation per unit time represents the amount G of fine particles that can be removed by oxidation per 10 minutes. As shown in FIG. 23, the amount G of fine particles that can be removed by oxidation without emitting a bright flame per hour increases as the temperature of the particulate filter 70 increases.
[0079]
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 smaller 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 catalyst are oxidized and removed sequentially in a short time without emitting a bright flame on the particulate filter 70.
[0080]
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. 23, the amount of active oxygen is insufficient to sequentially oxidize all the particulates. FIGS. 24A to 24C show the state of particulate oxidation in such a case.
[0081]
That is, when the amount of active oxygen is insufficient to oxidize all the particulates, if the particulates 62 adhere on the active oxygen release agent 61 as shown in FIG. Particulates that are only oxidized and not fully oxidized remain on the exhaust upstream side 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 is covered with the residual particulate portion 63.
[0082]
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.₂The active oxygen release action by the active oxygen release agent 61 is suppressed. As a result, the residual particulate portion 63 can be gradually oxidized over time. However, as shown in FIG. 24C, another particulate 64 is placed on the residual particulate portion 63 from the next to the next. And deposit. That is, when the particulates are deposited in a laminated form, these particulates are separated from platinum Pt and the active oxygen release agent, so that even if the particulates are easily oxidized, they are not oxidized by active oxygen. Absent. 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, the particulates are deposited on the particulate filter in a stacked manner.
[0083]
In this way, in the region I in FIG. 23, the particulates are oxidized on the particulate filter within a short time without emitting a bright flame, and in the region II in FIG. 23, 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, the engine output reduction can be kept to a minimum. However, this is not always realized, and if nothing is done, particulates may accumulate on the particulate filter.
[0084]
In the present embodiment, the accumulation of particulates on the particulate filter is prevented by controlling the operation of the valve body 71a according to the first flowchart shown in FIG. 25 by the electronic control unit 30 described above. This flowchart is repeated every predetermined time. First, in step 101, it is determined whether it is time to switch the valve body 71a. The switching time is set time or set travel distance. When this determination is denied, the process is terminated as it is. When the determination is affirmed, the process proceeds to step 102, and the valve body 71a is rotated from the current blocking position to the other blocking position.
[0085]
FIG. 26 is an enlarged cross-sectional view of the partition wall 54 of the particulate filter. As described above, the exhaust upstream surface of the partition wall 54 where the exhaust gas mainly collides and the exhaust gas flow facing surface in the pores collide and collect the particulates as one of the collection surfaces, and the active oxygen release agent Although the collected particulates are oxidized and removed by the released active oxygen, the operation in the region II in FIG. 23 may be performed while traveling for the set time or the set travel distance. FIG. As indicated by the lattice, particulate removal may remain due to insufficient oxidation removal. The exhaust resistance of the particulate filter accompanying such accumulation of particulates does not adversely affect the running of the vehicle, but if particulates accumulate further, problems such as a significant decrease in engine output occur. However, if the exhaust upstream side and the exhaust downstream side of the particulate filter are reversed at this point, no further particulates accumulate on the particulate remaining on one of the collecting surfaces of the partition wall 54. Residual particulates are gradually oxidized and removed by the active oxygen released from one of the collecting surfaces. In addition, the particulates remaining in the pores of the partition walls are easily broken and fragmented by the exhaust gas flow in the reverse direction, and move downstream as shown in FIG.
[0086]
As a result, many of the finely divided particulates are dispersed in the pores of the partition walls, i.e., the particulates flow and directly with the active oxygen release agent supported on the pore inner surfaces of the partition walls. There are many opportunities for contact and oxidation removal. In this way, by supporting the active oxygen release agent in the pores of the partition walls, the residual particulates can be remarkably easily removed by oxidation. Further, in addition to this oxidation removal, the other collection 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 side opposite to the one collecting surface), new particulates in the exhaust gas adhere and are oxidized and removed by the active oxygen released from the active oxygen release agent. . A part of the active oxygen released from the active oxygen release agent during the oxidation removal moves to the downstream side together with the exhaust gas, and the remaining particulates are oxidized and removed by the backflow of the exhaust gas.
[0087]
That is, the residual particulates on one collecting surface in the partition wall are not only 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 exhaust gas. The remaining active oxygen used comes from 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.
[0088]
The switching of the valve body may be performed irregularly even if it is not performed periodically, such as every set time or set travel distance. Further, the valve body may be switched every time the engine is decelerated. For the determination at the time of engine deceleration, it is possible to detect an operation that the driver intends to decelerate the vehicle, for example, release of an accelerator pedal, depression of a brake pedal, fuel cut, or the like.
[0089]
Further, the valve body may be switched when the amount of particulate accumulation on the particulate filter reaches a set amount. For the estimation of the particulate deposition amount, for example, the differential pressure between the upstream side and the downstream side of the particulate filter 70 that increases with the increase in the particulate deposition amount can be used. The electrical resistance value on the predetermined partition wall of the particulate filter that decreases with an increase in the amount of deposited particulate may be used, and light transmission on the predetermined partition wall of the particulate filter that decreases with an increase in the amount of particulate deposition. Rate or reflectance may be used. Further, based on the graph of FIG. 23, when the amount M of discharged particulate estimated from the current engine operating state exceeds the amount G of particulate that can be removed by oxidation considering the temperature of the particulate filter estimated from the current engine operating state. The difference (M−G) may be integrated as the particulate deposition amount.
[0090]
Further, when the air-fuel ratio of the exhaust gas is made rich, that is, when the oxygen concentration in the exhaust gas is lowered, the active oxygen O is released from the active oxygen release agent 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.
[0091]
On the other hand, when the air-fuel ratio is maintained lean, 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_XAs a result, the active oxygen release amount from the active oxygen release agent 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, if the air-fuel ratio is switched from rich to lean again, NO_XNO to increase the oxidation effect on_XAs a result, the amount of active oxygen supplemented from the active oxygen release agent 61 increases.
[0092]
Therefore, if the air-fuel ratio is temporarily switched from lean to rich when the air-fuel ratio is maintained lean, oxygen poisoning of platinum Pt is eliminated each time, so that active oxygen release when the air-fuel ratio is lean Thus, the amount of the agent increases, and thus the oxidation action of the particulates on the particulate filter 70 can be promoted.
[0093]
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 collection surface in the particulate filter partition wall where no particulates remain is obtained. Since the active oxygen is easily released as compared with the one collecting surface, the remaining particulates on the one collecting surface can be more reliably oxidized and removed by the active oxygen released in a larger amount. Of course, the air-fuel ratio of the exhaust gas may be occasionally enriched regardless of the switching of the valve body 71a, so that it is difficult for the particulates to remain and accumulate on the particulate filter.
[0094]
As a method for enriching the air-fuel ratio of the exhaust gas, for example, the low-temperature combustion described above may be performed. Alternatively, the combustion air-fuel ratio may be simply made rich. 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.
[0095]
By the way, calcium Ca in the exhaust gas is SO._ThreeIn the presence of calcium sulfate CaSO_FourIs generated. This calcium sulfate CaSO_FourIs hardly removed by oxidation and remains as ash on the particulate filter. Therefore, in order to prevent the particulate filter from being clogged due to residual calcium sulfate, 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 as the active oxygen release agent 61. , Thereby diffusing into the active oxygen release agent 61_ThreeBinds potassium K and potassium sulfate K₂SO_FourCalcium Ca is SO_ThreeIt 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, as the active oxygen release agent 61, 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, strontium Sr is used. Is preferred.
[0096]
In addition, even if only a noble metal such as platinum Pt is supported on a particulate filter as an active oxygen release agent, NO retained on the surface of platinum Pt.₂Or SO_ThreeActive oxygen can be released from However, in this case, the solid line indicating the amount G of fine particles that can be removed by oxidation moves slightly to the right as compared with the solid line shown in FIG. It is also possible to use ceria as the active oxygen release agent. Ceria absorbs oxygen when the oxygen concentration in the exhaust gas is high, and releases active oxygen when the oxygen concentration in the exhaust gas decreases. Therefore, ceria in the exhaust gas is removed for oxidation removal of particulates. It is necessary to enrich the air-fuel ratio regularly or irregularly. Instead of ceria, iron or tin may be used.
[0097]
In this embodiment, the particulate filter itself carries the active oxygen release agent, and the particulates are oxidized and removed by the active oxygen released by the active oxygen release agent. However, this limits the present invention. It is not a thing. For example, the active oxygen and the particulate oxidation component such as nitrogen dioxide that functions in the same way as the active oxygen may be released from the particulate filter or a substance carried on the particulate filter or may flow into the particulate filter from the outside. . Even when the particulate oxidation component flows from the outside, in order to collect the particulates, by alternately using the first collection surface and the second collection surface of the collection wall, Particulates are not newly deposited on one of the collected surfaces, and the deposited particulates are gradually oxidized and removed by the particulate oxidizing component flowing in from the other collecting surface. Can be sufficiently oxidized and removed in a certain time. In the meantime, the other collecting surface is oxidized by the particulate oxidation component together with the collection of the particulates, and thus the same effect as described above is brought about.
[0098]
The diesel engine of the present embodiment is switched between low-temperature combustion and normal combustion, but this does not limit the present invention. Of course, a diesel engine or particulate that performs only normal combustion is used. The present invention is also applicable to a gasoline engine that discharges.
[0099]
By the way, the particulate filter of the present embodiment, as described above, the NO in the exhaust gas._XAs described above, the structure of the particulate filter is a wall flow type in which the exhaust gas passes through the pores of the collection wall, and the exhaust gas generally flows along the partition wall supporting the catalyst. In order to allow the same amount of exhaust gas to pass through in comparison with a typical catalyst device, the size between the collection walls must be larger than the size between the partition walls. Thereby, the opportunity for the exhaust gas to come into contact with the active oxygen release agent carried on the surface of the collection wall in the particulate filter is less than the chance to come into contact with the catalyst in the monolith type catalyst device. Further, when the exhaust gas passes through the pores of the collection wall, it comes into contact with the active oxygen release agent carried in the pores, but mainly the active gas carried on the surface of the collection wall. Contact with oxygen release agent only. 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 an active oxygen release agent that absorbs oxygen is supported on the particulate filter, NO in the exhaust gas_XCan not be sufficiently purified.
[0100]
In the present embodiment, as described above, the NO on the upstream side of the switching unit 71 for reversing the exhaust upstream side and the exhaust downstream side of the particulate filter._XA purification catalyst device 76 is arranged. As a result, NO in the exhaust gas_XPart of NO_XPurified by the purification catalyst device 76 and combined with the particulate filter into the atmosphere NO_XIt is possible to sufficiently reduce the discharge amount. In this particulate filter 70, as described above, NO is removed for the oxidation removal of particulates._XNO to use_XA large amount of NO is upstream of the particulate filter 70 by the purification catalyst device 76._XMust not be purified.
[0101]
NO_XThe purification catalyst device 76 is NO._XNo matter which catalyst is used for purifying NO_XNO purification by reducing agent_XThat is, accompanied by oxidation of the reducing agent. In this embodiment, this reaction heat is generated by using NO_XThe case 77 that crosses the exhaust passage immediately downstream of the purification catalyst device 76 is heated satisfactorily. Thereby, the particulate filter 70 arranged in the case 77 is also heated well. In the particulate filter 70 thus heated, the amount of particulates that can be removed by oxidation increases as shown in FIG. 23, so that the collected particulates can be satisfactorily removed by oxidation.
[0102]
NO in this example_XThe purification catalyst device 76 uses, for example, alumina or the like for the partition walls of the honeycomb structure carrier, and will be described below._XAn absorbent and a noble metal catalyst such as platinum Pt are supported. NO_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.
[0103]
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 partition walls, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0104]
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. 27 (A), a part of is absorbed in the absorbent while being oxidized on platinum Pt and combined with barium oxide BaO._Three ^-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 neighborhood₂Is produced and NO in the absorbent_XUnless the absorption capacity is saturated, NO₂Is absorbed into the absorbent and nitrate ion NO._Three ^-Is generated.
[0105]
On the other hand, when the air-fuel ratio in the vicinity 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 proceeds in the reverse direction (NO_Three ^-→ NO₂) And thus nitrate ion NO in the absorbent_Three ^-Is NO₂Is released from the absorbent in the form of NO at this time_XNO released from the absorbent_XAs shown in FIG. 27 (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.
[0106]
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 released only gradually_XTotal NO absorbed by the purification catalyst unit_XIt takes a little longer time to release.
[0107]
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_XNO absorbed by the purification catalyst device_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. 28A 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. 28B as a function of required load L and engine speed N, and the NO per unit time is obtained._XBy accumulating the absorption amounts A and B, NO_XNO absorbed by the purification catalyst device_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_XNO when the amount of absorption exceeds a predetermined tolerance_XIn order to regenerate the purification catalyst device, the low-temperature combustion at the stoichiometric air-fuel ratio or rich air-fuel ratio is performed, or fuel is injected into the cylinder in the exhaust stroke, etc._XThe atmosphere in the vicinity of the purification catalyst device is set to the stoichiometric air-fuel ratio or rich air-fuel ratio, and this state is maintained at least for the time until regeneration is completed (the shorter the air-fuel ratio in the vicinity atmosphere is, the shorter is this). This regeneration process is NO_XThe heat of reaction is generated as the reducing agent is oxidized. In the present embodiment, as described above, this reaction heat can be effectively used for heating the particulate filter.
[0108]
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. Thereby, as in the present embodiment, the NO disposed on the upstream side of the particulate filter 70_XIf an oxidation catalyst such as platinum Pt is supported on the purification catalyst device, SOF in the exhaust gas is burned out upstream of the particulate filter, and the promotion of clogging of the particulate filter by SOF can be prevented.
[0109]
By the way, the fuel of the internal combustion engine contains sulfur, and the SO_XIs generated. SO_XIs NO_XNO to purification catalyst unit_XIt is absorbed in the form of sulfate by the same mechanism. This sulfate can release active oxygen by the same mechanism as nitrate. However, since sulfate is a stable substance, even if the ambient atmosphere is rich air-fuel ratio, it is NO._XIn fact, NO is hardly released from the purification catalyst device._XThe amount of occlusion increases gradually as it remains in the purification catalyst device. NO_XThe amount of nitrate or sulfate that can be stored in the purification catalyst device is finite, NO_XIf the storage amount of sulfate in the purification catalyst device 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.
[0110]
In this embodiment, the upstream side NO_XThe purification catalyst device 76 is SO_XAlthough it is poisoned, the active oxygen releasing agent on the downstream particulate filter is SO._XIs difficult to absorb. NO_XSO of the purification catalyst device 76_XThe recovery from poisoning is performed by the following procedure.
[0111]
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_XIn the regeneration process of the purification catalyst device, 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.
[0112]
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.
[0113]
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 between the two shut-off positions in the switching unit 71._XSO released from the purification catalyst device_XIs not absorbed by the active oxygen release agent of the particulate filter 70 in order to bypass the particulate filter 70. 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 time, SO_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. This SO_XNO in the poisoning recovery process_XHeat generated during the temperature rise of the purification catalyst device and SO_XThe combustion heat of the reducing substance during the reduction purification heats the case 77 well and effectively raises the temperature of the particulate filter 70 in the case 77.
[0114]
NO_XSO of purification catalyst equipment_XAt the same time as or immediately after completion of the poisoning recovery process, NO_XSO released from the purification catalyst device_XIs discharged to the exhaust pipe 73, the switching valve 71a of the switching unit 71 is set as one shut-off position, and the NO is brought to a high temperature (about 600 ° C.)._XBy guiding the exhaust gas heated through the purification catalyst device to the particulate filter, the temperature of the particulate filter can be further increased.
[0115]
【The invention's effect】
An exhaust gas purification apparatus for an internal combustion engine according to the present invention includes NO disposed in an exhaust passage._XPurification catalyst device and NO_XA case that penetrates the exhaust passage in the transverse direction on the downstream side of the purification catalyst device, a particulate filter that is disposed in the case and collects particulates, and that is arranged in the exhaust passage on the downstream side from the penetration position of the case. And the exhaust gas that has passed around the case in the exhaust passage passes through the particulate filter in the case through the switching unit, and the switching unit allows the exhaust gas upstream of the particulate filter to be exhausted. The exhaust side and the exhaust downstream side can be reversed, and the collected particulates are oxidized in the particulate filter.
[0116]
Thus, by reversely switching the exhaust gas upstream side and the exhaust gas downstream side of the particulate filter by the switching unit, the oxidation of the collected particulates in the particulate filter becomes relatively good._XNO in purification catalyst equipment_XThe reaction heat at the time of purification is not via the switching part, but NO_XIn order to heat the case penetrating the exhaust passage in the transverse direction on the downstream side of the purification catalyst device well and to effectively raise the temperature of the particulate filter in this case, the temperature of the particulate filter is increased by this temperature rise. The oxidation of the collected particulate can be further improved.
[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 a structure of a particulate filter.
FIG. 22 is a diagram for explaining the oxidizing action of particulates.
FIG. 23 is a graph showing the relationship between the amount of fine particles that can be removed by oxidation and the temperature of the particulate filter.
FIG. 24 is a diagram for explaining the particulate deposition action.
FIG. 25 is a flowchart for preventing accumulation of a large amount of particulates on a particulate filter.
FIG. 26 is an enlarged sectional view of the partition wall of the particulate filter.
FIG. 27: NO_XIt is a figure for demonstrating the absorption-and-release function of.
FIG. 28: NO per unit time_XIt is a figure which shows the map of absorption amount.
[Explanation of symbols]
6 ... Fuel injection valve
16 ... Throttle valve
71 ... switching part
70 ... Particulate filter
76 ... NO_XPurification catalyst device

Claims (5)

A NO _x purification catalyst device disposed in the exhaust passage, a case penetrating the exhaust passage in the transverse direction on the downstream side of the NO _x purification catalyst device, and a particulate matter disposed in the case for collecting particulates A curative filter, and a switching portion disposed in the exhaust passage on the downstream side of the penetrating position of the case, and the exhaust gas that has passed around the case in the exhaust passage passes through the switching portion to the case. The particulate filter passes through the particulate filter, and the switching unit can reverse the exhaust upstream side and the exhaust downstream side of the particulate filter. In the particulate filter, the collected particulate is An exhaust purification device for an internal combustion engine, characterized by being oxidized. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein an active oxygen release agent is supported on the particulate filter, and the active oxygen released from the active oxygen release agent oxidizes the particulate. 3. The active oxygen release agent according to claim 2, wherein oxygen is taken in and retained when excess oxygen is present in the surroundings, and the retained oxygen is released in the form of active oxygen when the surrounding oxygen concentration is lowered. 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. The active oxygen release agent binds and holds NO _x with oxygen when excess oxygen is present in the surrounding area, and decomposes the combined NO _x and oxygen into NO _x and active oxygen when the surrounding oxygen concentration decreases. The exhaust emission control device for an internal combustion engine according to claim 2, wherein the exhaust gas purification device is discharged. The NO _X purification catalyst device carries a NO _X storage reduction catalyst that absorbs NO _X when the ambient atmosphere is a lean air-fuel ratio and releases and reduces NO _X when the atmosphere is a stoichiometric or rich air-fuel ratio. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas purification apparatus is an internal combustion engine.

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