JP3558015B2 - Exhaust gas purification method for internal combustion engine - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification method for an internal combustion engine.
[0002]
[Prior art]
In an exhaust system of an internal combustion engine, a catalytic converter for purifying exhaust gas by converting harmful substances such as hydrocarbon HC and carbon monoxide CO contained in the exhaust gas into harmless substances is arranged. In the catalytic converter, an oxidation catalyst such as platinum Pt is supported on a carrier formed of a porous material such as ceramic or alumina. The carrier of the catalytic converter generally has a number of axial spaces subdivided by a number of axially extending partitions, the exhaust gas passing through each of these partitions when passing through these axial spaces. It is intended to be in contact with a catalyst supported on
[0003]
However, the exhaust gas passing through the center in each axial space of the carrier is not contacted with the partition wall, that is, the catalyst, and is discharged from the catalytic converter without being purified. It has been proposed to close one of two adjacent axial spaces in the carrier upstream of the exhaust and close the other downstream of the exhaust. In a wall-flow type catalytic converter having such a carrier, the exhaust gas always passes through the partition and surely comes into contact with the catalyst supported on the partition, so that the purification rate can be improved.
[0004]
By the way, the exhaust gas of an internal combustion engine, especially a diesel engine contains particulates. Although the main component of the particulates is soot, it also contains a soluble organic component (SOF). Due to the adhesiveness of the SOF, the particulates adhere to the exhaust upstream surface of each partition of the wall-flow type catalytic converter. I do.
[0005]
Since the main component of the particulates is soot, that is, carbon, if the oxidation reaction of hydrocarbons HC and carbon monoxide CO in the exhaust gas actively occurs in the oxidation catalyst of the catalytic converter, this reaction heat Thereby, the particulates can be easily burned off.
[0006]
[Problems to be solved by the invention]
However, a part of the oxidation catalyst supported on the exhaust gas upstream side of each partition of the catalytic converter is in a state where its function cannot be sufficiently exhibited due to adhesion of particulates by SOF, that is, poisoned by SOF. It cannot cause an active oxidation reaction. Thus, although gradually, particulates accumulate in the catalytic converter, and finally, the exhaust resistance is considerably increased, and problems such as a decrease in engine output occur. Of course, it is conceivable to forcibly burn off particulates deposited and deposited on the catalytic converter by a heating means such as a heater, but this requires a large amount of energy.
[0007]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an exhaust gas purification method for an internal combustion engine that can easily and reliably burn off particulates adhering to a wall-flow type catalytic converter having an oxidizing function.
[0008]
[Means for Solving the Problems]
An exhaust gas purifying method for an internal combustion engine according to claim 1 of the present invention includes a wall-flow type catalytic converter having at least an oxidizing function, and allows exhaust gas to flow from one side of the catalytic converter to the catalytic converter. In order to burn out the trapped particulates, exhaust gas containing a reducing substance is introduced from the opposite side of the catalytic converter.An inert gas supply unit for supplying the inert gas into the cylinder by burning the reducing substance by the oxidizing function of the catalytic converter and utilizing the heat of combustion for burning out the particulates. And a first combustion in which a larger amount of the inert gas than the worst inert gas amount that maximizes the generation amount of soot is supplied into the cylinder to perform combustion, and the first inert gas that is smaller than the worst inert gas amount. Exhaust gas containing the reducing substance, which is provided with switching means for selectively switching between a second combustion in which a gas is supplied into a cylinder and a combustion is performed, and which is caused to flow from the opposite side of the catalytic converter by the first combustion. GenerateIt is characterized by the following.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic longitudinal sectional view of a four-stroke diesel engine provided with an exhaust gas purifying apparatus used in the exhaust gas purifying method according to the present invention, and FIG. 2 is an enlarged longitudinal sectional view of a combustion chamber of the diesel engine of FIG. FIG. 3 is a bottom view of the cylinder head of the diesel engine of FIG. 1 to 3, reference numeral 1 denotes an engine body, 2 denotes a cylinder block, 3 denotes a cylinder head, 4 denotes a piston, 5a denotes a cavity formed on the top surface of the piston 4, and 5 denotes a cavity formed in the cavity 5a. A 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 arranged 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.
[0012]
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. Further, 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 engine cooling water cools the EGR gas.
[0013]
On the other hand, each fuel injection valve 6 is connected via a fuel supply pipe 25 to a fuel reservoir, a so-called common rail 26. Fuel is supplied into the common rail 26 from an electric control type variable discharge fuel pump 27, 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. The fuel pump 27 is controlled so that the fuel pressure in the common rail 26 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 28. Is controlled.
[0014]
An electronic control unit 30 receives an output signal of the air-fuel ratio sensor 21 and an output signal of the fuel pressure sensor 28. A load sensor 41 that generates an output voltage proportional to the amount of depression 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 the crank angle sensor 42 that generates an output pulse every time the motor rotates by 30 ° 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 19 disposed in the exhaust pipe 18 based on various signals. The switching valve 19 will be described later.
[0015]
As shown in FIGS. 2 and 3, in the embodiment according to the present invention, the fuel injection valve 6 comprises 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 scatter along the lower surface of the valve body of each exhaust valve 9. 2 and 3 show a case where 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 burned.
[0016]
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, a case is shown in which 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 valve body direction of the exhaust valve 9 goes between the rear surface of the bulk portion of the exhaust valve 9 and the exhaust port 10. That is, in other words, two of the six nozzle ports of the fuel injection valve 6 emit the fuel spray F when additional fuel Qa is injected when the exhaust valve 9 is opened. The valve 9 is formed so as to extend between the rear surface of the bulk portion of the valve 9 and the exhaust port 10. In this case, in the embodiment shown in FIG. 4, at this time, the fuel spray F collides with the back of the bulk of the exhaust valve 9, and the fuel spray F colliding with the back of the bulk of the exhaust valve 9 is reflected on the back of the bulk of the exhaust valve 9. Then, it goes into the exhaust port 10.
[0017]
Normally, no additional fuel Qa is injected, and only the main injection Qm is performed. FIG. 6 shows a change in the output torque when the air-fuel ratio A / F (horizontal axis in FIG. 6) is changed by changing the opening degree of the throttle valve 16 and the EGR rate during the low load operation of the engine, and smoke, HC , CO, NO_x4 shows an experimental example showing a change in the emission amount of the gas. 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 higher when the air-fuel ratio is equal to or less than the stoichiometric air-fuel ratio (比 14.6).
[0018]
As shown in FIG. 6, when the air-fuel ratio A / F is reduced by increasing the EGR rate, the amount of smoke generated when the EGR rate becomes about 40% and the air-fuel ratio A / F becomes about 30 is reduced. Start growing. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the amount of smoke generated sharply increases and reaches a peak. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the smoke is sharply reduced. When the EGR rate is increased to 65% or more, and the air-fuel ratio A / F becomes about 15.0, the smoke becomes almost zero. . That is, almost no soot is generated. At this time, the output torque of the engine slightly decreases, and NO_xThe amount of generation is considerably low. On the other hand, at this time, the generation amounts of HC and CO begin to increase.
[0019]
FIG. 7A shows a change in the combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is around 21 and the amount of generated smoke is the largest. FIG. 7B shows an air-fuel ratio A / F of 18. The graph shows changes in the combustion pressure in the combustion chamber 5 when the amount of generated smoke is almost zero in the vicinity. As can be seen by comparing FIG. 7 (A) and FIG. 7 (B), FIG. 7 (B) in which the amount of smoke generation is almost zero is shown in FIG. 7 (A) where the amount of smoke generation is large. It can be seen that the combustion pressure is lower than in the case.
[0020]
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 generated smoke is almost zero, as shown in FIG._xThe amount of generation of methane is considerably reduced. NO_xThe decrease in the amount of the generated gas means that the combustion temperature in the combustion chamber 5 has decreased. Therefore, it can be said that the combustion temperature in the combustion chamber 5 is low when little soot is 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 the combustion temperature in the combustion chamber 5 is low at this time.
[0021]
Second, when the amount of generated smoke, that is, the amount of generated soot becomes substantially zero, the amount of HC and CO emissions increases as shown in FIG. This means that hydrocarbons are emitted without growing to soot. That is, the linear hydrocarbons and aromatic hydrocarbons contained in the fuel as shown in FIG. 8 are thermally decomposed when the temperature is increased in a state of lack of oxygen, soot precursors are formed, and then mainly, A soot consisting of a solid aggregate of carbon atoms is produced. In this case, the actual soot generation process is complicated, and it is not clear what form the soot precursor takes, but in any case, the hydrocarbon as shown in FIG. It will grow to soot. Therefore, as described above, when the amount of soot generation becomes substantially zero, the emission amounts of HC and CO increase as shown in FIG. 6, but HC at this time is a soot precursor or a hydrocarbon in a state before the soot. .
[0022]
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 generated soot becomes almost zero. The hydrocarbon will be discharged from the combustion chamber 5. As a result of further detailed experimental research on this fact, when the temperature of the fuel and the surrounding gas in the combustion chamber 5 is lower than a certain temperature, the growth process of the soot is stopped halfway. It was found that no soot was generated, and soot was generated when the temperature of the fuel and its surroundings in the combustion chamber 5 became lower than a certain temperature.
[0023]
By the way, the temperature of the fuel and its surrounding when the process of producing hydrocarbons is stopped in the state of the soot precursor, that is, the above-mentioned certain temperature varies depending on various factors such as the type of fuel, the air-fuel ratio, the compression ratio, and the like. I can not say how many times, but this certain temperature is NO_xIs closely related to the amount of NOx generated, so that this certain temperature is NO_xCan be defined to some extent from the amount of occurrence of. That is, as the EGR rate increases, the fuel temperature during combustion and the gas temperature around the fuel decrease, and the NO_xGeneration amount decreases. NO at this time_xIs 10 p. p. When it is less or equal to or less than m, almost no soot is generated. Therefore, the above certain temperature is NO_xIs 10 p. p. m The temperature almost coincides with the temperature when the temperature becomes lower or higher.
[0024]
Once soot is produced, it cannot be purified by post-treatment using a catalyst having an oxidizing function. On the other hand, the soot precursor or the hydrocarbon in the state before the soot can be easily purified by a post-treatment using a catalyst having an oxidation function. Thus, NO_xIt is extremely effective in purifying exhaust gas to reduce the generation amount of methane and to discharge hydrocarbons from the combustion chamber 5 in the state of a precursor of soot or before the soot.
[0025]
Now, in order to stop the growth of hydrocarbons before soot is generated, it is necessary to suppress the temperature of the fuel and the surrounding gas during combustion in the combustion chamber 5 to a temperature lower than the temperature at which soot is generated. There is. In this case, it has been found that the endothermic effect of the gas around the fuel when the fuel burns has an extremely large effect on suppressing the temperature of the fuel and the gas around the fuel.
[0026]
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 separated 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 separated from the fuel hardly absorbs the heat of combustion heat of the fuel. In this case, since the combustion temperature becomes extremely high locally, the unburned hydrocarbons that have received the heat of combustion will generate soot.
[0027]
On the other hand, when fuel is present in a mixed gas of a large amount of inert gas and a small amount of air, the situation is slightly different. In this case, the evaporated fuel diffuses to the surroundings, 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 effect of the inert gas.
[0028]
In this case, controlling the temperature of the fuel and its surrounding gas to a temperature lower than the temperature at which soot is generated requires an amount of an inert gas that can absorb a sufficient amount of heat to do so. Therefore, if the amount of fuel increases, the required amount of inert gas increases accordingly. In this case, the endothermic effect becomes stronger as the specific heat of the inert gas increases, so that the inert gas preferably has a higher specific heat. In this regard, CO₂Since EGR gas has a relatively large specific heat, it can be said that it is preferable to use EGR gas as the inert gas.
[0029]
FIG. 9 shows the relationship between the EGR rate and the smoke when the EGR gas is used as the inert gas and the degree of cooling of the EGR gas is changed. That is, in FIG. 9, the curve A shows the case where the EGR gas is cooled strongly and the EGR gas temperature is maintained at approximately 90 ° C., and the curve B shows the case where the EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0030]
As shown by the curve A in FIG. 9, when the EGR gas is strongly cooled, the soot generation amount peaks at a point where the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. Then, almost no soot is generated. On the other hand, as shown by the curve B in FIG. 9, when the EGR gas is slightly cooled, the amount of soot generation peaks at a point where the EGR rate is slightly higher than 50%, and in this case, the EGR rate is increased to about 65% or more. So that almost no soot is generated.
[0031]
When the EGR gas is not forcibly cooled as shown by the curve C in FIG. 9, the amount of soot generation reaches a peak near the EGR rate of 55%. Above a percentage, soot is hardly generated. FIG. 9 shows the amount of smoke generated when the engine load is relatively high. When the engine load decreases, the EGR rate at which the amount of soot peaks slightly decreases, and the EGR rate at which almost no soot is generated Also lowers slightly. As described above, the lower limit of the EGR rate at which almost no soot is generated varies depending on the degree of cooling of the EGR gas and the engine load.
[0032]
FIG. 10 shows a mixed gas amount of the EGR gas and the air necessary for setting the fuel temperature during combustion and the surrounding gas temperature to a temperature lower than the temperature at which soot is generated when EGR gas is used as the inert gas; Further, the ratio of air in the mixed gas amount and the ratio of EGR gas in the mixed gas are shown. In FIG. 10, the vertical axis indicates the total intake gas amount drawn into the combustion chamber 5, and the chain line Y indicates the total intake gas amount that can be drawn 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.
[0033]
Referring to FIG. 10, the proportion 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 amount of air and the amount of injected fuel is the stoichiometric air-fuel ratio. On the other hand, in FIG. 10, the ratio of the EGR gas, that is, the amount of the EGR gas in the mixed gas, is set so that when the injected fuel is burned, the temperature of the fuel and the surrounding gas is made lower than the temperature at which soot is formed. The minimum required EGR gas amount is shown. This EGR gas amount is approximately 55% or more in terms of the EGR rate, and is 70% or more in the embodiment shown in FIG. That is, the total intake gas amount sucked into the combustion chamber 5 is indicated by a solid line X in FIG. 10, and the ratio between the air amount and the EGR gas amount in the total intake gas amount X is as shown in FIG. The temperature of the fuel and the gas around it will be lower than the temperature at which soot is produced, so that no soot is generated. In this case, NO_xThe amount generated is 10 p. p. m around or below and therefore NO_xIs extremely small.
[0034]
As 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 its 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 EGR gas amount must be increased as the injected fuel amount increases. That is, the EGR gas amount needs to increase as the required load increases.
[0035]
On the other hand, in the load region Z2 in FIG. 10, the total intake gas amount X necessary to prevent the generation of soot exceeds the total intake gas amount Y that can be sucked. Therefore, in this case, it is necessary to supercharge or pressurize both the EGR gas and the intake air or the EGR gas in order to supply the total intake gas amount X necessary for preventing the generation of soot into the combustion chamber 5. When the EGR gas or the like is not supercharged or pressurized, the total intake gas amount X matches 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 reduced to increase the amount of EGR gas, and the fuel is burned under a rich air-fuel ratio.
[0036]
As described above, FIG. 10 shows the case where the fuel is burned under the stoichiometric air-fuel ratio. However, 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 made rich, NO_xIs 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 soot generation even when lean_xIs 10 p. p. m can be around or below.
[0037]
That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but since the combustion temperature is suppressed to a low temperature, the excess fuel does not grow to soot, and thus no soot is generated. At this time, NO_xOnly 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 when the combustion temperature increases, but in the present invention, the soot is suppressed to a low temperature, so that the soot is reduced. Not generated at all. Furthermore, NO_xOnly a very small amount is generated.
[0038]
As described above, in the engine low load operation region Z1, no soot is generated regardless of the air-fuel ratio, that is, regardless of whether the air-fuel ratio is rich, the stoichiometric air-fuel ratio, or the average air-fuel ratio is lean, and NO_xIs 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.
[0039]
By the way, the temperature of the fuel and the surrounding gas at the time of combustion in the combustion chamber can be suppressed below the temperature at which the growth of hydrocarbons stops halfway, only when the heat generation amount due to combustion is small and the engine load is relatively low. Therefore, in the embodiment according to the present invention, when the engine load is relatively low, the first combustion, that is, the low-temperature combustion is performed by suppressing the temperature of the fuel during combustion and the gas around it to a temperature below the temperature at which the growth of hydrocarbons stops halfway. Then, when the engine load is relatively high, the second combustion, that is, the combustion that is usually performed conventionally, is performed. Here, the first combustion, that is, low-temperature combustion, as is clear from the description so far, the amount of inert gas in the combustion chamber is larger than the worst inert gas amount at which the generation amount of soot is maximum, and soot is almost generated. The second combustion, that is, the combustion that is usually performed in the past, is the combustion in which the amount of inert gas in the combustion chamber is smaller than the worst inert gas amount that produces the largest amount of soot. Say
[0040]
FIG. 11 shows a first operation region I in which first combustion, that is, low-temperature combustion is performed, and a second combustion region II in which second combustion, that is, combustion by a conventional combustion method, is performed. In FIG. 11, the vertical axis L indicates the depression amount 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 a 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. The second boundary with the area II is shown. The determination of the change of the operation region from the first operation region I to the second operation region II is made 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 determination of the change of the operating region is performed based on the second boundary Y (N).
[0041]
That is, if the required load L exceeds a first boundary X (N), which is a function of the engine speed N, when the operating state of the engine is in the first operating region I and low-temperature combustion is being performed, the operating region Is shifted to the second operation region II, and combustion is performed by the conventional combustion method. Next, when the required load L becomes lower than a second boundary Y (N) which is a function of the engine speed N, it is determined that the operation region has shifted to the first operation region I, and low-temperature combustion is performed again.
[0042]
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, the operation control in the first operation region I and the second operation region II will be schematically described with reference to FIG.
[0043]
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 operating region I where the required load L is low, the opening of the throttle valve 16 is gradually increased from almost fully closed to about half-open as the required load L increases, and the EGR control valve 23 Is gradually increased from near fully closed to fully open as the required load L increases. In the example shown in FIG. 13, in the first operation region I, the EGR rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio.
[0044]
In other words, in the first operating region I, the opening of the throttle valve 16 and the opening of the EGR control valve 23 are controlled such that the EGR rate becomes approximately 70% and the air-fuel ratio becomes a slightly lean air-fuel ratio. At this time, the air-fuel ratio is controlled to the target lean air-fuel ratio by correcting the opening 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 delayed as the injection start timing θS is delayed.
[0045]
At the time of idling operation, the throttle valve 16 is closed to almost fully closed, and at this time, the EGR control valve 23 is also closed to almost fully closed. When the throttle valve 16 is closed close to the fully closed position, the pressure in the combustion chamber 5 at the start of compression decreases, so that the compression pressure decreases. When the compression pressure decreases, the compression work by the piston 4 decreases, so that the vibration of the engine body 1 decreases. That is, at the time of idling operation, the throttle valve 16 is closed to almost fully closed in order to suppress the vibration of the engine body 1.
[0046]
On the other hand, when the operating region of the engine changes from the first operating region I to the second operating region II, the opening degree of the throttle valve 16 is increased stepwise from the half-open state to the fully open state. At this time, in the example shown in FIG. 13, the EGR rate is reduced stepwise from approximately 70% to 40% or less, and the air-fuel ratio is increased stepwise. That is, since the EGR rate jumps over the EGR rate range (FIG. 9) in which a large amount of smoke is generated, a large amount of smoke is generated when the operation region of the engine changes from the first operation region I to the second operation region II. There is no.
[0047]
In the second operation region II, the conventional combustion is performed. In this combustion method, soot and NO_xIs generated, but the thermal efficiency is higher than that of low-temperature combustion. Therefore, when the operating region of the engine changes from the first operating region I to the second operating region II, the injection amount decreases stepwise as shown in FIG. I'm sullen.
[0048]
In the second operating region II, the throttle valve 16 is held in a fully open state except for a part, and the opening of the EGR control valve 23 is gradually reduced as the required load L increases. In this operating 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 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.
[0049]
FIG. 14 shows the air-fuel ratio A / F in the first operation 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. 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 operating region I, and in the first operating region I, the air-fuel ratio A / F becomes leaner as the required load L decreases.
[0050]
That is, the lower the required load L, the smaller the amount of heat generated by combustion. Therefore, as the required load L decreases, low-temperature combustion can be performed even if the EGR rate is reduced. When the EGR rate is reduced, the air-fuel ratio increases. Therefore, as shown in FIG. 14, as the required load L decreases, the air-fuel ratio A / F increases. As the air-fuel ratio A / F increases, the fuel consumption rate increases. Therefore, in order to make the air-fuel ratio as lean as possible, in this embodiment, the air-fuel ratio A / F is increased as the required load L decreases.
[0051]
It should be noted that the target opening ST of the throttle valve 16 necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG. 14 is represented as a function of the required load L and the engine speed N as shown in FIG. The target load SE of the EGR control valve 23 required 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 in the form of a map as a function of the engine speed N in the ROM 32 in advance.
[0052]
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, curves indicated by A / F = 24, A / F = 35, A / F = 45, and A / F = 60 indicate target air-fuel ratios 24, 35, 45, and 60, respectively. As shown in FIG. 17A, the target opening ST of the throttle valve 16 necessary for setting the air-fuel ratio to the target air-fuel ratio is stored in the ROM 32 in advance in the form of a map as a function of the required load L and the engine speed N. The target opening SE of the EGR control valve 23 required to set the air-fuel ratio to the target air-fuel ratio is calculated 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 the form of a map in advance.
[0053]
Thus, in the diesel engine of the present embodiment, the first combustion, that is, the low temperature combustion, and the second combustion, that is, the normal combustion are switched based on the depression amount L of the accelerator pedal 40 and the engine speed N, In each combustion, the opening degree control of the throttle valve 16 and the EGR valve is performed based on the depression amount L of the accelerator pedal 40 and the engine speed N by using a map shown in FIG. 15 or FIG.
[0054]
Referring to FIG. 1, a switching valve 19 is disposed in an exhaust pipe 18 connected to a downstream side of an exhaust manifold 17. A C-shaped exhaust passage 18a is connected to the switching valve 19, and a catalytic converter 20 is disposed in the exhaust passage 18a. The catalytic converter 20 will be described later in detail. The switching valve 19 includes a valve body 19 a for shutting off the exhaust pipe 18. At one shutoff position of the valve element 19a indicated by a solid line, exhaust gas flows counterclockwise in FIG. 1 through the exhaust passage 18a from the upstream side of the switching valve 19 of the exhaust pipe 18 as indicated by a white arrow. It flows into the pipe 18 downstream of the switching valve 19.
[0055]
Further, at the other shutoff position of the valve element 19a indicated by the dashed line, the exhaust gas flows clockwise in FIG. It flows into the exhaust pipe 18 downstream of the switching valve 19. In this way, by switching the switching valve 19, the direction of the exhaust gas flowing into the catalytic converter 20 can be reversed, that is, the exhaust upstream side and the exhaust downstream side of the catalytic converter 20 can be reversed.
[0056]
As shown in the partially enlarged axial sectional view of FIG. 18, the carrier of the catalytic converter 20 includes a plurality of axially extending partitions 20a formed of a porous material, and a plurality of axial directions subdivided by these partitions. The wall flow type includes a space 20b and a closing member 20c that closes one of two adjacent axial spaces on the exhaust upstream side and closes the other on the exhaust downstream side. The catalyst is carried on the whole partition 20a of the carrier. In such a wall flow type carrier, since the exhaust gas always passes through the partition wall 20a, the exhaust gas surely comes into contact with the catalyst carried on the partition wall 20a, so that the purification rate can be improved.
[0057]
In the present embodiment, the catalyst supported on the partition wall 20a is, for example, potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, barium Ba, an alkaline earth such as calcium Ca, lanthanum La, or yttrium Y. At least one selected from rare earths and a noble metal such as platinum Pt. This catalyst has NO_xReferred to as an absorbent, the engine intake passage, the combustion chamber 5 and the NO_xThe ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the absorbent is set to NO._xWhen this is referred to as the air-fuel ratio of the exhaust gas flowing into the absorbent, this NO_xThe absorbent is NO when the air-fuel ratio of the inflowing exhaust gas is lean_xNO when the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich._xReleases NO_xPerforms the absorption and release action.
[0058]
This NO_xIf the absorbent is arranged in the engine exhaust passage, NO_xThe absorbent is actually NO_x, But there is a portion where the detailed mechanism of the absorption and release is not clear. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking platinum Pt and barium Ba supported on a carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0059]
In the diesel engine shown in FIG. 1, combustion is normally performed with the air-fuel ratio in the combustion chamber 5 being lean. When the combustion is performed in such a manner that the air-fuel ratio is lean, the oxygen concentration in the exhaust gas is high. At this time, as shown in FIG.₂Is O₂ ⁻Or O^2-On the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas becomes O 2 on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). NO generated next₂Is absorbed in the absorbent while being oxidized on the platinum Pt and combined with barium oxide BaO, and as shown in FIG.₃ ⁻Diffuses into the absorbent in the form of NO in this way_xIs NO_xAbsorbed in the absorbent. NO on the surface of platinum Pt as long as the oxygen concentration in the inflowing exhaust gas is high₂Is generated, and the NO_xNO unless absorption capacity is saturated₂Is absorbed in the absorbent and nitrate ion NO₃ ⁻Is generated.
[0060]
On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas decreases, and as a result, NO on the surface of the platinum Pt is reduced.₂Is reduced. NO₂The reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus the nitrate ions NO in the absorbent₃ ⁻Is NO₂Released from the absorbent in the form of NO at this time_xNO released from absorbent_x18B reacts with a large amount of unburned HC and CO contained in the inflowing exhaust gas to be reduced as shown in FIG. Thus, NO on the surface of platinum Pt₂When no longer exists, NO is changed from one absorbent to the next₂Is released. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, NO_xNO from absorbent_xIs released, and the released NO_xNO in the atmosphere to be reduced_xIs not emitted.
[0061]
In this case, even if the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO_xNO from absorbent_xIs released. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO_xNO from absorbent_xIs released only slowly_xTotal NO absorbed by absorbent_xIt takes a slightly longer time to release.
[0062]
NO as described above_xThe absorbent contains a noble metal such as platinum Pt and therefore NO_xThe absorbent has an oxidizing function. On the other hand, as described above, when the operating state of the engine is in the first operating region I and low-temperature combustion is being performed, soot is hardly generated, and instead, the unburned hydrocarbon is a precursor of soot or a former soot. It is discharged from the combustion chamber 5 in the form of a state. However, as described above, NO_xThe absorbent has an oxidizing function, and the unburned hydrocarbon discharged from the combustion chamber 5 at this time is NO._xIt can be oxidized well by the absorbent.
[0063]
By the way, NO_xAbsorbent NO_xAbsorption capacity is limited, NO_xAbsorbent NO_xNO before absorption capacity is saturated_xNO from absorbent_xMust be released. NO for that_xNO absorbed by absorbent_xThe quantity needs to be estimated. Therefore, in this embodiment, NO per unit time during the first combustion is performed._xThe amount of absorption A is determined in advance in the form of a map as shown in FIG. 20A as a function of the required load L and the engine speed N, and the NO per unit time during the second combustion is performed._xThe absorption amount B is obtained in advance as a function of the required load L and the engine speed N in the form of a map as shown in FIG._xNO by integrating absorption amounts A and B_xNO absorbed by absorbent_xThe amount ΣNOX is estimated.
[0064]
In the fourth embodiment, this NO_xNO when the absorption amount ΣNOX exceeds a predetermined allowable maximum value MAX._xNO from absorbent_xIs to be released. That is, when low-temperature combustion is being performed, NO_xWhen the absorption amount ΣNOX exceeds the permissible maximum value MAX, the air-fuel ratio in the combustion chamber 5 is temporarily made rich, whereby the NO_xNO from absorbent 19_xIs released. As described above, even when the air-fuel ratio is made rich during low-temperature combustion, almost no soot is generated.
[0065]
On the other hand, when the second combustion is being performed, NO_xWhen the absorption amount ΣNOX exceeds the allowable maximum value MAX, additional fuel Qa is injected when the exhaust valve 9 is opened as shown in FIGS. The amount of this additional fuel Qa is NO_xThe air-fuel ratio of the exhaust gas flowing into the absorbent is set to be rich, so that when additional fuel Qa is injected, NO_xNO from absorbent_xWill be released.
[0066]
As described above, when the first combustion is performed, little soot is generated, but when the second combustion is performed, a relatively large amount of soot is still generated. This soot is discharged from the combustion chamber 5 as particulate P together with the soluble organic component (SOF), and as shown in FIG. 18A, the wall-flow type carrier of the catalytic converter 20 due to the adhesion of the SOF. Adhere to the exhaust upstream surface of the partition wall 20a.
[0067]
Since the main component of the particulates is soot, that is, carbon, if HC and CO in the exhaust gas are actively oxidized by the oxidizing function of the catalytic converter 20, a large amount of heat of reaction is generated to generate particulates. Can be completely burned out.
[0068]
However, on the exhaust gas upstream side of the partition wall 20a in the carrier of the catalytic converter, a sufficient oxidizing function cannot be exerted due to the adhesion of the particulates by the SOF, that is, the SOF is partially poisoned, Even if a relatively large amount of HC and CO is discharged from the combustion chamber 5 during combustion, an active oxidation reaction cannot be caused. In this way, every time the second combustion is performed, the particulates that have not been completely burned are gradually deposited on the catalytic converter 20, and finally, the exhaust resistance is considerably increased. appear.
[0069]
In the present embodiment, in order to solve this problem, when the first combustion is being performed, the switching valve 19 is switched so that the exhaust upstream side and the exhaust downstream side of the catalytic converter 20 are reversed. I have. That is, in order to allow exhaust gas to flow in from one side of the catalytic converter and burn off the particulates collected in the catalytic converter, exhaust gas containing a reducing component is caused to flow in from the opposite side of the catalytic converter. Thereby, as shown in FIG. 18B, when the exhaust gas passes through the partition wall 20a of the carrier of the catalytic converter 20, the exhaust gas flows into the partition wall 20a from the side where the particulate P is not attached. Since no SOF poisoning has occurred on this side of the partition wall 20a, a relatively large amount of HC and CO generated during the first combustion is actively oxidized, and a large amount of reaction heat is generated. The thickness of the partition wall 20a is relatively thin, and this large amount of reaction heat satisfactorily transfers heat to sufficiently burn out the particulates P attached to the opposite side of the partition wall. Thus, at the time of the first combustion, the particulates P adhering to the partition wall 20a are completely burned off, and the problem of increasing the exhaust resistance does not occur.
[0070]
Incidentally, the particulates also contain oxides and sulfides such as calcium and phosphorus, which are combustion products of engine oil that has entered the cylinders. Since these are very difficult to burn, even if the particulates are burned off, they remain on the partition walls 20a as ash. If this is left unchecked, it becomes a solid mass equal to or larger than the particulates, and the exhaust resistance of the catalytic converter 20 increases. In the present embodiment, the ash generated by the burning out of the particulates is discharged from the catalytic converter 20 by the exhaust gas that passes through the partition 20a in the reverse direction, so that the ash does not form a solid mass on the partition 20a.
[0071]
In the present embodiment, the valve element 19a of the switching valve 19 is set to a shut-off position indicated by a dashed line in FIG. 1 during low-temperature combustion, and to a shut-off position indicated by a solid line in FIG. 1 during normal combustion, or vice versa. However, the switching valve 19 may not be switched when switching from low-temperature combustion to normal combustion, and the valve element 19a of the switching valve 19 may be switched only when switching from normal combustion to low-temperature combustion.
[0072]
In the present embodiment, a diesel engine capable of switching between low-temperature combustion and normal combustion is used as the internal combustion engine, but this does not limit the present invention. For example, in the case of a diesel engine that does not perform low-temperature combustion, NO_xAbsorbent is used and, as before, NO_xNO to absorbent_xWhen the absorption amount reaches the maximum allowable value, NO_xTo release and purify, that is, NO_xIn order to regenerate the absorbent, control is performed to make the air-fuel ratio of the exhaust gas rich. At this time, if the switching valve 19 is switched to reverse the exhaust upstream side and the exhaust downstream side of the catalytic converter, the exhaust gas containing the reducing substance flows into the partition wall from the side where the particulates do not adhere. A vigorous oxidation reaction is brought about, and the same effect as described above can be obtained.
[0073]
Such exhaust gas enrichment control is performed, for example, by enriching the combustion air-fuel ratio, injecting fuel into a cylinder in an exhaust stroke, or by supplying a reducing agent or fuel upstream of a catalytic converter in an engine exhaust system. Or supply. Needless to say, this enrichment control is performed by using NO in order to burn out the particulates._xThis may be performed regardless of the regeneration of the absorbent, and the upstream side and the downstream side of the exhaust of the catalytic converter may be reversed.
[0074]
In the case of a gasoline engine, a three-way catalytic converter is generally used as a catalytic converter. In this case, HC and CO are always contained in the exhaust gas. Accordingly, if the switching valve 19 is occasionally switched to reverse the exhaust upstream side and the exhaust downstream side of the catalytic converter, HC and CO can be obtained. Since the exhaust gas containing CO flows into the partition from the side where no particulates are attached, a vigorous oxidation reaction is brought about, and the same effect as described above can be obtained.
[0075]
Until now, the case where the particulates adhere to the wall-flow type catalytic converter has been described. However, the collision trapping particulate filter whose main purpose is to trap the particulates in the exhaust gas has pores. The exhaust gas is passed through the collecting wall to collect particulates. If the collecting wall carries an oxidation catalyst or the like and has an oxidizing function, the concept of the present invention can be applied. Thus, it is clear that such particulate filters are also included in the catalytic converter used herein. The wall flow type means that exhaust gas passes through pores provided in partition walls. In the present embodiment, the partition walls are arranged parallel to the exhaust system axis, but this does not limit the present invention. For example, the partition walls having pores may be perpendicular or oblique to the exhaust system axis. May be arranged.
[0076]
【The invention's effect】
As described above, according to the exhaust gas purifying method for an internal combustion engine according to the present invention, the wall-flow type catalytic converter having at least the oxidizing function is provided. Exhaust gas containing reducing substances was introduced from the opposite side of the catalytic converter to burn the burned particulates., The reducing substance is burned by the oxidizing function of the catalytic converter, and this combustion heat is used for burning out the particulatesIt has become.This internal combustion engine supplies inert gas to the cylinder and supplies an inert gas larger than the worst inert gas amount that maximizes soot generation into the cylinder and burns it. And a switching means for selectively switching between the first combustion for performing the combustion and the second combustion for performing the combustion by supplying an inert gas smaller than the worst inert gas amount into the cylinder, and performing the catalyst by the first combustion. An exhaust gas containing a reducing substance is generated for inflow from the opposite side of the converter.On the other side of the catalytic converter, since the SOF poisoning due to the adhesion of particulates has not occurred, the oxidation function of the catalytic converter can actively burn the reducing substances in the exhaust gas and generate a large amount of combustion heat. it can. Thus, the particulate matter collected on one side of the catalytic converter can be heated by the combustion heat to be surely burned off. Thus, a problem such as a decrease in engine output does not occur due to an increase in exhaust resistance of the catalytic converter due to accumulation of particulates.
[Brief description of the drawings]
FIG. 1 is a schematic longitudinal sectional view of a diesel engine provided with an exhaust gas purification device used in an exhaust gas purification method according to the present invention.
FIG. 2 is an enlarged vertical 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 lift and fuel injection of intake and exhaust valves.
FIG. 6: Smoke and NO_xFIG.
FIG. 7 is a diagram showing a combustion pressure.
FIG. 8 is a diagram showing fuel molecules.
FIG. 9 is a diagram showing a relationship between a generation amount of smoke and an EGR rate.
FIG. 10 is a diagram showing a relationship between an injected fuel amount and a mixed gas amount.
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.
FIG. 14 is a diagram showing an air-fuel ratio in a first operation region I.
FIG. 15 is a diagram showing a map of a target opening degree of a throttle valve and the like.
FIG. 16 is a diagram showing an air-fuel ratio in the second combustion.
FIG. 17 is a view showing a target opening degree of a throttle valve and the like.
FIGS. 18A and 18B are partial enlarged axial sectional views of the catalytic converter, wherein FIG. 18A shows the flow of exhaust gas at one shut-off position of the switching valve, and FIG. 18B shows the flow of exhaust gas at the other shut-off position of the switching valve; Shows the flow.
FIG. 19_xIt is a figure for explaining the absorption-release action of.
FIG. 20: NO per unit time_xIt is a figure showing a map of an amount of absorption.
[Explanation of symbols]
6 ... Fuel injection valve
16 ... Throttle valve
19 ... Switching valve
20: catalytic converter

Claims (1)

A catalytic converter of a wall flow type having at least an oxidizing function, and the other side of the catalytic converter in order to allow exhaust gas to flow in from one side of the catalytic converter and burn off particulates collected in the catalytic converter. Exhaust gas containing a reducing substance flows from the catalytic converter, and the reducing substance is burned by the oxidizing function of the catalytic converter, and the combustion heat is used for burning out the particulates. An inert gas supply means for supplying the inert gas, a first combustion for supplying a larger amount of the inert gas to the cylinder than the worst inert gas amount for maximizing the generation amount of soot into the cylinder, and performing the combustion, Switching means for selectively switching between a second combustion in which the inert gas is supplied into the cylinder in a smaller amount than the amount of the inert gas to perform the combustion, and Exhaust gas purification method for an internal combustion engine, characterized in that to generate the exhaust gas containing the reducing agent for flowing from the opposite side of the catalytic converter by a combustion.

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