JP2001082128A - Exhaust gas purification method for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying method for an internal combustion engine which can easily and surely burn off particulates adhering to a wall-flow type catalytic converter having at least an oxidizing function. SOLUTION: Exhaust gas containing a reducing substance is introduced from the opposite side of the catalytic converter in order to allow exhaust gas to flow from one side of the catalytic converter 20 to burn out particulates collected by the catalytic converter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art A catalytic converter for purifying exhaust gas by converting harmful substances such as hydrocarbon HC and carbon monoxide contained in the exhaust gas into harmless substances is disposed in an exhaust system of an internal combustion engine. Have been. The catalytic converter is one in which 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, and the exhaust gas passes through these axial spaces as it passes through each of the partitions. It is intended to be in contact with a catalyst supported on

However, the exhaust gas passing through the center of the carrier in each axial space does not come into contact with the partition walls, that is, the catalyst, and is discharged from the catalytic converter without being purified. To this end, 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 carried on the partition, so that the purification rate can be improved.

[0004] By the way, the exhaust gas of an internal combustion engine, in particular, 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 particulates is soot, ie, carbon, if the oxidation reaction of hydrocarbons HC and carbon monoxide CO in the exhaust gas occurs actively in the oxidation catalyst of the catalytic converter, The particulates can be easily burned off by the reaction heat.

[0006]

However, a part of the oxidation catalyst carried 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 the adhesion of the particulates by the SOF. , Are poisoned by SOF and 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.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for purifying exhaust gas of an internal combustion engine that can easily and reliably burn off particulates adhering to a wall-flow type catalytic converter having at least an oxidizing function.

[0008]

According to a first aspect of the present invention, there is provided a method for purifying exhaust gas of an internal combustion engine, comprising a wall-flow type catalytic converter having at least an oxidizing function, wherein exhaust gas is exhausted from one side of the catalytic converter. An exhaust gas containing a reducing substance is introduced from the opposite side of the catalytic converter in order to allow gas to flow in and burn out the particulates collected in the catalytic converter.

According to a second aspect of the present invention, there is provided the exhaust gas purifying method for an internal combustion engine according to the first aspect, wherein the reduction flowed from the opposite side of the catalytic converter is provided. A substance is burned by the oxidizing function of the catalytic converter, and the heat of combustion is used for burning out the particulates.

According to a third aspect of the present invention, there is provided a method for purifying an exhaust gas of an internal combustion engine according to the first or second aspect.
An inert gas supply means for supplying an inert gas into the cylinder; and a second means for supplying a larger amount of the inert gas to the cylinder than the worst inert gas amount that maximizes the amount of generated soot to perform combustion. Switching means for selectively switching between one combustion and a second combustion in which a smaller amount of the inert gas than the worst inert gas is supplied into the cylinder to perform the combustion; The method is characterized in that exhaust gas containing the reducing substance is generated for flowing in from the opposite side of the converter.

[0011]

FIG. 1 is a schematic longitudinal sectional view of a four-stroke diesel engine provided with an exhaust gas purifying apparatus used in an exhaust gas purifying method according to the present invention, and FIG. 2 shows combustion of the diesel engine shown in FIG. FIG. 3 is an enlarged vertical sectional view of the chamber, and FIG. 3 is a bottom view of a 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
Is a piston, 5a is a cavity formed on the top surface of the piston 4, 5 is a combustion chamber formed in the cavity 5a,
Reference numeral 6 denotes an electrically controlled fuel injection valve, 7 denotes a pair of intake valves, 8 denotes an intake port, 9 denotes a pair of exhaust valves, and 10 denotes 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.

As shown in FIG. 1, the exhaust manifold 17
Inside, an air-fuel ratio sensor 21 is arranged. The exhaust manifold 17 and the surge tank 12 are connected to each other via an EGR passage 22, and an electrically controlled EGR is provided in the EGR passage 22.
A control valve 23 is arranged. A cooling device 24 for cooling the EGR gas flowing in the EGR passage 22 is arranged 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.

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 a fuel pump 27 of an electrically controlled 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 provided 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.

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, for example, 30 ° is also input. Thus, the electronic control unit 30 controls the fuel injection valve 6, the electric motor 15, the EGR control valve 2 based on various signals.
3. Operate the fuel pump 27 and the switching valve 19 arranged in the exhaust pipe 18. This switching valve 19 will be described later.

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 openings, and the nozzle opening of the fuel injection valve 6 is slightly downwardly directed to the horizontal plane. Are injected at equal angular 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. FIGS. 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.

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 is directed between the back surface of the bulk 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 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.

Usually, no additional fuel Qa is injected,
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 , C
O, it represents an experimental example illustrating changes in emissions of NO _x.
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 stoichiometric air-fuel ratio (≒ 1
At 4.6 or less, the EGR rate is 65% or more.

As shown in FIG. 6, when the air-fuel ratio A / F is reduced by increasing the EGR rate, the smoke is reduced when the EGR rate becomes close to 40% and the air-fuel ratio A / F becomes about 30. The generation starts to increase. 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 is around 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 N
The generation amount of O _x is considerably reduced. On the other hand, at this time, the generation amounts of HC and CO begin to increase.

FIG. 7 (A) 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. 7 (B) shows the air-fuel ratio A / F. The graph shows the change in the combustion pressure in the combustion chamber 5 when the smoke generation amount is substantially zero when F is around 18. As can be seen by comparing FIG. 7A and FIG. 7B, in the case of FIG. 7B where the amount of smoke generation is almost zero, FIG.
It can be seen that the combustion pressure is lower than in the case shown in (A).

The following can be said from the experimental results shown in FIGS. That is, first, the air-fuel ratio A / F is 1
FIG. 6 when the amount of generated smoke is almost zero at 5.0 or less.
As shown in ₍₁₎ , the generation amount of NOx is considerably reduced. N
That the generation amount of O _x produced falls means that the combustion temperature in the combustion chamber 5 is reduced, thus the combustion temperature in the combustion chamber 5 becomes low when the soot is hardly generated I can say. 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.
The combustion temperature inside is low.

Second, when the amount of generated smoke, that is, the amount of generated soot becomes almost zero, as shown in FIG.
Emissions increase. 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, so that a precursor of soot is formed. 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. .

Summarizing these considerations based on the experimental results shown in FIGS. 6 and 7, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generation becomes almost zero. Is discharged from the combustion chamber 5. As a result of further detailed experimental study on this, if 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, that is, the soot is 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.

The temperature of the fuel and its surroundings when the process of producing hydrocarbons is stopped in the state of the soot precursor, ie, the above-mentioned certain temperature, depends on various factors such as the type of fuel, the air-fuel ratio and the compression ratio. Although the change can not be said that how many times since this certain temperature has a generation amount and the closely related of the nO _x, therefore this certain temperature is defined to a certain degree from the generation amount of the nO _x be able to. That is, the fuel and the gas temperature surrounding it at the time of combustion and the greater the EGR rate, decreases, the amount of the NO _x is reduced. Generation amount at this time NO _x is soot is hardly generated when it is around or less 10 ppm. Therefore, the above certain temperature is NO
It almost coincides with the temperature when the amount of generated _x is about 10 p.pm or less.

Once soot has been 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 a state before the soot can be easily purified by a post-treatment using a catalyst having an oxidation function. Thus, it is discharged from the combustion chamber 5 hydrocarbons in the precursor or its previous state of the soot while reducing the generation amount of the NO _x is extremely effective for purification of exhaust gas.

Now, in order to stop the growth of hydrocarbons before the soot is generated, the temperature of the fuel and the surrounding gas during combustion in the combustion chamber 5 is set to a temperature lower than the temperature at which the soot is generated. It needs to be suppressed. 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.

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.

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 fuel vapor diffuses to the surroundings, reacts with oxygen mixed in the inert gas, and burns. In this case, the combustion temperature is not increased so much because the combustion heat is absorbed by the surrounding inert gas. 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.

In this case, in order to suppress the temperature of the fuel and its surrounding gas to a temperature lower than the temperature at which the soot is formed, an amount of the inert gas that can absorb a sufficient amount of heat to do so is required. . Therefore, if the fuel amount 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, and therefore, the inert gas preferably has a higher specific heat. In this regard, it can be said that it is preferable to use EGR gas as the inert gas since CO ₂ and EGR gas have relatively large specific heats.

FIG. 9 shows the relationship between the EGR rate and the smoke when 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 that the EGR gas is cooled
Curve B shows the case where the EGR gas is cooled by a small cooling device, and curve C shows the case where the EGR gas is not forcibly cooled.

As shown by the curve A in FIG. 9, when the EGR gas is strongly cooled, the amount of soot generation reaches a peak when the EGR rate is slightly lower than 50%. In this case, the EGR rate is reduced to approximately 55%. Above a percentage, little 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. If so, almost no soot is generated.

As shown by a curve C in FIG.
When the R gas is not forcibly cooled, the EGR rate becomes 5
The soot generation amount peaks near 5%, and in this case, if the EGR rate is set to approximately 70% or more, 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.

FIG. 10 shows a mixture of air and EGR gas required to make the temperature of the fuel during combustion and the surrounding gas lower than the temperature at which soot is generated when EGR gas is used as the inert gas. It shows the gas amount, the ratio of air in the mixed gas amount, and the ratio of EGR gas in the mixed gas. In FIG. 10, the vertical axis indicates the total intake gas amount drawn into the combustion chamber 5, and the dashed 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.

Referring to FIG. 10, the proportion of air, that is, the amount of air in the mixed gas, indicates the amount of air required 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 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. Minimum EGR required
Shows the gas volume. 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.
When the ratio between the air amount and the EGR gas amount in the total intake gas amount X is set as shown in FIG. 10, the temperature of the fuel and the surrounding gas becomes lower than the temperature at which soot is generated. No soot is generated. Further, the amount of generated NO _{x at} this time is about 10 p.pm or less, and therefore, the amount of generated NO _x is extremely small.

If the fuel injection amount 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 the soot is generated, the heat generated by the EGR gas is required. 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.

On the other hand, in the load region Z2 of FIG. 10, the total intake gas amount X required to prevent 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, both the EGR gas and the intake air, or E
It is necessary to supercharge or pressurize the GR gas. 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.

As described above, FIG.
FIG. 10 shows a case where combustion is performed under
FIG. 10 shows the air amount in the low load operation region Z1.
Even if it is less than the amount of air
NO while preventing soot generation _x10p.p.m before
Later or less, and as shown in FIG.
In the low load region Z1, the amount of air
Even if it is larger than the air volume, that is, the average value of the air-fuel ratio is increased from 17
Even with a lean of 18, NO while preventing the generation of soot_xDeparture
The production amount can be around 10p.p.m or less
You.

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, thus producing soot. There is no. Further, at this time NO _x even only an extremely small amount of generated. On the other hand, when the average air-fuel ratio is lean, or even when the air-fuel ratio is the stoichiometric air-fuel ratio, a small amount of soot is generated if the combustion temperature 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 _x
Only very small amounts are generated.

As described above, in the engine low load operation region Z1, soot is generated 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. not, the amount of the NO _x becomes 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.

By the way, the temperature of the fuel and its surrounding gas during combustion in the combustion chamber can be suppressed to a temperature lower than the temperature at which the growth of hydrocarbons stops halfway, only when the calorific value due to combustion is small and the engine load is relatively low. Can be 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. When the engine load is relatively high, the second combustion, that is, the combustion that is conventionally performed normally, 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 amount of soot is maximized, 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

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. The vertical axis L in FIG.
, 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. First operating region I
Is determined based on the first boundary X (N), and the change determination of the operation region from the second operation region II to the first operation region I is performed based on the first boundary X (N). The determination is performed based on the second boundary Y (N).

That is, when the operating state of the engine is in the first operating region I
When the required load L exceeds a first boundary X (N), which is a function of the engine speed N, during low-temperature combustion, it is determined that the operation region has shifted to the second operation region II, Combustion is performed by a 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.

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.

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 degree 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 is performed.
The opening of the control valve 23 is gradually increased from near full closure to full opening as the required load L increases. FIG.
In the example shown in FIG. 3, in the first operating region I, the EGR rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio.

In other words, in the first operation region I, the EGR
The opening of the throttle valve 16 and the opening of the EGR control valve 23 are controlled such that the 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.

During the idling operation, the throttle valve 16 is closed until the valve is almost fully closed. At this time, the EGR control valve 23 is also closed almost completely. Throttle valve 1
When the valve 6 is closed close to the fully closed state, 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.

On the other hand, the operating region of the engine is the first operating region I.
From 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.
The air-fuel ratio is reduced in steps from 40 percent to less than 40 percent, and the air-fuel ratio is increased in steps. 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.

In the second operation region II, the conventional combustion is performed. Thermal efficiency is high, therefore the engine the second operating region operating region from the first operating region I in comparison to but soot and NO _x occurs slightly low temperature combustion in the combustion process
When the state changes to II, the injection amount is reduced stepwise as shown in FIG.

In the second operating region II, the throttle valve 16 is maintained 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.

FIG. 14 shows the air-fuel ratio A / F in the first operating region I. In FIG. 14, A / F = 15.
5, the curves indicated by 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 the air-fuel ratio A / F is leaner in the first operating region I as the required load L decreases.

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 decreases. When the EGR rate is reduced, the air-fuel ratio increases. Therefore, 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 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.

The target opening degree ST of the throttle valve 16 necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG.
As shown in FIG. 5 (A), a map is stored in advance in the ROM 32 as a function of the required load L and the engine speed N, and the EGR necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG. The target opening SE of the control valve 23 is as shown in FIG.
As shown in (1), it 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.

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, A / F = 24 and A / F = 3.
Curves indicated by 5, A / F = 45, and A / F = 60 indicate target air-fuel ratios 24, 35, 45, and 60, respectively. Throttle valve 1 required to set air-fuel ratio to this target air-fuel ratio
The target opening ST of No. 6 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 as shown in FIG. 17 (A), and the air-fuel ratio is set as this target air-fuel ratio. Opening SE of EGR control valve 23 required for
Are 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 as shown in FIG.

Thus, in the diesel engine of this embodiment, the first combustion, that is, the low temperature combustion, and the second combustion, that is, the normal combustion, are performed based on the depression amount L of the accelerator pedal 40 and the engine speed N. The switching is performed, and 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 according to the map shown in FIG. 15 or FIG.

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. The switching valve 19 has a C-shaped exhaust passage 18a.
Is connected to the catalytic converter 2 in the exhaust passage 18a.
0 is arranged. 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. Valve element 19 indicated by solid line
1, the 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. 19 flows downstream.

At the other shut-off position of the valve element 19a indicated by the dashed line, the exhaust gas flows from the exhaust pipe 18 upstream of the switching valve 19 of the exhaust pipe 18 as indicated by the hatched arrow.
Flows clockwise in FIG. 1 and flows into the exhaust pipe 18 downstream of the switching valve 19. By switching the switching valve 19 in this manner, 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.

As shown in the partially enlarged sectional view in the axial direction of FIG. 18, the carrier of the catalytic converter 20 has a plurality of axially extending partitions 20a formed of a porous material and a plurality of subdivided partitions formed by these partitions. An axial space 20b,
The wall flow type includes 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 entirely carried on each partition 20a of the carrier. In such a wall-flow type carrier, the exhaust gas must be supplied to the partition wall 20a.
In order to pass through, the purification rate can be improved in order to surely come into contact with the catalyst carried on the partition wall 20a.

In the present embodiment, the catalyst supported on the partition wall 20a is an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium. At least one selected from rare earths such as Y and a noble metal such as platinum Pt. This catalyst is N
_Ox absorber, referred to as the engine intake passage, combustion chamber 5 and N
When the ratio of O _x absorbent the exhaust passage upstream of the supply air and fuel into the (hydrocarbon) is referred to as the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent, empty the _the NO _x absorbent inflow exhaust gas When the fuel ratio is lean, NO _x is absorbed, and when the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich, the absorbed N
The O _x performs the absorbing and releasing action of NO _x to be released.

This NO_xAbsorbent placed in engine exhaust passage
NO_xThe absorbent is actually NO _xPerform the absorption and release action of
However, the detailed mechanism of this absorption / release effect is clarified.
Some parts are not clear. However, this absorption / release action
It is assumed that the operation is performed by the mechanism shown in FIG.
available. Next, regarding this mechanism, platinum P
The case where t and barium Ba are supported is explained as an example.
Other precious metals, alkali metals, alkaline earth, rare
The same mechanism can be obtained by using earth.

In the diesel engine shown in FIG.
Normally, combustion is performed with the air-fuel ratio in the combustion chamber 5 being lean. When the combustion is performed with the air-fuel ratio lean, the oxygen concentration in the exhaust gas is high. At this time, as shown in FIG.
₂ O ₂ ^- it is attached to or O the surface of the platinum Pt in ^2-form.
On the other hand, NO in the inflowing exhaust gas becomes O ₂ on the surface of platinum Pt.
^- or reacts with O ^2-, the _{NO 2 (2NO + O 2 →} 2
NO ₂ ). Next, a part of the generated NO ₂ is absorbed in the absorbent while being oxidized on platinum Pt, and barium oxide Ba is absorbed.
As shown in FIG. 18 (A), it combines with O and diffuses into the absorbent in the form of nitrate ion NO ₃ ⁻ . In this way, NO _x is absorbed in the NO _x absorbent. The oxygen concentration in the inflowing exhaust gas is NO ₂ on the surface of as high as platinum Pt is generated, as long as NO ₂ of absorption of NO _x capacity of the absorbent is not saturated
There is absorbed in the absorbent and nitrate ions NO ₃ ^- are produced.

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, the amount of NO ₂ generated on the surface of the platinum Pt decreases. The amount of NO ₂ is lowered and the reaction is reverse (NO ₃ ^- → NO ₂₎
Advances to, thus to nitrate ions in the absorbent NO ₃ ^- is NO
It is released from the absorbent in the form of ₂ . In this case _the NO _x absorbent NO _x released from caused to reduction by reaction with a large amount of unburned HC and CO contained in the inflowing exhaust gas as shown in FIG. 18 (B). In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO _{x
Since x} is released, and the released NO _x is reduced, NO _x is not discharged into the atmosphere.

[0061] In this case, NO _x is released from _the NO _x absorbent and the air-fuel ratio of the inflowing exhaust gas the stoichiometric air-fuel ratio. All NO _x, however that in the case where the air-fuel ratio of the inflowing exhaust gas the stoichiometric air-fuel ratio is absorbed in _the NO _x absorbent to NO _x is not only released gradually from _the NO _x absorbent
It takes a slightly longer time to release.

As described above, the NO _x absorbent contains a noble metal such as platinum Pt, and therefore, the NO _x absorbent has an oxidizing function. On the other hand, as described above, when the operation state of the engine is in the first operation 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 soot. It is discharged from the combustion chamber 5 in the form of a state. However _the NO _x absorbent as described above has an oxidation function, therefore unburned hydrocarbon discharged from the combustion chamber 5 at this time will be induced to better oxidized by _the NO _x absorbent.

By the way, NO_xAbsorbent NO_xTo the absorption capacity
Has a limit, NO_xAbsorbent NO _xAbsorption capacity saturates
NO before_xNO from absorbent_xNeed to be released
You. NO for that_xNO absorbed by absorbent_x
The quantity needs to be estimated. Therefore, in this embodiment, the first combustion
Per unit time when is performed_xAbsorption amount A
FIG. 20 as a function of required load L and engine speed N
It is obtained in advance in the form of a map as shown in FIG.
NO per unit time during combustion_xAbsorption
B as a function of required load L and engine speed N
This is obtained in advance in the form of a map as shown in FIG.
NO per unit time_xTo integrate the absorption amounts A and B
Therefore NO_xNO absorbed by absorbent_xQuantityΣNOX
Is to be estimated.

[0064] This Example 4 is NO _x when exceeding the allowable maximum value MAX of the absorption of NO _x amount ΣNOX reaches a predetermined
And so as to release the NO _x from the absorbent. That is,
When absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX when the low temperature combustion is being performed is the air-fuel ratio is temporarily made rich in the combustion chamber 5, whereby the NO _x from the absorbent 19 NO _x release Is done. As described above, even when the air-fuel ratio is made rich during low-temperature combustion, almost no soot is generated.

On the other hand, when the second combustion is being performed, NO
_{When the x} 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 the additional fuel Qa is determined so that the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent becomes rich, thus additional fuel Qa when is injected _the NO _x absorbent from the NO _x release Will be done.

As described above, when the first combustion is performed, almost no 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 a particulate P together with a soluble organic component (SOF),
As shown in FIG. 18A, the SOF adheres to the exhaust upstream surface of the partition wall 20a of the wall flow type carrier of the catalytic converter 20, as shown in FIG.

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 reaction heat is generated. , Can completely burn out the particulates.

However, the upstream side of the exhaust of the partition wall 20a in the carrier of the catalytic converter is in a state where a sufficient oxidizing function cannot be exhibited 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 the first 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 but gradually deposited on the catalytic converter 20, and finally, the exhaust resistance is considerably increased, and problems such as a decrease in engine output are caused. appear.

In the present embodiment, in order to solve this problem, the switching valve 19 is switched during the first combustion to reverse the exhaust upstream side and the exhaust downstream side of the catalytic converter 20. It has become. That is, in order to allow the exhaust gas to flow from one side of the catalytic converter and burn out the particulates collected by the catalytic converter,
The exhaust gas containing the reducing component is caused to flow in from the opposite side of the catalytic converter. As a result, FIG.
As shown in (B), the exhaust gas is supplied to the catalytic converter 20.
When passing through the partition 20a of the carrier, the particulate P flows into the partition 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 is sufficiently transferred to sufficiently burn out the particulate 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.

By the way, the particulates also contain oxides and sulfides such as calcium and phosphorus, which are combustion products of the engine oil that has entered the cylinders.
Since these are very hard to burn, even if the particulates are burned off, the partition walls 20 are still ashed.
a. If this is left unchecked, it becomes a solid mass equal to or larger than 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 wall 20a in the reverse direction, and therefore does not form a firm mass on the partition wall 20a.

In this embodiment, the valve body 19 of the switching valve 19
a is a cut-off position indicated by a dashed line in FIG. 1 during low-temperature combustion, and a cut-off position indicated by a solid line in FIG. 1 during normal combustion, or vice versa. The valve 19a of the switching valve 19 may be switched only when switching from normal combustion to low temperature combustion without switching the switching valve 19 when switching to combustion.

In this embodiment, a low-temperature combustion and
Uses a diesel engine that switches between normal combustion
However, this is not a limitation of the present invention. example
For example, if a diesel engine does not perform low-temperature combustion,
Represents NO as a catalytic converter_xAbsorbent material is used,
As before, NO_xNO to absorbent_xMaximum absorption capacity
When the value is reached, NO_xTo release and purify
In other words, NO _xExhaust to regenerate absorber
Control for enriching the air-fuel ratio of the gas is performed. This and
The switching valve 19 is switched to release the exhaust gas from the catalytic converter.
By reversing the flow side and the exhaust downstream side, exhaust containing reducing substances
Gas gas is separated from the side where no particulates adhere.
Due to the flow into the wall, a vigorous oxidation reaction results,
The same effect as described above can be obtained.

Such exhaust gas enrichment control includes, for example, enriching the combustion air-fuel ratio, injecting fuel into the cylinder during the exhaust stroke, or reducing the reducing agent upstream of the catalytic converter in the engine exhaust system. Or, it is performed by supplying fuel. Of course, the enrichment control, in order to burn off the particulates, be carried out regardless of the reproduction of the NO _x absorbent, may be to reverse the exhaust upstream of the catalytic converter and the exhaust downstream side.

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. Therefore, 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 wall from the side where the particulates are not attached, a vigorous oxidation reaction is brought about, and the same effect as described above can be obtained.

The case where particulates adhere to the wall-flow type catalytic converter has been described above.
The collision trapping particulate filter whose main purpose is to trap the particulates in the exhaust gas, the exhaust gas is passed through a trapping wall having pores, and the particulates are collected. The concept of the present invention can be applied if an oxidizing function is provided by supporting an oxidation catalyst or the like on the collecting wall. Thus, it is clear that such a particulate filter is also included in the catalytic converter used herein. Also, the wall flow type
This means that the exhaust gas passes through pores provided in the partition. 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]

As described above, according to the exhaust gas purifying method for an internal combustion engine according to the present invention, a wall flow type catalytic converter having at least an oxidizing function is provided, and the exhaust gas is introduced from one side of the catalytic converter. In order to burn out the particulate matter collected in the catalytic converter,
An exhaust gas containing a reducing substance flows from the opposite side of the catalytic converter. On the other side of the catalytic converter, since SOF poisoning due to the attachment of particulates has not occurred, the catalytic converter's oxidizing function can actively combust 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 is a diagram showing amounts of smoke and NO _x generated, and the like.

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 view 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.

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 is a view for explaining the NO _x absorption / release action.

FIG. 20 is a diagram showing a map of the NO _x absorption amount per unit time.

[Explanation of symbols]

 6: Fuel injection valve 16: Throttle valve 19: Switching valve 20: Catalytic converter

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F01N 3/08 F01N 3/24 E 3/24 ZABS ZAB F02D 21/08 301B F02D 21/08 301 F02M 25 / 07 570J F02M 25/07 570 B01D 53/36 103C

Claims (3)

[Claims] 1. A catalyst comprising a wall-flow type catalytic converter having at least an oxidizing function, wherein the exhaust gas flows from one side of the catalytic converter to burn out particulates collected by the catalytic converter. An exhaust gas purification method for an internal combustion engine, wherein exhaust gas containing a reducing substance is caused to flow in from the opposite side of a catalytic converter. 2. The method according to claim 1, wherein the reducing substance introduced from the opposite side of the catalytic converter is burned by the oxidizing function of the catalytic converter, and the heat of combustion is used for burning out the particulates. The exhaust gas purifying method for an internal combustion engine according to Claim 1. 3. An internal combustion engine, comprising: an inert gas supply unit for supplying an inert gas into a cylinder; and a cylinder that supplies a larger amount of the inert gas than the worst inert gas amount that maximizes soot generation. Switching means for selectively switching between the first combustion in which the combustion is performed by supplying the gas into the cylinder and the combustion in which the inert gas is supplied into the cylinder and the amount of the inert gas is smaller than the worst inert gas amount. The exhaust gas containing the reducing substance for flowing in from the opposite side of the catalytic converter is provided by the first combustion.
The exhaust gas purifying method for an internal combustion engine according to Claim 1.