JP3424573B2 - Internal combustion engine - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, in an internal combustion engine, for example, a diesel engine, an engine exhaust passage and an engine intake passage are connected by an exhaust gas recirculation (hereinafter referred to as EGR) passage in order to suppress the generation of NO _x. Exhaust gas, that is, EGR gas, is recirculated into the engine intake passage via the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount of heat.
The combustion temperature in the combustion chamber decreases as the GR gas amount increases, that is, as the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)) increases. When the combustion temperature decreases, the amount of NO _x generated decreases. Therefore, the higher the EGR rate, the lower the amount of NO _x generated.

As described above, it has been known that the amount of NO _x generated can be reduced by increasing the EGR rate. However, when the EGR rate is increased, when the EGR rate exceeds a certain limit, the amount of soot generated, that is, the smoke starts to increase rapidly. In this regard, it has been conventionally thought that if the EGR rate is further increased, the smoke will increase infinitely, and therefore the smoke will start to increase rapidly.
The GR rate is considered to be the maximum allowable limit for the EGR rate.

Therefore, conventionally, the EGR rate is set within a range that does not exceed the maximum allowable limit. The maximum allowable limit of this EGR rate is approximately 30 to 50 percent, though it varies considerably depending on the engine type and fuel.
Therefore, in the conventional diesel engine, the maximum EGR rate is 3
It is suppressed from 0% to 50%.

As described above, in the past, it was considered that the maximum allowable limit exists for the EGR rate.
The R rate is NO within the range that does not exceed this maximum allowable limit.
_It was stipulated that the amount of _x and smoke generated should be as small as possible. However, in this way EGR
Even if the rate is set so that the amount of NO _x and smoke produced is as small as possible, there is a limit to the reduction in the amount of NO _x and smoke produced, and in reality, a considerable amount of N 2 is still left.
The O _x, and smoke is generated at present.

However, if the EGR rate is made larger than the maximum permissible limit in the process of studying the combustion of a diesel engine, the smoke increases sharply as described above, but there is a peak in the amount of smoke produced, and the peak is exceeded. EGR
If the rate is further increased, the smoke will start to decrease sharply this time. If the EGR rate is set to 70% or more during idling, and if the EGR rate is set to 55% or more when the EGR gas is strongly cooled, the smoke will be reduced. It becomes almost zero. That is, it was found that soot is hardly generated. At this time, N
It has also been found that the amount of O _x generated is extremely small.
After that, the reason why soot was not generated was examined based on this finding, and as a result, soot and NO
_This led to the construction of a new combustion system capable of simultaneously reducing _x . This new combustion system will be explained in detail later, but in short, it is basically based on stopping the growth of hydrocarbons in the middle of the process until the hydrocarbons grow into soot.

[0007] That is, as a result of repeated experimental research, it was found that when the temperature of the fuel and the gas around it during combustion in the combustion chamber were below a certain temperature, the growth of hydrocarbons stopped in the middle of the process before reaching soot. However, if the temperature of the fuel and the gas around it rises above a certain temperature, the hydrocarbons will suddenly grow to soot. In this case, the temperature of the fuel and its surrounding gas is greatly affected by the endothermic action of the gas around the fuel when the fuel burns, and the endothermic amount of the gas around the fuel is adjusted according to the amount of heat generated during fuel combustion. Thus, the temperature of the fuel and the gas around it can be controlled.

Therefore, if the temperature of the fuel and the gas around it during combustion in the combustion chamber is suppressed below the temperature at which the growth of hydrocarbons stops halfway, soot will not be generated and the fuel and the surroundings during combustion in the combustion chamber will be eliminated. It is possible to control the gas temperature in the range below the temperature at which the growth of hydrocarbons stops halfway by adjusting the heat absorption amount of the gas around the fuel. On the other hand, hydrocarbons whose growth has stopped before reaching soot can be easily purified by post-treatment using an oxidation catalyst or the like. This is the basic idea of the new combustion system. The applicant has already applied for an internal combustion engine that employs this new combustion system (Japanese Patent Application No. 9-305850).

[0009]

By the way, in this new combustion system, the EGR rate needs to be approximately 55% or more. However, the EGR rate can be increased to approximately 55% or more when the intake air amount is relatively small, that is, when the engine load is relatively low. If the intake air amount exceeds a certain limit, the intake air amount is reduced unless the EGR rate is reduced. The amount of air cannot be increased. Therefore, when the intake air amount exceeds a certain limit, it is necessary to switch to the conventional combustion.

When the combustion is switched to the conventional one, a considerable amount of NO _x is released from the engine. Further, under the conventional combustion, the combustion is performed under excess air. Therefore, in the above internal combustion engine, when the air-fuel ratio of the inflowing exhaust gas is lean, it absorbs NO _x contained in the exhaust gas and when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich, it releases the absorbed NO _x . A NO _x absorbent is placed in the engine exhaust passage to prevent NO _x from being released into the atmosphere.

In the above internal combustion engine, when NO _x is to be released and reduced, the air-fuel ratio is made rich under new combustion,
The air-fuel ratio of the exhaust gas flowing into the NO _x absorbent is made rich, and NO _x is released and reduced. Since new combustion is performed with the stoichiometric air-fuel ratio or a slight lean, the amount of fuel to be increased in order to make the air-fuel ratio rich requires a very small amount.

[0012] However, even the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent when it should be reduced to release the NO _x trying the stoichiometric air-fuel ratio or rich, in fact the time deterioration of the engine components to do so In some cases, the air-fuel ratio of the exhaust gas may not be as stoichiometric or rich as desired. In this case, it is necessary to control so that the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent will be sure to become the stoichiometric air-fuel ratio or rich.

An object of the present invention is to control the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent so that it becomes rich when it should be rich.

[0014]

In order to achieve the above object, in the first invention, as the amount of the inert gas supplied into the combustion chamber is increased, the amount of soot generated gradually increases and reaches a peak. When the amount of the inert gas supplied to the combustion chamber is further increased, the temperature of the fuel during combustion in the combustion chamber and the gas temperature around it become lower than the soot generation temperature, and soot is hardly generated. In the above, when the air-fuel ratio of the inflowing exhaust gas is lean, N contained in the exhaust gas
NO _x to absorb O _x and the air-fuel ratio of the inflowing exhaust gas to release NO _x absorbed and becomes the stoichiometric air-fuel ratio or rich
The absorbent is placed in the exhaust passage of the engine, and the first combustion in which the amount of inert gas supplied to the combustion chamber is larger than the amount of inert gas at which the soot generation peaks and soot is hardly generated, and the soot A switching means is provided for selectively switching between the second combustion in which the amount of the inert gas supplied to the combustion chamber is smaller than the amount of the inert gas which produces a peak, and the air-fuel ratio of the exhaust gas in the engine exhaust passage. The air-fuel ratio detection means for detecting
_The NO _x absorbent air-fuel ratio detecting means the air-fuel ratio of the combustion chamber in order to release or after when a rich, NO _x from the when not detecting a rich corrected so as to be richer the air-fuel ratio in the combustion chamber. That is, when the air-fuel ratio detecting means does not detect rich, it is determined that the air-fuel ratio in the combustion chamber is not actually rich, and the air-fuel ratio in the combustion chamber is corrected to become richer. That is, for example, the injection fuel amount is corrected with respect to the intake air amount when the air-fuel ratio is set to the stoichiometric air-fuel ratio, and the rich degree of the air-fuel ratio is determined based on the corrected injection fuel amount.

According to a second aspect of the present invention, in the first aspect, the air-fuel ratio detecting means is arranged in the engine exhaust passage downstream of the NO _x absorbent, and the air-fuel ratio in the combustion chamber is made rich before the air-fuel ratio is increased. ratio detecting means detects the time until detecting a rich, NO from _the NO _x absorbent the longer said time
Try to delay when _x should be emitted. That is, the time from when the air-fuel ratio in the combustion chamber is rich until the air-fuel ratio detecting means detects rich is NO in the NO _x absorbent.
This is the period during which _x is released and reduced, and this NO _x
The longer the release / reduction period, the more N the NO _x absorbent can absorb.
Determining the amount of O _x is large, when releasing the NO _x from the NO _x absorbent, i.e. delaying the timing of the air-fuel ratio in the combustion chamber rich.

[0016] To delay the time the more third amount of exhaust gas flowing into the _the NO _x absorbent in the time in the second aspect according to the invention from _the NO _x absorbent should be released NO _x . 4 th invention to delay when the second to be released the NO _x greater the degree of richness of the air-fuel ratio of the absorbent to the inflowing exhaust gas _the NO _x absorbent from the NO _x in the time in the invention according To

According to a fifth aspect, in the first aspect, a recirculation device for recirculating exhaust gas discharged from the combustion chamber into the engine intake passage is provided, and the inert gas is recycled from the exhaust gas. Become. According to the sixth invention, in the fifth invention, the exhaust gas recirculation rate is about 55% or more.

According to a seventh aspect, in the first aspect, the operating region of the engine is divided into a first operating region on the low load side and a second operating region on the high load side, and in the first operating region, First combustion is performed, and second combustion is performed in the second operation region.

[0019]

FIG. 1 shows the case where the present invention is applied to a four-stroke compression ignition type internal combustion engine. Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, and 9 is an intake port. Indicates an exhaust valve, and 10 indicates an exhaust port, respectively. 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 via an intake duct 13 and an intercooler 14 to a supercharger, for example, an outlet portion of a compressor 16 of an exhaust turbocharger 15. Be connected. The inlet of the compressor 16 is connected to an air cleaner 18 via an air suction pipe 17, and a throttle valve 20 driven by a step motor 19 is arranged in the air suction pipe 17. In addition, the air suction pipe 17 upstream of the throttle valve 20.
A mass flow rate detector 21 for detecting the mass flow rate of the intake air is arranged therein.

On the other hand, the exhaust port 10 is connected to the exhaust manifold 2
Exhaust turbine 2 of exhaust turbocharger 15 via 2
3 and the outlet of the exhaust turbine 23 is connected via an exhaust pipe 24 to a catalytic converter 26 containing a catalyst 25 having an oxidizing function. Exhaust manifold 22
An air-fuel ratio sensor 27 is arranged inside. The exhaust pipe 28 connected to the outlet of the catalytic converter 26 and the throttle valve 2
Exhaust gas recirculation (hereinafter referred to as E
(Referred to as GR) are connected to each other via a passage 29,
In the passage 29, E driven by a step motor 30
The GR control valve 31 is arranged. An intercooler 32 for cooling the EGR gas flowing in the EGR passage 29 is arranged in the EGR passage 29. In the embodiment shown in FIG. 1, the engine cooling water is guided into the intercooler 32, and the engine cooling water cools the EGR gas.

On the other hand, the fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 34, via a fuel supply pipe 33. Fuel is supplied into the common rail 34 from an electrically controlled variable discharge fuel pump 35, and the fuel supplied into the common rail 34 is supplied to the fuel injection valve 6 via each fuel supply pipe 33. A fuel pressure sensor 36 for detecting a fuel pressure in the common rail 34 is attached to the common rail 34, and a fuel pump 35 is arranged so that the fuel pressure in the common rail 34 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 36. Is controlled.

The electronic control unit 40 is composed of a digital computer, and has a ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an input port 45 which are connected to each other by a bidirectional bus 41. The output port 46 is provided. The output signal of the mass flow rate detector 21 is input to the input port 45 via the corresponding AD converter 47, and the output signals of the air-fuel ratio sensor 27 and the fuel pressure sensor 36 are also input ports via the corresponding AD converter 47. 45 is input. A load sensor 51 that generates an output voltage proportional to the depression amount L of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. . Also,
The input port 45 has a crank angle sensor 52 that generates an output pulse each time the crankshaft rotates, for example, 30 °.
Are connected. On the other hand, the output port 46 is connected via the corresponding drive circuit 48 to the fuel injection valve 6, the throttle valve control step motor 19, and the EGR control valve control step motor 3.
0 and the fuel pump 35.

FIG. 2 shows the throttle valve 2 during engine low load operation.
The change in output torque when the air-fuel ratio A / F (horizontal axis in FIG. 2) is changed by changing the opening degree of 0 and the EGR rate, and the change in the amount of smoke, HC, CO, and NO _x emissions. It represents the experimental example shown. As can be seen from FIG. 2, in this experimental example, the EGR rate becomes larger as the air-fuel ratio A / F becomes smaller, and when the air-fuel ratio is equal to or less than the theoretical air-fuel ratio (≈14.6), the EGR rate becomes 65% or more.

As shown in FIG. 2, when the air-fuel ratio A / F is reduced by increasing the EGR rate, the EGR rate becomes around 40%, and when the air-fuel ratio A / F becomes about 30, the smoke of The amount of generation begins to increase. Next, when the EGR rate is further increased and the air-fuel ratio A / F is made smaller, 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 sharply decreases this time, the EGR rate is increased to 65% or more, and the smoke becomes almost zero when the air-fuel ratio A / F is around 15.0. . That is, soot is hardly generated. At this time, the output torque of the engine slightly decreases, and N
The amount of O _x generated is considerably low. On the other hand, at this time, HC,
The amount of CO generated starts to increase.

FIG. 3 (A) shows the change in combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is around 21 and the amount of smoke generated is the largest, and FIG. 3 (B) shows the air-fuel ratio A / F. It shows a change in the combustion pressure in the combustion chamber 5 when F is around 18 and the amount of smoke generated is almost zero. As can be seen by comparing FIG. 3 (A) and FIG. 3 (B), in the case shown in FIG. 3 (B) where the amount of smoke generated is almost zero, the amount of smoke generated is large.
It can be seen that the combustion pressure is lower than in the case shown in (A).

From the experimental results shown in FIGS. 2 and 3, the following can be said. That is, first of all, the air-fuel ratio A / F is 1
When the amount of smoke generated is 5.0 or less and the amount of smoke is almost zero,
As shown in (3), the amount of NO _x generated is considerably reduced. N
The decrease in the amount of generated O _x means that the combustion temperature in the combustion chamber 5 is decreased, and therefore, when the soot is hardly generated, the combustion temperature in the combustion chamber 5 is decreased. I can say. The same can be said from FIG. That is, the combustion pressure is low in the state shown in FIG. 3 (B) where almost no soot is generated.
The combustion temperature inside is low.

When the amount of the second smoke, that is, the amount of the soot, becomes almost zero, as shown in FIG. 2, HC and CO
Emissions will increase. This means that hydrocarbons are discharged without growing to soot. That is, linear hydrocarbons and aromatic hydrocarbons contained in the fuel as shown in FIG. 4 are thermally decomposed to form soot precursors when the temperature is raised in a state of oxygen deficiency, and then mainly soot is formed. Soot consisting of a solid with carbon atoms gathered is produced. In this case, the actual soot production process is complicated, and it is not clear what form the soot precursor takes, but in any case, the hydrocarbon as shown in FIG. After that, it will grow to soot. Therefore, as described above, when the amount of soot generated becomes almost zero, the emission amounts of HC and CO increase as shown in FIG. 2. At this time, HC is a soot precursor or a hydrocarbon in the state before it. .

Summarizing these considerations based on the experimental results shown in FIGS. 2 and 3, when the combustion temperature in the combustion chamber 5 is low, the soot generation amount becomes almost zero, and at this time, the soot precursor or the soot precursor The hydrocarbons in this state are discharged from the combustion chamber 5. As a result of further detailed experimental research on this, when the temperature of the fuel and the gas around it in the combustion chamber 5 is below a certain temperature, the soot growth process stops halfway, that is, the soot is generated. It was found that soot was not generated at all and soot was generated when the temperature of the fuel and its surroundings in the combustion chamber 5 reached a certain temperature or higher.

By the way, the temperature of the fuel and its surroundings when the hydrocarbon production process is stopped in the state of the soot precursor, that is, the above-mentioned certain temperature depends on various factors such as the type of fuel and the compression ratio of the air-fuel ratio. It cannot be said how many times it changes, but this certain temperature has a deep relationship with the amount of NO _x produced, and therefore this certain temperature is defined to some extent from the amount of NO _x produced. be able to. That is, as the EGR rate increases, the temperature of the fuel during combustion and the gas around it decreases, and the amount of NO _x generated decreases. At this time, soot is hardly generated when the amount of NO _x generated is about 10 p.pm or less. Therefore, the above certain temperature is NO
It is almost the same as the temperature when the amount of _x generation is around 10 p.pm or less.

Once soot is produced, this soot cannot be purified by a post-treatment using a catalyst having an oxidizing function. On the other hand, the soot precursor or the hydrocarbon in the state before it can be easily purified by a post-treatment using a catalyst having an oxidizing function. Considering the post-treatment with a catalyst having an oxidizing function as described above, regarding whether the hydrocarbon is discharged from the combustion chamber 5 in the state of the soot precursor or in the state before it, or is discharged from the combustion chamber 5 in the form of soot, There is a huge difference. The new combustion system employed in the present invention allows hydrocarbons to be discharged from the combustion chamber 5 in the form of soot precursors or pre-presence conditions without producing soot in the combustion chamber 5 The core is to oxidize with a catalyst having an oxidizing function.

In order to stop the growth of hydrocarbons before the soot is generated, the temperature of the fuel and the gas around it in the combustion chamber 5 at the time of combustion 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, in order to suppress the temperature of the fuel and the gas around it, the endothermic action of the gas around the fuel when the fuel burns has an extremely large effect.

That is, when only air exists 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 locally becomes extremely high. That is, at this time, the endothermic action of the combustion heat of the fuel of the air separated from the fuel is hardly performed. In this case, since the combustion temperature locally becomes extremely high, the unburned hydrocarbons that have received this heat of combustion generate soot.

On the other hand, the situation is slightly different when the fuel is present in a mixed gas of a large amount of inert gas and a small amount of air.
In this case, the evaporated fuel diffuses into the surroundings, reacts with oxygen mixed in the inert gas, and burns. In this case, the combustion heat is absorbed by the surrounding inert gas, so that 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 suppressed low by the endothermic action of the inert gas.

In this case, in order to suppress the temperature of the fuel and the gas around it to a temperature lower than the temperature at which soot is produced, an amount of inert gas sufficient to absorb the amount of heat required to do so is required. . Therefore, if the fuel amount increases, the required amount of inert gas also increases accordingly. In addition,
In this case, the larger the specific heat of the inert gas, the stronger the endothermic action, so that the inert gas is preferably a gas having a large specific heat. In this respect, since CO ₂ and EGR gas have relatively large specific heat, it can be said that it is preferable to use EGR gas as the inert gas.

FIG. 5 shows the relationship between the EGR rate and smoke when EGR gas is used as the inert gas and the cooling degree of the EGR gas is changed. That is, in FIG. 5, the curve A strongly cools the EGR gas to bring the EGR gas temperature to about 9
The curve B shows the case where the EGR gas is cooled by a small cooling device, and the curve C shows the case where the temperature is maintained at 0 ° C.
Indicates the case where the EGR gas is not forcibly cooled.

As shown by the curve A in FIG. 5, when the EGR gas is strongly cooled, the soot generation amount peaks when the EGR rate is slightly lower than 50%, and in this case, the EGR rate is almost 55. Almost no soot is generated if the percentage is exceeded. On the other hand, as shown by the curve B in FIG. 5, when the EGR gas is slightly cooled, the soot generation amount reaches a peak when the EGR rate is slightly higher than 50%. In this case, the EGR rate is approximately 65% or more. If so, soot is hardly generated.

Further, as shown by the curve C in FIG. 5, EG
When the R gas is not forcibly cooled, the EGR rate is 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. Note that FIG. 5 shows the amount of smoke generated when the engine load is relatively high, and the EGR rate at which the amount of soot generated peaks when the engine load decreases and the EGR rate at which soot almost does not occur decreases. The lower limit of is also slightly lowered. The lower limit of the EGR rate at which such soot hardly occurs changes depending on the cooling degree of the EGR gas and the engine load.

FIG. 6 shows a mixture of EGR gas and air required to bring the temperature of the fuel and its surrounding gas at the time of combustion to a temperature lower than the temperature at which soot is produced when EGR gas is used as the inert gas. The amount of gas, the ratio of air in this mixed gas amount, and the ratio of EGR gas in this mixed gas are shown. Note that, in FIG. 6, the vertical axis represents the total intake gas amount sucked into the combustion chamber 5, and the chain line Y represents the total intake gas amount that can be sucked into the combustion chamber 5 when supercharging is not performed. ing. The horizontal axis shows the required load.

Referring to FIG. 6, the ratio of air, that is, the amount of air in the mixed gas, shows the amount of air required to completely burn the injected fuel. That is, in the case shown in FIG. 6, the ratio between the air amount and the injected fuel amount is the theoretical air-fuel ratio. On the other hand, in FIG. 6, the ratio of EGR gas, that is, the amount of EGR gas in the mixed gas is set so that when the injected fuel is burned, the temperature of the fuel and its surrounding gas is 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 when expressed by the EGR rate, and is 7 in the embodiment shown in FIG.
It is 0% or more. That is, the total intake gas amount sucked into the combustion chamber 5 is shown by a solid line X in FIG. 6, and the ratio of the air amount and the EGR gas amount in the total intake gas amount X is shown in FIG.
When the ratio is as shown in (1), the temperature of the fuel and the gas around it becomes lower than the temperature at which soot is generated, and thus soot is not generated at all. Further, the amount of NO _x generated at this time is around 10 p.pm or less, so N
The amount of O _x generated is extremely small.

When the fuel injection amount increases, the heat generation amount when the fuel burns increases. Therefore, in order to maintain the temperature of the fuel and the gas around it at a temperature lower than the temperature at which soot is generated, heat generated by the EGR gas is used. The amount of absorption must be increased. Therefore, as shown in FIG. 6, the EGR gas amount must be increased as the injected fuel amount is increased.
That is, the EGR gas amount needs to increase as the required load increases.

By the way, when supercharging is not performed,
The upper limit of the total intake gas amount X drawn into the baking chamber 5 is Y.
Therefore, the required load is L in FIG.₀Greater than
In the region, the EGR gas ratio is increased as the required load increases.
It is possible to maintain the air-fuel ratio at the theoretical air-fuel ratio unless it is decreased.
Can not. In other words, it is necessary when supercharging is not done.
Load demand is L₀The theoretical air-fuel ratio in a region larger than
If you try to maintain the fuel ratio, the required load will increase.
As a result, the EGR rate decreases, so the required load is L ₀Than
In a large area, the temperature of the fuel and the gas around it is sooted
It cannot be maintained below the temperature at which it is produced.

However, as shown in FIG. 1, when EGR gas is recirculated to the inlet side of the supercharger, that is, the air suction pipe 17 of the exhaust turbocharger 15 via the EGR passage 29, the required load is larger than L _0. EGR rate is 5 in the region
It can be maintained above 5 percent, for example 70 percent, thus maintaining the fuel and surrounding gas temperatures below the temperature at which soot is produced. That is, if the EGR gas is recirculated so that the EGR rate in the air suction pipe 17 becomes, for example, 70%, the EGR rate of the intake gas boosted by the compressor 16 of the exhaust turbocharger 15 also becomes 70%. The temperature of the fuel and the gas around it can be maintained below the temperature at which soot is produced, up to the limit that can be boosted by the compressor 16. Therefore, the operating range of the engine capable of producing the low temperature combustion can be expanded. When the EGR rate is set to 55% or more in the region where the required load is larger than L ₀ , E
The GR control valve 31 is fully opened and the throttle valve 20 is slightly closed.

As described above, FIG. 6 shows the case where the fuel is burned under the stoichiometric air-fuel ratio. However, even if the air amount is made smaller than that shown in FIG. 6, that is, the air-fuel ratio is made rich. However, while preventing the generation of soot, the amount of NO _x generated is 10 _p .
It can be around pm or less, and even if the air amount is made larger than that shown in FIG. 6, that is, the average value of the air-fuel ratio is lean from 17 to 18, the soot generation is prevented. Meanwhile, the amount of NO _x generated can be set to around 10 p.pm or less.

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 excessive fuel does not grow to soot, and soot is generated. There is no. Further, at this time, a very small amount of NO _x is generated. On the other hand, when the average air-fuel ratio is lean, or even when the air-fuel ratio is the stoichiometric air-fuel ratio, a small amount of soot is generated if the combustion temperature becomes high, but in the present invention the combustion temperature is suppressed to a low temperature, soot Not generated at all. Furthermore, NO _x
Also produces only a very small amount.

Thus, when low temperature combustion is performed, 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. However, the amount of NO _x generated is extremely small. Therefore, considering the improvement of the fuel consumption rate, it can be said that it is preferable to make the average air-fuel ratio lean at this time.

By the way, the temperature of the fuel and the gas around it during combustion in the combustion chamber can be suppressed to a temperature below the temperature at which the growth of hydrocarbons stops halfway only when the engine is operating at low load, where the calorific value of combustion is relatively small. To be Therefore, in the embodiment according to the present invention, when the engine is operated at low load, the temperature of the fuel and the gas around it during combustion is suppressed below the temperature at which the growth of hydrocarbons stops halfway, and the first combustion, that is, low temperature combustion is performed. In addition, the second combustion, that is, the combustion normally performed from the conventional one, is performed during the engine high load operation. It should be noted that here, the first combustion, that is, low temperature combustion, as is clear from the above description, the amount of inert gas in the combustion chamber is larger than the amount of inert gas at which the amount of soot generated peaks, and soot is almost generated. The second combustion, that is, the combustion that is normally performed conventionally, is the combustion in which the amount of inert gas in the combustion chamber is less than the amount of inert gas at which the soot generation peaks. Say

FIG. 7 shows the first operating region I where the first combustion, that is, the low temperature combustion is performed, and the second operating region II where the second combustion, that is, the combustion by the conventional combustion method is performed. There is. In FIG. 7, the vertical axis L represents the depression amount of the accelerator pedal 50, that is, the required load, and the horizontal axis N represents the engine speed. Further, in FIG. 7, X (N) is the first
Shows the first boundary between the operating region I and the second operating region II, where Y (N) is the first operating region I and the second operating region.
The second boundary with II is shown. The determination of the change of the operating region from the first operating region I to the second operating region II is made based on the first boundary X (N), and the change from the second operating region II to the first operating region II is performed.
The determination of the change of the operating range to the operating range I of the second boundary Y
It is performed based on (N).

That is, the operating state of the engine is the first operating region I.
If the required load L exceeds the first boundary X (N) which is a function of the engine speed N during low temperature combustion, it is determined that the operating region has moved to the second operating region II. Combustion is performed by a conventional combustion method. Next, when the required load L becomes lower than the second boundary Y (N) which is a function of the engine speed N, it is determined that the operating region has moved to the first operating region I, and low temperature combustion is performed again.

In this way, the two boundaries of the first boundary X (N) and the second boundary Y (N) on the low load side of the first boundary X (N) are provided as follows. For one reason. The first reason is that the combustion temperature is relatively high on the high load side of the second operating region II, and at this time, even if the required load L becomes lower than the first boundary X (N), low temperature combustion cannot be immediately performed. Because. That is, the low temperature combustion does not start immediately unless the required load L becomes considerably low, that is, when it becomes lower than the second boundary Y (N). The second reason is that hysteresis is provided for changes in the operating region between the first operating region I and the second operating region II.

By the way, when the engine operating region is in the first operating region I and low temperature combustion is performed, soot is scarcely generated, and instead, unburned hydrocarbon is in a soot precursor or a state before it. It is discharged from the combustion chamber 5 in shape. At this time, the unburned hydrocarbons discharged from the combustion chamber 5 are oxidized and purified by the NO _x absorbent described in detail later. FIG. 8 shows the output of the air-fuel ratio sensor 27. As shown in FIG. 8, the output current I of the air-fuel ratio sensor 27 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 27.

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. 9 shows the throttle valve 2 for the required load L.
The opening degree of 0, the opening degree of the EGR control valve 31, the EGR rate, the air-fuel ratio, the injection timing and the injection amount are shown. As shown in FIG. 9, in the first operating region I where the required load L is low, the opening degree of the throttle valve 20 is gradually increased from near full closing to about 2/3 opening degree as the required load L increases. E
The opening degree of the GR control valve 31 is gradually increased from near full close to full open as the required load L increases. Also,
In the example shown in FIG. 9, the EGR rate is set to approximately 70% in the first operating region I, and the air-fuel ratio is made slightly lean. In other words, in the first operating region I, the EGR rate becomes approximately 70%, and the opening degree of the throttle valve 20 and the EGR control valve 31 are adjusted so that the air-fuel ratio becomes a slightly lean lean air-fuel ratio.
The opening degree of is controlled. Further, 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 becomes late as the required load L becomes high, and the injection completion timing θE also becomes late as the injection start timing θS becomes late.

During the idling operation, the throttle valve 20 is closed close to the fully closed state, and at this time, the EGR control valve 31 is also closed close to the fully closed state. Throttle valve 2
When 0 is closed to near full closure, the pressure in the combustion chamber 5 at the beginning of compression becomes low and the compression pressure becomes small. When the compression pressure becomes small, the compression work by the piston 4 becomes small, so that the vibration of the engine body 1 becomes small. That is, during idling operation, the throttle valve 20 is closed close to the fully closed state 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.
When changing from the second operating range II to the second operating range II, the opening degree of the throttle valve 20 is increased stepwise from about 2/3 opening degree toward the full opening direction. At this time, in the example shown in FIG. 9, the EGR rate is reduced stepwise from approximately 70% to 40% or less, and the air-fuel ratio is increased stepwise. That is, the EGR that produces a large amount of smoke with an EGR rate
The operating range of the engine is the first because the rate range (Fig. 5) is skipped.
A large amount of smoke does not occur when changing from the operating region I to the second operating region II.

In the second operating region II, the conventional combustion is performed. In the second operating region II, the throttle valve 20 is kept fully open except for a part, and the opening degree of the EGR control valve 31 is gradually reduced as the required load L increases. Further, in this operating region II, the EGR rate becomes lower as the required load L becomes higher, and the air-fuel ratio becomes smaller as the required load L becomes higher. However, the air-fuel ratio is a lean air-fuel ratio even if the required load L becomes high. Further, in the second operation region II, the injection start timing θS is set near the compression top dead center TDC.

FIG. 10A shows the target air-fuel ratio A / F in the first operating region I. In FIG. 10A, A / F = 15.5, A / F = 16, A / F = 17,
Each of the curves indicated by A / F = 18 has a target air-fuel ratio of 1
The values are 5.5, 16, 17, and 18, and the air-fuel ratio between the curves is determined by proportional distribution. Figure 10
As shown in (A), the air-fuel ratio is lean in the first operating region I, and in the first operating region I, the target air-fuel ratio A / F is leaner as the required load L is lower.

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 decreased. When the EGR rate is decreased, the air-fuel ratio becomes large, so that the target air-fuel ratio A / F becomes larger as the required load L becomes lower as shown in FIG. 10 (A). The fuel consumption rate increases as the target air-fuel ratio A / F increases, and therefore, in order to make the air-fuel ratio as lean as possible, in the embodiment of the present invention, the target air-fuel ratio A / F decreases as the required load L decreases.
Is increased.

The target air-fuel ratio A / F shown in FIG. 10A is a ROM 4 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. 10B.
It is stored in 2. In addition, the throttle valve 2 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG.
The target opening degree ST of 0 is stored in the ROM 42 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.
EGR required to achieve the target air-fuel ratio A / F shown in (A)
The target opening degree SE of the control valve 31 is previously stored in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. 11 (B), and the air-fuel ratio is shown in FIG. The target injection fuel amount Q required to obtain the target air-fuel ratio A / F shown in () is stored 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.
It is stored in 42.

FIG. 13A shows the target air-fuel ratio A when the second combustion, that is, the normal combustion by the conventional combustion method is performed.
/ F is shown. In addition, in FIG.
= 24, A / F = 35, A / F = 45, and A / F = 60, the respective curves indicated by the target air-fuel ratios of 24, 35, 45, 6 respectively.
0 is shown. Target air-fuel ratio A shown in FIG.
13 / F is a ROM 42 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.
It is stored in. In addition, the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG.
The target opening degree ST is stored in advance in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. 14 (A), and the air-fuel ratio is shown in FIG. 13 (A).
The target opening degree SE of the EGR control valve 31 required to achieve the target air-fuel ratio A / F shown in FIG. 14 is previously shown in the form of a map as a function of the required load L and the engine speed N as shown in FIG. 14 (B). It is stored in the ROM 42 and the air-fuel ratio is shown in FIG.
The target injection fuel amount Q required to achieve the target air-fuel ratio A / F shown in (A) is a ROM 42 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.
It is stored in.

By the way, in FIG. 1, the NO _x absorbent 25 is arranged in the casing 26. NO _x absorbent 2
5 is, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, and cesium C are supported on the carrier.
Alkali metals such as s, barium Ba, calcium C
At least one selected from alkaline earths such as a, lanthanum La, and rare earths such as yttrium Y and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbons) supplied into the engine intake passage, the combustion chamber 5 and the exhaust passage upstream of the NO _x absorbent 25 is calculated as NO _x.
When called the air-fuel ratio of the exhaust gas flowing into the absorbent 25, this N
O _x absorbent 25 absorbs the NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas to release NO _x absorbed and becomes the stoichiometric air-fuel ratio or rich NO _x
It absorbs and releases.

If this NO _x absorbent 25 is arranged in the engine exhaust passage, the NO _x absorbent 25 actually performs the action of absorbing and releasing NO _x , but the detailed mechanism of this action of absorbing and releasing some parts is not clear. . However, it is considered that this absorbing and releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

In the compression ignition type internal combustion engine shown in FIG. 1, combustion is normally performed in a state where the air-fuel ratio in the combustion chamber 5 is lean. Thus the oxygen concentration in the exhaust gas when the air-fuel ratio is performed is combusted in a lean state is high, these oxygen O ₂ as is shown in FIG. 16 (A) at this time O
It attaches to the surface of platinum Pt in the form of ₂ ⁻ or O ²⁻ . on the other hand,
NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ^{2 −} on the surface of platinum Pt to become NO ₂ (2NO + O ₂ → 2N
O ₂ ). Then, a part of the generated NO ₂ is oxidized on the platinum Pt and absorbed in the absorbent to be barium oxide BaO.
As shown in FIG. 16 (A), it diffuses into the absorbent in the form of nitrate ion NO ₃ ⁻ while being bound with. In this way, NO _x is absorbed in the NO _x absorbent 25. As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is produced on the surface of platinum Pt, and unless the NO _x absorption capacity of the absorbent is saturated, NO ₂ is generated.
₂ is absorbed in the absorbent to produce nitrate ion NO ₃ ⁻ .

On the other hand, the air-fuel ratio of the inflowing exhaust gas becomes rich.
Oxygen concentration in the inflowing exhaust gas decreases, resulting in white
NO on the surface of gold Pt₂The production amount of is reduced. NO₂of
If the amount of production decreases, the reaction goes in the opposite direction (NO₃ ^-→ NO₂)
And thus the nitrate ion NO in the absorbent₃ ^-Is NO
₂Is released from the absorbent in the form of. NO at this time_xAbsorbent
NO released from 25_xIs shown in Figure 16 (B).
A large amount of unburned HC and CO contained in the inflowing exhaust gas
Reacted and reduced. In this way, the platinum Pt
NO on the surface₂When there is no longer any absorbent
Heto NO₂Is released. Therefore, the air-fuel ratio of the inflowing exhaust gas
Is made rich, it will be NO in a short time. _xAbsorbent 25?
Et NO_xIs released, and this released NO_xIs returned
NO in the atmosphere to be sourced_xIs never emitted
Yes.

In this case, the air-fuel ratio of the inflowing exhaust gas is
NO even at the theoretical air-fuel ratio_xAbsorbent 25 to NO_xIs released
To be done. However, the air-fuel ratio of the inflowing exhaust gas is
NO in case of ratio_xAbsorbent 25 to NO_xGradually
NO because it is only released _xAbsorbed by the absorbent 25
All NO_xIt takes a little longer time to release.

As described above, the NO _x absorbent 25 is platinum P
It contains a noble metal such as t and is therefore a NO _x absorbent 2
5 has an oxidizing function. On the other hand, as described above, when the engine operating condition is in the first operating region I and the low temperature combustion is performed, soot is hardly generated, and instead, the unburned hydrocarbon is the precursor of soot or the soot precursor. It is discharged from the combustion chamber 5 in the form of a state. However, as described above, the NO _x absorbent 25 has an oxidizing function, and therefore the unburned hydrocarbons discharged from the combustion chamber 5 at this time are satisfactorily oxidized by the NO _x absorbent 25.

By the way the absorption of NO _x capacity of the NO _x absorbent 25 is limited, absorption of NO _x capacity of the NO _x absorbent 25 needs to release the NO _x from the NO _x absorbent 25 before saturation. For this purpose it is necessary to estimate the amount of NO _x is absorbed in the NO _x absorbent 25. Therefore, in the embodiment according to the present invention, the NO _x absorption amount A per unit time when the first combustion is being performed is plotted as a function of the required load L and the engine speed N on a map as shown in FIG. 17B as a function of the required load L and the engine speed N, the NO _x absorption amount B per unit time when the second combustion is being performed is calculated as shown in FIG. The NO _x amount ΣNOx absorbed in the NO _x absorbent 25 is estimated by previously obtaining the NO _x absorption amounts A and B per unit time.

In the embodiment according to the present invention, when the NO _x absorption amount ΣNOX exceeds the predetermined allowable maximum value, N
From O _x absorbent 25 so that to release NO _x. Next, this will be described with reference to FIG. Referring to FIG. 18, in the embodiment according to the present invention, there are two allowable maximum values, that is, an allowable maximum value MAX1 and an allowable maximum value MA.
X2 and are set. The maximum allowable value MAX1 is NO _x
It is set to about 30% of the maximum NO _x absorption amount that the absorbent 25 can absorb, and the allowable maximum value MAX2 is set to about 80% of the maximum absorption amount that the NO _x absorbent 25 can absorb. NO _x when the first combustion is taking place
When the absorption amount ΣNOX exceeds the maximum allowable value MAX1, the air-fuel ratio is made rich to release NO _x from the NO _x absorbent 25, and when the second combustion is being performed, the NO _x absorption amount ΣNOX is the maximum allowable value. Second when the value MAX1 is exceeded
Air-fuel ratio in order to release the NO _x from the NO _x absorbent 25 when it is switched from the combustion to the first combustion is made rich,
NO _x absorption amount ΣNOx when the second combustion is performed
Exceeds the maximum allowable value MAX2, NO _x absorbent 2
Additional fuel is injected in the latter half of the expansion stroke or during the exhaust stroke to release NO _x from 5.

That is, in FIG. 18, the period X is the required load L.
Is lower than the first boundary X (N) and shows the case where the first combustion is performed. At this time, the air-fuel ratio is a lean air-fuel ratio that is slightly leaner than the stoichiometric air-fuel ratio. When the first combustion is being performed, the amount of NO _x generated is extremely small. Therefore, at this time, the NO _x absorption amount ΣNO _x rises very slowly as shown in FIG. 18. NO _x absorption amount ΣN when the first combustion is performed
OX is an air-fuel ratio A / F exceeds the allowable maximum value MAX1 is temporarily rich, whereby NO _x from the NO _x absorbent 25 is released. At this time, NO _x absorption amount ΣNOX
Is zero.

As described above, when the first combustion is performed, soot is not generated regardless of whether the air-fuel ratio is lean, the stoichiometric air-fuel ratio, or the rich, so the first combustion is performed. Even if the air-fuel ratio A / F is made rich in order to release NO _x from the NO _x absorbent 25 at this time, soot is not generated at this time. Next, at time t ₁ , when the required load L exceeds the first boundary X (N), the first combustion is switched to the second combustion. Second as shown in FIG.
When the combustion is being performed, the air-fuel ratio A / F becomes considerably lean. The amount of NO _x generated is larger when the second combustion is being performed than when the first combustion is being performed. Therefore, when the second combustion is being performed, the NO _x amount ΣNOX is relatively rapid. Rise to.

The air-fuel ratio A when the second combustion is being performed
When / F is made rich, a large amount of soot is generated, and therefore the air-fuel ratio A / F cannot be made rich while the second combustion is being performed. Thus the air-fuel ratio in order to release the NO _x from _the NO _x absorbent 25 as the second of the NO _x absorption amount ΣNOX when combustion is being performed has exceeded the allowable maximum value MAX1, as shown in FIG. 18 A / F is not considered rich.
The required load L as at time t ₂ in FIG. 18 when the _the NO _x absorbent when is switched from the second combustion is lower in the first combustion than the second boundary Y (N) 25 The air-fuel ratio A / F is temporarily made rich in order to release NO _x from.

Next, at time t ₃ in FIG. 18, the first combustion is switched to the second combustion, and the second combustion is continued for a while. The time of absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX1, then in the second half or the exhaust stroke of the expansion stroke so as to release the NO _x assuming that exceeds the allowable maximum value MAX2 at time t ₄ at this time from _the NO _x absorbent 25 Is injected with additional fuel, and the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 25 is made rich.

The additional fuel injected during the latter half of the expansion stroke or during the exhaust stroke does not contribute to the generation of engine output, so it is preferable to minimize the chance of injecting the additional fuel. Therefore, when the NO _x absorption amount ΣNOX exceeds the allowable maximum value MAX1 when the second combustion is performed, the second
The air-fuel ratio A /
F is temporarily made rich, and additional fuel is injected only in a special case where the NO _x absorption amount ΣNOX exceeds the maximum allowable value MAX2.

FIG. 19 shows a processing routine of a NO _x releasing flag which is set when NO _x should be released from the NO _x absorbent 25, and this routine is executed by interruption at regular time intervals. Referring to FIG. 19, first, at step 100, it is judged if the graph I indicating that the engine operating region is the first operating region I is set or not. When the flag I is set, that is, when the engine operating region is the first operating region I, the routine proceeds to step 101, where the NO _x absorption amount A per unit time is calculated from the map shown in FIG. 17 (A). It Next, at step 102, A is added to the NO _x absorption amount ΣNOX. Next, at step 103, it is judged if the NO _x absorption amount ΣNOX exceeds the allowable maximum value MAX1. ΣN
When OX> MAX1, the routine proceeds to step 104, where NO _x
The process of setting the emission flag 1 is performed.

On the other hand, when it is determined in step 100 that the flag I is reset, that is, when the operating region of the engine is the second operating region II, step 106 is performed.
Then, the NO _x absorption amount B per unit time is calculated from the map shown in FIG. 17 (B). Next, at step 107, B is added to the NO _x absorption amount ΣNOX. Next, at step 108, the NO _x absorption amount ΣNOX is the allowable maximum value MA.
It is determined whether or not X1 has been exceeded. ΣNOX> MAX1
If so, the routine proceeds to step 109, where processing for setting the NO _x release flag 1 is performed. Next, at step 110, it is judged if the NO _x absorption amount ΣNOX exceeds the allowable maximum value MAX2. When ΣNOX> MAX2, the routine proceeds to step 111, where the NO _x release flag 2 is set.

[0074] Incidentally downstream air-fuel ratio sensor 54 for detecting an air-fuel ratio of the exhaust gas flowing out from _the NO _x absorbent 25 in the exhaust pipe 24 on the downstream side of the NO _x absorbent 25 in the present invention
Are placed. The downstream air-fuel ratio sensor 54 is similar to the air-fuel ratio sensor 27, and its output signal is input to the input port 45 via the corresponding AD converter 47.

In the present invention, these air-fuel ratio sensors 27,
Based on the output of 54, the maximum allowable values MAX1 and MAX2 used in the processing of the NO _x release flag shown in FIG. 19 are changed and optimized. That is, from the NO _x absorbent 25 to the NO _x
If the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 25 is made rich in order to release NO, the air-fuel ratio sensor 27 detects that the air-fuel ratio of the exhaust gas is rich. Immediately after this, NO _x
Since emission-reducing action of the absorbent 25 in the NO _x takes place downstream air-fuel ratio sensor 54 indicates that the air-fuel ratio of the exhaust gas is not a rich, that is, the stoichiometric air-fuel ratio. After this, for a while, the downstream air-fuel ratio sensor 54 continues to show that the air-fuel ratio of the exhaust gas is not rich. The downstream air-fuel ratio sensor 54 when the release-reduction action of the NO _x in _the NO _x absorbent 25 is completed indicates that the air-fuel ratio of the exhaust gas is rich. Here, the period from when the air-fuel ratio sensor 27 first outputs that the exhaust gas air-fuel ratio is rich to when the downstream side air-fuel ratio sensor 54 first outputs that the exhaust gas air-fuel ratio is rich is NO _x. Absorbent 25 to NO
_{period x} is released (hereinafter, referred to as _the NO _x releasing duration) corresponds to, and is absorbed in _the NO _x absorbent 25 N
The larger the amount of O _x, the longer the length. That is, the NO _x release continuation period is a parameter indicating the amount of NO _x absorbed by the NO _x absorbent 25.

Further, the higher the average air-fuel ratio A / F of the exhaust gas flowing into the NO _x absorbent 25 during the NO _x release continuation period, the larger the amount of hydrocarbons flowing into the NO _x absorbent 25. That is, the amount of NO _x that has reacted with the hydrocarbon is large. Therefore, the average air-fuel ratio A / F of the exhaust gas flowing into the NO _x absorbent 25 during the NO _x release continuation period is N absorbed by the NO _x absorbent 25.
This is a parameter indicating the amount of O _x .

Further NO_xNO within the release duration_xabsorption
The larger the amount of exhaust gas flowing into the agent 25, the NO_xAbsorbent
The amount of hydrocarbons flowing into is large. I.e. reacted with hydrocarbons
NO _xThere is also a large amount of. Therefore NO_xNO within the release duration
_xThe amount of exhaust gas flowing into the absorbent 25 is NO_xAbsorbent 2
NO absorbed in 5_xThis is a parameter indicating the amount.
Therefore, in the present invention, these NO_xRelease duration, NO_xRelease
NO during the continuation period_xOf the exhaust gas flowing into the absorbent 25
Air-fuel ratio, NO_xNO during the release period_xFlowing into the absorbent 25
Maximum allowable value MAX1, MA based on the amount of exhaust gas
Correct X2. Therefore, the corrected maximum allowable value MAX
1, MAX2 is NO at that time_xAbsorbent 25 can be absorbed
NO_xIt will accurately indicate the maximum allowable value of. In the present invention
Is NO_xSet the maximum allowable values MAX1 and MAX2 of the absorbent 25
The correction coefficient J for correcting to the actual allowable maximum value
NO as shown in FIG._xWith a function of the release duration Δt
Then, if it is stored in the ROM 42 in advance in the form of a map,
In both cases, the correction coefficient K is set to NO as shown in FIG._xRelease succession
NO in the next period_xExhaust gas amount AE flowing into the absorbent 25
And as a function of the average air-fuel ratio A / F in the form of a map
Store in ROM 42, NO_xAbsorbent 25
The maximum values MAX1 and MAX2 are corrected. This makes it
When the maximum allowable value is less than the maximum allowable value
NO_xIs NO _xIt is not absorbed by the absorbent 25 and flows out
It is prevented from sprinkling. Also, the actual maximum allowable value is set.
NO absorbed when the maximum allowable value is exceeded_x
Before the amount exceeds the actual maximum allowable value_xRelease / return of
It is prevented from doing the original. That is, NO_xEmission of
・ Reduction process is less frequent.

NO_xExhaust gas flowing into the absorbent 25
The average air-fuel ratio and amount of
And the mass flow rate detector 21. However,
Estimate the air-fuel ratio of the air-gas from the air-fuel ratio in the combustion chamber 5.
Yes. The maximum allowable value is NO_xDepending on the temperature of the absorbent 25
Also changes, so NO_xNO using the temperature of the absorbent 25 _x
The release duration may be corrected.

FIG. 22 is a flow chart showing the control for correcting the allowable maximum value of the NO _x absorbent 25. First, at step 200, the air-fuel ratio of exhaust gas A /
It is determined whether or not the rich flag that is set when the air-fuel ratio sensor 27 detects that F has become rich is set. When the rich flag is set, the routine proceeds to step 201, where the correction coefficients J and K are calculated from the maps of FIGS. Then step 202
In step 203, the maximum allowable value MAX1 is corrected by the correction factors J and K, and then in step 203, the maximum allowable value MAX1.
X2 is corrected by the correction coefficients J and K. Next, at step 204, the rich flag is reset.

Next, the operation control will be described with reference to FIG. Referring to FIG. 23, first, step 3
At 00, it is determined whether or not the flag I indicating that the operating state of the engine is in the first operating region I is set. When the flag I is set, that is, when the operating state of the engine is in the first operating region I, step 30
In step 1, it is determined whether the required load L has become larger than the first boundary X1 (N). When L ≦ X1 (N), the routine proceeds to step 303, where low temperature combustion is performed.

That is, at step 303, the target opening degree ST of the throttle valve 20 is calculated from the map shown in FIG. 11A, and the opening degree of the throttle valve 20 is made this target opening degree ST. Next, at step 304, the target opening degree SE of the EGR control valve 31 is calculated from the map shown in FIG. 11 (B),
The opening degree of the EGR control valve 31 is set to this target opening degree SE.
Next, at step 305, it is judged if the NO _x releasing flag 1 is set. When the NO _x releasing flag is not set, the routine proceeds to step 306 and FIG.
The fuel injection of the amount Q calculated from the map of FIG. 12 is performed so that the air-fuel ratio shown by 0 is obtained. At this time, low temperature combustion is performed under a lean air-fuel ratio.

On the other hand, when it is judged at step 305 that the NO _x releasing flag 1 is set, the routine proceeds to step 307, where the injection control for making the average air-fuel ratio in the combustion chamber 5 rich is carried out. In this case _the NO _x absorbent 25 from the NO _x is released, absorption of NO _x amount ΣNOX is made zero at step 307a. On the other hand, step 3
When it is determined at 01 that L> X (N), the routine proceeds to step 302, where the flag I is reset, then the routine proceeds to step 310, where the second combustion is performed.

That is, at step 310, the target opening degree ST of the throttle valve 20 is calculated from the map shown in FIG. 14 (A), and the opening degree of the throttle valve 20 is made this target opening degree ST. Next, at step 311, the target opening degree SE of the EGR control valve 31 is calculated from the map shown in FIG. 14 (B),
The opening degree of the EGR control valve 31 is set to this target opening degree SE.
Next, at step 312, it is judged if the NO _x releasing flag is set. When the NO _x release flag is not set, the routine proceeds to step 313 and FIG.
The fuel injection of the amount Q calculated from the map of FIG. 15 is performed so that the air-fuel ratio shown in FIG. At this time, the second combustion is performed under the lean air-fuel ratio.

On the other hand, when it is judged at step 313 that the NO _x releasing flag is set, the routine proceeds to step 314, where the amount Q of fuel calculated from the map of FIG. 15 is set so that the air-fuel ratio shown in FIG. 13 is obtained. Injection is performed and additional fuel is injected in the latter half of the expansion stroke of the engine or in the exhaust stroke. As a result, the injection control is performed so that the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 25 becomes rich, and N
NO _x is released from the O _x absorbent 25. Next, at step 314a, the NO _x absorption amount ΣNOX is made zero.

When the flag I is reset, in the next processing cycle, the routine proceeds from step 300 to step 308, where it is judged if the required load L has become lower than the second boundary Y (N). When L ≧ Y (N), step 310
And the second combustion is performed under a lean air-fuel ratio.
On the other hand, if it is determined at step 308 that L <Y (N), then the routine proceeds to step 309, where the flag I is set. Next, at step 303, low temperature combustion is performed.

In the present invention, step 30 in FIG.
The rich flag may be set after 7a and 314a. In this case, the air-fuel ratio sensor 27 becomes unnecessary. By the way, when the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 25 should be the stoichiometric air-fuel ratio or rich, there is a possibility that it will not actually become the stoichiometric air-fuel ratio or rich due to factors such as deterioration of the fuel injection valve 6. In this case, the air-fuel ratio of the exhaust gas should be controlled so as to become the desired air-fuel ratio. Therefore, in the present invention, when the amount of injected fuel injected from the fuel injection valve 6 is controlled so that the air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio or rich, the air-fuel ratio sensor 27 causes the air-fuel ratio of the exhaust gas to be theoretical. When it is not detected that the air-fuel ratio or rich is detected, correction is performed to increase the amount of injected fuel so that the air-fuel ratio of the exhaust gas becomes the desired air-fuel ratio. Thus according to the invention N
O _x The air-fuel ratio of the exhaust gas flowing into the absorbent 25 is the air-fuel ratio of the exhaust gas when it should be the stoichiometric air-fuel ratio or rich reliably becomes the stoichiometric air-fuel ratio or rich. Further, the increase correction performed at this time is used as it is for the correction of the injected fuel amount when the air-fuel ratio in the first combustion and the second combustion is lean. In this case, the desired air-fuel ratio can be obtained in all engine operations.

In the present invention, when the air-fuel ratio sensor 53 is excluded, the downstream side air-fuel ratio sensor 54 is used to control the injected fuel amount. That is, the downstream side air-fuel ratio sensor 54 does not detect that the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or rich even if a predetermined period elapses after the air-fuel ratio in the combustion chamber 5 is set to the stoichiometric air-fuel ratio or rich. In some cases, the amount of injected fuel is increased and corrected.

FIG. 24 shows a processing routine for correcting the injected fuel amount, which is performed when the air-fuel ratio of the exhaust gas should be the stoichiometric air-fuel ratio or rich. Referring to FIG. 24, first, step 4
At 00, it is determined whether or not the rich flag, which is set when the air-fuel ratio in the combustion chamber 5 is controlled to be rich (or stoichiometric air-fuel ratio), is set. When the rich flag is set in step 400, the routine proceeds to step 401, where it is judged if the air-fuel ratio sensor 27 has detected that the air-fuel ratio of the exhaust gas is rich (or the theoretical air-fuel ratio). Here, when it is not detected that the air-fuel ratio of the exhaust gas is rich (or the theoretical air-fuel ratio), the routine proceeds to step 402, where the injection fuel amount is increased and corrected.

The EGR passage 29 of the present invention extends substantially vertically upward from the exhaust pipe 28 for the following reason. The intercooler is preferably NO _x absorbent 25 or EG
It is arranged at a position higher than the height of the connecting portion between the R passage 29 and the exhaust pipe 28. In the present invention, the soot precursor and the like may flow out from the NO _x absorbent 25 when the temperature is not high enough to purify the soot precursor and the like in the exhaust gas. This is because if 29 extends vertically downward from the exhaust pipe 28, soot precursors and the like may be deposited on the intercooler 32, causing the intercooler 32 to malfunction. That is, when the EGR passage 29 extends vertically upward from the exhaust pipe 28 as in the present invention, the soot precursor and the like do not deposit on the intercooler 32, and the soot precursor and the like flow into the EGR passage 29. Even if it does, it is immediately discharged to the exhaust pipe 29. It should be noted that the soot precursor or the like discharged from the EGR passage 29 is the NO _x absorbent 25.
If configured to return to, precursors such as soot temperature of the NO _x absorbent 25 is discharged when the increase is purified by _the NO _x absorbent 25.

[0090]

According to the first to seventh inventions, NO_x
The air-fuel ratio of the exhaust gas flowing into the absorbent should be rich
The exhaust gas air-fuel ratio is surely made rich. Second
According to the fourth invention from_xN absorbed by the absorbent
O_xIs NO _xNO that the absorbent can absorb at that time_xamount
When the air-fuel ratio of the exhaust gas becomes
Be taken as

[Brief description of drawings]

FIG. 1 is an overall view of a compression ignition type internal combustion engine.

FIG. 2 is a diagram showing amounts of smoke and NO _x generated, and the like.

FIG. 3 is a diagram showing a combustion pressure.

FIG. 4 is a diagram showing a fuel molecule.

FIG. 5 is a diagram showing a relationship between a smoke generation amount and an EGR rate.

FIG. 6 is a diagram showing a relationship between an injected fuel amount and a mixed gas amount.

FIG. 7 is a diagram showing a first operating region I and a second operating region II.

FIG. 8 is a diagram showing an output of an air-fuel ratio sensor.

FIG. 9 is a diagram showing an opening degree of a throttle valve and the like.

FIG. 10 is a diagram showing an air-fuel ratio and the like in a first operating region I.

FIG. 11 is a diagram showing a map of a target opening degree of a throttle valve or the like.

FIG. 12 is a diagram showing a map of a fuel injection amount.

FIG. 13 is a diagram showing an air-fuel ratio and the like in a second operating region.

FIG. 14 is a diagram showing a map of a target opening degree of a throttle valve or the like.

FIG. 15 is a diagram showing a map of a fuel injection amount.

FIG. 16 is a diagram for explaining the action of absorbing and releasing NO _x .

FIG. 17 is a diagram showing a map of the amount of NO _x absorbed per unit time.

FIG. 18 is a diagram for explaining NO _x release control.

FIG. 19 is a flowchart for processing a NO _x releasing flag.

FIG. 20 is a diagram showing a map of a correction coefficient of an allowable maximum value.

FIG. 21 is a diagram showing a map of a correction coefficient for an allowable maximum value.

FIG. 22 is a flowchart for processing an allowable maximum value correction.

FIG. 23 is a flowchart for controlling the operation of the engine.

FIG. 24 is a flowchart for controlling the correction of the injected fuel amount.

[Explanation of symbols]

6 ... Fuel injection valve 15 ... Exhaust turbocharger 20 ... Throttle valve 29 ... EGR passage 31 ... EGR control valve

Front page continuation (51) Int.Cl. ⁷ Identification code FI F01N 3/24 F01N 3/24 S F02D 21/08 301 F02D 21/08 301D (72) Inventor Takekazu Ito 1 Toyota-cho, Toyota-shi, Aichi Toyota Motor Co., Ltd. (72) Inventor Hiroki Murata 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (56) Reference JP-A-7-4287 (JP, A) JP-A-8-177654 ( JP, A) JP-A-8-86251 (JP, A) JP-A-9-287527 (JP, A) JP-A-9-287528 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F02D 41/00-41/40

Claims (7)

(57) [Claims] 1. When the amount of inert gas supplied to the combustion chamber is increased, the amount of soot generated gradually increases and reaches a peak, and the amount of inert gas supplied to the combustion chamber is further increased. In an internal combustion engine in which the temperature of the fuel and the gas around it during combustion in the combustion chamber are lower than the soot generation temperature and soot is hardly generated, it is included in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean. A NO _x absorbent that absorbs the stored NO _x and releases the absorbed NO _{x when} the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or becomes rich, soot generation peaks The first combustion, in which the amount of inert gas supplied to the combustion chamber is larger than the amount of inert gas, and soot is hardly generated, and the amount of inert gas supplied to the combustion chamber is smaller than the amount of inert gas at which the amount of generated soot peaks. Low amount of active gas And a second combustion, the air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust gas is arranged in the engine exhaust passage, and NO _x
An internal combustion engine that corrects the air-fuel ratio in the combustion chamber to be richer when the air-fuel ratio in the combustion chamber is made rich in order to release NO _x from the absorbent or when the air-fuel ratio detecting means does not detect rich later. 2. The air-fuel ratio detecting means is NO_xUnder absorbent
It is arranged in the engine exhaust passage on the flow side and has an air-fuel ratio in the combustion chamber.
The air-fuel ratio detection means detects the rich
Detects the time until it goes out, and the longer the time, the more NO_xSucking
From scavenger to NO _xTo delay when it should release
The internal combustion engine according to claim 1. 3. An internal combustion engine according to claim 2 which is adapted retard when releasing the NO _x from the more _the NO _x absorbent the NO _x exhaust gas amount flowing into the absorber is more in the time. 4. The larger the rich degree of the air-fuel ratio of the exhaust gas that has flown into the NO _x absorbent within the time period, the higher the NO _x.
The internal combustion engine of claim 2 which is adapted retard when releasing the NO _x from the absorbent. 5. The internal combustion engine according to claim 1, further comprising a recirculation device for recirculating exhaust gas discharged from the combustion chamber into an engine intake passage, wherein the inert gas is recirculation exhaust gas. 6. The internal combustion engine according to claim 5, wherein the exhaust gas recirculation rate is approximately 55% or more. 7. The engine operating region is divided into a first operating region on the low load side and a second operating region on the high load side, and first combustion is performed in the first operating region to perform the second operating region. The internal combustion engine according to claim 1, wherein the second combustion is performed in the region.