JP2000110670A - Internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure stable low temperature combustion by switching first combustion having rate of inert gas in a combustion chamber much more than a rate of inert gas where a soot generating rate becomes a peak level, and second combustion having few inert gas rate, and moving a first operating region to a high load side toward reducing an air-fuel ratio. SOLUTION: In a diesel engine 1 wherein EGR gas is re-circulated to an intake passage 17, a soot generating rate is gradually increased in the case where a rate of inert gas in a combustion chamber 5 is increased, a fuel and gas temperatures at the time of combustion are reduced lower than a soot generating temperature, and soot is hardly generated. First combustion wherein there is a rate of inert gas in the combustion chamber much more than a rate of inert gas where a soot generating rate becomes a peak level and the soot is hardly generated, and second combustion having few inert gas rate are freely switched. An operating region is divided into a first operating region of a low load side where first and second combustion are carried out, and a second operating region of a high load side, and the first operating region is moved to the high load side toward reducing an air-fuel ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an internal combustion engine.

[0002]

Conventionally than internal combustion engines, for example exhaust gas recirculation and engine exhaust passage and the engine intake passage in order to suppress the generation of the NO _x in the diesel engine (hereinafter, referred to as EGR) connected by passages, the Exhaust gas, that is, EGR gas, is recirculated through the EGR passage into the engine intake 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 is lowered to decrease the generated amount of NO _x, thus the generation amount of the more NO _x to be increased EGR rate is lowered.

[0003] It has been found that can reduce the generation amount of the NO _x Thus conventionally increasing the EGR rate. However, when the EGR rate is increased, the soot generation amount, that is, smoke, starts to increase rapidly when the EGR rate exceeds a certain limit. In this regard, it has conventionally been considered that if the EGR rate is further increased, the smoke will increase indefinitely. Therefore, the smoke starts to increase rapidly.
The GR rate is considered to be the maximum allowable limit of the EGR rate.

Therefore, conventionally, the EGR rate is set within a range not exceeding the maximum allowable limit. The maximum allowable EGR rate varies considerably depending on the type of engine and fuel, but is approximately 30 to 50%.
Therefore, in a conventional diesel engine, the EGR rate is at most 3
It is reduced from 0% to about 50%.

As described above, conventionally, it has been considered that the maximum allowable limit exists for the EGR rate.
If the R rate is within the range not exceeding this maximum allowable limit, NO
_It was set so that the amount of _x and smoke generated was as small as possible. However, in this way EGR
Rate that there is a limit to the reduction of the NO _x and the amount of generated NO _x and the amount of smoke produced also defined to be as small as possible of smoke, in fact still a significant amount of N
At present, O _x and smoke are generated.

However, if the EGR rate is made larger than the maximum allowable limit in the course of research on the combustion of a diesel engine, the smoke rapidly increases as described above. However, the amount of generated smoke has a peak, and the peak exceeds this peak. EGR
When the rate is further increased, the smoke starts to decrease rapidly, and when the EGR rate is increased to 70% or more during idling operation, and when the EGR gas is cooled strongly, the smoke is reduced to about 55% or more. It was found that it was almost zero, that is, almost no soot was generated. In this case, N
Generation amount of O _x is also found that a very small amount.
After that, the reason why no soot was generated was examined based on this finding, and as a result, unprecedented soot and NO
_This has led to the construction of a new combustion system capable of simultaneously reducing _x . This new combustion system will be described in detail later, but in short, it is basically based on stopping the growth of hydrocarbons in the middle stage until the hydrocarbons grow into soot.

That is, as a result of repeated experimental studies, it has been found that when the temperature of the fuel and the surrounding gas during combustion in the combustion chamber is lower than a certain temperature, the growth of hydrocarbons is stopped at a halfway stage before reaching soot. However, when the temperature of the fuel and the gas around it rises above a certain temperature, the hydrocarbons grow into soot at a stretch. In this case, the temperature of the fuel and the surrounding gas is greatly affected by the heat absorbing action of the gas around the fuel when the fuel is burned, and the amount of heat absorbed by the gas around the fuel is adjusted according to the calorific value at the time of burning the fuel. As a result, the temperature of the fuel and the surrounding gas can be controlled.

Accordingly, if the temperature of the fuel and the surrounding gas during combustion in the combustion chamber is suppressed to a temperature below the temperature at which the growth of hydrocarbons stops halfway, soot will not be generated, and the fuel during combustion in the combustion chamber and its surroundings will not be generated. Can be suppressed to a temperature below the temperature at which the growth of hydrocarbons stops halfway, by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, hydrocarbons whose growth has stopped halfway before reaching soot can be easily purified by post-treatment using an oxidation catalyst or the like. This is the basic idea of a new combustion system. An internal combustion engine employing this new combustion system has already been filed by the present applicant (Japanese Patent Application No. 9-305850).

[0009]

However, in this new combustion system, the EGR rate needs to be approximately 55% or more, and the EGR ratio can be approximately 55% or more when the intake air amount is relatively small. It is. That is, if the amount of intake air exceeds a certain amount, this new combustion cannot be performed. Therefore, if the amount of intake air exceeds a certain amount, the combustion is switched to the conventionally performed combustion. In this case, almost no NO _x and soot are generated under the new combustion, and therefore, it is preferable to perform the new combustion in an operation range as wide as possible.

By the way, under the new combustion, when the air-fuel ratio increases, that is, when the amount of air around the fuel increases, the combustion becomes active, and as a result, the combustion temperature increases. On the other hand, when the air-fuel ratio is reduced, that is, when the amount of air around the fuel decreases, the combustion becomes less active, and as a result, the combustion temperature decreases. Hence it is possible to perform a new combustion does not occur of the NO _x and soot even when increasing the fuel injection quantity as the air-fuel ratio becomes smaller. In other words, as the air-fuel ratio decreases, the operating range in which new combustion can be performed can be expanded to the higher load side.

[0011]

Therefore, in the first invention, when the amount of inert gas in the combustion chamber is increased, the amount of soot generated gradually increases and reaches a peak, and the amount of inert gas in the combustion chamber is reduced. As the temperature increases, the temperature of the fuel during combustion in the combustion chamber and the temperature of the surrounding gas become lower than the temperature at which soot is generated. The first combustion in which the amount of inert gas in the combustion chamber is larger than the amount and little soot is generated, and the second combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the amount of generated soot reaches a peak. Switching means for selectively switching between the first operating region on the low load side where the engine operates in the first combustion and the second operating region on the high load side where the second combustion takes place. And as the air-fuel ratio decreases, The first operating region is moved to the high load side.

[0012] In the second invention, in the first invention,
As the air-fuel ratio decreases, the high load side limit and the low load side limit of the first operation region are shifted to the high load side. 3
In the second invention, in the second invention, when the air-fuel ratio is lean, there is no low-load limit in the first operating region, and when the air-fuel ratio is rich, the low-load limit in the first operating region is low. Appear.

In a fourth aspect, in the first aspect,
Control means for controlling the first operation range is provided, and the control means controls the first operation range in accordance with the target air-fuel ratio. In a fifth aspect based on the first aspect, the first operating region is shifted to a higher load side as the temperature of the fuel and the surrounding gas during the first combustion decreases.

In the sixth invention, in the fifth invention,
When the first combustion is performed under a rich air-fuel ratio, as the temperature of the fuel and the surrounding gas during the combustion decreases, the high-load limit and the low-load limit of the first operation region become high load. Moved to the side. In a seventh aspect based on the fifth aspect, there is provided a control means for controlling the first operation range based on the value of a parameter which changes the temperature of the fuel and the surrounding gas during the first combustion. The means moves the first operation range to the high load side when it is determined from the parameter values that the fuel and the gas temperature around the first combustion are reduced.

In the eighth invention, in the seventh invention,
The parameters comprise at least one of the temperature of the gas flowing into the combustion chamber, the temperature of the engine cooling water, the pressure in the engine intake passage or the humidity of the intake air. According to a ninth aspect, in the first aspect, there is provided an exhaust gas recirculation device for recirculating exhaust gas discharged from the combustion chamber into the engine intake passage, wherein the inert gas comprises recirculated exhaust gas.

In a tenth aspect based on the ninth aspect, the exhaust gas recirculation rate during the first combustion is substantially 55% or more. In an eleventh aspect based on the first aspect, a catalyst having an oxidation function is disposed in the engine exhaust passage. In a twelfth aspect based on the eleventh aspect, the catalyst comprises an oxidation catalyst or a three-way catalyst.

[0017] In 11 th invention the 13th invention, the catalyst is, when the air-fuel ratio of the inflowing exhaust gas lean absorbs NO _x contained in the exhaust gas and air-fuel ratio is the stoichiometric air inflow to the exhaust gas consisting _the NO _x absorbent to release the NO _x absorbed to become fuel ratio or rich. In 13 th invention is a 14 th invention, the air-fuel ratio in order to release the NO _x from the NO _x absorbent is made rich when the engine operating state is in the first operating region when the air-fuel ratio is rich You.

[0018]

FIG. 1 shows a 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, 9 Denotes an exhaust valve, 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 a supercharger, for example, an outlet of a compressor 16 of an exhaust turbocharger 15 via an intake duct 13 and an intercooler 14. Be linked. An 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.

On the other hand, the exhaust port 10 is connected to the exhaust manifold 2.
1 and the exhaust turbocharger 15 via the exhaust pipe 22
The exhaust gas turbine 23 is connected to a catalytic converter 26 having a built-in catalyst 25 having an oxidizing function via an exhaust pipe 24. The exhaust pipe 28 connected to the outlet of the catalytic converter 26 and the air suction pipe 17 downstream of the throttle valve 20 are connected to the EGR passage 2.
An EGR control valve 31 which is connected to each other via a motor 9 and driven by a step motor 30 is disposed in the EGR passage 29. Further, the EGR passage 29 is provided in the EGR passage 29.
An intercooler 32 for cooling the EGR gas flowing therein is arranged. 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 electric control type variable discharge fuel pump 35, and the fuel supplied into the common rail 34 is supplied to the fuel injection valve 6 through each fuel supply pipe 33. A fuel pressure sensor 36 for detecting the fuel pressure in the common rail 34 is attached to the common rail 34, and the fuel pump 35 is controlled so that the fuel pressure in the common rail 34 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 36. Is controlled.

The electronic control unit 40 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 42, a RAM (Random Access Memory) 43, a CPU (Microprocessor) 44, an input port 45, An output port 46 is provided. A water temperature sensor 60 for detecting an engine cooling water temperature is arranged in the engine body 1, and an output signal of the water temperature sensor 60 is input to an input port 45 via a corresponding AD converter 47. A pressure sensor 61 for detecting an absolute pressure in the surge tank 12 and a temperature sensor 62 for detecting a temperature of a mixed gas of the intake air and the EGR gas are arranged in the surge tank 12. The output signal of the sensor 62 is input to the input port 45 via the corresponding AD converter 47.

On the other hand, a humidity sensor 63 for detecting the humidity of the intake air is disposed in the air suction pipe 17 upstream of the throttle valve 20, and the output signal of the humidity sensor 63 is transmitted via a corresponding AD converter 47. It is input to the input port 45. The output signal of the fuel pressure sensor 36 also corresponds to the corresponding AD signal.
The data is input to the input port 45 via the converter 47. A load sensor 51 that generates an output voltage proportional to the amount of depression L of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is adjusted by the corresponding AD converter 4.
7 is input to the input port 45. The input port 45 is connected to a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, by 30 °. On the other hand, the output port 46 is connected to the corresponding drive circuit 48
Are connected to the fuel injection valve 6, the step motor 19 for controlling the throttle valve, the step motor 30 for controlling the EGR control valve, and the fuel pump 35.

FIG. 2 shows the throttle valve 2 when the engine is operating at a low load.
Change in the output torque when changing the air-fuel ratio A / F (abscissa in FIG. 2) by changing the opening and the EGR rate of 0, and smoke, HC, CO, a change in emission of the NO _x It shows the experimental example shown. As can be seen from FIG. 2, in this experimental example, the smaller the air-fuel ratio A / F, the higher the EGR rate. When the air-fuel ratio A / F is smaller than the stoichiometric air-fuel ratio (≒ 14.6), the EGR rate is 65% or more.

As shown in FIG. 2, when the air-fuel ratio A / F is decreased by increasing the EGR rate, the smoke is reduced when the air-fuel ratio A / F becomes about 30 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, HC,
The amount of generated CO starts to increase.

FIG. 3 (A) shows the 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. 3 (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. 3 (A) and FIG. 3 (B), in the case of FIG. 3 (B) 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. 2 when the smoke generation amount 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. 3B 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 substantially 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. 4 are thermally decomposed when the temperature is increased in a state of lack of oxygen, soot precursors are formed, and then mainly, Soot consisting of a solid aggregate of carbon atoms 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. It will grow to soot. Therefore, as described above, when the amount of generated soot becomes substantially zero, the emission amounts of HC and CO increase as shown in FIG. 2, but HC at this time is a precursor of soot or a hydrocarbon in a state before it. .

These considerations based on the experimental results shown in FIGS. 2 and 3 are summarized as follows. When the combustion temperature in the combustion chamber 5 is low, the amount of generated soot 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 exceeded a certain temperature.

Incidentally, the temperature of the fuel and its surroundings when the process of producing hydrocarbons 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, air-fuel ratio and 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. Considering the post-treatment with a catalyst having an oxidation function as described above, it is extremely difficult to discharge hydrocarbons from the combustion chamber 5 in the state of a precursor of soot or in the state before the soot or in the form of soot from the combustion chamber 5. There is a big difference. The new combustion system employed in the present invention discharges hydrocarbons from the combustion chamber 5 in the form of a soot precursor or previous state without producing soot in the combustion chamber 5 and removes the hydrocarbons. The core is to oxidize with a catalyst having an oxidation function.

In order to stop the growth of hydrocarbons before soot is generated, the temperature of fuel and the surrounding gas during combustion in the combustion chamber 5 are set to a temperature lower than the temperature at which 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 the surrounding gas to a temperature lower than the temperature at which the soot is generated, 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, as the specific heat of the inert gas increases, the endothermic effect becomes stronger. 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. 5 shows the relationship between the EGR rate and the smoke when the EGR gas is used as the inert gas and the degree of cooling of the EGR gas is changed. That is, in FIG. 5, a curve A indicates that the EGR gas temperature is substantially 9
Curve B shows the case where the EGR gas is cooled by a small cooling device, and curve C shows the case where the temperature is maintained at 0 ° C.
Indicates a case where the EGR gas is not forcibly cooled.

As shown by the curve A in FIG. 5, when the EGR gas is cooled strongly, the amount of soot generation peaks at a position where the EGR rate is slightly lower than 50%. Above a percentage, little soot is generated. 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 at a point where the EGR rate is slightly higher than 50%. In this case, the EGR rate is increased to about 65% or more. If so, almost no soot is generated.

As shown by the 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. 5 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 soot is hardly generated is reduced. 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. 6 shows the mixing of EGR gas and air necessary to make the fuel during combustion and the gas temperature around it 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. 6, the vertical axis indicates the total intake gas amount sucked into the combustion chamber 5, and the dashed line Y indicates the total intake gas amount that can be sucked into the combustion chamber 5 when supercharging is not performed. ing. The horizontal axis indicates the required load.

Referring to FIG. 6, the proportion of air, that is, the amount of air in the mixed gas, indicates the amount of air necessary to completely burn the injected fuel. That is, in the case shown in FIG. 6, the ratio between the air amount and the injected fuel amount is the stoichiometric air-fuel ratio. On the other hand, in FIG. 6, 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 lower than the temperature at which soot is formed. The required minimum EGR gas amount is shown. This EGR gas amount is approximately 55% or more in terms of the EGR rate. In the embodiment shown in FIG.
0% or more. That is, the total intake gas amount sucked into the combustion chamber 5 is indicated by a solid line X in FIG. 6, and the ratio of the air amount to the EGR gas amount in the total intake gas amount X is shown in FIG.
When the ratio is as shown in the following, the temperature of the fuel and the surrounding gas is lower than the temperature at which soot is generated, and thus no soot is generated. Further, the NO _x generation amount at this time is around 10 p.pm or less.
The amount of O _x generated 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. 6, 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.

When the supercharging is not performed, the upper limit of the total intake gas amount X sucked into the combustion chamber 5 is Y. Therefore, in FIG. 6, in the region where the required load is larger than Lo, the required load is reduced. As the ratio increases, the air-fuel ratio cannot be maintained at the stoichiometric air-fuel ratio unless the EGR gas ratio is reduced. In other words, when the supercharging is not performed and the required air-fuel ratio is maintained at the stoichiometric air-fuel ratio in an area where the required load is larger than Lo, the EGR rate decreases as the required load increases, and In the region where the required load is larger than Lo, the temperature of the fuel and the surrounding gas cannot be maintained at a temperature lower than the temperature at which soot is generated.

However, as shown in FIG. 1, when the EGR gas is recirculated through the EGR passage 29 to the inlet side of the supercharger, that is, into the air suction pipe 17 of the exhaust turbocharger 15, the required load is larger than Lo. In EGR rate 5
It can be maintained at 5% or more, for example 70%, so that the temperature of the fuel and its surrounding gas can be kept 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 suction gas boosted by the compressor 16 of the exhaust turbocharger 15 also becomes 70%. The temperature of the fuel and the surrounding gas can be maintained at a temperature lower than the temperature at which soot is generated, to the extent that the pressure can be increased by the compressor 16. Therefore, the operating range of the engine that can generate low-temperature combustion can be expanded.

In this case, when the EGR rate is set to 55% or more in a region where the required load is larger than Lo, the EGR 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 smaller than the air amount shown in FIG. 10p.pm the generation amount of the NO _x while preventing the occurrence of longitudinal or can below, also be more than the amount of air shown the amount of air in FIG. 6, that is, the average of the air-fuel ratio Even when leaning from 17 to 18, the amount of generated NO _x can be reduced to around 10 ppm or less while preventing the generation of soot.

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 into soot, and soot is generated. 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, 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. Sarezu, 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 the 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 during the low load operation in the engine where the calorific value due to combustion is relatively small. Can be Therefore, in the embodiment according to the present invention, the first combustion, that is, the low-temperature combustion is performed by suppressing the temperature of the fuel during combustion and the gas temperature around the same at or below the temperature at which the growth of hydrocarbons stops halfway during the low load operation in the engine. In addition, the second combustion, that is, the combustion that is usually performed conventionally, is performed during the high load operation of the engine. Here, the first combustion, that is, the low-temperature combustion, has a larger amount of the inert gas in the combustion chamber than the amount of the inert gas at which the soot generation amount is at a peak, as is clear from the description so far. The second combustion, that is, the combustion that has been performed normally in the past, is a combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the amount of soot is peaked. Say that.

Next, the operation range of the engine in which the first combustion, that is, the low-temperature combustion can be performed, will be described with reference to FIGS. 7A and 7B. In FIGS. 7A and 7B, the vertical axis TQ indicates the required torque, and the horizontal axis N indicates the engine speed. First, referring to FIG. 17 (B), FIG. 17 (B) shows that the air-fuel ratio is substantially equal to the stoichiometric air-fuel ratio or the first operating region I in which low-temperature combustion can be performed under a lean condition, and the air-fuel ratio is A second operation region II is shown in which low-temperature combustion cannot be performed substantially under the stoichiometric air-fuel ratio or lean, and conventional combustion must be performed.

In FIG. 7B, X (N) represents a first operating region I in which low-temperature combustion can be performed at an air-fuel ratio of approximately the stoichiometric air-fuel ratio or lean, and an air-fuel ratio of approximately the stoichiometric air-fuel ratio. A first boundary with the second operation region II in which the conventional combustion must be performed under the fuel ratio or the lean state is shown, and Y (N) represents the first operation region I and the first operation region. The second boundary with the second operating region II is shown. The determination of the change of the operation region from the first operation region I to the second operation region II is performed based on the first boundary X (N), and the second operation region II is determined.
Is determined based on the second boundary Y (N).

That is, in the embodiment according to the present invention, when the operating state of the engine is in the first operating region I shown in FIG. 7B, low-temperature combustion is performed. At this time, when the required torque TQ exceeds a first boundary X (N), which is a function of the engine speed N, it is determined that the operation region has shifted to the second operation region II, and combustion is performed by a conventional combustion method. Next, the required torque TQ
Is lower than a second boundary Y (N) which is a function of the engine speed N, it is determined that the operating region has shifted to the first operating region I, and low-temperature combustion is performed again. As described above, the first boundary X (N) and the second boundary on the lower torque side than the first boundary X (N).
The two boundaries with the boundary Y (N) are provided for the following two reasons. The first reason is that the combustion temperature is relatively high on the high torque side of the second operating region II, and the required torque T
This is because even if Q becomes lower than the first boundary X (N), low-temperature combustion cannot be performed immediately. That is, the required torque T
This is because the low-temperature combustion does not start immediately unless Q becomes considerably low, that is, when it becomes lower than the second boundary Y (N). The second reason is that hysteresis is provided for a change in the operation range between the first operation range I and the second operation range II.

On the other hand, in FIG. 7A, in addition to the first boundary X (N) shown in FIG. 7B, when the air-fuel ratio is made considerably rich, for example, when the air-fuel ratio becomes approximately 13.5. Operating region Z in which good low-temperature combustion can be performed when the pressure is also reduced, and a high-load-side limit Z of the first operating region Z.
1 (N) and the low load limit Z2 (N) are shown. As can be seen from FIG. 7A, these limits Z1 (N)
And Z2 (N) are functions of the engine speed N.

As can be seen from FIG. 7 (B), there is no limit on the low load side in the first operation region I where the low-temperature combustion can be performed when the air-fuel ratio is almost the stoichiometric air-fuel ratio or lean. On the other hand, the low-load-side limit Z2 (N) in the first operating region Z in which the low-temperature combustion can be performed when the air-fuel ratio is considerably rich appears when the required torque TQ is negative. Therefore, it can be seen that the lower the lower the air-fuel ratio, the lower the load-side limit of the first operating region where the low-temperature combustion can be performed moves to the higher load side.

As shown in FIG. 7A, the high load side limit Z1 (N) of the first operation region Z in which low-temperature combustion can be performed when the air-fuel ratio is considerably rich is determined by the air-fuel ratio. The first that can perform low-temperature combustion at almost the stoichiometric air-fuel ratio or lean
Is higher than the high load side limit X (N) of the operation region I. Therefore, it can be seen that the lower the air-fuel ratio, the lower the first operating region in which low-temperature combustion can be performed moves to the higher load side.

That is, as described above, low-temperature combustion can be performed regardless of whether the air-fuel ratio is rich or lean.
However, when the fuel injection amount is extremely small, misfiring occurs if the air-fuel ratio is made considerably rich, so that good low-temperature combustion cannot be performed. That is, even when the fuel injection amount is extremely small, if the air-fuel ratio is lean, the fuel is actively burned because there is sufficient air around the fuel particles. On the other hand, when the air-fuel ratio is made considerably rich, the combustion of the fuel particles is not so active because there is not enough air around the fuel particles. At this time, if the fuel injection amount is extremely small, the combustion temperature and the combustion pressure will not sufficiently increase, and thus misfire will occur.

In FIG. 7A, the area where the required torque TQ is negative indicates the time of deceleration operation, and at this time, the fuel injection amount becomes extremely small. Therefore, in the region where the required torque TQ is negative, the low load side limit Z2 (N) of the first operation region Z appears. On the other hand, if the air-fuel ratio is made rich while low-temperature combustion is being performed near the first boundary X (N), the torque increases by the amount of fuel increase, and therefore Z1
(N) is on the higher load side than X (N).

By the way, when the operating state of the engine is in the first operating region I or Z and low-temperature combustion is being performed, soot is hardly generated, and instead, the unburned hydrocarbon is a precursor of soot or a former soot. It is discharged from the combustion chamber 5 in the form of a state. At this time, the unburned hydrocarbon discharged from the combustion chamber 5 is oxidized well by the catalyst 25 having an oxidizing function.

[0056] As the catalyst 25 can be used for the oxidation catalyst, three-way catalyst, or _the NO _x absorbent. _The NO _x absorbent when the mean air-fuel ratio in the combustion chamber 5 of the lean NO _x
And releases NO _x when the average air-fuel ratio in the combustion chamber 5 becomes rich. This _the NO _x absorbent is a carrier such as alumina, on the support such as potassium K, sodium Na, lithium Li, cesium Cs
Alkali metals such as barium Ba, calcium Ca
Alkaline earth, lanthanum La, yttrium Y
At least one selected from rare earths such as
And a noble metal such as

In addition to the oxidation catalyst, the three-way catalyst and the NO
_{The x} absorbent also has an oxidizing function, so that a three-way catalyst and a NO _x absorbent can be used as the catalyst 25 as described above. Next, with reference to FIG. 8, the operation control in the first operation region I and the second operation region II when the low-temperature combustion is performed under the condition that the air-fuel ratio is substantially the stoichiometric air-fuel ratio or lean will be schematically described. explain.

FIG. 8 shows the opening degree of the throttle valve 20, the opening degree of the EGR control valve 31, the EGR rate, the air-fuel ratio, the injection timing, and the injection amount with respect to the required torque TQ. As shown in FIG. 8, in the first operating region I where the required torque TQ is low, the opening of the throttle valve 20 is gradually increased from almost fully closed to about 2/3 as the required load L increases. The opening degree of the EGR control valve 31 is gradually increased from near full close to full open as the required load L increases.
Further, in the example shown in FIG.
The R rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio.

In other words, in the first operating region I, the EGR
The opening of the throttle valve 20 and the opening of the EGR control valve 31 are controlled such that the rate becomes approximately 70% and the air-fuel ratio becomes a slightly lean air-fuel ratio. 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 20 is closed until the valve is almost fully closed. At this time, the EGR control valve 31 is also closed almost completely. Throttle valve 2
If the valve is closed close to 0, 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 20 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 to the second operating region II, the opening of the throttle valve 20 is increased stepwise from about 2/3 opening toward the full opening direction. At this time, in the example shown in FIG. 8, 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 rate at which the EGR rate generates a large amount of smoke
The engine operating range is the first because it jumps over the rate range (Fig. 5).
A large amount of smoke does not occur when changing from the operating region I to the second operating region II.

In the second operation region II, the second combustion, that is, the conventional combustion is performed. In this combustion method generates little soot and NO _x, but the heat efficiency is higher than the low temperature combustion, thus as the operating region of the engine is shown in FIG. 8 from the first operation area I changes to the second operating region II Thus, the injection amount is reduced stepwise. In the second operation region II, the throttle valve 20 is held in a fully open state except for a part, and the opening of the EGR control valve 31 is gradually reduced as the required torque TQ increases. In addition, this operation area II
In this case, the EGR rate decreases as the required torque TQ increases, and the air-fuel ratio decreases as the required torque TQ increases. However, the air-fuel ratio is a lean air-fuel ratio even when the required torque TQ increases. In the second operation region II, the injection start timing θS is set near the compression top dead center TDC.

FIG. 9A shows the relationship among the required torque TQ, the depression amount L of the accelerator pedal 50, and the engine speed N. In FIG. 9A, each curve represents an equal torque curve, the curve indicated by TQ = 0 indicates that the torque is zero, and the remaining curves are TQ = a, TQ
The required torque gradually increases in the order of Q = b, TQ = c, and TQ = d. Further, TQ = -f and TQ = -g indicate the case where the required torque is negative, that is, the time of deceleration operation.
The required torque is smaller when Q = -g than when TQ = -f. The required torque TQ shown in FIG.
As shown in (2), a map is stored in advance in the ROM 42 as a function of the depression amount L of the accelerator pedal 50 and the engine speed N. In the embodiment according to the present invention, FIG.
The required torque TQ corresponding to the depression amount L of the accelerator pedal 50 and the engine speed N is first calculated from the map shown in FIG. 7, and the target air-fuel ratio and the like are calculated based on the required torque TQ.

By the way, the first operating region I where low temperature combustion is possible
On the high load side varies according to the gas temperature in the combustion chamber 5 at the start of compression, the cylinder wall surface temperature, and the like. That is, when the required torque TQ increases and the amount of heat generated by combustion increases, the temperature of fuel and the surrounding gas during combustion increases, and thus low-temperature combustion cannot be performed. On the other hand, when the gas temperature TG in the combustion chamber 5 at the start of compression decreases, the gas temperature in the combustion chamber 5 immediately before the start of combustion decreases, so that the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, if the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower, even if the amount of heat generated by combustion increases, that is, the required torque T
Even if Q increases, the temperature of the fuel and the surrounding gas during combustion does not increase, and low-temperature combustion is performed. In other words, the gas temperature T in the combustion chamber 5 at the start of compression
As G decreases, the first operating region I in which low-temperature combustion can be performed expands to a higher load side.

The smaller the temperature difference (TW-TG) between the cylinder inner wall surface temperature TW and the gas temperature TG in the combustion chamber 5 at the start of compression, the greater the amount of heat that escapes through the cylinder inner wall surface during the compression stroke. Therefore, the smaller the temperature difference (TW-TG), the smaller the temperature rise of the gas in the combustion chamber 5 during the compression process, and thus the lower the temperature of the fuel and the surrounding gas during combustion. Therefore, the temperature difference (TW
-TG) is smaller in the first operating region I where the lower temperature combustion is possible.
Will expand to the high load side.

On the other hand, in the intake passage, for example, the surge tank 1
The compression pressure in the combustion chamber 5 decreases as the pressure in the combustion chamber 2 decreases, and thus the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, as the pressure in the surge tank 12 becomes lower, the first operating region I in which low-temperature combustion can be performed expands to the higher load side. Further, as the humidity in the intake air increases, the amount of heat absorbed by the moisture contained in the intake air increases, and thus the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, as the humidity in the intake air increases, the first operating region I in which low-temperature combustion can be performed expands to the high load side.

In the embodiment according to the present invention, when the gas temperature TG in the combustion chamber 5 at the start of compression becomes low, the first boundary is moved from Xo (N) to X (N) as shown in FIG. When the difference (TW-TG) becomes smaller, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Further, in the embodiment according to the present invention, when the pressure PM in the surge tank 12 decreases, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Becomes higher, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Here, Xo (N) indicates a first boundary serving as a reference. First boundary Xo to be a reference
(N) is a function of the engine speed N, and X (N) is
It is calculated based on the following equation using o (N).

X (N) = Xo (N) + C1 · K (T) · K (N) K (T) = K (T)₁+ K (T)_Two+ K (T)_Three+ K
(T)_Four Where C1 is a constant, K (T)₁Is shown in FIG.
The relationship between the gas temperature TG in the combustion chamber 5 at the beginning of compression
K (T)₁Is the value at the beginning of compression
The temperature increases as the gas temperature TG in the chamber 5 decreases. Also,
K (T)_TwoIs the temperature difference (T) as shown in FIG.
W-TG), and this K (T) _TwoIs the temperature difference
It becomes larger as (TW-TG) becomes smaller. Also, K
(T)_ThreeIs a surge tank as shown in FIG.
12, which is a function of the pressure PM, this K (T)_ThreeThe value of
The lower the pressure PM in the surge tank 12, the greater
You. Also, K (T)_FourIs as shown in FIG.
It is a function of the humidity DF, and this K (T)_FourIs the humidity DF
The higher the value, the larger. It should be noted that FIG.
T at 11 (D)₁Is the reference temperature, T_TwoIs the reference temperature
Difference, PM_ThreeIs the reference pressure, DF_FourIs the reference humidity and TG
= T₁, (TW−TG) = T_Two, PM = PM_ThreeAnd DF
= DF_FourIn this case, the first boundary is Xo (N) in FIG.
You.

On the other hand, K (N) is a function of the engine speed N as shown in FIG. 11E, and the value of K (N) decreases as the engine speed N increases. That is, when the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower than the reference temperature T _1, the first boundary X (N) becomes Xo (N) as the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower. To the high load side, and the temperature difference (TW-TG) becomes equal to the reference temperature difference T.
_If it is lower than _2, the first boundary X (N) moves to a higher load side with respect to Xo (N) as the temperature difference (TW-TG) becomes smaller. Further, as the pressure PM in the surge tank 12 is pressure PM of comprising a surge tank 12 lower than the reference pressure PM ₃ becomes lower first boundary X (N) is Xo (N)
To the high load side, and the humidity DF becomes the reference humidity DF ₄
As the humidity DF increases, the first boundary X (N) moves toward a higher load with respect to Xo (N). Further, the moving amount of X (N) with respect to Xo (N) decreases as the engine speed N increases.

FIG. 12A shows the air-fuel ratio A / F in the first operating region I when the first boundary is the first boundary Xo (N) as a reference. In FIG. 12A, A / F = 15, A / F = 16, A / F = 17, A / F
Each curve shown by F = 18 has an air-fuel ratio of 15, 16,
17 and 18, and the air-fuel ratio between the curves is determined by proportional distribution. As shown in FIG. 12A, 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. As the EGR rate is reduced, the air-fuel ratio increases. Therefore, as shown in FIG. 12A, as the required load L decreases, the air-fuel ratio A / F increases. As the air-fuel ratio A / F increases, the fuel consumption rate increases. Accordingly, in order to make the air-fuel ratio as lean as possible, in the embodiment according to the present invention, the air-fuel ratio A / F increases as the required load L decreases.

FIG. 12B shows the air-fuel ratio A / F in the first operating region I when the first boundary is X (N) shown in FIG. As can be seen by comparing FIGS. 12 (A) and 12 (B), when the first boundary X (N) moves toward the high load side with respect to Xo (N), A / F = 15, A / F = 16, A / F = 17, A / F
The curve of F = 18 also moves to the high load side. Therefore, it can be seen that when the first boundary X (N) moves toward the higher load side with respect to Xo (N), the air-fuel ratio A / F at the same required load L and the same engine speed N increases. That is, when the first operation region I is expanded toward the high load side, not only is the operation region where soot and NO _x are hardly generated generated, but also the fuel consumption rate is improved.

In the embodiment according to the present invention, the first boundary X
The target air-fuel ratio in the first operating region I when (N) changes variously, that is, the target air-fuel ratio in the first operating region I for various values of K (T) is shown in FIG.
As shown in FIG. 3 (D), it is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N. That is, FIG. 13A shows that the value of K (T) is K
FIG. 1 shows the target air-fuel ratio AFKT1 at the time of T1, and FIG.
3 (B) is the target air-fuel ratio A when the value of K (T) is KT2.
FIG. 13C shows the target air-fuel ratio AFKT3 when the value of K (T) is KT3, and FIG. 13D shows the target air-fuel ratio when the value of K (T) is KT4. An air-fuel ratio AFKT4 is shown.

On the other hand, the target opening degree of the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio is shown in FIG.
As shown in FIG. 4 (D), the EGR control valve 31 is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N, and is necessary for setting the air-fuel ratio to the target air-fuel ratio. The target opening degree of FIG.
As shown in FIG. 5 (D), a map is stored in advance in the ROM 42 as a function of the required torque TQ and the engine speed N.

That is, FIG. 14A shows the target opening ST15 of the throttle valve 20 when the air-fuel ratio is 15;
FIG. 15A shows the EGR control valve 31 when the air-fuel ratio is 15.
Shows the target opening degree SE15. FIG. 14B
Is the target opening S of the throttle valve 20 when the air-fuel ratio is 16.
FIG. 15B shows the target opening SE16 of the EGR control valve 31 when the air-fuel ratio is 16.

FIG. 14C shows the target opening ST17 of the throttle valve 20 when the air-fuel ratio is 17.
FIG. 15B shows the EGR control valve 31 when the air-fuel ratio is 17.
Shows the target opening degree SE17. FIG. 14 (D)
Is the target opening S of the throttle valve 20 when the air-fuel ratio is 18.
FIG. 15 (B) shows the target opening degree SE18 of the EGR control valve 31 when the air-fuel ratio is 18.

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 2 required to set air-fuel ratio to this target air-fuel ratio
The target opening ST of 0 is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N as shown in FIG. 17A, and the air-fuel ratio is set as the target air-fuel ratio. 17B, the target opening SE of the EGR control valve 31 required for this is stored in a ROM in advance in the form of a map as a function of the required torque TQ and the engine speed N.
42.

On the other hand, in the first operating region Z where low-temperature combustion can be performed when the air-fuel ratio is considerably rich, the gas temperature TG in the combustion chamber 5 at the start of compression and the temperature difference between the cylinder inner wall surface temperature TW and the gas temperature TG are also determined. (TW-TG), and changes depending on the pressure PM in the surge tank 12 and the humidity DF in the intake air. In this case, as in the first operation region I, the first operation region Z is also moved to a higher load side as the temperature of the fuel and the surrounding gas during combustion becomes lower.

That is, in FIG. 18, when Zo is a first operation region as a reference, Z1o (N) is a high load limit as a reference, and Z2o (N) is a low load limit as a reference, these reference values are satisfied. When the temperature of the fuel and the surrounding gas during combustion is lower than in the case of (1), both the high load side limit Z1 (N) and the low load side limit Z2 (N) are moved to the high load side. The operation area Z is also moved to the high load side.

At this time, the high load side limit Z 1 (N) and the low load side limit Z 2 (N) are also the respective values K shown in FIG.
(T) ₁ , K (T) ₂ , K (T) ₃ , K (T) ₄ , K
It is calculated from the following equation using (N). Z1 (N) = Z1o (N) + C2 · K (T) · K (N) Z2 (N) = Z2o (N) + C3 · K (T) · K (N) K (T) = K (T) ₁ + K (T) ₂ + K (T) ₃ + K
(T) ₄ where C2 and C3 are constants.

Therefore, when the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower than the reference temperature T ₁ (FIG. 11), the lower the gas temperature TG in the combustion chamber 5 at the start of compression becomes, the lower Z1 (N) and Z2 ( N) moves to the higher load side with respect to Z1o (N) and Z2o (N), respectively, and the temperature difference (TW
−TG) becomes lower than the reference temperature difference T ₂ (FIG. 11), the smaller the temperature difference (TW−TG), the more Z1 (N) and Z2 (N) are compared to Z1o (N) and Z2o (N), respectively. Move to high load side. When the pressure PM in the surge tank 12 becomes lower than the reference pressure PM ₃ (FIG. 11), the lower the pressure PM in the surge tank 12 becomes, the lower the pressure PM becomes.
(N) and Z2 (N) are Z1o (N) and Z2, respectively.
When the humidity DF is higher than the reference humidity DF ₄ (FIG. 11), the higher the humidity DF, the more Z1 (N) and Z2 (N) become Z1o.
(N) and move to the high load side with respect to Z2o (N).

As described above, an oxidation catalyst, a three-way catalyst, or a NO _x absorbent can be used as the catalyst 25. Hereinafter, a case where the NO _x absorbent is used as the catalyst 25 will be described. Engine intake passage, NO of Toko called air-fuel ratio of exhaust gas flowing the ratio of the combustion chamber 5 and _the NO _x absorbent 25 upstream of the exhaust passage supplying air and fuel into the (hydrocarbon) to _the NO _x absorbent 25 _x absorbent 25 absorbs the NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, perform absorption and release action of the NO _x that releases NO _x the air-fuel ratio is absorbed and becomes the stoichiometric air-fuel ratio or rich of the inflowing exhaust gas .

If this NO _x absorbent 25 is disposed in the exhaust passage of the engine, the NO _x absorbent 25 actually performs the absorption and release of NO _x , but the detailed mechanism of this absorption and release is not clear in some parts. . However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, 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 performed with the air-fuel ratio in the normal combustion chamber 5 being 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. 19 (A) at this time O
₂ ^- or O ^2- shape is deposited on the surface of the platinum Pt. on the other hand,
NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2N).
O ₂ ). Next, a part of the generated NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt, and the barium oxide BaO
As shown in FIG. 19 (A), it diffuses into the absorbent in the form of nitrate ion NO ₃ ⁻ while being combined. In this way, NO _x is absorbed in the NO _x absorbent 25. NO ₂ is produced on the surface of the platinum Pt so long as the oxygen concentration in the inflowing exhaust gas is high, NO unless absorption of NO _x capacity of the absorbent is not saturated
₂ is absorbed in the absorbent and nitrate ions NO ₃ ^- are produced.

On the other hand, the air-fuel ratio of the inflowing exhaust gas is made rich.
The concentration of oxygen in the incoming exhaust gas decreases,
NO on the surface of gold Pt_TwoIs reduced. NO_Twoof
When the amount of production decreases, the reaction reverses (NO_Three ^-→ NO_Two)
And thus nitrate ion NO in the absorbent_Three ^-Is NO
_TwoReleased from the absorbent in the form of NO at this time_xAbsorbent
NO released from 25_xIs shown in FIG. 19 (B).
A large amount of unburned HC and CO contained in the inflow exhaust gas
It is reduced by reaction. In this way, platinum Pt
NO on surface_TwoWhen no longer exists, the next
NO_TwoIs released. Therefore, the air-fuel ratio of the incoming exhaust gas
Is enriched in a short time, NO _xAbsorbent 25?
NO_xIs released, and the released NO_xIs returned
NO in the atmosphere to be removed_xWill not be released
No.

In this case, the air-fuel ratio of the inflowing exhaust gas is
NO even at stoichiometric air-fuel ratio_xNO from absorbent 25_xIs released
Is done. However, the air-fuel ratio of the inflowing exhaust gas is
NO if ratio_xNO from absorbent 25_xGradually
NO because only _xAbsorbed by the absorbent 25
All NO_xIt takes a slightly longer time to release.

[0087] Incidentally there is a limit to the absorption of NO _x capacity of the NO _x absorbent 25, 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 this embodiment of the present invention of a map as shown in FIG. 20 (A) as a function of the NO _x absorption amount A of the required torque TQ and engine speed N per unit time when it is performed first combustion is previously obtained in the form, 20 of a map as shown in (B) as a function of per unit time of the NO _x absorption amount B of the required torque TQ and engine speed N when the second combustion is being performed In advance, the N per unit time
O _x absorption amount A, so that to estimate the amount of NO _x ΣNOX being absorbed in the NO _x absorbent 25 by integrating the B.

[0088] In the embodiment according to the present invention N when exceeding the allowable maximum value that this absorption of NO _x amount ΣNOX reaches a predetermined
NO _x is released from the O _x absorbent 25. Next, this will be described with reference to FIG. Referring to FIG. 21, in the embodiment according to the present invention, there are two allowable maximum values, namely, an allowable maximum value MAX1 and an allowable maximum value MA.
X2 is set. The maximum allowable value MAX1 is NO _x
The maximum absorption amount MAX2 is about 30% of the maximum NO _x absorption amount that the absorbent 25 can absorb, and the allowable maximum value MAX2 is about 80% of the maximum absorption amount that the NO _x absorbent 25 can absorb. NO _x during the first combustion
NO fuel ratio in order to release the NO _x from _x absorbent 25 is made rich, NO _x absorption amount ΣNOX permissible maximum when the second combustion is being performed when the absorption amount ΣNOX has exceeded the allowable maximum value MAX1 When the value exceeds MAX1, the second
When switched from the combustion to the first combustion, the air-fuel ratio is made rich so as to release the NO _x from the NO _x absorbent 25 for example during deceleration operation, N when the second combustion is being performed
O _x absorption amount ΣNOX additional fuel late or during the exhaust stroke of the expansion stroke so as to release the NO _x from the NO _x absorbent 25 when exceeding the maximum allowable value MAX2 is injected.

That is, in FIG. 21, the period X shows a case where the required torque TQ is lower than the first boundary X (N) and the first combustion is performed, and the air-fuel ratio at this time is the stoichiometric air-fuel ratio. The air-fuel ratio is slightly leaner than that. First when combustion is being performed extremely small generation amount of the NO _x is therefore of absorption of NO _x amount ΣNOX as is shown in Figure 21 at this time increases very slowly. Absorption of NO _x amount ΣN when first combustion is being 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 set to zero.

As described above, when the first combustion is being performed, no soot is generated regardless of whether the air-fuel ratio is lean, the stoichiometric air-fuel ratio, or the rich, so that the first combustion is performed. and does not soot generated at this time is also the air-fuel ratio a / F in order to release the nO _x from the nO _x absorbent 25 is made rich when being. Then the required torque TQ at the time t ₁ is switched from the first combustion exceeds the first boundary X (N) to the second combustion. As shown in FIG. 21, when the second combustion is performed, the air-fuel ratio A / F becomes considerably lean. When the second combustion is being performed many generation amount of the NO _x as compared with the case where the first combustion is being performed, therefore when the second combustion is being performed NO _x
The amount ΣNOX rises relatively quickly.

When the second combustion is being performed, the air-fuel ratio A
When / F is made rich, a large amount of soot is generated, and therefore, the air-fuel ratio A / F cannot be made rich during the second combustion. Thus the air-fuel ratio in order to release the NO _x from the NO _x absorbent 25 even absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX1 when the second combustion is being performed as shown in FIG. 21 A / F is not made rich.
The required torque TQ in the case where the second boundary Y (N) from so as to release the NO _x from the NO _x absorbent 25 after being switched from the second combustion is lower in the first combustion also the air-fuel ratio A
/ F is temporarily made rich.

By the way, at time t _{2 in} FIG. 21, a case is shown in which the deceleration operation is performed, whereby the second combustion is switched to the first combustion. When the deceleration operation is performed, the required torque TQ becomes negative. As a result, as shown in FIG. 18, the case where the air-fuel ratio can be made rich by the position of the low load side limit Z2 (N) in the first operation region Z, May not be rich.

Therefore, in the embodiment according to the present invention, when the air-fuel ratio is to be made rich, it is determined whether or not the operating state of the engine is in the first operating area Z, and the operating state of the engine is set in the first operating area Z. air-fuel ratio a / F in order to release the NO _x from the NO _x absorbent 25 when it is switched from the second combustion to the first combustion, as shown at time t ₂ in FIG. 21 when located within
Is temporarily enriched.

[0094] Then the first combustion is switched to the second combustion at the time t ₃ in FIG. 21, the second combustion during interim pleasure continues. 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 additional fuel is injected, the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent 25 is made rich.

[0095] The additional fuel injected during the second half of the expansion stroke or during the exhaust stroke does not contribute to the generation of engine power, and therefore it is preferable to inject as little additional fuel as possible. Thus when the absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX1 when the second combustion is performed second
When the air-fuel ratio A /
Temporarily rich, F, and so as to inject additional fuel only if special where absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX2.

FIG. 22 shows a routine for processing a NO _x release flag which is set when NO _x is to be released from the NO _x absorbent 25. This routine is executed by interruption every predetermined time. Referring to FIG. 22, first, at step 100, it is determined whether or not a flag I indicating that the operating region of the engine is the first operating region I is set. When the flag I is set, that the operating region of the engine 20 from the map shown in (A) per unit time of absorption of NO _x amount A is calculated proceeds to step 101 when a first operating region I You. Then A is added to the absorption of NO _x amount ΣNOX step 102. Next, at step 103 NO _x absorption amount ΣNOX whether exceeds the allowable maximum value MAX1 is determined. ΣN
Becomes the OX> MAX1 proceeds to step 104, NO _x releasing flag 1 indicating that it should release the NO _x is set when the first combustion is being performed.

On the other hand, when it is determined in step 100 that the flag I has been reset, that is, when the operation region of the engine is the second operation region II, step 106
Figure 20 NO _x absorption amount B per from the map shown in (B) per unit time is calculated willing to. Next, at step 107 NO _x absorption amount ΣNOX is added to B. Next, at step 108 NO _x absorption amount ΣNOX permissible maximum value MA
It is determined whether or not X1 has been exceeded. ΣNOX> MAX1
Advances to become the step 109, NO _x releasing flag 1 indicating that it should release the NO _x is set when the first combustion is being performed.

[0098] On the other hand, in step 110, whether absorption of NO _x amount ΣNOX has exceeded the allowable maximum value MAX2 is determined. .SIGMA.NOX> becomes the MAX2 proceeds to step 111, it is a set _the NO _x releasing flag 2 indicating that it should release the second half or NO _x in the exhaust stroke of the expansion stroke. FIG.
Reference numeral 3 denotes a routine for controlling the low-temperature combustion region, that is, the first operation regions I and Z.

Referring to FIG. 23, first, at step 200, the gas temperature T in the combustion chamber 5 at the start of compression is set.
G, the cylinder inner wall surface temperature TW, the pressure PM in the surge tank 12, and the humidity DF in the intake air are calculated. In this embodiment, the temperature of the mixed gas of the intake air and the EGR gas detected by the temperature sensor 62 is used as the gas temperature TG in the combustion chamber 5 at the start of compression, and the engine cooling water temperature detected by the temperature sensor 60 is the cylinder inner wall surface temperature. TW. The pressure PM in the surge tank 12 is detected by a pressure sensor 61, and the humidity DF is detected by a humidity sensor 63. Next, in step 201, FIG.
From the relationship shown in FIG. 1 (D), K (T) ₁ , K (T) ₂ , K
(T) ₃ and K (T) ₄ are obtained. By adding K (T) ₄ from K (T) ₁ , K (T) (= K
(T) ₁ + K (T) ₂ + K (T) ₃ + K (T) ₄ ) is calculated.

Next, at step 202, K (N) is calculated from the relationship shown in FIG. Next, at step 203, the value of the first boundary X (N) is calculated based on the following equation using the value of the first boundary Xo (N) stored in advance. X (N) = Xo (N) + C1 · K (T) · K (N) Next, at step 204, the difference ΔL (N) between X (N) and Y (N) that changes according to the engine speed N is calculated. Is calculated.
Next, in step 205, the value of the second boundary Y (N) (= X
(N) −ΔL (N)) is calculated. Then step 2
At 06, the high-load-side limit Z1o (N) stored in advance.
Is used to calculate the high load limit Z1 (N) from the following equation.

Z1 (N) = Z1o (N) + C2 · K (T) · K (N) Next, in step 207, the low load side limit Z2o (N) stored in advance is used to calculate the low load Side limit Z
2 (N) is calculated. Z2 (N) = Z2o (N) + C3 · K (T) · K (N) Next, the operation control will be described with reference to FIG.

Referring to FIG. 24, first, at step 300, it is determined whether or not a flag I indicating that the operating state of the engine is in the first operating region I is set. If the flag I is set, that is, if the operating state of the engine is in the first operating region I, step 3
In step 01, 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, in step 303, FIG.
The target opening ST of the throttle valve 20 is calculated from the map shown in FIG. 2, and the opening of the throttle valve 20 is set as the target opening ST. Next, at step 304, the target opening SE of the EGR control valve 31 is calculated from the maps shown in FIGS. 15A to 15D, and the opening of the EGR control valve 31 is set to the target opening SE. Next, at step 305 NO _x releasing flag 1 is whether it is set or not. N
When the _Ox release flag 1 has not been set, the routine proceeds to step 307, where fuel injection is performed. At this time, low-temperature combustion is performed under a lean air-fuel ratio.

[0103] On the other hand, the operating state of the engine proceeds to step 306 when the _the NO _x releasing flag 1 in step 305 is determined to have been set first operating region Z
Is determined. If the operating state of the engine is not in the first operating region Z, the routine proceeds to step 307, where low-temperature combustion is performed under a lean air-fuel ratio. On the other hand, when the operating state of the engine is in the first operating region Z, the routine proceeds to step 308, where the air-fuel ratio is made rich for a predetermined period. NO _x is released from the NO _x absorbent 25 during this time. Next, the NO _x release flag 1 is reset, and ΣNO
X is cleared.

On the other hand, in step 301, L> X
When it is determined that (N) has been reached, the routine proceeds to step 302, where the flag I is reset.
And the second combustion is performed. That is, step 311
In FIG. 17A, the target opening ST of the throttle valve 20 is calculated from the map shown in FIG. 17A, and the opening of the throttle valve 20 is set to the target opening ST. Next, at step 312, the target opening SE of the EGR control valve 31 is calculated from the map shown in FIG. 17B, and the opening of the EGR control valve 31 is set as the target opening SE. Then whether _the NO _x releasing flag 2 in step 313 is set or not. N
When the _Ox release flag 2 is not set, the routine proceeds to step 314, where fuel injection is performed so as to achieve the air-fuel ratio shown in FIG. At this time, the second combustion is performed under the lean air-fuel ratio.

[0105] On the other hand, predetermined period proceeds to step 315 when it is judged to have been set _the NO _x releasing flag 2 in step 313, additional fuel is injected during the latter half of the expansion stroke or exhaust stroke. Then N
Air-fuel ratio of the exhaust gas flowing into the O _x absorbent 25 becomes rich, NO _x is released from the NO _x absorbent 25 during this time. Then _the NO _x releasing flag 1 and 2 are reset, .SIGMA.NOX is cleared.

[0106]

The stable low-temperature combustion according to the air-fuel ratio can be ensured by changing the region where low-temperature combustion is possible according to the air-fuel ratio.

[Brief description of the 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 fuel molecules.

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

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

FIG. 7 shows a first operation region I, Z and a second operation region II.
FIG.

FIG. 8 is a view showing an opening degree of a throttle valve and the like.

FIG. 9 is a diagram showing a required torque.

FIG. 10 is a diagram illustrating a first boundary Z (N).

FIG. 11 is a diagram showing K (T) ₁ to K (T) ₄ and K (N).

FIG. 12 is a diagram showing a target air-fuel ratio in a first operation region I.

FIG. 13 is a diagram showing a map of a target air-fuel ratio.

FIG. 14 is a view showing a map of a target opening of a throttle valve.

FIG. 15 is a view showing a map of a target opening of the EGR control valve.

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.

FIG. 18 is a diagram showing a first operation region Z.

FIG. 19 is a view for explaining the NO _x releasing action.

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

FIG. 21 is a diagram for explaining NO _x release control.

FIG. 22 is a flowchart for processing a NO _x release flag.

FIG. 23 is a flowchart for controlling a low-temperature combustion region.

FIG. 24 is a flowchart for controlling the operation of the engine.

[Explanation of symbols]

 6. Fuel injection valve 20 Throttle valve 31 EGR control valve

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 21/08 F02D 21/08 L 41/14 310 41/14 310C (72) Inventor Takekazu Ito Aichi 1 Toyota Town, Toyota City Inside Toyota Motor Co., Ltd. (72) Inventor Yoshi ▲ saki Koji 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Hiroki Murata 1 Toyota Town, Toyota City, Aichi Prefecture Address Toyota Motor Corporation (72) Inventor Tsukasa Abe 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F Term (Reference) 3G062 AA01 AA05 AA06 BA02 BA04 BA05 BA06 CA07 CA08 ED08 GA01 GA04 GA05 GA06 GA10 GA15 GA21 3G091 AA10 AA11 AA13 AA18 AA28 AB02 AB03 AB05 BA01 BA14 CB02 CB03 CB07 EA01 EA02 EA07 EA08 EA09 EA34 FA07 FA13 FA14 FB09 GB12W GB13W GB14W GB15W GB16W GB20X HB05 3G092 AA02 AA06 AA09 AA17 AA18 AB03 BA01 BA03 BA04 BB01 BB06 BB08 DB03 DC01 DE03S EA05 EA06 EA07 FA17 FA18 GA05 GA06 GA17 GA18 HA01X HA04X HA06X HA15X HB03HE03 HEB03XB033 KA09 KA24 KA25 LA01 LB04 LB11 LB13 MA01 MA11 MA18 NC04 PA01A PA10A PA11A PA17A PB03A PB05A PB08A PD15A PE01A PE03A PE06A

Claims (14)

[Claims] When the amount of inert gas in the combustion chamber increases, the amount of soot generated gradually increases and reaches a peak. When the amount of inert gas in the combustion chamber further increases, the amount of soot generated during combustion in the combustion chamber increases. In an internal combustion engine in which the temperature of the fuel and its surrounding gas is lower than the soot generation temperature and soot is hardly generated, the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the amount of soot generated peaks. Switching means for selectively switching between first combustion in which a large amount of soot is hardly generated and second combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the amount of generated soot becomes a peak; The first operating region on the low load side where the operating region of the engine can perform the first combustion and the second operating region.
The internal combustion engine is divided into a high load side second operation region in which combustion of the first fuel cell is performed, and the first operation region is moved to a high load side as the air-fuel ratio decreases. 2. The internal combustion engine according to claim 1, wherein the higher load limit and the lower load limit of the first operating range are shifted to the higher load side as the air-fuel ratio decreases. 3. A low-load limit in the first operating range does not exist when the air-fuel ratio is lean, and a low-load limit in the first operating range appears when the air-fuel ratio is rich. An internal combustion engine as described. 4. The internal combustion engine according to claim 1, further comprising control means for controlling a first operation range, wherein the control means controls the first operation range in accordance with a target air-fuel ratio. 5. The internal combustion engine according to claim 1, wherein the first operating region is shifted to a higher load side as the temperature of the fuel and the surrounding gas during the first combustion decreases. 6. When the first combustion is performed under a rich air-fuel ratio, as the temperature of the fuel and the surrounding gas during combustion decreases, the high load limit and the low load of the first operation region are reduced. 6. The internal combustion engine according to claim 5, wherein the side limit is shifted to a high load side. 7. A control device for controlling a first operating range based on a value of a parameter that changes the temperature of fuel and gas around the fuel during the first combustion, wherein the control device performs control based on the value of the parameter. 6. The internal combustion engine according to claim 5, wherein when it is determined that the temperature of the fuel and the surrounding gas during the first combustion decreases, the first operation range is shifted to the high load side. 8. The internal combustion engine according to claim 7, wherein said parameters comprise at least one of a temperature of gas flowing into the combustion chamber, a temperature of engine cooling water, a pressure in an engine intake passage, and a humidity of intake air. 9. An exhaust gas recirculation device for recirculating exhaust gas discharged from a combustion chamber into an engine intake passage,
The internal combustion engine of claim 1, wherein said inert gas comprises recirculated exhaust gas. 10. The internal combustion engine according to claim 9, wherein an exhaust gas recirculation rate during the first combustion is substantially equal to or greater than 55%. 11. The internal combustion engine according to claim 1, wherein a catalyst having an oxidation function is disposed in the engine exhaust passage. 12. The internal combustion engine according to claim 11, wherein said catalyst comprises an oxidation catalyst or a three-way catalyst. 13. The catalyst, the air-fuel ratio of the inflowing exhaust gas absorbed and the air-fuel ratio of the exhaust gas absorb and flowing the NO _x contained in the exhaust gas becomes the stoichiometric air-fuel ratio or rich when the lean NO the internal combustion engine of claim 11 consisting of _the NO _x absorbent to release the _x. Claim operating condition of 14. engine air-fuel ratio is the air-fuel ratio in order to release the NO _x from the NO _x absorbent when in the first operating region when it is rich is made rich 13
An internal combustion engine according to claim 1.