JP3427731B2 - 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 In a compression ignition type internal combustion engine in which a lean mixture is burned, NO _x is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and when the inflowing exhaust gas becomes a rich or stoichiometric air-fuel ratio. the _the NO _x absorbent to release the absorbed NO _x arranged in the engine exhaust passage, the NO _x generated when the lean air-fuel mixture is burned is absorbed by _the NO _x absorbent, NO from _the NO _x absorbent There is known a compression ignition type internal combustion engine in which a throttle valve is closed to a constant opening when _x is to be released, and a fuel injection amount is increased to temporarily make an air-fuel ratio rich (Patent Publication No. 2600492). No.).

[0003]

By the way, NO_xAbsorbent
To NO_xWhen the air-fuel ratio is made rich to release
When the air-fuel ratio rich degree is small or the air-fuel ratio is rich
NO if the time is short _xTotal N absorbed in the absorbent
O_xNO because it cannot release_xAbsorbent NO_xAbsorption capacity
The force gradually decreases and, on the other hand, the degree of air-fuel ratio rich
Is large or the air-fuel ratio rich time is long, NO
_xAbsorbent to all NO_xRich air-fuel ratio even after is released
During this period, unburned HC and CO are discharged into the atmosphere.
Will be issued. That is, NO_xAbsorbent to NO_xTo
The degree of richness of the air-fuel ratio when releasing and the air-fuel ratio
Rich time is NO_xAbsorbent to all NO_xLet's just release
Rich degree and rich time
It

In the compression ignition type internal combustion engine, the optimum throttle valve opening and the fuel injection amount are usually obtained as a function of the operating state of the engine, for example, as a function of the required load and the engine speed. The optimum throttle opening and fuel injection amount that have been obtained are stored in the form of a map, and the throttle valve opening and fuel injection amount are controlled to the values stored in the map during engine operation.

However, if the engine is used for a long period of time, the actual throttle valve opening and fuel injection amount will deviate from their normal values. As a result, when the air-fuel ratio is made rich based on the throttle opening and the fuel injection amount stored in the map, the degree of richness of the air-fuel ratio and the rich time of the air-fuel ratio deviate from the normal values, thus NO
absorption _of NO _x capacity of the _x absorbent is prevented from lowering and unburnt HC, while preventing the CO is discharged to the atmosphere NO _x
There is a problem that it is not possible to release all the NO _x absorbed in the absorbent.

[0006]

In order to solve the above problems, in the first invention, when the air-fuel ratio of the inflowing exhaust gas is lean, the exhaust gas that absorbs NO _x contained in the exhaust gas and flows in. _the NO _x absorbent when the air-fuel ratio is arranged in _the NO _x absorbent the engine exhaust passage that releases NO _x absorbed and becomes the stoichiometric air-fuel ratio or rich, burning fuel under a lean air-fuel ratio has been made In the internal combustion engine in which the fuel injection amount is increased to temporarily make the air-fuel ratio rich when releasing NO _x from the
The amount of soot generated gradually increases as the amount of inert gas increases.
Reaches its peak and further increases the amount of inert gas in the combustion chamber
As the fuel is burned in the combustion chamber and its surroundings
The gas temperature in the enclosure is lower than the soot formation temperature and the soot is almost
It consists of a compression ignition type internal combustion engine that does not generate
The amount of inert gas in the combustion chamber is more
The first combustion with a large amount of somatic gas and almost no soot,
The amount of soot generated peaks in the combustion chamber rather than the amount of inert gas
Second combustion with a small amount of inert gas is selectively switched
And switching means that, should release the NO _x from the NO _x absorbent
Determination means for determining whether or not there exists, detection means for detecting the intake air amount, and the detected intake air amount / fuel injection amount to be supplied when NO _x should be released from the NO _x absorbent =
The target rich air-fuel ratio from the relational expression, the calculating means for calculating either the target fuel injection amount or the target intake air amount necessary to make the air-fuel ratio the target rich air-fuel ratio.
It was judged by the disconnecting means that NO _x should be released.
When the first combustion is performed, the fuel injection amount or the intake air amount is set to the target fuel injection amount or the target intake air amount , whereby the air-fuel ratio is set to the target rich air-fuel ratio.

[0007]

[0008]

[0009] In the first aspect of the present invention is in the second invention,
NO from the NO _x absorbent when the second combustion is taking place
When it is determined that _x should be released, the air-fuel ratio is temporarily made rich when the second combustion is switched to the first combustion. A third aspect of the present invention is provided with an estimating means for estimating the NO _x amount absorbed in the NO _x absorbent in the first aspect , and the NO estimated by the estimating means when the second combustion is being performed. First with a predetermined _x amount
When the maximum allowable value is exceeded, the air-fuel ratio is temporarily made rich when the second combustion is switched to the first combustion, and is estimated by the estimating means when the second combustion is being performed. When the NO _x amount exceeds the second allowable maximum value that is larger than the first allowable maximum value, additional fuel is injected and flows into the NO _x absorbent during the latter half of the expansion stroke or during the exhaust stroke. The air-fuel ratio of the exhaust gas is made to be the stoichiometric air-fuel ratio or rich.

[0010] In the first aspect of the present invention in the fourth aspect of the invention,
A recirculation device for recirculating exhaust gas discharged from the combustion chamber into the engine intake passage is provided, and the inert gas is recirculated exhaust gas. In the fifth aspect in the fourth invention, the exhaust gas recirculation rate in the first combustion state is substantially 5
It is 5% or more.

[0011] In the first aspect of the present invention is in the sixth invention,
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, first combustion is performed in the first operating region, and second operation region is performed in the second operating region. I try to burn it. In the seventh invention, in the first invention,
A catalyst having an oxidizing function is arranged in the engine exhaust passage upstream of the NO _x absorbent.

[0012]

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 to an outlet portion of a compressor 16 of an exhaust turbocharger 15 via an intake duct 13 and an intercooler 14. Compressor 16
The inlet part of is connected to an air cleaner 18 via an air suction pipe 17. On the other hand, the exhaust port 10 is the exhaust manifold 1
Exhaust turbine 2 of exhaust turbocharger 15 via 9
The exhaust turbine 20 has an outlet connected to a casing 23 containing a NO _x absorbent 22 via an exhaust pipe 21.

A step motor 24 is provided in the intake duct 13.
A throttle valve 25 driven by the above is arranged, and an intake air amount sensor 26 for detecting a mass flow rate of intake air is arranged in the air suction pipe 17. The exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 27, and the EGR passage 27
An EGR control valve 29 driven by a step motor 28 is arranged inside. In addition, E around the EGR passage 27
A cooling device 30 for cooling the EGR gas flowing in the GR passage 27 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 30, and the engine cooling water cools the EGR gas.

On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 32, via a fuel supply pipe 31. Fuel is supplied into the common rail 32 from an electrically controlled variable fuel discharge pump 33, and the fuel supplied into the common rail 32 is supplied to the fuel injection valve 6 via each fuel supply pipe 31. A fuel pressure sensor 34 for detecting the fuel pressure in the common rail 32 is attached to the common rail 32, and the fuel pump 33 is arranged so that the fuel pressure in the common rail 32 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 34. 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 intake air amount sensor 26 is the corresponding AD
It is input to the input port 45 via the converter 47, and the output signal of the fuel pressure sensor 34 is also input to the input port 45 via the corresponding AD converter 47. 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.
Is output to the input port 45 via the corresponding AD converter 47. A crank angle sensor 52 that generates an output pulse each time the crankshaft rotates, for example, 30 ° is connected to the input port 45. On the other hand, the output port 46 is connected to the fuel injection valve 6, the throttle valve control step motor 24, the EGR control valve control step motor 28, and the fuel pump 33 via the corresponding drive circuit 48.

Conventionally, in an internal combustion engine, for example, a compression ignition type engine, an engine exhaust passage and an engine intake passage are connected by an EGR passage in order to suppress the generation of NO _x ,
Exhaust gas, that is, EGR gas, is recirculated through the EGR passage into the engine intake passage. In this case, since the EGR gas has a relatively high specific heat and can absorb a large amount of heat, the EGR gas amount increases, that is, the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)). Is increased, the combustion temperature in the combustion chamber decreases. 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 not exceeding 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 compression ignition type internal combustion engine, the EGR rate is suppressed to about 30% to 50% at the maximum.

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 allowable limit in the process of studying the combustion of the compression ignition type engine, the smoke increases sharply as described above, but there is a peak in the amount of smoke generated, and this peak is present. Beyond EGR
When the rate is further increased, the smoke starts to decrease sharply this time. When the EGR rate is 70% or more during idling operation, and when the EGR gas is strongly cooled, the EGR rate is almost 55% or more. It was found that it was almost zero, that is, soot was 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.

That is, it was found as a result of repeated experimental studies that when the temperature of the fuel during combustion in the combustion chamber and the temperature of the gas around it were lower than a certain temperature, the growth of hydrocarbons stopped in the middle stage 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 are 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 disappear. 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.

FIG. 1 shows a compression ignition type internal combustion engine which employs this new combustion system. FIG. 2 shows that in the compression ignition type internal combustion engine shown in FIG. 1, the air-fuel ratio A / F (horizontal axis in FIG. 2) is changed by changing the opening degree of the throttle valve 25 and the EGR rate during engine low load operation. Change of output torque at the time of smoke, HC, CO, NO
_It shows an experimental example showing the change in the emission amount of _x . As can be seen from FIG. 2, in this experimental example, the smaller the air-fuel ratio A / F, the larger the EGR rate, and the theoretical air-fuel ratio (≈14.6)
In the following cases, the EGR rate is 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. 3A shows the change in the combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is around 18 and the amount of smoke generated is the largest, and FIG. 3B shows the air-fuel ratio A / F. It shows a change in the combustion pressure in the combustion chamber 5 when F is around 13 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.

Secondly, when the amount of smoke produced, that is, the amount of soot produced, becomes almost zero, HC and CO are generated as shown in FIG.
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 in the combustion chamber 5 and its surroundings fell below a certain temperature.

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, the air-fuel ratio and the compression 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, it is extremely difficult to determine 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 big 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 air separated from the fuel hardly absorbs the combustion heat of the fuel. 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 this case, the larger the specific heat of the inert gas, the stronger the endothermic action. Therefore, the inert gas is preferably a gas having a large specific heat. 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. Thus, the lower limit of the EGR rate at which soot is hardly generated changes depending on the cooling degree of 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. In addition, the horizontal axis represents the required load, and Z1 represents the low load operation region.

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.

On the other hand, in the load region Z2 of FIG. 6, the total intake gas amount X required to prevent the generation of soot exceeds the total intake gas amount Y that can be inhaled. Therefore, in this case, in order to supply the total intake gas amount X required to prevent the generation of soot into the combustion chamber 5, both the EGR gas and the intake air, or EG
It is necessary to supercharge or pressurize the R gas. When the EGR gas or the like is not supercharged or pressurized, the total intake gas amount X matches the total intakeable gas amount Y in the load region Z2. Therefore, in this case, in order to prevent the generation of soot, the air amount is slightly decreased to increase the EGR gas amount and the fuel is burned under the rich air-fuel ratio.

As described above, FIG. 6 shows the case where the fuel is burned under the stoichiometric air-fuel ratio. However, in the low load operation region Z1 shown in FIG. 6, the air amount is smaller than the air amount shown in FIG. At least, that is, even if the air-fuel ratio is made rich, the generation amount of NO _x can be reduced to around 10 p.pm or less while preventing the generation of soot, and the air can be reduced in the low load region Z1 shown in FIG. Even if the amount is made larger than the air amount shown in FIG. 6, that is, even if the average value of the air-fuel ratio is lean from 17 to 18, the amount of NO _x generated is 10 while the generation of soot is prevented.
It can be around or below ppm.

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.

As described above, in the engine low load operation region Z1, soot is generated regardless of the air-fuel ratio, that is, whether the air-fuel ratio is rich, the stoichiometric air-fuel ratio, or the average air-fuel ratio is lean. Therefore, 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 below the temperature at which the growth of hydrocarbons stops halfway only when the amount of heat generated by combustion is small and the engine load is relatively low. Therefore, in the embodiment according to the present invention, when the engine load is relatively low, the temperature of the fuel and the gas around it during combustion are suppressed below the temperature at which the growth of hydrocarbons stops halfway, and the first combustion, that is, low temperature combustion is performed. Thus, when the engine load is relatively high, the second combustion, that is, the combustion that is more commonly performed than before is performed. 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 a first operation region I in which the first combustion, that is, low temperature combustion is performed, and a second combustion region II in which the second combustion, that is, combustion by the conventional combustion method is performed. 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 condition 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.

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. 8 shows the throttle valve 2 for the required load L.
5, the opening of the EGR control valve 29, the EGR rate, the air-fuel ratio, the injection timing and the injection amount are shown. As shown in FIG. 8, in the first operating region I where the required load L is low, the opening degree of the throttle valve 25 is gradually increased from near fully closed to about half opened as the required load L becomes higher, and the EGR control valve 29 The opening degree of is gradually increased from near full close to full open as the required load L increases. Further, in the example shown in FIG. 8, the EGR rate is approximately 7 in the first operating region I.
The air-fuel ratio is set to 0% and the air-fuel ratio is slightly lean. In other words, in the first operating region I, the EGR rate is approximately 70%, and the opening of the throttle valve 25 and the opening of the EGR control valve 29 are controlled so that the air-fuel ratio becomes a lean air-fuel ratio. 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 25 is closed to the fully closed state, and at this time, the EGR control valve 29 is also closed to the fully closed state. Throttle valve 2
When the valve 5 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, in idling operation, the throttle valve 25 is closed to 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 region II to the second operating region II, the opening degree of the throttle valve 25 is increased stepwise from the half open state to the full open 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. Second
In the operation area II, the conventional combustion is performed. In the second operating region II, the throttle valve 25 is kept fully open except for a part, and the opening degree of the EGR control valve 29 is gradually reduced as the required load L increases. Also,
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.

As shown in FIG. 8, the injection amount is the required load L.
Increases with increasing. This injection amount Q is stored in advance in the ROM 42 as a function of the required load L and the engine speed N, as shown in FIG. FIG. 10 shows the air-fuel ratio A / F in the first operating region I. Figure 10
, A / F = 15.5, A / F = 16, A / F =
Each of the curves indicated by 17 and A / F = 18 has an 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. As shown in FIG. 10, the air-fuel ratio is lean in the first operating region I, and in the first operating region I, the air-fuel ratio A / F 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 as shown in FIG. 10, the air-fuel ratio A / F is made larger as the required load L becomes lower. The fuel consumption rate increases as the air-fuel ratio A / F increases. Therefore, in order to make the air-fuel ratio as lean as possible, the air-fuel ratio A / F is increased as the required load L decreases in the embodiment of the present invention.

It should be noted that the target opening degree ST of the throttle valve 25 required to bring the air-fuel ratio to the target air-fuel ratio shown in FIG.
As shown in FIG. 1 (A), the EGR 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, and is required to set the air-fuel ratio to the target air-fuel ratio shown in FIG. The target opening degree SE of the control valve 29 is shown in FIG.
As shown in (4), it 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.

FIG. 12 shows the target air-fuel ratio when the second combustion, that is, the normal combustion by the conventional combustion method is performed. In FIG. 12, A / F = 24, A / F = 3
5, each curve shown by A / F = 45 and A / F = 60 shows the target air-fuel ratios 24, 35, 45, 60, respectively. Throttle valve 2 required to set the air-fuel ratio to this target air-fuel ratio
The target opening degree ST of 5 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. 13 (A), and the air-fuel ratio is set to this target air-fuel ratio. Target opening SE of EGR control valve 29 required for
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. 13 (B).

On the other hand, it is built in the casing 23.
NO_xThe absorbent 22 uses, for example, alumina as a carrier and
For example, potassium K, sodium Na, lithium L on the body
i, alkali metal such as cesium Cs, barium B
a, alkaline earth such as calcium Ca, lanthanum L
a, at least selected from rare earths such as yttrium Y
One and a noble metal such as platinum Pt are supported.
It Engine intake passage, combustion chamber 5 and NO_xOn the absorbent 22
Air and fuel (carbonized water) supplied in the exhaust passage of the flow
Ratio) to NO_xAir-fuel ratio of exhaust gas flowing into the absorbent 22
This is NO_xThe absorbent 22 is the air-fuel of the inflowing exhaust gas
NO when the ratio is lean_xAbsorbs the inflowing exhaust gas
NO absorbed when the air-fuel ratio becomes stoichiometric or rich_x
Releases NO _xIt absorbs and releases.

If this NO _x absorbent 22 is arranged in the exhaust passage of the engine, the NO _x absorbent 22 will actually perform 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. 14 (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. 14 (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 22. 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 22_xIs shown in Figure 14 (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 22
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 22 to NO_xIs released
To be done. However, the air-fuel ratio of the inflowing exhaust gas is
NO in case of ratio_xAbsorbent 22 to NO_xGradually
NO because it is only released _xAbsorbed by the absorbent 22
All NO_xIt takes a little longer time to release.

As described above, the NO _x absorbent 22 is platinum P
It contains a noble metal such as t and is therefore a NO _x absorbent 2
2 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 22 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 22.

By the way the absorption of NO _x capacity of the NO _x absorbent 22 is limited, absorption of NO _x capacity of the NO _x absorbent 22 needs to release the NO _x from the NO _x absorbent 22 before saturation. For this purpose it is necessary to estimate the amount of NO _x is absorbed in the NO _x absorbent 22. 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. In advance, the NO _x absorption amount B per unit time when the second combustion is being performed is calculated as a function of the required load L and the engine speed N of a map as shown in FIG. The NO _x amount ΣNOx absorbed in the NO _x absorbent 22 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 22 so that to release NO _x. Next, this will be described with reference to FIG. Referring to FIG. 16, in the embodiment according to the present invention, 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 22 can absorb, and the allowable maximum value MAX2 is set to about 80% of the maximum absorption amount that the NO _x absorbent 22 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 in order to release NO _x from the NO _x absorbent 22, and when the second combustion is 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 22 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
2 from in order to release the NO _x additional fuel late or during the exhaust stroke of the expansion stroke is injected.

That is, in FIG. 16, 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. 16. NO _x absorption amount Σ when the first combustion is performed
NOX is the air-fuel ratio A / F exceeds the allowable maximum value MAX1 is temporarily rich, whereby _the NO _x absorbent 22
Release NO _x . At this time, the NO _x absorption amount ΣNO
X is set to 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 that 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 22 during 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.

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 while the second combustion is being performed. Thus the air-fuel ratio in order to release the NO _x from the NO _x absorbent 22 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. 16 A / F is not considered rich.
In this case, when the required load L becomes lower than the second boundary Y (N) and the second combustion is switched to the first combustion as at time t ₂ in FIG. 16, the NO _x absorbent 22 is changed. The air-fuel ratio A / F is temporarily made rich in order to release NO _x from.

Next, at time t ₃ in FIG. 16, 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 22 Is injected with additional fuel to make the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 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 opportunity to inject 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.

[0067] Then the air-fuel ratio of the exhaust gas flowing from _the NO _x absorbent 22 in _the NO _x absorbent 22 to be released NO _x when while referring to the first combustion 17 has been performed on the rich The injection control in the case of doing will be described. As shown in FIG. 17, when the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 is made rich during the first combustion, the throttle opening and the EGR rate are not changed and the fuel is not changed. The air-fuel ratio is made rich by increasing the injection amount. In this case, conventionally, when the air-fuel ratio is operated at (A / F) _L , the air-fuel ratio is set to the target rich air-fuel ratio (A / F) _R
When the injection amount should be [(A / F) _L / (A / F)
_R ] ・ Q. In the case shown in FIG. 17, the injection amount is conventionally set to (17/12) · Q. Here, Q is shown in FIG.
Is the fuel injection amount calculated from

However, as in the present invention,
The opening degree ST of the torque valve 25 and the opening degree SE of the EGR control valve 29
And the fuel injection amount Q is determined based on the map value.
The actual lean air-fuel ratio is not always shown in FIG.
Target lean air-fuel ratio (A / F) _LDoes not match exactly. Servant
Therefore, the current air-fuel ratio is shown in FIG.
Target lean air-fuel ratio (A / F) determined from the number of revolutions N_LTo
Assuming that the injection amount is [[A / F)_L/
(A / F)_R] Even if it is calculated from the formula Q, the air-fuel ratio must be
Target rich air-fuel ratio (A / F)_RTo not match
Become.

By the way, the intake air amount G _a and the fuel injection amount Q _R
And the target rich air-fuel ratio (A / F) _R have the following relationship. By the intake air amount G _a / fuel injection amount Q _R = target rich air-fuel ratio (A / F) _R That is, the fuel injection amount Q _R = intake air amount G _a / target rich air-fuel ratio (A / F) _R The present invention In the first embodiment, the fuel injection amount Q _R is obtained by dividing the actual intake air amount G _a detected by the intake air amount sensor 26 by the target rich air-fuel ratio (A / F) _R , and this fuel injection amount so that the fuel is injected only Q _R. In this case, the air-fuel ratio exactly matches the target rich air-fuel ratio (A / F) _R.

Next, the relationship among the intake air amount G _a , the fuel injection amount Q _R, and the target rich air-fuel ratio (A / F) _R will be described in more detail. That is, the intake air amount sensor 26 detects the mass flow rate of the intake air amount per unit time, for example, the mass flow rate (g / sec) of the intake air amount per second. On the other hand, the fuel injection amount Q _R represents a volume of slightly injection. Therefore, if the specific gravity of the fuel is C and the engine speed is N, the injection amount per second (g / sec) is expressed by the following equation.

C · Q _R · (N / 60) · (number of cylinders / 2) Therefore, the relationship between the intake air amount G _a , the fuel injection amount Q _R and the target rich air-fuel ratio (A / F) _R is as follows. become. G _a / [C ・ Q _R・ (N / 60) ・ (number of cylinders / 2)] =
(A / F) _R Therefore, the fuel injection amount Q _R is calculated from the following equation.

Q _R = G _a / [C · (N / 60) · (number of cylinders / 2) · (A / F) _R ] FIG. 18 is empty when the second combustion is switched to the first combustion. The case where the fuel ratio is set to the target rich air-fuel ratio (A / F) _R is shown. Also at this time, the fuel injection amount Q
_R is calculated. Next, the air-fuel ratio is set to the target rich air-fuel ratio (A /
F) The period for maintaining _R is explained.

First, the air-fuel ratio is set to the theoretical air-fuel ratio (A / F).
_The fuel injection amount Q _ST required to achieve _ST is calculated. Then the air-fuel ratio from the fuel injection amount Q _R required for the target rich air-fuel ratio (A / F) _R, an excess fuel quantity ΔQ by subtracting the fuel injection amount Q _ST described above (= Q _R -Q _ST ) Is calculated. The cumulative value ΣΔQ of this excess fuel amount is the set value X1.
Until the air-fuel ratio becomes larger than the target rich air-fuel ratio (A /
F) _R

The set value X1 represents the amount of excess fuel required to release all the NO _x absorbed in the NO _x absorbent 22, so the set value X1 is the NO _x absorbed amount ΣNO.
It increases as X increases. Figure 19 is set to when releasing the NO _x from the NO _x absorbent 22 NO _x
The discharge flag processing routine is shown, and this routine is executed by interruption at regular intervals.

Referring to FIG. 19, first of all, step 1
At 00, it is determined whether or not the 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 is, when the operating region of the engine is the first operating region I, step 10
15, the NO _x absorption amount A per unit time is calculated from the map shown in FIG. Then step 102
Then, A is added to the NO _x absorption amount ΣNOX. Next, at step 103, the NO _x absorption amount ΣNOX is the allowable maximum value M.
It is determined whether or not AX1 has been exceeded. ΣNOX> MAX
Becomes 1 when the process 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 is reset, that is, when the engine operating region is the second operating region II, step 105
Then, the NO _x absorption amount B per unit time is calculated from the map shown in FIG. 15 (B). Next, at step 106, B is added to the NO _x absorption amount ΣNOX. Next, at step 107, the NO _x absorption amount ΣNOX is the allowable maximum value MA.
It is determined whether or not X1 has been exceeded. ΣNOX> MAX1
Becomes the flow proceeds to step 108, the first combustion is set _the NO _x releasing flag I indicating that it should release the NO _x when being performed.

On the other hand, at step 109, the NO _x absorption amount Σ
It is determined whether or not NOX exceeds the maximum allowable value MAX2. Becomes a .SIGMA.NOX> MAX2 proceeds to step 110, NO _x releasing flag II indicating that it should release the NO _x in the second half or the exhaust stroke of the expansion stroke is set. Next, the operation control will be described with reference to FIG.

Referring to FIG. 20, first, at step 200, it is judged if the flag I indicating that the engine operating condition is the first operating region I is set or not. When the flag I is set, that is, when the operating state of the engine is in the first operating region I, step 2
The routine proceeds to 01 where it is judged if the required load L has become larger than the first boundary X (N). When L ≦ X (N), the routine proceeds to step 203, where low temperature combustion is performed.

That is, at step 203, the target opening degree ST of the throttle valve 25 is calculated from the map shown in FIG. 11A, and the opening degree of the throttle valve 25 is made this target opening degree ST. Next, at step 204, the target opening degree SE of the EGR control valve 29 is calculated from the map shown in FIG. 11 (B),
The opening degree of the EGR control valve 29 is set to this target opening degree SE.
Next, at step 205, it is judged if the NO _x releasing flag I is set. When the NO _x release flag I is not set, the routine proceeds to step 206, where fuel injection is performed so that the air-fuel ratio shown in FIG. 10 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 205 that the NO _x releasing flag I is set, the routine proceeds to step 207, where the injection control I shown in FIG. 21 is performed. On the other hand, if it is determined at step 201 that L> X (N), then the routine proceeds to step 202, where the flag I is reset, and then the routine proceeds to step 210, where the second combustion is performed.

That is, at step 210, the target opening degree ST of the throttle valve 25 is calculated from the map shown in FIG. 13 (A), and the opening degree of the throttle valve 25 is made this target opening degree ST. Next, at step 211, the target opening degree SE of the EGR control valve 29 is calculated from the map shown in FIG. 13 (B),
The opening degree of the EGR control valve 29 is set to this target opening degree SE.
Next, at step 212, it is judged if the NO _x releasing flag II is set. When the NO _x release flag II is not set, the routine proceeds to step 213, where fuel injection is performed so that the air-fuel ratio shown in FIG. 12 is obtained. At this time, the second combustion is performed under the lean air-fuel ratio.

On the other hand, when it is judged at step 212 that the NO _x releasing flag II is set, the routine proceeds to step 214, where the injection control II shown in FIG. 22 is performed. Next, the injection control I will be described with reference to FIG. Referring to FIG. 21, first, at step 300, the mass flow rate G _a of the intake air detected by the intake air amount sensor 26 is taken in. Next, at step 301, the fuel injection amount Q _R for making the air-fuel ratio the target rich air-fuel ratio (A / F) _R is calculated based on the following equation.

Q _R = G _a / [C · (N / 60) · (number of cylinders / 2) · (A / F) _R ] Here, C represents the specific gravity of the fuel as described above, and N
Represents the engine speed. Next, at step 302, the injection timing is retarded so that the output torque of the engine does not fluctuate when the air-fuel ratio is switched from lean to rich. In the embodiment according to the present invention, the retarded injection timing is stored in advance in the ROM 4 in the form of a map as a function of the engine operating state.
2 is stored, and in step 302, the injection timing is calculated based on this map. Then step 30
In 3, the process for injecting the fuel injection amount Q _R calculated in step 301 at the injection timing calculated in step 302 is performed. Thus, the air-fuel ratio is switched from the lean air-fuel ratio to the target rich air-fuel ratio (A / F) _R.

If an air-fuel ratio sensor is installed in the engine exhaust passage and feedback control is performed so that the air-fuel ratio becomes the target air-fuel ratio (A / F) _R based on the output signal of this air-fuel ratio sensor, the NO _x absorbent 22 The air-fuel ratio can be maintained at the target air-fuel ratio (A / F) _R when NO _x is to be released from the engine. However, when such feedback control is used, the air-fuel ratio immediately after the air-fuel ratio is switched from lean to rich cannot be accurately controlled to the target rich air-fuel ratio.

On the other hand, in the present invention, the fuel injection amount Q _R into the cylinder into which the intake air flows is calculated based on the mass flow rate G _a of the intake air supplied into the engine cylinder. That is, feed-forward control is performed.
Therefore, in the present invention, as soon as the air-fuel ratio is switched from lean to rich, the air-fuel ratio is accurately set to the target rich air-fuel ratio (A / F).
Controlled by _R.

In Step 304 and thereafter, the NO _x absorbent 2 is used.
A process for maintaining the air-fuel ratio at the target rich air-fuel ratio (A / F) _R is performed until all NO _x absorbed in 2 is released. That is, at step 304, the fuel injection amount Q _ST required to make the air-fuel ratio the stoichiometric air-fuel ratio (A / F) _ST is calculated based on the following equation. Q _ST = G _a / [C ・ (N / 60) ・ (number of cylinders / 2) ・
(A / F) _ST ] Next, at step 305, the excess fuel amount ΔQ (= Q _R −Q
_ST ) is calculated, and then, in step 306, the cumulative value ΣΔQ (= ΣΔQ + ΔQ) of the excess fuel amount is calculated. Next, at step 307, it is judged if the cumulative value ΣΔQ of the excess fuel amount has exceeded the set value X1. When ΣΔQ> X1, the routine proceeds to step 308, where the NO _x release flag I is reset, then ΣNOX is made zero at step 309, and then ΣΔQ is made zero at step 310.

Next, the injection control II will be described with reference to FIG. Referring to FIG. 22, first step 4
At 00, the fuel injection amount Q is calculated from the map shown in FIG. Next, at step 401, the intake air mass flow rate G _a detected by the intake air amount sensor 26 is taken. Then the fuel injection amount Q _R for step 402 of the air-fuel ratio to the target rich air-fuel ratio (A / F) _R is calculated on the basis of the following equation.

Q _R = G _a / [C · (N / 60) · (number of cylinders / 2) · (A / F) _R ] Here, C represents the specific gravity of the fuel, as described above, and N
Represents the engine speed. Next, at step 403 the air-fuel ratio the target rich air-fuel ratio (A / F) the amount of fuel added by subtracting the fuel injection amount Q from the fuel injection amount Q _R necessary to the _R Q _dd (= Q _R -Q ) Is calculated.
Next, at step 404, a process for injecting the fuel injection amount Q near the compression top dead center and performing the injection of the additional fuel amount Q _dd in the latter half of the expansion stroke or during the exhaust stroke is performed. At this time, the second combustion is performed with the lean air-fuel ratio shown in FIG. 12, and the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 by the additional fuel is made the target rich air-fuel ratio (A / F) _R. It

In Step 405 and the subsequent steps, NO _x absorbent 2
A process for maintaining the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 at the target rich air-fuel ratio (A / F) _R is performed until all the NO _x absorbed in 2 is released. That is, in step 405, the air-fuel ratio is changed to the theoretical air-fuel ratio (A / F).
_The fuel injection amount Q _ST required to be _ST is calculated based on the following equation.

Q _ST = G _a / [C · (N / 60) · (number of cylinders / 2) · (A / F) _ST ] Next, at step 406, the excess fuel amount ΔQ (= Q _R −Q
_ST ) is calculated, and then, in step 407, the cumulative value ΣΔQ (= ΣΔQ + ΔQ) of the excess fuel amount is calculated. Next, at step 408, it is judged if the cumulative value ΣΔQ of the excess fuel amount has exceeded the set value X2 (> X1). ΣΔQ
When> X2, the routine proceeds to step 409, where the NO _x releasing flags I and II are reset, then at step 410 ΣNOX is made zero, and at step 411, ΣΔQ is made zero.

The second embodiment is shown in FIGS. 23 to 26. In the second embodiment, as shown in FIG. 23, a bypass passage 35 that connects the intake duct 13 upstream of the throttle valve 25 and the surge tank 12 is provided.
A bypass control valve 37 driven by a step motor 36 is arranged in the unit 5. Further, in this second embodiment, in order to oxidize unburned hydrocarbons better, at the outlet of the exhaust manifold 19, that is, in the exhaust passage upstream of the NO _x absorbent 22, an oxidizing function such as a three-way catalyst or an oxidation catalyst is provided. A catalytic converter 39 containing a catalyst 38 having a is arranged.

In the second embodiment, the lean air-fuel ratio (A /
F) NO _x when the first combustion is performed under _L
In order to release the NO _x from the absorbent 22 an air-fuel ratio the target rich air-fuel ratio (A / F) fuel injection amount Q _R is when the _R is calculated based on the following equation. Q _R = _{[(A / F) L / (} A / F) R ] · Q where Q is a fuel injection amount calculated from the map of FIG.

However, in this case, as described above, the actual air-fuel ratio is not necessarily the lean air-fuel ratio (A / F) _L shown in FIG. 10, and therefore this fuel injection amount Q
_{Even if R} injection is performed, the air-fuel ratio does not necessarily become the target rich air-fuel ratio (A / F) _R. So this also in the second embodiment, using the intake air quantity Q _a / fuel injection amount Q _R = target rich air-fuel ratio (A / F) _R becomes relation, first fuel injection amount Q _R (= [(A / F) _L / (A / F) _R ] · Q) is injected, the air-fuel ratio becomes the target rich air-fuel ratio (A /
F) The intake air amount G _O that becomes _R is calculated.

That is, as in the first embodiment, the specific gravity of fuel is C
And the engine speed is N, the injection amount per second (g / sec) when the air-fuel ratio is a rich air-fuel ratio is expressed by the following equation as described above. C · Q _R · (N / 60) · (number of cylinders / 2) Therefore, the relationship between the intake air amount G _O , the fuel injection amount Q _R, and the target rich air-fuel ratio (A / F) _R is as follows.

G _O / [C · Q _R · (N / 60) · (number of cylinders / 2)] = (A / F) _R Therefore, the intake air amount G _O is calculated from the following equation. G _O = _{[C · Q R · (N /} 60) · ( number of cylinders / 2)] -
(A / F) _R where Q _R = Substituting _{[(A / F) L / (} A / F) R ] _{· Q G O = C · Q} · (N / 60) · ( number of cylinders / 2)・ (A /
F) _{L, that} is, if the intake air amount is set to G _O , the air-fuel ratio becomes the target rich air-fuel ratio (A / F) _R. Therefore, in this embodiment, the mass flow rate G of the intake air detected by the intake air amount sensor 26
The opening degree of the bypass control valve 37 is feedback-controlled so that _a becomes G _O.

The opening degree of the bypass control valve 17 can be controlled in a short time. For example, the air-fuel ratio is set to the target rich air-fuel ratio (A
To complete the opening control of the bypass control valve 17 before starting them if the actual fuel injection opening control of the bypass control valve 17 is performed when the / F) to calculate the fuel injection amount Q _R for the _R be able to. Next, the injection control I will be described with reference to FIG.

Referring to FIG. 24, first, step 5
00 fuel injection amount Q _R for the air-fuel ratio to the target rich air-fuel ratio (A / F) _R is calculated using the following formula. Q _R = [(A / F) _L / (A / F) _R ] ・ Q where (A / F) _L is the lean air-fuel ratio obtained from the map shown in FIG. 10 according to the operating state of the engine. (A / F) _R represents the target rich air-fuel ratio, and Q
Shows the fuel injection amount calculated from the map shown in FIG. 9 according to the operating state of the engine.

Next, at step 501, the injection timing is retarded so that the output torque of the engine does not fluctuate when the air-fuel ratio is switched from lean to rich. In the embodiment according to the present invention, the retarded injection timing is stored in advance in the ROM 42 in the form of a map as a function of the engine operating state, and in step 501 the injection timing is calculated based on this map. Then, in step 502, step 50
A process for injecting the fuel injection amount Q _R calculated in step 500 at the injection timing calculated in 1 is performed. Thus, the air-fuel ratio is switched from the lean air-fuel ratio to the target rich air-fuel ratio (A / F) _R. Next, at step 503, the intake control flag is set.

In steps 504 and below, the NO _x absorbent 2
A process for maintaining the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 at the target rich air-fuel ratio (A / F) _R is performed until all the NO _x absorbed in 2 is released. That is, in step 504, the fuel injection amount Q _ST required to make the air-fuel ratio the stoichiometric air-fuel ratio is calculated based on the following equation. Q _ST = [(A / F) _L / (A / F) _ST ] · Q where (A / F) _L is the lean air-fuel ratio obtained from the map shown in Fig. 10 according to the operating condition of the engine. (A / F) _ST indicates the stoichiometric air-fuel ratio, and Q indicates the fuel injection amount calculated from the map shown in FIG. 9 according to the operating state of the engine.

Next, at step 505, the excess fuel amount ΔQ
(= Q _R -Q _ST) is calculated, then the accumulated value [Sigma] [Delta] Q of the step 506 excess fuel quantity (= ΣΔQ + ΔQ) is calculated. Next, at step 507, it is judged if the cumulative value ΣΔQ of the excess fuel amount has exceeded the set value X1. ΣΔ
When Q> X1, the routine proceeds to step 508, where the NO _x releasing flag I is reset, then ΣNOX is made zero at step 509, and then Σ at step 510.
ΔQ is set to zero, and then in step 511, the intake control flag is reset.

FIG. 25 shows an interrupt routine executed, for example, every 2 msec. Referring to FIG. 25, first, at step 600, it is judged if the intake control flag is set or not. When the intake control flag is set, the routine proceeds to step 601, where the intake air amount G required to bring the air-fuel ratio to the target rich air-fuel ratio (A / F) _R.
O is calculated based on the following equation.

G _O = C · Q · (N / 60) · (number of cylinders /
2) · (A / F) _L Here, C represents the specific gravity of the fuel, Q represents the fuel injection amount calculated from the map shown in FIG. 9 according to the operating state of the engine, and (A / F) _L indicates the lean air-fuel ratio obtained from the map shown in FIG. 10 according to the operating state of the engine.

Next, at step 602, the mass flow rate G _{a of} intake air detected by the intake air amount sensor 26 is taken in. Next, at step 603, it is judged if the intake air amount G _a is larger than the intake air amount G _o . G _a ＞
When G _O , the target step number of the step motor 36 is decremented by 1 in order to reduce the opening degree of the bypass control valve 37. On the other hand, when G _a ≤G _o, the routine proceeds to step 605, where the target step number of the step motor 36 is incremented by 1 to increase the opening degree of the bypass control valve 37. In this way, the intake air amount G _a
Is controlled to be G _O.

Instead of controlling the opening degree of the bypass control valve 37, the opening degree of the throttle valve 25 can be controlled. Next, the injection control II will be described with reference to FIG. Referring to FIG. 26, first, at step 700, the fuel injection amount Q according to the engine operating state is calculated from the map shown in FIG. Next, at step 701, the fuel injection amount Q _R for making the air-fuel ratio the target rich air-fuel ratio (A / F) _R is calculated based on the following equation.

Q _R = [(A / F) _L / (A / F) _R ] · Q where (A / F) _L is the lean obtained from the map shown in FIG. 10 according to the operating state of the engine. The air-fuel ratio is shown, and (A / F) _R is the target rich air-fuel ratio. Next, at step 702, the air-fuel ratio is set to the target rich air-fuel ratio (A
/ F) _R by subtracting the fuel injection amount Q from the fuel injection amount Q _R required to obtain the additional fuel amount Q _dd (= Q _R
-Q) is calculated. Next, at step 703, the process for injecting the fuel injection amount Q near the compression top dead center and performing the injection of the additional fuel amount Q _dd in the latter half of the expansion stroke or during the exhaust stroke is performed. At this time, the second combustion is performed with the lean air-fuel ratio shown in FIG. 12, and the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent 22 by the additional fuel is made the target rich air-fuel ratio (A / F) _R. It Next, at step 704, the intake control flag is set.

In steps 705 and below, the NO _x absorbent 2
A process for maintaining the air-fuel ratio at the target rich air-fuel ratio (A / F) _R is performed until all NO _x absorbed in 2 is released. That is, at step 705, the fuel injection amount Q _ST required to make the air-fuel ratio the stoichiometric air-fuel ratio is calculated based on the following equation. Q _ST = [(A / F) _L / (A / F) _ST ] · Q where (A / F) _L is the lean air-fuel ratio obtained from the map shown in Fig. 10 according to the operating condition of the engine. (A / F) _ST indicates the stoichiometric air-fuel ratio.

Next, at step 706, the excess fuel amount ΔQ
(= Q _R -Q _ST) is calculated, then the accumulated value [Sigma] [Delta] Q of the excess fuel amount at step 707 (= ΣΔQ + ΔQ) is calculated. Next, at step 708, it is judged if the cumulative value ΣΔQ of the excess fuel amount has exceeded the set value X2. ΣΔ
When Q> X2, the routine proceeds to step 709, where the NO _x releasing flags I and II are reset and then step 710.
In step 711, ΣNOX is made zero, and in step 711, ΣΔQ is made zero, and then in step 712, the intake control flag is reset.

[0108]

As described above, when the air-fuel ratio is made rich so as to release the NO _x from the NO _x absorbent, the air-fuel ratio can be accurately matched with the predetermined target rich air-fuel ratio.

[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 opening of a throttle valve and the like.

FIG. 9 is a diagram showing a map of an injection amount.

FIG. 10 is a diagram showing an air-fuel ratio 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 an air-fuel ratio in the second combustion.

FIG. 13 is a diagram showing a target opening of a throttle valve or the like.

FIG. 14 is a diagram for explaining the action of absorbing and releasing NO _x .

FIG. 15 is a diagram showing a map of the amount of NO _x absorbed per unit time.

FIG. 16 is a diagram for explaining NO _x release control.

FIG. 17 is a diagram showing changes in the injection amount and changes in the air-fuel ratio when switching from the lean air-fuel ratio to the rich air-fuel ratio.

FIG. 18 is a diagram showing changes in the injection amount and changes in the air-fuel ratio when switching from the lean air-fuel ratio to the rich air-fuel ratio.

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

FIG. 20 is a flowchart for controlling the operation of the engine.

FIG. 21 is a flow chart for executing injection control I.

FIG. 22 is a flow chart for executing injection control II.

FIG. 23 is an overall view showing another embodiment of the compression ignition type internal combustion engine.

FIG. 24 is a flowchart for executing injection control I.

FIG. 25 is a flowchart showing a time interruption routine.

FIG. 26 is a flowchart for executing injection control II.

[Explanation of symbols]

6 ... Fuel injection valve 22 ... NO _x absorbent 25 ... Throttle valve 26 ... Intake air amount sensor 29 ... EGR control valve

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI F02D 45/00 301 F02D 45/00 301G F02M 25/07 550 F02M 25/07 550R (56) Reference JP-A-9-217645 ( JP, A) JP 7-4287 (JP, A) JP 8-177654 (JP, A) JP 8-86251 (JP, A) JP 9-287527 (JP, A) JP Hei 9-287528 (JP, A) Patent 2600492 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/00-41/40

Claims (7)

(57) [Claims] 1. 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 . the _the NO _x absorbent arranged in the engine exhaust passage, the air-fuel ratio by increasing the amount of fuel injection when to release the NO _x from the NO _x absorbent when burning fuel under a lean air-fuel ratio has been performed one o'clock Of an internal combustion engine that is made to be rich, the internal combustion engine is inactive in the combustion chamber.
As the amount of gas increases, the amount of soot generated gradually increases
The amount of inert gas in the combustion chamber is increasing further.
Fuel and its surrounding gas during combustion in the combustion chamber
The soot temperature is lower than the soot formation temperature, and soot is almost generated
It consists of a compression ignition type internal combustion engine that does not
Inert gas in the combustion chamber rather than the peak amount of inert gas
The first combustion, which produces a large amount of soot and almost no soot, and the generation of soot
The amount of inert gas in the combustion chamber is more
Switching for selectively switching between the second combustion with a small amount of characteristic gas
Means, or should be released NO _x from the NO _x absorbent
Determining means for determining whether a detection unit for detecting an intake air quantity, to when releasing the NO _x from the NO _x absorbent,
Calculate either the target fuel injection amount or the target intake air amount required to make the air-fuel ratio the target rich air-fuel ratio from the relational expression: detected intake air amount / fuel injection amount to be supplied = target rich air-fuel ratio calculation means; and a, the determining means for
When it is judged that NO _x should be released by
Fuel injection amount or the intake air amount is with the target fuel injection amount or the target intake air amount when the first combustion is being performed, its
An internal combustion engine in which the air-fuel ratio is set to the target rich air-fuel ratio . 2. NO _x when the second combustion is being performed.
When it is determined that NO _x should be released from the absorbent
Is empty when the second combustion is switched to the first combustion.
The internal combustion engine according to claim 1, wherein the fuel ratio is temporarily made rich . 3. The amount of NO _x absorbed by the NO _x absorbent.
Is equipped with an estimation means for estimating
The NO _x amount estimated by the estimating means is determined in advance when
Second combustion when the first allowable maximum value set is exceeded
When the air-fuel ratio is changed from 1 to 1
When the second combustion is performed, the estimation is made rich.
The NO _x amount estimated by the means is the first allowable maximum value.
And exceeds the second allowable maximum value determined Rimodai heard previously
The additional fuel during the latter half of the expansion stroke or during the exhaust stroke.
Theory of the air-fuel ratio of the exhaust gas that shines and flows into the NO _x absorbent
The internal combustion engine according to claim 1, wherein the air-fuel ratio is made rich . 4. The exhaust gas discharged from the combustion chamber is sucked into the engine.
It is equipped with a recirculation device for recirculating in the air passage,
The internal combustion engine of claim 1, wherein the volatile gas comprises recirculated exhaust gas . 5. Exhaust gas reforming in the first combustion state
The circulation rate according to claim 4, which is approximately 55% or more.
Internal combustion engine. 6. A first operation on the low load side of the engine operating region
Area and the second operation area on the high load side, the first operation
The first combustion is performed in the region and the second combustion is performed in the second operation region.
The internal combustion engine according to claim 1, wherein combustion is performed . 7. An acid in the engine exhaust passage upstream of the NO _x absorbent.
The internal combustion engine according to claim 1, further comprising a catalyst having a gasification function .