JP4165036B2 - Internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an internal combustion engine.
[0002]
[Prior art]
Conventionally, an internal combustion engine that includes a turbocharger and reduces the intake air amount to shift the air-fuel ratio to the rich side is known. Further, when the amount of inert gas supplied into the combustion chamber is increased, the amount of soot generated gradually increases and reaches a peak, and when the amount of inert gas supplied to the combustion chamber is further increased. An internal combustion engine in which the temperature of fuel and its surrounding gas during combustion in the combustion chamber is lower than the generation temperature of soot and soot is hardly generated, and the temperature of fuel and its surrounding gas during combustion in the combustion chamber Low-temperature combustion where soot is hardly generated at a temperature lower than the generation temperature of the soot, and normal combustion in which the amount of inert gas supplied into the combustion chamber is smaller than the amount of inert gas at which soot generation peaks. There is known an internal combustion engine including a switching means for switching and a turbocharger. An example of this type of internal combustion engine is disclosed in Japanese Patent Application Laid-Open No. 11-35016, for example. In the internal combustion engine described in Japanese Patent Application Laid-Open No. 11-35016, the intake air amount is reduced in order to reduce the NOx of the NOx catalyst by shifting the air-fuel ratio to the rich side. Further, in the internal combustion engine described in JP-A-11-35016, the intake air amount is reduced in order to switch from normal combustion to low-temperature combustion.
[0003]
[Problems to be solved by the invention]
However, in the internal combustion engine described in Japanese Patent Laid-Open No. 11-35016, the intake air amount is reduced in order to reduce the NOx of the NOx catalyst by shifting the air-fuel ratio to the rich side, or to switch from normal combustion to low temperature combustion. It is not taken into consideration that the rotational speed of the turbine of the turbocharger is reduced when it is reduced. Specifically, it is considered that the turbine speed cannot be increased immediately even if the intake air quantity is increased after the turbine speed has decreased as the intake air quantity has decreased. Absent. Therefore, in the internal combustion engine described in Japanese Patent Application Laid-Open No. 11-35016, after reducing the intake air amount, when the intake air amount is to be increased, the torque increases as the increase in the intake air amount is delayed. There may be a shortage or an increase in smoke.
[0004]
In view of the above problems, the present invention reduces the intake air amount and then increases the intake air amount when the intake air amount is increased, resulting in insufficient torque or increased smoke. It is an object of the present invention to provide an internal combustion engine that can suppress the occurrence of the problem. Furthermore, according to the present invention, when the intake air amount is increased in order to switch from the low temperature combustion to the normal combustion, the increase in the intake air amount is delayed, resulting in a shortage of torque or an increase in smoke. It aims at providing the internal combustion engine which can be suppressed.
[0005]
[Means for Solving the Problems]
According to the first aspect of the present invention, in the internal combustion engine that includes the turbocharger and reduces the intake air amount and shifts the air-fuel ratio to the rich side, the auxiliary means for increasing the rotational speed of the turbine of the turbocharger. And reducing the intake air amount to shift the air-fuel ratio to the rich side, and then increasing the intake air amount by operating the auxiliary means when the air-fuel ratio is shifted to the lean side. An internal combustion engine is provided.
[0006]
According to the second aspect of the invention, the air-fuel ratio is shifted to the rich side by decreasing the intake air amount and increasing the fuel injection amount so that the torque does not decrease, and then the air-fuel ratio is shifted to the lean side. 2. The intake air amount is increased by actuating the auxiliary means so that the torque is not decreased as the fuel injection amount is decreased and the fuel injection amount is decreased. An internal combustion engine as described is provided.
[0007]
In the internal combustion engine according to claims 1 and 2, even if an attempt is made to increase the intake air amount after the turbine rotation speed has decreased as the intake air amount has decreased, the turbine rotation speed immediately increases. In view of this point, auxiliary means for increasing the rotational speed of the turbocharger turbine is provided, the intake air amount is decreased to shift the air-fuel ratio to the rich side, and then the air-fuel ratio is shifted to the lean side. When this is done, the amount of intake air is increased by operating the auxiliary means. Specifically, after reducing the intake air amount and increasing the fuel injection amount so that the torque does not decrease and shifting the air-fuel ratio to the rich side, the fuel injection amount is decreased when the air-fuel ratio is shifted to the lean side. In addition, the intake air amount is increased by operating the auxiliary means so that the torque does not decrease as the fuel injection amount decreases. Therefore, after reducing the intake air amount, when increasing the intake air amount, the increase of the intake air amount is delayed, so that the torque shortage and the smoke increase are suppressed. be able to.
[0008]
According to the third aspect of the present invention, as the amount of inert gas supplied into the combustion chamber increases, the amount of soot generated gradually increases and reaches a peak, and the inert gas supplied into the combustion chamber. If the amount of the fuel is further increased, the temperature of the fuel and the surrounding gas in the combustion chamber becomes lower than the generation temperature of soot, and soot is hardly generated. Low temperature combustion in which the temperature of the fuel and surrounding gas is lower than the soot generation temperature and soot is hardly generated, and the inertness supplied to the combustion chamber rather than the amount of inert gas at which soot generation peaks It is possible to switch between normal combustion with a large amount of gas and almost no soot, and in an internal combustion engine equipped with a turbocharger, auxiliary means for increasing the rotational speed of the turbine of the turbocharger is provided, When switched to the normal combustion from the internal combustion engine, characterized in that increasing the amount of intake air is provided by actuating the auxiliary means.
[0009]
In the internal combustion engine according to claim 3, even if an attempt is made to increase the intake air amount after the turbine rotation speed has decreased as the intake air amount has decreased, the turbine rotation speed can immediately increase. In view of this, auxiliary means for increasing the rotational speed of the turbine of the turbocharger is provided, and the intake air amount is increased by operating the auxiliary means when switching from low temperature combustion to normal combustion. Therefore, when increasing the intake air amount in order to switch from low temperature combustion to normal combustion, it is possible to suppress a shortage of torque or an increase in smoke as the increase in the intake air amount is delayed. be able to.
[0010]
According to a fourth aspect of the present invention, the intake air amount is increased by operating the auxiliary means at the time of switching from low temperature combustion to normal combustion and during engine acceleration operation. Is provided.
[0011]
In the internal combustion engine according to claim 4, in the internal combustion engine capable of switching between the low temperature combustion and the normal combustion, the low temperature combustion is executed during the engine steady operation, the normal combustion is executed during the engine acceleration operation, and the required intake air amount is reduced. In view of the considerable increase, the intake air amount is increased by operating the auxiliary means at the time of switching from low temperature combustion to normal combustion and during engine acceleration operation. Therefore, when the required intake air amount increases significantly, the intake air amount can be rapidly increased as required.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
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 Is an exhaust valve, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to an outlet portion of a compressor 16 of an exhaust turbocharger 15 via an intake duct 13 and an intercooler 14. An inlet portion of the compressor 16 is connected to an air cleaner 18 via an air suction pipe 17. On the other hand, the exhaust port 10 is connected to an inlet portion of an exhaust turbine 20 of an exhaust turbocharger 15 via an exhaust manifold 19, and the outlet portion of the exhaust turbine 20 contains a NOx absorbent 22 as a NOx catalyst via an exhaust pipe 21. To the casing 23. The exhaust turbocharger 15 is provided with electrical auxiliary means for increasing the rotational speed of the exhaust turbine 20.
[0013]
A throttle valve 25 driven by a step motor 24 is disposed in the intake duct 13, and an intake air amount sensor 26 for detecting the mass flow rate of the intake air is disposed 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 an EGR control valve 29 driven by a step motor 28 is disposed in the EGR passage 27. . A cooling device 30 for cooling the EGR gas flowing in the EGR passage 27 is disposed around the EGR passage 27. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 30, and the EGR gas is cooled by the engine cooling water.
[0014]
On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 32, via a fuel supply pipe 31. Fuel is supplied into the common rail 32 from an electrically controlled fuel pump 33 with variable discharge amount, and the fuel supplied into the common rail 32 is supplied to the fuel injection valve 6 through 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 a fuel pump 33 is set 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. The discharge amount is controlled.
[0015]
The electronic control unit 40 comprises a digital computer and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 are connected. It comprises. The output signal of the intake air amount sensor 26 is input to the input port 45 via the corresponding AD 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, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. . The input port 45 is connected to a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, 30 °. 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 through corresponding drive circuits 48.
[0016]
By the way, in an internal combustion engine, for example, a compression ignition type engine, in order to suppress generation of NOx, an engine exhaust passage and an engine intake passage are connected by an EGR passage, and exhaust gas, that is, EGR gas is connected to the engine through the EGR passage. The air is recirculated in the 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)). ) Increases, the combustion temperature in the combustion chamber decreases. As the combustion temperature decreases, the amount of NOx generated decreases, and as the EGR rate increases, the amount of NOx generated decreases.
[0017]
Thus, it has been known that the amount of NOx generated can be reduced if the EGR rate is increased. However, when the EGR rate is increased, when the EGR rate exceeds a certain limit, the generation amount of soot, that is, the smoke starts to increase rapidly. With respect to this point, it is conventionally considered that if the EGR rate is further increased, the smoke will increase as much as possible. Therefore, the EGR rate at which the smoke starts to increase rapidly is considered to be the maximum allowable limit of the EGR rate. It has been.
[0018]
Therefore, the EGR rate is conventionally determined within a range not exceeding the maximum allowable limit. The maximum allowable limit for this EGR rate is roughly 30 to 50 percent, although 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.
[0019]
As described above, since it has been conventionally considered that there is a maximum allowable limit for the EGR rate, the EGR rate is conventionally determined so that the generation amount of NOx and smoke is minimized within a range not exceeding the maximum allowable limit. It was. However, even if the EGR rate is determined in this way so that the amount of NOx and smoke generated becomes as small as possible, there is a limit to the reduction in the amount of NOx and smoke generated. In fact, a considerable amount of NOx and smoke is still generated. This is the current situation.
[0020]
However, if the EGR rate is made larger than the maximum allowable limit in the course of research on combustion in a compression ignition engine, smoke increases rapidly as described above, but there is a peak in the amount of smoke generated, and this peak is exceeded. When the EGR rate is further increased, the smoke starts to rapidly decrease. When the EGR rate is increased to 70% or higher during idling operation, and when the EGR gas is strongly cooled, the smoke is increased to approximately 55% or higher. Was found to be almost zero, i.e., almost no wrinkles occurred. At this time, it has been found that the amount of NOx generated is extremely small. After that, the reason why soot is not generated has been studied based on this knowledge, and as a result, a new combustion system capable of simultaneously reducing soot and NOx has been constructed. Although this new combustion system will be described in detail later, it is basically based on stopping the growth of hydrocarbons in the middle of the course until the hydrocarbons grow into soot.
[0021]
That is, as a result of repeated experimental research, it has been found that when the temperature of the fuel during combustion in the combustion chamber and the surrounding gas is below a certain temperature, the growth of the hydrocarbon stops before it reaches the soot. And when the temperature of the gas around it exceeds a certain temperature, the hydrocarbon grows up to a soot. In this case, the endothermic effect of the gas around the fuel when the fuel burns greatly affects the temperature of the fuel and the surrounding gas, and the endothermic amount of the gas around the fuel is adjusted according to the amount of heat generated during fuel combustion. As a result, the temperature of the fuel and the surrounding gas can be controlled.
[0022]
Therefore, if the temperature of the fuel during combustion in the combustion chamber and the surrounding gas temperature is suppressed below the temperature at which hydrocarbon growth stops halfway, soot will not be generated, and the temperature of the fuel during combustion in the combustion chamber and the surrounding gas temperature It is possible to suppress the temperature below the temperature at which hydrocarbon growth stops halfway by adjusting the endothermic amount of the gas around the fuel. On the other hand, hydrocarbons that have stopped growing before reaching soot can be easily purified by post-treatment using an oxidation catalyst or the like. This is the basic concept of the new combustion system.
[0023]
FIG. 1 shows a compression ignition type internal combustion engine employing this new combustion system. FIG. 2 shows the compression ignition type internal combustion engine shown in FIG. 1, in which the air-fuel ratio A / F (horizontal axis in FIG. 2) is changed by changing the opening of the throttle valve 25 and the EGR rate during engine low load operation. The experiment example which shows the change of the output torque at the time, and the change of discharge | emission amount of smoke, HC, CO, NOx is represented. As can be seen from FIG. 2, in this experimental example, the EGR rate increases as the air-fuel ratio A / F decreases, and the EGR rate is 65% or more when the air-fuel ratio is less than the theoretical air-fuel ratio (≈14.6).
[0024]
As shown in FIG. 2, when the air-fuel ratio A / F is decreased by increasing the EGR rate, the EGR rate becomes around 40%, and the amount of smoke generated when the air-fuel ratio A / F becomes about 30. Start to increase. Next, when the EGR rate is further increased and the air-fuel ratio A / F is decreased, the amount of smoke generated increases rapidly and reaches a peak. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the smoke suddenly decreases, the EGR rate is increased to 65% or more, and when the air-fuel ratio A / F is near 15.0, the smoke becomes almost zero. . That is, almost no wrinkles occur. At this time, the output torque of the engine slightly decreases, and the amount of NOx generated becomes considerably low. On the other hand, the generation amount of HC and CO starts to increase at this time.
[0025]
FIG. 3A shows the change in 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 that the air-fuel ratio A / F is 13 The change in the combustion pressure in the combustion chamber 5 when the amount of smoke generated in the vicinity is almost zero is shown. As can be seen from a comparison between FIG. 3A and FIG. 3B, the case of FIG. 3B where the amount of smoke generated is almost zero is shown in FIG. 3A where the amount of smoke generated is large. It can be seen that the combustion pressure is lower than the case.
[0026]
The following can be said from the experimental results shown in FIGS. That is, first, when the air-fuel ratio A / F is 15.0 or less and the amount of smoke generated is almost zero, the amount of NOx generated decreases considerably as shown in FIG. A reduction in the amount of NOx generated means that the combustion temperature in the combustion chamber 5 has decreased. Therefore, it can be said that the combustion temperature in the combustion chamber 5 is low when soot is hardly generated. . 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, and therefore the combustion temperature in the combustion chamber 5 is low at this time.
[0027]
Secondly, when the amount of smoke generated, that is, the amount of soot is substantially zero, the HC and CO emissions increase as shown in FIG. This means that the hydrocarbons are discharged without growing to the soot. That is, linear hydrocarbons and aromatic hydrocarbons as shown in FIG. 4 contained in the fuel are thermally decomposed to form soot precursors when the temperature is raised in an oxygen-deficient state, A soot made of a solid in which carbon atoms are assembled is produced. In this case, the actual soot formation process is complicated, and it is not clear what form the soot precursor will take, but in any case the hydrocarbons shown in FIG. After that, it will grow up to heels. Therefore, as described above, when the generation amount of soot becomes almost zero, the emission amount of HC and CO increases as shown in FIG. 2. At this time, HC is a precursor of soot or a hydrocarbon in the previous state. .
[0028]
Summarizing these considerations based on the experimental results shown in FIG. 2 and FIG. 3, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generated becomes almost zero, and at this time, the soot precursor or its previous state Hydrocarbons are discharged from the combustion chamber 5. As a result of repeated experimental studies in more detail, when the temperature of the fuel in the combustion chamber 5 and the gas surrounding it is below a certain temperature, the soot growth process stops midway, that is, soot It has been found that soot is not generated at all, and soot is generated when the temperature of the fuel in the combustion chamber 5 and the surrounding temperature are below a certain temperature.
[0029]
By the way, when the hydrocarbon production process stops in the state of soot precursor, the temperature of the fuel and its surroundings, that is, the above-mentioned certain temperature changes depending on various factors such as the type of fuel, the air-fuel ratio and the compression ratio. Although it cannot be said how many times, this certain temperature has a deep relationship with the amount of NOx generated, and therefore this certain temperature can be defined to some extent from the amount of NOx generated. That is, as the EGR rate increases, the temperature of the fuel during combustion and the surrounding gas temperature decrease, and the amount of NOx generated decreases. At this time, almost no soot is generated when the amount of NOx generated is about 10 p.p.m or less. Therefore, the above-mentioned certain temperature substantially coincides with the temperature when the amount of NOx generated is around 10 p.p.m or less.
[0030]
Once soot is produced, it cannot be purified by post-treatment using a catalyst having an oxidizing function. On the other hand, the soot precursor or the hydrocarbon in the previous state can be easily purified by post-treatment using a catalyst having an oxidation function. Considering the post-treatment with a catalyst having an oxidation function in this way, it is extremely important whether hydrocarbons are discharged from the combustion chamber 5 in the soot precursor or in the previous state or 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 in a previous state without generating soot in the combustion chamber 5. The core is to oxidize with a catalyst having an oxidation function.
[0031]
Now, in order to stop the growth of hydrocarbons before the soot is generated, it is necessary to suppress the temperature of the fuel and the surrounding gas in the combustion chamber 5 to a temperature lower than the temperature at which soot is generated. There is. In this case, in order to suppress the temperature of the fuel and the surrounding gas, it has been found that the endothermic action of the gas around the fuel when the fuel burns has a great influence.
[0032]
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 away from the fuel does not increase so much, and only the temperature around the fuel becomes extremely high locally. That is, at this time, the air away from the fuel hardly performs the endothermic action of the combustion heat of the fuel. In this case, since the combustion temperature becomes extremely high locally, the unburned hydrocarbons that have received this heat of combustion produce soot.
[0033]
On the other hand, the situation is slightly different when 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 around and reacts with oxygen mixed in the inert gas and burns. In this case, since the combustion heat is absorbed by the surrounding inert gas, the combustion temperature does not rise so much. That is, the combustion temperature can be kept low. That is, the presence of the inert gas plays an important role in suppressing the combustion temperature, and the combustion temperature can be kept low by the endothermic action of the inert gas.
[0034]
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 soot is generated, an amount of inert gas that can absorb a sufficient amount of heat is required. Therefore, if the amount of fuel increases, the amount of inert gas required increases accordingly. In this case, the greater 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. This point, CO₂Since EGR gas has a relatively large specific heat, it can be said that it is preferable to use EGR gas as an inert gas.
[0035]
FIG. 5 shows the relationship between the EGR rate and smoke when the EGR gas is used as the inert gas and the cooling degree of the EGR gas is changed. That is, in FIG. 5, curve A shows the case where the EGR gas is strongly cooled and the EGR gas temperature is maintained at about 90 ° C., and curve B shows the case where the EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0036]
As shown by curve A in FIG. 5, when the EGR gas is cooled strongly, soot generation peaks when the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. If you do, almost no wrinkles will occur. On the other hand, as shown by the curve B in FIG. 5, when the EGR gas is slightly cooled, the amount of soot peaks when the EGR rate is slightly higher than 50%. In this case, the EGR rate is about 65% or more. If this is done, almost no wrinkles will occur.
[0037]
Further, as shown by the curve C in FIG. 5, when the EGR gas is not forcibly cooled, the amount of soot generated reaches a peak when the EGR rate is around 55%. In this case, the EGR rate is approximately 70%. If the percentage is exceeded, almost no wrinkles occur. FIG. 5 shows the amount of smoke generated when the engine load is relatively high. When the engine load is small, the EGR rate at which the amount of soot reaches a peak slightly decreases, and the EGR rate at which soot hardly occurs. The lower limit is also slightly reduced. Thus, the lower limit of the EGR rate at which soot hardly occurs varies depending on the degree of cooling of the EGR gas and the engine load.
[0038]
FIG. 6 shows the amount of mixed gas of EGR gas and air required to make the temperature of the fuel and the surrounding gas lower than the temperature at which soot is generated when EGR gas is used as the inert gas. And the ratio of the air in this gas mixture amount, and the ratio of the EGR gas in this gas mixture are shown. In FIG. 6, the vertical axis indicates the total intake gas amount sucked into the combustion chamber 5, and the chain 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, and Z1 indicates the low load operation region.
[0039]
Referring to FIG. 6, the ratio of air, that is, the amount of air in the mixed gas indicates the amount of air necessary for completely burning the injected fuel. That is, in the case shown in FIG. 6, the ratio of the air amount and the injected fuel amount is the stoichiometric 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 used to make the temperature of the fuel and the surrounding gas lower than the temperature at which soot is formed when the injected fuel is burned. The minimum EGR gas amount is shown. This EGR gas amount is approximately 55% or more in terms of EGR rate, and is 70% or more in the embodiment shown in FIG. That is, if the total intake gas amount sucked into the combustion chamber 5 is 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 a ratio as shown in FIG. The temperature of the fuel and the surrounding gas is lower than the temperature at which soot is produced, so that no soot is generated. Further, the amount of NOx generated at this time is about 10 p.p.m or less, and therefore the amount of NOx generated is extremely small.
[0040]
If the amount of fuel injection increases, the amount of heat generated when the fuel burns increases. Therefore, in order to maintain the temperature of the fuel and the surrounding gas at a temperature lower than the temperature at which soot is generated, the amount of heat absorbed by the EGR gas Must be increased. Therefore, as shown in FIG. 6, the EGR gas amount must be increased as the amount of injected fuel increases. That is, the amount of EGR gas needs to increase as the required load increases.
[0041]
On the other hand, in the load region Z2 of FIG. 6, the total intake gas amount X necessary for preventing the generation of soot exceeds the total intake gas amount Y that can be sucked. Therefore, in this case, it is necessary to supercharge or pressurize both the EGR gas and the intake air, or the EGR gas in order to supply the entire intake gas amount X necessary for preventing the generation of soot into the combustion chamber 5. When EGR gas or the like is not supercharged or pressurized, the total intake gas amount X coincides with the total intake gas amount Y that can be sucked in the load region Z2. Therefore, in this case, in order to prevent the generation of soot, the amount of air is slightly decreased to increase the amount of EGR gas, and the fuel is burned under a rich air-fuel ratio.
[0042]
As described above, FIG. 6 shows the case where the fuel is burned under the stoichiometric air-fuel ratio, but even if the air amount is smaller than the air amount shown in FIG. 6 in the low load operation region Z1 shown in FIG. That is, even if the air-fuel ratio is rich, the generation amount of NOx can be reduced to around 10 ppm or less while preventing the generation of soot, and the air amount in the low load region Z1 shown in FIG. Even if the air amount is larger than that shown in FIG. 1, that is, even if the average value of the air-fuel ratio is 17 to 18, the generation amount of NOx can be reduced to around 10 p.pm or less while preventing the generation of soot. it can.
[0043]
That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but the combustion temperature is suppressed to a low temperature, so that the excess fuel does not grow to soot, and so no soot is generated. At this time, only a very small amount of NOx 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 is high, but in the present invention the soot is suppressed to a low temperature so Not generated at all. Furthermore, only a very small amount of NOx is generated.
[0044]
Thus, in the engine low load operation region Z1, no 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. The amount of 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 during combustion in the combustion chamber and the surrounding gas temperature can be suppressed to a temperature equal to or lower than the temperature at which hydrocarbon growth stops midway only when the engine load is small and the combustion 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 during combustion and the surrounding gas temperature is suppressed to a temperature below the temperature at which the growth of hydrocarbons stops midway, 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 normal combustion that is normally performed conventionally is performed. Note that the first combustion, that is, low-temperature combustion, here, 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 generated soot peaks, and soot is almost generated. The second combustion, that is, the normal combustion that is normally performed conventionally, is a combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas in which the generation amount of soot peaks. Say that.
[0045]
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 normal combustion by the conventional combustion method is performed. In FIG. 7, the vertical axis L indicates the amount of depression of the accelerator pedal 50, that is, the required load, and the horizontal axis N indicates the engine speed. In FIG. 7, X (N) indicates the first boundary between the first operation region I and the second operation region II, and Y (N) indicates the first operation region I and the second operation region. A second boundary with region II is shown. The change determination 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 change from the second operation region II to the first operation region I is performed. The change determination of the operation region is performed based on the second boundary Y (N).
[0046]
That is, if the required load L exceeds the first boundary X (N) that is a function of the engine speed N when the engine operating state is in the first operating region I and low-temperature combustion is being performed, the operating region Is determined to have moved to the second operation region II, and combustion by the conventional combustion method is performed. Next, when the required load L becomes lower than the second boundary Y (N) that 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.
[0047]
Next, 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 opening degree of the throttle valve 25, the opening degree of the EGR control valve 29, the EGR rate, the air-fuel ratio, the injection timing, and the injection amount with respect to the required load L. As shown in FIG. 8, in the first operation region I where the required load L is low, the opening degree of the throttle valve 25 is gradually increased from nearly fully closed to about half-opened as the required load L increases, and the EGR control valve 29 As the required load L increases, the degree of opening is gradually increased from near full close to full open. Further, in the example shown in FIG. 8, in the first operating region I, the EGR rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio. In other words, in the first 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 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 also delayed as the injection start timing θS is delayed.
[0048]
During the idling operation, the throttle valve 25 is closed to near full closure, and at this time, the EGR control valve 29 is also made to close to full close. When the throttle valve 25 is closed to close to the fully closed state, the pressure in the combustion chamber 5 at the start of compression becomes low, so the compression pressure becomes small. When the compression pressure is reduced, the compression work by the piston 4 is reduced, so that the vibration of the engine body 1 is reduced. That is, during idling operation, the throttle valve 25 is closed to close to the fully closed state in order to suppress vibration of the engine body 1.
[0049]
On the other hand, when the engine operating region changes from the first operating region I to the second operating region II, the opening of the throttle valve 25 is increased stepwise from the half-open state to the fully-open direction. At this time, in the example shown in FIG. 9, the EGR rate is decreased in a step-like manner from approximately 70% to 40% or less, and the air-fuel ratio is increased in a step-like manner. Conventional combustion is performed in the second operation region II. In the second operation region II, the throttle valve 25 is held in a fully open state except for a part thereof, and the opening degree of the EGR control valve 29 is gradually reduced as the required load L increases. In this operation region II, the EGR rate decreases as the required load L increases, and the air-fuel ratio decreases as the required load L increases. However, the air-fuel ratio is made a lean air-fuel ratio even when the required load L increases. In the second operation region II, the injection start timing θS is set near the compression top dead center TDC.
[0050]
As shown in FIG. 8, the injection amount increases as the required load L increases. The injection quantity 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. In FIG. 10, the curves indicated by A / F = 15.5, A / F = 16, A / F = 17, and A / F = 18 have air-fuel ratios of 15.5, 16, 17, and 18, respectively. 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 operation region I, and in the first operation region I, the air-fuel ratio A / F is leaner as the required load L becomes lower.
[0051]
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 is reduced. When the EGR rate is lowered, the air-fuel ratio increases, so the air-fuel ratio A / F increases as the required load L decreases as shown in FIG. As the air-fuel ratio A / F increases, the fuel consumption rate improves. Therefore, 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.
[0052]
It should be noted that the target opening ST of the throttle valve 25 necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG. 10 is a function of the required load L and the engine speed N as shown in FIG. The target opening degree SE of the EGR control valve 29 necessary for setting the air-fuel ratio to the target air-fuel ratio shown in FIG. 10 is stored in the ROM 42 in advance as shown in FIG. And stored in the ROM 42 in advance in the form of a map as a function of the engine speed N.
[0053]
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, the curves indicated by A / F = 24, A / F = 35, A / F = 45, and A / F = 60 indicate the target air-fuel ratios 24, 35, 45, and 60, respectively. The target opening ST of the throttle valve 25 necessary for setting the air-fuel ratio to the target air-fuel ratio is previously stored in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. The target opening degree SE of the EGR control valve 29 necessary for making the air-fuel ratio the target air-fuel ratio is stored as a function of the required load L and the engine speed N as shown in FIG. It is stored in advance in the ROM 42 in the form of a map.
[0054]
On the other hand, the NOx absorbent 22 as a NOx catalyst built in the casing 23 has, for example, alumina as a carrier, and an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, barium Ba, At least one selected from alkaline earth such as calcium Ca, lanthanum La, and rare earth such as yttrium Y, and a noble metal such as platinum Pt are supported. The ratio of the air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 5 and the exhaust passage upstream of the NOx absorbent 22 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 22. 22 absorbs and releases NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and performs NOx absorption / release action of releasing the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich.
[0055]
If this NOx absorbent 22 is arranged in the engine exhaust passage, the NOx absorbent 22 actually performs the NOx absorption / release action, but there are some unclear parts about the detailed mechanism of this absorption / release action. However, it is considered that this absorption / release action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on the support, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0056]
In the compression ignition internal combustion engine shown in FIG. 1, combustion is normally performed with the air-fuel ratio in the combustion chamber 5 being lean. In this way, when combustion is performed with the air-fuel ratio being lean, the oxygen concentration in the exhaust gas is high. At this time, as shown in FIG.₂Is O₂ ^-Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas is O on the surface of platinum Pt.₂ ^-Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). Then the generated NO₂As shown in FIG. 14 (A), a part of is absorbed in the absorbent while being oxidized on platinum Pt and combined with barium oxide BaO._Three ^-Diffuses into the absorbent in the form of In this way, NOx is absorbed into the NOx absorbent 22. As long as the oxygen concentration in the inflowing exhaust gas is high, NO on the surface of platinum Pt₂As long as the NOx absorption capacity of the absorbent is not saturated₂Is absorbed into the absorbent and nitrate ion NO._Three ^-Is generated.
[0057]
On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas decreases, and as a result, NO on the surface of platinum Pt.₂The production amount of is reduced. NO₂When the production amount of NO decreases, the reaction reverses (NO_Three ^-→ NO₂) And thus nitrate ion NO in the absorbent_Three ^-Is NO₂Is released from the absorbent in the form of At this time, the NOx released from the NOx absorbent 22 reacts with a large amount of unburned HC and CO contained in the inflowing exhaust gas and is reduced as shown in FIG. 14B. In this way, NO on the surface of platinum Pt.₂NO from the absorbent to the next when no longer exists₂Is released. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 22 in a short time, and NOx is discharged into the atmosphere because the released NOx is reduced. Absent.
[0058]
In this case, NOx is released from the NOx absorbent 22 even if the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NOx is only gradually released from the NOx absorbent 22, so that it takes a little longer time to release all the NOx absorbed in the NOx absorbent 22. Cost.
[0059]
As described above, the NOx absorbent 22 contains a noble metal such as platinum Pt, and therefore the NOx absorbent 22 has an oxidizing function. On the other hand, as described above, when the engine operating state is in the first operating region I and low temperature combustion is being performed, soot is hardly generated, and instead, unburned hydrocarbon is used as the precursor of soot or before that. It is discharged from the combustion chamber 5 in the form of a state. However, as described above, the NOx absorbent 22 has an oxidation function, and therefore, unburned hydrocarbons discharged from the combustion chamber 5 at this time are satisfactorily oxidized by the NOx absorbent 22.
[0060]
Incidentally, the NOx absorption capacity of the NOx absorbent 22 is limited, and it is necessary to release NOx from the NOx absorbent 22 before the NOx absorbent capacity of the NOx absorbent 22 is saturated. For that purpose, it is necessary to estimate the amount of NOx absorbed in the NOx absorbent 22. Accordingly, in the embodiment according to the present invention, the NOx absorption amount A per unit time when the first combustion is performed is shown as a map form as shown in FIG. 15A as a function of the required load L and the engine speed N. The NOx absorption amount B per unit time when the second combustion is performed is obtained as a function of the required load L and the engine speed N in the form of a map as shown in FIG. The NOx amount ΣNOX absorbed in the NOx absorbent 22 is estimated by calculating in advance and integrating the NOx absorption amounts A and B per unit time.
[0061]
In the embodiment according to the present invention, NOx is released from the NOx absorbent 22 when the NOx absorption amount ΣNOX exceeds a predetermined allowable maximum value. 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 MAX2 are set. The allowable maximum value MAX1 is about 30 percent of the maximum NOx absorption amount that can be absorbed by the NOx absorbent 22, and the allowable maximum value MAX2 is about 80 percent of the maximum absorption amount that can be absorbed by the NOx absorbent 22. . When the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1 during the first combustion (low temperature combustion), the air-fuel ratio is made rich to release NOx from the NOx absorbent 22, and the second combustion ( When the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1 during normal combustion), the air-fuel ratio is released so that NOx is released from the NOx absorbent 22 when the second combustion is switched to the first combustion. Is rich, and when the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX2 when the second combustion is being performed, additional NOx is released from the NOx absorbent 22 in the latter half of the expansion stroke or during the exhaust stroke. Fuel is injected.
[0062]
That is, the period X in FIG. 16 shows a case where the required load L is lower than the first boundary X (N) and the first combustion is being performed. At this time, the air-fuel ratio is slightly lower than the stoichiometric air-fuel ratio. It is just a lean lean air-fuel ratio. When the first combustion is being performed, the amount of NOx generated is extremely small. Therefore, at this time, the NOx absorption amount ΣNOX increases very slowly as shown in FIG. If the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1 during the first combustion, the air-fuel ratio A / F is temporarily made rich, whereby NOx is released from the NOx absorbent 22. At this time, the NOx absorption amount ΣNOX is set to zero.
[0063]
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 when the first combustion is being performed. Even if the air-fuel ratio A / F is made rich so as to release NOx from the NOx absorbent 22, no soot is generated at this time. Then time t₁When the required load L exceeds the first boundary X (N), the first combustion is switched to the second combustion. As shown in FIG. 16, when the second combustion is performed, the air-fuel ratio A / F becomes considerably lean. When the second combustion is performed, the amount of NOx generated is larger than when the first combustion is performed. Therefore, when the second combustion is performed, the NOx amount ΣNOX increases relatively rapidly. To do.
[0064]
When the air-fuel ratio A / F is made rich when the second combustion is being performed, a large amount of soot is generated, and therefore making the air-fuel ratio A / F rich when the second combustion is being performed is Can not. Therefore, as shown in FIG. 16, even when the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1 during the second combustion, the air-fuel ratio A / F is rich so as to release NOx from the NOx absorbent 22. And not. In this case, the time t in FIG.₂When the required load L becomes lower than the second boundary Y (N) and is switched from the second combustion to the first combustion, the air-fuel ratio A / F is released to release NOx from the NOx absorbent 22 as shown in FIG. Is temporarily rich.
[0065]
Next, time t in FIG._ThreeIs switched from the first combustion to the second combustion, and the second combustion is continued for a while. At this time, the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1, and then the time t_FourIf the maximum allowable value MAX2 is exceeded, additional fuel is injected during the second half of the expansion stroke or during the exhaust stroke to release NOx from the NOx absorbent 22, and the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 22 is Rich.
[0066]
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 minimize the chance of injecting additional fuel. Therefore, if the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX1 when the second combustion is performed, the air-fuel ratio A / F is temporarily made rich when the second combustion is switched to the first combustion. The additional fuel is injected only in a special case where the NOx absorption amount ΣNOX exceeds the allowable maximum value MAX2.
[0067]
Next, referring to FIG. 17, the injection control when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 22 is made rich so as to release NOx from the NOx absorbent 22 during the first combustion is performed. explain. As shown in FIG. 17, when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 22 is made rich when the first combustion is being performed, the fuel injection is performed without changing the throttle opening and the EGR rate. The air / fuel ratio is made rich by increasing the amount. In this case, conventionally, the air-fuel ratio is (A / F)._LThe air / fuel ratio is set to the target rich air / fuel ratio (A / F)_RThe injection amount is [(A / F)_L/ (A / F)_R] Q. In the case shown in FIG. 17, the injection amount is conventionally (17/12) · Q. Here, Q is the fuel injection amount calculated from FIG.
[0068]
However, when the opening degree ST of the throttle valve 25, the opening degree SE of the EGR control valve 29, and the fuel injection amount Q are determined based on the map values as in the present invention, the actual lean air-fuel ratio is not necessarily shown in FIG. Target lean air-fuel ratio (A / F)_LDoes not match exactly. Therefore, the target lean air-fuel ratio (A / F) in which the current air-fuel ratio is determined from the required load L and the engine speed N in FIG._LBased on the premise that it is, the injection amount [(A / F)_L/ (A / F)_R] Even if calculated from the equation Q, the air-fuel ratio is not necessarily the target rich air-fuel ratio (A / F)_RWill not match.
[0069]
By the way, intake air amount Ga and fuel injection amount Q_RAnd target rich air-fuel ratio (A / F)_RHas the following relationship:
Intake air amount Ga / fuel injection amount Q_R= Target rich air-fuel ratio (A / F)_R
That is, the fuel injection amount Q_R= Intake air amount Ga / Target rich air-fuel ratio (A / F)_R
Therefore, in the first embodiment according to the present invention, the actual intake air amount Ga detected by the intake air amount sensor 26 is used as the target rich air-fuel ratio (A / F)._RFuel injection amount Q by dividing by_RThis fuel injection amount Q_RJust try to inject fuel. In this case, the air-fuel ratio is the target rich air-fuel ratio (A / F)_RWill match exactly.
[0070]
Next, the intake air amount Ga and the fuel injection amount Q_RAnd target rich air-fuel ratio (A / F)_RThe relationship between and will be explained 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_RRepresents the volume around one injection. Therefore, if the specific gravity of the fuel is C and the engine speed is N, the injection amount (g / sec) per second is expressed by the following equation.
[0071]
C ・ Q_R・ (N / 60) ・ (Number of cylinders / 2)
Therefore, the intake air amount Ga and the fuel injection amount Q_RAnd target rich air-fuel ratio (A / F)_RThe relationship is as follows.
Ga / [C ・ Q_R(N / 60) (Number of cylinders / 2)] = (A / F)_R
Therefore, the fuel injection amount Q_RIs calculated from the following equation.
[0072]
Q_R= Ga / [C ・ (N / 60) ・ (Number of cylinders / 2) ・ (A / F)_R]
FIG. 18 shows the target rich air-fuel ratio (A / F) when the air-fuel ratio is switched from the second combustion to the first combustion._RShows the case. Also at this time, the fuel injection amount Q_RIs calculated.
Next, the air-fuel ratio is set to the target rich air-fuel ratio (A / F)_RThe period to be maintained will be described.
[0073]
First, the air-fuel ratio is set to the theoretical air-fuel ratio (A / F)._STFuel injection amount Q required to make_STIs calculated. Next, the air-fuel ratio is set to the target rich air-fuel ratio (A / F)_RFuel injection amount Q required to make_RFrom the above fuel injection amount Q_STIs subtracted from the excess fuel amount ΔQ (= Q_R-Q_ST) Is calculated. The air-fuel ratio becomes the target rich air-fuel ratio (A / F) until the cumulative value ΣΔQ of the excess fuel amount becomes larger than the set value X1._RIt is said.
[0074]
The set value X1 represents the excess fuel amount required to release all NOx absorbed in the NOx absorbent 22, and therefore the set value X1 increases as the NOx absorption amount ΣNOX increases.
FIG. 19 shows a processing routine of a NOx release flag that is set when NOx should be released from the NOx absorbent 22, and this routine is executed by interruption every predetermined time.
[0075]
Referring to FIG. 19, first, at step 100, it is judged if the flag I indicating that the engine operating region is the first operating region I is set. When the flag I is set, that is, when the engine operating region is the first operating region I, the routine proceeds to step 101 where the NOx absorption amount A per unit time is calculated from the map shown in FIG. . Next, at step 102, A is added to the NOx absorption amount ΣNOX. Next, at step 103, it is judged if the NOx absorption amount ΣNOX has exceeded the allowable maximum value MAX1. When ΣNOX> MAX1, the routine proceeds to step 104, where the NOx release flag 1 indicating that NOx should be released when the first combustion is being performed is set.
[0076]
On the other hand, when it is determined in step 100 that the flag I has been reset, that is, when the engine operating range is the second operating range II, the routine proceeds to step 105 and the unit time per unit time is determined from the map shown in FIG. NOx absorption amount B is calculated. Next, at step 106, B is added to the NOx absorption amount ΣNOX. Next, at step 107, it is judged if the NOx absorption amount ΣNOX has exceeded the allowable maximum value MAX1. When ΣNOX> MAX1, the routine proceeds to step 108 where a NOx release flag I indicating that NOx should be released when the first combustion is being performed is set.
[0077]
On the other hand, at step 109, it is judged if the NOx absorption amount ΣNOX has exceeded the allowable maximum value MAX2. When ΣNOX> MAX2, the routine proceeds to step 110 where the NOx release flag II indicating that NOx should be released during the latter half of the expansion stroke or during the exhaust stroke is set.
Next, the operation control will be described with reference to FIG.
[0078]
Referring to FIG. 20, first, at step 200, it is judged if the flag I indicating that the engine operating state is the first operating region I is set. When the flag I is set, that is, when the engine operating state is the first operating region I, the routine proceeds to step 201, where it is determined whether or not the required load L has become larger than the first boundary X1 (N). Is done. When L ≦ X1 (N), the routine proceeds to step 203 where low temperature combustion is performed.
[0079]
That is, in step 203, the target opening ST of the throttle valve 25 is calculated from the map shown in FIG. 11A, and the opening of the throttle valve 25 is set as this target opening ST. Next, at step 204, the target opening degree SE of the EGR control valve 29 is calculated from the map shown in FIG. 11B, and the opening degree of the EGR control valve 29 is set as this target opening degree SE. Next, at step 205, it is judged if the NOx release flag I is set. When the NOx 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. At this time, low-temperature combustion is performed under a lean air-fuel ratio.
[0080]
On the other hand, when it is determined at step 205 that the NOx release 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, when it is determined in step 201 that L> X (N), the routine proceeds to step 202 where the flag I is reset, and then the routine proceeds to step 210 where second combustion (normal combustion) is performed.
[0081]
That is, at step 210, the target opening ST of the throttle valve 25 is calculated from the map shown in FIG. 13A, and the opening of the throttle valve 25 is set as this target opening ST. Next, at step 211, the target opening degree SE of the EGR control valve 29 is calculated from the map shown in FIG. 13B, and the opening degree of the EGR control valve 29 is set as this target opening degree SE. Next, at step 212, it is judged if the NOx release flag II is set. When the NOx release flag II is not set, the routine proceeds to step 213, 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 a lean air-fuel ratio.
[0082]
On the other hand, when it is determined at step 212 that the NOx release 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, the intake air mass flow rate Ga detected by the intake air amount sensor 26 in step 300 is taken. Next, at step 301, the air-fuel ratio is set to the target rich air-fuel ratio (A / F)._RFuel injection amount Q for_RIs calculated based on the following equation.
[0083]
Q_R= Ga / [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 42 in the form of a map as a function of the engine operating state. In step 302, the injection timing is calculated based on this map. Next, at step 303, the fuel injection amount Q calculated at step 301 at the injection timing calculated at step 302._RThe process for injecting is performed. Thus, the air-fuel ratio is changed from the lean air-fuel ratio to the target rich air-fuel ratio (A / F)._RIt will be switched to.
[0084]
By the way, an air-fuel ratio sensor is installed in the engine exhaust passage, and the air-fuel ratio is set to the target air-fuel ratio (A / F) based on the output signal of this air-fuel ratio sensor._RIf feedback control is performed so that NOx is released from the NOx absorbent 22, the air-fuel ratio is set to the target air-fuel ratio (A / F)._RCan be maintained. 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.
[0085]
In contrast, in the present invention, the fuel injection amount Q to the cylinder into which the intake air flows is based on the mass flow rate Ga of the intake air to be supplied into the engine cylinder._RIs calculated. 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)._RControlled.
[0086]
In step 304 and subsequent steps, the air-fuel ratio is set to the target rich air-fuel ratio (A / F) until all NOx absorbed in the NOx absorbent 22 is released._RA process for maintaining the above is performed. That is, in step 304, the air-fuel ratio is changed to the theoretical air-fuel ratio (A / F)._STRequired fuel injection amount Q_STIs calculated based on the following equation.
Q_ST= Ga / [C ・ (N / 60) ・ (Number of cylinders / 2) ・ (A / F)_ST]
Next, at step 305, the excess fuel amount ΔQ (= Q_R-Q_STThen, 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 NOx release flag I is reset, then at step 309, ΣNOX is made zero, then at step 310, ΣΔQ is made zero.
[0087]
Next, the injection control II will be described with reference to FIG. Referring to FIG. 22, first, at step 400, the fuel injection amount Q is calculated from the map shown in FIG. Next, at step 401, the mass flow rate Ga of the intake air detected by the intake air amount sensor 26 is taken. Next, at step 402, the air-fuel ratio is set to the target rich air-fuel ratio (A / F)._RFuel injection amount Q for_RIs calculated based on the following equation.
[0088]
Q_R= Ga / [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 is set to the target rich air-fuel ratio (A / F)._RRequired fuel injection amount Q_RBy subtracting the fuel injection amount Q from the additional fuel amount Qadd (= Q_R-Q) is calculated. Next, at step 404, the fuel injection amount Q is injected in the vicinity of the compression top dead center, and the process for performing the injection of the additional fuel amount Qadd during 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 NOx absorbent 22 by the additional fuel is the target rich air-fuel ratio (A / F)._RIt is said.
[0089]
In step 405 and subsequent steps, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 22 is set to the target rich air-fuel ratio (A / F) until all the NOx absorbed by the NOx absorbent 22 is released._RA process for maintaining the above is performed. That is, in step 405, the air-fuel ratio is changed to the theoretical air-fuel ratio (A / F)._STRequired fuel injection amount Q_STIs calculated based on the following equation.
[0090]
Q_ST= Ga / [C ・ (N / 60) ・ (Number of cylinders / 2) ・ (A / F)_ST]
Next, at step 406, the excess fuel amount ΔQ (= Q_R-Q_STThen, 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). When ΣΔQ> X2, the routine proceeds to step 409, where the NOx release flags I and II are reset, then at step 410, ΣNOX is made zero, then at step 411, ΣΔQ is made zero.
[0091]
FIG. 23 is a diagram for explaining the turbine rotational speed assist control method according to the first embodiment. As shown in FIG. 23, before time t11, the throttle opening is fully opened and the EGR control valve opening is fully closed, and normal combustion is performed under a high load condition. At this time, the rotational speed of the turbine is a relatively high value. Next, when a request to release NOx from the NOx absorbent 22 is made at time t12, the throttle valve opening is switched from fully open to half open, and the EGR control valve opening is switched from fully closed to fully open, and the fuel injection amount. Is increased. When increasing the fuel injection amount, the fuel injection amount may be increased and the fuel injection timing may be retarded, or the sub-injection is performed before or after the main injection in addition to the main injection near the compression top dead center. May be executed. Between time t11 and time t12, since the throttle valve opening is reduced, the intake air amount is reduced, and the rotational speed of the turbine is also reduced. However, the generated torque does not decrease because the fuel injection amount is increased.
[0092]
When the control for releasing NOx from the NOx absorbent 22 ends at time t12, the throttle valve opening is fully opened again, and the EGR control valve opening is also fully closed again. Further, since it is not necessary to release NOx from the NOx absorbent 22, the fuel injection amount is reduced. Furthermore, in the first embodiment, the assist control for rapidly increasing the rotational speed of the turbine is executed by the turbocharger 15 with the electric auxiliary means. As described above, in the first embodiment, when the control for releasing NOx from the NOx absorbent 22 is completed, the assist control for quickly increasing the rotational speed of the turbine is executed, so that the generated torque is temporarily reduced. Can be avoided.
[0093]
In other words, according to the first embodiment, even if an attempt is made to increase the intake air amount after the turbine rotational speed has decreased as the intake air amount has decreased, the turbine rotational speed can immediately increase. In view of the above, an electrical auxiliary means for increasing the rotational speed of the turbine 20 is provided in the turbocharger 15, and after the intake air amount is decreased and the air-fuel ratio is shifted to the rich side at time t11, the air-fuel ratio is increased. At time t12 when the fuel ratio is shifted to the lean side, the intake air amount is increased by operating the electrical auxiliary means. Specifically, after reducing the intake air amount at time t11 and increasing the fuel injection amount so as not to reduce the torque to shift the air-fuel ratio to the rich side, at time t12 to shift the air-fuel ratio to the lean side, The intake air amount is increased by operating the electric auxiliary means so that the torque is not reduced as the fuel injection amount is reduced and the fuel injection amount is reduced. Therefore, after reducing the intake air amount, when increasing the intake air amount, the increase of the intake air amount is delayed, so that the torque shortage and the smoke increase are suppressed. be able to.
[0094]
FIG. 24 is a diagram for explaining the turbine rotation speed assist control method according to the second embodiment. As shown by a solid line in FIG. 24, before the time t21, the low temperature combustion described above is executed and the engine steady operation is performed. At this time, the rotational speed of the turbine is lower than when the above-described normal combustion is performed. Next, when a request for accelerating the engine is issued at time t21, the turbocharger 15 with the electric auxiliary means executes assist control for quickly increasing the rotational speed of the turbine. As described above, in the second embodiment, when the engine steady operation is switched to the engine acceleration operation, the assist control for quickly increasing the rotational speed of the turbine is executed, so that the generated torque necessary for accelerating the engine Can be increased quickly.
[0095]
That is, according to the second embodiment, for example, even if an attempt is made to increase the intake air amount after the rotational speed of the turbine has decreased as the intake air amount is decreased in order to switch from normal combustion to low temperature combustion, the turbine In view of the fact that the rotational speed of the turbocharger 15 cannot be increased immediately, electrical auxiliary means for increasing the rotational speed of the turbine 20 is provided in the turbocharger 15, and at time t21 when switching from low temperature combustion to normal combustion is performed. The amount of intake air is increased by operating the electrical auxiliary means. Therefore, when increasing the intake air amount in order to switch from low temperature combustion to normal combustion, it is possible to suppress a shortage of torque or an increase in smoke as the increase in the intake air amount is delayed. be able to.
[0096]
Further, according to the second embodiment, in an internal combustion engine capable of switching between low temperature combustion and normal combustion, low temperature combustion is executed during engine steady operation, normal combustion is executed during engine acceleration operation, and the required intake air amount is reduced. In view of the considerable increase, the intake air amount is increased by operating the electrical auxiliary means at time t21 when the engine is switched from low temperature combustion to normal combustion and during engine acceleration operation. Therefore, when the required intake air amount increases significantly, the intake air amount can be rapidly increased as required.
[0097]
【The invention's effect】
According to the first and second aspects of the present invention, after the intake air amount is decreased, when the intake air amount is increased, the increase in the intake air amount is delayed, resulting in insufficient torque. Smoke can be suppressed from increasing.
[0098]
According to the third aspect of the present invention, when the intake air amount is increased in order to switch from the low temperature combustion to the normal combustion, an increase in the intake air amount is delayed. Can be prevented from increasing.
[0099]
According to the fourth aspect of the present invention, when the required intake air amount increases significantly, the intake air amount can be rapidly increased as required.
[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 the amount of smoke and NOx generated.
FIG. 3 is a diagram showing combustion pressure.
FIG. 4 is a diagram showing fuel molecules.
FIG. 5 is a diagram showing the relationship between the amount of smoke generated and the 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 operation region I and a second operation region II.
FIG. 8 is a diagram showing an opening degree of a throttle valve and the like.
FIG. 9 is a diagram showing a map of injection amounts.
10 is a diagram showing an air-fuel ratio in a first operating region I. FIG.
FIG. 11 is a view 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 second combustion.
FIG. 13 is a diagram showing a target opening degree of a throttle valve or the like.
FIG. 14 is a view for explaining the NOx absorption / release action;
FIG. 15 is a diagram showing a map of NOx absorption per unit time.
FIG. 16 is a diagram for explaining NOx release control;
FIG. 17 is a diagram showing a change in injection amount and a change in air-fuel ratio when switching from a lean air-fuel ratio to a rich air-fuel ratio.
FIG. 18 is a diagram showing a change in injection amount and a change in air-fuel ratio when switching from a lean air-fuel ratio to a rich air-fuel ratio.
FIG. 19 is a flowchart for processing a NOx release flag;
FIG. 20 is a flowchart for controlling the operation of the engine.
FIG. 21 is a flowchart for executing injection control I;
FIG. 22 is a flowchart for executing injection control II.
FIG. 23 is a diagram for explaining a turbine rotation speed assist control method according to the first embodiment;
FIG. 24 is a diagram for explaining a turbine rotational speed assist control method according to a second embodiment;
[Explanation of symbols]
6 ... Fuel injection valve
15 ... exhaust turbocharger
20 ... exhaust turbine
22 ... NOx absorbent
25 ... Throttle valve
26. Intake air amount sensor
29 ... EGR control valve

Claims (4)

In an internal combustion engine equipped with a turbocharger and reducing the intake air amount to shift the air-fuel ratio to the rich side, auxiliary means for increasing the rotational speed of the turbine of the turbocharger is provided, and the intake air amount is reduced. An internal combustion engine characterized in that the intake air amount is increased by operating the auxiliary means when the air-fuel ratio is shifted to the lean side after the air-fuel ratio is shifted to the rich side. After reducing the intake air amount and increasing the fuel injection amount so that the torque does not decrease and shifting the air-fuel ratio to the rich side, when shifting the air-fuel ratio to the lean side, reducing the fuel injection amount, and 2. The internal combustion engine according to claim 1, wherein the intake air amount is increased by operating the auxiliary means so that the torque does not decrease as the fuel injection amount decreases. As the amount of inert gas supplied to the combustion chamber increases, the amount of soot generated gradually increases and reaches a peak, and when the amount of inert gas supplied to the combustion chamber increases further, Is an internal combustion engine in which the temperature of fuel and its surrounding gas is lower than the generation temperature of soot and soot is hardly generated, and the temperature of fuel and its surrounding gas in the combustion chamber generates soot Switching between low-temperature combustion, where soot is hardly generated at a temperature lower than the temperature, and normal combustion, where the amount of inert gas supplied to the combustion chamber is smaller than the amount of inert gas at which soot generation peaks And an auxiliary engine for increasing the rotational speed of the turbine of the turbocharger, and when the low temperature combustion is switched to the normal combustion, the auxiliary hand is provided. Internal combustion engine, characterized in that increasing the amount of intake air by activating the. 4. The internal combustion engine according to claim 3, wherein the intake air amount is increased by operating the auxiliary means when the low temperature combustion is switched to the normal combustion and the engine is accelerating.

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