JP3551785B2 - Internal combustion engine - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an internal combustion engine.
[0002]
[Prior art]
Conventionally, in an internal combustion engine, for example, a diesel engine, NO_xThe engine exhaust passage and the engine intake passage are connected by an exhaust gas recirculation (hereinafter, referred to as EGR) passage in order to suppress the generation of the exhaust gas, that is, the exhaust gas, that is, the EGR gas is introduced into the engine intake passage through the EGR passage. I try to recirculate. In this case, the EGR gas has a relatively high specific heat and can absorb a large amount of heat. Therefore, as the EGR gas amount increases, the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)) Increases, the combustion temperature in the combustion chamber decreases. NO when combustion temperature drops_xThe more the EGR rate increases, the lower the NO_xWill decrease.
[0003]
If the EGR rate is increased as compared with the conventional case, NO_xIt has been known that the amount of generation of phenol can be reduced. However, when the EGR rate is increased, when the EGR rate exceeds a certain limit, the amount of generated soot, that is, smoke starts to increase rapidly. In this regard, it has been conventionally considered that if the EGR rate is further increased, the smoke will increase infinitely. Therefore, the EGR rate at which the smoke starts to increase rapidly is considered to be the maximum allowable limit of the EGR rate. Has been.
[0004]
Therefore, conventionally, the EGR rate is set within a range not exceeding the maximum allowable limit. The maximum allowable EGR rate varies substantially depending on the type of engine and fuel, but is approximately 30 to 50%. Therefore, in the conventional diesel engine, the EGR rate is suppressed to about 30% to 50% at the maximum.
[0005]
As described above, conventionally, it has been considered that the maximum allowable limit exists for the EGR rate. Therefore, the EGR rate is conventionally set to a value within the range not exceeding the maximum allowable limit._xAnd the amount of smoke generated was determined to be as small as possible. However, in this way, the EGR rate is set to NO._xNO even if the amount of smoke generated is minimized_xAnd the reduction in the amount of smoke generated is limited, and in practice, a considerable amount of NO_xAt present, smoke is generated.
[0006]
However, if the EGR rate is made larger than the maximum permissible limit in the course of research on the combustion of a diesel engine, the smoke rapidly increases as described above. However, a peak exists in the amount of generated smoke. When the EGR rate is further increased, the smoke starts to decrease rapidly, and when the EGR rate is increased to 70% or more during idling operation, and when the EGR rate is increased to about 55% or more when the EGR gas is cooled strongly, the smoke is almost completely reduced. It was found that it was zero, ie, little soot was generated. In this case, NO_xIt has also been found that the amount of generation is extremely small. After that, based on this finding, the reason why soot was not generated was examined, and as a result, soot and NO_xThis has led to the construction of a new combustion system that can simultaneously reduce the combustion. This new combustion system will be described in detail later, but in short, it is basically based on stopping the growth of hydrocarbons in the middle of the process until the hydrocarbons grow into soot.
[0007]
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 gas temperature around it are below a certain temperature, the growth of hydrocarbons stops at a stage before reaching soot, and the fuel When the temperature of the gas surrounding the gas exceeds a certain temperature, hydrocarbons grow to soot at a stretch. In this case, the temperature of the fuel and the surrounding gas is greatly affected by the heat absorbing action of the gas around the fuel when the fuel is burned, and the amount of heat absorbed by the gas around the fuel is adjusted according to the calorific value at the time of burning the fuel. As a result, the temperature of the fuel and the surrounding gas can be controlled.
[0008]
Therefore, if the temperature of the fuel and the surrounding gas during combustion in the combustion chamber is suppressed to a temperature at which the growth of hydrocarbons stops halfway, soot will not be generated, and the temperature of the fuel and the surrounding gas during combustion in the combustion chamber will be reduced. Can be suppressed below the temperature at which the growth of hydrocarbons stops halfway, by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, hydrocarbons whose growth has stopped halfway before reaching soot can be easily purified by post-treatment using an oxidation catalyst or the like. This is the basic idea of a new combustion system. An internal combustion engine employing the new combustion system has already been filed by the present applicant (Japanese Patent Application No. 9-305850).
[0009]
[Problems to be solved by the invention]
By the way, in this new combustion system, the EGR rate needs to be about 55% or more, and the EGR rate can be made about 55% or more when the intake air amount is relatively small. That is, if the amount of intake air exceeds a certain amount, this new combustion cannot be performed. Therefore, if the amount of intake air exceeds a certain amount, the combustion is switched to the conventionally performed combustion. In this case, NO under new combustion_xIt is preferable that new combustion is performed in a wide operating range as much as possible.
[0010]
By the way, under the new combustion, when the air-fuel ratio increases, that is, when the amount of air around the fuel increases, the combustion becomes active, and as a result, the combustion temperature increases. On the other hand, when the air-fuel ratio is reduced, that is, when the amount of air around the fuel decreases, the combustion becomes inactive, and as a result, the combustion temperature decreases. Therefore, even if the fuel injection amount increases as the air-fuel ratio decreases, NO_xIn addition, new combustion without generation of soot can be performed. In other words, as the air-fuel ratio decreases, the operating range in which new combustion can be performed can be expanded to the higher load side.
[0011]
[Means for Solving the Problems]
Therefore, in the first invention, when the amount of inert gas in the combustion chamber increases, the amount of soot generated gradually increases and reaches a peak, and when the amount of inert gas in the combustion chamber further increases, the amount of soot in the combustion chamber increases. In an internal combustion engine in which the temperature of the fuel during combustion and its surrounding gas is lower than the soot generation temperature and soot is hardly generated, the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the amount of soot is peaked. Switching means for selectively switching between first combustion in which the amount of soot is hardly generated and second combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas in which the amount of generated soot is peaked; The engine operating region is divided into a first operating region on the low load side where the first combustion can be performed and a second operating region on the high load side where the second combustion is performed, and the air-fuel ratio is small. As the first operating area becomes higher, It is caused to dynamic.
[0012]
According to a second aspect, in the first aspect, as the air-fuel ratio decreases, the high load limit and the low load limit of the first operation range are shifted to the high load side.
In a third aspect based on the second aspect, the low load side limit of the first operation region does not exist when the air-fuel ratio is lean, and the low load side limit of the first operation region when the air-fuel ratio is rich. Appears.
[0013]
According to a fourth aspect, in the first aspect, a control means for controlling the first operation area is provided, and the control means controls the first operation area in accordance with the target air-fuel ratio.
In a fifth aspect based on the first aspect, the first operating region is shifted to a higher load side as the temperature of the fuel and the surrounding gas during the first combustion decreases.
[0014]
In a sixth aspect based on the fifth aspect, when the first combustion is performed under a rich air-fuel ratio, as the temperature of the fuel during combustion and the gas around it decrease, the first operating range increases. The load side limit and the low load side limit are moved to the high load side.
According to a seventh aspect, in the fifth aspect, there is provided a control means for controlling the first operation range based on a value of a parameter which changes the temperature of the fuel and the surrounding gas during the first combustion. The means moves the first operation range to the higher load side when it is determined from the parameter values that the temperature of the fuel and the surrounding gas during the first combustion decreases.
[0015]
In an eighth aspect based on the seventh aspect, the parameters comprise at least one of a temperature of the gas flowing into the combustion chamber, a temperature of the engine cooling water, a pressure in the engine intake passage, and a humidity of the intake air.
In a ninth aspect based on the first aspect, there is provided an exhaust gas recirculation device for recirculating exhaust gas discharged from the combustion chamber into the engine intake passage, wherein the inert gas comprises recirculated exhaust gas.
[0016]
In a tenth aspect based on the ninth aspect, the exhaust gas recirculation rate during the first combustion is approximately 55% or more.
In an eleventh aspect based on the first aspect, a catalyst having an oxidation function is disposed in the engine exhaust passage.
In a twelfth aspect based on the eleventh aspect, the catalyst comprises an oxidation catalyst or a three-way catalyst.
[0017]
In a thirteenth aspect based on the eleventh aspect, in the eleventh aspect, when the air-fuel ratio of the inflowing exhaust gas is lean, the catalyst includes NO contained in the exhaust gas._xAnd the NO absorbed when the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich._xReleases NO_xConsists of an absorbent.
According to a fourteenth aspect, in the thirteenth aspect, when the operating state of the engine is in the first operating range when the air-fuel ratio is rich, NO_xNO from absorbent_xThe air-fuel ratio is made rich to release.
[0018]
BEST MODE FOR CARRYING OUT 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 Denotes an exhaust valve, and 10 denotes an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch 11, and the surge tank 12 is connected to a supercharger, for example, an outlet of a compressor 16 of an exhaust turbocharger 15 via an intake duct 13 and an intercooler 14. Be linked. An inlet of the compressor 16 is connected to an air cleaner 18 via an air suction pipe 17, and a throttle valve 20 driven by a step motor 19 is arranged in the air suction pipe 17.
[0019]
On the other hand, the exhaust port 10 is connected to an inlet of an exhaust turbine 23 of the exhaust turbocharger 15 via an exhaust manifold 21 and an exhaust pipe 22, and an outlet of the exhaust turbine 23 is connected to a catalyst 25 having an oxidation function through an exhaust pipe 24. Is connected to a catalytic converter 26 having a built-in.
The exhaust pipe 28 connected to the outlet of the catalytic converter 26 and the air suction pipe 17 downstream of the throttle valve 20 are connected to each other via an EGR passage 29, and EGR control driven by a step motor 30 is provided in the EGR passage 29. A valve 31 is arranged. In the EGR passage 29, an intercooler 32 for cooling the EGR gas flowing in the EGR passage 29 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided into the intercooler 32, and the engine cooling water cools the EGR gas.
[0020]
On the other hand, the fuel injection valve 6 is connected via a fuel supply pipe 33 to a fuel reservoir, a so-called common rail 34. Fuel is supplied into the common rail 34 from an electric control type variable discharge fuel pump 35, and the fuel supplied into the common rail 34 is supplied to the fuel injection valve 6 through each fuel supply pipe 33. A fuel pressure sensor 36 for detecting the fuel pressure in the common rail 34 is attached to the common rail 34, and the fuel pump 35 is controlled so that the fuel pressure in the common rail 34 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 36. Is controlled.
[0021]
The electronic control unit 40 is composed of a digital computer, and is connected to each other by a bidirectional bus 41 such as 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. Is provided. A water temperature sensor 60 for detecting an engine cooling water temperature is disposed in the engine body 1, and an output signal of the water temperature sensor 60 is input to an input port 45 via a corresponding AD converter 47. A pressure sensor 61 for detecting an absolute pressure in the surge tank 12 and a temperature sensor 62 for detecting a temperature of a mixed gas of the intake air and the EGR gas are arranged in the surge tank 12. The output signal of the sensor 62 is input to the input port 45 via the corresponding AD converter 47.
[0022]
On the other hand, a humidity sensor 63 for detecting the humidity of the intake air is disposed in the air suction pipe 17 upstream of the throttle valve 20, and an output signal of the humidity sensor 63 is supplied to an input port 45 via a corresponding AD converter 47. Is input to The output signal of the fuel pressure sensor 36 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, by 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 6, the step motor 19 for controlling the throttle valve, the step motor 30 for controlling the EGR control valve, and the fuel pump 35 via the corresponding drive circuit 48.
[0023]
FIG. 2 shows a change in the output torque when the air-fuel ratio A / F (horizontal axis in FIG. 2) is changed by changing the opening degree and the EGR rate of the throttle valve 20 during the low load operation of the engine, and smoke, HC, CO, NO_x4 shows an experimental example showing a change in the emission amount of the gas. As can be seen from FIG. 2, in this experimental example, the smaller the air-fuel ratio A / F, the higher the EGR rate. When the air-fuel ratio A / F is smaller than the stoichiometric air-fuel ratio (．14.6), the EGR rate is 65% or more.
[0024]
As shown in FIG. 2, when the air-fuel ratio A / F is reduced by increasing the EGR rate, the amount of smoke generated when the EGR rate becomes about 40% and the air-fuel ratio A / F becomes about 30 is reduced. Start growing. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the amount of smoke generated sharply increases and reaches a peak. Next, when the EGR rate is further increased and the air-fuel ratio A / F is reduced, the smoke is sharply reduced. When the EGR rate is increased to 65% or more, and the air-fuel ratio A / F becomes about 15.0, the smoke becomes almost zero. . That is, almost no soot is generated. At this time, the output torque of the engine slightly decreases, and NO_xThe amount of generation is considerably low. On the other hand, at this time, the generation amounts of HC and CO begin to increase.
[0025]
FIG. 3A shows a change in the combustion pressure in the combustion chamber 5 when the air-fuel ratio A / F is around 21 and the amount of generated smoke is the largest, and FIG. The graph shows changes in the combustion pressure in the combustion chamber 5 when the amount of generated smoke is almost zero in the vicinity. As can be seen by comparing FIG. 3 (A) and FIG. 3 (B), FIG. 3 (B) in which the amount of smoke generation is almost zero is shown in FIG. 3 (A) where the amount of smoke generation is large. It can be seen that the combustion pressure is lower than in 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 generated smoke is almost zero, NO as shown in FIG._xThe amount of generation of methane is considerably reduced. NO_xThe decrease in the amount of the generated gas 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 little soot is 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 the combustion temperature in the combustion chamber 5 is low at this time.
[0027]
Second, when the amount of generated smoke, that is, the amount of generated soot becomes almost zero, the amount of HC and CO emissions increases as shown in FIG. This means that hydrocarbons are emitted without growing to soot. That is, the linear hydrocarbon and the aromatic hydrocarbon contained in the fuel as shown in FIG. 4 are thermally decomposed when the temperature is increased in a state of lack of oxygen, so that a precursor of soot is formed. A soot consisting of a solid aggregate of carbon atoms is produced. In this case, the actual soot generation process is complicated, and it is not clear what form the soot precursor takes, but in any case, the hydrocarbon as shown in FIG. It will grow to soot. Therefore, as described above, when the generation amount of soot becomes almost zero, the emission amounts of HC and CO increase as shown in FIG. 2, but HC at this time is a soot precursor or a hydrocarbon in a state before it. .
[0028]
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 amount of generated soot becomes almost zero. The hydrocarbon will be discharged from the combustion chamber 5. As a result of further detailed experimental research on this fact, when the temperature of the fuel and the surrounding gas in the combustion chamber 5 is lower than a certain temperature, the growth process of the soot is stopped halfway. It was found that no soot was generated, and soot was generated when the temperature of the fuel and its surroundings in the combustion chamber 5 exceeded a certain temperature.
[0029]
By the way, the temperature of the fuel and its surrounding when the process of producing hydrocarbons is stopped in the state of the soot precursor, that is, the above-mentioned certain temperature varies depending on various factors such as the type of fuel, the air-fuel ratio, the compression ratio, and the like. I can not say how many times, but this certain temperature is NO_xIs closely related to the amount of NOx generated, so that this certain temperature is NO_xCan be defined to some extent from the amount of occurrence of. That is, as the EGR rate increases, the fuel temperature during combustion and the gas temperature around the fuel decrease, and the NO_xGeneration amount decreases. NO at this time_xIs 10 p. p. When it is less or equal to or less than m, almost no soot is generated. Therefore, the above certain temperature is NO_xIs 10 p. p. m The temperature almost coincides with the temperature when the temperature becomes lower or higher.
[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 state before the soot can be easily purified by a post-treatment using a catalyst having an oxidation function. Considering the post-treatment with the catalyst having the oxidation function as described above, it is extremely difficult to discharge hydrocarbons from the combustion chamber 5 in the state of the precursor of soot or in the state before the soot or in the form of soot from the combustion chamber 5. There is a big difference. The new combustion system employed in the present invention discharges hydrocarbons from the combustion chamber 5 in the form of a precursor or previous soot without producing soot in the combustion chamber 5 and removing the hydrocarbons. The core is to oxidize with a catalyst having an oxidation function.
[0031]
Now, in order to stop the growth of hydrocarbons before soot is generated, it is necessary to suppress the temperature of the fuel and the surrounding gas during combustion in the combustion chamber 5 to a temperature lower than the temperature at which soot is generated. There is. In this case, it has been found that the endothermic effect of the gas around the fuel when the fuel burns has an extremely large effect on suppressing the temperature of the fuel and the gas around the fuel.
[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 separated from the fuel does not rise so much, and only the temperature around the fuel becomes extremely high locally. That is, at this time, the air separated from the fuel hardly absorbs the heat of combustion heat of the fuel. In this case, since the combustion temperature becomes extremely high locally, the unburned hydrocarbons that have received the heat of combustion will generate soot.
[0033]
On the other hand, when fuel is present in a mixed gas of a large amount of inert gas and a small amount of air, the situation is slightly different. In this case, the evaporated fuel diffuses to the surroundings, 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 effect of the inert gas.
[0034]
In this case, controlling the temperature of the fuel and its surrounding gas to a temperature lower than the temperature at which soot is generated requires an amount of an inert gas that can absorb a sufficient amount of heat to do so. Therefore, if the amount of fuel increases, the required amount of inert gas increases accordingly. In this case, the larger the specific heat of the inert gas, the stronger the endothermic action, and therefore, the inert gas is preferably a gas having a large specific heat. In this regard, CO₂Since EGR gas has a relatively large specific heat, it can be said that it is preferable to use EGR gas as the 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 degree of cooling of the EGR gas is changed. That is, in FIG. 5, the curve A shows a case where the EGR gas is cooled strongly and the EGR gas temperature is maintained at approximately 90 ° C., and the curve B shows a 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 the curve A in FIG. 5, when the EGR gas is cooled strongly, the amount of soot generation peaks at a point where the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to approximately 55% or more. Then, almost no soot is generated. On the other hand, as shown by the curve B in FIG. 5, when the EGR gas is slightly cooled, the amount of soot generation peaks at a point where the EGR rate is slightly higher than 50%, and in this case, the EGR rate is increased to about 65% or more. So that almost no soot is generated.
[0037]
As shown by the curve C in FIG. 5, when the EGR gas is not forcibly cooled, the amount of soot generation reaches a peak when the EGR rate is around 55%. Above a percentage, soot is hardly generated. FIG. 5 shows the amount of smoke generated when the engine load is relatively high. When the engine load decreases, the EGR rate at which the amount of soot peaks slightly decreases, and the EGR rate at which almost no soot is generated Also lowers slightly. As described above, the lower limit of the EGR rate at which almost no soot is generated varies depending on the degree of cooling of the EGR gas and the engine load.
[0038]
FIG. 6 shows a mixed gas amount of the EGR gas and the air necessary for setting the fuel temperature during combustion and the surrounding gas temperature to a temperature lower than the temperature at which soot is generated when EGR gas is used as the inert gas; Further, the ratio of air in the mixed gas amount and the ratio of EGR gas in the mixed gas are shown. In FIG. 6, the vertical axis indicates the total intake gas amount drawn into the combustion chamber 5, and the dashed line Y indicates the total intake gas amount that can be drawn into the combustion chamber 5 when supercharging is not performed. ing. The horizontal axis shows the required load.
[0039]
Referring to FIG. 6, the proportion of air, that is, the amount of air in the mixed gas, indicates the amount of air required to completely burn the injected fuel. That is, in the case shown in FIG. 6, the ratio between the air amount and the injected fuel amount is the stoichiometric air-fuel ratio. On the other hand, in FIG. 6, the ratio of the EGR gas, that is, the amount of the EGR gas in the mixed gas, is set so that when the injected fuel is burned, the temperature of the fuel and the surrounding gas is lower than the temperature at which the soot is formed. The minimum required EGR gas amount is shown. This EGR gas amount is approximately 55% or more in terms of the EGR rate, and is 70% or more in the embodiment shown in FIG. That is, the total intake gas amount sucked into the combustion chamber 5 is indicated by a solid line X in FIG. 6, and the ratio between the air amount and the EGR gas amount in the total intake gas amount X is as shown in FIG. The temperature of the fuel and the gas around it will be lower than the temperature at which soot is produced, so that no soot is generated. In this case, NO_xThe amount generated is 10 p. p. m around or below and therefore NO_xIs extremely small.
[0040]
As 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 its 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. Accordingly, as shown in FIG. 6, the EGR gas amount must be increased as the injected fuel amount increases. That is, the EGR gas amount needs to increase as the required load increases.
[0041]
By the way, when the supercharging is not performed, the upper limit of the total intake gas amount X sucked into the combustion chamber 5 is Y, and therefore, in FIG. 6, in the region where the required load is larger than Lo, as the required load increases, Unless the EGR gas ratio is reduced, the air-fuel ratio cannot be maintained at the stoichiometric air-fuel ratio. In other words, when the supercharge is not performed, and when the required air-fuel ratio is maintained at the stoichiometric air-fuel ratio in a region where the required load is larger than Lo, the EGR rate decreases as the required load increases. In a region where the required load is larger than Lo, the temperature of the fuel and the surrounding gas cannot be maintained at a temperature lower than the temperature at which soot is generated.
[0042]
However, as shown in FIG. 1, when the EGR gas is recirculated through the EGR passage 29 into the inlet side of the supercharger, that is, into the air suction pipe 17 of the exhaust turbocharger 15, the EGR rate is increased in a region where the required load is larger than Lo. Can be maintained at or above 55 percent, for example, 70 percent, and thus the temperature of the fuel and its surrounding gas can be maintained below the temperature at which soot is produced. That is, if the EGR gas is recirculated so that the EGR rate in the air suction pipe 17 becomes, for example, 70%, the EGR rate of the suction gas boosted by the compressor 16 of the exhaust turbocharger 15 also becomes 70%. The temperature of the fuel and the surrounding gas can be maintained at a temperature lower than the temperature at which soot is generated, to the extent that the pressure can be increased by the compressor 16. Therefore, the operating range of the engine that can generate low-temperature combustion can be expanded.
[0043]
In this case, when the EGR rate is set to 55% or more in a region where the required load is larger than Lo, the EGR control valve 31 is fully opened and the throttle valve 20 is slightly closed.
As described above, FIG. 6 shows the case where the fuel is burned under the stoichiometric air-fuel ratio. However, even if the air amount is smaller than the air amount shown in FIG. NO while preventing generation of_xIs 10 p. p. m or less, and even if the air amount is larger than the air amount shown in FIG. 6, that is, the average value of the air-fuel ratio is 17 to 18 lean, soot generation is prevented. NO_xIs 10 p. p. m can be around or below.
[0044]
That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but since the combustion temperature is suppressed to a low temperature, the excess fuel does not grow to soot, and thus no soot is generated. At this time, NO_xOnly a very small amount 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 when the combustion temperature increases, but in the present invention, the soot is suppressed to a low temperature, so that the soot is reduced. Not generated at all. Furthermore, NO_xOnly a very small amount is generated.
[0045]
Thus, when low-temperature combustion is being performed, soot is not 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, NO_xIs 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.
[0046]
By the way, the temperature of the fuel and the surrounding gas at the time of combustion in the combustion chamber can be suppressed to a temperature at or below the temperature at which the growth of hydrocarbons stops halfway, only during low load operation in the engine, which generates a relatively small amount of heat by combustion. Therefore, in the embodiment according to the present invention, the first combustion, that is, the low-temperature combustion is performed by suppressing the temperature of the fuel during combustion and the gas temperature around the same at or below the temperature at which the growth of hydrocarbons stops halfway during the low load operation in the engine. In addition, the second combustion, that is, the combustion that has been performed conventionally, is performed during the high load operation of the engine. Here, the first combustion, that is, the low-temperature combustion, has a larger amount of the inert gas in the combustion chamber than the amount of the inert gas at which the generation amount of soot is a peak, as is apparent from the description so far, and almost all the soot is generated. The second combustion, that is, the combustion that has been performed conventionally, is a combustion in which the amount of the inert gas in the combustion chamber is smaller than the amount of the inert gas at which the amount of soot is peaked. Say that.
[0047]
Next, an operation region of the engine that can perform the first combustion, that is, the low-temperature combustion, will be described with reference to FIGS. 7A and 7B. 7A and 7B, the vertical axis TQ indicates the required torque, and the horizontal axis N indicates the engine speed.
First, referring to FIG. 17 (B), FIG. 17 (B) shows that the air-fuel ratio is substantially equal to the stoichiometric air-fuel ratio or the first operating region I in which low-temperature combustion can be performed under a lean condition. A second operation region II is shown in which low-temperature combustion cannot be performed substantially under the stoichiometric air-fuel ratio or lean, and conventional combustion must be performed.
[0048]
In FIG. 7B, X (N) represents a first operating region I in which low-temperature combustion can be performed with an air-fuel ratio substantially equal to the stoichiometric air-fuel ratio or lean, and an air-fuel ratio substantially equal to the stoichiometric air-fuel ratio or lean. , A first boundary with a second operation region II in which combustion conventionally performed must be performed, and Y (N) indicates the first boundary between the first operation region I and the second operation region II. 2 shows a second boundary with the operating region II. The determination of the change of the operation region from the first operation region I to the second operation region II is made 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 determination of the change of the operating region is performed based on the second boundary Y (N).
[0049]
That is, in the embodiment according to the present invention, when the operating state of the engine is in the first operating region I shown in FIG. 7B, low-temperature combustion is performed. At this time, when the required torque TQ exceeds a first boundary X (N) which is a function of the engine speed N, it is determined that the operation region has shifted to the second operation region II, and combustion is performed by a conventional combustion method. Next, when the required torque TQ becomes lower than a second boundary Y (N) which is a function of the engine speed N, it is determined that the operation region has shifted to the first operation region I, and low-temperature combustion is performed again.
The two boundaries of the first boundary X (N) and the second boundary Y (N) on the lower torque side than the first boundary X (N) are provided for the following two reasons. . The first reason is that the combustion temperature is relatively high on the high torque side of the second operation region II, and even if the required torque TQ becomes lower than the first boundary X (N), low-temperature combustion cannot be performed immediately. Because. That is, low temperature combustion is not immediately started unless the required torque TQ becomes considerably low, that is, when the required torque TQ becomes lower than the second boundary Y (N). The second reason is that hysteresis is provided for a change in the operation range between the first operation range I and the second operation range II.
[0050]
On the other hand, in FIG. 7A, in addition to the first boundary X (N) shown in FIG. 7B, when the air-fuel ratio is made considerably rich, for example, the air-fuel ratio is made smaller than approximately 13.5. A first operating region Z in which good low-temperature combustion can be performed when the load is high, and a high-load-side limit Z1 (N) and a low-load-side limit Z2 (N) of the first operating region Z are shown. . As can be seen from FIG. 7A, these limits Z1 (N) and Z2 (N) are functions of the engine speed N.
[0051]
As can be seen from FIG. 7B, there is no limit on the low load side in the first operation region I where the low-temperature combustion can be performed when the air-fuel ratio is substantially the stoichiometric air-fuel ratio or lean. On the other hand, the low-load-side limit Z2 (N) of the first operating region Z where the low-temperature combustion can be performed when the air-fuel ratio is considerably rich appears when the required torque TQ is negative. Accordingly, it can be seen that the lower the lower the air-fuel ratio, the lower the load-side limit of the first operating region where the low-temperature combustion can be performed moves to the higher load side.
[0052]
Also, as shown in FIG. 7A, the high load side limit Z1 (N) of the first operating region Z in which low-temperature combustion can be performed when the air-fuel ratio is considerably rich is substantially equal to the stoichiometric air-fuel ratio. The load is higher than the high load limit X (N) of the first operating region I where low temperature combustion is possible when the fuel ratio is lean or lean. Therefore, it can be seen that the lower the air-fuel ratio, the lower the first operating region in which low-temperature combustion can be performed moves to the higher load side.
[0053]
That is, as described above, low-temperature combustion can be performed regardless of whether the air-fuel ratio is rich or lean. However, when the fuel injection amount is extremely small, misfiring occurs if the air-fuel ratio is made considerably rich, so that good low-temperature combustion cannot be performed. That is, even when the fuel injection amount is extremely small, if the air-fuel ratio is lean, the fuel is actively burned because there is sufficient air around the fuel particles. On the other hand, when the air-fuel ratio is made considerably rich, the combustion of the fuel particles is not so active because there is not enough air around the fuel particles. At this time, if the fuel injection amount is extremely small, the combustion temperature and the combustion pressure will not be sufficiently increased, thus causing a misfire.
[0054]
In FIG. 7A, the region where the required torque TQ is negative indicates the time of deceleration operation, and at this time, the fuel injection amount is extremely small. Therefore, in the region where the required torque TQ is negative, the low-load-side limit Z2 (N) of the first operation region Z appears.
On the other hand, if the air-fuel ratio is made rich while the low-temperature combustion is being performed near the first boundary X (N), the torque increases by the amount of fuel increase, and therefore Z1 (N) is larger than X (N). On the high load side.
[0055]
By the way, when the operating state of the engine is in the first operating region I or Z and low-temperature combustion is being performed, soot is hardly generated, and instead, the unburned hydrocarbon is in the form of a precursor of soot or a state before it. Thus, it is discharged from the combustion chamber 5. At this time, the unburned hydrocarbon discharged from the combustion chamber 5 is oxidized well by the catalyst 25 having an oxidizing function.
[0056]
As the catalyst 25, an oxidation catalyst, a three-way catalyst, or NO_xAn absorbent can be used. NO_xThe absorbent is NO when the average air-fuel ratio in the combustion chamber 5 is lean._xNO when the average air-fuel ratio in the combustion chamber 5 becomes rich_xHas the function of releasing.
This NO_xThe absorbent is, for example, alumina as a carrier. On this carrier, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, yttrium Y, or the like. And at least one selected from rare earths and a noble metal such as platinum Pt.
[0057]
Not only oxidation catalyst, but also three-way catalyst and NO_xThe absorbent also has an oxidizing function, and therefore, as described above, the three-way catalyst and the NO_xAn absorbent can be used as the catalyst 25.
Next, the operation control in the first operation region I and the second operation region II when the low-temperature combustion is performed under the condition that the air-fuel ratio is substantially the stoichiometric air-fuel ratio or lean with reference to FIG. explain.
[0058]
FIG. 8 shows the opening degree of the throttle valve 20, the opening degree of the EGR control valve 31, the EGR rate, the air-fuel ratio, the injection timing, and the injection amount with respect to the required torque TQ. As shown in FIG. 8, in the first operating region I where the required torque TQ is low, the opening of the throttle valve 20 is gradually increased from almost fully closed to about 2/3 as the required load L increases. The opening degree of the EGR control valve 31 is gradually increased from near full closure to full opening as the required load L increases. In the example shown in FIG. 8, in the first operation region I, the EGR rate is set to approximately 70%, and the air-fuel ratio is set to a slightly lean air-fuel ratio.
[0059]
In other words, in the first operating region I, the opening of the throttle valve 20 and the opening of the EGR control valve 31 are controlled such that the EGR rate becomes approximately 70% and the air-fuel ratio becomes a slightly lean air-fuel ratio. In the first operation region I, fuel injection is performed before the compression top dead center TDC. In this case, the injection start timing θS is delayed as the required load L is increased, and the injection completion timing θE is delayed as the injection start timing θS is delayed.
[0060]
At the time of idling operation, the throttle valve 20 is closed to almost fully closed, and at this time, the EGR control valve 31 is also closed to almost fully closed. When the throttle valve 20 is closed close to the fully closed state, the pressure in the combustion chamber 5 at the start of compression decreases, so that the compression pressure decreases. When the compression pressure decreases, the compression work by the piston 4 decreases, so that the vibration of the engine body 1 decreases. That is, at the time of idling operation, the throttle valve 20 is closed to almost fully closed in order to suppress the vibration of the engine body 1.
[0061]
On the other hand, when the operating region of the engine changes from the first operating region I to the second operating region II, the opening of the throttle valve 20 is increased stepwise from about 2/3 opening toward the full opening direction. At this time, in the example shown in FIG. 8, the EGR rate is reduced stepwise from approximately 70% to 40% or less, and the air-fuel ratio is increased stepwise. That is, since the EGR rate jumps over the EGR rate range (FIG. 5) in which a large amount of smoke is generated, a large amount of smoke is generated when the engine operating region changes from the first operating region I to the second operating region II. There is no.
[0062]
In the second operation region II, the second combustion, that is, the conventional combustion is performed. In this combustion method, soot and NO_xIs generated, but the thermal efficiency is higher than that of low-temperature combustion. Therefore, when the operating region of the engine changes from the first operating region I to the second operating region II, the injection amount decreases stepwise as shown in FIG. I'm sick. In the second operation region II, the throttle valve 20 is held in a fully open state except for a part, and the opening of the EGR control valve 31 is gradually reduced as the required torque TQ increases. In this operating region II, the EGR rate decreases as the required torque TQ increases, and the air-fuel ratio decreases as the required torque TQ increases. However, the air-fuel ratio is a lean air-fuel ratio even when the required torque TQ increases. In the second operation region II, the injection start timing θS is set near the compression top dead center TDC.
[0063]
FIG. 9A shows the relationship between the required torque TQ, the depression amount L of the accelerator pedal 50, and the engine speed N. In FIG. 9A, each curve represents an equal torque curve, a curve indicated by TQ = 0 indicates that the torque is zero, and the remaining curves are TQ = a, TQ = b, The required torque gradually increases in the order of TQ = c and TQ = d. Further, TQ = -f and TQ = -g indicate the case where the required torque is negative, that is, the time of deceleration operation. In this case, the required torque is smaller in TQ = -g than in TQ = -f. The required torque TQ shown in FIG. 9A is stored in advance in the ROM 42 as a function of the depression amount L of the accelerator pedal 50 and the engine speed N as shown in FIG. 9B. In the embodiment according to the present invention, the required torque TQ according to the depression amount L of the accelerator pedal 50 and the engine speed N is first calculated from the map shown in FIG. 9B, and the target air-fuel ratio is calculated based on the required torque TQ. Are calculated.
[0064]
Incidentally, the high-load limit of the first operating region I where low-temperature combustion can be performed changes according to the gas temperature in the combustion chamber 5 at the start of compression, the cylinder inner wall surface temperature, and the like. That is, when the required torque TQ increases and the amount of heat generated by combustion increases, the temperature of the fuel and the surrounding gas during combustion increases, and it becomes impossible to perform low-temperature combustion. On the other hand, when the gas temperature TG in the combustion chamber 5 at the start of compression decreases, the gas temperature in the combustion chamber 5 immediately before the start of combustion decreases, so that the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, if the gas temperature TG in the combustion chamber 5 at the start of compression decreases, the amount of heat generated by combustion increases, that is, even if the required torque TQ increases, the temperature of the fuel and the surrounding gas during combustion does not increase. Thus, low-temperature combustion is performed. In other words, as the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower, the first operating region I in which low-temperature combustion can be performed expands to a higher load side.
[0065]
Further, the smaller the temperature difference (TW-TG) between the cylinder inner wall surface temperature TW and the gas temperature TG in the combustion chamber 5 at the start of compression, the greater the amount of heat that escapes through the cylinder inner wall surface during the compression stroke. Therefore, the smaller the temperature difference (TW-TG), the smaller the temperature rise of the gas in the combustion chamber 5 during the compression process, and thus the lower the temperature of the fuel and the surrounding gas during combustion. Therefore, as the temperature difference (TW-TG) is smaller, the first operating region I in which low-temperature combustion can be performed expands to the higher load side.
[0066]
On the other hand, as the pressure in the intake passage, for example, in the surge tank 12, decreases, the compression pressure in the combustion chamber 5 decreases, and thus the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, as the pressure in the surge tank 12 becomes lower, the first operating region I in which low-temperature combustion can be performed expands to the higher load side. Further, as the humidity in the intake air increases, the amount of heat absorbed by the moisture contained in the intake air increases, and thus the temperature of the fuel and the surrounding gas during combustion decreases. Therefore, as the humidity in the intake air increases, the first operating region I in which low-temperature combustion can be performed expands to the high load side.
[0067]
In the embodiment according to the present invention, when the gas temperature TG in the combustion chamber 5 at the start of compression becomes low, the first boundary is moved from Xo (N) to X (N) as shown in FIG. When −TG) becomes smaller, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Further, in the embodiment according to the present invention, when the pressure PM in the surge tank 12 decreases, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Becomes higher, the first boundary is moved from Xo (N) to X (N) as shown in FIG. Here, Xo (N) indicates a first boundary serving as a reference. The reference first boundary Xo (N) is a function of the engine speed N, and X (N) is calculated using this Xo (N) based on the following equation.
[0068]
X (N) = Xo (N) + C1 · K (T) · K (N)
K (T) = K (T)₁+ K (T)₂+ K (T)₃+ K (T)₄
Where C1 is a constant, K (T)₁Is a function of the gas temperature TG in the combustion chamber 5 at the start of compression as shown in FIG.₁Increases as the gas temperature TG in the combustion chamber 5 at the start of compression decreases. Also, K (T)₂Is a function of the temperature difference (TW-TG) as shown in FIG.₂Becomes larger as the temperature difference (TW-TG) becomes smaller. Also, K (T)₃Is a function of the pressure PM in the surge tank 12 as shown in FIG.₃Becomes larger as the pressure PM in the surge tank 12 becomes lower. Also, K (T)₄Is a function of the humidity DF as shown in FIG.₄Increases as the humidity DF increases. Note that T in FIGS. 11A to 11D₁Is the reference temperature, T₂Is the reference temperature difference, PM₃Is the reference pressure, DF₄Is the reference humidity, TG = T₁, (TW−TG) = T₂, PM = PM₃And DF = DF₄In this case, the first boundary is Xo (N) in FIG.
[0069]
On the other hand, K (N) is a function of the engine speed N as shown in FIG. 11E, and the value of K (N) decreases as the engine speed N increases. That is, the gas temperature TG in the combustion chamber 5 at the start of the compression becomes equal to the reference temperature T.₁If the temperature is lower than the lower limit, the lower the gas temperature TG in the combustion chamber 5 at the beginning of the compression, the lower the first boundary X (N) moves toward the higher load side with respect to Xo (N), and the temperature difference (TW-TG) is used as a reference. Temperature difference T₂If the temperature is lower than the lower limit, the first boundary X (N) moves toward the higher load side with respect to Xo (N) as the temperature difference (TW-TG) becomes smaller. Further, the pressure PM in the surge tank 12 is equal to the reference pressure PM.₃As the pressure PM in the surge tank 12 becomes lower, the first boundary X (N) moves to a higher load side with respect to Xo (N), and the humidity DF becomes lower than the reference humidity DF.₄As the humidity DF increases, the first boundary X (N) moves toward a higher load with respect to Xo (N). Further, the moving amount of X (N) with respect to Xo (N) decreases as the engine speed N increases.
[0070]
FIG. 12A shows the air-fuel ratio A / F in the first operation region I when the first boundary is the first boundary Xo (N) as a reference. In FIG. 12A, curves indicated by A / F = 15, A / F = 16, A / F = 17, and A / F = 18 are obtained when the air-fuel ratio is 15, 16, 17, and 18, respectively. And the air-fuel ratio between the curves is determined by proportional distribution. As shown in FIG. 12A, the air-fuel ratio is lean in the first operating region I, and the air-fuel ratio A / F is leaner in the first operating region I as the required load L decreases. .
[0071]
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 reduced. When the EGR rate is reduced, the air-fuel ratio increases. Therefore, as shown in FIG. 12A, the air-fuel ratio A / F increases as the required load L decreases. As the air-fuel ratio A / F increases, the fuel consumption rate increases. Therefore, in order to make the air-fuel ratio as lean as possible, in the embodiment according to the present invention, the air-fuel ratio A / F increases as the required load L decreases.
[0072]
FIG. 12B shows the air-fuel ratio A / F in the first operation region I when the first boundary is X (N) shown in FIG. As can be seen by comparing FIGS. 12A and 12B, when the first boundary X (N) moves toward the high load side with respect to Xo (N), A / F = The curve of 15, A / F = 16, A / F = 17, A / F = 18 also moves to the high load side. Therefore, it can be seen that when the first boundary X (N) moves toward the higher load side with respect to Xo (N), the air-fuel ratio A / F at the same required load L and the same engine speed N increases. That is, when the first operation region I is expanded to the high load side, soot and NO_xNot only is the operating region in which the occurrence of stagnation rarely occurs expanded, but also the fuel consumption rate is improved.
[0073]
In the embodiment according to the present invention, the target air-fuel ratio in the first operation region I when the first boundary X (N) changes variously, that is, the target in the first operation region I for various values of K (T). The air-fuel ratio is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N as shown in FIGS. 13 (A) to 13 (D). That is, FIG. 13A shows the target air-fuel ratio AFKT1 when the value of K (T) is KT1, and FIG. 13B shows the target air-fuel ratio AFKT2 when the value of K (T) is KT2. 13C shows the target air-fuel ratio AFKT3 when the value of K (T) is KT3, and FIG. 13D shows the target air-fuel ratio when the value of K (T) is KT4. AFKT4 is shown.
[0074]
On the other hand, as shown in FIGS. 14A to 14D, the target opening degree of the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio is mapped as a function of the required torque TQ and the engine speed N as shown in FIGS. The target opening of the EGR control valve 31 required for setting the air-fuel ratio to the target air-fuel ratio is stored in the ROM 42 in advance as shown in FIGS. 15 (A) to 15 (D). It is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N.
[0075]
That is, FIG. 14A shows the target opening ST15 of the throttle valve 20 when the air-fuel ratio is 15, and FIG. 15A shows the target opening SE15 of the EGR control valve 31 when the air-fuel ratio is 15. Is shown.
FIG. 14B shows the target opening ST16 of the throttle valve 20 when the air-fuel ratio is 16, and FIG. 15B shows the target opening SE16 of the EGR control valve 31 when the air-fuel ratio is 16. Is shown.
[0076]
FIG. 14C shows the target opening ST17 of the throttle valve 20 when the air-fuel ratio is 17, and FIG. 15B shows the target opening SE17 of the EGR control valve 31 when the air-fuel ratio is 17. Is shown.
FIG. 14 (D) shows the target opening ST18 of the throttle valve 20 when the air-fuel ratio is 18, and FIG. 15 (B) shows the target opening SE18 of the EGR control valve 31 when the air-fuel ratio is 18. Is shown.
[0077]
FIG. 16 shows the target air-fuel ratio when the second combustion, that is, the normal combustion by the conventional combustion method is performed. Note that, in FIG. 16, each curve indicated by A / F = 24, A / F = 35, A / F = 45, and A / F = 60 indicates target air-fuel ratios 24, 35, 45, and 60, respectively. As shown in FIG. 17A, the target opening degree ST of the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio is stored in the ROM 42 in advance in the form of a map as a function of the required torque TQ and the engine speed N. The target opening SE of the EGR control valve 31 required for setting the air-fuel ratio to the target air-fuel ratio is calculated as a function of the required torque TQ and the engine speed N as shown in FIG. It is stored in the ROM 42 in the form of a map in advance.
[0078]
On the other hand, in the first operating region Z where low-temperature combustion is possible when the air-fuel ratio is considerably rich, the gas temperature TG in the combustion chamber 5 and the temperature difference (TW− TG), the pressure PM in the surge tank 12 and the humidity DF in the intake air. In this case, as in the first operation region I, the first operation region Z is also moved to a higher load side as the temperature of the fuel and the surrounding gas during combustion becomes lower.
[0079]
That is, in FIG. 18, when Zo is a first operation region as a reference, Z1o (N) is a high load limit as a reference, and Z2o (N) is a low load limit as a reference, Z0 (N) is a low load limit as a reference. In contrast, when the temperature of the fuel and the surrounding gas during combustion becomes lower, both the high load side limit Z1 (N) and the low load side limit Z2 (N) are moved to the high load side, so that the first operating region Z Is also moved to the high load side.
[0080]
At this time, the high load side limit Z1 (N) and the low load side limit Z2 (N) are also the respective values K (T) shown in FIG.₁, K (T)₂, K (T)₃, K (T)₄, K (N) using the following equation.
Z1 (N) = Z1o (N) + C2 · K (T) · K (N)
Z2 (N) = Z2o (N) + C3 · K (T) · K (N)
K (T) = K (T)₁+ K (T)₂+ K (T)₃+ K (T)₄
Here, C2 and C3 are constants.
[0081]
Therefore, the gas temperature TG in the combustion chamber 5 at the start of compression becomes equal to the reference temperature T.₁When the temperature is lower than (FIG. 11), as the gas temperature TG in the combustion chamber 5 at the start of compression becomes lower, Z1 (N) and Z2 (N) move toward the higher load side with respect to Z1o (N) and Z2o (N), respectively. And the temperature difference (TW-TG) is equal to the reference temperature difference T₂When the temperature difference (TW-TG) is smaller than (FIG. 11), Z1 (N) and Z2 (N) move to the higher load side with respect to Z1o (N) and Z2o (N), respectively. Further, the pressure PM in the surge tank 12 is equal to the reference pressure PM.₃When the pressure is lower than (FIG. 11), the lower the pressure PM in the surge tank 12 is, the more Z1 (N) and Z2 (N) move toward the higher load side with respect to Z1o (N) and Z2o (N), respectively. Is the reference humidity DF₄When it is larger than (FIG. 11), as the humidity DF increases, Z1 (N) and Z2 (N) move to the higher load side with respect to Z1o (N) and Z2o (N), respectively.
[0082]
As described above, the catalyst 25 may be an oxidation catalyst, a three-way catalyst or NO._xAn absorbent can be used._xThe case where an absorbent is used will be described.
Engine intake passage, combustion chamber 5 and NO_xThe ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the absorbent 25 is set to NO._xWhen the air-fuel ratio of the exhaust gas flowing into the absorbent 25 is called this NO_xThe absorbent 25 is NO when the air-fuel ratio of the inflowing exhaust gas is lean._xNO when the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric or rich._xReleases NO_xPerforms the absorption and release action.
[0083]
This NO_xIf the absorbent 25 is arranged in the engine exhaust passage, NO_xAbsorbent 25 is actually NO_xHowever, there is a part where the detailed mechanism of the absorption / release action is not clear. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking platinum Pt and barium Ba supported on a carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0084]
In the compression ignition type internal combustion engine shown in FIG. 1, combustion is performed in a state where the air-fuel ratio in the normal combustion chamber 5 is lean. When the combustion is performed with the lean air-fuel ratio, the oxygen concentration in the exhaust gas is high. At this time, as shown in FIG.₂Is O₂ ⁻Or O^2-On the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas becomes O 2 on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). NO generated next₂Is absorbed in the absorbent while being oxidized on the platinum Pt and combined with barium oxide BaO, as shown in FIG.₃ ⁻Diffuses into the absorbent in the form of NO in this way_xIs NO_xIt is absorbed in the absorbent 25. NO on the surface of platinum Pt as long as the oxygen concentration in the inflowing exhaust gas is high₂Is generated, and the NO_xNO unless absorption capacity is saturated₂Is absorbed in the absorbent and nitrate ion NO₃ ⁻Is generated.
[0085]
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 the platinum Pt is reduced.₂Is reduced. NO₂The reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus the nitrate ions NO in the absorbent₃ ⁻Is NO₂Released from the absorbent in the form of NO at this time_xNO released from absorbent 25_xIs reduced by reacting with a large amount of unburned HC and CO contained in the inflowing exhaust gas as shown in FIG. 19 (B). Thus, NO on the surface of platinum Pt₂When no longer exists, NO is changed from one absorbent to the next₂Is released. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, NO_xNO from absorbent 25_xIs released, and the released NO_xNO in the atmosphere to be reduced_xIs not emitted.
[0086]
In this case, even if the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO_xNO from absorbent 25_xIs released. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NO_xNO from absorbent 25_xIs released only slowly_xTotal NO absorbed in absorbent 25_xIt takes a slightly longer time to release.
[0087]
By the way, NO_xNO of absorbent 25_xAbsorption capacity is limited, NO_xNO of absorbent 25_xNO before absorption capacity is saturated_xNO from absorbent 25_xMust be released. NO for that_xNO absorbed by absorbent 25_xThe quantity needs to be estimated. Therefore, in the embodiment according to the present invention, NO per unit time when the first combustion is performed_xThe amount of absorption A is determined in advance in the form of a map as shown in FIG. 20A as a function of the required torque TQ and the engine speed N, and the NO per unit time during the second combustion is performed._xThe amount of absorption B is determined in advance as a function of the required torque TQ and the engine speed N in the form of a map as shown in FIG._xNO by integrating absorption amounts A and B_xNO absorbed by absorbent 25_xThe amount ΣNOX is estimated.
[0088]
In the embodiment according to the present invention, this NO_xNO when the absorption amount ΣNOX exceeds a predetermined allowable maximum value._xNO from absorbent 25_xIs to be released. Next, this will be described with reference to FIG.
Referring to FIG. 21, 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. Maximum allowable value MAX1 is NO_xMaximum NO that can be absorbed by the absorbent 25_xIt is about 30% of the absorption amount, and the allowable maximum value MAX2 is NO_xIt is about 80% of the maximum absorption amount that the absorbent 25 can absorb. NO when the first combustion is being performed_xNO when the absorption amount ΣNOX exceeds the allowable maximum value MAX1._xNO from absorbent 25_xWhen the air-fuel ratio is made rich and the second combustion is performed, NO_xWhen the absorption amount ΣNOX exceeds the permissible maximum value MAX1, when switching from the second combustion to the first combustion is performed, for example, NO during the deceleration operation_xNO from absorbent 25_xWhen the air-fuel ratio is made rich and the second combustion is performed, NO_xNO when the absorption amount ΣNOX exceeds the allowable maximum value MAX2._xNO from absorbent 25_xAdditional fuel is injected during the second half of the expansion stroke or during the exhaust stroke to release.
[0089]
That is, FIG. 21 shows a case where the required torque TQ is lower than the first boundary X (N) and the first combustion is performed in the period X, and the air-fuel ratio at this time is slightly smaller than the stoichiometric air-fuel ratio. It has a lean air-fuel ratio. NO when first combustion is being performed_xIs extremely small. Therefore, at this time, as shown in FIG._xThe absorption amount ΣNOX rises very slowly. NO when the first combustion is being performed_xWhen the absorption amount ΣNOX exceeds the allowable maximum value MAX1, the air-fuel ratio A / F is temporarily made rich, whereby the NO_xNO from absorbent 25_xIs released. NO at this time_xThe absorption amount ΣNOX is set to zero.
[0090]
As described above, when the first combustion is performed, no soot is generated regardless of whether the air-fuel ratio is lean, the stoichiometric air-fuel ratio, or rich, and therefore, when the first combustion is performed. NO_xNO from absorbent 25_xAt this time, no soot is generated even if the air-fuel ratio A / F is made rich in order to release the air.
Then at time t₁When the required torque TQ exceeds the first boundary X (N), the first combustion is switched to the second combustion. As shown in FIG. 21, when the second combustion is being performed, the air-fuel ratio A / F becomes considerably lean. NO is performed when the second combustion is performed, compared to when the first combustion is performed._xIs large when the second combustion is being performed._xThe amount ΣNOX rises relatively quickly.
[0091]
If the air-fuel ratio A / F is made rich while the second combustion is being performed, a large amount of soot is generated. Therefore, it is difficult to make the air-fuel ratio A / F rich when the second combustion is being performed. Can not. Therefore, when the second combustion is being performed as shown in FIG._xEven if the absorption amount ΣNOX exceeds the allowable maximum value MAX1, NO_xNO from absorbent 25_xThe air-fuel ratio A / F is not made rich in order to release the fuel. In this case, after the required torque TQ becomes lower than the second boundary Y (N) and switching from the second combustion to the first combustion is performed, NO is determined._xNO from absorbent 25_xThe air-fuel ratio A / F is temporarily made rich to release the air.
[0092]
By the way, at time t in FIG.₂Shows the case where the deceleration operation is performed and the second combustion is switched to the first combustion. When the deceleration operation is performed, the required torque TQ becomes negative. As a result, as shown in FIG. 18, the case where the air-fuel ratio can be made rich by the position of the low load side limit Z2 (N) in the first operation region Z, and the case where the air-fuel ratio May not be rich.
[0093]
Therefore, in the embodiment according to the present invention, when the air-fuel ratio is to be made rich, it is determined whether or not the operation state of the engine is within the first operation area Z, and the operation state of the engine is within the first operation area Z. Sometimes time t in FIG.₂NO when switching from the second combustion to the first combustion as shown in FIG._xNO from absorbent 25_xThe air-fuel ratio A / F is temporarily made rich to release the air.
[0094]
Next, at time t in FIG.₃Is switched from the first combustion to the second combustion, and the second combustion is continued for a while. NO at this time_xThe absorption amount ΣNOX exceeds the allowable maximum value MAX1, and then the time t₄In this case, if the allowable maximum value MAX2 is exceeded, then NO_xNO from absorbent 25_xAdditional fuel is injected during the second half of the expansion stroke or during the exhaust stroke to release_xThe air-fuel ratio of the exhaust gas flowing into the absorbent 25 is made rich.
[0095]
The additional fuel injected during the second half of the expansion stroke or during the exhaust stroke does not contribute to the generation of engine power, so it is preferable to inject the additional fuel as little as possible. Therefore, when the second combustion is performed, NO_xWhen the absorption amount ΣNOX exceeds the allowable maximum value MAX1, the air-fuel ratio A / F is temporarily made rich when the second combustion is switched to the first combustion, and NO_xAdditional fuel is injected only in special cases where the absorption amount ΣNOX exceeds the allowable maximum value MAX2.
[0096]
FIG. 22 shows NO_xNO from absorbent 25_xSet when to release_xThis shows a routine for processing the release flag, and this routine is executed by interruption every predetermined time.
Referring to FIG. 22, first, at step 100, it is determined whether or not a flag I indicating that the operation region of the engine is the first operation region I is set. When the flag I is set, that is, when the operation region of the engine is the first operation region I, the routine proceeds to step 101, where NO per unit time is obtained from the map shown in FIG._xThe absorption amount A is calculated. Next, at step 102, NO_xA is added to the absorption amount ΣNOX. Next, at step 103, NO_xIt is determined whether or not the absorption amount ＸNOX has exceeded the allowable maximum value MAX1. If ΣNOX> MAX1, the routine proceeds to step 104, where NO is set when the first combustion is being performed._xNO indicating that the gas should be released_xRelease flag 1 is set.
[0097]
On the other hand, when it is determined in step 100 that the flag I has been reset, that is, when the operating region of the engine is in the second operating region II, the routine proceeds to step 106, where the per-unit time is determined from the map shown in FIG. NO_xThe absorption amount B is calculated. Next, at step 107, NO_xThe absorption amount ΣNOX is added to B. Next, at step 108, NO_xIt is determined whether or not the absorption amount ＸNOX has exceeded the allowable maximum value MAX1. If ΣNOX> MAX1, the routine proceeds to step 109, where NO is set when the first combustion is being performed._xNO indicating that the gas should be released_xRelease flag 1 is set.
[0098]
On the other hand, in step 110, NO_xIt is determined whether or not the absorption amount ＸNOX has exceeded the allowable maximum value MAX2. When ΣNOX> MAX2, the routine proceeds to step 111, where NO is determined in the latter half of the expansion stroke or during the exhaust stroke._xNO indicating that the gas should be released_xRelease flag 2 is set.
FIG. 23 shows a routine for controlling the low temperature combustion region, that is, the first operation regions I and Z.
[0099]
Referring to FIG. 23, first, in step 200, the gas temperature TG in the combustion chamber 5, the cylinder wall surface temperature TW, the pressure PM in the surge tank 12, and the humidity DF in the intake air at the start of compression are calculated. In this embodiment, the mixed gas temperature of the intake air and the EGR gas detected by the temperature sensor 62 is used as the gas temperature TG in the combustion chamber 5 at the start of compression, and the engine cooling water temperature detected by the temperature sensor 60 is used as the cylinder inner wall surface temperature. TW. The pressure PM in the surge tank 12 is detected by a pressure sensor 61, and the humidity DF is detected by a humidity sensor 63. Next, in step 201, K (T) is obtained from the relationship shown in FIGS.₁, K (T)₂, K (T)₃, K (T)₄Is required, and these K (T)₁To K (T)₄K (T) (= K (T)₁+ K (T)₂+ K (T)₃+ K (T)₄) Is calculated.
[0100]
Next, at step 202, K (N) is calculated from the relationship shown in FIG. Next, in step 203, the value of the first boundary X (N) is calculated based on the following equation using the value of the first boundary Xo (N) stored in advance.
X (N) = Xo (N) + C1 · K (T) · K (N)
Next, at step 204, the difference ΔL (N) between X (N) and Y (N) that changes according to the engine speed N is calculated. Next, at step 205, the value of the second boundary Y (N) (= X (N) -ΔL (N)) is calculated by subtracting ΔL (N) from X (N). Next, in step 206, the high load side limit Z1 (N) is calculated from the following equation using the value of the high load side limit Z1o (N) stored in advance.
[0101]
Z1 (N) = Z1o (N) + C2 · K (T) · K (N)
Next, at step 207, the low load limit Z2 (N) is calculated from the following equation using the value of the low load limit Z2o (N) stored in advance.
Z2 (N) = Z2o (N) + C3 · K (T) · K (N)
Next, the operation control will be described with reference to FIG.
[0102]
Referring to FIG. 24, first, in step 300, it is determined whether or not a flag I indicating that the operation state of the engine is in the first operation region I is set. When the flag I is set, that is, when the operating state of the engine is in the first operating region I, the routine proceeds to step 301, 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 303, where low-temperature combustion is performed.
That is, in step 303, the target opening ST of the throttle valve 20 is calculated from the maps shown in FIGS. 14A to 14D, and the opening of the throttle valve 20 is set to the target opening ST. Next, at step 304, the target opening SE of the EGR control valve 31 is calculated from the maps shown in FIGS. 15A to 15D, and the opening of the EGR control valve 31 is set as the target opening SE. Next, at step 305, NO_xIt is determined whether the release flag 1 is set. NO_xIf the release flag 1 has not been set, the routine proceeds to step 307, where fuel injection is performed. At this time, low-temperature combustion is performed under a lean air-fuel ratio.
[0103]
On the other hand, in step 305, NO_xWhen it is determined that the release flag 1 is set, the routine proceeds to step 306, where it is determined whether or not the operating state of the engine is in the first operating region Z. If the operating state of the engine is not in the first operating region Z, the routine proceeds to step 307, where low-temperature combustion is performed under a lean air-fuel ratio. On the other hand, when the operating state of the engine is in the first operating region Z, the routine proceeds to step 308, where the air-fuel ratio is made rich for a predetermined period. NO during this time_xNO from absorbent 25_xIs released. Then NO_xThe release flag 1 is reset, and ΣNOX is cleared.
[0104]
On the other hand, when it is determined in step 301 that L> X (N), the routine proceeds to step 302, where the flag I is reset, and then proceeds to step 311 to perform the second combustion.
That is, in step 311, the target opening ST of the throttle valve 20 is calculated from the map shown in FIG. 17A, and the opening of the throttle valve 20 is set to the target opening ST. Next, at step 312, the target opening SE of the EGR control valve 31 is calculated from the map shown in FIG. 17B, and the opening of the EGR control valve 31 is set as the target opening SE. Next, at step 313, NO_xIt is determined whether the release flag 2 is set. NO_xIf the release flag 2 has not been set, the routine proceeds to step 314, where fuel injection is performed so as to achieve the air-fuel ratio shown in FIG. At this time, the second combustion is performed under the lean air-fuel ratio.
[0105]
On the other hand, in step 313, NO_xWhen it is determined that the release flag 2 is set, the routine proceeds to step 315, where additional fuel is injected during the second half of the expansion stroke or during the exhaust stroke for a predetermined period. NO at this time_xThe air-fuel ratio of the exhaust gas flowing into the absorbent 25 becomes rich._xNO from absorbent 25_xIs released. Then NO_xThe release flags 1 and 2 are reset, and ΣNOX is cleared.
[0106]
【The invention's effect】
By changing the region where low-temperature combustion is possible according to the air-fuel ratio, stable low-temperature combustion according to the air-fuel ratio can be ensured.
[Brief description of the drawings]
FIG. 1 is an overall view of a compression ignition type internal combustion engine.
FIG. 2 Smoke and NO_xFIG.
FIG. 3 is a diagram showing a combustion pressure.
FIG. 4 is a diagram showing fuel molecules.
FIG. 5 is a diagram showing a relationship between a generation amount of smoke and an EGR rate.
FIG. 6 is a diagram showing a relationship between an injected fuel amount and a mixed gas amount.
FIG. 7 is a diagram showing a first operation region I, Z and a second operation region II.
FIG. 8 is a view showing an opening degree of a throttle valve and the like.
FIG. 9 is a diagram showing a required torque.
FIG. 10 is a diagram showing a first boundary Z (N).
FIG. 11: K (T)₁To K (T)₄And K (N).
FIG. 12 is a diagram showing a target air-fuel ratio in a first operation region I.
FIG. 13 is a diagram showing a map of a target air-fuel ratio.
FIG. 14 is a view showing a map of a target opening of the throttle valve.
FIG. 15 is a view showing a map of a target opening of the EGR control valve.
FIG. 16 is a diagram showing an air-fuel ratio in the second combustion.
FIG. 17 is a view showing a target opening degree of a throttle valve and the like.
FIG. 18 is a diagram showing a first operation region Z.
FIG. 19_xIt is a figure for explaining the release operation of.
FIG. 20: NO per unit time_xIt is a figure showing a map of an amount of absorption.
FIG. 21 NO_xIt is a figure for explaining discharge control.
FIG. 22_xIt is a flowchart for processing a release flag.
FIG. 23 is a flowchart for controlling a low-temperature combustion region.
FIG. 24 is a flowchart for controlling operation of the engine.
[Explanation of symbols]
6 ... Fuel injection valve
20 ... Throttle valve
31 ... EGR control valve

Claims (14)

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

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