JP4042649B2 - Internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an internal combustion engine.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an internal combustion engine, for example, a diesel engine, an engine exhaust passage and an engine intake passage are connected by an exhaust gas recirculation (hereinafter referred to as EGR) passage in order to suppress the generation of NOx, and the exhaust gas is exhausted through the EGR passage. Gas, that is, EGR gas is recirculated into the engine intake passage. In this case, since the EGR gas has a relatively high specific heat and can absorb a large amount of heat, the EGR gas amount increases, that is, the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)). As the value 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.
[0003]
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.
[0004]
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 diesel engine, the EGR rate is suppressed to about 30% to 50% at the maximum.
[0005]
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.
[0006]
However, if the EGR rate is made larger than the maximum allowable limit in the course of research on combustion of diesel engines, smoke increases rapidly as described above, but there is a peak in the amount of smoke generated, and the EGR rate exceeds this peak. If the value is further increased, the smoke starts to decrease abruptly. When the EGR rate is increased to 70% or higher during idling operation, and when the EGR gas is strongly cooled, the smoke is almost reduced when the EGR rate is increased to approximately 55% or higher. Becomes zero. That is, it has been found that almost no wrinkles occur. 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.
[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 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.
[0008]
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.
[0009]
(For example, see Patent Document 1 to Patent Document 6.)
[Patent Document 1]
JP 2000-008835 A
[Patent Document 2]
JP 2000-008964 A
[Patent Document 3]
Japanese Patent No. 3356075
[Patent Document 4]
JP 2000-130270 A
[Patent Document 5]
JP 2001-140703 A
[Patent Document 6]
JP 2002-221102 A
[0010]
[Problems to be solved by the invention]
However, in this internal combustion engine, a part of the exhaust gas discharged into the exhaust manifold is directly passed through the EGR passage as EGR gas and recirculated to the intake passage. The EGR passage also has an EGR cooler for cooling the EGR gas, and an EGR for purifying SOF (soluble organic substances) and hydrocarbons (HC) in a large amount of PM (particulate matter) in the exhaust gas. A pre-cooler catalyst (for example, an oxidation catalyst) is provided. However, when the internal combustion engine employing this new combustion system is used at a high load, the supercharging pressure also increases accordingly. Therefore, since the pressure in the intake passage is high, it becomes difficult to recirculate EGR gas having a lower pressure to the intake passage, and as a result, new combustion cannot be performed due to the lack of EGR gas. Further, in such a high load region, the combustion temperature also rises so that smoke is generated, and it is difficult to perform low temperature combustion itself.
[0011]
[Means for Solving the Problems]
According to the first aspect of the invention, the exhaust gas recirculation device that recirculates the exhaust gas discharged from the combustion chamber into the engine intake passage is provided, and the amount of the recirculated exhaust gas supplied to the combustion chamber is reduced. As the amount of soot increases, the amount of soot gradually increases and reaches a peak, and when the amount of recirculated exhaust gas supplied into the combustion chamber is further increased, the fuel in the combustion chamber and its surroundings are combusted. Low temperature combustion where the gas temperature is lower than the soot generation temperature and soot is hardly generated, and the amount of inert gas supplied into the combustion chamber is less than the amount of inert gas at which soot generation peaks In an internal combustion engine having switching means for switching between normal combustion, the exhaust gas circulation device includes a low pressure exhaust gas recirculation passage in which the pressure of the recirculated exhaust gas is low, and a high pressure exhaust gas recirculation in which the pressure of the recirculated exhaust gas is high Passage and A first recirculation exhaust gas control valve capable of controlling the amount of recirculation exhaust gas flowing through the low pressure exhaust gas recirculation passage, and a first recirculation exhaust gas capable of controlling the amount of recirculation exhaust gas flowing through the high pressure exhaust gas recirculation passage. And the amount of recirculated exhaust gas flowing through the low pressure exhaust gas recirculation passage based on the engine demand load and the high pressure exhaust gas recirculation when the low temperature combustion is performed. An internal combustion engine is provided that controls the amount of recirculated exhaust gas flowing through a passage.
[0012]
That is, according to the first invention, when the engine required load is relatively low, the pressure in the intake passage of the internal combustion engine is also low, so that the second recirculation exhaust gas control valve of the high pressure exhaust gas recirculation passage is opened. The high-pressure recirculated exhaust gas can be recirculated only by the pressure difference between the pressure in the exhaust passage and the pressure in the intake passage. When the engine demand load is relatively low, it is preferable to supply high-temperature recirculation exhaust gas. However, since the recirculation exhaust gas in the high-pressure exhaust gas recirculation passage is naturally hot, the high-temperature recirculation exhaust gas Thus, stable low-temperature combustion can be performed. The high-pressure exhaust gas recirculation passage preferably extends from the upstream side of the exhaust turbine of the turbocharger to the downstream side of the compressor, whereby the high-pressure recirculation exhaust gas discharged from the combustion chamber is transferred to the high-pressure exhaust gas recirculation passage. Can flow inside. Further, when the engine demand load is high, the pressure in the intake passage is also high, so the above-mentioned pressure difference is small. However, the turbo is in a state where the first recirculation exhaust gas control valve of the low pressure exhaust gas recirculation passage is opened. By utilizing the supercharging of the charger, a large amount of the low-pressure recirculation exhaust gas in the low-pressure exhaust gas recirculation passage can be recirculated. When the engine demand load is high, it is preferable to supply low-temperature recirculation exhaust gas, but the low-pressure exhaust gas recirculation passage is inevitably longer than the high-pressure exhaust gas recirculation passage due to the structure of the internal combustion engine. When flowing in the low pressure exhaust gas recirculation passage, the recirculation exhaust gas can be at a relatively low temperature. Therefore, according to the first invention, when the engine required load is low, naturally, low temperature combustion can be performed even when the engine required load is high, and low temperature combustion can be performed in a higher load region than before. It becomes possible.
[0013]
According to a second invention, in the first invention, a turbocharger is further provided, and the low-pressure exhaust gas recirculation passage is connected to a downstream side of the exhaust turbine of the turbocharger and an upstream side of the compressor of the turbocharger. Connected.
That is, according to the second invention, the low-pressure exhaust gas recirculation passage is structurally longer than the high-pressure exhaust gas recirculation passage, so that the same operation and effect as in the first invention can be obtained.
[0014]
According to the third invention, in the first or second invention, a cooling device is provided only in the low-pressure exhaust gas recirculation passage.
That is, according to the third aspect of the invention, the recirculated exhaust gas can be actively cooled by providing a cooling device such as an EGR cooler in the low pressure exhaust gas recirculation passage.
[0015]
According to a fourth invention, in any one of the first to third inventions, the high-pressure exhaust gas recirculation passage is provided with a catalyst having an oxidation function or a filter carrying the catalyst.
That is, according to the fourth aspect of the invention, when the recirculated exhaust gas passes through the catalyst, CO, HC, etc. in the recirculated exhaust gas are oxidized, thereby increasing the temperature of the recirculated exhaust gas, and accordingly, the engine required load. When the temperature is low, higher-temperature recirculated exhaust gas can be supplied. The catalyst may be an oxidation catalyst or a NOx catalyst.
[0016]
According to a fifth aspect, in any one of the first to fourth aspects, the first recirculation exhaust gas control valve and the second recycle exhaust gas control valve are located in the vicinity of the boundary between the region where the engine required load is low and the region where the engine required load is high. The recirculation exhaust gas control valve was opened.
That is, according to the fifth aspect, by opening both the first and second recirculation exhaust gas control valves, a transition from a low engine demand load region to a high region, and a high engine demand load region to a low region. The transition to can be performed smoothly. In particular, since the high-pressure exhaust gas recirculation passage that can extend from the upstream side of the turbocharger exhaust turbine to the downstream side of the compressor has a high pressure, the recirculated exhaust gas at the time of transition between the low and high demand areas Can increase the responsiveness. In addition, when one recirculation exhaust gas control valve and the other recirculation exhaust gas control valve are simultaneously opened, the recirculation exhaust gas does not move when the required load shifts from a low region to a high region. Although it may temporarily stop flowing, such a situation can be avoided by opening both recirculation exhaust gas control valves.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following drawings, the same members are denoted by the same reference numerals. In order to facilitate understanding, the scales of these drawings are appropriately changed.
FIG. 1 shows a first embodiment in which 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 of a compressor 16 of a supercharger, for example, an exhaust turbocharger 15, via an intake duct 13 and an intercooler 14. Connected. An inlet portion 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 disposed in the air suction pipe 17. A mass flow rate detector 21 for detecting the mass flow rate of the intake air is disposed in the air suction pipe 17 upstream of the throttle valve 20.
[0018]
On the other hand, the exhaust port 10 is connected to an inlet portion of an exhaust turbine 23 of an exhaust turbocharger 15 via an exhaust manifold 22, and an outlet portion of the exhaust turbine 23 is connected to a filter 25 carrying a catalyst having an oxidation function via an exhaust pipe 24. Is connected to a catalytic converter 26 containing the. In this embodiment, a filter carrying an NOx storage reduction catalyst is used. A catalyst having a honeycomb structure (straight flow type) having an oxidation function may be used instead of the filter. An air-fuel ratio sensor 27 is disposed in the exhaust manifold 22.
[0019]
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 and upstream of the compressor 16 are connected via a first exhaust gas recirculation (hereinafter referred to as EGR) passage 61. A first EGR control valve 64 that is connected to each other and is driven by a first step motor 63 is disposed in the first EGR passage 61. Further, an EGR cooler 62 for cooling the EGR gas flowing in the first EGR passage 61 is disposed in the first EGR passage 61. In the embodiment shown in FIG. 1, the engine cooling water is guided into the EGR cooler 62, and the EGR gas is cooled by the engine cooling water. Further, the exhaust manifold 22 and the surge tank 12 positioned upstream of the exhaust turbine 23 are connected to each other via a second EGR passage 29, and are driven by a second step motor 30 in the second EGR passage 29. A second EGR control valve 31 is disposed. The EGR cooler is not provided in the second EGR passage 29, and the EGR gas flowing through the second EGR passage 29 is cooled by air cooling. Therefore, the second EGR passage 29 has a lower EGR gas cooling capacity than the first EGR passage 61. As shown in the figure, the second EGR passage 29 is provided with a catalytic converter 32 containing a catalyst having an oxidation function. The catalyst (not shown) in the catalytic converter 32 may be the same as the catalyst in the catalytic converter 26 described later. When the EGR gas passes through the second EGR passage 29, CO, HC, etc. in the EGR gas are oxidized, so that the temperature of the EGR gas after passing through the catalytic converter 32 is higher than that before passing through the catalytic converter 32. Becomes higher.
[0020]
On the other hand, the fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 34, through a fuel supply pipe 33. Fuel is supplied into the common rail 34 from an electrically controlled fuel pump 35 with variable discharge amount, 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 a fuel pump 35 is set so that the fuel pressure in the common rail 34 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 36. The discharge amount is controlled.
[0021]
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 mass flow detector 21 is input to the input port 45 via the corresponding AD converter 47, and the output signals of the air-fuel ratio sensor 27 and the fuel pressure sensor 36 are also input to the input port via the corresponding AD converter 47. 45. 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 19, the EGR control valve control step motors 30 and 63, and the fuel pump 35 through corresponding drive circuits 48.
[0022]
FIG. 2 shows the output when the air-fuel ratio A / F (horizontal axis in FIG. 2) is changed by changing the opening degree of the throttle valve 20 and the EGR rate during engine low load operation with the fuel injection timing fixed. The experiment example which shows the change of the change of a torque and the 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).
[0023]
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.
[0024]
The following can be said from the experimental results shown in FIG. 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. .
[0025]
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. In other words, linear hydrocarbons and aromatic hydrocarbons contained in fuel are thermally decomposed to form soot precursors when the temperature is raised in an oxygen-deficient state, and then from solids mainly composed of carbon atoms. A cocoon is generated. 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 hydrocarbons will grow to soot via the soot precursor. Become. 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. .
[0026]
Summarizing these considerations based on the experimental results shown in FIG. 2, when the combustion temperature in the combustion chamber 5 is low, the amount of soot generated becomes almost zero, and at this time, the precursor of soot or the hydrocarbon in the previous state is not present. It is 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 rise above a certain temperature.
[0027]
By the way, when the hydrocarbon generation 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 and the compression ratio of the air-fuel 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, the amount of NOx generated is 10 p. p. When it becomes around m or less, wrinkles hardly occur. Therefore, at a certain temperature described above, the amount of NOx generated is 10 p. p. It almost corresponds to the temperature when it becomes around m or less.
[0028]
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.
[0029]
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.
[0030]
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.
[0031]
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.
[0032]
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, as 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, CO ₂ Since EGR gas has a relatively large specific heat, it can be said that it is preferable to use EGR gas as an inert gas.
[0033]
FIG. 3 shows the relationship between the EGR rate and smoke when EGR gas is used as the inert gas and the cooling degree of the EGR gas is changed. That is, in FIG. 3, curve A shows the case where EGR gas is cooled strongly and the EGR gas temperature is maintained at about 90 ° C., and curve B shows the case where EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0034]
As shown by curve A in FIG. 3, when the EGR gas is cooled strongly, the amount of soot 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.
[0035]
On the other hand, as shown by the curve B in FIG. 3, 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.
[0036]
In addition, as shown by the curve C in FIG. 3, 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.
[0037]
FIG. 3 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. 4 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. 4, the vertical axis represents the total intake gas amount sucked into the combustion chamber 5, and the chain line Y represents the total intake gas amount that can be sucked into the combustion chamber 5 when supercharging is not performed. ing. The horizontal axis indicates the required load.
[0039]
Referring to FIG. 4, the ratio of air, that is, the amount of air in the mixed gas, indicates the amount of air necessary to completely burn the injected fuel. That is, in the case shown in FIG. 4, the ratio between the air amount and the injected fuel amount is the stoichiometric air-fuel ratio. On the other hand, in FIG. 4, 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 its 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. 4, 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 soot is hardly generated. The amount of NOx generated at this time is 10 p. p. 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. 4, 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]
When supercharging is not being performed, the second EGR control valve 31 is opened, and the outlet side of the compressor 16 of the exhaust turbocharger 15 via the second EGR passage 29, in FIG. The EGR gas is recirculated to the downstream side. In the region where the required load is smaller than LA, the EGR gas flowing in the second EGR passage 29 is recirculated only by the differential pressure between the intake pressure in the intake port 8 and the exhaust pressure in the exhaust port 10. For this reason, when the required load increases, there is a limit in recirculating the EGR gas only with such a differential pressure. The upper limit of the total intake gas amount X sucked into the combustion chamber 5 is Y. Accordingly, in the region where the required load is larger than LA in FIG. 4, the EGR gas ratio decreases as the required load increases and is maintained at the stoichiometric air-fuel ratio. Can not do it. In other words, when supercharging is not performed and an attempt is made to maintain the air-fuel ratio at the stoichiometric air-fuel ratio in a region where the required load is greater than LA, the EGR rate decreases as the required load increases, and thus In regions where the required load is greater than LA, the temperature of the fuel and the surrounding gas cannot be maintained below the temperature at which soot is produced.
[0042]
However, when EGR gas is recirculated through the first EGR passage 61 as shown in FIG. 1 into the inlet side of the turbocharger, that is, into the air suction pipe 17 of the exhaust turbocharger 15, the required load is greater than LA. The EGR rate can be maintained at 55 percent or higher, for example 70 percent, so that the fuel and surrounding gas temperatures can be maintained at a temperature lower than the temperature at which soot is produced. That is, if the EGR gas is recirculated so that the EGR rate in the air suction pipe 17 becomes, for example, 70%, the EGR rate of the intake gas boosted by the compressor 16 of the exhaust turbocharger 15 also becomes 70%. The temperature of the fuel and the surrounding gas can be maintained at a temperature lower than the temperature at which soot is produced, up to a limit that can be increased by the compressor 16. Therefore, the operating range of the engine that can cause low temperature combustion can be expanded. When the EGR rate is 55% or more in the region where the required load is larger than LA, the first EGR control valve 64 is fully opened and the throttle valve 20 is slightly closed. When the required load further increases and exceeds the required load LB, the second EGR control valve 31 is closed and only the first EGR control valve 64 is opened.
[0043]
As described above, FIG. 4 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. The amount of NOx generated is 10 p. p. m or less, and even if the air amount is made larger than the air amount shown in FIG. 4, that is, even if the average air-fuel ratio is lean from 17 to 18, soot formation is prevented. However, the amount of NOx generated is 10 p. p. It can be around m or less.
[0044]
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 Almost no generation. Furthermore, only a very small amount of NOx is generated.
[0045]
Thus, when low-temperature combustion is performed, soot is hardly 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. , NOx generation 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.
[0046]
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 the growth of hydrocarbons stops midway, only when the engine medium-low load operation has a relatively small amount of heat generated by combustion. Therefore, in the embodiment according to the present invention, at the time of engine low load operation, the temperature of the fuel at the time of combustion and the surrounding gas temperature is controlled to be equal to or lower than the temperature at which hydrocarbon growth stops halfway, and low temperature combustion is performed. In some cases, normal combustion, which is conventionally performed, is performed. Note that the low-temperature combustion here refers to combustion in which the amount of inert gas in the combustion chamber is larger than the amount of inert gas at which soot generation peaks, and soot is hardly generated, as is apparent from the above description. Ordinary combustion, which is normally performed conventionally, refers to 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 reaches a peak.
[0047]
FIG. 5 is a diagram showing the smoke concentration FSN, the low temperature combustion region and the normal fuel region in a specific operating state, and FIG. 6 is a conceptual diagram showing the low temperature combustion region and the normal combustion region. Using FIG. 5 and FIG. 6 and the like, low-temperature combustion, which is a combustion in which the amount of inert gas in the combustion chamber is larger than the amount of inert gas at which soot generation peaks, and soot is hardly generated, and the amount of soot generation is The normal combustion in which the amount of inert gas in the combustion chamber is smaller than the peak amount of inert gas will be described. As shown in FIG. 5, there are a low temperature combustion region and a normal combustion region on the high EGR rate side and the low EGR rate side of the EGR rate where the generation amount of soot peaks. Further, when low temperature combustion is performed, the injection timing is advanced compared to when normal combustion is performed. Therefore, as shown in FIG. 5, the low temperature combustion region is advanced compared to the normal combustion region.
[0048]
In FIG. 6, the broken line P indicates that the amount of soot generation peaks when the EGR rate is increased, and the solid line Q indicates that the amount of soot generation becomes almost zero when the EGR rate is further increased. Show. As can be seen from FIGS. 2 and 6, when the EGR rate is increased or decreased to switch between normal combustion and low-temperature combustion on the more advanced side than normal combustion, it passes through a point where the amount of soot generated reaches a peak. Further, when the EGR rate is increased with the injection timing kept constant as shown in FIG. 2, a peak occurs in the amount of smoke generated as shown by the broken line P in FIG. In FIG. 6, if the EGR rate is increased to switch from the normal combustion region to the low temperature combustion region on the more advanced side than the normal combustion region, it passes over the broken line P, so that a peak occurs in the amount of smoke generated during the increase of the EGR rate. Will occur.
[0049]
FIG. 7 shows a first operation region I in which low-temperature combustion is performed and a second operation region II in which normal combustion 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).
[0050]
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 operating region II, and normal combustion 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.
[0051]
The reason for providing the two boundaries of the first boundary X (N) and the second boundary Y (N) on the lower load side than the first boundary X (N) is as follows. . The first reason is that the combustion temperature is relatively high on the high load side of the second operation region II, and at this time, even if the required load L becomes lower than the first boundary X (N), the low temperature combustion cannot be performed immediately. Because. That is, low temperature combustion is not started immediately unless the required load L is considerably low, that is, when the required load L is lower than the second boundary Y (N). The second reason is to provide hysteresis with respect to a change in the operation region between the first operation region I and the second operation region II.
[0052]
Further, X1 (N) in FIG. 7 is a region in which only the first EGR control valve 64 is opened in the first operation region, and a region in which both the first and second EGR control valves 31 and 64 are opened. Indicates the boundary. Further, X2 (N) represents a boundary between a region where both the first and second EGR control valves 31, 64 are opened and a region where only the second EGR control valve 31 is opened. That is, in the present invention, when the load L is between X1 (N) and X2 (N), both the first and second EGR control valves 31, 64 are opened.
[0053]
By the way, when the engine operating region is in the first operating region I and low-temperature combustion is performed, soot is hardly generated, and instead, the unburned hydrocarbon is in the form of soot precursor or the previous state. It is discharged from the combustion chamber 5. At this time, the unburned hydrocarbons discharged from the combustion chamber 5 are satisfactorily oxidized by the catalyst having an oxidation function carried on the filter 25. At the same time, exhaust particulates are collected by the filter 25.
[0054]
As the catalyst 25, an oxidation catalyst, a three-way catalyst, or an occlusion reduction type NOx catalyst that releases and reduces the stored NOx can be used. The NOx catalyst has a function of storing NOx when the average air-fuel ratio in the combustion chamber 5 is lean and releasing NOx when the average air-fuel ratio in the combustion chamber 5 becomes rich.
[0055]
This NOx catalyst has, for example, alumina as a carrier, and on this carrier, for example, alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y, etc. At least one selected from such rare earths and a noble metal such as platinum Pt are supported.
[0056]
FIG. 8 shows the output of the air-fuel ratio sensor 27. As shown in FIG. 8, the output current I of the air-fuel ratio sensor 27 varies according to the air-fuel ratio A / F. Therefore, the air-fuel ratio can be known from the output current I of the air-fuel ratio sensor 27.
[0057]
Next, operation control in the first operation region I and the second operation region II will be schematically described with reference to FIG.
[0058]
FIG. 9 shows the opening degree of the throttle valve 20, the opening degree of the second EGR control valve 31, the opening degree of the first EGR control valve 64, the EGR rate, the air-fuel ratio, the injection timing, and the injection amount with respect to the required load L. Yes. As shown in FIG. 9, in the first operation region I where the required load L is low, the opening degree of the throttle valve 20 is gradually increased from nearly fully closed to about 2/3 opening degree as the required load L increases. As the required load L increases until the required load L reaches X2 (N), the opening degree of the second EGR control valve 31 is gradually increased from nearly fully closed to fully opened. When the required load L exceeds X1 (N), the opening degree of the second EGR control valve 31 decreases from fully open to almost fully closed. The opening degree of the first EGR control valve 64 is maintained so as to be in a fully closed state until the required load L reaches X2 (N). Will be increased. Even when the required load L exceeds X1 (N), the opening degree of the first EGR control valve 64 is maintained in the fully open state. In the example shown in FIG. 9, in the first operating region I, the EGR rate is approximately 70%, and the air-fuel ratio is a slightly lean air-fuel ratio.
[0059]
In other words, the opening degree of the throttle valve 20 and the opening degree of the second EGR control valve 31 are controlled so that the EGR rate is approximately 70% in the first operation region I and the air / fuel ratio becomes a slightly lean air / fuel ratio. Is done. 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.
[0060]
During idle operation, the throttle valve 20 is closed to near full close, and at this time, the second EGR control valve 31 is also closed to close to full close. When the throttle valve 20 is closed to near full close, 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 idle operation, the throttle valve 20 is closed to close to the fully closed state in order to suppress vibration of the engine body 1.
[0061]
On the other hand, when the operating range of the engine is changed from the first operating range I to the second operating range II, the opening degree of the throttle valve 20 is increased stepwise from about 2/3 opening degree to the full opening direction. At this time, in the example shown in FIG. 9, the EGR rate is decreased in a stepped manner from approximately 70% to 40% or less, and the air-fuel ratio is increased in a stepped manner. That is, since the EGR rate exceeds the EGR rate range (FIG. 3) that generates a large amount of smoke, 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, normal combustion is performed. In the second operation region II, the throttle valve 20 is held in a fully open state except for a part thereof, and the opening degree of the first EGR control valve 64 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.
[0063]
FIG. 10A shows the target air-fuel ratio A / F in the first operation region I. In FIG. 10A, each of the curves indicated by A / F = 15.5, A / F = 16, A / F = 17, A / F = 18 has a target air-fuel ratio of 15.5, 16, 17 respectively. , 18, and the air-fuel ratio between the curves is determined by proportional distribution. As shown in FIG. 10 (A), the air-fuel ratio is lean in the first operating region I, and in the first operating region I, the target air-fuel ratio A / F is made lean as the required load L becomes lower. The
[0064]
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 decreased, the air-fuel ratio increases, and therefore the target air-fuel ratio A / F is increased as the required load L decreases as shown in FIG. As the target 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 target air-fuel ratio A / F increases as the required load L decreases. .
[0065]
The target air-fuel ratio A / F shown in FIG. 10 (A) is stored in advance in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. 10 (B). . Further, the target opening ST of the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG. 10A is the required load L and the engine speed as shown in FIG. As a function of N, it is stored in advance in the ROM 42 in the form of a map, and the opening degree SEL of the first EGR control valve 64 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG. The target total opening degree SE, which is the sum of the opening degree SEH of the second EGR control valve 31 and the ROM 42 in advance in the form of a map as a function of the required load L and the engine speed N as shown in FIG. Is stored within. The ratio between the opening degree SEL of the first EGR control valve 64 and the opening degree SEH of the second EGR control valve 31 will be described in detail later.
[0066]
FIG. 12A shows the target air-fuel ratio A / F when normal combustion is performed. In FIG. 12A, 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. ing. The target air-fuel ratio A / F shown in FIG. 12 (A) is stored in advance in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG. 12 (B). Further, the target opening ST of the throttle valve 20 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG. 12A is the required load L and the engine speed as shown in FIG. As a function of N, it is stored in advance in the ROM 42 in the form of a map, and the opening degree SEL of the first EGR control valve 64 necessary for setting the air-fuel ratio to the target air-fuel ratio A / F shown in FIG. As shown in FIG. 13 (B), the target opening degree SE is stored in advance in the ROM 42 in the form of a map as a function of the required load L and the engine speed N.
[0067]
When normal combustion is being performed, the fuel injection amount Q is calculated based on the required load L and the engine speed N. The fuel injection amount Q is stored in advance in the ROM 42 in the form of a map as a function of the required load L and the engine speed N as shown in FIG.
[0068]
Next, the relationship between the temperature of the EGR gas, the air-fuel ratio A / F, the required load L, and the fuel injection timing when low-temperature combustion is performed will be described. FIG. 15 is a graph showing the relationship between the required load L and the injection timing in each case where the air-fuel ratio A / F and the EGR gas cooling method are changed when low-temperature combustion is performed. In FIG. 15, the solid line A indicates the relationship between the required load L and the injection timing when the air-fuel ratio A / F is 17 and the air-cooled EGR gas is supplied into the combustion chamber, and the broken line B indicates the air-fuel ratio A The relationship between the required load L and the injection timing when / F is 12 and air-cooled EGR gas is supplied into the combustion chamber, and the solid line C is water-cooled with an air-fuel ratio A / F of 17 The relationship between the required load L when the EGR gas is supplied into the combustion chamber and the injection timing is shown. The broken line D is when the air-fuel ratio A / F is 12 and the water-cooled EGR gas is supplied into the combustion chamber. The relationship between the required load L and the injection timing is shown.
[0069]
As shown in FIG. 15, the optimum injection timing changes when the temperature of the EGR gas supplied into the combustion chamber changes. Even if the air-fuel ratio A / F during combustion changes, the optimal injection timing changes. Also, the optimal injection timing changes depending on the required load L. Specifically, when the EGR gas is cooled by air cooling, soot is generated when the required load L becomes high. That is, when the cooling capacity of the EGR gas is low, low-temperature combustion cannot be performed unless the load is medium or low. On the other hand, when the EGR gas is cooled by water cooling, soot is not generated even under a high load, and low temperature combustion can be performed. However, when the air-fuel ratio A / F becomes rich, the required injection timing becomes the leading side, and the injected combustion adheres to the bore without being used for combustion. If the injection timing cannot be advanced, fuel supply will be delayed and misfire will occur. Further, when the EGR gas is cooled by water cooling, the injection timing when the air-fuel ratio A / F is 12 and the air-fuel ratio A / F are 17 in the low load region, compared to when the EGR gas is cooled by air cooling. The difference from the time of injection becomes too large. For this reason, when the EGR gas is cooled by water cooling in the low load region, the injection timing is excessively advanced when the combustion is switched from the low temperature combustion to the normal combustion, and a knock noise is likely to be generated.
[0070]
For the above reasons, when low temperature combustion is performed, it is necessary to adjust the temperature of the EGR gas supplied into the combustion chamber according to the required load L to an appropriate temperature. Therefore, in the present embodiment, as shown in FIG. 1, the first EGR passage 61 where the pressure of the EGR gas is low, and the first EGR that can control the amount of EGR gas flowing through the first EGR passage 61. A control valve 64, a second EGR passage 29 having a high EGR gas pressure, and a second EGR control valve 31 capable of controlling the amount of EGR gas flowing through the second EGR passage 29 are provided. When combustion is performed, the amount of low-pressure EGR gas flowing through the first EGR passage 61 and the amount of high-pressure EGR gas flowing through the second EGR passage 29 are controlled based on the required load L.
[0071]
Specifically, when the required load L is low, the amount of low-pressure EGR gas flowing through the first EGR passage 61 is decreased and the amount of high-pressure EGR gas flowing through the second EGR passage 29 is increased, that is, The ratio SEL / SEH between the opening degree SEL of the first EGR control valve 64 and the opening degree SEH of the second EGR control valve 31 is reduced, and relatively high-pressure EGR gas is supplied into the combustion chamber. Since the second EGR passage 29 extends immediately after the exhaust port 10, the temperature of the EGR gas in the second EGR passage 29 is relatively high. Thereby, the temperature of the air-fuel mixture in the combustion chamber can be increased, and low-temperature combustion can be performed in a stable state at low load.
[0072]
On the other hand, when the required load L is high, the amount of low-pressure EGR gas flowing through the first EGR passage 61 is increased and the amount of high-pressure EGR gas flowing through the second EGR passage 29 is decreased, that is, the first The ratio SEL / SEH between the opening degree SEL of the EGR control valve 64 and the opening degree SEH of the second EGR control valve 31 is increased, and relatively low pressure EGR gas is supplied into the combustion chamber. As can be seen from FIG. 1, the temperature of the EGR gas from the first EGR passage 61 is relatively low because it passes through the EGR cooler 62 and the intercooler 14, and as a result, soot is generated by lowering the combustion temperature. Can be avoided.
[0073]
Next, the operation control of this embodiment will be described with reference to FIG. Referring to FIG. 16, first, at step 100, 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 101, where it is determined whether or not the required load L has become larger than the first boundary X (N). Is done. When L ≦ X (N), the routine proceeds to step 105 where low temperature combustion is performed. When it is determined in step 101 that L> X (N), the routine proceeds to step 102 where the flag I is reset, and then the routine proceeds to step 113 where the second combustion is performed.
[0074]
On the other hand, when the flag I is not set in the throttle 100, that is, when the engine operating state is the second operating region II, the routine proceeds to step 103, where the required load L becomes lower than the second boundary Y (N). It is determined whether or not. When L ≧ Y (N), the routine proceeds to step 113, where the second combustion is performed under the lean air-fuel ratio. When it is determined in step 103 that L <Y (N), the routine proceeds to step 104 where the flag I is set, and then the routine proceeds to step 105 where low temperature combustion is performed.
[0075]
In step 105, the target opening ST of the throttle valve 20 is calculated from the map shown in FIG. 11A, and the opening of the throttle valve 20 is set as the target opening ST. Next, at step 106, the target opening degree SE that is the opening degree SE1 of the first EGR control valve 64 or the opening degree SE2 of the second EGR control valve 31 is calculated from the map shown in FIG. FIG. 17 is a map similar to FIG. 11B showing the relationship among the engine speed N, the required load L, and the target opening degree SE.
[0076]
Next, at step 1001, it is determined whether the required load L is higher than L1 (see FIG. 17), that is, whether the required load L is high or low. When the required load L is high (when the required load L is positioned above X1 (N) in FIG. 7), the opening degree SE2 of the second EGR control valve 31 is made zero at step 1002, and step 1003 The opening degree SE1 of the first EGR control valve 64 is set to the target opening degree SE. That is, when the required load L is high, the EGR gas is supplied into the combustion chamber 5 through the first EGR passage 61 in which the EGR gas intake port is disposed on the upstream side of the back pressure exhaust turbocharger 15. Therefore, a large amount of EGR gas can be easily supplied into the combustion chamber 5. Further, since the EGR gas passing through the first EGR passage 61 is cooled by the EGR cooler 62 and further cooled by the intercooler 14 even after being mixed with air, a relatively large amount of EGR gas at a relatively low temperature enters the combustion chamber. Supplied. Therefore, the combustion temperature in the combustion chamber can be lowered, thereby suppressing the generation of smoke. Further, in FIG. 1, since the EGR gas passing through the first EGR passage 61 has been taken out from the downstream side of the catalyst 25, it has already been purified, and the amount of deposit in the first EGR passage 61 can be reduced. it can.
[0077]
On the other hand, when the required load L is equal to or less than L1, it is determined in step 1004 whether the required load L is higher than L0 (see FIG. 17) smaller than L1, that is, whether the required load L is between L0 and L1. (L0 <L <L1) is determined. When the required load L is less than or equal to L0 (when the required load L is less than or equal to X2 (N) in FIG. 7), the routine proceeds to step 1007, where the opening degree SE2 of the second EGR control valve 31 is the target opening degree. In step 1008, the opening degree SE1 of the first EGR control valve 64 is made zero. That is, when the required load is low, the high-pressure EGR gas flowing through the second EGR passage 29 is supplied to the combustion chamber. Since the second EGR passage 29 is taken out from the downstream side of the exhaust port 10, the EGR gas flowing through the second EGR passage 29 is considerably hot. Since the EGR gas in the first EGR passage 29 in FIG. 1 generates heat due to oxidation in the catalytic converter 32, it is supplied to the combustion chamber at a higher temperature. Therefore, since the temperature of the air-fuel mixture supplied to the combustion chamber can be increased, low-temperature combustion can be favorably performed when the required load is low. When the required load L is between L0 and L1 (when the required load L is between X1 (N) and X2 (N) in FIG. 7), the routine proceeds to step 1005 and the second EGR. The opening degree SE2 of the control valve 31 is set to the target opening degree SE, and in step 1006, the opening degree SE1 of the first EGR control valve 64 is set to the target opening degree SE. That is, in this case, both the first and second EGR control valves 64 and 31 are opened. Even in such a case, the EGR gas in the second EGR passage 29 only flows due to the differential pressure between the intake pressure in the intake branch pipe 11 and the exhaust pressure in the exhaust manifold 22. A very large amount of EGR gas does not flow into the combustion chamber from the second EGR passages 61 and 29. Further, when the other EGR control valve is opened simultaneously with the closing of one EGR control valve, for example, when the second EGR control valve 31 is closed simultaneously with the opening of the first EGR control valve 64. The EGR gas may be temporarily absent from the compressor 16 of the exhaust turbocharger 15 to the intake duct 13. However, by setting a region in which both the first and second EGR control valves 64 and 31 are opened in this way, a region in which only one EGR control valve is opened and only the other EGR control valve is opened. The transition between the valved areas can be performed smoothly.
[0078]
Next, at step 110, the mass flow rate of intake air (hereinafter simply referred to as intake air amount) Ga detected by the mass flow detector 21 is taken in, and then at step 111, the target air-fuel ratio is calculated from the map shown in FIG. A / F is calculated. Next, at step 112, based on the intake air amount Ga and the target air-fuel ratio A / F, the fuel injection amount Q required to make the air-fuel ratio the target air-fuel ratio A / F is calculated.
[0079]
When the required load L or the engine speed N changes during low temperature combustion in this way, the opening degree of the throttle valve 20 and the opening degree of the first EGR control valve 64 or the opening degree of the second EGR control valve 31 are changed. Immediately, the target opening degree ST, SE corresponding to the required load L and the engine speed N is matched. Therefore, for example, when the required load L is increased, the amount of air in the combustion chamber 5 is immediately increased, and thus the generated torque of the engine is immediately increased.
[0080]
On the other hand, when the opening degree of the throttle valve 20, the opening degree of the first EGR control valve 64, or the opening degree of the second EGR control valve 31 is changed to change the intake air amount, the change of the intake air amount Ga is changed to the mass flow rate. The fuel injection amount Q is controlled based on the intake air amount Ga detected by the detector 21 and detected. That is, the fuel injection amount Q is changed after the intake air amount Ga actually changes.
[0081]
In step 113 in which the second combustion is performed, the target fuel injection amount Q is calculated from the map shown in FIG. 14, and the fuel injection amount is set as the target fuel injection amount Q. Next, at step 114, the target opening ST of the throttle valve 20 is calculated from the map shown in FIG. Next, at step 115, the target opening degree SE that is the opening degree of the first EGR control valve 64 is calculated from the map shown in FIG. Next, at step 1006, the opening degree SE2 of the second EGR control valve 31 is made zero, and at step 1007, the opening degree SE1 of the first EGR control valve 64 is made the target opening degree SE.
[0082]
Next, at step 116, the intake air amount Ga detected by the mass flow rate detector 21 is taken. Next, at step 117, the actual air-fuel ratio (A / F) is calculated from the fuel injection amount Q and the intake air amount Ga. _R Is calculated. Next, at step 118, the target air-fuel ratio A / F is calculated from the map shown in FIG. Next, at step 119, the actual air-fuel ratio (A / F) _R Is greater than the target air-fuel ratio A / F. (A / F) _R When> A / F, the routine proceeds to step 120 where the throttle opening correction value ΔST is decreased by a constant value α, and then the routine proceeds to step 112. On the other hand (A / F) _R When ≦ A / F, the routine proceeds to step 121 where the correction value ΔST is increased by a fixed value α, and then the routine proceeds to step 122. In step 122, the final target opening ST is calculated by adding the correction value ΔST to the target opening ST of the throttle valve 20, and the opening of the throttle valve 20 is made the final target opening ST. That is, the actual air-fuel ratio (A / F) _R Of the throttle valve 20 is controlled so that becomes the target air-fuel ratio A / F.
[0083]
When the required load L or the engine speed N changes during normal combustion in this way, the fuel injection amount is immediately matched with the target fuel injection amount Q corresponding to the required load L and the engine speed N. For example, when the required load L is increased, the fuel injection amount is immediately increased, and thus the generated torque of the engine is immediately increased.
[0084]
On the other hand, when the fuel injection amount Q is increased and the air-fuel ratio deviates from the target air-fuel ratio A / F, the opening degree of the throttle valve 20 is controlled so that the air-fuel ratio becomes the target air-fuel ratio A / F. That is, the air-fuel ratio is changed after the fuel injection amount Q is changed.
[0085]
In the embodiments described so far, the fuel injection amount Q is open-loop controlled when low-temperature combustion is performed, and the air-fuel ratio is controlled by changing the opening of the throttle valve 20 when normal combustion is performed. Is done. However, the fuel injection amount Q can be feedback-controlled based on the output signal of the air-fuel ratio sensor 27 when the low-temperature combustion is performed, and the air-fuel ratio is set to the first EGR when the normal combustion is performed. Control can also be performed by changing the opening of the control valve 64 or the opening of the second EGR control valve 31.
[0086]
Next, the operation control of the internal combustion engine according to another embodiment of the present invention will be described with reference to FIG. In FIG. 18, steps indicated by the same reference numerals as those in FIG. 16 are the same as those in FIG.
[0087]
In the present embodiment, for example, pressure sensors (not shown) are arranged in the intake branch pipe 11 and the exhaust manifold 22, respectively. These pressure sensors measure the intake pressure P1 and the exhaust pressure P2, respectively, and calculate a pressure difference ΔP (= P2−P1) between the exhaust pressure P2 and the intake pressure P1. Next, at step 2001, it is judged if the pressure difference ΔP is higher than P1, that is, whether the pressure difference ΔP is high or low. When the pressure difference ΔP is high according to the map (not shown) relating to the target opening degree of the EGR control valve based on the pressure difference ΔP and the engine speed N and / or the required load L, the second difference is made in step 2002 when the pressure difference ΔP is high. The opening degree SE2 of the EGR control valve 31 is set to the target opening degree SE, and in step 2003, the opening degree SE1 of the first EGR control valve 64 is made zero. That is, when the pressure difference ΔP is high, EGR gas can be supplied into the combustion chamber 5 through the EGR passage 29 only by the pressure difference ΔP. Since this EGR gas is taken out from the downstream of the exhaust port 10, it is very hot. In FIG. 18, since this EGR gas generates heat due to oxidation in the catalytic converter 32, it is supplied to the combustion chamber at a higher temperature. Therefore, since the temperature of the air-fuel mixture supplied to the combustion chamber can be increased, low-temperature combustion can be favorably performed when the required load is low.
[0088]
On the other hand, when the pressure difference ΔP is equal to or smaller than P1, it is determined in step 2004 whether the pressure difference ΔP is higher than P0 smaller than P1, that is, whether the pressure difference ΔP is between P0 and P1 (P0 <L < P1) is determined. When the pressure difference ΔP is equal to or smaller than P0, the routine proceeds to step 2007, where the opening degree SE2 of the second EGR control valve 31 is made zero, and at step 2008, the opening degree SE1 of the first EGR control valve 64 is The target opening degree SE is set. That is, when the pressure difference ΔP is small, EGR gas is supplied into the combustion chamber 5 through the first EGR passage 61. In addition, the EGR gas passing through the first EGR passage 61 is cooled by the EGR cooler 62 and further cooled by the intercooler 14 after becoming the air-fuel mixture, so that the relatively low temperature EGR gas is put into the combustion chamber. Supplied. Therefore, the combustion temperature can be lowered, thereby suppressing the generation of smoke. Further, in FIG. 18, since the EGR gas passing through the first EGR passage 61 has been taken out from the downstream side of the catalyst 25, it has already been purified, and the amount of deposit in the first EGR passage 61 can be reduced. it can. When the pressure difference ΔP is between P0 and P1, the routine proceeds to step 2005 where the opening degree SE2 of the second EGR control valve 31 is set to the target opening degree SE, and at step 2006, the first EGR control is performed. The opening degree SE1 of the valve 64 is set to the target opening degree SE. That is, in this case, the first and second EGR control valves 64 and 31 are opened. Even in such a case, the EGR gas in the second EGR passage 29 only flows due to the differential pressure between the intake pressure in the intake branch pipe 11 and the exhaust pressure in the exhaust manifold 22. A very large amount of EGR gas does not flow into the combustion chamber from the second EGR passages 61 and 29. Further, when the other EGR control valve is opened simultaneously with the closing of one EGR control valve, for example, when the second EGR control valve 31 is closed simultaneously with the opening of the first EGR control valve 64. In the vicinity of the intercooler 14, EGR gas may be temporarily absent. However, by setting a region in which both the first and second EGR control valves 64 and 31 are opened in this way, a region in which only one EGR control valve is opened and only the other EGR control valve is opened. The transition between the valved areas can be performed smoothly. For this reason, also in the case of this embodiment, the effect similar to embodiment mentioned above can be acquired.
[0089]
In step 113 where normal combustion is performed, the target fuel injection amount Q is calculated from the map shown in FIG. 14, and the fuel injection amount is set as this target fuel injection amount Q. Next, at step 114, the target opening ST of the throttle valve 20 is calculated from the map shown in FIG. Next, at step 115, the target opening degree SE that is the opening degree of the first EGR control valve 64 is calculated from the map shown in FIG. Next, at step 2006, the opening degree SE2 of the second EGR control valve 31 is made zero, and at step 2007, the opening degree SE1 of the first EGR control valve 64 is made the target opening degree SE.
[0090]
【The invention's effect】
According to each invention, it is possible to achieve a common effect that low temperature combustion is possible even when the required load is high.
[0091]
Furthermore, according to the second invention, it is possible to obtain an effect that the same operation and effect as in the case of the first invention can be obtained.
Furthermore, according to the third aspect of the present invention, it is possible to provide an effect that even lower temperature recirculated exhaust gas can be supplied when the required engine load is high.
Furthermore, according to the fourth aspect of the invention, it is possible to positively cool the recirculated exhaust gas.
Furthermore, according to the fifth aspect of the present invention, it is possible to provide an effect that even higher temperature recirculated exhaust gas can be supplied when the required engine load is low.
Further, according to the sixth aspect of the invention, it is possible to achieve an effect that the transition between the high engine demand load area and the low engine load area can be smoothly performed.
[Brief description of the drawings]
FIG. 1 is an overall view of a compression ignition type internal combustion engine of a first embodiment.
FIG. 2 is a diagram showing the amount of smoke and NOx generated.
FIG. 3 is a diagram showing the relationship between the amount of smoke generated and the EGR rate.
FIG. 4 is a diagram showing a relationship between an injected fuel amount and a mixed gas amount.
FIG. 5 is a diagram showing a smoke concentration FSN, a low-temperature combustion region, and a normal fuel region in a specific operation state.
FIG. 6 is a conceptual diagram showing a low temperature combustion region and a normal combustion region.
FIG. 7 is a diagram showing a first operation region I and a second operation region II.
FIG. 8 is a diagram showing an output of an air-fuel ratio sensor.
FIG. 9 is a diagram showing an opening degree of a throttle valve and the like.
10 is a diagram showing an air-fuel ratio and the like in a first operating region I. FIG.
FIG. 11 is a diagram showing a map of target opening degrees of the throttle valve and the like according to the first embodiment.
FIG. 12 is a diagram showing an air-fuel ratio and the like in the second combustion.
FIG. 13 is a view showing a map of a target opening degree of a throttle valve or the like.
FIG. 14 is a diagram showing a map of fuel injection amounts.
FIG. 15 is a graph showing the relationship between the required load L and the injection timing in each case where the air-fuel ratio A / F and the cooling method of EGR gas are changed when low-temperature combustion is performed.
FIG. 16 is a flowchart for controlling the operation of the engine of the first embodiment.
FIG. 17 is a view showing a map of a target opening degree of the EGR control valve according to the first embodiment.
FIG. 18 is a flowchart for controlling the operation of an engine according to another embodiment.
[Explanation of symbols]
1. Internal combustion engine
5 ... Combustion chamber
6 ... Fuel injection valve
14 ... Intercooler
15 ... Turbocharger
16 ... Compressor
20 ... Throttle valve
23 ... Exhaust turbine
25. Filter carrying a catalyst having an oxidation function (NOx storage reduction catalyst)
29 ... Second EGR passage (high-pressure exhaust gas recirculation passage)
31 ... Second EGR control valve (second recirculation exhaust gas control valve)
32 ... Catalytic converter
61 ... 1st EGR passage (low pressure exhaust gas recirculation passage)
64 ... 1st EGR control valve (1st recirculation exhaust gas control valve)
62 ... EGR cooler (cooling device)

Claims (5)

An exhaust gas recirculation device that recirculates exhaust gas discharged from the combustion chamber into the engine intake passage is provided, and when the amount of recirculated exhaust gas supplied to the combustion chamber is increased, the amount of soot generated is increased. As the amount of recirculated exhaust gas supplied into the combustion chamber increases further and gradually reaches a peak, the temperature of the fuel and the surrounding gas in the combustion chamber is lower than the soot generation temperature. Therefore, there is provided switching means for switching between low-temperature combustion in which soot is hardly generated and normal combustion in which the amount of inert gas supplied to the combustion chamber is smaller than the amount of inert gas in which the amount of soot reaches a peak. In internal combustion engines,
The exhaust gas circulation device includes a low pressure exhaust gas recirculation passage having a low recirculation exhaust gas pressure, a high pressure exhaust gas recirculation passage having a high recirculation exhaust gas pressure, and a recirculation flowing through the low pressure exhaust gas recirculation passage. A first recirculation exhaust gas control valve capable of controlling the amount of exhaust gas; and a second recirculation exhaust gas control valve capable of controlling the amount of recirculation exhaust gas flowing through the high-pressure exhaust gas recirculation passage. And
If the engine demand load is higher than a threshold when the low temperature combustion is performed, the amount of recirculated exhaust gas flowing through the low pressure exhaust gas recirculation passage is increased and the recirculation flowing through the high pressure exhaust gas recirculation passage Reduce the amount of exhaust gas,
If the engine required load is not higher than the threshold when the low temperature combustion is performed, the amount of recirculated exhaust gas flowing through the low pressure exhaust gas recirculation passage is reduced and the high pressure exhaust gas recirculation passage is An internal combustion engine that increases the amount of recirculated exhaust gas that flows .   The internal combustion engine according to claim 1, further comprising a turbocharger, wherein the low-pressure exhaust gas recirculation passage connects a downstream side of the exhaust turbine of the turbocharger and an upstream side of the compressor of the turbocharger.   The internal combustion engine according to any one of claims 1 to 2, wherein a cooling device is provided only in the low-pressure exhaust gas recirculation passage.   The internal combustion engine according to any one of claims 1 to 3, wherein the high-pressure exhaust gas recirculation passage is provided with a catalyst having an oxidation function or a filter carrying the catalyst.   5. The system according to claim 1, wherein the first recirculation exhaust gas control valve and the second recirculation exhaust gas control valve are opened near a boundary between a region where the engine required load is low and a region where the engine required load is high. An internal combustion engine according to claim 1.

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