JP2009191659A - Control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely prevent preignition, by quickly reducing the air-fuel ratio of exhaust gas, when performing a rich spike by low temperature combustion, in a control device of an internal combustion engine. <P>SOLUTION: This control device of the internal combustion engine includes an external EGR device for performing external EGR, an internal EGR quantity variable means capable of varying an internal EGR quantity, an actual compression ratio variable means capable of varying the actual compression ratio by changing the closing timing of an intake valve, a low temperature combustion means for transferring the total EGR ratio by the total of internal EGR and the external EGR to a low temperature combustion range higher than the EGR ratio of becoming the peak in a smoke discharge quantity by controlling the internal EGR quantity variable means so as to suddenly increase the internal EGR quantity when performing the rich spike for temporarily setting the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or less, and an actual compression ratio reducing means for controlling the actual compression ratio variable means so as to reduce the actual compression ratio when performing the low temperature combustion by the low temperature combustion means. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a control device for an internal combustion engine.

  Diesel engines are typically operated at lean air / fuel ratios that are significantly leaner than the stoichiometric air / fuel ratio. Therefore, a large amount of oxygen is contained in the exhaust gas. For this reason, NOx cannot be purified by the three-way catalyst. Therefore, an exhaust purification system in which a NOx catalyst is installed in the exhaust passage and NOx in the exhaust gas is stored in the NOx catalyst is used. There is a limit to the amount of NOx that can be stored in the NOx catalyst. For this reason, in the above-described system, the NOx stored in the NOx catalyst is desorbed and reduced and purified by periodically performing a rich spike in which the air-fuel ratio of the exhaust gas is temporarily made equal to or less than the stoichiometric air-fuel ratio. It is necessary.

  As a method of executing the rich spike, conventionally, post-injection in which fuel is additionally injected from the in-cylinder injector in the expansion stroke or exhaust stroke, or exhaust in which fuel is injected into the exhaust gas by a fuel addition valve provided in the exhaust system. System fuel addition. However, these methods have a problem that fuel consumption and emission are likely to deteriorate.

  Japanese Patent Application Laid-Open No. 2004-360484 discloses a technique for performing rich spike by low-temperature combustion (also referred to as low-temperature rich combustion) (“in-cylinder rich” in the same publication corresponds to low-temperature combustion). Low temperature combustion is combustion performed by lowering (enriching) the air-fuel ratio by performing a large amount of EGR and lowering the combustion temperature to a temperature at which smoke is not generated. According to this low temperature combustion, the diesel engine can be operated at an air-fuel ratio close to the stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio without discharging smoke. For this reason, the deterioration of fuel consumption and emission can be suppressed by executing the rich spike by low temperature combustion.

JP 2004-360484 A JP 2003-148225 A JP 2001-200715 A JP 2004-19462 A

  In the technique disclosed in the above publication, low-temperature combustion is performed by so-called external EGR in which exhaust gas (EGR gas) is recirculated through an EGR passage connecting the exhaust passage and the intake passage. However, when performing low temperature combustion with external EGR during a rich spike, there are several problems as follows.

  When starting a rich spike due to low-temperature combustion, it is important to rapidly lower the air-fuel ratio below the stoichiometric air-fuel ratio. If the air-fuel ratio declines slowly, it will pass through the combustion region where soot is produced in large quantities, and a large amount of smoke will be discharged, or NOx in the NOx catalyst will be desorbed without being purified. is there. In order to rapidly reduce the air-fuel ratio, it is necessary to increase the EGR rate instantaneously. However, in the case of external EGR, since EGR gas passes through the EGR passage, the EGR cooler, and the like and recirculates into the cylinder, it takes time until the EGR gas reaches the cylinder. For this reason, the EGR rate cannot be increased rapidly. Therefore, there is a problem that the air-fuel ratio cannot be lowered rapidly and smoke and NOx are discharged.

  Further, since a large amount of rich EGR gas containing a large amount of HC passes through the EGR passage, deposits due to HC or the like accumulate on the inner wall of the EGR passage, and the EGR passage is likely to be clogged. In order to prevent this clogging, it is necessary to install an HC removal catalyst in front of the EGR cooler, but this is difficult to realize for reasons such as cost.

  As a method for solving the above problems, a method of establishing low-temperature combustion by using the internal EGR can be considered. However, according to the knowledge of the present inventor, there are the following problems when performing low-temperature combustion using internal EGR. In the internal EGR, the high-temperature internal EGR gas immediately after combustion returns to the cylinder (residual) without being cooled. For this reason, when a large amount of internal EGR is performed, the in-cylinder temperature increases, and the compression end temperature tends to become excessively high. When the compression end temperature becomes excessively high, early ignition of the fuel is likely to occur, and the fuel falls out of the intended low-temperature combustion region, fuel consumption and emission are deteriorated, or smoke is increased.

  The present invention has been made in order to solve the above-described problems. When rich spike is performed by low-temperature combustion, the air-fuel ratio of exhaust gas can be rapidly reduced and early ignition can be reliably prevented. It is an object of the present invention to provide a control device for an internal combustion engine that can be used.

In order to achieve the above object, a first invention is a control device for an internal combustion engine,
An external EGR device for performing external EGR;
An internal EGR amount varying means for varying the internal EGR amount;
An actual compression ratio variable means for varying the actual compression ratio by changing the closing timing of the intake valve;
When executing a rich spike in which the air-fuel ratio of the exhaust gas is temporarily less than or equal to the stoichiometric air-fuel ratio, the internal EGR amount variable means is controlled so that the internal EGR amount suddenly increases, so that the internal EGR and external EGR Low-temperature combustion means for shifting the total EGR rate by summation to a low-temperature combustion range higher than the EGR rate at which smoke emission peaks,
An actual compression ratio reducing means for controlling the actual compression ratio variable means so that the actual compression ratio is lowered when the low temperature combustion is performed by the low temperature combustion means;
It is characterized by providing.

The second invention is the first invention, wherein
When low temperature combustion is performed by the low temperature combustion means, comprising early ignition determination means for determining whether there is a risk of early ignition,
The actual compression ratio reducing means reduces the actual compression ratio when it is determined that there is a risk of early ignition.

The third invention is the first or second invention, wherein
The actual compression ratio reducing means reduces the actual compression ratio as the engine load is higher.

According to a fourth invention, in any one of the first to third inventions,
A turbocharger,
A supercharging pressure adjusting actuator for adjusting the supercharging pressure;
A supercharging pressure amplifying means for operating the supercharging pressure adjusting actuator so that a supercharging pressure increases when the actual compression ratio is reduced by the actual compression ratio reducing means;
It is characterized by providing.

  According to the first invention, rich spike can be performed by low temperature combustion. For this reason, deterioration of fuel consumption and emission can be suppressed. Further, when the rich spike is executed, the total EGR rate can be rapidly increased to a temperature equal to or higher than the low temperature combustion threshold by rapidly increasing the internal EGR amount. As a result, the EGR rate region where the smoke discharge amount reaches a peak can be instantaneously passed, so that smoke discharge at the start of the rich spike can be reliably suppressed. Further, since the total EGR rate can be increased rapidly, the air-fuel ratio can be rapidly decreased below the stoichiometric air-fuel ratio. For this reason, it is possible to reliably prevent NOx occluded in the NOx catalyst from leaving without being purified. Furthermore, according to the first invention, when the low temperature combustion is executed, the compression end temperature can be lowered by reducing the actual compression ratio. For this reason, even if a large amount of internal EGR is performed, the compression end temperature does not become excessively high, and early ignition of the fuel can be reliably prevented. Therefore, it is possible to reliably avoid deterioration of fuel consumption and emission due to early ignition.

  According to the second invention, when low temperature combustion is performed, it is determined whether or not there is a possibility of early ignition, and when it is determined that there is a possibility of early ignition, the actual compression ratio is reduced. As a result, when it is not necessary to reduce the actual compression ratio (for example, at a light load), it is possible to avoid the control that lowers the actual compression ratio. it can.

  According to the third invention, when the low temperature combustion is performed, the actual compression ratio can be reduced as the engine load is higher. Thereby, even in a high load region where the in-cylinder temperature tends to be high, the combustion temperature can be sufficiently lowered, and low temperature combustion can be realized. Therefore, the region where the rich spike by low temperature combustion is possible can be expanded to the high load side.

  According to the fourth invention, when the actual compression ratio is reduced during the low temperature combustion, the supercharging pressure can be increased by the supercharging pressure adjusting actuator. Control for reducing the actual compression ratio is performed by closing the intake valve late or early. For this reason, since the amount of in-cylinder gas decreases, the amount of intake air (fresh air amount) also decreases. Therefore, the intake air amount may be insufficient particularly in the high rotation and high load region. If the amount of intake air is insufficient, the air-fuel ratio becomes too rich and combustion becomes unstable, or smoke is discharged. According to the fourth aspect of the invention, the above shortage of intake air can be reliably avoided, so that the above-described adverse effects can be reliably prevented.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted.

Embodiment 1 FIG.
[Description of system configuration]
FIG. 1 is a diagram for explaining a system configuration according to the first embodiment of the present invention. The system shown in FIG. 1 includes a diesel engine (compression ignition internal combustion engine) 10. The diesel engine 10 is mounted on a vehicle or the like and used as a power source. Although the diesel engine 10 shown in FIG. 1 is an in-line four-cylinder type, in the present invention, the number of cylinders and the cylinder arrangement are not limited thereto.

  Each cylinder of the diesel engine 10 is provided with a fuel injector 12 for directly injecting fuel into the cylinder. The fuel injectors 12 of the respective cylinders are connected to a common common rail 14. Fuel in a fuel tank (not shown) is pressurized to a predetermined fuel pressure by a supply pump 16, stored in the common rail 14, and supplied from the common rail 14 to each fuel injector 12.

  The exhaust gas discharged from each cylinder of the diesel engine 10 is collected by the exhaust manifold 20 and flows into the exhaust passage 18. The diesel engine 10 of the present embodiment includes a turbocharger 24 that performs supercharging with the energy of exhaust gas. The turbocharger 24 has an exhaust turbine 24a that rotates by the energy of the exhaust gas, and an intake compressor 24b that rotates by being driven by the exhaust turbine 24a. The exhaust turbine 24 a is arranged in the middle of the exhaust passage 18, and the intake compressor 24 b is arranged in the middle of the intake passage 28.

  The turbocharger 24 of the present embodiment further includes a variable nozzle 24c that makes the inlet area of the exhaust turbine 24a variable. The variable nozzle 24 c opens and closes when driven by the actuator 22. The smaller the opening of the variable nozzle 24c, the faster the flow rate of the exhaust gas flowing into the exhaust turbine 24a. Therefore, by reducing the opening degree of the variable nozzle 24c, the rotational speed of the turbocharger 24 increases, so that the supercharging pressure by the turbocharger 24 can be increased.

  A NOx catalyst 26 is installed in the exhaust passage 18 on the downstream side of the exhaust turbine 24a. When the diesel engine 10 is normally operated at a lean air-fuel ratio, NOx in the exhaust gas is captured and stored by the NOx catalyst 26. Thereby, the discharge | release of NOx to air | atmosphere can be suppressed.

In this system, in order to purify NOx occluded in the NOx catalyst 26, a rich spike in which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 26 is temporarily set to a theoretical air-fuel ratio or less is periodically executed. By flowing rich exhaust gas containing a reducing agent such as HC into the NOx catalyst 26, NOx can be desorbed from the NOx catalyst 26 and reduced to N ₂ for purification. A specific implementation method of the rich spike will be described later.

  An air cleaner 30 is provided near the inlet of the intake passage 28 of the diesel engine 10. The air sucked through the air cleaner 30 is compressed by the intake compressor 24 b of the turbocharger 24 and then cooled by the intercooler 32. The intake air that has passed through the intercooler 32 passes through the intake manifold 34 and flows into each cylinder.

  An intake throttle valve 36 is installed between the intercooler 32 and the intake manifold 34 in the intake passage 28. The intake throttle valve 36 is an electronically controlled throttle valve whose opening is adjusted by a motor. Further, an air flow meter 38 for detecting the amount of intake air is installed in the vicinity of the intake passage 28 downstream of the air cleaner 30.

  The diesel engine 10 also includes an external EGR device for performing EGR (Exhaust Gas Recirculation) for recirculating a part of the exhaust gas to the intake passage 28. The external EGR device of this embodiment includes an EGR passage 40 that connects the exhaust manifold 20 and the intake passage 28, an EGR cooler 42 that is installed in the middle of the EGR passage 40, and an EGR that is installed downstream of the EGR cooler 42. And a valve 44. Recirculation of exhaust gas (EGR gas) through the EGR passage 40 is hereinafter referred to as “external EGR”. Further, the EGR gas by the external EGR (that is, the exhaust gas recirculated into the cylinder through the EGR passage 40) is referred to as “external EGR gas”.

  The system of the present embodiment further includes an accelerator opening sensor 48 that detects the position (accelerator opening) of an accelerator pedal provided in the driver's seat of the vehicle, and an ECU (Electronic Control Unit) 50. The ECU 50 is electrically connected to the various sensors and actuators described above. The ECU 50 controls the operating state of the diesel engine 10 by driving each actuator according to a predetermined program based on the output of each sensor.

  FIG. 2 is a view showing a cross section of one cylinder of the diesel engine 10 in the system shown in FIG. Hereinafter, the diesel engine 10 will be further described. As shown in FIG. 2, a crank angle sensor 62 that detects the rotation angle (crank angle) of the crankshaft 60 is attached in the vicinity of the crankshaft 60 of the diesel engine 10. The crank angle sensor 62 is electrically connected to the ECU 50.

  Further, in the diesel engine 10 of this embodiment, the variable intake valve operating device 54 that varies the valve opening characteristics (opening timing and closing timing) of the intake valve 52 and the valve opening characteristics (at least the closing timing) of the exhaust valve 56. And an exhaust variable valve operating device 58 that makes the variable. The operation of the intake variable valve operating device 54 and the exhaust variable valve operating device 58 is controlled by the ECU 50.

The specific configuration of the intake variable valve operating device 54 and the exhaust variable valve operating device 58 is not particularly limited, and a mechanism exemplified below can be used.
(1) By changing the phase of the camshaft that drives the valve, the phase is variable so that the opening timing and closing timing can be continuously advanced or retarded while the operating angle (valve opening period) remains constant mechanism.
(2) A working angle variable mechanism that continuously varies the working angle (valve opening period) by interposing a swing cam or the like between the valve and the camshaft.
(3) A mechanism capable of opening and closing a valve at an arbitrary time by rotationally driving a camshaft by an electric motor.
(4) An electromagnetically driven valve that can be opened and closed at any time.

(Internal EGR)
In the diesel engine 10, the internal EGR can be executed by controlling the intake variable valve operating device 54 and the exhaust variable valve operating device 58 so that a positive or negative valve overlap occurs. FIG. 3 is a diagram showing the valve opening characteristics of the intake valve 52 and the exhaust valve 56 when a positive valve overlap is provided. As shown in this figure, the positive valve overlap is a period in which the exhaust valve 56 and the intake valve 52 are both open near the intake and exhaust top dead centers. By making the opening timing of the intake valve 52 earlier than normal and closing the exhaust valve 56 later than normal, the positive valve overlap can be expanded.

  FIG. 4 is a diagram showing the valve opening characteristics of the intake valve 52 and the exhaust valve 56 when a negative valve overlap is provided. As shown in this figure, the negative valve overlap is a period in which the exhaust valve 56 and the intake valve 52 are both closed near the intake and exhaust top dead centers. By setting the closing timing of the exhaust valve 56 earlier than the normal time and before the top dead center, and opening the intake valve 52 later than the normal time and after the top dead center, a negative valve overlap is provided. be able to.

  In the diesel engine 10, when executing the internal EGR, the intake variable valve operating device 54 and the exhaust variable valve operating device 58 are controlled so that the positive or negative valve overlap as described above occurs. In this case, the amount of internal EGR can be increased as the positive or negative valve overlap is increased. That is, the amount of internal EGR can be controlled by controlling the magnitude of the positive or negative valve overlap.

  Hereinafter, the EGR gas generated by the internal EGR (that is, the exhaust gas remaining in the cylinder) is referred to as “internal EGR gas”.

(Actual compression ratio variable control)
In the diesel engine 10, the actual compression ratio (substantial compression ratio) can be changed (decreased) by changing the closing timing of the intake valve 52 (hereinafter referred to as "intake valve closing timing"). FIG. 5 is a diagram showing the valve opening characteristics of the intake valve 52 when the control for reducing the actual compression ratio is executed.

  The intake valve 52 is normally closed near the bottom dead center (solid line in FIG. 5). In this case, the compression stroke starts as the piston 64 changes from descending to ascending. On the other hand, when the intake valve closing timing is made later than usual (hereinafter also referred to as “slow closing of the intake valve 52”), the intake valve 52 is open even after the piston 64 starts to rise. Part of the sucked in-cylinder gas is returned to the intake port through the intake valve 52. Then, after the intake valve 52 is closed, a substantial compression stroke starts. For this reason, compared with the case of the normal intake valve closing timing, the substantial compression stroke becomes shorter and the actual compression ratio becomes smaller.

  Further, when the intake valve closing timing is made earlier than usual and before the bottom dead center (hereinafter also referred to as “early closing of the intake valve 52”), the intake valve 52 is moved while the piston 64 is being lowered. Close. For this reason, in-cylinder gas expands as the piston 64 descends between the time when the intake valve 52 is closed and the bottom dead center. Thereafter, the piston 64 starts to rise after passing through the bottom dead center, and the in-cylinder gas returns to the original state at the position where the piston 64 is raised to the same height as the intake valve 52 is closed. Therefore, the substantial compression stroke is from this position to the top dead center. For this reason, compared with the case of the normal intake valve closing timing, the substantial compression stroke becomes shorter and the actual compression ratio becomes smaller.

  Thus, in the diesel engine 10, the actual compression ratio can be made smaller than the normal compression ratio by controlling the intake variable valve operating apparatus 54 so that the intake valve closing timing is later or earlier than usual.

(Low temperature combustion)
As described above, in the present embodiment, when NOx stored in the NOx catalyst 26 is reduced and purified, a rich spike is executed in which the air-fuel ratio of the exhaust gas is temporarily made equal to or lower than the stoichiometric air-fuel ratio. Further, when the NOx catalyst 26 is poisoned with sulfur, a rich spike may be executed to regenerate the NOx catalyst 26. In the present embodiment, when rich spike is executed, the air-fuel ratio of the exhaust gas discharged from the diesel engine 10 is reduced by performing low temperature combustion. This low temperature combustion will be described below.

  EGR gas (exhaust gas) has a higher specific heat than air, and therefore can absorb a large amount of heat. Accordingly, the combustion temperature in the combustion chamber decreases as the EGR rate (EGR gas amount / (EGR gas amount + intake fresh air amount)) increases. When the combustion temperature decreases, the amount of NOx generated decreases. Therefore, the amount of NOx generated decreases as the EGR rate increases.

  However, as the EGR rate increases, the amount of soot, that is, smoke, starts to increase rapidly when the EGR rate exceeds a certain limit. Therefore, normally, the EGR rate at which smoke starts to increase rapidly is the maximum allowable limit of the EGR rate, and the EGR rate is controlled within a range not exceeding this maximum allowable limit. The maximum allowable limit for the EGR rate is approximately 30% to 50%, although it varies considerably depending on the engine type and fuel.

  However, recent research has found the following. That is, if the EGR rate is made larger than the maximum allowable limit, the smoke increases rapidly as described above, but there is a peak in the amount of smoke generated, and if the EGR rate is further increased beyond this peak, this time the smoke is increased. Begins to decrease rapidly. When the EGR rate is set to a certain threshold value or more (for example, 70% or more), the smoke becomes almost zero. At this time, the amount of NOx generated is extremely small. In this way, a new combustion system capable of simultaneously reducing smoke and NOx by further increasing the EGR rate beyond the conventional maximum allowable limit is referred to as low temperature combustion.

  The basic principle of low temperature combustion is to stop the growth of hydrocarbons in the middle of the course until the hydrocarbons grow into soot. That is, as a result of repeated experimental research, it was found that the fuel during combustion in the combustion chamber and the surrounding gas temperature were below a certain temperature, and stopped at a mid-stage before hydrocarbon growth reached soot, And when the temperature of the gas around it exceeds a certain temperature, the hydrocarbon grows up to soot all at once. 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.

  Therefore, if the temperature of the fuel during combustion in the combustion chamber and the temperature of the gas surrounding the fuel are controlled to be equal to or lower than the temperature at which hydrocarbon growth stops halfway, soot will not be generated. The temperature of the fuel and the surrounding gas during combustion in the combustion chamber can be suppressed to a temperature at which hydrocarbon growth stops midway or less by adjusting the endothermic amount of the gas around the fuel. This is the basic idea of low temperature combustion.

  In the low temperature combustion as described above, since the EGR rate is extremely high, the amount of oxygen in the cylinder is small. Therefore, the in-cylinder air-fuel ratio can be significantly reduced (enriched) compared to that during normal combustion, and can be made lower than the stoichiometric air-fuel ratio. Therefore, it is possible to execute rich spike by operating the diesel engine 10 at low temperature combustion. When rich spike is performed using low-temperature combustion, there is an advantage that deterioration of emission and fuel consumption can be suppressed compared to the case where rich spike is performed by other methods such as exhaust fuel addition and post injection. is there.

  Incidentally, in the diesel engine 10, since it is necessary to sufficiently reduce the NOx concentration at the engine outlet in the normal operation state, the external EGR is executed in a wide range of operation. In this case, the EGR rate is set to the maximum allowable limit (for example, 50%) or less. In order to execute a rich spike by low temperature combustion from such a normal operation state, it is necessary to increase the EGR rate to a predetermined value (for example, 80%) in the low temperature combustion range. That is, in this embodiment, at the start of the rich spike, it is necessary to increase the EGR rate from a value below the maximum allowable limit to above the low temperature combustion threshold. In this case, it is required to rapidly increase the EGR rate. As described above, the smoke emission peak exists between the maximum allowable limit of the EGR rate and the low temperature combustion threshold. For this reason, if the increase in the EGR rate is slow, it takes a long time to pass through the peak range, and a large amount of smoke is discharged.

  It is also important to rapidly reduce the air-fuel ratio to the stoichiometric air-fuel ratio or less at the start of the rich spike. Since the increase in the EGR rate and the decrease in the air-fuel ratio are two sides of the same, if the increase in the EGR rate is slow, the decrease in the air-fuel ratio is also slow. If the decrease of the air-fuel ratio is slow at the start of the rich spike, the NOx stored in the NOx catalyst 26 is likely to be released to the atmosphere without being purified. The reason is as follows. If the decrease in the air-fuel ratio is slow at the start of the rich spike, the time during which the exhaust gas slightly leaner than the stoichiometric air-fuel ratio flows into the NOx catalyst 26 becomes longer. The NOx catalyst 26 does not release NOx all at once when the air-fuel ratio of the exhaust gas becomes equal to or lower than the stoichiometric air-fuel ratio, but does not release NOx even if the air-fuel ratio of the exhaust gas is slightly leaner than the stoichiometric air-fuel ratio. There is. On the other hand, when the atmosphere in the NOx catalyst 26 is slightly leaner than the stoichiometric air-fuel ratio, the desorbed NOx is not purified because the reducing agent is insufficient. For this reason, when exhaust gas slightly leaner than the stoichiometric air-fuel ratio flows into the NOx catalyst 26, the stored NOx is discharged without being purified.

  As described above, in order to suppress smoke and NOx emission, it is important to rapidly increase the EGR rate to the temperature equal to or higher than the low temperature combustion threshold at the start of the rich spike.

  In the case of external EGR, in order to increase the EGR amount, the opening degree of the EGR valve 44 is increased or the opening degree of the intake throttle valve 36 is decreased. In this case, the external EGR gas recirculates into the cylinder via a long EGR path (EGR passage 40, EGR cooler 42, intake manifold 34, etc.). For this reason, even if the opening degree of the EGR valve 44 or the intake throttle valve 36 is changed, it takes time for the external EGR gas flowing into the cylinder to increase. For this reason, when the EGR rate is raised to the low temperature combustion threshold by increasing the external EGR amount at the start of the rich spike, the increase in the EGR rate becomes slow. As a result, there is a problem that a lot of smoke and NOx are discharged.

  Further, if the EGR during the low temperature combustion is all covered by the external EGR, there is a problem that the inside of the EGR passage 40 and the EGR cooler 42 is easily clogged. That is, if EGR during low-temperature combustion is all covered by external EGR, a large amount of rich EGR gas containing a lot of HC flows through the EGR passage 40. For this reason, the deposits resulting from HC or the like are likely to accumulate on the inner wall of the flow path, resulting in clogging. In order to prevent this clogging, it is necessary to install an HC removal catalyst in front of the EGR cooler 42, but this is difficult to realize for reasons such as cost.

  In order to solve the problems as described above, in the present embodiment, the EGR rate is increased to a value equal to or higher than the low-temperature combustion threshold by using the internal EGR at the start of the rich spike. The internal EGR amount, that is, the residual gas amount, increases immediately from the next cycle when the intake variable valve device 54 and the exhaust variable valve device 58 are operated to expand the positive or negative valve overlap. For this reason, in the case of internal EGR, the amount of EGR can be increased very quickly compared to the case of external EGR. Therefore, it is possible to rapidly increase the total EGR rate (EGR rate based on the sum of the internal EGR and the external EGR), that is, to rapidly decrease the air-fuel ratio.

  For example, when a rich spike is started from a normal operation state where the external EGR rate is 50% and the internal EGR rate is 0%, a positive or negative valve overlap, for example, an internal EGR rate of 30% can be obtained. What is necessary is just to enlarge to a magnitude | size. As a result, the internal EGR rate rapidly increases from 0% to 30%. Accordingly, the total EGR rate rapidly increases from 50% below the maximum allowable limit to 80% above the low temperature combustion threshold. Therefore, smoke and NOx emission can be reliably suppressed.

  Moreover, according to this embodiment, the ratio of external EGR can be reduced by performing low temperature combustion in combination with internal EGR. For this reason, clogging of the EGR passage 40 and the EGR cooler 42 can be reliably suppressed.

  As described above, in the present embodiment, when low temperature combustion is performed for a rich spike, the air-fuel ratio can be quickly reduced below the stoichiometric air-fuel ratio by rapidly increasing the internal EGR amount. However, in this case, according to the knowledge of the present inventors, the following new problem arises. The internal EGR gas is a gas immediately after combustion and is not cooled by the EGR cooler 42, and therefore is extremely hot compared to the external EGR gas. Therefore, as the proportion of the internal EGR gas in the cylinder increases, the temperature of the in-cylinder gas (fresh air + external EGR gas + internal EGR gas) increases, and as a result, the compression end temperature (the cylinder near the compression top dead center). (Inner gas temperature) increases. For this reason, when low temperature combustion is performed with a large amount of internal EGR, the compression end temperature becomes too high, and early ignition (premature ignition) of the fuel tends to occur. Due to this early ignition, there is a problem that fuel consumption and emissions deteriorate.

  In the present embodiment, in order to prevent the early ignition as described above, the control for lowering the actual compression ratio in accordance with the increase of the internal EGR amount during the rich spike (that is, the intake variable valve operating device 54 takes in the intake air). The control for closing the valve 52 late or early) is executed. By reducing the actual compression ratio, the compression end temperature is lowered. For this reason, early ignition can be avoided reliably and deterioration of fuel consumption and emission can be prevented.

[Specific Processing in Embodiment 1]
FIG. 6 is a flowchart of a routine executed by the ECU 50 in the present embodiment in order to realize the above function. According to the routine shown in FIG. 6, first, it is determined whether or not there is a request to execute rich spike (step 100). The determination method in step 100 is not particularly limited, and can be performed by a known method. For example, the relationship between the operating state (engine speed and load) of the diesel engine 10 and the NOx emission amount is stored in the ECU 50 as a map. According to this map, the amount of NOx flowing into the NOx catalyst 26 can be sequentially calculated based on the operating state. Then, the NOx occlusion amount of the NOx catalyst 26 can be calculated by integrating the NOx inflow amount from the execution of the previous rich spike to the present. When this NOx occlusion amount exceeds a predetermined threshold, it can be determined that a rich spike is necessary.

  If it is determined in step 100 that a rich spike is not required, the current routine is terminated.

  On the other hand, if it is determined in step 100 that it is necessary to execute the rich spike, control for increasing the internal EGR amount, that is, control for expanding the positive or negative valve overlap is executed (step 102). ). Specifically, in step 102, first, the target internal EGR rate is calculated by subtracting the current external EGR rate from the target total EGR rate during low-temperature combustion. Next, a positive or negative valve overlap amount necessary to obtain the target internal EGR rate is calculated based on the map. Then, the intake variable valve operating device 54 or the exhaust variable valve operating device 58 is controlled so that the valve overlap amount is realized.

  Following the processing in step 102, it is determined whether there is a risk of early ignition (step 104). In step 104, specifically, the following processing is performed. The ECU 50 stores in advance a map for estimating the compression end temperature based on the operating state and a map for estimating an increase in the compression end temperature due to the influence of the internal EGR. In step 104, first, the compression end temperature corresponding to the current operating state is calculated based on the former map, and then the increment of the compression end temperature corresponding to the current internal EGR rate is calculated based on the latter map. Is calculated. Then, the current compression end temperature estimated value in consideration of the influence of the internal EGR is calculated by adding both of them. If this estimated compression end temperature exceeds a predetermined threshold, it is determined that there is a risk of early ignition, and if it does not exceed the threshold, it is determined that there is no risk of early ignition. Is done.

  If it is determined in step 104 that there is a risk of early ignition, control for reducing the actual compression ratio, that is, the intake variable valve operating device 54 is operated so that the intake valve 52 is closed late or early. Control is performed (step 106). By this control, the compression end temperature can be lowered. Therefore, early ignition can be avoided reliably and fuel consumption and emission can be prevented from deteriorating.

  If the actual compression ratio has been reduced by the processing in step 106, it is next determined whether or not the intake air amount is insufficient (step 108). The ECU 50 stores in advance a map that defines the relationship between the operating state and the lower limit value of the intake air amount. In this step 108, first, an intake air amount lower limit value corresponding to the current operating state is calculated based on the map. When the intake air amount detected by the air flow meter 38 is smaller than the intake air amount lower limit value, it is determined that the intake air amount is insufficient.

  If it is determined in step 108 that the intake air amount is insufficient, control for increasing the supercharging pressure is executed (step 110). In this step 110, specifically, control for reducing the opening of the variable nozzle 24c is executed. Thereby, the rotation speed of the turbocharger 24 increases, and the supercharging pressure can be increased. When the supercharging pressure increases, the in-cylinder gas amount increases, so that the shortage of the intake air amount can be solved.

  When the intake valve 52 is closed late or early in step 106, the in-cylinder gas amount decreases, so the intake air amount (fresh air amount) also decreases. For this reason, the intake air amount may be insufficient particularly in a high rotation and high load region. If the amount of intake air is insufficient, the air-fuel ratio becomes too rich and combustion becomes unstable, or smoke is discharged. According to the present embodiment, the process of steps 108 and 110 can quickly solve the shortage of the intake air amount, so that the above adverse effects can be reliably prevented.

  If it is determined in step 108 that the intake air amount is sufficient, it is not necessary to increase the supercharging pressure, and thus the processing in step 110 is skipped. If it is determined in step 104 that there is no risk of early ignition, the processing in steps 104 to 110 is not necessary and is skipped.

  Subsequent to the processing described above, processing for advancing the injection timing of the fuel injector 12 is executed (step 112). When performing low temperature combustion, since a large amount of EGR gas exists in the cylinder, the fuel may be difficult to ignite. Therefore, ignition can be facilitated by advancing the fuel injection timing in this step 112.

  By the above processing, low temperature combustion is established (step 114). As a result, the in-cylinder air-fuel ratio becomes less than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas also becomes less than the stoichiometric air-fuel ratio. Therefore, a rich spike is performed on the NOx catalyst 26, and the stored NOx can be released and reduced and purified (step 116). In this step 116, if necessary, a small amount of fuel may be added to the exhaust gas by adding exhaust system fuel, post-injection, or the like.

  As described above, according to the present invention, when the rich spike is performed, the total EGR rate can be rapidly increased to a temperature equal to or higher than the low-temperature combustion threshold by rapidly increasing the internal EGR amount. As a result, the air-fuel ratio can be rapidly lowered to the stoichiometric air-fuel ratio or less at the start of the rich spike. Therefore, smoke and NOx emission can be reliably suppressed. In addition, when switching from normal combustion to low-temperature combustion, it is not necessary to change the external EGR rate greatly, so that it is not easily affected by the delay in transport of external EGR gas. For this reason, when switching from normal combustion to low temperature combustion, the total EGR rate can be accurately controlled. Therefore, it is possible to sufficiently suppress deterioration of fuel consumption, emission, torque fluctuation, and the like, and smoothly shift to low temperature combustion.

  Furthermore, according to the present invention, it is possible to instantaneously switch from normal combustion to low-temperature combustion, so low-temperature combustion may be performed only at the timing of rich spike. For this reason, since the continuation time of low-temperature combustion can be minimized, deterioration of fuel consumption and emission can be minimized.

  Further, according to the present invention, it is possible to reliably avoid the compression end temperature from becoming excessively high by reducing the actual compression ratio during normal temperature combustion with internal EGR, as compared with the normal time. For this reason, it can prevent reliably that early ignition generate | occur | produces.

  In addition, according to the present invention, there is an advantage that the region where low temperature combustion is possible can be expanded to the high load side by reducing the actual compression ratio during normal temperature combustion to be smaller than normal. Conventionally, in a high load region, the in-cylinder temperature is high, so the combustion temperature is also high, and it is difficult to establish low-temperature combustion. On the other hand, according to the present invention, since the combustion temperature can be lowered by reducing the actual compression ratio, low temperature combustion is possible even in a high load region. Therefore, a rich spike using low temperature combustion can be executed even in a high load range. In the present embodiment, in step 106, the actual compression ratio may be controlled to be smaller as the engine load is higher. Thereby, the area | region in which low temperature combustion is possible can be expanded to the higher load side.

  In the present embodiment, when the positive or negative valve overlap is expanded to increase the internal EGR amount, the valve opening characteristics of both the intake valve 52 and the exhaust valve 56 are changed. In the present invention, the positive or negative valve overlap may be enlarged by changing one of the valve opening characteristics.

  In this embodiment, in step 110, the boost pressure is increased by reducing the opening of the variable nozzle 24c. However, in the present invention, the means for increasing the boost pressure is limited to this. It is not a thing. For example, in the case of a turbocharger equipped with an assist mechanism using an electric motor, the supercharging pressure may be increased by operating the electric motor.

  In the first embodiment described above, the intake variable valve operating device 54 and the exhaust variable valve operating device 58 are the “internal EGR amount variable means” in the first invention, and the intake variable valve operating device 54 is the first variable valve operating device. The variable nozzle 24c and the actuator 22 correspond to the “supercharging pressure adjusting actuator” according to the fourth aspect of the present invention. Further, when the ECU 50 executes the process of step 102, the “low temperature combustion means” in the first invention executes the process of step 106, and the “actual compression ratio reducing means in the first invention makes it possible. ”By executing the process of step 104, the“ early ignition determination means ”in the second invention, and by executing the process of step 110,“ supercharging pressure amplifying means ”in the fourth invention. Are realized.

It is a figure for demonstrating the system configuration | structure of Embodiment 1 of this invention. It is a figure which shows the cross section of one cylinder of the diesel engine in the system shown in FIG. It is a figure which shows the valve opening characteristic of the intake valve at the time of providing a positive valve overlap. It is a figure which shows the valve opening characteristic of an intake valve at the time of providing a negative valve overlap. It is a figure which shows the valve opening characteristic of the intake valve in the case of performing control which reduces an actual compression ratio. It is a flowchart of the routine performed in Embodiment 1 of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Diesel engine 12 Fuel injector 14 Common rail 18 Exhaust passage 20 Exhaust manifold 22 Actuator 24 Turbocharger 24c Variable nozzle 26 NOx catalyst 28 Intake passage 34 Intake manifold 36 Inlet throttle valve 38 Air flow meter 40 EGR passage 44 EGR valve 50 ECU
52 Intake valve 56 Exhaust valve 62 Crank angle sensor 64 Piston

Claims (4)

An external EGR device for performing external EGR;
An internal EGR amount varying means for varying the internal EGR amount;
An actual compression ratio variable means for varying the actual compression ratio by changing the closing timing of the intake valve;
When executing a rich spike in which the air-fuel ratio of the exhaust gas is temporarily less than or equal to the stoichiometric air-fuel ratio, the internal EGR amount variable means is controlled so that the internal EGR amount suddenly increases, so that the internal EGR and external EGR Low-temperature combustion means for shifting the total EGR rate by summation to a low-temperature combustion range higher than the EGR rate at which smoke emission peaks,
An actual compression ratio reducing means for controlling the actual compression ratio variable means so that the actual compression ratio is lowered when the low temperature combustion is performed by the low temperature combustion means;
A control device for an internal combustion engine, comprising: When low temperature combustion is performed by the low temperature combustion means, comprising early ignition determination means for determining whether there is a risk of early ignition,
2. The control apparatus for an internal combustion engine according to claim 1, wherein the actual compression ratio reducing means reduces the actual compression ratio when it is determined that there is a risk of early ignition.   3. The control device for an internal combustion engine according to claim 1, wherein the actual compression ratio reducing means reduces the actual compression ratio as the engine load is higher. A turbocharger,
A supercharging pressure adjusting actuator for adjusting the supercharging pressure;
A supercharging pressure amplifying means for operating the supercharging pressure adjusting actuator so that a supercharging pressure increases when the actual compression ratio is reduced by the actual compression ratio reducing means;
The control apparatus for an internal combustion engine according to any one of claims 1 to 3, further comprising:

Images