JP2009052504A - Control device for internal combustion engine - Google Patents

Abstract

The present invention relates to a control device for an internal combustion engine, and an object thereof is to stably perform low-temperature combustion even in a light load region.
It is determined whether or not a rich spike is necessary (step 100). If a rich spike is necessary, the rich spike is executed by low-temperature combustion with a large amount of EGR (step 102). Next, it is determined whether or not the engine load is smaller than a predetermined load (step 104). If the engine load is smaller than the predetermined load, impulse supercharging is executed (step 106).
[Selection] Figure 3

Description

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

In a diesel engine (compression ignition internal combustion engine), since a large amount of oxygen is usually contained in exhaust gas, NOx cannot be purified by a three-way catalyst. For this reason, 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. In such a system, in order to reduce and purify the NOx stored in the NOx catalyst and release it as N ₂ , it is possible to intermittently perform a rich spike that temporarily makes the exhaust air-fuel ratio rich below the stoichiometric air-fuel ratio. Necessary.

  A technique for performing rich spike by low temperature combustion (also called low temperature rich combustion) is disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-360484 (“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. For this reason, according to low temperature combustion, a diesel engine can be operated with an air-fuel ratio close to the stoichiometric air-fuel ratio or with an air-fuel ratio richer than the stoichiometric air-fuel ratio without discharging smoke.

JP 2004-360484 A JP 2000-248946 A JP 2006-283653 A JP 2006-283659 A

  In order to perform low-temperature combustion, a large amount of EGR is required, so it is necessary to throttle the throttle valve in the intake passage so that the intake manifold pressure becomes sufficiently lower than the exhaust manifold pressure. However, when the throttle valve is throttled, the compression end pressure decreases, so that ignition tends to be difficult to stabilize. Such a tendency becomes more prominent as the load is lighter. For this reason, conventionally, it has been difficult to stably perform low-temperature combustion in a light load region.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a control device for an internal combustion engine that can stably perform low-temperature combustion even in a light load region.

In order to achieve the above object, a first invention is a control device for an internal combustion engine,
An EGR device capable of executing EGR;
An impulse supercharger capable of performing impulse supercharging;
Combustion switching means capable of switching between normal combustion and low-temperature combustion that enriches the air-fuel ratio from the normal combustion and lowers the combustion temperature from the normal combustion by performing a large amount of EGR;
Impulse supercharging control means for operating the impulse supercharging device when performing the low temperature combustion;
It is characterized by providing.

The second invention is the first invention, wherein
The impulse supercharging control means operates the impulse supercharging device when the low temperature combustion is performed and the engine load is smaller than a predetermined load.

The third invention is the first or second invention, wherein
A NOx catalyst disposed in the exhaust passage;
The impulse supercharging control means operates the impulse supercharging device when a rich spike that temporarily sets the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a theoretical air-fuel ratio or less is implemented by the low-temperature combustion. It is characterized by that.

According to a fourth invention, in any one of the first to third inventions,
A NOx catalyst disposed in the exhaust passage;
The impulse supercharging control means is configured to perform the impulse from the middle of the rich spike when a rich spike that temporarily sets the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a theoretical air-fuel ratio or less is implemented by the low-temperature combustion. An impulse supercharging start timing control means for starting the operation of the supercharging device is included.

The fifth invention is the fourth invention, wherein
The impulse charge device includes an impulse charge valve that opens and closes an intake passage,
The impulse charge control means includes valve closing period control means for gradually increasing the valve closing period of the impulse charge valve after the impulse charge apparatus starts operating from the middle of the rich spike. Features.

  According to the first aspect of the invention, impulse supercharging can be executed when performing low temperature combustion. In order to perform low temperature combustion with a large amount of EGR, it is necessary to reduce the opening of the intake throttle valve. However, if the opening degree of the intake throttle valve is reduced, the intake manifold pressure decreases, so the amount of intake air-fuel mixture (a mixture of fresh air and EGR gas) decreases and the compression end pressure decreases. As a result, ignition tends to be difficult to stabilize, particularly in light load regions. According to the first aspect of the invention, by performing the impulse supercharging during the low temperature combustion, the charging efficiency can be improved even when the intake manifold pressure is low, so that a sufficient intake air mixture amount can be ensured. it can. For this reason, even if it is a case where low temperature combustion is performed in a light load region, ignition can be stabilized.

  According to the second invention, impulse supercharging can be executed when low-temperature combustion is performed and the engine load is smaller than the predetermined load. When impulse supercharging is performed, the pump loss increases, so that the thermal efficiency tends to decrease. In the second aspect of the invention, when performing low temperature combustion, if the engine load is large enough to obtain stable ignition without performing impulse supercharging, impulse supercharging can be avoided. For this reason, deterioration of fuel consumption can be suppressed.

  According to the third invention, when the rich spike is performed by low temperature combustion, the impulse charge can be executed. As a result, the impulse supercharging is executed during a rich spike that is intermittently performed only for a short time, so that there is little influence of a decrease in thermal efficiency due to the impulse supercharging. Therefore, deterioration of fuel consumption can be reliably suppressed.

  According to the fourth aspect of the invention, when the rich spike is performed by low temperature combustion, it is possible to start impulse supercharging from the middle of the rich spike. Immediately after the start of the rich spike due to low temperature combustion, a large amount of oxygen remains in the EGR gas. Therefore, if the amount of intake air-fuel mixture is increased by performing impulse supercharging immediately after the start of the rich spike, the amount of oxygen in the cylinder rises and the air-fuel ratio in the cylinder rises (lean). For this reason, immediately after the start of the rich spike, it becomes difficult to rapidly reduce the air-fuel ratio below the stoichiometric air-fuel ratio, and exhaust gas slightly leaner than the stoichiometric air-fuel ratio flows into the NOx catalyst. As a result, the amount of NOx discharged to the downstream of the NOx catalyst is likely to increase or the rich spike time is likely to be increased. According to the fourth aspect of the invention, in the initial stage of the rich spike, impulse supercharging is deactivated, thereby reducing the amount of oxygen in the cylinder and quickly reducing the air-fuel ratio in the cylinder to the stoichiometric air-fuel ratio or less. be able to. For this reason, generation | occurrence | production of the above bad effects can be prevented more reliably.

  According to the fifth aspect of the present invention, after the impulse supercharging is started in the middle of the rich spike, the valve closing period of the impulse supercharging valve can be gradually lengthened. Thereby, after the start of the rich spike due to the low temperature combustion, the intake air amount can be gradually increased as the air-fuel ratio of the EGR gas gradually approaches the stoichiometric air-fuel ratio. For this reason, the air-fuel ratio in the cylinder at the time of rich spike can be controlled more appropriately.

  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, the number of cylinders and the cylinder arrangement of the diesel engine in the present invention are not limited to this.

  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.

  An exhaust passage 18 of the diesel engine 10 is branched by an exhaust manifold 20 and connected to an exhaust port 22 of each cylinder. The diesel engine 10 according to this embodiment includes a turbocharger 24. The exhaust passage 18 is connected to the exhaust turbine of the turbocharger 24.

A NOx catalyst 26 is installed downstream of the turbocharger 24 in the exhaust passage 18. The NOx catalyst 26 stores NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is leaner than the stoichiometric air-fuel ratio, and stores the stored NOx when the air-fuel ratio of the inflowing exhaust gas is less than the stoichiometric air-fuel ratio. It has a function of discharging and reducing and purifying to N ₂ . The NOx catalyst 26 of the present embodiment may have other functions such as a function of capturing PM (Particulate Matter), for example.

  During normal combustion of the diesel engine 10, the air-fuel ratio of the exhaust gas becomes much leaner than the stoichiometric air-fuel ratio. Therefore, at this time, NOx in the exhaust gas is captured by the NOx catalyst 26 and stored. There is a limit to the amount of NOx that can be stored in the NOx catalyst 26. For this reason, in the system of the present embodiment, the rich spike for removing and purifying NOx stored in the NOx catalyst 26 is performed intermittently (periodically). The rich spike is to temporarily set the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 26 to be equal to or lower than the stoichiometric air-fuel ratio.

  An air cleaner 30 is provided near the inlet of the intake passage 28 of the diesel engine 10. The air drawn through the air cleaner 30 is compressed by the intake compressor of the turbocharger 24 and then cooled by the intercooler 32. The intake air that has passed through the intercooler 32 is distributed by the intake manifold 34 to the intake ports 35 (see FIG. 2) of the respective cylinders.

  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.

  One end of an EGR passage 40 is connected to the intake passage 28 in the vicinity of the intake manifold 34. The other end of the EGR passage 40 is connected to the vicinity of the exhaust manifold 20 of the exhaust passage 18. In this system, a part of the exhaust gas (burned gas) can be recirculated to the intake passage 28 through the EGR passage 40, that is, EGR (Exhaust Gas Recirculation) can be performed. Hereinafter, the exhaust gas recirculated through the EGR passage 40 is referred to as “EGR gas”.

  An EGR cooler 42 for cooling the EGR gas is provided in the middle of the EGR passage 40. An EGR valve 44 is provided downstream of the EGR cooler 42 in the EGR passage 40. By changing the opening of the EGR valve 44, the amount of EGR gas passing through the EGR passage 40, that is, the amount of EGR can be adjusted.

  An impulse supercharging valve 46 is installed between the joining portion of the intake passage 28 and the EGR passage 40 and the intake manifold 34. The impulse supercharging valve 46 can be displaced at high speed, for example, by a drive of an electric actuator such as a rotary solenoid, in a closed state in which the intake passage 28 is closed and the intake passage is blocked, and an open state in which the intake passage is allowed. It has become.

  Although the illustrated impulse charge valve 46 is configured as a butterfly valve, it may be configured as another type of valve such as a shutter valve. In the illustrated configuration, one impulse charge valve 46 common to all cylinders is provided, but an impulse charge valve may be provided for each cylinder individually.

  Furthermore, the system of the present embodiment includes an accelerator opening sensor 48 that detects an accelerator pedal position (accelerator opening) and an ECU (Electronic Control Unit) 50. From the detection signal of the accelerator opening sensor 48, the engine load (required load) can be calculated. The ECU 50 is 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 connected to the ECU 50.

  Further, the diesel engine 10 of the present embodiment includes an intake variable valve mechanism 54 that varies the valve opening characteristic of the intake valve 52 and an exhaust variable valve mechanism 58 that varies the valve opening characteristic of the exhaust valve 56. Is provided. These intake variable valve mechanism 54 and exhaust variable valve mechanism 58 are connected to the ECU 50. In the present invention, the intake variable valve mechanism 54 and the exhaust variable valve mechanism 58 may be omitted. That is, the valve opening characteristic of the intake valve 52 and the valve opening characteristic of the exhaust valve 56 may be fixed.

[Features of Embodiment 1]
(Impulse supercharging)
In the diesel engine 10 of this embodiment, the impulse charge can be performed using the impulse charge valve 46. When performing the impulse supercharging, the impulse supercharging valve 46 is in a closed state at the start of the opening of the intake valve 52 and is opened later than the intake valve 52 is opened (for example, the intake valve 52 is opened). Controlled to open later in the valve period). When the impulse charge valve 46 is controlled in this way, the interval between the impulse charge valve 46 and the intake valve 52 is between the opening start time of the intake valve 52 and the opening start time of the impulse charge valve 46. A negative pressure is formed in the intake passage. Thereafter, when the impulse charge valve 46 is instantaneously opened, the intake gas existing upstream of the impulse charge valve 46 flows into the cylinder at once due to the negative pressure. At this time, the intake valve 52 or the intake valve 52 and the impulse charge valve 46 are closed, so that a large amount of intake gas can be filled into the cylinder by a kind of inertial supercharging effect. That is, in this impulse supercharging, the differential pressure formed upstream and downstream of the impulse supercharging valve 46 is released at an appropriate time to actively generate air column vibration of the intake air and cause pulsation in the intake air. Then, supercharging is performed. The diameter and length of the intake passage of each cylinder are set so as to generate air column vibration suitable for impulse supercharging.

(Low temperature combustion)
In the diesel engine 10 of the present embodiment, the combustion temperature can be lowered to a temperature at which smoke is not generated by performing a large amount of EGR and increasing the EGR rate. Such combustion is referred to herein as “low temperature combustion”. Hereinafter, this low temperature combustion will be further described.

  The EGR gas recirculated into the intake passage 28 via the EGR passage 40 has a higher specific heat than air and can therefore absorb a large amount of heat. Therefore, the combustion temperature in the combustion chamber decreases as the EGR gas amount increases, that is, as the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)) increases. As the combustion temperature decreases, the amount of NOx generated decreases, and as the EGR rate increases, the amount of NOx generated decreases.

  Thus, it has been known that increasing the EGR rate can reduce the amount of NOx generated. However, when the EGR rate is increased, when the EGR rate exceeds a certain limit, the generation amount of soot, that is, smoke starts to increase rapidly. In this regard, 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 the maximum allowable limit of the EGR rate. It is thought that there is.

  Therefore, conventionally, the EGR rate is set 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.

  As described above, it has been conventionally considered that there is a maximum allowable limit with respect to the EGR rate, so that the amount of NOx and smoke generated is reduced as much as possible within the range in which the EGR rate does not exceed the maximum allowable limit. It was stipulated. However, even if the EGR rate is determined in this way so that the generation amount of NOx and smoke becomes as small as possible, there is a limit to the reduction in the generation amount of NOx and smoke, and in fact, a considerable amount of NOx and smoke is still present. Is currently occurring.

  However, in the course of research on diesel engine combustion, if the EGR rate is made larger than the maximum allowable limit, the smoke increases rapidly as described above. However, there is a peak in the amount of smoke generated, exceeding this peak. When the EGR rate is further increased, the smoke begins to rapidly decrease. When the EGR rate is increased to 70% or higher during idling operation, and when the EGR gas is strongly cooled, the smoke is increased to approximately 55% or higher. Was found to be almost zero, i.e., almost no wrinkles occurred. At this time, it has been found that the amount of NOx generated is extremely small. Thereafter, 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. In short, this new combustion system, that is, low-temperature combustion, is based on stopping 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 when the temperature of the fuel during combustion in the combustion chamber and the surrounding gas temperature is lower than a certain temperature, the growth of hydrocarbons stops in the middle before reaching soot, This means that when the temperature of the fuel and the surrounding gas rises above 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.

  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 idea of low temperature combustion.

  By the way, when performing EGR, the air-fuel ratio in the cylinder is determined by the amount of fresh air flowing into the cylinder, the amount of oxygen remaining in the EGR gas flowing into the cylinder, and the fuel injection amount from the fuel injector 12. It will be determined by. As the EGR rate increases, the in-cylinder air-fuel ratio decreases (enriches). In low temperature combustion, since the EGR rate is high, the in-cylinder air-fuel ratio can be significantly reduced (enriched) as compared with normal combustion (lean combustion) of the diesel engine 10, and it can be made lower than the stoichiometric air-fuel ratio. Is possible. Therefore, in the present embodiment, when a rich spike is required, the rich spike is performed by performing this low-temperature combustion so that the air-fuel ratio is equal to or lower than the stoichiometric air-fuel ratio.

  However, in general, there is a problem that it is difficult to perform low-temperature combustion in the light load region. The reason is as follows. In order to perform low temperature combustion, a large amount of EGR is required. Therefore, it is necessary to reduce the opening of the intake throttle valve 36 so that the intake manifold pressure (pressure downstream of the EGR passage 40) is sufficiently lower than the exhaust manifold pressure (pressure upstream of the EGR passage 40). is there. However, if the opening degree of the intake throttle valve 36 is reduced, the amount of intake air-fuel mixture (a mixture of fresh air and EGR gas) is reduced and the compression end pressure (pressure at the end of the compression stroke) is reduced. Tends to be less stable. Such a tendency becomes more prominent as the engine load is lower. For this reason, there is a limit on the light load side in the operation region where low temperature combustion can be performed while ensuring stable ignition. Therefore, when a rich spike is required when the engine load is in a region where the engine load is lower than the limit, there is a problem that low temperature combustion cannot be performed with stable ignition.

  In order to solve this problem, in this embodiment, when performing low-temperature combustion for a rich spike in a region where the engine load is lower than the above limit, impulse supercharging is performed in parallel. When impulse supercharging is performed, the charging efficiency can be improved, so that a sufficient amount of intake air-fuel mixture can be secured even if the intake manifold pressure is low. That is, when low-temperature combustion is performed with impulse supercharging, the EGR rate increases even when the intake manifold pressure is equivalent, and as a result, the compression end pressure also increases. For this reason, even if it is a case where low temperature combustion is performed in a light load region, ignition can be stabilized.

  On the other hand, unlike the present invention, when low-temperature combustion is performed without impulse supercharging, the amount of intake air-fuel mixture decreases, so the amount of exhaust gas also decreases and the exhaust manifold pressure decreases. For this reason, in order to flow a large amount of EGR gas while ensuring a differential pressure from the intake manifold pressure, it is necessary to further throttle the intake throttle valve 36 to further reduce the intake manifold pressure.

  On the other hand, when low-temperature combustion is performed with impulse supercharging as in the present embodiment, it is possible to suppress a decrease in the amount of intake air-fuel mixture, thereby suppressing a decrease in exhaust manifold pressure. . Therefore, since the differential pressure between the exhaust manifold pressure and the intake manifold pressure can be ensured without reducing the opening of the intake throttle valve 36 so much, the intake manifold pressure can be kept high. For this reason, even when low-temperature combustion is performed in a light load region, a sufficient amount of intake air-fuel mixture can be secured, so that the compression end pressure can be sufficiently increased and ignition can be reliably stabilized.

  By the way, when the impulse charge is performed, the intake air-fuel mixture flows into the cylinder vigorously, so that the swirl in the cylinder becomes strong. Conventionally, when impulse supercharging is performed in a light load region, that is, a region where the fuel injection amount is small, there is a problem that overswirl is likely to occur. In other words, mixing of fuel and air is excessively promoted by strong swirls, and the fuel is excessively diluted, thereby facilitating misfire and extinguishing. As a result, there is a problem that HC emissions are likely to increase. . For this reason, conventionally, it is normal not to perform impulse supercharging in a light load range.

  In contrast, in the present embodiment, impulse supercharging is performed together with low-temperature combustion with a low in-cylinder air-fuel ratio, so even if mixing of the fuel and air proceeds, the fuel is excessively diluted even to the theoretical air-fuel ratio. There is nothing. For this reason, there is no possibility that the above-described adverse effect of over swirling will occur. In particular, in the case of the present embodiment, since low temperature combustion is performed for the rich spike, there is no problem even if the HC emission amount is temporarily increased.

[Specific Processing in Embodiment 1]
FIG. 3 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. 3, it is first determined whether or not a rich spike is necessary (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 of the diesel engine 10 and the NOx emission amount is stored in the ECU 50 as a map, and the amount of NOx flowing into the NOx catalyst 26 after the previous rich spike is integrated according to the map, When the integrated value exceeds a predetermined value, 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 a rich spike is necessary, the combustion in the diesel engine 10 is switched from normal combustion to low-temperature combustion in which the air-fuel ratio is equal to or lower than the stoichiometric air-fuel ratio. A spike is performed (step 102). That is, a large amount of EGR is executed by increasing the opening of the EGR valve 44 and decreasing the opening of the intake throttle valve 36. Moreover, you may correct | amend fuel injection quantity, fuel injection timing, etc. as needed.

  Next, it is determined whether or not the engine load is smaller than a predetermined load (step 104). The predetermined load is set in advance as a lower limit value of the engine load that can perform low-temperature combustion while obtaining stable ignition even without impulse supercharging.

  If the engine load is larger than the predetermined load in step 104, it can be determined that stable ignition can be obtained even if low temperature combustion is performed without impulse supercharging. Therefore, in this case, the execution of the current routine is terminated as it is without executing the impulse supercharging.

  On the other hand, when the engine load is smaller than the predetermined load in step 104, the impulse charge valve 46 is operated and the impulse charge is executed (step 106). As a result, as described above, even when the engine load is low, a sufficient amount of intake air-fuel mixture can be secured, and a decrease in compression end pressure can be avoided. For this reason, the rich spike by low temperature combustion can be performed, ensuring the stable ignition.

  In addition, when impulse supercharging is performed, pump loss increases, and thus thermal efficiency tends to decrease. In this embodiment, impulse supercharging is performed during a rich spike that is intermittently performed only for a short time, and therefore, there is little influence of a decrease in thermal efficiency due to impulse supercharging. Therefore, deterioration of fuel consumption can be reliably suppressed. In particular, in the present embodiment, even when a rich spike occurs, impulse supercharging is not performed when the engine load is greater than the predetermined load, so that deterioration of fuel consumption due to impulse supercharging can be minimized.

  In the first embodiment described above, the case where the impulse supercharging is performed when the low temperature combustion is performed for the rich spike in the light load range is described as an example. However, in the present invention, the low temperature combustion is an object other than the rich spike. Impulse supercharging may be performed when it is performed in step (b). For example, the present invention can also be applied to a case where impulse supercharging is also performed when low-temperature combustion is regularly performed for the purpose of maintaining a high temperature of the exhaust purification catalyst in a light load region.

  In the first embodiment described above, the case where the low temperature combustion is realized by the external EGR has been described as an example. However, the present invention also applies to the case where the low temperature combustion is performed by using the internal EGR together or only by the internal EGR. Applicable. The internal EGR amount can be adjusted by changing the magnitude of the positive or negative valve overlap between the intake valve 52 and the exhaust valve 56 using a variable valve device.

  In the first embodiment described above, the ECU 50 executes the process of step 102, so that the “combustion switching means” in the first invention executes the process of the routine shown in FIG. The “impulse supercharging control means” in the first, second and third inventions is realized, respectively.

Embodiment 2. FIG.
Next, the second embodiment of the present invention will be described with reference to FIG. 4 to FIG. 6. The description will focus on the differences from the first embodiment described above, and the same matters will be described. Simplify or omit. The present embodiment can be realized by causing the ECU 50 to execute a routine shown in FIG. 6 to be described later using the hardware configuration shown in FIG.

[Features of Embodiment 2]
In the present embodiment, when impulse supercharging is performed in accordance with a rich spike caused by low temperature combustion, impulse supercharging is not started simultaneously with the start of the rich spike, but is started in the middle of the rich spike. It is characterized by.

  FIG. 4 is a diagram showing the behavior of the air-fuel ratio (exhaust air-fuel ratio) at the time of rich spike and the NOx concentration in the exhaust gas exiting the NOx catalyst 26, and FIG. It is a figure which shows the behavior of an EGR rate. The thick line in FIG. 4 and FIG. 5 shows the behavior when impulse supercharging is started in the middle of the rich spike, that is, in the case of the present embodiment. On the other hand, an example indicated by a thin line is a behavior when impulse supercharging is started simultaneously with the start of rich spike (low temperature combustion).

  When the diesel engine 10 is normally combusting, the air-fuel ratio is much leaner than the stoichiometric air-fuel ratio, so that the exhaust gas contains a lot of oxygen. Therefore, immediately after the rich spike is started and the normal combustion is switched to the low temperature combustion and the EGR rate rapidly increases, a large amount of oxygen remains in the EGR gas recirculated in the cylinder. And since EGR gas recirculates through the EGR passage 40, a transport delay occurs. For this reason, the air-fuel ratio of EGR gas does not fall below the stoichiometric air-fuel ratio unless a certain amount of time has elapsed after the start of rich spike.

  That is, immediately after the start of the rich spike, an air-fuel mixture composed of fresh air and EGR gas in which a large amount of oxygen remains is sucked into the cylinder. Therefore, if the amount of intake air-fuel mixture is increased by impulse supercharging, the amount of oxygen in the cylinder increases, and as a result, the air-fuel ratio in the cylinder increases (lean). For this reason, when the impulse charge is started simultaneously with the start of the rich spike, as shown in FIG. 4, immediately after the start of the rich spike, the air-fuel ratio cannot be rapidly lowered to the stoichiometric air-fuel ratio or less. Exhaust gas slightly leaner than the air-fuel ratio will flow into the NOx catalyst 26.

  The NOx catalyst 26 does not release the stored NOx at once when the air-fuel ratio of the inflowing exhaust gas becomes equal to or lower than the stoichiometric air-fuel ratio. It has the property of releasing NOx. When NOx is released from the NOx catalyst 26 by exhaust gas that is slightly leaner than the stoichiometric air-fuel ratio, there is no reducing agent in the NOx catalyst 26, so no reduction purification reaction occurs, and the NOx is downstream of the NOx catalyst 26. It flows as it is. For this reason, when the impulse charge is started simultaneously with the start of the rich spike, the exhaust gas slightly leaner than the stoichiometric air-fuel ratio is shown in FIG. When the gas flows into the exhaust gas, a large amount of NOx tends to be discharged downstream of the NOx catalyst 26.

  On the other hand, in this embodiment, impulse supercharging is deactivated at the initial stage of rich spike. For this reason, in the present embodiment, as shown in FIG. 5, the intake air amount at the initial stage of the rich spike can be reduced as compared with the case where the impulse supercharging is started simultaneously with the start of the rich spike. As a result, the amount of oxygen in the cylinder can be reduced, and as shown in FIG. 4, immediately after the start of the rich spike, the air-fuel ratio can be quickly lowered to the stoichiometric air-fuel ratio or less. Therefore, since the period during which the exhaust gas slightly leaner than the stoichiometric air-fuel ratio flows into the NOx catalyst 26 can be reliably shortened, it is possible to reliably prevent NOx from being discharged downstream of the NOx catalyst 26.

  As shown in FIG. 5, the behavior of the EGR rate is almost the same between the case of the present embodiment and the case of starting the impulse supercharging simultaneously with the start of the rich spike. Even if the behavior of the EGR rate is the same, the amount of oxygen in the cylinder is reduced by reducing the intake air-fuel mixture amount by deactivating impulse supercharging at the beginning of the rich spike, so that the air-fuel ratio in the cylinder is quickly increased. It can be lowered to the stoichiometric air-fuel ratio or less.

  By the way, in the rich spike, it is necessary to maintain the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 26 below the theoretical air-fuel ratio for a certain time. In the present embodiment, the air-fuel ratio can be quickly made equal to or lower than the stoichiometric air-fuel ratio as compared with the example in which the impulse supercharging is started simultaneously with the start of the rich spike. Therefore, as shown in FIG. Even rich spikes can be terminated early. That is, in this embodiment, the duration time of the rich spike can be shortened. As a result, fuel efficiency can be improved.

  Further, in the present embodiment, it is preferable to control so that the valve closing period of the impulse charge valve 46 gradually becomes longer every cycle after the impulse charge is started in the middle of the rich spike. The longer the valve closing period of the impulse charge valve 46 is, the more effective the impulse charge is, so that the intake air amount can be increased. By controlling so that the closing period of the impulse supercharging valve 46 gradually becomes longer every cycle after the start of the impulse supercharging, as shown in FIG. 5, the air-fuel ratio of the EGR gas gradually becomes theoretical every cycle. As the air-fuel ratio approaches, the intake air amount can be gradually increased for each cycle. For this reason, the air-fuel ratio in the cylinder can be controlled more appropriately.

[Specific Processing in Second Embodiment]
FIG. 6 is a flowchart of a routine executed by the ECU 50 in the present embodiment in order to realize the above function. Hereinafter, in FIG. 6, the same steps as those shown in FIG. 3 are denoted by the same reference numerals, and the description thereof is omitted or simplified.

  According to the routine shown in FIG. 6, it is first determined whether or not a rich spike is necessary (step 100). If it is determined that a rich spike is necessary, a rich spike due to low-temperature combustion is executed. (Step 102).

  Next, it is determined whether or not the engine load is smaller than a predetermined load (step 104). If it is determined that the engine load is smaller than the predetermined load, it is determined whether or not a predetermined time has elapsed since the start of the rich spike (step 108). This predetermined time corresponds to the start delay time of impulse supercharging required to quickly reduce the in-cylinder air-fuel ratio to the stoichiometric air-fuel ratio or less after the start of the rich spike, and is set to an appropriate period. It is set in advance. The predetermined time may be a function of the engine speed or the engine load.

  If it is determined in step 108 that the elapsed time from the start of the rich spike has reached the predetermined time, the impulse charge valve 46 is actuated and impulse charge is started (step 110). After the start of the impulse supercharging, the valve closing period of the impulse supercharging valve 46 is controlled so as to gradually become longer every cycle.

  According to the present embodiment described above, in addition to the same effects as those of the first embodiment described above, the amount of NOx discharged from the NOx catalyst 26 downstream during the rich spike can be reduced. Furthermore, since the duration time of the rich spike can be shortened, fuel efficiency can be improved.

  In the above-described second embodiment, the ECU 50 executes the process of step 108, so that the “impulse supercharging start timing control means” in the fourth invention executes the process of step 110. The “valve closing period control means” according to the fifth aspect of the present invention is 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 flowchart of the routine performed in Embodiment 2 of this invention. It is a figure which shows the behavior of the air fuel ratio at the time of rich spike, and the NOx concentration in the exhaust gas which exited the NOx catalyst. It is a figure which shows the behavior of the intake air amount at the time of a rich spike, and an EGR rate. It is a flowchart of the routine performed in Embodiment 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Diesel engine 12 Fuel injector 14 Common rail 18 Exhaust passage 20 Exhaust manifold 24 Turbo supercharger 26 Exhaust purification device 28 Intake passage 34 Intake manifold 36 Intake throttle valve 38 Air flow meter 40 EGR passage 44 EGR valve 46 Impulse supercharge valve 50 ECU
52 Intake valve 56 Exhaust valve 62 Crank angle sensor 64 Piston

Claims (5)

An EGR device capable of executing EGR;
An impulse supercharger capable of performing impulse supercharging;
Combustion switching means capable of switching between normal combustion and low-temperature combustion that enriches the air-fuel ratio from the normal combustion and lowers the combustion temperature from the normal combustion by performing a large amount of EGR;
Impulse supercharging control means for operating the impulse supercharging device when performing the low temperature combustion;
A control device for an internal combustion engine, comprising:   2. The control apparatus for an internal combustion engine according to claim 1, wherein the impulse supercharging control means operates the impulse supercharging device when the low temperature combustion is performed and the engine load is smaller than a predetermined load. . A NOx catalyst disposed in the exhaust passage;
The impulse supercharging control means operates the impulse supercharging device when a rich spike that temporarily sets the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a theoretical air-fuel ratio or less is implemented by the low-temperature combustion. 3. The control device for an internal combustion engine according to claim 1, wherein the control device is an internal combustion engine. A NOx catalyst disposed in the exhaust passage;
The impulse supercharging control means is configured to perform the impulse from the middle of the rich spike when a rich spike that temporarily sets the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a theoretical air-fuel ratio or less is implemented by the low-temperature combustion. 4. The control device for an internal combustion engine according to claim 1, further comprising impulse supercharging start timing control means for starting the operation of the supercharging device. The impulse charge device includes an impulse charge valve that opens and closes an intake passage,
The impulse charge control means includes valve closing period control means for gradually increasing the valve closing period of the impulse charge valve after the impulse charge apparatus starts operating from the middle of the rich spike. 5. The control device for an internal combustion engine according to claim 4, wherein the control device is an internal combustion engine.

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