Control device for internal combustion engine

An operating range boundary for switching a cam for driving an intake valve (drive cam) is changed in a direction of increasing an engine load if a target EGR rate is predicted to increase across the contour line of the EGR rate during an acceleration operation. By changing to such a high load direction, a range in which a large cam is selected is enlarged. That is, switching of the drive cam from the large cam to a small cam is delayed.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority under 35 U.S.C. § 119 to Japanese Patent Applications No. 2017-26422, filed on Feb. 15, 2017. The contents of these applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

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

BACKGROUND

JP 2013-72342 A discloses a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system. In such a conventional control device, an opening degree of an EGR valve is controlled based on a map defining a relationship between an operating range defined by engine speed and engine load and a target amount of external EGR gas (hereinafter referred to as a “target EGR amount”). In the map, the operating ranges are partitioned by contour lines of the target EGR amount. According to the map, the target EGR amount is set to a highest value in a partitioned range including a middle-engine-speed-and-middle-engine-load range, and decreases from this partitioned range toward a peripheral partitioned range.

The target EGR amounts in the map are obtained by an experiment or simulation performed in advance. According to the map, an actual external EGR amount (hereinafter also referred to as an “actual EGR amount”) can be maintained at an optimum value during a steady operation in which the engine operating state stays in a partitioned range having an equal target EGR amount. On the other hand, the actual EGR amount is largely affected by time lag during a transition operation in which the engine operating state is transferred across the contour line of the target EGR amount. When the engine operating state is transferred from a partitioned range with large target EGR amount to a partitioned range with low target EGR amount, for example, the large influence by time lag causes a period during which the actual EGR amount becomes excessive with respect to the target EGR amount. Then, the combustion in this excessive period tends to deteriorate easily. Accordingly, a countermeasure against such deterioration of combustion during the transition operation is needed.

The present disclosure addresses the above described problem, and an object of the present disclosure is to suppress to occur deterioration in combustion when the engine operating range and the engine operating state is transferred from the partitioned range with high target amount to the partitioned range with low target amount, in a case where an opening degree of an EGR valve is controlled based on the map defining a relationship between the target amount of external EGR gas and the engine operating range.

A first aspect of the present disclosure is a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system,

wherein the control device comprising:

an EGR map the defines a relationship between an operating range defined by engine speed and engine load and a target value of external EGR rate, and has a predetermined partitioned range in which the target value is set to a highest value; and

an operating angle map that defines a relationship between the operating range and an operating angle of an intake cam for driving an intake valve of the engine,

wherein the operating angle map being set so that

a large operating angle is selected in a first region including a region corresponding to the predetermined partitioned range, the large operating angle being capable of closing the intake valve in a first crank angle section, and

a small operating angle is selected in a second region in which the engine load is higher than that of the first region, the small operating angle being capable of closing the intake valve in a second crank angle section that is located nearer to a bottom dead center side than the first crank angle section,

wherein the control device is configured to:

select the operating angle in accordance with the operating angle map when it is predicted that the engine operating state stays in a partitioned range having the equal target value in the EGR map; and

when it is predicted that the engine operating state is transferred from a partitioned range with the high target value to a partitioned range with the low target value in the EGR map, in a case where the engine operating state is transferred in a direction of increasing the engine load, change a boundary between the first region and the second region in a direction of increasing the engine load, and then select the operating angle in accordance with the operating angle map.

A second aspect of the present disclosure is the control device for an internal combustion engine according to the first aspect,

wherein the control device is further configured to, when changing the boundary in the direction of increasing the engine load, increase a degree of a change of the boundary as a change rate of an accelerator opening degree of the engine becomes larger.

A third aspect of the present disclosure is the control combustion engine according to the first aspect,

wherein the control device is further configured to when changing the boundary in the direction of increasing the engine load:

calculate a time interval from a change point of the target value set in accordance with the EGR map to an increase starting point of an actual external EGR rate; and

increase the degree of the change of the boundary as the time interval is larger.

A fourth aspect of the present disclosure is the control device for an internal combustion engine according to any one of the first to third aspects,

wherein the engine comprising a turbocharger including a compressor and a turbine, and

the external EGR gas is recirculated from a downstream side of the turbine to an upstream side of the compressor.

According to the first aspect, when it is predicted that the engine operating state is transferred from a partitioned range with high target value of the external EGR rate to a partitioned range with low target value of the external EGR rate in the EGR map, in a case where the engine operating state is transferred in a direction of increasing the engine load, the boundary between the first region and the second region is changed in a direction of increasing the engine load, and then the operating angle can be selected in accordance with the operating angle map. When the boundary is changed in the direction of increasing the engine load, the first region in which the large operating angle is selected is enlarged. Accordingly, it is possible to continue to select the large operating angle even in the period in which the small operating angle is originally selected. Here, when selecting the large operating angle, the turbulence in the cylinder is larger than in a case of selecting the small operating angle. Therefore, according to the first aspect, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive with respect to the target EGR rate.

According to the second aspect, the degree of the change of the boundary in the direction of increasing the engine load can be changed as the change rate of the accelerator opening degree becomes larger. Accordingly, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive in accordance with the change rate of the accelerator opening degree.

According to the third aspect, the time lag from the change point of the target value of the external EGR rate set in accordance with the EGR map to the increase starting point of the actual value of the external EGR rate is directly calculated, and the degree of the change of the boundary can be increased as the time lag is larger. Therefore, it is possible to suppress deterioration of combustion in the period during which the actual EGR rate becomes excessive.

According to the fourth aspect, it is possible to suppress deterioration of combustion of the engine provided with an LPL-EGR device in the period during which the actual EGR rate becomes excessive.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure are described based on the drawings. It is to be noted that common elements in each figure are designated by the same reference numerals, and duplicated description thereof are omitted herein. It is also to be noted that the following embodiments do not limit the present disclosure.

First Embodiment

A first embodiment of the present disclosure is described with reference toFIGS. 1 to 8.

[Description of System Configuration Example]

FIG. 1is a diagram illustrating a configuration example of a system according to the first embodiment of the present disclosure. The system illustrated inFIG. 1is a system for an internal combustion engine mounted in a vehicle. The system illustrated inFIG. 1includes an internal combustion engine10as a driving source. The internal combustion engine10is a four-stroke reciprocating engine, and also an in-line three cylinder engine. It is to be noted that the number and arrangement of cylinders of the internal combustion engine10are not particularly limited to the above-described number and arrangement. Each cylinder of the interval combustion engine10communicates with an intake pipe12and an exhaust pipe14.

An intake system of the internal combustion engine10is described. An air cleaner16is attached in the vicinity of an inlet of the intake pipe12. A compressor18aof a turbocharger18is provided downstream of the air cleaner16. The compressor18ais driven by rotation of a turbine18bthat is provided in the exhaust pipe14, to compress intake air. An electronic control throttle valve20is provided downstream of the compressor18a. An intake manifold22that is connected to intake ports of each cylinder is provided downstream of the throttle valve20. A water-cooled type intercooler24is incorporated in the intake manifold22. Intake air flowing in the intercooler24is cooled by heat exchange with cooling water flowing in a cooling pipe26.

Next, an exhaust system of the internal combustion engine10is described. The turbine18bof the turbocharger18is attached to the exhaust pipe14. The turbine18bis connected to the compressor18a. The turbine18bis rotated by energy of exhaust gas flowing in the exhaust pipe14. A bypass pipe28that bypasses the turbine18bis provided in a middle of the exhaust pipe14. A WGV (waste gate valve)30is provided in the bypass pipe28. The WGV30is opened when an exhaust pipe pressure (back pressure) on an upstream side of the turbine18bis higher than a predetermined value. When the WGV30is opened, a part of exhaust gas flowing in the upstream side of the turbine18bflows into the downstream side of the turbine18bthrough the bypass pipe28. Catalysts32and34for cleaning exhaust gas are provided in the downstream side of the turbine18b.

Next, an EGR system for the internal combustion engine10is described. The internal combustion engine10includes an LPL-EGR (low pressure loop-EGR) device36. The LPL-EGR device36includes an EGR pipe38that connects the exhaust pipe14between the catalysts32and34, and the intake pipe12on the upstream side of the compressor18a. A water-cooled type EGR cooler40is provided in the middle of the EGR pipe38. Exhaust gas flowing in the EGR cooler40(i.e., external EGR gas) is cooled by heat exchange with cooling water flowing in a cooling pipe42. An electronic control EGR valve44is provided on the downstream side of the EGR cooler40. A change of an opening degree of the EGR valve44causes a change of a flow amount of the external EGR gas that flows from the EGR pipe38into the intake pipe12. When the opening degree of the EGR valve44becomes larger, an external EGR rate increases.

Next, a valve system for the internal combustion engine10is described.FIG. 2is an exemplary graph describing cam profiles (meaning at least one of a lift amount and an operating angle, the same shall apply hereinafter) of two types of intake cams that are provided in the system according to the first embodiment of the present disclosure. As illustrated inFIG. 2, the system according to the first embodiment includes a large cam and a small cam as the two types of intake cams. The small cam has an operating angle and a lift amount that are smaller than those of the large cam. The large cam and the small cam are carried on a camshaft that rotates in synchronization with a crankshaft. Two pair of large and small cams are carried on one cylinder because two intake valves are provided per cylinder. However, the number of intake valves per cylinder in the present disclosure may be one, or three or more. One of the intake earns is used as an intake cam for driving the intake valve (hereinafter also referred to as “drive cam”). The drive cam is switched between the large cam and the small cam by a switching operation of a switching mechanism.

The camshaft carrying the large cam and the small cam is provided with a VVT (variable valve timing mechanism). The VVT is a mechanism that varies a rotational phase difference of the camshaft with respect to the crankshaft thereby to vary a valve opening characteristic of the intake valve. The VVT includes a housing that is connected to the crankshaft through a timing chain or the like, and a valve body that is provided in the housing and attached to an end portion of the camshaft. Hydraulic pressure is supplied into a hydraulic chamber partitioned by the housing and the valve body, to thereby enable the valve body to be relatively rotated with respect to the housing, and further enable the rotational phase difference of the camshaft with respect to the crankshaft to be varied. The hydraulic pressure supplied to the VVT is controlled by a hydraulic pressure control valve provided in a hydraulic pressure supply line. A system of the VVT is known, and a configuration of the system is not limited in the present disclosure, and thus the further descriptions of the VVT are omitted.

Returning toFIG. 1, the configuration example of the system is continuously described. The system illustrated inFIG. 1includes an ECU (Electronic Control Unit)50as a control device. The ECU50includes a RAM (Random Access Memory), a ROM (Read Only Memory), a CPU (microprocessor) and the like. The ECU50takes in and processes signals from various sensors mounted in a vehicle. The various sensors include an air flow meter52, a crank angle sensor54, an accelerator opening degree sensor56, and a supercharging pressure sensor58. The air flow meter52is provided in the vicinity of the air cleaner16, and detects an intake air amount. The crank angle sensor54outputs a signal according to a rotation angle of the crankshaft. The accelerator opening degree sensor56detects a step-on amount of an accelerator pedal by a driver. The supercharging pressure sensor58detects an intake pipe pressure (supercharging pressure) on the upstream side of the throttle valve20. The ECU50takes in and processes the signals from the various sensors to operate various actuators in accordance with a predetermined control program. The various actuators include the above-described throttle valve20and WGV30. The various actuators also include a VVT60and a cam switching mechanism62.

[Characteristic Control in First Embodiment]

FIG. 3is an exemplary graph showing a relationship between an engine operating range and a target EGR rate. The relationship inFIG. 3is created based on a simulation performed in advance. It is to be noted that the target EGR rate means a target value obtained by dividing an external EGR amount by an intake air amount, or can be also referred to as a value obtained by dividing the above-described target EGR amount by the intake air amount. Among ranges partitioned by contour lines shown inFIG. 3, the target FOR rate is set to the highest value in the partitioned range including a middle-engine-speed-and-middle-engine-load range. Thus, the external EGR rate is increased in the middle-engine-speed-and-middle-engine-load range that is used with particularly high frequency, to decrease an intake air temperature, thereby improving the heat efficiency. The target EGR rate is set to a lower value in an operating range for which the frequency of use becomes relatively lower. Specifically, the target EGR rate is set to a lower value in the partitioned ranges including a high engine load range and a low engine load range compared with a value in the partitioned ranges including a middle engine load range. Similarly, the target EGR rate is set to a lower value in the partitioned ranges including a high engine speed range and a low engine speed range compared with a value in the partitioned ranges including a middle engine speed range. In the first embodiment, the relationship shown inFIG. 3is stored in the ROM of the ECU as a map, and an actual operating state is applied to the map to thereby control an opening degree of the EGR valve.

In the first embodiment, the engine is controlled by combining an intake valve closing timing with the above-described target EGR rate.FIG. 4is an exemplary graph showing a relationship between the engine operating range and the cam for driving the intake valve. As shown inFIG. 4, the large cam is selected in the middle-engine-speed-and-middle-engine-load range and the low-engine-speed-and-low-engine-load range, and the small cam is selected in the high-engine-speed-and-high-engine-load range. In the first embodiment, the relationship shown inFIG. 4is stored in the ROM of the ECU as a map, and an actual operating state is applied to the map to thereby control the switching operation of the cam switching mechanism.

FIG. 5is an exemplary graph describing an intake valve closing timing. As shown inFIG. 5, when a drive cam is a large cam, the intake valve is closed in a crank angle section CA1retarded with respect to a bottom dead center (ABDC=0). On the other hand, when a drive cam is a small cam, the intake valve is early closed in a crank angle section CA2including the bottom dead center. Widths of the crank angle sections CA1, CA2as shown inFIG. 5are provided to change the intake valve closing timing by the VVT. However, when the large cam is selected as the drive earn to increase the engine output, the crank angle section CA1is set so as to include a crank angle at which the suction efficiency is maximized. On the other hand, when the small cam having a small lift amount is selected as the drive earn, the crank angle section CA2is set so as not to include the crank angle at which the suction efficiency is maximized. It is to be noted that the suction efficiency shown inFIG. 5can be obtained under operating conditions in which the engine speed is fixed, for example.

When the EGR valve is controlled based on the relationship shown inFIG. 3, an actual external EGR rate (hereinafter also referred to as an “actual EGR rate”) can be controlled to an optimum value during a steady operation in which the engine operating state stays in a partitioned range having an equal target EGR rate. On the other hand, the actual EGR rate is largely affected by time lag during a transition operation in which the engine operating state is transferred across the contour line of the target FOR rate. This problem is described with reference toFIG. 6.FIG. 6is an exemplary graph showing a change in the engine operating state during the transition operation.FIG. 6is a graph in which the relationships shown inFIG. 3andFIG. 4are combined.FIG. 6shows an example of a change in the engine operating state during an acceleration operation. This example assumes that the engine operating state is changed from an operating point PA to an operating point PB. When the operating point is transferred from PA to PB, the operating point is transferred from a partition range R1through a partition range R2to a partitioned range R3.

The target EGR rate is set to the highest value in the partitioned range R1, and becomes lower in order of the partitioned ranges R2, R3, and R4. Thus, when the operating point is transferred from PA to PB, the target EGR rate continues to decrease. However, the time lag produces a period during which the actual EGR rate becomes excessive with respect to the target EGR rate. When such an excessive period of the actual EGR rate occurs, combustion state in the cylinder tends to became unstable. Furthermore, when the drive cam is switched from the large cam to the small cam during the excessive period, turbulence in the cylinder is reduced. Therefore, deterioration of the combustion state in the cylinder cannot be avoided.

In consideration of these reasons, in the first embodiment, an operating range boundary for switching the drive cam (hereinafter also referred to as a “switching boundary”) is changed in a direction of increasing the engine load if the target EGR rate is predicted to increase across the contour line shown inFIG. 3during the acceleration operation.FIG. 7is a graph describing a method of changing the switching boundary in the first embodiment of the present disclosure. Operating points PA, PB and partitioned ranges R1to R4that are shown inFIG. 7correspond to the operating points PA, PB and partitioned ranges R1to R4that are shown inFIG. 6, respectively. As can be seen from comparingFIG. 6andFIG. 7, a switching boundary inFIG. 7is changed in a higher load direction than that inFIG. 6. By changing to such a high load direction, a range in Which the large cam is selected is enlarged. That is, switching of the drive cam from the large earn to the small cam is delayed. Therefore, it is possible to suppress deterioration of the combustion state in the cylinder due to the switch of the drive cam during the excessive period.

FIG. 8is time charts describing a relationship between transitions of an accelerator opening degree, an engine load, and an external EGR rate, and a drive, respectively, when the operating point is transferred from PA to PB as shown inFIG. 6. As shown inFIG. 8, the accelerator opening degree starts to increase at a time t1. When the accelerator opening degree increases, the engine load increases from LA to LB. It is to be noted that the engine load. LA shown inFIG. 8represents the engine load at the operating point PA shown inFIG. 6, and the engine load LB represents the engine load at the operating point PB shown inFIG. 6.

Since the target EGR rate follows the relationship shown inFIG. 3, when the engine load increases after the time t1, the target EGR rate decreases. When the target EGR rate decreases, the EGR valve opening degree becomes smaller, and the actual EGR rate also decreases. The actual EGR rate starts to increase after the time t1because this is affected by the above-described time lag. It is to be noted that the target EGR rate starts to decrease after reaching the maximum valve because the operating point is transferred from the partitioned range R1to the partitioned range R2as shown in FIG.

The ECU predicts whether the target EGR rate increases across the contour line shown inFIG. 6based on a change rate of the accelerator opening degree after the time t1. For example, the ECU stores in the ROM a threshold set in advance based on intervals between the contour lines of the target EGR to compare between the threshold and the change rate of the accelerator opening rate. When the ECU determines that the change rate of the accelerator opening degree exceeds the threshold after the time t1, the ECU predicts that the target EGR rate increases across the contour line shown inFIG. 3.

InFIG. 8, it is predicted at the time t2that the target EGR rate increases across the contour line shown inFIG. 3. When the drive cam is switched, at the time t2, based on the relationship shown inFIG. 4from the large cam to the small cam, the combustion state in the cylinder easily deteriorates. In this respect, in the first embodiment, since the switching boundary is changed in the direction of increasing the engine load, it is possible to continue to select the large cam until a time t3being later than the time t2. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period.

In the above-described first embodiment, the map defining the relationship shown inFIG. 3corresponds to an “EGR map” in a first aspect. The map defining the relationship shown inFIG. 4corresponds to an “operating angle map” of the first aspect. The partitioned range R1shown inFIG. 6corresponds to a “predetermined partitioned range” of the first aspect. The crank angle section CA1described inFIG. 5corresponds to a “first crank angle section” of the first aspect. The crank angle section CA2corresponds to a “second crank angle section” of the first aspect.

Second Embodiment

A second embodiment of the present disclosure is described with reference toFIGS. 9 to 10.

[Characteristic Control in Second Embodiment]

In the above-described first embodiment, when it is predicted during the acceleration operation that the target EGR rate increases across the contour line shown inFIG. 3, the switching boundary is changed in the direction of increasing the engine load. The second embodiment takes a change rate of an accelerator opening degree during the acceleration operation into consideration.

FIG. 9is time charts describing transitions of the accelerator opening degrees, the engine load and the external EGR rates during the acceleration operation, respectively.FIG. 9shows a case of rapid acceleration in which the rate of change in the accelerator opening degree is large and a case of slow acceleration in which the rate of change in the accelerator opening degree is small. As shown inFIG. 9, in the case of rapid acceleration, the increase in the engine load is slower than that in the case of slow acceleration, and the decrease in the target EGR rate is also slower than that in the case of slow acceleration.

In the second embodiment, therefore, a position of the switching boundary is adjusted in accordance with the change rate of the accelerator opening degree during the acceleration operation.FIG. 10is a graph describing a method of adjusting the switching boundary in the second embodiment of the present disclosure. As shown inFIG. 10, in the second embodiment, the engine load at the switching boundary is set to a higher value as the change rate becomes larger. That is, the degree of the change of the switching boundary is increased as the change rate becomes larger. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period in accordance with the change rate of the accelerator opening degree. It is to be noted that the engine load value when the change rate of the accelerator opening degree equals zero as shown inFIG. 10corresponds to the engine load value at the switching boundary during the steady operation shown inFIG. 4.

Third Embodiment

A third embodiment of the present disclosure is described with reference toFIG. 1.

[Characteristic Control in Third Embodiment]

In the first embodiment, the switching boundary is changed in the direction of increasing the engine load when the target EGR rate increases across the contour line shown inFIG. 3. As described above, this reason is because the time lag produces the period during which the actual EGR rate becomes excessive with respect to the target EGR rate. In the third embodiment, the ECU calculates the actual EGR rate during the acceleration operation to predict the time lag. The actual EGR rate is calculated based on an intake air amount, a supercharging pressure and an actual opening degree of the EGR valve, for example. The time lag corresponds to a time interval from a change point of the target EGR rate during the acceleration operation to an increase starting point of the actual EGR rate. Therefore, the actual EGR rate during the acceleration operation is calculated to obtain the above-described time interval, and the actual time lag Δt can be thereby predicted.

In the third embodiment, a position of the switching boundary is adjusted based on the predicted time lag Δt.FIG. 11is a graph describing a method of adjusting the switching boundary in the third embodiment of the present disclosure. As shown inFIG. 11, the degree of the change of the switching boundary is increased as the predicted time lag Δt becomes larger. That is, the engine load at the switching boundary is set to a higher value as the predicted time lag Δt becomes larger. Therefore, it is possible to suppress the deterioration of the combustion state in the cylinder during the excessive period in accordance with the time lag Δt. It is to be noted that the engine load value when the time lag Δt equals zero as shown inFIG. 11corresponds to the engine load value at the switching boundary during the steady operation shown inFIG. 4.

Other Embodiment

The above-described first to third embodiments are described on the assumption that the internal combustion engine is provided with an LPL-EGR device. However, the internal combustion engine may be provided with an HPL-EGR (high pressure loop-EGR) device instead of the LPL-EGR device. The internal combustion engine may be provided with a non-supercharging EGR device instead of the LPL-EGR device. The internal combustion engine may be provided with both of the LPL-EGR device and the HPL-EGR device. However, considering that influence by the time lag of the external EGR rate becomes the largest in the LPL-EGR device, the methods described in the above-described first to third embodiments are particularly effective in the internal combustion engine provided with the LPL-EGR device.