JP2015113717A - Control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a turbo lag on acceleration request during lean combustion operation.SOLUTION: A control device is applied for a lean-combustion-operational internal combustion engine which has a supercharger and a low-pressure EGR mechanism. When a supercharging pressure is lower than a target supercharging pressure during acceleration in lean combustion operation, the control device for the internal combustion engine controls an EGR valve of the low-pressure EGR mechanism to be opened for a period before the supercharging pressure reaches the target supercharging pressure, and controls the EGR valve to be closed after the supercharging pressure reaches the target supercharging pressure. The control device also controls a fuel injection amount so that a change of a gas-fuel ratio is within a certain range, for a predetermined period including the period before the supercharging pressure reaches the target supercharging pressure.

Description

  The present invention relates to a control device for an internal combustion engine, and more particularly to a control device for an internal combustion engine having a supercharger and a low pressure EGR mechanism.

  Conventionally, for example, Patent Literature 1 discloses a control device for an internal combustion engine with a supercharger having a wastegate valve. In order to suppress turbo lag during transient operation due to acceleration of the internal combustion engine, this control device executes control for operating the wastegate valve to the closed side when a predetermined acceleration request for the internal combustion engine is detected. Further, by operating the wastegate valve to the closed side, the differential pressure between the back pressure upstream of the turbocharger and the supercharging pressure rises transiently, so that the EGR amount may increase. For this reason, in the control device of Patent Document 1, control for reducing the opening degree of the EGR valve from the target opening degree is performed while the wastegate valve is operated to the closed side, thereby suppressing an increase in the EGR amount. Has been.

JP 2010-096049 A JP 2012-202315 A JP 2008-223710 A

  When there is a request for acceleration from low-load operation, the acceleration response may be delayed due to a delay until the boost pressure increases. In particular, during the lean combustion operation in which the air-fuel mixture leaner than the stoichiometric air-fuel ratio is burned, the ratio of the fuel injection amount to the air amount is small, so the exhaust energy is small. For this reason, the turbo lag until the same intake air amount is reached at the time of acceleration request is larger than that in the stoichiometric combustion operation in which the air-fuel mixture is burned near the stoichiometric air-fuel ratio. In addition, the amount of intake air necessary to achieve equal torque is greater in the lean combustion operation than in the stoichiometric combustion operation. For this reason, in the case of the lean combustion operation, it is necessary to increase the supercharging pressure, so that the influence of the turbo lag becomes larger.

  In order to improve acceleration response, switching from lean combustion to stoichiometric combustion may be considered. However, when switching between the lean combustion operation and the stoichiometric combustion operation, the NOx emission amount increases by passing through the weak lean region. It is conceivable that the frequency of rich spike control increases to purify the increased NOx, and the fuel consumption deteriorates. In order to suppress an increase in the frequency of rich spike control, it is also conceivable that the torque being switched is connected by stoichiometric combustion and ignition timing retardation without using the weak lean region. However, this control tends to lower the torque efficiency, and it is conceivable that the fuel consumption will decrease.

  As described above, it is not preferable to improve the acceleration response by switching from the lean combustion operation to the stoichiometric combustion operation from the viewpoint of improving fuel consumption. Therefore, a system capable of ensuring high acceleration response responsiveness while maintaining the lean combustion operation as much as possible in response to the acceleration request during the lean combustion operation is desired.

  In this regard, according to the control of Patent Document 1, the turbo lag at the time of acceleration request is suppressed by closing the waste gate valve. However, the technique of Patent Document 1 is premised on including a high-pressure EGR mechanism, and even if the technique of Patent Document 1 is directly applied to an internal combustion engine having a low-pressure EGR mechanism, it is difficult to obtain an effect of suppressing turbo lag. .

  SUMMARY OF THE INVENTION An object of the present invention is to provide an improved control device for an internal combustion engine so that turbo lag can be suppressed when acceleration is requested during lean combustion operation.

In order to achieve the above object, the present invention is a control device for an internal combustion engine capable of a lean combustion operation for burning an air-fuel mixture leaner than a stoichiometric air-fuel ratio,
A turbocharger having a compressor disposed in an intake passage of an internal combustion engine and a turbine disposed in an exhaust passage, and supercharging intake air using exhaust pressure;
A low pressure EGR passage for recirculating part of the exhaust gas from the downstream side of the turbine to the upstream side of the compressor;
An EGR valve installed in the low-pressure EGR passage, for changing a flow passage cross-sectional area of the low-pressure EGR passage;
Means for controlling the EGR valve to the valve opening side during a period until the boost pressure reaches the target boost pressure when the boost pressure is lower than the target boost pressure during acceleration in the lean combustion operation;
Means for controlling the EGR valve to the valve closing side after the supercharging pressure reaches the target supercharging pressure;
Means for controlling the fuel injection amount so that the change of the gas fuel ratio is within a predetermined range for a predetermined period including a period until the supercharging pressure reaches the target supercharging pressure;
Is provided.

  According to the present invention, the back pressure downstream of the supercharger can be reduced by controlling the EGR valve to the valve opening side at the time of acceleration during the lean combustion operation of the internal combustion engine having the low pressure EGR mechanism. Therefore, the differential pressure between the upstream and downstream of the turbine of the supercharger can be increased, and the supercharging pressure can be increased early. Further, at least while the EGR valve is controlled to the valve closing side, the control is performed so that the change of the gas fuel ratio (G / F) is within a certain range. Thereby, the torque fluctuation at the time of lean combustion operation can be suppressed.

It is a schematic diagram for demonstrating the whole structure of the system in embodiment of this invention. It is a timing chart for demonstrating the control in embodiment of this invention. It is a flowchart for demonstrating the routine of control which ECU performs in embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof is simplified or omitted.

Embodiment.
FIG. 1 is a schematic diagram for explaining an overall configuration of a system according to Embodiment 1 of the present invention. In the system of FIG. 1, the internal combustion engine 2 is capable of a lean combustion operation in which an air-fuel mixture leaner than the stoichiometric air-fuel ratio is burned and a stoichiometric combustion operation in which an air-fuel mixture near the stoichiometric air-fuel ratio is combusted. Is an institution. The internal combustion engine 2 also includes a supercharger 4 that supercharges intake air using exhaust pressure, and an LPL-EGR mechanism (low pressure EGR mechanism) 6 that recirculates a part of the exhaust gas to the intake side as EGR gas. ing.

  An intake manifold 12 communicates with the intake port of each cylinder of the internal combustion engine 2, and an intake passage 14 communicates with the intake manifold 12. In the intake passage 14, an air cleaner 16, a compressor 18 of the supercharger 4, and a throttle valve 20 are installed in order from the intake upstream side. An exhaust manifold 22 communicates with the exhaust port of each cylinder of the internal combustion engine 2, and an exhaust passage 24 communicates with the exhaust manifold 22. A turbine 26 of the supercharger 4 is installed in the exhaust passage 24, and a bypass passage 28 that bypasses the turbine 26 is connected to the exhaust passage 24. A WGV (waist gate valve) 30 is installed in the bypass passage 28. An S / C catalyst 32 is installed downstream of the turbine 26 in the exhaust passage 24.

  In the system of FIG. 1, the LPL-EGR mechanism 6 includes an EGR passage 40 (low pressure EGR passage). The end of the EGR passage 40 on the inlet side is connected to the exhaust passage 24 downstream of the turbine 26 and downstream of the S / C catalyst 32. An end portion on the outlet side of the EGR passage 40 is connected to the downstream side of the air cleaner 16 of the intake passage 14 and to the upstream side of the compressor 18. An EGR cooler 42 and an EGR valve 44 are installed in the EGR passage 40. The EGR valve 44 is a valve that changes the flow path cross-sectional area of the EGR passage 40 by changing the opening thereof.

  The system in FIG. 1 includes an ECU 50. The ECU 50 is a control device for the internal combustion engine 2. The ECU 50 takes in and processes signals from various sensors including the air-fuel ratio sensor 52. In addition to the air-fuel ratio sensor 52, the sensor is attached to each part of the internal combustion engine 2 and the vehicle. For example, an air flow meter (not shown) is attached to the inlet of the intake passage 14. The ECU 50 processes the signals of the acquired sensors and operates the actuators according to a predetermined control program. In addition to the throttle valve 20, the WGV 30, and the EGR valve 44, the actuator operated by the ECU 50 includes a fuel injection valve and a spark plug (not shown) of the internal combustion engine 2. Many actuators and sensors connected to the ECU 50 exist in addition to the above, but the description thereof is omitted in this specification.

  The control executed by the ECU 50 in the present embodiment includes control of the throttle valve 20, the WGV 30, and the EGR valve 44 when a slow acceleration request is made during the lean combustion operation. Here, the “slow acceleration request” means a torque request and a supercharging region that can be realized within a range not exceeding the lean combustion operation region. If there is an acceleration request other than the slow acceleration request, for example, a strong acceleration request from the driver such as WOT acceleration, the air-fuel ratio is set to the theoretical air-fuel ratio or the output air-fuel ratio, and rapid acceleration is executed.

  FIG. 2 is a timing chart for explaining the control at the time of request for slow acceleration in the embodiment of the present invention. 2, (a) is the opening degree of the throttle valve 20, (b) is the opening degree of the WGV 30, (c) is the opening degree of the EGR valve 44, (d) is the EGR rate, (e) is the air-fuel ratio A / F, (f) is the gas fuel ratio G / F, (g) is the front pressure of the turbine 26 (pressure upstream of the turbine 26), (h) is the rear pressure of the turbine 26 (back pressure downstream of the turbine 26), (I) represents the supercharging pressure. Further, in each of (g) to (i), a solid line A indicates a case where the control of the present embodiment is executed, and a broken line B indicates a case where the control of the present embodiment is not executed. FIG. 2 shows an example in which a slow acceleration request is made at time t0.

  When a slow acceleration request is recognized during the lean combustion operation, the throttle valve 20 is fully opened as shown in (a). The WGV 30 is fully closed as shown in (b). Further, the EGR valve 44 is fully opened. The pressure at the outlet of the EGR passage 40 (that is, the connection portion between the EGR passage 40 and the intake passage 14) matches the pressure obtained by subtracting the pressure loss of the air cleaner 16 from the atmospheric pressure. Therefore, the back pressure after the turbine 26 can be reduced by fully opening the EGR valve 44. From a comparison between the solid line A and the broken line B in (h) of FIG. 2, it can be seen that the back pressure downstream of the turbine 26 is greatly reduced by fully controlling the EGR valve 44.

  By fully opening the EGR valve 44, the turbine expansion ratio can be increased by reducing the back pressure downstream of the turbine. Accordingly, as shown in FIG. 2I, the supercharging pressure can be increased at an early stage as compared with the case of the broken line B in which the control of the present embodiment is not performed.

By the above control, the EGR valve 44 is fully closed again at time t1 when the boost pressure has increased to the target boost pressure. Thereby, it returns to air lean combustion. In the lean combustion operation in the steady state, the gas fuel ratio (G / F) can be increased and the lean limit can be extended by stopping the introduction of the EGR gas rather than introducing the EGR gas. Further, since the specific heat ratio κ is larger than CO _{2 in} air, the air lean operation has higher thermal efficiency and better fuel efficiency. Therefore, after the supercharging pressure rises to the target supercharging pressure, the fuel efficiency can be further improved by closing the EGR valve 44 and switching to the air lean combustion operation.

  In the control at the time of the slow acceleration request, as shown in FIG. 2 (f), after the EGR valve 44 is fully opened, until at least the EGR valve 44 is closed and the EGR rate returns to near zero. The fuel injection amount is controlled so that the fuel ratio G / F is constant. That is, the fuel injection amount is determined so that the ratio of the amount of fresh air sucked into the cylinder and the amount of fuel combined with the EGR gas amount and the fuel injection amount is constant with respect to the amount of fresh air GA. Is done. At this time, for example, if the gas fuel ratio G / F is 24 and the EGR rate is 20%, the air-fuel ratio A / F is 19.2. While the gas fuel ratio G / F is controlled to be substantially constant, as shown in FIGS. 2D and 2E, when the EGR rate increases, the air-fuel ratio A / F becomes the value on the stoichiometric air-fuel ratio side. As the EGR rate decreases, the air-fuel ratio A / F increases to the lean side.

  If the EGR gas is introduced with the lean air-fuel ratio A / F set during the lean combustion operation, combustion may become unstable and torque fluctuation or misfire may occur. On the other hand, during the lean combustion operation, while the EGR gas is being introduced, the gas fuel ratio G / F is controlled to be constant so that the combustion during that time is stabilized and torque fluctuation and misfire are suppressed. Can do.

  FIG. 3 is a flowchart for illustrating a control routine executed by ECU 50 in the embodiment of the present invention. The routine of FIG. 3 shows a routine of control of the EGR valve 44 executed by the ECU 50, and is repeatedly executed at regular intervals during the operation of the internal combustion engine 2.

  In the routine of FIG. 3, it is first detected whether or not the requested target torque is within the lean combustion operation region. If it is recognized that the fuel is in the lean combustion operation region, then the target boost pressure is calculated (S104). The target boost pressure is calculated according to the target torque and the target air / fuel ratio.

  Next, it is determined whether a turbo lag is currently occurring (S106). Here, based on whether or not the target boost pressure is greater than the actual boost pressure (current boost pressure), it is determined whether or not a turbo lag has occurred. If it is determined in step S106 that a turbo lag has occurred, then the fully open request for the EGR valve 44 is turned ON (S108). When the full open request of the EGR valve 44 is turned ON, the EGR valve 44 is fully opened. Thereafter, the current process is temporarily terminated.

  As described above, the routine of FIG. 3 is repeatedly executed at regular intervals during the operation of the internal combustion engine 2. After the request to fully open the EGR valve 44 is turned ON and the EGR valve 44 is fully opened, in step S106, when it is not recognized that the turbo lag is generated, that is, the actual boost pressure is the target boost pressure. Is reached, the full open request of the EGR valve 44 is turned off (S110). Thereby, the EGR valve 44 is returned to the control according to the normal operating condition. For example, when the above-described air lean combustion operation is performed, the EGR valve 44 is controlled to be fully closed according to a separately set control routine. Thereafter, the current process is temporarily terminated. If it is not recognized in step S102 that the target torque is in the lean combustion operation region, the request for fully opening the EGR valve 44 is turned off (S110), and the current process is temporarily terminated.

  As described above, according to the present embodiment, the supercharging pressure can be increased early by fully opening the EGR valve 44 when there is a request for slow acceleration. Therefore, the delay in the acceleration response during the lean combustion operation can be reduced, and the lean combustion operation region can be expanded.

  In the present embodiment, the gas / fuel ratio G / F is controlled to be constant for a predetermined period including at least a period in which the EGR valve 44 is fully opened. Thereby, the combustion at the time of the acceleration request | requirement at the time of lean combustion operation can be stabilized, and generation | occurrence | production of a torque fluctuation and misfire can be suppressed. In the present embodiment, after starting the control for fully opening the EGR valve 44 due to the acceleration request, the period until the EGR valve 44 is fully closed again and the EGR rate decreases to near zero is set as a predetermined period. The case where the gas fuel ratio G / F is controlled to be constant has been described. However, in the present invention, the predetermined period is not limited to this period, and may be a period including a period in which the EGR valve 44 is fully opened in response to the acceleration request during the lean combustion operation.

  In the present embodiment, the gas fuel ratio G / F is assumed to be constant. However, in the present invention, the control of the gas fuel ratio G / F only needs to be controlled within a certain range in which the change of the gas fuel ratio is allowed.

  Further, in the present embodiment, the case where the EGR valve 44 is fully opened at the time of request for slow acceleration has been described. However, the present invention is not limited to this, as long as the EGR valve 44 is controlled to open. By controlling the EGR valve 44 to the open side, the effect of lowering the back pressure of the turbine 26 can be obtained to some extent, and turbo lag can be suppressed.

  In the present embodiment, the case where the EGR valve 44 is fully opened and the EGR valve 44 is fully closed after the supercharging pressure reaches the target supercharging pressure has been described. However, the present invention is not limited to this, and after the EGR valve 44 is fully opened, it may be controlled to the closing side, for example, according to the target opening.

  In the above embodiment, when referring to the number of each element, quantity, quantity, range, etc., the reference is made unless otherwise specified or the number is clearly specified in principle. The invention is not limited to the numbers. The structures, steps, and the like described in this embodiment are not necessarily essential to the present invention unless otherwise specified or clearly specified in principle.

2 Internal combustion engine 4 Supercharger 6 LPL-EGR mechanism 12 Intake manifold 14 Intake passage 16 Air cleaner 18 Compressor 20 Throttle valve 22 Exhaust manifold 24 Exhaust passage 26 Turbine 28 Bypass passage 30 WGV
32 S / C catalyst 40 EGR passage 42 EGR cooler 44 EGR valve 50 ECU
52 Air-fuel ratio sensor

Claims (1)

A control device for an internal combustion engine capable of a lean combustion operation for burning an air-fuel mixture leaner than a theoretical air-fuel ratio,
A turbocharger having a compressor disposed in an intake passage of an internal combustion engine and a turbine disposed in an exhaust passage, and supercharging intake air using exhaust pressure;
A low pressure EGR passage for recirculating part of the exhaust gas from the downstream side of the turbine to the upstream side of the compressor;
An EGR valve installed in the low-pressure EGR passage, for changing a flow passage cross-sectional area of the low-pressure EGR passage;
Means for controlling the EGR valve to the valve opening side during a period until the boost pressure reaches the target boost pressure when the boost pressure is lower than the target boost pressure during acceleration in the lean combustion operation;
Means for controlling the EGR valve to the valve closing side after the supercharging pressure reaches the target supercharging pressure;
Means for controlling the fuel injection amount so that the change of the gas fuel ratio is within a predetermined range for a predetermined period including a period until the supercharging pressure reaches the target supercharging pressure;
A control device for an internal combustion engine, comprising:

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