Control device for internal combustion engines

Abstract

To improve fuel economy in an internal combustion engine that can recirculate EGR gas to an intake passage.SOLUTION: An ECU 20 of an internal combustion engine 10 includes a control section 21 that performs control to close a throttle valve 124 and an EGR valve 172 when a deceleration request of vehicle 1 is made and to cause valve closing timing of an intake valve 118 to come close to an intake bottom dead center and delay valve closing timing of an exhaust valve 119 to increase overlap amount during deceleration of the vehicle 1. Since it is unnecessary to lower a target EGR rate during a steady operation by considering combustion stability during deceleration, fuel economy can be improved.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a control device for an internal combustion engine.

Background Art

[0002] An exhaust gas recirculation (EGR) device is known that can improve fuel efficiency by recirculating a part of the exhaust gas as EGR gas into the intake passage and burning it in the combustion chamber. In an internal combustion engine equipped with an EGR device, if the ratio of EGR gas in the gas in the combustion chamber is increased too much, combustion may become unstable and misfire may occur. Therefore, in an internal combustion engine equipped with an EGR device, in order to prevent misfire, when the vehicle decelerates, the throttle valve is closed and at the same time, the EGR valve is closed to control the inflow of EGR gas into the combustion chamber.

[0003] However, EGR gas remains between the downstream of the EGR valve in the EGR passage and the connection part of the intake passage, and between the connection part of the EGR passage in the intake passage and the combustion chamber. Therefore, even if the control of closing the throttle valve and closing the EGR valve at the same time is performed, the remaining EGR gas may flow into the combustion chamber, causing the ratio of EGR gas to temporarily rise rapidly and making it impossible to prevent misfire.

[0004] Patent Document 1 discloses that a control device for an internal combustion engine equipped with a variable compression ratio mechanism controls to increase the mechanical compression ratio when a deceleration request of the vehicle is made in order to improve misfire tolerance during deceleration of the vehicle.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, even if the mechanical compression ratio is increased during vehicle deceleration using a variable compression ratio mechanism as described in Patent Document 1, it is insufficient to prevent misfires when considering cases where an amount of EGR gas exceeding the misfire limit flows in. Therefore, the EGR rate during steady-state operation must be set to a level with a sufficient margin above the misfire limit, which results in a decrease in the EGR effect and a deterioration in fuel efficiency.

[0007] The present invention has been made in view of the problems described above, and aims to improve fuel efficiency in an internal combustion engine that can recirculate EGR gas into the intake passage. [Means for solving the problem]

[0008] The present invention relates to a control device for an internal combustion engine, comprising: a catalyst for purifying exhaust gas discharged from the combustion chamber of the internal combustion engine; an EGR passage connecting the exhaust passage and the intake passage of the internal combustion engine; an EGR valve for adjusting the flow rate of EGR gas through the EGR passage; and a throttle valve provided in the intake passage for adjusting the amount of intake air flowing into the combustion chamber, wherein the throttle valve and the EGR valve are closed when the vehicle is requested to decelerate. If it is determined that the misfire limit is exceeded due to the inflow of the EGR gas into the combustion chamber at the start of vehicle deceleration, the closing timing of the intake valve is set to intake bottom dead center. The system includes a control unit that controls the closing timing of the exhaust valve to retard it and increase the overlap amount. The control unit controls the intake valve closing timing to be set to intake bottom dead center and retarding the exhaust valve closing timing to increase the overlap amount, thereby causing the misfire limit EGR rate to start to decrease, and then gradually returning the closing timing of the intake valve and the exhaust valve according to the misfire limit EGR rate. It is characterized by the following: [Effects of the Invention]

[0009] According to the present invention, fuel efficiency can be improved in an internal combustion engine that can recirculate EGR gas into the intake passage. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram showing a partial configuration of a vehicle equipped with an internal combustion engine control device. [Figure 2] This flowchart shows the operation by the control device. [Figure 3]This diagram shows the opening and closing timing of the intake valve and exhaust valve. [Figure 4] This figure shows various timing charts. [Modes for carrying out the invention]

[0011] The internal combustion engine 10 according to the embodiment of the present invention includes a catalyst 133 for purifying exhaust gas discharged from the combustion chamber 112 of the internal combustion engine 10, an EGR passage 171 connecting the exhaust passage 131 and the intake passage 121 of the internal combustion engine 10, an EGR valve 172 for adjusting the flow rate of EGR gas flowing through the EGR passage 171, and a throttle valve 124 provided in the intake passage 121 for adjusting the amount of intake air flowing into the combustion chamber 112. The control unit 21 of the internal combustion engine 10 closes the throttle valve 124 and the EGR valve 172 when the vehicle 1 requests deceleration, and controls the closing timing of the intake valve 118 to approach the intake bottom dead center and retards the closing timing of the exhaust valve 119 to increase the overlap amount when the vehicle 1 decelerates. In this way, by controlling the opening and closing timing of the intake valve 118 and the exhaust valve 119 to prevent misfires, it is not necessary to lower the target EGR rate during steady operation to account for combustion instability during deceleration, and thus fuel efficiency can be improved. [Examples]

[0012] The control device for an internal combustion engine according to the present invention will be described below with reference to the drawings. Figure 1 is a schematic diagram showing a part of the configuration of a vehicle 1 equipped with an internal combustion engine control device.

[0013] Vehicle 1 comprises an internal combustion engine 10, an ECU (Electronic Control Unit) 20 that controls the internal combustion engine 10, and a sensor unit 30. Vehicle 1 also includes other devices that are typically found in vehicles, and their illustration and description are omitted.

[0014] The internal combustion engine 10 comprises an engine 11 as a power source, an intake system 12 for taking in air for combustion of the engine 11, an exhaust system 13 for discharging exhaust gas from the engine 11 to the outside, and a fuel system 14 for supplying fuel to the engine 11. The internal combustion engine 10 also includes an intake VVT ​​(Variable Valve Timing) device 15, an exhaust VVT (Variable Valve Timing) device 16, an exhaust gas recirculation device (EGR device) 17, etc.

[0015] Engine 11 performs a series of four strokes: an intake stroke, a compression stroke, a combustion stroke, and an exhaust stroke. Engine 11 has a combustion chamber 112 in which a piston 111 is reciprocally housed, a crank chamber 113 in which a crankshaft 116 is rotatably housed, an intake port 114 which is the inlet for combustion air, and an exhaust port 115 which is the outlet for exhaust gas. Engine 11 also has a spark plug 117 positioned so that its tip is located inside the combustion chamber 112, an intake valve 118 located between the combustion chamber 112 and the intake port 114, and an exhaust valve 119 located between the combustion chamber 112 and the exhaust port 115. The configuration of engine 11 is not particularly limited, and various known engines can be applied. Furthermore, engine 11 may be either a gasoline engine or a diesel engine.

[0016] The intake system 12 includes an intake passage 121, an air cleaner 123, and a throttle valve 124. The intake passage 121 is a passage that guides intake air taken in from outside the vehicle 1 to the combustion chamber 112 via the intake port 114. The intake passage 121 is mainly composed of an intake pipe 122. The air cleaner 123 is located in the intake passage 121 and purifies the intake air by removing foreign matter such as dust and dirt contained in the intake air. The throttle valve 124 is located in the intake passage 121 and adjusts the intake air flow rate by opening and closing it. The throttle valve 124 adjusts the intake air flow rate based on control by the ECU 20.

[0017] The exhaust system 13 includes an exhaust passage 131 and a catalyst (catalytic converter) 133. The exhaust passage 131 is a passage for exhausting the exhaust gas burned in the combustion chamber 112 to the outside of the vehicle 1 via the exhaust port 115. The exhaust passage 131 is mainly constituted by an exhaust pipe 132. The catalyst 133 purifies harmful substances contained in the exhaust gas. For the catalyst 133, for example, various known three-way catalysts and the like can be applied.

[0018] The fuel system 14 supplies fuel to the engine 11. The fuel system 14 includes a fuel tank, a fuel pump, and an injector 141. The fuel pump supplies the fuel stored in the fuel tank to the injector 141. The injector 141 injects the fuel supplied from the fuel tank into the intake passage 121. The injector 141 adjusts the injection amount of the fuel based on the control by the ECU 20. Also, the injector 141 is not limited to injecting the fuel into the intake passage 121, and may be configured to inject the fuel into the combustion chamber 112.

[0019] The intake VVT device 15 varies the opening and closing timing of the intake valve 118. The intake VVT device 15 changes the opening and closing timing of the intake valve 118 to advance or retard based on the control by the ECU 20. The intake VVT device 15 may be hydraulic or electric, and known devices can be used.

[0020] The exhaust VVT device 16 varies the opening and closing timing of the exhaust valve 119. The exhaust VVT device 16 changes the opening and closing timing of the exhaust valve 119 to advance or retard based on the control by the ECU 20. The exhaust VVT device 16 may be hydraulic or electric, and known devices can be used.

[0021] The exhaust gas recirculation device 17 recirculates a part of the exhaust gas burned in the combustion chamber 112 as EGR gas to the intake passage 121 and burns it again in the combustion chamber 112. The exhaust gas recirculation device 17 has an EGR passage 171 and an EGR valve 172. The EGR passage 171 connects the intake passage 121 and the exhaust passage 131, allowing a portion of the exhaust gas to be recirculated into the intake passage 121. Here, the exhaust gas flowing through the EGR passage 171 is the EGR gas. One end of the EGR passage 171 is connected to the exhaust pipe 132 downstream of the catalyst 133, and the other end is connected to the intake pipe 122 at a position between the throttle valve 124 and the engine 11.

[0022] The EGR valve 172 is located in the EGR passage 171 and adjusts the amount of EGR gas (recirculation rate) that flows from the exhaust passage 131 to the intake passage 121 by opening and closing it. The EGR valve 172 adjusts the amount of EGR gas based on control by the ECU 20. In addition, the exhaust gas recirculation device 17 may include an EGR cooler for cooling the EGR gas, which is placed in the EGR passage 171.

[0023] The ECU 20 controls the entire vehicle 1. The ECU 20 corresponds to the control unit of the internal combustion engine. The ECU 20 has a control unit 21. The control unit 21 includes, for example, a CPU, ROM, and RAM. The ROM pre-stores programs and predetermined information for controlling the engine 11, intake system 12, exhaust system 13, intake VVT ​​device 15, exhaust VVT device 16, and exhaust recirculation device 17. RAM is a work memory that temporarily stores programs and data. The CPU reads the programs stored in ROM, loads them into RAM, and executes them to control the engine 11, intake system 12, exhaust system 13, intake VVT ​​device 15, exhaust VVT device 16, and exhaust recirculation device 17.

[0024] The sensor unit 30 detects the driving conditions of the vehicle 1 and transmits the detected results to the ECU 20. The sensor unit 30 includes an accelerator pedal position sensor 31, a crank angle sensor 32, a throttle position sensor 33, and the like. The accelerator pedal position sensor 31 detects the amount of accelerator pedal operation performed by the driver of vehicle 1. The information of the operation amount detected by the accelerator pedal position sensor 31 is transmitted to the ECU 20. The crank angle sensor 32 detects the angle of the crankshaft 116. The angle information detected by the crankshaft 116 is transmitted to the ECU 20. The throttle position sensor 33 detects the opening degree of the throttle valve 124. The information of the opening degree detected by the throttle position sensor 33 is transmitted to the ECU 20. Furthermore, the sensor unit 30 detects information necessary for the ECU 20 to control the internal combustion engine 10 and transmits the detected information to the ECU 20.

[0025] In this embodiment, in the internal combustion engine 10 configured as described above, the control unit 21 controls the intake VVT ​​device 15 and the exhaust VVT device 16 to prevent misfires caused by an increase in the proportion of EGR gas in the combustion chamber due to EGR gas remaining in the intake passage 121 and the EGR passage 171. The specific operations of the control unit 21 will be explained below with reference to the flowchart in Figure 2. The flowchart in Figure 2 is executed repeatedly at regular intervals after the engine 11 is started. The flowchart in Figure 2 is realized when the CPU of the control unit 21 reads the program stored in ROM, loads it into RAM, and executes it. Here, it is assumed that the vehicle 1 is moving because the engine 11 is running.

[0026] In S10, the control unit 21 determines whether or not EGR gas is being recirculated into the intake passage 121. Specifically, the control unit 21 can determine that EGR gas is being recirculated into the intake passage 121 if the EGR valve 172 is open. If EGR gas is being recirculated, the process proceeds to S11; otherwise, it proceeds to S15.

[0027] In S11, the control unit 21 determines whether deceleration has started in response to the deceleration request. The driver requests deceleration from the vehicle 1 by releasing or loosening the accelerator pedal. The control unit 21 receives the deceleration request based on the accelerator pedal opening information detected by the accelerator pedal opening sensor 31 and controls the throttle valve 124 and injector 141 to decelerate the vehicle 1. First, the control unit 21 controls the throttle valve 124 and EGR valve 172 to close when a deceleration request is made in order to prevent misfires in the combustion chamber 112. Closing the EGR valve 172 prevents EGR gas from flowing into the EGR passage 171. The control unit 21 may determine whether deceleration has started by receiving information from a sensor that detects the rotation speed of the vehicle 1's tires, or it may determine that deceleration has started by receiving a deceleration request. If deceleration has started, the process proceeds to S12; otherwise, the process proceeds to S15.

[0028] In S12, the control unit 21 determines whether the misfire limit is exceeded. Specifically, the control unit 21 determines whether (misfire limit EGR rate - estimated EGR rate) is less than a threshold. Here, the EGR rate is the proportion of EGR gas flowing into the combustion chamber 112, i.e., the amount of EGR gas / (amount of intake air + amount of EGR gas). The estimated EGR rate is the estimated EGR rate at the present time. The misfire limit EGR rate is the EGR rate considered to be the limit of misfire. The estimated EGR rate and the misfire limit EGR rate are calculated in advance based on design values ​​and test results, taking into account the operating conditions (engine speed, engine load, etc.), and the control unit 21 stores these values ​​in a table. Therefore, the control unit 21 can obtain the estimated EGR rate and the misfire limit EGR rate at the present time by extracting them from the table based on the current operating conditions. The threshold is a predetermined value. If it is determined that the misfire limit has been exceeded, specifically if (misfire limit EGR rate - estimated EGR rate) is smaller than the threshold, the process proceeds to S13; otherwise, it proceeds to S15.

[0029] In S13, the control unit 21 controls the intake VVT ​​device 15 so that the opening and closing timing of the intake valve 118 is retarded compared to the normal control in S15, which will be described later, according to the misfire limit EGR rate. In this embodiment, the control unit 21 controls the closing timing of the intake valve 118 to be closer to the intake bottom dead center than in the normal control.

[0030] Figure 3 shows the opening and closing timing of the intake and exhaust valves. In Figure 3, the horizontal axis represents the stroke of the piston during its reciprocating motion, and the vertical axis represents the valve lift. On the horizontal axis, the top dead center is the exhaust top dead center of the piston, and the bottom dead center is the intake bottom dead center of the piston. In Figure 3, the opening and closing timing 41a of the intake valve under normal conditions (normal control) is shown by a solid line, and the opening and closing timing 41b under control by S13 is shown by a dashed line. Here, the valve closing time IVCa under normal conditions (opening and closing timing 41a) is before the intake bottom dead center, whereas the valve closing time IVCb under controlled conditions (opening and closing timing 41b) is at the intake bottom dead center.

[0031] As in the S13, by bringing the closing timing of the intake valve 118 closer to the intake bottom dead center than under normal control, the effective compression ratio can be increased and misfires can be prevented. Furthermore, by bringing the closing timing of the intake valve 118 closer to the intake bottom dead center than under normal control, the amount of fresh air (amount of intake air) increases, which reduces the EGR rate and thus improves combustion stability. In Figure 3, the closing timing IVCb of the intake valve 118 is set to intake bottom dead center. On the other hand, if the closing timing of the intake valve 118 is advanced beyond intake bottom dead center, the intake valve 118 closes in the middle of the intake stroke, resulting in a decrease in the amount of fresh air compared to when the intake valve 118 is closed at intake bottom dead center. Also, if the intake valve 118 is retarded beyond intake bottom dead center, the fresh air in the combustion chamber 112 is pushed back into the intake passage 121 during the compression stroke, resulting in a decrease in the amount of fresh air compared to when the intake valve 118 is closed at intake bottom dead center. Therefore, the effect of improving combustion stability can be maximized by setting the closing timing of the intake valve 118 to intake bottom dead center. However, if the estimated EGR rate has a margin over the misfire limit EGR rate, the control unit 21 may control the intake valve 118 to approach intake bottom dead center rather than setting it to intake bottom dead center.

[0032] In S14, the control unit 21 controls the exhaust VVT device 16 so that the opening and closing timing of the exhaust valve 119 is delayed compared to the normal control in S15, which will be described later, according to the misfire limit EGR rate. In this embodiment, the control unit 21 controls the closing timing of the exhaust valve 119 to be delayed compared to the normal control in order to increase the overlap amount.

[0033] In Figure 3, the opening and closing timing 42a of the exhaust valve under normal conditions (normal control) is shown by a solid line, and the opening and closing timing 42b under control by S14 is shown by a dashed line. Here, the closing timing EVCa at the normal opening and closing timing 42a of the exhaust valve is at exhaust top dead center, whereas the closing timing EVCb at the controlled opening and closing timing 42b is beyond exhaust top dead center, and the overlap amount between the intake valve and the exhaust valve is increased compared to normal control. In other words, as shown in Figure 3, the amount of angle retardation of the exhaust valve under control by S14 is greater than the amount of angle retardation of the intake valve under control by S13.

[0034] As in S14, by retarding the closing timing of the exhaust valve 119 and increasing the overlap amount, exhaust gas is blown back into the intake passage 121 during the overlap period, increasing the amount of so-called internal EGR gas in the combustion chamber 112. Also, as the exhaust gas is blown back into the intake passage 121, the pressure inside the intake passage 121 increases and the negative pressure decreases. This makes it possible to reduce the amount of external EGR flowing into the combustion chamber 112. By reducing the amount of external EGR at low temperatures and increasing the amount of internal EGR at high temperatures, the in-cylinder temperature increases and combustion can be stabilized. In addition, as the high-temperature exhaust gas is blown back into the intake passage 121, the temperature inside the intake passage 121 increases, the saturated water vapor partial pressure increases, and the amount of condensate can be reduced. By reducing the amount of condensate, misfires can be prevented.

[0035] On the other hand, in S15, the control unit 21, since the possibility of misfire is low at this point, omits the processes in S13 and S14 described above and performs normal control. Specifically, the control unit 21 controls the intake valve 118 to open and close at the opening and closing timing 41a shown by the solid line in Figure 3, and controls the exhaust valve 119 to open and close at the opening and closing timing 42a shown by the solid line in Figure 3. After the processes in S14 and S15 are completed, the system returns to S10 and repeats the process described above.

[0036] Figure 4 shows various timing charts. Figure 4(a) shows the throttle valve opening. Figure 4(b) shows the EGR valve opening. Figure 4(c) shows the opening and closing timing of the intake valve and exhaust valve. Figure 4(d) shows the overlap amount. Figure 4(e) shows the estimated EGR rate. Figure 4(f) shows the misfire limit EGR rate. In addition, the solid lines in Figures 4(c) to 4(f) show the timing chart when the control is left as normal (comparative example), and the dashed lines show the timing chart when the opening and closing timing of the intake valve and exhaust valve is controlled as described above (example).

[0037] In Figure 4, as deceleration begins at time T, the throttle valve shown in Figure 4(a) transitions from an open state to a closed state, and the EGR valve shown in Figure 4(b) also transitions from an open state to a closed state. In Figure 4(c), at time T, as shown by the dashed line, the intake valve closing timing is controlled to be at intake bottom dead center, and the exhaust valve closing timing is retarded. Consequently, in Figure 4(d), as shown by the dashed line, the overlap amount between the intake valve and the exhaust valve increases. Subsequently, as shown in Figures 4(c) and 4(d), the intake valve closing timing and the exhaust valve closing timing are gradually reversed according to the misfire limit EGR rate.

[0038] This control method allows the estimated EGR rate of the embodiment shown by the dashed line in Figure 4(e) to be reduced compared to the estimated EGR rate of the comparative example shown by the solid line. Furthermore, as shown in Figure 4(f), the misfire limit EGR rate of the embodiment shown by the dashed line can be maintained higher than the misfire limit EGR rate of the comparative example shown by the solid line.

[0039] As described above, the control unit 21 of this embodiment closes the throttle valve 124 and the EGR valve 172 when the vehicle 1 requests deceleration, and controls the closing timing of the intake valve 118 to approach the intake bottom dead center and retard the closing timing of the exhaust valve 119 to increase the overlap amount when the vehicle 1 decelerates. By bringing the closing timing of the intake valve 118 closer to the intake bottom dead center, the actual compression ratio can be increased to prevent misfires, and by retarding the closing timing of the exhaust valve 119 to increase the overlap amount, the amount of condensate can be reduced to prevent misfires. In this way, by controlling the opening and closing timing of the intake valve 118 and the exhaust valve 119, misfires can be prevented without using a variable compression ratio mechanism, which has a complex structure and is costly. Furthermore, since there is no need to lower the target EGR rate during steady-state operation to account for combustion instability during deceleration, fuel efficiency can be improved.

[0040] In this embodiment, the control unit 21 changes the amount of change in the closing timing of the intake valve 118 and the exhaust valve 119 according to the misfire limit, that is, according to the misfire limit EGR rate. For example, if there is sufficient margin in the misfire limit, the control unit 21 can control the closing timing of the intake valve 118 and the exhaust valve 119 to return to the normal control side. In this way, by changing the amount of change in the closing timing of the intake valve 118 and the exhaust valve 119 according to the misfire limit, misfire prevention can be appropriately performed.

[0041] In this embodiment, the control unit 21 controls the closing timing of the intake valve 118 to intake bottom dead center and retards the closing timing of the exhaust valve 119 to increase the overlap amount when the vehicle 1 is decelerating. If the intake valve 118 is advanced beyond intake bottom dead center, the intake valve 118 will be closed in the middle of the intake stroke, reducing the amount of fresh air flowing into the combustion chamber 112. Conversely, if the intake valve 118 is retarded beyond intake bottom dead center, the amount of fresh air in the combustion chamber 112 will decrease due to the pushback when the air-fuel mixture is compressed. As in this embodiment, by setting the closing timing of the intake valve 118 to intake bottom dead center, the amount of fresh air in the combustion chamber 112 can be increased.

[0042] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and modifications can be made within the scope of the present invention. In the embodiment described above, the case in which the control unit 21 controls the opening and closing timing of the intake valve 118 and the exhaust valve 119 was explained. However, the opening and closing timing of either the intake valve 118 or the exhaust valve 119 may be controlled to prevent misfires. [Explanation of Symbols]

[0043] 1: Vehicle 10: Internal combustion engine 15: Intake VVT ​​system 16: Exhaust VVT system 17: Exhaust gas recirculation system (EGR system) 20: ECU (control unit) 21: Control unit 112: Combustion chamber 118: Intake valve 119: Exhaust valve 121: Intake passage 124: Throttle valve 131: Exhaust passage 171: EGR passage 172: EGR valve

Claims

[Claim 1] A catalyst that purifies exhaust gases emitted from the combustion chamber of an internal combustion engine, An EGR passage connecting the exhaust passage and the intake passage of the aforementioned internal combustion engine, An EGR valve that adjusts the flow rate of EGR gas flowing through the aforementioned EGR passage, A control device for an internal combustion engine, comprising a throttle valve provided in the intake passage for adjusting the amount of intake air flowing into the combustion chamber, The vehicle includes a control unit that closes the throttle valve and the EGR valve when the vehicle decelerates, and when it is determined that the flow of EGR gas into the combustion chamber at the start of vehicle deceleration exceeds the misfire limit, controls the closing timing of the intake valve to the intake bottom dead center and retards the closing timing of the exhaust valve to increase the overlap amount. The control unit is characterized by controlling the intake valve to close at intake bottom dead center, retarding the closing timing of the exhaust valve to increase the overlap amount, thereby causing the misfire limit EGR rate to start to decrease, and then gradually returning the closing timing of the intake valve and the exhaust valve in accordance with the misfire limit EGR rate.

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