Method and device for controlling internal combustion engine of hybrid vehicle

Abstract

According to the present invention, a series hybrid vehicle comprises: a power-generation motor generator (1) driven by an internal combustion engine (2); a travel motor generator (4); and a battery (5). Non-power generation operation is performed when the vehicle is in a low water temperature state, in which combustion operation of the internal combustion engine (2) as a heat source is required, and the SOC of the battery (5) is higher than a charging prohibition level (L1). The non-power generation operation is performed by retarding the ignition timing while limiting the amount of air intake to a small amount so that the shaft torque applied to the motor generator (1) approaches 0, and making valve overlap smaller than in a normal power generation mode by means of the advance angle of an exhaust-side variable valve timing mechanism.

Description

Control Method and Control Device for Internal Combustion Engine of Hybrid Vehicle

[0001] This invention relates to the control of an internal combustion engine in a hybrid vehicle, particularly to the control when the SOC of the battery is close to full charge.

[0002] In a hybrid vehicle that drives a generator by the output of an internal combustion engine and stores the generated electric power in a battery, if the battery becomes overcharged, the battery will deteriorate, which is not preferable.

[0003] Patent Document 1 discloses that in a series hybrid vehicle, when the SOC of the battery becomes a predetermined level or higher, in order to avoid overcharging, the combustion operation of the internal combustion engine is stopped. Further, as a second embodiment, the combustion of some cylinders is stopped, and the remaining cylinders are operated near the misfire limit by significantly reducing the substantial intake air amount by so-called late closing control of the intake valve and setting the fuel injection amount to a very small amount.

[0004] However, in the former method of completely stopping the internal combustion engine, for example, when the internal combustion engine is in a cold state, the heating in the vehicle interior cannot be performed, and the warming of the catalyst cannot be performed, which causes problems.

[0005] On the other hand, in the latter method of limiting the intake air amount by late closing control of the intake valve and operating near the misfire limit, for example, if the fuel injection amount is reduced until the substantial power generation amount becomes 0, the combustion becomes unstable and misfire is likely to occur.

[0006] Japanese Patent Application Laid-Open No. 11-141364

[0007] This invention relates to a control method for an internal combustion engine in a hybrid vehicle, comprising an internal combustion engine, a generator driven by the output of the internal combustion engine, a traction motor, and a battery that supplies power to the traction motor, wherein the internal combustion engine is equipped with a variable valve timing mechanism that changes the valve opening timing and valve closing timing of at least one of the exhaust valve and the intake valve. In this method, under conditions where combustion operation of the internal combustion engine as a heat source is required, and when the state of charge of the battery is higher than a predetermined charge prohibition level close to full charge, the ignition timing is retarded while limiting the intake air amount to a small amount so that the shaft torque supplied to the generator approaches zero, and furthermore, the variable valve timing mechanism is controlled so that the valve overlap is reduced compared to normal power generation operation.

[0008] By maintaining the shaft torque supplied to the generator at near zero while continuing combustion operation of the internal combustion engine, it is possible to avoid an increase in the battery's state of charge (SOC) and to utilize the internal combustion engine as a heat source. For example, the internal combustion engine can be used as a heat source to heat the vehicle interior or warm up the catalytic converter.

[0009] Torque limiting through ignition timing retardation significantly reduces the generated torque even with relatively large intake and fuel injection volumes, allowing for a state where shaft torque is zero, in balance with friction and other factors. At this time, the variable valve timing mechanism reduces valve overlap, thus reducing internal EGR and stabilizing combustion with ignition timing retardation. Consequently, it is possible to maintain the shaft torque supplied to the generator at near zero while avoiding misfires.

[0010] Diagram illustrating the configuration of a series hybrid vehicle according to one embodiment. Time chart showing the operation of one embodiment. Valve timing charts for the intake valve and exhaust valve during (a) normal power generation operation and (b) non-power generation operation.

[0011] Hereinafter, an embodiment of this invention will be described in detail with reference to the drawings. Figure 1 schematically shows the configuration of a series hybrid vehicle as an example of a hybrid vehicle to which this invention is applied.

[0012] A series hybrid vehicle comprises a power generation motor generator 1 that primarily operates as a generator, an internal combustion engine 2 used as a power generation internal combustion engine to drive the power generation motor generator 1 in response to power generation demands, a drive motor generator 4 that primarily operates as a motor to drive the drive wheels 3, and a battery 5 consisting of a secondary battery such as a lithium-ion battery that temporarily stores the generated electricity. In one embodiment, the power generation motor generator 1 is driven by the internal combustion engine 2 via a gear train 10. The electricity obtained by the internal combustion engine 2 driving the power generation motor generator 1 is stored in the battery 5 via an inverter device (not shown). The drive motor generator 4 is driven and controlled using the power from the battery 5. The electricity generated during regeneration by the drive motor generator 4 is also stored in the battery 5 via an inverter device (not shown).

[0013] The operation of the motor generators 1 and 4, the charging and discharging of the battery 5, and the operation of the internal combustion engine 2 are controlled by the controller 6. The controller 6 consists of multiple controllers connected to each other so that they can communicate with one another, including a motor controller 7 that controls the motor generators 1 and 4, an engine controller 8 that controls the internal combustion engine 2, and a battery controller 9 that manages the battery 5. Information such as the opening of the accelerator pedal (not shown) and vehicle speed is input to the controller 6. The battery controller 9 also determines the State of Charge (SOC) of the battery 5 based on the voltage and current of the battery 5. The vehicle is equipped with an air conditioning system capable of both cooling and heating.

[0014] The driving modes of such a series hybrid vehicle include an EV driving mode in which the vehicle runs on the power of the battery 5 without combustion operation (i.e., power generation or charging) of the internal combustion engine 2, and an HEV driving mode in which the vehicle runs while generating power through combustion operation of the internal combustion engine 2. The battery controller 9 manages the charging and discharging of the battery 5 so that the State of Charge (SOC) of the battery 5 is maintained between a predetermined upper SOC target value and a lower SOC target value. For example, if the SOC decreases due to EV driving and falls below the lower SOC target value, the internal combustion engine 2 is started via the engine controller 8 and power generation is performed. This power generation by the internal combustion engine 2 ends, for example, when the SOC approaches the upper SOC target value. During this power generation, the internal combustion engine 2 is usually operated at a specific operating point (combination of torque and rotational speed) that provides the best fuel efficiency. This is called the normal power generation mode.

[0015] Furthermore, when the vehicle's required driving force is high, the power supplied from the battery 5 is insufficient to meet the vehicle's driving force requirements. In such cases, the vehicle enters HEV driving mode, and power is generated by the internal combustion engine 2. To increase the power output, the operation of the internal combustion engine 2 at a rotational speed higher than the optimal fuel efficiency point may be permitted.

[0016] The internal combustion engine 2 is a four-stroke, spark-ignition internal combustion engine, also known as a gasoline engine. As shown in Figure 1, the crankshaft of the internal combustion engine 2 is always connected to the rotating shaft of the power generation motor generator 1, and the two rotate together. In Figure 1, the two are connected via a gear train 10, but the crankshaft of the internal combustion engine 2 and the rotating shaft of the power generation motor generator 1 may be directly connected. The operation of the internal combustion engine 2, including starting and stopping, is controlled by the engine controller 8. The engine controller 8 is connected to various sensors that are generally necessary for controlling the internal combustion engine 2 (air flow meter, crank angle sensor, collector pressure sensor, air-fuel ratio sensor, coolant temperature sensor, atmospheric pressure sensor, outside temperature sensor, etc.). Based on these detection signals, the engine controller 8 optimally controls the fuel injection timing, fuel injection amount, ignition timing, EGR rate, etc.

[0017] The internal combustion engine 2 is equipped with a variable valve timing mechanism in at least one of the exhaust valve and intake valve to variably control the so-called valve overlap, where the exhaust valve opening period and the intake valve opening period overlap. In one preferred embodiment, the exhaust valve side is equipped with an exhaust-side variable valve timing mechanism that changes the exhaust valve opening time (EVO) and the exhaust valve closing time (EVC). The variable valve timing mechanism in one embodiment is a general hydraulic variable valve timing mechanism in which the relationship between the phase of the crankshaft and the phase of the exhaust camshaft is delayed by a rotary hydraulic actuator, so that the opening time (EVO) and the closing time (EVC) are delayed by the same angle. The intake valve is not equipped with a variable valve timing mechanism, and the intake valve opening time (IVO) and intake valve closing time (IVC) are fixed valve timings.

[0018] In normal power generation mode, as described above, the internal combustion engine 2 is operated at a specific operating point that provides the best fuel efficiency. At this time, the exhaust side variable valve timing mechanism is controlled to the retarded position as shown in the valve timing chart in Figure 3(a). The exhaust valve closing time (EVC) is after top dead center (TDC), and the intake valve opening time (IVO) is before top dead center (TDC). The valve overlap between the two is at its largest. Consequently, a relatively large internal EGR (exhaust gas recirculation due to exhaust gases remaining in the cylinder) occurs, improving the fuel efficiency of the internal combustion engine 2.

[0019] Next, the non-power generation operation of the internal combustion engine 2, which is the core part of the present invention, will be described. As mentioned above, overcharging of the battery 5 is undesirable because it is a factor in the deterioration of the battery 5. For example, on a long downhill slope, the regenerative operation of the traction motor generator 4 may cause the State of Charge (SOC) of the battery 5 to rise above a predetermined charging prohibition level. Alternatively, the charging prohibition level may be reached during power generation by the operation of the internal combustion engine 2. The charging prohibition level is generally set higher than the above-mentioned upper limit target value of SOC that stops the normal power generation mode, for example, to a level close to 100%. The normal upper limit target value of SOC may also be used as the charging prohibition level. In this way, when the SOC is higher than the charging prohibition level, the operation of the internal combustion engine 2 (i.e., power generation) is basically prohibited. However, under predetermined conditions where combustion operation of the internal combustion engine 2 as a heat source is necessary, non-power generation operation is performed, which involves combustion operation while avoiding substantial power generation (i.e., charging of the battery 5). For example, non-power generation operation is required when the outside temperature is low and the vehicle needs heating, or when the internal combustion engine 2 is in a cold state and the catalyst in the exhaust system of the internal combustion engine 2 needs to be warmed up. In one preferred embodiment, when the coolant temperature of the internal combustion engine 2 is below a predetermined temperature (for example, set to about 10°C), it is determined that combustion operation of the internal combustion engine 2 as a heat source is necessary.

[0020] Non-power generation operation is achieved by combining three methods: limiting the intake volume by reducing the throttle valve opening, retarding the ignition timing, and reducing valve overlap by using a variable valve timing mechanism on the exhaust side. The intake volume is limited to a level that provides a relatively large margin over the misfire limit. The ignition timing is then retarded so that the shaft torque supplied by the internal combustion engine 2 to the power generation motor generator 1 becomes virtually zero. In other words, by retarding the ignition timing from the basic ignition timing (which is essentially set along the MBT point) that is set in advance for each operating point of the internal combustion engine 2, the torque generated under the same intake volume and fuel volume is reduced, so that the shaft torque output to the power generation motor generator 1 becomes zero.

[0021] Furthermore, as shown in the valve timing chart in Figure 3(b), the exhaust-side variable valve timing mechanism is simultaneously controlled to the advance side (for example, the most advanced position). This reduces valve overlap and internal EGR, thereby stabilizing combustion and suppressing misfires. In other words, if the torque generated by combustion under conditions of ignition timing retardation is set to a level that balances with friction, etc., the shaft torque supplied to the power generation motor generator 1 will be zero. However, when comparing the magnitude of the torque, the larger the internal EGR, the greater the combustion variation from cycle to cycle (for example, σPI), and the more likely misfires are to occur. Conversely, reducing internal EGR reduces the combustion variation from cycle to cycle, making misfires less likely.

[0022] Figure 2 is a time chart showing the operation of each part when transitioning from normal power generation mode to non-power generation operation. From top to bottom, the figures show: (a) rotational speed of the internal combustion engine 2 (which is also effectively the rotational speed of the power generation motor generator 1), (b) SOC of the battery 5, (c) on / off status of the charge prohibition flag indicating that charging is prohibited, (d) operating angle of the exhaust side variable valve timing mechanism (upper part of the figure is the advance side), (e) ignition timing (upper part of the figure is the advance side), (f) shaft torque of the internal combustion engine 2, and (g) σPI (combustion variation). The temperature state of the internal combustion engine 2 is not shown, but the internal combustion engine 2 is in the predetermined low-temperature state described above.

[0023] In the example shown in Figure 2, for the sake of simplicity of explanation, it is shown that the State of Charge (SOC) of the battery 5 reaches the charging prohibition level L1 due to power generation by the operation of the internal combustion engine 2, and as a result, the system switches to non-power generation operation. In other words, at time t1, the SOC exceeds the charging prohibition level L1, causing the system to switch to non-power generation operation. Simultaneously, the throttle valve opening (not shown) decreases, the exhaust side variable valve timing mechanism advances, and ignition timing retardation is performed. The ignition timing retardation is performed so that the shaft torque becomes zero. By reducing the valve overlap and decreasing internal EGR in this way, torque reduction is performed by retarding the ignition timing, misfires are suppressed, and a stable rotational speed can be maintained.

[0024] As shown in Figure 3(b), the exhaust valve opening timing (EVC), which is the advanced position for non-power generation operation, is after top dead center (TDC), and a moderate predetermined amount of valve overlap still exists. If the valve overlap is too small and the residual gas becomes excessively low, the effective intake air volume will increase, and the shaft torque supplied to the power generation motor generator 1 will increase. In other words, it becomes difficult to perform non-power generation operation with zero shaft torque.

[0025] Therefore, the exhaust valve closing timing (EVC) during ignition timing advance is set to a residual exhaust volume that provides sufficient combustion stability to prevent misfires when the ignition timing is retarded until the shaft torque becomes zero.

[0026] The restart of the internal combustion engine 2 when transitioning from EV mode to HEV mode is performed with the exhaust-side variable valve timing mechanism controlled to the retarded side, that is, with a large valve overlap. In one preferred embodiment, the restart is performed by motorizing the internal combustion engine 2 at a relatively high rotational speed using the power generation motor generator 1 connected to the internal combustion engine 2. By motoring at a relatively high rotational speed in this way, it becomes possible to restart while the exhaust-side variable valve timing mechanism is in the retarded state. Therefore, after restarting, the normal power generation mode can be started without moving the exhaust-side variable valve timing mechanism.

[0027] Furthermore, when stopping the internal combustion engine 2 from a state where it is operating in normal power generation mode, it is preferable to stop the internal combustion engine 2 while maintaining the exhaust-side variable valve timing mechanism, which is positioned on the retarded side. For example, under conditions where combustion operation of the internal combustion engine 2 as a heat source is unnecessary, if the State of Charge (SOC) of the battery 5 rises above the charging prohibition level while the internal combustion engine 2 is operating, the combustion operation of the internal combustion engine 2 is stopped while the exhaust valve closing timing (EVC) is controlled to the retarded side. This eliminates the need to operate the exhaust-side variable valve timing mechanism when restarting the engine the next time.

[0028] Conversely, when the internal combustion engine 2 is operating in non-power generation mode with the SOC higher than the charging prohibition level, and for example the coolant temperature of the internal combustion engine 2 rises and non-power generation operation of the internal combustion engine 2 becomes unnecessary, it is desirable to change the exhaust-side variable valve timing mechanism, which is in the advanced position, to the retarded position before stopping the internal combustion engine 2. This eliminates the need to operate the exhaust-side variable valve timing mechanism the next time the engine is restarted.

[0029] Furthermore, the exhaust-side variable valve timing mechanism may be controlled to an appropriate advance position according to the operating conditions of the internal combustion engine 2, even outside of the non-power generation operation described above. For example, when generating power at high output when the required driving force of the vehicle is high, as mentioned above, the valve timing may be controlled to the advance side to increase the filling efficiency in the cylinder.

[0030] Although one embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above embodiment, a variable valve timing mechanism is provided on the exhaust valve side, but it is also possible to have a configuration in which the variable valve timing mechanism is provided on the intake valve side, or a configuration in which both the exhaust valve and the intake valve have variable valve timing mechanisms. Furthermore, although the above embodiment described an example applied to a series hybrid vehicle, the present invention is not necessarily limited to the internal combustion engine for power generation in a series hybrid vehicle, but can also be applied to the internal combustion engine of other types of hybrid vehicles that may have similar problems in avoiding overcharging.

Claims

1. A control method for an internal combustion engine of a hybrid vehicle, comprising an internal combustion engine, a generator driven by the output of the internal combustion engine, a traction motor, and a battery that supplies power to the traction motor, wherein the internal combustion engine is equipped with a variable valve timing mechanism that changes the valve opening timing and valve closing timing of at least one of the exhaust valve and the intake valve, wherein, under conditions where combustion operation of the internal combustion engine as a heat source is required, and when the state of charge of the battery is higher than a predetermined charge prohibition level close to full charge, the ignition timing is retarded while limiting the intake amount to a small amount so that the shaft torque supplied to the generator approaches zero, and further, the variable valve timing mechanism is controlled so that the valve overlap is reduced compared to normal power generation operation.

2. A control method for an internal combustion engine of a hybrid vehicle according to claim 1, wherein the exhaust valve side has a variable valve timing mechanism, and the valve overlap is reduced by advancing the exhaust valve closing timing.

3. A control method for an internal combustion engine of a hybrid vehicle according to claim 2, wherein the exhaust valve closing timing is controlled to be relatively retarded during normal power generation operation.

4. A control method for an internal combustion engine of a hybrid vehicle according to claim 3, wherein, under conditions where combustion operation of the internal combustion engine as a heat source is unnecessary, and when the state of charge (SOC) of the battery becomes higher than a predetermined charge prohibition level close to full charge, the combustion operation of the internal combustion engine is stopped while controlling the exhaust valve closing timing to the retarded side.

5. A control method for an internal combustion engine of a hybrid vehicle according to claim 4, wherein when the starting of the internal combustion engine is requested, the internal combustion engine is motored by the power of the generator to start combustion operation.

6. The intake valve side has a fixed valve timing, a control method for an internal combustion engine of a hybrid vehicle according to claim 2.

7. The control method for an internal combustion engine of a hybrid vehicle according to claim 2, wherein the timing of exhaust valve closure during ignition timing advance is set to be delayed compared to intake top dead center.

8. The control method for an internal combustion engine of a hybrid vehicle according to claim 2, wherein the timing of exhaust valve closure during ignition timing is set to a residual exhaust volume that provides sufficient combustion stability to prevent misfires when the shaft torque becomes zero.

9. The control method for an internal combustion engine of a hybrid vehicle according to claim 7, wherein the intake valve opening timing is fixedly set to advance beyond the intake top dead center.

10. A control method for an internal combustion engine of a hybrid vehicle according to claim 3, wherein when the conditions change from those requiring combustion operation of the internal combustion engine as a heat source to those requiring it, the exhaust valve closing timing is changed to the retarded side used for normal power generation operation, and the combustion operation of the internal combustion engine is stopped.

11. A control device for an internal combustion engine of a hybrid vehicle, comprising an internal combustion engine, a generator driven by the output of the internal combustion engine, a traction motor, and a battery that supplies power to the traction motor, wherein the internal combustion engine is equipped with a variable valve timing mechanism that changes the valve opening timing and valve closing timing of at least one of the exhaust valve and the intake valve, wherein the control device retards the ignition timing while limiting the intake amount to a small amount so that the shaft torque supplied to the generator approaches zero when combustion operation of the internal combustion engine as a heat source is required and the SOC of the battery is higher than a predetermined charge prohibition level close to full charge, and further controls the variable valve timing mechanism so that the valve overlap is reduced compared to normal power generation operation.

Images