INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2018-232238 filed on Dec. 12, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a hybrid vehicle including a clutch disposed between an engine and a motor.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 2011-213310 (JP 2011-213310 A) discloses a hybrid vehicle including a motor provided in a power transmission path from an engine to wheels, and a clutch provided between the engine and the motor in the power transmission path. The hybrid vehicle disclosed in JP 2011-213310 A is configured to switch between EV travel in which the power of the motor is transmitted to the wheels to run the vehicle in the state where the clutch is released and the engine is stopped, and engine travel in which the power of the engine is transmitted to the wheels to run the vehicle in the state where the clutch is engaged.

SUMMARY

When the clutch is released fast in the engine travel, the engine speed increases rapidly or a torque shock occurs so that the drivability is deteriorated. Therefore, it is desirable that the release of the clutch when switching from the engine travel to the EV travel be performed moderately while gradually decreasing an engagement force of the clutch. On the other hand, in order to quickly shift from the engine travel to the EV travel, the release of the clutch should be performed in a short time.

In the meantime, the gradual decrease of the engagement force when releasing the clutch as described above can be achieved by employing, as a control valve of the clutch oil pressure, not an on-off switching valve that switches between supplying the oil pressure and stopping the supply of the oil pressure in a binary manner, but a pressure regulating valve such as a linear solenoid valve that can regulate the oil pressure. However, the operation of the pressure regulating valve requires greater electric power than in the case of the on-off switching valve. Therefore, when the clutch oil pressure is controlled solely by the pressure regulating valve, the electricity consumption becomes large.

An aspect of the present disclosure relates to a hybrid vehicle including an engine, a motor, a clutch, an oil pressure generator, a pressure regulating valve, a first oil passage, a second oil passage, a switching valve, and an electronic control unit. The motor is provided in a power transmission path from the engine to a wheel. The clutch is provided between the engine and the motor in the power transmission path. The oil pressure generator is configured to generate a clutch engagement oil pressure. The pressure regulating valve is configured to apply pressure regulation to the clutch engagement oil pressure according to an electric power supply amount and output an oil pressure regulated, and configured to stop output of the oil pressure in response to a stoppage of electric power supply. The first oil passage serves as an oil passage for supplying the clutch engagement oil pressure generated by the oil pressure generator to the clutch. The second oil passage serves as an oil passage for supplying the oil pressure output by the pressure regulating valve to the clutch. The switching valve is configured to switch between a first state to connect the first oil passage to the clutch and interrupt connection of the second oil passage to the clutch, and a second state to connect the second oil passage to the clutch and interrupt connection of the first oil passage to the clutch. The electronic control unit is configured to perform travel control of the hybrid vehicle by selecting one travel mode from a plurality of travel modes including an engine travel mode in which the hybrid vehicle travels by transmitting power of the engine to the wheel, and an EV travel mode in which the hybrid vehicle travels by power of the motor in a state where the engine is stopped. The electronic control unit is configured to switch the travel mode to the EV travel mode when a predetermined engine stop condition is established while the engine travel mode is selected. The electronic control unit is configured to, while the EV travel mode is selected, maintain a released state of the clutch by setting the switching valve to the second state in a state where electric power supply to the pressure regulating valve is stopped, and configured to, while the engine travel mode is selected, maintain an engaged state of the clutch by setting the switching valve to the first state in a state where electric power supply to the pressure regulating valve is stopped. The electronic control unit is configured to determine whether or not, although the engine stop condition is currently not established, a possibility is high that the engine stop condition will be established afterwards. The electronic control unit is configured to perform filling control to supply electric power to the pressure regulating valve to increase the oil pressure in the second oil passage in a state where the switching valve is set to the first state. The electronic control unit is configured to perform clutch release control to release the clutch while performing oil pressure control by the pressure regulating valve in a state where the switching valve is set to the second state. The electronic control unit is configured to perform switching from the engine travel mode to the EV travel mode through a first stage to start the filling control when it is determined that the possibility is high that the engine stop condition will be established afterwards while the engine travel mode is selected, a second stage to start the clutch release control by switching the switching valve to the second state in response to establishment of the engine stop condition, and a third stage to stop operation of the engine after completion of the clutch release control.

With the hybrid vehicle according to the aspect of the present disclosure, when the switching valve is set to the first state to connect the clutch to the first oil passage in the state where the electric power supply to the pressure regulating valve is stopped, the clutch engagement oil pressure is supplied to the clutch through the first oil passage. On the other hand, when the switching valve is set to the second state to connect the clutch to the second oil passage in the state where the electric power supply to the pressure regulating valve is stopped, the oil pressure supply to the clutch is stopped. Consequently, in the hybrid vehicle described above, maintaining the engagement of the clutch while the engine travel mode is selected and maintaining the release of the clutch while the EV travel mode is selected are both performed in the state where the electric power supply to the pressure regulating valve is stopped. On the other hand, when the electric power supply to the pressure regulating valve is performed in the state where the switching valve is set to the second state to connect the clutch to the second oil passage, the oil pressure regulated by the pressure regulating valve is supplied to the clutch. Therefore, although the control of the supply oil pressure to the clutch (hereinafter referred to as a “clutch oil pressure”) by the pressure regulating valve is performed at the time of switching transition of the travel mode, it is possible to stop the electric power supply to the pressure regulating valve, of which operating electric power is greater than that of the switching valve, while the engine travel mode or the EV travel mode is selected, and therefore, the electricity consumption can be suppressed.

In the hybrid vehicle thus configured, the clutch engagement oil pressure is supplied to the clutch through the first oil passage while the engine travel mode is selected. On the other hand, in this state, the electric power supply to the pressure regulating valve is stopped so that the second oil passage is in a state where the oil pressure is lost. Even by starting the electric power supply to the pressure regulating valve in this state, the oil pressure in the second oil passage is not increased immediately. Therefore, when, in order to control the clutch oil pressure at the time of switching from the engine travel mode to the EV travel mode in response to establishment of the engine stop condition, the electric power supply to the pressure regulating valve is started and the switching valve is switched to the second state to connect the second oil passage to the clutch, the clutch oil pressure rapidly decreases immediately thereafter. As a result, the engagement force of the clutch decreases and therefore the load of the engine decreases, leading to a rapid increase in the engine speed, and further, since the power of the engine cannot be fully transmitted to the wheel due to the decrease in the engagement force, there is a possibility of the occurrence of a torque shock.

In this regard, in the hybrid vehicle described above, the establishment of the engine stop condition is predicted in advance. When it is determined that the possibility is high that the engine stop condition will be established afterwards, the filling control to supply the electric power to the pressure regulating valve is started while maintaining the switching valve in the first state. During the filling control, the oil pressure is introduced into the second oil passage in the state where the clutch engagement oil pressure is supplied to the clutch through the first oil passage. Therefore, when the engine stop condition is established afterwards, the second oil passage is in a pressure-increased state so that even when the second oil passage is connected to the clutch in response to the establishment of the engine stop condition, the clutch oil pressure does not decrease rapidly. Note that since the filling control in this event is performed before the establishment of the engine stop condition, the implementation of the filling control does not cause a delay in the completion of switching to the EV travel mode.

In the case where the engine stop condition is established in a short time after it is determined that the possibility is high that the engine stop condition will be established afterwards, there are cases where, even by starting the filling control when it is determined that the possibility is high that the engine stop condition will be established afterwards, the second oil passage cannot be fully increased in pressure until the establishment of the engine stop condition. In such a case, when the second oil passage is connected to the clutch in response to the establishment of the engine stop condition, there are cases where it is not possible to fully suppress a decrease in the clutch oil pressure. In view of this, the electronic control unit in the hybrid vehicle described above may be configured to, when the engine stop condition is established before the pressure increase of the second oil passage by the filling control is completed, delay the start of the clutch release control in the second stage until the pressure increase of the second oil passage is completed. Consequently, the second oil passage is connected to the clutch after the completion of the pressure increase even in such a case.

In the hybrid vehicle according to the aspect of the present disclosure, the electronic control unit may be configured to, when the clutch release control is started in the second stage, start torque replacement control to increase motor torque while decreasing shaft torque of the engine. With the hybrid vehicle thus configured, the motor torque is increased in parallel to decreasing the shaft torque of the engine toward the operation stop so that the driving force of the vehicle can be maintained even during switching of the travel mode.

Since it is difficult to perfectly predict whether or not the possibility is high that the engine stop condition will be established afterwards, there are cases where, according to the situation, the engine stop condition is established before the prediction is made. Therefore, in the hybrid vehicle according to the aspect of the present disclosure, the electronic control unit may be configured to, when the engine stop condition is established in a state where it is not determined that the possibility is high that the engine stop condition will be established afterwards while the engine travel mode is selected, perform switching from the engine travel mode to the EV travel mode through a fourth stage to start the filling control in response to establishment of the engine stop condition, a fifth stage to start the clutch release control by switching the switching valve to the second state in response to completion of the pressure increase of the second oil passage by the filling control, and a sixth stage to stop operation of the engine after completion of the clutch release control. With the hybrid vehicle thus configured, even when the establishment of the engine stop condition cannot be predicted in advance, it is possible to suppress a rapid decrease in the clutch oil pressure when the clutch is connected to the second oil passage for controlling the clutch oil pressure by the pressure regulating valve.

In the hybrid vehicle according to the aspect of the present disclosure, the electronic control unit may be configured to, when a predetermined engine start condition is established while the EV travel mode is selected, perform start control to start the engine in a state where the switching valve is set to the second state and electric power is supplied to the pressure regulating valve. While performing the start control in such a case, the oil pressure is supplied from the pressure regulating valve to the second oil passage. Therefore, the electronic control unit when performing such start control may be configured to, when the engine stop condition is established while performing the start control, start the clutch release control in response to the establishment of the engine stop condition.

