CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2020-061531 filed on Mar. 30, 2020, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a control device of a hybrid vehicle including an engine, engine connecting and disconnecting device, electric motor, and automatic transmission, and in particular to control performed when the engine is started during shift control of the automatic transmission.

2. Description of Related Art

In connection with a hybrid vehicle including an engine, an electric motor to which power is transmitted from the engine via an engine connecting and disconnecting device of a frictional engagement type, and an automatic transmission provided on the downstream side of the motor on a power transmission path, a control device having an engine start controller and a shift controller is known. The engine start controller performs engine start control to start the engine during traveling such that the engine can rotate by itself, and fully engage the engine connecting and disconnecting device. The shift controller switches a gear position of the automatic transmission between a plurality of gear positions, according to predetermined shift conditions. One example of the control device is described in Japanese Unexamined Patent Application Publication No. 2014-151907 (JP 2014-151907 A), in which a clutch K0is the engine connecting and disconnecting device. According to an engine starting method described in JP 2014-151907 A, the engine performs ignition and rotates by itself after the engine speed is increased through slipping engagement of the engine connecting and disconnecting device. Also, according to a technology described in JP 2014-151907 A, when the engine is started along with a downshift of the automatic transmission, the downshift of the automatic transmission proceeds after the engine speed is increased to start the engine, and the engine connecting and disconnecting device is fully engaged, so that heat and damage generated due to slipping engagement of the engine connecting and disconnecting device are reduced.

SUMMARY

However, when the engine start control is started by the engine start controller during shift control of the automatic transmission by the shift controller, such as when the accelerator pedal is depressed and the engine is started after shift control of a power-OFF downshift with the accelerator pedal released is started, for example, the engine cannot be started and the engine connecting and disconnecting device cannot be fully engaged, before the gearshift, as in JP 2014-151907 A. In this case, if the shift control for a further downshift is executed according to the operation to depress the accelerator pedal, the motor speed is increased, and a speed difference between the motor speed and the engine speed increases; therefore, it takes a long time to raise the engine speed to the synchronous rotation speed corresponding to the motor speed. As a result, a thermal load due to slipping engagement of the engine connecting and disconnecting device may be increased, and the engine connecting and disconnecting device may be damaged due to heat. In particular, when a skip gearshift for shifting down the automatic transmission while skipping one or more gear positions is performed according to the accelerator operation, the amount of increase of the motor speed is increased, and the above problem becomes more significant. To cope with the problem, it may be considered to set shift conditions so that the gear position as a destination of the downshift is restricted to a relatively high gear position, when the engine is started during a downshift of the automatic transmission. However, it takes time to shift down the automatic transmission to a target gear position, and the driveability may deteriorate.

In this connection, an engine starting method for self-rotating the engine through ignition from a stage of low-speed rotation including a stopped state of the engine is known (see, for example, Japanese Unexamined Patent Application Publication No. 2006-348863 (JP 2006-348863 A)). According to the engine starting method, slipping engagement of the engine connecting and disconnecting device is reduced or not required, and therefore, damage due to a thermal load of the engine connecting and disconnecting device can be prevented. However, when the motor speed is relatively low, for example, shock due to the inertia of the engine may be generated when the engine connecting and disconnecting device is fully engaged and the engine is connected to the motor after the start of the engine. Thus, it is not appropriate to employ the engine starting method unconditionally.

The present disclosure provides a control device of a hybrid vehicle, which appropriately determines a gear position to which an automatic transmission is to be shifted down, according to a thermal load of an engine connecting and disconnecting device, when engine start control is started during a downshift of the automatic transmission.

A first aspect of the present disclosure is applied to a hybrid vehicle including an engine, an electric motor configured to receive power from the engine via an engine connecting and disconnecting device of a frictional engagement type, and an automatic transmission provided on a downstream side of the electric motor on a power transmission path of the hybrid vehicle, and is concerned with a control device of the hybrid vehicle including an electronic control unit. The electronic control unit is configured to (a) execute engine start control to control the engine and the engine connecting and disconnecting device so as to start the engine during traveling such that the engine rotates by itself, and fully engage the engine connecting and disconnecting device, (b) control the automatic transmission to switch a gear position of the automatic transmission between a plurality of gear positions, according to predetermined shift conditions, (c) control the engine under the engine start control to start the engine in a selected one of a first starting method in which the engine performs ignition and rotates by itself after an engine speed as a rotational speed of the engine is increased to be equal to or higher than a predetermined rotational speed through slipping engagement of the engine connecting and disconnecting device, and a second starting method in which the engine performs ignition and rotates by itself from a stage before the engine speed reaches the predetermined rotational speed, and (d) control the automatic transmission to permit a lower gear position to be established according to the shift conditions in a case where the engine is started in the second starting method during a downshift of the automatic transmission, as compared with a case where the engine is started in the first starting method during the downshift of the automatic transmission. The downshift of the automatic transmission means a gearshift from a high-speed gear position, i.e., a gear position having a small speed ratio (=input rotational speed/output rotational speed), to a low-speed gear position, i.e., a gear position having a large speed ratio. Also, the lower gear position means a gear position having a larger speed ratio, namely, a lower-speed gear position.

In the control device according to the first aspect of the present disclosure, when the engine is started in the second starting method, the gear position as a destination of downshift, which is determined based on the shift conditions, is permitted to be a lower gear position than that in the case where the engine is started in the first starting method. Namely, in the second starting method in which the engine performs ignition and rotates by itself, from the stage where the engine speed is low, slipping engagement of the engine connecting and disconnecting device is relatively small or not required. Thus, even when the gear position as the destination of downshift is a relatively low gear position, and the amount of increase of the motor speed caused by the downshift is large, resulting in a large speed difference between the motor speed and the engine speed, the engine connecting and disconnecting device is unlikely to be damaged due to a thermal load caused by slipping engagement. Thus, the gear position determined based on the shift conditions is allowed to be a low gear position, and the automatic transmission can be promptly shifted down to a target gear position. Also, since the motor speed becomes relatively high, due to the downshift to a relatively low gear position, shock generated due to the inertia of the engine is curbed when the engine connecting and disconnecting device is fully engaged after the start of the engine, and the engine is connected to the motor.

In the control device according to the first aspect of the present disclosure, the electronic control unit may be configured to control the engine to complete the engine start control by executing the first starting method when a motor speed as a rotational speed of the electric motor is lower than a predetermined first determination value at a start of the engine start control, and complete the engine start control by executing the second starting method when the motor speed is equal to or higher than the first determination value at the start of the engine start control.

The control device as described above completes the engine start control by executing the second starting method when the motor speed is equal to or higher than the first determination value at the start of the engine start control. Thus, the engine start control is performed in the second starting method in a condition where the motor speed is relatively high, while the gear position determined based on the shift conditions is allowed to be a low gear position in the second starting method, and the gear position as a destination of downshift is set to the low gear position. As a result, the motor speed is further increased, and shock generated due to the inertia of the engine when the engine connecting and disconnecting device is fully engaged and the engine is connected to the motor is appropriately reduced or curbed. On the other hand, when the motor speed at the start of the engine start control is lower than the first determination value, the engine start control is completed by executing the first starting method. Thus, the gear position as a destination of downshift is set to a relatively high gear position according to the shift conditions, and the engine start control is performed by the first starting method in a condition where the motor speed is relatively low. As a result, damage due to the thermal load of the engine connecting and disconnecting device is reduced.

In the control device according to the first aspect of the present disclosure, the electronic control unit may be configured to control the engine to switch to the second starting method when a motor speed as a rotational speed of the electric motor becomes equal to or higher than a predetermined second determination value, before the engine start control is completed through execution of the first starting method. Also, the electronic control unit may be configured to control the automatic transmission such that, when a starting method of the engine is switched from the first starting method to the second starting method during shift control of the automatic transmission, the shift conditions permit a lower gear position than that based on the shift conditions in the first starting method to be established, in response to switching of the starting method.

When the motor speed becomes equal to or higher than the predetermined second determination value before the engine start control is completed, during starting of the engine in the first starting method, namely, when the motor speed is increased to be higher than the second determination value, in the process of increasing the engine speed through slipping engagement of the engine connecting and disconnecting device, the engine starting method is switched to the second starting method. As a result, damage due to a thermal load of the engine connecting and disconnecting device is appropriately reduced or curbed, irrespective of increase of the motor speed during the engine start control. Also, when the engine starting method is switched from the first starting method to the second starting method, a low gear position is determined based on the shift conditions, upon switching of the starting method; therefore, the automatic transmission can be promptly shifted down to a target gear position, and shock, which would be generated due to the inertia of the engine when the engine connecting and disconnecting device is fully engaged, can be appropriately reduced since the motor speed becomes high.

A second aspect of the present disclosure is applied to a hybrid vehicle including an engine, an electric motor configured to receive power from the engine via an engine connecting and disconnecting device of a frictional engagement type, and an automatic transmission provided on a downstream side of the electric motor on a power transmission path of the hybrid vehicle, and is concerned with a control device of the hybrid vehicle including an electronic control unit. The electronic control unit is configured to: (a) execute engine start control to control the engine and the engine connecting and disconnecting device so as to start the engine during traveling such that the engine rotates by itself, and fully engage the engine connecting and disconnecting device, (b) control the automatic transmission to switch a gear position of the automatic transmission between a plurality of gear positions, according to predetermined shift conditions, by switching engaged and released states of a plurality of frictional engagement devices of the automatic transmission, and (c) control the automatic transmission when the engine start control is started during a downshift of the automatic transmission, to permit a lower gear position to be established according to the shift conditions in a case where an inertia phase in which an input rotational speed of the automatic transmission is increased has not started, and the automatic transmission is able to stand by in a state before start of the inertia phase, as compared with a case where the downshift is in the inertia phase or a case where the inertia phase has not started, and the automatic transmission is unable to stand by in the state before start of the inertia phase.

In the control device according to the second aspect of the present disclosure, when the engine start control is started during a downshift of the automatic transmission, and the inertia phase has not started and the automatic transmission can stand by in a state before start of the inertia phase, the gear position as a destination of the downshift based on the shift conditions is allowed to be a lower gear position than that in the case where the downshift is in the inertia phase, or the case where the inertia phase has not started and the automatic transmission cannot stand by in the state before start of the inertia phase. Namely, when the inertia phase has not started, and the automatic transmission can stand by in the state before start of the inertia phase, the motor speed is kept relatively low if the transmission stands by in the state before start of the inertia phase. Thus, even when the engine is started in the first starting method, a thermal load due to slipping engagement of the engine connecting and disconnecting device is small, and damage of the engine connecting and disconnecting device is reduced. Also, the engine connecting and disconnecting device is fully engaged after the engine start control is completed, and the gear position as a destination of the downshift need not be restricted; therefore, the gear position determined based on the shift conditions is allowed to be the lower gear position, and the automatic transmission can be promptly shifted down to a target gear position.

In the control device according to the second aspect of the present disclosure, the electronic control unit may be configured to perform shift control of the downshift, by reducing an engaging pressure of a release-side frictional engagement device as one of the frictional engagement devices, and permitting increase of the input rotational speed, and the electronic control unit may be configured to enable the automatic transmission to stand by in the state before start of the inertia phase, by generating a command to stop reduction of the engaging pressure.

In the case where a downshift proceeds by reducing the engaging pressure of the release-side frictional engagement device as one of the frictional engagement devices, and permitting increase of the input rotational speed, a command to interrupt reduction of the engaging pressure is generated, to keep the automatic transmission in the state before start of the inertia phase. Thus, the motor speed is kept at a relatively low speed, and the engine can be appropriately started even when it is started in the first starting method, while a thermal load of the engine connecting and disconnecting device is reduced.

A third aspect of the present disclosure is applied to a hybrid vehicle including an engine, an electric motor configured to receive power from the engine via an engine connecting and disconnecting device of a friction engagement type, and an automatic transmission provided on a downstream side of the electric motor on a power transmission path of the hybrid vehicle, and is concerned with a control device of the hybrid vehicle including an electronic control unit. The electronic control unit is configured to (a) execute engine start control to control the engine and the engine connecting and disconnecting device so as to start the engine during traveling such that the engine rotates by itself, and fully engage the engine connecting and disconnecting device, (b) control the automatic transmission to switch a gear position of the automatic transmission between a plurality of gear positions, according to predetermined shift conditions, by switching engaged and released states of a plurality of frictional engagement devices of the automatic transmission, and (c) control the automatic transmission when the engine start control is started during a downshift of the automatic transmission, to permit a lower gear position to be established according to the shift conditions in a case where an inertia phase in which an input rotational speed of the automatic transmission is increased has not started, and the engine connecting and disconnecting device has been synchronized, as compared with a case where the inertia phase has not started, and the engine connecting and disconnecting device has not been synchronized. The synchronization of the engine connecting and disconnecting device means a relationship between the engine-side rotational speed and the motor-side rotational speed, which provides a condition where the engine connecting and disconnecting device can be fully engaged. Where the engine connecting and disconnecting device is a clutch, the relationship is that the engine-side rotational speed is equal to the motor-side rotational speed.

In the control device according to the third aspect of the present disclosure, when the engine start control is started during a downshift of the automatic transmission, the gear position as a destination of the downshift based on the shift conditions is allowed to be a lower gear position in the case where the inertia phase has not started and the engine connecting and disconnecting device has been synchronized, as compared with the case where the inertia phase has not started and the engine connecting and disconnecting device has not been synchronized. Namely, before the inertia phase starts and after the engine connecting and disconnecting device is synchronized, there is no fear of damage due to a thermal load caused by slipping engagement of the engine connecting and disconnecting device, and the gear position as a destination of downshift need not be restricted; therefore, the gear position determined based on the shift conditions is allowed to be the lower gear position, and the automatic transmission can be promptly shifted down to a target gear position.

In the control device according to the second or third aspect of the present disclosure, the electronic control unit may be configured to control the engine and the engine connecting and disconnecting device to start the engine in a first starting method in which the engine performs ignition and rotates by itself after an engine speed as a rotational speed of the engine is increased to be equal to or higher than a predetermined rotational speed through slipping engagement of the engine connecting and disconnecting device.

When the engine can be started in the first starting method in which the engine performs ignition and rotates by itself after the engine speed is increased to be equal to or higher than the predetermined rotational speed through slipping engagement of the engine connecting and disconnecting device, the thermal load of the engine connecting and disconnecting device is increased if the motor speed is increased due to a downshift; therefore, the shift conditions are set so that the gear position as a destination of the downshift is restricted to a high gear position when the engine is started. Accordingly, the electronic control unit permits the gear position determined based on the shift conditions to be a relatively low gear position, under a given condition, at the start of the engine, so as to promptly shift down the automatic transmission to a target gear position, while reducing damage due to the thermal load of the engine connecting and disconnecting device. Thus, the effects of the second and third aspects of the present disclosure can be appropriately obtained.

