CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2020-029904 filed on Feb. 25, 2020, which is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a vehicle power transmission device, and in particular relates to a lockup control device and a lockup control method for a vehicle power transmission device that is provided with a fluid transmission device including a lockup clutch.

2. Description of Related Art

With regard to a vehicle power transmission device provided with a fluid transmission device including a lockup clutch, there is known a lockup control device for a vehicle power transmission device that executes flex lockup control for slip-engagement of the lockup clutch such that the differential rotation of an input-side rotating member and an output-side rotating member of the fluid transmission device is a target differential rotation set in advance. A device described in Japanese Unexamined Patent Application Publication No. 2017-211027 (JP 2017-211027 A) is an example of the lockup control device, and this literature proposes a technology that performs flex lockup control taking into consideration the amount of heat generated by friction members of the lockup clutch during slip-engagement thereof under flex lockup control, since the heat generated may compromise durability.

SUMMARY

However, there is a possibility that flex lockup control based on the amount of heat generated will restrict flex lockup control beyond necessity, or conversely not sufficiently protect friction members and result in compromised durability.

The disclosure proposes a lockup control device and a lockup control method for a vehicle power transmission device, in which friction members are appropriately protected from deterioration due to heat and durability is improved, without restricting flex lockup control beyond necessity.

A first aspect of the disclosure relates to a lockup control device for a vehicle power transmission device provided with a fluid transmission device including a lockup clutch. The lockup control device is provided with an electronic control unit configured as follows. The electronic control unit is configured to execute flex lockup control to cause slip-engagement of the lockup clutch such that a differential rotation between an input-side rotating member and an output-side rotating member of the fluid transmission device is a target differential rotation set in advance, and to correct the target differential rotation based on transition of a heat accumulation amount applied to friction members of the lockup clutch.

With the lockup control device for the vehicle power transmission device according to the first aspect, the target differential rotation is corrected based on the transition of the heat accumulation amount applied to the friction members of the lockup clutch, and accordingly the friction members can be appropriately protected. That is to say, the heat accumulation amount is obtained by subtracting a heat dissipation amount from a heat generation amount, and is a parameter that more appropriately reflects the effects of heat on the friction members, and accordingly the friction members can be appropriately protected from deterioration due to heat and durability can be improved, without restricting flex lockup control beyond necessity.

In the lockup control device for the vehicle power transmission device according to the above aspect, the electronic control unit may be configured to calculate the heat accumulation amount, to calculate a long-term change that is a change in a cumulative heat accumulation amount obtained by cumulating the heat accumulation amount, as the transition of the heat accumulation amount, and to correct the heat accumulation amount to make the target differential rotation smaller when the long-term change is large, as compared to when the long-term change is small.

With the lockup control device of the above configuration, correction of the heat accumulation amount is performed based on the long-term change of the heat accumulation amount that is a change in a cumulative heat accumulation amount obtained by cumulating the heat accumulation amount, to make the target differential rotation smaller when the long-term change is large, as compared to when the long-term change is small. That is to say, even though the effects of heat by performing flex lockup control once may be small, the friction members deteriorate due to fatigue by repeated execution of flex lockup control. However, correcting the target differential rotation based on the long-term change of the heat accumulation amount enables deterioration of the friction members due to excessive fatigue to be suppressed, and durability to be improved.

In the lockup control device for the vehicle power transmission device according to the above configuration, the long-term change may be a change in the cumulative heat accumulation amount with respect to a driving distance of a vehicle, and the electronic control unit may be configured to correct the heat accumulation amount to make the target differential rotation smaller when the cumulative heat accumulation amount is large and the driving distance is short, as compared to when the cumulative heat accumulation amount is small and the driving distance is long.

With the lockup control device of the above configuration, the magnitude in the long-term change of the heat accumulation amount is specifically defined, and the heat accumulation amount is corrected to make the target differential rotation smaller when the cumulative heat accumulation amount is large and the driving distance is short, i.e., when the long-term change of the heat accumulation amount is large, as compared to when the cumulative heat accumulation amount is small and the driving distance is long, i.e., when the long-term change of the heat accumulation amount is small, which enables deterioration of the friction members due to excessive fatigue to be appropriately suppressed, and durability to be improved.

In the lockup control device for the vehicle power transmission device according to the above configuration, the electronic control unit may be configured to correct the heat accumulation amount following a correction rule set in advance for each of a plurality of regions set in advance with the driving distance and the cumulative heat accumulation amount as parameters, to make the target differential rotation smaller in a region where the cumulative heat accumulation amount is large and the driving distance is short, as compared to a region where the cumulative heat accumulation amount is small and the driving distance is long.

With the lockup control device of the above configuration, the heat accumulation amount is corrected following the correction rule set in advance for each of a plurality of regions set in advance with the driving distance of the vehicle and the cumulative heat accumulation amount as parameters, to make the target differential rotation smaller in the region where the cumulative heat accumulation amount is large and the driving distance is short, as compared to the region where the cumulative heat accumulation amount is small and the driving distance is long. Accordingly, the target differential rotation can be appropriately corrected in accordance with the transition (magnitude of change) of different cumulative heat accumulation amounts for each of the regions, and durability of the friction members can be improved.

In the lockup control device for the vehicle power transmission device according to the above aspect, the electronic control unit may be configured to calculate the heat accumulation amount, to calculate a change rate of the heat accumulation amount per unit time at a current point in time, as the transition of the heat accumulation amount, and to correct the heat accumulation amount to make the target differential rotation smaller when the change rate is large, as compared to when the change rate is small.

With the lockup control device of the above configuration, the heat accumulation amount is corrected based on the change rate of the heat accumulation amount per unit time at the current point in time, to make the target differential rotation smaller when the change rate is large, as compared to when the change rate is small. Accordingly, the target differential rotation is appropriately corrected based on the change rate of the heat accumulation amount, which enables deterioration of the friction members due to heat to be suppressed, and durability to be improved.

In the lockup control device for the vehicle power transmission device according to the above aspect, the electronic control unit may be configured to, in addition to correcting the target differential rotation based on the transition of the heat accumulation amount, correct the target differential rotation in accordance with a degree of oxidative deterioration of lubricating oil that lubricates the friction members following a lubricating oil deterioration correction rule set in advance, to make the target differential rotation smaller when the degree of oxidative deterioration is large, as compared to when the degree of oxidative deterioration is small.

With the lockup control device of the above configuration, when correcting the target differential rotation in accordance with the degree of oxidative deterioration of the lubricating oil following the lubricating oil deterioration correction rule, separately from correction of the heat accumulation amount, increase of thermal load on the friction members from lower lubrication performance due to deterioration of lubricating oil can be suppressed, and durability can be improved.

In the lockup control device for the vehicle power transmission device according to the above aspect, the electronic control unit may be configured to, in addition to correcting the target differential rotation based on the transition of the heat accumulation amount, correct the target differential rotation in accordance with an amount of inclusion of foreign matter in lubricating oil that lubricates the friction members following a foreign matter inclusion correction rule set in advance, to make the target differential rotation smaller when the amount of inclusion of foreign matter is large, as compared to when the amount of inclusion of foreign matter is small.

With the lockup control device of the above configuration, when correcting the target differential rotation in accordance with the amount of inclusion of foreign matter in the lubricating oil following the foreign matter inclusion correction rule, separately from correction of the heat accumulation amount, increase of thermal load on the friction members from lower lubrication performance due to inclusion of foreign matter can be suppressed, and durability can be improved.

In the lockup control device for the vehicle power transmission device according to the above aspect, the electronic control unit may be configured to, in addition to correcting the target differential rotation based on the transition of the heat accumulation amount, correct the target differential rotation for each of a plurality of localities where a vehicle in which the vehicle power transmission device is installed is used, following a locality-based correction rule set in advance, to make a correction amount of the target differential rotation larger in a locality where the correction amount of the target differential rotation due to correction of the heat accumulation amount is large as compared to a locality where the correction amount is small.

With the lockup control device of the above configuration, when correcting the target differential rotation following the locality-based correction rule for each of the localities where the correction amount of the target differential rotation due to correction of the heat accumulation amount differs among the localities, separately from correction of the heat accumulation amount, the target differential rotation is appropriately corrected in accordance with the difference in the correction amount of the target differential rotation due to correction of the heat accumulation amount for each of the localities. That is to say, the target differential rotation is appropriately corrected in accordance with difference in transition of the heat accumulation amount occurring due to differences in vehicle driving conditions of each of the localities and so forth, whereby deterioration of the friction members due to heat can be suppressed and durability can be improved.

A second aspect of the disclosure relates to a lockup control method for a vehicle power transmission device provided with a fluid transmission device including a lockup clutch. The lockup control method includes controlling the lockup clutch to perform slip-engagement such that a differential rotation between an input-side rotating member and an output-side rotating member of the fluid transmission device is a target differential rotation set in advance; and correcting the target differential rotation based on the transition of a heat accumulation amount applied to friction members of the lockup clutch.

With the lockup control method for the vehicle power transmission device according to the second aspect, the target differential rotation is corrected based on the transition of the heat accumulation amount applied to the friction members of the lockup clutch, and accordingly the friction members can be appropriately protected. That is to say, the friction members can be appropriately protected from deterioration due to heat and durability can be improved, without restricting flex lockup control beyond necessity.

