A transmission may include a torque converter. The torque converter provides a fluidic coupling between a torque source such as an engine and a transmission input shaft. The torque converter allows the engine to idle without having to rotate wheels of a vehicle while the transmission is engaged in drive mode. The torque converter also multiplies torque input to the torque converter when there is a speed difference between a torque converter impeller and a torque converter turbine. However, the torque converter may transfer power from the torque source to the transmission less efficiently than is desired. Consequently, vehicle fuel economy may be less than is desired.
The inventors herein have recognized the above-mentioned issues and have developed a method for operating a driveline of a vehicle, comprising: adjusting a torque capacity of a torque converter clutch to provide a non-zero torque capacity in response to a vehicle speed of zero; and opening the torque converter clutch in response to a transmission input torque request greater than the non-zero torque capacity after adjusting the torque capacity of the torque converter clutch to provide the non-zero torque capacity.
By providing a non-zero torque capacity via a torque converter clutch, it may be possible to provide the technical result of improving driveline efficiency. For example, if a driver or controller requests a low transmission input shaft torque, the torque converter clutch may be held closed or closed, including partially closed, to increase the torque converter clutch's torque capacity. Closing the torque converter clutch allows the transfer of engine torque through the torque converter without having to transfer torque through fluid in the torque converter. Consequently, vehicle driveline efficiency may be increased and vehicle fuel consumption may decrease. Nevertheless, if the driver or controller requests an increase in transmission input torque that exceeds a threshold torque, the torque converter clutch is opened to reduce the possibility of torque converter clutch degradation and increase torque multiplication through the torque converter. In this way, the present method may increase driveline efficiency during low load vehicle launch conditions. Further, the method may provide significant launch torque during conditions of high load vehicle launch conditions.
If a vehicle does not have an infinitely variable transmission and the powertrain torque source does not have sufficient function at zero speed and sub-idle speeds, a slip element may be positioned between the powertrain torque source and drive wheels. The slip element may be either a fluid clutch (perhaps enhanced to also be a torque converter) or a friction clutch. Conventionally, a fluid clutch is used both during zero vehicle speed, initial acceleration, and some other conditions. The idea is that if the vehicle has a fluid clutch, you use it for all slip conditions. All modern conventional automatic transmission also come with a lock-up clutch to eliminate the constant power loss of the fluid clutch. Thus, the typical launch sequence becomes: 1) idle with torque converter providing creep torque at a power loss, 2) accelerate using the slip and torque multiplication of the fluid clutch/torque converter, 3) lock up torque converter clutch eliminating the constant power loss associated with the fluid clutch after achieving a threshold speed. The inventors recognize that there are light torque launches where the use of the fluid clutch can be avoided with the result being less time spent with slip elements engaged, thereby increasing powertrain efficiency. Transmission power loss occurs while slip elements are being used. Thus, for light launch, the invented launch sequence becomes: 1) idle with torque converter clutch engaged, 2) accelerate using the slip provided by engaging the forward clutch (or equivalent, such as gear clutches), 3) fully lock up torque converter clutch once the torque flow through the torque converter's fluidic torque path is not expected to be needed. In an alternate embodiment, the vehicle may launch by adjusting the torque converter lock-up clutch capacity based on accelerator pedal position, but that does not yield the “neutral idle” benefit.
The present description may provide several advantages. In particular, the approach may increase vehicle driveline efficiency. Additionally, the approach may provide an expected vehicle launch during conditions of high transmission input torque demand. Additionally, the approach may automatically control the torque converter clutch when a vehicle is stopped in response to road grade to further reduce the possibility of torque converter clutch degradation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.