TECHNICAL FIELD

This disclosure relates to the field of control systems for automatic transmissions for motor vehicles. More particularly, the disclosure relates to a control strategy in which the off-going clutch continues to have torque capacity during the inertia phase.

BACKGROUND

Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.

Discrete ratio transmissions are capable of transmitting power via various power flow paths, each associated with a different speed ratio. A particular power flow path is established by engaging particular shift elements, such as clutches or brakes. Shifting from one gear ratio to another involves changing which shift elements are engaged. In many transmissions, the torque capacity of each shift element is controlled by routing fluid to the shift elements at controlled pressure. A controller adjusts the pressure by sending electrical signals to a valve body.

SUMMARY

According to one embodiment, a powertrain includes a transmission having an input shaft, an output shaft, and a plurality of clutches engageable in various combinations to establish power flow paths between the input and output shafts. A controller is programmed to, responsive to a shift of the transmission: reduce torque capacity of an off-going one of the clutches and increase torque capacity of an oncoming one of the clutches during a torque transfer phase of the shift, and, in response to an inertia phase of the shift, continue to command non-zero torque capacity to the off-going clutch such that the off-going clutch brakes the output shaft throughout an entire duration of the inertia phase.

According to another embodiment, a powertrain includes an engine and a transmission having an input shaft, an output shaft, and a plurality of clutches engageable in various combinations to establish power flow paths between the input and output shafts. A controller is programmed to, responsive to a shift of the transmission: during a torque transfer phase of the shift, increase torque capacity of an oncoming one of the clutches and decrease torque capacity of an off-going one of the clutches, and, during an inertia phase of the shift, command a series of sequential torque capacities, each having different non-zero magnitudes, to the off-going clutch such that the magnitudes increase and decrease relative to each other based on measured or inferred torque of the output shaft, slip of the off-going clutch, and inertia torque associated with the input shaft.

According to yet another embodiment, a method of shifting a transmission includes, during a torque transfer phase of the shift, decreasing torque capacity of an off-going clutch to a non-zero value to initiate slip across the off-going clutch, and, during an inertia phase of the shift, continuing to maintain slip across the off-going clutch by commanding a series of sequential nonlinear torque capacities that gradually reduce to zero at termination of the inertia phase.

