To realize better fuel economy benefits from transmissions, car makers have started the application of torque converter clutch control in second gear and beyond, resulting in greater demand on the torque converter clutch (TCC) and its control system. This paper focuses on one aspect of the control of the torque converter clutch to improve fuel economy and faster response of the transmission.
A TCC is implemented to control the slip between the pump and turbine of the torque converter, thereby increasing its energy transfer efficiency and increasing vehicle fuel economy. However, due to the non-linear nature of the torque converter fluid coupling, the slip feedback control has to be very active to handle different driver inputs and road-load conditions, such as different desired slip levels, changes in engine input torques, etc. This non-linearity requires intense calibration efforts to precisely control the clutch slip in all the scenarios.
In this paper, a model based control method is developed to calculate the feedforward portion of the pressure control signal. This feedforward pressure calculation is so accurate that when it is used in conjunction with the slip feedback signal, the control of the torque converter clutch becomes extremely effective in controlling the slip to desired small values and at the same time preventing clutch flares and lock ups. The proposed feedforward algorithm applies the Kotwicki model or an equivalent table lookup to predict torque across the clutch using the desired slip, current estimated engine torque, and turbine speed. This new control strategy provides more accurate control, and reacts faster during the transient conditions, such as engine torque changes and reference slip changes. The strategy has been applied and demonstrated in various torque converters in both RWD and FWD GM transmissions.