Shift Modulation of Allison Automatic and Powershift Transmissions 730839
The need for continuous and smooth power flow during shifting of power shift and automatic transmissions has long been recognized. In addition, over the years, vehicle users have demanded continuing transmission improvement in such areas as shift quality, clutch durability, overspeed protection, and performance.
The factors-clutch size, clutch coefficient variation, gear ratio step, and clutch apply pressure characteristics-which influence shift quality (or smoothness) have been identified. Compensation for the first three of these can be achieved by controlling the fourth, the clutch apply pressure characteristics, by means described as clutch capacity modulation.
A fifth factor which has an influence on shift smoothness in automatic transmissions is the proper scheduling of range shifts with respect to vehicle speed, engine speed, and engine throttle position. Several methods of shift point modulation have been developed, each with its own advantages and disadvantages.