730839 Shift Modulation of Allison Automatic and Powershift Transmissions K. B. Harmon and R. H. Schaefer Detroit Diesel Allison Div., General Motors Corp. WHEN DETROIT DIESEL ALLISON entered the heavy duty transmission business nearly 30 years ago, manually shifted units were the predominant type of transmission. At that time customer demands for increased vehicle productivity led to the introduction of powershift transmissions. Since then customers have made further demands for transmission im­ provements in the areas of passenger-car shift quality, im­ proved clutch durability, downshift overspeed protection, and maximum performance, as needed. Incorporation of these improvements has resulted in increas­ ing sophistication in transmission control systems. The new sophisticated controls, for both manual powershift and auto­ matic transmissions, place heavier demands, in turn, on the transmission industry. Many current transmissions with con­ trols which meet customer demands require ultra-clean hy­ draulic systems superior clutch materials, improved hydraulic fluids, and refined manufacturing and quality control tech­ niques. This paper will present a discussion of shift modulation in manual and automatic powershift transmissions. First the concept of shift smoothness will be discussed, then the de­velopment of the types of controls which have been employed on Allison transmissions and the effect of various shift point control systems on shift smoothness in automatic transmis­ sions will be described. SHIFT SMOOTHNESS Shift smoothness is as important in an automatically shifted transmission as it is with the conventional manual shift or the manual powershift transmission—perhaps even more so. In shifting the manual powershift transmission, the vehicle driver anticipates the shift and its associated "feel." However, with an automatic the shifts are not necessarily anticipated and, when a shift does occur, it must be extra smooth to be satis­ factory to the driver. In technical terms, shift smoothness in a powershift trans­ mission—regardless of whether it is a manual or automatic unit—means a minimum torque disturbance in the driveline during the shift from one gear range to another. The contrast between a smooth shift and a harsh shift can be seen in Fig. 1. While both shifts may have nearly the same peak torque, the ABSTRACT 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 in­ fluence 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 characteris­ tics, 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 dis­ advantages. 2781 Downloaded from SAE International by Purdue University, Monday, August 20, 20182782 K. B. HARMON AND R. H. SCHAEFER harsh shift invariably has a very high rate of torque change at the engaging clutch with respect to time. This results in a rapid change of acceleration which the driver feels as a "jolt" or "shock." From the driver's point of view, shift shock is a relative thing and may run from being a mere nuisance to being strongly objectionable. In any case, shift shock fatigues the driver and after a long day may tend