Automatic Transmission Shift Control

Performance of a direct-drive transmission with six shifts (six gears) is compared to a torque converter/automatic transmission with three shifts in Fig. 6.9. Note that the torque converter “broadens” the power transfer bands with the three shifts such that the power line is equally as smooth as the power line produced with six shifts. However, the automatic transmission power line is below the straight transmission power line, indicating that there is a higher power loss through the automatic transmission. As with any fluid device, there is an efficiency cost for any benefits.

Activation of the clutches in the automatic transmission (to achieve the shifts) is accomplished by activating solenoid valves to port pressurized fluid to the clutch actuator. (Typical activation pressures are in the 500 psi range.) Automatic transmissions going into vehicles today, particularly very heavy vehicles, have complex strategies to sequence activation of the solenoid valves.

A safety feature is built into valves to prevent the engine from being driven at too high a speed by an overrunning load. Other features are included to sense an operator error (or poor decision) and shift in an optimal manner to prevent damage to the engine or transmission. Using fluid power to implement these control strategies is an important fluid power application.


An engineer should understand the characteristics of direct-drive and automatic shift transmissions and compare these characteristics with the characteristics of hydrostatic transmissions presented in this chapter to select the best power transmission technology for the vehicle being designed. Sometimes an automatic shift transmission is the best choice based on cost (initial and operating), reliability, and service life; sometimes a hydrostatic transmission or direct-drive transmission is the best choice. This limited information on automatic shift transmissions should provide the needed background for interpretation of the following discussion of hydrostatic transmissions.

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