Adapting automatic transmissions for use in trucking has proven to be difficult. One reason was the inability of mechanical engines and transmissions to “talk” to one another and ensure optimal performance across applications and road conditions.
Early transmissions had a hard time figuring out what drivers wanted. Complaints concerning both frequent and uneven shifts were common, as were problems with “searching” – when transmissions would struggle in hilly terrain to find and stick with an optimal gear. Other problems along those lines emerged at low speeds – in traffic and while docking.
Even so, AMTs continued to make inroads in trucking. Now, engines and transmissions are sharing more data, so performance issues are getting smoothed out. AMTs now are being spec’d on a significant share of new trucks for reasons including fuel economy and easier driver training. The trend is likely to continue.
Two types of automatic transmissions are available for heavy-duty trucks: Automated manual transmissions and automatic transmissions. Both are two-pedal designs. Most drivers probably would be hard-pressed to tell much of a difference between them in real-world driving situations.
The basic difference is that automated manuals are manual gearboxes, with all the clutch actuation and gearshifts handled by electronically controlled systems. A true automatic transmission “for a Class 8 truck is usually fully automatic, like a typical car transmission, with planetary gearing with several multi-disc packs for clutches,” says Ed Saxman, product marketing manager with Volvo Trucks North America. These transmissions have a torque converter to enable powershifts of the planetary epicyclic gearing units that provide the various gear ratios.
An automated manual transmission uses the gearbox of a manual transmission and shifts it by computer, using a computer-operated single large-diameter disc clutch, otherwise similar to a manual clutch.
Allison Transmission’s automatic is designed with the company’s Continuous Power Technology, meaning it never interrupts torque and power to the wheels, says Steve Spurlin, executive director of international application engineering and vehicle integration. “It also uses a torque converter as the starting device,” he says.
An automated manual has incorporated electronic controls with basic manual transmission architecture to facilitate automated shifting of the gears and the input clutch-starting device. Both the automated manual and the basic manual interrupt power and torque every time a shift is made, whether automated or manually, Spurlin says.