Automatic or Manual?

| November 6, 2013

More choices — and it’s not just what you ‘feel’ like

Allison’s automatic transmission features fluid drive and a torque converter. Adherents argue these provide 100-percent power and double rim pull, making them ideal for vocational applications.

Allison’s automatic transmission features fluid drive and a torque converter. Adherents argue these provide 100-percent power and double rim pull, making them ideal for vocational applications.

Revolutions can be subtle things. Unless there’s an army or angry mob storming down Main Street, it’s not always easy to tell when you’re in one.

That’s what makes trucks today so interesting. The advent of automatic and automated transmissions in heavy-duty commercial vehicles is changing things. In many ways, the explosion in popularity of these units during the past decade is an offshoot of the rapid evolution being seen in onboard vehicle computers.

Automatic transmissions are nothing new. They’ve been around in automotive applications in one form or the other since the 1940s. But adapting them for use in heavy-duty trucks proved to be difficult for a variety of reasons – not the least of which was the inability of mechanical engines and transmissions to “talk” to one another and ensure optimal vehicle performance in the bewildering array of applications, road conditions and cargo/payload configurations.

Early attempts to develop and introduce them met with less-than-acceptable results. There were durability issues. But most complaints tended to focus on performance issues. Before the arrival of today’s powerful and compact electronic-control modules, early transmissions had a hard time figuring out what drivers wanted. Complaints concerning frequent, uneven shifts while accelerating 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.

Dry Clutches and Torque Convertors

Although the term “automatic” is being used to refer to all non-manual gearboxes, it’s important to note there are actually two types of automatic transmissions available for heavy-duty trucks: automated manual and automatic transmissions. Both types are two-pedal designs, and most drivers would be hard-pressed to tell much of a difference between them.

The basic difference is automated manuals are manual gearboxes with all the clutch actuation and gearshifts handled by electronically controlled systems. True automatic transmissions feature planetary gearing with disc packs for clutches and with torque converters.

“In this context, an automatic 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.” It uses a single computer-operated disc clutch, otherwise similar to a manual clutch.

“The Allison Automatic transmission has Continuous Power Technology, which means it never interrupts torque and power to the wheels,” explains Steve Spurlin, executive director, international application engineering and vehicle integration for Allison Transmission. “It also uses a torque converter as the starting device. An automated manual has incorporated electronic controls with basic manual transmission architecture to facilitate automated shifting of gears and of 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.”

Steve Rutherford, powertrain marketing manager, Caterpillar OEM Solutions Group, says the advantages of a pure automatic design are a perfect fit for the company’s vocational truck line – although he points out both work well in specific applications. “Both types of transmissions have two pedals and a shift pad instead of a gearshift lever,” he says. “It took a lot of work to perfect automated manuals to engage a dry clutch and generate well-synchronized shifts.”

But in construction applications, Rutherford says, with lots of starts and stops in muddy terrain with high rolling resistance, or having smooth, controllable startup power in front of a paver, an automatic transmission is the better choice in Caterpillar’s estimation. “You just cannot beat a torque converter in those applications when it comes to maintaining 100-percent boost engine power up and down through the gears,” he says. “Additionally, there’s not a dry clutch to be abused or overheat because an automatic is 100-percent fluid drive power. However, in a steady-state cruise application in pure highway mode, you simply cannot beat an automated manual.”

Despite their reputation as slow to change, it appears users are gravitating toward automatic transmissions in surprisingly high numbers.

David McKenna, director of powertrain sales for Mack Truck, says the mDrive take rate in Pinnacle trucks is currently about 38 percent, which aligns with other OEMs. In some medium-duty applications, AMTs are now spec’d in more than half of new trucks sold. Likewise, Saxman says more than 50 percent of new Volvos sold this year have been spec’d with the company’s iShift AMT.

There are several reasons for this significant shift, says Shane Groner, manager of product development, NAFTA, for Eaton. Currently, the company’s UltraShift Plus AMT is enjoying a 25-percent take rate in heavy-duty trucking applications, he says.

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Eaton’s UltraShift Plus automated manual transmission is a computer-controlled manual gearbox that features a dry clutch but only two pedals to excel in long-haul trucking applications.

The appeal of automatics can go far beyond driver preferences, with safety and fuel economy topping the list. “Automatics allow less-skilled (novice) drivers to be productive,” says Allison’s Spurlin. “They are also a large help with safety because the drivers can stay focused on the road or the task at hand instead of shifting gears. Additionally, drivers do not get as fatigued with an automatic.”

The way a drivetrain is being spec’d is changing, with automatic transmissions being a key part of the formula, says Brad Williamson, powertrain marketing manager for Daimler Trucks North America. “The industry at large is now spec’ing lower gear ratios with direct-drive powertrains becoming more common.”

At the same time, Williamson says, the concept of “downspeeding” – or operating a diesel at lower rpms while at cruise speeds – is being driven by the push for better fuel economy. “An automatic transmission can do these things better than all but the best drivers out there. Information is key: If you designed both the engine and the transmission, then they can ‘talk’ to one another and share critical information. The transmission can manage all this information and deliver the best performance possible given all those criteria. A driver can’t do that consistently.”

Williamson’s point addresses the latest development in the evolution of automatics in heavy-duty trucking: complete powertrain integration. Volvo, Mack, Daimler and Eaton (with a partnership with Cummins) have developed highly integrated powertrains that share unprecedented levels of proprietary information between the engine and transmission to deliver optimal fuel economy at all times.

“It all boils down to the electronics,” Mack Truck’s McKenna argues. “When you have Vendor A supplying an engine and Vendor B supplying a transmission, rarely do those two components share 100 percent of their information 100 percent of the time. Typically, the transmission in those instances ends up making decisions with about 75 percent of the data it needs to make an optimal shift.”

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