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Test Drive: Daimler's DT12 Automated Transmission

January 2013, TruckingInfo.com - Department

by Jim Park, Equipment Editor, Equipment Editor - Also by this author

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The arrival of Detroit's DT12 automated manual transmission wasn't unexpected. Daimler AG, the parent company of Daimler Trucks North America and Detroit Inc., has been using such transmissions in Europe for many years in the Mercedes Actros line and earlier products.

With the company's push toward global commonality in powertrain components, it was inevitable.

The first official inkling we had was a comment by Andreas Renschler, head of Daimler Trucks at Daimler AG, during the 2011 American Trucking Associations annual management conference. It wasn't so much his words as the twinkle in his eye that suggested we were in for something good.

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Volvo was the first to make North American truckers comfortable with 12-speed transmissions with its automated I-Shift, and with the change in torque and horsepower profiles in North American engines over the past five years, the 12-speed configuration has proven quite successful.

The steps between the gears match nicely with the broad torque plateaus we see today, rather than the torque peaks of previous engine generations. And when the transmission is doing the shifting and the thinking, drivers really don't have to concern themselves with shift timing to optimize torque and fuel efficiency. Ten, 12, 13, 18 speeds? It hardly matters anymore except in highly specialized applications.

So the DT12 arrived here in mid-2012 with the ice already broken. If what I found on a day-long test drive in August is any indication, I'm sure beads of sweat are already forming on competitors' brows.

DT1 2 Features

The DT12 is a 12-speed non-synchronized manual transmission with pneumatic X-Y shift and clutch actuators. It features a single countershaft and an aluminum housing to minimize weight.

It will be available in two input-torque ratings: 2,050 pounds-feet (789 pounds dry weight) and 1,650 pounds-feet (646 pounds). In direct-drive configuration, it has an overall drive ratio of 14.93:1, in overdrive (0.78) the overall drive ratio is 14.96:1. There are several modes:

eCoast: disengages the transmission from the drivetrain while the truck is coasting, reducing mechani cal drag.
Creep: modulates clutch engagement to improve low-speed operation. In traffic, the transmission can be up- and down-shifted at engine idle.
Kick down: functions like a passing gear, initiating a downshift to increase horsepower while passing.
Eco/Performance: changes the shifting strategy to match conditions or fleet/driver preference.
Auto-neutral: automatically returns to neutral if the parking brakes are set or the key is turned off.

According to Brad Williamson, manager, engine and component marketing for Daimler Trucks North America, the first model out the gate in May 2013 will be the 2,050-pound-foot version to mate up with the DD15 engine. In November the 1,650 pounds-foot version will appear matched with a DD13 engine. Sometime in 2014, the DT12 will be available with the DD16 engine.

Performance evaluation

The official test drive of the new Detroit transmission in a Freightliner Cascadia Evolution started with a couple of trips around an 18-mile loop from Yountville, Calif., into Napa on four-lane Route 29, and then back to Yountville on the two-lane Silverado Trail. That gave me an inkling of how it shifted on flat roads and in city traffic.

Its propensity for skip-shifting was immediately obvious. I think I was into 8th gear in three shifts, before I even had the truck pulled fully onto the highway from the parking area where the trucks were staged.

The shifts were done at remarkably low engine speed, 1,400 to 1,600 rpm. I knew then and there I was going to like this transmission. Under normal driving conditions, the engine revs seldom got above 1,600 and often dipped as low as 1,050.

The DT12 will skip-shift more often than not, even when decelerating, which really smoothes things out. I found it useful to shut off the engine brake below fifth gear, because it got a little "torque-y" in the cab at low speed.

I played around with various driving styles, from hard on the throttle to laid back and lazy. The DT12 was comfortable either way. It tended to skip-shift just as often under heavy throttle, but at higher rpm. In Performance mode, the engine ran about 300 rpm higher on most shifts. The shifting was somewhat dependent on how aggressive I was with the throttle pedal.

In the stop-and-go traffic in Napa proper, the smoothness of the clutch actuation was most obvious. At launch and in the moment just before the wheels stopped during braking, it felt much like a car with an automatic transmission. I think it's the smoothest clutch action of any automated gearbox out there.

The next day, I put the boots to the DT12.

Hunter Hill, I-80

What a great place to put an automated transmission through its paces.

Hunter Hill is located just east of Vallejo, Calif., on I-80. From the parking area at the top of the hill, I ran downhill into Vallejo, turned around at Tennessee Street and came back up the hill to Red Top Road, turned around and then headed back to the parking area. It's about a 15-mile loop with a 6% grade, and a much steeper on-ramp at Red Top Road - maybe 8% or 9% - that tested its shifting prowess while climbing under a 75,000-pound load.

I made several trips around the loop, comparing Eco mode with Performance mode, and making the most of eCoast on the moderate grades. I also tried the Advanced Cruise Control on the downhill grade set at Low to maintain my descent speed, the engine brake engaging at 3 mph above the set speed. However, because I was already close to 50 mph at a lazy 1,100 rpm in 12th gear when the hill got serious, I had to manually downshift to get some extra engine braking.

Remarkably, I didn't need to, but I was able to drop two gears on the hill, getting the revs up to 2,200, which the transmission let me do. There's some serious engine braking happening there.

Once I was off the steepest section, I flipped back and forth in the gears, finding I had a three-gear range to choose from. I could descend the hill in 10th, 1 1th, or 12th gear, with engine speed varying accordingly. Talk about options! With the engine brake in position 1, 2, or 3, and in a given gear, I could safely ease down the hill at about any speed I chose.

Climbing the hill was equally exciting. Left to its own devices in Eco mode, the engine pulled down to about 1,100 rpm on each downshift, re-engaging at about 1,400 rpm. I downshifted like that from 12th to 9th gear before the engine was able to maintain 1,150 rpm in the climb. As the grade leveled out near the top, the transmission upshifted to 10th gear at 1,250 rpm, reengaging at about 1,050 and digging deep for the pull. There's lots of torque at 1,050 rpm on the DD15, and the DT12 took advantage of every pound-foot available.

Repeating the climb on a subsequent loop in Performance mode, the engine shifted at about 300 rpm higher than it did in Eco mode, consistently. You'll get up the hill a little faster, but you'll burn more fuel.

In manual mode, when I became the brains of the operation, the shift points set in Eco mode seemed to me to be the logical place for a downshift. Even in manual, the transmission will upshift automatically if the revs drop too low. On one occasion, I grabbed two gears on an upshift, and the transmission let me do it. Nearing the top on leveler ground, I was able to get the engine down to 900 rpm by backing off the throttle a little and letting the engine think power demand was easing. I may have lost a few seconds, but I'm sure I saved a little fuel too.

Coming up the Red Top Road on-ramp was interesting. It's a steeper grade than the highway, and I started at the bottom in 6th gear, 1,400 rpm, at 10 mph. It shifted into 7th at 1,600 rpm, re-engaging at 1,100 rpm. It grabbed 8th gear at 1,600 rpm, re-engaging at 1,200 rpm and so on, until I was up to highway speed. Very low engine speed on each shift in Eco-Mode. Again, Performance mode gave me about 300 additional rpm on each upshift.

ECoast kicked in a few times at various points on the loop, and it works amazingly well. Having the transmission disengage from the driveline is an interesting way of saving fuel, but the good part is the re-engagement. Even going from idle speed to 1,900 rpm with the engine brake full-on, the transition was unbelievably smooth. Usually re-engagement isn't that dramatic, so drivers will not have any problems with this feature.

It's worth noting, too, that during the cross-country fuel economy test Daimler did with this truck in the spring, where it averaged 9.3 mpg, the truck was in eCoast mode for an astonishing 650 miles of the 2,400-mile trip from San Diego, Calif., to Gastonia, N.C.

Conclusions

If reduced engine speed is a key to improved fuel economy, this transmission is going to win Daimler many fans. It upshifts at consistently low engine speed, even when skip-shifting, which it does most of the time.

The clutch engagement is impossibly smooth, during both upshifts and downshifts, and even under extreme conditions going from idle to 1,900 rpm with full engine braking.

The controls are dead simple to use, and provide a ton of functionality and driver selection - although some will be equally happy just to stick it in D and go.

It's now easy to see why Renschler had that cat-that-ate-the-canary smile when he announced the DT12.

Driver Interface

The DT12 shifter stalk is a model of functionality, simplicity and even elegance. For starters, it's on the right side of the steering column where the driver's right hand naturally rests - in the absence of a gearshift knob. Second, it's a paddle-shaped device, so it's easy to hit without reaching, and the control surfaces are large and easy to manipulate.

The drive and reverse selector, obvious in the photo, is a rotary switch that goes right around the stalk so it can be gripped easily. It switches between manual and automatic operation with a press of the button at the end of the stalk. In manual mode, up-and down-shifting is done by pulling or pushing the stalk fore and aft, accordingly. Moving the stalk up or down engages a three-stage engine brake.

A dash display provides gear selection, operating mode and other information to keep drivers in the loop. Together, the display and the shifter are very intuitive and really easy to use-just like its predecessor, the right-arm rest.

From the December issue of HDT.

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