Todd Dygert, Allison’s TC10 specialist, estimates the rear axle ratio the transmission will use in over-the-road, linehaul applications at about 2.64:1 with the transmission’s 0.86:1 overdrive. The I-shift, Saxman says, uses 2.64-2.69:1 rear axles with its 0.78:1 top gear, depending on whether the truck is a 6×4 or a 4×2 single-drive-axle tractor.
To hold cruise speeds pulling hills with an automated drivetrain that incorporates engine downspeeding, the transmission must be able either to downshift as soon as power is demanded, as the Allison does, or earlier than a standard 10-speed would when shifted manually. In the second case, the engine must also deliver full torque down to a lower rpm. Because the I-Shift has 12 speeds, it has narrower steps than a 10-speed, which helps keep the engine in the sweet spot.
Saxman says the Volvo XE13’s 425-hp rating hides the fact that the engine produces constant torque until it’s turning more than 1,500 rpm, behaving like the Volvo engine’s maximum rating where the truck cruises and holding that torque down to a lower rpm.
So an automated drivetrain with an ultra-fast rear axle is practical because of more frequent downshifts and, best of all, the operator doesn’t have to do that shifting.
Addressing low rpm’s torsional vibration
One potential problem with chugging along at lower rpms is torsional vibration in the transmission gears or elsewhere. This can create noise or even accelerated wear. The problem arises because of differing vibration frequencies — the drivetrain equivalent of the opera singer holding a certain note to make a glass shatter.
Eaton has taken a further step, Shane Groner says, to adapt the clutch dampening system to the new conditions. “Our clutch designs will have larger springs because the peak of torsional vibration passes right through the sweet spot under cruise conditions,” he says.
When it comes to vibration problems, Volvo’s Ed Saxman says, “Our engine has a stiff bottom end, which helps. And we have a viscous damper on the camshaft. This reduces the stress on all the components from vibration transmitted from the engine. ”
The “input damper” on Allison’s TC10, says Todd Dygert, “was specifically designed based on our plan to run at very low engine speeds.”
Cummins anticipates no problem with torsional vibration, says Cummins’ Bill Kendrick. However, “engine mounts and torsional dampers are designed to limit vibrations within specific frequency ranges and lower engine speeds may reduce the effectiveness of these components. In light of that, Cummins is working with powertrain suppliers and vehicle manufacturers to ensure the entire system is well designed. Different components may be required for a specific vehicle.”
With Detroit Diesel, “Significant measurable fuel efficiency benefits are available” from configurations already tested, says Brad Williamson, director of engine and component marketing for Detroit owner Daimler Trucks North America. Detroit is “evaluating the correct driveline specifications for even more significant downspeeding,” he adds.
Another concern is the possibility that running at low rpm could cause problems with combustion and create deposits or higher emissions. But Dygert notes that the “‘on again, off again, on again’ transient cycles dictated by the power-interrupting shifts” of transmission systems in prevalent use today keep the typical turbocharged diesel from operating optimally. “Allison powershifting keeps the engine under a relatively constant load,” he notes, suggesting a boon to emissions reduction.
Kendrick says Cummins designs engines to operate at lower speeds. “When operated as designed, these engines will be fully emissions compliant. We do not anticipate any reduction in reliability or durability as a result of lower speed operation.”