Engine Downspeeding Dictates Drive-Axle Design

Justin Ide/Bloomberg News

This story appears in the Oct. 24 print edition of Transport Topics.

To slow the speed of the engine at cruise — called downspeeding, in which every 100 rpm slowed equates to 1% fuel savings — axle ratios are getting numerically lower. To wring the most miles from a gallon of fuel, some fleets are increasingly spec’ing direct-drive instead of overdrive transmissions, which is driving the trend to lower numerical final drive ratios.

The direct-drive transmission is gaining acceptance because of its high gear ratio, which is the cruising gear, and has a direct path for the engine torque through the transmission. An overdrive transmission passes the torque through two sets of gears resulting in a small, but measurable, loss of fuel economy through added friction.

However, downspeeding and direct drives result in higher torques in the whole drivetrain. As a result, along with lower axle ratios comes the need to beef up drivelines and the axle’s internal components.

Generally, downspeed ratios start around 2.5-to-1. The fastest currently sit at 2.26 for Dana Holding Corp. and 2.28 for Meritor Inc., the axle and driveline manufacturers.

Daimler Trucks North America, though, recently announced a 2.18 ratio is on the way for its upcoming Cascadia AeroX in its own-brand Detroit axle. This Cascadia is due to go on sale in the second quarter of 2017 as a 2018 model year truck.

Daimler also is adding fast ratios to its Detroit axles with a 2.28 for the Cascadia Evolution direct drive and 2.42 for the overdrive DT12 transmissions, which are part of the extreme economy spec for the current Evolution.

At Volvo, where downspeeding was first promoted in its XE drivetrain package, the ratio is 2.47-to-1 but is used in combination with an overdrive iShift transmission. At Mack, the downspeed package uses the overdrive mDrive transmission and a 2.66 axle ratio. Navistar’s fuel-efficient ProStar ES is offered with Dana or Meritor fast ratio axles.

But 2.26 is not final when it comes to downspeeding.

Steve Slesinski, global product planning director at Dana, said the truck and engine OEMs are looking at 900 rpm cruise engine speeds. That means still lower ratios if the axle manufacturers are to stay ahead of the curve.

In the Dana pipeline is a 2.05-1 ratio set for the AdvanTEK. The 6×2 EconoTREK will feature a 1.91 ratio in its single drive axle. And for its Dual Range Disconnect axle now in prototype testing, the ratio on the high-side 6×2 configuration may be as tall as 1.91-to-1.

To support the greenhouse gas fuel-economy mandates, this ratio may go as low as 1.7-to-1 as engines downspeed to 900 rpm to meet the anticipated targets of the Phase 2 mandate.

Dana and Meritor re-engineered their fast-ratio axles several years ago. Meritor was first with the 14X five years ago, said Ken Hogan, Meritor’s vice president of rear drivetrain. ”In the spec process, customers don’t think axle ratio first,” he said, noting they consider where a vehicle will run, desired vehicle speed, tire size and transmission choice, among other issues. “Given that, they decide on axle ratio,” Hogan said. “But in fuel-economy spec’ing, the important ratios are 2.28, 2.41. 2.47.”

One Meritor customer with a reputation for extreme fuel economy spec’ing is New Mexico- based Mesilla Valley Transportation, which ranks No. 66 on the Transport Topics Top 100 list of the largest U.S. and Canadian for-hire carriers.

Jimmy Ray, equipment and economy vice president, has supported downspeeding for years. Mesilla Valley likes to run at a fleet preferred cruising speed of 52 to 65 mph. Its target for tall gearing is two revolutions of the engine to equal one revolution of the drive tires, he said. “That gives us three power pulses times two per-tire revolution,” Ray said. “It reduces the Btu it takes to drive the [engine] accessories such as the water pump.”

Ray said he specs a 2.50 axle ratio but runs it with an overdrive transmission. He is experimenting with a ratio close to 2.3-to-1 and direct-drive transmissions. But shifting to lower number ratios “puts more stress on the metal,” Ray said.

It’s not the only fuel-saving technology MVT uses — for example, that low gear ratio in the single drive axle of Ray’s preferred 6×2 configuration. He’s a proponent of super single tires and other fuel-saving devices.

Another early adopter of 6×2 configuration in over-the-highway operation is Hudson, Illinois-based Nussbaum Transportation. “The 2010 model year trucks were the test trucks set up with some 6×2 versus 6×4, including some running with wide base versus duals,” Tony Morthland, Nussbaum’s director of maintenance, wrote in an e-mail. The company has had success especially when the 6×2 is combined with a tall ratio in the drive axle, he said. “The key is to match up the transmission with the right rear ratio to peak engine rpm performance and torque.”

He provided an example of a 2.50 ratio with a direct drive transmission that would give about a 1,400-rpm range at cruise. With a 2.28 ratio, the direct drive transmission ratio “would give you a 1,270 rpm,” Morthland said.

Many fleets that are reluctant to try the same combination fear the effects on resale value and maintenance expense. But Nussbaum’s experience shows, in its operation at least, such fears are unfounded. “We have not seen any added costs to our fleet to date,” he wrote. “We have been running lower ratios since 2010, but we run automated manual transmissions and believe that has helped with lowered maintenance and abuse costs.”

“We have fewer spinouts, and there’s no dumping of the clutches to twist drivelines or break an axle,” he wrote, adding there’s been no effect on trade-in value.

Meanwhile, the potential for driveline damage stacks up as the gear ratio in the axle lowers and the torques increase, said Dana’s Slesinski. Dana offers the 2.26 ratio, but the slower drive shaft rotation requires more strength in the universal joints.

“Downspeeding from 1,450 rpm to 1,125 rpm [from a 4.42 ratio to a 2.64 ratio] can increase driveline torque by 29%,” he said. “Going from a 3.55 to a 2.26 increases torque by 57%.”

As a result, the company developed the Spicer AdvanTEK D40 to be the “most efficient, durable and reliable in the industry,” Slesinski said. “To reiterate, 100-rpm reduction in engine speed equals a 1% efficiency improvement. Even at today’s lowered fuel price, that equals $427 in annual fuel cost per truck per year.”

Downspeeding, however, is not for all trucks.

Vocational and city drayage, where low speeds and stops and starts are common, would not realize the fuel savings, which can be 2% to 3% in linehaul. In addition, because of the high torque issues introduced into the drivetrain at ever-lower drive shaft speeds, there are concerns with a driveline’s reliability, said Mike Roeth, executive director of the North American Council for Freight Efficiency and operation lead for Trucking Efficiency at the Carbon War Room.

“A few fleets are having failures [with downspeeding], but they are usually in day-cab operations which see more hitting of docks and backing under trailers,” he said. “And we heard from one mixed-operation fleet that runs tractors on linehaul during the day and city deliveries during the night that drivelines are an issue.”