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The Drivetrain Spells Relief

Besides higher initial cost and potentially higher (but still unknown) lifetime costs, the big sticking point to the latest emissions-compliant engines is their drain on fuel economy. But according to OEMs, careful attention to drivetrain spec'ing can help mitigate any fuel efficiency hit from '02 and newer engines without any negative side effects. Relying on their own expertise and that of truck

Besides higher initial cost and potentially higher (but still unknown) lifetime costs, the big sticking point to the latest emissions-compliant engines is their drain on fuel economy.

But according to OEMs, careful attention to drivetrain spec'ing can help mitigate any fuel efficiency hit from '02 and newer engines without any negative side effects.

Relying on their own expertise and that of truck dealers and component suppliers, most fleet owners spec drivetrains with reliability in mind. And most, naturally enough, are reluctant to monkey with what works so wholesale changes to drivetrain specs are rare indeed.

Fortunately, what most industry experts suggest be done does not by any means require fleets to jettison their favored component choices — just to adjust them in one way or another.

Jim Fancher, marketing product manager for Volvo Trucks North America, says the “biggest change brought by '02 and newer engines is a decrease in the rpm economy band,” long known as the sweet spot.

“It used to be that with most engines, you got the best fuel efficiency and ample torque by running in a 200-rpm band, from 1400 to 1600 rpm plus or minus 50 rpm. That's where everyone was.


“But starting with the '02 engines,” Fancher continues, “that band narrowed by 50 rpm so the sweet spot became either 1400-1550, with Volvo engines, or 1450-1600 with other EGR engines such as the Cummins ISX we also offer.”

He explains that the sweet spot is smaller because “the power curve and torque bands of the engine have been changed by the need to meet today's tighter emissions regulations, so the truck buyer must be more aware of where they cruise at, what speed and over what terrain.

“As a general rule of thumb,” Fancher, continues, “specs can be adjusted for the new sweet spot by going to a faster — by one step — gear ratio. For example, a fleet that formerly used a 3.73 axle ratio to cruise at 68 mph would now be able to stay in the sweet spot by spec'ing a 3.58 ratio.”

He points out that some customers opt instead for a transmission with more speeds or go to one with a different differential overdrive rating.

According to Fancher, yet another way to keep hitting the new sweet spots is to change tire size from 24.5 to 22.5. He notes this is seen as a more fitting solution for the owner-operator or very small fleet operator who can cost-effectively switch all tires at once on new tractors to avoid a mixed batch of tires.

He cautions that besides the sweet spot shifting, the new engines also have “higher heat rejection — so more heat must be dissipated. So buyers may find larger or faster fans on engines. These create larger parasitic loss, especially when running in mountainous areas. A fan setup that once drew 34 hp. may be pulling 15 hp. more. When spec'ing, therefore, fleets should be sure that a comfortable speed will be maintained comparable to what they had before.”

Fancher says “proper attention to drivetrain spec'ing with the new engines could mitigate some of the fuel economy loss” experienced with these powerplants.

And he reports that Volvo is seeing increased spec'ing of electronically controlled (automated) transmissions with the new engines. “We think this is partly because vehicles now operate so quietly that it is harder for drivers to hear to shift properly. We even stay below the 80-decibel rating when our engine brake is operating.”

Whatever the components chosen, Fancher points out that “almost all truck makers have spec'ing tools available that will look at a drivetrain setup to see how much of a change in specs should occur. It may take two hours to go over a spec [with a dealer] but that time is worth it when you consider the fleet will keep the truck for a minimum of three years. The key is to remember one size really does not fit all.”

Jeff Sass, Kenworth's on-highway product manager, says whether or not drivetrain specs should be adjusted will depend on which engine a fleet runs. “In our case,” he says, “the choice is between the Cat ACERT and the Cummins EGR engines. Cat recommends dropping to a lower numerical ratio, from 3.55 to 3.36. But with Cummins, gearing is at the same level [as before].

“With both EGR and ACERT,” he continues, “the fuel economy is not what it was, so fleets may want to rethink how much horsepower they really need. In many cases 475 or even 450 hp. may be more than enough. Also, fleets with a broad range of driver skill levels may want to consider switching to automated or automatic transmissions for best across-the-board fuel economy.”

Tony Cook, International Truck & Engine's chief engineer for frame systems, who formerly was engineering chief for powertrain systems and before that worked for another OEM and another engine maker, says engine regs have wrought a major change in how the operating rpm range is leveraged by drivetrain spec'ing.


“In the past,” he explains, “you could trade performance for fuel economy in the operating range. But now these must be more average across the operating range. Because of the lower, narrower sweet spots, it's crucial for fleets to know their speeds. Best thing a fleet can do is query electronic devices to learn their average road speeds.

“If you know the average road speed for a fleet,” Cook continues, “you can tell how to set up the new powertrain properly. Essentially you work backwards from the average speed and the engine's sweet spot. Then you select the rear axle ratio, transmission top gear ratio and tire size. Most fleets won't want to change tire size so it becomes a matter of changing the transmission or axle ratio.”

He points out that even if a fleet selects an automated transmission, the gear ratio must be spec'd. “An automated box will minimize shifting errors so better fuel economy should result as well as less wear and tear on the drivetrain.”

As for how he'd go about spec'ing a drivetrain today, Cook says: “I'd look at the transmission top gear first. You can get a more exact ‘fit’ and a little more flexibility if you run a 13 instead of a 10 speed transmission. There are simply more gears with shorter steps between them.

“But,” he continues, “you must weigh this approach against the higher initial cost of the 13-speed and whether or not extra driver training would be needed to support this switch in specs. In general, just remember more gears means more gears to select from to stay in the sweet spot.”

As for the rear axle, Cook relates that the lower the numerical ratio (that is, the faster the gearing), the larger the pinion gear will be. “The larger unit will last longer than a smaller one doing the same job.”

When it comes to tire size, he says a fleet might go up or down “depending on what looks best for revs per mile, again to get to that sweet spot they need to hit.”

WHAT WORKS BEST Cook says “it all comes down to understanding what works best” in a given operation, whether rear axle ratio, number of transmission speeds or tire size.

While it may seem an engineering degree is a almost a prerequisite to spec'ing a drivetrain, Cook says not to worry as “most OEMs work with the engine makers to provide computerized analysis. Once specs are entered, these programs will post a red flag if anything is not matching up properly.”

Providing a different perspective on the issue of drivetrain selection is Mack Trucks, which like sister firm Volvo, manufacturers heavy-duty engines as well as Class 8 trucks.

According to David McKenna, product manager-marketing for Mack engines, transmissions and axles, the manufacturer “has not changed where we want users to operate the engine.

“No change is needed to either drivetrain specs or driving habits with '04 Mack-powered vehicles for highway applications,” say McKenna.


“Our Mack ASET cooled EGR engines were designed to meet the new operating profile without changing what fleets were used to.”

He says doing so “certainly took a concerted effort that resulted in Mack offering what I characterize as a better overall package of engine, transmission, axles/chassis and cab, including drivetrain components from other suppliers.”

As for vocational vehicles, McKenna says Mack's application-specific approach led it to develop an ASET engine with internal EGR. “In a transient, or work site, state, these engines are very fuel efficient but are less so in steady state, over-the-road operation.

“These engines have higher torque ranges so we've increased their operating range from 1020-1800 rpm to 1200-2100 rpm,” he continues. “That means we must lower the gear set. So in spec'ing these trucks you may go from a 4.17 rear axle ratio to a 4.42 ratio to increase engine rpm as needed. This is what's typically done because these customers generally do not want to change their transmission specs. In effect, changing the axle spec is viewed as more ‘transparent,’ if you will.

“Remember, though,” McKenna adds, “never says ‘one size fits all.’ What all customers should do when spec'ing is work with their salespeople at the dealers. Above all, ask questions.”

Box solutions

Transmission choices abound

According to John Hinesley, western regional manager, ArvinMeritor's FreedomLine automated transmission “always shifts at the optimum point within the rpm range” of any engine.

He says that's significant because ArvinMeritor “has seen customers negate the fuel economy lost to EGR” by spec'ing these automated transmissions. “They're getting back up to where they were [in MPG] before.”

As for manual transmission choices, Hinesley contends “the industry has pretty much eliminated the 9-speed” from the fleet market.

“I think a 10-speed transmission will give a fleet much of the same startability as a 13-speed. But it will cost less initially and have a longer warranty in linehaul applications. Lots of people used to spec a 13-speed for its resale value. But by opting for a 10-speed, they can save on initial cost, fuel and weight savings and gain more warranty.”

“There is much more emphasis now on getting the fuel mileage [lost to EGR] back via using different rear axle ratios,” remarks J. Scott Steurer, product line manager, performance & vocational transmission products for Roadranger. “Usually, this means running with a lower numerical, which means a faster ratio.

“We recommend using a 13-speed transmission to give the operator the most flexibility to run the engine at a lower speed, “ he continues. “Before this, since electronic engines began coming out, the industry had been moving toward 10-speeds away from the old favorite, the 9-speed.

“Now, the industry's move toward automated transmissions, including our AutoShift and UltraShift units, is helping fleets reduce the impact of the drivers' variability on fuel economy. At the same time, having these transmissions make it easier to attract drivers and to protect them from fatigue on duty.”

“Down the road,” says Brian Coe, Roadranger's manager of application / OEM engineering, “we see the ultimate system consisting of drivetrains and powerplants that are integrated or [at least] tuned to each other. Some people might view that as vertical integration but we see it as [coming from] working more closely with engine makers.”

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