Driving fuel savings

June 1, 2008
When it comes to spec'ing drivetrain components, David McKenna is a big believer in splitting hairs. By that, he means that fleets must focus closely on their operating style and spec a drivetrain to match driving needs in order to cast a big enough net to snare the largest amount possible in fuel savings. Because in today's low-emissions world, obtaining fuel economy improvements is getting harder

When it comes to spec'ing drivetrain components, David McKenna is a big believer in splitting hairs. By that, he means that fleets must focus closely on their operating style and spec a drivetrain to match driving needs in order to cast a big enough net to snare the largest amount possible in fuel savings. Because in today's low-emissions world, obtaining fuel economy improvements is getting harder and harder all the time.

“I have always approached complete vehicle spec'ing from a holistic standpoint — I know, too much time living in southern California,” says McKenna, powertrain sales and marketing manager for Mack Trucks. “But if the very best and most fuel-efficient engine is selected, yet is not mated with a compatible transmission; if it is set up to gear fast/run slow and the latter is not met; if the incorrect rear axle ratio is chosen and poor tires selected; then the negative affect is very costly.”

He notes that OEMs used to call the optimum engine rpm for best fuel economy the “sweet spot.” However, “over the years, as we reduced emissions to near zero, this spot has evolved into a ‘sweet dot,’ meaning the engine rpm best suited for fuel economy is much smaller today than in the past,” he explains.

The key is to design drivetrain components to work together as a complete and efficient system, adds Loren Dreier, FreedomLine engineering manager for ZF, meaning the drivetrain should work in conjunction with other components to maintain optimum fuel efficiency.


“Rising fuel costs are having a substantial impact on how trucks are spec'd,” Dreier says. “An improperly specified axle ratio, which, for example, results in engine rpm at on-highway cruise speeds outside the ideal optimum fuel island, result[s] in reduced fuel economy.”

“Correctly matching the drivetrain is absolutely critical to obtain, not only the maximum fuel economy, but also to have acceptable performance, including startability and gradeability,” notes Cam Seger, manager of powertrain development and validation at Navistar Truck Group.

“That being said, there is really a window of opportunity to achieve this goal,” Seger says. “There are usually a couple of axle ratios and transmission selections that will be okay; however, based on the application of the vehicle, those specs can be refined to work with the engine to obtain maximum efficiency. Depending on engine fuel map characteristics, different engines will respond more or less favorably to the same axle ratios and transmissions, so it is very important to work with your OEM to understand what spec makes the most sense.”

One key to remember is that you can't have maximum performance and fuel economy in the same package, Seger stresses. “You can have a compromise that maximizes or improves your fuel economy while still delivering mission-critical performance,” he states.

Yet it's not all about how the drivetrain components go together. There are a lot of other factors to consider, says Jeff Sass, marketing planning manager for Kenworth Truck Co. “There are a lot of variables you have to factor in, such as the terrain your fleet operates in,” he says. “Are you driving mostly on flat roads or on hills? Are your trucks on highway asphalt all the time, or do they go off road on gravel and dirt? Are you running long haul or urban pickup and delivery routes? All that information helps you truly optimize the drivetrain spec.”

Figuring out the truck's operating style helps drive component selection for the drivetrain, such as choosing proper axle ratios to maximize fuel efficiency, says Steve Slesinski, director of product management for the commercial vehicle products group of Dana Holding.

“Selecting an axle ratio to reach optimum performance around a certain engine rpm is more important than ever because of the high cost of fuel today,” he explains. “That's why it is very important to specify a transmission and axle system that is geared to be most efficient when the engine is operating in its most typical range of operating speed.”

For instance, in a P&D operation, Slesinski says fleets generally operate in a wide speed range. “So here you will want to go with a slower axle ratio to help make the vehicle easier to launch,” he notes. “Conversely, you will want to specify a faster ratio for over-the-road applications, or a ratio that will optimize engine rpms at higher speeds for the best fuel consumption.”


Another factor that needs to be considered is tire pressure, Slesinski stresses. “Under-inflated tires, especially those on the drive axle, can definitely impact operating efficiency and fuel economy,” he notes.

The biggest factor of all, contends Navistar's Seger, is the recommended speed ranges formulated by the engine OEMs. Over-the-road (OTR) applications tend to have higher average vehicle speeds, he says, so in order to keep the engine in the recommended range, this application would tend to run a lower numeric axle ratio than pickup and delivery operations.

“These applications tend to spend most time in the top gear of the transmission and most efficient transmission uses a direct drive top gear,” says Seger. “If the startability, gradeability and cruise speed requirements allow for spec'ing of a low numeric axle and a direct drive, it will provide the best fuel economy in OTR applications. If a deeper [higher numeric] axle ratio is required to meet startability and gradeability requirements, an overdrive transmission may be necessary to still achieve vehicle cruise speed. This will allow for operation in the proper engine speed range, minimizing the efficiency loss of the overdrive and maximizing the engine's potential.”

Long story short, incorrect drivetrain and other component spec'ing can cost a fleet well over 10% in fuel economy, Seger says. “Conversely, refining specs based on the application can net several percent in improvements,” he notes.

The important thing for fleets to recognize here is that fuel economy gains can be realized in all vocations, says Mack's McKenna, though each duty cycle will dictate differing specifications.

“Say a typical highway tractor with a cruise speed limit of 65 mph might have a rear axle ratio of 3.5 to 3.7:1, whereas a typical refuse collection chassis may have a rear axle gear set of 4.9:1,” he explains. “Yet both have been optimized for fuel economy in each of their respective duty cycles. But if the chassis roles were exchanged, both performances would significantly suffer. That's why it's never been more important than now to work with your dealer and technical support to optimize your new truck for your specific operation.”


Another drivetrain trend that's gaining steam due to rising fuel prices is the movement to automated and automatic transmissions. Class 8 operators especially are taking a much harder look at switching to automated mechanical transmissions (AMTs) despite a $5,000 or more premium versus manuals because AMTs can deliver more consistent fuel economy gains over time.

“The reason AMTs are picking up some speed in terms of wider use in this industry is simply because they don't get tired,” says Charlie Allen, national service director for commercial vehicle systems at ArvinMeritor. “The key here is that the electronics take over managing the drivetrain and begins making shifts based on a consistent set of rules. That consistency of operation is where you pick up fuel savings.”


Allen stresses, however, that a fleet's best driver can still beat an AMT in terms of delivering superior fuel economy. What an AMT does, though, is bring all of its other drivers up to that “best driver” fuel efficiency standard and keeps delivering it, day in and day out, despite the weather, the terrain and the load.

“You're getting more consistent miles per gallon across all of the fleet's vehicles, regardless of the driver behind the wheel,” he explains. “And these gains can be had in both long haul and pickup and delivery environments. And when we're talking about fuel costs of around 70 cents a mile for trucks getting six miles to the gallon, gaining even a couple percentage points in fuel economy can translate into saving some real money.”

“Safety is the huge driver behind AMTs, but just as equally important today is getting the best fuel economy across the entire fleet,” says Gerard Devito, director of engineering for heavy-duty transmission at Eaton Corp. “If you get even a one-tenth of a mile improvement in fuel economy, with fuel at $3.45 per gallon, you're talking $1,000 per year per truck. For a fleet with 1,000 trucks, that's $1 million in savings. That's significant.”

And at that rate, adds Devito, fleets can easily get an 18-month payback to cover the extra cost for an AMT versus a manual package. “The fuel savings easily offset the AMT upcharge,” he notes.

“In the past, the amount of fuel savings all depended on the driver,” adds Ed Saxman, drivetrain product manager for Volvo Trucks North America. “That included how the driver shifted on hills, managed poor weather and bad road conditions. Now you have an AMT in place, not only programmed to manage those conditions, but also tuned with the engine to operate in the fuel economy ‘sweet spot’ as much as possible.”

Saxman pointed to Volvo's I-shift automated transmission as an example. “Its electronics package measures road and load conditions constantly, so it knows exactly when to shift. And it skips gears like nobody's business,” he notes. “The key here is shifting when you need to, not when you think you need to.”

As an example, Saxman refers to a test drive he conducted last year in Savannah, GA, over the city's big 185-ft.-high bridge, which has a 5% grade. “In cruise control, the I-shift changed gears at 1,100 rpm, whereas I would have shifted at 1,200 rpm,” he explains. “So I would have made two extra shifts over that bridge, costing me extra fuel, because of how I felt behind the wheel — not based on what the truck really needed.”

The automatic angle

Though they remain the priciest transmission option for heavy-duty truck drivetrains, the value automatics can return in terms of fuel savings is rising exponentially across several operating characteristics, says Lou Gilbert, director, North American marketing for Allison Transmission.

“A fully automatic transmission and a truck spec that is appropriate for the duty cycle can provide optimum fuel economy and efficiency,” he explains. “To fully understand this, a truck's duty cycle must be broken into four components: acceleration, cruise speeds, deceleration and idle.”

However, in terms of just fuel economy gains, Gilbert says the first two components of drivetrain operation — acceleration and cruise speed — can benefit the most from a fully automatic transmission.

Acceleration: Automatics provide full-power shifts, which are the most fuel-efficient way to put engine power to the ground, he says, as manual and automated manual transmissions must interrupt engine power to the ground every time a shift is made. “Analysis of two North American pickup and delivery fleets shows an average of over 600 upshifts in an eight-hour timeframe,” he notes. With full-power shifts, fleet managers can recognize higher average speeds versus manual or automated manual transmissions, and higher average speeds over a day mean more work is accomplished with proportionately less fuel, says Gilbert.

Cruise: The most critical component to achieving maximum fuel economy during cruising is engine rpm, Gilbert stresses, so whether a truck cruises at 10, 40 or 65 mph, having the engine operating in its recommended area is the best way to reduce fuel consumption and increase mileage per gallon. A fully automatic transmission has a torque converter that multiplies engine torque in 1st and 2nd gear, Gilbert notes. With that multiplication, the torque converter allows all internal ranges in a fully automatic transmission to be higher (lower numerically) than a comparable manual or automated manual transmission. “This causes reduced engine rpm at lower cruise speed and reduces fuel consumption,” he says. “At higher cruise speeds, the Allison transmission overdrive ratios keep engine rpm in the optimum range producing optimum fuel economy.”

Lube performance

Can the type of lubricant used in transmissions and axles improve performance? Especially in terms of fuel savings? On-highway tests conducted by Roadranger, a marketing partnership between Eaton and Dana Corp., indicate that using synthetic lubricant in both components can translate into fuel economy gains of more than 1%.

Two linehaul fleets were involved in the tests, which compared new Roadranger Synthetic Lubricants to traditional synthetic blends and semi-synthetic blends. A third test was conducted by an independent research facility. Using an SAE Type III 1526 test, a TMC/SAE J1321 test, along with a modified J1376 test, the tests showed both linehaul carriers, on average, improved fuel economy 1.029%, with the independent research facility generating fuel savings of 1.112%.

“This is a significant finding when you take into account that every 1% improvement amounts to about $500 in annual fuel savings per truck,” says Richard “Rick” Muth, manager of lubricants for Roadranger. “Our previous Roadranger lubricant blends were already producing fuel-saving benefits of 2% and 4% over mineral-based lubricants, so the additional savings is a further bonus.”

Roadranger-approved synthetics are required in Eaton Fuller transmissions and Dana Spicer axles in order to qualify for extended warranty coverage and up to 500,000-mi. initial drain approvals, Muth notes.

He adds, by the way, that Road-ranger is now starting research work on ways to make transmission fluid thinner (reducing its viscosity) without compromising life expectancy, as thinner lubricant makes it easier for the gears to “move,” thus improving fuel economy further.

Highway drivetrain rules

David McKenna, powertrain sales and marketing manager for Mack Trucks, has a few simple drivetrain spec'ing rules for the typical highway fleet operation to follow that should net it improved fuel economy over the life cycle of its trucks:

  • Determine the horsepower required to perform the work.

  • Buy no more horsepower than required.

  • Gear the engine to operate at around 300 to 350 rpm above peak torque.

  • Transmission gear step percentages cannot exceed engine torque rise percentage.

  • Use all available fuel-saving features offered with the chassis and engine electronic packages.

  • Ask for advice on all updates and new technologies.

“One last bit of advice is to not overpower the truck,” notes Cam Seger, manager of powertrain development and validation at Navistar Truck Group. “If the application only requires 425 hp., don't run engines rated for more. Also, keep displacement in mind. If you can do the job with a 13-liter engine rather than 15-liter, the lower displacement engine will generally be better for fuel consumption.”

View more Fleet Owner news relating to alternative fuels, fuel conservation, fuel economy and diesel fuel prices.

About the Author

Sean Kilcarr | Editor in Chief

Sean previously reported and commented on trends affecting the many different strata of the trucking industry. Also be sure to visit Sean's blog Trucks at Work where he offers analysis on a variety of different topics inside the trucking industry.

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