Move the most freight consuming the least amount of fuel. The concept is simple, the benefits obvious and multiple, the value irresistible. Even the theoretical path to achieving maximum fuel efficiency in a truck is fairly clear. The problem is you have to do it in the real world, a place where there are huge variations in driver skills, road conditions, load characteristics, and hundreds of other variables big and small that all contrive to consume their extra little piece of the fuel pie.

But if you believe it's a worthwhile goal, you need to start somewhere. It might be a few decades behind fleet owners, but the federal government now believes in that goal. And it also believes it knows where to start — with the truck itself. The Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA) have been ordered to come up with performance standards that will push truck fuel efficiency as much as 20% above today's levels in graduated steps starting as soon as 2014.

The regulators have wisely listened to a panel of trucking experts assembled by the National Research Council (NRC), which cautioned that “truck” covers a huge array of vehicles performing a broad range of functions, each with its own fuel consumption patterns and measures of fuel efficiency. Unlike the automobile industry, trucking can have no meaningful corporate average fuel economy, or CAFE, standard based on a simple measure like mpg. Instead, trucks will need to be measured by productivity yardsticks designed for individual applications.

One of the major truck applications identified by the NRC is the over-the-road heavy-duty tractor-trailer combination. “There are so many commercial vehicle applications, but tractor-trailers use 75% of the fuel consumed [by CVs],” says Michael Roeth, executive director of the North American Council for Freight Efficiency (NACFE).

Formed last year by fleets, manufacturers and researchers to foster new approaches to freight efficiency, the council sees a focus on the tractor-trailer as offering the opportunity for the biggest and most immediate impact on fuel consumption. “If a tractor averages 120,000 mi. a year and diesel is at $4/gal., that translates into $900 for every 1% saved in fuel consumption,” Roeth says.

Apparently, the EPA and NHTSA agree. Their initial proposal for fuel efficiency standards addresses individual goals for heavy trucks, vocational vehicles and large pickups, but looks for the largest improvements to come from OTR tractor-trailers — a 20% reduction in fuel consumption by 2018.

Fuel economy is nothing new for fleets, which have long appreciated the value of even small improvements. Nor is it new for truck manufacturers and suppliers, which have responded to fleet pressure with steady advancement in fuel efficiency. So how do you build a tractor that will deliver another 20%? What will it look like? More importantly, how will it make that leap without damaging its ability to serve its single function — deliver freight?

Fleet Owner posed those questions to many of the engineers who will be responsible for meeting the ambitious EPA/NHTSA goal. We asked them, “What technology is going to be required? What will that tractor and that combination look like? How will it impact the industry's ability to move freight?”

As befits the practical nature of engineers, their answers make the individual changes required to get another 20% out of an over-the-road combination seem quite straightforward. But in aggregate, those elements add up to significant, if not always obvious, changes in the relatively near term for trucking's workhorse. Here's what they had to say on how to build the most fuel-efficient tractor and trailer.

NOTHING RADICAL

I think we have to realize that small things will add up,” says Roeth. “Sometimes we want the big home run we can point to. But with a lot of little improvements, I believe we can get [a tractor-trailer combination] that's twice as efficient.”

One critical element in OTR fuel efficiency is aerodynamics. “Generically, North American tractors are already the most aerodynamic in the world,” says Preston Feight, chief engineer for Kenworth Truck. “We understand tractor aerodynamics quite well. Windshield angles, corner radius, fairings have all evolved, and cabs are pretty slippery today. There will continue to be refinements in the shapes of things like grilles and roofs, but we still have to design the hood to fit around the engine and the cab around the doors. We could build a bullet train, but you have to inject realism into this.” The tractors that meet the 2018 goal of a 20% improvement “will have a strong family resemblance to today's designs,” says Feight.

Ramin Younessi, Navistar's group vice president for product development and strategy, also believes tractor appearance won't change radically, largely because all the truck makers — Navistar included — have rolled out completely new trucks and engines in 2010 and 2011.

“We've all launched new aerodynamic packages; we've all taken our products through extreme changes,” he says. “So don't expect major changes in the next three to five years. What you'll see are more minor tweaks and incremental advances, such as new mirrors, headlamp designs, and so on.”

“Since the front of the truck design is driven by the cooling module size, some new investment might lead to improving engine heat rejection and that could decrease the size of the cooling package,” says Landon Sproul, chief engineer of Peterbilt Motors. Still, it's smaller refinements like minor adjustments to the front bumper and hood gaps, and perhaps replacing mirrors with cameras that are more likely to add up to measurable aerodynamic improvements on the tractor exterior, he says.

For some fleets, there may be even more obvious places to gain aerodynamic improvements. Current options “like full-width bug deflectors, oversize fender-mounted convex mirrors, West Coast mirrors and mounts” will disappear, says David McKenna, director of powertrain sales and marketing for Mack Trucks. “Exterior sun visors will stay but be optimized for aerodynamics.”

A less visible aspect of tractor aerodynamics holds a bit more potential. “We understand external aerodynamics pretty well,” says Sproul. “But we're still learning about underhood air flow, about getting air through the cooling system and engine compartment. We're [computer] modeling underhood flow, but there's a lot of stuff under there and that makes it difficult to model. There's more work to be done, and I think some good improvement is possible.”

Looking to the other half of the vehicle combination, effective trailer skirts like those approved by the EPA SmartWay program have already proven their value in cutting drag and improving fuel economy, according to Garrick Hu, an engineer who's worked for a number of the major truck and component manufacturers and who helped write the NRC's initial study on truck fuel efficiency. Although not widely used at present, “boat tail” designs hold promise, too, in Hu's view “if they can be made practical.”

THE GAP GAP

We need the square shape of dry van trailers to haul the required load, so there's not much we can do about that,” says Younessi. “You won't be seeing ‘bullet shaped’ trailers anytime soon. But the trailer guys are pushing hard on fuel economy, too. So we'll see a lot of incremental design changes to improve [trailer] aerodynamics, as well as lower weight to make fuel economy improvements.”

But the largest potential for aerodynamic gains probably lies between the tractor and trailer, Hu says.

Actually, that's the consensus of every engineer interviewed. Sproul, for example, calls the tractor-trailer gap “a hog when it comes to drag.” And Dr. Wilfried Achenbach, senior vice president of engineering and technology for Daimler Trucks North America (DTNA), quantifies that opportunity: “You can get a 2 to 3% loss in fuel efficiency if the gap is too big.”

However, views on how to deal with that gap practically are still far from settled. “We've all done design concepts that eliminate the gap using things like sliding fifth wheels, air pressurization in that area, sliding fairings,” says Feight. “It comes down to cost benefit. An active system has to have reliability to make commercial sense. We can't create some complicated design that isn't practical in the real world.”

NACFE's Roeth comes at the gap problem from a different angle. “Our current [distribution] model calls for dropping trailers, which means a tractor has to be able to haul every trailer,” he says. “If we moved to a dedicated trailer model, we could work on that gap and other features to make the entire combination more efficient.”

Roeth speculates that such a dedicated combination might also allow truck makers to design an OTR tractor cab from a clean sheet of paper. “Truck cabs are designed to service many applications, but if we really focused on the long-haul tractor-trailer, that cab might look quite different and be more efficient,” he says. “For example, we wouldn't have to worry about having a cab wide enough to fit three people, or a front axle capable of carrying really heavy weights.”

Others like Feight aren't convinced that dedicated cabs are necessary, at least not in the next five to seven years. “The levers we need to pull [to maximize fuel efficiency] don't require separate OTR and truck cabs,” he says. “I'm not sure there's true value there. You could give back any aerodynamic benefit gained [from the tractor] before you get to the end of the trailer. And there's a risk to residual value if you get too specific, which is part of your true operating cost.”

POWER TO BURN

Of course, aerodynamics are just part of the equation when it comes to building the most fuel-efficient tractor-trailer. The powertrain is, after all, where the fuel is actually consumed.

Starting at the business end, choosing the right engine is the place to begin the drive for maximum fuel efficiency. The recent expansion of 13L diesel options for OTR tractors has fostered a good deal of debate over the inherent fuel efficiency advantages of the smaller displacement over the more traditional 15L heavy-duty diesel.

Given the sophistication of modern fuel systems and electronic controls, that may be a red herring. Calling 13L and 15L engines “virtually the same” when it comes to fuel consumption in a properly spec'd drivetrain, Achenbach still believes selecting the right engine for the application is key. The bigger displacement will be appropriate for more demanding applications that require higher horsepower and/or longer life, but the 13L will offer lower weight and all the power needed for many OTR applications, he explains.

While lowering weight is widely understood to improve miles per gallon, heading into the future mpg will not be the true measure of efficiency, according to Feight. “Ton-miles/gallon is the real measure of the work accomplished [by a freight-carrying truck], and the 400 lbs. you save with a 13L is meaningful if you can use the payload,” he says. (Not coincidently, ton-miles/gallon is likely to be the metric used by EPA and NHTSA to judge the mandated fuel efficiency improvement for an OTR tractor. )

No matter what the displacement, the most fuel-efficient truck “will keep the engine in the optimum point of its fuel map as much as it can,” says Sproul. “That's about matching the entire powertrain, getting the right axle ratios for actual driving cycles, not what a fleet thinks it might need.”

Technology will help engines stay in that “sweet spot” more consistently in a few ways. At the engine hardware level, there are already mass-based variable torque systems that automatically adjust available engine power to match the actual work at hand, says McKenna.

“How many times have we witnessed an empty trailer or bobtail tractor accelerating so hard that the torque raises the frame a few inches?” says McKenna. “That's nothing more than very expensive fuel being converted into noise.” Having engine controls supply just the power needed eliminates that waste.

Having the truck let the driver know when they're operating in the engine's sweet spot is another less direct but also effective strategy, says Ed Saxman, drivetrain product manager for Volvo Trucks North America. “Having them watch time in the sweet spot is more important than watching mpg because there are so many things affecting mpg that the driver has no control over,” he says. “If you have them aim for the sweet spot as much as possible, you'll maximize fuel efficiency.”

In the near future, automating other functions will further improve powertrain optimization. For example, with new three-dimensional GPS data becoming available, automated transmissions will be able to combine that information with engine data “to adjust shift points on the fly according to actual conditions,” says Achenbach. Similarly, such data could power even smarter cruise control systems that never tire of trying to keep the engine in the most fuel-efficient part of its fuel map, he adds.

THE LITTLE THINGS

While 3D GPS and mass-sensing torque delivery are exciting technologies, smaller and less glamorous changes will also play an important part in maximizing the powertrain's fuel efficiency. For example, according to both Roeth and Achenbach, many fleets could today switch from 6×4 to 6×2 drive axles, trading torque many don't actually require for better fuel efficiency through lower weight and reduced parasitic losses.

Reducing parasitic losses from compressors, steering pumps and other auxiliaries by activating them only when needed are among the many easy things that could be implemented at relatively low cost, explains Roeth. Even switching to all synthetic lubricants might improve fuel efficiency by 0.5%, he says. “These little things might not be as noticeable, but we have to realize they're just as important because all these tiny improvements will add up.”

In a similar vein, the SmartWay program has already pointed to currently available efficiency aides like fuel-efficient tires that will be a big part of building the most fuel-efficient tractor-trailer, adds Hu. “Fundamentally for the near term, SmartWay has already identified the major elements,” he says.

DOWN THE ROAD

Looking beyond the three- to five-year timeframe, there are some promising technologies that could take tractor efficiency even higher. One mentioned by a number of the engineers is a hybrid diesel electric specifically designed for a heavy-duty over-the-road application.

“There are some interesting results coming out of WalMart's testing of a hybrid for over the road,” says Hu. The WalMart project uses the hybrid power just for hotel loads, but Hu envisions a system that is optimized for OTR, running the tractor on electric power only up to 30 mph, moving to a blend of diesel and electric over 30, and switching to pure mechanical diesel power in the top two gears. “If you look at the big picture, hybridization needs to be explored for over the road, not just pickup and delivery,” says Hu.

Younessi is also a believer in the potential of diesel-electric hybrids for tractors. “It would allow us to downsize the diesel without lowering available power and also provide electric power to run a variety of onboard systems without having to rely on the diesel for that muscle,” he says.

The other longer term technology being explored by a number of truck makers is waste heat recovery. It's probably more than five years out, but “you're going to see a lot of focus on waste heat recovery, the ability to take heat lost through engine exhaust and convert it into electric power that can then be returned to the vehicle to reduce parasitic loads on the engine,” Younessi says.

Even variable ride-height control holds potential for delivering advanced but still practical increases in fuel efficiency by lowering drag at higher speeds. “In such a system, the chassis would lower by 1.5 in. to 2 in. at the maximum cruise speed setting, reducing frontal air size,” McKenna says. “I think we could also see electronically controlled fairing trim tabs, allowing for differences in trailer size and design.”

Whether any of these technologies find their way onto working trucks, one thing is certain: The coming mandate to boost tractor-trailer fuel efficiency by 20% in the next seven years is not just achievable, but it's really just one more step in the evolution of the most fuel-efficient truck.