For as long as can be remembered, getting a tractor-trailer to attain 10 mpg represented the Holy Grail of efficiency for the trucking industry. It was a magic number targeted by everyone from government scientists and private sector engineers to drivers and fleets alike.
And for just as long, breaking that mythical 10 mpg barrier proved to be difficult if not impossible, in no small part due to the complex emissions control systems added to diesel-fired commercial vehicles since 2002. Those systems in many cases reduced fuel economy.
Just don’t tell Henry Albert that.
Two years ago, Albert, owner of Statesville, NC-based Albert Transport, clocked 10.053 mpg on a 2.5-day, 1,438-mi. run from Lafayette, LA, to Laredo, TX, and then to Meridian, MS. Piloting a 2014 Freightliner Cascadia Evolution tractor equipped with a Detroit DD15 engine connected to a DT12 automated manual transmission (AMT), Albert says his loaded weight ranged from 60,000 to 65,000 lbs. during that trip, while his cruising speed hovered around 63 mph.
He also credits the super-aerodynamic profile of his tractor and trailer for helping him break the 10 mpg barrier, pointing in particular to the covers adorning his tractor and trailer wheels, 6x2 tractor axle configuration, trailer side fairings, and trailer boat tail aerodynamic device.
“I’ve made everything on my vehicle as aerodynamic as possible, especially the tractor,” Albert explains. “The tractor is the most valuable player in this battle with the air as it’s literally at the front line of this fight,” he points out. “By having the air flow smoothly across the tractor, it allows for maximum benefit from the aerodynamic devices that have been added to the trailer.”
Yet Albert stresses that he’s changed his driving style as well to better match the gear-shifting particulars of the AMT driving his rig.
“The key is still the driver; how he or she takes that aerodynamic vehicle and matches the sweet spots on the fuel map and torque curve,” Albert notes. “It’s also about knowing your operation; what you are hauling, how you haul it, the traffic, the weather—all the environmental conditions.”
Despite those many challenges, Albert says beating 10 mpg is no longer a remote possibility with today’s equipment, and he’s continuing to prove that on a consistent basis.
“I’ve done it for a day, for an entire tankful, for a week, and for a month,” he says. “Now, I aim to do it for an entire quarter. Right now, the closest I’ve gotten is 9.995 mpg—but I’m close.”
Mary Aufdemberg, director of product marketing at Freightliner Trucks, notes that Albert is an example of how aerodynamic specifications and driver skills can be combined to achieve such significant fuel mileage targets and also of just how hard it is to sustain such numbers over the long term.
“Terrain, weather, and application are definitely the biggest uncontrollable factors impacting fuel economy,” she says. “Although we make products that mitigate these factors, they can never be overcome completely. For example, the aerodynamic enhancements found on the Freightliner Cascadia Evolution can reduce the impact of crosswinds, but those conditions still hurt mpg.”
Aufdemberg adds that Freightliner continues to “aggressively pursue” engine downspeeding as well, crafting its DT12 AMT shift patterns into tighter gear steps to better control the engine’s sweet spot for fuel economy. “We can engineer the power and torque delivery much lower into the rpm range and hold the truck there in top gear to make the most of it,” she notes.
Properly driving the vehicle to attain the best fuel mileage remains a highly variable part of the 10 mpg equation, she says. “Fleets are becoming better at driver training, but it’s an uphill battle because turnover is so high and the pool of experienced drivers is shrinking,” she explains.
Ken Damon, manager of vehicle performance for Peterbilt Motors, says it’s going to be a combination of many things—optimized components and systems, enhanced aerodynamic efficiencies, and maximized driver performance—that will enable fully loaded tractor-trailers to eventually dial in consistent fuel economy beyond 10 mpg.
“The SuperTruck is a great example of these things working together,” he points out. SuperTruck is a project started by the U.S. Dept. of Energy almost five years ago.
In Peterbilt’s case, the OEM combined a Model 587 highway tractor, a higher-efficiency engine built by Cummins, and a super-aerodynamic trailer to try and hit higher fuel mileage targets.
“During testing, we did run the SuperTruck at 80,000 lbs. and achieved an average of 10.4 mpg,” Damon notes. “While the driver will always be essential to vehicle operation and fuel efficiency, automation does help immensely. Using technologies such as predictive cruise control has a significant impact on fuel economy as the vehicle optimizes acceleration, coasting, and braking based on terrain such as road inclines far ahead of what the driver can anticipate.”
He adds that trailers still remain an overlooked piece of this fuel economy puzzle. “It’s not all about the tractor; it’s the combination of the tractor and trailer. To reach the full potential for fuel economy, you need to treat the trailer equally,” Damon stresses. “OEMs can do everything possible to the tractor to improve fuel economy, but you will plateau if you don’t also work on the trailer.”
Damon also points out that the majority of the available fuel-saving technologies in production today are designed to work best with vehicles that spend the majority of their time at highway speeds.
“Aerodynamics become less and less important in stop-and-go operations and non-highway applications,” he emphasizes. “That’s why in city, pickup and delivery, and vocational applications, fuel economy improvement is a much greater challenge due to relative high idle time and the business of getting the load moving repeatedly.”
Jason Spence, Volvo Trucks product marketing manager for long haul, echoes that issue more succinctly: The biggest problem in terms of getting higher and more consistent fuel efficiency in trucking is that there is nothing consistent about trucking.
Drawbacks to higher mpg
“Even in an application where it might be possible to achieve 10 mpg consistently, doing so likely would require some undesirable trade-offs,” he says. “For example, closing the gap between the tractor and trailer completely or creating a smooth underside for the whole combination would improve aerodynamics, but the costs involved could negate the benefits.”
Accelerating slowly onto Interstates would save fuel, but this could be frustrating for truck drivers and for automobile drivers having to maneuver around slower trucks, potentially harming the public’s acceptance of truck traffic, Spence notes.
John Moore, Volvo Trucks product marketing manager for powertrains, points out that because operational consistency and aerodynamics play such a large role in fuel efficiency, it will be very challenging to achieve 10 mpg in applications other than long-haul highway trucks pulling van trailers.
“All other applications experience additional stops and starts and less aerodynamic trailer loads, and heavy-haul and vocational applications face gearing challenges,” he explains.
That’s partly why Moore thinks there really isn’t a “next big thing” on the horizon in terms of gaining more fuel economy. Rather, it will actually be the combination of a lot of little things that together yield big fuel savings.
“Truck owners and their suppliers will also have to balance out the technology costs relative to the return on investment (ROI),” he says.
One new avenue being explored in that ROI debate is the role data analytics can play, especially in terms of correcting driver behaviors, says Mike McQuade, chief technology officer for Zonar.
“Regarding driver behavior, this is something that we are seeing big breakthroughs in,” he explains. “As all trucks become connected, and additional data is pumped into the cloud, new analytics methods to help drivers operate the truck are delivering big wins.”
By contrast, McQuade feels that a lack of control and monitoring systems to ensure that the truck is at peak health and that the driver is operating it at maximum efficiency can cripple attempts to reach and surpass the 10 mpg barrier.
“From our perspective, data analytics are needed to maximize the return on all of these areas,” he explains. “Connecting every tractor to the Internet, collecting a high definition stream of data from the truck to maximize the truck health in terms of uptime and fuel economy—all while helping fine-tune driver techniques—will be key to getting and keeping this Holy Grail.”
Bruce Berger, director of operations for Aim NationaLease, which operates 500 power units, says that’s a lot easier said than done.
“We’re just not close enough to 10 mpg to do it on a consistent basis. That is partly because we run a variety of equipment—straight trucks to highway tractors—and run fully loaded out to 80,000 lbs. with tankers, flatbeds and dry van trailers.”
In a perfect world, with proper driving and a tailwind, Berger says his fleet could get to that 10 mpg mark. “But we’re not seeing that. Rather, we’re getting between 6.8 and 7.5 mpg,” he notes. “But we’re pretty happy with that; I never thought we’d see those kinds of numbers.”
Berger also thinks drivers are why fuel economy is better than in the past yet are also why it’s not improving further.
“We’re not driving trucks like we did 10 or even five years ago, and it’s hard to teach old dogs new tricks,” he says. “You just can’t tell drivers to slow down; you must show them why. For example, most are used to running at higher rpm. They want to downshift as soon as they see those rpm drop. That is a huge change in how trucks are operated.”
From Berger’s perspective, driver variability is slowly being removed from the fuel economy equation through the use of AMTs and fully automatic transmissions.
“As a result, we’re getting much more consistent fuel economy,” he says. “Yet [AMTs and automatics] are expensive. It’s a lot easier to see a return at $4/gal. compared to $2.85/gal. But we know [fuel prices] are not going to stay [low].”
And the everyday vagaries of truck operation will also continue to be a thorn in the side of attempts to reach higher fuel economy numbers.
“We still must negotiate city traffic, and it is still hard to reduce idle time,” Berger points out. “When drivers sleep, they need to stay warm or cool and so they need to idle the engine. APUs [auxiliary power units] remain expensive and they still burn fuel, though less than an engine.”
Trying to successfully compensate for such truck operation variability via equipment specifications is exceedingly difficult as well, says Aaron Peterson, chief performance engineer for on-highway at Navistar.
“One major factor is duty cycle. That will [prevent] fleets from hitting 10 mpg consistently,” he says. “Weather, cold, heat, snow, and rain plus traffic affects that as well. While we’re more than capable of high fuel mileage in steady-state highway driving, especially dedicated operators who are less interrupted by severe weather, others such as vocational fleets remain hindered by their operating environment.”
Peterson adds that it also takes very little time spent at zero mpg—idling in traffic—to ruin a 10 mpg trip average.
He believes some of the new paths the industry can pursue to get out of its current “box” on fuel economy include vehicle-to-infrastructure [V2I] and vehicle-to-vehicle [V2V] technology.
“V2I allows the truck to consistently look ahead to better predict needed engine load, traffic conditions, and weather,” Peterson points out. “It’s about fine-tuning truck efficiency further—turning off accessories when not needed and taking advantage of ‘free’ energy.”
Implementing the draft
V2V opens up another opportunity to create better vehicle-to-vehicle aerodynamics via platooning and drafting, he explains.
“Those types of things are more in the distant future than near term, but they are control strategies that will open up further opportunity for fuel savings,” he notes.
Roy Horton, director of product marketing at Mack Trucks, stresses that there isn’t one silver bullet that’s going to get commercial trucks to surpass the 10 mpg threshold day in and day out.
“No one should expect a 500-hp. tractor running at 75 mph to get 10 mpg,” he points out. “But just as there isn’t one solution to get you to 10 mpg, there isn’t one issue keeping you from it either. Excessive engine idling is a challenge, but it can be addressed with solutions like idle shutdown and APUs.”
While Horton never likes to think of anything as impossible, getting a Class 8 vehicle primarily working in an urban setting with stop-and-start driving to 10 mpg is challenging, to say the least.
“It can take up to three-quarters of a gallon of fuel to get a loaded Class 8 tractor-trailer up to highway speed from rest,” he emphasizes. “If they’re doing that repeatedly all day long, that’s a lot of extra energy being expended.”
Kevin Baney, chief engineer at Kenworth Truck, believes it will continue to demand an “all-encompassing” engineering view of equipment specs and driving styles in order to gain higher fuel economy levels.
“What our product engineering efforts have taught us is that 10 mpg trucks require total vehicle optimization,” he says. “It involves not only lowering engine speeds and optimizing fuel maps, but also considering smaller engine displacement rightsized for the loads carried. It involves less gear shifting through use of AMTs. It involves significant truck and tractor aerodynamic improvements, including virtually closing out the gap between tractor and trailer.”
In short, Baney believes attaining and staying at the 10 mpg level is perhaps the most significant engineering effort undertaken from a total truck perspective in a number years.
“The key to a 10 mpg truck is to keep as much of the truck’s operation optimized for the operating environment and automated as much as possible, and minimizing the ability of drivers to counteract the fuel-saving technologies purchased by the fleet,” he says.
“Predictive features are the next wave, followed closely by other look-ahead technologies,” Baney adds. “The goal of all of those technologies is to bring the behaviors of your very best drivers to every truck, regardless of driver experience. Eventually, the sum of these predictive technologies will become your ‘best driver,’ as they will be able to see more of the road ahead faster than any driver could.”
Checklist for a 10-mpg tractor-trailer
To consistently achieve 10 mpg on an 80,000-lb. tractor-trailer combination, Mary Aufdemberg, director of product marketing at Freightliner Trucks, believes a host of factors must be addressed, as some will help and some will hinder attempts to gain higher fuel mileage. Those factors include the following:
- Aerodynamics (tractor and trailer): The tractor must have an aerodynamic shape and include items such as an aero bumper, hood-to-bumper fill, internal antennas, chassis side fairings, side extenders, and wheel covers. Trailer aerodynamic enhancements must include trailer skirts and could benefit by a trailer tail, nose cone, and wheel covers. The trailer gap, i.e., the distance between back of cab and front of trailer, should not exceed 48 in.
- Drivetrain: The engine, transmission and axles must be designed to maximize fuel economy with components such as an amplified common rail system, friction-optimized pistons, automated manual transmission with coasting and skip shift features, direct-drive transmission with optimized gearing, Intelligent Powertrain Management system to predict the terrain ahead and adjust speeds and torque load requirements accordingly, and a 6x2 configuration for reduced parasitic loss.
- Tires: Must be low-rolling resistance tires and operated at recommended air pressures. The axle alignment on the tractor and trailer is also critical to maximum fuel economy.
- Driver skill: An experienced driver is crucial to ensure maximum fuel economy.
- Speed: There is a compromise between what is a reasonable speed to achieve maximum fuel economy and getting freight delivered on time; however, highway speeds in the area of 62 mpg have been found to provide an excellent fuel economy return.
- Weather: Excessive wind, snow and cold climates will reduce fuel economy numbers significantly.
- Terrain: If you are based in the West and consistently haul over mountain passes, it would be difficult to average 10 mpg; however, flatter Midwest hauls will be much friendlier to fuel-savvy haulers.
- Traffic: The benefits of aerodynamics are diminished at lower speeds. Stop-and-go traffic is also detrimental to fuel economy.