When Daimler Trucks North America set to work meeting the U.S. Department of Energy’s SuperTruck program goal of a 50% improvement in freight efficiency, it started off with a clean sheet — actually, eight of them.
The SuperTruck team was organized into eight different workstreams, each emphasizing different areas of the vehicle, including engine, aerodynamics, powertrain integration, energy management, parasitic losses, weight reduction, waste heat recovery and hybrid, according to Derek Rotz, principal investigator for DTNA’s SuperTruck project. “Each workstream was given explicit efficiency goals to meet, so that the overall SuperTruck would reach or exceed the 50% goal.”
Each of these global Daimler teams focused its energies on getting the most out of its platform to create a truck that would not only meet the DOE’s expectations, but drive right past them. The teams started off spending a considerable amount of time using computational tools for analysis and detailed 3-D modeling. According to Rotz, this made the preliminary stages faster and more cost effective than building and testing prototype vehicles.
“They investigated several innovative and unconventional concepts, and we took integration to an entirely new level,” Rotz says. “The vehicle was essentially proven out in the digital world before physical prototypes were built, which gave us an increased sense of confidence that the target on the final SuperTruck would be met.”
In the end, the Daimler SuperTruck program, funded half by the U.S. Department of Energy and half by the company itself ($40 million from each side), achieved a 115% freight efficiency improvement, hit 50.2% brake thermal efficiency (the goal was 50%), and achieved 12.2 mpg in real-world operation.
There were many features and technologies involved, some of which we explore here.
Several commercially viable technologies developed in conjunction with the SuperTruck program have been introduced in DTNA production vehicles, including 6x2 optimization and the aerodynamic components found on the Freightliner Cascadia Evolution and the integrated Detroit Powertrain.
Strategies such as downspeeding with a custom engine rating and using the predictive capabilities of Intelligent Powertrain Management components, such as pre-loaded 3-D digital maps to control shifting and eCoast events, also increased efficiency and economy.
Other technologies explored by the SuperTruck team are not commercially viable. But many will make their way onto Daimler trucks in the next few years.
Catching the heat, beating the air
When a tractor rolls down the road, less than 50% of diesel fuel energy is converted to torque. Much of the wasted energy is dissipated as heat through radiators and exhaust. When creating their SuperTruck, engineers at DTNA designed a waste heat recovery system that captures a portion of the exhaust heat energy and delivers it as drive power back to the truck. Since this type of new technology and components would add to the cost of the truck, when first approaching this concept, the engineering team imagined a system that could offer a reasonable payback period to the customer in the future.
“Significant design changes to both engine and vehicle are anticipated due to packaging of new components,” explains Sandeep Singh, senior development engineer. “For example, if the power generated by the heat recovery system is fed back to the engine crankshaft mechanically, then that could impact the accessory drive.”
But waste heat recovery was far from the only change made under the hood, Singh says. Freightliner also made improvements to the piston kit for friction reduction and implemented a viscous clutch water pump to reduce accessory load. By experimenting with low-viscosity oils and liner thermal barrier coatings, engineers saw an increase in mid-stroke oil film temperatures and a reduction in viscosity and friction.
The engine, of course, also needs air for cooling. The fan uses hydraulics rather than a belt. Speed and operation can be closely modulated so it’s on only when needed.
One of the most visible changes is another newly designed component, the aero grille, the result of managing the tradeoff between aerodynamics and cooling.
“An open grille is obviously required to provide airflow for the cooling package to keep the engine cool, especially at slower speeds while climbing a grade,” Rotz explains. “Our analysis showed that closing the grille, however, would reduce aerodynamic drag and improve fuel economy.”
The aero grille automatically opens and closes in relation to conditions based on vehicle speed and coolant temperature. While the truck is moving at high speeds, the grille closes to maximize fuel economy. At lower speeds, it opens to provide maximum airflow.
Because the SuperTruck goal is about freight efficiency, not just fuel efficiency, reducing weight in order to carry more payload was another strategy. Close to 50% of the frame’s weight reduction was saved by using aluminum frame rails with reinforcement strips. First, the cross section of the frame rail was optimized to reduce material, then reinforcement strips were applied to regions where high stresses occur, while still meeting strength and stiffness requirements with less material. The overall chassis weight reduction, including the frame, suspension, fifth wheel and driveline, contributed to approximately a 2% freight efficiency improvement.
However, Rotz notes, because of not only the cost of aluminum but also reliability and long-term durability requirements, the SuperTruck aluminum frame design doesn’t look like a commercially feasible solution.
In the future, truck manufacturers might also see more weight reductions — and aerodynamic improvement — if DTNA has its say. While the SuperTruck has side view mirrors, as mandated by current Federal Motor Vehicle Safety Standards, the engineers added monitors within the vehicle that act as mirrors, giving drivers an improved look at the possible blind spots to the left and right sides of the truck and trailer.
So DTNA has petitioned NHTSA to change the regulations to eliminate required exterior mirrors when replaced by cameras. “Optimal visibility would be achieved and fuel efficiency of new trucks hitting North American roads could be improved by up to 1.5%,” Rotz says.
Beyond the tractor
You can’t talk freight efficiency without a trailer, so engineers continued where they left off on the truck. Looking at the tractor and trailer as a single unit, engineers integrated the design by using computational fluid dynamics and 1/8th-scale wind tunnel testing.
“The integrated design matched tractor components such as side extenders, roof spoiler and drive wheel covers to trailer components such as the nose cone and trailer skirts,” Rotz says. “The testing indicates that over two-thirds of the aerodynamic benefits actually come from the trailer.”
Another one of those items that would be difficult to implement in most real-world fleets was the use of solar panels on the trailer to help provide electrical power.
With everything in place, the builders wanted to be able to gauge the different systems as simply as checking their social media statuses, so tablets were developed as an engineering and driver support tool. These allow Rotz and his team to monitor the performance and status of each system.
“There are several complex systems operating underneath the hood and not readily visible, which made a visual aid necessary,” Rotz explains. “Over time we discovered the usefulness of the tablets during ride and drive events to showcase SuperTruck technologies, so the displays were built out in a more intuitive and aesthetic layout.”
Using a wi-fi server on the truck connected to the networks that link the various computers, data is broadcast wirelessly to up to four tablets per vehicle. (The wireless communications system is only installed for demonstration purposes. It is also only a display tool and does not have the ability to control any vehicle functions.)
A team effort
There were many moving pieces in the creation of Freightliner’s SuperTruck, which Rotz described as best characterized as a full vehicle program in as much as a redesign of cab, chassis, powertrain and electronics was in scope. This effort not only required the campaign of the entire Freightliner team, but also the support of global partners within Daimler, as well as external partners such as suppliers, universities and national testing labs
“With SuperTruck it was important to have partners on-board who are professional, engaging and genuinely interested in pushing technology forward and to try something new,” Rotz says. “It takes a village to make a SuperTruck.”