In America, we move most of our stuff by road—trucks carried almost five times as much cargo as trains in 2017, according to the Bureau of Transportation Statistics. While it’s true that personal vehicles contribute the majority of our transport-related carbon emissions, a quarter still comes from bigger trucks and buses. When it comes to the biggest trucks on our roads—the Class 8 trucks that can pull up to 80,000 lbs (36.2 tonnes)—there’s a lot of room for improvement, which is where Super Truck II comes in. The Department of Energy funded a challenge to double the efficiency of 18-wheeler trucks, and the big machine you see in the photos here is Freightliner’s response.
In fact, the story dates back to 2010 and the first DoE Super Truck program, which eventually funded four truck makers (including Daimler Trucks, which owns Freightliner) to develop a heavy truck with 50 percent better efficiency than anything then in production.
Super Truck II got going in 2017, a couple of years after the end of the first program. Having already demonstrated that big efficiency savings were possible, Super Truck II has been about developing them into something production-ready.
“Class 8 long hauls typically try to do 700 miles a day of driving, so it’s mostly a highway kind of thing,” explained Darek Villeneuve, manager of advanced vehicle systems at Daimler Trucks North America. “So the routes are a little different than what your typical automotive kind of vehicle will go after, and it’s a difficult segment of the market to transition into alternative energies.”
“We learned a lot in Super Truck 1, really diving in deep in terms of, ‘Here’s all the energy usages, and what could potentially be done,’ and the goal of Super Tuck 1 was less commercial; it was a little more research-focused. At least that was our basis,” Villeneuve continued.
The most important factor in terms of efficiency at highway speeds was aerodynamics. There’s certainly a lot of low-hanging fruit to be picked here—just drive an EV past a tractor-trailer and listen to all that wind roar for evidence of wasted energy.
Daimler Trucks North America and Freightliner are still in the business of selling trucks, and that’s why the cab you see here isn’t some wacky lay-flat design that could scare off a conservative customer base. Unlike the first go around, for Super Truck II, that meant bringing the design team in at the start.
“From a design standpoint, I know I’m trying to make an aerodynamic-looking truck,” said Jeff Cotner, chief designer at DTNA. “I want this thing to communicate the performance that we’re trying to achieve so that when [people] see it whether standing still or driving, it’s giving the product message that we’re hoping for.”
The design brief from management asked for something commercially viable. “You can make a spaceship, and some of our competitors did. But to turn that around and actually deliver it and have it do everything it needs to do for all of our customers is very difficult. It takes a lot longer than a Supertruck project to figure all that out,” Cotner told me.
Achieving the required efficiency meant a lot of detail work to address flow separation—keeping the airflow attached to the body surfaces and not forming turbulent wakes is critical when fighting drag. Cotner’s team also took away the conventional mirrors, instead using rear-view cameras mounted high up above the cab’s doors.
Some other ideas that were no-brainers for the aerodynamicists didn’t survive contact with the intended industry. “One thing that we looked at and we tried to sell was these full covered drive wheel fairings,” said Villeneuve, referring to the pair of axles at the rear of the cab.
Shrouding those wheels would make a big impact on the cab’s drag coefficient, and for Super Truck I, the company did just that. “But we talked to customers, and they said, you know, part of the drive every day before a driver gets in there, they have to go around and inspect the tires. And we don’t want to have that stuff,” Villeneuve told me.
There might be no wheel fairings, and the team paid no real attention to the trailer side of the equation—there are at least 12 million trailers already in service, and no one is champing at the bit to replace them all with aero-efficient ones—but at highway speeds, the Freightliner Super Truck II pulls its trailer in a little closer, reducing the gap between cab and trailer to about four inches.
After aero came better tires, courtesy of a collaboration with Michelin. Each of the three axles on Super Truck II needs slightly different tires. The wheels on the front axle do the steering and have a very low-rolling resistance tire that features a new carcass construction (involving 3D-printed metal).
But there’s clever stuff happening in the drivetrain, too. The middle axle is the permanent drive axle, and its tires were optimized for wear and low rolling resistance. The third axle only contributes drive at lower speeds, so its tires can have even lower rolling resistance. The truck also uses its air suspension to reduce the weight load on this axle at speed. The variable 6×4 or 6×2 power transmission switches automatically at speed.
The engine is a 13 L diesel with two-stage turbocharging and a 13-speed transmission. Villeneuve says DTNA learned a lot about how to make the engine breathe better. “Air handling [has] a huge effect on how efficient a diesel engine can operate, so we executed it a certain way,” he said. But there was also a lot of work done on reducing friction loss in the engine, and there’s an all-new combustion design, plus coatings in the cylinders to keep heat where it should be.
Add in stuff like 48 V ancillaries, driven from a lithium-ion battery, that improve driver comfort while still cutting fuel consumption, and the result is a truck that’s 50 percent more efficient in terms of freight miles than a normal Freightliner.
At least for now—as noted earlier, the point of this project was to move some of these strategies closer to production, and Villeneuve and Cotner both said the lessons from Super Truck II are already being applied to future commercial products under development.