It takes a lot of energy to move an 80,000 pound truck through its day. Retail giant Walmart has a fleet of thousands of these in the U.S., logging millions of miles a day. For a company determined to be a leader in environmental sustainability, this requires using the most fuel-efficient vehicles in the industry, employing the latest freight logistics technologies and investigating (and investing in) future technologies.
Early last year Walmart introduced the Walmart Advanced Vehicle Experience—a truck that may never even hit the road in any fleet, yet the technologies exhibited in this single prototype may spark hundreds of other innovations that make a sustainable difference in greenhouse gas reduction and energy savings. This truck is a testament to what supply chain partnerships can accomplish in U.S. manufacturing. A few of the key partners on this project were Peterbilt (truck supplier), Capstone Turbine (powertrain supplier) and Parker Hannifin (motor supplier).
An Unusual Hybrid
Walmart’s truck supplier on this project, Peterbilt, happens to do powertrain research for the retailer, trying to validate 1% and 2% gains in everything from electrification of components to alternate fuels. Walmart challenged this OEM to help them demonstrate a double-digit improvement in fuel economy. Peterbilt started by looking at the powertrains and alternative fuels that were available, then tailored a vehicle to Walmart’s duty cycle. Then it teamed with Roush Engineering in Livonia, Mich., to build the body of the truck. The result was a truck body made out of composite materials that is about 20% more aerodynamic than its current trucks—resulting in a 10% improvement in fuel economy.
The powertrain was where the heavy lifting had to be done. According to Bill Kahn, principal engineer and manager of advanced concepts for Peterbilt, the diesel combustion engine had been optimized as much as possible without adding significant cost and complexity for very little additional improvement.
“We really wanted to look at trying to get a turbine engine in the truck because they have tremendous fuel economy potential, thermal efficiency potential, they’re low maintenance, they don’t require a coolant, and are mostly fuel-neutral. But there’s really not a transportation-based turbine engine at this time, so we decided that, by combining a smaller sized turbine with a hybrid powertrain, we could demonstrate the potential for such a combination.”
When news of this truck first came out, people’s imaginations were captured by its aerodynamic look and ultralight construction. However, its real sustainability impact was under the hood—in its hybrid power system. People normally associate hybrids with tiny sports cars, but Walmart’s concept truck may advance battery technology’s role as a prime mover.
A battery pack is the most expensive component in the drivetrain of any hybrid vehicle. The design of this truck demonstrates that industry could reduce the sheer number of cells needed to power a vehicle by improving energy-per-mile usage. Doing that even by only 1% can knock thousands of dollars off the cost of a battery pack while yielding the same range.
Global Vehicle Motor
The key to making a hybrid powertrain work on this size truck was the motor. That’s where Parker came in.
“There wasn't really a 400 horsepower transportation hybrid motor out there,” says Kahn, adding that the form factor and weight of the truck convinced Peterbilt to use Parker’s global vehicle motor (GVM) (see photo below). “We've used the same hybrid powertrain on a conventional truck with a standard diesel engine on it, and it was undersized compared to what we needed on this truck.”
Parker’s traction motors for electric and hybrid electric powertrain motors use highly engineered magnetics to achieve high efficiencies in peak regions not obtainable in other designs. Its cooling system has minimal impact on the size and weight of the motor and its scalability allows a wide performance range.
Normally a diesel powered internal combustion engine spins the generator that would charge the battery pack in a hybrid vehicle. But the micro-turbine in this motor is very small and very clean, even if it burns diesel, because it is so hot. It destroys pollutants and the greenhouse gases more effectively than an internal combustion engine does. The motor on the Walmart truck is a prototype 310mm diameter.
Capstone was Peterbilt’s micro-turbine supplier on this project. This serves as the range extender on a hybrid electric vehicle. The fact that there is a micro-turbine in a truck this size is news. There are certainly plenty of electric-only trucks being demonstrated around the world, and plenty of hybrids sharing mechanical power from the internal combustion engine to power the vehicle forward, but in this case, the internal combustion engine only charges the batteries.
The truck can drive under battery power alone, and when energy storage gets low, the turbine will automatically start and recharge the batteries. The batteries can be charged constantly while driving to extend the range of the vehicle over battery-alone power.
The turbine engine running on diesel doesn’t require after-treatment, and it meets the emissions standards of current engines. The turbine is also capable of being fuel-neutral, meaning it can run on natural gas, JP-8, kerosene, etc. with only minor adjustments.
The winding is very important to tune the motor to the exact demand of a vehicle’s operating cycles. For the Walmart concept truck, it was tuned exactly for a Class 8 truck. Class 8 stop and go would have a different winding than a Class 8 long-haul. The motor is cooled with a water glycol cooling circuit.
More Cooperation Needed on Communication Standards
Capstone’s Steve Gillette says the Walmart project was not only a good opportunity to showcase the kind of sustainability achievable in a Class-8 truck, but to meet its own product development goals as well.
“In trying to expand our presence in the hybrid vehicle market, we were looking for system integrators that could take our product and others and put them into a complete hybrid drive train,” Gillette explains. “One of the companies we worked with on a different project was Artisan Vehicle Systems, which was also using Parker inverters and motors for the mobile industry. Parker was working on a new 300 kilowatt motor, and it seemed like a reasonable fit to work with them for the Walmart project.”
Parker’s prototype eventually became the GVM, which is what moves the Walmart vehicle, replacing a traditional internal combustion engine. It is connected to a transmission that provides torque and power for the wheels.
The cost of battery power in this size vehicle is still the main challenge—and why we’re not seeing this truck on the road right now.
“So much more energy storage is required on a truck versus an automobile that the cost of that storage wasn't coming down to meet our product plan,” says Kahn. “So, we've kind of tabled those efforts while we wait for the hybrid technology to mature. But this truck shows us that as the next round of development occurs, we've got the motor solution we need. We just need the energy storage to allow it to pay for itself in the three to four-year timeframe. Right now hybrids are looking at six- and seven-year time frames, which still makes the cost of ownership a little bit high.”
He adds that some cooperation from the automotive industry on electronic communication standards would help in readying the technologies in the Walmart concept truck for prime time.
“Even though we used systems you may be very familiar with in other applications, when you put them together in a brand new configuration like this, without a communication backbone for them to talk to each other, it was really a big challenge for us. This speaks to the fact that we need turnkey solutions in the future. On the previous hybrid systems we did with Eaton, they gave us the electronics and the hardware, and all we had to do was install it on the vehicle and connect it to our existing computer network on the truck. This one was a lot more challenging because we had systems that hadn’t talked to each other before and actually didn’t even speak the standard controller area network [CAN bus] messaging. We had to develop that all ourselves. It really shows the automotive industry and the automotive suppliers that there are needs out there for electrified components capable of supporting class A trucks, be it powertrains, electrified accessories, power steering, air conditioning, etc.”
All those etceteras represent a lot more work but a great potential payback. $2 gas won’t last forever.
Jay Schultz is business development manager-vehicle electrification with Parker Hannifin Corp.