Battery of Issues Surrounds Large-Vehicle Electrification
Converting heavy-duty vehicles to battery power will require advances in weight reduction and thermal management.
Electrification has made major inroads in light trucks and compact off-highway machines, but its adoption in long-distance trucking and many large off-highway vehicles remains more promise than market-ready. Still, there is plenty of interest in replacing diesel and gasoline power units with battery power. A flurry of activity in recent months highlights the high level of interest in heavy-duty electric trucks, though these actions also spotlight the lingering engineering challenges in electrifying larger vehicles.
General Motors and startup truck maker Nikola teamed up in a deal worth $2 billion, but SEC investigations of Nikola executives emerged shortly after the announcement, raising questions about ongoing efforts to work together. Microsoft mogul Bill Gates started a debate when he questioned the viability of electrified large trucks, prompting fellow billionaire Elon Musk to fire back, saying the Tesla Semi remains set for 2021 shipments.
Battery-electric drives are rapidly replacing combustion engines and even hybrid diesel-electric drivetrains in delivery trucks and mobile machines such as aerial work platforms and forklifts. For equipment that can be recharged overnight, plug-in options are making strong inroads. For larger vehicles and those that cannot be charged on regular cycles, engineers still are trying to find the right balance that will meet user demands. Factors such as torque and battery weight pose major challenges.
“The next round of electrification challenges lies within the larger machines, such as the heavy-duty construction, agricultural and working machines that typically use diesel engines for their drivetrains and hydraulics to power their implements,” said Terry Hershberger, director, sales product management at Bosch Rexroth Corp. “A fundamental issue is the torque required to do work. Whether that torque is provided by a certain level of voltage and current, the pressure and flow of a hydraulic system or the rotational energy from a diesel motor through a gearbox to a mobile machine’s axles, creating and consistently delivering the power needed to move the machine and/or its load lies at the heart of mobile-machine engineering.”
Subsystem suppliers are ramping up to help OEMs advance the capabilities of electric power. Startups have become important players as new technologies like autonomy and electrification transform the industry. Some OEMs and suppliers form partnerships with these newcomers, while others bring startups into their tents. “We didn’t have an electronics portfolio, so we acquired three companies,” said Markus Plassmann, manager, Electric Drive Solutions at Danfoss Editron. “Now we have a portfolio for off-highway and materials handling, marine equipment and for on-highway buses and trucks.”
For commercial trucks, the size and weight of batteries becomes a greater issue. Moving tons of freight over long distances requires large battery packs. Marketers may create different battery pack sizes for vehicles so buyers can tailor vehicles for their distance and payload requirements. “In some applications, mass of the batteries and e-axle is critical as every kilogram saved means more payload capacity and higher vehicle revenue,” said Alexander Schey, chief commercial officer, electrification at Allison Transmission.
“This would point to a focus on mass reduction as a key criterion. However, the same vehicle may also be used to deliver parcels and boxes, where the vehicle rarely, if ever, comes close to its payload capacity. In this case, the mass of key components is far less of a concern and component cost is likely a greater priority,” Alexander noted. Design teams are working to trim weight and volume requirements throughout the vehicle. Factors like tires are included, as are techniques for altering axle positions.
“Electric trucks are heavier than their diesel counterparts because of battery weight,” said Brett Pope, director of electric trucks, Volvo Trucks North America. “To keep this weight delta as low as possible, lightweight components are standard, from air tanks to fifth wheel angles and crossmembers. We will offer a Volvo VNR Electric 6x2 tractor with a liftable pusher as an alternative to a 6x4. That saves 300 additional pounds, with the option to save 300 more pounds if wide-based tires are specified. Lifting the axle when lightly loaded lowers road resistance and decreases battery load.”
Weight is not the only challenge facing engineers. Batteries and high-voltage components generate a lot of heat, so cooling is an essential design consideration. “Along with challenges of weight and the size of the batteries, cooling is more challenging than with internal combustion engines,” Plassmann said. “High power-density motors and components may need water cooling and batteries also need cooling.”
Commercial trucks may also benefit by borrowing a technology that hasn’t been used in electric cars or buses: gears. Smaller EVs do not gain much by using gears. But as larger vehicles carry heavy loads up steep hills, shifting gears can bring significant benefits. “We propose using selectable gears as electrification expands to heavy- and medium-duty trucks,” said David Genise, director of engineering at Eaton, during a panel at SAE’s 2020 COMVEC Digital Summit. “Using selectable gears allows using smaller motors. You do need purpose-built electrified transmissions. When they’re properly matched, you need fewer gears than with diesels; each gear is used much more frequently, so reliability can be a factor.”
Pricing differentials are critical when emerging alternatives like batteries and fuel cells vie with petroleum, but availability is perhaps a larger issue. Infrastructure remains an obstacle for electric vehicles in many commercial applications – and it’s an even greater obstacle for fuel cells. Recharging is straightforward in environments where vehicles can return to their home base regularly. But for long-haul trucking and other fields that rely on a network of charging stations, availability still is a huge barrier.
“Charging infrastructure is key to gaining market penetration,” said Brett Pope, director of electric trucks, Volvo Trucks North America. “The commitment of local and regional power providers to help subsidize charging upgrades to potential electric vehicle customers certainly plays into investment decisions. But it is important that the investments and incentives keep coming to steadily increase the availability of electric truck charging stations to the final goal of full infrastructure.”
Even when there’s an acceptable charging network, the weight of batteries will be a barrier in commercial trucking. Heavy batteries reduce payloads, so engineers are looking for ways to power smaller battery packs while vehicles are traveling their routes. Fuel cells are seen as a way to generate onboard electricity and extend the range of zero-emission vehicles, functioning much like today’s hybrid cars. However, providing the hydrogen used by most fuel cells brings its own infrastructure challenges.
“ZF expects fuel cells to be perhaps the only viable option in the long-haul applications,” said Andre Kohl, North American business development manager, ZF. “The battery-electric option will face challenges to promote an extended-range lightweight battery to support long-distance application (interstates in the U.S. and China – large countries and inter-countries in Europe). With that said, charging stations for hydrogen might be a solution; however, another challenge presents itself, which is clean hydrogen cost and available fuel stations.”
The technology-development period remains a question, one that is compounded by the challenges of rolling out a distribution network. Commercial practicality will depend heavily on the widespread availability of hydrogen. “Allison does view fuel cells as a viable technology, particularly for long-haul heavy-duty applications where a pure EV solution may be impractical or too costly,” said Alexander Schey, chief commercial officer, electrification at Allison Transmission. “Precisely when fuel cells become viable from a commercial point of view depends in large part to the availability of the hydrogen refueling infrastructure.”
This infrastructure rollout will also depend on the regulations surrounding diesel fuel and gasoline. Unless these fuels are restricted or more-heavily taxed, their cost and ubiquitous availability give them an edge. Hydrogen will have a tough time making the cost and volume curve work in its favor. “The fuels we’re using now are very cheap; it’s a cost game to implement fuel cells,” said Antti Väyrynen, director, off-highway business line at Danfoss Editron. “The biggest obstacle will be the infrastructure. Fuel cells require hydrogen and making hydrogen available is not as easy as for fuels of today.”