Sage Kokjohn is the Phil and Jean Myers Professor of Mechanical Engineering at the University of Wisconsin-Madison. A member of the College of Engineering’s Engine Research Center, Kokjohn researches energy conversion for transportation and power generation, with a focus on internal combustion engines in heavy-duty trucks, off-road and marine vehicles. He and his students also study electric vehicle batteries and tools to evaluate vehicle architectures across duty cycles—the patterns of how, and how long, vehicles are used.
Fuel costs are soaring, with little end in sight, and a handful of manufacturers—Tesla, Volvo and Freightliner among them—have debuted electric semis. But are they really practical for efficiently hauling our goods across the country? In this Q&A, Kokjohn talks about the future of heavy-duty transportation, which he says likely depends on matching the right technologies to the right applications—from electrifying short-haul trucks to hybrid systems and alternative fuels for long-haul, off-road, and marine vehicles.
Will today’s technology allow us to actually make and operate an electric semi?
It’s very difficult to electrify something that needs to provide power over a long period, and that is what an over-the-road truck does. The most interest in that area for fully electric vehicles has been for last-mile or short-haul deliveries that would be operating around more dense urban environments. Long-haul purely electric vehicles are very, very challenging. That’s from a cost perspective, because the battery is very expensive, and from a duty cycle perspective, because the battery is going to be very heavy.
Trucking is weight-limited. Electric semi-trucks have a weight limit of up to 82,000 pounds, while non-electric vehicles have a limit of 80,000 pounds. The battery can take up a substantial share of the vehicle’s weight limit, and that can be very challenging depending on what a truck is hauling.
Some of Volvo’s trucks are in the 275-mile range, which is suitable for short-haul and local delivery. There you have shorter duty cycles, which make a lot more sense for an electric truck. When you get to long-haul, the vehicles need to be able to operate for 11 hours a day, which is the U.S. Department of Transportation limit (or possibly more if you have driver teams where one person sleeps and the other takes over).
All that said, electrification and hybridization for local delivery makes a ton of sense because there’s a lot of stop and start, which helps with regenerative braking. Electrification reduces criteria pollutants like nitrogen oxides and particulate matter.
What are challenges associated with electrifying heavy vehicles?
If electricity is really cheap, then there can be some operational cost savings. That being said, if you want to be able to recharge in any reasonable amount of time, you’re probably talking megawatt-level charging, and that usually ends up being pretty expensive from an operational standpoint.
You have to be pretty careful about making broad assumptions about carbon reduction, especially for long-haul routes that cross regions with different power grid mixes. You can potentially see more benefit if you’re somewhere like the Pacific Northwest, which has a low-carbon-intensity grid. We’ve done calculations where we looked at a route from California to Chicago and back. Initial charging in California uses a relatively low-carbon grid. You can make it to about Wyoming before you need to charge again, where the grid relies on a substantial amount of coal. Another challenge in that aspect is producing a massive battery—mostly outside of the United States—through a very energy-intensive process, so there are a lot of economic and environmental factors to consider.
A key difference between this market—whether it’s heavy-duty, off-road or marine—and the light-duty market (passenger vehicles, SUVs, minivans, small pickup trucks) is that people who are buying heavy vehicles are buying them to do a job. And they’re likely going to buy the vehicle that can do the job for the lowest overall cost. That’s what’s going to drive the sales of these vehicles. If an electric vehicle can’t compete, meaning if it can’t do the full duty cycle of the vehicle it is replacing, then it doesn’t make a lot of sense to replace that vehicle.
If internal combustion engines will be around in heavy vehicles for a while, how can we continue to increase their efficiency?
There’s been a lot of work in that space over the last 20-plus years as we’ve seen oil prices rise and fall. As oil prices climb, we’ve seen more interest in alternative fuels, so things like ethanol and methanol get more attention when oil’s more expensive. There’s more interest in technology that can improve efficiency because there’s a payback over time for the initial investment in that technology.
Every two years, the college’s Engine Research Center hosts a symposium on engine technology. You might see incremental improvements from one symposium to the next, but when you do this for more than 20 years, those really start to stack up, and what starts as a small improvement in efficiency can make a big difference.
On the engine side, work done 10 or more years ago on combustion system design is starting to make it into production. That includes changes to compression ratios, improved combustion chamber design, and improvements to heat rejection—or, moving heat away from the engine to prevent material failure. A lot of the work that’s really improved efficiency has been looking at the vehicle or the powertrain as a whole to be able to optimize it.
I think we’ll also see more growth in hybridization, including engines designed specifically for hybrid operations. In those systems, the electric machine and battery augment the engine, enabling it to operate in a more efficient regime than would otherwise be possible. It also makes the vehicle very drivable because you have an effectively instantaneous torque response from the electric machine.
What long-term challenges are UW-Madison engineers working to address?
In this field, things that we’re preparing to get into production in five to 10 years basically have to be ready today.
A lot of the work we do is in alternative fuels and looking at how we are going to maintain an energy density similar to diesel fuel or gasoline without relying on oil. It’s a very conservative market, so that’s not an easy question to answer. There are a lot of alternative fuels on the light-duty side, which uses spark ignition. It’s very difficult to do that on the heavy-duty side because of how those vehicles operate, so we look at how to make compression-ignition (i.e., diesel) engines operate on suitable alternative fuels.
There’s been substantial interest in methanol. It can be used, effectively, as an energy carrier and can be produced from a variety of methods. If you’re producing electricity and combine it with captured carbon, you can use that to create methanol. Then the question becomes how can we use that in a diesel engine? We’re seeing a significant interest in that in the marine space, where manufacturers are moving heavily toward methanol-ready ships. That’s generally a dual-fuel process, where the engine can still take diesel as a backup and as an ignition source. There’s ongoing work to move to a pure methanol process.
Featured image caption: Sage Kokjohn discusses heavy duty vehicles for a question-and-answer feature. Kokjohn is the Phil and Myers Professor of Mechanical Engineering. Credit: Joel Hallberg.