Hybrid Powertrains in the Product Mix

Hybridization and alternative fuels catching on as carbon-reduction targets intensify and infrastructure challenges persist for on-highway trucks.

January 28, 2025

Ryan Gehm

Cummins recently unveiled an integrated drivetrain concept featuring a diesel-hybrid powertrain. (Cummins)

Perfect is the enemy of good. That aphorism embodies some of the frustrations voiced by trucking-industry companies and associations when it comes to satisfying lower greenhouse gas (GHG) and NOx emissions.

Hydrogen-fueled combustion engines, such as Cummins’ X15H slated for production toward the end of this decade, could play a role in emissions-reducing product portfolios. (Cummins)

Chris Bitsis, assistant director for research and innovation at Southwest Research Institute (SwRI), moderated a powertrain panel at SAE COMVEC 2024 and showed a slide comparing California’s Advanced Clean Trucks (ACT) rules as well as EPA Phase 3 proposed rulemaking and final rulemaking for the different vehicle segments.

“Some of these could be very aggressive for ZEV adoption,” Bitsis said. “From an EPA standard, this is not a mandate in technology, this is an overall fleet average. Of course, some level of zero-emission vehicles will be averaged in there, but it leaves the door open for other technologies, like hybridization and alternative fuels.”

Low-carbon fuels increasingly are helping to reduce emissions. International announced in January 2025 that its S13 Integrated Powertrain is now approved for use with renewable diesel (R100 or HVO). (International)

Another introductory slide illustrated examples of ways to comply for 2027 to 2032 model years for the various categories, including a hydrogen internal combustion engine (H2-ICE) sales scenario and a plug-in hybrid (PHEV) scenario.

“If we had an H2-ICE focus, we would need somewhere between fifteen and twenty-five percent of sales by 2032 being H2-ICE, which is a steep hill to climb,” Bitsis said. “If we look at that PHEV focus, meaning still hydrocarbon-based fuel and not up to 100-percent CO₂ reduction, we could need up to 60-percent PHEV adoption, which has its own challenges for infrastructure and customer acceptance.

“The key is that each OEM has to develop a strategy for this product mix in order to meet this fleet-average greenhouse gas regulation,” Bitsis added. “I don’t envy the work that takes place at the OEMs to try to do this product planning.”

Cummins’ approach: Make an impact now

Brad Sutton, executive director of powertrain engineering at Cummins, offered some insights into the company’s strategy to reduce emissions over the next three decades.

48V mild hybridization was a popular choice for SuperTruck II programs, but International opted for a full 650V P2/P3 hybrid for its demo truck. The combustion engine with high-voltage accessories and technologies was developed with Bosch. (International)

“It’s very important for Cummins and our strategy to do something now to reduce emissions,” Sutton said at SAE COMVEC. “Not to wait on a perfect solution, not to wait for all infrastructure to be in place, not to wait for the perfect ZEV approach. But to start making an impact now.”

The decade ahead will see Cummins advancing its current technologies. “That means more optimization on our internal combustion diesel engines, it means newer fuels, it means more natural gas and the development potentially of hydrogen ICE as we move forward,” Sutton said.

Cummins started full production of its first heavy-duty natural gas engine in September 2024. The X15N is the first on the Cummins HELM platform that will eventually include diesel and hydrogen models. “It is our first step towards that alternative-fuel future that’s not just completely BEV or other zero-emissions options,” Sutton said. “Again, trying to reduce CO₂ today.”

As infrastructure becomes more mature in the following decade, there will be an “extremely competitive landscape,” Sutton said. “Lots of options, depending on the particular duty cycle or application you have. And we plan to play in all of them.”

The third decade, when infrastructure catches up at scale, zero-emissions solutions will prevail.

“Transportation is absolutely critical to the economy, and we need to make sure those customers that are part of that have affordable, justified, low-emission solutions the entire way,” Sutton said.

Hybrid alternatives

SwRI developed a natural-gas-fueled medium-duty truck outfitted with a hybrid powertrain that achieved a 25% reduction in GHG emissions vs. standard diesel powertrains. Isuzu Motors, Woodward Inc. and South Coast Air Quality Management District collaborated on the project. (Southwest Research Institute)

One low-emission solution that could find greater use in the coming years is hybrid technology, possibly providing a path to compliance if infrastructure or vehicle cost limits ZEV adoption. Higher customer acceptance for hybrid vehicles could be a benefit as well.

“Maybe there’s an opportunity for hybrids as we move forward towards greenhouse gas Phase 3,” Sutton said. “I’m pretty excited about that being a powertrain person because it brings a lot of new technology opportunities for us and the OEMs.”

Diesel engines are calibrated for a wide range of operating conditions and there can be regions of inefficiency. Diesel hybrids provide an opportunity to operate the engine more consistently at a very efficient point. But regulatory practices need to support that technology pathway, the COMVEC panelists agreed.

Cummins unveiled an integrated drivetrain concept featuring the HELM engine platform, including a diesel-hybrid option, at IAA Transportation in September 2024. Its X10 engine, with ratings from 240-335 kW (320-450 hp) and peak torque of 2,300 Nm (1,696 lb-ft), is utilized in the hybrid drivetrain concept.

“As the commercial vehicle industry in Europe approaches Euro 7 and CO₂ Heavy Duty Vehicles regulations, the need for diverse power solutions is more critical than ever,” Jane Beaman, VP of global on-highway and pick-up business at Cummins, said at IAA in Hanover, Germany. “As we look toward the next decade, when carbon-reduction targets intensify and infrastructure remains challenging, hybrid powertrains could help to eliminate range anxiety, enhance operational flexibility and contribute significantly to vehicle manufacturers’ CO₂-reduction targets.”

SwRI’s demonstration vehicle integrates a P2 hybrid architecture that places a 100-kW electric motor between the spark-ignited natural gas engine and the transmission, powered by a 40-kWh battery, enabling an EV mode, engine-only mode or a combined hybrid mode. (Southwest Research Institute)

Thomas Howell, segment lead for conventional powertrain at AVL in the U.S., posed the question, “In medium-duty, why diesel?” “If we’re worried about cost, which is absolutely a major driver of penetration, then should we be looking at a potentially cheaper energy source,” Howell said. “That may well be spark-ignited. There are dramatic improvements in gasoline-fueled engine efficiency, particularly when coupled into a hybrid powertrain. But there’s a lot of development to go into that.”

Programs such as SuperTruck II have conducted extensive development efforts to determine what type of powertrain makes sense for an over-the-road truck. “We performed quite a lot of those studies and ended up with mild hybrid,” Howell said. “Mild hybrid will charge the battery so that when the sleeper cab is used for twelve hours, you can run accessories off that. It really doesn’t make sense to have a large battery pack which is taking away from the freight in an over-the-road truck.”

Navtej Singh, director of powertrain engineering at International, noted that one out of every 10 vehicles sold in 2027 must be electric to meet OEM emissions targets under EPA’s Phase 3 limits. By 2032, that figure becomes four out of 10. International’s goal is more aggressive yet: 50% ZEV sales by 2030.

Plug-in hybrids can provide reductions in GHG emissions of 10-30%, Singh said, noting that hybrid technology is better suited to medium-duty vehicle applications than for long-haul trucks. “Basically, it can slow down your adoption to ZEVs, but it cannot change the inevitable,” he said.