Even as emissions rules tighten, diesel engines are far from standing still.

That was the message from engine makers and OEMs during a Clean Diesel Innovation panel at ACT Expo, where speakers emphasized that the next generation of engines will focus on efficiency, uptime, and simplicity — not just compliance.

“This is a battle to win for fleets,” said Mehdi Ferhan, head of powertrain technology at Volvo Group. “It’s about meeting emissions while staying within the boundaries of performance and profitability.”

Evolution, not reinvention

Across the board, manufacturers stressed that EPA27 won’t bring a wholesale redesign of diesel engines, but rather an evolution of proven platforms.

Navtej Singh, director of engineering at International, said about 94% of the company’s S13 engine carries over into its 2027 version.

“We already have more than 10 million miles on those drive cycles,” Singh said. “We’re not reinventing; we’re improving what customers already trust.”

That theme was echoed by others on the panel, who pointed to targeted upgrades in combustion, air management, and aftertreatment systems.

Lyle Kocher, director of technical programs and planning at Cummins, said the biggest gains aren’t coming from a single breakthrough, but from how the entire system works together. “It’s really about how you bring the system together,” he said.

Aftertreatment takes center stage

As NOx limits tighten under EPA27, aftertreatment systems are doing more of the heavy lifting, and becoming more sophisticated in the process.

Steve Collins, director of field sales engineering at Daimler Truck North America (DTNA), said improvements in SCR technology are allowing engineers to rethink other parts of the engine.

“The robustness of the aftertreatment system allows us to simplify the engine itself,” Collins said.

For example, DTNA has been able to move to a simpler fuel system design thanks to gains in emissions control downstream.

International is also focusing on simplification, Singh added, including eliminating external EGR and avoiding additional complexity where possible.

“What the engine should do is generate power and torque,” he said. “Aftertreatment should clean the emissions.”

Focus on uptime and simplicity

Despite tighter emissions standards, OEMs insisted fleets shouldn’t expect added maintenance headaches.

“Our service intervals are improving; they’re not going backward,” Singh said.

Panelists repeatedly emphasized uptime, predictive maintenance, and reduced complexity as key priorities. That includes expanded use of prognostics and over-the-air updates to catch issues before they lead to downtime.

“If a customer is about to have a fault, dealers will be notified ahead of it,” Singh said.

Collins said the goal is to keep added complexity behind the scenes.

“The complexity is real,” he said. “But we’re trying to manage that on our side so fleets don’t have to.”

Diesel still leads on TCO

Despite growing interest in alternative powertrains, panelists agreed diesel remains the benchmark in many applications.

“It still offers the best total cost of ownership today,” Kocher said.

Infrastructure, service networks, and power density all continue to give diesel an edge — especially in longhaul and high-utilization applications.

At the same time, OEMs are developing multi-energy platforms that can support diesel, natural gas, electric, and eventually hydrogen.

“We believe there is not one single path,” Ferhan said.

For fleets evaluating next-generation equipment, speakers pointed to a few key factors beyond fuel economy: uptime and reliability; service intervals; diagnostic capability; and real-world performance in specific duty cycles.

“You should test those drive cycles,” Singh said. “Make sure the technology matches your operation.”

EPA27 may be the biggest emissions shift since 2010, but for fleets, the experience behind the wheel shouldn’t change much.