The Environmental Protection Agency is proposing its first new rules governing heavy-truck nitrogen oxide emissions in two decades, but engine manufacturers say the proposals would be difficult to meet and counterproductive because they could slow the move to cleaner technology.

As state and federal clean-air regulators move forward with plans to further reduce nitrogen oxide and particulate matter emissions from commercial trucks, diesel engine makers are exploring new technologies to meet those targets in the coming years.

Almost since Donald Trump moved into the White House, the Environmental Protection Agency and California Air Resources Board have been in a political war over the direction of U.S. environmental policy. Can there be a thaw in the works now that EPA has announced plans to begin an “environmental review” to reduce nitrogen oxide emissions for heavy trucks?

With publication of the Phase 2 federal greenhouse-gas final rule for trucks expected soon, 12 agencies from state and local government have petitioned the Environmental Protection Agency to quickly produce a national rule that would tighten nitrogen-oxide emissions from trucks.

California environmental regulators said they plan to begin development of tougher nitrogen oxide standards for heavy trucks, possibly as early as 2017.