Engine manufacturers take note as EPA revisits NOx emissions threshold

Through its just-announced Cleaner Trucks Initiative (CTI), the U.S. Environmental Protection Agency aims to further decrease nitrogen oxide (NOx) emissions from on-highway heavy-duty trucks and update a current engine exhaust standard in an early-2020 rulemaking. Set in 2001, the standard culminated in 2010 with a 0.2-gram brake-horsepower-hour NOx threshold for which engine manufacturers deployed selective catalytic reduction (SCR) equipment and urea-based diesel exhaust fluid (DEF). SCR injects DEF into the engine exhaust stream; the ensuing chemical reaction converts NOx into nitrogen, water vapor and traces of carbon dioxide.

The post-EPA 2010 era has yielded single exhaust aftertreatment packages combining nitrogen oxide-treating selective catalytic reduction and diesel exhaust fluid dosing devices with diesel particulate filters. Volvo Trucks North America offers the One-Box Exhaust Aftertreatment System for VHD and companion models.

CTI will see EPA streamline compliance and certification requirements tied to any new NOx threshold through a “deregulatory” focus on onboard diagnostic requirements; cost-effective means of reassuring compliance by using modern and advanced technologies; deterioration factor testing process; and, concerns regarding annual engine family recertification.


Although geared to off-highway machines, the new Caterpillar C13 engine exhibits similar methodology to the Volvo One-Box (page 10): It utilizes SCR and diesel oxidation catalyst, plus a DPF; attains EPA Tier 4 emissions thresholds without the need for exhaust gas recirculation; and, is equipped with NOx sensors to reduce interface connections.

“The Initiative will help modernize heavy-duty truck engines, improving efficiency and providing cleaner air for all Americans,” said Acting EPA Administrator Andrew Wheeler, who unveiled the CTI in an early-November gathering with the American Trucking Associations, equipment manufacturers and industry stakeholders. “The U.S. has made major reductions in NOx emissions, but it’s been nearly 20 years since EPA updated these standards. Through rulemaking and a comprehensive review of existing requirements, we will capitalize on these gains and incentivize new technologies to ensure heavy-duty trucks are clean and remain a competitive method of transportation.”

“[CTI] makes clear that reducing NOx emissions from heavy-duty vehicles is a clean air priority for this administration,” added EPA Office of Air and Radiation Assistant Administrator Bill Wehrum.

NOx emissions dropped by more than 40 percent from 2007 to 2017, owing partly to the advent of SCR exhaust aftertreatment on heavy-duty trucks. But there is more work to be done, contends EPA, citing estimates that such vehicles will be responsible for one-third of the transportation sector ‘s NOx emissions in 2025. The agency expects that any update to the standards will result in significant mobile source NOx reductions, assisting communities across the country in ozone and particulate matter standards attainment.