NOx emissions inventories - Role of improved HD ICE technologies

Heavy-duty vehicles (HDVs) are a major source of nitrogen oxides (NOₓ), harmful pollutants that contribute to smog formation and respiratory illnesses—especially in communities near ports, freeways, and warehouses. It is expected that improvements in diesel engines, including ongoing advances in emissions control technologies and engine efficiency, and cleaner fuels, coupled with engines manufactured after 2027 being required to meet a much stricter EPA NOx standard of 0.035 g/bhp-h—nearly ten times lower than the limit set in 2010, will lead to a reduction in the overall emissions inventory for HDVs, but this topic has not been extensively evaluated.

In a recent study published in Science of The Total Environment, the University of California at Riverside’s (UCRs) Bourns College of Engineering – Center for Environmental Research and Technology evaluated how modern diesel and natural gas engines are already performing in the real world and what their adoption could mean for future emissions inventories.

UCR analyzed emissions data from 63 HDVs spanning five vocations: refuse, school and transit buses, delivery, and goods movement. The vehicles included diesel engines from 2010 and later equipped with selective catalytic reduction (SCR) systems, as well as natural gas vehicles. Emissions were measured using portable emissions measurement systems (PEMS) over a normal day of operation in their fleet duty cycles. Data were processed using the EPA’s two-bin moving average window (MAW) method, which will be used for in-use compliance testing starting in 2027.