The next challenge in emissions control for heavy-duty diesel vehicles: From NO_x to N₂O

Vehicle emissions are a major source of greenhouse gases globally. Dual selective catalytic reduction (SCR), an advanced version of single SCR, is crucial under stricter nitrogen oxide (NO_x) emission standards for heavy-duty diesel vehicles (HDDVs). However, the emission characteristics of nitrous oxide (N₂O), a byproduct of SCR and a potent greenhouse gas, remain unclear. This study investigates the N₂O emissions from HDDVs equipped with single or dual SCR systems using heavy-duty chassis dynamometers under various ambient temperatures, altitudes, and loading masses. The results showed that the brake-specific emissions (EF_b) of N₂O from HDDVs with single and dual SCRs were 76.28-269.65 mg/kWh and 147.50-170.22 mg/kWh, respectively. Notably, the dual SCR-equipped HDDV emitted 6-22 times more N₂O than NO_x under all tested conditions. As ambient temperature increased from -10 °C to 25 °C and from 25 °C to 40 °C, the average distance-based emission factors (EF_d) of N₂O for the single SCR-equipped HDDV increased by 87.73% and 48.26%, respectively. However, the variation was not significant for the dual SCR-equipped HDDV. Under half- and full-load conditions, the average EF_d of N₂O for the single SCR-equipped HDDV increased by 47.57% and 110.92%, respectively, compared to those without loading. Similarly, N₂O emissions for dual SCR-equipped HDDV increased by 41.40% and 65.37% under the same loading variations. As altitude increased from 0 m to 3000 m, the average EF_d of N₂O for the single SCR-equipped HDDV decreased by 64.31%. Additionally, N₂O emissions were significantly affected by SCR temperature, engine power, and nitric oxide (NO)/nitrogen dioxide (NO₂) ratio. These findings are crucial for setting future greenhouse gas limits of HDDVs and informing carbon reduction strategies.