The influence of exhaust thermal management on the realization of ultra-low NOx emissions from diesel engines

Heavy-duty diesel engines incorporating a single-stage selective catalytic reduction (SCR) system and calibrated to meet China VI standards are unable to attain the ultra-low NOx emission levels mandated by Euro VII and the forthcoming China VII regulations through calibration adjustments alone, and thus this study employs a China VI-compliant heavy-duty diesel engine as the test platform to analyze the effect of exhaust heat management strategies on ultra-low NOx emissions over the worldwide harmonized transient cycle (WHTC). The findings indicate that conventional NOx control techniques capable of meeting China VI requirements alone are insufficient for Euro VII compliance. The first 700 s of the WHTC cycle constitute the period of highest NOx emissions, owing predominantly to the low exhaust temperature, which leads to a marked reduction in the conversion efficiency of the SCR system. Three exhaust thermal management approaches, namely exhaust gas recirculation (EGR), a burner, and electric heating, were implemented, resulting in specific NOx emissions of 0.248, 0.075, and 0.108 g/(kW·h) respectively over the WHTC cycle. Among these, only the burner and electric heating strategies achieved the Euro VII emission limits. Enhanced exhaust thermal management exerted a pronounced effect on NOx abatement. Under cold start conditions, the burner and electric heating rapidly elevated the exhaust temperature, sustaining high SCR conversion efficiency and thereby delivering ultra-low NOx emissions. Consequently, the selection of suitable exhaust thermal management strategies is essential for meeting future ultra-low NOx regulations.