Controlling in-cylinder airflow is critical for enhancing fuel-air mixing, improving combustion efficiency, and reducing pollutant emissions in diesel engines. This study established a steady-flow intake test bench and used a four-valve diesel engine equipped with a helical/tangential dual intake system as the research subject. By integrating steady-flow measurements with computational fluid dynamics (CFD) simulations, the effects of helical/tangential intake valve lift asymmetry on in-cylinder flow, combustion, and emission characteristics were systematically investigated. The results establish clear design criteria for helical/tangential valve lift asymmetry: one intake valve should operate at the maximum lift (12 mm), while the other must be no less than two-thirds of the maximum (≥8 mm), with a lift difference exceeding 2 mm. Three representative asymmetry combinations—H9/T12, H12/T8, and H10/T12—were examined, all of which significantly enhanced intake performance. Compared with the baseline (H12/T12), these combinations increased the flow coefficient by up to 14.3% and the swirl ratio by up to 11.0%. Furthermore, they improved turbulent kinetic energy (TKE), reduced pumping losses, and enhanced overall intake efficiency, thereby promoting more uniform fuel–air mixing. In terms of combustion and emissions, the H9/T12 combination demonstrated the most favorable trade-off. Relative to the baseline, it increased peak cylinder pressure by 15.6% and cumulative heat release by 8.1%. Although nitrogen oxides (NO _x ) emissions rose by 6.3%, soot and carbon monoxide (CO) emissions were simultaneously reduced by 21.4% and 12.0%, respectively. The helical/tangential intake valve lift asymmetry strategy provides an effective pathway for simultaneously improving combustion efficiency and optimizing emission characteristics. This study offers valuable technical guidance for the design of high-efficiency, low-emission diesel engines, contributing to future advancements in clean and sustainable engine technologies.

Graphene-based Fe single-atom catalysts coordinated by N (Fe/N-GN) exhibit promising selectivity and catalytic activity for automotive exhaust purification. However, the role of N coordination in regulating the selective adsorption of NOX on Fe/GN catalysts under complex exhaust conditions remains unclear. An in-depth understanding of N-coordination-induced adsorption mechanisms is therefore essential for elucidating the selective NOx adsorption behavior of Fe/NX-GN catalysts in the presence of competing exhaust components. Herein, density functional theory (DFT) calculations are employed to systematically investigate the adsorption behaviors of key exhaust components (NOX, O2, N2, H2O) on Fe/NX-GN (x=0−4) catalysts. The results reveal that increasing the N coordination number enhances the structural stability of Fe single atoms by increasing the local charge density at the Fe active sites. Besides, the Fe/N3-GN catalyst shows comparable adsorption strengths toward NO, NO2, and O2, indicating its suitability for standard selective catalytic reduction (SCR) reactions. Furthermore, the Fe/N4-GN catalyst preferentially adsorbs NO and NO2 while effectively suppressing the adsorption of impurity gases, demonstrating strong potential for fast SCR processes under realistic exhaust conditions. This study provides mechanistic insights into N-coordination-regulated NOX adsorption and offers theoretical guidance for the rational design of the Fe/NX-GN catalysts for advanced automotive SCR applications.