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 (NOx) 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.
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