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Abstract
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hipboard diesel engines operate under unique marine conditions including high humidity, salt-laden
air, variable ambient pressure/temperature, and confined engine-room ventilation, which can
significantly affect combustion and emissions. This study investigates the effect of varying intake air
humidity on combustion and emissions in a marine diesel engine using three-dimensional computational
fluid dynamics (CFD) simulations (1D AVL-Boost and 3D AVL-FIRE with ECFM-3Z combustion
chemistry). Intake relative humidity levels of 25, 50, and 75% are imposed while keeping operating
conditions and combustion chamber geometry fixed at their baseline values. The simulation model is
first validated against experimental data, then used to compare in-cylinder pressure, temperature, heatrelease
rate, equivalence ratio, indicated work, and pollutant outputs under different humidity conditions.
Results show that increased intake humidity slightly raises the hydrogen content of the charge, leading
to a marginally higher peak in-cylinder pressure. Heat-release rate and equivalence ratio distributions
are largely unaffected by humidity. Importantly, increased intake humidity significantly reduces NOx
emissions. Soot formation, however, changes only slightly, with no clear trend (any minor increase from
higher local equivalence and temperature is offset by reduced oxygen). Overall, adding humidity reduces
NOx by altering oxygen availability and flame temperature, while having only minor impacts on engine
performance and soot.
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