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Abstract
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Shipboard 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 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, heat-release 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 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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