Strategies for nitrogen oxides mitigation in hydrogen–diesel dual-fuel engines

This study investigates two NOx-mitigation strategies, namely exhaust gas recirculation (EGR) and water–diesel emulsification, in a diesel–hydrogen dual-fuel compression ignition engine. A validated three-dimensional CFD model is employed to reproduce the combustion process of a single-cylinder engine operating at 1800 rpm and 5 bar of IMEP. The model is first calibrated and validated against experimental data for conventional diesel operation and two dual-fuel conditions (8% and 18% hydrogen energy share) conducted on a test bench, showing good agreement in terms of in-cylinder pressure, heat-release rate, and NOx emissions. Simulations demonstrate that hydrogen enrichment increases mixture reactivity and peak temperature, leading to higher NOx formation. EGR rates of 5–15% effectively reduce peak temperatures and suppress NOx formation, although they increase CO emissions and hydrogen slip due to the reduced mixture reactivity. Water–diesel emulsions containing up to 8% water (E8) also reduce NOx emissions by lowering local peak temperatures through evaporative cooling, but unlike EGR they promote a more uniform combustion process. As a result, the 8% emulsion (E8) leads not only to the largest NOx reduction but also to a slight decrease in hydrogen slip compared to the baseline conventional diesel case.