Simulation study of N2/Ar dilution gas to improve thermal efficiency and reduce NOX emissions in hydrogen engines

Abstract

Although N2 dilution is effective in reducing NOX emissions in hydrogen engines, it decreases thermal efficiency. Utilizing Ar with N2 as a dilution gas can improve thermal efficiency while reducing NOX emissions. However, the specific effects and mechanisms of the mixed dilution of Ar and N2 on the combustion and emission characteristics of hydrogen engines remain uncertain. This study examined the influence of Ar and N2 on the combustion and NOX emissions by adding Ar and N2 to the intake gas at various dilution rates. The examined cases were as follows: Case 1 (N2 = 100%), case 2 (N2:Ar = 80%:20%), case 3 (N2: Ar = 50%:50%), case 4 (N2:Ar = 20%:80%), and case 5 (Ar = 100%). The findings indicate that increasing the N2 ratio in the dilution significantly reduces NOX emissions, but it also reduces the combustion rate. In contrast, increasing the Ar proportion enhances combustion, resulting in higher peak values of the in-cylinder pressure, temperature, and heat release rate. In case 1 (N2 = 100%), the NOX emissions decreased from 255 to 3 ppm when the dilution rate increased from 5% to 20%. However, this also caused a significant decrease in the thermal efficiency from 44.4% to 22%. This limitation can be mitigated by appropriately increasing the proportion of Ar. Optimal results, with lower NOX emissions (250–500 ppm) and higher thermal efficiency (47.6%–49.3%), were achieved with a dilution rate of 17%–27% and an Ar proportion of 80%–90%.