Characteristics of high-pressure liquid ammonia sprays and combustion process in ammonia/diesel HPDI dual-fuel engines

Abstract

In the context of the “carbon emission peak and carbon neutrality” framework, ammonia has garnered considerable attention as a promising zero-carbon fuel. This study employs backlighting and schlieren optical diagnostics within a constant-volume visualization setup to investigate the effect of environmental temperature on the macroscopic behavior of liquid ammonia sprays. Additionally, it contrasts the spray characteristics of liquid ammonia with those of diesel under extreme thermal and pressure conditions. Through the application of computational fluid dynamics (CFD) simulations, the research elucidates the nuanced differences in microscopic spray dynamics between ammonia and diesel under these conditions. The study also explored how to reduce the negative impact of in-cylinder direct injection of liquid ammonia on combustion through the optimization of the longitudinal distribution of diesel and ammonia sprays in a dual-fuel HPDI engine. The results reveal that liquid ammonia sprays exhibit significant sensitivity to temperature. At higher temperatures, ammonia sprays are approximated to high-density gas injections, with the liquid phase proportion being less than 5 %. Under high-temperature and high-pressure conditions, the liquid phase penetration distance of ammonia is approximately 60 % shorter than that of diesel, which is unfavorable for the flame to propagate upstream of the spray. CFD simulations further indicate that in the HPDI mode, the longitudinal spacing between ammonia and diesel sprays plays a crucial role in the relative positioning of ammonia sprays within the stable ignition zone, thereby influencing the in-cylinder combustion process. Optimizing this spacing can enhance the diesel-induced ignition effect and thus improve the overall combustion performance.