Abstract: Ammonia is considered a potential carbon-free fuel for internal combustion engines (ICEs). However, its unfavorable combustion characteristics and a propensity to form air pollutants pose a challenge for engine designers. Recent investigations revealed that ammonia spray flames fail to stabilize at a fixed distance from the injector nozzle under diesel engine-relevant conditions, which results in increasing lift-off lengths over time. This work investigates the effect of diesel fuel post-injections (PIs) on the pollutant formation characteristics of diesel-piloted ammonia sprays in a rapid compression-expansion machine (RCEM). Emission measurement results are complemented with an analysis of the heat release rate (HRR), as well as shadowgraphy (SG) and OH*-chemiluminescence (CL) imaging. A 1-D spray model is used to gain insight into equivalence ratios in ammonia sprays during injection ramp-down. In addition, Φ-T maps are introduced to understand NOx formation characteristics of ammonia combustion. It is shown that diesel PIs decrease unburned ammonia emissions by up to 2/3. The largest reduction is obtained for diesel PI timings, which minimize the lift-off length of ammonia sprays during the end of injection (EOI) transient. The amount of diesel fuel in the PI is found to have only a minor effect. In addition, PIs promote the conversion of NO to NO2, which is attributed to an altered radical pool composition due to diesel combustion. Furthermore, it is shown that entrainment waves during ammonia injection rate ramp-downs create a lean wake region upstream of the lift-off length. If combustion of this lean zone is induced by diesel PIs, additional NOx is formed.     «

Abstract: Ammonia is considered a potential carbon-free fuel for internal combustion engines (ICEs). However, its unfavorable combustion characteristics and a propensity to form air pollutants pose a challenge for engine designers. Recent investigations revealed that ammonia spray flames fail to stabilize at a fixed distance from the injector nozzle under diesel engine-relevant conditions, which results in increasing lift-off lengths over time. This work investigates the effect of diesel fuel po...     »