Abstract
<div class="section abstract"><div class="htmlview paragraph">Ammonia shows promise as an alternative fuel for internal combustion engines (ICEs) in reducing CO<sub>2</sub> emissions due to its carbon-free nature and well-established infrastructure. However, certain drawbacks, such as the high ignition energy, the narrow flammability range, and the extremely low laminar flame speed, limit its widespread application. The dual fuel (DF) mode is an appealing approach to enhance ammonia combustion. The combustion characteristics of ammonia-diesel dual fuel mode and ammonia-PODE<sub>3</sub> dual fuel mode were experimentally studied using a full-view optical engine and the high-speed photography method. The ammonia energy ratio (ER<sub>a</sub>) was varied from 40% to 60%, and the main injection energy ratio (ER<sub>Inj1</sub>) and the main injection time (SOI<sub>1</sub>) were also varied in ammonia-PODE<sub>3</sub> mode. The findings demonstrate that ammonia-PODE<sub>3</sub> mode exhibits better ignition characteristics than ammonia-diesel mode, resulting in an earlier ignition start, a larger flame area, a larger flame expansion speed, a shorter ignition delay time (IDT) and a shorter combustion duration (CD) due to the higher cetane number (CN) and a greater injection mass of PODE<sub>3</sub> at the same energy input. Ammonia-PODE<sub>3</sub> mode achieves larger maximum cylinder pressure (P<sub>max</sub>) and peak heat release rate (HRR) compared to ammonia-diesel mode. Furthermore, almost no soot was observed in ammonia-PODE<sub>3</sub> mode throughout the entire combustion process. The results also indicate the start of the ignition is delayed and the first peak flame expansion speed decreases at a higher ammonia energy ratio. The maximum flame area is the largest at ER<sub>a</sub> = 60%, but the peak cylinder pressure and IMEP reach their highest values at ER<sub>a</sub> = 50%. Both the first peak flame area and the first peak flame expansion speed increase with the increase of ER<sub>Inj1</sub>, and the P<sub>max</sub> and first peak HRR also increase. The IDT slightly extends with the increase of ER<sub>Inj1</sub>, while CA50 advances and CD reduces. The start of the ignition is delayed with the advancement of the main injection time, and the largest flame area is observed at SOI<sub>1</sub> = -12.5 °CA. Both the first P<sub>max</sub> and the maximum IMEP occur at SOI<sub>1</sub> = -15 °CA. This is because the cylinder temperature and pressure are lower during combustion when the main injection time is early, while a later main injection time leads to inadequate fuel-air mixing and a delayed combustion phase.</div></div>