Effects of ammonia energy fraction and injection timings on combustion and emission characteristics of an ammonia/biodiesel dual direct injection engine

• Effects of AEF and injection timings on combustion and emissions of an ammonia/biodiesel dual direct injection engine. • High AEF increases mixture stratification and worsens combustion efficiency. • Advancing SOAI reduces ammonia slip but weakens combustion intensity. • Shorter ammonia-biodiesel injection interval promotes faster and more complete combustion. The effects of ammonia energy fraction (AEF) and the injection timings of both fuels on combustion and emissions of an ammonia/biodiesel dual direct injection engine were systematically investigated. Results show that increasing AEF delays combustion phasing and extends combustion duration due to low reactivity and strong cooling effect of ammonia. A maximum indicated thermal efficiency (ITE) of 37.37% is achieved at 50% AEF, approximately 7% higher than pure biodiesel operation. However, further increases in AEF deteriorate ITE and combustion efficiency (CE), while significantly increasing N 2 O and unburned ammonia emissions. Advancing the start of ammonia injection (SOAI) enhances ammonia/air mixing and reduces ammonia slip as low as 3,275 ppm at −25°CA, but weakens combustion intensity. Moderate retardation of SOAI increases in-cylinder temperature and NO formation, while excessive retardation suppresses NO but results in high ammonia slip. Similarly, delaying the start of biodiesel injection (SOBI) shortens the ignition delay and promotes ignition, but reduces biodiesel/air mixing time and flame propagation. Optimal performance is achieved when SOBI is set to −16°CA, yielding maximum CE and ITE of 88% and 35.5%, respectively, along with the lowest unburned ammonia emission of 8,157 ppm. These findings underscore the critical role of injection timings in achieving efficient, low-emission combustion under high-AEF conditions.