Effects of hydrogen direct-injection timing and angle on combustion and emissions of a large-bore ammonia-hydrogen marine engine: A numerical study

Ammonia-hydrogen (NH 3 /H 2 ) fuel blends have emerged as promising zero-carbon energy carriers for marine engines due to their complementary combustion characteristics. Addressing the insufficient understanding in existing research regarding the coupled effects of direct hydrogen injection in the main chamber of large-bore NH 3 /H 2 -fueled marine engines on mixture stratification, combustion processes, and emission formation under H 2 -enriched active pre-chamber conditions, this study numerically investigates the effects of hydrogen injection timing (−50 to −150°CA ATDC) and injection angle on mixture stratification, combustion behavior, and emissions characteristics. The results show that hydrogen injection timing is the dominant factor governing the mixture spatial distribution prior to ignition, as it regulates the extent of turbulent diffusion during the compression process, and thereby alters the location and concentration gradients of hydrogen-rich regions. A moderately advanced injection timing improves mixture stratification in the main chamber and maintains sufficient reactivity in the ignition region, leading to a more concentrated and efficient combustion process, albeit with increased thermal-NO formation. In contrast, excessively late injection results in overly strong stratification and insufficient reactivity in a large portion of the main chamber, while very early injection leads to excessive mixture homogenization and weakened local reactivity, both of which are unfavorable for combustion enhancement. Compared with injection timing, the injection angle has a minor influence on overall combustion intensity, and its effects are mainly associated with jet penetration and dispersion characteristics, which in turn affect the spatial distribution of high-temperature zones and the formation behavior of NO and N 2 O.