Effects of lean burn, oxygen enrichment, and hydrogen enrichment on an ammonia engine

Ammonia is a promising carbon-free fuel for internal combustion engines in transportation and energy sectors. However, its inherent properties, such as low laminar flame speed and high ignition energy requirement, hinder stable combustion, particularly under low-load conditions. To address these challenges, enriching the intake charge with small amounts of hydrogen or oxygen has been proposed to improve combustion stability and efficiency. Compared with hydrogen, oxygen is easier to liquefy and store, making oxygen-enrichment strategies a more practical solution for ammonia-fueled engines. This study investigates lean-burn, oxygen-rich, and hydrogen-rich strategies in a passive pre-chamber jet ignition ammonia engine with a high compression ratio of 17.3. The results demonstrate that under standard ammonia-air combustion, the engine operates stably in lean conditions (λ ≤ 1.10) and achieves an indicated thermal efficiency (ITE) exceeding 42.5%. Lean burn was found to reduce the proportion of fuel-derived NO in total NO emissions, thereby weakening the negative correlation between NO emissions and in-cylinder pressure. Both oxygen-rich and hydrogen-rich strategies successfully extended the engine's stable operating range. Notably, oxygen enrichment yielded a higher ITE than an equivalent molar addition of hydrogen. At an intake oxygen concentration of 23%, the engine reached an ITE of 43.9% on pure ammonia. A chemical kinetic analysis revealed that the hydrogen-rich strategy promotes the conversion of NH to N2H2, which inhibits HNO formation and suppresses NO production. Conversely, the oxygen-rich strategy increases NO concentration while reducing NH3 emission, resulting in a lower NH3-to-NO ratio that is beneficial for selective catalytic reduction (SCR) aftertreatment.