Experimental study on lean combustion in an ammonia engine with a hydrogen pre-chamber

Hydrogen pre-chamber ammonia engines are a promising zero‑carbon engine concept for transportation decarbonization. However, they still face two key challenges: first, at low loads, the minimum hydrogen energy ratio (HER) required for stable combustion, denoted as mHER, is relatively high; second, improvements in brake thermal efficiency (BTE) at high loads are limited by heat-transfer losses. Given the flow communication between the main chamber and the pre-chamber through the jet orifices, changes in main-chamber mixture leanness are expected to alter the oxygen availability of the mixture entering the pre-chamber, thereby affecting pre-chamber reactivity, jet characteristics, and subsequent main-chamber combustion. Experiments were conducted under various combinations of load, excess air ratio (λ), and HER. The experimental results demonstrate that appropriately increasing main-chamber mixture leanness accelerates the combustion of the pre-chamber mixture and strengthens jets, thereby promoting faster main-chamber combustion, higher BTE, and lower mHER. However, excessively lean ammonia-air mixtures in the main chamber were more difficult to ignite, burned more slowly, and were more susceptible to quenching, thereby increasing the mHER and unburned ammonia emissions. At λ = 1.2, the mHER decreased by 49% compared with that at 1.0. Lean combustion also reduced heat-transfer and throttling losses, resulting in higher BTE across all loads. Under mHER conditions, the BTE increased by at least 5.5 absolute percentage points as λ increased from 1.0 to 1.4. Therefore, main-chamber mixture leanness can serve as an effective actuator for pre-chamber reactivity management, enabling simultaneous reductions in hydrogen demand and improvements in thermal efficiency.