A new study published in Engineering has explored a pilot-ignition reactivity-stratification (PIRS) combustion strategy for ammonia-fueled compression ignition engines, realizing a high ammonia energy fraction (AEF) while maintaining efficient combustion and reduced emissions, offering a viable technical path for decarbonization in the transportation sector. Ammonia, a zero-carbon fuel that produces no carbon dioxide during combustion, is regarded as a promising alternative for traditional fossil fuels in internal combustion engines, yet its high ignition energy requirement and slow flame propagation speed have long hindered its practical application, necessitating reactive pilot fuels for ignition.

Conducted by researchers from Shanghai Jiao Tong University, the study used a modified 12.8 L four-stroke compression ignition engine with a compression ratio of 17.0 and an operating speed of 1500 rpm for high-load experiments, focusing on the impacts of excess air coefficient and diesel injection strategies on engine combustion and emissions at AEF no less than 80%. The team first examined how intake air mass affects the engine under 80% and 90% AEF, finding that the excess air coefficient λ closely influences diesel ignition delay: a higher λ advances the combustion phase of pre-injected diesel but causes separation between ammonia and diesel combustion, while a λ below 1.2 leads to incomplete diesel combustion with a sharp rise in CO emissions. A λ range of 1.3–1.4 was identified as the optimal interval, achieving a brake thermal efficiency (BTE) close to 44.5% and reducing unburned ammonia emissions by approximately 35% when λ decreased from 1.6 to 1.1.

The study then compared the PIRS combustion mode with the single diesel direct injection compression ignition (DICI) mode at a fixed λ of 1.3 and 80% AEF. The PIRS mode involves early pre-injection of most diesel at 60 °CA BTDC for full mixing with ammonia and intake air, with a small amount of diesel injected near top dead center for ignition. This design enhances the overall reactivity of the in-cylinder mixture, shortens combustion duration, and improves ammonia combustion efficiency. In contrast to DICI mode, PIRS mode reduced unburned ammonia emissions by more than 50%, and when the second diesel injection timing was optimized to 14 °CA BTDC, the BTE reached 46.1%, with unburned ammonia emissions at only 26.8% of that in DICI mode. With a pre-injection diesel mass of 30 mg, unburned ammonia emissions were further controlled below 2000 ppm and N₂O emissions down to 11.3 ppm.

By testing the minimum diesel injection mass for stable operation, the research achieved a maximum AEF of 99.1%, with CO₂ emissions at this point accounting for just 1.3% of those from pure diesel combustion at the same load, and total equivalent greenhouse gas emissions reduced to 13.2% of the diesel baseline when considering N₂O’s CO₂ equivalence factor of 298:1. The study also identified a globally optimized operating condition at 90% AEF in PIRS mode, where 15 mg of diesel is pre-mixed at 60 °CA BTDC and 5 mg of micro-diesel is injected for ignition at 15 °CA BTDC under a brake mean effective pressure of 2.26 MPa. This condition balances ammonia and NOₓ emissions at similar concentrations, avoids the need for additional reductants in selective catalytic reduction (SCR) aftertreatment, and maintains low greenhouse gas emissions with CO₂ emissions at 1.2% of the diesel baseline and N₂O emissions at 23.6 ppm.

The research notes that NOₓ emissions show a negative correlation with unburned ammonia emissions in ammonia-fueled engines, with NOₓ emissions exceeding 6000 ppm when ammonia combustion efficiency is improved, posing challenges for aftertreatment systems. The combination of multi-stage SCR and ammonia slip catalyst (ASC) is proposed as an effective solution for emission treatment under ultra-high AEF conditions. This study clarifies the substitution rate boundary of diesel-ignited ammonia engines and provides a technical basis for the optimization of combustion and emission control in ammonia-fueled engines.

The paper “Pilot-Ignition Reactivity-Stratification Combustion for Ammonia Fueled Engines,” is authored by Yuxiao Qiu, Yanyuan Zhang, Yingnan Yang, You Zhang, Dong Han, Zhen Huang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.10.024. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.