Combustion and emission characteristics of stratified premixed and diffusion modes in direct-injection ammonia-hydrogen SI engines

Ammonia-hydrogen spark-ignition engines offer a promising zero-carbon pathway, but the combustion behavior associated with direct ammonia injection remains insufficiently understood. This study performs a comprehensive investigation of stratified premixed combustion (SPC) and diffusion combustion (DC) modes in an ammonia-hydrogen engine with direct ammonia injection. Results demonstrate that the two modes display different combustion mechanisms: SPC promotes near-simultaneous burning of the premixed NH3-H2 mixture, whereas DC follows a hydrogen-ignited two-stage combustion pathway that accelerates ammonia oxidation and shifts combustion phasing. These differences govern the contrasting trends of indicated thermal efficiency (ITE) and pollutant emissions. Optimization of SPC through advanced ignition and dual post-injection strategies increases the ITE to 40.1 % while simultaneously reducing NH3, NO, and N2O emissions. For the DC mode, a 30 % dual pre-injection mass split achieves the best compromise between ITE and pollution emissions, reducing unburned NH3 and NO by 32.4 % and 62.6 %, respectively, compared with the baseline SPC, at a cost of only 0.36 % efficiency penalty. These findings demonstrate a dual-mode control strategy that enables switching between a high-efficiency/low-N2O mode (optimized SPC) and a low-NO/low-NH3 mode (optimized DC), providing new guidance for flexible combustion control in ammonia-fueled engines.