A comparative study of premixed and diffusion combustion strategies in a high-speed ammonia/diesel dual-direct-injection engine

Ammonia, as a carbon-free energy carrier, has garnered significant global attention as a promising alternative fuel. The increasing demand for cleaner fuels in internal combustion engines has driven intensive research into ammonia–diesel dual-fuel systems. However, the combustion strategies for ammonia remain underexplored. In this study, a comprehensive experimental comparison was conducted between premixed combustion and diffusion combustion modes. The comparison was carried out on a high-speed, four-stroke direct-injection dual-fuel engine platform, using identical fuel injection quantities. The effects of ammonia injection mass on combustion and emission characteristics were systematically investigated. The results indicate that in both combustion modes, increasing ammonia injection significantly suppresses complete diesel combustion, resulting in elevated CO emissions. Diffusion combustion, characterized by lower combustion temperatures and suboptimal ammonia–air mixing, effectively reduces thermal NOx and N2O formation. At low ammonia injection levels (lean equivalence ratios), diffusion combustion outperforms premixed combustion in terms of indicated thermal efficiency (ITE) and combustion stability. However, as the ammonia injection increases, excessive equivalence ratios severely hinder complete ammonia combustion. In contrast, the premixed mode maintains higher torque output, despite exhibiting inferior combustion stability and emission performance. This is due to enhanced combustion phasing and a higher degree of constant-volume combustion, even under fuel-rich conditions. This study provides practical insights for optimizing ammonia–diesel dual-fuel engine strategies, supporting the development of sustainable power solutions based on low-carbon or carbon-free fuels.