Analysis of the influence of ammonia energy ratio and injection timing on the formation of ultrafine particles in an Ammonia-Diesel Dual-Fuel (ADDF) engine

Global environmental pollution and energy shortages have focused considerable attention on ammonia as a promising zero-carbon fuel. This study explores the particulate emission characteristics of ammonia-diesel dual-fuel engines, revealing the complex microscopic mechanisms involved in particulate matter (PM) generation and oxidation. The results demonstrate that particulate matter emissions do not exhibit a simple linear relationship with ammonia energy ratio (AER). Notably, under late injection timing conditions, emissions display a non-linear "inflection point" pattern, where they initially increase and then decrease. Furthermore, the competition between NH3 and diesel for OH radicals during the dehydrogenation process results in particulate emissions that exceed those of pure diesel combustion. However, as the AER rises to 40 % or higher, the diesel injection volume decreases, lowering the local diesel equivalence ratio and subsequently reducing the formation of soot precursors. Concurrently, the high-temperature ammonia dehydrogenation reaction facilitates a shift towards hydrochemical combustion, increasing the molar ratio of hydrogen at the flame front. This, in turn, boosts OH radical formation and their oxidizing effect on soot precursors. At an AER of 60 %, soot emissions are dramatically reduced by 97.1 %. These findings provide a solid theoretical basis and practical guidelines for optimizing the combustion strategy of ammonia-diesel dual-fuel engines.