Effects of Pd/Pt-based soluble catalysts on the combustion characteristics of kerosene droplets

Soluble catalysts that are readily blended with fuels render significant potentials in long-term storage and engine compatibility to improve ignition and combustion reliability under demanding conditions. However, the mechanistic role of such additives under droplet combustion scenarios that involve evaporation-reaction coupling remains unclear. In this study, single-droplet combustion experiments were conducted using RP-3 aviation kerosene doped with two different soluble catalysts, palladium acetylacetonate (Pd(acac) 2 ) and platinum acetylacetonate (Pt(acac) 2 ), over a range of ppm-level concentrations. Ignition delay time, burning rate and droplet lifetime were systematically quantified to elucidate the influence of catalysts on the fuel combustion behaviors. A stage-resolved analysis was conducted to differentiate ignition onset from quasi-steady burning. The results revealed that both catalysts improve ignition and combustion performance, yet exhibit distinct catalytic roles governed by their thermochemical properties and oxidation states. Earlier ignition is promoted by Pd(acac) 2 due to its low-temperature sublimation and the formation of catalytically active Pd and PdO. The former, however, also competes with the fuel for oxygen at the flame front, inducing an inhibiting effect on droplet burning. These effects together result in a non‑monotonic concentration dependence of the burning rate. In contrast, steadier burning is sustained by Pt(acac) 2 through its stable metallic state Pt. These findings clarify how catalyst decomposition characteristics and oxidation-state stability govern ignition onset and subsequent burning behavior. The present study provides mechanistic insights into fuel-catalyst interactions and guidance for developing fuel-soluble additives to enhance ignition reliability and combustion efficiency in advanced aero-engine systems. Novelty and significance statement This study advances the understanding of fuel-soluble noble-metal catalytic additives by revealing how their transient physicochemical evolution affects single-droplet combustion behaviors. By comparatively resolving the thermochemical evolution of two catalysts, this work demonstrates that decomposition pathways, oxidation-state stability, and catalytic activity collectively modulate stage-specific responses during droplet evaporation, ignition and burning. This stage-resolved perspective moves beyond treating soluble catalysts as static combustion enhancers and clarifies the intrinsic coupling between catalyst evolution and droplet combustion. These mechanistic insights establish transferable principles for the rational design of next-generation fuel-soluble catalytic additives and support an engine-compatible strategy for enhancing high-altitude ignition reliability and combustion efficiency without structural modifications to existing aero-engines. In addition, a U-Net-based deep learning framework is introduced for automatic droplet contour recognition, enabling accurate and efficient droplet combustion diagnostics.