Chemiluminescence-Based Analysis of Syngas/Diesel Dual-Fuel Combustion in an Optically Accessible Engine

Syngas (synthesis gas) is a promising gaseous biofuel for small-scale power generation, but its highly variable composition, which depends on the biomass source and gasification process, poses challenges for engine optimization. This study investigates syngas–diesel dual-fuel combustion in an optically accessible engine using chemiluminescence imaging of OH*, CH*, and CH2O* to characterize ignition and flame development. Three representative syngas compositions—Downdraft, Updraft, and Fluidbed—were examined. The Fluidbed composition exhibited the weakest OH* signal, approximately one-third of that observed for the other two, primarily due to its higher CO2 dilution and lower H2 content. Ignition delay trends were strongly correlated with dilution level: Downdraft and Updraft showed similar delays despite different H2/CO ratios, while larger CO2 shares led to longer delays and flattened heat-release rates. CH* and CH2O* chemiluminescence showed better agreement with combustion timing than OH*. Methane enrichment enhanced flame propagation and reduced ignition delay, partially offsetting CO2 dilution effects.