Investigation on the dual-fuel active-thermal atmosphere combustion strategy based on optical diagnostics and numerical simulations

The dual-fuel active-thermal atmosphere combustion strategy has the potential to make the homogeneous combustion compression ignition more controllable, achieving low fuel consumption and engine emissions. However, the detailed combustion process and chemical interaction between the high- and low- reactivity fuels have not been fully understood. This work investigated the premixed iso-octane combustion assisted by the two-stage reaction of n-heptane on an optical engine. High-speed imaging technique was applied to visualize the natural flame luminosity and a data-processing method based on Cantera was proposed to evaluate the detailed chemical kinetics process. Results indicate that the high-temperature heat release stage changes from partially premixed combustion with distinct blue flames into the mixing-controlled combustion, showing evident signs of soot radiation when the two-stage ignition transits into three-stage ignition. Heat release from the premixed n-heptane is initially dominated by reaction HCO + O2 = CO + HO2 and turns into hydrogen-oxygen reactions with increasing temperature. In the primary reacting regions for the cases with latter injection timings, reactions C3H5 + H(+M) = C3H6(+M) and C3H3 + H(+M) = C3H4(+M) play the major role, which promotes the formation of soot precursors. Formaldehyde can represent the mixture reactivity. Reaction pathway of premixed n-heptane generated reactive species that promote the consumption of iso-octane. Reactions of iso-octane also enhance the formation of reactive species that further accelerate the combustion process. Retardation of the iso-octane injection timing reduces the local mixture reactivity and weakens the chemical interaction between iso-octane and n-heptane. This lowers the heat release rate, and, objectively, results in the transition of two-stage ignition to three-stage ignition