Pilot Injection Impact on Ignition of F-24/ATJ Blends Under Various Conditions

This study experimentally investigates the effects of pilot injection on ignition performance for blends of conventional petroleum-derived aviation fuel (F-24) and an isobutanol-derived alcohol-to-jet (ATJ) fuel. The fuel blends are injected into a constant flow combustion chamber under various thermodynamic conditions to simulate the ignition process in compression-ignition (CI) engines. Simultaneous imaging of planar laser-induced fluorescence of formaldehyde, chemiluminescence, and schlieren are performed to analyze the detailed ignition process. Variations in fuel blend, ambient conditions, and injection strategies yield distinct ignition behaviors. Blends with higher ratios of ATJ and lower thermodynamic conditions reduce overall heat release and increase both first stage and overall ignition delays, occasionally leading to misfire. Compared to a single injection, the addition of a pilot injection to the main injection enhances overall heat release and reduces both first stage and overall ignition delays across all fuel blends and ambient conditions. This acceleration of the ignition process results from early formation of intermediate combustion radicals from the pilot mixture, elevating local temperature and promoting the chemical reaction of the main mixture. This effect is enhanced in blends with lower ATJ fractions, reducing the ignition delay up to 60% compared to single injection. These findings demonstrate that the pilot injection can expand the operational window for CI engines, especially with low cetane number fuels at lower temperature and pressure conditions.