Dynamic characteristics of pilot injection: Spray evaporation, mixture formation, and combustion under engine-like conditions

In dual-fuel engines, pilot diesel injection critically influences combustion dynamics by initiating alternative fuel ignition, yet excessive replacement ratios may introduce disturbances within the nozzle, impacting external jet atomization and subsequent combustion processes. This study employs high-speed UV-LAS and OH* chemiluminescence imaging techniques to resolve transient mixture formation and flame development, comparing pilot injection (simulated with minimal needle valve activation) against main injection under varied injection pressures and ambient temperatures. Results demonstrate that pilot injection dominates liquid length control—surpassing ambient temperature effects and showing negligible pressure influence—where internal nozzle flow variations emerge as the primary driver of enhanced atomization when ambient enthalpy suffices for evaporation. Auto-ignition timing exhibits strong sensitivity to ambient temperature but minimal pilot-main differences due to comparable stoichiometric mixtures at ignition sites. Flame propagation dynamically self-regulates by targeting vapor-phase penetration, transitioning from entrainment-controlled to combustion-driven expansion. Quantitative analysis confirms pilot injection strategically modulates spatial mixture distribution: enriching near-stoichiometric/fuel-rich zones (φ > 0.8) to enhance low-reactivity fuel (e.g., ammonia) ignition, while rapid post-injection homogenization establishes >90 % lean mixtures (φ ≤ 0.8) within 0.5  ms after EOF. These findings establish pilot injection as a precision tool for in-cylinder equivalence ratio control, providing actionable strategies to optimize nozzle dynamics and combustion phasing in advanced dual-fuel systems.