Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (D_noz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different D_noz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τ_HTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when D_noz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τ_phy), low-temperature pre-reaction time (Δτ₁), low-temperature ignition delay time (τ_LTI), high-temperature pre-reaction time (Δτ₂), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing D_noz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H₂O₂ and inhibiting flame propagation. The combined effect of decreasing D_noz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.