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

Abstract

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz 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 (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 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 (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), 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 Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.