Effects of ambient density and injection pressure on ignition and combustion characteristics in diesel spray under plateau cold-start conditions

•The trends in ignition delay, overall flame temperature, and overall KL factor with varying injection pressure have changed under different ambient densities.•The competition between soot formation and oxidation was assessed by computing the time scale of soot evolution within the flame.•The optimal reference injection pressure for different ambient densities was proposed by comprehensively considering the ignition and combustion characteristics. Gaining insights into the multi-parameter coupling dominated in-cylinder combustion process takes on critical significance to controlling and enhancing the performance of diesel engines under plateau cold-start conditions. In this study, the effect of varying injection pressure and ambient density on ignition and combustion characteristics of No. -50 diesel spray was investigated through high-speed direct photography and the two-color imaging technique, respectively. Regarding ignition characteristics, the increase of the ambient density led to the variation of the non-monotonic trend of ignition delay time with the rise of the injection pressure to a monotonous decrease, whereas the correlation between the axial ignition location and the injection pressure remained unaffected. Under three different ambient densities, the transition modes of the flame structure varied with the increase of injection pressure, subsequently impacting combustion characteristics. For combustion characteristics, the average level of flame temperature and soot were inversely proportional to the injection pressure for all three ambient densities. Higher ambient density facilitated the heat release from combustion chemical reactions, influencing the impact of increasing turbulence intensity on the ignition and combustion processes of diesel spray. The overall temperature level of the flame displayed an inflection point with the rise of the injection pressure, and this inflection point is delayed with the increase of the ambient density. The local high-temperature regions in flames (＞2300 K) were identified under higher ambient densities, such that soot generation and oxidation were subjected to significant competition. At ρam = 16.6 kg/m3, the oxidation dominant time gradually decreased with the rise of the injection pressure, leading to the integral KL factor maximum at Pinj = 80 MPa case. The optimal fuel injection pressures for cold starting at different altitudes were determined by considering the ignition and combustion characteristi