Dynamic Characteristics of the Droplet Impact on the Ultracold Surface under the Engine Cold Start Conditions

The impact of liquid droplets on the ultracold surface affects significantly the cold start performance of internal combustion engines but the splash and spreading characteristics after impacting on the ultracold surface are not clearly understood. Therefore, droplets with various physical parameters impacting on the Al–Si alloy surface have been selected for the study under various surface temperatures (–40°C ≤ \(~{{T}_{s}}~\) ≤ 25°C) and droplet impact velocities (0.96 m/s ≤ \(~{{V}_{0}}~\) ≤ 3.52 m/s). The ultracold surface (\({{T}_{s}}\) = –40°C) is beneficial for corona splash, and droplets with the higher Oh number impacting on the ultracold surface easily produce corona splash as the main splash pattern. The ultracold surface assisted in enhancing the stability of the levitated lamella formation, and avoided the effects of rough surfaces, so the upper splash criterion is established to predict the transition from spreading to splash. The decreasing surface temperature reduces the maximum spreading diameter (\({{{{\beta }}}_{{{\text{max}},{\text{lt}}}}}\)) of low solidification point droplets (ethanol, n-propanol, and winter diesel). Based on the assumptions of qualitative temperature, the empirical correlation of the \({{{{\beta }}}_{{{\text{max}},{\text{lt}}}}}\) is created for the T_s from 25 to –40°C.