Study on free and impinging diesel spray macroscopic characteristics under extreme-low fuel temperature conditions

Spray characteristics have a significant regulatory effect on the combustion process of internal combustion engines, especially under cold start conditions. The macroscopic characteristics of free spray and impinging spray are investigated experimentally. Darker coloured enriched clusters appear at the bottom of free spray. The splits at the bottom of free sprays are severe, slight and none split at 20 °C, -20 °C and -40 °C, respectively. In the 0 to 466.62 μs after start of injection (ASOI), the spray area is decreased by >70 % at a low injection pressure of 25 MPa when the temperature drops from 20 °C to -40 °C, and by <30 % when the injection pressure >50 MPa. The impinging spray is divided into four regions: liquid core region, free jet region, wall jet region, and transition region. The spray in the transition region should be the focus of the combustion optimisation. A new dimensionless number s is defined as the shape coefficient of impinging spray. B-shaped when s < 0.1, K-shaped when 0.1 < s < 1, and M-shaped when s > 1. At 1333.20 µs after impinged the wall (AIMW), the maximum diffusion distance decrease by 19.94 % and the maximum entrainment height decrease by 24.59 % at 25 MPa when the fuel temperature decreases from 20 °C to -40 °C, for 37.73 % and 35.89 % at 100 MPa. The results can provide new insights for multiphase flow modelling, and a theoretical basis can be provided for the design of fuel injection systems for low-temperature suitability.