Experimental Quantitative Analysis of Spray Velocity Field Characteristics of Four Compression-Ignition Engine Oxygenated Alternative Fuels

As the suitable substitutes for diesel in compression-ignition (CI) piston engines, hydrotreated vegetable oil (HVO), polyoxymethylene dimethyl ethers (PODEs), and bio-aviation fuel (BAF), among other oxygenated alternative fuels have been widely recognized due to higher cetane values. To explore the in-cylinder fuel spray dynamics and subsequent fuel–air entrainment of these fuels, experimental studies on near-field and full-field spray characteristics were carried out by the diffuser back-illumination imaging (DBI) method within a constant-volume chamber. The local velocity was inferred by momentum flux conservation and Gaussian radial profile assumption, and the dimensionless J_et number was introduced to qualify the strength of interaction within two-phase flow. It was found that the initial spray transitions from a “needle” to a larger spray head structure as injection pressure rises, especially with PODE_3-5 exhibiting a stable “mushroom” structure due to its higher surface tension. Superior axial penetration and velocity are achieved by PODE_3-5 due to higher density (ρ = 1), resulting in the smallest spray cone angle. The largest J_et numbers for PODE_3-5 at the center axis signify stronger friction between the fuel and gas, while the weaker droplet fragmentation and atomization were indicated by the lower J_et numbers at the spray periphery. BAF was characterized by the maximum cone angles and highest radial spray velocities near the nozzle due to the cavitation effect caused by the maximum saturated vapor pressure (ρ = 1), which promoted radial spray development. HVO exhibited smaller near-nozzle cone angles resulting from its higher viscosity (ρ = −0.6), though comparable cone angles to BAF were achieved in downstream regions owing to the second-highest saturated vapor pressure. Finally, a more accurate modified empirical spray penetration model is derived by incorporating fuel density.