Effects of the CH₂O chain length on spray combustion characteristics of PODE/diesel blends

Spray and combustion of the fuel can dominate the formation of fuel/air mixture, even subsequently the in-cylinder combustion and pollutant generation. Polyoxymethylene dimethyl ether (PODE) have emerged as promising alternative fuels for diesel engines, in order to decarbonize agricultural applications. This study aims to investigate the impact of blending PODE with various CH₂O chain lengths on the spray combustion of diesel fuel. Three fuel blends were compared on the diesel/PODE₂, diesel/PODE₃, and diesel/PODE₄ under various ambient conditions and different injection pressures. The experiments were conducted in a visualized constant-volume combustion chamber. Thus the spray development images were captured via the schlieren and combustion images through direct photography. Homemade software was utilized to extract the spray parameters, such as spray tip penetration, spray cone angle, and spray area from the schlieren images, while spray combustion parameters were derived from the combustion images, including ignition delay times, flame lift-off length, and integrated natural flame luminosity. The results revealed that the addition of PODE increased the spray tip penetration, but decreased both the spray cone angle and spray area. The spray tip penetration of the diesel/PODE blends further increased, as the CH₂O chain length in PODE increased. The higher density of the fuels was attributed to the fuel atomization, rather than kinematic viscosity. Meanwhile, the spray cone angle decreased with the increasing CH₂O chain length, indicating that the kinematic viscosity was dominated. The spray area exhibited an increasing trend with the longer CH₂O chain lengths, thus resulting from the combined effects of spray tip penetration and spray cone angle. Additionally, the increasing injection pressure led to a rising trend in the spray tip penetration, spray cone angle, and spray area for the blended fuels. The integrated natural flame luminosity decreased at an ambient temperature of 823 K and the air atmosphere, as the CH₂O chain length in PODE increased from 2 to 4. The reason was that the oxidation of free radicals was enhanced to reduce the soot generation, due to the increased oxygen content in the blended fuel. Furthermore, both ignition delay times and flame lift-off length were shortened significantly. The integrated natural flame luminosity and ignition delay decreased significantly for the tested fuels, with the higher injection pressures. Notably, the integrated natural flame luminosity, ignition delay, and flame lift-off length all decreased significantly with increasing CH₂O chain length. In conclusion, the PODE was incorporated to enhance the atomization of diesel fuel, in order to improve the ignition of diesel fuel sprays for less soot emissions. Moreover, the longer CH₂O chain lengths in PODE were used to further reduce the soot emissions. This finding can provide valuable insights into applying the PODE as a suitable and promising alternative fuel in high-pressure diesel engines in modern agriculture.