Combustion and soot characteristics of hydrotreated vegetable oil compression-ignited spray flames

Quantitative measurements of combustion and in-flame soot in hydrotreated vegetable oil (HVO), biodiesel (rapeseed methyl ester [RME]) and diesel fuel spray flames are presented in this study. The measurements were performed in a compression ignition chamber with optical access having engine-like thermodynamic conditions. The simultaneously employed optical techniques were high-speed diffuse back-illuminated light extinction imaging for measuring the optical depth (KL) of in-flame soot, and OH* chemiluminescence for imaging high temperature reaction zones. The results show that diesel produces a considerably higher amount of in-flame soot compared to RME and HVO, which is likely due to the aromatics present in diesel, while RME and HVO produce similar levels. RME was found to produce slightly less soot, which is likely due to the fuel-bound oxygen, since HVO and RME have a similar number of carbon-carbon bonds. The soot production during the premixed and mixing-controlled period were found to have similar trends. The time lag between start of ignition and soot formation was found to be constant for varying ambient temperatures for individual fuels, where soot was detected earliest after ignition for diesel, followed by HVO and RME, matching the measured sooting tendencies. The stabilised flame lift-off length of HVO was found to be shorter than that of diesel, while RME had the shortest of them all.