Experimental correlation of laminar flame pollutant emission indices with methyl ester fuel degree of unsaturation and equivalence ratio

Fuel unsaturation has been widely cited as a reason for the increased nitric oxide emissions from compression ignition engines when fueled with methyl esters (biodiesels) and their blends with petroleum counterparts. In an earlier study, a parameter called degree of unsaturation (DOU) was established to serve as a common platform across different fuel families (esters/alkanes/aromatics) to quantify the effects of fuel unsaturation, particularly with petroleum/biodiesel blends. DOU can be evaluated based on the average molecular formula of the fuel alone without involving complex and expensive experimental procedures such as those involved in the measurement of iodine number and bromine number. In this article, the fuel unsaturation effects on the emission characteristics from the laminar flames of blends of various pre-vaporized methyl esters, such as methyl oleate, neat biodiesels (from methyl esters of soy, canola, palm and rapeseed feedstock) and biodiesel blends were investigated at four burner-exit equivalence ratios of 0.9, 1.0, 1.2 and 1.5. The selection of these fuels for this study was an attempt to isolate the fuel unsaturation effect of biodiesels, and their blending effect with petroleum fuels. Experimental correlations were developed between DOU (over a range of 1.7-2.5) and global NO and CO emission indices as a function of equivalence ratio. The effects of DOU on EINO were significantly influenced by the equivalence ratio, with the maximum influence at an equivalence ratio of 1.2.