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The European biodiesel standard EN-14214 calls for determining the oxidation stability (OS) at 110 °C with a minimum induction time of 6 h by the Rancimat method (EN-14112). The ASTM standard D-6751 has recently introduced a minimum induction period of 3 h. Palm methyl ester (PME) has been successfully evaluated as a diesel substitute in summer and with an additive in winter due to its poor cold-flow properties. Neat PME exhibited an OS of 9.24 h; thus, it was highly stable. Research was conducted to investigate the effect of the presence of transition metals, likely to be present in the metallurgy of storage tanks and barrels, on the highly stable PME. It was found that the influence of metal was detrimental and catalytic even for stable PME. Small concentrations of metal contaminants showed nearly the same influence on OS as large amounts. Copper showed the strongest detrimental and catalytic effect. Antioxidants, namely, tert-butylated hydroxytoluene (TBHT), tert-butylated phenol derivative (TBP), octylated butylated diphenyl amine (OBPA), and tert-butylhydroxquinone (TBHQ) were doped to improve the OS of metal-contaminated PME. It was found that the antioxidant TBHQ was most effective among all of the antioxidants used.

To minimize use of biodiesels synthesized from edible oils, such as palm oil, because of the raising food versus fuel issue, palm biodiesel (PBD) was blended in different weight ratios with biodiesels synthesized from tree-borne non-edible oil seeds, jatropha and pongamia, to examine the effects on the cold filter plugging point (CFPP) of PBD. The CFPP of PBD improved significantly after blending with jatropha biodiesel (JBD) and pongamia biodiesel (PoBD). The dependence of CFPP upon the esters of fatty acid composition was also examined. Good correlations between the CFPP and palmitic acid methyl ester (PAME) were obtained. A correlation between CFPP and total unsaturated fatty acid methyl ester (X) was also determined. Using these two correlations, the CFPP of different biodiesel blends can be determined.

According to the proposed National Mission on biodiesel in India, we have undertaken studies on the stability of biodiesel from tree-borne non-edible oil seeds like Pongamia pinnata. Neat Pongamia methyl ester (PoME) exhibited an oxidation stability (OS) of 2.54 h and research was conducted to investigate the effect of the presence of transition metals likely to be present in the metallurgy of storage tanks and barrels, on the OS of PoME. It was found that the influence of metal was detrimental to OS and was catalytic, as even small concentrations of metal contaminants showed nearly the same influence on OS as large amounts. Copper showed the strongest detrimental and catalytic effect on OS. The OS of metal-contaminated PoME was found to increase with an increase in the dosage of antioxidant but the dosage required for copper-contaminated PoME became approximately four times than required for neat PoME. The dependence of the OS on the type of metal showed that long-term storage tests in different types of metal containers for examining the influence of container material on OS of biodiesel may be replaced by the significantly faster Rancimat test serving as an accelerated storage test.