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The emergence of biodiesel fuels as diesel fuel substitutes has led to several studies on their properties. Surface tension, which plays a role in atomization, has lacked attention compared to other properties. This paper presents a method to predict the surface tension of biodiesel fuels based on the fatty acid composition. Several binary, ternary, and quaternary mixtures of fatty acid ethyl ester gas chromatographic (GC) standards were prepared, and we found that a mass-average equation predicted the surface tension of these mixtures within ±3.5% of their measured values. Six complex mixtures of fatty acid methyl ester GC standards that simulated typical oils used as biodiesel fuels were also prepared. For these complex mixtures the predicted surface tensions of the mixtures, calculated from a mass-average equation, were 2-6% higher than the measured values. A mass-average equation was developed in which we used a weighted surface tension for the individual components, and we found that this method predicted the surface tension of the simulated oils within ±4.5% of their measured values. Five natural vegetable oils were used to produce biodiesel fuels by the transesterification process. The predicted surface tensions of these fuels were all within ±3.5% of their measured values. The surface tensions of 15 biodiesel types were then predicted, based on their fatty acid composition as published in the literature. These results show that the differences in surface tension between biodiesel types are not the main cause of the reported differences in engine tests.Paper no. J8912 in JAOCS 76, 317-323 (March 1999).KEY WORDS: Biodiesel, fatty acid esters, properties, surface tension.Biodiesel fuels, which are generally made up of methyl or other esters of animal and vegetable oils, have received much attention since the apparent fuel crisis in the 1970s. There are several researchers actively involved in testing various properties and performance parameters of biodiesel fuels, but surface tension has received little attention (1). Surface tension of a liquid is considered to be one of the fundamental properties that affects its atomization characteristics (2). The atomization process is the initial stage of the combustion of a fuel in a diesel engine, and thus the surface tension of a fuel has a role in the fuel's combustion. The results of studies on the various performance parameters of biodiesel fuels (e.g., power and specific fuel consumption) have varied from one researcher to another (1,3), and some of these variations may be a result of the variation of the fuel's surface tension due to differences in atomization. This paper presents a procedure for predicting the surface tension of biodiesel fuels comprised of fatty acid ester mixtures, based on their fatty acid composition.The reader may question at the outset whether results of tests carried out at temperatures and pressures not identical to engine conditions are relevant. In an operating engine, the injectors are cooled by the fuel passing through them, and h...

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