Viscosity affects the atomization of a fuel upon injection into the com­bustion chamber and, thereby, ultimately the formation of engine deposits. The higher the viscosity, the greater the tendency of the fuel to cause such problems. The viscosity of a transesterified oil, i. e., biodiesel, is about an order of magnitude lower than that of the parent oil [1, 2]. High viscosity is the major fuel property why neat vegetable oils have been largely abandoned as alternative DF. Kinematic viscosity has been included in most biodiesel standards. It can be determined by standards such as ASTM D445 or ISO 3104. The difference in viscosity between the parent oil and the alkyl ester derivatives can be used in monitoring biodiesel production [67]. The effect on viscosity of blending biodiesel and petrodiesel has also been investigated [68], and an equation has been derived, which allows calculating the viscosity of such blends.

The prediction of viscosity of fatty materials has received considerable attention in the literature. Viscosity values of biodiesel/mixtures of fatty esters have been predicted from the viscosities of the individual components by a logarithmic equation for dynamic viscosity [10]. Viscosity increases with chain length (number of carbon atoms) and with increasing degree of saturation. This holds also for the alcohol moiety as the viscosity of ethyl esters is slightly higher than that of methyl esters [11]. Factors such as double bond configuration influence viscosity (cis double bond con­figuration giving a lower viscosity than the trans configuration), while the double bond position affects viscosity less [11]. Thus, a feedstock such as used frying oils, which is more saturated and contains some amounts of trans fatty acid chains, has a higher viscosity than its parent oil. Branching in the ester moiety, however, has little or no influence on viscosity, again showing that this is a technically promising approach for improving low-temperature properties without significantly affecting other fuel properties. Values for dynamic viscosity and kinematic viscosity of neat fatty acid alkyl esters are included in Table 5.1.