In contrast, unsaturated fatty acids give better lubricity and cold flow characteris­tics than saturated fatty acids (Knothe, 2005). Desulfurization of conventional diesel leads to a considerable loss in lubricity, which is required for the functioning of the engines’ pumps and injectors. A number of studies have shown that the addi­tion of small quantities (5%) of biodiesel can increase the lubricity of conventional diesel (Hu et al., 2005; Knothe, 2005). The polar elements in biodiesel help to form a layer on the metal parts of the engine, reducing engine wear. Lubricity is measured by the ball-on-cylinder lubricity evaluator (BOCLE) test or the high frequency reciprocating rig (HFRR) test and those fuels with good lubricity give values of 4500-5000 g. Figure 7.19 shows the effect on lubricity of adding biodiesel to con­ventional diesel.

Therefore, the fatty acid content of biodiesel has to be a compromise between the best cetane and oxidative stability and cold flow characteristics. It has been suggested that the ideal fatty acid composition for biodiesel would be 10% myristic (C14:0), 50% palmitoleic (C16:1) and 40% oleic (C18:1) acids. This composition is perhaps the aim of all forms of plant breeding or some form of blend of oil.