The straight use of vegetable oils as fuel in diesel engines entails adjusting several physical
properties such as density and viscosity. By adequately heating the vegetable oil before
entering the injection system, its physical parameters can reach values very close to that of
diesel fuel. Consequently, by properly adjusting the temperature of vegetable oils used as
fuel, it is possible to improve their combustion performance, thus avoiding premature
engine aging due to incomplete burning. In this study the density and viscosity of several
vegetable oils are studied within a wide variety of temperatures. The optimal range of
temperatures at which each vegetable oil should operate in order to adjust its properties to
those of automotive diesel and biodiesel is then found. Additionally an empirical relationship between the dependence of viscosity with density is presented. Thus, by means of
the above-described relationship, through measuring the density of a given oil, its viscosity
can be directly deduced.