The main problem with directly using vegtable oil is that it is too thick at temperatures below 15C, in fact clouding as small grains of solid form that clog filters and injectors. You can use it directly in warm temperatures.
Bio fats like veg and animal oils are glycerine molecules with long fatty acid chains replacing the hydrogen in the three alcohol (-OH) groups on the 3 carbons of the glycerine molecule:
Transesterification is the easy reaction that is used to turn any bio-oils or fats into far more fluid mixtures of lower cloud temps called "biodiesel"
methanol, ethanol or other longer chain alcohols are added at ratios like 20% methanol to 80% oils. Acids or Bases catalyse this reaction replacing the glycerine with the alkyl group of the alcohol on the fatty acid chains. this giving you a monoester and some glycerine contamination that is fairly easy to remove due to its water solubility as are remaining traces of catalyst and free carboxilic acids.
Its easy stuff to do at home or in the boot of your car to prepare things like chip fryer waste oil etc.
