Vegetable oil can be obtained from more than 300 different plant species. Oil is contained mainly in fruits and seeds, yet still other origins exist. The highest oil yields can be obtained from tree crops, such as palms, coconuts, and olives, but there are a number of field crops containing oils. Climatic and soil conditions, oil content, yields and the feasibility of farm operations, however, limit the potential use of vegetable oils to a reduced number of crops.
Apart from the previously mentioned semirefined oils, vegetable oils can also be used in the esterificated form. Diesel engines malfunction if an excess of carbon is present in the combustion process. It becomes necessary to split the glycerides causing an excess in the carbon composition. This can be achieved by treating oil with alcoholtransesterification or by cracking procedures. Ideally, transesterification is potentially a less expensive way of transforming the large, branched molecular structure of the bio-oils into smaller, straight-chain molecules of the type required in regular diesel combusion engines. The so called biodiesel fuels are oil esters of a biological origin. Rape oil methyl-ester (RME) and sunflower methyl-ester (SME) are two biodiesels derived from their corresponding oil seeds.