Biodiesel is defined as a mixture of fatty acid esters, normally methyl esters, produced from plant oils, animal fats and waste cooking oils. The mixture of fatty acid esters (biodiesel) has properties very similar to those of diesel and can be used with very little modification in a diesel engine. A comparison between conventional diesel, plant oils and biodiesel is shown in Table 7.13.

As can be seen, the main differences between diesel and the widely used plant oil from rapeseed are higher viscosity and higher flash point. The elevated flash point is an advantage as it makes the oil safer, but the higher viscosity makes the long-term use of the oil in engine difficult, especially in cold weather. However, by forming a fatty acid ester mixture, the viscosity is considerably reduced to a level where the biodiesel can be used in diesel engines without modification. The European standard for 100% biodiesel is EN 14214 and the values are given in Table 7.13.