Vegetable oils and animal fats are triglycerides: esters of glycerol with three fatty acid chains.  Although some unmodified vegetable oils have been used as fuel in internal combustion engines, in general the viscosity is significantly higher than that of conventional diesel and a number of modifications are required to a vehicle's fuel system (including heaters, additional filters and modified injectors) to use them.

Even then, the lack of a transport fuel specification for straight vegetable oil (SVO) and concerns about long term engine reliability using SVO as a fuel, make this inadvisable for the present.

Instead, SVO and filtered waste vegetable oil (WVO) can be reacted with methanol (or ethanol) to change the triglyceride esters into methanol (or ethanol) monoesters, each with single fatty acid chains making fatty acid methyl ester (FAME), commonly known as biodiesel. 

Although not all oils and fats are suitable for this process, depending on such chemical characteristics as degree of saturation of the fatty acid chains (iodine number), melting point, etc., many oils have been successfully used, including rape seed oil, palm oil and soybean oil.

Both virgin oil and waste oil can potentially be used. However WVO needs both to be filtered before use and assayed for iodine number and free fatty acid concentration before use.

The biodiesel produced can be used on its own or mixed with petroleum based diesel fuel as a 5% biodiesel/95% fossil diesel blend and used by unmodified, conventional diesel engines.

The European standard for FAME biodiesel is EN14214, which sets out minimum cetane number, maximum iodine number and water content, viscosity range, etc. and has helped considerably to reduce the variability in biodiesel between different producers and feedstocks.  However at present not all small scale producers meet it. 

Even for biodiesel meeting EN14214, however, there can be material compatibility issues, particularly with nitrile and natural rubber based hoses and seals and some metals.  It also has an increased tendency to oxidise and biodegrade in storage compared with mineral diesel.

Many modern vehicles have viton seals and hoses that are compatible with biodiesel, and can be used entirely satisfactorily with EN14214 fuel provided it is not left unused for too long.

For agricultural users with farm equipment that may be left unused for several months at a time, fuel degradation in storage may be an issue.