Vegetable oils may have inherited properties of certain fuel oils like diesel. Some common oil properties that may be comparable to vegetable oil are viscosity which is quite high than many normal liquids, and stability to oxidate which is pretty low because of higher viscosity and density.
But vegetable oils may not be directly used as fuel.They may have to be processed.In other words their viscosity has to be reduced to allow automisation of fuel,which is the most suitable condition for use of vegetable oils as automobile fuels.
Else, there may be incomplete combustion of these fuels which may create thick black carbon leading to not only environmental pollution but also decreasing the durability of your vehicles.
Modern techniques and research have proved that these vegetable oils can also be blend with a very little quantity of diesel to obtain automobile fuel quickly.
There are many cases that the vehicle engine is started initially with diesel, when it warms up the regulator is switched on to vegetable oils. This may be an advantageous thing since you may not have to start the engine with diesel which is not only scarce but also pretty expensive.
Vegetable oils are not restricted only as automobile fuels. They extend their support to home heating like boilers, furnaces etc. with minimum modification in their compositions.
Both pure as well as waste vegetable oils can be used for this purpose to serve as alternative fueling