Vegetable oils like rapeseed oil make good fuels but they can't be burned directly in engines.
The oils must be converted into biodiesel first.
This involves reacting them with methanol, using a strong alkali catalyst such as potassium hydroxide or sodium hydroxide.
You get a mixture of methyl esters of fatty acids - this us biodiesel.
Is biodiesel 100% carbon neutral?
Biodiesel can be thought of as carbon neutral because when crops grow they absorb the same amount of CO2 as they produce when they are burned.
But it is not quite that simple - energy us used to make the fertiliser to grow crops, and it is used in planting, harvesting and converting the oil.
If this energy comes from fossil fuels, then the process won't be carbon neutral overall.