Sorry Ugo , I have to agree with jens on this "Commercial diesels, including cars, cannot run on triglycerides, the main component of vegetable oils, that need to be chemically modified in order to get glycerin out from them."
There are 2 types of bio diesel in use currently, 1/ is just filtered waste/new cooking oils that is simply filtered to remove solids. diesels run fine on this , but it has 2 disadvantages/problems, 1/ is the likelyhood of containing water that can destroy the injection system and 2/ a higher wax point -due to containing triglycerides. they overcome these by 1/ starting and stopping on regular diesel fuel to stop water sitting in the injection pump/injectors, and 2/ if nessessary due to climate , a heated fuel tank, mostly all diesels useing waste oil fuel have a heated filter just befor the injection pump to lower the viscosity of the oil befor injection.
and secondly the converted waste/new cooking oils that have the glycerine stripped out , this runs in a std injection/fuel tank setup, but even this biodiesel suffers from a higher wax point so in colder climates is generaly sold or used blended with regular mineral diesel.
Stewart