Diesel Particulate Filters

Newer diesel vehicles being sold often have Diesel Particulate Filters (DPFs) which reduce particulate emissions by trapping them in a filter in the exhaust system. To prevent the filter from plugging, fuel is sent into the exhaust system at intervals to burn off the particulates.

There are a couple problems with this system:

1. extra fuel is used to burn the particulates, which reduces overal MPG.
2. extra CO2 emissions are released when burning up the particulates.
3. Biodiesel, which vaporizes a higher temperature than petroleum diesel, doesn't function the same way in the system, and fuel ends up in the engine crankcase rather than clearing the DPF. [1]
   

If you have a newer diesel vehicle you may want to investigate these issues before running biodiesel. A good discussion of the issues, with links to more information, can be found here:

http://biodiesel.infopop.cc/eve/forums/a/tpc/f/419605551/m/2721003282

[1] It depends on how the system is designed, of course. The problem is caused by using the fuel injectors in the cylinder to place additional fuel into the exhaust gases. Since biodiesel doesn't vaporize per the system design, some of it stays in the cylinder and leaks past the piston rings into the crankcase. If the additional fuel is squirted into the exhaust pipe, and not the cylinder, then you bypass the problem of fuel getting into the crankcase. But that is more complex and maintenance intensive (additional dedicated fuel injectors).