a small enough diesel is more the challenge
TBH, personally I'm not really interested in the biodiesel aspect... I mean it's cool an' all, but w/our application even just pump diesel would be amazing methinks
BTW, few to no diesels will run well or for very long off of just plain old "cooking oil" or vegetable oil & certainly not
the "used fry oil" ya hear so much about;
Biodiesel.org - Biodiesel 101
Vegetable oil fuel - Wikipedia, the free encyclopedia
Additional modifications such as a preheater, a secondary tank w/petrodiesel, and/or adding a mix of kerosene are commonplace methods of reducing the viscosity & help combat poor combustion & carbon buildup - the "used fry oil" thing bugs the heck out me, while it's a viable source ya really do need to re-refine it at the very least as the water contamination alone is massively excessive, not to mention the other organics.