Penn State researchers recently published a paper where they found that
vegetable oils, without any additives to aid in lubrication, do NOT offer
the lubricating qualities required for long life of mechanical injection
pumps. They did find that, by adding the same sulfur-containing additives
used in normal diesel fuel, this situation was easily remedied. However,
the very same sulfur is the fraction of the exhaust gasses that are
currently being objected to by the public. The researchers have identified
other compounds that can be blended with the fuels, and seem to provide
acceptable lubrication, but research is still ongoing.