Vegetable oil can be used as diesel fuel just as it is, without being converted to biodiesel.
The downside is that straight vegetable oil (SVO) is much more viscous (thicker) than conventional diesel fuel or biodiesel, and it doesn't burn the same in the engine -- many studies have found that it can damage engines.
BUT it can be done properly and safely -- IF you get a professional engine conversion. (See below.)
There are other approaches, here are the main ones:
- Just put it in and go.
- Mix it with diesel fuel or kerosene then just put it in and go.
- Blend it with an organic solvent additive or with what some companies call "our secret ingredient that we'll tell you about if you pay us" (several versions) or with up to 20% gasoline (petrol), just put it in and go.
- The only way to use veg-oil is in a properly installed two-tank system where the oil is pre-heated and you start up and shut down on diesel fuel (or biodiesel).
We've never had much time for Nos. 1 to 3 and we've had a two-tank SVO kit for a couple of years that pre-heats the oil and switches the fuel, but we never used it. They do work, but we just didn't think it solved the problem very well, and the more we learnt about it the more we didn't think so.
Along with many others, especially in Europe, we think pre-heating the oil is still not enough to ensure that it will combust properly inside the engine. It needs a complete system including specially made injector nozzles and glow plugs optimised for veg-oil, such as the professional single-tank SVO kits from Germany. Then you really can just put it in and go.
For more info: http://www.journeytoforever.org/biodiesel_svo.html