I have been researching detailed websites and though it costs a lot, I believe that it can be done. I have relatives that own them, and some even get their oil fuel for free,from restaurants that deep fry food in vegetable oil.They would gladly give them away instead of paying for disposal.I couldn't really believe that it could be done until I had seen it.It costs a large amount, but it can be done.
I have worked on my father in laws diesel while working on it he explained that you can take filter veg oil, add a chemical or two, I know that lye is one of them, and the engine will run it with no issues. It actually is fairly easy and inexpensive since you can get filtered oil from fast food places for free.
I don't know how effective this method is,but I know you can do this in order to run the engine. I have never tried but it can be done. It has to be converted and I know that alone is going to cost you a pretty penny, you can't just dump it in.
If vegetable oil has the right viscosity to operate within a diesel engine then it can work but that's the hitch. If the oil is too thick it won't be able to move through the system. As a work around you can start the engine with diesel, heat it up and then switch to an alternative fuel.
I do think that diesel engines can be powered vegetables. I do not know the exact science here, but I have read that this is more than possible. It would be a great alternative to diesel fuel, and just a great fuel alternative in general. This is a possibility now.
No, diesel engines cannot be powered with vegetable oils, because they are not vegetable oil engines. Vegetable oil just doesn't have the composition to ignite. If the technology can be developed, the technology just isn't there yet. We need more development in order to make use of vegetable oil-powered engines.