Can you use vegetable oil as a fuel?

By Marshall Brain


http://youtube.com/watch?v=YOGIh3yRltE

In the video, Adam and Jamie use vegetable oil as a direct replacement for diesel fuel in an old Mercedes Benz. They do not modify the car in any way, and they don't modify the oil in any way besides filtering it.

How can they do that? How can a car burn vegetable oil just like it is diesel fuel? If you know a little about the chemical structure of diesel fuel and vegetable oil, this mystery is easily solved.

Diesel fuel is known as a Hydrocarbon. That means that it is made from hydrogen and carbon atoms. The carbon atoms form a chain. In diesel fuel, the chain is normally 16 or 17 carbons long, like this:

    H H H H H H H H H H H H H H H H H
    | | | | | | | | | | | | | | | | |
  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
    | | | | | | | | | | | | | | | | |
    H H H H H H H H H H H H H H H H H

Now compare that to vegetable oil. Vegetable oil is made out of hydrocarbon chains too. A vegetable oil molecule actually has three chains that are bonded together. That's why another name for a fat molecule is a triglyide. A vegetable oil molecule looks like this:

     H     O H H H H H H H H H H H H H H H H H
      \    | | | | | | | | | | | | | | | | | |
       C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
      /|     | | | | | | | | | | | | | | | | |
     H |     H H H H H H H H H H H H H H H H H
       |
       |
       |   O H H H H H H H H H H H H H H H H H
       |   | | | | | | | | | | | | | | | | | |
     H-C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
       |     | | | | | | | | | | | | | | | | |
       |     H H H H H H H H H H H H H H H H H
       |
       |
     H |   O H H H H H H H H H H H H H H H H H
      \|   | | | | | | | | | | | | | | | | | |
       C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
      /      | | | | | | | | | | | | | | | | |
     H       H H H H H H H H H H H H H H H H H

That is one big molecule with three legs attached together. But if you look at the legs, you see they look exactly like diesel fuel molecules! So these legs burn in the engine just like diesel fuel does. The thing on the left that holds the three legs together is called a glycerol, and it burns in the engine too without any trouble.

So now you can see why Adam and Jamie were able to burn vegetable oil in their engine. Vegetable oil looks almost exactly like diesel fuel at the molecular level.

You may have heard of biodeisel. To make biodiesel, you use heat and a little chemistry to take off the glycerol part and release the three legs. This turns the vegetable oil into actual diesel fuel. The advantage is that:

  • Biodiesel flows a little better (diesel fuel is thinner than vegetable oil
  • Biodiesel doesn't "go rancid" like vegetable oil can
  • Biodiesel works better in the winter.
But in the summer, vegetable oil works pretty well in its natural form! Have fun performing your own science experiments!