This article is for those who are a little more experienced at making biodiesel at home and who wish to progress from using straight vegetable oil to waste oil. As users of biodiesel fuel know vegetable oil will react very similarly to diesel fuel in a diesel engine. Like diesel fuel, biodiesel will ignite when compressed causing air in the engine cylinders to expand. This is the energy that pushes the engine pistons into motion.

The reason you must make biodiesel is that if you burned straight unprocessed oils in the diesel engine it would simply congeal and then clot your fuel injectors and engine like cholesterol hardening human arteries.

Read more on how to Make Your Own Biodiesel

There are many biodiesel recipes for removing the fatty acids from oil.

Here is a very common and reputable recipe for making biodiesel at home from waste oil.

You will need:

Used or fresh vegetable oil (that has been strained so that it is free of particulates such as food crumbs)
Lye
Methanol
Isopropyl Alcohol (99% IPA)
eyedropper- or other type of 1 milliliter dropper
PH paper (to test for acidity )
an old blender
plastic or glass measuring cups or beakers, with metric measures-- and/or a gram scale
plastic, glass or stainless stirrers and spoons,
plastic buckets

1. Dissolve 1 gram NaOH (Sodium Hydroxide- Lye) in 1000 ml. of water. This is your NaOH solution.
2. Dissolve 1ml. of wasted vegetable oil  in 10 ml. Isopropyl Alcohol(IPA).
3. With an eyedropper, drop the diluted NaOH into wasted vegetable oil mixture a milliliter at a time. Count the drops. After each drop check the ph level of the WVO solution with standard ph paper-- you will see an eventual rise in the ph level. Continue to add the NaOH solution into the WVO solution, a drop at a time, until it reaches a ph of 8-9.

To determine your proportions, figure:

The number of drops of NaOH needed for the WVO solution to reach a ph of 8-9 1 ml. plus 3.5 g. NaOH to catalyze the oil.

Then go ahead and prepare a larger batch in your blender. Don't forget to clarify, settle and bubble wash this mixture as usual and remember to follow the usual safety precautions for making biodiesel at home.