How to Make Biodiesel

Traditional diesel fuel is made from petroleum, and in a post-apocalyptic world that will be hard to come by.  Thankfully, biodiesel is an alternative that can be made from any organic oil or fat that doesn’t require any kind of modification to your diesel engine.

Generally, you are adding a catalyst to a triglyceride-rich liquid in order to break a glycerin molecule off of the fatty acid chains in the oil and forcing each of three fatty acid chains to recombine with the introduced alcohol to essentially create a new alcohol.  This process is called transesterification.

Transesterification: ester + alcohol = different ester + different alcohol
Transesterification: ester + alcohol = different ester + different alcohol


Organic Oil/Fat

This can be nearly any kind of oil or fat (I’ve seen biodiesel made with rendered pig fat).  There are a few considerations to take into account though:

  • Peanut oil, coconut oil, palm oil, tallow, and lard all have a higher clouding point than other oils, meaning that they start to crystallize and gel at a higher temperature.  This means that they will work perfectly fine in warmer weather, but may cause problems in cooler temperatures.
  • Olive oil, peanut oil, palm oil, tallow, and lard have a higher acidity.  This can interfere with the transesterification process and means you probably want to titrate a sample (explained later) to determine if extra lye will be needed.
  • If you are using used oil, you need to process it before going forward with the recipe.

Rapeseed (or canola) oil, corn oil, soy oil, and sunflower oil are considered to be the preferable choice for biodiesel production.


It is possible to use either methyl alcohol or ethyl alcohol for biodiesel.  Methyl alcohol is preferable because there is less work involved with methyl alcohol.  Either way, you want as close to 100% pure as possible.


There are two types of lye potassium hydroxide (KOH) and sodium hydroxide (NaOH).  Either can be used, but KOH is preferable since it dissolves easier in alcohol.  Also, conveniently enough, it can be made per the instructions I give in How to Make Lye.


  • 10 parts oil
  • 2 parts methyl alcohol or 2.7 parts ethyl alcohol
  • 3.5 grams NaOH or 4.9 grams KOH per liter of oil used (plus any excess lye as indicated in titration for used oil)

The Process

Biodiesel and Glycerine separated into layers
Biodiesel and Glycerin separated into layers

Mix your alcohol and lye in an HDPE container (like a milk jug) and swirl occasionally until all the lye is fully dissolved.  This could take as little as 10 minutes for KOH and as much as overnight for NaOH.  This creates you methyl or ethyl esters.

Blend the ester mixture with oil/fat heated to 55C for roughly 30 minutes.

Let the mixture settle for 24 hours.  In this time, transesterification will occur, leaving behind glycerin as a by-product.  Three distinct layers will form, the heavy (bottom) layer is the glycerin, the light (top) layer is the biodiesel, and the middle layer is a soapy emulsion created by the reaction of lye with oil.  You can keep the glycerin, as it is useful in other situations, but unneeded for the rest of this recipe.  Move the biodiesel into a different container, ensuring that no glycerin or soap is carried along, and either store the glycerin or through it out.

Quality Testing

  • The Wash Test – Put a small amount of fuel in a PET bottle with water and shake vigorously for  approximately 10 seconds.  Let it sit for a half an hour.  If water separates from fuel with a very thin, foamy layer between (or no layer at all), then you’ve produced quality fuel.  If they don’t separate or there is a thick foamy layer, then your fuel is of poor quality.  This can be caused by too much lye or contaminants present acting as emulsifier.
  • The Methanol Test – Mix 25ml of biodiesel with 225ml of methanol.  If anything is going to separate, it will happen nearly instantly.  Each milliliter of biodiesel that separates from the methanol equals a 4% impurity.  Ideally, nothing will separate, meaning your fuel is 100% pure, but a little bit won’t hurt.


After testing to ensure your fuel is good you need to “wash” it.  This process removes any physical impurities or unconverted ingredients from the fuel, as these can all cause problems in your engine down the line (lye can corrode the fuel injectors and fuel tank, glycerin and soap can clog any number of parts, etc).  Mix 1 part fresh, clean water with 2 parts biodiesel until it appears homogenous.  Let the mixture settle for several hours, then drain water.  Move fuel to new receptacle and repeat process 2-3 times.  Let the fuel sit for several days. once it is no longer cloudy, it is “dry” and ready to use.  If it doesn’t clear up, you can try washing it again.

Processing Used Oil

You can use “certified pre-owned” oil to make biodiesel, you just need to do some things to it first.

Cleaning Old Oil

Some people recommend filtering the used oil first, but I say that it is unnecessary.  All the gunk and goo in the old oil will sink to the bottom and since you are usually working with the top layer of a separated liquid, you are naturally filtering it as you work with it.  However, there is a significant amount of water suspended in used oil (typically from the food cooked in it) and that can be a problem.

To remove the water, bring the oil to a boil at 100C and leave there until boiling slows, then boil at 130C for approximately 10 minutes.  This should ensure that most of the water is removed.


Every time you use or heat oil you create free fatty acids, which are basically broken-down triglycerides.  This means that there is more work required to convert your oil into biodiesel than with new oil.  This work is done by adding extra lye to the process.  To find out how much more lye to add, we use a process called titration.  This process should also be used if you are using ethyl alcohol instead of methyl alcohol, or an oil with a higher acidity.

First, make a 0.1% lye solution by mixing 1g of lye into 1 liter of distilled water.  Now dissolve 1ml of oil in 10ml of isopropyl alcohol.  At this point you need to choose a way to determine the pH of the oil/alcohol mixture.  You can use a pH tester, phenolphthalein droplets, or (if push comes to shove) red cabbage juice – seriously it indicates pH really well..  Add the lye solution drop by drop until pH is around 8-9.  If you’re using phenolphthalein, this is indicated by the liquid turning a pinkish color, if you are using red cabbage juice you are looking for a blue/blue-green color.  The number of milliliters of lye solution added to the oil solution equals the additional number of grams of lye per liter of oil to use in the transesterification process.


If you want to get super technical in your measurements, the amount of KOH used depends on the strength.

Purity Measurement (in grams)
99% 4.9
92% 5.3
90% 5.5
85% 5.8

Red Cabbage Juice

pH 2 4 6 8 10 12
Color Red Purple Violet Blue Blue-Green Greenish Yellow