Aluminium:

Driving on Vegetable Oil

Like the original diesel engine (which ran on peanut oil) many diesel engines will run on vegetable oils (or even used, filtered cooking oil) with very little adaptation.

Starting the engine can be a problem, as cold vegetable oils are too thick, and need to be heated to over 70°C to work properly with the fuel injector. One adaptation uses refined diesel from a separate tank to start the engine, and the driver switches to vegetable oil once the engine is hot enough to pre-heat the oil. You also have to remember to switch back to refined diesel for about two minutes before you stop the engine, or the oil can congeal in the fuel pipes! Electrical heating of the fuel oil is another option.

Diagram: Adapted diesel engine

Biodiesel

Chemical treatment of vegetable oil changes it into a much more runny liquid which can be used in diesel engines. This is biodiesel fuel, and it is already in common use as a 5% additive to petrochemical diesel. At this level no engine modification is needed, so it can be used in any diesel vehicle.

Biodiesel chemistry
Vegetable oils (and animal fats) contain mainly triglycerides. These are esters formed from long chain fatty acids and propane-1, 2, 3-triol ("glycerol"). Click here to find out more about their structure.

Processing

The vegetable oil is processed by treating it with excess methanol, which releases the fatty acid chains as methyl esters, leaving glycerol. The reaction is described as transesterification. A strong base, for example sodium hydroxide, is used as a catalyst. The methanol and sodium hydroxide are prepared together first (giving sodium methoxide, CH₃ONa), as otherwise the sodium hydroxide is likely to react with fatty acids to produce soap!

The methyl esters are the main components of biodiesel, but the precise composition will depend on the vegetable oil used. The glycerol is separated, purified and sold for use in food, pharmaceuticals and cosmetics.

Diagram: Chemical equation

Used Cooking Oil

It is also possible to make biodiesel from used cooking oil, but there are two important differences:

Used oil will contain much more debris and so it must be filtered thoroughly to remove all suspended particles

One of the effects of heat is to partially decompose triglycerides and produce free fatty acid molecules. These will react with sodium ions to produce soap, and so consume the transesterification catalyst. The amount of catalyst must be adjusted to compensate for this, and the biodiesel must be washed to remove the soap.

Photo: Cooking oil