Cleaner fuels

Author: Ana Perea Angulo

Not everything labelled as biodiesel is the same, and there are some things we should know before buying it. This article is intended to help you find the best way to reduce the amount of diesel consumed by your car.

There are a number of ways to reduce consumption. First and foremost: use the vehicle less. Share your car; go on foot, by bicycle or on public transport.

Secondly, there are a number of options for filling the tank of a diesel car with fuels based on recycled vegetable oils: biodiesel (sold in some petrol stations or home-produced), mixing oil with diesel, and using oil as the only fuel (this involves modifying the engine).


Biodiesel is produced by subjecting new or recycled vegetable oils to a chemical process. It can currently be found in some filling stations mixed with diesel in proportions ranging from 5% to 30%.

Home-made biodiesel

Oil can be converted into biodiesel using a fairly complex chemical process.

Mixing diesel with oil

Another way to reduce diesel consumption is to add vegetable oil to the tank. This process requires certain resources and takes some time. If you plan to do it on a regular basis, it is a good idea to pool the time and equipment involved with others to produce a greater quantity of oil.

There are three steps:

  1. Obtaining the oil The best option is to reuse frying oil. You will need much more than that used in your own kitchen. Collect it from around the neighbourhood, from restaurants, schools, fried food stands, etc. Legally, you should be a registered waste collector, so you will have to collect the oil through private agreements.
    Some farmers produce oil for their own use or for local users with whom they have reached an agreement (to sell it they would have to declare it as a commercial activity).
  2. Filtering It is vital to filter the oil properly to prevent the fuel lines in your vehicle from getting blocked. See the How to filter used oil box for instructions.
  3. Mixing the fuels See the How much vegetable oil to add table for correct proportions for each type of diesel car (from 10% to 80%). The best method is to collect the diesel in a fuel can and mix it well at home.
    It is advisable to start with just a little (10%) and after two refills start increasing the proportion up to the recommended limit, monitoring carefully for any changes in how the vehicle performs (start-up, noise, fumes, etc.).


Oil only!

If you mix vegetable oil with diesel you are reducing the pollutants contained in the exhaust fumes, but you are still using diesel. A diesel engine can run on vegetable oil alone (in fact the inventor of the system used peanut oil), but it will require some modifications (to heat the oil and to give it the same qualities as diesel). Kits to convert engines are sold by a number of companies, some of which have been in the business for over 20 years. You can install the kit yourself or get a mechanic to do it.

+INFO: (in French), (in Spanish), (in English, see Link section).

How to filter used oil

  • You will need a large barrel with a tap 10 centimetres up from the bottom.
  • Pour the oil in through a cloth (a blanket or t-shirt) to remove food fragments.
  • Leave it to settle for at least a week so that water and solids sink to the bottom (below the tap).

It is very important that there is no sugar or salt in the oil. To clean it, add water and stir vigorously: the salt and sugar will dissolve in the water, which will then settle in the bottom of the barrel. When you have used all the oil in the barrel, you will have to take the residue to a recycling centre.

  • Make sure there is no water in the oil when you add it to the diesel tank. When you draw off the oil you need into a container (by opening the tap), heat a small quantity in a frying pan. If it sizzles there is still water in it. If this is the case, heat the oil to evaporate the water, and repeat the frying pan test.
  • Pass the oil through a fine filter, such as an old pair of jeans, and then through a 1- to 5-micron swimming pool filter, which should be changed after every 1,000 litres.
  • Transfer the oil from a higher container to a lower one during filtering so that gravity will supply the pressure needed to push it through the filter. The results of the filtering process will be better the colder the oil, but extreme cold will make it very dense, in which case you may have to heat it a little so it flows better.

It is vital to use the oil within one month of filtering to ensure no solid particles form, which could block the car’s fuel filter.

How much vegetable oil to add

Type of engine Summer Winter
Indirect injection, in-line pump 80% 50%
Indirect injection, axial-piston pump 50% 30%
Indirect injection, radial-piston pump 30% 10%
Indirect injection or new generation diesel (electronic injection, common rail or injection pump) 30% 10%

Precautionary measures

  • Before you start to use vegetable oil, consult sources of information and read up about other people’s experiences, such as this forum: (in English).
  • Change the fuel filter after the first 2,000 kilometres, as both biodiesel and vegetable oil flush dirt out from the engine, which may then accumulate in the filter.
  • If you use oil, check your car’s compression ratio. This will prevent oil entering the sump.
  • If your car has rubber parts in the injection system (only used pre-1994), change these for modern Viton plastic parts.