When you heat oils - they become less viscous. Cooking oil can be made to flow and combust with similar properties to diesel fuel if it is heated sufficiently. So, by heating vegetable oil, it can be burned in a diesel engine.

There are however some obstacles to cross since vehicles are not designed to run on vegetable oil. The main obstacle is cold starting; how to start a cold vehicle with thick vegetable oil in it's fuel tank, filter and injector pump. There are a number of ways to overcome this issue, including a) blending the oil with diesel to thin it, b) converting the vehicle to start and run on cold oil (known as single tank conversion) and c) starting and warming up on diesel then switching over to a second tank with vegoil when the engine is warm enough (twin tank conversion). There are advantages and disadvantages with each method.

We very much recommend the twin tank system. Here are the reasons why;

• The engine's fuel system gets a clean with mineral diesel every time it goes through the purge and warm-up period. This keeps the fuel system healthy for longer.
• The engine is always started from cold on fuel with the correct viscosity, which means it flows through the filter and fuel system correctly, minimising strain on the injector pump and filter. This also ensures better fuel vaporisation and cleaner burning, which in turn minimises the risk of coking the injectors (clogging the nozzles with sooty deposits) and ring gumming (see below).
• The pump is lubricated properly during the warm-up period; the time when pump axles tend to break with other systems due to poor lubrication.
• If there is a problem with the vegoil fuel system on a journey, (eg the filter becomes blocked), it is possible to switch to the diesel system to complete the journey rather than undertake repairs on the side of the road

Some vehicles are happier being powered by vegoil than others. Generally, indirect injection engines are best. Most cars from the early 1990s will have indirect injection engines although many modern pickups and 4x4s will also be powered by IDI engines. They have swirl chambers where the fuel is injected giving very good atomisation of the fuel before it is burned. Many cars of this vintage have Bosch VE rotary pumps or equivalent which are great with vegetable oil. However some have more fragile LUCAS/CAV injector pumps which are particularly sensitive and may break, especially if run on cold or insufficiently heated oil. Direct injection engines are also good with vegetable oil, especially if using a Bosch VP37 pump or equivalent. The fuel is injected directly into the combustion chamber. The problem of ring gumming is an issue with DI engines. This is caused by unburned vegoil running between cylinder walls and the pistons. Over time this can become rubbery, preventing a good seal between the piston rings and cylinder, resulting in loss of compression. A secondary issue of ring gumming is engine oil polymerisation. This is where vegoil gets into the engine oil and a reaction takes place which thickens the engine oil. Ring gumming mostly occurs during cold starts when poorly atomised fuel is injected into cold combustion chambers. The best way to avoid this is to use a twin tank system where the engine is started and warmed up on diesel before vegoil is introduced to the engine. Later, common rail vehicles have higher fuel pressures and complicated engine management systems which make them more difficult to convert successfully. They are also expensive to repair. In our experience it is best not to convert common rail engines.

The service intervals of a vegoil vehicle should be reduced - the engine oil and filter should be changed more regularly to prevent engine damage. This is particularly important with direct injection vehicles to avoid engine oil polymerisation as discussed above. The fuel filter should be changed more regularly to avoid it clogging. It is sensible to always carry a spare fuel filter in the vehicle.

And finally this leads us on to the fuel. The biggest savings by far, both financially and environmentally are to be had with used cooking oil. This is a case of collecting the oil from a local restaurant, pub, hotel, canteen etc. filtering it to remove the microscopic particles (to say 5 microns) and pouring it straight into your fuel tank. You need to make sure the oil is 'lightly' used to prevent it from being fatty, watery or acidic. It must also be kept clean and dry. It can take some effort to start with but this can be quite a slick operation with the cooperation of an understanding chef. You visit regularly to drop off some empty containers and collect some full ones. Since you can use 2,500 litres per year before paying duty then it is potentially completely free, a saving of £60 to £100 per fill up in a typical diesel car.

Another option is to buy vegetable oil in bulk like you would heating oil (either pumped into your storage tank or delivered in a 1m³ container). We do not recommended that you buy cooking oil from the cash and carry or supermarkets (for many reasons including the amount of waste packaging, extra journeys, inconvenience to other shoppers and for ecological reasons).

For some really good, clear information about driving on vegetable oil, please visit Geoff's fantastic website where you will find information and videos describing how a twin tank system works, driving on vegetable oil and filtering waste oil as well as pages and pages of background, hints and tips. All useful information if you are thinking of venturing into the world of vegoil motoring