Unlike crude oil, which needs refining before it can be used in a modern car, vegetable oil doesn't. Vegetable oil is a perfectly good fuel, but it's just too thick to pass through the components of a modern diesel engine, and it doesn't vaporize well, when sprayed through a diesel injector that was designed for nice runny fossil diesel. 

Instead of refining oil to make it more runny, as is done chemically when you make biodiesel, you can simply heat the oil. All liquids become runnier, less viscous and less dense when they are heated up. At around 70°C, vegetable oil has about the same viscosity as diesel on a cold and frosty morning. Hot vegetable oil will, therefore, do no more damage and put no more strain on an engine and its components than very cold diesel.

Fit a fuel pre-heater between the fuel tank and the fuel pump to heat the vegetable oil and you can fill your tank with vegetable oil and drive around without the hassle and cost of having to convert it to biodiesel. This does give rise to a few problems, however. Whatever heat you apply, the oil already in the fuel pump and fuel lines and injectors will be thick, viscous and stone cold. Starting the engine with this in the system is not ideal. Most people therefore, install a twin tank system.

Twin Tank: A second, smaller tank is installed in the boot, filled with diesel, and is connected into the main fuel pipe by way of an electrically controlled 3 way solenoid valve, operated by a switch on the dashboard. A heat exchanger is installed in the fuel line immediately before the fuel pump, with coolant water from the radiator passing one way, heating the cold vegetable oil coming fro the tank passing the other. The engine is started on diesel in the normal way. Once the engine is nice and warm, and the heat exchanger is up to temperature, the solenoid switch is operated, selecting vegetable oil from the main tank. Just before reaching your destination, the solenoid switch is used to switch back to the smaller tank full of diesel, flushing the system, ensuring that next time the engine is started, the fuel pump and pipe lines are full of diesel once again.

Single Tank: Instead of waiting for the engine to warm up and the heat exchanger to get hot, which can take many minutes, you can install an electrically powered fuel pre-heater before the fuel pump. When the ignition is switched on, it begins heating the fuel. Although the engine will always start on the cold thick oil already in the system, the hot oil will replace it within seconds.  This offers a much simpler and cheaper system.

Other systems are used, including electric immersion heaters fitted into the fuel tank and the engine block, with electrically heated fuel pipes. The car is plugged in to the mains and remains hot all night, meaning you always start and run on hot oil. Performance when using 100% oil is almost identical to driving on diesel. Again the main difference is that the exhaust smells very much nicer.

Suitable cars: Generally running on 100% oil in either a single or twin tank system would be limited to cars manufactured before 2004.