The fuel pump and injectors fitted to diesel cars are complex pieces of engineering which operate on very fine tolerances. The only lubrication for them is the diesel fuel itself, which is just one reason why the correct grade of fuel is so important. All modern diesel engine fuel pumps have to be able to cope with five per cent biodiesel blended with normal mineral diesel, and this is the mixture we are currently buying at fuel stations. Mixtures of more than 5 per cent biodiesel may not have the right lubricant properties and so may cause damage. Manufacturers can easily tell if the wrong fuel has caused a failure and they will definitely reject any warranty claim – you have invalidated the warranty by using non-standard fuel.

There are ways of converting some older engines to run on alternatives to diesel, provided they have the right type of injector pump. Sunflower oil falls into the category of fuels known as Straight Vegetable Oil (SVO). SVO is much more viscous (thicker) than normal diesel. This means the pump has to work much harder to pump it and the injectors will have difficulty ejecting it as the fine spray needed for efficient burning. So, the oil really needs to be warmed, to around 80 degrees. This means you need a fuel heater. Ideally, the engine should start and finish the journey on normal diesel because the pump and injectors also rely on the detergents in mineral diesel to help clean them and prevent a build up of carbon which stops the injectors spraying properly. So, the system also needs a changeover valve to change between fuels, and of course, two tanks. You can avoid the changeover valve and dual tanks etc and run exclusively on SVO but you should really give the heater a couple of minutes or so to heat it up before you start the engine, and you’ll need to add injection system cleaner to a tank of fuel every so often to keep the carbon deposits from building up. The injector nozzles need to be changed as well (for those with long memories, it’s the equivalent of changing the burners on your gas cooker to cope with North Sea Gas). And you’ll need a different fuel filter and glow plug heaters.

After all that, you need the right sort of injector pump in the first place – not all older pumps can cope with SVO. As I’ve said, the injector pump is a very accurately made piece of complex engineering and if your car is old enough to have one, the cost of replacing it is probably more than the car’s value, so it’s important to get this bit right. If the engine is fitted with a pump made by Bosch you can use a mix of vegetable oil and ordinary diesel in a ration of about 10 per cent vegetable oil to 90 per cent mineral diesel without any modification. For any higher proportion of vegetable oil you will need to get a conversion kit which involves the fuel heater, different filter, different injector nozzles and glow plugs. If you feel it’s worth the effort (and about £500 for the parts) you can run on good quality vegetable oil.

For engines with pumps made by Lucas CAV or Rotodiesel, forget it – you can’t convert it.

HMRC have confirmed that they class SVO as biodiesel for taxation purposes so if you use less than 2500 litres a year there is no tax to pay and it’s not illegal.