There are thousands, or possibly hundreds of thousands of different types of oil, ranging from very thick – tar is an oil – to very thin, like paraffin or petrol. They all burn. Some will burst into flame very easily, some have to be persuaded with a spark or high temperature. When they burn they produce carbon dioxide, water and a small amount of other stuff, from tarry residues left behind, to sooty particles to noxious gases that disperse into the atmosphere. Almost all oils could be used as fuel if you wanted to, but generally the lighter versions, diesel, petrol, paraffin and so on are used.
Once you’ve decided what fuel you want to use, you design pumps and injectors for the engine that can cope with the thickness of it, you make sure the internal bits generate enough heat to set fire to the fuel and you work out ways of dealing with the leftover bits – the “other stuff” – so that they don’t gum up the works and bring everything to a halt or release noxious fumes into the atmosphere. All this amounts to quite a lot of fine tuning and general tweaking to get things just right. For much of the hundred years or so that we’ve been driving cars, petrol and diesel were was made to roughly the right specification, but they did vary quite a bit from country to country and even from garage to garage. This wasn’t a huge problem because the engines weren’t made that accurately either, when compared to what modern production methods can achieve, so they generally chugged along on whatever they were given.
The trouble is, if you have to cope with a range of fuels you have to compromise on efficiency. The more accurately you know what the fuel will be, the more accurately you can tweak the engine to get the most out of the fuel. And that’s exactly how modern engines are made.
With reducing oil supplies, rising costs and increasing environmental concerns, we need to get all the energy we can from every litre of fuel, so both diesel and petrol sold at filling stations anywhere in Europe have to conform to very precise specifications covering things like how quickly the fuel burns, its viscosity at different temperatures, flash point, flame temperature and so on. (for those who won’t sleep without knowing, EN 228 is for 95 octane petrol and EN 590 is for diesel).
Manufacturers know they can rely on the fuel to have exactly the right properties so they can make engines that are far more efficient those of even a decade ago. However, this does mean that if you use a different fuel you are in danger of seriously upsetting the engine. Vegetable oil is much thicker than mineral diesel, especially at low temperatures.
What does all this mean for well intentioned drivers? Well, any diesel engine made in the last three or four years will be using technology known as “common rail”. All you need to know about common rail technology is that vegetable oil will quickly bring it to a very expensive halt and any warranty claim WILL be rejected, so don’t even think about it – it’s very easy to tell you’ve used non-standard fuel.
How about older vehicles? Now we get to slightly more technical areas because it all depends on the make of the injection pump. The injection 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. They are SERIOUSLY expensive, 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% vegetable oil to 90% mineral diesel without any modification. For any higher proportion of vegetable oil you will need to get a conversion kit which mainly involves a 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.
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So, if you have an ancient Mercedes that’s been running perfectly well on recycled chip oil for a million miles, congratulations, but please don’t e-mail me to tell me about it. And if you want to keep your friends, don’t tell them to use it in their nice new diesel cars.
One final point. You used to have to go and tell the nice man at Her Majesty’s Customs and Excise how much vegetable oil you were using to power a road vehicle and pay duty on it. That changed in 2007 and now if you use less than 2,500 litres per year, you do not need to let HMRC know or pay any duty. That will make it a very cheap fuel if you have an older car with the right pump and do all the converting. And you’ll be the greenest motorist on your street.
Words: Tim Shallcross