Older diesel engines will burn a wide range of fuels. In the warmer weather they will run happily on sunflower oil, olive oil or engine oil. But be warned -The first two are illegal (you're avoiding excise duty) and if you're unwise enough to put engine oil in the fuel tank, it will run smoothly enough but will emit horrendous amounts of blue smoke from the exhaust, clog the exhaust catalyst, and result in a large bill from your mechanic.

Unknown to most people, however, oil companies are increasing the amount of biodiesel in UK diesel. They are adding a 5% blend of biologically-derived fuel (mainly from rapeseed) to mineral diesel. The public not been consulted about this, or told; it's just happening. This is being done in response to the government's renewable transport fuel obligation. You probably won't notice any difference in performance if you have an ordinary family saloon, but there are some long-term drawbacks to using bio-fuel which have not been publicised.

Firstly, natural oils are quite viscous, so they can't be used satisfactorily in high-performance diesel engines.

Secondly, even if your engine will burn, say, sunflower oil, this is very prone to waxing in cold weather. Vegetable oils need to be processed into something thinner.

Processed oils ("fatty acid methyl esters", or FAME) have problems too. They are solvents - they dissolve some of the accumulated grime and debris from fuel tank walls and carry it around the fuel system, which can cause flow problems or damage to injectors.

Bio-fuel can be attacked by bugs, which semi-digest it to form slime which clogs filters and fuel lines.

Fuel tank corrosion has also been noted: as well giving your fuel tank an unwanted clean, biofuel tends to absorb small quantities of moisture, which attacks steel.

According to the biofuels directive issued by the EU in 1999, no extra labelling is necessary if the fuel contains 5% or less of "fatty acid methyl ester" - processed vegetable oil fuel.

So a supplying oil company need not tell its customers that a fuel contains, say, 5% biodiesel. This has led to the situation where a refiner supplied a fuel company with a biofuel blend without saying so. The distributor then added 5% biofuel to sell the mix as an advertised biodiesel, but it was 10%, not 5%. This fuel attacked storage tanks and blocked fuel filters in service station dispensers. One pump burst the filter, squirting the contamination into motorists' tanks. You may remember - it got into the papers - "dirty fuel" which resulted in breakdowns, repair bills and insurance claims.

Even without accidents like these, there's another problem. Mixtures containing biodiesel are less stable than ordinary diesel, so we might expect power loss, exhaust catalyst damage, engine oil dilution and more rapid engine wear unless the engine oil is changed more frequently. There is also a worry about the solvent effect on some rubber and plastic seals.

I mustn't overstate the case - many cars won't notice the difference. But yours might be the one to be affected. In the words of Peter Barlow, a chemist at Shell with 32 years experience, the European Commission, unconfused by the facts, is pressing ahead with its political objective for 10% biodiesel to be widespread across Europe by 2020. As he says - perhaps we should be told a bit more about this fuel before we are forced to use it.

N.D., habitat21 website

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