How much alcohol is too much?

In mogas, that is: the flying magazines and forums have been alive with of subjective accounts of how mogas containing ethanol has supposedly caused problems with aircraft fuel systems, but rather less has been reported in the way of objective study.

Certainly, pure ethanol, or ethanol that has separated out of from a fuel blend can attack certain materials and – in the presence of water – cause corrosion. However, these things may occur in automotive fuel systems too - something the reputable oil companies are anxious to avoid.

In fact, the wider use of ethanol – and use of it in higher percentage blend - as a consequence of the EU mandate to use more ‘biofuel’ content may not be such a bad thing as it is commonly portrayed. Responding to questions posed by British Motor Heritage Ltd, reported in the Spring issue of its journal Motoring Classics, BP spokesman Wolfgang Dormer explained why fuel containing ten per cent ethanol (E10) may actually be more tolerant of small quantities water getting into fuel systems than E5 (mogas containing up to five per cent and not requiring to be labelled a such at the pumps).

‘The ethanol used in petrol is not hygroscopic,’ said Dormer. ‘It is miscible with water, though more of which can in fact be harmlessly dissolved by E10 fuel than E5 – in this state there is no risk of corrosion.’ The caveat is that ‘in such extreme circumstances as an excess of water content… the water can separate from the fuel – in which case steel and even plastic or coated tanks are vulnerable to corrosion’.

To avoid such problems, as well sticking to quality fuel, car owners are recommended to use a recognised aftermarket inhibitor (see the fuel information section of the Federation of British Historic Vehicle Clubs’ website fbhvc.co.uk for info), try to maintain a regular throughput of fuel and ‘avoid leaving tanks low on petrol for extended periods’.

Dormer also notes that premium (i.e. 97-99 RON) fuels do not contain ethanol.

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