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Classic Engines, Modern Fuel

Comments on Topic: Modern Fuel and Wick / Surface Carburettors

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Submitted by Anonymous

Hi - I have a 1911 Lanchester with a wick carburettor. Most, if not all, early Lanchesters used the same type of carburettor.

Anecdotally, these cars ran perfectly well on pump fuel up to (and possibly including) the 1970s. Thereafter, owners have been forced to convert to spray carburettors in order to overcome some of the difficulties that they were experiencing with more modern fuel.

The spray carburettor conversions are made more difficult by the mechanical layout. They are rarely entirely successful and are always detrimental to the originality of the vehicle - the "Lanchester Wick Carburettor" being something of a defining feature of the cars.

Your book suggests that the use of super grade + ethanol mixed with Kerosene might reduce the low temperature volatility of modern fuel so that it more closely resembles that of 1960s fuel. I was wondering whether this might be sufficient to enable our cars to run with their wick carburettors ?

If the petrol / Kerosene mix did work acceptably with wick carburettors, do you think that mixing super grade + ethanol with diesel might result in a similar outcome ?

The reason for the question is that it would be easier, on a longer journey, to replenish a store of diesel than Kerosene.

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Submitted by The Author

This is a very interesting comment.

If my understanding is correct the wick carburettor worked by “raising” the petrol from a float chamber using a wick to a surface which was warmed by a hot air inlet where it evaporated. This would suggest the engine inducted an air / petrol vapour mix, rather than with a more traditional carburettor where the engine inducts mainly liquid petrol.

If this is the case then one would expect the increased lower temperature volatility of modern petrol to make things better not worse. However I suspect the operation of the wick carburettor is not that simple.

As the petrol evaporates, it cools the wick reducing the volume of petrol that will evaporate. Less petrol evaporating, the temperature of the wick would increase, allowing more petrol to evaporate. This would act as a “sort of” mixture control. But the mixture control would heavily depend on the volatility profile of the petrol.

If this assumption is correct, then I would expect the problem with the wick carburettor using modern fuel is that the engine runs too rich because of the lower temperature volatility.

In the case of my TC, the kerosene improves the way the engine runs because it improves the combustion, rather than the mixture. I suggest with a wick carburettor, the kerosene would change the profile of the volatility curve and hence the mixture.

I assume the Lancaster has a very low compression ratio (5:1 or 6:1) this means that the octane rating of the petrol is not really an issue. Even with my 7.25:1 compression ratio, I could not detect any pinking during my tests using kerosene. This means you should be able to mix kerosene or even diesel with your petrol to “tune” the evaporation curve.

My suggestion would be to:

  1. Buy some Anglo American oil R Storage plus – this has a very close volatility curve to classic petrol. Assess how well the engine runs with the wick carburettor using this petrol. If it runs fine then your problem is almost certainly due to the volatility profile.
  2. You could then try different mixes of, I suggest a premium quality petrol, with kerosene or diesel until you find a cocktail that works. By doing this you will be “tuning” the volatility profile of your fuel and hence your mixture.

One caveat, you may find the engine starts better with “pure” modern petrol because it will evaporate better in a cold carburettor.

BTW: The ethanol only improves the combustion with a traditional carburettor because of the oxygen contained in the liquid petrol. As the wick carburettor only delivers petrol vapour, I would imagine the ethanol has little effect on combustion.

Best of luck and please let me know how you get on.


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