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Classic Engines, Modern Fuel

Comments on Topic: French fuel test

Submitted by Anonymous


Since petrol brands are different with country, I decided to perform boiling tests of different fuels easily available in France.This first result is preliminary since it was performed with fuels taken from my cars tank. But anyway, they show some interesting hints.

Figure 2-1 of the book shows the volume of front end component Vs temperature. My test evaluates the flow of gases for different temperatures. I think that this flow, is more representative of the hot restart problem than the evaporated volume itself. Micro bubbles can escape slowly from the jet without drowning the inlet manifold. A single big bubble is enough to eject liquid petrol in the inlet manifold.

3 jars in a pan of water heated by an electric portable stove, outside of course.

Fuel A = French big petrol company 98 E5 (in the TD tank since last summer)

Fuel B = supermarket 98 E5 (fresh from the pump, summer mix)

Fuel C = supermarket 95 E5 (filled in a tank this winter)

Nota : only B does not contain lead substitute additive. Any effect on front end components ?

The first micro bubbles (size less than of a small pin) appear at 40°C only in A and B

At 50°C, all fuels have small bubbles (sparkling champagne). Fuel C being the worst

From 55°C to 65°C, the fuel ranking is clear. B is the best (still micro and small bubbles), C is the worst with huge flow of big bubbles, and A is in between.

At 70°C, all fuels have similar big bubbles. Letting it cool down very slowly and the volume loss in the 3 jars is similar. about 40%, roughly compliant to the figure 2-1. Nevertheless the 3 fuels will have different behaviuor regarding hot restart issue.

I think that B is really preferred for its high temperature boiling point.

Conclusion : the storage of fuels during winter and even during few days in hot sunshine for fuel A, does not evaporates the front end components. They remain the worst despite storage.

Another hint, as the temperature was stabilized in the pan for several minutes the observed bubble flows remain constant. So I don't think that we can hope any front end removal in hoses between pump and carb bowl when driving. I even doubt for carb bowls. What is the mean time for fuel in the bowl ?

I plan to perform similar benchmark on 3 fresh fuels from the pump, summer mix.


Submitted by The Author

I advise anybody trying to reproduce Laurent’s test to exercise extreme care. At certain concentrations petrol vapour can auto-ignite. Petrol fires are very difficult to put out. You cannot use water. The burning petrol floats on the water and the fire spreads.

After saying that, I think Laurent’s tests are very helpful and certainly support the findings from Manchester that super grade fuel appears to be less volatile at low temperatures.

The statement that “Micro bubbles can escape slowly from the jet without drowning the inlet manifold. A single big bubble is enough to eject liquid petrol in the inlet manifold.” is probably correct. However, micro bubbles will affect the way the engine runs.

Carburettors are volumetric devices. They measure the volume of air flowing into the engine and add a given volume of petrol. Petrol vapour is approximately 14 times the volume of that quantity of petrol. This means that ANY bubbles in the petrol will significantly weaken the mixture. Needless to say this process is complex. The volume of petrol flowing into the airstream depends on i) the annulus between the jet and needle; ii) the pressure differences between the choke and atmospheric; iii) the viscosity of the petrol. Petrol vapour is less viscous than liquid petrol increasing the volume of vapour leaving the jet. This will offset the weakening effect.

My experience on the hot day in a queue, reported above, does not match the assumption. “So I don't think that we can hope any front end removal in hoses between pump and carb bowl when driving”. My fuel pump was clicking very regularly in bursts. The only explanation for this is that there was petrol vapour in the hoses between the pump and float bowls. Even so, the engine ticked over and ran without any problems suggesting there was no vapour getting into the jet. As the petrol sprays into the float chamber through the needle valve, the vapour will escape. This will remove the low temperature boiling point fractions tbefore they reach the jet.

Submitted by Anonymous

Paul is right. Extreme care. I even add some precision. Very low volume. The electric stove was powered on and off at the wall plug with a long chord, not at the stove itsellf. And my powder extinguisher was not far.

Second test serie.

Supermarket 95 E5

Supermarket 98 E5

French Petrol company 98 E5 (the same as B in previous experiment)

Now the 3 fuels are summer blends, fresh from the pump.

Conclusion :

Summer fuel is less volatile. About 5°C

98 is preferred to 95

Big company is a little bit better than supermarket.

These test are qualitative and based on gases flows rather than evaporated volumes. But I see that the results from my tests are consistent with the volatility curves shown in the book.

Hope this helps for all french Ttypers.


Submitted by Anonymous

Another complementary test today. Try to boil lead substitute with the same process. No bubble was observed till 85°C. Then I stopped the test.

So no early vaporisation with this additive. No issue.


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