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Classic Engines, Modern Fuel

Comments on Topic: The volatility profile of modern petrol causes a number of problems

Submitted by The Author

Possible solutions to help reduce the problems caused by the volatility of modern petrol.

Submitted by Wim

Dear Paul,

Thank you for writing your book, I was truly positively amazed and impressed.

I was wondering about the infra red temperature pictures in the chapter “Keep the fuel system cool”. As I understand it these infrared based temperature devices are critical to the proper setting, specifying the correct emissivity coefficient for the body which has its temperature measured. A black radiating body has an emissivity coefficient of 1, polished, shiny aluminium could be at 0.2 – 0.3. Measuring by infrared the carburettor dash pot with an emissivity of 1 (which is way to high) would give a false reading for instance of 40 degrees C where as in reality the temperature could be 20 to 30 degrees C higher.

Not all handheld infra red temperature guns have a functionality to set the emissivity coefficient. These guns (with-out the function of setting the emissivity coefficient) are easy and simple to use, however they do introduce an inherent danger of wrong temperatures and subsequently confusion.

Could you comment on the fuel viscosity change as a function of the temperature for a classic ethanol free fuel and for E5 ? The Strombergs from the late sixties on claimed to have a feature to correct for visicoitry changes due to temperature changes.



Submitted by The Author

Dear Wim,

Thanks for the positive comments about the book. To answer your questions:

Infrared photographs. These were taken with a professional FLIR i7 camera. The emissivity was set to “Semi-matt 0.8”. This gave reasonable temperature measurements for all the parts of the carburettor and braided fuel hoses. However, the pictures were only used for illustration purposes to show the relative temperatures of the fuel system components. The actual temperature measurements were taken using thermocouples which do not depend emissivity.

The second part of your question is interesting. There are three physical characteristics of petrol that will affect the operation of the carburettor:

  1. Specific gravity (SG). A carburettor is a volumetric device. Fuel with a higher specific gravity (i.e. denser) will change the mixture. There is no difference in SG between classic and modern fuels. Average SG is 0.74 gm/cc, including fuels containing ethanol. For both classic and modern fuels, the SG can vary by about 7% between brands and deliveries. This is much higher than the differences caused by temperature change.

    Differences in SG have far less impact than may be expected. The flow rate through the jet is proportional to 1 / Sqrt (SG). As SG increases and the fuel contains more hydrocarbons which make the mixture richer, the flow rate drops, weakening the mixture. These effects offset each other. As a result, changes in SG have a small effect on mixture.
  2. Viscosity. As you point out, the volume of fuel flowing through a jet is inversely dependent on viscosity. Unfortunately, I have no modern published data on the viscosity of petrol and how it changes with temperature. We did not investigate this at Manchester.

    However, it appears that temperature effects on fluid flow for less viscous fluids such as petrol are small:

    “changes in viscosity have too slight an influence upon the time of flow. For example, with the Engler instrument, the time for gasoline at 10° C (50° F) was found to be 48.7 seconds, and at 50° C (122° F) it was 46.1 seconds.” (Herschel, W. 1919).

    Unfortunately, this data is very old and may not apply to modern petrol. The Fuel Temperature (viscosity) compensation system fitted to later SU and Stromberg carburettors were used to reduce emissions. This suggests that viscosity changes did have an effect on the mixture. In practice I am not sure how effective these systems would be. As I found at Manchester, the fuel flow through the system and evaporation in the carburettor kept its temperature constant at around around 45° C. It is possible these compensation systems were more effective when the engine was warming up than under normal running conditions.
  3. Surface tension. It requires more energy to break up liquids with a higher surface tension into droplets. This will impact the atomisation and dispersion of the petrol, something the tests suggested was very important.

    Again, I do not have any data on the surface tension of petrol. My attempts to measure it in my garage were not successful. Some people claim adding acetone to petrol reduces its surface tension and improves engine performance. It is also possible that kerosene has the same effect. However, I do not have any evidence to support this.

Kind regards


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