Hard Hot Start Fuel Composition Question

[Roger_McWilliams]

Can people throw in some opinions about hard hot starting versus fuel composition?

Occasionally someone will remark about ethanol in modern automotive fuel leading to hard hot starts. There may also be a remark about ethanol having a low boiling point which contributes to hard hot start problems.

Gasoline typically is a mix of pentanes (boiling point around 36 deg C), hexanes (boiling point 68 deg C), heptanes (boiling point 98 deg C), thousands of other carbon compounds, and nowadays ethanol (boiling point 78 deg C). I can find literature which shows gasoline boils starting at 39 deg C, with the boiling point rising as the distillation clears out the lower boiling point elements, reaching boiling point of 110 deg C at 50% distillation.

Vapor pressure versus temperature curves can be found showing ethanol-free gasoline has a higher vapor pressure than ethanol until reaching ethanol boiling point. I also have the impression that vapor lock issues are reduced in racing conditions when using alcohol fuels. And finally on vapor pressure, don’t fuel suppliers vary the gasoline vapor pressure with the season, so a car rarely driven could have a high vapor pressure winter blend in use on a hot summer day?

Both the boiling point temperatures and the vapor pressures suggest that ethanol reduces hard hot start problems resulting from vapors compared to gasoline without ethanol.

What are your observations? What sources of evidence can help understand whether ethanol blend fuels create harder hot start problems than ethanol-free gasoline?

[Ed_Nantes]

I’m not sure about the connection with ethanol
I previously had hot start problems, util I istalled a return capillary .But then, I have never used ethanol fuels. so it could not have been ethanol. The issue for our cars is that SU fuel pups have a non return valve which effectively traps fuel between the pump[s] and the carb float bowls. After the car has stopped, this relatively cool fuel is trapped and starts to warm and expand. The only place it can go is pushing past the needle in the float bowl top and so flooding the carbies.

This is not helped by he otherwise perfect design of the SS motor car, having the petrol pumps under the bonnet.

If one takes a kettle up a mountain it boils at a lower temp. A petrol pump at the front works by lowering the pressure in the line from the tank causing petrol to flow into it But also that lower pressure drops the boiling [ or vapourisation ] point. Conversely, putting the pump at the back , works by increasing the pressure in the line and raising the boiling point and so lessening the vapourisation problem.
The other factor may be the entirely different overflow arrangement sin the float bowl tops of early and later SU carbs.

[Wiggles (Paul Wigton)]

@Ed_Nantes

This is not helped by he otherwise perfect design of the SS motor car,

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…Eddy, you funny man! Me love you long time!!