Quote:
Originally Posted by Doug
Boiling mogas is trapped between the fuel pump outlet check valve and the carby float valve, it continues to boil, the pressure increases to a point where the float valve is overcome .....
The pressure is only relieved by cooling the fuel line below the boiling point of its lowest temperature fraction, or by some active pressure relief mechanism such as a purge valve.
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Exactly, Doug. Inadvertantly, the OP is doing a fractional distillation in the fuel line. The winter fuel has some components with low boiling points- makes your car start easier in cold temps, but also makes vaporization in a hot soaked fuel line more likely. And, as Dan said, the fuel pump springs sets pump pressure. Boiling fuel in the line and the resulting high pressure has the pump diaphragm pushed as far toward the crankcase side as possible and the spring is not stiff enough to move the diaphragm until fuel side pressure drops.
Interesting failure mode. One might call it vaporization induced overpressure.
The other mode is fuel starvation. No pressure and no fuel as in when all the fuel pumps have to work with is vapor, and the vapor compresses and doesn't work it's way through the system- vapor lock.
Both failure modes are a very good reason for a fuel recirc line. It may only be needed to evacuate the fuel lines post hot soak. When the airplane has continuous fuel flow and air moving through the cowl, the fuel may never get hot enough.
Vaporization-Induced Overpressure
Linear Mode
