Bubbles in Return Fuel Stream

Bill,

After getting the idea from your post, I built an apparatus to test gasoline RVP, and followed the ASTM D323 procedure.  I’m not going to do anything earth shaking with it, it was just an interesting experiment to help better understand fuel vaporization.  The result was a value of 10 using the fuel in question, but it has been decanting for a few weeks so I’m sure it was 11 or 12 at the time of the vapor lock incident . 

There is something I’m having trouble understanding.  RVP is measured with a standard gauge that reads 0-PSI at 1-atm and references ambient pressure, and the conversion from RVP to TVP does not adjust for atmospheric pressure.  The test chamber was exposed to ambient pressure, then closed,  and the fuel vapor raised that pressure 10 psi above ambient.  So, intuitively it feels like the total pressure required to keep the fluid (gasoline) at equilibrium should be gauge pressure plus atmospheric pressure (absolute pressure).  So let’s call that 10 psi + 14.7 psi = 24.7 psi.   Yet, we calculate the fuel vaporization to occur at an altitude where TVD = atmospheric pressure, which in this case would be 10 psi.  I know the math works and I accept it,  but I can’t square it in my head.  

 

Yes, it is a weird principle to get your head around. Try THIS for more Info. 

 The 10 psi you measured is the pressure of the vapor phase of the fuel, actually the sum of all the partial pressures of all the individual compounds within the fuel, pressing against the liquid phase of all those same compounds.
The ambient atmospheric pressure is just the Zero reference point for the gauge before you started the measurement and is Not part of the measured pressure.
If you attempted to perform the same experiment at altitude, you would get the same 10psi result.
What you are measuring initially is a sealed container with 14.7 psi Ambient within the container and 14.7 psi Ambient outside of the container. Net gauge pressure is zero.
Once you seal the container, the Fuel evaporates to the point of equilibrium and its pressure adds to the pressure of the Atmospheric Pressure that you started with.
Yes, the Internal pressure is 24.7 psi Absolute referenced to the 14.7 psi ambient.  10 psi differential.  (Gauge Pressure)
If you changed altitude AFTER sealing the Test Chamber, the results would vary with altitude because your reference pressure is changing.
Although nearly impossible to set up, you could perform the test in the vacuum of space and still get the same 10psi result.

In an open container (Vented Fuel Tank) the vapor pressure component futilely flies out the vent in an attempt to come to equilibrium with the Earth's entire atmosphere.
Therefore the pressure in the tank is the ambient atmospheric pressure only.
The end result is that as long as there is more than 10 psi of atmospheric pressure keeping the "Boiling Genie" in the liquid in the bottle, it will - not  Boil (Vapor Lock).
- - -
This is the reason fuel goes "Bad" if stored in an unsealed container.
The vapors continually attempt to come to equilibrium with the atmosphere causing the lighter, Higher-pressure, components to evaporate at a higher rate than the heavier, lower-pressure, components.
Eventually, you end up with a fuel that is closer to Turpentine than Gasoline.

Keep the vents closed on your fuel storage can no matter how much they may seem to bulge at the seams on a hot day.

This is also why modern cars have fuel caps that seal the tanks and your "Check Engine" Light come ON if it doesn't

RVP is a difficult issue and for aircraft to use auto fuels we have to accept some compromises.  Given that the RVP level on almost all auto fuels is higher than Avgas, sometimes even double, it has risk at altitude.  This same risk factor applies in hot conditions.   

With the injected engine we have a pressurised fuel system however that only is in respect to the pressure after thte fuel pumps and once released into the return line the atmospheric pressure then may allow the fuel to off gas or vaporise.  The best advice for auto fuel users is to avoid any winter blend auto fuels in hot conditions, say over 75 to 80 F.  If you fly over say 8000 ft I would also avoid it as the pressure is not enough with a high RVP fuel to stop vapor from forming.  

The tribal knowledge most use for hot or high conditions is to at the very least mix 100LL at about 50% if you have to fly over mountains or are unsure of the blend of fuel you are using.  Avgas 100LL, ASTM D910 fuel, is about 7 PSI and UL94 is about 9 PSI as I recall.  Summer auto fuels will be down to 7 or 8, up to 9 with 10% ethanol and for the most part good to use.  Winter blends are a real problem if you hit hot weather as they may be as high as 15 PSI ( a lot of Butane in it) as Bill pointed out it is very important to keep it in a sealed container.  Metal ones are better than plastic as plastics can "breath" the light elements out over time.  The fuel companies state that fuel at its max time should be OK for up to 6 months however if it contains ethanol that time is 50% or only 3 months.  Given that in many states we have winter blends with higher RVP in winter it might be a really good idea to not store your tanks full.  (that is an old idea with aircraft tanks where the idea was to prevent condensation in the fuel...water...from contaminating the fuel tank due to hot/cold cycles on the ramp.  Avgas sucks up a lot of water that drops out right away, auto fuel not so much as it always contains some amount of alcohol that will absorb the water and burn it harmlessly.  

For anyone testing RVP there are some good videos online of how to test it and how to make the equipment.  This is done by racing engine builders where the fuel RVP must be known when testing on dynos.  

https://www.youtube.com/watch?v=p8UoX7gwCdM

Cheers