Ok I can't hold back any longer ;-)

I used to own a ford tempo. The "geniuses" at ford ran the fuel return line
and the exhaust in the same tunnel in the floor pan. They also used an in
tank submerged pump. No problem normally. Any day where the temp was over 95
and the trip was longer than a couple hours and you had serious problems!
The fuel would get hotter and hotter until it finally began to boil. When
this happened the car would buck and surge like a rodeo bull. At first I
thought I would be smart and stop and put some cooler fuel in the car. This
worked for a while until I had a tank FULL of boiling fuel. Then I stopped
at a car wash and tried to cool the tank with water. That got me about 30
miles down the road. The next thing that happens in this wonderful chain of
events, under the blazing sun, is the pump, which is cooled and lubricated
by the fuel, dies. Not a pretty picture.

Most aircraft which use fuel for a coolant (including the SR71) do so with
the fuel before it is injected into the engine. This actually helps increase
the efficiency of the engine (marginally). In our case, hot fuel into the
engine does not help since it goes in before the mixture is compressed, and
therefore reduces volumetric and thermodynamic efficiency.

Now, all this being said, if you are cruising at 14500 with nice cold air
flowing over the tanks, it is a very different situation than the one in my
bedraggled ford tempo cruising along in the blistering summer heat, but
something to keep in mind none the less. A few gallons of hot fuel left in
the tanks at low altitude in summer could pose a problem with vapor lock.
The other thing that makes me nervous is the possibility of a tank
explosion. In tank pumps are tested rigorously by the auto guys to eliminate
this problem. I'm not sure that we have the resources to make sure that our
installations are safe in this regard. A loose wire due to fatigue or
chaffing, and a tank full of volatile hot fuel vapor and air could ruin your
day. Remember Flight 800? (conspiracy theories aside, there have been
documented fuel tank explosions due to everything from lightning to ground
fueling accidents)

Monty

 Can't blow up a gasoline tank with liquid fuel still in the tank by adding a
spark down in the tank.  Too rich to burn.  Look closer at the fuel guage and
pump - zero, zip, none, nada effort to isolate the 12 V from anything.  Insulate
electrically so it will work, yeah, make sparkproof, no.  Gas tanks do not
blow up while tooling down the road.  A leak under the hood is very bad,
but a spark inside the tank is just fine, as long as you keep some liquid
fuel in
the tank.  Just don't drain it!

You can get a nice fire going at the inlet where there is enough air, or in an
"empty" tank, where there is too little liquid fuel to make the required (safe)
excessively rich mixture.  Welding on an "empty" metal fuel tank is EXTREMELY
dangerous.  Welding on a FULL metal fuel tank is fine, altho you can get a nice
external fire if there is an active leak.  Spooky, but true.

Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor

 
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