"Leon" wrote in message
...

"scouter3" wrote in message
. ..


I worked as a Metrologist in a fuel pump test facility for one of the big
US car companies for 10 years, so I might have the long sought answer to
your question.

The somewhat simplified answer, without going into all kinds of details
is:

When you run out of fuel there are still a lot of vapors in the tank.
Fuel vapors are heavier than air and so they pool in the tank. That
leaves an oxygen lean environment.

The chances of getting a combustable mixture into the fuel pump and then
a hot enough spark from the brush at the that instance are extremely
unlikely.

I suppose it is possible for that big bang to happen but probably only
likely on a full moon, during the vernal equinox, South of the equator.

I'll take your word for it as it is indeed beyond me and you seem to have
the credentials to back up the explanation. BUT while it seems like it
would be a once in a blue moon condition that may lend itself to an
explosion these type pumps have been used for decades concerning GM cars.
It seems likely with the millions of cars produced during that time there
surely would have been several once in a blue moons occourances. I have
never heard of that happening.


Hi Leon,

There are some other safety features designed in that take the probability
of a major thermal event occuring to near zero. I just didn't want to go
into them all.

The commutators and brushes are positioned and selected to provide a
minimal spark magnitude to further reduce the likelyhood of combustion. The
area inside the pump where any combustion would begin is very small. It
would be a little bang so to speak. The pump body is a heavy steel and there
is a check valve to prevent any flame from the little bang from escaping the
pump body and getting into the tank.

Further because the fuel vapors are heavier than air it would take a long
long time for them to escape from the tank and allow enough oxygen to enter
and form a combustable mix. The car would likely have to sit out of gas with
the gas cap off for months or years. The pump would have to pull the air
mix into the brush chamber. That would again take a long time to draw the
air in because the fuel pump isn't designed to move air. So it would take a
long long long time to get the right conditions for combustion.

I add that I was not a fuel pump engineer so there are more things that I
am forgetting about or probably don't even know about. I designed
calibration systems to ensure that the measurements made by the fuel pump
testers were accurate and precise, so I was somewhat on the perifery of the
fuel pump's inner workings. I spent some time on a team trying to solve why
ethanol was destroying the early electric fuel pumps and got to know the
inside of a fuel pump pretty well from that experience.

And ethanol comes from plants and wood does too, so now we're back on
topic...

--
Lloyd Baker