I don't think it is totally fool-proof except that the conditions for
flammability are much more limited than you think owing to the high
vapor pressure that does, as another poster noted, keep positive
pressure in the tank as long as there's any fuel surface at all.
And, I'd venture that most of the cases you're talking about there's
still a residual of fuel in the tank, only that it is below the pickup
Added to that that the brushes are undoubtedly built to be non-arcing so
that there really isn't a continuous ignition source, the conditions
just aren't satisfied.
That makes sense to me.
According to a table I have, for 100 octane gasoline vapor, when mixed with
air, the explosive (flammable) range is from 1.4% to 7.6%. vapor/air
mixture, - a condition which, as you say, would rarely be met.
And rarely is the event that I am talking about. Rarely does not equal
absolutley not, and with millions upon millions of these vehicled out there
you still don't hear of a problem. There has to be a stop gap safe guard
that takes care of those rare situations such as when there is an accident
with a ruptured tank and no explosion by fault of the fuel pump. There are
simply way too many conditions that can and do occour that would create the
right rare situation.
I don't think there's any evidence to support that hypothesis that the
conditions can be made right so easily as you suppose--in fact, I think
the evidence clearly demonstrates the opposite--that is, despite the
number of vehicles, the conditions under which the fuel pump is an
ignition source _and_ the tank vapors are in an explosive mixture state
is simultaneously, are, simply not events that occur with any frequency
Again, all these scenarios you've brought up simply don't lead to
nothing but a dilute air:vapor mix in just the right
proportions--there's still far to much fuel.
Again, I don't think it is impossible, only highly improbable.
Ok you and he are both missing the point. I realize that the with and
absence of air the possibility of an explosion is nil. I am specifically
looking at the situation where there is air present, ie. a rupture in the
tank where air absolutely will displace the gasoline that is leaking out.
We saw this all the time in the shop. Car died on the freeway, completely
If there is a hole in the bottom of the tank, it is absolutley below the
pick up level. Out of the millions of vehicles built with this
configuration you have to know that there will be a deviation that will
negate the situation that relies on the lack of air to prevent an explosion.
Because I have not ever heard of a tank explosion caused by an electronic
fuel pump I know that the system works under normal conditions but normal
conditions do not always exist.
That may be the case. Non arcing brushes and again the brushes were in a
strongly built contained steel enclosure behind a steel ball check ball.
But are the pumps mounted in a well? Are there not formed
hollows/ridges for structural rigidity in the tanks? Even if it leaks
to the point of not running, I'd expect there to be a little gas in
almost all cases left in such areas. If the tanks were a perfect sphere
and you punched a hole at the very bottom, all the liquid would run out,
yes. But there still wouldn't be air intrusion in most cases, only
almost pure vapor.
So, there's the design feature(s) you're after which are simply part of
the design. I'm not saying the pumps are built w/ no thought of
preventing an ignition source; only that I'm convinced there's no
secondary protective device or system other than the base design and the
physics of gasoline combustibility.
It may also be that even when the fuel/air ratio conditions for combustion
are met, the arcing from the brushes may not provide a high enough
temperature to instigate ignition. (The article in the url below gives an
ignition temperature of 600F for gasoline vapor.)
And there in lies the probable answer as a fowled spark plug will spark but
not necessarily ignite the fuel mixture. Gasoline running past the brushes
keeps every thing cooled down under normal operating conditions. And with
electrical wiring running into the fuel tanks even for the tank units to
measure the level of fuel you have to know that sooner or later there is
going to be a short and that does not seem to present a problem either.
that has veen a fact long before electric fuel pumps wer even being used.
There is absolutely air in the tank otherwise the tanks would colapse or
fuel would not exit. Fuel vapor may be too dense to burn unless the
conditions are right. Vapor literally does not exist with out air, vapor is
simply difused matter suspended in air.
Either way I believe your explanation that the spark in not hot enough is
the stop gap safety measure. That is indeed a constant vs air and vapor
conditions around the fuel pump.
Or, more visible way to think of it, if were to start w/ a liquid in a
closed container w/ a movable wall and if retract that wall creating a
vacuum there will become a new equilibrium of two phases--liquid and
gas. The concentration will be dependent on the specific liquid.
There's no air needed...
It might seem so, but that is not the case.
Gasoline vapor is heavier than air. With air given a vapor density of 1, the
vapor density of gasoline is between 3 and 4.
In an undisturbed tank, vented to the outside, as gasoline evaporates, the
vapor will displace the air.
In the case where all of the gasoline has been drained and air has entered
the tank, the remaining vapor will be concentrated at the bottom.
Disturbed, I suspect that a shaken tank would not qualify as undisturbed.
There is an awful lot of shoshing around going on in there when the vehicle
is moving and if there is a hole in the bottom of the tank from a damaging
impact the vapors will "grain out" after the fuel.
Sadly, it does happen.
Many years ago a young neighbor of mine was using an oxy-acetylene torch to
cut open a fuel tank which he had emptied some time beforehand and thought
was safe. It exploded, leaving him with serious permanent brain damage.
Friend of mine was working on a gas tank from a 1979 Ford Bronco washed
the gas tank out several times was trying to solder the tank and all of
a sudden there was a big bang what had happened the tank actually
expanded,he finished soldering it but we had a hellova time getting it
back onto the truck as it had expanded so much. Dan
No. They are typically flat and generally tilt towards a low end of the tank
for all the gunk to collect at. The pump is suspended by the tank unit up
from that position. There are ridges for rigidity on some tanks but those
ridges generally run in a direction so as to drain to the low spot of the
tank and the low spot is typically the forward end facing towards the front
of the vehicle. This allows for more ground clearance under the back of the
vehicle. That area is also the most vulnerable and typically will be the
damaged/ruptured area when a driver drives over an obstacle.
Are there not formed
I would say that a pint of fuel could collect, maybe depending ont he
damaged area but if the damage is located on the front bottom it is likely
very very little will remain. Keep in mind a puntcture on the bottom of the
tank will act exactly like a can of juice. Hold the juice can and poke a
hole in it from the bottom. Juice will come out and as it comes out air
will replace the leaking juice otherwise the fluid would not drain.
Gasiline works the same way.
Yeah I am sure there is a secondary back up measure there are countless
others in the fuel lines and in the old days the carburetors had
preventatice features to limit fuel flow.
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