Large spark in CMS motor

[krw]

Nonsense. There is no air in there at all, even without a gas cap the vapor pressure of the gas will push any out.

Explanation for what?

[Nova]

I've toured the Robert Moses power generation station in Niagara Falls a number of times. The turbines are at the bottom of the dam to take advantage of the water's pressure. The generators are on the top floor of the structure to facilitate installation/replacement. The two are connected by a massive shafts. You're correct, the water (hopefully) doen't come near the generators.

[dpb]

Actually, large-capacity turbine-generators are typically H cooled to enhance conductivity. One does not smoke in the vicinity... :)

[Puckdropper]

"Leon" wrote in news:h4CdnTDTjrOECiLXnZ2dnUVZ [e-mail]:

I'll keep an eye on it. I expect some sparks (nature of the beast), but this was a rather large one that filled the motor chamber. Now it only does it when I let go of the switch before it's come up to speed. (I'm curious now. If it starts again I'll have to put an ammeter on there, take a few readings, then clean the brushes.)

Puckdropper

[Leon]

Obviousely clueless.

[Leon]

My Delta does it, at night with a not so well lit garage the motor becomes pretty lit up when I release the switch.

[dpb]

...

Not quite, no...not _exactly_ "none" wrt to air but the high vapor pressure (very low flash point) of gasoline _is_ the key. To save having to dig up the flammability and explosive mixture data, the most cogent explanation I found on the web quickly was at a site discussing the Flight 800 explosion that went on to compare the situation w/ jet A and an airline tank and the fuel pump in automobile tank and gasoline. From that site--

In short, the answer is that the high vapor pressure of gasoline does create an atmosphere that is too rich for explosion even when the tank is uncapped if there is any liquid fuel at all in the tank at anything above very cold ambient temperature.

I will agree it _is_ something of a mental twist that the very volatility of gasoline is in fact the property that mitigates the fire hazard suppressant from the fuel pump when the tank is low (although I presume the pumps are mounted in a well or at least at the lowest point in the tanks in order to keep them submerged until absolutely all fuel that can be picked up has been. I suppose one could even raise the inlet pipe a little to ensure that; don't know if they do or not.)

hth....

-dpb

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[Leon]

While the explanation is valid and works for 99.999% of the time my question is about the other .0001 percent of the time. When the lost gas cap is never replaced and the car is parked and sits for several months with little fuel. Eventually some one cranks it up and the tank has little fuel to submerge the pump. And if you think that is not common, repositions of abandoned cars filled the bill at the dealerships and many were often stripped.

Because the fuel pump is relatively tiny and totally enclosed, IIRC there was a check ball valve on the inlet side, perhaps in some instances of a low fuel condition there is an actual flash when the pump motor in energized, but the flash is contained with in the motor casing. I do recall the motor housing being rather substantial compared to most electric motors found on automobiles.

Thanks for the time spent gathering the information.

[dpb]

While perhaps it is relatively common, it's still highly unlikely to be a problem. If there's enough fuel to crank and start the engine, there's enough to suppress the spark and have high enough vapor pressure to prevent flammability at any temperature above about -10F. Remember the flashpoint of gasoline is -40C (which iirc is the magic point where

-40C=-40F???).

About the only time above about 0F one could get a real problem would be an essentially dry tank w/ residual vapor and run the pump dry. I'd think that if vehicles were so poor condition as to be left w/ empty tanks and no lid there would be little likelihood of them being cranked in any near-term time frame. Hence the evaporation over a period of time would likely imo serve to remove even the vapors.

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[dpb]

dpb wrote: ...

And if your contention is that it is _possible_ under specific circumstances and there's no failsafe protection against that very dilute vapor case that is in the flammability region then yes, it is possible.

I hadn't actively thought about the Arctic conditions increasing risk, I don't know if there's anything done for the really cold areas out of the ordinary or not. I'd guess not; rely on folks in inclement weather areas having enough sense to keep adequate fuel on hand I'd guess.

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[Leon]

The problem is though people run out of gas, people run over things on the highway, Houston Freeways, that bounce up and puncture the bottoms of the tanks. Tanks commonly run dry with the pump running.

That is possible but GM has been using a fuel tank pump since the "early

70's I worked directly and exclusively with GM vehicles for 17 years and never ever heard of a problem. And I am not in doubt that what works, "works" effectively. I was and still am curious as to exactly what makes the set up "fool proof".

[keithw86]

Wrong again. Unlike you, I'm not guessing.

[dpb]

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 level.

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.

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[litteratuer]

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.

[Leon]

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 empty tank.

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.

[Leon]

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.

[dpb]

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 at all.

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.

[dpb]

Leon wrote: ...

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.

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[litteratuer]

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.)

[Lew Hodgett]

RE: Subject

First liar doesn't have a chance.

Lew