Why don't submerged fuel pumps catch fire?

[paulsco]

Ok,

There is something that I have been concerned about for a long time and wonder if you can enlighten me. Why don?t submerged fuel pumps catch fire? The motor brushes are likely to spark, after all. And what about fuel tank senders; a current of electricity passes through them and the resistance shows the level of the fuel on the gauge. I have never heard of any of these causing a fire, but why?

Paul

[David Oslo]

To achieve combustion, you need all three of these, and in the right A/F mixture (which for fuel is quite a narrow band)

Fuel + Air (Oxygen) + Heat

So even if you air sloshing around, and happen to acheive the perfect fuel-air ratio (however unlikely), there is not enough heat from the motor or tank-sender-unit to achieve ignition. The fuel acts as a coolant in this case.

[paulsco]

but what about when the tank is nearly empty? There is a lot of air and fuel vapor at the top of the tank and the sender and presumably the sparking motor carbon brushes are above the fuel level in the vapor rich environment. there will also be air/oxygen? The heat could be caused by the sparking carbon, or the resistance or failure of the coils in the sender?

Paul

[Moeregaard]

Even with an almost-empty tank, the petrol vapor concentration relative to the amount of oxygen in the tank is too great to allow combustion. My last chemistry class was 35 years ago, but I believe this is due to the relatively high vapor pressure of petrol. Diesel and its relative, Jet A, is an entirely different breed of cat, and measures are taken to ensure that no potential sources of ignition exist within tanks containing those fuels. TWA Flight 800 was brought down by a electrical issue that initiated an explosion in the 747's center fuel tank. Most modern cars use submerged pumps and these are actually cooled by being immersed in petrol. Many premature fuel pump failures I've seen are from people who allow the tank to almost run dry before refilling.

[SEJohnson95]

Damnit gents you all beat me to the answer! Yes it's down to the correct air-fuel mixture required for combustion, and such a lot of fuel is contained in such a small volume there is never going to be enough oxygen for combustion. Remember reading about TWA flight 800 that went down over the Atlantic because of a wiring loom fault that produced a spark in the centre wing tank. The fuel tank was somehow heated to the flash point of the fuel which also melted the wires. The tank was almost empty (think of the volume of air in a 747 centre wing tank) and that critical zone was reached. Inevitable result unfortunately :/ but don't let that put you off!

Edit: Mark, clearly we're both on the same wavelength here, it's always a good idea to read what someone else has posted