|MadSci Network: Engineering|
From your question, I assume you are talking about the modern electric fuel pumps which are located inside the fuel tank. This kind of fuel pump consists of a small turbine and electric motor, and fuel flows thru the actual motor's armature, including the commutator. The commutator is the switch which determines which set of coils becomes energized, depending on the angle of rotation. In most electric motors (especially high powered ones), sparks can be seen comming from the commutator. The size of those sparks depends on the current flow, mechanical load, and the design of the commutator itself. Sparks occur because the current inside a coil has a tendency to continue flowing even after an interruption, even if that involves elevating the voltage enough to cause a spark (jumping thru the air). There are multiple factors that prevent fires from occuring in fuel tanks: 1. The commutator and choice of brush materials are designed to minimize the size of sparks 2. The power load on the motor is low, and so is the current flowing thru the motor, which reduces the size of sparks well below that required for ignition 3. The motor is entirely submerged in gasoline, with no oxigen to produce ignition 4. In the event of running out of fuel, the vapor pressure of automotive gasoline inside the tank is too high to form a mixture that is ignitable under such circumstances. By the time the vapor pressure goes low enough for a good ignition, the fuel pump has shut down because there is no fuel (which further reduces the load on the pump as well as the current draw. It is interesting to note that electrostatic charge buildup seems to be a greater concern, especially in aircrafts. For that reason, the kinds of fuel used contain additives that increase its electrical conductivity so that any charge buildup disipates long before it has any real chance to become dangerous. (Remember: static buildup is greater in dry weather because dry air is a better insulator than damp air.) Your Mad Scientist, -Aurelio R. Ramos
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