MadSci Network: Engineering

Re: why don't electric fuel pumps cause fires in cars?

Date: Wed Mar 27 23:20:48 2002
Posted By: Aurelio Ramos, Grad student, Computer Engineering, Not a member of any institution
Area of science: Engineering
ID: 1016677312.Eg

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 
3. The motor is entirely submerged in gasoline, with no oxigen to produce 
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

Current Queue | Current Queue for Engineering | Engineering archives

Try the links in the MadSci Library for more information on Engineering.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2002. All rights reserved.