Theo, et Listers,
There is usually some confusion with regard to the terms "flash point" and
"ignition temperature". The "flash point" refers to an equilibrium
temperature of the liquid at which temperature the vapor pressure is
sufficient to create a combustible mixture in air. I don't have the exact
numbers in front of me, but gasoline has a very wide range of combustible
mixtures; more-or-less all the way between 10% and 90% I believe. If the
lowest concentration that will burn is 10%, then the flash point is the
minimum temperature that will produce this 10% mixture in a controlled test
under (I believe) quiescent conditions. However, in order to ignite the
mixture, it needs to be exposed to the ignition temperature, which is
unrelated to the flash point and usually quite a bit hotter; i.e., a spark
or a match, etc. Again, I don't know what the ignition temp of gas/air
mixture is, but consider the conditions inside the combustion chamber where
you have the ideal gas/air mixture, compressed, and in contact with some
pretty hot surfaces, and still no combustion until the spark is fired.
Bottom line, I wouldn't worry about the muffler itself catching the gas on
fire. You'd need something a lot hotter, not that that isn't possible; say
a leak in the exhaust coupled with a backfire or such.
Recalling my freshman chemistry in San Diego,
Bob
At 11:41 AM 2/18/99 -0700, Theo Smit wrote:
>I think the heat shield is a good idea, but the insulation is a bad idea. The
>last thing you want is for a leak to soak a whole bunch of gas into the
>insulation, and _then_ have it set on fire by the muffler. The heat shield
>would
>act as a deflector to keep gas from dripping on the muffler, if nothing else.
>
>Obligatory War Story:
>Years (about 12) ago, I had a Civic set up with a single DCOE and a Japanese
>(dual-outlet) exhaust manifold, which I piped into a collector using a pair of
>24" long secondaries. The secondaries ran directly underneath the carburetor,
>with the carb located about 15 inches above the two pipes. Because of the lack
>of vibration isolators (no room between the carb and the firewall), the carb
>float would eventually develop a crack and sink, thus flooding the carb.
On the
>DCOE, there's a 1/2" float vent hole that runs into the air cleaner, and of
>course from there the fuel would drip down onto the two exhaust pipes. It
would
>just evaporate when it hit the pipes - no smoke, no flames.
>I don't know how hot the mufflers on the Tiger actually get, and how close to
>the flashpoint of gasoline that is.
>
>Theo Smit
>tsmit@novatel.ca
>B382002705
Robert L. Palmer
Dept. of AMES, Univ. of Calif., San Diego
rpalmer@ames.ucsd.edu
rpalmer@cts.com
|