Re: fuel vapor pressure

["John S. Denker" <jsd@MONMOUTH.COM>]

At 11:06 PM 6/3/01 -0400, Geoff Nunes wrote:
> Gasoline is safe because it has a high vapor pressure and makes up 95% or
> so of the gas in the tank.
> ....
> (The fun things you learn at safety meetings!)


Hmmmmmm.  I don't believe everything I hear at safety meetings.

The vapor pressure of gasoline does not reach 0.95 Atm until the
temperature exceeds 125 C or so.  We've had a heat-wave lately, but it
hasn't been THAT hot :-)

More relevant would be a discussion of
        lower flammability limit:  1.3 or 1.4 volume %
        upper flammability limit:  7.1 or 7.6 volume % (for gasoline)

This is a reasonably narrow range, compared to (say) acetylene.
  http://www.e-cats.com/databook/Page%2023.htm

The vapor pressure at room temperature is closer to the _lower_
flammability limit than the upper flammability limit.

Indeed there is a wide range of ordinary temperatures that produce vapor
pressures in this range (around 20 C to over 50 C).  This is calculated
using the vapor-pressure numbers for octane in the CRC _Handbook of
Chemistry and Physics_.  (I checked that the formula predicts roughly the
right boiling point, which gives it a little extra credibility.)

The physics of the lower flammability limit is fairly obvious.

We can understand the physics of the upper flammability limit roughly as
follows:  Gasoline is a reasonably large molecule.  It takes many moles of
oxygen to react with one mole of gasoline.  And air is not 100% oxygen,
either.  Stoichiometry is about 2% by volume.  Extra gasoline impedes the
reaction;  to first order this is because the extra gasoline undergoes
decomposition, and this bond-breaking siphons off some of the energy.

> This effect is what puts the upper limit on the
> ethanol content in gasoline/ethanol mixtures.

I doubt it.

According to my calculations, the range of temperatures for which ethanol
vapor is flammable is very similar to the range for gasoline.

Am I missing something here?