Re: Need Help

[mel1523@acs.tamu.edu (Mitch Loescher)]

>I moved here back in March, so this is my first winter.  I would expect water
>vapor for a while here because of the temps but even after she's good and hot??
>The car performs great so I'm not too concerned at this point.  I hope it is
>just water vapor.
>
>Water is one of the primary products of combustion, and is there
>just as much in the summer as it is in the winter. It's visible
>in the winter because it condenses when the exhaust gas hits the
>cold ambient air.
>
>Exhaust system temp doesn't have much to do with it. In fact, there
>will be more water in the exhaust gas when the exhaust system
>is hot, becuase less of it will condense out on the metal before
>leaving the tailpipe. Whether you *see* more depends on how cold
>the ambient air is and how fast the exhaust gas dissipates.

I can depend on exhaust gas temperature.  Here's how...  When the gas hits
the air surrounding the tail pipe, it mixes.  This does two things...it
dilutes the water vapor, and cools the mixture.  The dilution unfavors the
formation of droplets by lowering the dew point of the mixture (not of the
air).  Cooling favors the formation by bringing the mixture closer to its
dew point.  If it hits the dew point before the dew point moves too low
(ultimately to the dew point of the ambient air at total dispersion) then
you get dew (visible water droplets).  Eventualy it blends until there is no
mixture anymore and you're at the dewpoint and temperature of the air (or
close enough).  That's why it forms, then quickly disappears.  Cold exhaust
helps the formation by starting out closer to the dew point.  Cold air helps
by cooling more for a given amount of blending.  Wet cold air is the best.
Is is possible to have your car do this all the time on cold wet days and if
your exhaust cools the gas alot.   It's the same chemistry that causes
con-trails on jets (even though those are ice crystals).

 - Mitch