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From: scw@cepu.UUCP (Stephen C. Woods)
Newsgroups: net.physics
Subject: Re: Visible automobile exhaust on a cold day
Message-ID: <421@cepu.UUCP>
Date: Fri, 4-Jan-85 16:33:57 EST
Article-I.D.: cepu.421
Posted: Fri Jan  4 16:33:57 1985
Date-Received: Mon, 7-Jan-85 02:43:23 EST
References: <174@dmcnh.UUCP>
Reply-To: scw@cepu.UUCP (Stephen C. Woods)
Distribution: net
Organization: VA Wadsworth Med. Center; LA CA
Lines: 25
Summary: 

In article <174@dmcnh.UUCP> gts@dmcnh.UUCP writes:
>Can anyone out there tell me why a car has visible exhaust while warming
>up on a cold day; then as the car warms up, it becomes invisible.  It is
>possible to tell how long a car has been driving by noticing how much of
>its exhaust is visible.  Why is this?

The 2 major components of automobile exaust are CO2 and H2O. The H2O is
in the form of steam (vapor), this will condense into visible liquid
when the air temp is low enough (in fact it will freeze into ice before
it disipates if the tempature is low enough (~-30F)).  The formation of
visable condensation requires that vapor be in a high enough
concentration to form droplets while condensing.  The reason that the
visible condensation vanishes as the engine/exaust system gets hot is
that the concentration of vapor gets too low before the tempature drops
enough to condense it.

  As an interesting side light, burning 1 Gal of gasoline (Petrol for our
English friends) produces about 1.5 Gal of water (I misremember the exact
amount but I know that it was more than the volume of fuel).  Airships
(rigid ones, that is Zepllens or Dirigables) condensed some of the water
out of their exaust gas to replace the weight of the burned fuel.
-- 
Stephen C. Woods (VA Wadsworth Med Ctr./UCLA Dept. of Neurology)
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