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From: marcus@pyuxt.UUCP (M. G. Hand)
Newsgroups: net.physics
Subject: Re: Visible automobile exhaust on a cold day
Message-ID: <234@pyuxt.UUCP>
Date: Tue, 8-Jan-85 16:15:22 EST
Article-I.D.: pyuxt.234
Posted: Tue Jan  8 16:15:22 1985
Date-Received: Wed, 9-Jan-85 06:22:33 EST
References: <421@cepu.UUCP>
Organization: Bell Communications Research, Piscataway N.J.
Lines: 46

In Article-I.D.: cepu.421 scw@cepu.UUCP (Stephen C. Woods) writes:

> The 2 major components of automobile exaust are CO2 and H2O.... 

>   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.

Thats about the right order of magnitude, but of course the precise figure
depends on the fuel being burned, and the extent of combustion:

Lets assume we're burning pure iso-octane (with an octane rating, therefore
of 100).  The combustion reaction is:

	C8H18 + 12.5O2 ---> 8CO2 + 9H2O

which we all know never quite gets achieved (hence that noxious CO floating
around).

So 1 mole of octane produces 9 moles of water.

The molecular weights of these are 8*12 + 18*1 = 114,  and 2*1 + 16 = 18,
and lets further assume that the densities of the two liquids are
	water	-	1000 kg m-3
	octane  -	 980 kg m-3	(actually this is a wild guess
			but as everyone knows, gas floats on water, so in
			the absence of a Handbook of Physical Constants...)

Therefore,
	114/980 m-3 octane generates 18*9/1000 m-3 water

	or 1 volume of octane will produce 18*9*98/11400 = 1.39 volumes of water

"m-3" means "per cubic metre", ie m raised to the power of -3.

If, as is likely the density of octane were a little higher more water would be
produced.  The presence of longer chain homologues would tend to  decrease
the amount produced, as would cyclic and more unsaturated molecules.

A-level chemistry i should think!!!

-- 

		Marcus Hand	{ihnp4!}pyuxt!marcus