Water in the Exhaust

     Have you ever noticed white clouds of exhaust billowing out of a car’s tailpipe on a cold day? When hydrocarbon fuels (like gasoline, diesel fuel, oil, propane, natural gas, etc.) are burned in air, normally the vast majority of the exhaust consists of carbon dioxide, water vapor, and nitrogen.  All three of these gases are colorless and transparent, so when you see white exhaust coming out of a car or see an airplane’s contrail, it is light reflecting off the surfaces of tiny droplets of liquid water that have condensed from the water vapor.  In this post we’ll examine the amount of water produced by combustion, and the conditions under which it condenses.
      Before going further, we should note that one exception to the statement above about exhaust visibility occurs when there is a problem with an automobile engine, and soot particles from burning oil are included.  Usually these particles form a black or gray exhaust.  Another exception occurs when a diesel engine is under a heavy load, like accelerating away from a stop, and incomplete combustion of the diesel fuel results in soot particles in that exhaust.
     Now, let’s look at a stoichiometric combustion of C₈H₁₈ (commonly used to represent gasoline) in air.  The balanced reaction looks like this:
       C₈H₁₈ + 12.5*(O₂ + 3.76*N₂) --> 8*CO₂ + 9*H₂O + 47*N₂
     Normally the nitrogen, which was in the air originally, just passes through the combustion process without chemically participating, for the most part.  The carbon and hydrogen from the gasoline are completely burned to create carbon dioxide and water.  For every mole of gasoline, the combustion requires 59.5 moles of air (consisting of 12.5 moles of oxygen, and 47 moles of nitrogen) and produces 8 moles of carbon dioxide and 9 moles of water vapor.
With those constituents, the partial pressure of the water vapor in the exhaust gases would be about 2.07 psia (assuming atmospheric pressure of 14.7 psia) which corresponds to a saturation temperature of 127.2 deg F.  So, if the exhaust gases get cooled below 127 deg F, the water vapor will start to condense, and a cloud will appear.  Of course, the exhaust gases rapidly mix and disperse in the atmosphere, which is why sometimes you see no cloud at all.  Even when a cloud forms, the very tiny water droplets will re-evaporate quickly which is why the white exhaust clouds usually don’t hang around long. Contrast that with the behavior of the gray or black exhaust clouds composed of soot particles; they usually linger in the air for much longer before they disperse enough to become invisible.
     Other hydrocarbon fuels behave similarly.  The table below shows the saturation temperature of the water vapor in the exhaust for several other common fuels.  The fuel is approximated by the hydrocarbon shown in parentheses.  In all cases, it was assumed that the combustion was with stoichiometric, dry air at one atmosphere.

Water in the Exhaust, Part 2