Water in the Exhaust, Part 2

     In the last post we examined the water vapor that is present after the combustion of hydrocarbon fuels.  This time let’s look a little closer at the effects of excess air and water vapor in the combustion air on the amount of water vapor in the exhaust.

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.

Adiabatic Flame Temperature

Normally when a fuel burns, the energy of combustion goes partly into raising the temperature of the exhaust gases, and partly into heat that is removed from the reactants via conduction, convection, and radiation.  It is often helpful to consider the two extremes of this process:
(1) How much heat would be obtainable if you could remove so much that the exhaust gases came out at the same temperature as the reactants started at? 
(2) How hot would the exhaust gases get if you removed NO heat whatsoever?
The answer to that second question is called the adiabatic flame temperature, and it is the subject of our post today.