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.
     We’ll use the combustion of C₈H₁₈ (our approximation for gasoline) in dry stoichiometric air as our baseline.  Recall from the last post that for that case, the saturation temperature for the water vapor in the exhaust was 127.2 deg F.
     For the case of leaner mixtures (excess air over stoichiometric) you would expect that the water vapor would constitute less (proportionally) of the total exhaust mixture, and so the partial pressure and hence, saturation temperature would go down.  That is exactly what happens as can be seen in this figure:

The saturation temperature of the water vapor in the exhaust is plotted as a function of theoretical air (the ratio of actual air to stoichiometric air).
     For the case of water vapor included in the combustion air, we assume that the water vapor is chemically non-reactive in the combustion process, and just rides along like the nitrogen does.  Of course, if there is some water vapor to start with, there will be more at the end.  This figure shows that effect:      

 The saturation temperature of the water vapor in the exhaust is plotted as a function of the humidity ratio (mass of the water vapor per unit mass of dry air) of the combustion air.  A value of zero for humidity ratio corresponds to completely dry air.  A humidity ratio of 0.03 would be pretty high for typical environmental conditions corresponding to 110 deg F and about 53% relative humidity.
     The variation in saturation temperature among the fuels that we considered in the last post, as well as the variations due to atmospheric humidity and excess air that we looked at today are not particularly large: each independent variation covers less than 10 deg F. 

Water in the Exhaust, Part 1