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Dew Point of Exhaust Gas
Name: Carol
Status: Student
Grade: 9-12
Location: TX
Country: United States
Date: April 2007
Question:
What is the dew point of water vapor in the exhaust
gas leaving a catalytic converter of an automobile?
Replies:
Hi Carol,
Your question is unfortunately nearly impossible to
calculate, and is something that would need to be actually
measured. Basically, the presence of the catalytic converter
itself has no significant effect on the answer at all. A
standard 3-way catalytic converter simply oxidizes any
residual amounts of unburned carbon monoxide and hydrocarbons
in the exhaust passing through it, to carbon dioxide and
water. In addition it reduces any residual amounts of
nitrogen oxides back to nitrogen and oxygen. But almost all
of the water vapor that comes out of a catalytic converter,
is created by the engine's combustion process, and would be
there whether the "cat" was present or not.
Modern "cat" equipped engines use an oxygen sensor in the
exhaust, that constantly senses the presence of any residual
oxygen and feeds back this information to the fuel injection
system controller. The controller constantly fine-tunes the
fuel being fed to the engine, to ensure that the air-fuel
mixture is "stoichiometric"; that is, a "perfect" mixture
which results in almost no excess oxygen or unburned fuel.
This requires a ratio of 14.7 parts of air to one part of
fuel by weight. Very little carbon monoxide, unburned
hydrocarbons, or nitrogen oxides, are formed when the fuel-
air ratio is so tightly controlled, but what small amounts of
these pollutants do result, are eliminated by the "cat".
The exhaust therefore is essentially just a mixture of carbon
dioxide and water vapor, whose proportions are determined by
the ratio of carbon and hydrogen atoms in the particular
gasoline being burned. Since gasoline is a mixture of
different hydrocarbons, and this mixture changes from one
brand of gasoline to another, and is even adjusted by the oil
companies from winter to summer, there is no way to
accurately know the exact ratio of carbon to hydrogen
contained in the fuel, other than to analyze is. Therefore,
there is no way to know the exact percentage of water vapour
contained in the engine's exhaust without actually analyzing
the exhaust.
Thus, the dew point of water vapour, in a gaseous mixture
composed of an unknown ratio of water vapor and carbon
dioxide, could only be determined by measurement.
Regards,
Bob Wilson.
Your inquiry does not have a simple answer because the term "dew
point" depends upon the water vapor content in the exhaust, which
from vary from car to car and ignition conditions. "Dew point" is
defined as the temperature (the temperature could be in degrees F,
C, or K) at which the partial pressure of water in the exhaust
equals to the vapor pressure of water at that temperature (which you
can find in most any chemical or physics handbook). So the more
water produced in the combustion of the automobile, the higher the
dew point of the exhaust gas will be. The dew point will be further
complicated if the ambient air already contains water vapor. Then
this too will contribute to the value of the dew point.
Vince Calder
Revised April 2007
If you make the reasonable assumptions of stoichiometric combustion
(or any reasonable AFR ratio for a lean burn engine) and atmospheric
pressure (perfect gas) of the exhaust gas mixture out of the catalyst,
you can easily determine the partial pressure of the water vapor in
the mixture, and using saturation tables determine the dew point.
For our natural gas engines (methane) the dew point of stoichiometric
exhaust products at atmospheric pressure after a three-way catalyst is
calculated at about 140 deg F. Tests have verified the accuracy of the
calculations. I haven’t done the calculation for gasoline stoichiometric
exhaust but I suspect it would be similar. Just need mole fractions,
molecular weights, etc, and an engineering thermo text book. Also, the
exhaust constituents don’t vary all that much, contrary to what the
answer seems to imply.
David Watson
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