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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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