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Water vapor from hot/cold exhaust?

Water vapor from hot/cold exhaust?

Water vapor from hot/cold exhaust?

When we start the engine on a cold day (below freezing point) there is condensed water vapor (mist) coming from the exhaust. When the engine and the exhaust get hot there is no more water vapor mist coming from the exhaust. Why? When moist hot gases exit the hot exhaust they enter very cold air and water vapor starts to condense, isn't it? Why we don't see it? We see it while it comes out from cold exhaust?

RE: Water vapor from hot/cold exhaust?

Exhaust gases contain water but are not pure steam. If you cool undiluted exhaust enough, the water will condense and be visible as mist. Hot exhaust gas can mix with air and be diluted faster than it cools so that the dew point of the exhaust is never reached. That is, the dew point of the exhaust falls faster than the temperature when mixing with air, if the temperature starts high enough. Dew point is a function of water concentration and temperature.

RE: Water vapor from hot/cold exhaust?

For every pound of fuel burned, you get about a pound and a third of water, as a combustion product, in the exhaust gas.  No way to prevent it; it's right there in the chemical equation.  You only see it when the engine is cold, or the gas flows over cold surfaces or into cold air, where the water becomes liquid.


Mike Halloran
Pembroke Pines, FL, USA

RE: Water vapor from hot/cold exhaust?

Compositepro, thanks for great post!
Let me see if I understand, if we don't want to see water vapor mist we should have as hot gas as we can (we can't influence gas humidity) when it is leaving tailpipe, so that it has enough time to mix with air (so the relative humidity of mixture gas/air drops) before it is cooled down to dew point.

Does someone know how much water vapor content is in gasoline exhaust gases and how much in diesel exhaust gases when idle? If diesel and gasoline fuels have almost equal hydrogen content, and on idle diesel engine uses about 3 times more air than gasoline engine (1:45 in diesel, 1:15 in gasoline engine) and if exhaust temperatures are similar, than diesel should have 3 times lower relative humidity of exhaust gas than gasoline engine. Because H2O production is almost the same if the amount of injected fuel is the same and there is about 3 times more air mass in diesel engine. Is this theory correct? Because I noticed that gasoline cars make more water vapor mist compared to diesel cars!

RE: Water vapor from hot/cold exhaust?

Your theory is pretty good but there are some errors in your assumptions.  Diesel fuel has heavier hydrocarbon atoms so there are fewer hydrogen atoms per gallon.  Diesel exhaust is cooler at idle.  Diesel also uses less fuel at idle.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Water vapor from hot/cold exhaust?

Yes you are correct. Diesel contains less hydrogen than gasoline and the air/fuel ratio is greater so the absolute humidity of the exhaust gas is lower.

RE: Water vapor from hot/cold exhaust?

At the other end of the spectrum are methanol and natural gas fuelled engines.  These engines, during warmup operatation, can tax the moisture handling ability of exhaust systems that were designed for a different fuel.

RE: Water vapor from hot/cold exhaust?


Here are some fuel properties where we can see that in the same mass of fuel diesel and gasoline have very similar amount of hydrogen. As we can see in the same mass gasoline can even contain little less hydrogen than diesel!? Only natural gas has higher amount of hydrogen.

RE: Water vapor from hot/cold exhaust?

It depends on the amount of aromatics present.

From the point of view of the aliphatics present, the higher the molecular weight, the lower the hydrogen to carbon ratio. The two end carbons have 3 hydrogens, but all the middle ones have 2 hydrogens. Diesel is based on higher molecular weight hydrocarbons.

Since the swing away from lead, greater amounts of aromatics are used as octane boosters. Due to the double bonds between some carbons in aromatics, the hydrogen ratio drops.


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