Water vapor from hot/cold exhaust?
Water vapor from hot/cold exhaust?
(OP)
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?
RE: Water vapor from hot/cold exhaust?
Mike Halloran
Pembroke Pines, FL, USA
RE: Water vapor from hot/cold exhaust?
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?
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RE: Water vapor from hot/cold exhaust?
RE: Water vapor from hot/cold exhaust?
RE: Water vapor from hot/cold exhaust?
I think you meant heavier hydrocarbon molecules, not atoms. ie there are more carbons with only 2 hydrogens between the two carbons with 3 hydrogens.
Regards
Pat
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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?
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.
Regards
Pat
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