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Thread: Gas

  1. #1 Gas 
    Forum Masters Degree thyristor's Avatar
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    There's something I don't really understand. When it's cold out side you can often smell the gases from the cars but since they're so hot I think they would rise high up in the air very fast due to their low density. I mean, there's no chance that they will be cooled down so fast.


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  3. #2  
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    The exhaust from cars is affected much more by air currents than by convection.


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    Forum Masters Degree bit4bit's Avatar
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    Not to mention that a car is constantly emitting a stream of gasses from the exhaust, so even on a still day, the exhaust gasses may rise as they come out, but they are in constant supply from car, and constantly present if you are in the vacinity of any vehicles. If you turned the car off, and waited a minute, most of the gasses would have risen.
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  5. #4  
    Forum Junior Zitterbewegung's Avatar
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    Come to think about it there actually might be an effect but this is not directly related to the temperature of the exhaust. But on cold days the engine controll injects more fuel in the combustion chamber i.e the mix is running on "fat". This is done so the engine does not sputter when you hit the gas. Downside: more unburned fuel is released via the exhaust. Thus yo usmell it more intense than on a warm day when the fuel-air mix is closer to eta=1 (stoichiometric mix of fuel and air) when almost all of the fuel is burned.
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    I have been reading this thread just wondering where it would go. Too many variables to know for sure. So far I like currents, or wind direction.

    Atmospheric conditions have many components besides cold. If it is calm then an inversion can form close to the surface and trap fumes near the surface.

    Vincent
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  7. #6  
    Universal Mind John Galt's Avatar
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    Why is everyone assuming that the density of hot nitrous oxides is lower than that of a cool nitrogenxygen mix?
    Why has everyone forgotten that emitted gases will not segregate in the atmosphere, but will mix in the atmosphere?
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  8. #7  
    Forum Masters Degree thyristor's Avatar
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    But if the would have higher density then the possibility that it would rise would be increased if the air is cold and thus have higher density.
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  9. #8  
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    Ophiolite, your two statements are contradictory. We agree with your second statement that gaseous components to not segregate themselves in an air current. That makes the density of its components totally irrelevant.




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  10. #9  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by SteveF
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    Ophiolite, your two statements are contradictory. We agree with your second statement that gaseous components to not segregate themselves in an air current. That makes the density of its components totally irrelevant.*
    Steve, your typographical errors have made your reply ambiguous. I think you are saying that, yes, the exhaust fumes do mix in the atmosphere and therefore their density, relative to the air, is irrelevant.

    However, they take time to mix. Moreover in still air conditions some density stratification may occur - hence the LA smog of today and the London fog of yesterday. So while the mechanisms are in opposition, the statements are not - I think - contradictory.
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  11. #10  
    Forum Junior Zitterbewegung's Avatar
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    ummmmmm.......did anyone bother to read my above post. I double checked with my mechanic and he agrees. The smell you notice has nothing to do with density or temperature of the exhaust gasses but rather with the fuel-air mixture the engine is running on when it is colder outside.

    Enriched fuel mix when it is cold --> more unburned fuel through the exhaust pipe --> smell
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  12. #11  
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    Quote Originally Posted by Zitterbewegung
    ummmmmm.......did anyone bother to read my above post. I double checked with my mechanic and he agrees. The smell you notice has nothing to do with density or temperature of the exhaust gasses but rather with the fuel-air mixture the engine is running on when it is colder outside.
    Yes, I read it. I disliked your absolute 'nothing to do with' statement. Had you rephrased that to read 'may also be influenced by' I think we could have ended the thread quite nicely here.
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  13. #12  
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    Quote Originally Posted by Zitterbewegung
    ummmmmm.......did anyone bother to read my above post. I double checked with my mechanic and he agrees. The smell you notice has nothing to do with density or temperature of the exhaust gasses but rather with the fuel-air mixture the engine is running on when it is colder outside.

    Enriched fuel mix when it is cold --> more unburned fuel through the exhaust pipe --> smell
    One other reason might be that on a cold day, the gases may condense to a liquid vapor, giving off heat to the atmosphere. Right at the outlet. They could get very cold, and vapor ridden, by this process, and lay low.

    At the same time I will admit, because of the dry air, lack of humidity, you have the liquids, as they mix with more air, evaporating, taking heat from the atmosphere, and becoming a gas again. As they absorb heat, they do cool the area. The gases do not just go straight up. It forms a layer of dense gases and or ice particles, that lay low.

    We have experienced this. On the job. It is the refrigeration effect. As gasses expand rapidly they cool instantly. These cooled gases and ice particles might very well, lay near the ground.

    On a very cold day the liquids that still exist in the exhaust are instantly evaporated or instantly frozen. Either way it causes the gases to lay low.

    You can also on very, very cold days look at this effect. It is amazing to see over a warm river near a city. An almost frozen white emission is given off by the river that does not rise and dissipate as you would imagine it would. It actually just lays on the river. It is amazing to see. A must see.

    I did like the choke, causing more gas to be pumped into the engine, that is a good one. Excellent hands on observation.

    Sincerely,


    William McCormick
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