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Aviation, Temperature vs. Density Question

  1. Sep 24, 2011 #1
    Hey guys, I'm an aircraft maintenance apprentice and I've been wondering about the air that is squeezed through a carburetor vs the carb heat air that is put through the carburetor to prevent icing. The air going through the venturi of the carburetor increases in velocity, lowers in density and is very cool. The hot air that is used for carb heat is very warm but is also low density. My question is how can they both be low density and vastly different temperatures, is it the velocity of the air that makes it cool or warm? Sorry if this is really basic kinda stuff, it's just been confusing me for a little while and I'd be grateful for someone to clear this up for me.
     
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  3. Sep 24, 2011 #2

    mathman

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    Obviously the hot air is being heated up, presumably by engine heat.
     
  4. Sep 24, 2011 #3
    The hot air is just ambient air that is warmed up inside a shroud around the muffler and then piped into the carb to remove ice. My question is just about both the hot and cold air being a lower density than the ambient air though. I've always understood that when something is cold it's more dense than the same thing when it is warm I.E. air. The only exception I've known of is water.
     
  5. Sep 24, 2011 #4

    mathman

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    Your understanding is correct up to the point where something is heated up.
     
  6. Sep 24, 2011 #5
    Alright, I apologize my knowledge is pretty limited in physics as you probably guessed, would you be able to offer me a brief explanation on this, or can you point me in the right direction to find an answer?
     
  7. Sep 24, 2011 #6

    rcgldr

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    The density is only changed by a tiny amount. The purpose of the venturi is to reduce the pressure on a carbureted engine to draw fuel. There may be a two stage venturi as well as a tube the fuel flows out of that creates a vortice to further reduce pressure and draw more fuel. There's a mixture control that the pilot controls that also affects fuel flow, but I'm not sure how it's implemented.

    The main effect of the heated air would be the amount of increase in pressure in the engine during combustion. Hotter air would gain less heat and the engine would not produce as much power, so the carburetor heater is only used to prevent icing in cold conditions.
     
  8. Sep 25, 2011 #7
    The density of air is a function of both its temperature AND its pressure. Keep temperature the same, and density increases with pressure. Keep pressure the same, and density decreases with temperature. Change them both, and the results are complex.

    When you take cold outside air and heat it in the shroud, you decrease its density. You may or may not increase its ambient pressure. This depends on the configuration of the heating system. When you pass that same cold outside air through a venturi, you increase its density and decrease its pressure on the throat of the venturi (the Bernoulli effect). This drop in pressure allows fuel to flow into the carb.
     
  9. Sep 25, 2011 #8

    rcgldr

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    I should have stated changed by a small amount. The temperature factor would be related to the relative change in Kelvins (equal to celsius + about 273).

    I'm not sure how much density or temperature is reduced when the presssure is reduced going through a venturi. How different are the temperatures on the Kelvin scale of the heated and non heated air? Assuming the pressure of the heated air is the same as the unheated air, then the density will be less by the ratio of the Kelvin temperatures (cold/hot), (and volume per unit mass would increase by hot/cold ratio) assuming air behaves like an ideal gas within that range of Kelvin temperature.
     
    Last edited: Sep 25, 2011
  10. Sep 25, 2011 #9
    Thanks Klimatos, that was what I was looking for. I believe my confusion may have come from my misunderstanding of density and pressure.
     
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