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Air Density in ICE

  1. Feb 19, 2007 #1
    How would you estimate the charge density (air actually entering the cylinder) in an ICE? Assume the outside is standard pressure and temperature of 15*C (60* F) and 101.3kPa (14.7psi). At high airflow speeds (Wide Open Throttle), pressure in the manifold should approximate atmospheric conditions. There is also less time for heat soak while the air travels to the cylinder.

    What about a lower throttle (air speeds)? Assume the manifold pressure is 50kPa, but we keep the temperature constant. Is the density really half of the atmospheric conditions (using 2.7 * P / T ; P in psi and T in *R)?
  2. jcsd
  3. Feb 20, 2007 #2
    Anyone have a thought? :)
  4. Mar 1, 2007 #3
    Nobody. :frown:

    Let me ask the question this way: :)

    What if I had a long tube, and I was pushing/pulling air through this tube. Assume that there are signifcant temperature differences along the tube:

    100* -20* 50*

    -> -> -> Airflow -> -> ->


    If I could "capture" part of the air at the temperature points above, is the density different at each point? Assume that a fan is pushing air through the tube at a rate of 200cfm and the air entering is 60*F and 101.3kPa.
  5. Mar 5, 2007 #4
    What is ICE?
  6. Mar 5, 2007 #5
    Internal Combustion Engine

    Is that why no one has responded? Where have all the IC engine gurus gone? :)

    Just curious about thoughts on the effects of heat and pressure relating to airflow into an engine. Since the engine is not static nor a 'closed system'(I am sure that I am not using that term correctly. e.g. like a balloon filled with air; heat it, it expands, same mass but now more volume, thus less dense), I was curious about the effects of heat (and pressure) on the air entering the engine. The intake manifold, intake ports, etc. can't expand like the balloon, but the air can flow backwards through the system. i.e. back out through the TB the wrong direction. As air enters the engine and is heated, does the air 'expand', does this alter (slow) the airflow rate, etc.
  7. Mar 10, 2007 #6


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    Science Advisor
    Gold Member

    Yes, as the air picks up heat during the induction process, it expands, and you get less charge into the cylinders. This is exactly why air inlets on performance cars are sited to ensure the source is as cold as possible, why intercoolers work so well, and why your car seems to have better performance on a cold day.

    I'd not done any replying cos I've just bought a house and been moving in!
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