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How much does air expand with heat

  1. Oct 27, 2008 #1
    So I'm just wondering how much air expands with heat eg if you have 1000cc of air at 20C how many cc's will it be at say 40C is there a formula to work this out?

    Another question if you compressed 2000cc of air in to a 1000cc bottle what would the pressure be? my guess would be 2 bar 29psi is this right?

    thanks for the help
  2. jcsd
  3. Oct 27, 2008 #2


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    Welcome to pf, the formula is PV = nRT (n is the amount of gas and R is a constant)

    What this means is that pressure * volume is proportional to temperature.
    (The temperature has to be absolute ie. = celcius + 273)

    So if you go from 20->40 C, the pressure * volume increases by (40+273)/(20+273) or about 6%. So if you keep the pressure the same the 1000cc will become about 1060cc.
    And so you are correct - assuming the temperature is the same, if you half the volume you double the pressure.
    Last edited: Oct 27, 2008
  4. Oct 27, 2008 #3


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    You need to make assumptions about heat flow here. If you compress 2000 cc ino 1000 cc and then let the temperature equalize, you'll have 2 bar. If you don't let heat transfer, you'll end up with more pressure because the temperature will go up: you're doing work on the gas when you compress it.

    In any case: P1V1/T1=P2V2/T2
  5. Oct 28, 2008 #4
    Thanks so much for the warm welcome.

    The reason I want to know is because I'm thinking of building a cold intake system for my car and trying to work out if the effort is worth it.

    Ok this is going to be a bit of rough math.

    So (273+40)/(273+20) = 1.0689 so 6.89% increase.

    1 bar = 14.5 psi

    So 100/14.5 = 6.89

    So roughly dropping the intake temp 20C will have the same affect as adding 1 psi of boost.

    Is this close to correct? or have I missed some thing?

    thanks for the help :)
  6. Oct 28, 2008 #5
    Mechanical engineers have been chilling air intake to engines forever...cold air expands more upon combustion and boosts HP....
    apparently it's not very efficient or car manufacturers would be using air conditioning to chill the engine air intake and reduce fuel consumption.
  7. Oct 28, 2008 #6
    Well I put a temp sensor in the intake just after the filter and it reads an average of 53C and maxed out at 62C. Outside temp was about 26C. So a 20C drop does not seem like that much of an effort.
  8. Oct 28, 2008 #7


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    The difficult part is cooling the air without restricting the flow. The air only cools when it isin contact with something so simply cooling the outside of the intake isn't going to cool air in the middle. To cool all the air passing through you need something like a radiator - but putting a radiator or fins accross the air intakeis going to restrict the flow a lot.
  9. Oct 28, 2008 #8
    I was thinking for making a custom intake box and cutting a hole in the bonnet for a scoop so. air will get scooped in while driving along at normal temp then straight in to the engine.
  10. Oct 29, 2008 #9
    Depending on the rest of the design (carburetor or fuel injection, etc), you may end up doing more harm than good with respect to fuel mileage or power. If a simple hood scoop was "worth it" dont you think that the manufacturer would have also figured that out. On the other hand, I really like the idea that there are still guys out there who like this kind of tinkering and questioning and trying things out. So, go for it - and let us know what you find.
  11. Nov 16, 2008 #10
    Cooling the intake air will give you more power for a number of reasons. The general rule of thumb is that a ten degree F drop in air temp will gain you about 1% more power if I remember correctly.

    The main reason that manufacturers don't have more direct air intake has to do with noise restrictions. Go back far enough and you find quite a variety of cold air systems that also feed the aural senses.
  12. Feb 14, 2009 #11
    Hi there. I am researching exactly the same thing at the present time. I've gone past thinking about putting holes in the lid and have done it... I searched using google for ten minutes with the search ""how much does air expand due to temperature?"" as I've done all sorts over the last few months on a very limited budget and the most important thing for air intake is really air pressure at intake mouth (in my opinion - not being a scientist or mechanic of any note.)




    This thread is very interesting to me but it's late and I'm tired. Hopefully I will see you here again soon. (EDIT - Am in New Zealand, so time is relative)
    Last edited: Feb 14, 2009
  13. Feb 14, 2009 #12
    Have also made this...
    As you can see it is a Phenolic spacer of 8mm thickness that I designed to stop heat transfer from the alloy head to the alloy Broad Band Manifold (BBM) system. A BBM provides a long air path at low engine revs and uses a vacuum line to move butterflies and provide a short air path from throttle to combustion chamber at mid - high revs. It is all computer controlled from 3D tables, but you can see an example of it kicking in by looking closely at a dyno reading...
    This example is probably not the clearest, but you can see a fluctuation around the 4000rpm mark. It may drop or rise in revs depending on throttle position, load, etc.

    Where was I?

    Oh yes - I have been testing this spacer installation of mine for a while now and am happy to report that the highest temp reading from the middle of the throttle butterfly has dropped from the high 60's (degrees in Celsius) to the mid 30's. This astounded me, and it is simply too good to believe so I am now lurking around with pocket protector wearing Physics Dudes on the net rather than telling the skinned knuckle and tattoo brigade over at http://www.fordmods.com !

    Obviously temperature drops of this nature are going to be good for an internal combustion engine. I am actually on this site as a result of searching for air expansion due to temperature so I can be armed and ready for the disbelieving masses...

    It appears as tho there's a few here too...

    """If it worked then car manufacturers would have been doing it already""" etc etc etc.

    CAR MANUFACTURERS do not put scoops out their bonnets etc etc for a number of reasons. One of which is simply SALES. Middle aged families who buy cars don't really want scoops and holes in the bonnet. Simple!

    Virtually all serious race or performance cars (even from manufacturer) have such vents near front or mid point of bonnet and intakes further up the bonnet for Cold Air Induction (CAI) or intercoolers. As previous writer, above, states """you need fins to get a large contact area and this would impede airflow""". Yeah mate, quite right. That is why Forced Induction (Turbo, Supercharger) use an alloy "radiator" in the air supply as the air is heated by the process of forcing it into engine at 20PSI.

    Another reason car manufactures don't actually do things to full extent they could is cost of manufacture. There are very few people who will pay the thousands more per vehicle for the 10% jump in performance. Most people wouldn't even notice! Some people buy "racing cars" just for the stickers, I guess...

    The only real way to see examples of things done properly is to go to your local "production car" race days. Guarantee you most of these guys will have changed the CAI and exhaust outlets.

    Not all of them would have spent days or weeks researching their own vehicles properties first and simply copied their mates, and even fewer would have spent hours on https://www.physicsforums.com !

    I do not know what I am doing. I hadn't even changed a brake pad on a car until a year ago. People shouldn't just take stuff for granted but re-think, re-design and attempt to improve upon.

    "Hey, that apple fell on my head."
    "So what, they always do that. It's normal."
    "Oh. Right, fair enough. What's for lunch?"
  14. Feb 14, 2009 #13
    I had ambient of 21, temp at pod filter of 30 and throttle body at over 60 after a half hour run out the motorway with some spirited driving. With the alterations I have made to mine (see some attachments, above) it is now struggling to reach 40 degrees Celsius, and when it does it drops temperature a million trillion * times faster than it did before all these alterations.

    This should, in theory, equate to your 1psi boost argument (or 6% more bang for your buck).

    * actual mathematics not relevant
  15. Feb 14, 2009 #14
    I am merely a novice, but my guess is you would not use air con to reduce induction temperatures as the amount of energy air con uses would outweigh the potential energy saving by having cold air.
  16. Feb 14, 2009 #15
    Fiend, I fear you misread my post. I didn't say "if it worked the car manufacturers would do it." I was just trying to point out that there is more to improving performance than simply getting more air into the engine. You need to get more gas in, too, and get it all out, get it to burn completely and at the right time, and if you're building production vehicles, you have to do it within the sound level restrictions, blah blah blah,

    If you want to make your own car go faster, then by all means hang around the local tracks and see what the fast ones do to make their cars faster. And I guarantee the fast ones will tell you too - "its not as simple as just getting more air in..."

    If you surf around this site you can find many threads where some crackpot claims he can vastly improve the performance / gas mileage / etc of the production units by "simple" devices attached to the intake to add water / ionize the charge / whatever... Nothing is simple, and you will find most of the people on this site have little tolerance for crackpots. That's why you see the perhaps "prickly" reception such posts can get. That's not the same as a respectful appreciation of people who are modifying / tinkering with their machines.
  17. Feb 14, 2009 #16
    Mate, I think you've missed the point of mine (and I have re-read yours and there is no mention of other processes involved in combustion of fuels and conversion into Rear Wheel Kilowatts).

    I have, now you mention it, also put on some extraction pipes which work by increasing gas speed (velocity) and therefore helping the next exhaust cycle expel waste gases... I have also done things to the drive train. I have also put in slightly bigger injectors.

    I am getting a larger throttle body, arguable how this will work, but at high revs I expect improvement. I am also getting a different cam and have a head off another motor to shave a little off and up the compression, along with opening up the exhaust ports slightly.

    As the EEC-V system does things such as adjust spark timing I am also aiming to put in a programmable chip and purchase the programmer required to make alterations to tables controlling fuel, air, ignition timing.

    But, by far the most important thing of all --- Installing the laptop with blue tooth remote control for playing media through the roof mounted 15" and in dash 7" monitors. That will be the day that I can simply forget about efficient combustion and sit in the car in a car park rather than bother driving it.

    Good on you for replying though. I was simply talking here about Cold Air Induction and how much effect it would have on the air supply. Thanks for taking the time to view my ramblings.

  18. Feb 14, 2009 #17
    The good news is, while the police can put speed limits on the roads, they haven't yet thought of "acceleration limits." Leaving the field open to hood scoops, cams, pipes, gear ratios, ...

    Have fun !
  19. Feb 14, 2009 #18
    Am not a big fan of scoops as such, but the holes in my bonnet work very well indeed at letting heated air out of the engine bay. Has anyone else ever wondered why all the waste air from the radiators and oil cooling systems is expelled straight into an engine bay (and more often than not - directly onto an area affecting the air intake temperature? Now, that is something that manufacturers should do something about.

    Or sponsor this site and get everyone working on "alternative" power sources.

    Thanks for the thoughts once again dude... I will, no doubt, be back to test some hair brained theory. Thing with my theories is that they'd damn well better work - I don't have enough money to produce duplicates for proper experimentation, so I ask everyone to look at everything from every angle.

    I then don't listen and do it anyway.

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