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In critical applications tires are inflated using pure nitrogen (link).

But carbon dioxide (CO2) is a larger molecule (will leak slower from the tire), as well as inert (will not oxidize rim and will not combust in case of an accident). (link)

Also, from the ideal gas law, pressure is proportional to the density, individual gas constant and temperature. Assuming constant volume (i.e. P ∝ R*T), any change in temperature is "amplified" by the individual gas constant. And the gas constant is in favor of CO2: R(CO2) < R(O2) < R(N2). (ref) So CO2 seems to give better pressure stability over temperature fluctuations.

Also, CO2 storage is cheaper than nitrogen storage.

So why tires are inflated with nitrogen and not CO2?

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    I think I've heard of CO2 cartridges being used for bicycle tyres, though they lose pressure quite fast. I doubt the validity of the small molecules leaking through the tyre theory. I'd say the main advantages of nitrogen over air are pressure stability over a larger range of temperatures and lack of corrosive properties.
    – I have no idea what I'm doing
    Commented Jul 4, 2016 at 14:01
  • If pressure is proportional to density as you suggest, then either hydrogen balloons aren't buoyant (because they're the same density as the surrounding air) or they collapse (because the pressure is lower than that of the surrounding air). Observation suggests otherwise.
    – user19347
    Commented Jul 4, 2016 at 19:02
  • Not just critical applications but the GT-R
    – cat
    Commented Jul 4, 2016 at 22:11
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    "well as inert (will not oxidize rim and will not combust in case of an accident)" - just make sure you don't tell magnesium that - it has a tendency to disagree. Mg will burn when surrounded by solid dry-ice, such is it's ability to grab oxygen, it can pull the O₂ out of CO₂ - Here's a sample video: youtube.com/watch?v=2oQ_9nFe9HU
    – enhzflep
    Commented Jul 4, 2016 at 22:28
  • The source that you cite suggests that water vapor present in "normal" air used for inflating tires is one of the big factors and that a larger molecule also helps. Beyond that, I would guess that the answer has to do with the cost of production of nitrogen vs. other reasonable gas choices.
    – dlu
    Commented Jul 6, 2016 at 7:44

5 Answers 5

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Nitrogen is inert and doesn't affect rubber or the steel rim. Carbon dioxide is highly reactive and affects both the rubber by causing swelling in the rubber. It also would cause corrosion in iron based rims (particularly Carbon- Steel).

Edit: When CO2 is mixed with moisture becomes Carbolic acid which is corrosive. It all depends on concentration. It is like comparing Acetic Acid commonly referred to as Vinegar. At 3% you can put on french fries. At industrial strength 97% not only will it dissolve the french fry but also human flesh right down to the bone very fast. As for CO2 affect on rubber I cited a industry source-(Air Liquide) a company that specialized in compressed gases.

In the case of automotive tires using CO2 would probably cause catastrophic failure. My experience with Nitrogen is that I have noticed it to be much more quieter while driving and less pressure variations when the temperature varies from season to season.

http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&GasID=26

Check under Material Compatibility under the heading of Elastomers- Quite illuminating.....

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    Highly reactive may be overstating it. It is no Cl2 etc.
    – Frames Catherine White
    Commented Jul 5, 2016 at 1:47
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    It is totally overstating it. CO₂ is not very reactive at all, it certainly doesn't corrode carbon steel. (In fact, steel is produced by reducing ore with carbon monoxide to Fe and CO₂!) If that article warns about swelling then there's surely a point to this, and it may well be a reason why it's not used for car tyres, but it's not like any rubber components immediately fail when you bring them in contact with CO₂. In fact, I think CO₂ is quite commonly used for bicycle tyres; perhaps the easier storage outweighs the worse properties here.
    – leftaroundabout
    Commented Jul 5, 2016 at 8:58
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    Carbon dioxide is so highly reactive it is even used in fire extinguishers.
    – Dmitry Grigoryev
    Commented Jul 5, 2016 at 11:05
  • Nitrogen is not inert, perhaps not very reactive with the materials involved, but it is not inert. It would be nice to have some citations, for example why would CO2 cause catastrophic failure – what component would fail, under what conditions?
    – dlu
    Commented Jul 6, 2016 at 7:38
  • I would expect that CO2 used for filling tires to be dry (free of water vapor) since the is one of the big reasons for not just using "normal" compressed air (the water changes state and the difference in volume between liquid water and water vapor results in pressure changes in the tires). This leads me to think that the creation of carbonic acid is not the reason for not using CO2 – although the affect on rubber might well be.
    – dlu
    Commented Jul 6, 2016 at 17:06
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Nitrogen makes up around 80% of Air - therefore its more readily reclaimed and separated than the smaller amounts of other gases in air. I.e. the process for reclaiming nitrogen from air could be less efficient than that of reclaiming CO2 from air and still be cost effective. Additionally its stability at higher temperatures means its behaves more predictably and means the tyres handling characteristics are consistent.

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  • so you're saying nitrogen for this purpose can be separated on-the-spot? i.e. no need to store it in liquid form?
    – Sparkler
    Commented Jul 4, 2016 at 14:35
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    You can create CO2 easily, you don't need to reclaim it. You cannot create nitrogen, but it should be easy to pull out of the air, I'd bet.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 4, 2016 at 14:49
  • Mauro, see my updated question regarding temperature stability.
    – Sparkler
    Commented Jul 4, 2016 at 14:50
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    You don't need to store nitrogen or CO2 in liquid form, just compressed. Nitrogen also has the advantage of being inert, meaning it won't react with anything. CO2 can combine with other molecules, for example water + CO2 makes carbonic acid, which isn't what you want in a tire. Probably small enough quantities not to matter, but that's just an example.
    – GdD
    Commented Jul 4, 2016 at 16:35
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    @dlu: N2 nitrogen is inert at normal temperatures.
    – Joshua
    Commented Nov 1, 2016 at 15:25
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Nitrogen is cheaper because it's readily available in the air, and can be extracted with a rather compact nitrogen generator. Extraction of carbon dioxide from the air is inefficient, so it has to be produced by burning fuels (such as methane) or via thermal decomposition of limestone.

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Nitrogen is a very predictable gas. All of its behaviors have been tested rigorously over many many years. As such they can predict the behavior much more easily than a mixed gas. Also, its performance is very similar to air. If you were unable to have access to a bottle of nitrogen, you could still use normal air and still be close to the original performance.

From a safety perspective, nitrogen contains no oxygen. If there were an accident there is no oxygen to propagate a fire (air contains ~21% oxygen).

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    This doesn't seem right. Nitrogen is a very predictable gas. All of its behaviors have been tested rigorously over many many years. The properties of CO2 are also known. As such they can predict the behavior much more easily than a mixed gas. CO2 isn't a mixture. *If there were an accident there is no oxygen to propagate a fire (air contains ~21% oxygen). * This answer seems to be comparing nitrogen with air, but the question is asking about nitrogen compared to CO2.
    – user19542
    Commented Jul 5, 2016 at 0:27
  • CO2 is a mixed gas, per se - one carbon, two oxygens. Although I don't see the oxygen in your tires being a huge fire hazard... Any more that the air we breathe
    – Cullub
    Commented Jul 5, 2016 at 20:23
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Primarily two reasons:

  1. Over 78% of air is comprised of N2, while less than 0.1% is CO2. So, we have a lot more nitrogen readily available.
  2. CO2 has a molar mass of 44g/mol, while N2 is significantly lighter at 28g/mol. Seeing as how everyone wants less unsprung mass, nitrogen is also the clear winner in this category as well -- those nitrogen tires will be about 36% lighter inside than the CO2 ones!

Of course, you can argue that since CO2 is a bigger and heavier molecule then it will leak more slowly from your tires (about 20% more slowly per Graham's Law!), but I believe that to be largely overshadowed by the two points mentioned above.

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  • "36% lighter" - the entire gas mass is under 100 grams so this argument is insignificant
    – Sparkler
    Commented Aug 13, 2020 at 14:11
  • Ahh yes, I meant the gas inside the tires will be lighter, not the "entire" tire itself! : )
    – ManRow
    Commented Aug 18, 2020 at 5:05

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