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NEOWatcher
2006-Jul-31, 07:30 PM
According to the guy in this article
Motorist turning to nitrogen to save gase (http://www.msnbc.msn.com/id/14121979/)
He's getting 5-10% better mileage in his pickup by filling the tires with nitrogen.
I can see the benefits of the tires staying fully inflated between air checks.
And, maybe I can see some slight differences in pressure changes due to temperature.
But; To what degree is this valid or not valid?

Swift
2006-Jul-31, 07:47 PM
There is a discussion about the topic in this engineering forum (http://www.eng-tips.com/viewthread.cfm?qid=120996&page=1). Two ideas about why it might do good is no oxygen (oxygen might oxidize the rubber and might diffuse out more) and moisture (which has a different heat capacity). The consensus seems to be that it is harmless bunk (it won't hurt your tires, but it won't do any good). The post from 20 June 2005 seems to cover the points best.

publius
2006-Jul-31, 08:09 PM
NEO,

Do a Google on nitrogen in tires. This question is complex, and I'm very skeptical that using pure nitrogen will be boost mileage, about *properly maintaining tire pressure with regular air*.

The claim that nitrogen reduces pressure variations with temperature has nothing to do with the pressure vs temp curves. Dry air and nitrogen are about the same, but moisture in air will skew the behavior, but it still doesn't make that much difference. Figure about 1psi per 10F. What is claimed is nitrogen's heat *transfer* characteristics are better, and so doesn't get as hot as with normal air. It will also improve tire life as there is no chemical attack from the oxygen.

Note that mileage improves with higher pressures. So a hotter tire should get better mileage, although only to a point. An overinflated tire will give you better mileage than one at rated pressure. But it does this by making a smaller footprint on the road, which reduces traction (all this applies to dry road -- with standing water, things get more complex -- and underinflated tire will flex rather than displace water, reducing total footprint, while a harder tire won't. Recommened pressure is a balance of all these various factors).

Overinflation will increase steering responsive a bit as well, and the ride will be stiffer.

What kills mileage is underinflation. If you pump up your tires hot, they will mostly likely be underinflated in the morning by a couple PSI or so.

Best thing to do is check your tires early in the morning about once a week, and overinflate them slightly if you want better mileage but don't overdo it. If it's going to rain, however, don't overinflate.

That will give you all the supposed benefits of nitrogen.


-Richard

pghnative
2006-Jul-31, 08:10 PM
I'm a bit befuddled by the claim that nitrogen is a bigger molecule than oxygen. Both are di-atomic, but N2 has a slightly lower MW than O2 (28 vs 32.

Is there an inorganic chemist out there who can explain this? Perhaps the electron cloud pattern in triple bonded N2 (N=N) somehow gives it an effectively larger radius than double bonded O2 (O=O).

George
2006-Jul-31, 08:21 PM
It sure looks bunky to me. However, there is a psychological element. If you use nitrogen, you will be attentive to your tire pressure. This attention will, likely, help maintain tire pressure; better mileage would be inevitable. The nitrogen would be a pneumatic (or would that be nitromatic?) placebo. :)


There is a discussion about the topic in this engineering forum (http://www.eng-tips.com/viewthread.cfm?qid=120996&page=1). Two ideas about why it might do good is no oxygen (oxygen might oxidize the rubber and might diffuse out more) and moisture (which has a different heat capacity).
The racing car post was interesting since the additional heat from racing causes concern that the moisture would expand and, possibly, unbalance the sensitive tire pressures. Reportedly, a change of 1/4 psi is a concern.

Since liquid water expands 1700x its normal volume (at 1 Atm), I can see why it would be an issue for them. It also tells me that those tires are high dollar since most rubber does not do well in the 200F range.

NEOWatcher
2006-Jul-31, 08:23 PM
There is a discussion about the topic in this engineering forum (http://www.eng-tips.com/viewthread.cfm?qid=120996&page=1). ...
Well that pretty much confirmed what I was thinking. For the anecdotal evidence...I wonder if by being more aware of the tire pressure, it's just properly inflated more often.

pghnative
2006-Jul-31, 08:44 PM
The racing car post was interesting since the additional heat from racing causes concern that the moisture would expand and, possibly, unbalance the sensitive tire pressures. Reportedly, a change of 1/4 psi is a concern.

Since liquid water expands 1700x its normal volume (at 1 Atm), I can see why it would be an issue for them. With the caveat that you'd normally expect the water in the tires to already be in the form of vapor.

I suppose you could have a problem if you filled your tires on a hot, humid day, and then had the water condense when the temperature cooled. But even then the difference would only be about 4% (assuming 90F, 90% humidity daytime, 60F nighttime -- won't actually get that cool if that humid, but no matter), or a little over 1 psi out of a 30 psi tire.

Would be cheaper to just use dry compressed air, which is surely less expensive than nitrogen.

publius
2006-Jul-31, 08:56 PM
All you need to do is put a dryer on your compressor's output. For small CFM requirements, these things are cheap and a good idea for your air tools as well. A regulator is a good thing to have as well.

One of my biggest complaints about tire shops is the lack of regulators. They'll have a big 2-stage compressor putting out 150-175psi, and some flunky will plug his air wrench into that unregulated and put that on your lug nuts. He hold the trigger until it hammers, greatly overtightening. Max pressure for most air tools is 90psi, and one should adjust the torque screw as needed with that. The flunkies don't bother and just go wide open.

If he's a real idiot, he won't tighten them in steps following the pattern, and will warp your wheels, even rotors. That happened to a '93 Crown Vic of mine when I had some new tires put on. The idiot warped my front rotors.

-Richard

snarkophilus
2006-Jul-31, 10:45 PM
I'm a bit befuddled by the claim that nitrogen is a bigger molecule than oxygen. Both are di-atomic, but N2 has a slightly lower MW than O2 (28 vs 32.

Is there an inorganic chemist out there who can explain this? Perhaps the electron cloud pattern in triple bonded N2 (N=N) somehow gives it an effectively larger radius than double bonded O2 (O=O).

Heh heh... well, it's the same number of electrons overall, but oxygen is more electronegative... but then they're covalently bonded, so that pushes things out a bit... they're likely very close to the same size, with nitrogen being slightly (like, at most 0.5 A) bigger. You can figure it out for sure if you want by filling a balloon with nitrogen, and another with oxygen, and seeing which goes flat faster.

But it doesn't matter. In terms of heat/pressure, they both behave exactly the same way. To first order, the size of the molecule is completely irrelevant when considering pressure: only the number of molecules matters. A bit of heat can be stored in the vibrational and rotational modes of the molecules, which is why water has a high heat capacity (many degrees of freedom), but nitrogen and oxygen, having similar energy levels, won't be very different from each other.

As to oxygen excaping, suppose you fill your tires with normal air. Then a bunch of the oxygen escapes, making them go a little flat. Now you have pure nitrogen in your tires. So you top them off with normal air. It's just a little oxygen now, which escapes. So you top them off again. It doesn't take many refills until your tires would be full of nitrogen anyway, if oxygen really did escape faster.

If in step 1 you had 0.2 oxygen in your tires (and 0.8 nitrogen), then at re-fill number 1 you'd have 0.04 oxygen, at 3 you'd have 0.008 oxygen, et cetera.

pghnative
2006-Aug-01, 02:15 PM
Heh heh... well, it's the same number of electrons overall...Nope -- O2 has 16 electrons, N2 only 14.

they're likely very close to the same size, with nitrogen being slightly (like, at most 0.5 A) bigger. why do you say that? Seems to me that O2 should be slightly bigger.

But it doesn't matter. In terms of heat/pressure, they both behave exactly the same way. To first order, the size of the molecule is completely irrelevant when considering pressure: Understood -- the "size" discussion was with respect to diffusion (leak) rates. With regard to diffusion, size matters.

snarkophilus
2006-Aug-01, 09:46 PM
Nope -- O2 has 16 electrons, N2 only 14.
why do you say that? Seems to me that O2 should be slightly bigger.

Heh heh sorry... stupid me, and thanks for catching that. That's some really basic chemistry. When I was composing the post, I even changed it to that from the right answer, too. :doh:

In addition to there being extra electrons, oxygen's two extra electrons are in different orbitals (unpaired), meaning we have two extra populated orbitals, not just the one. However, those orbitals are about the same size as the lower energy ones. The only difference is that they are mostly outside the nuclei, rather than between them. Although this pushes the nuclei close together, you still get a bit of extra electron density to the outside. That means that the oxygen molecule is a little bit longer than the nitrogen one.

But again, it's not much of a difference, because the increased electronegativity of oxygen pulls all the electrons closer (which is the point I meant to make when I wrote the last post).

trinitree88
2006-Aug-01, 11:04 PM
snippet .....Understood -- the "size" discussion was with respect to diffusion (leak) rates. With regard to diffusion, size matters.

pghnative. With regards to escape of a gas through a pinhole..(molecular effusion). At the same temperature, two gases of differing molecular weights have the same average kinetic energy...1/2 MV2. If they have differing masses, then Gas 1 = 1/2M1(V1)2

Gas 2 = 1/2M2(V2)2

setting the two kinetic energies equal yields Grahams Law of Effusion...diffusion. V1/ V2 = sqrt(m2/m1).
The relative velocity of escape is inversely proportional to the square root of the reciprocal of their masses. Mass matters more than size as the relative separation is roughly a factor of one thousand....and the velocity determines the frequency of hits per second. Pete

BioSci
2006-Aug-01, 11:39 PM
Permeability of a gas through a barrier (such as rubber) is actually a complicated process. It is determined by the diffusivity of the gas (size) and the solubility of the gas in the barrier or polymer:

See these two links for more information than needed:
http://www.cus.cam.ac.uk/~jae1001/teaching/materials/M6_Lecture_6.pdf
http://www.diffusion-polymers.com/Gas%20Diffusion.htm

N2 gas is a "larger" molecule than O2 and O2 is even much more permeable through rubber. This difference is interesting but unlikley to be of much significance for normal automotive uses. Simply checking and maintaining tire pressure (with air) will maintain nearly all benefits of N2.
Use of N2 may also reduce oxidation/aging of the interior of the tire - but such benefit is not likely to be significant for normal usage - the exterior of the tire seems to be more susceptible to such oxidation damage.

trinitree88
2006-Aug-02, 01:39 AM
[QUOTE=BioSci]Permeability of a gas through a barrier (such as rubber) is actually a complicated process. It is determined by the diffusivity of the gas (size) and the solubility of the gas in the barrier or polymer:

BioSci. The comment was for diffusion through a leak I thought...so Graham's law applies...which is for escape of a gas through a pinhole. It's experimentally determined from equimolar volumes of gas in latex balloons. You put a piece of scotch tape on the surface, insert a pin,remove the pin, and time the collapse from the pinhole....light gases escape faster...it's used in UF6 isotope separation . Oak Ridge, terra cotta plates, Fat Man...etc.Pete.

pghnative
2006-Aug-02, 05:51 PM
pghnative.
The relative velocity of escape is inversely proportional to the square root of the reciprocal of their masses.
aarrrggh:o

I knew that, I really did, I swear. Unfortunately my brain went right to the conclusion (lower MolecularWeight materials diffuse faster) and forgot the reasoning.

With regard to BioSci's comment, I can see that O2 may diffuse through rubber faster due to solubility, but I still question this:

N2 gas is a "larger" molecule than O2
Regardless, we are in agreement that O2 loss due to permeation is likely small compared to leakage past the rim seal and the valve seat. Leakage there is due to standard diffusion, which as you pointed out, is driven primarily by MW.

pghnative
2006-Aug-02, 05:54 PM
With regard to BioSci's comment, ...I still question this:
N2 gas is a "larger" molecule than O2

Hmmm -- I looked at BioSci's links, and N2 is listed as having a larger "Lennard-Jones Diameter" . I will need to investigate what that means.