Filling Your Tires With Nitrogen

[AirFarce1]

In Cecil’s column **Should I Put Nitrogen In My Tires? ** he talks about the temperature not getting high enough to make a difference. I tend to disagree. Even in common automobiles, the temperature differential between at rest tires and a vehicle that has been on the road for an hour or so make a difference in handling, so much so that tire are required by the NTHSA to mark tires with a rating. I am not advocating nitrogen in tires for everyone, but it might be a good idea if you commonly drive a vehicle in environments where there is a great difference in temperature
between at rest tires and warmed up tires.
Link to original article: http://www.straightdope.com/columns/070216.html

[Machine_Elf]

@AirFarce1

In Cecil’s column **I am not advocating nitrogen in tires for everyone, but it might be a good idea if you commonly drive a vehicle in environments where there is a great difference in temperature
between at rest tires and warmed up tires.
Link to original article: http://www.straightdope.com/columns/070216.html

In motorcycling, we typically shoot for a cold tire pressure such that after an extended period of riding, the tire pressure increases by about ten percent - implying that the absolute temperature also increases by about ten percent, or goes from say 70F to 120F (this is off a bit, since we need to use absolute instead of gage pressure, but it’s a reasonable rule of thumb).

Depending on how much water you’ve got in your tires, it can make a notable difference in final pressure for a given temperature rise, but if you’re comparing dry nitrogen with dry air, the difference ain’t squat.

Last fall I did a comprehensive analysis for a BMWSportTouring website I moderate. The UBB-marked-up text doesn’t display well here, so rather than copy and paste, I have to just give you a link to the post; click here to see the whole thing, complete with pretty plots that shows what happens to different gas mixtures over a range of temperatures.

[zut]

A couple earlier GQ threads on this very subject:

Is filling tires with nitrogen a gimmick?
nitrogen-inflated tyres?

Also, despite Cecil’s characterization of oxygen and nitrogen molecules as “skinny” and “fat,” the size difference isn’t all that much, and doesn’t really explain the permeation rate difference between oxygen and nitrogen.

Nice analysis, Joe.

[matt_mcl]

The tires used on the Montreal metro are filled with nitrogen for the reason Cecil mentioned - to prevent combustion.

[WarmNPrickly]

Also, despite Cecil’s characterization of oxygen and nitrogen molecules as “skinny” and “fat,” the size difference isn’t all that much, and doesn’t really explain the permeation rate difference between oxygen and nitrogen.

Without looking at any actual numbers, I was fairly suprised to find that oxygen was the smaller molecule. N2 has a strong triple bond that ought to hold the two atoms together tightly while O2 merely has a puny single bond (its a diradical) that ought to make the distance between the two atoms larger.

[John_W.Kennedy]

But the oxygen atom is smaller, and apparently binds more tightly. (In general, atoms get smaller going from left to right in the periodic table.)

[WarmNPrickly]

Yes, an oxygen atom is smaller, but both of these molecules are diatomic. There is no way, that oxygen binds to itself more tightly than nitrogen does to itself. Oxygen is a single bond. Nitrogen is a triple bond.

[E.Lefraw]

The greater the bond order (Single, double or triple), The shorter and stronger the bond.

[WarmNPrickly]

Therefore nitrogen (triple bond) has the shortest bond.

[Una_Persson]

As it happens, I got into an argument with someone over this a while ago. I did some Googling to find the links I looked up then, and they might be helpful here. I’m finding a few places talking about nitrogen having a larger kinetic diameter, which is critical for diffusion through media.

A process which relies on the smaller kinetic diameter of oxygen.

“PSA” stands for Pressure Swing Adsorption. This is the separation technology which separates Nitrogen and Oxygen Molecules from rich compressed air. Commercial nitrogen production process usually employ Carbon Molecular Sieves as adsorbents. These process depends on kinetic separation accomplished by a faster diffusion of oxygen molecules through the pore structure of the adsorbent. The Oxygen is adsorbed to its equilibrium value much faster than Nitrogen. This is because Oxygen which is smaller in size (kinetic diameter 3.46 A.o) diffuses much faster in Carbon Molecular Sieves than Nitrogen (kinetic diameter 3.64 A.o).

Here’s a less authoritative note, but it prompted me to do some other searching, and I can’t find much wrong with the summary: http://www.physorg.com/news73584032.html

Like many chemists and engineers that don’t deal with gas molecular size on a regular basis would probably presume, I mistakenly reasoned that this was hogwash because an O2 molecule should be larger than a N2 molecule based simply on molecular weight. But that is not the case…I learned quite a bit from just a couple minutes of Googling the topic.

The kinetic diameter of N2 is, in fact, larger than O2 (3.8 nm versus 3.64 nm!) That is why carbon based mol sieves can preferentially remove O2 from an air stream, and why membrane based units (such as those made by Permea) work as well as they do. I had also mistakenly presumed that the N2 permeated the membrane preferentially, but what are referred to as the “fast gases” (O2, CO2, H2O) all permeate the membrane faster than does N2. The N2 remains on the high pressure side.

And since pressure loss from a tire would involve diffusion through a polymer matrix–functionally the same as a membrane–it seems that N2 may very well not leak as fast as does O2.

This US Patent relies on a claim that nitrogen will not pass through a space as easily as oxygen: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4,144,196.PN.&OS=PN/4,144,196&RS=PN/4,144,196

The minimum kinetic diameter of a water molecule has been reported at 2.65A and the minumum kinetic diameters of oxygen and nitrogen molecules, respectively, at 3.46 and 3.64A.

Just something to consider - I think Cecil got it right. IIRC, there’s an article out there by a tire maker or the NHTSA about this too, but damned if I can find it again.

[Terminus_Est]

Note that O[sub]2[/sub] has a double bond. Remember H[sub]2[/sub]O?

[Enola_Straight]

I was under the impression filling with nitrogen has two possible benefits:

Since nitrogen is relatively inert, oxidation would not degrade the interior of the tire, and, since the gas is dry, no liquid water would enter the tire…air compressors can let moisture condense in the pumping operation. Cold winter months would freeze inside the tire, really throwing the wheel balance off.

[WarmNPrickly]

Note that O2 has a double bond. Remember H2O?

Remember, O2 is a ground state triplet. Single bond. Diradical.

[WarmNPrickly]

Just something to consider - I think Cecil got it right.

I have no doubt Cecil got this right. I am merely sating my suprise at the result based on what I know. I wonder if oxygen is smaller in all dimensions. I still expect that O2 (really its a single bond) is longer than N2. I don’t know anything about kinetic diameter which must be what is important here.

[Terminus_Est]

@Christopher

Remember, O2 is a ground state triplet. Single bond. Diradical.

Cite?

[WarmNPrickly]

I am actually corrected here. Oxygen does have a bond order of two, but it is still a ground state triplet. Lewis dot structures fail this molecule.

Cite

look towards the bottom.

Wiki

[WarmNPrickly]

Ironically, if I had to draw the lewis structure of O2 now, I would draw it as a triple bond (maybe I would dash the third bond) with a radical and a lone pair on each oxygen. This makes much more sense when considering the smaller size of the molecule.

[Common_Tater]

@Enola Straight

I was under the impression filling with nitrogen has two possible benefits:

Since nitrogen is relatively inert, oxidation would not degrade the interior of the tire, and, since the gas is dry, no liquid water would enter the tire…air compressors can let moisture condense in the pumping operation. Cold winter months would freeze inside the tire, really throwing the wheel balance off.

Yes, this has to be one of the primary reasons that nitrogen is used in aviation and aerospace application. It is a known quantity, inert - and completely “dry”. Consider the rolling speed of an airplane on landing or takeoff - and a couple pounds of ice inside the tire thumping around and you get the idea. It’s just not good practice to have all that water sloshing around in there and anyone who has changed an automobile tire can attest they build up astonishing quantities.

[Jinx]

@Christopher

Without looking at any actual numbers, I was fairly suprised to find that oxygen was the smaller molecule. N2 has a strong triple bond that ought to hold the two atoms together tightly while O2 merely has a puny single bond (its a diradical) that ought to make the distance between the two atoms larger.

This is all very confusing on three accounts on Nitrogen, plus one for H2O:

1. Cecil claims oxygen is smaller than nitrogen; therefore, it diffuses more readily. Yet, O is #8 on the Periodic Table while N is #7; hence, intuition says nitrogen is smaller. But, I recognize both are diatomic in nature, so we go on to #2 below.

2. You say “Without looking at actual numbers…N2 has a strong triple bond that ought to hold the two atoms together tightly”. Well, does it or doesn’t it? Could it simply be N2 smaller because N is smaller to start??? Or, are you trusting Cecil is correct and speculating about the bonds to justify his logic?

3. Air is 78% Nitrogen. So, if N2 is really smaller, maybe it really is the N2 diffusing out of my air-inflated more quickly than the O2, afterall? Then, the N2 deal is all a hoax sponsered by the National N2 Association, right? (Helium will be next to improve gas mileage, right? )

4. Water vapor: Why is N2 any drier? Wouldn’t H20 still evaporate into N2? Regardless, all compressors (well, all compressor designs I know), have an in-line receiver/drier to knock out the moisture when compressing any gas. So, this issue Cecil mentions is a moot point for tire inflation, right?

Dazed and confused,

• Jinx

[Jinx]

The editing time on my previous post (above) expired, but I will be man enough to admit that I see my points 1-3 (in my post above) were already discussed in greater detail within this thread (although hybrid cloud models and antibonding blow my mind). But! I still hold argument #4 that water vapor in a compressed air line, for one, is always undesirable. Thus, water vapor is removed by a dessicant drier device, for one.

Dazed and confused,

• Jinx