Physico-chemical theory of tire inflation

Recently as suggestion came across my desk that there might be a benefit to inflating tires with pure nitrogen gas, as opposed to the compressed air that is normally used (at least for passenger cars and most commercial trucks.) After some research, I found out that some race-car drivers and all (or most) commercial airliners use nitrogen to inflate their tires.

A number of reasons were advanced to explain this:

1) In applications where the tires heat up, using N2 instead of air reduces the amount of oxygen available to fuel a fire in the event of a crash and tire blowout.

2) Compressed air contains small amounts of water vapor, which may condense at inflation pressure, and as tires heat and cool, this may result in unpredictable effetcs. (think of an airliner tires that are exposed to temperatiures at 10,000 meter altitude and then, upon landing, hit the ground at start spinning as theplane is going a couple hundred km/hr, and then braking very suddenly.)

3) Using Nitrogen in the tire prevents oxidation of the rubber in the tire carcass (that's a technical industry term, by the way), at least on the inside of the tire, anyway. This increases tire life, compared to the use of compressed air (which is essentially a 80% N2 - 20% O2 mix).

4) Using N2 in the tire prevents oxidation of the steel wheel rim (assuming you're using steel rims), thus preventing rust flakes that might insinuate their way into the valve seat and cause slow leaks.

5) N2 diffuses more slowly than air through the tire, thus allowing tires inflated with nitrogen to hold their inflation pressure longer than tires inflated with compressed air.

Because air is 80% N2 and the molecular mass of N2 and O2 are so similar (28 for N2 vs. 32 for O2 and 29 for air), I'm somehwat skeptical of these claims. I've tried googling on the subject, but didn't find the enlightenment for which ZI'm seeking. There was some stuff on membrane gas permeation, which reintroduced me to something called "Graham's Law of effusion," which then sent me to my 22-year old P-chem textbook to look it up. This didn't help, Graham's Law states that effusion (or diffusion) rates are inversely proportional to the molecular mass, which kind of messes up (5), because N2, having a lower molecular mass, would diffuse through the tire slightly faster than either oxygen or air. Now maybe if you inflated tires with CO2, which has a molecular mass of 44, you might see some reduced diffusion.

As far as (2) goes, from my college day recollection of hooking up compressed air to analytical instruments, it's possible to provide water-free compressed air, so why bother with nitrogen?

I couldn't come up with any theoretical reason or find anything in the engineering literature to evaluate any of the other supposed benefits of inflating tires with nitrogen instead of air. If anyone here can point me to some citable technical sources, I'd sure appreciate it.

Thanks in advance,

Joe

Reply to
Joe Bachman
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NASCAR racer use nitrogen supposedly because of the second reason you stated. On oval courses, the tire inflation pressure is a very important parameter. Teams adjust handling by changing the pressure in 1/2 lb increments. Anything that makes the pressure build-up (as the tire heats up) more consistent and predictable is considered beneficial. I'd love to conduct a double blind test to see if it really matters.

Ed

Joe Bachman wrote:

Reply to
C. E. White

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That's the best I've found, currently. The final page discusses using Nitrogen, and states that in general, there appear to be no benefits, except in special high load or high speed applications. Of cours,e this is all referring to truck tires, and doesn't mention Nascar or other racing tires at all.

My understanding , as far as Nascar goes, is that Nitrogen inflated tires are easier to predict pressures of while out on the track. And yes, this apparently has to do with moisture, however, as the article I linked to states, moisture is more related to your equipment than to the substance you fill your tire with.

Reply to
cbromley

Slightly off topic: bicycle racers will fill their tires with helium, but that is more of a weight issue than anything else. A lighter tire can be accelerated faster, especially if the lightness is at the rim.

John

Reply to
John Spevacek

How do they keep the pressure up? Helium has a bad habit of diffusing through rubber at a much higher rate than "air." Also, do bicycle racers use tubeless tires? All the ones I see still appear to have wire spoke wheels which probably precudes the use of tubless tires.

Ed

Reply to
C. E. White

This discussion has been around for a while, as any search of Usenet archives will indicate. IIRC, only the powered wheels on commercial aircraft have to be inflated with nitrogen.

That reason, along with corrosion of wheel, appear to be the main reasons for the commercial aircraft requirement. Motor racing also uses nitrogen, perhaps for the same reason, but also because many (all?) of their air tools are powered by nitrogen, not compressed air.

IIRC, powered wheels are already spinning. Air used fill automobile tires has usually be compressed to at least 50% higher pressure and the water allowed to condense from the receiver vessel. I suspect aircraft air may also have driers in the line before being used - but I haven't checked.

The potential for the water vapour to cause corrosion of the light metal alloys used in wheels is substantial, given the thermal cycling of the wheel and tire during normal use. The possibility

It's been noted that oxidation of tire carcasses adversely affects retreading, and the inner surfaces are going to be a lot hotter. IIRC, the recall of Firestone Wilderness tires was found to be due to oxidative damage to the rubber, which is indicated by exponential increase in cross-link density of the tire ( increase in cross-link density results in decrease of elongation at break property ).

The problem has manifested because of the much larger SUV vehicles impose higher loads on tires ( temperature cycling is greater, partly because the thicker tires facilitate heat buildup ), resulting in faster oxidation of the tire rubber. The recall of some tires was apparently because the real world oxidation was more extensive than testing had revealed.

One major cause of excessive internal heating is under-inflation of tires, as the deformation results in rapid heat buildup which then increases the oxidation.

It's been suggested that approval testing conditions of tires can result in anaerobic degradation, whilst real world degradation of tires ( on road wheels and as spare ) is oxidative. IIRC, a new testing regime and specification for tires used in USA is due in a few years, and they may be planning on using a 50/50 O2/N2 mixture to ensure testing changes to the tire are aerobic.

Given that the tires are often generously lubricated with water when they are installed on the wheel, it's unlikely the small amount of water in the compressed air is going to add much more. Don't know about US, but most new wheels are alloy, and steel wheels have a fairly good protective coating.

Permeability of oxygen through many common rubbers is around 1-4 times that of nitrogen. Obviously the gas mixture, water content, fillers and additives have an effect, as does the rubber type ( eg for butyl rubber air ( not dried ) =

45, nitrogen = 50, oxygen = 200; but for neoprene air = 300, nitrogen = 160, oxygen = 600 ). I don't have data for actual tires, but IIRC, some newer ones have a butyl-based inner layer. ).

Permeability is usually a combination of solubility and diffusion, and does have an effect on the ability of the tire to retain pressure over time. The effect isn't large, but in some applications, consistently is critical, so nitrogen is specified.

I suspect a search of the WWW will find data that shows nitrogen is beneficial, especially for some extreme applications, but for most cases the hassle to vehicle owners isn't worth the marginal benefits. That's especially likely for the new generation of tires that comply with the impending regulations.

Bruce Hamilton

Reply to
Bruce Hamilton

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For one thing there are a lot more commercial sources for dry nitrogen because there are many many uses for it. There's no reason dry air can't be made and compressed into a cylinder, but there's not enough demand that makes air more useful than nitrogen so in the end compressed air won't be any cheaper.

Joe Bachman wrote:

Reply to
AC/DCdude17

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