Nitrogen inflation

nitrogenNitrogen inflation (nitrogen filled tyres) is one of those topics that gets discussed in car circles a lot. Some people swear by it, whilst others consider it to be an expensive rip off. So what's the big idea? Well there are two common theories on this.

Theory 1: nitrogen molecules are larger than oxygen molecules so they won't permeate through the rubber of the tyre like oxygen will, and thus you'll never lose pressure over time due to leakage. The fact is any gas will leak out of a tyre if its at a higher pressure than the ambient pressure outside. The only way to stop it is a non-gas-permeable membrane lining the inside of the tyre.
The science bit: Water is about half the size of either nitrogen or oxygen, so it might diffuse out of the tyre faster, but it would have to be much, much faster to make a difference. Tyres can leak 1-2 psi a month at the extreme end of the scale although it's not clear how much of that is by permeation through the rubber, and how much is through microscopic leaks of various sorts. For a racing tyre to lose significant water during its racing lifetime (maybe an hour or so for Formula 1), the permeation rate would have to be hundreds of times faster than oxygen or nitrogen, so that pretty much cancels out the idea that it's the molecule size that makes the difference.

Theory 2: Nitrogen means less water vapour. This is more to do with the thermal properties than anything else. Nitrogen is an inert gas; it doesn't combust or oxidise. The process used to compress nitrogen eliminates water vapor and that's the key to this particular theory. When a tyre heats up under normal use, any water vapour inside it also heats up which causes an increase in tyre pressure. By removing water vapor with a pure nitrogen fill, you're basically going to allow the tyre to stay at a more constant pressure irrespective of temperature over the life of the tyre. In other words, your tyre pressures won't change as you drive.
The science bit: The van der Waals gas equation provides a good estimate for comparing the expansions of oxygen and nitrogen to water. If you compare moist air (20°C, 80% RH) to nitrogen, you'll find that going up as far as 80°C results in the moist air increasing in pressure by about 0.01 psi less per litre volume than nitrogen. Moist air will increase in pressure by 7.253psi whereas nitrogen will increase in pressure by 7.263psi. Even humid air has only a small amount of water in it (about 2 mole % which means about 2% by volume), so that all puts a bit of a blunt tip on the theory that it's the differences in thermal expansion rates that give nitrogen an advantage. In fact it would seem to suggest that damp air is marginally better than nitrogen. Go figure.

So which option is right - smaller molecules, or less water vapour? It would seem neither. A reader of this site had a good thought on the whole nitrogen inflation thing. He wrote: Some racer who did not know the details of chemistry and physics thought that nitrogen would be better because (insert plausible but incorrect science here) and he started using nitrogen. He won some races and word got out that he was using nitrogen in his tires. Well, it is not expensive to use nitrogen in place of air, so pretty soon everyone was doing it. Hey, until I hear a reason that makes good scientific sense, this explanation seems just as good.

Nitrogen inflation is nothing new - the aerospace world has been doing it for years in aircraft tyres. Racing teams will also often use nitrogen inflation, but largely out of conveience rather than due to any specific performance benefit, which would tend to fit with the armchair science outlined above. Nitrogen is supplied in pressurised tanks, so no other equipment is needed to inflate the tyres - no compressors or generators or anything.

So does it make a difference to drivers in the real world? Well consider this; The air you breathe is already made up of 78% nitrogen. The composition is completed by 21% oxygen and tiny percentages of argon, carbon dioxide, neon, methane, helium, krypton, hydrogen and xenon. The kit that is used to generate nitrogen for road tyres typically only gets to about 95% purity. To get close to that in your tyres, you'd need to inflate and deflate them several times to purge any remaining oxygen and even then you're only likely to get about 90% pure nitrogen. So under ideal conditions, you're increasing the nitrogen content of the gas in the tyre from 78% to 90%. Given that nitrogen inflation from the average tyre workshop is a one-shot deal (no purging involved) you're more likely to be driving around with 80% pure nitrogen than 90%. That's a 2% difference from bog standard air. On top of that, nitrogen inflation doesn't make your tyres any less prone to damage from road debris and punctures and such. It doesn't make them any stronger, and if you need to top them up and use a regular garage air-line to do it, you've diluted whatever purity of nitrogen was in the tyres right there. For $30 a tyre for nitrogen inflation, do you think that's worth it? For all the alleged benefits of a nitrogen fill, you'd be far better off finding a tyre change place that has a vapour-elimination system in their air compressor. If they can pump up your tyres with dry air, you'll get about the same benefits as you would with a nitrogen inflation but for free.