Tyre (Tire) Inflation with Nitrogen (N2)

Tyres(tire) usually inflated with compressed air and most of us doing it while fueling our vehicle at the fuel station. Recently, I went to a workshop for wheel alignment and to change new tyres for my vehicle. The workshop owner claimed that by inflating the tyres with nitrogen gas, we will have a more comfy ride and less need for tyers inflation. He also claimed it I can save a bit more on the fuel consumption.

See here what I've found on the net:

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.

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 convenience 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.

Filling your tyres with nitrogen mainly does two things: it eliminates moisture, and it replaces skinny oxygen molecules with fat nitrogen molecules, reducing the rate at which compressed gas diffuses through porous tyre walls. That means, theoretically at least, that a tire filled with nitrogen retains optimal pressure longer, leading to more uniform tyre wear and save the fuel consumption. The commonly quoted figure is that tyres inflated to 32 psi get 3.3 percent better mileage than at 24 psi.

Some benefit on using Nitrogen gas for a conclusion: 

-N2 is an inlet gas and dry, hence no moisture and minimize tyres & rim oxidation. 

-N2 insensitive to temperature and remain stable pressure/not expanding when hot. 

-N2 has bigger molecular size than O2, therefore air 'leaking' through tyre wall is much slower, good for those who not checking air pressure regularly.

-Tire runs cooler because N2 retains less heat

Overall, filling up with nitrogen won't hurt and may provide some minimal benefit

Search Amazon.com for nitrogen gas