Just heard from a car service guy that some electric cars specify N in the tyres… nobody knows why.
fill car tyres with strawberry-flavoured air for an extra charge. Quote
I can see some merit in that – if you think that you could be the person having to get a whiff of the air the next time the tire is being deflated!
Pilot_DAR wrote:
As for filling civilian tires, I opine that the use of nitrogen over air is maintenance elitism, with no benefit other than a basis for bragging.
I heard of one enterprising serviceman who was offering to fill car tyres with strawberry-flavoured air for an extra charge.
Pilot_DAR wrote:
As for filling civilian tires, I opine that the use of nitrogen over air is maintenance elitism
I lately took a car to a little race track. The tires of this car had been filled with air. As the car can display tire pressures it took about 15 minutes with 2 tonnes, around 600 horsepower and huge ceramic disks to increase the tire pressure by about 0,5 bar. At this point the heat from the wheels was so extreme I has to slow down and drive many kilometers on the street to get everything back to normal. I could not measure the exact temeratures as they did exceed the maximum of what my laser device was able to measure.
So I doubt any small GA plane will ever get to the point where the expansion might matter. The tiny steel brake disks are a joke and will melt long before.
Peter wrote:
And is it physically possible to ignite oil or grease purely by compression in air
Just oil or grease in bulk or on a metal or rubber surface at room temperature, extremely unlikely. Much more likely in pure oxygen (hence the prohibition of lubricants in oxygen systems, though actually only some lubricants are susceptible to that) or with oily rags (a lot more exposed surface per unit weight + a lot less heat dissipation; they first heat slowly through oxidation and only then flash up).
From my experience maintaining Cessna struts, I did not ever encounter a circumstance where there was any defect, damage nor operational difference between charging with bottled nitrogen rather than dry air. There are so many paths to a problem with a GA airplane strut, that by the time you find a defect which could possibly have originated with the difference between nitrogen and air, you’ll probably have destroyed the strut anyway!
I’m pretty confident in observing that nitrogen is used to charged the struts of larger planes because it is much more commonly available at the required high pressure. Scuba and SCBA bottles aside, it is much less common that “industry” has compressed air available at pressures exceeding what a shop compressor can produce.
As for filling civilian tires, I opine that the use of nitrogen over air is maintenance elitism, with no benefit other than a basis for bragging. If heat transfer within a static volume of compressed gas is a factor, you’re working into a really tiny performance corner! But I don’t credit this factor as being at all important in a tire in a moving vehicle, in which the gas will be sloshing and changing local pressure within the wheel constantly. Piston engine compression/combustion does suffer these factors (detonation) if something is wrong – but the shape of the compressed chamber of gas & vapour is changed by only piston movement. Inside a tire of a moving car, only the inside of the rim is not constantly changing shape!
I use compressed air.
Compressors used to fill gas cylinders are equipped with moisture traps.
Indeed, but there is still a fair bit of water in the air, AIUI, whereas cryogenics produces a gas with negligible water levels. That said, I got defeated by google because there are many different purity levels. But for sure you can get below 100ppm total impurities.
Anecdotally, scuba cylinders can show internal rust.
Whether this actually matters I don’t know, because homebuilders routinely use diving air not only for tyres (like we all do, below the jet level) but also for gas struts.
And is it physically possible to ignite oil or grease purely by compression in air? Gas struts are either all gas or they contain Fluid 41 (hydraulic fluid).
Apparently you can do it with residual hydrogen in the air but that takes a lot more work 
I don’t have a degree in physics but I do have one in electronics, and since that was my major and I got 100% in everything, they could not chuck me out despite failing every maths exam in the 3 years 
gallois wrote:
Nitrogen is believed to more follow the rules of thermodynamics in that the heat from the area closest to the heat source transfers itself to the next layer without convection and with even changes in pressure.
I am sorry, but having a degree in physics, I feel compelled to declare this statement to be nonsense. The only piece of truth in the argumentation for nitrogen is that it does not support combustion (and isn’t combustible itself, of course). It is also true that compressed nitrogen from a cylinder is dry compared to ambient air, but it has nothing to do with it being nitrogen, it could well be air or any other gas.
Stable in the way that if you put air in a sealed box and then heat the bottom of it you will ceeate a convective flow.
Nitrogen is believed to more follow the rules of thermodynamics in that the heat from the area closest to the heat source transfers itself to the next layer without convection and with even changes in pressure. So the process by which the wheel hub is heated is slower and more spread around the wheel rather than producing hot spots which would weaken the tyre in patches and be more likely to burst.
How much of this is true but it is my understanding of an explanation I received from a Pirelli tyre technician who was part of the crew that provide the tyres for the formula 1 teams.
Peter wrote:
AIUI the reason for nitrogen in gas struts is to avoid getting water in there, which is one advantage of cryogenic distillation used to manufacture most industrial gases.
Nitrogen in gas struts is used to avoid possibility for oil to combust. Water is more problematic in systems where compressed gas expands like oxygen regulators.
