Does this actually work? Noticed Kwik Fit are saying nitrogen extends tyre life by 25% and fuel economy by 10%. They are chargin £1.50 per tyre to inflate it with nitrogen.
I remember when I was a Liney in the RAF, we used nitrogen for aircraft tyres, but that was to blow out any flames from when the brakes used to explode and rip through the tyres.
They're probably careful not to actually say that, because without any qualification it's a lie. The improvements are comparing correctly inflated tyres with underinflated ones, and it's entirely possible to do that with boring normal air.
Compressed nitrogen is _really_ cheap, being essentially a byproduct of the oxygen business. They'd love you to use it.
Sensible application there. Fortunately car brakes tend not to explode like that :-)
Camdor gurgled happily, sounding much like they were saying:
To a certain degree, it stabilises pressures across wide heat ranges, which is why it's used in aviation. F1 teams tend to use neat CO2 for the same reason.
They also claim that the larger N2 molecules can't get through rubber by osmosis, but since the tyres on my cars rarely loose even a single psi from being fitted to being replaced, that's of dubious merit.
But if you bear in mind that "raw" air contains somewhere just shy of 80% Nitrogen, you might get a hint of the - umm - applicability of transferring those claims to road cars.
If you read the small print, they'll probably actually be saying that having properly inflated tyres does that - which is blindingly obvious.
Mmm. Do they mention how much it extends their profit margins by?
FROM THE KWIK FIT SITE :-
" Nitrogen
Filling your tyres with nitrogen may seem odd but that?s exactly what motor sport and aviation professionals have been doing for years. Nitrogen is completely safe. And by using it in a mixture with oxygen to inflate your tyres the theory is that it?s possible to negate the issue of slow deflation, which is caused by oxygen slowly infusing through the tyre wall from the atmosphere.
Having a tyre that does not deflate means you will improve fuel consumption and will probably improve safety standards too. It?s not yet standard practice but Nitrogen could well be here to stay as a result. "
Andy C
Read the last paragraph:-)
Yes, and do you believe everything you read on the kwik fit site? :-)
The second half of the first paragraph is written by somebody who really doesn't have a clue about what they're talking about. They could at least have had somebody proof read it so it actually says what they want it to.
That sounds like exceptionally clever marketing. First they tell you why nitrogen is good for your tyres, then they offer to sell you nitrogen mixed with some oxygen. They don't say how much oxygen they've mixed in.
Could it be they're selling you just ordinary compressed air? I wonder if their waiting room has a vending machine selling bottles of Peckham Spring water.
I never said I believed it ! ;-)
I was quite happy with your explanation. I simply pasted it for reference.
Andy C
Cool. So where am I going to get my hands on this new wonder-mix of nitrogen and oxygen, then? I'd definitely be prepared to pay a premium for that, rather than just using poxy air.
David
Sounds bit like ordinary air to me?
How can something coming into the tyre from outside cause deflation?
I've seen some completey wrong stuff in the past but that really does take the biscuit.
THey buy it form the Snake Oil Nitrogen Company.
It's bollocks.
MBQ
whose tyres decay before they wear out anyway?
My camper, yes on 3 toons at 300 miles a year..
But the sunlight got them anyway.
Sounds BS to me.
well largely air is a mixture of nitrogen, and oxygen. With a few things like CO2 thrown in as well.
No shit, Sherlock...!?
Mine vary with general ambient temperature, yes measured when cold, but they are big tyres with a lot of air in them. At a guesstimate I'd say 1.5 psi difference for every 10C change in temperature.
CostCo also offer Nitrogen filling, but as standard IIRC. They may wibble about extending tyre life as without oxygen in the tyre it can't degrade the rubber.
OK, I can just about make out some valid physics amongst the bullshit.
If we assume there's no oxygen in the tyre, then an oxygen molecule outside the tyre will see a vacuum inside the tyre. The partial pressure of oxygen outside the tyre will be 20% of air pressure (15 lb/sq.inch), i.e. 3 lb/sq.inch, so there will be about 3 lb/sq.inch of pressure forcing oxygen to diffuse in through the tyre.
Now consider the nitrogen. Assume the tyre is pumped up to, say,
30 lb/sq.inch nitrogen above air pressure, which is 45 lb/sq.inch absolute. The partial pressure of nitrogen outside the tyre will be 80% of 15 = 12 lb/sq.inch, so the net difference is 45 - 12 = 33 lb/sq.inchSo there's 3 lb/sq.inch of oxygen pressure outside the tyre against 33 lb/sq.inch net of nitrogen pressure inside the tyre (which by way of confirmation, cancels to give the 30 lb/sq.inch observed tyre pressure). The ratio of the oxygen to nitrogen pressures is 1:11.
Now, if the rate of diffusion of oxygen through the tyre is more than 11 times the rate of diffusion of nitrogen through the tyre, the pressure in the tyre will rise as more oxygen will diffuse in, than nitrogen diffuses out.
So this boils down to the relative rate of diffusion of Oxygen and Nitrogen through the tyre, and I don't know this so I can only guess from now on. The two gasses are quite similar in terms of molecule size, and I don't believe that would cause a difference anywhere near 11:1. Oxygen is potentially quite reactive, and there maybe some other mechanism for getting it through the tyre if the molecule can be pulled apart and reassembled by chemical reaction, so it's actually the atoms diffusing through rather than the molecule, whereas Nitrogen is less likely to have such a mechanism, being more inert.
So it would be good to know what the mechanism at play here really is.
If anyone has access to a nitrogen cylinder, it would be interesting to blow up a balloon with pure nitrogen, seal, and then see if it slowly gets bigger. (Need to cancel out effects of size change due to temperature change.) Diffusion through a balloon may not work same way as diffusion through a tyre though, so it still won't necessarily confirm or refute this theory. Also, diffusion through the rubber may not be the primary mechanism and certainly won't be the only mechanism for gas exchange in/out of a tyre. Valve seals, rim seal, etc are also potential candidates, and diffusion/leaks through these may behave quite differently.
to produce oxygen rich gas rather than Nitrogen rich - up to 40-50% O2 depending on the equipment... And who mght use it - Scuba divers who want Nitrox...
Gordon
Funny, the reason nitrogen is supposed to be better than air is because the oxygen tends to leak through the tyre leaving an underinflated nitrogen filled tyre.
If you actually check your tyres weekly it probably wont make any difference, if you do it every year it might.
I wonder how they get the air out to fill it with nitrogen? Do they vacuum pump it?
I think they may already know that. ;-)
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