Thread: Nitrogen in tyres

Originally Posted by Redbuck40

Ran into a fellow the other day who was driving a newish disco, he told me that he used to put nitrogen into his tyres of his old disco (the same model as mine) and it improved handling and fuel economy.

Any one heard of such?

Regular air is 78% nitrogen anyway, you won't notice any difference unless your tyres are on fire. Save your cash and just keep your pressures up to spec.

The nissian gtr came with nitrogen inflated tyres. As it is inert it does not react with the heat of the tyre. Maybe give you a slight advantage on the track but fairly useless on the road.

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Filling tyres with nitrogen is great for the guy selling it

What they told me, was that the nitrogen molecules are larger than the oxygen ones, and thus, don't tend to leak as easily thru the rubber of the tyre. Plus, the tyre pressure doesn't change as the rubber heats up.

As said, good for the race track, expensive & pointless on a normal road car. If you need to adjust your tyre pressures, as soon as you add normal air, you have defeated the purpose.

As pointed out, air is 80% nitrogen anyway. The only possible perceptible advantage, even in racing, is that the nitrogen is free of moisture, which is significantly lighter and lower heat capacity than nitrogen (but the proportion of moisture is rarely significant anyway).

The use of nitrogen for tyres probably derives from the use of nitrogen in aviation, originally for filling suspension struts, and only more recently for tyres. The use in aviation is originally for fire safety reasons - with high pressures and temperatures in suspension struts it is preferable to have a gas filling it that does not support combustion. Same with tyres, that often operate at far higher pressures and temperatures than do car tyres.

As far as I can see, for cars, the only reason for using nitrogen is to increase the income of the tyre service that supplies it. But the only drawback is financial - it won't hurt your tyres.

John

What we're really trying to compare is the (specific) heat capacity of both gases, air and nitrogen.

Heat capacity is the amount of heat, denoted by a measurable quantity, required to change the temperature of a given gas/solid/liquid by a given amount. Usually expressed in joules per kilogram per unit Kelvin.

It changes with both volume/mass and pressure.

The specific heat of air and nitrogen at both constant pressure (cP) and constant volume (cV), in kj/Kg @ 1ata/14,7psi.

Air cP 1.01 cV 0.718
N2 cP 1.04 cV 0.743

The lower the figure the more heat capacity it has. For instance argon has a cP pf 0.52 and is quite insulative (it's used as a drysuit inflation gas for deep/cave diving for long exposure times).

These figures are so close when used with the volumes we're talking about that it makes near enough to no difference. When broken down The N2 change for both gases is within 21% of each other (has to be because of partial pressure mass of the gas). When you look at the "other" gases in air they are both lower and higher and balanced out the difference is newar to nothing. Also industrial nitrogen is usually a by-product of oxygen generation and so doesn't cost the producers anything (separation can be done by membrane systems which allows N2 to pass through but oxygen not to and so the output gas can be oxygen "enriched" through the removal of some of the nitrogen or pure by removal of all of the N2. Either way a cost free product sold to you.

So as also stated, the N2 is a different sized molecule. But molecular size is a very strange thing. Oxygen has a higher molecular mass than nitrogen but nitrogen is smaller. The difference is size is actually 0.00000000003 meters.

IF, nitrogen was large enough to not pass through rubber, which is porous, but oxygen was small enough to, a loss of 21% of tyre pressure (for a 36psi pressure that's down by 7.5psi) accounts for loosing all the O2 leaving just the N2. Top up and deflate to the point of complete O2 loss enough times and you'll have a pure enough N2 fill.

The change in pressure you see could be mostly accounted to water vapor in the gas when filled (face it how many servos and tyre facilities are providing dry air). The water vapor is more sensitive to temperature and will affect pressure (& volume) more greatly than the difference between N2 and air.

Originally Posted by clubagreenie

What we're really trying to compare is the (specific) heat capacity of both gases, air and nitrogen.

Heat capacity is the amount of heat, denoted by a measurable quantity, required to change the temperature of a given gas/solid/liquid by a given amount. Usually expressed in joules per kilogram per unit Kelvin.

It changes with both volume/mass and pressure.

The specific heat of air and nitrogen at both constant pressure (cP) and constant volume (cV), in kj/Kg @ 1ata/14,7psi.

Air cP 1.01 cV 0.718
N2 cP 1.04 cV 0.743

The lower the figure the more heat capacity it has. For instance argon has a cP pf 0.52 and is quite insulative (it's used as a drysuit inflation gas for deep/cave diving for long exposure times).

These figures are so close when used with the volumes we're talking about that it makes near enough to no difference. When broken down The N2 change for both gases is within 21% of each other (has to be because of partial pressure mass of the gas). When you look at the "other" gases in air they are both lower and higher and balanced out the difference is newar to nothing. Also industrial nitrogen is usually a by-product of oxygen generation and so doesn't cost the producers anything (separation can be done by membrane systems which allows N2 to pass through but oxygen not to and so the output gas can be oxygen "enriched" through the removal of some of the nitrogen or pure by removal of all of the N2. Either way a cost free product sold to you.

So as also stated, the N2 is a different sized molecule. But molecular size is a very strange thing. Oxygen has a higher molecular mass than nitrogen but nitrogen is smaller. The difference is size is actually 0.00000000003 meters.

IF, nitrogen was large enough to not pass through rubber, which is porous, but oxygen was small enough to, a loss of 21% of tyre pressure (for a 36psi pressure that's down by 7.5psi) accounts for loosing all the O2 leaving just the N2. Top up and deflate to the point of complete O2 loss enough times and you'll have a pure enough N2 fill.

The change in pressure you see could be mostly accounted to water vapor in the gas when filled (face it how many servos and tyre facilities are providing dry air). The water vapor is more sensitive to temperature and will affect pressure (& volume) more greatly than the difference between N2 and air.

Well, yea. That's the other way to put it...

and FWIW, dehydrated air is used in F1.

I used to use dehydrated air or dry nitrogen several lifetimes ago when I raced as I had both in the workshop.
No difference in performance.

Because I got bored arguing with an idiot on the net at 0130 this morning I went outside, stripped a tyre and checked volume and made some mental calcs. For a 265/75/16 BFG AT, using the comparison, it would take approx 27 "refills" lost non N2 air components to reduce the half values of said components to a point where the component loss is less than 0.5psi.