Nitrogen?

This has been discussed ad infinitum on this forum!

Consider the percentage of nitrogen in air (78%) and then consider whether it's worth paying to have 99% nitrogen in your tyres.

Upto you, it's your money. Personally I think it's just a way for the garage to make some extra money for nothing.

 

I've used Nitrogen in all of my vehicles....all my life.
I use the N-78 blend, just like Grumpy.

If you think that paying for N100 is worth the money?
I guess there are more stupid things that you can spend your money on...but not very many.

Take the $20 and go buy lottery tickets. It will have the same effect on your car in the end....unless you win, that is.
Then.....in 99.44 cases out of 100????
The "Change Automobile" light will be very quickly illuminated and you won't have to worry about people fleecing you out of money for (more) nitrogen for your G3.


YMMV.

 

I think it is a great idea, will it save you mpg? No, will your tire wear more evenly, yes, and when you deal with -35, you know it will go down, I think it is a no brainer. I have it in my tires and really happy that I don't have to worry with temp fluctuation

 

The pressure will vary with nitrogen just as with air. The difference is that nitrogen is much drier, and the moisture in normal compressed air makes the pressure variations more extreme. If you have access to dry compressed air, it would work in a manner similar to nitrogen.

 

I love how this pops up every 6 months or so..... I think Nitrogen should be a sticky

Yes Nitrogen will help a great deal if you are driving a 767, and are coming in for a landing from 30k feet.
Driving on a freeway, not so much!
Stick with the 73% mixture, it's not a snobby, but works just fine!

 

I think you are probably correct, that moisture increases temperature/tire pressure sensitivity. However, Popular Mechanics says that N2 has less temperature dependence than air (they do not offer any explanation for why), and additionally moisture compounds this effect.

Nitrogen vs Air In Tires - Why Nitrogen in Tires - Popular Mechanics

The standard rule of thumb is that you lose 1 psig tire pressure for every 10 deg F outside temperature. Consumer Reports says 30 psi@80F goes to 25 psi@30F, which fits the rule of thumb. If you try to calculate this via ideal gas law you would calc as follows: 30 psi x (30+460)/(80+460) = 27.2 psig or in other words the observed loss in tire pressure (25 psig) is greater than one would calculate for the temperature drop. I presume this "extra" pressure loss might be due to some shrinking of the tire volume as the pressure goes down?

In any case we are apparently left to speculate whether or not the rule of thumb includes the moisture effect or not. I could not find any data on impact of moisture content on tire pressure temp sensitivity but I found several references including PM eHOW state that say it is true.

PS- It is apparently true that a nitrogen molecule (N2 @28 mole wt) is physically slightly larger than an oxygen molecule (O2@32 mole wt) so N2 does leak more slowly thru the sidewalls. This effect can be compensated by checking tire pressure more often with regular air.

 

^ If that's true.....and I think that it's is as "true" as getting more gas/$ if you pump in the cooler AM/PM hours than you will during the middle of the day since cooler fuel is more dense......

Won't those pesky O2 molecules leech out of the tires on their own, leaving you with a higher N2 concentration as you refill??? After a few top-offs.....wouldn't you have mostly N2 molecules left without getting ripped off by paying $$$$ for....air???
And....while we're at it. Tires are pretty rigid. when you deflate them to (attempt to) get rid of all of the "contaminated" air, aren't you REALLY just getting rid of most of it? What about the air (and moisture!) that's left inside the tire when it's deflated? Unless you take a vacuum pump and evacuate most the air and moisture from the wheel then you're not getting N100, and you're not getting all of the moisture out of the wheel.......unless you expect us to believe that they un-mount the tire, remove the moisture, remount it, evacuate the air out of the wheel without breaking the tire bead, and then put "pure" N2 into the wheel.
Really???


Face it.
Unless you're a pilot...it's snake oil.
If you ARE a pilot...you're only getting the O2 out of the tires to help prevent combustion from tires bursting when they go from zero to 130mph in about 0.1 seconds.....something that most Prius drivers REALLY do not have to concern themselves with.
I'm a free market kinda guy, and snake oil salespersons have to make their buck too.
However (comma!) they're not going to make it off of ME.

Good Luck!!

 

Exercise for readers: How long will this process take to reach the ~95% N2 concentration that the hucksters are selling?

I haven't done this myself, but from the differential diffusion rates mentioned in previous threads, it seems that the target wouldn't be reached soon enough to be useful.

 

This is the WORST thing about Nitrogen. It gives people the feeling that they don't have to worry about their tire pressure, so they don't check it. Bad idea. Also Charles' Law states the the volume of a gas is directly proportional to the temperature (in Kelvin). No real difference between Oxygen and Nitrogen. (Water vapor is another matter as mentioned above.) That means that one will ALWAYS need to worry about tire pressure with fluctuating temperatures.

 

I think you are correct Resident Skeptic, that the air in tire will become enriched with N2 anyways, since the O2 leaks out 3-4 times faster according to one reference. Presumably the rule of thumb (1 psig per 10F temp) applies to N2 as well as air. It is not snake oil, N2 does leak more slowly and "dry" N2 is better than wet air. But as Consumer Reports says it is probably not worth the extra cost.

However, I am still interested in the science. Why does low temp reduce tire pressure more than one would calc? Is that because the tire vol shrinks as pressure goes down? and why does the water content impart even more temp sensitivity and what equations show this?

 

Both Oxygen and Nitrogen leak so slowly that you need to adjust for seasonal variation before you would notice any leakage (of gas through rubber). So it doesn't matter. If one lives in a mono-climate, checking the pressure loses more gas than is lost in the interim, so again it doesn't matter.

See the Ideal Gas Law.

Definitely. And Dry air is better than wet N2.

Water vapor condenses in the temperature range we are talking about, (and freezes but that is not terribly relevant). Water going from a gas to a liquid causes a large drop in volume.

 

...right but OK say you have 1% water in the gas it's only 1% drop if it all condenses, so does not seem to be a huge impact. One source says worse case scenario is 5% H2O vapor in the air.

 

Let's say you pump your tires up on a hot humid day in the summer, (90°F, 90% Relative Humidity).
That is 305°K and 0.0272 Ratio of water to dry air (by weight).
Michelin Energy Savers in 185/65/15 hold about 11.15 liters, and max pressure is 44 PSI (plus 15 for atmosphere) is 4.05 Atm. R constant = 0.08206 L * Atm / Mol * K
Using the Ideal Gas Law we get: moles = PV/RT => 4.05 * 11.15 / 0.08206 * 305 = 1.804 moles.

Come winter, cooling down to -35°F (brrr), the pressure becomes P = nRT / V => 1.804 * 0.08206 * 235 / 11.15 = 3.12 Atm. Or 30.85 PSI. So that is best case for a perfect gas. N2 or Air is going to be worse.

Now consider the water in the air (or Nitrogen for that matter). 90% Relative humidity at 90°F is an water / air ratio of 2.72% by weight. Comparing molecular weights gives us 4.2% by moles. So after the water has all condensed, there is 1.727 moles of air left. P=nRT / V => 1.727 * 0.08206 * 235 / 11.15 = 2.98 Atm or 28.79 PSI.

Hopefully, I got that all correct.
44 PSI at 90°F = 30.9 PSI at -35°F for dry air or N2.
44 PSI at 90%RH at 90° = 28.8 PSI at -35°F for damp air.

Conclusion: Air (dry or wet), or Pure Nitrogen, one should check one's tire pressure with significant changes in temperature.

 

Other references have claimed much narrower differentials.

 

Not being a scientific person, can somebody answer me the following;

When the tyre is seated it is full of air (78% nitrogen) at atmospheric pressure (5 psi?) and is then pumped up to 36 psi using 99% Nitrogen. What percentage of nitrogen is one left with after pumping pure nitrogen into the air already in the tyre?

Surely it can't be more than 90% or so? Or do they suck all the 'air' out first before filling fully with Nitrogen.

 

Atmospheric pressure is roughly 15 PSI. The tire is then pumped up BY 36 PSI, for a total of 51 PSI. A quick check for this is look at your pressure gauge, in the open atmosphere it reads 0 (not 15).

My understanding is that some places claim to fill with N2 then empty then fill again. They might even do that.

 

^
No prob.
One thing you might consider is running your tires a little stiffer than OEM specs. That is...bumping the pressures up a little bit not to exceed the max sidewall rating on the tire.
You'll get scads of opinions on this as well and usually I'm not what you would call a huge fan of overinflating tires however (comma!) in your case you may want to consider bumping them up about 5 to 10-percent.
It will lengthen your trip interval to the pay-to-pump, and you will probably get a barely measureable increase in your fuel efficiency. Also....IIRC the TPMS works off of a pressure delta before it illuminates that little light on your dash. I think I read somewhere where they will alert the dizzy-whizz computer if there is about a ten percent downward shift in your tire pressures, but I could be wrong about that one. It might be a 10-psi change that will cause this to happen.
Anyway....and either way...you'll have more air remaining in the tires when (if) the TPMS ever pops a flare over your dash if there's more air in the tire to begin with.
Just don't do what humans are wont to do and inflate your tires to the point where you can run over a coin and tell whether it's heads or tails thinking (incorrectly) that you'll save loads of fuel.
You won't.
When I got "my" car, I inflated my tires to max sidewall pressure for a tank and I really didn't see that big a difference....certainly *not* worth shaking your kidneys down into your butt over.
You’re probably looking at about a 1 MAYBE 2 MPG gain, and of course…..YMMV.

Loft a thread with the following title:
"Over inflating tires is stupid and it doesn’t save ANY fuel"

Then grab a bag of popcorn, sit back and watch the fun.
There are people in these hallowed halls that sit up at night and wait for threads like that one….
Don’t try to use the search function in this forum.
As you have already discovered….it’s REALLY good at searching…..not so much at finding.

Like I said. If you stuff a few extra psi into each tire, you'll save a little money in the loooooooong run, but mostly it will be from not having to shove whatever form of currency or credit you use to feed the air machine.

Best of Luck!

 

Obviously most practical idea is (like Costco) if the new tires are initially filled with dry N2 or dry air, then when we top off with regular air the O2 and moisture have a chance to diffuse out. We have not yet talked about the relative molecular size of H2O vs. N2 and O2 but there's always another day.