Nitrogen in tires

Love the BS

If the N2 folklore about the difference in leakage rates were true, one could simply overfill their tires by 28% and the O2 would magically leak out leaving pure N2 at the desired pressure.

 

Deleted dupe

 

It would only stay at 42 if there is no leaks and temperature is the same in a month and then in 2 month. Otherwise you are braking laws of physics.

- Alex

 

N2 leaks more slowly (4x more slowly than O2) from car tires. Therefore it is nice to have, but I would not pay extra for it. In race cars, trucks, planes there are additional benefits (less fire protential). So in those cases the extra cost may be worth it.

 

Good to know. Thank you, this is helpful.

 

^^^yes 4x sounds impressive (and it is) but since the air is 78% N2 anyway what you actually see is only 1 to 2-lbs less leakage over 6 months or so. Consumer Reports verified it.

 

Here's the study at Consumer Reports. Sadly, the images there have faded away since 2007.

 

...not really! N2 has the same temperature response as an ideal gas, laws of physics as someone stated.
Race car drivers like to use dry (no moisture) N2 because, if you have moisture (liquid water) in your tires, as your tire temperature approaches 212 deg F (boiling point of water) the water could evaporate quickly and cause a more rapid pressure increase. Race cars drivers are all about keeping the tire pressure increase on a predictable linear path as tire temperature increases.

 

The boiling point of water at 35psig is 280F, so the boiling point at atmospheric pressure (212F) isn't a problem. The issue is vapor pressure and that issue is usually greatly exaggerated. Exagerated because of the very small amount of water that is normally in a tire.
Vapour pressure of water - Wikipedia, the free encyclopedia

 

Yes thank you I am sure that is correct as far as 280F but any water does start to evaporate by 212F. Key point is the race car drivers are not saying N2 tire pressure is constant with temp, they are saying it (dry N2) follows the normal ideal gas temperature/pressure relationship.

 

That is certainly the claim. Do you have evidence that it is true and significant? I did the chemistry and math, the last time this topic came up, and concluded that Pure Nitrogen or air, wet or dry, significant changes in temperature require adjustment to tire pressure to maintain consistent pressure. The difference between dry air and dry Nitrogen did not significantly change whether the pressure should be checked, neither was stable enough to alleviate worry. In fact, if having Nitrogen in your tires causes you to not worry about your tire pressure it is probably doing more harm than good, just based on how often you check tire pressure.

 

...you are correct Corwyn, the water is only a big deal at very hot tire temps.
Cold temps reduce tire pressure no mater what.
However, I think it would be nice if new car tires were filled with N2 since it holds better pressure.

 

Read the report. Thank you. Is tire leakage the reason dealers use to justify the use of nitrogen. If so...I'm glad I don't have to pay for it. For some reason I was thinking it was a good idea. Hate being wrong but after 62 years I guess I should be getting used to it.

 

And again I will ask, do you have evidence that that is both true and significant. How much better does it hold pressure?

Consumer Reports says: "The test started on September 20, 2006 and the final measurements were taken on September 20, 2007. The results show nitrogen does reduce pressure loss over time, but the reduction is only a 1.3 psi difference from air-filled tires. The average loss of air-filled tires was just 3.5 psi from the initial 30 pressure setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial 30 psi setting. More important, all tires lost air pressure regardless of the inflation medium, so consumers should check their tires' air pressure routinely."

What I take from that is that I know I will need to be adjusting my tire pressure at least four times a year due to temperature changes (which cause differences in the tens of PSI in my climate), whether leakage has been 1 PSI or 0.5 PSI in the meantime is not a huge deal. The really important thing is the last bit "...consumers should check their tires' air pressure routinely." If one does that, air vs nitrogen is largely irrelevant, if one does not, nitrogen is not going to save you.

Here is a good question to ask oneself: "If i check my tires, and find them 1.5 PSI below optimum, am I more likely to fill them if I have air in them or Nitrogen?" [no inference about 'right answer' should be made. Whatever works for you]

 

Water evaporates at ANY temperature (above absolute zero). I dry my clothes on a line outside in the winter. Ice evaporates (technically sublimates). For any given temperature and pressure there will an equilibrium humidity above which water will condense (or freeze), below which it will evaporate.

 

Corwyn, are you really an Energy Curmudgeon? I seriously feel EPA should consider some way to encourage N2 in new tires becuase it saves fuel due to better MPG's. I have previously posted US gov't DOE or DOT documentation (attached) I think quoting that N2 is 4x slower than O2 to diffuse through a tire. This is the science of membrane separation. Just the chemistry of the situation.

What you are saying, since no one is actually filling tires with 100% O2, who cares if O2 goes thru the tire 4x faster? And that's a fairly good point, since air is only 21% O2 all we really see is a 1-2 lbs faster leakage with air over a year. And since O2 goes thru faster, the air inside the tire does become somehwat more concentrated in N2 anyway. In the end it is just a subtle but observable benefit for N2. Note I have never filled my tires with 100% N2. But I would take N2 if offered up for free (eg at Costco).

We need a sticky for some of these generic issues since we keep re-discussing.

 

Relative difference is useless. What we care about is absolute leakage. And since NO ONE puts O2 in tires, let's consider actual usage. N2 (roughly pure) vs Air (78% N2 21% O2), means N2 leaking 1 arbitrary unit, air will leak 1.62 arbitrary units. So instead of 400%, the theoretical difference is actually 62%. How does that compare with empirical measurements? CR measured 2.2 and 3.5 PSI respectively. What do you know, that is within measurement error to the theoretical result we just calculated.

Where did you get that impression? Where did I even mention O2?

So let's take the result above and see where it gets us in the real world (or at least mine). Every month, I will be losing 0.18 PSI in my tires for N2, and 0.29 PSI for air. On the other hand, the average temperature in any given month changes by 10 degrees. A tire with either dry gas varies .105 PSI per degree, so 1.05 per 10 degrees. Adding leakage (i.e. cooling), we get 1.23 PSI for N2, and 1.34 PSI for air. If my threshold for monthly variation is 1 PSI, I am filling either way. The difference in pressure loss due to N2 vs air is thus really only 9% of total losses. My daily variation in temperature (and thus pressure) is more likely to be twice the monthly. So if it is really a concern I should be adjusting my tires for every trip, or scheduling my drives to always be at the same ambient temperature.


With a correlation between pressure and mileage we could convert this to energy units, if you like. Then in order to know whether N2 in tires saves energy, we need to compare that to how much energy it takes to produce (isolate), transport, store, and deliver the Nitrogen, and how far out of your way are you traveling to get it. Then I will make my curmudgeonly pronouncement.

The simple cash-proxy analysis, for me, is that there is a Nitrogen station within 1/2 mile of my house on a route I take a couple of times a week. If they were giving away Nitrogen (and it could thus be assumed to have a fairly low embodied energy), I would probably keep my tires filled with N2. If they charge $20 (one time per set of tires) and gave free fill-ups, the embodied energy of N2 makes it a loss energy wise. Note: "Free" in this case, =/= "included in the price whether I want it or not".

Yup.

 

Doesn't the TPMS requirement already harvest the great bulk of savings that N2 could provide?

Those of us who check pressure frequently get little to no fuel saving benefit from either TPMS or N2. With TPMS alone, those who rarely or never check will continue ignoring the tires until the TPMS light triggers. Their average pressure over time will be the same, half way between full and the TPMS, regardless of N2 or air mixture. N2 will lengthen the cycle but not change the average.

N2 will help the average pressure of semi-conscientious operators who don't check often enough, but whose declining pressure is usually caught by them or their mechanic before the TPMS lights up. Will this fraction save enough fuel to justify the cost of fleetwide N2? I doubt it.

 

Not quite that simple. Again, using me for an example, for 6 months of the year, the increase in tire pressure due to temperature variability exceeds the loss from leakage. So let's say I fill up in January and again in July. And if we assume optimum mileage comes from pressures higher than sidewall recommendations (say 40 PSI) then I want to fill up in January at sidewall max - (temperature increase to July * 0.105 PSI/degree) + (leakage / month * 6 months). This keeps me below sidewall max until July. With a 50 degree swing in temperature, I would pump to 35.8 PSI for Nitrogen, and 36.5 PSI for air. Thus, for people who rarely check their tire pressure, 6 months of the year air gives a pressure closer to optimum than Nitrogen. And other 6 months, Nitrogen does better (by more than the air gains in its season). So the savings for Nitrogen in is merely the difference between the cooling and heating scenarios.

 

...minor point, but if Consumer Reports is reporting 3 psi/yr leak rate at 30 psi, then the leak rate at 40 psi would be 40/30 x 3 = 4 psia.