Nitrogen?

Seems that's a moot point since normal air is not 100 percent oxygen but rather only 21 percent and 78 percent nitrogen.

If they want to compare, it needs to be with normal air.... Not pure oxygen

SCH-I535 ? 2

 

This (N2 in Tires) is a really interesting topic.
Basically there is trend since about 2000, supported by Auto companies and Tire companies to go to N2 in tires, for various advantages including slower diffusion of N2 and other benefits. Obviously these benefits are secondary or tertiary. For some reason it appears many consumers (such as Prius Chat posters) do not accept that there is an advantage for N2. There are advantages, but they are not huge. One tertiary benefit is better MPG due to better inflation pressure, so one might think Prius owners would be early adopters to the N2-in-tire movement. Interestingly, the reason N2 is more popular is improved availability of cheaper N2 from membrane separation from air. So OK your tires accomplish the same membrane process passing O2 more quickly than N2.

1. Consumer Reports
Tires - Nitrogen air loss study
Consumer Reports did a careful test showing N2 leaked more slowly from car tires.
They received a ton of email from skeptical consumers and CR tried to answer the questions.

2. Goodyear

3. Michelin

4. Air Products discussion of Diffusion (attached)

Note that although Consumers Reports, Goodyear, and Michelin do not specifically state that O2 diffuses 3-4x faster than N2, their observations of improved pressure maintenance with N2 could only be true if O2 diffused out quite a bit faster, and 3-4x seems about right as shown in my sample calc posted earlier.

 

I can accept that there is a benefit, but I refuse to pay more than that benefit is worth. Outside of Costco, that 'cheaper N2' for the vendor is not translating into cheap N2 for the consumer.

 

Here is a FORD technical paper discussing N2 in Tires.
It talks about tire rubber/quality maintentance.
But I wanted to highlight to the reason for race car drivers to use N2 in tires is better pressure maintenance with high temperature (I am augmenting here) is presumably because racing tires can get well over 100F at this point any moisture would evaporate and could increase tire pressure unacceptably or unpredictably. So the moisture in air is bad for race car drivers, but more minor impact for regular cars. So the idea of "dry" N2 or dry air being better for pressure maintenance at low outside temperature is probably a semi-bogus argument, although moisture consensation at low temps could possibly account for 0.5 psia add'l drop at low temps (very minor). The major reason for lower tire pressure at lower temperature remains the Ideal Gas Law (PV=nRT).

I should add the reason why race car drivers use dry N2 is to get a linear tire pressure response with tire temperature. Many people "hear" this wrong, the race car drivers are not saying no temperature response to tire pressure, they are simply saying predictable tire pressure increase (due to no moisture).

 

Better inflation pressure is not a quantitative statement, so let's see if we can make it one. How about "closer to the optimal pressure for existing conditions"? But one can fill their tires to any pressure. So there is no way that N2 can be said to be 'better inflation pressure' in that sense.

How about "stays closer to optimal pressure for varying conditions"? Since dry air and nitrogen are close in how the are affected by the gas laws, they are going to be fairly close in how they react to temperature (see my previous analysis). However, since there is variation from optimal in changing conditions, a much better strategy is to check and change the pressure whenever conditions change. This is where Prius owners are getting an advantage out of using air. They check and change their pressure more often, and thus are closer to optimal in all conditions.

How about "maintains pressure against loss"? Here is where the claims for N2 put the advantage, but the leakage is very slow (either N2 or air), and thus it is surpassed by the frequency of pressure checks and changes. And again we can see why Prius owners are benefiting more from their policy of frequent checks and changes, over a system with perhaps a tiny bit more stability but which is more susceptible to changing conditions (by virtue of producing a feeling in the owner of not needing to do anything).

So one should think that Prius owners would be slow adopters of N2 until such time as it is ubiquitous, and they can be guaranteed of finding a N2 station whenever they want to check and change their tire pressure.

 

If O2 diffuses out that much more rapidly than N2, then it's self correcting problem. The partial pressures of O2 inside and outside the tire will approach equilibrium after a few top offs whether you fill your tires with air or N2.

My very unscientific undocumented impression is that after my tires are a year old they lose pressure slower than when they are new. However, even when they are new, I have had to let some air out to maintain pressure when the weather got warmer in the spring and summer, and I live in a climate with mild winters. I also need to add air as the weather gets colder, about 1 psi per 10 degrees F temperature change.

 

^^^this is true and I said it myself a couple of posts ago. I suggest best approach if someone like COSTCO fills your new tires with N2 then you could refill with regular air. For some reason, a lot of people say incl. CR you must refill with N2 if you start out with N2. No way I am doing that especially if O2 diffused out faster anyways.

 

That is an interesting paper, but if a person believes in the big difference between O2 and N2 permeability through tires, then all tires end up with about the same partial pressure of O2 in the tires after a few months.

 

^^ Are you should equilibrium would be reached in months, not years?

 

Another incentive for car clinics is to get you back in for refilling/checking tire air means you will most likely buy something else too.
That's also the incentive for very low cost oil changes.

SCH-I535 ? 2

 

A proper calculation would require deriving differential equations and that makes my head hurt.
With the large differences in permeability claimed, getting within a few percentage points of equilibrium shouldn't take years.

Also note that O2 will permeate into a tire filled with pure N2. A tire filled with pure N2 (not that that will ever happen in the consumer world) and maintained at 40 psi will end up at about 5% N2 over time. Also note that water vapor permeates far faster than O2 or N2 so a tire filled with dry air or N2 will pick up moisture from the atmosphere and a tire with high inside humidity will lose water vapor to a dryer atmosphere through the tire.

 

See page #13 of this paper.

Sounds inconclusive to me.

http://pubs.its.ucdavis.edu/download_pdf.php?id=1044

 

Why? Because it has no scientific basis. We could also agree that the Prius gets good mileage from the clever use of butterfly kisses, but that wouldn't be true either.

Science isn't a popularity contest or even a democracy. Somehow we have lost our way in this modern world, where many if not most people believe that the true test of a scientific theory or fact is how many people believe it is true. Popularity is not the discriminator, nor is profitability. Science is (or should be) based on facts.

Tom

 

Thanks for this.
Looks like they are mostly quoting the FORD article however.
I did not understand (thought it might have been a mistake) when the FORD guy said "
nitrogen’s lower water retention" and they repeated it. N2 retains as much moisture as any gas, given the chance, right?

 

Bill,

Here is the info I gave you earlier. I'll post it here for others to read as well.

http://www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs/Crash%20Avoidance/2009/811094.pdf

 

^^^that's a good one. Last sentence confirms the 4x faster loss rate of O2 vs. N2.
I finally think I have this area mastered. If someone wants to sneak over and put N2 in my tires, please do so. But I am not going to pay to flush the air out. The last question I had was - how much does the volume of a tire change with tire pressure? The one answer I saw somewhere was no change in radial tire volume vs. tire pressure.

 

No change in volume over the range of that pressure a person would intentionally use is a good approximation. You could carefully measure from the inside rim over the tire to the outside rim at say 20 and 40 psi to verify. There is also a small volume change to the slight flattening of the bottom of the tire, but it's not very significant.

 

The change in volume is very tiny as xs650 stated. I will add, however, that the shape of the tire does change with higher pressure. Start off with 35psi in your front tires and measure the height at the lowest point on the front bumper. Now air your front tires to 45psi and remeasure the height.

 

Their may be a 4x faster loss rate of O2 vs N2 but, I don't know of anyone, anywhere that puts straight O2 in their tires.

 

Yeah, just spend 100 bucks and keep an air compressor in you garage, it's a lot less stress than going and dropping off your car or waiting for them to fill it.

Whether you use Nitrogen or normal air, you still have to check it yourself from time to time.

Alan... The second mouse gets the cheese!
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