Nitrogen in Prius Tires?

<div class='quotetop'>QUOTE(Georgia @ Jul 2 2007, 03:49 PM) [snapback]471755[/snapback]</div>

Seeing that the air we get for free is 78% nitrogen and the other major gas, oxygen (21%), is similar in density, I would seriously doubt that the hassle you have to go through to get pure nitrogen in your tires would be worth it.

That's my guess.

 

<div class='quotetop'>QUOTE(avdavsim @ Jul 2 2007, 02:02 PM) [snapback]471771[/snapback]</div>

Thank God, the nitrogen in the tires scam has not made it's way to my region yet, I don't think. I have never heard of it before, other than on Priuschat. And I agree, stay with the free 78 percent nitrogen available from any air compressor near you.

 

The conclusions I've come to from reading this site is that nitrogen is better, but the differences are much smaller than the nitrogen hawkers would have you believe. From what I've read, people say that the nitrogen molecule is larger than the oxygen molecule, so it won't leak out through the rubber tires. This means that you don't have to fill up your tires with air as often.

So, if not checking your tire pressure every month is worth the cost of the nitrogen, go for it. If not, then keep filling up with regular air. As far as I can tell (again, this is just gleaned from reading stuff here), there is no additional advantage for "street driving" of the Prius.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 2 2007, 06:06 PM) [snapback]471825[/snapback]</div>

If "air" leaks out of the tire but nitrogen doesn't then it must be the oxygen leaving. By this reasoning, if you keep your tp up, sooner or later you will have only nitrogen in the tire anyway. Pure nitrogen will help if you drive at 150mph or at 50,000 feet. But Joker gas (from Keaton's BATMAN) is the best.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 2 2007, 06:06 PM) [snapback]471825[/snapback]</div>

Oh no! Not another Nitrogen thread!

The diffusion rates for nitrogen and oxygen are very similar, and as already posted, if it was a diffusion problem then the oxygen would diffuse out of the air and diffuse back into nitrogen until an equilibrium was reached. All tires would end up with the same nitrogen/oxygen mixture after a short period of time, regardless of their original fills (if this doesn't make sense to you, look up the term "partial pressure" as it relates to gasses). The real issue is water vapor, which has a considerable volumetric change when it condenses into liquid water. Dry nitrogen contains very little water vapor, so tires filled with dry nitrogen will show less pressure fluctuation verses temperature. Dry air will do the same thing. The "nitrogen" offered by the tire shops is not pure nitrogen, nor is it any drier than air from a good compressor with a water separator. Nitrogen from the tire shops will not hurt your tires, nor will there be any measurable gain. Don't pay extra for it.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Jul 2 2007, 10:02 PM) [snapback]472019[/snapback]</div>

Are you sure about this? What you said sounds plausible, but my little bit of Wikipedia-ing leads me to "Dalton's law of partial pressures" which states that the total pressure of a bunch of ideal gases is the sum of the pressures that the gases would have on their own. OK. But the tires are at a much higher pressure than the surrounding air, so it seems to me that there should be no diffusion into the tire (which would INCREASE the total pressure in the tire and seems completely opposite the direction that diffusion should happen.) But, I'm willing to be proven wrong if you're willing to explain it to me.

Regardless of whether this is correct, I think we both agree that the advantage to using nitrogen is somewhere between zero (you) and very slight (me).

<div class='quotetop'>QUOTE(JimN @ Jul 2 2007, 08:49 PM) [snapback]471972[/snapback]</div>

Yes. Assuming that my idea of diffusion across the porous rubber membrane is correct, I think this is true; the nitrogen concentration will increase with time if you just keep adding air.

 

If you buy your tires at Costco, then they will automatically fill them with nitrogen. However, I don't know how they respond to requests to fill your tires to 42/40 or 44/42 for improved MPG.

If you are getting your nitrogen for free or it is included in the price of new tires, then I say go for it. But, if it is something that you have to pay for, then I say take a pass, because there is absolutely no tangible benefit from running nitrogen in the tires on a passenger car.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 3 2007, 12:09 AM) [snapback]472050[/snapback]</div>

Absolutely sure. What you just quoted is true: the diffusion rate of individual gases is completely independent of the total pressure or the partial pressures of the other gases in the mixture. Looking at our example, increasing the pressure of the nitrogen in the tire only increases the rate at which the nitrogen diffuses out of the tire. It does not reduce the rate at which oxygen diffuses back into the tire (or out, for that matter). Only the relative pressures of oxygen inside and outside of the tire, and the permeability of the tire, control the diffusion rate of the oxygen. Once the partial pressure of oxygen is the same inside and outside of the tire, oxygen stops diffusing through the tire, or more correctly, oxygen diffuses at the same rate into and out of the tire, resulting in zero net diffusion. Double the amount of nitrogen and it makes no difference to the oxygen. The converse is also true, as it is with any ideal gas (in this environment, oxygen and nitrogen both behave as an ideal gas). Water vapor is a non-ideal gas at common temperatures. It easily condenses into water causing large volumetric changes. This is where most of the trouble comes from with using "air" in tires. Air from compressors and tire pumps may contain a fair amount of water.

Now before I get beat up by the nitrogen supporters, let me state that there are advantages to using dry nitrogen in tires in some circumstances:

1) Dry nitrogen contains very little water vapor. As a consequence, it is easier to predict pressure changes as a function of tire temperature. Note that the pressure still changes with temperature, but it does so at a known rate. Race car drivers use this advantage to allow them to tune their tire pressures to withing a fraction of a PSI. They are able to predict tire temperatures based on previous racing experience, which allows them a fine degree of control over tire inflation. They could do the same thing with a gas containing water vapor, but they would need to know how much water vapor the gas contained. It's the uncertainty of the mixture that causes them problems with air (some days are more humid than others).

2) Aircraft also use dry nitrogen to avoid ice build up on the inside of tires. Aircraft tires go through temperature extremes, with hot tires sitting in freezing conditions, leading to off balance ice if any water is present.

3) Nitrogen does not support combustion, which can be important with aircraft and race cars. Normal car tires are not subjected to these extremes.

4) Nitrogen does not promote corrosion. Once again, this is not a big issue with car tires and wheels, which seldom if ever rust or rot from the inside out.

5) Dry nitrogen is readily available as liquid nitrogen. It is easy to have a large amount of high pressure nitrogen on hand. This is one of the biggest reasons it is used in high pressure tires.

The "nitrogen" supplied by tire stores is almost never dry nitrogen from a liquid nitrogen tank. Most stores use a "nitronizer" which increases the nitrogen content of ordinary air. It's still air, just with more nitrogen. We can hope they do a good job filtering out the water vapor so that there is some benefit from running it through the "nitronizer". Plain compressed air from a good system will work just as well, but you don't get the cool green dust caps.

Tom

 

Nitrogen is still too reactive.

I recommend using only ARGON for your tires.

And, with a molecular weight of 39.948 for ARGON versus 28.0134 for lowly Nitrogen it will diffuse through the tire walls much more slowly.

 

<div class='quotetop'>QUOTE(qbee42 @ Jul 3 2007, 10:42 AM) [snapback]472284[/snapback]</div>

I understand your argument, but there must be something I'm still missing. Why should physics care about what gas molecules are providing the pressure? In other words, pressure is basically a result of the number of collisions molecules (or atoms) make with a membrane. More specifically, it's force per area. In the case of a nitrogen-filled car tire, the force of the nitrogen on the inside is greater than the force of the air (i.e. nitrogen/oxygen mix) on the outside; otherwise the tire wouldn't inflate. If oxygen diffuses into the tire from the outside, it must increase the pressure inside (since partial pressures are additive). Again, since pressure is just force per unit area, the air seems to be flowing opposite the direction of the force: in your scenario, oxygen would be flowing from an area of lower pressure (force) to an area of greater pressure (force).

I didn't study much chemistry in college, but I was a physics major and this just doesn't quite make sense to me yet; I just can't understand how something can flow against the force vector. If you can point out the flaw in my argument, I'd be happy to learn something.

<div class='quotetop'>QUOTE(qbee42 @ Jul 3 2007, 10:42 AM) [snapback]472284[/snapback]</div>

But, by your logic (which I'm not sure I buy yet), shouldn't oxygen diffuse into the tire, leading to no difference between nitrogen-filled tires and air-filled tires?

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 3 2007, 01:10 PM) [snapback]472338[/snapback]</div>

Exactly! Now you get it.

BTW, It's not my logic, it's simply physics. You need to argue with Dalton about this one, but those little gas molecules don't give a red rat's a$$ about the other gas molecules. They only care about their own. Being a physics major, I'm sure you studied Dalton's law of partial pressures. It's the same reason a fart can travel upwind (well, sort of; we have to get into diffusion speeds for this). It's also similar to osmosis in cell membranes. I know it's counter intuitive, but that's the way it works.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Jul 3 2007, 12:58 PM) [snapback]472367[/snapback]</div>

OK. Then what's the point of putting Nitrogen in race car tires? You've said that there is a benefit for the car not catching on fire, but then you also said that oxygen will diffuse into the tire, thereby negating that benefit. So, is there a benefit, or isn't there?

<div class='quotetop'>QUOTE(qbee42 @ Jul 3 2007, 12:58 PM) [snapback]472367[/snapback]</div>

But this doesn't make sense. How do gas molecules only care about other alike gas molecules? A collision is a collision is a collision and whether an oxygen molecule is hitting up against a nitrogen molecule or another oxygen molecule shouldn't matter to that oxygen molecule. In fact, I would argue that Dalton's law supports my point of view: if you add gas to a mixture, the pressure will increase by an amount equal to the pressure that the gas would exert alone in the same volume. The salient part of that argument is that the pressure would go up. Maybe I'm wrong for another reason, but I definitely don't buy the idea that the composition of the molecule makes any difference for these the pressure calculations (except to the extent that there's a mass difference between different molecules). So, if a oxygen molecule is trying to diffuse in through a small hole in a nitrogen-filled tire, the force of being bashed by 100 fast-moving nitrogen molecules on the inside will be greater than the force of being bashed by 1 nitrogen molecule and 1 oxygen molecule on the outside, so shouldn't the oxygen molecule have a net force vector away from the tire?

For what it's worth, most of the stuff I've been able to find on the web support my point of view. Now, I'm fully aware that there are a lot of specious arguments out there that gain a lot of traction on the web, but I'm surprised that I haven't yet found a physically-motivated debunking of the nitrogen diffusion theory.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 3 2007, 01:25 PM) [snapback]472388[/snapback]</div>

What they're filled with only matters during the race (if at all). Presumably any race team that cares about the matter will empty and re-fill the tires immediately before each race. Whatever diffusion may occur between races is of no consequence. The pressure inside my 42 PSI tires drops no more than 5% per month when measured at the same temperatures. Whatever diffusion is occuring does not have enough of an effect to worry about.

OP, if whoever services your tires puts dry nitrogen into them it does no harm. But it's not worth paying for and not worth driving out of your way for it. There's far more benefit for your tires and your fuel economy if you check their pressures frequently (whenever the daily average outside temp changes more than 20'F or so, or at least once a month) and adjust them as needed.

 

Many race car teams use nitrogen instead of air in their tires because nitrogen has a much more consistent rate of expansion and contraction than does regular air. Often, a half pound of pressure will radically affect traction and handling. With track and tire temperatures varying over the duration of a race, the consistency of nitrogen is needed.

Nitrogen pressure is more consistent than normal air pressure, because air typically contains varying amounts of moisture due to changes in the relative humidity on race day. Water causes air to be inconsistent in its rate of expansion and contraction. So, a humid race in Atlanta or a dry race in Arizona could make for unpredictable tire pressures if "dry" nitrogen were not used.

However, the point is really moot because we aren't talking about race cars, we are talking about the Prius. None of these advantages is important to the average driver.

 

I'm still not sure that I buy into the argument that oxygen molecules pass through the tires, and nitrogen molecules don't.

For the sake of understanding partial pressures and behaviors of an ideal gas, lets assume that for the moment that this is true.

Using your example of gas molecules bouncing off the membrane, occasionally an oxygen molecule will pass through a "small hole" at a moment when there is no nitrogen molecule blocking the path. How frequently this happens is dependant on how permeable the membrane is and what the partial pressure of the oxygen is. Since the partial pressure of the oxygen outside the tire is higher than the partial pressure of the oxygen inside the tire, there will be more oxygen molecules bouncing off each square inch of the outside of the tire than the inside. Therefore there will be more oppurtunities for oxygen to enter the tire than leave it. As the partial pressure of the oxygen inside the tire increases, the number of molecules per unit of time leaving the tire will increase. Eventually, when the partial pressure of the oxygen inside the tire is equal to the partial pressure of the oxygen in the atmosphere, the amount of oxygen molecules leaving the tire will be equal to the amount of oxygen molecules entering the tire per unit of time. At this time the amount of oxygen in the tire at any time will no longer significantly increase.

If the tire was filled with a nitrogen-oxygen mixture to start with, and the percentages of each gas were roughly equivalent to the percentages in the atmosphere, the partial pressure of oxygen in the tire would be higher than the partial pressure of oxygen in the atmosphere. Some oxygen molecules would still pass into the tire through the "small holes" and other oxygen molecules would pass out of the tire at the same time through the "small holes". However, due to the higher partial pressure of the oxygen inside the tire, it there would be more oppurtunities for oxygen molecules to pass out of the tire, and pressure in the tire would drop until the partial pressure of the oxygen in the tire exactly matched the partial pressure of the oxygen outside the tire. Adding more nitrogen-oxygen mixture to the tire would repeat the process. If you did this enough times you eventually reach the point where the tire is at the pressure you want, and the partial pressure ot the oxygen in the tire matches the partial pressure of the oxygen in the atmosphere with the rest of the pressure coming from the partial pressure of the nitrogen.

Note that gas pressure is not a force at this scale. Gas pressure is a measurement of the speed and frequency that collisions occur between the membrane and the molecules. So it is possible for a molecule to pass from a lower pressure region to a higher pressure region, it just doesn't happen as frequently as the inverse.

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 3 2007, 03:23 PM) [snapback]472428[/snapback]</div>

They don't. Oxygen and nitrogen molecules pass through tires about equally well, which is to say not well at all. Tires are obviously designed with low permeability materials. The whole explanation in the previous posts about oxygen diffusion was to make the point that if it were an issue of oxygen diffusing more quickly, then it wouldn't really matter because it would diffuse in as well as out, so all tires would end up with the same gas composition after some period of time.

Thanks for the good explanation of partial pressure at work. It's often hard for people to visualize the world at atomic levels.

Tom

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 3 2007, 02:23 PM) [snapback]472428[/snapback]</div>

Thanks for the description. I'm fine with oxygen molecules not passing through tires much (that sounds reasonable and I can believe that it is true). However, again, I'd just like to continue this discussion assuming that oxygen molecules can pass through the tire wile. I'm still hung up on something, so let me try to articulate myself more clearly.

You comment that:

That makes sense to me, except for the part where you say that it's dependent on the partial pressure of oxygen. From the physics perspective, it seems like it should depend on the total pressure of the gas, since that's has to do with the number of times that the incoming oxygen molecules will be hit molecules (be them nitrogen or oxygen).

AHHHHH! LIGHT BULB!!!!!

Since it's an ideal gas, there are almost never actually collisions between individual molecules. It's just basically the number of molecules of gas that pass through the membrane in each direction. When the partial pressure of oxygen on the inside equals the atmospheric pressure of oxygen (i.e. the pressure on the outside), you have as many oxygen molecules passing out through the membrane at a given time as you have passing in through the membrane.

So, assuming that the rubber membrane of the tire is permeable and you fill it with pure nitrogen, you will have oxygen diffuse into the tire. However, the eventual partial pressure of the oxygen in the tire will only be able to reach the partial pressure of oxygen in the atmosphere. Therefore, the percentage of gas in the tire that is oxygen will be much lower than the percentage of oxygen in the (non-pressurized) atmosphere. So, the nitrogen under pressure will, by far, dominate the gas fraction (both by number and mass).

So, while I now understand how oxygen could diffuse into a tire, I think I still stand by my original statement on this (with an important (italicized) qualification): Assuming that oxygen diffuses through the walls of a tire at a faster rate than nitrogen, your tire pressure will go down faster with air-filled tires than it will with nitrogen-filled tires.

EDIT: Credit where credit is due. My "revelation" is pretty much exactly what Danny said in the penultimate paragraph of his post.

 

HA HA HA!!! You poor fools - face the massive superiority of my pure Xenon 133 filled tires and despair!!!

What? They're only a little radioactive....

 

So now we are in agreement as to how oxygen could pass into vessel closed with membrane that is permeable to oxygen and not to nitrogen and pressurized above local atmospheric pressure with pure nitrogen.

I think we are also in agreement that initially such a vessel would lose pressure if filled with a nitrogen-oxygen mixture equivalent to "air" but at higher than local atmospheric pressure, and that it would gain a slight amount of pressure if filled with pure nitrogen to higher than local atmospheric pressure.

What is left to understand is that either way, over time, if you always adjusted the pressure to keep it at some predetermined pressure (44psi?), and always made that adjustment the same way (adding the same nitrogen-oxygen mixture every time, or venting excess total pressure from the initially pure nitrogen vessel), over time the amount of oxygen would eventually equalize, and the vessel would cease to lose/gain total pressure. Either method eventually ends up with nearly the same quantity of each gas in the vessel.