DansYellow66

I heard a local tire dealer advertising today that when you buy and mount tires at their store, they can fill them with nitrogen to improve gas mileage. Never heard this sales pitch before. How does that work to improve mileage?

Dan

Plasticman

Theory is that less change in pressure due to temp. changes, so the tires are less prone to go too low when cold, or inflate too much when hot.

The NASCAR racers have been using nitrogen for many years.

I "won" a door prize for a nitrogen fill at a local Vette show held at a Chevy dealer, and declined (they gave me a socket set instead). I did not want to "like" it too much and have to come back for a nitrogen fill evey time a tire was low (I have an air compressor for that).

Plasticman

Ironcross

I believe that nitrogen was used in my Cessna 210 and also in other aircraft applications. I dont know of any benifits for ground pieces though. Maybe just a sales gimmick.

SherpaPilot

The reason nitrogen is commonly used in aircraft tires is because normal air contains moisture which could freeze at upper altitudes. If the frozen water failed to melt on landing, the tire would be out of balance and at very high speeds could fail. Nitrogen does not contain any moisture sufficient enough to freeze and cause a problem. This information and about 5 bucks should get you a cup of coffee at Starbucks.

rgs

The sales pitch is based upon the tire maintaining a more controlled pressure change over temperature than compressed air. I really doubt that any increase in mileage would be realized. On the other hand, almost all top road race teams use it. Partly because it is readily available since it is used to power their air tools, partly because it has a more stable pressure versus temperature reaction, and lastly the nitrogen ages the tire slower than air.

Dry nitrogen reacts to temperature changes per the ideal gas law PV=nRT or pressure x volume = (moles of gas) x (gas constant) x temperature. In a closed container (tire) if we ignore the expansion of the tire, then the v,n, and R are held constant leaving P=xT where x is the combinatin of the constants. So the pressure in the tire changes in direct proportion to the temperature.
Air from a compressed atmospheric air source contains nitrogen and other ideal gasses, but it also contains water vapor. The water vapor does not respond to volume changes linearly as does the gases so tire pressures change less linearly.

pletner

What he said.

Cobra66

How about nitrous oxide in tires?

If things get a bit dull, one can always suck a bit out of the valve stem .

GaryS

Probably not appropriate to auto tires but N is added to aircraft tires MAINLY for the safety factor if you have overheated brakes. That way, if the fuse plugs on the wheel don't do their job (melt and let the air out slowly) you don't get a catastrophic explosion; you just get a tire that bursts without the added potential for fire.

Fwiw, I currently fly an Airbus 320 but I have flown five other commercial aircraft, as well as the C130 shown in my avatar. And I am retired Air Force.

The mechanics tell us that N is more stable, expands and contracts less, and, thus, leaks less. Makes their job easier and my job safer!

If someone offered to fill my auto tires with N for free, I would do it. It wouldn't be the deciding factor in causing me to spend more money on tires though.
Gary

al329

Taken from Popular Mechanics site.....http://www.popularmechanics.com/auto...tml?page=2&c=y

"Another cool thing is this Ingersoll-Rand (www.irgaragesolutions.com) nitrogen generator I have. People don't realize how corrosive the air is--it's especially bad for a car's aluminum wheels. A lot of times, when you see old aluminum wheels, they're pitted on the inside. If you fill the tires with pure nitrogen, the wheels stay like new. Also, the pressure of a nitrogen-filled tire doesn't rise or fall like one filled with air. Put 32 psi in your Corvette tires, go out and do a few burnouts, and now you have 38 psi. But nitrogen won't do that. It stays where you set it. The nice thing about this Ingersoll-Rand nitrogen inflation system is that it's a generator, so it extracts the nitrogen right out of the air--for free. You don't have to call a guy to bring a huge tank of nitrogen. I'm running nitrogen in everything now."

Hey if Leno said it....it must be true!!

kidvette

With:

Sherpa pilot
GaryS
Al329

SWCDuke

Water vapor behaves as an ideal gas, too!

So as long as water vapor does not condense, there is no difference in behavior relative to dry air or dry nitrogen, but that's the rub. If you fill your tires with air at 80F and the dew point is 60F, water vapor will condense out at of below 60, which will drop pressure.

Most tire shops with decent compressor systems have driers, but the two-bit air compressors at gas stations nowadays do not.

If you have a water trap on your home compressor, it will probably keep water vapor content low enough to prevent condensation. Fill your compressor then let it cool to ambient. Set the regulator to 40-50 psi. As the air flows thorugh the regulator, temperature and pressure will drop, and if the temperature drops to below the dew point, water will condense, but a decent water trap will keep it from flowing into the tires.

Dry nitrogen is primarily marketing hype and likely a high profit item. If you understand water vapor and dew point, remain cognizent of this issue, and take steps to minimize the possiblilty of condensation, you don't need nitrogen.

There is also a story that nitrogen is less likely to leak out because the molecules are smaller. This is pure BS. Nitrogen has and atomic number of 14 and atomic weight of 28 compared to oxygen, 16/32 and both form diatomic molecules at normal temperatures and pressures.

If anything, N2 molecules are smaller than O2 molecules.

Duke

vark_wso

Nitrogen (atomic #7) has a significantly larger atomic radius than oxygen (atomic #8). Recall that atoms in the same period -- in this case Period 2 -- get smaller as atomic number increases from left to right across the periodic table. The covalent bond gas N2 is also larger than O2 by about 5% and would be less permeable than O2.

Since compressed 'air' is ~80% nitrogen, the net affect of going to 100% N2 would be small when leakage through a membrane of sorts is considered.

Vipermike

Popular Mechanics is trying to fill their readers full of B.S. in order to sell a product. The pressure within a tire which contains nitrogen will increase as it heats up just like a tire which contains air. The only difference (and the reason race teams use nitrogen) is that the air pressure in each tire containing nitrogen will increase by the same amount when those tires are heated where tires containing air may not all experience the same pressure increases over the same heat range (due to varying amounts of water in each tire).

People assume that because race teams use nitrogen it must be better for their tires, which is foolish. The only reason race teams use nitrogen is because it provides consistency and repeatability, not becaue it is some kind of performance secret.

split_window

Costco sells tires at very good rates and does rotation and balancing for the life of the tires that you buy from them for free. This was the reason I got my tires there for my Caddy CTS.

As a side, they had just started using Nitrogen. For what it is worth, here are my observations from my on-board pressure gauge:

1. The N pressure does rise as the tires get hot. duh! This is Nitrogen, not some "unobtainium" gas. Take any gas, warm it up, it will expand. Period. End of discussion. Does it go as high as "air" tires? Can't say for sure, but I would guess not. We are splitting hairs though because the difference is only a couple psi max.

2. The N filled tires' pressure first thing in the morning seems to be more stable than that of the "air" filled tires. By that I mean that the cold tire pressure doesn't fluctuate as much AND does not decrease over the longer term as much as with "air". It may be molecular size. It may be that "air" dances on the dew point borderline (dew in the morning, vaporized as the tires warm). For whatever reason, the N does a better job of staying in the tire. It is, however, not perfect. I still have to have the Costco folks top off the N on occasion, just not as frequently as with "air". Yes, I am **** about keeping tire pressure where it should be.

Long story short: N does do a better job than "air" of staying in the tires, but it is not perfect. As for the corrosive effect of vapor, I can't say either way.

Denney

Beat me to it. I was going to say I already run 76% N2 in my tires and that seems to be good enough for me...

Grebs

I'd think twice before filling my tires with nitrogen. "The mesonetric resonance of the neutrons surrounding the hydrogen atoms is affected severely by competiing vibrations of the elastopolymer atoms contained in the nearby tire wall. Normally this not a problem when the wheel is at rest or reasonably low speeds, but add the counter-rotational effects of a wheel rolling at precisely 88.6 mph and you have a recipe for disaster. The one time I took my Delorean to this speed, it lifted a foot off the ground and rotated horizontally3 times before touching down again. Luckily I was pointed towards the nearest restroom when I landed. Needless to say, I'm back to filling my tires with helium".
Dr. Emmett Brown, physicist.

chris ritchie

You'll lose judging points if you use nitrogen. Your car's tires were delivered with air from St. Louis. There is no discernible difference between today's St. Louis air and that from the 1960s or 1970s. So you can get away with today's air for judging. Consider it repro air. But nitrogen is definitely verboten. Sorta like Pertronix or DOT5 brake fluid.

The Black Knight

Chris I love it!!!!!



Too funny!!!!!

rgs

I'm afraid I'll have to disagree. Take a look at a Handbook of Chemistry and Physics for vapor pressure of water. It definitely is not linear. Here is a link to an online table.

http://www.chemistrycoach.com/vapor_...e_of_water.htm

DansYellow66

Thanks for the info. Guess I'll pass for now and keep my air compressor drained.

Dan