Nitrogen in tires?

[Bruce Dorsi]

Some tire dealers, and service stations, are starting to recommend filling passenger car tires with nitrogen.

Ingersoll-Rand, who makes a system to separate nitrogen from compressed air, claims 40% better pressure retention, 30% longer tire life, and up to 6% increase in fuel economy, if tires are filled with nitrogen.  

These sound like claims which would be hard to substantiate!  ....Of course, the "up to 6% increase in fuel economy" could mean .001 mpg!

I know nitrogen has been used for many years to fill tires in aviation and race-car tires.  ....Now there is a move to promote it for street-use tires.

Snake oil?  ....Or, valid claims?

I'd like to hear your opinions.

I'm here to learn!

[enjenjo]

It just so happens that I was involved in some real world testing of nitrogen for tires.

The trucking company I worked for tried it. After all, when you spend $25,000,000 a year on fuel, even 1 percent starts to run into real money. On an 8 month test, using 8 identical tractors, on the same route, 4 with air and 4 with nitrogen, the same drivers, but not always in the same truck, there was no statisical difference in fuel economy. In fact there was more difference from truck to truck in each group of 4, than there was between the two groups.

We did not test tire life, of course truck tire casings usually outlast three recaps anyway. And tire pressure is checked every thirty days. During the course of the test, we had 3 tire failures, all from road debris. That doesn't include tires that were replaced for tread wear, balance, or similar problems

[docchevy]

It sounds like a spoof to me.  The nitrogen molecule is smaller than the oxygen, so it would leak better.  The heat capacity is about the same.  The chemical activity of the oxygen would be rather insignificant in something as inert as rubber.  The only possible benefit that I could see would possibly be less rusting of the rim.  
Perhaps it's just a scam to get us to pay for the gas you put in the tires as well as the tank!
The next thing you know they will be advocating the use of CaCl2 in the tires like farmers use!

[Crosley.In.AZ]

nitrogen is suppose to leak out slower.  usually a green valve stem cap indicates nitrogen in the tire.

[HOTRODSRJ]

Some tire dealers, and service stations, are starting to recommend filling passenger car tires with nitrogen.

Ingersoll-Rand, who makes a system to separate nitrogen from compressed air, claims 40% better pressure retention, 30% longer tire life, and up to 6% increase in fuel economy, if tires are filled with nitrogen.  

These sound like claims which would be hard to substantiate!  ....Of course, the "up to 6% increase in fuel economy" could mean .001 mpg!

I know nitrogen has been used for many years to fill tires in aviation and race-car tires.  ....Now there is a move to promote it for street-use tires.

Snake oil?  ....Or, valid claims?

I'd like to hear your opinions.

I'm here to learn!

Actually, I submit that the nitrogen atom to be larger than oxygen?  And "air" is made up of several molecules and nitrogen will leak alot slower from the tire and therefore......... they diffuse less easily through a tire's sidewall. As a result, tires filled with nitrogen tend to remain properly inflated longer.

In addition, nitrogen disperses heat more quickly than compressed air, resulting in cooler-running tires, which helps make them last longer. This temperature stability will keep the tires inflated to the exacting pressure instead of expanding and contracting with temperature.

The advantages are obvious. Tires that remain properly inflated experience longer tread life and exhibit lower rolling resistance, improving fuel mileage. Nitrogen also makes tires safer simply by keeping them fully inflated longer and does not support combustion. Air-filled tires, in contrast, tend to lose pressure over time, which becomes a problem only because so few drivers check their air pressure regularly.

Typically, tires filled with air leak about 2.8 lbs per month and filled with nitrogen...only .7lbs per month.  

But, nitrogen costs alot to fill up with...about $20 per fill I have seen.

[Bob Paulin]

In addition, nitrogen disperses heat more quickly than compressed air, resulting in cooler-running tires, which helps make them last longer. This temperature stability will keep the tires inflated to the exacting pressure instead of expanding and contracting with temperature.

Where does the heat disperse to?

You've still got the same steel/aluminum/composite wheel and tire surface areas from which the heat actually radiates and disperses away from the tire. How does nitrogen cause them to disperse heat more quickly?

The only reason we ever used nitrogen in oval-track race car tires is that it is "dry" and, therefore, not as subject to heat-related air pressure changes as "regular" air - which in itself is 78 percent nitrogen - whose moisture content expands upon being heated.

The average boulevard bomber doesn't begin to generate the sorts of 200° to 250° tire temperatures that an oval-track racing tire develops, so the non-expansion "advantages" of heated nitrogen are substantially diminished for street cars as opposed to race cars.

The advantages are obvious. Tires that remain properly inflated experience longer tread life and exhibit lower rolling resistance, improving fuel mileage.

Air-filled tires, in contrast, tend to lose pressure over time, which becomes a problem only because so few drivers check their air pressure regularly.

As Enjenjo pointed out, monthly tire pressure checks also work well.....and are much cheaper.

I believe this whole nitrogen craze runs along the same lines as "computerized, four-wheel alignment" where there are more benefits (spelled p-r-o-f-i-t-s) for the shop offering the service than the consumer purchasing the service.....

.....at least that's the impression I get from the advertisements I see for nitrogen systems in the weekly "Tire Review" newspaper I receive.


B.P.

[PeterR]

For temperatures in excess of 0degreesF the thermal conductivity of air is slightly higher than nitrogen.   Not that it matters, heat generated in the tire carcase has to be dissipated into the surroundings not into the enclosed air.

If the oxygen molecule was significantly smaller than nitrogen (which it is not) and it bled quickly through the tire, then as oxy only makes up 20% of the original mix, after a few top ups all the oxy would be gone leaving the tire filled with the remaining pure nitrogen.  --And at no cost to the driver.

A typical tire has less than 0.5lb of air when fully inflated. Leakage at a rate of 2.8lb/month corresponds to about 0.1lb/day. At that loss rate we would be topping up tires about twice a day.

Nitrogen, air, or human generated "bean gas" has exactly the same temperature/expansion properties, so a tire filled with any of these would behave the same.  As Bob points out the real culprit is moisture in the gas, and this can be eliminated by using compressed breathing grade air at a fraction of the cost of nitrogen.

[Bruce Dorsi]

Thank you for your responses, gentlemen!  .....You have reinforced my skepticism!


Follow-up questions:

Nitrogen, air, or human generated "bean gas" has exactly the same temperature/expansion properties, so a tire filled with any of these would behave the same.  As Bob points out the real culprit is moisture in the gas, and this can be eliminated by using compressed breathing grade air at a fraction of the cost of nitrogen.

Since the moisture present in the compressed air is in the form of water vapor, and NOT liquid, isn't the vapor subject to the same gas laws?
.....Any gas will expand when heated, as previously pointed out.  Does water vapor have unique properties which make it immune to the gas laws?

Is it a false assumption to think that the gas temperature inside the tire ( especially on a race car) is sufficient to keep any water vapor from condensing into a liquid state?


Thanks again, for your replies.  ....I do not wish to be a pest, but instead, I hope to gain more knowledge of this subject.

[jaybee]

Since the moisture present in the compressed air is in the form of water vapor, and NOT liquid, isn't the vapor subject to the same gas laws?
.....Any gas will expand when heated, as previously pointed out.  Does water vapor have unique properties which make it immune to the gas laws?

Excellent point and I will be very interested to hear some additional input on this point.  I had heard that the whole point of using Nitrogen in race tires is that it doesn't build up pressure as much as atmospheric air and is more predictable.  That said, it's also the reason why I've always been skeptical about Nitrogen inflation of street tires.  They have very little heat build up compared to racing tires, so the benefits should be minimal if not inconsequential.  If there's any benefit it's surely outweighed by the expense.

[HOTRODSRJ]

I think this product is really aimed at the average joe...and not us per sei.  I know folks that NEVER  check their tire pressure....leaving it to the quick change places only.  But, I submit that it's based on good science and not BS.  Truck fleets are going to this in droves.

I also respectfully challange the notion that heat doesn't build up in STREET tires. It's obviously not like racing tires....but it happens.  I also may have misspoke when I used the word "dissipate" when actually it's more like "stable" in stead.....as prudently pointed out.

I have gotten off the interstate here in Hotlanta before only to find out that you could not hold your hand on the tire much to my surprise?  After replicating this...I shot the iR and tires can be a sultry 120+ plus (that was 55 degrees delta from that morning ambient temps)! The reason is simple.  Our cars blow heat down at the pavement and we all know these black surfaces suck heat up too from sunlight (ever try walking across a blacktop parking lot in your bare feet?) which gets transfered to your tires .......so the combination of heated air and moisture (vapor as correctly pointed out) will change the pressure drastically with temps.  This is the MAIN feature with this product......temperature and migration stability.

Supposidly by all accounts.......a pultry 5lb underinflation can cost you as much as 20% in fuel? So, with losing a couple of pounds of pressure month....you can see where this is going.  I frequently find my daughters car tires at less than 20psi???  And my wife won't ever check! So go figure.

[PeterR]

Any gas will expand when heated, as previously pointed out.  Does water vapor have unique properties which make it immune to the gas laws?

.......Excellent point and I will be very interested to hear some additional input on this point.

If all the moisture remained as vapour all the time it would follow the gas laws.    However that is not the case and as the moisture passes through a phase change its volume changes considerably causing disproportionate pressure variations.

Any dry gas will solve this problem.

[EMSjunkie]

Any dry gas will solve this problem.

not entirely true, Oxygen is a "dry" gas.
it should NEVER be used to inflate ANYTHING  
or run pneumatic tools. the consequences could be catastrophic  


Vance

[entodad]

I always thought that N2 was used to inflate tires in high stress situations like on an airliner so that if sufficient heat were to build up inside the tire, there would be no oxidizing agent to react with the rubber, or any other flamable substance inside the tire causing a fast rise in pressure and possibly causing a catastrophic failure of the tire.  
However in everyday service in a car, since atmospheric gas is about 80 percent nitrogen to begin with, it should make little difference in performance.  Just keep a gauge on the visor...and check you wife's pressure for her...cause God knows she won't!

Pure O2 plus a little heat will cause quite an explosion.  Just play with your oxy-acetylene torch to see that.

Doug

[

[PeterR]

Any dry gas will solve this problem.

not entirely true, Oxygen is a "dry" gas.
it should NEVER be used to inflate ANYTHING  
or run pneumatic tools. the consequences could be catastrophic  
Vance

For the purpose of the discussion relating to improving pressure consistency, any dry gas even the highly unlikely "bean gas" will suffice. However as you quite rightly state, compressed oxygen should never be used for general inflation or to drive power tools.  

The handling of any gas under pressure is a serious business and particularly so with oxygen.   Unfortunately there has been so much blatantly incorrect folklore about oxygen the importance of correct handling has been undermined.  

Oxygen itself is not explosive, but even small quantities of a combustible material in an oxygen rich environment may burn with such ferocity that the heat generated causes an increase in gas pressure sufficient to tun the container into a grenade.   The warning "No Oil" on the face of an oxy gauge is there for a reason

[HOTRODSRJ]

I always thought that N2 was used to inflate tires in high stress situations like on an airliner so that if sufficient heat were to build up inside the tire, there would be no oxidizing agent to react with the rubber, or any other flamable substance inside the tire causing a fast rise in pressure and possibly causing a catastrophic failure of the tire.

Good observation!

As an A&P, I can tell you that nitrogen is used widely and almost exclusively (most aircraft manufacturers will let you top off with dry compressed air (if N is not available)........


First, yes...temperature stability from the "dry" nitrogen is a wanted feature to stabilize tire pressures at wider operational temperature ranges...and yes the "dry" part is the most important here.. There is considerable heat generated by stopping an aircraft, so the temp ranges far exceed that of a car.

"PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES 25.733 (e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels." MOre to follow.
 
Secondly, flying at high altitudes...any moisture will/can be rung out of the air and freeze inside the tire! This is a no-no and a very unwanted aspect.  The ice can produce destruction inside the tires.  Third....the decrease in migration from the tires is a wanted feature as well even tho this is probably the least aspect. Forth.....brake fires are frequent on aircraft and N is a safer and more stable gas should the tire fail (see para above).  Fifth, common pressures in aircraft tires are in excess of 200psi - 300psi and oxidation rates at these pressures are far more than automotive applications. So, the reduction of oxidation is a wanted feature as well.  Sixth point.....convenience.  You can easily liquify N in a bottle and make it very portable.....where as you can't do that with air per se.

Just tooooo much info.