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Saturday, April 9, 2011

Inflate with Nitrogen and there will be MAGIC

Ok so I may have exaggerated a bit, but not by much.   


Previously I started a series on proper inflation of tires. Before I continue lets cover the Nitrogen inflation question.

Nitrogen, Symbol  N  Atomic Number 7   Mass 14.01

Important concept: A claim of “up to” 30% savings can be met with a 0.01% savings. Telling the truth but leaving out the whole truth can be misleading.
When it comes to the suggestion of inflating with Nitrogen there are many claims. Am I suggesting all the claims are false? Not at all, but I believe many are based on assumptions not stated or significant exaggerations of the data that does exist.
Would I see some benefits if I inflate my tires with Nitrogen? The quick and short answer is yes. But, would these improvements be meaningful or cost effective? The answer to this question is probably not.
Warning: What follows is a detailed engineering discussion. Only read if interested. Otherwise you can skip to the end summary.
Let’s look at some claims
Nitrogen is a dry gas and does not contain moisture. A more accurate statement might be that all compressed gas purchased in a metal tank at very high pressure could be considered dry. This would include a tank of CO2, Argon or even a tank of Air.
Oxygen in compressed air contains moisture and is known to cause oxidation. It’s not the O2 that contains moisture. The compressed air we get from an air-line at the local gas station contains more moisture than high pressure gas.
Tires lose air pressure at about 1% to 3% per month when temperature and barometric pressure are held constant. This is true.
Tires inflated with Nitrogen lose pressure more slowly than when inflated with air but they will still lose pressure. True, but they will seldom tell you in the sales pitch that tires inflated with N2 will still loose pressure. The big selling point for Nitrogen is that if you don't check your tires it is a better inflation gas than regular air. But no matter which gas you use, you need to check to ensure you do not have a puncture or leaking valve.
Would you accept the concept that if I use synthetic engine oil I don't have to check the oil level because synthetic lasts longer?
Nitrogen-filled tires run cooler and can improve tire life up to 25%. Engineer Speak Warning. The Thermal Conductivity of pure N2 is 9.8 miliwatts per meter kelvin and O2 is 9.3 making the N2 more thermally conductive by 5%. So the heat generated by the tire due to flexing can be transferred 5% better to the metal wheel for dissipation to outside air. What they fail to mention is that if N2 conducts heat better it will also conduct heat from bearings and brakes to the tire.
Some have claimed fuel economy gains of over 20%. Well here is another claim that is hard to justify if you know how a tire works. It is well established that increase in inflation will lower the fuel usage because the tire deflects less. So, if the claim is that N2 run cooler, how does that not result in a lower pressure in the tire than when the tire runs hotter? Doesn’t it follow that if the pressure is lower, the tire will deflect more which results in greater energy loss due to tire deflection? The only way for a tire inflated with N2 to yield better fuel economy is if you compare it to a tire that has not had its inflation pressure maintained.
Nitrogen does not expand at the rate compressed air does while heating up as you travel down the road.OK now we are really getting technical. First the Gas law needs to be considered. As pointed out this is theoretical but "the ideal gas law is a good approximation for most gases under moderate pressure and temperature." The link also points out "If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature". Now the tire is a closed system so the number of gas molecules does not change. There is a slight increase in tire volume as pressure increases and of course this would offset some of the theoretical pressure increase but we can assume that is essentially negligible for the few degrees temperature difference we might be considering. This take us right back to the question of the properties of pure Nitrogen vs. Dry air. This also brings up the question of what percent Nitrogen do you end up with even after a purge with N2. The numbers I have seen indicate you would be going from 78% to 95% N2 not 100%.
Some claim you can reduce tire failures by 50 percent. If this were true don't you think the tire companies would all have been inflating tires with N2 for decades? Where's the data?
Some like to point to aircraft tires. They use N2 but for a different reason. The Autoignition temperature for rubber is 260 - 316°F. Unless there is some mechanical failure you will not see this temperature. I have seen many dozens of tires where components have exceeded 390°F and there has been no fire so it takes a special set of circumstances for this to be an issue.
End Summary
Now having said all this, would it be better to inflate with Nitrogen than air? If there was no cost and you could somehow have N2 available all the time, my answer is yes.
The problem is this is not a perfect world and I don't want to carry around a tank of Nitrogen to inflate my tires or run my air tools as I did with my race car. Even if I was given the tank and regulator for free, hauling it around will take up space and cost me fuel, most likely much more than I might theoretically save with the almost magic properties attributed to this "Wonder Gas".
A lower cost thing to do is to inflate your tires at least 5 psi over that needed to carry the actual load while not exceeding the max for the tire or wheel. If you are concerned about moisture buy some inexpensive air dryer and inflate your tires with dryer air in your line just before your air chuck.. You will gain almost all the anti-corrosion benefits by eliminating the moisture and you can keep these pocket size "filters" almost anywhere if kept in their original packaging. The filter #68215 at Harbor freight is OK for 90 psi and costs less than $8 and another $10 or so will allow you to make up the needed air chuck adapters.

UPDATE:5-17-12 I will do some research and try and find similar dryers for 125 psi application and make a new post.

The questions supporters don’t address are; What is the initial cost? How will you refill your tire pressure as time goes on? Will you be faced with the decision to drive on an underinflated tire to get to some place to inflate with Nitrogen?
What is not stated with all the claims is that if you check your tires for damage and inflation and adjust as you should ie before each trip, you will not see the performance improvements claimed.

13 comments:

  1. The air we breath has 29% nitrogen in it

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  2. Well Anonymous, not sure where you got your info but according to Wikipedia, "Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%."

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  3. I think the point was that if air is already 4/5 pure nitrogen, then all the claims need to be discounted by 4/5. Although you touched on this, it should have been a core part of your argument, and by not making it so, you have been led into errors. Consider the thermal conductivity argument. You state that the thermal conductivity of nitrogen is 9.8 mw/meter-kelvin, and oxygen is 9.3. The difference is 5%, so you go on to state that with nitrogen, heat transfer to the metal wheel will be 5% better. Better than what? It is 5% better only if you originally filled with pure oxygen, and then switched to pure nitrogen. But no one fills with pure oxygen! The proper answer is that since air is already 4/5 pure nitrogen, the difference has to be discounted by 4/5, and the difference in conductivity of pure nitrogen over air is 1%, not 5%.

    Nitrogen leaks more slowly that air? This claim makes no sense at all. Presumably the leakage is through microscopic pinholes in the tire. And since the oxygen molecule is physically bigger than the nitrogen molecule, it will pass more slowly through a tiny pinhole, not more rapidly. If I have two identical buckets with a variety of holes in them, and I fill one with golf balls and one with baseballs, and I shake them, which bucket will empty faster? My guess is the one with the golf balls, because they are smaller and pass more easily through the holes.

    None of this really makes any difference. The people who believe in nitrogen believe it with almost religious fervor, and the nitrogen sellers are their prophets. Facts are irrelevant.

    Bill (a different Anonymous)

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  4. Although airplane tires are made of rubber, the use of nitrogen in them is so it does not add to the fire if such happens. And, why do you think dry powder fire extinguishers are filled with nitrogen for a propellant? Two cents from an old fireman.

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  5. Thanks for the info. Adds up to what I've been telling people all along. I am about to buy new tires and after checking prices and offerings I will probably end up in that N2 inflation place, but only for the better deal. If they install the "I'm a hype eating moron" green valve stems we will go to war... ;)

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  6. So, if I bought a car that had tires on it already inflated with Nitrogen, does that mean that I cannot at some later date, add compressed air to the mix?

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  7. Jerry. I think that with Nitrogen the least expensive inert gas it is the obvious choice when you don't want more air (with its Oxygen) added to the fire.



    Linda. Don't worry about it. Nothing wrong with tires being inflated with Nitrogen. A car my wife bought had it. I have used my relatively dry shop air to add the few psi she needed. Since I retired from racing I no longer have a bottle of Nitrogen in the shop so I use air.

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  8. It seems to me that, with tires needing to be replaced every five years, nitrogen is not cost effective. Here, in AZ, tires weather check badly in a few years. The tires on my truck are less than three and are already checking, only just over 15,000 miles and only 20% tread wear.

    Tire manufacturers only guaranty against road hazard, and/or sidewall blowout for five years, as I understand after talking to Goodyear, and Michelin.

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  9. Drived a truck with tires inflated with nitrogen and I must admit it drives better. Tom

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  10. There are a lot of rumors about why airplanes use nitrogen in their tires...(Incluidng fire safety...also B.S.)
    The simple reason is because airplanes also have high-pressure oleo-struts in their landing gears that require 1,000 psi and higher pressures. The internal components of the oleos require DRY air. Nitrogen bottles are dry nitrogen. Mechanics who service those landing gear oleos also service the tires....some of which also commonly require higher pressures (160 psi and higher) than portable compressors are capable... so the mechanics have high-press nitrogen bottles of dry nitrogen at 2,200 psi. Those bottles are usually carried in portable wheeled carts and sometimes in multiple-bottled manifolds, which are easy to tow around the ramp to service the aircraft, and can be used for both landing gear and tires. A side benefit is dry air/nitrogen in tires will not contain frost/ice within them at cruising altitudes where outside air temps are in the -50 degree range, and upon landing when that tire accelerates from zero to 120 mph+ doesn't create steam within to create yet another hazard to pressures and failures.
    Simple as that.

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  11. At one time tires lost air 1-3% per month. Tire technology and rubber chemistry has improved and the loss is now significantly less than that. I check mine frequently and I do not lose more than 1-2 psi per year. Most of that is probably from the checking itself. This makes the advantage of pure nitrogen even weaker.

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  12. I think this is a great read. I believe I would inflate with nitrogen rather then air but thats just my opinion.

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