• May 3, 2006
The picture at right nicely captures the sales message at many service centers. But will you get better gas mileage if you inflate your car's tires with nitrogen instead of compressed air? That's one of the claims of advocates of nitrogen inflation systems, like N2Revolution, and auto service centers that offer nitrogen inflation services (often for a fee).
The idea stems from the fact that underinflated tires increase rolling resistance, adversely affecting fuel economy (among other things). Any gas mileage improvement through using nitrogen in tires would come from nitrogen's reported ability to maintain tire pressure at the correct level for longer, because nitrogen is supposed to leak through the walls of the tire more slowly than oxygen. Well, perhaps, but air is already about 78 percent nitrogen, so devotees won't really realize a huge benefit from switching to pure nitrogen. Not only that, but leakage is likely to be more significant at the tire bead or around the valve than through the body of the tire.

Other nitrogen benefits are sort of "inherited" from its use in aircraft tires, truck tires and race car tires. (If it's good for the pros it should be good for us, right?) The use of nitrogen is required in the braked wheels of aircraft over a certain weight, because the inert gas reduces explosion risk under combinations of extreme loads and high temperatures that far exceed the conditions experienced by car tires. As an inert gas, nitrogen may also extend the life of the tire carcass - important if tires are retreaded, as is common for trucks and aircraft, but insignificant for car and light vehicle tires, where tread wear determines longevity. Racers know that the most important benefit of using nitrogen is that it is "dry" - the water vapor in compressed air causes too great a pressure change as the tire heats up, a big deal when a half-pound of air pressure change can affect the handling of a racecar at the outer limits of performance.

The bottom line? Nitrogen is no substitute for checking your tire's inflation regularly. If it's free, go ahead and use it. If you have to pay for it, save your money.

[N2Revolution via CNET]


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  • 15 Comments
      • 8 Years Ago
      But how much energy is used to extract O2 and all the other stuff from the air we breathe, ship it to car centers, etc.?

      On another note, the only time I have had problems with mags leaking is when I only had one set of rims for summer and winter tires, eh. I don't know if it's corrosion or the mount-dismount.
      • 8 Years Ago
      1. Michelin Tecnical Bulletin PM-03-05 "Michelin supports the use of nitrogen based on its ability to better retain air over a period of time."
      2 Bridgestone Real Answers, Volume 8, Issue 3 "Nitrogen permeates through rubber more slowly. It might take six months to lose 2 psi with nitrogen, compared to a month with air."
      3 from the same article "water vapor in compressed air acts as a catalyst, accelerating rust and corrision. Water vapor also absorbs and holds heat. And,when it changes from liquid to vapor, water expands tremendously in volume. So tires inflated with air tend to run hotter and fluctuate in pressure more.
      4.Goodyear Product Service Bulletin PSB 2004-09
      "Goodyear supports the use of nitrogen....., based on the ability to retain pressure for a longer period of time."
      5.High quality Generators can produce 99.9% nitrogen, high quality installers quarantee 95%+ in the tire, as recommended by the "Rubber and Plastic News, Vol 34, No 4, pp14-19, 2004
      6."Every day we are losing over 2 millions gallons of gaoline to a seemingly unimportant thing---low air pressure." US Department of Energy

      just documented facts--Bridgestone, Michelin and Goodyear do not sell Nitrogen Generating equipment.
      • 8 Years Ago
      Tire pressure is measured as relative to atmospheric. If atmospheric is 14.7 and your tires are inflated to 32, the actual pressure inside is 46.7. Thus your tire is not already half-inflated. If you put a gauge on an empty tire, it will read 0, not 14.7 or atmospheric pressure.
      • 8 Years Ago
      [quoted from pischemist at eng-tips forums]

      The question about nitrogen in tires has come up in other lists on the web as well. Let me clarrify a few points.

      1. Air is 78% nitrogen, N2, and 21% oxygen, O2. So even if you put air in the tire, it's already 78% nitrogen. Many of the so called nitrogen generators don't produce much more than 90% nitrogen.

      2. At relatively low pressures (ie tire pressures) N2, O2 and water vapor will all behave as ideal gases, and follow PV=nRT. Pressure will increase or decrease to the same extent as the temperature increases or decreases regardless of which gas is in the tire. (Even at 300 psi, which is about 20 atm, there is little deviation from ideality.) Therefore the comments about N2 not changing in pressure as the temperature changes are without merit.

      3. The rate of effusion (or diffusion) of a gas through a porous membrane depends on the molar mass and to some degree on the molecular diameter. N2 and O2 are almost the same size and N2 is lighter than O2 (28 g/mol vs 32 g/mol) so if either gas were to effuse out of the tire, nitrogen would do it more quickly. Luckily, tires are designed not to be porous membranes.

      4. N2 and O2 both have essentially the same specific heat capacity, about 1.0 J/gK, and thermal conductivity, about 0.00026 W/cmK. Water vapor has a specific heat capacity of about 2 J/gK. But remember, water vapor will constitute less than 1% of the air in the tire. So the idea that N2 has different heat handling properties is also without merit.

      5. The ozone, O3, in the atmosphere, which is a ground level pollutant, will do a great deal more damage to your tires than the O2 inside the tire. For instance, don't leave a condom out in the air in Los Angeles for a few days. It will develop lots of tiny holes and weaken.

      spdracer22 says that dry air is preferably to air with a lot of water vapor. As a tire heats up, the very small amount of H2O present will be in the vapor state which may contribute to the overall pressure very slightly.

      Several have suggested that N2 in a high pressure tank is more portable and requires no electricity. That would make sense, particularly for aircraft tires.

      I find no reason to believe that N2 is going to produce a "better ride" or "better handling".

      The bottom line is that for general passenger car tires or truck tires there is nothing to be gained (other than portability) by using nitrogen rather than air. The biggest gain will be $$$ by the companies that sell nitrogen handling equipment and the tire merchants that appeal to ignorant customers. And who is the biggest loser? Yep, the consumer.

      Regardless of how much you want a gas not to increase in pressure when heated, its just not going to happen. The laws of chemistry and physics apply all the time. PV=nRT is true all the time, for every gas, be it air, nitrogen or water vapor. Saying that a gas does not increase in pressure as temperature increases is nonsense.

      Also, dont worry about the water in the air. It exists as water vapor, a gas, and is simply part of the total pressure in the tire. As the tire heats up, the water vapor will continue to be a gas and be governed by the equation PV=nRT, just like air, which is 78% nitrogen.

      Every gas is going to escape from a tire, regardless, as long as the pressure inside the tire is greater than the pressure outside the tire. The gas molecules are going to diffuse through the walls of the tire at a very slow rate. It doesnt make any difference what the gas is, although there are small differences between the diffusion rates of oxygen and nitrogen. Since nitrogen is slightly less massive than oxygen (28 g/mol vs 32 g/mol) Grahams law predicts that nitrogen will diffuse slightly more rapidly than oxygen.

      The bottom line is that you cant set aside science in favor of hype and wishful thinking.
      [the whole thread can be found at: http://www.eng-tips.com/viewthread.cfm?qid=120996&page=2 , hope this puts an end to the myth. ]
      • 8 Years Ago
      jm,
      Thanks for posting that...
      • 8 Years Ago
      #7, Racers at all levels use nitrogen in their tires, from F1 to go-karts. The nitrogen you source from a gas supplier is very "dry" (most of the water vapor is removed) and always the same, while compressed air contains more water vapor and the amount varies. From personal experience, race tires "air up" (gain pressure when hot) more if you use air than they do if you use nitrogen, but more important, because the nitrogen always has the same water content, the tires will respond the same to heating from one day to the next. This lets you predict accurately what the hot pressure will be when you set the pressure "cold" in the pits.

      An alternative to bottled nitrogen is using a drying system for compressed air, but nitrogen is widely available (any welding supply dealer) and cheap, so most racers just keep a gas bottle in the trailer.
      • 8 Years Ago
      I wanted to reply to Mike - the guy that responded first.

      Aluminum wheels have a clear coat on them when they are made. Tire centers that do not take care not to scratch this will do just that, and then the aluminum starts to corrode.

      At this point, you can only try to clean it up when the tire is installed or have it clear coated again. I have had good luck getting the tire guys to take a wire wheel to the back side of the rim where the bead seats, and removing all the corrosion and flaking clear coat paint. This allows the tire to seat well and generally lasts for a year or two.

      Eventually, you will have the corrosion wrap around to the front of the wheel and it will look very bad. Then you only choice will be to replace it or bead blast it and have it recoated.

      Hope that helps.
      Mike
      • 8 Years Ago
      If you are the type that checks air pressure on a regular basis just use regular air, if you don't check your air much it is a wise idea to fill it with nitrogen.
      • 8 Years Ago
      Costco does it for free. If you are low, you simply fill up on air, and take it in to costco when you get a chance (and then they'll purge it and refill for free). BTW, they generate the gas onsite, not transport it.

      So, just out of curiosity, if it is all bunk, why DO they use it in racing? In a place like F1 racing where every single thing is calculated, I find it hard to believe they jsut do it for no reason at all. (note I am not saying it provides any benefit for the average user, but there must be a reason it's done...?)
      • 8 Years Ago
      Theoretically, if you always put air in your tires, but the oxygen leaks out more easily, then you'll end up with mostly nitrogen over time anyway. No doubt if this really happens.

      One further possible advantage is that nitrogen might reduce the tendency of some rims, especially chrome rims, to corrode. All four wheels in my wife's four-year-old car are currently leaking air because the rims have corroded where the rubber touches them, in places you cannot see. One loses about 7 pounds a day.

      Through my site I'm currently studying how common this problem is. If it is a common problem, nitrogen still might not help.
      Robert
      • 8 Years Ago
      Inflate nitrogen-in-tires mixed with helium and you will have one happy ride! LoL... Hey, don't laugh ballons with helium float and rise rapidly when released, why not tires on autos???
      • 8 Years Ago
      I work at a dealership that just spent an enormous amount of money on one of those "nitrogen generators"
      Most of my peers agree that it is a waste of money for both the company and the consumer. ebing that air pressure is pressure whether it be air or nitrogen
      32 psi is 32 psi, right ?
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