Why Bother with Nitrogen in Tires?

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Introduction

Figure 1: Nitrogen has advantages for inflating tires..

Figure 1: Nitrogen has advantages tire inflation. (Source)

Every Friday afternoon, the hardware and software engineers sit down in our lunch room and chat about what occurred during the week. The discussion is always lively and includes management and engineers. It is my favorite time of the week at work. This week we discussed why automotive shops put "dry nitrogen" in car tires (Figure 1) – I am pretty sure all compressed nitrogen is dry because of the way it is processed.

A couple of our engineers are motorheads and they use nitrogen to fill the tires of their personal vehicles. This post is an extension of my earlier post on the variation of tire pressure with temperature.

We normally fill our tires with pressurized air, which is 78% nitrogen. There are four main reasons why professional car people use 100% nitrogen:

  • Nitrogen-filled tires retain their pressure longer.

    All tires eventually lose air pressure with time as the gas diffuses through the tire. Nitrogen has a lower rate of diffusion from tires than oxygen, so the tire retains it pressure longer. The permeability of a gas through a material is a function of the gas and the material. For example, the permeabilities of nitrogen and oxygen through poly-isoprene are (Source)

    • Oxygen: {{\kappa }_{{{{O}_{2}}}}}=4.6\cdot {{10}^{{-13}}}\frac{{\text{c}{{\text{m}}^{3}}\left( {\text{STP}} \right)\cdot \text{cm}}}{{\text{c}{{\text{m}}^{2}}\cdot \text{s}\cdot \text{Pa}}}

    • Nitrogen: {{\kappa }_{{{{N}_{2}}}}}=1.6\cdot {{10}^{{-13}}}\frac{{\text{c}{{\text{m}}^{3}}\left( {\text{STP}} \right)\cdot \text{cm}}}{{\text{c}{{\text{m}}^{2}}\cdot \text{s}\cdot \text{Pa}}}

    Where STP standpoints for standard temperature and pressure. As you can see, the permeability of oxygen through synthetic rubber is substantially greater than that of nitrogen.

  • Nitrogen gas is dry -- it has virtually no water in it.

    We isolate nitrogen from the air industrially by the fractional distillation of liquified air. This process removes all water from gas. Using a dry gas provides two advantages:

    • Removing water eliminates a source of corrosion.

    • Removing water results in less pressure variation with temperature. See Figure 2 for an example (Source).

      Figure 1: Variation of Tire Pressure with Moist Air.

      Figure 2: Variation of Tire Pressure with Moist Air.

  • Unlike oxygen, nitrogen gas does not promote corrosion.

    At least for iron compounds, oxygen promotes rust. Oxygen and water together are bad for iron-based materials.

  • Nitrogen is relatively cheap.

    There are lots of dry gasses available. Nitrogen is about as cheap as you can get.

After we completed our discussion of tire pressure, we then discussed how we need to liven up our discussions. Sitting around on a Friday afternoon discussing tire pressure sounds really boring. I guess that's what life is like with a bunch of engineers around.

When I worked on torpedoes, we used to backfill them with nitrogen. The main concern there was corrosion.

 
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One Response to Why Bother with Nitrogen in Tires?

  1. Joe Webb says:

    Sorry, but in all practicality, this is misleading and inaccurate. It ignores other variables which commonly have a greater effect on air/N2 escaping from a pressurized tire than diffusion through sidewall/tread. Furthermore, as your graph misleadingly portrays, liquid water would absolutely *have* to form inside the "Moist air" tire in order for the graph results to differ by the 33% indicated (about 33psi vs. 44 psi!). A closed volume of merely unsaturated "moist air" without the formation of any actual liquid condensate, will not cause any such change in pressure as temperature varies. This information reminds me of the attempted sales gimmick my wife and I were dragged through for Nitrogen-charging the tires when we bought a new car a few years ago (they wanted $150!), after the salesman promoted their magic 10-year clear wax miracle for $499! Fortunately, a little wisdom and some education saves one money. That applies here, too.
    If you're going to present things as facts, then include *all* the facts, relevantly and honestly.
    As non-dry air is compressed, clearly the water vapor within will be forced to eventually precipitate if/when the pressure achieved get high enough. Simply take a gander at a psychrometric chart, apply the ideal gas law, and forecast what happens and when to the water vapor->liquid in the closed volume. You'll find that at common and typical RH where we drive and ride, the relative pressure differential in a tire isn't adequate to form effective levels of condensate at the temperature ranges indicated by your chart. Unless it's monsoon-ing, there's practically no need to use Nitrogen, period, or even dried processed air, to avoid significant condensation in tires (unless you're airing military plane tires). Granted, a wet air compressor will contribute significantly to the RH going in (it can even inject straight liquid water whilst filling if it's contaminated/wet enough - seen it happen), but you don't indicate or mention *that*. The REAL and important rule of thumb here is: Keep your compressors clean and dry, and don't fill tires from unmaintained compressors.
    If you air your tires with either ambient air (i.e. a compact on-demand compressor) or air from a clean compressor tank, there will be no condensate, and hence no corrosion inside your wheels.
    Regarding the N2 vs. O2 permeability argument, gas diffusion occurs very minimally through a modern multi-layered motor vehicle tire casing, even over months. You would probably find it to be insignificant. Much more air/nitrogen/gas escapes through the schrader valve during the process of checking tire pressure, and especially around the tire-rim bead seal (quite an imperfect seal indeed, is variable among vehicles, is a hugely depends on the rim's condition, the ingress of contamination, a proper cleaning prior to mounting the tire, and inclusion of decent sealant). Again, there's no honest empirical data to support the handy diffusion constants for O2 and N2 shown. Also, let's see any (assumed) calculation: per volume 20% O2, permeability constants shown 4.6/1.6=2.88, a difference factor or 0.2x2.88=0.57 (now that's a much smaller number than 2.88!), and then consider the actual *rate* of the permeation through the thick rubber (can't count the metal rim; it's zero) at which this occurs *practically*; it's pretty darn insignificant in a motor vehicle tire (note: not so in a bicycle tire, esp. like with latex-based and racing tubes).
    Personally, I've never used N2 in any tires. Currently live in a relatively arid climate though, and have always kept my compressors clean and drained. When changing tires for new ones, I make sure the rim bead is nice and clean and free of old rubber skids-marks and remains, dirt, scale and rust or corrosive ingress near the outer edge of the bead from road salt. This requires proper cleaning with steel wool and an use of an agent (like 409 or something), then rinsing/drying, then a bead sealant. And I will kindly but insistently accompany the techs at the tire store in doing this ($5 tip helps for forthcoming and future cooperation - and encouraging clean work in general). I've never had a problem with losing air, even over months. If this cleaning task is not performed, Nitrogen will leak from your tires just as readily and apparently as regular old air; there would be no discernible difference. On my motorcycles, I do my own work, and so don't have to monitor or convince techs to perform a proper job.
    One last point, one of further practicality: If one does indeed N2 fill their tires, and then happens to need to top-off (whilst on the road and away from one's N2 source; a common expectation), then what? How do you know whether/not there's been condensate introduced? Upon returning to your reliable N2 source, if you choose to drain/refill with N2, then there's *still* going to be liquid H2O in your tires, unless you dismount and dry/remove the water. If not, the N2 in your tire is no longer dry; all that bother, effort, and extra $ for nothing. The wise solution is to take along a little 12V tire pump; compact, cheap, re-usable, lasts forever, no condensate, easy to use, always fresh air.
    -Joe

     

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