For a while now, I've been debating back and forth on the use of Nitrogen gas for filling tires. Race teams almost exclusively use it for that slight advantage that it brings over air filled tires, which justifies the costs. But that's racing, where the difference between a more aerodynamic hood latch is cause for controversy. Then Nissan announced that the new GTR would come with Nitrogen filled tires to enhance its high speed stability. Maybe that's just supercar hoopla, but the fact that Nitrogen is being considered for use on passenger cars by the OEMs says something. The matter was brought up again as some local BMW dealers are now offering to fill tires with Nitrogen as a free service, which means now it's dirt cheap.
So, does that mean everyone should be running on Nitrogen just because racers use it, and now that it's affordable? I'm still on the fence about this even though there are definite advantages to using Nitrogen gas. We have to get some background on what gas does in order to understand why Nitrogen might be advantageous.
Regardless of what gases we fill our tires with, the purpose of the gas is to both support the weight of the vehicle, add some spring and damping, and help maintain the tire's toroid shape. Anything in a gaseous state does this well since a gas can conform to any shape and will distribute pressure forces uniformly. We could even use liquids to evenly distribute the force, but that would make for a lot of unsprung weight. Liquids are also essentially incompressible, which would be an issue if the tire hits a bump that reduces its internal volume, turning your wheels and tires into water grenades from hell.
Gasses are lighter and can compress, which is why we use them. There is exactly the same pressure pushing against every square inch inside the tire and against the wheels. The pressure pushing out is always uniform, so that the tire is always evenly loaded. Having a consistent pressure distribution is what maintains the even tension on the hoop that the rubber forms around the wheel; it's this hoop stress that really carries the weight of the car, not the sidewalls.
But because gasses can compress, it also means they can expand. Since the gases inside a tire are held under constant volume and mass conditions, it means tire pressure can only go up as temperatures increase, which we see all the time at the track.
This brings us to the physical properties of Nitrogen. For most intents and purposes, we treat Nitrogen as an inert gas, even though it's not a real noble gas. It has a lower molecular weight than air, by about one gram for every 22.4 liters under standard temperature and pressure conditions. It's also not a melting pot of other gasses like air, which is made up of 78.08 percent Nitrogen, 20.95 percent Oxygen, 0.93 percent Argon, and a bunch of other trace elements. Even though air is mostly Nitrogen, the Oxygen in it, which are larger molecules and have more atomic mass, is what makes air just slightly heavier than Nitrogen. In most other aspects, air shares almost identical properties with Nitrogen since that's what it's mostly made of.
So, let's do some back of the envelope calculations just to see what the differences might be between air and Nitrogen. Take a hypothetical 205/50/15 track tire that has a tread surface temperature of 170 F. Without knowing the heat transfer rates of the tire, we'll just assume that the internal gas temperature is about 120 F. A rough calculation of the volume inside comes with about 19.6 liters of gas volume inside the wheel recess and tire. Under the Ideal Gas Law, this comes to about 0.103 pounds of gas if filled with Nitrogen and 0.107 pounds if filled with air, a hair splitting difference of 0.004 pounds or 1.6 grams in unsprung and rotational mass.
If we looked at it from a pressure stability perspective, air has just a slightly higher specific heating value. This means that it takes just a little more energy (btu) to increase the temperature of the same amount of gas by one degree. If it takes more energy (even though its splitting hairs) to heat up air than Nitrogen, then that means the same heat generated at the tread would increase the temperature of the Nitrogen gas just slightly more. This would then translate to the fact that Nitrogen fill tires should increase with pressure more than air fill tires.
So, who in their right mind would put Nitrogen into their tires instead of free cheap air? What we've ignored so far is water, or water vapor, to be exact. All of the calculations so far were based on the assumption that air is dry, which is far from the case. The amount of water in the air greatly varies from 4 percent to its lowest of 0.5 percent. When large amounts of water are present in the air, other elements are present in lower amounts, which lowers the overall density and weight of air since water has a lower molecular weight than Nitrogen or Oxygen, the major components of air.
If you've ever worked around a compressor, you'll know that under pressure, water collects at the bottom of the tank. This comes from humidity which, under the pressures inside a compressor, becomes liquid water. When that's pumped into a tire, it immediately expands back into vapor due to the much lower pressure inside the tire.
Unfortunately, the conditions inside a tire make it very easy for water to change between gaseous and liquid states since it has such a low vapor pressure and boiling point. Under cold conditions, the humidity inside the tire takes up very little space. Heat it up to full operating temps, and the water vapor expands and increases pressure far more than dry air.
This is where the real world advantages of Nitrogen come from. Nitrogen in a bottle is free of moisture, unlike the air you get from a compressor. Nitrogen molecules are also much larger than Helium and water molecules, which are present in air and can permeate through rubber causing pressure loss over time. The low permeability of Nitrogen and lesser effect of heat on tire pressure make Nitrogen filled tires require less service and allow racers to get out on track and be at full operating pressures faster. High speed supercars like the GTR also don't have to worry about pressure changes from the extreme heats of high speed runs. Some claim the slightly lighter weight of Nitrogen also reduces rolling resistance and rotational inertia for better fuel economy. How much is your guess?
But there are some practical disadvantages. Filling pure Nitrogen into a tire isn't easy since you have to get all the air and moisture out. Most race teams with single valve stem wheels have to fill a wheel and then purge it three times to get it close to pure Nitrogen. That can get costly. Some wheel manufacturers like Enkie make a dual valve stem wheel like the NT03+M where one valve is use to fill with Nitrogen, while the other valve is for simultaneously purging the air. It might have been better if they placed the stems 180 degrees from each other for better purging instead of adjacently.
Is Nitrogen better? Maybe, but I'll stick with air, which is almost 80 percent Nitrogen. And being in the low humidity desert climate of Southern California means that the air in my tires isn't mostly water. The minuscule weight and rotational inertia gains are a questionable matter since you'd end up with a lot better fuel economy by just bumping up your commuting tire pressures by 5 psi.
|Molecular Weight (g/mol)||28.96|
|Critical Temp. (°F)||N/A|
|Critical Pressure (psia)||N/A|
|Boiling Point (°F)||-317.8|
|Melting Point (°F)||N/A|
|Gas Density @ 70°F 1 atm (lb/ft3)||0.075|
|Specific Volume @ 70°F 1 atm (ft3/lb)||13.3|
|Specific Heat @ 70°F (Btu/gmol-°F)||6.96|
|Molecular Weight (g/mol)||28.01|
|Critical Temp. (F)||-232.5|
|Critical Pressure (psia)||492.3|
|Boiling Point (F)||-320.5|
|Melting Point (F)||-345.9|
|Gas Density @ 70F 1 atm (lb/ft3)||0.073|
|Specific Volume @ 70F 1 atm (ft3/lb)||13.8|
|Specific Heat @ 70F (Btu/gmol-F)||6.97|