Why is nitrogen gas preferred in car tires?

[Jack Ryan]

[James Thomas Rook Jr.]

100% Nitrogen is a molecule than "slowly" reacts with the tires "rubber", whereas normal atmospheric "air, is only 78% Nitrogen. 

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Clearing the Air About Nitrogen Tire Inflation

One of a tire's primary tasks is to carry the weight of the vehicle. But anyone who's ever had a flat tire knows that the tire doesn't really carry the load...the inflation pressure inside it does! Using the correct inflation pressure not only provides the appropriate load capacity, it also enhances the tire's performance, durability and contribution to vehicle fuel economy.

Tires are typically inflated with air that's a combination of roughly 78% nitrogen (N₂), 21% oxygen (O₂) and 1% miscellaneous gases. And since all gasses expand when heated and contract when cooled, tire inflation pressures rise and fall with changes in temperature by about one psi (pound per square inch) for every 10° Fahrenheit change in temperature. This is one of the reasons it's recommended that tire pressures be checked early in the morning before ambient temperatures, the sun's radiant heat, or the heat generated by driving causes the tire pressure to rise.

And while tires appear solid, if you could see their microscopic structure you would find that rubber looks a bit like strands of cooked spaghetti stuck together. These molecular strands continuously stretch to and from their relaxed state every time the tire rolls and conspire to allow some of the gas to escape through the microscopic spaces between the rubber molecules (called permeation or diffusion). It's been estimated that up to one psi of pressure may escape each month a tire is in service.

Fortunately compressed air is often available at gas stations, tire stores and auto repair shops. Sometimes it's free, while other times it's only available from coin-operated compressors. Unfortunately the compressed air often provided contains varying degrees of moisture depending on relative humidity and the compressor system's ability to dry the air by removing moisture.

"Rather than pay extra for nitrogen, most drivers would be better off buying an accurate tire pressure gauge and checking and adjusting their tire pressures regularly."

So what can we do to help maintain more constant tire pressures? We could change what we inflate our tires with.

Pure nitrogen has been used to inflate critical tire applications for years, primarily because it doesn't support moisture or combustion. These include racing tires (IndyCar, Formula 1, NASCAR), aircraft tires (commercial and military) and heavy-duty equipment tires (earthmovers and mining equipment). The challenge facing nitrogen inflation hasn't been its application, it's been its method of supply and cost.

Nitrogen molecules have a more difficult time escaping through the microscopic spaces that exist between a tire's rubber molecules. Nitrogen is a "slow" inactive gas labeled as an inert gas due to its nonreactive nature with many materials. Oxygen on the other hand is a "fast" active gas that reacts with many materials called "oxidation." Additionally nitrogen is a dry gas that doesn't support moisture while oxygen combined with hydrogen makes water (H₂O).

What are the effects of using pure nitrogen to inflate tires?

• Nitrogen is a gas and is still affected by changes in ambient temperature (about one psi for every 10° Fahrenheit). Nitrogen filled tires will require pressure be added during the fall/winter months as ambient temperatures and tire pressures drop. Nitrogen is good but can't change the laws of physics.
• Nitrogen reduces the loss of tire pressure due to permeation through rubber over time by about 1/3. This helps maintain the vehicle's required tire pressures a little longer, but doesn't eliminate the need for monthly tire pressure checks. This is good for people who don't maintain their vehicles well.
• Nitrogen is a dry gas and will not support moisture that could contribute to corrosion of the tire's steel components (bead, sidewall reinforcement and belts) due to the absence of moisture over extended periods of time. However it's important to remember that atmospheric pressure is constantly pushing oxygen and moisture into the rubber from the outside of the tire. This is especially good for low mileage drivers who don't wear out their tires quickly or those that run average annual mileages but use long wearing radial (60K and 80K warranted) tires.
• Nitrogen assures more consistent pressure increases due to increases in operating temperatures in a racing environment because of the absence of moisture. This is especially good for participants in track days, high-performance drivers education schools and road racing.
• Drivers should use standard air if pressure adjustments are required when a local source of nitrogen can't be found during a trip. While this reduces the benefit of higher nitrogen content, it is far better than running the tires underinflated in search of a source. Often the original nitrogen provider will refill the tires for free or a nominal cost when the driver returns to his hometown.

Several service equipment manufacturers have developed small, on-site nitrogen generator systems that use the selective permeation principle to separate oxygen and moisture from the shop's compressed air lines to capture nitrogen. The key component is a membrane that separates the gasses. Each module contains hollow fibers that allow the oxygen and water vapor to be selectively removed, resulting in a source of nearly pure nitrogen that is kept in a separate storage tank until it is used to inflate tires.

The nitrogen generator, storage tank and filling system aren't free and the dealer is entitled to some return on his investment. It's time-consuming for a technician to bleed air from the tires (sometimes requiring several purges during the initial inflation) to achieve the desired nitrogen purity, however some of the latest equipment automatically goes through several purge cycles without requiring the technician's participation.

While inflating tires with nitrogen never results in 100% purity, most nitrogen service equipment providers advise that reaching at least a 93% to 95% purity is necessary to receive the desired benefits. This ratio is normally achieved by initially purging the tires of existing air (down to just a few psi) and then refilling them with nitrogen. The purge/fill cycle is often repeated to achieve the desired level of nitrogen purity.

Note: Tires should never be subjected to a vacuum in an effort to eliminate the oxygen. Distorting the tire as shown in the accompanying picture can be as detrimental to the internal structure of the tire as running over potholes and road hazards.

So what should drivers do?

Overall, inflating tires with nitrogen won't hurt them and may provide some minimal benefits.

Is it worth it? If you go someplace that provides free nitrogen with new tires, why not? Additionally we've seen some service providers offering reasonable prices of about $5 per tire (including periodic adjustments for the life of the tire) to a less reasonable $10 per tire (with additional costs for subsequent pressure adjustments) or more as part of a service contract, which we believe exceeds the value of nitrogen's benefit.

Rather than pay extra for nitrogen, most drivers would be better off buying an accurate tire pressure gauge and checking and adjusting their tire pressures regularly.