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Nitrogen in your car tires

11 April 2011

Although putting nitrogen instead of air makes sense for aircraft tires, it doesn't seem worth it for the common commuter car.

My wife and I bought a new car on Saturday, a 2011 Honda Fit. Given that my previous car, a 1993 Honda Civic, was equipped with Spartan disdain for distracting comfort, I expected the new car to abound in features that my old car lacked, such as electric windows, a radio, and air conditioning.

But when Peter the Honda salesman was touting the Fit's features, I didn't expect him to say that the car we ended up buying had nitrogen in its tires (indicated by the green tire stem cap in the photo).

Tire.jpg

"But air is 80% nitrogen," I pointed out, "What's the advantage of using pure nitrogen?" Peter explained that nitrogen is used in aircraft tires to reduce the effect of temperature fluctuations on pressure.

I didn't press him on the issue, but I remained puzzled. Under everyday conditions, the gas in a tire should behave like an ideal gas—that is, its pressure should be proportional to its temperature regardless of its molecular composition. Curious, I looked into the matter.

Aircraft tires indeed are filled with nitrogen to mitigate temperature fluctuations, but not because nitrogen has any special heat-absorbing qualities. Rather, it's the presence of water that makes standard, commercially available compressed air a poor, even dangerous choice for aircraft tires.

At low temperatures, such as a plane might encounter at Chicago's O'Hare airport in midwinter, the water in an air-filled tire exists as liquid droplets. But if that plane had to land and brake suddenly, heat from the screeching tires would vaporize the droplets, adding an extra pressure-exerting component to the gas in the tires.

The sudden increase in pressure from the water vapor can be fatal. On 31 March 1986, a tire on Mexicana Flight 940 was mistakenly filled with air, not nitrogen. Fifteen minutes after takeoff from Mexico City, an overheated landing gear brake caused the tire to explode. The resulting crash killed all 167 passengers and crew.

Pure nitrogen has other advantages over air besides its dryness. When tires get very hot, oxygen, the second most abundant component of air, can react with volatile chemicals in the rubber and cause an explosion. Even at lower, everyday temperatures, oxygen reacts with rubber, weakening it.

So will I refill my tires with nitrogen? Not if it costs more than a few dollars. Having owned the same car for 18 years, I know that worn treads will prompt me to replace the tires long before oxidation sets in.

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