Tuesday, May 24, 2011

When air just isn't good enough

While munching on my Cheerios yesterday morning, I learned from reading the cereal box that NASCAR teams fill their tires with pure nitrogen gas, instead of regular air. (I also found a toy car prize inside the box which exponentially improved my Monday.)

NASCAR teams aren't the only ones to pump up their rubber with pure nitrogen. The gas is also used in bicycle tires on the Tour de France, in Formula 1 car tires, Space Shuttle orbiter tires and some aircraft tires.

But why nitrogen? What's wrong with regular old air?

To understand why it matters what gas is used in a tire, we have to learn a little about pressure first. Tires are designed to operate under a certain pressure in order to support the vehicle they are carrying. The tire pressure is determined by the amount of air (or other gas) filling the tire. It is measured in psi - pounds per square inch.

An under-filled tire will sag on the ground, creating lots of friction between the tire and the road surface, making it hard for the vehicle to move. An over-filled tire can minimize contact with the road, making handling worse. So, naturally, there is a happy middle ground of ideal tire pressure.
Pressure is also related to temperature. As the tires roll around the race track, the rubber heats up, warming the gas inside the tire. The gas, in turn, becomes more active and pushes against the inside walls of the tire more, increasing the tire pressure. In fact, after two laps around a race track, the pressure in some NASCAR tires can increase by 10 psi. This is a lot when you consider some tires start out filled to a little over 20 psi.

The air we breath is made of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon and a fraction of a percent of carbon dioxide and other gasses. One of those other gasses is water vapor - water in gaseous form. The pressure of water molecules can fluctuate a lot with changes in temperature, especially around water's boiling point. Even small changes in temperature can result in big changes in pressure. Therefore, even that small amount of water vapor in regular old air can have a big impact on the pressure inside a tire.

To get around that, race car teams opt to use a "dry" gas - a gas stripped of pesky water vapor. Dry ordinary air is available, but nitrogen is the most common choice since it has even less water than dry air and can be had for a similar price. The nitrogen's pressure also doesn't fluctuate as much with changes in temperature, making it more predictable.

Lastly, nitrogen is also non-flammable. This makes it very attractive for use in riskier environments, like racetracks. Imagine what could happen during a blowout to a tire filled with pure oxygen or hydrogen, both very flammable gases. Argon is another non-flammable gas that some race teams use, but it's more expensive than nitrogen without being much more efficient. So nitrogen is the clear winner.

What if I wanted to try filling my tires with nitrogen? As we've learned, it's more stable and predictable and having properly-filled tires improves gas mileage and safety. Having properly-filled tires, though, is something that will require maintenance no matter what you put in your tires. Having the stability provided by nitrogen is certainly a concern for racing and aviation tires that undergo huge temperature and pressure fluctuations, but unless you take your car to the track a lot, it probably isn't worth it.

Just to be sure, I called a nitrogen provider whose gas can be found at dealerships and automotive shops across the U.S. The woman on the phone told me the cost of filling four automotive tires with their nitrogen varies from dealer to dealer, but that it can range from $34 to $200 per car. When you compare that to the cost of a few quarters down at the corner gas mart, regular old air, I think, will be all right for me.

9 comments:

  1. We have a Toyota Prius with nitrogen in the tires. It cost $39 to have nitrogen instead of regular air, and it seems to improve gas mileage somewhere between 5 and 10%.

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  2. Pardon me, but the last time I checked, Oxygen is NOT flammable. Supports combustion absolutely, but actually combustible, not so much.

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  3. Ted is correct but it would certainly allow the tyre to catch alight a lot more easily.
    I think the main point here is the toy and the fact that I didn't get one this morning has lead me to ask 'when did I last get a toy in my breakfast and have something interesting to read?' The answer is a good few years ago now. The solution buy a new cereal.

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  4. @Anonymous, although using nitrogen in your tires makes the pressure more consistent, it's not the nitrogen itself that improves your mileage. It expands and contracts less than damp air, but not much less.

    Properly pressurized tire, regardless of the gas inside, is the important thing. I use free air at my local gas station, and check my tire pressure often (because I race autocross on the weekends, and have to raise the pressure for races), and I rarely see fluctuations of more than a few psi. So a 5-10% improvement in mileage with nitrogen seems a bit hard to swallow.

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  5. According to my quick, back-of-the-envelope calculation, the pressure change in a tire that starts out at 30 psi at freezing temps and is heated to 70 degrees Fahrenheit (25 C) should be about 3.5 psi. Of that, 3 psi is due to the dry air, and .5 psi is due to water vapor (if you had LOTS of water in your tires). With pure nitrogen, you would expect about 3 psi rise over that range.

    I did the calculation assuming nitrogen is an ideal gas, and follows the ideal gas law (PV=nRT). It's not ideal because it's a diatomic gas, but I think the error that comes from the assumption is small.

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  6. This is actually the very thing we discuss on our blog, Nitronomics.org. Nitrogen tire inflation, and why it's a good choice for the consumer market.

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  7. By maintaining proper inflation pressure longer, nitrogen inflated tires get better gas mileage and have a longer tread life. Goodyear recognizes that underinflation is a major contributor to poor gas mileage and tire life, and are running a summertime air inflation program to combat these issues.

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  8. By maintaining proper inflation pressure longer, nitrogen inflated tires get better gas mileage and have a longer tread life. Goodyear recognizes that underinflation is a major contributor to poor gas mileage and tire life, and are running a summertime air inflation program to combat these issues.

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  9. I do a portable Nitrogen business part time at cruises etc. Most all feedback from customers is positive with an improved ride, longer psi retention and most have gone over a year with no psi loss.

    I did an 18 wheeler that usually gets 300 to 300k on his driver tires.I put in nitrogen and he went 475k before changing tires and did not loose psi or have to rotate due to uneven wear when he used compressed air. The tire shop said they could sell the take off tires to a local delivery truck.

    For my family, friends and customers have had positive experience using nitrogen as inflation. I charge $20.00 for 4 tires. I understand most the science and dont agree with price gouging. Someday Nitrogen will be the norm of inflation and free.

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