This seems pretty definitive, copied from an engineering board (my apologies to the original writer for not copying your info before I closed the site) - it also corrects me on the size issue (and has provided a little embarrassment for previously cited chem teacher who looked it up and went, and I quote "ooops".)


The question about nitrogen in tires has come up in other lists on the web as well. Let me clarrify a few points.

1. Air is 78% nitrogen, N2, and 21% oxygen, O2. So even if you put air in the tire, it's already 78% nitrogen. Many of the so called nitrogen generators don't produce much more than 90% nitrogen.

2. At relatively low pressures (ie tire pressures) N2, O2 and water vapor will all behave as ideal gases, and follow PV=nRT. Pressure will increase or decrease to the same extent as the temperature increases or decreases regardless of which gas is in the tire. (Even at 300 psi, which is about 20 atm, there is little deviation from ideality.) Therefore the comments about N2 not changing in pressure as the temperature changes are without merit.

3. The rate of effusion (or diffusion) of a gas through a porous membrane depends on the molar mass and to some degree on the molecular diameter. N2 and O2 are almost the same size and N2 is lighter than O2 (28 g/mol vs 32 g/mol) so if either gas were to effuse out of the tire, nitrogen would do it more quickly. Luckily, tires are designed not to be porous membranes.

4. N2 and O2 both have essentially the same specific heat capacity, about 1.0 J/gK, and thermal conductivity, about 0.00026 W/cmK. Water vapor has a specific heat capacity of about 2 J/gK. But remember, water vapor will constitute less than 1% of the air in the tire. So the idea that N2 has different heat handling properties is also without merit.

5. The ozone, O3, in the atmosphere, which is a ground level pollutant, will do a great deal more damage to your tires than the O2 inside the tire. For instance, don't leave a condom out in the air in Los Angeles for a few days. It will develop lots of tiny holes and weaken.

spdracer22 says that dry air is preferably to air with a lot of water vapor. As a tire heats up, the very small amount of H2O present will be in the vapor state which may contribute to the overall pressure very slightly.

Several have suggested that N2 in a high pressure tank is more portable and requires no electricity. That would make sense, particularly for aircraft tires.

I find no reason to believe that N2 is going to produce a "better ride" or "better handling".

The bottom line is that for general passenger car tires or truck tires there is nothing to be gained (other than portability) by using nitrogen rather than air. The biggest gain will be $$$ by the companies that sell nitrogen handling equipment and the tire merchants that appeal to ignorant customers. And who is the biggest loser? Yep, the consumer.

 

I have been using nitrogen in my various vehicles for about a year (5 total). It works! Is it worth the small cost, about $30? Definately - especially now since my dealer includes a great roadside assistance program free of charge - so I am saving about $100 a year on that alone.

While it is true that many nitrogen systems do not have high levels of nitrogen, the dealer where I got my tires filled uses a system called NitroFill. Their system fills tires with almost pure nitogen, up to 99+% acording to their literature. They manually check the purity level after they fill your tires and will refill them if you don't have close to 99% nitrogen. I saw them test mine and it was indeed 99%. My tire pressure dropped only 3 psi over a 4 month period. They refill the tires for free as well any time you want.

I find it amazing that we can spend $20k+ on a vehicle and not invest $30 in something that gives additional safety and potential fuel savings. If its good enough for the space shuttle, its good enough for my shuttle! Its worth getting just for the savings from the roadside assistance program!

Here is a link to their site:

http://nitrofillnow.com/

 

When I started this post I was thinking that maybe at some point the batterys would basicly go dormant (about 0 degrees or lower) and hurt the MPG until they were both charged and warmed. The posts about tires and nitrogen have been just as intresting, so thanks for that.But what about batterys. Is there a web site or a battery company I could call or does someone on this post know A little aboue NiMH batteries. JOE

 

And another poor mpg day occured yesterday. We drove 200 miles round trip ( a trip we have done before in good weather) at an average of 36 degrees and the Relative Humidity was 100 percent. foggy and drizzley. would this cause mpg to drop to 38 from our normal 41 ish.

 

Yes you encountered factor No 4 from my prior post.

Before my Prius I had little or no knowledge of how these outside forces impacted driving. Now with the means to measure these variable forces ( MFD ) and a good baseline of data to compare them against it's an unexpected benefit for me to learn how much the hybrids have increased the awareness of driving.

Situations that in the past would have been 'Huh?' are easily understood. The knowledge base on sites like GH are also invaluable in tying it all together.

 

Loonbeam — Thanks for the copy of the article on nitrogen vs air that you posted. It confirms what I thought to be the case. I personally am not willing to try what appear to be snake oil "solutions" even if they are cheap. To answer a previous question of yours, that I see I failed to address, yes, I was referring to the oxygen:nitrogen ratio decreasing inside the tire if oxygen migrated out through the rubber faster than nitrogen. I asked a chemistry prof. the size question yesterday, and he thought off-the-cuff (correctly, according to your post) that nitrogen was probably a smaller molecule than oxygen.

Stan

 

A coworker fills his tires propane, and no his name is not Hank Hill.

I don't agree with it, but he has not had a problem yet. But all it takes is one.

 

You would think that helium would be best because the car would be lighter.

 

Ummm, is that not a tad bit like driving a car with a bomb on each corner?

I would even be scared that scraping a curb could blow up my car!

 

talk about insane.... Hope no one is smoking when they check his air pressure.

I don't even want to think about what could happen when a shop replaces his tires