Nitrogen

[Adnillien]

As everyone remembers from high school physics PV=nRT. To first order the volume of the tire does not change with pressure. Hence, the pressure is directly proportional to temperature and R (gas constant). Everyone is spot on with respect to Air, N2 and water vapor changes in pressure with temperature. Here are the gas constant values:

N2 = 117​

Air = 287​

H2O = 461​

​

 

[hmp10]

I used to have nitrogen put in my 2015 Tesla Model S with every tire change and then refill only with nitrogen at the same tire shop. However, they quit offering nitrogen, whereupon I switched to regular air. I never noticed any difference in mileage. Of course, I check tire pressures every couple of weeks and always keep tires at optimum pressure. I found I had to top off the pressure a bit more often with regular air, but as I could top off with an electric pump in my garage instead of making a stop at a tire shop for nitrogen, it actually turned out to be more convenient. Also, by topping off at home in the morning, I could be sure I was topping off to the exact cold air pressure instead of guessing about the adjustment necessitated by the miles-long drive to the tire shop during which pressure built due to heat expansion.

Atmospheric air is almost 80% nitrogen, anyway. I really wonder if replacing that ~20% of oxygen with nitrogen makes all that big a difference?

 

[pam4_AGT]

As everyone remembers from high school physics PV=nRT. To first order the volume of the tire does not change with pressure. Hence, the pressure is directly proportional to temperature and R (gas constant). Everyone is spot on with respect to Air, N2 and water vapor changes in pressure with temperature. Here are the gas constant values:

N2 = 117​

Air = 287​

H2O = 461​

​

N2 is 296.8J/kg K
Air is 287.05J/kg K

 

[Adnillien]

N2 is 296.8J/kg K
Air is 287.05J/kg K

Yes I read the wrong line in my table.

 

[idiot900]

As everyone remembers from high school physics PV=nRT. To first order the volume of the tire does not change with pressure. Hence, the pressure is directly proportional to temperature and R (gas constant). Everyone is spot on with respect to Air, N2 and water vapor changes in pressure with temperature. Here are the gas constant values:

N2 = 117​

Air = 287​

H2O = 461​

​

No, R is the universal molar gas constant. It does not vary among ideal gases. If you include these other constants, you must also include the molecular weight somewhere, so they cancel out.

The ideal gas law is only an approximation - there are correction factors that are irrelevant in "regular" conditions such as this.

The volume of the tire does indeed change with pressure, if the number of moles of gas and temperature remain constant: this is what the equation says.

 

[pam4_AGT]

No, R is the universal molar gas constant. It does not vary among ideal gases. If you include these other constants, you must also include the molecular weight somewhere, so they cancel out.

What you are referring to is Ru "Universal gas constant" which indeed is same for every gas.

The individual gas constant "R" for each gas can be calculated from the following and indeed takes into account the molecular weight as you mentioned:

The individual gas constants for various gases (R and not Ru) is listed in a table here:

Individual & Universal Gas Constants: Definitions, Values, and Applications

The Universal and Individual Gas Constants in fluid mechanics and thermodynamics. Individual gas constants for the most common gases.

www.engineeringtoolbox.com

The volume of the tire does indeed change with pressure, if the number of moles of gas and temperature remain constant: this is what the equation says.

The equation says the product PV increases with temperature. If you use a solid metal cylinder to store the gas, volume is constant and pressure has to increase. If you use a balloon, then it expands and both P and V will change but the product PV is governed by the equation PV=nRT.

 

[idiot900]

The equation says the product PV increases with temperature. If you use a solid metal cylinder to store the gas, volume is constant and pressure has to increase. If you use a balloon, then it expands and both P and V will change but the product PV is governed by the equation PV=nRT.

Correct, and complementary to what I said (the tire is the "balloon" case).

 

[pam4_AGT]

Correct, and complementary to what I said (the tire is the "balloon" case).

Not really. The tire is stiff enough to prevent that from happening. For all practical purposes, the volume of a tire is relatively constant. Here are some pointers:

13.3 The Ideal Gas Law | Texas Gateway

www.texasgateway.org

Here is another dude who actually did quite a few measurements and came to the same conclusion:

https://www.coyoteents.com/temperature-and-tire-pressure/

 

[idiot900]

Not really. The tire is stiff enough to prevent that from happening. For all practical purposes, the volume of a tire is relatively constant. Here are some pointers:

"Relatively" but not absolutely. I don't think we actually disagree.

 

[MoniputerLM]

Tires that I have bought from Costco come inflated with nitrogen. Of course, it gets diluted when I add "air" at home.

 

[hmp10]

Tires that I have bought from Costco come inflated with nitrogen. Of course, it gets diluted when I add "air" at home.

Tires inflated with "air" are still being inflated with 78% nitrogen. I used nitrogen for several years before switching back to regular air for convenience, and I never noticed any difference in mileage, handling, or tread wear. I really think the use of nitrogen to inflate passenger car tires for normal driving on public roads is largely a marketing gimmick.

 

[pam4_AGT]

I really think the use of nitrogen to inflate passenger car tires for normal driving on public roads is largely a marketing gimmick.

The only thing that might remotely make a difference is water vapor (conjecture on my part). I have no idea how nitrogen is made for this is made, but I'm guessing water vapor gets separated out(?)

 

[Codyroo]

Thank you for those posting up the Ideal Gas Law, means I don't have to do it. Nitrogen is pure marketing for passenger cars/trucks. Now, if you are racing cars, and dialing in 1/4 pound of pressure makes the difference between a car that is too tight/loose vs perfectly balanced, then yes, you want Nitrogen to remove minor variability of other gases AND to remove moisture vapor.

For passenger cars, the variability is negligible. The coyoteents article seems to summarize that in the final bit where it says "PSI is PSI". Stick with air, you'll be fine. If you are worried about range and rates, slowing down and not tailgating will make WAY more difference than Nitrogen filled tires.

 

[Codyroo]

The only thing that might remotely make a difference is water vapor (conjecture on my part). I have no idea how nitrogen is made for this is made, but I'm guessing water vapor gets separated out(?)

They would just run the Nitrogen through a silica gel drying tube to remove moisture (most likely) as it is cheap.

 

[Steveinarizona]

Costco uses nitrogen whenever you buy a tire or get it serviced there. I haven't noted any differences in my Prius (same make/model tire) or ICE vehicle. Some Costco's even have nitrogen refill pumps in their parking lot to make it easy to use nitrogen to maintain tire pressure (the one I go to has one next to the tire center). If you choose to go this route, you need to be able to refill with nitrogen. As with most things, YMMV - it would be interesting if another owner did a nitrogen conversion.

@niejelow How important do you think the vacuum evacuation of the last remaining air/moisture was to the process?

Yes...the one near me has the nitrogen pumps in the parking lot near the tire center.

 

[hmp10]

@niejelow How important do you think the vacuum evacuation of the last remaining air/moisture was to the process?

How can you draw a vacuum on a mounted tire without breaking the rim seal?

 

[MikeTz]

I believe both the Nitrogen fans and the natural air fans will agree on one thing. Check and set the air pressure often. You will be safer, get better miles/kWh and the tires will last longer.