Nitogen in Tyres - A con or OK ??

[Clarky]

Originally Posted by chrisk

Hi All,
Picked the car up yesterday with 5 new Cooper AT3's fitted. Went up to 245/70R17's in light truck spec. as I tow a fairly heavy van. My Jeep feels fantastic considering the short drive home. I'll be hooking the van on this afternoon as we are going away for a short trip this weekend. I can report back next week as to how it all went.

I for one am curious to know how much the tyre pressure changes in your tyres after a few hours towing?
I have just got back from QLD towing a 2000kg van (approx. 150kg ball weight). Rear 38 psi cold after a few hours on the road at 100 kmh pressure went to 42 psi (according to the dash readout)

[drover]

4 psi is generally whats used as figure, so your dash was pretty right.


Classic one Fester.......

[mudpig]

Originally Posted by Banshee

Sorry, I missed your post & haven't been back here for a while.

Yes the (relative) pressure would be higher, but I don't know exactly what, but it would be between your initial 200psi (commercial aircraft run 300psi btw ), and 214.7psi, given that standard atmospheric pressure is 14.7. The pressure would then drop due to temperature, so assuming a temp of 27C to start with, vacuum at altitude and the volume of the tyre remains unchainged, max pressure in the tyre would be:

P2 = P1-P1(T1-T2)/T1

So


P2 = 214.7 - 214.7(300-244)/300 = 174.6psi

Haha very good
but I do remember filling tyres to around 220psi on the 744 about 20 years ago. Obviously some things have changed since then. BTW are you still in the game and if so who are you working for these days? Cheers
Jase

[layback40]

Originally Posted by hills46

Absolutely N2 is bigger. Enriched air as used in diving (i.e. gas with greater than 21% oxygen) is commonly produced by using a compressor fitted with a membrane which filters out the nitrogen. So it is conceivable that you will get less leakage from a tyre but I don't think my tyres last long enough to notice.

As has already been pointed out, the pressure of a gas is directly proportional to temperature regardless of the make up of the gas. The good news is we're talking absolute temperature so the % increase is not all that much. Might make a difference in an F1 car where tyres are everything but not on a 4wd.

Any compressed gas which contains oxygen can be an extreme fire risk. If you compress air to 2 atmospheres, which is only roughly 12psi, you've got twice as many oxygen molecules to feed a fire. In recompression chambers for example which may go to 5 or 6 atmospheres there are very strict procedures to prevent any chance of a spark. I believe this may be the primary reason why nitrogen is used in areas such as aviation.

A good explanation. All to do with electronegativity & size of electron cloud.

http://www.getnitrogen.org/pdf/graham.pdf

[Banshee]

To get a 10% pressure change, (to make it easy, start at 27C or 300K (close enough to absolute for this)), the temp needs to go up by 30 degrees, (C & K degrees are the same, unlike Fahrenheight), so the tyre is now at 57C.

Actually, I was thinking about it afterwards & I got it wrong. It should be as follows:

PV~T so:

V~T/P and if the volume is constant

T1/P1 = T2/P2 so

P2 = T2*P1/T1

P2 = 244*300/300 therefore

P2 = 244psi down from 300

or if it was 200 to start with

P2 = 162.67 psi

My bad!

edit: Forgot to add the 14.7 for assumed vacuum at altitude!

So P2 = 162.67 + 14.7 = 177.37psi

[drover]

Brilliant.......... but I will just stay with a tyre gauge and compressor.