The Naked Scientists

The Naked Scientists Forum

Author Topic: Is it better to inflate car tyres with nitrogen?  (Read 15405 times)

Offline Sprool

  • Sr. Member
  • ****
  • Posts: 120
    • View Profile
Is it better to inflate car tyres with nitrogen?
« on: 11/01/2012 15:49:20 »
Heres some dodgy science promoted by Qwikfit, proposing massive increases in fuel economy if you use Nitrogen to inflate your car tyres. What are your views? Do they realise how much nitrogen is already in compressed air? When invetigating further it seems the company supplying the N2 machines to the garages say its about maintaining a proper tyre pressure that boosts your fuel consumption, as well as all the safety benefits of having inert gas in  your tyre instead of all that evil combusible oxygen in case of a crash - that's "why F1 cars and aeroplanes all use N2". No it isn't. Discuss.
« Last Edit: 13/01/2012 23:26:37 by chris »


 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Nitrogen in car tyres
« Reply #1 on: 11/01/2012 16:37:09 »
This was discussed here earlier. 
All gasses have some weight with Oxygen being about 15% heavier than Nitrogen, and the weight would become more significant if pressurizing a large tire.  But, as you mention, the difference would be minimal considering that the mix in the air is already about 80% Nitrogen and 20% Oxygen.
I would have to ask why they don't put in Helium, other than leakage.  Tire weight being wide and low is good for F1 cars, but rolling weight at least resists change in speeds.

Anyway, I think the issue may be water.  Nitrogen would be absolutely DRY.  Air would have a small amount of water content.  Pressurizing it would tend to knock the water out of the air so it becomes liquid.  And, with slow leakage and repeated filling, one could potentially pump more water in (which is why one is supposed to periodically drain air compressors and air brake systems.).

It would seem that a much cheaper alternative would be to add a high quality desiccant to one's air systems, and make sure that one always uses dry air to refill the tires.

Of course, it is not as easy to advertize "dry air" as NITROGEN ::)
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #2 on: 11/01/2012 19:16:46 »
The problem with using natural air to inflate tires is the water vapour content, this leads to a much larger variation of pressure with temperature than if dry Nitrogen is used.
All my local tire suppliers advertise that they use Nitrogen to inflate the new tires that they supply.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Nitrogen in car tyres
« Reply #3 on: 11/01/2012 20:06:05 »
It's a good way for the tire shops to sell you something else you don't really need.

Of course, you could also fill them with anti-freeze, like the ones on my tractor.
 

Offline Sprool

  • Sr. Member
  • ****
  • Posts: 120
    • View Profile
Re: Nitrogen in car tyres
« Reply #4 on: 12/01/2012 13:19:24 »
The problem with using natural air to inflate tires is the water vapour content, this leads to a much larger variation of pressure with temperature than if dry Nitrogen is used.
I'd like to see some data on this to see if it really is significant - I suspect in normal motoring conditions it makes diddly squat diff. There is also a load of stuff argued about differences in diffusion rate of oxygen and nitrogen molecules, they say N2 is bigger molecule than O2 so will retain stable tyre pressure longer. Well we are comparing compressed air with N2, not O2 with N2.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8657
  • Thanked: 42 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #5 on: 12/01/2012 18:38:29 »
The problem with using natural air to inflate tires is the water vapour content, this leads to a much larger variation of pressure with temperature than if dry Nitrogen is used.
All my local tire suppliers advertise that they use Nitrogen to inflate the new tires that they supply.

How?
To a damned good approximation air and water vapour are perfect gases.
There's not usually a lot of water there because the air is compressed to, and stored at, a higher pressure than the tires are filled to.
 

Offline SeanB

  • Neilep Level Member
  • ******
  • Posts: 1118
  • Thanked: 3 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #6 on: 12/01/2012 19:19:05 »
Compressed air is normally saturated with water, just from the ambient humidity. This excess water condenses out in the air receiver, and has to be drained off. I often see tyre gauges spray water out that has condensed in the piping, so adding liquid water to the tyre will not help in maintaining balance.

The major reason to use nitrogen is to stop the reactions between the inside rubber and other tyre components with oxygen which will over time weaken them. Long life tyres may be on the car for many years, and this weakening may cause failure before the tread is worn out from normal wear.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8657
  • Thanked: 42 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #7 on: 12/01/2012 20:33:01 »
A competently designed system will compress the air to a high pressure- say 10 bar then leave it in a tank to cool and let the water drop out and be drained off ( automatically with a float valve is the best way to do it).

That air is saturated with water but it's at 10 bar.
When it expands to, say, 3 bar to fill a tyre it's only about 30% of saturated so there should never be any condensation in the pipe feeding to the tyre.
If you know somewhere that has a badly maintained compressor system, I strongly suggest you go somewhere else to get your tyres filled (and I wouldn't trust them with nitrogen either).

As you say, adding liquid water is a very bad idea.

I also question the possible merit of filling the tyre with nitrogen to prevent reaction with oxygen.
For a start, what reaction?
Rubber perishes in air, but that's largely due to ozone which is produced photochemically.
Since it's dark inside a tyre...

The other obvious glitch in the logic is that, if rubber reacted with oxygen (under normal conditions), then the outside of the tyre would be destroyed anyway.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #8 on: 12/01/2012 22:31:01 »
When I was at school we were told that bicycle tires could burst if left in the sun, doing the sums for a perfect gas this seemed improbable so I assumed that it was the moisture content of the air that was the problem.
The tires on aircraft and race cars are always inflated with Nitrogen to eliminate as much water vapour as possible.
Not all tire inflation machines store compressd air a lot trigger a compresser when you make contact with the tire valve.
F1 race car tires normally operate at 100°C while aircraft tires are subject to -55°C making water vapour a greater concern that regular car tires.
 

Offline Sprool

  • Sr. Member
  • ****
  • Posts: 120
    • View Profile
Re: Nitrogen in car tyres
« Reply #9 on: 12/01/2012 23:05:01 »
I was told it was the temperature extremes and moisture content that drove the choice for aircraft tyres, as well as the inert nature in case of fire risk on blow-out. For F1 cars I read it was the lower thermal coefficient of expansion which enabled them to predict more accurately the pressure for an expected average running temperature.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Nitrogen in car tyres
« Reply #10 on: 12/01/2012 23:26:36 »
Isn't the expansion more or less related to the Ideal Gas Law.

PV=nRT

Fixing everything else, the Pressure is directly proportional to the Temperature.

The biggest difference is if you also have a phase change in there...

So, if water is liquid at one temperature, and a gas at another, then it could throw off all of the calculations.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Nitrogen in car tyres
« Reply #11 on: 13/01/2012 03:27:47 »
Why not use hydrogen? Think of the weight saving.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Nitrogen in car tyres
« Reply #12 on: 13/01/2012 03:48:07 »
Why not use hydrogen? Think of the weight saving.
Indy fans would really get excited to see the wrecks then  :o

I suggested helium instead.  It would at least help with rotational weight.  If one is to have vehicle weight, the lower and the wider the better (like in tires).
 

Offline Sprool

  • Sr. Member
  • ****
  • Posts: 120
    • View Profile
Re: Nitrogen in car tyres
« Reply #13 on: 13/01/2012 10:39:17 »
Helium is expensive. Hydrogen can be explosive, Nitrogen is inert and cheap. 78% of air is nitrogen anyway.
Qwikfit's bold claims about fuel saving and safety as well as better tyre longevity are entirely linked with keeping your tyres at the correct inflation level, there is nothing whatsoever to do with replacing compressed air with compressed nitrogen. If you keep your tyres filled with compressed air at the optimum pressure then you will get same benefits of tyre wear and fuel efficiency. It is blatant misuse of science to pull the wool over peoples eyes and shell out a little extra for somehting that will make no discernable difference.
 

Offline Mazurka

  • Hero Member
  • *****
  • Posts: 510
    • View Profile
Re: Nitrogen in car tyres
« Reply #14 on: 13/01/2012 11:50:47 »
as well as being expensive helium would also leak away more quickly than air...
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #15 on: 13/01/2012 16:22:54 »
Quikfit are correct in as much as tires inflated with dry Nitrogen will as opposed to those inflated with normal air containing a proportion of water vapour maintain their correct pressure better under conditions of varying temperature.
Off course most any gas would do as long as water vapour was avoided provided no phase changes occured during the normal range of temperatures encountered
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8657
  • Thanked: 42 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #16 on: 13/01/2012 18:05:49 »
"Quikfit are correct in as much as tires inflated with dry Nitrogen will as opposed to those inflated with normal air containing a proportion of water vapour maintain their correct pressure better under conditions of varying temperature."
Once again, I'm asking why?
As Clifford and I have pointed out, to a very good approximation, the pressure is proportional to the absolute temperature no matter what the gas is.
If there's liquid in your tyres then you have another problem.

In any event, even if water vapour is a problem, dry air is cheaper than nitrogen.
« Last Edit: 13/01/2012 18:13:21 by Bored chemist »
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Re: Nitrogen in car tyres
« Reply #17 on: 13/01/2012 22:14:09 »
No the pressure is not proportional to to the aboslute temperature if a phase change takes place which occurs in water vapouur at 273 and 373°K in water vapour which is relevent to aircraft and race car tires although only the lower temperate is encountered under normal condtions on the road.
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5336
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Re: Is it better to inflate car tyres with nitrogen?
« Reply #18 on: 13/01/2012 23:29:29 »
Why not inflate the tyres with a vacuum; think of the huge savings on weight then...
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Re: Is it better to inflate car tyres with nitrogen?
« Reply #19 on: 14/01/2012 01:57:03 »
Why not inflate the tyres with a vacuum; think of the huge savings on weight then...

Nope! I tried to inflate my tyres tires with my Hoover and it didn't work.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: Is it better to inflate car tyres with nitrogen?
« Reply #20 on: 14/01/2012 03:45:07 »
Why not inflate the tyres with a vacuum; think of the huge savings on weight then...
To inflate with a vacuum, your tires would have to have either a filler, or some kind of a structure that would prevent them from collapsing.     

There are "Run Flat" tires that can be used, at least for a bit, at little or no air pressure.

Here, laws require pneumatic tires, and prohibit solid rubber tires on road vehicles.  I presume some 100 yr old vehicles are exempt though.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8657
  • Thanked: 42 times
    • View Profile
Re: Is it better to inflate car tyres with nitrogen?
« Reply #21 on: 14/01/2012 11:20:28 »
No the pressure is not proportional to to the aboslute temperature if a phase change takes place which occurs in water vapouur at 273 and 373°K in water vapour which is relevent to aircraft and race car tires although only the lower temperate is encountered under normal condtions on the road.

I think we are all agreed that you shouldn't have liquid water in your tyres because of the adverse effect on balancing and the corrosion of the rims, so there should not be a phase change.

Anyway, as I said, dry air is cheaper than nitrogen.
 

Offline techmind

  • Hero Member
  • *****
  • Posts: 934
  • Un-obfuscated
    • View Profile
    • techmind.org
Re: Is it better to inflate car tyres with nitrogen?
« Reply #22 on: 14/01/2012 12:17:53 »
If you regularly topped up your tyres on warm wet days (from a simply domestic tyre pump), then in the worst case you could fill them with air saturated at (say) 20°C (at 1 atmosphere).

From the graph on the Wikipedia page http://en.wikipedia.org/wiki/Relative_humidity this would contain 15g water per kilogram of air.
Now consider on a cold frosty morning (say 0°C) 10g of that water will condense out.
Molar mass H2O = 1x2+16 = 18.
Molar mass of dry air (approximate as nitrogen) N2 = 14x2 = 28

Grrr - I haven't got time to follow the calculations through now, but I think it will make a few percent difference (on top of the simple ideal-gas temperature dependence).
Anyone else care to complete for me?
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8657
  • Thanked: 42 times
    • View Profile
Re: Is it better to inflate car tyres with nitrogen?
« Reply #23 on: 14/01/2012 12:26:39 »
Certainly, if you use a foot pump on a warm wet day to fill a tyre then wait till a cold day you may well get condensation in the tyre.
That's a bad thing.

So you shouldn't use a foot pump to fill a tyre on a warm wet day.

It will also slightly affect the pressure in the tyre.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Re: Is it better to inflate car tyres with nitrogen?
« Reply #24 on: 14/01/2012 14:48:02 »
I think the phase change that occurs at 373°K in race car tires is the more important in F1 car tires as they operate at about this temperature and the pressure and assosiated ground clearence is very important.
I wonder if dry Nitrogen purchased in cylinders is not cheaper than having your own system for producing dry compressed air, it certainly would be much more convinient for a F1 team travelling.
« Last Edit: 14/01/2012 15:38:07 by syhprum »
 

The Naked Scientists Forum

Re: Is it better to inflate car tyres with nitrogen?
« Reply #24 on: 14/01/2012 14:48:02 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length