Naked Science Forum
Non Life Sciences => Technology => Topic started by: Sprool on 11/01/2012 15:49:20
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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.
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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 [::)]
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Why not use hydrogen? Think of the weight saving.
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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).
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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.
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as well as being expensive helium would also leak away more quickly than air...
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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
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"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.
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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.
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Why not inflate the tyres with a vacuum; think of the huge savings on weight then...
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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.
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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.
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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.
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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?
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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.
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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.
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Since F1 teams don't fill their tyres up at quick fit I think we can leave them out of the discussion.
The economics of buying N2 vs drying air would depend on the number of customers (and how much you could rip them off) .
A garage that does engineering work might well have a compressed air system anyway in which case adding a dryer might be fairly cheap.
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http://www.uniflate.com/ Uniflate is the company that sells the N2 gear via qwikfit and other garages, it's not cylinder stuff they use, it's some sort of compressor and N2 filter with a water trap. Begs the question - why not make use of the compressor and water trap anyway just give me the old fashioned 21/78 mix compressed air.
What is a bit naughty is garages just taking the decision for you that its better (tyre wear, longevity, safety, fuel economy) and slipping the extra £5.50 into the bill without your consent whenever you get a new tyre fitted. Most people won't bother when its a one off payment but obviously to qwikfit if they are doing this to 10,000 tyres per month up and down the country thats an awful lot of extra revenue for no real significant benefit to the consumer.
Here's Uniflate's claims:
1. Better handling and road holding
2. Reduces tread wear and increases tyre life by up to 25%
3. Correct inflation pressures reduce puncture risk by up to 33%
4. Reduced rolling resistance improves miles per gallon by 2%
5. Improved pressure retention
http://www.tyrebaydirect.com/35-Nitrogen_Tyre_Inflation.html
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The first 4 of those are benefits of correct tyre pressure rather than nitrogen.
Incidentally, since they don't remove the air (and water) that's already in the tyre then there's a limit to how much difference this can make.
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The improved pressure retention is an interesting one. Uniflate sent me some academic studies about the migration rate of N2 vs O2 through rubber. Depending on what model you want to believe, either O2 is faster than N2 or they are both about the same. However, the studies did not compare O2/N2 mixture (air) against N2 so in my view this is at best misleading. Sounds like cherry picking the bits that best suit the story.
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As far as Oxygen and Rubber.
I've seen 50 yr old inner tubes that as long as they are protected from light & elements, they are often in quite good shape. The rest of the tire may be completely shot though.
F1 racing can't be too concerned about gradual leakage, as I would hope they check the tire pressure before every race (probably setting each tire differently). And, the air won't leak out in 500 miles and a couple of hours.
My guess that the N2 is a bunch of marketing hype. How much can you sell "dry air" for when one should be using it anyway? It wouldn't be too hard to make concentrated N2 though. Just take a hospital surplus O2 concentrator, and run it backwards.
And, what better way to convince people to buy a product than convince them that it is the same thing as used by F1 cars and jets.
The next time your mechanic suggests using Indy technology in your car, you should ask them if it makes any difference if the average lifespan of an Indy engine is about 501 miles!!! Not all of them make it that far! And, of course, your goal is to get a bit better than the Indy average of about 1.92 mpg.
How much effort do they make to ensure you actually are using N2 in the tire? If you inflate the tire at 1ATM with and 80/20 mix of N2/O2. Then you increase the tire pressure to 2ATM. Don't you end up with a mix of about 90/10 N2/O2?
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Good point. Come to think of it, has anyone ever checked to see how pure the N2 really is?
I'd like to see it in liquid form so I can take its temperature before I pay for it.
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Good point. Come to think of it, has anyone ever checked to see how pure the N2 really is?
I'd like to see it in liquid form so I can take its temperature before I pay for it.
Just don't fill your tires with liquid N2.
The first big pothole... then...
Well, not exactly. The tires would explode with liquid N2 in them. But, poorly processed gas from the liquid could be mighty cold, and would cause the tires to become brittle.
My point isn't the purity from the system.
When your tire is flat (but round, as if you just installed it on the rim), it is actually full of air at 1ATM.
If you fill it up to 30 lbs... that is equivalent to adding 2ATM worth of additional pressure (sorry, I was off with my conversion). So, the air in your tire is now whatever the ambient air was in the tire shop, diluted to about 1/3 original air, 2/3 N2.
You should really order an "air flush" for your tires too. [;D]
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Good point. Come to think of it, has anyone ever checked to see how pure the N2 really is?
I'd like to see it in liquid form so I can take its temperature before I pay for it.
Just don't fill your tires with liquid N2.
The first big pothole... then...
Well, not exactly. The tires would explode with liquid N2 in them. But, poorly processed gas from the liquid could be mighty cold, and would cause the tires to become brittle.
My point isn't the purity from the system.
When your tire is flat (but round, as if you just installed it on the rim), it is actually full of air at 1ATM.
If you fill it up to 30 lbs... that is equivalent to adding 2ATM worth of additional pressure (sorry, I was off with my conversion). So, the air in your tire is now whatever the ambient air was in the tire shop, diluted to about 1/3 original air, 2/3 N2.
You should really order an "air flush" for your tires too. ;D
What you'd need to do is fill them using one if Chris' Hoovers to suck all the air out (assuming that didn't actually destroy the tire) before inserting the efficacious pure nitrogen.
Come to think of it, that won't work either. The low pressure will pop the seal at the bead between the rim and the tire, so it's not that easy to get the air out. You'd have to flush it a few times with pure nitrogen as you suggest.
You could use steam to get rid of the air instead, but that would seem to defeat the purpose of the entire exercise.
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Good Point.
If you mounted one of these from the 70's... you'd be all set [::)]
http://www.ratwell.com/mirror/jrivers/AspareVWtire.htm
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.ratwell.com%2Fmirror%2Fjrivers%2Fdonut_-spare.jpg&hash=a931b662dda385c60e0dec5fb2a620e6)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.ratwell.com%2Fmirror%2Fjrivers%2Fdonut-spare.jpg&hash=2dce5566c1ef0e2a810850d3753cc155)
All you need is to add an LN2 dewar to feed the little blue pump.
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I have attended the Indy500 race for the last 7 years and find that engine failures are very rare, it is a rather disapointing feature of modern auto racing that modern engine control system and deliberate power restriction has made them very reliable.
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Good Point.
If you mounted one of these from the 70's... you'd be all set ::)
http://www.ratwell.com/mirror/jrivers/AspareVWtire.htm (http://www.ratwell.com/mirror/jrivers/AspareVWtire.htm)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.ratwell.com%2Fmirror%2Fjrivers%2Fdonut_-spare.jpg&hash=a931b662dda385c60e0dec5fb2a620e6)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.ratwell.com%2Fmirror%2Fjrivers%2Fdonut-spare.jpg&hash=2dce5566c1ef0e2a810850d3753cc155)
All you need is to add an LN2 dewar to feed the little blue pump.
Blimey! I wouldn't put my big toe under the tire using that jack.
Looks like VW bus, but the odd thing is that the wheel is attached by a combination of bolts and studs. I've never seen that on a VW before. All the ones I had only used bolts which makes it a real pain to line up the holes.
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Not sure how relevant indy car engine lifetimes or vw wheel fixings are to the N2 debate in tyres. :-\