Straight Dope on Nitrogen?

[swann2001]

I have been using Nitrogen for over 10 years and would never go back to using air again which by the way is no longer free in most places. First Cecil seems to think the only reason to get Nitrogen is for the cool green valve stems. I totally disagree and the end of his tirade against Nitrogen even gives the main reason why people should replace air with Nitrogen, that better than 60% of driver's rarely if ever check their tires, and probably make that 95% of all women. However a recent Government report stated that as much as 700 million barrels of oil per year could be saved if every driver just kept their tires properly inflated. The way to do that is Nitrogen, nothing else works as well and at Tire Warehouse the cost is only $5 per tire and right now 7/14/11 they have a 20% discount to encourage more people to try it. I have driven my jeep for over 3 years and never once needed a boost or refill and my tires are wearing evenly which means they will last much longer, saving landfills space from tires that still had plenty of good tread but had bald spots due to under or over inflation. If people reading this are concerned about saving the whales, global warming or the environment and yet won't spend $20 to get better gas mileage and save money on premature tire replacements then so be it. But don't complain about the high cost of fuel or global warming or put bumper stickers on your car about saving the whales because that would just make you shallow and something of a hypocrite. Don't take my word for it, try it and I will bet $20 you will never want just air in your tires again.

*************************
LINK TO COLUMN: http://www.straightdope.com/columns/...instead-of-air

[Chronos]

Proper tire inflation is important. Using nitrogen instead of air has nothing to do with proper tire inflation.

[Stranger On A Train]

So the argument of the o.p. is that nitrogen is a magical gas assures that your tires always remain properly inflated? Or is this just a viral marketing campaign for Tire Warehouse?

Stranger

[kayaker]

Originally Posted by Stranger On A Train

Or is this just a viral marketing campaign for Tire Warehouse?

Until shown otherwise. IMHO.

[penumbrage]

The only minuses are cost and availability, although more and more shops are offering the option for less and less (some for free) and several companies sell small, portable tank/regulator units.
On the plus side, N2 has slight to moderate advantages over hydrous air in thermal conductivity, tire oxidization, rim corrosion, pressure variability over temperature and pressure loss over time (Consumer Reports long term leak down testing averaged 62% that of air).
The major plus and deciding factor for me was the same reason it's used in emergency vehicles, race cars and commercial aircraft, simple safety.
Your $20 car fire extinguisher is much more likely to save some other person's life than your own (the more severe the crash the greater the chance of fire and the more likelihood you'll be unconscious or too dazed to use it).
In the unfortunate event you do crash and burn - or some fool generously includes you in his carbecue - I think $20 spent on a poor man's fire suppression system (OK, oxygen displacement system) that auto-deploys either at impact (when the tank is rupturing and metal is grinding and sparking) or after impact (if there's a fire) is obviously a far better choice than the admittedly cheaper automatic fire enhancement system - driving around on a hundred cubic feet of pressurized oxidizer.

[Stranger On A Train]

Originally Posted by penumbrage

In the unfortunate event you do crash and burn - or some fool generously includes you in his carbecue - I think $20 spent on a poor man's fire suppression system (OK, oxygen displacement system) that auto-deploys either at impact (when the tank is rupturing and metal is grinding and sparking) or after impact (if there's a fire) is obviously a far better choice than the admittedly cheaper automatic fire enhancement system - driving around on a hundred cubic feet of pressurized oxidizer.

Please provide a single example of a passenger or commercial vehicle bursting into flame due to air escaping from ruptured tires and/or an automotive fire being suppressed due to release of pure nitrogen inflated tires.

Also, what kind of gigantor monster truck tires are you driving around on that have a total 100 cu ft air capacity?

Stranger

[Measure for Measure]

I'm interested in Stranger's question as well.

Originally Posted by penumbrage

(Consumer Reports long term leak down testing averaged 62% that of air).

Here are the cites:
http://news.consumerreports.org/cars...nitrogen-.html
http://news.consumerreports.org/cars...en-tires-.html

Consumer Reports compared all nitrogen inflated tires to those inflated with air: they took the same tire models, inflated them both ways, and placed them outdoors for a year. They did not attach them to vehicles.

The N2 tires bled 2.2 psi. The air tires bled 3.5 psi. So there was a difference - a 37% difference. But that hardly eliminates the need to check the air on your tires. Checking it every 40 days rather than every 30 days seems hardly worth the bother.

Originally Posted by Consumer Reports

Q: Seems to me that since the difference is so small and nitrogen is difficult to get and more costly that it is not worth the effort and the cost.
A: That's a logical conclusion.
...
Q: A flawed study and analysis. And sadly quiet on the advantages of using nitrogen in heavy trucks where 18 tires need to be maintained weekly to pressures of 100 psi.
A: The positive benefits of nitrogen in high(er) service pressure applications, such as used in large truck tires, has been documented in the industry. Our test centered on passenger tires, only. We are not discrediting the use of nitrogen, but it is not a substitute for regular inflation checks.

[Rain Soaked]

Nitrogen is indeed very useful in tires but I like to use a mixture of different gases. I have found that 78% Nitrogen, 21% Oxygen plus a bit of Argon also give me really good mileage when I use it to inflate my tires to the proper pressure. I also put a small bit of CO2 in there to help the environment.

[voltaire]

Originally Posted by Rain Soaked

Nitrogen is indeed very useful in tires but I like to use a mixture of different gases. I have found that 78% Nitrogen, 21% Oxygen plus a bit of Argon also give me really good mileage when I use it to inflate my tires to the proper pressure. I also put a small bit of CO2 in there to help the environment.

But what if that were to leak out and harm the environment?!?

[Crazyhorse]

Originally Posted by voltaire

But what if that were to leak out and harm the environment?!?

Very valid point, but it does remove almost all traces of Dihydrogen Monoxide from the tires.

[Stranger On A Train]

Only on the inside.

Stranger

[Scuba_Ben]

Originally Posted by Rain Soaked

Nitrogen is indeed very useful in tires but I like to use a mixture of different gases. I have found that 78% Nitrogen, 21% Oxygen plus a bit of Argon also give me really good mileage when I use it to inflate my tires to the proper pressure. I also put a small bit of CO2 in there to help the environment.

I've found that at places charging for a tire gas fill, I can get the N₂-O₂-Ar blend that Rain Soaked uses for 10% of the price swann2001 normally pays for the all-N₂ gas fill. Some places even give me this gas fill for free!

Now if I can only get dive shops to provide clean EAN21 for the same price, I'll be set.

[Crazyhorse]

I think this is the column in question. I wouldn't exactly call it a scathing 'tirade against nitrogen'.

Filling your tires with nitrogen mainly does two things: it eliminates moisture, and it replaces skinny oxygen molecules with fat nitrogen molecules, reducing the rate at which compressed gas diffuses through porous tire walls. That means, theoretically at least, that a tire filled with nitrogen retains optimal pressure longer, leading to more uniform tire wear and better gas mileage. [snip] Overall, filling up with nitrogen won't hurt and may provide some minimal benefit.

[Rick]

The OP says that the tire store fills tires filled with Nitrogen for five bucks a pop.
At my dealership we will fill your tires with 78% Nitrogen for free.
And people say car dealers are ripoffs.

[crucible]

Originally Posted by swann2001

I have been using Nitrogen for over 10 years and would never go back to using air again which by the way is no longer free in most places. First Cecil seems to think the only reason to get Nitrogen is for the cool green valve stems. I totally disagree and the end of his tirade against Nitrogen even gives the main reason why people should replace air with Nitrogen, that better than 60% of driver's rarely if ever check their tires, and probably make that 95% of all women. However a recent Government report stated that as much as 700 million barrels of oil per year could be saved if every driver just kept their tires properly inflated. The way to do that is Nitrogen, nothing else works as well and at Tire Warehouse the cost is only $5 per tire and right now 7/14/11 they have a 20% discount to encourage more people to try it. I have driven my jeep for over 3 years and never once needed a boost or refill and my tires are wearing evenly which means they will last much longer, saving landfills space from tires that still had plenty of good tread but had bald spots due to under or over inflation. If people reading this are concerned about saving the whales, global warming or the environment and yet won't spend $20 to get better gas mileage and save money on premature tire replacements then so be it. But don't complain about the high cost of fuel or global warming or put bumper stickers on your car about saving the whales because that would just make you shallow and something of a hypocrite. Don't take my word for it, try it and I will bet $20 you will never want just air in your tires again.

*************************
LINK TO COLUMN: http://www.straightdope.com/columns/...instead-of-air

filling with nitrogen has nothing at all to do with not having leaks. We used to do it with truck tires, the rationale being that we had, at the time, lots of tire fires caused by ply seperations, the resultant friction, etc. Not having any oxygen inside the tire was a matter of 'every little bit helps'. We also stocked our truck cabs with water fire extinguishers because the powder or foam ones didn't do any good on ply fires.

IMHO, nitrogen for tires is a bit of a scam.

[picunurse]

Originally Posted by swann2001

...probably make that 95% of all women...

Excuse me? I check all my gauges every time I drive, including tire pressure.

[Stranger On A Train]

Originally Posted by picunurse

Excuse me? I check all my gauges every time I drive, including tire pressure.

Clearly you are an outlier.

Especially if your tires don't spontaneously explode with tens of cubic feet of air every time you brake.

Stranger

[Strassia]

Originally Posted by Stranger On A Train

Clearly you are an outlier.

Especially if your tires don't spontaneously explode with tens of cubic feet of air every time you brake.

Stranger

Not to mention how unusual it is to have a tire gauge in a passenger car dashboard. I have a low pressure light, but it comes on at about 65% of nominal, so is pretty useless.

[Labtrash]

Originally Posted by Strassia

Not to mention how unusual it is to have a tire gauge in a passenger car dashboard. I have a low pressure light, but it comes on at about 65% of nominal, so is pretty useless.

My wife's 2011 GMC Terrain has an in-dash pressure display - the individual pressures in all four tires are displayed.

Pretty sure it was standard - we didn't go all "bells & whistles" when we bought it.

[picunurse]

Originally Posted by Stranger On A Train

Clearly you are an outlier.

Especially if your tires don't spontaneously explode with tens of cubic feet of air every time you brake.

Stranger

I have a read out one the head's up display. I don't have to physically check them unless the gauge says one or more are low.
It gives the pressure of each tire. I check it against a physical gauge once a year. It's always right on.

[penumbrage]

Originally Posted by Stranger On A Train

Please provide a single example of a passenger or commercial vehicle bursting into flame due to air escaping from ruptured tires and/or an automotive fire being suppressed due to release of pure nitrogen inflated tires.

Stranger

This FAA CFR Final Ruling banning the use of air in Part 25 aircraft tires cites three separate tire explosions (not just superheated, explosive, fire feeding de-pressurizations) resulting from the violent combustion of hot rubber gasses and oxygen inside the tire.
http://rgl.faa.gov/Regulatory_and_Gu...pandSection=-4

Originally Posted by Stranger On A Train

Also, what kind of gigantor monster truck tires are you driving around on that have a total 100 cu ft air capacity?

Stranger

Stock tires, about 1.2 cu.ft. each times 4 times tire burst pressure (can be over 300 psi, less a bit for fire compromised strength) call it 18 atmospheres, which makes about 88 cu.ft. for my little truck. You get a different answer?

any

Originally Posted by crucible

We used to do it with truck tires, the rationale being that we had, at the time, lots of tire fires caused by ply seperations, the resultant friction, etc. Not having any oxygen inside the tire was a matter of 'every little bit helps'.

IMHO, nitrogen for tires is a bit of a scam.

I'm curious if you know whether they had fewer separations after they switched to N2?
The N2 proponents (and some studies) claim that nitrogen is making tires last longer (longer than keeping 1/2 of the surface area from oxidizing would account for) and they say it's because of the internal rubber damage oxygen atoms cause as they migrate through the tire. They say a 94% nitrogen / 6% oxygen mix will balance the internal and atmospheric oxygen partial pressures (at 32psi) and minimize rotting the tire from the inside.
Then again, some are claiming nitrogen can make a truck tire (with appropriate re-treadings) last a million miles.

[Stranger On A Train]

Originally Posted by penumbrage

This FAA CFR Final Ruling banning the use of air in Part 25 aircraft tires cites three separate tire explosions (not just superheated, explosive, fire feeding de-pressurizations) resulting from the violent combustion of hot rubber gasses and oxygen inside the tire.
http://rgl.faa.gov/Regulatory_and_Gu...pandSection=-4

So...your daily driver is a 737? Or are you intentionally attempting to mislead by comparing coconuts to acorns? You do understand that the loads, environments, hazards, and even basic construction of aircraft landing gear tires are completely different from tires used in automotive applications, right?

Originally Posted by penumbrage

Stock tires, about 1.2 cu.ft. each times 4 times tire burst pressure (can be over 300 psi, less a bit for fire compromised strength) call it 18 atmospheres, which makes about 88 cu.ft. for my little truck. You get a different answer?

I want to see these amazing tires that you have on your car that are inflated at 300 psi. Personally, I have ultra-high performance all-season radials on my car with an XL load range and a speed rating of 93W (rated for speeds up to 168 mph). The maximum pressure rating for the tire is 51 psi, and the recommended running pressure for this vehicle application is 38 psi. Using your 1.2 cubic foot per tire (mine are low profile and closer to about 0.8 cubic feet, but we'll be conservative) that gives about 8 lbs of air, which if course is 78% oxidatively inert diatomic nitrogen. There is a greater danger that my socks will spontaneously combust.

Stranger

[Stranger On A Train]

Sorry, did the volume unit conversions wrong. That is about 0.8 lb of air at 38 psi times four tires, or a volume of about 10 cubic feet of air at STP, or which ~2.2 cubic feet is an oxidizer.

Stranger

[penumbrage]

Originally Posted by Stranger On A Train

I want to see these amazing tires that you have on your car that are inflated at 300 psi.

Stranger

However did you manage to get to 'inflated at 300psi'? My normal pressure is 34psi fronts, 32psi rears, but when you set them on fire (my fire safety point was what you objected to, wasn't it?) the tire pressure keeps on increasing until the (decreasing) burst pressure is reached.

Originally Posted by Stranger On A Train

So...your daily driver is a 737? Or are you intentionally attempting to mislead by comparing coconuts to acorns?

Stranger

I was pointing out that the hot rubber gasses combined with oxygen inside a burning tire can actually explode (given the higher pressures and containment of aircraft tires) to quash any possible argument from you that they wouldn't make a car fire more severe.
You seem to be saying the over-pressurized blast of these same superheated flammable gasses and oxygen from a burning car tire bursting is just as safe as your socks.
What do you wear, legwarmers woven from guncotton?

[Stranger On A Train]

Originally Posted by penumbrage

However did you manage to get to 'inflated at 300psi'? My normal pressure is 34psi fronts, 32psi rears, but when you set them on fire (my fire safety point was what you objected to, wasn't it?) the tire pressure keeps on increasing until the (decreasing) burst pressure is reached.

From your own words, to wit:

Stock tires, about 1.2 cu.ft. each times 4 times tire burst pressure (can be over 300 psi, less a bit for fire compromised strength) call it 18 atmospheres, which makes about 88 cu.ft. for my little truck.

No automotive tire is going to hold 300 psi before bursting; the bead will let go long before that, and in any case, increasing the temperature doesn't magically increase the amount of gas in the tire, which is constant regardless of thermal inputs.

Originally Posted by penumbrage

I was pointing out that the hot rubber gasses combined with oxygen inside a burning tire can actually explode (given the higher pressures and containment of aircraft tires) to quash any possible argument from you that they wouldn't make a car fire more severe.

Again, you are comparing aircraft tires, which are of a different construction (bias ply versus radial), pressurized to much higher pressure (~200 psi), and subject to temperature extremes and impact loads that are far in excess of anything seen by automotive tires, even on heavy haul tractors.

Without dodging the question, please provide evidence that indicates that automotive tires are substantially inclined to this supposed energetic decomposition when filled with a gas composed of ambient air versus pure nitrogen, which was your assertion.

Stranger

[crucible]

We were in the nitrogen business (industrial gases, in general), and it was easy for us to use nitrogen. To my knowledge, testing was done in a qualified facility to see the difference in burning characteristics, ignition, rate of burn, etc. between normall aspirated tires and nitrogen ones...I think I recall seeing film. I don't think the frequency of these fires could be gauged and controlled for tire manufacturer, ambient temperature, OTR vs. local, etc. What I'm saying is that in my fleet of 30 vehicles, using mostly 11/22.5 radials, we didn't have any tire fires either before or after the change to nitrogen. Comparing results for miles of use per tire was a very long process and we had little data to compare to at our local location.

In other words, no startling results, only theoretically positive ones.

Why would a dipole molecule like oxygen leak more easily than a monopole like nitrogen? I suspect the oxygen reacting with the gummy stuff in the bead where the tire meets the rim, or, even, the seals in the valve stems.

to me, the technology that lets you keep an eye on tire pressure on all 18 tires on a rig is rather wonderful. I'm sure it pays for itself, sooner or later, independent of the gas used.

An aside. When inflating balloons, our old expert used to make a mixture of helium and carbon dioxide (99/1) claiming the carbon dioxide molecules would plug the interstices in the balloon, sealing it better. Our easy tests of his method revealed that he was right, inflation lasting twice as long with his mix.

Today, they use mylar to produce the same effect. I think a balloon mix helium would be worth considering if you owned a balloon inflating franchise....

[Cheshire Human]

I have a set of valve stem caps that show green when you are at the proper pressure, yellow when you should be concerned, and red when you should be alarmed. A quick walk-around and glance at each tire... There, I've checked my pressure.

[Nancarrow]

Originally Posted by swann2001

I have been using Nitrogen for over 10 years and would never go back to using air again...

This thread promised to be a lot more interesting before I realised it was about tires.

[gamerunknown]

This thread promised to be a lot more interesting before I realised it was about tires.

Then again I wouldn't really like to be reading Frank Booth's take on events...

[Stranger On A Train]

Originally Posted by penumbrage

If you have a source you can cite I'd be happy to take a look.
Then you can contact the ASTM and inform them the Draves and Skolnik paper they reviewed and published requires correction.
Since the entire Society failed to catch this egregious error you may have to type slowly for them.

As you did not bother to provide a specific cite to the ""Draves and Skolnik" paper you reference I cannot evaluate what it claims. But regardless, the burst strength of the tire does not reflect the capability of the bead or seal to retain pressure.

Originally Posted by penumbrage

I see nothing in the download indicating any slight differences in rubber compounds would cause significant differences in volumes or compositions of pyrolytic gasses generated when burning aircraft tires are compared to burning higher speed rated Bonneville tires, higher load rated earth mover tires or even mundane automotive tires

I don't know how you missed this in the previous post, but I'll state it again:

Pages 33 and 51 clearly demonstrate that combination of service loads and speeds for aircraft are much more severe than for any other tire application, including high speed race cars and industrial heavy mobile equipment.

[quote=penumbrage;14428647Now you're getting it. As the temperature goes up (even past the point where the combustible gasses from the outer surface facing the heat have ignited), the pressure goes up (including significant quantities of the same combustible gasses lacking an ignition source) and the reinforcement strength goes down - when they meet everything inside suddenly comes out.[/QUOTE]The quantity of oxygen does not increase. If you don't understand the basic principle of conservation of mass, there is no basis for rational discussion.

Originally Posted by penumbrage

If you burn them very, very fast (mixed with oxygen and heated to actual ignition like the higher initial pressure and stronger containment of an aircraft tire sometimes allows) you get an explosion that send pieces of rubber and rim through the plane.
If you just burn them fast (mixed with oxygen and released by a burning tire to an ignition source) you get an energetic combustion with a heat and pressure spike as they burn at the speed of ignition propagation through the explosive mixture.
If you burn them as slow as possible (mixed with N2 and released to an ignition source) they compete with the rest of the flammable vapors in the oxygen lean fire environment and only burn when the leading edge can find some, releasing their energy in the slowest, coolest, least energetic and safest manner possible.

I'll repeat the query I first stated in post #12:

Please provide a single example of a passenger or commercial vehicle bursting into flame due to air escaping from ruptured tires and/or an automotive fire being suppressed due to release of pure nitrogen inflated tires.

Stranger

[Vlad/Igor]

Let's go back to Physics 101 and do some calculations.

Definitions:

Ideal gas law: PV = nRT
Where P = pressure in atm; V = volume in L; n = moles of air; R = gas constant in L.atm/K/mol; T = temperature in K

Standard room temperature = 68 deg F or 20 deg C or 293 K.

Conversions: psi to atm: atm = psi x 0.0680
Deg F to deg C = (F - 32) x 0.556

A sealed tire mounted on a rim, holding a steady pressure of 32 psi has 32 x
0.0680 = 2.18 atm pressure. Assuming a tire volume of 10 L pressure, the number of moles of air (n) = PV/RT. Plugging in numbers, we get:

(2.18 atm x 10 L)/(0.082 L.atm/k/m x 293 K) = 21.8/24.0 = 0.9 moles of air.

To check values, the equation P = nRT/V is used:

P = (0.9 m x 0.082 x 293)/10 L = 21.8/10 = 2.18 atm.

A sealed tire heated to 470 deg F (244 deg C, 517 K) will develop an
internal pressure of:

(0.9 m x 0.082 x 517 K)/10 L = 38.2/10 = 3.8 atm, or 3.8/0.0680 = 56.1 psi.

This 56.1 psi is well below 300 psi, making me suspect that pyrolitic tire
failure is due to the thin bead giving way, not the tread. NOTE, however,
that the volume and molar quantity of air in the tire does not change. A
sealed tire on a rim that loses no appreciable pressure represents a closed
system. Thus, there's no way a tire, sealed on the rim has gained "quantities
of the same combustible gasses" because the increase is pressure is due to an
increase in temperature only.

The number of oxygen molecules in the tire at 20 deg C is exactly the same at 244 deg C, or 0.21 x 0.9 = 0.19 moles of oxygen. Meanwhile, you have essentially an infinite number of oxygen molecules outside of the tire feeding the fire. Likewise, an airliner tire catching on fire while speeding on the ground at, say, 100 MPH is going to run into a lot more oxygen molecules than nitrogen molecules inside the tire. Putting pure N2 into a tire as a fire suppressant would be like pissing on a tree to stop a forest fire.

[Irishman]

Originally Posted by Vlad/Igor

This 56.1 psi is well below 300 psi, making me suspect that pyrolitic tire
failure is due to the thin bead giving way, not the tread. NOTE, however,
that the volume and molar quantity of air in the tire does not change. A
sealed tire on a rim that loses no appreciable pressure represents a closed
system. Thus, there's no way a tire, sealed on the rim has gained "quantities
of the same combustible gasses" because the increase is pressure is due to an
increase in temperature only.

What about breakdown of the rubber at 470 deg F, causing outgassing of combustible gases inside (and also outside) the tire? Wouldn't those gases also increase the internal pressure of the tire? But they probably won't increase the amount of oxygen in the tire - I don't think oxygen is a breakdown product of rubber and latex - I could be wrong.

Likewise, an airliner tire catching on fire while speeding on the ground at, say, 100 MPH is going to run into a lot more oxygen molecules than nitrogen molecules inside the tire. Putting pure N2 into a tire as a fire suppressant would be like pissing on a tree to stop a forest fire.

This is a valid observation.

[Tndd]

exellent

[Vlad/Igor]

I suppose the pressure might be slightly higher due to outgassing of the hot tire, but you are right that the molar concentration of oxygen inside the tire would not change. Tires are made of synthetic rubber compounds that don't contain oxygen.

[penumbrage]

Originally Posted by Vlad/Igor

This 56.1 psi is well below 300 psi, making me suspect that pyrolitic tire failure is due to the thin bead giving way, not the tread.

A sealed tire on a rim that loses no appreciable pressure represents a closed
system. Thus, there's no way a tire, sealed on the rim has gained "quantities
of the same combustible gasses" because the increase is pressure is due to an
increase in temperature only.

If no combustible gasses are gained inside the tire by pyrolitic breakdown then what was it that the oxygen violently reacted with in the explosions that led the FAA to ban air?
From the FAA final ruling defining burst and explosion.
'A tire explosion is a completely different phenomenon. It results from the autoignition and explosion of a mixture of explosive vapors released from the innerliner of a severely overheated or abused tire, and any oxygen that may be present inside the tire.'
http://rgl.faa.gov/Regulatory_and_Gu...pandSection=-4
How do you boil these explosive vapors off the innerliner in a closed system without increasing the pressure?

I never said 300psi fire burst pressure, I said '... times [cold] tire burst pressure (can be over 300 psi, less a bit for fire compromised [hot] strength) call it 18 atmospheres...' and I'm willing admit my estimate of 18 is high (that's why I'm here, to get intelligent feedback).
I had assumed the pressure was much higher than 56psi (plus whatever the combustible gasses add) due to reports from the firemen I checked with that failing tires in car fires don't go whoosh or whump - they go BOOM!, distort wheel wells and send trim pieces flying. Since road hazard tire blowouts seldom require fender replacement, if the pressures are truly that low then the noise and damage would have to come from a much faster, more violent combustion of the explosive mixture than I previously thought.

The FAA ruling doesn't address how many moles of combustible gasses are generated internally at pyrolitic temperatures or how much they increase the pressure or what the oxygen/fuel ratio was but it does clearly state how reducing the oxygen to 5% or less downgrades the mixture from explosive to merely flammable.

Originally Posted by Vlad/Igor

Likewise, an airliner tire catching on fire while speeding on the ground at, say, 100 MPH is going to run into a lot more oxygen molecules than nitrogen molecules inside the tire. Putting pure N2 into a tire as a fire suppressant would be like pissing on a tree to stop a forest fire.

While hot airliner brakes were a major factor in the FAA ruling (the part where evacuating passengers and fire crews didn't get riddled with shrapnel helped), Stranger and I were discussing whether N2 provides any degree of safety a in car fire.
If your car fire is moving at 100MPH you have more immediate concerns than how hot or how fast it's burning.
In (non-moving) open fires the available oxygen that convection and turbulence provide is far outweighed by the combustible gasses (much of which rise unburned as smoke).
When merely releasing 2 cu ft of pressurized oxygen in this oxidizer poor environment will make the fire burn faster and hotter, I simply can't agree with Stranger's opinion that releasing this same quantity of oxygen combined with the combustible gasses that make it an explosively ignitable mixture can possibly be as safe as releasing inert N2.

[Vlad/Igor]

Originally Posted by penumbrage

If no combustible gasses are gained inside the tire by pyrolitic breakdown then what was it that the oxygen violently reacted with in the explosions that led the FAA to ban air?

I have conceded in the above post that combustible gasses could be present inside the tire at high temperatures. The reaction of oxygen and gasses requires a certain amount of heat energy (in chemistry this is known as activation energy) to start the reaction. After that, the energy released from the reaction is more than the activation energy, so another molecule of oxygen has enough energy to react with another molecule of flammable gas, releasing energy, etc. This can easily result in an explosion because the tire is already pressurized, not because a fireball is created. This scenario is not the same as the outside surface of the tire burning at normal air pressure and temperature.

...Stranger and I were discussing whether N2 provides any degree of safety a in car fire. ...When merely releasing 2 cu ft of pressurized oxygen in this oxidizer poor environment will make the fire burn faster and hotter, I simply can't agree with Stranger's opinion that releasing this same quantity of oxygen combined with the combustible gasses that make it an explosively ignitable mixture can possibly be as safe as releasing inert N2.

Ok, so the oxygen in a tire would contribute to the burning tire. Above I calculated that there are 0.19 moles of oxygen in a 10 liter tire inflated to 32 psi at 20 degrees C at a standard atmospheric pressure of 760 mm Hg. I don't know what you mean by 2 cu ft of pressurized oxygen. No one fills their tires with pure oxygen. They fill tires with air, which contains 21% oxygen. An "oxidizer poor environment" doesn't make sense in this discussion. A burning tire indicates plenty of oxidizer is present. If you think that air escaping the tire is going to fan the flames, performing some basic chemistry calculations will address that idea.

Going to the inside of the tire, if we assume incomplete oxidation of synthetic rubber to carbon monoxide (CO), we need one molecule of oxygen to combine with one molecule of carbon. Therefore if we have 0.19 moles of oxygen, we will burn at most 0.19 moles of carbon. Carbon has a molecular weight of 12 g/mole, thus 12 g/mole x 0.19 moles = 2.2 grams of tire. Given that a tire is at least 4 kg (~ 9 lbs), this represents at most 0.06% of the entire mass of the tire. So, yes, the oxygen inside the tire will contribute to the burning tire, in the same way that throwing in an unlit match contributes to a bonfire.

[Irishman]

I think the Straight Dope sounds funnier on helium.

[Vlad/Igor]

Well, sure, if you want to sound like Donald Duck discoursing on the Existence of God.

[Stranger On A Train]

Originally Posted by penumbrage

If no combustible gasses are gained inside the tire by pyrolitic breakdown then what was it that the oxygen violently reacted with in the explosions that led the FAA to ban air?
From the FAA final ruling defining burst and explosion.
'A tire explosion is a completely different phenomenon. It results from the autoignition and explosion of a mixture of explosive vapors released from the innerliner of a severely overheated or abused tire, and any oxygen that may be present inside the tire.'
http://rgl.faa.gov/Regulatory_and_Gu...pandSection=-4
How do you boil these explosive vapors off the innerliner in a closed system without increasing the pressure?

I will say this one more time; the loads, pressures, temperature variations, and basic construction of tires used in commerical aircraft are completely different that those used in automotive applications, including high speed racing and heavy haul cargo. In aircraft applications, users have to be concerned about undetected damage to the tire and shock-induced combustion ("dieseling") within the tire causing failure upon a hard landing, and the subsequent loss of stability and potential for damage to delicate landing gear and the unprotected undercarriage of the aircraft as well as shrapnel being inducted into the turbines. None of this is a practical concern for passenger automobile applications where the most dangerous scenario is a sidewall blowout, or a cargo hauler where tread failure is most common.

I have provided objective evidence, maintenance information from an aircraft tire manufacturer highlighting the differences between automotive and aircraft tire applications, and basic physics of tire inflation and failure modes. You have provided nothing but insinuation and errant speculation. You have ignored repeated requests to provide anything like evidence of any actual accident or injury as the result of these incredible exploding tires that you suggest are the result of inflation with atmospheric air. There is no indication that any recognized authority (SAE, IIHS, NHTSA) has promoted pure gaseous nitrogen as prefereable on a safety bases, nor that any legislative or regulatory body has attemtped to legislate or require the use of gaseous nitrogen as a necessary and prudent safety measure in passenger automobiles. You are either patently dishonest, severely misguided, or more likely both, in your dogged pursuit of this position.

Stranger

[Rick]

I would just like to add that I have been near car fires and I have never seen a tire explode. Melt yes, explode, no.

[Vlad/Igor]

I've been present when two different cars were burning, and both times, the tires "exploded." The first had those big ol' mudding tires, the other had normal sized tires. They "exploded" by producing a loud pop! and then sagged to the ground like a flat tire. There was no fireball. There was no increase in combustion. There was no shrapnel. Women and children were not evacuated from the vicinity. The tires stayed on the rims and there was no visible damage to the tire.

[Una Persson]

I actually saw an overheated tire explode where a brief flash of orange flame was produced. It lasted a very short fraction of a second and extinguished. There was sort of a soft "whoomph" sound.

[Vlad/Igor]

I would guess, then, that you saw what I calculated above in post #39. I imagine whether or not an exploding tire produces a flame depends on how it is heated, for how long and the composition of the rubber. With a N of 4, I think we can say that data suggests that exploding automobile tires do not significantly contribute to a car already on fire. More study is needed, though. SO WHO'S UP FOR SETTING INFLATED TIRES ON FIRE!!!???

[Cheshire Human]

Originally Posted by Vlad/Igor

More study is needed, though. SO WHO'S UP FOR SETTING INFLATED TIRES ON FIRE!!!???

I suggest asking the French. I hear setting cars on fire is very popular over there. They must have lots of data.

[constanze]

I think Mythbusters tested exploding tires once, and they, too, found that tires under stress will rupture first, releasing the gas, instead of exploding.

[penumbrage]

Except for old butane Fix-A-Flat formulas and fools who weld on rims with mounted tires (with a posthumous nod to the Darwin Award nominee who did BOTH), actual automotive tire explosions are probably quite rare.
I’m just talking about the difference between releasing a pressurized burst of explosive gasses when the tire fails (even without the containment and internal ignition required for an explosion) compared to merely releasing more flammable gasses into the combustible gas rich environment of a car fire – basically the same difference as using a lit match to pop a balloon filled with acetylene and air or popping one filled with acetylene and nitrogen - the same combustible gas energy is released much more rapidly and violently when pre-mixed with oxygen, generating more heat and pressure to intensify and accelerate your car fire.
This demo has a good example of the explosive gasses that a severely overheated, air filled tire generates (http://www.youtube.com/watch?v=HiLeji8bLOk) although the physics differ from a car fire since the rim is the heat source for generating the gasses and the ignition source (the temperature gauge shows a few seconds of internal combustion before sufficient temperature, heat and gas volumes build to explode).
I don't know if the loud bangs reported at car fires are just pinhole failures allowing ignition to propagate inside the partial containment of the tire or if it's possible for a high quality tire and a thermally conductive aluminum rim to actually explode, but I do know (as per the FAA) that keeping the inflation gas oxygen level at 5% or less never generates an explosive mixture and eliminates the possibility of either one.

[postpic200]

On the car board that I'm on beat nitrogen to death. Basically it came down to this there is a small benefit from using nitrogen, but unless filling the tires with nitrogen was free or less than a buck or two, it wasn't worth it for the average driver. If you race cars or engage is some kind of high stress, high temp driving there basically no benefit to you. You also have to remember that when filling a tire with nitrogen it must be done correctly to get the benefits, the tire must be filled and emptied several times to purge as much air as possible out of the tire, and even then some plain old air will remain. If they just mount the tire and fill it with nitrogen, the tire is NOT considered filled with nitrogen. And even if you managed to get all the air out of a tire oxygen will migrate it's way back into the tire.

The point of less pressure loss overtime is true but it doesn't stop pressure loss and you will lose pressure so you still need to check your tires.

They use it in aircraft, NASA, NASCAR so it must be good argument. As has been point out aircraft, NASA, NASCAR have different loads placed on their tires and falls into the high stress high temp exception. But a car isn't an aircraft, NASA isn't using it in their car tires, and odds are you're not driving a NASCAR. Just because it good for a aircraft or whatever doesn't make it good for you. Did you know for example that some hoses used on aircraft don't meet DOT standards for cars and aren't supposed to be used on cars? Should they then not be allowed on aircraft, even though they are far stronger than anything used on a car. If I remember right the aircraft hose fail during the "whip-resistance" test. But that doesn't mean it a bad hose it just means it should be used on cars according to the DOT.

But, don't let me or anyone else stop you from using nitrogen in your car if you wish, just know the benefits are small for the average driver.

[postpic200]

But that doesn't mean it a bad hose it just means it should be used on cars according to the DOT.

Sorry it should have read

But that doesn't mean it a bad hose it just means it shouldn't be used on cars according to the DOT.

[sfbiker]

Originally Posted by swann2001

I totally disagree and the end of his tirade against Nitrogen even gives the main reason why people should replace air with Nitrogen, that better than 60% of driver's rarely if ever check their tires, and probably make that 95% of all women.
*************************
LINK TO COLUMN: http://www.straightdope.com/columns/...instead-of-air

I don't know why you single out women, but most female drivers I know are religious about getting their oil changed every 3000 miles, and I haven't run across any oil-change place that doesn't top up your tires (with plain air) for free during the service.

So while getting tire pressure checked every 3 months may not be ideal, it's not like they never get it checked.

[penumbrage]

Originally Posted by postpic200

But, don't let me or anyone else stop you from using nitrogen in your car if you wish, just know the benefits are small for the average driver.

Did your in depth car board discussions uncover any additional advantages N2 provides over hydrous air besides thermal conductivity, innerliner oxidization, rim corrosion, pressure variability over temperature, fire safety, less than 2/3 the leak down rate or nearly doubling the tire body life?
Or did you find any nitrogen disadvantages other than cost and availability (I get it for free half a block off my commute so it's not a hard call for me)?
It's not magic, of course they have to be filled correctly and regularly checked for pressure (they just get slightly better gas mileage between checks).
Your board missed an important point, however - the recommended ratio is not 100%, it's 95% N2 (which equalizes oxygen partial pressure inside and out, minimizing oxygen migration and rubber degradation inside the tire body).
Fortunately, nitrogen has no such aircraft only limitations. It provides the same advantages in any natural/styrene-butadiene/polybutadiene rubber tire, from a 500lb rated wheelbarrow tire to a 140,000lb rated, 300psi space shuttle tire.
When the advantages are great enough that more and more major truck fleets are switching over (Wal-mart, after extensive in-house testing and cost analysis, being the latest), rest assured, you haven't stopped me from using nitrogen.