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.
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.
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_Guidance_Library/rgFinalRule.nsf/0/e18b9cadcf1a66da86256a3100713354!OpenDocument&ExpandSection=-4
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
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.
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…
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?
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
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
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.
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?
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.
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
I’ll throw my take into this:
Nitrogen does offer the advantage of removing water vapour, as Cecil pointed. Whether this is significant, I’m not sure. I’ve had experience with a customer whose tyres ended up grossly overinflated when hot. This was due to about half a cup of water being inside their tyres (inexperienced fitters were using it as lubricant…). While the pressure rise was extreme, it was also on a car going around a race track, so not a normal application.
Secondly, if Nitrogen is so great, since it makes up ~80% of air, if all the other gases leaked out/oxidised inside the tyre, whatever, there should only be a 20% pressure reduction…say from 36 psi to 28-30 psi.
Once you reinflate your tyres, 20% of the replaced volume will be non nitrogen, making 4% total non nitrogen. If that all leaks out, its ~ a 2% pressure drop.
So after you’ve checked your tyre pressures a few times, your tyres will be close to 100% nitrogen (I say a few times, because hopefully you don’t let your tyres drop so low as 28 psi.)
So, in my opinion, instead of nitrogen, the use of dry air (from an air line with a water trap) is just as good.
The tire engineers disagree with you.
“What should be the minimum burst strength of a new tire at ambient temperature? For bias passenger tires at 20 degrees C we generally find a factor of safety for burst of 10.” Tire Reinforcement and Tire Performance ASTM
Tire makers won’t admit to anything over 160-180psi (as they have lawyers) but after de-rating for temperature, load and age it agrees with the unciteable claims from yahoos who fight boredom with NDT (Non-scientific Destructive Testing) that modern tires range from 200 to 300psi with premium tires like ours at the top since bigger safety margins are the only way they get such a reputation for reliability.
Again, I’m comparing reinforced, tire compound, rubber pressure vessels and the difference between oxygen and nitrogen when they burn.
The only item on your list relevant to the flammable gasses generated by rubber at high temperature is pressure, and it works the wrong way to support your argument (the higher initial pressure of the aircraft tire will mean fewer combustible gasses generated than in a low pressure auto tire at the same temperature).
If you’ve got the physics and calculus to show the differences between the effects on a car fire of a 300psi tire burst generated by 20 atmospheres of ambient temperature air compared to one caused by 2.3 atmospheres heated to tire combustion temperatures, fly at it. I know they are identical at the instant of tire failure but I am curious how much faster the volume and pressure of the hot burst falls off as it expands and cools (however much cooling it gets to do in a car fire).
While increasing the temperature only increases the pressure of the nitrogen/oxygen inside the tire you seem to be ignoring the substantial quantities of additional volatile gasses generated by heating the rubber past 470 degrees.
My assertion was that nitrogen was safer than oxygen in an accident or a car fire. You may not agree with me, the FAA, fire and police departments, ambulance companies, race teams and major corporations (WalMart is the latest to convert their fleet) that the benefits are worth the cost, but you can’t deny the physics of feeding a dangerous fire unnecessary extra oxygen.
We’re just arguing whether fire safety is a moderate or slight (or perhaps in your view, insignificant) advantage for N2. That’s not physics, that’s just opinion - some people try to stack the deck in favor of their survival, some are happy with house odds and some go out of their way to take extra risks.
Let’s try a different tack and see if we can agree.
Given that I get N2 for free at the Costco I drive by ten times a week, can you think of even a single disadvantage of N2 (moderate, slight or insignificant - I don’t grade as hard as you), just so air doesn’t lose the comparison in a complete shut-out?
If you re-run your numbers assuming N2 leaks out at 63% the rate of oxygen you’ll find it takes longer than you think, but (if my info above on partial pressures is correct) the tires will eventually reach equilibrium at about 94% N2.
Which means anyone who insists on driving with 80/20 air will have to empty and refill their tires every few years.
I w i l l t y p e s l o w l y t h i s t i m e s o t h a t y o u c a n u n d e r s t a n d: the bead of an automotive tire (the part that makes a seal between the rubber of the tire and the metal of the wheel) will not retain 300 psi of pressure.
And I’m pointing out–again–that tires used in large aircraft landing applications see higher loads, greater temperature extremes, and are pressurized to greater internal pressure than anything seen in any passenger or commercial land vehicle application. See The Goodyear Aviation Tire Care and Maintenance Manual. Pages 33 and 51 have graphical explanations for the reading impaired.
Permit me to introduce you to the concept of conservation of mass; just because you add additional temperature to the air in the tire does not increase the molar content of oxidizer inside the tire.
If the rubber of the tire is heated “past 470 degrees” (I’ll allow the benefit of the doubt by assuming you mean degrees Fahrenheit) then your tire is probably already on fire, and is certainly undergoing pyrolitic breakdown, even if it is inflated with nitrogen, and will doubtless soon rupture. Also, an explosive rupture–one that causes volatile compounds to combust–will release at about 1000 psi. I have personally been around both flexible bladders and metallic pressure vessels that have ruptured at approximately this pressure level or higher. While it makes a very noisy bang and may produce a few fragments, it doesn’t do anything that would injure an occupant inside of a car or a bystander more than a few feet away.
That “unnecessary extra oxygen” (which is, again only 21% of the gas content of the tire) will expand essentially uniformly and either be consumed or drop to ambient pressure within milliseconds. You make it sound as if the tire is inflated with liquid oxygen.
Make sure you buy a lottery ticket, too; you don’t want to miss out on the possibility of being America’s next PowerBall winner.
Now you are just being dishonest in trying to characterize the argument in a false dichotomy. I never stated that there is any disadvantage (other than cost) to inflation with nitrogen. There is just no significant safety, longevity, or maintenance advantage over inflation with dry ambient air.
Thank you, but I don’t think I’ll go back and recalculate anything based on numbers pulled out of your ass.
Stranger
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.
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.
‘While increasing the temperature only increases the pressure of the nitrogen/oxygen inside the tire…’
Thanks kindly for the offer, but it seems we already have a passing acquaintance.
Now 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.
And that 2 cu ft of oxygen in your tires would behave just like the atmospheric oxygen feeding the fire if it weren’t pressurized and mixed with combustible gasses.
Until we develop non-flammable tires the gasses are generated regardless, all we can do is try to control the rate we burn them at.
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.
Since we couldn’t agree on the actual degree of fire safety I thought agreeing there was no good reason not to enjoy whatever degree of safety N2 provided was an honest enough compromise.
Benefits other than fire safety are whole different discussion.
That reply was actually to cooky173 to help with a missing variable in his analysis.
The source for the 63% was Consumer Reports testing (seconded by Measure for Measure and other studies) but I’ll be sure to pass along your assessment of the production quality of their nether regions if I ever get out that way.
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.
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.
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
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.
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.
This is a valid observation.
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.
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_Guidance_Library/rgFinalRule.nsf/0/e18b9cadcf1a66da86256a3100713354!OpenDocument&ExpandSection=-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.
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.
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.
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.
I think the Straight Dope sounds funnier on helium.