Nitrogen in the tires. The dew point and partial pressure.

Around how's the car running, there are so many myths. Whether in the extreme situation of harm to the stabilization system? What type of drive safer? Is it possible to include neutral when driving, is it possible to brake in a turn, whether on a slippery road, apply the engine brake? And so on, the list is endless. I like questions no longer interested in, and write about them, I was not going to. But the Internet is once again someone wrong :), and today I somehow decided to distribute a note on one of those.

And this issue is nitrogen in the tires. Why are the wheels pumped with nitrogen? I did a search on the drive, which found a huge number of posts, but no matter how much I read, no one right answer I have not seen (I'm sure they are, I just don't have the patience to Goritsa to them), and wrong is getting pretty aggressive.

All that I have mastered reading, was reduced almost literally to these texts: www.drive2.ru/l/4899916394579211090/. Be sure to read that before you continue to read my article — there are not many and quite useful.

Why do this theme proved to be so tenacious? On the one hand, "any adequate person it is obvious" that this divorce is kidding and forgot. But on the other hand, there is the Motorsport. Where the car build dozens of engineers who know about physics all. And where "pumped up wheels with nitrogen".

Here is a man at the link above great making a point. Remember even the laws of Charles and Gay-Lussac. And concludes: "In the "Formula-1" is pumped nitrogen in tires ONLY! for fire-safety. Pure nitrogen, if the damage to the wheel, leaving the bus, do not conducive to burning." That statement is incorrect.

Now to the point. In Motorsport really pumped up wheels with nitrogen. I'll tell you why.

First, it is not always nitrogen, it can be a mixture of such gases or just the dry air. Specifically, in the Formula-1 now banned by the rules to swing something other than air, and therefore pump air – keyword – drained. (By the way, as you can see, the risk of fires :) didn't prevent it to place the item in regalement.)

A simple Google search finds us an interview with Paul Hembry, head of Pirelli Motorsport, with the answer to this question.

— What gas do you use for inflating tires?
— Previously allowed the use of special gases, but now it is illegal and we pumped the tire with dry air – moisture is removed to provide the necessary stability.

But the link above we read:
Proposition 1. Increase stability of tire pressure.
– This statement contradicts the laws of physics, namely the law of Charles (the pressure of the gas at constant volume is directly proportional to temperature) and the law of Gay-Lussac (coefficient of volume expansion of all gases are the same) that we were in the 9th grade of comprehensive school.

Really the head of Pirelli Motorsport was in the ninth grade? Paul, Charles Gay-Lussac and Mr divulgane — I suggest you right now to bet, one of them "something is catching up". :)

In fact, everything is simple. The essence of the problem in contained in the gas moisture.

Indeed, "the pressure of the gas at constant volume is directly proportional to temperature" (the law of Charles sounds like a P(pressure of gas)/T(temperature)=const and correct for an ideal gas, which with a very high degree of accuracy is air with the nitrogen in the tire). That is, in this case, the higher the temperature inside the tire, the higher the pressure. So when the car starts to go and she starts to warm up the wheels – the pressure in the tires increases. Two to four tenths of an atmosphere already warmed up even before the start of the bus is gaining easy. Nitrogen is no exception to this law — even if you pump the wheels with nitrogen, the tire attains the same two or four dozens of pressure that "normal air".

No need to explain how tire pressure is important for a racing car. Moreover, it is important that pressure "hot". What pressure cold wheel – no difference, it should be correct when the wheel will go — will heat up and start to work. The problem is that if the wheel to pump moist air when cooling and heating the wheels the amount of gas in the tire will start to randomly change and will start randomly jumping pressure

In practice it looks very simple – before the start (during the collection) to the wheels of the machine put the same pressure of, say, 2.0 ATM. The car drives a few laps (or in rally advanced), pressure is measured, and it turns out that one wheel pressure 2.2, and in the other 2.4. (At this point the technicians who collected the wheels, give the brain – was violated the procedure for the collection of wheels.)

Why is this happening? There is a concept of dew point. This is the temperature to which air must be cooled below its contained water vapor reached saturation and began to condense. Dew point is determined by the relative humidity, the higher the relative humidity, the dew point is higher and closer to the actual air temperature. If the relative humidity is 100%, the dew point corresponds to the actual temperature.

That is 100% humidity means that the volume of gas more water vapor to stay – then he will begin to condense. When we pumped air into the tire, that is, raise its pressure, and then give the wheel to cool, water vapor condenseries inside the tire in water droplets. Checking the pressure in this wheel, we see, for example, 2 day. When the machine will go and the wheel will start to heat up, some of these water droplets will start to evaporate, adding additional tire gas. The additional amount of gas will add pressure (see "partial pressure" is the total pressure of gas mixture is sum of partial pressures of its components). How much water will evaporate, it is impossible to predict (as it was in the wheel when inflating?), so the pressure will no longer linearly depend on the temperature. And when the car will go – there could be 2.2 bar and 2.4 bar.

For race car this difference in pressure is dramatic.

Therefore, a large wheel sport assemble for a specific procedure. And do a pump it with nitrogen. Other teams use compressors that are air-dried.

In time attack that, where more than half of the participants own team of mechanics and wheels and assemble pump conventional pump often, even without dryer (or simple), just have to check the pressure after each session and, accordingly, blow/pump up the tire, exposing the optimal pressure until it "calms down" at the right level. If the "civilian" tyre provides the service "pumping tires with nitrogen," then enjoy it, too, is "caution". In order to pump up the wheel with nitrogen, we need to begin to blow. When deflation, that is reduction of gas pressure, it is cooled, which can also lead to condensation of some part of the moisture inside the tyre. Therefore, in professional sport to minimize the finding of moisture inside of the wheel leave it open for a few hours, then pump dry air (nitrogen) and left for a few hours again to equalize the temperature, and then put the final pressure. If such a procedure is already possible to count on the fact that the driving pressure in the tire will increase linearly, proportional to the temperature, and this increase can be predicted.

So in conclusion, does all this to a civilian car? Virtually no. Bus civilian car, almost no heat while driving. (By the way, many people think that the tires produce more heat just from the friction of the road, but in fact they primarily heat up from the brakes through the wheels, the friction of the road is the second factor, even for a racing car, and for civil wheels the second important factor is the internal friction in the tire, which arises from the deformation of the rolling wheel).

The only time the temperature changes significantly, and accordingly, the pressure in the civil wheel is if you pumped the wheel in the spring when +5 and didn't check the pressure until the summer, when it became +30... and even in direct sun left wheel. Then do some variation in the pressure can be in the wheels (although critical for civilian drive it still will not). But it's easier to just once a week to check the pressure in the wheels, than to bother with nitrogen or dry air.

P. S. All of the above honestly "at loggerheads". I absolutely do not claim authorship of this explanation, exactly the opposite is what everybody knows a good mechanic, engineer, pilot, etc., and lit more than once, but for some reason completely unknown to civilian drivers. Now let it be and drive.