There’s a new trend in automotive – instead of air that is normally used to pump the tires up, somebody has initiated an idea that it must be substituted with nitrogen. Arguments behind this are various, for instance, because it is done for Formula 1 bolides, a part of heavy-duty commercial vehicles and.. a Nissan GTR supercar!
In addition to these, there are few other reasons:
- If the tire gets punctured, nitrogen flows out slower than air
- Slower tire wear-our due to more evenly distributed pressure inside the tire
- Pressure in the tire is not affected by temperature
- Risk of tire blowing is fully mitigated
- More stable pace of movement
- Improved grip with the road surface
It seems everything is perfect: so many benefits for a ridiculous amount of money. But now let’s get to the basics of physics and try to understand what exactly we are trying to compare here. Air is a substance mixture that consists of: 78% nitrogen, 21% oxygen and 1% carbon dioxide and various other components. Specialists recommend filling up the wheels with a 95% nitrogen and 5% oxygen mixture. And now let’s examine each myth individually.
Myth no 1: more even tire pressure
Nitrogen thermal expansion coefficient is much smaller than the one of air, so the tire heat-up or cooling is almost not affected by the pressure. So the argument behind this is simple: nitrogen, unlike air, is an ideal gas that does not expand at all.
But this assertion is clearly in contradiction to Charles’s Law – without going into the details (formulas and calculations), nitrogen and air expansion coefficient difference is only 0.0001 or 0.00025 atmospheres. Typical standard tire pressure is ~2 atmospheres, so you see that the difference is insignificant.
Myth no 2: tires filled with nitrogen leak slower
Nitrogen molecules are larger than air molecules, so nitrogen slower escapes the tire rubber in case of puncture.
Indeed, nitrogen molecular size is 0.364 nm, while oxygen – 0.346 nm. But again, this difference is insignificant, because if a tire loses pressure, it’s typically trough a valve or tire and rim joints. Similarly, if the tire surface is cracked from age, it will not hold its shape no matter which of both substances fills it. Maybe the trick lies in the fact that nitrogen molecules block the tire’s pores and prevents from pressure releasing out? Even if it was so, remember that air-sellers offer mixtures that contain only 17% more nitrogen than a free air does.
Myth no 3: lowers the risk of blowing
Nitrogen is an inert gas that does not burn. So, at high speeds, tire does not heat up and there is no oxygen that could ignite the fire. Let’s start with Mendeleev’s table: nitrogen is in the fifth group, but inert gases – in the eighth. However, this is not the most important. Sound we can hear, when a tire blows up, actually is caused by rupture, not explosion. To explode a tire, it must be heated up to around 1000 degrees after Celsius.
Myth no 4: fuel economy
Nitrogen filled tire, even for a 4×4 ride, is lighter than a tire filled with air. Respectively, this is supposed to reduce the load on the suspension and significantly reduce the fuel consumption. In first moment, it seems like a reasonable and logical argument. Now let’s examine the numbers.
A cubic meter of air containing 78% nitrogen, weights 1.28 kg of which 1.25kg is the weight of nitrogen. Let’s take a 165 / 70R13 size tire. Its volume is about 20 liters, overpressure – 2 kg per square centimeter, so the combined volume is 60 liters of gas. This means that the weight of nitrogen in the particular tire would be 0.0750kg, but the weight of air in the same tire – 0.0774kg. We get an astonishing saving of 2.4 grams per tire. Considering the total mass of a vehicle, this is completely negligible saving!
Myth no 5: better adhesion to road surface
Compared with air, which is usually subject to rather high temperature and pressure variations, nitrogen indeed is more stable. It’s impossible to find any argument to back up the statement above as the adhesion with road depends completely on tread rubber characteristics, tire structure, pressure distribution and, last but not least, the quality of road.
Nitrogen in auto workshops is more a fashion trend than a real breakthrough. But, as we know, fashion is not friends with science!