Few controversies in the automotive world have raged as fiercely, and for as long as the debate about the pros and cons of filling car tyres with nitrogen gas has. On the one hand, there are many online resources perpetuating half-truths, distortions, and downright misleading information, while on the other hand, many users report increased tyre life, improved handling, and measurable fuel savings after converting to nitrogen in their tyres. Whom to believe? In this article, we will take a closer look at the practice of filling car tyres with nitrogen, and also discuss some of the pros and cons of using nitrogen, starting with this question-
In gaseous form, nitrogen is a colourless, tasteless, and odourless non-reactive gas that accounts for about 78% of the volume of Earth's atmosphere. It is also the seventh-most abundant element in both the Milky Way Galaxy and the Solar System, and it occurs naturally in all living organisms; the human body contains about 3% nitrogen, making it the fourth most abundant element in humans after oxygen, carbon, and hydrogen.
Depending on your point of view, the above may or may not be interesting or relevant, but it does beg this question-
It is somewhat less than clear exactly when, where, and how the practice started, but advocates of the practice are at some pains to point out that since nitrogen is used in (amongst other applications) heavy aircraft tyres, it must be a good idea to fill car tyres with it, too. However, there are very good reasons why nitrogen, and not atmospheric air, is used to inflate aircraft tyres.
Atmospheric air contains about 20% oxygen and other gasses that can all contain varying concentrations of water vapour, which means that at the altitudes most commercial aircraft fly at, the water vapour in their tyres can, and does freeze. This can, and does, produce large imbalances in aircraft wheels. In practice, these imbalances can be large enough to cause aircraft tyres to burst upon (or soon after) landing, so for commercial aviation applications, it makes good sense to inflate aircraft tyres with pure nitrogen, since all water vapour is excluded from aircraft tyres.
In practice though, passenger vehicles and their tyres are never operated at temperatures as low as -300C to -400C, which is the range of temperatures commercial airliners operate in for hours at a time. So, what other claims do proponents of filing car tyres with nitrogen make? Let us look at a few of the more common claims-
While this is true, it is so only up to a point and moreover, this argument presupposes that the tyre bead/rim seal is perfect, that the valve does not allow any nitrogen to escape, and that the tyre itself is perfect and free of pinhole leaks.
When compared to oxygen molecules (that comprise the bulk of atmospheric air after nitrogen), nitrogen molecules are 2.7% larger than oxygen molecules, which means that nitrogen molecules find it harder to escape through the rubber of a tyre as compared to oxygen molecules. To prove this theory, Consumer Reports, an authoritative consumer protection organisation, filled some tyres with nitrogen, and an equal number of tyres with atmospheric air.
Both sets of tyres were then stored, and checked for pressure loss after a year. The results were as follows: the tyres that were filed with nitrogen lost an average of 0.002896 bar per week, while the air-filled tyres lost an average of 0.004619 bar per week. In practice, this means that the pressure loss in the air-filled tyres came to 0.068948 bar every 15 weeks, while the nitrogen-filled tyres lost an equal amount of pressure every 24 weeks.
The con
This is a pretty insignificant difference by any standard, but the problem with this is that based on this observed rate of pressure loss, many advocates of nitrogen are telling people that they only need to check their tyre pressures once every five or six months, which is ridiculous, if not downright dangerous, to say the least.
As technicians, we all know that the vast majority of our customers drive on tyres that are chronically underinflated, and sometimes dangerously so, despite the presence of tyre pressure monitoring systems on many vehicles. There are many reasons for this, including incorrect readings from notoriously inaccurate tyre pressure monitoring systems, poorly calibrated garage forecourt tyre pressure gauges; slow leaks in tyres and valves, and people that simply neglect to check their tyre pressures even on a semi-regular basis.
Tyres lose pressure for a variety of reasons, and under real-world conditions, the use of nitrogen in tyres offers a motorist no protection against pressure losses caused by leaking valves, punctures by nails, screws, and other road debris, or structural damage to tyres caused by hitting potholes and other obstacles.
Add to this the fact that if a motorist is under the false impression that his tyres can never deflate simply because they are filled with nitrogen, that motorist may allow his tyres to deflate to the point where tyre failure becomes almost inevitable, with possibly fatal consequences for all involved in the resulting accident.
This is also true, but again, only up to a point and only under conditions and in applications that normal passenger vehicle tyres will never experience.
The cooler-tyre argument goes something like this: since nitrogen does not expand as much as atmospheric air (due to the high oxygen content of atmospheric air), the thermal stability of nitrogen means that pressure fluctuations, and hence temperature fluctuations in tyres are eliminated, thus making tyres run cooler. Moreover, this claim often includes the statement that since nitrogen-filled tyres are thermally stable, tyre life can be increased by as much as 50%.
Note that the 50% increase in tyre life applies mostly to heavy vehicle and construction equipment tyres, since they can be repaired and/or retreaded more often if they can be prevented from running hot due to under inflation. As such, the increase does not apply to passenger vehicle tyres.
The con-
The cooler-tyre argument is based on the fact that since race cars (NASCAR cars, amongst others) use nitrogen in their tyres, this fact translates into definitive proof that passenger car tyres that are filled with nitrogen will also be as thermally stable as the tyres on race cars are.
However, the fact is that the specific heat* of nitrogen differs by only 4% vs. that of atmospheric air. Thus, assuming that a NASCAR racer with air-filled tyres starts a race with tyre pressures of 2.206322 bar, and a temperature of 200C, its tyres will have a pressure of 2.356904 bar when they reach a temperature of 400C.
* Specific heat is defined as the amount of heat energy that is required to increase the temperature of a substance by one degree Celsius.
Bear in mind that the above example assumes that no pressure losses occur due to leaks, and that the heat input parameters stay the same, i.e., average speed around the track remains constant, braking occurs at exactly the same point and at the same intensity each time, and that the car always follows exactly the same line through each curve.
Now, if we fill the tyres with nitrogen and repeat the race under the same conditions and parameters, the 4% difference in specific heat between atmospheric air and nitrogen means that the nitrogen-filled tyres will end the race at temperatures of 39.20C, and pressures of 2.350836 bar, which represents a pressure difference of 0.2574536%.
While this small difference may be of some import on racetracks, the above example cannot be applied to passenger vehicles because the heat inputs on the tyres of passenger vehicles are never constant. Moreover, since no passenger vehicle tyre is specifically designed to be filled with nitrogen, the small, almost negligible advantage in terms of thermal stability that nitrogen brings is of no consequence to the average driver.
The argument goes that since nitrogen contains neither water vapour nor oxygen, rims cannot rust, and tyres cannot sustain oxidative damage by the oxygen in atmospheric air.
The con-
The fact is that rims are protected against corrosion by a layer of extremely tough paint in the case of steel rims, and a layer of clear lacquer in the case of alloy rims. To prove how effective these layers of paint are against corrosion, try to remember when last you saw evidence of rust and corrosion on the inside of any type of rim when you dismounted a tyre.
Additionally, most proponents of nitrogen often fail to mention that to prevent damage to tyres by atmospheric oxygen, all the atmospheric air must be purged from the tyre prior to filling with nitrogen, and that the nitrogen has to be 100% pure, which is neither possible, nor available to the automotive trade. In fact, most nitrogen that is available in tanks, or, nitrogen that can be generated with nitrogen generators varies in purity from about 93% (and often considerably less) to about 96% at best. Therefore, unless all the atmospheric air is purged from a tyre, there will always be some water vapour in any tyre that is inflated with nitrogen.
Nonetheless, as a practical matter, the purity (or otherwise) of nitrogen that makes it into tyres hardly matters in any meaningful way. The point is this; the inside of all high-quality tyres are treated with advanced coatings to both resist the effects of oxygen on rubber, and to limit the flux of oxygen molecules to the outside of the tyre through the thinnest parts of a tyre, which are the sidewalls.
Also consider this fact: even though the outsides of tyres are not protected against attack by the 20% oxygen component of atmospheric air, industrial pollutants, and/or road grime in any way, most tyres seem to cope just fine for the four years or so it takes for them to wear down to the point where they are replaced, anyway.
This is one of the most oft-repeated claims made by proponents of nitrogen in tyres, but unlike some other claims that are partially true, there is no truth in this one.
The con-
There are many factors that influence and determine how a car handles in any given situation, including the road surface, road speed, suspension characteristics and design, the mass of the vehicle, the skill (or lack thereof) of the driver, and many more, as we know.
Nevertheless, as far as the role tyres play in all of this goes, the most important aspects are the size of the contact patch of the tyre as a function of tyre design and construction, tyre width, aspect ratio, diameter, circumference, but most importantly, the inflation pressure of the tyre.
In terms of practicalities, it makes no difference to a tyre if it is filled with nitrogen, atmospheric air, or within reason, any other gas. Provided the tyre is inflated to its recommended inflation pressure, is properly aligned and not worn unevenly it will perform as intended, regardless of the gas that is in it. In fact, a nitrogen-filled tyre that is underinflated is just as dangerous as a tyre that is underinflated with atmospheric air, and the mere fact that a tyre contains nitrogen offers a driver no protection at all against loss of control, or tyre failure as the result of the tyre’s underinflated state.
This statement is also true, but only up to a point, and only under laboratory conditions. The argument goes that since nitrogen is lighter than air, a vehicle with nitrogen in its tyres has a lower unsprung weight, and it will therefore consume less fuel.
The con-
The fact is that while nitrogen is indeed lighter than atmospheric air, the mass difference amounts to less than two percent, which is nothing to get excited about. Let us put this into perspective-
An empty tyre that is not under a vacuum contains atmospheric air at a pressure that is equal to the ambient atmospheric pressure. Thus, if we add two atmospheric pressures to the tyre, we have three atmospheric pressures’ worth of air in the tyre, which, depending on the water content of the air charge, weighs about 76.5 grams if we assume that the tyre has a volume of 20 litres. By way of comparison, the volume of nitrogen required to bring the same tyre up to the same pressure weighs only 75 grams.
In practice then, the atmospheric air in four of our example tyres will weigh a total of 306 grams, while the same volume of nitrogen weighs just 300 grams; a difference of 6 grams, which is extremely unlikely to produce measureable improvements in fuel consumption on any vehicle.
When all of the above is viewed objectively, it should be obvious that the practice of filling passenger vehicle tyres with nitrogen offers no significant advantages, and could in fact, encourage some drivers to neglect the checking of their tyre pressures even more than they are doing now.
One other disadvantage involves the high cost of nitrogen tyre fills, which could run to about $50 (or sometimes more) for four tyres. Given that there are no tangible benefits to filling tyres with nitrogen, this is perhaps the biggest drawback of nitrogen of all, since in most locations compressed atmospheric can be had either free of charge, or for a nominal fee.
